JPH06212369A - Method for heat-treating hydrogen storage alloy for ni-hydrogen battery - Google Patents

Abstract

PURPOSE:To increase the initial activity of a hydrogen storage alloy for a Ni-hydrogen battery by heat treatment. CONSTITUTION:The hydrogen storage alloy for a Ni hydrogen battery is held to 550 to 1200 deg.C for 2 to 8hr in a hydrogen gas atmosphere having <=30ppm oxygen concn. or in a rare gas atmosphere having <=30ppm oxygen concn. and >=1vol% hydrogen concn. and is thereafter cooled to <=50 deg.C at <=20K/min cooling rate in the same atmosphere, and the hydrogen storage alloy for a Ni-hydrogen battery is subjected to heat treatment. The hydrogen storage alloy solidified at >=50K/sec cooling rate from the melting state is subjected to the heat treatment preferably by executing holding to 550 to 950 deg.C for 2 to 5hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for hydrogen storage alloys for Ni-hydrogen batteries, and more particularly to a heat treatment method for hydrogen storage alloys that facilitates initial activation and contributes to improved productivity of Ni-hydrogen battery production. Regarding

[0002]

2. Description of the Related Art At present, Ni-Cd batteries are the mainstream of small secondary batteries used for memory backup of AV equipment and notebook type personal computers, mobile cellular phones and the like. However, Cd has a pollution problem, and Cd is produced as a by-product of zinc refining, and the world's annual output is 7,000 tons.

From the viewpoint of these problems and the development of a secondary battery having a higher capacity, a secondary battery called Ni-hydrogen battery has been developed which uses a hydrogen storage alloy as a material for the cathode (negative electrode) instead of Cd. It was Secondary batteries using this hydrogen storage alloy have a higher capacity than Ni-Cd batteries and Ni-Zn batteries, so secondary batteries for electric vehicles are expanding their use as pollution-free vehicles due to global environmental problems. It is also being considered for use as, and mass production is about to begin.

However, some problems have been newly highlighted at the start of mass production. One of the problems is that the initial activation takes a very long time, which is a factor that hinders productivity. That is, in a Ni-hydrogen battery, hydrogen absorption / desorption occurs at the hydrogen storage alloy electrode during charging / discharging, but since the efficiency of hydrogen absorption / desorption is poor at the initial stage, the original performance of the battery cannot be exhibited.
Therefore, after assembling the battery, it is necessary to perform an activation treatment in advance until a predetermined discharge capacity is obtained.
The current initial activation process is a method of repeating long-time charging and discharging (charging for 15 to 20 hours, discharging for several hours) at low current several times. Therefore, even after assembling the battery, it was necessary to repeat charging and discharging for several days within the factory before shipping.

As a means for solving this problem, Japanese Patent Application Laid-Open No. 3-219036 discloses that a specific element such as B is added to promote pulverization at the time of absorbing and desorbing hydrogen and increasing the specific surface area. Therefore, it has been proposed to improve the initial activation characteristics. However, in this method, the second phase generated to cause pulverization has a small amount of reversibly absorbing and releasing hydrogen, so that the discharge capacity of the alloy as a whole is reduced and the pulverization proceeds. There is also a problem that the current collecting property of the electrode is deteriorated due to the excess and the discharge capacity is deteriorated at an early stage.

[0006]

Therefore, in the practical application of hydrogen storage alloys for Ni-hydrogen batteries, new means for improving the initial activation characteristics without impairing the original performance of the alloys are required. An object of the present invention is to provide a means for improving the initial activation characteristics of the hydrogen storage alloy for Ni-hydrogen battery without adding a new element and without sacrificing the hydrogen storage amount or the life. .

[0007]

[Means for Solving the Problems] The present inventors have studied various factors that affect the activation characteristics of hydrogen storage alloys.
The following findings were obtained. The factor that hinders the initial absorption and desorption of hydrogen in hydrogen storage alloys is the oxide film formed on the alloy surface. become. The oxygen film is formed on the surface during the heat treatment process and the mechanical crushing process.

From these findings, heat treatment of a hydrogen storage alloy in a hydrogen-containing atmosphere and subsequent cooling prevent the formation of an oxide film on the surface of the alloy during the heat treatment and, at the same time, the alloy is in a gas phase state during the cooling process. It was found that this alloy absorbs hydrogen and activates in the gas phase, and when this alloy is used for the negative electrode active material of Ni-hydrogen secondary battery, it was confirmed that the activation property of the battery is improved. Completed the invention.

Here, the gist of the present invention is to provide a hydrogen storage alloy at 550 to 1200 ° C. in a hydrogen gas atmosphere having an oxygen concentration of 30 ppm or less, or in a rare gas atmosphere having an oxygen concentration of 30 ppm or less and a hydrogen concentration of 1 vol% or more. The method for heat treating a hydrogen storage alloy for a Ni-hydrogen battery is characterized in that it is held at the temperature of 2 to 8 hours and then cooled to a temperature of 50 ° C. or less in the atmosphere at a cooling rate of 20 K / min or less. .

When the hydrogen storage alloy is obtained by solidifying it from the molten state at a cooling rate of 50 K / sec or more, the heat treatment of the hydrogen storage alloy is performed under the above atmosphere at a temperature of 550 to 950 ° C.
After holding for 5 hours, it is preferable to perform cooling to a temperature of 50 ° C. or less in the atmosphere at a cooling rate of 20 K / min or less.

[0011]

According to the present invention, the heat treatment of the hydrogen storage alloy and the subsequent cooling are performed in a hydrogen gas atmosphere or an inert rare gas atmosphere containing hydrogen to prevent oxidation of the alloy surface and to prevent vaporization during the cooling process. It absorbs phase hydrogen and activates the alloy.

Since the hydrogen dissociation pressure of a hydrogen storage alloy generally increases as the temperature rises, the hydrogen storage alloy has the property of releasing hydrogen at high temperatures and absorbing hydrogen at low temperatures. When cooled in a hydrogen-containing atmosphere according to the heat treatment method of the present invention, the hydrogen storage alloy absorbs hydrogen due to this property, and the vapor absorption of hydrogen increases the activity of the alloy for hydrogen. Therefore, the initial activation of the Ni-hydrogen battery using this alloy as the negative electrode active material becomes easy.

The heat treatment method of the present invention can be applied to any hydrogen storage alloy for Ni-hydrogen batteries. A typical hydrogen storage alloy for Ni-hydrogen batteries is LaNi ₅ series or MmNi ₅ series.
(Mm is a misch metal that is a mixture of lanthanoid rare earth metals containing La as a main component) Hydrogen storage alloys and Laves phase ZrV ₂ -based hydrogen storage alloys. In LaNi ₅ or MmNi ₅ , a part of Ni is Co, Mn, Al, Fe, V, Cu, B, Mo, W,
It may be substituted with one or more metals such as Ta. In ZrV ₂ , part of Zr is one or two metals of Ti and Hf, and part of V is Ni, Mn, Fe, Co, Mo,
It may be substituted with one or more metals such as Cr, W and Al.

The hydrogen storage alloy can be manufactured by any conventionally known method. The alloy manufacturing method is, for example,
Ingot method in which alloy melted by high frequency induction heating is cast into a mold and cooled (in this case, the cooling rate is usually 10 K
/ sec or slower) or gas atomizing method,
Any method capable of solidification (quick solidification) at a cooling rate of 50 K / sec or more, such as a rotating electrode method or a roll quenching method, may be used. If necessary, the obtained hydrogen storage alloy may be ground in an inert gas atmosphere before heat treatment. The crushing may be performed after the heat treatment. Further, when the alloy is obtained in the form of powder as in the gas atomizing method, pulverization may not be necessary.

According to the present invention, the hydrogen storage alloy is placed in a hydrogen gas atmosphere having an oxygen concentration of 30 ppm or less or at an oxygen concentration of 30 p.
Heat treatment is performed in a rare gas atmosphere having a hydrogen concentration of 1 vol% or more and pm or less. He, Ar, Ne and Kr gases can be used as the rare gas, but the cheapest Ar gas is usually preferable. The reason for using such a gas is that the heat treatment atmosphere must be a reducing atmosphere in order to prevent the formation of an oxide film on the alloy surface. The most common nitrogen gas atmosphere as an inert gas atmosphere is that when N ₂ and H ₂ coexist, the catalytic action of the hydrogen storage alloy may generate NH _{3 and} poison the alloy. Not used in the method.

The oxygen concentration in the heat treatment atmosphere gas containing hydrogen is set to 30 ppm or less in order to prevent surface oxidation during the heat treatment. To get this effect, 30 p
An oxygen concentration of pm or less is required, and it is more effective if it is desirably 10 ppm or less.

When the heat treatment is carried out in a rare gas atmosphere, the atmosphere gas must contain at least 1 vol% hydrogen. This is to obtain the lowest hydrogen partial pressure needed to activate the alloy by hydrogen absorption during heat treatment and subsequent cooling. Desirably, if the hydrogen concentration is 3 vol% or more,
This effect is further enhanced.

The above heat treatment in the hydrogen-containing gas atmosphere is carried out by holding the alloy at a temperature of 550 to 1200 ° C. for 2 to 8 hours. This makes it possible to homogenize the alloy while preventing oxidation of the alloy surface during heat treatment. 10 K / s
In the ingot method of solidifying at a cooling rate of ec or less, the preferable heat treatment condition is 800 to 1100 ° C x 6 to 8 hours.

When the hydrogen storage alloy is manufactured by rapid solidification at a cooling rate of 50 K / sec or more (when a melting method such as a gas atomizing method or a rotating electrode method is adopted), strain during solidification cooling is removed. Therefore, it is preferable to perform heat treatment for the purpose of strain relief annealing. Therefore, in this case, it is preferable to carry out the heat treatment by maintaining the temperature at 550 to 950 ° C. for 2 to 5 hours. By heat treatment under such conditions, strain can be removed and the crystal grain size can be controlled without causing segregation. The more preferable heat treatment condition in this case is 7
00 to 900 ° C x 3 to 4 hours.

After the heat treatment in the hydrogen-containing gas atmosphere under the above-mentioned conditions, it is cooled to a temperature of 50 ° C. or less at a cooling rate of 20 K / min or less. The atmosphere during this cooling is also the same as the atmosphere during the heat treatment. In the hydrogen-containing gas atmosphere described above, the atmosphere gas may be changed during the heat treatment and during the cooling, but it is usually simple to perform the cooling in the atmosphere during the heat treatment.

During the heat treatment or during the cooling after the heat treatment, the alloy starts absorbing hydrogen and activation occurs. Although the amount of absorbed hydrogen increases as the temperature decreases, the hydrogen storage alloy has the property of generating intense heat when absorbing hydrogen. For this reason,
If the temperature is drastically lowered, a large amount of hydrogen is absorbed at one time, causing a sudden rise in heat generation, and the temperature rises above the desired heat treatment temperature, self-heating causes sintering, and self-heating causes the alloy composition to heat up. The problem of disassembly occurs. To avoid such heat generation, the cooling rate after heat treatment is 20 K
Slow cooling less than / min. The desired cooling rate is 10 K / min
It is the following.

The reason for cooling to 50 ° C. or lower by this slow cooling is as follows. In many hydrogen storage alloys, the hydrogen storage amount increases as the temperature decreases. It is desirable to store more hydrogen in order to activate the alloy. Many of the alloys used for Ni-hydrogen batteries have a hydrogen absorption equilibrium pressure of 1 to 5 atm or less at 50 ° C, and if cooled to 50 ° C or less, the hydrogen absorption equilibrium pressure of the alloy becomes about 1 to 5 atm. Hydrogen is absorbed until it reaches the temperature and the activation of the alloy proceeds. Therefore, it is necessary to cool it to 50 ° C or lower, preferably 40 ° C or lower. Further, if the alloy heat-treated in a hydrogen atmosphere is taken out into the atmosphere at a high temperature, an oxide film may be formed on the surface and the activation may be delayed. To prevent this, 50 ℃
It is necessary to cool to below, preferably to 40 ° C, and then take out to the atmosphere.

[0023]

EXAMPLES Table 1 shows the compositions of the alloys used in the examples. These alloys are flaky electrolytic Ni with a purity of 99.9%,
99.8% electrolytic Co, 99.9% shot-like Al, 99.8% plate-like electrolytic Mn, Ni-56.91% V mother alloy, 99.5% or more sponge-like Z
r, Mm with a rare earth metal purity of 99.8% or more (Misch metal)
(La = 28 wt%, Ce = 48 wt%, Nd = 18 wt%, Pr = 6 wt%)
75 kg / ch Ar gas atomization method using
(I), 0.1 kg / ch Ar arc button melting method (II) or 50 kg / ch high frequency vacuum induction heating melting method (III). Table 2 shows the cooling rate of the alloy in each melting method.

[0024]

[Table 1]

[0025]

[Table 2]

The resulting donations were heat treated as described in each example. The heat-treated match gold was crushed to 74 μm or less and 32 μm or more in an Ar gas atmosphere, and a binder (polyvinyl alcohol 5% aqueous solution) was added and kneaded. Nickel foam metal porous body made of pasty alloy powder
(For example, Sumitomo Electric Celmet) was filled and dried, and then pressurized at a pressure of 1.5 ton / cm ³ to support the powder in Celmet to form a negative electrode. The amount of alloy carried at this time was about 12 g.

A commercially available Ni positive electrode plate of nominal 2000 mA was used for 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 obtain a battery of nominal 2000 mA. . This battery was hermetically sealed in a C-type case to obtain a battery used for the test. This battery is a positive electrode capacity regulated battery having a large negative electrode capacity.

The atomized ingot material was crushed as described above in an Ar gas atmosphere after heat treatment.
A different battery was constructed by using both the (melting method IA) and the atomized as-melted powder of 74 to 32 μm that had not been pulverized (melting method IB).

(Example 1) In order to prove the validity of the heat treatment condition range of the present invention, various conditions shown in Table 4 were used by using match gas as the gas A and gas F (conventional gas) shown in Table 3. Heat treatment was performed below, and the batteries were configured as described above, and the ease of initial activation was compared by an initial activation test. The heat treatment was performed in a closed atmosphere furnace filled with A gas or F gas at the temperature and holding time shown in Table 4, then cooled to 20 ° C. at a cooling rate of 20 K / min, and then taken out of the furnace.

The initial activation test was carried out at 25 ° C. for the assembled battery.
After 3 hours of charging at 1000 mA at 2000 mA, a repeated charging / discharging test was conducted in which the terminal voltage was discharged to 0.9 V at 2000 mA. The discharge capacity at the 1st time and the discharge capacity at the 10th time were measured, and the ratio (1st discharge capacity / 10th discharge capacity × 10
0%) was used to evaluate the ease of initial activation. The results are also shown in Table 4.

[0031]

[Table 3]

[0032]

[Table 4]

From the results shown in Table 4, the following facts became clear. No. 1, No. 2, No. 7, heat treatment conditions are outside the scope of the present invention,
In No. 12, even when A gas having a composition within the range of the present invention was used, the initial discharge capacity (first cycle discharge capacity) was 75% or less at the 10th cycle, and the initial activation characteristics were not good. It was

The alloy materials obtained by any of the melting methods were heat treated under the heat treatment conditions within the scope of the present invention using A gas, and the initial discharge capacity was 75 at the 10th cycle.
% Or more, the initial activation characteristics were improved.

Ar gas atomizing method (melting method IA and I
B) or button melting method (melting method II)
For alloy materials that have been rapidly cooled at a cooling rate of 0 K / sec or more, (55
When heat treatment is performed within the range of (0 to 950 ° C) x (2 to 5 hours),
The initial discharge capacity was 90% or more at the 10th cycle, and the initial activity was particularly excellent.

With respect to the gas atomized ingot material, when heat-treated under the conditions within the scope of the present invention, a powder having sufficient initial activity was obtained even if the battery was constructed without being ground (melting method IB). Therefore, when melted by the gas atomization method, it is advantageous in that the crushing step can be omitted.

(Example 2) In order to investigate the influence of the heat treatment gas composition, gases having compositions A to F shown in Table 3 were used.
Heat treatment was performed at 4 ° C for 4 hours, the temperature was cooled to 20 ° C at a cooling rate of 20 K / min, and the product was taken out of the heat treatment furnace. Table 5 shows the results of investigating the initial activity of the heat-treated alloy material obtained.

[0038]

[Table 5]

From the results shown in Table 5, the following facts became clear. When heat-treated using B gas with an oxygen concentration of 35 ppm, no significant difference is observed compared to the case of using normal Ar gas (comparison between No. 14 and No. 18). Even if the heat treatment is performed under the condition that the hydrogen concentration in the gas is 1 vol% or less, no significant improvement effect is observed (No. 15 and No. 18).

Example 3 For alloys having the alloy compositions shown in Table 1, the gas A shown in Table 2 was used to obtain 900 ° C. × 4 hr.
The effects of the cooling rate and the temperature at which the alloy was removed from the furnace on the initial activation characteristics of the alloy during the heat treatment of were investigated. After the heat treatment, it was cooled to 40 ° C at a cooling rate of 20 K / min and then taken out of the heat treatment furnace. The test results are shown in Table 6.

[0041]

[Table 6]

From the results shown in Table 6, the following facts became clear. When the cooling rate is 20 K / min or less and the temperature is 50 ° C or less, the gas atomized material has an initial activity of 90% or more, and the high frequency induction heating melting material has an initial activity of 85% or more.

When the cooling rate was higher than 20 K / min (No. 23) or the take-out temperature was higher than 50 ° C. (No. 28), the initial activity was 75% or less. When X-ray diffraction was performed on both of these, rare earth metals (La, Ce,
The diffraction line of the oxide of Nd) was observed, and it is considered that the oxidation reduced the initial activity of this alloy.

[0044]

According to the method for heat treating a hydrogen storage alloy of the present invention, in addition to preventing oxidation of the alloy surface during heat treatment, hydrogen absorption occurs during subsequent cooling to activate the alloy, so that Ni-hydrogen is used. The initial activity is very high when the battery is constructed. Specifically, the initial discharge capacity in the first cycle is at least 75% of the discharge capacity in the 10th cycle (predetermined discharge capacity).
That is all. In particular, when the alloy rapidly solidified at a cooling rate of 50 K / sec or more is heat treated under the condition of (550 to 950 ℃) × (2 to 5 hours), the initial discharge capacity is very high, 90% or more at the 10th cycle. Initial activity is obtained. Therefore, the number of times of charging / discharging required for initial activation of the Ni-hydrogen battery can be reduced, and the productivity of the battery can be significantly improved.

Front page continuation (72) Sumio Toyosumi 4-53-3 Kitahama, Chuo-ku, Osaka City Sumitomo Kinen Kogyo Co., Ltd. (72) Yuuki Takeshita 4-53-3 Kitahama, Chuo-ku, Osaka Sumitomo Kinen Industry Co., Ltd.

Claims (2)

[Claims] 1. A hydrogen storage alloy in a hydrogen gas atmosphere having an oxygen concentration of 30 ppm or less, or in a rare gas atmosphere having an oxygen concentration of 30 ppm or less and a hydrogen concentration of 1 vol% or more at a temperature of 550 to 1200 ° C. for 2 to 8 hours. A method for heat treatment of a hydrogen storage alloy for a Ni-hydrogen battery, which is characterized by cooling in the atmosphere at a cooling rate of 20 K / min or less to a temperature of 50 ° C or less after the holding. 2. A hydrogen storage alloy solidified from a molten state at a cooling rate of 50 K / sec or more in a hydrogen gas atmosphere with an oxygen concentration of 30 ppm or less, or with an oxygen concentration of 30 ppm or less and a hydrogen concentration of 1
2 to 550 to 950 ℃ under noble gas atmosphere of vol% or more
After holding for 5 hours, the atmosphere is cooled to a temperature of 50 ° C. or less at a cooling rate of 20 K / min or less.
-Heat treatment method for hydrogen storage alloys for hydrogen batteries.