JP2007266548A - Electric double layer capacitor using carbon nanotube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric double layer capacitor where there is no possibility of a leaked liquid due to a deformation caused from an impact, bending load etc., and which is thin and can storage a large capacity of electricity. <P>SOLUTION: The electric double layer capacitor is constructed in a manner such that a pair of positive/negative electrodes, in which a gelled electrolytic membrane 3 is formed on a collector 1 so as to cover a group 2 of carbon nanotubes located on the collector 1, are located so that a group of positive nanotubes and a group of negative nanotubes face each other in a non-contact manner. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric double layer capacitor capable of storing a large amount of electricity without fear of leakage.

  Conventionally, in an electric double layer capacitor, a separator such as a polypropylene non-woven fabric is sandwiched between a pair of polarizable electrode layers in which a polarizable electrode layer mainly composed of activated carbon is formed on a current collector, and an electrolytic solution is applied to the electrode layer. The element was housed in a metal container, and the metal container was sealed with a sealing plate and a gasket (see Patent Document 1).

In addition, activated carbon having a large specific surface area generally has low electrical conductivity, and the activated carbon alone increases the internal resistance of the polarizable electrode, and a large current cannot be taken out. Therefore, in order to reduce the internal resistance, carbon nanotubes are included in the polarizable electrode. Attempts have been made to increase the capacity by adding groups to increase the electrical conductivity (see Patent Document 2).
JP-A-4-154106 JP 2000-1224079 A

  The electric double layer capacitor as described above has a larger capacity than a conventional electrolytic capacitor and is excellent in instantaneous charge / discharge characteristics. Therefore, it has been widely used as a backup power source for IC circuits. Development of smaller, lighter and larger capacity electric double layer capacitors is being promoted for application to equipment.

  However, in an electric double layer capacitor using a normal organic electrolyte, a porous separator is used as an electrode constituent member, and in view of the electrolyte retention and safety, it is about 50 μm. Thickness is needed. Moreover, since it is a liquid electrolyte solution, when a damage | wound or destruction arises in the exterior of a device, electrolyte solution will leak outside and it is dangerous.

  Therefore, an object of the present invention is to provide a thin electric double layer capacitor that is capable of storing a large amount of electricity without fear of leakage due to deformation due to impact or bending load.

  An electric double layer capacitor according to the present invention comprises a pair of positive and negative electrodes formed by forming a gel electrolyte membrane on a current collector so as to cover a group of carbon nanotubes provided on the current collector, The carbon nanotube group and the carbon nanotube group of the negative electrode are arranged so as to face each other and in a non-contact manner.

  In the present invention, an organic electrolytic solution that is a conventional electrode constituent member is not used, and a separator for separating the positive electrode and the negative electrode is not used. Instead, a gel electrolyte is used. By combining an electrode using carbon nanotubes with a gel electrolyte, an unprecedented thin sheet-shaped electric double layer capacitor can be produced.

  In the electric double layer capacitor using the gel electrolyte, the electrolyte component is held in the polymer matrix, and the liquid component does not leak to the outside even when some destruction occurs in the exterior material. Furthermore, the gel electrolyte can easily produce a thin film which is difficult with a porous separator. For this reason, the electric double layer capacitor using the gel electrolyte is extremely safe and the thickness of the product can be extremely reduced.

  The carbon nanotube group is preferably formed so as to be inclined with respect to the substrate serving as a current collector.

Thus, by inclining the brush-like carbon nanotube group,
(A) The thickness of the carbon nanotube group can be reduced, whereby the electrode thickness can be reduced. As a result, a thin-film electric double layer capacitor can be realized.

(B) By increasing the carbon density per volume (carbon nanotube density), the carbon surface area per volume increases, and both the energy density per volume and the output density per volume increase.

(C) Since the distance between the positive electrode current collector and the negative electrode current collector is reduced, a direct current resistance component such as a solution resistance that is a resistance component between the electrodes is reduced, and the resistance of the electric double layer capacitor is reduced.

  The gel electrolyte is obtained by heat-gelling a gel precursor based on an ionic liquid.

An ionic liquid refers to a salt that shows a colorless and transparent liquid state at room temperature (about 15 to 25 ° C.) while being an ionic substance.
Ethylmethylimidazolium tetrafluoroborate (EMI • BF ₄ )
Ethylmethylimidazolium perfluorosulfonimide (EMI / TFSI)
Butylpyridium tetrafluoroborate (BP / BF ₄ )
Butylpyridium perfluorosulfonimide (BP / TFSI)
Trimethylpropylammonium tetrafluoroborate (TMPA · BF ₄ )
Trimethylpropylammonium perfluorosulfonimide (TMPA / TFSI)
Ethylmethylpyrrolidinium tetrafluoroborate (P12 · BF ₄ )
Ethylmethylpyrrolidinium perfluorosulfonimide (P12 / TFSI)
The ionic liquid is not limited to these.

  In the present invention, an organic electrolytic solution that is a conventional electrode constituent member is not used, and a separator for separating the positive electrode and the negative electrode is not used. Instead, a gel electrolyte is used. By combining an electrode using carbon nanotubes with a gel electrolyte, an unprecedented thin sheet-shaped electric double layer capacitor can be produced.

  In the electric double layer capacitor using the gel electrolyte, the electrolyte component is held in the polymer matrix, and the liquid component does not leak to the outside even when some destruction occurs in the exterior material. Furthermore, the gel electrolyte can easily produce a thin film which is difficult with a porous separator. For this reason, the electric double layer capacitor using the gel electrolyte is extremely safe and the thickness of the product can be extremely reduced.

  Furthermore, according to the present invention, a synergistic effect of the carbon nanotube electrode structure and the gel electrolyte can be obtained. That is, an electrode using normal activated carbon has a large surface area and a large number of pores having a pore diameter of 2 nm or less on the carbon surface. Such pores are not infused with electrolyte ions as well as gel electrolytes. In other words, the gel electrolyte does not penetrate into the pores, resulting in pores that cannot be used.

  Since the carbon nanotubes are not having a pore structure but are brush-like, there are no spaces smaller than 2 nm. Rather, carbon nanotubes exist at intervals of 10 nm or more. This may allow the gel precursor of the gel electrolyte to be impregnated between the carbon nanotubes.

  Usually, the gel electrolyte has lower ionic conductivity as compared with an electrolytic solution using a solution. When such an electrolyte is used, in a normal electrode using activated carbon, the development of capacity with respect to a large current is deteriorated (capacity is not output during high rate characteristics). As long as it is a gel electrolyte, the same phenomenon occurs in a carbon nanotube electrode. Even if it exists, since the moving direction is aligned or not disturbed, the capacity for a large current is superior to that of activated carbon.

(1) The capacitor electrode is produced, for example, as follows.

  1) A group of brush-like carbon nanotubes is grown on a substrate by a known method such as chemical vapor deposition, and transferred from the substrate to a current collector to produce an electrode (see, for example, WO03 / 073440).

2) In order to adjust the thickness of the carbon nanotube group, the brush-like carbon nanotube group is inclined with respect to the current collector. Specifically, an electrode made of a current collector with brush-like carbon nanotubes is set in a press device and sandwiched between two stainless steel plates, pressure: 400 to 1000 kg / cm ² , holding time: 0.5 to 2 minutes Press the brush-like carbon nanotubes to the extent.

As for the tilt angle, when the vertically aligned state is 90 °, the tilt angle may be less than 90 °, but the minimum tilt angle is
Sinθ> 10 / L ₁ (I)
Is an angle θ satisfying (see FIG. 1).

Here, Sinθ = X / L ₁
θ: Inclination angle
X: Distance between carbon nanotubes
L ₁ : Distance (pitch) between centers of adjacent carbon nanotubes (2) on the current collector (1)

  The above formula (I) is introduced as follows.

  As a prerequisite, X must be at least 10 nm. This is because the distance between the carbon nanotubes (2) needs to be 10 nm apart from the distance of the electric double layer.

Therefore, X = L ₁ × Sinθ> 10
That is, Sinθ> 10 / L ₁
It becomes.

  The inclination of the bristle-like carbon nanotube group may be omitted (see FIG. 3).

  3) Production of the gel electrolyte membrane is performed according to the following steps.

  Preparation of gel precursor → Pour of gel precursor onto carbon nanotube (2) group → Gelation by heating → Cooling

  The gel precursor is obtained by mixing an ionic liquid and a gelling agent with stirring. The ratio of the ionic liquid and the gelling agent varies depending on the degree of polymer polymerization of the gelling agent, but the former: the latter = 50 to 95:50 to 5 (weight ratio) is preferable. A gelling agent of 50% by weight is sufficient, and if it is too much, the concentration of the ionic liquid is lowered and the capacitance of the capacitor is lowered. If the gelling agent is less than 5% by weight, the ionic liquid may not gel.

  In order to remove bubbles mixed in the gel precursor, this is left to stand under vacuum and defoamed. As the ionic liquid, those described above are used alone or in combination of two. A polymer material is suitable as the gelling agent, and a material that forms a polymer network by heating or a crosslinking reaction is used. Typical examples of the gelling agent include polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) and the like, but the gelling agent is limited to these polymer materials. It is not a thing.

  The gel precursor preparation step is performed in an environment having a dew point temperature of −60 ° C. or lower, for example, in a dry room or a glove box.

  The electrolyte concentration in the gel electrolyte is much higher than the concentration of the organic electrolyte solution using a liquid solvent, and contributes greatly to the increase in capacity of the electric double layer. For this reason, the electric double layer capacitor using the gel electrolyte can provide a larger capacity than the electric double layer capacitor using the organic electrolyte.

  The step of pouring the gel precursor onto the carbon nanotube (2) group is also performed in an environment having a dew point temperature of −60 ° C. or lower, for example, in a dry room or a glove box. First, the gel precursor is dropped on the surface of the current collector on the carbon nanotube (2) side, and the carbon nanotube (2) group is covered with the gel precursor. Next, this is left to stand for 60 minutes under vacuum to be defoamed. In this way, the gel precursor film (3) is formed on the current collector so as to cover the carbon nanotubes (2) provided on the current collector, and the thickness of the gel precursor film (3) is reduced by a baker applicator or the like. adjust.

  Then, in order to gelatinize a gel precursor, this is heat-processed. Preferred treatment conditions are 100 to 200 ° C. and 0.5 to 10 hours. The heating temperature is appropriately determined depending on the type of the gelling agent, but if it is less than 100 ° C., gelation of the gel precursor does not occur. If the heating temperature exceeds 200 ° C, the gelling agent may be deteriorated due to decomposition or the like.

  Next, the obtained gel electrolyte membrane (3) is cooled. In this way, the gel electrolyte membrane (3) is formed on the current collector so as to cover the carbon nanotubes (2) provided on the current collector.

4) Fabrication of an electric double layer capacitor A pair of positive and negative electrodes made of a current collector having a carbon nanotube (2) group are placed in, for example, a packaging container, and the positive carbon nanotube (2) group and the negative carbon nanotube (2) Arrange so that the groups face each other. Since the carbon nanotube (2) group of the positive electrode and the carbon nanotube (2) group of the negative electrode are both covered with the gel electrolyte membrane (3), they do not contact each other. The container is then sealed to obtain an electric double layer capacitor.

  Next, in order to describe the present invention specifically, examples of the present invention will be given.

Example 1
In FIG. 2, brush-like carbon nanotubes (2) grown vertically on a silicon substrate by a known method were transferred to a current collector (1) made of aluminum foil to produce an electrode. At this time, the length of the bristle-like carbon nanotube was 60 μm.

The transferred current collector (1) with brush-like carbon nanotubes (2) is set in a hydraulic press and sandwiched between two stainless steel plates, pressure: 500 kg / cm ² , holding time: about 1.5 minutes Then, the brush-like carbon nanotube (2) was pressed and inclined with respect to the current collector (1). At this time, the aluminum foil was not wrinkled. The thickness of the layer made of the inclined carbon nanotube (2) group was 30 μm.

12 g of ethylmethylimidazolium tetrafluoroborate (EMI · BF ₄ ) and 3 g of polyvinylidene fluoride-hexafluoride propylene in an environment having a dew point temperature of −60 ° C. or less (specifically, in a dry room or in a glove box) The copolymer (PVdF-HFP) was mixed for 3 hours with stirring in a container to obtain a gel precursor.

  Next, in order to remove bubbles mixed in the gel precursor, this was left to stand for 60 minutes under vacuum to perform defoaming treatment.

  Next, in an environment having a dew point temperature of −60 ° C. or lower, for example, in a dry room or a glove box, first, a gel precursor is dropped on the surface of the current collector (1) on the carbon nanotube (2) side, and the carbon nanotube (2) The group was impregnated and the group of carbon nanotubes (2) was covered with a gel precursor. Subsequently, this was left to stand for 60 minutes under vacuum to perform defoaming treatment. Thus, the gel precursor film (3) was formed on the current collector (1) so as to cover the carbon nanotubes (2) group on the current collector (1). The thickness of the gel precursor film (3) was adjusted to 35 μm with a Baker applicator.

  Then, in order to gelatinize a gel precursor, this was heat-processed at about 120 degreeC for 1 hour.

  Next, the obtained gel electrolyte membrane (3) was cooled.

  Thus, the gel electrolyte membrane (3) was formed on the current collector (1) so as to cover the carbon nanotubes (2) group on the current collector (1).

  Gel electrolyte membrane (3) Coated carbon nanotube (2) A pair of positive and negative electrodes made of a current collector (1) group is placed in a packaging container made of an aluminum laminate and the positive electrode carbon nanotube (2) group and the negative electrode The carbon nanotubes (2) were arranged so as to face each other.

  Thereafter, the packaging container was sealed with a sealing machine such as an impulse sealer.

Example 2
9.5 g of ethylmethylimidazolium tetrafluoroborate (EMI · BF ₄ ) and 0.5 g of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) were mixed to obtain a gel precursor, The same operation as in Example 1 was performed to produce an electric double layer capacitor.

  Thus, by increasing the proportion of the ionic liquid, the amount of effective ions can be increased, and the resistance component (PVdF-HFP is an insulator) can be decreased.

Example 3
In FIG. 3, an electric double layer capacitor was produced in the same manner as in Example 1 except that the step of tilting the bristle-like carbon nanotubes (2) group with respect to the current collector (1) was omitted. The thickness of the layer composed of the group of brush-like carbon nanotubes (2) was 60 μm, and the thickness of the gel electrolyte membrane (3) covering the layer was 65 μm.

It is a vertical sectional view showing a state in which carbon nanotubes provided perpendicular to the current collector are inclined. It is a vertical sectional view showing an electrode in which carbon nanotubes inclined with respect to a current collector are covered with a gel electrolyte. It is a vertical sectional view showing an electrode in which brush-like carbon nanotubes provided perpendicular to the current collector are covered with a gel electrolyte.

Explanation of symbols

(1): Current collector
(2): Carbon nanotube
(3): Gel precursor membrane (gel electrolyte membrane)

Claims (3)

  A pair of positive and negative electrodes formed by forming a gel electrolyte membrane on the current collector so as to cover the carbon nanotube group provided on the current collector, and the positive carbon nanotube group and the negative carbon nanotube group in the container An electric double layer capacitor using carbon nanotubes, characterized in that they are arranged so as to face each other and in a non-contact manner.   The electric double layer capacitor using carbon nanotubes according to claim 1, wherein the carbon nanotube group is inclined with respect to the current collector. The electric double layer capacitor using carbon nanotubes according to claim 1 or 2, wherein the gel electrolyte is obtained by gelation of a gel precursor based on an ionic liquid.

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