Janus Ag Nanoskin‐Armored Polyimide Bilayer Separator Enables Efficient Li‐Nucleation Regulation for Dendrite‐Free Lithium Metal Battery

ABSTRACT Uncontrolled lithium (Li) dendrite growth has been regarded as the most critical obstacle for the widespread adoption of Li metal batteries (LMBs), stemming from the preferential nucleation of Li ions (Li + ) at low‐energy‐barrier sites that trigger heterogeneous deposition. Therefore, it is pivotal yet significantly challenging to effectively control Li nucleation and deposition in LMBs. Here, we engineer a Janus Ag nanoskin‐armored polyimide (Ag@PI/PI) bilayer separator by in situ growth Ag nanoparticles on the surface of PI nanofiber and integrating a high‐strength PI nanofiber membrane as the dorsal substrate. Specifically, the Ag nanoskin acts as a three‐dimensional lithiophilic host to seize Li and induce sequential Li nucleation, thereby preventing Li dendrite formation. As a result, the Ag@PI/PI‐Li||Cu cell exhibits a low overpotential of 31.0 mV, high Coulombic efficiency of 98.9%, and prolonged Sand's time of 26.3 h. Significantly, the Ag@PI/PI separator delivers a high Li + transference number (0.70) and exceptional cycling stability (1600 h) in a Li||Li cell. Moreover, the Ag@PI/PI‐based Li||NCM523 cell achieves a remarkable specific capacity of 108.32 mAh g −1 at 5 C while retaining a satisfying capacity retention of 88.7% at 1 C. These results underscore the viability of this universal and efficient Ag nanoskin‐armored strategy for high‐security LMBs applications.