JP2007273889A - Electrode for electric double layer capacitor and method for manufacturing the same - Google Patents

Abstract

A method of manufacturing a high-density and homogeneous electrode for an electric double layer capacitor, and an electrode for an electric double layer capacitor having a low internal resistance, particularly for an application requiring a high capacity and a large current flow. An electrode for a multilayer capacitor is provided.
A molded body obtained by extruding a mixture containing a carbonaceous material, a processing aid and polytetrafluoroethylene into a sheet shape or a rod shape is cut into a certain length along the extrusion direction to obtain a molded body piece. Thereafter, the molded pieces are stacked along the thickness direction, extruded into a sheet shape along the stacking direction, and the obtained sheet-shaped formed body is rolled to a predetermined thickness and then cut into an electrode shape. A method for producing a double layer capacitor electrode, and an electrode for an electric double layer capacitor, each having a tensile strength in a rolling direction and a direction perpendicular to the rolling direction of 2.0 kg / cm ² or more.
[Selection] Figure 3

Description

  The present invention relates to an electrode for an electric double layer capacitor made of a carbon material, and an electric double layer capacitor using this electrode.

  In recent years, demand for an electric double layer capacitor using an electric double layer formed at a polarizable electrode and an electrolyte interface, particularly a coin type capacitor, has been rapidly increasing as a memory backup power source. On the other hand, high energy density and high power density electric double layer capacitors with large capacity per unit volume and low internal resistance for applications that require large capacity such as power supplies for various transportation equipment including automobiles Development is desired. In addition, the electric double layer capacitor for memory backup is also required to reduce the internal resistance, and in order to realize such characteristics, the electrode for the double layer capacitor must be a thin and long sheet. Necessary. Various methods have been proposed for producing this type of sheet-like electrode.

  For example, it is known that carbon fine powder, a conductive assistant and a raw material made of a fluororesin as a binder are mixed and kneaded to obtain a kneaded product, and then the kneaded product is formed into a sheet-like molded body having a predetermined thickness by a roll press. (See Patent Document 1). However, in the electrode obtained by this method, the binder is non-uniformly fiberized by kneading after mixing, and when the sheet is formed and rolled, the density of the sheet density is increased, and the tensile strength of the electrode is reduced. In addition, there has been a problem that formation of a thin sheet makes cracking and the like difficult to form.

  As a method for producing a thin and long sheet-like electrode for an electric double layer capacitor, a mixture of a fine carbon powder, a fluoropolymer resin, and a liquid lubricant is preformed into a sheet. A method is proposed in which the preforms are rolled and joined at the edges to form a continuous long preform, the liquid lubricant is removed, and then the preform is molded with a heated rolling roll. (See Patent Document 2). However, in order to obtain a long sheet-like electrode by this method, a paste-like viscous mixture is prepared and rolled to prepare a plurality of sheet-like preforms having a thickness of about 1 mm. It is necessary to produce a long preform through a complicated process of stacking the preforms at the ends and joining them with a rolling roll, resulting in a problem of poor production efficiency. In addition, the sheet-like preform by such a manufacturing method often has variations in thickness and hardness, and when the end portions are overlapped and joined, the joint portion does not become homogeneous with other parts, The joined long sheet-like preform has variations in thickness and hardness. Therefore, the thin sheet-like electrode obtained by rolling such a preform has a problem that the thickness and hardness are not uniform.

  Also, the activated carbon powder and polytetrafluoroethylene are mixed to form a paste, applied to the current collector, dried, then heated to a temperature higher than the melting point of polytetrafluoroethylene, press-molded to reduce the thickness of the electrode and increase the density. A method has also been proposed (see Patent Document 3). However, this method has a problem in that the internal resistance of the electrode is increased because the polytetrafluoroethylene binder is not highly fiberized and exists in a lump.

JP 2001-307964 A JP-A-2-235320 JP 9-36005 A

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide a method for producing a high-density and homogeneous electrode for an electric double layer capacitor. Another object of the present invention is to provide an electrode for an electric double layer capacitor having a low internal resistance, particularly an electrode for an electric double layer capacitor for applications that require a large current to flow with a high capacity.

In order to achieve the above object, the present invention provides the following electrode for an electric double layer capacitor and a method for producing the same.
(1) A first extrusion step in which a mixture containing a carbonaceous material, a processing aid and polytetrafluoroethylene is extruded to form a sheet or rod shape, and the molded body obtained in the first extrusion step in the extrusion direction A molded body piece cut to a certain length along the thickness direction, laminated along the thickness direction, and extruded into a sheet shape along the lamination direction, and a sheet shape obtained in the second extrusion process A method of manufacturing an electrode for an electric double layer capacitor, comprising: a rolling step of rolling the formed body to a predetermined thickness.
(2) In the second extruding step, before the cylinder for extrusion becomes empty, a new formed body piece is replenished, and a long sheet-shaped formed body is extruded. Manufacturing method of electrode for double layer capacitor.
(3) The polytetrafluoroethylene is 1 to 50% by mass of the total amount of the polytetrafluoroethylene and the carbonaceous material, and the processing aid is based on the total amount of the polytetrafluoroethylene and the carbonaceous material. The method for producing an electrode for an electric double layer capacitor as described in (1) or (2) above, wherein a mixture of 20 to 90% by mass is used.
(4) The extrusion ratio is 5 to 500 in the first extrusion process, and the extrusion ratio is 1 to 300 in the second extrusion process. Any one of (1) to (3) above A manufacturing method of the electrode for electric double layer capacitors given in 2.
(5) A carbonaceous material, a processing aid, and polytetrafluoroethylene, which are obtained by the production method according to any one of (1) to (4) above, and in the width direction and length on the sheet surface An electrode for an electric double layer capacitor, wherein each of the tensile strengths in the direction is 2.0 kg / cm ² or more.

  According to the present invention, a mixture containing a carbonaceous material, a processing aid, and polytetrafluoroethylene is formed into a sheet by a two-stage extrusion process with different extrusion directions. Is obtained. Further, the internal resistance of the electrode for an electric double layer capacitor of the present invention is reduced, and an electric double layer capacitor having a large internal capacitance can be obtained by using the electrode for an electric double layer capacitor of the present invention.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

  In the present invention, a mixture containing a carbonaceous material, a processing aid and polytetrafluoroethylene is used as a starting material for molding.

As the carbonaceous material, a powder made of activated carbon, carbon black or the like and having a specific surface area of 200 to 3500 m ² / g is preferable. Further, fibers or powders such as carbon fibers, carbon whiskers, and graphite can also be preferably used if the specific surface area is 200 to 3500 m ² / g. As activated carbon, coconut husk type, petroleum pitch type, petroleum coke type or phenol resin type can be used, and when the particle size is 0.1 to 100 μm, particularly 1 to 20 μm, it is easy to form a thin film and the capacity density tends to be high. Therefore, it is preferable. Carbon black is also preferably used as a conductive material mixed with other carbonaceous materials. Carbon black having a particle size of 0.001 to, particularly preferably 0.01 to 0.5 [mu] m, also, that as the specific surface area of 200-2000 m ² / g, preferably from 500 to 1500 ² / g. Further, in fibers such as carbon fiber and carbon whisker, the fiber diameter is preferably 0.01 to 50 μm, particularly preferably 0.1 to 30 μm, the fiber length is preferably 1 to 500 μm, particularly preferably 5 to 100 μm, and the particle size of the graphite powder. Is preferably 0.1 to 100 μm, particularly preferably 1 to 50 μm. With such a fiber diameter, fiber length, or particle diameter, it is easy to make a thin film and the internal resistance can be reduced.

In particular, the combined use of carbon black and activated carbon having a specific surface area of 200-3500 m ² / g and a particle size of 0.1-100 μm can reduce the internal resistance of the electrode for the electric double layer capacitor, This is preferable because the capacity can be maintained high.

  Polytetrafluoroethylene includes not only a homopolymer of tetrafluoroethylene but also a copolymer obtained by copolymerizing another monomer with 0.5 mol% or less of tetrafluoroethylene. If other monomer is 0.5 mol% or less, melt flowability is not imparted to polytetrafluoroethylene, and it is made into a fiber like a tetrafluoroethylene homopolymer and has a high strength and low resistance in the form of a sheet-like electrode Can be produced. Examples of other monomers include hexafluoropropylene, chlorotrifluoroethylene, perfluoroalkyl vinyl ether, trifluoroethylene, and perfluoroalkylethylene.

In addition, since polytetrafluoroethylene has a low molecular weight and is in a liquid or gel form and is difficult to be fiberized, it preferably 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. . In addition, polytetrafluoroethylene is preferable because the one obtained by emulsion polymerization is easily fiberized.

  The processing aid is added so that the polytetrafluoroethylene is moderately fibrillated and plastically deformed, and any liquid that can easily wet the polytetrafluoroethylene and easily remove it from the molded electrode. Can be used. Specifically, hydrocarbons such as ethanol, methanol, kerosene, solvent naphtha, white naphtha, isoparaffin, alcohol solvents such as methyl alcohol, ethyl alcohol, isopropyl alcohol, ethylene glycol, propylene glycol, dipropylene glycol, glycerin, Examples include ether solvents such as ethylene glycol monobutyl ether, and particularly, solvent naphtha or isoparaffin can be preferably used.

  Also, fluororesin dispersions such as polytetrafluoroethylene aqueous dispersion can be used as a processing aid, and may be used alone or in combination with other processing aids.

  The mixing ratio of the polytetrafluoroethylene, the carbonaceous material and the processing aid is such that the polytetrafluoroethylene is 1 to 50% by mass, more preferably 3 to 30% by mass with respect to the total amount of the carbonaceous material and polytetrafluoroethylene. % Is preferred. Since polytetrafluoroethylene is a component contained in the electrode as a binder that maintains the shape of the electrode, if the polytetrafluoroethylene is less than 1% by mass, the strength is weak, and if it exceeds 50% by mass, the internal resistance of the electrode increases. It's easy to do. Further, the processing aid is 20 to 90 mass%, preferably 40 to 80 mass%, based on the total amount of the carbonaceous material and polytetrafluoroethylene. If the processing aid is less than 20% by mass, the extrusion pressure becomes too high and molding becomes difficult. When it exceeds 90% by mass, the extrusion pressure does not increase, and the polytetrafluoroethylene is not sufficiently fiberized and tends to be a low-strength sheet.

  In preparing the mixture, the carbonaceous material and polytetrafluoroethylene may be mixed and then a processing aid may be added and further mixed, or the carbonaceous material, polytetrafluoroethylene and the processing aid may be mixed simultaneously. Also good.

  In order to produce an electrode for an electric double layer capacitor, first, a mixture of a carbonaceous material, polytetrafluoroethylene, and a processing aid is preformed, then placed in an extruder 10 shown in FIG. A molded body 2 having a shape such as a rod or a sheet is obtained (first extrusion step).

  Next, as shown in FIG. 2, the molded body 2 obtained in the first extrusion step is cut into a fixed length along the extrusion direction to form a molded body piece 2a.

  Next, as shown in FIG. 3, the molded body piece 2 a is filled in the cylinder of the extruder 20 in a state of being laminated along the thickness direction, and is pressed along the lamination direction to form the sheet-like molded body 3. To obtain (second extrusion step). Therefore, the molded body 3 obtained in the second extrusion process is pressed in a direction orthogonal to the pressing direction in the first extrusion process. In this second extrusion step, the molded body piece 2a is preferably rectangular in a planar shape from the viewpoint of stability in the cylinder of the extruder 20 and ease of pressurization. In the above, it is preferable that the molded body 2 is formed into a long sheet shape and cut in the longitudinal direction.

  Also, in the second extrusion step, a continuous long sheet-like sheet is formed by replenishing a new molded body piece 2a before the cylinder of the extruder 20 is emptied and pressurizing and intermittently extruding. A molded body 3 is obtained.

  In the first extrusion step, the extrusion ratio, that is, the value obtained by dividing the cross-sectional area of the cylinder into which the preform is put by the cross-sectional area of the extrusion nozzle in the extruder 10 is preferably 5 to 500, more preferably 10 ~ 100. If the extrusion ratio is less than 5, the molded body 2 is too soft to keep the shape. Exceeding 500 makes extrusion difficult, the molded body 2 is brittle, and it becomes difficult to form a continuous sheet in the second extrusion step.

  In the second extrusion step, the extrusion ratio is preferably 1 to 300, more preferably 3 to 100. If the extrusion ratio is less than 1, the molded body 3 is too soft and it is difficult to make the shape continuous. If it exceeds 300, extrusion becomes difficult, the molded body 3 is brittle, and continuous sheeting becomes difficult.

  Next, the sheet-like formed body 3 is rolled between rolling rolls to obtain a sheet having a predetermined thickness (rolling step). The electric double layer capacitor preferably has a small sheet thickness in order to increase the capacity density. However, if the electric double layer capacitor is too thin, the thickness of the electric double layer capacitor is difficult to handle due to insufficient strength of the sheet. Is preferably 0.002 to 0.35 mm, and more preferably 0.003 to 0.30 mm. Therefore, in this rolling step, the molded body 3 is rolled to such a sheet thickness. In addition, 20-350 degreeC is preferable and, as for the temperature of the rolling roll at the time of roll-rolling, ie, the sheet | seat which touches a sheet | seat, More preferably, it is 40-180 degreeC. When the rolling temperature is less than 20 ° C., polytetrafluoroethylene is not sufficiently fiberized and tends to be a brittle sheet. When the rolling temperature exceeds 350 ° C., the processing aid evaporates rapidly, so that cracking, peeling, etc. are likely to occur on the sheet surface.

  The sheet formed by roll rolling is then dried to remove the processing aid. The drying temperature is preferably a temperature not lower than the boiling point of the processing aid and not higher than the melting point of polytetrafluoroethylene. Moreover, you may extend | stretch the sheet-like material after drying or the semi-drying which removed some processing aids. Further, after the stretching treatment, roll rolling may be performed again.

  In the series of steps described above, the molded body 2 extruded in the first extrusion step has a polytetrafluoroethylene fiberized in the same direction as the extrusion direction. Further, in the second extrusion process, the molded body 2 is fiberized with polytetrafluoroethylene in a direction perpendicular to the extrusion direction in the first extrusion process. And by carrying out roll rolling in the same direction as the extrusion direction of the second extrusion step, fiberization in the rolling direction proceeds, and a network structure in which polytetrafluoroethylene is fiberized longitudinally and laterally is formed, and the carbonaceous material is this network structure. Held by. Therefore, even in a small amount of polytetrafluoroethylene, the shape retention is excellent, the film can be easily formed into a thin film, and the sheet has a high resistance to cracking and breakage and has a high strength. In addition, the amount of the carbonaceous material is relatively increased, the conductivity is increased, and the resistance is reduced. Or even if it is a small amount of carbonaceous material, sufficient electroconductivity can be ensured.

  Next, the rolled sheet is cut into a predetermined shape to obtain a sheet piece, and the electric double layer capacitor electrode of the present invention is obtained (cutting step).

  Moreover, you may bake a sheet piece as needed. The firing may be complete firing at a temperature equal to or higher than the melting point of polytetrafluoroethylene or incomplete firing at a temperature lower than the melting point of polytetrafluoroethylene.

The electric double layer capacitor electrode of the present invention thus obtained has a tensile strength of 2.0 kg / cm ² or more in both the vertical and horizontal directions, and has high strength. More preferably, it is 4.0 kg / cm ² or more. The tensile strength means that after a sheet-like electrode is dried at 250 ° C. for 1 hour, it is punched into the shape of No. 1 dumbbell-shaped test piece defined in JIS-K6301 and pulled at an ambient temperature of 25 ± 2 ° C. A value obtained by dividing the maximum load when the tensile test is performed at a speed of 20 mm / min by the cross-sectional area (electrode thickness × width of parallel part).

  Next, examples of the present invention will be described. However, this invention is not limited only to those Examples.

[Example 1]
80% by mass of an activated carbon having a specific surface area of 1800 m ² / g, which has been vacuum-dried at 120 ° C. for 2 hours in advance, 10% by mass of carbon black and 10% by mass of polytetrafluoroethylene powder are put into a mixer container and mixed. With respect to the total amount of the powder body, 62% by mass of an isoparaffin-based processing aid was added and mixed in the container to obtain a mixture.

Next, the mixture was preformed at a pressure of 40 kg / cm ² for the first extrusion step. This preform was extruded with an extruder having a cylinder with an inner diameter of 40 mm, a nozzle having a 7 mm square, and a rectangular nozzle with an extrusion ratio of 25 to obtain a 7 mm square molded body (see FIG. 1). Next, the obtained molded body was cut to a length of 100 mm to form a molded body piece (see FIG. 2), and the 10 molded body pieces were extruded at an extrusion ratio of 7 so that the length direction thereof was perpendicular to the extrusion direction of the extruder. The second extrusion process was performed (see FIG. 3). At that time, 10 new molded body pieces were replenished while the molded body piece was left in the nozzle portion of the cylinder, and extrusion molding was performed. By this second extrusion step, a long sheet having a thickness of 1 mm, a width of 100 mm, and a length of about 600 mm was obtained.

  Next, the obtained long sheet was roll-rolled at a roll diameter of 200 mm and a temperature of 80 ° C., dried at 250 ° C. for 1 hour, and the processing aid was removed to form a sheet having a thickness of 300 μm.

The obtained sheet was punched into the shape of a No. 1 dumbbell-shaped test piece defined in JIS-K6301, and a tensile test was performed at an ambient temperature of 23 ° C. and a tensile speed of 20 mm / min to measure the maximum load. This measurement was repeated three times, and the maximum load was divided by the sheet cross-sectional area before the tensile test to obtain the tensile strength. The average value was taken as the tensile strength of this sheet. In addition, in order to measure the tensile strength of the sheet | seat of the extrusion direction in a 1st extrusion process, the dumbbell-shaped test piece was pierced so that the length direction might turn into a rolling direction. The tensile strength of the sheet according to the above test was 7.5 kg / cm ² in the rolling direction and 5.4 kg / cm ² in the direction orthogonal to the rolling direction.

  In addition, a conductive adhesive was uniformly applied in a film form on one side of an etching aluminum foil having a lead terminal and a rectangular shape having a width of 4 cm and a length of 4 cm and a thickness of 40 μm. Next, the sheet was cut into an area of 4 cm × 4 cm to form an electrode sheet, and the electrode sheet was joined to form an electrode body. Two electrode bodies were prepared, the electrode surfaces of the two electrode bodies were opposed to each other, and a cellulose fiber separator having a thickness of 30 μm was interposed therebetween, and the thickness was 2 mm, the width was 5 cm, and the length was 5 cm. The capacitor element was fixed with two glass holding plates. The total thickness of the two electrode bodies and the separator was 0.73 mm as measured by a digital caliper.

As an electrolytic solution, an electrolytic solution in which 1.0 mol / L tetraethylammonium tetrafluoroborate was dissolved in propylene carbonate was prepared. Then, the device is vacuum-heated at 200 ° C. for 2 hours to remove the impurity of the device, and this electrolytic solution is impregnated with vacuum and accommodated in a square envelope container made of aluminum foil to produce an electric double layer capacitor. did. The capacity and DC resistance were measured at a current density of 20 mA / cm ² , and the capacity per unit volume (capacity density) and the resistance per unit volume were calculated. The results are shown in Table 1.

[Example 2]
A sheet was produced in the same manner as in Example 1 except that the thickness was changed to 200 μm by rolling. And when the tensile strength of this sheet | seat was measured similarly to Example 1, the rolling direction was 8.5 kg / cm < ² > and the direction orthogonal to a rolling direction was 5.7 kg / cm < ² >. Further, an electric double layer capacitor was produced in the same manner as in Example 1 except that an electrode sheet was obtained from this sheet, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

[Example 3]
A sheet was produced in the same manner as in Example 1 except that the thickness was changed to 80 μm by rolling. And when the tensile strength of this sheet | seat was measured similarly to Example 1, the rolling direction was 11.7 kg / cm < ² > and the direction orthogonal to a rolling direction was 7.1 kg / cm < ² >. Further, an electric double layer capacitor was produced in the same manner as in Example 1 except that an electrode sheet was obtained from this sheet, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

[Example 4]
A sheet was produced in the same manner as in Example 1 except that the thickness was 120 μm by rolling. Then, the measured tensile strength of the sheet in the same manner as in Example 1, the rolling direction is 6.9 kg / cm ^2, a direction perpendicular to the rolling direction was 4.5 kg / cm ^2. Further, an electric double layer capacitor was produced in the same manner as in Example 1 except that an electrode sheet was obtained from this sheet, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

[Comparative Example 1]
A sheet as in Example 1 except that only the first extrusion process was performed and the second extrusion process was not performed, and the thickness was 250 μm by roll rolling in the same direction as the extrusion direction of the first extrusion process. Was made. And when the tensile strength of this sheet | seat was measured similarly to Example 1, the rolling direction was 3.8 kg / cm < ² > and the direction orthogonal to a rolling direction was 1.1 kg / cm < ² >. Further, an electric double layer capacitor was produced in the same manner as in Example 1 except that an electrode sheet was obtained from this sheet, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

[Comparative Example 2]
The sheet was formed in the same manner as in Example 1 except that only the first extrusion process was performed and the second extrusion process was not performed, and the roll rolling was performed in the same direction as the extrusion direction of the first extrusion process to make the thickness 130 μm. Produced. And when the tensile strength of this sheet | seat was measured similarly to Example 1, the rolling direction was 4.3 kg / cm < ² > and the direction orthogonal to a rolling direction was 0.9 kg / cm < ² >. Further, an electric double layer capacitor was produced in the same manner as in Example 1 except that an electrode sheet was obtained from this sheet, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

  As shown in Table 1, by performing the first extrusion process and the second extrusion process according to the present invention, an electrode for an electric double layer capacitor having excellent tensile strength in both the longitudinal direction and the transverse direction can be obtained. Further, by using the electrode for an electric double layer capacitor according to the present invention, an electric double layer capacitor having a small internal resistance and a large capacitance can be obtained.

It is a figure for demonstrating a 1st extrusion process. It is a figure for demonstrating a cutting process. It is a figure for demonstrating a 2nd extrusion process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mixture 2 Molded body 2a Molded body piece 3 Molded body 10 Extruder 20 Extruder

Claims (5)

  A first extrusion process in which a mixture containing a carbonaceous material, a processing aid and polytetrafluoroethylene is extruded to form a sheet or rod, and the molded body obtained in the first extrusion process is constant along the extrusion direction. A second extruding step of laminating shaped pieces cut into lengths along the thickness direction and extruding them into a sheet shape along the laminating direction, and a sheet-like shaped product obtained by the second extruding step A method for producing an electrode for an electric double layer capacitor, comprising: a rolling step of rolling to a predetermined thickness.   2. The electric double layer capacitor according to claim 1, wherein, in the second extruding step, a new molded body piece is replenished before the extrusion cylinder becomes empty, and a long sheet-shaped molded body is extruded. For manufacturing an electrode.   The polytetrafluoroethylene is 1 to 50% by mass of the total amount of the polytetrafluoroethylene and the carbonaceous material, and the processing aid is 20 to 90% with respect to the total amount of the polytetrafluoroethylene and the carbonaceous material. The method for producing an electrode for an electric double layer capacitor according to claim 1, wherein a mixture having a mass% is used.   The electric double layer according to any one of claims 1 to 3, wherein the extrusion ratio is 5 to 500 in the first extrusion step, and the extrusion ratio is 1 to 300 in the second extrusion step. A method for manufacturing a capacitor electrode. Each of the tensile strengths in the width direction and the length direction on the sheet surface is obtained by the production method according to any one of claims 1 to 4, including a carbonaceous material, a processing aid, and polytetrafluoroethylene. Is an electrode for an electric double layer capacitor, characterized in that all are 2.0 kg / cm ² or more.

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