In most conventional sodium-sulfur cells, which have the configuration Na/b-alumina/S, the molten sodium electrode is contained within the b-alumina tube, typically called the central sodium design. The molten sulfur electrode is contained on the outside between the tube and the inner walls of the metal container, which are usually coated with chromium to resist attack from the highly corrosive sulfur electrode. Cells can also be constructed with a central sulfur design, but this design provides lower power and is less tolerant to freeze-thaw cycling (i. e., cooling and heating of the battery during which the electrodes are allowed to solidify and melt repeatedly).
During discharge of Na/b-alumina/S cells, the sodium reacts electrochemically with sulfur to form a series of polysulfide compounds according to the general reaction
2Na + xS — Na2Sx
over the range 3<x<5. The reaction can be considered to take place in two steps. The initial reaction produces Na2S5, which is immiscible with the molten sulfur. This reaction is therefore a two – phase process; it yields a constant cell voltage of 2.08 V. Thereafter, the sulfur electrode discharges in a single-phase process to form Na2Sx compositions between Na2S5 and Na2S3, during which the cell voltage decreases linearly. The open circuit voltage at the end of discharge is 1.78 V. The composition Na2S3 represents the fully discharged composition because further discharge yields Na2S2, which is a solid, insoluble product that leads to an increase in the internal resistance of the cell. Moreover, it is difficult to recharge a Na2S2 electrode. In practice, therefore, it is common for the end voltage of sodium-sulfur cells to be restricted to between 1.9 and 1.78 V not only because it prevents overdischarge in local areas of the cells but also because it reduces the severity of the corrosion reactions that occur for Na2Sx products with lower values of x.
Because sodium and sulfur are relatively light elements, the sodium-sulfur battery offers a high theoretical specific energy (754mAh/g). However, the robust battery construction, heating systems, and thermal insulation that are required for efficient and safe operation place a penalty on the specific energy that can be delivered in practice (117Wh/kg). The molten state of the electrodes and high-temperature operation ensure an acceptable rate capability; a specific power of 240W/kg for electric vehicle batteries has been reported (Table I).
One of the major disadvantages of sodium-sulfur cells is that they are inherently unsafe; any fracture of the b-alumina tube that allows the intimate contact of molten sodium and molten sulfur can result in a violent reaction that is difficult to control. Furthermore, damaged cells that fail in an open-circuit mode need to be electrically bypassed to ensure continued operation of the battery. These limitations, the need for hermetic seals in a very corrosive environment, and the difficulty of meeting cost goals were among the major reasons why support for the development of sodium-sulfur batteries for electric vehicle applications was terminated worldwide in the mid-1990s. Nevertheless, considerable development of this technology is under way, particularly in Japan, for stationary energy storage applications.