JP2002075797A - Electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric double-layer capacitor which is safe and large in electrostatic capacity. SOLUTION: In the electric double-layer capacitor, having, as its main constituent materials, positive and negative polarizable electrodes positioned opposite to each other via a separator, quaternary ammonium fluoride HF salt is contained in an electrolyte.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electric double layer capacitor. More specifically, the present invention relates to an electric double layer capacitor exhibiting high capacity.

[0002]

2. Description of the Related Art An electric double layer capacitor is an electrode element comprising a positive electrode and a negative electrode which are disposed opposite to each other with a separator interposed therebetween, wherein both the positive electrode and the negative electrode use activated carbon fibers, a molded article of activated carbon particles, a coating film of activated carbon particles, or the like. This is a polarizable electrode constituted by the above, and an electrolyte is included in the electrode element. In the electric double layer capacitor having the above configuration, electric charges are stored in the electric double layer generated at the interface between the polarizable electrode and the electrolytic solution.

[0003] The above-mentioned electrolytes are roughly classified into liquid electrolytes (electrolyte) and solid electrolytes according to their properties. Further, the above-mentioned electrolytes are classified into aqueous electrolytes and non-aqueous electrolytes according to the types of solvents and salts used. Divided into As the aqueous electrolyte, an aqueous solution of sulfuric acid, an aqueous solution of potassium hydroxide or the like is used (Uesei, Electrochemical, 66, 904 (1998)). However, since the aqueous electrolyte has a lower withstand voltage than the non-aqueous electrolyte, it is constant. Current
The following equation (1) when discharging from voltage Vi to Vf at I
Electric vehicles that have attracted attention in recent years have low energy W per unit cell of the electric double layer capacitor represented by
For power applications such as hybrid vehicles and electric power storage, those using a non-aqueous electrolyte are suitable.

[0004]

W = 1/2 · C · (Vi ² −Vf ² ) (1) (where C is the capacitance (F)). Is an electrolyte obtained by dissolving a quaternary ammonium salt of perchloric acid in an organic solvent such as propylene carbonate (Japanese Patent Publication No.
No. 4-9704), an electrolytic solution in which BF ₄ salt and PF ₆ salt of quaternary ammonium are dissolved (Japanese Patent Publication No. 52-40025, Japanese Patent Application Laid-Open No. 63-17312), and an electrolyte in which quaternary phosphonium salt is dissolved. Liquid (Japanese Patent Publication No. 6-66233), an electrolytic solution in which a salt of a compound having an amidine group is dissolved (WO 95/15572), and an electrolytic solution using a salt containing an imidazolium compound as a cation component (WO9)
9/08299, JP-A-8-32439), and those using a room temperature molten salt without using an organic solvent as an electrolyte (JP-A-5-74665, WO97 / 02).
252) is known. Also, Japanese Patent Application Laid-Open No.
Japanese Patent No. 4656 discloses that a mixture of an inorganic salt and an organic salt is used as a room temperature molten salt. Chloride or bromide of quaternary ammonium or quaternary phosphonium salt is used as an organic salt, and AlCl ₃ , Ti is used as an inorganic salt.
A chloride selected from the group consisting of Cl ₃ , TiCl ₄ , and BeCl ₂ is described as being preferred. However, 1-ethyl-3-methylimidazolium (EMI) AlCl _4, which is described in Examples as a mixture of chloride of quaternary ammonium salt and AlCl ₃ , is unstable to oxygen and moisture in the air and is difficult to handle. It was limited and was not satisfactory as an electrolyte.

As an improvement, several fluorine-based room-temperature molten salts that are stable even in the air have been reported. J. Chem. So
c., Chem. Commun., 965 (1992) that Wilkes et al.
It announced F _4, room temperature molten salt and its electrolyte by a combination of anion having a van der Waals volume of the 100 Å ³ or more annular quaternary ammonium cation in JP WO97 / 02,252, and electrochemical cell using the electrolyte solution It is shown.

[0006] WO95 / 15572 discloses an electrolytic solution in which a salt having a quaternary salt of a cyclic amidine compound such as imidazolium as a cation is dissolved in an organic solvent, and an electrochemical device using the same. In addition, WO99 /
The 08299 discloses cyclic quaternary ammonium cation and _{^{PF 6 -, BF 4 -,}} AsF 6 - and CF ₃ SO ₃ ^- electrolysis of salt which is a combination of anion having a 100 Å ³ below the van der Waals volume is dissolved in an organic solvent such as A liquid as well as an electrochemical capacitor using the electrolyte are shown.

[0007]

The non-aqueous electrolyte using the above-mentioned organic solvent has a high capacitance, but is flammable, so that the electrolyte leaks due to damage caused by an increase in internal pressure due to shock or deterioration. It was feared that there would be a risk of fire if this occurred. On the other hand, the conventional room temperature molten salt that does not use an organic solvent has high safety because it is nonflammable, but has a drawback that its capacitance is lower than that of a non-aqueous electrolyte using an organic solvent.

Accordingly, an object of the present invention is to provide an electric double layer capacitor which is safe and has a large capacitance.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, a high capacity electric double layer capacitor can be obtained by using a quaternary ammonium fluoride HF salt as an electrolyte. This led to the completion of the present invention. That is, the gist of the present invention is to provide an electric double layer capacitor in which a polarizable electrode is used for both a positive electrode and a negative electrode that are arranged to face each other with a separator therebetween as a main constituent material.
The present invention relates to an electric double layer capacitor comprising a quaternary ammonium fluoride HF salt in an electrolytic solution.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The electric double layer capacitor of the present invention, as a main constituent material,
A polarizable electrode is used for both a positive electrode and a negative electrode which are arranged to face each other with a separator interposed therebetween, and a quaternary ammonium fluoride HF salt is contained in an electrolytic solution impregnated in the electrode.

Specific examples of the quaternary ammonium cation of the quaternary ammonium fluoride HF salt include tetramethylammonium, methyltriethylammonium, dimethyldiethylammonium, trimethylethylammonium, tetraethylammonium, tetrabutylammonium, benzyltrimethylammonium, 1-dimethylpyrrolidinium, 1-methyl-1-ethylpyrrolidinium, 1,1-dimethylpiperidinium, 1-ethylpyridinium, 1,3-dimethylimidazolium, 1-
Ethyl-3-methylimidazolium, 1,3-dimethylbenzimidazolium, 1,3-dimethylimidazolinium, 1-ethyl-3-methylimidazolinium, 1-
Ethyl-2,3-dimethylimidazolinium, 1,2,2
Examples include, but are not limited to, 3,4-tetramethylimidazolinium.

Preferred quaternary ammonium fluoride HF salts include cyclic quaternary ammonium fluoride HF.
Salts are more preferable, and among the quaternary ammonium fluoride HF salts, an imidazolium salt represented by the following formula (1) is preferable.

[0013]

Embedded image

(Wherein R ₁ and R ₃ each independently represent an alkyl group having 1 to 4 carbon atoms, and R ₂ , R ₄ , and R ₅
Each independently represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Further, a part or all of R _{1 to} R ₅ may be mutually bonded to form a ring. n represents the numerical value of 1-4. Examples of the alkyl group having 1 to 4 carbon atoms include a methyl group, an ethyl group, an n-propyl group, an i-propyl group, and an n-butyl group. Preferably it represents a group. In the present invention, it is more preferable that R ₁ and R ₃ are asymmetric, since the viscosity becomes low. R ₂ , R ₄ and R ₅ are a hydrogen atom,
It preferably represents a methyl group or an ethyl group, more preferably a hydrogen atom.

Specific examples of the compound represented by the formula (1) include 1,3-dimethylimidazolium salt,
3,4-trimethylimidazolium salt, 1-ethyl-3
-Methylimidazolium salt and the like, and most preferred is 1-ethyl-3-methylimidazolium salt which is a room temperature molten salt. Part or all of R _{1 to} R ₅ may be mutually bonded to form a ring. As a specific example,
3-dimethylbenzimidazolium salt, 1-ethyl-3
-Methylbenzimidazolium salt and the like.

N is a numerical value from 1 to 4, and need not necessarily be an integer. The value of n is calculated from the elemental analysis value of the compound. As a method for producing the quaternary ammonium fluoride HF salt represented by the above formula (1), as an example, 1-ethyl-3-methylimidazolium fluoride HF salt, which is a room temperature molten salt, is prepared by the present inventors. J. by the work. Fluorine Chemistry, 99,1-
3 (1999), but the production method is not limited thereto.

The above quaternary ammonium fluoride H
The F salts may be used in combination with each other or with a quaternary ammonium salt composed of another anion. Specific examples of other anions include PF ₆ ⁻ , BF ₄ ⁻ , AsF ₆ ⁻ and triflate anion. Water content in the electrolyte causes a decrease in withstand voltage, so the water content is 300 ppm or less,
Preferably 100 ppm or less, particularly preferably 30 pp
m or less.

An organic solvent such as propylene carbonate, ethylene carbonate or γ-butyrolactone may be added for the purpose of improving the impregnating property and the like, as long as the properties of the present invention are not impaired. However, the smaller the content of the organic solvent in the electrolytic solution, the more incombustible and the improved safety is preferable, and
Most preferred is when no organic solvent is contained. The main component of the polarizable electrode used as the positive electrode and the negative electrode of the electric double layer capacitor according to the present invention is electrochemically inert to the electrolytic solution, and has a moderate electric conductivity. It is preferable to use activated carbon in view of a large electrode interface where charges are accumulated, particularly.

Hereinafter, the case where activated carbon is used as the carbonaceous substance will be described in detail, but the present invention is not limited to this. The specific surface area of the activated carbon cannot be determined unconditionally because of the capacitance per unit area (F / m ² ) due to the carbonaceous species and the decrease in the bulk density due to the increase in the specific surface area. the specific surface area is preferably 500~2500m ² / g as determined by the law, especially, a specific surface area of activated carbon 1000 to 2000 ² / g, the capacitance per volume greater preferred.

The method for producing the activated carbon used in the present invention is not particularly limited, but generally, plant-based wood, sawdust, coconut shell, pulp waste liquid, fossil fuel-based coal, heavy petroleum oil, or Coal and petroleum pitch, petroleum coke, carbon aerogel, tar pitch spun fiber, synthetic polymer, phenol resin, furan resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, polyamide resin It is manufactured by carbonizing and activating a wide variety of raw materials such as liquid crystal polymers, plastic waste, and waste tires. As the activation method, a gas activation method in which the carbonized raw material is brought into contact with steam, carbon dioxide, oxygen, and other oxidizing gases at a high temperature, and zinc chloride, phosphoric acid, sodium phosphate, calcium chloride, Potassium sulfide, potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, sodium sulfate, potassium sulfate,
There is a chemical activation method in which calcium carbonate or the like is evenly impregnated, heated in an inert gas atmosphere, and activated carbon is obtained by dehydration and oxidation reaction of the chemical, and any of them can be used.

The activated carbon after the activation treatment is replaced with nitrogen, argon,
500 to 2 under an inert atmosphere such as helium or xenon
Heat treatment at 500C, preferably 700-1500C,
The electron conductivity may be increased by removing unnecessary surface functional groups or by developing the crystallinity of carbon. The shape of activated carbon is
Crushing, granulation, granules, fibers, felts, woven fabrics, various shapes such as sheets can be used without any particular limitation, but in the case of granules, in terms of improving the bulk density of the electrode and reducing the internal resistance,
The average particle size is preferably 30 μm or less.

The positive electrode and the negative electrode are usually composed of a carbonaceous substance, a conductive agent and a binder substance, and are used as a thin coating film, a sheet-like or plate-like molded body. As the conductive agent,
At least one conductive agent selected from the group consisting of carbon black such as acetylene black and Ketjen black, natural graphite, thermally expanded graphite, carbon fiber, and metal fiber such as ruthenium oxide, titanium oxide, aluminum and nickel is preferable. Acetylene black and Ketjen black are particularly preferred in that the conductivity is effectively improved with a small amount, and the blending amount with activated carbon is different depending on the bulk density of the activated carbon. , 5 to 50% by weight of activated carbon, especially 10 to 30%
The degree is preferred. In addition, as the binder substance, at least one or more of polytetrafluoroethylene, polyvinylidene fluoride, carboxymethyl cellulose, fluoroolefin copolymer crosslinked polymer, polyvinyl alcohol, polyacrylic acid, polyimide, petroleum pitch, coal pitch, and phenol resin It is preferably used.

The amount of the binder substance in the electrode varies depending on the type and shape of the carbonaceous substance. For example, when the carbonaceous substance is activated carbon, if the amount is too large, the ratio of the activated carbon decreases, and if the amount is too small, the capacity decreases. Since the binding property is deteriorated and the strength is reduced, 0.5 to 30% by weight of the activated carbon is preferable,
2-30% is particularly preferred. The positive electrode and the negative electrode can be formed by a conventionally known method. For example, it is obtained by adding and mixing polytetrafluoroethylene to a mixture of activated carbon and acetylene black, followed by press molding. Also, a sintered body is obtained by mixing and molding activated carbon and a binder substance such as pitch, tar, and phenolic resin, and then performing a heat treatment in an inert atmosphere. Furthermore, it is also possible to sinter activated carbon and a binder or only activated carbon to form a polarizable electrode.

In the present embodiment, the same activated carbon and the same molding method are used for the positive electrode and the negative electrode. However, the activated carbon and the molding method for the positive electrode and the negative electrode are not necessarily the same. What is necessary is just to have the specific surface area of. As the type of the separator, a separator made of a material having high hydrogen fluoride resistance such as paper, cellulose fiber, polypropylene, or polyethylene is preferable. Conversely, glass fiber separators are not suitable because they corrode and dissolve.

It is preferable to use a hydrogen fluoride-resistant material for the current collector, and a conductive rubber sheet in which carbon is dispersed in butyl rubber or stainless steel can be used. The former is more preferable. Gasket is heat and acid resistant A
It is preferably made of BS resin or polypropylene. In order to improve airtightness, it is also preferable that the gasket, the current collector, and the case are bonded with a highly acid-resistant epoxy adhesive, and the surroundings are fixed with bolts and nuts.

Although the working voltage of the electric double layer capacitor of the present invention is not particularly limited, it is suitably used for applications having a voltage of 2.5 V or less. As a method for evaluating the safety of the electric double layer capacitor, a burning rate of a manila paper impregnated with an electrolytic solution was employed.

FIGS. 1 to 3 show general coin-type, wound-type, and square-type electric double-layer capacitors. The present invention can be used for any type of electric double-layer capacitor. It is not limited.

[0028]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the scope of the invention. (Example 1) Fluorine Chemistr
y, 99, 1-3 (1999), a compound having the structural formula (2) (EMIF. (HF) _2.3 ) was synthesized and used as an electrolyte. The electrolyte was handled in a dry argon atmosphere in order to prevent water from being mixed into the electrolyte. Structural formula (2)

[0029]

Embedded image

In order to evaluate the performance as an electric double layer capacitor, it was manufactured as follows. Coconut activated carbon powder (specific surface area 17) obtained by steam activation of carbonaceous material
00m ² / g, average particle size 10 [mu] m) 80 wt%, acetylene black 10 wt%, was kneaded mixture of polytetrafluoroethylene 10 wt%, 50 kgf / at a pressure of cm ² pressure-molded to a diameter of 10 mm, A disk-shaped molded body having a thickness of 0.5 mm was obtained and used as a polarizable electrode. By repeating this molding operation, one more polarizable electrode having the same composition and shape was obtained. After drying the obtained two molded bodies at 300 ° C. for 3 hours in a vacuum of 0.1 torr or less, they were transferred to a glove box in an argon atmosphere. After cooling, the two polarizable electrode bodies (activated carbon molded bodies) were impregnated with the above electrolyte under reduced pressure. A polypropylene separator is sandwiched between two polarizable electrodes impregnated with the electrolytic solution,
An electric double layer capacitor as shown in FIG. 1 was obtained by caulking and sealing in a stainless steel case via a polypropylene gasket.

The obtained electric double layer capacitor was heated to 70 ° C.
After charging at a constant current of 5 mA, constant voltage charging was performed at a predetermined voltage for 50 minutes from the start of charging, followed by discharging at a constant current of 5 mA, and the capacitance density was measured as an index of performance. The predetermined voltage was tested from 0.8V to 4.7V in 0.1V steps. The capacitance density was calculated by dividing the capacitance obtained from the total energy at the time of discharge by the total volume of the positive electrode and the negative electrode, and the maximum value of the capacitance density is shown in Table 1.

On the other hand, in order to evaluate the safety of the obtained electric double layer capacitor, the used electrolyte was 15 mm wide and 32 cm long.
0mm, thickness 40μ, density 0.6g / cm3 Manila paper soaked for 1 minute, suspended vertically for 3 minutes to remove excess electrolyte, 25
The sample was fixed horizontally to a sample holder having supporting needles at intervals of mm, and one end of the sample was ignited with a match. The length and time of the burning were measured, and the burning rate was determined and shown in Table 1. (Example 2) In this example, a propylene carbonate (PC) solvent was used as an electrolytic solution and EMI having a concentration of 1.0 mole as a solute was used.
An electric double layer capacitor having the structure shown in FIG. 1 was fabricated and the capacitance density was measured in exactly the same manner as in Example 1, except that a solution in which F · (HF) _2.3 was dissolved was used. The maximum values are shown in Table 1.

The safety evaluation was performed in the same manner as in Example 1, and the burning speed is shown in Table 1. (Comparative Example 1) In this comparative example, the same procedure as in Example 1 was performed except that 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF ₄ ), which is a room temperature molten salt, was used as an electrolytic solution. An electric double layer capacitor having the structure shown in FIG. 1 was manufactured, the capacitance density was measured, and the maximum value was shown in Table 1.

The safety evaluation was performed in the same manner as in Example 1, and the burning speed is shown in Table 1. (Comparative Example 2) In this comparative example, a 1.0 molar concentration of triethylmethylammonium borofluoride (TEMAB) was used as an electrolyte in a propylene carbonate (PC) solvent and as a solute.
An electric double layer capacitor having the structure shown in FIG. 1 was prepared, the capacitance density was measured, and the maximum value was determined in the same manner as in Example 1 except that a solution in which F ₄ was dissolved was used. The results are shown in Table 1.

The safety evaluation was performed in the same manner as in Example 1, and the burning rates are shown in Table 1.

[0036]

[Table 1]

[0037]

Since the electric double layer capacitor of the present invention has a large capacitance and excellent safety, it is suitable for use as a memory backup for various electronic devices and for an electric vehicle which may leak due to collision.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cell structure of a typical coin-type electric double layer capacitor.

FIG. 2 is a diagram showing a cell structure of a typical wound electric double layer capacitor.

FIG. 3 is a diagram showing a cell structure of a typical square electric double layer capacitor.

[Explanation of symbols]

 1. Case 2. Positive electrode 3. Gasket 4. Separator 5. Negative electrode 6. Top lid

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Akiko Toriumi 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Pref.

Claims (6)

[Claims] In an electric double layer capacitor using a polarizable electrode for both a positive electrode and a negative electrode which are disposed opposite to each other with a separator interposed therebetween, a quaternary ammonium fluoride HF salt is contained in an electrolytic solution. Characteristic electric double layer capacitor. 2. The electric double layer capacitor according to claim 1, wherein the quaternary ammonium fluoride HF salt is a cyclic quaternary ammonium fluoride HF salt. 3. A quaternary ammonium fluoride HF salt,
The electric double layer capacitor according to claim 1, wherein the electric double layer capacitor is represented by the following formula (1). Embedded image (Wherein, R ₁ and R ₃ each independently represent a carbon number of 1 to 4)
R ₂ , R ₄ , and R ₅ each independently represent a hydrogen atom or an alkyl group having 1 to 4 carbon atoms. Further, a part or all of R _{1 to} R ₅ may be mutually bonded to form a ring. n represents the numerical value of 1-4. ) 4. The quaternary ammonium fluoride is 1-ethyl-3-methylimidazolium fluoride.
4. The electric double layer capacitor according to any one of items 3 to 3. 5. The electric double layer capacitor according to claim 1, wherein the separator is made of paper, cellulose fiber, polypropylene or polyethylene. 6. The electric double layer capacitor according to claim 1, wherein the voltage of the electric double layer capacitor is 2.5 V or less.