Impact of Liquid Phase Formation on Microstructure and Conductivity of Li-Stabilized Na-beta "-alumina Ceramics

Na-beta "-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-beta "-alumina ceramics. Depending on sintering temperature and sintering time, we measure conductivities between 0.04 and 0.37 S/cm at 300 degrees C on ceramics prepared from identical starting powders. During sintering, formation of a liquid phase is observed above 1500 degrees 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 degrees 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 mu m, we obtain conductivities between 0.17 and 0.27 S/cm at 300 degrees 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-beta "-alumina ceramic electrolytes into next-generation batteries.