Molecular-Ratchet Like Cation-Anion Coupling Dynamics Assist Ion Migration in Room Temperature Ionic Conductors.

Abstract

A deep understanding of ion transport mechanisms in solids is crucial to the rational design and optimization of ionic conductors. In this work, ab initio molecular dynamics (AIMD) simulations at room temperature were employed to elucidate the cation-anion dynamic coupling and its critical role in promoting cation migration in lithium ionic conductors. Polyanion groups exhibit only small-amplitude motions under ambient conditions, with no detectable full rotations. Anion sublattice relaxations significantly enhance Li-ion hopping frequency despite shortening Li-anion bonds, contracting lithium polyhedra, and narrowing diffusion channels, indicating that ion transport is highly affected by the cation-anion dynamic coupling rather than the favorable local structural changes upon anion framework relaxation. Velocity cross-correlation analysis reveals strong short-time scale couplings between Li ion velocities and anion velocities. These velocity correlations oscillate rapidly between positive and negative values, enabling successive forward and backward polyanion motions to cooperatively propel adjacent Li ions in a consistent net direction across multiple cycles, similar to the molecular ratchet mechanism. This positive-negative alternating correlation pattern is observed universally across the common sulfides and oxides. The molecular ratchet mechanism with the coupled cation-anion dynamics rather than the classical paddle-wheel rotation serves as the intrinsic cooperative mechanism to enhance Li-ion diffusivity in ionic conductors at room temperature. Our findings refine the mechanistic understanding of fast ion conduction and provide new theoretical guidance for the design of high-performance ionic conductors.