Material Design Strategy for Halide Solid Electrolytes Li₃MX₆(X = Cl, Br, and I) for All-Solid-State High-Voltage Li-Ion Batteries

Although several solid electrolyte (SE) candidates have been explored, achieving the necessary combination of performance, stability, and processability has been challenging. Recently, several lithium ternary halides have attracted increasing attention for SEs because of their favorable combination of high ionic conductivity and wide electrochemical window. This study aims to provide a material design strategy for lithium halides Li3MX6 (X = Cl, Br, and I) for high-voltage all-solid-state Li-ion batteries, achieved by the systematic investigation of crystal structures, phase and electrochemical stabilities, electronic and mechanical properties, and ionic conductivities. Calculation results reveal that the electronegativity difference between M and X affects structural properties and stabilities. Weak Coulomb interactions in Li3MX6 result in the preference of the monoclinic phase, and the oxidation potential and chemical stability against the cathode materials of Li3MX6 increase for relatively small X. Chlorides exhibit the highest oxidation potential (∼4.3 V) among Li3MX6, suggesting that chlorides are appropriate SEs for high-voltage cathodes. The band gap and elastic moduli increase for relatively small X, suggesting the relatively low electronic conductivity and elastic deformability of chlorides. Chlorides with transition metals typically exhibit trigonal phases, a wider electrochemical stability window, a larger band gap, and higher elastic moduli compared to other types of halides. Additionally, chloride Li3MCl6 is expected to have relatively high ionic conductivities with the aliovalent substitution of M3+ to Zr4+ and the anion mixing of Cl with Br. The findings of this study will provide fundamental guidelines for the development of lithium halide SEs for high-voltage all-solid-state Li-ion batteries.