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Certain highly concentrated electrolytes (HCEs) enhance the charge-transfer reaction rate at the electrode/electrolyte interface in Li-ion batteries. The solvation structure of Li+ in HCEs significantly affects the electrochemical interfacial reaction kinetics. However, the effects of different anions on these kinetics have not yet been fully understood. Therefore, in this study, we investigated the effects of anionic species on the liquid structure and electrochemical reactions of HCEs composed of various Li salts and propylene carbonate (PC). Raman spectra revealed that both PC and anions were coordinated to Li+ ions in HCEs and that the concentration of uncoordinated (free) PC changed depending on the anionic species. Consequently, the activity of Li+ in the electrolyte changed depending on the anionic species. The use of Li salts with weakly Lewis basic anions increased the activity of Li+ and decreased the concentration of free PC in HCEs. The activity of Li+ in the electrolyte significantly affected the Li+ intercalation/deintercalation reaction rate of the LiCoO2 thin-film electrode. Electrochemical impedance spectroscopy revealed that the interfacial reaction rate of LiCoO2 was enhanced in the HCEs with anions having weaker Lewis basicity owing to the higher Li+ ion activity.

Electrochemical separation methods utilizing quinones as CO2 capture agents have gained considerable interest owing to their low-energy requirements for near-isothermal CO2 separation. However, the low solubility of quinones in nonaqueous solvents limits their total CO2 carrying capacity in the system. In this study, we synthesized an ionically modified anthraquinone, 1-butyl-3-((2′-anthraquinoyl)methyl)imidazolium bis(trifluoromethanesulfonyl)amide ([BAQMIM][TFSA]), which exhibited a 360-fold increase in solubility compared to pristine anthraquinone in dimethyl sulfoxide. The synthesized ionic quinone derivative can also serve as a supporting electrolyte, offering both high ionic conductivity (12.2 mS cm–1) and quinone concentration (200 mM). Constant-potential electrolysis demonstrated a high CO2 carrying capacity in a highly concentrated [BAQMIM][TFSA] solution. The predominantly reversible CO2 capture and subsequent release processes were successfully monitored through in situ spectroscopic analysis and density functional theory calculations. These findings provide a promising approach toward large-scale volumetric CO2 separation through the functionalization of quinones.

Localized high-concentration electrolytes (LHCEs), which are mixtures of highly concentrated electrolytes (HCEs) and non-coordinating diluents, have attracted significant interest as promising liquid electrolytes for next-generation Li secondary batteries, owing to their various beneficial properties both in the bulk and at the electrode/electrolyte interface. We previously reported that the large Li+-ion transference number in sulfolane (SL)-based HCEs, attributed to the unique exchange/hopping-like Li+-ion conduction, decreased upon dilution with the non-coordinating hydrofluoroether (HFE) despite the retention of the local Li+-ion coordination structure. Therefore, in this study, we investigated the effects of HFE dilution on the Li+ transference number and the solution structure of SL-based LHCEs via the analysis of dynamic ion correlations and molecular dynamics simulations. The addition of HFE caused nano-segregation in the SL-based LHCEs to afford polar and nonpolar domains and fragmentation of the polar ion-conducting pathway into smaller clusters with increasing HFE content. Analysis of the dynamic ion correlations revealed that the anti-correlated Li+?Li+ motions were more pronounced upon HFE addition, suggesting that the Li+ exchange/hopping conduction is obstructed by the non-ion-conducting HFE-rich domains. Thus, the HFE addition affects the entire solution structure and ion transport without significantly affecting the local Li+-ion coordination structure. Further studies on ion transport in LHCEs would help obtain a design principle for liquid electrolytes with high ionic conductivity and large Li+-ion transference numbers.

Sulfones are polar molecules that can be used as thermally stable electrolyte solvents for Li-ion batteries (LIBs). Li salts form stoichiometric solvates with sulfones in the electrolytes. The melting points of the solvates tend to be higher than room temperature, thereby limiting the operating temperature range of the batteries. In this study, the applicability of ternary eutectic mixtures of LiN(SO2F)2 (LiFSA), sulfolane (SL), and dimethyl sulfone (DMS) as LIB electrolytes was assessed. Relative to the binary LiFSA–sulfone electrolytes, the ternary eutectic electrolytes remained liquid over a wide temperature range due to the increased entropy of mixing. Sulfone-bridged Li+–sulfone–Li+ and anion-bridged Li+–FSA––Li+ network structures were formed in the eutectic electrolyte with a composition of [LiFSA]/[SL]/[DMS] = 1/1.5/1.5. Pulsed-field gradient NMR measurements revealed that the Li+ ion dynamically exchanges sulfones and anions and diffuses more rapidly than these ligands, resulting in the relatively high Li+ transference number of the electrolyte. Highly reversible charge–discharge processes of the LiCoO2 and graphite electrodes were attained using the ternary eutectic electrolyte. The rate capability of the Li/LiCoO2 cell in the eutectic electrolyte was comparable to that of the cell in the conventional 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate solution despite its lower ionic conductivity.

Electrolytes with a high Li-ion transference number (tLi) have attracted significant attention for the improvement of the rapid charge–discharge performance of Li-ion batteries (LIBs). Nonaqueous polyelectrolyte solutions exhibit high tLi upon immobilization of the anion on a polymer backbone. However, the transport properties and Li-ion solvation in these media are not fully understood. Here, we investigated the Li salt of a weakly coordinating polyanion, poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)amide] (poly(LiSTFSA)), in various ethylene carbonate and dimethyl carbonate mixtures. The highest ionic conductivity was unexpectedly observed for the lowest polar mixture at the highest salt concentration despite the low dissociation degree of poly(LiSTFSA). This was attributed to a unique conduction phenomenon resulting from the faster diffusion of transiently solvated Li ions along the interconnected aggregates of polyanion chains. A Li/LiFePO4 cell using such an electrolyte demonstrated improved rate capability. These results provide insights into a design strategy of nonaqueous liquid electrolytes for LIBs.

Understanding the ion-conduction mechanism in concentrated electrolyte solutions of Li salts is helpful in designing electrolytes for advanced Li batteries. In certain highly concentrated electrolytes, the Li+ ion hopping/ligand exchange mechanism contributes to ionic conduction, and the transference number of Li+ becomes higher than 0.5, which mitigates the concentration polarization in Li batteries. However, details of the hopping mechanism remain unclear. In this study, we investigated the temperature dependence of the transport properties of Li salt/sulfolane (SL) electrolytes. Using pulsed-field gradient nuclear magnetic resonance spectroscopy, we found that Li+ ion diffuses faster than SL and anion in highly concentrated electrolytes, suggesting that Li+-ion-hopping conduction occurs. The apparent activation energy (Ea) of the diffusivity of Li+ in highly concentrated electrolytes is slightly lower than Ea of the fluidity of liquid, which indicates that Li+-ion conduction is partially decoupled from the viscosity. The ratio of the actual molar conductivity to the calculated one based on the Nernst–Einstein relationship increases with decrease in temperature, leading to a lower Ea for ionic conduction compared to that of the fluidity and the ion diffusivity.