Polymer Electrolytes Containing Solvate Ionic Liquid and Beyond

Certain molten solvates of Li salts can be regarded as solvate ionic liquids (SILs). A typical example is equimolar mixtures of glymes and Li[TFSA]([TFSA]=[NTf₂]). The amount (activity) of free glyme is a trace in [Li(glyme)][TFSA], and thereby can be regarded as solvate ionic liquids. Unlike conventional electrolytes, the solvation of Li⁺ by the glyme forms stable and discrete solvate ions ([Li(glyme)]⁺) in the SILs. Polymer electrolytes composed of ABA-triblock copolymers and [Li(glyme)][TFSA] SILs are proposed to simultaneously achieve high ionic conductivity, thermal stability, and a wide potential window. Intriguing points of the polymer electrolytes are decoupled ion transport from segmental motion of the matrix polymer and the persistence of solvate structure in the polymeric phase. Recently, we find that Li⁺ hopping conduction, which cannot be explained by conventional Stokes law, emerges in certain highly concentrated molten solvate electrolytes. Li⁺ diffuses faster than the solvent and anion, and thus the evolution of Li⁺ hopping conduction is confirmed, which leads to a higher Li⁺ transference number. Possible application of these new electrolytes will be discussed.