Glyme-based Solvate Ionic Liquids and Their Electrolyte Properties

Certain concentrated mixtures of lithium salt and solvent (ligand) are no longer simple solutions but are categorized as solvate ionic liquids (SILs), wherein the solvent strongly coordinates to the cation to form a solvate. A negligible amount of free solvent remains, thus constituting the SIL as the solvate cation and the anion. Typical examples include one-to-one mixtures of lithium bis(trifluoromethane sulfonyl)amide (Li[TFSA]) and certain glymes (CH3O–(CH2CH2–O)n–CH3). The successful formation of a SIL greatly depends on both the ligand and anion structures. To obtain robust and long-lived solvates, a ligand exhibiting a chelate effect is essential, with n = 3 and 4 (i.e., triglyme and tetraglyme) being suitable for lithium solvate formation. The weak Lewis basicity of the anions promotes SIL formation, as the ligand–Li+ interaction predominates over the Li+–anion interaction. SILs exhibit unique features as electrolytes, including enhanced oxidation stability of glyme components, unique Li+ transport via ligand exchange during interfacial electrochemical reactions, poor solubility towards ionic electroactive materials, and electrochemical graphite intercalation reactions. Additionally, new polymer electrolytes containing SILs have been proposed, enabling film processability, high ionic conductivity, thermal stability, and a wide potential window simultaneously.