Atomistic Insights into the Hydration of LaCl 3 -Based Lithium Superionic Conductors

Halide-based solid electrolytes are attracting significant attention for next-generation solid-state batteries due to their high ionic conductivity and compatibility with high-voltage positive electrodes. However, their sensitivity to moisture remains a major challenge for practical applications. In this study, the interaction between water molecules and lithiated LaCl3 surfaces is investigated using density functional theory and ab initio molecular dynamics simulations. Two representative low-index surfaces, (001) and (100), are analyzed to examine the influence of crystallographic orientation on water adsorption behavior. Surface energy calculations show that the (100) surface is thermodynamically more stable than the (001) surface. Water adsorption exhibits clear site preferences, with O−La coordination dominating on the (001) surface and O−Li coordination favored on the (100) surface, resulting in the potential formation of LaCl3·nH2O and LiCl·nH2O hydrates, respectively, as confirmed experimentally. The surfaces are revealed to be highly dynamic with libration of the LaClx and LiClx polyhedra and migration of the Li ions across the surface and to and from the bulk. These results provide atomistic insights into water–surface interactions in LaCl3-based electrolytes and contribute to understanding their moisture stability.