Synergy of I-Cl co-occupation on halogen-rich argyrodites and resultant dual-layer interface for advanced all-solid-state Li metal batteries

The (electro)chemical stability and Li dendrite suppression capability of sulfide solid electrolytes (SEs) need further improvement for developing all-solid-state Li batteries (ASSLBs). Here, we report advanced halogen-rich argyrodites via I and Cl co-occupation on the crystal lattice. Notably, a proper I content forms a single phase, whereas an excessive I causes precipitation of two argyrodite phases like a superlattice structure. The resultant synergistic effect of the optimized composition allows to gain high ionic conductivities at room temperature and −20 °C, and enhances the (electro)chemical stability against Li and Li dendrite suppression capability. The Li|argyrodite interface is very sensitive to the ratio of I and Cl. A LiCl- and LiI-rich double-layer interface is observed from the cell using the SE with optimized composition, whereas too high I content forms only a single interface layer with a mixture of LiI and LiCl. This double-layer interface is found to effectively mitigate the Li/SE reaction. The proper designed argyrodite enables ASSLBs to achieve good electrochemical properties at a broad temperature range regardless of the electrode materials. This co-occupation strategy provides a novel exploration for advanced halogen-rich argyrodite system.