Non-sintered nickel electrode with excellent over-discharge characteristics, an alkaline storage cell having the non-sintered nickel electrode, and a manufacturing method of the non-sintered nickel electrode

A alkali storage cell includes a non-sintered type nickel electrode which includes a highly efficient nickel hydroxide active material and which causes no capacity decrease during an over-discharge operation. The nickel electrode contains an active material composed of nickel hydroxide, a solid solution of at least one of zinc, cadmium, magnesium, and calcium which are added to the nickel hydroxide, and cobalt compound layers which are formed over the surfaces of particles of the nickel hydroxide. The cobalt compound layers have an oxidation number of larger than 2 and a disordered crystal structure. Such an active material can be manufactured by mixing nickel hydroxide powder containing a solid solution of at least one of zinc, cadmium, magnesium, and calcium with either metallic cobalt or a cobalt compound, and subjecting the mixture to heat treatment in the presence of oxygen and alkali. Another production method is to precipitate a cobalt compound over the surfaces of the nickel hydroxide powder containing a solid solution of at least one of zinc, cadmium, magnesium, and calcium so as to form cobalt compound layers, before subjecting it to the heat treatment in the same conditions.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a non-sintered nickel electrode and a 
manufacturing method thereof, and further to an alkaline storage cell 
which includes the non-sintered nickel electrode. 
(2) Related Art 
Nickel electrodes for use in alkaline storage cells are classified into 
sintered type and non-sintered type. 
A sintered type nickel electrode is manufactured by repeating the following 
two soaking operations: firstly a porous sintered nickel substrate used as 
an active material holder is soaked in a solution of an acid nickel salt, 
such as nickel nitrate, so as to fill the pores with nickel salt, and 
secondly the substrate is soaked in an alkali solution so as to convert 
the nickel salt into nickel hydroxide. 
In contrast, a non-sintered type nickel electrode is manufactured by 
applying a nickel active material which is manufactured separately onto an 
electrode substrate. One well-known method of manufacturing such an 
electrode is to mix a nickel active material which is mainly composed of 
nickel hydroxide with a conductive agent, a binder, water, and the like 
into a paste, to apply this paste onto an electrode substrate such as a 
punching metal, and to dry the coated substrate. Another well-known method 
is to fill an electrode substrate which is made from an open pore metal or 
a sintered metallic fiber with a nickel active material slurry. 
Of these two types, sintered type nickel electrodes have a disadvantage 
that the manufacturing process is complex since the soaking operations 
must be repeated several times to obtain a sufficient amount of active 
material. Another disadvantage is that the substrate has low porosity, 
which sets limits the capacity of the electrode. 
Non-sintered type nickel electrodes, on the other hand, are free from these 
drawbacks, while they are inferior to the sintered type nickel electrodes 
in the utilization factor of nickel hydroxide as an active material. 
Through various studies on the utilization factor of active materials, it 
is now known that the addition of a highly conductive high-order cobalt 
compound to the active material used in a non-sintered nickel electrode 
leads to an improvement in efficiency. 
For example, Japanese Laid-open Patent Application No. 1-200555 describes a 
technique of manufacturing highly-conductive high-order cobalt compound 
layers such as CoOOH Co.sub.2 O.sub.3, by forming cobalt hydroxide layers 
over the surfaces of nickel hydroxide active material particles and then 
subjecting the cobalt hydroxide layers to heat treatment in the presence 
of alkali. 
The utilization factor of active materials can be improved by adding an 
active material containing a highly-conductive high-order cobalt compound 
to a nickel electrode because the high-order cobalt compound layers form a 
conductive network within the electrode. 
However, such non-sintered nickel electrodes still have a drawback that 
their capacity is greatly decreased during an over-discharge operation. 
SUMMARY OF THE INVENTION 
A first object of the present invention is to provide a non-sintered type 
nickel electrode which has a high utilization factor of a nickel hydroxide 
active material and effectively restricts a capacity decrease during an 
over-discharge operation. 
A second object of the present invention is to provide a manufacturing 
method of such a non-sintered type nickel electrode. 
A third object of the present invention is to provide an alkali storage 
cell which includes such a non-sintered type nickel electrode. 
The first object can be achieved by a non-sintered nickel electrode 
supplied with an active material containing nickel hydroxide, a cobalt 
compound, and at least one of zinc, cadmium, magnesium, and calcium. At 
least one of zinc, cadmium, magnesium, and calcium is added in a form of a 
solid solution to the nickel hydroxide, and the cobalt compound is formed 
into layers over surfaces of particles of the nickel hydroxide. The cobalt 
compound has an oxidation number of larger than 2 and a disordered crystal 
structure. 
The third object can be achieved by using the above-constructed nickel 
electrode, an alkali electrolyte, a separator which is mainly composed of 
unwoven polyolefin resin fiber, and a negative electrode which is composed 
of a MmNi.sub.5 system hydrogen-absorbing alloy as components of a cell. 
In the nickel electrode which is constructed as explained above, the 
utilization factor of active materials is remarkably improved. This 
improvement results from the fact that the high-order cobalt compound 
which has an oxidation number of larger than 2 and a disordered crystal 
structure has an extremely high conductivity, so that a conductive network 
is formed in the electrode. 
In the nickel electrode which is constructed as explained above, a capacity 
decrease to be caused by an over-discharging operation is also restrained. 
This restraint seems to result from the following: 
In a conventional nickel active material having high-order cobalt compound 
layers on the surfaces, the cobalt compound penetrates into the particles 
of the nickel active material when the cell is being over-discharged. As a 
result, the amount of cobalt on the surfaces lessens, decreasing the 
conductive network function within the electrode, thereby decreasing the 
capacity of the cell. 
In contrast, when a solid solution of a metal such as zinc, cadmium, 
magnesium, and calcium is added to nickel hydroxide powder, the metal 
works to restrain the penetration of the cobalt compound into the nickel 
hydroxide power, so that the reduction of the amount of cobalt on the 
surfaces is restrained when the cell is being over-discharged. 
Furthermore, when at least one of these metals is added in the form of 
being liberated from the nickel hydroxide active material, the 
chargeability at a high temperature is improved because the oxygen 
generation potential during a charging operation is shifted to being 
noble. 
There are two methods of manufacturing such an active material. 
One method is to mix nickel hydroxide powder containing a solid solution of 
at least one of zinc, cadmium, magnesium, and calcium with either metallic 
cobalt or a cobalt compound, and to subject the mixture to heat treatment 
in the presence of oxygen and alkali. A preferable amount of the metallic 
cobalt and the cobalt compound is 5 mol % to 14 mol % to the nickel 
hydroxide. This method is much easier. 
The other method is to precipitate a cobalt compound over the surfaces of 
the nickel hydroxide powder containing a solid solution of at least one of 
zinc, cadmium, magnesium, and calcium so as to form cobalt compound 
layers, before subjecting it to the heat treatment in the same conditions. 
This method allows the cobalt compound layers to be formed more uniformly. 
In either method, a preferable concentration of the alkali aqueous solution 
in the alkali heat treatment would be 15% by weight to 40% by weight, and 
a preferable temperature of the alkali heat treatment would be 50.degree. 
C. to 150.degree. C. 
In addition, the use of an alkali solution including lithium ions for the 
alkali heat treatment contributes to the restriction of the capacity 
decrease to be caused by an over-discharging operation. 
Therefore, an alkali storage cell including the nickel electrode of the 
present invention has a high utilization factor of active materials and 
restricts a capacity decrease during an over-discharge operation. In other 
words, such a cell has a great industrial value because of its large 
capacity and operational stability.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
&lt;Embodiment 1&gt; 
FIG. 1 shows the manufacture process of a nickel electrode of the present 
embodiment, and the state of nickel hydroxide particles in each stage of 
the manufacture process. 
(Manufacture of a positive electrode) 
A sodium hydroxide aqueous solution and a zinc sulfate aqueous solution 
were gradually added to a nickel sulfate aqueous solution whose specific 
gravity was about 1.33, with the pH being constantly adjusted using an 
ammonia aqueous solution, so as to precipitate nickel hydroxide in which 
zinc is added as a solid solution. The zinc sulfate aqueous solution was 2 
mol % to the nickel sulfate aqueous solution. 
As a result, nickel hydroxide powder in which 2 mol % of zinc was added as 
a solid solution was obtained. The ratio between nickel and zinc in the 
obtained nickel hydroxide powder in which 2 mol % of zinc is added as a 
solid solution could be measured with an ICP spectrometer by dissolving it 
in a hydrochloride aqueous solution. 
Then, a commercially available cobalt hydroxide powder was added to and 
mixed with the obtained nickel hydroxide powder in which 2 mol % of zinc 
was added as a solid solution so as to manufacture a mixture powder. The 
amount of the cobalt hydroxide powder added was 10 mol % to the nickel 
hydroxide in the nickel hydroxide powder in which 2 mol % of zinc was 
added as a solid solution. 
The mixture powder was then mixed with a 25% by weight sodium hydroxide 
aqueous solution, applied alkali heat treatment at a temperature of 
100.degree. C. in the air, washed in water, and dried. As a result, an 
active material a1 was obtained. 
The alkali heat treatment was performed as follows. 
While stirring the mixture powder in a beaker, the sodium hydroxide aqueous 
solution was added. While further stirring the mixture powder, it was 
heated for 0.5 hour. The amount of sodium hydroxide added was 5 parts by 
weight as a solid against 95 parts by weight of the mixture powder. 
As another method of the alkali heat treatment, a sodium hydroxide aqueous 
solution may be applied to the mixture powder in the form of spray and 
then the mixture powder may be heated in the air. 
During the alkali heat treatment, some of the cobalt hydroxide powder 
including cobalt whose oxidation number is 2 is converted into a cobalt 
compound whose oxidation is 3. As a result, a high-order cobalt compound 
whose average oxidation number is greater than 2 is manufactured, and 
adheres onto the surfaces of the nickel hydroxide particles. 
Furthermore, some of the cobalt hydroxide dissolves in the sodium hydroxide 
aqueous solution and precipitates again during the alkali heat-treatment. 
As a result, high-order cobalt compound layers are formed onto the 
surfaces of the nickel hydroxide particles. 
Therefore, the active material a1 consists of nickel hydroxide particles in 
which zinc is added as a solid solution and high-order cobalt compound 
layers which are formed over the surfaces of the nickel hydroxide 
particles. 
The high-order cobalt compound layers have a disordered crystal structure 
as shown in FIG. 2. 
FIG. 2 shows an X-ray diffraction chart (a) of cobalt hydroxide which has 
been applied alkali heat treatment, and an X-ray diffraction chart (b) of 
cobalt hydroxide which has not been applied alkali heat treatment. 
Most peaks including two outstanding peaks around 19 degrees and 38 degrees 
which are seen in the chart (b) are not seen in the chart (a). This 
indicates that the alkali heat treatment makes cobalt hydroxide be 
converted into a cobalt compound having a disordered crystal structure. 
100 parts by weight of the active material a1 was mixed with 50 parts by 
weight of a 0.2% by weight methyl cellulose aqueous solution into a 
slurry. The slurry was used to fill foam nickel of thickness 1.6 mm and of 
a 95% porosity, before this was dried and rolled so as to manufacture a 
nickel electrode A1 of thickness 0.6 mm. 
(Manufacture of a negative electrode) 
Misch metal (a mixture of rare earth elements), nickel, cobalt, aluminum, 
and manganese were mixed at a ratio of 1.0:3.6:0.6:0.2:0.6, with this 
mixture then being melted into an alloy in an argon gas atmosphere in a 
harmonic induction furnace. This molten alloy was then cooled so as to 
manufacture a hydrogen-absorbing alloy ingot expressed by the formation 
equation Mm.sub.1.0 Ni.sub.3.6 Co.sub.0.6 Al.sub.0.2 Mn.sub.0.6. This 
ingot was then pulverized into hydrogen-absorbing alloy powder whose 
average particle diameter is 100 .mu.m. 
This hydrogen-absorbing alloy powder was then mixed with a binder such as a 
polyethylene oxide and an appropriate amount of water into a 
hydrogen-absorbing alloy paste. This paste was applied onto both sides of 
a punching metal, dried, and rolled into a hydrogen-absorbing alloy 
electrode with a thickness of 0.4 mm. 
(Assembly of an alkali storage cell) 
The nickel electrode A1, the hydrogen-absorbing alloy electrode, and a 
separator which was mainly composed of an unwoven polyolefin resin fiber 
were cut into respective predetermined lengths. The nickel electrode A1 
and the hydrogen absorbing alloy electrode were stacked with the separator 
therebetween, rolled up, and inserted into an outer casing into which 
alkali electrolyte (a potassium hydroxide aqueous solution of 7N to 8.5N) 
was poured. As a result, a nickel-hydrogen storage cell was manufactured, 
which is hereinafter referred to as cell (A1). 
It should be noted here that the nominal capacity of the cell (A1) is 1200 
mAh, which is set by the positive electrode, and the capacity of the 
negative electrode is set around 1.5 times that figure. 
Cells (A2)-(A7) were manufactured by using nickel electrodes A2-A7, 
respectively, in the same manner as the cell (A1). 
The nickel electrodes A2-A7 contain active materials a2-a7, respectively, 
which were manufactured by partially modifying the formula of the active 
material a1 as follows: 
The active material a2 was manufactured in the same manner as the active 
material a1 except that a cadmium sulfate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder with a 2 mol % cadmium solid solution instead of the 
nickel hydroxide powder in which 2 mol % of zinc was added as a solid 
solution. The active material a2 is composed of nickel hydroxide powder in 
which cadmium is added as a solid solution and high-order cobalt compound 
layers which are formed onto the surfaces of the nickel hydroxide powder. 
The active material a3 was manufactured in the same manner as the active 
material a1 except that a magnesium nitrate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder in which 2 mol % of magnesium was added as a solid 
solution instead of the nickel hydroxide powder in which 2 mol % of zinc 
was added as a solid solution. The active material a3 is composed of 
nickel hydroxide powder in which magnesium is added as a solid solution 
and high-order cobalt compound layers which are formed onto the surfaces 
of the nickel hydroxide powder. 
The active material a4 was manufactured in the same manner as the active 
material a1 except that a calcium nitrate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder in which 2 mol % of calcium was added as a solid solution 
instead of the nickel hydroxide powder in which 2 mol % of zinc was added 
as a solid solution. The active material a4 is composed of nickel 
hydroxide powder in which calcium is added as a solid solution and 
high-order cobalt compound layers which are formed onto the surfaces of 
the nickel hydroxide powder. 
The active material a5 was manufactured in the same manner as the active 
material a1 except that a 1 mol % zinc sulfate aqueous solution and a 1 
mol % cadmium sulfate aqueous solution were used in place of the 2 mol % 
zinc sulfate aqueous solution so as to manufacture nickel hydroxide powder 
in which 1 mol % of zinc and 1 mol % of cadmium were added as a solid 
solution instead of the nickel hydroxide powder in which 2 mol % of zinc 
was added as a solid solution. The active material a5 is composed of 
nickel hydroxide powder in which zinc and cadmium are added as a solid 
solution and high-order cobalt compound layers which are formed onto the 
surfaces of the nickel hydroxide powder. 
The active material a6 was manufactured in the same manner as the active 
material a1 except that 10 mol % metallic cobalt powder was added in place 
of the cobalt hydroxide powder to the nickel hydroxide powder. The active 
material a6 is composed of nickel hydroxide powder in which zinc is added 
as a solid solution and high-order cobalt compound layers which are formed 
onto the surfaces of the nickel hydroxide powder. 
The active material a7 was manufactured in the same manner as the active 
material a1 except that a mixture solution of sodium hydroxide and lithium 
hydroxide at a mole ratio of 9:1 was added in place of the 25% by weight 
sodium hydroxide to the mixture powder. The mixture solution has the same 
number of moles as the 25% by weight sodium hydroxide. 
The active material a7, like the active material a1, is composed of nickel 
hydroxide powder in which zinc is added as a solid solution and high-order 
cobalt compound layers which are formed onto the surfaces of the nickel 
hydroxide powder. In the alkali heat treatment of the active material a7, 
however, the alkali solution contained lithium ions. 
&lt;Embodiment 2&gt; 
FIG. 3 shows the manufacture process of nickel electrodes of the present 
embodiment, and the state of nickel hydroxide particles in each stage of 
the manufacture process. 
(Manufacture of a positive electrode) 
Nickel hydroxide in which 2 mol % of zinc was added as a solid solution was 
precipitated in the same manner as in the first embodiment. The solution 
which contains nickel hydroxide powder in which 2 mol % of zinc was added 
as a solid solution was mixed with a cobalt sulfate aqueous solution and a 
sodium hydroxide aqueous solution while keeping the pH at 10. As a result, 
cobalt compound layers were formed onto the surfaces of the nickel 
hydroxide powder. 
It should be noted here that the cobalt compound layers were made 10 mol % 
by adjusting the amount of the cobalt sulfate aqueous solution. 
The mixture powder of nickel hydroxide, a zinc solid solution, and cobalt 
compound were taken, washed in water, and dried. 
An active material b1 was manufactured by mixing the mixture powder with a 
25% by weight sodium hydroxide aqueous solution, and subjecting it to 
alkali heat treatment in the same conditions as the first embodiment. 
Since most of the cobalt compound layers are converted into high-order 
cobalt compounds during the alkali heat treatment, high-order cobalt 
compound layers are formed over the surfaces of the nickel hydroxide 
powder. 
The active material b1 thus manufactured is, like the active material a1, 
composed of nickel hydroxide powder in which zinc is added as a solid 
solution and high-order cobalt compound layers which are formed over the 
surfaces of the nickel hydroxide powder. It is believed, however, that the 
high-order cobalt compound layers of the active material b1 are more 
uniform. 
The ratio among nickel, zinc, and cobalt in the obtained mixture powder 
could be measured with an ICP spectrometer by dissolving it in a 
hydrochloride aqueous solution. 
A nickel electrode B1 was manufactured by using the active material b1, in 
the same formula as the nickel electrode A1 of the first embodiment. 
(Assembly of an alkali storage cell) 
A cell (B1) was manufactured by using a nickel electrode B1, in the same 
manner as the cell (A1) of the first embodiment. 
Cells (B2)-(B5) were manufactured by using nickel electrodes B2-B5, 
respectively, in the same manner as the cell (B1). 
The nickel electrodes B2-B5 contain active materials b2-b5, respectively, 
which were manufactured by partially modifying the formula of the active 
material b1 as follows: 
The active material b2 was manufactured in the same manner as the active 
material b1 except that a cadmium sulfate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder in which 2 mol % of cadmium was added as a solid solution 
instead of the nickel hydroxide powder in which 2 mol % of zinc was added 
as a solid solution. The active material b2 is composed of nickel 
hydroxide powder in which cadmium is added as a solid solution and 
high-order cobalt compound layers which are formed onto the surfaces of 
the nickel hydroxide powder. 
The active material b3 was manufactured in the same manner as the active 
material b1 except that a magnesium nitrate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder with a 2 mol % magnesium solid solution instead of the 
nickel hydroxide powder in which 2 mol % of zinc was added as a solid 
solution. The active material b3 is composed of nickel hydroxide powder in 
which magnesium is added as a solid solution and high-order cobalt 
compound layers which are formed onto the surfaces of the nickel hydroxide 
powder. 
The active material b4 was manufactured in the same manner as the active 
material b1 except that a calcium nitrate aqueous solution was used in 
place of the zinc sulfate aqueous solution so as to manufacture nickel 
hydroxide powder with a 2 mol % calcium solid solution instead of the 
nickel hydroxide powder in which 2 mol % of zinc was added as a solid 
solution. The active material b4 is composed of nickel hydroxide powder in 
which calcium is added as a solid solution and high-order cobalt compound 
layers which are formed onto the surfaces of the nickel hydroxide powder. 
The active material b5 was manufactured in the same manner as the active 
material b1 except that a 1 mol % zinc sulfate aqueous solution and a 1 
mol % cadmium sulfate aqueous solution were used in place of the 2 mol % 
zinc sulfate aqueous solution so as to manufacture nickel hydroxide powder 
in which 1 mol % of zinc and 1 mol % of cadmium were added as a solid 
solution instead of the nickel hydroxide powder in which 2 mol % of zinc 
was added as a solid solution. The active material b5 is composed of 
nickel hydroxide powder in which zinc and cadmium were added as a solid 
solution and high-order cobalt compound layers which are formed onto the 
surfaces of the nickel hydroxide powder. 
&lt;Embodiment 3&gt; 
An active material a8 was manufactured by mixing 100 parts by weight of the 
active material a1 of the first embodiment with 4 parts by weight of zinc 
oxide. Then, a nickel electrode A8 was procured in the same manner as the 
nickel electrode A1, by using the active material a8, and a cell (A8) was 
manufactured in the same manner as (A1), by using the nickel electrode A8. 
An active material a9 was manufactured in the same manner as the active 
material a1 except that a 6 mol % zinc sulfate aqueous solution in place 
of the 2 mol % zinc sulfate aqueous solution was added to the nickel 
sulfate aqueous solution so as to manufacture nickel hydroxide powder in 
which 6 mol % of zinc was added as a solid solution. Then, a nickel 
electrode A9 was procured in the same manner as the nickel electrode A1, 
by using the active material a9, and a cell (A9) was manufactured in the 
same manner as (A1), by using the nickel electrode A9. 
An active material b6 was manufactured by mixing 100 parts by weight of the 
active material b1 of the second embodiment with 4 parts by weight of zinc 
oxide. Then, a nickel electrode B6 was procured in the same manner as the 
nickel electrode B1, by using the active material b6, and a cell (B6) was 
manufactured in the same manner as (B1), by using the nickel electrode B6. 
An active material b7 was manufactured in the same manner as the active 
material b1 of the second embodiment except that a 6 mol % zinc sulfate 
aqueous solution in place of the 2 mol % zinc sulfate aqueous solution was 
added to the nickel sulfate aqueous solution so as to manufacture nickel 
hydroxide powder in which 6 mol % of zinc was added as a solid solution. 
Then, a nickel electrode B7 was procured in the same manner as the nickel 
electrode B1, by using the active material b7, and a cell (B7) was 
manufactured in the same manner as (B1), by using the nickel electrode B7. 
A comparison between the active material a8 and the active material a9 
indicates that both materials contain approximately the same amount of 
zinc. However, some zinc is added in the form of a solid solution in the 
nickel hydroxide powder and some zinc is in the state of being liberated 
therefrom in the active material a8, whereas all zinc is added in the form 
of a solid solution in the nickel hydroxide powder in the active material 
a9. 
A comparison between the active material b6 and the active material b7 
indicates the same relationship. 
&lt;COMATIVE EXAMPLE 1&gt; 
In the present comparative example, no solid solution is added to nickel 
hydroxide powder in the production of active materials. Comparative cells 
(C1), (C6), (D1), and (D6) were manufactured as follows. 
The cell (C1) was manufactured in the same manner as the cell (A1) of the 
first embodiment except that no zinc sulfate was added to the nickel 
sulfate aqueous solution in the production of the active material a1. The 
cell (C1) has the same structure as the cell (A1) except that the nickel 
active material has no zinc added as a solid solution. 
The cell (C6) was manufactured in the same manner as the cell (A1) except 
that zinc oxide was added to nickel sulfate in the state of being 
liberated therefrom, instead of adding the zinc sulfate aqueous solution 
to the nickel sulfate aqueous solution in the production of the active 
material a1. The zinc oxide was 2 wt % of the nickel sulfate. The cell 
(C6) has the same structure as the cell (A1) except that the nickel active 
material has zinc oxide liberated from nickel hydroxide, not a solid 
solution. 
The cell (D1) was manufactured the same manner as the cell (B1) of the 
second embodiment except that no zinc oxide was added to the nickel 
sulfate aqueous solution in the production of the active material b1. The 
cell (D1) has the same structure as the cell (B1) except that the nickel 
active material has no zinc solid solution. 
The cell (D6) was manufactured in the same manner as the cell (B1) of the 
second embodiment except that zinc oxide was added to nickel sulfate in 
the state of being liberated therefrom, instead of adding the zinc sulfate 
aqueous solution to the nickel sulfate aqueous solution in the production 
of the active material b1. The zing oxide was 2 wt % of the nickel 
sulfate. The cell (D6) has the same structure as the cell (B1) except that 
the nickel active material has zinc oxide liberated from nickel hydroxide, 
not a solid solution. 
&lt;COMATIVE EXAMPLE 2&gt; 
The present comparative example deals with modifications of the alkali heat 
treatment. 
Comparative active materials c2-c5 and d2, and comparative nickel 
electrodes C2-C5 and D2 were manufactured as follows. 
The active material c2 was manufactured in the same manner as the active 
material a1 of the first embodiment except that when the mixture powder 
was heated at the temperature of 100.degree. C. in the air, no sodium 
hydroxide aqueous solution was added to the mixture powder. In other 
words, the mixture powder was applied heat treatment without alkali. Then, 
the nickel electrode C2 was manufactured in the same manner as the nickel 
electrode A1, by using the active material c2. 
The active material c3 was manufactured in the same manner as the active 
material a1 except that the mixture powder was applied oxidation treatment 
with H.sub.2 O.sub.2 instead of the alkali heat treatment. Then, the 
nickel electrode C3 was manufactured in the same manner as the nickel 
electrode A1, by using the active material c3. 
The active material c4 was manufactured in the same manner as the active 
material a1 except that the alkali heat treatment was applied to the 
cobalt hydroxide powder mixed with a 25% sodium hydroxide aqueous solution 
at the temperature of 100.degree. C. in the air, and then it was mixed 
with nickel hydroxide powder in which zinc was added as a solid solution. 
Then, the nickel electrode C4 was manufactured in the same manner as the 
nickel electrode A1, by using the active material c4. 
The active material c5 was manufactured in the same manner as the active 
material a1 except that no alkali heat treatment was applied to the 
mixture powder. Then, the nickel electrode C5 was manufactured in the same 
manner as the nickel electrode A1, by using the active material c5. 
The active material d2 was manufactured in the same manner as the active 
material b1 of the second embodiment except that when the mixture powder 
was heated at the temperature of 100.degree. C. in the air, no sodium 
hydroxide aqueous solution was added. In other words, the mixture powder 
was applied heat treatment without alkali. Then, the nickel electrode D2 
was manufactured in the same manner as the nickel electrode B1, by using 
the active material d2. 
&lt;EXPERIMENTS&gt; 
The following seven experiments were conducted with the nickel electrodes 
and cells of the first, second, and third embodiments and comparative 
examples 1 and 2. 
&lt;Experiment 1&gt; 
(over-discharge characteristics) 
The over-discharge characteristics of the cells (A1)-(A7) of the first 
embodiment, the cells (B1)-(B5) of the second embodiment, and the cells 
(C1), (C6), (D1), and (D6) of the comparative example 1 were measured. 
The measurement was carried out under the following conditions. A charging 
operation was started at a current of 1 C (1200 mA) at a room temperature 
and suspended for an hour when the value of -.DELTA.V reached 10 mV. The 
value indicates the amount of voltage which dropped from the maximum 
charging voltage. Then, a discharging operation was started at a current 
of 1 C, and terminated when the discharge termination voltage of 1V was 
attained. After the discharging operation, a forcibly discharging 
operation was carried out for 16 hours at a current of 0.05 C (60 mA). 
While the process was repeated for cycles, the discharge capacity after the 
first cycle and the discharge capacity after the fifth cycle were 
measured, The discharge capacity after the fifth cycle is shown as a 
percentage of the discharge capacity after the first cycle of each cell in 
Table 1. 
TABLE 1 
__________________________________________________________________________ 
CELL 
SOLID SOLUTION 
ADDITIVES TO NiOH 
DISCHARGE CAITY(%) 
__________________________________________________________________________ 
(A.sub.1) 
Zn Co(OH).sub.2 powder 
91.2 
(A.sub.2) 
Cd Co(OH).sub.2 powder 
92.3 
(A.sub.3) 
Mg Co(OH).sub.2 powder 
90.4 
(A.sub.4) 
Ca Co(OH).sub.2 powder 
88.6 
(A.sub.5) 
Zn + Cd Co(OH).sub.2 powder 
91.0 
(A.sub.6) 
Zn Co powder 92.1 
(A.sub.7) 
Zn Co(OH).sub.2 powder 
94.9 
(B.sub.1) 
Zn cobalt sulfate + NaOH 
91.4 
(B.sub.2) 
Cd cobalt sulfate + NaOH 
92.2 
(B.sub.3) 
Mg cobalt sulfate + NaOH 
90.0 
(B.sub.4) 
Ca cobalt sulfate + NaOH 
87.4 
(B.sub.5) 
Zn + Cd cobalt,sulfate + NaOH 
90.7 
(C.sub.1) 
none Co(OH).sub.2 powder 
77.3 
(D.sub.1) 
none cobalt sulfate + NaOH 
77.8 
(C.sub.6) 
none Co(OH).sub.2 powder 
80.5 
(liberalized Zn) 
(D.sub.6) 
none cobalt sulfate + NaOH 
80.7 
(liberalized Zn) 
__________________________________________________________________________ 
Table 1 indicates that the cells (A1)-(A7) of the first embodiment and the 
cells (B1)-(B5) of the second embodiment, and especially the cell (A7) 
exhibited higher discharge capacity than the cells (C1), (D1), (C6), and 
(C6) of the comparative example 1. 
The results indicate that it is important to add zinc, cadmium, magnesium, 
or calcium to the nickel active material in the form of a solid solution 
in order to restrain a capacity decrease which is caused by an 
over-discharging operation. If they are added in the form of being 
liberated from the nickel hydroxide powder, they do not effectively 
function for the restriction. 
The results further indicate that the use of an alkali solution including 
lithium ions for the alkali heat treatment also contributes to the 
restriction of the capacity decrease to be caused by an over-discharging 
operation. 
&lt;Experiment 2&gt; 
(the relationship between the presence of alkali heat treatment and the 
utilization factor of active materials) 
Electrodes each having a theoretical capacity of 1200 mAh were manufactured 
by using the active materials a1 and b1 of the embodiments 1 and 2, and 
the active materials c2-c5 and d2 of the comparative example 2, and the 
utilization factor of these active materials were measured. 
The measurement was carried out as follows. 
A simple open cell was manufactured for each nickel electrode by using a 
nickel plate as the opposing electrodes, and a 25% by weight potassium 
hydroxide aqueous solution. 
The simple open cell was charged for 24 hours at a current of 120 mA, 
discharged at a current of 400 mA until the discharge termination voltage 
of -0.8V was reached at the nickel plate. The discharge capacity at this 
moment was measured and the utilization factor of the active material was 
calculated according to the equation 1 below. 
##EQU1## 
The calculation results are shown with the features of these nickel 
electrodes in Table 2. 
TABLE 2 
__________________________________________________________________________ 
UTILIZATION FACTOR OF 
ELECTRODES 
FEATURES OF TREATMENTS 
ACTIVE MATERIAL 
__________________________________________________________________________ 
A.sub.1 
apply alkali heat treatment to mixture powder 
92.9 
B.sub.1 
apply alkali heat treatment to Ni(OH).sub.2 
93.4 
having cobalt compound layers 
C.sub.2 
apply heat treatment to mixture powder 
69.8 
without alkali 
C.sub.3 
apply oxidation treatment to mixture powder 
71.2 
with H.sub.2 O.sub.2 
C.sub.4 
apply alkali heat treatment to Co(OH).sub.2 and 
68.3 
mix it with Ni(OH).sub.2 
C.sub.5 
apply no oxidation treatment to mixture powder 
84.2 
D.sub.2 
apply no oxidation treatment to Ni(OH).sub.2 
86.4 
having cobalt compound layers 
__________________________________________________________________________ 
Table 2 indicates that the nickel electrodes A1 and B1 exhibited remarkably 
high utilization factor of the active materials as compared with the 
nickel electrodes C2-C5 and 
It is believed that the low utilization factor of the active materials in 
the electrodes C2-C4 is due to the unsuccessful formation of the 
high-order cobalt compound layers over the surfaces of the nickel 
hydroxide particles in the manufacture of the electrodes C2-C4. The 
results indicate that it is important for the improvement of the 
utilization factor of the active materials to form the high-order cobalt 
compound layers through alkali heat treatment. 
The utilization factor of the active materials in the nickel electrodes 
C2-C4 is still lower than that of the nickel electrodes C5 and D2. 
It is believed that the low utilization factor of the active materials in 
the electrodes C2-C4 was resulted from the high-order cobalt compound 
layers formed have poor conductivity because oxidation was conducted in 
the absence of alkali. 
&lt;Experiment 3&gt; 
(the relationship between the amount of cobalt hydroxide to be added and 
the capacity per unit active material) 
Using the same manufacturing formula as the active material a1 of the first 
embodiment, five different active materials were manufactured varying the 
amount of cobalt hydroxide powder to be added to the nickel hydroxide 
powder. Then, five different nickel electrodes were manufactured by sing 
these active materials, respectively. Then, the discharge capacity of each 
electrode was measured in the same manner as in the second experiment, and 
the capacity per unit active material of each electrode was calculated 
according to the following equation 2. 
##EQU2## 
The calculated results are shown in Table 3 and FIG. 4. 
TABLE 3 
______________________________________ 
CAITY OF PER UNIT 
COBALT COMPOUNDS 
ACTIVE MATERIAL 
(mol %) (mAh/g) 
______________________________________ 
3 212.5 
5 233.2 
10 236.9 
14 232.8 
16 214.2 
______________________________________ 
Table 3 and FIG. 4 indicate that when the mixture powder consisting of 
nickel hydroxide powder and cobalt hydroxide powder is applied alkali heat 
treatment, a preferable amount of the cobalt hydroxide powder is 5 mol % 
to 14 mol %. 
It is believed that when the cobalt hydroxide powder is less than 5 mol %, 
the high-order cobalt compound layers are not formed successfully due to 
the lack of the cobalt hydroxide, whereas when the amount exceeds 14 mol 
%, the small ratio of the nickel hydroxide to the active material has a 
more effect on the capacity than on the formation of the high-order cobalt 
compound layers. 
It is believed that when metallic cobalt powder is added in place of the 
cobalt hydroxide powder, its preferable amount is also 5 mol % to 14 mol 
%. 
Using the same manufacturing formula as the active material b1 of the 
second embodiment, six different active materials were manufactured by 
varying the amount of the cobalt sulfate salt aqueous solution, thereby 
varying the amount of cobalt hydroxide to be precipitated over the 
surfaces of the nickel hydroxide particles. Then, six different nickel 
electrodes were manufactured by using these active materials, 
respectively. Then, the capacity per unit active material of each 
electrode was calculated according to the equation 2. The calculated 
results are shown in Table 4 and FIG. 5. 
TABLE 4 
______________________________________ 
CAITY OF PER UNIT 
COBALT COMPOUNDS 
ACTIVE MATERIAL 
(mol %) (mAh/g) 
______________________________________ 
2 223.2 
3 230.5 
5 238.2 
10 242.4 
14 234.2 
16 218.2 
______________________________________ 
Table 4 and FIG. 5 indicate that when the nickel hydroxide powder having 
cobalt hydroxide layers on their surfaces are applied alkali heat 
treatment like in the second embodiment the preferable amount of the 
cobalt hydroxide powder to be added to the nickel hydroxide powder is 3 
mol % to 14 mol %. 
It is believed that when the cobalt hydroxide powder is less than 3 mol %, 
the high-order cobalt compound layers are not formed successfully due to 
the lack of the cobalt hydroxide, whereas when the amount exceeds 14 mol 
%, the small ratio of the nickel hydroxide to the active material has a 
more effect on the capacity than on the formation of the high-order cobalt 
compound layers. 
It is believed that when layers made from a cobalt compound other than 
cobalt hydroxide are formed over the surfaces of the nickel hydroxide 
powder, its preferable amount is also 3 mol % to 14 mol %. 
&lt;Experiment 4&gt; 
(the relationship between the concentration of alkali for use in alkali 
heat treatment and the utilization factor of active material) 
Using the same manufacturing formula as the active material a1 of the first 
embodiment, five different active materials were manufactured by varying 
the concentration of the sodium hydroxide aqueous solution when the alkali 
heat treatment was applied. Then, five different nickel electrodes were 
manufactured by using these active materials, respectively. Then, the 
utilization factor of the active material of each electrode was measured 
in the same manner as in the second experiment. The calculated results are 
shown in Table 5 and FIG. 6. 
TABLE 5 
______________________________________ 
CONCENTRATION UTILIZATION FACTOR OF 
OF NaOH ACTIVE MATERIAL 
(wt %) (%) 
______________________________________ 
10 81.2 
15 89.8 
25 92.9 
40 90.2 
45 80.2 
______________________________________ 
Using the same manufacturing formula as the active material b1 of the 
second embodiment, five different active materials were manufactured by 
varying the concentration of the sodium hydroxide aqueous solution when 
the alkali heat treatment was applied. Then, five different nickel 
electrodes were manufactured by using these active materials, 
respectively. Then, the utilization factor of the active material of each 
electrode was measured in the same manner as in the second experiment. The 
calculated results are shown in Table 6 and FIG. 7. 
TABLE 6 
______________________________________ 
CONCENTRATION UTILIZATION FACTOR OF 
OF NaOH ACTIVE MATERIAL 
(wt %) (%) 
______________________________________ 
10 82.2 
15 89.9 
25 93.4 
40 90.5 
45 81.6 
______________________________________ 
Tables 5 and 6 and FIGS. 6 and 7 indicate that the preferable concentration 
of the alkali aqueous solution to be used for the alkali heat treatment is 
15 mol % to 40 mol % in any of the nickel electrodes. 
It is believed that when the concentration of the sodium hydroxide aqueous 
solution is less than 15 mol %, the solubility of the cobalt hydroxide in 
the sodium hydroxide aqueous solution is insufficient, and as a result, 
the effects of the alkali heat treatment become insufficient, whereas when 
the concentration exceeds 40 mol %, the excessive viscosity of the sodium 
hydroxide aqueous solution decreases its permeability to the mixture 
powder particles, and as a result, the effects of the alkali heat 
treatment become insufficient. 
&lt;Experiment 5&gt; 
(the relationship between the temperature of alkali heat treatment and the 
utilization factor per unit active material) 
Using the same manufacturing formula as the active material a1 of the first 
embodiment, five different active materials were manufactured by varying 
the temperature of the alkali heat treatment. Then, five different nickel 
electrodes were manufactured by using these active materials, 
respectively. Then, the utilization factor of the active material of each 
electrode was measured in the same manner as in the second experiment. The 
calculated results are shown in Table 7 and FIG. 8. 
TABLE 7 
______________________________________ 
TEMPERATURE OF UTILIZATION FACTOR OF 
HEAT TREATMENT ACTIVE MATERIAL 
(.degree.C.) (%) 
______________________________________ 
25 80.5 
50 90.7 
100 92.9 
150 91.4 
175 70.5 
______________________________________ 
Using the same manufacturing formula as the active material b1 of the 
second embodiment, five different active materials were manufactured by 
varying the temperature of the alkali heat treatment. Then, five different 
nickel electrodes were manufactured by using these active materials, 
respectively. Then, the utilization factor of the active material of each 
electrode was measured in the same manner as in the second experiment. The 
calculated results are shown in Table 8 and FIG. 9 
TABLE 8 
______________________________________ 
TEMPERATURE OF UTILIZATION FACTOR OF 
HEAT TREATMENT ACTIVE MATERIAL 
(.degree.C.) (%) 
______________________________________ 
25 81.5 
50 91.3 
100 93.4 
150 92.6 
175 72.5 
______________________________________ 
Tables 7 and 8 and FIGS. 8 and 9 indicate that the preferable temperature 
of the alkali heat treatment is 50.degree. C. to 150.degree. C. in any of 
the nickel electrodes. 
It is believed that when the temperature of the alkali heat treatment is 
below 50.degree. C., the solubility of the cobalt hydroxide in alkali is 
low, and as a result, the effects of the alkali heat treatment are not 
sufficient, whereas when the temperature is over 150.degree. C., the 
crystal structure of the nickel hydroxide itself is changed. 
&lt;Experiment 6&gt; 
(the relationship between the state of zinc to be added and the 
high-temperature charge characteristics) 
The high-temperature charge characteristics of each of the cells (A8) and 
(A9) of the third embodiment were measured under the following conditions. 
A charging operation was carried out for 16 hours at a current of 0.1 C 
(120 mA) at a temperature of either 25.degree. C., 40.degree. C., or 
60.degree. C., and suspended for 3 hours at 25.degree. C. Then, a 
discharging operation was carried out at a current of 1 C until the 
discharge termination voltage of 1V was attained, and their discharge 
capacities were measured. The ratio of the discharge capacity obtained at 
40.degree. C. and 60.degree. C. to the discharge capacity obtained at 
25.degree. C. which is set at 100% was calculated. The results are shown 
in Table 9. 
TABLE 9 
__________________________________________________________________________ 
ZINC OXIDE 
DISCHARGE CAITY(%) 
DISCHARGE CAITY(%) 
CELLS 
ADDED (charge at 40.degree. C.) 
(charge at 60.degree. C.) 
__________________________________________________________________________ 
(A.sub.8) 
YES 86.1 46.9 
(A.sub.9) 
NO 82.2 42.5 
__________________________________________________________________________ 
The high-temperature charge characteristics of each of the cells (B6) and 
(B7) of the third embodiment were measured in the same manner as the cells 
(A8) and (A9) of the third embodiment. 
TABLE 10 
__________________________________________________________________________ 
ZINC OXIDE 
DISCHARGE CAITY(%) 
DISCHARGE CAITY(%) 
CELLS 
ADDED (charge at 40.degree. C.) 
(charge at 60.degree. C.) 
__________________________________________________________________________ 
(B.sub.6) 
YES 87.3 47.8 
(B.sub.7) 
NO 82.9 43.7 
__________________________________________________________________________ 
Tables 9 and 10 indicate that the cells (A8) and (B6) are superior to the 
cells (A9) and (B7) in the high-temperature charge characteristics. The 
superiority indicates the chargeability of these cells increases at a high 
temperature. 
It is believed that the increase in the chargeability at a high temperature 
resulted from the oxygen generation potential during a charging operation 
having been shifted to be noble due to the presence of zinc oxide in the 
form of being liberated from the nickel hydroxide. 
&lt;Experiment 7&gt; 
(the relationship between pH and the utilization factor of the active 
material) 
Using the same manufacturing formula as the active material b1 of the 
second embodiment, five different active materials were manufactured by 
varying the pH when the cobalt compound layers were formed over the 
surfaces of the nickel hydroxide powder. Then, five different nickel 
electrodes were manufactured by using these active materials, 
respectively. Then, the utilization factor of the active material of each 
electrode was measured in the same manner as in the second experiment. The 
calculated results are shown in Table 11 and FIG. 10. 
TABLE 11 
______________________________________ 
pH OF NaOH UTILIZATION FACTOR OF 
AQUEOUS SOLUTION 
ACTIVE MATERIAL(%) 
______________________________________ 
7.5 78.5 
8.0 92.1 
10.0 93.4 
12.0 91.8 
12.5 72.5 
______________________________________ 
Table 11 and FIG. 10 indicate that the preferable pH when the cobalt 
compound layers are formed is 8.0 to 12.0. 
It is believed that the precipitation of the cobalt compound proceeds 
gradually and continuously in the vicinity of the surfaces of the nickel 
hydroxide particles, and as a result, uniform coating layers are formed 
when the pH is within the range. In contrast, such circumstances suitable 
for the precipitation cannot be generated outside the range. 
It is further believed that the precipitation proceeds too rapid to form 
uniform coating layers when the pH is over 12.0. 
(Others) 
Although the aforementioned experiments 2-7 were conducted by using the 
nickel hydroxide in which zinc was added as a solid solution, the same 
results would be obtained when at least one of zinc, cadmium, magnesium, 
and calcium in the form of a solid solution is added to the nickel 
hydroxide. 
Although a sodium hydroxide aqueous solution was used in the alkali heat 
treatment in the embodiments, the same results would be obtained when one 
of a potassium hydroxide aqueous solution, a sodium hydroxide aqueous 
solution which contains LiOH, and a potassium hydroxide aqueous solution 
which contains LiOH is used. In either case, a preferable concentration of 
the alkali aqueous solution would be 15% by weight to 40% by weight, and a 
preferable temperature of the alkali heat treatment would be 50.degree. C. 
to 150.degree. C. like the aforementioned experiments. 
Although the nickel electrodes of the embodiments were manufactured by 
filling foam nickel with an active material, they may be manufactured by 
applying an active material onto an electrode substrate such as a punching 
metal to obtain same effects. 
Although the present invention has been fully described by way of examples 
with reference to the accompanying drawings, it is to be noted that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless such changes and modifications depart from the 
scope of the present invention, they should be construed as being included 
therein.