Molecular dynamics simulations were employed to investigate the structure and dynamics of [LiTFSA]:[FEC]:[HFE] (TFSA, bis(trifluoromethanesulfonyl)amide; FEC, fluoroethylene carbonate; HFE, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether), which acts as an optimal electrolyte for lithium-sulfur batteries; however, it exhibits high performance (ratio of 1:6:3) or low performance (ratio of 1:4:5) depending on the ratio of the constituents. Simulations revealed that both electrolyte systems spontaneously liquid-liquid phase-separate into two distinct domains, namely, a large LiTFSA-rich microphase and an HFE-rich phase; however, the microphase properties significantly vary with the constituent composition. For the high-performance electrolyte, the flexible microphase facilitates the formation of small Li-ion-containing clusters in the HFE-rich phase, promoting enhanced Li-ion diffusion. Furthermore, the Li ions diffuse via different mechanisms on the 0.001, 0.1, 10, and 100 ns time scales. In contrast, the low-performance electrolyte exhibited a more rigid microphase with fewer small clusters, restricting Li-ion transport mainly to the microphase. These variations alter the pathways for ionic conduction, implying that even when identical chemical species are used, changes in the constituent composition can result in minor modifications in the physical properties of the microphase, significantly altering the performance. This insight underscores a key consideration for electrolyte design, not only for lithium-sulfur batteries but also for other battery systems.

For the practical use of polymer gel electrolytes in advanced battery systems, simultaneous improvement of mechanical strength, ionic conductivity, and Li-ion transport properties remains challenging. In this study, a mechanically reliable and ionically conductive gel electrolyte was developed by cross-linking tetra-arm poly(methyl acrylate) (tetra-PMA), a weakly coordinating polymer to Li+, in a sulfolane (SL)-based highly concentrated electrolyte ([Li(SL)3][TFSA]) using a copper-free click reaction. Tetra-PMA was synthesized via atom transfer radical polymerization and converted to azide-terminated precursors, enabling efficient gelation with an electron-deficient dialkyne. The resulting tetra-PMA gel formed a uniform, defect-minimized network structure, allowing membrane formation at a low polymer content of 10 wt %, where conventional nonuniform PMA gels failed to form self-standing membranes. Mechanical testing revealed that the tetra-PMA gel exhibited a relatively high Young's modulus and fracture energy, comparable to those of tetra-arm poly(ethylene glycol) (tetra-PEG) gels. Raman spectroscopy confirmed that the tetra-PMA gel preserved the Li+ solvation structure of the parent SL-based concentrated electrolyte, in contrast to PEG-based gels, which disrupted Li+ coordination owing to the strong coordinating ability of the ether chains. Electrochemical measurements demonstrated that the tetra-PMA gel retained a high Li-ion transference number (t Li abc = 0.59), comparable to that of the SL-based concentrated electrolyte, and exhibited sufficient oxidative stability up to 4.5 V vs Li/Li+. In Li||LiCoO2 cells, the gel enabled stable cycling and delivered discharge capacities exceeding 120 mA h g-1 at 2 C, attributable to suppressed concentration polarization resulting from the high t Li abc. These results demonstrate the effectiveness of a weakly coordinating, homogeneous polymer network in achieving both mechanical robustness and favorable Li-ion transport properties in gel electrolytes, offering a promising platform for next-generation high-rate lithium-ion polymer batteries.

Highly concentrated electrolytes (HCEs) exhibit unique ion-transport properties that fundamentally differ from those of conventional electrolytes; however, the role of anion species in governing Li+ transport remains unknown. Herein, Li+-transport properties in lithium salt/propylene carbonate (LiX/PC) mixtures were systematically investigated by varying the basicity of the Lewis base anion: PF6 -, N(SO2F)2 -, N(SO2CF3)2 -, ClO4 -, BF4 -, and SO3CF3 - (TfO-). Ionic conductivity, viscosity, self-diffusion coefficients, and Li+ transference numbers under anion-blocking conditions were evaluated and correlated with molecular-scale structures obtained from molecular dynamics simulations. Weak Lewis-base anions exhibited high ionic conductivity and coupled Li+-solvent diffusion at high salt concentrations. Conversely, strong Lewis-base anions promoted ion-pair and aggregate formation, resulting in structural diffusion of Li+ and high transference numbers. Notably, Li+ transference numbers increased with anion Lewis basicity and concentration, attaining 0.83 for LiTfO/PC = 1/2.5, while conductivity decreased, revealing an intrinsic tradeoff between these transport descriptors. Therefore, anion Lewis basicity critically governs ion association, correlated motion, and Li+-transport mechanisms in HCEs.

In lithium-ion batteries, the desolvation of Li+ is widely regarded as the activation barrier for interfacial charge-transfer reactions; however, the governing factors remain unclear. This study investigated the effect of solvent properties on charge-transfer kinetics for a Li1.05Mn1.95O4 thin-film electrode using lithium bis(trifluoromethanesulfonyl)amide-based electrolytes with H2O, tetrahydrofuran, and n-dibutyl ether. Electrochemical impedance spectroscopy revealed that the concentrated aqueous electrolyte exhibited a significantly lower charge-transfer resistance (R-ct) than the monoether-based systems, contrary to expectations based on viscosity. Analysis of Li+ activity indicated that the high activity in the concentrated aqueous system contributed to the reduced R-ct; however, comparisons using 1 mol dm(-3) (1 M) electrolytes showed that neither viscosity nor Li+ activity alone could explain the observed trends. These results suggest that dynamic solvent properties play a critical role, and that the rapid molecular reorientation of water facilitates faster solvation and desolvation of Li+ at the interface.

The charge-transfer kinetics at the interface between LiMn2O4 thin-film electrodes and ether-based concentrated electrolytes were studied. The [Li(triglyme)1][N(SO2CF3)2] electrolyte exhibited higher charge-transfer resistance and activation energy barrier compared to monoglyme and tetrahydrofuran-based concentrated electrolytes. Li salt concentration and viscosity are the primary factors affecting the electrochemical reaction kinetics.

Interfacially-localized high-concentration electrolytes were developed using an anionic surfactant and a magnesium( ii ) salt to achieve selective Li ion transport, high electrochemical stability and superior SEI formation in aqueous electrolytes.

Surfactants possess unique properties in bulk solutions and at interfaces, naturally forming self-assembled structures. Herein, cetyltrimethylammonium trifluoroacetate (CTATFA) was incorporated into aqueous electrolytes as a cationic surfactant to enhance their ionic conductivity and electrochemical stability. The presence of CTATFA widened the electrochemical stability windows of both Li-based and Zn-based electrolytes. The Zn-based electrolyte exhibited high ionic conductivity and low viscosity in the bulk solution. In a Zn symmetric cell, the electrolyte containing 1 M Zn(TFA)2-0.5 M CTATFA demonstrated excellent Zn plating/stripping reversibility for over 800 h at 1 mAh cm-2 and 1 mA cm-2. A Zn-Cu cell with 1 M Zn(TFA)2-0.5 M CTATFA exhibited excellent reversibility, achieving over 300 plating/stripping cycles at 5 mA cm-2 and 5 mAh cm-2. The Zn/MnO2 cell using the Zn-based electrolyte also demonstrated a specific capacity of 105 mAh g-1 over 750 cycles at a current density of 0.5 A g-1. This study provides insight into the design of high-performance aqueous electrolytes based on the self-assembly and surface adsorption of cationic surfactants.

Lithium metal is a promising anode material for next-generation rechargeable batteries. However, significant challenges remain, including the growth of Li dendrites and electrolyte decomposition on the Li electrode. LiNO3 has been widely studied as an electrolyte additive because of its ability to form a Li3N-rich solid-electrolyte interphase (SEI) on Li, thereby suppressing electrolyte decomposition. In this study, sulfolane (SL) was employed as a solvent having high LiNO3 solubility, and the electrochemical properties of LiNO3/SL electrolytes were investigated systematically. The LiNO3/SL electrolytes facilitated the formation of an SEI derived from LiNO3 and enabled the uniform deposition of Li. However, the ionic conductivity of LiNO3/SL electrolyte was low due to the low degree of dissociation of LiNO3. The ionic conductivity was improved by mixing a highly dissociative LiN(SO2CF3)2 (LiTFSA) with LiNO3 in SL, and the coulombic efficiency for Li deposition/dissolution was improved simultaneously. Further, the electrode potential of Li in the SL-based electrolyte became more positive with the incorporation of LiTFSA, weakening the reducing power of Li. Consequently, the LiNO3-derived SEI and the weak reducing power of Li synergistically suppressed the reductive decomposition of SL-based electrolyte and enhanced the reversibility of Li deposition/dissolution.

The endurance of lithium-sulfur (Li-S) cells depends on the stability of lithium (Li) metal anodes and their consistent efficiency during extended Li dissolution and deposition cycles. Electrolytes containing Li[N(SO2F)2] (Li[FSA]) have shown potential in enhancing Li anode reversibility by promoting the formation of a favorable inorganic-rich solid-electrolyte interphase (SEI) on the Li metal electrode. However, the use of Li[FSA] as the primary electrolyte salt in Li-S batteries is hindered by the spontaneous side reactions of [FSA]- anions with soluble lithium-polysulfides (Li2S x , 2 <= x <= 8). To overcome this challenge, we have developed a localized high-concentration electrolyte (LHCE) with reduced Li2S x solubility, composed of Li[TFSA0.8LiFSA0.2] ([TFSA]: [N(SO2CF3)2]) binary salts dissolved in sulfolane (SL) and diluted by 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE). This LHCE solution demonstrates superior stability of [FSA]- anions, due to the restricted dissolution of Li2S x within the LHCE. We experimentally evaluated the critical factors affecting reversibility of Li dissolution/deposition in electrolytes containing Li[TFSA0.8LiFSA0.2]. Increased salt concentration, combined with HFE dilution, widens the reduction potential gap between the anion and Li+, which thermodynamically promotes anion reduction, controls SEI composition, and improves Li reversibility. We demonstrate the operation of a Li-S pouch cell under practical conditions with a high sulfur loading of 5.5 mg(S) cm-2 and an extremely low electrolyte/sulfur (E/S) ratio of 3.0 mu L mg(S) -1. The battery delivers a high energy density of 280 Wh kg-1. Our findings provide insights into the critical factors for achieving prolonged Li dissolution/deposition reversibility, particularly under practical Li-S pouch cell conditions, through electrolyte formulation design.

Understanding the effects of non-coordinating diluents on the physicochemical properties of localized high-concentration electrolytes (LHCEs) is essential for the rational design of battery electrolytes. In this study, we examined the effect of a hydrofluoroether (HFE), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, on the liquid structure, transport properties, and electrochemical reaction kinetics of a model LHCE containing lithium bis(fluorosulfonyl)amide (LiFSA), 1,2-dimethoxyethane (DME), and HFE. Raman spectroscopy revealed that the Li+ solvation structure in the model LHCE remained largely unchanged upon dilution with HFE. The ion-pairing environment involving FSA- was also preserved, consistent with the weak coordinating ability of HFE. Although HFE did not coordinate with Li+, molecular dynamics simulations indicated strong interactions between HFE protons and FSA-, supporting its miscibility with the concentrated [LiFSA]/[DME] = 1/2 electrolyte. With increasing HFE content, viscosity decreased, while ionic conductivity reached a maximum at an intermediate LiFSA concentration owing to the trade-off between ion concentration and mobility. The diffusion coefficients increased with dilution; however, the decreasing molar conductivity/diffusivity ratio indicated a dynamic domain structure and prolonged ion-pair lifetime in the LHCEs. Electrochemical impedance analysis revealed that the charge-transfer reaction resistance at the LiMn2O4 electrode reached a minimum at an intermediate concentration ([LiFSA]/[DME]/[HFE] = 1/2/1), while the activation energy remained nearly constant. This finding indicates that HFE lowers viscosity without affecting the energy barrier for Li+ desolvation at the electrode-electrolyte interface. These findings demonstrate that non-coordinating diluents modulate the liquid structure, ion transport, and interfacial properties of LHCEs.

Although Li metal batteries have attracted much attention as high energy density rechargeable batteries, poor cycle ability and safety remain great concerns for practical application. The development of suitable electrolytes is the key to improving the performance of Li metal batteries, and research on electrolytes has been vigorously pursued. In this study, weakly coordinating properties of linear monoether-based electrolytes that regulate energy level of Li ion coordination to be energetically less stable, frustrated state were exploited to improve Li ion transport via rapid Li ion exchange between the weak-coordination sites (solvent and anion). In addition to the reductive stability of the monoether solvents, formation of anion-derived inorganic-rich solid electrolyte inter-phase (SEI), and the improved Li ion mass transport, a significant positive shift of electrode potential of Li deposition/dissolution was responsible for high reversibility of Li metal electrode in the monoether-based electrolytes. The charge-discharge rate capabilities of Li/LiFePO4 cells with n-butyl methyl ether (BME)-based electrolytes were superior to those of cells with highly concentrated electrolytes (HCEs) of dimethoxyethane (DME) containing lithium bis(fluorosulfonyl)amide (LiFSA). Li/LiFePO4 cells with BME-based electrolytes delivered a high discharge capacity of 110 mAh g-1 even at a high current density of 15 mA cm-2. This study highlights that less-stabilized Li ion coordination in the weakly coordinating electrolytes enables the enhanced Li ion transport and highly reversible deposition/dissolution of Li metal, which in turn leads to the greater charge-discharge performance of Li metal batteries at high current densities.

The desolvation of Li+ ion is generally considered to be the rate-determining step of Li+ insertion/extraction reactions at the negative and positive electrodes in the electrolyte solutions of Li-ion batteries (LIBs). However, specific factors that affect the charge-transfer kinetics at the electrode/electrolyte interface remain to be clearly understood. In this study, we investigated the interfacial charge-transfer reaction rate at LiMn2O4 thin-film electrodes in LiN(SO2CF3)2/monoglyme electrolytes. Our analysis revealed that the Li+ activity and electrolyte viscosity significantly affect the interfacial charge-transfer reaction rate. A higher Li-salt concentration enhances Li+ activity, accelerating the reaction rate. However, increasing the salt concentration beyond a certain level increases the electrolyte viscosity, which retards the interfacial reaction kinetics. A trade-off between these two factors results in the fastest reaction rate at an intermediate concentration (similar to 1.8 mol L-1). In the electrolytes with higher LiN(SO2CF3)2 concentrations, the chemical potential of Li+ increases, which facilitates the desolvation of Li+. However, the activation energy of the electrolyte viscosity increases with increasing salt concentration. Consequently, the activation energy of the interfacial charge-transfer reaction reached the minimum at approximately 1.8 mol L-1. These insights into the factors that affect the Li+ insertion reaction kinetics can help design optimal electrolytes for high-power LIBs.

To achieve high safety of lithium-ion batteries (LIBs), non-flammable electrolytes and prevention of liquid electrolytes are desirable. Sulfolane (SL) is a thermally stable and non-flammable solvent, and can dissolve high concentration Li salts. Recently, highly concentrated electrolytes (HCEs) containing Li salts over 3 mol dm −3 have attracted much attention for their attractive properties such as high thermal stability and wide electrochemical windows. We previously reported that SL-based HCEs exhibit Liion hopping conduction mechanism. 1,2 In SL-based HCEs, a unique solvation structure is formed in which different Li + ions are cross-linked by the SL and anions. In the cross-linked structure, Li + ions dynamically exchange the ligands (SL and anion) and diffuse/migrate faster than SL solvent and anion, leading to high transference numbers of Li + ion (>0.5). The high transference number of Li + is effective in suppressing the concentration polarization during high-rate charging and discharging of a LIB. Gelation of the HCEs can prevent the leakage of liquids and further improve the safety of LIBs. In the case of polymer gel electrolytes, liquid electrolytes are incorporated in the polymer network. Ionic conductivity of a gel electrolyte is generally lower than that of its parent liquid electrolyte. However, to prepare self-standing and mechanically robust gel electrolytes high polymer concentrations (typically over 20 wt%) are required. To achieve both high ionic conductivity and mechanical toughness of a gel electrolyte, the use of tetra-arm poly(ethylene glycol) (TPEG) has been proposed. 3 TPEG gels obtained by the end-coupling reaction of two symmetrical TPEGs with different terminals exhibit excellent mechanical properties. 4 TPEG forms a homogeneous polymer network, allowing the resulting gels to uniformly disperse external stresses. Consequently, TPEG gels possess excellent mechanical toughness even at low polymer concentrations (<10 wt%). In this work, we prepared self-standing TPEG gel membranes containing a high concentration LiN(SO 2 CF 3 ) 2 /SL electrolyte. 5 The mechanical, ion transport, and electrochemical properties of the gels were characterized. TPEG gels exhibited high transference numbers of Li + . The low polymer concentration and homogeneous polymer network in the gel electrolyte were useful in achieving both mechanical reliability and high Li + transport ability. Li/LiCoO 2 cell with a TPEG gel electrolyte membrane could discharge at a current density of 2.9 mA cm −2 despite the low ionic conductivity (0.26 mS cm −1 ) of the gel electrolyte. Acknowledgements : This study was partially supported by the JSPS KAKENHI (Grant No. JP19H05813, JP22H00340, and JP23K17370). References K. Dokko, et al, J. Phys. Chem. B, 2018, 122 , 10736–10745. A. Nakanishi, et al., J. Phys. Chem. C, 2019, 123 , 14229–14238. K. Fujii, et al., Soft Matter, 2012, 8 , 1756–1759 T. Sakai, et al., Macromolecules, 2008, 41 , 5379–5384. N. Tasaki, et al., Phys. Chem. Chem. Phys., 2023, 25 , 17793–17797

Abstract Li–S batteries have attracted attention as the next‐generation secondary batteries. While substantial progress is made in understanding Li–S chemistry at a fundamental level, only a limited number of studies are dedicated to achieving high energy density at the practical pouch cell level. The challenge lies in attaining high‐energy‐density Li–S batteries under harsh conditions, which involve a minimal amount of electrolyte and a relatively high areal S‐loading cathode. This discrepancy creates a substantial gap between fundamental material research and comprehensive cell‐level investigations. In this study, it is investigated how the morphology and properties of two carbon materials, namely Ketjen black (KB) and mesoporous carbon nano‐dendrites (MCND), influence the composite cathode architecture and determine the performance of Li–S batteries. Unlike KB, MCND allows for a higher sulfur‐loading cathode without evident cracks in the composite cathode. This achievement can be attributed to the high porosity, excellent wettability, and high conductivity exhibited during an identical electrode preparation procedure. Furthermore, large‐format Li–S pouch cells incorporating MCND/S cathodes are successfully fabricated. These cells demonstrate an energy density surpassing 250 Wh kg−1 and an initial discharge capacity of 3.7 Ah under challenging conditions (S‐loading > 5 mg cm−2 and E/S < 3.5 µL mg−1).

Solvate ionic liquids (SILs) are promising electrolytes for Li metal batteries. In this study, Li plating-stripping reactions in SILs were investigated using electrochemical quartz crystal microbalance measurements and X-ray photoelectron spectroscopy (XPS). SILs were prepared by mixing Li salt and glyme (triglyme or tetraglyme) in a 1:1 molar ratio. During the Li plating-stripping reaction, a reversible mass change was observed in LiN(SO 2 F) 2 (LiFSA) based SILs. In contrast, a considerably higher mass change than the theoretical value calculated from the Faraday current was observed in the LiN(SO 2 CF 3 ) 2 (LiTFSA) based SILs, owing to the accumulation of decomposition products of electrolytes on the electrode surface. XPS depth profiling for the deposited Li suggested the formation of thin solid-electrolyte interphase (SEI) in the LiFSA-based SILs while considerably thicker SEI was formed in the LiTFSA-based SILs. The SEI formed in LiFSA-based SILs effectively suppressed the decomposition of electrolytes. Thus, LiFSA-based SILs are favorable for achieving highly reversible charge–discharge of Li electrodes in Li metal batteries.

Highly concentrated Li salt electrolyte solutions have attracted attention recently due to the unique physicochemical and electrochemical properties.1 Recently, our group reported that highly concentrated Li salt/sulfolane electrolytes exhibit Li+ ion hopping conduction mechanism.2 Gel electrolytes, which incorporate liquid electrolytes within polymer network generally maintain ion transport properties derived from liquid electrolytes. The gelation of liquid electrolytes can prevent the leakage of liquids and improve the safety of batteries. However, certain gel electrolytes containing high concentration Li salt solution have been reported that the ion transport properties can be affected by the participation of the polymer matrix in the solvation structure.3 In this study, we focused on the effects of polymer side chains on ion transport properties of gels. We prepared novel gel electrolytes using polymers with different side chains as a polymer matrix containing high concentration Li salt/sulfolane solutions. Gel electrolytes incorporating lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and sulfolane (SL) at 1:3 molar ratio, which exhibits a high Li+ ion transference number (t Li+ ), were prepared by free radical polymerization using four types of methacrylate monomers (methyl methacrylate: MMA, propyl methacrylate: PMA, 2,2,3,3-tetrafluoropropyl methacrylate: TFPMA, diethylene glycol monomethyl ether methacrylate: DEGMA). Ionic conductivity and of PMMA gel were comparable to those of PPMA gel with extended alkyl side chains, while in PDEGMA gel with multiple coordination sites for Li+ showed lower t Li+ because Li+ ions were trapped by the ether moiety of the side chain. On the other hand, PTFPMA gel with partially fluorinated propyl side chains showed a higher t Li+ . The self-diffusion coefficients of Li+, TFSA−, and SL in PTFPMA gel measured with pulsed field gradient (PFG) NMR suggested lower diffusivity of TFSA− compared to that in PPMA gel. Moreover, 19F NMR revealed that the signal derived from TFSA− shifted to downfield side in PPMA gel and upfield shift in PTFPMA gel compared to that in the liquid electrolyte of [LiTFSA]/[SL] =1:3, indicating a change of solvation structure due to the polymer side chain. Finally, we demonstrated rate capability tests for gel electrolytes with LiCoO2/Li cells and found that the cell with PTFPMA gel having high t Li+ exhibited better rate performance than one with PPMA gel. Acknowledgements This study was partially supported by Japan Science and Technology Agency (JST) GteX Program (Grant No. JPMJGX23S0) and JSPS KAKENHI (Grant No. 22H00340) from the Japan Society for the Promotion of Science (JSPS). References Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423. Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi and M. Watanabe, Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes, J. Phys. Chem. B, 2018, 122, 10736–10745. Tasaki, Y. Ugata, K. Hashimoto, H. Kokubo, K. Ueno, M. Watanabe and K. Dokko, Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers, Phys. Chem. Chem. Phys., 2023, 25, 17793–17797.

High-concentration Li salt/sulfone solutions have attracted attention as promising liquid electrolytes for Li batteries owing to their high oxidative stability, nonflammability, and high Li+ ion transference number (t Li+). Herein, we report the temperature-dependent electrolyte properties of a sulfone-based ternary mixture composed of LiN(SO2F)2, sulfolane, and dimethyl sulfone, which enables Li batteries to operate in a wide temperature range. At -20 degrees C, the rate capability of a Li/LiCoO2 cell with the sulfone-based electrolyte was comparable to that with a conventional carbonate-based electrolyte, even though the ionic conductivity of the electrolyte was significantly lower in the former case (0.11 versus 2.92 mS cm-1). This is because the former electrolyte has a higher t Li+ value, effectively suppressing the concentration overpotential during cell charging and discharging. Moreover, the vapor pressure was much lower for the sulfone-based electrolyte than for the carbonate-based one, and the Li/LiCoO2 cell with the former electrolyte was successfully operated at 60 degrees C. This study provides insights into the characteristics of high-concentration electrolytes that affect the temperature dependence of Li battery performance.

The charge transfer resistances (R ct) of Li+ intercalation/deintercalation at the electrodes in lithium-ion batteries (LIBs) are affected by several factors. Decreasing R ct is essential to improve the power density of LIBs. Desolvation of Li+ is the rate-determining step of the interfacial charge transfer reaction in electrolytes containing 1 mol dm− 3 of Li salts.1 Furthermore, decreasing viscosity (η) of electrolytes decreases R ct.2 The high-concentration electrolytes (HCEs) exhibit high η and the different liquid structure from that of 1 mol dm− 3. In this work, we investigated the charge transfer kinetics at the intercalation electrode/HCE interface. Here, we use lithium bis(fluorosulfonyl)amide (LiFSA)/monoglyme (G1)/1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) ternary mixtures as electrolytes. The molar ratio of LiFSA and G1 in the ternary electrolytes was set to be 1:2 and the amount of HFE was altered. Adding HFE into HCE of LiFSA/G1 = 1:2 can decrease η while HFE is not involved in the coordination of Li+ due to its weekly coordinating ability.3 Such electrolytes are called the localized high-concentration electrolytes (LHCEs). We evaluated R ct at LiMn2O4 thin-film electrode (LMO)/LHCE interface to reveal the effects of liquid structure and η on the charge transfer kinetics. If the Sumi-Marcus theory is valid for Li+ intercalation reaction, R ct is proportional to dielectric longitudinal relaxation time of solvent (τ L).4 τ L is roughly proportional to η, therefore, R ct is assumed to be proportional to η. However, the salt concentration dependence of R ct in the LHCE showed the minimum value at around 2.5 mol dm− 3 while η continuously decreases with increasing the salt concentration. Therefore, the electrode/LHCE interfacial charge transfer kinetics is not solely dominated by η. To estimate the electrochemically reactive area of the LHCE/electrode interface, we evaluated electric double layer capacitance (C dl) by electrochemical impedance spectroscopy. C dl decreased gradually at salt concentration lower than 2.5 mol dm− 3, indicating that the electrochemically reactive area decreases at lower than 2.5 mol dm− 3. As aforementioned, HFE is not involved in the coordination of Li+, and the liquid structure of LHCE consists of high-concentration electrolyte domain and HFE domain.5 This domain structure is assumed to be dynamic, however, the volume fraction of HFE domain becomes larger in LHCE with increasing the HFE (decreasing the salt concentration). In summary, η of the electrolyte decreases with increasing HFE concentration, which enhance the interfacial kinetics, however, the electrochemically reactive area also decreases with high volume fraction of HFE, resulting in the minimum value of R ct at 2.5 mol dm− 3. Acknowledgements This study was partially supported by JSPS KAKENHI (Grant Numbers 22H00340 and 23K17370) from the Japan Society for the Promotion of Science (JSPS). References (1) T. Abe et al., J. Electrochem. Soc. 151, A1120 (2004). (2) Y. Uchimoto et al., Solid State Ionics, 176, 2377 (2005). (3) K. Dokko et al., J. Electrochem. Soc. 160, A1304 (2013). (4) H. Sumi and R. A. Marcus, J. Chem. Phys., 84, 4894–4914 (1986). (5) S. Lin et al., ACS Appl. Mater. Interfaces., 12, 33710–33718 (2020).

Abstract The demand for innovative batteries with high specific energy densities has increased. Li‐metal batteries employing Li‐metal anodes, regarded as the ultimate anodes with a high theoretical capacity, have been extensively studied over the past few decades. However, the poor reversibility and safety concerns regarding Li‐metal anodes remain unresolved. The importance of the electrode/electrolyte interface, especially the solid electrolyte interphase (SEI), for achieving reversibility of Li metal anodes has been extensively studied. Herein, we focused on the impact of the Li ion transport properties in oligoether (glyme)‐based electrolytes on the deposition/dissolution efficiency of Li metal anodes. Analysis of the low‐frequency impedance spectra of Li‐plated Cu/Li cells revealed that the diffusion resistance of Li ions (Rdiffusion) may be a dominant contributor to the internal resistance of the cells employing glyme‐based electrolytes. A higher Rdiffusion in poor‐mass‐transport electrolytes with a lower Li ion transference number resulted in larger polarization during Li deposition/dissolution, leading to more pronounced unfavorable side reactions and lower Coulombic efficiency. Rdiffusion rather than interfacial resistance affected the reversibility of the Li metal anode. Enhancing the Li ion mass transport ability of electrolytes is important for achieving highly reversible charge‐discharge performance of Li metal anodes at high current densities.

Li metal batteries have attracted much attention as the next-generation rechargeable batteries owing to the exceptionally high theoretical capacity (3860 mAh g⁻¹) and the lowest electrode potential of Li metal electrode. Molten Li salt solvate electrolytes or highly concentrated electrolytes (HCEs) are an emerging class of electrolytes having a similarity to ionic liquids (ILs) in high ionic nature, and indeed some of which are found to behave like an IL.1 This type of the electrolytes have attracted considerable attention as prospective candidates for electrolyte materials in future Li metal batteries owing to their favorable properties both in the bulk and at the electrochemical interface. In this study, linear ether-based molten Li salt solvates were investigated for potential effects of weakly coordinating linear ethers on transport properties and battery performance. We focused on the electrolytes using lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or lithium bis(fluorosulfonyl)amide (LiFSA) as the salt and linear chain monoethers such as methylpropyl ether (MPE), n-butylmethyl ether (BME), and ethyl propyl ether (EPE) as the solvent for lithium-sulfur and lithium metal batteries. Owing to the low lithium polysulfide solubility, low viscosity, relatively high ionic conductivity, high Li ion transference number, high reductive stability, and low specific gravity, all of which are favorable for achieving high-energy density batteries, linear ether-based molten Li salt solvates were found to be effective electrolytes. The correlation between Li ion solvation and ionic transport properties of the linear ethers with different alkyl chain length were further studied to optimize the electrolyte compositions. The chemical structure of the ethers had a strong impact on the Li ion coordination structure and ionic conductivity: the monomethyl ether such as MPE and BME showed more-pronounced Li ion coordination and higher ionic conductivity whereas steric hinderance of longer alkyl chains in ethers with longer alkyl chain length such as ethyl propyl ether (EPE) resulted in lower solvation number, enhanced ion pairing and lower ionic conductivity. The improved Li ion transport property of the linear ether-based electrolytes led to the better rate performance. Furthermore, a pouch-type cell demonstrated an energy density exceeding 300 Wh kg-1 under lean electrolyte conditions.2 Shigenobu, T. Sudoh, J. Murai, K. Dokko, M. Watanabe, K. Ueno, Chem. Rec., 2023, 23, e202200301. Ishikawa, S. Haga, K. Shigenobu, T. Sudoh, S. Tsuzuki, W. Shinoda, K. Dokko, M. Watanabe, K. Ueno, Faraday Discuss., 2024, Accepted Manuscript. DOI: 10.1039/D4FD00024B