Fireproof Solid Polymer Electrolyte with Chemically Bonded Phosphorus Toward Stable and Safe Lithium‐Metal Battery

Abstract Lithium‐metal batteries (LMBs) are considered one of the most promising next‐generation high‐energy‐density battery systems. However, the leakage problem and fire hazard of commercial liquid electrolytes hinder their practical applications. Herein, a flame‐retardant solid polymer electrolyte (FRSPE) is fabricated by in situ polymerization of methyl methacrylate (MMA), allyl diglycol carbonate (ADC), and flame‐retardant monomer, i.e., diethyl vinyl phosphonate (DEVP), in which the phosphorus is chemically bonded to the polymer matrix to avoid the parasitic reaction between flame‐retardant molecules with lithium (Li) anode. Compared with the previously reported solid polymer electrolytes (SPEs) possessing free phosphorus, significantly increased efficiency of flame‐retardant and improved electrochemical performance can be achieved. With a wide electrochemical window (≈4.4V), high ionic conductivity (1.8 × 10⁻⁴ S cm⁻¹), and superior compatibility with Li anode, the assembled Li/FRSPE/LiFePO₄ (LFP) cell exhibits stable cycling over a wide temperature range (capacity retention of 70.9%@1000 cycles and 69.1%@200 cycles at 25 and 80 °C, respectively). Furthermore, the high‐voltage full cells, e.g., Li/LiCoO₂ (LCO) and Li/LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), with FRSPE also deliver excellent cycling performance. The current design principle with flame‐retardant chemically bonded in the polymer framework can provide a new pathway for developing practical, safe, and stable LMBs.