Compatibility of Halide Electrolytes in Solid-State Li-S Battery Cathodes

The utilization of earth-abundant and high-capacity sulfur in solid-state batteries presents a promising strategy to circumvent the use of rare transition metals and enhance achievable specific energy. However, numerous challenges remain. The transport limitation within the cathode composite, particularly with sulfide electrolytes during charging, has been identified as a major degradation mechanism in solid-state Li-S batteries. This degradation is linked to electrolyte oxidation and a concomitant reduction in the effective ionic conductivity of the cathode composite. Inspired by the sufficiently high oxidation stability of halide-based electrolytes, we investigated their compatibility with solid-state Li-S batteries in this work. The electrochemical stability of halides in contact with conductive additives, the stability window of fast ion transport in the composite electrodes, and chemical compatibility with sulfur-active materials (e.g., S and Li₂S), in addition to the cyclability of the halide-based composite electrodes, are explored. Three halides were employed as model electrolytes: Li₃InCl₆, Li₃YCl₆, and Li₃YBr₆. Despite its high oxidation stability, Li₃InCl₆ exhibited rapid degradation due to electrolyte reduction. The composite with Li₃YCl₆ lost its capacity because of chemical incompatibility, especially with Li₂S, resulting in the formation of LiYS₂ at the interface. In contrast, Li₃YBr₆ demonstrated superior performance, maintaining a capacity of 1100 mAh g_S^-1 for 20 cycles (normalized to the sulfur content in the cathode material). This study elucidates the degradation mechanisms of halide-based solid-state Li-S batteries and proposes potential design strategies to mitigate chemical incompatibility issues.