Multifunctional Phosphate Monomer Enabling LiNO ₃ Solvation and In Situ Formation of Flame‐Retardant Gel Polymer Electrolyte for High‐Voltage Lithium Metal Batteries

ABSTRACT Preparing electrolytes that simultaneously enable high oxidation stability, interfacial compatibility, and intrinsic safety remains a major challenge for high‐voltage lithium metal batteries (LMBs). Herein, we report a phosphorus‐containing multifunctional monomer, ethyl di(2‐(methacryloyloxy)ethyl) phosphate, which enables both LiNO ₃ dissolution and in situ polymerization within a liquid electrolyte (LE) to form a flame‐retardant gel polymer electrolyte (GPE). The resulting GPE exhibits excellent ionic conductivity (3.19 × 10 ⁻³ S cm ⁻¹ at 25°C), a wide electrochemical stability window (> 4.6 V), and superior flame retardancy. The LiNO ₃ in GPE can promote the formation of Li ₃ N and LiF in the solid electrolyte interphase (SEI) layer on the Li metal anode, facilitating Li ⁺ transport and promoting dense and smooth Li deposition. When applied in Li||LiNi _0.6 Co _0.2 Mn _0.2 O ₂ and Li||LiNi _0.8 Co _0.1 Mn _0.1 O ₂ cells, the GPE system delivers remarkable cycling stability with capacity retentions of 83.1% after 400 cycles and 91.5% after 200 cycles, respectively. Spectroscopic and structural analyses reveal that the polymer matrix in GPE stabilizes cathode–electrolyte interfaces, mitigates transition‐metal dissolution, and suppresses Li/Ni cation mixing. This work establishes a molecular‐level electrolyte design strategy that integrates LiNO ₃ solvation, flame retardancy, and interfacial stabilization, offering a promising pathway toward safe, high‐voltage LMBs.