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

Abstract

A room temperature ionic liquid (RTIL), N, N -diethyl- N -methyl- N -(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)amide ([DEME][TFSA]), was used as an electrolyte solvent for lithium–sulfur (Li–S) batteries. Li[TFSA] was dissolved into [DEME][TFSA] to prepare the electrolytes, and a molecular solvent—tetraethylene glycol dimethyl ether (TEGDME)—was used for Li[TFSA] 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 (Li 2 S m ), 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 [TFSA] − anion, whereas conventional ether-based molecular solvents such as TEGDME have high donor ability. The dissolution of Li 2 S m was significantly suppressed owing to the weak donor ability of RTIL. In the RTIL electrolyte, Li 2 S m was immobilized on the electrode, and the electrochemical reaction of the S species occurred exclusively 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.