Certain stoichiometric mixtures of molecular solvents (ligands) and Li salts are known to yield low melting Li salt solvates. The molten state of Li salt solvates (or highly concentrated electrolyte) has become an important class of electrolytes for energy storage devices owing to their favourable properties: strong solvent binding to Li ions at extreme salt concentrations improves the thermal and electrochemical stabilities of the electrolytes, and a high concentration of Li ions allows fast electrode reactions in batteries. We report two types of Li salt solvate electrolytes that show unusual Li ion transport behavior: Li-glyme and Li-sulfone systems. In the equimolar mixtures of triglyme (G3) or tetraglyme (G4) and LiTFSA, self-diffusion coefficients of Li ion and glymes measured by PFG NMR were found to be identical. This suggests that Li-glyme complex cation remains stable in the molten state. The long-lived complex cation was responsible for ionic liquid-like behavior of the molten Li-glyme solvates. In molten solvates of Li salt and sulfolane (SL), diffusivity of Li ions was the highest among the diffusion species (Li, counter anions, and SL), suggesting that hopping-like diffusion is dominant for Li ions. The unique Li ion transport behavior is discussed in relation to Li ion solvation structure in these molten Li salt solvate electrolytes. Moreover, we show how Li ion solvation and transport link to the electrolyte properties of the molten Li salt solvates and the battery performance of Li secondary batteries.

Solvation structure and Li ion transport in molten Li salt solvate electrolytes

2019 Spring National Meeting

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