Theoretical Insights on the Comparison of Li-Ion Conductivity in Halide Superionic Conductors Li3MCl6, Li2M2/3Cl4, and LiMCl4 (M = Y, Sc, Al, and Sm)

Recently, halide materials for solid electrolytes have received modest research interest. A variety of new halide electrolytes, such as Li3YCl6, Li3InCl6, and Li2Sc2/(3)Cl(4 )halospinel structure, and LiAlCl4, have been experimentally prepared with high Li-ionic conductivities close to 10(-3) S/cm and low activation energies. Although much effort (experimental and theoretical) has been devoted to uncovering the best combination of Li-M-X, less attention has been paid to the structural effects on the ionic conductivities and electrochemical stabilities. In this article, DFT and AIMD simulations are performed to do a comparative study on several halide electrolytes with selected structures, including the most common rock-salt Li3MCl6, spinel Li2M2/3Cl4, and LiMCl4. It is revealed that halospinel Li2M2/3Cl4 structures with cubic symmetry are three-dimension-ally conducting and mechanically stable superionic conductors. They exhibit an excellent Li-ionic conductivity within the range 0.26-19.0 mS/ cm with an activation energy lower than 0.20 eV (0.342-0.195 eV). Among them, Li2Sm2/3Cl4 is predicted to have an outstanding balance between the ionic conductivity and stability. The room-temperature ionic conductivity is calculated to be as high as 15.3 mS/cm, whereas the band gap and electrochemical stability window vs Li/Li+ reach 4.26 V, respectively, making it a promising candidate for practical applications as a superionic conductor in solid-state batteries.