LAUSR

Solid‐state Li–S and Li–Se batteries are promising devices that can address the safety and electrochemical stability issues that arise from liquid‐based systems. However, solid‐state Li–Se/S batteries usually present poor cycling stability due to the high resistance interfaces and decomposition of solid electrolytes caused by their narrow electrochemical stability windows. Here, an integrated solid‐state Li–Se battery based on a halide Li3HoCl6 solid electrolyte with high ionic conductivity is presented. The intrinsic wide electrochemical stability window of the Li3HoCl6 and its stability toward Se and the lithiated species effectively inhibit degeneration of the electrolyte and the Se cathode by suppressing side reactions. The inherent thermodynamic mechanism of the lithiation/delithiation process of the Se cathode in solid is also revealed and confirmed by theoretical calculations. The battery achieves a reversible capacity of 402 mAh g−1 after 750 cycles. The electrochemical performance, thermodynamic lithiation/delithiation mechanism, and stability of metal‐halide‐based Li–Se batteries confer theoretical study and practical applicability that extends to other energy‐storage systems.