Degradation of beta″-alumina electrolyte by calcium impurities
Abstract Calcium impurities, incorporated into the beta″-alumina electrolyte during its production, are known to cause a resistance rise of sodium/sulfur cells with time of cycling, decreasing the efficiency and leading to a short cell life. This paper deals with the effect of calcium impurities which are either contained in the sodium or introduced into the solid electrolyte by ion exchange. It is demonstrated that calcium impurities above about 50 ppm in the sodium are detrimental to the long term operation. The effect of calcium incorporated by ion exchange is even stronger than that of calcium impurities of 500 ppm in the sodium. The influence of calcium impurities in the sodium is due to a Ca-rich layer on the inner surface without Ca penetration into the bulk. In contrast, ion exchange does not only produce the Ca-rich layer, but also leads to a uniform distribution of calcium in the beta″ -alumina without preferential segregation in grain boundaries or the formation of second phase.