JPH09302431A - Production of hydrogen storage alloy powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hydrogen storage alloy powder, remarkably improved in hydrogen occluding velocity, by melting a misch metal - nickel hydrogen storage alloy, atomizing the molten alloy by the use of an inert gas containing specific amounts of gaseous nitrogen, and subjecting the resultant powder to surface treatment with an acid solution. SOLUTION: A misch metal - nickel hydrogen storage alloy is melted, and the resultant molten alloy is gas-atomized by using an inert gas (argon) containing 0.1-<50vol.% gaseous nitrogen. A nitride film is formed on the surface of the powder prepared by atomizing, and this nitride film is composed essentially of rare earths contained in misch metal. When this powder is acid- treated by using a solution of hydrochloric acid, the nitride film is removed from the surface and the Ni-enriched phase right under the surface is allowed to appear at the surface. By this method, the atomized powder excellent in hydrogen occluding velocity can be obtained. This powder can remarkably improve the electrochemical properties when used for electrode.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a hydrogen absorbing alloy powder, and more particularly to a method of manufacturing the AB ₅ type hydrogen-absorbing alloy powder for the negative electrode material for a nickel-hydrogen battery.

[0002]

In recent years, nickel-hydrogen battery is attracting attention as a secondary battery in place of nickel-cadmium battery, a hydrogen absorbing alloy powder therefor have been studied, among others AB ₅
Hydrogen storage alloy powders are used with excellent characteristics as negative electrode materials for batteries. This is, for example, Ce50
%, La25%, Nd15%, balance Pr, and other misch metal Mm and nickel alloy containing, for example, Mn, Al, Co, etc. mixed and melted.
_{1.0 is} an intermetallic compound of the type such as Ni (5-x-y-z) MnxAlyCoz. Conventionally, this has been pulverized by various methods such as pulverization of a cast material, pulverization of a rapidly solidified thin ribbon brought into contact with a rotating drum, and atomization of an inert gas such as argon.

[0003]

Among the various powdering methods described above, the method of pulverizing a cast material is not uniform in the composition of each powder particle due to segregation of the material and the like. Is inferior to powders obtained by gas atomization or crushing of rapidly solidified ribbons. And since the powder obtained by the gas atomization method has a spherical particle shape, the packing density when incorporated into a battery electrode is superior to the powder obtained by crushing a cast material or a quenched ribbon, and the same hydrogen storage characteristics. Even when the powder having the above is used for the electrode, the energy density of the electrode can be increased, and a battery having a large capacity can be manufactured.

[0004] When the hydrogen storage alloy powder is used for a secondary battery, the performance required is that the hydrogen storage amount is large,
It is that the absorption and release of hydrogen are rapid, and the decrease in hydrogen storage amount due to repeated absorption and release is small. The magnitude of the hydrogen storage capacity is related to the capacity of the battery, the rate of absorption and release is related to the discharge efficiency of the battery and the increase in the internal pressure of the battery during charging, and the decrease in the hydrogen storage capacity is related to the life of the secondary battery. I do.

The amount of hydrogen absorbed and the speed of absorption and desorption are greatly influenced by the oxide layer on the surface of the alloy powder. However, the hydrogen storage alloy powder described above is easily oxidized because it contains a large amount of rare earth elements, and even when powdered by gas atomization of argon, an oxide layer is formed on the surface due to a slight oxygen partial pressure in the atmosphere. The oxide layer is often thicker than the powder obtained by crushing the cast material.

[0006] In the case of such a powder in which the surface of most of the particles is covered with an oxide layer, an activation step for bringing the powder into a state capable of absorbing hydrogen is required, or the powder is used as it is without activating the battery. In the case where is manufactured, it is necessary to repeatedly charge and discharge over a long period of time to increase the capacity of the battery, which significantly impairs productivity. As a method for further improving the battery characteristics and productivity, it has been proposed to treat the powder with an acid to remove the oxide layer on the particle surface.

The above-mentioned decrease in the hydrogen storage amount is caused by the fact that the powder particles are crushed more finely than necessary by repeated charging and discharging. Such crushing is caused by uneven distribution of microscopic alloy composition inside the particles, residual strain during casting, etc., so that volume expansion / contraction during hydrogen absorption / release is performed uniformly. This is partly due to the absence. Then, oxidation proceeds from the crushed surface, and the hydrogen storage capacity is gradually lost.

Therefore, in order to prolong the life of the battery, it is required that the alloy composition of the powder particles is microscopically uniform and no strain remains. Therefore, conventionally, a long-time heat treatment is performed at a high temperature during the casting / crushing process. In the case of gas atomized powder, the alloy composition is considerably more uniform and the residual amount of strain is smaller than in the case of the cast material. Therefore, the heat treatment also produces a powder having a good structure at a lower temperature and in a shorter time than in the case of the cast material. For the above reasons, it is considered that the alloy powder obtained by gas atomization is currently heat-treated, and the heat-treated powder is the most suitable powder for batteries. In order to further accelerate the reaction, it is desired to improve the hydrogen storage rate of the atomized powder after acid treatment.

The present inventors have prepared a powder by mixing nitrogen gas with a gas used for atomizing, and then, regarding the surface state of the powder subjected to the acid treatment, the powder atomized without mixing the nitrogen gas is treated with an acid. As a result of a detailed comparison with the case of treatment, after atomizing by mixing nitrogen gas, the hydrogen absorption rate of the powder was significantly improved when the powder was acid-treated, and the electrochemical when the powder was used as an electrode It was also found that the characteristics were significantly improved as compared with the powder produced by the gas containing no nitrogen.

[0010]

The gist of the present invention is as follows. (1) After melting a misch metal nickel-based hydrogen storage alloy, 0.1 vol. % Or more 50 vol. % Of nitrogen gas and the rest is an inert gas, which is gas atomized, and then the powder is surface-treated in an acid solution to produce a hydrogen storage alloy powder. (2) In the method for producing a hydrogen storage alloy powder according to the above (1), the gas atomized powder is heat-treated and then acid-treated.

Hereinafter, the present invention will be described in detail. The nitrogen gas content according to the present invention is 0.1 vol. % Or more is limited to 0.1 vol. This is because if the mixing ratio is less than 100%, the effect of mixing nitrogen gas is not sufficient, and a remarkable improvement in hydrogen storage speed is not observed.
l. % Less than 50 vol. % Or more, the hydrogen storage alloy powder obtained by gas atomization has an indefinite shape instead of a spherical shape, so when used for an electrode, the improvement in filling density, which is one of the features of gas atomized powder, is not noticeable. Is.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION A nitride film is formed on the surface by using an inert gas mixed with nitrogen gas when atomizing the above-mentioned mischmetal-nickel hydrogen storage alloy. The metal as the main component of this nitrided film is a rare earth contained in misch metal. Since the rare earths forming the nitrided film are preferentially collected on the surface, the metal element immediately below the surface is in a state in which the rare earths are deficient compared to the compounding components, and as a result, constituent elements such as Ni, Co, Mn, and Al, especially other than rare earth The main component of Ni is a large amount and solidifies. By treating the powder with an acid to remove the nitride film on the surface, the Ni-rich phase immediately below the surface is exposed on the surface. N
It has been pointed out that the i-rich phase functions as a catalyst when hydrogen is taken into the hydrogen storage alloy, and the hydrogen storage alloy atomized powder produced as described above has an excellent hydrogen storage rate, and the powder was used for the electrode. In this case, the electrochemical characteristics are also significantly improved as compared with the powder made of a gas containing no nitrogen.

[0013]

[Example] Mm _1.0 Ni _3.5 Co _0.7 Mn _0.5 Al _0.3
The metal raw material blended so as to constitute the above was housed in an alumina crucible and melted by induction melting, and then a molten metal at 1500 ° C. was dropped through a nozzle having a diameter of 2 mm, and gas was sprayed on this to rapidly cool to produce a gas atomized powder. . As gas atomization, gases obtained by mixing 5N argon and 5N nitrogen as shown in Table 1 were used. All the obtained powders were classified with a sieve having a mesh size of 106μ, then placed in an iron container, and heat-treated in an argon stream at 700 ° C for 2 hours. Further, the heat-treated powder was immersed in a solution of a hydrochloric acid solution having a pH of 1 with respect to a powder volume of 1 and stirred to perform surface treatment. After the surface treatment, the dried powder sample was subjected to shape observation by a scanning electron microscope, filling density measurement, and initial hydrogen gas absorption rate. For the measurement of the initial storage rate, a Sibelts apparatus for measuring hydrogen storage characteristics (PCT characteristics) is used, 1 g of the sample is stored in a container having a capacity of 30 cm ³ , heated to 80 ° C., and vacuum suction is performed for 30 minutes. Two
It was kept at 0 ° C., 10 atmospheres of hydrogen gas was enclosed, and the time required for the hydrogen gas pressure to fall to 9 atmospheres was determined. Table 2 summarizes the obtained results.

[0014]

[Table 1]

[0015]

[Table 2]

Regarding the results of SEM observation and packing density in Table 2, the atomized powder has a spherical shape, and in order to take advantage of the fact that the packing density is high when compared with the cast pulverized powder having an irregular shape, at least the packing density is 4 g. / Cc or more is required. A perfect spherical powder is obtained when atomized with 100% argon gas, and the packing density is also a high value, but as nitrogen gas is mixed with argon gas, the shape changes from spherical to irregular shape such as cast pulverized powder. It will change. In particular, when a gas having a nitrogen gas content ratio of 50% by volume or more is used, an apparently irregular shape is formed and the packing density is reduced to 4 g / cc or less.
Use of a gas of more than 0% by volume leads to impairing the packing density of the gas atomized powder.

Regarding the results of the initial hydrogen gas absorption rate in Table 2, it is possible to produce a nickel-hydrogen battery having characteristics that can be sufficiently put to practical use even when the powder having the initial hydrogen gas absorption rate of Sample 1 is used. Is. However, when it is used in an attempt to further improve the battery characteristics, a powder having a higher storage speed is required. In order to manufacture a battery whose characteristics are clearly improved, it is desirable that the initial hydrogen gas absorption rate is at least twice as fast as that of Sample 1, that is, Table 2
The gas mixing ratios of Samples 3 to 9 are desirable. As described above, from the viewpoint of the packing density and the hydrogen gas storage rate, it is desirable to use, as the atomizing gas, a gas containing 1% by volume or more and less than 50% by volume of nitrogen gas in argon gas.

[0018]

As described above, according to the present invention, when the powder is acid-treated after being mixed with nitrogen gas and atomized, the hydrogen absorption rate of the powder is significantly improved, and the powder is used for the electrode. In this case, the electrochemical characteristics also have a remarkably improved effect, which is significantly improved as compared with the powder prepared with a gas containing no nitrogen.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shunichiro Nishikawa 3007 Nakajima-ji, Nakajima, Himeji-shi, Hyogo Prefecture Sanyo Special Steel Co., Ltd.

Claims (2)

[Claims] 1. After melting the misch metal / nickel-based hydrogen storage alloy, 0.1 vol. % Or more 50 vol. % Of nitrogen gas and the rest is an inert gas, which is gas atomized, and then the powder is surface-treated in an acid solution to produce a hydrogen storage alloy powder. 2. The method for producing a hydrogen storage alloy powder according to claim 1, wherein the gas atomized powder is heat-treated and then acid-treated.