Block Copolymer Solid Electrolytes Based on Comb-Like Poly(ethylene glycol) Plasticized Poly(ionic liquid)s for Lithium-Ion Batteries†

Electrolytes based on poly(ionic liquid)s (PILs) have attracted great attention in the fields of next-generation solid lithium-ion batteries. However, the low ionic conductivity prevents their practical applications. Herein, we report novel solid electrolytes based on block copolymers composed of PILs and comb-like poly(ethylene glycol) (PEG), which were synthesized via ring-opening metathesis polymerization of 3-(2-(bicyclo[2.2.1]hept-5-en-2-yl)ethyl)-1-butyl-1H-imidazol-3-ium bis((trifluoromethyl)sulfonyl) amide and poly(ethylene glycol monomethyl ether) bicyclo[2.2.1]hept-5-ene-2-carboxylate. Comb-like PEG acts as plasticizers in block copolymers dominated by PILs to promote the mobility of PILs segments. Effects of the copolymer composition and length of the comb-like PEG chain on ionic conductivity were investigated. The optimized electrolyte delivers the highest ionic conductivity of 1.5 × 10^–5 S·cm^–1 at 30 °C, and robust electrochemical stability up to 4.6 V. A solid-state Li/LiFePO₄ cell using the optimized electrolyte demonstrates good cycle performance at 0.2 C with high capacity retention of 92% after 70 cycles at 50 °C.