JP2682162B2 - Nickel electrode active material for alkaline storage batteries - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a nickel electrode active material for an alkaline storage battery.

2. Description of the Related Art In recent years, along with the reduction in size and weight of portable electronic devices, it has been desired to increase the energy density of batteries that are the power source thereof. In addition, due to the expansion of the use range of the device, a battery having stable performance in a wide temperature range, especially at a high temperature, is required.

Regarding the first problem of higher energy, the electrode of the conventional alkaline battery is called a sintered electrode, and the energy density of its positive electrode is limited to about 400 mAh / cc. Furthermore, paste type electrodes have been developed over the last few years for the purpose of increasing the capacity. For example, a nickel fiber porous body or a foamed metal porous body having a porosity of 95% or more is used as an electrode substrate, and a mixture of nickel hydroxide powder and cobalt monoxide powder or metal cobalt powder that is an additive for improving the utilization rate is used. There is an attempt to make a paste liquid and directly fill the substrate to make a positive electrode. In addition, the conventional nickel hydroxide powder is a very porous particle, and its internal pore volume ratio reaches as high as 30%. Development of nickel hydroxide powder having a higher density, which suppresses the development of the internal pore volume, is under way. With these developments, the energy density of the positive electrode is 500
It has been increased to ~ 600mAh / cc.

As the second measure against high temperature, conventionally, a measure has been taken in which cobalt is added as a solid solution to nickel hydroxide powder to improve charging efficiency at high temperature.

However, when a nickel hydroxide powder, especially a high-density nickel hydroxide powder in which the development of the internal pore volume is suppressed, is used for the paste type electrode, repeated charge / discharge cycles cause electrode swelling, which shortens the battery life. . Therefore, in order to prevent the electrode swelling, various additives such as cadmium hydroxide and zinc hydroxide are added alone as a solid solution, but the electrode swelling is not completely suppressed. Also, as the charge and discharge cycle is repeated,
The internal pore volume increases, and the apparent density (tapping density) of the nickel hydroxide powder decreases, resulting in a decrease in the density of the active material. As a result, high energy density of the electrode is hindered and reliability is low. I am making it.

Although high temperature performance can be improved by adding a small amount of solid solution of cobalt, electrode swelling cannot be suppressed, and at least 20% or more of solid solution must be added to nickel hydroxide powder. Therefore, there is a problem in that the capacity is reduced and the discharge voltage is significantly reduced.

OBJECT OF THE INVENTION The present invention has been made in view of the above conventional problems, and suppresses an increase in internal pore volume of nickel hydroxide powder due to charge and discharge, and prevents electrode swelling which is a main factor of electrode life. Another object of the present invention is to provide an active material for a nickel electrode for an alkaline storage battery, which has excellent temperature characteristics.

In order to achieve the above object, the present invention provides a nickel hydroxide powder containing zinc which is a Group II element of the periodic law,
A nickel electrode active material for an alkaline storage battery, wherein at least one kind of cadmium and magnesium and cobalt are simultaneously added as a solid solution and a cobalt oxyhydroxide layer is formed on the surface.

Action There are two crystalline forms in the nickel hydroxide charge product,
That is, β-NiOOH (density: 4.68 g / cc) and γ-NiOOH (density:
3.79g / cc) is available. Electrode swelling is due to this low density of γ-NiOO
It occurs because the crystal strain generated when H is generated increases the internal pore volume of the active material. Therefore, γ-NiOOH
Swelling of the electrode can be prevented by suppressing the increase in the volume of the internal pores due to the generation of

γ-NiOOH is produced in the following process. Β-NiOOH is the first product generated during the charging process.
OH has a crystal structure in which cadmium iodide type layers are hydrogen-bonded to each other. As the charge (oxidation) progresses further, cations and water molecules in the electrolytic solution intercalate between layers (intercalation into layers), and the structure moves to a rhombohedral system structure in which the interlayer distance is expanded, resulting in low density γ-. NiOOH is generated. That is, the intercalation of cations and water molecules between the crystal layers is γ-
This is the cause of NiOOH generation, and if it is prevented, it becomes possible to prevent the increase of the internal pore volume and the electrode swelling accompanying the production of γ-NiOOH.

Therefore, the following two measures are effective measures to prevent the electrode swelling.

(1) The hydrogen bond between the crystal layers of the nickel hydroxide active material itself is strengthened to prevent cations and water molecules from entering the layers.

(2) A third layer that prevents cations and water molecules from passing is provided between the nickel hydroxide active material and the electrolytic solution.

According to the present invention, (1) by adding a solid solution of zinc, cadmium and magnesium, which are Group II elements of the periodic law, to a solid solution of nickel hydroxide powder, hydrogen bonding between layers of the active material can be strengthened and maintained, and (2) ) By forming a cobalt oxyhydroxide layer on the surface of the nickel hydroxide powder,
The two synergistic effects of preventing the passage of cations and water molecules from the outside to the active material prevent the increase of the internal pore volume and the electrode swelling. Further, due to this synergistic effect, even if a small amount of a solid solution of cobalt is added, it is possible to obtain a nickel electrode having excellent high temperature performance which does not swell the electrode.

Examples Hereinafter, details of the present invention will be described with reference to Examples.

Ammonium sulfate salt is added to an aqueous solution of nickel sulfate with a small amount of zinc sulfate and cobalt sulfate salt to form an ammine complex ion of nickel, cobalt and zinc. This solution was dropped into an aqueous solution of sodium hydroxide to deposit nickel hydroxide powder in which cobalt and zinc were solid-dissolved. Next, as one method for forming a cobalt oxyhydroxide layer on the surface of this powder, this powder was added to an aqueous solution of a cobalt sulfate, and cobalt hydroxide was deposited and coated on the surface in the same manner as described above. Then, the powder was treated with an oxidizing agent, for example, an aqueous solution of hydrogen peroxide, to oxidize only cobalt hydroxide in the surface layer to cobalt oxyhydroxide to obtain the active material powder of the present invention. Alternatively, after leaving it in an alkaline aqueous solution in which a cobalt complex salt is dissolved, and coating the surface of the nickel hydroxide particles with a cobalt hydroxide layer,
You may perform the same oxidation treatment as the above. FIG. 1 shows a structural model diagram of the active material powder of the present invention. Here, 1 is a nickel hydroxide powder to which zinc, cadmium and cobalt are added as a solid solution, and 2 is a cobalt oxyhydroxide layer.

For comparison, an active material powder to which cobalt or zinc was not added and an active material powder having no cobalt oxyhydroxide layer on the surface were also prepared in the same process.

The active material powder thus prepared was uniformly mixed with 30% by weight of an aqueous solution in which 2% by weight of methylcarboxycellulose (CMC) was dissolved as a thickener to prepare a paste liquid. A nickel fiber substrate having a porosity of 95% was filled with this paste solution, dried and pressed to a predetermined thickness to obtain a nickel electrode for testing. A single cell was constructed by using a cadmium electrode as a counter electrode for this nickel electrode, and charging and discharging were repeated. The electrolyte is an aqueous potassium hydroxide solution with a specific gravity of 1.26, and the charge is the theoretical capacity.
The discharge was 150%, and 0 V was applied to the mercuric oxide reference electrode. In addition, the capacity test was conducted at charge / discharge temperatures: 0 ° C, 20 ° C and 45 ° C.

FIG. 2 shows various changes when an undoped nickel hydroxide electrode having no cobalt oxyhydroxide layer on its surface was charged and discharged. It can be seen that the repetition rate of charge and discharge increases the production rate of γ-NiOOH, and proportionally increases the internal pore volume of the active material, causing an increase in electrode thickness (electrode swelling). Further, the tapping density is lowered due to the increase of the internal pore volume.

FIG. 3 shows the internal pore volume distribution (pore size distribution) of the active material at that time, and a remarkable increase in the pore volume of the pore radius of 30 to 100 Å is recognized. Thus, the internal pore volume of the active material is a direct measure of the electrode swelling degree, and the electrode swelling degree can be evaluated in detail by looking at the size and the pore size distribution.

In order to see the effect of the addition of a solid solution such as zinc to the nickel hydroxide powder and the effect of the surface layer of cobalt oxyhydroxide on the electrode swelling degree, the internal pore volume distribution of the active material after charging and discharging was measured for the nickel electrode. Examined. The results are shown in FIG. Here, A represents the pore volume distribution before charging, and B, C, D, and E represent the pore volume distribution after charging. A
Are almost the same regardless of the powder composition. B is zinc solid solution addition + cobalt oxyhydroxide surface layer, C is zinc solid solution addition only, D is cobalt oxyhydroxide surface layer only, and E is nickel hydroxide (untreated) only. Pore radius, which is a measure of the degree of electrode swelling, the increase in pore volume of 30 to 100 Å can be suppressed to a certain extent only by the addition of a solid solution of zinc or only by the cobalt oxyhydroxide surface layer. You can see that is large. However, the suppressing effect is insufficient, and it is not possible to completely suppress the increase of the internal pore volume. On the other hand, as in the present invention, zinc is added as a solid solution to nickel hydroxide, and in an active material provided with a cobalt oxyhydroxide surface layer, the synergistic effect thereof causes a pore radius of 30 to 100 Å. The electrode swelling degree is almost completely suppressed. In addition, generation of γ-NiOOH was not confirmed by X-ray analysis. The same was true for the addition of solid solution in the presence of cadmium, magnesium and cobalt other than zinc.

Further, the nickel hydroxide powder to which zinc and cadmium were simultaneously added as a solid solution was almost the same as the one to which each was added alone.

FIG. 5 shows changes in tapping density with charge / discharge cycles of the active material according to the present invention. Without adding zinc or the like to the nickel hydroxide powder, or without providing the cobalt oxyhydroxide surface layer, the density of the active material decreases significantly with cycling, and high energy density as an electrode cannot be maintained. . On the other hand, although the density of the product of the present invention was slightly decreased at the initial stage, the extent of the decrease was small, and the high energy density property was maintained even in the practical use of 300 cycles. Particularly, in the high-density powder in which the development of the internal pore volume of nickel hydroxide particles is suppressed, as in the present invention,
It can be seen that the addition of a solid solution such as zinc and the nickel oxyhydroxide layer are indispensable for suppressing electrode swelling.

The total addition amount of zinc or the like to the nickel hydroxide powder is 3 to 10% by weight based on the nickel hydroxide, and the coating amount of cobalt oxyhydroxide is 1 to 10% by weight, which is effective in suppressing the electrode swelling. there were.

FIG. 6 shows the temperature characteristics of the active material of the present invention in which zinc and cobalt are added to nickel hydroxide as a solid solution to form a cobalt oxyhydroxide surface layer. The product of the present invention showed an active material utilization rate of 90% or more even at a high temperature of 45 ° C, and showed stable performance in the temperature range of 0 to 45 ° C without causing electrode swelling.

Instead of zinc, cadmium, magnesium, or a combination thereof may be used. Adding only solid solution of cobalt alone produced no γ-NiOOH at low temperature and suppressed the swelling of the electrode.

EFFECTS OF THE INVENTION As described above, the present invention suppresses an increase in internal pore volume of nickel hydroxide powder due to charge and discharge, prevents electrode swelling which is a main factor of electrode life, and has excellent temperature characteristics. Since it is possible to provide an active material for a nickel electrode for industrial use, its industrial value is extremely large.

[Brief description of the drawings]

FIG. 1 is a structural model diagram of the active material of the present invention. FIG. 2 is a diagram showing changes in various physical properties of a conventional nickel hydroxide electrode due to charging and discharging. FIG. 3 is a diagram showing changes in the internal pore volume distribution of a conventional nickel hydroxide active material. FIG. 4 is a diagram comparing changes in the internal pore volume distributions of the active material of the present invention and the conventional active material. FIG. 5 is a diagram showing charge / discharge cycle characteristics of the active material of the present invention and the conventional active material. FIG. 6 is a diagram showing temperature characteristics of the active material of the present invention.

Claims (1)

(57) [Claims] 1. A nickel hydroxide powder, to which at least one of zinc, cadmium and magnesium, which are Group II elements of the periodic law, and cobalt are simultaneously added as a solid solution, and a cobalt oxyhydroxide layer is formed on the surface. Nickel electrode active material for alkaline storage batteries.