Simultaneously Enhance Humidity Tolerance and Ionic Conductivity of Halide Electrolytes by Cation–Anion Co‐Doping

Abstract Halide solid‐state electrolytes (SSEs) with high ionic conductivity and oxidation stability show significant potentials for the commercialization of all‐solid‐state batteries (ASSBs). However, their practical applications are limited by poor humidity tolerance and decreased ionic conductivity. Herein, the Li_3/3‐xIn_1‐xM_xCl_5.6F_0.4 (M = Hf, Zr, Fe, and Y; x = 0.015, 0.02, 0.025, 0.075, 0.1, 0.125, and 0.15) are synthesized by co‐doping Li₃InCl₆ with M and F ions for high‐performance ASSBs. Li_2.98In_0.98Hf_0.02Cl_5.6F_0.4 achieves the highest ionic conductivity of 1.04 mS cm⁻¹ at room temperature and good humidity tolerance of 87.96% conductivity retention after 24 h exposure. The analysis of experiments combining BVSE and AIMD simulations indicates that it benefits from the enhanced rigid structure that is thermodynamically stable to water caused by Cl⁻ doping with F⁻ and higher Li⁺ vacancies concentration that facilitates Li⁺ conduction generated by In³⁺ doping with Hf⁴⁺. Moreover, an all‐solid‐state battery ensembled with Li_2.98In_0.98Hf_0.02Cl_5.6F_0.4, LiCoO₂ and Li–In alloy, exhibits a high discharge capacity of 83.6 mAh g⁻¹ and 78.6% capacity retention after 100 cycles under 0.5 C at 25 °C. This work provides a reliable strategy for designing advanced halide SSEs for commercial applications in ASSBs by cation–anion co‐doping.