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

Halide superconductors (e.g., Li3YCl6 and Li3YBr6) 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 Li3MX6 (M = Sc, Y, Er; X = halogen) using first-principles calculations. The results show that Li3MX6 materials, especially chlorides, exhibit an ideal bandgap and excellent elastic and thermal properties. Meanwhile, it is further revealed that Li3MX6 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 Li3YCl6 and Li3YBr6 have been further uncovered. The calculated results demonstrate that the ordered Li3YCl6 and Li3YBr6 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.