Abstract:
Cobalt/cobalt oxide powder having the composition Co x  (CoO) 1-x , where x is from 0.02 to 5, and a process for its production and use in nickel hydroxide electrodes for secondary batteries to enhance capacity.

Description:
This is a continuation of application Ser. No. 08/279,948, filed Jul. 25, 1994 now abandoned. 
    
    
     This invention relates to a new cobalt/cobalt oxide powder having the composition Co x  (CoO) 1-x , to a process for its production and to its use. 
     BACKGROUND OF THE INVENTION 
     The principal electrochemically active constituents of nickel electrodes for nickel/cadmium or nickel/metal hydride secondary batteries are nickel hydroxide and--as additives thereto--such cobalt materials as, for example, cobalt metal powder, cobalt(II) oxide and/or cobalt(II) hydroxide. These constituents are processed together with other auxiliary components to form pastes which are then incorporated in an electrically conductive electrode support. The electrodes thus obtained are subjected to further processing steps, such as drying and/or sintering, to find electrode forms for batteries differing in their design. 
     For the production of button cells in particular, the electrochemically active electrode constituents are pressed together with auxiliary components, such as graphite or nickel powder for example, to form tablets differing in size. The cobalt component is typically used in the form of a mixture of cobalt metal powder and cobalt(II) oxide or cobalt(II) hydroxide. 
     The cobalt materials, which are generally present in the electrode material in a concentration of 2 to 10% by weight, perform various functions. 
     Thus, it is postulated in European Patent Application 353 837 that, in the formulation of the electrode, i.e. in the initial charging of the battery, the cobalt metal is first oxidized in accordance with its potential to form divalent cobalt which is soluble in the alkaline electrolyte. Co 2+   ions from the cobalt metal and any Co 2+   ions already present from the monooxide or hydroxide diffuse towards the surface of the nickel hydroxide. As the charging of the battery continues, they are oxidized to Co +3  ions in the form of CoO(OH) which is deposited as a layer onto the surface of the nickel hydroxide particles. This layer establishes the necessary electrical conductivity of the electrode material in the subsequent charging and discharging cycles of the battery. In addition, the Co 2+   ion can migrate into the lattice of the nickel hydroxide layer where it modifies the electrochemical properties of the nickel hydroxide layer in such a way that the electrode material assumes a higher charging efficiency. In addition, the cobalt component acts as a safety reserve in the event of excessive discharging in that the Co 2+   ion is electrochemically reduced and the elimination of hydrogen is prevented. 
     Suitable cobalt compounds are disclosed in U.S. Pat. No. 5,032,475 and U.S. Pat. No. 5,053,292 and in European Patent Application 523,284. 
     In the electrochemical oxidation process, the cobalt metal powder is made available for the charging and discharging processes in the electrode to a level of only 50% because a considerable proportion of the metal becomes coated with a firmly adhering oxide layer and more cobalt in the form of Co 2+   ions is prevented from diffusing into the electrolyte. Finally, approximately 50% of the cobalt metal is converted into insoluble compact cobalt oxides. Hitherto, this problem has been overcome by the addition of partly soluble cobalt compounds, such as cobalt hydroxide or cobalt monoxide, to the electrode material. Even before the electrochemical forming process, the partly dissolved Co 2+   ion disperses in the electrolyte and is deposited in the required form on the surface of the nickel hydroxide (Matsumo et al.: The 162nd ECS Fall Meeting Detroit, 18 (1982)). 
     The cobalt(II) oxide hitherto used for the described applications is industrially produced by thermal decomposition of cobalt carbonate, cobalt hydroxide or higher cobalt oxides. Unfortunately, this gives rise to the formation of cobalt oxides which--in accordance with the thermodynamic equilibrium--contain an excess of oxygen (see Gmelin, Co, Erganzungsband (Supplement) 58, pages 470-479). The oxygen excess decreases with increasing calcination temperature. 
     Traces of Co 3+   in the cobalt(II) oxide accelerate the further oxidation of the divalent cobalt under the effect of atmospheric oxygen and atmospheric moisture. Because of its insolubility in the electrolyte, the trivalent cobalt formed is not available as an active component of the electrode material in the battery and, accordingly, eliminates the effectiveness of the cobalt compounds used for the electrochemical process. 
     The cobalt hydroxide often used shows poorer solubility in the electrolyte than cobalt(II) oxide and is also highly sensitive to further oxidation by atmospheric oxygen, particularly under the influence of atmospheric moisture. The effectiveness of the cobalt from cobalt (II) hydroxide in the electrode is lower than that of the cobalt from cobalt(II) oxide. 
     Accordingly, the problem addressed by the present invention is to provide a cobalt material as an active component in the electrode material for secondary cells of nickel hydroxide which do not have any of the disadvantages of the prior art and which is more effective and more useful than the hitherto known mixtures of cobalt metal powder and cobalt(II) oxide. 
     SUMMARY OF THE INVENTION 
     It has now been found that these requirements are satisfied by a cobalt/cobalt oxide powder having the composition Co x  (CoO) 1-x , where x=preferably 0.02 to 0.5, and an average particle size of less than 20 μm. The cobalt/cobalt oxide powder according to the invention is a solid dark brown powder in which the cobalt(II) oxide and cobalt metal are homogeneously distributed in the form of extremely fine particles. The cobalt/cobalt oxide powder according to the invention is free from Co 3+ . The presence of the cobalt metal protects the cobalt/cobalt oxide powder according to the invention against unwanted oxidation to trivalent cobalt oxides. 
     According to the invention, the average particle size of the primary particles of the cobalt/cobalt oxide powder should be selected in dependence upon the particle size of the nickel hydroxide in the battery. In general, it should be smaller than that of the nickel hydroxide because highly uniform mixtures can be obtained in this way. In addition, a small particle size and large BET surface of the cobalt/cobalt oxide provide for easier and more rapid solubility in the electrolyte. The average particle size of the primary particles is preferably in the range from 0.5 to 10 μm. 
     As mentioned at the beginning, conventional electrode mixtures contain mixtures of cobalt metal powder and cobalt(II) oxide in addition to nickel hydroxide. If the cobalt(II) oxide is replaced by equal amounts of the cobalt/cobalt oxide powder as taught herein, it is possible to obtain at least 10% higher effectiveness of the cobalt used for the electrochemical charging and discharging processes in the battery. 
     By increasing the percentage content of cobalt metal in the cobalt/cobalt oxide powder, the percentage content of cobalt metal powder in the electrode can be further reduced, enabling the cobalt used in the electrode material to be saved. The choice of the cobalt metal content is critically determined by the type of battery and the applications envisaged. 
     Cobalt/cobalt oxide powders having the composition Co x  (CoO) 1-x , where x has a value of 0.02 to 0.2, are particularly suitable. 
     The present invention also relates to a process for the production of the cobalt/cobalt oxide powders according to the invention, in which cobalt oxide and/or cobalt compounds which form cobalt oxides on thermal decomposition are reduced at temperatures of 500° to 850° C. under conditions leading to the formation of the cobalt/cobalt oxide powders. CoO, Co 2  O 3  and/or Co 3  O 4  are advantageously used as the cobalt oxides while hydroxides, oxalates and/or carbonates are used as the cobalt compounds forming cobalt oxides. Suitable reducing agents are, in particular, carbon monoxide, hydrogen, carbon, natural gas and/or hydrocarbons. 
     The reaction is advantageously carried out continuously in rotary kilns or fluidized beds at temperatures of, preferably, 580° to 650° C. The particle size distribution can be varied over a broad range through the choice of the calcination conditions and the reaction parameters. Particularly homogeneous cobalt/cobalt oxide powders can be obtained in a spiral rotary kiln of the kind disclosed in European Patent Application 135 078. 
     The present invention also relates to the use of the cobalt/cobalt oxide powders according to the invention as a constituent of electrode materials for secondary batteries. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following Examples are intended to illustrate the invention without limiting it in any way. 
    
    
     EXAMPLES 1 TO 13 
     Production of cobalt/cobalt oxide in a rotating tube furnace 
     Various cobalt/cobalt oxides produced in the following rotating tube furnace are listed in Table 1 along with their physical and chemical properties and the production conditions: 
     
         ______________________________________Length of the heating zone:                1600 mmCooling zone (cooling water):                1000 mmElectrical heating capacity:                36 kWRotational speed:    1.5-7.5 r.p.m.,                infinitely variableInclination of the rotating tube:                up to 8°Tube diameter:       154 mmMaterial of the rotating tube:                chrome-molybdenum                steel______________________________________ 
    
     
                                           TABLE 1__________________________________________________________________________                            Laser   CO-                      Residence                            diffra-  Apparent   content  Temper-       Nitrogen            Hydrogen                 Throughput                       time tion BET*                                     densityEx.    %! ature  °C.!        m.sup.3 /h!             m.sup.3 /h!                  kg/h!                        mins.!                            (D.sub.50 -μm!                                  m.sup.2 /g!                                      g/cm.sup.3 !__________________________________________________________________________1. 80.44  800  9    120  0.96  35   37.0 0.8 1.472. 79.94  800  9    120  2     30   27.0 0.8 1.303. 79.6  600  5    192  2.4   25   1.1  7.5 0.814. 79.94  650  5    192  1.7   25   2.4  3.9 1.025. 80  700  5    192  1.96  25   2.8  2.2 1.176. 79.4  750  5    192  1.98  25   2.9  1.7 1.287. 80.3  600  5    192  1.91  25   3.2  9.9 0.728. 78.7  620  5    192  1.91  25   3.2  11.7                                     0.719. 81.3  640  5    192  1.91  25   3.5  7.6 0.7510.   81.4  600  5    400  2.06  25   3.3  12.6                                     0.75   78.6  600  5    120  2.02  25   3.1  9.9 0.7212 79.7  600  5    192  1.1   25   3.5  11.6                                     0.7813 80.5  580  5    192  1.9   25   3.2  11.7                                     0.71__________________________________________________________________________ *As determined by the nitrogen 1point method (DIN 66131) 
    
     EXAMPLE 14 
     Comparison of electrodes containing cobalt monoxide (comparison) and a cobalt/cobalt oxide (according to the invention) 
     A nickel hydroxide electrode in tablet form was produced as follows: 
     
                       TABLE 2______________________________________Hydroxide         Co/CoO  Co/Co.sub.0.05 (CoO).sub.0.95______________________________________Nickel hydride, regular             100 g   100 gBET 61 m.sup.2 /g, doped with 1%Co(OH).sub.2Cobalt metal powder, FSSS              2 g     2 g1.5 μmCobalt monoxide, FSSS              4 g1.5 μmCo.sub.0.05 (CoO).sub.0.95  FSSS 1.5 μm                      3.9 gfrom Example 12Graphite           40 g    40 g______________________________________ 
    
     The percentage content of the cobalt monoxide or cobalt/cobalt oxide is gauged in such a way that the same quantity of cobalt is present in the electrode. 30 charging and discharging cycles with 1/5 of the capacity is charged/discharged per hour 1/5 C were measured with these electrodes. The capacity of the nickel hydroxide electrode of Example 14 is plotted against the number of cycles in FIG. 1. 
     For the same quantity of cobalt in the form of cobalt metal powder and cobalt monoxide or cobalt/cobalt oxide, a higher capacity and hence higher utilization of the battery electrode are obtained in the case of cobalt/cobalt oxide.