JP2002110472A - Electric double layer capacitor - Google Patents

Abstract

[PROBLEMS] To provide an electric double layer capacitor having high withstand voltage and high capacitance. SOLUTION: An electric double layer capacitor in which a positive electrode and a negative electrode contain activated carbon as an electrode active material and use a room temperature molten salt as an electrolytic solution, wherein the cation constituting the room temperature molten salt is N, N'-dialkyl. Imidazolium, for which an anion has a van der Waals volume of 0.0
It is an anion in the range of 10~0.060nm ^3,
And the activated carbon is activated carbon having an average pore diameter of 2.5 nm or less.

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. More specifically, the present invention is an electric double layer capacitor in which the positive and negative electrode active materials contain a specific activated carbon, and use a specific room-temperature molten salt containing no organic solvent as an electrolytic solution. By adopting this, an electric double layer capacitor having a high withstand voltage and a high capacity is provided.

[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, Electrochemistry, Vol. 66, p. 904, 1998). However, since the aqueous electrolyte has a lower withstand voltage than the non-aqueous electrolyte. , a constant current I, low energy W per unit cell of the electric double layer capacitor of the following formula (1) the time of discharging the voltage V _i to V _f, in recent years, electric vehicles have been attracting attention For a power application such as a hybrid vehicle and electric power storage, a device using a non-aqueous electrolyte is suitable.

[0004] W = 1/2 × C × (V i 2 -V f 2) ...... (1) ( where C is the capacitance (F).) The non-aqueous electrolytic solution, propylene carbonate Such as an electrolyte solution in which a quaternary ammonium salt of perchloric acid is dissolved in an organic solvent such as (JP-B-54-9704); JP-A-52-40025, JP-A-6
No. 3-173312), an electrolytic solution in which a quaternary phosphonium salt is dissolved (Japanese Patent Publication No. 6-66233), and an electrolytic solution in which a salt of a compound having an amidine group is dissolved (WO95 / 1)
No. 5572), and an electrolytic solution using a salt containing an imidazolium compound as a cation component as a solute (WO99 / 08299).
And Japanese Patent Application Laid-Open (JP-A) No. 8-321439) and those using a room temperature molten salt that does not use an organic solvent (Japanese Patent Application Laid-Open No.
4656, JP-A-6-61095, WO9
7/02252) and the like.

[0005] Room temperature molten salt is a general term for salts that exhibit a liquid appearance at room temperature because the salt has a low boiling point, and exhibits high electrical conductivity because it is an ionic liquid. Unlike an electrolytic solution containing an organic solvent, the solvent does not evaporate or decompose, so that deterioration with time is small, and no harmful substances are discharged, so that the burden on the environment is small. For this reason, room temperature molten salts are attracting attention as solvents for various conductive materials and various chemical reactions.

[0006] JP-A-5-74665 and JP-A-6-61095, which are proposals for using the above-mentioned room temperature molten salt, disclose the use of a mixture of an inorganic salt and an organic salt as a room temperature molten salt. I have. Chloride or bromide of quaternary ammonium or quaternary phosphonium salt is used as an organic salt, and AlCl ₃ , TiCl ₃ ,
Chloride selected from the group consisting of TiCl ₄ and BeCl ₂ is described as being preferred.

However, 1-ethyl-3-methylimidazolium (hereinafter referred to as EMI) described in Examples as a mixture of chloride of quaternary ammonium salt and AlCl _3.
AlCl ₄ is not stable as an electrolyte for electric double layer capacitors because AlCl ₄ is unstable to oxygen and moisture in the air and is therefore limited in handling. To improve this, several room temperature molten salts having a fluorine-containing anion that is stable even in the air have been reported. J. Ch
em. Soc. Chem. Commun, 965 (1
992). S. Wilkes et al. Published EMI tetrafluoroborate and WO 97/02252 states that heterocyclic cations containing EMI and 0.1 nm ³
A room-temperature molten salt (for example, bis (trifluoromethanesulfonyl) imide salt) formed by a combination of anions having the above van der Waals volume and an electrochemical device using the same are shown.

[0008] Further, it has been studied to prepare an electrolytic solution obtained by dissolving the room temperature molten salt in an organic solvent and use it for an electrochemical element such as an electric double layer capacitor. Examples of these are WO 95/15572 and WO 97/15572.
/ 02250 and WO99 / 08299.

[0009]

In recent years, the performance requirements for electric double layer capacitors have been becoming severer, and the development of electric double layer capacitors having higher withstand voltage and higher capacitance has been demanded. An object of the present invention is to provide an electric double layer capacitor using a room temperature molten salt containing no organic solvent and having a higher withstand voltage and a higher capacitance.

[0010]

Means for Solving the Problems As a result of intensive studies to achieve the above object, the present inventors have found that a cation having a specific structure and a van der Waals in a specific range are contained as constituents of a room temperature molten salt. The inventors have found that an electric double layer capacitor having a high withstand voltage and a high capacitance can be obtained by using a combination of an anion having a volume and an activated carbon having an average pore diameter in a specific range, and completed the present invention.

That is, the present invention relates to an electric double layer capacitor in which the positive electrode and the negative electrode contain activated carbon as an electrode active material and use a room temperature molten salt as an electrolyte, wherein the cations constituting the room temperature molten salt are N, N '- dialkyl imidazolium, which for an anion anion is in the range of 0.010～0.060Nm ³ in van der Waals volume, and wherein the activated carbon is average pore diameter 2.5nm
An electric double layer capacitor characterized by the following activated carbon is provided.

In the present invention, the N, N'-dialkylimidazolium is preferably 1-ethyl-3-methylimidazolium, and the anion constituting the room-temperature molten salt is tetrafluoroboric acid. It is preferably an ion.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION A cation constituting a room-temperature molten salt containing no organic solvent as an electrolyte used in the electric double layer capacitor of the present invention has an alkyl group bonded to two nitrogen atoms of an imidazolium ring. N, N'-dialkylimidazolium which is a monovalent cation. Preferred as the alkyl group bonded to the N-position and N'-position of the imidazolium ring are each independently an alkyl group having 1 to 4 carbon atoms, specifically, a methyl group, an ethyl group, an n-propyl group, Examples thereof include an i-propyl group and an n-butyl group.

The 2-, 4-, and 5-positions of the imidazolium ring may be unsubstituted, that is, a hydrogen atom may be bonded, and a preferred substituent, an alkyl group having 1 to 4 carbon atoms, may be bonded. May be. In addition, some or all of the alkyl groups bonded to the imidazolium ring may be bonded to each other to form a ring structure. As a specific example,
Dimethylbenzimidazolium, 1-ethyl-3-methylbenzimidazolium and the like.

The choice of the alkyl substituent on the imidazolium ring is very important and care must be taken to obtain a room temperature molten salt with higher electrical conductivity. First,
It is preferable to use a methyl or ethyl group having a relatively low molecular weight as the alkyl group, so that an imidazolium ion having high mobility can be selected. Also, N
When the alkyl groups bonded to the N- and N'-positions are different from each other, the symmetry of the molecule is reduced, so that the degree of dissociation of the room-temperature molten salt is improved, and the number of carriers (free ions) responsible for ionic conduction is increased. Is preferred.

Preferred examples of the N, N'-dialkylimidazolium used in the present invention from the above viewpoints include 1,3-dimethylimidazolium, 1,3,4
-Trimethylimidazolium, 1,2,3,4-trimethylimidazolium, 1-ethyl-3-methylimidazolium, most preferred is 1-ethyl-3-methylimidazolium.

The room temperature molten salt used in the present invention is an anion having a van der Waals volume in a specific range. Here, the van der Waals volume of the anion of the room temperature molten salt defined in the present invention can be calculated from a model in which spherical atoms constituting the ion are bonded at a predetermined bonding distance and bonding angle and overlap each other ( M. Ue,
J. Electrochem. Soc. , 141, 33
36 (1994)). The radius of the sphere of each atom is A.
Bondi, J .; Phys. Chem. , 68,441
(1964), adopted by the Chemical Society of Japan, “Basic Chemistry Handbook II”, H: 0.120 nm, B: 0.1
90 nm, C; 0.170 nm, N; 0.155 nm,
F: 0.147 nm, S; 0.180 nm)
For the conformation, a value measured by X-ray diffraction or neutron diffraction of an ionic crystal containing the anion was used. Further, a conformation obtained by “MM2 calculation” known as a molecular force field calculation program may be used. According to the "MM2 calculation", the value of the obtained volume is slightly different depending on the size of the lattice spacing at the time of numerically integrating the volume. Therefore, in the present invention, the lattice spacing is fixed at 0.075 nm.

In the present invention, an anion having a van der Waals volume of 0.010 to 0.060 nm ³ is used. It is presumed that this allows the anions to diffuse quite freely in the pores of the activated carbon in the positive electrode. The van der Waals volume is 0.01
If it is less than 0 nm ³ , the interaction with the cation becomes strong, the degree of dissociation decreases, and the electric conductivity decreases, which is not preferable. If it exceeds 0.060 nm ^{3, it} is considered that the ions themselves are too large, so that the diffusion of ions in the pores is hindered and the amount of electric charge stored per unit area decreases, resulting in a decrease in capacity.

Anions satisfying the above conditions include:
A tetrafluoroborate ion having a van der Waals volume of 0.051 nm ³ satisfies high withstand voltage and electric conductivity. The most preferable combination of a cation and an anion in the room temperature molten salt used in the present invention is tetrafluoroborate (1-ethyl-3-acid).
Methylimidazolium).

The water content in the room-temperature molten salt causes a decrease in the withstand voltage of the electric double layer capacitor. Therefore, the water content is usually 1% or less, preferably 1000 ppm or less, more preferably 500 ppm or less, particularly 100 ppm or less. . The activated carbon used for the positive electrode and the negative electrode of the electric double layer capacitor of the present invention preferably has an average pore diameter of 2.5 n.
m or less. The average pore size of activated carbon is B
It is measured by the ET method.

The specific surface area of the activated carbon cannot be unconditionally determined because of the capacitance per unit area (F / m ² ) due to the carbonaceous species and the decrease in bulk density due to the increase in the specific surface area. Surface area determined by the BET method according to
Preferably, the specific surface area is 1000 m ² / g.
Activated carbon of ２０００2000 m ² / g is preferred because of its large capacitance per volume.

The method for producing the activated carbon used in the present invention is not particularly limited. Generally, plant-based wood, sawdust, coconut shell, pulp waste liquid, fossil fuel-based coal, heavy petroleum oil, or Coal and petroleum pitch pyrolyzed from them,
Petroleum coke, carbon aerogel, mesophase carbon, fiber spun from tar pitch, synthetic polymer, phenolic resin, furan resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, polyamide resin,
It is manufactured by carbonizing and activating various kinds of raw materials such as ion exchange resin, liquid crystal polymer, plastic waste, and waste tire.

As the activation method, a gas activation method in which a carbonized raw material is brought into contact with steam, carbon dioxide, oxygen, other oxidizing gas or the like 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,
Evenly impregnate calcium carbonate, boric acid, nitric acid, etc.,
There is a chemical activation method of heating in an inert gas atmosphere to obtain activated carbon 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 atmospheres under an inert gas atmosphere such as helium, xenon, etc.
Heat treatment may be performed at ２2500 ° C., preferably 700-1500 ° C., to remove unnecessary surface functional groups or to improve the crystallinity of carbon to increase electron conductivity. The shape of the activated carbon can be used without particular limitation, such as various shapes such as crushing, granulation, granules, fibers, felts, woven fabrics, sheets, and the like. The average particle diameter is preferably 30 μm or less.

In addition to activated carbon, it is also possible to use the above-mentioned carbon material having a high specific surface area, for example, carbon nanotubes or diamond produced by plasma CVD. The configuration of the positive electrode and the negative electrode is not particularly limited and may be a known one, and is usually formed of the above-described activated carbon as an electrode active material, a conductive agent and a binder material, and is used as a thin coating film, a sheet-like or plate-like molded body. I do.

The conductive agent is at least one 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. One type of conductive agent is preferred.
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. The weight of activated carbon is preferably 5 to 50%, particularly preferably about 10 to 30%.

Further, as the binder substance, polytetrafluoroethylene, polyvinylidene fluoride, carboxymethyl cellulose, a crosslinked polymer of a fluoroolefin copolymer, polyvinyl alcohol, polyacrylic acid,
It is preferable to use at least one of polyimide, petroleum pitch, coal pitch, and phenol resin. The amount of the binder material in the electrode varies depending on the type and shape of the carbonaceous material, but if the amount is too large, the ratio of the activated carbon decreases, the capacity decreases, and if the amount is too small, the binding property deteriorates and the strength decreases. 0.5 to 30% by weight of the activated carbon is preferred, and 2 to 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 an electrode. When an activated carbon fiber cloth obtained by activating a carbon fiber cloth is used, it may be used as an electrode without using a binder.

As the electric double layer capacitor, a porous thin film between the positive electrode and the negative electrode, that is, a so-called separator can be used. As the material of the separator, paper, polypropylene, polyethylene, glass fiber, or the like is used. The shape of the electric double layer capacitor may be any shape such as a coin shape, a wound shape, a square shape, and an aluminum laminate type, and is not limited to these shapes. Various known methods can be applied to the method of manufacturing the electric double layer capacitor as the electrochemical device of the present invention.

In the examples described later, 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 a specific surface area of about.

[0031]

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 present invention. The following examples,
In the comparative example, the average pore diameter was calculated from the adsorption isotherm of nitrogen gas measured using a gas adsorption measurement device “Autosorb 1” manufactured by Yuasa Ionics Inc.

Example 1 Activated carbon powder having an average pore diameter of 2.0 nm (specific surface area of 170 nm) obtained by subjecting coconut shell activated carbon to steam activation treatment.
0 m ² / g, average particle diameter 10 μm) 80% by weight, acetylene black 10% by weight, polytetrafluoroethylene 1
After kneading a mixture consisting of 0% by weight, 60 kgf / c
Pressure molding with a pressure of m ² , diameter 10 mm, thickness 0.6 m
m was prepared as a positive and negative electrode of an electric double layer capacitor. Remove the two molded products from 0.
After drying at 300 ° C. for 3 hours in a vacuum of 1 torr or less, they were transferred into a glove box under an argon atmosphere. After cooling, the two activated carbon molded bodies were impregnated with the above-mentioned room temperature molten salt tetrafluoroborate (1-ethyl-3-methylimidazolium) (a product obtained by dehydrating Tokyo Kasei under reduced pressure) under reduced pressure. The van der Waals volume of the tetrafluoroborate ion as an anion was 0.051 nm ³ by MM2 calculation.

2 impregnated with the room temperature molten salt obtained above
A coin-type electric double-layer capacitor as shown in FIG. 1 was obtained by sandwiching a polypropylene separator between two polarizable electrodes and sealing them in a stainless steel case via a polypropylene gasket. The obtained electric double layer capacitor was charged at a constant current of 5 mA at 70 ° C., then was charged at a predetermined voltage for 50 minutes from the start of charging, and then was discharged at a constant current of 5 mA to obtain an electrostatic capacity as an index of performance. The capacity density was measured. The predetermined voltage is:
The test was performed from 0.8 V to 4.9 V in 0.1 V steps. The capacitance density was calculated by dividing the capacitance obtained from the total energy at the time of discharging from a predetermined voltage by the total volume of the positive electrode and the negative electrode.

Comparative Examples 1 to 3 In Comparative Example 1, bis (trifluoromethanesulfonyl) imide (1-ethyl-3-methylimidazo) having an anion (0.144 nm ³ ) having a large van der Waals volume as a room temperature molten salt was used. An electric double layer capacitor was produced in exactly the same manner as in Example 1 except that (I) was used.

In Comparative Example 2, activated carbon powder having an average pore diameter of 2.6 nm (specific surface area: 1000 m ² /
g, an average particle diameter of 12 μm) was prepared in the same manner as in Example 1 except that an electric double layer capacitor was used. Comparative Example 3 was performed in the same manner as in Example 1 except that a propylene carbonate solution of tetrafluoroboric acid (1-ethyl-3-methylimidazolium) (solute concentration: 1.0 mol / dm ³ ) was used instead of the room-temperature molten salt. An electric double layer capacitor was produced in exactly the same manner as described above.

Table 1 shows the anionic species of the common molten salt used in Examples and Comparative Examples and their van der Waals volumes, the average pore diameter of the activated carbon used, and the maximum value of the capacitance density of the produced electric double layer capacitor. And the charging voltage at that time are displayed.

[0037]

[Table 1]

[0038]

The electric double layer capacitor of the present invention has a large capacitance, a high withstand voltage and a high energy density, and is therefore suitable for power applications such as electric vehicles, hybrid vehicles and electric power storage.

[0039]

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a typical electric double layer capacitor.

[Explanation of symbols]

 1 Case 2 Positive electrode 3 Gasket 4 Separator 5 Negative electrode

Claims (3)

[Claims] 1. An electric double layer capacitor in which a positive electrode and a negative electrode contain activated carbon as an electrode active material and a room temperature molten salt is used as an electrolyte, wherein the cation constituting the room temperature molten salt is N, N'-dialkylimidazo. And the anion for this is 0.00 in van der Waals volume.
It is an anion in the range of 10~0.060nm ^3,
And the activated carbon is activated carbon having an average pore diameter of 2.5 nm or less. 2. The electric double layer capacitor according to claim 1, wherein said N, N'-dialkylimidazolium is 1-ethyl-3-methylimidazolium. 3. An anion constituting the room-temperature molten salt,
The electric double layer capacitor according to claim 1, wherein the electric double layer capacitor is a tetrafluoroborate ion.