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

Comprehensive Summary 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‐1 H ‐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/LiFePO 4 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.