JP2011066275A - Method of manufacturing activated carbon sheet electrode for electric double-layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing an activated carbon sheet electrode that is useful in constructing an electric double-layer capacitor having a small internal resistance, while maintaining high capacity density. <P>SOLUTION: The method of manufacturing the activated carbon sheet electrode includes adding water to, and kneading a raw material mixture of activated carbon powder, carbon black and an organic binder mixed at a predefined ratio which is formed into a sheet-like stuff, by being extended through applying pressure; and then, drying it so as to make water evaporate, wherein assuming that additive amount of the water is a specific additive amount, when the kneaded object of the raw material mixture and water cannot maintain its integrity accompanying addition of the water, thus turning into a slurry form, with the amount of water added to the raw material mixture being controlled in a range of specific additive amount, up to -5 mass% of the specific additive amount. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing an activated carbon sheet electrode for an electric double layer capacitor, and more particularly to a method for producing an activated carbon sheet electrode for an electric double layer capacitor having a high capacity density and a low internal resistance.

  Electric double layer capacitors (capacitors) using an electric double layer formed at the interface between a polarizable electrode and an electrolyte have already been used in several fields. Especially, coin-shaped capacitors are widely used as memory backup power supplies for electronic circuits. in use. In addition, the possibility of substituting a battery has been examined, and applications that require a large capacity, such as a power source for electric vehicles, have been studied.

  The electric double layer capacitor is based on the principle that an electric double layer is formed at the interface between a sheet-like electrode mainly composed of activated carbon and an electrolytic solution, and charges are stored in the electric double layer. In recent years, with the expansion of the field of application of electric double layer capacitors, it is desired to further reduce the internal resistance while increasing the capacitance density.

  For this reason, various attempts have been made. For example, in Japanese Patent Application Laid-Open No. 2002-175950 (Patent Document 1), when manufacturing a sheet-like electrode for an electric double layer capacitor containing a carbonaceous material and a binder, roll rolling is performed. Thus, a method for reducing the internal resistance by increasing the adhesion between the current collector and the electrode is described.

  In addition, JP 2000-235938 A (Patent Document 2) and JP 2004-031986 A (Patent Document 3) paste a mixture containing activated carbon, carbon black and a binder in order to reduce internal resistance. It describes a method for producing a sheet-shaped electrode for an electric double layer capacitor by extruding and then rolling with a rolling roll.

  Furthermore, in Japanese Patent Application Laid-Open No. 2008-252023 (Patent Document 4), when forming a sheet by rolling, the capacity density is high by performing at least three or more rolling steps while gradually setting a roll interval. A method for producing a sheet-like electrode suitable for an electric double layer capacitor having a low internal resistance is described.

  However, any of the above-described methods is characterized by rolling when a mixture containing a carbonaceous material such as activated carbon or carbon black and a binder is formed into a sheet-like electrode. The capacitor could not be said to have a sufficiently reduced internal resistance.

JP 2002-175950 A JP 2000-235938 A JP 2004-031986 JP 2008-252023 A

  The present invention has been made to solve the above-described conventional problems, and provides a method for producing an activated carbon sheet electrode useful for the construction of an electric double layer capacitor having a low internal resistance while maintaining a high capacity density. For the purpose.

  In order to achieve the above object, the present inventors have conducted extensive research on a method for producing an activated carbon sheet electrode. As a result, the activated carbon, carbon black, and organic binder constituting the electrode are kneaded and formed into a sheet shape. In addition, the inventors have found that the activated carbon sheet electrode obtained by adding a specific amount of water as a molding aid is suitable for the construction of an electric double layer capacitor having a high capacity density and a low internal resistance.

  That is, the method for producing an activated carbon sheet electrode for an electric double layer capacitor according to the present invention includes adding water to a raw material mixture of activated carbon powder, carbon black, and organic binder blended in a predetermined ratio, and forming into a sheet by rolling. After that, the method for producing an activated carbon sheet electrode for drying and evaporating water, wherein the amount of water added to the raw material mixture is maintained, and the kneaded mixture of the raw material mixture and water maintains the integrity as the water is added. When the added amount of water immediately before becoming a slurry without being made into a specific added amount is defined as a specific added amount, the specific added amount is set in a range from -5% by mass.

  Moreover, in the manufacturing method of the activated carbon sheet electrode for electric double layer capacitors of the present invention, the blending amount of the carbon black is 5 to 30% by mass with respect to the activated carbon, and the blending amount of the organic binder is between activated carbon and carbon black. It is preferable to set it as 1-50 mass% with respect to the total amount.

  According to the present invention, an activated carbon sheet electrode for an electric double layer capacitor having a high capacity density and a low internal resistance can be produced. Therefore, by using this activated carbon sheet electrode, an electric double layer capacitor having a high capacity density and a low internal resistance, particularly an electric double layer capacitor suitable for applications requiring a high capacity and a large current to flow is manufactured. can do.

The activated carbon sheet electrode for electric double layer capacitors in the present invention contains activated carbon powder, carbon black, and an organic binder, which are carbonaceous materials, as essential components. As the activated carbon powder, phenol type, rayon type, acrylic type, pitch type or coconut shell type activated carbon powder can be used. The specific surface area of the activated carbon powder is preferably in the range of 200 to 3500 m ² / g. The average particle diameter of the activated carbon powder is preferably 0.1 to 100 μm, and more preferably 1 to 20 μm because a high capacity density can be obtained.

Further, in place of the activated carbon powder or in combination with the activated carbon powder, fibers or powders such as carbon fiber, carbon whisker, graphite, polyacene, etc. are also preferable if the specific surface area is in the range of 200 to 3500 m ² / g. Can be used.

Carbon black, which is an essential component, is used as a conductive additive mixed with activated carbon. The average particle size of carbon black is preferably 0.001 to 1 μm, and more preferably 0.01 to 0.5 μm because the resistance of the electrode can be reduced even if only a small amount is contained in the electrode. The specific surface area of the carbon black is preferably ^{200~1500m 2 / g, 500~1200m 2 /} g is more preferable.

  The blending amount of carbon black is preferably in the range of 5% by mass to 30% by mass with respect to the activated carbon powder, and more preferably in the range of 10% by mass to 25% by mass. If it is less than 5% by mass, the specific resistance of the electrode cannot be lowered sufficiently, and therefore, the internal resistance of the electrode cannot be reduced even if the constituent powders are sufficiently dispersed. On the other hand, if it exceeds 30% by mass, the amount of carbon black that does not contribute to the capacity of the electrode becomes excessive, so that a high capacity density required for an electric double layer capacitor cannot be obtained.

  As an essential organic binder, polytetrafluoroethylene (hereinafter abbreviated as PTFE) can be preferably used. As PTFE, not only a homopolymer of tetrafluoroethylene but also a copolymer obtained by copolymerization by adding other monomers of 0.5 mol% or less to tetrafluoroethylene can be used. If the other monomer is 0.5 mol% or less, the melt fluidity is not imparted to PTFE, and it can be made into a fiber in the same manner as a tetrafluoroethylene homopolymer to produce a high-strength and low-resistance electrode sheet. It is.

Examples of other monomers copolymerized with the tetrafluoroethylene include hexafluoropropylene, chlorotrifluoroethylene, perfluoro (alkyl vinyl ether), trifluoroethylene, and (perfluoroalkyl) ethylene. In general, PTFE has a low molecular weight and is liquid or gel-like and hardly fiberized. Therefore, it is more preferable that PTFE contains 50% by mass or more of a polymer having a molecular weight calculated from the standard specific gravity of 1 × 10 ⁶ or more. Further, PTFE is preferred because it can be easily made into a fiber obtained by emulsion polymerization.

  The amount of the organic binder such as PTFE is preferably in the range of 1 to 50% by mass and more preferably in the range of 5 to 30% by mass with respect to the carbonaceous material (activated carbon powder and carbon black). Since the organic binder is contained in the electrode sheet in order to maintain the shape of the electrode sheet, if the amount is less than 1% by mass, the strength becomes weak and the shape cannot be maintained. Since resistance is likely to increase, it is not preferable.

  In the present invention, a predetermined amount of water is added to the raw material mixture composed of the above-mentioned activated carbon powder, carbon black, and organic binder. Water easily wets PTFE as a molding aid, moderately fiberizes or plastically deforms, and simultaneously acts as a dispersant for activated carbon powder and carbon black. The water added to the raw material mixture can be removed by evaporating it after forming it into a sheet by rolling.

  The amount of water added to the raw material mixture is determined in advance by the following procedure. First, activated carbon powder, carbon black, and an organic binder are blended in a predetermined ratio, and water is added in small amounts to a predetermined amount of the raw material mixture, for example, 0.01 to 0.1 g per 1.00 g of the raw material mixture. Knead. As the amount of water added increases, the raw material mixture gradually becomes a monolithic mixture from powder to clay or mud. Further, the amount of water added is increased, and when the amount exceeds a certain amount, the raw material mixture cannot rapidly maintain the integrity and changes into a slurry having fluidity. The amount of water added immediately before the predetermined amount of the raw material mixture cannot maintain the integrity (maximum value in a state where the integrity is maintained) is defined as the specific addition amount.

  Next, in an actual electrode manufacturing process, water is added within a range from the specific addition amount to −5 mass% of the specific addition amount with respect to a predetermined amount of the raw material mixture blended at a predetermined ratio. If the amount of water added is less than -5% of the specific amount added, the viscosity of the raw material mixture is too high, so the activated carbon and carbon black are not sufficiently dispersed, and the activated carbon cannot be sufficiently contacted via the carbon black. As a result, the internal resistance of the activated carbon sheet electrode increases. On the other hand, if the amount added exceeds a specific amount, the raw material mixture becomes a slurry, so that the raw material mixture is not subjected to sufficient shear stress, and the dispersibility of the activated carbon and carbon black deteriorates. Will become bigger.

  Thus, by controlling the amount of water added to the predetermined amount of the raw material mixture within a range of ± 5% with respect to the specific addition amount, the activated carbon, the carbon black, and the organic binder can be sufficiently dispersed. In addition, the specific addition amount is determined in advance according to the type and combination of the activated carbon, carbon black and organic binder to be used, so high capacity density is maintained regardless of the type of activated carbon, carbon black and organic binder that can be used. However, it is possible to produce an activated carbon sheet electrode that can reduce internal resistance.

  Next, the manufacturing method of the activated carbon sheet electrode for electric double layer capacitors in this invention is demonstrated concretely. First, a predetermined amount of activated carbon and carbon black are weighed and mixed. For mixing, a known mixer such as a rocking mixer or a V blender horizontal cylindrical mixer can be used.

  Subsequently, a predetermined amount of an organic binder and a specific addition amount ± 5 mass of water determined experimentally in advance as described above are weighed, added and kneaded. For kneading, a known kneader or mixer such as a pressure kneader, an extruder, or a planetary mixer can be used. In addition, since the effect of this invention may not be acquired when water evaporation progresses, it is necessary to be careful so that water may not evaporate in the process from kneading to the next rolling.

  Next, the obtained kneaded material is rolled by a known rolling machine such as a calender roll to be formed into a sheet shape to obtain an activated carbon sheet. At this time, the temperature of the rolling roll may be room temperature, but may be heated to facilitate rolling. The thickness of the activated carbon sheet is preferably 80 μm or more and 700 μm or less. If the thickness is less than 80 μm, a sufficient capacity density cannot be obtained, and a high-capacity capacitor cannot be manufactured. On the other hand, when the thickness is greater than 700 μm, the resistance in the thickness direction increases, and as a result, the internal resistance increases.

  Since the activated carbon sheet formed by rolling contains water, moisture is removed by drying. The removal of water from the activated carbon sheet may be dried after rolling and forming, or may be dried simultaneously with the rolling to remove a part or all of the water. Drying is preferably performed by heating to 60 ° C. or higher in a vacuum and above the boiling point of water in the air, and a temperature at which an organic binder such as PTFE does not evaporate.

  The dried activated carbon sheet can be made into an activated carbon sheet electrode by sticking to a current collector using a conductive adhesive. As the conductive adhesive used for bonding, an adhesive that does not cause a decomposition reaction when charging and discharging as an electric double layer capacitor is preferable, and specifically, a graphite-based conductive adhesive is preferable.

  The current collector may be any material that is excellent in electrical conductivity and electrochemically durable. For example, metals such as aluminum, titanium, and tantalum, and stainless steel are usually used. Among them, aluminum is preferable because it has a low specific gravity, excellent conductivity, and excellent electrochemical stability. In order to further reduce the internal resistance and improve the applicability of the conductive adhesive, it is preferable to use an aluminum foil whose surface has been etched in advance.

  When moisture is adsorbed in the obtained activated carbon sheet electrode, the water inside the electrode undergoes electrolysis and gasifies when charged / discharged, increasing the pressure inside the cell, and in the worst case the cell Cause damage. In order to prevent such a situation, it is necessary to dry the activated carbon sheet electrode in a vacuum for a sufficiently long time. In addition, it is preferable that the electrode is not exposed to the atmosphere after drying.

  The produced activated carbon sheet electrode is opposed to each other with a separator interposed therebetween, and an electrolytic solution is impregnated in the electrode and accommodated in an airtight container to constitute an electric double layer capacitor. There is no limitation on the structure of the electric double layer capacitor, and a separator is interposed between a coin type housed in a metal case, a laminate type housed in an aluminum laminate film, and a pair of strip electrode bodies in which electrode layers are formed on both sides of the current collector. A cylindrical type that is rolled and housed in a bottomed cylindrical container, and a stacked type in which a plurality of activated carbon sheet electrodes with activated carbon sheets pasted on both sides of the current collector are alternately stacked via a separator and accommodated in a bottomed rectangular container For example, a known structure can be used.

[Example 1]
The specific surface area as the activated carbon powder is 1500~2000m ² / g, average particle diameter 15μm powder (Kuraray Chemical Co., RP-20) using a specific surface area of carbon black is 800 m ² / g, an average particle diameter of primary particles Used 40 nm ketjen black (manufactured by Lion, carbon ECP). Moreover, as PTFE of the organic binder, PTFE-containing aqueous dispersion (manufactured by Daikin Industries, D-1E, containing 60% PTFE) was used.

  After mixing 40.00 g of the activated carbon powder, 5.00 g of carbon black, and 8.33 g of PTFE-containing aqueous dispersion, kneading while adding a predetermined amount (0.5 g) of pure water to the obtained raw material mixture. The operation was repeated. With the addition of water, the monolithic material that was gathered into clay or mud changed to a fluid slurry without maintaining the integrity when the next predetermined amount of water was added.

  The amount of water added just before the raw material mixture could not maintain the integrity, that is, the specific amount added was 70.03 g (including 3.33 g of water in the PTFE-containing aqueous dispersion). The specific addition amount is 1.40 g per 1.00 g of the raw material mixture.

  Next, as sample 1, 40.00 g of the activated carbon powder and 5.00 g of carbon black were weighed and dry mixed for 10 minutes with a rocking mixer (RM-10-2, manufactured by Aichi Electric). Thereafter, 8.33 g of the above PTFE-containing aqueous dispersion and 66.67 g of pure water were added and mixed for 3 minutes with a multi blender mill (manufactured by Nippon Seiki Seisakusho, BLA-102). The amount of pure water added at this time is 70.03 g which is the same as the above-mentioned specific added amount when the amount of pure water 66.7 g and the amount of water contained in the PTFE-containing aqueous dispersion 3.33 g are combined. Become.

Thereafter, the raw material mixture is put in a unipack so that water does not evaporate, and rolled to a thickness of 140 μm using a roll mill (Oono Roll, 6-inch: 2 / 4-high mill) to obtain an activated carbon sheet. And dried at 60 ° C. to evaporate the water. This activated carbon sheet was cut out into a square with an area of 10 cm ² , and an etching aluminum foil (made by KDK, C208, thickness) as a current collector using a graphite-based conductive adhesive (made by Nippon Graphite, Bunny Height T-602). : 20 μm, core part: 15 μm) and dried in vacuum at 180 ° C. for 12 hours to obtain an activated carbon sheet electrode of Sample 1.

  Insert two activated carbon sheet electrodes, positive and negative electrodes facing each other via a separator (Nippon Kogyo Paper Industries Co., Ltd., TF4050) inside the laminated aluminum laminate film (Showa Denko Packaging Co., ALR-CPP80), Sealed leaving a vacuum port. The electrolytic solution was injected into the inside from the vacuum port and the inside of the system was evacuated to impregnate the electrolyte solution, and then the vacuum port was sealed to obtain a laminate type electric double layer capacitor of Sample 1.

  Further, 8.33 g of PTFE-containing aqueous dispersion was added to 40.00 g of the same activated carbon powder as sample 1 and 5.00 g of carbon black, and the addition amount of pure water was 3.6% less than the specific addition amount. 67.50 g (sample 2), 7.2% less 65.00 g (sample 3), and 2.0% more 71.43 g (sample 4). An electrode was manufactured and further an electric double layer capacitor was constructed.

Each activated carbon sheet electrode of Samples 1 to 4 obtained as described above had a total thickness of 280 μm and an electrode volume of 0.28 cm ³ . The electrolyte used was a propylene carbonate solution (CPG-00005, manufactured by Kishida Chemical Co., Ltd.) in which tetraethylammonium borofluoride was dissolved at 1 mol / l. Moreover, the separator and aluminum laminated film which are the members of a cell were used by previously drying for 12 hours or more at 150 degreeC in the vacuum.

  The cycle in which the electric double layer capacitors of Samples 1 to 4 described above were set in a charge / discharge measurement device, charged to 2.5 V, held for 30 minutes, and then discharged to 1.0 V was repeated three times. In each cycle, the capacity density and internal resistance of the activated carbon sheet-like electrode were calculated from the IV curve at the time of discharge according to the standard RC-2377 of the Japan Electronics and Information Technology Industries Association. The measurement results of the capacity density and the internal resistance at the third cycle for each of the electric double layer capacitors of Samples 1 to 4 are shown in Table 1 below along with the amount of water added.

As can be seen from the above results, each of the electric double layer capacitors of Samples 1 and 2 of the present invention in which the amount of water added to the raw material mixture is controlled within a range of ± 5% by mass of the specific amount added has a capacity density of about It was 20 F / cm ³ , and the internal resistance was good at about 1Ω or less. On the other hand, in Samples 3 to 4 which are comparative examples, the amount of water added to the raw material mixture is out of the range from the specific addition amount to −5 mass% of the specific addition amount. At the same time, the internal resistance doubled.

Claims (2)

  This is a method for producing an activated carbon sheet electrode in which water is added to a raw material mixture of activated carbon powder, carbon black, and organic binder blended at a predetermined ratio, kneaded, formed into a sheet by rolling, and then dried to evaporate water. The amount of water added to the raw material mixture is defined as the specific amount of water added immediately before the raw material mixture and the water kneaded mixture cannot maintain integrity with the addition of water and become a slurry. When manufacturing, the manufacturing method of the activated carbon sheet electrode for electric double layer capacitors characterized by setting it as the range from specific addition amount to -5 mass% of specific addition amount.   The blending amount of the carbon black is 5 to 30% by mass with respect to the activated carbon powder, and the blending amount of the organic binder is 1 to 50% by mass with respect to the total amount of the activated carbon powder and carbon black, The manufacturing method of the activated carbon sheet electrode for electric double layer capacitors of Claim 1.