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Nanostructured LiMnO2 with Li3PO4 Integrated at the Atomic Scale for High-Energy Electrode Materials with Reversible Anionic Redox

  • Miho Sawamura
    Miho Sawamura
    Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan
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    Sho Kobayakawa
    Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan
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    Jun Kikkawa
    National Institute for Materials Science (NIMS), Namiki, Tsukuba, Ibaraki 305-0044, Japan
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    School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia
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    Mark Wainwright Analytical Centre, UNSW Sydney, Sydney, NSW 2052, Australia
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    Nanaka Shimada
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
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    Kentaro Yamamoto
    Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
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    Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
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    Yingying Zhou
    Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
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    Yoshiharu Uchimoto
    Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
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    SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
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    Kei Mitsuhara
    SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
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  • Koji Ohara
    Koji Ohara
    Diffraction and Scattering Division, Center for Synchrotron Radiation Research, Japan Synchrotron Radiation Research Institute (JASRI, SPring-8), Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
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    Jiwon Park
    Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) and KAIST Institute for NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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    Orcidhttp://orcid.org/0000-0002-2258-310X
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    Hye Ryung Byon
    Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) and KAIST Institute for NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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    Orcidhttp://orcid.org/0000-0003-3692-6713
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  • Hiroaki Koga
    Hiroaki Koga
    Research Organization for Information Science and Technology (RIST), 1-18-16 Hamamatsucho, Minato-ku, Tokyo 105-0013, Japan
    Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
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  • Masaki Okoshi
    Masaki Okoshi
    Research Organization for Information Science and Technology (RIST), 1-18-16 Hamamatsucho, Minato-ku, Tokyo 105-0013, Japan
    Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
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    Toshiaki Ohta
    SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
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  • Naoaki Yabuuchi*
    Naoaki Yabuuchi
    Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
    Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
    Advanced Chemical Energy Research Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
    *E-mail: [email protected]
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    Orcidhttp://orcid.org/0000-0002-9404-5693
Cite this: ACS Cent. Sci. 2020, 6, 12, 2326–2338
Publication Date (Web):December 15, 2020
https://doi.org/10.1021/acscentsci.0c01200

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Abstract

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Nanostructured LiMnO2 integrated with Li3PO4 was successfully synthesized by the mechanical milling route and examined as a new series of positive electrode materials for rechargeable lithium batteries. Although uniform mixing at the atomic scale between LiMnO2 and Li3PO4 was not anticipated because of the noncompatibility of crystal structures for both phases, our study reveals that phosphorus ions with excess lithium ions dissolve into nanosize crystalline LiMnO2 as first evidenced by elemental mapping using STEM-EELS combined with total X-ray scattering, solid-state NMR spectroscopy, and a theoretical ab initio study. The integrated phase features a low-crystallinity metastable phase with a unique nanostructure; the phosphorus ion located at the tetrahedral site shares faces with adjacent lithium ions at slightly distorted octahedral sites. This phase delivers a large reversible capacity of ∼320 mA h g–1 as a high-energy positive electrode material in Li cells. The large reversible capacity originated from the contribution from the anionic redox of oxygen coupled with the cationic redox of Mn ions, as evidenced by operando soft XAS spectroscopy, and the superior reversibility of the anionic redox and the suppression of oxygen loss were also found by online electrochemical mass spectroscopy. The improved reversibility of the anionic redox originates from the presence of phosphorus ions associated with the suppression of oxygen dimerization, as supported by a theoretical study. From these results, the mechanistic foundations of nanostructured high-capacity positive electrode materials were established, and further chemical and physical optimization may lead to the development of next-generation electrochemical devices.

Synopsis

Li3PO4 is successfully integrated into rocksalt LiMnO2. The integrated phase delivers a large reversible capacity with anionic redox, which is used for high-energy electrode materials of Li-ion batteries.

Introduction

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Lithium insertion materials, which are electronic or ionic conductors, are indispensable to rechargeable lithium batteries extensively used in our daily life. The understanding of the chemistry of lithium insertion materials has rapidly developed in the past four decades, (1) and the technological progress on rechargeable lithium batteries has resulted in the electrific automobile applications. For electric vehicle applications, Ni/Co-containing layered oxides are the dominant positive electrode material, coupled with graphite as a negative electrode material. (2) Critically, the market for electric vehicles is rapidly growing worldwide, and this expansion is likely to result in the shortage of Co resources. (3) Therefore, high-energy and high-performance positive electrode materials without Co ions (and preferably without Ni ions as well) are required. In the past decade, Li2MnO3-based electrode materials have been extensively studied as potential high-capacity positive electrode materials. (4−7) Compared with other layered materials such as LiMO2, where M are transition metals, Li2MnO3-based electrodes feature an enriched lithium content that results in higher theoretical capacities (at both the electrode and battery level) because the amount of the extractable Li content is larger. Although pure Li2MnO3 is electrochemically less active (or inactive), solid-solution samples such as Li1.2Co0.13Ni0.13Mn0.54O2 deliver large reversible capacities as electrode materials. (8) Charge compensation at high levels of lithium extraction has been demonstrated by the reversible oxidation of oxide ions in addition to the conventional redox of transition metal ions. (9) However, a continuous phase transition associated with the formation of trivalent Mn ions, presumably coupled with oxygen loss on the charge, restricts its use for practical applications, (7,10) especially in such rigid layered structures.
Redox reactions of such anionic species dramatically change the requirements of the host materials and hence the material design strategies for energy storage applications. (11−13) New lithium-excess and high-capacity positive electrode materials have appeared in the past several years. (14−17) Among them, binary systems with LiMnO2 have been proposed as a new class of electrode materials, including Li3NbO4–LiMnO2, (18) Li2TiO3–LiMnO2, (7) and LiF–LiMnO2, (16,19) and these Li-excess electrode materials effectively utilize the anionic redox coupled with the cationic redox of Mn3+/Mn4+. In these cases, the end-members of Li3NbO4, Li2TiO3, and LiF are known as electrochemically inactive because of the absence of conductive electrons. Nevertheless, the crystal structures of these compounds are classified as (cation-ordered) rocksalt-type structures and are considered to feature a structural compatibility with LiMnO2, which possesses a cubic close packed (ccp) oxygen lattice. These binary systems, indeed experimentally, crystallize into cation- or anion-disordered rocksalt phases as solid solution samples of both end-members. Consequently, Li ions in these electrochemically inactive compounds are activated with anionic redox. (11,20)
In this study, Li3PO4 with a hexagonal close-packed oxygen lattice is targeted for the end member of the binary system with LiMnO2. Unlike previous studies, LiMnO2 (21) and Li3PO4 (22) feature different oxygen packings, with Li and Mn ions located at octahedral sites in LiMnO2 compared to the Li and P ions located at tetrahedral sites in Li3PO4. Therefore, the formation of a thermodynamically stable solid solution between the end members is an implausible scenario. Interestingly, a lower molar mass of Li3PO4 is advantageous in increasing the electrode theoretical capacity, and the cationic and anionic redox electrochemical potentials are expected to increase due to the inductive effect of PO43– ions. (23) Although synthesis by conventional calcination failed, presumably from the structural incompatibility of the phases, we demonstrate the formation of integrated phases of LiMnO2 and Li3PO4 that were obtained by a mechanical milling route. The milling route allows the synthesis of thermodynamically metastable phases using mechanical energy, including friction and shear stress on mixing. (24−27) The incorporation of phosphorus ions to rocksalt LiMnO2 is clearly evidenced by a combined analysis with STEM-EELS, total X-ray scattering, solid-state NMR, and theoretical ab initio studies. Moreover, anionic redox is activated in this system, leading to large reversible capacities with a higher operating voltage. From these results, the possibility of a new series of high-capacity metastable electrode materials, which are integrated at the atomic-to-nano scale, are discussed.

Results and Discussion

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Synthesis of Materials and Detailed Phase Characterization

Figure 1a shows X-ray diffraction patterns of mixtures of LiMnO2, Li3PO4, and various compositions in (1 – x)LiMnO2·xLi3PO4 after mechanical milling. After mechanical milling, the well-defined diffraction pattern of LiMnO2 (x = 0) with a zigzag-type layered structure is completely lost, and low crystallinity phases with cation-disordered rocksalt structures are observed, consistent with our previous report. (28) In contrast, no change in the crystal structure for Li3PO4 (x = 1) was observed after milling under the same experimental conditions; however there was a noticeable decrease in the crystallinity. For the binary system, where x = 0.1 and 0.2, the diffraction pattern of Li3PO4 was completely lost, and a cation disordered rocksalt phase formed. The phase evolution process of x = 0.2 with different milling times, which was reformulated as Li7/6P1/6Mn2/3O2, is shown in Supporting Figure S1. After milling, TEM analysis revealed that a primary particle size reduced into less than 20 nm (Figure 1b and Supporting Figure S2), consistent with the observation of broad reflections in the diffraction patterns. Moreover, such nanoparticles are tightly agglomerated around each other, forming relatively larger secondary particles. Therefore, nanosized samples prepared by mechanical milling generally have a relatively small surface area. (29) Note that a uniform distribution of Mn and P is evidenced from scanning transmission electron microscopy (STEM) coupled with elemental mapping by energy dispersive X-ray spectroscopy (EDX), as shown in Supporting Figure S2. Nevertheless, when the fraction of Li3PO4 increases to x = 0.3, a reflection characteristic of Li3PO4 with low-crystallinity at 2θ = 22° appears. Phase segregation was also observed by EDX mapping, as shown in Supporting Figure S3. These results clearly indicate that the “solubility” of Li3PO4 in LiMnO2 is limited in a relatively narrow range using the milling method.

Figure 1

Figure 1. Characterization of a binary system of (1 – x)LiMnO2·xLi3PO4. (a) XRD patterns of LiMnO2 and Li3PO4 before and after mechanical milling as well as of mixtures of LiMnO2 and Li3PO4 with different compositions, (1 – x)LiMnO2·xLi3PO4, after mechanical milling. Schematic illustrations of crystal structures were drawn using the VESTA program. (59) (b) Phosphorus and manganese distributions obtained by STEM-EELS elemental mapping; a STEM image of the sample x = 0.2 in (1 – x)LiMnO2·xLi3PO4 is also shown. Observed P L-edge EELS spectra of points α and β are also shown. Averaged and full EELS spectra of the sample and the data for Li3PO4 after milling are also shown for comparison. (c) Clear lattice fringes can be seen in a high-resolution TEM image (also see Supporting Figure S7b).

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Although “apparently” single-phase samples for x = 0.1 and 0.2 were successfully obtained by mechanical milling, the detailed structure of the binary phase is still not clear. To further examine the crystal structures in the LiMnO2–Li3PO4 binary system, cation distributions in the crystalline phase were determined using Rietveld analysis. Earlier work by Hayashi and co-workers on electrodes for solid-state batteries examined mechanical milling in the LiCoO2 and Li2SO4 binary system. (24) They proposed that nanosize LiCoO2 was embedded in an amorphous matrix of Li2SO4, forming a nanocomposite. Further work associated with solid-state batteries examined a sputtered thin amorphous layer of Li3PO4 on LiNiO2. (30) A question arises; amorphous Li3PO4 cannot be obtained by mechanical milling, as shown in Figure 1a, and crystalline Li3PO4 only shows a reduction in the crystallinity with milling. Notably, synchrotron XRD data show a higher crystallinity for the rocksalt phase for x = 0.2 when compared with that of x = 0 without Li3PO4 (Supporting Figure S4). Therefore, the possibility of PO43– incorporation into crystalline domains with Mn ions cannot be eliminated.
To validate this hypothesis, a detailed structure of nanosize and rocksalt LiMnO2 without phosphorus ions was first analyzed. An ideal rocksalt model in which both cations, Li and Mn ions, are located at the same octahedral 4a site is not suitable (Supporting Figure S5) as the intensity of the 111 reflection at 2θ = 12° is unmatched. The March distribution function was used along the 111 direction, resulting in a significantly better fit. Note that this model contains a preferred orientation term. For Li7/6P1/6Mn2/3O2, phosphorus ions were modeled at tetrahedral 8c sites with the preferred orientation term, and a good fitting result was obtained (Supporting Figure S5). The lattice parameter of the sample increased from a = 4.156 to 4.181 Å by phosphorus integration. This model assumes the formation of an atomic incorporation of phosphorus ions into LiMnO2 and a solid-solution-like phase, Li1+xPxMn1–2xO2. It should be noted that this is likely a metastable phase with enriched structural defects, and is therefore segregated into LiMnO2 and Li3PO4 upon heating (Supporting Figure S6a); however, the metastable phase is stable after one-year storage in the glovebox at room temperature (Supporting Figure S6b). In the solid-solution model, a large repulsive electrostatic interaction is unavoidable for face-shared sites between octahedral and tetrahedral sites (discussed later).
To clarify the structural analysis, detailed nanostructures were analyzed by STEM combined with electron energy loss spectroscopy (EELS), with atomic distributions observed with a 1.5 nm spatial resolution. Typical EELS spectra of the P L-edge are also shown in Figure 1b, and Mn L-edge and Li K-edge spectra are shown in Supporting Figures 7a; atomic distributions of P and Mn ions obtained from the intensity in the EELS spectra are also shown in Figure 1b. EELS analysis reveals that P ions are well integrated with Mn ions at the nanoscale, and uniform distributions of cationic species are also noted. The major phase appears to be a nanosize crystalline phase with a size of less than 5 nm (Figure 1b and Supporting Figure S7b), and the XRD and TEM observations and data are inconsistent with the scenario of the formation of amorphous Li3PO4 and a composite structure with crystalline LiMnO2. The redistribution of both cationic species clearly proceeds by mechanical milling in this binary system. Surface conversion reactions were also proposed for the nanocomposite of LiF and MnO, (31) but a more uniform distribution of the cationic species is noted in the binary system of LiMnO2–Li3PO4. In addition, the incorporation of P ions into the crystalline LiMnO2 phase is further supported from the profile of P L-edge EELS spectra. P L-edge EELS spectra collected from different spots are clearly different from those of Li3PO4 (Figure 1b), with a small but notable energy shift in the spectra relative to Li3PO4. A similar trend is also noted in the Li K-edge and Mn L-edge EELS spectra (Supporting Figure S7a).
The incorporation of P ions into the crystalline LiMnO2 phase is further evidenced by total X-ray scattering measurements, with patterns in reciprocal space for different samples shown in Supporting Figure S8. The reduced pair distribution functions (PDFs) for rocksalt LiMnO2 and Li7/6P1/6Mn2/3O2 are shown in Figure 2a. Although both samples show similar PDFs in the r-range over 3 Å, a clear difference is visible in first and second peaks at 1.6 and 2.0 Å, respectively. The intensity of the 2.0 Å peak originating from Mn–O bonds decreases, while that of the 1.6 Å peak increases; this originates from P–O bonds, as clearly found in reference PDF data for Li3PO4. Note that no change in the PDF data before and after milling was observed for Li3PO4 (Supporting Figure S9), indicating that amorphous Li3PO4 was not formed by mechanical milling. Li7/6P1/6Mn2/3O2 is a metastable phase (Supporting Figure S6a) that transforms to zigzag LiMnO2 and Li3PO4 during heating to 550 °C, as is clearly observed in the PDF by taking the sum of the data for LiMnO2 and Li3PO4 (Figure 2a). The clustering of PO43– ions in the 550 °C heated structure clearly influences the profile of the PDF data in the wide r-range (Figure 2a) and is in contrast with milled Li7/6P1/6Mn2/3O2 and rocksalt LiMnO2, where the wide r-range data essentially overlap. This is strong evidence that phosphorus ions are nonclustered and diluted to rocksalt LiMnO2 in the milled Li7/6P1/6Mn2/3O2. A solid solution was formed rather than clusters.

Figure 2

Figure 2. Analysis of local structures for the as-prepared sample x = 0.2, Li7/6P1/6Mn2/3O2. (a) Experimental X-ray pair distribution function (PDF), (b) Raman spectra, and (c) solid-state NMR spectra. The data for the reference materials are also shown. From the all data, the clustering of phosphorus ions is not seen for Li7/6P1/6Mn2/3O2, indicating that phosphorus is diluted to crystalline rocksalt LiMnO2.

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These observations are also supported by Raman and solid-state NMR spectroscopy. Figure 2b compares the Raman spectra of LiMnO2, Li3PO4, and Li7/6P1/6Mn2/3O2. Two clearly resolved peaks were observed for the sample after milling. The peak at 623 cm–1 is assigned as the vibration of the Mn–O bonds in the MnO6 octahedra, and a similar peak is also visible for rocksalt LiMnO2 at 614 cm–1. (28) In the binary system, a minute energy shift (wavenumber) compared with that of rocksalt LiMnO2 was noted. In addition, another peak at 930 cm–1 is present, originating from the symmetric stretching mode of PO4 tetrahedra. However, a clear shift in the wavenumber was also compared with the spectrum of pure Li3PO4 (955 cm–1). Note that the peak shift to 930 cm–1 wasis observed for the high-temperature phase of Li3PO4, originating from differences in the P–O interatomic distances (1.55 Å for the low-temperature phase and 1.86 Å for the high-temperature phase). (32) The P–O distance for Li7/6P1/6Mn2/3O2 was calculated to be 1.81 Å, assuming that phosphorus ions were located at tetrahedral sites in the ccp lattice (also see Supporting Figure S12a). The P–O interatomic distance is almost the same with that of the high-temperature phase of Li3PO4; therefore, the Raman peak shift for Li7/6P1/6Mn2/3O2 is also consistent with the solid solution model. In addition, the increase of the P–O distance after mixing with LiMnO2 was also found for the PDF data (Figure 2a). Such differences in the local environments are further supported by solid-state NMR spectroscopy. 7Li and 31P NMR spectra collected, including reference materials, are shown in Supporting Figure S10. There is a positive chemical shift (36 ppm) in the 7Li NMR spectra for zigzag-type LiMnO2, originating from the interaction of electrons in Mn3+ ions via oxygen (Li–O–Mn bonding). (33) After milling, a small chemical shift for the 7Li NMR spectra was noted (Figure 2c). In the disordered rocksalt structure, a number of different local environments for Li ions are created; therefore, the total spin density transferred to Li ions from Mn ions may be canceled out. Note that a broad peak centered at 36 ppm is also present, which may originate from the presence of defects in the structure that resemble LiMnO2. No difference in the chemical shift was noted for Li3PO4 for 7Li and 31P NMR spectra before and after milling, as shown in Figure 2c and Supporting Figure S11, respectively. This is also consistent with the data obtained by Raman scattering (Supporting Figure S12b). Peak broadening was observed for Li3PO4 after milling, probably associated with the lowered crystallinity by milling. For Li7/6P1/6Mn2/3O2, the chemical shift of the 7Li NMR spectra is the same as that of rocksalt LiMnO2; however, there is a modification of chemical shift in 31P NMR spectra, indicating that there is interaction of the PO4 tetrahedra with Mn ions with d electrons. These results are also consistent with the observation from PDF analysis, EELS spectra, and Raman spectra, and it was concluded that PO43– ions are nonclustered for Li7/6P1/6Mn2/3O2 and successfully dissolved into LiMnO2.

Electrochemistry

Electrochemical properties of the binary system of LiMnO2–Li3PO4 as metastable phases were examined as positive electrode materials in Li cells. Galvanostatic charge–discharge curves of the binary system are shown in Figure 3. Rocksalt LiMnO2 shows ca. 90% of the theoretical capacity (286 mA h g–1) calculated from the Li content and Mn3+/Mn4+ redox. (28) Reversible capacities of the binary system increase relative to the parent with the addition of Li3PO4. The highest reversible capacity iwas obtained for the x = 0.1 sample of over 300 mA h g–1. The reversible capacity was slightly reduced for x = 0.2 (Li7/6P1/6Mn2/3O2). However, the reversible capacity reached nearly 300 mA h g–1, which clearly exceeds the theoretical capacity based on Mn3+/Mn4+ redox (218 mA h g–1). This fact suggests that anionic redox is activated in this system, which also supports the successful integration of LiMnO2 and Li3PO4. Li ions, which are originally found in the structure of Li3PO4, are effectively extracted by the electrochemical oxidation of oxide ions for the integrated phase (anionic redox in this system is discussed further in a later section). The initial discharge capacity is slightly larger than that of the initial charge capacity. A similar trend was also observed for nanosized samples prepared by mechanical milling, originating from the partial oxidation of the sample during milling (also see the Supporting Information). (27) Interestingly, the further enrichment of Li3PO4 in the binary system results in the reduction of the reversible capacities. This observation is also consistent with the results obtained by XRD, where phase segregation of Li3PO4 wasis observed for x = 0.3. Li ions cannot be extracted from the segregated phase because Li3PO4 is an electronic insulator. Nevertheless, the well-integrated phase, x = 0.2, shows a relatively good rate capability and delivers 190 mA h g–1 at a rate of 640 mA g–1, (Figure 3b), which is significantly better than those of rocksalt LiMnO2 prepared by mechanical milling without Li3PO4. (28) Electrochemical properties of the sample of x = 0.2 were further tested with different cutoff voltages in Li cells. Initial discharge capacities are shown in Figure 3c, and charge–discharge curves and capacity retention are also compared in Supporting Figure S13. The discharge capacity increased to ca. 320 mA h g–1 with a 5.2 V cutoff. However, cyclability as electrode materials is not acceptable, and nearly 50% of the reversible capacity was lost after 30 cycles. In contrast, good capacity retention is observed with a 4.5 V cutoff, and the sample retainsed 80% of the reversible capacity. Although the reversible capacity wasis reduced to 180 mA h g–1, excellent capacity retention was achieved with a 4.1 V cutoff. Note that the reversible capacities and capacity retention are also significantly influenced by the operating temperature (Supporting Figure S14). A large reversible capacity of 320 mA h g–1 was observed at 50 °C, but a deterioration of the capacity retention was observed. In contrast, good capacity retention wasis realized at 0 °C even though the reversible capacity was reduced to 235 mA h g–1. This observation suggests that irreversible oxygen loss occurs on electrochemical cycles and is accelerated at elevated temperatures, as observed in Li2MnO3-based electrode materials. (7)

Figure 3

Figure 3. Electrochemical properties of the (1 – x)LiMnO2·xLi3PO4 binary system. (a) Galvanostatic charge–discharge curves of the samples with different compositions prepared by mechanical milling, (b) rate-capability of the sample x = 0.2, Li7/6P1/6Mn2/3O2, and (c) dependency of cutoff voltages on eversible capacities and capacity retention. The electrode loading was ca. 6.0–6.5 mg cm–2 for these electrochemical tests.

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Charge Compensation Mechanisms

Reaction mechanisms of the binary system of LiMnO2–Li3PO4 as electrode materials were examined by synchrotron X-ray diffraction (SXRD) combined with hard–soft X-ray absorption spectroscopy (XAS) and online gas analysis focusing on x = 0.2, Li7/6–yP1/6Mn2/3O2. On charge, a clear shift of the reflection positions and a contraction of the crystal lattice were noted as shown in Figure 4a, indicating that a topotactic reaction proceeds. All the Bragg diffraction lines continuously shift to higher angles on charge to 220 mA h g–1 (Li extraction), and the lattice parameters change from 4.178 to 4.061 Å. However, only a minor change in the reflection positions and the lattice parameter were noted for the further charge to 4.8 V (i.e., similar lattice parameters were noted for the fully charged sample and the sample charged to 220 mA h g–1). On discharge, reflection broadening was clearly observed, and the lattice parameters of the fully discharged sample were much larger than those of the as-prepared sample, suggesting that irreversible reactions occur on electrochemical cycles (at least for the first cycle). The origin of said degradation is further discussed in a later section.

Figure 4

Figure 4. Reaction mechanisms of Li7/6–yP1/6Mn2/3O2. (a) Changes in synchrotron XRD patterns. (b and c) Charge compensation processes observed by an operando soft XAS study shown by (b) a schematic illustration of the experimental setup, (34) and (c) variations in the Mn L-edge and O K-edge XAS spectra. (d and e) In situ partial gas pressures of CO2, H2, and O2 and the corresponding charge curves from nanosized Li2MnO3 and Li7/6P1/6Mn2/3O2, respectively. The dashed line is the baseline of the gas pressure, and the filled area indicates the change of the partial gas pressure through the gas evolution.

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To understand the charge compensation process, soft X-ray absorption spectroscopy was applied to study changes in electronic structures based on previous work indicating oxygen redox activity for Li-excess electrode materials using this technique. (15) However, ex-situ soft XAS spectroscopy requires high-vacuum conditions that, when coupled with possible extrinsic changes induced in the samples during washing, drying, and long exposure to vacuum, can provide misleading information. Therefore, operando soft X-ray absorption spectroscopy was developed (34,35) and utilized in this study (Figure 4b). Instead of a conventional carbonate electrolyte, lithium bis(trifluoromethanesulfonyl)amide (TFSA) dissolved in acetonitrile was utilized as the electrolyte to reduce X-ray absorption. Although TFSA anions contain oxygen in the structure, the major absorption was observed above 533.5 eV, as shown in Supporting Figure S15a; therefore, changes in O K-edge XAS spectra near pre-edge regions from the electrode materials could be clearly detected. Aluminum-sputtered Si3N4 was used as windows to allow the transmittance of the synchrotron X-rays. Acetonitrile is reactive with metallic Li; therefore, a delithiated phase of lithium iron phosphate, Li0.5FePO4, was used as a counter electrode. A charge curve taken during the operando measurement is shown in Supporting Figure S15b. The irreversible capacity was relatively large, probably originating from electrolyte decomposition. Therefore, the chemical composition and amount of extracted Li ions (equaling the charge capacity) were estimated and calibrated by comparing the cutoff voltages. A clear shift of the O K-edge spectra was noted on charge, as shown in Figure 4c, and a similar trend was observed with the ex situ study shown in Supporting Figure S16. After charging to 110 mA h g–1 (corresponding to 4 V vs Li+/Li), Mn was in the tetravalent state, and no further change was observed beyond the 4 V region for the oxidation state of Mn ions. Pronounced changes in the O K-edge XAS spectra were subsequently noted, and peak intensity around 532 eV increased as the charge capacity and voltage increased. A similar trend is noted with other Li-excess materials with Mn ions. (7) This peak is proposed to originate from the hole creation and stabilization by a π-type interaction with Mn t2g orbital. (36) The appearance of a clear peak at 532 eV is noted for Li1.2–yTi0.4Mn0.4O2, (15) suggesting that the stability of the holes created by electrochemical oxidation is energetically less stable for the nanosize samples prepared by mechanical milling. This finding is also consistent with the non-negligible deterioration of cyclability after charging to 5.2 V and at elevated temperatures (Supporting Figures S13 and S14). Significantly better capacity retention was achieved with a 4.5 V cutoff as shown in Figure 3c. The changes in electronic structures of phosphorus ions were also studied, and P K-edge XAS spectra were collected, as shown in Supporting Figure S16d. A gradual shift of the spectra to a higher energy region was noted on the charge process. A similar phenomenon was also observed for Li1–xFePO4, (37) and the peak shift was expected to originate from changes in local structures (shrinkage of the crystal lattice) and bonding characters associated with the oxidation of Mn ions. These changes in P K-edge spectra also support the successful integration of phosphorus ions into LiMnO2.
Note that on the discharge process the partial Mn reduction to a divalent state, associated with minor oxygen loss on charge, is evidenced by the ex situ XAS study (Supporting Figure S16). Such phenomena are generally observed in Li2MnO3-based high-capacity electrode materials. (7) Nevertheless, the profiles of O K-edge XAS spectra for second charge–discharge are superimposed with those for first charge–discharge as shown in Supporting Figure S17, indicating the high reversibility in the anionic redox for Li7/6P1/6Mn2/3O2 on the charge–discharge processes.
The suppression of oxygen loss for the phosphorus-integrated composition is also supported by online electrochemical mass spectrometry (OEMS) in Figure 4d and e. (38) For comparison, nanosized Li2MnO3 has been prepared by mechanical milling (Supporting Figure S18). For nanosized Li2MnO3, the CO2 partial pressure drastically increases on lithium extraction (Figure 4d), which is consistent with published data. (39) CO2 is considered to originate from the side reaction between the electrolyte solvent and the highly active singlet oxygen that is released from the electrode materials by irreversible anionic redox. (40) Irreversible oxygen loss from the active material is a dominative process for nanosized Li2MnO3. In addition, hydrogen gas was detected as the charge capacity and voltage increased, which was previously detected for Li1.2–yMn0.4Ti0.4O2 on charge and likely originated from the oxidative decomposition of the electrolyte solution at >4.7 V. (35) In contrast, gas evolution for both CO2 and H2 was notably suppressed for Li7/6–yP1/6Mn2/3O2 on charge to 4.5 V (Figure 4e), although a small amount of CO2 was detected in the higher voltage region; this amount was significantly smaller than nanosized Li2MnO3. This finding is consistent with the better electrochemical reversibility for Li7/6P1/6Mn2/3O2 with a 4.5 V cutoff (Figure 3c). To further increase the cyclability as electrode materials with anionic redox, a surface coating, (41) functional binder, (42) electrolyte additive, (43) and high-voltage-tolerated electrolyte (44) would be effective to suppress unfavorable side reactions with electrolyte.

Theoretical Understanding of Crystal Structure and Reaction Mechanisms

To provide a unified picture of the phosphorus integration into LiMnO2, theoretical calculations were conducted. Models used were derived from the experimental data that P ions are diluted in LiMnO2 and no P-based clusters occur. First, a large unit cell of Li16Mn16O32 was constructed, and then two Mn ions were replaced by Li and P ions, leading to the formation of Li17Mn14PO32. After structural relaxation, the formation of the unique nanostructure was noted. The P ion at the tetrahedral site shares faces with three lithium ions originally located at octahedral sites. An electrostatic repulsive interaction between P and Li ions results in the displacement of Li ions from the original octahedral position, as shown in Figure 5a. The total energy of Li17Mn14PO32 is 3.11 eV higher than that of a simple mixture of Li16Mn16O32 (16 units of LiMnO2) and Li3PO4, which is consistent with the fact that the P-integrated phase is the metastable phase (Supporting Figure S6). Nevertheless, the total energy is relatively stabilized through the structural relaxation associated with the displacement of the face-shared Li ions. Note that when the trivalent Mn ion is forced to share the face with the P ion instead of the Li ion, the energy is further destabilized by 1.4 eV. These results indicate that the formation of the Li-rich phase and the short-range ordering of one P and three Li ions are necessary to energetically stabilize such unique metastable phases.

Figure 5

Figure 5. Theoretical calculation of Li7/6P1/6Mn2/3O2. (a) An optimized crystal structure of rocksalt LiMnO2 after P-integration, and a local structure of the P ion in the rocksalt phase. Changes in the number of valence electrons for (b) Mn, P, and (c) O ions upon the delithiation process from Li9Mn6PO16 (∼Li7/6P1/6Mn2/3O2). Li ions are extracted from the model one by one, which is the lowest energy in the structure. Electron density changes during delithiation are also visualized in panel d. Yellow and cyan isosurfaces (drawn at 0.01 e– Bohr–1) (3) indicate electron accumulation and depletion, respectively; □ denotes vacant sites. The data for −3, – 7, and −9 Li are also shown in Supporting Figure S20.

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Li extraction processes from the P-integrated LiMnO2 were further studied by theoretical calculations, and the model with a smaller unit cell of Li9Mn6PO16 was used (also see the Supporting Information). Note that the unit cell volume of Li9Mn6PO16 is larger (approximately 2.3%) than that of Li8Mn8O16 without phosphorus ions, and a similar trend in the unit cell volume change was observed from experimental observations (Supporting Figure S4). The calculated voltage upon Li extraction (Supporting Figure S19) shows two different regionsat 3 and 4 V, which likely correspond to cationic Mn and anionic O redox, respectively. Changes in the oxidation states of the P-integrated phase were evaluated by the Bader method; the number of valence electrons on each atomic site upon Li extraction summarized is Figure 5b and c, and variations in the charge density are visualized in Figure 5d and Supporting Figure S20. From the valence electrons, oxide ions are classified as oxygen bonded or oxygen nonbonded to phosphorus. The covalency of the P–O bonds is much higher than those of the Li–O and Mn–O bonds, resulting in the difference in the number of valence electrons. On the charge process, Mn ions are oxidized and coupled with Li extraction; therefore, the number of valence electrons is reduced for Mn ions, forming □6Li3Mn6PO16 (□ denotes vacant sites). Upon further oxidation and Li extraction, a clear reduction of the number of valence electrons was observed for oxygen (Figure 5c). This finding is consistent with experimentally observed operando XAS data (Figure 4c). Note that oxygen bonded to phosphorus is not responsible for charge compensation and oxygen bonded to Mn ions is selectively oxidized, as shown in the number of valence electron and differential charge density distributions. Moreover, no dimerization of oxygen for the fully charged sample, □9Mn6PO16, was observed even after structural relaxation. It is proposed that the presence of phosphorus ions in the structure suppresses the “excess” oxidation of oxide ions; therefore, the possibility of oxygen dimerization around phosphorus ions would be reduced in this system, leading to the better reversibility of anionic redox. This is consistent with better performance for the P-integrated phase relative to that of the nanosized Li2MnO3 demonstrated above.

Conclusions

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In this study, a new series of electrode materials has been proposed, i.e., rocksalt-type metastable crystalline phases with atomically incorporated P ions. The successful integration of phosphorus ions with excess Li ions into nanosize and crystalline LiMnO2 was demonstrated by the combination of different characterization techniques. The P ion is located at the tetrahedral site in the CCP lattice, which shares faces with adjacent Li ions, and a slight distortion from the ideal octahedral site for Li sites energetically stabilizes this unique nanostructure. This phase delivers a large reversible capacity of ∼320 mA h g–1 that originates from the contribution from cationic redox reaction of Mn ions coupled with oxygen anionic redox, and the superior reversibility of the anionic redox is realized by the presence of the P ion in this phase. The use of such materials with unique nanostructures is a conceptually new approach to electrode design, which may lead to the next generation of materials. This metastable system opens the possibility for a new path to design high-capacity electrode materials. Moreover, this new concept for material design could also extended to different systems, such as sodium or potassium battery applications, leading to the development of a sustainable energy society in the future.

Experimental Section

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Synthesis of Materials

A nanoscale integrated LiMnO2–Li3PO4 binary system was prepared by mechanical milling using a planetary ball mill (PULVERISETTE 7; FRITSCH). For the sample of x = 0.2 in the (1 – x)LiMnO2·xLi3PO4 binary system, 1.146 g of LiMnO2 and 0.354 g of Li3PO4 (97%, Wako) were initially mixed using a zirconia pot (45 mL) and zirconia balls (14.8 g) at 600 rpm for 12 h. After milling for 12 h, the mixture was removed from the container and mixed with a mortar and pestle to ensure sample uniformity during the milling. The mixture was again milled using the zirconia pot and balls at 600 rpm for 12 h. This process was repeated three times, and the mixture was milled for 36 h in total. LiMnO2 was prepared from Li2CO3 (98.5%, Kanto Kagaku) and Mn2O3 at 900 °C for 12 h in an Ar atmosphere. Mn2O3 was obtained by heating MnCO3 (Kishida Chemical Co., Ltd.) at 850 °C for 12 h in air. After the preparation, the samples were stored in an Ar-filled glovebox. The sample handling before and after the mechanical milling was also carried out in an Ar-filled glovebox without exposure to moist air.

Electrochemical Measurement

After synthetic milling, the samples were further mixed with acetylene black (HS-100; Denka Co., Ltd., LiMnO2–Li3PO4/AB 90:10 wt %) using the planetary ball mill at 300 rpm for 12 h with the zirconia container and balls. Better electrode performance was achieved through this process even though a relatively higher content of carbon sacrifices the volumetric energy density. Composite positive electrodes comprised of 76.5 wt % LiMnO2–Li3PO4, 13.5 wt % acetylene black, and 10 wt % polyvinylidene fluoride (KF 1100; Kureha Co. Ltd.) dispersed in N-methylpyrrolidone were pasted on aluminum foil as a current collector. The electrodes were dried at 80 °C for 2 h in vacuum and then heated at 120 °C for 2 h. Metallic lithium (Honjo Metal Co., Ltd.) was used as a negative electrode. The electrolyte solution used was 1.0 mol dm–3 LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (3:7 by volume, battery grade; Kishida Chemical Corp., Ltd.). Two-electrode cells (TJ-AC; Tomcell Japan) were assembled in an Ar-filled glovebox. The cells were cycled at a rate of 10 mA g–1 at room temperature.

Characterization of Materials

X-ray diffraction (XRD) patterns of the samples were collected using an X-ray diffractometer (D2 PHASER; Bruker Corp., Ltd.) equipped with a one-dimensional X-ray detector using Cu Ka radiation generated at 300 W (30 kV and 10 mA) with a Ni filter. An airtight sample holder was used for the XRD measurement. Synchrotron XRD data were collected on the beamline BL16XU at SPring-8 synchrotron facility in Japan. The measurement was conducted using an automatic powder diffraction system for the Debye–Scherrer geometry by placing the sample in a glass capillary. The wavelength used was 0.5003 Å, which was calibrated with CeO2 as a reference sample. Structural analysis was carried out using RIETAN-FP. (45)
STEM–EELS was conducted using an electron microscope (Titan Cubed; Thermo Fisher Scientific) equipped with a spectroscope (GIF Quantum ERS; Gatan). The electron beam energy and EELS resolution were 300 keV and 1.3 eV, respectively. The P/Mn and Li/Mn atomic ratios were calculated with P-L2,3, Mn-M2,3, and Li–K edges at each pixel using software (DigitalMicrograph; Gatan). Calculated Li/Mn ratios were corrected using the similarly calculated ratios of the reference materials, Li2MnO3 and LiMnO2.
X-ray total scattering measurements was performed with an incident X-ray energy of E = 61.4 keV at the BL04B2 beamline in SPring-8, Japan, to study the local structure by pair distribution function (PDF) analysis. Hybrid detectors of Ge and CdTe were employed for the data collection of the total structure factor S(Q). The reduced PDF G(r) was obtained by the conventional Fourier transform of S(Q). (46)
The solid-state NMR spectra were acquired on a Bruker Avance III 300 MHz spectrometer with a 7 T superconducting magnet operating at a frequency of 116.6 MHz for the 7Li nucleus and 121.4 MHz for the 31P nucleus. Pristine powders of the positive electrode materials were packed into 2.5 mm zirconia rotors in an argon-filled glovebox before being transferred to the spectrometer. The rotors were spun to 30 kHz MAS under a nitrogen atmosphere. Single-pulse excitation was used to detect the 7Li NMR spectra with a 2 μs hard pulse for excitation and 1 s recycle delays. The spectra were referenced to the 7Li spectrum of solid LiF set to 0 ppm. For 31P NMR, the spectra were acquired with a 2.65 μs 90° pulse and a 120 s recycle delay. The 31P chemical shift was referenced using the 31P signal of NH4H2PO4 set to 1 ppm.
Raman spectra of the samples were collected by using a Raman spectrometer (NRS-4100, JASCO Ltd.) with a 532 nm laser. The measurement was conducted with a sample holder (LIBcell; Nanophoton) without exposure to air.
Hard X-ray absorption spectroscopy (XAS) at the Mn K-edge was performed at beamline BL-12C of the Photon Factory Synchrotron Source in Japan. Hard XAS spectra were collected with a silicon monochromator in the transmission mode. The intensities of the incident and transmitted X-rays were measured using an ionization chamber at room temperature. Composite electrode samples were prepared using the two-electrode cells at a rate of 10 mA g–1. The composite electrodes were rinsed with dimethyl carbonate and sealed in a water-resistant polymer film in the Ar-filled glovebox. Normalization of the XAS spectra was carried out using the program code IFEFFIT. (47) The postedge background was determined using a cubic spline procedure.
The ex situ and operando XAS measurements for O K-edge and Mn L-edge were collected at BL27SU in SPring-8, Japan. For ex situ measurements, the same sample preparation method was used as that for the hard XAS measurement. The absorption spectra were collected with the partial fluorescence mode. The detailed experimental setup of the operando XAS measurement is found in literature. (34)
An online mass electrochemical mass spectrometry (OEMS) system was established as described in previous work. (38) The custom-designed OEMS cell (Tomcell Japan) was comprised of electrode materials, a lithium foil as the negative electrode, and 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC, 3:7 by volume) as the electrolyte solution. The assembled OEMS cell was rested in an Ar-filled glovebox for 2 h and linked to the OEMS apparatus in a constant-temperature container maintained at 25 °C. The OEMS cell was purged with an Ar gas (99.9999%) for a total 10 min to test the system. The cell pressure (16–18 psi) and the mass spectrum were also monitored at an open-circuit voltage at room temperature. The CO2 gas evolution from DMC evaporation stabilized after 3 h, which was then set as background. The galvanostatic charging process was carried out at a current density of 10 mA g–1. The gas analysis was conducted after collecting gas products every 1 h.
Spin-polarized DFT + U calculations were performed using the GPU-port (48,49) of the Vienna Ab Initio Simulation Package (50,51) (VASP). Exchange–correlation effects and ion–electron interactions were described by the Perdew–Burke–Ernzerhof (PBE) functional (52) and the projector-augmented wave (PAW) method, (53,54) respectively. Dispersion interactions were described by DFT-D3. (55) The cutoff energy for plane-wave expansion was 520 eV, and an onsite Coulomb interaction correction of U – J = 3.9 eV (56) was applied to Mn 3d using the method by Dudarev et al. (57) In structural relaxation calculations, atomic coordinates as well as cell parameters were optimized. The Bader method was used to determine the number of valence electrons on each atomic site. (58)

Safety Statement

No unexpected or unusually high-safety hazards were encountered.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.0c01200.

  • DFT calculation details; structural evolutions of the LiMnO2–Li3PO4 system; STEM images; EDX maps; SEM images; comparison of SXRD patterns; Reitveld analysis of rocksalt LiMnO2 and Li7/6P1/6Mn2/3O2; phase segregation of Li7/6P1/6Mn2/3O2 after heating; analysis of the Mn distrivution; X-ray diffraction patterns, X-ray pair distribution functions; Raman, XAS, and solid-state NMR spectra; charge–discharge and charge curves; charge density differences, and synthesis and electrochemical properties of nanosize rocksalt Li2MnO3 (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Author
    • Naoaki Yabuuchi - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan;  Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan;  Advanced Chemical Energy Research Center, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan;  Orcidhttp://orcid.org/0000-0002-9404-5693;  Email: [email protected]
  • Authors
    • Miho Sawamura - Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan
    • Sho Kobayakawa - Department of Applied Chemistry, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan
    • Jun Kikkawa - National Institute for Materials Science (NIMS), Namiki, Tsukuba, Ibaraki 305-0044, Japan
    • Neeraj Sharma - School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia;  Orcidhttp://orcid.org/0000-0003-1197-6343
    • Damian Goonetilleke - School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia;  Present Address: D.G. Battery and Electrochemistry Laboratory (BELLA), Institute of Nanotechnology, Karlsruhe Institute for Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany;  Orcidhttp://orcid.org/0000-0003-1033-4787
    • Aditya Rawal - Mark Wainwright Analytical Centre, UNSW Sydney, Sydney, NSW 2052, Australia;  Orcidhttp://orcid.org/0000-0002-5396-1265
    • Nanaka Shimada - Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan
    • Kentaro Yamamoto - Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan;  Orcidhttp://orcid.org/0000-0002-8739-4246
    • Rina Yamamoto - Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
    • Yingying Zhou - Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan
    • Yoshiharu Uchimoto - Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-honmachi, Sakyo, Kyoto 606-8501, Japan;  Orcidhttp://orcid.org/0000-0002-1491-2647
    • Koji Nakanishi - SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
    • Kei Mitsuhara - SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
    • Koji Ohara - Diffraction and Scattering Division, Center for Synchrotron Radiation Research, Japan Synchrotron Radiation Research Institute (JASRI, SPring-8), Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan;  Orcidhttp://orcid.org/0000-0002-3134-512X
    • Jiwon Park - Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) and KAIST Institute for NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea;  Orcidhttp://orcid.org/0000-0002-2258-310X
    • Hye Ryung Byon - Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST) and KAIST Institute for NanoCentury, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea;  Orcidhttp://orcid.org/0000-0003-3692-6713
    • Hiroaki Koga - Research Organization for Information Science and Technology (RIST), 1-18-16 Hamamatsucho, Minato-ku, Tokyo 105-0013, Japan;  Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
    • Masaki Okoshi - Research Organization for Information Science and Technology (RIST), 1-18-16 Hamamatsucho, Minato-ku, Tokyo 105-0013, Japan;  Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, f1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
    • Toshiaki Ohta - SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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N.Y. acknowledges the partial support from JSPS, Grant-in-Aid for Scientific Research (Grants 15H05701, 18H02076, and 19H05816), and the MEXT program “Elements Strategy Initiative to Form Core Research Center (no. JPMXP0112101003)” from MEXT, Ministry of Education Culture, Sports, Science and Technology, Japan. N.Sharma and D.G. thanks the Australian Research Council (ARC) for support through the projects DE160100237, DP170100269, and the research training scheme. K.O. thanks the support from JSPS, Grant-in-Aid for Scientific Research (Grant 19H05814). H.R.B. acknowledges partial support from the KAIST-funded Global Singularity Research Program. The synchrotron X-ray absorption work was done under the approval of the Photon Factory Program Advisory Committee (Proposal no. 2019G033). The synchrotron radiation experiments were performed at the BL04B2, BL19B2, and BL27SU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal nos. 2018B1811, 2019A2058, 2016A1460, and 2016A1463). The TEM observation was supported by Dr. Keisuke Shinoda, NIMS, Battery Research Platform with support by NIMS microstructural characterization platform as a program of the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The computational resource of the AI Bridging Cloud Infrastructure (ABCI) provided by National Institute of Advanced Industrial Science and Technology (AIST) was used. N.Y. also thanks Prof. Masanori Tachikawa, Assoc. Prof. Takayoshi Ishimoto, and Assist. Prof. Makito Takagi (Yokohama City University, Japan) for the fruitful discussion about structural models for the theoretical calculation.

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    Lu, Zhonghua; MacNeil, D. D.; Dahn, J. R.
    Electrochemical and Solid-State Letters (2001), 4 (11), A191-A194CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    The structure, synthesis, and electrochem. behavior of layered Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 for x = 1/3, 5/12, and 1/2 are reported for the first time. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 is derived from Li2MnO3 or Li[Li1/3Mn2/3]O2 by substitution of Li+ and Mn4+ by Ni2+ while maintaining all the remaining Mn atoms in the 4+ oxidn. state. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 5/12 can deliver steady capacities of 150 and 160 mAh/g at 30 and 55°C, resp., between 3.0 and 4.4 V using a c.d. of 30 mA/g. Differential scanning calorimetry expts. on charged electrodes of Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 for x = 5/12 indicate that this material should be safer than LiCoO2. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 1/3, 5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, resp., at 30°C. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 1/3 gives a capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a c.d. of 30 mA/g.
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  6. 6
    Robertson, A. D.; Bruce, P. G. Mechanism of electrochemical activity in Li2MnO3. Chem. Mater. 2003, 15, 1984– 1992,  DOI: 10.1021/cm030047u
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    6
    Mechanism of Electrochemical Activity in Li2MnO3
    Robertson, Alastair D.; Bruce, Peter G.
    Chemistry of Materials (2003), 15 (10), 1984-1992CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Lithium intercalation compds. based on lithium manganese oxides are of great importance as cathodes for rechargeable lithium batteries. It is widely accepted that Li+ may be extd. (deintercalated) from such lithium manganese oxides accompanied by oxidn. of Mn up to a max. oxidn. state of +4. However, it has been suggested recently that further Li+ removal may be possible. Among the mechanisms that have been proposed to charge balance the removal of Li+ are Mn oxidn. beyond +4 or loss of O2-. To investigate this phenomenon we have selected Li2MnO3, a layered compd. Li[Li1/3Mn2/3]O2 with a ready supply of mobile Li+ ions but with all Mn already in the +4 oxidn. state. We show that a substantial quantity of Li (at least 1.39 Li) may be removed. At 55° this occurs exclusively by oxidn. of the nonaq. electrolyte, thus generating H+ which exchange one-for-one with Li+ to form Li2-xHxMnO3. The presence of H+ between the oxide layers results in a change of the layer stacking from O3 to P3, the latter being more stable for O-H-O bonding. At 30° initial Li removal is accompanied by oxygen loss (effective removal of Li2O) but further Li+ removal involves the same proton exchange mechanism as obsd. at 55°. The reaction is partially reversible. On extended cycling the material converts to spinel.
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  7. 7
    Yabuuchi, N.; Kubota, K.; Aoki, Y.; Komaba, S. Understanding Particle-Size-Dependent Electrochemical Properties of Li2MnO3-Based Positive Electrode Materials for Rechargeable Lithium Batteries. J. Phys. Chem. C 2016, 120, 875– 885,  DOI: 10.1021/acs.jpcc.5b10517
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    7
    Understanding Particle-Size-Dependent Electrochemical Properties of Li2MnO3-Based Positive Electrode Materials for Rechargeable Lithium Batteries
    Yabuuchi, Naoaki; Kubota, Kei; Aoki, Yoshinori; Komaba, Shinichi
    Journal of Physical Chemistry C (2016), 120 (2), 875-885CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Electrochem. properties of Li-excess electrode materials, Li1.2Co0.13Ni0.13Mn0.54O2, with different primary particle sizes are studied in Li cells, and phase transition behavior on continuous electrochem. cycles is systematically examd. Although the nanosize (<100 nm) sample delivers a large reversible capacity of 300 mAh g-1 at the initial cycle, capacity retention is not sufficient as a pos. electrode material. Moreover, unfavorable phase transition, gradual enrichment of trivalent manganese ions, and lowering structural symmetry is not avoidable on electrochem. cycles for a nanosize sample, which is confirmed by combined techniques of synchrotron X-ray diffraction, X-ray absorption spectroscopy, and XPS. A submicron-size sample also delivers a large reversible capacity of 250 mAh g-1 even though a slow activation process is obsd. accompanied with partial oxygen loss and migration oxide ions in the crystal lattice coupled with transition metal migration on the initial charge process. Such an unfavorable phase transition at room temp. is effectively suppressed by the use of a submicrosize sample with low surface area. However, suppression of the phase transition is found to be a kinetically controlled phenomena and is, therefore, unavoidable at elevated temps.
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  8. 8
    Johnson, C. S.; Li, N.; Lefief, C.; Vaughey, J. T.; Thackeray, M. M. Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 – x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7). Chem. Mater. 2008, 20, 6095– 6106,  DOI: 10.1021/cm801245r
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    8
    Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 - x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7)
    Johnson, Christopher S.; Li, Naichao; Lefief, Christina; Vaughey, John T.; Thackeray, Michael M.
    Chemistry of Materials (2008), 20 (19), 6095-6106CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Li- and Mn-rich layered electrode materials, represented by the formula xLi2MnO3·(1 - x)LiMO2 in which M is Mn, Ni or Co are of interest for high-power and high-capacity Li ion batteries. The synthesis, structural and electrochem. characterization of xLi2MnO3·(1 - x)LiMn0.333Ni0.333Co0.333O2 electrodes for the range 0 ≤ x ≤ 0.7 was studied. Changes that occur to the compositional, structural and electrochem. properties of the electrodes as a function of x and the importance of using a relatively high Mn content and a high charging potential (>4.4 V) to obtain a capacity of >200 mA-h/g are highlighted. Attention is given to the electrode compn. 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 which, if completely de-lithiated during charge, yields Mn0.533Ni0.233Co0.233O2, in which the Mn ions are tetravalent and, when fully discharged, LiMn0.533Ni0.233Co0.233O2, in which the av. Mn oxidn. state (3.44) is marginally below that expected for a potentially damaging Jahn-Teller distortion (3.5). Acid treatment of 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 composite electrode structures with 0.1M HNO3 chem. activates the Li2MnO3 component and eliminates the 1st cycle capacity loss but damages electrochem. behavior, consistent with earlier reports for Li2MnO3-stabilized electrodes. Differences between electrochem. and chem. activation of the Li2MnO3 component are discussed. Electrochem. charge/discharge profiles and cyclic voltammogram data suggest that small spinel-like regions, generated in cycled Mn-rich electrodes, serve to stabilize the electrodes, particularly at low Li loadings (high potentials). The study emphasizes that, for high values of x, a relatively small LiMO2 concn. stabilizes a layered Li2MnO3 electrode to reversible Li insertion and extn. when charged to a high potential.
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  9. 9
    Koga, H.; Croguennec, L.; Ménétrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C. Reversible Oxygen Participation to the Redox Processes Revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 2013, 160, A786– A792,  DOI: 10.1149/2.038306jes
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    9
    Reversible oxygen participation to the redox processes revealed for Li1.20Mn0.54Co0.13Ni0.13O2
    Koga, Hideyuki; Croguennec, Laurence; Menetrier, Michel; Douhil, Kais; Belin, Stephanie; Bourgeois, Lydie; Suard, Emmanuelle; Weill, Francois; Delmas, Claude
    Journal of the Electrochemical Society (2013), 160 (6), A786-A792CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    Materials prepd. by chem. Li deintercalation with NO2BF4 from Li1.20Mn0.54Co0.13Ni0.13O2 and chem. Li reinsertion with LiI show very similar chem. compn., oxidn. state of each transition metal ion, structural properties and electrochem. performance to those of the material recovered after the 1st electrochem. cycle. Investigations combining redox titrn., magnetic measurement, neutron diffraction and chem. analyzes reveal that uncommon redox processes are involved during the first charge at high voltage and explain the charge overcapacity and large reversible discharge capacity obtained for this material. This further assesses our proposal that oxygen, in addn. to nickel and cobalt, participates to the redox processes in charge: within the bulk oxygen is oxidized without oxygen loss, whereas at the surface oxygen is oxidized to O2 and irreversibly lost from the structure. During the subsequent discharge, in addn. to nickel, cobalt and oxygen, manganese is also slightly involved in the redox processes (redn.) to compensate for the initial surface oxygen loss.
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  10. 10
    Chen, C.-J.; Pang, W. K.; Mori, T.; Peterson, V. K.; Sharma, N.; Lee, P.-H.; Wu, S.-h.; Wang, C.-C.; Song, Y.-F.; Liu, R.-S. The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-ray Microscopy Study. J. Am. Chem. Soc. 2016, 138, 8824– 8833,  DOI: 10.1021/jacs.6b03932
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    10
    The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-ray Microscopy Study
    Chen, Chih-Jung; Pang, Wei Kong; Mori, Tatsuhiro; Peterson, Vanessa K.; Sharma, Neeraj; Lee, Po-Han; Wu, She-huang; Wang, Chun-Chieh; Song, Yen-Fang; Liu, Ru-Shi
    Journal of the American Chemical Society (2016), 138 (28), 8824-8833CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The mechanism of capacity fade of the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) composite pos. electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallog., and morphol. evolution of the material during electrochem. cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R‾3m LiMO2 with the remaining 27(1) wt % C2/m Li2MnO3 likely existing as an intergrowth. Cracking in the Li2MnO3·LiMO2 electrode particle under operando microscopy observation was revealed to be initiated by the solid-soln. reaction of the LiMO2 phase on charge to 4.55 V vs Li+/Li and intensified during further charge to 4.7 V vs Li+/Li during the concurrent two-phase reaction of the LiMO2 phase, involving the largest lattice change of any phase, and oxygen evolution from the Li2MnO3 phase. Notably, significant healing of the generated cracks in the Li2MnO3·LiMO2 electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally assocd. with the solid-soln. reaction of the LiMO2 phase. This work reveals that while it is the redn. of lattice size of electrode phases during charge that results in cracking of the Li2MnO3·LiMO2 electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-soln. reaction occurring during discharge. Importantly, it is the phase sepn. during the two-phase reaction of the LiMO2 phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase sepn., such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode.
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  11. 11
    Yabuuchi, N. Material Design Concept of Lithium-Excess Electrode Materials with Rocksalt-Related Structures for Rechargeable Non-Aqueous Batteries. Chem. Rec. 2019, 19 (4), 690,  DOI: 10.1002/tcr.201800089
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    11
    Material Design Concept of Lithium-Excess Electrode Materials with Rocksalt-Related Structures for Rechargeable Non-Aqueous Batteries
    Yabuuchi, Naoaki
    Chemical Record (2019), 19 (4), 690-707CODEN: CRHEAK; ISSN:1528-0691. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Dependence on lithium-ion batteries for automobile applications is rapidly increasing, and further improvement, esp. for pos. electrode materials, is indispensable to increase energy d. of lithium-ion batteries. In the past several years, many new lithium-excess high-capacity electrode materials with rocksalt-related structures have been reported. These materials deliver high reversible capacity with cationic/anionic redox and percolative lithium migration in the oxide/oxyfluoride framework structures, and recent research progresses on these electrode materials are reviewed. Material design strategies for these lithium-excess electrode materials are also described. Future possibility of high-energy non-aq. batteries with advanced pos. electrode materials is discussed for more details.
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  12. 12
    Ben Yahia, M.; Vergnet, J.; Saubanère, M.; Doublet, M.-L. Unified picture of anionic redox in Li/Na-ion batteries. Nat. Mater. 2019, 18, 496– 502,  DOI: 10.1038/s41563-019-0318-3
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    12
    Unified picture of anionic redox in Li/Na-ion batteries
    Ben Yahia, Mouna; Vergnet, Jean; Saubanere, Matthieu; Doublet, Marie-Liesse
    Nature Materials (2019), 18 (5), 496-502CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
    Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy d. of rechargeable batteries. Ever since, numerous electrodes delivering extra anionic capacity beyond the theor. cationic capacity have been reported. Unfortunately, most often the anionic capacity achieved in charge is partly irreversible in discharge. A unified picture of anionic redox in A-rich-TMOs is designed here to identify the electronic origin of this irreversibility and to propose new directions to improve the cycling performance of the electrodes. The electron localization function is introduced as a holistic tool to unambiguously locate the O lone pairs in the structure and follow their participation in the redox activity of A-rich-TMOs. The charge-transfer gap of transition metal oxides is proposed as the pertinent observable to quantify the amt. of extra capacity achievable in charge and its reversibility in discharge, irresp. of the material chem. compn. From this generalized approach, the reversibility of the anionic capacity is limited to a crit. no. of O holes per O, hO ≤ 1/3.
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  13. 13
    Sudayama, T.; Uehara, K.; Mukai, T.; Asakura, D.; Shi, X.-M.; Tsuchimoto, A.; Mortemard de Boisse, B.; Shimada, T.; Watanabe, E.; Harada, Y.; Nakayama, M.; Okubo, M.; Yamada, A. Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes. Energy Environ. Sci. 2020, 13 (5), 1492,  DOI: 10.1039/C9EE04197D
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    13
    Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes
    Sudayama, Takaaki; Uehara, Kazuki; Mukai, Takahiro; Asakura, Daisuke; Shi, Xiang-Mei; Tsuchimoto, Akihisa; Mortemard de Boisse, Benoit; Shimada, Tatau; Watanabe, Eriko; Harada, Yoshihisa; Nakayama, Masanobu; Okubo, Masashi; Yamada, Atsuo
    Energy & Environmental Science (2020), 13 (5), 1492-1500CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    High-energy-d. batteries have been a long-standing target toward sustainability, but the energy d. of state-of-the-art lithium-ion batteries is limited in part by the small capacity of the pos. electrode materials. Although employing the addnl. oxygen-redox reaction of Li-excess transition-metal oxides is an attractive approach to increase the capacity, an at.-level understanding of the reaction mechanism has not been established so far. Here, using bulk-sensitive resonant inelastic X-ray scattering spectroscopy combined with ab initio computations, we demonstrate the presence of a localized oxygen 2p orbital weakly hybridized with transition metal t2g orbitals that was theor. predicted to play a key role in oxygen-redox reactions. After oxygen oxidn., the hole in the oxygen 2p orbital is stabilized by the generation of either a (σ + π) multiorbital bond through strong π back-donation or peroxide O22- through oxygen dimerization. The multiorbital bond formation with σ-accepting and π-donating transition metals can thus lead to reversible oxygen-redox reaction.
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  14. 14
    Zhao, W.; Yamaguchi, K.; Sato, T.; Yabuuchi, N. Li4/3Ni1/3Mo1/3O2 – LiNi1/2Mn1/2O2 Binary System as High Capacity Positive Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 2018, 165, A1357– A1362,  DOI: 10.1149/2.0661807jes
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    14
    Li4/3Ni1/3Mo1/3O2 - LiNi1/2Mn1/2O2 binary system as high capacity positive electrode materials for rechargeable lithium Batteries
    Zhao, Wenwen; Yamaguchi, Kazuma; Sato, Takahito; Yabuuchi, Naoaki
    Journal of the Electrochemical Society (2018), 165 (7), A1357-A1362CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    A binary system of x Li4/3Ni1/3Mo1/3O2 - (1-x) LiNi1/2Mn1/2O2 is studied as high-capacity pos. electrode materials for rechargeable lithium batteries. Structural and electrochem. properties of oxides with different compns. in this binary system are examd. Mo ordering is retained for 1 ≤ x ≤ 1/3 with a monoclinic symmetry and disappears for x ≤ 1/6 with a rhombohedral symmetry. Compared with Li4/3Ni1/3Mo1/3O2, partial substitution of Mn for Mo lead to the improvement of reversible capacity and redn. of polarization. For Li6/5Ni2/5Mn1/5Mo1/5O2 (x = 1/3) and Li9/8Ni7/16Mn5/16Mo1/8O2 (x = 1/6), high reversible capacities of around 200 mAh g-1 are obtained. Improved cycling performance is achieved through the optimization of voltage ranges. Further structural characterization by ex-situ XRD reveals that the improved reversibility for the Mn-substituted samples mainly results from the suppression of Mo migration during cycling, probably assocd. with partial oxygen loss.
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  15. 15
    Yabuuchi, N.; Nakayama, M.; Takeuchi, M.; Komaba, S.; Hashimoto, Y.; Mukai, T.; Shiiba, H.; Sato, K.; Kobayashi, Y.; Nakao, A.; Yonemura, M.; Yamanaka, K.; Mitsuhara, K.; Ohta, T. Origin of Stabilization and Destabilization in Solid-State Redox Reaction of Oxide Ions for Rechargeable Lithium Batteries. Nat. Commun. 2016, 7 (1), 13814,  DOI: 10.1038/ncomms13814
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    15
    Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries
    Yabuuchi, Naoaki; Nakayama, Masanobu; Takeuchi, Mitsue; Komaba, Shinichi; Hashimoto, Yu; Mukai, Takahiro; Shiiba, Hiromasa; Sato, Kei; Kobayashi, Yuki; Nakao, Aiko; Yonemura, Masao; Yamanaka, Keisuke; Mitsuhara, Kei; Ohta, Toshiaki
    Nature Communications (2016), 7 (), 13814CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Further increase in energy d. of lithium batteries is needed for zero emission vehicles. However, energy d. is restricted by unavoidable theor. limits for pos. electrodes used in com. applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extn. results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic cond. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g-1 based on solid-state redox reaction of oxide ions.
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  16. 16
    House, R. A.; Jin, L.; Maitra, U.; Tsuruta, K.; Somerville, J. W.; Förstermann, D. P.; Massel, F.; Duda, L.; Roberts, M. R.; Bruce, P. G. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ. Sci. 2018, 11, 926– 932,  DOI: 10.1039/C7EE03195E
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    16
    Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox
    House, Robert A.; Jin, Liyu; Maitra, Urmimala; Tsuruta, Kazuki; Somerville, James W.; Forstermann, Dominic P.; Massel, Felix; Duda, Laurent; Roberts, Matthew R.; Bruce, Peter G.
    Energy & Environmental Science (2018), 11 (4), 926-932CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    The quantity of charge stored in transition metal oxide intercalation cathodes for Li or Na batteries is not limited by transition metal redox reactions but can also access redox reactions on O; examples include Li1.2Ni0.13Mn0.54Co0.13O2, Li2Ru0.75Sn0.25O3, Li1.2Nb0.3Mn0.4O2, Na2RuO3 and Na2/3Mg0.28Mn0.72O2. Here we show that oxyfluorides can also exhibit charge storage by O-redox. We report the discovery of lithium manganese oxyfluoride, specifically the compn., Li1.9Mn0.95O2.05F0.95, with a high capacity to store charge of 280 mA h g-1 (corresponding to 960 W h kg-1) of which almost half, 130 mA h g-1, arises from O-redox. This material has a disordered cubic rocksalt structure and the voltage-compn. curve is significantly more reversible compared with ordered Li-rich layered cathodes. Unlike lithium manganese oxides such as the ordered layered rocksalt Li2MnO3, Li1.9Mn0.95O2.05F0.95 does not exhibit O loss from the lattice. The material is synthesized using a simple, one-pot mechanochem. procedure.
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  17. 17
    Lee, J.; Kitchaev, D. A.; Kwon, D.-H.; Lee, C.-W.; Papp, J. K.; Liu, Y.-S.; Lun, Z.; Clément, R. J.; Shi, T.; McCloskey, B. D.; Guo, J.; Balasubramanian, M.; Ceder, G. Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials. Nature 2018, 556, 185– 190,  DOI: 10.1038/s41586-018-0015-4
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    17
    Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials
    Lee, Jinhyuk; Kitchaev, Daniil A.; Kwon, Deok-Hwang; Lee, Chang-Wook; Papp, Joseph K.; Liu, Yi-Sheng; Lun, Zhengyan; Clement, Raphaele J.; Shi, Tan; McCloskey, Bryan D.; Guo, Jinghua; Balasubramanian, Mahalingam; Ceder, Gerbrand
    Nature (London, United Kingdom) (2018), 556 (7700), 185-190CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    There is an urgent need for low-cost, resource-friendly, high-energy-d. cathode materials for lithium-ion batteries to satisfy the rapidly increasing need for elec. energy storage. To replace the nickel and cobalt, which are limited resources and are assocd. with safety problems, in current lithium-ion batteries, high-capacity cathodes based on manganese would be particularly desirable owing to the low cost and high abundance of the metal, and the intrinsic stability of the Mn4+ oxidn. state. Here we present a strategy of combining high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn2+/Mn4+ double redox couple into lithium-excess cathode materials. The lithium-rich cathodes thus produced have high capacity and energy d. The use of the Mn2+/Mn4+ redox reduces oxygen redox activity, thereby stabilizing the materials, and opens up new opportunities for the design of high-performance manganese-rich cathodes for advanced lithium-ion batteries.
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  18. 18
    Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S. High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 7650– 7655,  DOI: 10.1073/pnas.1504901112
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    18
    High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure
    Yabuuchi, Naoaki; Takeuchi, Mitsue; Nakayama, Masanobu; Shiiba, Hiromasa; Ogawa, Masahiro; Nakayama, Keisuke; Ohta, Toshiaki; Endo, Daisuke; Ozaki, Tetsuya; Inamasu, Tokuo; Sato, Kei; Komaba, Shinichi
    Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (25), 7650-7655CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for elec. vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy d. of lithium batteries. In the past decade, lithium-excess compds., Li2MeO3 (Me = Mn4+, Ru4+, etc.), have been extensively studied as high-capacity pos. electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co3+, Ni3+, etc.). Herein, as a compd. with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examd. as the host structure of a new series of high-capacity pos. electrode materials for rechargeable lithium batteries. Approx. 300 mAh·g-1 of high-reversible capacity at 50 °C is exptl. obsd., which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochem. inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.
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  19. 19
    Ji, H.; Wu, J.; Cai, Z.; Liu, J.; Kwon, D.-H.; Kim, H.; Urban, A.; Papp, J. K.; Foley, E.; Tian, Y.; Balasubramanian, M.; Kim, H.; Clément, R. J.; McCloskey, B. D.; Yang, W.; Ceder, G. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nat. Energy 2020, 5, 213– 221,  DOI: 10.1038/s41560-020-0573-1
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    Ultrahigh power and energy density in partially ordered lithium-ion cathode materials
    Ji, Huiwen; Wu, Jinpeng; Cai, Zijian; Liu, Jue; Kwon, Deok-Hwang; Kim, Hyunchul; Urban, Alexander; Papp, Joseph K.; Foley, Emily; Tian, Yaosen; Balasubramanian, Mahalingam; Kim, Haegyeom; Clement, Raphaele J.; McCloskey, Bryan D.; Yang, Wanli; Ceder, Gerbrand
    Nature Energy (2020), 5 (3), 213-221CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
    The rapid market growth of rechargeable batteries requires electrode materials that combine high power and energy and are made from earth-abundant elements. Here we show that combining a partial spinel-like cation order and substantial lithium excess enables both dense and fast energy storage. Cation overstoichiometry and the resulting partial order is used to eliminate the phase transitions typical of ordered spinels and enable a larger practical capacity, while lithium excess is synergistically used with fluorine substitution to create a high lithium mobility. With this strategy, we achieved specific energies greater than 1,100 Wh kg-1 and discharge rates up to 20 A g-1. Remarkably, the cathode materials thus obtained from inexpensive manganese present a rare case wherein an excellent rate capability coexists with a reversible oxygen redox activity. Our work shows the potential for designing cathode materials in the vast space between fully ordered and disordered compds.
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  20. 20
    Kobayashi, Y.; Sawamura, M.; Kondo, S.; Harada, M.; Noda, Y.; Nakayama, M.; Kobayakawa, S.; Zhao, W.; Nakao, A.; Yasui, A.; Rajendra, H. B.; Yamanaka, K.; Ohta, T.; Yabuuchi, N. Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3–LiMnO2 binary system. Mater. Today 2020, 37, 43,  DOI: 10.1016/j.mattod.2020.03.002
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    20
    Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3-LiMnO2 binary system
    Kobayashi, Yuki; Sawamura, Miho; Kondo, Sayaka; Harada, Maho; Noda, Yusuke; Nakayama, Masanobu; Kobayakawa, Sho; Zhao, Wenwen; Nakao, Aiko; Yasui, Akira; Rajendra, Hongahally Basappa; Yamanaka, Keisuke; Ohta, Toshiaki; Yabuuchi, Naoaki
    Materials Today (Oxford, United Kingdom) (2020), 37 (), 43-55CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    A binary system of Li2TiO3-LiMnO2 is systematically examd. by joint exptl. and theor. studies as electrode materials for Li storage applications. Increase in a fraction of Li2TiO3 effectively activates anionic redox, and thus holes are reversibly formed on oxygen by electrochem. oxidn. Such holes are energetically stabilized through π-type interaction with Mn t2g orbital as suggested by theor. calcn. However, excess enrichment of Li2TiO3 fractions in this binary system results in the oxygen loss as an irreversible process on delithiation because of a non-bonding character for Ti-O bonds coupled with the formation of O-O dimers, which are chem. and electrochem. unstable species. Detailed electrochem. study clearly shows that Li migration kinetics is relatively slow, presumably coupled with low electronic cond. Nevertheless, nanosizing of primary particles is an effective strategy to overcome this limitation. The nanosized sample prepd. by mech. milling delivers a large reversible capacity, ∼300 mA h g-1, even at room temp. and shows much improved capacity retention. Formation and stabilization of holes for the nanosized sample are also directly evidenced by soft X-ray absorption spectroscopy. From these results, factors affecting the reversibility of anionic redox as emerging new chem. and its possibility for energy storage applications are discussed in more details.
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    Croguennec, L.; Deniard, P.; Brec, R. Electrochemical Cyclability of Orthorhombic LiMnO2: Characterization of Cycled Materials. J. Electrochem. Soc. 1997, 144, 3323– 3330,  DOI: 10.1149/1.1838013
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    Electrochemical cyclability of orthorhombic LiMnO2. Characterization of cycled materials
    Croguennec, L.; Deniard, P.; Brec, R.
    Journal of the Electrochemical Society (1997), 144 (10), 3323-3330CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The electrochem. removal of Li from orthorhombic LiMnO2 (o-LiMnO2) leads to a phase transition with a first plateau at ∼3.7 V. This corresponds to the formation of a spinel-like material; a possible transition to a rhombohedral Li2MnO2 phase was ruled out through structural and crystal-site energy considerations. Several electrochem. cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals, the fewer the no. of cycles needed. The capacity difference between large and small crystallite/crystal compds. is ascribed to kinetic reasons as shown by ex-situ x-ray diffraction analyses and quasi-equil. electrochem. studies. Capacities as high as 200 A-h/kg were found for ≈0.3 μm crystal size materials. Contrary to the spinel prepd. at high temp., the electrochem. obtained spinel-like phase cycled very well in the 2.5-4.3 V range, suggesting structural differences between the 2 materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the Mn second neighbors for 2 compds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO2.
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    Keffer, C.; Mighell, A.; Mauer, F.; Swanson, H.; Block, S. CRYSTAL STRUCTURE OF TWINNED LOW-TEMPERATURE LITHIUM PHOSPHATE. Inorg. Chem. 1967, 6, 119,  DOI: 10.1021/ic50047a027
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    Crystal structure of twinned low-temperature lithium phosphate
    Keffer, Charles; Mighell, Alan D.; Mauer, Floyd; Swanson, Howard E.; Block, Stanley
    Inorganic Chemistry (1967), 6 (1), 119-25CODEN: INOCAJ; ISSN:0020-1669.
    Li3PO4 prepd. by pptn. from an aq. soln. differs from the form that has been described in the literature. When heated it transforms irreversibly at 502 ± 5° to the familiar form. The low-temp. form crystallizes in space group Pmn21 with a0 6.1150 ± 0.0010, b0 5.2394 ± 0.0011, and c0 4.8554 ± 0.0010 A.; Z = 2. It exhibits merohederal twinning with the twin plane normal to the z axis. The multiplicity of the predominant image is 0.75. All atoms are tetrahedrally coordinated. The final reliability index is 0.054.
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    Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. Effect of Structure on the Fe3 + /Fe2 + Redox Couple in Iron Phosphates. J. Electrochem. Soc. 1997, 144, 1609– 1613,  DOI: 10.1149/1.1837649
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    Effect of structure on the Fe3+/Fe2+ redox couple in iron phosphates
    Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B.
    Journal of the Electrochemical Society (1997), 144 (5), 1609-1613CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    To understand the role of structure on the position of the octahedral Fe3+/Fe2+ redox couple in compds. having the same polyanions, four iron phosphates: Li3Fe2(PO4)3, LiFeP2O7, Fe4(P2O7)3, and LiFePO4 were studied. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3+/Fe2+ redox couple in the above compds. lies at 2.8, 2.9, 3.1, and 3.5 eV, resp., below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the cryst. elec. field at the iron sites. The electrochem. characteristics of the iron phosphates are described.
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    Nagao, K.; Hayashi, A.; Deguchi, M.; Tsukasaki, H.; Mori, S.; Tatsumisago, M. Amorphous LiCoO2Li2SO4 active materials: Potential positive electrodes for bulk-type all-oxide solid-state lithium batteries with high energy density. J. Power Sources 2017, 348, 1– 8,  DOI: 10.1016/j.jpowsour.2017.02.038
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    Amorphous LiCoO2-Li2SO4 active materials: Potential positive electrodes for bulk-type all-oxide solid-state lithium batteries with high energy density
    Nagao, Kenji; Hayashi, Akitoshi; Deguchi, Minako; Tsukasaki, Hirofumi; Mori, Shigeo; Tatsumisago, Masahiro
    Journal of Power Sources (2017), 348 (), 1-8CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Newly amorphous Li2-x/100Cox/100S1-x/100O4-x/50 (xLiCoO2·(100-x)Li2SO4 (mol%)) pos. electrode active materials are synthesized using mechanochem. techniques. SEM observation indicates that av. radii of the Li1.2Co0.8S0.2O2.4 (80LiCoO2·20Li2SO4 (mol%)) particles are about 3 μm. HR-TEM images indicate that the particles comprise nano-cryst. and amorphous phases. The cryst. phase is attributable to cubic LiCoO2 phase. These active materials exhibit a high electronic cond. of around 10-5-10-1 S cm-1 and an ionic cond. of around 10-7-10-6 S cm-1 at room temp. Bulk-type all-oxide solid-state cells (Li-In alloy/Li3BO3-based glass-ceramic electrolyte/amorphous Li2-x/100Cox/100S1-x/100O4-x/50) are fabricated by pressing at room temp. without high temp. sintering. Although the cell with the milled LiCoO2 shows no capacity, the cell using the Li1.2Co0.8S0.2O2.4 electrode with no conductive components (ca. 150 μm thickness) operates as a secondary battery at 100°, with an av. discharge potential of 3.3 V (vs. Li+/Li) and discharge capacity of 163°mAh°g-1. A pos. electrode with large amts. of active materials is suitable for achieving high energy d. in all-solid-state batteries. These newly synthesized amorphous Li2-x/100Cox/100S1-x/100O4-x/50 electrodes with ionic and electronic conductivities and good processability meet that demand.
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    Kitajou, A.; Kobayashi, E.; Okada, S. Electrochemical Performance of a Novel Cathode material “LiFeOF” for Li-ion Batteries. Electrochemistry 2015, 83, 885– 888,  DOI: 10.5796/electrochemistry.83.885
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    Electrochemical performance of a novel cathode material "LiFeOF" for Li-ion batteries
    Kitajou, Ayuko; Kobayashi, Eiji; Okada, Shigeto
    Electrochemistry (Tokyo, Japan) (2015), 83 (10), 885-888CODEN: EECTFA; ISSN:1344-3542. (Electrochemical Society of Japan)
    Iron-based conversion cathode, FeOF is attractive, because of the low cost and the large specific capacity. However, the synthesis is not easy and it cannot be used as cathode against carbonaceous anode. To overcome these drawbacks, we focused on the LiFeOF phase, which has the same chem. compn. as the discharged intermediate product of FeOF cathode. LiFeOF can be easily synthesized from LiF and FeO by the dry ball-milling method at room temp. The reversible capacity was 292 mAh·g-1 with an av. voltage of 2.5 V and an energy d. over 700 Wh·kg-1, which is higher than that of LiFePO4. In addn., we confirmed the feasibility of LiFeOF cathode against Li44Ti55O12 anode.
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    Chen, R. Y.; Ren, S. H.; Knapp, M.; Wang, D.; Witter, R.; Fichtner, M.; Hahn, H. Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage. Adv. Energy Mater. 2015, 5, 1401814,  DOI: 10.1002/aenm.201401814
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    Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage
    Chen, Ruiyong; Ren, Shuhua; Knapp, Michael; Wang, Di; Witter, Raiker; Fichtner, Maximilian; Hahn, Horst
    Advanced Energy Materials (2015), 5 (9), 1401814/1-1401814/7CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    This article discusses about the disordered lithium-rich oxyfluoride as stable host for enhanced Li+ intercalation storage.
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    Hoshino, S.; Glushenkov, A. M.; Ichikawa, S.; Ozaki, T.; Inamasu, T.; Yabuuchi, N. Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries. ACS Energy Letters 2017, 2, 733– 738,  DOI: 10.1021/acsenergylett.7b00037
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    Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries
    Hoshino, Satoshi; Glushenkov, Alexey M.; Ichikawa, Shinnosuke; Ozaki, Tetsuya; Inamasu, Tokuo; Yabuuchi, Naoaki
    ACS Energy Letters (2017), 2 (4), 733-738CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    To increase the energy d. of lithium batteries, the development of high-capacity pos. electrode materials is essential. Herein, the use is proposed of a three-electron redox reaction of Mo3+/Mo6+ for a new series of high-capacity lithium insertion materials. In this study, a binary system of LiMoO2-Li3NbO4 is targeted, and nanosize and metastable Li9/7Nb2/7Mo3/7O2 is successfully prepd. by a mech. milling process. The sample delivers a large reversible capacity of ∼280 mAhg-1 in a Li cell with good capacity retention. On the basis of these results, the future possibility of high-capacity electrode materials with a three-electron Mo3+/Mo6+ redox reaction is discussed.
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    Sato, T.; Sato, K.; Zhao, W.; Kajiya, Y.; Yabuuchi, N. Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO2 and NaMnO2. J. Mater. Chem. A 2018, 6, 13943– 13951,  DOI: 10.1039/C8TA03667E
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    Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO2 and NaMnO2
    Sato, Takahito; Sato, Kei; Zhao, Wenwen; Kajiya, Yoshio; Yabuuchi, Naoaki
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (28), 13943-13951CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Stoichiometric LiMnO2 and NaMnO2 with a cation-disordered rock salt-type structure as metastable polymorphs were successfully prepd. by mech. milling. Although cation-disordered rock salt phases with a stoichiometric compn. (Li:Mn molar ratio = 1:1) are expected to be electrochem. less active, both samples show superior performance as electrode materials when compared with thermodynamically stable layered phases in Li/Na cells. Both metastable samples deliver large reversible capacities, which correspond to >80% of their theor. capacities, with relatively small polarization on the basis of reversible Mn3+/Mn4+ redox. Moreover, for rock salt LiMnO2, the phase transition into a spinel phase is effectively suppressed compared with a thermodynamically stable phase. The electrode reversibility of NaMnO2 is also drastically improved by the use of the metastable phase with good capacity retention. Metastable phases with unique nanostructures open a new path for the design of advanced electrode materials with high energy d., and thus a broad impact is anticipated for rechargeable Li/Na battery applications.
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    Tsuchiya, Y.; Glushenkov, A. M.; Yabuuchi, N. Effect of Nanosizing on Reversible Sodium Storage in a NaCrO2 Electrode. ACS Applied Nano Materials 2018, 1, 364– 370,  DOI: 10.1021/acsanm.7b00207
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    29
    Effect of Nanosizing on Reversible Sodium Storage in a NaCrO2 Electrode
    Tsuchiya, Yuka; Glushenkov, Alexey M.; Yabuuchi, Naoaki
    ACS Applied Nano Materials (2018), 1 (1), 364-370CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
    The effect of nanosizing on the sodium storage performance in NaCrO2 is systematically examd. Cation-disordered rock-salt-type and nanosized NaCrO2 is prepd. by mech. milling, and layered O3-type and nanosized NaCrO2 is prepd. by heat treatment of the rock-salt phase. The observation by high-resoln. transmission electron microscopy reveals that secondary particles consist of highly cryst. and nanosized NaCrO2 primary particles with enriched grain boundaries. Such morphol. features affect the voltage profiles in sodium cells, leading to an S-shaped profile with a single-phase reaction even for layered NaCrO2, in which a biphasic reaction dominates because of a large repulsive interaction between Na ions. Moreover, the O3-P3 phase transition is suppressed for the heat-treated sample with the presence of enriched grain boundaries. The suppression of the phase transition is proposed to be due to the cancellation of CrO2 layers gliding for the incoherently aligned grain boundaries. Thus, good capacity retention as electrode materials is realized compared with as-prepd. bulk O3 NaCrO2. Nanotechnol. potentially changes materials design strategies for sodium insertion materials, leading to the development of innovative rechargeable sodium batteries in the future.
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    Sabi, Y.; Sato, S.; Hayashi, S.; Furuya, T.; Kusanagi, S. A new class of amorphous cathode active material LixMyPOz (M = Ni, Cu, Co, Mn, Au, Ag, Pd). J. Power Sources 2014, 258, 54– 60,  DOI: 10.1016/j.jpowsour.2014.02.021
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    30
    A new class of amorphous cathode active material LixMyPOz (M = Ni, Cu, Co, Mn, Au, Ag, Pd)
    Sabi, Yuichi; Sato, Susumu; Hayashi, Saori; Furuya, Tatsuya; Kusanagi, Susumu
    Journal of Power Sources (2014), 258 (), 54-60CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    A new class of amorphous cathode active materials LixMyPOz (LiMPO) is proposed. The materials are sputter deposited to form a thin film using Li3PO4 together with metal or metal oxide targets. Among several materials tested as thin-film battery, materials suitable as working material were found to be with M = Ni, Cu, Co, Mn, Au, Ag, Pd. The property is intensively studied for LixCuyPOz (LiCuPO) and LixNiyPOz (LiNiPO). Those materials show a wide compn. margin, such as y of 1-3, and a high capacity for LiNiPO with a max. value of 330 mAh g-1. The capability to charge and discharge at a high rate is shown up to 30 C. This preliminary report reveals its high potentiality for further optimization.
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    Jung, S.-K.; Kim, H.; Cho, M. G.; Cho, S.-P.; Lee, B.; Kim, H.; Park, Y.-U.; Hong, J.; Park, K.-Y.; Yoon, G.; Seong, W. M.; Cho, Y.; Oh, M. H.; Kim, H.; Gwon, H.; Hwang, I.; Hyeon, T.; Yoon, W.-S.; Kang, K. Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries. Nat. Energy 2017, 2, 16208,  DOI: 10.1038/nenergy.2016.208
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    Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries
    Jung, Sung-Kyun; Kim, Hyunchul; Cho, Min Gee; Cho, Sung-Pyo; Lee, Byungju; Kim, Hyungsub; Park, Young-Uk; Hong, Jihyun; Park, Kyu-Young; Yoon, Gabin; Seong, Won Mo; Cho, Yongbeom; Oh, Myoung Hwan; Kim, Haegyeom; Gwon, Hyeokjo; Hwang, Insang; Hyeon, Taeghwan; Yoon, Won-Sub; Kang, Kisuk
    Nature Energy (2017), 2 (2), 16208CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
    Lithium-ion batteries based on intercalation compds. have dominated the advanced portable energy storage market. The pos. electrode materials in these batteries belong to a material group of lithium-conducting crystals that contain redox-active transition metal and lithium. Materials without lithium-conducting paths or lithium-free compds. could be rarely used as pos. electrodes due to the incapability of reversible lithium intercalation or the necessity of using metallic lithium as neg. electrodes. These constraints have significantly limited the choice of materials and retarded the development of new pos. electrodes in lithium-ion batteries. Here, we demonstrate that lithium-free transition metal monoxides that do not contain lithium-conducting paths in their crystal structure can be converted into high-capacity pos. electrodes in the electrochem. cell by initially decorating the monoxide surface with nanosized lithium fluoride. This unusual electrochem. behavior is attributed to a surface conversion reaction mechanism in contrast with the classic lithium intercalation reaction. Our findings will offer a potential new path in the design of pos. electrode materials in lithium-ion batteries.
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    Popović, L.; Manoun, B.; de Waal, D.; Nieuwoudt, M. K.; Comins, J. D. Raman spectroscopic study of phase transitions in Li3PO4. J. Raman Spectrosc. 2003, 34, 77– 83,  DOI: 10.1002/jrs.954
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    Raman spectroscopic study of phase transitions in Li3PO4
    Popovic, L.; Manoun, B.; de Waal, D.; Nieuwoudt, M. K.; Comins, J. D.
    Journal of Raman Spectroscopy (2003), 34 (1), 77-83CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
    Although three forms of Li phosphate are known, a complete structural description of the highest temp. polymorph has not yet been given. The phase transitions of Li phosphate were studied at high temps. using Raman microscopy and x-ray powder diffraction. Both transitions were obsd. by following the temp. dependence of the totally sym. Raman stretching vibration of PO43-. Currently available structural information on the a form, resulting in P-O bond lengths of 1.787-1.899 Å, as detd. by valence bond calcns., are disputed here. A correlation between Raman wavenumber and bond length in inorg. phosphates ests. the P-O bond length in α-Li3PO4 to be around 1.57(1) Å, which is in closer agreement with values for other orthophosphates of between 1.50 and 1.58 Å.
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  33. 33
    Lee, Y. J.; Grey, C. P. 6Li Magic-Angle Spinning (MAS) NMR Study of Electron Correlations, Magnetic Ordering, and Stability of Lithium Manganese(III) Oxides. Chem. Mater. 2000, 12, 3871– 3878,  DOI: 10.1021/cm000469t
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    33
    6Li Magic-Angle Spinning (MAS) NMR Study of Electron Correlations, Magnetic Ordering, and Stability of Lithium Manganese(III) Oxides
    Lee, Young Joo; Grey, Clare P.
    Chemistry of Materials (2000), 12 (12), 3871-3878CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    Three different lithium manganese(III) oxides (the orthorhombic LiMnO2 phase, monoclinic layered LiMnO2 and tetragonal Li2Mn2O4) were studied with 6Li magic-angle spinning (MAS) NMR spectroscopy. Much smaller shifts of 36-143 ppm are obsd. for the Mn(III) phases, in comparison to the other lithium manganese oxides with manganese oxidn. states varying from +3.5 to +4. The NMR shift of this system is governed by the Fermi-contact interaction and consequently, is controlled by the lithium local environment. For orthorhombic LiMnO2, one resonance at 36 ppm is obsd. between -39 and 283° and a single resonance at -5 ppm is seen <-39°, indicating a magnetic phase transition involving a change from short-range electronic correlations to long-range antiferromagnetic ordering. The 6Li NMR shift of the resonances of the monoclinic and tetragonal phases show very little change with temp. in the range studied (-136 to 283°), implying that short-range antiferromagnetic interactions between Mn cations also exist for these phases. No evidence for a magnetic phase transition to three-dimensional ordering, however, is obsd. Two different lithium sites are identified in Li2Mn2O4, which are assigned to lithium on the 8c octahedral and 4a tetrahedral sites. Samples with lithium on the 8c site only were obtained using mild synthesis conditions, whereas occupation of both sites was obtained with more stringent conditions or at high temps.
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  34. 34
    Hattori, M.; Yamamoto, K.; Matsui, M.; Nakanishi, K.; Mandai, T.; Choudhary, A.; Tateyama, Y.; Sodeyama, K.; Uchiyama, T.; Orikasa, Y.; Tamenori, Y.; Takeguchi, T.; Kanamura, K.; Uchimoto, Y. Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode. J. Phys. Chem. C 2018, 122, 25204– 25210,  DOI: 10.1021/acs.jpcc.8b08558
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    Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode
    Hattori, Masashi; Yamamoto, Kentaro; Matsui, Masaki; Nakanishi, Koji; Mandai, Toshihiko; Choudhary, Ashu; Tateyama, Yoshitaka; Sodeyama, Keitaro; Uchiyama, Tomoki; Orikasa, Yuki; Tamenori, Yusuke; Takeguchi, Tatsuya; Kanamura, Kiyoshi; Uchimoto, Yoshiharu
    Journal of Physical Chemistry C (2018), 122 (44), 25204-25210CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Mechanism of Mg ion alloying reaction into Bi electrode in Mg bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/MeCN (AN) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF) electrolyte was examd. by a combination of operando soft x-ray absorption spectroscopy (XAS), Raman spectroscopy, and d. functional theory (DFT) calcns. In 0.5M Mg(TFSA)2/AN, the Mg ions alloying reaction occurred, whereas the alloying reaction did not occur in 0.5M Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that <15% of [TFSA]- coordinates with Mg ions in 0.5M Mg(TFSA)2/AN, while >90% of [TFSA]- coordinates with Mg ions in Mg(TFSA)2/2-MeTHF. Using operando XAS measurements, electronic and local structure of Mg ion changed similarly upon cathodic polarization in both electrolytes. The difference of the behavior of alloy formation should be affected by the difference of coordinate structure of [TFSA]- in both electrolytes. The authors' DFT calcn. results indicates [TFSA]- coordinated to Mg ions undergoes redn. decompn. more easily than [TFSA]- uncoordinated to Mg ions. In 0.5M Mg(TFSA)2/2-MeTHF, the [TFSA]- coordinating to Mg ions undergoes redn. decompn., which inhibits the alloying reaction into the Bi electrode. However, in 0.5M Mg(TFSA)2/AN, the [TFSA]- redn. decompn. occurs relatively slowly because of the weak coordination between [TFSA]- and Mg ions, which allows the Mg ions alloying into the Bi electrode in the electrolyte.
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    Yamamoto, K.; Zhou, Y.; Yabuuchi, N.; Nakanishi, K.; Yoshinari, T.; Kobayashi, T.; Kobayashi, Y.; Yamamoto, R.; Watanabe, A.; Orikasa, Y.; Tsuruta, K.; Park, J.; Byon, H. R.; Tamenori, Y.; Ohta, T.; Uchimoto, Y. Charge Compensation Mechanism of Lithium-Excess Metal Oxides with Different Covalent and Ionic Characters Revealed by Operando Soft and Hard X-ray Absorption Spectroscopy. Chem. Mater. 2020, 32, 139– 147,  DOI: 10.1021/acs.chemmater.9b02838
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    Charge Compensation Mechanism of Lithium-Excess Metal Oxides with Different Covalent and Ionic Characters Revealed by Operando Soft and Hard X-ray Absorption Spectroscopy
    Yamamoto, Kentaro; Zhou, Yingying; Yabuuchi, Naoaki; Nakanishi, Koji; Yoshinari, Takahiro; Kobayashi, Takanori; Kobayashi, Yuki; Yamamoto, Rina; Watanabe, Aruto; Orikasa, Yuki; Tsuruta, Kazuki; Park, Jiwon; Byon, Hye Ryung; Tamenori, Yusuke; Ohta, Toshiaki; Uchimoto, Yoshiharu
    Chemistry of Materials (2020), 32 (1), 139-147CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    The charge/discharge capacity of current lithium-ion battery cathode materials is limited by the charge compensation of transition-metal redox during the charge/discharge processes. Recently, the use of oxide ion redox for charge compensation has been proposed to realize a higher charge/discharge capacity than that obsd. for transition-metal redox. Different stabilization mechanisms of the reversible oxide ion redox have been proposed. To clarify the mechanism, anal. of the electronic and local structures around oxygen is required. Because of the high-voltage region in which the oxide ion redox occurs, several reactions such as oxygen gas evolution and/or electrolyte oxidn. are often included. Thus, operando measurements are required to directly prove this concept and generalize the understanding of the oxide ion redox. This study employs operando soft/hard X-ray absorption spectroscopy combined with X-ray diffraction spectroscopy for four lithium-excess electrode materials with different chem. bond natures. The exptl. data together with online anal. of the generated on-charge gas reveal two extreme cases: significantly enhanced covalent or ionic characters in the metal-oxygen chem. bonds, which are necessary conditions to stabilize the oxidn. of the oxide ions. This finding provides new insights with exciting possibilities for designing high energy d. cathode materials based on anion redox.
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    Okubo, M.; Yamada, A. Molecular Orbital Principles of Oxygen-Redox Battery Electrodes. ACS Appl. Mater. Interfaces 2017, 9, 36463– 36472,  DOI: 10.1021/acsami.7b09835
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    Molecular Orbital Principles of Oxygen-Redox Battery Electrodes
    Okubo, Masashi; Yamada, Atsuo
    ACS Applied Materials & Interfaces (2017), 9 (42), 36463-36472CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    A review. Li-ion batteries are key energy-storage devices for a sustainable society. The most widely used pos. electrode materials are LiMO2 (M: transition metal), in which a redox reaction of M occurs in assocn. with Li+ (de)intercalation. Recent developments of Li-excess transition-metal oxides, which deliver a large capacity of >200 mA-h/g using an extra redox reaction of O, introduce new possibilities for designing higher energy d. Li-ion batteries. For better engineering using this fascinating new chem., it is necessary to achieve a full understanding of the reaction mechanism by gaining knowledge on the chem. state of O. A summary of the recent advances in O-redox battery electrodes is provided, followed by a systematic demonstration of the overall electronic structures based on MOs with a focus on the local coordination environment around O. A π-type MO plays an important role in stabilizing the oxidized O that emerges upon the charging process. MO principles are convenient for an at.-level understanding of how reversible O-redox reactions occur in bulk, providing a solid foundation toward improved O-redox pos. electrode materials for high energy-d. batteries.
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    Yoon, W.-S.; Chung, K. Y.; McBreen, J.; Zaghib, K.; Yang, X.-Q. Electronic Structure of the Electrochemically Delithiated Li[sub 1–x]FePO[sub 4] Electrodes Investigated by P K-edge X-Ray Absorption Spectroscopy. Electrochem. Solid-State Lett. 2006, 9, A415,  DOI: 10.1149/1.2216619
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    Electronic Structure of the Electrochemically Delithiated Li1-xFePO4 Electrodes Investigated by P K-edge X-Ray Absorption Spectroscopy
    Yoon, Won-Sub; Chung, Kyung Yoon; McBreen, James; Zaghib, Karim; Yang, Xiao-Qing
    Electrochemical and Solid-State Letters (2006), 9 (9), A415-A417CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    P K-edge x-ray absorption spectroscopy (XAS) was used to study the electronic structure of electrochem. de-lithiated Li1-xFePO4 for Li rechargeable batteries. The gradual shift of main edge features to higher energies showed that P-O bonds become less covalent during de-lithiation due to the more covalent Fe3+-O bonds via the inductive effect. A principal component anal. of P K-edge XAS spectra of the electrochem. de-lithiated Li1-xFePO4 reveals that this set of spectra can be represented by 2 primary components, in agreement with a 1st-order phase transition involving the LiFePO4 and FePO4 phases. From the observation of pre-edge peaks, electrochem. de-lithiation of Li1-xFePO4 results in the hybridization of P 3p states with the Fe 3d states.
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    Bonnet-Mercier, N.; Wong, R. A.; Thomas, M. L.; Dutta, A.; Yamanaka, K.; Yogi, C.; Ohta, T.; Byon, H. R. A structured three-dimensional polymer electrolyte with enlarged active reaction zone for Li–O2 batteries. Sci. Rep. 2015, 4, 7127,  DOI: 10.1038/srep07127
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    Rana, J.; Papp, J. K.; Lebens-Higgins, Z.; Zuba, M.; Kaufman, L. A.; Goel, A.; Schmuch, R.; Winter, M.; Whittingham, M. S.; Yang, W.; McCloskey, B. D.; Piper, L. F. J. Quantifying the Capacity Contributions during Activation of Li2MnO3. ACS Energy Letters 2020, 5, 634– 641,  DOI: 10.1021/acsenergylett.9b02799
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    Quantifying the Capacity Contributions during Activation of Li2MnO3
    Rana, Jatinkumar; Papp, Joseph K.; Lebens-Higgins, Zachary; Zuba, Mateusz; Kaufman, Lori A.; Goel, Anshika; Schmuch, Richard; Winter, Martin; Whittingham, M. Stanley; Yang, Wanli; McCloskey, Bryan D.; Piper, Louis F. J.
    ACS Energy Letters (2020), 5 (2), 634-641CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Though Li2MnO3 was originally considered to be electrochem. inert, its obsd. activation has spawned a new class of Li-rich layered compds. that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compds., the underlying origin of the initial charge capacity of Li2MnO3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidn. of Mn4+, and surface degrdn. for Li2MnO3 cathodes displaying capacities exceeding 350 mAh g-1. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidn. of Mn4+ and bulk oxygen redox during activation of Li2MnO3. Quant. gas-evolution and titrn. studies reveal that O2 and CO2 release accounted for a large fraction of the obsd. capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li2MnO3 is considered crit. for promoting bulk anionic redox in Li-rich layered oxides, Li2MnO3 by itself does not exhibit bulk oxygen redox or manganese oxidn. beyond its initial Mn4+ valence.
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    Wandt, J.; Freiberg, A. T. S.; Ogrodnik, A.; Gasteiger, H. A. Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries. Mater. Today 2018, 21, 825– 833,  DOI: 10.1016/j.mattod.2018.03.037
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    Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries
    Wandt, Johannes; Freiberg, Anna T. S.; Ogrodnik, Alexander; Gasteiger, Hubert A.
    Materials Today (Oxford, United Kingdom) (2018), 21 (8), 825-833CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
    For achieving higher energy d. lithium-ion batteries, the improvement of cathode active materials is crucial. The most promising cathode materials are nickel-rich layered oxides LiNixCoyMnzO2 (NCM) and over lithiated NCM (often called HE-NCM). Unfortunately, the full capacity of NCM cannot be utilized due to its limited cycle-life at high state-of-charge (SOC), while HE-NCM requires high voltages. By operando emission spectroscopy, we show for the first time that highly reactive singlet oxygen is released when charging NCM and HE-NCM to an SOC beyond ≈80%. In addn., online mass-spectrometry reveals the evolution of CO and CO2 once singlet oxygen is detected, providing significant evidence for the reaction between singlet oxygen and electrolyte to be a chem. reaction. It is controlled by the SOC rather than by potential, as would be the case for a purely electrochem. electrolyte oxidn. Singlet oxygen formation therefore imposes a severe challenge to the development of high-energy batteries based on layered oxide cathodes, shifting the focus of research from electrochem. stable 5 V-electrolytes to chem. stability toward singlet oxygen.
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    Hu, S.; Li, Y.; Chen, Y.; Peng, J.; Zhou, T.; Pang, W. K.; Didier, C.; Peterson, V. K.; Wang, H.; Li, Q.; Guo, Z. Insight of a Phase Compatible Surface Coating for Long-Durable Li-Rich Layered Oxide Cathode. Adv. Energy Mater. 2019, 9, 1901795,  DOI: 10.1002/aenm.201901795
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    Zhang, S.; Gu, H.; Pan, H.; Yang, S.; Du, W.; Li, X.; Gao, M.; Liu, Y.; Zhu, M.; Ouyang, L.; Jian, D.; Pan, F. A Novel Strategy to Suppress Capacity and Voltage Fading of Li- and Mn-Rich Layered Oxide Cathode Material for Lithium-Ion Batteries. Adv. Energy Mater. 2017, 7, 1601066,  DOI: 10.1002/aenm.201601066
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    Hekmatfar, M.; Kazzazi, A.; Eshetu, G. G.; Hasa, I.; Passerini, S. Understanding the Electrode/Electrolyte Interface Layer on the Li-Rich Nickel Manganese Cobalt Layered Oxide Cathode by XPS. ACS Appl. Mater. Interfaces 2019, 11, 43166– 43179,  DOI: 10.1021/acsami.9b14389
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    Understanding the Electrode/Electrolyte Interface Layer on the Li-Rich Nickel Manganese Cobalt Layered Oxide Cathode by XPS
    Hekmatfar, Maral; Kazzazi, Arefeh; Eshetu, Gebrekidan Gebresilassie; Hasa, Ivana; Passerini, Stefano
    ACS Applied Materials & Interfaces (2019), 11 (46), 43166-43179CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Layered lithium-rich nickel manganese cobalt oxide (LR-NMC) represents one of the most promising cathode materials for application in high energy d. lithium-ion batteries. The extraordinary capacity delivered derives from a combination of both cationic and anionic redox processes. However, the latter ones lead to oxygen evolution which triggers structural degrdn. and electrode/electrolyte interface (EEI) instability that hinders the use of LR-NMC in practical application. In this work, we investigate the surface chem. of LR-NMC and its evolution upon different conditions to give further insights into the processes occurring at the EEI. XPS studies reveal that once the org. component of the layer is formed, it remains stable independently on the higher cutoff voltage applied, while continuous growth of inorgs. along with oxygen evolution occurs. The results performed on lithiated and delithiated LR-NMC surfaces indicate an instability of the EEI layer formed at high voltages, which undergoes a partial decompn. Furthermore, the tris(pentafluorophenyl)borane electrolyte additive simultaneously prevents excess LiF formation and changes the chem. compn. of the EEI layer. The latter is characterized by a higher amt. of poly(ethylene oxide) oligomer species and LixPOyFz formation. In addn., the presence of boron-contg. compds. in the EEI layer cannot be excluded, which may be also responsible of the increased thickness of the EEI layer. Finally, fast kinetics at elevated temps. exacerbate the salt decompn. which results in the formation of an EEI which is thicker and richer in LiF.
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    Doi, T.; Shimizu, Y.; Matsumoto, R.; Hashinokuchi, M.; Inaba, M. Suppression of Mn–Ion-Dissolution of LiNi0.5Mn1.5O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures. ChemistrySelect 2017, 2, 8824– 8827,  DOI: 10.1002/slct.201701668
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    Suppression of Mn-Ion-Dissolution of LiNi0.5Mn1.5O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures
    Doi, Takayuki; Shimizu, Yusuke; Matsumoto, Ryo; Hashinokuchi, Michihiro; Inaba, Minoru
    ChemistrySelect (2017), 2 (28), 8824-8827CODEN: CHEMUD; ISSN:2365-6549. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Mn-based active materials, such as LiMn2O4, are widely used for pos. electrodes in lithium ion batteries, and spinel LiNi0.5Mn1.5O4 is drawing much attention to realize 5-V class batteries. However, the oxidative decompn. of electrolyte soln. at high voltages and Mn-dissoln. of LiNi0.5Mn1.5O4 are serious problems to be solved. These two drawbacks are more marked at elevated temps., and should be caused by free solvent mols. in electrolyte soln. In this study, highly concd. electrolyte soln., which contains few free solvent mols., was investigated to solve the problems. LiNi0.5Mn1.5O4 electrodes worked at 50oC in nearly satd. 7.25 mol kg-1 LiBF4/ propylene carbonate (PC) electrolyte soln., whereas not in the nearly satd. 4.30 mol kg-1 LiPF6/PC. In addn., Mn-ion dissoln. from LiNi0.5Mn1.5O4 was significantly suppressed in highly concd. electrolyte solns., and correlated to the fraction of free PC mols. in them.
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    Izumi, F.; Momma, K. Three-Dimensional Visualization in Powder Diffraction. Solid State Phenom. 2007, 130, 15– 20,  DOI: 10.4028/www.scientific.net/SSP.130.15
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    Three-dimensional visualization in powder diffraction
    Izumi, Fujio; Momma, Koichi
    Diffusion and Defect Data--Solid State Data, Pt. B: Solid State Phenomena (2007), 130 (Applied Crystallography XX), 15-20CODEN: DDBPE8; ISSN:1012-0394. (Trans Tech Publications Ltd.)
    A multi-purpose pattern-fitting system, RIETAN-2000, has been extensively utilized to contribute to many structural studies. It offers a sophisticated structure-refinement technique of whole-pattern fitting based on the max.-entropy method (MEM) in combination with a MEM anal. program PRIMA. We have recently completed a successor system, RIETAN-FP, adding new features such as standardization of crystal-structure data, an extended March-Dollase preferred-orientation function, and automation of imposing restraints on bond lengths and angles. Further, we have been developing a new three-dimensional visualization system, VESTA, using wxWidgets as a C++ application framework. VESTA excels in visualization, rendering, and manipulation of crystal structures and electron/nuclear densities detd. by X-ray/neutron diffraction and electronic-structure calcns. VESTA also enables us to display wave functions and electrostatic potentials calcd. with part of these programs.
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    Ohara, K.; Tominaka, S.; Yamada, H.; Takahashi, M.; Yamaguchi, H.; Utsuno, F.; Umeki, T.; Yao, A.; Nakada, K.; Takemoto, M.; Hiroi, S.; Tsuji, N.; Wakihara, T. Time-resolved pair distribution function analysis of disordered materials on beamlines BL04B2 and BL08W at SPring-8. J. Synchrotron Radiat. 2018, 25, 1627– 1633,  DOI: 10.1107/S1600577518011232
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    Time-resolved pair distribution function analysis of disordered materials on beamlines BL04B2 and BL08W at SPring-8
    Ohara Koji; Tominaka Satoshi; Yamada Hiroki; Takahashi Masakuni; Nakada Kengo; Takemoto Michitaka; Hiroi Satoshi; Tsuji Naruki; Yamaguchi Hiroshi; Utsuno Futoshi; Umeki Takashi; Yao Atsushi; Wakihara Toru
    Journal of synchrotron radiation (2018), 25 (Pt 6), 1627-1633 ISSN:.
    A dedicated apparatus has been developed for studying structural changes in amorphous and disordered crystalline materials substantially in real time. The apparatus, which can be set up on beamlines BL04B2 and BL08W at SPring-8, mainly consists of a large two-dimensional flat-panel detector and high-energy X-rays, enabling total scattering measurements to be carried out for time-resolved pair distribution function (PDF) analysis in the temperature range from room temperature to 873 K at pressures of up to 20 bar. For successful time-resolved analysis, a newly developed program was used that can monitor and process two-dimensional image data simultaneously with the data collection. The use of time-resolved hardware and software is of great importance for obtaining a detailed understanding of the structural changes in disordered materials, as exemplified by the results of commissioned measurements carried out on both beamlines. Benchmark results obtained using amorphous silica and demonstration results for the observation of sulfide glass crystallization upon annealing are introduced.
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    Newville, M. IFEFFIT: interactive XAFS analysis and FEFF fitting. J. Synchrotron Radiat. 2001, 8, 322– 324,  DOI: 10.1107/S0909049500016964
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    IFEFFIT: interactive XAFS analysis and FEFF fitting
    Newville, Matthew
    Journal of Synchrotron Radiation (2001), 8 (2), 322-324CODEN: JSYRES; ISSN:0909-0495. (Munksgaard International Publishers Ltd.)
    IFEFFIT, an interactive program and scriptable library of XAFS algorithms is presented. The core algorithms of AUTOBK and FEFFIT were combined with general data manipulation and interactive graphics into a single package. IFEFFIT comes with a command-line program that can be run either interactively or in batch-mode. It also provides a library of functions that can be used easily from C or Fortran, as well as high level scripting languages such as Tcl, Perl and Python. Using this library, a Graphical User Interface for rapid 'online' data anal. is demonstrated. IFEFFIT is freely available with an Open Source license. Outside use, development, and contributions are encouraged.
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    Hacene, M.; Anciaux-Sedrakian, A.; Rozanska, X.; Klahr, D.; Guignon, T.; Fleurat-Lessard, P. Accelerating VASP electronic structure calculations using graphic processing units. J. Comput. Chem. 2012, 33, 2581– 2589,  DOI: 10.1002/jcc.23096
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    Accelerating VASP electronic structure calculations using graphic processing units
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    Figure 1

    Figure 1. Characterization of a binary system of (1 – x)LiMnO2·xLi3PO4. (a) XRD patterns of LiMnO2 and Li3PO4 before and after mechanical milling as well as of mixtures of LiMnO2 and Li3PO4 with different compositions, (1 – x)LiMnO2·xLi3PO4, after mechanical milling. Schematic illustrations of crystal structures were drawn using the VESTA program. (59) (b) Phosphorus and manganese distributions obtained by STEM-EELS elemental mapping; a STEM image of the sample x = 0.2 in (1 – x)LiMnO2·xLi3PO4 is also shown. Observed P L-edge EELS spectra of points α and β are also shown. Averaged and full EELS spectra of the sample and the data for Li3PO4 after milling are also shown for comparison. (c) Clear lattice fringes can be seen in a high-resolution TEM image (also see Supporting Figure S7b).

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    Figure 2

    Figure 2. Analysis of local structures for the as-prepared sample x = 0.2, Li7/6P1/6Mn2/3O2. (a) Experimental X-ray pair distribution function (PDF), (b) Raman spectra, and (c) solid-state NMR spectra. The data for the reference materials are also shown. From the all data, the clustering of phosphorus ions is not seen for Li7/6P1/6Mn2/3O2, indicating that phosphorus is diluted to crystalline rocksalt LiMnO2.

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    Figure 3

    Figure 3. Electrochemical properties of the (1 – x)LiMnO2·xLi3PO4 binary system. (a) Galvanostatic charge–discharge curves of the samples with different compositions prepared by mechanical milling, (b) rate-capability of the sample x = 0.2, Li7/6P1/6Mn2/3O2, and (c) dependency of cutoff voltages on eversible capacities and capacity retention. The electrode loading was ca. 6.0–6.5 mg cm–2 for these electrochemical tests.

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    Figure 4

    Figure 4. Reaction mechanisms of Li7/6–yP1/6Mn2/3O2. (a) Changes in synchrotron XRD patterns. (b and c) Charge compensation processes observed by an operando soft XAS study shown by (b) a schematic illustration of the experimental setup, (34) and (c) variations in the Mn L-edge and O K-edge XAS spectra. (d and e) In situ partial gas pressures of CO2, H2, and O2 and the corresponding charge curves from nanosized Li2MnO3 and Li7/6P1/6Mn2/3O2, respectively. The dashed line is the baseline of the gas pressure, and the filled area indicates the change of the partial gas pressure through the gas evolution.

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    Figure 5

    Figure 5. Theoretical calculation of Li7/6P1/6Mn2/3O2. (a) An optimized crystal structure of rocksalt LiMnO2 after P-integration, and a local structure of the P ion in the rocksalt phase. Changes in the number of valence electrons for (b) Mn, P, and (c) O ions upon the delithiation process from Li9Mn6PO16 (∼Li7/6P1/6Mn2/3O2). Li ions are extracted from the model one by one, which is the lowest energy in the structure. Electron density changes during delithiation are also visualized in panel d. Yellow and cyan isosurfaces (drawn at 0.01 e– Bohr–1) (3) indicate electron accumulation and depletion, respectively; □ denotes vacant sites. The data for −3, – 7, and −9 Li are also shown in Supporting Figure S20.

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      The electrochem. behavior of 0.3Li2MnO3·0.7LiMn0.5Ni0.5O2 "composite' electrodes, when charged to potentials ≥ 4.5 V in lithium cells, was compared with the behavior of electrodes that were preconditioned by acid treatment. When charged to 5 V, all the lithium can be extd. from 0.3Li2MnO3·0.7LiMn0.5Ni0.5O2 in two distinct steps to yield a Mn0.65Ni0.35O2 product, the 1st step corresponding predominantly to lithium extn. from the electrode structure with the concomitant oxidn. of Ni2+ to Ni4+, and the 2nd to the electrochem. removal of Li2O from the structure. The electrode delivers a rechargeable capacity >250 mAh/g when cycled between 5.0 and 2.0 V vs. Li0; the high capacity and cycling stability are attributed to the high manganese (IV) content in the electrode over this voltage range. Acid-treatment significantly reduces the coulombic inefficiency of the initial charge/discharge cycle of the cells. The electrochem. behavior of 0.3Li2MnO3·0.7LiMn0.5Ni0.5O2 is compared with that of std. Li2MnO3 electrodes. The advantage of using the two-component notation xLi2MnO3·(1 - x)LiMn0.5Ni0.5O2 instead of the equiv. layered notation Li[Lix/(2 + x)Mn(1 + x)/(2 + x)Ni(1-x)/(2 + x)]O2 to monitor the compositional changes that occur in the electrode during electrochem. charge and discharge is highlighted.
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      Layered cathode materials Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 for lithium-ion batteries
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      Electrochemical and Solid-State Letters (2001), 4 (11), A191-A194CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      The structure, synthesis, and electrochem. behavior of layered Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 for x = 1/3, 5/12, and 1/2 are reported for the first time. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 is derived from Li2MnO3 or Li[Li1/3Mn2/3]O2 by substitution of Li+ and Mn4+ by Ni2+ while maintaining all the remaining Mn atoms in the 4+ oxidn. state. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 5/12 can deliver steady capacities of 150 and 160 mAh/g at 30 and 55°C, resp., between 3.0 and 4.4 V using a c.d. of 30 mA/g. Differential scanning calorimetry expts. on charged electrodes of Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 for x = 5/12 indicate that this material should be safer than LiCoO2. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 1/3, 5/12, and 1/2 can be cycled between 2.0 and 4.6 V to give capacities of about 200, 180, and 160 mAh/g, resp., at 30°C. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O2 with x = 1/3 gives a capacity of 220 mAh/g at 55°C between 2.0 and 4.6 V using a c.d. of 30 mA/g.
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      Robertson, A. D.; Bruce, P. G. Mechanism of electrochemical activity in Li2MnO3. Chem. Mater. 2003, 15, 1984– 1992,  DOI: 10.1021/cm030047u
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      Mechanism of Electrochemical Activity in Li2MnO3
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      Chemistry of Materials (2003), 15 (10), 1984-1992CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Lithium intercalation compds. based on lithium manganese oxides are of great importance as cathodes for rechargeable lithium batteries. It is widely accepted that Li+ may be extd. (deintercalated) from such lithium manganese oxides accompanied by oxidn. of Mn up to a max. oxidn. state of +4. However, it has been suggested recently that further Li+ removal may be possible. Among the mechanisms that have been proposed to charge balance the removal of Li+ are Mn oxidn. beyond +4 or loss of O2-. To investigate this phenomenon we have selected Li2MnO3, a layered compd. Li[Li1/3Mn2/3]O2 with a ready supply of mobile Li+ ions but with all Mn already in the +4 oxidn. state. We show that a substantial quantity of Li (at least 1.39 Li) may be removed. At 55° this occurs exclusively by oxidn. of the nonaq. electrolyte, thus generating H+ which exchange one-for-one with Li+ to form Li2-xHxMnO3. The presence of H+ between the oxide layers results in a change of the layer stacking from O3 to P3, the latter being more stable for O-H-O bonding. At 30° initial Li removal is accompanied by oxygen loss (effective removal of Li2O) but further Li+ removal involves the same proton exchange mechanism as obsd. at 55°. The reaction is partially reversible. On extended cycling the material converts to spinel.
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      Yabuuchi, N.; Kubota, K.; Aoki, Y.; Komaba, S. Understanding Particle-Size-Dependent Electrochemical Properties of Li2MnO3-Based Positive Electrode Materials for Rechargeable Lithium Batteries. J. Phys. Chem. C 2016, 120, 875– 885,  DOI: 10.1021/acs.jpcc.5b10517
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      Understanding Particle-Size-Dependent Electrochemical Properties of Li2MnO3-Based Positive Electrode Materials for Rechargeable Lithium Batteries
      Yabuuchi, Naoaki; Kubota, Kei; Aoki, Yoshinori; Komaba, Shinichi
      Journal of Physical Chemistry C (2016), 120 (2), 875-885CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Electrochem. properties of Li-excess electrode materials, Li1.2Co0.13Ni0.13Mn0.54O2, with different primary particle sizes are studied in Li cells, and phase transition behavior on continuous electrochem. cycles is systematically examd. Although the nanosize (<100 nm) sample delivers a large reversible capacity of 300 mAh g-1 at the initial cycle, capacity retention is not sufficient as a pos. electrode material. Moreover, unfavorable phase transition, gradual enrichment of trivalent manganese ions, and lowering structural symmetry is not avoidable on electrochem. cycles for a nanosize sample, which is confirmed by combined techniques of synchrotron X-ray diffraction, X-ray absorption spectroscopy, and XPS. A submicron-size sample also delivers a large reversible capacity of 250 mAh g-1 even though a slow activation process is obsd. accompanied with partial oxygen loss and migration oxide ions in the crystal lattice coupled with transition metal migration on the initial charge process. Such an unfavorable phase transition at room temp. is effectively suppressed by the use of a submicrosize sample with low surface area. However, suppression of the phase transition is found to be a kinetically controlled phenomena and is, therefore, unavoidable at elevated temps.
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      Johnson, C. S.; Li, N.; Lefief, C.; Vaughey, J. T.; Thackeray, M. M. Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 – x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7). Chem. Mater. 2008, 20, 6095– 6106,  DOI: 10.1021/cm801245r
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      Synthesis, Characterization and Electrochemistry of Lithium Battery Electrodes: xLi2MnO3·(1 - x)LiMn0.333Ni0.333Co0.333O2 (0 ≤ x ≤ 0.7)
      Johnson, Christopher S.; Li, Naichao; Lefief, Christina; Vaughey, John T.; Thackeray, Michael M.
      Chemistry of Materials (2008), 20 (19), 6095-6106CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Li- and Mn-rich layered electrode materials, represented by the formula xLi2MnO3·(1 - x)LiMO2 in which M is Mn, Ni or Co are of interest for high-power and high-capacity Li ion batteries. The synthesis, structural and electrochem. characterization of xLi2MnO3·(1 - x)LiMn0.333Ni0.333Co0.333O2 electrodes for the range 0 ≤ x ≤ 0.7 was studied. Changes that occur to the compositional, structural and electrochem. properties of the electrodes as a function of x and the importance of using a relatively high Mn content and a high charging potential (>4.4 V) to obtain a capacity of >200 mA-h/g are highlighted. Attention is given to the electrode compn. 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 which, if completely de-lithiated during charge, yields Mn0.533Ni0.233Co0.233O2, in which the Mn ions are tetravalent and, when fully discharged, LiMn0.533Ni0.233Co0.233O2, in which the av. Mn oxidn. state (3.44) is marginally below that expected for a potentially damaging Jahn-Teller distortion (3.5). Acid treatment of 0.3Li2MnO3·0.7LiMn0.333Ni0.333Co0.333O2 composite electrode structures with 0.1M HNO3 chem. activates the Li2MnO3 component and eliminates the 1st cycle capacity loss but damages electrochem. behavior, consistent with earlier reports for Li2MnO3-stabilized electrodes. Differences between electrochem. and chem. activation of the Li2MnO3 component are discussed. Electrochem. charge/discharge profiles and cyclic voltammogram data suggest that small spinel-like regions, generated in cycled Mn-rich electrodes, serve to stabilize the electrodes, particularly at low Li loadings (high potentials). The study emphasizes that, for high values of x, a relatively small LiMO2 concn. stabilizes a layered Li2MnO3 electrode to reversible Li insertion and extn. when charged to a high potential.
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      Koga, H.; Croguennec, L.; Ménétrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C. Reversible Oxygen Participation to the Redox Processes Revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 2013, 160, A786– A792,  DOI: 10.1149/2.038306jes
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      Reversible oxygen participation to the redox processes revealed for Li1.20Mn0.54Co0.13Ni0.13O2
      Koga, Hideyuki; Croguennec, Laurence; Menetrier, Michel; Douhil, Kais; Belin, Stephanie; Bourgeois, Lydie; Suard, Emmanuelle; Weill, Francois; Delmas, Claude
      Journal of the Electrochemical Society (2013), 160 (6), A786-A792CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      Materials prepd. by chem. Li deintercalation with NO2BF4 from Li1.20Mn0.54Co0.13Ni0.13O2 and chem. Li reinsertion with LiI show very similar chem. compn., oxidn. state of each transition metal ion, structural properties and electrochem. performance to those of the material recovered after the 1st electrochem. cycle. Investigations combining redox titrn., magnetic measurement, neutron diffraction and chem. analyzes reveal that uncommon redox processes are involved during the first charge at high voltage and explain the charge overcapacity and large reversible discharge capacity obtained for this material. This further assesses our proposal that oxygen, in addn. to nickel and cobalt, participates to the redox processes in charge: within the bulk oxygen is oxidized without oxygen loss, whereas at the surface oxygen is oxidized to O2 and irreversibly lost from the structure. During the subsequent discharge, in addn. to nickel, cobalt and oxygen, manganese is also slightly involved in the redox processes (redn.) to compensate for the initial surface oxygen loss.
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      Chen, C.-J.; Pang, W. K.; Mori, T.; Peterson, V. K.; Sharma, N.; Lee, P.-H.; Wu, S.-h.; Wang, C.-C.; Song, Y.-F.; Liu, R.-S. The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-ray Microscopy Study. J. Am. Chem. Soc. 2016, 138, 8824– 8833,  DOI: 10.1021/jacs.6b03932
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      The Origin of Capacity Fade in the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) Microsphere Positive Electrode: An Operando Neutron Diffraction and Transmission X-ray Microscopy Study
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      Journal of the American Chemical Society (2016), 138 (28), 8824-8833CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The mechanism of capacity fade of the Li2MnO3·LiMO2 (M = Li, Ni, Co, Mn) composite pos. electrode within a full cell was investigated using a combination of operando neutron powder diffraction and transmission X-ray microscopy methods, enabling the phase, crystallog., and morphol. evolution of the material during electrochem. cycling to be understood. The electrode was shown to initially consist of 73(1) wt % R‾3m LiMO2 with the remaining 27(1) wt % C2/m Li2MnO3 likely existing as an intergrowth. Cracking in the Li2MnO3·LiMO2 electrode particle under operando microscopy observation was revealed to be initiated by the solid-soln. reaction of the LiMO2 phase on charge to 4.55 V vs Li+/Li and intensified during further charge to 4.7 V vs Li+/Li during the concurrent two-phase reaction of the LiMO2 phase, involving the largest lattice change of any phase, and oxygen evolution from the Li2MnO3 phase. Notably, significant healing of the generated cracks in the Li2MnO3·LiMO2 electrode particle occurred during subsequent lithiation on discharge, with this rehealing being principally assocd. with the solid-soln. reaction of the LiMO2 phase. This work reveals that while it is the redn. of lattice size of electrode phases during charge that results in cracking of the Li2MnO3·LiMO2 electrode particle, with the extent of cracking correlated to the magnitude of the size change, crack healing is possible in the reverse solid-soln. reaction occurring during discharge. Importantly, it is the phase sepn. during the two-phase reaction of the LiMO2 phase that prevents the complete healing of the electrode particle, leading to pulverization over extended cycling. This work points to the minimization of behavior leading to phase sepn., such as two-phase and oxygen evolution, as a key strategy in preventing capacity fade of the electrode.
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      Yabuuchi, N. Material Design Concept of Lithium-Excess Electrode Materials with Rocksalt-Related Structures for Rechargeable Non-Aqueous Batteries. Chem. Rec. 2019, 19 (4), 690,  DOI: 10.1002/tcr.201800089
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      Material Design Concept of Lithium-Excess Electrode Materials with Rocksalt-Related Structures for Rechargeable Non-Aqueous Batteries
      Yabuuchi, Naoaki
      Chemical Record (2019), 19 (4), 690-707CODEN: CRHEAK; ISSN:1528-0691. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Dependence on lithium-ion batteries for automobile applications is rapidly increasing, and further improvement, esp. for pos. electrode materials, is indispensable to increase energy d. of lithium-ion batteries. In the past several years, many new lithium-excess high-capacity electrode materials with rocksalt-related structures have been reported. These materials deliver high reversible capacity with cationic/anionic redox and percolative lithium migration in the oxide/oxyfluoride framework structures, and recent research progresses on these electrode materials are reviewed. Material design strategies for these lithium-excess electrode materials are also described. Future possibility of high-energy non-aq. batteries with advanced pos. electrode materials is discussed for more details.
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      Ben Yahia, M.; Vergnet, J.; Saubanère, M.; Doublet, M.-L. Unified picture of anionic redox in Li/Na-ion batteries. Nat. Mater. 2019, 18, 496– 502,  DOI: 10.1038/s41563-019-0318-3
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      Unified picture of anionic redox in Li/Na-ion batteries
      Ben Yahia, Mouna; Vergnet, Jean; Saubanere, Matthieu; Doublet, Marie-Liesse
      Nature Materials (2019), 18 (5), 496-502CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
      Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy d. of rechargeable batteries. Ever since, numerous electrodes delivering extra anionic capacity beyond the theor. cationic capacity have been reported. Unfortunately, most often the anionic capacity achieved in charge is partly irreversible in discharge. A unified picture of anionic redox in A-rich-TMOs is designed here to identify the electronic origin of this irreversibility and to propose new directions to improve the cycling performance of the electrodes. The electron localization function is introduced as a holistic tool to unambiguously locate the O lone pairs in the structure and follow their participation in the redox activity of A-rich-TMOs. The charge-transfer gap of transition metal oxides is proposed as the pertinent observable to quantify the amt. of extra capacity achievable in charge and its reversibility in discharge, irresp. of the material chem. compn. From this generalized approach, the reversibility of the anionic capacity is limited to a crit. no. of O holes per O, hO ≤ 1/3.
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      Sudayama, T.; Uehara, K.; Mukai, T.; Asakura, D.; Shi, X.-M.; Tsuchimoto, A.; Mortemard de Boisse, B.; Shimada, T.; Watanabe, E.; Harada, Y.; Nakayama, M.; Okubo, M.; Yamada, A. Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes. Energy Environ. Sci. 2020, 13 (5), 1492,  DOI: 10.1039/C9EE04197D
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      Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes
      Sudayama, Takaaki; Uehara, Kazuki; Mukai, Takahiro; Asakura, Daisuke; Shi, Xiang-Mei; Tsuchimoto, Akihisa; Mortemard de Boisse, Benoit; Shimada, Tatau; Watanabe, Eriko; Harada, Yoshihisa; Nakayama, Masanobu; Okubo, Masashi; Yamada, Atsuo
      Energy & Environmental Science (2020), 13 (5), 1492-1500CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      High-energy-d. batteries have been a long-standing target toward sustainability, but the energy d. of state-of-the-art lithium-ion batteries is limited in part by the small capacity of the pos. electrode materials. Although employing the addnl. oxygen-redox reaction of Li-excess transition-metal oxides is an attractive approach to increase the capacity, an at.-level understanding of the reaction mechanism has not been established so far. Here, using bulk-sensitive resonant inelastic X-ray scattering spectroscopy combined with ab initio computations, we demonstrate the presence of a localized oxygen 2p orbital weakly hybridized with transition metal t2g orbitals that was theor. predicted to play a key role in oxygen-redox reactions. After oxygen oxidn., the hole in the oxygen 2p orbital is stabilized by the generation of either a (σ + π) multiorbital bond through strong π back-donation or peroxide O22- through oxygen dimerization. The multiorbital bond formation with σ-accepting and π-donating transition metals can thus lead to reversible oxygen-redox reaction.
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      Zhao, W.; Yamaguchi, K.; Sato, T.; Yabuuchi, N. Li4/3Ni1/3Mo1/3O2 – LiNi1/2Mn1/2O2 Binary System as High Capacity Positive Electrode Materials for Rechargeable Lithium Batteries. J. Electrochem. Soc. 2018, 165, A1357– A1362,  DOI: 10.1149/2.0661807jes
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      Li4/3Ni1/3Mo1/3O2 - LiNi1/2Mn1/2O2 binary system as high capacity positive electrode materials for rechargeable lithium Batteries
      Zhao, Wenwen; Yamaguchi, Kazuma; Sato, Takahito; Yabuuchi, Naoaki
      Journal of the Electrochemical Society (2018), 165 (7), A1357-A1362CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A binary system of x Li4/3Ni1/3Mo1/3O2 - (1-x) LiNi1/2Mn1/2O2 is studied as high-capacity pos. electrode materials for rechargeable lithium batteries. Structural and electrochem. properties of oxides with different compns. in this binary system are examd. Mo ordering is retained for 1 ≤ x ≤ 1/3 with a monoclinic symmetry and disappears for x ≤ 1/6 with a rhombohedral symmetry. Compared with Li4/3Ni1/3Mo1/3O2, partial substitution of Mn for Mo lead to the improvement of reversible capacity and redn. of polarization. For Li6/5Ni2/5Mn1/5Mo1/5O2 (x = 1/3) and Li9/8Ni7/16Mn5/16Mo1/8O2 (x = 1/6), high reversible capacities of around 200 mAh g-1 are obtained. Improved cycling performance is achieved through the optimization of voltage ranges. Further structural characterization by ex-situ XRD reveals that the improved reversibility for the Mn-substituted samples mainly results from the suppression of Mo migration during cycling, probably assocd. with partial oxygen loss.
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      Yabuuchi, N.; Nakayama, M.; Takeuchi, M.; Komaba, S.; Hashimoto, Y.; Mukai, T.; Shiiba, H.; Sato, K.; Kobayashi, Y.; Nakao, A.; Yonemura, M.; Yamanaka, K.; Mitsuhara, K.; Ohta, T. Origin of Stabilization and Destabilization in Solid-State Redox Reaction of Oxide Ions for Rechargeable Lithium Batteries. Nat. Commun. 2016, 7 (1), 13814,  DOI: 10.1038/ncomms13814
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      15
      Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries
      Yabuuchi, Naoaki; Nakayama, Masanobu; Takeuchi, Mitsue; Komaba, Shinichi; Hashimoto, Yu; Mukai, Takahiro; Shiiba, Hiromasa; Sato, Kei; Kobayashi, Yuki; Nakao, Aiko; Yonemura, Masao; Yamanaka, Keisuke; Mitsuhara, Kei; Ohta, Toshiaki
      Nature Communications (2016), 7 (), 13814CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Further increase in energy d. of lithium batteries is needed for zero emission vehicles. However, energy d. is restricted by unavoidable theor. limits for pos. electrodes used in com. applications. One possibility towards energy densities exceeding these limits is to utilize anion (oxide ion) redox, instead of classical transition metal redox. Nevertheless, origin of activation of the oxide ion and its stabilization mechanism are not fully understood. Here we demonstrate that the suppression of formation of superoxide-like species on lithium extn. results in reversible redox for oxide ions, which is stabilized by the presence of relatively less covalent character of Mn4+ with oxide ions without the sacrifice of electronic cond. On the basis of these findings, we report an electrode material, whose metallic constituents consist only of 3d transition metal elements. The material delivers a reversible capacity of 300 mAh g-1 based on solid-state redox reaction of oxide ions.
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    16. 16
      House, R. A.; Jin, L.; Maitra, U.; Tsuruta, K.; Somerville, J. W.; Förstermann, D. P.; Massel, F.; Duda, L.; Roberts, M. R.; Bruce, P. G. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ. Sci. 2018, 11, 926– 932,  DOI: 10.1039/C7EE03195E
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      16
      Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox
      House, Robert A.; Jin, Liyu; Maitra, Urmimala; Tsuruta, Kazuki; Somerville, James W.; Forstermann, Dominic P.; Massel, Felix; Duda, Laurent; Roberts, Matthew R.; Bruce, Peter G.
      Energy & Environmental Science (2018), 11 (4), 926-932CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      The quantity of charge stored in transition metal oxide intercalation cathodes for Li or Na batteries is not limited by transition metal redox reactions but can also access redox reactions on O; examples include Li1.2Ni0.13Mn0.54Co0.13O2, Li2Ru0.75Sn0.25O3, Li1.2Nb0.3Mn0.4O2, Na2RuO3 and Na2/3Mg0.28Mn0.72O2. Here we show that oxyfluorides can also exhibit charge storage by O-redox. We report the discovery of lithium manganese oxyfluoride, specifically the compn., Li1.9Mn0.95O2.05F0.95, with a high capacity to store charge of 280 mA h g-1 (corresponding to 960 W h kg-1) of which almost half, 130 mA h g-1, arises from O-redox. This material has a disordered cubic rocksalt structure and the voltage-compn. curve is significantly more reversible compared with ordered Li-rich layered cathodes. Unlike lithium manganese oxides such as the ordered layered rocksalt Li2MnO3, Li1.9Mn0.95O2.05F0.95 does not exhibit O loss from the lattice. The material is synthesized using a simple, one-pot mechanochem. procedure.
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    17. 17
      Lee, J.; Kitchaev, D. A.; Kwon, D.-H.; Lee, C.-W.; Papp, J. K.; Liu, Y.-S.; Lun, Z.; Clément, R. J.; Shi, T.; McCloskey, B. D.; Guo, J.; Balasubramanian, M.; Ceder, G. Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials. Nature 2018, 556, 185– 190,  DOI: 10.1038/s41586-018-0015-4
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      17
      Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials
      Lee, Jinhyuk; Kitchaev, Daniil A.; Kwon, Deok-Hwang; Lee, Chang-Wook; Papp, Joseph K.; Liu, Yi-Sheng; Lun, Zhengyan; Clement, Raphaele J.; Shi, Tan; McCloskey, Bryan D.; Guo, Jinghua; Balasubramanian, Mahalingam; Ceder, Gerbrand
      Nature (London, United Kingdom) (2018), 556 (7700), 185-190CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      There is an urgent need for low-cost, resource-friendly, high-energy-d. cathode materials for lithium-ion batteries to satisfy the rapidly increasing need for elec. energy storage. To replace the nickel and cobalt, which are limited resources and are assocd. with safety problems, in current lithium-ion batteries, high-capacity cathodes based on manganese would be particularly desirable owing to the low cost and high abundance of the metal, and the intrinsic stability of the Mn4+ oxidn. state. Here we present a strategy of combining high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn2+/Mn4+ double redox couple into lithium-excess cathode materials. The lithium-rich cathodes thus produced have high capacity and energy d. The use of the Mn2+/Mn4+ redox reduces oxygen redox activity, thereby stabilizing the materials, and opens up new opportunities for the design of high-performance manganese-rich cathodes for advanced lithium-ion batteries.
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    18. 18
      Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S. High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 7650– 7655,  DOI: 10.1073/pnas.1504901112
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      18
      High-capacity electrode materials for rechargeable lithium batteries: Li3NbO4-based system with cation-disordered rocksalt structure
      Yabuuchi, Naoaki; Takeuchi, Mitsue; Nakayama, Masanobu; Shiiba, Hiromasa; Ogawa, Masahiro; Nakayama, Keisuke; Ohta, Toshiaki; Endo, Daisuke; Ozaki, Tetsuya; Inamasu, Tokuo; Sato, Kei; Komaba, Shinichi
      Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (25), 7650-7655CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Rechargeable lithium batteries have rapidly risen to prominence as fundamental devices for green and sustainable energy development. Lithium batteries are now used as power sources for elec. vehicles. However, materials innovations are still needed to satisfy the growing demand for increasing energy d. of lithium batteries. In the past decade, lithium-excess compds., Li2MeO3 (Me = Mn4+, Ru4+, etc.), have been extensively studied as high-capacity pos. electrode materials. Although the origin as the high reversible capacity has been a debatable subject for a long time, recently it has been confirmed that charge compensation is partly achieved by solid-state redox of nonmetal anions (i.e., oxide ions), coupled with solid-state redox of transition metals, which is the basic theory used for classic lithium insertion materials, such as LiMeO2 (Me = Co3+, Ni3+, etc.). Herein, as a compd. with further excess lithium contents, a cation-ordered rocksalt phase with lithium and pentavalent niobium ions, Li3NbO4, is first examd. as the host structure of a new series of high-capacity pos. electrode materials for rechargeable lithium batteries. Approx. 300 mAh·g-1 of high-reversible capacity at 50 °C is exptl. obsd., which partly originates from charge compensation by solid-state redox of oxide ions. It is proposed that such a charge compensation process by oxide ions is effectively stabilized by the presence of electrochem. inactive niobium ions. These results will contribute to the development of a new class of high-capacity electrode materials, potentially with further lithium enrichment (and fewer transition metals) in the close-packed framework structure with oxide ions.
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    19. 19
      Ji, H.; Wu, J.; Cai, Z.; Liu, J.; Kwon, D.-H.; Kim, H.; Urban, A.; Papp, J. K.; Foley, E.; Tian, Y.; Balasubramanian, M.; Kim, H.; Clément, R. J.; McCloskey, B. D.; Yang, W.; Ceder, G. Ultrahigh power and energy density in partially ordered lithium-ion cathode materials. Nat. Energy 2020, 5, 213– 221,  DOI: 10.1038/s41560-020-0573-1
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      19
      Ultrahigh power and energy density in partially ordered lithium-ion cathode materials
      Ji, Huiwen; Wu, Jinpeng; Cai, Zijian; Liu, Jue; Kwon, Deok-Hwang; Kim, Hyunchul; Urban, Alexander; Papp, Joseph K.; Foley, Emily; Tian, Yaosen; Balasubramanian, Mahalingam; Kim, Haegyeom; Clement, Raphaele J.; McCloskey, Bryan D.; Yang, Wanli; Ceder, Gerbrand
      Nature Energy (2020), 5 (3), 213-221CODEN: NEANFD; ISSN:2058-7546. (Nature Research)
      The rapid market growth of rechargeable batteries requires electrode materials that combine high power and energy and are made from earth-abundant elements. Here we show that combining a partial spinel-like cation order and substantial lithium excess enables both dense and fast energy storage. Cation overstoichiometry and the resulting partial order is used to eliminate the phase transitions typical of ordered spinels and enable a larger practical capacity, while lithium excess is synergistically used with fluorine substitution to create a high lithium mobility. With this strategy, we achieved specific energies greater than 1,100 Wh kg-1 and discharge rates up to 20 A g-1. Remarkably, the cathode materials thus obtained from inexpensive manganese present a rare case wherein an excellent rate capability coexists with a reversible oxygen redox activity. Our work shows the potential for designing cathode materials in the vast space between fully ordered and disordered compds.
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    20. 20
      Kobayashi, Y.; Sawamura, M.; Kondo, S.; Harada, M.; Noda, Y.; Nakayama, M.; Kobayakawa, S.; Zhao, W.; Nakao, A.; Yasui, A.; Rajendra, H. B.; Yamanaka, K.; Ohta, T.; Yabuuchi, N. Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3–LiMnO2 binary system. Mater. Today 2020, 37, 43,  DOI: 10.1016/j.mattod.2020.03.002
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      20
      Activation and stabilization mechanisms of anionic redox for Li storage applications: Joint experimental and theoretical study on Li2TiO3-LiMnO2 binary system
      Kobayashi, Yuki; Sawamura, Miho; Kondo, Sayaka; Harada, Maho; Noda, Yusuke; Nakayama, Masanobu; Kobayakawa, Sho; Zhao, Wenwen; Nakao, Aiko; Yasui, Akira; Rajendra, Hongahally Basappa; Yamanaka, Keisuke; Ohta, Toshiaki; Yabuuchi, Naoaki
      Materials Today (Oxford, United Kingdom) (2020), 37 (), 43-55CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      A binary system of Li2TiO3-LiMnO2 is systematically examd. by joint exptl. and theor. studies as electrode materials for Li storage applications. Increase in a fraction of Li2TiO3 effectively activates anionic redox, and thus holes are reversibly formed on oxygen by electrochem. oxidn. Such holes are energetically stabilized through π-type interaction with Mn t2g orbital as suggested by theor. calcn. However, excess enrichment of Li2TiO3 fractions in this binary system results in the oxygen loss as an irreversible process on delithiation because of a non-bonding character for Ti-O bonds coupled with the formation of O-O dimers, which are chem. and electrochem. unstable species. Detailed electrochem. study clearly shows that Li migration kinetics is relatively slow, presumably coupled with low electronic cond. Nevertheless, nanosizing of primary particles is an effective strategy to overcome this limitation. The nanosized sample prepd. by mech. milling delivers a large reversible capacity, ∼300 mA h g-1, even at room temp. and shows much improved capacity retention. Formation and stabilization of holes for the nanosized sample are also directly evidenced by soft X-ray absorption spectroscopy. From these results, factors affecting the reversibility of anionic redox as emerging new chem. and its possibility for energy storage applications are discussed in more details.
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      Croguennec, L.; Deniard, P.; Brec, R. Electrochemical Cyclability of Orthorhombic LiMnO2: Characterization of Cycled Materials. J. Electrochem. Soc. 1997, 144, 3323– 3330,  DOI: 10.1149/1.1838013
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      21
      Electrochemical cyclability of orthorhombic LiMnO2. Characterization of cycled materials
      Croguennec, L.; Deniard, P.; Brec, R.
      Journal of the Electrochemical Society (1997), 144 (10), 3323-3330CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The electrochem. removal of Li from orthorhombic LiMnO2 (o-LiMnO2) leads to a phase transition with a first plateau at ∼3.7 V. This corresponds to the formation of a spinel-like material; a possible transition to a rhombohedral Li2MnO2 phase was ruled out through structural and crystal-site energy considerations. Several electrochem. cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals, the fewer the no. of cycles needed. The capacity difference between large and small crystallite/crystal compds. is ascribed to kinetic reasons as shown by ex-situ x-ray diffraction analyses and quasi-equil. electrochem. studies. Capacities as high as 200 A-h/kg were found for ≈0.3 μm crystal size materials. Contrary to the spinel prepd. at high temp., the electrochem. obtained spinel-like phase cycled very well in the 2.5-4.3 V range, suggesting structural differences between the 2 materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the Mn second neighbors for 2 compds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO2.
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      Keffer, C.; Mighell, A.; Mauer, F.; Swanson, H.; Block, S. CRYSTAL STRUCTURE OF TWINNED LOW-TEMPERATURE LITHIUM PHOSPHATE. Inorg. Chem. 1967, 6, 119,  DOI: 10.1021/ic50047a027
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      22
      Crystal structure of twinned low-temperature lithium phosphate
      Keffer, Charles; Mighell, Alan D.; Mauer, Floyd; Swanson, Howard E.; Block, Stanley
      Inorganic Chemistry (1967), 6 (1), 119-25CODEN: INOCAJ; ISSN:0020-1669.
      Li3PO4 prepd. by pptn. from an aq. soln. differs from the form that has been described in the literature. When heated it transforms irreversibly at 502 ± 5° to the familiar form. The low-temp. form crystallizes in space group Pmn21 with a0 6.1150 ± 0.0010, b0 5.2394 ± 0.0011, and c0 4.8554 ± 0.0010 A.; Z = 2. It exhibits merohederal twinning with the twin plane normal to the z axis. The multiplicity of the predominant image is 0.75. All atoms are tetrahedrally coordinated. The final reliability index is 0.054.
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      Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B. Effect of Structure on the Fe3 + /Fe2 + Redox Couple in Iron Phosphates. J. Electrochem. Soc. 1997, 144, 1609– 1613,  DOI: 10.1149/1.1837649
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      23
      Effect of structure on the Fe3+/Fe2+ redox couple in iron phosphates
      Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Okada, S.; Goodenough, J. B.
      Journal of the Electrochemical Society (1997), 144 (5), 1609-1613CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      To understand the role of structure on the position of the octahedral Fe3+/Fe2+ redox couple in compds. having the same polyanions, four iron phosphates: Li3Fe2(PO4)3, LiFeP2O7, Fe4(P2O7)3, and LiFePO4 were studied. They vary in structure as well as in the manner in which the octahedral iron atoms are linked to each other. The Fe3+/Fe2+ redox couple in the above compds. lies at 2.8, 2.9, 3.1, and 3.5 eV, resp., below the Fermi level of lithium. The reason for the difference in the position of the redox couples is related to changes in the P-O bond lengths as well as to changes in the cryst. elec. field at the iron sites. The electrochem. characteristics of the iron phosphates are described.
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      Nagao, K.; Hayashi, A.; Deguchi, M.; Tsukasaki, H.; Mori, S.; Tatsumisago, M. Amorphous LiCoO2Li2SO4 active materials: Potential positive electrodes for bulk-type all-oxide solid-state lithium batteries with high energy density. J. Power Sources 2017, 348, 1– 8,  DOI: 10.1016/j.jpowsour.2017.02.038
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      24
      Amorphous LiCoO2-Li2SO4 active materials: Potential positive electrodes for bulk-type all-oxide solid-state lithium batteries with high energy density
      Nagao, Kenji; Hayashi, Akitoshi; Deguchi, Minako; Tsukasaki, Hirofumi; Mori, Shigeo; Tatsumisago, Masahiro
      Journal of Power Sources (2017), 348 (), 1-8CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Newly amorphous Li2-x/100Cox/100S1-x/100O4-x/50 (xLiCoO2·(100-x)Li2SO4 (mol%)) pos. electrode active materials are synthesized using mechanochem. techniques. SEM observation indicates that av. radii of the Li1.2Co0.8S0.2O2.4 (80LiCoO2·20Li2SO4 (mol%)) particles are about 3 μm. HR-TEM images indicate that the particles comprise nano-cryst. and amorphous phases. The cryst. phase is attributable to cubic LiCoO2 phase. These active materials exhibit a high electronic cond. of around 10-5-10-1 S cm-1 and an ionic cond. of around 10-7-10-6 S cm-1 at room temp. Bulk-type all-oxide solid-state cells (Li-In alloy/Li3BO3-based glass-ceramic electrolyte/amorphous Li2-x/100Cox/100S1-x/100O4-x/50) are fabricated by pressing at room temp. without high temp. sintering. Although the cell with the milled LiCoO2 shows no capacity, the cell using the Li1.2Co0.8S0.2O2.4 electrode with no conductive components (ca. 150 μm thickness) operates as a secondary battery at 100°, with an av. discharge potential of 3.3 V (vs. Li+/Li) and discharge capacity of 163°mAh°g-1. A pos. electrode with large amts. of active materials is suitable for achieving high energy d. in all-solid-state batteries. These newly synthesized amorphous Li2-x/100Cox/100S1-x/100O4-x/50 electrodes with ionic and electronic conductivities and good processability meet that demand.
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      Kitajou, A.; Kobayashi, E.; Okada, S. Electrochemical Performance of a Novel Cathode material “LiFeOF” for Li-ion Batteries. Electrochemistry 2015, 83, 885– 888,  DOI: 10.5796/electrochemistry.83.885
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      Electrochemical performance of a novel cathode material "LiFeOF" for Li-ion batteries
      Kitajou, Ayuko; Kobayashi, Eiji; Okada, Shigeto
      Electrochemistry (Tokyo, Japan) (2015), 83 (10), 885-888CODEN: EECTFA; ISSN:1344-3542. (Electrochemical Society of Japan)
      Iron-based conversion cathode, FeOF is attractive, because of the low cost and the large specific capacity. However, the synthesis is not easy and it cannot be used as cathode against carbonaceous anode. To overcome these drawbacks, we focused on the LiFeOF phase, which has the same chem. compn. as the discharged intermediate product of FeOF cathode. LiFeOF can be easily synthesized from LiF and FeO by the dry ball-milling method at room temp. The reversible capacity was 292 mAh·g-1 with an av. voltage of 2.5 V and an energy d. over 700 Wh·kg-1, which is higher than that of LiFePO4. In addn., we confirmed the feasibility of LiFeOF cathode against Li44Ti55O12 anode.
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      Chen, R. Y.; Ren, S. H.; Knapp, M.; Wang, D.; Witter, R.; Fichtner, M.; Hahn, H. Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage. Adv. Energy Mater. 2015, 5, 1401814,  DOI: 10.1002/aenm.201401814
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      Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage
      Chen, Ruiyong; Ren, Shuhua; Knapp, Michael; Wang, Di; Witter, Raiker; Fichtner, Maximilian; Hahn, Horst
      Advanced Energy Materials (2015), 5 (9), 1401814/1-1401814/7CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      This article discusses about the disordered lithium-rich oxyfluoride as stable host for enhanced Li+ intercalation storage.
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      Hoshino, S.; Glushenkov, A. M.; Ichikawa, S.; Ozaki, T.; Inamasu, T.; Yabuuchi, N. Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries. ACS Energy Letters 2017, 2, 733– 738,  DOI: 10.1021/acsenergylett.7b00037
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      27
      Reversible Three-Electron Redox Reaction of Mo3+/Mo6+ for Rechargeable Lithium Batteries
      Hoshino, Satoshi; Glushenkov, Alexey M.; Ichikawa, Shinnosuke; Ozaki, Tetsuya; Inamasu, Tokuo; Yabuuchi, Naoaki
      ACS Energy Letters (2017), 2 (4), 733-738CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      To increase the energy d. of lithium batteries, the development of high-capacity pos. electrode materials is essential. Herein, the use is proposed of a three-electron redox reaction of Mo3+/Mo6+ for a new series of high-capacity lithium insertion materials. In this study, a binary system of LiMoO2-Li3NbO4 is targeted, and nanosize and metastable Li9/7Nb2/7Mo3/7O2 is successfully prepd. by a mech. milling process. The sample delivers a large reversible capacity of ∼280 mAhg-1 in a Li cell with good capacity retention. On the basis of these results, the future possibility of high-capacity electrode materials with a three-electron Mo3+/Mo6+ redox reaction is discussed.
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      Sato, T.; Sato, K.; Zhao, W.; Kajiya, Y.; Yabuuchi, N. Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO2 and NaMnO2. J. Mater. Chem. A 2018, 6, 13943– 13951,  DOI: 10.1039/C8TA03667E
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      28
      Metastable and nanosize cation-disordered rocksalt-type oxides: revisit of stoichiometric LiMnO2 and NaMnO2
      Sato, Takahito; Sato, Kei; Zhao, Wenwen; Kajiya, Yoshio; Yabuuchi, Naoaki
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (28), 13943-13951CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Stoichiometric LiMnO2 and NaMnO2 with a cation-disordered rock salt-type structure as metastable polymorphs were successfully prepd. by mech. milling. Although cation-disordered rock salt phases with a stoichiometric compn. (Li:Mn molar ratio = 1:1) are expected to be electrochem. less active, both samples show superior performance as electrode materials when compared with thermodynamically stable layered phases in Li/Na cells. Both metastable samples deliver large reversible capacities, which correspond to >80% of their theor. capacities, with relatively small polarization on the basis of reversible Mn3+/Mn4+ redox. Moreover, for rock salt LiMnO2, the phase transition into a spinel phase is effectively suppressed compared with a thermodynamically stable phase. The electrode reversibility of NaMnO2 is also drastically improved by the use of the metastable phase with good capacity retention. Metastable phases with unique nanostructures open a new path for the design of advanced electrode materials with high energy d., and thus a broad impact is anticipated for rechargeable Li/Na battery applications.
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      Tsuchiya, Y.; Glushenkov, A. M.; Yabuuchi, N. Effect of Nanosizing on Reversible Sodium Storage in a NaCrO2 Electrode. ACS Applied Nano Materials 2018, 1, 364– 370,  DOI: 10.1021/acsanm.7b00207
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      29
      Effect of Nanosizing on Reversible Sodium Storage in a NaCrO2 Electrode
      Tsuchiya, Yuka; Glushenkov, Alexey M.; Yabuuchi, Naoaki
      ACS Applied Nano Materials (2018), 1 (1), 364-370CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      The effect of nanosizing on the sodium storage performance in NaCrO2 is systematically examd. Cation-disordered rock-salt-type and nanosized NaCrO2 is prepd. by mech. milling, and layered O3-type and nanosized NaCrO2 is prepd. by heat treatment of the rock-salt phase. The observation by high-resoln. transmission electron microscopy reveals that secondary particles consist of highly cryst. and nanosized NaCrO2 primary particles with enriched grain boundaries. Such morphol. features affect the voltage profiles in sodium cells, leading to an S-shaped profile with a single-phase reaction even for layered NaCrO2, in which a biphasic reaction dominates because of a large repulsive interaction between Na ions. Moreover, the O3-P3 phase transition is suppressed for the heat-treated sample with the presence of enriched grain boundaries. The suppression of the phase transition is proposed to be due to the cancellation of CrO2 layers gliding for the incoherently aligned grain boundaries. Thus, good capacity retention as electrode materials is realized compared with as-prepd. bulk O3 NaCrO2. Nanotechnol. potentially changes materials design strategies for sodium insertion materials, leading to the development of innovative rechargeable sodium batteries in the future.
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    30. 30
      Sabi, Y.; Sato, S.; Hayashi, S.; Furuya, T.; Kusanagi, S. A new class of amorphous cathode active material LixMyPOz (M = Ni, Cu, Co, Mn, Au, Ag, Pd). J. Power Sources 2014, 258, 54– 60,  DOI: 10.1016/j.jpowsour.2014.02.021
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      30
      A new class of amorphous cathode active material LixMyPOz (M = Ni, Cu, Co, Mn, Au, Ag, Pd)
      Sabi, Yuichi; Sato, Susumu; Hayashi, Saori; Furuya, Tatsuya; Kusanagi, Susumu
      Journal of Power Sources (2014), 258 (), 54-60CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      A new class of amorphous cathode active materials LixMyPOz (LiMPO) is proposed. The materials are sputter deposited to form a thin film using Li3PO4 together with metal or metal oxide targets. Among several materials tested as thin-film battery, materials suitable as working material were found to be with M = Ni, Cu, Co, Mn, Au, Ag, Pd. The property is intensively studied for LixCuyPOz (LiCuPO) and LixNiyPOz (LiNiPO). Those materials show a wide compn. margin, such as y of 1-3, and a high capacity for LiNiPO with a max. value of 330 mAh g-1. The capability to charge and discharge at a high rate is shown up to 30 C. This preliminary report reveals its high potentiality for further optimization.
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    31. 31
      Jung, S.-K.; Kim, H.; Cho, M. G.; Cho, S.-P.; Lee, B.; Kim, H.; Park, Y.-U.; Hong, J.; Park, K.-Y.; Yoon, G.; Seong, W. M.; Cho, Y.; Oh, M. H.; Kim, H.; Gwon, H.; Hwang, I.; Hyeon, T.; Yoon, W.-S.; Kang, K. Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries. Nat. Energy 2017, 2, 16208,  DOI: 10.1038/nenergy.2016.208
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      Lithium-free transition metal monoxides for positive electrodes in lithium-ion batteries
      Jung, Sung-Kyun; Kim, Hyunchul; Cho, Min Gee; Cho, Sung-Pyo; Lee, Byungju; Kim, Hyungsub; Park, Young-Uk; Hong, Jihyun; Park, Kyu-Young; Yoon, Gabin; Seong, Won Mo; Cho, Yongbeom; Oh, Myoung Hwan; Kim, Haegyeom; Gwon, Hyeokjo; Hwang, Insang; Hyeon, Taeghwan; Yoon, Won-Sub; Kang, Kisuk
      Nature Energy (2017), 2 (2), 16208CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
      Lithium-ion batteries based on intercalation compds. have dominated the advanced portable energy storage market. The pos. electrode materials in these batteries belong to a material group of lithium-conducting crystals that contain redox-active transition metal and lithium. Materials without lithium-conducting paths or lithium-free compds. could be rarely used as pos. electrodes due to the incapability of reversible lithium intercalation or the necessity of using metallic lithium as neg. electrodes. These constraints have significantly limited the choice of materials and retarded the development of new pos. electrodes in lithium-ion batteries. Here, we demonstrate that lithium-free transition metal monoxides that do not contain lithium-conducting paths in their crystal structure can be converted into high-capacity pos. electrodes in the electrochem. cell by initially decorating the monoxide surface with nanosized lithium fluoride. This unusual electrochem. behavior is attributed to a surface conversion reaction mechanism in contrast with the classic lithium intercalation reaction. Our findings will offer a potential new path in the design of pos. electrode materials in lithium-ion batteries.
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    32. 32
      Popović, L.; Manoun, B.; de Waal, D.; Nieuwoudt, M. K.; Comins, J. D. Raman spectroscopic study of phase transitions in Li3PO4. J. Raman Spectrosc. 2003, 34, 77– 83,  DOI: 10.1002/jrs.954
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      Raman spectroscopic study of phase transitions in Li3PO4
      Popovic, L.; Manoun, B.; de Waal, D.; Nieuwoudt, M. K.; Comins, J. D.
      Journal of Raman Spectroscopy (2003), 34 (1), 77-83CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
      Although three forms of Li phosphate are known, a complete structural description of the highest temp. polymorph has not yet been given. The phase transitions of Li phosphate were studied at high temps. using Raman microscopy and x-ray powder diffraction. Both transitions were obsd. by following the temp. dependence of the totally sym. Raman stretching vibration of PO43-. Currently available structural information on the a form, resulting in P-O bond lengths of 1.787-1.899 Å, as detd. by valence bond calcns., are disputed here. A correlation between Raman wavenumber and bond length in inorg. phosphates ests. the P-O bond length in α-Li3PO4 to be around 1.57(1) Å, which is in closer agreement with values for other orthophosphates of between 1.50 and 1.58 Å.
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    33. 33
      Lee, Y. J.; Grey, C. P. 6Li Magic-Angle Spinning (MAS) NMR Study of Electron Correlations, Magnetic Ordering, and Stability of Lithium Manganese(III) Oxides. Chem. Mater. 2000, 12, 3871– 3878,  DOI: 10.1021/cm000469t
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      33
      6Li Magic-Angle Spinning (MAS) NMR Study of Electron Correlations, Magnetic Ordering, and Stability of Lithium Manganese(III) Oxides
      Lee, Young Joo; Grey, Clare P.
      Chemistry of Materials (2000), 12 (12), 3871-3878CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      Three different lithium manganese(III) oxides (the orthorhombic LiMnO2 phase, monoclinic layered LiMnO2 and tetragonal Li2Mn2O4) were studied with 6Li magic-angle spinning (MAS) NMR spectroscopy. Much smaller shifts of 36-143 ppm are obsd. for the Mn(III) phases, in comparison to the other lithium manganese oxides with manganese oxidn. states varying from +3.5 to +4. The NMR shift of this system is governed by the Fermi-contact interaction and consequently, is controlled by the lithium local environment. For orthorhombic LiMnO2, one resonance at 36 ppm is obsd. between -39 and 283° and a single resonance at -5 ppm is seen <-39°, indicating a magnetic phase transition involving a change from short-range electronic correlations to long-range antiferromagnetic ordering. The 6Li NMR shift of the resonances of the monoclinic and tetragonal phases show very little change with temp. in the range studied (-136 to 283°), implying that short-range antiferromagnetic interactions between Mn cations also exist for these phases. No evidence for a magnetic phase transition to three-dimensional ordering, however, is obsd. Two different lithium sites are identified in Li2Mn2O4, which are assigned to lithium on the 8c octahedral and 4a tetrahedral sites. Samples with lithium on the 8c site only were obtained using mild synthesis conditions, whereas occupation of both sites was obtained with more stringent conditions or at high temps.
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    34. 34
      Hattori, M.; Yamamoto, K.; Matsui, M.; Nakanishi, K.; Mandai, T.; Choudhary, A.; Tateyama, Y.; Sodeyama, K.; Uchiyama, T.; Orikasa, Y.; Tamenori, Y.; Takeguchi, T.; Kanamura, K.; Uchimoto, Y. Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode. J. Phys. Chem. C 2018, 122, 25204– 25210,  DOI: 10.1021/acs.jpcc.8b08558
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      Role of Coordination Structure of Magnesium Ions on Charge and Discharge Behavior of Magnesium Alloy Electrode
      Hattori, Masashi; Yamamoto, Kentaro; Matsui, Masaki; Nakanishi, Koji; Mandai, Toshihiko; Choudhary, Ashu; Tateyama, Yoshitaka; Sodeyama, Keitaro; Uchiyama, Tomoki; Orikasa, Yuki; Tamenori, Yusuke; Takeguchi, Tatsuya; Kanamura, Kiyoshi; Uchimoto, Yoshiharu
      Journal of Physical Chemistry C (2018), 122 (44), 25204-25210CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Mechanism of Mg ion alloying reaction into Bi electrode in Mg bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/MeCN (AN) and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF) electrolyte was examd. by a combination of operando soft x-ray absorption spectroscopy (XAS), Raman spectroscopy, and d. functional theory (DFT) calcns. In 0.5M Mg(TFSA)2/AN, the Mg ions alloying reaction occurred, whereas the alloying reaction did not occur in 0.5M Mg(TFSA)2/2-MeTHF. Raman spectroscopy showed that <15% of [TFSA]- coordinates with Mg ions in 0.5M Mg(TFSA)2/AN, while >90% of [TFSA]- coordinates with Mg ions in Mg(TFSA)2/2-MeTHF. Using operando XAS measurements, electronic and local structure of Mg ion changed similarly upon cathodic polarization in both electrolytes. The difference of the behavior of alloy formation should be affected by the difference of coordinate structure of [TFSA]- in both electrolytes. The authors' DFT calcn. results indicates [TFSA]- coordinated to Mg ions undergoes redn. decompn. more easily than [TFSA]- uncoordinated to Mg ions. In 0.5M Mg(TFSA)2/2-MeTHF, the [TFSA]- coordinating to Mg ions undergoes redn. decompn., which inhibits the alloying reaction into the Bi electrode. However, in 0.5M Mg(TFSA)2/AN, the [TFSA]- redn. decompn. occurs relatively slowly because of the weak coordination between [TFSA]- and Mg ions, which allows the Mg ions alloying into the Bi electrode in the electrolyte.
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    35. 35
      Yamamoto, K.; Zhou, Y.; Yabuuchi, N.; Nakanishi, K.; Yoshinari, T.; Kobayashi, T.; Kobayashi, Y.; Yamamoto, R.; Watanabe, A.; Orikasa, Y.; Tsuruta, K.; Park, J.; Byon, H. R.; Tamenori, Y.; Ohta, T.; Uchimoto, Y. Charge Compensation Mechanism of Lithium-Excess Metal Oxides with Different Covalent and Ionic Characters Revealed by Operando Soft and Hard X-ray Absorption Spectroscopy. Chem. Mater. 2020, 32, 139– 147,  DOI: 10.1021/acs.chemmater.9b02838
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      Charge Compensation Mechanism of Lithium-Excess Metal Oxides with Different Covalent and Ionic Characters Revealed by Operando Soft and Hard X-ray Absorption Spectroscopy
      Yamamoto, Kentaro; Zhou, Yingying; Yabuuchi, Naoaki; Nakanishi, Koji; Yoshinari, Takahiro; Kobayashi, Takanori; Kobayashi, Yuki; Yamamoto, Rina; Watanabe, Aruto; Orikasa, Yuki; Tsuruta, Kazuki; Park, Jiwon; Byon, Hye Ryung; Tamenori, Yusuke; Ohta, Toshiaki; Uchimoto, Yoshiharu
      Chemistry of Materials (2020), 32 (1), 139-147CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      The charge/discharge capacity of current lithium-ion battery cathode materials is limited by the charge compensation of transition-metal redox during the charge/discharge processes. Recently, the use of oxide ion redox for charge compensation has been proposed to realize a higher charge/discharge capacity than that obsd. for transition-metal redox. Different stabilization mechanisms of the reversible oxide ion redox have been proposed. To clarify the mechanism, anal. of the electronic and local structures around oxygen is required. Because of the high-voltage region in which the oxide ion redox occurs, several reactions such as oxygen gas evolution and/or electrolyte oxidn. are often included. Thus, operando measurements are required to directly prove this concept and generalize the understanding of the oxide ion redox. This study employs operando soft/hard X-ray absorption spectroscopy combined with X-ray diffraction spectroscopy for four lithium-excess electrode materials with different chem. bond natures. The exptl. data together with online anal. of the generated on-charge gas reveal two extreme cases: significantly enhanced covalent or ionic characters in the metal-oxygen chem. bonds, which are necessary conditions to stabilize the oxidn. of the oxide ions. This finding provides new insights with exciting possibilities for designing high energy d. cathode materials based on anion redox.
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    36. 36
      Okubo, M.; Yamada, A. Molecular Orbital Principles of Oxygen-Redox Battery Electrodes. ACS Appl. Mater. Interfaces 2017, 9, 36463– 36472,  DOI: 10.1021/acsami.7b09835
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      36
      Molecular Orbital Principles of Oxygen-Redox Battery Electrodes
      Okubo, Masashi; Yamada, Atsuo
      ACS Applied Materials & Interfaces (2017), 9 (42), 36463-36472CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      A review. Li-ion batteries are key energy-storage devices for a sustainable society. The most widely used pos. electrode materials are LiMO2 (M: transition metal), in which a redox reaction of M occurs in assocn. with Li+ (de)intercalation. Recent developments of Li-excess transition-metal oxides, which deliver a large capacity of >200 mA-h/g using an extra redox reaction of O, introduce new possibilities for designing higher energy d. Li-ion batteries. For better engineering using this fascinating new chem., it is necessary to achieve a full understanding of the reaction mechanism by gaining knowledge on the chem. state of O. A summary of the recent advances in O-redox battery electrodes is provided, followed by a systematic demonstration of the overall electronic structures based on MOs with a focus on the local coordination environment around O. A π-type MO plays an important role in stabilizing the oxidized O that emerges upon the charging process. MO principles are convenient for an at.-level understanding of how reversible O-redox reactions occur in bulk, providing a solid foundation toward improved O-redox pos. electrode materials for high energy-d. batteries.
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    37. 37
      Yoon, W.-S.; Chung, K. Y.; McBreen, J.; Zaghib, K.; Yang, X.-Q. Electronic Structure of the Electrochemically Delithiated Li[sub 1–x]FePO[sub 4] Electrodes Investigated by P K-edge X-Ray Absorption Spectroscopy. Electrochem. Solid-State Lett. 2006, 9, A415,  DOI: 10.1149/1.2216619
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      Electronic Structure of the Electrochemically Delithiated Li1-xFePO4 Electrodes Investigated by P K-edge X-Ray Absorption Spectroscopy
      Yoon, Won-Sub; Chung, Kyung Yoon; McBreen, James; Zaghib, Karim; Yang, Xiao-Qing
      Electrochemical and Solid-State Letters (2006), 9 (9), A415-A417CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      P K-edge x-ray absorption spectroscopy (XAS) was used to study the electronic structure of electrochem. de-lithiated Li1-xFePO4 for Li rechargeable batteries. The gradual shift of main edge features to higher energies showed that P-O bonds become less covalent during de-lithiation due to the more covalent Fe3+-O bonds via the inductive effect. A principal component anal. of P K-edge XAS spectra of the electrochem. de-lithiated Li1-xFePO4 reveals that this set of spectra can be represented by 2 primary components, in agreement with a 1st-order phase transition involving the LiFePO4 and FePO4 phases. From the observation of pre-edge peaks, electrochem. de-lithiation of Li1-xFePO4 results in the hybridization of P 3p states with the Fe 3d states.
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      Bonnet-Mercier, N.; Wong, R. A.; Thomas, M. L.; Dutta, A.; Yamanaka, K.; Yogi, C.; Ohta, T.; Byon, H. R. A structured three-dimensional polymer electrolyte with enlarged active reaction zone for Li–O2 batteries. Sci. Rep. 2015, 4, 7127,  DOI: 10.1038/srep07127
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      Rana, J.; Papp, J. K.; Lebens-Higgins, Z.; Zuba, M.; Kaufman, L. A.; Goel, A.; Schmuch, R.; Winter, M.; Whittingham, M. S.; Yang, W.; McCloskey, B. D.; Piper, L. F. J. Quantifying the Capacity Contributions during Activation of Li2MnO3. ACS Energy Letters 2020, 5, 634– 641,  DOI: 10.1021/acsenergylett.9b02799
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      Quantifying the Capacity Contributions during Activation of Li2MnO3
      Rana, Jatinkumar; Papp, Joseph K.; Lebens-Higgins, Zachary; Zuba, Mateusz; Kaufman, Lori A.; Goel, Anshika; Schmuch, Richard; Winter, Martin; Whittingham, M. Stanley; Yang, Wanli; McCloskey, Bryan D.; Piper, Louis F. J.
      ACS Energy Letters (2020), 5 (2), 634-641CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Though Li2MnO3 was originally considered to be electrochem. inert, its obsd. activation has spawned a new class of Li-rich layered compds. that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compds., the underlying origin of the initial charge capacity of Li2MnO3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidn. of Mn4+, and surface degrdn. for Li2MnO3 cathodes displaying capacities exceeding 350 mAh g-1. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidn. of Mn4+ and bulk oxygen redox during activation of Li2MnO3. Quant. gas-evolution and titrn. studies reveal that O2 and CO2 release accounted for a large fraction of the obsd. capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li2MnO3 is considered crit. for promoting bulk anionic redox in Li-rich layered oxides, Li2MnO3 by itself does not exhibit bulk oxygen redox or manganese oxidn. beyond its initial Mn4+ valence.
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    40. 40
      Wandt, J.; Freiberg, A. T. S.; Ogrodnik, A.; Gasteiger, H. A. Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries. Mater. Today 2018, 21, 825– 833,  DOI: 10.1016/j.mattod.2018.03.037
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      Singlet oxygen evolution from layered transition metal oxide cathode materials and its implications for lithium-ion batteries
      Wandt, Johannes; Freiberg, Anna T. S.; Ogrodnik, Alexander; Gasteiger, Hubert A.
      Materials Today (Oxford, United Kingdom) (2018), 21 (8), 825-833CODEN: MTOUAN; ISSN:1369-7021. (Elsevier Ltd.)
      For achieving higher energy d. lithium-ion batteries, the improvement of cathode active materials is crucial. The most promising cathode materials are nickel-rich layered oxides LiNixCoyMnzO2 (NCM) and over lithiated NCM (often called HE-NCM). Unfortunately, the full capacity of NCM cannot be utilized due to its limited cycle-life at high state-of-charge (SOC), while HE-NCM requires high voltages. By operando emission spectroscopy, we show for the first time that highly reactive singlet oxygen is released when charging NCM and HE-NCM to an SOC beyond ≈80%. In addn., online mass-spectrometry reveals the evolution of CO and CO2 once singlet oxygen is detected, providing significant evidence for the reaction between singlet oxygen and electrolyte to be a chem. reaction. It is controlled by the SOC rather than by potential, as would be the case for a purely electrochem. electrolyte oxidn. Singlet oxygen formation therefore imposes a severe challenge to the development of high-energy batteries based on layered oxide cathodes, shifting the focus of research from electrochem. stable 5 V-electrolytes to chem. stability toward singlet oxygen.
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      Hu, S.; Li, Y.; Chen, Y.; Peng, J.; Zhou, T.; Pang, W. K.; Didier, C.; Peterson, V. K.; Wang, H.; Li, Q.; Guo, Z. Insight of a Phase Compatible Surface Coating for Long-Durable Li-Rich Layered Oxide Cathode. Adv. Energy Mater. 2019, 9, 1901795,  DOI: 10.1002/aenm.201901795
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      Zhang, S.; Gu, H.; Pan, H.; Yang, S.; Du, W.; Li, X.; Gao, M.; Liu, Y.; Zhu, M.; Ouyang, L.; Jian, D.; Pan, F. A Novel Strategy to Suppress Capacity and Voltage Fading of Li- and Mn-Rich Layered Oxide Cathode Material for Lithium-Ion Batteries. Adv. Energy Mater. 2017, 7, 1601066,  DOI: 10.1002/aenm.201601066
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      Hekmatfar, M.; Kazzazi, A.; Eshetu, G. G.; Hasa, I.; Passerini, S. Understanding the Electrode/Electrolyte Interface Layer on the Li-Rich Nickel Manganese Cobalt Layered Oxide Cathode by XPS. ACS Appl. Mater. Interfaces 2019, 11, 43166– 43179,  DOI: 10.1021/acsami.9b14389
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      43
      Understanding the Electrode/Electrolyte Interface Layer on the Li-Rich Nickel Manganese Cobalt Layered Oxide Cathode by XPS
      Hekmatfar, Maral; Kazzazi, Arefeh; Eshetu, Gebrekidan Gebresilassie; Hasa, Ivana; Passerini, Stefano
      ACS Applied Materials & Interfaces (2019), 11 (46), 43166-43179CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Layered lithium-rich nickel manganese cobalt oxide (LR-NMC) represents one of the most promising cathode materials for application in high energy d. lithium-ion batteries. The extraordinary capacity delivered derives from a combination of both cationic and anionic redox processes. However, the latter ones lead to oxygen evolution which triggers structural degrdn. and electrode/electrolyte interface (EEI) instability that hinders the use of LR-NMC in practical application. In this work, we investigate the surface chem. of LR-NMC and its evolution upon different conditions to give further insights into the processes occurring at the EEI. XPS studies reveal that once the org. component of the layer is formed, it remains stable independently on the higher cutoff voltage applied, while continuous growth of inorgs. along with oxygen evolution occurs. The results performed on lithiated and delithiated LR-NMC surfaces indicate an instability of the EEI layer formed at high voltages, which undergoes a partial decompn. Furthermore, the tris(pentafluorophenyl)borane electrolyte additive simultaneously prevents excess LiF formation and changes the chem. compn. of the EEI layer. The latter is characterized by a higher amt. of poly(ethylene oxide) oligomer species and LixPOyFz formation. In addn., the presence of boron-contg. compds. in the EEI layer cannot be excluded, which may be also responsible of the increased thickness of the EEI layer. Finally, fast kinetics at elevated temps. exacerbate the salt decompn. which results in the formation of an EEI which is thicker and richer in LiF.
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    44. 44
      Doi, T.; Shimizu, Y.; Matsumoto, R.; Hashinokuchi, M.; Inaba, M. Suppression of Mn–Ion-Dissolution of LiNi0.5Mn1.5O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures. ChemistrySelect 2017, 2, 8824– 8827,  DOI: 10.1002/slct.201701668
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      44
      Suppression of Mn-Ion-Dissolution of LiNi0.5Mn1.5O4 Electrodes in a Highly Concentrated Electrolyte Solution at Elevated Temperatures
      Doi, Takayuki; Shimizu, Yusuke; Matsumoto, Ryo; Hashinokuchi, Michihiro; Inaba, Minoru
      ChemistrySelect (2017), 2 (28), 8824-8827CODEN: CHEMUD; ISSN:2365-6549. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Mn-based active materials, such as LiMn2O4, are widely used for pos. electrodes in lithium ion batteries, and spinel LiNi0.5Mn1.5O4 is drawing much attention to realize 5-V class batteries. However, the oxidative decompn. of electrolyte soln. at high voltages and Mn-dissoln. of LiNi0.5Mn1.5O4 are serious problems to be solved. These two drawbacks are more marked at elevated temps., and should be caused by free solvent mols. in electrolyte soln. In this study, highly concd. electrolyte soln., which contains few free solvent mols., was investigated to solve the problems. LiNi0.5Mn1.5O4 electrodes worked at 50oC in nearly satd. 7.25 mol kg-1 LiBF4/ propylene carbonate (PC) electrolyte soln., whereas not in the nearly satd. 4.30 mol kg-1 LiPF6/PC. In addn., Mn-ion dissoln. from LiNi0.5Mn1.5O4 was significantly suppressed in highly concd. electrolyte solns., and correlated to the fraction of free PC mols. in them.
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      A multi-purpose pattern-fitting system, RIETAN-2000, has been extensively utilized to contribute to many structural studies. It offers a sophisticated structure-refinement technique of whole-pattern fitting based on the max.-entropy method (MEM) in combination with a MEM anal. program PRIMA. We have recently completed a successor system, RIETAN-FP, adding new features such as standardization of crystal-structure data, an extended March-Dollase preferred-orientation function, and automation of imposing restraints on bond lengths and angles. Further, we have been developing a new three-dimensional visualization system, VESTA, using wxWidgets as a C++ application framework. VESTA excels in visualization, rendering, and manipulation of crystal structures and electron/nuclear densities detd. by X-ray/neutron diffraction and electronic-structure calcns. VESTA also enables us to display wave functions and electrostatic potentials calcd. with part of these programs.
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      Ohara, K.; Tominaka, S.; Yamada, H.; Takahashi, M.; Yamaguchi, H.; Utsuno, F.; Umeki, T.; Yao, A.; Nakada, K.; Takemoto, M.; Hiroi, S.; Tsuji, N.; Wakihara, T. Time-resolved pair distribution function analysis of disordered materials on beamlines BL04B2 and BL08W at SPring-8. J. Synchrotron Radiat. 2018, 25, 1627– 1633,  DOI: 10.1107/S1600577518011232
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      A dedicated apparatus has been developed for studying structural changes in amorphous and disordered crystalline materials substantially in real time. The apparatus, which can be set up on beamlines BL04B2 and BL08W at SPring-8, mainly consists of a large two-dimensional flat-panel detector and high-energy X-rays, enabling total scattering measurements to be carried out for time-resolved pair distribution function (PDF) analysis in the temperature range from room temperature to 873 K at pressures of up to 20 bar. For successful time-resolved analysis, a newly developed program was used that can monitor and process two-dimensional image data simultaneously with the data collection. The use of time-resolved hardware and software is of great importance for obtaining a detailed understanding of the structural changes in disordered materials, as exemplified by the results of commissioned measurements carried out on both beamlines. Benchmark results obtained using amorphous silica and demonstration results for the observation of sulfide glass crystallization upon annealing are introduced.
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      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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      The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived. The total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addn., crit. tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed-core all-electron methods. These tests include small mols. (H2, H2O, Li2, N2, F2, BF3, SiF4) and several bulk systems (diamond, Si, V, Li, Ca, CaF2, Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
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      Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104,  DOI: 10.1063/1.3382344
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      VESTA is a 3D visualization system for crystallog. studies and electronic state calcns. It was upgraded to the latest version, VESTA 3, implementing new features including drawing the external morphpol. of crysals; superimposing multiple structural models, volumetric data and crystal faces; calcn. of electron and nuclear densities from structure parameters; calcn. of Patterson functions from the structure parameters or volumetric data; integration of electron and nuclear densities by Voronoi tessellation; visualization of isosurfaces with multiple levels, detn. of the best plane for selected atoms; an extended bond-search algorithm to enable more sophisticated searches in complex mols. and cage-like structures; undo and redo is graphical user interface operations; and significant performance improvements in rendering isosurfaces and calcg. slices.
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    • DFT calculation details; structural evolutions of the LiMnO2–Li3PO4 system; STEM images; EDX maps; SEM images; comparison of SXRD patterns; Reitveld analysis of rocksalt LiMnO2 and Li7/6P1/6Mn2/3O2; phase segregation of Li7/6P1/6Mn2/3O2 after heating; analysis of the Mn distrivution; X-ray diffraction patterns, X-ray pair distribution functions; Raman, XAS, and solid-state NMR spectra; charge–discharge and charge curves; charge density differences, and synthesis and electrochemical properties of nanosize rocksalt Li2MnO3 (PDF)


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