Positive and negative interactive electrode formulation for a zinc-containing cell having an alkaline electrolyte

#This invention relates to alkaline galvanic cells having zinc electrodes and an alkaline electrolyte. More particularly, the present invention relates to high energy density rechargeable cells having a zinc or zinc-based negative electrodes, an alkaline electrolyte, and positive electrodes which may be nickel, silver, air, or iron.

#The provision of rechargeable zinc batteries having alkaline electrolytes is well known. Leaving aside the question of zinc/manganese dioxide cells, which find dominance in commercial fields supplying cells (batteries) for use in flashlights, toys, low drainage devices such as electric clocks, and the like, there is also a very large market and requirement for high energy density, high capacity cells and batteries such as nickel-zinc, silver-zinc, and zinc-air batteries, as well as a recently introduced super iron-zinc battery. A requirement is, however, that such cells and batteries must be cycled many times through discharge/charge cycles; leading in turn to several further requirements. The first is that the capacity of the rechargeable cell should not diminish significantly over a number of cycles, there should be no significant shape change—particularly of the zinc electrode—and no significant dendrite formation. Most especially, newly developed high energy density rechargeable zinc cells should be free or substantially free of toxicity, so as to be environmentally benign. This means, in particularly, that a robust, long-lasting, rechargeable battery must be brought to the market which contains no cadmium, no lead, and no mercury.

#In the following discussion, the terms “cell” and “battery” may be used interchangeably. Of course, it is recognized that a cell comprises two electrodes, one positive and one negative, and an electrolyte; and a battery may comprise a number of cells which are joined together in series, parallel, or series/parallel. In many batteries, of course, there are a plurality of negative and positive plates and a common electrolyte all contained in a single casing; and in some cases, the plates may be bipolar. In other batteries, there may be a plurality of self-contained cells, each having their own positive and negative electrodes and electrolyte. Moreover, cells and batteries may be cylindrical or rectangular, they may comprise flat plates or rolled plates, and they may have a relatively low voltage of one or two volts. Batteries may have a relatively high voltage, in the range of twelve to sixteen volts, sometimes much higher. The present invention applies to any of the above considerations in respect of cells and batteries, as it relates to the structure of the electrodes.

#The performance of rechargeable zinc electrodes in alkaline electrolytes has been the subject of many studies that encompass the zinc electrode composition and the interaction with the electrolyte. A performance inhibiting disfigurement of the zinc electrode occurs as cycling progresses. The most obvious effect is a lower than acceptable amp-hour capacity delivered at useable voltages. This tendency has been arrested by a number of approaches. The combination of more dilute potassium hydroxide electrolyte together with the addition of calcium hydroxide to the Zinc electrode appears to be particularly effective (U.S. Pat. No. 4,358,517 issued Nov. 9, 1982 to JONES). Alternate approaches have used buffered electrolytes with and without fluoride additions that have also resulted in increased zinc electrode life span. Noteworthy among these approaches is that described in U.S. Pat. No. 5,453,336 issued Sep. 26, 1995 to ADLER et al. which teaches a mixture of alkaline electrolyte (2-12M) combined with a carbonate of 0.5-4M and a fluoride of 0.5-4M is particularly effective. U.S. Pat. No. 4,273,841 issued Jun. 16, 1981 to CARLSON describes another mixture that employs 5-10% hydroxide, 10-20% phosphate and 5-15% fluoride. EISENBERG describes two electrolyte formulations in U.S. Pat. No. 4,224,391 issued Sep. 23, 1980 and U.S. Pat. No. 5,215,836 issued Jun. 1, 1993. Both employ mixtures of potassium hydroxide and boric, phosphoric or arsenic acid. However the latter patent describes advantages of alkali fluorides in the range of 0.01 to 1M. This should be implemented in combination with a more alkaline electrolyte.

#Electrode additions of numerous oxide additives have been investigated and proved to be beneficial (U.S. Pat. No. 5,556,720 issued Sep. 17, 1996 to CHARKEY and U.S. Pat. No. 4,084,047 issued Apr. 11, 1978 to HIMY et al.). Some oxides are claimed to result in a conductive metal matrix that enhances electrode conductivity; others may result in the formation of beneficial alloys and a reduction in hydrogen evolution. Zinc fluoride and zinc titanate (U.S. Pat. No. 4,304,828 issued Dec. 8, 1981 to VAIDYANATHAN) and the direct additions of alkali fluoride to the zinc electrode have also been found beneficial (U.S. Pat. No. 4,017,665 issued Apr. 12, 1977 to SANDERA et al.).

#It should also be noted that lead, cadmium, and mercury oxides are particularly favoured as electrode enhancement additives. However, these oxides impart a toxicity to the electrode that is undesirable—and, in some instances, unacceptable.

#The novel features which are believed to be characteristic of the present invention, as to its structure, organization, use and method of operation, together with further objectives and advantages thereof, will be better understood from the following discussion.

#It has not previously been recognized that there is considerable interaction between the positive and the negative electrodes—particularly in a cell having an alkaline electrolyte. Such interaction can lead to deleterious contamination of the negative electrode, with resulting shortened cycle life. In particular, the cobalt materials that are commonly added to a nickel positive electrode often become soluble; and this permits them to diffuse to the negative electrode where they are reduced. This, in turn, increases the hydrogen evolution tendency of the negative electrode throughout the life of the cell. The result of the increased decomposition of the electrolyte is that the cell will dry out prematurely. Even more dramatically, these compounds can cause soft short circuits in the cell, that prematurely drain the battery.

#It is common practice to add cobalt oxides to a nickel hydroxide paste to affect additional conductivity between the active material particles (see U.S. Pat. Nos. 5,759,718 & 5,489,314, for example). As the CoO material slowly dissolves and is oxidized to a conductive insoluble CoOOH, a conductive network is set up in situ. In some cases, external chemical oxidants are used to promote the reaction (see U.S. Pat. No. 5,489,314). The tendency for the leaching cobalt materials to generate short circuits was referred to in U.S. Pat. No. 5,435,055. The answer appeared to be to oxidize the electrode to an extent equal to 5% of the cell rated capacity. However, it was not recognized that the problem is not primarily associated with cobalt metal inclusion, but rather it comes as a consequence of the solubility of the cobalt oxide.