Electrode for electric double-layer capacitor, and manufacturing method thereof

The present invention relates to an electrode for an electric double layer capacitor having good life characteristics and a method for manufacturing the same.

Conventionally, an electric double layer capacitor composed of a pair of polarized electrodes and an electrolytic solution is composed of a pair of polarized electrodes, a separator existing between them, and a current collecting layer of each polarized electrode. Activated carbon powder, activated carbon fiber, and the like are used as typical polarized electrodes.

In this method for producing a polarized electrode, a conductive substance such as acetylene black and a resin such as polytetrafluoroethylene or tetrafluoroethylene resin are added as a binder to activated carbon powder, which is a typical polarized electrode, and mixed. Later, a method of pressure molding and a method of applying this mixture to the current collector can be mentioned.

Such an electric double layer capacitor has a problem of a decrease in capacity during high temperature standing, which is considered to be caused by a reaction due to a functional group on the surface of activated carbon. Proposals have been made to solve this problem (Patent Document 1), but this is not sufficient. Further, it is known that carbon nanotubes have better life characteristics than activated carbon because they have few such functional groups.

On the other hand, for the purpose of improving conductivity, there is an attempt to use a polar electrode containing activated carbon and carbon nanotubes (Patent Document 2), but in the present invention, since a binder which is a resin component is used, the capacitance characteristics There was a problem that a good electrode could not be obtained.<patcit num="1"><text>Japanese Unexamined Patent Publication No. 2001-237149</text></patcit><patcit num="2"><text>Japanese Patent Application Laid-Open No. 2007-200979</text></patcit>

<p> The present invention is to obtain an electrode for an electric double layer capacitor having good life characteristics, being able to be manufactured by a simpler method, and having excellent capacitance characteristics, and a method for manufacturing the same.</p>

<p>In the electrode for an electric double layer capacitor of the present invention, a sheet obtained by paper-molding activated carbon and carbon nanotubes is integrated with the etching foil by the uneven portion via the uneven portion formed on the surface of the etching foil constituting the current collector. It is characterized by being.</p><p> The sheet obtained by paper-molding the activated carbon and carbon nanotubes is characterized in that the sheet is paper-molded without using a binder.</p><p> Further, in the method for manufacturing an electric double layer capacitor of the present invention, a sheet obtained by paper-molding activated carbon and carbon nanotubes is pressed against an uneven portion formed on the surface of an etching foil constituting a current collector to etch the carbon nanotubes. It is characterized by being integrated with the foil.</p>

<p> In the present invention, a polarized electrode in which activated carbon and carbon nanotubes are mixed can provide good life characteristics, and further, the carbon nanotubes and the carbon nanotubes are etched in a state where the uneven portion of the etching foil bites into the activated carbon and the carbon nanotubes that have been made. Since it is integrated with the foil, it is not necessary to use other materials such as resin-based binders and conductive auxiliary materials, and the conductive materials come into direct contact with the current collector, so electrical resistance can be reduced. An electrode for an electric double-walled capacitor having excellent capacitance characteristics can be obtained.</p>

The outline of this method is that activated carbon (hereinafter referred to as AC) and carbon nanotubes (hereinafter referred to as CNT) are made into paper by molding (paper making) without using a binder, and a sheet (also called paper) composed of activated carbon and CNT is formed. ) Is formed, and this sheet (hereinafter referred to as AC / CNT sheet) is pressed against a foil (hereinafter referred to as an etched foil) of a current collector such as an etched aluminum foil to form an AC / CNT sheet and an etched foil. The electrodes are manufactured by integrating the above. Details are shown below.

Activated carbon and single-walled carbon nanotubes (hereinafter referred to as SWCNTs) are mixed with methanol and stirred with a mixer for about 30 seconds to prepare an AC / SWCNT / methanol dispersion. This dispersion is filtered under reduced pressure using a PTFE filter paper (diameter: 35 mm, average pores 0.2 μm) to obtain a paper-formed AC / SWCNT sheet. The thickness of this SWCNT sheet is such that the thickness of the polar electrode layer formed after pressing is about 1 to 60 μm (more specifically, the thickness of the polar electrode layer required for the electrode of the electric double layer capacitor). ), The amount of deformation due to the press shall be considered in advance.

Cut this AC / SWCNT sheet to the same size as the current collector, place it on the etched aluminum foil that is the current collector, and sandwich it with a separately prepared aluminum foil with a flat surface that has not been etched. , 10t / cm from the top and bottom of the foil²Press for 1 minute at the pressure of.

Pressing pressure is 0.1 ~ 100t / cm²By this pressing, pressure is applied to the enlarged uneven portion of the etched aluminum foil, and as a result, pressure is applied to the convex portion to form a bent state, and the convex portion becomes a paper-molded SWCNT sheet. By biting into it, excellent bondability can be imparted.

As a pressing method, 0.1 ~ 100t / cm²As long as the pressure can be applied, other methods such as a roll press can be used in addition to a flat press.

As this etching foil, a current collector of a metal foil such as an aluminum foil whose surface has been enlarged by a chemical etching method or an electrochemical etching method is used. In this case, the thickness of the etching foil, which is a current collector, is preferably 10 μm to 100 μm. If it is 10 μm or less, it is difficult to construct a capacitor cell due to insufficient strength of the current collector, and if it is too thick, the capacitance density per cell decreases.

Further, as the shape of the uneven portion formed on the etching foil, it is desirable that the tip of the convex portion bites into the sheet during pressing, or the tip of the convex portion is deformed and entangled with the fibers constituting the sheet, and the uneven portion is formed. A sharp rod-shaped (needle-shaped) one is preferable to one having a gentle mound-like shape.

Further, the height of the uneven portion (distance from the tip of the convex portion to the bottom of the concave portion) is set to be lower than the thickness of the carbon nanotube layer (for example, about 1 to 60 μm), and the current collector layer is pressed. It is set to be surely covered with the carbon nanotube layer formed later. In addition, it is necessary to consider the thickness of the etching foil that is the current collector (about 10 μm to 100 μm), and for a thin etching foil, it should be within 2/3 of the thickness to maintain the strength of the etching foil. Preferred above.

As the activated carbon, it is preferable to use natural plant tissues such as palm shavings, synthetic resins such as phenol, and fossil fuels such as coal, coke, and pitch as raw materials, which are activated and used. The activated carbon activation method varies depending on the raw material used, but usually includes an alkaline activation method such as a steam activation method or a KOH activation method. In the present invention, activated carbon obtained by either the steam activation method or the alkaline activation method is preferably used. The specific surface area of activated carbon is 1000 to 4000 m.²/ g, preferably 2000-3000m²Those in the range of / g are desirable. The average particle size of the activated carbon is preferably 0.1 to 50 μm, preferably 1 to 10 μm. If it is smaller than this range, the activated carbon particles deviate from the electrode and diffuse into the electrolytic solution, which is not preferable because the capacitance density decreases. On the other hand, if it is larger than this range, CNTs are less likely to be entangled with the activated carbon particles and good papermaking becomes difficult, which is not preferable.

Carbon nanotubes (CNTs) are single-walled carbon nanos with a single layer of graphene sheet. The tube may be a multi-walled carbon nanotube (MWCNT) in which two or more layers of graphene sheets are coaxially rolled and the tube wall is multi-walled, or they may be mixed. The outer diameter of the CNT is preferably in the range of 1 to 100 nm, preferably 2 to 70 nm, and further preferably 3 to 40 nm. The length of the carbon nanotubes is preferably in the range of 50 to 500 μm, preferably 70 to 400 μm, and more preferably 100 to 200 μm. Further, as the number of layers of the graphene sheet of CNT is smaller, the capacitance density of CNT itself is higher. Therefore, CNT having 50 or less layers, preferably 10 or less layers is preferable from the viewpoint of capacitance density.

SWCNT is a cylindrical fiber made of a single-layer graphite layer, but SWCNT is a fiber. Some T have an open tip, while others are seamlessly sealed with single-layer graphite. Since the electric double layer is more likely to be formed at the graphite edge than the graphite surface, SWCNTs with an open tip have a higher capacitance per unit area, which is preferable from the viewpoint of capacitance density. Furthermore, if MWCNT with an open tip is used, the capacitance per unit area can be further increased, which is preferable from the viewpoint of capacitance density. SWCNTs and MWCNTs, whose tips are seamlessly sealed with single-layer graphite, have few graphite edges and therefore have few functional groups present at the graphite edges. Therefore, the capacity is unlikely to decrease during high-temperature standing, which is thought to be caused by the reaction between the functional group and the electrolytic solution, which is preferable in terms of the life characteristics of the capacitor.

The content of activated carbon and CNT is such that the content of CNT is 3 to 95% by weight, preferably 5 to 80% by weight, and more preferably 8 to 50% by weight, based on the total amount of activated carbon and CNT.

(Example 1) Weigh 50 ml of steam-activated activated carbon and carbon nanotubes (outer diameter, 20 nm, length 150 μm, 5-layer MWCNT) so that the AC weight ratio is 90% and the CNT weight ratio is 10%. The mixture was mixed with methanol and stirred with a mixer for about 30 seconds to prepare an AC / CNT / methanol dispersion. This dispersion was filtered under reduced pressure using a PTFE filter paper (diameter: 35 mm, average pore size 0.2 μm) to obtain an AC / CNT sheet (papermaking AC / CNT sheet) molded by papermaking. Cut this to the same size as the current collector, place it on the etched aluminum foil as a current collector, sandwich them with another aluminum foil, and 10 tcm from the vertical direction of the foil.^-2The electrode (papermaking AC / CNT electrode) was obtained by pressing at the pressure of 1 for 1 minute. Papermaking Using two AC / CNT electrodes for both electrodes, an electric double layer capacitor element was manufactured via a cellulosic separator (electrode area: 2.1 cm).²). And 1M (= 1mol / dm)³) Was impregnated into the device with a propylene carbonate solution containing tetraethylammonium borate tetrafluoride as an electrolytic solution, and then heat-sealed using a laminate film to prepare an evaluation cell (papermaking AC / CNT cell). (Comparative example 1) Weigh a total of 50 mg of CNT with the same weight ratio as in Example 1, a binder (PTFE) dispersion with a solid content of 5% and activated carbon with a weight ratio of 85%, and use the same method as in Example 1 to evaluate the cell (evaluation cell). Papermaking AC / CNT / PTFE cell) was prepared. (Comparative example 2) Weigh 50 mg of Ketjen Black (KB) having the same weight ratio as in Example 1, a binder (PTFE) dispersion with a solid content of 5% and activated carbon having a weight ratio of 85%, and mix them, and knead them in a dairy pot. did. Then, it was stretched with a biaxial roller to obtain an AC sheet. This is cut to the same size as the current collector, and the same etching-treated aluminum foil as in Example 1 is used as the current collector, carbon paste is applied to the adhesive surface of the cut AC sheet, and AC is applied thereto. The sheets were adhered and dried at 120 ° C. under normal pressure for 1 hour to obtain an electrode (AC electrode). Using this electrode for both electrodes, an evaluation cell (AC cell) was prepared by the same method as in Example 1. (Test results) Although the papermaking AC / CNT sheet prepared in Example 1 did not contain a binder, a good sheet could be formed as in the sheet prepared in Comparative Example 1. It was also found that the papermaking AC / CNT electrodes produced in Example 1 were in good contact with each other, as in the sheet produced in Comparative Example 1, although they did not use an adhesive such as carbon paste.

From the discharge curves of the cells of Example 1, Comparative Example 1 and Comparative Example 2 when a voltage was applied to 3 V by constant current charging, the voltage was held for 30 minutes, and then discharged to 0 V by constant current discharge. , The capacity density of each cell was calculated. The results are shown in Table 1. As is clear from Table 1, it was found that Example 1 had a higher volume density than Comparative Example 1. It is presumed that this is because there is no capacity reduction due to the binder in Example 1 as compared with Comparative Example 1. Moreover, the capacitance density per weight of the polarizable electrode of Example 1 is higher than that of Comparative Example 2. It is presumed that this is because the capacitance density of CNTs used in Example 1 is higher than that of KB used in Comparative Example 2.

In order to examine the life characteristics of the cells of Example 1 and Comparative Example 2, after applying a voltage up to 3.7 V by constant current charging, when the applied voltage is applied for a certain period of time, the voltage is loaded. The relationship between the time and the capacity density was measured. The results are shown in Fig. 1. As is clear from FIG. 1, it was found that the capacity of Example 1 can be maintained higher than that of Comparative Example 2. It is presumed that this is because the capacity decrease due to the decomposition of the binder contained in the electrode of Example 2 is large.

<tables num="1"><img file="JP2009246306A_D0001.tif" /></tables>

From the results shown in Table 1 and FIG. 1, it was found that the electric double layer capacitor obtained by the present invention has good life characteristics, can be manufactured by a relatively simple process, and has excellent capacitance characteristics.

<figref num="1">The figure which shows the life characteristic of the electric double layer capacitor of this invention.</figref>