TPFPP@PMMA core-shell polymer modified flame retardant separator achieved by aqueous technique for high-voltage lithium metal batteries

The safety of lithium metal batteries (LMBs) is a critical barrier to their further development towards achieving higher energy densities exceeding 400 Wh kg⁻¹. Although the incorporation of flame-retardant additives into the liquid electrolyte can enhance the safety of LMBs, it often compromises electrochemical performance. To achieve a balance, a flame-retardant separator with thermally responsive properties has been developed by coating a core-shell flame-retardant polymer (poly(methyl methacrylate) (PMMA) as the shell and tris (pentafluorophenyl) phosphine (TPFPP) as the core) onto a polyethylene (PE) separator using an aqueous technique. Compared to the original PE separator, the TPFPP@PMMA flame-retardant separator exhibits superior mechanical strength and electrolyte wettability, along with significantly improved flame retardancy and thermal stability. Due to the excellent compatibility of PMMA polymer with lithium anode, a stable cycle life of over 500 hours for the Li||Li symmetrical coin cell has been achieved. The capacity retention of the Li||LiNi_0.8Co_0.1Mn_0.1O₂ battery is higher than that of the PE separator (80.0% vs. 65.8%) after 100 cycles between 3.0 V and 4.35 V, ascribed to the formation of a stable and robust cathode electrolyte interface (CEI) film, primarily composed of rich LiF and poor Li₂CO₃ on the cathode surface, induced by the PT/PE separator. Thus, the developed flame-retardant separator offers new prospects to the revival of high-energy-density LMBs.