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

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.