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 LiXFePO4 (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.
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Acknowledgments
We thank Reginaldo J. Gomes from Prof. Chibueze Amanchukwu’s group for providing 2 M 6LiOH(aq). We thank Evguenia Karapetrova from Argonne National Laboratory for assistance with synchrotron XRD measurements at the 33-BM beamline of the Advanced Photon Source (APS). We thank Vivek Thampy, Molleigh B. Preefer, and Christopher J. Takacs from SLAC National Accelerator Laboratory for setting up synchrotron XRD measurements at the SSRL 11–3 beamline. This work made use of instruments in the Electron Microscopy Core, Research Resources Center, at the University of Illinois at Chicago.
Funding
G.Y. and C.L. acknowledge support from the Energy Storage Research Alliance (ESRA, DE-AC02-06CH11357), an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). P.O. and M.W. acknowledge support from Shell Global Solutions International B.V. Z.T., and Q.C. acknowledge support from the U.S. National Science Foundation (NSF) under award No. 2427924 for 4D-STEM data collection and analysis. M.W. and H.Z. acknowledge use of the Stanford Synchrotron Radiation Lightsource (SSRL), SLAC National Accelerator Laboratory, supported by the U.S. DOE, Office of Science, BES, under Contract No. DE-AC02-76SF00515. B.L. and E.E.A. acknowledge use of the Advanced Photon Source (APS), a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. B.L. further acknowledges support from the National Science Foundation, Division of Earth Sciences (EAR), through SEES under award No. EAR-2223273.
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Yan, G., Ombrini, P., Tang, Z. et al. Crossover dynamics of non-Fickian ionic diffusion in solids. Nat Commun (2026). https://doi.org/10.1038/s41467-026-73937-w
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DOI: https://doi.org/10.1038/s41467-026-73937-w
