Glyme-Li Salt Solvate Ionic Liquids for Advanced Lithium Batteries

Certain molten complexes of Li salts and solvents can be regarded as ionic liquids. In this study, the local structure of Li + ions in equimolar mixtures ([Li(glyme)]X) of glymes (G3:triglyme and G4: tetraglyme) and different Li salts was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectroscopic analysis allowed us to estimate the fraction of the free glyme in [Li(glyme)]X. The amount of free glyme was estimated to be a few percent in [Li(glyme)]X with perfluorosulfonylamide-type anions, and thereby could be regarded as solvate ionic liquids. Other equimolar mixtures of [Li(glyme)]X were found to contain a considerable amount of free glyme, and they were categorized as traditional concentrated solutions. The activity of Li + in the glyme-Li salt mixtures was also evaluated by measuring the electrode potential of Li/Li + as a function of concentration. At a higher concentration of Li salt, the amount of free glyme diminished in the solvate ionic liquids, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the solvation of Li + by the glyme forms stable and discrete solvate ions ([Li(glyme)] + ) in the solvate ionic liquids. This anomalous Li + solvation had a great impact on the electrolyte properties and electrode reactions, which enhanced the utility of the solvate ionic liquids in advanced lithium batteries such as Li-S batteries.