JPH11126734A - Carbon material for electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon material providing an excellent electric conductivity and a high performance. SOLUTION: Carbon fibers having thicknesses not less than 0.05 μm and not larger than 5 μm are condensed in carbon powder having a specific surface area of 10<2> /g or less by a vapor phase growth process, and then added with a 0.1-30 weight% of carbon fibers which are fixedly contacted at least at their parts with each other by carbide and which have block or string-ball shaped structures of sizes not less than 5 μm and not larger than 500 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitor, and more particularly to a carbon material used for an electrode plate for an electric double layer capacitor.

[0002]

2. Description of the Related Art In recent years, charge accumulation based on electric double layers,
That is, an electric double layer capacitor using the electric double layer principle has been developed and commercialized. Since the capacitor can provide a large capacitance, a small capacitor is used for a backup power supply for a semiconductor memory of an electronic device, and a large capacitor is used for a part of a lead battery used in a vehicle.

In general, a carbon electrode is used in an electric double layer capacitor. Since this electric double layer capacitor utilizes the electric double layer generated at the interface between the electrode surface and the electrolyte, it is necessary to increase the surface area of such a carbon electrode. Therefore, for example, Japanese Patent Application Laid-Open No. 63-187
No. 61, JP-A 1-227417, activated carbon particles having fine pores, and JP-A-7-24955.
As disclosed in Japanese Patent Application Laid-Open No. 1 (1993) -1995, there has been proposed a method in which a polyvinylidene chloride resin is heated in a non-oxidizing atmosphere, thereby causing atomic and molecular defects to form pores.

On the other hand, in order to manufacture a high-capacity electrode plate, it is necessary to provide not only the surface area of the electrode, that is, the specific surface area, but also sufficient electric conductivity and thermal conductivity. However,
The activated carbon particles as described above have low crystallinity and high electrical resistance of the material itself. Therefore, when an electrode is formed using the activated carbon particles, the contact electric resistance between the activated carbon particles increases, and it is difficult to obtain sufficient electric conductivity. Therefore, recently, an electric double layer capacitor having an electrode containing activated carbon particles and carbon whiskers has been proposed (Japanese Patent Laid-Open No. 7-1995).
No. 307250).

[0005]

However, the filler of the single-fiber carbon whisker can obtain the effect of the one-dimensional conductive distance, but cannot sufficiently obtain the three-dimensional conductive distance. As the electrode plate of the electric double layer capacitor, one having good three-dimensionally uniform conductivity is preferable and desirable. In order to further improve the conductivity, a filler having a structure that can exhibit the same characteristics in the three-dimensional direction and has a conductive distance as long as possible is preferable, and the filler has a network-like structure capable of sufficiently holding activated carbon particles. Is desirable.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention has a specific surface area of 500 m ² / g.
Vapor-grown carbon fibers having a thickness of not less than 0.05 μm and not more than 5 μm are agglomerated on the above carbon powder, and at least a part of the contact points between the fibers is fixed by a carbide of a carbon substance.
0.1 wt% to 30 watts of carbon fiber mainly composed of a floc-like or thread-like structure having a size of not less than μm and not more than 500 μm.
Provided is a carbon material for an electrode for an electric double layer capacitor, wherein the carbon material is added by t%.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Various carbon powders can be used as the carbon powder having a specific surface area of 500 m ² / g or more used for the electric double layer capacitor of the present invention. Examples thereof include petroleum coke, coal coke, coconut shell, and activated carbon powder made from phenol resin powder, polyvinylidene chloride resin powder, or the like. Regarding the physical properties of the activated carbon powder, the specific surface area is preferably 500 m ² / g or more. If it is less than 500 m ² / g, a sufficient surface area cannot be secured as an electrode of the electric double layer capacitor, so that the ion adsorption area is small and the capacity of the capacitor is small.

[0008] Next, the carbon fiber as a filler used in the present invention has a branched portion in a part, and agglomerates into a floc shape or a thread ball shape (hereinafter referred to as a floc shape or the like) of a predetermined size. And a structure in which the fibers are entangled and at least some of the contacts are bonded with carbide. This carbon fiber is produced by molding, heat-treating and pulverizing vapor-grown carbon fiber.

[0009] In order to convert a vapor-grown carbon fiber into a structure such as a floc having a three-dimensional structure, first, a fiber assembly containing a vapor-grown carbon fiber having a high branching rate is formed by compression to increase the density. Connect the fibers together. The vapor grown carbon fiber used in this case may be a single fiber that is not branched, but it is preferable to include a branched fiber because it is easily entangled.
The raw material vapor grown carbon fiber has a fiber diameter (thickness) of 0.1 mm.
Fibers having a size of from 05 μm to 5 μm, preferably from 0.1 μm to 1 μm are good.

When the fiber diameter is 5 μm or more, the activated carbon particles, which are the active material of the main agent, usually have an average particle diameter of 20 μm or less, so that it becomes difficult to enter the gap between the activated carbon particles to form a stitch structure. It becomes difficult to uniformly distribute on a thin electrode plate. Further, when the thickness is 0.05 μm or less, the specific surface area is large, so that the number of defects increases during firing or graphitization, so that the strength of the fiber itself is also weakened, it is difficult to form a stitch structure when formed into an electrode, and the electric resistance is reduced. The effect is reduced, which is not preferable.

The length of the fiber is preferably 5 μm or more and 100 μm or less. If the fiber length exceeds 100 μm, the bulk density becomes too small and it is difficult to handle, and it is difficult to disperse uniformly in the electrode plate. When the particle diameter is 5 μm or less, the activated carbon particles as the main agent are 20 μm or less, so that the activated carbon particles
Since it is difficult to coordinate more than one element, the effect of addition is not increased, which is not preferable.

The vapor-grown carbon fiber used for molding may be a fiber which has been once heat-treated, but it is more effective to start with a crude vapor-grown carbon fiber which has not been subjected to heat treatment. is there.

If the fiber after heat treatment is used as a starting material, or if the amount of organic substances such as pitch contained in the aggregate of the crude vapor-grown carbon fiber before treatment is small, it is easy to perform heat treatment. It is advisable to add a binder substance such as pitch, which is easily carbonized, to the molded product.

[0014] The crude vapor grown carbon fiber has a portion where crystals are not sufficiently developed on the surface, and furthermore, unreacted organic compounds such as tar components are adsorbed and the fiber surface is covered. The amount of the unreacted organic compound varies depending on the production conditions, but is generally about 5 to 20% by weight. The unreacted organic material plays a role of a binder when the crude vapor grown carbon fiber is molded, and is easily carbonized by heat treatment to form a carbide that bonds the fibers together.

Unbranched monofilaments and branched vapor grown carbon fibers have compressive properties, and are easily entangled and easily aggregated. Therefore, the fiber aggregate is compressed to increase the bulk density and is easily formed. Can be. Any molding method may be used as long as pressure is applied. For example, a compression granulation method, an extrusion granulation method, or the like can be used, but a method of compressing the fiber aggregate in the system is easy. Any method may be used as long as pressure is applied to the fiber as a compression molding method, for example, a method using a simple compression plate or a method using a plunger, a screw method,
Although any method such as a disk pelletizer method can be used, in particular, a plunger method and an extrusion method using a compression plate are relatively easy.

The aggregate of vapor grown carbon fibers has good moldability and can be compression molded into various shapes. For example, a columnar or rectangular parallelepiped shape is easy. The shape of the molded body may be any shape, but industrially, a simple shape that is easy to handle as much as possible is preferable, and a columnar shape and a prismatic shape that are easily heat-treated are preferable. If the size of the molded article is too small, the ratio of fiber breakage due to pressure increases, and for example, the diameter is at least 3 mm or more, preferably 5 mm or more in the case of a cylindrical shape. In the present invention, it was formed into a 150 mmφ cylindrical shape.

The pressure to be applied at the time of forming the vapor grown carbon fiber may be selected optimally from the relationship between the pressure and the bulk density of the formed body. Specifically, a pressure of 0.1 kg / cm ² or more, preferably 1.0 kg / cm ² or more is sufficient. Although the pressure may be high, it is preferable that the fiber does not collapse. Also, the higher the pressure, the higher the equipment cost of the pressurizing system itself, so it is about 1 to 100 kg / cm ² , preferably about 1 to 50 kg / cm ^2. Is good.

[0018] The density of the compact is also related to the material density of the floc-like carbon fiber to be a product. The bulk density of the original vapor grown carbon fiber is very small, 0.001 to 0.005 g / cm.
It is about ³ and this is molded to increase the density. The bulk density as a molded body that improves performance and is easy to handle is 0.02 g /
cm ³ or more is suitable. If it is smaller than that, heat treatment is difficult to perform and the efficiency is poor, and sufficient performance as a filler cannot be obtained.

The heat treatment is carried out at normal pressure or under pressure by transferring the compact obtained by compression molding to, for example, a heating zone. At the time of heating, the residence time is set so that the temperature reaches the target value or more to the center of the compact. The heating method may be a commonly used method, for example, an external heating method using a high-frequency furnace, a resistance furnace or a high-temperature gas, a direct heating method using a high-temperature gas, an energization heating method, or any method as long as the target temperature can be achieved. But it is good.

The heat treatment temperature may be selected depending on the target temperature of the physical properties of the final use, and is preferably 600 ° C. or more, preferably 800 ° C. or more and 3500 ° C. or less. The above is the case where the molding and the heat treatment are performed in different steps, but they can be performed simultaneously. For example, in compression molding, there is a method in which a heating device is provided in a molding device and heating is performed while applying pressure at the above-described pressure. Heat treatment temperature is 60
At a temperature of 0 ° C. or higher, particularly 800 ° C. or higher, the reactivity of carbon itself increases. Therefore, in order to prevent the reaction of the vapor grown carbon fiber and the ambient gas, adjusting the atmosphere in the heating portion helium, argon, xenon, krypton, optionally or other inert gas atmosphere to a reducing atmosphere such as H ₂ Is preferred.

A feature of the present invention resides in that an aggregate of vapor grown carbon fibers is formed, and the formed body is heat-treated and then crushed to a certain size to maintain a three-dimensional structure. The crushed material has a floc or thread-like shape, and has a size (maximum diameter portion) of 5 μm or more and 500 μm or less, preferably 10 μm or more and 200 μm or less, and has a three-dimensional structure in which fibers are intertwined. .

The entangled fibers are branched, and at least a part of the contacts of the entangled fibers is fixed with a carbide of a carbon substance such as tar or pitch. Then, compression molding and heat treatment are crushed, so molding without pressure,
Although heat-treated, there are more contacts and more strongly adhered parts than crushed products. Therefore, even when a composite material is formed, the shape such as a floc shape is maintained and dispersed.

If the size of the floc or other vapor-grown carbon fibers is less than 5 microns, contact between the activated carbon particles as the main agent is difficult to obtain even if the particles enter between the activated carbon particles, and sufficient conductivity cannot be obtained. On the other hand, when the size of the floc is 500 μm or more, the floc is too large, causing steric hindrance, and the clogging of the particles is deteriorated, and the density of the particles is not increased and the capacity is reduced. Further, it is difficult to make a contact between particles, so that an effect of lowering electric resistance cannot be obtained.

The crushing method may be any method that can maintain the floc shape or the like at a fixed size.
For the crushing, the compact may be crushed in one stage, but a method of crushing by combining two or more crushers is also effective. For example, it is crushed in advance to a shape of several tens of mm or less by a compression crusher, a shear crusher, an impact crusher, and the like, and then is crushed by a high-speed rotating pin mill, a screen mill, a hammer mill or the like to a crusher of 5 μm to 500 μm. Disintegrate into the following range of shapes.

The powder resistance (in the case of a powder density of 0.08 g / cm ³ ) of the vapor-grown carbon fiber having a floc shape or the like is 0.5.
It is preferably 3 Ωcm or less, more preferably 0.1 Ωcm or less. If the resistance value is 0.3 Ωcm or more, the effect of lowering the resistance value of the electrode when formed into an electrode plate is unfavorably small.

If the specific surface area of the carbon material, which is a structure of a vapor-grown carbon fiber such as a floc, is not less than 20 m ² / g, electric conductivity is undesirably deteriorated.

The amount of the carbon material which is the structure of the vapor-grown carbon fiber such as the floc is 0.05 wt% or more and 30 wt% or more.
wt% or less. If it is added in an amount of 30 wt% or more, the capacity of the carbon fiber is large, so that the amount of activated carbon particles of the active material, which is the main agent, is reduced, and the capacity as a capacitor is reduced. On the other hand, if the content is 0.05 wt% or less, the effect of increasing the number of contacts with the activated carbon particles is small, so that the electrode resistance cannot be reduced and consequently the effect of improving the capacity cannot be recognized.

[0028]

In such an electrode material, activated carbon particles as an electrode active material are incorporated into aggregates of vapor-grown carbon fibers having a floc-like structure, or a network of vapor-grown carbon fibers having branches. Since it is entangled with the flaky structure, the probability of contact between the individual activated carbon particles and the carbon fibers increases, and the number of electrical contacts increases.
As a result, the current becomes extremely easy to flow and the internal resistance decreases,
In addition, the activated carbon particles can be effectively used for the electrolytic reaction without waste, and the battery capacity increases. Furthermore, since the activated carbon particles are entangled and held between the carbon fibers of the floc-like or network-like aggregates of vapor-grown carbon fibers, they absorb the swelling of the activated carbon particles due to the repetition of expansion and contraction due to heat radiation of charge and discharge. , Further separation,
Since the falling off can be prevented, the collapse of the electrode plate can be prevented. As a result, a decrease in battery capacity can be prevented, and a long battery life can be achieved.

[0029]

The present invention will be described below in more detail with reference to examples. (Example 1) Average particle size is 13 µm and specific surface area is 1500
m ² / g activated carbon has a specific surface area of 17 m ² / g, a fiber diameter of 0.15 μm, a floc size or the like of 15 μm, and a powder density of 0.08 g / cm ³ . 016
Add 5.0 wt% of carbon fibers such as flocks of Ωcm and mix well. Then, as a binder, Shonor dissolved in alcohol (BRL-12 manufactured by Showa Polymer Co., Ltd.)
0Z) 7 wt% was added and molded into 12 cm × 12 cm, and carbonized and sintered at a temperature of 850 ° C. to obtain a carbon plate having a thickness of 1 mm. An electrode plate was prepared by cutting out a square plate of 4 cm × 4 cm from the carbon plate. Using this electrode plate, a sulfuric acid-based electric double layer capacitor was assembled and its volume capacity was measured. As a result, the volume specific capacity at a current of 0.5 A was 21 F / cc.

Example 2 The specific surface area was 9 μm and the specific surface area was 9 μm.
2000m^Two / G of activated carbon with a specific surface area of 13m ^Two /
g, fiber diameter 0.15 μm. The size of flock etc. is 12
0 μm, powder density 0.08 g / cm^Three Powder resistance at
5% by weight of 0.021Ωcm floc-like carbon fiber
Add and mix well. Then, as a binder, Alco
(BRL manufactured by Showa Polymer Co., Ltd.)
-120Z) Add 7wt% to form 12cm x 12cm
Mold and carbonized and sintered at a temperature of 850 ° C.
A bon plate was used. An electrode plate is created from this carbon plate and
An electric double layer capacitor similar to that of the first embodiment is used, and the volume specific capacity is
It was measured. As a result, the volume specific capacity at the time of 0.5 A current is 21.
F / cc.

(Example 3) Activated carbon having an average particle size of 3.5 μm and a specific surface area of 1900 m ² / g, and a specific surface area of 13
m ² / g, fiber diameter 0, 20 μm, floc size, etc., 400 μm, powder density 0.08 g / cm ³ , powder resistance 0.014 Ωm, 5 wt.
% And mix well. Thereafter, polytetrafluoroethylene corresponding to 10 wt% as a binder and an alcohol are added and sufficiently kneaded. The kneaded mixture was roll-rolled to a thickness of 0.7 mm and then dried at 130 ° C. for 3 hours to form a sheet-like plate. An electrode plate was prepared from this sheet-shaped plate, and the volume specific capacity was measured as an electric double layer capacitor similar to that in Example 1. As a result, 0.5A
The volume specific capacity at the time was 20 F / cc.

(Example 4) Instead of the floc-like carbon fiber of Example 1, the specific surface area is 13 m ² / g, the fiber diameter is 0.15 µm, the floc-like size is 120 µm, and the powder density is 0. Powder resistance at 0.08 g / cm ³ is 0.019Ω
cm, except that 10% of floc-like carbon fiber was added.
An electrode plate was prepared in the same manner as in Example 1, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A is 19 F / c.
c.

(Example 5) An electrode plate was prepared in the same manner as in Example 4 except that the amount of floc-like carbon fibers added was 3%, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A was 18 F / cc.

(Example 6) Instead of the floc-like carbon fibers of Example 2, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
Electrode plate in the same manner as in Example 2 except that 28 wt% of floc-like carbon fiber having a powder resistance of 0.017 Ωcm at a powder size of 0.08 g / cm ³ and a powder resistance of 0.01 μm, a floc-like size of 200 μm, and the like was added. Was prepared and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A is 17 F / cc.
Met.

Example 7 Instead of the floc-like carbon fibers of Example 1, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
Example except that the addition amount of the floc-like carbon fiber having a particle size of 20 µm, a floc-like size of 120 µm, a powder density of 0.07 g / cm ³ and a powder resistance of 0.017 Ωm was 0.1 wt%. An electrode plate was prepared in the same manner as in Example 1, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A is 1
It was 6F / cc.

(Example 8) Instead of the floc-like carbon fibers of Example 1, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
Example except that the addition amount of the floc-like carbon fiber having a size of 15 µm, a floc-like shape of 50 µm, a powder density of 0.08 g / cm ³ and a powder resistance of 0.10 Ωcm was 5.0 wt%. An electrode plate was prepared in the same manner as in Example 1, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A is 16
F / cc.

Comparative Example 1 As a binder, activated carbon having a specific surface area of 2100 m ² / g was dissolved in alcohol as a binder, and 7 wt% of Shonor (BRL-120Z manufactured by Showa Polymer Co., Ltd.) was used.
%, And carbonized and sintered at a temperature of 850 ° C. to obtain a carbon plate having a thickness of 1 mm. A 4 cm × 4 cm electrode plate was prepared from this carbon plate, and a sulfuric acid-based electric double layer capacitor was assembled using this electrode plate, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A is 15
F / cc.

(Comparative Example 2) Instead of the floc-like carbon fiber of Example 1, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
15 μm, floc-like carbon fiber having a size of 20 μm, powder resistance of 0.07 g / cm ³ , and powder resistance of 0.017 Ωcm are added to the lower limit of the addition amount of 0.07 or less.
An electrode plate was prepared in the same manner as in Example 1 except that 5 wt% was added, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A was 15 F / cc.

(Comparative Example 3) Instead of the floc-like carbon fibers of Example 1, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
The addition amount of the floc-like carbon fiber having a powder size of 2 µm, a floc-like size of 25 µm, a powder density of 0.07 g / cm ³ , and a powder resistance of 0.017 Ωcm is 35 wt% exceeding the upper limit. An electrode plate was prepared and the volume specific capacity was measured in the same manner as in Example 1 except for the above. As a result, 0.5
The volume specific capacity at the time of A was 15 F / cc.

(Comparative Example 4) Instead of the floc-like carbon fibers of Example 1, the specific surface area was 13 m ² / g, and the fiber diameter was 0.1 mm.
15 μm, floc size etc. are 15 μm, powder density is 0.08 g / cm ³ , powder resistance is 0.31 Ωcm. An electrode plate was prepared in the same manner as in Example 1, and the volume specific capacity was measured. As a result, the volume specific capacity at 0.5 A was 15 F / cc.

Comparative Example 5 Example 1 was repeated except that a single fiber of vapor grown carbon fiber having a fiber diameter of 6 μm and a fiber length of 117 μm was added in place of the floc-shaped carbon fiber of Example 1 in an amount of 5.0 wt%. An electrode plate was prepared in the same manner as described above, and the volume specific capacity was measured. As a result, the volume specific capacity at the time of 0.5 A is 15 F / c.
c.

(Comparative Example 6) Instead of the floc-shaped carbon fiber of Example 3, a single fiber of vapor grown carbon fiber having a fiber diameter of 0.2 μm and a fiber length of 4 μm, and a powder density of 0.08 g
An electrode plate was prepared and the volume specific capacity was measured in the same manner as in Example 3, except that 5 wt% of carbon fiber having a powder resistance at 0.031 Ωcm / cm ³ was added. As a result, 0.5A
The volume specific capacity at that time was 14 F / cc. Table 1 shows the above results.

[0043]

[Table 1]

(Note) The size of the floc and the like was measured by dispersing the crushed floc and the like carbon fibers in alcohol, dropping the solution on a glass plate, drying and observing with an optical microscope. The measurement averaged about 100 pieces in several fields.

[0045]

As described above, the carbon material for an electric double layer capacitor of the present invention contains a floc-shaped vapor-grown carbon fiber having a high electric conductivity, so that the carbon material is three-dimensionally electrically conductive as an electrode plate. The properties such as volume specific capacity are excellent. In addition, since activated carbon having a large specific surface area is held in a stitch structure by the floc-shaped carbon fibers, the strength is also excellent.

Claims (3)

[Claims] 1. A vapor-grown carbon fiber having a thickness of 0.05 μm or more and 5 μm or less agglomerates on a carbon powder having a specific surface area of 500 m ² / g or more, and at least a part of a contact point between the fibers is made of a carbon material. Characterized in that 0.1 wt% to 30 wt% of a carbon fiber mainly composed of a flock-like or thread-like structure having a size of 5 μm or more and 500 μm or less, which is fixed by a carbide, is added. material. 2. The powder resistance of a carbon fiber mainly composed of a floc or thread-like structure at a powder density of 0.08 g / cm ³ or more is 0.3 Ωcm or less. 2. The carbon material for an electric double layer capacitor according to 1. 3. The carbon material for an electric double layer capacitor according to claim 1, wherein the specific surface area of the carbon fiber mainly composed of a floc-like or thread-like structure is 20 m ² / g or less. .