Sodium-sulphur

It has been reported that although pyrite is only slightly floatable in self-induced flotation, the floatability is pronounced if sodium sulphur is added, which is called sulphur-induced collectorless flotation. As can be seen from... 

During the past two decades, the sodium-sulphur battery which... 

Table 11.3. Characteristics of the sodium-sulphur cell (Hitachi)...

Sodium-sulphur batteries with /3-alumina electrolyte ( beta batteries )... 

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 ... 

Almost all practical sodium-sulphur cells are based on electrolytes formed as closed tubes. These are usually manufactured by isostatic pressing, or electrophoretic deposition of powdered /3-aluminas (or their precursors)... 

For EV applications (e.g. urban delivery vans) the target figures for a commercially viable sodium-sulphur battery would be for a cycle life of... 

Fig. 8.16 Schematic diagram of a sodium-sulphur cell for electric vehicle applications...

Fig. 8.18 Charge-discharge charaderisiics Cor a sodium-sulphur cell. (By permission of Elsevier Sequoia F.G.R. Zobel.. /. Power Saitrrrx. 1978, 3. 29.)...

A fleet of Ford Ecostar vans is now operating in the US and Europe, powered by 34 kWh sodium-sulphur batteries. There have been a number of incidents in which sodium-sulphur batteries have been destroyed by the exothermic reaction which occurs when sodium and sulphur react chemically. Despite this, field trials are continuing indicating that these problems may not be insurmountable. 

Strong contenders for automotive power are the sodium/sulphur and sodium/ nickel chloride batteries, the latter known as the ZEBRA cell. ZEBRA was originally (c. 1979) an acronym devised for commercial security reasons but now it stands for the very apt Zero Emissions Batteries Research Activity . Several European car manufacturers including BMW and Mercedes have incorporated the ZEBRA cell into prototype cars, vans and buses. The performance of the battery far outstrips that of the lead/acid counterpart, as is evident from Fig. 

It follows from this brief outline that if maximum useful power is to be extracted from the cell then the electrolyte resistance must be kept to a minimum. This is dependent not only on the material but also on the geometry of the membrane which must be as thin as is practicable. As in the case of the fuel cell, the e.m.f can be calculated from thermodynamic principles. In fact the chemical reactions occurring when the sodium ions react with the sulphur are rather complex and the sodium sulphur ratios change as the battery discharges. The first product as the cell discharges is the compound Na2S5. 

When all the sulphur has been converted to Na2S5 this compound is gradually converted to Na2S3 as the discharge proceeds. The sodium/sulphur cell operates at a temperature of approximately 350 °C (the melting points of both Na2S5 and Na2S3 are close to 240 °C). 

Amongst other new systems under study are the sodium/sulphur battery with sodium / -alumina solid electrolyte operating at 300-375°C and Li-FeS batteries operating at about 450°C. Long-term battery research is directed towards batteries that can operate at room temperature with aqueous electrolyte, such as zinc-halogen, aluminium-air, and iron-air. 

Review of the British Rail Sodium Sulphur Battery Programme, October 1980 R. M. Dell, J. L. Sudworth, and I. Wynn Jones, Proceedings of the 11th lECEC, State Line, 1976, p. 503. 

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