JPH0514018B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] 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 use over a long period of time, and further exhibits little decrease in discharge capacity even during large current discharge. [Conventional technology] In order to improve energy storage capacity, a hydrogen storage alloy that reversibly stores and releases hydrogen is used as a negative electrode, and a nickel alloy that uses the stored hydrogen as an active material is used as a negative electrode.
Metal hydride secondary batteries have been proposed and are being rapidly developed. The hydrogen storage alloy used for this purpose is required to meet the following requirements. (1) The effective hydrogen storage capacity, that is, the electrical capacity, is large. (2) Hydrogen equilibrium dissociation pressure at battery operating temperature (-20 to 60
℃) and 10 ^-3 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 electrode reactions. (6) High hydrogen diffusion rate and reaction resistance (overvoltage)
is small. (7) It must be inexpensive. Mitsushi Metal (Mm) has long been known as an inexpensive raw material containing rare earth elements. It is a mixture of rare earth metals, usually La25~35% by weight, Ce45~55% by weight, Nd10~
Consists of 15% by weight. In this hydrogen storage alloy made from Mm, the hydrogen dissociation pressure is high because the amount of Ce in the rare earth metal is large. In MmNi-based hydrogen storage alloys, in order to make the hydrogen dissociation pressure about 1 atm at the battery operating temperature, it is necessary to partially replace Ni with elements such as Mn, Al, and Co. In MmNiCoAl-based hydrogen storage alloys in which a portion of Ni is replaced with Co and Al, the larger the Co substitution amount, 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. In addition, Al forms a dense oxide film on the surface of the alloy powder, thereby suppressing oxidation of the alloy and improving cycle life. However, Al substitution at high temperatures causes the formation of an oxide film, which contributes to increasing the reaction resistance of the electrode, and has the effect of deteriorating rapid discharge characteristics and low-temperature discharge characteristics. Therefore, it is advisable to keep the Al substitution amount to the minimum amount necessary to obtain appropriate hydrogen dissociation pressure and cycle life characteristics.
Alloys made with this in mind
There is a MmNi _3.5 Co _0.7 Al _0.8 alloy, but its discharge capacity is about 254 mA·h/g, which is not necessarily large. [Problem to be solved by the invention] In this way, in the MmNiCoAl system, 260mA・h/
Since it is not possible to obtain an alloy that has both a large discharge capacity of more than g and good cycle life characteristics,
MmNiCoMnAl alloys, in which part of Ni is replaced with Mn, are considered promising. This Mn substitution is useful in reducing the hydrogen dissociation pressure without reducing the hydrogen storage capacity (discharge capacity) of the alloy, and Mn can be used as a substitution element in place of Co and Al. 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 shortening the cycle life. Therefore, when performing Mn substitution, it is necessary to simultaneously search for measures to prevent the elution of Mn and prevent the deterioration of cycle life characteristics, but no reliable method has been found to date. Nakatsuta. The present invention aims to provide an alloy for hydrogen storage electrodes with high capacity and long life by replacing a part of Mm with a small amount of Zr. [Means for Solving the Problems] The present invention that achieves the above object has the general formula Mm _1-X
It is characterized by using a hydrogen storage alloy represented by Zr _X Ni _A Co _B Mn _C Al _D. However, in the above general formula, Mm is Mitsushi 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. Among ores containing rare earth elements, there are various types of Mitsushimetal used in the present invention, 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. Mixed rare earths (Mitsushimetal) that are not subjected to this process or mixed rare earths that have been separated from useful elements such as Ce, Eu, Sm, Dy, Gd, Tb, and Yb are used. Sample A shown in Table 1 below is a commonly used final separated Mitsushi Metal.
Due to the high Ce content, the capacitance is low, but the cycle life is long, and it is the cheapest. Sample B,
C and D are separated elements such as Ce, but as the La content increases, the electric capacity increases, but the cycle life tends to become shorter.
Prices will also rise. Therefore, these Mitsushi metals can be used depending on the purpose of the battery. 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.
Although it has a high concentration of impurities such as Mg, Cl, Fe, and Si, it is inexpensive. However, using Mitsushi Metal, which has a high content of Mg and Cl, causes problems such as damage to the electric furnace during alloy production, and the total cost cannot be compared. Mm manufactured from chloride was used as sample E, but depending on the alloy composition, it may cause a significant decrease in electric capacity and cycle life, which may be caused by impurities. However, as shown in the examples below, the effect of impurities added with Mn can be weakened to some extent, so there is no problem in practice.

[Table] As shown in the above composition formula as a hydrogen storage alloy, part of Mm is Zr, and part of Ni is Co, Mn,
By using an alloy substituted with Al for the electrode,
Because of the cycle life improvement effect of Zr and the effect of suppressing Mn elution on the alloy surface, we were able to create a hydrogen storage electrode whose characteristics do not deteriorate even after repeated charging and discharging over a long period of time. This effect of Zr 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 Mitsushi Metal. After the alloy was mechanically pulverized, a copper coating layer equivalent to about 20% by weight was formed on the surface of the alloy powder by electroless copper plating. FEP (tetrafluoroethylene/fluoropropylene copolymer) resin is added to this alloy powder as a binder.
Approximately 300 mg of powder mixture (alloy weight: approx. 216 mg) was added in an amount equivalent to 10% by weight and was cold pressed to a diameter of 13 mm.
It was molded into an electrode pellet with a shape of 0.4 mm x 0.4 mm.
This is used together with the nickel mesh that becomes the current collector.
Alloy electrodes for testing were prepared by hot pressing at a temperature of 300°C. A test battery was constructed using this hydrogen storage electrode as a negative electrode, a nickel oxide electrode similar to that used in a 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 mA in a constant temperature room at a temperature of 20°C, and after resting for 0.5 hours, the potential difference between the reference electrode and the hydrogen storage electrode decreased to -0.6 V at a discharge current of 20 mA. A long-term charge-discharge cycle test was conducted using a cycle in which the battery was discharged until 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 cycle life characteristics. In each alloy shown in Table 2, the amount of Al substitution is adjusted so that the equilibrium hydrogen dissociation pressure at room temperature is approximately equal to 1 atm or less.

【table】

[Table] The alloys of Comparative Examples 1 and 2 are MmNiCoAl-based alloys, and have different amounts of Co and Al substitution. Compared to the alloy of Comparative Example 1, the alloy of Comparative Example 2 has a smaller amount of Co and Al substitution, so the initial discharge capacity is larger, but for practical purposes it is only 260 mA.
It is desirable that it is higher than h/g, and some improvement measures are necessary. 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 that of the alloy of Comparative Example 2 with this alloy system, it is necessary to further reduce the amount of Co and Al substitution, 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 Al substitution. On the other hand, the alloy of Comparative Example 3 has a composition in which Mn substitution is added to the alloy of Comparative Example 1, and although the initial discharge capacity is increased, the cycle life characteristics are deteriorated. In addition, the alloy of Comparative Example 4 is the alloy of Comparative Example 2 with Mn substitution added,
The same tendency as above is observed. In the alloy shown in Comparative Example 4, in which the discharge capacity is increased by Mn substitution, the cycle life characteristics deteriorate, and it cannot be said to be practically sufficient. 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 contain a portion of Mm.
This is an alloy related to the patent of this application in which Zr is substituted (substitution ratio 0.05). The alloy of Example 2 is compositionally similar to Comparative Example 4.
Although some of the Mm in the alloy was replaced with Zr, the initial discharge capacity was reduced, but the cycle life characteristics were significantly improved. On the other hand, in the alloy of Comparative Example 5, a part of Mm in the alloy of Comparative Example 3 was replaced with Zr, and although the initial discharge capacity was similarly increased, it was still sufficient from a practical standpoint. It cannot be said that it has the capacity. It is considered that in this alloy, the amount of Co substitution is larger than that in the alloy of Example 3, which causes a relative decrease in capacity. Furthermore, although the Co substitution amount of the alloy of Example 1 was set to an intermediate value between those of the alloy of Comparative Example 5 and the alloy of Example 2, the capacity was also set to a value between them. Comparative example 3 shows cycle life characteristics of this alloy.
The reason why this alloy is better than the alloy of Example 2 is probably due to the fact that the amount of Mn substitution is somewhat reduced and the amount of Al substitution is increased. The alloy of Example 3 has Co and Co in order to increase its initial discharge capacity within a range that has almost practical cycle life characteristics.
Comparative example 4 has a reduced amount of Mn substitution.
The cycle life is considerably improved compared to the alloy with the same discharge capacity. 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 results in the formation of metal compounds (such as ZrNi ₂ ) with Zr and other elements, which reduces the effective hydrogen storage capacity. An example thereof is shown in Comparative Example 6. The alloy of Comparative Example 6 is a similar alloy with a Zr substitution rate of 0.1, but the initial discharge capacity is similar to that of Example 2.
It is considerably smaller than the alloy. According to the data obtained by the inventors, it is appropriate to set the substitution ratio of Zr to La (Mm) at 0.08 or less as long as other unnecessary intermetallic compounds do not appear. From the above results, the general formula Mm _1-X Zr _X Ni _A Co _B Mn _C
The newly developed alloy for hydrogen storage electrodes denoted by Al _D has a composition of 0.05≦X≦0.06, Y=5.0 (for example, Y=A+B+C+D), 3.2≦A≦3.6, 0.5≦B
By satisfying ≦1.0, 0.3≦C≦0.5, and 0.4≦D≦0.5, good electrode characteristics are exhibited. Also, although it was not mentioned in the previous tests, the composition of the alloy represented by the general formula La(Mm)Ni _Y is
When (Mm):Ni deviates from the stoichiometric composition of 1:5 (Y=5), cycle life characteristics deteriorate. The problem is the degree of deterioration, but La ₂ Nd _0.15 Zr _0.95 Ni _3.8
When investigating a Co _0.7 Al _0.5 series alloy, Z = 0.78 compared to an alloy with a stoichiometric composition (Z = 0.80).
The discharge capacity retention rate after 300 cycles decreased by 14% for the non-stoichiometric alloy with Z = 0.82, and by 15% for the non-stoichiometric alloy with Z = 0.82. If we set the permissible limit as the rate of decrease in capacity retention rate of 15% or less, then the value of Y in the alloy expressed by the above La(Mm)Ni _Y formula should be in the range of 4.9 to 5.1. It is necessary that there be. This means that the value of Y in the above composition formula is
If the Y value is less than 5.0, intermetallic compounds such as La ₂ Ni ₇ will appear in the alloy, and if the Y value exceeds 5.0, a single Ni phase will appear in the alloy, leading to deterioration of cycle life characteristics. It is thought that there are. Therefore, it is considered that there is no problem in extending the above optimum composition range to 0.01≦X≦0.08 and 4.9≦Y≦5.1, as long as a certain degree of decrease in initial capacity and deterioration in cycle life are allowed. In addition, the above test results are at a temperature of 20℃, and for batteries used at temperatures as low as -20℃, it is necessary to reduce the amount of Al substitution to avoid the resulting increase in overvoltage. The appropriate range of Al substitution amount is 0.3≦D≦0.5. As mentioned earlier, if the amount of Al substitution is too small, it may deteriorate the cycle life characteristics, but when using Mitsushi Metal containing a lot of Ce and Nd, which are effective in improving life, it is 0.4≦D≦0.3. However, it is thought that sufficient cycle life can be secured. In this way, part of Mm is Zr and part of Ni is
By replacing Co with Mn and Al and adopting an appropriate composition design that minimizes the amount of each substitution, it is possible to create an alloy for hydrogen storage electrodes with high capacity and long life. Examples 4 to 7, Comparative Examples 7 to 14 Test alloy electrodes were manufactured in the same manner as in Examples 1 to 3, except that Sample E in Table 1 was used as Mitsushi Metal, and a charge/discharge cycle test was conducted.
The results are shown in Table 3 below.

〔Effect of the invention〕

Part of Mm is Zr, part of Ni is Co, Mn, Al
The hydrogen storage alloy represented by the general formula Mm _1-X Zr _X Ni _A Co _B Mn _C Al _D has an alloy composition of 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, so that the electrode has excellent electrode characteristics such as large charge/discharge capacity and high cycle life characteristics. be able to. In addition, it can be used for inexpensive raw materials such as Mm, which contains impurities such as Mg and Cl at 0.5% by weight or more of the total amount, and the content of expensive Co is low, making it a low-cost hydrogen storage material. It has become highly practical as an electrode.

Claims (1)

[Claims] 1. A hydrogen storage electrode characterized by using a hydrogen storage alloy represented by the general formula Mm _1-X Zr _X Ni _A Co _B Mn _C Al _D. However, in the above general formula, Mm is Mitsushi 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.