JP5850493B2 - Hydrogen storage alloy and nickel metal hydride secondary battery using this hydrogen storage alloy - Google Patents

Description

  The present invention relates to a hydrogen storage alloy and a nickel metal hydride secondary battery using the hydrogen storage alloy.

  Nickel metal hydride secondary batteries are used in various applications such as various portable devices and hybrid electric vehicles because they have higher capacity and better environmental safety than nickel cadmium secondary batteries. It has become.

The hydrogen storage alloy used in the negative electrode of the nickel-hydrogen secondary batteries, for example, rare earth -Ni-based hydrogen storage alloy having a structure as a main phase an AB ₅ phase, i.e., LaNi ₅ type hydrogen absorbing alloy is used in general Has been.
However, this hydrogen storage alloy does not necessarily have sufficient hydrogen storage capability, and it has been difficult to increase the capacity in the case of a nickel metal hydride secondary battery using this hydrogen storage alloy as a negative electrode. For this reason, in order to further increase the capacity of the nickel hydrogen secondary battery, it is desired to improve the hydrogen storage capacity of the hydrogen storage alloy.

  Here, as a hydrogen storage alloy with improved hydrogen storage capacity, for example, a hydrogen storage alloy disclosed in Patent Document 1 is known. This hydrogen storage alloy is a rare earth-Ca-Mg-Ni hydrogen storage alloy in which a part of the rare earth element contained in the rare earth-Ni system hydrogen storage alloy is replaced with Mg and Ca. In this rare earth-Ca-Mg-Ni-based hydrogen storage alloy, Ca, which is an element having a relatively large atomic radius, exists, so that a large gap between adjacent elements, that is, a gap in the crystal lattice can be secured, More hydrogen can be stored in these gaps. For this reason, rare earth-Ca-Mg-Ni-based hydrogen storage alloys can store a larger amount of hydrogen than conventional rare-earth-Ni-based hydrogen storage alloys. Contributes to higher capacity.

JP-A-11-217643

By the way, the rare earth-Ca-Mg-Ni-based hydrogen storage alloy of Patent Document 1 has an AB ₃ phase as a main phase. This main phase is likely to be distorted in the crystal structure due to the expansion of the crystal lattice during hydrogen storage. Then, when the occlusion and release of hydrogen are repeated and the residual amount of strain increases, the crystal lattice itself becomes a form incapable of releasing hydrogen, making it difficult to release hydrogen from the alloy.

  On the other hand, the large gap in the crystal lattice caused by Ca tends to stabilize the stored hydrogen. Stabilized hydrogen is difficult to release from the alloy, so as hydrogen is stored and released repeatedly, hydrogen that cannot be released accumulates in the alloy, and the amount of hydrogen that can be released gradually decreases. .

  As described above, although the rare earth-Ca-Mg-Ni-based hydrogen storage alloy can increase the amount of hydrogen stored, it is difficult to release the stored hydrogen if the storage and release of hydrogen are repeated. . That is, deterioration due to repeated occlusion and release of hydrogen is likely to occur. Therefore, in a battery using this hydrogen storage alloy as a negative electrode element, the capacity can be increased but the cycle life is short.

  The present invention has been made to solve the above-described problems in a nickel-metal hydride secondary battery including a rare earth-Ca-Mg-Ni-based hydrogen storage alloy as a negative electrode element. An object of the present invention is to provide a hydrogen storage alloy capable of achieving both higher capacity and improved cycle life characteristics, and a nickel hydride secondary battery using this hydrogen storage alloy.

In order to achieve the above object, according to the present invention, a rare-earth-Ca-Mg-Ni-based hydrogen storage alloy comprising an A ₂ B ₇ phase as a main phase, wherein the A site of the main phase is a rare earth element consists Ca and Mg, B site of the main phase consists of Ni, the rare earth element is at one or more selected Pr, and Nd and Sm, the number of atoms of the element of the a site And the ratio of the number of atoms of the B site element to B / A, this B / A satisfies the relationship of 3.4 <B / A ≦ 3.6, and the Ca content is Provided is a hydrogen storage alloy characterized by being 20 atomic% or more and 30 atomic% or less with respect to the total number of atoms of the rare earth element, the Ca and the Mg (Claim 1).

Preferably, the crystal structure of the main phase is one of Ce ₂ Ni ₇ type and Gd ₂ Co ₇ type (Claim 2).

  Moreover, according to this invention, the nickel hydride secondary battery provided with the negative electrode element containing the hydrogen storage alloy of Claim 1 or Claim 2 is provided (Claim 3).

The hydrogen storage alloy according to the present invention, the A ₂ B ₇ phase as a main phase, A site rare earth element of the main phase consists of Ca and Mg, B site of the main phase is composed of Ni, the rare earth element It has a configuration that is one or more selected from Pr, Nd, and Sm. With this configuration, while increasing the hydrogen storage amount, the hydrogen release is inhibited even when the hydrogen is stored and released repeatedly. This can be suppressed. Moreover, the nickel-metal hydride secondary battery of this invention provided with the negative electrode element containing this hydrogen storage alloy has a high capacity and excellent cycle life characteristics.

It is sectional drawing which showed schematic structure of the open-type nickel-hydrogen secondary battery.

The hydrogen storage alloy according to an embodiment of the present invention is a rare earth -Ca-Mg-Ni-based hydrogen storage alloy as the main phase an A ₂ B ₇ phase. In this A ₂ B ₇ phase, the A site is composed of rare earth elements, calcium and magnesium , and the B site is composed of nickel. Then, the rare-earth element is one or more selected Pr, and Nd, and Sm.

Here, the A ₂ B ₇ phase has a laminated structure in which the basic units in which the AB ₂ phase and the AB ₅ phase are arranged in a ratio of AB ₂ : AB ₅ = 1: 2 are repeatedly laminated. Although this AB ₂ phase has a large amount of occlusion of hydrogen, the crystal structure is easily distorted. On the other hand, the AB ₅ phase has a crystal structure that is not easily distorted and has stable hydrogen storage and release. In the hydrogen storage alloy according to the present invention, since these AB ₂ phase and AB ₅ phase are contained in the above-described ratio, the strain of AB ₂ phase is relaxed by the AB ₅ phase, and the balance is as a whole. It is thought that it has a structure that is not easily distorted. Therefore, the hydrogen storage alloy according to the present invention can suppress deterioration due to distortion of the crystal structure even if hydrogen storage / release is repeated.

Moreover, since the hydrogen storage alloy which concerns on this invention employ | adopts the 1 type, or 2 or more types selected from Pr, Nd, and Sm as a rare earth element combined with Ca and Mg, by adding Ca, hydrogen of While obtaining the merit that the amount of occlusion can be increased, it is possible to suppress the occurrence of problems that hydrogen cannot be released due to excessive stabilization.

Here, when one or more selected from Pr, Nd, and Sm are contained, it is considered that the occluded hydrogen can be prevented from being overly stabilized for the following reason.

  In the hydrogen storage alloy, when the distance between the elements constituting the crystal lattice increases and a gap is generated in the crystal lattice, hydrogen is stored in the gap. However, the larger the gap, the easier the hydrogen is stabilized. On the other hand, if this gap is small, the amount of occlusion of hydrogen cannot be improved. In the rare earth-Ca-Mg-Ni based hydrogen storage alloy, it is considered that the size of the gap is determined by the rare earth element combined with Ca and Mg. When at least one selected from the group consisting of Pr, Nd and Sm is used as the rare earth element combined with Ca and Mg, the distance between the elements of the crystal lattice can be controlled, and a gap larger than necessary is formed. It is thought that this can be suppressed. That is, by adopting at least one selected from the group consisting of Pr, Nd, and Sm, it is possible to suppress excessive stabilization of hydrogen in the hydrogen storage alloy while ensuring a relatively large amount of hydrogen storage. It is thought that a gap of a size can be formed.

Here, among rare earth elements, La and Ce having a relatively large atomic radius and Y having a relatively small atomic radius, when combined with Ca and Mg, do not provide a gap of an appropriate size, and the hydrogen storage amount. It is impossible to achieve both improvement of the hydrogen content and suppression of hydrogen stabilization. For this reason, the rare earth element combined with Ca and Mg is selected from one or more selected from Pr, Nd and Sm. Preferably, at least one of Pr and Nd is selected . The content of Pr, Nd and Sm as the rare earth element, rare earth element, is preferably 20 atomic% to 80 atomic% or less based on the sum of the numbers of atoms of Ca and Mg.

The hydrogen storage alloy according to the present invention is easily corroded if there is too much Ca, and conversely if it is too little, the effect of increasing the capacity by adding Ca cannot be obtained. on the sum of the numbers of atoms of Ca and Mg you 20 atomic% to 30 atomic% or less.

In addition, the hydrogen storage alloy according to the present invention cannot obtain the A ₂ B ₇ phase even if the Mg contained therein is too much or too little. Therefore, it is preferable that the Mg content is 10 atom% or more and 30 atom% or less with respect to the total number of atoms of the rare earth element, Ca and Mg.

Furthermore, in the hydrogen storage alloy according to the present invention, in order to obtain the A ₂ B ₇ phase as the main phase, when the total number of atoms of rare earth elements, Ca and Mg is 1, the ratio of the number of Ni atoms is: 3.4 shall be the the more than 3.6 or less ((rare earth elements + Ca + Mg): Ni = 1: (3.6 from more than 3.4)). A more preferable ratio of the number of Ni atoms is 3.5 ((rare earth element + Ca + Mg): Ni = 1: 3.5).

In the hydrogen storage alloy according to the present invention, the crystal structure of the main phase is preferably one of Ce ₂ Ni ₇ type and Gd ₂ Co ₇ type .

Next, the hydrogen storage alloy of this invention is obtained as follows, for example.
First, metal raw materials are weighed and mixed so as to have a predetermined composition, and this mixture is melted in, for example, a high-frequency induction melting furnace and then cooled to an ingot. The obtained ingot is heated to 900 to 1200 ° C. under an inert gas atmosphere and subjected to heat treatment for 5 to 24 hours to obtain the hydrogen storage alloy of the present invention. Thereafter, the ingot is pulverized and classified to a desired particle size by sieving to obtain hydrogen storage alloy particles.

(1) Production of hydrogen storage alloy electrode (negative electrode) and production of nickel-hydrogen secondary battery using this negative electrode

Example 1
Pr, Ca, Mg and Ni are weighed and mixed so as to have a predetermined composition, and the obtained mixture is melted in a high-frequency induction melting furnace in an argon gas atmosphere, and the molten metal is poured into a mold and cooled to room temperature. Thus, an ingot of a hydrogen storage alloy was obtained.
Then, the hydrogen storage alloy ingot was subjected to a heat treatment of heating to 1000 ° C. and holding for 10 hours in an argon gas atmosphere.

Then, the heat-treated ingot was mechanically pulverized in an argon gas atmosphere to obtain a rare earth-Ca—Mg—Ni-based hydrogen storage alloy powder having an average particle size of 60 μm.
1 part by weight of the obtained hydrogen storage alloy powder and 3 parts by weight of nickel powder having an average particle size of 2.5 μm as a conductive agent are mixed, and the resulting mixture is pressure-molded to give 1 g of a pellet-shaped hydrogen storage alloy. A negative electrode 14 was produced.

  Next, in order to measure the electrochemical capacity (alloy capacity) of the hydrogen storage alloy, an open-type liquid-rich nickel-hydrogen secondary battery 10 shown in FIG. 1 was produced. In addition, this nickel metal hydride secondary battery 10 is negative electrode capacity regulation. As is clear from FIG. 1, this nickel metal hydride secondary battery 10 includes a container 12 made of polypropylene, and the negative electrode 14 of the hydrogen storage alloy obtained as described above in the container 12, A reference electrode 16 made of a mercury oxide electrode was disposed at a position near the negative electrode 14. Furthermore, a cylindrical positive electrode 18 was disposed so as to surround the negative electrode 14 and the reference electrode 16. The positive electrode 18 is a sintered nickel positive electrode having a sufficiently large capacity with respect to the negative electrode 14. Then, an alkaline electrolyte 20 made of a 7N KOH solution was poured into the container 12 so that the negative electrode 14, the reference electrode 16, and the positive electrode 18 were completely immersed, and the open-type nickel-hydrogen secondary battery 10 was produced. In FIG. 1, reference numerals 22, 24, and 26 represent leads connected to the negative electrode 14, the reference electrode 16, and the positive electrode 18, respectively. These leads 22, 24, and 26 are a charging power source and a measuring instrument (not shown). Etc. are connected.

Example 2
A hydrogen storage alloy negative electrode similar to that in Example 1 was prepared except that Nd was used instead of Pr. And the open-type nickel-hydrogen secondary battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

Example 3
A hydrogen storage alloy negative electrode similar to that of Example 1 was prepared except that Sm was used instead of Pr. And the open-type nickel-hydrogen secondary battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

Comparative Example 1
A hydrogen storage alloy negative electrode similar to that of Example 1 was prepared except that La was used instead of Pr. And the open-type nickel-hydrogen secondary battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

Comparative Example 2
A hydrogen storage alloy negative electrode similar to that in Example 1 was prepared except that Ce was used instead of Pr. And the open-type nickel-hydrogen secondary battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

Comparative Example 3
A hydrogen storage alloy negative electrode similar to that of Example 1 was prepared except that Y was used instead of Pr. And the open-type nickel-hydrogen secondary battery 10 was produced like Example 1 except having used the negative electrode obtained in this way.

(2) Evaluation of hydrogen storage alloy (i) Composition analysis Samples for composition analysis were collected in advance from the hydrogen storage alloy ingots after heat treatment in each example and each comparative example, and a high frequency plasma spectroscopic analysis method was applied to this sample. Composition analysis by (ICP) was performed. The results are shown in Table 1 as the composition of the hydrogen storage alloy.

(Ii) Analysis of crystal structure Samples for crystal structure analysis were collected in advance from the hydrogen storage alloy powders of the examples and comparative examples, and X-ray diffraction measurement (XRD measurement) was performed on the samples. As a result, the crystal structures of the main phases of the hydrogen storage alloys of each Example and each Comparative Example were all Gd ₂ Co ₇ type.

(3) Evaluation of nickel hydride secondary battery (i) The obtained open-type negative electrode capacity-restricted nickel hydride secondary battery 10 was heated at a temperature of 25 ° C. for 210 minutes at a current of 300 mA with respect to 1 g of the hydrogen storage alloy. Charging and then resting for 10 minutes, then discharging until the voltage of the negative electrode 14 with respect to the reference electrode 16 (mercury oxide electrode) becomes −0.7 V at the same current as charging, and then resting for 10 minutes is one cycle. This operation was repeated 50 times. And the discharge capacity for every cycle was measured, and this measured value was made into cycle capacity. Here, the maximum capacity, which is the maximum value among all the cycle capacities, was converted to 1 g of hydrogen storage alloy and shown as an alloy capacity in Table 1. The cycle capacity at the 50th cycle was set to 50 cycle capacity. And the ratio (capacity maintenance factor) of 50 cycle capacity | capacitance with respect to the maximum capacity | capacitance shown by (I) Formula was calculated | required. The results are shown in Table 1 as the capacity retention after 50 cycles.
Capacity maintenance ratio after 50 cycles (%) = (50 cycle capacity / maximum capacity) × 100 (I)

(4) From Table 1, the following is clear.
(I) The hydrogen storage alloys of Examples 1 to 3 show high values for both the alloy capacity and the capacity retention after 50 cycles. From this, it can be seen that the cycle life is long because the amount of hydrogen occlusion is large and the rate of decrease in capacity is low even after repeated charge and discharge. In other words, it can be said that the hydrogen storage alloy of the present application has excellent characteristics capable of achieving both a high capacity of the battery and an improvement in cycle life characteristics when used in a nickel metal hydride secondary battery.

This is because the hydrogen storage alloys of Examples 1 to 3 use one or more selected from Pr, Nd, and Sm as rare earth elements combined with Ca and Mg, so that the gap between the crystal lattices is appropriately set. This is probably because the amount of hydrogen occluded was increased, the hydrogen stabilization was suppressed, and the amount of hydrogen released could be kept high even when the number of cycles progressed. As a result, it is considered that the capacity of the battery could be maintained high and the cycle life could be extended.

(Ii) The hydrogen storage alloy of Comparative Example 1 has a high alloy capacity, but has a low capacity retention rate after 50 cycles.
This is because when La is selected as the rare earth element, the hydrogen storage amount can be increased, so that the alloy capacity can be increased, but the stored hydrogen is easily stabilized, and it becomes difficult to release hydrogen as the number of cycles increases. It is thought that the capacity maintenance rate has decreased.

(Iii) The hydrogen storage alloy of Comparative Example 2 has a low alloy capacity, but the capacity retention after 50 cycles shows a high value.
This is because when Ce is selected as the rare earth element, stabilization of the stored hydrogen can be suppressed, and even when the number of cycles progresses, hydrogen can be easily released, so that the capacity retention rate is increased, but the hydrogen storage amount is Since it cannot be increased, the alloy capacity is considered to be low.

(Iv) The hydrogen storage alloy of Comparative Example 3 shows a low value for both the alloy capacity and the capacity retention rate after 50 cycles.
This is presumably because when Y is selected as the rare earth element, the hydrogen storage amount cannot be increased and the stored hydrogen is easily stabilized.

(V) As described above, when a hydrogen storage alloy containing rare earth elements other than Pr, Nd, and Sm is used as in Comparative Examples 1 to 3, both the increase in capacity and the improvement in cycle life characteristics of the nickel hydrogen secondary battery are achieved. Can not be planned.
Also from this, like the hydrogen storage alloy of the present invention, it is possible to increase the capacity of the nickel-metal hydride secondary battery by including one or more kinds selected from Pr, Nd and Sm as rare earth elements. It can be seen that this is effective in improving both cycle life characteristics.

  In addition, this invention is not limited to an above-described Example, A various deformation | transformation is possible. For example, the nickel metal hydride secondary battery using the negative electrode element including the hydrogen storage alloy according to the present invention is not limited to the above-described open nickel metal hydride secondary battery, but is used for a sealed nickel metal hydride secondary battery. It doesn't matter.

10 Open Nickel Metal Hydride Battery 12 Container 14 Negative Electrode 16 Reference Electrode 18 Positive Electrode 20 Alkaline Electrolyte 22, 24, 26 Lead

Claims (3)

A rare earth-Ca-Mg-Ni-based hydrogen storage alloy,
A ₂ B ₇ phase as the main phase,
A site of the main phase is made from rare earth elements, Ca and Mg,
B site of the main phase consists of Ni,
The rare earth element is one or more selected from Pr, Nd and Sm,
When the ratio of the number of atoms of the element at the A site and the number of atoms of the element at the B site is B / A, this B / A satisfies the relationship of 3.4 <B / A ≦ 3.6. ,
The hydrogen storage alloy characterized in that the content of Ca is 20 atom% or more and 30 atom% or less with respect to the total number of atoms of the rare earth element, Ca and Mg. The crystal structure of the main phase is
The hydrogen storage alloy according to claim 1, which is one of Ce ₂ Ni ₇ type and Gd ₂ Co ₇ type.   A nickel metal hydride secondary battery comprising a negative electrode element comprising the hydrogen storage alloy according to claim 1.

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