Solvate Ionic Liquids for Li, Na, K, and Mg Batteries

From the viewpoint of element strategy, non‐Li batteries with promising negative and positive electrodes have been widely studied to support a sustainable society. To develop non‐Li batteries having high energy density, research on electrolyte materials is pivotal. Solvate ionic liquids (SILs) are an emerging class of electrolytes possessing somewhat superior properties for battery applications compared to conventional ionic liquid electrolytes. In this account, we describe our recent efforts regarding SIL‐based electrolytes for Li, Na, K, and Mg batteries with respect to structural, physicochemical, and electrochemical characteristics. Systematic studies based on crystallography and Raman spectroscopy combined with thermal/electrochemical stability analysis showed that the balance of competitive cation−anion and cation−solvent interactions predominates the stability of the solvate cations. We also demonstrated battery applications of SILs as electrolytes for non‐Li batteries, particularly for Na batteries. Solvate ionic liquids particularly those incorporating cost‐effective earth‐abundant non‐Li metal ions such as Na+, K+, and Mg2+, are promising electrolytes to realize safe and highly efficient large‐scale electrochemical energy storage systems. In this account, the fundamental aspects and characteristics for electrolyte applications of SILs consisting of glymes and non‐Li salts are reported. The balance of competitive interactions of anions and ligands with metal ions was found to predominate the stability of the solvate cations. Suitable combinations of salts, ligands, and diluents resulted in electrolytes showing excellent stability and remarkable battery performance.