Speciation Analysis and Thermodynamic Criterion of Solvate Ionic Liquids; Whether Ionic Liquids or Super-Concentrated Solutions?

Lithium-glyme solvate ionic liquids (Li-G SILs) and super-concentrated electrolyte solutions (SCESs) are expected as a promising electrolyte for the next generation lithium secondary batteries. The former consists of only the oligoether glyme solvated lithium ion and its counter anion and the latter contains no full solvated Li(+) ion by the solvents due to the extremely high Li salt concentration. Although both of them are similar with each other, it is still unclear that both should be room-temperature ionic liquids (RTILs). To reveal the distinct definition for them, speciation analyses were performed to the Li-G SIL and the aqueous SCES to evaluate the free solvent concentration in these solutions with a new Raman/IR spectral analysis technique called complementary least square analysis (CLSA). Furthermore, from a thermodynamic point of views, we investigated the solvent activity/activity coefficient in the gas phase equilibrated with sample solutions and found they can be a good criterion for SILs.