Properties of Na-beta-alumina solid electrolyte with sodium bromide as the additive

Abstract

Sodium bromide is used as an additive to modify the sintering and conductive properties of sodium beta alumina solid electrolyte (BASE) through the halogen effect. The present study investigates the effect of NaBr on the sintering behavior, phase composition, ionic conductivity and mechanical strength of BASE. BASE with a high content of NaBr showed a high open porosity, and retarded densification at higher temperature. BASE with a high proportion of NaBr also exhibited a low relative β''-phase content and a large c-lattice parameter. BASE with 1% ratio of Br anions was found to possess the largest proportion of abnormal grains in the microstructure among other BASEs, with the ratio of Br anions ranging from 0 to 2%. BASE with 1% ratio of Br anions also exhibited the highest conductivity and a better bending strength. These results are attributed to the volatilization of NaBr and the possible reaction between the Br anion and the Al–O bond at the particle surface. NaBr could effectively tailor the grain size and ionic transport at the grain boundary in BASE. Due to the less amount of grain-boundary phase and a suitable proportion of abnormal grains, BASE with 1% ratio of Br anions showed an excellent ionic conductivity of 1.01 mS cm−1 at 20 °C and a moderate bending strength of 186 MPa. This study suggests that the ionic conductivity in BASE can be increased by incorporating Br anions, and thus, offers some insights into the effect of anions on the properties of BASE.