Electric double-layer capacitor, its manufacturing method, and electronic device using same

Provided are an electric double-layer capacitor which can have a reduced internal resistance and an improved withstand voltage with a simple structure, and a method for manufacturing the capacitor. The electric double-layer capacitor uses an alloy of carbon and aluminum as a material of the electrode. The electrode is formed by applying carbon to an aluminum foil and heating the aluminum foil with carbon thereon to a temperature at which the aluminum foil and the carbon are alloyed.

TECHNICAL FIELD

The present invention relates to an electric double-layer capacitor (hereinafter, EDLC) used in a variety of electronic devices, a method for manufacturing the EDLC and an electronic device having the EDLC.

BACKGROUND ART

Japanese Patent Unexamined Publications No. H11-288849 (Document 1) and No. 2001-297952 (Document 2) disclose methods for producing an electrode metal material which can reduce the internal resistance of a capacitor. The electrode metal material is used in contact with an electrolytic solution in a capacitor such as an EDLC. These methods reduce the internal resistance of an electrode by fixing carbon particles on a valve metal such as aluminum so as to secure the electric connection between the aluminum and the active carbon on the electrode. Japanese Patent Unexamined Publication No. 2000-269095 (Document 3) discloses a method for reducing the internal resistance of an EDLC by covering the uneven surface of a collector with carbon black particles so as to form a conductive layer. In the three methods, the collectors and the electrodes are all made of pure aluminum and pure carbon.

In Documents 1 and 2 mentioned above, the aluminum portion is covered with an oxide film caused by the water in the electrolytic solution. The potential to form the oxide film can be detected by the reaction potential on the oxidizing side, which can be measured by CV (cyclic voltammetry) or the like. An example of the CV measurement is shown inFIG. 20. InFIG. 20, the horizontal and vertical axes represent potential and current, respectively. The reference electrode is an Ag/Ag+ electrode and the counter electrode is Pt. As the working electrodes, an aluminum electrode and an aluminum electrode with carbon particles fixed thereon are used for comparison. The results show that the aluminum electrode, and the aluminum electrode with the carbon particles fixed thereon have nearly the same reaction potential. This means that both electrodes have an oxide film covering the aluminum.

These EDLCs, which have a capacitance large enough to supply a large current, can be used in electronic devices such as electric vehicles (EV) as disclosed in Japanese Patent Unexamined Publication No. H10-271611 (Document 4).

However, the aforementioned conventional EDLCs require a complicated and difficult-to-control process to from electrodes as follows. Carbon particles are fixed on aluminum and the aluminum is etched so that the carbon particles are halfway fixed on and slightly exposed from the aluminum.

The electric connection entirely depends on carbon particles, so that the reliability in fixing the carbon particles is very important for securing conduction. On the other hand, the aluminum portion covered with the oxide film caused by the water in the electrolytic solution does not contribute to conduction. Since the conductive portion (the carbon particle portion) and the nonconductive portion (the oxide film portion) are formed on the same aluminum foil surface, it is difficult to meet both conductivity and withstand voltage at the same time.

Because of being covered with the oxide film caused by the water in the electrolytic solution, the aluminum portion has a potential window whose size is limited by the reaction at the time of forming the oxide film. This results in a reduction in the withstand voltage.

In Document 4, a large number of EDLCs must be connected in series when used as the power supply unit of the EV because the withstand voltage cannot be increased. For example, if each EDLC has a withstand voltage of 2V and the EV requires a voltage of 400V, then as many as 200 EDLCs are required. This results in an increase in the size of the power supply unit. In other words, it is inevitable to improve the withstand voltage of each EDLC for the size reduction of the power supply unit. On the other hand, it is also being tried to improve the withstand voltage of electrolytic solutions, and electrolytic solutions with comparatively high withstand voltages are being developed.

However, the low withstand voltage of EDLCs results from the deterioration of the aluminum electrode foils. Therefore, deteriorated aluminum electrode foils cause a reduction in the withstand voltage of the EDLCs even with an electrolytic solution having a comparatively high withstand voltage.

SUMMARY OF THE INVENTION

The present invention provides an electric double-layer capacitor including an element which is composed of a pair of polarizable electrodes either wound or laminated with a separator disposed therebetween and which is sealed in a case with an electrolytic solution, wherein the polarizable electrodes are made of a material including an alloy of carbon and aluminum.

The present invention further provides an electric double-layer capacitor including polarizable electrodes that are electrode foils at least one of which is made of aluminum, and the at least one of the electrode foils is coated on the front and rear sides with aluminum fluoride.

The present invention further provides an electric double-layer capacitor including a case which is coated on at least the inner surface thereof with aluminum fluoride.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detail as follows with reference to accompanying drawings. Note that the drawings are only schematic and do not show the exact dimensions of each component.

First Exemplary Embodiment

FIG. 1is a production flowchart showing a method for manufacturing an electrode of an EDLC of a first embodiment of the present invention.

As shown inFIG. 1, first of all, an aluminum foil is prepared. The aluminum foil is coated with a carbon material containing carbon black having an average particle diameter of 0.3 μm. Then, the aluminum foil coated with the carbon material is rolled out at temperatures of not less than 300° C. to form an aluminum-carbon alloy layer with a composition of Al4C3.

The heating temperature has only to be not less than the alloying temperature. The Al4C3alloy layer is found to have a thickness of about 1 μm by SIMS analysis. The Al4C3alloy layer is formed with some degree of variation in stoichiometry, and the variation is conspicuous at the interface between aluminum and Al4C3.

By using the aluminum electrode containing the Al4C3alloy layer formed in this manner, a wound EDLC shown inFIGS. 2A and 2Bis manufactured. InFIGS. 2A and 2B, each electrode body3includes collector1of an aluminum electrode containing the aforementioned Al4C3alloy layer, and electrode layer2mainly made of active carbon and formed on the front and rear surfaces of collector1. The EDLC further includes separators4, lead wires5, ring packing6, sealant7and case8, which is made of aluminum.

The wound EDLC with the aforementioned structure is manufactured as follows. First of all, as described above withFIG. 1, each collector1is formed by applying the 1 μm-thick Al4C3alloy layer on the front and rear surfaces of the aluminum foil, which is 30 μm thick. Then, a coating solution for forming electrodes is applied on the front and rear surfaces of collector1so as to form electrode layers2, each of which is 85 μm thick. As a result, electrode bodies3each having a total thickness of 200 μm are complete. The coating solution for forming the electrodes is prepared as follows. Active carbon is added with 8.1 wt % of a binder such as PTFE and 10.8 wt % of a conductive additive such as acetylene black, kneaded together with an appropriate amount of water, and then evenly granulated using a pressure homogenizer.

In order to increase the electrode density so as to improve the strength, electrode bodies3are press-molded to have a thickness of 195 μm each. Then, a pair of lead wires5are respectively connected to the positive and negative electrodes bodies3. The positive and negative electrodes have separators4disposed therebetween and are wound together. Separator4is 35 μm thick and made of a cellulose material. As a result, an element, which is 10 mm in winding diameter and 40 mm in width, is complete. The element is inserted in a dehumidified atmosphere into case8together with an electrolytic solution. The electrolytic solution has a concentration of 0.69 mol/kg, and contains propylene carbonate (PC) as a solvent and tetraethylammonium tetrafluoroborate (TEABF4) as a solute. Case8is 12 mm in diameter and 48 mm in height. Finally, sealant7is applied so as to complete the EDLC.

Ten EDLCs of the present embodiment having the aforementioned structure and ten conventional EDLCs for comparison are manufactured and measured for capacity and internal resistance. The mean values of the results are shown in Table 1 below.

The conventional EDLCs are manufactured based on the technique of Documents 1 and 2 mentioned above. The capacity and internal resistance are calculated from the behavior of the voltage between terminals after performing a constant current charge at 1.0 A, a constant voltage charge at 2.0V, a leave for 6 minutes 20 seconds, and a constant current discharge at 1.0 A in this order.

The resistance is calculated from the initial IR drop during the constant current discharge.

As apparent from Table 1, the EDLC of the present embodiment, which includes the collectors containing the aluminum-carbon alloy layer with a composition of Al4C3, has only about ⅔ as high an internal resistance as the EDLC including the conventional collectors. The EDLC of the present embodiment and the conventional EDLC have nearly the same capacity at the time. The results indicate that the use of the collectors containing the aluminum-carbon alloy layer with a composition of Al4C3can achieve an EDLC with a low resistance almost without reducing the capacity.

The EDLCs of the present embodiment and the conventional EDLCs are also subjected to CV measurement to examine the reaction potential of the collecting electrodes containing the Al4C3alloy layer of the present embodiment. The results are shown inFIG. 3. The reference electrode is an Ag/Ag+ electrode and the counter electrode is Pt. As the working electrodes, a collecting electrode containing the Al4C3alloy layer, an aluminum electrode having carbon particles fixed thereon, and an aluminum electrode are used for comparison. The results show that the collecting electrode containing the Al4C3alloy layer has a more noble reaction potential than the aluminum electrode having carbon particles fixed thereon and the other aluminum electrode. In other words, the use of the electrodes containing the Al4C3alloy layer as the collectors can make the potential window larger than in the conventional electrodes. This seems to indicate that an EDLC can have a high withstand voltage by using the collecting electrodes containing the Al4C3alloy layer.

As described above, the present embodiment achieves an EDLC including the collecting electrodes containing the Al4C3alloy layer lower in resistance and higher in withstand voltage than an EDLC having the conventional collecting electrodes. The collecting electrodes containing the Al4C3alloy layer in the present embodiment are formed by applying carbon to the aluminum foil and heating it. However, the present invention is not limited to this method: the carbon can be applied to the aluminum foil by a vacuum technique such as vacuum deposition, sputtering or CVD.

It is alternatively possible to vacuum-deposit aluminum onto a carbon electrode and to heat it. The vacuum deposition can be replaced by sputtering or CVD.

Second Exemplary Embodiment

FIG. 4is a partially broken perspective view showing a structure of an EDLC of a second embodiment of the present invention.FIG. 5is a perspective view showing the EDLC.

InFIGS. 4 and 5, the EDLC includes case9made of aluminum, an electrolytic solution filled in case9and two electrode foils10made of aluminum and soaked in the electrolytic solution. Electrode foils10are alternately laminated with separators11and wound together as shown inFIG. 4. Two electrode foils10are connected with each of lead wires12. Lead wires12are drawn out of case9through sealing rubber13. Each aluminum electrode foil10is coated on the front and rear surfaces with aluminum fluoride14and further with active carbon15as shown inFIG. 8.

The principle of operation of the EDLC will be described as follows with reference toFIG. 6.

FIGS. 6A and 6Bare sectional views of the EDLC in charging conditions and in discharging conditions, respectively. In charging conditions shown inFIG. 6A, electrostatic attraction attracts anions17in electrolytic solution16toward anode active carbon15aand cations18toward cathode active carbon15b. Then, ion layers called electric double layers are respectively formed in the vicinity of anode active carbon15aand cathode active carbon15bso as to accumulate electric charges there. On the other hand, in discharging conditions shown inFIG. 6B, anions17and cations18are released from anode active carbon15aand cathode active carbon15b, respectively, and dispersed into electrolytic solution16.

The EDLC of the present embodiment is characterized in that the surface of each electrode foil10is coated with aluminum fluoride14as shown inFIG. 8.

The electrodes of conventional EDLCs are made of active carbon-coated aluminum, and the aluminum elutes during the voltage application as shown inFIG. 6A, thereby deteriorating the electrodes. The cause of the aluminum elution seems to be the weak bonding between the aluminum atoms and the oxygen atoms in an oxide film, that is, an aluminum oxide film, which usually exists on the surface of aluminum.

On the other hand, the bonding between the aluminum atoms and the fluorine atoms in aluminum fluoride is stronger than the bonding between the aluminum atoms and the oxygen atoms in aluminum oxide. This strong bonding seems to prevent the aluminum from eluting into electrolytic solution16.

One method for producing aluminum fluoride is plasma treatment.FIG. 7show sectional views of processes of the plasma treatment.

FIG. 7Ashows a sectional view of a plasma treatment chamber, andFIG. 7Bshows a sectional view of a plasma generation chamber. Plasma treatment chamber19and plasma generation chamber20are connected with each other via chamber connection hole21. InFIG. 7B, plasma is generated by injecting a gas mixture consisting of argon and carbon tetrafluoride through gas inlet hole22into between electrodes24A and24B connected to high frequency power source23. The plasma is supplied from plasma outlet holes25into plasma treatment chamber19through plasma inlet holes26.

InFIG. 7A, plasma is introduced into plasma treatment chamber19through plasma inlet holes26. Plasma treatment chamber19includes a roll of electrode foil27. Electrode foil27which is unwound as untreated electrode foil27afrom the electrode foil unwinding side is subjected to plasma treatment, and is rewound as treated electrode foil27bon the electrode foil rewinding side. Plasma treating both the front and rear surfaces of electrode foil27at the same time in this manner can improve the productivity compared with the case of treating each surface separately. Electrode foil27shown inFIG. 7is cut into length and used as electrode foils10shown inFIG. 4.

In the present embodiment, the plasma treatment apparatus is composed of plasma treatment chamber19and plasma generation chamber20. It goes without saying that plasma generation and plasma treatment can be performed in the same chamber by providing a pair of plasma-generating electrodes with an electrode foil sandwiched therebetween. This arrangement can achieve efficient plasma treatment.

If the plasma treatment is performed before active carbon15shown inFIG. 8is fixed to electrode foil27shown inFIG. 7, aluminum electrode foil10and active carbon15shown inFIG. 8have aluminum fluoride14therebetween, thereby increasing the resistance value of the electrode. Therefore, it is preferable to perform the plasma treatment after active carbon15is fixed to electrode foil10as shown inFIG. 8.

FIG. 8is a sectional view showing electrode foil10, which is plasma-treated after active carbon15is fixed to the front and rear surfaces thereof. InFIG. 8A, the symbol “F” represents fluorine atoms. In order to fix active carbon15to electrode foil10, conductive additive28and binder29are added to active carbon15so as to form a conductive composition. The conductive composition consists of active carbon15, conductive additive28and binder29in a weight ratio of 80:10:10. Active carbon15has at its terminal hydrophilic groups such as hydroxyl groups and carboxyl groups as shown inFIG. 8B. Therefore, active carbon15has a low affinity with the hydrophobic electrolytic solution, which means that the electrolytic solution has a low wettability. However, the low wettability can be improved by the plasma treatment because it substitutes fluorine for the hydrophilic groups. In other words, active carbon15improves affinity with hydrophobic electrolytic solution16, thereby facilitating the permeation of electrolytic solution16into fine pores of electrode foil10. As a result, the real electrode area increases, resulting in the improvement in capacitance.

FIG. 9is a withstand voltage characteristic of the anode of the EDLC of the present embodiment. Here, the potential at a current value of 0.01 mA is defined as the withstand voltage of the anode. As electrolytic solution16, TEABF4 is used.

In a conventional EDLC, the electrode foil not subjected to the plasma treatment with fluorine has a withstand voltage of 0.9V (“A” inFIG. 9). In contrast, electrode foil3subjected to the plasma treatment has an improved withstand voltage of 1.5V (“B” inFIG. 9).

A withstand voltage comparison shows that the conventional EDLC and the EDLC of the present invention have withstand voltages of 2.0 V and 2.6 V, respectively. This means that the withstand voltage has improved by 30%.

As described above, the use of electrode foil10whose surface is coated with aluminum fluoride14can improve the withstand voltage of an EDLC.

Third Exemplary Embodiment

FIGS. 10A and 10Bare sectional views showing an electrode body in an EDLC of a third embodiment of the present invention before and after the electrode body is plasma-treated, respectively. The electrode body is formed in the same manner as described in the first embodiment as follows. First, 2 μm-thick aluminum-carbon alloy layer1awith a composition of Al4C3is formed on the surface of collector1made of a 20 μm-thick aluminum foil, and then electrode layer2mainly composed of active carbon2ais formed on Al4C3alloy layer1a. Note that electrode layer2mainly composed of active carbon2afurther contains conductive additive2band binder2c.

The electrode body thus formed is subjected to a plasma treatment in accordance with the requirements shown in Table 2 below. In the electrode body subjected to the plasma treatment, as shown inFIG. 10B, the portion of Al4C3alloy layer1athat is in contact with collector1made of aluminum foil and active carbon2ahas no change before and after the plasma treatment. The unit “sccm” in flow rate represents a gas flow rate (cc/min.) in normal operating conditions.

On the other hand, the portion of Al4C3alloy layer1athat is exposed without being sandwiched between collector1and active carbon2ais fluorinated by the plasma treatment, and changed into AlF3alloy layer1b. Thus, the plasma treatment can be applied to previously change the aluminum component into a composition of AlF3. As a result, the aluminum elution can be avoided when the electrode body is soaked in the electrolytic solution and charge-discharge is performed. This is how a reduction in capacity and resistance of the EDLC is prevented. More specifically, the electrode body which is untreated with plasma (the electrode body including aluminum-carbon alloy layer1awith a composition of Al4C3, and electrode layer2mainly made of active carbon2aformed on Al4C3alloy layer1a) can reduce the contact resistance between electrode layer2and collector1by being disposed therebetween. However, when charge-discharge is performed with the electrode body soaked in the electrolytic solution, aluminum elutes from Al4C3alloy layer1aand reacts with the fluorine component contained in the electrolytic solution as to form an AlF compound. The AlF compound is attached to the surface of active carbon2a. This reduces the active carbon area, and thus the capacity of the EDLC.

The AlF compound, which is not a good conductor, also causes an increase in the resistance as the reaction advances. However, plasma-treating the electrode body as in the present embodiment can reduce the aluminum elution so as to prevent the deterioration in capacity and resistance. Table 3 below shows the properties of an EDLC including the electrode body of the present embodiment in comparison with the properties of the conventional product. The term “DCR” stands for direct current resistance.

As apparent from Table 3, the EDLC of the present embodiment is excellent in both capacity and resistance in the initial stages, and shows little deterioration after the test.

In the requirements of the plasma treatment shown in Table 2, the Ar gas can be replaced by other rare gases to obtain the same results.

The CF4gas can be replaced by a fluorocarbon gas such as C2F6, C3F8, C4F8, C5F8, C3F6or CH3F to obtain the same results.

The RF frequency is 20 kHz in Table 2, but it has only to be in the range of not less than 20 kH and not more than 20 MH. For example, it can be 40 kHz or 13.56 MHz.

Fourth Exemplary Embodiment

FIGS. 11A and 11Bare sectional views showing a case in an unprocessed state and a case in a processed state, respectively, which are used in an EDLC of a fourth embodiment of the present invention.

FIGS. 12A to 12Care sectional views showing structures of EDLCs using these cases.FIGS. 11 and 12show unprocessed aluminum case30(having a diameter of 18 mm), and processed aluminum case31and aluminum fluoride layer31aformed on the inner surface of case31. Aluminum fluoride layer31ais formed by applying a plasma treatment in accordance with the requirements described with Table 2 in the second embodiment. Aluminum fluoride layer31acan be formed also on the outer surface of case31without causing any troubles later on.

In order to confirm the advantages of aluminum fluoride layer31aformed on case31thus structured, the EDLCs shown inFIGS. 12A to 12Care manufactured.

The EDLC shown inFIG. 12Ais manufactured by vacuum-impregnating conventional element32with an electrolytic solution consisting, for example, of TEABF4 in PC solution; inserting it into unprocessed case30shown inFIG. 11A; and sealing it with sealing rubber33. This EDLC is referred to as the conventional product.

The EDLC shown inFIG. 12Bis manufactured by vacuum-impregnating conventional element32with an electrolytic solution consisting, for example, of TEABF4 in PC solution; inserting it into processed case31having aluminum fluoride layer31ashown inFIG. 11B; and sealing it with sealing rubber33. This EDLC is referred to as Structure 1.

The EDLC shown inFIG. 12Cis manufactured using element34whose surface is coated with aluminum fluoride by the plasma treatment described in the third embodiment. Element34is vacuum-impregnated with an electrolytic solution consisting, for example, of TEABF4 in PC solution; inserted into processed case31having aluminum fluoride layer31ashown inFIG. 11B; and sealed with sealing rubber33. This EDLC is referred to as Structure 2.

The comparison results of the properties of these EDLCs are shown in Table 4 below.

As apparent from Table 4, the EDLCs of the present embodiment are excellent in both capacity and resistance in the initial stages, and show little deterioration in capacity and little increase in resistance after the test.

The present embodiment uses an electrolytic solution consisting of TEABF4 in PC solution; however, this is not the only electrolytic solution to be used in the present invention. The same advantages can be obtained by using an electrolytic solution containing an amidine-based electrolyte such as 1-ethyl-3-methylimidazole, 1-ethyl-2,3-dimethylimidazole, or other organic electrolytes.

The PC can be replaced by an organic solvent such as γ-butyrolactone, or by a mixture solvent containing PC and an organic solvent such as dimethyl carbonate to obtain the same advantage.

Although the described element is cylindrical, it can be flat, laminated or the like.

Fifth Exemplary Embodiment

The EDLC of the present embodiment is identical to the EDLC of the fourth embodiment except for the element structure. Therefore, the same components as those in the fourth embodiment will be referred to the same numerals and symbols as those in the fourth embodiment and not described in detail again. The following description will be focused on the different portions with reference to drawings.

FIGS. 13A and 13Bare sectional views showing a case in an unprocessed state and a case in a processed state, respectively, which are used in an EDLC of the fifth embodiment of the present invention.

FIGS. 14A and 14Bare sectional views showing structures of EDLCs using these cases.FIGS. 13 and 14show unprocessed aluminum case35(having a diameter of 35 mm), and processed aluminum case36and aluminum fluoride layer36aformed on the inner surface of aluminum case36. Aluminum fluoride layer36ais formed by applying a plasma treatment in the same manner as in the fourth embodiment. Element37is provided with anode collector38aand cathode collector39a. Element37will be described in detail as follows with reference toFIGS. 15Aand15B.

FIGS. 15A and 15Bare a partially developed view and a sectional view (the part in the broken line inFIG. 15A) showing the structure of element37.

Anode electrode38band cathode electrode39bare opposed to each other via separators40and are wound together to form element37.

Anode collector38aand cathode collector39aare exposed respectively at the bottom and top ends of element37. Anode collector38ais laser-welded to cases35and36. Cathode collector39aof element37is laser-welded to lead plate42which will be described later.

Sealing member41seals the openings of cases35and36. Sealing member41is provided with lead plate42to which cathode collector39aof element37is bonded, and also with anode terminal43and cathode terminal44for external connection. Anode terminal43is bonded to cases35and36via connection bar45. Cathode terminal44is bonded to lead plate42.

Pressure control valve47is inserted in such a manner that inlet46for injecting the electrolytic solution is sealed after the injection. Although it is not illustrated, an electrolytic solution consisting, for example, of TEABF4 in PC solution is used as the electrolytic solution.

EDLCs are manufactured using unprocessed case35shown inFIG. 14A, and case36provided with aluminum fluoride layer36ashown inFIG. 14B. The former EDLC is referred to as the conventional product and the latter as Structure 3. The comparison results of the properties of these EDLCs are shown in Table 5.

As apparent from Table 5, the EDLC of the present embodiment is excellent in both capacity and resistance in the initial stages, and shows little deterioration in capacity and little increase in resistance after the test.

The present embodiment uses an electrolytic solution consisting of TEABF4 in PC solution; however, this is not the only electrolytic solution to be used in the present invention. The same advantages can be obtained by using an electrolytic solution containing an amidine-based electrolyte such as 1-ethyl-3-methylimidazole, 1-ethyl-2,3-dimethylimidazole, or other organic electrolytes.

The PC can be replaced by an organic solvent such as γ-butyrolactone, or by a mixture solvent containing PC and an organic solvent such as dimethyl carbonate to obtain the same advantage.

Although the described element is cylindrical, it can be flat, laminated or the like.

Sixth Exemplary Embodiment

The EDLC of the present embodiment differs from the EDLC of the fifth embodiment only in how to form the aluminum fluoride layer in the case. Since the other structure is identical, the same components as those in the fifth embodiment will be referred to using the same numerals and symbols as those in the fifth embodiment and not described in detail again. The following description will be focused on the different portions with reference to the drawings.

FIG. 16is a sectional view showing a method for producing a case used in an EDLC of the present embodiment. As shown inFIG. 16, aluminum case48is filled with fluorine-containing solution49. Fluorine-containing solution49used in the present embodiment consists of TEABF4 in PC solution. Fluorine-containing solution49has counter electrode50inside, which is preferably made of an electrochemically stable metal such as platinum. DC power source51is provided to apply a current between case48and counter electrode50, and it is preferable that case48and counter electrode50have a potential difference of 3 to 5V.

Applying a current from DC power source51in this manner allows the elution of aluminum ions from case48. The aluminum ions then react with the fluorine ions contained in fluorine-containing solution49to form aluminum fluoride. As a result, aluminum fluoride layer48ais formed on the inner surface of case48. The aforementioned potential difference is determined by selecting the most efficient requirements.FIGS. 17A and 17Bare sectional views showing the structures of EDLCs having case48thus produced.

FIG. 17Ashows an EDLC having element37of the fifth embodiment. This EDLC is referred to as Structure 4.FIG. 17Bshows an EDLC having element52including the electrode body coated with aluminum fluoride by a plasma treatment in the same manner as in the third embodiment. This EDLC is referred to as Structure 5, and provided with anode collector53aand cathode collector54a.

The properties of these EDLCs are shown in Table 5 together with the properties of the EDLC of the fifth embodiment for comparison.

As apparent from Table 5, the EDLCs of the present embodiment are excellent in both capacity and resistance in the initial stages, and show little deterioration in capacity and little increase in resistance after the test.

The present embodiment uses an electrolytic solution consisting of TEABF4 in PC solution; however, this is not the only electrolytic solution to be used in the present invention. The same advantages can be obtained by using an electrolytic solution containing an amidine-based electrolyte such as 1-ethyl-3-methylimidazole, 1-ethyl-2,3-dimethylimidazole, or other organic electrolytes.

The PC can be replaced by an organic solvent such as γ-butyrolactone, or by a mixture solvent containing PC and an organic solvent such as dimethyl carbonate to obtain the same advantage.

Although the described element is cylindrical, it can be flat, laminated or the like.

Seventh Exemplary Embodiment

The present embodiment shows an example where the EDLCs shown in the first to sixth embodiments are mounted on an electronic device. As the electronic device, an EV is used.FIG. 18is a system chart of the EV of the present embodiment.FIG. 19is a circuit diagram showing a capacitor unit of the EV.

The EV is composed of motor56linked with axle55, fuel cell57for supplying current to motor56, and capacitor unit58connected to the current supply path. Capacitor unit58includes a plurality of EDLCs of the present embodiment connected in series.

The capacitor unit shown inFIG. 19includes charge/discharge control circuit60A.

InFIG. 19, capacitor unit58is required to have a voltage of 400V. This corresponds to 200 conventional EDLCs.

On the other hand, when the EDLC of the second embodiment of the present invention is used, the withstand voltage is 2.6V as mentioned above, so that only 151 EDLCs are required. This results in a reduction in the size of capacitor unit58, and thus in the size of the electronic device.

The EDLCs can be connected to the current supplying path of motor56either in parallel or in series as needed.

As described above, the EDLC of the present invention can have a reduced internal resistance. The EDLC can also have an efficiently improved withstand voltage by making the reaction potential of the anode noble.

Applying aluminum fluoride to the front and rear surfaces of the aluminum electrode foil or to at least an inner surface of the case can provide the following advantage. Aluminum fluoride, which has a strong bonding between the fluorine atoms and the aluminum atoms, reduces the elution of aluminum into the electrolytic solution during voltage application. This prevents the deterioration of the electrode foil.

Although the present embodiment uses an EV as the electronic device, it goes without saying that EVs are not the only example to be used as the electronic device in the present invention.

INDUSTRIAL APPLICABILITY

The EDLC and its manufacturing method according to the present invention can have a reduced internal resistance and an improved withstand voltage, and can also prevent deterioration of the electrode foil. These advantages enable the EDLC to be used widely as, for example, a power source in a variety of electronic devices.