JPH09302401A - Treatment of hydrogen storage alloy powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce hydrogen storage alloy powder easily capable of initial activation and having a long service life by applying ultrasonic waves of specified frequency to hydrogen storage alloy powder in which a specified ratio of surface area is coated with an oxidized film while it is immersed in a nonoxidizing acidic aq. soln. SOLUTION: A hydrogen storage alloy is treated by a rapid solidifying method, and according to necessity, heating is executed at about 550 to 950 deg.C to produce hydrogen storage alloy powder in which >=20% area of the surface is coated with an oxidized film. This hydrogen storage alloy powder is treated at about 0 to 80 deg.C for prescribed time by about 0.5 to 5% of hydrochloric acid, about 0.5 to 3% of hydrofluoric acid or about 0.5 to 5% of mixed acid of chlorine and hydrofluoric acid. Simultaneously, in the process of the acid treatment, ultrasonic waves in the frequency of 20 to 100kHz is applied to effectively remove an oxidized film on the surface of the hydrogen storage alloy powder. In this way, the hydrogen storage alloy powder in which initial activating characteristics are remarkably improved, self-discharge at a high temp. is suppressed, and the service life of an electrode is improved can be obtd.

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 secondary batteries such as molds (especially for Ni-hydrogen secondary batteries). The treatment method of the present invention facilitates initial activation, reduces self-discharge at high temperatures,
A hydrogen storage alloy powder for a battery having a long electrode life can be obtained.

[0002]

2. Description of the Related Art As a secondary battery used for memory backup of AV equipment and notebook type personal computers, conventionally
Ni-Cd batteries have been used. But the Cd pollution problem,
From the viewpoint of the resource limitation problem that Cd is a by-product of zinc refining and the development of secondary batteries with higher electric capacity, a secondary battery called Ni-hydrogen battery that uses a hydrogen storage alloy as the cathode active material instead of Cd has been developed. It has been developed and used. Since the Ni-hydrogen battery has a high capacity, its use as a secondary battery for electric vehicles is also being considered, and mass production has already started.

The main alloy system that has been studied as a hydrogen storage alloy for Ni-hydrogen batteries has an AB ₅ type crystal structure represented by LaNi ₅ and MmNi ₅ (Mm is a misch metal which is a rare earth metal mixture). It is an intermetallic compound having an AB ₂ type Laves phase structure represented by ZnV _0.4 Ni _1.6 . AB ₅ type hydrogen storage alloys are more advanced in practical use, but AB ₂ type hydrogen storage alloys are also promising because they show high capacity.

However, some problems have emerged in the mass production of Ni-hydrogen batteries. One of them is Ni-
This means that the initial activation process (the process of increasing the discharge capacity of the battery to a predetermined steady value) after the hydrogen battery is constructed takes a very long time, and the productivity is significantly impaired.

The initial activation process currently performed is a process of discharging after a long time charging with 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 was necessary to repeat charging and discharging in the factory for several days as an initial activation process from the time the battery was assembled to the time it was shipped.

In order to solve this problem, Japanese Patent Laid-Open No.
Japanese Patent No. 219036 proposes that boron is added to a hydrogen storage alloy to generate a boron-rich phase that is easily pulverized and the specific surface area is increased by the pulverization to improve the initial activation efficiency. However, in this method, the amount of the generated boron-rich phase that reversibly absorbs and releases hydrogen is small, and the hydrogen storage amount and therefore the discharge capacity of the entire alloy decreases. Moreover, pulverization progresses too much to reduce the current collecting ability as an electrode, and the discharge capacity deteriorates at an early stage.

[0007] In Japanese Patent Laid-Open No. 4-179055, an oxide layer or a hydroxide layer can be removed by immersing the hydrogen-absorbing alloy powder in an acidic aqueous solution having 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 has been reported that the activation cycle of the electrode can be shortened and the electrode utilization rate and the cycle life can be improved.

Further, the inventors of the present invention disclosed in Japanese Unexamined Patent Publication No. 6-223827 that a hydrogen storage alloy powder formed 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, when the surface oxide film of the hydrogen storage alloy powder is chemically removed using a strong acidic aqueous solution, more Ni (OH) ₂ is produced on the alloy surface.
A Ni-hydrogen battery using a hydrogen storage alloy powder having Ni (OH) ₂ adhered on its surface as an active material of a negative electrode has a large self-discharge at high temperature. This is a substance in which the OH ⁻ ions of Ni (OH) ₂ are involved in the discharge reaction of the negative electrode, and when Ni (OH) ₂ is present on the alloy surface of the negative electrode, OH ⁻ ions are liberated from this at high temperature and the negative electrode discharge occurs. 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, for example, a hydrogen absorbing alloy powder is obtained by a method of crushing alloy flakes obtained by the roll rapid cooling method or directly obtaining a powder from a molten alloy by an atomizing method. Since the form when solidifying from the molten metal is a form with a large surface area such as flakes and powder, many oxide films are formed on the alloy surface during solidification. Further, the obtained hydrogen storage alloy powder removes or alleviates quenching strain.
Since this leads to an increase in discharge capacity, heat treatment is often performed after manufacturing, 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 even by the roll rapid cooling method, the pulverization is much less than that of the ingot method. Therefore, the proportion of the alloy surface newly appearing by pulverization is small, and the proportion of the oxide film generated during solidification or heat treatment remaining on the surface of the alloy powder is much higher 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 existing on the surface becomes a strong oxide film generated 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 Japanese Patent Laid-Open No. 6-223827, but a large amount of Ni (OH) ₂ is formed on the powder surface during this treatment. As described above, the self-discharge at a high temperature becomes remarkable and the electrode life (storage capacity) is reduced.

On the other hand, in the ingot method, most of the powder surface is the surface newly formed by pulverization. Although a surface oxide film is formed even during pulverization, since the degree of formation is small, most of the oxide film on the surface of the hydrogen storage alloy powder produced by the ingot method is a weak oxide film produced by pulverization and cannot be removed. It's easy.

JP-A-5-195007 describes that when a dense oxide film is formed on the surface, ultrasonic treatment in an alkaline aqueous solution is effective for improving the alloy activity. However, even though this treatment method is effective for the oxide film formed on the powder surface during pulverization, it does not give a strong solidification that occurs during solidification from the molten metal or during heat treatment like the powder obtained by the rapid solidification method. It was found that a sufficient effect could not be obtained when many oxide films were present on the powder surface.

The object of the present invention is to remove the strong oxide film found in the hydrogen storage alloy powder produced by the rapid solidification method, and to produce Ni (OH) _{2 on} the alloy surface (that is, It is an object of the present invention to provide a method for treating a hydrogen storage alloy powder, which can suppress the increase in oxygen concentration) and can further improve the initial activity and the electrode life of a Ni-hydrogen battery.

[0016]

Means for Solving the Problems As a result of studies to achieve the above object, the inventors of the present invention have found that a combination of acid treatment and ultrasonic treatment can be used to produce a hydrogen storage alloy powder in a relatively low concentration aqueous acid solution. The strong oxide film on the surface of can be removed,
Moreover, the inventors have found that the combined use of ultrasonic waves significantly suppresses the generation of Ni (OH) ₂ , and that the high temperature self-discharge is reduced to improve the electrode life.

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

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

The initial activity of the hydrogen storage alloy (that is, the ease of initial activation of the Ni-hydrogen battery) depends 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 phenomenon of the decrease in the electrode reaction activity due to the oxide film on the alloy surface, and the volume change at the time of hydrogen absorption / desorption at the beginning of the charge / discharge cycle. The crushing occurs, the specific surface area increases, and the initial activity improves.

As described in Japanese Patent Laid-Open No. 4-179055,
When the hydrogen-absorbing alloy powder obtained by simply crushing the melted alloy is immersed in an acidic aqueous solution, the initial activity is improved by the amount by which the oxide film on the alloy surface can be removed by the above, but the alloy bulk structure according to the above It cannot be expected to improve the initial activity due to.

On the other hand, when the hydrogen-absorbing 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 of (1), the effect of crushing the grain boundary starting point in the initial cycle of (1) and the synergistic effect on the initial activation by increasing the surface area can be obtained, so that the effect of improving the initial activity becomes large.

Further, an alloy obtained at a normal solidification cooling rate
(Crystal grain size ≧ 80 to 100 μm), many crystal grains are crushed in the crushing process, and as a result, individual crystal units are destroyed, so that the life of the electrode tends to be shortened. If the surface oxide film is removed in an acidic aqueous solution after manufacturing an alloy with a fine grain size without crushing by a rapid solidification method, especially an atomizing method, the proportion of crystal units remaining without being destroyed increases. In addition, because of the rapid cooling, a uniform structure with less component segregation can be obtained, and the life of the electrode is extended.

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

According to the present invention, a strong oxide film on the surface of the hydrogen-absorbing alloy powder can be removed with an aqueous acid solution having a relatively low concentration by using the ultrasonic treatment together. By this effect, generation of Ni (OH) _{2 on} the alloy surface is suppressed, good self-discharge characteristics at high temperature are maintained, and the electrode life is extended.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The hydrogen storage alloy treated in the present invention is not particularly limited as long as it contains Ni.
A typical example is an AB ₅ type or AB ₂ type hydrogen storage alloy, but an AB type or A ₂ B type hydrogen storage alloy may be used.

The specific composition of the alloy is not particularly limited, either.
Examples of AB ₅ type alloys are LaNi _x or MmNi _{x, where} x is 4.7-5.
2) as a basic structure, with some of Ni as Co, Mn, Al, Fe, Cr, C
It is substituted with one or more elements such as u, V, Be, Zr, Ti and Mo. LaNi _x is on a high price, since reduction in life is fast, the practical MmNi _x preferred.

An example of an AB ₂ type alloy is ZrNi _y (y is 1.9 to 2.
25) is a basic structure, and a part of Ni is V, Mn, Cr, Co, Fe,
It is substituted with one or more elements such as Al, Mo, Cu and Be. As a specific example, 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 storage alloy powder treated in the present invention is such that 20% or more 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 powders treated in the present invention are not limited to those manufactured by the rapid solidification method. .

In the rapid solidification method, since the solidified product is solidified from the molten state at a high cooling rate of 50 ° C./sec or more, preferably 500 ° C./sec or more, the obtained hydrogen storage alloy powder has little component segregation and good corrosion resistance. Therefore, there is a feature that the capacity decrease due to repeated charging and discharging is very small and the electrode life is long.

As the rapid solidification method, for example, a rotating electrode method, rapid cooling by pouring on a rotating drum, thin casting on a water-cooled steel sheet, gas atomizing method, etc. can be adopted. Of these, the gas atomization method is preferable because it can directly obtain spherical alloy powder. According to the gas atomizing method, the crushing process of the alloy at the time of manufacturing the electrode is not always necessary, and since the alloy is a spherical powder, it is possible to obtain the high capacity electrode by the closest packing of the alloy particles at the time of forming the electrode. Because you can

Preferably, the hydrogen storage alloy powder is heated at 550 to 950 ° C. for 2 to 2 after the rapid solidification and before the treatment with the acidic aqueous solution.
Heat treatment for 5 hours. As a result, the alloy is recrystallized, quenching strain is removed, and the discharge capacity is improved. This heat treatment is generally performed in vacuum or in an inert atmosphere. Preferred heat treatment conditions are 750 to 900 ° C. for 3 to 4 hours, and 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) is preferred.

If the hydrogen storage alloy produced is not in powder form, the alloy is ground before the treatment according to the invention.
This is because when the pulverization is performed later, an oxide film is formed on the alloy surface during the pulverization and the initial activation characteristics are deteriorated.

The surface of the hydrogen storage alloy powder produced in this manner is covered with an oxide film. For example, when hydrogen-absorbing alloy powder is manufactured by the gas atomizing method, oxidation is caused by the influence of a trace amount of oxygen contained in the gas in the gas atomizing process or the residual oxygen content in the atomizing chamber, and further in the heat treatment furnace in the heat treatment process after alloy production. An oxide film is formed on the surface due to the oxidation caused by a trace amount of oxygen contained in the inert gas.

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

The oxide film formed at a high temperature among the oxide films, that is, the oxide film formed during solidification from the molten metal or during the heat treatment of the alloy is particularly strong and difficult to remove by surface treatment, and is removed only by acid treatment. Attempts to do so require 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 an acid aqueous solution having a relatively low concentration. When an alkaline aqueous solution is used for the surface treatment, the strong oxide film formed at high temperature cannot be efficiently removed even if ultrasonic waves are used together.

The treatment of the present invention is to remove a strong and hard-to-remove oxide film formed at high temperature during solidification or heat treatment.
It is a hydrogen storage alloy powder that occupies 20% or more of the surface area. The ratio (area ratio) of the oxide film formed during solidification or heat treatment is calculated from the surface area (A) of the alloy after heat treatment (after solidification if not heat treated) and the surface area (B) of the alloy after pulverization.
It can be calculated by (A) / (B) × 100.

For example, in the case of the non-pulverized hydrogen storage alloy powder produced by the atomization method, the ratio of the oxide film formed during solidification or heat treatment is 100%. Even if manufactured by the rapid solidification method, if the degree of crushing is large,
The percentage of oxide film formed at high temperature during solidification or heat treatment is
It may be less than 20%. When the ratio of the strong oxide film formed at a high temperature is less than 20% of the powder surface, the conventional method can sufficiently remove the oxide film on the powder surface.

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. With other acids (e.g. nitric acid, sulfuric acid, etc.), the oxidation function of the acid makes it easier for a new oxide film to form on the alloy surface during the immersion treatment in an acidic aqueous solution, and the effect of improving the initial activation characteristics of the alloy is not sufficient. Is not available.

The acidic aqueous solution used for the treatment is a special grade reagent or a first grade reagent or a stock solution having a concentration similar to that (35% for hydrochloric acid).
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% with hydrochloric acid, 0.5 to 3% with hydrofluoric acid, and 0. 0 with a 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 preferable 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 mixed acid of hydrochloric acid and hydrofluoric acid.

If the acid concentration is too high, the surface of the alloy will have Ni (OH) ₂
Are generated in a large amount, and the self-discharge characteristics at high temperature are likely to deteriorate.
However, when the acid concentration is high, generation of Ni (OH) ₂ can be suppressed by shortening the treatment time and the effect of the present invention can be achieved, so the concentration of the acid aqueous solution is not particularly limited.

The preferable treatment time with the acidic aqueous solution is shown below for each concentration. If it is longer than the time shown here, a large amount of Ni (OH) ₂ is likely to be formed 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 to 5.0
% Concentration = 5 minutes or less.

The temperature of the acid treatment is preferably in the range of 0 to 80 ° C. At temperatures lower than 0 ° C, the same as when 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, if the treatment temperature exceeds 80 ° C, as in the case where the acid concentration is too high, the pickling reaction proceeds too fast, and the pickling tends not only to remove the oxide film on the alloy surface but also to the inside of the alloy. . Further, at a temperature higher than 80 ° C, due to the hydrogen absorption equilibrium pressure, the hydrogen generated in the pickling reaction may not be absorbed by the alloy and may be released into the atmosphere, resulting in insufficient promotion of activation. .

In the present invention, ultrasonic waves are applied to the hydrogen storage alloy powder in the aqueous acid solution during the acid treatment. However, the ultrasonic treatment can also be performed after the acid treatment. The ultrasonic wave used is 20 to 100 kH, which is used for normal ultrasonic cleaning.
A sufficient effect can be obtained by using the frequency of z. 20 k
Below Hz, it is an audible sound wave and the effect of removing the oxide film on the surface is small. Ultrasonic waves with a frequency above 100 kHz are effective for the purpose of removing deposits on the surface, but are less effective for removing the oxide film on the alloy surface. The preferred frequency is 28-80 kHz.

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

The ultrasonic wave 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 surface modification is small.
Desirably, when the ultrasonic wave intensity is 0.35 W / cm ² or more, cavitation occurs in water, and oxides on the alloy surface can be removed more efficiently, and the surface modification effect is large.

The ultrasonic waves do not have to be applied during the immersion of the hydrogen storage alloy powder in the aqueous acid solution, and may be applied only for a part of the immersion time. In that case, it is preferable to apply ultrasonic waves in the latter half of the immersion. Further, although there is some effect by applying ultrasonic waves during washing with water after the acid treatment, 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
It may be carried out so that the acid attached to the alloy is almost completely removed. For example, the decimal point is the change in pH of the solution before and after washing.
It can be washed with water until it becomes less than the digit.

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,
By removing this oxide film, the absorption and release of hydrogen necessary for the activation of the hydrogen storage alloy can be easily performed from the initial stage. Further, hydrogen is generated on the surface of the alloy during the acid treatment, and the generated hydrogen is absorbed inside the alloy from which the oxide film is removed and the gas easily permeates. Combined with these effects, hydrogen is sufficiently absorbed inside the alloy from the initial stage of the activation treatment, the activation is accelerated, and the initial activation characteristics are remarkably improved.

Furthermore, by performing the acid treatment together with the ultrasonic treatment, the amount of Ni (OH) ₂ formed on the alloy surface during the acid treatment is remarkably reduced, and the self-treatment at high temperature due to the hydroxide is caused. As a result of suppressing the discharge, the electrode life is improved.

An electrode was 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.
Used as a negative electrode for hydrogen batteries. The electrode is formed, for example, by mixing hydrogen storage alloy powder with an appropriate binder (resin such as polyvinyl alcohol) and water (or other liquid) to form a paste, filling the nickel porous body and drying it, and then It can be manufactured by pressure forming into the electrode shape.

[0051]

EXAMPLES The effects of the hydrogen storage alloy powder processing method of the present invention will be 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 above-mentioned undiluted solution [that is, reagent grade 35% hydrochloric acid (HCl) or 46% hydrofluoric acid (HF)].

(Production Example 1) Hydrogen storage alloy powder of 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 for the production of the alloy are flake Ni having a purity of 99.9%, electrolytic Co having a purity of 99.8%, shot Al having a purity of 99.9%, plate electrolytic Mn having a purity of 99.8%, and Ni-56.9% V mother. Alloy, sponge Zr with a purity of 99.5% or more, rare earth metal purity
99.8% or more of 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 rapid 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 in thickness (cooling rate from molten metal 9 × 10 ³ ℃ / sec) or about 250 μm (also 3 × 10 ³ deg / sec) A hydrogen storage alloy having a predetermined composition was produced by adopting a double roll quenching method. further,
Hydrogen storage alloy powder was also produced by the conventional 100 kg / ch ingot method (cooling rate from molten metal 1.0 ° C / sec).

Each of the obtained hydrogen storage alloys had a purity of 9
The heat treatment was performed under a predetermined condition 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 crushed into powder by a stainless ball mill in an Ar atmosphere after heat treatment.
Each hydrogen storage alloy powder including the powder obtained by the atomization method was classified to -74 μm.

In the surface oxide film of the obtained hydrogen storage alloy powder, an oxide film formed at high temperature during solidification or heat treatment
The ratio (hereinafter referred to as high temperature oxide film) was determined from the specific surface area measured by the BET method before pulverization and after pulverization / classification. However, since the powder produced by the atomization method does not go through the crushing process, the proportion of the high temperature oxide film is 100%. The proportion of the high temperature oxide film thus obtained 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 a rapid solidification method, the flakes show 500 pieces.
When the thickness is as thick as μm, the surface exposed by crushing increases and the ratio of the high temperature oxide film becomes less than 20%.

(Example 1) As described in Production Example 1, the powder of the AB ₅ type hydrogen storage alloy species produced by each of the methods (a) to (d) in Table 2 was prepared as follows. )-(C), and the surface treatment was carried out by the various methods of the present invention. The heat treatment after the hydrogen storage alloy was manufactured was the same under the same conditions of 850 ° C. × 4 hours. The surface treatment conditions will be described below.

(A) Alkali treatment + ultrasonic wave (Japanese Patent Laid-Open No. 5-19
No. 5007) KO with a specific gravity of 1.30 at a ratio of 1 liter to 1 kg of alloy powder
Immerse the alloy powder in the H 2 aqueous solution and stir at 100 rpm while maintaining the liquid temperature at 80 ° C, and install an immersion ultrasonic transducer in the liquid to generate ultrasonic waves (frequency: 26 kHz, output: 20 W / Processing volume of 1 m ³ ) was applied and alkali treatment was carried out for 60 minutes. Then, the alkaline aqueous solution was removed, the alloy powder was washed with water until the pH of the washing liquid was 9 or less, and dried in a vacuum atmosphere.

(B) Treatment with high-concentration acidic aqueous solution (JP-A-6-22
(No. 3827) The alloy powder was immersed for 8 minutes at a liquid temperature of 30 ° C. in a 7% HCl aqueous solution in a ratio of 2 liters to 1 kg of the alloy powder. The treatment liquid was stirred at 100 rpm during the immersion, but no ultrasonic wave was applied. After that, the acidic aqueous solution is removed and the weight of the alloy is reduced to 20%.
It was washed with twice the amount 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 for 5 minutes at a liquid temperature of 30 ° C. in a 1% HCl aqueous solution in a ratio of 2 liters to 1 kg of the alloy powder. The treatment liquid was stirred at 60 rpm during the immersion, 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). After that, the acidic aqueous solution was removed, washed with 20 times the weight of water of the alloy, and dried in a vacuum atmosphere.

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

(Test method) Hydrogen storage alloy powder was used as a binder.
(Polyvinyl alcohol 5% aqueous solution) 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 amount of the hydrogen storage alloy powder supported was about 12 g.

A commercially available Ni electrode with a nominal value of 2000 mA was used as the positive electrode, and a nylon non-woven fabric impregnated with an alkaline electrolyte of 6N-KOH was sandwiched between the positive electrode and the negative electrode as a separator to give a nominal value of 20%.
A Ni-hydrogen secondary battery of 00 mA was constructed. This battery was hermetically sealed in a C / D type case to obtain a battery to be tested. This battery is a positive electrode capacity regulated battery having a large negative electrode capacity.

(1) Initial activity The prepared positive electrode capacity regulated Ni-hydrogen battery was tested at 25 ° C, 400 mA.
After charging for 6 hours at 400 mA, the battery was repeatedly charged and discharged 10 times by discharging at 400 mA to a terminal voltage of 0.9 V. The discharge capacity at the first time and the discharge capacity at the 10th time were measured, and the initial activity was determined by the ratio (1st 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 above Ni-hydrogen battery was repeatedly charged and discharged at 25 ° C. with the same amount of charge and discharge current as described above, and the test was temporarily stopped in the fully charged state of the 20th cycle. This battery is then 10
It was stored for one day, then returned to 25 ° C., discharged at 400 mA, and the remaining capacity was measured. The remaining capacity at this time and the 19th cycle (25
The high-temperature self-discharge characteristics were evaluated by determining the ratio (° C) to the discharge capacity (called storage capacity or capacity after storage). The larger this value, the less self-discharge at high temperature, and the better the self-discharge characteristics of the battery. If this value is 85% or more, it can be evaluated as a battery with a small self-discharge amount.

[0069]

[Table 3]

From Table 3, the following results were found. (i) Without surface treatment, the initial performance and storage capacity do not reach the target performance. (ii) The combination of the treatment with the alkaline aqueous solution and the ultrasonic treatment described in JP-A-5-195007 is sufficiently effective for the hydrogen storage alloy powder having an area ratio of the high temperature oxide film on the surface of less than 20%. , 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 improvement of the initial activity is insufficient.

(Iii) In the surface treatment with the high-concentration acidic aqueous solution described in JP-A-6-223827, even if the hydrogen storage alloy powder has an area ratio of the high temperature oxide film of 20% or more,
Although the oxide film can be removed sufficiently and the initial activity reaches the target value, the 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 the low-concentration acidic aqueous solution and the ultrasonic treatment are used together, sufficient improvement effects can be obtained in both initial activity and storage capacity.

Example 2 As described in Production Example 1, (a) (ingot method), (c) (250 μm thick roll quenching method), and (d) (gas atomizing method) in Table 2 were performed. The alloy powder of the AB ₅ type hydrogen storage alloy species or the AB ₂ type hydrogen storage alloy species produced by each of the above methods was used for the surface treatment.

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

Surface treatment method (C) of Example 1 for each alloy powder
In the same manner as above, the surface treatment was performed by using both acid treatment and ultrasonic treatment in combination. The powders of the ingot materials of Test Nos. 1 and 2 were subjected to the test without any treatment. Table 4 summarizes the treatment conditions (concentration of the acid aqueous solution, treatment time and temperature, ultrasonic frequency and intensity). The stirring speed of the liquid during the treatment was 60 rpm. After the surface treatment, the alloy 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]

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[Table 4-2]

Even if the heat treatment conditions and the hydrogen storage alloy species change, the high temperature oxide film can be removed by the treatment method of the present invention to sufficiently enhance the initial activity, and at that time, Ni (OH) 2
_It can be seen that since the amount of ₂ generated is small, self-discharge after storage at high temperature is suppressed, and a battery having a sufficiently high storage capacity can be obtained. However, the ultrasonic frequency is less than 20 kHz or 100 kH
Above z, the surface modification effect is insufficient and the initial activity becomes poor.

[0078]

EFFECTS OF THE INVENTION According to the present invention, it is produced 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 have fine crystal grains and can improve the initial activity. It is possible to remove the oxide film while suppressing the generation of Ni (OH) ₂ by surface-treating the hydrogen-absorbing alloy powder whose oxide film covers 20% or more of the surface. As a result, when a Ni-hydrogen battery is produced by using the hydrogen storage alloy powder surface-treated by the method of the present invention as a negative electrode active material, the initial activity is high, the storage capacity at high temperature, and thus the electrode life is also good Ni-hydrogen battery. can get.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koki Takeshita 4-5-33 Kitahama, Chuo-ku, Osaka City Within Sumitomo Metal Industries, Ltd. (72) Inventor Koichi Jinshiro 4-5-33 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries Co., Ltd.

Claims (1)

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