Glyme–Lithium Salt Equimolar Molten Mixtures: Concentrated Solutions or Solvate Ionic Liquids?

TLDR

The authors introduce a new family of solvate ionic liquids by investigating the thermal, transport, electrochemical, Lewis basicity, and ionicity properties of equimolar mixtures of glymes (G3, G4) with nine lithium salts, proposing that this concept can be extended to many other metal–ligand combinations. The study characterizes equimolar glyme–lithium salt mixtures, showing that the liquid regime—ordinary concentrated solution or solvate IL—depends on competitive interactions among glymes, Li⁺ cations, and the counteranions. They find that mixtures with weakly Lewis basic anions form long‑lived [Li(glyme)]⁺ complexes, behaving as typical ionic liquids, and that these solvate ILs exhibit high ionicity, lithium transference number, Li⁺ concentration, and oxidative stability, making them promising lithium‑conducting electrolytes.

Abstract

To demonstrate a new family of ionic liquids (ILs), i.e., "solvate" ionic liquids, the properties (thermal, transport, and electrochemical properties, Lewis basicity, and ionicity) of equimolar molten mixtures of glymes (triglyme (G3) and tetraglyme (G4)) and nine different lithium salts (LiX) were investigated. By exploring the anion-dependent properties and comparing them with the reported data on common aprotic ILs, two different classes of liquid regimes, i.e., ordinary concentrated solutions and "solvate" ILs, were found in the glyme-Li salt equimolar mixtures ([Li(glyme)]X) depending on the anionic structures. The class a given [Li(glyme)]X belonged to was governed by competitive interactions between the glymes and Li cations and between the counteranions (X) and Li cations. [Li(glyme)]X with weakly Lewis basic anions can form long-lived [Li(glyme)](+) complex cations. Thus, they behaved as typical ionic liquids. The lithium "solvate" ILs based on [Li(glyme)]X have many desirable properties for lithium-conducting electrolytes, including high ionicity, a high lithium transference number, high Li cation concentration, and high oxidative stability, in addition to the common properties of ionic liquids. The concept of "solvate" ionic liquids can be utilized in an unlimited number of combinations of other metal salts and ligands, and will thus open a new field of research on ionic liquids.