Hard‐Acid High‐Entropy Stabilized Chloride Octahedra for Halide Solid Electrolytes at High‐Voltage of 4.8 V

Halide solid‐state electrolytes (HSSEs) are promising candidates for high‐energy‐density all‐solid‐state lithium batteries (ASSLBs). However, the instability of the metal‐chloride (M‐Cl) octahedra remains a major obstacle to their operation under ultrahigh‐voltage conditions. In chloride electrolytes such as Li 3 InCl 6 , the elongated weak M‐Cl bond can trigger structural degradation and induce Cl − oxidation during the charging process. Herein, guided by the strong hard acid‐hard base interaction between metal cations and Cl − (hard base), a hard‐acid high‐entropy halide electrolyte, Li 2.5 In 0.5 Y 0.1 Al 0.1 Zr 0.1 Ta 0.2 Cl 6 , is developed through the synergistic incorporation of multiple hard‐acid cations. Structural and electronic analyses reveal that the hard‐acid high‐entropy strategy can stabilize M‐Cl octahedra and chloride electronic states, thereby suppressing chloride oxidation and enhancing stability at 4.8 V. Meanwhile, the modified framework creates a more favorable Li + migration pathway, enabling a high ionic conductivity of 2.38 mS cm −1 . When paired with an ultrahigh‐nickel NCM955 cathode, the resulting ASSLB exhibits outstanding electrochemical performance, retaining over 80% of its capacity after 1400 cycles at 10 C and 4.8 V while maintaining stable operation at ‐20°C. This work demonstrates the effectiveness of hard‐acid high‐entropy engineering in simultaneously enhancing oxidative stability and Li + transport, providing a new crystal‐chemistry design strategy for ultrahigh‐voltage halide solid electrolytes.