WO2009091102A1

WO2009091102A1 - Electrode for electrochemical capacitor and process for preparing the same

Info

Publication number: WO2009091102A1
Application number: PCT/KR2008/002323
Authority: WO
Inventors: Young Jin Jung; Ki Sang Yoo; Jung Ho Choi; Kyoung Hee Kwon; Young Ho Kim
Original assignee: Nesscap Co., Ltd.
Priority date: 2008-01-14
Filing date: 2008-04-24
Publication date: 2009-07-23
Also published as: KR100917408B1; KR20090078058A

Abstract

Provided are electrode for an electrochemical capacitor and a process for preparing the same. Unlike in typical methods, a binder for an electric double layer capacitor having more favorable physical characteristics and thermal stability is used, in order to increase electrode density to realize a high-capacity capacitor, and increase yield by raising coating temperature and drying temperature to improve thermal resistance.

Description

ELECTRODE FOR ELECTROCHEMICAL CAPACITOR AND PROCESS FOR PREPARING THE SAME

Technical Field

[1] The present invention relates to an electrode for an electrochemical capacitor and a process for preparing the same, and more particularly, to an electrode for an electrochemical capacitor using a binder for an electric double layer capacitor having more favorable physical characteristics and thermal stability, instead of a related art soluble cellulose-based polymer having a low pyrolytic temperature, in order to increase electrode density to realize a high-capacity capacitor, and increase yield by raising coating temperature and drying temperature to improve thermal resistance, and to a process for preparing the same. Background Art

[2] Electrochemical capacitors are configured with two types of electrodes, and can be generally be classified into symmetric capacitors and hybrid capacitors depending on their electrode configurations. Symmetric capacitors can be further classified into electric double layer capacitors (that uses a plurality of activated carbon electrodes and has double layer capacitance occurring at the contacting interfaces between each electrode and liquid electrolyte), and pseudocapacitors (with pseudocapacitance through reduction/oxidation reaction (redox) that accompanies capacitance). Since an electric double layer capacitor induces electric charging and discharging through physical reactions of ion absorption and desorption on an active material surface, it has high output and an almost indefinite service life. Active materials that may be used include active carbon powder, active carbon fiber, carbon nanotubes, and carbon aero gel. A pseudocapacitor has a capacitance three or four times greater than an electric double layer capacitor. However, manufacturing a pseudocapacitor is difficult because expensive metal oxide or limiting conductive polymer is used as an electrode active material, and due to its high internal resistance, pseudocapacitors are not yet widely used.

[3] A hybrid capacitor has double layer capacitance at one electrode employing activated carbon material, and exhibits Faradaic pseudocapacitive behaviors at the other electrode that is formed, not of activated carbon, but of one or more selected from groups (a) Ru, Rh, Pd, Ta, Os, Mo, Mn, Ni, Co, Ir, W, and V; (b) an alloy, an oxide, a nitride, a carbide, and a hydroxide of the above metals; (c) a doped conductive polymer; and (d) a graphite-based carbon in which lithium ion is introduced. While hybrid capacitors are advantageous in their ability to surmount the small capacities of symmetric capacitors and their more favorable output characteristics over batteries, they are limited by reduced service life in comparison to symmetric capacitors.

[4] There are currently about two methods that are the most widely used to prepare electrodes for electrochemical capacitors employing activated carbon. One involves forming a paste mixture with active material, conductive material, and a binder, and then compressing and attaching the paste to a current collector, and the other involves forming slurry with active material and conductive material combined with a binder, and then coating the slurry on a current collector.

[5] Of these, the method of forming an electrode by coating slurry involves a comparatively simpler work process and lower cost for processing equipment. Also, when compared to the method of using a separate conductive binder and attaching a compressed electrode, the former method has a relatively lower internal resistance. For these reasons, the method for preparing electrodes using the slurry coating method is the most widely used for preparing electrodes for electrochemical capacitors.

[6] The method of forming electrodes for electrochemical capacitors by coating slurry involves coating a slurry including one or more binders on a current collector, and then drying and binding an active material with another active material and an active material with a current collector. The binder is one of the important elements that determine the capacitor's performance. If the performance of the binder is insufficient or not enough binder is contained for an electrode, it is difficult to coat a layer of uniform thickness when coating an electrode, and even after the capacitor is configured, active material may disengage from other active material or a current coll ector, so that the capacitor is given reduced capacitance or increased internal resistance. Conversely, if an excessive amount of binder is used, the amount of active material in the electrode is reduced so that the capacitance of the capacitor drops, or its internal resistance is raised by the electrical characteristics of polymer, which is largely an electric insulator.

[7] The most representative binder used to prepare electrodes for electrochemical capacitors in the related art is carboxymethyl cellulose (CMC). Japanese Laid-open Patent No. 1991-280518 discloses a method of preparing an electrode for an electric double layer capacitor, through dissolving CMC in water to mix and diffuse activated carbon, after which prepared slurry is coated on an aluminum current collector. Carboxymethyl cellulose is a binder that is commonly used to increase viscosity. It is also used to disperse active materials in many applications, and is an important binder that is widely used for coupling one active material with another active material, and an active material with a current collector. Due to its pyrolytic temperature of 25O⁰C or less, however, there are restrictions imposed on how high drying temperatures can be raised following layer coating, and also on temperatures employed in a drying process for removing moisture adsorbed in holes of activated carbon. Also, the above binder becomes very brittle after it is dried. Thus, when an electrode is wound after coating, the coated layer is prone to cracking or peeling. Specifically, it this can lead to a shortened service life of an electrochemical capacitor, which must maintain a stable internal capacitance. Finally, due to the characteristic of cellulose that forms a networked configuration, the density of active material tends to drop after slurry is coated. This leads to a capacitance reduction of a capacitor due to a lower density of its electrode.

[8] Korean Laid-open Patent No. 2001-84422 discloses a method for mixing and using the above binder with butadiene styrene. This has been proposed as an alternative to the typical use of only carboxymethyl cellulose for electrodes, due to the latter' s extreme lack of flexibility. Because butadiene styrene is composed of small particles approximately 130 nm in size and induces point contact between one active material and another active material and between an active material and a current collector, it is very effective at increasing the strength of an electrode. However, mixing too much into an electrode causes a sharp increase in internal resistance, and capacitance reduction results from a reduced amount of active material in the mixture. Disclosure of Invention Technical Problem

[9] To overcome the above problems of the related art, the present inventor employed an n- vinyl acetamide -based polymer with excellent thermal stability and dispersive properties as a binder for an electrochemical capacitor, which demonstrated abilities to increase electrode density to realize a high capacitance capacitor, and increase heat resistance to raise coating temperature and drying temperature and thus improve productivity in the preparing of electrodes and provide a stable capacitor service life.

[10] Compared to the glass transition temperatures of most electrochemical capacitor binders that range from -100°C to 100°C or below, an n-vinyl acetamide-based binder is characterized in having a glass transition temperature of 16O⁰C or higher. The present inventor discovered that problems in the related art could be solved by using a binder formed of the n-vinyl acetamide-based polymer.

[11] Accordingly, an object of the present invention is to provide an electrode for an electrochemical capacitor capable of increasing electrode density to realize a high capacitance capacitor, improving heat resistance to raise coating temperature and drying temperature in order to improve productivity in electrode preparation, and a process for preparing the same. Technical Solution

[12] According to an aspect of the present invention, there is provided an electrode for an electrochemical capacitor, including a compound coated on a current collector, the compound including an active material, a conductive material and a binder, wherein the binder includes an n-vinyl acetamide polymer.

[13] In order to improve physical properties such as flexibility, the binder may further include butadiene styrene.

[14] The binder may have a higher glass transition temperature than the related art, which may be 16O⁰C or higher - preferably, 350°C.

[15] 1 to 20 parts by weight of the binder may be included for every 100 parts by weight of the active material, and 1 to 10 parts by weight of the n-vinyl acetamide polymer and 1 to 10 parts by weight of the butadiene styrene may be used for every 100 parts by weight of the active material as the binder.

[16] The capacitor may be an electric double layer capacitor including a plurality of symmetrical electrodes, or may be a hybrid capacitor including a plurality of asymmetrical electrodes.

[17] The electric double layer capacitor may be configured with two of the electrodes both including the active material that is an activated carbon powder, activated carbon fiber, carbon nanotubes, carbon aero gel, or a combination thereof, and one of the electrodes may have a capacitance that is 1 to 3 times that of the other electrode.

[18] The hybrid capacitor may be configured with a first of the electrodes including the active material that is an activated carbon powder, activated carbon fiber, carbon nanotubes, carbon aero gel, or a combination thereof, and a second of the electrodes including the active material that is one or more selected from the groups: (a) Ru, Rh, Pd, Ta, Os, Mo, Mn, Ni, Co, Ir, W, and V; (b) an alloy, an oxide, a nitride, a carbide, and a hydroxide of the metals in group (a); (c) a doped conductive polymer; and (d) a graphite-based carbon in which lithium ion is introduced, and the first or the second of the electrodes may have a capacitance 10 times or greater than that of the other electrode.

[19] According to another aspect of the present invention, a method of preparing an electrode for an electrochemical capacitor is provided to include an active material, a conductive material, a binder for binding the active material and the conductive material on a current conductor, and a solvent and dispersing agent for dispersing the active material.

[20] Specifically, the method of preparing an electrode for an electrochemical capacitor, includes: forming a slurry through mixing a compound including an active material, a conductive material and a binder; and forming the electrode through coating the slurry on a current collector, where the binder may include an n-vinyl acetamide polymer, butadiene styrene, or a compound thereof. Advantageous Effects

[21] Instead of carboxymethyl cellulose used as a binder in related art electrochemical capacitors, the present invention uses an n- vinyl acetamide polymer to enable an electrode density increase of 12% or more, as well as an equivalent series resistance (ESR) service life by approximately two-fold. Most importantly, by reducing the amount of solvent used in the related art by approximately 68% and improving reliability under high temperature, preparation time can be reduced, and moisture inside active material can efficiently be removed, to increase the service life of a capacitor. That is, the issues of realizing high capacitance, low cost, and extended service life for an electrochemical capacitor can be resolved. Brief Description of Drawings

[22] FIG. 1 is a flowchart of a process for preparing an electrode for an electrochemical capacitor according to the present invention.

[23] FIG. 2 illustrates an electrode strength test.

[24] FIG. 3 illustrates an electrode bent to test its flexibility.

[25] FIG. 4 is a graph comparing electrode strengths for different electrode temperatures of electrodes according to a present embodiment and a comparative embodiment.

[26] FIG. 5 is a graph comparing pyrolysis temperatures of carboxymethyl cellulose and n- vinyl acetamide polymer.

[27] FIG. 6 is a graph comparing viscosity stability of a present embodiment and a comparative embodiment.

[28] FIG. 7 is a graph illustrating results of a service life test for electrodes according to present and comparative embodiments. Best Mode for Carrying out the Invention

[29] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[30] FIG. 1 is a flowchart of a process for preparing an electrode for an electrochemical capacitor according to the present invention, where the preparing process is constituted of a first mixing stage 1, a second mixing stage 2, a slurry forming stage 3, a current collector coating stage 4, and an electrode drying stage 5.

[31] First, activated carbon Ia and conductive material Ib are mixed as an active material in a dry mixing process. For the active material, in addition to activated carbon powder, activated carbon fiber, carbon nanotubes, carbon aero gel, etc. may be used.

[32] The activated carbon Ia may be selected from material with an average grain diameter of 1 to 100 μm and a specific surface area of 500 to 3000 m /g. More preferably, activated carbon with an average grain diameter of 1 to 50 μm and a specific surface area of 1000 to 3000 m /g may be used, and most preferably, activated carbon with an average grain diameter of 1 to 10 μm and a specific surface area of 2000 to 3000 m /g may be used, and phenol, peach, cokes, bamboo, charcoal, palm, sugar, etc. may be used as a starting material.

[33] The conductive material Ib is formed of fine conductive powder such as Ketjen black, Acetylene Black, or graphite powder that have an average particle size smaller than activated carbon. Such fine conductive powdered materials fit in gaps between activated carbon to enable uninterrupted flow of electrons, and may be introduced in a quantity of 1 to 30 parts by weight for 100 parts by weight activated carbon, and more preferably, in a quantity of 3 to 20 parts by weight for 100 parts by weight activated carbon. If the introduced quantity is insufficient, while the capacitance of the capacitor may be increased, its internal resistance is also increased abruptly, and the density is reduced to produce a weaker electrode. Conversely, when the introduced quantity is excessive, while the capacitor's internal resistance can be reduced, cell capacitance also drops due to a reduction in the amount of activated carbon contained. Thus, a suitable quantity by weight must be introduced.

[34] Next, after the activated carbon Ia and the conductive material Ib have been dry- mixed, wet mixing of binder 2b and solvent 2a is performed to obtain a secure coating on a current collector. Through this process, slurry is formed in the slurry forming stage 3.

[35] De-ionized water may be used as the solvent 2a.

[36] For the binder 2b, carboxymethyl cellulose (used in the related art), butadiene styrene, and n- vinyl acetamide polymer (provided in the present invention) may be used.

[37] The binder 2b may be introduced in a quantity of 1 to 20 parts by weight for 100 parts by weight of active material, and for the binder, 1 to 10 parts by weight of n- vinyl acetamide polymer and 1 to 10 parts by weight of butadiene styrene may be used for 100 parts by weight of active material.

[38] In particular, 1 to 5 parts by weight of n- vinyl acetamide polymer for every 100 parts by weight of activated carbon may be introduced in an electrode. As for butadiene styrene, using 1 to 10 parts by weight for every 100 parts by weight of activated carbon is preferable, and using 1 to 5 parts by weight is more preferable.

[39] Because n-vinyl acetamide-based polymer is a binder that has favorable physical properties and thermal stability, by using a binder that includes an n-vinyl acetamide- based monomer unit and a polymer with a glass transition temperature of 16O⁰C or higher, the present invention discovered that problems in the related art could be solved. It is preferable to introduce 1 to 10 parts by weight for every 100 parts by weight of activated carbon in an electrode, and more preferable to introduce 1 to 5 parts by weight. When comparing this binder to carboxymethyl cellulose, it has a lower moisture absorption rate. It has exceptional ability to disperse active material in slurry with even a small amount of water. In other words, less water is used to obtain the same slurry viscosity. This suggests that a denser electrode can be obtained after coating. Also, because the amount of water within the slurry is small, drying time can be reduced. Therefore, by using an n-vinyl acetamide-based binder, electrode density can be increased to realize a high-capacitance capacitor, and an increase in yield can be anticipated from the reduced drying time. Finally, because n-vinyl acetamide-based binder has a pyrolysis temperature of 350°C or higher, its stability in high temperatures is much greater than carboxymethyl cellulose. This greatly aids improving productivity by allowing the drying temperature to be raised following coating, thereby increasing processing speed. Also, drying temperature can be raised for effectively removing moisture adsorbed in voids in activated carbon to reduce drying time.

[40] Next, slurry is coated on a metal current collector in the current collector coating stage 4. The metal current collector used may be virtually any thin metal layer of Al, Cu, Ni, etc., and is an Al current collector, which is the most widely-used in electrochemical capacitors. To raise the adhering force between the electrode and current collector, a method for increasing surface area using a chemical to etch the current collector may be employed, or physical abrasion may be used to roughen the surface. The above metal current collector may have a thickness of 10 to 100 μm. If it is insufficiently thick, it may be unable to support an electrode coated thereon, and if it is excessively thick, when configuring the capacitor, the capacitance of the capacitor may be lowered on account of the increased volume taken up inside by the current collector that limits the size of the electrode.

[41] Next, after being coated, the electrode is immediately dried in the electrode drying stage 5. The drying temperature may be varied according to the thickness and width of the coated layer and the amount of solvent contained in the slurry. It may also be varied according to the thermal stability of the binder.

[42] While a detailed description will be provided below on an electrode for an electrochemical capacitor and a method of preparing the electrode, according to preferred embodiments of the present invention, the present invention is not limited to or restricted by the embodiments described below.

[43] [EMBODIMENTS/COMPARATIVE EXAMPLES]

[44] As shown in Table 1 below, after different slurries were prepared by mixing predetermined quantities of carboxymethyl cellulose having a portion substituted with Na ions, n-vinyl acetamide polymer and/or butadiene styrene with 100 parts by weight of activated carbon powder (with an average grain diameter of 10 μm and a specific surface area of 2300 m /g), the slurry was coated on an Al current collector (with a 20 μm thickness) to form an electrode. Here, 10 parts by weight of Acetylene Black (with a 1 μm or less average grain diameter) as a conductive material, and 300 parts by weight of de-ionized water as a solvent were introduced to 100 parts by weight of activated carbon, and the coating layer was formed 250 μm thick.

[45] Table 1 [Table 1] [Table ]

[46] [TEST EXAMPLE 1] [47] In the first and second embodiments and comparative examples 1 and 2, electrodes were compared in terms of preparation and properties of coating layers, and specifically, tests were conducted for electrode strength and flexibility according to weight.

[48] The electrode strength tests were measured with a coated layer strength tester (RHESCA; FPR-2000), and the testing method is outlined below. Referring to FIG. 2, first, a compression ball 8 was used to press the surface of a coated layer 7 measuring 3 cm across and 4 cm vertically. Here, a load 11 of 500 g was applied over ten counterclockwise rotations per minute. The test duration was 120 seconds, in which scratches in the shape of a circle 9 appeared, followed by the development of cracks in the coated layer 7 after a predetermined time. The times at which cracks first began appearing were used as electrode strength comparison indicators. Zero point was adjusted by a support 10 and a counter load 6.

[49] In the flexibility test, a coated electrode 12 was bent, as shown in FIG. 3, to see whether or not cracks appeared in the coated layer. In this test, flexibility was evaluated with X (poor flexibility) when a coated layer separated from a current collector, Δ (when fine cracks appeared), and O (good flexibility) when cracks did not appear. The coated layer was 250 μm thick.

[50] Table 2 [Table 2] [Table ]

[51] As shown by the tests results in Table 2, the electrode strength increased with an increase in the amount of binder used. While two types of binders both failed to exhibit good flexibility, complete separation from conductors occurred in the cases of the comparative examples in which only carboxymethyl cellulose was added, while only fine cracks occurred in the electrodes (embodiments) in which only an n- vinyl acetamide polymer was added.

[52] [TEST EXAMPLE 2] [53] In the third and fourth embodiments and comparative examples 3 and 4, electrodes were compared in terms of electrode strength and flexibility according to weight, and the test methodology was the same as in test example 1.

[54] Table 3 [Table 3] [Table ]

[55] As shown by the test results in Table 3, electrode strength was increased over test example 1 by adding butadiene styrene. Furthermore, as the added amount of butadiene styrene was increased, electrode strength increased. With respect to the flexibility test results also, a marked improvement in flexibility over test example 1 was observed when butadiene styrene was added. However, in the case of comparative example 3, there was a limit to how much electrode flexibility could be improved by adding only 2 parts by weight of butadiene styrene. Specifically, it was confirmed that carboxymethyl cellulose has poorer flexibility than n- vinyl acetamide polymer.

[56] [TEST EXAMPLE 3]

[57] In test example 2 whose results are depicted in Table 3, electrode strength variation was tested according to drying temperature changes for electrodes in comparative example 4 and the fourth embodiment (which were materials that displayed good flexibility), and the results are shown in FIG. 4. In FIG. 4, reference numeral 13 represents drying results over 12 hours for an electrode of comparative example 4, and reference numeral 14 represents drying results over 24 hours for the electrode of comparative example 4. Reference numeral 15 represents drying results over 12 hours for an electrode of the fourth embodiment, and reference numeral 16 represents drying results over 24 hours for the electrode of the fourth embodiment. The test was conducted under -1 atmospheric pressure (-760 mmHg).

[58] FIG. 4 shows that electrode strength rapidly dropped with an increase in drying temperatures 13 and 14 for carboxymethyl cellulose. It is also apparent that the longer drying is prolonged, the more quickly a drop occurs in electrode strength according to drying temperature. On the other hand, in the case of the n-vinyl acetamide polymer 15 and 16, electrode strength was relatively stable over varying drying temperatures and drying durations.

[59] FIG. 5 is a graph comparing pyrolysis temperatures of carboxymethyl cellulose having a portion substituted with Na ions, and n-vinyl acetamide polymer. A thermo- gravimetric analysis method was used to observe changes in mass of each binder within a nitrogen (N ) atmosphere, where temperature was raised by 5⁰C per minute increments up to 500°C.

[60] In FIG. 5, reference numeral 17 represents changes in mass of carboxymethyl cellulose, and reference numeral 18 represents changes in mass of n-vinyl acetamide polymer. The results indicate that carboxymethyl cellulose began pyrolysis at approximately 25O⁰C, and n-vinyl acetamide polymer began pyrolysis at approximately 350°C. It can therefore be concluded that n-vinyl acetamide polymer has a thermal stability threshold that is approximately 100°C higher.

[61] These results indicate that many benefits can be realized in the manufacture of electrochemical capacitors. First, processing time can be reduced to increase productivity. Because coating of slurry on a current collector is immediately followed by a drying process to evaporate solvent, by increasing the drying temperature, coating time can ultimately be reduced. Second, removal of moisture from within activated carbon is facilitated. Because activated carbon has high adsorbing ability, it readily adsorbs moisture from air. If such moisture is not properly removed, it may become a determining factor in causing non-response within an electrochemical capacitor. This non-response may cause gas generation that shortens the service life of the capacitor. Therefore, an activated carbon electrode for an electrochemical capacitor must always undergo a drying process before being assembled. Here, because an n- vinyl acetamide polymer has higher temperature stability than carboxymethyl cellulose, the drying temperature can be raised to increase efficiency.

[62] [TEST EXAMPLE 4] [63] Two types of slurry were prepared by adding different binders in the same ratio to activated carbon powder, after which viscosities were determined and the slurries were coated, and then the densities of electrodes were observed. The results are shown in Table 4. Water was used as solvent, and the water was controlled in quantity in each test to yield similar viscosities. The slurry viscosities were measured using Brookfield Co' s 44DV-II+ PRO VISCOMETER model, and the measurements were recorded approximately 3 minutes later when the measurements were stabilized.

[64] Table 4 [Table 4] [Table ]

[65] In the results shown in Table 4, while a viscosity of 1680 cP was measured when 200 g of water was introduced for slurry in which carboxymethyl cellulose was added, for a slurry in which an n- vinyl acetamide polymer was introduced to yield a similar viscosity measurement, only 135 g of water was used. This is approximately 68% of comparative example 4.

[66] FIG. 6 shows viscosity stability of slurry, in which reference numeral 19 represents a viscosity change of slurry with carboxymethyl cellulose added, and reference numeral 20 represents a viscosity change of slurry with an n- vinyl acetamide polymer added. No difference was observed between the stabilities of the two slurries over a 24-hour period. That is, it was possible to prepare a stable slurry with less water with the n- vinyl acetamide polymer, which displayed benefits in two aspects that are described below over a related art carboxymethyl cellulose slurry.

[67] First, through dispersion of an active material that is made both uniform and stable with a small quantity of solvent, electrode density increases after drying. As shown in Table 4, embodiment 4 has an approximately 12% higher electrode density than comparative example 4. This indicates that a higher capacitance can be expected when an electrochemical capacitor cell is configured.

[68] Second, because the quantity of solvent within the slurry is small, the drying time following coating can be shortened. That is, through reducing drying time, yield can be increased. It is anticipated that under the same drying conditions, the reduction in solvent can produce a drying time that is reduced to approximately 68%.

[69] [TEST EXAMPLE 5] [70] Using coated electrode slurries of comparative example 4 and embodiment 4 to configure cells, the respective performances were observed. The cells were made into cylindrical type cells with 18 mm diameters, and electrodes were made 200 μm thick. An acetonitrile (AN) organic electrolyte with 1 M TEABF 4 added was used. After initial performance was tested, a service life test was performed in a 7O⁰C oven after 2.7 V was applied. Performance was checked after 200 hours and 500 hours, respectively.

[71] Table 5 [Table 5] [Table ]

[72] The test results in Table 5 show that a cell with an electrode having an n- vinyl acetamide polymer added yielded a capacitance measurement approximately 12% higher than a cell with an electrode having carboxymethyl cellulose added. This is the same value as the electrode density ratio in test example 4. That is, it was noted that through an increase in electrode density, the capacitance of an electric double layer capacitor also increased. ESR was also reduced by approximately 3%. More important results were derived from the service life test.

[73] FIG. 7 shows results of a service life test. Reference numerals 21 and 22 represent capacitance changes of comparative example 4 and embodiment 4, and reference numerals 23 and 24 represent ESR changes of comparative example 4 and embodiment 4. From these results, capacitance reductions due to differences between the two binders are almost identical. This is because the capacitance of an electrochemical capacitor changes due to activated carbon. However, ESR was considerably more stable with the fourth embodiment. The ESR increase rate with the fourth embodiment was measured at about half that of comparative example 4. This is believed to be due to superior electrochemical stability of an n- vinyl acetamide polymer over car- boxymethyl cellulose. That is, when used over prolonged periods under extreme conditions, a cell configured with an electrode having an n- vinyl acetamide polymer added is projected to have a longer service life than a cell configured with an electrode having carboxymethyl cellulose added.

Claims

[1] An electrode for an electrochemical capacitor, comprising a compound coated on a current collector, the compound comprising an active material and a binder, wherein the binder comprises an n-vinyl acetamide polymer.

[2] The electrode of claim 1, wherein the binder further comprises butadiene styrene.

[3] The electrode of claim 1, wherein the binder has a glass transition temperature of

16O⁰C or higher. [4] The electrode of claim 1, wherein 1 to 20 parts by weight of the binder is included for every 100 parts by weight of the active material. [5] The electrode of claim 2, wherein 1 to 10 parts by weight of the n-vinyl acetamide polymer and 1 to 10 parts by weight of the butadiene styrene are used for every 100 parts by weight of the active material as the binder. [6] The electrode of claim 1, wherein the capacitor is an electric double layer capacitor comprising a plurality of symmetrical electrodes, or is a hybrid capacitor comprising a plurality of asymmetrical electrodes. [7] The electrode of claim 6, wherein the electric double layer capacitor is configured with two of the electrodes both comprising the active material that is an activated carbon powder, activated carbon fiber, carbon nanotubes, carbon aero gel, or a combination thereof, and one of the electrodes has a capacitance that is 1 to 3 times that of the other electrode. [8] The electrode of claim 6, wherein the hybrid capacitor is configured with a first of the electrodes comprising the active material that is an activated carbon powder, activated carbon fiber, carbon nanotubes, carbon aero gel, or a combination thereof, and a second of the electrodes comprising the active material that is one or more selected from the groups: (a) Ru, Rh, Pd, Ta, Os, Mo, Mn, Ni, Co, Ir, W, and V;

(b) an alloy, an oxide, a nitride, a carbide, and a hydroxide of the metals in group

(a); (c) a doped conductive polymer; and (d) a graphite-based carbon in which lithium ion is introduced, and the first or the second of the electrodes has a capacitance 10 times or greater than that of the other electrode. [9] A method of preparing an electrode for an electrochemical capacitor, comprising: forming a slurry through mixing a compound comprising an active material and a binder; and forming the electrode through coating the slurry on a current collector. [10] The method of claim 9, wherein the binder comprises an n- vinyl acetamide polymer, butadiene styrene, or a compound thereof.