JP2002270168A - Active material for nickel electrode, method for producing the same, and nickel electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an alkaline storage battery exhibiting enhanced charge efficiency and discharge efficiency even when actuating at a high temperature. SOLUTION: This active material for the nickel electrode in a form of a powder used for the alkaline storage battery is constituted by a core layer mainly consisting of nickel hydroxide, and a conductive surface layer consisting of a cobalt compound. The active material contains a rare earth element, and a density of the rare earth element is higher on the surface than at the inside of the particle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active material for a nickel electrode used in an alkaline storage battery such as a nickel hydride battery, a nickel cadmium battery and a nickel zinc battery, a method for producing the same, and a nickel electrode using the same.

[0002]

2. Description of the Related Art There is an increasing demand for alkaline storage batteries as power sources for portable electronic devices such as mobile phones, small electric tools and small personal computers, and as power sources for electric vehicles.

[0003] An alkaline storage battery uses a nickel electrode whose active material is particles mainly composed of nickel hydroxide as a positive electrode. When an alkaline storage battery is used near room temperature, the potential at which oxygen is generated at the positive electrode due to decomposition of the electrolyte during the charging process (oxygen generation potential) and the potential at which nickel hydroxide, which is a positive electrode active material, is oxidized to nickel oxyhydroxide (Oxidation reaction potential) is large. In this case, since the generation of oxygen is suppressed, generally high charging efficiency can be expected.

[0004] However, when the battery temperature rises due to heat generated during charging or discharging, the above-mentioned potential difference becomes small, and the charging efficiency tends to decrease. Usually, an alkaline storage battery is often stored in a narrow space where heat is hardly dissipated, such as in a portable electronic device or an automobile. Therefore, it is often difficult to avoid a decrease in charging efficiency due to an increase in battery temperature. When the charging efficiency decreases, adverse effects such as an increase in the internal pressure of the battery at the time of charging and a decrease in cycle performance occur.

[0005] Therefore, various improvements for suppressing a decrease in the charging efficiency of an alkaline storage battery at high temperatures have been studied. For example, Japanese Patent Application Laid-Open No. 3-78965 discloses that a part of nickel of nickel hydroxide as a positive electrode active material is periodically regulated.
By substituting with a group II element, cobalt or both of them, the oxygen generation potential is shifted noblely, or the oxidation reaction potential of nickel hydroxide is shifted to a lower level,
This publication discloses that the difference between the oxygen generation potential and the oxidation reaction potential is increased even at a high temperature. However, the replacement with cobalt has the disadvantage of lowering the discharge potential of the nickel electrode and consequently lowering the battery output. For this reason, the substitution amount by cobalt cannot be increased, and the effect of expanding the difference between the oxygen generation potential and the oxidation reaction potential is small.

Japanese Patent Application Laid-Open No. 7-45281 discloses that lithium hydroxide is added to an aqueous solution of potassium hydroxide, which is an alkaline electrolyte commonly used in nickel-metal hydride storage batteries, thereby shifting the oxygen generation potential noblely. An arrangement is disclosed. However, since the solubility of lithium hydroxide in an alkaline electrolyte is small, the obtained effect is small.

Further, Japanese Patent Application Laid-Open No. 9-92279 discloses a cathode obtained by adding ytterbium or an ytterbium compound (eg, an oxide such as Yb ₂ O ₃ ) to a nickel hydroxide of a positive electrode. Also, in Japanese Patent Application Laid-Open No. 5-89992,
Similarly, there is disclosed a cathode nickel hydroxide in which yttrium or an yttrium compound (for example, Y ₂ O ₃ or Y (OH) ₃ ) is added. According to the publication, the effect of shifting the oxygen generation potential of the positive electrode to the noble side is remarkable, and the difference between the potential and the oxidation reaction potential can be increased, so that charging efficiency at high temperatures can be improved. .

The above-mentioned ytterbium, ytterbium compound, yttrium or yttrium compound is usually not obtained in the form of fine particles, so that it is difficult to uniformly disperse it in the electrode. Therefore, in order to shift the oxygen generation potential by adding these elements or compounds, it is necessary to add at least several% by weight. Because the amount of rare earth element must be increased,
There are disadvantages that the electrode becomes expensive and that the filling amount of the active material must be relatively reduced.

Since nickel hydroxide itself has poor conductivity, it is necessary to impart conductivity to the electrode by some means. As disclosed in Japanese Patent Application Laid-Open No. 61-138458, when a cobalt compound such as cobalt monoxide is used as a conductivity-imparting agent, the cobalt compound mixed and added to the nickel hydroxide particles contains After dissolving in the alkaline electrolyte by the above, it precipitates as cobalt hydroxide, which changes into conductive cobalt oxyhydroxide by charging, and forms a conductive network in the electrode.

However, if a cobalt compound and a rare earth compound such as yttrium hydroxide are added simultaneously, a cobalt compound once dissolved is prevented from being precipitated as cobalt hydroxide in the electrode, so that a good conductive network is not formed. In addition, there is a possibility that the utilization rate of the active material may decrease.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and has a high charging efficiency by suppressing the generation of oxygen at the positive electrode during charging.
Another object of the present invention is to provide a nickel electrode for an alkaline storage battery having a high active material utilization rate by forming a good conductive network in the electrode.

[0012]

In the present invention, a nickel electrode active material is used for a particulate nickel electrode having a core layer mainly composed of nickel hydroxide and a conductive surface layer made of a cobalt compound. An active material containing a rare earth element and having a higher concentration of the rare earth element on the surface of the particle than inside the particle.

Then, a conductive surface layer made of a cobalt compound is formed on the surface of the particles mainly composed of nickel hydroxide, and the hydroxide of the rare earth element is deposited on the outermost surface, whereby the nickel having the above structure is formed. Manufacture active materials for electrodes.

In the present invention, a nickel electrode is provided in which the active material particles are supported on an alkali-resistant substrate.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The core layer of the active material particles of the nickel electrode for an alkaline storage battery according to the present invention is not particularly limited as long as it has nickel hydroxide as a main component. For example, nickel hydroxide containing elements other than nickel, such as zinc and cobalt, in a solid solution or a mixed state can also be applied.

The active material particles of the nickel electrode of the alkaline storage battery according to the present invention have a conductive surface layer containing cobalt oxyhydroxide as a main component on the surface of the core layer. The surface layer forms a conductive network within the nickel electrode.

The active material particles of the nickel electrode of the alkaline storage battery according to the present invention include erbium (Er), thulium (T
m), at least one kind of rare earth element selected from ytterbium (Yb), lutetium (Lu) and yttrium (Y), and the concentration of the rare earth element in the particles is higher on the particle surface than in the particles. .

The ratio of the rare earth element contained in the active material particles is preferably set to 0.1 wt% or more in terms of weight in terms of metal. When the content ratio is less than 0.1 wt%,
The effect of preciously shifting the oxygen generation potential is not sufficient, and the effect of increasing the charging efficiency of the alkaline storage battery at high temperatures may not be obtained. Further, it is desirable that the ratio is set to 20 wt% or less. When the ratio exceeds 20 wt%, the thickness of the layer containing the rare earth becomes large, impairing the conductivity, and the filling amount of nickel hydroxide decreases relatively.

The rare earth compound contained in the active material particles of the present invention is not particularly limited, but is preferably an Er, Tm, Yb, or Lu compound. Further, a composite compound containing two or more rare earth elements containing at least one of the above four kinds of rare earth elements may be used. The composite compound has an advantage that it can be procured at a lower cost than a pure rare earth compound.

The active material for a nickel electrode according to the present invention is synthesized, for example, according to the following steps. In the first step, a surface layer mainly composed of cobalt hydroxide is formed on the surface of the core layer particles mainly composed of nickel hydroxide having an average particle diameter of several tens of μm. In the second step, a hydroxide containing the rare earth element is deposited on the outer surface of the particles. Finally, after being incorporated as a nickel electrode into an alkaline storage battery, the active material is generated in the course of repeated charging and discharging operations.

The nickel hydroxide in the surface layer is oxidized by the initial charge at a low rate after being incorporated in the battery, and is converted into conductive nickel oxyhydroxide. The surface layers of the individual particles continue to form a conductive network throughout the electrode.

In the course of the repetition operation of the subsequent discharge and charge, the hydroxide of the rare earth element on the outermost surface of the particle is repeatedly dissolved and precipitated as described later, and diffuses inside the particle to form active material particles having a high rare earth element on the surface of the particle. I do.

The nickel electrode according to the present invention has the active material particles carried on an alkali-resistant metal substrate.
Specifically, the active material particles are mixed with an aqueous solution of a binder such as carboxymethyl cellulose to form a paste, and the paste is applied and filled on a substrate. As the alkali-resistant metal substrate, a substrate generally used as a substrate for a paste electrode of an alkaline storage battery can be applied. Specific examples include a sponge-like metal, a non-woven metal, and a perforated plate made of nickel or a nickel-plated steel plate.

The process of synthesizing the active material particles after being incorporated into the battery will be described in detail below. After the battery is assembled and charged for the first time, the discharging and charging operation is repeated several times. The initial charge converts the cobalt hydroxide to conductive cobalt oxyhydroxide. Further, the rare earth element is caused to penetrate into the particles by a repetitive operation of charging and discharging thereafter. Since the rare earth element is mainly present on the surface of the particles even after the rare earth element has penetrated into the inside of the particle, the concentration of the rare earth element is higher on the surface of the particle than on the inside of the particle.

FIG. 1 is a schematic diagram for explaining a cross-sectional structure of active material particles applied to a nickel electrode according to the present invention. In the figure, for the sake of clarity, the outermost portion containing a high concentration of rare earth elements is shown as a clear layer, but in reality the concentration of rare earth elements is high on the particle surface and the concentration is low inside the particles. And there are no strict boundaries.

Core layer particles 1 mainly composed of nickel hydroxide
The surface layer 2 made of cobalt hydroxide provided outside is changed to a layer of conductive cobalt oxyhydroxide by the initial charge. By the subsequent repetition of discharge charging, the hydroxide or oxide of the rare earth element deposited on the outermost surface 3 of the particles diffuses inside the particles.

Generally, nickel hydroxide particles have a fine structure, and one particle (secondary particle) is an aggregate of a plurality of fine particles (primary particles). It is considered that the rare earth element penetrates into the nickel hydroxide particles mainly due to dissolution and precipitation in the alkaline electrolyte, and thus penetrates into the inside of the particles along the surface of the fine particles (primary particles). Therefore, it is considered that the rare earth element that has entered the inside of the particle mainly exists on the surface of the fine particles (primary particles) and the surface of the pores of the active material particles (secondary particles).

In the nickel electrode of the present invention, the presence of a small amount of a rare earth element has a remarkable effect on the shift of the oxygen generation potential of the nickel electrode. Oxygen is generated mainly at the solid-liquid interface between the active material particles and the electrolyte. According to the present invention, the rare earth element is uniformly distributed in the electrode. In addition, the same element at the interface between the active material particles containing pores and the electrolyte,
It exists as a separate phase from nickel hydroxide and cobalt compounds. It is considered that the rare earth element existing at this interface is effectively acting to suppress the generation of oxygen.

In the present invention, since the rare earth element is mainly present on the surface of the active material particles as a separate phase from the core layer and the conductive surface layer, nickel hydroxide constituting the core layer and a precursor of the conductive surface layer are formed. After forming the precursors of the core layer and the conductive surface layer while avoiding coprecipitation with the cobalt compound constituting the above, a rare earth element or a compound thereof is further precipitated outside thereof.
As a result, a remarkable effect can be obtained with a small amount of a rare earth element.

(Examples) Hereinafter, details of the present invention will be described.
The present invention will be described by way of examples, but the present invention is not limited to these examples.

(Preparation of Core Layer Particles of Positive Electrode Active Material) Core layer particles mainly composed of nickel hydroxide are disclosed in JP-A-2-3006.
No. 1 was synthesized by a known method. That is, a mixed aqueous solution of nickel sulfate, cobalt sulfate and zinc sulfate having a predetermined concentration is adjusted, ammonium sulfate is added to the aqueous solution to adjust the pH to 11 to 12 to generate an ammine complex, and the aqueous solution is vigorously stirred. While adding the aqueous sodium hydroxide solution, the pH of the aqueous solution was adjusted to 11 to 12 to precipitate particles mainly composed of nickel hydroxide. The obtained particles were obtained by converting cobalt and zinc into 1.5 wt.
wt%.

(Formation of Precursor for Conductive Surface Layer) A precursor for the conductive surface layer was produced by a known method described in JP-A-62-234867. The core layer particles containing nickel hydroxide as a main component were dispersed in an aqueous solution adjusted to pH 8 to 13 by adding ammonium sulfate and sodium hydroxide. While stirring the aqueous solution, a cobalt sulfate solution was added dropwise to form a surface layer composed of cobalt hydroxide. During this time, the pH of the solution was adjusted to fall within the range of 8 to 13 by simultaneously dropping an aqueous solution of sodium hydroxide. It was left for about 5 hours after the completion of the dropping. Filtration, washing,
The active material particles which were dried to form a precursor of the conductive surface layer were obtained. The ratio of the surface layer contained in the obtained particles was 4% by weight.

(Precipitation of Rare Earth Element on Particle Surface) The particles comprising the core layer and the surface layer were dispersed in an aqueous solution adjusted to pH 8 to 13 by adding ammonium sulfate and sodium hydroxide. While stirring the aqueous solution, a ytterbium sulfate solution was added dropwise to precipitate ytterbium hydroxide on the particle surfaces. During this time, the pH of the solution was adjusted to fall within the range of 8 to 13 by simultaneously dropping an aqueous solution of sodium hydroxide. It was left for about 5 hours after completion of the dropping. Filtration, washing,
Drying yielded active material particles having a ytterbium hydroxide precipitated on the surface. The amount of ytterbium contained in the outer particles was 2.5 wt% in terms of metal.

(Comparative Example 1) The core layer particles were the same as those in the above-mentioned Example. Further, a precursor of a conductive surface layer made of cobalt hydroxide was formed on the particle surface in the same manner as in the example. The particles were used as active material particles of Comparative Example 1. Comparative Example 2 97 parts by weight of the active material particles described in the comparative example and ytterbium oxide (Yb ₂₎ having an average particle size of about 4.5 μm were used.
₃ parts by weight of O ₃ ) powder were mixed.

(Preparation of Nickel Electrode) (Example) 80 parts by weight of the active material particles described in the above example and carboxymethyl cellulose (CMC) as a binder were used.
A paste was prepared by mixing 20 parts by weight of the aqueous solution. The outer paste was uniformly applied to a nickel porous substrate. After drying the electrode plate, it was subjected to pressure and cutting to obtain a nickel electrode plate. The capacity (design value) of the produced positive electrode plate is 1000 m
Ah.

Comparative Example 1 A paste was prepared by mixing 80 parts by weight of the active material particles described in Comparative Example 1 and 20 parts by weight of a carboxymethyl cellulose (CMC) aqueous solution as a binder. The outer paste was uniformly applied to a nickel porous substrate. After drying the electrode plate, it was subjected to pressure and cutting to obtain a nickel electrode plate. The capacity (design value) of the produced positive electrode plate is 1000 mAh. Comparative Example 2 A paste was prepared by mixing 80 parts by weight of the mixed particles and 20 parts by weight of an aqueous solution of carboxymethyl cellulose (CMC) as a binder. The paste was uniformly applied to a nickel porous substrate. After drying the electrode plate, it was subjected to pressure and cutting to obtain a nickel electrode plate. The capacity of the produced positive electrode plate is 1000 mAh.

(Preparation of Negative Electrode Plate) MmN having CaCu ₅ type structure
i _3.5 Co _0.8 Mn _0.4 Al _0.3 composition (Mm means a misch metal which is a mixture of rare earth elements such as La, Ce, Pr, and Nd).
The same applies to the following embodiments. A thickener was added to the hydrogen storage alloy powder shown in (1) to prepare a paste. This paste was applied to a perforated steel plate and dried, and then the perforated steel plate was pressed and cut into an AA size battery electrode size. Thus, a negative electrode was obtained. The capacity (design value) of the negative electrode plate was set to 1.6 times the capacity of the positive electrode plate.

(Preparation of Nickel-Hydrogen Storage Battery for Test) A negative electrode plate was disposed on both sides of the above-mentioned positive electrode plate with a separator made of a polypropylene resin-based nonwoven fabric on which acrylic acid was graft-polymerized interposed therebetween. Insert the electrode plate into the container, the concentration is 6.8mol
An electrolytic solution composed of a mixed solution of a 1 / l aqueous solution of potassium hydroxide and a 0.5 mol / l aqueous solution of lithium hydroxide was injected to prepare an open cell for testing.

(Evaluation of Charging Efficiency) The cells using the electrodes of Examples and Comparative Examples according to the present invention were charged at a temperature of 20 ° C. and a rate of 0.1 C for 15 hours. The initial charge changes the precursor (cobalt hydroxide) of the conductive surface layer provided as the surface layer of the active material particles into conductive cobalt oxyhydroxide. After charging and resting for 1 hour, the battery was discharged to a rate of 0.2 C and a final potential (vs. Hg / HgO) of 0 mV. This charge / discharge operation was repeated five times, and after confirming that the discharge capacity was stabilized,
The charging efficiency at temperatures of 40 ° C. and 60 ° C. was examined. The charging efficiency was evaluated by the ratio of the discharge capacity after charging at the temperature to the discharge capacity after charging at 20 ° C. The charging and discharging rates and the final potential in the charging efficiency evaluation test were the same as those described above.

FIG. 2 shows the results. From FIG. 2, it can be seen that both the example electrode and the comparative example electrode according to the present invention maintain high charging efficiency. This is due to the effect that the ytterbium compound shifts the oxygen generation potential of the nickel electrode in a noble direction and suppresses the generation of oxygen.

(Evaluation of High Rate Discharge Performance) The batteries of Example and Comparative Examples were subjected to a high rate discharge test. Test temperature is 2
0 ° C. The discharge rates were 1C and 3C. The results are shown in FIG. As shown in FIG. 3, the cell using the example electrode according to the present invention has higher performance than the cell using the comparative example electrode.

This is because, in the case of the comparative example electrode, the mixed and added ytterbium oxide inhibits the cobalt once dissolved in the electrolytic solution from re-precipitating as cobalt hydroxide, whereas the comparative example electrode has It is considered that a good conductive network of cobalt hydroxide was formed.

Although ytterbium has been described as an example of the rare earth element, the rare earth element applied to the present invention is not limited to ytterbium. Said E
r, Tm, Yb, Lu, Y and composite compounds containing at least one of these elements are suitable materials.

[0044]

According to the first aspect of the present invention, it is possible to provide a nickel electrode for an alkaline storage battery having a high charging efficiency at a high temperature, an excellent oxygen gas generation suppressing function and a high rate discharge performance. According to the second and third aspects of the present invention, it is possible to provide a nickel electrode having the particularly excellent performance described in the first aspect. According to claim 4 of the present invention, the nickel electrode active material described in claim 1 and claim 3 can be synthesized with high reliability. According to claim 5 of the present invention, according to claim 5 of the present invention, there is provided an alkaline storage battery having high charging efficiency, particularly when charged at a high temperature, excellent in battery internal pressure suppressing function, and excellent in high rate discharge performance. Enable.

[0045]

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a cross-sectional structure of an active material particle for a nickel electrode according to the present invention.

FIG. 2 is a graph showing the charging efficiency of cells using the example electrode and the comparative example electrode according to the present invention.

FIG. 3 is a graph showing high-rate discharge performance of cells using an example electrode and a comparative example electrode according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 core layer 2 conductive surface layer 3 rare earth element high concentration layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Minoru Kurokuzuhara 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka Prefecture Inside Yuasa Corporation (72) Inventor Masaharu Watada 2-3-3, Furusobe-cho, Takatsuki-shi, Osaka No. 21 F-term in Yuasa Corporation (reference) 5H050 AA02 AA08 BA11 CA01 CA02 CA03 CB16 DA02 EA22 FA12 FA17 FA18 GA06 GA07

Claims (5)

[Claims] 1. A particulate nickel electrode active material having a core layer containing nickel hydroxide as a main component and a conductive surface layer made of a cobalt compound, comprising a rare earth element, An active material for a nickel electrode, wherein a rare earth element concentration is high on a particle surface. 2. The active material for a nickel electrode according to claim 1, wherein said rare earth element includes at least one selected from erbium, thulium, ytterbium, lutetium and yttrium. 3. The rare earth element is present mainly as a hydroxide or oxide on the particle surface, and the hydroxide or oxide is present as a phase different from the nickel hydroxide and the conductive cobalt compound. 2. The method according to claim 1, wherein
The active material for a nickel electrode according to claim 2 or 3. 4. A conductive surface layer made of a cobalt compound is formed on the surface of a core layer particle mainly composed of nickel hydroxide, and the hydroxide of the rare earth element is deposited on the outermost surface of the particle. The method for producing an active material for a nickel electrode according to claim 1, 2, 3, or 4. 5. A nickel electrode, wherein the active material particles according to claim 1, 2 or 3 are carried on an alkali-resistant substrate.