JPH06163041A - Hydrogen storage alloy electrode - Google Patents

Abstract

PURPOSE:To provide an electrode by which internal stress of hydrogen storage alloy powder becomes small and which is hard to be pulverized and has the long cycle service life. CONSTITUTION:In a hydrogen storage alloy electrode for a metal-hydride secondary battery formed integrally of hydrogen storage alloy, the hydrogen storage alloy is composed of hydrogen storage alloy powder where a value (a/b) of a ratio of major axis length (a) of crystallite to minor axis length (b) is not more than 2. Such hydrogen storage alloy powder can be obtained by a method of cooling rapidly mixture melted solution of a hydrogen storage allay component at cooling speed equal to or faster than 1X10<4> deg.C/sec.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy electrode, and more particularly to improvement of hydrogen storage alloy powder for obtaining a metal / hydride secondary battery having a long cycle life.

[0002]

2. Description of the Related Art In recent years,
A metal / hydride secondary battery has been proposed in which a metal compound such as nickel hydroxide is used for the positive electrode and a new material hydrogen storage alloy is used for the negative electrode, which has a high energy density per unit weight and unit volume and high capacity. Therefore, it is in the spotlight as a next-generation alkaline storage battery that replaces the nickel-cadmium secondary battery.

Conventionally, the hydrogen storage alloy used for the negative electrode of this metal / hydride secondary battery is generally a cooling rate of about 100 ° C./second after the alloy component metals are mixed and melted. It is slightly different depending on the species and the manufacturing method).

However, the metal / hydride secondary battery using the hydrogen storage alloy obtained by the above method for the negative electrode has a problem that the cycle life is generally short.

As a result of intensive studies, the present inventors have found that there is a close relationship between the shape of the crystallite of the hydrogen storage alloy used for the negative electrode and the cycle life.

The present invention has been made on the basis of such findings, and an object of the present invention is to provide a hydrogen storage alloy electrode which makes it possible to obtain a metal / hydride secondary battery having a long cycle life. It is in.

[0007]

A hydrogen storage alloy electrode for a metal / hydride secondary battery according to the present invention for achieving the above object (hereinafter referred to as "electrode of the present invention") is a hydrogen storage alloy. A hydrogen storage alloy electrode for a metal / hydride secondary battery, wherein the hydrogen storage alloy has a ratio of a major axis length a and a minor axis length b of a crystallite (hereinafter For convenience, it is referred to as "aspect ratio".) A / b is made of a hydrogen storage alloy powder having a value of 2 or less.

FIG. 4 shows a crystal structure of a hydrogen storage alloy having an aspect ratio a / b of the crystallite C1 of 2 or less in the present invention (a / b = 1 in the figure), and FIG. The crystallite C2 has a large aspect ratio a / b of 2 or more, and shows a crystal structure of a hydrogen storage alloy.

A hydrogen storage alloy having an aspect ratio a / b of 2 or less is
For example melt of a mixture of the hydrogen storage alloy component 10 ⁴ °
It can be obtained by slow cooling at a cooling rate of C / sec or more.

In the present invention, the aspect ratio a / b of the crystallites (microcrystals that can be regarded as a single crystal) of the hydrogen storage alloy powder is regulated to 2 or less because the hydrogen storage alloy powder is accompanied by the progress of charge / discharge cycles. This is for suppressing the oxidative deterioration due to the pulverization of.

Incidentally, in a hydrogen storage alloy electrode of a conventional metal / hydride secondary battery, a hydrogen storage alloy powder having a crystallite aspect ratio a / b of about 3 to 4 has been used.

As the hydrogen storage alloy in the present invention, for example, LaNi ₅ , TiNi _2, etc., as well as La containing Mm can be used.
Examples thereof include those partially substituted with (Misch metal: mixture of rare earth metals), those partially substituted with Ni by Co, Mn, Al, etc., but are not particularly limited thereto.

As described above, the present invention provides a hydrogen storage alloy electrode capable of obtaining a metal / hydride secondary battery having a long cycle life, in order to provide a crystallite aspect ratio a /.
A feature is that a hydrogen storage alloy powder having b of 2 or less is used. Therefore, other materials constituting the electrode, such as the binder used in the production of the electrode of the present invention and the conductive agent added as necessary, have hitherto been practically used or proposed for hydrogen storage alloy electrodes. It is possible to use the various materials mentioned without limitation.

[0014]

The hydrogen storage alloy powder according to the present invention has a smaller crystallite aspect ratio a / b than the conventionally used hydrogen storage alloy powder, and therefore has a small internal stress which causes cracks. Therefore, even if the charge / discharge cycle is repeated, cracks are less likely to occur and the powder is less likely to be pulverized.

[0015]

EXAMPLES The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited by the examples described below, and various modifications may be made without departing from the scope of the invention. Is possible.

(Production Examples 1 to 6) Mm, Ni, Co, Al
And Mn alloy component metals (commercial pure metal having a purity of 99.9% or more) are weighed and mixed in a predetermined amount, and after induction heating and melting in a high-frequency melting furnace in an inert gas (argon) atmosphere. , Various cooling rates (1 × 10 ² , 1 × 10 ³ , 3
× 10 ³ , 1 × 10 ⁴ , 1 × 10 ⁵ , 1 × 10 ⁶ ° C /
Second), pulverized and then compositional formula MmNi _3.2 CoAl
Hydrogen storage alloy powders A1 to A6 represented by _0.3 Mn _0.5 were produced. In addition, in the case of cooling at 1 × 10 ² ° C / sec, by changing the thickness of the casting mold,
In cooling at 10 ³ ° C / sec or more, the cooling rate was adjusted by changing the rotation number of the roll in the roll method.

(Relationship between Cooling Rate and Aspect Ratio a / b) For the hydrogen storage alloy powders A1 to A6 produced in Production Examples 1 to 6, the relationship between the cooling rate during production and the crystallite aspect ratio a / b. I checked. The aspect ratio a / b of crystallites was determined as an average value by randomly selecting 50 crystallites by SEM (scanning electron microscope) after chemically etching the alloy cross section. The results are shown in Fig. 1.

FIG. 1 shows the cooling rate and the crystallite aspect ratio a / b.
Is a graph in which the vertical axis represents the crystallite aspect ratio a / b, and the horizontal axis represents the cooling rate (° C / sec).
From the figure, it can be seen that the aspect ratio a / b of the crystallite becomes 2 or less when the cooling rate is set to 1 × 10 ⁴ ° C / sec or more.

(Relationship between Aspect Ratio of Crystallite and Cycle Life) Hydrogen storage alloy powders A1 to A produced in Production Examples 1 to 6.
6, hydrogen storage alloy electrodes E1 to
E6 was produced.

That is, first, 1 g of each hydrogen storage alloy powder
In addition, polytetrafluoroethylene (PT
0.2 g of FE) and 1.2 g of carbonyl nickel as a conductive agent were mixed and rolled to obtain an alloy paste.

Then, a predetermined amount of this alloy paste is wrapped with nickel mesh and pressed to form disk-shaped hydrogen storage alloy electrodes (paste electrodes) E1 to E6 having a diameter of 20 mm.
Was produced.

Test cells were assembled using the hydrogen storage alloy electrodes E1 to E6 thus produced as negative electrodes, and the cycle life of each test cell was examined.

FIG. 2 is a schematic perspective view of the assembled test cell. The illustrated test cell 1 includes a disk-shaped paste electrode (test alloy electrode) 2, a cylindrical sintered nickel electrode 3, and an insulation. It is composed of a hermetically sealed container 4 and the like.

The sintered nickel electrode 3 is held by a positive electrode lead 5 connected to the upper surface 6 of the hermetically sealed container 4, and the paste electrode 2 is perpendicular to the substantially center of the sintered nickel electrode 3 in the cylinder. It is held by a negative electrode lead 7 attached to the upper surface 6 of the closed container 4 so as to be positioned.

Each end of the positive electrode lead 5 and the negative electrode lead 7 penetrates the upper surface 6 of the closed container 4 and is exposed to the outside, and is connected to the positive electrode terminal 5a and the negative electrode terminal 7a, respectively.

The paste electrode 2 and the sintered nickel electrode 3 are immersed in an alkaline electrolyte (30% by weight potassium hydroxide aqueous solution; not shown) contained in a closed container 4,
The space above the alkaline electrolyte is filled with nitrogen gas so that a predetermined pressure is applied to the paste electrode 2.

Further, in the central portion of the upper surface 6 of the closed container 4,
In order to prevent the internal pressure of the closed container 4 from rising above a predetermined pressure, a pressure gauge 8 and a relief pipe 10 having a relief valve 9 are inserted.

The cycle life is 5 at room temperature (25 ° C).
50m after charging at 0mA / g for 8 hours and resting for 1 hour
A charging / discharging cycle test in which one cycle includes a step of discharging at A / g to a discharge final voltage of 1.0 V and resting for 1 hour was evaluated by the number of cycles (times) until the capacity decreased to 50% of the initial capacity. . The results are shown in Fig. 3.

FIG. 3 is a graph showing the cycle life characteristics of each test cell, with the vertical axis representing the number of cycles (times) and the horizontal axis representing the crystallite aspect ratio a / b. When the aspect ratio a / b of the crystallite is 2 or less, compared with the case where the aspect ratio a / b of the crystallite is 3 to 4 (corresponding to a conventional cell),
It can be seen that the cycle life is significantly longer.

[0030]

EFFECTS OF THE INVENTION Since the hydrogen storage alloy powder of the present invention has a small internal stress and is difficult to be pulverized, the present invention has excellent unique effects such as a long cycle life.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a cooling rate and a crystallite aspect ratio a / b.

FIG. 2 is a schematic perspective view of a test cell assembled in an example.

FIG. 3 is a graph showing the relationship between the crystallite aspect ratio a / b and the cycle life.

FIG. 4 is a partially enlarged sectional view schematically showing a crystal structure of a hydrogen storage alloy according to the present invention.

FIG. 5 is a partially enlarged cross-sectional view schematically showing a crystal structure of a hydrogen storage alloy that has been conventionally used.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Toshihiko Saito 2-18 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A hydrogen storage alloy electrode for a metal / hydride secondary battery, which is formed by integrating a hydrogen storage alloy, wherein the hydrogen storage alloy has a major axis length a and a minor axis length of a crystallite. A hydrogen storage alloy electrode comprising a hydrogen storage alloy powder having a ratio value a / b of 2 or less. 2. The hydrogen storage alloy powder is obtained by rapidly cooling a melt of a mixture of hydrogen storage alloy components at a cooling rate of 1 × 10 ⁴ ° C / sec or more and then pulverizing the melt. Hydrogen storage alloy electrode.