Multi‐Field Coupling Driven in Situ Interfacial Growth of COF‐functionalized Membranes for Lithium–Sulfur Batteries

Abstract Conventional modification methods for the separator of high‐performance lithium‐sulfur batteries often struggle to realize the functional materials well adhering on and across the substrate. Herein, we employ a scalable top‐down strategy in an oil–water–oil system to interfacially polymerize COF nanoparticles across a polyolefin substrate as a self‐standing separator for Li─S batteries. By providing a suitable environment for the membrane to remain flat during polymerization, COF particles can penetrate from the surface into internal pores, forming a uniform and robust architecture. The resulting COF‐functionalized membrane incorporates abundant ‐OH groups that effectively promote Li + transport and restrict polysulfide shuttling. Benefiting from this synergy, the modified separator achieves high ionic conductivity (0.92 mS cm −1 ), an elevated polysulfide diffusion barrier (0.390 versus 0.283 eV), and excellent electrochemical performance, including 678 mAh g −1 after 500 cycles at 1.0 C. This in situ interface polymerization strategy offers a new path for more comprehensively functionalizing COF‐based membrane to achieve high‐performance lithium‐sulfur batteries.