JPH1074512A - Nickel hydrogen secondary battery and positive electrode thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain high energy density by using nickel oxyhydroxide particles whose surfaces are coated with cobalt oxyhydroxide, or solid solution particles mainly comprising nickel oxyhydroxide particles as active material particles. SOLUTION: In the manufacture of a positive electrode, nickel hydroxide powder is stirred in an aqueous solution containing an oxidizing agent to oxidize 'the nickel hydroxide, nickel oxyhydroxide obtained is washed, stirred in an alkaline aqueous solution, a cobalt salt aqueous solution is dropped therein to obtain nickel oxyhydroxide particles whose surfaces are coated with cobalt oxyhydroxide, obtained particles are washed and dried, then formed in a pasty material, and the pasty material is filled in a substrate. By coating the surfaces of the nickel oxyhydroxide particles of an active material with the cobalt oxyhydroxide of a conductive material, the amount of conductive material is reduced compared with the use of only powdery cobalt hydroxide as the conductive material, the physical filling capability of the active material is enhanced, irreversible electric capacity is reduced, and energy density is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nickel-hydrogen secondary battery and its positive electrode.

[0002]

2. Description of the Related Art In recent years, with the spread of portable devices, high-capacity secondary batteries have been demanded as power sources for portable devices. In particular, nickel-hydrogen batteries are secondary batteries consisting of a positive electrode made of an active material mainly composed of nickel hydroxide and a negative electrode made of a hydrogen storage alloy as an active material. It is becoming popular.

Hereinafter, the above-mentioned conventional positive electrode for an alkaline storage battery will be described. The positive electrode for an alkaline storage battery is roughly classified into a sintered type and a non-sintered type. In the former, a porous nickel sintered substrate obtained by sintering nickel powder and having a porosity of about 80% is impregnated with a nickel salt solution such as a nickel nitrate aqueous solution and then immersed in an alkaline aqueous solution. It is manufactured by producing a nickel hydroxide active material in a sintered substrate. Since it is difficult to further increase the porosity of the substrate in this electrode, the amount of the filled active material cannot be increased, and there is a limit to increasing the capacity.

As the latter non-sintered type positive electrode, for example, a sponge-like porous substrate made of nickel metal and having a three-dimensionally continuous porosity of 95% or more disclosed in Japanese Patent Application Laid-Open No. 50-36935 is disclosed. In addition, a battery filled with nickel hydroxide, which is an active material, has been proposed, and is now widely used as a positive electrode of a high-capacity secondary battery. In this non-sintered positive electrode, it has been proposed to fill spherical nickel hydroxide from the viewpoint of increasing the capacity. The pore (pore) size of the sponge-like porous substrate is about 200 to 500 μm, and since the pores are filled with spherical nickel hydroxide having a particle size of several μm to several tens μm, a nickel metal skeleton that maintains a conductive network The charge / discharge reaction proceeds smoothly in the vicinity of nickel hydroxide, but the reaction of nickel hydroxide separated from the skeleton does not sufficiently proceed. Therefore, in order to improve the utilization rate of the filled nickel hydroxide in this non-sintered positive electrode, a conductive agent is used in addition to the active material nickel hydroxide, thereby electrically connecting the spherical nickel hydroxide particles. Connected.
As the conductive agent, a cobalt compound such as cobalt hydroxide or cobalt monoxide, metallic cobalt, metallic nickel or the like is used. Thus, the non-sintered positive electrode can be filled with the active material at a high density, and the capacity can be increased as compared with the sintered positive electrode.

[0005]

However, even in the non-sintered positive electrode having the above structure, the addition of these conductive agents improves the utilization of the active material, but the conductive agent itself does not work as the active material. As a result, the packing density of the active material in the substrate is substantially reduced, and as a result, the capacity density of the positive electrode plate becomes 6
It becomes about 00 mAh / cc.

When cobalt hydroxide is used as a conductive agent, the cobalt hydroxide dissolves in a strongly alkaline electrolyte as cobaltate ions. The dissolved cobaltate ion is re-deposited on the surface of the nickel hydroxide particles or the surface of the cobalt hydroxide particles to become cobalt hydroxide, and further becomes cobalt oxyhydroxide, which is a conductor by initial charging, and causes the nickel hydroxide particles to be separated from each other. It is believed that they are electrically coupled. Also, the undissolved cobalt hydroxide changes into cobalt oxyhydroxide upon initial charging, and this also acts as a conductive agent.

However, the solubility of cobalt hydroxide and the like is about 100 to 200 ppm with respect to the electrolytic solution, and the cobalt layer formed on the surface of the nickel hydroxide particles by the dissolution and deposition is thin and weak. The cobalt layer is formed not only on the surface of the nickel hydroxide particles, but also on the surface of the sponge-like porous substrate skeleton which does not participate in the conductive network between the nickel hydroxides. Moreover, since the conductive network made of cobalt oxyhydroxide is thin and weak, the current collection cannot catch up with the discharge reaction under high discharge rate conditions, or when stored at a high temperature, etc., it dissolves in an alkaline electrolyte to form a conductive layer. For this reason, it has been difficult to maintain stable battery characteristics.

The utilization factor is also affected by the physical mixing state when nickel hydroxide and cobalt hydroxide or the like added as a conductive agent are mixed and filled in a sponge-like porous substrate, and stable characteristics are obtained. There is a problem that you can not always get.

In a sealed nickel-metal hydride secondary battery,
When the positive electrode is fully charged, an electrode plate having an electric capacity ratio of the negative electrode to the positive electrode of 1.5 to 1.8 times is used in order to prevent the internal pressure from increasing due to the generation of oxygen gas from the positive electrode. This is to allow excess hydrogen to be stored in the hydrogen storage alloy of the negative electrode at the time of full charge, and that the hydrogen reacts with oxygen generated from the positive electrode to increase the capacity to reduce the gas pressure inside the battery. Because you have to put it. Here, the fact that the excess amount of the negative electrode capacity to that of the positive electrode must be increased by 1.5 to 1.8 times is that nickel hydroxide, which is the active material of the positive electrode, is in the operating range of a normal nickel-metal hydride secondary battery. In this case, it is possible to discharge only in the vicinity of 2.2 in terms of the average valence of nickel hydroxide in the discharged state.

That is, the negative electrode opposite to the positive electrode also absorbs hydrogen equivalent to 0.2 valence at the time of discharging, and its electric capacity is irreversible and does not contribute to the battery capacity. Therefore, in order to improve the capacity of the sealed nickel-metal hydride secondary battery, it is necessary to reduce the irreversible capacity and to substantially improve the battery capacity.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a nickel hydrogen secondary battery having a high energy density and a positive electrode thereof.

[0012]

In order to solve the above problems, a positive electrode for a nickel-metal hydride secondary battery according to the present invention is provided on a sponge-like porous substrate by using oxyhydroxide coated with cobalt oxyhydroxide or cobalt hydroxide. It has a structure in which nickel oxide particles or solid solution particles containing nickel oxyhydroxide as a main component are filled as an active material. Here, the average valence of nickel oxyhydroxide is preferably from 2 to 2.3.

According to the above-mentioned structure, the surface of the nickel hydroxide particles as the active material is coated in advance with cobalt oxyhydroxide as the conductive agent. The physical filling property of the active material is improved as compared with the case where only particulate cobalt hydroxide or the like is used as the agent, and furthermore, since this positive electrode is partially oxidized, a battery having reduced irreversible electric capacity is constituted. As a result, stable battery characteristics with high energy density can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION According to the first aspect of the present invention, active material particles are nickel oxyhydroxide particles coated with cobalt oxyhydroxide or solid solution particles containing nickel oxyhydroxide as a main component. Therefore, the present invention provides a positive electrode for a nickel-metal hydride secondary battery, which has high energy density and stable battery characteristics.

(Embodiment) A process for manufacturing a positive electrode for an alkaline storage battery according to the present invention will be described below. The first step is a step of mixing and stirring the nickel hydroxide powder in an aqueous solution containing an oxidizing agent to oxidize the nickel hydroxide. The second step is a step of washing the nickel oxyhydroxide powder with water, and the third step is a step of dropping a predetermined amount of a cobalt salt aqueous solution into the alkaline aqueous solution while stirring the powder in the alkaline aqueous solution. The fourth step is a step of washing and drying the nickel hydroxide powder, and the fifth step is a step of filling the nickel oxyhydroxide powder into a porous metal substrate.

Next, a process for producing a positive electrode for a nickel-metal hydride secondary battery and a nickel-hydrogen secondary battery using the positive electrode will be described in more detail.

[0017]

【Example】

(Example 1) First, in the first step, a predetermined amount of pure water was charged into a reaction vessel, and 100 g of nickel hydroxide powder was charged therein, and the nickel hydroxide powder was stirred and dispersed by a stirrer. In order to oxidize nickel hydroxide to nickel oxyhydroxide while stirring the dispersion, sodium hypochlorite was added as an oxidizing agent and stirred sufficiently. The amount of sodium hypochlorite added was 10% by weight of divalent nickel hydroxide.
% To be oxidized to trivalent. At this time, it was confirmed by chemical analysis that the average oxidation valence of nickel hydroxide was 2.1.

In the second step, the obtained nickel oxyhydroxide was washed with pure water to near neutrality to remove impurities.

In the third step, the washed nickel oxyhydroxide powder was put into an aqueous solution of 5 mol / l sodium hydroxide, and the aqueous solution was heated to 65 ° C. by a heater while being stirred by a stirrer. This aqueous solution contains 1 mol of cobalt sulfate.
The mixture was sufficiently stirred while a 1 / l aqueous solution was added dropwise.

[0020] added dropwise a solution of cobalt sulfate in sodium hydroxide was heated in this process, cobalt cobalt ion HCoO ₂ ^- next, a solution noble potential than the standard hydrogen electrode potential in the same manner alkali hydroxide The oxidation-reduction reaction shown in the following (Chemical Formula 1) occurs with the resulting nickel oxyhydroxide, and cobalt oxyhydroxide having high conductivity grows on the surface of the nickel oxyhydroxide.

[0021]

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At this time, the amount of the dropped cobalt sulfate affects the amount to be coated. Here, the amount of the cobalt oxyhydroxide to be coated is 10 wt% with respect to the nickel oxyhydroxide.

If the alkali temperature, the concentration of cobalt sulfate, and the dropping rate are not set to appropriate values, cobalt hydroxide particles precipitate in the alkaline solution. The precipitated cobalt hydroxide particles are oxidized when they come into contact with nickel oxyhydroxide particles in an alkaline solution, and are dispersed in the solution as cobalt oxyhydroxide particles. Since the cobalt oxyhydroxide does not dissolve to form cobaltate ions, the surface of the nickel oxyhydroxide is not coated with the oxidation-reduction reaction. Also, when using cobalt hydroxide particles in an alkali as a raw material for cobalt acid ions, the surface of nickel oxyhydroxide is coated with cobalt oxyhydroxide in the same manner as when the cobalt sulfate solution is not properly dropped. I can't.

In the fourth step, the nickel oxyhydroxide particles coated with cobalt oxyhydroxide were washed with pure water and dried to remove impurities.

In the fifth step, 2 wt% of zinc oxide is added to the obtained powder, the water content is adjusted with pure water to form a paste,
This was filled into a sponge-like porous substrate to obtain a positive electrode for a nickel-metal hydride secondary battery.

A nickel-metal hydride battery was constructed by combining a positive electrode made of an active material whose surface was coated with 10% by weight of cobalt oxyhydroxide and a negative electrode made of a hydrogen storage alloy.
The battery of the present invention was prepared as A. For comparison, a nickel hydride battery was prepared by using a positive electrode made of an active material obtained by adding 10 wt% of cobalt hydroxide to nickel oxyhydroxide not coated with cobalt oxyhydroxide. A battery B was prepared by adding 10 wt% of cobalt hydroxide to a nickel hydroxide active material to form a positive electrode, and using this to form a nickel hydrogen battery.

The batteries A, B and C were charged at a charge rate of 0.1 C.
The battery capacity was measured when the battery was charged at mA for 15 hours and discharged at a discharge rate of 0.2 CmA to a final voltage of 1.0 V five times. The respective battery capacities were calculated as the theoretical capacities (electricity of 289 mAh / g when nickel hydroxide reacts one electron with the weight of the hydroxide active material filled in the positive electrode).
(A value multiplied by) is shown in (Table 1).

[0028]

[Table 1]

Next, the charging efficiency of the batteries A and C at a high temperature was measured. Charging at a temperature of 45 ° C and a charging rate of 0.1 Cm
A was charged for 15 hours at A, and the discharge rate was 0.2 CmA at 20 ° C. to a final voltage of 1.0 V.

[0030]

[Table 2]

As described above, according to the present embodiment, a highly conductive cobalt oxyhydroxide selectively chemically bonded to the surface of nickel oxyhydroxide can be formed by using an oxidation-reduction reaction, so that a high energy density is obtained. As a result, stable battery characteristics were obtained.

In the example, sodium chlorite was used as the oxidizing agent in the first step, but the oxidizing agent was K ₂
S ₂ O ₈ , Na ₂ S ₂ O ₈ , (NH ₄ ) ₂ S ₂ O ₈ or H ₂ O ₂
Any neutral or alkaline oxidizing agent may be used. The oxidation amount is 10w of divalent nickel hydroxide.
The amount of oxidation to be trivalent by t% was used, but it may be 30 wt% as long as it is equal to or more than the coating amount of cobalt oxyhydroxide to be coated.
The following is desirable.

In the third step, an aqueous solution of sodium hydroxide was used, but potassium hydroxide or lithium hydroxide may be used, and its concentration is 1 mol / l to 8.5 mol / l.
The temperature may be in the following range, and the temperature may be 80 ° C. or less. Although cobalt sulfate was used as a raw material for cobalt, any cobalt salt aqueous solution such as cobalt nitrate or cobalt chloride may be used. The concentration is desirably 2 mol / l or less at which precipitation does not occur at room temperature.

In the fifth step, zinc oxide is used as an additive, but cadmium oxide, zinc hydroxide, or the like may be used, and there is no particular limitation.

Example 2 In the first step, nickel oxyhydroxide was produced in the same manner as in the first and second steps of Example 1.

In the second step, 10 wt% of cobalt hydroxide particles are mixed with the nickel oxyhydroxide particles prepared in the first step, and the surface of the nickel oxyhydroxide particles is hydroxylated by dry mechanical mixing. Cobalt particles were coated.

In the third step, the nickel oxyhydroxide coated with cobalt hydroxide obtained in the second step was used as an active material in the same manner as the fifth step of Example 1 for a nickel-hydrogen secondary battery. A positive electrode was produced.

In the fourth step, an electrode group was formed in a spiral shape using the above-mentioned positive electrode for a nickel-hydrogen secondary battery, a negative electrode made of a separator and a hydrogen storage alloy.

In the fifth step, the above-mentioned electrode group was inserted into a battery case, a predetermined electrolytic solution was injected, and the battery was sealed with a sealing plate to produce a sealed nickel-metal hydride secondary battery.

In the sixth step, after the electrolyte was injected and sealed, the battery was left to stand at room temperature or higher and 80 ° C. or lower within 24 hours, and then subjected to initial charge and discharge.

During the standing time of this step, the cobalt hydroxide coated on the surface of the nickel oxyhydroxide is dissolved in the electrolytic solution to form cobaltate ions. To form cobalt oxyhydroxide.

A positive electrode was prepared from an active material in which nickel oxyhydroxide particles were coated with cobalt hydroxide particles by the dry mechanical mixing method of the present invention. For this purpose, a positive electrode was prepared from an active material obtained by adding 10 wt% of cobalt hydroxide particles to nickel oxyhydroxide particles and mixing in a mortar, and a nickel-hydrogen secondary battery was formed as a battery E, and nickel hydroxide was formed by dry mechanical mixing. A positive electrode was prepared from an active material in which 10% by weight of cobalt hydroxide was coated on the particle surface, and a nickel-hydrogen secondary battery was designated as battery F.

The batteries D, E and F were charged at a charge rate of 0.1 C.
The battery capacity was measured when the battery was charged at 15 mA for 15 hours and discharged at a discharge rate of 0.2 CmA to a final voltage of 1.0 V.
The utilization rate obtained by dividing each battery capacity by the theoretical capacity (a value obtained by multiplying the weight of the hydroxide active material filled in the positive electrode by the amount of electricity of 289 mAh / g when nickel hydroxide performs a one-electron reaction) (Table 3) Shown in

[0044]

[Table 3]

From the above results, it is possible to obtain high energy density and stable battery characteristics by using cobalt oxyhydroxide having a chemical bond and cobalt hydroxide uniformly and physically dispersed by a mechanical mixing method.

Here, in the second step, the addition amount of cobalt hydroxide is set to 10 wt%, but is not limited to 10 wt% and may be less.

(Example 3) In the first step, an active material of nickel oxyhydroxide coated with cobalt oxyhydroxide was prepared in the same manner as in the first to fourth steps of Example 1.

In the second step, the nickel oxyhydroxide coated with cobalt oxyhydroxide in the same step as in Example 2 was used by using the nickel oxyhydroxide particles coated with cobalt oxyhydroxide. Further, a nickel-metal hydride secondary battery was manufactured using the active material coated with cobalt hydroxide.

In this manner, 5
A nickel-hydrogen secondary battery G was constructed by combining a positive electrode using an active material coated with 5% by weight and further 5% by weight of cobalt hydroxide with a negative electrode made of a hydrogen storage alloy. The battery was charged at a charge rate of 0.1 CmA for 15 hours, and the battery capacity when discharged at a discharge rate of 0.2 CmA to a final voltage of 1.0 V was measured. The battery capacity was calculated as the theoretical capacity (electrical quantity of 289 mAh / g when nickel hydroxide was subjected to a one-electron reaction with the weight of the hydroxide active material filled in the positive electrode).
Was multiplied by 97.6%.

As described above, in this embodiment, high energy density and stable battery characteristics can be obtained by using cobalt oxyhydroxide having a chemical bond and cobalt hydroxide physically disposed.

[0051]

As described above, according to the present invention, a strong and uniform physical arrangement of the conductive agent is achieved by previously coating the surface of nickel hydroxide particles as the active material with cobalt oxyhydroxide as the conductive agent. In addition, the amount of the conductive agent can be reduced as compared with the case where only particulate cobalt hydroxide or the like is used as the conductive agent, the physical filling of the active material is improved, and the battery has a further reduced irreversible electric capacity. Therefore, it is possible to provide a nickel-metal hydride secondary battery capable of obtaining stable battery characteristics at a high energy density.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Koji Yuasa 1006 Kazuma Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (11)

[Claims] 1. A positive electrode for a nickel-metal hydride secondary battery, characterized in that nickel oxyhydroxide particles whose surface is coated with cobalt oxyhydroxide or solid solution particles containing nickel oxyhydroxide as a main component are used as active material particles. . 2. A nickel-hydrogen secondary battery positive electrode comprising nickel oxyhydroxide particles whose surface is coated with cobalt hydroxide or solid solution particles containing nickel oxyhydroxide as a main component, as active material particles. 3. An active material particle, wherein nickel oxyhydroxide particles whose surface is coated with cobalt oxyhydroxide or solid solution particles mainly containing nickel oxyhydroxide are further coated with cobalt hydroxide. Positive electrode for nickel-metal hydride secondary batteries. 4. Obtained by oxidizing nickel hydroxide with an oxidizing agent,
3. The positive electrode for a nickel-metal hydride secondary battery according to claim 1, wherein nickel oxyhydroxide particles are used. 5. The positive electrode for a nickel hydride secondary battery according to claim 4, wherein the average valence of nickel oxyhydroxide is 2.3 or less. 6. A nickel-hydrogen secondary battery comprising a combination of a positive electrode using nickel oxyhydroxide surface-coated with cobalt oxyhydroxide as an active material, a negative electrode made of a hydrogen storage alloy, a separator, and an alkaline electrolyte. . 7. The nickel oxyhydroxide having an oxidation valence of 2.3.
7. The nickel-metal hydride secondary battery according to claim 6, wherein the secondary battery has a charge of not more than the valence. 8. A nickel-hydrogen secondary battery comprising a combination of a positive electrode made of nickel oxyhydroxide whose surface is coated with cobalt hydroxide as an active material, a negative electrode made of a hydrogen storage alloy, a separator, and an alkaline electrolyte. . 9. The nickel oxyhydroxide having an oxidation valence of 2.3.
9. The nickel-metal hydride secondary battery according to claim 8, wherein the secondary battery has a charge of not more than the valence. 10. A positive electrode further coated with cobalt oxyhydroxide-coated nickel oxyhydroxide particles or solid solution particles containing nickel oxyhydroxide as a main component, a negative electrode made of a hydrogen storage alloy, and a separator. And a nickel-hydrogen secondary battery obtained by combining an alkaline electrolyte. 11. The nickel oxyhydroxide having an oxidation valence of 2.
The nickel-metal hydride secondary battery according to claim 10, wherein the secondary battery has three or less valences.