Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2- xTeO6 Solid Electrolytes with Solid-State NMR

Abstract

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 (²³Na, ¹²⁵Te, ²⁵Mg, and ⁶⁷Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na₂Mg_xZn_2-xTeO₆ 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 Na₂MgZnTeO₆ 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 P6₃22 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using ¹²⁵Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) ²³Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of ²⁵Mg and ⁶⁷Zn 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 × 10⁹ Hz) by using a laser to achieve variable high-temperature (∼860 K) ²³Na NMR, which are sensitive to different Mg/Zn ratios. The Na₂MgZnTeO₆ with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.