Li-Ion Transport and Solution Structure in Sulfolane-Based Localized High-Concentration Electrolytes

Localized high-concentration electrolytes (LHCEs), whichare mixturesof highly concentrated electrolytes (HCEs) and non-coordinating diluents,have attracted significant interest as promising liquid electrolytesfor next-generation Li secondary batteries, owing to their variousbeneficial properties both in the bulk and at the electrode/electrolyteinterface. We previously reported that the large Li+-iontransference number in sulfolane (SL)-based HCEs, attributed to theunique exchange/hopping-like Li+-ion conduction, decreasedupon dilution with the non-coordinating hydrofluoroether (HFE) despitethe retention of the local Li+-ion coordination structure.Therefore, in this study, we investigated the effects of HFE dilutionon the Li+ transference number and the solution structureof SL-based LHCEs via the analysis of dynamic ion correlations andmolecular dynamics simulations. The addition of HFE caused nano-segregationin the SL-based LHCEs to afford polar and nonpolar domains and fragmentationof the polar ion-conducting pathway into smaller clusters with increasingHFE content. Analysis of the dynamic ion correlations revealed thatthe anti-correlated Li+-Li+ motions weremore pronounced upon HFE addition, suggesting that the Li+ exchange/hopping conduction is obstructed by the non-ion-conductingHFE-rich domains. Thus, the HFE addition affects the entire solutionstructure and ion transport without significantly affecting the localLi(+)-ion coordination structure. Further studies on iontransport in LHCEs would help obtain a design principle for liquidelectrolytes with high ionic conductivity and large Li+-ion transference numbers.