JP2001035754A - Separator for electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for an electric double layer capacitor, where even in use for long-period cycle, no performance deterioration such as rising of internal resistance, dry out, or short results. SOLUTION: Related to an electric double-layer capacitor, a pair of polarizable electrodes where sulfuric acid is used as an electrolyte is separated with a separator. Here, the separator comprises a wet nonwoven fabric whose main components are acid-proof single fabric and acid-proof fibrillation fabric, while the thickness of separator is set to 300 μm or smaller, porosity is 40-90%, and the liquid-reservation coefficient at 40% impregnated with sulfuric acid is 200 wt.% or higher relative to the weight of separator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric double layer capacitor in which a pair of polarizable electrodes are separated by a separator, and more particularly to an electric double layer capacitor separator using sulfuric acid as an electrolyte.

[0002]

2. Description of the Related Art An electric double layer capacitor is a capacitor having a large capacitance of the farad order utilizing an electric double layer generated at an interface between a polarizable electrode and an electrolyte. Charge / discharge is a phenomenon of physical adsorption / desorption of electrolyte ions to / from the electrode surface, and thus has a feature that energy density is small but deterioration due to charge / discharge is extremely small as compared with a secondary battery utilizing a chemical reaction. Therefore, the larger the area on which ions can be adsorbed, the larger the capacitance can be obtained. Therefore, activated carbon having a large surface area is widely used for polarizable electrodes. Such electric double layer capacitors are roughly classified into aqueous solution type and organic solvent type depending on the type of electrolytic solution.

[0003] The important role of the separator in the electric double layer capacitor is firstly prevention of short circuit due to physical contact between electrodes (prevention of internal short circuit), and secondly, not to hinder movement of ions in the electrolyte (internally. Resistance suppression). The separator for an electric double layer capacitor according to the present invention uses 30 to 60% by weight of sulfuric acid as an electrolytic solution in order to cope with high capacity, and has sufficient ion conductivity in this concentration range. Electrolyte retention is required. Further, there is a demand for sheet strength that suppresses penetration of activated carbon serving as a pair of polarizable electrodes and prevents short circuit.

Conventionally, as a separator for an electric double layer capacitor using sulfuric acid as an electrolytic solution, a wet nonwoven sheet mainly composed of glass fibers and a dry polypropylene nonwoven nonwoven fabric by a melt blow method have been used. These separators generally have a thickness of 600 to 800 μm. Also, since there is no hydrogen bond, the strength is weak. Therefore, simply reducing the thickness of these sheets is the object of the present invention.
At present, it is impossible to obtain a separator having a thickness of not more than 00 μm and a satisfactory porosity, liquid retention rate, and strength.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a separator for an electric double layer capacitor using a sulfuric acid (about 40%) electrolyte, which has a thickness of 300 μm or less and has a thickness of 10,000 times or more. An object of the present invention is to provide a separator that does not cause performance deterioration such as an increase in internal resistance, dryout, and short circuit even when used for a long cycle.

[0006]

That is, a first invention of the present invention is an electric double layer capacitor in which a pair of polarizable electrodes using sulfuric acid as an electrolytic solution are separated by a separator. It is made of a wet nonwoven fabric containing a single fiber and an acid-resistant fibrillated fiber as main components, and has a separator thickness of 300 μm or less and a porosity of 40 to 90.
% And a liquid retention rate by impregnation with 40% sulfuric acid is 200% by weight or more with respect to the weight of the separator.
The second invention of the present invention relates to a separator for an electric double layer capacitor, wherein the wet nonwoven fabric according to the first invention is a wet nonwoven fabric composed of acrylic single fibers and partially fibrillated acrylic fibers. Third of the present invention
The present invention relates to a separator for an electric double layer capacitor, wherein the wet nonwoven fabric according to the first invention is a wet nonwoven fabric composed of polyolefin single fibers and partially fibrillated polyolefin fibers.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each embodiment of the electric double layer capacitor will be described. The fibers selected for the separator vary depending on whether the electrolyte is acidic or alkaline. In the present invention, the electrolytic solution has been studied by focusing on H ₂ SO ₄ , and the separator is made of a wet nonwoven fabric containing acid-resistant single fibers and acid-resistant fibrillated fibers as main components. Polyolefins as acid-resistant monofilaments and fibrillated fibers
Fibers such as acrylic, PPS fiber, and aramid fiber are preferred. When a wet nonwoven fabric is made of these fibers, it can be made of different types of single fibers and different types of fibrillated fibers. For example, a combination of acrylic short fibers and fibrillated polyolefin fibers, or polyolefin short fibers and fibrillated acrylic fibers may be used. However, for the purpose of minimizing the variation in hydrophilicity on the sheet, the combination of the same type of fiber is preferable for the single fiber and the fibrillated fiber. A part of the single fiber or a part of the fibrillated fiber can be replaced with a different kind of fiber.

In the present invention, in order to obtain the wet strength of the wet nonwoven fabric, a single fiber and a fibrillated fiber are combined. The blending amount of the fibrillated fiber is preferably about 5 to 40 parts with good balance to the dry weight of the wet nonwoven fabric. If the amount is less than 5 parts, the wet strength is insufficient, while if it exceeds 40 parts, the dry strength decreases, which is not preferable. Further, a resin binder may be added to obtain wet strength of the wet nonwoven fabric. However, when acrylic fiber is used, a fusing phenomenon appears when heat of 90 ° C. or more is applied in a wet state.
Since a similar effect appears at 0 ° C. or higher, a sheet having a thickness of 300 μm or less can be sufficiently prepared without binder.

As the acid resistant fiber of the present invention, polyolefin and acrylic fiber are particularly preferred. The acrylic single fiber is not particularly limited, but is preferably a fiber cut to 0.5 to 30 mm and having a fiber diameter of 1 to 30 μm. The fibrillated acrylic fiber can be obtained, for example, by the following method. First, the raw acrylic fiber is suspended in water. The pulp concentration of the suspension is preferably from 0.5% by weight to 5% by weight. It can be fibrillated with a device such as a beater, disc refiner, sand grinder, homogenizer and the like. The fibrillated acrylic fiber may be partially fibrillated by observation with an electron microscope.
Those which are fibrillated to have a F freeness (Canadian freeness value) of 600 to 50 cc are particularly preferred. In addition, since the acrylic fiber has an appropriate wettability, it is not necessary to perform a hydrophilic treatment required for a polyolefin-based nonwoven fabric.

The polyolefin monofilament is not particularly limited, but may be polyethylene, polypropylene, a copolymer of polyethylene and polypropylene, or a resin of these, cut to 0.5 to 30 mm and having a fiber diameter of 1 to 30 μm. Preferred are side-by-side type and core-sheath type composite fibers. The fibrillated polyolefin fiber can be obtained by the same method as the fibrillated acrylic fiber, but can also be obtained by the SWP method and further by the method of miniaturization using a homogenizer described in JP-A-6-240595. The fibrillated polyolefin fiber may be partially fibrillated by observation with an electron microscope, but it is particularly preferable that the fibrillated polyolefin fiber has a CSF freeness of 600 to 50 cc.

The polyolefin-based nonwoven fabric has insufficient electrolyte retention properties as it is, and needs to be subjected to a hydrophilic treatment. The method for hydrophilization is not particularly limited, and includes a method of introducing a sulfone group, a method of graft-polymerizing a vinyl monomer by UV irradiation, a method of corona, plasma, and a method of impregnating or coating a hydrophilic resin. And other known methods.

The separator for an electric double layer capacitor of the present invention has a thickness of 300 μm or less. When the thickness exceeds 300 μm, the internal resistance increases and the object of the present invention cannot be achieved. Furthermore, the separator of the present invention has a porosity
It needs to be 40-90%. If it is less than 40%, the internal resistance of the capacitor increases. If it exceeds 90%, the strength of the separator sheet itself cannot be maintained. The porosity can be obtained by creating a PV curve and calculating the porosity since the volume (V) of mercury penetrating into the pores can be directly measured by a mercury porosimetry measuring device as a function of the applied pressure. Further, the separator of the present invention requires a liquid retention rate of 200% by weight or more based on the weight of the separator when impregnated with 40% sulfuric acid. If the content is less than 200% by weight, it is difficult to obtain the expected high capacity by using a thin separator, and the internal resistance of the capacitor tends to increase, which is not preferable.

In the present invention, a wet papermaking method is preferred for producing a separator substrate. The wet nonwoven fabric manufacturing method has an advantage that fibers having different fiber diameters and a plurality of types of fibers can be mixed at an arbitrary ratio in the same apparatus. In addition, the form of the fiber can be selected from single fibers, staples, pulp, etc., and the usable fiber diameter can be used from ultra-fine fibers of 7 μm or less to thick fibers. This is how the web is obtained. As a wet papermaking method for obtaining a wet nonwoven fabric, a specified amount of fibrillated fiber and a specified amount of single fiber are independently or mixed and dispersed in water, preferably
A method in which the slurry whose concentration has been adjusted to 0.5% or less is applied to a wet paper machine such as a fourdrinier type or a circular net type, dewatered in a continuous wire mesh dewatering part, and then dried by a dryer to obtain a sheet. General.

In the present invention, the wet-type nonwoven fabric is (heat)
The separator can be obtained by partially compressing and thermally fusing the constituent short fibers by a (heating) compression device such as a calender. This operation can be used as a means for adjusting the thickness and porosity of the separator. At that time, a thermosetting binder or the like may be further sprayed on the wet nonwoven fabric and then cured to increase the sheet strength of the wet or dry wet nonwoven fabric. However, when using acrylic fiber or polyolefin fiber,
In some cases, sufficient strength can be obtained by heat fusion of the fibers themselves without using a binder.

As the polarizable electrode, activated carbon having a very large specific surface area and a small electric resistance is often used. The current collector has a role of extracting the electric charge of the polarizable electrode to the outside and a role of sealing the electrolytic solution together with the gasket. In most cases, activated carbon uses its high specific surface area (1000 m ² / g or more). In order to do so, it is necessary that the target substance is adsorbed in the micropores and no adsorption occurs. Since only an outer surface area of at most several m ² / g can be used, high performance cannot be exhibited. Activated carbon generally has a specific surface area, pore distribution,
It is evaluated by the pore volume and the isotherm of the target substance. Of these, the pore distribution is the most important. As activated carbon, activated carbon fibers of rayon type, acrylic type, pitch type and phenol resin type, and solidified powdered activated carbon such as ogre carbon and pitch carbon are used.

As the aqueous electrolyte for the electric double layer capacitor, a concentrated aqueous solution of a strong acid or strong alkali, particularly 20 to
An aqueous solution of H ₂ SO ₄ and KOH of about 50 wt% is preferably used. The reason for this is that the electric conductivity of a strong acid or strong alkali aqueous solution is higher than the electric conductivity of a neutral salt aqueous solution.
H ₂ SO ₄ shows the maximum electrical conductivity in a concentration of about 40 wt%. The separator of the present invention is used in a sulfuric acid aqueous solution.

[0017]

EXAMPLES The present invention will be described more specifically with reference to examples below, but the present invention is of course not limited to these. In the following examples,% is% by weight unless otherwise specified.

Example 1 Acrylic fiber chop (trade name: R56F) having a fineness of 3 d and a cut length of 4 mm was placed in a stainless steel vat with 1000 g of water.
Toyobo Co., Ltd.) was dispersed in water to a concentration of 1%, and the CSF freeness was 250 using a single disk refiner (Super Hazegawa 600-A manufactured by Hasegawa Iron Works).
ml of fibrillated acrylic fiber dispersion was obtained.
The concentration of the resulting dispersion was 1%. (Dispersion A) Next, 1000 g of water was placed in a stainless steel vat, and the fineness was set to 1.2.
d, Acrylic fiber chop with a cut length of 6 mm (Product name:
SKS T941 (manufactured by Toyobo Co., Ltd.) to give a concentration of 1% to obtain an acrylic fiber dispersion. (Dispersion B)

The dispersion A: dispersion B = 1: 3 was collected at a weight ratio, further diluted 10 times with water, and polyethylene oxide (trade name: PEO-PF3, manufactured by Sumitomo Seika Co., Ltd.) Was added by hand and a wet nonwoven sheet was hand-made. The obtained wet nonwoven fabric sheet was dried by a drum dryer to obtain a separator base material having a basis weight of 15 g / m ² and a thickness of 100 μm. The obtained base material was further subjected to a pressure treatment using a calender, and the porosity was adjusted to 70% (thickness: 50 μm) to obtain a separator for an electric double layer capacitor.

Example 2 Weight average fiber length 0.45 mm, fiber diameter distribution 0.5 μm or more
Dispersion A was prepared by dispersing 5 g of a polyolefin pulp (trade name: Tiara KY-430M; manufactured by Daicel Corporation) having a CSF freeness of 400 ml at a concentration of 1% with a pulp paper at 7.0%.
I got Next, 1500 g of water was weighed into a stainless steel vat, and 5 g of a polypropylene chop (trade name: PZ, manufactured by Daiwabo Co., Ltd.) with a fineness of 0.7 d and a cut length of 5 mm, and a polypropylene / polyethylene core sheath with a fineness of 0.9 d and a cut length of 5 mm 10 g of fiber (trade name: NBF Star 220; manufactured by Daiwabo Co., Ltd.) was weighed and dispersed at a dispersion concentration of 1% to obtain a polyolefin fiber dispersion (dispersion B).

The dispersion A and the dispersion B are mixed at a ratio of 5: 5 (weight ratio), and further diluted 10 times with water. In addition, polyethylene oxide (trade name: PEO-P)
F, manufactured by Sumitomo Seika Co., Ltd.), and the wet nonwoven sheet was hand-made. The obtained wet nonwoven fabric sheet was dried with a drum dryer, and had a basis weight of 15 g / m ² and a thickness of 100 μm.
m was obtained. The obtained substrate is subjected to a pressure treatment using a super calender, and a porosity of 70% (thickness of 50 μ
m). Further, the interval between the parallel plate electrodes was adjusted to 5 mm using a batch type atmospheric pressure plasma surface treatment apparatus (high-frequency high-voltage power supply: PHF-2K type; manufactured by Heiden Lab.) Using N ₂ gas. The sample was scissored and irradiated with plasma to obtain an electric double layer capacitor separator.

Comparative Example 1 Weight average fiber length 0.45 mm, fiber diameter distribution 0.5 μm or more
5 g of a polyolefin pulp (trade name: Tiara KY-430M; manufactured by Daicel Corporation) having a freeness of 7.0 μm and a CSF of 400 ml was dispersed with a pulp at a concentration of 1% to obtain a dispersion C.
I got Next, 1500 g of water was weighed into a stainless steel vat, and 5 g of a polypropylene chop (trade name: PZ, manufactured by Daiwabo Co., Ltd.) with a fineness of 0.7 d and a cut length of 5 mm, and a polypropylene / polyethylene core sheath with a fineness of 0.9 d and a cut length of 5 mm 10 g of fiber (trade name: NBF Star 220; manufactured by Daiwabo Co., Ltd.) was weighed and dispersed at a dispersion concentration of 1% to obtain a polyolefin fiber dispersion (dispersion D).

The dispersion C and the dispersion D are mixed at a ratio of 1: 3 (weight ratio), and further diluted 10 times with water. In addition, polyethylene oxide (trade name: PEO-P)
F, manufactured by Sumitomo Seika Co., Ltd.) in an extremely small amount, and the basis weight is 15 g.
/ M ² , 100 μm wet nonwoven sheet was hand-made.
After drying the obtained wet nonwoven sheet with a drum dryer,
73% porosity and 50μm thickness by super calender
To obtain a separator for an electric double layer capacitor.

The three types of electric double layer capacitor separators thus obtained were evaluated by the following test methods. Table 1 shows the results.

Using the above separator, a unit cell of a simple electric double layer capacitor was manufactured, and its equivalent series resistance (ESR) was measured. The following describes the cell fabrication method, ES
The method for measuring R will be described. (Method of Manufacturing Unit Cell) A sample punched out to 70 × 50 mm was sandwiched between two 68 × 48 mm electrodes, and immersed in a 40% sulfuric acid electrolyte. In addition, sandwich the electrode with gold foil from both ends.
A pressure of 0 kg was applied. (Measurement method of ESR) An AC voltage of 10 mVrms was applied to the fixed cell, and the ESR was measured from the real part of the impedance at 1 kHz. (Calculation of the resistance value of the separator part) The above ESR value (Ω ·
cm) × thickness (cm) ÷ area (cm ² ) ÷ porosity (%) (Measurement of liquid retention) A sample of 10 cm × 10 cm for a separator is collected and weighed to four decimal places. (W1) Next, the sample is immersed in 40% sulfuric acid. After 2 minutes, the sample is pulled up, the liquid is cut off for 2 minutes, and the weight is measured. (W2) Retention rate (%) = (W2−W1) / W1 × 100

[0026]

[Table 1]

[0027]

The separator for an electric double layer capacitor according to the present invention thus constituted is a good separator having a small internal resistance.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4L031 AA14 AA17 AB01 AB34 CB05 DA00 4L055 AF15 AF16 AF17 AF29 AF44 AF46 AF47 BE02 BE20 EA08 EA18 EA19 EA25 FA19 FA30 GA37 GA39 GA50

Claims (3)

[Claims] 1. An electric double layer capacitor in which a pair of polarizable electrodes using sulfuric acid as an electrolytic solution are separated by a separator, wherein the separator is mainly composed of an acid-resistant single fiber and an acid-resistant fibrillated fiber. Made of wet non-woven fabric, separator thickness is 300 μm or less, porosity is 40
A separator for an electric double layer capacitor, characterized in that a liquid retention rate by impregnation with sulfuric acid of up to 90% and 40% is 200% by weight or more based on the weight of the separator. 2. The separator for an electric double layer capacitor according to claim 1, wherein the wet nonwoven fabric is a wet nonwoven fabric comprising an acrylic single fiber and an acrylic fiber partially fibrillated. 3. The separator for an electric double layer capacitor according to claim 1, wherein the wet nonwoven fabric is a wet nonwoven fabric composed of a polyolefin single fiber and a partially fibrillated polyolefin fiber.