In Situ Polymerized Flame Retardant Gel Electrolyte for High-Performance and Safety-Enhanced Lithium Metal Batteries

A flame retardant gel electrolyte (FRGE) is deemed as one of the most promising electrolytes to relieve the problems of safety hazards and interfacial incompatibility of Li metal batteries. Herein, a novel solvent triethyl 2-fluoro-2-phosphonoacetate (TFPA) with outstanding flame retardancy is introduced in the polymer skeleton synthesized by in situ polymerization of the monomer polyethylene glycol dimethacrylate (PEGDMA) and the cross-linker pentaerythritol tetraacrylate (PETEA). The FRGE exhibits superb interfacial compatibility with Li metal anodes and inhibits uncontrolled Li dendrite growth. This can be ascribed to the restriction of free phosphate molecules by the polymer skeleton, thus realizing a stable cycling performance over 500 h at 1 mA cm^-2 and 1 mAh cm^-2 in the Li||Li symmetric cell. In addition, the high ionic conductivity (3.15 mS cm^-1) and Li⁺ transference number (0.47) of the FRGE further enhance the electrochemical performance of the correspondent battery. As a result, the LiFePO₄|FRGE|Li cell exhibits excellent long-term cycling life with a capacity retention of 94.6% after 700 cycles. This work points to a new pathway for the practical development of high-safety and high-energy-density Li metal-based batteries.