Synergistic Effects of Solid Electrolyte Mild Sintering and Lithium Surface Passivation for Enhanced Lithium Metal Cycling in All‐Solid‐State Batteries

The argyrodite-type solid electrolyte (SE) Li₆PS₅Cl (LPSCl), recognized for its high ionic conductivity and low-temperature processability, offers substantial potential for enabling lithium metal anodes in all-solid-state batteries (ASSBs), promising high energy densities with enhanced safety. However, lithium dendrite penetration and unstable solid electrolyte interphase (SEI) formation hinder stable cycling at high current densities. This work presents a synergistic strategy to address these challenges by combining mild sintering of LPSCl pellets with the deposition of a lithium fluoride (LiF) passivation layer on 50 µm thick lithium metal. Optimized sintering at 80°C improves surface uniformity and densifies the LPSCl pellets, reducing porosity and increasing ionic conductivity. Complementarily, the deposition of a uniform 65 nm LiF layer on lithium via electron beam evaporation, reduces interfacial resistance, and stabilizes SEI formation. This dual modification doubles the critical current density of lithium symmetric cells from 1.1 to 2.2 mA cm^-2. In full cells configurations with LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathodes, remarkable cycling stability is achieved over 2700 cycles (at 1 mA cm^-2, 1.5 mAh cm^-2), with 75% capacity retained after 1500 cycles. This study provides a practical approach for improving both SE pellet quality and lithium-SE interfacial stability, paving the way for the reliable implementation of thin lithium metal in next-generation ASSBs.