JPH02148568A - Hydrogen occludent electrode - Google Patents

Abstract

PURPOSE:To manufacture a low-cost electrode with large discharge capacity and no characteristic deterioration for a long term by having a mischmetal- nickel-aluminium hydrogen occludent alloy as a base while adding a small amount of Zr and V. CONSTITUTION:A part of an Mm quantity of a general formula to be expressed by Mm1-xZrxNiaAlbVc (provided that Mm denotes mischmetal) is Zr, and a part of an Ni quantity is replaced by Al and V, while being included respectively in the range of 0.05<=x<=0.2, 4.8<=a+b+c<=5.2, 3.9<=a<4.2, 0.6<=b<=0.9, 0.05<=c<=0.3. This hydrogen occludent alloy has a large discharge capacity and no characteristic deterioration is caused in the long-term repetition of a occlusion-discharge cycle of hydrogen. Further, its manufacture is based on the relatively low-priced mischmetal and it contains no expensive Co, and therefore it is formed inexpensively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a sealed alkaline storage battery using a hydrogen storage alloy that electrochemically stores and releases hydrogen.

[Conventional technology] Traditionally, the most common storage batteries have been those using lead or nickel cadmium, but recently, with the expectation that they will be lighter and have higher capacity than these, hydrogen storage alloys have been used as negative electrodes. A metal-hydrogen alkaline storage battery using, for example, nickel hydroxide for the positive electrode has been announced.

The conditions required for a storage battery are that it has a high charge/discharge capacity and that its characteristics do not deteriorate even after repeated use. Deterioration becomes a problem. This problem of deterioration of electrode properties due to oxidation includes alloy oxidation during the electrode manufacturing process, but mainly occurs when oxygen gas released from the positive electrode side diffuses to the negative electrode at the end of battery charging or during excessive charging. This is caused by partially oxidizing the hydrogen storage alloy of the negative electrode. In addition, the problem of deterioration of electrode properties due to pulverization is that due to the expansion and contraction of the hydrogen storage alloy as the battery is charged and discharged, the alloy is fractured at the grain boundaries, and as this process is repeated, some alloys become pulverized. falls off from the electrode molded body. Any of these deterioration phenomena results in a decrease in battery capacity and shortens cycle life.

Many proposals have been made to improve this problem.

For example, "hydrogen storage alloy" (Japanese Patent Application Laid-Open No. 62-11986
Giant N1xCOyAlz alloy, "sealed alkaline storage battery" in JP-A-60-250558@
MynN"+xC07Mz (where M is Atl, Sn, sb, Cu, Fe, Mn.

There are many materials including at least one selected from the group consisting of Cr, Mo, V, Nb, Ta, Zn, and M9).

Also, in academia or industry, L
'lr V & alloys include LaN i 2.5 co2.4 A I developed by Phillips
20.1, 1-ao, 8 N d□, 2 N i 2.
5 CO2,4S i □,1 is well known.

LiteratureNigney G.G.WilliamsrLa
Stability of hydrogen storage batteries of Nrs-related compositions (JJ,
G, Willems”Metal Hydride E
Electrodes Stability of L
aN i 5-related Compounds”
, Phi 1ips Journal of Re5
earth Vol, 395upp1. No, 1.19
[Problems to be Solved by the Invention] As mentioned above, there are a wide variety of known techniques for using La-Ni5 as the basic material of a hydrogen storage battery and improving the desired functions of the battery using this as a starting point. A common feature can be found in that the focus is on Co.

In other words, the addition of Co improves the corrosion resistance and oxidation resistance of the alloy, increases the grain boundary strength of the alloy, and also reduces the hydrogen storage capacity of the alloy to suppress expansion and contraction associated with hydrogen absorption and release. It will be suppressed. Because of this, Co
Addition is considered to be one of the most suitable methods for preventing the deterioration of electrode characteristics as described above.

However, Co, which provides this effectiveness, is very expensive, and furthermore, the treatment applied to prevent the properties of the alloy from deteriorating also increases the cost of manufacturing the electrode. Therefore, hydrogen storage electrodes are currently relatively expensive compared to cadmium electrodes and the like, and this situation has been a very big problem that hinders the widespread use of nickel-metal hydride secondary batteries.

In order to solve the above-mentioned problems, the present invention uses Mitsushi metal, which is relatively inexpensive because it contains various elements such as La, Nd, and Ce as rare earth elements and does not undergo a purification or separation process, and replaces the expensive Co element. The objective is to provide an alloy system that does not contain

[Means for Solving the Problems] The hydrogen storage electrode according to the present invention has a general formula of Myn+-x.
It is expressed as ZyxNiaA7bVc (however, Mm is Mitsushi metal), and a part of Mmffi is Zr and Ni
Part of the amount is replaced with Ae and ■, and 0.05≦x
≦0.2゜468≦a+b+c≦5.2.3.9≦a≦4.2゜0
．． The above problem was solved by using hydrogen storage alloys falling within the ranges of 6≦b≦0.9 and 0.05≦c≦0.3.

That is, from the viewpoint of suppressing self-discharge of the battery, the composition of the alloy used as the hydrogen storage electrode material must be determined so that the equilibrium hydrogen dissociation pressure of the hydrogen storage alloy is equal to or lower than atmospheric pressure. When Mitshu metal is used, the equilibrium dissociation pressure will be high if the composition is similar to that of a lanthanum-based alloy, and it is essential to add alloying elements to lower the equilibrium dissociation pressure. In order to lower the equilibrium dissociation pressure without impairing the charge/discharge capacity or cycle life, substitution with AQ is most suitable, and the basic composition of the alloy is Mm Ni, t-a Al! It was set as a.

The main problems that arise when using such alloys as electrode materials are a decrease in the initial charge/discharge range and a decrease in charge/discharge capacity due to repeated long-term charging and discharging as described above. Improvements were made by adding elements.

First, we have found from many experimental facts that it is very effective to replace part of the Ni in MmNi5 as a basic composition with ■ in order to lower the initial charge/discharge capacity. It has been found that depending on the replacement date (1) at this time, there is some effect on cycle life, but adding it alone cannot be said to be practically sufficient. We discovered that the most effective way to improve this lifespan is to replace a part of the Mitshu metal side with Zr, and by using it in conjunction with the above-mentioned V replacement, the initial charge and discharge capacity is high, and the charge and discharge speed is also increased. We were able to invent a hydrogen storage alloy that shows little deterioration due to repeated cycles.

The newly developed alloy for hydrogen storage electrodes has the composition formula %, and each index in the composition formula is as follows: 0.05≦x≦0.2 4.8≦a+b+c≦5.23
．． 9≦a≦4.2 0.6≦b≦0.90.05≦c≦
It is shown in each range of 0.3.

[Operation/Example] In order to confirm the operation of the present invention, the hydrogen storage alloys shown in Table 1 were melted in a vacuum arc furnace, these alloys were mechanically pulverized, and then alloy powder was formed by electroless copper plating. A copper coating layer corresponding to about 20% by weight of the alloy after plating was formed on the surface. The resulting copper-coated alloy was coated with fluorocarbon resin (
Tetrafluoroethylene/propylene fluoride copolymer, resin addition amount: equivalent to 10%) is added and molded by cold press.
This was heated to 300°C along with the nickel mesh as a current collector.
Alloy electrodes were prepared by pot pressing.

These electrode molded bodies have a diameter of 13 mm and a thickness of 0.2 mm.
The thickness is 5 to 0.30 mm, and the weight excluding the nickel mesh material is approximately 300 my.

A test battery was constructed using this hydrogen storage electrode as a negative electrode, the same nickel oxide electrode as the nickel-cadmium storage battery as a positive electrode, and a 6M potassium hydroxide solution as an electrolyte. Note that a mercury oxide electrode was used as the reference electrode. This test battery was charged in a constant temperature room at a temperature of 20°C for 285 hours at a charging current of 40 mA, rested for 0.5 hour, and then discharged at a discharge current of 20 mA until the voltage dropped to 0.6 V. , a long-term repeated charge-discharge test was conducted. The initial maximum discharge capacity obtained in this test and 3
Table 1 does not show the results regarding the residual rate of discharge capacity after 00 cycles. The latter discharge capacity remaining rate is an index of the 1-night life of the battery. Although not shown here, the gas phase P for each alloy
A CT test was carried out, and from the hydrogen dissociation pressure-composition isotherm diagram (PCT diagram) in an equilibrium state at a temperature of 20 ° C.
We are looking for the hydrogen storage capacity of an alloy at atmospheric pressure.

This table first shows an experiment to understand the effect of adding Zr and V alone, and MmN i 4.2 A ff104
Comparative Example 1 is based on an alloy, Comparative Examples 2 to 4 are alloy systems in which V is added alone, Comparative Examples 2 and 3 are alloys in which ■ is replaced with a part of Ni, and Comparative Example 4 is a part of Mitshu metal. It has been replaced with .

Comparative Examples 5 and 6 are alloy systems in which Zr is added alone; in Comparative Example 5, Zr is replaced by a part of Ni, and in Comparative Example 6, Zr is replaced by a part of Mitshu metal.

Comparative Example 7 is a hydrogen storage alloy belonging to the same series, and is an alloy belonging to the prior art in which CO is blended in order to enhance the action as a battery.

Below are the margins: First, MmN i 4.2 A ffi□ of Comparative Example 1,
Let's look at the effects of adding ■ and Zr alone based on alloy No. 6. When part of Ni is replaced with ■, Comparative Example 21,
As compared with Comparative Example 3, the initial discharge capacity also increased as the substitution ratio increased. In addition, the remaining discharge capacity after 300 cycles is Comparative Example 1, Comparative Example 2, and Comparative Example 3.
and ■As the substitution ratio increases, it increases, albeit slightly. It can be seen that when the ratio of replacing part of Ni with V is within a small range, it is somewhat effective not only for increasing the initial discharge capacity but also for improving the cycle life.

However, from the results of the PCT test, it is known that when the V substitution ratio increases beyond a certain limit, the hydrogen storage capacity of the alloy decreases. You need to be careful as it can cause problems. In addition, when a part of Mitsushimetal is replaced with ■, as shown in Comparative Example 4, the initial discharge @ mother is reduced, and no significant influence on the cycle life is observed.

On the other hand, in the alloy of Comparative Example 6 in which a portion of Mitshu metal was replaced with a small amount of Zr, the initial discharge capacity was increased compared to the alloy of Comparative Example 1 in which no substitution was made. However, in this case too, if the Zr substitution ratio becomes too large, the hydrogen storage capacity determined by the PCT test will decrease, so it would be unreasonable to expect that excessive Zr substitution will increase the initial discharge capacity. .

As is clear from Comparative Example 6, the discharge capacity residual rate was significantly improved by substituting a small amount of Zr.

In this way, when a portion of the Mitshu metal is replaced with Zr in a small proportion, it is expected that the initial discharge capacity will increase somewhat and the cycle life will be greatly improved. On the other hand, when part of Ni was replaced with Zr, the initial charge capacity of Comparative Example 5 did not change much, and the life span was only slightly improved.

As mentioned above, it is possible to increase the initial discharge capacity by replacing a portion of Ni with a suitable amount of V, and it is possible to improve the cycle life by replacing a portion of Mitshu metal with a suitable amount of Zr. In order to find out the upper and lower limits of the appropriate amount to maximize the synergistic effect of V and Zr substitution, the same treatment and the same measurements were carried out using the components shown in Table 2 to specify the range.

In the alloy system in which a portion of the Ni margin is replaced with V, the initial discharge capacity is highest at a substitution ratio of around 0.2, as in the case of the single substitution described above. For example, among the three alloys of Example 2, Example 3, and Example 40, the alloy of Example 3 with a substitution ratio of 0.2 is the alloy of Example 3, and the alloy of Example 3 has a substitution ratio of 0.2. Among the alloys, the alloy of Example 6, which also had a substitution ratio of 0.2, had the highest initial discharge capacity. In addition, as seen in examples such as Example 4, Example 7, and Comparative Example 8, in alloy systems in which Ⅰ substitution was performed such that the substitution ratio exceeded 0.2, Comparative Example 1 without Ⅰ substitution The initial discharge capacity is lower than that. The results of the PC test also show that the hydrogen storage capacity of the alloy tends to decrease when the substitution ratio exceeds 0.2, indicating a good correspondence of 1? It is being In this way, it is very effective to replace a part of Ni with V from the viewpoint of improving the initial discharge capacity, and the highest initial discharge capacity can be obtained at a substitution ratio of around O32. It can be seen that if the value is exceeded, the initial discharge capacity decreases. On the other hand, in the alloy of Example 1 in which part of Mitsushi metal was replaced with a small amount of Zr, the initial discharge capacity was increased compared to that of Comparative Example 2. However, when the amount of Zr substitution exceeds a certain amount, Example 1
As seen in the cases of Example 2 relative to Example 3 and Example 5 relative to Example 3, the initial discharge capacity decreases.

A similar tendency was observed in the PCT test regarding the amount of hydrogen storage, and the amount of hydrogen storage and initial discharge capacity correspond well. If the Zr substitution ratio is approximately 0.1 or more, the initial discharge capacity will be reduced by 'rB<. Focusing on the increase in initial discharge capacity, it is possible to reduce some of the Ni to V
It is considered appropriate to set the ratio of substitution with Zr to about 0.2, and to set the ratio of substitution of part of Mitshu metal with Zr to 0.1 or less.

Regarding the discharge capacity remaining rate after 300 cycles, an alloy system in which part of Ni was replaced with V at the same ratio,
As the substitution ratio of Zr to Comparative Example 2, Example 1, and Example 2 and Mitsushi Metal increases, and similarly as the substitution ratio of Zr to Comparative Example 3, Example 3, and Example 5 increases, the remaining discharge capacity decreases. rate is rising. It can be seen that by substituting part of the Mitshu metal with Zr in this way, the cycle life is improved. however,
In Examples 4 and 7, where the ratio of replacing part of N1 with V is as high as 0.3, an opposite tendency appears in which the discharge capacity remaining rate decreases as the Zr replacement ratio increases, so the V replacement ratio is Criticality must be identified.

In general, even in the case of Comparative Example 8 in which the substitution ratio (1) was 0.4, the residual discharge capacity was still low despite the relatively high Zr substitution ratio. These things mean that if the ratio of replacing part of Ni with V is high, such as 0.3 or more, the cycle life improvement effect obtained by replacing part of Mitsushmetal/L with Zr will be lost. It means being lost. Therefore, if we focus on improving cycle life, it is best to set the ratio of replacing part of Ni with V to about 0.2, and at most 0.3.
It is proven that the

The two alloys of Example 5 and Example 6 having the same V substitution ratio of 0.2 have some difference in discharge capacity remaining rate. It is originally believed that the addition of Affi has a positive effect on cycle life, and it is presumed that this factor is also a result. As mentioned earlier, the cycle life improves as the substitution ratio of Zr that partially replaces Mitsushi metal increases.It is difficult to find an appropriate Zr substitution ratio, but from the viewpoint of balance with the initial discharge capacity, the cycle life is improved. 10-0.15 is appropriate.

From the experimental facts shown above, a hydrogen storage alloy that has both high discharge capacity and long cycle life as a battery has the compositional formula t'lyn 1-* described in the claims.
ZY
The requirement is that it does not exceed 0.3 at most.

Comparative Example 7 uses Mitsushmetal, LaN i 2.5 Ga4.4 A E□ developed by Philips,
Although the alloy has a composition close to 1, this alloy has very good cycle life characteristics but has a fairly low discharge capacity. Among the examples shown, the newly developed hydrogen storage alloy shown in Example 6, which has both high discharge capacity and good cycle life characteristics, has a significant performance advantage over conventional alloys. There is. In addition, we use the new U■ metal alloy, which is relatively inexpensive, and furthermore, we use the expensive Co metal.
Since it does not contain hydrogen, it can be used as an inexpensive hydrogen storage electrode.

[Effect of the invention] Expensive C using relatively inexpensive Mitsushi metal.

By applying an alloy containing small amounts of Zr and V to the electrode material based on a Mitsushmetal-nickel-aluminum hydrogen storage alloy that does not contain hydrogen, it has a large charge/discharge capacity and an electrochemical hydrogen storage alloy. It has become possible to provide a hydrogen storage electrode at a low cost whose characteristics do not deteriorate even after repeated storage/desorption cycles over a long period of time.

Claims (1)

[Claims] The general formula is Mm_1_-_xZr_xNi_aAl_bV
c (however, Mm is Mitsushi Metal), a part of the Mm amount is replaced with Zr, a part of the Ni amount is replaced with Al and V, and 0.05≦x≦0.2, 4.8 ≦a+b+c
≦5.2, 3.9≦a≦4.2, 0.6≦b≦0.9,
A hydrogen storage electrode characterized in that it uses a hydrogen storage alloy that falls within the range of 0.05≦c≦0.3.