Impact of Liquid Phase Formation on Microstructure and Conductivity of Li-Stabilized Na-β″-alumina Ceramics

Na-β″-alumina ceramics are archetypical ion conductors with excellent sodium-ion conductivity. Their processing is, however, challenging and results in large variations in reported conductivity measurements. We systematically reexamine the impact of sintering conditions on microstructure and sodium-ion conductivity of Na-β″-alumina ceramics. Depending on sintering temperature and sintering time, we measure conductivities between 0.04 and 0.37 S/cm at 300 °C on ceramics prepared from identical starting powders. During sintering, formation of a liquid phase is observed above 1500 °C, which promotes densification but leads to abnormal grain growth for extended sintering times. While such conditions result in the highest conductivities measured for our sample series (0.37 S/cm at 300 °C), the corresponding microstructures are mechanically fragile. For mechanically robust, densely sintered samples, we identify the average grain size as the dominating factor controlling ion conductivity. For average grain sizes between 1 and 6 μm, we obtain conductivities between 0.17 and 0.27 S/cm at 300 °C. The influence of porosity in undersintered, highly porous samples is well accounted for by Archie’s law and results in low ion conductivities down to 0.04 S/cm at 68% density. Our insights into microstructural factors controlling ionic conductivity such as grain size and density are instrumental for the successful integration of Na-β″-alumina ceramic electrolytes into next-generation batteries.