Low-Content Bromine Substitution Accelerates Li⁺ Conduction in Chloride Electrolytes for All-Solid-State Batteries

Chloride-based solid-state electrolytes (SSEs) have attracted significant attention due to their favorable combination of ionic conductivity and electrochemical stability. However, chloride SSEs exhibit lower ionic conductivity than sulfides and liquid electrolytes, along with Li metal instability, hindering their high-rate all-solid-state battery (ASSB) applications. Previous studies have emphasized a cation substitution strategy, particularly high-entropy design, to enhance ionic conductivity, while anion substitution remains an underexplored yet promising alternative. Herein, through low-content Br substitution, Li 3 InCl 5.9 Br 0.1 is synthesized via mechanical ball milling and achieves a room-temperature ionic conductivity of 1.30 mS cm –1, which represents a 48% enhancement over pristine Li 3 InCl 6 (0.88 mS cm –1 ). Combined experimental and theoretical analyses reveal that the enhanced ionic conductivity stems from moderate local lattice distortion and optimized Li–Cl/Br bond lengths that facilitate Li + conduction. The assembled LiCoO 2 |Li 3 InCl 5.9 Br 0.1 |Li 6 PS 5 Cl|Li–In ASSBs significantly demonstrate 81.89% capacity retention after 100 cycles at 0.2C (vs 70.55% for ASSBs with pristine Li 3 InCl 6 ), along with improved rate performance. This work provides a reliable strategy of low-content Br – substitution to develop advanced chloride SSEs for application in ASSBs.