Electrochemical capacitor

Abstract

Disclosed is an electrochemical capacitor provided with a pair of electrodes and an electrolytic solution. At least one of the pair of electrodes includes a current collector and an active layer provided on the surface of the current collector. The active layer contains activated carbon and a dispersant. The activated carbon includes a first metal. The ratio of the first metal in the activated carbon is 10 ppm or more on a mass basis. The dispersant contains at least one selected from the group consisting of carboxyalkyl cellulose and an ammonium salt of carboxyalkyl cellulose.

Description

electrochemical capacitor

[0001] The present disclosure relates to an electrochemical capacitor including an electrode containing activated carbon and a dispersant in the electrode layer configuration.

[0002] An electrochemical capacitor comprises a pair of electrodes and an electrolyte. At least one of the pair of electrodes contains an active material capable of adsorbing and desorbing ions. An electric double layer capacitor, which is one example of an electrochemical capacitor, has a longer life, is capable of rapid charging, and has superior output characteristics compared to secondary batteries. For these reasons, electric double layer capacitors are widely used as backup power sources and the like.

[0003] Porous carbon particles (hereinafter also referred to as activated carbon) obtained by carbonizing and activating a carbon raw material such as coconut shells are used as electrode active materials for electrochemical capacitors. Various studies have been conducted on activated carbon for use in electrochemical capacitors. For example, Patent Document 1 proposes activated carbon with a total heavy metal content of 20 ppm or less and a total alkali metal content of 200 ppm or less.

[0004] Japanese Patent Application Laid-Open No. 2008-297201

[0005] Even if the amount of metal contained in activated carbon is limited as in Patent Document 1, the float characteristics of the electrochemical capacitor may be easily deteriorated. As such, the relationship between the metal contained in activated carbon and the performance of the electrochemical capacitor has not yet been fully investigated.

[0006] In view of the above, one aspect of the present disclosure relates to an electrochemical capacitor including a pair of electrodes and an electrolyte solution, wherein at least one of the pair of electrodes includes a current collector and an active layer provided on a surface of the current collector, the active layer includes activated carbon and a dispersant, the activated carbon includes a first metal, the ratio of the first metal in the activated carbon is 10 ppm or more by mass, and the dispersant includes at least one selected from the group consisting of carboxyalkyl cellulose and ammonium salts of carboxyalkyl cellulose.

[0007] According to the present disclosure, the floating characteristics of an electrochemical capacitor can be improved.

[0008] FIG. 1 is a schematic perspective view of an electrochemical capacitor according to an embodiment of the present disclosure, with a portion cut away.

[0009] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0010] Technique (1) An electrochemical capacitor according to one aspect of the present disclosure includes a pair of electrodes and an electrolyte. At least one of the pair of electrodes includes a current collector and an active layer provided on the surface of the current collector. The active layer includes activated carbon and a dispersant. The activated carbon includes a first metal. The ratio of the first metal in the activated carbon is 10 ppm or more by mass. The dispersant includes at least one selected from the group consisting of carboxyalkyl cellulose and ammonium salts of carboxyalkyl cellulose. Hereinafter, ppm by mass may be referred to as "ppm by mass." Furthermore, at least one selected from the group consisting of carboxyalkyl cellulose and ammonium salts of carboxyalkyl cellulose may be referred to as the "first carboxyalkyl cellulose component." The first metal may be contained in any form in the activated carbon. The ratio of the first metal is expressed in terms of the first metal element. An electrode including such an active layer may be referred to as the "first electrode."

[0011] In electrochemical capacitors, when ions are adsorbed onto activated carbon in an electrolyte, an electric double layer is formed, thereby generating capacitance. When ions are desorbed from the activated carbon, a non-Faradic current flows. The electrodes of the electrochemical capacitor according to the present disclosure utilize this phenomenon.

[0012] In an electrode (first electrode) of an electrochemical capacitor according to one aspect of the present disclosure, the ratio of a first metal in activated carbon is 10 ppm by mass or more, and the dispersant contains a first carboxyalkyl cellulose component. This configuration can improve the float characteristics of the electrochemical capacitor.

[0013] The float characteristics are an index of the degree of deterioration of an electrochemical capacitor when float charging is performed, maintaining a constant voltage using an external DC power source. The smaller the increase in internal resistance during float charging, the better the float characteristics. The increase in internal resistance is thought to be caused in part by cracks that occur in parts of the active layer during float charging. Furthermore, the more uniform the binding strength in the thickness direction of the active layer, the better the float characteristics.

[0014] In the electrochemical capacitor of the present disclosure, the active layer contains the activated carbon and the dispersant. This configuration improves the dispersion stability of a slurry containing the activated carbon and the dispersant. The active layer formed by applying the slurry to the surface of a current collector exhibits more uniform binding strength in its thickness direction. Therefore, the present disclosure provides an electrochemical capacitor with excellent float characteristics. The active layer is formed, for example, by applying the slurry to the surface of a current collector to obtain a coating, drying the coating, and rolling it. In conventional technologies, the dispersion stability of the slurry is lower than that of the above-described slurry used in the present disclosure, and therefore the uniformity of the binding strength in the thickness direction of the active layer is thought to be lower. When an electrochemical capacitor using an active layer with insufficient uniformity of binding strength in the thickness direction is subjected to float charging, cracks that occur in parts of the active layer increase the internal resistance.

[0015] Technique (2) In the technique (1), the activated carbon may contain mesopores. The ratio of the volume of the mesopores to the total pore volume of the activated carbon (=mesopore volume / total pore volume×100) may be 30% or more.

[0016] Technique (3) In the technique (1) or (2), the activated carbon may contain mesopores. The ratio of the volume of the mesopores to the total pore volume of the activated carbon (=mesopore volume / total pore volume×100) may be 40% or more.

[0017] Technique (4) In any one of the techniques (1) to (3) above, the first metal may include at least one selected from the group consisting of magnesium, calcium, and iron.

[0018] Technique (5) In any one of the techniques (1) to (4), the active layer may contain a second metal. The ratio of the second metal in the active layer may be 500 mass ppm or less. The second metal may be an alkali metal. The second metal may be contained in any form in the active layer. The ratio of the second metal is expressed as a ratio converted into the second metal element.

[0019] Technique (6) In the technique (5), the ratio of the second metal in the active layer may be 100 mass ppm or less.

[0020] Technique (7) In any one of the techniques (1) to (6) above, the carboxyalkyl cellulose and the carboxyalkyl cellulose constituting the ammonium salt may each be carboxymethyl cellulose.

[0021] Technique (8) In any one of the techniques (1) to (7), the active layer may contain a binder. The binder may contain styrene-butadiene rubber.

[0022] Technique (9) In any one of the techniques (1) to (8), the ratio of the first metal in the activated carbon may be 50 ppm by mass or more.

[0023] Technique (10) In any one of the techniques (1) to (9), the activated carbon may include a first activated carbon and a second activated carbon. The mass ratio of the first activated carbon to the total mass of the first activated carbon and the second activated carbon may be 1 mass% or more and 50 mass% or less. The first activated carbon may have an integrated volume of pores having a pore diameter of less than 20 Å obtained by an MP method of 0.5 cm3 or less. ３ / g or less, and the cumulative volume B of pores having a pore diameter of 20 Å or more and 300 Å or less obtained by the BJH method is 0.5 cm ３ The second activated carbon has an integrated volume of pores having a pore diameter of less than 20 Å obtained by the MP method of 0.5 cm ３ / g or more, and the cumulative volume B of pores having a pore diameter of 20 Å or more and 300 Å or less obtained by the BJH method is 0.5 cm ３ / g.

[0024] The electrochemical capacitor of the present disclosure will be described in more detail below, including the above-mentioned techniques (1) to (10), with reference to the drawings as necessary. At least one of the above-mentioned techniques (1) to (10) may be combined with at least one of the elements described below, provided that no technical contradiction exists. Note that the drawings are schematic illustrations, and the dimensional ratios (e.g., thickness) of each component may differ from the actual ratios.

[0025] The electrochemical capacitor of the present disclosure comprises a pair of electrodes and an electrolyte.

[0026] (Electrode) At least one of the pair of electrodes includes a current collector and an active layer. The active layer includes a dispersant and activated carbon. The active layer may further include a binder and a conductive agent.

[0027] (Active Layer) (Dispersant) The dispersant contained in the active layer contains at least one selected from the group consisting of carboxyalkyl cellulose and ammonium salt. The ammonium salt is preferably an ammonium salt of carboxyalkyl cellulose. The active layer (or dispersant) preferably contains at least an ammonium salt of carboxyalkyl cellulose. This is because in these cases, better float characteristics can be obtained. The active layer (or dispersant) may contain an ammonium salt of carboxyalkyl cellulose and carboxyalkyl cellulose.

[0028] Examples of carboxyalkyl celluloses (including carboxyalkyl celluloses that form ammonium salts) include carboxymethyl cellulose (CMC), carboxymethyl ethyl cellulose, and carboxyethyl cellulose. The active layer (or dispersant) may contain one type of carboxyalkyl cellulose or a combination of two or more types. The number of carbon atoms in the alkyl of the carboxyalkyl cellulose is preferably 1 to 4, more preferably 1 or 2. CMC is more preferred from the viewpoints of easy availability and easy acquisition of excellent properties.

[0029] The ammonium salt may be at least one selected from the group consisting of primary ammonium salts, secondary ammonium salts, tertiary ammonium salts, and quaternary ammonium salts. The active layer (or dispersant) may contain one or more ammonium salts. In each of the secondary ammonium salts, tertiary ammonium salts, and quaternary ammonium salts, at least one of the counter anions of the ammonium cation is preferably a carboxyalkyl cellulose anion. The remaining counter anions are, for example, selected from the group consisting of carboxyalkyl cellulose anions, other organic anions, and inorganic anions.

[0030] The ammonium salt is at least a primary ammonium salt of carboxyalkyl cellulose (especially CMC) (specifically, CMC-NH ４ ). A carboxyalkyl cellulose component (sometimes referred to as a second carboxyalkyl cellulose component) containing an alkali metal salt (such as a sodium salt), such as a sodium salt of carboxyalkyl cellulose (CMC-Na), has high dispersion stability as a dispersant. Therefore, when the second carboxyalkyl cellulose component is used as a dispersant, even if part of the slurry dries, it has good re-solubility in water, making it easy to prepare a slurry with little undissolved matter. However, alkali metal salts such as sodium salts remaining in the active layer are prone to deliquescence. Alkali metal salts such as sodium salts containing moisture may corrode the aluminum foil that serves as the current collector. Therefore, it is preferable to use alkali metal salts such as sodium salts in the range of the ratio (ppm by mass) of the second metal, as described below, in terms of alkali metal element. CMC-NH ４ In the case of ammonium salts of carboxyalkyl cellulose such as CMC-NH, part of the ammonium salt volatilizes as ammonia during the drying process of the slurry, and deliquescent salts do not remain in the active layer. ４When an ammonium salt of carboxyalkyl cellulose such as carboxyalkyl cellulose is used as a dispersant in an electrode-forming slurry, the water resistance of the active layer is improved and corrosion of the aluminum foil current collector is suppressed. Deterioration of the internal resistance of the electrochemical capacitor is also suppressed. The same effect as above can be obtained when using a free carboxyalkyl cellulose such as CMC. From the perspective of ensuring higher dispersibility in the slurry, it is preferable to use an ammonium salt of carboxyalkyl cellulose in the preparation of the slurry. In this case, the active layer of the electrochemical capacitor may contain the ammonium salt of carboxyalkyl cellulose, or it may be contained in the form of carboxyalkyl cellulose due to the volatilization of ammonia. Furthermore, some carboxy groups of carboxyalkyl cellulose such as CMC that have lost the ammonium salt due to the volatilization of ammonia form hydrogen bonds with molecules of the carboxyalkyl cellulose (e.g., CMC). A strong binding layer is formed so that the hydrogen-bonded molecules surround the activated carbon particles, thereby improving the binding strength of the entire active layer. From the above, it is preferable to use a primary ammonium salt of carboxyalkyl cellulose (e.g., CMC) in order to achieve both dispersion stability and water resistance, and CMC-NH ４ As the dispersant, the first carboxyalkyl cellulose component may be used alone or in combination of two or more types.

[0031] (Activated Carbon) The ratio of the first metal in the activated carbon is 10 mass ppm or more, preferably 50 mass ppm or more, and more preferably 100 mass ppm or more. When the ratio of the first metal in the activated carbon is 10 mass ppm or more, a part of the first metal dissolves in the slurry and becomes a cation. Generally, CMC-NH ４When using ammonium salts such as those mentioned above, ammonia gas may volatilize during the process of forming the active layer, which may result in a decrease in dispersion stability. However, in the present invention, the activated carbon contains the first metal within the above range, so that the first metal acts as a cation of the carboxyalkyl cellulose, thereby maintaining the dispersion stability of the carboxyalkyl cellulose. As a result, even when using free carboxyalkyl cellulose or when using an ammonium salt and ammonia gas volatilizes, the binding strength in the thickness direction of the active layer is made uniform.

[0032] The first metal may include at least one selected from the group consisting of magnesium, calcium, and iron. When the first metal includes or is the above metal species, the free carboxyalkyl cellulose is likely to function as a metal salt, and the dispersion stability of the carboxyalkyl cellulose is likely to be maintained. Furthermore, the first metal has less effect on the corrosion of the aluminum foil used in the current collector than alkali metals such as sodium, and is less likely to cause deterioration of float characteristics.

[0033] The activated carbon may contain micropores. Micropores refer to pores with a pore diameter of less than 20 Å obtained by the MP method described below. The capacity of an electrochemical capacitor is proportional to the amount of ions adsorbed to the activated carbon. Therefore, activated carbon with a larger ratio of micropore volume to total pore volume can obtain a larger capacity.

[0034] On the other hand, the float characteristics of an electrochemical capacitor are related to the pore structure of the activated carbon (total pore volume, mesopore volume, etc.) and the ease of movement of ionic substances contained in the electrolyte. Mesopores refer to pores with a pore diameter of 20 Å or more and 300 Å or less, obtained by the BJH method described below.

[0035] The ratio of mesopore volume to total pore volume (= mesopore volume / total pore volume × 100) is preferably 30% or more, and more preferably 40% or more. When the ratio of mesopore volume to total pore volume of activated carbon is within the above range, the mobility of ions in the electrolyte within the pores increases, improving float characteristics. In other words, ions in the electrolyte are more easily diffused within the pores of the activated carbon, making the pores less likely to clog, thereby improving float characteristics. The ratio of mesopore volume to total pore volume (= mesopore volume / total pore volume × 100) may be 70% or less.

[0036] The pore volume of the activated carbon is determined by measuring the pore distribution of the activated carbon sample. When collecting a sample from an electrochemical capacitor, a fully discharged, unused or initial electrochemical capacitor is used. The electrochemical capacitor is disassembled to remove the electrodes, and the active layer is peeled off from the current collector. The peeled active layer is pulverized to obtain a pulverized sample (particle group). The collected sample is heated and dried at 150°C to obtain a sample used for measuring the pore distribution. An initial electrochemical capacitor is an electrochemical capacitor that has been assembled, aged, or subjected to break-in charging and discharging, and then brought to a fully discharged state. When using a commercially available electrochemical capacitor, a fully discharged electrochemical capacitor is used as the initial electrochemical capacitor.

[0037] The pore size distribution is measured by a gas adsorption method using nitrogen gas. For example, an automatic specific surface area / pore size distribution measuring device "Tristar II 3020" manufactured by Shimadzu Corporation is used as the measuring device. Note that, in order to remove impurities, the sample is pretreated by heating and evacuating to a vacuum (for example, 250°C and 50 mTorr or less) before the measurement. The pore size distribution is analyzed by the BJH method (Barrett-Joyner-Halenda method), which uses the Harkins & Jura equation. Using the cumulative pore volume distribution obtained by the BJH method, the mesopore volume (cm) per 1 g of the sample is calculated. ３On the other hand, the MP method utilizes "t-plot" (B.C. Lippens, J.H. de Boer, J.Catalysis, 4319 (1965)) to obtain the micropore distribution and the micropore volume (cm ３ ) is required.

[0038] Although samples taken from electrochemical capacitors may contain binders and conductive agents in addition to activated carbon, the amount of binders is small, and the effect on the pore distribution of activated carbon is small. Specifically, the shape of the cumulative pore volume distribution curve for samples taken from electrochemical capacitors is almost the same as that for samples containing only activated carbon, except that the cumulative pore volume distribution curve is slightly shifted downward (the cumulative volume is slightly smaller) compared to samples containing only activated carbon.

[0039] Activated carbon can be produced, for example, by carbonizing a raw material through heat treatment and then activating the resulting carbonized material to make it porous. Examples of raw materials include wood, coconut shells, pulp waste liquid, coal or coal-based pitch obtained by thermal decomposition thereof, heavy oil or petroleum-based pitch obtained by thermal decomposition thereof, phenolic resin, petroleum coke, and coal coke. Examples of activation treatments include gas activation using gas such as steam, activation using zinc chloride (ZnCl ２ Examples of chemical activation include the use of an acid such as ammonium hydroxide (Ammonium hydroxide) or an alkali such as potassium hydroxide (KOH). The activated carbon obtained by the activation treatment may be subjected to a pulverization treatment. After the pulverization treatment, classification may be performed. For example, a ball mill, a jet mill, or the like is used for the pulverization treatment.

[0040] The pore distribution of activated carbon can be adjusted by the raw material, heat treatment temperature, activation temperature during gas activation, degree of pulverization, etc. Chemical activation of carbonized material obtained by heat-treating wood-based raw materials such as sawdust makes it easier to obtain activated carbon with a large ratio of mesopore volume to total pore volume. On the other hand, steam activation of carbonized material obtained by heat-treating coconut shells makes it easier to obtain activated carbon with a large ratio of micropore volume. In the present disclosure, activated carbon is not limited by the raw material or activation method of activated carbon. For activated carbon, the production conditions, such as raw materials and activation method, may be appropriately selected so that the ratio of mesopore volume to total pore volume is within the range of 30% or more, for example. Furthermore, two or more types of activated carbon may be used in combination.

[0041] (Second Metal) The active layer may contain a second metal. The ratio of the second metal in the active layer is preferably 500 ppm by mass or less, more preferably 100 ppm by mass or less. The second metal may also be an alkali metal. A portion of the second metal contained in the active layer also dissolves in the slurry to form a salt, which can improve the dispersion stability of the first carboxyalkyl cellulose component. However, when the second metal is an alkali metal, the aluminum foil used as the current collector may be easily corroded. The ratio of the second metal in the active layer is preferably 500 ppm by mass or less, more preferably 100 ppm by mass or less, from the viewpoint of suppressing corrosion of the aluminum foil. When the ratio of the second metal in the active layer is within this range, better float characteristics can be obtained.

[0042] (Other Components) The active layer may contain a binder. Examples of binders include resin binders. Examples of resin binders include thermoplastic resins and thermosetting resins. Examples of binders include fluororesin materials such as polytetrafluoroethylene (PTFE), acrylic resin materials such as polyacrylic acid, and rubber materials such as styrene-butadiene rubber (SBR). The active layer may contain one type of binder, or two or more types of binders. From the viewpoint of further improving the binding strength of the active layer, the binder preferably contains styrene-butadiene rubber. For example, 50% by mass or more (preferably 80% by mass or more) of the binder may be styrene-butadiene rubber. The proportion of styrene-butadiene rubber in the binder is 100% by mass or less. It is also preferable that the binder is styrene-butadiene rubber. When styrene-butadiene rubber is used as the binder, cracks that may occur in the active layer and peeling at the interface between the active layer and the current collector are suppressed, thereby suppressing an increase in internal resistance.

[0043] The conductive agent may be conductive carbon or the like. For example, carbon black such as acetylene black is used as the conductive agent. The active layer may contain one type of conductive agent or two or more types of conductive agents.

[0044] (Other) As described above, the electrochemical capacitor of the present disclosure includes a pair of electrodes and an electrolyte. At least one of the pair of electrodes includes a current collector and an active layer provided on the surface of the current collector. The active layer includes activated carbon and a dispersant. The activated carbon includes a first metal. The ratio of the first metal in the activated carbon is 10 mass ppm or more. The dispersant includes at least one selected from the group consisting of carboxyalkyl cellulose and ammonium salts of carboxyalkyl cellulose. In an electrode (first electrode) including such an active layer, the active layer may include a binder and a conductive agent in addition to the activated carbon and the dispersant. That is, the active layer may be formed of a mixture (composite) of activated carbon, dispersant, binder, and conductive agent. When the active layer is formed of a composite, the ratio of activated carbon in the active layer is, for example, 88 mass% or more. The ratio of activated carbon in the active layer may be less than 100 mass%.

[0045] The electrode (first electrode) of the electrochemical capacitor of the present disclosure is obtained by forming an active layer on the surface of a current collector. The active layer can be obtained, for example, by applying a slurry containing the components of the active layer (such as activated carbon and a dispersant) to the surface of the current collector to obtain a coating, and then drying and rolling the coating. The slurry may further include a binder and a conductive agent.

[0046] The current collector may be made of a metal foil such as aluminum foil, aluminum alloy foil, etc. The surface of the current collector may be roughened by etching or other techniques.

[0047] Examples of electrochemical capacitors include electric double layer capacitors (EDLCs) and lithium ion capacitors. When the electrochemical capacitor is an EDLC, a first electrode (preferably a first electrode containing activated carbon having a mesopore volume ratio to the total pore volume of 30% or more) can be used for at least one of a pair of electrodes. When the electrochemical capacitor is an EDLC, both of the pair of electrodes may be first electrodes. When the electrochemical capacitor is a lithium ion capacitor, the first electrode can be used for one of the pair of electrodes (positive electrode), and a negative electrode used in a lithium ion secondary battery can be used for the other of the pair of electrodes (negative electrode). The negative electrode used in a lithium ion secondary battery includes, for example, a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

[0048] (Electrolyte) The electrolyte contains a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and contains cations and anions. The ionic substance may contain, for example, a low-melting-point compound (ionic liquid) that can exist as a liquid at around room temperature. The concentration of the ionic substance in the electrolyte is, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0049] The solvent is preferably a high-boiling solvent. For example, lactones such as γ-butyrolactone, carbonates such as propylene carbonate, polyhydric alcohols such as ethylene glycol and propylene glycol, cyclic sulfones such as sulfolane, amides such as N-methylacetamide, N,N-dimethylformamide, and N-methyl-2-pyrrolidone, esters such as methyl acetate, ethers such as 1,4-dioxane, ketones such as methyl ethyl ketone, and formaldehyde can be used. The electrolytic solution may contain one solvent or a combination of two or more solvents.

[0050] Ionic substances include, for example, organic salts. Organic salts are salts in which at least one of the anion and cation contains an organic substance. Examples of organic salts in which the cation contains an organic substance include quaternary ammonium salts. Examples of organic salts in which the anion (or both ions) contains an organic substance include trimethylamine maleate, triethylamine borodisalicylate, ethyldimethylamine phthalate, mono-1,2,3,4-tetramethylimidazolinium phthalate, and mono-1,3-dimethyl-2-ethylimidazolinium phthalate.

[0051] From the viewpoint of improving the withstand voltage characteristics, the anion preferably includes an anion of a fluorine-containing acid. Examples of the anion of a fluorine-containing acid include BF ４ － and / or PF ６ － The organic salt preferably contains, for example, a tetraalkylammonium cation and a fluorine-containing acid anion. Specifically, diethyldimethylammonium tetrafluoroborate (DEDMABF ４ ), triethylmethylammonium tetrafluoroborate (TEMABF ４ ) etc.

[0052] The electrolyte may contain one or more ionic substances, one or more anions, and one or more cations.

[0053] (Separator) It is desirable to interpose a separator between the pair of electrodes. The separator has ion permeability and serves to physically separate the pair of electrodes to prevent short-circuiting. For example, a nonwoven fabric, a glass fiber mat, or a microporous film of a polyolefin such as polyethylene is used as the separator. The nonwoven fabric may be a nonwoven fabric whose main component is cellulose, or a nonwoven fabric made of synthetic fibers. A laminate of a nonwoven fabric and a microporous film may also be used as the separator.

[0054] An electrochemical capacitor according to an embodiment of the present disclosure will be described below with reference to Fig. 1. Fig. 1 is a schematic perspective view of an electrochemical capacitor according to an embodiment of the present disclosure, with a portion cut away. Note that the present disclosure is not limited to the electrochemical capacitor of Fig. 1.

[0055] The electrochemical capacitor 10 in Fig. 1 is an electric double layer capacitor and includes a wound-type capacitor element 1. The capacitor element 1 is configured by winding a sheet-like first electrode 2 and a sheet-like second electrode 3 with a separator 4 interposed therebetween. The first electrode 2 and the second electrode 3 include a first current collector and a second current collector made of metal, respectively, with a first active layer and a second active layer carried on the surfaces thereof. The electrochemical capacitor 10 generates capacitance when the electrodes 2 and 3 adsorb and desorb ions.

[0056] The current collector may be made of, for example, aluminum foil. The surface of the current collector may be roughened by etching or other techniques. The separator 4 may be made of, for example, a nonwoven fabric primarily composed of cellulose. A first lead wire 5a and a second lead wire 5b are connected to the first electrode 2 and the second electrode 3, respectively, as lead members. The capacitor element 1 is housed in a cylindrical outer case 6 together with an electrolyte (not shown).

[0057] Examples of materials for the exterior case 6 include metals such as aluminum, stainless steel, copper, iron, and brass. The opening of the exterior case 6 is sealed with a sealing member 7. The first lead wire 5a and the second lead wire 5b are led out to the outside so as to pass through the sealing member 7. The sealing member 7 is made of, for example, a rubber material such as butyl rubber.

[0058] In the above embodiment, a wound type capacitor has been described, but the scope of application of the present invention is not limited to the above, and it can also be applied to capacitors of other structures, such as stacked type or coin type capacitors.

[0059] [Examples] Hereinafter, the electrochemical capacitor of the present disclosure will be described in more detail based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0060] (Preparation of activated carbon) Activated carbons A to I used in the examples and comparative examples were produced using the raw materials shown in Table 1. More specifically, the raw materials were carbonized by heat treatment, and the resulting carbonized material was activated to make it porous, and then washed with hydrochloric acid to produce activated carbon. Table 1 shows the conditions used for activated carbons A to I, such as the raw materials, carbonization temperature, activation method, and washing method.

[0061]

[0062] Examples 1 to 7 As electrochemical capacitors, wound-type electric double layer capacitors with a rated voltage of 2.7 V were fabricated. A specific method for fabricating the electrochemical capacitors will be described below.

[0063] (Electrode Preparation) 88 parts by mass of the active material shown in Table 2, 2 parts by mass of binder, 4 parts by mass of dispersant (solid content ratio 5% by mass) swollen in water, and 6 parts by mass of conductive agent were dispersed in water to prepare a slurry. The obtained slurry was applied to an aluminum foil to form a coating film. The obtained coating film was vacuum dried at 110°C and rolled to form an active layer on the surface of the aluminum foil. In this way, an electrode was obtained.

[0064] (Preparation of Electrolyte) A solution of diethyldimethylammonium tetrafluoroborate (DEDMABF) in γ-butyrolactone (GBL) was prepared. ４ ) was dissolved in the electrolyte to prepare an electrolyte solution. ４ The concentration was set to 1.0 mol / L.

[0065] (Preparation of Electrochemical Capacitor) A pair of electrodes was prepared, each connected to a lead wire, and wound with a cellulose nonwoven separator to form a capacitor element. The capacitor element was housed in a specified outer case together with an electrolyte solution and sealed with a sealing material to complete an electrochemical capacitor (electric double layer capacitor). Subsequently, an aging treatment was performed at 60°C for 5.5 hours while applying a rated voltage.

[0066] Comparative Examples 1 to 4 In Comparative Examples 1, 3, and 4, a mixture slurry was prepared in the same manner as in Example 1, except that the types of activated carbon and dispersant were changed, and electrochemical capacitors were fabricated using the obtained electrodes.

[0067] In Comparative Example 2, no dispersant was added. As a result, the viscosity of the mixture slurry was low, which deteriorated the coating properties, and the mixture could not be uniformly coated on the current collector to the desired thickness. Because of the above, it was not possible to prepare an electrode y2 corresponding to Comparative Example 2, and therefore evaluation of the electrochemical capacitor of Comparative Example 2 was discontinued.

[0068] In the preparation of the above electrodes, electrodes x1 to x7 were obtained using an active material in which activated carbon, a dispersant, a binder, and a conductive agent were mixed in a predetermined mass ratio (see Table 2 below). Examples 1 to 7 are electrochemical capacitors each including a pair of electrodes x1 to x7.

[0069] Activated carbon, a dispersant, a binder, and a conductive agent were mixed in a predetermined mass ratio to obtain active materials y1 to y4 (see Table 2 below). Comparative Examples 1 to 4 are electrochemical capacitors each having a pair of electrodes y1 to y4.

[0070] Table 2 shows the types of activated carbon, dispersant, binder, and conductive agent used in the active layer of each electrode of the electrochemical capacitors of Examples 1 to 7 and Comparative Examples 1 to 4.

[0071]

[0072] Table 3 also shows the ratio of the mesopore volume to the total pore volume of the activated carbon contained in the active layer of each electrode of the electrochemical capacitors of Examples 1 to 7 and Comparative Examples 1 to 4, the type and amount of the first metal contained in the activated carbon, and the type and amount of the second metal in the active layer.

[0073]

[0074] The electrochemical capacitors obtained above were evaluated as follows.

[0075] [Evaluation 1] (Measurement of binding strength in the thickness direction of the active layer) The binding strength was evaluated using the SAICAS (Surface and Interfacial Cutting Analysis System) method, which involves cutting the sample surface with a fine, sharp cutting blade while controlling the depth position, and measuring the stress on the blade.

[0076] <Evaluation equipment> Cycas surface and interface analysis equipment Equipment company name: Daipla Wintes Co., Ltd. Equipment model name: DN-GS

[0077] <Measurement Method> The electrode was cut into 20 mm squares to prepare samples for SAICAS evaluation. The cutting strength of the evaluation samples was measured using a cutting strength measuring device, Cycas (registered trademark) DN-GS (manufactured by Daipla Wintes Co., Ltd.). A ceramic blade made of Borazon material was used as the cutting blade, with a blade width of 2.0 mm, a blade angle of 20 degrees, a clearance angle of 10 degrees, and measurement speeds of 0.5 μm / sec vertically and 10 μm / sec horizontally at constant speeds. The Borazon blades used were new, and all were confirmed to be defect-free in advance. The measurement temperature was room temperature (25°C), and the sample (electrode) temperature was also 25°C. Cutting strength measurements were performed on the electrode active layer in a predetermined region at least 0.2 μm away from the surface of the electrode active layer, for example, a predetermined region 0.2 μm away.

[0078] The cutting strength was calculated by calculating the average value and the population standard deviation σ from 50 seconds to 200 seconds after a certain depth using the data software of the Cycas device. The coefficient of variation was calculated by dividing the average value by the population standard deviation σ.

[0079] [Evaluation 2] (Measurement of internal resistance of electrochemical capacitor) In an environment of −30° C., constant current charging was performed at a current of 200 mA until the voltage reached 2.35 V, and then the state in which a voltage of 2.35 V was applied was maintained for 7 minutes. Thereafter, in an environment of −30° C., constant current discharging was performed at a current of 100 mA until the voltage reached 0 V.

[0080] Using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained from the discharge, a linear approximation line is calculated in the range of 0.5 to 2 seconds after the start of discharge on the discharge curve, and the voltage V at the intercept of the linear approximation line is calculated. Ｓ The voltage V at the start of discharge (0 seconds after the start of discharge) was calculated. ０ to voltage V Ｓ The value obtained by subtracting (V ０ -V Ｓ ) was calculated as ΔV. ΔV (V) and the current value during discharge (current density per positive electrode area 2 mA / cm ２ × positive electrode area) Id was used to calculate the internal resistance (DCR) R1 (Ω) of the electrochemical capacitor according to the following formula (A).

[0081] Internal resistance R1=ΔV / Id (A)

[0082] (Float Test of Electrochemical Capacitor) In an environment of 50°C, constant current charging was performed at a current of 100 mA until the voltage reached 2.7 V, and then the voltage of 2.7 V was maintained for 3000 hours. The electrochemical capacitor was stored in this state with a voltage of 2.7 V applied. Thereafter, constant current discharging was performed at a current of 20 mA in an environment of 25°C until the voltage reached 0 V.

[0083] (Measurement of Internal Resistance of Electrochemical Capacitor After Float Test) Thereafter, charge and discharge were performed in an environment of −30° C. in the same manner as before the float test, and the internal resistance (DCR) R2 (Ω) of the electrochemical capacitor after the float test was determined. The internal resistance R2 (Ω) was determined in accordance with (A) above.

[0084] The coefficient of variation of the binding strength of the active layer and the internal resistance DCR after the float test for Examples 1 to 7 each including a pair of electrodes X1 to X7 and Comparative Examples 1 to 4 each including a pair of electrodes Y1 to Y4 are shown in Table 4. The coefficient of variation of the binding strength of the active layer is expressed as a relative value, with the value for Comparative Example 1 taken as 1. The DCR after the float test is expressed as a relative value, with the value for Comparative Example 1 taken as 100.

[0085]

[0086] In Example 1, the active layer of the electrode x1 was made of CMC-NH ４ Furthermore, since activated carbon A containing 10 ppm or more of the first metal was used, it exhibited better float characteristics than Comparative Example 1, which used activated carbon H containing less than 10 ppm of the first metal. ４ The active layer of electrode x1, in which the dispersibility of the active layer was improved, had a smaller coefficient of variation of the binding strength than that of electrode y1, and a more uniform binding strength was obtained in the thickness direction of the active layer, thereby suppressing deterioration of the internal resistance.

[0087] In Example 2, the mesopore volume of the activated carbon B contained in the active layer of the electrode x2 was 30% or more, which was larger than the mesopore volume of the electrode x1. Therefore, good ion movement in the electrolyte was ensured within the pores of the activated carbon, and better float characteristics were exhibited than in Example 1.

[0088] In Example 3, the amount of the second metal contained in the active layer was 500 ppm or less, which was less than the amount of the second metal contained in the active layer in Example 2. Therefore, the influence of corrosion of the aluminum foil by the second metal in the active layer was small, and deterioration of the float characteristics was suppressed.

[0089] In Example 4, activated carbon D containing 50 ppm or more of the first metal was used in the active layer of electrode x4, and therefore, better float characteristics were exhibited than in Example 3, which used activated carbon C containing less than 50 ppm of the first metal. The active layer of electrode x4 had an even smaller coefficient of variation in binding strength than that of electrode x3, and a more uniform binding strength was obtained in the thickness direction of the active layer, thereby suppressing deterioration of internal resistance.

[0090] In Example 5, the amount of the second metal contained in the active layer was smaller than the amount of the second metal contained in the active layer of Example 4, so the corrosion effect of the second metal in the active layer on the aluminum foil was less, and the deterioration of the float characteristics was suppressed.

[0091] In Example 6, the mesopore volume of the activated carbon F contained in the active layer of the electrode x6 was 40% or more, which was larger than the mesopore volume of the activated carbon E contained in the active layer of the electrode x5. Therefore, good ion movement in the pores of the activated carbon was ensured in the electrolyte, and better float characteristics were exhibited than in Example 5.

[0092] In Example 7, the binder contained in the active layer of electrode x7 was SBR, which improved the binding strength of the active layer compared to the PTFE binder contained in the active layers of electrodes x1 to x6. Furthermore, the coefficient of variation of the binding strength was even smaller than that of electrodes x1 to x6, and a more uniform binding strength was obtained in the thickness direction of the active layer, thereby suppressing deterioration of internal resistance.

[0093] In Comparative Example 3, the active layer of the electrode y3 was CMC-NH ４ It is believed that the lack of CMC-NH as a dispersant in the electrode-forming slurry resulted in insufficient bonding strength of the entire active layer, and this led to the deterioration of the internal resistance due to cracks. ４ Since CMC-Na was used instead of , the amount of the second metal in the active layer of electrode y3 was 500 ppm or more, and the float characteristics were deteriorated due to the corrosion of the aluminum foil by the second metal in the active layer.

[0094] In Comparative Example 4, the active layer of the electrode y4 was CMC-NH ４ It is believed that the lack of CMC-NH as a dispersant in the electrode-forming slurry resulted in insufficient bonding strength of the entire active layer, and this led to the deterioration of the internal resistance due to cracks. ４ However, because CMC-Na was used instead of CMC-Na, the amount of the second metal in the active layer of electrode y4 was 500 ppm or more, and the float characteristics deteriorated due to the corrosion of the aluminum foil by the second metal in the active layer. Furthermore, the mesopore volume of activated carbon I contained in the active layer of electrode y4 was less than 30%, making it difficult for ions in the electrolyte to diffuse within the pores, and good ion movement could not be ensured, resulting in a significant deterioration in the float characteristics.

[0095] From the above, it was found that in Examples 1 to 7, the active layer contained activated carbon and a dispersant, the activated carbon contained a first metal, the ratio of the first metal in the activated carbon was 10 ppm by mass or more, and the dispersant contained a first carboxyalkyl cellulose component, and therefore excellent float characteristics were obtained.

[0096] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0097] The electrochemical capacitor according to the present disclosure is suitable for use in applications requiring excellent float characteristics, but the applications of the electrochemical capacitor are not limited to this.

[0098] 1: capacitor element, 2: first electrode, 3: second electrode, 4: separator, 5a: first lead wire, 5b: second lead wire, 6: exterior case, 7: sealing member, 10: electrochemical capacitor

Claims

1. An electrochemical capacitor comprising a pair of electrodes and an electrolytic solution, wherein at least one of the pair of electrodes includes a current collector and an active layer provided on the surface of the current collector, the active layer includes activated carbon and a dispersant, the activated carbon contains a first metal, the ratio of the first metal in the activated carbon is 10 ppm or more on a mass basis, and the dispersant includes at least one selected from the group consisting of carboxyalkyl cellulose and an ammonium salt of carboxyalkyl cellulose.

2. The electrochemical capacitor according to claim 1, wherein the activated carbon includes mesopores, and the ratio of the volume of the mesopores to the total pore volume is 30% or more.

3. The electrochemical capacitor according to claim 1 or 2, wherein the activated carbon includes mesopores, and the ratio of the volume of the mesopores to the total pore volume is 40% or more.

4. The electrochemical capacitor according to claim 1 or 2, wherein the first metal includes at least one selected from the group consisting of magnesium, calcium, and iron.

5. The electrochemical capacitor according to claim 1 or 2, wherein the active layer contains a second metal, the ratio of the second metal in the active layer is 500 ppm or less on a mass basis, and the second metal is an alkali metal.

6. The electrochemical capacitor according to claim 5, wherein the ratio of the second metal in the active layer is 100 ppm or less on a mass basis.

7. The electrochemical capacitor according to claim 1 or 2, wherein the carboxyalkyl cellulose and the carboxyalkyl cellulose constituting the ammonium salt are each carboxymethyl cellulose.

8. The electrochemical capacitor according to claim 1 or 2, wherein the active layer contains a binder, and the binder includes styrene-butadiene rubber.

9. The electrochemical capacitor according to claim 1 or 2, wherein the ratio of the first metal in the activated carbon is 50 ppm or more on a mass basis.

10. The activated carbon includes first activated carbon and second activated carbon. In the total mass of the first activated carbon and the second activated carbon, the ratio of the mass of the first activated carbon is 1% by mass or more and 50% by mass or less. The first activated carbon has a cumulative volume of pores with a pore diameter of less than 20 Å obtained by the MP method of less than 0.5 cm ３ / g, and a cumulative volume B of pores with a pore diameter of 20 Å or more and 300 Å or less obtained by the BJH method of 0.5 cm ３ / g or more. The second activated carbon has a cumulative volume of pores with a pore diameter of less than 20 Å obtained by the MP method of 0.5 cm ３ / g or more, and a cumulative volume B of pores with a pore diameter of 20 Å or more and 300 Å or less obtained by the BJH method of less than 0.5 cm ３ / g. The electrochemical capacitor according to claim 1 or 2.

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