Redox active glyme-li salt solvate ionic liquids based on tetrabromoferrate(III)

Abstract

An equimolar mixture of oligoether solvents, glymes, and Li salt yields a new subclass of ionic liquids (ILs), referred to as solvate ILs, when using an appropriate combination of glyme and Li salt. In this paper a triglyme-Li tetrabromoferrate complex [Li(G3)][FeBr4] is prepared by combining a crystalline glyme-Li salt complex, [Li(G3)]Br, and an iron(III) bromide Lewis acid, FeBr3, to demonstrate a solvate IL comprised entirely of complex ions. [Li(G3)][FeBr4] is characterized in terms of thermal properties, coordination structure, and transport properties. This study reveals that the melting point of [Li(G3)][FeBr4] is lower than 100°C and that highly dissociated complex ions, [Li(G3)]+ and [FeBr4]-, are present both in the molten state and in polar acetonitrile solutions. [Li(G3)][FeBr4] is thus deemed a good solvate IL. The reversible redox property of the [FeBr4]-/[FeBr4]2- couple is exploited for the catholytes of rechargeable lithium batteries. The catholyte based on this redox active solvate IL exhibits a highly reversible charge-discharge behavior.