A multifunctional organic-inorganic hybrid flame-retardant separator for high-safety lithium-ion batteries

As lithium-ion batteries (LIBs) continue to pursue higher energy density, safety concerns have intensified, with the limited thermal stability of commercial polyolefin separators emerging as a critical bottleneck. Although numerous strategies have been proposed to improve separator safety, most suffer from an intrinsic trade-off between enhanced thermal stability and preserved electrochemical performance. Herein, we report an organic-inorganic hybrid flame-retardant (HFR) separator in which a hierarchical porous layer composed of inorganic boehmite particles embedded within a phosphorus-based flame-retardant polymer network is introduced onto a polyethylene (PE) substrate via a vapor-induced phase separation (VIPS) process. Despite the introduction of an additional coating layer, ion transport is preserved by the hierarchical porosity and anion-interacting boehmite, while thermal shrinkage of the PE substrate is also effectively suppressed. Moreover, excellent flame retardancy originates from the cooperative interplay between phosphorus-based radical quenching and the endothermic decomposition of boehmite. Beyond thermal safety, the HFR layer preferentially decomposes to form a thin, phosphorus-rich cathode-electrolyte interphase (CEI) that stabilizes the cathode surface, resulting in improved cycling stability. This work demonstrates a scalable separator design that simultaneously enhances thermal safety and electrochemical performance, offering a practical pathway toward next-generation high-safety LIBs.