Electrolyte for electric double layer capacitor and electric double layer capacitor

Abstract

To provide an electrolyte for an electric double layer capacitor, and an electric double layer capacitor, in which the solvent does not decompose even when used at a high voltage of 3.0 V or more and which can exhibit capacitance even in a low-temperature environment such as -30°C.SOLUTION: An electrolyte for an electric double layer capacitor contains sulfolane and a specific carboxylate as a solvent. The specific carboxylate has two or more ester bonds. In the molecular structure, the specific carboxylate has a linear carbon skeleton with two or more carbon atoms along the linear chain. Carboxylate ester groups are bonded to both ends of the linear carbon skeleton, that is, the linear carbon skeleton is sandwiched between two ester bonds.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an electrolytic solution for an electric double layer capacitor and an electric double layer capacitor comprising the electrolytic solution. [Background technology]

[0002] An electric double layer capacitor is composed of a pair of polarizable electrodes impregnated with an electrolyte, housed in a container, and utilizes the charge storage effect of the electric double layer formed at the interface between the polarizable electrodes and the electrolyte. That is, at the positive electrode, the anions of the electrolyte are aligned at the interface with the polarizable electrode, and pair with the pores in the polarizable electrode at an extremely short distance. This forms a potential barrier at the positive electrode. At the negative electrode, the cations of the electrolyte are aligned at the interface with the polarizable electrode, and pair with the electrons in the polarizable electrode at an extremely short distance, forming a potential barrier at the negative electrode.

[0003] In an electric double layer capacitor, activated carbon powder is typically used as the polarizable electrode material, a metal having valve action such as aluminum is used as the current collector, and an aprotic electrolyte is used as the electrolyte. Quaternary ammonium salts are mainly used as the electrolyte for the electrolyte. Typically, carbonate solvents such as propylene carbonate, or carboxylates such as γ-butyrolactone are used as the solvent for the electrolyte (see, for example, Patent Document 1).

[0004] However, when propylene carbonate or gamma-butyrolactone is used as the solvent for the electrolyte and the electric double layer capacitor is charged at a voltage of 3.0 V or more, the low voltage resistance characteristics of propylene carbonate and gamma-butyrolactone cause an electrochemical reaction and decomposition, resulting in the generation of gas or deposits in the electric double layer capacitor. The generation of gas or deposits can cause the capacity of the electric double layer capacitor to deteriorate, the resistance to increase, or the case to open.

[0005] In order to obtain high voltage characteristics without generating gas and deposits, it is necessary to connect multiple electric double layer capacitors in series, which increases the number of electric double layer capacitors and requires a cell voltage equalization circuit to suppress voltage variations, resulting in a larger system.

[0006] In view of this, an electric double layer capacitor using sulfolane as a solvent for the electrolyte has been proposed (see, for example, Patent Document 2). Sulfolane is one of the most electrochemically stable solvents, and it has been reported that by combining it with activated carbon fiber, which has a low impurity content, it is possible to improve the storage stability, charge / discharge characteristics, and voltage resistance of the electric double layer capacitor. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2001-217150 A [Patent Document 2] Japanese Patent Application Publication No. 06-275468 Summary of the Invention [Problem to be solved by the invention]

[0008] Electric double layer capacitors physically store electric charge without using chemical reactions for charging and discharging. Therefore, electric double layer capacitors are less susceptible to deterioration of their constituent materials and have an excellent charge and discharge cycle life. Therefore, electric double layer capacitors are often used, for example, as backup power sources. In recent years, with the advancement of electrification of automobiles, electric double layer capacitors are also being used as on-board backup power sources for safety reasons.

[0009] Electric double layer capacitors for in-vehicle use are expected to be used in low-temperature environments, such as -30° C. Because the freezing point of sulfolane is 28° C., an electric double layer capacitor that uses sulfolane as a solvent for the electrolyte at -20° C. will lose its chargeable / dischargeable capacitance to zero, making it unusable.

[0010] The present invention has been proposed to solve the above problems, and an object of the present invention is to provide an electrolyte for an electric double layer capacitor, in which the solvent does not decompose even when used at a high voltage of 3.0 V or more, and which is capable of exhibiting capacitance even in a low-temperature environment such as -30°C, and an electric double layer capacitor. [Means for solving the problem]

[0011] In order to solve the above problems, the electrolytic solution for electric double layer capacitors of the present embodiment is an electrolytic solution used in electric double layer capacitors, and contains sulfolane and a carboxylic acid ester represented by the following chemical formula (1) as a solvent, the carboxylic acid ester having two or more ester bonds, and the X structure in the following chemical formula (1) has a linear carbon skeleton with two or more carbon atoms along the linear chain. [ka] [In the formula, n and m are each 1 or more.]

[0012] The solvent has a SO band of 2.05 cm as observed by Raman spectroscopy compared to sulfolane alone. -1 The peak may be shifted in this manner.

[0013] The carboxylate ester may have the carbon skeleton having 3 to 6 carbon atoms along a straight chain within the X structure in the chemical formula (1).

[0014] The solvent may contain two or more kinds of the carboxylate ester.

[0015] Each of the two or more kinds of carboxylate esters may have the carbon skeleton having 3 or more and 6 or less carbon atoms along a straight chain in the X structure in the chemical formula (1).

[0016] The solvent may further include a carboxylic acid ester having one ester bond.

[0017] Propylene carbonate and γ-butyrolactone may be absent.

[0018] The carboxylate ester may have a solubility parameter, SP, of 9 or more.

[0019] The carboxylate may have a methyl group, an ethyl group, a propyl group or a butyl group as a branched chain in the X structure in the chemical formula (1).

[0020] The carboxylate may have a carboxylate group in the X structure of the chemical formula (1).

[0021] The solvent may further include dimethylsulfone.

[0022] The carboxylate may have 3 or less ester bonds.

[0023] A quaternary ammonium salt may further be included.

[0024] In order to solve the above problems, the electric double layer capacitor of the present embodiment includes such an electrolyte for an electric double layer capacitor, and positive and negative electrode assemblies.

[0025] The capacitance ratio of the positive electrode to the negative electrode may be greater than 1.13. Effect of the Invention

[0026] According to the present invention, the electric double layer capacitor does not decompose the solvent even when used at a high voltage of 3.0 V or more, and can exhibit capacitance even in a low-temperature environment such as -30°C. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the preferred embodiments. However, the present invention is not limited to the preferred embodiments described below.

[0028] (electrolyte) The electrolytic solution of this embodiment is an electrolytic solution for an electric double layer capacitor. An electric double layer capacitor is a passive element that utilizes the charge storage action of an electric double layer formed at the interface between a polarizable electrode and an electrolytic solution to store and discharge electric charges by electrostatic capacitance. An electric double layer capacitor element includes a positive electrode, a negative electrode, and a separator. The electrolytic solution is filled in the voids of the electric double layer capacitor element.

[0029] (solvent) The solvent of this electrolyte is a mixed solvent containing sulfolane and a specific carboxylate ester (hereinafter, referred to as the specific carboxylate ester). The specific carboxylate ester is represented by the following chemical formula (1) and has two or more ester bonds. [ka] [In the formula, n and m are each 1 or more.]

[0030] The X structure in the above chemical formula (1) is a partial structure sandwiched between carboxylate groups at both ends. This X structure has a linear carbon skeleton. An example of a linear carbon skeleton is a chain hydrocarbon. This carbon skeleton has two or more carbon atoms along the linear chain. In other words, the specific carboxylate ester has a chain hydrocarbon with two or more carbon atoms along the linear chain, and both ends of the chain hydrocarbon are substituted with a carboxylate ester group.

[0031] The linear carbon skeleton may have a methyl group, an ethyl group, a propyl group, a butyl group, or a plurality of these groups bonded as a branched chain. The linear carbon skeleton may also have a carboxylate bonded as a branched chain, and the specific carboxylate may have a total of three or more ester bonds.

[0032] This specific carboxylate has a solubility parameter SP value of 9 or more. Carboxylate esters with an SP value of 9 or more tend to have good electron acceptor properties, and generate a large interaction between the ester group and the sulfonyl group of sulfolane. This interaction lowers the freezing point of sulfolane, improving the capacitance of an electric double layer capacitor in a low-temperature environment such as -30°C. Although this is speculation and is not intended to be limiting, it is speculated that the freezing point depression of sulfolane follows the van't Hoff coefficient.

[0033] However, this specific carboxylate ester has a carbon number of C=2 or more along the straight chain of the chain hydrocarbon. When the carbon number of C=1 along the straight chain of the chain hydrocarbon, this carbon becomes an α-carbon relative to the carboxylate ester groups at both ends. The α-carbon is a carbon that is prone to decomposition reactions because α-hydrogen is easily eliminated. Therefore, if the α-carbons relative to the carboxylate ester groups at both ends are the same carbon, when used at a high voltage of 3.0 V or more, hydrogen from the carboxylate ester is eliminated in the solvent to generate an enolate, and the enolate is decomposed to generate gas that was not observed when the solvent was sulfolane alone. On the other hand, when the carbon number of C=2 or more along the straight chain of the chain hydrocarbon, gas generation is suppressed.

[0034] Therefore, when the solvent of the electrolyte is a mixed solvent containing sulfolane and a specific carboxylate, the solvent does not decompose even when the electric double layer capacitor is used at a high voltage of 3.0 V or more, and the capacitance can be exhibited even in a low temperature environment such as −30° C. Examples of the specific carboxylate include dimethyl succinate, diethyl succinate, dimethyl glutarate, diethyl glutarate, dimethyl pimelate, diethyl pimelate, dimethyl 2-methylglutarate, diethyl 2-methylglutarate, trimethyl 1,2,3-propanetricarboxylate, triethyl 1,2,3-propanetricarboxylate, and triethyl o-acetylcitrate.

[0035] The amount of sulfolane in the solvent is preferably 50 vol% or more and 90 vol% or less based on the total amount of the solvent in the electrolyte. The amount of the specific carboxylic acid ester in the solvent is preferably 1 vol% or more and 50 vol% or less based on the total amount of the solvent in the electrolyte. If the amount of the specific carboxylic acid ester is too small, the peak shift of the sulfonyl group of sulfolane becomes small, and the capacitance expression effect in a low temperature environment decreases. If the amount of sulfolane is too small, gas is easily generated when used at a high voltage of 3.0 V or more.

[0036] The number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is preferably 3 or more and 6 or less. When the number of carbon atoms is in the range of 3 or more and 6 or less, the SP value is much higher than 9. For example, dimethyl succinate has an SP value of 9.52, dimethyl glutarate has an SP value of 10.36, dimethyl adipate has an SP value of 9.90, dimethyl pimelate has an SP value of 9.87, dimethyl suberate has an SP value of 9.69, and dimethyl azelate has an SP value of 7.59. When the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is 3 or more and 6 or less, the SO2 peak shift of the sulfonyl group of sulfolane is 2.05 cm. -1 The electric double layer capacitor exhibits capacitance even in a low-temperature environment of -40°C.

[0037] The reference value is the Raman shift of the sulfonyl group measured by Raman spectroscopy using only sulfolane as a sample. The measured value is the Raman shift of the sulfonyl group measured by Raman spectroscopy using a mixed solvent of sulfolane and a specific carboxylic acid ester. The SO2 peak shift is the difference between this measured value and the reference value. Specifically, the reference value is the Raman shift of 1140 cm -1 This is a peak that appears near the reference value. In a mixed solvent of sulfolane and a specific carboxylic acid ester, the Raman shift of the sulfonyl group changes from the reference value due to the interaction between sulfolane and the specific carboxylic acid ester. The peak after this change is the measured value.

[0038] The method for calculating the SP value is as follows, where ΔH is the molar evaporation energy and V is the molar molecular volume [cm 3 / mol], R is the gas constant, and T is the absolute temperature. The molar volume was calculated from the molecular weight and specific gravity. δ = [(ΔH-RT) / V] 1 / 2

[0039] Also, preferably, the branched chain bonded to the chain hydrocarbon in the X structure does not contain an acetoxy group. In other words, the branched chain of the carboxylate group is preferably bonded to the chain hydrocarbon by carbon-carbon bond, not to the chain hydrocarbon via an oxygen atom. For example, triethyl 1,2,3-propanetricarboxylate exhibits better capacitance in a low-temperature environment than triethyl o-acetylcitrate.

[0040] As a solvent for an electrolyte, it is preferable to mix two or more specific carboxylate esters. A mixed solvent containing two or more specific carboxylate esters has a larger SO2 peak shift than a mixed solvent containing a single specific carboxylate ester due to the interaction between molecules having ester bonds. This further improves the capacitance in a low-temperature environment. For example, even if a single type has zero capacitance in a temperature environment of -40°C, two or more types will exhibit capacitance in a low-temperature environment of -40°C.

[0041] More preferably, all the specific carboxylates in the solvent have 6 or less carbon atoms along the linear chain of the chain hydrocarbon in the X structure. When the specific carboxylates in the X structure, in which the number of carbon atoms along the linear chain of the chain hydrocarbon is 7 or more, constitute a part of the solvent, the capacitance at a low temperature of -40°C tends to be zero or to decrease. This is because the chain hydrocarbon in the X structure becomes longer and has a smaller effect on the SO2 peak shift. In other words, the longer chain hydrocarbon works in the direction of weakening the interaction with the sulfonyl group of sulfolane. Therefore, when two or more types of specific carboxylates are mixed, it is preferable not to include dimethyl azelate or dimethyl sebacate, in which the number of carbon atoms along the linear chain in the X structure is C=7 or C=8, in the group.

[0042] In addition, it is preferable to further contain a carboxylic acid ester having one ester bond as a solvent for the electrolyte in addition to sulfolane and the specific carboxylic acid ester. An example of a carboxylic acid ester having one ester bond is ethyl propionate. Even if the combination of sulfolane and the specific carboxylic acid ester has zero capacitance in a temperature environment of -40°C, the addition of a carboxylic acid ester having one ester bond will produce capacitance even in a low-temperature environment of -40°C.

[0043] That is, even if a carboxylic acid ester having one ester bond is combined with sulfolane to form a two-type mixed solvent, the capacitance of the electric double layer capacitor is not expressed in a low-temperature environment. However, when this carboxylic acid ester having one ester bond is added, the interaction between molecules having ester bonds is strengthened, and the SO2 peak shift becomes larger. Furthermore, the capacitance in a low-temperature environment is improved compared to the case of a two-type mixed solvent combining sulfolane and a specific carboxylic acid ester.

[0044] Such a solvent may be mixed with other solvents different from the carboxylate ester having one ester bond. Examples of other solvents include dimethyl sulfone and dimethyl 1,4-cyclohexanedicarboxylate. However, when propylene carbonate and γ-butyrolactone are used at a high voltage of 3.0 V or more, even when used in combination with sulfolane and a specific carboxylate ester, the amount of gas generated increases. When 0.1 vol% of propylene carbonate or γ-butyrolactone was added to the solvent and used at a high voltage of 3.0 V or more, a large amount of gas was generated. Therefore, it is preferable to add a solvent other than propylene carbonate and γ-butyrolactone. It is also preferable not to include propylene carbonate and γ-butyrolactone in the solvent.

[0045] (electrolyte) Next, the electrolyte of the electrolytic solution may contain one or more selected from the group consisting of quaternary ammonium salts and lithium salts. Any quaternary ammonium salt or lithium salt can be used as long as it is an electrolyte capable of generating quaternary ammonium ions or lithium ions.

[0046] The electrolyte may be present in an amount of 0.5 to 3.5 moles per 1 L of solvent. Preferably, the amount of electrolyte added is in the range of 2.5 to 3.5 moles per 1 L of solvent. Within this range, the capacitance at a low temperature of -40°C is better than when the amount is outside this range.

[0047] Examples of the quaternary ammonium salt include tetramethylammonium, ethyltrimethylammonium, tetraethylammonium, triethylmethylammonium, diethyldimethylammonium, methylethylpyrrolidinium, and spirobipyrrolidinium as cationic components, and BF4 - , PF6 - , ClO4 - , AsF6 - , SbF6 - , AlCl4 - , or RfSO3- , (RfSO2)2N - , RfCO2 - (Rf is a fluoroalkyl group having 1 to 8 carbon atoms).

[0048] Typically, the quaternary ammonium salts include tetramethylammonium BF4, ethyltrimethylammonium BF4, diethyldimethylammonium BF4, triethylmethylammonium BF4, tetraethylammonium BF4, spirobipyrrolidinium BF4, methylethylpyrrolidinium BF4, tetramethylammonium PF6, ethyltrimethylammonium PF6, diethyldimethylammonium PF6, triethylmethylammonium PF6, tetraethylammonium PF6, spirobipyrrolidinium PF6, methylethylpyrrolidinium PF 6、 Tetramethylammonium bis(oxalato)borate, ethyltrimethylammonium bis(oxalato)borate, diethyldimethylammonium bis(oxalato)borate, triethylmethylammonium bis(oxalato)borate, tetraethylammonium bis(oxalato)borate, spirobipyrrolidinium bis(oxalato)borate, methylethylpyrrolidinium bis(oxalato)borate, tetramethylammonium difluorooxalatoborate, ethyltrimethylammonium difluorooxalatoborate, diethyldimethylammonium difluorooxalatoborate, triethylmethylammonium difluorooxalatoborate, tetraethylammonium difluorooxalatoborate, spirobipyrrolidinium difluorooxalatoborate, methylethylpyrrolidinium difluorooxalatoborate, and the like can be used.

[0049] The lithium salt may be LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(SO2C2F5)2, CF3SO3Li, LiC(SO2CF3)3, and LiPF3(C2F5)3, or mixtures thereof.

[0050] The electrolyte may contain various additives. Examples of additives include phosphoric acids and their derivatives (phosphoric acid, phosphorous acid, phosphoric esters, phosphonic acids, etc.), boric acids and their derivatives (boric acid, boric acid oxide, boric esters, complexes of boron with a hydroxyl group, a carboxyl group, or a compound having a plurality of these, etc.), nitrates (lithium nitrate, etc.), nitro compounds (nitrobenzoic acid, nitrophenol, nitrophenetole, nitroacetophenone, aromatic nitro compounds, etc.), etc. The electrolyte may also contain a gas absorbent. There are no particular limitations on the absorbent for gas generated from the electrodes, so long as it does not react with each component of the electrolyte (solvent, electrolyte salt, various additives, etc.) and does not remove (adsorb, etc.). Specific examples include zeolite, silica gel, etc.

[0051] The method for preparing the electrolyte is not limited, but for example, it can be prepared as follows. First, a mixed solvent is prepared by mixing sulfolane and a specific carboxylate ester in an arbitrary ratio. In a dry room, an electrolyte is added to a 1 L measuring flask under normal temperature and pressure conditions (1 atmosphere and 20°C), and then the mixed solvent is added until the total volume reaches 1 L. Additives other than the specific carboxylate ester are added at the same time as the specific carboxylate ester is added, and mixed.

[0052] (Electric double layer capacitor) In an electric double layer capacitor, the positive and negative electrode bodies are formed by laminating polarizable electrodes on a current collector. The current collector can be made of a metal having valve action, such as aluminum foil, platinum, gold, nickel, titanium, steel, or carbon. The shape of the current collector can be any shape, such as a film, foil, plate, mesh, expanded metal, or cylinder. The surface of the current collector may be unevenly etched or may be plain. Furthermore, phosphorus may be attached to the surface of the current collector by surface treatment.

[0053] A cylindrical collector is an electric double layer capacitor element that is wound. In a wound electric double layer capacitor, the positive and negative electrode collectors have a long strip shape and are wound in a spiral shape with a separator between them. In the positive and negative electrodes, the direction along the spiral is called the strip length direction, and generally, the positive and negative electrodes are wound along the long length direction, which is the strip length direction.

[0054] The polarizable electrode contains an active material having an electric double layer capacity. The main active material is preferably activated carbon. The activated carbon may be made from natural plant tissue such as coconut shell, synthetic resin such as phenol, or fossil fuel such as coal, coke, or pitch, and may be subjected to activation treatment such as steam activation, alkali activation, zinc chloride activation, or electric field activation, as well as aperture treatment.

[0055] The polarizable electrode may be formed, for example, by applying a slurry of activated carbon and a binder onto a current collector by a doctor blade method or the like, and drying the electrode. The mixture of the carbon material and the binder may be molded into a sheet and pressed onto the current collector. Examples of the binder include rubbers such as fluorine-based rubber, diene-based rubber, and styrene-based rubber; fluorine-containing polymers such as polytetrafluoroethylene and polyvinylidene fluoride; celluloses such as carboxymethyl cellulose and nitrocellulose; and polyolefin resins, polyimide resins, acrylic resins, nitrile resins, polyester resins, phenolic resins, polyvinyl acetate resins, polyvinyl alcohol resins, and epoxy resins. These binders may be used alone or in combination of two or more.

[0056] The polarizable electrode may contain a conductive assistant. Examples of the conductive assistant include carbon black such as Ketjen black, acetylene black, and channel black, carbon nanohorns, amorphous carbon, natural graphite, artificial graphite, graphitized Ketjen black, mesoporous carbon, carbon nanotubes, and carbon nanofibers. The carbon nanotube may be a single-walled carbon nanotube (SWCNT) having one graphene sheet, or a multi-walled carbon nanotube (MWCNT) having two or more graphene sheets rolled coaxially to form multiple tube walls, or a mixture of these.

[0057] Also, phosphorus may be attached to the surface of the current collector to suppress hydration and oxidation of the current collector. For example, the current collector is immersed in an aqueous phosphoric acid solution or an aqueous phosphate solution. Furthermore, a carbon coating layer containing a conductive agent such as graphite may be provided between the current collector and the polarizable electrode layer. The carbon coating layer can be formed by applying a slurry containing a conductive agent such as graphite, a binder, etc. to the surface of the current collector and drying it.

[0058] When the electrode areas of the polarizable electrodes of the electric double layer capacitor are made different, the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance is preferably more than 1.13 times. That is, the polarizable electrode layers are adjusted so that the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance is more than 1.13 times. For example, the capacitance of the positive electrode and the capacitance of the negative electrode may be adjusted by the thickness of each polarizable electrode layer.

[0059] When the capacitance ratio of the positive electrode to the negative electrode is 1.13 times or less, even if sulfolane is used as a solvent for the electrolyte, a large amount of gas is generated in the electric double layer capacitor. For example, in an electric double layer capacitor using a wound element, the strip lengths of the positive electrode and the negative electrode may be different, and the electrode areas of the polarizable electrodes may be different. In particular, when the negative electrode is arranged on the outermost periphery of the wound element so that the strip length of the negative electrode is longer than the strip length of the positive electrode, there is a risk of exceeding the potential window on the oxidation side of the electrolyte. Therefore, even if the electrode areas of the positive electrode and the negative electrode are different, it is desirable that the amount of gas generated is even less than the amount of gas generated when the capacitance ratio of the positive electrode to the negative electrode is 1.13 times or less.

[0060] When the capacitance ratio of the positive electrode to the negative electrode exceeds 1.13, the amount of gas generated is reduced to half or less compared to when the capacitance ratio of the positive electrode to the negative electrode is 1.13. For example, when the capacitance ratio of the positive electrode to the negative electrode is 1.35 or more, the amount of gas generated is reduced to half or less compared to when the capacitance ratio of the positive electrode to the negative electrode is 1.13.

[0061] The capacitance of the positive electrode and the capacitance of the negative electrode are measured as follows. That is, a test piece of a specified area is cut out, and a platinum plate is used as the counter electrode, and the test piece is immersed in a capacitance measuring solution in a glass measuring tank, and the capacitance is measured using a capacitance meter. For example, the specified area is 1 cm. 2 The capacitance measurement liquid is an aqueous solution of ammonium pentaborate at 30°C, the capacitance meter is a potentiostat and a frequency response analyzer, an electrochemical impedance analyzer, or an LCR meter, etc., and the measurement conditions are a DC bias voltage of 1.5 V and an AC amplitude of 1 V.

[0062] The separator interposed between the positive and negative electrodes is a cellulose or a mixture thereof, a polyester resin, a polytetrafluoroethylene resin, a polyvinylidene fluoride resin, a vinylon resin, a polyamide resin, a polyimide resin, a polyethylene resin, a polypropylene resin, a trimethylpentene resin, a polyphenylene sulfide resin, an acrylic resin, or the like, and these resins can be used alone or in combination. Examples of cellulose include kraft, Manila hemp, esparto, hemp, and rayon. Examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, and derivatives thereof. Examples of polyamide resins include polytetrafluoroethylene resin, polyvinylidene fluoride resin, vinylon resin, aliphatic polyamide, and aramid. Aramid includes semi-aromatic polyamide and fully aromatic polyamide. EXAMPLES

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0064] Example 1 The electrolytic solution for the electric double layer capacitor and the electric double layer capacitor of Example 1 were prepared as follows. First, a mixed solvent of sulfolane and dimethyl glutarate was prepared. Sulfolane (A) and dimethyl glutarate (B) were mixed in a volume ratio of A:B=70:30. 1.5 moles of spirobipyrrolidinium tetrafluoroborate (SBPBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolytic solution for the electric double layer capacitor of Example 1 was prepared.

[0065] Next, activated carbon that had been activated with water vapor was used as the active material, and this activated carbon was mixed with carbon black as a conductive assistant, carboxymethyl cellulose as a dispersant, SBR emulsion as a binder, and pure water to obtain a slurry. The ratio of carbon black to the total amount of activated carbon and carbon black was 5 wt%. This slurry was applied to an aluminum current collector and dried to produce positive and negative electrode bodies. The polarizable electrodes of activated carbon and the aluminum current collector were pressed together using a press.

[0066] A separator made of nonwoven fabric was sandwiched between the positive and negative electrode bodies to form an element in which the positive electrode, the separator, and the negative electrode were laminated. This element was impregnated with the electrolyte for the electric double layer capacitor of Example 1. After the element was impregnated with the electric double layer capacitor, it was laminated and sealed to produce the electric double layer capacitor of Example 1. A constant voltage of 3.1 V was applied to this laminate cell in a temperature environment of 25° C., and aging treatment was performed for 12 hours.

[0067] (Comparative Example) Electrolyte solutions for electric double layer capacitors of Comparative Examples 1 to 5 were prepared. The electrolytic solution of Comparative Example 1 contained only sulfolane as a solvent, and the type and amount of electrolyte added were the same as those of Example 1. The electrolytic solution of Comparative Example 2 contained only γ-butyrolactone as a solvent, and the type and amount of electrolyte added were the same as those of Example 1. The electrolytic solution of Comparative Example 3 contained only propylene carbonate as a solvent, and the type and amount of electrolyte added were the same as those of Example 1.

[0068] The solvent of the electrolytic solution of Comparative Example 4 was a mixed solvent of sulfolane (A) and dimethyl malonate (B) in a volume ratio of A:B=70:30.

[0069] The solvent of the electrolytic solution of Comparative Example 5 was a mixed solvent of sulfolane (A) and ethyl propionate (B) in a volume ratio of A:B=70:30.

[0070] Using the electrolytic solutions for electric double layer capacitors of Comparative Examples 1 to 5, electric double layer capacitors were fabricated with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 1.

[0071] (Characteristics test 1) The electric double layer capacitors of Example 1 and Comparative Examples 1 to 5 were placed in a temperature environment of 60° C. and held for 1500 hours while applying a constant voltage of 3.1 V. The amount of gas generated was measured after 0 hours, 150 hours, 300 hours, 600 hours, 1000 hours, and 1500 hours. The gas generated inside the laminate cell was removed with a syringe, and the volume when the pressure was returned to atmospheric pressure was regarded as the amount of gas generated.

[0072] The amounts of gas generated at each elapsed time for the electric double layer capacitors of Example 1 and Comparative Examples 1 to 5 are summarized in Table 1 below. In the table, SLF is sulfolane, GBL is γ-butyrolactone, and PC is propylene carbonate. Also, in the table, MeGlu is dimethyl glutarate, EtPro is ethyl propionate, and MeMal is dimethyl malonate.

[0073] (Table 1) TIFF2025013304000003.tif81161

[0074] In addition, the electric double layer capacitors of Example 1 and Comparative Examples 1 to 5 were placed in temperature environments of 20°C, -20°C, -30°C, and -40°C, and the capacitance was measured under each temperature environment. Specifically, in each temperature environment, the electric double layer capacitor was charged by applying a constant voltage of 3.1V after constant current discharge to 0.1V. After the charging was completed, the constant current discharge was performed immediately. Then, the capacitance was measured according to the constant current discharge method. During the discharge at the constant current I, the required time T required for the voltage to drop from the measurement start voltage to the measurement end voltage of 0.1V was measured, excluding the IR drop. Then, the product of the constant current I and the required time T was divided by the difference (V) between the measurement start voltage and the measurement end voltage to calculate the capacitance (F). Furthermore, the percentage of the capacitance at each temperature relative to the capacitance at 20°C was calculated as the capacity retention rate.

[0075] The capacitance retention rates at each temperature of the electric double layer capacitors of Example 1 and Comparative Examples 1 to 5 are summarized in Table 2 below. (Table 2) TIFF2025013304000004.tif81161

[0076] As shown in Table 1 above, the electric double layer capacitor using the electrolyte for electrolytic capacitors of Example 1 had a suppressed amount of gas generation compared to Comparative Examples 2 to 4. In addition, as shown in Table 2 above, the electric double layer capacitor using the electrolyte for electrolytic capacitors of Example 1 had a better capacity retention rate in a low temperature environment compared to Comparative Examples 1 and 5.

[0077] On the other hand, in Comparative Example 4, in which dimethyl malonate, in which the carbon number of the chain hydrocarbon in the X structure is 1, was mixed, the amount of gas generated was worse than in Comparative Examples 2 and 3, in which γ-butyrolactone or propylene carbonate was used. In Comparative Example 5, in which ethyl propionate, in which one ester bond was mixed, capacitance was generated in a temperature environment of -30°C, but the capacity retention rate was low. In Comparative Example 2, the measurement of the amount of gas generated was stopped after 1000 hours because γ-butyrolactone was deteriorated and measurement was impossible.

[0078] In contrast, in Example 1, the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is 3, and dimethyl glutarate having carboxylate groups at both ends of the X structure is mixed in. In Example 1, the amount of gas generated was suppressed to the same level as in Comparative Example 1, which used sulfolane alone as a solvent, and the capacitance retention rate in a temperature environment of -30°C was also good.

[0079] (Examples 2 to 9) As described below, the electrolytic solution for electric double layer capacitors and the electric double layer capacitors of Examples 2 to 9 were prepared. The mixed solvent of the electrolytic solution of Example 2 was prepared by mixing sulfolane and dimethyl succinate. Sulfolane (A) and dimethyl succinate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolytic solution for electric double layer capacitors of Example 2 was prepared.

[0080] The mixed solvent of the electrolyte solution of Example 3 was prepared by mixing sulfolane and diethyl succinate. Sulfolane (A) and diethyl succinate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte solution for the electric double layer capacitor of Example 3 was prepared.

[0081] The mixed solvent of the electrolyte of Example 4 was prepared by mixing sulfolane and dimethyl glutarate. Sulfolane (A) and dimethyl glutarate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 4 was prepared.

[0082] The mixed solvent of the electrolyte solution of Example 5 was prepared by mixing sulfolane and diethyl glutarate. Sulfolane (A) and diethyl glutarate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte solution for the electric double layer capacitor of Example 5 was prepared.

[0083] The mixed solvent of the electrolyte solution of Example 6 was prepared by mixing sulfolane and dimethyl pimelate. Sulfolane (A) and dimethyl pimelate (B) were mixed in a volume ratio of A:B=70:30. 1.5 moles of spirobipyrrolidinium tetrafluoroborate (SBPBF4) was added as an electrolyte to this mixed solvent per 1 L of solvent. Thus, the electrolyte solution for the electric double layer capacitor of Example 6 was prepared.

[0084] The mixed solvent of the electrolyte of Example 7 was prepared by mixing sulfolane and dimethyl 2-methylglutarate. Sulfolane (A) and dimethyl 2-methylglutarate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 7 was prepared.

[0085] The mixed solvent of the electrolyte of Example 8 was prepared by mixing sulfolane and trimethyl 1,2,3-propanetricarboxylate. Sulfolane (A) and trimethyl 1,2,3-propanetricarboxylate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 8 was prepared.

[0086] The mixed solvent of the electrolyte of Example 9 was prepared by mixing sulfolane and trimethyl o-acetyl citrate. Sulfolane (A) and trimethyl o-acetyl citrate (B) were mixed in a volume ratio of A:B=70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 9 was prepared.

[0087] Using the electrolytic solutions for electric double layer capacitors of Examples 2 to 9, electric double layer capacitors were fabricated with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 1.

[0088] (Capacity test) For the electric double layer capacitors of Examples 2 to 9, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance measurement and the capacity retention rate calculation were performed using the same measurement method and conditions as in Example 1.

[0089] The capacitance retention rates at each temperature of the electric double layer capacitors of Examples 2 to 9 are summarized in Table 3 below. In the table, SLF is sulfolane, MeSuc is dimethyl succinate, EtSuc is diethyl succinate, MeGlu is dimethyl glutarate, EtGlu is diethyl glutarate, MePim is dimethyl pimelate, 2MethylMeGlu is dimethyl 2-methylglutarate, CAEMeGlu is trimethyl 1,2,3-propanetricarboxylate, and AcMeCit is trimethyl o-acetylcitrate.

[0090] (Table 3) TIFF2025013304000005.tif101161

[0091] As shown in Table 3, the capacitance is good in a temperature environment of -30°C by mixing a specific carboxylate other than dimethyl glutarate of Example 1 with sulfolane. Moreover, Example 4 has a different type of electrolyte compared to Example 1 containing SBPBF4 as the electrolyte, but has a good capacitance in a temperature environment of -30°C like Example 1.

[0092] Therefore, it was confirmed that when the solvent of the electrolyte for an electric double layer capacitor is a mixed solvent of sulfolane and a specific carboxylic acid ester, gas generation is suppressed even when the electric double layer capacitor is used at a high voltage of 3.0 V or more, and the capacitance of the electric double layer capacitor is good even in a low temperature environment such as -30°C.

[0093] In particular, in Examples 4 to 9, capacitance is exhibited even in a temperature environment of -40°C. In dimethyl glutarate and diethyl glutarate contained in Examples 4 and 5, the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is C=3. In diethyl pimelate contained in Example 6, the number of carbon atoms of the chain hydrocarbon in the X structure is C=5. In Examples 7 to 9, the number of carbon atoms of the chain hydrocarbon in the X structure is also C=3.

[0094] The SP value of each specific carboxylate ester and the SO2 peak shift when mixed with sulfolane are shown in Table 4 below. In the table, the carbon number is the number of carbons along the straight chain of the chain hydrocarbon in the X structure. The SO2 peak shift was evaluated by Raman spectroscopy with the volume ratio of sulfolane (A) to specific carboxylate (B) in the solvent being A:B=70:30.

[0095] (Table 4) TIFF2025013304000006.tif81161

[0096] As shown in Table 4, the specific carboxylate ester in which the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is 3 to 6 has an SP value that is significantly greater than 9. Therefore, when the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is 3 to 6, the SO2 peak shift is also 2.05 cm. -1 As a result, in Examples 4 to 9 in Table 3, the capacitance is good in a temperature environment of -30°C, and the capacitance is also exhibited in a temperature environment of -40°C.

[0097] In this way, it was confirmed that when the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure is 3 to 6, the capacitance is good in a temperature environment of -30°C, and capacitance is also exhibited in a temperature environment of -40°C. In addition, compared to when the solvent is sulfolane alone, the SO2 band observed by Raman spectroscopy is 2.05 cm -1It was confirmed that the peak-shifted sulfolane not only improved the capacitance in a temperature environment of -30°C, but also demonstrated capacitance in a temperature environment of -40°C.

[0098] (Gas generation test) The following electrolytic solutions for electric double layer capacitors of Comparative Examples 6 and 7 were prepared, and gas generation from the electric double layer capacitors was measured. Comparative Example 6 differs from Example 2 in that propylene carbonate was further added as a solvent to the electrolytic solution of Example 2. In Comparative Example 6, sulfolane (A), dimethyl succinate (B), and propylene carbonate (C) were mixed in a volume ratio of A:B:C=70:20:10. Using this electrolytic solution for electric double layer capacitors of Comparative Example 6, an electric double layer capacitor was prepared with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 2.

[0099] Comparative Example 7 differs from Example 4 in that propylene carbonate was further added as a solvent to the electrolytic solution of Example 4. In Comparative Example 7, sulfolane (A), dimethyl glutarate (B), and propylene carbonate (C) were mixed in a volume ratio of A:B:C=70:20:10. Using this electrolytic solution for an electric double layer capacitor of Comparative Example 7, an electric double layer capacitor was produced with the same configuration and by the same production method as the electric double layer capacitor of Example 4.

[0100] The gas generation was measured by the same method and under the same conditions as in Example 1, and the electric double layer capacitor was placed in a temperature environment of 60° C. and held for 1500 hours while applying a constant voltage of 3.1 V. The results are shown in Table 5 below, together with the same test results for Examples 2 and 4.

[0101] (Table 5) TIFF2025013304000007.tif61161

[0102] As shown in Table 5 above, the amount of gas generated in Comparative Example 6 is slightly less than five times that of Example 2, and the amount of gas generated in Comparative Example 7 is slightly less than three times that of Example 4. Compared to Comparative Example 3 in which the solvent was composed only of propylene carbonate, the amount of gas generated is suppressed, but it was confirmed that the amount of gas generated can be further suppressed by using a mixed solvent of sulfolane and a specific carboxylic acid ester as the solvent for the electrolyte for electric double layer capacitors and excluding propylene carbonate and γ-butyrolactone from the electrolyte.

[0103] (Examples 10 to 13) As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 10 to 13 were prepared. The mixed solvent of the electrolytic solutions of Examples 10 to 13 was a mixed solvent of sulfolane and two kinds of dialkyl succinate. Then, using the prepared electrolytic solutions, electric double layer capacitors were prepared with the same configuration and the same manufacturing method as the electric double layer capacitor of Example 1.

[0104] The mixed solvent of the electrolyte of Example 10 was prepared by mixing sulfolane, dimethyl succinate, and diethyl succinate. Sulfolane (A), dimethyl succinate (B), and diethyl succinate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 10 was prepared.

[0105] The mixed solvent of the electrolyte of Example 11 was prepared by mixing sulfolane, dimethyl succinate, and diethyl succinate. Sulfolane (A), dimethyl succinate (B), and diethyl succinate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 11 was prepared.

[0106] The mixed solvent of the electrolyte of Example 12 was prepared by mixing sulfolane, dimethyl succinate, and dipropyl succinate. Sulfolane (A), dimethyl succinate (B), and dipropyl succinate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 12 was prepared.

[0107] The mixed solvent of the electrolyte of Example 13 was prepared by mixing sulfolane, dimethyl succinate, and dibutyl succinate. Sulfolane (A), dimethyl succinate (B), and dibutyl succinate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 13 was prepared.

[0108] (Capacity test) For the electric double layer capacitors of Examples 10 to 13, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are shown in Table 6 below, together with the results of Example 2.

[0109] (Table 6) TIFF2025013304000008.tif71161

[0110] In Table 6 above, MeSuc is dimethyl succinate, EtSuc is diethyl succinate, PrSuc is dipropyl succinate, and BuSuc is dibutyl succinate.

[0111] As shown in Table 6, in Examples 10 to 13, the capacitance is improved in a temperature environment of -30°C compared to Example 2, and further, the capacitance is well expressed in a temperature environment of -40°C. Examples 10 to 13 are mixed solvents of sulfolane and two types of specific carboxylate esters. In addition, the two types of specific carboxylate esters in Examples 10 to 13 have two carbon atoms along the straight chain of the chain hydrocarbon in the X structure, and do not exhibit capacitance in a temperature environment of -40°C when used alone.

[0112] In this way, it was confirmed that by including sulfolane and two or more specific carboxylic acid esters as the solvent of the electrolyte, the capacitance is improved in a temperature environment of -30° C., and further the capacitance is satisfactorily exhibited in a temperature environment of -40° C. Furthermore, it was confirmed that by adding two specific carboxylic acids having two carbon atoms along the straight chain of the chain hydrocarbon in the X structure, the capacitance at -40° C., which was not exhibited by the single substance, is satisfactorily exhibited.

[0113] (Examples 14 to 19) As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 14 to 19 were prepared. The mixed solvent of the electrolytic solutions of Examples 14 to 19 was a mixed solvent of sulfolane and two types of specific carboxylate esters. Using the prepared electrolytic solutions, electric double layer capacitors were prepared with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 1.

[0114] The mixed solvent of the electrolyte of Example 14 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl succinate. Sulfolane (A), dimethyl glutarate (B), and dimethyl succinate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 14 was prepared.

[0115] The mixed solvent of the electrolyte of Example 15 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl succinate. Sulfolane (A), dimethyl glutarate (B), and dimethyl succinate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 15 was prepared.

[0116] The mixed solvent of the electrolyte of Example 16 was prepared by mixing sulfolane, dimethyl glutarate, and diethyl glutarate. Sulfolane (A), dimethyl glutarate (B), and diethyl glutarate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 16 was prepared.

[0117] The mixed solvent of the electrolyte of Example 17 was prepared by mixing sulfolane, dimethyl glutarate, and diethyl glutarate. Sulfolane (A), dimethyl glutarate (B), and diethyl glutarate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) per 1 L of solvent was added to this mixed solvent as an electrolyte. Thus, the electrolyte for the electric double layer capacitor of Example 17 was prepared.

[0118] The mixed solvent of the electrolyte of Example 18 was prepared by mixing sulfolane, dimethyl glutarate, and diethyl glutarate. Sulfolane (A), dimethyl glutarate (B), and diethyl glutarate (C) were mixed in a volume ratio of A:B:C = 70:10:20. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 18 was prepared.

[0119] The mixed solvent of the electrolyte of Example 19 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl adipate. Sulfolane (A), dimethyl glutarate (B), and dimethyl adipate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 19 was prepared.

[0120] (Capacity test) For the electric double layer capacitors of Examples 14 to 19, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are shown in Table 7 below, together with the results of Example 4.

[0121] (Table 7) TIFF2025013304000009.tif126161

[0122] In Table 7 above, MeGlu is dimethyl glutarate, MeSuc is dimethyl succinate, EtGlu is diethyl glutarate, and MeAdi is dimethyl adipate.

[0123] As shown in Table 7, Examples 14 to 19 exhibit good capacitance in temperature environments of -30°C and -40°C, which is comparable to Example 4. Examples 16 to 18 have the same mixed solvent composition but different composition ratios. However, even though the composition ratios are different, Examples 16 to 18 exhibit good capacitance in temperature environments of -30°C and -40°C, which is comparable to Example 4. In this way, it was confirmed that by including sulfolane and two or more specific carboxylic acid esters as the solvent of the electrolyte, the capacitance in a temperature environment of -30°C is improved, and further, the capacitance in a temperature environment of -40°C is exhibited well.

[0124] (Examples 20 to 24) As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 20 to 24 were prepared. The mixed solvent of the electrolytic solutions of Examples 20 to 24 was a mixed solvent of sulfolane and two types of specific carboxylate esters. Then, using the prepared electrolytic solutions, electric double layer capacitors were prepared with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 1.

[0125] The mixed solvent of the electrolyte of Example 20 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl sebacate. Sulfolane (A), dimethyl glutarate (B), and dimethyl sebacate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 20 was prepared.

[0126] The mixed solvent of the electrolyte of Example 21 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl sebacate. Sulfolane (A), dimethyl glutarate (B), and dimethyl sebacate (C) were mixed in a volume ratio of A:B:C=70:15:15. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 21 was prepared.

[0127] The mixed solvent of the electrolyte of Example 22 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl azelate. Sulfolane (A), dimethyl glutarate (B), and dimethyl azelate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 22 was prepared.

[0128] The mixed solvent of the electrolyte of Example 23 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl azelate. Sulfolane (A), dimethyl glutarate (B), and dimethyl azelate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) per 1 L of solvent was added to this mixed solvent as an electrolyte. Thus, the electrolyte for the electric double layer capacitor of Example 23 was prepared.

[0129] The mixed solvent of the electrolyte of Example 24 was prepared by mixing sulfolane, dimethyl glutarate, and dimethyl adipate. Sulfolane (A), dimethyl glutarate (B), and dimethyl adipate (C) were mixed in a volume ratio of A:B:C = 70:15:15. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 24 was prepared.

[0130] (Capacity test) For the electric double layer capacitors of Examples 20 to 24, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are shown in Table 8 below, together with the results of Example 4.

[0131] (Table 8) TIFF2025013304000010.tif111161

[0132] In Table 8 above, MeGlu is dimethyl glutarate, MeSeb is dimethyl sebacate, MeAze is dimethyl azelaate, and MeAdi is dimethyl adipate.

[0133] As shown in Table 8, in Example 4 in which dimethyl glutarate was mixed, the number of carbon atoms along the straight chain of the chain hydrocarbon in the X structure of dimethyl glutarate was 3, and the SP value was 10.36, so that the capacitance was well expressed even in a temperature environment of -40°C. However, in Examples 20 to 23 in which dimethyl glutarate was mixed, the capacitance was not expressed or was smaller than that of Example 4 in a temperature environment of -40°C. Dimethyl sebacate has 8 carbon atoms, and dimethyl azelate has 7 carbon atoms, so that the chain hydrocarbon in the X structure is elongated.

[0134] This confirmed that the inclusion of sulfolane and two or more specific carboxylates as a solvent for the electrolyte solution has a favorable effect on the capacitance in a low-temperature environment, but it is preferable that all of the specific carboxylates in the solvent have 6 or less carbon atoms along the straight chain of the chain hydrocarbon in the X structure.

[0135] (Examples 25 to 28) As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 25 to 28 were prepared. The mixed solvent of the electrolytic solutions of Examples 25 to 28 was a mixed solvent in which ethyl propionate, a carboxylic acid ester having one ester bond, was added to sulfolane and a specific carboxylic acid ester. Then, using the prepared electrolytic solutions, each electric double layer capacitor was prepared with the same configuration and the same manufacturing method as the electric double layer capacitor of Example 1.

[0136] The mixed solvent of the electrolyte of Example 25 was prepared by mixing sulfolane, dimethyl succinate, and ethyl propionate. Sulfolane (A), dimethyl succinate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 25 was prepared.

[0137] The mixed solvent of the electrolyte of Example 26 was prepared by mixing sulfolane, diethyl succinate, and ethyl propionate. Sulfolane (A), diethyl succinate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 26 was prepared.

[0138] The mixed solvent of the electrolyte of Example 27 was prepared by mixing sulfolane, dimethyl glutarate, and ethyl propionate. Sulfolane (A), dimethyl glutarate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 27 was prepared.

[0139] The mixed solvent of the electrolyte of Example 28 was prepared by mixing sulfolane, diethyl glutarate, and ethyl propionate. Sulfolane (A), diethyl glutarate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 70:20:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) was added to this mixed solvent as an electrolyte per 1 L of solvent. Thus, the electrolyte for the electric double layer capacitor of Example 28 was prepared.

[0140] (Capacity test) For the electric double layer capacitors of Examples 25 to 28, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are shown in Table 9 below, together with the results of Examples 2 and 4.

[0141] (Table 9) TIFF2025013304000011.tif81161

[0142] In Table 9 above, MeSuc is dimethyl succinate, EtSuc is diethyl succinate, MeGlu is dimethyl glutarate, EtGlu is diethyl glutarate, and EtPro is ethyl propionate.

[0143] As shown in Table 9, in Example 25 and Example 26, the capacitance was improved in a temperature environment of -30 ° C. compared to Example 2, and the capacitance was well expressed in a temperature environment of -40 ° C. In Example 27 and Example 28, the capacitance was improved in both temperature environments of -30 ° C. and -40 ° C. compared to Example 4. In this way, it was confirmed that the capacitance was improved in a temperature environment of -30 ° C. and the capacitance was well expressed in a temperature environment of -40 ° C. by including a carboxylate having one ester bond in addition to sulfolane and a specific carboxylate as a solvent of the electrolyte.

[0144] (Examples 29 and 30) As described below, electrolytic solutions for the electric double layer capacitors and electric double layer capacitors of Examples 29 and 30 were prepared. Then, using the prepared electrolytic solutions, electric double layer capacitors were prepared with the same configuration and by the same manufacturing method as the electric double layer capacitor of Example 1.

[0145] The mixed solvent of the electrolyte of Example 29 was prepared by mixing sulfolane, dimethyl glutarate, ethyl propionate, and dimethyl sulfone. Sulfolane (A), dimethyl glutarate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D=72:10:10:8. 2.0 moles of spirobipyrrolidinium tetrafluoroborate (SBPBF4) per 1 L of solvent was added to this mixed solvent as an electrolyte. Thus, the electrolyte for the electric double layer capacitor of Example 29 was prepared.

[0146] The mixed solvent of the electrolyte of Example 30 was prepared by mixing sulfolane, dimethyl glutarate, diethyl glutarate, and ethyl propionate. Sulfolane (A), dimethyl glutarate (B), diethyl glutarate (C), and ethyl propionate (d) were mixed in a volume ratio of A:B:C:D=70:10:10:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) per 1 L of solvent was added to this mixed solvent as an electrolyte. Thus, the electrolyte for the electric double layer capacitor of Example 30 was prepared.

[0147] (Capacity test) For the electric double layer capacitors of Examples 29 and 30, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are summarized in Table 10 below.

[0148] (Table 10) TIFF2025013304000012.tif41161

[0149] In Table 10 above, MeGlu is dimethyl glutarate, EtGlu is diethyl glutarate, EtPro is ethyl propionate, and DMS is dimethyl sulfone.

[0150] As shown in Table 10, in Examples 29 and 30, the capacitance was well expressed in both temperature environments of -30 ° C. and -40 ° C. As a result, it was confirmed that the capacitance was improved in a low temperature environment such as -30 ° C. even if the electrolyte solvent contained a carboxylate having one ester bond in addition to sulfolane and two specific carboxylates. It was also confirmed that the capacitance was improved in a low temperature environment such as -30 ° C. even if the electrolyte solvent contained another solvent such as dimethyl sulfone in addition to sulfolane and one specific carboxylate.

[0151] (Examples 31 to 33) As follows, the electrolytic solutions for electric double layer capacitors and the electric double layer capacitors of Examples 31 to 33 were prepared. The electrolytic solutions of Examples 31 to 33 have different electrolyte concentrations from the electrolytic solution of Example 27. The electrolytic solutions of Examples 27 and Examples 31 to 33 are all common except for the electrolyte concentrations. That is, the mixed solvent of the electrolytic solutions of Examples 27 and Examples 31 to 33 was prepared by mixing sulfolane, dimethyl glutarate, and ethyl propionate. Sulfolane (A), dimethyl glutarate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C=70:20:10. The electrolytic solutions of Examples 27 and Examples 31 to 33 were prepared by adding methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) as an electrolyte to this mixed solvent.

[0152] The electrolyte of Example 27 was added at a ratio of 2.0 mol per 1 L of solvent. The electrolyte of Example 31 was added at a ratio of 2.5 mol per 1 L of solvent. The electrolyte of Example 32 was added at a ratio of 3.0 mol per 1 L of solvent. The electrolyte of Example 27 was added at a ratio of 3.5 mol per 1 L of solvent. In this way, the electrolyte solutions for electric double layer capacitors of Examples 31 to 33 in addition to Example 27 were prepared.

[0153] (Capacity test) For the electric double layer capacitors of Examples 31 to 33, the capacitance was measured under each temperature environment, and the capacity retention rate was calculated based on the capacitance at 20° C. The capacitance was measured and the capacity retention rate was calculated using the same measurement method and conditions as in Example 1. The results are summarized in Table 11 below, together with the results of Example 27. In Table 11 below, MeGlu is dimethyl glutarate, and EtPro is ethyl propionate.

[0154] (Table 11) TIFF2025013304000013.tif61161

[0155] As shown in Table 11, Examples 31 to 33 exhibited better capacitance in low-temperature environments of -30°C and -40°C than Example 27. This confirmed that the electrolyte concentration is preferably 2.5M or more and 3.5M or less.

[0156] (Examples 34 to 37) As follows, the electrolytic solution and the electric double layer capacitor of Examples 34 to 37 were prepared. The mixed solvent of the electrolytic solution for the electric double layer capacitor of Examples 34 to 37 was prepared by mixing sulfolane, dimethyl glutarate, ethyl propionate, and dimethyl sulfone. Sulfolane (A), dimethyl glutarate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D=72:10:10:8. 1.9 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF4) per 1 L of the solvent was added as an electrolyte to this mixed solvent. Thus, the electrolytic solution for the electric double layer capacitor of Examples 34 to 37 was prepared.

[0157] Furthermore, electric double layer capacitors were produced using the electrolytic solutions for electric double layer capacitors of Examples 34 to 37. The current collectors of the positive and negative electrodes were long aluminum foils, and both surfaces of the aluminum foils were coated with paint containing graphite. A slurry of a polarizable active material layer was obtained by mixing steam-activated activated carbon, carbon black, carboxymethyl cellulose as a dispersant, SBR emulsion as a binder, and pure water. The slurry was applied to both sides of the current collector and dried.

[0158] In Examples 34 to 37, the strip length of the negative electrode is 1.05 times longer than the strip length of the positive electrode. Therefore, the capacitance ratio Cp:Cn of the capacitance Cp of the positive electrode and the capacitance Cn of the negative electrode was adjusted by varying the coating thickness of the polarizable electrode layer. In Examples 34 to 37, the coating thickness of the positive electrode was set to be greater than or equal to the coating thickness of the negative electrode, thereby adjusting the capacitance ratio Cp:Cn of the capacitance Cp of the positive electrode and the capacitance Cn of the negative electrode as shown in Table 12 below.

[0159] (Table 12) TIFF2025013304000014.tif21161

[0160] The center lines of the positive and negative electrodes in the strip length direction were aligned, and the electrodes were stacked with a rayon separator between them to form a wound element. The wound element was impregnated with the electrolyte of Examples 34 to 37. The electrolyte-impregnated element was then placed in a case having a diameter of 18 mm and a height of 40 mm and sealed with a seal.

[0161] (Gas generation test) Gas generation was measured for the electric double layer capacitors of Examples 34 to 37. The electric double layer capacitors were placed in a temperature environment of 65°C and held for 500 hours while applying a constant voltage of 3.0V. The amount of swelling of the case after 500 hours was then measured. The amount of swelling of the case was obtained by subtracting the maximum longitudinal dimension of the case before the test from the maximum longitudinal dimension of the case after 500 hours.

[0162] The results of the amount of case swelling of the electric double layer capacitors of Examples 34 to 37 are shown in Table 13 below. (Table 13) TIFF2025013304000015.tif81161

[0163] As shown in Table 13 above, the amount of case swelling in Examples 35 to 37 was suppressed to less than half of the amount of case swelling in Example 34. The capacity ratio of the positive electrode capacitance to the negative electrode capacitance in Example 34 was 1.13 times, and the capacity ratio of the positive electrode capacitance to the negative electrode capacitance in Example 35 was 1.35 times. That is, the capacity ratio of the positive electrode capacitance to the negative electrode capacitance in Examples 35 to 37 was more than 1.13 times.

[0164] It was confirmed that the amount of gas generated can be further suppressed by using sulfolane and a specific carboxylic acid ester as a solvent for the electrolyte used in an electric double layer capacitor and by setting the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance to be more than 1.35 times and not more than 1.90 times.

Claims

1. An electrolytic solution for use in an electric double layer capacitor, The solvent contains sulfolane and a carboxylic acid ester represented by the following chemical formula (1), The carboxylic acid ester has two or more ester bonds, The X structure in the following chemical formula (1) has a linear carbon skeleton having two or more carbon atoms along the linear chain. An electrolyte for an electric double layer capacitor, comprising: 【Chemistry 1】 [In the formula, n and m are each 1 or more.]

2. The solvents have a significantly lower SO ２ The band is 2.05 cm －１ Peak shift is more than 2. The electrolyte for an electric double layer capacitor according to claim 1,

3. The carboxylate ester has the carbon skeleton having 3 to 6 carbon atoms along a straight chain in the X structure of the chemical formula (1); 2. The electrolyte for an electric double layer capacitor according to claim 1,

4. The solvent contains two or more of the carboxylic acid esters; 2. The electrolyte for an electric double layer capacitor according to claim 1,

5. each of the two or more carboxylates has the carbon skeleton having 3 or more and 6 or less carbon atoms along a straight chain in the X structure in the chemical formula (1); 5. The electrolyte for an electric double layer capacitor according to claim 4,

6. the solvent further comprises a carboxylic acid ester having one ester bond; 2. The electrolyte for an electric double layer capacitor according to claim 1,

7. The carboxylic acid ester has a solubility parameter SP value of 9 or more; 7. The electrolyte for an electric double layer capacitor according to claim 1,

8. The carboxylate ester has a methyl group, an ethyl group, a propyl group, or a butyl group as a branched chain in the X structure of the chemical formula (1); 7. The electrolyte for an electric double layer capacitor according to claim 1,

9. The carboxylate has a carboxylate group in the X structure of the chemical formula (1); 7. The electrolyte for an electric double layer capacitor according to claim 1,

10. The solvent further comprises dimethyl sulfone; 7. The electrolyte for an electric double layer capacitor according to claim 1,

11. the carboxylic acid ester has 3 or less ester bonds; 7. The electrolyte for an electric double layer capacitor according to claim 1,

12. Further comprising a quaternary ammonium salt; 7. The electrolyte for an electric double layer capacitor according to claim 1,

13. The electrolyte for an electric double layer capacitor according to any one of claims 1 to 6, A positive electrode and a negative electrode assembly; An electric double layer capacitor comprising:

14. The capacitance ratio of the positive electrode to the negative electrode is more than 1.13 times; The electric double layer capacitor according to claim 13,

Images