JPH0931573A - Hydrogen storage alloy and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hydrogen storage alloy electrode with high capacitance having a long service life by incorporating specified amounts of misch metal, Ni, Co, Mn, Al, Mo, Cr and La into an alloy to improve deterioration in its capacitance after a charging-discharging cycle. SOLUTION: In the formula, Mm denotes misch metal and X denotes Mo and/or Cr: 3.5<a<4.0, 0.001<e<0.3 and 4.8 <=a+b+c+d<5.4. The content of La in Mm is regulated to 20 to 80wt.%. The metallic raw materials of each component are melted and mixed, which is poured on a flat board or a mold cooled by water to rapidly solidify the molten body. The compsn. of MmNi3.55 Co0.75 Mn0.4 Al0.3 Mo0.01 or MmNi3.55 Co0.75 Mn0.4 Al0.3 Cr0.01 is preferable. In the case the content of La in Mm is less than 20wt.%, its capacitance and cycle characteristics deteriorate. In the case of >80wt.%, the cycle characteristics deteriorate though the capacitance is high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy used as a negative electrode of a nickel-hydrogen storage battery and a method for producing the same, and more particularly to a hydrogen storage alloy having improved cycle life of a nickel-hydrogen storage battery and a method for producing the same.

[0002]

2. Description of the Related Art A nickel-hydrogen storage battery using a hydrogen storage alloy as a negative electrode and nickel hydroxide as a positive electrode has recently been rapidly spread as a power source for small electronic devices. Conventionally, nickel-hydrogen storage batteries have solved many problems in order to replace the widely used nickel-cadmium storage batteries with nickel-hydrogen storage batteries because of their pollution-free and high capacity (for example, “Ota et al. Electrochemistry and Industrial Physical Chemistry 60, No. 8 (1992), Nos. 689-692
page").

However, sufficient performance has not yet been obtained to meet the higher requirements of electronic equipment. One of the requirements is to obtain a larger initial capacity, and the other requirement is to improve the cycle life so that the capacity is not lowered so much even after long-term charging and discharging.

Since these performances are mainly determined by the properties of the hydrogen storage alloy of the negative electrode, various attempts have been made to improve the hydrogen storage alloy. However, increasing the capacity and the cycle life give conflicting results in adjusting the alloy composition. In many cases, it was in an unsolved state (for example, "Sakai et al., Osaka Industrial Technology Laboratory, Quarterly Report 42 [2] (1991), Nos. 23-55).
page").

At present, the hydrogen storage alloy used in nickel-hydrogen storage batteries that have been put into practical use is the AB ₅ type hydrogen storage alloy, and the alloy composition of misch metal (Mm), nickel, cobalt, manganese, and aluminum is adjusted. By doing so, the capacity and the life are balanced, and a capacity of about 260 mAh / g is obtained as a hydrogen storage alloy (“Yuasa et al., National Technical.
Report, Vol. 37, no. 1, Feb. 19
91, pp. 44-51 ").

However, in order to meet the high requirements for applications such as electronic devices and electric vehicles, at least 320 alloys are required.
There is a need for a hydrogen storage alloy as a negative electrode having a capacity of mAh / g or more, preferably 350 mAh / g or more, and having less capacity deterioration even after several 100 charge / discharge cycles. Has not been obtained yet.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a hydrogen storage alloy having an initial capacity of 320 mAh / g or more and having a small capacity deterioration even after charge / discharge cycles when used as a negative electrode of a nickel-hydrogen battery. It is an object of the present invention to provide a hydrogen storage alloy capable of improving the life and a method for producing the same.

[0008]

The above object of the present invention is to use molybdenum and / or chromium as the sixth component in addition to the alloy components of misch metal, nickel, cobalt, manganese, and aluminum. This is achieved by specifying the compounding ratio and keeping the lanthanum content in the misch metal constant.

That is, the present invention provides the following formula:

Embedded image (In the formula, Mm is misch metal, X is molybdenum and / or chromium, 3.5 <a <4.0, 0.001 <e
<0.3, 4.8 ≦ a + b + c + d <5.4), wherein the lanthanum content in the Mm is 20 to 80% by weight.

The hydrogen storage alloy of the present invention has the following formula:

Embedded image Indicated by

Here, Mm represents misch metal, and Mm
20-80% by weight of lanthanum, preferably 30-7
It is necessary to contain 5% by weight. When the lanthanum content in Mm is less than 30% by weight, the discharge capacity and cycle characteristics are deteriorated. The lanthanum content in Mm is 80% by weight.
When it exceeds, the discharge capacity is in a high range, but the cycle characteristics are deteriorated.

Further, nickel needs to be in the range of a = 3.5 to 4.0 (molar ratio) with respect to the misch metal. When a is less than 3.5, the discharge capacity and cycle characteristics deteriorate. When a exceeds 4.0, the discharge capacity and cycle characteristics deteriorate.

X represents molybdenum and / or chromium, and this molybdenum and / or chromium needs to have e = 0.001 to 0.3 (molar ratio) with respect to the misch metal. When e is less than 0.001, the discharge capacity is in a high range, but the cycle characteristics are deteriorated. Moreover, when e exceeds 0.3, both the discharge capacity and the cycle characteristics deteriorate.

The total amount (a + b + c + d) of nickel, cobalt, manganese and aluminum is a + b + c + d = 4.8 to less than 5.4 (molar ratio) with respect to the misch metal. When a + b + c + d is less than 4.8, both the discharge capacity and the cycle characteristics are deteriorated. Further, even if a + b + c + d is 5.4 or more, both the discharge capacity and the cycle characteristics are deteriorated.

The method for producing a hydrogen storage alloy of the present invention is
The metal raw material of each component constituting the hydrogen storage alloy represented by the above formula is melt-mixed, and each component composition is 3.5 <a <4.
0, 0.001 <e <0.3, 4.8 ≦ a + b + c + d
<5.4, lanthanum content in Mm is 20 to 80% by weight
A uniform melt is prepared as follows, and then the melt is poured onto a water-cooled flat plate or into a mold for rapid solidification.

[0016]

EXAMPLES The present invention will be specifically described below based on examples. Examples 1 to 3 and Comparative Examples 1 to 3 Commercially available misch metal (La content 70
% By weight), nickel, cobalt, manganese, aluminum, molybdenum, or chromium metal raw materials, and after melting in an argon inert atmosphere, high frequency melting furnace,
It was poured into a water-cooled mold made of copper and rapidly solidified to prepare hydrogen storage alloys having various compositions shown in Table 1.

The hydrogen storage alloy thus obtained is
Mechanically coarsely pulverized to a particle size of 500 μm or less and using a PCT (hydrogen storage property) measuring device, the hydrogen storage capacity (PC
The T capacity) was measured and the results are shown in Table 1.

Next, a test cell was prepared to measure the capacity of the hydrogen storage alloy electrode and the charge / discharge cycle life. First, the coarsely crushed hydrogen storage alloy was further -45.
After finely pulverized to μm, the mixture was mixed with a binder and nickel powder, and then pressure-molded into a pellet to prepare an electrode. A charge / discharge cycle test was performed in a potassium hydroxide electrolyte using nickel hydride as the counter electrode. Table 1 shows the electrode capacity after 200 cycles of charge and discharge.

[0019]

[Table 1]

[0020] As apparent from Table 1, nickel other than molybdenum or chromium, cobalt, manganese, total amount and Misch molar ratio between the metal of the aluminum, i.e. 1 in the AB _5: largely shifted in the ratio of 5 either side However, the electrode capacity after 200 cycles becomes small. That is, the cycle life becomes short. Therefore, it is understood that a hydrogen storage alloy having a + b + c + d of 4.8 or more and less than 5.4 is a suitable electrode.

Examples 4 to 6 and Comparative Examples 4 to 6 In Table 2, the composition ratio of nickel was changed to prepare a hydrogen storage alloy and a test cell in the same manner as in Example 1, and the PCT capacity and 2 were prepared.
The electrode capacity after 00 cycles was measured, and the results are shown in Table 2. Here, the composition other than nickel is MmNi _a Co _4.3-a M
n _0.4 Al _0.3 (Mo, Cr) _0.1 and the lanthanum content in the misch metal is 33% by weight.

[0022]

[Table 2]

As is clear from Table 2, when nickel is too small, cobalt increases, so the discharge capacity decreases, and when nickel is excessive, the equilibrium pressure for hydrogen absorption / desorption increases, so the discharge capacity is also high. And the cobalt content decreases, the corrosion resistance decreases, and the cycle life deteriorates. Therefore, nickel is 3.5 <a <4.0.
It can be seen that a hydrogen storage alloy within the range is a suitable alloy electrode.

Examples 7 to 14 and Comparative Examples 7 to 10 The hydrogen storage alloy and the test cell were prepared in the same manner as in Example 1 except that the composition of molybdenum and / or chromium was changed as shown in Table 3, and the PCT capacity and after 200 cycles were obtained. The electrode capacity was measured and the results are shown in Table 3. Here, the composition other than molybdenum and chromium is MmNi _3.55 Co _0.75 Mn _0.40 Al _0.30 X
a _e, lanthanum content in mischmetal was 33 weight%.

[0025]

[Table 3]

As can be seen from Table 3, molybdenum and /
Further, it can be seen that the inclusion of chromium has a remarkable effect particularly in increasing the electrode capacity, that is, improving the cycle life. From this result, it is found that the content of molybdenum and / or chromium is preferably 0.001 <e <0.3. If the content of molybdenum and / or chromium is small, there is no effect, and if the content is excessive, segregation of molybdenum and chromium deteriorates corrosion resistance and cycle life.

Examples 15 to 19 and Comparative Examples 12 to 14 In Table 4, the lanthanum content in misch metal was changed,
A hydrogen storage alloy and a test cell were prepared in the same manner as in Example 1, the PCT capacity and the electrode capacity after 200 cycles were measured, and the results are shown in Table 4. The composition of the hydrogen storage alloy used here is MmNi _3.55 Co _0.75 Mn _0.40 Al _0.30 (Mo,
Cr) was set to _0.10 .

The lanthanum content in the misch metal is adjusted by adding lanthanum metal when the lanthanum is insufficient in the used misch metal, and by adding other cerium, neodymium, praseodymium or the like when the lanthanum is excessive. I did.

[0029]

[Table 4]

From Table 4, it can be seen that the PCT capacity increases as the lanthanum content in the misch metal increases, and the electrode capacity after 200 cycles also increases. Comparative Example 1
As shown in FIG. 4, in the conventional alloy composition containing no molybdenum and / or chromium, it was said that if the amount of lanthanum was increased, the electrode capacity after the cycle was decreased and it was not practical. However, as is clear from Examples 15 to 19, it can be seen that even if the amount of lanthanum is increased, the electrode capacity after the cycle is kept high, and an alloy electrode having a higher capacity and a longer cycle life can be obtained.

Examples 20-21 Table 5 shows the composition Mm (La content 33% by weight) Ni _3.55 Co.
Presence or absence of manganese segregation obtained when solidifying an alloy melt of _0.75 Mn _0.40 Al _0.30 Mo _0.10 by changing the cooling method,
The metal composition and the discharge capacity after 200 cycles are shown.

[0032]

[Table 5]

As shown in Table 5, in Examples 20 to 21 using the rapid solidification method which is the manufacturing method of the present invention, the corrosion resistance is not deteriorated by the segregation of manganese, and columnar crystals are obtained. It was found that an electrode having a long cycle life with little cycle deterioration in was obtained.

[0034]

EFFECTS OF THE INVENTION The hydrogen storage alloy of the present invention improves the reduction in discharge capacity after charge / discharge cycles of the misch metal hydrogen storage alloy electrode, which is a drawback of the prior art, and has a higher capacity and a longer life. Alloy electrodes can be provided.

Claims (5)

[Claims] 1. The following formula; (In the formula, Mm is misch metal, X is molybdenum and / or chromium, 3.5 <a <4.0, 0.001 <e
<0.3, 4.8 ≦ a + b + c + d <5.4), wherein the lanthanum content in the Mm is 20 to 80% by weight. 2. The hydrogen storage alloy according to claim 1, wherein a + b + c + d = 5.0. 3. The formula is MmNi _3.55 Co _0.75 Mn
_0.4 Al _0.3 Mo _0.01 or Mm Ni _3.55 Co _0.75 Mn
The hydrogen storage alloy according to claim 1, which is _0.4 Al _0.3 Cr _0.01 . 4. The lanthanum content in Mm of the alloy is 3
The hydrogen storage alloy according to claim 1, which is 0 to 75% by weight. 5. A metal raw material of each component constituting the hydrogen storage alloy represented by the formula of claim 1 is melt-mixed, and each component composition is 3.5 <a <4.0, 0.001 <. e <0.
3,4.8 ≦ a + b + c + d <5.4, a uniform melt was prepared so that the lanthanum content in Mm was 20-80 wt%, and then the melt was placed on a water-cooled flat plate or A method for producing a hydrogen storage alloy, which comprises pouring into a mold and rapidly solidifying.