Developing New Functionalities of Superconcentrated Electrolytes for Lithium-ion Batteries

Abstract

The nature of electrolyte solutions is dominated by the three factors of Li salts, solvents, and their mixing ratios (salt concentrations). Conventionally, the selections of Li salts and solvents have been considered of prime importance for Li-ion battery electrolytes, while the salt concentrations have been always fixed to approximately 1 mol dm⁻³ based on maximized ionic conductivities. Recently, however, the salt concentrations are increasingly recognized as a key to developing new functionalities for battery electrolytes in the wake of various unusual interfacial/bulk properties discovered in superconcentrated (highly concentrated) electrolytes. For example, highly concentrated electrolytes i) passivate effectively negative electrodes, ii) facilitate rapid Li⁺ intercalation reactions, iii) show high oxidative stabilities, iv) prevent the corrosion of Al current collectors, and v) suppress the dissolution of transition metals from positive electrodes, all of which are beneficial for battery applications. This article discusses those unique functionalities of highly concentrated electrolytes from the viewpoint of their ion-solvent and ion-ion coordination structures.