Solvent Effect of Room Temperature Ionic Liquids on Electrochemical Reactions in Lithium-Sulfur Batteries

A room temperature ionic liquid (RTIL), N,N-diethyl-Nmethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide aDEMENTESAD, was used as an electrolyte solvent for lithium-sulfur (Li-S) batteries. Li[TFSA] was dissolved into [DEME] [TESA] to prepare the electrolytes, and a molecular solvent tetraethylene glycol dimethyl ether (TEGDME) was used for Li[TESA] as a reference. Discharge-charge tests of Li-S cells using these electrolytes were carried out. The discharge-charge cycle stability and Coulombic efficiency of a cell with an RTIL electrolyte were found to be surprisingly superior to those of a cell with TEGDME electrolyte. The poor cycle stability of the cell with the TEGDME electrolyte was attributed to the dissolution of lithium polysulfides (Li2Sm), which were generated as reaction intermediates through a redox process at the S cathode in the Li-S cell. RTIL has low donor ability owing to the weak Lewis basicity of [TESA](-) anion, whereas conventional ether-based molecular solvents such as TEGDME have high donor ability. The dissolution of Li2Sm was significantly suppressed owing to the weak donor ability of RTIL. In the RTIL electrolyte, Li2Sm was immobilized on the electrode, and the electrochemical reaction of the S species occurred exdusively in the solid phase. These results clearly prove a novel solvent effect of RTILs on the electrochemical reactions of the S cathode in Li-S cells.