Energetically preferred Li⁺ ion jump processes in crystalline solids: Site-specific hopping in β-Li₃VF₆ as revealed by high-resolution ⁶Li 2D EXSY NMR

The visualization of atomic or ionic jump processes on the Ångström length scale is important to identify the preferred diffusion pathways in solid electrolytes for energy storage devices. Two-dimensional high-resolution ⁶Li nuclear magnetic resonance (NMR) spectroscopy is highly suited to yield unprecedented site-specific insights into local Li⁺ exchange processes within a single measurement. Here, the beta-modification of Li₃VF₆ is used as a model system for such an investigation as it provides a range of important Li⁺ geometric environments in one and the same crystal structure useful to elucidate qualitatively a ranking of energetic preferences of the Li⁺ exchange processes. In Li₃VF₆ the Li⁺ ions are subject to diffusive exchange processes among five crystallographically and magnetically inequivalent Li sites: LiF_n (n = 6, 4). By using a sample with a natural concentration of the ⁶Li isotope, we suppressed unwanted spin-diffusion processes and visualized the various exchange processes on the ms time scale. We were able to verify the following ranking experimentally: Li⁺ ion jumps between face-shared polyhedra are preferred, followed by Li⁺ exchange between edge-shared configurations for which interstitial sites are needed to jump from site to site. Surprisingly, Li⁺ exchange between corner-shared polyhedra and Li⁺ hopping involving almost isolated LiF₄ polyhedra do contribute to overall Li⁺ self-diffusion as well. In this sense, the current study experimentally verifies current predictions by theory but also extends our understanding of ion dynamics between corner-shared Li-bearing polyhedra.