JP2000114105A - Electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a capacitor having low inner resistance and large capacitance per unit volume. SOLUTION: A separator, which is composed of paper formed by using at least 50 wt.% of fiber formed by beating regenerated cellulose fiber, is arranged between a positive electrode and a negative electrode which are constituted of carbonaceous electrodes. The separator is impregnated with a nonaqueous electrolyte, using triethyl monomethyl ammonium tetrafluoroborate as the solute. It is preferable that the thickness of the separator be 20-60 μm and that the density be 0.30-0.60 g/cm3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electric double layer capacitor having a high output and a high energy density.

[0002]

2. Description of the Related Art Conventionally, separators disposed between a positive electrode and a negative electrode of an electric double layer capacitor include electrolytic paper, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyester nonwoven fabric, kraft paper, rayon fiber and sisal fiber mixed sheet, manila hemp. Sheets, glass fiber sheets and the like are known (JP-A-9-45586, JP-A-1-3047).
19 etc.). The role of the separator is to electrically insulate between the positive electrode and the negative electrode, while facilitating the movement of ions in the electrolyte caused by charging and discharging.

In recent years, an electric double layer capacitor for charging and discharging a large current has been receiving attention. However, organic fiber-based separators such as polyethylene have low liquid absorption and retention properties of the electrolytic solution, and therefore have low ionic conductivity and high internal resistance of the electric double layer capacitor. Therefore, when an instantaneous large current discharge, which is one of the great characteristics of the electric double layer capacitor, is performed, the voltage drop is large and is not practical.

A conventional paper separator is excellent in heat resistance and tensile strength. Therefore, it is not suitable for an electric double layer capacitor used for a power supply which does not discharge a large current, such as a hybrid power supply with a solar cell. May be valid. However, when a paper separator is used for an electric double layer capacitor in which the electrolytic solution that discharges a large current is non-aqueous, the ion permeability is insufficient.

[0005] Further, in the electric double layer capacitor, since it is desired to reduce the resistance and increase the capacity per unit volume, it is necessary to reduce the thickness of the separator as much as possible. Insufficient insulation between them may cause micro-short-circuiting, self-discharge is likely to occur, and the production yield of the capacitor may be reduced.

[0006]

DISCLOSURE OF THE INVENTION The present invention is intended to reduce the resistance of an electric double layer capacitor by using a separator which is excellent in heat resistance and ion permeability and has high strength even though it is thin and excellent insulation. The purpose is to realize high capacity.

[0007]

According to the present invention, there is provided an electric double layer capacitor comprising a separator disposed between a positive electrode comprising a carbonaceous electrode and a negative electrode and impregnated with a non-aqueous electrolyte. An electric double-layer capacitor, which is a paper made by containing 50% by weight or more of fibers obtained by beating cellulose fibers, and wherein the solute of the non-aqueous electrolyte is triethyl monomethyl ammonium tetrafluoroborate. provide.

According to the present invention, as a raw material for a separator of an electric double layer capacitor using a non-aqueous electrolyte, a refining cellulose fiber or a solvent-spun rayon having a high degree of polymerization is beaten with a beater installed in a usual paper making process. Possible regenerated cellulose fibers are beaten and used.

If the fiber obtained by beating the regenerated cellulose fiber is less than 50% by weight, the resistance of the separator increases or the strength of the separator decreases. In order to maintain the strength of the separator, the thickness of the separator may be increased,
It is not preferable because the resistance increases. In the present invention, the fiber obtained by beating the regenerated cellulose fiber is preferably at least 65% by weight, more preferably at least 80% by weight.

[0010] The regenerated cellulose fibers that can be beaten are uniformly fibrillated by beating treatment, and the flexibility is increased. Therefore, a separator made of paper made by containing 50% by weight or more of these fibers has high density and excellent tensile strength. Further, since the fibers that have been beaten and fibrillated are very dense and the cross section of the fibrils is almost a perfect circle, the separator made of paper made by containing the fibers in an amount of 50% by weight or more has low resistance. .

[0011] The regenerated cellulose fiber in the present invention is J
Canadian standard freeness (Ca) specified in IS-P8121
nadian Standard Freeness,
(Hereinafter referred to as CSF) is preferably beaten until the volume becomes 0 to 600 ml. Regenerated cellulose fibers such as solvent-spun rayon have a CSF of about 800 ml in an unbeaten state. If the degree of beating is weak, the fibers are not sufficiently fibrillated.
It is preferable to beat until the water content becomes 600 ml or less.
In addition, since the regenerated cellulose fiber is improved in denseness as it is beaten, high strength can be expected. Therefore, it may be beaten until the CSF becomes 0 ml.

Other materials to be added to the beaten regenerated cellulose fiber are not particularly limited, and fibers such as manila hemp, sisal hemp, kraft pulp and the like can be used. Further, it is preferable that these materials are beaten in accordance with the degree of beating of the regenerated cellulose fiber.

The separator according to the present invention is obtained, for example, as follows. First, beatenable regenerated cellulose fibers cut to several mm are beaten by a beater until an appropriate CSF value is obtained. On the other hand, the fibers to be mixed raw materials are similarly beaten appropriately and then appropriately mixed so as to contain 50% by weight or more of regenerated cellulose fibers to form paper having a predetermined thickness. The paper thus obtained is used as a separator disposed between the positive electrode and the negative electrode.

The separator according to the present invention has a thickness of 20
密度 60 μm, and a density of 0.30 to
It is preferably 0.60 g / cm ³ . 20μ thick
If it is less than m, the electrodes may not be sufficiently insulated,
If it exceeds 60 μm, the capacity density of the electric double layer capacitor may not be sufficiently increased. The density is 0.3
If it is less than 0 g / cm ³ , the strength of the separator tends to be weak, and if the density exceeds 0.60 g / cm ³ , the ion permeability tends to be insufficient. In particular, the thickness is 30-50
μm is preferable, and the density is 0.35 to 0.50 g /
A range of cm ³ is preferred.

[0015] A separator made of paper made from paper containing at least 50% by weight of fibers obtained by beating regenerated cellulose fibers usually contains 3 to 10% by weight of water. In order to reduce the leakage current and secure a high withstand voltage in the non-aqueous electric double layer capacitor, it is preferable to remove this moisture. When used as a separator, the water content in the paper is preferably 1% by weight or less.

In order to efficiently remove water, a separator must be placed in advance before placing the separator between the positive electrode and the negative electrode.
It is preferred to heat at ~ 250 ° C. Particularly, in order to obtain a large-capacity electric double layer capacitor, a cylindrical type in which a pair of long electrodes are wound through a separator and impregnated with an electrolytic solution in an electrolytic solution and housed in a bottomed cylindrical container, Alternatively, in the case where a device formed by alternately laminating a plurality of positive electrodes and negative electrodes with a separator interposed therebetween is impregnated with an electrolytic solution and formed into a rectangular container or the like and housed in a rectangular container, the element is formed by the electrodes and the separator. After that, efficient water removal cannot be performed.

If the heat treatment temperature is lower than 90 ° C., the removal of water from the separator becomes insufficient, and the effect of reducing leakage current and the like is reduced. When the temperature exceeds 250 ° C., thermal decomposition of the separator itself starts to decrease the strength or generate moisture. More preferably, the heat treatment temperature is from 120 to 230C. The heat treatment time is appropriately selected depending on the relationship with the heat treatment temperature, but is usually 3 seconds or more.

As a method of the heat treatment, a method such as contact with a heated heater, infrared irradiation, and heated air is appropriately selected. The separator is usually obtained in a wound state,
In a wound state, it is difficult to effectively dehydrate in a short time by heating. It is preferable to heat the separator in a state where it does not overlap tightly, because dehydration can be performed effectively. Specifically, the separator roll is heated in a dry atmosphere while being rewound while being wound to produce a dewatered separator roll, or a plurality of separator sheets are cut in advance from the separator roll, and the separator sheets are separated from each other. Dehydration may be performed by heating with, for example, a heat-resistant spacer network interposed so as not to overlap tightly.

In the electric double layer capacitor of the present invention, both the positive electrode and the negative electrode are carbonaceous electrodes mainly composed of a carbon material, and charge is stored in the electric double layer formed at the interface between the electrode and the electrolyte. The principle is to do. In order to increase the capacity of the electric double layer capacitor, the carbon material preferably has a large specific surface area, and the carbonaceous electrode has a specific surface area of 700.
It is preferable that the carbon material comprises up to 2500 m ² / g and an organic binder.

Activated carbon, carbon black, polyacene and the like can be used as the carbon material. A conductive material may be added to the carbonaceous electrode as needed to increase conductivity,
An organic binder is added to form a sheet on a metal current collector to form an electrode unit integrated with the current collector. As the organic binder used here, polyvinylidene fluoride,
Polytetrafluoroethylene, a polyimide resin, a polyamideimide resin and the like are preferable. Further, as the metal current collector, a foil, a net, or the like of aluminum, stainless steel, or the like can be used. Aluminum is particularly preferred because it is lightweight and has low resistance.

The electrolytic solution used for the electric double layer capacitor includes an aqueous electrolytic solution and a non-aqueous electrolytic solution. The withstand voltage is about 0.8 V in an aqueous system and about 2.5 V in a non-aqueous system. Since the electrostatic energy of the electric double layer capacitor is proportional to the square of the withstand voltage, it is preferable to use a non-aqueous electrolyte because the use of a non-aqueous electrolyte solution can increase about 9 times in terms of energy density.

As the solute of the non-aqueous electrolyte of the electric double layer capacitor, R ¹ R ² R ³ R ⁴ N ⁺ or R ¹ R ² R ³ R ⁴ P ⁺ (where R ¹ , R ² , R ³ , R ⁴ each independently has 1 to 1 carbon atoms
A quaternary onium cation represented by BF ₄ ⁻ , PF ₆ ⁻ , CF ₃ SO ₃ ⁻ , AsF ₆ ⁻ , N (S
One or a mixture of two or more of salts formed with anions such as O ₂ CF ₃ ) ₂ ⁻ and ClO ₄ ⁻ can be preferably used. In the present invention, triethyl monomethyl ammonium tetrafluoroborate is used.

Examples of the organic solvent used for the non-aqueous electrolyte include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate, chain carbonates such as dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate, sulfolane and sulfolane derivatives. Are preferred. In particular, at least one selected from the group consisting of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, sulfolane and methyl sulfolane is preferable.

As the shape of the electric double layer capacitor of the present invention, a pair of long electrode bodies are wound through a long separator and impregnated with a non-aqueous electrolyte to form a bottomed cylindrical case. A plurality of cylindrical and rectangular electrode bodies housed therein are alternately stacked as a positive electrode body and a negative electrode body via a separator, impregnated with a non-aqueous electrolyte, and housed in a bottomed square case. Is preferable since a large capacity can be obtained. In particular, in the case of a cylindrical shape, sufficient tensile strength of the electrode body and the separator is required when winding, but the separator of the present invention is preferable because it has a high strength even if it is thin.

[0025]

[Example 1] Solvent-spun rayon with CSF of 20 m
The paper was beaten until it became 1 and was used to form paper having a thickness of 40 μm and a density of 0.40 g / cm ³ , which was used as a separator. The breaking strength was 0.70 kg / cm width. This separator contained 7% by weight of water. This separator is cut into 5 cm x 7 cm,
It was dried at 50 ° C. for 1 hour and dehydrated to reduce the water content to 1% by weight or less.

An electrode formed into a sheet having a composition of 80% by weight of activated carbon having a specific surface area of 1500 m ² / g, 10% by weight of carbon black and 10% by weight of polytetrafluoroethylene (electrode area: 24 cm ² , electrode thickness: 0.15 mm) ),
4cm wide, 6cm high, 50μ thick with lead terminals
m is bonded to one side of a rectangular aluminum foil via a conductive adhesive, and the adhesive is thermally cured by heating.
To obtain an electrode assembly.

In a glove box having a dew point of 50 ° C., the two electrode bodies are opposed to each other with the electrode surfaces interposed therebetween through the dehydrated separator, and two glass holding plates having a thickness of 2 mm, a width of 5 cm, and a height of 7 cm are provided. To form a capacitor element. The total thickness of the electrode body and the separator was 0.44 mm. Next, the element was heated in vacuum at 200 ° C. for 3 hours to further remove impurities and moisture in the electrode.

As the electrolytic solution, a solution prepared by dissolving 1.5 mol / l of triethylmonomethylammonium tetrafluoroborate in propylene carbonate was used. The above element was vacuum-impregnated with this electrolyte to form an electric double layer capacitor cell, and the DC resistance and capacity were determined at a current density of 20 mA / cm ² . DC resistance is 0.20Ω, capacity is 13.2
F. The leakage current at a voltage of 2.5 V was 8 μA. Capacitance per 1 cc element is 12.5F, element 1
The internal resistance per cc was 0.21Ω.

[Example 2] A solvent-spun rayon having a CSF of 10
70% by weight of fibers beaten to 0 ml and 30% of fibers beaten to Manila hemp to CSF of 400 ml.
Weight 50%, density 0.3
A capacitor element was assembled in the same manner as in Example 1 except that paper of 5 g / cm ³ was made and used as a separator, and the separator was dried at 200 ° C. for 1 hour in advance. The breaking strength of the separator was 0.96 kg / cm width.

An electric double layer capacitor cell was prepared in the same manner as in Example 1 except that the above-mentioned element was used, and the performance was evaluated.
The total thickness of the electrode body and the separator was 0.45 mm. The DC resistance was 0.24Ω and the capacity was 12.1F. The leakage current at a voltage of 2.5 V was 6 μA.
The capacitance per 1 cc of the device was 11.2 F, and the internal resistance per 1 cc of the device was 0.26 Ω.

Example 3 An electric double layer capacitor cell was produced in the same manner as in Example 1 except that the separator was not preliminarily dried, and its performance was evaluated. The total thickness of the electrode body and the separator was 0.45 mm. DC resistance is 0.24Ω,
The capacity was 12.1F. The leakage current at a voltage of 2.5 V was 13 μA. The capacity per 1 cc element is
The internal resistance per 1.2 F and 1 cc of the element was 0.26Ω.

Example 4 (Comparative Example) A mixture of 70% by weight of unbeaten solvent-spun rayon (800 ml of CSF) and 30% by weight of fibers obtained by beating Manila hemp to 200 ml of CSF was mixed to a thickness of 60 μm and a density of 60%. 0.35 g / cm ³
Was made and used as a separator. The breaking strength of the separator was 0.83 kg / cm width.

An electric double layer capacitor cell was prepared in the same manner as in Example 1 except that the above-mentioned element was used, and the performance was evaluated.
The total thickness of the electrode body and the separator was 0.46 mm. The DC resistance was 1.30Ω and the capacity was 8.2F.
The leakage current at a voltage of 2.5 V was 5 μA. The capacitance per 1 cc of the device was 7.4 F, and the internal resistance per 1 cc of the device was 1.44 Ω.

Example 5 (Comparative Example) A separator made of a nonwoven fabric made of polypropylene (thickness: 160 μm, basis weight: 52 g / m ² ) was used. The breaking strength of the separator is 1.53k
g / cm width. In order to prevent thermal degradation of the polypropylene, an electric double layer capacitor cell was prepared in the same manner as in Example 1 except that the temperature of the vacuum heating of the capacitor element was set to 120 ° C., and the performance was evaluated. The DC resistance was 2.0Ω and the capacity was 6.5F. The leakage current at a voltage of 2.5 V was 14 μA. The capacity per 1 cc element is 4.85
F, the internal resistance per cc of the element was 2.68Ω.

Example 6 (Comparative Example) Solvent-spun rayon was C
It was beaten until the SF became 500 ml, and sisal was blended with the beaten to produce paper having a thickness of 70 μm and a density of 0.48 g / cm ³ (solvent-spun rayon / sisal 40/60 by weight). A capacitor element was assembled in the same manner as in Example 1 except that this was used as a separator. The breaking strength of the separator was 1.56 kg / cm width.

An electric double layer capacitor cell was prepared in the same manner as in Example 1 except that the above-mentioned element was used, and the performance was evaluated.
The total thickness of the electrode body and the separator was 0.47 mm. The DC resistance was 1.10Ω and the capacity was 9.3F.
The leakage current at a voltage of 2.5 V was 8 μA. The capacity per 1 cc of the device was 8.2 F, and the internal resistance per 1 cc of the device was 1.24 Ω.

[0037]

According to the present invention, an electric double layer capacitor having a low internal resistance, a low leakage current and a high capacitance density can be obtained. In addition, since the separator of the present invention has high strength, it has sufficient strength to be wound, and a wound electric double layer capacitor can be easily manufactured. The electric double layer capacitor according to the present invention has a discharge capacity of 50 to 2 in particular.
0000F, or a large discharge capacity of 1 to 1000 A,
It is suitable for wound type and multilayer electric double layer capacitors for large currents.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Katsuharu Ikeda 1150 Hazawacho, Kanagawa-ku, Yokohama-shi, Kanagawa-ken Asahi Glass Co., Ltd.

Claims (4)

[Claims] 1. An electric double layer capacitor in which a separator is arranged between a positive electrode and a negative electrode comprising a carbonaceous electrode and impregnated with a non-aqueous electrolyte, wherein the separator comprises fibers obtained by beating regenerated cellulose fibers. , Wherein the solute of the nonaqueous electrolyte is triethylmonomethylammonium tetrafluoroborate. 2. The electric double layer capacitor according to claim 1, wherein the fiber obtained by beating the regenerated cellulose fiber has a Canadian standard freeness defined by JIS-P8121 of 0 to 600 ml. 3. The electric double layer capacitor according to claim 1, wherein the separator has a thickness of 20 to 60 μm and a density of 0.30 to 0.60 g / cm ³ . 4. The carbonaceous electrode has a specific surface area of 700 to 2500 m.
² / g of a carbon material and an organic binder, wherein the solvent of the electrolytic solution is at least one selected from the group consisting of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, sulfolane and methyl sulfolane. Item 1,
4. The electric double layer capacitor according to 2 or 3.