JP3837866B2 - Electric double layer capacitor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electric double layer capacitor having excellent charge / discharge cycle durability and durability during voltage application.
[0002]
[Prior art]
The electric double layer capacitor is an electrochemical element that can be charged / discharged with a large current, which includes a polarizable electrode, an electrolyte, a separator, and a current collector, and is promising for applications such as an electric vehicle and an auxiliary power source. Therefore, it is desired to realize an electric double layer capacitor that can be rapidly charged and discharged and has excellent durability when a voltage is applied for a long period of time and excellent charge / discharge cycle durability.
[0003]
The energy stored in the single cell of the capacitor is calculated by 1/2 · C · V ² , where C is a capacity per single cell (F), and V is a voltage (V) that can be applied to the single cell. Since the square of the value of the applicable voltage V is reflected in the energy, it is effective to increase the voltage applied to the capacitor to improve the energy density, but the electrolytic solution is decomposed at a large voltage.
[0004]
Therefore, the withstand voltage per unit cell of a conventional electric double layer capacitor is about 2.4 V in the case of an electric double layer capacitor of a non-aqueous electrolyte (Japanese Patent Laid-Open No. 7-50001), and a high voltage of 2.5 V or more When used in, an increase in internal series resistance or a decrease in capacitance occurs in a short time. Therefore, it is attempted to apply a voltage of 2.5 V to 2.8 V by examining in detail the positive and negative electrodes, separator, electrolyte, container, and the like. As for the improvement of the electrode, for example, as a result of selecting a raw material and a method for improving durability by heat-treating an electrode using activated carbon obtained by KOH activation of phenol resin, petroleum coke, etc. in an inert atmosphere, In the case of phenol resin, furan resin, and polyacrylonitrile resin, the durability was slightly improved (Japanese Patent Laid-Open No. 8-16375), and a method for improving durability by using porous aluminum for the current collector of the capacitor (special feature) No. 8-339941) is known.
[0005]
For non-aqueous electrolytes, a method is known in which the solvent of the electrolyte is sulfolane in order to improve the withstand voltage compared to the propylene-carbonate solution of tetraethylammonium tetrafluoroborate, which is a typical electrolyte (Japanese Patent Laid-Open No. Hei. 9-205041).
[0006]
[Problems to be Solved by the Invention]
However, these examples were not satisfactory to any extent. Regarding the electrode, for example, the method of heat-treating the electrode using activated carbon obtained by KOH activation of phenol resin, petroleum coke, etc., in an inert atmosphere has a problem that the initial capacitance is simultaneously reduced. . Further, it can be said that the durability cannot be fundamentally improved by the methods disclosed in JP-A-8-16375 and JP-A-8-339941. As for the electrolyte, the electric conductivity and dielectric constant of the electrolyte using sulfolane as a solvent are both smaller than that of propylene carbonate, so there is a problem that the capacitance decreases, and the capacitor is downsized and increased in capacity. Had a problem.
[0007]
In the activated carbon electrode used in the conventional electric double layer capacitor, the gas generation or the deposition of the reaction product on the polarizable electrode occurred due to the continuous application of a high voltage exceeding 2.5 V. This has the disadvantage of causing a significant increase in internal resistance or a decrease in capacitance. As for the electrolyte, the electric conductivity and dielectric constant of the electrolyte using sulfolane as a solvent are both smaller than that of propylene carbonate, so there is a problem that the capacitance decreases, and the capacitor is downsized and increased in capacity. Had a problem.
[0008]
In view of this, the present inventors, in Japanese Patent Application No. 9-183670, added a metal such as an alkali metal or an inorganic substance to the carbonaceous electrode to arbitrarily adjust the natural potential, thereby adjusting the potential at the time of charging of the electrolytic solution. It has been proposed that the decomposition of the electrolytic solution is suppressed and the voltage that can be applied and the durability of the electric double layer capacitor can be improved by making the voltage lower than the substantial decomposition start voltage on the high potential side (oxidation side).
[0009]
This will be briefly described.
When an electrode made of a substantially carbonaceous material of a quaternary alkyl ammonium salt propylene carbonate solution, which is a typical non-aqueous electrolyte, is used, the decomposition initiation voltage on the oxidation side of the electrolyte is 4.4 V (vs. Li / Li ⁺ ) is said to be near. On the other hand, the natural potential of a normal activated carbon electrode is around 3 V (vs. Li / Li ⁺ ), and when the applied voltage of the capacitor is 2.8 V, the polarization on the positive electrode side after charging is about 1.4 V, and the potential on the oxidation side is 4.4V (vs. Li / Li ⁺ ) or higher, indicating that electrochemical decomposition of the electrolyte occurs. As a result, when a conventional activated carbon electrode is used, the capacity decreases due to gas generated by the decomposition of the electrolytic solution, and thus there has been a problem in durability when used for a long time. When the current electric double layer capacitor is used at an applied voltage of 2.5 V or more, the durability is low because of the relationship between the potential change of the positive and negative electrodes of the capacitor and the decomposition voltage of the electrolyte. Therefore, in the invention of Japanese Patent Application No. 9-183670, by reducing the natural potential of the activated carbon electrode, the potential on the positive electrode side after charging was made lower than the oxidative decomposition starting voltage of the electrolytic solution, and the effect of improving the durability was obtained.
However, an appropriate combination of the natural potential of the carbonaceous electrode and the non-aqueous electrolyte composition, which shows high capacity and high durability, has not been known.
[0010]
[Means for Solving the Invention]
Therefore, the present inventors diligently studied to examine the above problems, and as a result, in an electric double layer capacitor having an applied voltage of 2.5 V or more, polarizability in which a natural potential is adjusted by adding a metal such as an alkali metal. Combined with an electrode body and an electrolyte solution with high electrical conductivity and high withstand voltage, and an electrochemically stable electrolyte solution that does not react with metals or inorganic substances in the electrode body, it has a high capacity and excellent durability. The inventors have found that a double layer capacitor can be obtained and have reached the present invention. That is, an object of the present invention is to provide an electric double layer capacitor that is excellent in durability and high charge / discharge durability when a high voltage of 2.5 V or higher is applied, and has a large capacitance. The solvent of the liquid is mainly composed of a mixture of propylene carbonate and ethylene carbonate, and a polarizable electrode body made of a carbonaceous material is used. Further, by incorporating lithium by an electrochemical method, its natural potential is reduced. It is easily achieved by setting it to 1.7 V or more and 2.7 V or less (vs. Li / Li ⁺ ).
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The most significant feature of the present invention is that the solvent of the electrolyte is mainly a mixture of propylene carbonate and ethylene carbonate, and the natural potential of the polarizable electrode body is 1.7 V or more and 2.7 V or less (vs. Li / Li ⁺ ). Therefore, when a high voltage of 2.5 V or higher is applied, a high capacitance is exhibited, and durability and long-term durability when a voltage is applied for a long time are greatly improved.
[0012]
Specifically, the present invention relates to an electric double layer capacitor in which an applied voltage using a non-aqueous electrolyte and a polarizable electrode body at both electrodes is 2.5 V or more, and the solvent of the electrolyte is a mixture of propylene carbonate and ethylene carbonate. And the natural potential of the polarizable electrode body in the electrolyte is 1.7 V or more and 2.7 V or less (Li / Li ⁺ ).
[0013]
The measurement of the natural potential of the polarizable electrode body in the present invention is performed using a normal electrochemical technique. In non-aqueous potential measurement, a potential reference such as a standard hydrogen electrode in an aqueous solution is not strictly defined, but in practice, an electrode such as a silver-silver chloride electrode, a platinum electrode, or a lithium electrode is used. Generally done widely. In the present invention, it can be measured by the same method.
[0014]
As the polarizable electrode body used in the present invention, a carbonaceous electrode is particularly preferable. However, with pure carbon, the natural potential does not fall within the range of 1.7 V to 2.7 V, so some adjustment is required.
The method for adjusting the natural potential of the carbonaceous electrode to 1.7 V or more and 2.7 V or less (vs. Li / Li ⁺ ) is not particularly limited, but it is possible to add at least one substance selected from inorganic substances (including metals). preferable. For inorganic materials, ruthenium oxide, platinum oxide, osmium oxide, iridium oxide, tin oxide, manganese oxide, titanium oxide, vanadium oxide, chromium oxide, strontium oxide, tungsten oxide, cobalt oxide , Nickel oxide, zinc oxide, cadmium oxide, copper oxide, iron oxide, niobium oxide, molybdenum oxide, rhenium oxide, rhodium oxide, lithium oxide, rare earth oxide, ruthenium composite oxide , Platinum composite oxide, osmium composite oxide, iridium composite oxide, tin composite oxide, manganese composite oxide, titanium composite oxide, vanadium composite oxide, chromium composite oxide, strontium composite oxide, tungsten composite oxide , Cobalt composite oxide, nickel composite oxide, zinc composite oxide, cadmium Complex oxide, copper complex oxide, iron complex oxide, niobium complex oxide, molybdenum complex oxide, rhenium complex oxide, rhodium complex oxide, lithium complex oxide, rare earth complex oxide, at least one Although it is possible to use a semiconductor oxide or a conductive oxide composed of two or more metal oxides or composite oxides, it is simpler and more effective to use a metal. The state of the inorganic substance is not particularly limited as long as the natural potential of the electrode body changes, whether it is ionized or not. The natural potential of the carbonaceous electrode can be adjusted to 1.7 V or more and 2.7 V or less (vs. Li / Li ⁺ ) by introducing a base metal into the carbonaceous electrode. In particular, a substance containing an alkaline metal element such as lithium, sodium, potassium, rubidium and cesium having a lower natural potential than a carbonaceous substance, an alkaline earth metal such as calcium and magnesium, and a rare earth metal such as yttrium and neodymium is preferable. In particular, a substance containing lithium element that exhibits the lowest potential is more preferable. The substance containing lithium is not particularly limited. For example, lithium-containing alloys such as lithium-aluminum alloys and lithium-magnesium alloys, lithium intermetallic compounds, manganese-containing lithium oxides, cobalt oxides, nickel It is preferable to use at least one substance selected from oxides, composite oxides such as vanadium oxide, titanium sulfide containing lithium, chalcogenite such as niobium selenide and molybdenum sulfide, and carbon containing lithium.
[0015]
These inorganic substances are not particularly limited, but can be added to the electrode body by an electrochemical technique, a chemical technique, a physical technique, or the like. For example, as one simple method, an electrochemical cell composed of a lithium-containing electrode, a carbonaceous electrode, a separator, and a non-aqueous electrolyte made of metallic lithium or a substance containing lithium having a very low potential on the positive electrode side The lithium-containing electrode and the carbonaceous electrode can be short-circuited, or lithium can be introduced into the carbonaceous electrode by charging with the lithium-containing electrode as a positive electrode and the carbonaceous electrode as a negative electrode. Further, the lithium content in the carbonaceous electrode is about 0.01% by weight to 2% by weight, although it varies depending on the bulk density, specific surface area, surface properties, etc. of the carbonaceous material.
[0016]
An electric double layer capacitor is assembled using the carbonaceous electrode to which the metal or inorganic substance thus obtained is added as both electrodes.
In order to increase the capacity of the electric double layer capacitor, it is preferable to use activated carbon having a large specific surface area for the electrode of the carbonaceous material before adding a metal or the like. If the specific surface area of the activated carbon is too large, the bulk density is lowered and the energy density is lowered. Therefore, it is preferably 200 m ² / g or more and 3000 m ² / g or less, more preferably 300 m ² / g or more and 2300 m ² / g. It is as follows. As raw materials for activated carbon, plant-based wood, sawdust, coconut husk, pulp waste liquor, fossil fuel-based coal, heavy petroleum oil, or pyrolyzed coal, petroleum-based pitch, and tar pitch are spun. They are used in a wide variety of applications, including fibers, synthetic polymers, phenolic resins, furan resins, polyvinyl chloride resins, polyvinylidene chloride resins, polyimide resins, polyamide resins, liquid crystal polymers, plastic waste, and waste tires. These raw materials are activated after carbonization, and activation methods are roughly classified into gas activation and chemical activation. The gas activation method is also called physical activation while chemical activation is chemical activation, and the carbonized raw material is contacted with water vapor, carbon dioxide, oxygen, other oxidizing gases, etc. at high temperatures. Activated carbon is obtained. The chemical activation method is a method in which an activated chemical is impregnated uniformly in a raw material, heated in an inert gas atmosphere, and activated carbon is obtained by dehydration and oxidation reaction of the chemical. Examples of chemicals used include zinc chloride, phosphoric acid, sodium phosphate, calcium chloride, potassium sulfide, potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, sodium sulfate, potassium sulfate, and calcium carbonate. Various methods for producing activated carbon have been described above, but there is no particular limitation. The activated carbon has various shapes such as crushing, granulation, granule, fiber, felt, woven fabric, and sheet shape, any of which can be used in the present invention. Among these activated carbons, activated carbon obtained by chemical activation using KOH is particularly preferred because it tends to have a larger capacity than steam activated products.
[0017]
The activated carbon after the activation treatment is heat-treated at 500 to 2500 ° C., preferably 700 to 1500 ° C. in an inert atmosphere such as nitrogen, argon, helium, xenon, etc. to remove unnecessary surface functional groups, The electronic conductivity may be increased by developing the sex.
Furthermore, heat treatment of activated carbon in a gas containing ammonia, hydrogen, water vapor, carbon dioxide, oxygen and air introduces heteroatoms such as hydrogen, oxygen and nitrogen into the carbon skeleton and introduces surface functional groups. Thus, the natural potential may be controlled.
[0018]
In the case of granular activated carbon, the average particle diameter is preferably 30 μm or less in terms of improving the bulk density of the electrode and reducing the internal resistance.
A polarizable electrode mainly composed of activated carbon is composed of activated carbon, a conductive agent and a binder. The polarizable electrode can be formed by a conventionally known method. For example, it can be obtained by adding and mixing polytetrafluoroethylene to a mixture of activated carbon and acetylene black, followed by press molding. Moreover, it is also possible to sinter only activated carbon without using a conductive agent and a binder to form a polarizable electrode. The electrode may be a thin coating film, a sheet-shaped or plate-shaped molded body, or a plate-shaped molded body made of a composite.
[0019]
The conductive agent used for the polarizable electrode is selected from the group consisting of carbon black such as acetylene black and ketjen black, natural graphite, thermally expanded graphite, carbon fiber, ruthenium oxide, titanium oxide, aluminum, nickel, and other metal fibers. At least one conductive agent is preferred. Acetylene black and ketjen black are particularly preferable in that the conductivity is effectively improved in a small amount. Particularly when activated carbon is used as the polarizable electrode, the blending amount varies depending on the bulk density of the activated carbon, but if too much, Since the ratio decreases and the capacity decreases, it is preferably about 5 to 50%, particularly about 10 to 30% of the weight of the activated carbon.
As the binder, at least one of polytetrafluoroethylene, polyvinylidene fluoride, carboxymethylcellulose, fluoroolefin copolymer crosslinked polymer, polyvinyl alcohol, polyacrylic acid, polyimide, petroleum pitch, coal pitch, and phenol resin is used. preferable.
[0020]
The current collector need only be electrochemically and chemically corrosion resistant, and is not particularly limited. For example, there are stainless steel, aluminum, titanium, and tantalum for the positive electrode, and stainless steel, nickel, copper, and the like are suitable for the negative electrode. Used for.
The solute of the nonaqueous electrolytic solution is not particularly limited, but R ₄ N ⁺ , R ₄ P ⁺ (where R is an alkyl group represented by C _n H _{2n + 1} ), triethylmethylammonium ion, etc. It is preferable to use a salt in which a quaternary onium cation, an alkali metal cation such as lithium ion or potassium ion, and an anion of BF ₄ ⁻ , PF ₆ ⁻ , ClO ₄ ⁻ , or CF ₃ SO ₃ ⁻ are combined. The concentration of these salts in the non-aqueous electrolyte solution is preferably 0.1 to 2.5 mol / liter, particularly 0.3 to 2.0 mol / liter so that the characteristics of the electric double layer capacitor can be sufficiently extracted.
[0021]
As the solvent for the non-aqueous electrolyte, a solvent mainly composed of a mixed solvent of propylene carbonate and ethylene carbonate is used. The mixing ratio of propylene carbonate and ethylene carbonate is not particularly limited, but the volume of ethylene carbonate is preferably 0.1 or more and 4 or less with respect to volume 1 of propylene carbonate. In addition, in order to improve the electrical conductivity, electrochemical stability, and chemical stability of the electrolyte, a mixed solvent of propylene carbonate and ethylene carbonate is used in butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane. One or more organic solvents selected from methylsulfolane, γ-butyrolactone, γ-valerolactone, N-methyloxazolidinone, dimethyl sulfoxide, trimethyl sulfoxide, acetonitrile, and propionitrile may be added. In order to obtain a high withstand voltage, the water content in the non-aqueous electrolyte is preferably 200 ppm or less, more preferably 50 ppm or less.
[0022]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated with a specific Example, this invention is not limited by a following example.
Example 1
First, a method for adjusting the natural potential of the carbonaceous electrode will be described.
After kneading a mixture of 80% by weight of phenol resin activated carbon powder (specific surface area 1900m ² / g, average particle size 10μm) obtained by KOH activation treatment, 10% by weight of acetylene black and 10% by weight of polytetrafluoroethylene Using a tablet molding machine manufactured by JASCO Corporation, a disk-shaped molded body was obtained by pressure molding with a hydraulic press at a pressure of 50 kgf / cm ² so as to have a diameter of 10 mm and a thickness of 0.5 mm. This molded body was dried in a vacuum of 0.1 torr or less at 300 ° C. for 3 hours to obtain an electrode body. After putting a polyethylene separator made by Mitsubishi Chemical between two electrodes made by this method, sandwich the whole with two platinum plates used for the current collector, and the current collector, activated carbon electrode, and separator are in good contact As described above, an open cell type capacitor was assembled by sandwiching two Teflon plates having a thickness of 5 mm and four bolt holes from the outermost side. The thus obtained open cell capacitor and a lithium electrode produced by pressing a metal lithium foil on the tip of a platinum plate were immersed in a 1 mol / liter LiBF4 ₄ propylene carbonate solution in a beaker. Next, the lithium electrode and the activated carbon electrode were connected by a lead wire and short-circuited for about 1 hour. After being short-circuited, the natural potential of the activated carbon electrode measured by connecting a voltmeter between the activated carbon electrode and the lithium electrode was 2.26 V (vs. Li / Li ⁺ ).
[0023]
Next, a method for producing an electric double layer capacitor will be described. Two activated carbon electrodes with a natural potential of 2.26V obtained by the above method are fully impregnated with a solution of (C ₂ H ₅ ) ₄ NBF ₄ in propylene carbonate + ethylene carbonate (volume mixing ratio 1 + 1) at a concentration of 1 mol / liter. Each of these was used as a positive electrode and a negative electrode, and a polyethylene separator was placed between the two electrodes to assemble a coin-type cell as shown in FIG. 1 to obtain an electric double layer capacitor. In this case, the container is a current collector. To the obtained electric double layer capacitor, a voltage of 2.8 V was applied for 1 hour at room temperature using a charge / discharge device “HJ201-B” manufactured by Hokuto Denko, and then the initial static charge obtained by constant current discharge at 1.16 mA was obtained. The electric capacity was 1.90F. In order to evaluate the long-term operational reliability of the capacitor under voltage application in an accelerated manner, the rate of change in capacitance after 500 hours when a voltage of 2.8V was applied and the capacitor was placed in a thermostat at 70 ° C Showed -2.8%.
[0024]
(Example 2)
An electric double layer capacitor similar to that of Example 1 was constructed except that a short-circuit treatment between the lithium electrode and the activated carbon electrode was performed for 30 minutes. The natural potential of the activated carbon electrode was 2.58V. The initial capacitance of the electric double layer capacitor was 1.88F. The capacitance change rate after 500 hours was -2.7%.
[0025]
(Comparative Example 1)
An electric double layer capacitor was constructed in the same manner as in Example 1 except that the electrolytic solution of the electric double layer capacitor was a propylene carbonate solution of (C ₂ H ₅ ) ₄ NBF _{4 having} a concentration of 1 mol / liter. The initial capacitance of the electric double layer capacitor was 1.79F. The capacitance change rate after 500 hours was -5.0%.
[0026]
(Comparative Example 2)
An electric double layer capacitor was constructed in the same manner as in Example 2 except that the electrolytic solution of the electric double layer capacitor was a propylene carbonate solution of (C ₂ H ₅ ) ₄ NBF _{4 having} a concentration of 1 mol / liter. The initial capacitance of the electric double layer capacitor was 1.81F. The capacitance change rate after 500 hours was -4.4%.
[0027]
(Comparative Example 3)
An electric double layer capacitor similar to that of Example 1 was configured except that the short-circuit treatment between the lithium electrode and the activated carbon electrode was not performed. The natural potential of the activated carbon electrode was 3.05V. The initial capacitance of the obtained electric double layer capacitor was 1.89F. The capacitance change rate after 500 hours was -13%.
[0028]
(Comparative Example 4)
Example 1 except that the short-circuit treatment between the lithium electrode and the activated carbon electrode was not performed, and the electrolytic solution of the electric double layer capacitor was a (C ₂ H ₅ ) ₄ NBF ₄ propylene carbonate solution having a concentration of 1 mol / liter. Similarly, an electric double layer capacitor was constructed. The natural potential of the activated carbon electrode was 3.05V. The initial capacitance of the obtained electric double layer capacitor was 1.83F. The capacitance change rate after 500 hours was -23%.
[0029]
(Comparative Example 5)
The electric double layer capacitor was the same as in Example 2 except that the electrolyte of the electric double layer capacitor was a solution of (C ₂ H ₅ ) ₄ NBF ₄ in sulfolane + ethylene carbonate (volume mixing ratio 1 + 1) at a concentration of 1 mol / liter. Configured. The initial capacitance of the electric double layer capacitor was as low as 1.55F.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a coin-type cell used in Example 1. FIG.
[Explanation of symbols]
1 Stainless steel container case 2 Activated carbon molded body 3 Gasket 4 Separator 5 Activated carbon molded body 6 Upper cover of stainless steel container

Claims (4)

In the electric double layer capacitor using the polarizable electrode body comprising a carbonaceous material in a non-aqueous electrolyte and both electrodes, wherein the solvent of the electrolyte solution is composed mainly of a mixture of propylene carbonate and ethylene carbonate, and polarizable electrode body there so also the lithium is contained by electrochemical technique, natural potential to counter the Li / Li ⁺ in the electrolytic solution is 1.7V
The electric double layer capacitor is characterized by being 2.7 V or less.   The electric double layer capacitor according to claim 1, wherein the carbonaceous material is activated carbon. 3. The electric double layer capacitor according to claim 1, wherein the carbonaceous material is activated carbon having a specific surface area of 300 m ² / g or more and 2300 m ² / g or less. The electrochemical method of containing lithium in the polarizable electrode body shorts the metal lithium or the substance containing lithium and the polarizable electrode body in the non-aqueous electrolyte, or charges the former as the positive electrode and the latter as the negative electrode The electric double layer capacitor according to claim 1, wherein the electric double layer capacitor is a method.

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