Revealing cation-disorder effects on lithium transport in halide superionic conductors

High Li-ion conductivity solid electrolytes are pivotal for the development of all-solid-state batteries (ASSBs), which promise increased volumetric energy density and enhanced safety over conventional liquid-electrolyte-based batteries. Among the chloride-based Li-ion solid electrolytes, zirconium-based systems such as Li₂ZrCl₆ (LZC) have emerged as a potential candidate due to their affordability, moisture stability, and high ionic conductivity. However, the crystal structure of LZC and the effects of cation disorder on ionic transport are not well understood yet. In this computational study, we used cluster expansion Monte Carlo and molecular dynamics derived from ab-initio calculations to explore the ionic conductivity of alpha- and beta-LZC in relation to cation disorder. We found that disordered phases form primarily at elevated temperatures, explaining their synthesis through mechanochemical ball-milling. Additionally, our study indicates that superionic conductivity is enabled by the activation of Li-vacancy disorder, contingent upon the extent of the Zr-disorder. The insights obtained from this study can contribute to the optimization of halide superionic conductor performance as a technologically significant material in ASSBs.