Electrolytic solution for electric double layer capacitor and electric double layer capacitor

Abstract

Provided are an electrolytic solution for an electric double layer capacitor and an electric double layer capacitor, in which a solvent does not decompose even when said electrolytic solution is used under a high voltage of 3.0 V or more, and an electrostatic capacity can be realized even in a low temperature environment such as -30°C. The electrolytic solution used in the electric double layer capacitor contains sulfolane and a carboxylic acid ester as a solvent. The carboxylic acid ester has an annular carbon skeleton and two or more ester bonds, such as a cyclohexanedicarboxylic acid diester and a cyclohexanetricarboxylic acid triester.

Description

Electrolyte for electric double layer capacitor and electric double layer capacitor

[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.

[0002] An electric double layer capacitor consists of a pair of polarizable electrodes impregnated with an electrolyte solution housed in a container, and utilizes the charge-storing effect of the electric double layer formed at the interface between the polarizable electrodes and the electrolyte solution. Specifically, at the positive electrode, the anions of the electrolyte align at the interface with the polarizable electrode and pair with pores in the polarizable electrode across an extremely short distance, forming a potential barrier at the positive electrode. At the negative electrode, the cations of the electrolyte align at the interface with the polarizable electrode and pair with electrons in the polarizable electrode across an extremely short distance, forming a potential barrier at the negative electrode.

[0003] Electric double layer capacitors typically use activated carbon powder as the polarizable electrode material, a valve metal such as aluminum as the current collector, and an aprotic electrolyte solution, which mainly uses a quaternary ammonium salt as the electrolyte. Typical solvents for the electrolyte solution include carbonate solvents such as propylene carbonate and carboxylic acid esters such as γ-butyrolactone (see, for example, Patent Document 1).

[0004] However, when propylene carbonate or γ-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 higher, the propylene carbonate and γ-butyrolactone undergo an electrochemical reaction and decompose, generating gas and deposits inside the electric double layer capacitor. The generation of gas and deposits can cause a decrease in the capacity of the electric double layer capacitor, an increase in resistance, and opening of the case valve.

[0005] To obtain high voltage characteristics without generating gas or 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] Therefore, 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 combining it with activated carbon fiber, which has a low impurity content, can improve the storage stability, charge / discharge characteristics, and voltage resistance of the electric double layer capacitor.

[0007] JP 2001-217150 JP 06-275468

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

[0009] It is expected that electric double layer capacitors for automotive use will 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 in the electrolyte will lose its chargeable / dischargeable capacitance at -20° C., rendering it unusable.

[0010] The present invention has been proposed to solve the above-mentioned problems, and an object of the present invention is to provide an electrolyte solution 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.

[0011] In order to solve the above problems, the electrolytic solution for an electric double layer capacitor of the present embodiment is an electrolytic solution used in an electric double layer capacitor, and contains sulfolane and a carboxylic acid ester as a solvent, and the carboxylic acid ester has a cyclic carbon skeleton and two or more ester bonds.

[0012] Propylene carbonate and γ-butyrolactone may be absent.

[0013] The solvent may further contain a carboxylic acid ester having one or more ester bonds.

[0014] The solvent may further include dimethyl sulfone.

[0015] The solvent may further contain a carboxylic acid ester having one or more ester bonds and dimethyl sulfone, in which the carboxylic acid ester having a cyclic carbon skeleton and two or more ester bonds may be contained in an amount of 3 vol % or more of the total amount of the solvent.

[0016] A quaternary ammonium salt may also be included.

[0017] The carboxylic acid ester may be in the cis or trans form and may be dimethyl 1,4-cyclohexanedicarboxylate, dimethyl 1,2-cyclohexanedicarboxylate, diethyl 1,2-cyclohexanedicarboxylate, dimethyl 1,3-cyclohexanedicarboxylate, dimethyl 1,3-cyclopentanedicarboxylate, or trimethyl 1,3,5-cyclohexanetricarboxylate.

[0018] In order to solve the above problems, the electric double layer capacitor of this embodiment includes such an electrolytic solution for an electric double layer capacitor and positive and negative electrode bodies.

[0019] The capacitance ratio of the positive electrode to the negative electrode may be greater than 1.13.

[0020] The electrolytic solution for an electric double layer capacitor may contain 2.0 moles or more and 2.8 moles or less of an electrolyte per liter of the solvent.

[0021] 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.

[0022] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the embodiments described below.

[0023] (Electrolyte) The electrolyte of this embodiment is an electrolyte for an electric double layer capacitor. An electric double layer capacitor is a passive element that stores and discharges electric charges by capacitance, utilizing the charge storage action of an electric double layer formed at the interface between a polarizable electrode and an electrolyte. An electric double layer capacitor element includes a positive electrode, a negative electrode, and a separator. The electrolyte is filled in the voids of the electric double layer capacitor element.

[0024] (Solvent) The solvent of this electrolytic solution is a mixed solvent containing sulfolane and a specific carboxylic acid ester (hereinafter referred to as the specific carboxylic acid ester). The specific carboxylic acid ester has a cyclic carbon skeleton and two or more ester bonds. Here, the cyclic carbon skeleton is a cycloalkane structure having four or more carbon atoms forming the skeleton. That is, the specific carboxylic acid ester is a cyclohexanedicarboxylic acid diester, a cyclopentanedicarboxylic acid diester, or a cyclohexanetricarboxylic acid triester, and is preferably a compound represented by any one of the following chemical formulas (1) to (4).

[0025] (Chem.1)

[0026] (Case 2)

[0027] (Case 3)

[0028] (Case 4)

[0029] The specific carboxylic acid ester of the above chemical formula (1) is a 1,4-cyclohexanedicarboxylic acid diester. The specific carboxylic acid ester of the above chemical formula (2) is a 1,2-cyclohexanedicarboxylic acid diester. The specific carboxylic acid ester of the above chemical formula (3) is a 1,3-cyclopentanedicarboxylic acid diester. The specific carboxylic acid ester of the above chemical formula (4) is a 1,3,5-cyclohexanetricarboxylic acid triester. In the formula, n, m, and p are 1 or more and 4 or less, and the alkyl group may have a branched chain. Furthermore, these specific carboxylic acid esters may be in either a cis form or a trans form. Furthermore, an example of a specific carboxylic acid ester other than those of the above chemical formulas (1) to (4) is dimethyl 1,3-cyclohexanedicarboxylate.

[0030] This specific carboxylic acid ester has good electron acceptor properties, which causes a strong interaction between the ester group and the sulfonyl group of sulfolane. This interaction lowers the freezing point of sulfolane and improves the capacitance of the electric double layer capacitor in low-temperature environments such as −30°C.

[0031] For example, in the case of dimethyl malonate, where the carbon number of the hydrocarbon linking the two carboxylic acid ester groups is C=1, there is one α-carbon sandwiched between the carboxylic acid ester groups at both ends. Since the α-hydrogen bonded to this α-carbon is easily eliminated, dimethyl malonate is susceptible to decomposition reactions.

[0032] On the other hand, in this specific carboxylic acid ester, the two carboxylic acid ester groups are not bonded to the same carbon, and the α-carbons are separate. Furthermore, the cyclic carbon skeleton provides a steric hindrance that inhibits the elimination of the α-hydrogen. Therefore, when a mixed solvent containing this specific carboxylic acid ester and sulfolane is used in an electric double layer capacitor at a high voltage of 3.0 V or higher, the solvent does not decompose, and capacitance can be exhibited even in a low-temperature environment such as -30°C.

[0033] The amount of sulfolane in the solvent is preferably 50 vol% or more and 90 vol% or less relative to the total amount of solvent in the electrolyte solution. Furthermore, the amount of the specific carboxylic acid ester in the solvent is preferably 1 vol% or more and 50 vol% or less relative to the total amount of solvent in the electrolyte solution. If the amount of the specific carboxylic acid ester is too small, the Raman shift of the sulfonyl group of sulfolane becomes small, and the capacitance development effect in a low-temperature environment decreases. Furthermore, if the amount of sulfolane is too small, gas is likely to be generated when used at a high voltage of 3.0 V or more.

[0034] In addition to sulfolane and the specific carboxylic acid ester, the solvent for the electrolyte preferably further contains a carboxylic acid ester having one or more ester bonds. An example of a carboxylic acid ester having one ester bond is ethyl propionate. A mixed solvent containing sulfolane, the specific carboxylic acid ester, and a carboxylic acid ester having one ester bond further improves the capacitance in a low-temperature environment of −30°C. Examples of a carboxylic acid ester having one or more ester bonds include dimethyl succinate, dimethyl glutarate, dimethyl adipate, and dimethyl pimelate.

[0035] It is also preferable that the electrolyte further contains dimethyl sulfone as a solvent in addition to sulfolane and the specific carboxylic acid ester. Although the combination of sulfolane and the specific carboxylic acid ester does not exhibit capacitance in a temperature environment of −40°C, the addition of dimethyl sulfone enables the electrolyte to exhibit capacitance even in a low-temperature environment of −40°C.

[0036] More preferably, the electrolyte solvent further contains both a carboxylic acid ester having one ester bond and dimethyl sulfone in addition to sulfolane and the specific carboxylic acid ester. This electrolyte significantly improves capacitance at low temperatures of both −30° C. and −40° C. It is particularly preferable that the amount of the specific carboxylic acid ester be 3 vol% or more relative to the total amount of solvent, as this significantly improves capacitance at low temperatures of −40° C.

[0037] Furthermore, even when propylene carbonate and γ-butyrolactone are used in combination with sulfolane and a specific carboxylic acid ester, a large amount of gas is generated when used at a high voltage of 3.0 V or higher. When 0.1 vol % of propylene carbonate or γ-butyrolactone was added to a solvent and used at a high voltage of 3.0 V or higher, a large amount of gas was generated. Therefore, it is preferable not to include propylene carbonate or γ-butyrolactone in the solvent.

[0038] (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. The electrolyte may be contained in an amount of 0.5 mol to 3.5 mol per 1 L of solvent.

[0039] Examples of the quaternary ammonium salt include tetramethylammonium, ethyltrimethylammonium, tetraethylammonium, triethylmethylammonium, diethyldimethylammonium, methylethylpyrrolidinium, and spirobipyrrolidinium as cationic components, and BF as anionic components. ４ － , P.F. ６ － , ClO ４ － , AsF ６ － , SbF ６ － , AlCl ４ － , or RfSO ３ － , (RfSO ２ ) ２ N － , RfCO ２ － (Rf is a fluoroalkyl group having 1 to 8 carbon atoms).

[0040] Typically, the quaternary ammonium salt is tetramethylammonium BF ４, ethyltrimethylammonium BF4, diethyldimethylammonium BF4, triethylmethylammonium BF4, tetraethylammonium BF4, spirobipyrrolidinium BF4, methylethylpyrrolidinium BF4, tetramethylammonium PF ６ , 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.

[0041] The lithium salt is LiPF ６ , LiBF ４ , LiClO ４ , LiN(SO ２ CF ３ ) ２ , LiN(SO ２ C ２ F ５ ) ２ , C.F. ３ SO ３ Li, LiC(SO ２ CF ３ ) ３ , and LiPF ３ (C ２ F ５ ) ３or a mixture thereof can be used.

[0042] The electrolyte may contain various additives. Examples of additives include phosphoric acids and their derivatives (phosphoric acid, phosphorous acid, phosphate esters, phosphonic acids, etc.), boric acids and their derivatives (boric acid, boric acid oxide, borate esters, complexes of boron with a hydroxyl group, a carboxyl group, or a compound having a plurality of these), nitrates (lithium nitrate, etc.), and nitro compounds (nitrobenzoic acid, nitrophenol, nitrophenetole, nitroacetophenone, aromatic nitro compounds, etc.). The electrolyte may also contain a gas absorbent. The absorbent for gas generated from the electrode is not particularly limited as long as it does not react with the components of the electrolyte (solvent, electrolyte salt, various additives, etc.) and does not remove (e.g., adsorb). Specific examples include zeolite and silica gel.

[0043] The method for preparing the electrolyte solution is not limited, but may be, for example, as follows: First, a mixed solvent is prepared by mixing sulfolane and a specific carboxylic acid ester in any ratio. In a dry room, an electrolyte is added to a 1 L volumetric 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 carboxylic acid ester are added and mixed together when the specific carboxylic acid ester is added.

[0044] (Electric double layer capacitor) In an electric double layer capacitor, the positive and negative electrode bodies are formed by laminating polarizable electrodes on current collectors. The current collectors can be made of valve metals such as aluminum foil, platinum, gold, nickel, titanium, steel, and carbon. The current collectors can have any shape, such as a film, foil, plate, mesh, expanded metal, or cylinder. The surface of the current collector may be textured by etching or may be plain. Furthermore, surface treatment may be performed to deposit phosphorus on the surface of the current collector.

[0045] A cylindrical current collector is a wound type electric double layer capacitor element. In a wound type electric double layer capacitor, the positive and negative electrode current collectors have a long strip shape and are wound in a spiral shape on an intervening separator. In the positive and negative electrodes, the direction along the spiral is called the strip length direction. Generally, the positive and negative electrodes are wound along the longitudinal direction, and this longitudinal direction is the strip length direction.

[0046] 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 derived from natural plant tissue such as coconut husk, 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.

[0047] A polarizable electrode may be formed, for example, by applying a slurry of activated carbon and a binder onto a current collector using a doctor blade method or the like and drying the applied mixture. A mixture of the carbon material and the binder may be formed into a sheet and then pressure-bonded to the current collector. Examples of binders 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.

[0048] The polarizable electrode may contain a conductive additive. Examples of the conductive additive 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 nanotubes may be single-walled carbon nanotubes (SWCNTs) that have a single graphene sheet, or multi-walled carbon nanotubes (MWCNTs) that have two or more graphene sheets rolled coaxially to form multiple tube walls, or a mixture of these.

[0049] Furthermore, phosphorus may be attached to the surface of the current collector to inhibit hydration and oxidation of the current collector. For example, the current collector may be 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.

[0050] When the electrode areas of the polarizable electrodes of an 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. 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. 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.

[0051] When the capacitance ratio of the positive electrode to the negative electrode is 1.13 or less, a large amount of gas is generated in the electric double layer capacitor, even if sulfolane is used as the solvent for the electrolyte. For example, in an electric double layer capacitor using a wound element, the strip lengths of the positive and negative electrodes may be different, resulting in different electrode areas for the positive and negative electrodes. In particular, when the negative electrode is arranged at the outermost periphery of the wound element and 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 when the electrode areas of the positive and negative electrodes are different, it is desirable that the amount of gas generated be even smaller than the amount of gas generated when the capacitance ratio of the positive electrode to the negative electrode is 1.13 or less.

[0052] When the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance is more than 1.13, the amount of gas generated is reduced to half or less compared to when the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance is 1.13. For example, when the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance 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 capacitance to the negative electrode capacitance is 1.13.

[0053] The capacitance of the positive electrode and the capacitance of the negative electrode are measured as follows: a test piece having 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 measurement 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 frequency response analyzer, an electrochemical impedance analyzer, an LCR meter, or the like, and the measurement conditions are a DC bias voltage of 1.5 V and an AC amplitude of 1 V.

[0054] Here, increasing the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance reduces the amount of gas generated in the electric double layer capacitor, but reduces the capacitance development effect in low-temperature environments. Therefore, in order to further reduce the amount of gas generated and simultaneously achieve the capacitance development effect in low-temperature environments, it is preferable that the amount of electrolyte added be in the range of 2.0 mol to 2.8 mol per liter of solvent. As long as the amount of electrolyte added is in the range of 2.0 mol to 2.8 mol per liter of solvent, the capacitance development effect in low-temperature environments can be maintained even if the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance is increased.

[0055] However, preferably, the ratio of the capacitance of the positive electrode to the capacitance of the negative electrode is less than 1.90. Furthermore, particularly preferably, the amount of electrolyte added is 2.0 mol or more and less than 2.8 mol per liter of solvent. When the ratio of the capacitance of the positive electrode to the capacitance of the negative electrode is 1.90 or less and the amount of electrolyte added is 2.0 mol or more and less than 2.8 mol per liter of solvent, the capacitance development effect is maintained even in a low-temperature environment of -40°C.

[0056] The separator interposed between the positive and negative electrodes may be made of cellulose or a mixture thereof, polyester resin, polytetrafluoroethylene resin, polyvinylidene fluoride resin, vinylon resin, polyamide resin, polyimide resin, polyethylene resin, polypropylene resin, trimethylpentene resin, polyphenylene sulfide resin, or acrylic resin, and these resins may 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. Aramids include semi-aromatic polyamides and fully aromatic polyamides.

[0057] 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.

[0058] Example 1 An electrolytic solution for an electric double layer capacitor and an electric double layer capacitor of Example 1 were prepared as follows. First, a mixed solvent of sulfolane and dimethyl 1,4-cyclohexanedicarboxylate was prepared. Sulfolane (A) and dimethyl 1,4-cyclohexanedicarboxylate (B) were mixed in a volume ratio of A:B = 70:30. 75% of the dimethyl 1,4-cyclohexanedicarboxylate was in the cis form and 25% in the trans form. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) was added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 1.

[0059] Next, activated carbon activated with steam was used as the active material. This activated carbon was mixed with carbon black as a conductive additive, 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 assemblies. The activated carbon polarizable electrodes and the aluminum current collector were pressed together using a press.

[0060] A separator made of nonwoven fabric was sandwiched between the positive and negative electrode bodies, and an element was formed by laminating the positive electrode, separator, and negative electrode. This element was impregnated with the electrolyte solution 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 laminated cell in a temperature environment of 25°C, and an aging treatment was performed for 12 hours.

[0061] Comparative Examples Electrolyte solutions for electric double layer capacitors were prepared in Comparative Examples 1 to 5. The electrolyte solution of Comparative Example 1 contained only sulfolane as a solvent, and 1.5 mol of spirobipyrrolidinium tetrafluoroborate (SBPBF) was used per 1 L of the solvent.４ ) was added as an electrolyte. The electrolytic solution of Comparative Example 2 contained only γ-butyrolactone as a solvent, and the type and amount of the electrolyte added were the same as those of Comparative Example 1. The electrolytic solution of Comparative Example 3 contained only propylene carbonate as a solvent, and the type and amount of the electrolyte added were the same as those of Comparative Example 1.

[0062] The solvent of the electrolyte 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, and the type and amount of electrolyte added were the same as those of Comparative Example 1.

[0063] 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, and the type and amount of electrolyte added were the same as those of Comparative Example 1.

[0064] Using the electrolytic solutions for electric double layer capacitors of Comparative Examples 1 to 5, electric double layer capacitors were fabricated using the same configuration and the same manufacturing method as the electric double layer capacitor of Example 1.

[0065] (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 maintained for 1500 hours while a constant voltage of 3.1 V was applied. 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 taken as the amount of gas generated.

[0066] 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, PC is propylene carbonate, PE is ethyl propionate, and MeMal is dimethyl malonate.

[0067] (Table 1)

[0068] Furthermore, 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, the electric double layer capacitors were charged under each temperature environment by applying a constant voltage of 0.1 V to 3.1 V. After charging was completed, a constant current discharge was immediately performed. The capacitance was then measured according to the constant current discharge method. During discharge at a constant current I, the time T required for the voltage to drop from the measurement start voltage to the measurement end voltage of 0.1 V was measured, excluding the IR drop. The product of the constant current I and the required time T was then divided by the difference (V) between the measurement start voltage and the measurement end voltage to calculate the capacitance (F). Furthermore, the capacitance retention rate was calculated as a percentage of the capacitance at each temperature relative to the capacitance at 20°C.

[0069] The capacitance retention rates at various temperatures of the electric double layer capacitors of Example 1 and Comparative Examples 1 to 5 are summarized in Table 2 below. (Table 2)

[0070] As shown in Table 1 above, the electric double layer capacitor using the electrolyte solution for electrolytic capacitors of Example 1 exhibited a reduced amount of gas generation compared to Comparative Examples 2 to 5. Moreover, Example 1 exhibited a better reduction in the amount of gas generation than the case where sulfolane was used as the solvent. Furthermore, as shown in Table 2 above, Comparative Example 1 was unable to exhibit capacitance in a low-temperature environment of -30°C, but the electric double layer capacitor using the electrolyte solution for electrolytic capacitors of Example 1 exhibited a good capacitance in a low-temperature environment of -30°C.

[0071] That is, when sulfolane is used as the solvent as in Comparative Example 1, no capacitance is generated in a temperature environment of −30° C. When γ-butyrolactone or propylene carbonate is used as the solvent as in Comparative Examples 2 and 3, the solvent decomposes and gas is generated. On the other hand, when a mixed solvent of sulfolane and cyclohexanedicarboxylic acid diester is used as in Example 1, capacitance can be generated in a temperature environment of −30° C., and further, gas generation is suppressed even when used at a high voltage of 3.0 V or more.

[0072] In this way, it was confirmed that by using cyclohexane and a specific carboxylic acid ester having two or more ester bonds together with sulfolane as a solvent, gas generation is suppressed even when an 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 improved in a low-temperature environment such as −30°C.

[0073] In Comparative Example 2, the measurement of the amount of gas generated was discontinued after 1000 hours because the γ-butyrolactone had deteriorated and was no longer measurable. In Comparative Example 4, which used a mixed solvent of sulfolane and dimethyl malonate, the amount of gas generated was worse than in Comparative Examples 2 and 3, which used γ-butyrolactone or propylene carbonate. In Comparative Example 5, which did not contain cyclohexane and used ethyl propionate with one ester bond, capacitance was exhibited in a temperature environment of −30° C., but the capacity retention rate was low.

[0074] Examples 2 to 4 As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 2 to 4 were prepared. The electrolytic solutions of Examples 2 to 4 contained a specific carboxylic acid ester of a different type than that of Example 1. Using these electrolytic solutions for electric double layer capacitors of Examples 2 to 4, 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.

[0075] The mixed solvent of the electrolyte solution in Example 2 was prepared by mixing sulfolane and cis-dimethyl 1,2-cyclohexanedicarboxylate. Sulfolane (A) and dimethyl 1,2-cyclohexanedicarboxylate (B) were mixed in a volume ratio of A:B = 70:30. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) were added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 2.

[0076] The mixed solvent of the electrolyte solution of Example 3 was prepared by mixing sulfolane and diethyl 1,2-cyclohexanedicarboxylate containing both cis and trans isomers. Sulfolane (A) and diethyl 1,2-cyclohexanedicarboxylate (B) were mixed in a volume ratio of A:B = 70:30. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) were added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 3.

[0077] The mixed solvent of the electrolyte solution in Example 4 was prepared by mixing sulfolane and dimethyl 1,3,5-cyclohexanedicarboxylate. Sulfolane (A) and trimethyl 1,3,5-cyclohexanetricarboxylate (B) were mixed in a volume ratio of A:B = 70:30. 2.0 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) was added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 4.

[0078] (Characteristics Test 2) For the electric double layer capacitors of Examples 2 to 4, 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 calculation of the capacity retention rate were performed using the same measurement method and conditions as in Example 1. The capacitance retention rates at each temperature of the electric double layer capacitors of Examples 2 to 4 are shown in Table 3 below.

[0079] (Table 3)

[0080] In addition, the electric double layer capacitors of Examples 2 to 4 were placed in temperature environments of 20°C, -20°C, -30°C, and -40°C, and the direct current internal resistance (DCIR) was measured under each temperature environment. Specifically, two cycles of charge and discharge were repeated in the range of 0.1V to 3.1V under each temperature environment. Then, the DCIR was calculated from the voltage drop one second after the start of discharge during the second cycle. The DICRs of the electric double layer capacitors of Examples 2 to 4 at each temperature are shown in Table 4 below.

[0081] (Table 4)

[0082] As shown in Table 3, Examples 2 to 4 are electrolyte solutions prepared by mixing a specific carboxylic acid ester different from that of Example 1 with sulfolane, but they have good capacitance in a temperature environment of −30° C., similar to Example 1. Therefore, it was confirmed that when the solvent of the electrolyte solution 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 in a low-temperature environment such as −30° C.

[0083] In particular, Examples 2 to 4 exhibited capacitance even in a temperature environment of −40° C. Furthermore, as shown in Table 4, Example 2 had better DICR in temperature environments of −30° C. and −40° C. than the other Examples.

[0084] Examples 5 to 10 As described below, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 5 to 10 were prepared. The electrolytic solutions of Examples 5 to 10 contained other types of solvents in addition to sulfolane and a specific carboxylic acid ester. Using these electrolytic solutions for electric double layer capacitors of Examples 5 to 10, 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.

[0085] The mixed solvent of the electrolyte solution of Example 5 was prepared by mixing sulfolane, dimethyl 1,4-cyclohexanedicarboxylate, and dimethyl sulfone. 75% of the dimethyl 1,4-cyclohexanedicarboxylate was in the cis form and 25% in the trans form. Sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), and dimethyl sulfone (C) were mixed in a volume ratio of A:B = 72:20:8. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) were added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 5.

[0086] The mixed solvent of the electrolyte solution of Example 6 was prepared by mixing sulfolane, dimethyl 1,4-cyclohexanedicarboxylate, and ethyl propionate. 75% of the dimethyl 1,4-cyclohexanedicarboxylate was in the cis form and 25% in the trans form. Sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), and ethyl propionate (C) were mixed in a volume ratio of A:B:C = 80:10:10. 2.0 moles of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) were added to this mixed solvent per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 6.

[0087] The mixed solvent of the electrolyte solution of Example 7 was prepared by mixing sulfolane, dimethyl 1,4-cyclohexanedicarboxylate, ethyl propionate, and dimethyl sulfone. The dimethyl 1,4-cyclohexanedicarboxylate was 75% cis-isomer and 25% trans-isomer. Sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D = 72:10:10:8. To this mixed solvent, 2.0 moles of spirobipyrrolidinium tetrafluoroborate (SBPBF) were added per 1 L of solvent. ４ ) was added as an electrolyte to prepare an electrolytic solution for an electric double layer capacitor of Example 7.

[0088] The mixed solvent of the electrolyte solution in Example 8 uses the same four types of solvents as in Example 7, but the composition ratio is different, and the amount of dimethyl 1,4-cyclohexanedicarboxylate is reduced compared to Example 7. In Example 8, sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D = 76.5:5:10:8.5.

[0089] The mixed solvent of the electrolyte solution in Example 9 uses the same four types of solvents as in Example 7, but the composition ratio is different, and the amount of dimethyl 1,4-cyclohexanedicarboxylate is reduced compared to Example 8. In Example 9, sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D = 78.3:3:10:8.7.

[0090] The mixed solvent of the electrolyte solution in Example 10 uses the same four types of solvents as in Example 7, but the composition ratio is different, and the amount of dimethyl 1,4-cyclohexanedicarboxylate is reduced compared to Example 9. In Example 10, sulfolane (A), dimethyl 1,4-cyclohexanedicarboxylate (B), ethyl propionate (C), and dimethyl sulfone (D) were mixed in a volume ratio of A:B:C:D = 80.1:1:10:8.9.

[0091] (Capacitance Test) For the electric double layer capacitors of Examples 5 to 10, 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 calculation of the capacity retention rate were performed using the same measurement method and conditions as in Example 1. The capacitance retention rates at each temperature of the electric double layer capacitors of Examples 5 to 10 are shown in Table 5 below, together with the results of Example 1.

[0092] (Table 5)

[0093] As shown in Table 5, Example 5, unlike Example 1, exhibited capacitance in a low-temperature environment of −40° C. This confirmed that by using a mixed solvent of sulfolane, a specific carboxylic acid ester, and dimethyl sulfone, capacitance was exhibited even in a low-temperature environment of −40° C. In the case of a mixed solvent of sulfolane and dimethyl sulfone, capacitance was exhibited in a low-temperature environment of −30° C., but no capacitance was exhibited in a low-temperature environment of −40° C.

[0094] Furthermore, Example 6 exhibited a significantly improved capacitance in a low-temperature environment of −30° C. compared to Example 1. This confirmed that the use of a mixed solvent of sulfolane, a specific carboxylic acid ester, and ethyl propionate significantly improved the capacitance in a low-temperature environment of −30° C.

[0095] As shown in Table 5, Examples 7 to 10 exhibited capacitance improvements in a low-temperature environment of −30° C. that were equal to or greater than that of Example 6. Furthermore, Examples 7 to 10 exhibited improved capacitance in a low-temperature environment of −40° C., even compared to Example 5. In particular, Examples 7 to 9, in which the amount of the specific carboxylic acid ester was 3 vol % or more relative to the total amount of the solvent, exhibited a remarkable effect of improving capacitance in a low-temperature environment of −40° C.

[0096] This confirmed that when the electrolyte solvent further contains both a carboxylic acid ester having one ester bond and dimethyl sulfone in addition to sulfolane and a specific carboxylic acid ester, the capacitance is significantly improved in low-temperature environments of both −30°C and −40°C.

[0097] (Examples 11 to 16) As follows, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 11 to 14 were prepared. The electric double layer capacitors of Examples 11 to 14 differ in the capacitance ratio Cp:Cn of the positive electrode capacitance Cp to the negative electrode capacitance Cn. The electrolytic solutions for electric double layer capacitors of Examples 11 to 14 are the same. The mixed solvent of the electrolytic solutions for electric double layer capacitors of Examples 11 to 14 is the same as that of Example 7. The electrolytic solutions for electric double layer capacitors of Examples 11 to 14 differ from those of Example 7 in the type and amount of electrolyte added. The electrolytic solutions for electric double layer capacitors of Examples 11 to 14 contained 2.2 mol of methylethylpyrrolidinium tetrafluoroborate (MEPyBF) per 1 L of the mixed solvent. ４ ) was added as an electrolyte.

[0098] Furthermore, electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 15 and 16 were prepared. The electric double layer capacitors of Examples 15 and 16 differ in the capacitance ratio Cp:Cn of the positive electrode capacitance Cp to the negative electrode capacitance Cn. The electrolytic solutions for electric double layer capacitors of Examples 15 and 16 are the same. The electrolytic solutions for electric double layer capacitors of Examples 15 and 16 are the same as those of Example 7, except for the amount of electrolyte added. The electrolytic solutions for electric double layer capacitors of Examples 15 and 16 contained 2.0 mol of spirobipyrrolidinium tetrafluoroborate (SBPBF) per 1 L of the mixed solvent. ４ ) was added as an electrolyte.

[0099] Furthermore, electric double layer capacitors were fabricated using the electrolytic solutions for electric double layer capacitors of Examples 11 to 16. The current collectors for the positive and negative electrodes were long lengths of aluminum foil, and both surfaces of the aluminum foil were coated with a paint containing graphite. A slurry for the 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.

[0100] In Examples 11 to 16, the strip length of the negative electrode was 1.05 times or more longer than the strip length of the positive electrode. Therefore, the ratio of the positive electrode capacity to the negative electrode capacity was adjusted by varying the coating thickness of the polarizable electrode layer. In Examples 11 to 16, the coating thickness of the positive electrode was made equal to or greater than the coating thickness of the negative electrode, thereby adjusting the capacitance ratio Cp:Cn of the positive electrode capacitance Cp to the negative electrode capacitance Cn as shown in Table 6 below.

[0101] (Table 6)

[0102] The centerlines of the positive and negative electrodes were aligned along the strip length, and the electrodes were stacked with a rayon separator between them to form a wound element. The wound element was impregnated with the electrolyte solution of Examples 34 to 37. The electrolyte-impregnated element was then placed in a case with a diameter of 18 mm and a height of 40 mm and sealed with a seal.

[0103] (Gas Generation Test) Gas generation was measured for the electric double layer capacitors of Examples 11 to 16. The electric double layer capacitors were placed in a temperature environment of 65°C and maintained for 500 hours while a constant voltage of 3.0 V was applied. The amount of swelling of the case after 500 hours was 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.

[0104] The results of the amount of case swelling of the electric double layer capacitors of Examples 11 to 16 are shown in Table 7 below. (Table 7)

[0105] As shown in Table 7 above, the amount of case swelling in Examples 12 to 14 was reduced to less than half of the amount of case swelling in Example 11. The capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Example 11 was 1.13, and the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Example 12 was 1.35. That is, the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Examples 12 to 14 was more than 1.13.

[0106] Furthermore, as shown in Table 7 above, the amount of case swelling in Example 16 was reduced to one-third or less of the amount of case swelling in Example 15. The capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Example 15 was 1.13 times, and the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Example 16 was 1.35 times. That is, the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance in Example 16 was more than 1.13 times.

[0107] This confirmed that the amount of gas generation can be further suppressed by using cyclohexane and a specific carboxylic acid ester having two or more ester bonds together with sulfolane as the 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. Furthermore, it was confirmed that this effect of suppressing the amount of gas generation is independent of the type of electrolyte.

[0108] (Capacitance Test) For the electric double layer capacitors of Examples 11 to 14, 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 calculation of the capacity retention rate were performed using the same measurement method and conditions as in Example 1. The capacitance retention rates at each temperature of the electric double layer capacitors of Examples 11 to 14 are shown in Table 8 below.

[0109] (Table 8)

[0110] As shown in Table 8, Examples 11 to 13 maintained particularly good capacitance retention rates in a low-temperature environment of -40°C. However, Example 14 also exhibited capacitance in a low-temperature environment of -40°C. In Examples 11 to 13, the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance was less than 1.90. This confirmed that when the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance was less than 1.90, the amount of gas generation was further suppressed while the capacitance was also satisfactorily exhibited in a low-temperature environment of -40°C.

[0111] Examples 17 to 19 The electrolytic solutions for electric double layer capacitors and electric double layer capacitors of Examples 17 to 19 were prepared as follows. The electrolytic solutions for electric double layer capacitors of Examples 17 to 19 differ from Example 13 only in the solute concentration. The electrolytic solutions for electric double layer capacitors of Examples 17 to 19 had the same composition and composition ratio as Example 13, except for the solute concentration, and were prepared by the same manufacturing method and under the same manufacturing conditions. Furthermore, the electric double layer capacitors of Examples 17 to 19 had the same configuration as Example 13, including the capacitance ratio Cp:Cn of the positive electrode capacitance Cp to the negative electrode capacitance Cn, and were prepared by the same manufacturing method and under the same conditions as Example 13.

[0112] In the electrolytic solution for electric double layer capacitors of Example 17, the solute concentration is 2.0 M (mol / L) where 2.0 moles are added to 1 L of solvent. In the electrolytic solution for electric double layer capacitors of Example 13, the solute concentration is 2.2 M (mol / L) where 2.2 moles are added to 1 L of solvent. In the electrolytic solution for electric double layer capacitors of Example 18, the solute concentration is 2.5 M (mol / L) where 2.5 moles are added to 1 L of solvent. In the electrolytic solution for electric double layer capacitors of Example 19, the solute concentration is 2.8 M (mol / L) where 2.8 moles are added to 1 L of solvent.

[0113] (Capacitance Test) For the electric double layer capacitors of Example 13 and Examples 17 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 measurement and calculation of the capacity retention rate were performed using the same measurement method and conditions as in Example 1. The capacitance retention rates at each temperature of the electric double layer capacitors of Example 13 and Examples 17 to 19 are shown in Table 9 below.

[0114] (Table 9)

[0115] As shown in Table 9, Examples 17, 13, and 18 all exhibited good capacitance in low-temperature environments of -30°C and -40°C. However, Example 19 also exhibited good capacitance in a low-temperature environment of -40°C. In Examples 17, 13, and 18, the electrolyte concentration was less than 2.8 mol per liter of solvent. This confirmed that when the capacitance ratio of the positive electrode capacitance to the negative electrode capacitance was more than 1.13 times and less than 1.90 times, and the electrolytic solution for electric double layer capacitors contained 2.0 mol to 2.8 mol of electrolyte per liter of solvent, the amount of gas generation was further suppressed, while good capacitance was also exhibited in a low-temperature environment of -40°C.

Claims

1. An electrolyte for an electric double layer capacitor, comprising a solvent, sulfolane and a carboxylic acid ester, the carboxylic acid ester having a cyclic carbon skeleton and two or more ester bonds.

2. The electrolyte for an electric double layer capacitor according to claim 1, further comprising a carboxylic acid ester having one or more ester bonds as the solvent.

3. The electrolyte for an electric double layer capacitor according to claim 1, further comprising dimethyl sulfone as the solvent.

4. The electrolyte for an electric double layer capacitor according to claim 1, further comprising, as the solvent, a carboxylate having one or more ester bonds and dimethylsulfone.

5. The electrolyte for an electric double layer capacitor according to claim 4, wherein the carboxylic acid ester having a cyclic carbon skeleton and two or more ester bonds is contained in an amount of 3 vol % or more based on the total amount of the solvent.

6. The electrolyte for an electric double layer capacitor according to any one of claims 1 to 5, further comprising a quaternary ammonium salt.

7. The electrolyte for electric double layer capacitors according to claim 1 or 5, wherein the carboxylate ester is in cis or trans form and is 1,4-dimethyl cyclohexanedicarboxylate, 1,2-dimethyl cyclohexanedicarboxylate, 1,2-diethyl cyclohexanedicarboxylate, 1,3-dimethyl cyclohexanedicarboxylate, 1,3-dimethyl cyclopentanedicarboxylate, or 1,3,5-trimethyl cyclohexanetricarboxylate.

8. An electric double layer capacitor comprising: the electrolytic solution for an electric double layer capacitor according to any one of claims 1 to 5; and positive and negative electrode bodies.

9. The electric double layer capacitor according to claim 8, wherein the ratio of the capacitance of the positive electrode to the capacitance of the negative electrode is more than 1.13 times.

10. The electric double layer capacitor according to claim 9, wherein the electrolytic solution for the electric double layer capacitor contains 2.0 moles or more and 2.8 moles or less of an electrolyte per 1 L of the solvent.

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