Elucidating Anisotropic Ionic Diffusion Mechanism in Li₃YCl₆ with Molecular Dynamics Simulations

Halide-based solid electrolytes have emerged as promising materials for the development of solid-state batteries, due to their high ionic conductivity and excellent chemical properties. Li3YCl6 is a prototype halide-based superionic material that features anisotropic ionic diffusion. Elucidating the ionic transport and optimizing the conductivity in such anisotropic materials are crucial for enhancing the performance of solid-state batteries. In this work, by using molecular dynamics simulations with a machine learning force field, we systematically study the anisotropic ion diffusion behavior, including directional conductivity contribution, concerted migration, and disorder–order transition in Li3YCl6. Our results prove that the fast c-direction is the major contributor to total diffusivity, especially at room temperature. The hexagonal close-packed anion arrangement leads to anisotropic diffusion mechanism. Lithium diffusion along the c-direction exhibits a highly concerted feature, which is absent in the ab-plane diffusion. A disorder–order transition of the lithium sublattice can occur below a critical temperature. Our results show that the ordering occurs with a regular pattern of lithium ions. The lithium sublattice ordering is strongly influenced by yttrium cation arrangement and can be suppressed if a small amount of Li/Y antisite defects are present. These understandings can help to provide guidance for the future development of anisotropic superionic materials.