Positive nickel electrode

The positive nickel electrode is provided having a structure of cellular nickel foam filled with a paste based on nickel hydroxide. The paste contains (in dry matter and per 100 parts by weight nickel hydroxide) 7 to 8 parts by weight powder-form nickel metal, 5 to 12 parts by weight of a cobalt hydroxide and/or salt, the parts by weight being expressed as equivalents of cobalt metal.

This invention relates to a positive nickel electrode, more particularly 
for an alkaline battery, and to a process for its production. 
BACKGROUND OF THE INVENTION 
In recent years, positive nickel electrodes intended for alkaline 
batteries, such as Ni-Cd, Ni-Zn, Ni-Fe, etc., have been developed by two 
methods. 
The first and earlier method comprises packing the active material, namely 
hydroxide Ni(OH).sub.2, mixed with an additional conductor into a metal 
container of which the walls are perforated so that the electrolyte is 
able to impregnate the active material without the active material being 
able to escape from the container. Although electrodes such as these can 
be produced at relatively low cost, they are attended by the disadvantage 
that they have unfavourable weight characteristics (Ah/kg) and are 
unsuitable for severe charging and discharging conditions. 
Progress was also made by development of the process for making electrodes 
having sintered or fibrous support in which the active material is 
introduced by chemical or electrochemical precipitation. Electrodes of 
this type were found to be capable of restoring a significant fraction of 
their nominal capacity even when they are subjected to discharging rates 
as rapid as 17 C (discharge in 1/17th of an hour). 
However, it must be emphasized that impregnation by chemical precipitation 
takes a considerable time (several tens of hours). Electrochemical 
impregnation is advantageous in this regard insofar as it can be carried 
out in about 1 hour for surface capacities of the order of 40 
mAh/cm.sup.2. 
Over the last decade, numerous efforts have been made to develop a 
continuous process for introducing the active material into a support by 
mechanical filling with paste. U.S. Pat. Nos. 4,217,939 and 4,251,603 
(Matsushita) and FR-PS 2 618 949 (Sanyo) describe processes for the 
introduction of a paste based on Ni(OH).sub.2 into three-dimensional 
structures of the foam type. In addition, it is clear from many 
publications, particularly those cited above and French Patent No. 2 567 
326 (Wonder), that a good yield of active material can only be obtained 
providing an additionnal conductor, generally Ni powder, is added to the 
Ni hydroxide. 
Japanese patents JP-A-52036732 (Matsushita) and JP-A-6251157 (Shin Kobe 
Electric Match) describe the use of a paste based on nickel hydroxide and 
nickel carbonyl or metallic nickel containing cobalt hydroxide. 
In the case of Wonder, the Ni(OH).sub.2 powder is accompanied by a 
conductive powder of nickel carbonyl or graphite. Finally, the advantage 
of immersing the electrode in cobalt sulfate with regard to the charging 
efficiency of Ni(OH).sub.2 is also mentioned. 
SUMMARY AND DETAILED DESCRIPTION OF THE INVENTION 
The present invention is based on the discovery by Applicants that, after 
several charging and discharging cycles, the cobalt added is in fact 
converted into cobalt hydroxide Co(OH).sub.2 intimately mixed with 
Ni(OH).sub.2, 
Accordingly, the present invention relates to a positive nickel electrode 
having a structure of cellular nickel foam filled with a paste based on 
nickel hydroxide. The paste contains (in dry matter and per 100 parts by 
weight nickel hydroxide) 
7 to 18 parts by weight powder-form nickel metal, 
5 to 12 parts by weight of a cobalt salt alone or in admixture with cobalt 
hydroxide, the parts by weight being expressed as equivalents of cobalt 
metal. 
For reasons of mechanical strength and viscosity of the paste, the paste 
also advantageously contains 
3 to 8 parts by weight polytetrafluoroethylene, 
1 to 3 parts by weight methyl cellulose. 
Accordingly, determination of the optimal cobalt content represents one 
aspect of the invention. Another aspect of the invention is a process for 
incorporating cobalt salt in the paste. 
This process is characterized in that the paste is introduced into the 
cellular nickel foam by mechanical pressing, the cobalt salt alone or in 
admixture with cobalt hydroxide being in the aqueous phase. 
The paste is preferably heated to a temperature of 50.degree. to 80.degree. 
C. before introduction into the nickel foam for the purpose of 
stabilization. 
The properties of the positive nickel electrode, particularly in regard to 
its efficiency based on Ni(OH).sub.2 for severe discharging conditions, 
are essentially determined by the possibilities of draining the electrons 
towards the collector (the structure of the electrode) at the reaction 
site. The use of a three-dimensional collector of the foam type is more 
advantageous in this regard than the use of a collector of the perforated 
plate type. 
However, the use of a foam collector is not sufficient for ensuring correct 
collection of the charges in the absence of an Ni powder dispersed in 
Ni(OH).sub.2 which facilitates percolation of the electrons. Since Ni 
powder is a relatively onerous component, it is advisable to minimize its 
content in the active material. 
According to another aspect of the invention, it has been found that, by 
using Ni foams of sufficiently small cell diameter, it is possible to 
minimize the Ni powder content and, at the same time, to obtain 
performance characteristics for severe discharging conditions. It appears 
that this performance is better for a minimal content of Ni powder, the 
smaller the mean cell diameter of the foam. 
On the other hand, the smaller the diameter of the cells, the more 
difficult it is to fill the foam rapidly and completely with the paste. 
Accordingly, a compromise and optimum solution has been found whereby the 
cells of the nickel foam preferably have a mean diameter of 0.1 to 0.4 
millimetres, for example 0.2 mm; in other words, the foam contains 100 
pores per inch (ppi). 
Another compromise and optimum solution has been found in this regard 
whereby the powder-form nickel metal is advantageously in the form of a 
powder having a mean diameter of 1 to 5 micrometers. 
The cobalt derivative introduced into the paste may be the sulfate, for 
example in the form of CoSO.sub.4 .multidot.7H.sub.2 O, or a mixture of 
this sulfate with the hydroxide Co(OH).sub.2. 
The cobalt sulfate is added in the form of an aqueous solution while the 
cobalt hydroxide, where it is used, is added in the form of a more or less 
thick aqueous suspension. 
The 5 to 12 parts cobalt hydroxide and/or cobalt salt comprise (in 
equivalents of cobalt metal) 
2.5 to 4.5 parts by weight cobalt hydroxide, 
2.5 to 7.5 parts by weight sulfate in the form of CoSO.sub.4 
.multidot.7H.sub.2 O. 
The following Examples are intended to illustrate the invention without 
limiting it in any way.

EXAMPLES 1 AND 2 
The starting material is a support of Ni .foam of the Metapore MN100 (100 
ppi) type marketed by EPCI which has a very high porosity with mean cell 
openings of the order of 0.2 mm. The initial thickness of the Ni foam is 
selected in dependence upon the required surface capacity, namely: 
for 25 mAh/cm.sup.2, e.sub.-- 1 7 mm (Example 1) 
for 46 mAh/cm.sup.2, e.sub.-- 2.5 mm (Example 2). 
A paste having the following composition is prepared in a mixer: 
66.8% Ni(OH).sub.2 suitable for electrochemical applications corresponding, 
for example, to the types marketed by MHO under the name of "Hoboken". 
This hydroxide contains 3.5% Co in the form of the hydroxide, 
8.3% Ni powder having a mean diameter of 3 micrometers 
19.9% CoSO.sub.4 .multidot.7H.sub.2 O in the form of a 500 g/dm.sup.3 
solution, 
1% methyl cellulose, 
4% polytetrafluoroethylene (PTFE) in the form of a 60% aqueous suspension. 
In a second phase, the paste obtained is heated in an oven to a temperature 
of 50.degree. to 80.degree. C. The object of this is to stabilize the 
paste by fibrillization of the PTFE. 
In a third phase, the paste is rehomogenized by kneading, after which 16 g 
H.sub.2 O are added per 40 g dry mixture so that the paste is of a 
suitable consistency for introduction into the foam by pressing and/or 
immersion, the foam advantageously being filled with the paste from each 
face. 
The electrodes produced in accordance with Example 1 have a surface 
capacity, as determined at a discharge rate of 0.2 C, of the order of 23 
mAh/cm.sup.2. At the nominal discharge rate (0.2 C), the efficiency based 
on Ni(OH).sub.2 is 1. At a discharge rate of 15 C, the restored capacity 
is 13.8 mAh/cm.sup.2 (i.e. 60% of the nominal capacity). 
The electrodes produced in accordance with Example 2 have a surface 
capacity under the same conditions of 46 mAh/cm.sup.2. 
COMISON EXAMPLE 1 
The procedure is as described in Example 2 (thickness 2.5 mm) except that 
the paste containing CoSO.sub.4 .multidot.7H.sub.2 O is replaced by a 
paste containing a Co powder in a quantity of 5% by weight Co based on 
Ni(OH).sub.2. 
The surface capacity obtained, as determined at a discharge rate of 0.2 C, 
is only 40 mAh/cm, and the efficiency only 0.9. 
COMISON EXAMPLE 2 
The procedure is as in Example 2 (thickness 2.2 mm) using a paste 
containing Co(OH).sub.2 instead of CoSO.sub.4 .multidot.7 H.sub.2 O and 
having the following composition: 
81.1% Ni(OH).sub.2 suitable for electrochemical applications, 
10.0% Ni powder, 
3.9% Co(OH).sub.2, 
1% methyl cellulose and 
5% polytetrafluoroethylene. 
As for Comparison Example the surface capacity obtained, as determined at a 
discharge rate of 0.2 C, is only 40 mAh/cm.sup.2 and the efficiency only 
0.9.