Core–Shell Structured Flame‐Retardant Separator Mediated with Metal–Organic Framework Armor Enables Self‐Acceleration Mechanism for Dendrite‐Free and Safer Lithium Metal Batteries

Developing an optimal multifunctional flame‐retardant separator is crucial for enhancing lithium metal battery (LMB) safety. However, this task poses challenges due to the inferior electrochemical stability and limited ion transport of most fire retardant‐based coatings. In this work, the core–shell structured flame‐retardant matrix is elaborated by in situ growing a thin layer of MOF armor onto flame‐retardant ammonium polyphosphate (APP) bulk materials to further manufacture a multifunctional separator (APP@ZIF‐8@PP). The MOF armor acts as both a protector and ion transport regulator, effectively safeguarding APP from side reactions and optimizing the efficiency, selectivity, and deposition behavior of ion flux. Notably, such separator demonstrates a self‐acceleration mechanism, i.e., APP and ZIF‐8 can promote the decomposition of each other mutually, activating the flame‐retardant effect at lower temperatures. Additionally, APP@ZIF‐8 aids in forming a dense char layer during pyrolysis, which can insulate against the transfer of oxygen and heat. Finally, the LMBs assembled with such a multifunctional separator exhibit heightened safety and optimized electrochemical performance. This work provides valuable insights into the development of advanced porous materials‐based flame‐retardant separators, contributing to safer and more reliable energy storage devices. Core–shell structured APP@MOF matrix is constructed by in situ growing MOF armor onto flame‐retardant APP to manufacture multifunctional separator further. The MOF serves as protector and ion flux rectifier. Moreover, the self‐acceleration mechanism of APP@MOF allows triggering flame retardancy at lower temperatures and forming a dense char layer. These merits afford lithium metal batteries with higher safety and optimized electrochemical performance.