JP2002050355A - Alkaline secondary battery coated nickel hydroxide powder and its manufacturing method and evaluation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluation method of cobalthydroxide coated nickel hydroxide powder as an alkaline secondary battery positive electrode active material, and to provide coated nickelhydroxide powder that has excellent preservation property, oxidization resistance and a superior battery property and its manufacturing method. SOLUTION: The coated nickelhydroxide powder which has a chromaticity parameter 'a' value of -8.5 or less when evaluated by the chromaticity measurement by Lab method is most desirable as a positive electrode active material. The desirable coated nickelhydroxide powder which has an excellent preservation property and can maintain its chromaticity parameter 'a' value at -8.5 or less is obtained by treating in the water solution containing an antioxidant and by containing the antioxidant. As an antioxidant, a salt made of at least one kind selected from ascorbic acid, erythorbic acid and citric acid and an alkaline metal is desirable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a cobalt hydroxide-coated nickel hydroxide powder suitable as a positive electrode active material for non-sintered alkaline secondary batteries, a method for producing the same, and a method for evaluating the same.

[0002]

2. Description of the Related Art As portable electronic devices have become smaller and lighter, batteries as power sources have been required to have higher energy densities. In addition, as equipment has become more multifunctional and higher performance, battery performance has been improved in high-temperature conditions resulting from heat generation of the equipment, or output has been increased and increased for use under high load. It is desired to extend the life.

Under such circumstances, demand for nickel-metal hydride secondary batteries has rapidly increased as an alkaline secondary battery replacing nickel-cadmium batteries. In nickel-metal hydride secondary batteries, nickel hydroxide is used as the active material of the positive electrode, and a hydrogen storage alloy is used as the active material of the negative electrode. It is stored in a container together with the liquid.

In such an alkaline secondary battery,
In response to the demand for higher energy density, non-sintered (paste) alkaline secondary batteries have been developed. That is, a nickel electrode plate in which nickel hydroxide powder is pasted and filled into a support made of a metal fiber sintered body having a high porosity of 95% is used as a positive electrode of an alkaline secondary battery. In addition, as a manufacturing method of nickel hydroxide powder,
For example, a method described in JP-A-9-017429 is known.

[0005] However, this non-sintered alkaline secondary battery has a problem that the utilization rate of the battery is lowered in exchange for the advantage that nickel hydroxide as an active material can be filled at a high density.

[0006] In order to improve the utilization, it is necessary to increase the conductivity between the support and the nickel hydroxide particles.
Therefore, conventionally, a cobalt compound such as cobalt hydroxide was mixed and added during the preparation of the paste of nickel hydroxide powder, and as a highly conductive cobalt oxyhydroxide by an electrode reaction,
Methods have been taken to improve the conductivity between the support and the nickel hydroxide particles. However, at present, various effects can be obtained even with a small amount of a cobalt compound at the same time as increasing the dispersibility. Therefore, various cobalt compounds are coated on the surface of nickel hydroxide.

As a method for coating the surface of a nickel hydroxide particle with a cobalt compound, for example, Japanese Patent Application Laid-Open No. 3-9316
No. 1, Japanese Patent Application Laid-Open No. 6-1879, which discloses a method of coating cobalt using electroless plating or the like.
As disclosed in Japanese Patent Publication No. 84-84, there is a method of coating cobalt or the like by a mechanochemical reaction, but a method of coating with cobalt hydroxide is generally employed.

As a method for coating the surface of nickel hydroxide particles with cobalt hydroxide, for example, JP-A-2000-07
There is a method described in Japanese Patent No. 7070. In this method, an aqueous solution of a cobalt salt such as cobalt sulfate and a caustic alkali are simultaneously added to a suspension of nickel hydroxide particles, and p
H is adjusted to the range of 10.5 to 13 and the coating is performed while keeping the ammonium ion concentration in the range of 7 to 25 g / l.

[0009]

The use of such cobalt hydroxide-coated nickel hydroxide powder provides a highly dispersed state of cobalt, and at the same time reduces the amount of cobalt compound used during electrode fabrication. Became possible.

However, as a result of reducing the amount of the cobalt compound used, there is a problem that the properties of the electrode and the battery are greatly affected by the properties of cobalt hydroxide, which is the coating layer on the particle surface. In particular, when the coating layer, cobalt hydroxide, is oxidized, there is a problem that the battery utilization rate decreases and a desired battery capacity cannot be obtained.

Moreover, battery characteristics such as battery utilization and battery capacity cannot be evaluated unless a battery is actually manufactured and measured. It was difficult to predict battery characteristics.

The present invention has been made in view of such a conventional situation,
As a positive electrode active material for alkaline secondary batteries, it provides a method for evaluating cobalt hydroxide-coated nickel hydroxide powder that provides excellent battery characteristics, and has excellent storage characteristics, is hardly oxidized, and has excellent battery characteristics. It is an object of the present invention to provide a cobalt hydroxide-coated nickel hydroxide powder and a method for producing the same.

[0013]

In order to achieve the above object, the present invention firstly provides a coated nickel hydroxide powder having a coating layer of a hydroxide mainly composed of cobalt on the surface of nickel hydroxide particles. Is selected as the positive electrode active material of the alkaline secondary battery, the chromaticity parameter a value is evaluated by chromaticity measurement in the Lab method, and the use of the coated nickel hydroxide powder having a value of -8.5 or less is performed. Provided is a method for evaluating a coated nickel hydroxide for an alkaline secondary battery.

Further, the coated nickel hydroxide powder for an alkaline secondary battery provided by the present invention has a coating layer of a hydroxide mainly composed of cobalt on the surface of nickel hydroxide particles.
The chromaticity parameter a value obtained by the chromaticity measurement in the method b is-
It is characterized in that it is not more than 8.5, and preferably further contains an antioxidant.

Further, in the method for producing the coated nickel hydroxide powder according to the present invention, a coated layer of a hydroxide mainly composed of cobalt is formed on the surface of the nickel hydroxide particles, and then the obtained coated nickel hydroxide powder is obtained. Is treated with an aqueous solution containing an antioxidant, thereby containing an antioxidant.

In the above coated nickel hydroxide powder for an alkaline secondary battery and the method for producing the same according to the present invention, the antioxidant comprises at least one acid selected from ascorbic acid, erythorbic acid and citric acid, and an alkali metal. Is preferred.

In the above coated nickel hydroxide powder for an alkaline secondary battery according to the present invention, the content of the antioxidant is 0.05 to 0.18% by weight in terms of carbon in ascorbate and in terms of alkali metal. 0.01 to 0.04% by weight, and for erythorbate and citrate, 0.05 to 0.2% by weight of carbon and 0.02 to 0.2% by weight of alkali metal.
0.04% by weight.

In the above-mentioned coated nickel hydroxide powder for an alkaline secondary battery and the method for producing the same according to the present invention, the concentration of the antioxidant in the aqueous solution is 5 to 25 g / l for ascorbate, In the case of salts and citrates, it is characterized by 8 to 25 g / l.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have investigated the battery characteristics of an alkaline secondary battery and the coating of nickel hydroxide powder coated with cobalt hydroxide (hereinafter also simply referred to as coated nickel hydroxide powder) as its positive electrode active material. As a result of intensive studies on the properties of the layered cobalt hydroxide, the chromaticity parameter a of the coated nickel hydroxide evaluated by chromaticity measurement in the Lab method
It has been found that when the value is -8.5 or less, it is suitable as a coated nickel hydroxide powder for an alkaline secondary battery.

That is, the coated nickel hydroxide powder is
Is evaluated by the chromaticity measurement in the method, and the chromaticity parameter a value is -8.5 or less.
An alkaline secondary battery having excellent characteristics such as a utilization factor and a battery capacity can be manufactured. The chromaticity parameter a value is -8.
In the case of the coated nickel hydroxide powder exceeding 5, the oxidation of the coating layer of cobalt hydroxide is progressing, so that the battery using this as a positive electrode active material has a low utilization rate and obtains a desired battery capacity. Can not.

Here, the chromaticity parameter a value used in the present invention will be described. The colors are tristimulus values X, Y,
It is determined by the value of Z and is represented three-dimensionally. But,
It is difficult to perform the display in a plan view.
Display is performed by a combination of dimensional display.
Therefore, based on the tristimulus values X, Y, and Z described above, these are measured and converted into parameters by various mathematical expressions. In the present invention, the chromaticity parameter a obtained by performing the conversion by the Lab method using the following equation 1 is used.
The values are used as evaluation criteria for the coated nickel hydroxide powder.

[0022]

L = Y1 / 2 a = 17.5 (1.02XY) / Y1 / 2 b = 7.0 (Y-0.847Z) Y1 / 2

Here, the chromaticity L represents lightness, that is, lightness or darkness, in other words, it represents whiteness.
Further, the chromaticity a and the chromaticity b show a color distribution as shown in FIG. 1 in terms of image, and the smaller the value of the chromaticity parameter a used in the present invention (the larger-), the stronger the green / blue color. It is shown that.

Thus, according to the evaluation method of the present invention,
When selecting a coated nickel hydroxide powder suitable for the positive electrode active material of an alkaline secondary battery,
It can be selected and evaluated by the chromaticity parameter a value by the method. That is, the coated nickel hydroxide powder itself can be evaluated without producing a battery and measuring the battery characteristics.

However, such a coated nickel hydroxide powder having a chromaticity parameter a value of -8.5 or less is oxidized by a commonly used drying method.
It proved difficult to manufacture. That is, in the stationary drying, the coating layer is oxidized by moisture contained in the surface and the inside, and in the natural drying, the drying time becomes longer and the coating layer surface is oxidized, so that the chromaticity parameter a value tends to be larger than -8.5. Because.

Accordingly, the value of the chromaticity parameter a is -8.5.
In order to obtain the following coated nickel hydroxide powder, it is necessary to dry forcibly in a short time, and to perform an antioxidant treatment even after the drying. Specifically, it is necessary to dry at a low temperature and in a short time by using a flash dryer or the like, and to prevent oxidation by air by treatment such as vacuum packaging after drying. If a suitable powder is evaluated and selected for the coated nickel hydroxide powder thus manufactured using the above-described evaluation method of the present invention, an alkaline secondary battery having excellent characteristics can be manufactured.

The present invention also provides a coating water which can prevent the progress of oxidation without using any special method during drying and storage, and can maintain the chromaticity parameter a value at -8.5 or less. It provides a nickel oxide powder. That is, the coated nickel hydroxide of the present invention contains an antioxidant, so that Lab lab under ordinary drying and storage methods
The chromaticity parameter a value by the chromaticity measurement in the method is -8.
It can be easily maintained below 5.

As the antioxidant to be contained in the coated nickel hydroxide powder, a salt of an alkali metal with ascorbic acid, erythorbic acid or citric acid is preferable, and one or more of them are used. Ascorbic acid,
The alkali metal forming a salt with erythorbic acid or citric acid is not particularly limited, but is preferably sodium or potassium in view of cost.

When the antioxidant is ascorbate, the content of the antioxidant contained in the coated nickel hydroxide powder is 0.05 to 0.18% by weight in terms of carbon and 0 in terms of the amount of alkali metal. It is preferable to contain 0.01 to 0.04% by weight. In the case of erythorbate or citrate, it is preferable to contain 0.05 to 0.2% by weight of carbon and 0.02 to 0.04% by weight of alkali metal. When the content of each antioxidant exceeds the upper limit of the above range, the battery characteristics are adversely affected, and the battery capacity is reduced. Conversely, when the value is below the lower limit, the antioxidant effect becomes insufficient, and the oxidation proceeds during drying or storage, so that the chromaticity parameter a value becomes -8.5 or more, and the battery capacity decreases.

Next, a method for producing the coated nickel hydroxide powder containing the antioxidant of the present invention will be described. First, a nickel hydroxide powder is produced, for example, by the method described in JP-A-9-017429. That is, an aqueous solution containing nickel ions and sulfate ions, caustic alkali and aqueous ammonia are simultaneously and continuously supplied to a reaction tank equipped with a stirrer, and the nickel ion concentration in the reaction solution is 1 to 5
0 mg / l, a reaction temperature of 40 to 60 ° C., a fluctuation range of the reaction temperature ± 1 ° C., and a discharge head of a stirring blade of a stirrer of 14 to 70
The reaction is performed under the conditions of m ² / sec ² and a residence time of the produced nickel hydroxide powder of 6 hours or more in the reaction tank to produce a nickel hydroxide powder. The tap density of the nickel hydroxide powder is desirably 2.05 g / ml or more.

The nickel hydroxide powder particles thus produced are coated with cobalt hydroxide, for example, by the method described in JP-A-2000-077070. That is, a suspension solution containing nickel hydroxide powder maintained at 30 to 60 ° C. was adjusted to pH 10.5 to 13 with caustic alkali, and an aqueous solution containing a cobalt ion and an ammonium ion donor was replaced with an ammonium ion in the reaction solution. Concentration 10
By continuously supplying a fixed amount of ２５25 g / l, cobalt hydroxide-coated nickel hydroxide powder can be obtained.

The coating amount of cobalt hydroxide is not particularly limited, but is preferably 1 to 10% by weight. When the coating amount of cobalt hydroxide exceeds 10% by weight, the amount of nickel hydroxide as an active material is excessively reduced, so that the battery capacity is extremely reduced and the advantage of the paste electrode is lost. In addition, coating with a large amount of cobalt contradicts the original purpose of forming a conductive path efficiently by uniformly dispersing a small amount of a cobalt compound in an electrode. If the amount of cobalt hydroxide to be coated is less than 1% by weight, the advantage of the coating cannot be obtained.

The obtained cobalt hydroxide-coated nickel hydroxide powder is separated by filtration, washed, and dried to obtain a coated nickel hydroxide powder. In order to prevent deterioration due to oxidation or the like of the coating layer during storage, by treating with an aqueous solution containing an antioxidant during the washing,
What is necessary is just to contain an antioxidant. In particular, in the washing step after the filtration, a washing treatment with a washing solution containing an antioxidant is convenient because the antioxidant can be contained simultaneously with the washing of the coated nickel oxide powder.

The content of the antioxidant in the aqueous solution used is 5 to 25 g when the antioxidant is ascorbate.
/ L, 8 to 8 for erythorbate and citrate
A range of 25 g / l is preferred. If the concentration of the antioxidant exceeds the upper limit of these ranges, the content of the antioxidant increases and battery characteristics deteriorate. On the other hand, if the concentration is lower than the lower limit of the range, an antioxidant having a content exhibiting an antioxidant effect cannot be carried.

[0035]

EXAMPLES [Production of Nickel Hydroxide Powder] An aqueous solution of nickel sulfate, alkali hydroxide and ammonium water were simultaneously and continuously supplied to a reaction vessel equipped with a stirrer to reduce the nickel ion concentration in the reaction solution. 3 mg / l, reaction temperature 55
C., the fluctuation range of the reaction temperature was ± 1 ° C., the discharge head of the stirring blade of the stirrer was 40 m ² / sec ² , and the residence time of the produced nickel hydroxide in the reaction tank was 10 hours.
The obtained nickel hydroxide powder has a tap density of 2.17 g.
/ L.

[Production of coated nickel hydroxide powder] 5 kg of the obtained high bulk density nickel hydroxide powder is suspended in a mixed solution of 19 liters of pure water and 1.1 liters of ammonia water heated to 50 ° C. I let it. While stirring this suspension, the pH was adjusted to 12.0 with a sodium hydroxide solution while simultaneously adding an aqueous solution of cobalt sulfate and aqueous ammonia. The addition rate of the aqueous cobalt sulfate solution was 3.0 g / min, and the addition rate of the aqueous ammonia was 4.7 ml / min.

The aqueous cobalt sulfate solution, sodium hydroxide solution and aqueous ammonia were prepared as follows. That is, the cobalt sulfate aqueous solution is obtained by dissolving a reagent grade cobalt sulfate manufactured by Wako Pure Chemical Industries, Ltd. in pure water and having a cobalt concentration of 1.6 mol.
/ L. The caustic soda solution was prepared at 3.2 mol / l by dissolving reagent grade 1 caustic soda manufactured by Wako Pure Chemical Industries, Ltd. in pure water. As the aqueous ammonia, reagent-grade aqueous ammonia was used as it was.

After a lapse of time (46 minutes) at which the cobalt content in the charged nickel hydroxide powder is calculated to be 4% by weight,
The stirring was stopped to terminate the reaction. The resulting precipitate is immediately filtered by suction through a Buchner funnel, washed twice with 5 liters of pure water, and the coated nickel hydroxide powder cake is dried for about 6 k.
g (about 15% by weight of water). As a result of sampling and analyzing the reaction solution, the ammonium ion concentration in the solution was 9.2 g / l.

[Antioxidant treatment] 1 kg of the coated nickel hydroxide powder cake prepared as described above was treated as an antioxidant with sodium ascorbate (A) having a concentration shown in Table 1 below.
Sodium B-acid) or sodium erythorbate (EB
The solution was poured into 1 liter of pure water containing sodium acid, stirred, filtered, and dried at 60 ° C. in place. The powder (sample 5) treated with pure water containing no antioxidant was analyzed by atomic absorption analysis after dissolving the dried product in nitric acid, containing 3.99% by weight of cobalt and having a tapping density of 2.99%. The average particle size was 14 g / ml and 10.8 μm.

[Evaluation of storage stability] The chromaticity of each of the obtained coated nickel hydroxide powders of Samples 1 to 8 was measured by a Lab method using a color difference meter (manufactured by Nippon Denshoku Industries Co., Ltd.), and the color was measured. The value of the degree parameter a was determined. Next, 20 g of the coated nickel hydroxide powder of each sample except for Sample 5 was taken into a plastic bag and sealed to prepare a sample for a storage test. With respect to the sample 5 which was not treated with antioxidant, 1 kg of the powder was weighed into a plastic bag and sealed similarly. Each sample was stored in an atmosphere at 40 ° C. and a relative humidity of 80% using a constant temperature and humidity tester (PR-1KP, manufactured by Tabai Espec Corp.)
The chromaticity was measured in the same manner as described above, and the change with time of the chromaticity parameter a was examined. The relationship between the number of storage days and the value of the chromaticity parameter a is also shown in Table 1 below.

[0041]

[Table 1]

As can be seen from Table 1, the coated nickel hydroxide powder of Sample 5 containing no antioxidant had a storage life of 1 day.
Within 5 days, the chromaticity parameter a value exceeds -8.5, and even with a coated nickel hydroxide powder containing an antioxidant, the lower the content, the lower the storage stability. For example, the coated nickel hydroxide powder of Sample 7 treated with a 6 g / l aqueous solution of sodium erythorbate can be used within 30 days of storage or 6 days.
It can be seen that the value of the chromaticity parameter a of Sample 1 treated with the aqueous sodium ascorbate solution of g / l exceeded -8.5 within 60 days of storage.

[Evaluation of Battery Characteristics] Of each of Samples 1 to 8 in Table 1 above, each coated nickel hydroxide powder on storage day 0, and each coated water on storage test samples 5 and 15 of Sample 5 Using the nickel oxide powder, a positive electrode was produced as follows. That is, cobalt hydroxide (produced by Ise Chemical Co., Ltd.) was added to each coated nickel hydroxide powder so that the amount of Co in the positive electrode was 6.5.
% By weight, and the amount of hydroxypropylcellulose (1000 to 1000%) is adjusted so that the binder amount becomes 2% by weight.
4000cp; Wako Pure Chemical Industries, Ltd., Reagent 1st grade) aqueous solution is added, and non-babbling kneader (Nippon Seiko Co., Ltd.)
Into a paste. This paste was filled into foamed nickel (manufactured by Sumitomo Electric Industries, Ltd., Celmet: porosity: 95%), dried, and then subjected to isostatic pressing at a pressure of ² ton / cm ² to obtain a positive electrode.

A battery was constructed by using a cadmium electrode as a negative electrode, a sulfonated polyolefin nonwoven fabric as a separator, and a 7.2 mol / l aqueous solution of potassium hydroxide as an electrolyte together with the obtained positive electrode. Charging was performed at 0.1 C to 150% of the theoretical capacity (289 mAh / g), and discharging was performed at 0.2 C to 1.0 V. At this time, the utilization is expressed as a percentage of the discharge capacity with respect to the theoretical capacity.

Table 2 below shows the contents (Na content and C content) of the antioxidant and the chromaticity parameter a in each of the coated nickel hydroxide powders of Samples 1 to 8, and also shows that Sample 1 was prepared as described above. The storage efficiency of the third cycle in the batteries in which the positive electrodes were prepared using the powders of the storage days 0 to 8 and the storage tests of the sample 5 on the fifth and fifteenth days of the sample 5 are shown together. The contents of Na and C were determined by measuring the contents of the coated nickel hydroxide powder of each sample, and measuring the content of Sample 5 containing no antioxidant.
The value was obtained by subtracting the measured value of the powder (the number of storage days was 0).

[0046]

[Table 2]

As can be seen from Table 2, the batteries of Samples 1 to 4 of the present invention have a high utilization factor. However, a comparison of these samples shows that the battery utilization rate decreases with an increase in the content of the antioxidant, and in particular, samples 6 and 8 which were subjected to antioxidation treatment with a liquid having a high antioxidant concentration of 30 g / l. In the sample, the battery utilization rate was reduced by 5% or more as compared with the sample 5 not subjected to the antioxidation treatment (the number of storage days was 0).
Regarding the sample 5, the chromaticity parameter a value of the sample on the 15th storage day is -8.5 or more,
At the same time, it can be seen that the utilization rate has also rapidly deteriorated.

[0048]

According to the present invention, there is provided a cobalt hydroxide-coated nickel hydroxide powder which has excellent storage stability, is hardly oxidized, and has excellent battery characteristics when used as a positive electrode active material for an alkaline secondary battery. In addition, the nickel hydroxide powder coated with cobalt hydroxide, which can provide excellent battery characteristics, can be easily evaluated and selected.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating color attributes of chromaticity a and chromaticity b.

Continued on the front page (72) Inventor Minoru Shiraoka 3-5-3 Nishihara-cho, Niihama-city, Ehime Pref. Inside the Besshi Works, Sumitomo Metal Mining Co., Ltd. 3 In the Besshi Works of Sumitomo Metal Mining Co., Ltd. (72) Inventor Kuniyasu Kuniyasu 3-5-3 Nishiharacho, Niihama-shi, Ehime Prefecture In the Besshi Business Center of Sumitomo Metal Mining Co., Ltd. 3-5-3 Nishiharacho, Sumitomo Metal Mining Co., Ltd. Besshi Office (72) Inventor Hidekazu Sasaki 3-5-3 Nishiharacho, Niihama-shi, Ehime Prefecture Sumitomo Gold Mining Co., Ltd. Besshi Office F-term (reference) 4G048 AA02 AB04 AC06 AD03 AE05 5H050 AA08 AA19 BA11 CA04 CB14 CB16 FA17 FA18 GA10 GA13 GA22 HA00 HA01 HA10

Claims (8)

[Claims] 1. A nickel hydroxide particle having on its surface a coating layer of a hydroxide mainly composed of cobalt, wherein a value of a chromaticity parameter a by chromaticity measurement by a Lab method is -8.5 or less. Coated nickel hydroxide powder for alkaline secondary batteries. 2. The method according to claim 1, wherein the nickel hydroxide particles have a coating layer of a hydroxide mainly composed of cobalt on the surface thereof, and the chromaticity parameter a measured by the chromaticity measurement in the Lab method is -8.5 or less. A coated nickel hydroxide powder for an alkaline secondary battery, comprising an inhibitor. 3. The alkali antioxidant according to claim 2, wherein the antioxidant is a salt of at least one acid selected from ascorbic acid, erythorbic acid, and citric acid with an alkali metal. Coated nickel hydroxide powder for secondary batteries. 4. The content of the antioxidant in ascorbate is 0.05 to 0.18% by weight in terms of carbon and 0.01 to 0.04% by weight in terms of alkali metal, and erythorbic acid For salts and citrates, the carbon content is 0.05 to 0.2.
4. The coated nickel hydroxide powder for an alkaline secondary battery according to claim 2, wherein the content is 0.02 to 0.04% by weight and an alkali metal amount. 5. 5. After forming a coating layer of a hydroxide mainly composed of cobalt on the surface of the nickel hydroxide particles, the obtained coated nickel hydroxide powder is treated with an aqueous solution containing an antioxidant to thereby oxidize. A method for producing a coated nickel hydroxide powder for an alkaline secondary battery, comprising an inhibitor. 6. The alkali antioxidant according to claim 5, wherein the antioxidant is a salt of at least one acid selected from ascorbic acid, erythorbic acid, and citric acid with an alkali metal. Method for producing coated nickel hydroxide powder for secondary battery. 7. The antioxidant concentration in the aqueous solution is 5 to 25 g / l for ascorbate, and 8 to 25 g / l for erythorbate and citrate. The method for producing a coated nickel hydroxide powder for an alkaline secondary battery according to claim 5 or 6. 8. When a coated nickel hydroxide powder having a coating layer of a hydroxide mainly composed of cobalt on the surface of nickel hydroxide particles is selected as a positive electrode active material of an alkaline secondary battery, a color in a Lab method is selected. A method for evaluating a coated nickel hydroxide powder for an alkaline secondary battery, comprising using a coated nickel hydroxide powder having a chromaticity parameter a value of -8.5 or less as evaluated by a degree measurement.