JP2008192758A - Electrode for electric double-layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarizable electrode for an electric double-layer capacitor which can be increased in capacitance and can be reduced in internal resistance, and to provide an electric double-layer capacitor using the same. <P>SOLUTION: The polarizable electrode for an electric double-layer capacitor is made by conducting a conductive treatment, electrolytic nickel plating treatment, and chrome plating treatment on a foaming resin in this order, removing the foaming resin, and then conducting annealing in a reduced atmosphere to obtain a current collector, and finally filling the current collector with activated carbon. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a novel electrode for an electric double layer capacitor and an electric double layer capacitor using the same.

  The electric double layer capacitor has recently attracted attention because of its large capacitance among various capacitors. For example, capacitors are widely used for memory backup of electrical equipment, but the use of electric double layer capacitors is also promoted for this purpose. More recently, it is also expected to be used for vehicles such as hybrid vehicles and fuel vehicles.

  There are various types of electric double layer capacitors such as a button type, a cylindrical type, and a square type. For example, the button type has a pair of polarizable electrodes with an activated carbon electrode layer on a current collector, and a separator is placed between the electrodes to form an electric double layer capacitor element that is housed in a metal case together with the electrolyte. It is manufactured by sealing with a sealing plate and a gasket that insulates both. In the cylindrical type, a pair of polarizable electrodes and a separator are overlapped and wound to form an electric double layer capacitor element. The element is impregnated with an electrolytic solution and stored in an aluminum case, and sealed with a sealing material. It is manufactured by. The basic structure of the square type is almost the same as the button type or cylindrical type.

The polarizable electrode used for this electric double layer capacitor is usually manufactured by applying activated carbon to a current collector that is an aluminum foil. For example, Patent Documents 1 to 3 disclose various current collectors constituting the polarizable electrode for non-aqueous electrolyte electric double layer capacitors. Patent Document 1 discloses aluminum, stainless steel and the like as a metal current collector. Patent Document 2 discloses a material in which a stainless steel fiber mat is electrically welded to a stainless steel foil. Patent Document 3 discloses a porous body made of at least one metal selected from tantalum, aluminum, and titanium.
Japanese Patent Laid-Open No. 11-274012 Japanese Unexamined Patent Publication No. 09-232190 Japanese Patent Laid-Open No. 11-150042

  By the way, electric double layer capacitors used for applications such as memory backup and automobiles are required to have a higher capacity. That is, reduction of the capacity per unit volume and internal resistance is required. In order to achieve this, attempts have been made to add a conductive agent such as carbon black or carbon fiber to the activated carbon in the polarizable electrode, or to replace the current collector with a metal foil to make a porous body (three-dimensional structure). ing.

  However, in the trial using the conductive auxiliary agent, if a large amount of conductive auxiliary agent is added to lower the electric resistance, the content of the activated carbon in the polarizable electrode is reduced, and conversely, the capacitance of the capacitor is reduced. Arise.

  On the other hand, regarding current collectors, attempts have been made to use screens, punching metals, laths, etc. as porous bodies, but the structure is essentially a two-dimensional structure, and a significant improvement in capacitance is expected. Can not.

  Currently, foamed nickel is a three-dimensional structure current collector that can be mass-produced, and is widely used as a current collector for an alkaline electrolyte secondary battery. However, in an electric double layer capacitor using a non-aqueous electrolyte, nickel cannot be used because it is oxidized or corroded by the non-aqueous electrolyte. In addition, it is difficult to mass-produce current collectors having a three-dimensional structure with a high porosity using metals such as stainless steel other than nickel.

  In view of the above problems, the present inventors have intensively researched and, as a result, have adopted the current collector having a specific structure and have solved the above problems. That is, the present invention relates to the following polarizable electrode for an electric double layer type capacitor and a capacitor using the same.

  Item 1. Conductive treatment, electrolytic nickel plating treatment and chromium plating treatment are sequentially performed on the foamed resin, the foamed resin is removed, and the current collector obtained by annealing in a reducing atmosphere is filled with activated carbon. A polarizable electrode for an electric double layer capacitor.

  Item 2. Item 2. The polarizable electrode according to Item 1, wherein the foamed resin has an average pore diameter of 30 μm to 80 μm.

  Item 3. Item 3. The polarizable electrode according to Item 1 or 2, wherein the conductive treatment is an electroless nickel plating treatment or a nickel sputtering treatment.

Item 4. Basis weight of the nickel plating layer formed by electroless nickel plating treatment is ^{10g / m 2 ~250g / m 2} , polarizable electrode according to any one of Items 1 to 3.

Item 5. Basis weight of the chromium plating layer formed by the chromium plating treatment is ^{50g / m 2 ~300g / m 2} , polarizable electrode according to any one of Items 1 to 4.

  Item 6. Item 6. The polarizable electrode according to any one of Items 1 to 5, wherein the conductive additive is contained at a ratio of 0.2 to 5 parts by weight with respect to 100 parts by weight of the activated carbon.

  Item 7. Item 7. An electric double layer capacitor comprising the polarizable electrode according to any one of Items 1 to 6.

  Item 8. Item 8. The electric double layer capacitor according to Item 7, comprising a non-aqueous electrolyte as the electrolyte.

  The polarizable electrode for an electric double layer capacitor of the present invention is obtained by sequentially conducting conductive treatment, electrolytic nickel plating treatment and chromium plating treatment on a foamed resin, removing the foamed resin, and annealing in a reducing atmosphere. The obtained current collector is filled with activated carbon.

Current collector The current collector used in the present invention is obtained by sequentially conducting a conductive treatment, electrolytic nickel plating treatment and chromium plating treatment on a foamed resin, removing the foamed resin, and annealing in a reducing atmosphere. It is done. Since the current collector of the present invention is a porous body, it can be filled with more activated carbon and the capacitance can be improved. In addition, since the activated carbon is enclosed in the voids in the porous body, the content of a binder or the like (insulator) for binding the activated carbon and the current collector can be reduced, and the internal resistance can be lowered. it can. Further, since the current collector is excellent in oxidation resistance, the life of the capacitor can be extended.

  As the foamed resin, any known or commercially available one can be used as long as it is porous, and examples thereof include foamed urethane and foamed styrene. Among these, urethane foam is preferable from the viewpoint of particularly high porosity.

  The porosity of the foamed resin is not limited, and is usually about 85 to 97 vol%, preferably about 90 to 96 vol%. The average pore diameter is usually about 20 μm to 200 μm, preferably about 30 μm to 80 μm.

  The thickness of the foamed resin is not limited, and is appropriately determined according to the use of the electric double layer capacitor, but is usually about 300 μm to 1500 μm, preferably about 400 μm to 1200 μm.

  The foamed resin is sequentially subjected to conductive treatment, electrolytic nickel plating treatment and chromium plating treatment.

  The conductive treatment is not limited as long as a conductive layer can be provided. Examples of a material constituting the conductive layer (conductive plating layer) include graphite, in addition to metals such as nickel, titanium, and stainless steel. Among these, nickel is particularly preferable.

  As specific examples of the conductive treatment, for example, when nickel is used, electroless plating treatment, sputtering treatment, and the like are preferably exemplified. For example, in the case of using a material such as titanium, stainless steel, or a material such as graphite, a treatment obtained by applying a mixture obtained by adding a binder to fine powder of these materials to a foamed resin is preferable. The binder in this case can be the same as the active material paste described later, and the amount of the binder added is not limited and may be the same as that of the active material paste.

  As the electroless plating treatment using nickel, for example, the foamed resin may be immersed in a known electroless nickel plating bath such as a nickel sulfate aqueous solution containing sodium hypophosphite as a reducing agent. If necessary, the foamed resin may be immersed in an activation liquid containing a trace amount of palladium ions (cleaning liquid manufactured by Kanigen Co., Ltd.) or the like before immersion in the plating bath.

  As a sputtering process using nickel, for example, after attaching a foamed resin to a substrate holder, ionization is performed by applying a DC voltage between the holder and a target (nickel) while introducing an inert gas. The nickel particles blown off may be deposited on the foamed resin surface by colliding the inert gas with nickel.

  Next, electrolytic nickel plating is applied to the foamed resin for forming a conductive plating layer. What is necessary is just to perform an electrolytic nickel plating process in accordance with a conventional method. As the plating bath used for the electrolytic nickel plating treatment, a known or commercially available bath can be used, and examples thereof include a watt bath, a chloride bath, a sulfamic acid bath, and the like.

  Next, a chromium plating treatment is applied to the conductive plating layer / nickel plating layer forming foamed resin. The chromium plating process may be performed according to a conventional method, and the chromium plating process is not limited, and examples thereof include an electrolytic plating method, an electroless plating method, and a sputtering method. In the present invention, a chromium plating layer formed by an electrolytic plating method is preferable.

As the electroplating bath used in the electroplating method, a known or commercially available bath can be used. For example, a sergeant bath (typical composition is CrO ₃ : 250 g / l and sulfuric acid H ₂ SO ₄ : 2.5 g / l). And a fluorination bath (typical composition is CrO ₃ : 250 g / l and fluorine component F: 0.6 g / l (or SiF ₆ : 2.5 g / l)).

  The basis weight (adhesion amount) of each of these plating layers is not particularly limited. The conductive plating layer only needs to be continuously formed on the foamed resin surface, the electrolytic nickel plating layer only needs to be formed on the conductive plating layer to the extent that the conductive plating layer is not exposed, and the chromium plating layer is electrolytic. What is necessary is just to be formed on the said electrolytic nickel plating layer to such an extent that a nickel plating layer is not exposed. In addition, when a conductive plating layer is a nickel layer, the chromium plating layer should just be formed so that the said conductive plating layer (nickel layer) and an electrolytic nickel plating layer may not be exposed.

The basis weight of the conductive plating layer is not limited, and is usually about 5 to 15 g / m ² , preferably about 7 to 10 g / m ² .

The basis weight of the electrolytic nickel plating layer is not limited, and is usually about 10 to 250 g / m ² , preferably about 50 to 200 g / m ² .

The basis weight of the chromium plating layer is not limited, and is usually about 50 to 300 g / m ² , preferably about 80 to 200 g / m ² .

The total weight of the conductive plating layer, the electrolytic nickel plating layer, and the chromium plating layer is preferably about 150 to 500 g / m ² , more preferably about 300 to 450 g / m ² . When the total amount is below this range, the strength of the current collector may be reduced. On the other hand, if the total amount exceeds this range, the filling amount of the polarizable material is reduced or the cost is disadvantageous.

  Next, the foamed resin component in the conductive plating layer / nickel plating layer / chromium plating layer forming foamed resin obtained as described above is removed. Although the removal method is not limited, it may be removed by incineration. Specifically, the heating may be performed in an oxidizing atmosphere such as air of about 600 ° C. or higher. Moreover, you may heat at about 750 degreeC or more in reducing atmosphere, such as hydrogen. Thereby, the porous body which consists of a conductive plating layer, a nickel plating layer, and a chromium plating layer is obtained.

  Next, the current collector of the present invention can be produced by annealing the porous body in a reducing atmosphere. The obtained current collector is a three-dimensional structure having a large porosity. By this annealing treatment, the porous body made of the conductive layer, nickel plating and chromium plating can be reduced, and by this reduction action, nickel and chromium are alloyed to further improve the electrolytic solution resistance and oxidation resistance. be able to.

  Although it does not specifically limit as reducing atmosphere, Preferably decomposition | disassembly ammonia etc. are mentioned. Decomposed ammonia is a mixed gas obtained by decomposing known ammonia and having a mixing ratio of nitrogen and hydrogen of about 3: 1 (volume ratio).

  The heating temperature is not limited as long as it is a temperature at which the chromium and nickel are reduced, but is usually about 600 to 1100 ° C, preferably about 700 to 950 ° C. Moreover, you may carry out by a normal pressure and you may carry out under reduced pressure.

A polarizable electrode polarizable electrode is produced by filling the current collector of the present invention with activated carbon.

  The activated carbon can use what is generally marketed for electric double layer capacitors.

  Examples of the raw material for the activated carbon include wood, coconut shell, pulp waste liquid, coal, heavy petroleum oil, coal / petroleum pitch obtained by pyrolyzing them, and resins such as phenol resins. The activation is generally performed after carbonization, and examples of the activation method include a gas activation method and a chemical activation method. The gas activation method is a method in which activated carbon is obtained by contact reaction with water vapor, carbon dioxide gas, oxygen or the like at a high temperature. The chemical activation method is a method in which activated carbon is obtained by impregnating the above-mentioned raw material with a known activation chemical and heating it in an inert gas atmosphere to cause dehydration and oxidation reaction of the activation chemical. Examples of the activation chemical include zinc chloride and sodium hydroxide.

The particle size of the activated carbon is not limited, but is preferably 20 μm or less. The specific surface area is not limited and is preferably about 800 to 3000 m ² / g. By setting this range, the capacitance of the capacitor can be increased, and the internal resistance can be decreased.

  Moreover, you may contain additives, such as a conductive support agent and a binder, as needed.

  The conductive auxiliary agent is not limited, and known or commercially available ones can be used. Examples thereof include acetylene black, ketjen black, carbon fiber, natural graphite (scaly graphite, earthy graphite, etc.), artificial graphite, ruthenium oxide and the like. Among these, acetylene black, ketjen black, carbon fiber and the like are preferable. Thereby, the electrical conductivity of the capacitor can be improved. The content of the conductive assistant is not limited, but is preferably about 0.1 to 10 parts by weight with respect to 100 parts by weight of the activated carbon. If it exceeds 10 parts by weight, the capacitance may decrease.

  The binder is not limited, and known or commercially available binders can be used. Examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl pyrrolidone, polyvinyl chloride, polyolefin, styrene butadiene rubber, polyvinyl alcohol, carboxymethyl cellulose and the like.

  The content of the binder is not limited, but is preferably 0.5 to 5 parts by weight with respect to 100 parts by weight of the activated carbon. By setting this range, the binding strength can be improved while suppressing an increase in electrical resistance and a decrease in capacitance.

The filling amount (content) when the current collector is filled with activated carbon is not particularly limited, and may be determined as appropriate according to the thickness of the current collector, the shape of the capacitor, and the like. 40 mg / cm ² or so, it may be the preferred 16~32mg / cm ² approximately.

  As a method for filling the current collector of the present invention with activated carbon or the like, for example, a known method such as press-fitting activated carbon paste may be used.

  The activated carbon paste should just contain activated carbon and a solvent, and the mixture ratio is not limited. The solvent is not limited, and examples thereof include N-methyl-2-pyrrolidone and water. In particular, when polyvinylidene fluoride is used as a binder, N-methyl-2-pyrrolidone may be used as a solvent. When polytetrafluoroethylene, polyvinyl alcohol, carboxymethyl cellulose, or the like is used as a binder, water may be used as a solvent. Moreover, you may contain additives, such as the said conductive support agent and a binder, as needed.

  Examples of the press-fitting method include a method of immersing the current collector in activated carbon paste and reducing the pressure as necessary, and a method of filling the activated carbon paste while applying pressure from one side of the current collector with a pump or the like.

  After the polarizable electrode of the present invention is filled with the activated carbon paste, the solvent in the paste may be removed by performing a drying treatment as necessary. Further, if necessary, after the activated carbon paste is filled, it may be compression-molded by pressurizing with a roller press or the like. The thickness before and after compression is not limited, but the thickness before compression is usually 300 μm to 1500 μm, preferably 400 μm to 1200 μm, and the thickness after compression molding is usually about 150 μm to 700 μm, preferably about 200 μm to 600 μm. And it is sufficient.

  The polarizable electrode may be provided with a lead terminal. The lead terminal may be attached by welding or applying an adhesive.

Electric Double Layer Capacitor The capacitor of the present invention comprises a pair of two polarizable electrodes of the present invention, a separator disposed between these electrodes, and a separator impregnated with an electrolyte solution.

  A known or commercially available separator can be used. For example, an insulating film made of polyolefin, polyethylene terephthalate, polyamide, polyimide, cellulose, glass fiber or the like is preferable. The average pore diameter of the separator is not particularly limited, and is usually about 0.01 μm to 5 μm, and the thickness is usually about 10 μm to 100 μm.

  As the electrolytic solution, a known or commercially available one can be used, and either an alkaline aqueous solution or a non-aqueous electrolytic solution can be used. Examples of the alkaline electrolyte include alkaline aqueous solutions such as an aqueous potassium hydroxide solution and an aqueous sodium hydroxide solution. Examples of the non-aqueous electrolyte include a propylene carbonate solution in which tetraalkylphosphonium tetrafluoroborate is dissolved, a propylene carbonate solution or sulfolane solution in which tetraalkylammonium tetrafluoroborate is dissolved, and a propylene carbonate solution in which triethylmethylammonium tetrafluoroborate is dissolved. Etc.

  Among these, a nonaqueous electrolytic solution is preferable in the present invention. By using such a non-aqueous electrolyte, the capacitance can be improved.

  According to the polarizable electrode of the present invention, it is possible to provide an electric double layer type capacitor having a large capacitance, a small internal resistance, and an excellent durability.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. The present invention is not limited to the following examples.

Example 1
(Preparation of current collector)
As the foamed resin, a urethane foam resin (commercially available product, average pore diameter 80 μm, thickness 800 μm, porosity 95%) was used.

By performing sputtering treatment using nickel as a target for the foamed urethane resin, a conductive plating layer (nickel layer) was formed on the surface of the foamed urethane resin. The basis weight of the conductive plating layer was 8 g / m ² .

Subsequently, the electroplating process was performed to the obtained electroconductive plating layer formation foaming urethane resin. As an electrolytic nickel plating bath, a Watt bath (nickel sulfate 330 g / l, nickel chloride 50 g / l, boric acid 40 g / l) was used. A titanium basket containing nickel pieces was used as the counter electrode. The electrodeposition conditions were a bath temperature of 60 ° C. and a current density of 30 A / dm ² . The basis weight of the electrolytic nickel plating layer was 192 g / m ² .

Next, electrolytic chrome plating was performed. As the plating bath, a surf bath (chromic acid 250 g / l and sulfuric acid 2.5 g / l) was used. As the counter electrode, an electrode coated with a lead alloy with copper as a core was used. The electrodeposition conditions were a bath temperature of 50 ° C. and a current density of 40 A / dm ² . The basis weight of the chromium plating layer was 100 g / m ² . The average thickness of the current collector at this time was 520 μm.

  Next, the current collector was washed with water and dried, and then heated to about 600 ° C. in air to incinerate and remove the urethane resin. Furthermore, a current collector having a three-dimensional structure was produced by performing reduction and annealing at about 750 ° C. in a decomposed ammonia atmosphere.

(Preparation of polarizable electrode)
100 parts by weight of activated carbon powder (specific surface area 2500 m ² / g, average particle size of about 5 μm), 2 parts by weight of ketjen black as a conductive aid, 4 parts by weight of polyvinylidene fluoride powder as a binder, and 15 weights of N-methylpyrrolidone as a solvent The activated carbon paste was prepared by adding a part and stirring with a mixer.

The activated carbon paste was filled into the current collector using a pump so that the activated carbon content was 30 mg / cm ² . Next, the surface was smoothed, dried at 80 ° C. for 20 minutes, and pressure-molded with a roller press to obtain a polarizable electrode. The thickness of this polarizable electrode was 480 μm.

(Production of test coin-type electric double layer capacitor)
Two polarizable electrodes thus obtained were punched out to a diameter of 14 mm, and a cellulose fiber separator (thickness 45 μm, density 450 mg / cm ³ , porosity 70%) was sandwiched between these polarizable electrodes. In this state, it was dried under reduced pressure at 180 ° C. for 5 hours. After that, the cell case was housed using a stainless steel spacer, and the polarizable electrode and the separator were impregnated with a non-aqueous electrolyte (a propylene carbonate solution in which tetraethylphosphonium tetrafluoroborate was dissolved to 1 mol / L). Further, the case lid was tightened and sealed through an insulating gasket made of propylene to produce a coin-shaped test electric double layer capacitor. This capacitor had a diameter of 20 mm and a thickness of 3.2 mm. The rated voltage was 2.8V.

Comparative Example 1
A capacitor was produced in the same manner as in Example 1 except that the current collector was foamed nickel (nickel amount: 400 g / m ² ). However, since oxidation and corrosion of nickel occurred violently, characteristics as a capacitor could not be obtained.

Comparative Example 2
An aluminum foil (thickness 20 μm) was used as a current collector. The same activated carbon paste as in Example 1 was prepared and applied to the side facing the counter electrode, but the activated carbon did not adhere to the aluminum foil. Therefore, an activated carbon paste having a binder content of 8 parts by weight and a conductive additive of 3 parts by weight was prepared, and this paste was applied to the surface of the aluminum foil so that the activated carbon content was 8 mg / cm ² . . Next, the electrode was dried at 80 ° C. for 20 minutes and pressure-molded with a roller press to obtain a polarizable electrode. The thickness of the polarizable electrode was 180 μm including the aluminum foil.

  A coin-type test electric double layer capacitor of a comparative example was produced in the same manner as in the example except that the polarizable electrode of this comparative example was used. The rated voltage was 2.8V.

Evaluation of Capacitance Ten capacitors similar to those in Example 1 and Comparative Example 2 were produced, respectively, and the capacitance per unit area, unit volume, and average value of internal resistance are shown in Table 1.

  As is apparent from Table 1, the capacitor of the example has a larger capacity per unit volume and a lower internal resistance than the capacitor of the comparative example. In particular, in terms of capacitance, the capacitor of the example exhibits a capacitance three times or more that of the capacitor of the comparative example. Therefore, it can be seen that the capacitor (particularly the polarizable electrode portion) of the present invention can be achieved with a length of 1/3 or less in order to obtain the same capacitance as the conventional capacitor of the comparative example.

  In addition, it can be seen that the present invention can improve the energy density because the amount of the material (binder) that does not contribute to the capacitance can be reduced.

Durability test 1
Next, durability was examined as capacitor characteristics. First, aging was performed by applying a voltage of 2.8V at 65 ° C for 6 hours, and then discharging was performed at a current of 1mA with 3.0V as the starting voltage at 25 ° C. Investigated for. Subsequently, it was maintained for 2000 hours while applying a voltage of 2.7 V at 65 ° C. Thereafter, the capacitance and internal resistance were measured at 25 ° C., and the rate of change in capacitance and internal resistance from the beginning was examined. The results are shown in Table 2.

  As is clear from Table 2, the change in capacitance and internal resistance of the example was small after 2000 hours as compared with the comparative example. Therefore, it was found that the electric double layer capacitor of the present invention has a high capacitance and is excellent in durability.

Durability test 2
The charge / discharge cycle characteristics were examined as another durability evaluation method. As conditions, charge and discharge cycles with a constant current of 1 mA at an ambient temperature of 45 ° C between 0.5 and 3.0 V were repeated 10,000 times, and the discharge capacity and internal resistance after 10,000 cycles were measured and evaluated in comparison with the initial characteristics. Went. As a result, the decrease rate of the capacitance in the example was 10%, whereas in the comparative example, the decrease rate was 12%. The internal resistance increased by 10% in this example, but increased by 15% in the comparative example.

Claims (8)

  Conductive treatment, electrolytic nickel plating treatment and chromium plating treatment are sequentially performed on the foamed resin, the foamed resin is removed, and the current collector obtained by annealing in a reducing atmosphere is filled with activated carbon. A polarizable electrode for an electric double layer capacitor.   The polarizable electrode according to claim 1, wherein the foamed resin has an average pore diameter of 30 μm to 80 μm.   The polarizable electrode according to claim 1 or 2, wherein the conductive treatment is an electroless nickel plating treatment or a nickel sputtering treatment. Basis weight of the nickel plating layer formed by electroless nickel plating treatment is ^{10g / m 2 ~250g / m 2} , polarizable electrode according to any of claims 1 to 3. Basis weight of the chromium plating layer formed by the chromium plating treatment is ^{50g / m 2 ~300g / m 2} , polarizable electrode according to claim 1.   The polarizable electrode according to any one of claims 1 to 5, wherein the conductive additive is contained at a ratio of 0.2 to 5 parts by weight with respect to 100 parts by weight of the activated carbon.   An electric double layer capacitor comprising the polarizable electrode according to claim 1.   The electric double layer capacitor according to claim 7, comprising a non-aqueous electrolyte as the electrolyte.