Competing characters of Li+-Glyme complex in a solvate ionic liquid: High stability in the bulk and rapid desolvation on an electrode surface

Abstract

Solvate ionic liquids (SILs) composed of glymes and Li salts are potential electrolyte materials for Li ion batteries. Safety and durability of SIL-based batteries can be realized by competing characters of solvate cations. Solvate structure of Li+ should be stable in the bulk to be nonvolatile, and its desolvation should occur rapidly on electrode surfaces which is considered as a rate-limiting step in charging or discharging. To investigate such characters, potential-dependent interfacial structure of [Li(G4)][TFSA], equimolar mixture of tetraglyme and lithium bis(trifluoromethanesulfonyl)amide, on an Au electrode has been investigated at the molecular scale by using surface-enhanced infrared absorption spectroscopy. Despite discrete solvate ion [Li(G4)]+ is stable in the bulk, partial desolvation of Li+ on the electrode was observed at around the potential of zero charge, much more positive than Li+ reduction reactions. The driving force of the partial desolvation can be ascribed to the breaking of isotropic dielectric environment in the vicinity of the electrode, where image charge induced on the electrode surface stabilizes the partially desolvated structure enabling shorter Li+-electrode distance.