JP2000269095A - Electrode for electric double layer capacitor - Google Patents

Abstract

[PROBLEMS] To effectively reduce the internal resistance of a capacitor by reducing the contact resistance at the interface between a current collector and an active material layer. A carbon black includes a current collector having irregularities on its surface, and carbon black particles formed on a surface of the current collector having irregularities and having a particle size smaller than the width of the irregularities. Layer 2 and an active material layer 3 formed on carbon black layer 2 and containing at least activated carbon particles 3a
Consists of The gap between the surface of the current collector 1 and the activated carbon particles 3a of the active material layer 3 which causes the contact resistance therebetween to increase due to the carbon black particles 2a entering the recesses of the unevenness 1a. Can be reliably buried with the carbon black particles 2a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for an electric double layer capacitor, and more particularly to an electrode for an electric double layer capacitor which can contribute to improvement of energy density by reducing internal resistance.

[0002]

2. Description of the Related Art In recent years, as a backup power supply for a microcomputer, an IC memory, or the like, an electric double layer capacitor which has a relatively large capacitance, and is excellent in charge / discharge cycle characteristics and rapid chargeability has attracted attention. An electric double layer capacitor is based on the principle that electric charges are accumulated in an electric double layer as a charge separation layer formed at an interface between an electrode and an electrolytic solution. Charge / discharge is performed with adsorption / desorption on the surface.

[0003] Such an electric double layer capacitor generally includes a pair of current collectors, a pair of active material layers formed on opposing surfaces of the current collectors facing each other, and an active material layer between the active material layers. A basic cell is constituted by the provided separator, and each active material layer and the separator are impregnated with an electrolytic solution. That is, the electrode for an electric double layer capacitor includes a current collector such as an aluminum foil and an active material layer formed on the surface of the current collector and functioning as a polarizable electrode. As the active material, generally, activated carbon, which is a material having a high specific surface area on the order of several thousand m ² / g, is used. An active material such as activated carbon forms an active material layer by being supported on the surface of the current collector via a binder such as methylcellulose or polytetrafluoroethylene (PTFE).

[0004] Here, the contact area between the current collector surface and the active material layer is increased to increase the adhesion between the two and the active material layer is prevented from being peeled off from the current collector surface by etching. Forming irregularities of about 3 μm or less on the surface of an electric body has been performed. On the other hand, in the electric double layer capacitor electrode, if the electric resistance at the electrode, that is, the contact resistance between activated carbons and the contact resistance between the activated carbon and the current collector are reduced, the internal resistance of the capacitor is reduced and ions of the activated carbon are reduced. Since the adsorptivity is improved, the capacitance of the capacitor can be increased. If the capacity of the capacitor can be increased in this manner, the energy density can be improved, which is advantageous for reducing the size and weight of the electric double layer capacitor.

Therefore, by adding a conductive material for improving the conductivity of the electrode, for example, carbon black or graphite to the active material layer, the resistance of the electrode is reduced and the internal resistance of the capacitor is reduced. (Japanese Patent Laid-Open No. Hei 10
-4037). For example, carbon black particles having a small particle size of, for example, about 0.1 μm or less as a conductive material are wet-kneaded with a binder together with activated carbon, applied to the surface of a current collector, and then dried to form an active material layer. I do.
This allows carbon black particles having a small particle size of about 0.1 μm or less to enter gaps formed between activated carbons having a relatively large particle size of about 3 to 10 μm. For this reason, the carbon black particles can assist the transfer of the electric charges in the gap where the electric charges cannot move in the space. Therefore, it is possible to reduce the contact resistance between the activated carbons and reduce the internal resistance of the capacitor.

[0006]

However, it has been difficult to sufficiently reduce the internal resistance of a capacitor even by a technique of adding graphite or carbon black as a conductive material to an active material layer. In this regard, the present inventor has studied and found that the contact resistance at the interface between the current collector and the active material layer considerably controls the internal resistance of the capacitor. It has been found that this is a cause of insufficient reduction in resistance.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and by reducing the contact resistance at the interface between the current collector and the active material layer, the internal resistance of the capacitor can be effectively reduced, thereby reducing the capacitance of the capacitor. It is an object of the present invention to provide an electrode for an electric double layer capacitor capable of increasing the energy density and improving the energy density.

[0008]

According to the present invention, there is provided an electrode for an electric double layer capacitor, comprising: a current collector having an uneven surface; and a current collector formed on the uneven surface of the current collector. It is characterized by comprising a carbon black layer containing carbon black particles having a particle diameter smaller than the width of the irregularities, and an active material layer formed on the carbon black layer and containing at least activated carbon.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrode for an electric double layer capacitor of the present invention has a carbon black layer interposed between the current collector surface and the active material layer. The contact resistance at the interface with the carbon black layer can be reduced by the carbon black layer, whereby the internal resistance of the capacitor can be effectively reduced. That is, by first forming a carbon black layer on the uneven surface of the current collector, carbon black particles having a particle size smaller than the width of the unevenness are introduced into the unevenness of the current collector, and the carbon black layer is formed. Can be filled with particles. Then, by forming an active material layer thereon, a gap that exists between the surface of the current collector and the activated carbon of the active material layer and causes an increase in contact resistance therebetween is formed by carbon black particles. It can be reliably buried and lost. That is, since the carbon black particles can be surely present between the current collector and the active material layer including the inside of the concave portion on the current collector surface, the contact area is increased due to the unevenness. Activated carbon can be contacted via carbon black particles. Therefore, the contact resistance at the interface between the current collector surface and the active material layer can be reduced,
Thereby, the internal resistance of the capacitor can be effectively reduced.

When the carbon black layer is formed by supporting carbon black particles on the surface of the current collector via a binder as described later, the collector is formed via the binder in the carbon black layer. Since the activated carbon of the active material layer can be reliably carried on the current collector surface, the adhesion of the active material layer to the current collector surface can be improved.

The current collector has a thickness of 5 to 100 μm.
Metal foil (aluminum foil, nickel foil, copper foil, etc.)
Can be used. The shape of the current collector may be a band or a square depending on the shape of the electric double layer capacitor, and the size may be arbitrary depending on the capacity or the like. Irregularities are formed on the surface of the current collector by etching or the like in order to increase the adhesion between the surface of the current collector and the carbon black layer formed on the surface of the current collector. The size of the unevenness is about 3 μm or less, and specifically, for one recess (or projection), the depth (or height) is about 0.5 to 2 μm on average, and the width is 1 to 5 μm on average.
m. Note that the unevenness is formed on one surface or both surfaces of the current collector.

The carbon black layer formed on the uneven surface of the current collector contains carbon black particles having a particle size smaller than the width of the unevenness. The particle size of the carbon black particles can be about 0.1 μm or less, specifically, about 0.02 to 0.05 μm in average particle diameter. Further, the thickness of the carbon black layer can be about 5 to 20 μm. When the thickness is less than 5 μm, it is difficult to form a stable film, and the effect of forming the carbon black layer cannot be sufficiently exerted. On the other hand, 20
Even if the thickness is larger than μm, the effect of forming the carbon black layer cannot be expected any more, leading to an increase in the thickness of the electric double layer capacitor and an increase in cost.

This carbon black layer can be formed by adding a binder to carbon black particles in an amount of about 10 to 20 wt% to form a paste, applying the paste on the surface of a current collector, and then drying the paste. . If the amount of the binder added to the carbon black particles at this time is less than 10 wt%, it is difficult to form a stable film of the carbon black layer due to the lack of the binder, and the active material on the current collector surface The adhesion of the layer may be reduced. On the other hand, the amount of the binder added to the carbon black particles is 20 wt.
%, The shortage of carbon black particles causes
It is difficult to sufficiently exert the effect of the carbon black particles that lowers the contact resistance at the interface between the current collector surface and the active material layer. Examples of the binder used to form the carbon black layer include water-soluble methylcellulose and polyvinyl alcohol.

[0014] The active material layer contains at least activated carbon. In addition to activated carbon, a conductive material such as carbon black or graphite may be included in the active material layer in an appropriate amount. However,
From the viewpoint of improving the adhesion between the carbon black layer and the active material layer, it is preferable to employ carbon black as the conductive material contained in the active material layer. The average particle size of the activated carbon can be about several μm to several tens μm. Further, the thickness of the active material layer can be about 30 to 100 μm.

The active material layer contains activated carbon particles,
If necessary, a paste obtained by adding conductive material particles can be applied onto the carbon black layer, and then dried to form a paste.

[0016]

EXAMPLES Specific examples of the electrode for an electric double layer capacitor according to the present invention will be described below. (Embodiment) The electrode for an electric double layer capacitor of this embodiment schematically shown in FIG. 1 includes a current collector 1, a carbon black layer 2 formed on the surface of the current collector 1, and a carbon black layer 2. And the active material layer 3 formed thereon.

The current collector 1 has a square shape with a thickness of 20 μm and a side length of 50 mm, and is made of a metal foil (aluminum foil). On one surface of the current collector 1, irregularities 1a are formed by an etching process. The size of the unevenness 1a is about 3 μm or less, and specifically, the depth (or height) of one concave (or convex) is about 1 μm on average,
The width is about 3 μm on average.

The carbon black layer 2 is provided with the irregularities 1
carbon black particles 2 having a particle size smaller than the width of a
a) and methylcellulose (not shown) as a binder. The average particle size of the carbon black particles 2a is 0.3 μm. The thickness of the carbon black layer 2 is 10 μm. The active material layer 3 is made of activated carbon particles 3a.
And carbon black particles 2a as a conductive material, and methylcellulose (not shown) as a binder. The average particle size of the activated carbon particles 3a is 6 μm. The thickness of the active material layer 3 is 50 μm.

This electrode for an electric double layer capacitor was manufactured as follows. First, a current collector 1 having a predetermined shape was prepared, and one surface of the current collector 1 was subjected to an etching process to form irregularities 1 a on one surface of the current collector 1. Then, 10% by weight of water-soluble methylcellulose is added to the carbon black particles 2a to form a paste, which is applied to the surface of the current collector 1 having the irregularities 1a, and then dried to form a paste. The carbon black layer 2 was formed on the surface having the irregularities 1a. Finally, activated carbon particles 3
a, and 15 wt% of the water-soluble methylcellulose was added to 10 wt% of the carbon black particles 2 a with respect to the activated carbon particles 3 a to form a paste, which was coated on the carbon black layer 2. By drying, the active material layer 3 was formed on the carbon black layer 2, and the electrode for an electric double layer capacitor of the present example was completed.

Comparative Example An electrode for an electric double layer capacitor of a comparative example was manufactured in the same manner as in the above example except that the carbon black layer 2 was not formed. (Evaluation of Internal Resistance of Capacitor) The effect of reducing the internal resistance of the capacitor was evaluated for each of the electric double layer capacitor electrodes of the above Examples and Comparative Examples.

That is, the electrode according to the embodiment in which the carbon black layers 2 are formed on both surfaces of the current collector 1 and the active material layer 3 is formed on each of the carbon black layers 2 is used as a positive electrode, The two electrodes according to the embodiment in which the carbon black layer 2 is formed on one side and the active material layer 3 is formed on the carbon black layer 2 are used as negative electrodes, and the positive electrode is disposed between the negative electrodes and the separator is provided. , Three electrodes were overlapped so that the active material layers 3 face each other, and the outside of each of the negative electrodes was pressed and fixed with an insulating pressing plate, and incorporated into a capacitor experimental cell. Then, an electrolytic solution was injected into the experimental cell to impregnate each active material layer 3 and the separator with the electrolytic solution, thereby producing a capacitor experimental cell according to the example. In addition, using a microporous polypropylene film having a thickness of 25 μm as a separator,
Propylene carbonate was used as an electrolyte. Also,
After the injection of the electrolyte, the inside of the experimental cell was degassed to completely remove water.

With respect to the experimental cell according to this embodiment, charging and discharging were repeated at a constant current, and the internal resistance was calculated from the change in voltage at that time. Table 1 shows the results. An experimental cell according to a comparative example was manufactured in the same manner as described above, except that the carbon black layer 2 was not formed on the positive electrode and the two negative electrodes of the experimental cell according to the above example, and the internal resistance was examined in the same manner. Table 1 shows the results.

[0023]

[Table 1] As is clear from Table 1, by interposing the carbon black layer 2 between the current collector 1 surface and the active material layer 3,
The internal resistance of the capacitor was reduced by half.

(Relationship Between Carbon Black Added Amount and Internal Resistance of Capacitor) The relationship between the added amount of carbon black in the active material layer 3 and the internal resistance of the capacitor was examined. That is, for the electric double layer capacitor electrode of the comparative example in which the carbon black layer 2 was not formed, when the addition amount of the carbon black particles 2a to the activated carbon particles 3a in the active material layer 3 was variously changed, the capacitor was accordingly changed. Was examined how the internal resistance changes. Table 2 shows the results.

[0025]

[Table 2] As is clear from Table 2, it is found that the internal resistance of the capacitor decreases as the amount of the carbon black particles 2a added to the active material layer 3 increases. However, when the addition amount of the carbon black particles 2a to the activated carbon particles 3a exceeds 10 wt%, the carbon black particles 2a
Reduces the effect of reducing the internal resistance. This is considered to be because the contact resistance at the interface between the current collector 1 and the active material layer 3 considerably governs the internal resistance of the capacitor.

Further, in the electrode for an electric double layer capacitor of the above embodiment in which the carbon black layer 2 is formed, the amount of the carbon black particles 2a added to the active material layer 3 is 10 watts.
t%, the internal resistance of the capacitor is 5
Ωcm ²
In the electrode for an electric double layer capacitor of the comparative example in which the carbon black layer 2 was not formed, even if the addition amount of the carbon black particles 2a in the active material layer 3 was 20 wt%, the internal resistance of the capacitor was only 7 Ω · cm ^2. It could not be reduced. From this result, it can be seen that the current collector 1
It can be seen that the contact resistance at the interface between the capacitor and the active material layer 3 considerably governs the internal resistance of the capacitor. It is to be noted that the carbon black particles 2a excessively added to the active material layer 3 with respect to the effect of reducing the internal resistance have a particle diameter larger than the width of the unevenness 1a of the current collector 1. It is considered that it is difficult to preferentially cause the carbon black particles 2a to be present in the recesses of the unevenness 1a because the activated carbon particles 3a are adsorbed on the surface of the activated carbon particles 3a.

[0027]

As described in detail above, the electrode for an electric double layer capacitor according to the present invention employs a structure in which a carbon black layer is interposed between the surface of the current collector and the active material layer. The contact resistance at the interface between the capacitor and the active material layer can be reduced, and the internal resistance of the capacitor can be effectively reduced, so that the capacitance of the capacitor can be increased. Therefore, the improvement of the energy density is advantageous for reducing the size and weight of the electric double layer capacitor.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view schematically illustrating a structure of an electrode for an electric double layer capacitor according to the present embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Current collector 1a ... Unevenness 2 ... Carbon black layer 2a ... Carbon black particles 3 ... Active material layer 3a ... Activated carbon particles

Claims (1)

[Claims] 1. A current collector having an uneven surface, and a carbon black layer formed on the uneven surface of the current collector and including carbon black particles having a particle size smaller than the width of the unevenness. An electrode for an electric double layer capacitor, comprising: an active material layer containing at least activated carbon formed on the carbon black layer.