Speciation Analysis and Thermodynamic Criteria of Solvated Ionic Liquids: Ionic Liquids or Superconcentrated Solutions?
Nana Arai, Hikari Watanabe, Erika Nozaki, Shiro Seki, Seiji Tsuzuki, Kazuhide Ueno, Kaoru Dokko, Masayoshi Watanabe, Yasuo Kameda, Yasuhiro Umebayashi
Abstract
Lithium–glyme solvated ionic liquids (Li–G SILs) and superconcentrated electrolyte solutions (SCESs) are expected to be promising electrolytes for next-generation lithium secondary batteries. The former consists of only the oligoether glyme solvated lithium ion and its counteranion, 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 to each other, it is still unclear that both should be room-temperature ionic liquids. To distinctly define them, speciation analyses were performed with the Li–G SIL and the aqueous SCES to evaluate the free solvent concentration in these solutions with a new Raman/infrared spectral analysis technique called complementary least-squares analysis. Furthermore, from a thermodynamic point of view, we investigated the solvent activity and activity coefficient in the gas phase equilibrated with sample solutions and found they can be good criteria for SILs.