JPH03294444A - Hydrogen occluding electrode - Google Patents

Abstract

PURPOSE:To obtain an alloy for a hydrogen occluding electrode having a high capacity and a long service life, in an Mm-Ni-Co-Mn-Al series alloy, by substituting a part of Mm by Zr. CONSTITUTION:As a hydrogen occluding electrode, an alloy expressed by a general formula :Mm1-xZrxNiACoBMnCAlD is used. In the formula, Mm denotes a misch metal as well as 0.014<=x<=0.08, 4.9<=A+B+C+D<=5.1, 3.2<=A<=3.6, 0.5<=B<=1.0, 0.3<=C<=0.5 and 0.3<=D<=0.5 are satisfied. By the alloy having this compsn., an excellent electrode having a high charging-discharging capacity and high cycle life properties can be formed. In the starting material, inexpensive raw materials such as Mm including total >=0.5wt.% impurities such as Mg and Cl can be used as well as the content of expensive Co is low, so that the alloy is formed into a one having high practicality as an inexpensive hydrogen occluding electrode.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a nickel-metal hydride secondary battery that uses a hydrogen storage alloy as a negative electrode and a nickel oxide electrode as a positive electrode. The present invention relates to a hydrogen storage electrode that exhibits little deterioration in characteristics even after repeated charge/discharge cycles over a long period of time, and that exhibits little decrease in discharge capacity even during large current discharge.

[Conventional technology]

In order to improve energy storage capacity, a nickel-metal hydride secondary battery was proposed, using a hydrogen storage alloy that reversibly stores and releases hydrogen as the negative electrode, and uses the stored hydrogen as the active material, and development is urgently needed. ing. The hydrogen storage alloy used for this purpose is required to meet the following requirements.

-XZrXNiACoBMnCAlD) The effective hydrogen storage amount, that is, the electric capacity is large.

(2) The hydrogen equilibrium dissociation pressure is
C) should be 10-'' to several atmospheres.

(3) Excellent corrosion resistance in concentrated alkaline electrolyte.

(4) The speed of pulverization due to repeated electrode reactions is slow.

(5) There should be no change in composition due to elution of some specific elements due to repeated polar reactions.

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

(7) It should be inexpensive.

Mitshu metal (Ml) has been known as an inexpensive raw material containing rare earth elements. It is a mixture of rare earth metals, typically La 25-35
% by weight, 45 to 55% by weight of Ce, and 10 to 15% by weight of Nd. In a hydrogen storage alloy using h as a raw material, the hydrogen dissociation pressure becomes high because the amount of Ce in the rare earth metal is large.

In the MmNi hydrogen absorbing alloy storage, in order to make the hydrogen dissociation pressure about 1 atm at the battery operating temperature, part of the Ni is h,^1
It is necessary to replace it with an element such as %CO.

In a HINiCoAl-based hydrogen storage alloy in which a portion of Ni is replaced with Co and AI, the larger the amount of Co substitution, the smaller the effective hydrogen storage amount, and the smaller the discharge capacity when used as an electrode. Therefore, it is a good idea to keep the Co substitution amount as low as possible within a range that allows good cycle life characteristics to be maintained.

Furthermore, by forming a dense oxide film on the surface of the alloy powder, AI suppresses oxidation of the alloy and improves the cycle life. However, excessive AI replacement causes the formation of an oxide film, which contributes to increasing the reaction resistance of the electrode.
It acts in the direction of worsening rapid discharge characteristics and discharge characteristics at low temperatures. Therefore, it is advisable to keep the A11l exchange amount to the minimum amount necessary to obtain appropriate hydrogen dissociation pressure and cycle life characteristics. Alloys made with this in mind include MaNis, 5Coo, and ? A111
．． There is one alloy, but its discharge capacity is 254tsA -h
/g, and cannot necessarily be said to be large.

[Problem to be solved by the invention]

In this way, in the MmNiCoAl system, 260 mA-h/
Since it is not possible to obtain an alloy that has both a large discharge capacity of more than 100 g and good cycle life characteristics, some of the Ni
MmNiCoMnAl alloys substituted with n are considered to be promising. This Mn substitution is useful in lowering the hydrogen dissociation pressure without reducing the hydrogen storage capacity (discharge capacity) of the alloy.
n can be used as a substitution element in place of Co'pAl.

However, with repeated charging and discharging, it has been confirmed that Mn near the surface of the alloy powder is eluted into the electrolyte, which has the disadvantage of reducing the cycle life. Therefore, when performing Mn substitution, it is necessary to simultaneously search for measures to prevent the elution of Mn and prevent deterioration of cycle life characteristics, but no reliable method has been found to date. .

In the present invention, by replacing a part of h with a small amount of Zr,
The purpose of this invention is to provide an alloy for hydrogen storage electrodes with high capacity and long life.

[Means to solve the problem]

The present invention achieves the above object by general MI11. −.

It is characterized by using a hydrogen storage alloy represented by Zr, N1ACo*MncAlo.

However, in the above general formula, h■ is Mitsushmetal, and 0.01≦X≦0.08.4.9≦A+B+C+
D≦5.1.3.2≦A≦3.6.0.5≦B≦1.0
．． 0.3≦C≦0.5 and 0.3≦D≦0.5.

Among the ores containing rare earth elements, Mitsushmetal used in the present invention includes various types such as monazite and bastnasite, which contain a large amount of light rare earth, and xenotime, which contains a large amount of heavy rare earth, and they do not require any separation operation. Mixed powder ± (Mitushmetal), Ce, Eu, SIl,
A mixed rare earth or the like after separating useful elements such as Dy, Gd, Tb, and Yb is used.

Sample A shown in Table 1 below is unseparated Mitshu metal that is commonly used and has a high Ce content, so although it has a low electric capacity, it has a long cycle life and is the cheapest. Samples B1C and D are samples in which elements such as Ce are separated, but as the La content increases, the capacitance increases, but the cycle life tends to become shorter, and the price also increases. Therefore, depending on the purpose of the battery,
You can use these Mitsushi metals properly.

In addition, as methods for producing rare earth metals, there are oxide electrolysis using rare earth oxides as raw materials and chloride electrolysis using chlorides as raw materials. Chloride electrolysis is better for producing Mg, CI, Fe5S.
Although it has a high impurity concentration, it is inexpensive.

However, if misch metal with a high content of Mg or CI is used, problems such as damage to the electric furnace occur during alloy production, and the total cost cannot be compared.

Sample E was prepared from chloride, but depending on the alloy composition, the capacitance and cycle life may be significantly reduced, which may be due to impurities.

However, as shown in the examples below, adding Mn can weaken the influence of impurities to some extent, so there is no practical problem.

(Margins below this page) As shown in the above compositional formula as a hydrogen storage alloy, Mm
By using an alloy in which a part of Ni is replaced with Zr and a part of Ni with COlMn and AI for the electrode, Zr has an effect of improving the cycle life and suppressing Mn elution on the alloy surface, so that it can be used for long-term repeated charging and discharging. We were able to create a hydrogen storage electrode whose properties do not deteriorate. Z like this
The effect of r is thought to be exerted by forming a dense oxide film on the alloy surface.

Examples of the present invention will be described below.

Examples 1 to 3, Comparative Examples 1 to 6 In order to confirm the effect of the present invention, hydrogen storage alloys shown in Table 2 were obtained by arc melting in an argon atmosphere. Note that Sample A in Table 1 was used as the Mitsushi metal.

After the alloy was mechanically pulverized, a copper coating layer corresponding to about 20% overlap was formed on the surface of the alloy powder by electroless copper plating. To this alloy powder, an amount equivalent to 10% by weight of FEP (tetrafluoroethylene/fluorinated propylene copolymer) resin was added as a binder, and a powder mixture of about 300 cm (alloy weight: about 2
16o+g) was formed into an electrode pellet having a diameter of 13a x 0.4a and a thickness of 0.4a by cold pressing. This was hot pressed together with a nickel mesh serving as a current collector at a temperature of 300''C to obtain an alloy electrode for testing.

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 all test batteries were negative electrode regulated types in which the battery capacity depends on the capacity of the negative electrode, and a mercury oxide electrode was used as the reference electrode. This test battery was charged for 2.5 hours at a charging current of 40 amA in a constant temperature room at a temperature of 20″C.
A long-term charge/discharge cycle test was performed by stopping for 0.5 hours and then discharging at a discharge current of 20 mA until the potential difference between the reference electrode and the hydrogen storage electrode decreased to -0.6 V.

The test results for each alloy are shown in Table 2.

Here, the value obtained by dividing the discharge capacity 300 cycles after reaching the initial maximum capacity by the initial maximum capacity is treated as the capacity retention rate, and is treated as an index indicating the cycle life characteristics. In addition, in each alloy shown in Table 2, the amount of AI substitution is adjusted so that the equilibrium hydrogen dissociation pressure at room temperature is approximately equal to 1 atm or less.

(Margins below this page) The alloys of Comparative Examples 1 and 2 are Ms+NiCoAl-based alloys, and the amounts of Co and AI substituted are changed, respectively.

The alloy of Comparative Example 2 has less Co and AI than the alloy of Comparative Example 1.
Since the amount of replacement is small, the initial discharge capacity is large, but practically it is desirable that it be 260 mA-h/g or more, and some improvement measures are required. Note that the cycle life characteristics of the alloys of Comparative Examples 1 and 2 are both very good. In order to obtain a discharge capacity higher than Comparative Example 20 alloy with this alloy system, it is necessary to further reduce the amount of CO and AI replacement, but from the viewpoint of ensuring cycle life characteristics and adjusting the hydrogen dissociation pressure, It is practically impossible to further reduce the amount of Co and AI replaced.

On the other hand, the alloy of Comparative Example 3 is a composition obtained by adding Mn substitution to the alloy of Comparative Example 1, and although the initial discharge capacity is increased, the cycle life characteristics are deteriorated.

Moreover, Comparative Example 40 alloy is the alloy of Comparative Example 2 with Mn substitution added, and the same tendency as above is observed. Comparative Example 4 in which discharge capacity was increased by these Mn substitutions
The cycle life characteristics of alloys shown in the following are deteriorated and cannot be said to be sufficient for practical use. This is because, as described above, Mn near the surface of the alloy powder is eluted into the electrolyte with repeated charging and discharging.

The alloys of Examples 1.2 and 3 are alloys according to the present patent in which a part of Mm is replaced with Zr (substitution ratio 0.05). The alloy of Example 2 has the same compositional Ma as that of the alloy of Comparative Example 4.
Although a part of + was replaced with Zr, the initial discharge capacity was reduced, but the cycle life characteristics were significantly improved. On the other hand, the alloy of Comparative Example 5 is
It is an alloy in which a part of h is replaced with Zr, and similarly,
Although the initial discharge capacity has increased, it cannot be said that it has sufficient capacity from a practical standpoint. In this alloy, it is thought that the fact that the amount of Co substitution is greater than that in the alloy of Example 3 causes a relative decrease in capacity. Further, in the alloy of Example 1, the Co substitution amount was set to a value between those of the alloy of Comparative Example 5 and the alloy of Example 2, and the capacity was also set to a value between them. The cycle life characteristics of this alloy are better than those of Comparative Example 30 alloy and Example 2 alloy, probably due to the fact that the amount of Mni & replacement is somewhat reduced and the amount of A1 replaced is increased. The alloy of 3 is
In order to increase the initial discharge capacity within a range that has cycle life characteristics at a practical level, the amount of Co and Mn substitution is reduced, and compared to Comparative Example 40 alloy, the cycle life is shorter even with the same discharge capacity. It has improved considerably. These are thought to be because by substituting a portion of Mm with Zr, a strong oxide film is formed on the alloy surface, suppressing oxidation of the rare earth metal and suppressing the elution of Mn.

However, excessive Zr substitution only forms intermetallic compounds (such as ZrNiz) between Zr and other elements, resulting in a reduction in the effective hydrogen storage amount. An example thereof is shown in Comparative Example 6.

The alloy of Comparative Example 6 is a similar alloy with a Zr substitution ratio of 0.1, but the initial discharge capacity is considerably smaller than that of the alloy of Example 2. According to the data obtained by the inventor, La
It is appropriate that the substitution ratio of Zr to (Mid) be 0.08 or less.

From the above results, the general formula Mml-gZrxNiaco1
The newly developed hydrogen storage electrode alloy designated by MncAID has a composition of 0.05≦X≦0.06, Y=5.0 (
However, Y=A+B+C+D), 3.2≦A≦3.6.0
．． 5≦B≦1.0.0.3≦C≦0.5.0.4≦D≦
When the value is 0.5, good electrode characteristics are exhibited.

Also, although it was not mentioned in the previous tests, the general formula La
The composition of the alloy represented by (Mm)Niy is La(Ma+
): When Ni deviates from the stoichiometric composition of 1:5 (Y=5), the cycle life characteristics deteriorate. The problem is the degree of deterioration, but LazNdo, l5Zro,
95 Nis, *Coo, ? A10. When examining S-based alloys, it was found that compared to the stoichiometric (Z = 0.80) alloy, the non-stoichiometric alloy with Z = 0.78 had a 14% reduction in stoichiometry, and the non-stoichiometric alloy with Z = 0.82 had a 15% for non-stoichiometric alloys;
In each case, the discharge capacity retention rate after 300 cycles decreased. If the rate of decrease in capacity maintenance rate is set to be 15% or less as an acceptable limit, then the above La (Mm)
Ni, the value of Y in the alloy represented by the formula is 4.9 to 5.1
It is necessary to be within the range of . This means that when the value of Y in the composition formula shown below is less than 5.0, intermetallic compounds such as LazNit are used, and when the value of Y exceeds 5.0, N is used.
It is thought that the single i phase appears in the alloy, leading to deterioration in cycle life characteristics.

Therefore, if a certain degree of initial capacity reduction and cycle life deterioration are allowed, the above optimal composition range should be set to 0.01.
It is considered that there is no problem even if it is expanded to ≦X≦0.08.4.9≦Y≦5.1.

In addition, the above test results are at a temperature of 20℃, -2
For batteries that are used at low temperatures of around 0°C, it is necessary to reduce the amount of AI replacement to avoid the associated rise in overvoltage, so the appropriate range of the amount of AI replacement is set to 0.3≦D≦0.
5. As mentioned earlier, if the amount of AI replacement is too small, it may deteriorate the cycle life characteristics.
If Mitshu metal containing a large amount of Ce or Nd, which is effective in improving lifespan, is used, it is considered that sufficient cycle life can be ensured even if 0.4≧D≧0.3.

In this way, by replacing a portion of h- with Zr and a portion of Ni with CO, Mn, and AI, and by adopting an appropriate component design that minimizes the amount of each substitution, high capacity can be achieved. It can be made into an alloy for a long-life hydrogen storage electrode.

Examples 4 to 7, Comparative Examples 7 to 14 Test alloy electrodes were manufactured in the same manner as Examples 1 to 3, except that Sample E in Table 1 was used as Misoshu metal,
A charge/discharge cycle test was conducted. The results are shown in Table 3 below.

(Margins below this page) The alloy of Comparative Example 7 made using M (sample A) containing a large amount of Ce, which has been used in the past, has a lower initial discharge capacity than the alloy system using La. Although small, the cycle life characteristics are very good.

The alloy of the comparative example day was Ms containing many Mg and C1 impurities.
(Sample E in Table 1, written as 'Mm in the table to avoid confusion with conventionally commercially available Mm), and MmNi5.5CO of Comparative Example 7.
-XZrXNiACoBMnCAlD, the initial discharge capacity is small compared to the 7AI0.1 alloy, and the cycle life is extremely short. On the other hand, the alloys of Comparative Example 9.10 and Process 1 have a part of Ni replaced with Mn, and the alloys of Comparative Example 7
Both the initial discharge capacity and cycle life characteristics are greatly improved compared to the alloy. It should be noted that the larger the Ni composition ratio, that is, the latter in the order of Comparative Examples 9, 10, and 11.
The initial discharge capacity is large, and the cycle life characteristics tend to be poor. In general, Mn substitution increases the hydrogen storage capacity of the alloy and increases the discharge capacity, but it deteriorates the cycle life characteristics due to the elution of Mn near the surface of the alloy. However, as can be seen by comparing both the alloy of Comparative Example 8 and the alloy of Comparative Example 11, a tendency different from the conventional tendency of decreasing cycle life characteristics due to Mn substitution is observed. It is assumed that this is because Mn substitution forms a stable compound with Mg and CI impurities and is removed as floating substances, but the detailed reason is unknown. In any case, it is shown that Mn substitution has the effect of purifying the alloy.

Next, Comparative Example 12 is an alloy with a non-stoichiometric composition in which Mm was set to be included in excess and Mn substitution was performed in the same manner as above. This alloy is based on Comparative Example 10 in terms of composition, but has a slightly larger initial discharge capacity. On the other hand, in the alloy of Example 4, the excess of M- was slightly reduced,
With the addition of a small amount of Zr, the initial discharge capacity is further increased and the cycle life characteristics have also been improved to almost a practical level. Compared to the alloy of Example 4, the initial discharge capacity of the alloy of Comparative Example 12, which has a higher Ma+ abundance ratio, is lower because the excess Mm forms another phase such as MmJi7, which absorbs and releases hydrogen. This is because the phase of the Ma+N i s structure that contributes to decreases. This is considered to be the cause of the low cycle life of the alloy of Comparative Example 12.

The alloys of Examples 5.6 and 7 are alloys in which Zr is additionally added in place of excess Mm. According to the inventor's experience, Zr is partially substituted for h in terms of the crystal structure. This Zr
Due to the increased substitution amount, the alloy of Example 6 has a smaller initial discharge capacity than the alloy of Example 4, but has better cycle life characteristics. This is Zr
This is thought to be due to the formation of a strong oxide film on the alloy surface, which suppresses oxidation of the rare earth metal. The alloy of Example 5 is
This is an alloy in which good cycle life characteristics are ensured by adding a small amount of Zr and increasing the amount of Co substitution, while reducing the amount of Mn substitution.

In this alloy, the initial discharge capacity is somewhat lower than that of the alloy of Example 6 because the amount of Mn substitution is small. In addition, the alloy of Example 7 has a slightly lower amount of Co substitution than the alloy of Example 6, and the amount of hydrogen storage increases accordingly, so the initial discharge capacity increases and the cycle life characteristics are slightly reduced. It has become a degraded alloy.

The alloy of Comparative Example 13 is an alloy based on the alloy containing excessive Mm of Comparative Example 12, and a part of it is replaced with Zr,
Compared to the alloy of Comparative Example 12, although the discharge capacity is lower, the cycle life characteristics are somewhat improved. Further, although the alloy of Comparative Example 14 is based on the alloy of Example 6 with an increased amount of Zr added, the initial discharge capacity is significantly reduced. As shown in Comparative Examples 12, 13, and 14, alloy systems in which the combined amount of Mn + and Zr is too excessive, that is, 1.1 or more Mm + Zr with respect to 5 on the Ni side, have the highest capacity. An alloy with an appropriate composition that increases the (Example 4: N
The initial discharge capacity is smaller than that for the i-side 5 and the Mm f Zr side 1.05, and even if Zr has a life-improving effect, the cycle life characteristics generally tend to deteriorate. This is because, as mentioned earlier, when excessive H is added, metal compounds such as MmzNit are formed, and when excessive Zr is added, ZrN1z etc. are formed as intermetallic compounds, and effective hydrogen is not produced. This is because it results in a reduction in manufacturing and storage space. The total amount of impurities in the table including Mg and CI used in this test was about 0.6% by weight, and these impurities
Since it is removed as a composite compound with Mm by replacing H-, it is thought that good electrode characteristics were obtained with the alloy shown in the example in which the depleted H- was replenished. It is thought that the alloy shown in Example 4 containing 5% by weight of excess Mm + Zr has a composition close to the stoichiometric composition.

In order to prevent the appearance of other unnecessary intermetallic compounds, alloys not listed in Table 3 were investigated using means such as X-ray diffraction, and the excess on the Mm f Zr side was 0.08 or less. It is appropriate to

Usually, the composition of the alloy represented by the general formula La(Mm)Ni,f is La(Ma+):Ni of 1:5 (
If the composition deviates from the stoichiometric composition of Y=5), the cycle life characteristics deteriorate. For the time being, if the rate of decrease in capacity maintenance rate is set to be 15% or less as an acceptable limit, then the above La
The value of Y in the alloy expressed by the (Mm)Niy formula is 4.9
It is necessary to be within the range of ~5.1.

This means that when the value of Y in the above compositional formula is less than 5.0, intermetallic compounds such as LazNit are used when the value of Y is less than 5.0.
If it exceeds 0, a single Ni phase appears in the alloy, which is considered to cause deterioration of cycle life characteristics.

From the above results, the general formula MmZr, lNiaCOIMn
The newly developed alloy for hydrogen storage electrodes, denoted by cAln, is
Its composition is 0.01≦X≦-0,05,3,2≦A≦3
．． 6.0.6≦B≦1.0.0.3≦C≦0.5.0.
When 4≦D≦0.5, the best electrode characteristics are exhibited. However, if a certain degree of decrease in initial capacity and deterioration in cycle life is acceptable, the above optimum composition range can be expanded to 0. It is considered that there is no problem within the range of O1≦X≦0.08.4.9≦A0B0C+D≦5.1. All of the alloys shown in the examples used C) MmNi5.5Coo, Jio, *alloy compared to Comparative Example 1 using conventional h.
It has a very large discharge capacity while having practical cycle life characteristics.

It has been described above that the alloy of the present invention exhibits excellent electrode properties when used as a hydrogen storage electrode for nickel-metal hydride secondary batteries, including the mechanism of its effect. This electrode is common to secondary batteries that use an alkaline electrolyte, and can also be used for secondary batteries that use manganese dioxide as a material.

[Effect of the invention] Part of Mm is Zr, part of Ni is Co and Mn, AI
ZrXNiACOllM substituted with
n (The hydrogen storage alloy represented by AID has an alloy composition of 0.01≦X≦0.08.4.9≦A+B+C1D≦5
．． 1.3.2≦A≦3.6.0.5≦B≦1.0.0.
By setting the range of 3≦C≦0.5, it is possible to obtain an electrode that has excellent electrode properties such as a large charge/discharge capacity and a high cycle life characteristic.

Moreover, impurities such as Mg and CI are contained in the raw materials by 0.5 of the total amount.
It can be used as a cheap raw material such as h containing more than % by weight, and the content of expensive Co is small, making it highly practical as a low-cost hydrogen storage electrode.

Claims (1)

[Claims] General formula Mm_1_-_XZr_XNi_ACo_BMn
A hydrogen storage electrode characterized by using a hydrogen storage alloy represented by _CAl_D. However, in the above general formula, Mm is misch metal, 0.01≦X≦0.08, 4.9≦A+B+C+
D≦5.1, 3.2≦A≦3.6, 0.5≦B≦1.0
, 0.3≦C≦0.5 and 0.3≦D≦0.5.