1. Field of the Invention
The present invention relates to secondary batteries to be used for electric power load leveling or electric vehicles, and particularly to sodium sulfur cells adaptable for achieving prolongation of life thereof and a process for manufacturing the same.
2. Related Art Statement
Sodium sulfur cells are high temperature type secondary cells which operate at 300.degree. C.-350.degree. C. and are composed of molten sodium as an anode active material, molten sulfur and/or sodium polysulfide as a cathode active material, a sodium ion conductive ceramic as a solid electrolyte, and a metallic container. The structure of a typical sodium sulfur cell is shown in FIG. 1.
In FIG. 1, the numeral 1 indicates a sodium ion conductive beta alumina tube having a closed tip end, 2 is a metallic container functioning as a cathode, 3 is sulfur or sodium polysulfide, 4 is a metallic container functioning as an anode, 5 is sodium, 6 is a ring insulator such as an .alpha.-alumina, and 7 is a metallic lid. As the beta alumina 1, there are .beta."-alumina, .beta.-alumina, and mixture of the both, or the like.
Processes for manufacturing the above-described sodium sulfur cells generally comprise the steps of: bonding the open end periphery of the prepared .beta.-alumina tube 1 with the ring insulator 6 of .alpha.-alumina or the like, by means of glass-soldering or the like; further bonding the ring insulator 6 supporting the .beta.-alumina tube 1, with the metallic containers 2 and 4, by a solid phase reaction or the like at a high temperature under pressure, to form upper and lower spaces; then loading the metallic containers 4 and 2 with sodium 5 and sulfur or sodium polysulfide 3, respectively; and hermetically closing the metallic containers 2 and 4 with the lids 7, 7', by means of welding 8, 8' or the like, to provide a cell.
In the sodium sulfur cells having the above-described structure, if a current concentration, an overvoltage on the end of charging or discharging, or impurity inclusion in the active materials occurs, there have arisen problems of fracturing of the beta alumina tube and impairment of charging or discharging efficiency as the resistance increases. In particular, though the impurity inclusion into the active materials is a serious problem, no attention has so far been paid, in the manufacture of the sodium sulfur cells, in respect of what kind of impurities and what degree of inclusion will cause the problems.