It has been recognized that, when a rechargeable alkaline manganese dioxide-zinc cell--or, for that matter, other cells as discussed hereafter--is first assembled, it is fully charged so that the first cycle to which the cell is subjected in use is a discharge cycle. This is, of course, in contradistinction to such alkaline cells as nickel cadmium cells, which must be charged prior to their initial use.
However, it has also been noted that such cells as alkaline manganese dioxide-zinc cells have an inefficiency when first discharged and recharged, so that there is a loss of cumulative capacity in such cells. Of course, there has also been a negative impact to the storage capacity of such cells, which has been eliminated by limiting the discharge capacity of the zinc anode to a pretermined fraction of the discharge capacity of the cathode, as noted in Awano U.S. Pat. No. 3,530,496. See also Kordesch, "Batteries", published by Markkel Dekker, 1974, at page 288.
The following discussions are generally related to rechargeable manganese dioxide-zinc cells, being cells which have a cathode in which manganese dioxide is the active material, and an anode in which zinc is the active material, with an alkaline electrolyte. The specific nature of the cathode, and certain typical formulations therefore, are discussed hereafter. Likewise, typical examples of anodes that are formulated using powder zinc are discussed.
What the present invention provides, however, is a cathode which is preconditioned, so as to improve the cumulative capacity and the cycle life of the cell.
Generally, the electrolyte is a 6 N KOH to 12 N KOH. The anolyte--which is essentially electrolyte which is used for formulating the anode--is generally also 6 N to 12 N KOH, but may have zinc oxide dissolved in it, in an attempt to reduce corrosion of the zinc metal of the anode, and so as to provide an overcharge reserve.
The separator which is used between the cathode and the anode is generally a cellulose, non-woven material, which may have an optional fibre structure in it for reinforcement.
As to the anode, it is generally a zinc powder mixed with a gelling agent which may be such as NaCMC. Corrosion inhibitors such as mercury may also be included in the anode formulation, in an attempt to reduce hydrogen gassing within the cell.
The following discussion is directed to the characteristics of the anode and the cathode, relating to several observed and theoretical explanations for the reasons that rechargeable alkaline manganese-zinc cells experience decreasing capacity during the first few cycles of their life.