1. Field of the Invention
This invention relates to a method of electrolytically preparing a rechargeable modified manganese-containing material, as defined herein.
2. Background of the Invention and Description of the Prior Art
Materials for battery electrodes which have the capability of being recharged are of increasing usefulness in today's society due to the effort being made to develop energy alternatives. These efforts have resulted from increased awareness of environmental threats presented by use of conventional fossil fuels and depletion of resources of such fuels. In addition, certain fuels are impractical or impossible to use in applications such as space travel and satellites. Moreover, electric energy sources which are rechargeable such as rechargeable battery cells obviate the need for continuously replacing non-rechargeable cells which have been exhausted. Such energy sources are particularly useful as backups for solar energy systems.
A material which has dominated the primary "throw away" battery since the early 1950's is =modified form of manganese dioxide. This modified manganese dioxide is a high energy, low-cost material. As such, many processes have been tried to bring long life rechargeability to this material. For example, a method of making a birnessite compound which included one type modified manganese dioxide compound was disclosed in U.S. Pat. No. 4,520,005. The method disclosed is a batch method which is not suitable for use on a large scale in commercial applications. Additionally, U.S. Pat. No. 4,451,534 discloses a battery cell which includes a positive electrode composed of a modified manganese dioxide compound. The modified manganese dioxide compound disclosed in the patent is mixed with a percentage of bismuth, lead or mixtures thereof. The cell also includes a zinc negative electrode. Furthermore, the material disclosed can only be made in small batches over a 12 to 24 hour period. The amount of product developed during a relatively lengthy preparation time is generally unacceptable for any commercial application. In addition, the material disclosed in the two aforementioned patents has to cycle with a suitable anode. As noted, zinc is the preferred anode yet it has problems cycling because soluble zinc (zincate) in the electrolyte chemically reacts with the anode material yielding a non-rechargeable mixed anode.
Some of these types of methods have resulted in a rechargeable material. The materials show rechargeability on the order of about 250 cycles, but a continuous drop in capacity occurs as the number of cycles increases.
A method of producing traditional manganese dioxide, as opposed to a modified manganese dioxide material were disclosed in U.S. Pat. No. 3,535,217. The method disclosed includes using a chloride solution to produce electrolytic, anodic manganese dioxide. The method is suitable for treating naturally-occurring manganese ores. The resulting material is useful in dry cells, however, it is not rechargeable material. U.S. Pat. No. 4,048,027 discloses a method of producing electrolytic manganese dioxide from molten manganese nitrate hexahydrate. Higher current densities used in this method allow higher yields of depolarized material. Improved performance in dry cells is stated to be an advantage. Again, this involves traditional manganese dioxide.
Methods have been developed for ease of removal of electrolytically produced manganese dioxide powder. See U.S. Pat. Nos. 4,170,527 and 4,295,943.
Other uses of manganese dioxide have been known such as use of manganese dioxide as a catalytic coating for oxygen evolution electrodes. See U.S. Pat. Nos. 4,072,586 and 4,476,104. U.S. Pat. No. 4,863,817 discloses a method of making electrolytic gamma-MnO.sub.2 involving stirring the solution with a gas.
Manganese dioxide prepared by the prior art electrolytic methods mentioned hereinbefore is used generally in primary alkaline manganese dioxide-zinc cells. The cells are typically made in a charged state and then discarded. The use of the manganese dioxide-zinc system is limited to primary cells because the manganese dioxide material produced is not fully rechargeable. Although some rechargeability can be achieved, the capacity significantly reduces with each successive cycle. This is particularly true if the material has been deep discharged, as would be understood by those skilled in the art to be greater than about fifty percent of the theoretical two-electron capacity.
It has been shown that electrolytic manganese dioxide may be recharged in the range of 100 times, if it is discharged to less than about thirty percent of the theoretical one electron capacity, or less than about fifteen percent of the theoretical two electron capacity. See K. Kordesch, et. al., Electrochemica Acta, 26, 1495 (1981). However, the restriction of shallow discharge does limit the amount of energy which can be generated by such cells, and thus reduces rechargeability of the cells.
There remains a need, therefore, for a method of making a highly rechargeable modified manganese dioxide material which can be produced on a commercial scale in a feasible amount of time. There remains a further need for a method of making rechargeable modified manganese dioxide using an electrolytic process.