Unraveling Sodium-Ion Dynamics in Honeycomb-Layered NaMgZnTeO Solid Electrolytes with Solid-State NMR.

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (Na, Te, Mg, and Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide NaMgZnTeO solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10 S cm for NaMgZnTeO at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the 622 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of Mg and Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τ = 0.83 × 10 Hz) by using a laser to achieve variable high-temperature (∼860 K) Na NMR, which are sensitive to different Mg/Zn ratios. The NaMgZnTeO with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.