Capacitor and method for manufacturing the same

Abstract

This capacitor comprises: a first electrode including a first active material; a second electrode including a second active material; a separator interposed between the first electrode and the second electrode; and an electrolytic solution. The electrolytic solution includes an ionic liquid. In the pore size distribution of the separator, the total pore volume Va of pores in the range from 0.1 μm to 2 μm is at least 0.4 cm3 / g.

Description

Technical Field

[0001] The present invention relates to a capacitor and a method for manufacturing the same.

Background Art

[0002] In recent years, capacitors having a high energy density have been desired. Since a higher energy density can be obtained as the operating voltage of the capacitor is increased, combinations of electrolytes and electrode materials that operate at high voltages have been explored.

[0003] Patent Document 1 proposes a graphene sheet film in which two or more graphene sheets are stacked in parallel via carbon nanotubes, and the graphene sheet stack is three-dimensionally connected electrically and mechanically by carbon nanotubes to each other. A capacitance of 290.6 F / g has been reported when using this film as an electrode.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Although the capacitor described in Patent Document 1 has a high output density when compared with a lithium ion battery, its energy density is still low and its applications are limited. In order to expand the applications of capacitors, it is desired to further increase the energy density of capacitors.

[0006] The higher the operating voltage of the capacitor, the higher the energy density obtained. Therefore, combinations of electrolytes and electrode materials that operate at high voltages have been explored. However, in addition to electrolytes and electrode materials, it is also important to explore separators that can achieve a high energy density even under high voltages.

Means for Solving the Problem

[0007] In view of the above, one aspect of the present invention includes a first electrode containing a first active material, a second electrode containing a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution. The electrolytic solution contains an ionic liquid. In the pore size distribution of the separator, the total volume Va in the range of pore sizes from 0.1 μm to 2 μm is 0.4 cm 3 / g or more, and relates to a capacitor.

[0008] In view of the above, another aspect of the present invention includes a first electrode containing a first active material, a second electrode containing a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution. The electrolytic solution contains an ionic liquid. The separator includes a porous substrate and a reduction reaction product of the ionic liquid fixed to the porous substrate, and relates to a capacitor.

[0009] In view of the above, still another aspect of the present invention is a method for manufacturing a capacitor including a first electrode containing a first active material, a second electrode containing a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution. The method includes a step of preparing a porous substrate and an ionic liquid, and a step of obtaining a separator by fixing a reduction reaction product of the ionic liquid to the porous substrate, and relates to a method for manufacturing a capacitor.

Advantages of the Invention

[0010] According to the present invention, a capacitor with a high energy density can be realized.

[0011] The novel features of the present invention are described in the appended claims. However, the present invention will be better understood from the following detailed description in conjunction with the drawings, in relation to both the configuration and the content, along with other objects and features of the present invention.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present disclosure will be described. In the following description, embodiments of the present disclosure will be described with examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure can be obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "numerical value A or more and numerical value B or less". In the following description, when the lower limit and the upper limit of a numerical value regarding a specific physical property or condition are exemplified, any combination of any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not more than the upper limit. When a plurality of materials are exemplified, one of them may be selected and used alone, or two or more of them may be used in combination.

[0014] In addition, the present disclosure includes combinations of matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims. That is, as long as no technical contradiction occurs, matters described in two or more claims arbitrarily selected from a plurality of claims described in the appended claims can be combined.

[0015] Hereinafter, the capacitor means a capacitor having various power storage mechanisms, and for example, a power storage device having at least partially a power storage mechanism such as an electric double layer capacitor or a lithium ion capacitor. The power storage device or capacitor includes a pair of capacitor electrodes and an electrolytic solution. The electrode contains an active material.

[0016] The active material exhibits capacitance, for example, by doping and undoping ions. Doping of ions into the active material is a concept that includes adsorption of ions onto the active material, occlusion of ions by the active material, chemical interaction between the active material and ions, and the like. Further, undoping of ions from the active material is a concept that includes desorption of ions from the active material, release of ions from the active material, dissociation of chemical interaction between the active material and ions, and the like. However, here, doping of ions into the active material mainly refers to adsorption of ions onto the active material, and undoping of ions from the active material mainly refers to desorption of ions from the active material. When ions are adsorbed onto the active material, an electric double layer is formed, and capacitance is exhibited. That is, the electrode for the capacitor mainly means a polarizable electrode, but may also be an electrode that has the properties of a polarizable electrode and in which a Faraday reaction also contributes to the capacitance.

[0017] A capacitor according to an embodiment of the present disclosure includes a first electrode including a first active material, a second electrode including a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution. Either one of the first electrode and the second electrode is the positive electrode of the capacitor, and the other is the negative electrode. The first electrode and the second electrode may be the same electrode. The electrolytic solution includes an ionic liquid.

[0018] In the pore size distribution of the separator, the total volume Va in the range of pore sizes of 0.1 μm to 2 μm is 0.4 cm 3 / g or more.

[0019] A high voltage of, for example, 4.0 V or more can be applied between the first electrode and the second electrode of the capacitor. In this case, when a separator in which Va satisfies the above conditions is used, a capacitor with high capacitance and high energy density can be realized.

[0020] When Va is 0.4 cm 3 / g or more, the reason why the capacitance of the capacitor becomes significantly high is currently being elucidated, but is considered as follows. However, the present invention is not limited thereto.

[0021] The cations or anions contained in the ionic liquid can be reduced or oxidized by the application of a high voltage, and reaction products of the reduction reaction or oxidation reaction can be generated in the electrolyte. If these reaction products are present in the vicinity of the positive electrode or the negative electrode, they may prevent the formation of the electric double layer, which is considered to be one of the causes of capacity reduction. For example, the reaction products can react with the members constituting the electrode layer (e.g., functional groups of the active material, side chains of the binder, etc.), and side reaction products can be generated. When this side reaction product accumulates in the vicinity of the positive electrode or the negative electrode and in the voids of the electrode layer, it may reduce the active sites of the active material or prevent the movement (diffusion) of cations or anions, thereby preventing the formation of the electric double layer.

[0022] On the other hand, the separator can adsorb the reaction products. The reaction products adsorbed on the separator do not prevent the formation of the electric double layer at the positive electrode and the negative electrode, and high capacity can be maintained. However, if the pore diameter of the separator is larger than 2 μm, the reaction products easily pass through the pores, and the effect of adsorbing the reaction products is small. In contrast, minute pores with a pore diameter of 2 μm or less easily trap the reaction products in the pores, and have a high effect of adsorbing and fixing the reaction products. When Va is 0.4 cm 3 / g or more, the separator has sufficiently fine pores with a pore diameter of 2 μm or less, and easily fixes the reaction products. As a result, when Va is 0.4 cm 3 / g or more, it is considered that significantly high capacity can be maintained.

[0023] In this case, in the pore diameter distribution of the separator, it is more preferable that the total volume Vm in the range of a pore diameter of 0.01 μm to 6 μm is 0.7 cm 3 / g or more.

[0024] Specifically, for example, when 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) is used as the ionic liquid, during charging at a voltage of 4.0 V or higher, 1-ethyl-3-methylimidazolium cation (EMI +) is reduced at the negative electrode, and EMI radicals can be generated by the reaction shown in the following Reaction Formula 1.

[0025] Reaction Formula 1:

Chemical Formula

[0026] On the other hand, during discharge, the reverse reaction of Chemical Formula 1 proceeds, and a reaction (oxidation reaction) in which EMI radicals release one electron to return to EMI + cations can proceed. Therefore, it is considered that the capacitor exhibits capacitance not only by the adsorption of cations or anions but also by a redox reaction, and as a result, a significantly high capacitance can be obtained.

[0027] 1 EMI radicals can be changed into carbenes by the reaction shown in the following Reaction Formula 2. Also, dimers can be generated by the reaction shown in the following Reaction Formula 3. The reactions shown in Reaction Formulas 2 and 3 are equilibrium reactions. When the concentration of the ionic liquid (EMIBF4) contained in the electrolytic solution is low, the reaction shown in Reaction Formula 3 hardly proceeds, and the reaction shown in Reaction Formula 2 mainly proceeds. On the other hand, when EMIBF4 is present in a high concentration in the electrolytic solution, the reaction shown in Reaction Formula 3 easily proceeds. Note that the presence of EMI dimers can be confirmed by performing an analysis by

[0028] Reaction Formula 2:

Chemical Formula

[0029] Reaction Formula 3:

Chemical Formula

[0030] When the reaction products, carbene and dimer, are present near the positive or negative electrode, the reaction products may prevent the formation of the electric double layer and reduce the capacitance as described above. However, since the separator has the function of adsorbing and fixing the reaction products, the reaction products are suppressed from preventing the formation of the electric double layer, and the reduction of the capacitance is suppressed.

[0031] In addition, during discharge, the EMI radical is oxidized and returns to the EMI cation to contribute to the capacitance, while in the state of carbene and dimer, it does not directly contribute to the capacitance. Therefore, the capacitance of the capacitor decreases by the amount of carbene and dimer generated by the reaction of the EMI radical. However, in the capacitor according to an embodiment of the present disclosure, since the carbene and dimer are fixed to the separator, the concentration of the carbene and dimer near the negative electrode can be increased, and the equilibriums of Reaction Formulas 2 and 3 can be shifted to the left side. As a result, the generation of carbene and dimer is suppressed, so that a large number of EMI radicals can be oxidized and return to the EMI cation during discharge, and the capacitance exhibited by the redox reaction can be maintained high.

[0032] The pore volumes Va and Vm are obtained by measuring the differential pore volume distribution or the logarithmic differential pore volume distribution of the separator and integrating them in the pore diameter ranges of 0.1 μm to 2 μm and 0.01 μm to 6 μm, respectively. For the measurement of the pore volume distribution, a mercury porosimeter based on the mercury intrusion method, or a Palm porometer based on the bubble point method or the gas permeation method can be used.

[0033] The volume Va in the pore diameter range of 0.1 μm to 2 μm of the separator may be 0.6 cm 3 / g or more. The volume Va may be 1.0 cm 3 / g or less, 0.8 cm 3 / g or less, or 0.7 cm 3 / g or less.

[0034] The porosity of the separator is preferably 60% or more, more preferably 65% or more, and even more preferably 70% or more. The porosity of the separator is calculated by the following formula, where W (g / cm 2 ) is the mass per unit area of the separator, ρ (g / cm 3 ) is the true density of the material constituting the separator, and t (cm) is the thickness of the separator. A micrometer can be used to measure the thickness t. The thickness measurement method is to stack 10 separators, apply a certain load, and calculate the thickness per sheet from the total thickness. Porosity (%) = 100 - (W / (ρ × t)) × 100

[0035] The porosity of the separator may be measured, for example, by a pure water pressure porosimeter.

[0036] The density of the separator is, for example, 0.3 g / cm 3 or more and 0.8 g / cm 3 or less, and may be 0.35 g / cm 3 or more and 0.7 g / cm 3 or less. Here, the density of the separator means the bulk density and is calculated by the following formula. (Density of separator) = (Weight of separator) / [(Area of separator) × (Thickness of separator)]

[0037] As the material of the separator, it is preferable to include cellulose or its derivatives. Cellulose is stable against both acids and alkalis and is also stable in an environment where a high voltage is applied. In addition, it has high tensile strength and sufficient strength even when the first electrode and the second electrode are wound to form a wound electrode group. As the form of the separator, for example, a microporous membrane, a woven fabric, or a non-woven fabric can be used. The thickness of the separator is, for example, 8 to 50 μm, preferably 12 to 35 μm, and more preferably 14 to 35 μm or 16 to 35 μm.

[0038] Also, a capacitor according to another embodiment of the present disclosure includes a first electrode including a first active material, a second electrode including a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution. Either one of the first electrode and the second electrode is the positive electrode of the capacitor, and the other is the negative electrode. The first electrode and the second electrode may be the same electrode. The electrolytic solution includes an ionic liquid. The separator includes a porous substrate and a reduction reaction product of the ionic liquid fixed to the porous substrate.

[0039] Note that, in the above, the ionic liquid contained in the electrolytic solution of the capacitor and the ionic liquid before the reaction with the reduction reaction product fixed to the separator may be the same or different. Hereinafter, the ionic liquid before the reaction of the reduction reaction product may be referred to as "first ionic liquid", and the ionic liquid contained in the electrolytic solution of the capacitor may be referred to as "second ionic liquid".

[0040] Further, a method for manufacturing a capacitor according to an embodiment of the present disclosure is a method for manufacturing a capacitor including a first electrode including a first active material, a second electrode including a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution, the method having a step of preparing a porous substrate and an ionic liquid, and a step of obtaining a separator by fixing a reduction reaction product of the ionic liquid to the porous substrate.

[0041] By using a separator in which a reduction reaction product of an ionic liquid is fixed to a porous substrate, the capacitance of a capacitor using the ionic liquid as an electrolytic solution can be increased. This is presumably because, in addition to the capacitance being exhibited by doping and undoping of ions constituting the ionic liquid into the active material, an additional capacitance is exhibited by the oxidation-reduction reaction of the ions constituting the ionic liquid. By previously fixing the reduction reaction product to the separator, the oxidation-reduction reaction occurs efficiently and the capacitance increases. Also, the charge-discharge efficiency is improved.

[0042] Specifically, for example, when using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) as the ionic liquid, during charging at a voltage of 4.0 V or higher, the 1-ethyl-3-methylimidazolium cation (EMI + ) is reduced at the negative electrode, and EMI radicals can be generated by the reaction shown in Reaction Formula 1 above. On the other hand, during discharge, the reaction opposite to Reaction Formula 1 proceeds, and a reaction (oxidation reaction) in which EMI radicals release one electron and return to EMI + cations can proceed. EMI radicals can be changed into carbenes by the reaction shown in Reaction Formula 2 above. Also, dimers can be generated by the reaction shown in Reaction Formula 3 above.

[0043] During discharge, in the capacitor, additional capacitance is manifested when EMI radicals are oxidized and return to EMI cations. However, in the state of carbenes and dimers, it is difficult to release one electron and they cannot directly contribute to the capacitance. Therefore, if the number of carbenes and dimers generated by the reaction of EMI radicals is large, the increase in capacitance due to the redox reaction is suppressed accordingly. Also, since a part of the electric charge applied during charging is consumed for the generation of reduction reaction products, the charge-discharge efficiency decreases.

[0044] On the other hand, according to the capacitor and the method for manufacturing a capacitor according to an embodiment of the present disclosure, since a large number of reduction reaction products (for example, carbenes and dimers) are fixed to the separator, the concentration of reduction reaction products in the vicinity of the negative electrode in the capacitor using this separator is increased. Thereby, the equilibriums of Reaction Formula 2 and Reaction Formula 3 can be shifted to the left side, and the generation of reduction reaction products during charging of the capacitor is suppressed. Therefore, during discharge, a large number of EMI radicals can be oxidized and return to EMI cations, the capacitance manifested by the redox reaction can be maintained high, and the charge-discharge efficiency can be increased.

[0045] In addition, when the reduction reaction product is present in the vicinity of the positive electrode or the negative electrode, it may prevent the formation of the electric double layer and reduce the capacitance. However, by suppressing the generation of the reduction reaction product, the prevention of the formation of the electric double layer by the reduction reaction product is suppressed, and the reduction of the capacitance is suppressed. Furthermore, by suppressing the generation of the reduction reaction product, even after a large number of charge-discharge cycles are repeated, the increase in the concentration of the reduction reaction product in the electrolyte is suppressed, and the increase in the viscosity of the electrolyte is also suppressed. As a result, the reduction in capacitance after repeating a plurality of charge-discharge cycles is also suppressed.

[0046] The step of obtaining the separator includes, for example, a step of preparing a cell by disposing a porous substrate impregnated with a first ionic liquid between a pair of electrodes, and a step of applying a voltage to the cell to generate a reduction reaction product. Thereafter, the porous substrate is taken out from the cell and washed to obtain a separator in which the reduction reaction product is fixed to the porous substrate. An electrolyte in which the first ionic liquid is mixed with another liquid (solvent) may be impregnated into the porous substrate. The voltage applied to the cell is equal to or higher than the voltage that causes the reduction reaction of the first ionic liquid, and depends on the first ionic liquid. For example, it is 4.0 V or higher. The application time is, for example, 5 hours or longer. The voltage application cycle may be repeated a plurality of times so that the total voltage application time is 5 hours or longer.

[0047] When fixing the reduction reaction product to the porous substrate, the configuration of the cell (configuration of the electrode and electrolyte, size, etc.) may be the same as or different from the configuration of the capacitor to be manufactured. For example, the active materials used in the positive and negative electrodes of the cell may be the same as or different from the active materials used in the positive and negative electrodes of the capacitor to be manufactured. The electrolyte used in the cell and the electrolyte used in the capacitor to be manufactured both contain an ionic liquid, but the concentration of the ionic liquid in the electrolyte may be different. Also, the first ionic liquid contained in the electrolyte used in the cell and the ionic liquid (second ionic liquid) contained in the electrolyte used in the capacitor to be manufactured may be different. As described above, as long as the reduction reaction products are the same, the first ionic liquid and the second ionic liquid only need to contain the same cation, and the anions constituting the ionic liquid may be different.

[0048] The porous substrate preferably contains at least one selected from the group consisting of cellulose and its derivatives. Cellulose is stable to both acids and alkalis and is also stable in an environment where a high voltage is applied. In addition, it has a high tensile strength and sufficient strength even when wound together with the first and second electrodes of the capacitor to form a wound electrode group. As the form of the porous substrate, for example, a microporous membrane, a woven fabric or a non-woven fabric can be used. The thickness of the porous substrate is, for example, 8 to 50 μm, preferably 12 to 35 μm, more preferably 14 to 35 μm or 16 to 35 μm.

[0049] As the porous substrate, those having a structure with more fine pores are preferable in that it is easy to fix the reduction reaction product in the pores. More specifically, a porous substrate having well-developed micropores with a pore diameter of less than 2 μm is preferable. In pores with a pore diameter of 2 μm or more, the reduction reaction product easily passes through the inside of the pores and it is difficult for the reduction reaction product to adhere. In the pore size distribution of the porous substrate, the total volume Va in the range of pore diameter 0.1 μm to 2 μm may be 0.4 cm 3 / g or more. In addition, the total volume Vm in the range of pore diameter 0.01 μm to 6 μm may be 0.7 cm3 It is preferably 1 / g or more. When Va is 0.4 cm 3 A porous substrate with a pore volume Va of 0.4 cm3 / g or more has a high effect of fixing the reduction reaction product, and it is easy to realize a capacitor with a high energy density. The pore volume Va and Vm are obtained by measuring the differential pore volume distribution or logarithmic differential pore volume distribution of the separator and integrating in the pore diameter ranges of 0.1 μm to 2 μm and 0.01 μm to 6 μm, respectively. For the measurement of the pore volume distribution, a mercury porosimeter based on the mercury intrusion method, or a palm porometer based on the bubble point method or gas permeation method can be used.

[0050] As the first ionic liquid, a salt compound that exists in a liquid state under normal temperature (25 °C) and normal pressure (atmospheric pressure) can be preferably used. As the cation constituting the salt compound, imidazolium-based cations, pyrrolidinium-based cations, pyridinium-based cations, piperidinium-based cations, ammonium-based cations, phosphonium-based cations, etc. can be mentioned. As the anion, halide ions (Cl - , Br - , etc.), tetrafluoroborate ions (BF4 - ), hexafluorophosphate ions (PF6 - ), bis(fluorosulfonyl)imide ions ((FSO2)2N - ), bis(trifluoromethylsulfonyl)imide ions ((CF3SO2)2N - ), etc. can be mentioned.

[0051] Among these cations, when a cation having a heteroaromatic ring is used, when a carbon material having a graphene layer is used as an active material using a separator, a capacitor with an extremely high capacity can be realized. The reason for this is currently under investigation, but it is considered that the delocalized π orbitals derived from the aromatic ring easily bond to the π orbitals of graphene and form a particularly stable adsorption state.

[0052] The cation having a complex aromatic ring may be an imidazolium cation. The imidazolium cation may be a cation in which some of the hydrogens of the imidazole skeleton are substituted with an alkyl group or the like.

[0053] As an example of the imidazolium cation, the cation having a complex aromatic ring may be a 1-C 1-3 alkyl-3-C 1-3 alkylimidazolium cation, and more specifically, may include a 1-ethyl-3-methylimidazolium cation. 80 mol% or more of the cations may be 1-C 1-3 alkyl-3-C 1-3 alkylimidazolium cation or 1-ethyl-3-methylimidazolium cation.

[0054] The content rate of the reduction reaction product fixed in the separator (that is, the ratio of the mass of the reduction reaction product to the total mass of the separator) is, for example, 2% by mass or more, and may be 5% by mass or more, 10% by mass or more, 20% by mass or more, or 30% by mass or more. In the capacitor, a part of the reduction reaction product fixed in the separator can dissolve in the electrolyte solution, but most remain fixed to the separator.

[0055] The amount of the reduction reaction product fixed in the separator can be analyzed, for example, by the following method.

[0056] First, take out the separator to which the reaction product is fixed, wash the surface with dimethyl carbonate (DMC), and dry it for 2 hours or more under a reduced pressure atmosphere (for example, 0.1 MPa or less, preferably 10 Pa or less) at room temperature. Then, sufficiently extract the liquid containing the reaction product from inside the separator by centrifugation. By performing ESR analysis on the extract and analyzing the concentration of radicals and the like that are the reaction products, the minimum amount of the fixed amount of the reaction products can be defined. By drawing a calibration curve with a spin quantification standard sample solution (TEMPOL), it is possible to quantify the radicals that are the reaction products. Also, 1By performing H-NMR analysis, two peaks, one due to the cation (EMI + ), and the other due to the dimer, are separately detected. The ratio of the dimer contained in the extract can be quantified from the areas of the two peaks.

[0057] Examples of the ionic liquid or the second ionic liquid contained in the electrolytic solution of the capacitor include the compounds listed as the first ionic liquid above. The second ionic liquid contains the same cation as the cation constituting the first ionic liquid. The second ionic liquid may be the same as the first ionic liquid.

[0058] A high voltage of, for example, 4.0 V or more can be applied between the first electrode and the second electrode of the capacitor. In this case, a significantly high capacitance is realized.

[0059] In order to adjust the viscosity of the electrolytic solution and enhance the output characteristics, a solvent may be mixed with the second ionic liquid and used as the electrolytic solution. As the solvent, as long as it can be uniformly mixed with the ionic liquid, a solvent (non-aqueous solvent) conventionally used as the electrolytic solution of the capacitor can be used.

[0060] As the non-aqueous solvent, a high-boiling solvent is preferred. 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, formaldehyde, etc. can be used.

[0061] When the electrolytic solution contains a solvent other than the ionic liquid, the proportion of the ionic liquid in the whole electrolytic solution may be 75% by mass or more, or 80% by mass or more, and further may be 90% by mass or more.

[0062] As the material of the first active substance and / or the second active substance, a carbon material having a layer structure can be used. That is, the carbon material includes a laminated structure of layers (generally called graphene layers or graphene sheets) in which carbon atoms are bonded so as to form a hexagonal network in a plane. The carbon material may contain graphene. The graphene may be reduced graphene oxide or graphene having a three-dimensional structure.

[0063] Graphene is a carbon material with a graphene sheet having a thickness of one carbon atom as the minimum unit, and usually constitutes a laminate in which a plurality of graphene sheets are laminated. A graphene sheet is an aggregate or molecule composed of sp2-bonded carbon having a thickness of one carbon atom, and has a honeycomb lattice structure that spreads in a sheet shape.

[0064] At least the first active substance of the first active substance and the second active substance may contain a carbon material having a layer structure.

[0065] The carbon material having a layer structure can be used as either the active substance on the positive electrode side or the active substance on the negative electrode side. However, in one embodiment, by using an electrode (the first electrode or the second electrode) containing the carbon material as the active substance as the negative electrode, the capacity increases dramatically. Here, it will be described assuming that at least the first electrode of the first electrode and the second electrode is an electrode containing the carbon material as the active substance (the first active substance).

[0066] The carbon material may contain reduced graphene oxide. Reduced graphene oxide is obtained by reducing graphene oxide.

[0067] Graphene oxide (hereinafter also referred to as "GO") has a structure in which oxygen-containing groups are bonded to the graphene sheet. It is considered that the oxygen-containing groups are mainly bonded to the edge surfaces of the graphene sheet laminate. The oxygen-containing groups are hydrophilic groups such as hydroxyl groups, carbonyl groups, and carboxyl groups. Graphene oxide (GO) generally has dispersibility in polar solvents such as water. Since graphene oxide (GO) contains sp3-bonded carbon, it generally has insulating properties.

[0068] By reducing graphene oxide (GO), the oxygen-containing groups are removed, and reduced graphene oxide (hereinafter also referred to as "rGO") is obtained. Reduced graphene oxide (rGO) is a graphene analog having conductivity. Reduced graphene oxide may contain oxygen-containing groups that were not removed in the reduction process. Due to the presence of functional groups such as oxygen atoms in the graphene layer, the crystal structure of the carbon material deviates from the ideal graphene or graphite structure, and sp2 bonds and sp3 bonds coexist in the carbon atoms in the graphene layer. As a result, when a high voltage is applied, the space between the graphene layers is likely to expand, cations and anions are likely to be inserted between the graphene layers, and the region for forming the electric double layer is likely to expand. Therefore, by using reduced graphene oxide as the active material, the capacitance of the capacitor can be dramatically increased.

[0069] In addition, due to the presence of functional groups such as oxygen atoms, distortion occurs in the planar structure of graphene, and a three-dimensional structure can be formed by disorder of the layer structure (or disorder of the interlayer distance), or by bending or buckling of the graphene layer. As a result, the effective surface area on which cations and anions can be adsorbed increases, and high capacity can be realized.

[0070] General graphene usually has a flat sheet-like form. On the other hand, the graphene used in the capacitor of this embodiment may be a laminated body of graphene sheets in various forms having a disorder in the layer structure (or a disorder in the interlayer distance) (or having a three-dimensional structure), rather than a flat sheet-like form. By using graphene (graphene sheet laminated body) having a three-dimensional structure as the active material, the capacitance of the capacitor increases dramatically.

[0071] The three-dimensional structure mainly means a microscopic three-dimensional structure (i.e., fine structure) formed within flake-like particles. By having a three-dimensional structure, the overlap between graphene sheets is significantly suppressed compared to flat sheet-like graphene, and the large surface area of graphene can be effectively utilized. On the main surface (mainly the 002 plane (basal plane)) of the graphene sheet laminated body having a three-dimensional structure, a plurality of protrusions or a plurality of depressions (i.e., folds) are formed. Due to such a three-dimensional structure, the distance between graphene sheets can be appropriately controlled, and the overlap between graphene sheets can be effectively reduced.

[0072] The three-dimensional structure may include a bent structure in which a graphene sheet having folds is bent. Through the bent portion, a single graphene sheet can be folded in a direction intersecting the plane of the sheet to form a laminated body. The radius of curvature of the bent portion in the bent structure is, for example, in the range of 10 to 1000 nm. The interval between the folds is, for example, in the range of 10 to 100 nm.

[0073] The bent structure in the three-dimensional structure includes, for example, a crumpled structure or a folded structure in the sheet portion between the bent portions. At this time, each graphene sheet laminated body may have a fine porous structure by itself. Therefore, the diffusion of ions in the vicinity of the surface of the laminated body becomes better. The presence of the crumpled structure and the folded structure (i.e., the fold portion) can be confirmed by an electron microscope (SEM, TEM, etc.) photograph of the graphene sheet laminated body.

[0074] The wrinkled structure may be, for example, a structure having a plurality of randomly formed fold-like ridges and depressions. Further, the folded structure is a structure having a folded portion where a single graphene sheet laminate is partially folded a plurality of times, and is included in the category of the wrinkled structure. The height of the ridge or the depth of the depression formed in the folded portion may be larger than the thickness of the carbon portion of the graphene sheet laminate having the structure, and may be twice or more the thickness of the carbon portion.

[0075] Graphene having such a three-dimensional structure has a peak in the pore size range of 2 nm to 4 nm in the logarithmic differential pore volume distribution, and further, may have a distribution in which the volume increases quadratically with the increase in pore size in the pore size range of 4 nm to 50 nm. The total mesopore volume in the pore size range of 2 nm to 50 nm can be, for example, 0.20 cm 3 / g or more. The total mesopore volume may be in the range of 0.20 cm 3 / g to 0.5 cm 3 / g, in the range of 0.25 cm 3 / g to 0.4 cm 3 / g, or in the range of 0.25 cm 3 / g to 0.35 cm 3 / g. The total pore volume in the pore size range of 4 nm to 50 nm can be, for example, 15 times or more the total pore volume in the pore size range of 2 nm to 4 nm. For the measurement of the specific surface area and pore size distribution of graphene, a BELSORP 28SA device available from Nippon Bell Co., Ltd. can be used. As the analysis theory of mesopores, the Dollimore Heal method (DH method) calculated based on the capillary condensation theory (Kelvin's equation) is used.

[0076] With such graphene having a three-dimensional structure, an electrode for a capacitor can be obtained in which the logarithmic differential pore volume distribution measured by a mercury porosimeter has a maximum peak in the range of 0.3 μm or more and 6 μm or less. By having a three-dimensional structure, the overlap between graphene sheets is significantly suppressed as compared with flat sheet-like graphene, and the surface area of graphene can be effectively utilized for ion adsorption. Due to the three-dimensional structure in which a plurality of protrusions or a plurality of depressions are formed on the basal plane, the distance between graphene sheets is appropriately controlled, and the overlap between graphene sheets is effectively reduced. Therefore, the activity sites of the active material can be increased without suppressing the movement (diffusion) of ions, and high capacity can be exhibited. In addition, since more reaction products can be moved, it can contribute to the expression of high capacity.

[0077] According to the capacitor according to an embodiment of the present disclosure, specifically, a capacitor can be realized in which the capacitance per unit mass of the active material when a voltage of 4.0 V or more is applied is 300 F / g or more. Capacitors with a capacitance per unit mass of the active material of 400 F / g or more, 500 F / g or more, 800 F / g or more, or 1000 F / g or more are also realizable. Note that the capacitance per unit mass of the active material being 300 F / g or more when a voltage of 4.0 V or more is applied means that there exists a voltage range in which the capacitance when the capacitor is charged and then discharged at a voltage of 4.0 V or more is 300 F / g or more in a certain voltage range. The voltage range in which the capacitance becomes 300 F / g or more is not particularly limited as long as it is 4.0 V or more. The capacitance may be 300 F / g or more in a voltage range of 4.0 V or more and may also be 300 F / g or more in a certain voltage range less than 4.0 V.

[0078] The average number of stacked layers of the graphene sheet laminate is, for example, 10 layers or less, and may be 5 layers or less. The graphene sheet laminate is preferably closer to the minimum unit graphene sheet (i.e., single-layer sheet) having a thickness of one carbon atom.

[0079] The average number of layers is estimated from the interplanar distance (d002) calculated from the diffraction peak attributed to the 002 plane (basal plane) of the X-ray diffraction profile (for example, Proceedings of the 2015 Autumn Meeting of the Physical Society of Japan, Abstracts, p1014). Alternatively, an estimated value obtained from an electron microscope (SEM, etc.) photograph of graphene may be used. For example, the number of layers of the graphene sheet can be estimated from the scale of the SEM photograph of graphene and the interplanar distance of the 002 plane (basal plane) of the graphene sheet. For example, any 20 graphene sheet laminates can be selected, the number of layers of each can be estimated, the fifth to the largest and the fifth to the smallest values can be excluded, and the average value of the middle 10 values can be taken as the average number of layers.

[0080] The interlayer distance between graphene sheets (i.e., the basal plane distance) may vary randomly. A random change in the interlayer distance means that the crystallinity of the graphene sheet laminate is low. The greater the disorder in the stacking structure in the laminate, the more significant the change in the interlayer distance.

[0081] The X-ray diffraction profile of graphene usually has a diffraction peak B attributed to the 002 plane. The greater the overlap between graphene sheets and the higher the crystallinity of graphene, the sharper the diffraction peak B becomes.

[0082] On the other hand, when graphene has a three-dimensional structure, the diffraction peak B becomes broad and can be resolved into multiple peaks. A halo pattern attributed to the amorphous phase may be observed on the high-angle side of the diffraction peak B.

[0083] The interplanar distance d002 of the 002 plane of graphene calculated from the X-ray diffraction profile is 0.330 nm or more and 0.360 nm or less. d002 is calculated by resolving the diffraction peak observed in the region around 2θ = 26.38° into its components, calculating d002 for each component, and taking the average. The distance d002 of the 002 plane of graphene is preferably 0.340 nm (3.40 Å) or more, more preferably 0.360 nm (3.60 Å) or more, and even more preferably 0.370 nm (3.70 Å) or more.

[0084] Graphene having the above structure is used as an active material to produce a first electrode and / or a second electrode as an electrode for a capacitor. A binder may be included in the capacitor electrode. The binder serves to assist the bonding between graphenes and the bonding between graphene and the current collector when forming the graphene having the three-dimensional structure into an electrode layer.

[0085] Hereinafter, an example of a method for manufacturing graphene according to an embodiment of the present disclosure and a method for manufacturing an electrode for a capacitor using the graphene thus manufactured will be described.

[0086] ≪Method for Manufacturing Graphene≫ (i) Dispersion Preparation Step First, an aqueous dispersion containing graphene oxide is prepared. In addition to graphene oxide and water, the aqueous dispersion may contain a dispersant such as carboxymethyl cellulose (CMC). Graphene oxide can be exfoliated and generated from graphite in a single-layer or multi-layer state, for example, via oxidation of graphite.

[0087] The oxidation of graphite can be performed, for example, using an oxidizing agent in water. As the oxidizing agent, sulfuric acid, potassium permanganate, chromic acid, sodium dichromate, sodium nitrate, peroxide, persulfate, organic peracid, etc. can be used. A water-soluble solvent may be added to the water. Examples of the water-soluble solvent include alcohols, ketones such as acetone, ethers such as dioxane and tetrahydrofuran. An aqueous dispersion of graphene oxide is generated by the oxidation reaction in water.

[0088] The oxygen content of graphene oxide may be, for example, 10 to 60% by mass, may be 20 to 50% by mass, or may be 30 to 50% by mass.

[0089] (ii) Reduction Step Next, reduced graphene oxide is produced by reducing graphene oxide in an aqueous dispersion containing graphene oxide (first reduction step). As the reduction method, for example, hydrothermal treatment is preferable. For example, an aqueous dispersion may be sealed in an autoclave and hydrothermally treated to produce a gel-like product. The temperature of the hydrothermal treatment may be, for example, 150°C or higher, preferably 170°C or higher, and may be 200°C or lower.

[0090] Although it is possible to obtain reduced graphene oxide having a three-dimensional structure only by hydrothermal treatment, in order to further progress the reduction, the gel-like product may be brought into contact with a reducing agent (second reduction step). Examples of the reducing agent include metal hydrides, borohydrides, boranes, hydrazine or hydrazides, ascorbic acids, thioglycolic acids, cysteines, sulfites, thiosulfates, dithionites, etc. For example, the gel-like product may be immersed in an aqueous solution containing a water-soluble reducing agent such as sodium ascorbate. The temperature of the aqueous solution may be, for example, 20 to 110°C, may be 40 to 100°C, or may be 50 to 100°C. The amount of the reducing agent used may be appropriately adjusted according to the type of the reducing agent, the oxygen content of the first carbon raw material (graphene oxide), the amount of the gel-like product, etc.

[0091] Thereafter, the gel-like product may be freeze-dried. By freeze-drying, a dry gel (xerogel) in a state where the three-dimensional structure of graphene is highly maintained can be obtained. Freeze-drying may be performed, for example, at -50°C to 0°C, preferably at -50°C to -20°C, under a reduced pressure of 100 Pa or lower, and further 1 Pa or lower.

[0092] Next, the dry gel is thermally reduced in a non-oxidizing atmosphere to eliminate the remaining functional groups (third reduction step).

[0093] The non-oxidizing atmosphere may be a reduced pressure atmosphere (for example, 0.1 MPa or lower (preferably 10 Pa or lower)), a reducing atmosphere (for example, a hydrogen atmosphere of 0.01 MPa or lower), an inert gas atmosphere (for example, a flowing atmosphere of N2, Ar, Ne, He, etc.).

[0094] The heating temperature in a non-oxidizing atmosphere is 700 °C or higher, and may be 800 °C or higher, 900 °C or higher, 1000 °C or higher, or 1200 °C or higher. However, there is a limit to the reduction of the oxygen content of the reduced graphene oxide, and considering the production cost, the heating temperature in a non-oxidizing atmosphere may be 1800 °C or lower, 1400 °C or lower, or 1200 °C or lower. When defining the temperature range, these upper and lower limits may be arbitrarily combined. The temperature range may be, for example, 1000 °C to 1800 °C.

[0095] The heating time in a non-oxidizing atmosphere is appropriately selected according to the heating conditions and the amount of carbon to be treated, but may be, for example, about 0.1 to 5 hours.

[0096] The oxygen content of the dried gel after thermal reduction is less than 5% by mass, and may be 4% by mass or less, 3% by mass or less, preferably less than 2.6% by mass, and may be 2% by mass or less, or 1.5% by mass or less. When the oxygen content decreases to less than 5% by mass, the reactants generated by the redox reaction between the oxygen-containing groups and the electrolyte components decrease. Such reactants block the pores of the electrode, reduce the diffusivity of ions, and reduce the ion adsorption sites. Therefore, the fewer the reactants, the less the diffusivity of ions is reduced and the ion adsorption sites are not reduced, so that a high capacity can be exhibited.

[0097] The dried gel after thermal reduction is pulverized to obtain a powder of reduced graphene oxide (rGO powder). By the above method, graphene having a three-dimensional structure (reduced graphene oxide) is produced. Using the produced graphene, for example, a capacitor electrode can be produced by the following method.

[0098] ≪Method for manufacturing a capacitor electrode≫ (iii) Polarization process For example, rGO powder is dispersed in a dispersion medium such as water together with a binder to prepare a slurry. The obtained slurry is applied to a conductive substrate (current collector), and the coating film is dried, whereby an electrode layer supported on the current collector is formed, and an electrode for a capacitor is obtained. Thereafter, the electrode layer may be rolled.

[0099] As the binder, for example, fluororesins such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), vinylidene fluoride - hexafluoropropylene copolymer (PVdF - HFP), water - soluble resins such as carboxymethyl cellulose (CMC), polyethylene oxide (PEO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyacrylic acid (PAA), polyvinyl acetate, etc. can be used.

[0100] In addition to the above - mentioned graphene, the electrode layer may contain other active materials such as activated carbon. Further, the electrode layer may contain carbon fibers such as carbon nanotubes (CNT), carbon particles such as carbon black and graphite. However, from the viewpoint of achieving both high capacity and high reliability, it is desirable that graphene constitutes 50 mass% or more of the electrode layer, and more preferably 65 mass% or more.

[0101] As the current collector, a metal foil, a metal porous body, etc. can be used. As the material of the current collector, aluminum, copper, nickel, iron, stainless steel, platinum, etc. can be used. An alloy mainly composed of these metals may also be used. The metal foil may be a plain foil, but may also be a foil roughened by etching or the like, a foil subjected to plasma treatment, etc. The metal porous body has, for example, a three - dimensional network structure.

[0102] The mass per unit area of the metal porous body is, for example, 500 g / m 2 or less, and may also be 150 g / m 2 or less. The porosity of the metal porous body may be, for example, 80% to 98% by volume, and may also be 90% to 98% by volume.

[0103] The average pore diameter of the pores of the porous metal may be, for example, 50 μm or more and 1000 μm or less, may be 400 μm or more and 900 μm or less, or may be 450 μm or more and 850 μm or less.

[0104] <<Capacitor>> Next, an example of a capacitor including the above-described capacitor electrodes as a first electrode and a second electrode will be described. FIG. 1 is a partially cutaway perspective view of a capacitor 10.

[0105] The illustrated capacitor 10 includes a wound capacitor element 1. The capacitor element 1 is configured by winding a sheet-like first electrode 2 and a second electrode 3 with a separator 4 interposed therebetween. The first electrode 2 and the second electrode 3 each have a first current collector and a second current collector made of metal, and a first electrode layer and a second electrode layer supported on the surface thereof, and exhibit capacitance by adsorbing and desorbing ions. For example, aluminum foil is used for the first and second current collectors. The surface of the current collector may be roughened by a method such as etching. For example, a nonwoven fabric mainly composed of cellulose is used for the separator 4. Lead wires 5a and 5b are connected to the first electrode 2 and the second electrode 3 as lead-out members, respectively. The capacitor element 1 is housed in a cylindrical exterior case 6 together with an electrolytic solution (not shown). The material of the exterior case 6 may be, for example, a metal such as aluminum, stainless steel, copper, iron, or brass. The opening of the exterior case 6 is sealed by a sealing member 7. The lead wires 5a and 5b are led out to the outside so as to penetrate the sealing member 7. For example, a rubber material such as butyl rubber is used for the sealing member 7.

[0106] The electrode layer contains an active material as an essential component and may contain a binder, a conductive auxiliary, etc. as optional components. The active material includes, for example, graphene having the characteristics already described. The electrode layer is obtained, for example, by applying a slurry obtained by kneading an active material, a binder (for example, carboxymethyl cellulose (CMC)), etc. with water using a kneader onto the surface of the current collector, drying the coating film, and rolling it.

[0107] The separator preferably has the above-described Va determined from the pore size distribution of 0.4 cm 3 / g or more. A reduction reaction product may be fixed to the separator.

[0108] In the above embodiment, the wound capacitor has been described, but the application range of the present invention is not limited to the above, and it can also be applied to capacitors having other structures, for example, multilayer or coin-type capacitors.

[0109] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to the examples.

[0110] <Examples 1 to 6, Comparative Examples 1 to 4> (1) Fabrication of the capacitor electrode Graphite was oxidized in water using potassium permanganate as an oxidizing agent to obtain graphene oxide. A water dispersion containing 1% by mass of graphene oxide was hydrothermally treated at 180 ° C for 6 hours to obtain a gel-like product (the first reduction step).

[0111] Subsequently, the gel-like product was immersed in an aqueous solution of sodium ascorbate (sodium ascorbate concentration: 1.0 mol / L) as a reducing agent, heated to 100 ° C, and held for 2 hours to sufficiently reduce the carbon (the second reduction step).

[0112] Thereafter, the gel-like product was freeze-dried (freeze-dried) at -20 ° C under a reduced pressure of 100 Pa to obtain a xerogel. Subsequently, the xerogel was heat-treated at 1200 ° C for 2 hours under a nitrogen flow (the third reduction step). The xerogel after the heat treatment was pulverized to obtain a powder of reduced graphene oxide.

[0113] 100 parts by mass of the reduced graphene oxide powder and 10 parts by mass of CMC as a binder were dispersed in an appropriate amount of water to prepare a slurry. The obtained slurry was applied to a current collector made of an Al foil having a thickness of 30 μm, and the coating film was vacuum-dried at 110 ° C and rolled to form an electrode layer, thereby obtaining a capacitor electrode.

[0114] (2) Preparation of separator Nonwoven separators a1 to a4 made of cellulose fibers, nonwoven separators a5, a6, b1, b2 made of synthetic fibers containing cellulose fibers, nonwoven separator c1 made of polypropylene (PP), and polyolefin nonwoven separator d1 were prepared.

[0115] Table 1 shows the thickness, porosity, and density (bulk density) of each of the separators a1 to a6, b1, b2, c1, and d1. Table 1 also shows the results of the total volume Va in the pore diameter range of 0.1 μm to 2 μm and the total volume Vm in the pore diameter range of 0.01 μm to 6 μm, measured using a mercury porosimeter.

[0116] [Table 1]

[0117] (3) Fabrication of capacitor A pair of capacitor electrodes were prepared and punched into a square shape of 20 mm × 20 mm. Lead wires were connected to each of the capacitor electrodes, and the capacitor electrodes were overlapped so that the coated surfaces of the electrode layers faced each other with separator a1 in between, to obtain a laminated capacitor element. The capacitor element was housed in an outer package case made of Al laminate together with an electrolytic solution and sealed with a sealing member to complete capacitor A1 of Example 1. Thereafter, an aging treatment was performed at 60 °C for 6 hours while applying 2.5 V.

[0118] As the electrolytic solution for capacitor A1, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), which is an ionic liquid, was used. The chemical formula of EMIBF4 is shown below. [Chemical formula]

[0119] Similarly, by using separators a2 to a6, b1, b2, c1, and d1 instead of separator a1 respectively, capacitors A2 to A6, B1, B2, C1, and D1 were fabricated. Capacitors A1 to A6 correspond to Examples 1 to 6 respectively, and capacitors B1, B2, C1, and D1 correspond to Comparative Examples 1 to 4 respectively.

[0120] For capacitors A1 to A6, B1, B2, C1, and D1, the evaluations shown below were respectively performed.

[0121] <Evaluation> The capacitances of each of capacitors A1 to A6, B1, B2, C1, and D1 were evaluated by the following method.

[0122] After manufacturing, the capacitor was charged at a constant current with a current of 0.1 to 1 A / g with respect to the cell active material weight (the total of the positive electrode active material mass and the negative electrode active material mass) up to the set voltage V1 (= 2.8 V). Thereafter, constant voltage charging was performed at the set voltage V1 for 8 minutes.

[0123] After the constant voltage charging was completed, constant current discharging was performed to 0 V with a current of 0.1 to 1 A / g with respect to the cell active material weight. The capacitance was calculated from the slope of the voltage change until the voltage decreased from 2.1 V to 0.1 V during discharging. The capacitance is calculated by the following formula, where the constant current value during discharging is I (A) and the discharging time until the voltage decreases from 2.1 V to 0.1 V is t (sec). Capacitance (F) = It / (2.1 - 0.1)

[0124] From the calculated capacitance, the single - pole capacitance was calculated assuming that the positive and negative electrode capacitances (positive and negative electrode active material masses) were the same, and the single - pole capacitance C1 (F / g) per 1 g of the positive and negative electrode active material mass when 2.8 V was applied was obtained.

[0125] After manufacturing, the set voltage of the capacitor was changed to V2 (= 4.4 V), and charging and discharging were similarly performed in the above charge - discharge cycle, and the single - pole capacitance C2 (F / g) per 1 g of the positive and negative electrode active material mass when 4.4 V was applied was obtained.

[0126] Table 2 shows the evaluation results of the single-pole capacitances C1 and C2 and the ratio C2 / C1 of C2 to C1 in each of the capacitors A1 to A6, B1, B2, C1, and D1, together with the total volume Va in the range of the pore diameter of the separator used from 0.1 μm to 2 μm.

[0127]

Table 2

[0128] From Table 2, when the applied voltage of the capacitor is 2.8 V, the difference in capacitance C1 in each of the capacitors A1 to A6, B1, B2, C1, and D1 is small. On the other hand, when the applied voltage of the capacitor is 4.4 V, the difference in capacitance C2 becomes significantly large among the capacitors A1 to A6, B1, B2, C1, and D1. However, for the capacitors A1 to A6 with Va of the separator of 0.4 cm 3 / g or more, even when a voltage of 4.0 V or higher is applied, a high capacitance can be realized.

[0129] <Example 7> (1) Fabrication of Capacitor Electrodes In the same manner as in Example 1, capacitor electrodes were obtained.

[0130] (2) Fabrication of Test Cell A cellulose nonwoven separator (porous base material) X1 was prepared. A pair of capacitor electrodes was prepared and punched into a square shape of 20 mm × 20 mm. Lead wires were connected to each of the capacitor electrodes, and the capacitor electrodes were overlapped so that the coated surfaces of the electrode layers faced each other with the separator X interposed therebetween, to obtain a laminated capacitor element. The capacitor element was housed together with the electrolytic solution in an outer package case made of Al laminate and sealed with a sealing member to complete the test cell. As the electrolytic solution, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), which is an ionic liquid, was used.

[0131] Thereafter, a voltage of 4.4 V to 4.6 V was applied to the test cell for 5 hours.

[0132] The separator (porous substrate) was taken out from the test cell after voltage application, washed with dimethyl carbonate (DMC), dried for 2 hours in a normal temperature and reduced pressure atmosphere, and separator Y1 with the reduction reaction product fixed thereon was obtained. The electrolyte in the test cell after voltage application had turned brown, and separator Y1 was also colored brown.

[0133] For the electrolyte taken out from the test cell 1 1H-NMR, 13 13C-NMR, 11 11B-NMR and 19 19F-NMR analysis was performed. 1 From the results of 1H-NMR and 13 13C-NMR, the presence of the EMI dimer shown in Chemical Formula 3 was confirmed. On the other hand, 11 from the results of 11B-NMR and 19 19F-NMR, no oxidation reaction product derived from the anion of the ionic liquid was confirmed, and it was confirmed that BF4 - remained unchanged as it was. Further, ESR analysis was performed on the electrolyte taken out from the test cell, and the presence of EMI radical and EMI carbene was confirmed.

[0134] When the mass of separator Y1 was measured, a 33% mass increase was observed compared to the state before voltage application (separator X1). The increase in mass is considered to be due to the fixed reduction reaction product.

[0135] (3) Fabrication of capacitor A pair of capacitor electrodes was prepared. Using unused EMIBF4 as the electrolyte and separator Y1, a capacitor element was fabricated in the same manner as the fabrication of the test cell, and capacitor A7 was obtained.

[0136] <Example 8> A non-woven fabric separator (porous substrate) X2 made of mixed fibers containing cellulose was prepared. Otherwise, in the same manner as in Example 7, separator Y2 with the reduction reaction product fixed thereon was obtained from separator X2, and a capacitor element was fabricated using separator Y2, and capacitor A8 was obtained.

[0137] <Comparative Example 5> A pair of capacitor electrodes was prepared. Using unused EMIBF4 as the electrolytic solution and separator X1, a capacitor element was fabricated in the same manner as in the preparation of the test cell to obtain capacitor E1. In other words, the test cell using separator X1 before the voltage application process of 4.4V to 4.6V was used as capacitor E1 and was made the evaluation target.

[0138] <Comparative Example 6> A pair of capacitor electrodes was prepared. Using unused EMIBF4 as the electrolytic solution and separator X2, a capacitor element was fabricated in the same manner as in the preparation of the test cell to obtain capacitor E2. In other words, the test cell using separator X2 before the voltage application process of 4.4V to 4.6V was used as capacitor E2 and was made the evaluation target. Capacitors A7 and A8 correspond to Examples 7 and 8 respectively, and capacitors E1 and E2 correspond to Comparative Examples 5 and 6 respectively.

[0139] For capacitors A7, A8, E1 and E2, the evaluations shown below were respectively performed.

[0140] (Unipolar capacitance measurement) After manufacturing, the capacitor was charged at a constant current of 0.1 A / g to 1 A / g with respect to the cell active material weight (the total of the positive electrode active material mass and the negative electrode active material mass) up to the set voltage (4.6V). Thereafter, constant voltage charging was performed at the set voltage for 8 minutes.

[0141] After the constant voltage charging was completed, constant voltage discharge was performed to 0V at a current of 0.1 A / g to 1 A / g with respect to the cell active material weight. The capacitance was calculated from the slope of the voltage change until the voltage decreased from 2.1V to 0.1V during discharge. The capacitance is calculated by the following formula using the constant current value during discharge as I (A) and the discharge time until the voltage decreases from 2.1V to 0.1V as t (sec). Capacitance (F) = It / (2.1 - 0.1)

[0142] From the calculated capacity, the single-pole capacity was calculated assuming that the positive and negative electrode capacities (positive and negative electrode active material masses) were the same, and the single-pole capacity C0 (F / g) per gram of the positive and negative electrode active material mass was determined when 4.6 V was applied.

[0143] (Charge-discharge efficiency) For the manufactured capacitor, the following charge 1 and discharge 1 were defined as one cycle, and this was repeated 250 times. [Charge 1] Constant current charging was performed at a current of 0.1 A / g to 1 A / g with respect to the cell active material weight (the total of the positive electrode active material mass and the negative electrode active material mass) up to the set voltage (= 4.6 V), and then constant voltage charging was performed at the set voltage for 8 minutes. [Discharge 1] Constant current discharge was performed at a current of 0.1 A / g to 1 A / g with respect to the cell active material weight until 0 V.

[0144] In each cycle, the charge amount Q1 in charge 1 and the discharge amount Q2 in discharge 1 were measured, and R = (Q2 / Q1) × 100 was evaluated as the charge-discharge efficiency.

[0145] Table 3 shows the evaluation results of the single-pole capacity C0 and the charge-discharge efficiency Q2 / Q1 in the 250th cycle for each capacitor A7, A8, E1, and E2. Also, FIG. 2 shows the change in the charge-discharge efficiency Q2 / Q1 for each cycle for each capacitor A7, A8, and E1.

[0146]

Table 3

[0147] From Table 3 and FIG. 2, in capacitors A7 and A8 using a separator on which the reduction reaction product was previously fixed, the capacity C0 increased and the charge-discharge efficiency also increased compared to capacitors E1 and E2 in which the reduction reaction product was not fixed to the separator.

Industrial Applicability

[0148] According to the present invention, a capacitor with a high energy density can be obtained.

[0149] Although the present invention has been described with respect to preferred embodiments at the present time, such disclosure should not be construed in a limiting sense. Various modifications and alterations will doubtless become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims are to be construed as encompassing all modifications and alterations without departing from the true spirit and scope of the present invention.

Explanation of Signs

[0150] 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: Capacitor

Claims

1. A first electrode including a first active material, a second electrode including a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution, wherein the electrolytic solution contains an ionic liquid. In the pore size distribution of the separator, the total volume Va in the range of pore sizes from 0.1 μm to 2 μm is 0.4 cm ３ / g or more, a capacitor.

2. The capacitor according to claim 1, wherein a voltage of 4.0 V or more is applied between the first electrode and the second electrode.

3. In the pore size distribution of the separator, the total volume Vm in the range of pore sizes from 0.01 μm to 6 μm is 0.7 cm ３ / g or more, and the capacitor according to claim 1.

4. The capacitor according to claim 1, wherein the porosity of the separator is 60% or more.

5. The capacitor according to any one of claims 1 to 4, wherein at least the first active material of the first active material and the second active material contains reduced graphene oxide.

6. The capacitor according to claim 5, wherein the reduced graphene oxide has a peak in a range of a pore diameter of 2 nm to 4 nm in a logarithmic differential pore volume distribution, and has a distribution in which the volume increases quadratically with an increase in the pore diameter in a range of a pore diameter of 4 nm to 50 nm.

7. The capacitor according to any one of claims 1 to 4, wherein the ionic liquid contains a cation having a complex aromatic ring.

8. The capacitor according to claim 7, wherein the cation contains a 1-ethyl-3-methylimidazolium cation.

9. The capacitor according to any one of claims 1 to 4, wherein the capacitance per mass of the first active material when a voltage of 4.0 V or more is applied is 300 F / g or more.

10. A first electrode including a first active material, a second electrode including a second active material, a separator interposed between the first electrode and the second electrode, and an electrolytic solution, wherein the electrolytic solution contains an ionic liquid, wherein the separator includes a porous substrate and a reduction reaction product of an ionic liquid fixed to the porous substrate.

11. The capacitor according to claim 10, wherein the content of the reduction reaction product fixed in the separator is 2% by mass or more.

12. The capacitor according to claim 10, wherein at least one of the first active material and the second active material contains reduced graphene oxide.

13. The capacitor according to claim 12, wherein the reduced graphene oxide has a peak in a range of a pore diameter of 2 nm to 4 nm in a logarithmic differential pore volume distribution, and has a distribution in which the volume increases quadratically with an increase in the pore diameter in a range of a pore diameter of 4 nm to 50 nm.

14. The capacitor according to any one of claims 10 to 13, wherein the capacitance per mass of the first active material when a voltage of 4.0 V or more is applied is 300 F / g or more.

15. A first electrode including a first active material, A second electrode including a second active material, A separator interposed between the first electrode and the second electrode, An electrolytic solution, and a method for manufacturing a capacitor comprising: Preparing a porous substrate and an ionic liquid; A method for manufacturing a capacitor, comprising: fixing a reduction reaction product of the ionic liquid to the porous substrate to obtain a separator.

16. The step of obtaining the separator includes: Disposing the porous substrate impregnated with the ionic liquid between a pair of electrodes to form a cell; Applying a voltage to the cell to generate the reduction reaction product; The method for manufacturing a capacitor according to claim 15, comprising:

17. The method for manufacturing a capacitor according to claim 15, wherein the porous substrate includes at least one selected from the group consisting of cellulose and its derivatives.

18. The method for manufacturing a capacitor according to any one of claims 15 to 17, wherein the ionic liquid includes a cation having a heteroaromatic ring.

19. The method for manufacturing a capacitor according to claim 18, wherein the cation includes a 1-ethyl-3-methylimidazolium cation.

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