Multifunctional Phosphate Monomer Enabling LiNO₃ Solvation and In Situ Formation of Flame-Retardant Gel Polymer...

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 LiNO₃ 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 LiNO₃ in GPE can promote the formation of Li₃N 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_0.6Co_0.2Mn_0.2O₂ and Li||LiNi_0.8Co_0.1Mn_0.1O₂ 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 LiNO₃ solvation, flame retardancy, and interfacial stabilization, offering a promising pathway toward safe, high-voltage LMBs.