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C: Energy Conversion and StorageDecember 12, 2023

Tuning of Molecular Water Organization in Water-in-Salt Electrolytes by Addition of Chaotropic Ionic Liquids
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Aleksandar Tot*
Aleksandar Tot
Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-10044, Sweden
*Email: [email protected]
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https://orcid.org/0000-0002-1118-8788
Leiting Zhang
Leiting Zhang
Department of Chemistry − Ångström Laboratory, Uppsala University, Uppsala SE-751 21, Sweden
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https://orcid.org/0000-0003-4057-7106
Per H. Svensson
Per H. Svensson
Chemical and Pharmaceutical Development, RISE Research Institutes of Sweden, Södertälje SE-151 36, Sweden
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https://orcid.org/0000-0003-2410-7366
Lars Kloo
Lars Kloo
Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-10044, Sweden
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https://orcid.org/0000-0002-0168-2942

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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2023, 127, 50, 24065–24076

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https://doi.org/10.1021/acs.jpcc.3c07164

Published December 12, 2023

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Abstract

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Water-in-salt electrolytes (WISEs) have expanded the useful electrochemical stability of water, making the development of functional aqueous lithium-ion batteries more accessible. The implementation of additives in the formulation of WISEs can further improve the electrochemical stability of water and avoid potential lithium-ion salt solubility issues. Here, we have used Gemini-type ionic liquids to suppress water activity by designing the structure of ionic-liquid cations. The different water-organizing effects of ionic-liquid cations have been investigated and correlated to battery performance in LTO/LMO full cells. The champion device, containing the most chaotropic ionic liquid, retained at least 99% of its Coulombic efficiency after 500 charging cycles, associated with a final specific discharge capacity of 85 mA h·g^–1. These results indicated that water-rich Li⁺ solvation shells significantly contribute to the excellent device performance and long-term stability of the LTO/LMO-based full battery cells. This work shows that the fine-tuning of the Li⁺ solvation shell and water structure by the addition of chaotropic cations represents a promising strategy for generating more stable and effective lithium-ion-containing rechargeable aqueous batteries.

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Subjects

Introduction

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Over the past decade, rechargeable lithium-ion batteries (LIBs) have become increasingly used in vehicle electrification and grid storage markets. The high energy density and charge-cycling stability have qualified LIBs to a dominant position in current energy storage technologies. (1−3) Nevertheless, the safety risks associated with the current LIB technology, predominantly owing to the high flammability and toxicity of the electrolytes employed, have highlighted the need for urgent improvements in the LIB design. (4,5) Alternatives involve, for instance, completely different electrolytes, such as solid-state polymer electrolytes (SPEs), inorganic/ceramic electrolytes, or even (hybrid) metal–organic framework (MOF) electrolytes, or liquid electrolytes with different fundamental properties of the solvent used. (6−8) As recently demonstrated, replacing problematic solvents with water has emerged as a promising strategy for overcoming safety issues. (9−11) The main disadvantage of aqueous LIBs is the lower energy density recorded, originating from the intrinsically narrow electrochemical stability window (ESW) caused by the water present (limited to 1.23 V for pure water). This problem was initially solved by Suo et al., by increasing the concentration of the lithium-ion salt, thus pioneering the water-in-salt electrolyte (WISE) concept for aqueous LIBs. (12) At such high concentrations of Li-ion salt, there are not sufficient water molecules to fully solvate all ions in the electrolyte, leading to disruption of the water–water hydrogen bonding network (HBN) and predominant ion–ion interactions (ion-pair and ion-aggregate formation). (12−15) This leads to a significant decrease in the thermodynamic water activity followed by a smaller amount of “free” water molecules that can aggregate into water-like entities in the electrolyte. The high concentration of anions in these systems promotes their involvement in the Li-ion coordination, and together with the low water activity, this suppresses hydrogen evolution yielding both high Coulombic efficiency and charge-cycling stability. The initial hypotheses suggested that the formation of a solid electrolyte interphase (SEI) was the main reason for the high stability of WISE-containing LIBs. (12,16) However, the recent work of Ming et al. has challenged this viewpoint by “transfer experiments”, proving the importance of the thermodynamic effect (i.e., the stability of the O–H bond) on the water molecules. (17−19) In this hypothesis, instead, it is the lack of water clustering that promotes Li⁺–water interactions, in turn resulting in a higher strength of the intramolecular O–H bond in the water molecules. The stronger intramolecular O–H bonds in turn retard the water-splitting reaction manifested by the higher electrochemical stability observed for WISEs. (20) A slightly different viewpoint was presented in recent work by Teng and co-workers, pointing out the importance of also considering the solvation of OH^– and H⁺ with respect to the observed widening of the water ESW upon an increase of salt concentration. In essence, they emphasize that the full chemical reaction must be taken into account, not only the reactant side. In this aspect, the solvation of the products of water splitting, OH^– and H⁺, should be considered to allow a proper evaluation of the driving force for the overall reaction. In their view, the main contribution to the wider ESW of water upon increasing salt concentrations originates from the lower solvation energies of OH^– and H⁺. Notably, this effect is complementary to the hypothesis of thermodynamic effects on the O–H bond strength on the reactant side. (21)

Regardless of all the aforementioned effects, just increasing the salt content in aqueous electrolytes expands the ESW to around 2.5 V. (6,22) A further increase in electrochemical stability is hampered by solubility issues with respect to the Li-ion salt. (15,23) In order to overcome this issue, hybrid water-in-salt concepts have been developed. Despite higher Li-salt solubility in this type of additive-modified electrolyte, the added components themselves also influence the water activity, resulting in further enhanced electrochemical stability.

Ionic liquids (ILs) have emerged as promising additives in hybrid water-in-salt electrolytes, owing to their high electrochemical stability, nonvolatility, and nonflammability. (24) Because of their wide application in recent decades, ILs have been considered the “green solvents of the future”. The possibility of tuning their physicochemical properties by rather straightforward modifications of the cation–anion combinations and substitution on their core molecular structures promotes them as a promising molecular design platform in different fields. The IL molecular design nature can be effectively exploited in the formulation of hybrid WISE-type LIBs as well. It is well known that ILs exhibit different tendencies to influence water HBN depending on the type and size of the constituent ions. (25) The extensive work of Marcus and other groups has resulted in a detailed understanding of ion influence on the water structure for common ions. (25−27) The ions that are prone to promote a water structure are defined as cosmotropic (structure-makers), while in contrast, chaotropic ions (structure-breakers) tend to break the intrinsic water structure characterized by extensive water–water molecule hydrogen bonding. In a search for the best strategy for anion selection in WISE architectures, Reber et al. investigated a selection of anions exhibiting different chaotropic/cosmotropic effects. (28) It was shown that the chaotropic nature of the anion plays an important role in widening the ESW of aqueous electrolytes, with the [TFSI^–] anion as a promising candidate for implementation in WISE systems. In that work, cosmotropic cations in the formulations of salt were used, causing the strengthening of the HBN. Thus, an interesting prospect would be to combine [TFSI^–] with a chaotropic cation to form a cosalt that could benefit from the aforementioned effect by the [TFSI^–] anion. The addition of ILs to the structure of WISEs brings another variable to light exhibited as an indirect effect of the IL cations on the molecular water organization. The cosmotropicity and chaotropicity of ILs are, to a large extent, determined by the aromaticity of the cation ring and the type of cation substituents. Previous research has shown that aromatic imidazolium and pyridinium rings alone show a cosmotropic tendency that is more pronounced than in ammonium, piperidinium, and pyrrolidinium cations. (26,27) However, the cosmotropicity/chaotropicity of the cations is primarily determined by the length and type of cation side chains. Short alkyl chains (-methyl, -ethyl, and -propyl) tend to promote chaotropic behavior, while a further increase of the side-chain length causes “hydrophobic hydration” and instead boosts cosmotropicity. The water molecules around surfactant-type molecules tend to form transient clathrate-like clusters, leading to small alternation in water HBN (hydrophobic hydration). The presence of hydrophilic groups in the cation side chains promotes water organization through the formation of H-bonds. Therefore, the molecular design nature of ILs represents a useful tool for gaining a better understanding of the thermodynamic effects in WISE-type LIBs. Recently, different groups have developed cosalts of “high-entropy” aqueous electrolytes, leading to desolvation processes caused by the presence of hydrophobic cations at the electrode–electrolyte interface. (29,30) However, we anticipate distinct roles for entropic and chaotropic effects in solution, with the primary focus of this article being on the latter. While the effects of chaotropic ions on the HBN and the macroscopic consequences of such interactions are still unclear, the importance of chaotropicity in the reactivity of aqueous electrolytes has clearly been demonstrated. (28,31,32)

Recently, ILs have been used in a few formulations of hybrid WISEs for LIBs. In all those investigations, a combination of a nonflammable IL with a WISE has been proposed to both reduce the H₂O activity and to improve the ionic conductivity. Chen et al. increased the solubility of Li[TFSI] up to 63 m using 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide) added to an aqueous Li[TFSI] mixture. (33) The influence of imidazolium-type ILs on hybrid WISEs was studied by Becker and co-workers. (34) The addition of an IL boosted the solubility of Li[TFSI] and rendered an acceptable ionic conductivity of 1.2 mS cm^–1 at 25 °C and at the same time enabled stable charge cycling of a niobia-coated Li₄Ti₅O₁₂/NMC811 full battery cell. However, the reaction kinetics still needs improvement. The same research group formulated quaternary systems by the addition of a cosolvent in the form of succinonitrile to promote better lithium-ion mobility, resulting in promising performance of the LIBs during 250 charging cycles. (35) In our previous work, the use of Gemini-type ionic liquids (GILs) based on pyrrolidinium and piperidinium cations was demonstrated. (36) Pyrrolidinium cation rings interlinked with poly(ethylene glycol)-like spacers were shown to generate stable charge cycling of conventional Li₄Ti₅O₁₂/LiMn₂O₄ (LTO/LMO for short) cells during at least 500 cycles with a specific discharge capacity of up to 80 mA h·g^–1. Also, the inclusion of GILs in the architecture of the WISEs increased the ESW to >3.7 V. In our present work, we have investigated the influence of chaotropic/cosmotropic cations in GILs on the molecular organization of a new generation of WISEs and their resulting performance in aqueous LIBs. The comprehensive variation of the side chains of the core pyrrolidinium ring allows systematic tuning of chaotropicity and cosmotropicity, which was monitored by a detailed solvation structure investigation of the bulk electrolytes. Most of the new systems generated show good performance in battery devices based on commercial LTO and LMO electrode materials without the need for any additional protective coating.

Methods

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Synthesis of Gemini Ionic Liquids

The syntheses were conducted via a three-step procedure (Scheme S1). The initial phase involved alkylation of the pyrrolidine ring system with the appropriate chloroalkane analogues. An equimolar amount (0.1 mol) of pyrrolidine and chloroalkane was dissolved in acetone and mixed with solid potassium carbonate (0.2 mol). The mixture was stirred for 48–72 h at 50 °C under a protecting nitrogen atmosphere. After the removal of unreacted material and solvent, the resulting product was dried for 24 h under vacuum. The obtained N-alkylpyrrolidine (2 mol equiv) was mixed with 1 mol equivalent of bis(2-chloroethyl)ether in isopropanol. The mixture was stirred for 72 h at 80 °C under reflux. The obtained products were purified at least five times by recrystallization in acetone. The purified GILs were further dried under vacuum over P₂O₅ for at least 48 h. The purity of the halide-containing GILs was determined by argentometric potentiometric titration, and the water content was determined by Karl Fischer titration.

In order to obtain perfluorinated suflonylamide-type GILs, an ion exchange reaction was executed. One mol equivalent of the synthesized halide-containing GIL was dissolved in water or acetone and subsequently mixed with two mol equiv of Li[TFSI]. The solution was vigorously stirred for 24 h at room temperature, resulting in the separation of two phases. The choice of solvent highly depends on the hydrophobic character of the resulting GIL; water was used for hydrophobic GILs, while acetone was the preferable solvent for hydrophilic GILs.

For hydrophobic GILs, the aqueous top phase containing LiCl(aq) was removed and the resulting GILs were washed with dichloromethane several times. After the removal of dichloromethane and drying, the water and halide contents were determined as described above. In the case of hydrophobic GILs, the acetone phase consisted of a dissolved ionic liquid, while the precipitate was the halide salt of the lithium ions. After suitable mixing, the mixture was kept for 24 h in an ice bath to further decrease the solubility of the lithium salt. Thereafter, the top phase was removed, acetone was evaporated, and the remaining GILs were dried with subsequent determination of the water and halide contents. The yield and water content in all GILs are presented in Table 1. All GILs were stored under vacuum in the presence of P₂O₅. The molecular structures were confirmed using mass spectrometry (MS), and multinuclear NMR and FTIR spectroscopy (Figures S1–S6).

Table 1. Abbreviation of the Gemini Ionic Liquids Used, Along with Water Content and Synthesis Yield

GIL	abbreviation	yield (%)	water content (ppm)
(C₂Pip–C₂OC₂–C₂Pip)[TFSI]₂	C₂Pyr	94	8.6
(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂	C₄Pyr	93	10.2
(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂	C₂OC₂Pyr	86	7.9
(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂	HOC₃Pyr	81	37.0
(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂	HOC₂OC₂Pyr	84	25.2
(HOOCC₂Pyr–C₂OC₂–HOCC₂Pyr)[TFSI]₂	HOOCC₂Pyr	73	45.9

Electrolyte Preparation

Lithium bis(trifluoromethanesulfonyl)imide, Li[TFSI] (Sigma-Aldrich, CAS number: 90076-65-6, initial purity of 99.99%) was dried and stored under vacuum before use. The electrolytes were formulated by mixing Li[TFSI], GILs, and water in the molar ratio 1:1:3 (for all systems studied in this work). The water content of Li[TFSI] and the synthesized GILs was determined by Karl Fischer titration before the preparation of the electrolytes, and consequently, existing water was subtracted from the amount of water added to generate the target molar ratios. The added high-purity water (Millipore Mili-Q) was degassed and purged with argon before use.

Viscosity and Conductivity Investigations

The viscosities were determined by using a Brookfield CAP 2000+ viscometer. A Mettler-Toledo Seven Excellence conductometer S700 with platinum electrodes was used for the electric conductivity experiments. The analyses were performed in a Pyrex-type cell with an external thermostated water flow to maintain a constant temperature by using a Lauda thermostat. The conductivity cell constant was predetermined using a 0.01 M aqueous KCl standard solution, and validation was performed at all temperatures investigated.

Raman Spectroscopy

A BioRad FTS 6000 spectrometer with a Raman accessory was used for recording the Raman spectra. A liquid-nitrogen-cooled Ge detector was employed. The effective frequency range used was 100–4000 cm^–1. A spectral resolution of 4 cm^–1 was applied.

Molecular Dynamics Simulations

The molecular dynamics (MD) simulations were performed using the Tinker package, with an implemented many-body polarizable force field developed by Borodin. (37) The simulation cells were constructed to contain 128 molecules of Li[TFSI], 128 molecules of GIL, and 384 water molecules. All systems were initially equilibrated using an NPT ensemble at 100 °C for 10 ns, followed by 20 ns equilibration at 50 °C and an additional 20 ns at 25 °C. During NPT equilibration, the dimensions of the simulation cell were reduced to match experimental densities at 25 °C (Table S3). The obtained box size at 25 °C was used for further simulations. The production run was performed in an NVT ensemble using a Nose–Hoover thermostat set to a temperature of 25 °C. Multiple time step integration was employed with a time step of 0.5 fs for bonded interactions, a time step of 1.5 fs for all nonbonded interactions within a truncation distance of 8.0 Å, and an outer time step of 3.0 fs for all nonbonded interactions between 8.0 Å and the nonbonded truncation distance of 14 Å. Ewald summation was used for electrostatic interactions between set charges or induced dipole moments with k = 83 vectors. The reciprocal part of the Ewald model was computed at every 3.0 fs. Induced dipoles were found self-consistently with a convergence criterium of 10^–9.

Battery Assembly

The electrodes were prepared from commercially available active materials, lithium titanate (LTO) and spinel lithium manganate (LMO). Polyvinylidene fluoride (PVDF) was used as the binder material. Before preparation of the slurry, PVDF was dissolved in N-methyl pyrrolidone forming a 5 wt % solution. The electrode slurry consisted of the active material, conductive carbon Super C65, and PVDF, mixed in a mass ratio of 8:1:1. The slurry was coated onto stainless-steel current collectors and dried for 6 h at 80 °C in air and subsequently for 6 h at 80 °C under vacuum. The electrodes were punched to a diameter of 13 mm. The mass loading of LTO was 2.4 mg (expected theoretical capacity of 175 mA h·g^–1), while the corresponding mass of LMO was 3.0 mg (expected theoretical capacity of 148 mA h·g^–1). Active materials and the binder were purchased from Sigma-Aldrich, while the conductive carbon was provided by MSE Suppliers. The prepared electrodes were used in coin-type LIB full cells. A CR2032 stainless-steel battery casing was used, and the electrodes were separated using a Whatman GF/D glass microfiber (d = 15 mm) sheet. The prepared electrolytes were added in a volume of 55 μL. All the batteries were assembled in ambient conditions and were allowed to equilibrate for 24 h prior to characterization. For the electrochemical stability window determination, a Swagelok T-cell assembly was used. The same stainless-steel current collectors as for the coin cells were used as working and counter electrodes, while an extra reference electrode was introduced consisting of a miniature Ag/AgCl electrode (eDAQ).

Electrochemical Characterization

Linear sweep voltammetry was performed using Swagelok T-cells with a scan rate of 0.1 mV/s and an Autolab PG12 potentiostat. Galvanostatic cycling was performed with a Neware battery station by using coin cells. The voltage window was set to 0.5–3.0 V. The current density was adjusted according to the mass of the LTO electrode. Electrochemical characterization was performed without control of the temperature. All presented data are the averages of 8 coin cells.

Postmortem Analysis

The cycled LTO electrodes were investigated by using FTIR spectroscopy and X-ray absorption spectroscopy (XAS). All the electrodes were carefully disassembled after cycling, and the remaining GIL-based electrolytes were removed using a mixture of propylene carbonate and dimethyl carbonate. The solvents used for washing the electrodes were subsequently evaporated at an elevated temperature under vacuum. For the XAS measurements, all of the steps were performed in an inert atmosphere. The pristine electrode was washed by using the same procedure. FTIR spectra were recorded by a Thermo Nicolet spectrophotometer by using the ATR technique on a ZnSe crystal. XAS spectra were recorded at the PEAXIS beamline, BESSY II synchrotron at the Helmholtz Zentrum Berlin. The absorption spectra of the F K-edge in the energy range 685–710 eV were recorded, and the pristine electrode data were subtracted from the obtained spectra.

Results and Discussion

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According to our previous work, WISEs enriched with pyrrolidinium-based GILs offer superior results with respect to their piperidinium analogous in terms of transport properties and electrochemical performance, despite exhibiting lower electrochemical stability. (36) Therefore, a systematic change of substituents of the pyrrolidinium GIL cations included in the WISE composition was performed in this study (Figure 1 and Table 1).

Figure 1. Schematic representation of the GILs used in this work, along with their molecular water structure making (cosmotropic)/breaking (chaotropic) tendency. Cosmotropicity/chaotropicity of cations investigated are determined by a method based on viscosity as explained in the Supporting Material (Figures S7, S8, and Table S1).

The different substituents were chosen to tune the hydrophilic character of investigated GILs, from completely hydrophobic with butyl side chains to hydrophilic GILs with hydroxyl or carboxyl groups (Figure 1). This modification also tunes their chaotropicity (structure-breaking) and cosmotropicity (structure-making), resulting in different structural organizations of the water molecules and consequently differences in the Li⁺ solvation shell. By including GILs with ether-containing (−C₂OC₂−) spacers, the viscosity could be minimized and the conductivity maximized. All ternary electrolyte systems were composed to have the same molar ratio of Li[TFSI], corresponding GIL, and water (1:1:3), with no initial heating in the preparation process. Equilibration at higher temperatures was avoided to prevent the risk of generating thermodynamically unstable, oversaturated electrolyte systems. (15)

The viscosity and conductivity of the WISEs containing the different GILs were determined in the temperature range 298–323 K and the resulting data are shown in Figure 2 and Table S2. The consequence of introducing the ether-containing spacer (−C₂OC₂−) in the structure of the GIL cations is clearly observed by comparison with the analogous cations instead containing the −C₄– spacer (Figure S9 and Table S3 in the Supporting Material). The reduction in viscosity by the inclusion of the (−C₂OC₂−) spacer is in the range of 5–25%. The reduction of viscosity by introduction of the ether group in the alkyl side chain was ascribed to the increased free volume, which results from the high conformational flexibility of the ether moieties and provides more available holes for the convenience of mass transport and low viscosity. The same trend is observed in the corresponding conductivities, where 2–15% higher conductivities were recorded for electrolytes with the −C₂OC₂– spacer. Regardless of the spacer type, side-chain substitutions display a similar pattern with respect to the transport properties. Electrolytes containing C₂OC₂Pyr show the lowest viscosity (61 mPa·s at 298 K) and the highest conductivity (3.4 mS·cm^–1 at 298 K). The worst electrolyte in terms of transport properties is the one containing the carboxyl end-group, HOOCC₂Pyr, with a viscosity of 122 mPa·s, and a conductivity of 1.6 mS·cm^–1 at 298 K. As a general pattern, the viscosity decreases in the order HOOCC₂Pyr > HOC₃Pyr > HOC₂OC₂Pyr > C₄Pyr > C₂Pyr > C₂OC₂Pyr, and the same trend regarding conductivity can be observed (HOOCC₂Pyr < HOC₃Pyr < HOC₂OC₂Pyr < C₄Pyr < C₂Pyr < C₂OC₂Pyr). Overall, WISEs containing GILs with a chaotropic tendency (C₄Pyr, C₂Pyr, C₂OC₂Pyr) exhibit better transport properties as compared with systems with GILs that tend to promote molecular water structures.

Figure 2. Temperature-dependent viscosities (a) and electric conductivities (b) for the investigated systems. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.

Solvation Structures

In order to understand the impact of different GILs on the electrolyte performance, basic insights into the solvation structures of the bulk electrolytes were obtained by using Raman spectroscopy and molecular dynamics (MD) simulations. The most important regions of the Raman spectra for the investigated systems are the −OH stretching vibration modes of water (3300–3600 cm^–1) and the [TFSI]⁻ breathing mode (740–750 cm^–1). Figure 3 shows Raman spectra in the wavenumber region corresponding to the S–N–S vibration of the [TFSI]⁻. It is well-known that this breathing vibration is strongly influenced by the coordination environment of the anion in the WISEs. (12) In the highly concentrated 21 m aqueous Li[TFSI] system, that band displays a significant blueshift to close to 750 cm^–1, which is correlated to a strong Li⁺–[TFSI]⁻ interaction. The approach to quantify the cation–anion interaction, proposed by Suo et al., suggests that the cation–anion cluster can be linked to the wavenumber of the [TFSI]⁻ breathing vibration. (12) The proposed classification divides the cation–anion interactions into four groups: (I) free anion (740–743 cm^–1), (II) loose ion-pair (743–746 cm^–1), (III) intimate ion-pair (746–748 cm^–1), (IV) aggregated ion-pair (748–750 cm^–1). In our systems, the C₂Pyr-containing electrolyte offers the lowest wavenumber (≈743.6 cm^–1). Taking the lowest wavenumber limit of the range as a point of comparison, a blueshift of ≈1 cm^–1 is noted for electrolytes with C₄Pyr and C₂OC₂Pyr. The replacement of the terminal methyl group with −OH groups causes a significant blueshift. Electrolytes with HOC₃Pyr and HOC₂OC₂Pyr shift the Raman spectra up to 746.7 and 747.4 cm^–1, respectively. Even if using one decimal in the wavenumbers may be overoptimistic with respect to absolute values, they serve the purpose of illustrating consistent relative Raman shifts of the analogous GIL cations. A further increase of the blueshift was noted in the presence of a carboxyl end-group in the side chain (748.6 cm^–1). All the pure GILs display a Raman shift between 742 and 744 cm^–1, suggesting weak coordination between the ionic-liquid cations and [TFSI]⁻ (Figure S10). Based on these results, the Raman shift in water + Li[TFSI] + GIL systems can be assigned to the amount of Li⁺–[TFSI]⁻ interaction. Based on Suo’s classification, the formation of loose ion pairs between lithium ions and [TFSI]⁻ is noted in the electrolytes with chaotropic GILs (C₂Pyr, C₄Pyr, C₂OC₂Pyr). A more pronounced Li⁺–[TFSI]⁻ interaction is observed in HOC₃Pyr- and HOC₂OC₂Pyr and could be assigned as an intimate ion-pair formation. The largest blueshift is observed for electrolytes with the most cosmotropic GIL (HOOCC₂Pyr), which is on the borderline of aggregate ion pairs. The inclusion of GIL functional groups that promote molecular water structures correlates with a more pronounced Li⁺–[TFSI]⁻ interaction. In a typical WISE, water molecules constitute the only coordinator competitor that can displace [TFSI]⁻ from the Li-ion solvation shell, while in our more complex systems, the indirect effects of the GIL cation may also influence the Li-ion solvation shell. Therefore, the observation of the O–H stretch vibrations in the Raman spectra of water molecules in the electrolyte systems (Figure 3b) may provide additional information about the water HBN and the availability of water molecules to coordinate Li⁺. The broad signal that usually can be observed in pure water or dilute aqueous solutions is close to 3500 cm^–1 while it becomes sharper when the concentration of dissolved salt is increased. That has been explained by a disruption of the predominant water HBN, with a continuously more red-shifted and sharper band emerging at higher salt concentrations disrupting the inherent water structure. This phenomenon is noted in all highly concentrated aqueous electrolytes.

Figure 3. Normalized Raman spectra for (a) breathing [TFSI]⁻ mode at ≈750 cm^–1 and (b) water stretch vibration modes at ≈3500 cm^–1. Black: Li[TFSI]¹:(C₂Pyr-C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³; orange: water (intensity of the water band was divided by a factor of 5 for clarity purpose).

Since the GILs used in this work were designed to subtly change the HBN of water, a redshift of the O–H band should be expected as a result of reduced cosmotropicity. Electrolytes with carboxyl-substituted GIL cations show the broadest peak centered around 3547 cm^–1, followed by −OH containing GIL cations with peaks at around 3555 cm^–1. A large blueshift of ≈20 cm^–1 is observed for electrolytes with the C₄Pyr and C₂OC₂Pyr GILs, while the most prominent increase of wavenumber is observed for the ethyl-substituted GIL (≈3582 cm^–1).

The inclusion of hydrophilic groups (−COOH and −OH) in the structure of the GIL cations leads to them becoming incorporated in the HBN causing a similar shape and position of the −OH stretch band as compared with that in pure water. On the other hand, the hydrophobic groups in the C₂Pyr, C₄Pyr, and C₂OC₂Pyr GILs disturb the HBN resulting in a significant blueshift and sharpening of the Raman bands. Thus, the obtained results for the [TFSI]⁻ breathing vibration need to be closely correlated with the water structure effects and change in the HBN itself, as caused by the GILs. It is clear that systems based on GILs that increase the structural order of water, thus retaining the water HBN (HOC₃Pyr, HOC₂OC₂Pyr, and HOOCC₂Pyr), contain fewer available water molecules that can compete with [TFSI]⁻ for a position in the Li⁺-ion solvation shell. Therefore, the change in the S–N–S vibration band of the [TFSI]⁻ ion is the most prominent for these types of electrolytes, while the −OH band displays less deviation from that of pure water. On the other hand, disturbing the water structure makes water molecules more available to compete for a coordination position in the Li⁺-solvation shell, thus providing [TFSI]⁻ ions with stronger competition, and therefore, a smaller shift of S–N–S band is expected, and this is exactly what is noted for chaotropic GILs (C₂Pyr, C₄Pyr, C₂OC₂Pyr). In spite of the above logical reasoning, the presence of −OH and −COOH groups in the structures of some GIL cations needs to be taken into account as additional Li⁺-coordination competitors themselves. Deeper insights into the interplay between Li⁺-coordination competitors were revealed by molecular dynamics (MD) simulations. All the simulations were performed at 298 K, with the same molecular ratios for all electrolytes (Table S4).

The partial radial distribution functions (pRDFs) and coordination numbers (CNs) were used for the representation of the local structural environments. CNs were estimated as average coordination numbers during the simulation time using the equation:

CN = 4 πρ \int_{0}^{r^{'}} g (r) r^{2} d r

(1)

where ρ is the average density, g(r) is the radial distribution function (RDF), and r′ is the distance from the selected type of atom. As a better dynamic representation of the system, CN probability distribution was calculated using each time step in the simulations with a cutoff distance <4.0 Å. In typical WISE systems, the water local environment is characterized as water coordinated to a lithium ion or water solvated by other water molecules. The nature and structure of the GIL cations introduce another variable in the water organization structure that may have direct or indirect effects on the Li⁺ solvation shell.

A “free” water organization in GIL-containing WISEs was investigated by water–water pRDFs and the corresponding CNs (Figure 4a,d). The most important feature corresponds to the first peak in the pRDFs. The most intense peak at the shortest distance (≈2.9 Å) is observed for systems consisting of C₄Pyr and HOC₃Pyr. This peak is slightly shifted toward longer distances, 3.1 and 3.2 Å, for electrolytes containing HOOCC₂Pyr and HOC₂OC₂Pyr, respectively. The longest distances are noted for electrolytes formulated with the most chaotropic GILs, C₂Pyr, and C₂OC₂Pyr. Electrolytes with C₂Pyr display the lowest integral (i.e., CN) of the first peak appearing at a distance as long as 3.5 Å, while a more prominent peak can be observed at a similar distance for the analogous system with C₂OC₂Pyr.

Figure 4. pRDFs (a–c) and CNs (d–f) of water–water, Li–O(water), and Li–O(TFSI) distances. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.

By the investigation of the CN probability (Figure 4d), only systems with C₄Pyr and HOC₃Pyr show noticeable water–water clustering. In these electrolytes, around 40% of the water molecules retain some degree of structural organization corresponding to that observed in pure water. On the other hand, the presence of chaotropic GILs (C₂Pyr or C₂OC₂Pyr) causes an obvious disturbance in the intrinsic water structure with more than 95% of the water molecules no longer in the proximity of other water molecules. In electrolytes with the most prominent cosmotropic GILs (HOC₂OC₂Pyr and HOOCC₂Pyr), around 85% of the water molecules are not coordinated to any neighboring water molecules.

As a next step, the Li⁺ solvation structure was analyzed by generating the pRDFs and CNs for Li–O(H₂O) (Figure 4b,e) and Li–O(TFSI^–) and interactions (Figure 4c,f). For both types of coordination, the first peak appears at almost the same distance, regardless of which GIL is included in the system. The sharp first peak for the Li–O(TFSI^–) distance appears at ≈2 Å, with a significant difference in peak intensity (i.e., probability) between the electrolytes with cosmotropic and chaotropic GIL cations. The peak intensity follows the trend: HOOCC₂Pyr > HOC₂OC₂Pyr > HOC₃Pyr > C₄Pyr ≈ C₂OC₂Pyr > C₂Pyr. A similar peak position was obtained for Li–O(H₂O) (Figure 4b). The decrease in peak intensity in this case follows the order: C₂Pyr > C₂OC₂Pyr > C₄Pyr > HOC₃Pyr > HOC₂OC₂Pyr > HOOCC₂Pyr. The intensity of the first pRDF peak suggests that the Li⁺–[TFSI]⁻ interactions are the most prominent in electrolytes with cosmotropic GILs, while Li⁺–water interactions are more pronounced when electrolytes contain chaotropic GILs.

The most straightforward comparison of Li⁺-solvation shell structures in the presence of different GILs is obtained from the probability of the presence of CNs. Electrolytes enriched with cosmotropic GILs (HOC₂OC₂Pyr, HOC₃Pyr, HOOCC₂Pyr) are predominantly characterized by one water molecule in the solvation shell of the lithium ions, with almost 20% of lithium ions without any water in the solvation shell. With the increase of chaotropicity, CNs are gradually increasing with almost the same probability of finding 2 or 3 water molecules in the Li⁺ solvation shell. Analogously, almost no “water-free” Li⁺ solvation shell is observed for the most chaotropic GIL (C₂Pyr). The presence of [TFSI]⁻ in the Li solvation shell is more pronounced in all systems simulated (Figure 4c). In electrolytes with C₂Pyr, almost 40% of the lithium ions have 2 [TFSI]⁻ in the solvation shell, followed by slightly more than 30% of Li⁺ with 3 [TFSI]⁻. An increasing cosmotropicity of the GILs leads to a higher abundance of anions in the Li⁺ solvation shell, which is manifested by a continuous increase in the CNs to 3 or higher. Therefore, the WISE containing C₂OC₂Pyr displays a similar probability of finding 2 or 3 anions in the Li⁺ solvation shell, both with about 40% probability (Figure 4f). The Li ions in electrolytes with C₄Pyr, HOC₃Pyr, and HOC₂OC₂Pyr are characterized by predominantly three anions in the Li⁺ solvation shell, while the presence of the most cosmotropic GIL (HOOCC₂Pyr) increases the probability of finding even four anions in the Li-ion coordination shell.

In order to gain a better understanding of the overall liquid structure of GIL-enriched WISEs, the average CNs for the most probable interactions were evaluated and are presented in Figure 5. The visual interpretation of the solvation structure taken from the MD simulations from most cosmotropic and chaotropic GIL is presented in Figure S11. Upon addition of chaotropic GILs (especially C₂Pyr and C₂OC₂Pyr), the water structure in the WISEs becomes quite disturbed, resulting in small amounts of water clusters in the systems (lowest CNs and lowest intensity of the pRDFs peaks for O(water)–O(water) interactions). Disturbance of the water structure is manifested through the breaking of water–water hydrogen bonds, making water molecules more accessible for the solvation of Li⁺. The hydrophobic character of [TFSI]⁻ also contributes to pushing water more toward the Li⁺ solvation shell, inducing a higher CN for Li–O(water) in systems with C₂Pyr and C₂OC₂Pyr GILs. When the cosmotropicity is increased, for instance, represented by the C₄Pyr GIL, the water–water interaction peak in the pRDF becomes more prominent, as well as the corresponding CN, suggesting that water retains some archetypical water organization. Therefore, water molecules in these small clusters have H-bonds that need to be broken in order to contribute to Li⁺ solvation.

Figure 5. Average coordination number of the most probable interactions in simulated hybrid WISE systems. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³. Error bars correspond to ± one standard deviation estimated using the block-average technique.

Therefore, a smaller number of water molecules can contribute to Li⁺ solvation, reducing the resulting Li-water CNs. The inclusion of −OH groups in the structure of HOC₃Pyr also helps to retain the water organization level to some extent. The water–water pRDFs peak is only slightly shifted and is of lower intensity than that for systems containing C₄Pyr. It is important to note that the inclusion of cosmotropic cations contributes to additional competition with respect to water coordination (hydrophilic group in the side chain). Therefore, it is not surprising to observe that a small amount of water also binds to the −OH group of the GIL cation in the presence of the HOC₃Pyr. With prolongation of the side chain and increased flexibility in HOC₂OC₂Pyr, the probability of finding water molecules around the GIL cations becomes more prominent. This results in an additional decrease in water–water coordination (average CN decreases to 0.1). This effect is even more prominent in the presence of the carboxyl cation groups, which represent competitors not only for water coordination but also as potential ligands for Li-ion coordination. Owing to the presence of water-attracting groups in the structures of HOC₃Pyr, HOC₂OC₂Pyr, and HOOCC₂Pyr, water becomes distributed among the three available environments: Li-ion, hydrophilic groups of the GIL cations and other water molecules. The higher affinity of the GIL cations with respect to water molecules leads to less accessible water for participating in Li⁺ solvation, resulting in high Li–O(TFSI^–) CNs in the electrolytes containing such cations. Therefore, the electrolytes with cosmotropic GILs display [TFSI]⁻-rich Li⁺ solvation shells, while electrolytes with chaotropic GILs instead are characterized by Li⁺ solvation shells rich in water.

Electrochemical Performance

The electrochemical stability window (ESW) for GIL-enriched WISEs was investigated by linear sweep voltammetry (LSV, Figure 6), using a previously established procedure. (32) The presence of GILs increases the ESW as compared to non-GIL-containing WISEs by at least 1 V. The narrowest ESW (3.32 V) was recorded for the WISE containing the carboxyl-functionalized GIL, associated with both lower reductive and oxidative stability in comparison to those of the other investigated electrolytes. Other GIL-containing WISEs show similar oxidative stability but significant differences in reductive stability depending on the type of GIL present. All electrolytes with cosmotropic cations exhibit narrower ESWs, owing to a decrease in reductive stability in comparison with WISEs with chaotropic GILs. The most stable electrolyte contains the C₂Pyr GIL with a recorded ESW of 3.91 V. Overall, the inclusion of hydrophilic groups (−OH and −COOH) in the structure of the GIL cations results in a lower ESW by at least 0.2 V. Thus, the cations with a more pronounced chaotropic effect (C₂OC₂Pyr, C₂Pyr, C₄Pyr) lead to wider ESWs in the ternary-type WISEs.

Figure 6. Electrochemical stability window. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.

In Figure 6, a small peak can be noted at ≈−1 V vs Ag/AgCl, and it is the most prominent in the WISE system containing HOOCC₂Pyr. In contrast, it is almost invisible for the electrolyte based on C₂Pyr. In the literature, this peak has been interpreted as essential for the formation of an anion-derived solid electrolyte interphase (SEI) at the electrode, with two possible mechanisms (reduction of “free” water, or reduction of “free” TFSI^–). (38,39) The correlation between “free” water content in the electrolytes and the intensity of the peak at −1 V was identified by a comparison between the results from the MD simulations and the integral area from the LSV curve. A low peak intensity correlates well with high Li-water CNs, indicating the possible reduction of water molecules in these systems at −1 V vs Ag/AgCl leading to the high stability of electrolytes with more chaotropic GILs present (more water-rich Li⁺ solvation shell). In order to investigate the battery performance of the WISE electrolytes, full cells based on Li₄Ti₅O₁₂ (LTO) anodes and LiMn₂O₄ (LMO) cathodes were assembled. The wide ESW of all electrolytes should ensure stable charge cycling of both electrodes (LTO ≈ −1.5 V vs. Ag/AgCl in concentrated electrolytes, LMO ≈ +1.2 V vs Ag/AgCl in concentrated electrolytes). The obtained charge/discharge cycling performance of full cells in a voltage range of +0.5 to +3.0 V at 1C rate (corresponding to 90 mA·g^–1) is shown in Figure 7. The voltage profiles of full cells, estimated with respect to the active mass of LTO at 1C rate, for other electrolytes, are presented in the Supporting Material (Figures S12–S16). The batteries with HOOCC₂Pyr-based GILs showed a malfunction that possibly originates from a low reductive electrochemical stability close to that of the LTO electrode.

Figure 7. Battery performance of LTO (mass loading 2.4 mg)/LMO (mass loading 3.0 mg) cells based on the GIL-containing electrolytes. (a) Specific discharge capacity, (b) Coulombic efficiency, (c) voltage profiles for the 10th cycle at 1C-rate, and (d) rate performance. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.

In terms of cycling stability, an obvious difference was observed between electrolytes containing chaotropic and cosmotropic GILs. All cells with electrolytes based on chaotropic GILs (C₂Pyr, C₄Pyr, and C₂OC₂Pyr) showed stable cycling for more than 500 cycles, while cells based on WISEs containing cosmotropic GILs instead showed a fast decline in discharge capacity. The most promising coin cells in terms of discharge capacity and Coulombic efficiency (CE) are based on C₂Pyr. These cells retain 99% of the CE after 500 cycles, associated with a final specific discharge capacity of 85 mA h·g^–1. The average CE over this period of cycling was 99.5%, suggesting a very high stability of the investigated electrolyte.

In terms of capacity retention, batteries based on C₂Pyr GILs retain 90% of their initial discharge capacity. The second-best-performing cells were based on C₂OC₂Pyr, retaining just over 99% of the CE for more than 300 cycles. After 500 cycles, the cells retained 95% of the CE and 86% of its initial discharge capacity. Further decrease of chaotropicity of the GILs used in the WISEs leads to further decrease in CE and specific capacity. The batteries with electrolytes based on C₄Pyr display a final 90% CE and 82% initial discharging capacity. The inclusion of cosmotropic types of cations (HOC₃Pyr and HOC₂OC₂Pyr) caused a fast decline in the discharge capacity, leading to a decay in battery performance. The batteries with the HOC₂OC₂Pyr electrolyte faded away after only 100 cycles, with an almost linear decrease of the specific discharge capacity with cycle number. Slightly better performance was recorded for the formulation of WISEs with the less cosmotropic HOC₃Pyr, which rendered a CE of 80% after 200 cycles, followed by a significant loss of the initial discharge capacity.

The rate performance for battery cells based on GIL-enriched WISEs is presented in Figure 6d. For each discharge rate, 10 cycles were performed starting with the slowest discharge rate that allows 2 h until the full discharge of the battery (C/2), followed by 1C, 2C, 5C, 10C, and 20C, while in the last 10 cycles, battery cells returned to a C/2 rate. The electrolytes with chaotropic GILs outperform electrolytes based on cosmotropic GILs with respect to this parameter as well. Again, batteries containing C₂Pyr show the best results, being the most resilient with close to 100% of retained initial discharge capacity after changing the discharge rate from 20C back to C/2. The least viscous electrolyte, based on C₂OC₂Pyr, gave cells outperformed by those containing C₂Pyr, suggesting that the lowest viscosity and highest conductivity in the formulation of WISEs are important factors but not the only determining factors for good rate performance. Indeed, cells based on C₂OC₂Pyr show better robustness at larger C-rates, as compared to those based on the more viscous C₄Pyr and WISEs containing more cosmotropic GILs. However, the performance is similar to that of batteries based on C₂Pyr.

The presence of carboxyl-based groups in the GIL structure results in malfunction for all batteries investigated already during the initial charging cycle involving the LTO/LMO electrode system. From the perspective of the solvation structure of the bulk electrolytes, all electrolytes with cosmotropic GILs have an anion-rich solvation shell of Li⁺, with a significantly lower number of water-coordinated to the Li⁺ ions, as compared to electrolytes with chaotropic GILs. The presence of water-attracting groups in the GIL-cation structure clearly causes a significant amount of water molecules to be trapped within the nanodomains formed by the GIL cations, and this has consequences for the durability during battery cycling. On the other hand, electrolytes with chaotropic GIL cations have water-rich Li⁺ solvation shells with, statistically, about 1 molecule of water per lithium ion, more than electrolytes with cosmotropic GILs. The two electrolytes that combine the lowest percentage of “free” water molecules and the highest CN for the Li-ion-water interaction in the bulk electrolyte (C₂Pyr and C₂OC₂Pyr) also display the widest ESW and the best overall battery performance. However, we do take caution in correlating these findings to more quantitative kinetics interpretations. We hypothesize that the widening of the ESW and promising performance of our systems are primarily caused by the different chaotropic effects of our GIL cations. While we believe that chaotropicity is the main cause of the better performance observed in our systems, studying Li+ dynamics in the bulk and at the interfaces in the GIL + Li[TFSI] mixtures could provide additional insights into any entropic effects and will be studied advancing the project (preliminary results from DFT computations are included in the Supporting Information Material).

A postmortem analysis of cycled LTO electrodes was performed by FTIR spectroscopy and XAS (Figure 8). FTIR spectra show similar peaks around 1134 and 1054 cm^–1 for all electrodes originating from the GIL-based battery cells. These bands can be ascribed to S=O stretching or carboxylate stretching modes, suggesting the possible decomposition of [TFSI]⁻ giving rise to the formation of a sulfur-containing interphase, as has been proposed by Yamada and co-workers. (22)

Figure 8. FTIR (a) and XAS F K-edge (b) spectra of electrodes from charge-cycled batteries. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.

A significant difference between spectra from cells containing chaotropic and cosmotropic electrolytes was observed in the range of 1570–1650 cm^–1. Two peaks from the electrodes being exposed to electrolytes with GILs containing −OH or −COOH substituents appear in the 1570–1650 cm^–1 range, while these bands are absent in WISEs containing C₂Pyr, C₄Pyr, and C₂OC₂Pyr. These peaks can be assigned to C=O, C=C, and amine bond stretching vibration modes, instead suggesting a decomposition of the GIL cations. The proposed LiF formation was followed by XAS investigations of the F K-edge. A fluorine-containing absorption peak could be observed from all electrode surfaces of successfully cycled cells, with a broader peak appearance (690–700 eV) for the electrodes exposed to electrolytes containing HOC₃Pyr and HOC₂OC₂Pyr. However, we cannot exclude the contribution from the decomposition of the remaining Li[TFSI] salt by radiation damage as has been reported previously. (40) Therefore, the obtained results need to be verified by further experiments. An interesting feature is observed in the systems originating from cosmotropic GILs and C₄Pyr manifested by an additional peak close to 710 eV, which is characteristic of iron. This observation could suggest potential corrosion of the stainless-steel current collectors in GIL-enriched WISEs, which needs to be seriously considered in attempts to further increase Li-salt concentrations.

Conclusions

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The current study demonstrated the possibility of influencing the electrolyte water structure by adding different GILs to WISEs. The overall conclusion is that the Li⁺ solvation shell is water-rich in WISEs containing chaotropic GILs. Instead, anion-rich Li⁺ solvation shells are predominant in WISEs enriched with cosmotropic GILs. Weak water clustering is observed in the electrolyte systems with C₄Pyr and HOC₃Pyr, while in other systems, it is absent.

The addition of GILs to the WISE systems increases the electrochemical stability to almost 4 V. The stability window is significantly increased by the addition of chaotropic GILs. Only electrolytes containing HOOCC₂Pyr were not compatible with LTO/LMO-based full battery cells. The best cells, containing C₄Pyr, retained 99% of its CE after 500 cycles, associated with a final specific discharge capacity of 85 mAh·g^–1. Based on all results from the electrochemical characterization, we conclude that a water-rich Li⁺ solvation shell (i.e., in the presence of chaotropic GILs) significantly contributes to excellent performance and long-term stability of LTO/LMO-based full battery cells.

The fine-tuning of the Li⁺ solvation shell and water structure by the addition of chaotropic cations to the structure of ionic liquids qualifies as a promising strategy for generating more stable and effective aqueous Li-ion batteries. Since GILs offer a vast design space, we are planning to employ automated combinatorial screening methods to accelerate the identification of novel effective, secure, and environmentally friendly, water-based electrolytes.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c07164.

Schematic representation of synthetic pathway; NMR, FTIR, Raman, and MS spectra of synthesized GILs; experimental results of conductivity and viscosity of GIL-enriched electrolytes; snapshots of MD simulations; and voltage profiles of each electrolyte at selected cycles along with standard deviations (PDF)

jp3c07164_si_001.pdf (1.31 MB)

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Author Information

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Corresponding Author
- Aleksandar Tot - Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-10044, Sweden; https://orcid.org/0000-0002-1118-8788; Email: [email protected]
Authors
- Leiting Zhang - Department of Chemistry − Ångström Laboratory, Uppsala University, Uppsala SE-751 21, Sweden; https://orcid.org/0000-0003-4057-7106
- Per H. Svensson - Chemical and Pharmaceutical Development, RISE Research Institutes of Sweden, Södertälje SE-151 36, Sweden; https://orcid.org/0000-0003-2410-7366
- Lars Kloo - Applied Physical Chemistry, Department of Chemistry, KTH Royal Institute of Technology, Stockholm SE-10044, Sweden; https://orcid.org/0000-0002-0168-2942
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.

Acknowledgments

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The work was supported by the Swedish Energy Agency contract no. 50119-1, entitled “Be WiSE─Robotic screening of water-in-salt electrolytes (WiSE) for environmental batteries” and the Swedish Research Council contract no. 2020-06701.

References

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Goodenough, John B.; Kim, Youngsik
Chemistry of Materials (2010), 22 (3), 587-603CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
A review of challenges for further development of Li rechargeable batteries for elec. vehicles. Most important is safety, which requires development of a nonflammable electrolyte with either a larger window between its LUMO and HOMO or a constituent (or additive) that can develop rapidly a solid/electrolyte interface (SEI) layer to prevent plating of Li on a carbon anode during a fast charge of the battery. A high Li+-ion cond. (σLi > 10-4 S/cm) in the electrolyte and across the electrode/electrolyte interface is needed for a power battery. Important also is an increase in the d. of the stored energy, which is the product of the voltage and capacity of reversible Li insertion/extn. into/from the electrodes. It will be difficult to design a better anode than carbon, but carbon requires formation of an SEI layer, which involves an irreversible capacity loss. The design of a cathode composed of environmentally benign, low-cost materials that has its electrochem. potential μC well-matched to the HOMO of the electrolyte and allows access to two Li atoms per transition-metal cation would increase the energy d., but it is a daunting challenge. Two redox couples can be accessed where the cation redox couples are pinned at the top of the O 2p bands, but to take advantage of this possibility, it must be realized in a framework structure that can accept more than one Li atom per transition-metal cation. Moreover, such a situation represents an intrinsic voltage limit of the cathode, and matching this limit to the HOMO of the electrolyte requires the ability to tune the intrinsic voltage limit. Finally, the chem. compatibility in the battery must allow a long service life.
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Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical energy storage for the grid: a battery of choices. Science. 2011, 334, 928– 935, DOI: 10.1126/science.1212741
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Electrical Energy Storage for the Grid: A Battery of Choices
Dunn, Bruce; Kamath, Haresh; Tarascon, Jean-Marie
Science (Washington, DC, United States) (2011), 334 (6058), 928-935CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
A review. The increasing interest in energy storage for the grid can be attributed to multiple factors, including the capital costs of managing peak demands, the investments needed for grid reliability, and the integration of renewable energy sources. Although existing energy storage is dominated by pumped hydroelec., there is the recognition that battery systems can offer a no. of high-value opportunities, provided that lower costs can be obtained. The battery systems reviewed here include sodium-sulfur batteries that are com. available for grid applications, redox-flow batteries that offer low cost, and lithium-ion batteries whose development for com. electronics and elec. vehicles is being applied to grid storage.
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Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem. Rev. 2004, 104, 4303– 4417, DOI: 10.1021/cr030203g
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Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries
Xu, Kang
Chemical Reviews (Washington, DC, United States) (2004), 104 (10), 4303-4417CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review of progress in the research and development of electrolytes for lithium-based batteries. Since lithium ion chem. is by far the only commercialized rechargeable lithium-based technol., emphasis is placed on electrolytes developed for this system. Liq. electrolytes are important and the review includes their ionics, phase diagrams, interfaces with cathode and anode materials, long-term chem. stability in the device, thermal properties, performance at extreme temps., and safety characterization.
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Lebedeva, N.; Boon-Brett, L. Considerations on the chemical toxicity of contemporary Li-ion battery electrolytes and their components. J. Elec. Soc. 2016, 163, A821, DOI: 10.1149/2.0171606jes
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Considerations on the Chemical Toxicity of Contemporary Li-Ion Battery Electrolytes and Their Components
Lebedeva, Natalia P.; Boon-Brett, Lois
Journal of the Electrochemical Society (2016), 163 (6), A821-A830CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
This work evaluated chem. hazards and risks assocd. with the accidental release of Li ion battery electrolyte in an enclosed space. Due to the high volatility and reactivity of some components of contemporary Li ion battery electrolytes, work focused on inhalation toxicity of released and generated gas phase components, including evapd. solvents and HF as a decompn. product of the widely used LiPF6 salt. Calcns. showed at room temp., a small electrolyte release can result in formation of a toxic atm. with released compd. concns. reaching an acute exposure limit where irreversible and other serious health effects are expected to occur. For most contemporary electrolyte components, this corresponds to a release of <∼250 mL in a vol. occupied by a medium-size car with a clearance of 1 m, i.e., ∼62 m3. Addnl. research required for a thorough risk anal. is identified.
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Gao, Y.; Yan, Z.; Gray, J. L.; He, X.; Wang, D.; Chen, T.; Huang, Q.; Li, Y. C.; Wang, H.; Kim, S. H. Polymer–inorganic solid–electrolyte interphase for stable lithium metal batteries under lean electrolyte conditions. Nat. Mater. 2019, 18, 384– 389, DOI: 10.1038/s41563-019-0305-8
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Polymer-inorganic solid-electrolyte interphase for stable lithium metal batteries under lean electrolyte conditions
Gao, Yue; Yan, Zhifei; Gray, Jennifer L.; He, Xin; Wang, Daiwei; Chen, Tianhang; Huang, Qingquan; Li, Yuguang C.; Wang, Haiying; Kim, Seong H.; Mallouk, Thomas E.; Wang, Donghai
Nature Materials (2019), 18 (4), 384-389CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
The solid-electrolyte interphase (SEI) is pivotal in stabilizing Li metal anodes for rechargeable batteries. However, the SEI is constantly reforming and consuming electrolyte with cycling. The rational design of a stable SEI is plagued by the failure to control its structure and stability. Here the authors report a mol.-level SEI design using a reactive polymer composite, which effectively suppresses electrolyte consumption in the formation and maintenance of the SEI. The SEI layer consists of a polymeric Li salt, LiF nanoparticles and graphene oxide sheets, as evidenced by cryo-TEM, at. force microscopy and surface-sensitive spectroscopies. This structure is different from that of a conventional electrolyte-derived SEI and has excellent passivation properties, homogeneity and mech. strength. The use of the polymer-inorg. SEI enables high-efficiency Li deposition and stable cycling of 4 V Li|LiNi0.5Co0.2Mn0.3O2 cells under lean electrolyte, limited Li excess and high capacity conditions. The same approach was also applied to design stable SEI layers for Na and Zn anodes.
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Famprikis, T.; Canepa, P.; Dawson, J. A.; Islam, M. S.; Masquelier, C. Fundamentals of inorganic solid-state electrolytes for batteries. Nat. Mater. 2019, 18, 1278– 1291, DOI: 10.1038/s41563-019-0431-3
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Fundamentals of inorganic solid-state electrolytes for batteries
Famprikis, Theodosios; Canepa, Pieremanuele; Dawson, James A.; Islam, M. Saiful; Masquelier, Christian
Nature Materials (2019), 18 (12), 1278-1291CODEN: NMAACR; ISSN:1476-1122. (Nature Research)
A review. In the crit. area of sustainable energy storage, solid-state batteries have attracted considerable attention due to their potential safety, energy-d. and cycle-life benefits. This Review describes recent progress in the fundamental understanding of inorg. solid electrolytes, which lie at the heart of the solid-state battery concept, by addressing key issues in the areas of multiscale ion transport, electrochem. and mech. properties, and current processing routes. The main electrolyte-related challenges for practical solid-state devices include utilization of metal anodes, stabilization of interfaces and the maintenance of phys. contact, the solns. to which hinge on gaining greater knowledge of the underlying properties of solid electrolyte materials.
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Zettl, R.; Lunghammer, S.; Gadermaier, B.; Boulaoued, A.; Johansson, P.; Wilkening, H. M. R.; Hanzu, I. High Li⁺ and Na⁺ conductivity in new hybrid solid electrolytes based on the porous MIL-121 metal organic framework. Adv. Energy Mater. 2021, 11, 2003524 DOI: 10.1002/aenm.202003542
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Li, W.; Dahn, J. R.; Wainwright, D. S. Rechargeable lithium batteries with aqueous electrolytes. Science. 1994, 264, 1115– 1118, DOI: 10.1126/science.264.5162.1115
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Rechargeable lithium batteries with aqueous electrolytes
Li, Wu; Dahn, J. R.; Wainwright, D. S.
Science (Washington, DC, United States) (1994), 264 (5162), 1115-18CODEN: SCIEAS; ISSN:0036-8075.
Rechargeable lithium-ion batteries that use an aq. electrolyte were developed. Cells with LiMn2O4 and VO2(B-crystal) as electrodes and 5M LiNO3 in water as the electrolyte provide a fundamentally safe and cost-effective technol. that can compete with nickel-cadmium and lead-acid batteries on the basis of stored energy per unit of wt.
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Luo, J. Y.; Cui, W. J.; He, P.; Xia, Y. Y. Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte. Nat. Chem. 2010, 2, 760– 765, DOI: 10.1038/nchem.763
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Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte
Luo, Jia-Yan; Cui, Wang-Jun; He, Ping; Xia, Yong-Yao
Nature Chemistry (2010), 2 (9), 760-765CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
Aq. lithium-ion batteries may solve the safety problem assocd. with lithium-ion batteries that use highly toxic and flammable org. solvents, and the poor cycling life assocd. with commercialized aq. rechargeable batteries such as lead-acid and nickel-metal hydride systems. However, all reported aq. lithium-ion battery systems have shown poor stability: the capacity retention is typically less than 50% after 100 cycles. Here, the stability of electrode materials in an aq. electrolyte was extensively analyzed. The anodes of aq. lithium-ion batteries in a discharged state can react with water and oxygen, resulting in capacity fading upon cycling. By eliminating oxygen, adjusting the pH values of the electrolyte and using carbon-coated electrode materials, LiTi2(PO4)3/Li2SO4/LiFePO4 aq. lithium-ion batteries exhibited excellent stability with capacity retention >90% after 1000 cycles when being fully charged/discharged in 10 min and 85% after 50 cycles even at a very low current rate of 8 h for a full charge/discharge offering an energy storage system with high safety, low cost, long cycling life and appropriate energy d.
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Pasta, M.; Wessels, C. D.; Huggins, R. A.; Cui, Y. A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage. Nat. Commun. 2012, 3, 1149, DOI: 10.1038/ncomms2139
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A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage
Pasta Mauro; Wessells Colin D; Huggins Robert A; Cui Yi
Nature communications (2012), 3 (), 1149 ISSN:.
New types of energy storage are needed in conjunction with the deployment of solar, wind and other volatile renewable energy sources and their integration with the electric grid. No existing energy storage technology can economically provide the power, cycle life and energy efficiency needed to respond to the costly short-term transients that arise from renewables and other aspects of grid operation. Here we demonstrate a new type of safe, fast, inexpensive, long-life aqueous electrolyte battery, which relies on the insertion of potassium ions into a copper hexacyanoferrate cathode and a novel activated carbon/polypyrrole hybrid anode. The cathode reacts rapidly with very little hysteresis. The hybrid anode uses an electrochemically active additive to tune its potential. This high-rate, high-efficiency cell has a 95% round-trip energy efficiency when cycled at a 5C rate, and a 79% energy efficiency at 50C. It also has zero-capacity loss after 1,000 deep-discharge cycles.
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Suo, L.; Borodin, O.; Gao, T.; Ogluin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 2015, 350, 938– 943, DOI: 10.1126/science.aab1595
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"Water-in-salt" electrolyte enables high-voltage aqueous lithium-ion chemistries
Suo, Liumin; Borodin, Oleg; Gao, Tao; Olguin, Marco; Ho, Janet; Fan, Xiulin; Luo, Chao; Wang, Chunsheng; Xu, Kang
Science (Washington, DC, United States) (2015), 350 (6263), 938-943CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
Lithium-ion batteries raise safety, environmental, and cost concerns, which mostly arise from their nonaq. electrolytes. The use of aq. alternatives is limited by their narrow electrochem. stability window (1.23 V), which sets an intrinsic limit on the practical voltage and energy output. We report a highly concd. aq. electrolyte whose window was expanded to ∼3.0 V with the formation of an electrode-electrolyte interphase. A full lithium-ion battery of 2.3 V using such an aq. electrolyte was demonstrated to cycle up to 1000 times, with nearly 100% coulombic efficiency at both low (0.15 C) and high (4.5 coulombs) discharge and charge rates.
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Miyazaki, K.; Takenaka, N.; Watanabe, E.; Iizuka, S.; Yamada, Y.; Tateyama, Y.; Yamada, A. First-Principles study on the peculiar water environment in a hydrate-melt electrolyte. J. Phys. Chem. Lett. 2019, 10, 6301– 6305, DOI: 10.1021/acs.jpclett.9b02207
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First-Principles Study on the Peculiar Water Environment in a Hydrate-Melt Electrolyte
Miyazaki, Kasumi; Takenaka, Norio; Watanabe, Eriko; Iizuka, Shota; Yamada, Yuki; Tateyama, Yoshitaka; Yamada, Atsuo
Journal of Physical Chemistry Letters (2019), 10 (20), 6301-6305CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Aq. electrolytes have great potential to improve the safety and prodn. costs of Li-ion batteries. Our recent materials exploration led to the discovery of the Li-salt dihydrate melt Li(TFSI)0.7(BETI)0.3·2H2O, which possesses an extremely wide potential window. To clarify the detailed liq. structure and electronic states of this unique aq. system, a first-principles mol. dynamics study has been conducted. We found that water mols. in the hydrate melt exist as isolated monomers or clusters consisting of only a few (≤5) H2O mols. Both the monomers and clusters have electronic structures largely deviating from that in bulk water, where the lowest unoccupied states are higher in energy than that of the Li-salt anions, which preferentially cause anion redn. leading to formation of an anion-derived stable solid-electrolyte interphase. This clearly shows the role of characteristic electronic structure inherent to the peculiar water environment for the extraordinary electrochem. stability of hydrate melts.
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Zhang, Y.; Lewis, N. H. C.; Mars, J.; Wan, G.; Weadock, N. J.; Takacs, C. J.; Lukatskaya, M. R.; Steinrück, H. G.; Toney, M. F.; Tokmakoff, A. Water-in-salt LiTFSI aqueous electrolytes. 1. Liquids structure from combined molecular dynamics simulation and experimental studies. J. Phys. Chem. B 2021, 125, 4501– 4513, DOI: 10.1021/acs.jpcb.1c02189
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Water-in-Salt LiTFSI Aqueous Electrolytes. 1. Liquid Structure from Combined Molecular Dynamics Simulation and Experimental Studies
Zhang, Yong; Lewis, Nicholas H. C.; Mars, Julian; Wan, Gang; Weadock, Nicholas J.; Takacs, Christopher J.; Lukatskaya, Maria R.; Steinruck, Hans-Georg; Toney, Michael F.; Tokmakoff, Andrei; Maginn, Edward J.
Journal of Physical Chemistry B (2021), 125 (17), 4501-4513CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy d. in aq. Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform IR spectra measurements over a wide range of temps. and salt concns. are reported for the LiTFSI (lithium bis(trifluoromethane sulfonyl)imide)-based water-in-salt electrolyte. Classical mol. dynamics simulations are validated against the expts. and used to gain addnl. information about the electrolyte structure. Based on our analyses, a new model for the liq. structure is proposed. Specifically, we demonstrate that at the highest LiTFSI concn. of 20 m the water network is disrupted, and the majority of water mols. exist in the form of isolated monomers, clusters, or small aggregates with chain-like configurations. On the other hand, TFSI- anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies of this system.
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Tot, A.; Kloo, L. Water-in-salt electrolytes – molecular insights to the high solubility of lithium-ion salts. Chem. Commun. 2022, 58, 9528, DOI: 10.1039/D2CC03062D
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Water-in-salt electrolytes molecular insights to high solubility of lithium-ion salts
Tot, Aleksandar; Kloo, Lars
Chemical Communications (Cambridge, United Kingdom) (2022), 58 (68), 9528-9531CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
The recently established water-in-salt electrolyte (WISE) concept indicates the possible application of aq. electrolytes in lithium-ion batteries (LiBs). The application of this type of highly concd. electrolyte relies on a proper understanding of their thermodynamically stable solns. Therefore, fundamental insights regarding the Li[TFSI] soly. in water are important for the rational design of reproducible and stable WISE.
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Ko, S.; Yamada, Y.; Yamada, A. Formation of a solid electrolyte interphase in hydrate-melt electrolytes. ACS Appl. Mater. Interfaces. 2019, 11, 45554– 45560, DOI: 10.1021/acsami.9b13662
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Formation of a Solid Electrolyte Interphase in Hydrate-Melt Electrolytes
Ko, Seongjae; Yamada, Yuki; Yamada, Atsuo
ACS Applied Materials & Interfaces (2019), 11 (49), 45554-45560CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
Aq. Li-ion batteries using non-flammable aq. electrolytes have been continuously studied to achieve the ultimate safety of the battery system. However, they have a major drawback of low operation voltage resulting from the narrow potential window of aq. electrolytes. Recently, a room-temp. hydrate melt of Li salts was discovered as a new class of stable aq. electrolyte with widened potential window (> 3 V) that enables the reversible operation of high-voltage (3 V-class) aq. Li-ion batteries. An important factor contributing to the wide potential window is the formation of solid electrolyte interphase (SEI) on neg. electrodes, but its detailed mechanism has not been fully understood yet. Here, the SEI formation is studied in the hydrate-melt electrolyte in relation with the compn. and morphol. of the electrodes investigated via XPS and SEM. It is demonstrated that the formation of a stable SEI depends on the type of the electrodes used, as well as the electrolyte salt concns.
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Cheng, H.; Sun, Q.; Li, L.; Zou, Y.; Wang, Y.; Cai, T.; Zhao, F.; Liu, G.; Ma, Z.; Wahyudi, W.; Li, Q.; Ming, J. Emerging era of electrolyte solvation structure and interfacial model in batteries. ACS Energy Lett. 2022, 7, 490– 513, DOI: 10.1021/acsenergylett.1c02425
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Emerging Era of Electrolyte Solvation Structure and Interfacial Model in Batteries
Cheng, Haoran; Sun, Qujiang; Li, Leilei; Zou, Yeguo; Wang, Yuqi; Cai, Tao; Zhao, Fei; Liu, Gang; Ma, Zheng; Wahyudi, Wandi; Li, Qian; Ming, Jun
ACS Energy Letters (2022), 7 (1), 490-513CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
A review. The solid-electrolyte interphase (SEI) layer that formed on the electrode surface is believed to be pivotal for stabilizing the electrode performance in lithium-ion batteries (LIBs) over the past two decades. However, more and more researchers currently realized that the metal ion solvation structure (e.g., Li+) in electrolytes and the derived interfacial model (i.e., desolvation process) can affect the electrode performance significantly. Thus, herein we summary the recent researches on how to discover the importance of electrolyte solvation structure, develop a quant. model to describe solvation structure, construct an interfacial model to understand electrode performance, and apply these theories to the design of electrolytes. We provide a timely review on the scientific relationship between the mol. interactions of metal ions, anions, and solvents in the interfacial model and electrode performances, of which the viewpoint differs from the SEI interpretations before. These discoveries may herald a new era as the post-SEI due to the significance for guiding the design of LIBs and their performance improvement, as well as developing other metal ion batteries and beyond.
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Li, L.; Cheng, H.; Zhang, J.; Guo, Y.; Sun, C.; Zhou, M.; Li, Q.; Ma, Z.; Ming, J. Quantitative chemistry in electrolyte solvation design for aqueous batteries. ACS Energy Lett. 2023, 8, 1076– 1095, DOI: 10.1021/acsenergylett.2c02585
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Quantitative chemistry in electrolyte solvation design for aqueous batteries
Li, Leilei; Cheng, Haoran; Zhang, Junli; Guo, Yingjun; Sun, Chunsheng; Zhou, Min; Li, Qian; Ma, Zheng; Ming, Jun
ACS Energy Letters (2023), 8 (2), 1076-1095CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
A review. Aq. electrolyte design is pivotal for boosting the energy d. and lifespan of aq. batteries, because it can expand the electrochem. stability window and also mitigate the parasitic side reactions. Until now, three main kinds of electrolytes, i.e., water-in-salt, eutectic, and additives-modified electrolytes, have been developed by which the activity of H2O can be lowered and/or the formed specific solid-electrolyte interphase (SEI) can mitigate the decompn. of H2O. However, there is still a lack of a universal model to elucidate the reason for the improved performance, esp. as the SEI interpretation becomes ever more controversial. Herein, we present a quant. and graphical model of the electrolyte solvation structure and metal-ion (de)solvation process (i.e., interfacial model) to summarize a relationship between the electrolyte-electrode interfacial chem. and electrode performance. This Focus Review extends the solvation structure and interfacial model into the field of aq. electrolytes, revealing the essential influence of the solvation structure's properties on electrolyte stability and electrode performance, by which electrode performance and electrolyte design can be more quant. and accurately understood.
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Ming, J.; Cao, Z.; Wahyudi, W.; Li, M.; Kumar, P.; Wu, Y.; Hwang, J. Y.; Nejib Hedhili, M.; Cavallo, L.; Sun, Y. K.; Li, L. J. New insights on graphite anode stability in rechargeable: Li ion coordination structures prevail over solid electrolyte interphases. ACS Energy Lett. 2018, 3, 335– 340, DOI: 10.1021/acsenergylett.7b01177
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New Insights on Graphite Anode Stability in Rechargeable Batteries: Li Ion Coordination Structures Prevail over Solid Electrolyte Interphases
Ming, Jun; Cao, Zhen; Wahyudi, Wandi; Li, Mengliu; Kumar, Pushpendra; Wu, Yingqiang; Hwang, Jang-Yeon; Hedhili, Mohamed Nejib; Cavallo, Luigi; Sun, Yang-Kook; Li, Lain-Jong
ACS Energy Letters (2018), 3 (2), 335-340CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
Graphite anodes are not stable in most noncarbonate solvents (e.g., ether, sulfoxide, sulfone) upon Li ion intercalation, known as an urgent issue in present Li ions and next-generation Li-S and Li-O2 batteries for storage of Li ions within the anode for safety features. The solid electrolyte interphase (SEI) is commonly believed to be decisive for stabilizing the graphite anode. However, the solvation structure of the Li ions, detd. by the electrolyte compn. including lithium salts, solvents, and additives, plays a more dominant role than SEI in graphite anode stability. The Li ion intercalation desired for battery operation competes with the undesired Li+-solvent co-insertion, leading to graphite exfoliation. The increase in org. lithium salt LiN(SO2CF3)2 concn. or, more effectively, the addn. of LiNO3 lowers the interaction strength between Li+ and solvents, suppressing the graphite exfoliation caused by Li+-solvent co-insertion. The findings refresh the knowledge of the known SEI for graphite stability in metal ion batteries and also provide new guidelines for electrolyte systems to achieve reliable and safe Li-S full batteries.
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Zheng, J.; Tan, G.; Shan, P.; Liu, T.; Hu, J.; Feng, Y.; Yang, L.; Zhang, M.; Chen, Z.; Lin, Y. Understanding thermodynamic and kinetic contribution in expanding the stability window of aqueous electrolytes. Chem. 2018, 4, 2872– 2882, DOI: 10.1016/j.chempr.2018.09.004
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Understanding Thermodynamic and Kinetic Contributions in Expanding the Stability Window of Aqueous Electrolytes
Zheng, Jiaxin; Tan, Guoyu; Shan, Peng; Liu, Tongchao; Hu, Jiangtao; Feng, Yancong; Yang, Luyi; Zhang, Mingjian; Chen, Zonghai; Lin, Yuan; Lu, Jun; Neuefeind, Joerg C.; Ren, Yang; Amine, Khalil; Wang, Lin-Wang; Xu, Kang; Pan, Feng
Chem (2018), 4 (12), 2872-2882CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
Aq. electrolytes come with an intrinsic narrow electrochem. stability window (1.23 V). Expanding this window represents significant benefits in both fundamental science and practical battery applications. Recent breakthroughs made via super-concn. have resulted in >3.0 V windows, but fundamental understanding of the related mechanism is still absent. In the present work, we examd. the widened window (2.55 V) of a super-concd. (unsatd.) aq. soln. of LiNO3 through both theor. and spectral analyses and discovered that a local structure of intimate Li+-water interaction arises at super-concn., generating (Li+(H2O)2)n polymer-like chains to replace the ubiquitous hydrogen bonding between water mols. Such structure is mainly responsible for the expanded electrochem. stability window. Further theor. and exptl. analyses quant. differentiate the contributions to this window, identifying the kinetic factor (desolvation) as the main contributor. Such mol.-level and quant. understanding will further assist in tailor designing more effective approaches to stabilizing water electrochem.
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Tang, L.; Xu, Y.; Zhang, W.; Sui, Y.; Scida, A.; Tachibana, S. R.; Garaga, M.; Sandstrom, S. K.; Chiu, N.-C.; Stylianou, K. C. Strengthening aqueous electrolytes without strengthening water. Angew. Chem. Ind. Ed. 2023, 62, e202307212 DOI: 10.1002/anie.202307212
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Strengthening Aqueous Electrolytes without Strengthening Water
Tang, Longteng; Xu, Yunkai; Zhang, Weiyi; Sui, Yiming; Scida, Alexis; Tachibana, Sean R.; Garaga, Mounesha; Sandstrom, Sean K.; Chiu, Nan-Chieh; Stylianou, Kyriakos C.; Greenbaum, Steve G.; Greaney, Peter Alex; Fang, Chong; Ji, Xiulei
Angewandte Chemie, International Edition (2023), 62 (35), e202307212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Aq. electrolytes typically suffer from poor electrochem. stability; however, eutectic aq. solns.-25 wt.% LiCl and 62 wt.% H3PO4-cooled to -78°C exhibit a significantly widened stability window. Integrated exptl. and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl-, ice-like water clusters form, and H···Cl- bonding strengthens. Surprisingly, this low-temp. solvation structure does not strengthen water mols.' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temp. inertness in the electrolyte: less favorable solvation of OH- and H+, the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aq. Li-ion battery using LiMn2O4 cathode and CuSe anode with a high energy d. of 109 Wh/kg. These results highlight the potential of aq. batteries for polar and extraterrestrial missions.
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Yamada, Y.; Usui, K.; Sodeyama, K.; Ko, S.; Tateyama, Y.; Yamada, A. Hydrate-melt electrolytes for high-energy-density aqueous batteries. Nat. Energy. 2016, 1, 16129, DOI: 10.1038/nenergy.2016.129
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Hydrate-melt electrolytes for high-energy-density aqueous batteries
Yamada, Yuki; Usui, Kenji; Sodeyama, Keitaro; Ko, Seongjae; Tateyama, Yoshitaka; Yamada, Atsuo
Nature Energy (2016), 1 (10), 16129CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
Aq. Li-ion batteries are attracting increasing attention because they are potentially low in cost, safe and environmentally friendly. However, their low energy d. (<100 Wh kg-1 based on total electrode wt.), which results from the narrow operating potential window of water and the limited selection of suitable neg. electrodes, is problematic for their future widespread application. Here, we explore optimized eutectic systems of several org. Li salts and show that a room-temp. hydrate melt of Li salts can be used as a stable aq. electrolyte in which all water mols. participate in Li+ hydration shells while retaining fluidity. This hydrate-melt electrolyte enables a reversible reaction at a com. Li4Ti5O12 neg. electrode with a low reaction potential (1.55 V vs. Li+/Li) and a high capacity (175 mAh g-1). The resultant aq. Li-ion batteries with high energy d. (>130 Wh kg-1) and high voltage (∼2.3-3.1 V) represent significant progress towards performance comparable to that of com. non-aq. batteries (with energy densities of ∼150-400 Wh kg-1 and voltages of ∼2.4-3.8 V).
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Ding, M. S.; Xu, K. Phase diagram, conductivity, and glass transition of LiTFSI-H₂O binary electrolytes. J. Phys. Chem. C 2018, 122, 16624– 16629, DOI: 10.1021/acs.jpcc.8b05193
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Phase Diagram, Conductivity, and Glass Transition of LiTFSI-H2O Binary Electrolytes
Ding, Michael S.; Xu, Kang
Journal of Physical Chemistry C (2018), 122 (29), 16624-16629CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The aq. electrolyte system of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI-H2O, was systematically and accurately measured for a complete liq.-solid phase diagram and an extensive set of data on electrolytic cond. and glass-transition temp. The cond. data set was fitted with a VFT-based function (VFT: Vogel-Fulcher-Tammann), of which the three parameters were set to Laurent polynomial functions of compn. The fitting results were correlated with other quantities, and comparisons were made between the results of this study and those of other aq. and carbonate electrolyte systems. The results of these measurements, correlations, and comparisons strongly suggest a decoupling of cationic conduction from the movement of the bulk soln., in sharp contrast to what was obsd. in any nonaq. electrolytes so far.
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Welton, T. Ionic liquids: a brief history. Biophys. Rev. 2018, 10, 691– 706, DOI: 10.1007/s12551-018-0419-2
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Ionic liquids: a brief history
Welton, Tom
Biophysical Reviews (2018), 10 (3), 691-706CODEN: BRIECG; ISSN:1867-2450. (Springer)
A review. There is no doubt that ionic liqs. have become a major subject of study for modern chem. We have become used to ever more publications in the field each year, although there is some evidence that this is beginning to plateau at approx. 3500 papers each year. They have been the subject of several major reviews and books, dealing with different applications and aspects of their behaviors. In this article, I will show a little of how interest in ionic liqs. grew and developed.
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Zhao, H. Are ionic liquids kosmotropic or chaotropic? An evaluation of available thermodynamic parameters for quantifying the ion kosmotropicity of ionic liquids. J. Chem. Technol. Biotechnol. 2006, 81, 877– 891, DOI: 10.1002/jctb.1449
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Are ionic liquids kosmotropic or chaotropic? An evaluation of available thermodynamic parameters for quantifying the ion kosmotropicity of ionic liquids
Zhao, Hua
Journal of Chemical Technology and Biotechnology (2006), 81 (6), 877-891CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
A review. The hydration of ionic liqs. (a new type of org. salt) is not well understood. One property for characterizing the effect of ion hydration is kosmotropicity. To quantify the ion kosmotropicity, this review evaluated several thermodn. parameters including B-coeffs., structural entropies, structural vols., and ion mobility. The availability and reliability of exptl. data and various models for these parameters were examd. for ionic liqs. The relationship and comparison of these parameters are also discussed. Smaller pyridinium and imidazolium cations in ionic liqs. are more likely to be chaotropes or borderline ions owing to their hydrophobic hydration. The kosmotropicity of anions varies.
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Marcus, Y. Effect of ions on the structure of water: Structure making and breaking. Chem. Rev. 2009, 109, 1346– 1370, DOI: 10.1021/cr8003828
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Effect of Ions on the Structure of Water: Structure Making and Breaking
Marcus, Yizhak
Chemical Reviews (Washington, DC, United States) (2009), 109 (3), 1346-1370CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review.
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Marcus, Y. Ions in water and biophysical implications: From chaos to cosmos; Springer, 2012.
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Reber, D.; Grissa, R.; Becker, M.; Kühnel, R. S.; Battaglia, C. Anion selection criteria for water-in-salt electrolytes. Adv. Energy Mater. 2021, 11, 2002913 DOI: 10.1002/aenm.202002913
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Anion Selection Criteria for Water-in-Salt Electrolytes
Reber, David; Grissa, Rabeb; Becker, Maximilian; Kuehnel, Ruben-Simon; Battaglia, Corsin
Advanced Energy Materials (2021), 11 (5), 2002913CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
Water-in-salt electrolytes have enabled the development of novel high-voltage aq. lithium-ion batteries. This study explores the reasons why analogus sodium electrolytes have struggled to reach the same level of electrochem. stability. Soln. structure and electrochem. stability are compared for 11 sodium salts, selected among the major classes of salts proposed for highly concd. electrolytes. The water environment established for each anion is related to its position in the Hofmeister series and a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochem. stability of the electrolyte is found. The search for suitable sodium salts is complicated by the fact that higher salt concns. are needed than for their lithium equiv. Reaching such a high concn. of >25 mol kg-1 with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. Hence, alternative approaches such as multisolvent systems should be explored. The water soly. of NaTFSI can be increased from 8 to 30 mol kg-1 in the presence of ionic liqs. Such a ternary electrolyte enables stable cycling of a 2 V class sodium-ion battery based on the NaTi2(PO4)3/Na2Mn[Fe(CN)6] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.
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Chen, M.; Wu, J.; Ye, T.; Ye, J.; Zhao, C.; Bi, S.; Yan, J.; Mao, B.; Feng, G. Adding salt to expand voltage window of humid ionic liquids. Nat. Commun. 2020, 11, 5809, DOI: 10.1038/s41467-020-19469-3
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Adding salt to expand voltage window of humid ionic liquids
Chen, Ming; Wu, Jiedu; Ye, Ting; Ye, Jinyu; Zhao, Chang; Bi, Sheng; Yan, Jiawei; Mao, Bingwei; Feng, Guang
Nature Communications (2020), 11 (1), 5809CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
Humid hydrophobic ionic liqs.-widely used as electrolytes-have narrowed electrochem. windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed mol. dynamics simulations to explore effects of adding Li salt in humid ionic liqs. on the water adsorbed on the electrode surface. Results reveal that most of the water mols. are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the redn. of electrochem. windows of humid hydrophobic ionic liqs. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liqs. exhibit enlarged electrochem. windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liqs. from electrochem. performance degrdn.
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Yang, C.; Xia, J.; Cui, C.; Pollard, T. P.; Vataman, J.; Faraone, A.; Dura, J. A.; Tyagi, M.; Kattan, A.; Thimsen, E.; Xu, J.; Song, W.; Hu, E.; Ji, X.; Hou, S.; Zhang, X.; Ding, M. S.; Hwang, M. S.; Su, D.; Ren, Y.; Yang, X.-Q.; Wang, H.; Borodin, O.; Wang, C. All-temperature zinc batteries with high-entropy aqueous electrolyte. Nat. Sustainability 2023, 6, 325– 335, DOI: 10.1038/s41893-022-01028-x
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Ji, H.; Xie, C.; Wu, T.; Wang, H.; Cai, Z.; Zhang, Q.; Li, W.; Fu, L.; Li, H.; Wang, H. High-entropy solvent design enabling a universal electrolyte with a low freezing point for low-temperature aqueous batteries. Chem. Commun. 2023, 59, 8715– 8718, DOI: 10.1039/D3CC02214E
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High-entropy solvent design enabling a universal electrolyte with a low freezing point for low-temperature aqueous batteries
Ji, Huimin; Xie, Chunlin; Wu, Tingqing; Wang, Hao; Cai, Zhiwen; Zhang, Qi; Li, Wenbin; Fu, Liang; Li, Huanhuan; Wang, Haiyan
Chemical Communications (Cambridge, United Kingdom) (2023), 59 (56), 8715-8718CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Amide additives acting as hydrogen-bonding ligands effectively break the crosslinking structures between water mols. and increase the entropy of mixed solvents, thus enabling a mixed solvent with an ultralow f.p. of -98°C. Zinc-ion batteries using this hybrid solvent exhibit good cycling stability over a wide temp. range from -60°C to 50°C.
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Zafar, Z. A.; Abbas, G.; Knizek, K.; Silhavik, M.; Kumar, P.; Jiricek, P.; Houdková, J.; Frank, O.; Cervenka, J. Chaotropic anion based “water-in-salt” electrolyte realizes a high voltage Zn-graphite dual-ion battery. J. Mater. Chem. A 2022, 10, 2064– 2074, DOI: 10.1039/D1TA10122F
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Chaotropic anion based "water-in-salt" electrolyte realizes a high voltage Zn-graphite dual-ion battery
Zafar, Zahid Ali; Abbas, Ghulam; Knizek, Karel; Silhavik, Martin; Kumar, Prabhat; Jiricek, Petr; Houdkova, Jana; Frank, Otakar; Cervenka, Jiri
Journal of Materials Chemistry A: Materials for Energy and Sustainability (2022), 10 (4), 2064-2074CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
Aq. Zn-based batteries are promising candidates for grid energy storage due to their low cost, intrinsic safety, and environmental friendliness. Nevertheless, they suffer from limited energy d. due to the utilization of low-voltage cathodes and electrolytes. Graphite could be a viable high-voltage cathode material owing to its high redox potential (2.1-3.1 V vs. Zn/Zn2+). However, finding a suitable aq. electrolyte with high anodic stability remains a fundamental challenge. This work realizes a high-voltage and low-cost aq. Zn-graphite dual-ion battery based on a Zn(ClO4)2 water-in-salt electrolyte with a wide electrochem. window of 2.80 V. The implementation of the supersatd. Zn(ClO4)2 water-in-salt electrolyte contg. strong chaotropic ClO4- anions expands the oxidative stability of the aq. electrolyte beyond 1.65 V vs. Ag/AgCl or 2.60 V vs. Zn/Zn2+, and facilitates reversible plating/stripping of Zn2+ with a low overpotential of <50 mV at 1 mA cm-2 and a high upper cut-off potential of 2.5 V vs. Zn/Zn2+. Consequently, the Zn-graphite dual-ion battery delivers a max. discharge capacity of 45 mA h g-1 at 100 mA g-1 with a mean discharge voltage of ∼1.95 V and cycle life of over 500 cycles.
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Brown, J.; Forero-Saboya, J.; Baptise, B.; Karlsmo, M.; Rousse, G.; Grimaud, A. A guanidinium salt as a chaotropic agent for aqueous battery electrolytes. Chem. Commun. 2023, 59, 12266– 12269, DOI: 10.1039/d3cc03769j
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A guanidium salt as a chaotropic agent for aqueous battery electrolytes
Brown, John; Forero-Saboya, Juan; Baptiste, Benoit; Karlsmo, Martin; Rousse, Gwenaelle; Grimaud, Alexis
Chemical Communications (Cambridge, United Kingdom) (2023), 59 (82), 12266-12269CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
This study investigates a salt design principle for aq. battery electrolytes by combining chaotropic ions, guanidium cations (Gdm) and bis(trifluoromethanesulfonyl)imide anions (TFSI), forming GdmTFSI. This salt's crystal structure was solved via single-crystal X-ray diffraction and characterized using Fourier-transform IR spectroscopy. Study reveals that GdmTFSI salt disrupts the hydrogen bonding network of aq. solns., impacting water reactivity at electrochem. interfaces.
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Becker, M.; Rentsch, D.; Reber, D.; Aribia, A.; Battaglia, C.; Kühnel, R. S. The hydrotropic effect of ionic liquids in water-in-salt electrolytes. Angew. Chem., Int. Ed. 2021, 60, 14100– 14108, DOI: 10.1002/anie.202103375
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The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes
Becker, Maximilian; Rentsch, Daniel; Reber, David; Aribia, Abdessalem; Battaglia, Corsin; Kuehnel, Ruben-Simon
Angewandte Chemie, International Edition (2021), 60 (25), 14100-14108CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Water-in-salt electrolytes have successfully expanded the electrochem. stability window of aq. electrolytes beyond 2 V. Further improvements in stability can be achieved by partially substituting water with either classical org. solvents or ionic liqs. Here, we study ternary electrolytes composed of LiTFSI, water, and imidazolium ionic liqs. We find that the LiTFSI soly. strongly increases from 21 mol kg-1 in water to up to 60 mol kg-1 in the presence of ionic liq. The soln. structure is investigated with Raman and NMR spectroscopy and the enhanced LiTFSI soly. is found to originate from a hydrotropic effect of the ionic liqs. The increased reductive stability of the ternary electrolytes enables stable cycling of an aq. lithium-ion battery with an energy d. of 150 Wh kg-1 on the active material level based on com. relevant Li4Ti5O12 and LiNi0.8Mn0.1Co0.1O2 electrode materials.
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Reber, D.; Borodin, O.; Becker, M.; Rentsch, D.; Thienenkamp, J. H.; Grissa, R.; Zhao, W.; Aribia, A.; Brunklaus, G.; Battaglia, C. Water/ionic liquids/succinonitrile hybrid electrolytes for aqueous batteries. Adv. Funct. Mater. 2022, 32, 2112138 DOI: 10.1002/adfm.202112138
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Water/Ionic Liquid/Succinonitrile Hybrid Electrolytes for Aqueous Batteries
Reber, David; Borodin, Oleg; Becker, Maximilian; Rentsch, Daniel; Thienenkamp, Johannes Helmut; Grissa, Rabeb; Zhao, Wengao; Aribia, Abdessalem; Brunklaus, Gunther; Battaglia, Corsin; Kuehnel, Ruben-Simon
Advanced Functional Materials (2022), 32 (20), 2112138CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
The water-in-salt concept has significantly improved the electrochem. stability of aq. electrolytes, and the hybridization with org. solvents or ionic liqs. has further enhanced their reductive stability, enabling cell chemistries with up to 150 Wh kg-1 of active material. Here, a large design space is opened by introducing succinonitrile as a cosolvent in water/ionic liq./succinonitrile hybrid electrolytes (WISHEs). By means of succinonitrile addn., the soly. limits can be fully circumvented, and the properties of the electrolytes can be optimized for various metrics such as highest electrochem. stability, max. cond., or lowest cost. While excessive nitrile fractions render the mixts. flammable, careful selection of component ratios yields highly performant, nonflammable electrolytes that enable stable cycling of Li4Ti5O12-LiNi0.8Mn0.1Co0.1O2 full cells over a wide temp. range with strong rate performance, facilitated by the fast conformational dynamics of succinonitrile. The WISHEs allow stable cycling with a max. energy d. of ≈140 Wh kg-1 of active material, Coulombic efficiencies of close to 99.5% at 1C, and a capacity retention of 53% at 10C relative to 1C.
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Tot, A.; Zhang, L.; Berg, E. J.; Svensson, P. H.; Kloo, L. Water-in-salt electrolyte made saltier by Gemini ionic liquids for highly efficient Li-ion batteries. Sci. Rep. 2023, 13, 2154, DOI: 10.1038/s41598-023-29387-1
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Water-in-salt electrolytes made saltier by Gemini ionic liquids for highly efficient Li-ion batteries
Tot, Aleksandar; Zhang, Leiting; Berg, Erik J.; Svensson, Per H.; Kloo, Lars
Scientific Reports (2023), 13 (1), 2154CODEN: SRCEC3; ISSN:2045-2322. (Nature Portfolio)
The water-in-salt electrolytes have promoted aq. Li-ion batteries to become one of the most promising candidates to overcome safety concerns/issues of traditional Li-ion batteries. A simple increase of Li-salt concn. in electrolytes can successfully expand the electrochem. stability window of aq. electrolytes beyond 2 V. However, necessary stability improvements require an increase in complexity of the ternary electrolytes. Here, we have explored the effects of novel, Gemini-type ionic liqs. (GILs) as a co-solvent systems in aq. Li[TFSI] mixts. and investigated the transport properties of the resulting electrolytes, as well as their electrochem. performance. The devices contg. pyrrolidinium-based GILs show superior cycling stability and promising specific capacity in the cells based on the commonly used electrode materials LTO (Li4Ti5O12) and LMO (LiMn2O4).
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Borodin, O. Polarizable force field development and molecular dynamics simulations of ionic liquids. J. Phys. Chem. B 2009, 113, 11463– 11478, DOI: 10.1021/jp905220k
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Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids
Borodin, Oleg
Journal of Physical Chemistry B (2009), 113 (33), 11463-11478CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
A many-body polarizable force field was developed and validated for ionic liqs. (ILs) contg. 1-methyl-3-alkylimidazolium, 1-alkyl-2-methyl-3-alkylimidazolium, N-methyl-N-alkylpyrrolidinium, N-alkylpyridinium, N-alkyl-N-alkylpiperidinium, N-alkyl-N-alkylmorpholinium, tetraalkylammonium, tetraalkylphosphonium, N-methyl-N-oligoetherpyrrolidinium cations and BF4-, CF3BF3-, CH3BF3-, CF3SO3-, PF6-, dicyanamide, tricyanomethanide, tetracyanoborate, bis(trifluoromethane sulfonyl)imide (Ntf2- or TFSI-), bis(fluorosulfonyl)imide (FSI-) and nitrate anions. Classical mol. dynamics (MD) simulations were performed on 30 ionic liqs. at 298, 333, and 393 K. The IL d., heat of vaporization, ion self-diffusion coeff., cond., and viscosity were found in a good agreement with available exptl. data. The ability of the developed force field to predict ionic crystal cell parameters was tested on four ionic crystals contg. Ntf2- anions. The influence of polarization on the structure and ion transport was investigated for [emim][BF4] IL. A connection between the structural changes in IL resulting from turning off polarization and slowing down of ion dynamics was found. Developed force field provided accurate description/prediction of thermodn. and transport properties of alkanes, fluoroalkanes, oligoethers (1,2-dimethoxyethane), ethylene carbonate, propylene carbonate, di-Me carbonate, hydrazine, methyhydrazine, dimethylhydrazine, acetonitrile, di-Me amine, and di-Me ketone.
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Suo, L.; Oh, D.; Lin, Y.; Zhuo, Z.; Borodin, O.; Gao, T.; Wang, F.; Kushima, A.; Wang, Z.; Kim, H. C. How solid-electrolyte interphase forms in aqueous electrolytes. J. Am. Chem. Soc. 2017, 139, 18670– 18680, DOI: 10.1021/jacs.7b10688
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How Solid-Electrolyte Interphase Forms in Aqueous Electrolytes
Suo, Liumin; Oh, Dahyun; Lin, Yuxiao; Zhuo, Zengqing; Borodin, Oleg; Gao, Tao; Wang, Fei; Kushima, Akihiro; Wang, Ziqiang; Kim, Ho-Cheol; Qi, Yue; Yang, Wanli; Pan, Feng; Li, Ju; Xu, Kang; Wang, Chunsheng
Journal of the American Chemical Society (2017), 139 (51), 18670-18680CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Solid-electrolyte interphase (SEI) is the key component that enables all advanced electrochem. devices, the best representative of which is Li-ion battery (LIB). It kinetically stabilizes electrolytes at potentials far beyond their thermodn. stability limits, so that cell reactions could proceed reversibly. Its ad hoc chem. and formation mechanism has been a topic under intensive investigation since the first commercialization of LIB 25 years ago. Traditionally SEI can only be formed in nonaq. electrolytes. However, recent efforts successfully transplanted this concept into aq. media, leading to significant expansion in the electrochem. stability window of aq. electrolytes from 1.23 V to beyond 4.0 V. This not only made it possible to construct a series of high voltage/energy d. aq. LIBs with unprecedented safety, but also brought high flexibility and even "open configurations" that have been hitherto unavailable for any LIB chemistries. While this new class of aq. electrolytes has been successfully demonstrated to support diversified battery chemistries, the chem. and formation mechanism of the key component, an aq. SEI, has remained virtually unknown. In this work, combining various spectroscopic, electrochem. and computational techniques, we rigorously examd. this new interphase, and comprehensively characterized its chem. compn., microstructure and stability in battery environment. A dynamic picture obtained reveals how a dense and protective interphase forms on anode surface under competitive decompns. of salt anion, dissolved ambient gases and water mol. By establishing basic laws governing the successful formation of an aq. SEI, the in-depth understanding presented in this work will assist the efforts in tailor-designing better interphases that enable more energetic chemistries operating farther away from equil. in aq. media.
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Dubouis, N.; Lemaire, P.; Mirvaux, B.; Salager, E.; Deschamps, M.; Grimaud, A. The role of the hydrogen evolution reaction in the solid-electrolyte interphase formation mechanism for “Water-in-Salt” electrolytes. Energy Environ. Sci. 2018, 11, 3491– 3499, DOI: 10.1039/C8EE02456A
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The role of the hydrogen evolution reaction in the solid-electrolyte interphase formation mechanism for "Water-in-Salt" electrolytes
Dubouis, Nicolas; Lemaire, Pierre; Mirvaux, Boris; Salager, Elodie; Deschamps, Michael; Grimaud, Alexis
Energy & Environmental Science (2018), 11 (12), 3491-3499CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
Aq. Li-ion batteries have long been envisioned as safe and green energy storage technol., but have never been com. realized owing to the limited electrochem. stability window of water, which drastically hampers their energy d. Recently, Water-in-Salt electrolytes (WiSEs) in which a large amt. of org. salt is dissolved into water were proposed to allow for assembling 3 V Li-ion batteries. Hereby, our attention focused on the fate of water at the electrochem. interface under neg. polarization and the potential reactivity of TFSI anions with products originating from the water redn. Hence, combining anal. of bulk electrolytes with electrochem. measurements on model electrodes and operando characterization, we were able to demonstrate that hydroxides generated during the hydrogen evolution reaction can chem. react with TFSI and catalyze the formation of a fluorinated solid-electrolyte interphase (SEI) that prevents further water redn. Mastering this new SEI formation path with the chem. degrdn. of TFSI anions mediated by the electrochem. redn. of water can therefore open new avenues for the future development of not only WiSEs but also Li batteries functioning in org. electrolytes.
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Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate and Cyclic Ether Electrolytes. II. Surface Chemistry
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Chem. components of surface films of deposited lithium on nickel substrates in electrolytes with LiN(SO2CF3)2 (LiTFSI), LiN (SO2C2F5)2 (LiBETI), LiPF6 solutes, and THF solvents were characterized by FTIR, two-dimensional NMR (2-dimensional NMR), XPS, evolved gas anal., and ion chromatograph to understand the electrochem. performance of lithium imide/cyclic ether-based electrolytes. The top layers of the surface film were ROCO2Li, Li2CO3, polymer constituents, and LiF. The inner layers of the surface film consisted of Li2O and carbide species. In imide/cyclic ether-based electrolytes, Li2S2O4 and Li2SO3 as outer layers, and Li2S as the inner layer were formed on a nickel substrate as reductive constituents of imide solute. Org. surface layers consisted of lithium ethoxides, lithium ethylene dicarbonate (CH2OCO2Li)2, polyethylene oxide, and lithium ethylene dicarbonate contg. an oxyethylene unit by 1H, 13C, and 2-dimensional NMR. Li cycling efficiency affects not only the deposited lithium morphol. but also chem. components.
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2023, 127, 50, 24065–24076

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https://doi.org/10.1021/acs.jpcc.3c07164

Published December 12, 2023

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Abstract
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Figure 1
Figure 1. Schematic representation of the GILs used in this work, along with their molecular water structure making (cosmotropic)/breaking (chaotropic) tendency. Cosmotropicity/chaotropicity of cations investigated are determined by a method based on viscosity as explained in the Supporting Material (Figures S7, S8, and Table S1).
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Figure 2
Figure 2. Temperature-dependent viscosities (a) and electric conductivities (b) for the investigated systems. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.
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Figure 3
Figure 3. Normalized Raman spectra for (a) breathing [TFSI]⁻ mode at ≈750 cm^–1 and (b) water stretch vibration modes at ≈3500 cm^–1. Black: Li[TFSI]¹:(C₂Pyr-C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³; orange: water (intensity of the water band was divided by a factor of 5 for clarity purpose).
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Figure 4
Figure 4. pRDFs (a–c) and CNs (d–f) of water–water, Li–O(water), and Li–O(TFSI) distances. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.
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Figure 5
Figure 5. Average coordination number of the most probable interactions in simulated hybrid WISE systems. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³. Error bars correspond to ± one standard deviation estimated using the block-average technique.
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Figure 6
Figure 6. Electrochemical stability window. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.
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Figure 7
Figure 7. Battery performance of LTO (mass loading 2.4 mg)/LMO (mass loading 3.0 mg) cells based on the GIL-containing electrolytes. (a) Specific discharge capacity, (b) Coulombic efficiency, (c) voltage profiles for the 10th cycle at 1C-rate, and (d) rate performance. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.
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Figure 8
Figure 8. FTIR (a) and XAS F K-edge (b) spectra of electrodes from charge-cycled batteries. Black: Li[TFSI]¹:(C₂Pyr–C₂OC₂–C₂Pyr)[TFSI]₂¹:H₂O³; red: Li[TFSI]¹:(C₄Pyr–C₂OC₂–C₄Pyr)[TFSI]₂¹:H₂O³; blue: Li[TFSI]¹:(C₂OC₂Pyr–C₂OC₂–C₂OC₂Pyr)[TFSI]₂¹:H₂O³; green: Li[TFSI]¹:(HOC₂OC₂Pyr–C₂OC₂–HOC₂OC₂Pyr)[TFSI]₂¹:H₂O³; cyan: Li[TFSI]¹:(HOC₃Pyr–C₂OC₂–HOC₃Pyr)[TFSI]₂¹:H₂O³; magenta: Li[TFSI]¹:(HOOCC₂Pyr–C₂OC₂–HOOCC₂Pyr)[TFSI]₂¹:H₂O³.
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  Nature Chemistry (2010), 2 (9), 760-765CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)
  Aq. lithium-ion batteries may solve the safety problem assocd. with lithium-ion batteries that use highly toxic and flammable org. solvents, and the poor cycling life assocd. with commercialized aq. rechargeable batteries such as lead-acid and nickel-metal hydride systems. However, all reported aq. lithium-ion battery systems have shown poor stability: the capacity retention is typically less than 50% after 100 cycles. Here, the stability of electrode materials in an aq. electrolyte was extensively analyzed. The anodes of aq. lithium-ion batteries in a discharged state can react with water and oxygen, resulting in capacity fading upon cycling. By eliminating oxygen, adjusting the pH values of the electrolyte and using carbon-coated electrode materials, LiTi2(PO4)3/Li2SO4/LiFePO4 aq. lithium-ion batteries exhibited excellent stability with capacity retention >90% after 1000 cycles when being fully charged/discharged in 10 min and 85% after 50 cycles even at a very low current rate of 8 h for a full charge/discharge offering an energy storage system with high safety, low cost, long cycling life and appropriate energy d.
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11. 11
  Pasta, M.; Wessels, C. D.; Huggins, R. A.; Cui, Y. A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage. Nat. Commun. 2012, 3, 1149, DOI: 10.1038/ncomms2139
  11
  A high-rate and long cycle life aqueous electrolyte battery for grid-scale energy storage
  Pasta Mauro; Wessells Colin D; Huggins Robert A; Cui Yi
  Nature communications (2012), 3 (), 1149 ISSN:.
  New types of energy storage are needed in conjunction with the deployment of solar, wind and other volatile renewable energy sources and their integration with the electric grid. No existing energy storage technology can economically provide the power, cycle life and energy efficiency needed to respond to the costly short-term transients that arise from renewables and other aspects of grid operation. Here we demonstrate a new type of safe, fast, inexpensive, long-life aqueous electrolyte battery, which relies on the insertion of potassium ions into a copper hexacyanoferrate cathode and a novel activated carbon/polypyrrole hybrid anode. The cathode reacts rapidly with very little hysteresis. The hybrid anode uses an electrochemically active additive to tune its potential. This high-rate, high-efficiency cell has a 95% round-trip energy efficiency when cycled at a 5C rate, and a 79% energy efficiency at 50C. It also has zero-capacity loss after 1,000 deep-discharge cycles.
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12. 12
  Suo, L.; Borodin, O.; Gao, T.; Ogluin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 2015, 350, 938– 943, DOI: 10.1126/science.aab1595
  12
  "Water-in-salt" electrolyte enables high-voltage aqueous lithium-ion chemistries
  Suo, Liumin; Borodin, Oleg; Gao, Tao; Olguin, Marco; Ho, Janet; Fan, Xiulin; Luo, Chao; Wang, Chunsheng; Xu, Kang
  Science (Washington, DC, United States) (2015), 350 (6263), 938-943CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  Lithium-ion batteries raise safety, environmental, and cost concerns, which mostly arise from their nonaq. electrolytes. The use of aq. alternatives is limited by their narrow electrochem. stability window (1.23 V), which sets an intrinsic limit on the practical voltage and energy output. We report a highly concd. aq. electrolyte whose window was expanded to ∼3.0 V with the formation of an electrode-electrolyte interphase. A full lithium-ion battery of 2.3 V using such an aq. electrolyte was demonstrated to cycle up to 1000 times, with nearly 100% coulombic efficiency at both low (0.15 C) and high (4.5 coulombs) discharge and charge rates.
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13. 13
  Miyazaki, K.; Takenaka, N.; Watanabe, E.; Iizuka, S.; Yamada, Y.; Tateyama, Y.; Yamada, A. First-Principles study on the peculiar water environment in a hydrate-melt electrolyte. J. Phys. Chem. Lett. 2019, 10, 6301– 6305, DOI: 10.1021/acs.jpclett.9b02207
  13
  First-Principles Study on the Peculiar Water Environment in a Hydrate-Melt Electrolyte
  Miyazaki, Kasumi; Takenaka, Norio; Watanabe, Eriko; Iizuka, Shota; Yamada, Yuki; Tateyama, Yoshitaka; Yamada, Atsuo
  Journal of Physical Chemistry Letters (2019), 10 (20), 6301-6305CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Aq. electrolytes have great potential to improve the safety and prodn. costs of Li-ion batteries. Our recent materials exploration led to the discovery of the Li-salt dihydrate melt Li(TFSI)0.7(BETI)0.3·2H2O, which possesses an extremely wide potential window. To clarify the detailed liq. structure and electronic states of this unique aq. system, a first-principles mol. dynamics study has been conducted. We found that water mols. in the hydrate melt exist as isolated monomers or clusters consisting of only a few (≤5) H2O mols. Both the monomers and clusters have electronic structures largely deviating from that in bulk water, where the lowest unoccupied states are higher in energy than that of the Li-salt anions, which preferentially cause anion redn. leading to formation of an anion-derived stable solid-electrolyte interphase. This clearly shows the role of characteristic electronic structure inherent to the peculiar water environment for the extraordinary electrochem. stability of hydrate melts.
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14. 14
  Zhang, Y.; Lewis, N. H. C.; Mars, J.; Wan, G.; Weadock, N. J.; Takacs, C. J.; Lukatskaya, M. R.; Steinrück, H. G.; Toney, M. F.; Tokmakoff, A. Water-in-salt LiTFSI aqueous electrolytes. 1. Liquids structure from combined molecular dynamics simulation and experimental studies. J. Phys. Chem. B 2021, 125, 4501– 4513, DOI: 10.1021/acs.jpcb.1c02189
  14
  Water-in-Salt LiTFSI Aqueous Electrolytes. 1. Liquid Structure from Combined Molecular Dynamics Simulation and Experimental Studies
  Zhang, Yong; Lewis, Nicholas H. C.; Mars, Julian; Wan, Gang; Weadock, Nicholas J.; Takacs, Christopher J.; Lukatskaya, Maria R.; Steinruck, Hans-Georg; Toney, Michael F.; Tokmakoff, Andrei; Maginn, Edward J.
  Journal of Physical Chemistry B (2021), 125 (17), 4501-4513CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
  The concept of water-in-salt electrolytes was introduced recently, and these systems have been successfully applied to yield extended operation voltage and hence significantly improved energy d. in aq. Li-ion batteries. In the present work, results of X-ray scattering and Fourier-transform IR spectra measurements over a wide range of temps. and salt concns. are reported for the LiTFSI (lithium bis(trifluoromethane sulfonyl)imide)-based water-in-salt electrolyte. Classical mol. dynamics simulations are validated against the expts. and used to gain addnl. information about the electrolyte structure. Based on our analyses, a new model for the liq. structure is proposed. Specifically, we demonstrate that at the highest LiTFSI concn. of 20 m the water network is disrupted, and the majority of water mols. exist in the form of isolated monomers, clusters, or small aggregates with chain-like configurations. On the other hand, TFSI- anions are connected to each other and form a network. This description is fundamentally different from those proposed in earlier studies of this system.
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  125
15. 15
  Tot, A.; Kloo, L. Water-in-salt electrolytes – molecular insights to the high solubility of lithium-ion salts. Chem. Commun. 2022, 58, 9528, DOI: 10.1039/D2CC03062D
  15
  Water-in-salt electrolytes molecular insights to high solubility of lithium-ion salts
  Tot, Aleksandar; Kloo, Lars
  Chemical Communications (Cambridge, United Kingdom) (2022), 58 (68), 9528-9531CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  The recently established water-in-salt electrolyte (WISE) concept indicates the possible application of aq. electrolytes in lithium-ion batteries (LiBs). The application of this type of highly concd. electrolyte relies on a proper understanding of their thermodynamically stable solns. Therefore, fundamental insights regarding the Li[TFSI] soly. in water are important for the rational design of reproducible and stable WISE.
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16. 16
  Ko, S.; Yamada, Y.; Yamada, A. Formation of a solid electrolyte interphase in hydrate-melt electrolytes. ACS Appl. Mater. Interfaces. 2019, 11, 45554– 45560, DOI: 10.1021/acsami.9b13662
  16
  Formation of a Solid Electrolyte Interphase in Hydrate-Melt Electrolytes
  Ko, Seongjae; Yamada, Yuki; Yamada, Atsuo
  ACS Applied Materials & Interfaces (2019), 11 (49), 45554-45560CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  Aq. Li-ion batteries using non-flammable aq. electrolytes have been continuously studied to achieve the ultimate safety of the battery system. However, they have a major drawback of low operation voltage resulting from the narrow potential window of aq. electrolytes. Recently, a room-temp. hydrate melt of Li salts was discovered as a new class of stable aq. electrolyte with widened potential window (> 3 V) that enables the reversible operation of high-voltage (3 V-class) aq. Li-ion batteries. An important factor contributing to the wide potential window is the formation of solid electrolyte interphase (SEI) on neg. electrodes, but its detailed mechanism has not been fully understood yet. Here, the SEI formation is studied in the hydrate-melt electrolyte in relation with the compn. and morphol. of the electrodes investigated via XPS and SEM. It is demonstrated that the formation of a stable SEI depends on the type of the electrodes used, as well as the electrolyte salt concns.
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17. 17
  Cheng, H.; Sun, Q.; Li, L.; Zou, Y.; Wang, Y.; Cai, T.; Zhao, F.; Liu, G.; Ma, Z.; Wahyudi, W.; Li, Q.; Ming, J. Emerging era of electrolyte solvation structure and interfacial model in batteries. ACS Energy Lett. 2022, 7, 490– 513, DOI: 10.1021/acsenergylett.1c02425
  17
  Emerging Era of Electrolyte Solvation Structure and Interfacial Model in Batteries
  Cheng, Haoran; Sun, Qujiang; Li, Leilei; Zou, Yeguo; Wang, Yuqi; Cai, Tao; Zhao, Fei; Liu, Gang; Ma, Zheng; Wahyudi, Wandi; Li, Qian; Ming, Jun
  ACS Energy Letters (2022), 7 (1), 490-513CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
  A review. The solid-electrolyte interphase (SEI) layer that formed on the electrode surface is believed to be pivotal for stabilizing the electrode performance in lithium-ion batteries (LIBs) over the past two decades. However, more and more researchers currently realized that the metal ion solvation structure (e.g., Li+) in electrolytes and the derived interfacial model (i.e., desolvation process) can affect the electrode performance significantly. Thus, herein we summary the recent researches on how to discover the importance of electrolyte solvation structure, develop a quant. model to describe solvation structure, construct an interfacial model to understand electrode performance, and apply these theories to the design of electrolytes. We provide a timely review on the scientific relationship between the mol. interactions of metal ions, anions, and solvents in the interfacial model and electrode performances, of which the viewpoint differs from the SEI interpretations before. These discoveries may herald a new era as the post-SEI due to the significance for guiding the design of LIBs and their performance improvement, as well as developing other metal ion batteries and beyond.
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18. 18
  Li, L.; Cheng, H.; Zhang, J.; Guo, Y.; Sun, C.; Zhou, M.; Li, Q.; Ma, Z.; Ming, J. Quantitative chemistry in electrolyte solvation design for aqueous batteries. ACS Energy Lett. 2023, 8, 1076– 1095, DOI: 10.1021/acsenergylett.2c02585
  18
  Quantitative chemistry in electrolyte solvation design for aqueous batteries
  Li, Leilei; Cheng, Haoran; Zhang, Junli; Guo, Yingjun; Sun, Chunsheng; Zhou, Min; Li, Qian; Ma, Zheng; Ming, Jun
  ACS Energy Letters (2023), 8 (2), 1076-1095CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
  A review. Aq. electrolyte design is pivotal for boosting the energy d. and lifespan of aq. batteries, because it can expand the electrochem. stability window and also mitigate the parasitic side reactions. Until now, three main kinds of electrolytes, i.e., water-in-salt, eutectic, and additives-modified electrolytes, have been developed by which the activity of H2O can be lowered and/or the formed specific solid-electrolyte interphase (SEI) can mitigate the decompn. of H2O. However, there is still a lack of a universal model to elucidate the reason for the improved performance, esp. as the SEI interpretation becomes ever more controversial. Herein, we present a quant. and graphical model of the electrolyte solvation structure and metal-ion (de)solvation process (i.e., interfacial model) to summarize a relationship between the electrolyte-electrode interfacial chem. and electrode performance. This Focus Review extends the solvation structure and interfacial model into the field of aq. electrolytes, revealing the essential influence of the solvation structure's properties on electrolyte stability and electrode performance, by which electrode performance and electrolyte design can be more quant. and accurately understood.
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19. 19
  Ming, J.; Cao, Z.; Wahyudi, W.; Li, M.; Kumar, P.; Wu, Y.; Hwang, J. Y.; Nejib Hedhili, M.; Cavallo, L.; Sun, Y. K.; Li, L. J. New insights on graphite anode stability in rechargeable: Li ion coordination structures prevail over solid electrolyte interphases. ACS Energy Lett. 2018, 3, 335– 340, DOI: 10.1021/acsenergylett.7b01177
  19
  New Insights on Graphite Anode Stability in Rechargeable Batteries: Li Ion Coordination Structures Prevail over Solid Electrolyte Interphases
  Ming, Jun; Cao, Zhen; Wahyudi, Wandi; Li, Mengliu; Kumar, Pushpendra; Wu, Yingqiang; Hwang, Jang-Yeon; Hedhili, Mohamed Nejib; Cavallo, Luigi; Sun, Yang-Kook; Li, Lain-Jong
  ACS Energy Letters (2018), 3 (2), 335-340CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
  Graphite anodes are not stable in most noncarbonate solvents (e.g., ether, sulfoxide, sulfone) upon Li ion intercalation, known as an urgent issue in present Li ions and next-generation Li-S and Li-O2 batteries for storage of Li ions within the anode for safety features. The solid electrolyte interphase (SEI) is commonly believed to be decisive for stabilizing the graphite anode. However, the solvation structure of the Li ions, detd. by the electrolyte compn. including lithium salts, solvents, and additives, plays a more dominant role than SEI in graphite anode stability. The Li ion intercalation desired for battery operation competes with the undesired Li+-solvent co-insertion, leading to graphite exfoliation. The increase in org. lithium salt LiN(SO2CF3)2 concn. or, more effectively, the addn. of LiNO3 lowers the interaction strength between Li+ and solvents, suppressing the graphite exfoliation caused by Li+-solvent co-insertion. The findings refresh the knowledge of the known SEI for graphite stability in metal ion batteries and also provide new guidelines for electrolyte systems to achieve reliable and safe Li-S full batteries.
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20. 20
  Zheng, J.; Tan, G.; Shan, P.; Liu, T.; Hu, J.; Feng, Y.; Yang, L.; Zhang, M.; Chen, Z.; Lin, Y. Understanding thermodynamic and kinetic contribution in expanding the stability window of aqueous electrolytes. Chem. 2018, 4, 2872– 2882, DOI: 10.1016/j.chempr.2018.09.004
  20
  Understanding Thermodynamic and Kinetic Contributions in Expanding the Stability Window of Aqueous Electrolytes
  Zheng, Jiaxin; Tan, Guoyu; Shan, Peng; Liu, Tongchao; Hu, Jiangtao; Feng, Yancong; Yang, Luyi; Zhang, Mingjian; Chen, Zonghai; Lin, Yuan; Lu, Jun; Neuefeind, Joerg C.; Ren, Yang; Amine, Khalil; Wang, Lin-Wang; Xu, Kang; Pan, Feng
  Chem (2018), 4 (12), 2872-2882CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
  Aq. electrolytes come with an intrinsic narrow electrochem. stability window (1.23 V). Expanding this window represents significant benefits in both fundamental science and practical battery applications. Recent breakthroughs made via super-concn. have resulted in >3.0 V windows, but fundamental understanding of the related mechanism is still absent. In the present work, we examd. the widened window (2.55 V) of a super-concd. (unsatd.) aq. soln. of LiNO3 through both theor. and spectral analyses and discovered that a local structure of intimate Li+-water interaction arises at super-concn., generating (Li+(H2O)2)n polymer-like chains to replace the ubiquitous hydrogen bonding between water mols. Such structure is mainly responsible for the expanded electrochem. stability window. Further theor. and exptl. analyses quant. differentiate the contributions to this window, identifying the kinetic factor (desolvation) as the main contributor. Such mol.-level and quant. understanding will further assist in tailor designing more effective approaches to stabilizing water electrochem.
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21. 21
  Tang, L.; Xu, Y.; Zhang, W.; Sui, Y.; Scida, A.; Tachibana, S. R.; Garaga, M.; Sandstrom, S. K.; Chiu, N.-C.; Stylianou, K. C. Strengthening aqueous electrolytes without strengthening water. Angew. Chem. Ind. Ed. 2023, 62, e202307212 DOI: 10.1002/anie.202307212
  21
  Strengthening Aqueous Electrolytes without Strengthening Water
  Tang, Longteng; Xu, Yunkai; Zhang, Weiyi; Sui, Yiming; Scida, Alexis; Tachibana, Sean R.; Garaga, Mounesha; Sandstrom, Sean K.; Chiu, Nan-Chieh; Stylianou, Kyriakos C.; Greenbaum, Steve G.; Greaney, Peter Alex; Fang, Chong; Ji, Xiulei
  Angewandte Chemie, International Edition (2023), 62 (35), e202307212CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Aq. electrolytes typically suffer from poor electrochem. stability; however, eutectic aq. solns.-25 wt.% LiCl and 62 wt.% H3PO4-cooled to -78°C exhibit a significantly widened stability window. Integrated exptl. and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl-, ice-like water clusters form, and H···Cl- bonding strengthens. Surprisingly, this low-temp. solvation structure does not strengthen water mols.' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temp. inertness in the electrolyte: less favorable solvation of OH- and H+, the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aq. Li-ion battery using LiMn2O4 cathode and CuSe anode with a high energy d. of 109 Wh/kg. These results highlight the potential of aq. batteries for polar and extraterrestrial missions.
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22. 22
  Yamada, Y.; Usui, K.; Sodeyama, K.; Ko, S.; Tateyama, Y.; Yamada, A. Hydrate-melt electrolytes for high-energy-density aqueous batteries. Nat. Energy. 2016, 1, 16129, DOI: 10.1038/nenergy.2016.129
  22
  Hydrate-melt electrolytes for high-energy-density aqueous batteries
  Yamada, Yuki; Usui, Kenji; Sodeyama, Keitaro; Ko, Seongjae; Tateyama, Yoshitaka; Yamada, Atsuo
  Nature Energy (2016), 1 (10), 16129CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)
  Aq. Li-ion batteries are attracting increasing attention because they are potentially low in cost, safe and environmentally friendly. However, their low energy d. (<100 Wh kg-1 based on total electrode wt.), which results from the narrow operating potential window of water and the limited selection of suitable neg. electrodes, is problematic for their future widespread application. Here, we explore optimized eutectic systems of several org. Li salts and show that a room-temp. hydrate melt of Li salts can be used as a stable aq. electrolyte in which all water mols. participate in Li+ hydration shells while retaining fluidity. This hydrate-melt electrolyte enables a reversible reaction at a com. Li4Ti5O12 neg. electrode with a low reaction potential (1.55 V vs. Li+/Li) and a high capacity (175 mAh g-1). The resultant aq. Li-ion batteries with high energy d. (>130 Wh kg-1) and high voltage (∼2.3-3.1 V) represent significant progress towards performance comparable to that of com. non-aq. batteries (with energy densities of ∼150-400 Wh kg-1 and voltages of ∼2.4-3.8 V).
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23. 23
  Ding, M. S.; Xu, K. Phase diagram, conductivity, and glass transition of LiTFSI-H₂O binary electrolytes. J. Phys. Chem. C 2018, 122, 16624– 16629, DOI: 10.1021/acs.jpcc.8b05193
  23
  Phase Diagram, Conductivity, and Glass Transition of LiTFSI-H2O Binary Electrolytes
  Ding, Michael S.; Xu, Kang
  Journal of Physical Chemistry C (2018), 122 (29), 16624-16629CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The aq. electrolyte system of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI-H2O, was systematically and accurately measured for a complete liq.-solid phase diagram and an extensive set of data on electrolytic cond. and glass-transition temp. The cond. data set was fitted with a VFT-based function (VFT: Vogel-Fulcher-Tammann), of which the three parameters were set to Laurent polynomial functions of compn. The fitting results were correlated with other quantities, and comparisons were made between the results of this study and those of other aq. and carbonate electrolyte systems. The results of these measurements, correlations, and comparisons strongly suggest a decoupling of cationic conduction from the movement of the bulk soln., in sharp contrast to what was obsd. in any nonaq. electrolytes so far.
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24. 24
  Welton, T. Ionic liquids: a brief history. Biophys. Rev. 2018, 10, 691– 706, DOI: 10.1007/s12551-018-0419-2
  24
  Ionic liquids: a brief history
  Welton, Tom
  Biophysical Reviews (2018), 10 (3), 691-706CODEN: BRIECG; ISSN:1867-2450. (Springer)
  A review. There is no doubt that ionic liqs. have become a major subject of study for modern chem. We have become used to ever more publications in the field each year, although there is some evidence that this is beginning to plateau at approx. 3500 papers each year. They have been the subject of several major reviews and books, dealing with different applications and aspects of their behaviors. In this article, I will show a little of how interest in ionic liqs. grew and developed.
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25. 25
  Zhao, H. Are ionic liquids kosmotropic or chaotropic? An evaluation of available thermodynamic parameters for quantifying the ion kosmotropicity of ionic liquids. J. Chem. Technol. Biotechnol. 2006, 81, 877– 891, DOI: 10.1002/jctb.1449
  25
  Are ionic liquids kosmotropic or chaotropic? An evaluation of available thermodynamic parameters for quantifying the ion kosmotropicity of ionic liquids
  Zhao, Hua
  Journal of Chemical Technology and Biotechnology (2006), 81 (6), 877-891CODEN: JCTBED; ISSN:0268-2575. (John Wiley & Sons Ltd.)
  A review. The hydration of ionic liqs. (a new type of org. salt) is not well understood. One property for characterizing the effect of ion hydration is kosmotropicity. To quantify the ion kosmotropicity, this review evaluated several thermodn. parameters including B-coeffs., structural entropies, structural vols., and ion mobility. The availability and reliability of exptl. data and various models for these parameters were examd. for ionic liqs. The relationship and comparison of these parameters are also discussed. Smaller pyridinium and imidazolium cations in ionic liqs. are more likely to be chaotropes or borderline ions owing to their hydrophobic hydration. The kosmotropicity of anions varies.
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26. 26
  Marcus, Y. Effect of ions on the structure of water: Structure making and breaking. Chem. Rev. 2009, 109, 1346– 1370, DOI: 10.1021/cr8003828
  26
  Effect of Ions on the Structure of Water: Structure Making and Breaking
  Marcus, Yizhak
  Chemical Reviews (Washington, DC, United States) (2009), 109 (3), 1346-1370CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review.
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27. 27
  Marcus, Y. Ions in water and biophysical implications: From chaos to cosmos; Springer, 2012.
  There is no corresponding record for this reference.
28. 28
  Reber, D.; Grissa, R.; Becker, M.; Kühnel, R. S.; Battaglia, C. Anion selection criteria for water-in-salt electrolytes. Adv. Energy Mater. 2021, 11, 2002913 DOI: 10.1002/aenm.202002913
  28
  Anion Selection Criteria for Water-in-Salt Electrolytes
  Reber, David; Grissa, Rabeb; Becker, Maximilian; Kuehnel, Ruben-Simon; Battaglia, Corsin
  Advanced Energy Materials (2021), 11 (5), 2002913CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
  Water-in-salt electrolytes have enabled the development of novel high-voltage aq. lithium-ion batteries. This study explores the reasons why analogus sodium electrolytes have struggled to reach the same level of electrochem. stability. Soln. structure and electrochem. stability are compared for 11 sodium salts, selected among the major classes of salts proposed for highly concd. electrolytes. The water environment established for each anion is related to its position in the Hofmeister series and a surprisingly strong correlation between the chaotropicity of the anion and the resulting electrochem. stability of the electrolyte is found. The search for suitable sodium salts is complicated by the fact that higher salt concns. are needed than for their lithium equiv. Reaching such a high concn. of >25 mol kg-1 with one or a combination of multiple sodium salts that have the desired properties remains a major challenge. Hence, alternative approaches such as multisolvent systems should be explored. The water soly. of NaTFSI can be increased from 8 to 30 mol kg-1 in the presence of ionic liqs. Such a ternary electrolyte enables stable cycling of a 2 V class sodium-ion battery based on the NaTi2(PO4)3/Na2Mn[Fe(CN)6] electrode couple for 300 cycles at 1C with a Coulombic efficiency of >99.5%.
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29. 29
  Chen, M.; Wu, J.; Ye, T.; Ye, J.; Zhao, C.; Bi, S.; Yan, J.; Mao, B.; Feng, G. Adding salt to expand voltage window of humid ionic liquids. Nat. Commun. 2020, 11, 5809, DOI: 10.1038/s41467-020-19469-3
  29
  Adding salt to expand voltage window of humid ionic liquids
  Chen, Ming; Wu, Jiedu; Ye, Ting; Ye, Jinyu; Zhao, Chang; Bi, Sheng; Yan, Jiawei; Mao, Bingwei; Feng, Guang
  Nature Communications (2020), 11 (1), 5809CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  Humid hydrophobic ionic liqs.-widely used as electrolytes-have narrowed electrochem. windows due to the involvement of water, absorbed on the electrode surface, in electrolysis. In this work, we performed mol. dynamics simulations to explore effects of adding Li salt in humid ionic liqs. on the water adsorbed on the electrode surface. Results reveal that most of the water mols. are pushed away from both cathode and anode, by adding salt. The water remaining on the electrode is almost bound with Li+, having significantly lowered activity. The Li+-bonding and re-arrangement of the surface-adsorbed water both facilitate the inhibition of water electrolysis, and thus prevent the redn. of electrochem. windows of humid hydrophobic ionic liqs. This finding is testified by cyclic voltammetry measurements where salt-in-humid ionic liqs. exhibit enlarged electrochem. windows. Our work provides the underlying mechanism and a simple but practical approach for protection of humid ionic liqs. from electrochem. performance degrdn.
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30. 30
  Yang, C.; Xia, J.; Cui, C.; Pollard, T. P.; Vataman, J.; Faraone, A.; Dura, J. A.; Tyagi, M.; Kattan, A.; Thimsen, E.; Xu, J.; Song, W.; Hu, E.; Ji, X.; Hou, S.; Zhang, X.; Ding, M. S.; Hwang, M. S.; Su, D.; Ren, Y.; Yang, X.-Q.; Wang, H.; Borodin, O.; Wang, C. All-temperature zinc batteries with high-entropy aqueous electrolyte. Nat. Sustainability 2023, 6, 325– 335, DOI: 10.1038/s41893-022-01028-x
  There is no corresponding record for this reference.
31. 31
  Ji, H.; Xie, C.; Wu, T.; Wang, H.; Cai, Z.; Zhang, Q.; Li, W.; Fu, L.; Li, H.; Wang, H. High-entropy solvent design enabling a universal electrolyte with a low freezing point for low-temperature aqueous batteries. Chem. Commun. 2023, 59, 8715– 8718, DOI: 10.1039/D3CC02214E
  31
  High-entropy solvent design enabling a universal electrolyte with a low freezing point for low-temperature aqueous batteries
  Ji, Huimin; Xie, Chunlin; Wu, Tingqing; Wang, Hao; Cai, Zhiwen; Zhang, Qi; Li, Wenbin; Fu, Liang; Li, Huanhuan; Wang, Haiyan
  Chemical Communications (Cambridge, United Kingdom) (2023), 59 (56), 8715-8718CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Amide additives acting as hydrogen-bonding ligands effectively break the crosslinking structures between water mols. and increase the entropy of mixed solvents, thus enabling a mixed solvent with an ultralow f.p. of -98°C. Zinc-ion batteries using this hybrid solvent exhibit good cycling stability over a wide temp. range from -60°C to 50°C.
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32. 32
  Zafar, Z. A.; Abbas, G.; Knizek, K.; Silhavik, M.; Kumar, P.; Jiricek, P.; Houdková, J.; Frank, O.; Cervenka, J. Chaotropic anion based “water-in-salt” electrolyte realizes a high voltage Zn-graphite dual-ion battery. J. Mater. Chem. A 2022, 10, 2064– 2074, DOI: 10.1039/D1TA10122F
  32
  Chaotropic anion based "water-in-salt" electrolyte realizes a high voltage Zn-graphite dual-ion battery
  Zafar, Zahid Ali; Abbas, Ghulam; Knizek, Karel; Silhavik, Martin; Kumar, Prabhat; Jiricek, Petr; Houdkova, Jana; Frank, Otakar; Cervenka, Jiri
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2022), 10 (4), 2064-2074CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  Aq. Zn-based batteries are promising candidates for grid energy storage due to their low cost, intrinsic safety, and environmental friendliness. Nevertheless, they suffer from limited energy d. due to the utilization of low-voltage cathodes and electrolytes. Graphite could be a viable high-voltage cathode material owing to its high redox potential (2.1-3.1 V vs. Zn/Zn2+). However, finding a suitable aq. electrolyte with high anodic stability remains a fundamental challenge. This work realizes a high-voltage and low-cost aq. Zn-graphite dual-ion battery based on a Zn(ClO4)2 water-in-salt electrolyte with a wide electrochem. window of 2.80 V. The implementation of the supersatd. Zn(ClO4)2 water-in-salt electrolyte contg. strong chaotropic ClO4- anions expands the oxidative stability of the aq. electrolyte beyond 1.65 V vs. Ag/AgCl or 2.60 V vs. Zn/Zn2+, and facilitates reversible plating/stripping of Zn2+ with a low overpotential of <50 mV at 1 mA cm-2 and a high upper cut-off potential of 2.5 V vs. Zn/Zn2+. Consequently, the Zn-graphite dual-ion battery delivers a max. discharge capacity of 45 mA h g-1 at 100 mA g-1 with a mean discharge voltage of ∼1.95 V and cycle life of over 500 cycles.
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33. 33
  Brown, J.; Forero-Saboya, J.; Baptise, B.; Karlsmo, M.; Rousse, G.; Grimaud, A. A guanidinium salt as a chaotropic agent for aqueous battery electrolytes. Chem. Commun. 2023, 59, 12266– 12269, DOI: 10.1039/d3cc03769j
  33
  A guanidium salt as a chaotropic agent for aqueous battery electrolytes
  Brown, John; Forero-Saboya, Juan; Baptiste, Benoit; Karlsmo, Martin; Rousse, Gwenaelle; Grimaud, Alexis
  Chemical Communications (Cambridge, United Kingdom) (2023), 59 (82), 12266-12269CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  This study investigates a salt design principle for aq. battery electrolytes by combining chaotropic ions, guanidium cations (Gdm) and bis(trifluoromethanesulfonyl)imide anions (TFSI), forming GdmTFSI. This salt's crystal structure was solved via single-crystal X-ray diffraction and characterized using Fourier-transform IR spectroscopy. Study reveals that GdmTFSI salt disrupts the hydrogen bonding network of aq. solns., impacting water reactivity at electrochem. interfaces.
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34. 34
  Becker, M.; Rentsch, D.; Reber, D.; Aribia, A.; Battaglia, C.; Kühnel, R. S. The hydrotropic effect of ionic liquids in water-in-salt electrolytes. Angew. Chem., Int. Ed. 2021, 60, 14100– 14108, DOI: 10.1002/anie.202103375
  34
  The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes
  Becker, Maximilian; Rentsch, Daniel; Reber, David; Aribia, Abdessalem; Battaglia, Corsin; Kuehnel, Ruben-Simon
  Angewandte Chemie, International Edition (2021), 60 (25), 14100-14108CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Water-in-salt electrolytes have successfully expanded the electrochem. stability window of aq. electrolytes beyond 2 V. Further improvements in stability can be achieved by partially substituting water with either classical org. solvents or ionic liqs. Here, we study ternary electrolytes composed of LiTFSI, water, and imidazolium ionic liqs. We find that the LiTFSI soly. strongly increases from 21 mol kg-1 in water to up to 60 mol kg-1 in the presence of ionic liq. The soln. structure is investigated with Raman and NMR spectroscopy and the enhanced LiTFSI soly. is found to originate from a hydrotropic effect of the ionic liqs. The increased reductive stability of the ternary electrolytes enables stable cycling of an aq. lithium-ion battery with an energy d. of 150 Wh kg-1 on the active material level based on com. relevant Li4Ti5O12 and LiNi0.8Mn0.1Co0.1O2 electrode materials.
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35. 35
  Reber, D.; Borodin, O.; Becker, M.; Rentsch, D.; Thienenkamp, J. H.; Grissa, R.; Zhao, W.; Aribia, A.; Brunklaus, G.; Battaglia, C. Water/ionic liquids/succinonitrile hybrid electrolytes for aqueous batteries. Adv. Funct. Mater. 2022, 32, 2112138 DOI: 10.1002/adfm.202112138
  35
  Water/Ionic Liquid/Succinonitrile Hybrid Electrolytes for Aqueous Batteries
  Reber, David; Borodin, Oleg; Becker, Maximilian; Rentsch, Daniel; Thienenkamp, Johannes Helmut; Grissa, Rabeb; Zhao, Wengao; Aribia, Abdessalem; Brunklaus, Gunther; Battaglia, Corsin; Kuehnel, Ruben-Simon
  Advanced Functional Materials (2022), 32 (20), 2112138CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The water-in-salt concept has significantly improved the electrochem. stability of aq. electrolytes, and the hybridization with org. solvents or ionic liqs. has further enhanced their reductive stability, enabling cell chemistries with up to 150 Wh kg-1 of active material. Here, a large design space is opened by introducing succinonitrile as a cosolvent in water/ionic liq./succinonitrile hybrid electrolytes (WISHEs). By means of succinonitrile addn., the soly. limits can be fully circumvented, and the properties of the electrolytes can be optimized for various metrics such as highest electrochem. stability, max. cond., or lowest cost. While excessive nitrile fractions render the mixts. flammable, careful selection of component ratios yields highly performant, nonflammable electrolytes that enable stable cycling of Li4Ti5O12-LiNi0.8Mn0.1Co0.1O2 full cells over a wide temp. range with strong rate performance, facilitated by the fast conformational dynamics of succinonitrile. The WISHEs allow stable cycling with a max. energy d. of ≈140 Wh kg-1 of active material, Coulombic efficiencies of close to 99.5% at 1C, and a capacity retention of 53% at 10C relative to 1C.
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36. 36
  Tot, A.; Zhang, L.; Berg, E. J.; Svensson, P. H.; Kloo, L. Water-in-salt electrolyte made saltier by Gemini ionic liquids for highly efficient Li-ion batteries. Sci. Rep. 2023, 13, 2154, DOI: 10.1038/s41598-023-29387-1
  36
  Water-in-salt electrolytes made saltier by Gemini ionic liquids for highly efficient Li-ion batteries
  Tot, Aleksandar; Zhang, Leiting; Berg, Erik J.; Svensson, Per H.; Kloo, Lars
  Scientific Reports (2023), 13 (1), 2154CODEN: SRCEC3; ISSN:2045-2322. (Nature Portfolio)
  The water-in-salt electrolytes have promoted aq. Li-ion batteries to become one of the most promising candidates to overcome safety concerns/issues of traditional Li-ion batteries. A simple increase of Li-salt concn. in electrolytes can successfully expand the electrochem. stability window of aq. electrolytes beyond 2 V. However, necessary stability improvements require an increase in complexity of the ternary electrolytes. Here, we have explored the effects of novel, Gemini-type ionic liqs. (GILs) as a co-solvent systems in aq. Li[TFSI] mixts. and investigated the transport properties of the resulting electrolytes, as well as their electrochem. performance. The devices contg. pyrrolidinium-based GILs show superior cycling stability and promising specific capacity in the cells based on the commonly used electrode materials LTO (Li4Ti5O12) and LMO (LiMn2O4).
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37. 37
  Borodin, O. Polarizable force field development and molecular dynamics simulations of ionic liquids. J. Phys. Chem. B 2009, 113, 11463– 11478, DOI: 10.1021/jp905220k
  37
  Polarizable Force Field Development and Molecular Dynamics Simulations of Ionic Liquids
  Borodin, Oleg
  Journal of Physical Chemistry B (2009), 113 (33), 11463-11478CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  A many-body polarizable force field was developed and validated for ionic liqs. (ILs) contg. 1-methyl-3-alkylimidazolium, 1-alkyl-2-methyl-3-alkylimidazolium, N-methyl-N-alkylpyrrolidinium, N-alkylpyridinium, N-alkyl-N-alkylpiperidinium, N-alkyl-N-alkylmorpholinium, tetraalkylammonium, tetraalkylphosphonium, N-methyl-N-oligoetherpyrrolidinium cations and BF4-, CF3BF3-, CH3BF3-, CF3SO3-, PF6-, dicyanamide, tricyanomethanide, tetracyanoborate, bis(trifluoromethane sulfonyl)imide (Ntf2- or TFSI-), bis(fluorosulfonyl)imide (FSI-) and nitrate anions. Classical mol. dynamics (MD) simulations were performed on 30 ionic liqs. at 298, 333, and 393 K. The IL d., heat of vaporization, ion self-diffusion coeff., cond., and viscosity were found in a good agreement with available exptl. data. The ability of the developed force field to predict ionic crystal cell parameters was tested on four ionic crystals contg. Ntf2- anions. The influence of polarization on the structure and ion transport was investigated for [emim][BF4] IL. A connection between the structural changes in IL resulting from turning off polarization and slowing down of ion dynamics was found. Developed force field provided accurate description/prediction of thermodn. and transport properties of alkanes, fluoroalkanes, oligoethers (1,2-dimethoxyethane), ethylene carbonate, propylene carbonate, di-Me carbonate, hydrazine, methyhydrazine, dimethylhydrazine, acetonitrile, di-Me amine, and di-Me ketone.
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38. 38
  Suo, L.; Oh, D.; Lin, Y.; Zhuo, Z.; Borodin, O.; Gao, T.; Wang, F.; Kushima, A.; Wang, Z.; Kim, H. C. How solid-electrolyte interphase forms in aqueous electrolytes. J. Am. Chem. Soc. 2017, 139, 18670– 18680, DOI: 10.1021/jacs.7b10688
  38
  How Solid-Electrolyte Interphase Forms in Aqueous Electrolytes
  Suo, Liumin; Oh, Dahyun; Lin, Yuxiao; Zhuo, Zengqing; Borodin, Oleg; Gao, Tao; Wang, Fei; Kushima, Akihiro; Wang, Ziqiang; Kim, Ho-Cheol; Qi, Yue; Yang, Wanli; Pan, Feng; Li, Ju; Xu, Kang; Wang, Chunsheng
  Journal of the American Chemical Society (2017), 139 (51), 18670-18680CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Solid-electrolyte interphase (SEI) is the key component that enables all advanced electrochem. devices, the best representative of which is Li-ion battery (LIB). It kinetically stabilizes electrolytes at potentials far beyond their thermodn. stability limits, so that cell reactions could proceed reversibly. Its ad hoc chem. and formation mechanism has been a topic under intensive investigation since the first commercialization of LIB 25 years ago. Traditionally SEI can only be formed in nonaq. electrolytes. However, recent efforts successfully transplanted this concept into aq. media, leading to significant expansion in the electrochem. stability window of aq. electrolytes from 1.23 V to beyond 4.0 V. This not only made it possible to construct a series of high voltage/energy d. aq. LIBs with unprecedented safety, but also brought high flexibility and even "open configurations" that have been hitherto unavailable for any LIB chemistries. While this new class of aq. electrolytes has been successfully demonstrated to support diversified battery chemistries, the chem. and formation mechanism of the key component, an aq. SEI, has remained virtually unknown. In this work, combining various spectroscopic, electrochem. and computational techniques, we rigorously examd. this new interphase, and comprehensively characterized its chem. compn., microstructure and stability in battery environment. A dynamic picture obtained reveals how a dense and protective interphase forms on anode surface under competitive decompns. of salt anion, dissolved ambient gases and water mol. By establishing basic laws governing the successful formation of an aq. SEI, the in-depth understanding presented in this work will assist the efforts in tailor-designing better interphases that enable more energetic chemistries operating farther away from equil. in aq. media.
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39. 39
  Dubouis, N.; Lemaire, P.; Mirvaux, B.; Salager, E.; Deschamps, M.; Grimaud, A. The role of the hydrogen evolution reaction in the solid-electrolyte interphase formation mechanism for “Water-in-Salt” electrolytes. Energy Environ. Sci. 2018, 11, 3491– 3499, DOI: 10.1039/C8EE02456A
  39
  The role of the hydrogen evolution reaction in the solid-electrolyte interphase formation mechanism for "Water-in-Salt" electrolytes
  Dubouis, Nicolas; Lemaire, Pierre; Mirvaux, Boris; Salager, Elodie; Deschamps, Michael; Grimaud, Alexis
  Energy & Environmental Science (2018), 11 (12), 3491-3499CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  Aq. Li-ion batteries have long been envisioned as safe and green energy storage technol., but have never been com. realized owing to the limited electrochem. stability window of water, which drastically hampers their energy d. Recently, Water-in-Salt electrolytes (WiSEs) in which a large amt. of org. salt is dissolved into water were proposed to allow for assembling 3 V Li-ion batteries. Hereby, our attention focused on the fate of water at the electrochem. interface under neg. polarization and the potential reactivity of TFSI anions with products originating from the water redn. Hence, combining anal. of bulk electrolytes with electrochem. measurements on model electrodes and operando characterization, we were able to demonstrate that hydroxides generated during the hydrogen evolution reaction can chem. react with TFSI and catalyze the formation of a fluorinated solid-electrolyte interphase (SEI) that prevents further water redn. Mastering this new SEI formation path with the chem. degrdn. of TFSI anions mediated by the electrochem. redn. of water can therefore open new avenues for the future development of not only WiSEs but also Li batteries functioning in org. electrolytes.
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40. 40
  Ota, H.; Sakata, Y.; Wang, X.; Sasahara, J.; Yasukawa, E. Characterization of lithium electrode in lithium imides/ethylene carbonate and cyclic ether electrolytes. J. Electrochem. Soc. 2004, 151, A437, DOI: 10.1149/1.1644137
  40
  Characterization of Lithium Electrode in Lithium Imides/Ethylene Carbonate and Cyclic Ether Electrolytes. II. Surface Chemistry
  Ota, Hitoshi; Sakata, Yuuichi; Wang, Xianming; Sasahara, Jun; Yasukawa, Eiki
  Journal of the Electrochemical Society (2004), 151 (3), A437-A446CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
  Chem. components of surface films of deposited lithium on nickel substrates in electrolytes with LiN(SO2CF3)2 (LiTFSI), LiN (SO2C2F5)2 (LiBETI), LiPF6 solutes, and THF solvents were characterized by FTIR, two-dimensional NMR (2-dimensional NMR), XPS, evolved gas anal., and ion chromatograph to understand the electrochem. performance of lithium imide/cyclic ether-based electrolytes. The top layers of the surface film were ROCO2Li, Li2CO3, polymer constituents, and LiF. The inner layers of the surface film consisted of Li2O and carbide species. In imide/cyclic ether-based electrolytes, Li2S2O4 and Li2SO3 as outer layers, and Li2S as the inner layer were formed on a nickel substrate as reductive constituents of imide solute. Org. surface layers consisted of lithium ethoxides, lithium ethylene dicarbonate (CH2OCO2Li)2, polyethylene oxide, and lithium ethylene dicarbonate contg. an oxyethylene unit by 1H, 13C, and 2-dimensional NMR. Li cycling efficiency affects not only the deposited lithium morphol. but also chem. components.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c07164.
- Schematic representation of synthetic pathway; NMR, FTIR, Raman, and MS spectra of synthesized GILs; experimental results of conductivity and viscosity of GIL-enriched electrolytes; snapshots of MD simulations; and voltage profiles of each electrolyte at selected cycles along with standard deviations (PDF)
- jp3c07164_si_001.pdf (1.31 MB)
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Tuning of Molecular Water Organization in Water-in-Salt Electrolytes by Addition of Chaotropic Ionic LiquidsClick to copy article linkArticle link copied!

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Tuning of Molecular Water Organization in Water-in-Salt Electrolytes by Addition of Chaotropic Ionic Liquids
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