Decoupling Mechanical Strength and Ion Transport in Copolymer Electrolytes for Stable Lithium Metal Batteries

Solid polymer electrolytes (SPEs) are critical for lithium metal batteries (LMBs), yet their practical application is hindered by the intrinsic trade‐off between mechanical strength and ionic transport. Herein, a copolymer electrolyte (RFPE) integrating a rigid styrene sulfonate (SS) backbone and flexible ethylene glycol methyl ether methacrylate (EGMEM) domains is developed to simultaneously achieve high mechanical robustness and fast Li + transport. The flexible EGMEM domains facilitate rapid Li + migration and enhanced chain mobility, while the rigid SS backbone provides mechanical reinforcement and stabilizes the electrode/electrolyte interface. Benefiting from this synergistic and decoupled design, the RFPE‐1.0 delivers a higher ionic conductivity of 8.42 × 10 −4 S cm −1 and a Li + transference number of 0.737 at 30°C. The RFPE‐1.0 enables stable Li/Li symmetric cell cycling for over 2800 hours and maintains 131.09 mAh g −1 after 200 cycles at 0.5 C in LiFePO 4 full cells. Moreover, pouch cells further display stable output even under severe mechanical abuse. This work provides an effective strategy for designing high‐performance and safe polymer electrolytes for advanced LMBs.