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This paper briefly describes sodium sulfur (NAS) battery development with emphasis on the program to establish the technology for the use of a β-alumina solid electrolyte. Since the mid-1980s, NGK INSULATORS, LTD. (NGK) and the Tokyo Electric Power Company (TEPCO) have jointly conducted the NAS battery development program in Japan and, in April 2003, NGK launched production on a commercial scale. A critical aspect of NAS battery development has been achieving low resistance to the flow of sodium ions through the β-alumina solid electrolyte while ensuring material properties consistent with high mechanical strength, as well as with the dimensional stability required for automated manufacturing. The resistance to sodium ion flow within β-alumina depends on three factors: grain resistivity, grain boundary resistivity, and crystal orientation. NGK's approach to addressing these factors is explained in this paper.