JP2847873B2 - Hydrogen storage electrode - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nickel-metal hydride secondary battery in which a hydrogen storage alloy is used as a negative electrode and a nickel oxide electrode is used as a positive electrode. Also, the present invention relates to a hydrogen storage electrode in which the deterioration of characteristics is small even during long-term repetition of charge / discharge cycles, and the decrease in discharge capacity is small even during large current discharge.

Conventional technology and its problems To improve energy storage capacity, a nickel-metal hydride secondary battery using a hydrogen storage alloy that reversibly stores and releases hydrogen as the negative electrode and uses the stored hydrogen as an active material is proposed. And development is urgent. The hydrogen storage alloy used for this purpose must satisfy the following requirements.

(1) The effective hydrogen storage amount, that is, the electric capacity is large.

(2) The hydrogen equilibrium dissociation pressure is the battery operating temperature (-20 to 60 ° C)
10-3 to several atmospheres.

(3) Excellent corrosion resistance in a concentrated alkaline electrolyte.

(4) The rate of pulverization by repetition of the electrode reaction is low.

(5) There should be no change in composition due to elution of a specific element or the like due to repetition of the electrode reaction.

(6) High hydrogen diffusion rate and low reaction resistance (overvoltage).

(7) Inexpensive.

As an inexpensive raw material containing a rare earth element, a conventionally known material is misch metal (Mm). It is a mixture of rare earth metals, usually La25-35% by weight, C
e 45-55% by weight, Nd 10-15% by weight. In the hydrogen storage alloy using Mm as a raw material, the equilibrium hydrogen dissociation pressure increases because the amount of Ce in the rare earth metal is large. In order to reduce the hydrogen dissociation pressure to about 1 atm or less in the battery operating temperature range, it is necessary to partially replace Ni with an element such as Co or Al. In such an MmNiCoAl-based hydrogen storage alloy, generally, as the amount of Co increases, the discharge capacity decreases, but the cycle life improves. It has been experimentally confirmed that the minimum substitution amount of Co for obtaining good cycle life characteristics is 0.6 to 0.7. Also, it has recently been found that an alloy with a reduced amount of Co substitution and a higher Ni content has better rapid discharge characteristics.

Next, Al forms a dense oxide film on the surface of the alloy powder, thereby suppressing oxidation of the alloy and improving the cycle life. However, excessive Al substitution works in the direction of increasing the reaction resistance of the electrode because the oxide film has electrical insulation properties, and deteriorates rapid discharge characteristics and low-temperature discharge characteristics. Therefore, in this system, in order to obtain an alloy exhibiting good electrode characteristics, it is important to suppress the Al substitution amount and the Co substitution amount to the minimum necessary amounts.

MmNi _3.5 Co _0.7 Al was added to the alloy developed based on this concept.
There is a _0.8 alloy, which has a relatively high initial discharge capacity of 254 mA / g, as described later, and has good cycle life characteristics. However, due to the use of Mm containing a large amount of Ce, Al substitution required for lowering the hydrogen dissociation pressure inevitably increases, and this alloy is not necessarily good in terms of rapid discharge characteristics.

As an alloy having a larger discharge capacity than the MmNiCoAl-based alloy, an MmNiCoAl-based alloy in which a part of Ni is replaced with Mn has recently been considered. This Mn substitution is effective in increasing the capacity, but it has been confirmed that Mn near the surface of the alloy powder elutes into the electrolytic solution due to repeated charge and discharge, and the cycle life is shortened. It has been found that there is an adverse effect. Therefore, when performing Mn substitution, it is necessary to take measures to prevent the elution of Mn at the same time and to prevent deterioration of the cycle life characteristics, but such an effective treatment method has been found at present. Not.

As described above, when using inexpensive Mm containing a large amount of Ce as a raw material, various problems occur along with Al substitution and Mn substitution, so that the discharge capacity, cycle life characteristics, rapid discharge characteristics, and low-temperature discharge characteristics are reduced. It has been difficult to design alloys with due consideration for everything.

Means for Solving the Problems Mm containing 75% by weight or more of La is useful for improving the discharge capacity of the hydrogen storage electrode and reducing the cost. Using this raw material, the discharge capacity, cycle life characteristics, and rapid discharge characteristics were improved by substituting a small amount of Zr for part of Mm and minimizing the amount of Co substituted as part of Ni. Alloys for hydrogen storage electrodes could be developed.

The newly developed alloy for hydrogen storage electrodes has the general formula Mm
_1-X Zr _X Ni _YAB Co _A Al _B
Mm) is 75% by weight or more and 90% by weight or less and simultaneously Ce, Nd, Pr
Mm containing 10% by weight or less of each rare earth element is used as a raw material. A hydrogen storage alloy characterized by substituting a part of this Mm with Zr and a part of Ni with Co and Al, the alloy composition of which is 0.01 ≦ X ≦ 0.08,4.9 ≦ Y ≦ 5.1,1.0 ≦ A + B
≤1.5, 0.5≤A≤1.1, 0.3≤B≤0.6.

Action As shown in the above composition formula as a hydrogen storage alloy, Mm
By using an alloy in which a part of the alloy was replaced with Zr and a part of Ni was replaced with Co or Al, a hydrogen storage electrode whose characteristics were not deteriorated even after long-term charge / discharge cycles could be manufactured. This improvement in cycle life characteristics is achieved because Zr forms an oxide film on the alloy surface, which improves the corrosion resistance of the alloy without inhibiting the diffusion of H ⁺ ions and suppresses the oxidation of rare earth metals. It is thought that it was done.

In addition, since the amount of La contained in Mm is large, and a component design that suppresses the substitution amount of Co and Al has been performed, the charge / discharge capacity is large, and an electrode having excellent rapid discharge characteristics can be obtained. did it.

Example In order to confirm the effect of the present invention, a hydrogen storage alloy shown in Table 1 was obtained by arc melting in an argon atmosphere. After mechanically pulverizing these alloys, a copper coating layer corresponding to about 20% by weight was formed on the surface of the alloy powder by electroless copper plating. FEP as a binder to this alloy powder
(Ethylene tetrafluoride / propylene fluoride copolymer) Resin is added in an amount equivalent to 10% by weight, and about 300 mg of a powder mixture (alloy weight: about 216 mg) is cold-pressed to a diameter of 13 mm and a thickness of about 13 mm.
It was formed into a 0.4 mm-shaped electrode pellet. Then, this was hot-pressed at a temperature of 300 ° C. together with a nickel mesh serving as a current collector to obtain an alloy electrode for testing.

A test battery was constructed by using the hydrogen storage electrode as a negative electrode, the same nickel oxide electrode as a nickel-cadmium storage battery as a positive electrode, and a 6M potassium hydroxide solution as an electrolyte. In addition, each test battery was a negative electrode regulation type in which the battery capacity depends on the capacity of the negative electrode, and a mercury oxide electrode was used as a reference electrode. This test battery was charged in a constant temperature room at a temperature of 20 ° C for 2.5 hours at a charging current of 40 mA, and after resting for 0.5 hour, the potential difference between the reference electrode and the hydrogen storage electrode dropped to -0.6 V at a discharging current of 20 mA. A long-term charge / discharge cycle test was performed in such a cycle that the battery was discharged. Table 1 shows the test results for each alloy. Here, a value obtained by dividing the discharge capacity when 300 cycles have elapsed after reaching the long-term maximum capacity by the initial maximum capacity is used as an index indicating cycle life characteristics as a capacity retention ratio. In addition, each alloy shown in Table 1 is adjusted by the Al substitution amount in order to make the equilibrium hydrogen dissociation pressure at room temperature substantially equal to or less than 1 atm.

The alloy of Comparative Example 1 was prepared by using La alone as a rare earth metal, and although the initial discharge capacity was large,
The discharge capacity after the passage of the cycle becomes small, and the cycle life is not very good. In the alloys of Comparative Examples 2 and 3, La was partially replaced with Zr, and the initial discharge capacity was reduced as the replacement amount increased with Comparative Examples 2 and 3. However, the cycle life characteristics tend to improve as the Zr substitution amount increases. This is thought to be because Zr forms a strong oxide film on the surface of the alloy powder.

Next, Mm containing much Ce, which has been used more than before
Comparative Example 4 prepared using (La content: 30.0% by weight)
The alloy has a small initial discharge capacity due to the high content of Ce, but has very good cycle life characteristics.

On the other hand, La was 82.5% by weight, Ce was 2.6% by weight, and Nd was 8.9%.
Test results of alloys prepared using Mm containing 3.3% by weight of Pr by weight are shown in Comparative Examples 5, 6, and 7. Here, for the sake of convenience, the latter Mm is written as Lm for the purpose of distinguishing it from the aforementioned Mm. The alloy of Comparative Example 5 corresponds in composition to Comparative Examples 1 and 4, and although the initial discharge capacity is relatively high and the cycle life characteristics are slightly low, it can be said that it is at a practical level. In contrast, in the alloy of Comparative Example 6,
Although the initial discharge capacity is large, the cycle life is considerably short.

The alloys of Examples 1, 2 and 3 are alloys according to the present patent application in which a part of Lm is substituted by Zr (substitution ratio: 0.05). As in Examples 1, 2, and 3, as the amount of Co substitution decreases, the initial discharge capacity increases and the cycle life characteristics decrease.
Among them, even the alloy of Example 1 having the smallest initial discharge capacity has substantially the same cycle life characteristics as the alloy of Comparative Example 4, but the initial discharge capacity is considerably large. As shown in Example 1 for Comparative Example 5 and Example 2 for Comparative Example 6, although the initial discharge capacity was reduced by substituting a part of Lm with Zr,
The cycle life is greatly improved. This is presumably because, as described above, the oxide film formed on the alloy surface by the Zr substitution suppresses the oxidation of the rare earth metal. However, excessive Zr substitution only forms an intermetallic compound of Zr with another element (such as ZrNi ₂ ), resulting in a reduction in the effective hydrogen storage capacity. An example is shown in Comparative Example 7. Comparative Example 7 is a similar alloy with a Zr substitution rate of 0.1, but the initial discharge capacity is slightly smaller than that of the alloy of Example 2.
Similar results are observed when Comparative Example 1 and Comparative Example 3 are compared. Empirically, the substitution rate of Z steps with respect to La (Mm) is set to 0.08 or less, as a range in which other useless intermetallic compounds do not appear, and as a range in which the discharge capacity does not decrease much even if it appears to some extent. Is appropriate.

From the above results, the newly developed alloy for hydrogen storage electrode represented by the general formula Mm _1-X Zr _X Ni _YAB Co _A Al _B
= 0.05 (less than 0.08), Y = 5.0, 1.0 ≦ A + B ≦ 1.1, 0.5 ≦ A
By setting ≦ 1.5, 0.4 ≦ B ≦ 0.5, a material exhibiting good electrode characteristics can be obtained.

Although not mentioned in previous tests, the general formula La
(Mm) the composition of an alloy represented by Ni _P is, La (Mm): Ni is 1: 5
If the stoichiometric composition deviates from the stoichiometric composition (P = 5), the cycle life characteristics deteriorate. The problem is the degree of degradation, but La _Q N
d _0.15 Zr _0.05 Ni _3.8 Co _0.7 Al _0.5 alloy
Compared to alloys with stoichiometric composition (Q = 0.80), Q =
The non-stoichiometric alloy of 0.78 had 14%, and the non-stoichiometric alloy of Q = 0.82 had 15%, and the discharge capacity retention rate after 300 cycles respectively decreased. Temporarily, the rate of decrease in capacity maintenance rate is
If set% that less is acceptable limit, the value of P of an alloy represented by La (Mm) Ni _P expression from 4.9 to 5.
Must be in range 1. This is because when the value of P in the above composition formula is less than 5.0, an intermetallic compound such as La ₂ Ni ₇ appears in the alloy when the value of P exceeds 5.0. Therefore, it is considered that the cycle life characteristic is deteriorated.

The above test results are obtained at a temperature of 20 ° C.
In a battery used at a low temperature of about ℃ or a battery discharged with a large current, it is necessary to reduce the amount of Al substitution to avoid an increase in overvoltage due to formation of an insulating oxide film. In consideration of this, the appropriate range of the Al substitution amount is set to 0.3 ≦ B ≦ 0.6. As described above, the Al substitution amount may deteriorate the cycle life characteristics if it is too small, but 0.3 ≦
B ≦ 0.4 is obtained when the composition has a composition close to that of Example 1 having excellent properties. In the case of Lm, since Ce and Nd which are effective in improving the life are included, B ≦ 0.3 is sufficient. It is considered that the cycle life can be ensured. As will be described later, the rapid discharge characteristics do not deteriorate unless the Al substitution amount is excessively large. The range of B ≦ 0.6 is a range that does not adversely affect these points.

Therefore, if a certain reduction in initial capacity and deterioration in cycle life can be tolerated, the above optimum composition range is extended to 0.01 ≦ X ≦ 0.08, 4.9 ≦ Y ≦ 5.1, 1.0 ≦ A + B ≦ 1.
Even when 5,0.5 ≦ A ≦ 1.1 and 0.3 ≦ B ≦ 0.6, the alloy is superior in both capacity and life to the conventional alloy system as in Comparative Example 4.

Table 2 shows the rapid discharge characteristics of some of the alloys listed in Table 1. This test was carried out using the same test batteries as those used in the above-described charge / discharge cycle test, while varying the discharge current. In Table 2, only the value obtained by dividing the discharge capacity when discharging at a discharge current of 300 mA by the discharge capacity when discharging at a current of 20 mA is shown. In addition, discharge current 300m
The discharge condition at A depends on the discharge capacity,
This corresponds to a 0.2 hour discharge rate.

Although the alloy of Comparative Example 1 had difficulty in cycle life characteristics as described above, the decrease in discharge capacity was relatively small even by rapid discharge at a discharge rate of 0.2 hours. Compared with the previously known LaNi _2.5 Co _2.4 Al _0.1 alloy of 48% of the same capacity reduction rate, the rapid discharge characteristics are considerably better. This is probably because the more Ni element in the alloy, the higher the catalytic activity on the surface and the smoother the electrode reaction.

Further, the alloy of Comparative Example 4 had a good initial cycle capacity but a good cycle life, but the capacity drop during rapid discharge was considerably large. In the case of the alloy of Comparative Example 4, the substitution amount of Al is large, and the overvoltage is increased by the oxide film, which causes a decrease in capacity at the time of rapid discharge.

Next, in the case of the alloys of Comparative Examples 5 and 6, since these alloys also have a large Ni content, they show good rapid discharge characteristics as in Comparative Example 1. Of these, Ni
The reason why the alloy of Comparative Example 6 having a large content is superior in the rapid discharge characteristics is considered to be the same as that described in Comparative Example 1 above.

The alloys of Examples 1, 2 and 3 are at substantially the same level as the alloys of Comparative Examples 5 and 6, and all have excellent rapid discharge characteristics. When explaining the cycle life improvement effect of Zr substitution, it was considered that it depends on the oxide film formation as in the case of Al substitution. Despite the formation of the same oxide film, the rapid discharge characteristics do not deteriorate in the case of Zr substitution.
It is considered that the reasons are as follows. First of all,
This is probably because the oxide film formed by Al substitution is electrically insulating, whereas the oxide film formed by Zr substitution has porosity and does not inhibit the diffusion of H ⁺ ions. Secondly, the substitution of Zr is a substitution for La (Mm), and it is considered that the surplus Ni accompanying the formation of the oxide film is simultaneously liberated on the alloy surface, and this Ni exerts a catalytic activity. It is considered that the effect of increasing the temperature does not deteriorate the rapid discharge characteristics even in the presence of the oxide film.

By partially replacing La (Mm) with Zr in this way, the initial discharge capacity is slightly reduced, but by using a component system with a small amount of Co replacement, high capacity, long life, and
An alloy for a hydrogen storage electrode having excellent rapid discharge characteristics can be obtained.

As described above, it has been described that the alloy of the present invention exhibits excellent electrode characteristics as a material used for a hydrogen storage electrode of a nickel-metal hydride secondary battery, including the mechanism of its effect. The action is common to secondary batteries using an alkaline electrolyte, and the electrode can also be used for a secondary battery using manganese dioxide or the like for the positive electrode.

Effect of the Invention Mm having a La content (La / Mm) in the raw material of 75% by weight or more and 90% by weight or less was used as a raw material, part of Mm was replaced with Zr, and part of Ni was replaced with Co and Al. The hydrogen storage alloy represented by the general formula Mm _1-X Zr _X Ni _YAB Co _A Al _B has a large charge / discharge capacity and a high cycle life characteristic by having the alloy composition be within the above range. Further, an electrode having excellent characteristics such as a small capacity reduction even by rapid discharge can be obtained. In addition, since the raw material Mm is inexpensive and the content of expensive Co is small, it has become highly practical as a low-cost hydrogen storage electrode.

Claims (1)

(57) [Claims] [Claim 1] The general formula Mm _1-X Zr _X Ni _YAB Co _A Al _B (where M
m: misch metal) and the amount of La contained in Mm (La
/ Mm) is 75% by weight or more and 90% by weight or less, and at the same time, Ce, Nd, P
A hydrogen storage alloy characterized by substituting a part of Mm with Zr and a part of Ni with Co and Al using Mm containing 10% by weight or less of each of the rare earth elements of r as a raw material, and 0.01 ≦ X ≦ 0.
08,4.9 ≦ Y ≦ 5.1,1.0 ≦ A + B ≦ 1.5,0.5 ≦ B ≦ 1.1,0.3
A hydrogen storage electrode using an alloy having a composition represented by each range of ≦ B ≦ 0.6.