Polaron-Coupled Lithium-Ion Diffusion Rate Constant on MoS ₂ Surface: Nonadiabatic Effect

Ionic diffusion is a fundamental property of electrode materials in metal-ion batteries and is often coupled with charge transfer processes. In this study, lithium (Li)-ion diffusion on MoS2 surfaces is modeled using nonadiabatic transition-state theory combined with Landau–Zener-type transition probabilities. The parameters for the rate-constant calculations are obtained from constrained density functional theory (cDFT). Unlike traditional approaches that assume adiabatic diffusion, the present results reveal significant nonadiabatic effects arising from ion-coupled electron transfer, wherein Li-ion diffusion is accompanied by the formation of a small polaron on the MoS2 surface. These nonadiabatic effects reduce the diffusion rate constant by a factor of 0.43 compared with the adiabatic approximation. Under charge/discharge conditions, the Li ion remains in a cationic state, while the small polaron increases the diffusion barrier from 0.161 to 0.244 eV. This effect is further elucidated by analyzing vibrational motions of the Li ion perpendicular to the MoS2 surface. The findings underscore the importance of incorporating a nonadiabatic correction in modeling Li-ion transport and offer computational insights for the optimization of MoS2-based electrode materials.