Ultra-slow Li ion jump diffusion in Li₂SnO₃ studied by two-time ⁷Li spin-alignment echo NMR and ⁷Li NMR relaxometry

A deeper understanding of atomic-scale Li⁺ hopping in solid Li ion conductors requires the study of ion dynamics over a broad time scale and length scale. Here, we report a complementary study, applying ⁷Li spin-lattice relaxation (SLR) nuclear magnetic resonance (NMR) techniques together with spin-alignment echo (SAE) NMR to study slow Li⁺ self-diffusion in monoclinic Li₂SnO₃. The stannate serves as a model substance to quantify both local barriers of the elementary steps of ion diffusion as well as long-range ion transport. From spin-alignment echoes, if recorded at short preparation time of 20 μs, activation energies (0.46 eV) and Li jump rates were deduced. The same diffusion process is seen via SLR NMR (0.45 eV) performed in the rotating frame of reference at a locking frequency of 20 kHz. By comparing jump rates extracted from SAE NMR with those recently measured by high-resolution 1D ⁶Li exchange NMR we attribute the SLR and SAE NMR response to the Li(1,2)-Li(3) exchange process (5 s^{- 1}) taking place perpendicular to the ab-plane in Li₂SnO₃. SAE NMR measurements, dedicated to elaborate the influence of dipolar correlations on echo formation at t_p > 20 μs, reveal a strong dependence of the SAE rate constant on preparation time at low temperatures. This finding might points towards heterogeneous dynamics including localized jump processes which are likely to be caused by a slightly disordered structure of the oxide.