Bifunctional separators for safe Li metal batteries: Uniform fluorination interface engineering and flame retardancy

Traditional fluorination strategies have attracted significant attention due to their ability to construct a fluorine-rich solid-electrolyte interphase (SEI) at the anode-electrolyte interface. However, the inhomogeneous distribution of LiF within the SEI layer remains a critical challenge that hinders the effective suppression of lithium dendrite growth. This work explores the synergy between HDBDPE (degradation products of decabromodiphenyl ethane) and MgF2, developing a fluorinated, flame-retardant separator with optimized ion channels. HDBDPE acts as both a low-toxicity flame retardant and an “F-element guide” through hydrogen bonding with MgF2, forming a uniform LiF SEI layer on the Li anode for rapid reaction kinetics. Moreover, under thermal runaway conditions, the MgF2@HDBDPE/PP separator constructs a “high-efficiency flame-retardant barrier”, effectively quenching free radicals and promoting the formation of MgBr2, thereby suppressing electrolyte combustion. As a result, both symmetric and full cells assembled with the MgF2@HDBDPE/PP separator demonstrate superior cyclic stability and rate performance due to improved interfacial reactions. The MgF2@HDBDPE/PP separator exhibits rapid flame retardancy under thermal runaway conditions.