Cobaltous oxide containing finely-dispersed metallic cobalt,...

Details Cobaltous oxide containing finely-dispersed metallic cobalt,... Cobaltous oxide containing finely-dispersed metallic cobalt,...

1999-12-17

2001-08-21

Gupta, Yogendra N. (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Metal compound containing

C252S500000, C252S513000, C420S435000, C423S594120, C429S218200, C429S209000, C205S586000, C148S674000, C556S138000

Reexamination Certificate

active

06277305

ABSTRACT:

The present invention relates to cobalt(II) oxide containing metallic cobalt, the procedure for the production thereof and to the use thereof.
Cobalt(II) oxide is used mixed with metallic cobalt as an additive in the positive composition for rechargeable alkaline Ni batteries based on Ni/Cd or Ni/NiH. For this purpose, NI(OH)2 is processed with the Cobalt(II) oxide/metal mixture and auxiliary substances to yield pastes, which are then incorporated into an electrically conductive electrode carrier. The electrodes produced in this manner are further processed by drying and/or sintering in order to obtain batteries of the most varied types.
In the production of button cells, the electrochemically active electrode constituents are compressed, together with auxiliary substances, principally araphite or nickel powder, into tablets of various sizes. The proportion of the cobalt in the electrode compositions in such cases is between 2 and 10 wt. %.
According to EP-A 353 837, the principal action of the cobalt metal is based on the phenomenon that during the first charge cycles (forming cycles), the cobalt metal is initially oxidised in accordance with its potential to divalent cobalt and may thus dissolve in the alkaline electrolyte. The resultant Co
2+
ions and -nose possibly already present then diffuse towards the surface of the nickel hydroxide. Here, as battery charging, continues, the ions are oxidised to Co(III) in the form of CoO(OH). This in turn forms a layer on the surface of the nickel hydroxide particles and, in subsequent battery charging and discharging cycles, ensures the electrical conductivity of the electrode material.
Co
2+
ions may, however, also enter the layer lattice of the nickel hydroxide and there modify the properties of the hydroxide in such a manner that greater charging efficiency of the electrode material is achieved. In addition to the already stated properties, the cobalt used in the electrode composition may act as a safety reserve in the event of excessive discharging. In this case, Co
2+
ions are again reduced electrochemically and so prevent any evolution of hydrogen. Cobalt compounds with the above-stated properties are also disclosed in patents U.S. Pat. Nos. 5,032,475 and 5,053,292 and in European patent application EP-A 523 284.
Only up to approximately 50% of the cobalt metal powder may be utilised in the electrode for the charging and discharging cycles on electrochemical oxidation, as the predominant proportion of the cobalt is coated with a stable oxide layer. This protective layer in turn prevents the formation of Co
2+
ions which, as has already been mentioned, are necessary for activation of the electrodes. In order to circumvent this difficulty, soluble cobalt compounds such as cobalt hydroxide or monoxide have hitherto also been incorporated into the electrode composition. This ensured that Co
21
ions were already dissolved in the electrolyte prior to electrochemical forming and these ions could already be deposited on the surface of the nickel hydroxide (Matsumo et al.: The 162nd ECS Fall Meeting, Detroit 18, (1982)).
According to the prior art, the Cobalt(II) oxide used for the above-described applications was produced industrially by the thermal decomposition of cobalt carbonate, cobalt hydroxide or higher cobalt oxides. However, in line with the thermodynamic equilibrium, these always contain an excess of oxygen and thus residual amounts of Co(III),
However, slight traces of Co(III) in the Cobalt(II) oxide autocatatytically catalyse, the oxidation of divalent cobalt to trivalent cobalt. This latter compound does not, however, form any compounds soluble in the electrolyte so that the conductive layer cannot be formed by means of the mechanism described above. As a consequence, a high degree of electrode utilisation may be achieved only if the content of Co(III) is as low as possible.
One possibility for avoiding the presence of Co(III), and for producing a low cobalt metal content, is to calcine the above-stated starting materials such as cobalt carbonate, cobalt hydroxides and/or cobalt oxides under inert gas in the presence of appropriate quantities of hydrogen. However, this process entails complex process control, i.e. very thorough mixing and constant adjustment of hydrogen apportionment to fluctuations in throughput, which are difficult to avoid on an industrial scale. Only in this case is consistent product quality with uniform distribution of the metallic cobalt ensured.
The object of the present invention is thus to provide a Cobalt(II) oxide containing cobalt metal, which oxide does not exhibit the disadvantages described above.
Corresponding cobalt(II) oxides may be obtained by a process for the production of cobalt(II) oxide containing metallic cobalt, which process is based lipon reacting aqueous cobalt chloride and/or nitrate and/or sulphate solutions with alkali metal and/or alkaline earth metal and/or ammonium carbonate and/or hydrogen carbonate and/or hydroxide and an organic compound containing at least one carboxyl group, wherein a coprecipitate of the general formula
Co[(OH)
2
]a[O]
b
[CO
3
]
c
[R]
d
is obtained, wherein the sum is I:!~a+b+c+d:!~1.5 and R denotes two identical or different carboxylic acid residues, the molar ratio of which d/(a+b+c+d) is adjusted in accordance with their reductive capacity, and, once separated from the solution, the coprecipitate is calcined. The present invention provides such a process.
It is generally known that on thermal decomposition cobalt oxalate breaks down into cobalt metal and carbon dioxide.
Tartaric acid also behaves similarly.
The decomposition products of versatic acid residues are, for example, unsaturated hydrocarbons, which may themselves have a reducing action. The reductive capacity of acids with a complicated structure and the derivatives thereof must be determined experimentally. It is important that the only organic acid residues used are those which yield no solid decomposition products on calcination, in order to avoid contamination of the product with carbon.
Thanks to the incorporation of reducing anions into the crystal lattice of the coprecipitate, the component with the reducing action is optimally distributed and it is possible to achieve a much more uniform distribution of the metallic cobalt than is possible with gas phase reduction.
Carboxylic acids may preferably be used in the process according to the invention as the organic compound containing at least one carboxyl group. Suitable carboxylic acids are in particular
linear or branched, saturated or unsaturated monocarboxylic acids with a number of C atoms from 1 to 9 and/or
linear or branched, saturated or unsaturated polycarboxylic acids with a number of C atoms from 2 to 10 and/or
cyclic or heterocyclic, saturated or unsaturated mono- and polycarboxylic acids with a number of C atoms from 4 to 14 and/or
linear or branched, saturated or unsaturated mono- and polycarboxylic acids with a number of C atoms from 2 to 7 and/or
aromatic hydroxycarboxylic acids with a number of C atoms from 7 to 11 and/or
cyclic or aliphatic, saturated or unsaturated ketocarboxylic- acids With a number of C atoms from 2 to 14.
Adipic acid, succinic acid, glutaric acid, glyoxylic acid, maleic acid, malonic acid, lactic acid, oxalic acid, phthalic acids, mucic acid, sorbic acid, racemic taxtaric acid, versatic acid, tartaric acid and/or citric acid may also advantageously be used.
In another advantageous event of the process according to the invention, the carboxylic acids may also be used in partially esterified form, providing that they still have at least one active carboxyl group.
The reaction according to the invention is preferably performed in a temperature range from 20° C. to 100° C., preferably 25° C. to 85° C.
It is assumed that, as reduction begins in the crystal lattice, individual cobalt atoms are initially formed which combine by diffusion processes to yield clusters and finally fine deposits of cobalt

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