Imidazolium-based Ionic Liquid Electrolytes with Bis(fluorosulfonyl)imide Anions for Lithium Metal Anodes: Effects of Salt Concentration and Solvent Dilution (Supporting Information)

Ionic liquids (ILs) are attractive electrolytes for Li metal batteries because of their non-volatility, non-flammability, and wide electrochemical stability window. Among them, 1-ethyl-3-methylimidazolium (EMI)-based ILs exhibit high ionic conductivity but suffer from limited reductive stability, and their compatibility with Li metal anodes remains insufficiently explored in the literature. Herein, we report a systematic investigation of binary mixture electrolytes with bis(fluorosulfonyl)imide (FSI) anions, [EMI][FSI]–LiFSI, for Li metal anodes. Increasing the LiFSI concentration improves the Li deposition/dissolution Coulombic efficiency through robust FSI-derived solid electrolyte interphase (SEI) formation and a positive shift in the Li deposition/dissolution potential, which together contribute to the suppression of EMI+ decomposition. Furthermore, solvent dilution reveals contrasting effects: coordinating dimethoxyethane (G1) weakens the Li+–FSI− interactions and reduces the Coulombic efficiency, whereas the non-coordinating hydrofluoroether (HFE) preserves the interfacial stability while lowering the viscosity. These findings establish EMI-based ILs as a promising class of non-flammable electrolytes, providing an optimal design strategy for achieving highly reversible and safe Li metal batteries.