Sulfolane-Based Highly Concentrated Electrolytes of Lithium Bis(trifluoromethanesulfonyl)amide: Ionic Transport, Li-Ion Coordination, and Li–S Battery Performance

Following our recent study demonstrating predominant Li-ion hopping conduction in sulfolane (SL)-based highly concentrated electrolytes with LiBF4, LiClO4, and lithium bis(fluorosulfonyl)amide, herein a systematic study on transport properties and Li-ion coordination of SL-based electrolytes with lithium bis(trifluoromethanesulfonyl)amide was performed. In the highly concentrated region, Li ions clearly diffuse faster than SL and TFSA anions. The two oxygen atoms of the SL sulfonyl group tend to coordinate to two different neighboring Li ions and TFSA anions form ionic clusters with Li ions, verifying the previous observation of the unusual Li-ion conduction and its relevance to the SL- and anion-bridged, chainlike Li-ion coordination structure for the SL-based concentrated systems with other Li salts. Moreover, addition of hydrofluoroether (HFE) to the SL-based concentrated electrolytes greatly enhances diffusion coefficients but fragments the chainlike Li-ion coordination to smaller clusters, leading to a reduced contribution of Li-ion hopping to the overall Li-ion conduction. The SL-based concentrated electrolyte and its mixtures with HFE showed lower lithium polysulfide solubility and higher rate capability for lithium–sulfur (Li–S) cells compared with previously reported tetraglyme-based electrolytes. The SL-based electrolytes were found to manifest a significant improvement in Li-ion mass transfer as a sparingly solvating electrolyte, enabling the solid-state sulfur redox reactions in high-performance Li–S batteries.