Sodium sulfur cell

Sodium-Sulfur Batteries. The sodium-sulfur battery consists of molten sodium at the anode, molten sulfur at the cathode, and a solid electrolyte of a material that allows for the passage of sodium only. For the solid electrolyte to be sufficiently conductive and to keep the sodium and sulfur in a liquid state, sodium-sulfur cells must operate at 300°C to 350°C (570°F to 660°F). There has been great interest in this technology because sodium and sulfur are widely available and inexpensive, and each cell can deliver up to 2.3 volts. 

The molten salt electrolyte also contributes to the safety behavior of ZEBRA cells. The large amount of energy stored in a 700 g cell, which means about 30 kWh in a 300 kg battery, is not released suddenly as heat as be expected in a system with liquid electrodes such as the sodium sulfur cell. In the case of accidental destruction of ZEBRA cells, the sodium will react mainly with the molten salt, forming A1 sponge and NaCl. -The diffusion of the NaAICI ... 

Corrosion-Resistant Materials for Sodium/Sulfur Cells... 

A sodium-sulfur cell is one of the more startling batteries (Fig. 12.23). It has liquid reactants (sodium and sulfur) and a solid electrolyte (a porous aluminum oxide ceramic) it must operate at a temperature of about 320°C and it is highly dangerous in case of breakage. Because sodium has a low density, these cells have a very high specific energy. Their most common application is to power electric... 

The high ionic conductivity of sodium (3"-alumina suggested that it would form a suitable electrolyte for a battery using sodium as one component. Two such cells have been extensively studied, the sodium-sulfur cell and the sodium-nickel chloride (ZEBRA) cell. The principle of the sodium-sulfur battery is simple (Fig. 6.13a). The (3"-alumina electrolyte, made in the form of a large test tube, separates an anode of molten sodium from a cathode of molten sulfur, which is contained in a porous carbon felt. The operating temperature of the cell is about 300°C. 

Figure 6.13 Batteries using p"-alumina electrolyte, schematic (a) the sodium-sulfur cell and (b) the sodium-nickel chloride (ZEBRA) cell.

Sodium cells20-23 operate at fairly high temperatures (300-400 °C) and require an inert atmosphere (argon) in a sealed, corrosion-resistant vessel (e.g., Cr-coated steel). Furthermore, leakage of liquid Na could obviously have dire consequences. Nevertheless, sodium-sulfur cells have received serious consideration as rechargeable batteries ... 

Figure 3.5.4 (A) Sodium sulfur cell. Source NASA John Glenn Research Center, public domain. (B) Na/NiCh cell. Source [10],...

C, and sodium at 98°C). The sodium-sulfur cell (Fig. 17.10), for example, has an optimal operating temperature of 250°C. The half-cell reactions are... 

What makes the sodium-sulfur cell possible is a remarkable property of a compound called beta-alumina, which has the composition NaAlnOiy. Beta-alumina allows sodium ions to migrate through its structure very easily, but it blocks the passage of polysulfide ions. Therefore, it can function as a semipermeable medium like the membranes used in osmosis (see Section 11.5). Such an ion-conducting solid electrolyte is essential to prevent direct chemical reaction between sulfur and sodium. The lithium-sulfur battery operates on similar principles, and other solid electrolytes such as calcium fluoride, which permits ionic transport of fluoride ion, may find use in cells based on those elements. 

Sodium-sulfur cell — used to power electric vehicles, has all liquid reactants and products, a solid electrolyte, and relatively-high voltage. 

The discovery of a solid conductor of sodium ions by Kummer and Weber made possible the construction of sodium-sulfur cells which utilize molten or dissolved reactants separated by the ceramic electrolyte j3-(cf. Fig. 12), or, usually, j3"-alumina. The latter ceramic has a three Al-0 spinel block structure, a molar ratio of Al203-Na20 = 5, and contains 1-4% of MgO or Li20. The resistivity of the polycrystalline material at 350°C is about 5 H cm, -4 times lower than that of alumina. Other recently reported solid Na ion conductors containing phosphorus oxides do not seem to be stable in contact with sodium at elevated temperatures. ... 

Figure 13 Schematic of Ford sodium-sulfur cell.

The alkali metals—lithium, sodium, and potassium—are logical choices for anodes in a sulfur-based electrochemical cell. All three have been incorporated into cells, and lithium and sodium remain under serious consideration. The lithium-sulfur combination is the topic of another chapter in this volume and will not be discussed further. Two types of sodium-sulfur cells have been constructed. One type uses thin-walled glass capillaries as a cell divider, and the other uses various sorts of ionically conducting sodium aluminate for this purpose. Of the two, the latter seems to hold the most promise and certainly has generated the most interest and enthusiasm (1). Because of the unique properties of the solid electrolyte cell separator this battery is also probably the most interesting from a purely scientific point of view. 

This paper restricts itself to sodium-sulfur cells and batteries which use solid electrolyte cell dividers and provides a current picture of the state of scientific knowledge and technological achievement with respect to sodium-solid electrolyte-sulfur batteries. The references cited should not be construed as a complete review but should instead be viewed as an introduction to the relevant literature. 

A diagram of a recent sodium-sulfur cell is shown in Figure 1 along with a photograph of the actual cell. This cell was constructed from interchangeable hardware and was designed for laboratory studies rather... 

The construction of operational, hermetically sealed sodium-sulfur cells requires container materials, which are mechanically suitable and compatible with sodium and sulfur-sodium polysulfide, current leads to the sulfur-graphite electrode, and several kinds of seals. These requirements of course are in addition to those for the solid electrolytes and sulfur electrodes described earlier. The problem of satisfying these requirements has been summarized by Gratch and co-workers (3). Particular applications—utility load-leveling, traction power, and military— impose further design constraints dictated by performance, capacity, size and weight, life, cost, and safety requirements. 

Although the cited examples are far from being prototypes of production devices, they do validate the feasibility of fully packaged sodium-sulfur cells and provide a measure of projected performance. It seems that sufficient information is now available to permit researchers to speculate on the design of large cells and battery systems and to project the performance of such devices on the basis of anticipated technology. 

The principle of a sodium-sulfur cell is shown in Fig. 12. The solid electrolyte is a Na+ ion conductor, consisting of )0-Al2O3. It is generally used as a tube closed at one end and filled with liquid sodium as the anode. An iron sponge, which absorbs the liquid sodium, serves to improve the wetting of the electrolyte and to improve safety. A metal wire leads out of the anode to carry the current. The cathode consists of liquid sodium poly sulfide and sulfur inserted in porous graphite. The working temperature of the sodium-sulfur cell is around 300°C. 

In the cell reaction sodium ions pass through the electrolyte and electrons through the external circuit, so that sodium is dissolved in sodium polysulfide. In this way electrical energy can be liberated. The energy density of the sodium-sulfur cell is many times greater than that... 

---