There exists a critical need to provide very large capacity rechargeable battery systems to the electrical utilities. This need comes about because the Nation's electrical generating capacity is frequently insufficient to provide peak power requirements. Storage batteries of the type described in this invention can be charged during off-power periods or during the night and then utilized to supplement the peak power demands. This application of very large capacity batteries is referred to in the electrical utility industry as load leveling or peak shaving. Batteries for this type of application must provide energy storage capacity at an economical price so as to be able to compete with alternate storage or peak power systems. As examples, compressed air and pumped water are alternate approaches to satisfying the peaking energy demand. It is generally accepted that a battery system with preferred features will utilize inexpensive constructional materials, will have very long-term cycling capability, will be environmentally harmless, will be inherently safe and because of real estate costs, will have a minimal size. Of great importance is the turnaround energy conversion efficiency which, as is generally accepted, should be 70 percent or greater.
Certain efforts have already been made to develop systems which satisfy these requirements. The traditional lead-acid battery has been improved so as to be able to provide a minimum of 2500 cycles without deterioration. Even although these efforts have been partially successful, the cost figures of such batteries indicate that they will greatly exceed $30/kW-hr, which has been specified by the utility industry as the maximum allowable cost. Also, because of the nature of lead-acid batteries wherein the electrodes themselves are constantly being changed in composition over each charge-discharge cycle, it is difficult to accomplish very many cycles without some irreversible morphological change. To overcome this disadvantage, investigators have been developing a new type of battery referred to as a circulating electrolyte redox battery. In a redox battery the electrodes do not change in composition; they merely act as sites at which the electrochemical reduction and oxidation (redox) reactions of selected ions in solution can occur. Typical ion pairs may comprise titanium plus three ions and titanium plus four ions (Ti.sup.3+ /Ti.sup.4+) on one side of the battery and on the other, iron plus two ions and iron plus three ions (Fe.sup.2+ /Fe.sup.3+). In these redox cells, a corrosive acid supporting electrolyte is generally used, such as hydrochloric acid, and a separator which has ion selective properties is essential. The separator allows a common ion, such as the hydrogen ion, to transport the electrical charge from one side of the battery to the other while, at the same time, preventing the titanium ions and iron ions from mixing. If such mixing occurs, the cell becomes chemically short-circuited and the products of such mixing cannot be economically separated for reuse. Separators of the required degree of cationic discriminations suitable for this application have not yet been developed.
The open circuit voltage of redox cells is generally quite low, usually less than 1 volt, and during discharge this voltage is degraded by polarization. This polarization or degradation of potential is characteristic of many electrochemical systems, but in the redox system it increases with time as the ratios of the ion concentrations on each side of the battery change during discharge or charge. Because of the corrosive electrolyte, need for an ion selective membrane, and high chemical costs, it is unlikely that a redox system will satisfy the economic or durability requirements of electric utilities. Characteristics of redox load leveling battery systems are described in two recent publications; see, L. H. Thaller, Proceedings of the Symposium on Load Leveling, V 77-4, p. 353; N. P. Yao and J. R. Selman, Eds., The Electrochemical Society, Inc., Princeton, N.J., 1977; and J. Giner, L. Swette and K. Cahill, "Screening of Redox Couples and Electrode Materials," Final Report, Contract NAS 3-19760, Sept. 1, 1976, CR 134705. In U.S. Pat. No. 3,553,017, sealed storage batteries based on selected pairs of redox couples are described. All of these couples require acidic electrolytes for proper functioning.
Another approach to development of load leveling batteries utilizes a zinc electrode as the negative electrode and a chlorine/chloride cell as the positive electrode. This system is illustratively described in U.S. Pat. Nos. 3,713,888; 3,935,024; 3,940,283; 3,954,502; and 4,001,036 and utilizes a circulating acid electrolyte. This zinc-chlorine system has several advantages: (1) mixing of the electrolyte adjacent to the electrodes does not degrade performance as happens with the aforementioned redox battery; (2) the voltage of the cell is quite high, i.e., 2.0 V; (3) the cell voltage does not change with state of charge or discharge because the chemical composition at the electrodes remains constant; and (4) relatively inexpensive reactants are utilized. Nevertheless, the system suffers from a number of disadvantages such as the corrosive nature of the acid electrolyte, self-corrosion of the zinc electrode in the acid electrolyte and generation of explosive hydrogen gas, polarization of the chlorine gas/chloride ion positive electrode, and the requirement for a refrigeration system to store the chlorine reactant as its hydrate in a low temperature aqueous solution.