Operating sodium-sulfur battery

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. 

Zinc-Bromide. Unlike sodium-sulfur batteries, zinc-bromide batteries operate at ordinary temperatures. Although they use low-cost, readily available... 

The disadvantage of the heat loss which is caused by the high operating temperature is compensated by the advantage that the sodium/sulfur battery can be operated independently of the ambient temperature, which can vary from -40 to +50 °C. 

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. 

The Ecostar used high-temperature, sodium-sulfur batteries because of their range, but they also allowed the van to go from 0-60 in 12 seconds. The Ecostar had no trouble keeping up with gas vehicles, but Ford only built about 80 Ecostars. The sodium-sulfur batteries proved to be too sensitive to cold weather. They operated at 500° Fahrenheit and caused fires in several of the demonstrator cars. 

Another direction of battery development involves high temperature and larger units. NGK Insulators, Ltd., in Japan uses sodium-sulfur batteries operating at 427°C (800°F) that are able to deliver 1 mW for 7 hours from a battery unit. The size of these units is about the size of a bus. Such units could be used at electric filling stations that are not connected to the grid. 

American Electric Power is installing a 6 mW wind farm with battery storage for 27 million, or at a unit cost of 4,500/kW, using NGK Insulator s sodium-sulfur batteries made in Japan. The rationale for the installation is that although the wind turbines operate mostly at night when the value of electricity is low, by storing the electricity generated until the next peak period, its value is much increased. 

Sodium-sulfur battery— Secondary -+ battery employing molten sodium and molten sulfur/sodium sulfide) as active masses and a sodium-conducting aluminum oxide as solid electrolyte operating at about T = 350 °C. The electrode reactions are... 

Since the principle of the sodium sulfur battery was established in 1967, it has been under development throughout the world. The schematic set-up of a sodium sulfur battery, operated at 300 350 °C, is shown in Figure 22. Molten sodium, the anode active material, is placed in a sintered S-alumina solid electrolyte tube, and molten sulfur impregnated in the porous graphite cathode, outside. The... 

The sodium/sulfur battery operates around 570-620 K and consists of a molten sodium anode and a liquid sulfur cathode separated by a solid P-alumina electrol5he (see Section 27.3). The cell reaction is ... 

The sulfur-sodium polysulfide system has received the attention of electrochemists but few of the studies have been under conditions comparable to sodium-sulfur battery operating conditions. The thermodynamics of the system have been studied by means of open-circuit potentials (17,27), and dynamic measurements have been made in fused salts (28). The most pertinent studies are those of sulfur-polysulfide electrochemistry in the actual sulfur-polysulfide melts (24, 29, 35). The results of these studies seem to indicate that both the oxidation and reduction reactions are rapid, although the oxidation reaction is hindered by the formation of an insulating sulfur film. These studies also concluded that the electrode reaction sequences were quite complex because of the multitude of polysufide species. As the system becomes better characterized more quantitative descriptions are possible as evidenced by a recent work which modeled the resistive drop through an actual sulfur impregnated graphite electrode in order to correlate the spatial distribu-... 

Once the principles of operating in a molten salt environment have been grasped, suitable extrapolations or interpolations of materials requirements and cell and equipment designs can be made between different systems. In bringing a molten salt process into commercial operation, unique materials problems requiring special solutions often limit its progress, but practically never prevent it. Thus, if a desired result may not be achieved for theoretical reasons in any alternative electrolyte, because of electrochemical instability, for example, then initial development costs and difficulties become inconsequential. Such has been the case with thermal batteries, " sodium-sulfur batteries, molten fluoride nuclear reactors, and molten carbonate fuel... 

The principle of operation is illustrated on Figure 4. The fast ion conductor 3-aluminium has been developed as the basic component of the sodium sulfur battery cell. Whether it will give birth to a new technological process is too early to predict. 

A new generation of batteries is under development that has distinct advantages over the traditional lead-sulfuric acid batteries both in terms of weight and energy density, and which can be adapted to road or rail transport. Most attention to date has been applied to the sodium-sulfur battery, in which liquid sodium and liquid sulfur are separated by a diaphragm of )8-alumina. The cell is operated at 300-350°C, and the cell reaction is... 

Figure 12.2. Principle of operation of the sodium-sulfur battery...

The most advanced system of this complex is the sodium/sulfur battery. Cost estimates on high-temperature batteries show that after the development phase has been completed and prototypes tested, these systems may operate well inside economical margins, assuming that mass production starts. In case these vehicles and their batteries are only produced in small numbers, the same problem will be at hand, as already discussed with the lead-acid battery. A deficiency of mass production makes vehicles and batteries artificially expensive. 

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