Na+ self-diffusion and ionic transport in sodium β″-alumina

Fast ion transport is crucial for solid materials to serve as electrolytes, with only a few classes achieving the high ionic conductivity needed for practical use. The discovery of fast ion transport in Na β ″-alumina not only introduced a groundbreaking class of materials but also marked the advent of the research field now known as solid-state ionics . Since the early 1980s, Na β ″-alumina has been widely recognized as a solid electrolyte for high-temperature sodium-based batteries, such as Na‑sulfur batteries. Despite numerous studies on the diffusion and transport properties of layer-structured Na β ″-alumina, a comprehensive investigation combining conductivity and nuclear magnetic resonance (NMR) to study both short-range and long-range ion dynamics has been lacking. In this work, we used a commercially available, highly sintered Na β ″-alumina sample to explore Na + dynamics in detail through GHz conductivity spectroscopy and 23 Na nuclear spin relaxation. The two methods provide a rather consistent picture of ion transport. While long-range Na + transport, reaching a conductivity of 4 mS cm −1 at ambient temperature, is governed by an activation energy of 0.3 eV, short-range motions sense a barrier of 0.13 eV. The Arrhenius pre-factor obtained from characteristic electric relaxation frequencies is consist with that determined from diffusion-induced longitudinal NMR spin-lattice relaxation rates if analyzed with a modified BPP (Bloembergen, Purcell, Pound) spectral density function. • Long-range Na + transport in commercial Na-β″-alumina reaches 4 mS/cm, governed by an activation energy of 0.3 eV. • Short-range Na + motions sense a lower barrier of 0.13 eV as seen by both NMR (116 MHz) and ac conductivity spectroscopy (100 MHz). • The Arrhenius pre-factor aligns across methods when analyzing the NMR rate peak using a modified BPP-type model.