JP2001210554A - Separator for electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for electric double-layer capacitor which will not cause deterioration in performance even if it is used in a long-term cycle, such as rise in internal resistance, dry out, shout-circuiting, etc. SOLUTION: This separator for electric double-layer capacitor, which is made by isolating a pair of polarization electrode which uses sulfuric acid for electrolytic liquid by a separator, the separator is mutually bonded with a vinylidene chloride binder to fibers of a nonwoven fabric sheet which consist of acrylic short fiber as the main component, and the separator has a thickness of 300 μm or less, a keeping liquid reserve rate by 40% sulfuric acid is 150 weight % or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric double layer capacitor having a pair of polarizable electrodes separated by a separator, and more particularly to a separator for an electric double layer capacitor 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 in 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. Since the larger the area where ions can be adsorbed is, the larger the capacitance can be obtained.
Activated carbon having a large surface area is widely used for the polarizable electrode. Such electric double layer capacitors are roughly classified into an aqueous solution type and an organic solvent type depending on the type of the 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 electrolytic solution (internally. Resistance suppression). The separator for an electric double layer capacitor targeted by the present invention,
In order to cope with high capacity, sulfuric acid of 30 to 60% by weight is used as an electrolytic solution, and durability to sulfuric acid in this concentration range and liquid retaining property of an electrolytic solution having sufficient ionic conductivity are required. is necessary. 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 electrolyte, a wet nonwoven sheet mainly composed of glass fiber 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.

[0004] In recent years, electric double layer capacitors have attracted attention as backup power sources for memories for electric vehicles and large equipment, and higher capacity has been studied.
For this reason, separators are becoming thinner than conventional ones. However, in order to reduce the thickness of the conventional separator, it is difficult to manufacture the glass fiber nonwoven fabric because there is no hydrogen bond, and the meltblown nonwoven fabric has a limited strength and insufficient strength.

[0005] To obtain a thin separator, a nonwoven fabric made by wet papermaking is most suitable. However, since this method requires a certain degree of wet paper strength, fibers that easily form hydrogen bonds, fibers having self-adhesive properties, and entanglement of fibers are used. Sheets are formed by a method using fibers or the like, which easily generate odor, a method of adding a resin binder, or the like. The present inventors have proposed acrylic fibers as fibers suitable for a separator for a thinned electric double layer capacitor. (Japanese Patent Application No. 11-206278)
Acrylic fiber is excellent in sulfuric acid resistance and hydrophilicity and is suitable for a capacitor separator. However, acrylic fiber could be made into a single sheet by hydrogen bonding, but this alone has the drawback that the sheet is decomposed in high-temperature, high-concentration sulfuric acid and breaks apart due to insufficient bonding strength. Was. Therefore, it is necessary to increase the sulfuric acid resistance of the binder in order to adapt it to electric vehicles, which require high capacity and heat resistance, and memory backup of large equipment.

[0006]

The present invention has a thickness of 300 mm.
μm or less, and even when used over 10,000 cycles,
An object of the present invention is to provide a separator for an electric double layer capacitor using a sulfuric acid (about 40 to 60%) electrolytic solution which does not cause performance deterioration such as increase in internal resistance, dryout, and short circuit.

[0007]

Means for Solving the Problems A first invention of the present invention is:
A separator for use in an electric double layer capacitor in which a pair of polarizable electrodes using sulfuric acid as an electrolyte is separated by a separator, wherein the separator is made of a non-woven fabric sheet mainly composed of acrylic short fibers. It is bound with a vinylidene resin binder and has a thickness of 300μ.
The present invention relates to a separator for an electric double layer capacitor having a liquid retention of 40% sulfuric acid of 150% by weight or more when the weight is 40 m or less. The second invention of the present invention relates to a separator for an electric double layer capacitor, wherein the acrylic short fibers in the first invention are polyacrylonitrile fibers made of homoacrylonitrile. According to a third aspect of the present invention, in the first or second aspect, the vinylidene chloride-based resin binder has a vinylidene chloride content of 80% in monomer units.
The present invention relates to a separator for an electric double layer capacitor comprising a vinylidene chloride-based resin containing about 95% by weight. The fourth invention of the present invention relates to the separator for an electric double layer capacitor having a porosity of 40 to 90% in any one of the first to third inventions.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Each embodiment of the separator for an electric double layer capacitor of the present invention will be described. The separator of the present invention is used for an electric double layer capacitor in which sulfuric acid is used as an electrolytic solution and a pair of polarizable electrodes are separated by a separator. In the present specification, the term "nonwoven fabric sheet" refers to a sheet obtained by a wet papermaking method or the like before adding a vinylidene chloride-based resin. In the present specification, the “substrate for a separator” refers to a nonwoven fabric sheet to which a vinylidene chloride-based resin is added and dried. As used herein, the term “separator” refers to a material which can be used as a separator by subjecting a separator substrate to a pressure treatment with a calender.

[0009] The separator for an electric double layer capacitor of the present invention is obtained by binding fibers of a nonwoven fabric sheet mainly composed of acrylic fibers with a vinylidene chloride resin binder. Generally, as a resin binder,
Examples thereof include acrylonitrile-butadiene (NBR), styrene-butadiene (SBR), natural rubber, polyvinyl acetate, and polyacrylate, but the present inventors have high crystallinity and sulfuric acid-resistant chloride. Attention was paid to vinylidene resin.

[0010] Vinylidene chloride-based resin has strong adhesiveness to organic fibers, has high crystallinity, is excellent in chemical resistance, and is hardly attacked by strong acids, so it has very strong resistance to sulfuric acid as an electrolytic solution. There is. Therefore, the separator of the present invention using a vinylidene chloride-based resin is required to have a temperature of 80 ° C., 7 ° C., which is necessary for a separator for an electric double layer capacitor which is required to withstand high-temperature and high-concentration sulfuric acid, particularly for electric vehicles.
Durability of more than 1000 hours in 0% sulfuric acid is possible.

For use as a separator, it is necessary to have excellent sulfuric acid resistance and at the same time to be able to appropriately hold sulfuric acid as an electrolytic solution. Since vinylidene chloride resin is a highly polar substance, it is excellent in hydrophilicity. Although it does not allow water to pass through when it is formed into a film, the contact angle with water is reduced by appropriately covering the surface of the nonwoven fabric, and the moisture absorption and liquid retention performance of the entire sheet are improved. The liquid retention by 40% sulfuric acid impregnation needs to be 150% by weight or more based on the weight of the separator. If the content is less than 150% by weight, when incorporated in a capacitor, the internal resistance increases due to a shortage of the electrolyte,
As a result, the capacity is reduced. Due to these properties,
When a vinylidene chloride-based resin is used as a binder in a nonwoven fabric sheet, the effects of increasing strength, improving hydrophilicity, and increasing sulfuric acid resistance due to a binder effect are exhibited. For this reason, the separator can be made thinner, the liquid retention power as the separator, and the separator life can be prolonged due to increased sulfuric acid resistance.

The amount of the vinylidene chloride resin used is 1 to 75% based on 100% by weight of the nonwoven fabric sheet before the binder is added.
% By weight, and particularly preferably 10 to 40%. If the amount is less than 1% by weight, the effect of the binder effect, the increase in hydrophilicity, and the increase in sulfuric acid resistance due to the vinylidene chloride resin will not be exhibited. If it exceeds 75% by weight, the nonwoven fabric becomes a film, the porosity and the liquid retention rate decrease, and the internal resistance increases.

The vinylidene chloride-based resin is not particularly limited, but is a copolymer of a vinylidene chloride monomer and a monomer of another resin, and examples thereof include a vinylidene chloride-acrylate copolymer, a vinylidene chloride-acrylonitrile copolymer, and a chloride. Vinylidene-vinyl chloride copolymer and the like can be used. Incidentally, vinylidene chloride monomer cannot be used because it has very poor thermal stability. Among these resins, those containing from 80 to 95% by weight of vinylidene chloride in monomer units are desirable. When the ratio of vinylidene chloride exceeds 95%, crystallization becomes very fast, so that the film lacks flexibility and deteriorates in stability. If it is less than 80%, the crystallinity is low, so that the chemical resistance is poor.

In the present invention, the method of adding the vinylidene chloride-based resin to the nonwoven fabric sheet is not particularly limited, and examples thereof include a method of spraying an emulsion, a method of impregnation, and a method of coating with an coater.

As the fiber material constituting the nonwoven fabric sheet, polyolefin fiber, glass fiber and the like can be considered. The separator for the electric double layer capacitor of the present invention is preferably acrylic fiber having hydrophilicity and sulfuric acid resistance. Among them, fibers made of a homoacrylonitrile polymer in which a copolymer is not copolymerized are particularly preferred. Polyacrylonitrile is a substance with strong sulfuric acid resistance, but when it contains a copolymer such as acrylate or vinyl acetate, it is likely to be hydrolyzed in sulfuric acid, which may cause a decrease in sulfuric acid resistance. is there. In general, many acrylic fibers are copolymerized with a copolymer for the purpose of improving dyeability. Acrylic fibers can form a sheet by themselves because they generate hydrogen bonds, but they dissolve in high-concentration sulfuric acid because of their weak bonding power. However, when bound with a vinylidene chloride-based resin, it functions as a binder having a high sulfuric acid resistance, and thus cannot be dissolved even in sulfuric acid.

The porosity of the separator is preferably 40 to 90%. If it is less than 40%, the internal resistance of the capacitor increases. If it is larger than 90%, problems such as occurrence of a short circuit due to the through-hole and failure to maintain the strength of the separator sheet itself will occur. The porosity is measured by mercury porosimetry using a mercury volume (V)
Can be measured directly as a function of the pressure, so P-
It is obtained by creating a V curve and calculating the porosity.

In the present invention, in order to keep the porosity of the sheet at an optimum, fibrillated fibers such as acrylic fibers and polyolefin fibers can be used in addition to the acrylic short fibers as the main component. Above all, when fibrillated fibers of polyethylene are used in combination, drying at an appropriate temperature also works as a binder, which is preferable. Although polyolefin fibers have no hydrophilicity, moderate hydrophilicity can be maintained by using a vinylidene chloride-based resin binder. The amount of the fibrillated fiber used is 0 to 70% with respect to the acrylic short fiber as the main component. 70%
If it exceeds, the pore size becomes too small, which causes a decrease in the liquid retention amount.

A fibrillated polyolefin fiber,
Acrylic fibers can be obtained, for example, by the following method.
First, the raw 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. It is sufficient that a part is fibrillated by observation with an electron microscope, but the CSF freeness (Canadian freeness value) is 600 to 50 cc.
Particularly preferred are those which are fibrillated. The fibrillated polyolefin fiber can also be obtained by the SWP method and further by the method of pulverizing with a homogenizer described in JP-A-6-240595. It is sufficient that a part is fibrillated by observation with an electron microscope, but the CSF freeness (Canadian freeness value) is 600 to 50 cc.
Particularly preferred are those which are fibrillated.

The separator of the present invention can be adjusted to various thicknesses. However, in order to use the separator for a high-capacity capacitor, the separator must be thinner than 300 μm. In order to increase the capacity, it is necessary to stack several layers of cells in a limited space. However, the separator according to the present invention uses a combination of acrylic fiber and vinylidene chloride-based resin.
Even with a thickness of 0 μm or less, it is possible to improve the strength of wet paper necessary for wet papermaking and to develop sufficient mechanical strength for preventing short circuit.

In the present invention, a wet papermaking method is preferred for producing a nonwoven fabric sheet. The wet papermaking 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 fibers can be selected from single fibers, pulp, etc., and the usable fiber diameter can be used from ultra-fine fibers of 7 μm or less to thick fibers. Is obtained.

The wet papermaking method for obtaining a nonwoven fabric sheet means that each fiber is independently or mixed and dispersed in water.
The slurry, whose concentration is preferably adjusted to 0.5% or less, is applied to a wet paper machine such as a long net type or a circular net type, dewatered in a continuous wire mesh dewatering part, and then dried by a drier to form a sheet. Is a way to get

[0022]

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.

Example 1 Acrylic fiber chops (trade name: SKS T941 manufactured by Toyobo Co., Ltd.) having a fineness of 1.2 d and a cut length of 6 mm were dispersed at a concentration of 0.1% to obtain an acrylic fiber dispersion. . A very small amount of polyethylene oxide (trade name: PEO-PF3, manufactured by Sumitomo Seika Co., Ltd.) was added as a viscosity agent to the obtained acrylic fiber dispersion, and a wet nonwoven fabric sheet having a basis weight of 20 g / m ² was hand-made. On the other hand, a vinylidene chloride-acrylate copolymer emulsion (trade name: Diophan 192)
D, manufactured by BASF Dispersion Co., Ltd.) to prepare a binder solution adjusted to have a solid content concentration of 3%. After applying to 25% by weight, it was dried with hot air at 130 ° C. for 15 minutes to give a basis weight of 25 g.
/ M ² was obtained. The obtained base material is subjected to a pressure treatment with a calender to a thickness of 60 μm (porosity of about 60 μm).
%) To obtain the separator of the present invention.

Example 2 Acrylic fiber chops (trade name: SKS T941, manufactured by Toyobo Co., Ltd.) having a fineness of 1.2 d and a cut length of 6 mm were dispersed so as to have a concentration of 0.1% to obtain a dispersion liquid A. . Next, a weight average fiber length of 0.45 mm and a fiber diameter distribution of 0.5 μm
Dispersion B was prepared by dispersing 5 g of polyolefin pulp (trade name: Tiara KY-420M; manufactured by Daicel Corporation) having a thickness of 7.0 μm and a Canadian freeness of 400 ml to a concentration of 0.1%.
I got Dispersion A: Dispersion B = 8: 2 was mixed to obtain Dispersion C. A very small amount of polyethylene oxide (trade name: PEO-PF3, manufactured by Sumitomo Seika Co., Ltd.) was added to Dispersion C as a tackifier, and a wet nonwoven fabric sheet having a basis weight of 20 g / m ² was hand-made. On the other hand, a vinylidene chloride-acrylate copolymer emulsion (trade name: Diophan 192)
D, manufactured by BASF Dispersion Co., Ltd.) to prepare a binder solution adjusted to have a solid content concentration of 3%. After applying to 25% by weight, it was dried with hot air at 130 ° C. for 15 minutes to give a basis weight of 25 g.
/ M ² was obtained. The obtained base material is subjected to a pressure treatment with a calender to a thickness of 60 μm (porosity of about 60 μm).
%) To obtain the separator of the present invention.

Example 3 An acrylic fiber chop (trade name: SKS T941 manufactured by Toyobo Co., Ltd.) having a fineness of 1.2 d and a cut length of 6 mm was used at a concentration of 0.1%.
To obtain a dispersion D. Next, a fiber length of 3.0 to 6.5 mm and a Canadian freeness of 70 ml
Acrylic fiber pulp (trade name: R-56D, manufactured by Toyobo Co., Ltd.) was dispersed at a concentration of 0.1% to obtain Dispersion E. Dispersion D: Dispersion E = 8: 2 was mixed to obtain dispersion F. A very small amount of polyethylene oxide (trade name: PEO-PF, manufactured by Sumitomo Seika Co., Ltd.) was added to Dispersion F as a tackifier, and a wet nonwoven fabric sheet having a basis weight of 20 g / m ² was hand-made. On the other hand, a vinylidene chloride-acrylate copolymer emulsion (trade name: Diophan 192)
D, manufactured by BASF Dispersion Co., Ltd.) to prepare a binder solution adjusted to have a solid content concentration of 3%. 37.5% by weight, and then dried with hot air at 130 ° C. for 15 minutes.
7.5 g / m ² of the substrate for a separator was obtained. The obtained substrate was subjected to a pressure treatment with a calender to adjust the thickness to 60 μm (porosity: about 55%) to obtain the separator of the present invention.

Comparative Example 1 A polypropylene / polyethylene core-sheath fiber (trade name: NBF Star 220; manufactured by Daiwabo Co., Ltd.) having a fineness of 0.9 d and a cut length of 5 mm was dispersed at a concentration of 0.1% to obtain a dispersion G. Next, the weight average fiber length is 0.45 mm, and the fiber diameter distribution is 0.4 mm.
5 μm to 7.0 μm, Canadian freeness 400 m
l of polyolefin pulp (trade name: Tiara KY-43)
0M; manufactured by Daicel Corporation) at a concentration of 0.1% to obtain a dispersion H. Dispersion G: Dispersion H = 3: 2 (weight ratio) were mixed at a ratio, and a very small amount of polyethylene oxide (trade name: PEO-PF, manufactured by Sumitomo Seika Co., Ltd.) was added as a tackifier. A wet nonwoven sheet having a basis weight of 20 g / m ² was hand-made. After drying the obtained wet nonwoven fabric sheet with a drum dryer, a porosity of about 66% was obtained using a super calender.
The thickness was adjusted to 60 μm to obtain a separator for comparison.

Comparative Example 2 An acrylic fiber chop (trade name: SKS T941 manufactured by Toyobo Co., Ltd.) having a fineness of 1.2 d and a cut length of 6 mm was 0.1% in concentration.
To obtain a dispersion liquid I. Next, the fiber length3.
Acrylic fiber pulp of 0-6.5 mm, Canadian freeness 70 ml (trade name: R-56D, Toyobo Co., Ltd.)
Was dispersed at a concentration of 0.1% to obtain a dispersion J. Dispersion I: Dispersion J = 7: 3 was mixed to obtain Dispersion K. A very small amount of polyethylene oxide (trade name: PEO-PF, manufactured by Sumitomo Seika Co., Ltd.) was added to Dispersion K as a tackifier, and a wet nonwoven sheet having a basis weight of 20 g / m ² was hand-made. After drying the obtained wet nonwoven fabric sheet with a drum dryer, the porosity was adjusted to about 72% and the thickness was adjusted to 60 μm using a super calender to obtain a separator for comparison.

Comparative Example 3 An acrylic fiber chop (trade name: SKS T941 manufactured by Toyobo Co., Ltd.) having a fineness of 1.2 d and a cut length of 6 mm was 0.1% in concentration.
And a dispersion liquid L was obtained. Next, a fiber length of 3.0 to 6.5 mm and a Canadian freeness of 70 ml
Acrylic fiber pulp (trade name: R-56D, manufactured by Toyobo Co., Ltd.) was dispersed at a concentration of 0.1% to obtain a dispersion M. Dispersion liquid L: dispersion liquid M = 3: 1 was mixed to obtain dispersion liquid N. A very small amount of polyethylene oxide (trade name: PEO-PF3, manufactured by Sumitomo Seika Co., Ltd.) was added to Dispersion N as a tackifier, and a wet nonwoven fabric sheet having a basis weight of 20 g / m ² was hand-made. On the other hand, an acrylic resin emulsion (trade name: Boncoat AN-180H, manufactured by Dainippon Ink and Chemicals, Inc.) was prepared to have a binder solution adjusted to a solid concentration of 3%, and impregnated into a previously hand-woven nonwoven sheet. 25% by weight of the non-woven fabric with respect to the non-woven fabric, and then dried with hot air at 130 ° C. for 15 minutes to obtain a basis weight of 25 g / m 2.
^A substrate for separator ² was obtained. The obtained base material is subjected to a pressure treatment by a calender and a thickness of 60 μm (porosity of about 60%)
And a separator for comparison was obtained.

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

Test Method (basis weight) A sample of 10 cm × 10 cm is collected and weighed to four decimal places. Basis weight (g / m ² ) = sample weight × 100 (thickness) Measured at 10 places with a micrometer, and the average value is defined as the thickness. Using the above separator, a unit cell of a simple electric double layer capacitor was manufactured, and the equivalent series resistance (ESR) was measured. The following is a method for producing a cell,
The method for measuring ESR 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. (Method of Measuring ESR (Equivalent Series Resistance)) An AC voltage of 10 mVrms was applied to a fixed cell, and the ESR was measured from the real part of the impedance at 1 kHz. (Measurement of Liquid Retention) A 10 cm × 10 cm sample 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

(Measurement of Sulfuric Acid Resistance) A sample for a separator of 5 cm × 5 cm is taken, immersed in 40% and 70% sulfuric acid, and left in a constant temperature layer set at 85 ° C. Evaluation is performed based on the separator shape after standing for 1000 hours. Evaluation criteria ○: No change in sheet appearance △: Sheet shape remains, but coloring and partial disappearance of fibers are observed ×: Sheet shape does not remain

[0032]

[Table 1]

[0033]

The separator for an electric double layer capacitor of the present invention having the above-mentioned structure is a good separator which does not cause performance deterioration such as an increase in internal resistance, dryout, and short circuit.

Claims (4)

[Claims] 1. A separator for use in an electric double layer capacitor in which a pair of polarizable electrodes using sulfuric acid as an electrolyte is separated by a separator, wherein the separator is a non-woven fabric sheet mainly composed of acrylic short fibers. Fibers are bound by vinylidene chloride resin binder,
Thickness is less than 300μm and liquid retention rate by 40% sulfuric acid is 1
50% by weight or more of an electric double layer capacitor separator. 2. The separator for an electric double layer capacitor according to claim 1, wherein the acrylic short fiber is a polyacrylonitrile fiber composed of homoacrylonitrile. 3. The separator for an electric double layer capacitor according to claim 1, wherein the vinylidene chloride-based resin binder comprises a vinylidene chloride-based resin containing 80 to 95% by weight of vinylidene chloride in a monomer unit. 4. A porosity of 40 to 90%.
The separator for an electric double layer capacitor described in any one of the above.