Ether-Functionalized Tetrahydrothiophenium Ionic Liquids as Electrolytes for Lithium Batteries: Ion Transport, Solvation Structure, and Lithium Redox Behavior

Abstract Ionic liquids have attracted considerable attention as safe electrolytes for next-generation lithium batteries because of their nonflammability and high thermal stability. In this study, an ether-functionalized tetrahydrothiophenium ionic liquid, 1-(2-methoxyethyl)tetrahydrothiophenium bis(trifluoromethanesulfonyl)amide ([Tht2O1][TFSA]), was synthesized and evaluated as a lithium battery electrolyte. Neat [Tht2O1][TFSA] exhibited a high ionic conductivity of 3.2 mS cm–1 at room temperature and maintained favorable ion-transport properties upon the addition of LiTFSA. Raman spectroscopic analysis revealed that Li+ ions are primarily solvated by TFSA– anions, with a Li+ solvation number of 2.2 ± 0.54. Furthermore, the formation of Li+–TFSA– aggregate structures was observed at elevated LiTFSA concentrations. The electrolyte containing 5 mol % LiTFSA exhibited the highest Li transference number (tLi+ = 0.19) and Li-ion conductivity (σLi+ = 0.49 mS cm–1), outperforming previously reported ether-functionalized pyrrolidinium-based ionic liquid electrolytes under comparable low-salt conditions. Cyclic voltammetry and Li/Li symmetric-cell measurements further demonstrated reversible lithium deposition/dissolution and stable lithium plating/stripping behavior. These results indicate that combining ether-functionalized side chains with a tetrahydrothiophenium framework is an effective molecular design strategy for enhancing Li+ transport and provides new insights into the development of safe ionic liquid electrolytes for lithium–metal batteries.