From Ionic Liquids to Solvate Ionic Liquids: Challenges and Opportunities for Next Generation Battery Electrolytes

Certain concentrated mixtures of lithium salt and solvent ( ligand) are no longer simple solutions, but categorized as solvate ionic liquids (SILs), where the solvent strongly coordinates to the cation to form a solvate, a negligible amount of free solvent remains, and thus the SIL consists of the solvate cation and the anion. Typical examples are mixtures of lithium bis(trifluoromethane sulfonyl) amide (Li[TFSA]) and certain glymes (CH3-O-(CH2-CH2-O)(n)-CH3) The successful formation of a SIL greatly depends on both the ligand and lithium salt structures. To obtain robust and long-lived solvates, a ligand exhibiting a chelate effect is essential and n = 3 and 4 (i.e. triglyme and tetraglyme) are suitable for the formation of lithium solvates. The Lewis basicity of the lithium salt anion also significantly affects the formation of SILs. Specifically, a weak Lewis basicity promotes the formation of a SIL, since the ligand-Li+ interaction overwhelms the Li+-anion interaction. SILs can be diluted with rather low polarity solvents to increase the ionic conductivity, where the solvate structure is maintained even after the dilution. SILs exhibit unique features as electrolytes, including the enhancement of oxidation stability of the component glymes, unique Li+ transport through ligand exchange when interfacial electrochemical reactions are occurring, the inhibition of aluminum corrosion when Al foil is used as a cathode current collector, poorly-solubilizing towards ionic electroactive materials, and electrochemical graphite intercalation reactions. These features greatly enhance the possibility for application of SILs as next generation lithium battery electrolytes. Furthermore, new polymer electrolytes containing SILs have been proposed, simultaneously enabling film-processability, high ionic conductivity, thermal stability, and a wide potential window. Preservation of the solvate structure in the polymeric phases is pivotal to such achievements.