JPH0584026B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a nickel electrode for alkaline batteries.

Conventional technology and its problems Nickel electrodes in commonly used alkaline batteries are called sintered electrodes, and their manufacturing method involves impregnating a microporous substrate made by sintering ordinary nickel powder into a perforated steel plate, etc. with a nickel nitrate salt solution. In this method, a predetermined amount of nickel hydroxide is filled by repeating the step of converting the nickel hydroxide into nickel hydroxide in an alkaline solution several times.

However, this filling method is very complicated and involves repeating the steps many times, which is one of the causes of high costs. Moreover, the porosity of the substrate used is practically 80%.
Due to the following limitations, only electrodes with a low packing density of active material and a low energy density of about 400 mAh/cc can be produced.

In an attempt to improve these drawbacks, non-sintered electrodes have been widely developed. For example, JP-A-61
As disclosed in No. 138458, CoO powder that forms a conductive network between active materials is added to a nickel hydroxide powder active material prepared from a nickel nitrate aqueous solution and a sodium hydroxide aqueous solution, and carboxymethyl cellulose is added. It is created by adding a viscous liquid dissolved in water and filling it in a paste state into a fiber substrate. This nickel electrode is considerably cheaper than the sintered type and has a high energy density of around 500mAh/cc.

However, as portable electronic devices have become lighter in recent years, the market needs have increased to 600mAh/
A high energy density of about cc is required. In order to meet this need, it is necessary to increase the density of the nickel hydroxide powder itself since there is a limit to the porosity of the substrate.

The problem with conventional nickel hydroxide powder in densification is due to the disordered development of transition pores with a pore radius of 30 Å or more, as shown in A of the pore size distribution curve in Figure 1.

As is well known, nickel hydroxide particles are secondary particles made up of fine particles, and the pores of the primary particles are micropores with a radius of 10 to 20 Å, and larger transition pores are caused by the way the primary particles are packed. That is, it depends on how the primary particles grow.
In particular, the growth rate depends on the PH (alkali concentration) of the bath, which changes nickel ions into nickel hydroxide.

In conventional methods, it is difficult to control the pH of the bath at the particle level. However, a manufacturing method that has been developed in recent years and uses nickel hydroxide as a raw material for parkalizing iron plates can uniformly control the alkaline concentration of the bath at a low value. The manufacturing method involves dissolving nitric acid or nickel sulfate in a weakly basic ammonia aqueous solution, stabilizing it as a nickel ammine complex ion, and adding an aqueous sodium hydroxide solution at an appropriate pH to prevent the development of pores inside the particles. In this way, nickel hydroxide is gradually deposited and grown. Nickel hydroxide particles produced under appropriate conditions are particles in which transition pores are not developed at all, as shown in FIG. 1B, for example. As a result of adding cadmium or the like in solid solution to this powder without changing the pores and improving it as a battery active material, it is reaching a level where it can be used for practical purposes.

An electrode composed of this powder and the CoO additive has a capacity density of about 600 mAh/cc. However, in order to obtain this high capacity density, the electrode must be immersed in an alkaline electrolyte for more than a day, and conductivity will be established between the active material particles based on the reaction mechanism shown in Figure 2.
The process of connecting with CoOOH particles is essential.
It is desired to shorten this immersion-standing time and to make battery production smoother.

OBJECTS OF THE INVENTION The object of the present invention is to provide a nickel electrode for alkaline batteries in which the time required to form a CoOOH conductive network is shortened.

Structure of the Invention The present invention provides a paste-type nickel electrode in which a porous alkali-resistant metal fiber substrate is used as a current collector and nickel hydroxide powder is the main active material, and 〓-Co(OH) ₂ is applied to the surface of the nickel hydroxide particles. coating the particles;
The present invention is a nickel electrode for alkaline batteries characterized by having a triple layer structure in which nickel hydroxide particles are further coated on top of the nickel electrode.

In addition, the above cobalt hydroxide layer is not in a solid solution state with nickel hydroxide, but is in a free state, and the amount of cobalt hydroxide is 4 to 50% relative to the amount of nickel hydroxide.
It is in the range of 12wt%.

The reaction mechanism of paste-type nickel electrodes is different from that of conventional sinter-type nickel electrodes. In the case of sintered nickel electrodes, the current collector is made by sintering fine nickel powder, and its pore diameter is as small as 10-odd meters. For this reason, even with a nickel oxyhydroxide active material having low conductivity, a high active material utilization rate can be obtained. However, in the case of paste-type nickel electrodes, the active material, nickel hydroxide powder with a thickness of several tens of meters, must be directly filled into the current collector in the form of a paste liquid, which requires a large pore diameter of about 100 meters. I need. As the distance between the current collector and the active material increases, the active material utilization rate decreases significantly. Therefore, a means is needed to compensate for the electrical conductivity between the active material particles and the current collector. On the other hand, as disclosed in JP-A No. 61-138458, CoO which is soluble in alkaline electrolyte
A method has been proposed in which CoOOH particles are added and connected using conductive CoOOH particles. This reaction mechanism is based on the second
As shown in the modeling diagram of Fig. CoO→
In order to achieve uniform dispersion by the reaction of HCoOO ^- 〓-Co(OH) ₂ , a standing period of one day or more is required. In order to shorten this standing period, a method can be considered in which the surface of the nickel hydroxide particles is coated with -Co(OH) ₂ particles in advance and uniformly dispersed.
However, 〓−Co on the Ni(OH) ₂ particle surface
(OH) If only _two particles are coated, Ni
(OH) ₂ particles and 〓-Co(OH) ₂ particles are easily separated due to poor binding properties. Therefore, after coating the surface of nickel hydroxide particles with 〓-Co(OH) ₂ particles,
Furthermore, a thin uniform particle layer of porous nickel hydroxide particles was formed.

Examples Hereinafter, details of the present invention will be explained using examples.

Ammonium nitrate is added to an aqueous solution of nickel nitrate and a small amount of cadmium nitrate to form an ammine complex ion of nickel and cadmium.
This solution was dropped into an aqueous sodium hydroxide solution with vigorous stirring to gradually decompose the complex ions and form a solid solution of cadmium with an average particle diameter of 20〓m.
A certain amount of nickel hydroxide particles are precipitated. Conventional high-concentration alkaline solutions with a pH of 14 or higher precipitate nickel hydroxide particles with disordered particle sizes.
At a low alkaline concentration of about PH11 to 13, at a temperature of 20
Gradual precipitation was carried out in the range of ~90°C.

The higher the pH within the above pH range, the more porous the precipitated nickel hydroxide was.

FIG. 4 shows the particle size distribution of precipitated particles used in the present invention and conventional particles. These results show that the particles of the present invention have more uniform particle diameters than conventional powders. This indicates that the particles of the present invention do not precipitate in a disordered manner, but that the precipitation growth of the particles can be controlled.

Next, the liquid composition was made into a cobalt sulfate aqueous solution by almost the same operation as for obtaining the above-mentioned nickel hydroxide, and the A particles were coated with cobalt hydroxide in a range of 1 to 15 wt%. This amount is approximately several m
corresponds to the layer thickness of After that, using the above-mentioned nickel hydroxide precipitation growth process, approximately 3wt% was produced at around PH=13.
It was coated with C-layer nickel hydroxide particles. This layer requires a porous structure so that the internal 〓-Co(OH) ₂ particles can be dissolved. Fifth
The figure shows an electron micrograph of the surface of the C layer. This shows that the surface of the C layer has a porous structure.

A nickel-cadmium battery of the present invention was assembled from the nickel electrode obtained above using a cadmium electrode as a counter electrode and a caustic potassium aqueous solution having a specific gravity of 1.27 without undergoing a normal chemical conversion process. Using the battery of the present invention, the relationship between the storage period and the utilization rate of the active material was investigated. For comparison, a conventional battery using nickel hydroxide powder without a C layer as an active material and a conventional battery using an active material in which CoO powder was physically mixed with nickel hydroxide powder A were similarly assembled. Note that the amount of cobalt in all electrodes was the same.
The results are shown in FIG. The battery of the present invention is CoO
Although the utilization rate is somewhat lower than that of the mixed electrode, it shows a constant utilization rate after being left for about 3 hours. Conventional batteries without a C layer mostly peel off during the preparation of a paste solution, resulting in a low active material utilization rate and are not suitable for practical use.

In order for the active material to react, it is necessary to smoothly transfer electrons from the current collector to the surface of the active material particles.

As mentioned above, a network of electrically conductive CoOOH particles in a free state (existing on the particle surface without solid solution in nickel hydroxide) is essential. FIG. 7 shows the relationship between the amount of 〓-Co(OH) ₂ added, the active material utilization rate, and the energy density per electrode plate volume. From Fig. 7, as the amount of -Co(OH) ₂ is increased, the active material utilization rate also increases and converges to around 87%. However, since the additive itself only contributes to conductivity and does not participate in discharge, the upper limit of the actual plate energy density is 12 wt% of the amount of 〓-Co(OH) ₂ .

Effects of the Invention As described above, the present invention can provide a nickel electrode for alkaline batteries in which the time required to form a CoOOH conductive network is shortened, and therefore its industrial value is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a pore size distribution diagram of nickel hydroxide powder. FIG. 2 is a diagram showing a modeling of the conductive network formation mechanism of a cobalt compound additive in a paste-type nickel electrode. FIG. 3 is a modeling diagram of the structure of nickel hydroxide powder used in the present invention. FIG. 4 is a diagram showing the particle size distribution of nickel hydroxide powder. FIG. 5 shows an electron micrograph of the surface of the nickel hydroxide powder used in the present invention. FIG. 6 is a diagram showing the relationship between the standing time after injecting the electrolyte into the battery and the utilization rate of the active material. FIG. 7 is a diagram showing the relationship between the amount of 〓-Co(OH) ₂ additive, the active material utilization rate, and the energy density per electrode plate volume.

Claims (1)

[Claims] 1. An alkali-resistant metal porous substrate as a current collector,
A paste-type nickel electrode whose main active material is nickel hydroxide powder has a triple-layer structure in which the surface of the nickel hydroxide particles is coated with 〓-Co(OH) ₂ particles, and then the nickel hydroxide particles are further coated on top of that. A nickel electrode for alkaline batteries characterized by comprising: 2. The nickel electrode for an alkaline battery according to claim 1, wherein the cobalt hydroxide layer is not in a solid solution state with the nickel hydroxide but in a free state. 3. The nickel electrode for an alkaline battery according to claim 1, wherein the amount of cobalt hydroxide is in the range of 4 to 12 wt% based on the amount of nickel hydroxide. 4. The nickel hydroxide particles for alkaline batteries according to claim 1, wherein the nickel hydroxide particles coated with the cobalt hydroxide layer do not develop transition pores with a radius of 30 Å or more inside the particles and have a pore volume of 0.05 ml/g or less. electrode.