Toward Safe and Cost-Effective LiFePO ₄ Batteries: A Multifunctional Electrolyte with Flame-Retardant and Interfacial Protection

The safety challenges of lithium-ion batteries, particularly in LiFePO₄ systems, are rooted not only in the flammability of carbonate-based electrolytes but also in the early-stage exothermic reactions triggered by the breakdown of the solid electrolyte interphase on lithiated graphite. Conventional flame-retardant approaches primarily mitigate combustion in the late stage of thermal runaway yet offer limited protection against these initial interfacial reactions. To fill this critical gap, a two-dimensional electrolyte design was adopted to decouple safety functions from electrochemical behavior. By incorporating three lithiophobic, noncoordinating components─perfluoro-2-methyl-3-pentanone (FK), ethoxy(pentafluoro)cyclotriphosphazene (PFPN), and 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane (D₃F)─a multifunctional electrolyte, termed RDF, is developed, achieving both robust flame retardancy and protection of the graphite anode. D₃F undergoes in situ ring-opening polymerization to reconstruct a siloxane-rich protective interphase, effectively suppressing exothermic reactions, while FK and PFPN cooperatively inhibit combustion in the later stage. This multifunctional protection markedly enhances thermal safety in 4.8 Ah LiFePO₄∥Graphite pouch cells subjected to thermal abuse. The RDF electrolyte delays the onset of internal short circuit by nearly 50 min relative to a conventional carbonate electrolyte. Meanwhile, excellent electrochemical performance is retained, with commercial-loading LiFePO₄∥Graphite cells maintaining 80.2% capacity after 200 cycles and 1 Ah pouch cells delivering 93% capacity retention over 200 cycles. This work provides a practical and scalable electrolyte design strategy that integrates flame retardancy with interfacial protection, enabling intrinsically safer and high-performance lithium-ion batteries.