Session on Vehicle Propulsion Batteries
SODIUM-SULFUR SECONDARY BATTERY
Neill Weber and J. T. Kummer
Ford Motor Company
This secondary battery uses liquid sodium as an anode and liquid sulfur as the oxidant in the cathode compartment. The cathode can be porous carbon. The electrolyte is a solid ceramic membrane which is impervious to and resistant to chemical attack by sodium, sodium polysulfides, and sulfur. It operates in the temperature range of from 250 to 300°C.
The Ceramic Membrane
The operation of this battery is based on the electrochemical properties of a crystalline solid similar to beta-alumina. The parent material, beta-alumina, which is available commercially as a fusion cast brick, exhibits high ionic conductivity4 together with no electronic conductivity and a high resistance to chemical attack by many fused salts and metals. The high ionic conductivity of a single crystal of beta-alumina is observed in the directions perpendicular to the c axis of the crystal (hexagonal structure2) and not in the direction along the c axis. The value of this single crystal ionic resistivity is ~30 ohm-cm at 25°C and ~3.5 ohm-cm at 300°C. When these crystals are ground up to a fine powder, pressed, and sintered at a high temperature, the resultant ceramic exhibits a resistivity at room temperature of 200 to 250 ohm-cm as measured with a D.C. ohmmeter with Ag-Ag2O-5NNaOH electrodes, and a resistivity at 300°C of ~18 ohm-cm when measured with sodium electrodes and a D.C. ohmmeter or with an A.C. bridge at ~500 KC/sec using Pt electrodes. The difference between the single crystal conductivity and the conductivity of the polycrystalline ceramic, a factor of ~8 at 25°C, is larger than the estimated tortuosity of the conduction path of 2 or 3 so it is inferred that some interface resistance between the crystals of the polycrystalline ceramic may be present. This may be due to the fact that the density of the polycrystalline beta-alumina ceramic is not equal to the theoretical density of beta-alumina. (3.24 gm/cc for Na20·11A1203).
The ceramic material used in the bench cells described below (Fig. 3 and 4) and to be used in larger cells is similar to beta-alumina but has been modified to give better conductivity and sintering properties. The final sintered material is impervious as determined by a helium leak detector test. The ionic resistance of this modified material is ~3 to 5 ohm-cm at 300°C and ~250 ohm-cm at 25°C. This material has been prepared in the form of 12" tubes ~ 5 mm O.D. and as low as 0.7 mm wall thickness, and also in the form of flat disks. The composition of the modified material and the sintering techniques employed are not at this time (March, 1967) being released by the Ford Motor Company. When correctly prepared, this ceramic resists the corrosive action of various molten salts at 300°C and of sodium.
While the sodium form of this material is the one we are concerned with here, it is possible to prepare material with other monovalent cations as the mobile ion by various procedures.
The Choice of Reductant and Oxidant
The existence of an impervious nonreactive ceramic material with high ionic conductivity at relatively low temperatures gives one the opportunity to construct a variety of electrochemical devices. The major effort at the Ford Scientific Laboratory has been directed toward the development of a secondary storage battery with high energy and power density suitable for motive power applications.
The obvious selection for the anode material is sodium metal since it exhibits good electronic conductivity, shows (at 300°C) little or no polarization at the sodium ceramic interface with current densities up to several amps/cm2, and has a highelectrochemical potential.
When one uses sodium as the anode together with the ceramic membrane in a battery, sodium ions will pass through the membrane on discharge and appear in the cathode compartment. It is necessary for the sodium ion in the cathode compartment to have high ionic mobility in order to obtain a high power density battery. Two convenient ways to obtain this are by the use of molten salts or aqueous or organic solutions. Since the ceramic has exhibited poor stability to usual aqueous solutions and since most organic solutions have poor conductivity, the use of molten salts seemed indicated. The choice of molten salt is governed by the oxidizing agent used and by the desired temperature of operation. The temperature employed should be as low as possible for practical reasons and yet be sufficiently high to give adequate ceramic conductivity and power, and maintain both the reactants and products liquid.
Of a number of systems examined, the one:
2Na(l) + 3S(l) -> Na2S3(l)
offers a number of advantages. It requires only 2.1 grams of sulfur per gram of sodium and gives a theoretical energy density as written above of 346 watt-hrs per pound. The electrode reaction involved appears capable of high current density. We are not dealing with any thermodynamically unstable compounds (indeed both Na and S are elements) and since the polysulfide is the stable product there are no undesirable side reactions. The voltage of ~ 2 volts (Fig. 1) while not as high as that of some halide systems is adequate, and sulfur is inexpensive. The melting point of the polysulfide product is ~275°C which, while not as low as might be desired, is not unduly high. Since the sodium-sulfur system can be a completely sealed system with no auxiliary equipment we have favored this system and are developing a secondary battery based on it that will be cheap and have long life.37Figure1.gif)
Figure 1. Open circuit voltage of a sodium-sulfur cell vs. state of discharge.
The phase diagram for the Na-S system is shown in Fig. 2. Sulfur and sodium pentasulfide, Na2S5, are immiscible although sulfur will react with the lower polysulfides to give Na2S5. The positive electrode used in some cells is graphite felt (97% porous) which extends from the current collecting plate to the ceramic surface. Other forms of carbon such as powdered graphite can also be used. Contact of the sulfur impregnated felt with the ceramic surface is sufficient to enable discharge to take place and be maintained at a high current density. Characteristic charge-discharge curves are given in Fig. 3 for a cell of the type shown in Fig. 4 with a 3 mm thick layer of C + S and a 0.8 mm ceramic wall thickness. The voltage drop observed from the open circuit voltage can in most part be accounted for by the ohmic resistance in the leads and ceramic. These small glass (Kovar sealing glass 7052-7056 seals to beta-alumina directly) cells in which the center ceramic tube is ~ 10 cm long and 5 mm O.D. deliver ~ 20 watts maximum power (1 to 1.3 volt output) and weigh 60 grams. The energy density of these small cells which have a disproportionate weight of glass and leads is ~60 watt-hrs per pound.
37Figure2.gif)
Figure 2. Phase diagram of the system Na2S-S (after Pearson and Robinson(3)).
The graphite, polysulfide, and sulfur form a three-phase system with the electrochemical reaction of interest occurring at the three-phase interface similar, it is thought, to the operation of fuel cell electrodes. The high solubility of sulfur in the lower polysulfides Na2S3, Na2S4, at the interface meniscus as a higher polysulfide is thought to lead to sulfur transport across the meniscus from the sulfur to the carbon electrode and lead to the high current densities that are possible at this electrode. In molten Na2S4 at 300°C a smooth electrode polarizes due to concentration polarization at ~20 ma/cm2.
37Figure3.gif)
Figure 3. Terminal voltage vs. state of discharge for steady discharge and charge currents of 170, 340 and 680 milliamperes per sq. cm of ceramic membrane.
Because of the absence of irreversible electrode processes and because the current efficiency is 100%, this cell is capable of high charging efficiencies. A cell designed to completely discharge in two hours at 80% efficiency can be charged in two hours at 80% efficiency or in 10 hours at 96% efficiency.
37Figure4.gif)
Figure 4. The construction of a small sodium-sulfur cell using a glass envelope.
Battery Construction
No large batteries have as yet been constructed. Current work involves the development of several geometric configurations and the sealing techniques that go with them. One type of construction under consideration, which is just an enlargement of the laboratory test cells, would consist of a sodium reservoir from which plates of ceramic tubes would be suspended and to which they would be sealed with a seal- ing glass or glasses resistant to sodium and sodium polysulfide at 300°C. These ceramic tubes would be immersed in the sulfur and graphite felt which would in turn contact a current collection plate and the whole assembly would be enclosed in an aluminum can. Aluminum is quite stable toward sulfur and polysulfides at 300°C. Such a battery, it is estimated, would have an energy density of 148 watt hrs-lb and a power density of 11 lb per kw, both at 80% discharge efficiency.
The chief disadvantage of this battery is the high temperature of operation. At room temperature where the sodium and sulfur are solid, the battery does not deliver significant amounts of power. The solution to this problem for motive power applications is to keep the battery hot (>250°C) at all times by insulation. Provisions for cooling by air will be necessary during operation, and the small losses during charging will be able to maintain temperature. At other times the battery must use its own power to keep itself hot. It is estimated that a 60 kwh battery with 4 in. dead air insulation would be able to keep itself at temperature for one week on a full charge. This necessity for keeping itself warm degrades the energy density of the battery, particularly if long periods of non-use are involved.
The life of these batteries will, we feel, be limited by the corrosive action of sodium and polysulfide on the materials of construction at 250-300°C rather than any change in electrode morphology since these are liquid. We have at this time no estimate of this life, since our materials of construction which have not been explicitly described in this paper are under continued test and improvement.
REFERENCES
1. R. Ridgway, A. Klein, W. O'Leary, Trans. Electrochemical Soc. 70,
71 (1936).
2. W. L. Bragg, C. Gottfried, and J. West, Z. Krist. 77, 255 (1931)
and C. A. Beavers, M. A. S. Ross, Z. Krist. 97, 59 (1937).
3. Pearson and Robinson, J. Chem. Soc. 1473 (1930).
4. Y. Yao, J. T. Kummer, J. Inorg. and Nucl. Chem., in press.
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