Electrode for electrochemical device and electrochemical device

Abstract

Disclosed is an electrode for electrochemical devices that includes an active layer, the active layer containing porous carbon particles and a binder. The porous carbon particles have an average particle diameter of 0.5-1.0 μm. The binder contains: at least one type of carboxymethylcellulose selected from carboxymethylcellulose and salts of carboxymethylcellulose; and, rubber. The amount of mass depletion based on the carboxymethylcellulose and the rubber when thermogravimetric analysis of the active layer is performed is 8.2-21.3%, inclusive, of the mass of the active layer.

Description

Electrode for electrochemical device and electrochemical device

[0001] The present disclosure relates to an electrode for an electrochemical device and an electrochemical device.

[0002] An example of an electrochemical device used as an electricity storage device includes a pair of electrodes and an electrolyte. At least one of the pair of electrodes includes an active material capable of adsorbing and desorbing ions. Electrochemical devices such as electric double layer capacitors have a longer life, are capable of rapid charging, and have superior output characteristics compared to secondary batteries. For this reason, electrochemical devices are widely used as backup power sources and the like.

[0003] Improvements in the characteristics of electrochemical devices using porous carbon particles (such as activated carbon) have been studied. For example, Patent Document 1 (WO 2006 / 068291) discloses "an electric double layer capacitor comprising electrodes made of activated carbon, a separator, and a non-aqueous electrolyte, characterized in that the activated carbon has an average particle size of 0.1 μm or more and less than 1.0 μm."

[0004] Patent Document 2 (Japanese Patent No. 6569526) discloses "an electrode obtained by removing the solvent from a solution in which fibrous carbon and porous carbon powder having an average particle size of less than 100 nm are dispersed, by filtration under reduced pressure, wherein the particle size distribution of the aggregates of the carbon powder and fibrous carbon constituting the electrode has a single peak, and the ratio D90 / D50 of the particle size at the 50% cumulative value D50 to the particle size at the 90% cumulative value D90 of the particle size distribution is 2.1 or less." Patent Document 2 also describes the proportion of mesopores among the pores in the porous carbon powder.

[0005] International Publication No. 2006 / 068291 Patent No. 6569526

[0006] Currently, there is a demand for improved performance of electrochemical devices, particularly for achieving both high capacity and high reliability. In this situation, one of the objects of the present disclosure is to provide an electrochemical device that can achieve both high capacity (e.g., high capacity at low temperatures) and high reliability, and an electrode used therein.

[0007] One aspect of the present disclosure relates to an electrode for an electrochemical device including an active layer, wherein the active layer includes porous carbon particles and a binder, the porous carbon particles having an average particle size in the range of 0.5 μm to 1.0 μm, the binder including at least one carboxymethylcellulose selected from carboxymethylcellulose and a salt of carboxymethylcellulose, and a rubber, and when thermogravimetric analysis of the active layer is performed, the amount of mass loss due to the carboxymethylcellulose and the rubber is 8.2% or more and 21.3% or less of the mass of the active layer.

[0008] Another aspect of the present disclosure relates to an electrochemical device including a pair of electrodes and an electrolyte, wherein at least one of the pair of electrodes is the electrode for electrochemical devices according to any one of claims 1 to 4.

[0009] According to the present disclosure, an electrochemical device that can achieve both high capacity and high reliability, and an electrode used therein, can be obtained.

[0010] FIG. 1 is a perspective cutaway view of an electrochemical device according to one embodiment of the present disclosure.

[0011] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "greater than or equal to A and less than or equal to B." In the following description, when lower and upper limits are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits can be arbitrarily combined with any of the exemplified upper limits, as long as the lower limit is not greater than or equal to the upper limit.

[0012] (Electrode for Electrochemical Device) An electrode for an electrochemical device according to one embodiment of the present disclosure is an electrode including an active layer. This electrode may be referred to hereinafter as "electrode (E)." The active layer included in electrode (E) includes porous carbon particles and a binder. The porous carbon particles have an average particle size in the range of 0.5 μm to 1.0 μm. The binder includes at least one carboxymethylcellulose selected from carboxymethylcellulose and a salt of carboxymethylcellulose, and a rubber. When thermogravimetric analysis of the active layer is performed, the amount of mass loss due to the carboxymethylcellulose and the rubber is 8.2% or more and 21.3% or less of the mass of the active layer.

[0013] Examples of electrochemical devices will be described later. The electrode (E) may include an active layer and a current collector supporting the active layer. There are no particular limitations on the current collector, and any known current collector used in electrodes for electrochemical devices may be used.

[0014] Examples of the current collector include metal foils such as aluminum foil. The surface of the current collector is preferably roughened. There are no particular limitations on the roughening method, and the surface may be roughened by a known method (e.g., etching).

[0015] The electrode (E) may further include an aluminum foil (current collector) supporting the active layer, and the surface of the aluminum foil may be roughened. In this case, the capacitance of the aluminum foil (current collector) is 30 μF / cm ２There is no particular limitation on the upper limit of the capacitance, but it is preferably 120 μF / cm ２ It may be the following:

[0016] Capacitance (μF / cm ２ ) is the unit area of ​​the current collector (1 cm ２ ) is the capacitance per unit of the current collector (μF / cm ２ ) is the sum of the capacitance of one surface of the current collector and the capacitance of the other surface. In other words, if the capacitance of a current collector whose both surfaces are similarly roughened is X μF / cm ２ , the capacitance of one side is substantially 0.5×μF / cm ２ is.

[0017] The capacitance of the aluminum foil (current collector) (μF / cm ２ ) is measured by the following method. First, an aluminum foil test piece is prepared. The shape and size of the test piece are not particularly limited, but it has a measurement portion and a lead-out portion for measuring capacitance. The measurement portion is the portion immersed in the measurement solution. The shape and size of the measurement portion are not particularly limited, but a rectangular shape is preferable, with dimensions of, for example, 10 mm x 50 mm. The lead-out portion is the portion not immersed in the measurement solution. The lead-out portion may be a portion cut out integrally with the measurement portion. The shape and size of the lead-out portion are arbitrary. The measurement solution is an aqueous solution prepared by dissolving 80 g of ammonium pentaborate in 1 L of water. The capacitance measurement device complies with JIS C 5101-1. The accuracy is ±2% of the measured value, the measurement frequency is 120 Hz ±5%, and the measurement voltage is 0.5 Vrms or less. The measurement tank containing the measurement solution is a 200 mL or 300 mL tall glass beaker conforming to JIS R 3503. The temperature of the measurement solution is set to 30°C ± 1°C, and the measurement portions of a pair of test pieces of the same shape and size are immersed in the measurement solution, and the extended portions of each are connected to a capacitance meter conforming to JIS C 5101-1 to measure the capacitance. The test pieces (measurement portions) can be oriented in any direction in the measurement solution, but a distance of 5 mm ± 2 mm is provided between them. In this way, the capacitance of the current collector is measured.

[0018] The active layer contains porous carbon particles as the active material and a binder as essential components. The active layer may optionally contain other components such as a conductive agent. Examples of the conductive agent include carbon black such as acetylene black.

[0019] When ions are adsorbed onto the porous carbon particles in the electrolyte, an electric double layer is formed, and capacitance is generated. When ions are desorbed from the porous carbon particles, a non-Faradic current flows. The electrode (E) utilizes this phenomenon.

[0020] The electrode (E) uses porous carbon particles with a smaller average particle size than conventional electrodes. Furthermore, a unique combination of carboxymethyl cellulose and rubber is selected as the binder, and the content is further limited. Numerous substances are known as binders for electrodes in electrochemical devices, and countless combinations exist. After further investigation, the present inventors have newly discovered that by combining porous carbon particles with a small average particle size with the specific combination and specific content of the binder, it is possible to achieve both high capacity and high reliability. The present disclosure is based on this new finding.

[0021] When forming the active layer, the active layer may be formed so that the total proportion S of the carboxymethyl cellulose and the rubber in the active layer is 8.2% by mass or more. The proportion S may be 21.3% by mass or less. The proportion S may be in the range of 8.2 to 21.3% by mass.

[0022] (Binder) As described above, the binder contains at least one carboxymethylcellulose selected from carboxymethylcellulose and salts of carboxymethylcellulose. Examples of the salts of carboxymethylcellulose include alkali metal salts of carboxymethylcellulose (e.g., sodium salts). A preferred example of the carboxymethylcellulose is carboxymethylcellulose.

[0023] Examples of rubber contained in the binder include styrene butadiene rubber (SBR) and acrylic rubber.

[0024] Among the combinations of carboxymethyl cellulose and rubber, the combination of carboxymethyl cellulose and styrene-butadiene rubber is particularly preferred. That is, the rubber is preferably styrene-butadiene rubber. By using these combinations and setting the binder content within the above range, particularly high effects can be obtained, as will be described in the examples.

[0025] In the active layer, the mass of the carboxymethyl cellulose may be greater than the mass of the styrene-butadiene rubber. For example, the mass of the carboxymethyl cellulose may be in the range of 1.1 to 4.5 times the mass of the styrene-butadiene rubber. This configuration provides particularly high effects.

[0026] The binder improves the adhesion between the porous carbon particles and between the porous carbon particles and the current collector. Carboxymethyl celluloses are considered to be highly effective in improving the adhesion between the porous carbon particles. In this respect, it is particularly important to increase the proportion of carboxymethyl celluloses when using porous carbon particles with a small average particle size.

[0027] The active layer may contain binders other than carboxymethyl celluloses and rubber. However, in the active layer, the mass of the other binders is usually 10% or less (e.g., 5% or less) of the total mass of the carboxymethyl celluloses and rubber. Examples of other binders include polytetrafluoroethylene (PTFE). A preferred example of the active layer contains only carboxymethyl celluloses and rubber as binders.

[0028] (Porous Carbon Particles) The average particle size of the porous carbon particles may be 0.5 μm or more or 0.6 μm or more, or may be 1.0 μm or less or 0.8 μm or less. In this specification, the average particle size refers to the particle size at which the volume integrated value is 50% (median diameter, D50) in the volume-based particle size distribution measured by a laser diffraction / scattering method.

[0029] In the pore distribution of the porous carbon particles, the ratio B / A of the cumulative volume A of pores having a pore diameter of 1 nm or more and less than 2 nm to the cumulative volume B of pores having a pore diameter of 2 nm or more and 50 nm or less may be 1 or more (e.g., 1.2 or more). The ratio B / A may be 1 or more (e.g., 1.2 or more) and 1.5 or less.

[0030] In the pore distribution of the porous carbon particles contained in the active layer, pores having a pore diameter of 1 nm or more and less than 2 nm may be referred to hereinafter as "pore a" or "micropore." Also, pores having a pore diameter of 2 nm or more and 50 nm or less may be referred to hereinafter as "pore b" or "mesopore."

[0031] The cumulative volume A is the total volume (cm) of pores a per 1 g of porous carbon particles. ３ The cumulative volume B is the total volume (cm) of pores b per 1 g of porous carbon particles. ３ ) The volume-based particle size frequency distribution is a volume-based particle size distribution with the vertical axis representing frequency and the horizontal axis representing particle size.

[0032] The porous carbon particles are, for example, activated carbon. The porous carbon particles can be produced, for example, by carbonizing a raw material through heat treatment and then activating the resulting carbonized material to make it porous. The carbonized material may be crushed and sized before the activation treatment. The porous carbon particles obtained by the activation treatment may be pulverized. After the pulverization treatment, classification may be performed. Examples of activation treatments include gas activation using a gas such as water vapor and chemical activation using an alkali such as potassium hydroxide.

[0033] Examples of raw materials for the porous carbon particles include wood, coconut shells, pulp waste liquor, coal or coal-based pitch obtained by thermal decomposition thereof, heavy oil or petroleum-based pitch obtained by thermal decomposition thereof, phenolic resin, petroleum-based coke, and coal-based coke. Among these, petroleum-based coke and coal-based coke are preferred as raw materials. In this case, activation treatment can easily produce porous carbon particles with a large proportion of pores b.

[0034] Petroleum coke or coal-based coke may be heat-treated, the resulting carbonized product may be activated, and porous carbon particles having a large proportion of pores b may be obtained. The porous carbon particles may then be pulverized. However, excessive pulverization may result in a B / A ratio greater than 1.5. For example, a ball mill, a jet mill, or the like may be used for the pulverization.

[0035] The pore distribution and particle size distribution of the porous carbon particles can be adjusted by the raw material, heat treatment temperature, activation temperature in gas activation, degree of pulverization, etc. Alternatively, the pore distribution and particle size distribution of the porous carbon particles may be adjusted by mixing two types of porous carbon particles made from different raw materials.

[0036] When the ratio B / A is 1 or more, an electrochemical device having a large capacity (initial capacity) and excellent float characteristics can be obtained. The float characteristics are an index of the degree of deterioration of an electrochemical device when float charging is performed, maintaining a constant voltage using an external DC power source. The smaller the capacity decrease and the smaller the increase in internal resistance during float charging, the better the float characteristics can be said to be.

[0037] Pore ​​a mainly contributes to the specific surface area of ​​the porous carbon particles and mainly affects the capacity (especially the initial capacity). Pore b mainly contributes to the mobility of ions in the electrolyte within the pores and mainly affects the float characteristics and internal resistance. When the pore diameter is 2 nm or more, ions in the electrolyte easily diffuse within the pores, making the pores less likely to clog. In pores with a pore diameter of 2 nm or more, good ion mobility is ensured even at low temperatures.

[0038] When the B / A ratio is less than 1, the proportion of pores a increases and the proportion of pores b decreases. As a result, ions in the electrolyte solution are less likely to diffuse within the pores, and the pores are more likely to become clogged. For example, the pores may be more likely to become clogged by decomposition products of the electrolyte solution. This can increase internal resistance and degrade float characteristics. When the ratio B / A exceeds 1.5, the proportion of pores a decreases and the proportion of pores b increases. As a result, the electrode density decreases, and the initial capacity at low temperatures may decrease.

[0039] In the pore distribution of the porous carbon particles, the total proportion of the integrated volumes A and B to the total pore volume (the sum of the volumes of all pores in the range of 1 nm to 100 nm) is preferably, for example, 87% to 90%. In this case, pores a and pores b are distributed in large numbers, making it easy to obtain large capacity and excellent float characteristics.

[0040] From the viewpoint of ensuring the initial capacity, the BET specific surface area of ​​the porous carbon particles is 1500 to 2500 m ２ / g range (e.g., 1790 to 2240 m ２ / g).

[0041] (Evaluation Method) The pore distribution of porous carbon particles is measured by a gas adsorption method. Nitrogen gas is used as the gas. For example, a Shimadzu Corporation automatic specific surface area / pore distribution measuring device "Tristar II 3020" is used as the measuring device. Note that, in order to remove impurities, the porous carbon particle sample is pretreated by heating and evacuating to a vacuum (for example, 250°C and 50 mTorr or less) before measurement. The pore distribution is analyzed using the BJH method (Barrett-Joyner-Halenda method), which uses the Harkins & Jura equation. The cumulative volumes A and B are determined using the cumulative pore volume distribution obtained by the BJH method.

[0042] The BET specific surface area of ​​the porous carbon particles is measured by a gas adsorption method (BET single-point method). Nitrogen gas is used as the gas. The measurement device used is, for example, an automatic specific surface area / pore distribution measurement device "Tristar II 3020" manufactured by Shimadzu Corporation.

[0043] The particle size distribution of the porous carbon particles is measured by a laser diffraction / scattering method, for example, using a laser diffraction / scattering particle size distribution measuring device "MT3300EXII" manufactured by Microtrac Corporation.

[0044] The mass of the binder in the active layer can be determined by thermogravimetric analysis (TGA). Specifically, the active layer of the electrode is peeled off and the powder constituting the active layer is subjected to thermogravimetric analysis to measure the mass of the binder. For example, a NEXTA STA300 (Hitachi High-Tech Corporation) can be used as the analytical device. The sample (active layer) to be analyzed is dried before being subjected to thermogravimetric analysis. Specifically, the sample can be dried at 130°C for 2 hours before the analysis.

[0045] The type of binder in the electrode can be identified by, for example, pyrolysis gas chromatography / mass spectrometry (pyrolysis GC / MS), using an instrument such as an HP 5890 Series II GC with an HP5972 MSD and 7673 Autosampler (Hewlett Packard) or a Hewlett Packard 5890 GC Series II with an ALS 7673 Controller and a PC.

[0046] The method for producing the electrode (E) is not particularly limited, and known methods may be used. In one example of the production method, first, porous carbon particles, a binder, a dispersion medium, and, if necessary, other optional components are mixed to prepare a slurry. The porous carbon particles and binder are as described above. The materials are mixed so that the ratio of the components in the active layer is the ratio described above. Typically, the mass ratio in the slurry is reflected in the mass ratio in the active layer. The dispersion medium is not particularly limited, and a common dispersion medium (e.g., water) may be used. Next, the slurry is applied to the surface of a current collector to form a coating film, and the coating film is rolled to form the active layer. In this manner, the electrode (E) can be produced.

[0047] (Electrochemical Device) An electrochemical device according to an embodiment of the present disclosure includes a pair of electrodes and an electrolyte solution, at least one of which is the electrode (E) according to this embodiment.

[0048] Examples of electrochemical devices include electric double layer capacitors (EDLCs) and lithium ion capacitors (LICs). When the electrochemical device is an EDLC, the above-described electrode for an electrochemical device can be used for at least one of a pair of electrodes. When the electrochemical device is an LIC, the above-described electrode for an electrochemical device 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 contains, for example, a negative electrode active material (e.g., graphite) capable of absorbing and releasing lithium ions.

[0049] The electrolyte solution includes a solvent (non-aqueous solvent) and an ionic substance. The ionic substance is dissolved in the solvent and includes cations and anions. The ionic substance may include, 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 solution is, for example, 0.5 mol / L or more, and 2.0 mol / L or more.

[0050] The solvent is preferably a high-boiling solvent, and examples thereof include 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.

[0051] 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.

[0052] 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.

[0053] A separator may be disposed between the pair of electrodes. The separator has ion permeability and serves to physically separate the pair of electrodes to prevent short circuits. Examples of separators that can be used include cellulose-based nonwoven fabric, glass fiber mats, and microporous films of polyolefins such as polyethylene.

[0054] An example electrochemical device according to an embodiment of the present disclosure will be described with reference to Figure 1. Figure 1 is a perspective view of the electrochemical device with a portion cut away.

[0055] The electrochemical device 10 in Fig. 1 is an electric double layer capacitor and includes a wound 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 have a first current collector and a second current collector made of metal, respectively, and a first active layer and a second active layer carried on the surfaces thereof, and exhibit capacitance by adsorbing and desorbing ions. At least one of the first active layer and the second active layer is the active layer described above.

[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). The outer case 6 may be made of a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the outer case 6 is sealed with a sealing member 7. The lead wires 5a and 5b are routed to the outside by passing through the sealing member 7. The sealing member 7 may be made of, for example, a rubber material such as butyl rubber.

[0057] 1 shows an example of a wound capacitor, the present disclosure is not limited to this example, and may also be applied to capacitors of other structures, such as stacked or coin-type capacitors.

[0058] The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to the following examples.

[0059] [Electrochemical Device DA1] A wound type electric double layer capacitor (Device DA1) having a rated voltage of 2.7 V was fabricated as an electrochemical device. A specific method for fabricating Device DA1 will be described below.

[0060] (Preparation of Electrode) A slurry was prepared by dispersing 100 parts by mass of porous carbon particles (active material), a binder, and 6.66 parts by mass of acetylene black (conductive agent) in water. The porous carbon particles used had the average particle size and specific surface area shown in Table 1 below. The binder used was the one shown in Table 1. The porous carbon particles and binder were mixed to the parts by mass shown in Table 1. Note that the total mass loss of the binder in Table 1 is a measured value obtained by analyzing the prepared capacitor.

[0061] Next, the obtained slurry was applied to an aluminum foil (thickness: 30 μm, current collector) to form a coating film. The coating film was dried with hot air at 110°C and rolled to form an active layer (thickness: 40 μm). In this way, an electrode was obtained.

[0062] (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.

[0063] (Fabrication of Electrochemical Device DA1) Two of the above electrodes were fabricated, and lead wires were connected to each. Next, the two electrodes were wound with a separator (a nonwoven fabric made of cellulose) interposed therebetween to form a capacitor element. The capacitor element was housed in a predetermined exterior case together with an electrolyte solution and sealed with a sealing material. In this manner, an electric double layer capacitor was assembled. The resulting capacitor was subjected to an aging treatment at 60°C for 16 hours while applying a rated voltage. In this manner, device DA1 (electric double layer capacitor) was obtained.

[0064] (Electrochemical Devices DA2 to DA8 and DB1 to DB7) DA2 to DA8 and DB1 to DB7 were fabricated in the same manner and under the same conditions as device DA1, except that at least one of the porous carbon particles, binder, and aluminum foil (current collector) was changed as shown in Table 1. The average particle size of the porous particles was changed by crushing the porous particles.

[0065] The aluminum foil used for the electrodes of devices DA7, DA8, and DB7 had a roughened surface. The higher the degree of roughness, the stronger the adhesion between the active layer and the aluminum foil, and the higher the capacitance of the aluminum foil. The capacitance of the aluminum foil was measured using the method described above.

[0066] (Electrochemical Devices DA9 and DA10) Devices DA9 and DA10 were used to evaluate the influence of the ratio B / A. The values ​​of the ratio B / A in the active layer of each electrode are shown in Table 1. The ratio B / A of devices other than devices DA9 and DA10 was not evaluated.

[0067] Electrochemical devices DA1 to DA10 are electrochemical devices according to the present disclosure. Electrochemical devices DB1 to DB7 are comparative examples. These electrochemical devices, the electrodes used therein, and the porous carbon particles used therein were evaluated as follows.

[0068] (Evaluation of Porous Carbon Particles) The particle size distribution of the porous carbon particles was measured by the method described above, and the average particle size was determined from the particle size distribution. The specific surface area of ​​the porous carbon particles was determined by the BET method using nitrogen gas, as described above.

[0069] The pore distribution of the porous carbon particles was measured by the gas adsorption method using nitrogen gas, as described above. To remove impurities, the porous carbon particle samples were pretreated by heating and evacuating to a vacuum (for example, at 250°C and 50 mTorr or less) before the measurement. The BJH method (Barrett-Joyner-Halenda method) was used to analyze the pore distribution. The BJH method uses the Harkins & Jura equation. The cumulative volumes A and B were determined using the cumulative pore volume distribution obtained by the BJH method.

[0070] (Binder Evaluation) The mass of the binder in the electrode was determined by thermogravimetric analysis (TGA). Specifically, first, the active layer of the electrode was peeled off and dried at 130°C for 2 hours to obtain a sample. The mass of this sample was measured, and then the sample was analyzed by thermogravimetric analysis. This determined the mass loss due to the binder in the active layer. The thermogravimetric analysis was performed under the following conditions. Analytical device: NEXTA STA300 (Hitachi High-Tech Corporation) <Temperature, heating rate, atmosphere> 25°C to 650°C, 10°C / min, N ２ Gas flow: 650-900°C, 10°C / min, air; 900°C, hold for 5 minutes, air

[0071] The type of binder in the electrode was identified by pyrolysis gas chromatography / mass spectrometry (pyrolysis GC / MS) as described above.

[0072] From the above measurement results, the percentage of the mass of the active layer that was accounted for by the mass loss due to carboxymethyl celluloses, rubber, etc. when the active layer was subjected to thermogravimetric analysis was calculated.

[0073] (Evaluation of Adhesion Strength of Active Layer) Tape was applied to the active layer of the prepared electrode, and the degree to which the active layer peeled off from the current collector (aluminum foil) when the tape was pulled was observed. This allowed the adhesion strength of the active layer to be evaluated.

[0074] (Measurement of initial capacity of electrochemical device) In an environment of −30° C., constant current charging was performed at a current of 100 mA until the voltage reached 2.7 V, and then the state in which a voltage of 2.7 V was applied was maintained for 7 minutes. Thereafter, in an environment of −30° C., constant current discharging was performed at a current of 20 mA until the voltage reached 0 V.

[0075] During the above discharge, the time t (sec) required for the voltage to drop from 2.16 V to 1.08 V was measured. Note that 2.16 V is a voltage equivalent to 80% of 2.7 V (the voltage at full charge), and 1.08 V is a voltage equivalent to 40% of 2.7 V. Using the measured time t, the initial capacity (initial capacity) C1 (F) of the electrochemical device (before the float test) was calculated using the following formula (1): Capacity C1 = Id × t / V (1) (Note that in formula (1), Id is the current value during discharge (0.02 A), and V is the value obtained by subtracting 1.08 V from 2.16 V (1.08 V).)

[0076] Using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained from the above discharge, a linear approximation line was calculated for the range of 0.5 to 2 seconds after the start of discharge of the discharge curve, and the voltage VS at the intercept of the approximation line was calculated. ΔV was calculated as the value (V0-VS) obtained by subtracting the voltage VS from the voltage VO at the start of discharge (0 seconds after the start of discharge). ΔV (V) and the current value Id (0.02 A) during discharge were used to calculate the internal resistance (DCR) R1 (Ω) of the electrochemical device before the float test according to the following formula (2): Internal resistance R1=ΔV / Id (2)

[0077] (Float Test of Electrochemical Device) In an environment of 70°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 2000 hours. In this manner, the electrochemical device was stored with a voltage of 2.7 V applied. Thereafter, in an environment of 60°C, constant current discharging was performed at a current of 20 mA until the voltage reached 0 V.

[0078] (Measurement of Capacity and Internal Resistance of Electrochemical Device After Float Test) Thereafter, the electrochemical device was charged and discharged in an environment of −30° C. in the same manner as in the measurement of the capacity and internal resistance before the float test, and the capacity C2 (F) and internal resistance R2 (Ω) of the electrochemical device after the float test were determined.

[0079] (Measurement of the slope of capacity change and the slope of resistance change) Using the capacitances C1 and C2 of the electrochemical device before and after the float test obtained above, the slope of capacity change was calculated according to the following formula (3). A smaller slope of capacity change indicates that the decrease in capacity after the float test is more suppressed. In formula (3), T is the time (2000 hours) during which float charging was performed in the float test. Slope of capacity change = |(C2 / C1-1) × 100 / (T １／２ ) | (3)

[0080] The resistance change gradient was calculated from the internal resistances R1 and R2 of the electrochemical device before and after the float test obtained above using the following formula (4). A smaller resistance change gradient indicates that the increase in internal resistance after the float test is more suppressed. In formula (4), T is the time (2000 hours) during which float charging was performed in the float test. Resistance change gradient = |(R2 / R1-1) × 100 / (T １／２ ) | (4)

[0081] Some of the manufacturing conditions for the electrochemical device and the above evaluation results are shown in Table 1. In Table 1, the capacity density is calculated as the capacity per 1 cm of the active layer. ３ The capacity per unit volume (F) is the initial capacity density before the float test.

[0082]

[0083] The resistance change slope calculated by the above formula (4) was 1.2 for DA9 and 1.5 for DA10. In other words, by setting the ratio B / A to 1 or more, the float characteristics were improved. The reason for this is not clear, but it can be considered as follows. When charging and discharging are performed, decomposition products of the electrolyte are generated. These decomposition products tend to block micropores. Therefore, when the proportion of micropores is high (the value of the ratio B / A is low), the proportion of blocked pores increases, and the float characteristics are likely to deteriorate. As shown in the above examples, by setting the ratio B / A to 1 or more, the float characteristics can be improved.

[0084] As is clear from the comparison of Devices DB1 and DA1 to DA3, by reducing the particle size of the porous carbon particles, the capacity density (particularly the capacity density at low temperatures) can be significantly increased. On the other hand, reducing the particle size of the porous carbon particles reduces the adhesion strength of the active layer, so that the adhesion strength of the active layer cannot be maintained even when a conventional binder (e.g., only one type of binder) is used. As shown in Table 1, by using carboxymethyl cellulose and rubber (particularly styrene butadiene rubber) as the binder and limiting the amount of binder within a predetermined range, the adhesion strength of the active layer could be improved. Thus, the electrochemical device according to the present disclosure was able to achieve both high capacity and high reliability.

[0085] A comparison of devices DA7, DA8, and DB7 reveals that the capacitance of the aluminum foil is 30 μF / cm ２ To achieve the above, it is preferable to roughen the surface of the aluminum foil.

[0086] The present disclosure can be used for an electrode for an electrochemical device and an electrochemical device using the same.

[0087] 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 device

Claims

1. An electrode for an electrochemical device comprising an active layer, the active layer includes porous carbon particles and a binder; The average particle size of the porous carbon particles is in the range of 0.5 μm to 1.0 μm, The binder includes at least one carboxymethylcellulose selected from carboxymethylcellulose and a salt of carboxymethylcellulose, and a rubber, An electrode for an electrochemical device, wherein, when a thermogravimetric analysis is performed on the active layer, a mass loss due to the carboxymethyl cellulose and the rubber is 8.2% or more and 21.3% or less of the mass of the active layer.

2. The rubber is a styrene butadiene rubber, 2. The electrode for electrochemical devices according to claim 1, wherein in said active layer, the mass of said carboxymethyl cellulose is in the range of 1.1 to 4.5 times the mass of said styrene-butadiene rubber.

3. Further comprising an aluminum foil supporting the active layer; The surface of the aluminum foil is roughened, The capacitance of the aluminum foil is 30 μF / cm ２ The electrode for electrochemical devices according to claim 1 .

4. 2. The electrode for electrochemical devices according to claim 1, wherein in a pore distribution of the porous carbon particles, a ratio B / A of an integrated volume A of pores having a pore diameter of 1 nm or more and less than 2 nm to an integrated volume B of pores having a pore diameter of 2 nm or more and 50 nm or less is 1 or more.

5. A pair of electrodes and an electrolyte solution are included. An electrochemical device, wherein at least one of the pair of electrodes is the electrode for electrochemical devices according to any one of claims 1 to 4.