JPH03278512A - Electric double layer capacitor - Google Patents

Abstract

PURPOSE:To obtain a separator material having very little self-discharge and leakage current, when the material is used for an electric double layer capacitor, by constituting a separator from a film in a specific state obtained through molding and stretching a mixture including polyolefine resin and inorganic fine powders. CONSTITUTION:In an electric double layer capacitor equipped with a separator 3 impregnated with an electrolyte and with a pair of polarizable electrodes 2a, 2b facing each other through the separator, the separator 3 is composed of a film 10 obtained through molding and stretching a mixture including polyolefine resin 12 and inorganic fine powders 11. In that film 10, air-gaps 13 giving a multiple-aperture state in a layered manner are formed in a mutually communicating state and the inorganic fine powders 11 are supported by filiform bodies 14 formed by stretching the polyolefine resin 12 in a threadlike manner within those air-gaps 13. For example, 40-80wt.% polyolefine resin 12 such as polypropylene and 60-20wt.% inorganic fine powders 11 such as diatomaceous earth are contained in the film 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electric double layer capacitor, and particularly to a separator used in an electric double layer capacitor.

[Prior Art] An electric double layer capacitor may be composed of a single cell 1 shown in FIG. 4, or may be composed of two or more cells 1 stacked together.

The cell 1 includes a pair of polarizable electrodes 2a and 2b arranged opposite to each other, a separator 3 arranged between them, an annular gasket 4 surrounding these electrodes, and electrodes on the upper and lower surfaces of the gasket 4, respectively. It includes a pair of current collectors 5a and 5b that are heat-bonded.

[Problems to be Solved by the Invention] Electric double layer capacitors are mainly used as a minute current supply source for portable devices. However, when the current consumption becomes extremely small as in recent semiconductor products, the current consumed by self-discharge of the energy stored in the electric double layer capacitor is greater than that of the semiconductor product, which poses a serious problem in terms of product performance. It becomes. For this reason, there is a demand for electric double layer capacitors that are not only compact but also exhibit less self-discharge.

Things that can provide a self-discharge path for the electric double layer capacitor include, for example, separators, polarizable electrodes, and electrolytes, and impurities contained in these systems are thought to be the cause of self-discharge. Therefore, the inventor of the present invention focused on the fact that the electron conduction path formed inside the separator material is a major cause of self-discharge.

The functions required of a separator are to block electron conduction between a pair of polarizable electrodes facing each other via the separator,
The purpose is to allow only ions to pass through.

Usually, a separator is provided by an electrically insulating resin film having fine holes penetrating the front and back sides, and this separator is used by impregnating it with an electrolyte. Therefore, when selecting a separator material, the wettability to electrolyte, chemical and electrochemical stability, heat resistance,
Mechanical strength etc. are taken into consideration.

However, the following three points are the more important characteristics required of the separator material.

■ The pores are sufficiently small: self-discharge caused by carbon particles detached from the polarizable electrode entering the pores and forming electron conduction paths is prevented.

■ Thinness: If the separator is thick, it not only increases the resistance of the electrolyte between the front and back sides of the separator, but also becomes an obstacle to making the product smaller and thinner.

■ Large aperture ratio: The electrolyte impregnated into the openings becomes an ion conduction path, so if the aperture ratio is small, ion conductivity decreases and resistance increases.

Note that, as mentioned above, electric double layer capacitors are desired to be small and have little self-discharge, so it is especially important to satisfy the conditions (1) and (2) above.

As separators for electric double layer capacitors, polyzirconium phosphate, which is used as a separator in the battery field, as described in Japanese Patent Publication No. 55-41015 (US Pat. No. 3,536,963),
Porous polyvinyl chloride, Whatman GFA glass filter vapor, cellulose acetate, mixed esters of cellulose, and glass fiber cloth can be used.

Furthermore, as described in Japanese Patent Publication No. 61-29134, as a separator, electrolytic paper, polyethylene non-woven fabric, polypropylene non-woven fabric, polyester non-woven fabric, or a mixture thereof, and glass having a diameter smaller than the fiber diameter of these are added. Some are made of porous paper mixed with fibers.

Furthermore, as described in Japanese Patent Publication No. 63-23649, synthetic paper produced using polyester, polyethylene, polypropylene, etc. as a separator;
Alternatively, there is one in which the base material is a natural paper such as kraft paper or Manila paper, or a porous paper made from a mixture of both, and cellulose is attached to this base material.

Generally speaking, these conventional techniques use nonwoven fabric or paper as the separator.

Therefore, when we investigated electric double layer capacitors using nonwoven fabric or paper as a separator, which are typical examples of such electric double layer capacitors, good results were not obtained under any of the conditions (1), (2), and (2) described above. In other words, since materials such as non-woven fabrics or paper are made of tangled resin fibers, they need to have a thickness of 150 microns or more in order to obtain a usable pore size. .

In addition, the ionic conductivity was poor when impregnated with the electrolyte, and the carbon particles released from the polarizable electrodes entered the separator and formed an electron conduction path, resulting in large self-discharge and leakage current. .

Next, a microporous polypropylene film with holes formed through fibrillation during stretching of the polypropylene film (
We investigated the application of Celguard, manufactured by Celanese, USA, to separators.

According to this, the above-mentioned conditions (■) and (2) are at a sufficiently satisfactory level, and as for condition (■),
The pore diameter was extremely small and at a practically usable level.

However, the self-discharge and leakage current were much larger than expected from the pore size of the separator material and the particle size of the carbon particles forming the polarizable electrode. This was found to be due to the fact that carbon particles still penetrated into the pores and the insulation resistance decreased.

The separator materials mentioned above, as well as some of the separator materials mentioned before, are modeled to have fine pores penetrating perpendicularly to the film surface, and with such pore shapes, carbon It is thought that fine particles can easily enter.

Therefore, we investigated a separator material with a structure in which micropores meander through the film. An electric double layer capacitor using such a material as a separator is described in JP-A-62-205614.

This separator is porous and has inorganic substances added to a polyolefin resin base material, and is said to be able to improve self-discharge characteristics.

However, this publication does not disclose a specific method for making it porous, and possible methods include mixing the inorganic material with the resin and then dissolving the inorganic material. This includes the problem that dissolved inorganic substances gradually deteriorate the self-discharge characteristics.

Therefore, the characteristics required of a separator could not be fully satisfied, and there was also room for improvement in the shape of the micropores. Moreover, this material is very expensive due to its complicated manufacturing method, and is not acceptable from a practical point of view.

It is therefore an object of the present invention to provide a separator material that has extremely low self-discharge and leakage current when used in electric double layer capacitors.

[Means for Solving the Problems] According to the present invention, to put it simply, a separator material is provided in which fine pores pass through the separator material in a meandering manner and the shape of the pores is improved.

That is, the present invention includes a separator impregnated with an electrolytic solution and a pair of polarizable electrodes facing each other with the separator impregnated with the separator.
The separator is intended for electric double layer capacitors, and in order to solve the above-mentioned technical problems, the separator is composed of a film obtained by molding and stretching a mixture containing a polyolefin resin and an inorganic fine powder. Ru. This film is formed with interconnected voids that provide a layered porous state. Inside these voids, the inorganic fine powder is supported by filamentous bodies in which the polyolefin resin is stretched into filaments.

In another aspect of the present invention, a microporous film obtained by forming a mixture containing a polyolefin resin and an inorganic fine powder into a film and uniaxially or biaxially stretching the film is used as the separator.

Preferably, in the film as described above, the polyolefin resin is 40 to 80 wt%, and the inorganic fine powder is 60 to 80 wt%.
Contains 20 wt% gold.

In addition, as the polyolefin resin, for example, polypropylene resin is used, and as the inorganic fine powder,
Silk screen clay is used.

[Operations and Effects of the Invention] The microporous film constituting the separator used in the electric double layer capacitor according to the present invention is obtained by molding and stretching a mixture containing a polyolefin resin and an inorganic fine powder. Therefore, micropores, or voids, are formed where the inorganic fine powder was present, that is, at the interface between the inorganic fine powder and the polyolefin resin. Thus, the interconnected voids can provide serpentine pores within the film when viewed throughout the film. Therefore, it becomes difficult to form an electron conduction path penetrating the separator.

Furthermore, inside the voids of the film, the inorganic fine powder is supported in a suspended state by the filamentous filaments made of stretched polyolefin resin, so the effective diameter of the voids becomes extremely small. Therefore, the carbon particles separated from the polarizable electrode can be effectively prevented from entering into the pores of the separator.

For this reason, according to the electric double layer capacitor using the separator as described above, it is possible to significantly reduce self-discharge and leakage current.

[Embodiment] Koni's invention is characterized by the structure of the separator 3 included in the cell 1 constituting the electric double layer capacitor as shown in FIG. That is, a microporous resin film obtained by mixing inorganic fine powder in a polyolefin resin, forming it into a film shape, and then stretching it is used as a separator for an electric double layer capacitor.

More specifically, first, after melt-mixing a compound consisting of a polyolefin resin and an inorganic fine powder, this mixture is
It is molded into a film by an inflation molding method or a T-die extrusion molding method. Next, a microporous resin film is obtained by uniaxially or biaxially stretching the film-shaped molded product while heating it to a softening temperature below the melting point of the resin.

An electron micrograph of the microporous resin film obtained by the above method is shown in FIG. Also, Figure 2 shows the first
It is an explanatory view of a figure. FIG. 3 is an explanatory diagram showing a cross section of the film.

As shown in FIGS. 1, 2, and 3, voids 13 are formed in the microporous resin film 10 between the inorganic fine powder 11 and the matrix resin 12. , are in communication with each other inside the film 10. The voids 13 give the film 10 a porous layered state, as is particularly well shown in FIG. As a result, the film 10 as a whole has a structure in which micropores meander through the voids [13].

Further, inside the void 13, a filamentous body 14, which is a resin derived from the matrix resin 12 and is formed by being stretched into a filament, can be observed.

The inorganic fine powder 11 is supported in a suspended state inside the void 13 by such filamentous bodies 14 .

The above-mentioned voids 13, ie, micropores, are formed by stretching. The mechanism of action at this time can be considered as follows. That is, when a film-like molded product is stretched in a heated state, the matrix resin 12 stretches, but the inorganic fine powder 11 dispersed in such a film-like molded product
Since it cannot follow the elongation, voids 13 are formed at the interface between the inorganic fine powder 11 and the matrix resin 12. Such voids 13 are created by stretching the film-like molded product, so the microporous resin film 10
Since the inorganic fine powders 11 are present relatively close to each other, the spaces 1113 are in a state of communication with each other. In addition, since the matrix resin 12 has a property of adhering to the inorganic fine powder 11 to a greater or lesser extent, the matrix resin The resin derived from 12 is stretched into a thread-like state, thereby forming a thread-like body 14.

The reason why the inorganic fine powder 11 is supported by the filamentous bodies 14 is based on the origin of the formation of the filamentous bodies 14 as described above.

As the resin serving as the matrix in the microporous resin film according to the present invention, polyolefin resins such as polypropylene and polyethylene are excellent, and if necessary, plasticizers, Stabilizers, molding aids, etc. may also be added. Polyolefin resins not only have excellent chemical and electrochemical stability, but also have a low affinity with the surface of inorganic fine powder, so they tend to form voids during stretching. When considering heat resistance, polypropylene is particularly preferred among polyolefin resins.

Further, in the microporous resin film according to the present invention, as the inorganic fine powder mixed with the polyolefin resin, for example, diatomaceous earth, silica, alumina, zeolite, glass microballoon, talc, barium sulfate, etc. can be used. can. The particle size is preferably about 0.05 to 10 microns, particularly about 1 to 3 microns.

When selecting the material for the inorganic fine powder, it is preferable to take into consideration stability with respect to electrolyte solution, amount or behavior of impurities, etc. Particular attention must be paid to metal oxides contained as impurities, as they may dissolve into the electrolyte and cause serious defects.

As the material for the inorganic fine powder, diatomaceous earth is particularly desirable. This is because, due to the porosity of the diatomaceous earth particles and the active hydroxyl groups present on the surface, they have excellent wettability with electrolyte solutions and are characterized by little inhibition of ionic conduction.

The porosity of the microporous resin film according to the present invention can be adjusted mainly by the content of the inorganic fine powder and the stretching ratio. As the content of the inorganic fine powder increases, the porosity increases, but when it exceeds 60 wt%, the formability of the film decreases and the workability of film production becomes significantly worse.

On the other hand, if it is less than 20 wt%, it becomes difficult to obtain a practical level of porosity. Therefore, by setting the content of the inorganic fine powder within the range of 20 to 60 wt%, a practically good microporous resin film can be obtained. The porosity can also be increased by increasing the stretching ratio. Although it is possible to increase the areal stretching ratio up to about 15 times, it is appropriate to set it to 3 to 6 times in consideration of the uniformity and stability of the film.

Capacitance, E, S of an electric double layer capacitor using the microporous resin film according to the present invention as a separator
, R, (equivalent series resistance), leakage current, price, and thickness are qualitatively compared with those of electric double layer capacitors using nonwoven fabric or other conventional materials as separators, and Table 1 shows the results. .

Table 1 In each evaluation item shown in Table 1, ◎, 0
The degree of desirability is shown in the order of 1△ and ×.

As mentioned above, "Celguard" is a porous film manufactured by Celanese, and "High Bore" is a porous film manufactured by Fukaisei.

From Table 1, it can be seen that the examples according to the present invention are well-balanced across all evaluation items and are the most excellent separator materials.

Next, as shown in Table 2, various examples and comparative examples according to the present invention were prepared.

(Margin below) In Table 2, in Examples 1 to 4, polypropylene (PP) was used as the matrix resin, and in Example 5,
High density polyethylene (PE) was used. Further, in Examples 1 to 5, diatomaceous earth (average particle size 2 microns) was used as the inorganic fine powder, and the content was determined as follows:
It is shown in Table 2.

Further, Comparative Example 1 is Celguard (manufactured by Celanese Co., Ltd.) made of polypropylene, and Comparative Example 2 is Hi-Bore (manufactured by Fukaisei Co., Ltd.) made of polyethylene.

The porosity of each film is based on the air permeability (JIS-P8
117) and electrical resistance (JIS-C2313).

In addition, the product performance is determined by using separator 3 for each film.
A prototype obtained by connecting six cells 1 in Figure 1 in series (
5°5V, 0.047F, outer diameter 11mm, height 6.5~
7. These are the results of evaluation regarding 0mm). Note that the self-discharge time constant of the prototype is 5. After charging for 1 hour with Ov,
Measurements were taken by leaving the terminals open in an atmosphere at 25°C.

For convenience, the termination voltage is 2. It was Ov.

As is clear from Table 2, in all the examples according to the present invention, the effect of improving the self-discharge time constant is remarkable. In order to make the self-discharge time constant extremely long as in Examples 1 and 2, the equivalent series resistance (E, S, R,)
However, due to the nature of electric double layer capacitors, this degree of increase is not a problem at all.

[Brief explanation of drawings]

FIG. 1 is an electron micrograph of a microporous resin film used as a separator of an electric double layer capacitor according to an embodiment of the present invention. FIG. 2 is an explanatory diagram of FIG. 1. FIG. 3 is an explanatory view showing the microporous resin film shown in FIGS. 1 and 2 from a cross-sectional direction. FIG. 4 is a sectional view showing the cell 1 constituting the electric double layer capacitor. In the figure, 1 is a cell, 2a and 2b are polarizable electrodes, 3 is a separator, 4 is a gasket, 5a and 5b are current collectors, 1
0 is a microporous resin film, 11 is an inorganic fine powder, 12 is a matrix resin, 13 is a void, and 14 is a filamentous body. Figure 1 Figure 4-Figure July 25, 1990

Claims (4)

[Claims] (1) An electric double layer capacitor comprising a separator impregnated with an electrolytic solution and a pair of polarizable electrodes facing each other through the separator, in which the separator is formed by molding a mixture containing a polyolefin resin and an inorganic fine powder. The film is made of a film obtained by stretching, and the film is formed with interconnected voids giving a layered porous state;
An electric double layer capacitor characterized in that, inside the void, the inorganic fine powder is supported by a thread-like body made of the polyolefin resin stretched into a thread-like shape. (2) An electric double layer capacitor comprising a separator impregnated with an electrolytic solution and a pair of polarizable electrodes facing each other through the separator, in which the separator is made of a mixture containing a polyolefin resin and an inorganic fine powder in the form of a film. 1. An electric double layer capacitor characterized by using a microporous film obtained by molding the film and uniaxially or biaxially stretching the same. (3) In the film, the polyolefin resin is 40 to 80 wt%, and the inorganic fine powder is 60 to 20 wt%.
% of the electric double layer capacitor according to claim 1 or 2. (4) The electric double layer capacitor according to any one of claims 1 to 3, wherein the polyolefin resin is a polypropylene resin, and the inorganic fine powder is diatomaceous earth.