JP3800810B2 - Electric double layer capacitor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electric double layer capacitor that has excellent charge / discharge cycle durability and durability when a high voltage is applied, and has a large energy density.
[0002]
[Prior art]
Electric double layer capacitors that can be charged and discharged with a large current are promising for applications such as electric vehicles and auxiliary power supplies. Therefore, it is desired to realize an electric double layer capacitor having a high energy density, capable of rapid charge / discharge, and excellent durability during high voltage application and charge / discharge cycle durability.
[0003]
The energy stored in the capacitor cell is 1/2 · C · V.²Where C is the capacity per cell (F), and V is the voltage (V) that can be applied to the 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 in order to improve the energy density, but the electrolytic solution is decomposed at a large voltage.
[0004]
Therefore, the withstand voltage per unit cell is about 2.4 V in the case of a non-aqueous electrolyte electric double layer capacitor, although it depends on the type of solvent and solute of the electrolyte used in the conventional electric double layer capacitor. (Japanese Patent Laid-Open No. 7-14001), when used at a high voltage of 2.5 V or more, there is a problem that an increase in internal series resistance or a decrease in capacitance occurs in a short time.
[0005]
Therefore, it has been 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. For example, 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, and selecting raw materials, phenol resin, furan resin, poly Durability is improved by using a baked product such as acrylonitrile resin (Japanese Patent Laid-Open No. Hei 8-162375), and a method for improving durability by using porous aluminum for the current collector of the capacitor (Japanese Patent Laid-Open No. Hei 8-339994). Etc.) are known.
[0006]
[Problems to be solved by the invention]
However, none of these examples was satisfactory to some extent. For example, the method of heat-treating an electrode using activated carbon obtained by KOH activation of the above-mentioned phenol resin, petroleum coke, etc. in an inert atmosphere has a problem that the initial capacitance is simultaneously reduced. In addition, it can be said that the methods of JP-A-8-162375 and JP-A-8-339941 cannot fundamentally improve the durability. For the above reasons, it is practically impossible to set the applied voltage per unit cell to 3 V or more, which is a large barrier to the energy density.
[0007]
As a method of setting the applied voltage to 3 V or higher, JP-A-8-107068 discloses an electrolyte containing graphite containing lithium absorbed by contacting a lithium foil as a negative electrode, activated carbon as a positive electrode, and lithium ions as a solute. However, since this negative electrode is a non-polarizable electrode and involves an oxidation-reduction reaction between the electrode and the electrolytic solution, there is a problem in durability. In addition, since the negative electrode contains lithium, the positive electrode (polarizable electrode) is already about 3 V in an uncharged state, and the potential change due to charging when a voltage up to 4.3 V is applied as in the example described is It becomes about 1.3V. Therefore, the energy density when used as a capacitor is smaller than that of a normal capacitor.
For these reasons, there has been a demand for an electric double layer capacitor having a high energy density such that the charging potential of the positive electrode is equal to or lower than the decomposition voltage of the electrolytic solution and the potential difference due to charging and discharging is large, for example, 3 V or higher.
[0008]
[Means for Solving the Problems]
Therefore, as a result of intensive studies to solve the above-mentioned problems, the present inventors have drastically different from the conventional methods that make it difficult to decompose the electrolytic solution and reduce impurities in the electrode. As a simple solution, by arbitrarily adjusting the natural potential of the electrode, it is possible to fully use the range in which the decomposition due to oxidation or reduction of the electrolytic solution occurs, and as a result, the capacitance of the capacitor has changed. However, the present inventors have found that the energy density can be increased and reached the present invention. Such a concept has never been used in a capacitor that does not perform a redox reaction.
[0009]
An object of the present invention is to provide an electric double layer capacitor having a high energy density capable of greatly increasing the voltage capable of being charged / discharged as never before, and further to charge / discharge cycle durability and high voltage. An object of the present invention is to provide an electric double layer capacitor excellent in durability when applied.
[0011]
  That is,The gist of the present invention is an electric double layer capacitor using a non-aqueous solvent electrolyte and polarizable electrodes on both electrodes, and the polarizable electrode bodyConsists of activated carbon with lithium added by an electrochemical method.,And,LithiumBefore addingSelfThe potential is E1 [V],LithiumAdditiondidThe subsequent natural potential is E2 [V]., ElectricThe substantial decomposition start voltage on the oxidation side or high potential side of the solution is expressed as a [V]., ElectricWhen the substantial decomposition start voltage on the reducing side or low potential side of the liquid solution is b [V],
E1> (a + b) /2
a+ B-E1 <E2 <E1
Formula ofIt exists in the electric double layer capacitor characterized by satisfy | filling.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in detail below.
The greatest feature of the present invention is that in a non-aqueous electrolyte-based electric double layer capacitor using a polarizable electrode body, a polarizable electrode body in which a natural potential generated when the electrode is immersed in the non-aqueous electrolyte is adjusted as a positive electrode. More preferably, by using it for the positive electrode and the negative electrode, it is possible to greatly increase the voltage that can be charged and discharged, and to provide an electric double layer capacitor having a high energy density. In addition, since the potential at the time of charging does not exceed the decomposition voltage of the electrolyte, the charge / discharge cycle durability and the durability at the time of applying a high voltage are greatly improved.
[0013]
First, the principle will be described.
According to the study by the present inventors, the durability of the current electric double layer capacitor is not improved, and the chargeable voltage is limited to 2.8 V. The potential change of the positive electrode and the negative electrode of the electric double layer capacitor And the decomposition voltage of the electrolyte.
The decomposition voltage of the electrolytic solution varies depending on the solvent, solute, etc. of the electrolytic solution, but as a representative of those that are difficult to decompose, for example, in the case of a propylene carbonate solution of a quaternary alkyl ammonium salt, an electrode that is substantially composed of a carbonaceous material is used. When used, the decomposition start voltage on the oxidation side is 4.4 V (vs. Li / Li⁺) It is said to be near. On the other hand, the natural potential of a normal carbonaceous electrode is around 3 V (vs. Li / Li⁺Therefore, the potential on the oxidation side when the potential difference between the positive and negative electrodes due to polarization after charging is 2.8 V or more is 4.4 V (vs. Li / Li).⁺It is considered that the electrochemical decomposition of the electrolyte occurs. As a result, when a conventional carbon electrode is used, the capacity decreases due to gas generated by the decomposition of the electrode solution, and thus there is a problem in durability when used for a long time. That is, in principle, the voltage that can be applied to the electric double layer capacitor is about 2.8 V at the maximum, and since it is impossible to apply a voltage higher than this, there is a limit to improving the energy density.
[0014]
Therefore, the present inventors have used as the polarizable electrode body an electrode having a natural potential such that the potential at the time of charging reaches a potential at which a substantial decomposition start voltage of the electrolyte is generated on the high potential side (oxidation side). It has been found that by using it, it is possible to increase the potential difference during charging and discharging, and it is possible to realize an unprecedented electric durability with excellent durability and a remarkably high energy density by increasing the voltage that can be applied.
[0015]
Theoretically, as a polarizable electrode body, the natural potential is a substantial decomposition starting voltage on the oxidation side or high potential side of the nonaqueous solvent electrolyte, and a substantial potential on the reduction side or low potential side of the electrolyte. By using the one closer to the center of the decomposition start voltage, it becomes possible to maximize the potential difference during charge / discharge even when the same electrolyte is used, and to obtain a durable electric double layer capacitor. It is considered possible. However, in actuality, the performance or durability of the electric double layer capacitor, such as when gas is generated due to decomposition of the electrolyte, etc., when the potential during charging is higher than the decomposition start voltage on the high potential side (oxidation side) On the other hand, when the potential is lower than the decomposition start voltage on the low potential side (reduction side), there is a tendency that the decomposition is suppressed by forming a passive state around the electrode. Therefore, the natural potential of the polarizable electrode body is a substantial decomposition start voltage on the oxidation side or high potential side of the nonaqueous solvent electrolyte and a substantial decomposition start voltage on the reduction side or low potential side of the electrolyte. It is preferably near the center or less. Specifically, the natural potential of the polarizable electrode body is E [V], the substantial decomposition start voltage on the oxidation side or high potential side of the electrolytic solution is a [V], the reduction side or low potential side of the electrolytic solution. When the substantial decomposition start voltage is b [V],
(A + b) /2-1.0≦E≦ (a + b) /2+0.2
It is preferable that
[0016]
As the polarizable electrode body of the present invention, any electrode may be used as long as the natural potential satisfies the above-mentioned conditions, and an electrode composed of at least one substance selected from carbonaceous substances and inorganic substances is usually used. In addition, when the natural potential of a substance used as a polarizable electrode is outside the above conditions, the natural potential is adjusted by adding at least one substance selected from metals and inorganic substances to the polarizable electrode. Also good.
[0017]
When the natural potential of the polarizable electrode is adjusted by adding a substance in this way, the polarizable electrode body is used to which at least one substance selected from metals and inorganic substances is added, and The natural potential of the polarizable electrode body before addition is E1 [V], the natural potential after addition is E2 [V], and the substantial decomposition start voltage on the oxidation side or high potential side of the electrolyte is a [V]. And b [V] is a substantial decomposition start voltage on the reducing side or low potential side of the electrolyte solution,
(1) When E1> (a + b) / 2, E2 is a + b−E1 <E2 <E1
(2) When E1 <(a + b) / 2, E2 is a + b−E1> E2> E1
To satisfy the conditional expression of
[0018]
The decomposition initiation voltage of the electrolyte is measured by a normal electrochemical method such as cyclic voltammetry. In the present invention, a region having a very small redox current (for example, 0.1 mA / cm by cyclic voltammetry).²When exceeding the following range), it is considered that the electrolyte substantially starts to decompose, and this voltage is set as a substantial decomposition start voltage of the electrolyte. Although it cannot generally be said depending on measurement conditions such as temperature, in the case of a typical non-aqueous electrolyte described later, a substantial decomposition starting voltage from room temperature to 70 ° C. is 4.2 V to 4.5 V (on the oxidation side) Li / Li⁺), 0.1 V to 0.2 V on the reduction side (vs. Li / Li)⁺)
[0019]
The measurement of the natural potential of the polarizable electrode body is also 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. For example, if the substantial decomposition starting voltage of the non-aqueous electrolyte is as described above, the natural potential of the polarizable electrode is 1.5 V or more and less than 2.8 V (vs. Li / Li⁺Is preferably 1.7 V or more and less than 2.7 V (vs. Li / Li)⁺Is more preferable. Specifically, in the case of an electric double layer capacitor using a propylene carbonate solution of a quaternary alkyl ammonium salt as an electrolytic solution and a carbonaceous electrode having a natural potential adjusted to around 2.3 V as a polarizable electrode body, about 4 V Even when a large voltage is applied, the voltage drops below the decomposition start voltage on the oxidation side of the electrolyte, and the electrolyte does not decompose even at a high voltage. Charge / discharge cycle durability and durability when a high voltage is applied are greatly improved. The In addition, since the voltage that can be applied is increased from about 2.8V to about 4V, the maximum usable energy density of the electric double layer capacitor is greatly increased to about 2 to 6 times that of the conventional one. Therefore, the practical value is remarkable as an electric double layer capacitor which is used for a power source of an electric vehicle or the like and requires high energy density and durability.
[0020]
The polarizable electrode before adding at least one substance selected from metals and inorganic substances is preferably substantially composed of at least one substance selected from carbonaceous substances and inorganic substances. Carbon materials include activated carbon, activated carbon plant-based wood, sawdust, coconut husk, pulp waste liquor, fossil fuel-based coal, petroleum heavy oil, or pyrolyzed coal and petroleum-based tar and pitch, petroleum Coke, coal coke, tar pitch spun fiber, carbon fiber, natural graphite, artificial graphite, anthracite, fluid coke mesocarbon microbead, carbon black, fine graphite fiber, carbon aerogel, synthetic polymer, phenol resin, furan Resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, polyamide resin, polyacrylonitrile, liquid crystal polymer, plastic waste, waste tire, fullerene, carbon nanotube, and the like can be used. Inorganic materials include 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 At least one selected from a composite oxide, a copper composite oxide, an iron composite oxide, a niobium composite oxide, a molybdenum composite oxide, a rhenium composite oxide, a rhodium composite oxide, a lithium composite oxide, and a rare earth composite oxide A semiconductor oxide or a conductive oxide made of two or more metal oxides or metal composite oxides can be used. Among these, in particular, it is more preferable to use activated carbon having a large solid-liquid interface for expressing the electric double layer capacity, which is electrochemically and chemically relatively stable, for the carbonaceous material.
[0021]
The substance to be added is not particularly limited as long as the natural potential of the polarizable electrode body is changed by the addition. As a substance to be added, at least one selected from metals and inorganic substances may be used. 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 At least one selected from 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 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 metal and the inorganic substance is not particularly limited as long as the natural potential of the electrode body changes, whether it is ionized or not. A base metal can be added to lower the natural potential, and a noble metal can be added to increase the natural potential. For example, it is more effective to reduce the natural potential in current electrolytes. In this case, additives include alkali metals such as lithium, sodium, potassium, rubidium and cesium, alkaline earth metals such as calcium and magnesium, and yttrium. In addition, a rare earth metal such as neodymium or a substance containing these metals can be used. Among them, a substance containing an alkali metal element is preferable, and a substance containing a lithium element exhibiting a very basic potential is more preferable.
[0022]
  When the polarizable electrode is substantially a carbonaceous material, when lowering the natural potential, a base such as lithium or potassium that is easily occluded into carbon is used.NaA metal having a potential is preferably used. These metals are not particularly limited, but can be added to the electrode body by an electrochemical method, a chemical method, a physical method, or the like. For example, as one of the simple methods, a lithium-containing metal made of lithium or a metal containing lithium having a very low potential on the positive electrode side, a carbon electrode, a separator, and a non-aqueous electrolyte on the negative electrode side. Short chemical cellTangleThus, lithium can be added to the carbon electrode.
[0023]
Also, by introducing heteroatoms such as boron, nitrogen and oxygen into the carbon skeleton, or by introducing surface functional groups such as amino groups, carboxyl groups, phenol groups, carbonyl groups and sulfone groups into the carbon. Although it is conceivable to adjust the natural potential, it is more effective to adjust the natural potential by adding a metal as described above.
[0024]
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.²/ G is preferred, more preferably 300-2300 m²/ G. The raw materials for activated carbon include plant-based wood, sawdust, coconut husk, pulp waste liquor, fossil fuel-based coal, heavy petroleum oil, or pyrolyzed coal, petroleum pitch, and tar pitch fiber. , Synthetic polymer, phenolic resin, furan resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, polyamide resin, liquid crystal polymer, plastic waste, waste tire, etc. 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 uniformly impregnated 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, and any of them can be used in the present invention. Among these activated carbons, activated carbon obtained by chemical activation using KOH is particularly preferable because it tends to have a larger capacity than a steam activated product.
[0025]
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.
[0026]
As for the particle diameter of the activated carbon powder, the average particle diameter is preferably about 3 to 40 μm. If the average particle size exceeds 40 μm, the amount of binder can be reduced, but the internal resistance of the electric double layer capacitor increases due to factors such as a decrease in the impregnation property of the electrolytic solution into the activated carbon particles. When the average particle size is less than 3 μm, it is necessary to add about 70% by weight or more of a binder to obtain sufficient strength of the molded product, and the capacity of the electric double layer capacitor is reduced.
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 internal resistance.
[0027]
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 into a polarizable electrode without using a conductive agent and a binder. 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.
[0028]
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, and the blending amount with activated carbon varies depending on the bulk density of the activated carbon, but if the amount is too large, the proportion of activated carbon decreases and the capacity decreases. The weight of activated carbon is preferably 5 to 50%, particularly about 10 to 30%.
[0029]
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.
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.
[0030]
In order to obtain a capacitor having a large voltage that can be applied and a large energy density, the electrolyte is a non-aqueous electrolyte. The solute of the non-aqueous electrolyte solution is not particularly limited, but R_FourN⁺, R_FourP⁺(However, R is C_nH_{2n + 1}A quaternary onium cation composed of triethylmethylammonium ion and the like, an alkali metal cation such as lithium ion and potassium ion, and BF_Four ^-, PF₆ ^-, ClO_Four ^-Or CF_ThreeSO_Three ^-It is preferable to use a salt in combination with an anion. The concentration of these salts in the non-aqueous electrolytic 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. The solvent of the non-aqueous electrolyte solution is not particularly limited, but propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, γ-butyrolactone, γ-valerolactone, An organic solvent composed of one or more selected from N-methyloxazolidinone, dimethyl sulfoxide, and trimethyl sulfoxide is preferable. One or more kinds selected from propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, and γ-butyrolactone from the viewpoint of excellent electrochemical and chemical stability and electrical conductivity The organic solvent is particularly preferred. 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. Polyethylene oxide, polyvinylidene fluoride, polyvinylidene chloride, polyvinyl chloride, polyvinyl pyrrolidone, polyvinyl pyrrolidone-polyvinyl acetate copolymer, polyvinylidene fluoride-hexafluoropropylene, poly-1,5-diaminoanthquinone, polyaniline, An electric double layer capacitor containing a polymer electrolyte obtained by mixing a polymer such as polypyrrole or polythiophene with the above non-aqueous electrolyte is also encompassed in the present invention.
[0031]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated with a specific Example, this invention is not limited by a following example.
(Example 1)
Coal-based activated carbon powder (specific surface area 2270m) obtained by KOH activation treatment²/ G, average particle size 10 μm) After kneading a mixture of 80% by weight, acetylene black 10% by weight, polytetrafluoroethylene 10% by weight, using a tablet press made by JASCO, diameter 10 mm, thickness 50kg weight / cm to be 0.5mm²A disk-shaped molded body was obtained by pressure molding under the pressure of The molded body was dried at 300 ° C. for 3 hours in a vacuum of 0.1 torr or less to obtain an electrode body. Using the obtained electrode body mainly composed of activated carbon, a coin-type cell as shown in FIG. 1 was assembled in an argon atmosphere. LiBF with a concentration of 1 mol / liter on the activated carbon electrode body as the positive electrode 2_FourAfter being sufficiently impregnated with the propylene carbonate solution, it was adhered to the inner bottom of the stainless case 1. Furthermore, the above-mentioned LiBF_FourAfter pouring the propylene carbonate solution into case 1, a polyethylene separator 3 made by Mitsubishi Chemical Co., polypropylene gasket 4 and a negative electrode 5 having a metal lithium having a diameter of 10 mm and a thickness of 0.5 mm are obtained. Further, after overlaying the stainless steel upper lid 6, the coin-type container cell was caulked and sealed.
[0032]
In order to lower the natural potential of the activated carbon electrode, lithium was doped into the activated carbon electrode by a short circuit treatment. Specifically, the coin-type cell was short-circuited by bringing the lead wire into contact with the case 1 (positive electrode side) and the upper lid 6 (negative electrode side) of the obtained coin-type cell for about 10 seconds, respectively. When the potential difference between the positive electrode and the negative electrode after the short circuit was measured with a voltmeter, it was 2.47 V (vs. Li / Li⁺This represents the natural potential of the activated carbon electrode after lithium doping on the positive electrode side (vs. Li / Li).⁺). Next, this coin cell was charged with a constant current of 1.16 mA for 50 minutes using a charge / discharge device “HJ-201B” manufactured by Hokuto Denko, and the potential was 4.06V.
[0033]
(Comparative Example 1)
In Example 1, the natural potential of the coin-type cell assembled in the same manner as in Example 1 is 2.99 V (vs. Li / Li), except that the short-circuit treatment between the lithium electrode and the activated carbon electrode is not performed.⁺In addition, the potential after charging for 50 minutes at a constant current of 1.16 mA was 4.56V. The potential after this charge was a value exceeding the decomposition voltage (about 4.4 V) on the positive electrode side of the electrolyte. From Example 1 and Comparative Example 1, it can be seen that the capacitor of the present invention can increase the energy density compared to the conventional product.
[0034]
(Example 2)
After producing a coin cell having a reduced natural potential by short-circuiting in the same manner as in Example 1, this was decomposed in argon, and only the activated carbon molded body on the positive electrode side was taken out. Repeating the above operation, the natural potential is about 2.5 V (body Li / Li⁺Two activated carbon moldings were obtained. Next, (C) having a concentration of 1 mol / liter is applied to the two molded bodies.₂H_Five)_FourNBF_Four2 were respectively impregnated with a propylene carbonate solution sufficiently to form a positive electrode 2 and a negative electrode 5 and a separator was placed between both electrodes to assemble a coin-type cell in the same manner as described above to obtain an electric double layer capacitor as shown in FIG. It was. The initial capacitance obtained by applying a voltage of 2.8 V to the obtained electric double layer capacitor at room temperature for 1 hour and then discharging at a constant current of 1.16 mA was 1.24F. In order to evaluate the long-term operational reliability of the capacitor under voltage application in an accelerated manner, the capacitor is placed in a constant temperature bath at 70 ° C., a voltage of 2.8 V is applied, and the capacitance after 500 hours has elapsed. As a result, it was 1.22F, and there was almost no deterioration in capacity.
[0035]
(Comparative Example 2)
In Example 2, an electric double layer capacitor was assembled in the same manner as in Example 2 except that the activated carbon electrode not subjected to the short-circuit treatment of Comparative Example 1 was used. The initial capacitance of the obtained electric double layer capacitor was 1.23F. The capacitance after the acceleration evaluation was 1.00 F, and the capacitance was greatly deteriorated.
[0036]
(Example 3)
The initial capacitance obtained by applying a voltage of 4.0 V for 1 hour at room temperature to the electric double layer capacitor produced in the same manner as in Example 2 and discharging at a constant current of 1.16 mA is 1.28 F. Met. Furthermore, the electrostatic capacity after repeated charging and discharging 50 times under the same conditions is 1.27 F, there is almost no capacity deterioration even when a high voltage is repeatedly applied, the energy density is large, and the cycle repetition characteristics are improved. An excellent electric double layer capacitor was obtained.
[0037]
(Comparative Example 3)
In Example 3, evaluation was made in the same manner as in Example 3 except that an electric double layer capacitor produced in the same manner as in Comparative Example 2 was used. The initial capacitance after applying the 4.0 V voltage was 1.26 F, and the capacitance after 50 repetitions was 0.90 F.
[0038]
【The invention's effect】
According to the present invention, it is possible to provide a capacitor having a larger maximum capacity and less deterioration than before.
[Brief description of the drawings]
FIG. 1 is an explanatory view of a coin cell used for lithium doping of an electrode in Example 1 of the present invention.
FIG. 2 is a schematic view of a capacitor used in an example of the present invention.
[Explanation of symbols]
11 Stainless steel container case
12 Activated carbon molding (positive electrode)
13 Gasket
14 Separator
15 Lithium metal (negative electrode)
16 Top cover of stainless steel container
21 Stainless steel container case
22 Activated carbon molding
23 Gasket
24 Separator
25 Activated carbon molding
26 Top cover of stainless steel container

Claims (3)

In the electric double layer capacitor using the polarizable electrode in a non-aqueous solvent electrolyte and both electrodes, polarizable electrode body consists of activated carbon was added lithium by an electrochemical method, and natural prior to the addition of lithium the potential E1 [V], the natural potential after the addition of lithium E2 V, electrodeposition substantial decomposition start voltage solutions liquid oxidation side or high potential side of a V, electrolytic solution reduction side Alternatively, when the substantial decomposition start voltage on the low potential side is b [V],
E 1> (a + b) / 2
a + b-E1 <E2 < E 1
An electric double layer capacitor characterized by satisfying the formula: Natural potential, when the Li / Li ⁺ as a counter electrode, an electric double layer capacitor of claim 1 Symbol placement is less than 2.8V or 1.5V polarizable electrode body. The specific surface area of activated carbon is 200 meters ² / g or more, an electric double layer capacitor according to claim 1 or 2, wherein at most 3000 m ² / g.

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