Polymer Electrolytes Containing Solvate Ionic Liquids: A New Approach to Achieve High Ionic Conductivity, Thermal Stability, and a Wide Potential Window

We describe here the electrochemical properties and battery performance of polymer electrolytes composed of ABA-triblock copolymers and Li-glyme solvate ionic liquids (SILs), which consist of the [Li(glyme)]⁺ complex cation and bis(trifluoromethanesulfoly)amide ([TFSA]^-) anion, to simultaneously achieve high ionic conductivity, thermal stability, and a wide potential window. Three different block copolymers, consisting of a SIL-incompatible A segment (polystyrene, PSt) and SIL-compatible B segments (poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), and poly(butyl acrylate) (PBA)) were synthesized. The SILs were solidified with the copolymers through physical cross-linking by the self-assembly of the PSt segment. The thermal and electrochemical properties of the polymer electrolytes were significantly affected by the stability of the [Li(glyme)]⁺ complex in the block copolymer B segments, and the preservation of the SILs contributed 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 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- and PBA-based polymer electrolytes because of the weak interaction between Li⁺ and the polymer chains. By use of the PMMA- and PBA-based polymer electrolytes, 4-V class Li batteries with a LiCoO₂ cathode and a Li metal anode could be operated stably at 60 °C; in contrast, this was not possible using the PEO-based electrolyte.