Covalently Anchoring an Ultrathin Conformal SiO_x Coating on Polyolefin Separator for Enhanced Lithium Metal Battery Performance

Surface modification of separators with inorganic oxide ceramics such as SiO_x, Al₂O₃, and TiO₂ has emerged as a promising strategy to suppress lithium dendrite growth in lithium metal batteries, thereby enhancing safety and extending battery life. However, the binder-dependent nature of these modifications often leads to increased separator thickness and a heightened risk of detachment during lithium plating, ultimately compromising both battery performance and energy density. In this study, a conformal, ultrathin (≈20 nm) SiO_x coating with strong covalent bonding is grown on a porous separator through a liquid polymer-derived method. Compared to the pristine polyethylene (PE) separators, this SiO_x-co-PE separator exhibits significantly improved electrolyte wettability, mechanical strength, and thermal stability without any increase in thickness, leading to vastly enhanced cycling stability and dendrite resistance in cells with Li metal anodes. In a practical demonstration, a 402.9 Wh·kg⁻¹ Li-S pouch cell, with a high sulfur loading of 9 mg cm⁻² and a low E/S ratio of 3.3 µL mg⁻¹, is assembled with the SiO_x-co-PE separator, achieving stable cycling over 70 cycles at 25 °C.