Interfacial Engineering of Li Anode and Ni‐Rich Cathode via Anion‐Regulated SEI/CEI Layers in Flame‐Retardant Nanofiber‐Reinforced Polymer Electrolyte for Lithium Metal Batteries

Abstract To address safety concerns such as dendrite growth and thermal runaway in lithium metal batteries (LMBs), a localized highly concentrated gel polymer electrolyte (LHCE‐GPE) is proposed with dual flame‐retardant functionality. This electrolyte is fabricated through a synergistic electrospinning and in‐situ curing strategy: A high‐temperature‐resistant copoly (phthalazinone biphenyl ether sulfone) (PPBES) 3D nano fiber membrane (NFM) serves as the skeleton, while ethoxy(pentafluoro) cyclotriphosphazene (PFPN) functions as both a flame‐retardant diluent and a solvation structure modulator. The coordinated interaction between the 3D nanofiber and PFPN not only suppresses combustion but also facilitates anion‐dominated decomposition, enabling the formation of an inorganic‐rich, mechanically stable solid‐electrolyte interphase (SEI) on anodes. Concurrently, preferential decomposition of anions to form a stable cathodic electrolyte interphase (CEI) layer on the cathode, effectively mitigating side reactions and structural degradation of Ni‐rich cathodes under high voltages. The LHCE‐GPE delivers exceptional electrochemical performance: Li/LFP cells achieve ultralong cycling over 2000 cycles at 10 C with a minimal capacity decay of 0.0157% per cycle. More importantly, Li/NCM811 cells maintain stable operation at an ultrahigh voltage of 4.7 V, significantly surpassing conventional electrolytes. This research underscores the critical contribution of LHCE‐GPE to both electrode interfaces, presenting an innovative approach for developing safer electrolytes in high‐voltage LMBs.