JP2001076975A - Electric double-layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the reliability of performance for long time, to raise operation voltage and to enlarge energy density by incorporating one or more kinds of organic component where an inorganic value (I) and an organic value (O) satisfy a specified expression in electrolysis solution. SOLUTION: One or more kinds of organic compound where an inorganic value (I) and an organic value (O) satisfy expression I and II are contained in electrolytic solution. Alkylbenzene, bromobenzene, iodobenzene, benzotrichhalogenide, fluoroxylene and alkane having a normal chain or a side chain are used as organic compound. The amount of organic compound contained in electrolytic solution is set to about 0.1 to 30 wt.%. A positive electrode and a negative electrode contain carbonaceous materials which form electric double-layers on an interface with electrolytic solution and whose specific surface area is about 100 to 3000 m2/g. Thus, the electric double layer capacitor in which the voltage resistance of the electric double layer capacitor is improved and performance reliability for long time is superior even if high voltage is applied and whose energy density is high is manufactured.

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 excellent long-term reliability of performance, high withstand voltage and high energy density.

[0002]

2. Description of the Related Art Electric double layer capacitors have excellent characteristics in terms of long-term reliability and power density due to charge / discharge cycles, but have a problem that the energy density is lower than, for example, lithium ion secondary batteries. There is. The amount of energy stored in a capacitor is proportional to the capacitance of the capacitor,
It is proportional to the square of the applied voltage. Therefore, the energy density can be effectively increased by increasing the withstand voltage of the capacitor.

The withstand voltage of an electric double layer capacitor is basically limited by the electrochemical decomposition voltage of an electrolytic solution.
When an organic electrolytic solution having a higher withstand voltage than an aqueous electrolytic solution is used as the electrolytic solution, electrolysis occurs due to impurities, particularly moisture, contained in the electrolytic solution. Therefore, the organic electrolytic solution is used after being highly purified and dehydrated for both the solvent and the electrolyte salt.

On the other hand, the electrodes of the electric double layer capacitor
When an electrode material having a high specific surface area is used and combined with the above-mentioned organic electrolyte, activated carbon is generally used as the electrode material having a high specific surface area. Activated carbon is a porous material having micropores of about several nm, but has a high adsorptivity, and therefore easily adsorbs moisture in the environment. Therefore, activated carbon needs to be highly dehydrated in the manufacturing process of the electric double layer capacitor.

Usually, in order to completely remove water from the pores of activated carbon, a dehydration treatment at a high temperature of 300 ° C. or more in a vacuum or an inert gas atmosphere is required.
However, the activated carbon particles are usually formed in a layer on the current collector using a binder such as an organic polymer to constitute an electrode, and the binder is thermally decomposed at a high temperature treatment of 300 ° C. or more, and therefore, usually 200 ° C. or less. Only heat treatment. Therefore, to completely remove the water in the activated carbon electrode,
Have difficulty.

An electric double layer capacitor using an organic electrolyte is usually operated at a voltage of 2 V or higher, which is higher than the theoretical decomposition voltage of water (1.23 V), in order to increase the energy density. For this reason, in a use state in which a voltage is applied after assembling the capacitor cell, the remaining moisture in the pores is electrolyzed and gas is generated. The generated gas is
It was found that it was gradually accumulated in the activated carbon pores and was not discharged to the outside of the element body composed of electrodes facing each other via the separator impregnated with the electrolytic solution, but remained inside the element body.

In such a situation, when the electric double layer capacitor is used for a long period of time, the generated gas expels the electrolyte present in the pores, so that the capacity originally obtained cannot be obtained. Further, for this reason, since the electric conduction path due to the movement of ions in the pores is interrupted, there is a problem that the performance of the capacitor is reduced, such as a decrease in capacitance and an increase in internal resistance.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an electric double layer capacitor having excellent long-term reliability, high operating voltage and high energy density. To provide.

[0009]

According to the present invention, there is provided an element body in which a positive electrode and an negative electrode each comprising an electrode containing a carbonaceous material having a specific surface area of 100 to 3,000 m ² / g are opposed to each other via a separator. In an electric double layer capacitor in which electric charge is accumulated by forming an electric double layer on the surface of the electrode by impregnating an organic electrolytic solution, the electrolytic solution has an inorganic value (I) and an organic value (O) as follows: An electric double layer capacitor characterized by containing one or more organic compounds satisfying the following two formulas: (1) (I) / (O) ≦ 0.36 (2) (O) ≧ 160

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
In the present invention, the organic electrolytic solution of the electric double layer capacitor contains an organic compound having an inorganic value (I) and an organic value (O) within the specific ranges as described above. .

Here, the organic value is an index indicating the covalent bond of a certain organic compound molecule, and the inorganic value is an index indicating the ionic bond of the molecule. This is an introduced concept. The organic value is plotted on a two-dimensional diagram with the organic value on the horizontal axis and the inorganic value on the vertical axis, and the organic value and inorganic value are plotted on two dimensions. By doing so, the position of a certain compound is determined, and the position on the figure is related to the property of the compound. That is, specifically, an organic conceptual diagram is mainly divided into an organic property based on the number of carbon atoms and an inorganic property based on the properties and tendencies of the substituents, and orthogonal coordinates in which the organic compound is named an organic axis and an inorganic axis. To understand the outline of its properties (for example, Atsushi Fujita, Masami Akatsuka,
"Systematic organic qualitative analysis" (mixture edition), Kazama Publishing (19
74), Yoshio Koda, "Organic Conceptual Diagram-Basics and Applications-", Sankyo Publishing (1984), Hiroo Inoue, Haku Uehara, Mamoru Minamigo, "Organic Compound Separation Method", Shokabo (1990), Mitsuhiko Tobita, Yasuzo Uchida, "Fine Chemicals", Maruzen (1982), etc.).

The organic value (O) is defined as the value of one carbon atom being 2
Assuming that the value of the hydroxyl group is 100 in comparison with the organic value, the values of the other substituents are determined based on these values. For example, an aromatic monocycle such as a benzene nucleus has an organic (20) based on the number of its carbon atoms.
× 6) In addition to the above, it has an inorganic value of 15. Similarly, the alicyclic compound such as cycloalkane has an inorganic value of 10. Halogen, nitro group, and nitroso group are inorganic groups having organic properties, and their (O, I) values are represented by I (8
0, 10), Br (60, 10), Cl (40, 1)
0), F (5,5), NO ₂ (70,70), NO (5
0, 50) etc. are determined experimentally and empirically. The organic value and inorganic value of the compound can be determined by summing the organic value and inorganic value of atoms and substituents contained in the molecule.

In the present invention, the electrolyte contains one or more organic compounds whose inorganic value (I) and organic value (O) satisfy the following two formulas.

(1) (I) / (O) ≦ 0.36 (2) (O) ≧ 160

(I) / (O) is used as a measure of the balance between hydrophilicity and hydrophobicity. The larger the ratio (I) / (O), the higher the hydrophilicity, and the smaller the ratio, the higher the hydrophobicity. However, it is preferable to add an organic compound whose (I) / (O) is equal to or smaller than 0.36 to the organic electrolyte in order to improve the withstand voltage and durability of the capacitor. When an organic compound having (I) / (O) of more than 0.36 is mixed with the organic electrolytic solution, the effect on the improvement of the withstand voltage and the durability is not exhibited, and generally, it is larger than 0.36. However, there is a tendency that the deterioration of the performance tends to be more remarkable than when no mixing is performed, which is not preferable.

It is also preferable that (O) is an organic compound having 160 or more for improving the withstand voltage and the durability.
When an organic compound having less than 160 is added to the organic electrolytic solution, no effect on the withstand voltage or the durability is improved, and the performance often deteriorates, which is not preferable.

As described above, (I) / (O) is 0.36
When an organic electrolyte solution containing an organic compound having a content of (O) of 160 or more is used, the improvement of the withstand voltage of the capacitor and the deterioration with time are suppressed.

(I) / (O) ≦ 0.36, (O)
The region of ≧ 160 is as shown in the organic conceptual diagram of FIG. 1 of the accompanying drawings described later.

Examples of the organic compound that can be used in the present invention include the following. The numerical value in parentheses indicates the (O, I) value of the substance.

Alkylbenzene such as xylene, (may be any of o, m, and p; the same applies hereinafter); (160, 15), mesitylene (180, 15), cumene (180, 15);

Monobromobenzene (180, 25), dibromobenzene (240, 35), tribromobenzene (300, 45), tetrabromobenzene (360, 5)
5) bromobenzene such as pentabromobenzene (420, 65), hexabromobenzene (480, 75);
Monoiodobenzene (200,35), diiodobenzene (280,35), triiodobenzene (380,4
5) iodobenzenes such as tetraiodobenzene (440, 55), pentaiodobenzene (520, 65), hexaiodobenzene (600, 75);

Benzotrihalide such as benzotrichloride (260, 45), benzotribromide (320, 45), benzotriiodide (380, 45); fluoroxylene such as difluoroxylene (170, 25);

Decane (200,0), undecane (22
0,0), dodecane (240,0), tridecane (26
0,0), tetradecane (280,0), pentadecane (300,0), hexadecane (320,0), heptadecane (340,0), octadecane (360,0),
Alkanes having a straight or side chain, such as nonadecane (380,0);

Nonafluorohexane (175, 45);
Fluoroalkanes such as perfluorohexane (190, 70); trichloroethane (160, 30), pentachloroethane (240, 50), trichloropropane (1
80, 30), chloroalkane such as dichlorobutane (160, 20), trichlorobutane (200, 30); tribromoethane (220, 30), pentabromoethane (340, 50), dibromopropane (180, 2).
0), tribromopropane (240, 30), dibromobutane (200, 20), tribromobutane (260,
Bromoalkanes such as 30); triiodoethane (28
0,30), pentaiodoethane (440,50), diiodopropane (220,20), triiodopropane (300,30), diiodobutane (240,20),
Preferred are iodoalkanes such as triiodobutane (320, 30).

In the present invention, the amount of the organic compound contained in the electrolytic solution is appropriately adjusted depending on the pore characteristics and the residual water content of the carbonaceous material contained in the electrode. In addition, when an organic compound is excessively added, the ionic conductivity of the electrolytic solution tends to decrease. Therefore, it is preferable to suppress the ionic conductivity to a small range. Therefore, the organic compound is preferably contained in the electrolytic solution in an amount of 0.1 to 30% by weight, particularly 1 to 10% by weight.

The action of the organic compound added to the electrolyte in the present invention is not necessarily clear, but is presumed to be as follows.

As described above, in the pores of the activated carbon mainly constituting the electrodes of the electric double layer capacitor, moisture and organic substances which cannot be desorbed by heating or decompression treatment remain unavoidably. When a voltage is applied to the capacitor, the water and the like in the pores are electrolyzed, which is considered to be a cause of characteristic deterioration.

In the present invention, the organic compound to be added to the electrolytic solution has a higher adsorptivity to the activated carbon surface than the remaining water. It is thought that there is an effect of driving out water from the pores of the activated carbon. Where the organic compound mixed in the electrolyte solution is thought to require a certain degree of hydrophobicity in order to drive out the water in the pores of the activated carbon,
Quantitatively, it is estimated that this effect is obtained if the value of (I) / (O), which is an index of hydrophobicity, is 0.36 or less. Therefore, the organic compound to be added also needs to have an adsorbing force on the activated carbon surface, and the adsorbing force on the activated carbon is said to be dominated by the dispersing force in the van der Waals force. It is considered that the larger the size is, the larger the dispersing power becomes and the influence of the carbon number on the molecular size is large. Conceivable.

As described above, the residual moisture displaced and adsorbed by the added organic compound and expelled from the micropores is leached into the electrolyte outside the micropores, and the water is impregnated with the electrolyte. When a voltage is applied to the element element, it is considered that the element element is easily electrolyzed and gasified, and is discharged as bubbles to the outside of the element element. As a result, it is presumed that the performance degradation of the electric double layer capacitor can be suppressed slightly.

The electrode of the present invention has a specific surface area of 100 to 100 which forms an electric double layer at the interface between the positive electrode and the negative electrode with the electrolyte.
Contains 3,000 m ² / g of carbonaceous material. As the carbonaceous material, resin-based activated carbon such as phenol, coconut-based activated carbon, coke-based activated carbon, pitch-based activated carbon, carbon nanotube, carbon airgel, carbon black, polyacene, and the like can be preferably used.

The electrode may be provided with one or more of these carbonaceous materials,
It is preferable that the conductive material is added and an organic binder is added. For example, an organic binder such as polytetrafluoroethylene, polyvinylidene fluoride, carboxymethylcellulose, polyvinylpyrrolidone, polyvinyl alcohol, or polyacrylic acid is added to a carbonaceous material and kneaded in the presence of a solvent such as alcohol to form a sheet. And dried, and then joined to a current collector with a conductive adhesive or the like. Further, the powder is formed by mixing a powder of the carbonaceous material, the organic binder, and preferably a conductive material with a solvent to form a slurry, coating the slurry on a current collector, and drying the slurry.

The content of the binder in the electrode is preferably about 0.5 to 20% by weight based on the total amount of the carbonaceous material and the binder. If the amount of the binder is less than 0.5% by weight, the strength of the electrode is insufficient, and if it exceeds 20% by weight, the electric resistance increases and the capacity decreases, which is not preferable. From the viewpoint of electrode strength and capacity balance, the amount of the binder is more preferably 0.5 to 10% by weight. As the conductive material, powders of carbon black, natural graphite, artificial graphite, titanium oxide, ruthenium oxide and the like are used. Among them, it is preferable to use Ketjen black or acetylene black, which is a kind of carbon black, since the effect of improving conductivity is large even in a small amount.

The amount of the conductive material such as carbon black in the electrode is preferably 5% by weight or more, especially 10% by weight or more based on the total amount with the carbonaceous material powder so as to improve the conductivity. preferable. If the compounding amount of the conductive material is too large, the compounding ratio of the carbonaceous material decreases and the capacitance of the electrode decreases,
The amount of the conductive material in the electrode is 40% by weight or less, particularly 30% by weight.
% By weight or less.

As the solvent for forming the slurry, those capable of dissolving the above binder are preferable, and methanol, ethanol, N-methylpyrrolidone, dimethylformamide, toluene, xylene, isophorone, methyl ethyl ketone, ethyl acetate, methyl acetate, ethyl acetate, dimethyl Phthalate, isopropanol, butanol, water and the like are appropriately selected.

The current collector of the electrode may be any conductor that is electrochemically and chemically resistant to corrosion. Stainless steel, aluminum, titanium, tantalum, nickel, or the like is used as a current collector for an electrode mainly composed of a carbonaceous material. Among them, stainless steel and aluminum are preferred current collectors in terms of both performance and cost.

As the electrolyte for the electric double layer capacitor of the present invention, an organic electrolyte is used. Examples of the organic solvent used include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; sulfolane derivatives such as sulfolane and 3-methylsulfolane;
A solvent comprising at least one selected from butyrolactone, 1,2-dimethoxyethane, acetonitrile, glutaronitrile, valeronitrile, dimethylformamide, dimethylsulfoxide, tetrahydrofuran, dimethoxyethane and methylformate is preferred.

In the present invention, the electrolyte which is a solute contained in the organic electrolytic solution for forming the electric double layer is preferably a quaternary onium salt in view of electric conductivity, solubility and electrochemical stability.

In particular, R¹ R^Two R^Three R^Four N⁺Or R¹ 
R^Two R^Three R^Four P⁺(R¹ , R^Two , R ^Three , R^Four Are each
Independently, an alkyl group having 1 to 6 carbon atoms, or 6 to 10 carbon atoms
Is an aryl group. ) Quaternary onium click
On, BF_Four ^-, PF_Four ^-, PF₆ ^-, ClO_Four ^-,
AsF₆ ^-, N (SO_Two CF_Three )_Two ^-, CF_Three SO_Three ^-
And a salt comprising an anion selected from

Specifically, for example, (C ₂ H ₅ ) ₄ NB
F ₄ , (C ₂ H ₅ ) ₃ (CH ₃ ) NBF ₄ , (C ₂ H
₅ ) ₄ PBF ₄ and (C ₂ H ₅ ) ₃ (CH ₃ ) PBF ₄
And the like are preferred. The concentration of these onium salts in the electrolyte is 0.5 to 0.5 in order to secure the amount of ions necessary for forming the electric double layer and obtain sufficient electric conductivity.
It is preferably about 2 mol / l, more preferably about 1.0 to 2 mol / l.

The separator in the present invention is not particularly limited, but is preferably made of a porous material which is excellent in electric insulation and chemical stability with respect to an electrolytic solution, and has a large liquid absorbing capacity of an electrolytic solution and is excellent in liquid retention. . Specifically, glass fiber, silica fiber, alumina fiber, asbestos,
And inorganic fibers such as whiskers, natural fibers such as manila hemp, and organic fibers such as synthetic fibers such as polyolefin and polyester, and a sheet formed from these fibers is preferable. Also, polyolefins,
A microporous film in which fine holes are provided in a film made of polyester or the like by a stretching operation is also preferable.

The structure of the electric double layer capacitor of the present invention is not particularly limited. A coin-shaped structure in which a disk-shaped positive electrode and a negative electrode are opposed to each other via a separator and accommodated in a metal case together with an electrolytic solution, and a rectangular positive electrode and a negative electrode Are laminated alternately with a separator interposed therebetween, and a laminated structure in which the electrolytic solution is impregnated and accommodated in a square case, a pair of strip-shaped positive and negative electrodes are wound facing each other with the separator interposed therebetween, and the electrolytic solution is formed. Can be preferably employed, such as a wound-type structure in which a cylindrical case is impregnated and accommodated.

[0042]

Next, the present invention will be described with reference to Examples 1 to 5 and Comparative Example 1.
Although these will be described more specifically with reference to Nos. To 20, these are merely examples, and the technical scope of the present invention is not limited thereto.

Specific surface area 1,800 m ² / g, average particle size 1
After mixing 80 parts by weight of a high-purity phenol resin-based activated carbon powder of 0 μm, 10 parts by weight of carbon black as a conductive material, and 10 parts by weight of polytetrafluoroethylene powder as a binder, kneading the mixture while dripping ethanol, and rolling it with a roll, 200 After drying at 30 ° C. for 30 minutes to remove ethanol, an electrode sheet having a thickness of 140 μm was prepared.

The above-mentioned electrode sheet was joined to both sides of a portion having a width of 6 cm and a length of 13 cm of an aluminum foil current collector having a lead lead portion and having a thickness of 40 μm using a conductive adhesive, and further roll-pressed to form an electrode. An electrode body having a thickness of 330 μm in which the electrode and the current collector were integrated was produced, and this was used as a positive electrode body and a negative electrode body.

The glass fiber sheet was punched out into a rectangular shape having a width of 6.6 cm and a length of 13.6 cm to form a separator. The 18 positive electrodes and the 18 negative electrodes were alternately laminated with the separator interposed therebetween. An element body was obtained.

This element body is 15 cm in height, 7 cm in width,
It was inserted into a bottomed square-shaped aluminum metal case having a thickness of 2.2 cm. Next, the positive electrode terminal and the negative electrode electrode lead were collectively and ultrasonically welded to the positive electrode terminal and the negative electrode terminal, which were hermetically attached while being insulated from the aluminum lid. Thereafter, the lid was fitted into the opening of the metal case, and the periphery was sealed by laser welding. The lid has a small hole serving as a liquid inlet for the electrolyte.

The sealed case containing the element was dried at 200 ° C. for 72 hours in a vacuum. Next, the case is placed in a dry box (dew point −50 ° C.), the inside of the case is evacuated, and 1.5 mol / l of (C ₂ H ₅ ) ₃
(CH ₃ ) NBF ₄ was dissolved, and a propylene carbonate solution was injected as an organic electrolyte. Except for Comparative Example 1, this organic electrolyte was prepared by adding an organic compound shown in Table 1 to the electrolyte by 5% by weight.

A process of applying a voltage of 2.9 V to the cell after the injection in an open state for 72 hours was performed. The injection hole was plugged and sealed, and then taken out of the dry box to produce an electric double layer capacitor.

After measuring the initial discharge capacity and internal DC resistance of each of the obtained electric double layer capacitors, as an acceleration test of the performance reliability, the capacitors were placed in a 45 ° C. constant temperature bath at a voltage of 2.7 V. It was held for 500 hours while applying voltage, and a durability test was performed. The discharge capacity and the internal resistance after the durability test were measured, and the capacity change rate and the resistance increase rate after the durability test with respect to the initial characteristics were calculated. The results are summarized in Table 1.

[0050]

[Table 1]

It can be seen that all the organic compounds of the present invention containing an organic electrolyte have excellent durability.

Comparative Examples 13, 14, 16, and 17
Deterioration, especially the rise in internal pressure, was remarkable, and measurement after 500 hours was impossible.

When the organic values (O) and inorganic values (I) of the organic compounds used in the above Examples and Comparative Examples are plotted on an organic-inorganic graph (organic conceptual diagram), as shown in FIG. Become. As is clear from the figure, all the additives used in the examples exist in the region defined by (1) (I) / (O) ≦ 0.36 (2) (O) ≧ 160. Understand.

[0054]

According to the present invention, the withstand voltage of the electric double layer capacitor can be increased, and the electric double layer capacitor having excellent performance reliability and high energy density for a long time even when a high voltage is applied can be obtained. can get.

[Brief description of the drawings]

FIG. 1 is an organic conceptual diagram showing the values of organic and inorganic values of organic compounds used in Examples and Comparative Examples.

Claims (2)

[Claims] An element body comprising a positive electrode and a negative electrode, each comprising an electrode containing a carbonaceous material having a specific surface area of 100 to 3,000 m ² / g, opposed to each other with a separator interposed therebetween, is impregnated with an organic electrolytic solution. In an electric double layer capacitor in which electric charge is accumulated by forming an electric double layer on the surface, one kind of an organic compound having an inorganic value (I) and an organic value (O) satisfying the following two formulas is contained in the electrolytic solution. An electric double layer capacitor characterized by containing the above. (1) (I) / (O) ≦ 0.36 (2) (O) ≧ 160 2. The electric double layer capacitor according to claim 1, wherein said organic compound is contained in an amount of 0.1 to 30% by weight based on said electrolytic solution.