Insights into Bulk Properties and Transport Mechanisms in New Ternary Halide Solid Electrolytes: First-Principles Calculations

Halide superconductors (e.g., Li 3 YCl 6 and Li 3 YBr 6 ) reported recently have become promising candidate solid electrolytes for Li-ion batteries due to their simultaneous high ionic conductivity and electrochemical stability. However, the bulk properties and Li-ion diffusion mechanism of such materials are not well understood. To address this issue, we systematically evaluate the bulk properties of Li 3 MX 6 (M = Sc, Y, Er; X = halogen) using first-principles calculations. The results show that Li 3 MX 6 materials, especially chlorides, exhibit an ideal bandgap and excellent elastic and thermal properties. Meanwhile, it is further revealed that Li 3 MX 6 materials follow the trend that the larger the difference between halogen and rare-earth atoms in electronegativity, the better the properties, and vice versa, which could be a universal design principle for searching ternary halides with better bulk properties. In addition, the underlying mechanisms of Li-ion conduction in Li 3 YCl 6 and Li 3 YBr 6 have been further uncovered. The calculated results demonstrate that the ordered Li 3 YCl 6 and Li 3 YBr 6 are superionic conductors with a 1D diffusion channel of 0.19 eV energy barrier and a 3D diffusion channel of 0.25 eV energy barrier, respectively. However, the disorder of cations can significantly block the Li-ion migration. These results provide comprehensive insights into such halides for their practical application as solid electrolytes.