Crossover dynamics of non-Fickian ionic diffusion in solids.

Abstract

Ionic diffusion in solids underpins energy storage, electronics, and catalysis, yet conventional diffusion models often fail to capture complexities arising from confinement, crystallographic disorder, lattice distortions, and coupled transport with phonons or electrons. These challenges are particularly pronounced in battery materials, where ionic and electronic carriers move together, complicating the interpretation of electrochemical measurements. Here we employ tracer exchange as a direct, non-electrochemical probe to reveal rich ion dynamics in the model one-dimensional (1D) conductor olivine Li_XFePO₄ (0 ≤ X ≤ 1). ⁶Li-⁷Li isotope exchange confirms single-file diffusion (SFD), where 1D confinement prevents ion bypassing and preserves spatial order. Kinetic Monte Carlo (KMC) simulations and chronoamperometry further quantify Faradaic and non-Faradaic surface exchange, identifying electron transport as rate-limiting during electrochemical reactions. In contrast, Li-Na exchange exhibits apparent superdiffusion, where the exchange rate increases with Na content. Simulations attribute this behavior to surface-exchange limitation and Na⁺-enhanced Li⁺ cross-channel hopping that drives a dimensional crossover from 1D to quasi-2D transport, supported by 4D-STEM and in situ synchrotron XRD. These results establish tracer exchange as a powerful platform for probing coupled multi-ion and electron transport in solids.