(Keynote) Solvate Ionic Liquids and Their Polymer Electrolytes: A New Approach to Achieve Thermal Stability, High Ionic Conductivity, Unusual Solubility, and a Wide Potential Window
Authors: Watanabe Masayoshi 
Published: February 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 estimated by Raman spectroscopy and MD simulation is a trace in [Li(glyme)]X with perfluorosulfonylamide-type anions such as [TFSA]-, and thereby can be regarded as solvate ionic liquids. The activity of Li+ in the glyme-Li salt mixtures can also be evaluated by measuring EMF of the concentration cells. At a higher concentration of Li salt, the amount of free glyme diminishes in the solvate ionic liquids, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes, the solvation of Li+ by the glyme forms stable and discrete solvate ions ([Li(glyme)]+) in the solvate ionic liquids. This anomalous Li+ solvation has a great impact on the electrolyte properties and electrode reactions, which enhances the utility of the molten solvates in advanced lithium batteries.

An intriguing aspect of the solvate ionic liquids is unusual solubility. 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 redox reaction at 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.

Further, 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+. Pulsed-field gradient spin-echo nuclear magnetic resonance measurements of the polymer electrolytes and molecular dynamics (MD) simulation indicate that the [Li(glyme)]+ complex cation is unstable in the PEO matrix because of the competitive coordination of the PEO chain and glyme with Li+. On the other hand, the complex structure of [Li(glyme)]+ is stable in the PMMA-based polymer electrolytes owing to the weak interaction between Li+ and the polymer chains. 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.