Cell voltage sodium sulphur

Wh/kg. As indicated in the sodium-sulphur phase diagram given in Fig. 8.15, sodium pentasulphide and sulphur are not mutually soluble at the temperature of cell operation, so that two liquid phases are present in the cathode compartment and the cell voltage is invariant. As the discharge progresses and the available elemental sulphur is consumed, a series of reactions commences as the sodium pentasulphide is converted to lower polysulphides, all of which are mutually soluble ... 

Cells are assembled in the fully charged state and the positive electrode mix contains antimony trichloride (29%), sodium chloride (15%), aluminium chloride (48%), carbon black (8%) and small amounts of sulphur, which are added to give a higher average discharge voltage. 

The cell is based upon a liquid sodium anode and liquid sulphur cathode, separated by a beta alumina ceramic-type electrolyte which is an electronic insulator, but through which sodium ions diffuse rapidly at 300-400 C. During discharge the reaction 2Na + 5S —> Na S. leads to an open circuit voltage (GCV) of 2.08V. Continued reaction beyond Na S results in the formation of lower polysulphides in the range Na - Na S (OCV 1.78V), after which solid separates out. The polysulphides... 

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