JPH08138666A - Hydrogen storage alloy - Google Patents

Abstract

PURPOSE: To provide a hydrogen storage alloy which is useful for a negative electrode of an alkaline storage battery and has high discharge capacity and an excellent cycle life characteristic. CONSTITUTION: Regarding a Zr-V-Ni-Mn-Cr hydrogen storage alloy which absorbs and desorbs hydrogen in reversible way; the alloy composition is made to have 13.9-20.3 atomic % of the content of Mn, one of the component elements. and 3.4-9.8 atomic % of the content of Cr. Preferably, the alloy phase of the hydrogen storage alloy belongs to a Laves-phase of an intermetallic compound and the crystal structure is made to be C<15> -type cubic structure or C<14> -type hexagonal structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the composition of hydrogen storage alloys. More specifically, the present invention relates to a composition of a new hydrogen storage alloy capable of producing a hydrogen storage alloy having a large discharge capacity as an anode of an alkaline storage battery and excellent cycle life characteristics.

[0002]

2. Description of the Related Art In recent years, hydrogen storage alloys have been attracting attention as negative electrodes for alkaline storage batteries, and hydrogen storage alloys are being developed as electrode materials having a large discharge capacity. Since their properties and performances largely depend on the composition, the crystal structure, and the manufacturing method, these points have been studied in various aspects in the development of new alloys.

Although various ideas and improvements have been proposed so far, usually, it is necessary to melt the constituent elemental components of various hydrogen storage alloys at high temperature, cool the alloyed components, and then crush the alloy ingots to form them. Alternatively, the powder obtained as a mixture thereof is mixed with a conductive agent and a resin binder and bonded to a current collector to be used as an electrode. However, most of the hydrogen storage alloys used so far have large discharge capacities as electrodes, but most have poor cycle life characteristics.

In order to improve such a defect, regarding the composition of the alloy or the crystal structure, the alloy phase has a Laves phase of an intermetallic compound, and the crystal structure is C15 type of cubic crystal, or A C14 type has been proposed (JP-A-63-284758, JP-A-1-102).
855, JP-A-2-65060), and improvement in performance such as discharge capacity has been attempted, but there is still room for improvement in performance improvement.

In view of the problems of the prior art as described above, the present invention solves these drawbacks and provides a hydrogen storage alloy having a large discharge capacity and excellent cycle life characteristics, which have been a drawback so far. The purpose is to do.

[0006]

[Means for Solving the Problems] The composition of a hydrogen storage alloy is
The crystal structure belonging to the Laves phase of the intermetallic compound as the alloy phase is cubic C15 type or hexagonal C14 type, and can be achieved by setting the alloy composition to Zra-Vb-Nic-Mnd-Cre. a = 30 to 35 at%, b = 6 to 8 at%, c = 30 to
38 at% d = 13.9 to 20.3 at%, e = 3.4 to 9.8a
t%

The present invention relates to Mn and Cr as well as V, Z
An alloy containing r, Ni, and unavoidable impurities is also one of its modes. Regarding the above hydrogen storage alloy, more specifically, Zr _a V _b Ni _c Mn _d
The composition of Cr _f can be exemplified. The atomic ratio (at%) is usually a = 30 to 35, b = 6 to 8,
c = 30-38, d = 13.9-20.3, f = 3.4
It is desirable to set it to about 9.8.

Next, the reasons for limiting the amount of each component element will be described.
Regarding Mn and Cr, the content of Mn is 13.9 to 2
0.3 (at%), the content of Cr is 3.4 to 9.8 (a
t%) is preferable. Mn content is 20.3
If it is more than (at%), the corrosion resistance of the alloy in the alkaline electrolyte is lowered and the cycle life is shortened. Further, if the Mn content is less than 13.9 (at%), the discharge capacity decreases, which is not practical. Cr content is 9.8 (at%)
If the amount is larger, the passive film of Cr formed on the alloy surface in the alkaline electrolyte becomes excessive and interferes with the electrochemical reaction, so that the discharge capacity at high rate discharge is reduced. Further, when the Cr content is less than 3.4 (at%), the passivation film of Cr does not protect the alloy surface from the alkaline electrolyte, so that the amount of the alloy eluted into the alkaline electrolyte increases and the cycle The life is shortened.

When the proportions of Zr and Ni are too small or too large, it is difficult to form a Laves phase C15 type cubic crystal structure having a stable discharge capacity. V is for improving the discharge capacity, but when it is too small, the discharge capacity decreases and the corrosion resistance in the alkaline electrolyte at room temperature decreases. On the other hand, when the amount is excessive, the corrosion resistance in the high temperature alkaline electrolyte is lowered.
If Ni is too small, the conductivity and corrosion resistance of the alloy will decrease,
In the case of excess, the discharge capacity decreases.

The alloy electrode material of the present invention is manufactured by the following steps. Of course, it is not strictly limited to this step, and various modes are possible. First, each constituent element is melted in an electric arc furnace or a high frequency induction furnace so as to have a predetermined alloy composition, and an alloy powder is obtained by an atomizing method. Next, the alloy powder is heat-treated in a vacuum or in an inert gas to homogenize it. The heat treatment conditions are preferably, for example, vacuum treatment at a temperature of about 800 to 1100 ° C. for about 0.5 to 20 hours.

After this homogenizing heat treatment, hydrogenation pulverization or mechanical pulverization can be carried out and classification can be carried out to obtain an average particle size of about 20 to 150 μm, more preferably an average particle size of 30 to 75.
It is about μm. If the particle size is too small, the alloy will be easily oxidized, and if too large, the electrode will be broken when the alloy is pulverized in the alkaline electrolyte. The obtained powder or granule is molded into a predetermined electrode shape by press molding or the like using a binder resin such as polytetrafluoroethylene. It can be molded into any shape.

Regarding melting, homogenizing, crushing, classifying, and molding, it is not necessary to say that various devices and conditions can be adopted.

[0013]

According to the present invention, as described above, the content of Mn of the elements contained in the hydrogen storage alloy is 13.9 to 20.3 (at%),
The alloy composition has a Cr content of 3.4 to 9.8 (at%). With this composition, a passivation coating layer of Cr is formed on the alloy surface in the alkaline electrolyte and the alloy surface is protected from the alkaline electrolyte, so that elution of the alloy into the alkaline electrolyte is suppressed. Further, the amount of Mn, which is more likely to be eluted into the alkaline electrolyte than the conventional composition, is reduced as compared with the conventional composition. By improving the above two points,
It becomes possible to produce a hydrogen storage alloy as an electrode material with a small discharge capacity reduction, a large discharge capacity that could not be realized with conventional alloys, and an excellent cycle life characteristic.

Hereinafter, the present invention will be described in more detail with reference to examples.

【Example】

Examples 1-5 The alloy composition is Zr _32.2 V _6.8 Ni _37.3 Mn.
_20.3 Cr _3.4 , Zr _32.2 V _6.8 Ni
_37.3 Mn _18.7 Cr _5.0 , Zr _32.2 V
_6.8 Ni _37.3 Mn _17.1 Cr _8.6 , Zr
_32.2 V _6.8 Ni _37.3 Mn _15.5 C
r _8.2 , Zr _32.2 V _6.8 Ni _37.3 Mn
An alloy material having an AB ₂ Laves phase C15 type crystal structure of _13.9 Cr _9.8 was melted and sprayed in an argon or helium atmosphere in a melting furnace evacuated to a vacuum degree of 0.01 to 0.2 Torr. The alloy was heat-treated in vacuum (1080 ° C., hold for 10 hours) and then hydro-ground. Also, for comparison, Zr _32.2 V _6.8 Ni
An alloy having a composition of _37.3 Mn _23.7 was also manufactured by the same method (Comparative Example 1).

An alloy powder of 200 mesh or less was prepared, and this hydrogen storage alloy powder was used as Ni powder and PTFE as a binder.
(Polytetrafluoroethylene) in a weight ratio of 1: 3:
The mixture was mixed at a ratio of 0.167 and press-molded using a mold. A lead wire was connected to this molded body and used as a negative electrode of an alkaline storage battery cell. The electrolyte used was 30% KOH. Foamed nickel was used for the positive electrode and an Hg / HgO electrode was used for the reference electrode. Measuring device is P manufactured by Hokuto Denko
It was measured in a half-cell system using an otentio-Galvano stat.

Table 1 shows a measurement temperature of 25 ° C. and a charge / discharge current amount of 1
It shows the result of the discharge capacity at 7 mA / g. Further, FIG. 1 and Table 2 show the results of a cycle life test conducted under the conditions of a measurement temperature of 45 ° C. and a charge / discharge current amount of 110 mA / g. FIG. 1 is a correlation diagram of the discharge capacity and the number of cycles of each composition. Table 2 shows the amount of decrease in discharge capacity per cycle. The measurement temperature of 45 ° C. is a temperature set in order to make the corrosion of the alloy more noticeable than the normal temperature of 25 ° C. and accelerate the result.

[0017]

From Table 1, the discharge capacity at a measuring temperature of 25 ° C. and a charging / discharging current amount of 17 mA / g is 380 m, which is the same as that of the comparative example.
A value of Ah / g or more is shown, and in Example 2, 417 mAh
High capacity of / g can be obtained. In addition, from FIG. 1 and Table 2, the discharge capacity reduction amount per cycle in the cycle life test performed under the conditions of the measurement temperature of 45 ° C. and the charging / discharging current amount of 110 mA / g was the same as that of Comparative Example 1. In Example 1 and Example 2, it is about 3 times, and in Example 5, it is about 10 times. As is clear from these results, when the alloy having the composition of the present invention is used as the negative electrode, a discharge capacity equal to or higher than that of the composition of the comparative example is obtained, and the cycle life characteristic is better than that of the comparative example. I understand.

[0019]

According to the present invention, it becomes possible to produce a hydrogen storage alloy useful as a negative electrode of an alkaline storage battery, which has a large discharge capacity and is excellent in cycle life characteristics.

[Brief description of drawings]

FIG. 1 is a correlation diagram between discharge capacity and cycle number (measurement temperature 45 ° C., charge / discharge current amount 110 mA / g).

Claims (1)

[Claims] 1. A crystal structure belonging to the Laves phase of an intermetallic compound as an alloy phase capable of reversibly occluding and releasing hydrogen, is a cubic C15 type or a hexagonal C14 type, and has an alloy composition of Zra-Vb-. A hydrogen storage alloy comprising Nic-Mnd-Cre and inevitable impurities. a = 30 to 35 at%, b = 6 to 8 at%, c = 30 to
38 at%, d = 13.9 to 20.3 at%, e = 3.
4 to 9.8%