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.
The transition from fossil fuels to renewable energy sources requires economic, high-performance electrochemical energy storage. Hightemperature sodium-metal chloride batteries combine long cycle and calendar life, with high specific energy, no self-discharge, and minimum maintenance requirements, while employing abundant raw materials. However, largescale deployment in mobility and stationary storage applications is currently hindered by high production cost of the complex, commercial tubular cells and limited rate capability. The present study introduces sodium-metal chloride cells with a simple, planar architecture that provide high specific power while maintaining the inherent high specific energy. Rational cathode design, considering critical transport processes and the effect of cathode composition on the cell resistance, enables the development of high-performance cells with average discharge power of 1022 W kg −1 and discharge energy per cycle of 258 Wh kg −1 on cathode composite level, shown over 140 cycles at an areal capacity of 50 mAh cm −2 . This corresponds to a 3.2C discharge over 80% of full charge. Compared to the best performing planar sodium-metal chloride cells with similar cycling stability and mass loading in the literature, the presented performance represents an increase in specific power by more than a factor of four, while also raising the specific energy by 74%.
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Na–NiCl 2 thermal batteries have been developed for applications such as electric energy backup, energy storage, and automotive application. A typical Na–NiCl 2 battery consists of a molten sodium anode, a solid‐state electrolyte (β″‐alumina), and a secondary liquid electrolyte (NaAlCl 4 ) in the cathode side, with NiCl 2 as the active cathode materials. These systems, operating at high temperatures (about 270–350 °C), provide a battery completely independent of ambient temperature with high specific energy and high specific power. Special characteristics such as relatively low cost, long deep cycle life, long‐term cold‐storage life, abuse resistant, zero electrical self‐discharge, maintenance free, and safety place this technology as the best choice where environmentally strong conditions are present and other secondary battery systems fail.
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