Electrochemically Stable Li_3–xIn_1–xHf_xCl₆ Halide Solid Electrolytes for All-Solid-State Batteries

Halide solid electrolytes (SEs) stand out among the many different types of SEs owing to their high ionic conductivity and excellent oxidative stability. Aliovalent substitution is a common strategy to enhance the ionic conductivity of halide electrolytes, but this strategy significantly decreases their electrochemical stability. Herein, we report Hf-substituted Li 3 InCl 6 (Li 3– x In 1– x Hf x Cl 6, 0 ≤ x ≤ 0.7) SEs, in which a low concentration (0.1 ≤ x ≤ 0.5) of Hf enhances the ionic conductivity without affecting the electrochemical stability. Among them, Li 2.7 In 0.7 Hf 0.3 Cl 6 exhibits a high ionic conductivity of 1.28 mS cm –1 and a wide electrochemical stability window of 2.68–4.22 V. All-solid-state batteries fabricated using Li 2.7 In 0.7 Hf 0.3 Cl 6 SE present high discharge capacity and good cycling stability at 25 °C. Furthermore, we summarize the methods of crystal structure regulation by which aliovalent substitution of halide SEs is achieved and discuss potential research directions in the design of novel halide SEs with high ionic conductivity and electrochemical stability.