Additives for electrochemical cells

Details Additives for electrochemical cells Additives for electrochemical cells

1998-04-15

2002-05-28

Hardee, John (Department: 1751)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Include electrolyte chemically specified and method

Reexamination Certificate

active

06395422

ABSTRACT:

The present invention relates to additives for electrochemical cells which can eliminate or reduce undesirable effects which can arise during storage and abuse of such cells.
The history of electrochemical cells goes back to 1866 when Leclanché first discovered the principle on which they are based. The manufacture and design of electrochemical cells has come a long way since that time, but problems still remain. Cells (also known as batteries, although the term technically relates to a series of cells) essentially consist of an anode, a cathode and an electrolyte. In the present day version of the Leclancheé cell, the anode is zinc, the cathode is manganese dioxide and the electrolyte is an aqueous solution of varying proportions of zinc chloride and ammonium chloride. In other primary cells, the electrolyte is frequently an aqueous solution of potassium or sodium hydroxide. In any event, it is necessary to seal the various components into a can in order to prevent the possibly dangerous escape of the constituents, as well as to prevent the atmosphere from affecting the constituents.
The problem of leakage of the electrolyte and corrosion of the can (zinc in Leclanche cells) was very largely overcome by the addition of cadmium and mercury, but especially mercury, to the cell ingredients.
Thus, mercury was responsible for reducing perforation of the can during abuse conditions, reducing corrosion and preventing perforation during storage, and it also had the advantage that it assisted in discharge. However, now that mercury is viewed as a major environmental pollutant, there has been a very major push to develop cells with no added mercury and, to a lesser extent, cells with no added cadmium.
The essential problem with cells which have no added mercury is that no one has yet found any additive which is capable of recreating the advantages of cells which contain mercury. In fact, even the optimum selection of all of the currently known additives is not as good as mercury.
Some of the known additives which have been looked at include, for example: the arylsulphur compounds of EP-A-421660 (which prevent leakage and perforation but do not control gassing); the fluoroalkylpolyoxyethylene ether compounds of U.S. Pat. No. 4,606,984 (which control gassing but which have no effect on corrosion, leakage or perforation); the alkyl polyoxyethylene ethers of U.S. Pat. No. 3,847,669 and the alkyl polyoxyethylene phosphate ethers of GB-A-2170946 (which control gassing but nothing else); and the tetraalkyl and alkyl ammonium compounds of U.S. Pat. No. 3,945,849 (which prevent corrosion, leakage and perforation but not gassing and which also have poor electrical performance).
In addition, known additives, such as listed above, while having certain beneficial effects, have the unfortunate side-effect of reducing performance. In order to assay this, cells are kept at high temperatures for long periods (for example, 13 weeks at 45° C. and 50% r.h. [relative humidity]). Performance retention is then calculated as a measure of performance compared with similar batteries kept for two weeks at 20° C., the results ideally being in excess of 80%. In fact, the above listed additives provide performance retention generally in the region of only 75% by comparison with cells containing no additives, such cells typically having a performance retention in the region of 82%.
Apart from the necessary anode, cathode and electrolyte, practical considerations demand that a separator is provided between the anode and the cathode in order to avoid possible contact between the anode and cathode, which could lead to undesirable short circuiting.
In general, one of two types of separator is employed, and is either a gel/paste composition or coated paper. With the drive to greater efficiency and performance, the coated paper separators are particularly preferred, as they take up less space in the cell.
The coated paper separators are coated with starch which, in the presence of the electrolyte, is tonically conductive, but not electronically conductive.
We have now, surprisingly, established that polyoxyalkylene nitrogen containing compounds can be used as additives in electrochemical cells in order to alleviate or even overcome many of the problems associated with cells having no added mercury.
Thus, in a first aspect, the present invention provides an additive for use in an electrochemical call having an acidic electrolyte, characterised in that the additive is a polyoxyalkylene nitrogen containing compound.
Additives of the present invention are useful to help inhibit gassing and leakage in electrochemical cells, especially cells with no added mercury, as well an having minimal adverse effect on the performance of the cell.
In fact, we have found that polyoxyalkylene nitrogen containing compounds generally perform as well as, or better than, any single other additive intended to prevent gassing, leakage or corrosion. In addition, we have also found that performance is often enhanced. Thus, the compounds of the invention are generally useful to reduce corrosion, gassing and leakage, a combination which has not previously been observed for any single additive.
Furthermore, as stated above, arylalkylpolyoxyethylene ether and trimethylalkyl ammonium both have good antigassing properties, but a combination of these two compounds does not have the same effect as the compounds of the present invention. Whilst the combination of compounds is slightly better than arylalkylpolyoxyethylene ether alone, the compounds of the present invention are generally capable of reducing gassing by up to half, or more, of the levels observed with the arylalkylpolyoxyethylene ether additives.
The nitrogen containing compounds of the present invention may be of any type that is suitable to be substituted by one or more polyoxyalkylene groups. Whilst amine and ammonium compounds are preferred, especially the amine compounds, other compounds which have substitutable nitrogen bonds are also suitable, such as carbamoyl, diazo and aci-nitro compounds.
The individual alkylene moieties in the polyoxyalkylene substituents may be the same or different, but will generally be the same owing to the methods of manufacture employed for such compounds. Useful alkylene groups tend to be restricted to the ethylene and propylene groups, but the propylene groups are not as good as the ethylene groups at preventing gassing, so that polyoxyethylene nitrogen containing compounds are preferred, especially the polyoxyethylene amines. It will be appreciated that it is possible for any given polyoxyalkylene moiety to contain a mix of lower alkylene groups, such as methyl, ethyl and propyl. Where this is the case, then we prefer the average alkylene length to be two, or close to two, carbon atoms.
The compounds of the invention are commonly available as surfactants, typically as provided in “Industrial Surfactants Electronic Handbook” (published by Gower and edited by Michael and Irene Ash).
Regarding the nitrogen atom, it is particularly preferred that this is substituted by at least one polyoxyalkylene group and one alkyl group. The alkyl group may be substituted by one or more substituents, such as hydroxy groups and halogen atoms, but it is generally preferred that the alkyl group is unsubstituted. It is also preferred that the alkyl group should be straight chain and contain from 1 to 30 carbon atoms.
Compounds of the present invention may also contain more than one amine centre, in which case it is preferred that the individual amine groups are bridged by alkylene groups, preferably a short chain alkylene group such as a trimethylene group.
The chain length of the polyoxyalkylene group is not particularly important to the present invention, but we prefer that the chain length should be between 1 and 50, preferably with an average length of between 3 and 15 and especially around 10, an average length of 10 being the most preferred. Furthermore, compounds derived from coconut amines are preferred, and coconut alkyl groups contain between 6 and 18 (inclus

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