JPH1079247A - Surface treatment method for hydrogen storage alloy negative electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve cycle characteristic and initial activity by surface-treating a negative electrode active material by chromium oxide. SOLUTION: An oxide layer to be formed on the surface of a negative electrode and for impeding storage and discharge of hydrogen can be dissolved and removed by bringing chromium oxide solution in contact with the front surface of a hydrogen storage alloy negative electrode. Therefore, initial activity of a battery can be enhanced. A chromium oxide coating film is formed on the surface of surface-treated hydrogen storage alloy. Accordingly, an oxide layer is not easily generated in a negative electrode during the charge and discharge cycle in the battery. Accordingly, cycle characteristic of the battery can be improved. It is desirable that the chromium oxide solution contains hexavalent chromium ion and that pH is 3 or less. Therefore, an oxide layer formed on the alloy surface is easily removed, and a chromium oxide coating film excellent in corrosion resistance can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a surface treatment method for a hydrogen storage alloy negative electrode used as a negative electrode of various secondary batteries such as a nickel-metal hydride battery.

[0002]

Description of the Prior Art Conventionally, for example LaNi ₅ or MmNi
₅ (Mm represents a misch metal, lanthanum and specifically refers to a mixture of rare earth elements such as cerium) an AB ₅ type alloys typified or ZrV _0.4 Ni _1.6 etc. Ti,
A hydrogen storage alloy such as an AB ₂ type alloy represented by a ZrVNi-based Laves phase alloy can store and release hydrogen reversibly because the equilibrium pressure at room temperature is about 1 atm.

[0003] Therefore, the above-mentioned hydrogen storage alloy is applied as a negative electrode active material of a secondary battery which can be used repeatedly by charging. The negative electrode reaction in the battery in this case is considered as follows. M + H ₂ O + e ⁻ ← MH + OH ⁻ M indicates a hydrogen storage alloy, and the reaction proceeds rightward during charging and proceeds leftward during discharging. Examples of such a secondary battery include a nickel-metal hydride battery. This battery comprises a hydrogen storage alloy negative electrode, a nickel positive electrode, and an electrolyte filled between the two electrodes. In addition,
The electrolyte is an alkaline aqueous solution.

However, the above-mentioned hydrogen storage alloy has a problem that an oxide layer is easily formed on the surface thereof, and this oxide layer hinders the storage and release of hydrogen. In addition, the nickel-metal hydride battery has a problem that the negative electrode capacity of the battery is very small in an initial use stage (a stage in which the number of charge / discharge cycles is small).

That is, in this nickel-metal hydride battery, in the initial use stage where the number of charge / discharge cycles is small, the surface of the hydrogen-absorbing alloy negative electrode is provided with a catalyst layer for electrolyzing water and activating the generated hydrogen. (Eg Ni layer)
Is not formed. Therefore, the hydrogen storage / release reaction at the hydrogen storage alloy negative electrode hardly occurs, and the negative electrode capacity is in a small state. This problem is referred to as low initial activity.

To solve these problems, Japanese Patent Laid-Open No.
No. 13077, JP-A-4-137361 and the like have proposed a method for activating a hydrogen storage alloy negative electrode by immersing a hydrogen storage alloy powder in a high-temperature alkaline aqueous solution. That is, in this method, Mm, Co, Al, Mn, and the like are eluted from the hydrogen storage alloy into the alkaline aqueous solution, and a Ni catalyst layer is formed by Ni metal remaining on the surface.

[0007]

However, the above activation method has the following problems. That is, a large amount of oxygen is usually dissolved in the alkaline aqueous solution. For this reason, the dissolved oxygen and the eluted metal react with each other, and a large amount of an oxide layer is again formed on the surface of the hydrogen storage alloy with the passage of time. Therefore, in the above-described nickel-metal hydride battery, the capacity of the negative electrode decreases with time during the charge / discharge cycle. That is, the cycle characteristics of the hydrogen storage alloy negative electrode subjected to the activation treatment were low.

Therefore, Japanese Patent Application Laid-Open No. 3-49154 proposes that a salt of phosphoric acid or chromic acid is added to the negative electrode active material comprising the above-mentioned hydrogen storage alloy to simultaneously improve both the initial activation and the cycle characteristics. Have been. However, when the above-mentioned salt was used, a high initial activity could not be obtained.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a surface treatment method for a hydrogen storage alloy negative electrode which can provide a secondary battery having excellent cycle characteristics and excellent initial activity.

[0010]

According to a first aspect of the present invention, a negative electrode active material comprising a hydrogen storage alloy or a negative electrode active material comprising an activated hydrogen storage alloy is subjected to a surface treatment with a chromium oxide solution. An alloy negative electrode has a surface treatment method.

As the chromium oxide, Cr ₂ O ₃ , C
There are rO _{3 and the} like, and these are used in the state of an aqueous solution, for example. Further, it is preferable that the surface treatment is continued for 10 minutes or more. If the time is less than 10 minutes, the effects of the present invention may not be achieved.

Further, the surface treatment is performed by using LaNi ₅ or Mm
AB ₅ type alloy such as Ni ₅ (Mm represents a misch metal, specifically, a mixture of rare earth elements such as lanthanum and cerium), and AB such as Laves phase alloy such as (Zr, Ti) (V, Ni) ₂ Applicable to hydrogen storage alloy such as type ₂ alloy. Further, specific operations relating to the surface treatment include, for example, a method of immersing a powdery hydrogen storage alloy in a chromium oxide solution, and a method of spraying a chromium oxide solution on the powdery hydrogen storage alloy. .

In the hydrogen storage alloy after the activation treatment, the oxide layer formed on the surface is removed, and the metal Ni contained in the oxide layer remains to form a Ni catalyst layer. In addition, the formation of oxides on the alloy surface is suppressed by the chromium oxide film formed. By using the activated hydrogen storage alloy as the negative electrode of the secondary battery, the initial activation is quick and the cycle characteristics are excellent. A secondary battery can be obtained.

The above-mentioned hydrogen storage alloy negative electrode is made of nickel-nickel.
It can be used in metal hydride batteries, air-metal hydride batteries, and the like.

The operation of the present invention will be described below. The surface treatment method according to the present invention is to bring the chromium oxide solution into contact with the surface of the hydrogen storage alloy negative electrode. As a result, the oxide layer formed on the surface of the hydrogen storage alloy negative electrode and inhibiting the storage and release of hydrogen can be dissolved and removed. Therefore, the initial activity of the secondary battery using the hydrogen storage alloy negative electrode can be increased.

Further, a chromium oxide film is formed on the surface of the hydrogen storage alloy after the surface treatment. For this reason, in a secondary battery using a hydrogen storage alloy negative electrode, an oxide layer is less likely to be formed on the negative electrode during a charge / discharge cycle. Thereby, the cycle characteristics of the secondary battery using the hydrogen storage alloy negative electrode can be improved.

As described above, according to the present invention, it is possible to provide a surface treatment method for a hydrogen-absorbing alloy negative electrode capable of obtaining a secondary battery having excellent cycle characteristics and excellent initial activity.

Next, the chromium oxide solution is preferably a chromium oxide (VI) solution, that is, a solution containing hexavalent chromium ions. Thereby, the oxide layer formed on the surface of the hydrogen storage alloy can be easily removed, and a chromium oxide film having excellent corrosion resistance can be obtained.

The above chromium oxide (VI) solution contains 6
It is preferable that the solution contains a valent chromium ion and has a pH of 3 or less. If the pH value is higher than 3, a problem may occur that the oxide layer on the surface of the hydrogen storage alloy is not sufficiently removed.

Examples of the chromium (VI) oxide solution include chromates containing alkali metals such as potassium (such as potassium dichromate) and chromates containing ammonium ions (ammonium dichromate). Can also be used.

Next, the concentration of hexavalent chromium in the chromium oxide (VI) solution is preferably 0.1 to 50 wt%. Thereby, a hydrogen storage alloy negative electrode having more excellent initial activity can be obtained. The above concentration is 0.1
If the amount is less than wt%, the chromium oxide film formed on the surface of the alloy is thin, and it may be difficult to suppress the formation of an oxide layer. On the other hand, when the above concentration is 50 wt%
If the concentration is higher, the chromium oxide coating on the surface of the hydrogen storage alloy becomes too thick, and the initial activity may be reduced.

It is more preferable that the lower limit of the concentration be 0.5 wt% or more. As a result, the processing time of the surface treatment can be completed in about 10 minutes.
The surface treatment operation can be shortened.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment A surface treatment method for a hydrogen storage alloy negative electrode according to an embodiment of the present invention will be described with reference to FIGS. In this example, a secondary battery made of a sample 1 or 2 which is a hydrogen storage alloy negative electrode subjected to the surface treatment method according to the present invention, and an activation using the alkaline aqueous solution shown in the conventional example without the above surface treatment. The comparison is made as shown below for a secondary battery made of Sample 3 which is a hydrogen storage alloy negative electrode subjected to only the treatment.

First, the hydrogen storage alloy used for preparing each sample will be described. The hydrogen-absorbing alloy, the AB ₅ type, in the hydrogen storage alloy having a composition of _{MmNi 3.6 Co 0.7 Mn 0.35 Al 0.3} , were prepared as shown below. First, misch metal (Mm), nickel (Ni), cobalt (Co), manganese (Mn) and aluminum (A
1) was prepared. The above Mm is lanthanum (La) 4
4 wt%, cerium (Ce) 29 wt%, praseodymium (Pr) 6 wt%, and neodymium (Nd) 21 wt%.

Next, these raw materials are mixed at a predetermined ratio,
MmNi _3.6 Co _0.7 Mn _0.35 using high frequency melting furnace
A hydrogen storage alloy ingot having a composition of Al _0.3 was produced. This ingot was pulverized to obtain an alloy powder having an average particle size of 50 μm. Samples 1 to 3 were prepared from this alloy powder.

Next, the surface treatment or the activation treatment applied to the samples 1 to 3 will be described. Concentration of 5 from above alloy powder
The substrate was immersed in a wt% chromium (VI) oxide solution at room temperature for 30 minutes, and then thoroughly washed with water. By this treatment, the oxide layer formed on the surface of the hydrogen storage alloy was dissolved, and a chromium oxide film was formed instead.

To 4 g of the thus obtained alloy powder, a 2% by weight aqueous solution of methylcellulose was added in an amount of 1.2 g.
g and kneaded to prepare an alloy powder paste. Then,
Foamed Ni (Cermet # 7, manufactured by Sumitomo Electric Industries) cut into a 3 cm × 4 cm square was prepared, and spot-welded to this using a 60 μm thick Ni plate as a terminal. The alloy powder paste was filled in the pores of the foamed Ni. After drying this, pressing was performed to obtain a hydrogen storage alloy negative electrode according to Sample 1. The theoretical capacity of the hydrogen storage alloy negative electrode was 756 mAh.

In the same manner as described above, the above alloy powder is
It was immersed in a t% chromium oxide (VI) solution at room temperature for 10 minutes. The alloy powder thus obtained was used as a hydrogen storage alloy negative electrode in the same procedure as described above, and this was used as Sample 2.

Here, it was measured how much the oxide layer on the surface was removed by the surface treatment in the hydrogen-absorbing alloy powder used for producing the sample 2. In this measurement, the amount of oxygen contained in the hydrogen-absorbing alloy was measured using an analyzer for oxygen, nitrogen and hydrogen in metal (Horiba, EMGA-1)
300). As a result of the above measurement, in Sample 2, the amount of oxygen contained in the hydrogen storage alloy before the surface treatment was 0.32 wt%, and after the treatment, the oxygen amount was reduced by half to 0.17 wt%. The removal effect was clearly confirmed.

The above alloy powder was stirred at a temperature of 110.
℃ electrolyte (6.8M KOH + 0.8M LiOH)
For 1 hour and subjected to an alkali treatment. The alloy powder thus obtained was used as a hydrogen storage alloy negative electrode in the same procedure as described above, and this was used as Sample 3.

As described above, the samples 1 to 3 were combined with a nickel positive electrode to produce a nickel-metal hydride battery as shown below. The nickel positive electrode is Ni-Co
-Zn-based nickel hydroxide electrode (theoretical capacity = 1200 mA)
h) was used. The battery was sandwiched between the two nickel positive electrodes packed in a polypropylene-polyethylene nonwoven fabric having a fiber diameter of 10 μm or less as a separator, each of the samples 1 to 3 was sandwiched, and 6.8 M KOH + 0.8 M L
It was constituted by injecting an electrolytic solution composed of iOH.

Using the above nickel-metal hydride battery,
The discharge amount of the hydrogen storage alloy negative electrode according to Samples 1 to 3 was measured. In the above measurement, the battery was charged in a constant temperature bath at a temperature of 20 ° C. at a charge of 120% at 0.2 C, a pause of 30 minutes,
A charge / discharge test was performed with a discharge (discharge voltage = 0.8 V) as one cycle at 0.2 C, and the time change of the discharge capacity was determined. Since the capacity of the positive electrode used in the battery is three times or more the capacity of the negative electrode, the discharge capacity obtained by the above measurement is proportional to the capacity of the negative electrode. The above measurement results are shown in FIGS. In the figure, the horizontal axis indicates the number of cycles, and the vertical axis indicates the ratio of the measured negative electrode capacity to the theoretical capacity of the hydrogen storage alloy negative electrode in percentage.

FIG. 1 shows the relationship between the number of cycles and the anode capacity in a nickel-metal hydride battery using Samples 1 and 3. According to the figure, it was found that the negative electrode capacity of Sample 1 did not substantially decrease even after 300 cycles. In addition, it was found that the negative electrode capacity of Sample 3 gradually decreased as the cycle progressed. As described above, it was found that the hydrogen storage alloy negative electrode subjected to the surface treatment according to the present invention hardly formed an oxide layer even when immersed in or contacted with an alkaline aqueous solution, and was excellent in cycle characteristics.

FIG. 2 shows the relationship between the cycle characteristics and the negative electrode capacity in a nickel metal hydride battery using samples 1 to 3, especially when the number of cycles is less than 60. According to the figure, it was found that the negative electrode capacity of each of Samples 1 to 3 reached about 100% after 20 cycles. From the above, it was found that the hydrogen storage alloy negative electrode subjected to the surface treatment according to the present invention was excellent in initial activity.

[0035]

As described above, according to the present invention, it is possible to provide a surface treatment method for a hydrogen storage alloy negative electrode capable of obtaining a secondary battery having excellent cycle characteristics and excellent initial activity.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the negative electrode capacity of samples 1 and 3 and the number of cycles (up to 300 times) according to an embodiment.

FIG. 2 is a diagram showing the relationship between the negative electrode capacity of samples 1 to 3 and the number of cycles (up to 60 times) according to the embodiment.

Continuing from the front page (72) Inventor Shinya Morishita 41, Chukumi Yokomichi, Nagakute-machi, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. Address 1 Inside Toyota Central R & D Laboratories Co., Ltd. (72) Inventor Koji Muta 2-1-1 Toyota-cho, Kariya-shi, Aichi Prefecture Inside Toyota Industries Corporation (72) Inventor Kyoichi Kinoshita 2-Chome Toyota-cho, Kariya-shi, Aichi Prefecture No. 1 Inside Toyota Industries Corporation

Claims (1)

[Claims] 1. A negative electrode active material comprising a hydrogen storage alloy or a negative electrode active material comprising a hydrogen storage alloy having undergone an activation treatment.
A surface treatment method for a negative electrode of a hydrogen storage alloy, which comprises performing a surface treatment with a chromium oxide solution.