JP2001200324A - Hydrogen storage alloy and nickel hydrogen secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrogen storage alloy with a high capacity improved in high rate discharging characteristics while suppressing pulverization and furthermore exhibiting cycle life characteristics equal to the conventional ones or more while maintaining its corrosion resistance even in the case the content of cobalt is reduced. SOLUTION: In a hydrogen storage alloy having an LnNi5 series (wherein Ln denotes La-rich misch metal) as a CaCu5 type crystal structure as the main phase, the content of La in Ln is 70 to 100 wt.%, also, the substitution ratio of Fe to Ni in the alloy is controlled to the atomic ratio of 0.015 to 0.40, and also, Mg or/Ca is contained in the alloy by 0.1 to 1 wt.%. Moreover, in the above alloy, the substitution ratio of Co to Ni in the alloy is controlled to the atomic ratio of 0.50 or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a hydrogen storage alloy, and more particularly to a hydrogen storage alloy for a negative electrode used in a nickel-metal hydride secondary battery.

[0002]

2. Description of the Related Art In a nickel-metal hydride secondary battery, as a hydrogen storage alloy used for a negative electrode, a misch metal (hereinafter, referred to as "Mm") and a nickel-based alloy in which a part of nickel is replaced by various elements have been used. Widely used. Here, Mm is Ce, Pr, Nd, in addition to La.
It contains one or more selected from Sm and other rare earth elements. Among them, alloys containing cobalt have a relatively large hydrogen storage capacity, are hard to be pulverized when storing hydrogen, are excellent in corrosion resistance in alkali, and are used when used for the negative electrode of nickel-metal hydride secondary batteries. It has been found that there is an effect of extending the life of the battery. On the other hand, it has been found that a smaller cobalt content is better for improving the high rate discharge characteristics. The reason for this is that the reduction in the cobalt content promotes micronization and increases the surface area per weight. However, when the cobalt content is simply reduced, the high-rate discharge characteristics are improved, but the cycle life of the battery is reduced. This is because, as mentioned above, in addition to the promotion of micronization,
This is because the corrosion resistance of the alloy surface is reduced, so that the corrosion of the alloy surface progresses, the negative electrode takes in the electrolyte solution in the battery, and a dryout occurs to lower the battery capacity.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, which suppresses fine powdering, improves high-rate discharge characteristics while improving corrosion resistance, and reduces the cobalt content. The present invention provides a high-capacity hydrogen storage alloy that exhibits cycle life characteristics that are at least comparable to those of the prior art even when it is used.

[0004]

According to the present invention, an alloy contains a relatively large amount of La and a relatively small amount of Mg or Ca, which is an alkaline earth metal, in excess of impurities, and further contains Fe in the alloy. To improve the high-rate discharge characteristics despite suppressing pulverization while maintaining a high capacity, and suppress pulverization even when the cobalt content is reduced, and improve corrosion resistance. To find something to do. Specifically, the present invention relates to a hydrogen storage alloy having a main phase of an LnNi ₅ system (where Ln represents La-rich Mm) having a CaCu ₅ type crystal structure.
n is 70 to 100% by weight, and the alloy contains Mg and / or Ca together with Fe in the alloy;
And a hydrogen storage alloy containing 0.1 to 1% by weight of Mg and / or Ca in the alloy. Further, preferably, in a hydrogen storage alloy having a main phase of LnNi ₅ having a CaCu ₅ type crystal structure, the La content in Ln is 70 to 1
00% by weight, and Mg in the alloy together with Fe.
And / or Ca, further containing Co, an Fe substitution ratio with respect to Ni in an atomic ratio of 0.015 to 0.40, a Co substitution ratio with respect to Ni in an atomic ratio of 0.50 or less, And / or a hydrogen storage alloy containing 0.1 to 1% by weight of Ca in the alloy. Also, the present invention
The present invention relates to a nickel-hydrogen secondary battery using these hydrogen storage alloys for electrodes.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The AB ₅ type hydrogen storage alloy of the present invention contains 0.1 to 1.0 weight% of Mg and / or Ca in the alloy in order to improve high rate discharge characteristics while suppressing pulverization. % Of the hydrogen storage alloy in order to increase the hydrogen storage amount and control the hydrogen equilibrium pressure.
By setting the La content in m to 70 to 100% by weight, and preferably 75 to 95% by weight in terms of life, compared to a conventional alloy, it has a higher capacity, suppresses pulverization, and has a higher By improving the rate discharge characteristics, and further including the Fe substitution ratio with respect to Ni in the alloy of 0.015 to 0.40 atomic ratio, even in a hydrogen storage alloy having a small cobalt content, the pulverization resistance is improved, The corrosion resistance of the alloy can be improved. Further, the AB ₅ type hydrogen storage alloy of the present invention has the remaining constituents, and the A side contains La alone or one or more selected from Ce, Pr, Nd, Sm and other rare earth elements together with La; Is Ni alone or N
In addition to i, one or more selected from Co, Al, Mn, Cu, Cr and other transition metals are included. A side is preferably La alone or at least Ce together with La.
And the B side is preferably Ni,
It is a combination of Co, Mn and Al. B in the alloy
A / A may have a CaCu ₅ -based structure, but preferably B / A = 5 to 5.5 (not including Mg and Ca).
It is good to More preferably, it should be B-rich.
The AB ₅ type hydrogen storage alloy used in the present invention is preferably a hydrogen storage alloy having a CaCu ₅ type crystal structure as a main phase. CaC
A hydrogen storage alloy having a u ₅ type crystal structure as a main phase refers to an alloy phase which shows a CaCu ₅ type in a diffraction pattern by XRD, while partially observing a segregation phase in cross-sectional structure observation.

In the hydrogen storage alloy of the present invention, the Fe substitution ratio with respect to Ni in the alloy is 0.015 to 0.40 atomic ratio, and the alloy contains 0.1 to 1% by weight of Mg and / or Ca. It has characteristics that When the Mg or Ca content is less than 0.1% by weight, the effect of suppressing pulverization is small.
If it exceeds 0% by weight, the hydrogen storage amount will be too low.
Preferably, the substitution ratio of Fe is 0.05 to 0.3. When the Fe atomic ratio is less than 0.015, the effect of improving corrosion resistance is small, and when it exceeds 0.40, the hydrogen storage amount is excessively reduced.

Further, if Mg or Ca is contained only in an amount of 0.1 to 1.0% by weight, the equilibrium pressure at the time of storing and releasing hydrogen is increased.
The content of a was adjusted to 70 to 100% by weight. In the present invention, it is particularly preferable to add Mg.

[0008] Further, the present invention, as described above, contains a relatively small amount of Mg or Ca and partially replaces Ni with Fe, so that Ni in alloys that could not be obtained before can be obtained.
A long life was achieved when the substitution ratio of Co with respect to was 0.50 atomic ratio or less. That is, even when the cobalt content is reduced, pulverization is suppressed and corrosion resistance is improved.
In addition, the total substitution elements other than Ni and Co are preferably not more than 1.0 atomic ratio in terms of substitution ratio with respect to Ni.

The hydrogen storage alloy of the present invention can be obtained as follows. A predetermined amount of each element is weighed, and in an induction melting furnace, an inert gas such as Ar gas (200 to 1500) is used.
(Torr). At this time, when an element having a high vapor pressure such as Mg or Ca is added, an alloy with another element constituting the alloy is used. After melting, an ingot is cast at 1300 to 1600 ° C. in an iron mold or the like. In addition, when particularly necessary, it is preferable to use an inert gas such as Ar gas (600 to 15
(00 Torr) at 800 to 1200 ° C. for 5 to 20 hours. In the present invention, an alloy can be obtained by the above-mentioned melting casting, but the alloy may be produced by a roll quenching method or an atomizing method. The hydrogen storage alloy prepared by the above method is pulverized in an inert atmosphere such as Ar by a pulverizer such as an impact or attrition pulverizer or a jet mill to have an average particle size of 4 to 70 μm. A hydrogen storage alloy can be obtained.

The hydrogen-absorbing alloy powder thus obtained is kneaded into a paste by a known method, for example, using a binder such as polyvinyl alcohol and methyl cellulose, or a binder such as PTFE, polyethylene oxide or polymer latex. An electrode can be formed by filling a three-dimensional conductive support such as a nickel foam or a nickel fiber, or a two-dimensional conductive support such as a punching metal. The amount of the binder used is 100% by weight of the alloy.
It is preferable to use 0.1 to 20% by weight. If necessary, a conductive auxiliary such as carbon graphite, Ni, or Cu powder may be added in an amount of 0.1 to 10% by weight based on the alloy. An alkaline battery using the hydrogen storage alloy of the present invention as a negative electrode has a long cycle life and excellent high rate discharge characteristics and low temperature discharge characteristics even with low cobalt.

[0011]

Hereinafter, the present invention will be described in detail with reference to Examples.
The present invention is not limited to this. Examples 1 to 7, Comparative Examples 1 to 5 Mm, or each element of La, Ce, Pr, and Nd, and N
The elements i, Co, Mn, Al, and Fe, and Mg were weighed so that the compositions shown in Tables 1 and 2 were obtained. At this time, Mg
Was added using a Mg-Ni alloy. Each of the ingots was obtained by heating and melting in a high frequency melting furnace and casting in an iron mold. The ingot was heat-treated at 1050 ° C. for 6 hours in an Ar atmosphere, and the average particle size was 50 μm by a pulverizer.
m to obtain a hydrogen storage alloy powder. When the alloy powder was measured by XRD, it showed a CaCu ₅ type crystal structure.

An aqueous solution of 3% by weight of polyvinyl alcohol (average degree of polymerization: 2,000, saponification degree: 98 mol%) is mixed with 2.5 g of a 10 wt% powder to form a paste. 30 in the body
After filling with vol%, drying and pressure molding, the thickness is 0.5-1.0
mm electrode plate was prepared, and then a lead wire was attached to obtain a negative electrode. Using a sintered electrode for the positive electrode, the negative electrode is bonded to the negative electrode via a polypropylene separator, and 6N-KOH
The battery was fabricated by immersion in an electrolyte.

First, the obtained battery was charged at 120 ° C. at 20 ° C. at 0.3 C with respect to the capacity of the negative electrode, and after resting for 30 minutes,
The battery was discharged at 0.2 C until the battery voltage reached 0.6 V. The maximum discharge capacity when this cycle was repeated 20 times was defined as the "capacity" of the alloy. 120% at 0.3C afterwards
The capacity discharged at 2.0 C after charging was defined as “high-rate discharge capacity”. After that, in order to observe the progress of the pulverization, the electrode was disassembled, the alloy powder was separated from the current collector with an ultrasonic horn in water, and the particle size distribution after charge / discharge was measured with a microtrack, and averaged. A particle size D of ₅₀ μm was obtained. The “micronization maintenance rate” was determined by the following equation. Micronization maintenance rate (%) = (alloy particle size after charge / discharge / alloy particle size before charge / discharge) × 100 The results are shown in Tables 1 and 2. Incidentally, D ₅₀ is, when measuring the particle size distribution, in the cumulative frequency when detecting the individual particle size,
The particle diameter at which the value obtained by cumulative addition from the small-diameter particles corresponds to 50% of the entire distribution is defined as D ₅₀ .

Corrosion resistance test of alloy powder 3 g of alloy powder was immersed in 30 ml of 6N-KOH,
After filtering, washing and drying, the integrated intensity of the peak of the rare earth hydroxide (XRD: 201 plane) by XRD was determined. Further, the corrosion resistance was calculated by the following equation. Corrosion resistance (%) = (integral strength of Mg, Fe-containing alloy / integral strength without Mg, Fe) x 100

Cycle life After confirming the high rate discharge capacity, at 20 ° C., the capacity of the negative electrode was reduced to 0.
After charging for 30% at 3C and resting for 30 minutes, a charge / discharge cycle of discharging at 0.2C until the battery voltage becomes 0.6V is 20
After 0 cycles, the discharge capacity retention rate was determined by the following equation. Retention rate (%) = (discharge capacity after 200 cycles / discharge capacity after 20 cycles) x 100

Tables 1 and 2 show the results. "Lm" in Table 1
All have a composition of 80% by weight of La, 12% by weight of Ce, 4% by weight of Pr and 4% by weight of Nd. The compositions on the B side in Table 2 are all Ni _4.08 Co _0.40 Mn _0.37 Al _0.30 Fe _0.10
It is. "Corrosion resistance" and "micronization maintenance rate" in Tables 1 and 2
Represents Comparative Example 2 as 100. Table 2 (Examples 6 to
According to the result of 8), when La is less than 70% by weight, the corrosion resistance is maintained but the pulverization proceeds, so that the cycle life is reduced. Further, since the discharge capacity also decreases, it is considered that the La content is preferably 70% by weight or more.

[0017]

According to the present invention, when used as a negative electrode of an alkaline storage battery, the hydrogen storage alloy of the present invention can increase the capacity of the battery, improve the high rate discharge characteristics, and furthermore, despite the low cobalt, Since the pulverization can be suppressed and the corrosion resistance can be improved, the cost of the battery can be reduced.

[0018]

[Table 1]

[0019]

[Table 2]

Claims (3)

[Claims] 1. LnNi ₅ having a CaCu ₅ type crystal structure
System (where Ln represents La-rich misch metal)
A La content in the Ln is 70 to 100% by weight, and the hydrogen storage alloy contains Mg and / or Ca together with Fe, and the Fe substitution ratio with respect to Ni is atomic. 0.015 to 0.40 in ratio,
A hydrogen storage alloy containing Mg and / or Ca in an amount of 0.1 to 1% by weight in the hydrogen storage alloy. 2. The composition according to claim 1, further comprising Co, wherein C is
The hydrogen storage alloy according to claim 1, wherein the substitution ratio of o is 0.50 or less in atomic ratio. 3. A nickel-hydrogen secondary battery using the hydrogen storage alloy according to claim 1 for an electrode.