LAUSR

All‐solid‐state batteries (ASSBs) with sulfide‐based electrolytes, such as argyrodite (Li₆PS₅Cl, LPSCl), offer significant advantages regarding safety and energy density. However, conventional Cu current collectors with LPSCl suffer from corrosion, necessitating a deeper understanding of appropriate mechanisms and strategies to address them. This study investigates the impact of electrolyte decomposition on Cu degradation in sulfide‐based ASSBs. Accelerated experiments reveal that LPSCl decomposition forms an ineffective passive layer on Cu, resulting in significant corrosion above 2 V during delithiation. In addition, the corrosion potential implies that sulfide and chlorine species are involved in the corrosion reaction. Comparative analyses with Ni current collectors, which are known for their resistance to the corrosive species, demonstrate superior stability to Cu. Corrosion‐prevention strategies are proposed based on the elucidated mechanisms, with the Pilling–Bedworth ratio explaining why certain metal sulfide layers formed during electrolyte decomposition may fail to effectively prevent corrosion. These insights support the development of targeted protective strategies and alternative current collector materials to enhance the durability and performance of sulfide‐based ASSBs.