Cathode for molten salt batteries

#This invention was made in the course of, or under, a contract with the Energy Research and Development Administration. It relates in general to molten salt batteries and, in particular, to batteries having molten chloroaluminate solvents and cathodes comprising positive oxidation states of sulfur. Such a battery is described in commonly assigned U.S. Pat. No. 3,966,491 for "Molten Salt Electrochemical Systems for Battery Applications" issued June 29, 1976 to Gleb Mamantov, Roberto Marassi, and James Q. Chambers, the disclosure of which is herein incorporated by reference.

#The battery described in U.S. Pat. No. 3,966,491 comprised a cathode of sulfur having a positive oxidation state of +1 or less. It was therein disclosed that the molten salt electrolyte for both cathode and anode compartments was a mixture of AlCl.sub.3 /NaCl having a mole ratio of from 49/51 to 80/20. In a specific example, the electrolyte comprised a salt mixture of AlCl.sub.3 /NaCl mole ratio of 49.8/50.2, the cathode compartment containing 0.5 gram of sulfur. The cell was charged by anodizing a tungsten or carbon charge collector in the cathode compartment. The potential measured relative to an external aluminum reference rose from 1.1 to 2.2 volts during charging. A significant improvement in such a battery would be achieved if sulfur could be oxidized to a higher oxidation state than +1, thereby resulting in a higher power/weight ratio. A molten salt concentration cell with aluminum electrodes is described in U.S. Pat. No. 3,770,503 to Brabson et al.

#It is an object of this invention to provide a battery system of higher potential and higher energy density than achieved in the prior art.

#According to our invention, a battery having a high power-to-weight ratio comprises a partitioned cell having a cathode compartment and an anode conmpartment, the cathode compartment containing a molten chloroaluminate salt. The molten chloroaluminate salt is a mixture of AlCl.sub.3 and MCl, where M is an alkali metal. The temperature prevailing within our system need only be high enough to prevent the salt from freezing. Some compositions are liquid as low as 115.degree. C, however, 150.degree.-170.degree. C is required in many cases. In the cathode compartment, the cathode comprises sulfur in the tetravalent oxidation state dissolved in chloroaluminate salt having a molar concentration ratio of AlCl.sub.3 /MCl within the range of from above 50.0/50.0 up to 80/20. This concentration range is the acidic range for the salt mixture. For purposes of this system, an acid is defined as a Cl.sup.- acceptor and a base is a Cl.sup.- donor. It is critical to the efficient operation of our battery system that the AlCl.sub.3 /MCl mole ratio in the cathode compartment be more than 50.0/50.0. We have found that only when the AlCl.sub.3 /MCl mole ratio is more than 50.0/50.0 is the tetravalent sulfur species of our invention stable in the molten salt. If the AlCl.sub.3 /MCl mole ratio is 50.0/50.0 or below, the salt mixture is not acidic, with a Cl.sup.- concentration several orders of magnitude greater than in the acidic salts. It is believed that the stability of the tetravalent sulfur is adversely affected by an excess of chloride ions; hence, the observed instability in molten salts having AlCl.sub.3 /MCl mole ratios 50.0/50.0 or below. One method of supplying the tetravalent sulfur within the cathode compartment is by adding a tetravalent sulfur salt such as SCl.sub.3 AlCl.sub.4. This salt can be synthesized by contacting a stoichiometric mixture of S and AlCl.sub.3 at room temperature with excess chlorine and heating the mixture to about 150.degree. C in excess chlorine to complete the reaction. A detailed example of a synthesis for this salt can be found in Revue de Chimie Minerale t.6 (1969) pages 795 -815. The cathode compartment also contains a charge collector such as a tungsten coil or a carbon rod for conducting electric current from the battery. The charge collector can be any metal or conductor which does not react with the molten salt. Other possible materials include Au and Mo. Alternately, the tetravalent sulfur in the cathode compartment can be provided by dissolving elemental sulfur or a sulfur compound such as M.sub.2 S and S.sub.2 Cl.sub.2 (which add no extraneous ions to the melt) and then anodizing the charge carrier by passing current through the cell in the direction opposite of the battery output. To achieve full oxidation, 4 Faradays of charge per mole of elemental sulfur are needed. Of course, various salts would require various amounts of charge. Because tetravalent sulfur is present in our cathode compartment, the energy density of our present system is greatly enhanced over the system disclosed in U.S. Pat. No. 3,966,491. The equivalent weight of tetravalent sulfur is one-fourth the equivalent weight of monovalent sulfur and the energy density is inversely proportional to the equivalent weight.

#Of course, it is desirable that the cathode compartment contain as much oxidized sulfur as possible upon full charge and it is within the skill of the art to determine the maximum sulfur concentration compatible with a system.

#The anode can be any electropositive metal, however, greater energy densities are obtained with metals having low equivalent weights such as Al, Na, Li, Mg, and Ca. When an aluminum anode is used in our system, the anode compartment contains molten chloroaluminate salt in contact with the anode. The molten chloroaluminate salt in the anode compartment, also a mixture of AlCl.sub.3 and MCl, can have an AlCl.sub.3 /MCl mole ratio anywhere between the saturation concentration of MCl and up to 80/20. For example, the MCl saturation molar ratio for the AlCl.sub.3 /NaCl system is at 49.75/50.25 at 175.degree. C. When Al is the anode, the separator can be any porous material such as fritted glass. The function of the separator in our battery system is to minimize mixing of the salts between the compartments and provide an ionic pathway for charge transfer. As the battery discharges, M+ ions migrate from the anode compartment to the cathode compartment through the separator until the chloroaluminate salt in the cathode compartment becomes saturated with MCl. As mentioned earlier at an AlCl.sub.3 /MCl molar ratio of 50.0/50.0 or below, the tetravalent sulfur is unstable, resulting in a change in EMF as S.sup.4+ chemically decomposes to S(I). Therefore, the preferred chloroaluminate salt concentrations for the respective salts is a higher AlCl.sub.3 /MCl ratio in the cathode compartment and a lower AlCl.sub.3 /MCl ratio in the anode conpartment.