Categorizing molten salt complexes as ionic liquids and their applications to battery electrolytes

Abstract

Certain stoichiometric mixtures of salts and solvents (ligands) yield a salt complex. Salt complexes that are low melting and consist of discrete complex ions and their counter ions in the molten state can together form the essence of an ionic liquid (IL). This stable complex melt can now be categorized into a new subclass of ionic liquids, "solvate (or chelate) ILs." In this paper, we describe the current criteria for this new family of ILs. Concentrated mixtures of lithium salts and oligoether solvents are useful models for these solvate ionic liquids; the effects of the ion-ion and ion-solvent interactions on their structure and properties were investigated to contrast with classical concentrated electrolyte solutions. The performance of solvate ILs as electrolytes for lithium-sulfur (Li-S) batteries is also reported. The dissolution of lithium polysulfides (i.e., reaction intermediates of the sulfur cathode) into the electrolyte, which is a serious issue for the practical application of Li-S cells, was greatly suppressed in the solvate ILs. Therefore, a stable charge-discharge with high Coulombic efficiency was achieved with the solvate IL electrolytes.