(Invited) Solvate Ionic Liquids and Their Polymer Electrolytes: Possible Beyond LIB Electrolytes
Authors: Watanabe Masayoshi 
Published: December 2020

Certain molten solvates of Li salts can be regarded as solvate ionic liquids (SILs). A typical example is equimolar mixtures of glymes (G3: triglyme and G4: tetraglyme) and Li[TFSA]([TFSA]=[NTf2]) ([Li(glyme)][TFSA]). The amount of free glyme is a trace in [Li(glyme)][TFSA], and thereby can be regarded as solvate ionic liquids. Unlike conventional electrolytes, the solvation of Li+ by the glyme forms stable and discrete solvate ions ([Li(glyme)]+) in the solvate ionic liquids. The electrochemical oxidation of glyme in [Li(glyme)][TFSA] is greatly enhanced due to the donation of lone pairs of ether oxygen atoms to the Li+ cation, resulting in the HOMO energy level lowering of a glyme molecule. This anomalous Li+ solvation induces interesting transport properties when interfacial electrochemical reactions proceed, which is not transport of solvated ions based on Stokes’ law but a ligand (solvent)-exchange transport. Li+-intercalated graphite was electrochemically formed in [Li(G3)1][TFSA]. In contrast, the cointercalation of G3 and Li+ (intercalation of solvate [Li(G3)1]+ cation) into graphite occurred in [Li(G3)
x
][TFSA] electrolytes containing excess G3 (x > 1). In the solvate ionic liquid, the activity of the free solvent is very low, which would make the solvate ion unstable and the desolvation possible at the interface. Another intriguing aspect of the solvate ionic liquids is unusual solubility, which leads to the stable operation of the Li–S battery due to very low solubility of the discharge products (Li2S
x
). The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the discharge reaction of the S cathode. In the SILs, [Li(glyme)][TFSA], both cations and anions are weakly coordinating ions with low Lewis acidity and basicity, respectively. The [Li(glyme)][TFSA] molten complexes do not readily dissolve other ionic solutes due to the weak coordinating nature of the cation and anion, which leads to the stable operation of the Li–S battery.

Furthermore, polymer electrolytes composed of ABA-triblock copolymers and [Li(glyme)][TFSA] SILs are proposed to simultaneously achieve high ionic conductivity, thermal stability, and a wide potential window. Different block copolymers, consisting of a SIL-incompatible A segment (polystyrene, PSt) and SIL-compatible B segments (poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO)) are utilized. The SILs can be solidified with the copolymers through physical crosslinking by the self-assembly of the PSt segment. The thermal and electrochemical properties of the polymer electrolytes are significantly affected by the stability of the [Li(glyme)]+ complex in the block copolymer B segments, and the preservation of the SILs contributes to their thermal stabilities and oxidation stabilities greater than 4 V vs. Li/Li+. The [Li(glyme)]+ complex cation is unstable in the PEO matrix, whereas the complex structure of [Li(glyme)]+ is stable in the PMMA-based polymer electrolyte. Intriguing points of the PMMA-based polymer electrolytes are decoupled ion transport from segmental motion of the matrix polymer. By using the PMMA-based polymer electrolytes, a 4-V class Li batteries with a LiCoO2 cathode and a Li metal anode can be stably operated; in contrast, this is not possible using the PEO-based electrolyte.

Recently, we find that Li+ hopping conduction, which cannot be explained by conventional Stokes law, emerges in certain highly concentrated molten solvate electrolytes. Li+ diffuses faster than the solvent and anion, and thus the evolution of Li+ hopping conduction is confirmed, which leads to a higher Li+ transference number. Possible application of these new electrolytes will also be discussed.