Innovation in the design of electrolyte materials is crucial for realizing next-generation electrochemical energy storage devices such as Li–S batteries. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. Herein, we present simple solvate ionic liquids, glyme–Li salt molten complexes, as excellent electrolyte candidates because they greatly suppress the dissolution of lithium polysulfides. The molten complexes do not readily dissolve other ionic solutes, which leads to the stable operation of the Li–S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-sulfur−1 and with coulombic efficiencies higher than 98% throughout the cycles. Such high performance has not been realized to the best of our knowledge. Furthermore, the addition of a nonflammable fluorinated solvent, which does not break the solvate structure of the glyme–Li salt molten complexes, greatly enhances the power density of the Li–S battery. The strategic design of electrolyte properties provides opportunities for the development of new electrochemical devices with many different electrode materials.

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 (Li2S 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 Li2S m was significantly suppressed owing to the weak donor ability of RTIL. In the RTIL electrolyte, Li2S 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.

Electrochemical reactions of sulfur supported on three-dimensionally ordered macroporous carbon in glyme-Li salt molten complex electrolytes exhibit good reversibility and large capacity based on the mass of sulfur, which suggests that glyme-Li salt molten complexes are suitable electrolytes for Li-S batteries.