Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na 2 Mg x Zn 2- x TeO 6 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 ( 23 Na, 125 Te, 25 Mg, and 67 Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na 2 Mg x Zn 2- x TeO 6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10 -4 S cm -1 for Na 2 MgZnTeO 6 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 P 6 3 22 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125 Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23 Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25 Mg and 67 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 NMR = 0.83 × 10 9 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23 Na NMR, which are sensitive to different Mg/Zn ratios. The Na 2 MgZnTeO 6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.