JPH10265810A - Hydrogen occlusion alloy powder, its production and electrode consisting thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain hydrogen occlusion alloy powder which has an excellent discharge capacity and charge and discharge repetition life when the power is used for the battery by specifying the ratio of the average grain size of the alloy powder pulverized after a heat treatment to the average grain size of the substantially spherical hydrogen occlusion alloy powder before the heat treatment. SOLUTION: The hydrogen occlusion alloy powder at least partially sintered during the heat treatment is pulverized and the sintered compacts are nearly completely disintegrated. The disintegration is so executed that the ratio of the average grain size of the pulverized powder after the heat treatment to the average grain size of the powder before the heat treatment attains a range of 0.85 to 1.15. Namely, the powder having nearly the same average grain size as the average grain size of the powder before the heat treatment is obtd. Such pulverization is easily attained by using a pulverizing machine of which the main pulverizing mechanism is impact. The hydrogen occlusion alloy powder having the spherical shape can thus be obtd. Consequently, a high-density electrode packed with a large amt. of the hydrogen occlusion alloy by the peculiar high packing property of the spherical powder is obtd. and the capacity of the battery is made higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage material having a high filling rate, which is suitable as a material for a negative electrode of a nickel-hydrogen secondary battery (hereinafter referred to as Ni-H secondary battery or simply Ni-H battery). The present invention relates to an alloy powder and a method for producing the same.

[0002]

2. Description of the Related Art In today's energy situation, global environmental problems such as air pollution and global warming caused by fossil fuels have become issues, and hydrogen has been attracting attention as a clean fuel replacing these fossil fuels. This is because hydrogen uses water as a raw material, the combustion product is water, and it is a material that can be applied to an energy conversion system for electric power, heat, and power.

[0003] A hydrogen storage alloy exists for storing this hydrogen, and its application to storage, heat pumps, actuators and the like has been developed. In recent years, application to Ni-H secondary batteries has been put to practical use. In particular, demand in this field has grown sharply due to the mainstream pollution problems of Ni-Cd secondary batteries, resource problems of Cd, and the need for higher capacity due to portable equipment. At present, it has become a major part of the use of hydrogen storage alloys.

[0004] The major alloy systems that have been studied as a hydrogen storage alloy is a system or the like having a Mg-based, LaNi ₅ or MmNi ₅ like the AB ₅ type, AB ₂ type Laves phase as represented by the ZrV _2, etc. .
In applications for the secondary battery has progressed a practical, although AB ₅ system is the most advanced, AB ₂ system also is promising in that a high capacity can be achieved.

[0005] Ni-H secondary batteries have been several years after mass production has started, and their capacity has been increasing. However, with the demand for higher capacity from equipment manufacturers and the recent appearance of high capacity lithium ion batteries, There is a growing need for higher capacity Ni-H secondary batteries.

Various approaches to increasing the capacity have been studied, and a method for improving the filling rate of the alloy into the negative electrode by using a spherical hydrogen storage alloy is described in Japanese Patent Application Laid-Open No. 3-116655. ing. Such spherical hydrogen storage alloy powder
Examples of a method for obtaining spherical powder (hereinafter, also referred to as spherical powder) include a gas atomizing method and a rotating electrode method.

[0007] Spherical hydrogen storage alloy powder can be packed at a higher density than irregular shaped powder, and it is possible to fill a larger amount of alloy in a battery having a fixed volume (shape). Become. As a result, the capacity of the electrode is increased, that is, the capacity of the battery is increased.

[0008] Such a spherical hydrogen storage alloy powder is usually rapidly solidified. The rapidly solidified hydrogen-absorbing alloy powder has the characteristics of low segregation and small crystal grain size, and when used for the negative electrode of a Ni-H secondary battery, has high corrosion resistance and high-rate discharge. It has the advantage of improving the characteristics (JP-A-3-216959, JP-A-4-63959)
No. 207, etc.).

[0009]

As a method for further highlighting these advantages, there is a heat treatment. Although the rapidly solidified alloy generally has the above-mentioned features (small segregation and small crystal grain size), it has a lattice strain due to rapid cooling, and minute segregation remains. If there is such a lattice strain or minute segregation, hydrogen cannot be sufficiently absorbed, so that the discharge capacity of the Ni-H secondary battery is significantly reduced, and the alloy powder is easily collapsed and powdered during use. The repeated charge / discharge life of the battery also deteriorates. To eliminate lattice distortion and minute segregation in rapidly solidified alloys,
Heat treatment is effective.

Japanese Unexamined Patent Publication (Kokai) No. 3-116655 discloses a technique for increasing the capacity of a Ni-H secondary battery by improving the filling rate by using spherical powder, but discloses heat treatment. Absent.

[0011] The heat treatment of the hydrogen storage alloy powder is usually performed at a temperature of 550 to 1100 ° C, though it varies depending on the material. However, due to the heat treatment in the powder state, the alloy sinters during this heat treatment, and the spherical powder adheres to each other to form a sintered body, or several (eg, two to three) spherical particles. May become a connected temporary sintered body.
From the sintered or pre-sintered body of such a hydrogen storage alloy powder, pulverization is necessary to obtain separate powder again, but this pulverization breaks the precious spherical particle shape, resulting in high filling. In many cases, the superiority of the spherical powder in terms of ratio was lost. For this reason, the spherical hydrogen storage alloy powder could not fully enjoy the advantage of high capacity that it should originally have.

Japanese Patent Application Laid-Open No. 8-45505 discloses that a partially sintered alloy obtained by partially sintering a spherical hydrogen-absorbing alloy powder by heat treatment and, if necessary, crushing the powder is used for the electrode. Have been described. However, the optimal heat treatment temperature is to be determined from the viewpoint of alloy properties, and the temperature at which partial sintering is performed does not always match the optimal heat treatment temperature. Therefore, in the heat treatment at a temperature at which the alloy is partially sintered, there is a possibility that the performance of the alloy originally exhibited by the heat treatment cannot be secured. Further, since the alloy powder obtained by crushing is still partially in a sintered state, the high filling property of the spherical powder is impaired.

According to the present invention, the performance of a spherical (ie, rapidly solidified) hydrogen-absorbing alloy powder can be maximized by heat-treating it. An object of the present invention is to provide a hydrogen storage alloy powder excellent in both the discharge capacity and the charge / discharge repetition life, in which the reduction of the filling property caused is substantially avoided, and a method for producing the same.

[0014]

Means for Solving the Problems The inventors of the present invention heat-treat a spherical hydrogen-absorbing alloy powder and then pulverize a pre-sintered body or a sintered body with an impact-type pulverizer. It has been found that a powder having a shape close to the average particle diameter of the spherical powder and having a substantially spherical shape can be obtained, and that this means can solve the above problem.

Here, the present invention provides a hydrogen storage alloy which comprises heat-treating a substantially spherical hydrogen storage alloy powder under conditions at least partially sintering the powder and then pulverizing the sintered powder. A method for producing a powder, wherein the ratio of the average particle size of the powder crushed after the heat treatment to the average particle size of the hydrogen storage alloy powder before the heat treatment is in the range of 0.85 to 1.15 ''. .

The pulverization can be performed using a pulverizer whose main pulverizing mechanism is an impact. The hydrogen-absorbing alloy powder produced by the method of the present invention has a higher filling factor than the irregularly-shaped hydrogen-absorbing alloy powder of the alloy having the same composition, and thus has a high discharge capacity of a Ni-H secondary battery. Can be made.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a hydrogen storage alloy powder according to the present invention comprises heat-treating a substantially spherical hydrogen storage alloy powder and then pulverizing the powder. Where "substantially spherical"
Means that the aspect ratio of the powder is generally 2 or less. Hereinafter, for simplicity of description, this hydrogen storage alloy powder is simply referred to as “spherical” powder.

As described above, such a spherical powder of the hydrogen storage alloy can be obtained by a gas atomizing method, a rotating electrode method, or the like. However, the method for producing this powder is not particularly limited, and any method may be used as long as a spherical powder of the hydrogen storage alloy powder can be obtained.

In the method of the present invention, the alloy composition of the hydrogen storage alloy powder is not particularly limited. At present AB ₅ type and A
Although the B ₂ type hydrogen storage alloy is used as the negative electrode material of the Ni—H secondary battery, the present invention can be applied to other hydrogen storage alloy powders.

Before the heat treatment according to the present invention, the spherical powder of the hydrogen storage alloy may be subjected to an appropriate surface treatment. As such a surface treatment, treatment with an aqueous acid solution or aqueous alkali solution, nickel plating, and the like are known, and the initial activation characteristics and other characteristics may be improved in some cases.

For example, since the spherical powder of the hydrogen storage alloy powder obtained by the gas atomizing method has a relatively wide particle size distribution, it may be classified before heat treatment to adjust the particle size distribution and the average particle size. As the negative electrode material of the Ni-H secondary battery, a hydrogen storage alloy powder having an average particle diameter in the range of 10 to 100 μm is preferable. As will be described later, in the method of the present invention, a hydrogen storage alloy powder having substantially the same average particle size as before heat treatment is finally obtained, so that the average particle size of the spherical powder of the hydrogen storage alloy to be subjected to heat treatment is used. What is necessary is just to decide so that it may be optimal for a use.

In the present invention, the average particle size of the hydrogen-absorbing alloy powder means a value of 50% cumulative in the particle size distribution obtained by a laser diffraction type particle size distribution meter.

As described above, since the substantially spherical hydrogen-absorbing alloy powder is generally rapidly solidified, there is lattice distortion and minute segregation, and heat treatment is performed to remove these. The heat treatment conditions may be selected so as to release lattice strain and minute segregation, and vary depending on the alloy composition of the hydrogen storage alloy powder, the production method (particularly, rapid cooling rate), the particle size, and the like. For example,
The AB ₅ type hydrogen storage alloy, at a temperature from 550 to 1,100 ° C., preferably the heat treatment time of about 2-8 hours. Above all, in the case of rapidly solidified powder obtained by the gas atomizing method, 650 to 1
A temperature range of 000 ° C. is preferred.

The atmosphere for the heat treatment is preferably a condition under which the oxidation of the alloy powder is suppressed, that is, a non-oxidizing atmosphere.
For example, a vacuum or reduced pressure atmosphere, or an inert gas atmosphere is preferable.

Depending on the heat treatment conditions, the substantially spherical hydrogen-absorbing alloy powder may at least partially sinter and stick to each other. As a result, a sintered body in which the powder is solidified as a whole or a temporary sintered body in which several spherical particles are connected to each other is generated. The method of the present invention is directed to the case where the heat treatment is performed under such conditions (ie, under conditions where sintering occurs at least partially). If no sintering occurs during the heat treatment, the hydrogen storage alloy powder after the heat treatment maintains a substantially spherical shape before the heat treatment and can be used as it is.

According to the present invention, the hydrogen storage alloy powder at least partially sintered during the heat treatment is pulverized to almost completely dissolve the sintered body (including the temporarily sintered body, the same applies hereinafter). The pulverization at this time is performed so that the ratio of the average particle diameter of the powder pulverized after the heat treatment to the average particle diameter of the hydrogen storage alloy powder before the heat treatment (average particle diameter ratio) is in the range of 0.85 to 1.15. Do. That is, a powder having an average particle diameter substantially equal to the average particle diameter before the heat treatment is obtained by pulverization. Such pulverization can be easily achieved by using a pulverizer whose main pulverizing mechanism is impact.

The crushing mechanism of the crusher (external force applied to the solid for crushing) is mainly of four types: compression, shear, impact, and friction. Among these pulverizing mechanisms, when a spherical hydrogen storage alloy powder that is at least partially sintered is pulverized using a pulverizer mainly based on compression, shearing, and / or friction, the pulverized powder has a spherical shape. And an irregular shape. However, when this sintered powder is crushed by a crusher that crushes mainly by impact, the original (before heat treatment)
It was found that a powder having a spherical shape was obtained. The reason is considered as follows.

In the pulverization by the impact, since the external force is applied only instantaneously, the part to be broken (cut) is not the hydrogen storage alloy powder itself, but the part which is sintered during the heat treatment which is the weakest part. The connection part of the particles, which is usually called “neck”, is mainly used. Therefore, the powder obtained after the pulverization has the same spherical particle shape as before the sintering (that is, before the heat treatment). Further, since the powder particles before heat treatment are not broken, the average particle size of the powder after pulverization is close to the average particle size of the powder before heat treatment.

On the other hand, in the pulverization by compression, friction, and / or shearing, an external force is applied for a relatively long period of time, so that only the neck portion (the particle connecting portion of the sintered body) which is the weakest portion is bonded. In addition, the probability that the spherical powder particles themselves are broken or crushed is increased. As a result, when grinding is performed until the sintered powder particles are almost completely eliminated (i.e., almost completely disintegrated), a significant portion of the resulting powder becomes irregular due to cracking and flattening of the particles. It becomes impossible to obtain the characteristic of high filling property which is a characteristic of the spherical powder. Moreover, since most of the spherical powder before heat treatment is broken and divided, the average particle size becomes considerably smaller than before heat treatment.

Therefore, in the present invention, the spherical powder of the hydrogen-absorbing alloy, which is at least partially sintered during the heat treatment, is obtained by using a pulverizer that is mainly pulverized by impact (that is, the main pulverizing mechanism is the impact). Smash. By using this pulverizer, the sintered part (neck) of the sintered powder particles is preferentially destroyed, so that the sintered powder is almost completely disintegrated while maintaining the original spherical particle shape. And a hydrogen storage alloy powder having a spherical shape can be obtained. As a result, due to the high filling property inherent in spherical powder, a larger amount of hydrogen storage alloy can be filled in the electrode than the irregularly shaped powder,
A high-density electrode can be obtained, and the capacity of the Ni-H secondary battery can be increased.

Representative examples of the crusher that mainly crushes by impact are a hammer mill (a crusher that crushes with a hammer attached to a high-speed rotating rotor) and a screen mill that is a kind of a hammer mill. The crushing mechanism of a hammer mill or screen mill combines impact and shear,
Shock is the main.

Another example of a pulverizer mainly crushing by impact is a jet mill (a crusher in which powder is entrained in a jet stream and crushed by collision of particles), a rolling ball mill (rotation with a horizontal axis as a rotation axis). A crusher that puts a ball as a crushing medium or impactor in a container and crushes the ball by dropping the ball; also includes a pebble mill in which the crushing medium is a pebble shape and a rod mill in which a rod shape is used; Ball mills for pulverization), as well as disc mills and pin mills for pulverization with pins or grooves of a rotating disk facing each other. In these crushers, crushing is achieved by impact and friction, or by impact, friction and shear, but impact is the main crushing mechanism. Other grinders can also be used in the method of the present invention if the primary grinding mechanism is an impact.

In the pulverizer used in the present invention, it is sufficient that the pulverizing mechanism is mainly an impact. However, as the proportion of the pulverizing mechanism other than the impact increases, the possibility that spherical powder particles are broken increases, and The effect of improving the rate is reduced. Conversely, the higher the proportion of the impact on the crushing mechanism, the greater the effect obtained. Therefore, a pulverizer, which is a pulverizing mechanism in which impact is dominant, is preferable for obtaining a greater effect. For example, a hammer mill has a particularly large effect of improving the filling rate because impact is dominant in its pulverizing mechanism. vice versa,
In grinding with a rolling ball mill or vibrating ball mill, impact is mainly involved, but also shearing and friction are considerably involved, so
The effect is smaller than that of a hammer mill. Jet mills generally provide intermediate results between hammer mills and ball mills.

On the other hand, examples of pulverizers which mainly use a pulverizing mechanism other than an impact and which cannot be used in the method of the present invention include the following: pulverizers which mainly use compression and are subject to friction and shear.
[Example: Roll mill (pulverizer that rotates in the opposite direction, pulverizes between two rolls with different rotation speeds)], pulverizer that mainly applies compression, and is subject to impact and shearing [Example, stamp mill (punch type stamp) Crusher that crushes by falling (vertical movement)
Crushers mainly for friction and compression (eg low speed ball mills, mortars and pestle).

In the present invention, an object is to obtain a substantially spherical hydrogen storage alloy powder capable of obtaining a high filling rate by pulverization, so that the pulverized powder has a substantially spherical shape. You need to make sure that The most direct confirmation method is to directly observe the powder shape with an SEM or the like, but it is necessary to observe all powders, which is not practical in an actual manufacturing process.

The present inventors have found that when pulverizing using a pulverizer whose main pulverizing mechanism is impact, the ratio of the average particle diameter of the powder before heat treatment to the average particle diameter of the powder pulverized after heat treatment (hereinafter, referred to as
If the average particle size ratio is within the range of 0.85 to 1.15, the pulverized powder has a substantially spherical particle shape before heat treatment, and the sintered powder particles are almost completely eliminated. We have empirically determined that it has been crushed. This average particle size ratio is preferably from 0.90 to 1.10.

As described above, in a crusher in which the main crushing mechanism is an impact, a mechanically weak sintering portion (neck portion) is preferentially destroyed, and the original spherical powder itself is substantially destroyed. The powder itself is hardly destroyed even when complete disintegration has occurred. Therefore, since the average particle size of the powder after pulverization at this point is not so different from that before the heat treatment, the average particle size is ± 15% of the original value (that is, the average particle size ratio is 0.85 to 1.15). Based on this, it is considered that by controlling the progress of the pulverization, the powder is almost completely disintegrated into individual particles, and a powder having a substantially spherical powder shape before the heat treatment is obtained.

However, the average particle size of the powder often shows a slightly different value each time a sample is collected, even in the same lot of powder. In addition, even if the main pulverizing mechanism is pulverized using a pulverizer having an impact, it cannot be said that powder cracking or flattening does not occur at all, and a very small amount of powder undergoes cracking or flattening or cracks. It is common for powder to agglomerate. Even with such various phenomena, when a crusher whose main crushing mechanism is an impact is used, the progress of the crushing is controlled by the average particle diameter ratio as a whole to achieve the purpose and It was confirmed that a pulverization result could be obtained.

According to the method of the present invention, even if the spherical powder of the hydrogen-absorbing alloy is completely sintered during the heat treatment, it returns to a shape close to the original spherical powder by pulverization, so that the heat treatment conditions can be freely set. Therefore, the heat treatment can be performed under the optimal conditions for eliminating the lattice distortion and minute segregation of the rapidly solidified spherical powder. Then, since the powder returns to a substantially spherical powder shape after pulverization, the advantage of the high filling property inherent to the spherical powder can be enjoyed as it is, and the filling rate is higher than that of the irregularly shaped hydrogen storage alloy powder. Therefore, a Ni-H secondary battery having a large discharge capacity can be formed.

The hydrogen-absorbing alloy powder produced by the method of the present invention can produce an electrode using the alloy powder as an active material in the same manner as a conventional hydrogen-absorbing alloy powder. The electrode can be produced by sintering the hydrogen storage alloy powder by a powder metallurgy technique, but is usually produced by molding using an organic binder. As the organic binder, for example, polyvinyl alcohol, polytetrafluoroethylene, polyethylene oxide, carboxymethyl cellulose and the like can be used.

For the production of an electrode using a binder, a hydrogen storage alloy powder and an organic binder are mixed with a suitable solvent (eg, water,
A slurry or paste is formed using alcohols, ketones, etc., and this is a porous metal plate such as an electrode substrate (eg, a foamed metal plate, a perforated metal plate, a wire mesh), preferably a perforated iron plate plated with nickel. It can be performed by coating or filling a punching metal), drying and removing the solvent, and then rolling and molding. The hydrogen storage electrode thus manufactured is useful as a negative electrode of a Ni-H secondary battery.

[0042]

Hydrogen-absorbing alloy powder used in EXAMPLE was prepared by argon gas atomizing method, substantially of the hydrogen storage alloy of AB ₅ type having a composition shown in Table 1 (Alloy A) or AB ₂ type (Alloy B) Spherical powder (average particle size: 30μm)
Met. In the table, Mm is La: 27%, Ce: 48%, Pr: 7%,
Nd: Rare earth metal mixed alloy containing 17% (Misch metal)
It is. However, the present invention is not limited to the above composition, but is applicable to hydrogen storage alloys of any composition.

For comparison, an ingot material of each alloy was also manufactured by a conventional method. The pulverizer used in the examples, its main pulverizing mechanism and pulverizing conditions are as shown in Table 2 below.

[0044]

[Table 1]

[0045]

[Table 2]

Example 1 AB having the composition of alloy A shown in Table 1
Type ₅ substantially spherical hydrogen storage alloy powder (average particle size 30μ
m) was subjected to a heat treatment at 950 ° C. for 8 hours in an argon gas atmosphere. The hydrogen storage alloy powder after the heat treatment was sintered in a cake shape. The sintered body of the hydrogen storage alloy powder obtained by the heat treatment was pulverized by a pulverization method shown in Table 2. Milling was continued until the sintered powder particles were almost completely gone (ie, almost completely broken up).

Table 3 shows the average particle size, tap density, and results of shape observation by a scanning electron microscope (SEM) of the obtained hydrogen storage alloy powder together with the crusher, crushing conditions and crushing time. Here, the average particle size is a particle size at a cumulative 50% of the particle size distribution measured by a laser diffraction type particle size distribution meter. The tap density is a value that is an index of the closest packing density of the powder, and is obtained by dividing the powder weight when the powder uniformly compressed by tapping (moving up and down) the cylinder filled with the powder by the occupied volume. Desired. In this example, cylinder volume: 100 cc, stroke: 40 mm, tapping: 800 times
(2 times / second).

For reference, SEM of a hydrogen storage alloy powder which was jet-milled after heat treatment of Test No. 4 and a hydrogen storage alloy powder which was stamp-milled after heat treatment of Test No. 6
The photographs are shown in FIGS. 1 and 2, respectively.

From the obtained hydrogen storage alloy powder, an electrode was produced as follows. A 5% aqueous solution of polyvinyl alcohol as a binder was added to 100 parts by weight of the alloy powder and kneaded to prepare a paste of a hydrogen storage alloy powder, and this paste was applied to both surfaces of an electrode substrate (punching metal).
After drying, it was rolled by a roll rolling mill and cut into appropriate dimensions to obtain an electrode. The thickness of each electrode was made constant so that the volume occupied by the portion (alloy + binder) excluding the electrode substrate was 1 cm ³ . The alloy density (density excluding the electrode substrate) in the obtained electrode was measured and also shown in Table 3.

The electrode is connected to a non-woven polyamide fabric.
In combination with a known sintered nickel positive electrode having a capacity larger than that of the negative electrode, the battery was inserted into a battery container, and a 6N-KOH solution was injected to form a negative electrode capacity-controlled Ni-H secondary battery. Next, the alloy filling rate and the theoretical capacity of the alloy (about 320 mA for alloy A)
h / g), calculate the theoretical capacity of this battery, perform 110% overcharging with a 3-hour rate battery, and discharge to a terminal voltage of 0.9 V at the same 3-hour rate current. The discharge capacity was recorded. The results are also shown in Table 3.

For comparison, the hydrogen storage alloy powder before heat treatment and the hydrogen storage alloy powder obtained by heat-treating the ingot material under the same conditions as above and then pulverizing it with a mortar and pestle were also used. The same test was performed. The results are also shown in Table 3.

[0052]

[Table 3]

As can be seen from Table 3, according to the present invention,
The hydrogen storage alloy powder obtained by pulverizing until almost completely disintegrated with a pulverizer that is mainly subjected to impact after heat treatment has an average particle diameter of 0.85 to the average particle diameter (30 μm) before heat treatment.
As described above, it was within the range of 1.15 or less, and the shape was spherical. From the SEM photograph shown in FIG. 1, it can be seen that almost all the powder particles are spherical.

The tap density and the electrode density of the hydrogen-absorbing alloy powder become the highest in the atomized state before the heat treatment, and the powder obtained by the method of the present invention also shows a high density close to that. Above all, when crushed with a hammer mill where impact is dominant, both tap density and electrode density become highest,
It was very close to the value before heat treatment.

Further, the hydrogen storage alloy powder obtained by the method of the present invention has a lower electrode strain than an atomized alloy powder that has not been heat-treated, as a result of eliminating lattice distortion and minute segregation of the atomized powder by heat treatment. Although the density was low, the discharge capacity of the electrodeposition showed an improved value. Also in this case, the powder crushed by the hammer mill having the highest electrode density showed the highest discharge capacity. However, the discharge capacity of the powder milled by the jet mill or the ball mill was higher than that of the atomized powder, even though the electrode density was considerably lower than that of the atomized powder.

On the other hand, in the comparative example, since a pulverizer mainly comprising a pulverizing mechanism other than an impact was used for pulverization after the heat treatment, the spherical powder was broken during the pulverization, as shown in the SEM photograph of FIG. Some of the spherical powder also remained, but very many blasted balls were generated, and the resulting fine powder was aggregated. Therefore, the average particle size of the powder after pulverization was smaller than 0.85 times the average particle size before heat treatment. Also, despite the small average particle size, the tap density and electrode density were significantly reduced due to the irregular shape of the blast ball. Due to this decrease in packing density, the discharge capacity of the battery was significantly lower than that of the powder obtained by the method of the present invention, despite the heat treatment.
Among them, the powder obtained by grinding the ingot material with a mortar had the worst results in both the density and the discharge capacity.

Incidentally, in the comparative example of "as atomized powder" without heat treatment, the discharge capacity shows a relatively high value. However, because it has not been heat treated, its corrosion resistance is poor,
When charge and discharge are repeated, the discharge capacity is likely to decrease and the battery life is shortened, so that it is not practical. This is the same in Table 4 described later.

Example 2 As a hydrogen storage alloy, an AB ₂ type alloy having an alloy B composition shown in Table 1 (theoretical capacity is about 360
Example 1 was repeated except that mAh / g) was used. The heat treatment conditions were the same as in Example 1. Table 4 summarizes the pulverizer, the pulverization time, the average particle size, and the test results of the obtained powder.

[0059]

[Table 4]

AB ₂ type alloy B is AB ₅ type alloy A
Since the theoretical capacity was higher, the discharge capacity of the battery was higher than that of Example 1, but the test results showed the same tendency as that of Example 1. That is, when the heat-treated atomized powder was pulverized by a pulverizer mainly pulverized by impact according to the present invention, a spherical powder having an average particle diameter ratio of 0.85 to 1.15 was obtained. Among the examples of the present invention, the powder pulverized by the hammer mill had the highest tap density and the highest electrode density, and the discharge capacity was remarkably improved as compared with the powder that had been atomized. Also,
In the case of the jet mill and the ball mill, the discharge capacity was slightly higher, but the discharge capacity was still equal to or higher than that of the powder which was still atomized. On the other hand, when pulverization was performed by another pulverizer, the average particle size ratio was smaller than 0.85, and the powder shape was also blasted. Further, since the powder had an irregular shape, the density was lowered, and it was not possible to obtain the discharge capacity of the atomized powder despite the heat treatment.

[0061]

According to the present invention, a substantially spherical hydrogen storage alloy powder obtained by a gas atomization method, a rotating electrode method, or the like is heat-treated, and then the sintered powder is substantially broken into its original spherical shape. To obtain a hydrogen storage alloy powder having a spherical shape. Therefore, heat treatment can be performed under sufficient conditions to completely eliminate lattice distortion and minute segregation due to rapid solidification without regard to the degree of sintering due to heat treatment.

Further, the obtained spherical hydrogen-absorbing alloy powder shows a high filling property inherent to the spherical powder, so that an electrode having a high alloy filling density can be produced, and the discharge capacity can be reduced in combination with the effect of the heat treatment. A very high Ni-H secondary battery can be provided. Therefore, according to the present invention, the performance of the spherical powder of the hydrogen storage alloy can be maximized, and the performance of the Ni-H secondary battery is significantly improved.

[Brief description of the drawings]

FIG. 1 is an SEM photograph of a hydrogen storage alloy powder pulverized by a jet mill after heat treatment according to the present invention.

FIG. 2 is an SEM photograph of a hydrogen storage alloy powder pulverized by a stamp mill after heat treatment in a comparative example.

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[Procedure amendment]

[Submission date] March 26, 1997

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

FIG. 2

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

Claims (4)

[Claims] 1. A method for producing a hydrogen-absorbing alloy powder, comprising heat-treating a substantially spherical hydrogen-absorbing alloy powder under conditions at least partially sintering the powder, and pulverizing the sintered powder. Wherein the ratio of the average particle size of the hydrogen storage alloy powder before and after heat treatment to the average particle size of the hydrogen storage alloy powder before heat treatment is 0.85 to 1.15. 2. The method according to claim 1, wherein the crushing is performed using a crusher whose main crushing mechanism is an impact. 3. A hydrogen storage alloy powder produced by the method according to claim 1 having a higher filling factor than an irregularly shaped powder of an alloy of the same composition. 4. An electrode for a Ni-hydrogen secondary battery comprising the hydrogen storage alloy powder according to claim 3 as an active material.