Li+ Transport in Ca-Substituted β -Li3PS4: Role of PS43– Vibrations and Rotations from Machine-Learning Molecular Dynamics

Abstract

Sulfide-based solid electrolytes are key materials for all-solid-state batteries owing to their high Li+ conductivity. In particular, β-Li3PS4 with aliovalent Ca2+ substitution has been found experimentally to enhance Li+ conductivity. However, the microscopic mechanisms by which Ca2+-induced defects and local structural changes govern Li+ diffusion remain unclear. Here, we develop a system-specific machine learning potential (MLP) for Ca-substituted Li3–2xCaxPS4 and perform large-scale, long-time machine learning molecular dynamics (MLMD) simulations to elucidate the structural and dynamical origins for the enhancement of Li+ diffusivity. The MLP, trained on ab initio molecular dynamics (AIMD) data for both β-Li3PS4 and γ-Li3PS4 over a range of Ca2+ substitutions and temperatures, accurately reproduces AIMD-derived energies, forces, virial stresses, and radial distribution functions, validating its reliability for describing PS43– dynamics and Ca2+-induced defects. MLMD simulations reveal that each Ca2+ is coordinated by seven S atoms belonging to five surrounding PS43–. Ca2+ substitution induces anisotropic changes in the lattice parameters, in agreement with the experiment. The Li+ diffusion coefficient exhibits a nonmonotonic dependence on the Ca2+ substitution level x: the Li+ diffusivity and the effective porosity, i.e., the volume accessible to Li+ diffusion is maximized around x = 0.05, whereas excessive Ca2+ acts as a blocking center and fragments the conduction network. The dynamics of PS43– play a central role in mediating Li+ transport. With increasing Ca2+ substitution, the vibrational amplitudes of PS43– are enhanced, while their rotational motion is progressively suppressed. These results provide microscopic design guidelines for tuning aliovalent substitution and PS43– dynamics to achieve fast Li+ conduction in sulfide solid electrolytes.