Electrochemically Induced Interphase by Complex Hydride Anions in Argyrodite Solid Electrolytes for Stable Lithium Metal All‐Solid‐State Batteries

ABSTRACT Complex hydride anion substitution in the argyrodite solid electrolyte has emerged as a promising approach to enhance ionic conductivity and interfacial stability. Despite these advances, the influence of complex hydride anions on Li metal interfacial stability remains unclear. Here, we clarify that complex hydride anions drive distinct interfacial reaction pathways at Li metal under electrochemical operation. In the BH ₄ ⁻ ‐substituted argyrodite Li ₅ PS ₄ (BH ₄ ) ₂ , BH ₄ ⁻ species rapidly react with Li during electrochemical operation, forming a Li─B─H‐rich interphase. This interphase limits further decomposition of the sulfide framework while maintaining efficient Li ⁺ transport. In contrast, the conventional halide argyrodite Li ₆ PS ₅ Cl undergoes sustained interfacial decomposition into Li ₂ S and Li ₃ P, resulting in unstable cycling behavior. Based on these results, we developed all‐solid‐state Li metal batteries with a gradual current increase that promotes Li─B─H‐rich interphase formation, achieving stable cycling over 1000 cycles at high current densities up to 2.1 mA cm ⁻² . Collectively, our findings provide new insight into how complex hydride anions in solid electrolytes, when coupled with rationally engineered electrochemical operation, enable stable, high‐current all‐solid‐state Li metal batteries.