Rationally Designed Conversion‐Type Lithium Metal Protective Layer for All‐Solid‐State Lithium Metal Batteries

Abstract A stable interfacial design bridging Li metal and sulfide solid electrolytes is imperative for deploying practical all‐solid‐state Li metal batteries. Despite the extensive exploration of interlayer materials, including inorganic substances, lithiophilic metals, and their composites, a comprehensive understanding of their stability and chemo‐mechanical evolution, particularly those influenced by cell fabrication processes, remains unexplored. Herein, it is meticulously investigate the formation and evolution of LiF, Mg, and conversion‐type multicomponent MgF₂ ultrathin interlayers, each fabricated via thermal evaporation deposition. Unexpectedly, LiF and Mg fail to enhance cell performance, with LiF notably susceptible to external pressures during cell fabrication, leading to serious current constriction, while Mg deposition results in the formation of a Li‐rich solid solution. Remarkably, the MgF₂ coatings demonstrate substantially superior performance in both Li|Li₆PS₅Cl|Li symmetric cells (up to 2000 h) and LiNi_0.70Co_0.15Mn_0.15O₂|Li₆PS₅Cl|Li full‐cells (82% capacity retention after 800 cycles) at 30 °C. These results are attributed to the in‐situ formation of LiF and Li_xMg nanograins through a conversion reaction, which, after repeated cycling, maintains stability and a fixed position at the interface while ensuring uniform interfacial Li⁺ flux. Supported by comprehensive analyses, these findings highlight the pivotal role of conversion‐type interlayers in mitigating side reactions and preventing Li penetration.