Revealing Na⁺ Dynamics in the Na₄Sn₂Ge₅O₁₆ Solid Electrolyte Material Using ²³Na Solid-State NMR Spectroscopy and Computational Methods

Na₄Sn₂Ge₅O₁₆ is a novel Na⁺ conductor that can be utilized in the next generation of all-solid-state Na⁺ batteries (ASSNIBs). ²³Na ssNMR experiments were carried out on the Na₄Sn₂Ge₅O₁₆ phase to investigate the Na⁺ dynamics for the three unique crystallographic Na sites under multiple field strengths (20, 11.7, and 7 T). However, completely resolving the fast exchange Na2 and Na3 pair (4c sites) in the ²³Na NMR spectra (Na2-3 resonance) was nontrivial under the available experimental conditions. The subsequent relaxation study supports that the Na2-3 peaks are dynamically mediated, in contrast to immobile Na1 peaks. To further understand the dynamic effects on the ²³Na NMR spectra, 1D and 2D 3QMAS ²³Na spectra under various magnetic field strengths were analyzed to establish possible ranges of "averaged" quadrupolar parameter values for the two sites. With the assistance of density functional theory (DFT)-based CASTEP calculations, simulated lineshapes of the Na1 site (8d) are established, which exhibit excellent agreement with the experimentally determined ones. Subsequently, the EXchange Program for RElaxing Spin Systems (EXPRESS) script was employed to simulate the Na2-3 peaks under dynamic conditions, with a final estimated exchange rate of 2 × 10⁵ Hz. This work paves the way for future studies of Na⁺ transport at the molecular level in fast-conducting Na-based solid electrolytes (SEs).