Cobalt (II) oxide containing cobalt metal, a process for produci

Details Cobalt (II) oxide containing cobalt metal, a process for produci Cobalt (II) oxide containing cobalt metal, a process for produci

1997-07-09

1999-10-26

Kopec, Mark

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

429218, C01B 1314, H01M 458

Patent

active

059723068

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to cobalt(II) oxide containing metallic cobalt, to a process for producing it and to its use.
Cobalt(II) oxide is used in admixture with metallic cobalt as an additive in the positive paste material of rechargeable alkaline Ni batteries based on Ni/Cd or Ni/NiH. For this purpose Ni(OH).sub.2 is processed with the Co(II) oxide-metal mixture and auxiliary materials to form pastes which are subsequently incorporated in an electrically conductive electrode support. The electrodes produced by this route are further processed by drying and/or sintering in order thus to produce batteries of various designs.
Thus for the production of round button cells, for example, the electrochemically active electrode constituents are compacted together with auxiliary materials, predominantly graphite or nickel powder, to form tablets of various sizes. The content of cobalt in the electrode compositions is between 2 and 10% by weight in this application.
According to EP-A 353 837, the main effect of cobalt metal is due to the fact that during the first charging cycles (forming cycles) the cobalt metal first is oxidised, corresponding to its potential, to divalent cobalt, and is thus able to dissolve in alkaline electrolytes. The Co.sup.2+ ions which are thus obtained and those which may already be present then diffuse towards the surface of the nickel hydroxide. On further charging of the battery, they are oxidised here to Co.sup.3+ ions in the form of CoO(OH). This in turn is formed as a layer on the surface of the nickel hydroxide particles and gives rise to the electrical conductivity of the electrode material during the following charging and discharging cycles.
However, Co.sup.2+ ions can also enter the layer lattice of the nickel hydroxide and can modify the properties of the hydroxide there so that a higher charging efficiency of the electrode material is obtained. In addition to the properties which have already been mentioned, the cobalt used in the electrode paste material can act as a safety reserve if discharging is too intensive. In the course of this procedure, Co.sup.2+ ions are electrochemically reduced again and thus prevent the evolution of hydrogen. Cobalt compounds with the aforementioned properties are also disclosed in U.S. Pat. No. 5,032,475 and U.S. Pat. No. 5,053,292, and in European Patent Application EP-A 523 284.
Only up to about 50% of the cobalt in the electrode can be utilised for the charging and discharging cycles on electrochemical oxidation, since the pre-dominant fraction of the cobalt is coated with a stable oxide layer. This protective layer in turn prevents the formation of Co.sup.2+ ions which are necessary for the activation of the electrode, as mentioned above. In order to circumvent this difficulty, soluble cobalt compounds such as cobalt hydroxide or monoxide have hitherto been incorporated in the electrode paste material. The effect of this has been that Co.sup.2+ ions are present in dissolved form in the electrolyte even before electrochemical forming, and can already separate out at the surface of the nickel hydroxide (Matsumo et al.: The 162nd ECS Fall Meeting Detroit, 18 (1982).
According to this prior art, the Co(II) oxide used for the purposes of application described above is produced commercially by the thermal decomposition of cobalt carbonate, cobalt hydroxide or higher oxides of cobalt. In accordance with thermodynamic equilibrium considerations, however, these always contain an excess of oxygen and thus have residual contents of Co(III).
However, slight traces of Co(III) in Co(II) oxide catalyse the oxidation of divalent cobalt to trivalent cobalt. The latter does not form compounds which are soluble in the electrolyte, however, so that the formation of the conductive layer according to the mechanism described above is not possible. The result of this is that a high utilisability of the electrode can only be obtained if the Co(III) content of the starting material is as low as possible.
The object of the present invention is therefore to pro

REFERENCES:
patent: 5032475 (1991-07-01), Hasebe et al.
patent: 5053292 (1991-10-01), Hasebe et al.
I. Matsumoto et al., Ni-Fe Battery, Abstract No. 10, 162d ECS Fall Meeting, Detroit, p. 18. (1982).
International Preliminary Examination Report for International Application No. PCT/EP96/00337, International Filing Date Jan. 29, 1996, International Patent Classification IPC No. C01G51/04, dated Dec. 3, 1996.
N. M. Khoklacheva et al., Production and Thermal Decomposition of Formates of Some Nonferrous Metals, Journal of Applied Chemistry (Russian language monograph/deposited document 1980) VINITI Publishing Plant 4960-80 9pp., and English language translat.

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