Analysis of c-lattice parameters to evaluate Na2O loss from and Na2O content in β''-alumina ceramics

Abstract

Phase pure Na-β''-alumina ceramics are excellent electrolytes for molten-salt and all-solid-state batteries. However, controlled processing of this complex material is required to obtain the desired phase content, as Na-β''-alumina can accommodate variable amounts of sodium within its conduction slab. On the one hand, addition of dopant ions (e.g. Li+, Mg2+) influences stability and sodium content of Na-β''-alumina. On the other hand, β''-aluminas are prone to sodium evaporation, resulting in compositional changes during high-temperature sintering. Here, we show that while sodium evaporation indeed becomes significant at temperatures > 1300 °C, Na-β''-alumina ceramics of constant total composition can be prepared at strongly variable sintering conditions by applying an encapsulation. Nevertheless, our study reveals that the sodium content in the Na-β''-alumina crystal structure varies with the processing temperature. This is indicated by changes in the c-lattice parameter, which decreases with increasing sintering temperature, corresponding to an increase in sodium content.