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Metal–Organic Framework-Derived Magnesium Oxide@Carbon Interlayer for Stable Lithium–Sulfur Batteries

  • Hyeonmuk Kang
    Hyeonmuk Kang
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
  • Jaewook Shin
    Jaewook Shin
    Advanced Battery Center, KAIST Institute for NanoCentury, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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  • Tae-Hee Kim
    Tae-Hee Kim
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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  • Yongju Lee
    Yongju Lee
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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  • Daehee Lee
    Daehee Lee
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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  • Junho Lee
    Junho Lee
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
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  • Gyungtae Kim
    Gyungtae Kim
    Department of Measurement & Analysis, National NanoFab Center, Daejeon 34141, Republic of Korea
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  • , and 
  • EunAe Cho*
    EunAe Cho
    Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
    *Phone: +82-42-350-3317. Email: [email protected]
    More by EunAe Cho
Cite this: ACS Sustainable Chem. Eng. 2023, 11, 4, 1344–1354
Publication Date (Web):January 19, 2023
https://doi.org/10.1021/acssuschemeng.2c05064

Copyright © 2023 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

Lithium sulfur (Li–S) batteries represent a promising future battery technology. However, the low electrical conductivity of solid-state sulfur species (S, Li2S2, and Li2S) and the polysulfide shuttle effect deteriorate their practical capacity and cycling retention. Herein, we present an interlayer composed of magnesium oxide (MgO) nanoparticles and carbon matrix for the Li–S batteries. In the composite, MgO can capture dissolved polysulfides that diffuse to the carbon matrix along the oxide surface for further reduction reactions. As a novel precursor to produce the composite structure, a Mg metal–organic-framework, Mg-MOF-74, is adopted and synthesized on a free-standing carbon paper (MOF/C-paper). Through pyrolysis, Mg-MOF-74 is converted into highly porous carbon containing uniformly distributed MgO nanoparticles (MgO@C/C-paper). The Li–S cells assembled with MgO@C/C-paper and C-paper interlayer show significantly higher initial capacities (980 and 898 mAh g–1, respectively) than the interlayer-free cell (729 mAh g–1) owing to the conductive interlayers. After 200 cycles at 0.2 C, the MgO@C/C-paper cell presents a cycle retention (78.3%) superior to that of the C-paper cell (76.5%). With a higher sulfur loading of 3.3 mg cm–2, the MgO@C/C-paper cell exhibits an even higher capacity retention (80.1%) than the C-paper cell (54.6%) after 100 cycles. The excellent cycle stability of the MgO@C/C-paper cell over the C-paper cell demonstrates that the unique structure of the MOF-derived MgO@C is highly effective in anchoring and reutilizing dissolved polysulfides.

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Synopsis

MOF-derived MgO@C interlayer increases utilization of polysulfides and reversibility of the Li−S cell for sustainable energy storage.

Introduction

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Since the conception of the lithium ion battery (LIB), LIBs have played a key role in the revolution of mobile electronic devices owing to their high gravimetric energy density (260 Wh kg–1). However, the advancement of portable devices and large-scale applications such as electric vehicles (EVs) and energy storage systems (ESSs) requires higher energy density than an LIB can supply. Among post-LIB candidates, lithium–sulfur (Li–S) batteries have a better chance of reaching commercialization owing to their high theoretical energy density (2500 Wh kg–1), low cost, and the environmental friendliness of the sulfur cathode material. (1,2) Compared to conventional transition-metal oxide cathodes (275 mAh g–1), which involve a maximum of one electron per transition-metal atom, the sulfur cathode has a higher theoretical capacity (1675 mAh g–1), as it is a lightweight element and incorporates two electrons per sulfur atom to form lithium sulfide (Li2S). (3)
Despite these advantages, sulfur cathodes have several challenges that must be overcome. In particular, low electrical conductivity of solid-state sulfur species (S, Li2S2, and Li2S) and the dissolution of lithium polysulfides (Li2Sn, 4 ≤ n ≤ 8) hinder the full utilization of the theoretical capacity and result in poor cyclic retention. (4,5) Low electrical conductivity of sulfur leads to large overpotential and low practical capacity, limiting the sulfur loading mass as well. The dissolved polysulfides lose electric contact with the cathode and subsequently become inactive. In addition, they can move to the lithium anode and react to form S or Li2S2/Li2S, which passivates the anode surface and increases the cell overpotential due to the ensuing poor electrical conductivity.
In an effort to address the aforementioned issues, conductive host materials such as graphene, (6) graphene oxide, (7) and highly porous carbon (8−10) have been added into the sulfur cathodes to enhance the electrical conductivity. (11,12) However, addition of a host material to the cathode includes the extra step of a melt diffusion process (13) and reduces the sulfur content of the cathode. To increase sulfur loading mass and to use simple synthesis processes, free-standing interlayers have been widely developed. Starting with a multiwall carbon nanotube (MWCNT) interlayer, a variety of carbon-based interlayers have been explored. (14−18) Although those interlayers can enhance both the specific capacity and cycle stability of sulfur cathodes by providing electrical conductivity to the sulfur cathode, the nonpolar surface of carbon materials cannot effectively suppress polysulfide shuttling due to having a low affinity for polar polysulfide and limitations in improving the cycle stability. (19)
For efficient polysulfide capture, inorganic compounds such as transition-metal oxides, (11,20−23) sulfides, (24,25) and carbides (26,27) have been incorporated into carbon materials. Among them, metal oxides are known to have the highest affinity to polysulfides. (20) However, the anchored polysulfide on a metal oxide surface cannot react further due to low electrical conductivity of the metal oxides. To utilize the dissolved polysulfides, the anchored polysulfides should diffuse to neighboring conducting carbon. Thus, in addition to the chemical affinity with polysulfides, diffusion of anchored polysulfides on metal oxide is a crucial factor to be considered in designing an interlayer. A recent study showed that MgO is a promising material among different metal oxides considering strong binding and a low surface diffusion barrier through density functional theory (DFT) calculation and experimental results. (28)
In this study, we present a commercial carbon paper coated with porous carbon and magnesium oxide (MgO). As a novel precursor for the MgO and carbon composite, an Mg metal–organic framework, Mg-MOF-74, was synthesized on a free-standing carbon paper (MOF/C-paper). Through pyrolysis, Mg-MOF-74 is converted into highly porous carbon containing uniformly distributed MgO nanoparticles (MgO@C/C-paper). The unique structure of the MOF-derived MgO@C provides uniformly abundant anchoring sites and short diffusion pathways for dissolved polysulfides, thereby improving cycle retention of the Li–S cell. The roles of MgO@C/C-paper interlayer in capturing and reutilizing dissolved polysulfides are investigated using scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), and X-ray photoelectron spectroscopy (XPS).

Results and Discussions

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Figure 1a,b shows SEM images of a pristine carbon paper (C-paper) composed of carbon fibers with a diameter of approximately 5 μm and a smooth surface. The C-paper has a porous structure with a thickness of approximately 110 μm. An X-ray diffraction (XRD) pattern (Figure 2) reveals that the C-paper consists of graphitic carbon. (29) To produce a metal–organic framework (MOF) on the carbon fibers, the C-paper was soaked in a solution containing magnesium acetate tetrahydrate (Mg(CH3COO)2·4H2O) as a metal precursor and 2,5-dihydroxyterephthalic acid (H4DHTP) as an organic ligand and kept at 125 °C for 6 h (denoted as “MOF/C-paper”). As illustrated in Figure 1c,d, most of the carbon fibers are covered with the reaction products throughout the C-paper. The XRD pattern of MOF/C-paper displays distinct peaks at 6.7° and 11.7° (Figure 2) that correspond to (110) and (300) of space group R3, respectively, (30) in addition to the graphitic carbon peaks. These outcomes imply that crystalline Mg-MOF-74 was formed on graphitic carbon fibers throughout the C-paper. Then, the MOF/C-paper was heat-treated at 900 °C under an Ar gas condition for 1 h. (31) The XRD pattern of the heat-treated MOF/C-paper (Figure 2) exhibits only graphitic carbon peaks without the Mg-MOF-74 peaks observed from the MOF/C-paper, indicating that the organic ligands in Mg-MOF-74 were pyrolyzed into carbon. Similar to the MOF/C-paper, most of the carbon fibers in the heat-treated MOF/C-paper (Figure 1e,f) are covered with pyrolyzed MOF throughout the C-paper. The thicknesses of the MOF/C-paper and heat-treated MOF/C-paper were similar (∼110 μm) to that of the pristine C-paper.

Figure 1

Figure 1. Top-view and cross-sectional SEM images of (a, b) C-paper, (c, d) MOF/C-paper, and (e, f) heat-treated MOF/C-paper.

Figure 2

Figure 2. XRD patterns of C-paper, MOF/C-paper, and heat-treated MOF/C-paper.

Since the boiling point of Mg is 1091 °C, (32) Mg in Mg-MOF-74 would survive the heat treatment and be oxidized when the heat-treated MOF/C-paper was exposed to air. A strand of the heat-treated MOF/C-paper was carefully collected by cutting a fiber (Figure 3a) for elemental mapping. Unavoidably, pyrolyzed MOF was partially removed from the fiber during the cutting process. The elemental mapping image (Figure 3b) shows that Mg is uniformly distributed in the pyrolyzed MOF. Transmission electron microscopy (TEM) images of the pyrolyzed MOF (Figure 3c) demonstrate nanosized (approximately 2 nm) particles in an amorphous matrix. The fast Fourier transformed (FFT) images for a nanoparticle (Figure 3d) and for a carbon matrix (Figure 3e) illustrate the rock salt phase (Fmm) with a lattice parameter of 0.21 nm, which correspond to Mg-oxide (MgO) (33) and amorphous carbon, respectively. These results demonstrate that pyrolyzed MOF is composed of MgO nanoparticles in an amorphous carbon matrix (MgO@C). The absence of MgO peaks from the XRD pattern (Figure 2) of the heat-treated MOF/C-paper (denoted as “MgO@C/C-paper”) is related to the small crystallite size of MgO and strong intensity of graphitic carbon peaks from the C-paper.

Figure 3

Figure 3. (a) SEM and (b) EDS elemental Mg mapping images of a fiber collected from the heat-treated MOF/C-paper. (c) TEM image of pyrolyzed MOF and FFT images for (d) the nanoparticle marked with a red square and (e) carbon matrix marked with a blue square in (c).

To support that the MOF/C-paper was pyrolyzed into MgO@C/C-paper, Mg-MOF-74 was prepared in powder form (denoted as “MOF powder”, Figure S1a) and heat-treated in the same manner as MOF/C-paper (denoted as “heat-treated MOF powder”, Figure S1b). While the MOF powder exhibits the XRD peaks of Mg-MOF-74 peak distinctively at 6.7° and 11.7° (Figure S2), the heat-treated MOF powder presents only MgO-related XRD peaks at 43° and 62°, without Mg-MOF-74 peaks or carbon peaks. These results confirm that the heat-treated MOF powder is composed of crystalline MgO and amorphous carbon (denoted as “MgO@C powder”).
To estimate the composition of MgO, MOF-derived amorphous carbon, and graphitic C-paper in the MgO@C/C-paper, thermogravimetric analysis (TGA) was conducted for MgO@C powder and MgO@C/C-paper in an air condition to 900 °C. The MgO@C powder sample lost approximately 53.4% of its original weight at temperature around ∼300 °C (Figure S3a), confirming the MOF-derived carbon has an amorphous structure, (34) and retained 46.6% of its original weight up to 900 °C. Thus, the weight ratio of MgO to MOF-derived amorphous carbon in MgO@C is 46.6 to 53.4. The MgO@C/C-paper lost its weight slightly at a temperature around ∼300 °C and mostly at temperatures above ∼600 °C, where approximately 97.5% of graphitic carbon oxidized from its original weight (Figure S3b). (35) Accordingly, MgO@C/C-paper is composed of 2.5 wt % MgO, 2.9 wt % MOF-derived amorphous carbon, and 94.6 wt % of graphitic C-paper. The free-standing C-paper and MgO@C/C-paper have a weight of 14.1 and 14.4 mg, respectively. A simple calculation shows that MgO@C/C-paper only has 0.16 mg of extra MgO added from the original C-paper.
The electrical conductivity of MgO@C/C-paper was measured to be 0.71 S cm–2, which was slightly lower than that of the C-paper (0.94 S cm–2) due to the presence of insulating MgO. The Barrett–Joyner–Halenda (BJH) measurement (Figure S4) exhibits that MOF-derived carbon has mesopores with an average pore size of 5.2 nm. Owing to the highly porous MOF-derived carbon, the MgO@C/C-paper has a substantially larger Brunauer–Emmett–Teller (BET) surface area (0.70 m2/g) than that of the C-paper (0.29 m2/g).
With a cathode sulfur loading of 1.7 mg cm–2, three types of Li–S coin cells were assembled: a standard cell without an interlayer and also C-paper and MgO@C/C-paper that employ C-paper and MgO@C/C-paper interlayer, respectively, between the cathode and the separator. The ratio between electrolyte and sulfur (E/S) was 20 μL/g. The cycle test was performed at the current density 0.2 C, with cutoff voltages of 1.7 and 2.8 V (Figure 4a). The standard cell has an initial capacity of 729 and 543 mAh g–1 after 200 cycles with retention of 74.5%. On the other hand, the interlayer-containing cells, C-paper and MgO@C/C-paper, deliver substantially enhanced initial capacities of 898 and 980 mAh g–1, respectively. After 200 cycles, C-paper had a capacity of 687 mAh g–1 with retention 76.5%, and MgO@C/C-paper had a capacity of 767 mAh g–1 with retention 78.3%. In addition, the Coulombic efficiency of standard, C-paper, and MgO@C/C-paper after 200 cycles is 98.3, 98.7, and 99.0%, respectively. The high electrical conductivity and the large surface of the interlayers provide plenty of reaction sites for polysulfides, enhancing the polysulfide utilization. However, it is expected that the formation of Li2S2 and Li2S eventually passivates the surface of the C-paper, (36) whereas the MgO@C layer prevents the passivation.

Figure 4

Figure 4. Electrochemical performance of the Li–S cells without an interlayer (denoted as “standard”) and with C-paper and MgO@C/C-paper interlayer at 0.2 C. (a) Cycling performances with a sulfur loading mass of 1.7 mg cm–2. Charge–Discharge curves of (b) standard, (c) C-paper, and (d) MgO@C/C-paper.

In Figure 4b–d, the prepared coin cells demonstrate discharge voltage plateaus at approximately 2.3 and 2.1 V, corresponding to the reduction of elemental sulfur to high-order polysulfides (Li2Sx, 6 ≤ x ≤ 8) and a further reduction of these polysulfides to lower-order polysulfides (Li2Sx, 4 ≤ x ≤ 6) and lithium sulfide (Li2S), respectively. Comparing standard, C-paper, and MgO@C/C-paper cells, at the first cycle, the capacity coming from the first plateau is 318, 346, and 359 mAh g–1, respectively. The standard cell, without an interlayer, has the smallest capacity due to a lack of electrical conductivity. A conductive interlayer on the top of the sulfur cathode surface, where the electrochemical reaction predominantly occurs, enhances the utilization of nonconductive sulfur (S8) and lithium sulfides (Li2S2 and Li2S). (15,21,37) The effect of the MgO@C/C-paper interlayer is emphasized at the second plateau where higher capacity is extracted by reduction of polysulfides (Standard: 411 mAh g–1, C-paper: 552 mAh g–1, and MgO@C/C-paper: 621 mAh g–1). The voltage plateau at 2.1 V was extended for the MgO@C/C-paper over the C-paper implying that the MgO@C layer promotes the reduction of low-order polysulfides (at 2.1 V) more than the reduction of elemental sulfur to high-order polysulfides (at 2.3 V), resulting in the higher specific capacity compared to the C-paper.
After 20 cycles, the first plateau for the standard cell dramatically decreased having 192 mAh g–1 and moderately decreased capacity of 347 mAh g–1 for the second plateau. Repeated polysulfide dissolution and S/Li2S2/Li2S deposition result in the separation of sulfur and carbon in the cathode. Insulating sulfur species at the interface between the electrolyte and cathode passivates the cathode. It has been reported that the low electrical conductivity of the sulfur species and the loss of the active material related to polysulfide dissolution decreases the sulfur utilization. (25,38,39) For C-paper and MgO@C/C-paper, the change in the first plateau is relatively moderate (C-paper: 271 mAh g–1, MgO@C/C-paper: 285 mAh g–1). However, an abrupt voltage decay decreased the capacity derived from the second plateau of the C-paper 437 mAh g–1, whereas MgO@C/C-paper maintains the capacity of 529 mAh g–1. The difference in the second plateau capacity after 20 cycles is expected to come from the passivation of active sites of the C-paper interlayer.
In order to separate the role of MgO, MgO particles are removed by using 2 M HCl acid leaching (Figure S5). TEM high-angle annular dark-field (HAADF) images and energy-dispersive X-ray spectroscopy (EDS) mapping of heat-treated MOF powder show MgO nanoparticles and clear Mg peaks, whereas the acid-leached heat-treated MOF powder only demonstrates clear amorphous carbon matrix without any Mg peaks. MgO@C/C-paper after acid leaching, denoted as “C/C-paper”, is assembled in a Li–S full cell to compare the electrochemical performance (Figure S6). C/C-paper has an initial capacity of 891 mAh g–1, which is very similar to that of the C-paper (898 mAh g–1). Also, it retained its capacity of 680 mAh g–1 at 50 cycles showing a similar decay curve when compared to the C-paper. Such a result confirms that the presence of MgO nanoparticles can increase the capacity of the Li–S cell.
One of the crucial challenges in the commercialization of Li–S batteries is the limitation of sulfur loading. With an increased sulfur loading of 3.3 mg cm–2 (Figure 5), the initial specific capacity of sulfur dramatically drops to approximately 300 mAh g–1 at the first cycle at 0.05 C, whereas the interlayer-containing cells retain the cathode-specific capacities of approximately 1000 mAh g–1. During 100 cycles at 0.2 C, the capacity only retained at approximately 200 mAh g–1 without an interlayer (Figure 5b). The C-paper cell that initially had a high capacity (1st cycle: 987 mAh g–1) underwent a rapid capacity fading, delivering the specific capacity of 414 mAh g–1 at the 100th cycle (54.6% of the second cycle capacity, 759 mAh g–1). On the other hand, the MgO@C/C-paper cell shows an excellent cycle retention, presenting a cathode capacity of 584 mAh g–1 at the 100th cycle (80.1% of the second cycle capacity, 729 mAh g–1). Compared to the MgO@C/C-paper cell, the rapid capacity fading of the C-paper cell is related to the decay of the voltage plateau at 2.1 V (Figure 5a). These outcomes reflect that, with the higher sulfur loading, a greater amount of polysulfides is dissolved, accelerating passivation of the C-paper and capacity fading. In contrast, with the MgO@C/C-paper, passivation of the C-paper is effectively suppressed, thereby enabling reduction of polysulfides.

Figure 5

Figure 5. (a) Charge–Discharge curves and (b) electrochemical performance of the Li–S cells without an interlayer (denoted as “standard”) and with C-paper and MgO@C/C-paper interlayer at 0.2 C with sulfur loading of 3.3 mg cm–2.

There are different MOF and MOF-derived materials used in Li–S battery studies. For example, the HKUST-1 based MOF@GO separator shows an initial capacity of 1207 mAh g–1 and retains 870 mAh g–1 at the 100th cycle at 0.5 C, (40) and Y-FTZB-coated separator shows an initial capacity of 1101 mAh g–1 and retains its capacity of 557 mAh g–1 after 300 cycles at 0.25 C. (41) Although the overall capacity of this work is slightly lower than the previous results, considering the loading mass (0.6–1.0 mg cm–2) of previous results, this result is comparable.
The charge–discharge curves and rate performance of the three Li–S batteries are shown in Figure S7. As C-rate increases, the voltage gap between charge and discharge plateaus increases due to the limited conductivity of the standard sulfur cathode. However, the cell with MgO@C/C-paper has flatter plateaus and reduced overpotential. MgO@C-paper has a capacity of 1237 mAh g–1 at 0.05 C and 651 mAh g–1 at 1 C having higher rate capability compared to other cells. Also, it recovers up to 1011 mAh g–1 when the C-rate returns to 0.05 C, suggesting superior rate performance of MgO@C/C-paper.
To achieve high sulfur utilization and cycle stability, dissolved polysulfides should be anchored and reduced to Li2S2/Li2S. To estimate polysulfide-anchoring capacity, coin cells were disassembled after the first discharge (Figure S8). While the standard cell displays a yellowish electrolyte, the interlayer-containing cells show colorless electrolytes, implying yellow-colored polysulfides were captured by the interlayers. (42) Compared to the pristine states (Figure 6a,b), the pores in the C-paper and MgO@C/C-paper were severely clogged throughout the interlayers (Figure 5c,d). Elemental mapping images display sulfur signals uniformly distributed throughout the interlayers. These outcomes reflect that a considerable amount of polysulfide was captured by the interlayers in the first discharge (Figure 6e,f). In addition, the MgO@C/C-paper has stronger S signals than the C-paper. To estimate polysulfide capturing ability, pristine C-paper and MgO@C/C-paper were soaked in a 5 mM polysulfide (Li2S6) solution for an hour and washed with 1,2-dimethoxymethane (DME) solution (Figure S9). The sulfur contents measured using elemental analysis (EA) were found to be 278 and 408 ppm in the C-paper and MgO@C/C-paper, respectively. These results demonstrate that MgO@C enhance the polysulfide capturing capability, possibly due to the highly porous carbon and polar MgO nanoparticles. (28)

Figure 6

Figure 6. Top view SEM images of the interlayers (a), (b) before and (c, d) after the first discharge. Cross-section images and elemental EDS mapping images of (e) C-paper and (f) MgO@C/C-paper.

To evaluate the capabilities of reducing the anchored polysulfides, coin cells were prepared using the C-paper and MgO@C/C-paper as blank cathodes. Polysulfide (Li2S8) was dissolved in the electrolyte as an active material instead of sulfur cathode. Voltage–Time curves were measured at a constant current of 27 μA (Figure S10), displaying sloping discharge profiles above 2.1 V and voltage plateaus at 2.1 V, corresponding to the reduction of dissolved Li2S8 to Li2S6 and the reduction of Li2S6 to Li2S2 or Li2S, respectively. (43) Whereas the capacities from the reaction of Li2S8 to Li2S6 are similar for both blank cathodes, the MgO@C/C-paper cathode produces a longer voltage plateau and approximately 1.5 times higher capacity than the C-paper cathode. These results demonstrate that MgO@C promotes reduction of the anchored polysulfides to Li2S2 or Li2S.
To determine the state of the sulfur species on the blank cathodes, the discharged blank cathodes were disassembled, washed in DME solution, and dried in a vacuum chamber. The washed blank cathodes are transferred using a vacuum transfer vessel to collect X-ray photoelectron spectroscopy (XPS) S 2p spectra (Figure 7). R-SO2-R/SO42–, S2O32–/SO32–, ST–1, and S2 peaks are observed at binding energies of 169.0, 167.2, 162.0, and 160.4 eV, respectively. (44) Since the soluble polysulfides were removed by DME, R-SO2-R/SO42– and S2O32–/SO32– signals reflect the oxidation of sulfur species during the sample transfer or due to the residual bis(trifluoromethyl sulfonyl)imide (LiTFSI). Compared to the C-paper, MgO@C/C-paper exhibits the higher relative intensity of R-SO2-R/SO42– and S2O32–/SO32–, indicating a greater amount of polysulfides anchored in accordance with the EA results (Figure S9). The ST–1 and S2– species correspond to Li2S2 and Li2S, respectively. The peak areal ratios of S2– to ST–1 (S2–/ST–1) were calculated to be 5.9 and 1.8 for the MgO@C/C-paper and C-paper, respectively, implying that lithium persulfide (Li2S2) was further reduced to lithium sulfide (Li2S) in the MgO@C/C-paper. At a given state of discharge, Li2S2 would be more thinly formed in the MgO@C/C-paper than in the C-paper owing to the larger surface area of MgO@C layer, allowing further reduction to Li2S.

Figure 7

Figure 7. XPS spectra of S 2p of (a) C-paper and (b) MgO@C/C-paper cathode after the first discharge in the Li2S8 solution.

For stable Li–S battery cycling, the solid-state Li2S2/Li2S should be oxidized back to polysulfides to retain the cycle stability. In this regard, the C-paper and MgO@C/C-paper blank cathodes were observed at the discharged and charged states. The C-paper and MgO@C/C-paper cathodes were charged up to 2.27 V in order to oxidize solid-state Li2S2/Li2S to soluble polysulfides. Compared to the pristine state (Figure 8a,b), discharged C-paper and MgO@C/C-paper cathodes show different morphologies (Figure 8c,d), being covered with solid-state Li2S2 and/or Li2S, as identified by the XPS results. The S contents in the C-paper and MgO@C/C-paper were measured using EDS and found to be 25.2 and 22.2 atom %, respectively (Table S1). When charged, the C-paper cathode is still covered with Li2S2/Li2S, but the MgO@C/C-paper cathode seems similar to its pristine state (Figure 8e,f). In fact, the S contents were lowered to 13.5 and 3.1 atom % in the C-paper and MgO@C/C-paper after charging to 2.27 V. In other words, approximately 46% and 86% of the solid-state sulfur species (Li2S2/Li2S) were oxidized back to polysulfide, in the C-paper and MgO@C/C-paper, respectively. These results imply that the C-paper is passivated by the formation of Li2S2/Li2S where MgO@C/C-paper is capable of reutilizing the solid-state sulfur species.

Figure 8

Figure 8. SEM images of the blank cathodes. (a) Pristine C-paper, (b) pristine MgO@C/C-paper, (c) discharged C-paper, (d) discharged MgO@C/C-paper, (e) charged C-paper, and (f) charged MgO@C/C-paper.

The Nyquist plots of the standard, C-paper, and MgO@C/C-paper cells are shown in Figure S11. The first semicircle shown at the high-frequency region represents charge transfer resistance (Rct) at the interface between the cathode and the electrolyte. The next semicircle represents the interfacial contact resistance (Rint) due to the formation of an insulating layer (Li2S2/Li2S). (45) Before cycling, C-paper and MgO@C/C-paper show a very similar plot in the high-frequency region and have very small Rct compared to the standard cell. This result is due to the presence of a conductive interlayer at the cathode surface. All the cells have a small semicircle corresponding to Rint in the medium-frequency region due to self-discharge. After 10 cycles, MgO@C/C-paper cell has the smallest Rct and Rint among other cells. Such a low resistance confirms that the MgO@C layer prevents C-paper from passivation by high reutilization of the solid-state sulfur species.
Figure 9 presents the high-angle annular dark-field (HAADF) cross-sectional images and EDS mapping images of C-paper and MgO@C/C-paper. In the C-paper, as observed in SEM images, Li2S2/Li2S is observed on carbon fiber both at charged and discharged states. Inactive Li2S2/Li2S residue could not be oxidized back to polysulfides and passivates carbon surface. In the MgO@C/C-paper cathode, Mg and S signals are overlapped both at charged and discharged states. Theses outcomes reflect that polysulfides diffuse through the MgO@C layer and reduce to Li2S2/Li2S. In addition, compared to the discharged state (Figure 9c), sulfur signals are substantially weaker in the charged MgO@/C-paper (Figure 9d), implying oxidation of Li2S2/Li2S to the polysulfide.

Figure 9

Figure 9. STEM HAADF images and EDS mapping of (a) discharged and (b) charged C-paper and (c) discharged and (d) charged MgO@C/C-paper.

The unique structure of the MOF-derived MgO@C, which consists of highly porous carbon with MgO nanoparticles, efficiently improves the utilization and cycle retention of sulfur. Figure 10 schematically illustrates roles of the MgO@C layer in promoting the redox reactions of sulfur species on C-paper interlayer. The highly porous MOF-derived carbon and polar MgO provide abundant polysulfide capturing sites. The uniformly distributed nanosized MgO with low surface diffusion barrier provides the anchored polysulfides with fast and short diffusion pathways to carbon for the reduction to Li2S2/Li2S. Owing to the large surface area, the solid-state Li2S2/Li2S are deposited thinly on the MOF-derived carbon, thereby providing a short electron pathway from the carbon fiber to the Li2S2/Li2S surface. Thus, Li2S2 can be further reduced to Li2S during discharge, and Li2S/Li2S2 can be efficiently oxidized back to polysulfide.

Figure 10

Figure 10. Schematic diagram of redox reaction of polysulfide on C-paper and MgO@C/C-paper; MgO nanoparticle anchors dissolved polysulfides and diffused to carbon composite for reduction.

Conclusion

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MgO@C was synthesized on a free-standing carbon paper (MgO@C/C-paper) and presented as an interlayer between the cathode and the separator of a Li–S cell. By pyrolyzing a novel precursor for the MgO@C, highly porous carbon was obtained with uniformly distributed MgO nanoparticles. The Li–S cells assembled with C-paper and MgO@C/C-paper demonstrated initial discharge capacities (898 and 980 mAh g–1, respectively) much higher than that of the interlayer-free cell (729 mAh g–1), owing to the high electrical conductivity of the interlayers. The MgO@C/C-paper cell exhibited superior cycle stability of the C-paper cell; at the 200th cycle, capacity retention of MgO@C/C-paper and C-paper cell was 78.3% and 76.5%, respectively, with a sulfur loading of 1.7 mg cm–2. In the MgO@C, MgO nanoparticle anchors dissolved polysulfides that diffuse to carbon matrix for electrochemical reactions. The unique structure of MOF-derived MgO@C provides polysulfides with numerous anchoring sites and short diffusion pathways from MgO to carbon, enabling efficient utilization of dissolved polysulfides. In addition, passivation of the carbon substrate is prevented since the solid-state sulfur species (Li2S2/Li2S) deposited on MgO@C is efficiently oxidized to polysulfide during charging, increasing the utilization and the reversibility of the Li–S cell.

Experimental Section

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Material Preparation

Synthesis of Mg-MOF-74 and MgO@C/C-paper

Mg-MOF-74 was synthesized on carbon paper (denoted as “MOF/C-paper”) through a solvothermal method using magnesium acetate tetrahydrate (Mg(CH3COO)2·4H2O) as a metal precursor and 2,5-dihydroxyterephthalic acid (H4DHTP) as an organic ligand. (30) The magnesium acetate tetrahydrate was dissolved in a mixture of 1 mL of deionized (DI) water, 1 mL of ethanol, and 8 mL of N,N-dimethylformamide (DMF). H4DHTP was dissolved in 10 mL of DMF and stirred for 1 h. The fully dissolved H4DHTP solution was added dropwise to the magnesium acetate solution. The prepared solution was transferred into a Teflon-lined autoclave, and commercial carbon paper (t: 0.10 ± 0.01 mm, density: 0.78 g/cm3) was soaked in the solution. The C-paper was composed of carbon fibers with a diameter of approximately 5 μm with a highly porous structure (Figure 1a–c), allowing penetration of the solution. The autoclave was placed in a convection oven and kept at 125 °C for 6 h for the solvothermal reaction. The soaked carbon paper was washed with DMF and dried in a vacuum oven overnight to obtain MOF/C-paper. To convert Mg-MOF-74 to MgO and carbon, the prepared MOF/C-paper was heat-treated at 900 °C under an Ar gas condition for 1 h (denoted as “MgO@C/C-paper”).

Material Characterizations

The surface and cross-section of each sample were observed using a scanning electron microscope (SEM, Phillips, XL30S). An elemental analysis was conducted using energy-dispersive X-ray spectroscopy (EDS) attached to the SEM. Morphological and elemental analysis of the samples were conducted using a scanning transmission electron microscope (S/TEM, FEI Titan cubed G2 60-300). The S/TEM specimens were prepared via ion-beam milling using the focused ion beam (FIB, FEI Helios G4 UX) method. The structural phase was classified with a high-resolution powder X-ray diffractometer (XRD, RIGAKU, SmartLab) using Cu K-α-1 (λ = 1.5418 Å) radiation at a scanning speed of 0.5°/min in the 2θ range of 5–80°. The surface elemental information and corresponding valence states were confirmed by X-ray photoelectron spectrometry (XPS, ThermoFisher Scientific, K-Alpha+, MXP10). The conductivity of the carbon interlayer was measured using a manual probe station (SUMMIT11862B) with a Keithley instrument (4200-SCS/F). The surface area and pore size distribution of the interlayers were measured by a surface area and pore size analyzer (3Flex) using the Brunauer–Emmett–Teller (BET) method. The thermal stability and volatile components of the interlayer were monitored via a high-resolution thermogravimetric analysis (TGA, TG209 F1 Libra) in the temperature range of 30–900 °C with a step of 5 °C/min in air. The electrical conductivity of the interlayers was measured with a manual probe station (SUMMIT11862B) integrated with a Keithley instrument (4200-SCS/F).

Electrochemical Measurements

Li–S full cells were prepared without an interlayer and using the carbon paper and MgO@C/C as an interlayer. A Li metal disk (D: 15.6 mm, w: 0.45 mm, MTI Korea) was used as an anode, and a polyethylene separator (Tonen Chemical Corporation) was used as a separator. To fabricate the cathode, a slurry was prepared by mixing 70 wt % sulfur powder (325 mesh S8, Alfa Aesar), 20 wt % carbon black (Super-P), and 10 wt % poly(vinylidene fluoride) (PVDF, Sigma-Aldrich) in N-methyl-1,2-pyrrolidone (NMP, Sigma-Aldrich). The slurry was cast using the doctor-blade method onto an aluminum (Al) foil current collector and dried in a vacuum oven at 80 °C overnight. The cathode cast on Al foil had a loading mass of the active material of 1.7 mg cm–2. As an electrolyte, 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a mixture of 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 v/v) was used with 0.2 M LiNO3 as an additive. CR2032-type coin cells were assembled in an argon-filled glovebox (MOTech Co., Ltd.). Electrochemical tests were performed in a cutoff voltage range of 1.7–2.8 V (vs Li+/Li) using a battery cycler (WonATech) at room temperature (25 °C). The cells were cycled at 0.05 C in the first cycle for the activation step, and the remaining cycles were cycled at 0.2 C. To investigate the interaction between the dissolved polysulfide and the interlayers, the cells were assembled using carbon and MgO@C/C interlayers as a cathode and 50 μL of a 0.125 M Li2S8 solution in DOL/DME as an active material. The cell was discharged and charged at 0.027 mA. The electrochemical impedance spectroscopy (EIS) result was obtained using a VersaSTAT 4 Potentiostat Galvanostat with VersaStudio software. The measurement was taken at the open-circuit voltage with the frequency range from 1 MHz to 0.01 Hz with amplitude of 10 mV.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.2c05064.

  • SEM images of the MOF powder and the heat-treated MOF powder, XRD patterns of the MOF powder and heat-treated MOF powder, Thermogravimetric analysis (TGA) curve of (a) MgO@C powder and (b) MgO@C/C-paper, BJH pore size distribution of the MOF powder and MgO@C powder, TEM HAADF images, EDS mapping images, and EDS spectrum of heat-treated MOF powder and after acid etching, electrochemical performance of the Li–S cells without an interlayer and with C-paper, C/C-paper, and MgO@C/C-paper at 0.2C, charge–discharge curves of standard, C-paper, and MgO@C/C-paper at different C-rates, rate performance of cells with different interlayers, photo images of the coin cells without and with interlayers after the first discharge, schematic diagram of the interlayer interacting with a polysulfide solution, and elemental analysis of sulfur with C-paper and MgO@C/C-paper after soaking in 5 mM polysulfide solution, charge and discharge curve of the coin cells assembled with C-paper and MgO@C/C-paper as a blank cathode and polysulfide (Li2S8) dissolved in the electrolyte as an active material, electrochemical impedance spectroscopy (EIS) spectra of the standard coin cell, C-paper cell, and MgO@C/C-paper cell before cycling and after 10 cycles, and quantitative elemental composition of cycled interlayer obtained from the EDS spectra (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Hyeonmuk Kang - Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of KoreaOrcidhttps://orcid.org/0000-0003-0174-555X
    • Jaewook Shin - Advanced Battery Center, KAIST Institute for NanoCentury, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of KoreaOrcidhttps://orcid.org/0000-0002-7431-9255
    • Tae-Hee Kim - Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
    • Yongju Lee - Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
    • Daehee Lee - Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
    • Junho Lee - Department of Materials Science and Engineering, Korea Advanced Institute of Science & Technology, 291 Daehak-Ro, Yuseong-gu, Daejeon 34141, Republic of Korea
    • Gyungtae Kim - Department of Measurement & Analysis, National NanoFab Center, Daejeon 34141, Republic of Korea
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Thanks go to the KAIST Analysis Center for Research Advancement (KARA) and to the National NanoFab Center for granting access to their equipment. This research was supported by the KAIST-funded Global Singularity Research Program for 2022 and by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE), (P0017120, The Competency Development Program for Industry Specialist).

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  • Abstract

    Figure 1

    Figure 1. Top-view and cross-sectional SEM images of (a, b) C-paper, (c, d) MOF/C-paper, and (e, f) heat-treated MOF/C-paper.

    Figure 2

    Figure 2. XRD patterns of C-paper, MOF/C-paper, and heat-treated MOF/C-paper.

    Figure 3

    Figure 3. (a) SEM and (b) EDS elemental Mg mapping images of a fiber collected from the heat-treated MOF/C-paper. (c) TEM image of pyrolyzed MOF and FFT images for (d) the nanoparticle marked with a red square and (e) carbon matrix marked with a blue square in (c).

    Figure 4

    Figure 4. Electrochemical performance of the Li–S cells without an interlayer (denoted as “standard”) and with C-paper and MgO@C/C-paper interlayer at 0.2 C. (a) Cycling performances with a sulfur loading mass of 1.7 mg cm–2. Charge–Discharge curves of (b) standard, (c) C-paper, and (d) MgO@C/C-paper.

    Figure 5

    Figure 5. (a) Charge–Discharge curves and (b) electrochemical performance of the Li–S cells without an interlayer (denoted as “standard”) and with C-paper and MgO@C/C-paper interlayer at 0.2 C with sulfur loading of 3.3 mg cm–2.

    Figure 6

    Figure 6. Top view SEM images of the interlayers (a), (b) before and (c, d) after the first discharge. Cross-section images and elemental EDS mapping images of (e) C-paper and (f) MgO@C/C-paper.

    Figure 7

    Figure 7. XPS spectra of S 2p of (a) C-paper and (b) MgO@C/C-paper cathode after the first discharge in the Li2S8 solution.

    Figure 8

    Figure 8. SEM images of the blank cathodes. (a) Pristine C-paper, (b) pristine MgO@C/C-paper, (c) discharged C-paper, (d) discharged MgO@C/C-paper, (e) charged C-paper, and (f) charged MgO@C/C-paper.

    Figure 9

    Figure 9. STEM HAADF images and EDS mapping of (a) discharged and (b) charged C-paper and (c) discharged and (d) charged MgO@C/C-paper.

    Figure 10

    Figure 10. Schematic diagram of redox reaction of polysulfide on C-paper and MgO@C/C-paper; MgO nanoparticle anchors dissolved polysulfides and diffused to carbon composite for reduction.

  • References

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    This article references 45 other publications.

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    • SEM images of the MOF powder and the heat-treated MOF powder, XRD patterns of the MOF powder and heat-treated MOF powder, Thermogravimetric analysis (TGA) curve of (a) MgO@C powder and (b) MgO@C/C-paper, BJH pore size distribution of the MOF powder and MgO@C powder, TEM HAADF images, EDS mapping images, and EDS spectrum of heat-treated MOF powder and after acid etching, electrochemical performance of the Li–S cells without an interlayer and with C-paper, C/C-paper, and MgO@C/C-paper at 0.2C, charge–discharge curves of standard, C-paper, and MgO@C/C-paper at different C-rates, rate performance of cells with different interlayers, photo images of the coin cells without and with interlayers after the first discharge, schematic diagram of the interlayer interacting with a polysulfide solution, and elemental analysis of sulfur with C-paper and MgO@C/C-paper after soaking in 5 mM polysulfide solution, charge and discharge curve of the coin cells assembled with C-paper and MgO@C/C-paper as a blank cathode and polysulfide (Li2S8) dissolved in the electrolyte as an active material, electrochemical impedance spectroscopy (EIS) spectra of the standard coin cell, C-paper cell, and MgO@C/C-paper cell before cycling and after 10 cycles, and quantitative elemental composition of cycled interlayer obtained from the EDS spectra (PDF)


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