Molecularly Engineered Phosphorus‐Based Flame‐Retardant Solid Polymer Electrolyte for Solid‐State Lithium Batteries

ABSTRACT Developing solid electrolytes that simultaneously ensure high‐safety and electrochemical performance remains a critical challenge for next‐generation high‐energy‐density batteries. Herein, we design a phosphorus‐containing ionic liquid (POIL) and fabricate a flame‐retardant composite solid electrolyte (PIL‐SPEs) via in situ thermal polymerization. The incorporated POIL endows PIL‐SPEs with exceptional self‐extinguishing properties (< 1 s), and synergizes with the polymer matrix to facilitate Li + transport and salt dissociation. Consequently, PIL‐SPEs achieve a high room‐temperature ionic conductivity (3.50 × 10 −4 S cm −1 ) and Li + transference number (0.60). At 60°C, its conductivity rises to 1.28 × 10 −3 S cm −1 , coupled with a wide electrochemical stability window (4.7 V vs. Li + /Li). LiFePO 4 ||Li full cells employing PIL‐SPEs demonstrate excellent cycling stability at 60°C, retaining 76% capacity (108.6 mAh g −1 ) after 200 cycles. The rational molecular design of POIL and its synergistic effects within the composite matrix provide a viable material strategy for developing safe, high‐performance solid‐state lithium batteries.