JP3149783B2 - Processing method of hydrogen storage alloy powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates is, AB ₅ type, AB ₂
The present invention relates to a method for treating a hydrogen storage alloy powder for a secondary battery such as a mold (particularly for a Ni-hydrogen secondary battery). By the treatment method of the present invention, initial activation is easy, self-discharge at high temperature is small,
A hydrogen storage alloy powder for a battery having a long electrode life is obtained.

[0002]

2. Description of the Related Art Conventionally, as a secondary battery used for memory backup of AV equipment and notebook personal computers, it has been mainly used.
Ni-Cd batteries have been used. However, the pollution problem of Cd,
In view of the problem of resource limitation, which is a by-product of Cd zinc refining, and the development of secondary batteries with higher electric capacity, a secondary battery called Ni-Hydrogen battery using a hydrogen storage alloy instead of Cd as a cathode active material has been developed. Developed and used. Because of the high capacity of Ni-hydrogen batteries, their use as secondary batteries for electric vehicles is also being studied, and mass production has already begun.

A major alloy systems that have been studied as a hydrogen-absorbing alloy for a Ni- MH batteries, LaNi ₅ or MmNi ₅ (Mm is misch metal is a rare earth metal mixture) takes the AB ₅ type crystal structure represented by intermetallic compound, which is an intermetallic compound takes a Laves phase structure of AB ₂ type typified by ZnV _0.4 Ni _1.6. For practical use, the AB ₅ type hydrogen storage alloy is more advanced, but the AB ₂ type hydrogen storage alloy is also promising because of its high capacity.

[0004] However, several problems have emerged in the mass production of Ni-hydrogen batteries. One of them is Ni-
The initial activation process (the process of increasing the discharge capacity of the battery to a predetermined steady-state value) after configuring the hydrogen battery takes a very long time, and productivity is significantly impaired.

The initial activation process currently performed is a process of discharging after charging for a long time at a low current (charging for 15 to 20 hours and discharging for several hours) until a predetermined discharge capacity is obtained. This is done by repeating several times. For this reason, it is necessary to repeat charging / discharging in a factory for several days as an initial activation process before assembling and shipping the battery.

To solve this problem, Japanese Patent Laid-Open No.
Japanese Patent No. 219036 proposes that a boron-rich phase which is easy to be powdered is generated by adding boron to the hydrogen storage alloy, and the initial activation efficiency is improved by increasing the specific surface area due to the powdering. However, in this method, the amount of the generated boron-rich phase reversibly absorbing and releasing hydrogen is small, and the hydrogen storage amount, and thus the discharge capacity, of the alloy as a whole decreases. In addition, the powdering proceeds too much, and the current collecting properties of the electrode decrease, and the discharge capacity deteriorates early.

[0007] Japanese Patent Application Laid-Open No. 4-179055 discloses that an oxide layer or a hydroxide layer can be removed by immersing a hydrogen storage alloy powder in an acidic aqueous solution in a predetermined concentration range for a predetermined time and then washing with water. When an electrode is made from the hydrogen storage alloy powder,
It is reported that the activation cycle of the electrode can be shortened, and the electrode utilization rate and the cycle life are improved.

The present inventors have disclosed in Japanese Patent Application Laid-Open No. Hei 6-223827 that when a hydrogen storage alloy powder which has been made into fine crystal grains by a rapid solidification method is treated in an acidic aqueous solution to remove a surface oxide film,
It was reported that a more active hydrogen storage alloy powder was produced.

[0009]

However, if the surface oxide film of the hydrogen storage alloy powder is chemically removed using a strong acidic aqueous solution, more Ni (OH) ₂ is generated on the alloy surface.
A Ni-hydrogen battery using a hydrogen storage alloy powder having Ni (OH) ₂ adhered to the surface as an active material of a negative electrode has a large self-discharge at a high temperature. This is a substance in which the OH ^- ion of Ni (OH) ₂ participates in the discharge reaction of the negative electrode.If Ni (OH) ₂ is present on the alloy surface of the negative electrode, the OH ^- ion is released at a high temperature from this, and the negative electrode discharge reaction ^{_{(MH x + x OH - →}} M + x H 2 O + xe -) happening, because self-discharge reaction proceeds while consuming hydrogen. Therefore, a treatment method that does not generate Ni (OH) _{2 on} the surface of the hydrogen storage alloy powder is desirable.

In the rapid solidification method, a hydrogen storage alloy powder is obtained by, for example, pulverizing alloy flakes obtained by a roll quenching method, or obtaining powder directly from a molten alloy by an atomizing method. Since the form at the time of solidification from the molten metal is a form having a large surface area, such as a flake or a powder, many oxide films are formed on the alloy surface during the solidification. In addition, the obtained hydrogen storage alloy powder removes or reduces quenching strain
(It leads to an increase in discharge capacity.) Therefore, heat treatment is often performed after the production, but an oxide film is formed on the powder surface during this heat treatment.

When the hydrogen storage alloy powder is produced by the rapid solidification method, pulverization is hardly required as in the atomizing method, or the pulverization in the roll quenching method is much less than in the ingot method. Therefore, the proportion of the alloy surface newly appearing in the pulverization is small, and the proportion of the oxide film formed during solidification or heat treatment remaining on the surface of the alloy powder is much larger than that of the powder obtained by the ingot method.

Therefore, in the alloy powder obtained by the rapid solidification method, most of the oxide film present on the surface becomes a strong oxide film formed during solidification or heat treatment. This strong surface oxide film can be removed by treatment with a high-concentration acidic aqueous solution as proposed in JP-A-6-223827, but during this treatment, a large amount of Ni (OH) ₂ is present on the powder surface. As described above, self-discharge at a high temperature becomes remarkable, and the electrode life (storage capacity) decreases.

On the other hand, in the ingot method, most of the powder surface is a surface newly formed by pulverization. A surface oxide film is also generated during pulverization, but the degree of formation is small, so most of the oxide film on the surface of the hydrogen storage alloy powder produced by the ingot method is a weak oxide film generated by pulverization, and it is difficult to remove it. Easy.

Japanese Patent Application Laid-Open No. 5-195007 discloses that when a dense oxide film is formed on the surface, ultrasonic treatment in an aqueous alkali solution is effective for improving alloy activity. However, this treatment method is effective for an oxide film formed on the powder surface at the time of pulverization, but as in the case of a powder obtained by a rapid solidification method, a strong solidification generated during solidification from a molten metal or during heat treatment. It has been found that a sufficient effect cannot be obtained when a large amount of an oxide film is present on the powder surface.

An object of the present invention is to remove a strong oxide film as seen in a hydrogen storage alloy powder produced by a rapid solidification method, and to form Ni (OH) _{2 on} the alloy surface (ie, An object of the present invention is to provide a method for treating a hydrogen storage alloy powder capable of realizing further improvement in the initial activity and electrode life of a Ni-hydrogen battery in which the increase in oxygen concentration is suppressed.

[0016]

Means for Solving the Problems The inventors of the present invention have studied to achieve the above object, and as a result, by using an acid treatment and an ultrasonic treatment together, a hydrogen storage alloy powder was prepared with a relatively low concentration aqueous acid solution. Can remove the strong oxide film on the surface of
In addition, they have found that the production of Ni (OH) ₂ is remarkably suppressed by the combined use of ultrasonic waves, and the life of the electrode is improved by reducing the high-temperature self-discharge, and the present invention has been achieved.

According to the present invention, a hydrogen storage alloy powder, which is an oxide film having an area of 20% or more of the surface of the powder formed during solidification or heat treatment of the alloy, is immersed in a non-oxidizing acidic aqueous solution, A method for treating hydrogen storage alloy powder, characterized by applying ultrasonic waves having a frequency of 20 to 100 kHz.

The hydrogen storage alloy powder to be treated according to the present invention is one in which at least 20% of the surface of the powder is covered with a strong oxide film formed at a high temperature during solidification or heat treatment of the alloy. An example of such a powder is a hydrogen storage alloy powder produced by a rapid solidification method and optionally further subjected to a heat treatment as described above.

The initial activity of the hydrogen storage alloy (that is, the ease of initial activation of a Ni-hydrogen battery) changes depending on both the oxidation state of the alloy surface and the bulk structure of the hydrogen storage alloy. Specifically, the main phenomena that affect the initial activity are the origin of the grain boundary due to the decrease in electrode reaction activity due to the oxide film on the alloy surface and the volume change during hydrogen absorption and release at the beginning of the charge / discharge cycle. Crushing occurs, the specific surface area increases, and the initial activity increases.

As described in JP-A-4-179055,
When the hydrogen storage alloy powder obtained by simply pulverizing the melted alloy is immersed in an acidic aqueous solution, the initial activity is improved as much as the oxide film on the alloy surface can be removed by the above, but the alloy bulk structure according to the above No improvement in the initial activity due to the above can be expected.

On the other hand, when the hydrogen storage alloy powder is manufactured by the rapid solidification method, the crystal grains become fine (preferably 30 μm or less) and the structure has many crystal grain boundary portions.
In addition to the removal of the surface oxide film, a synergistic effect on the initial activation due to the crushing of the grain boundary starting point at the beginning of the cycle and the increase of the surface area is obtained, and the effect of improving the initial activity is increased.

An alloy obtained at a normal solidification cooling rate
In the case of (crystal grain size ≧ 80 to 100 μm), many crystal grains are crushed in the pulverizing step, and individual crystal units are destroyed, so that the life of the electrode tends to be shortened. After manufacturing an alloy with a fine crystal grain size without crushing by rapid solidification method, especially atomizing method, if the surface oxide film is removed in an acidic aqueous solution, the ratio of crystal units remaining without being destroyed increases. This, combined with the fact that a homogeneous structure is obtained with less component segregation due to quenching, prolongs the electrode life.

However, in order to remove a strong oxide film formed during rapid solidification and heat treatment, it has been considered necessary to treat with a strong (high concentration) acidic aqueous solution. When treated with a strong acidic aqueous solution, as described above, a large amount of Ni (OH) ₂ is generated on the alloy surface, which deteriorates the self-discharge characteristics of the Ni-hydrogen battery at high temperatures, and prolongs the electrode life. I was out.

According to the present invention, by using ultrasonic treatment together, it is possible to remove a strong oxide film on the surface of the hydrogen storage alloy powder with a relatively low concentration of an acid aqueous solution. The effect of (1) suppresses the production of Ni (OH) _{2 on} the alloy surface, thereby maintaining good self-discharge characteristics at high temperatures and extending the life of the electrode.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The hydrogen storage alloy to be treated in the present invention is not particularly limited as long as it contains Ni.
Representative examples include, but are AB ₅ type or AB ₂ type hydrogen storage alloy may be a AB type or A ₂ B type hydrogen storage alloy.

Although the specific composition of the alloy is not particularly limited,
Examples of AB type ₅ alloys are LaNi _x or MmNi _x (x is 4.7 to 5.
2) as the basic structure, and part of Ni is Co, Mn, Al, Fe, Cr, C
It is substituted with one or more elements such as u, V, Be, Zr, Ti, Mo and the like. Since LaNi _x is expensive and has a short life, MmNi _x is practically preferable.

An example of the AB ₂ type alloy is ZrNi _y (y is 1.9 to 2.
25) as the basic structure, and part of Ni is V, Mn, Cr, Co, Fe,
It is substituted with one or more elements such as Al, Mo, Cu, Be and the like. As specific examples, Zr _1.0 V _0.4 Ni _1.6 , Zr _1.0 M
n _0.6 Cr _0.2 Ni _1.2 , Zr _1.0 Ni _1.2 Mn _0.6 V _0.2 Co _0.1 , Zr _1.0 Ni
_1.2 Mn _0.6 V _0.2 Fe _0.1 , Zr _1.0 V _0.4 Ni _1.6, etc.

The hydrogen-absorbing alloy powder to be treated in the present invention is one in which at least 20% of the surface of the powder is covered with an oxide film formed during solidification or heat treatment of the alloy. As described above, many hydrogen storage alloy powders manufactured by the rapid solidification method satisfy this condition, but the hydrogen storage alloy powder to be treated in the present invention is not limited to those manufactured by the rapid solidification method. .

In the rapid solidification method, the hydrogen storage alloy powder is solidified at a high cooling rate of 50 ° C./sec or more, preferably 500 ° C./sec or more from the molten state, so that the obtained hydrogen storage alloy powder has little component segregation and good corrosion resistance. Thus, there is a feature that the capacity is not greatly reduced due to repeated charging / discharging, and the electrode life is long.

As the rapid solidification method, for example, a rotating electrode method, a rapid cooling method by pouring a molten metal onto a rotating drum, a method of thin casting on a water-cooled steel plate, a gas atomizing method, and the like can be used. Of these, a gas atomizing method that can directly obtain a spherical alloy powder is preferable. According to the gas atomizing method, a pulverizing step of the alloy at the time of manufacturing the electrode is not always necessary, and since the alloy is in the form of a spherical powder, the alloy particles are closest packed at the time of forming the electrode to obtain a high capacity electrode. Because it can be.

Preferably, after the rapid solidification, before the treatment with the acidic aqueous solution, the hydrogen storage alloy powder is heated at 550 to 950 ° C. for 2 to 2 hours.
Subject to heat treatment for 5 hours. Thereby, the alloy is recrystallized, the quenching strain is removed, and the discharge capacity is improved. This heat treatment is generally performed in a vacuum or an inert atmosphere. Preferred heat treatment conditions are 750 to 900 ° C for 3 to 4 hours, more preferably 750 to 800 ° C for 4 hours. The heat treatment atmosphere is more inert than vacuum from the viewpoint of preventing surface oxidation.
(Eg, argon, helium) are preferred.

If the produced hydrogen storage alloy is not in powder form, the alloy is pulverized before performing the treatment of the present invention.
This is because, after the pulverization, an oxide film is formed on the alloy surface during the pulverization, and the initial activation characteristics are deteriorated.

The surface of the thus produced hydrogen storage alloy powder is covered with an oxide film. For example, when the hydrogen storage alloy powder is manufactured by the gas atomization method, oxidation is caused by the influence of a trace amount of oxygen contained in the gas during the gas atomization process or the residual oxygen content in the atomization chamber. An oxide film is formed on the surface due to oxidation by a trace amount of oxygen contained in the inert gas.

Even when the hydrogen storage alloy powder is manufactured by another method, oxidation cannot be completely avoided during the alloy manufacturing process, heat treatment process (when heat treatment is performed), and alloy pulverization process. An oxide film is formed.

Of the oxide films, those formed at high temperatures, that is, oxide films formed during solidification from the molten metal or during heat treatment of the alloy, are particularly strong and difficult to remove by surface treatment, and are removed only by acid treatment. Attempting to do so requires a highly concentrated aqueous acid solution. In the present invention, by using the acid treatment and the ultrasonic treatment together, such a strong oxide film can be removed with a relatively low-concentration aqueous acid solution. When an alkaline aqueous solution is used for the surface treatment, a strong oxide film generated at a high temperature cannot be efficiently removed even with the use of ultrasonic waves.

In the present invention, a strong and hard to remove oxide film formed at a high temperature during solidification or heat treatment is used.
Hydrogen storage alloy powder occupying more than 20% of the surface area. The ratio (area ratio) of the oxide film generated during solidification or heat treatment is calculated from the surface area of the alloy after heat treatment (after solidification if no heat treatment) and the surface area of the alloy after pulverization (B).
It can be calculated by (A) / (B) × 100.

For example, in the case of unground pulverized hydrogen-absorbing alloy powder produced by the atomizing method, the proportion of the oxide film formed during solidification or heat treatment is 100%. Even if it is manufactured by the rapid solidification method, if the degree of pulverization is large,
The percentage of oxide film formed at high temperature during solidification or heat treatment
It can be less than 20%. When the ratio of the strong oxide film formed at such a high temperature is less than 20% of the powder surface, the oxide film on the powder surface can be sufficiently removed by the conventional method.

The acid used in the treatment of the present invention is hydrochloric acid and / or
Alternatively, a non-oxidizing acid such as hydrofluoric acid is preferable. For other acids (e.g., nitric acid, sulfuric acid, etc.), the oxidation function of the acid makes it easy for a new oxide film to be formed on the alloy surface during the immersion treatment with the acidic aqueous solution, and the effect of improving the initial activation properties of the alloy is sufficient. Is not obtained.

The acidic aqueous solution used for the treatment is a stock solution of the reagent grade or the first grade or a similar concentration thereof (hydrochloric acid is 35 to
(36%, hydrofluoric acid 44-46% concentration) can be prepared by diluting with water. The acid concentration of the acidic aqueous solution is 0.5 to 5% for hydrochloric acid, 0.5 to 3% for hydrofluoric acid, and 0.1% for mixed acid of hydrochloric acid and hydrofluoric acid as the content (% by weight) of this stock solution.
A concentration of 5-5% is preferred. More preferred acid concentrations are 1.0 to 2% for hydrochloric acid, 0.5 to 1.0% for hydrofluoric acid, and 0.5 to 2.0% for a mixed acid of hydrochloric acid and hydrofluoric acid.

If the acid concentration is too high, Ni (OH) ₂
Are generated in large quantities, and the self-discharge characteristics at high temperatures are likely to deteriorate.
However, when the acid concentration is high, the effect of the present invention can be achieved by shortening the treatment time to suppress the production of Ni (OH) ₂ , so that the concentration of the acid aqueous solution is not particularly limited.

The preferred treatment time with the acidic aqueous solution is shown below for each concentration. If the time is longer than this, a large amount of Ni (OH) ₂ is likely to be generated on the surface of the alloy. HCl: 0.5 to 2.0% concentration = 10 minutes or less, 2.0 to 5.0% concentration = 5 minutes or less, HF: 0.5 to 1.0% concentration = 5 minutes or less, 1.0 to 3.0% concentration = 3 minutes or less, HCl + HF mixed acid: 0.5 to 2.0 % Concentration = 10 minutes or less, 2.0-5.0
% Concentration = 5 minutes or less.

The temperature of the acid treatment is preferably in the range of 0 to 80 ° C. At a temperature lower than 0 ° C., as in the case where the acid concentration is too low,
The pickling reaction proceeds slowly, and it is often difficult to sufficiently remove the surface oxide film. On the other hand, when the treatment temperature exceeds 80 ° C., as in the case where the acid concentration is too high, the pickling reaction proceeds too quickly, and the pickling tends to progress not only to the removal of the oxide film on the alloy surface but also to the inside of the alloy. . Furthermore, at temperatures higher than 80 ° C, due to the hydrogen absorption equilibrium pressure, all the hydrogen generated in the pickling reaction is released to the atmosphere without being absorbed by the alloy, and the activation may not be sufficiently promoted. .

In the present invention, an ultrasonic wave is applied to the hydrogen storage alloy powder in the acid aqueous solution during the acid treatment. However, the ultrasonic treatment can be performed after the acid treatment. The ultrasonic wave used is 20 to 100 kHz which is used for normal ultrasonic cleaning.
A sufficient effect can be obtained by using the frequency of z. 20k
If the frequency is lower than Hz, the sound is audible and the effect of removing the oxide film on the surface is small. Ultrasonic waves having a frequency exceeding 100 kHz are effective for the purpose of removing deposits on the surface and the like, but have little effect for removing the oxide film on the alloy surface. Preferred frequencies are between 28 and 80 kHz.

The interaction between the ultrasonic wave and the pickling effect of the acidic aqueous solution makes it possible to efficiently remove the strong oxide film on the surface of the hydrogen storage alloy powder even when the acid aqueous solution has a relatively low concentration. Becomes It is not clear why the generation of Ni (OH) ₂ is small on the surface of the hydrogen storage alloy powder treated by the method of the present invention, but in addition to the effect of the concentration of the acid aqueous solution, Ni (OH) ₂ is generated by using ultrasonic waves. It is presumed that removal of ₂ by ultrasonic energy also contributes.

The ultrasonic intensity is preferably 0.05 W / cm ² or more. Below this, the ability to remove the oxide film on the surface is small and the effect of modifying the surface is reduced.
If the ultrasonic intensity is desirably 0.35 W / cm ² or more, cavitation occurs in water, so that oxide removal from the alloy surface can be performed more efficiently, and the surface modification effect is large.

The ultrasonic wave does not need to be applied during the immersion of the hydrogen storage alloy powder in the aqueous acid solution, but may be performed only for a part of the time during the immersion. In that case, it is preferable to apply ultrasonic waves in the latter half of the immersion. Although applying some ultrasonic waves during washing with water after the acid treatment has some effects, it is preferable to apply ultrasonic waves during the acid treatment.

The hydrogen storage alloy treated with the acidic aqueous solution is then washed with water and usually dried thereafter. Washing with water
What is necessary is just to implement so that the acid attached to the alloy may be almost completely removed. For example, the change in pH of the solution before and after washing is represented by decimal point 1
It can be washed with water until it is less than digits.

As described above, since the surface oxide film is a barrier that prevents hydrogen permeation, it is treated with an acid according to the present invention,
When this oxide film is removed, the absorption and release of hydrogen necessary for the expression of the activity of the hydrogen storage alloy are easily performed from the beginning. Further, hydrogen is generated on the surface of the alloy during the acid treatment, and the generated hydrogen is absorbed into the alloy in which the oxide film has been removed and the gas has become easier to permeate. In combination with these actions, hydrogen is sufficiently absorbed into the alloy from the early stage of the activation process to accelerate the activation, and the initial activation characteristics are remarkably improved.

Further, by performing the acid treatment in combination with the ultrasonic treatment, the amount of Ni (OH) ₂ generated on the alloy surface during the acid treatment is significantly reduced, and the self-heating at a high temperature caused by the hydroxide is reduced. As a result of suppressing the discharge, the electrode life is improved.

An electrode is manufactured from the hydrogen-absorbing alloy powder surface-treated by the method of the present invention by a method well known to those skilled in the art, and Ni-
Used as a negative electrode of a hydrogen battery. The electrode is formed, for example, by mixing a hydrogen storage alloy powder with a suitable binder (resin such as polyvinyl alcohol) and water (or other liquid) to form a paste, filling a nickel porous body, drying the mixture, and then drying the paste. It can be manufactured by pressure molding to the electrode shape of

[0051]

EXAMPLES The effects of the method for treating a hydrogen storage alloy powder of the present invention are demonstrated by the following examples. In Examples,% is% by weight unless otherwise specified. The acid concentration of the acidic aqueous solution used for the treatment is the content (% by weight) of the stock solution described above [namely, 35% hydrochloric acid (HCl) or 46% hydrofluoric acid (HF), which is a special grade of reagent].

(Production Example 1) A hydrogen storage alloy powder of an AB ₅ type or AB ₂ type alloy having the composition shown in Table 1 was produced by the method described below.

[0053]

[Table 1]

The raw materials used in the production of the alloy were flake-like Ni having a purity of 99.9%, electrolytic Co having a purity of 99.8%, shot-like Al having a purity of 99.9%, plate-like electrolytic Mn having a purity of 99.8%, and Ni-56.9% V Alloy, sponge Zr with purity over 99.5%, rare earth metal purity
99.8% or more misch metal (Mm) (La = 28%, Ce = 48
%, Nd = 18%, Pr = 6%), and La-rich misch metal (Lm) (La = 58%, Ce = 15%, Pr = 7%, Nd = 20%).

From these raw materials, a quenching solidification method of 75 k
g / ch Ar gas atomization method (cooling rate from molten metal 1 × 10
³ to 1 × 10 ⁴ ℃ / sec), or about 500 μm thick (cooling rate from molten metal 9 × 10 ³ ℃ / sec) or about 250 μm (3 × 10 ³ ℃ / sec) A hydrogen storage alloy having a predetermined composition was produced by employing a roll quenching method. further,
Hydrogen storage alloy powder was also prepared by the conventional method of ingot method of 100 kg / ch (cooling rate from molten metal 1.0 ° C / sec).

Each of the obtained hydrogen storage alloys had a purity of 9%.
Heat treatment was performed under predetermined conditions in an Ar atmosphere using 9.99% Ar gas. The hydrogen storage alloy produced by the ingot method and the roll quenching method was mechanically pulverized into a powder by a stainless steel ball mill under an Ar atmosphere after heat treatment.
Each hydrogen storage alloy powder, including the powder obtained by the atomizing method, was classified into -74 µm.

An oxide film formed at a high temperature during solidification or heat treatment on the surface oxide film of the obtained hydrogen storage alloy powder
(Hereinafter referred to as a high-temperature oxide film) was determined from the specific surface area measured by the BET method before and after pulverization and classification. However, since the powder produced by the atomizing method does not pass through the pulverizing step, the ratio of the high-temperature oxide film is 100%. The percentage of the high-temperature oxide film thus determined is summarized in Table 2 below.

[0058]

[Table 2]

As can be seen from Table 2, even when the hydrogen storage alloy is manufactured by the roll quenching method, which is the quenching and solidification method, 500 μm of flakes are obtained.
When the thickness was as large as μm, the surface that appeared during pulverization increased, and the proportion of the high-temperature oxide film was less than 20%.

[0060] as described in (Example 1) Production Example 1 of Table 2 (a) ~ AB ₅ -type hydrogen absorbing alloy species powder prepared in the method of (d), the following (A ) To (C) were surface-treated by the conventional techniques and various methods of the present invention. The heat treatment after the production of the hydrogen storage alloy was performed under the same conditions of 850 ° C. × 4 hours. Next, the surface treatment conditions will be described.

(A) Alkali treatment + ultrasonic wave (JP-A-5-19)
No.5007) KO with specific gravity of 1.30, 1 liter per 1 kg of alloy powder
The alloy powder is immersed in an aqueous solution of H and stirred at 100 rpm while maintaining the liquid temperature at 80 ° C., while immersion ultrasonic vibrators are installed in the liquid and ultrasonic waves (frequency: 26 kHz, output: 20 W) / Alkaline treatment was performed for 60 minutes while applying a treatment capacity of 1 m ³ ). Thereafter, the alkaline aqueous solution was removed, and the alloy powder was washed with water until the pH of the washing solution became 9 or less, and dried in a vacuum atmosphere.

(B) High-concentration acidic aqueous solution treatment (JP-A-6-22)
No. 3827) The alloy powder was immersed in a 7% aqueous solution of HCl at a rate of 2 liters per 1 kg of the alloy powder at a liquid temperature of 30 ° C. for 8 minutes. During the immersion, the treatment liquid was stirred at 100 rpm, but no ultrasonic wave was applied. After that, the acidic aqueous solution was removed and 20% of the alloy by weight was removed.
Washed with twice the volume of water and dried in a vacuum atmosphere.

(C) Low concentration acid treatment + ultrasonic wave (method of the present invention) The alloy powder was immersed in a 1% HCl aqueous solution at a rate of 2 liters per 1 kg of the alloy powder at a liquid temperature of 30 ° C. for 5 minutes. During the immersion, the treatment liquid was stirred at 60 rpm, and ultrasonic waves having a frequency of 48 kHz and an intensity of 0.50 W / cm ² were applied in the same manner as in the method (A). Thereafter, the acidic aqueous solution was removed, washed with water 20 times the weight of the alloy, and dried in a vacuum atmosphere.

The electrode characteristics of the hydrogen-absorbing alloy powder surface-treated by the methods (A) to (C) and the untreated hydrogen-absorbing alloy powder were tested as follows. Table 3 summarizes the test results.

(Test Method) A hydrogen storage alloy powder was used as a binder
(5% aqueous solution of polyvinyl alcohol) was added and kneaded. The obtained alloy powder paste is filled into a nickel foamed porous metal body (for example, Celmet manufactured by Sumitomo Electric), dried, and then pressed at a pressure of 1.5 ton / cm ² to hold the alloy powder in the Ni porous body. Thus, a negative electrode of the battery was formed. At this time, the supported amount of the hydrogen storage alloy powder was about 12 g.

A commercially available nominally 2000 mA Ni electrode was used for the positive electrode, and a nylon nonwoven fabric impregnated with 6N-KOH alkaline electrolyte was sandwiched between the positive and negative electrodes as a separator.
A 00 mA Ni-hydrogen secondary battery was constructed. This battery was hermetically sealed in a CASE case to obtain a battery to be tested. This battery is a positive electrode capacity regulating type battery having a large negative electrode capacity.

(1) Initial activity The prepared positive-electrode capacity-regulated Ni-hydrogen battery was heated at 25 ° C. and 400 mA.
, And then repeatedly charged and discharged 10 times at 400 mA to a terminal voltage of 0.9 V. The first discharge capacity and the tenth discharge capacity were measured, and the initial activity was determined by the ratio (first discharge capacity / 10th discharge capacity × 100%). If the initial activity is 95% or more, it can be evaluated as having excellent initial activity.

(2) High-Temperature Self-Discharge Characteristics The Ni-hydrogen battery was repeatedly charged and discharged at 25 ° C. with the same charge and discharge current amounts as described above, and the temporary test was interrupted in the fully charged state at the 20th cycle. The battery is then stored at 50 ° C for 10
After storing for 25 days, the temperature was returned to 25 ° C., the battery was discharged at 400 mA, and the remaining capacity was measured. The remaining capacity at this time and the 19th cycle (25
(° C.) with respect to the discharge capacity (called storage capacity or capacity after storage) to evaluate high-temperature self-discharge characteristics. The larger this value is, the less the self-discharge at high temperature is, and the more excellent the self-discharge characteristic is. If this value is 85% or more, the battery can be evaluated as having a small amount of self-discharge.

[0069]

[Table 3]

Table 3 shows the following results. (i) Unless surface treatment is performed, neither initial activity nor storage capacity reaches the target performance. (ii) The combination of treatment with an alkaline aqueous solution and ultrasonic treatment described in JP-A-5-195007 is sufficiently effective for hydrogen storage alloy powder having a surface high-temperature oxide film area ratio of less than 20%. , The initial activity and storage capacity reach the target value. However, with an alloy powder having an area ratio of the high-temperature oxide film of 20% or more, the oxide film cannot be sufficiently removed, so that the initial activity is insufficiently improved.

(Iii) In the surface treatment with a high-concentration acidic aqueous solution described in JP-A-6-223827, even if the hydrogen storage alloy powder has a high-temperature oxide film on its surface having an area ratio of 20% or more,
Although the oxide film can be sufficiently removed and the initial activity reaches the target value, self-discharge at high temperature increases and the storage capacity becomes lower than the target value. (iv) According to the present invention, when the treatment with a low-concentration acidic aqueous solution and the ultrasonic treatment are used in combination, a sufficient effect of improving both the initial activity and the storage capacity can be obtained.

(Example 2) As described in Production Example 1, (a) (ingot method), (c) (roll quenching method of 250 μm thickness) and (d) (gas atomizing method) in Table 2 hydrogen storage alloy species of the alloy powder of the hydrogen storage alloy species or AB ₂ type of AB ₅ type prepared in the method of using the surface treatment.

The heat treatment conditions after the preparation of this hydrogen storage alloy were as shown in Table 4. As shown in Table 2,
The alloy powder produced by the ingot method (abbreviated as IT) has an area ratio of the high-temperature oxide film on the surface of less than 20%, and the roll quenching method
(Abbreviated as RL) and the alloy powder produced by the gas atomizing method (abbreviated as AT) had an area ratio of the high-temperature oxide film on the surface of 20% or more.

Each alloy powder was subjected to the surface treatment method (C) of Example 1.
In the same manner as in the above, the surface treatment was performed by using both the acid treatment and the ultrasonic treatment. The ingot material powders of Test Nos. 1 and 2 were subjected to the test without any treatment. Table 4 summarizes the processing conditions (concentration of stock solution of acid aqueous solution, processing time and temperature, frequency and intensity of ultrasonic wave). The stirring speed of the liquid during the treatment was 60 rpm. After the surface treatment, the powder was washed with water in an amount 20 times the weight of the alloy powder, and dried in a vacuum atmosphere. Table 4 also shows the results of testing the electrode characteristics of the surface-treated hydrogen storage alloy powder in the same manner as in Example 1.

[0075]

[Table 4-1]

[0076]

[Table 4-2]

Even if the heat treatment conditions and the type of the hydrogen storage alloy are changed, the treatment method of the present invention can remove the high-temperature oxide film and sufficiently increase the initial activity.
_It can be seen that since the generation of ₂ is small, self-discharge after high-temperature storage is suppressed, and a battery with sufficiently high storage capacity can be obtained. However, the ultrasonic frequency is less than 20 kHz or 100 kHz
If it exceeds z, the surface modification effect is insufficient, and the initial activity becomes poor.

[0078]

According to the present invention, the crystal grains are formed at a high temperature as seen in the hydrogen storage alloy powder produced by the rapid solidification method such as the atomizing method and the roll quenching method, which can improve the initial activity. The oxide film can be removed while the production of Ni (OH) ₂ is suppressed by performing a surface treatment on the hydrogen-absorbing alloy powder in which the formed oxide film covers 20% or more of the surface. As a result, when a Ni-hydrogen battery is produced using the hydrogen-absorbing alloy powder surface-treated by the method of the present invention as a negative electrode active material, a Ni-hydrogen battery having a high initial activity, a high-temperature storage capacity, and thus a good electrode life is obtained. can get.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kouki Takeshita 4-5-33 Kitahama, Chuo-ku, Osaka City Inside Sumitomo Metal Industries, Ltd. (72) Inventor Koichi Jinshiro 4-5-33 Kitahama, Chuo-ku, Osaka Sumitomo (56) References JP-A-5-195007 (JP, A) JP-A-6-223827 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22F 1 / 00-1/02 H01M 4/38

Claims (1)

(57) [Claims] 1. A hydrogen storage alloy powder in which at least 20% of the surface of the powder is covered with an oxide film formed during solidification or heat treatment of the alloy, is immersed in a non-oxidizing acidic aqueous solution; A method for treating a hydrogen-absorbing alloy powder, characterized by applying ultrasonic waves having a frequency of up to 100 kHz.