JPH11135112A - Positive electrode for alkaline storage battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for an alkaline storage battery with a long service life, excellent storage characteristics, and a low cost. SOLUTION: A positive electrode for an alkaline storage battery with a long service life, superior storage characteristic and a low cost is obtained by using a porous metal plate having myriad recessed and projection parts on the surface as a substrate, adding 1-15 wt.% of carbon powder to metal powder as a conductive material or a conductive assistant, 1-15 wt.% of cobalt powder and/or cobalt oxide powder to a metal oxide, and 0.5-5 wt.% of fluororesin to the metal oxide as a binder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode for an alkaline storage battery, and more particularly to a positive electrode using a metal oxide mainly composed of nickel.

[0002]

2. Description of the Related Art In recent years, the use of portable cordless electric devices has been progressing, and there is a demand for a small-sized and higher-performance secondary battery. As a battery for such an application, in addition to a nickel-cadmium storage battery, the energy density is about 1.5 times.
Alkaline storage batteries, such as nickel-metal hydride storage batteries, are doubled. There is an increasing demand for these batteries to have higher capacity, higher energy density, and lower cost.

[0003] Conventional alkaline storage batteries such as nickel
As a positive electrode plate of a cadmium storage battery, a sintered nickel positive electrode has been used in which a carbonyl nickel powder or the like is sintered in a reducing atmosphere to form a porous substrate and an active material is retained therein. The electrode plate obtained by this method has a feature that it has excellent high-rate discharge characteristics and a long life due to a robust electrode structure. However, the sintered nickel positive electrode has the disadvantage that the cost is high because the substrate is sintered in a high-temperature reducing atmosphere and the active material filling step is relatively complicated. In addition, the maximum porosity of the porous substrate is about 85% in consideration of the strength of the substrate.
The filling amount of the active material is limited, and there is a limit in improving the packing density.

[0004] In order to improve these disadvantages, non-sintered nickel positive electrodes have been widely developed.
For example, a paste-type nickel positive electrode in which a foamed nickel porous body having a porosity of about 95% is used as a substrate instead of the above-described substrate and directly filled with a powder mainly composed of an active material has already been put into practical use. In addition, a fibrous nickel porous body or the like has been developed as a substrate for a paste-type nickel positive electrode. Since these nickel positive electrodes can use substrates with high porosity,
The filling amount of the active material can be increased as compared with the case of the sintering method, and the packing density of the electrode plate can be improved. Further, it has a feature that the filling step can be simplified. However, the substrate used for the paste nickel positive electrode is relatively expensive,
This is an issue in reducing the cost of batteries.

[0005]

Problems that are to be solved by the present invention
The substrate used for the strike-type nickel positive electrode is inferior in conductivity when formed into an electrode plate as compared with the sintered substrate, and therefore, cobalt and / or cobalt oxide is added as a conductive additive. By adding these conductive assistants, a conductive network is formed in the electrode plate, and a high initial positive electrode utilization rate of 90% or more can be obtained. However, a battery using such a positive electrode has a problem that the capacity is reduced compared to that before storage and the battery is not completely recovered even if normal charge and discharge are performed after long-term storage.

On the other hand, as a nickel positive electrode, a paste-coated nickel positive electrode using a metal porous plate such as an inexpensive punching metal for a substrate as an electrode support has already been proposed. However, in an electrode plate using such a substrate, the conductivity in the thickness direction of the electrode plate is higher than that of a paste-type positive electrode using a foamed nickel porous body or the like as a substrate. Further decline. For this reason, a satisfactory initial utilization cannot be obtained unless a large amount of a conductive material such as carbon is added in addition to a conductive aid such as cobalt. As a result, the energy density of the electrode plate has been reduced. It is also important that the mechanical strength of a paste-coated nickel positive electrode made of a porous metal such as the punched metal be close to that of a sintered electrode. For this purpose, the binder
It has been proposed to use a rubber-based resin such as styrene-butadiene rubber (hereinafter referred to as SBR). According to this method, although the strength of the electrode plate can be improved, such organic substances contained in the positive electrode are oxidized by oxygen generated during charging to generate carbonate ions. It's easy to do. Carbonate ions present in the electrolytic solution reduce the ionic conductivity of the electrolytic solution, and some ions reach the negative electrode and react with the negative electrode material to form different types of compounds. It often causes trouble. In other words, there is a problem that the discharge capacity and the cycle life characteristics are remarkably reduced as battery characteristics.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide a low-cost positive electrode for an alkaline storage battery having a long life and excellent storage characteristics.

[0008]

SUMMARY OF THE INVENTION In order to solve these problems, the present invention provides an electrode formed by coating or filling a mixture mainly composed of a metal oxide on a porous metal plate having a myriad of irregularities on its surface. In addition to the metal oxide, the carbon powder is 1 to 15 wt% based on the metal oxide, and the cobalt and / or the cobalt oxide powder is 1 to 15 wt% based on the metal oxide.
t%, fluororesin is 0.5 to 5 wt.
% Of a positive electrode for an alkaline storage battery using a mixture containing 0.1% by weight.

As a result, a low-cost positive electrode for an alkaline storage battery having a long life and excellent storage characteristics can be obtained.

In alkaline storage batteries such as nickel-cadmium and nickel-hydrogen storage batteries, the decrease in the utilization rate of the positive electrode after long-term storage is because the potential of the positive electrode decreases during storage and the added cobalt or cobalt oxide becomes alkaline. It is considered that the conductive network is dissolved in the electrolytic solution and the conductive network of cobalt is not sufficient. Even if the potential of the positive electrode decreases as a conductive material in addition to cobalt or cobalt oxide, this problem can be solved by adding stable carbon powder in an alkaline electrolyte.

On the other hand, by using a fluororesin having a high oxidation resistance also for the binder instead of the rubber-based resin, it is possible to suppress the formation of carbonate groups during the charge / discharge cycle and to improve the life characteristics.

A porous metal plate having a myriad of irregularities on its surface is inferior in conductivity when used as an electrode plate than a foamed nickel porous body, but is inexpensive and has a greater thickness direction than a perforated plate such as punched metal. Is excellent in conductivity, so that it is not necessary to newly add a large amount of conductive material. Therefore, by using these as a substrate, it is possible to obtain a more inexpensive positive electrode plate for an alkaline storage battery without significantly lowering the energy density of the electrode plate.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a schematic view of a cross section of an electrode plate according to the present invention. The substrate portion in FIG. 1 is constituted by a porous metal plate having a myriad of irregularities 3 formed on the surface thereof by sintering nickel particles applied to the surface of the punching metal 1 in a reducing atmosphere. ing. An electrode formed by coating or filling a mixture layer 4 mainly composed of nickel hydroxide powder 2, which is a metal oxide, on the substrate, wherein the mixture layer contains carbon oxide in addition to metal oxide. 1 to 15 wt% based on the material, and 1 to 15 w% of cobalt and / or cobalt oxide powder based on the metal oxide.
t%, fluororesin is 0.5 to 5 wt.
%.

The substrate in FIG. 2 is a porous metal plate formed by forming a nickel plate or a steel plate having a nickel-plated surface into a three-dimensional corrugated shape and a hole by die molding. It is composed of An electrode formed by coating or filling a mixture layer 4 mainly composed of nickel hydroxide powder 2, which is a metal oxide, on the substrate, wherein the mixture layer contains carbon oxide in addition to metal oxide. 1 to 15 wt% of cobalt and / or cobalt oxide powder to metal oxide
wt%, fluororesin is 0.5 to 5 w
t%.

By forming innumerable irregularities on the surface, the contact area between the metal oxide (nickel hydroxide in this case) as the active material and the porous metal as the current collector increases, and the surface becomes flat. As compared with the case where a substrate such as a punching metal is used as a current collector, the conductivity when an electrode plate is used is improved. Furthermore, when cobalt is present on the surface of these porous metal plates, it becomes cobalt oxyhydroxide having high conductivity as in the case where it is added to the active material as an additive, and further improves the conductivity between the substrate and the active material. be able to.

As shown in FIGS. 1 and 2, in addition to nickel hydroxide as an active material, carbon powder, cobalt and / or cobalt oxide powder (here, cobalt and water An example in which cobalt oxide was added was shown). Here, graphite powder was used as the carbon powder. In general, graphite powder is produced through a high-temperature heat treatment, and therefore has better oxidation resistance than other carbon powders. Further, since it is an inexpensive material, the cost of the electrode plate does not increase.

Further, as for cobalt and cobalt oxide, those which are more easily oxidized in order to form a conductive network as described above are desirable. The bivalent cobalt oxide such as CoO or Co (OH) 2 according to claim 7 has a higher efficiency of being oxidized to highly conductive cobalt oxyhydroxide at the time of the first charge than other cobalt species. Further, the carbonyl cobalt described in claim 6 and the Co (OH) 2 powder having a particle diameter of 1 μm or less as described in claim 8 have a large specific surface area due to a small particle diameter, and are liable to be oxidized more easily. Conceivable. In general,
Cobalt and cobalt oxide do not have sufficient conductivity, and the conductivity is improved by becoming trivalent cobalt oxyhydroxide as described above. In other words, in the uncharged state, a conductive network is not formed by cobalt, and a conductive network must exist in the pole beforehand in order for electrochemical oxidation of cobalt and cobalt oxide to occur. It is. In the present invention, since carbon powder is added as a conductive material in addition to cobalt and / or cobalt oxide powder, electrochemical oxidation of cobalt or cobalt oxide during initial charging easily occurs, and oxidation efficiency is also increased. .

On the other hand, as an effect of adding cobalt and cobalt oxide in addition to improving the conductivity, a discharge reserve may be formed in the negative electrode. The negative electrode used for the alkaline storage battery may have a lower utilization factor with respect to the theoretical capacity than the positive electrode depending on the discharge conditions, and in such a case, the expected sufficient battery capacity cannot be obtained. In other words, the discharge reserve means that the negative electrode is kept in a certain state of charge even if the positive electrode is completely discharged, depending on the type of negative electrode used and the expected use condition of the battery so that such a situation does not occur It is. For this purpose, there is a method in which only the negative electrode is pre-charged in advance. However, a new process is required for this, which leads to an increase in the battery manufacturing cost. The cobalt and cobalt oxide added here are oxidized during the initial charge and do not participate in the discharge.Therefore, there is almost no further change in normal charge and discharge. Becomes This quantity of electricity can be adjusted by the addition amounts and proportions of cobalt and cobalt oxide, and a required amount of charge reserve can be formed without providing a new process.

A relatively inexpensive substrate as described in claims 1 to 3 of the present invention has a higher active material holding force when used as an electrode than a sintered nickel substrate or a foamed nickel porous substrate. Inferior, a binder is indispensable for a practical positive electrode plate. As the binder, a fluororesin which is chemically stable from the viewpoint of oxidation resistance, particularly one containing polytetrafluoroethylene (hereinafter referred to as PTFE) as described in claim 9 is desirable. Further, PTFE fiberizes when a mechanical shearing force is applied when the active material and the additive are kneaded. Fiberized PTFE prevents aggregation of additives,
There is also an effect of improving the dispersibility of the additive and improving the battery characteristics.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific examples of the present invention will be described below.

(Example 1) Spherical nickel hydroxide powder,
Graphite powder, carbonyl cobalt powder, and P
To a mixture of TFE in various ratios as shown in Tables A to G, carboxymethyl cellulose (hereinafter referred to as CMC) as a thickener was added at 2 wt.
% And water were added and kneaded to form a paste. After thinly applying a layer of carbonyl nickel powder on the surface of the punching metal, sintering in a reducing atmosphere and applying these to a substrate having a myriad of irregularities on the surface, drying at 80 ° C. for 1 hour,
Pressing and cutting gave a positive electrode plate as shown in FIG.

[0023]

[Table 1]

These positive electrode plates are spirally formed through a general-purpose hydrogen storage alloy negative electrode and a separator made of a non-woven fabric made of polypropylene and subjected to a hydrophilic treatment to form an AA cylindrical closed type as shown in FIG. Nickel-metal hydride storage batteries A to G were prototyped. The nominal capacity of these batteries is 1000 mAh. The electrolytic solution was added to an aqueous KOH solution having a specific gravity of 1.30 (20 ° C.) at a rate of
A solution in which 0 g of LiOH was dissolved was used. In addition, as a comparative example, a battery H having the same configuration as the above battery was produced as a trial except that a positive electrode plate having the composition shown in Table 1H and filled with a powdered nickel foam was used.

When the utilization rates of these batteries were measured,
As shown in Table 2, in the batteries F and G in which the amount of carbon added was less than 3 wt%, the utilization rate of 90% or less was obtained.
It has been found to be disadvantageous for use in batteries requiring high energy density. The utilization was determined as shown in the following equation with respect to the theoretical capacity of the battery when the capacity of 1 g of nickel hydroxide was 289 mAh.

Utilization rate (%) = (0.2 C discharge capacity) / (theoretical capacity) × 100

[0027]

[Table 2]

In the battery A in which the amount of carbon added exceeds 15 wt%, although the utilization factor is high, the packing density of the electrode plate is low, so that the capacity density is as low as 550 mAh / cc or less. Therefore, it has been found that the addition of carbon exceeding 15 wt% is disadvantageous in increasing the energy density of the battery.

Next, these batteries were charged at 1000 mA for 1.5 hours in an atmosphere of 20 ° C.
The charge / discharge cycle was repeated under the condition of discharging the battery until the battery voltage reached 1.0 V in A, and a life test was performed. The measurement was performed for each of five batteries, and the average was taken. FIG. 4 shows changes in the discharge capacity of these batteries with charge / discharge cycles. When the battery life was defined as the time when the discharge capacity was reduced to 60% or less of the nominal capacity, the life characteristics tended to decrease as the amount of graphite added increased. However, when the amount of graphite added was 15 wt% or less. In this case, the capacity was maintained at 60% or more of the nominal capacity even after 500 cycles, whereas the capacity was sharply reduced in about 300 cycles in the case of the graphite addition amount of 20 wt%. The cause of the life degradation at this time is that the graphite powder added as a conductive agent is oxidized during charge and discharge to generate a carbonate group,
It is considered that the charge / discharge reaction of the battery was inhibited. on the other hand,
In the batteries F and G with a small amount of graphite added, the capacity gradually decreases from the beginning of the cycle, and the capacity retention rate becomes 60% or less in about 300 cycles. It is considered that the utilization rate of this battery is lower than that of the initial stage of the cycle, so that the amount of overcharged electricity increases as compared with other batteries, and the active material falls off from the substrate with the generation of oxygen gas.

Next, the following tests were performed to confirm the storage characteristics of these batteries. Separately from the batteries used in the life test shown above, batteries A to H using the positive electrode plates having the compositions shown in Table 1 were prototyped in exactly the same manner as in the above-described constitutional method. The capacity after storage for one month in an atmosphere of 45 ° C. was measured, and the capacity recoverability was compared. The results are shown in Table 2 together with the results of the standard capacity before storage. As a result, a battery in which graphite powder was added as a conductive material exhibited a high capacity recovery of 90% or more, while a battery in which graphite powder was not added used a foamed nickel porous body as a substrate. 80 for H
%. As described above, the cobalt and the cobalt compound added to the nickel positive electrode are oxidized to cobalt oxyhydroxide, which is a trivalent cobalt compound, at the time of the initial charge of the battery, so that the nickel hydroxide powder having poor conductivity is formed. It is considered that a conductive network is formed to greatly increase the utilization rate of the active material. However, when the battery is stored under the above conditions and the battery voltage is reduced to around 0 V, the cobalt oxyhydroxide is electrochemically reduced to a divalent cobalt compound having poor conductivity. Thereafter, even if the battery is charged, it does not completely return to the original state, so that the utilization rate does not recover. On the other hand, in a battery in which graphite powder is added as a conductive material, the conductive network in the positive electrode plate is not only a cobalt compound but also a conductive compound.
Since it is also formed of graphite powder, even if a part of the conductive network made of cobalt disappears, there is no great drop in utilization.

Example 2 Batteries I to N were produced in the same manner as in Example 1 except that a paste having the composition shown in Table 3 was applied.

[0032]

[Table 3]

When the utilization rates of these batteries relative to the theoretical capacity of the positive electrode were measured, the utilization rates tended to increase as the amount of metallic cobalt added increased.
%, It was almost constant. Considering that increasing the amount of additives other than nickel hydroxide, which is an active material, reduces the packing density of the electrode plate, the addition of metallic cobalt exceeding 15 wt% is disadvantageous in increasing the energy density of the battery. is there. Further, in the battery N to which no metallic cobalt is added, only a utilization rate of 80% or less is obtained, and as a result, the capacity density of the electrode plate is lowered, and the positive electrode plate of an alkaline storage battery requiring a high energy density is obtained. As a result, it was found that it was not suitable for practical use.

Next, batteries OT were produced in the same manner as in Example 1 except that a paste having the composition shown in Table 4 was applied.

[0035]

[Table 4]

The utilization of these batteries also tended to increase as the amount of cobalt hydroxide added increased. By the way, when the amount of cobalt hydroxide is considered as converted to cobalt atoms, the result is as shown in FIG. 5, and the same utilization rate is shown with a small addition amount as compared with the case where metal cobalt is added. This is presumably because the cobalt compound is more efficiently oxidized to cobalt oxyhydroxide during charging than the metallic cobalt when forming the above-mentioned conductive network of cobalt.

Next, a cycle life test of these batteries was performed in the same manner as in Example 1. The results are shown in FIGS. The lower the added amount of cobalt or the cobalt compound, the lower the capacity was observed. However, except for the batteries N and T, 60% or more of the nominal capacity was maintained even after 500 cycles. The cause of the deterioration of the life of a battery to which cobalt or a cobalt compound is not added is considered as follows. Since these batteries have a low utilization rate from the beginning of the cycle, the amount of overcharge increases and the amount of gas generated increases. As a result, the active material is detached from the electrode plate when gas is generated, the contact between the active material layer and the substrate is deteriorated, and the capacity is reduced.

On the other hand, as shown in Tables 5 and 6, the capacity recoverability in the same storage test as in Example 1 was as high as 90% or more in each of the batteries. This is considered to be due to the effect of adding graphite powder as a conductive material as described above.

[0039]

[Table 5]

[0040]

[Table 6]

Example 3 Batteries U to Y were produced in exactly the same manner as in Example 1 except that pastes having the compositions shown in Table 7 were applied.

[0042]

[Table 7]

When the utilization of the positive electrodes of these batteries with respect to the theoretical capacity was measured, the utilization tended to decrease as the amount of PTFE added increased. When the utilization exceeded 5 wt%, the utilization decreased to 90% or less. became. Further, the positive electrode plate of composition Y was considered to be unsuitable for mass production because the mechanical strength of the electrode plate was weak, so that the active material dropped off during processing such as cutting.

Next, the results of a cycle life test performed under the same conditions as in Example 1 are shown in FIG. It is considered that the cause of the deterioration in the battery using the positive electrode with a large amount of PTFE added is mainly due to the fall of the active material due to the increase in the amount of generated gas as described above because the initial utilization is low. on the other hand,
It is considered that the cause of the deterioration in the battery using the positive electrode to which the PTFE addition amount is small is that the mechanical strength of the electrode plate is weak, and the active material falls off when the electrode plate expands and contracts due to charge and discharge.

As shown in Table 8, the capacity recoverability during storage performed in the same manner as in Example 1 was as good as 90% or more in any of the batteries.

[0046]

[Table 8]

Here, as an example, a metal porous body having an infinite number of irregularities formed on the surface thereof with particulate nickel is used for the substrate, but a metal having an infinite number of irregularities formed on the surface thereof with short fiber nickel is used. When a porous body and a metal perforated plate processed into a three-dimensional corrugated shape are used as the substrate, the same results as those in Examples 1 to 3 can be obtained. Regarding the addition of cobalt and cobalt oxide, the same results as in the case of adding each alone can be obtained within the range shown in claims 1 and 3 even when they are added in combination.
Therefore, the required amount of discharge reserve can be arbitrarily determined by the addition ratio of cobalt and cobalt oxide.

[0048]

As described above, according to the present invention, in addition to the metal oxide, carbon powder is added to the relatively inexpensive porous metal plate having a myriad of irregularities on its surface. A mixture containing 1 to 15% by weight, 1 to 15% by weight of cobalt and / or cobalt oxide powder with respect to the metal oxide, and 0.5 to 5% by weight of the fluororesin with respect to the metal oxide. By filling, there is an effect that a low-cost positive electrode for an alkaline storage battery having a long life and excellent storage characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a cross section of a positive electrode plate when a porous metal body having countless irregularities on its surface is used as a substrate according to an embodiment of the present invention.

FIG. 2 is a schematic diagram of a cross section of a positive electrode plate when a three-dimensional corrugated metal plate according to an embodiment of the present invention is used as a substrate.

FIG. 3 is a diagram illustrating a sealed cylindrical battery according to an embodiment of the present invention.

FIG. 4 is a diagram showing cycle life characteristics in an example of the present invention.

FIG. 5 is a diagram showing a relationship between a converted cobalt amount and a utilization rate with respect to a theoretical capacity of a positive electrode in a second embodiment of the present invention.

FIG. 6 is a diagram showing cycle life characteristics in a second embodiment of the present invention.

FIG. 7 is a diagram showing cycle life characteristics in a second embodiment of the present invention.

FIG. 8 is a diagram showing cycle life characteristics in a third embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Punched metal 2 Nickel hydroxide powder 3 Particulate nickel 4 Cobalt and cobalt hydroxide, graphite, PTF
E mixed layer 5 hole 6 corrugated porous metal plate 7 hydrogen storage alloy negative electrode 8 positive electrode 9 separator 10 case 11 cap 12 safety valve 13 sealing plate 14 insulating gasket 15 positive electrode current collector

Claims (9)

[Claims] 1. A positive electrode for an alkaline storage battery in which a mixture mainly composed of a metal oxide is applied or filled on a porous metal plate having a myriad of irregularities on its surface, wherein the mixture comprises a metal oxide and a metal oxide. -Bon powder to metal oxide 1-15
A positive electrode for an alkaline storage battery, wherein the positive electrode contains 1 to 15 wt% of cobalt and / or cobalt oxide powder with respect to metal oxide and 0.5 to 5 wt% of fluororesin with respect to metal oxide. 2. The positive electrode for an alkaline storage battery according to claim 1, wherein the innumerable uneven portions are formed of fine particulate nickel or short fibrous nickel. 3. The positive electrode for an alkaline storage battery according to claim 1, wherein the innumerable uneven portions are formed by processing the metal plate into a three-dimensional wave shape. 4. The positive electrode for an alkaline storage battery according to claim 1, wherein the metal oxide is mainly nickel oxide. 5. The positive electrode for an alkaline storage battery according to claim 1, wherein the carbon powder is a graphite powder. 6. The positive electrode for an alkaline storage battery according to claim 1, wherein the cobalt powder is carbonyl cobalt. 7. The method according to claim 1, wherein the cobalt oxide powder is CoO and / or
Or Co (OH) _2.
The positive electrode for an alkaline storage battery according to the above. 8. The Co (OH) ₂ powder has an average particle size of 1 μm.
The positive electrode for an alkaline storage battery according to claim 1, wherein: 9. The positive electrode for an alkaline storage battery according to claim 1, wherein the fluororesin contains at least polytetrafluoroethylene.