Nickel active material for use in alkali storage cells and its manufactuiring method

A nickel active material for an alkali storage cell whose surface is covered with a cobalt compound, wherein the diffusion and permeation into the nickel hydroxide mother particles of cobalt compound during excessive discharging, which act to reduce the active material efficiency and the excessive discharging characteristics, are prevented. This is achieved by having a covering layer, including one or more of the following metal compounds; an aluminum compound, a magnesium compound, an indium compound and a zinc compound, in addition to a cobalt compound, formed on the surface of a mother particle of nickel hydroxide, and by heat treating the covered mother particles in the presence of alkali and oxygen so as to convert the cobalt compound into a compound of cobalt where an oxidization number of cobalt is greater than 2.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to alkali storage cells, and more 
specifically to a nickel hydroxide active material for use in the positive 
electrode of an alkali storage cell. 
2. Prior Art 
Nickel hydroxide electrodes which include nickel hydroxide as an active 
material are widely used as the positive electrode in nickel-cadmium 
storage cells, nickel-hydrogen storage cells and other such alkali storage 
cells. Along with increasing demands for improved storage capacity due, 
for example, to the use of these kinds of alkali storage cells in portable 
electronic equipment, there have also been demands for improved energy 
density for nickel hydroxide electrodes. 
One of the main conventional techniques for the formation of nickel 
hydroxide electrodes has been sintering wherein a multi-pore substrate, 
created by sintering powdered nickel into punching metal or the like, is 
impregnated with nickel hydroxide, although under said technique it is 
difficult to achieve a ratio of pores as high as 80% for such substrates. 
This places a limit on the amount of active material which can be 
impregnated into the substrate, making increases in the energy density of 
the electrode problematic. Such sintered electrodes also contain fine 
pores which are under 10 .mu.m, which limits the possible pore-filling 
methods to solution impregnation methods and electrodeposition methods, 
both of which require complex processes. 
There are, however, known techniques for producing non-sintered nickel 
hydroxide electrodes. Here, foam nickel which does not have a Central core 
is used as the substrate which is then directly filled using nickel 
hydroxide, this method having the advantages that a 95% ratio of pores can 
be achieved for the substrate and that there are improvements over 
sintering techniques in both energy density and the ease of the 
pore-filling process. 
However, there is the problem that the radius of the pores in foam nickel 
substrates is large, resulting in a reduction in the contact area between 
the nickel substrate which is the current collecting plate and the active 
material particles with which the substrate is filled. This means that 
there is poor electrical contact between the active material and current 
collecting plate which causes a reduction in the overall efficiency of the 
active material in the positive electrode. 
In order to overcome the above drawback with non-sintered foam nickel 
substrates, techniques for covering the surface of the nickel hydroxide 
mother particles with cobalt hydroxide or for covering the surface of the 
nickel hydroxide mother particles with a solid solution of nickel 
hydroxide and cobalt hydroxide have been proposed. 
Once nickel positive electrode plates adapted to these techniques have been 
installed into alkali storage cells, the cobalt component dissolves in the 
electrolyte and is evenly dispersed on the surface of the nickel 
hydroxide, being precipitated during the first charging of the cell 
between the particles of active material connecting them and between the 
particles of active material and the current collecting plate connecting 
them also. This precipitate is cobalt oxyhydroxide which forms conductive 
networks which improve the electrical conductivity between the particles 
of active material and between the particles of active material and the 
current collecting plate. This in turn improves the efficiency of the 
active material particles. 
Here, a technique for covering the surface of nickel hydroxide mother 
particles with cobalt hydroxide and then heat treating this covering layer 
in the presence of alkali solution is taught by Japanese Laid-Open Patent 
Application No. 1-200555, with an even greater improvement in conductivity 
being attained due to this process. 
However, if a cell containing an active material where the surface of 
nickel hydroxide particles is covered in cobalthydroxide is subjected to 
excessive discharging, this will result in the cobalt hydroxide forming 
the covering layer being diffused and permeating into the nickel hydroxide 
mother particles. This causes a reduction in the amount of cobalt 
hydroxide on the surface of the mother particles, and a reduction in the 
electrical conductivity of the conductive networks formed inside the 
electrode. This causes a drop in the capacity of the electrode. There can 
be variation in the degree to which this phenomenon occurs, but it is a 
recognized problem for the technique covering the mother particle with a 
solid solution of nickel hydroxide and cobalt hydroxide, even when the 
mother particles are heat treated in the presence of alkali solution, so 
that a solution to this problem is desired. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a nickel active 
material and an alkali storage cell using said nickel active material, 
wherein the nickel active material is a nickel hydroxide active material 
with its surface covered in cobalt compound, which achieves a large 
improvement in conductivity using only a small amount of covering material 
and which does not suffer from the diffusion and permeation of the cobalt 
compound in the covering layer into the nickel hydroxide mother particles. 
This object can be achieved by a nickel active material in particle form 
comprising a covering layer formed on a surface of one of a mother 
particle formed of nickel hydroxide and a mother particle formed with a 
main component of nickel hydroxide, wherein the covering layer includes a 
cobalt compound and at least one of the following metal compounds; an 
aluminum compound, a magnesium compound, an indium compound and a zinc 
compound, and the cobalt compound in the covering layer is converted into 
cobalt compound where an oxidization number of cobalt is higher than 2 by 
heat treatment of the nickel active material in the presence of oxygen and 
alkali. 
This object can also be achieved by an alkali storage cell, comprising: a 
positive electrode made up of a substrate filled with a nickel active 
material in particle form comprised of a covering layer formed on a 
surface of one of a mother particle formed of nickel hydroxide and a 
mother particle formed with a main component of nickel hydroxide, wherein 
the covering layer includes a cobalt compound and at least one of the 
following metal compounds; an aluminum compound, a magnesium compound, an 
indium compound and a zinc compound, and wherein the cobalt compound in 
the covering layer is converted into cobalt compound where an oxidization 
number of cobalt is higher than 2 by heat treatment of the nickel active 
material in the presence of oxygen and alkali; a negative electrode 
arranged so as to face the positive electrode with a separator in-between; 
and an alkali electrolyte which is in contact with both the positive 
electrode and the negative electrode. 
This object can also be achieved by a manufacturing method for nickel 
active material for an alkali storage cell, including: a dispersed 
solution adjustment step for adjusting a dispersed solution by dispersing 
one of mother particles formed of nickel hydroxide and mother particles 
formed with nickel hydroxide as a main component; a covering step for 
adding an alkali solution and a multiple component solution containing a 
cobalt compound and metal compound made up of at least one of an aluminum 
compound, a magnesium compound, an indium compound and a zinc compound to 
the dispersed solution whilst adjusting a weak base, thereby using the 
mother particles as a base and covering a surface of the mother particles 
by precipitating a multiple component precipitate made up of the cobalt 
compound and the metal compound; and an alkali heat treatment step for 
placing the mother particles covered in the covering step into alkali 
metal solution and heat treating with oxygen present. 
Furthermore, this object can also be achieved by a manufacturing method for 
an alkali storage cell, comprising: a positive electrode manufacturing 
step which comprises: dispersed solution adjustment step for adjusting a 
dispersed solution by dispersing one of mother particles formed of nickel 
hydroxide and mother particles formed with a main component of nickel 
hydroxide; a covering step for adding an alkali solution and a multiple 
component solution containing a cobalt compound and metal compound made up 
of at least one of an aluminum compound, a magnesium compound, an indium 
compound and a zinc compound to the dispersed solution whilst adjusting a 
weak base, thereby using the mother particles as a base and covering a 
surface of the mother particles by precipitating a multiple component 
precipitate made up of the cobalt compound and the metal compound; and an 
alkali heat treatment step for placing the mother particles covered in the 
covering step into alkali metal solution and heat treating with oxygen 
present, and a cell assembly step for arranging the positive electrode 
formed in the positive electrode manufacturing step and a negative 
electrode so as to face each other with a separator in-between and for 
assembling a cell by pouring in an alkali electrolyte. 
By means of the constructions of the invention, the surface of the mother 
particle which is composed of nickel hydroxide or which has nickel 
hydroxide as its main component is covered with cobalt compound where the 
oxidization number of cobalt exceeds 2. Since cobalt compounds where the 
oxidization number of cobalt exceeds 2 have excellent conductive 
characteristics, there is a clear improvement in the conductivity of the 
active material. 
When this nickel hydroxide active material of superior conductive 
characteristics is used to fill a substrate, the cobalt compound in the 
covering layer forms favorable conductive networks between neighboring 
particles of the nickel hydroxide active material. As a result, a greater 
number of particles of the nickel hydroxide active material can contribute 
to the electrode reaction, leading to a great improvement in the 
efficiency of the active material. 
Here, since the oxidization number of the cobalt in the cobalt compound in 
the covering layer on the mother particle is greater than 2, the 
conductivity of the active material can be improved using only a small 
amount of additive. 
Also, the covering layer includes a metal compound made up of at least one 
of an aluminum compound, a magnesium compound, an indium compound and a 
zinc compound, so that the capacity of the nickel hydroxide active 
material, especially the capacity after excessive discharging (which is to 
say excessive discharging characteristics) is improved. 
This is because if a metal compound made up of at least one of an aluminum 
compound, a magnesium compound, an indium compound and a zinc compound is 
added to the covering layer made up of cobalt compound, this metal 
compound is believed to act so as to suppress the diffusion and permeation 
of the cobalt compound into the inner part of the mother particles. 
Here, it is desirable for the amount of cobalt compound in the covering 
layer to be within a range of 1% to 15% by weight of the nickel hydroxide 
mother particle calculating in terms of hydroxides. 
In the same way it is desirable for the amount of the metal compound such 
as aluminum compound, magnesium compound, indium compound and zinc 
compound in the covering layer to be within a range of 0.5% to 25% by 
weight of the cobalt compound calculating in terms of hydroxides, with it 
also being desirable for said weight to be 3% or less of the weight of the 
nickel hydroxide in the mother particle calculating in terms of 
hydroxides. 
By means of the manufacturing method of the invention, a multiple component 
precipitate formed of a cobalt compound and a metal compound made up of at 
least one of an aluminum compound, a magnesium compound, an indium 
compound and a zinc compound is formed on the surface of the mother 
particles during the covering step, thereby forming a covering layer on 
the surface of the mother particles. 
Furthermore, since with these methods the composition and concentration of 
the multiple component solution can be adjusted and the pH, dispersion 
solution temperature, and strength of stirring can be changed for the 
solution into which the mother particles are dispersed (the dispersed 
solution), the composition of the covering layer, the thickness of the 
covering layer and condition of the covering layer can be adjusted, so 
that a desired covering layer can be easily produced with a higher yield 
rate. 
In the alkali heat treatment step, the cobalt compound contained in the 
covering layer is converted into cobalt compound where the oxidization 
number of cobalt is greater than 2 and the microstructure of the covering 
layer is converted into a porous state which is favorable for the 
electrode reaction. In this way, the conductivity of the covering layer is 
improved and the microstructure of the covering layer is changed so that 
the contact between the electrolyte and the mother particle is improved. 
Here, it is preferable for the concentration of the alkali metal solution 
in the alkali heat treatment to be 15% to 40% by weight and for the 
temperature during the heat treatment to be within a range of 50.degree. 
C. to 150.degree. C. 
By means of these kinds of manufacturing methods, nickel hydroxide active 
material and alkali cells for which the efficiency of the active material, 
the capacity per unit weight of active material and the excessive 
discharging characteristics are superior can be reliably manufactured with 
a higher yield rate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following is a description of the embodiments of the present invention 
and comparative examples, as well as the results obtained from experiments 
using the embodiments of the present invention and the comparative 
examples. 
First Embodiment 
FIG. 1 is a drawing showing the manufacturing process of the nickel 
hydroxide active material and the alkali storage cell to which the present 
embodiment relates, said drawing also showing the state of the nickel 
hydroxide particles at each stage of the manufacturing process. 
The following is an explanation of the manufacturing process of the nickel 
hydroxide active material and the alkali storage cell of the present 
embodiment and of the characteristics of said nickel hydroxide active 
material and alkali storage cell. 
Manufacture of the Nickel Hydroxide Mother Particles and the Covering Layer 
Firstly, an aqueous solution of 25% sodium hydroxide by weight was slowly 
mixed into an aqueous solution of nickel sulfate of specific gravity of 
around 1.33 so as to precipitate nickel hydroxide, with the pH being 
constantly adjusted using an aqueous solution of ammonia. This nickel 
hydroxide precipitate was then washed in water so that nickel hydroxide 
mother particles were produced. It should be noted here that the average 
particle radius of the nickel hydroxide mother particles was around 10 
.mu.m. 
Following this, the aforementioned nickel hydroxide mother particles were 
mixed and dispersed in approximately four times the amount of water 
(relatively by weight) and, while keeping the pH of this mixture (slurry) 
constant at 10 using sodium hydroxide solution, drops of solutions 
including a cobalt compound and an aluminum compound were added. In this 
embodiment, the multiple component solution is made up of a fixed amount 
of aluminum sulfate mixed into an aqueous solution of cobalt sulfate whose 
concentration is 10% by weight when calculated in terms of the weight of 
the metal (cobalt). 
By doing so, a multiple component precipitate was precipitated on the 
surface of the mother particles so as to cover the mother particles with a 
multiple component layer. These particles were then removed, washed and 
dried. 
These particles of nickel hydroxide whose surface is covered are called 
covered nickel particles. The covering layer of these covered nickel 
particles is made up of a solid solution of cobalt hydroxide and aluminum 
hydroxide, with it being possible to adjust the covering amount of cobalt 
hydroxide by changing the amount of drops mixed into the multiple 
component solution relative to the mother particles. That is to say, the 
relative amount of the dripped substances and the cobalt hydroxide in the 
multiple component solution can be found beforehand by experimentation, so 
that a desired covering amount can be attained by adjusting the amount of 
the dripped substances in the multiple component solution. 
In the present embodiment, the amount of cobalt hydroxide in the covering 
layer was set so as to be 10% by weight of the nickel hydroxide in the 
mother particle when calculating in terms of hydroxide amount. 
Also, by adjusting the amount of aluminum sulfate added to the aqueous 
solution of cobalt sulfate when adjusting the multiple component solution, 
the amount of aluminum hydroxide included in the covering layer can be 
adjusted relative to the amount of cobalt hydroxide. 
In the present embodiment, the amount of aluminum sulfate added to the 
multiple component solution was set so that aluminum hydroxide included in 
the covering layer is 5% by weight of hydroxide relative to the amount of 
cobalt hydroxide. 
The amounts of the nickel hydroxide, cobalt hydroxide and aluminum 
hydroxide in the covered nickel particles were measured according to the 
following method. 
The covered nickel particles were dissolved in diluted hydrochloric acid 
and the ratio between the nickel, cobalt and aluminum was measured using 
an ICP quantimeter. After this, the percentage by weight of nickel 
hydroxide, cobalt hydroxide and aluminum hydroxide were calculated based 
on the measured ratio between the nickel, cobalt and aluminum. 
It should be noted that although aluminum sulfate was used here as an 
additive into the aqueous solution of cobalt sulfate to form the multiple 
component solution, the same results can be achieved by adding aluminum 
nitrate to cobalt sulfate, by adding aluminum chloride to cobalt chloride 
or by other such combinations. 
Alkali Heat Treatment 
While stirring the covered nickel particles in a beaker, enough of an 
alkali metal solution (an aqueous solution of 25% by weight of sodium 
hydroxide) to dampen the particles was added so as to impregnate the 
covered nickel particles, before the particles were heat treated for 0.5 
hours at 80.degree. C. while stirring in the presence of oxygen. This 
process is called alkali heat treatment. 
This alkali heat treatment converts the cobalt compound in the covering 
layer into a compound of cobalt where the oxidization number of cobalt is 
greater than 2. That is to say, much of the cobalt hydroxide in the 
covering layer (the oxidization number of cobalt being 2) is converted 
during the alkali heat treatment into cobalt oxide where the oxidization 
number of cobalt is 3, so that the average oxidization number of the 
cobalt becomes greater than 2. 
The microstructure of the covering layer is also converted into a porous 
structure as is described later in this text, which improves the excessive 
charging characteristics of the cell and the contact between the mother 
particles and the electrolyte. 
It should be clear here that the aluminum compound is still present in the 
covering layer after the alkali heat treatment has been performed. 
The active material manufactured in this way is set as the active material 
A.sub.1. 
Manufacture of the Nickel Electrode 
100 parts by weight of the active material A.sub.1 and 50 parts by weight 
of an aqueous solution of hydroxypropyl cellulose (0.2% by weight) were 
mixed so as to produce an active material slurry, with this active 
material slurry being used to fill foam nickel of thickness 1.6 mm and of 
a 5% degree of porousness, before this was dried and rolled so as to 
produce an electrode which is 0.6 mm thick and which has a nominal 
capacity of 1200 mAh. 
It should be noted here that the amount of the filled active material is 
viewed in terms of only the nickel hydroxide in the active material 
A.sub.1, and is calculated based on a theoretical capacity (289 mAh/g) per 
unit weight of nickel hydroxide. 
Manufacture of the Negative Electrode 
Misch metal (Mm), 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 to form 
an alloy under argon gas in a harmonic induction furnace. This molten 
alloy was then cooled so as to produce an 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 is then pounded to form a hydrogen absorbing alloy whose average 
particle radius is 100 .mu.m. 
This hydrogen absorbing alloy was then mixed with a binding agent such as 
polyethylene oxide and an appropriate amount of water to form a hydrogen 
absorbing alloy paste which was spread on both sides of punching metal, 
dried, and then rolled to a thickness of 0.4 mm to form the negative 
electrode. 
Assembly of the alkali storage cell 
The nickel electrode described above was used as the positive electrode, 
with the positive electrode and negative electrode being used as layers 
with a separator in-between. The set of electrodes was then inserted into 
a cylindrical outer casing into which a fixed amount of alkali electrolyte 
was poured. The cylindrical nickel-hydrogen storage cell was then 
completed by sealing the outer casing. 
It should be noted here that the theoretical capacity (1200 mAh) of the 
cell is set by the positive electrode with the capacity of the negative 
electrode being set at 1.5 times that figure. 
The cell constructed as described above is called cell A.sub.1. 
Second Embodiment 
During the manufacturing process of the active material A.sub.1 in the 
first embodiment, one of magnesium sulfate, indium sulfate and zinc 
sulfate was used in place of aluminum sulfate as the metal salt added to 
the aqueous solution of cobalt sulfate in manufacturing the multiple 
component solution. In this way, the active materials A.sub.2, A.sub.3 and 
A.sub.4 were produced. 
Here, the covering layer of the manufactured active material A.sub.2 is 
formed of high-order converted cobalt along with a magnesium compound, the 
covering layer of the manufactured active material A.sub.3 is formed of 
high-order converted cobalt along with an indium compound and the covering 
layer of the manufactured active material A.sub.4 is formed of high-order 
converted cobalt along with a zinc compound. 
As before, cells A.sub.2, A.sub.3 and A.sub.4 were manufactured according 
to the same method as the cell in the first embodiment using the active 
materials A.sub.2, A.sub.3 and A.sub.4. 
It should be noted here that while in the present embodiment, magnesium 
sulfate, indium sulfate and zinc sulfate were added to the aqueous 
solution of cobalt sulfate in manufacturing the multiple component 
solution, other salts such as any of magnesium nitrate, indium nitrate and 
zinc nitrate can similarly be added to cobalt nitrate to achieve the same 
results. 
COMATIVE EXAMPLE 1 
During the manufacturing process of the active material A.sub.1 of the 
first embodiment, in place of the dripping of the multiple component 
solution, dripping of an aqueous solution of cobalt sulfate (of 
concentration 10% by weight in terms of the weight of the metal) was 
performed, with other than that the same method as the first embodiment 
being used to produce the active material X. 
The active material X has the cobalt compounds in the covering layer 
converted into higher-order cobalt but does not include a compound of a 
metal such as aluminum. 
Using the same method as the cell in the first embodiment, a cell X 
including active material X was manufactured. 
COMATIVE EXAMPLE 2 
Here, active materials Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 were 
manufactured using the same processes as active materials X and A.sub.1, 
A.sub.2, A.sub.3 and A.sub.4 of comparative example 1 and the first and 
second embodiments with the exception that the alkali heat treatment was 
not performed. 
Since these active materials Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 
were not subjected to alkali heat treatment, the cobalt compound in the 
covering layer is not converted into higher-order cobalt and the 
conversion of the microstructure of the covering layer does not occur. 
Using the same method as the cell in the first embodiment, cells Y.sub.0, 
Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 including these active materials 
Y.sub.0, Y.sub.1, Y.sub.2, Y.sub.3 and Y.sub.4 were manufactured. 
The composition and conditions of manufacturing the covering layer of the 
active materials in the first and second embodiments and comparative 
examples 1 and 2 are set out in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Alkali heat 
Composition of covered nickel active material 
treatment conditions 
particle and conditions for manufacture 
Concentration 
Nickel active 
covering layer composition* 
pH of solvent 
of alkali 
temp during 
material type 
Co Al Mg In Zn during covering 
metal solution 
treatment 
__________________________________________________________________________ 
Y.sub.0 
10 -- -- -- -- 10 
Y.sub.1 
10 0.5 
-- -- -- 10 
Y.sub.2 
10 -- 0.5 
-- -- 10 
Y.sub.3 
10 -- -- 0.5 
-- 10 
Y.sub.4 
10 -- -- -- 0.5 
10 
X 10 -- -- -- -- 10 25 80 
A.sub.1 
10 0.5 
-- -- -- 10 25 80 
A.sub.2 
10 -- 0.5 
-- -- 10 25 80 
A.sub.3 
10 -- -- 0.5 
-- 10 25 80 
A.sub.4 
10 -- -- -- 0.5 
10 25 80 
__________________________________________________________________________ 
*displayed as a percentage by weight of the mother particle 
Experiments 
The following experiments were carried out for the active materials in the 
first and second embodiments and comparative examples 1 and 2. 
Experiment 1 
The capacity per unit weight of active material and excess charging 
characteristics were measured. 
Here, the capacity per unit weight of active material and excess 
discharging characteristics were measured for the cells A.sub.1 -A.sub.4, 
X and Y.sub.0 -Y.sub.4. 
The electrical capacity per unit weight of the active material was 
calculated according to the following mathematical equation, with the 
discharging capacity being measured by having the aforementioned cells 
continuously charged for 16 hours using a current of 120 mA before 
discharging the cells with a current of 240 mA until the cell voltage 
falls to 1.0V. 
##EQU1## 
The excessive discharging characteristics were also evaluated for the 
aforementioned cells under the conditions described below. 
1. When charging at 1200 mA, the charging was stopped for one hour when the 
cell voltage dropped 10 mV (-.DELTA.V) after the maximum voltage had been 
attained. 
2. After stopping for one hour, the cell was discharged at a current of 
1200 mA until the discharge ending voltage of 1.0V was reached. 
3. After the above discharging, the cell was forcibly discharged at a 
current of 60 mA for sixteen hours. 
4. After repeating 1.-3. for ten cycles, the process 1. to 2. was repeated 
for five cycles. The discharge capacity before excess discharging and the 
discharge capacity after the final cycle were completed were measured, 
with the ratio between the two forming the excessive discharging 
characteristics. 
The results of Experiment 1 are shown in Table 2. 
TABLE 2 
______________________________________ 
Capacity per 
unit weight of 
Excess discharging 
Active active material 
characteristics 
material type 
(index number) 
(index number) 
______________________________________ 
Y.sub.0 100 (standard) 
100 (standard) 
Y.sub.1 102 121 
Y.sub.2 103 120 
Y.sub.3 103 119 
Y.sub.4 103 121 
X 104 113 
A.sub.1 105 133 
A.sub.2 104 133 
A.sub.3 104 132 
A.sub.4 105 135 
______________________________________ 
Note here that the values in Table 2 are expressed as index numbers with 
cell Y.sub.0 as the standard so that its cell capacity and excessive 
discharge characteristics are set at 100. 
As can be seen from the results in Table 2, the capacity per unit weight of 
active material and the excessive discharging characteristics of cells 
Y.sub.1 -Y.sub.4 are superior when compared to cell Y.sub.0, with cells 
A.sub.1 -A.sub.4 also being superior when compared to cell X. The 
improvement here in excessive discharging characteristics is especially 
noticeable. 
This is to say the excessive discharging characteristics of active material 
are improved by adding a compound of any of aluminum, magnesium, indium 
and zinc to the covering layer made up of a cobalt compound (hereinafter, 
this effect is known as the multiple component effect). 
Also, by comparing cell X and cells A.sub.1 -A.sub.4 to cells Y.sub.0 
-Y.sub.4, it can be seen that the former have superior capacity per unit 
weight of active material and excessive discharge characteristics, showing 
that the alkali heat treatment leads to an improvement in capacity per 
unit weight of active material and in excessive discharge characteristics. 
Accordingly, it can be understood that improvements in capacity per unit 
weight of active material and in excessive discharge characteristics can 
be made by forming the covering layer of a multiple component precipitate 
including a cobalt compound and an aluminum compound or the like. 
The multiple component effect and the effect of alkali heat treatment can 
be considered as follows. 
When a covering layer of a cobalt compound is formed on the surface of the 
nickel hydroxide mother particle, the covering layer acts to improve the 
conductivity of the active material particle and to form conductive 
networks between active material particles. Accordingly, the efficiency of 
the active material is improved. However, if the covering layer is formed 
of only a cobalt compound, this cobalt compound is diffused and permeates 
into the nickel hydroxide mother particle during excessive discharging, 
leading to a reduction in the amount of the cobalt compound on the surface 
of the particle, reducing the conductivity between the particles of active 
material after excessive discharging. 
It can also be considered that the further improvement in excessive 
discharging characteristics due to alkali heat treatment is due to the 
production of a higher-order cobalt compound of an oxidization number 
higher than 2 which is a superior conductor, and due to the conversion to 
a porous structure of the microstructure of the multiple component 
precipitate covering layer, so that the electrical contact between the 
mother particles and the electrolyte is improved. 
This is to say, the alkali heat treatment can be considered as having both 
a chemical and a physical effect on the covering layer so that the 
covering layer is converted into a more favorable state for the electrode 
reaction. 
It should be noted here that the metal compounds such as the aluminum 
compound, magnesium compound, indium compound and zinc compound have 
superior properties for electrode additives in that they can be easily 
formed into a multiple component precipitate with the cobalt compound and 
in that they do not affect the cell reaction. 
Experiment 2 
The relationship between amount of covering layer and the efficiency of the 
active material was investigated. 
Using the same manufacturing method as active material A.sub.4 in the 
second embodiment, active materials B.sub.1 -B.sub.8 were produced with 
the difference with active material A.sub.4 being that the amount of 
cobalt compound measured in terms of the nickel hydroxide in the mother 
particle was varied between 0.5% to 16% by weight of hydroxides, with the 
efficiency of the active material then being measured for these active 
materials B.sub.1 -B.sub.8. It should be noted here that the covering 
layer of these active materials B.sub.1 -B.sub.8 was formed of a two 
component precipitate of a cobalt compound and a zinc compound at a fixed 
ratio of 10:0.5. 
The efficiency of the active material was measured according to the 
following method. A simple open cell made up of a nickel electrode of 
theoretical capacity 360 mAh using one the active materials B.sub.1 
-B.sub.8, a nickel plate as an opposing electrode and an aqueous solution 
of 25% potassium hydroxide by weight was manufactured. It should be noted 
here that the amount of active material used to fill the nickel electrode 
was regarded here as consisting of only the nickel hydroxide component and 
was calculated using the theoretical capacity of (289 mAh/g) per unit 
weight of the nickel hydroxide. 
First, the efficiency of the active material was calculated according to 
the equation below, with the discharging capacity being measured by having 
the simple cell described above continuously charged for 24 hours using a 
current of 36 mA before discharging the cell with a current of 120 mA 
until the ending discharge voltage of -0.8V was reached at the nickel 
plate. 
##EQU2## 
The results of these experiments are shown in Table 3, with the amount of 
the cobalt covering layer being expressed as the percentage by weight of 
the cobalt compound relative to the nickel hydroxide calculated in terms 
of the weight of hydroxides. 
TABLE 3 
______________________________________ 
Operational efficiency 
Co % in terms 
Nickel active 
of active material 
of weight of 
material type 
index number! 
Ni hydroxide 
______________________________________ 
B.sub.1 92 0.5 
B.sub.2 97 1 
B.sub.3 98 3 
B.sub.4 100 7.5 
B.sub.5 100 (standard) 
10 
B.sub.6 98 12.5 
B.sub.7 98 15 
B.sub.8 90 16 
______________________________________ 
It should be noted here that the efficiency of the active materials shown 
in Table 3 are expressed as index numbers with the efficiency of the 
active material B.sub.5 being set at 100 as the standard. 
As can be clearly seen from Table 3, there is a large drop in the 
efficiency of the active material when the percentage by weight of cobalt 
is below 1% or above 15%. 
This can be considered as being caused by an insufficiency of the cobalt 
compound when the percentage by weight is below 1%, so that favorable 
conductive networks cannot be formed. On the other hand, when the 
percentage by weight is above 15%, the minus effect due to the relative 
decrease of the nickel hydroxide included in the active material which 
leads to a reduction in energy density can be considered as being more 
significant than the effect of the improvement in conductivity. 
From these results, it can be seen that it is desirable to have the amount 
of cobalt compound on the surface of the mother particles fall within a 
range of 1%-15% of the weight of the mother particle. 
Experiment 3 
In Experiment 3, the relationship between the proportionate amount of zinc 
compound to the cobalt compound and the excessive discharging 
characteristics was investigated. 
Using the same manufacturing method as active material A.sub.4 in the 
second embodiment, active materials C.sub.1 -C.sub.7 where the 
proportionate amount of zinc compound to cobalt compound in the covering 
is varied between 0.3% and 30% in terms of weight of the hydroxide were 
produced, with all other processes being the same as for active material 
A.sub.4. Using the same methods as Experiment 1, the capacity per unit 
active material and excessive discharging characteristics were measured 
for each active material C.sub.1 -C.sub.7. 
It should be noted here that for the covering layers of active materials 
C.sub.1 -C.sub.7, the proportion of the cobalt compound to the nickel 
hydroxide was fixed at 10% by weight calculated in terms of hydroxides. 
In Table 4, the capacity per unit active material and excessive discharging 
characteristics are expressed as index numbers with the capacity per unit 
active material and excessive discharging characteristics of active 
material C.sub.3 being set at 100 as the standard. 
TABLE 4 
______________________________________ 
electrical capacity 
excessive Co:Zn = 
per unit weight of 
discharging 
1:X 
Active the active material 
characteristics 
X is 
material type 
index no.! index no.! 
varied 
______________________________________ 
C.sub.1 101 88 0.3 
C.sub.2 100 98 0.5 
C.sub.3 100 (standard) 
100 (standard) 
5 
C.sub.4 99 100 10 
C.sub.5 99 99 15 
C.sub.6 98 100 25 
C.sub.7 92 100 30 
______________________________________ 
As can be clearly seen from Table 4, there is a sudden drop in excessive 
discharging characteristics once the proportion of the zinc compound in 
terms of the cobalt compound is below 0.5% by weight. This as can be 
considered as being caused by the decrease in the proportion of the zinc 
compound preventing sufficient realization of the multiple component 
effect, so that as a result the cobalt compound is diffused and permeates 
into the nickel hydroxide mother particles. 
On the other hand, once the proportion of the zinc compound in terms of the 
cobalt compound is above 25% by weight, there is a drop in the electrical 
capacity per unit weight of the active material. This can be considered as 
being caused by the decrease in the capacity per unit active material of 
the density of the cobalt compound on the surface of the active material 
which results from the increase in the amount of zinc compound. 
It should be noted here that the tendencies exhibited by the results in 
Table 4 were repeated when an aluminum compound, a magnesium compound or 
an indium compound were substituted for the zinc compound in the multiple 
component precipitate. 
From the above results, it can be seen that in the covering layer, it is 
desirable to have the ratio of one of the aluminum compound, the magnesium 
compound, the indium compound and the zinc compound to a cobalt compound 
fall within a range of 0.5% by weight to 25% by weight. 
Experiment 4 
In Experiment 4, the relationship between the proportion of zinc compound 
to cobalt compound and the operational efficiency of the active material 
was measured. 
The same manufacturing method as B.sub.7 was used to produce active 
materials D.sub.1 -D.sub.4 wherein the amount of cobalt compound in the 
covering layer is fixed at 15% by weight of the mother particle and the 
amount of zinc compound is varied and between 2% and 3.5% of the weight of 
the nickel hydroxide. 
The results of this experiment into the relationship between the proportion 
of the zinc compound to the nickel hydroxide (calculated in terms of 
hydroxides) and the efficiency of the active material are shown in Table 
5. 
TABLE 5 
______________________________________ 
composition 
in terms of nickel 
operational 
Active hydroxide active material 
material Co Zn index no.! 
______________________________________ 
D.sub.1 15 2 100 (standard) 
D.sub.2 15 2.5 100 
D.sub.3 15 3 99 
D.sub.4 15 3.5 96 
______________________________________ 
In Table 5, the efficiency of the active material is expressed as an index 
number with the efficiency of the active material D.sub.1 being set at 100 
as the standard. 
As can be seen from Table 5, there is a noticeable drop in the operational 
efficiency of the active material once the amount of zinc compound exceeds 
3% by weight of the nickel hydroxide mother particle compared to when the 
amount of the zinc compound is in the range of 2%-3% by weight. This can 
be thought to be due to the minus effect of the decrease in the amount of 
nickel hydroxide once the amount of zinc compound exceeds 3% by weight. 
From the above results and the results of Experiment 3, it can be seen that 
while it is desirable that the proportion of the zinc compound in the 
covering layer to the cobalt compound is between 0.5% to 25% by weight, 
when there is a relatively high proportion by weight of the cobalt 
compound to the nickel hydroxide, it is desirable that the amount of zinc 
compound in the covering be 3% by weight or below of the nickel hydroxide 
mother particle calculated in terms of hydroxides. 
Experiment 5 
In Experiment 5, the relationship of the operational efficiency of the 
active material and the pH of the solution during covering was 
investigated. 
Using the same manufacturing method as active material A.sub.4 in the 
second embodiment, active materials E.sub.1 -E.sub.6 were produced varying 
the pH of the solution during the formation of the covering layer within a 
range of 6.5 to 13 and the capacity per unit active material was measured 
in the same way as in Experiment 1. 
The results of this experiment into the relation between the capacity per 
unit active material and the pH of the solution are shown in Table 6. 
TABLE 6 
______________________________________ 
Active capacity per unit 
pH of solution 
material of active material 
during covering 
______________________________________ 
E.sub.1 81 6.5 
E.sub.2 94 7.5 
E.sub.3 95 9.5 
E.sub.4 100 (standard) 
10 
E.sub.5 94 12.5 
E.sub.6 80 13 
______________________________________ 
In Table 6, the capacity per unit active material is expressed as an index 
number with the capacity per unit active material of active material 
E.sub.4 being set at 100 as the standard. 
As can be clearly seen from Table 6, there is favorable capacity per unit 
active material when the pH of the solution during the formation of the 
covering layer is between 7.5 and 12.5, while when the pH of the solution 
is below 7.5 or above 12.5, there is a large drop in capacity per unit 
active material. 
When precipitation of the covering layer is performed within a pH range of 
7.5 to 12.5, the precipitation of the cobalt compound and zinc compound 
near the surface of the mother particles proceeds gradually and 
continuously, so that an even and precise covering layer can be formed. On 
the other hand, it is difficult for favorable precipitation to occur when 
the pH of the solution is below 7.5 or above 12.5. In particular, when the 
pH of the solution is above 12.5, the precipitation of the two-component 
precipitate becomes rapid and a covering layer cannot be properly formed. 
Experiment 6 
In Experiment 6, the relationship between the concentration of the alkali 
metal solution in the alkali heat treatment and the operational efficiency 
of the active materials was found. 
Using the same manufacturing method as the active material A.sub.4, active 
materials F.sub.1 -F.sub.6 were manufactured varying the concentration of 
the aqueous sodium hydroxide solution used in the alkali heat treatment 
between 12%-45%. The efficiency of the active materials F.sub.1 -F.sub.6 
was then measured in the same way as in Experiment 2. 
Table 7 shows the results of this experiment in which the relationship 
between the concentration of the aqueous sodium hydroxide solution and the 
efficiency of the active materials was investigated. 
TABLE 7 
______________________________________ 
operational efficiency 
concentration 
Active of active material 
of alkali metal 
material index number! 
solution (wt %) 
______________________________________ 
F.sub.1 89 12 
F.sub.2 96 15 
F.sub.3 100 (standard) 
25 
F.sub.4 100 35 
F.sub.5 98 40 
F.sub.6 91 45 
______________________________________ 
NB: Temperature during heat treatment: 80.degree. C. 
As can be clearly seen from Table 7, the efficiency of the active materials 
is favorable when the concentration of the sodium hydroxide solution is 
15% to 40% by weight, while when the concentration is below 15% by weight 
or above 40% by weight, there is a noticeable drop in the efficiency. 
This can be considered as being caused by the factors described below. 
When alkali heat treatment is performed using sodium hydroxide solution 
whose concentration is 15% to 40% by weight, the cobalt compound on the 
surface of the particles is evenly converted into cobalt compounds where 
the oxidization number of cobalt is above 2 (high-order cobalt compounds) 
which increases the conductivity of the covering layer. In this way, 
conducting networks are formed between the particles of active materials 
which increases the overall efficiency. 
On the other hand, if the concentration of the alkali metal solution is 
below 15% by weight, the solubility of the cobalt hydroxide in the alkali 
solution is reduced, resulting in problems in the reaction converting the 
cobalt hydroxide into higher order compounds. This can be considered as 
the reason why there is insufficient improvement in efficiency. 
Also, if the concentration of the alkali metal solution is above 40% by 
weight, it is thought that the increased viscosity of the solution 
prevents the alkali from permeating the covering layer, leading to 
unevenness in the reaction converting the cobalt hydroxide into higher 
order compounds. 
It should be noted here that it was checked that the same results were 
achieved by using other alkali compounds, such as potassium hydroxide, in 
place of sodium hydroxide. 
Experiment 7 
In Experiment 7, the relationship between the temperature during the alkali 
heat treatment and the efficiency of the active materials was 
investigated. 
Active materials G.sub.1 -C.sub.7 were manufactured using the same 
manufacturing method as active material A.sub.4 in the second embodiment, 
with the exception that the temperature during the alkali heat treatment 
was varied between 40.degree. C. and 160.degree. C., with the efficiency 
of the active material in these active materials G.sub.1 -G.sub.7 then 
being measured in the same way as in Experiment 2. 
The results are shown in Table 8. 
TABLE 8 
______________________________________ 
operational efficiency 
temperature in 
Nickel active 
of active material 
alkali heat 
material type 
index number! 
treatment (.degree.C.) 
______________________________________ 
G.sub.1 81 40 
G.sub.2 95 50 
G.sub.3 99 70 
G.sub.4 100 (standard) 
80 
G.sub.5 99 120 
G.sub.6 95 150 
G.sub.7 71 160 
______________________________________ 
NB: Concentration of sodium hydroxide in aqueous solution: 25% 
As can be clearly seen from Table 8, a range of heat treatment temperature 
between 50.degree. C. and 150.degree. C. is favorable for the efficiency 
of the active material, with there being a severe drop in efficiency below 
50.degree. C. and above 150.degree. C. 
The desirable range for the heating temperature during the alkali heat 
treatment being 50.degree. C. to 150.degree. C. can be considered as being 
due to this range allowing the smooth conversion of cobalt to a higher 
order and also due to the destruction of the microstructure of the 
covering layer composed of the multiple component precipitate during heat 
treatment and the formation of a covering layer with suitably-sized pores 
as the higher order cobalt compounds are generated. If the covering layer 
has suitably-sized pores, the covering layer does not hinder the contact 
between the electrolyte and the mother particle so that the 
electrochemical reaction can proceed smoothly. 
On the other hand, if the temperature during heat treatment is below 
50.degree. C., this will result in a decrease in the solubility of the 
cobalt hydroxide in the alkali solution as well as a reduction in the 
effect of the heat treatment on the covering layer. If the temperature is 
above 150.degree. C., it can be considered that the heat treatment has a 
detrimental effect on the nickel hydroxide forming the mother particle, so 
that the mother particle is converted into nickel oxide which is not an 
active material for an electrolytic cell. 
It should be noted here that the above embodiments have described a 
covering layer of a cobalt compound which contains one of an aluminum 
compound, a magnesium compound, an indium compound or a zinc compound, 
although the same results may be achieved if two or more of such compounds 
are used. 
Similarly, the above embodiments describe examples of foam nickel filled 
with active materials as the substrates of nickel electrodes, although the 
same results can be achieved by filling a substrate, such as punching 
metal, with active material in the same way as is described above. 
For the nickel hydroxide active materials of the present invention, the 
cobalt hydroxide in the covering layer is converted by means of alkali 
heat treatment into cobalt compounds where the oxidization number of 
cobalt exceeds 2, which acts to increase the conductivity of the active 
material. Further adding one or more of the following metal compounds; an 
aluminum compound, a magnesium compound, an indium compound and a zinc 
compound, also acts to suppress the diffusion and permeation of the cobalt 
compound into the nickel hydroxide mother particles during excessive 
discharging. 
Therefore, when this kind of nickel hydroxide active material is used to 
fill an electrode substrate which is then installed into an alkali storage 
cell, favorable conductive networks will be formed between the active 
material particles, leading to a large increase in the efficiency of the 
electrode active materials and an increase in the electrical capacity per 
unit weight of active material (energy density), as well as preventing any 
reduction in electrical capacity after excessive discharging. 
Furthermore, the manufacturing method of the present invention enables the 
safe and reliable manufacture of the above nickel active material and 
alkali storage cell by means of a relatively simple method. 
Accordingly, this method can provide nickel active material of high 
electrical capacity per unit weight and of superior excessive discharging 
characteristics at low cost. 
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.