Thermal, Physical, and Electrochemical Properties of Li[N(SO2F)2]‑[1-Ethyl-3-methylimidazolium][N(SO2F)2] Ionic Liquid Electrolytes for Li Secondary Batteries Operated at Room and Intermediate Temperatures

The Li­[FSA]-[C2C1im]­[FSA] (FSA–: bis­(fluorosulfonyl)­amide and C2C1im+: 1-ethyl-3-methylimidazolium) ionic liquids have been studied as electrolytes for Li secondary batteries, though their thermal, physical, and electrochemical properties have not been systematically characterized. In this study, the thermal and transport properties of Li­[FSA]-[C2C1im]­[FSA] ionic liquids as a function of the Li­[FSA] molar fraction and temperature, in view of their operation at both room and intermediate temperatures. Differential scanning calorimetric analysis revealed that this system has a wide liquid-phase temperature range from Li­[FSA] fractions of 0.0 to 0.4 and indicated the existence of the Li­[C2C1im]­[FSA]2 line compound. Single-crystal X-ray diffraction analysis was used to determine the crystal structure of Li­[C2C1im]­[FSA]2, which consists of Li+ octahedrally coordinated by six O atoms originating from four FSA– anions. The temperature dependences of the viscosity and ionic conductivity were fitted by the Vogel–Tammann–Fulcher equation, and the viscosity and molar ionic conductivity were connected by the fractional Walden rule. Lithium–metal deposition/dissolution efficiency decreased with increasing measurement temperature and decreasing Li­[FSA] fraction. Aluminum corrosion at positive potentials was investigated by a potential step method, which revealed that the stability of an aluminum electrode was improved at high Li­[FSA] fractions at 298 K, and the corrosion-limit potential decreased at elevated temperatures.