Direct Assessment of Ultralow Li⁺ Jump Rates in Single Crystalline Li₃N by Evolution-Time-Resolved ⁷Li Spin-Alignment Echo NMR

Diffusion processes of small cations and anions play important roles in many applications such as batteries and sensors. Despite the enormous progress we have witnessed over the past years in characterizing the irregular movement of ions such as Li⁺, new methods able to sharpen our view and understanding of fast and slow diffusion phenomena are steadily developed. Still, very few techniques are, however, available to directly sense extremely slow Li⁺ diffusion processes. Here, we took advantage of 1D evolution-time resolved ⁷Li spin-alignment echo NMR that is able to probe the extremely slow interlayer Li⁺ hopping process in layer-structured Li₃N, which served as a model substance for our purposes. The use of single crystals enabled us to study this translational process without being interfered by the fast intralayer Li⁺ motions. At 318 K the corresponding jump rate of interlayer dynamics turned out to be in the order of 2500(200) s⁻¹ resulting in a diffusion coefficient as low as 1×10⁻¹⁷ m² s⁻¹, which is in excellent agreement with results from literature. The method, comparable to 1D and 2D NMR exchange spectroscopy, relies on temporal fluctuations of electric interactions the jumping ions are subjected to. ⁷Li single crystal 1D SAE NMR offers new opportunities to precisely quantify slow Li⁺ diffusion processes needed to validate theoretical models and to develop design principles for new solid electrolytes.