Harnessing Machine Learning Potential to Assess Subtle Coupled Transrotational Ionic Dynamics in Solid State Electrolyte

Pursuing higher energy density and improved battery safety has drawn growing attention to solid electrolytes. In this study, we developed a machine learning potential (MLP) tailored for the complex lithium superionic conductor Li11AlP2S12 (LAPS). It enables molecular dynamics (MD) simulations that combine density functional theory (DFT)-level accuracy with extended simulation time scales, from picoseconds to nanoseconds. Using this machine learning molecular dynamics (MLMD) framework, we explored the thermal response of the [PS4]3– polyanions in detail. The prolonged simulation window allowed us to capture infrequent yet informative rotational dynamics, providing a deeper perspective on the structural evolution and transport characteristics of the system. Two different rotational motions were observed at different thermal conditions. At lower temperatures, the polyanions primarily exhibited localized vibrations or wiggling within their lattice sites. However, as the temperature increases, more pronounced rotational motion becomes apparent, signaling a transition to broader angular flexibility. These thermally activated rotations were analyzed to understand their influence on lithium-ion mobility. Although absent at lower temperatures (≤400 K), at high temperatures, the evidence of transrotational dynamics, a direct correlation between polyanion rotation and lithium-ion translation, often referred to as the paddlewheel effect, is observed. The dynamic tilting and orientation of the anion motifs appear to facilitate transient coordination environments that favor ion migration. The local structural rearrangements support smoother lithium hopping events within the crystal lattice. The LAPS material has a computed lithium-ion diffusion energy barrier of 0.20 eV, which is in close agreement with experimental measurements. Overall, the MLMD simulations captured subtle yet meaningful rotational phenomena of the [PS4]3– units at higher temperatures (≥500 K), and offer insights into their potential contributions to ion transport behavior in thiophosphate-based solid electrolytes.