Room-Temperature Ionic Liquid Electrolytes for Alkali Metal-Sulfur Batteries

Abstract

Bis (trifluoromethanesulfonyl) amide (TFSA) based room-temperature ionic liquids (RTILs) were used as electrolytes for lithium-sulfur (Li-S) and sodium-sulfur (Na-S) batteries. The discharge-charge cycle stability and coulombic efficiency of Li-S cells with TFSA-based RTILs were found to be surprisingly superior to those of a cell with a conventional organic electrolyte consisting of tetrahydrofuran (THF) and a Li slat. The poor cycle stability of the cell with the conventional organic electrolyte was attributed to the dissolution of lithium or sodium polysulfides (M2Sm, M＝ alkali metal, 2≤m≤8), which were generated as reaction intermediates through redox processes at the S cathode in the M-S cell. TFSA-based RTILs have low donor ability owing to the weak Lewis basicity of [TFSA] − anion, whereas conventional ether-based molecular solvents, such as THF, have high donor ability. The dissolution of M2 Sm was significantly suppressed owing to the weak donor ability of RTILs.