JP2007019491A - Electric double layer capacitor and electrode therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electricarry activated electric double layer capacitor in which the volumetric capacitance density, resistance, and withstand voltage are all enhanced. <P>SOLUTION: An electric double layer capacitor comprises a polarizable electrode that contains a carbon material including a microcrystalline carbon similar to graphite, and an electrolyte that contains a spiro compound represented by a general formula (1). In the formula, A represents a spiro atom having an sp<SP>3</SP>hybrid orbital, Z<SP>1</SP>and Z<SP>2</SP>respectively represent an atomic group forming a saturated or unsaturated ring having four or more ring atoms including A, and X<SP>-</SP>represents a counter anion. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrode for an electric double layer capacitor and an electric double layer capacitor.

  In recent years, electric double layer capacitors that can be charged and discharged with a large current are promising as power storage devices with high charge / discharge frequency, such as an auxiliary power source for electric vehicles, an auxiliary power source for solar cells, and an auxiliary power source for wind power generation. Therefore, an electric double layer capacitor having a high energy density, capable of rapid charge / discharge, and excellent in durability is desired.

  The electric double layer capacitor has a structure in which a pair of polarizable electrodes are opposed to each other with a separator interposed therebetween to form a positive electrode and a negative electrode. Each polarizable electrode is impregnated with an aqueous electrolyte solution or a non-aqueous electrolyte solution, and each polarizable electrode is joined to a collector electrode. The aqueous electrolyte solution can increase the volume capacitance density and reduce the resistance value, but it is necessary to make the working voltage below the voltage at which water electrolysis occurs, so it is not possible to increase the energy density. An aqueous electrolyte is used.

As a polarizable electrode material used for an electric double layer capacitor, a carbon material having microcrystalline carbon similar to graphite (hereinafter referred to as “graphite similar carbon material”) is known (Patent Documents 1 to 5). This carbon material was prepared so that the interlayer distance (d ₀₀₂ ) of the crystallites of microcrystalline carbon similar to graphite was within the range of 0.365 to 0.385 nm by controlling the activation treatment of the raw material. It is. When the microcrystalline carbon having such a specific interlayer distance is brought into contact with the electrolyte solution and a voltage higher than the voltage normally used (rated voltage) is applied, electrolyte ions are inserted between the carbon crystal layers and electrically activated. (Electric field activation) occurs, and as a result, a high capacitance is exhibited (electric field activation type capacitor). The graphite-like carbon material maintains a high capacitance even if it is repeatedly used at a rated voltage after the insertion of ions once to form pores. Graphite-like carbon materials have a higher withstand voltage and can significantly increase energy density compared to activated carbon that is generally used as a carbon material for electric double layer capacitors. Collecting.

  Conventionally, as an electrolytic solution for an electric field activation type capacitor, since it is suitable for an electric field activation type capacitor that applies a high voltage with a wide potential window, a tetraethylammonium salt that is a tetraalkyl quaternary ammonium, an asymmetric type triethyl. What used methylammonium salt etc. as the electrolyte was used (patent document 2). However, these electrolytes are difficult to insert between 0.365 nm and 0.385 nm of a graphite-like carbon material, and therefore, in terms of capacitance, direct current internal resistance, etc. The electrode performance could not be fully exploited.

  In the case of an electric field activated capacitor, unlike the conventional activated carbon type, the capacitor performance can be exhibited only after the micropores are formed by the insertion of electrolyte ions, so that the characteristics of the electrolytic ions at the time of electric field activation, especially the structure, can be obtained. Greatly affects performance. Paying attention to the structure of such electrolyte ions, it has been proposed to use an imidazolium salt having a planar molecular structure as the electrolyte of an electric field activated capacitor (Patent Document 6). Since such an electrolyte is easily inserted between the layers of microcrystalline carbon, it is possible to improve the initial value of the capacitance and direct current internal resistance of the obtained capacitor. However, the proposed imidazolium salt-based electrolyte has a potential window narrower than that of the tetraalkyl quaternary ammonium salt, and the electrolyte itself decomposes when a high voltage is applied, so that the electrode performance of the graphite-like carbon material can be sufficiently extracted. It cannot be used at the rated voltage.

As an electrolytic solution for an activated carbon capacitor, one using 1,1′-spirobipyrrolidinium compound salt as an electrolyte is known (Non-patent Document 1). The 1,1′-spirobipyrrolidinium compound salt-based electrolyte has high solubility in a solvent, realizes higher conductivity than a conventional quaternary ammonium salt-based electrolyte, and is thermally and electrically stable. It has been reported. However, there is no example in which such a spiro compound salt electrolyte is applied to an electric field activated capacitor. In such a spiro compound, since the spiro atom has sp ³ hybrid orbitals, the two rings are not arranged on one plane, and the whole molecule becomes bulky, so that it is inserted between very fine layers of a graphite-like carbon material. This is because it is contrary to the conventional teaching (Patent Document 6) in which a planar molecular structure is adopted and the substituted alkyl group is preferably not bulky.

JP 11-317333 A JP 2000-077273 A JP 2000-068164 A JP 2000-068165 A Japanese Patent Laid-Open No. 2000-1000066 JP 2004-289130 A Chiba et al., Abstracts of the 72nd Annual Meeting of the Electrochemical Society, page 242, 2005

  The present invention provides an electric field activated electric double layer capacitor that can simultaneously improve the volume capacitance density (energy density), resistance value, and withstand voltage by selecting an electrolyte, and thereby sufficiently bring out the electrode performance of a graphite-like carbon material. The purpose is to provide.

According to the present invention,
(1) An electric double layer capacitor comprising a polarizable electrode containing a carbon material having microcrystalline carbon similar to graphite and an electrolyte containing a spiro compound represented by the following general formula (1) is provided.

(In the above formula, A represents a spiro atom having an sp ³ hybrid orbital, and Z ¹ and Z ² are each independently an atom that forms a saturated or unsaturated ring having 4 or more ring atoms including A. Represents a group, and X ⁻ represents a counter anion.)

Furthermore, according to the present invention,
(2) The electric double layer capacitor according to (1), wherein the spiro atom bears a positive charge of the electrolyte.

Furthermore, according to the present invention,
(3) The electric double layer capacitor according to (2), wherein the spiro atom is nitrogen.

Furthermore, according to the present invention,
(4) The electric double layer capacitor according to any one of (1) to (3), wherein the number of ring atoms of Z ¹ and Z ² is the same.

Furthermore, according to the present invention,
(5) The electric double layer capacitor according to (4), wherein the number of ring atoms of Z ¹ and Z ² is 5.

Furthermore, according to the present invention,
(6) The electric double layer capacitor according to (4) or (5), wherein the ring structures of Z ¹ and Z ² are the same.

Furthermore, according to the present invention,
(7) The electric double layer capacitor according to (1), wherein the spiro compound is 1,1′-spirobipyrrolidinium.

Furthermore, according to the present invention,
(8) The carbon material having microcrystalline carbon similar to graphite has an uncharged specific surface area of 800 m ² / g or less by the BET one-point method and an interlayer distance d ₀₀₂ of 0.350 by the X-ray diffraction method. The electric double layer capacitor according to any one of (1) to (7), which is in a range of ˜0.385 nm, is provided.

  According to the present invention, by combining a polarizable electrode containing a graphite-like carbon material with an electrolyte containing a spiro compound represented by the general formula (1), the volume capacitance density of an electric field activated electric double layer capacitor is obtained. (Energy density), resistance value, and withstand voltage are improved at the same time, and the electrode performance of the graphite-like carbon material can be further enhanced.

  An electric double layer capacitor according to the present invention comprises a polarizable electrode containing a carbon material having microcrystalline carbon similar to graphite, and an electrolyte containing a spiro compound represented by the above general formula (1). It is what.

In the electrolyte according to the present invention, two cyclic structures (Z ¹ and Z ² ) are bonded by spiro atoms having sp ³ hybrid orbitals. Therefore, the plane formed by each ring is twisted at a constant angle with each other, and the two annular structures are not arranged on one plane. According to the conventional teaching, in order to effectively insert the electrolyte between the layers of the graphite-like carbon material, it is supposed that the constituent atoms of the electrolyte cation should have a molecular structure arranged on a plane (Patent Document 6). According to such teaching, it is predicted that a spiro compound having two cyclic structures that are not arranged on one plane is difficult to insert between layers of a graphite-like carbon material. Surprisingly, however, the electrolyte containing the spiro compound according to the present invention can be inserted between the graphite-like carbon materials more effectively than the conventional electrolyte having a molecular structure in which constituent atoms are arranged on a plane. all right. While not being bound by any particular theory, the electrolyte containing the spiro compound according to the present invention has one of the two cyclic structures as a first step with respect to the crystal plane of the graphite-like carbon material upon electric field activation. The graphite-like carbon material is inserted between the crystal layers in a substantially parallel state, and the interlayer of the graphite-like carbon material is expanded by the same action as that of the conventional planar molecule. It is considered that the film is further expanded by opening the first widened layer by inserting it between the crystal layers at a certain angle not parallel to the crystal plane. In a tetraalkyl quaternary ammonium salt that does not have a planar molecular structure, the entire ion is relatively bulky and insertion at the first stage is unlikely to occur. In addition, in the case of an imidazolium salt in which the planar molecular structure is formed only in one plane, the second-stage interlayer expansion does not occur. According to the present invention, since the specific surface area is increased by the second-stage interlayer expansion and the electrolyte ions easily move, higher capacitance and lower DC internal resistance can be realized than the conventional field activated capacitor using the electrolyte. . In addition, since the electrolyte containing the spiro compound according to the present invention has a wide potential window and can be used at a high voltage comparable to a quaternary ammonium salt, the energy density proportional to the square of the capacitance and the voltage is much higher than before. Can be increased.

  The electrolyte containing the spiro compound according to the present invention is represented by the following general formula (1).

In the above formula, A represents a spiro atom having an sp ³ hybrid orbital, and is specifically selected from nitrogen (N) and carbon (C). The positive charge of the spiro compound molecule is preferably localized on the spiro atom so that the influence of solvation is reduced by the charge being shielded by surrounding structures. The spiro atom is preferably nitrogen in that the atomic radius is relatively small.

Z ¹ and Z ² each independently represent an atomic group that forms a saturated or unsaturated ring having 4 or more ring atoms including A. The two cyclic structures represented by Z ¹ and Z ² have different types and / or numbers of ring atoms so that the ease of inserting the graphite-like carbon material between the crystal layers is not easily affected by the orientation of the molecules. It is preferable that they are similar, and more preferably, the kind and number of ring atoms are the same. The number of ring atoms is preferably 5 or more for convenience in the synthesis of the spiro compound, and 5 or more is most preferable because the conductivity decreases when 6 or more. The ring atoms can contain nitrogen, sulfur, oxygen, etc. in addition to carbon. When the spiro atom does not bear the positive charge of the electrolyte, it is necessary to arrange an atom that bears the positive charge on the ring atom other than the spiro atom, for example, quaternized nitrogen. The size of the electrolyte ion is an important factor that affects the ease of inserting the graphite-like carbon material between the crystal layers. In particular cations, van der Waals volume for having a very large ion diameter when compared to anion in the range of 0.01～0.06Nm ^3, is possible to reduce the ion diameter of the cation leads to the promotion of the electric field activation. Therefore, when the ring atom of the spiro compound according to the present invention has a substituent, the substituent is preferably small, and more preferably does not contain any substituent.

X ⁻ represents a counter anion. Counter anions include BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , ClO ₄ ⁻ , CF ₃ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₂ N from the viewpoint of electrochemical stability and molecular ion diameter. _{^{-, AlCl 4 -, SbF 6}} - preferably like, especially BF ₄ ^- are preferable.

上式中、Ｒ₁〜Ｒ₁₀は、水素または炭素数１〜５のアルキル基を表わす。 Hereinafter, specific examples of the electrolyte cation suitably used in the present invention will be listed.

In the above formula, R _{1 to} R ₁₀ represent hydrogen or an alkyl group having 1 to 5 carbon atoms.

  The electrolyte according to the present invention may be used as it is without being diluted when it is liquid at room temperature, but it is generally preferable to use it as an electrolytic solution dissolved in an organic solvent. By using an organic solvent, the viscosity of the electrolytic solution can be lowered and an increase in the DC internal resistance of the electrode can be suppressed. The organic solvent is selected depending on the solubility of the electrolyte, the reactivity with the electrode, etc., but propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dimethoxyethane, dimethoxyethane, Examples include ethoxyethane, γ-butyrolactone (GBL), acetonitrile (AN), propionitrile and the like. For example, when the electrolyte is a 1,1′-spirobipyrrolidinium compound, it has a solubility three or more times that of tetraethylammonium salt in propylene carbonate, so that high conductivity is secured by increasing the concentration of the electrolyte. Can do. An organic solvent may be used independently and may be used as a mixed solvent which combined 2 or more types. Since electrolyte ions inserted between the crystal layers of the graphite-like carbon material at the time of electric field activation are considered to be solvated with the surrounding solvent, it is preferable to use a solvent having a small molecular volume. The concentration of the electrolyte in the electrolytic solution is preferably 0.5 mol / L or more, and more preferably 1.0 mol / L or more. The upper limit of the electrolyte concentration is the solubility determined by the combination of the individual specific electrolyte and the organic solvent.

The graphite-like carbon material used as the polarizable electrode of the electric double layer capacitor according to the present invention has microcrystalline carbon. A graphite-like carbon material has a microcrystalline carbon interlayer distance d ₀₀₂ (according to X-ray diffractometry) in a specific range, that is, 0.350 to 0.385 nm. Ions are inserted between the crystal layers of microcrystalline carbon to exhibit a high capacitance as a polarizable electrode. When the interlayer distance d ₀₀₂ is in the range of 0.355～0.370Nm, more preferable because expression of capacitance due to insertion into the interlayer of the electrolyte ions conspicuous. When the interlayer distance d ₀₀₂ is below 0.350 nm, since the insertion into the interlayer of the electrolyte ions becomes difficult to occur, the rate of increase in the electrostatic capacitance decreases. When the interlayer distance d ₀₀₂ in the opposite is more than 0.385 nm, increasing the amount of functional groups present on the surface of the graphite-like carbon material, due to that these functional groups are decomposed during voltage application of the electric double layer capacitor This is not preferable because the performance is significantly reduced. The interlayer distance d _002, using the X-ray diffractometer "RINT2000" manufactured by Rigaku Corporation, a powder sample in air, CuKa line (target: Cu, excitation voltage 30 kV) is a value measured by.

The specific surface area of this graphite-like carbon material is preferably 800 m ² / g or less, more preferably 600m ² / g. When this specific surface area exceeds 800 m ² / g, sufficient electrostatic capacity can be obtained without depending on the electric field activation. In addition, the amount of functional groups present on the surface of the graphite-like carbon material increases, and the performance of the electric double layer capacitor is significantly lowered due to the decomposition of these functional groups when a voltage is applied. The specific surface area is a value measured by a BET 1-point method using “MONOSORB” manufactured by Yuasa Ionics Co., Ltd. (drying temperature: 180 ° C., drying time: 1 hour).

  The graphite-like carbon material can be a low-temperature calcined carbon material that has not been activated. Wood, coconut husk, pulp waste liquor, fossil fuel-based coal, heavy petroleum oil, and pyrolysis of them It can be produced by using various materials such as coal, petroleum-based pitch, coke, synthetic resin such as phenol resin, furan resin, polyvinyl chloride resin, and polyvinylidene chloride resin. When the carbon material is classified into graphitizable carbon and non-graphitizable carbon, it is preferable to use graphitizable carbon containing a large amount of graphite-like microcrystalline carbon from the viewpoint of capacitance. In order to suppress electrode expansion during electric field activation, it is also possible to use a carbon material that is a composite of non-graphitizable carbon and graphitizable carbon on a nanoscale (Katsuharu Ikeda, Nikkei Automotive Technology launch seminar, (Lecture material, May 21, 2004, Nikkei Electronics). Further, in order to balance the performance, two or more kinds of carbon materials having different raw materials and production methods can be mixed and used.

  At the time of producing the graphite-like carbon material, heat treatment can be performed in an inert atmosphere before activation so that the activation does not proceed greatly, or the activation operation can be performed for a short time. As the heat treatment temperature, those subjected to firing at a relatively low temperature of about 600 to 1000 ° C. are preferable. For other graphite-like carbon materials suitably used in the present invention and methods for producing the same, see Patent Documents 1 to 5.

  The graphite-like carbon material is contained in the polarizable electrode in the range of 50 to 99% by mass, preferably 65 to 95% by mass, based on the total mass of the conductive auxiliary material and binder described later. When the content of the graphite-like carbon material is less than 50% by mass, the energy density of the electrode is lowered. On the other hand, if the content exceeds 99% by mass, the binder is insufficient and it becomes difficult to hold the carbon material in the electrode.

  The electrode for an electric double layer capacitor contains a conductive auxiliary material for imparting conductivity to the graphite-like carbon material. As the conductive auxiliary material, carbon black such as ketjen black and acetylene black, vapor grown carbon fiber, fullerene, carbon nanotube, nanocarbon such as carbon nanohorn, powdery or granular graphite, and the like can be used. The conductive auxiliary material may be added in an amount of preferably 1 to 40% by mass, more preferably 3 to 20% by mass, based on the total mass of the graphite-like carbon material and the binder. When the addition amount of the conductive auxiliary material is less than 1% by mass, the DC internal resistance of the electric double layer capacitor is increased. On the contrary, when the addition amount exceeds 40% by mass, the energy density of the electrode is lowered.

  The electrode for an electric double layer capacitor contains a binder for binding the graphite-like carbon material and the conductive auxiliary material. As the binder, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene (PP), styrene butadiene rubber (SBR), acrylonitrile butadiene rubber (NBR), or the like can be used. The binder may be added in an amount of preferably 1 to 30% by mass, more preferably 3 to 20% by mass, based on the total mass of the graphite-like carbon material and the conductive auxiliary material. When the added amount of the binder is less than 1% by mass, it is difficult to hold the carbon material on the electrode. On the contrary, when the addition amount exceeds 30% by mass, the energy density of the electric double layer capacitor is lowered, and the direct current internal resistance is increased.

  The electrode for an electric double layer capacitor can be produced by the same sheet forming method and coating method (coating method) as in the case of using conventional activated carbon. For example, in the case of a sheet molding method, after adjusting the particle size of the graphite-like carbon material obtained by the above method so that the average particle size D50 is about 5 to 200 μm, a conductive auxiliary material and a binder are added thereto. Kneaded and rolled to form a sheet. In kneading, liquid auxiliaries such as water, ethanol and acetonitrile may be used alone or in combination as appropriate. The thickness of the electric double layer capacitor electrode is preferably 50 to 500 μm, more preferably 60 to 300 μm. When this thickness is less than 50 μm, the volume occupied by the current collector, which will be described later, increases when the capacitor cell is formed, and the energy density is lowered. On the other hand, if the thickness exceeds 500 μm, the density of the electrode cannot be increased, so that the energy density is similarly lowered and the DC internal resistance of the electric double layer capacitor is also increased. The thickness of the electrode is a value measured using a dial thickness gauge “SM-528” manufactured by Teclock Co., Ltd. in a state where no load other than the main body spring load is applied.

  The electrode for an electric double layer capacitor is generally used by being integrated with a current collector. As the current collector, various sheet materials including metal sheets such as aluminum, titanium and stainless steel, and nonmetal sheets such as conductive polymer films and conductive filler-containing plastic films can be used. . The sheet-like current collector may have a hole in part or the entire surface. When the sheet electrode and the current collector are integrated, it can function by simply crimping the both, but in order to reduce the contact resistance between them, the electrode and the current collector Or the electrode and the current collector may be pressure-bonded after the conductive paint is applied to the electrode or the current collector and dried. In the electrode produced by the coating method, the electrode is molded and the current collector is bonded at the same time.

  The electric double layer capacitor has a structure in which a pair of polarizable electrodes are opposed to each other with a separator interposed therebetween to form a positive electrode and a negative electrode. As the separator, an insulating material such as microporous paper, glass, or a plastic porous film such as polyethylene, polypropylene, polyimide, or polytetrafluoroethylene can be used. The thickness of the separator is generally about 10 to 100 μm. The separator may be used by stacking two or more insulating materials.

The electric field activation can be performed by applying a voltage higher than the rated voltage with a relatively small current value. For the method of electric field activation, refer to the conventional method (Patent Document 5). During electric field activation, the graphite-like carbon material exhibits a characteristic that it mainly expands in the direction of voltage application by both current collectors. For this reason, even if the capacitance of the electric double layer capacitor increases, the capacitance per unit volume (volume capacitance density) is reduced by the amount of expansion. Therefore, in order to enjoy the increase in the volume capacitance density, it is preferable to minimize the increase in the volume of the electric double layer capacitor due to the expansion of the graphite-like carbon material. In order to suppress the increase in volume of the electric double layer capacitor, a pressure that resists the pressure (expansion pressure) generated by the expansion of the graphite-like carbon material may be applied to the electrode from the outside. For example, the volume capacitance density can be increased by applying a pressure of 0.1 to 30 MPa to the electrode from the outside during charging. However, if the expansion of the electrode is completely suppressed, the insertion of electrolyte ions between the crystal layers of the graphite-like carbon material becomes insufficient, and the effect of improving the capacitance is reduced, so that the expansion of about 5 to 100% occurs. Thus, it is preferable to set the external pressure.
After the electrolytic activation is performed once, the polarity of the positive electrode and the negative electrode is reversed and the electrolytic activation is performed again so that the crystal layer of the graphite-like carbon material matches the cation (the ion diameter is larger than the anion) in both electrodes. As a result, the specific surface area is increased and the electrolyte ions are easily moved, which is preferable because a higher capacitance and a lower DC internal resistance can be realized.

Hereinafter, the present invention will be specifically described by way of examples.
Example 1
A petroleum pitch-based carbon material (500 g) was pulverized with a pulverizer to prepare a powder having a D50 of 20 μm, which was fired at a temperature of 800 ° C. in an inert atmosphere to obtain a carbonized material. This carbonized material was mixed with potassium hydroxide twice the mass ratio and subjected to activation treatment at 700 ° C. in an inert atmosphere. Thereafter, the mixture was cooled to room temperature and washed with water to remove the alkali and dried. The obtained graphite-like carbon material had a BET specific surface area of 50 m ² / g, and an interlayer distance d _{002 of the} microcrystalline carbon measured by the X-ray diffraction method was 0.355 nm. 85% by mass of this graphite-like carbon material, 5% by mass of Ketjen black powder (“EC600JD” manufactured by Ketjen Black International Co., Ltd.) as a conductive auxiliary material, and polytetrafluoroethylene powder (Mitsui Dupont Fluoro Chemical Co., Ltd.) as a binder Ethanol was added to a mixture of 10% by mass of “Teflon (registered trademark) 6J”) and kneaded, and then roll rolling was performed 5 times to obtain a polarizable sheet having a width of 100 mm and a thickness of 200 μm. A high-purity etched aluminum foil (“C512” manufactured by KDK Corporation) having a width of 150 mm and a thickness of 50 μm is used as a current collector, and a conductive adhesive solution (“GA-37” manufactured by Hitachi Powdered Metallurgy Co., Ltd.) is provided on both sides thereof. Was applied and the electrodes were stacked, and this was passed through a compression roll and crimped to obtain a laminated sheet in which the contact interfaces were bonded together. The laminated sheet was placed in an oven set at a temperature of 150 ° C. and held for 10 minutes, and the polarizable electrode was obtained by evaporating and removing the dispersion medium from the conductive adhesive liquid layer.

  As shown in FIG. 1, this laminated sheet has a 3 cm square dimension of the carbon electrode portion of the polarizable electrode, and a lead portion (a portion where the polarizable electrode is not laminated on the current collector) is 1 × 5 cm. A square-shaped polarizable electrode was punched out in a shape. Two polarizable electrode bodies are used as a positive electrode and a negative electrode, and an ePTFE sheet (“BSP070807-2” manufactured by Japan Gore-Tex Co., Ltd.) having a thickness of 80 μm and a 3.5 cm square is inserted as a separator between them. Cover the electrodes and separator with two 5 x 10 cm aluminum laminates ("PET12 / A120 / PET12 / CPP30 dry laminate" manufactured by Showa Denko Packaging Co., Ltd.) and heat-melt the three sides including the lead The aluminum pack cell was made by sealing by wearing. At that time, a part of the lead part was led out of the aluminum pack cell, and the joint part between the lead part and the aluminum pack cell was sealed by heat fusion of the lead part and the aluminum laminate material. This aluminum pack cell was vacuum-dried at 150 ° C. for 24 hours, and then brought into a glove box that maintained a dew point of −60 ° C. or lower in an argon atmosphere, and the opening (unsealed side) faced upward. 4 mL of a propylene carbonate solution of 1,1′-spirobipyrrolidinium tetrafluoroborate in mol / L was poured into an aluminum pack as an electrolyte, and allowed to stand for 10 minutes under a reduced pressure of −0.05 MPa. The gas was replaced with electrolyte. Finally, a single-layer electric double layer capacitor was fabricated by fusing and sealing the opening of the aluminum pack. The intended use voltage of this capacitor is 3.0-3.5V. This electric double layer capacitor was stored at 40 ° C. for 24 hours, and the electrolyte was aged to the inside of the electrode. Thereafter, pressurization was performed at 0.2 MPa from the surface direction of the capacitor, and this capacitor was referred to as Example 1.

Example 2
A capacitor was assembled in the same manner as in Example 1, except that a 1.5 mol / L 1,1′-spirobipiperidinium (6-membered ring structure) tetrafluoroborate propylene carbonate solution was used as the electrolyte. .

Comparative Example 1
A capacitor was assembled in the same manner as in Example 1 except that a 1.5 mol / L triethylmethylammonium tetrafluoroborate propylene carbonate solution was used as the electrolytic solution.

Comparative Example 2
A capacitor was assembled in the same manner as in Example 1 except that a 1.5 mol / L 1,3-dimethylimidazolium tetrafluoroborate propylene carbonate solution was used as the electrolytic solution.

Comparative Example 3
A capacitor was assembled in the same manner as in Example 1 except that a propylene carbonate solution of N, N-diethylpyrrolidinium tetrafluoroborate was used as the electrolytic solution.

Comparative Example 4
A capacitor was assembled in the same manner as in Example 1 except that activated carbon (“YP-17” manufactured by Kuraray Chemical Co., Ltd.) having a specific surface area of 1600 m ² / g was used as the carbon material.

Comparative Example 5
Example 1 except that activated carbon having the same specific surface area of 1600 m ² / g as in Comparative Example 4 was used as the carbon material, and a 1.5 mol / L propylene carbonate solution of triethylmethylammonium tetrafluoroborate was used as the electrolyte. A capacitor was assembled in the same manner as above.

Test 1 (Performance comparison under the same conditions)
The capacitor cells of Examples 1 and 2 and Comparative Examples 1 to 5 manufactured as described above were tested under the following conditions, and the volume capacitance density, DC internal resistance, leakage current (charging) at the start-up 20th cycle (End current) was measured.
Conditions for electric field activation at the first cycle and start-up of the 20th cycle (electric field activation)
Charge: 1 mA / cm ² , 4.0 V, 6 hours Discharge: 1 mA / cm ² , 0 V
Temperature: 25 ° C
Cycle: 1
(Start-up)
Charging: 5 mA / cm ² , 3.0 V, 30 minutes Discharging: 5 mA / cm ² , 0 V
Temperature: 25 ° C
Cycle: 20
In Comparative Examples 4 and 5, since electrolysis occurred when a high voltage was applied, only startup was performed. The measurement results are shown in Table 2 below.

Test 2 (Performance comparison according to withstand voltage)
Using each cell after completion of test 1, a charge / discharge test of one cycle is performed while increasing the charging voltage by 0.1 V from 2.7 to 4.0 V, and the voltage at which the leakage current becomes 5 mA (withstand voltage) Was measured. The measurement results are shown in Table 3 below. The volume capacitance density and energy density in Table 3 are values at withstand voltage.

(Volume capacitance density)
It calculated | required by the energy conversion method, and divided | segmented by the volume of the carbon electrode part of the positive / negative electrode which does not contain the electrical power collector before expansion | swelling, and computed.
(DC internal resistance)
The discharge curve from the discharge start time to 10% of the total discharge time was linearly approximated, and the DC internal resistance at the discharge start time was calculated.
(Leak current)
The current value (charge end current) required to maintain the charge voltage at the end of charge is shown.
(apparatus)
Charge / Discharge Test Equipment “CDT5R2-4” manufactured by Power Systems
Software for analysis "CDT Utility Ver. 2.02" manufactured by Power System Co., Ltd.

  As can be seen from Tables 2 and 3, the capacitor cell according to the present invention is significantly advantageous over the capacitor cells according to the respective comparative examples in that a high energy density, a low DC internal resistance and a high withstand voltage can be realized simultaneously. In particular, when Example 1 or 2 and Comparative Example 1 are compared, the volume capacitance density is 17% or more higher. Therefore, the electrolyte containing the spiro compound according to the present invention is more graphite than the conventional quaternary ammonium salt-based electrolyte when the electric field is activated. It turns out that it is easy to insert between the crystal | crystallization layers of a similar carbon material. In addition, since the conductivity of the electrolyte itself is high, it can be seen that the direct current internal resistance of the capacitor of the example was reduced by about 40%. Looking at Comparative Example 2, 1,3-dimethylimidazolium decomposes under the conditions of Test 1, and the DC internal resistance and leakage current become significantly large, so that the operating voltage cannot be increased, resulting in an energy density of It turns out that it becomes low. When Example 1 and Comparative Example 3 are compared, even if the types and numbers of constituent atoms of the electrolyte are substantially the same, the two annular structures are obtained from the high volumetric capacitance density and energy density of Example 1. It can be seen that electric field activation is promoted by bonding with spiro atoms. Comparative Example 4 and Comparative Example 5 show that there is no substantial difference in volume capacitance density compared to conventional quaternary ammonium salt electrolytes even when an electrolyte containing a spiro compound according to the present invention is combined with an activated carbon electrode. Is shown. That is, it can be said that the electrolyte containing the spiro compound exhibits a remarkable effect of improving the volume capacitance density when combined with the graphite-like carbon material. Looking at Example 1 and Example 2, 1,1′-spirobipyrrolidinium (5-membered ring) has a volume capacitance density higher than 1,1′-spirobipiperidinium (6-membered ring). It can be said that both the DC internal resistance and the DC resistance are slightly better.

For each sample of Example 1, Comparative Example 1, and Comparative Example 2, before and after Test 1, the crystal interlayer distance _d002 of the carbon material in the negative electrode on which the electrolyte cations adsorb was measured. Specifically, two capacitor cells were prepared for each of the above samples, one was decomposed before the test (after the electrolyte aging) and the other after the test, and the negative electrode was washed with propylene carbonate. Next, the negative electrode was heated at 250 ° C. for 12 hours to separate the carbon material sheet and the aluminum foil current collector, and then the carbon material sheet was heated at 400 ° C. in a nitrogen atmosphere for 3 hours to decompose the PTFE binder, The material sheet was pulverized. Table 4 shows the measurement result of the crystal interlayer distance _d002 of the powdered carbon material by the X-ray diffraction method.

  As can be seen from Table 4, 1,1′-spirobipyrrolidinium ion (Example 1) is more charcoal than tetraethylmethylammonium ion (Comparative Example 1) and 1,3-dimethylimidazolium ion (Comparative Example 2). The action of expanding the crystal layers of the material is strong, and this is considered to be related to the development of a high volume capacitance density according to the present invention.

It is the schematic which shows the preparation methods of the capacitor cell used in the Example.

Claims (8)

An electric double layer capacitor comprising a polarizable electrode containing a carbon material having microcrystalline carbon similar to graphite and an electrolyte containing a spiro compound represented by the following general formula (1).

(In the above formula, A represents a spiro atom having an sp ³ hybrid orbital, and Z ¹ and Z ² are each independently an atom that forms a saturated or unsaturated ring having 4 or more ring atoms including A. Represents a group, and X ⁻ represents a counter anion.)   The electric double layer capacitor according to claim 1, wherein the spiro atom carries a positive charge of the electrolyte.   The electric double layer capacitor according to claim 2, wherein the spiro atom is nitrogen. The electric double layer capacitor according to claim 1, wherein Z ¹ and Z ² have the same number of ring atoms. The electric double layer capacitor according to claim 4, wherein Z ¹ and Z ² have 5 ring atoms. The electric double layer capacitor according to claim 4 or 5, wherein the ring structures of Z ¹ and Z ² are the same.   The electric double layer capacitor according to claim 1, wherein the spiro compound is 1,1'-spirobipyrrolidinium. The carbon material having microcrystalline carbon similar to graphite has an uncharged specific surface area of 800 m ² / g or less by the BET 1-point method, and an interlayer distance d ₀₀₂ by the X-ray diffraction method of 0.350-0. The electric double layer capacitor according to claim 1, which is in a range of 385 nm.

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