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

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 LiNi0.70Co0.15Mn0.15O2|Li6PS5Cl|Li full-cells (82% capacity retention after 800 cycles) at 30 degrees C. These results are attributed to the in-situ formation of LiF and LixMg 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. MgF2, LiF, and Mg as protective interlayers for all-solid-state Li metal batteries, derived through thermal evaporation deposition, are investigated with a focus on their stability and (electro)chemical and chemo-mechanical evolution. MgF2-coated electrodes showcase superior performance in Li|Li6PS5Cl|Li symmetric cells and LiNi0.70Co0.15Mn0.15O2|Li6PS5Cl|Li full-cells owing to the stability and fixed position of the in situ converted LiF and LixMg nanograins.image