An Unexpected Pathway: ⁶Li-Exchange NMR Spectroscopy Points to Vacancy-Driven Out-of-Plane Li-Ion Hopping in Crystalline Li₂SnO₃

The development and engineering of new materials for modern electrochemical energy-storage systems requires an in-depth understanding of Li-ion dynamics, not only on the macroscopic length scale but also from an atomic-scale point of view. Hence, the study of suitable model systems is indispensable to understand the complexity of nonmodel systems already applied, for example, as active materials in rechargeable batteries. Here, Li₂SnO₃ served as such a model system to enlighten the elementary steps of ion hopping between the three magnetically distinct Li sites. Through high-resolution 1D and 2D NMR spectroscopies, we probed the favored exchange pathway. Both 1D and 2D NMR spectroscopies point to nonuniform ion dynamics and two independent exchange processes perpendicular to the ab plane, namely, between the sites 4e [Li(3)] and 8f [Li(1)] and between 4e and 4d [Li(2)]. ⁶Li selective-inversion NMR spectroscopy confirmed extremely slow Li exchange and yielded hopping rates on the order of 3 s^-1 for 4e-8f and 0.7 s^-1 for 4e-4d. Altogether, the findings provide evidence for a three-site, two-exchange model describing Li hopping along the c axis rather than in the Li-rich ab plane as one would expect at first glance. This unexpected result can, however, be understood when the site preference of Li vacancies is considered. Recent theoretical calculations predicted the preferred formation of Li vacancies at the Li(3) sites. This allows for localized Li-ion exchange involving Li(3), thus, perfectly corroborating the present findings obtained by ⁶Li MAS NMR spectroscopy.