JP3185351B2 - Positive electrode of metal-halogen battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode of a metal-halogen battery, for example, a zinc-bromine battery, and more particularly to a positive electrode active layer using activated carbon fibers.

[0002]

2. Description of the Related Art The positive electrode of a metal-halogen battery, for example, a zinc-bromine battery, comprises an electrode substrate made of carbon plastic and an active layer formed on the surface thereof. Activated carbon fibers are conventionally used for the active layer by thermocompression bonding to an electrode substrate. These activated carbon fibers may be in the form of cloth, felt, paper, or the like, and any of them can be used. The specific surface area of the activated carbon fiber itself is determined by its activation method, and the specific surface area increases as the activation increases. As for the positive electrode characteristics of a battery, it is common that the larger the specific surface area of the activated carbon fiber to be the positive electrode active layer, the better the characteristics.

[0003] However, activated carbon fibers having a large specific surface area and a high degree of activation may be broken and fall off because the mechanical strength is reduced or the fiber diameter is reduced. In addition, a disadvantage that fibers are brought into close contact with each other due to the electrolyte and the number of active points is reduced is also recognized. That is, in many cases, the positive electrode active layer composed only of the activated carbon fiber having a large specific surface area is used without sufficiently exhibiting its original characteristics.

[0004] Conventionally, in order to make use of active points which have good electrolyte penetration and are effective in battery reaction, for example, Japanese Patent Application No. Hei.
As in the positive electrode described in Japanese Patent No. 248706, it has been proposed that the positive electrode active layer be a laminate of a plurality of activated carbon fibers having different fiber diameters and specific surface areas. As shown in FIG. 3, in this positive electrode, a positive electrode active layer 1 integrally formed with an electrode substrate 2 by thermocompression bonding is a laminate of a plurality of activated carbon fibers having different fiber diameters and specific surface areas. Is configured so that the activated carbon fiber diameter of the first layer> the activated carbon fiber diameter of the second layer> the activated carbon fiber diameter of the third layer, and the specific surface area of each layer is set to the value of the activated carbon fiber of the first layer. The specific surface area <the specific surface area of the activated carbon fibers of the second layer <the specific surface area of the activated carbon fibers of the third layer.

[0005]

The positive electrode shown in FIG. 3 is formed by arranging activated carbon fibers having different fiber diameters in layers so that bromine, which is an active material in an electrolytic solution, is well penetrated and active points are not reduced. Therefore, carbon paste was used as a dispersion of the activated carbon fibers and a binder.

The carbon paste is easy to use and is suitable as a dispersant. However, the positive electrode surface treatment layer obtained by molding has low mechanical strength. In addition to corrosion, there is a drawback that the fibers fall off and the spacing between the fiber layers increases, and the activity gradually decreases.

Further, since the positive electrode of FIG. 3 is integrally formed by heat press, there is a disadvantage that the density becomes high until the fiber gap is closed.

The present invention has been made in view of the above points, and its object is to make it possible to make use of active sites which have good penetration of an electrolytic solution and are effective with respect to the reaction of a battery, and which have an increased adhesive strength and can be used for 1000 cycles or more. It is an object of the present invention to provide a positive electrode of a metal-halogen battery which has a longer life.

[0009]

According to the present invention, there is provided an electrode substrate made of carbon plastic, and a positive electrode active layer of a plurality of activated carbon fibers superposed on the electrode substrate. The fiber diameter of the n-th layer from the substrate side (n is a positive integer) is smaller than the fiber diameter of the n + 1-th layer from the substrate side, and the specific surface area of the n-th layer from the substrate side is n + 1 The positive electrode active layer configured to be larger than the specific surface area of the second layer, using a carbon paste and a powder mixture for carbon plastic molding having the same composition as the electrode substrate as a binder, using as a binder,
It is characterized by being dried by heating and integrally molded.

[0010]

The activated carbon fiber layers having different fiber diameters are well dispersed in a binder, for example, a carbon paste liquid in which polyethylene and carbon black are mixed, and the adhesion is increased. Since it is not the integral molding by the conventional heat press, the density is not increased until the fiber gap is closed, and the arrangement of the fiber layers is sufficiently smooth for the permeation of the electrolyte. The contact between the positive electrode active layer and the electrode substrate is improved, and the life is increased.

Among the positive electrode active layers, the uppermost active layer farthest from the electrode substrate has the smallest specific surface area,
The diameter of the activated carbon fiber is the largest. For this reason, since the fiber gap is large, the electrolyte easily penetrates and the wettability is high. The specific surface area of each layer gradually increases as the uppermost active layer approaches the electrode substrate, and the diameter of the activated carbon fiber gradually decreases. Therefore, the electrolyte easily penetrates, and the reactivity at the time of charge and discharge is high.

[0012] As described above, the positive electrode active layer is not a single layer structure composed of only the activated carbon fiber having a large specific surface area as in the related art.
Since several types of activated carbon fibers with different fiber diameters and specific surface areas are laminated stepwise, the activated carbon fibers do not break down as in the past, or the active points are reduced due to the close contact between the fibers. .

[0013]

An embodiment of the present invention will be described below with reference to the drawings. In the present invention, in order to further increase the effect of the carbon paste used as the dispersant and the binder, the same set as the carbon plastic of the electrode substrate is used.
After adding a small amount of a mixture of synthetic and similar polyethylene and carbon black, and arranging the activated carbon fiber layer stepwise on the electrode substrate in the same manner as in FIG. Alternatively, a method of heating and drying at about 130 ° C. in an electric furnace to integrate them was adopted.

According to such a method, the activated carbon fiber layers having different fiber diameters are well dispersed in a binder, for example, a carbon paste liquid in which polyethylene and carbon black are mixed, and the adhesion is increased. Since it is not the integral molding by the conventional heat press, the density is not increased until the fiber gap is closed, and the arrangement of the fiber layers is sufficiently smooth for the permeation of the electrolyte.

In the positive electrode active layer 1, activated carbon fibers having a large specific surface area are arranged in a third layer closest to the electrode substrate 2, and activated carbon fibers having a small specific surface area in the direction of contact with the electrolyte and having a large fiber diameter. Is provided. In the positive electrode active layer 1 having this structure, the specific surface area of the first layer is small, but the fiber diameter is large, the fiber gap is large, and the electrolyte is easily penetrated, so that the wettability is high. Although the fiber gap becomes smaller as approaching the substrate 2, it is gradual as compared with the conventional one-layer structure, so that although the specific surface area increases, the electrolyte easily penetrates and the reactivity is high. It has characteristics.

Next, a specific embodiment of the present invention will be described. In FIG. 1, a carbon plastic electrode (polyethylene / carbon black / graphite = 5) is formed on an electrode substrate 2.
0/15/35 weight ratio). And the positive electrode active layer 1
The following three types of fibrous activated carbon are used. That is, type A-10 (fiber diameter 14 μm, specific surface area 1000 m ² /
g) in the first layer, type A-15 (fiber diameter 12 μm, specific surface area 1500 m ² / g) in the second layer, type A-20
(Fiber diameter 11 μm, specific surface area 2000 m ² / g)
Used for each layer.

The fibrous activated carbons of the first to third layers are arranged stepwise as shown in the figure, and a powder mixture of polyethylene, carbon black and graphite having the same composition as the electrode substrate 2 is dispersed in a carbon paste solution. The mixture was molded as a binder 3 using a hot air drier for about 1 hour.
It was dried by heating at 30 ° C. and integrated.

The characteristics of this electrode were evaluated as (A), and for comparison, the characteristics of the conventional electrode integrated with a heat press using only carbon paste as a binder were examined as (B). A battery was constructed using these electrodes (A) and (B) as positive electrodes, respectively, at 30 ° C. and a current density of 36 mA / cm ^2.
, And an accelerated life test was performed under the conditions of discharging at the same current density. As a result, the relationship between the energy efficiency and the number of cycles is shown in FIG. In the conventional electrode (B), the electrode characteristics deteriorated after about 800 cycles, but in the electrode (A), the contact between the positive electrode active layer and the electrode substrate was good and the number of cycles increased.

In the above embodiment, the positive electrode active layer 1 is of type A
Although it has a three-layer structure of -10, type A-15, and type A-20, the present invention is not limited to this, and a multi-layer structure using other activated carbon fibers having different fiber diameters and specific surface areas may be used.

[0020]

As described above, according to the present invention, carbon paste obtained by adding a powder mixture for forming a carbon plastic having the same composition as an electrode substrate to a carbon paste is used as a binder. Since the positive electrode active layer formed by arranging two or more layers of activated carbon fibers in a stepwise manner is heated and dried and integrally molded, the adhesive strength between the positive electrode active layer and the electrode substrate is increased as compared with a conventional electrode, Further, since heat pressing is not performed as in the related art, a long-life positive electrode having no decrease in active points can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a positive electrode showing one embodiment of the present invention.

FIG. 2 is a characteristic diagram showing the relationship between energy efficiency and the number of cycles between a conventional battery and a battery using the positive electrode of the present invention.

FIG. 3 is a sectional view showing an example of a positive electrode of a conventional zinc-bromine battery.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Positive electrode active layer 2 ... Electrode substrate 3 ... Binder layer

Claims (1)

(57) [Claims] 1. An electrode substrate made of carbon plastic, and a positive electrode active layer of a plurality of activated carbon fibers superposed on the electrode substrate, wherein the fiber diameter and specific surface area of the plurality of activated carbon fibers are determined by the substrate. Nth from the side (n is a positive integer)
Layer is smaller than the fiber diameter of the (n + 1) th layer,
And a positive electrode active layer configured such that the specific surface area of the n-th layer from the substrate side is larger than the specific surface area of the (n + 1) -th layer. A positive electrode for a metal-halogen battery, wherein the positive electrode is formed by heating and drying to form an integral body, using a binder to which the compound (1) is added.