Li+ solvation in glyme–Li salt solvate ionic liquids

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 Li salts (LiX: lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), lithium bis(pentafluoroethanesulfonyl)amide (LiBETI), lithium trifluoromethanesulfonate (LiOTf), LiBF4, LiClO4, LiNO3, and lithium trifluoroacetate (LiTFA)) was investigated to discriminate between solvate ionic liquids and concentrated solutions. Raman spectra and ab initio molecular orbital calculations have shown that the glyme molecules adopt a crown-ether like conformation to form a monomeric Li(glyme)(+) in the molten state. Further, 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, by using concentration cells against a reference electrode. At a higher concentration of Li salt, the amount of free glyme diminishes and affects the electrode reaction, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes (dilute and concentrated solutions), the significantly high electrode potential found in the solvate ILs indicates that the solvation of Li(+) by the glyme forms stable and discrete solvate ions (Li(glyme)(+)) in the molten state. This anomalous Li(+) solvation may have a great impact on the electrode reactions in Li batteries.