An Ammonium Polyphosphate Nanoparticle‐Based Flame‐Retardant Separator Enhances Safety of Lithium‐Metal Batteries

Flame-retardant separators present a viable strategy to address safety concerns in lithium-metal batteries (LMBs). However, the high loading and inhomogeneous distribution of flame-retardant microparticles within these separators can significantly impede ion transport, thereby degrading the electrochemical performance of LMBs. Herein, a nanoparticle-based flame-retardant separator is reported that exhibits enhanced flame-retardant efficiency and high cycling stability for LMBs. The separator consists of a nanoporous poly(vinylidene fluoride) (PVDF) matrix and uniformly dispersed ammonium polyphosphate (APP) nanoparticles (151 ± 27 nm, 10 wt.% loading). The nano-sized APP facilitates the formation of a uniform and intact char layer from the PVDF matrix at lower temperatures, thereby enhancing its barrier effect. Consequently, in pouch cells employing the PVDF/nano-APP separator, the peak heat release rate and total heat release are reduced by 30.3% and 27.1%, respectively, compared to those using conventional PP separators. Additionally, the APP nanoparticles help preserve the nanoporous structure of PVDF, allowing the corresponding LMBs to maintain 94.6% capacity retention over 2000 cycles. The flame-retardant nanocomposite separators show great potential for developing high-safety LMBs.