Positive nickel electrode for alkaline storage battery

A positive electrode and a negative electrode for an alkaline storage battery and manufacturing methods thereof are provided. The positive electrode includes a nickel porous body formed on a plate, active material particles containing nickel hydroxide and additives filled up into the nickel porous body, a conductive layer coated on the surface of active material particle, and a protective layer coated on the surface of the conductive layer, for increasing the binding force between the active material particles and preventing the contact between the active material particles and an electrolytic aqueous solution.

BACKGROUND OF THE INVENTION 
The present invention relates to electrodes for a storage battery, and more 
particularly, to a positive nickel electrode and a negative electrode for 
an alkaline storage battery and manufacturing methods thereof, for use in 
a secondary battery such as an Ni-Cd, Ni-Fe, Ni-Zn, Ni-H or Ni-MH battery, 
which increase the capacity of the battery and reduce the self-discharge 
of the battery when open-circuited. 
A positive nickel electrode for an alkaline storage battery is made by 
filling up, so as to permeate, a porous nickel current collector with an 
active material, i.e., nickel hydroxide, by a sintering or paste method. 
In the fabrication of the positive electrode according to the sintering 
method, first, a porous nickel current collector is made by coating a 
nickel-plated steel plate with a slurry containing nickel powder as a main 
component, and drying and sintering the slurry-coated plate. Then, an 
active material containing nickel hydroxide is precipitated, chemically or 
electrochemically, at the pores of the nickel current collector, and 
treated in an alkaline solution. In this method, since the nickel current 
collector and the active material are strongly bonded and contact each 
other electrically over a large area, this type of positive nickel 
electrode exhibits the advantages of a high charging and discharging 
efficiency and a long life. Also, when additives to the active material 
are required, the amount of additives can be easily controlled by adding 
sodium nitrate containing a different element to a nickel nitrate solution 
and submerging the electrode in the solution. 
The sintering method, however, is complicated to perform and costly. The 
maximum porosity of the current collector is no more than 80% and thus the 
density of the precipitated active material is relatively low. 
On the other hand, a paste type positive nickel electrode is made by 
spraying or coating a porous nickel current collector of a strong 
alkali-proof foam metal with an active material in the form of paste and 
drying the current collector. 
Such a paste type positive nickel electrode is advantageous over the 
sintering type positive nickel electrode in terms of process simplicity 
and thus fitness for mass production. However, since the porous nickel 
current collector is directly filled with the active material in the form 
of paste, the active material contacts the current collector over a 
smaller area than in the sintering type positive nickel electrode, thus 
lowering the performance of the battery. 
FIG. 1 schematically illustrates a conventional alkaline battery and its 
positive electrode structure. 
Referring to FIG. 1, a nickel porous body 11 formed on a positive electrode 
plate 10 is filled with particles 12 of an active material containing 
nickel hydroxide and additives. The active material particles 12 each are 
coated with a conductive layer 13 of, for example, Co(OH).sub.2. Reactions 
take place at the positive nickel electrode during charge and discharge, 
as follows: 
EQU Ni(OH).sub.2 +OH.sup.- .revreaction.NiOOH+H.sub.2 O+e.sup.- 
The crystal structure of the nickel hydroxide, which depends on the 
manufacturing method thereof, experiences complicated changes during the 
reactions. Nickel hydroxide produced chemically in an aqueous solution is 
hexagonal .beta.-Ni(OH).sub.2, having a nickel ion interpositioned in an 
octahedral coordination between hydroxide ion layers. 
.beta.-Ni(OH).sub.2 and .beta.-NiOOH, formed after charging, each have a 
c-axis length of about 4.6-4.8 .ANG., since other interstitial ions or 
H.sub.2 O are not introduced between the layers in the crystal structure. 
The charge and discharge reaction between .beta.-Ni(OH).sub.2 and 
.beta.-NiOOH results in little change in structure and volume, since 
hydrogen ions are merely adsorbed and dissociated between layers. 
On the other hand, when .beta.-Ni(OH).sub.2 is overcharged, H.sub.2 O or 
interstitial ions are introduced between layers in .beta.-NiOOH, thereby 
producing .gamma.-NiOOH, which is changed into .alpha.-Ni(OH).sub.2 during 
discharging, in turn. Undesirable formation of this low-density 
.gamma.-NiOOH is accelerated when nickel hydroxide is filled in a high 
density to increase electron density, which obstructs diffusion of 
hydrogen ions into crystals. 
These .gamma.-NiOOH and .beta.-Ni(OH).sub.2 have a c-axis length of about 
7-8 .ANG., a 1.5 time-increase from that of .alpha.-Ni(OH).sub.2 or 
.beta.-NiOOH, since H.sub.2 O and interstitial ions between layers. Here, 
when charging, .alpha.-Ni(OH).sub.2 is changed into .beta.-Ni(OH).sub.2 of 
high density through chemical reaction, entailing a remarkable change in 
volume. This volume change causes swelling of the electrode and thus 
fall-off of the active material. As a result, a battery is charged in two 
stages. 
It is known that the major causes of degradation of a positive nickel 
electrode is the swelling-induced fall-off of the active material, 
destruction of the current collector, and corrosion of nickel used for the 
current collector. 
In an attempt to overcome the above problems, a method has been reported in 
which space for proton transfer is secured by transforming the lattice 
structure of nickel hydroxide. Thereby the conductivity of an active 
material is increased, leading to active transfer of electrons and 
suppression of .gamma.-NiOOH formation. For example, to induce 
transformation of the lattice, Zn or Mg is dissolved in a solid state in 
nickel hydroxide. To increase the conductivity, a conductive material such 
as a Co class compound is added. Co or preferably CoO is generally used as 
the conductive material. However, since the amount of the added compound 
reaches 10-20% of the total amount, the amount of nickel hydroxide used as 
the active material is relatively reduced, thus decreasing the capacity of 
the battery. Furthermore, it is difficult to completely prevent the 
electrode degradation and .gamma.-NiOOH formation caused during repeated 
charge and discharge processes. 
In addition, in the charge and discharge reactions of the conventional 
battery, the change in oxidation number is only one through the reaction 
of .beta.-Ni(OH).sub.2 .revreaction..beta.-NiOOH. One electron per nickel 
atom is exchanged during charge and discharge. Thus, the theoretical 
capacity is merely 289 mAhr/g. 
FIG. 2 schematically illustrates a conventional alkaline battery and its 
negative electrode structure. A negative electrode plate 20 for a cathode 
includes an active material structure 25 formed thereon and a hydrogen 
storage metal 26 filling the active material structure 25. 
The hydrogen storage metal 26 itself is a metal, thus having no problem 
related to conductivity. However, its hydrogen storage capacity 
sensitivity varies with temperature, and thus the self-discharge of the 
battery easily occurs at high temperatures. Moreover, the conventional 
negative electrode using Cd(OH).sub.2 as the active material has the 
problem of large self-discharge. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide electrodes for an 
alkaline storage battery and manufacturing methods thereof, which includes 
a non-aqueous, protective layer for preventing contact between an active 
material and an electrolytic aqueous solution, to prevent fall-off of the 
active material and reduce the self-discharge of the battery. 
To achieve the above object, there is provided a positive electrode for an 
alkaline battery comprising: a plate; a nickel porous body formed on the 
plate; active material particles containing nickel hydroxide and 
additives, filled up, so as to permeate, into the nickel porous body; a 
conductive layer coated on the surface of the active material particle; 
and a non-aqueous, protective layer coated on the surface of the 
conductive layer, for increasing the binding force between the active 
material particles and preventing contact between the active material 
particles and an electrolytic aqueous solution. 
There is provided a negative electrode for an alkaline battery comprising: 
a plate; an active material structure formed on the plate; hydrogen 
storage metal particles filled up, so as to permeate, into the active 
material structure; and a non-aqueous, protective layer coated on the 
surfaces of the hydrogen storage metal particles, for preventing contact 
between the hydrogen storage metal particles and an electrolytic aqueous 
solution. 
There is provided a method for manufacturing a positive electrode for an 
alkaline battery, comprising the steps of: permeating a porous nickel body 
formed on a plate with active material particles containing nickel 
hydroxide and additives; submerging the plate in an aqueous electrolytic 
solution to form a conductive layer on the surfaces of the active material 
particles; and forming a protective layer on the surface of the active 
material particles by submerging the plate for a predetermined time in a 
solution which is less conductive than the active material, and neither 
reacts nor mixes with an aqueous electrolytic solution, and drying the 
plate. 
There is provided a method for manufacturing a cathode, comprising the 
steps of: permeating an active material structure formed on a plate with 
hydrogen storage metal particles; and forming a protective layer on the 
surface of the hydrogen storage metal particle, by submerging the plate 
for a predetermined time in a liquid or solution which is less conductive 
than the active material and does not react with an electrolytic aqueous 
solution, and thereafter drying the plate.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 3, a positive electrode for an alkaline storage battery 
according to the present invention includes a porous nickel body 31 formed 
on a plate, and active material particles 32 containing nickel hydroxide 
and additives, such as conductive materials, for example, Co and CoO, Zn, 
and additives, such as bonding agents filling or permeating the pores of 
the porous body 31. Each of the active material particles 32 has a 
conductive layer 33 coating its surface. The conductive layer 33 is 
generally formed of Co(OH).sub.2 or CoO(OH), resulting from the reaction 
of Co or CoO in aqueous alkaline solution, such as KOH or NaOH solution. 
The surface of the conductive layer 33 is coated with a non-aqueous 
protective layer 34. The liquid protective substance or solution layer 34, 
which is characteristic of the present invention, serves to prevent 
fall-off of the active material 32 and reduce the self-discharge of the 
battery by preventing electrical contact between the active material 32 
and an aqueous electrolytic solution. Preferably, the protective layer 34 
is less conductive than Ni(OH).sub.2 used as the active material. It is 
preferably a liquid formed from one or more organic compounds which are 
immiscible or substantially immiscible with water and do not react or mix 
with the aqueous electrolytic solution. The protective layer 34 may be 
formed of at least one substance selected from the group including 
benzene, n-butylacetate, sec-butylacetate, n-butylchloride, carbon 
tetrachloride, chlorobenzene, chloroform, cyclohexane, cyclopentane, 
dichlorobenzene, ethyl ether, heptane, hexane, methylenechloride, toluene, 
trichloroethylene, and xylene. 
Referring to FIG. 4, a negative electrode for an alkaline storage battery 
according to the present invention includes an active material structure 
45, such as Cd(OH).sub.2, and hydrogen storage metal particles 46 
permeating the pores of the active material structure 45. The surfaces of 
the hydrogen storage metal particles 46, which may be any substance 
commonly used by the skilled artisan for such purposes (for example, 
Mg.sub.2 Ni, Mg.sub.2 Cu, CaNi.sub.5, ZrMn.sub.2, etc.), are coated with a 
protective layer 44, like the active material particles of the positive 
electrode. 
A method for manufacturing a positive electrode for an alkaline storage 
battery will be described. 
The porous nickel body 31 formed on the plate (reference numeral 10 in FIG. 
1) is filled with the active material particles 32, and then dried at an 
above-ambient temperature for a time sufficient to remove substantially 
all moisture and cause drying. After the active material particles 32 are 
completely dried, the plate is submerged into a KOH solution for a time 
sufficient to allow total permeation and reaction, e.g., on the order of 
about 12 hours. At this time, an additive, CoO, contained in the active 
material is changed into CoO(OH) or Co(OH).sub.2. Then, the plate is dried 
again at an above-ambient temperature for a time sufficient to remove 
substantially all moisture and effect drying, thus forming conductive 
layers 33 on the active material particles 32. Subsequently, the plate is 
submerged for a predetermined time in a non-aqueous liquid protective 
substance or solution which is less conductive than the active material. 
The liquid protective substance or solution is preferably a liquid formed 
from one or more organic compounds which are immiscible or substantially 
immiscible in water and do not react or mix with an aqueous electrolytic 
solution, and thereby the conductive layers 33 are coated with protective 
layers 34. The time required for the plate to be submerged in the 
protective liquid or solution can be adequately controlled in 
consideration of the performance of the battery, the kind of solution, and 
additives added to the solution. Preferably, the solution does not react 
with the KOH solution. Finally, when the coating of the active material 
particles 32 is completed, the plate is dried. With the coating of the 
protective layers 34 on the surfaces of the anode active material 
particles 32, the active material becomes less conductive with respect to 
the electrolytic aqueous solution, resulting in a flow of a relatively 
large amount of current between the active material and the nickel porous 
body 31. 
According to the present invention, the negative electrode manufacturing 
method is the same as the positive electrode manufacturing method, except 
for the step of forming the conductive layer. In other words, a negative 
electrode is made by submerging a fabricated plate in a liquid protective 
substance or solution which is less conductive than an active material and 
does not react or mix with the aqueous electrolytic solution, and drying 
the plate. 
The protective layers 34 and 44 coating on the active material particles 32 
and 42, respectively, increase the binding force between the active 
material particles 32 and 42, thereby preventing fall-off of the active 
materials caused during charge and discharge. Further, since the active 
materials are prevented from reacting with the aqueous electrolytic 
solution, the self-discharge of the battery is reduced and electron 
exchanges at the electrode are facilitated. Thus, the capacity of the 
battery is increased and its long lifetime is ensured. 
EXAMPLE 
2 g of water was mixed with 4.36 g of nickel hydroxide including 5 wt % of 
Zn dissolved in a solid state, 0.62 g of CoO, 0.15 g of Co, 0.03 g of 
(hydroxypropyl)methyl cellulose (HPMC), and 0.1 g of 
polytetrafluoroethylene (PTFE). The nickel porous body 31 formed on the 
plate was filled with the above mixture, submerged in a KOH solution, and 
completely dried. Then, the plate was submerged in 100% toluene for 30 
minutes and dried. The capacity of the battery fabricated from the 
electrode plate increased by 20% over that of the battery of a non-toluene 
treated electrode.