Method of manufacturing electrode group unit for lithium ion capacitor and lithium ion capacitor

A method of manufacturing an electrode group unit for lithium ion capacitor that allows reliable welding between a current collecting member and an electrode and that provides a welded portion with a low resistance is provided. A lithium ion capacitor is also provided. An unapplied portion 25 of a positive electrode 9 and an unapplied portion 33 of a negative electrode 11 are disposed to project outside of separators 13, 15 in directions opposite to each other. The resulting assembly is wound into a swirling shape in cross section about an axial core 7 to form an electrode group 5. A lithium metal support member 17 is disposed on the negative electrode 11 such that a layer in which the lithium metal support member 17 is wound is located in a radially middle region of the electrode group 5. A negative current collecting member 45 is placed on the unapplied portion 33, and welding is performed using a direct-collecting semiconductor laser device that continuously generates laser light to manufacture an electrode group unit 2 for lithium ion capacitor. The electrode group unit 2 for lithium ion capacitor is housed in a container 3 to obtain a lithium ion capacitor 1.

TECHNICAL FIELD

The present invention relates to a method of manufacturing an electrode group unit for lithium ion capacitor, and to a lithium ion capacitor including the electrode group unit.

BACKGROUND ART

Lithium ion capacitors that combine advantages of a lithium ion battery and advantages of an electric double-layer capacitor are developed as high-capacitance capacitors (with a capacitance of 500 F or more, for example). In recently developed lithium ion capacitors, in general, activated carbon is used as a positive active material, and a carbon material capable of occluding and releasing lithium ions is used as a negative active material. The potential of the negative electrode is kept lower (at approximately −3 V) in the lithium ion capacitor than in the normal electric double-layer capacitor (at approximately −1 V to −1.35 V) since lithium ions are occluded or doped in a negative electrode in advance. Therefore, the cell can be used in a higher voltage range (approximately 2.2 V to 3.8 V). In addition, because the lithium ion capacitor can also serve as a positive electrode charge/discharge mechanism that can adsorb positive ions, in addition to adsorbing negative ions which are utilized by the normal electric double-layer capacitor, the lithium ion capacitor can have a double capacity in principle. Further, while the lithium ion capacitor has a small capacitance compared to the lithium ion battery, the lithium ion capacitor advantageously has a low internal resistance, excellent output characteristics, and a long life [Japanese Unexamined Patent Application Publication No. 2010-141217 (Patent Document 1)]. Patent Document 1 discloses a structure in which a plurality of tabs are used to connect a current collector and a current collecting member to each other.

In order to reliably weld a negative current collector made of a copper foil and a negative current collecting member made of copper during manufacture of such a lithium ion capacitor, laser welding may be performed using YAG laser light [Japanese Unexamined Patent Application Publication No. 2010-93178 (Patent Document 2)]. Patent Document 2 discloses a structure in which a negative current collector is directly welded to a negative current collecting member without using tabs.

Patent Document

SUMMARY OF INVENTION

Technical Problem

In the welding technique which uses YAG laser light as taught in Patent Document 2 or CO2laser light, however, the energy of the collected laser light may be so high that a portion of the metal irradiated with the laser may be evaporated. As a reaction, the metal may be melted to form a so-called “keyhole”. Therefore, the molten metal may be scattered (sputtered), or gas may enter the hole in the molten metal and form a blowhole, leading to welding defects. Moreover, the YAG laser light and the CO2laser light has a small spot diameter, and the focal point of such light may vary according to a rise in temperature of a lens for collecting the laser light. Thus, the welding position may be easily displaced, leading to fluctuations in welding quality. If the current collecting member and the current collector of the electrode are not sufficiently welded to each other during manufacture of the lithium ion capacitor, the resistance at the welded portion may be increased to degrade the performance of the capacitor.

An object of the present invention is to provide a method of manufacturing an electrode group unit for lithium ion capacitor that allows reliable welding between a current collecting member and an electrode, that provides a welded portion with a low resistance, and that prevents a short circuit due to a molten metal, and to provide a lithium ion capacitor.

Another object of the present invention is to provide a method of manufacturing an electrode group unit for lithium ion capacitor that allows lithium metal to be reliably occluded during preliminary charging, and to provide a lithium ion capacitor.

Solution to Problem

The present invention provides a method of manufacturing an electrode group unit for lithium ion capacitor including, as its basic components, an electrode group formed by winding a laminated member in which a positive electrode formed by applying a positive active material mixture to an aluminum foil (positive current collector) and a negative electrode formed by applying a negative active material mixture to a copper foil (negative current collector) are laminated on each other via a separator, a positive current collecting member made of aluminum and electrically connected to the aluminum foil of the positive electrode, a negative current collecting member made of a metal material and electrically connected to the copper foil of the negative electrode, and a lithium metal support member including lithium metal and a support member made of a copper foil and electrically connected to the negative current collecting member, the support member having ion permeability and having a structure for supporting the lithium metal.

The lithium metal support member is disposed in or adjacent to the electrode group where the lithium metal support member is electrically insulated from the positive electrode and such that the lithium metal is occluded by (pre-doped into) a negative active material contained in the negative active material mixture of the negative electrode so that only the copper foil remains.

In the method of manufacturing an electrode group unit for lithium ion capacitor according to the present invention, the negative electrode that has a layer of the negative active material mixture on the copper foil and has an unapplied portion on which the negative active material mixture is not applied along the layer of the negative active material mixture is prepared. The positive electrode that has a layer of the positive active material mixture on the aluminum foil and has an unapplied portion on which the positive active material mixture is not applied along the layer of the positive active material mixture is prepared. The electrode group is formed such that the unapplied portion of the negative electrode and the unapplied portion of the positive electrode project outside of the separator in directions opposite to each other. The negative current collecting member is placed on the unapplied portion of the negative electrode of the thus configured electrode group and the end portion of the support member, and welding is performed using a direct-condensing semiconductor laser device.

A direct-collecting semiconductor laser device (Direct Diode Laser: DDL) used in the present invention is a semiconductor laser device that directly collects and outputs laser light from a semiconductor laser light source that emits laser diode (LD) light capable of high-efficiency oscillation (for example, high-power semiconductor laser devices available from Coherent Japan, Inc. and Laserline GmbH). The direct-collecting semiconductor laser device enables welding by melting metal using heat conducted by the laser light, and can continuously generate the laser light. Therefore, a so-called keyhole is not formed, the molten metal is not scattered, or a blowhole is not formed. Rather, performing laser welding using the direct-collecting semiconductor laser device allows the negative current collecting member to be efficiently melted, enables reliable welding, and reliably prevents an increase in resistance of the welded portion. Since little sputter due to scattered molten metal is produced, there is little possibility that a short circuit with a container is caused when the electrode group unit for lithium ion capacitor is housed in the container. In the present invention, further, the negative current collecting member is directly welded to the unapplied portion of the negative electrode of the wound electrode group. This increases the welding area, which reduces the contact resistance. In the present invention, in particular, the support member of the lithium metal support member made of a copper foil is also welded to the negative current collecting member. Therefore, the lithium metal support member and the negative current collecting member can be reliably welded to each other without increasing the resistance of the welded portion. This allows the lithium metal to be reliably occluded by the negative active material contained in the negative active material mixture of the negative electrode during preliminary charging. This also prevents the remaining support member from slipping off after the lithium metal is occluded.

The wavelength and the power of the laser light output from the direct-collecting semiconductor laser device used may be selected such that the negative current collecting member can be efficiently melted. In the present invention, a fiber-guided semiconductor laser device (FOLD) that collects output laser light in an optical fiber to facilitate handling may be used as the direct-collecting semiconductor laser device.

The direct-collecting semiconductor laser device used for the negative electrode side in the present invention can be also used to weld aluminum used for the positive current collecting member and the positive electrode. Therefore, the same welding device can be used to weld not only the negative current collecting member and the negative electrode but also the positive current collecting member and the positive electrode. That is, the positive current collecting member may be placed on the unapplied portion of the positive electrode of the electrode group, and the positive current collecting member may be locally melted by continuously irradiating the positive current collecting member with laser light continuously generated by a direct-condensing semiconductor laser device, to weld the unapplied portion of the positive electrode and the positive current collecting member to each other with a molten metal. Thus, welding work can be performed for both the negative electrode side and the positive electrode side using a single direct-collecting semiconductor laser device, improving the production efficiency.

In the manufacturing method according to the present invention, welding is preferably performed such that the molten metal does not extend radially outward beyond an electrode layer located on the radially outermost side of the electrode group. This is because if the molten metal extended radially outward beyond the electrode layer located on the radially outermost side of the electrode group, the molten metal might electrically contact the wall surface of a metal container and cause a short circuit when the electrode group unit for lithium ion capacitor manufactured by the manufacturing method according to the present invention is finally housed in the container.

In order to more reliably occlude the lithium metal, the electrode group is preferably formed such that the length of projection of an end portion of the support member from the separator is larger than the length of projection of the unapplied portion of the negative electrode from the separator. This configuration allows more reliable welding between the negative current collecting member and the support member, and allows the lithium metal to be reliably occluded without increasing the resistance value of the welded portion. In other words, the support member is deeply embedded into the melted negative current collecting member to increase the contact area. This helps suppress an increase in resistance value of the welded portion.

The lithium metal support member may be located at any position in the electrode group. For example, the lithium metal support member may be located such that a layer in which the lithium metal support member is wound is located outside of the electrode group. In this case, however, it is necessary to increase the diameter of the negative current collecting member to reliably weld the negative current collecting member and the support member of the lithium metal support member to each other. It is also necessary that welding should be performed to the outside of the negative current collecting member, which makes it highly likely that the molten metal due to the welding extends beyond an electrode layer located on the radially outermost side of the electrode group. Therefore, the lithium metal support member is preferably disposed such that a layer in which the lithium metal support member is wound is located in a radially middle region of the electrode group. If the lithium metal support member is disposed in this way, it is not necessary to substantially increase the diameter of the negative current collecting member. In addition, the support member of the lithium metal support member and the negative current collecting member can be reliably welded to each other even if welding is not performed to an end of the negative current collecting member.

A recessed portion or a projected portion extending linearly is preferably formed by pressing at a portion of each of the negative current collecting member and the positive current collecting member that is irradiated with the laser light. This configuration prevents thermal diffusion of heat produced by the laser light, and clearly indicates the position to be irradiated with the laser light, which allows accurate welding of a desired position.

The electrode group unit for lithium ion capacitor manufactured as described above can be applied to a lithium ion capacitor.

The present invention can also be implemented as a lithium ion capacitor. The present invention provides a lithium ion capacitor having the electrode group unit for lithium ion capacitor manufactured by the method discussed earlier. As discussed earlier, the electrode group unit for lithium ion capacitor includes an electrode group formed by winding a laminated member in which a positive electrode formed by applying a positive active material mixture to an aluminum foil and a negative electrode formed by applying a negative active material mixture to a copper foil are laminated on each other via a separator, a positive current collecting member made of aluminum and electrically connected to the aluminum foil of the positive electrode, and a negative current collecting member made of a metal material and electrically connected to the copper foil of the negative electrode. The electrode group unit for lithium ion capacitor also includes a lithium metal support member including lithium metal and a support member made of a copper foil and electrically connected to the negative current collecting member, the support member having ion permeability and having a structure for supporting the lithium metal. The lithium metal support member is disposed in or adjacent to the electrode group where the lithium metal support member is electrically insulated from the positive electrode and such that the lithium metal is occluded by a negative active material contained in the negative active material mixture of the negative electrode so that only the copper foil remains. In the lithium ion capacitor according to the present invention, the electrode group unit for lithium ion capacitor is housed in a bottomed container, an opening portion of which is sealed by a lid member serving also as a terminal electrode.

The negative electrode has a layer of the negative active material mixture on the copper foil and has an unapplied portion on which the negative active material mixture is not applied along the layer of the negative active material mixture, and the positive electrode has a layer of the positive active material mixture on the aluminum foil and has an unapplied portion on which the positive active material mixture is not applied along the layer of the positive active material mixture. The electrode group is formed such that the unapplied portion of the negative electrode and the unapplied portion of the positive electrode project outside of the separator in directions opposite to each other. The negative current collecting member is laser-welded to the unapplied portion of the negative electrode of the electrode group and an end portion of the support member. The metal material of the negative current collecting member is a material that is melted by laser light continuously output from a direct-collecting semiconductor laser device. Consequently, a lithium ion capacitor in which the negative electrode and the negative current collecting member are reliably welded to each other and which has a welded portion with a low contact resistance can be provided.

The metal material of the negative current collecting member melted by the laser light continuously output from the direct-collecting semiconductor laser device may be nickel or copper plated with nickel.

DESCRIPTION OF EMBODIMENTS

An embodiment in which the present invention is applied to a cylindrical lithium ion capacitor will be described below with reference to the drawings.

FIG. 1Ais a plan view of a lithium ion capacitor1(hereinafter simply referred as “capacitor1”) according to the embodiment with a positive electrode facing up, andFIG. 1Bis a cross-sectional view taken along the line IB-IB ofFIG. 1A. InFIG. 1B, the cross-sectional shape of an electrode group5is not shown, and cross-sectional portions are not hatched. The capacitor1includes a container (can)3made of steel plated with nickel and having the shape of a bottomed cylinder. An electrode group unit2for lithium ion capacitor is housed in the container3. The electrode group unit2for lithium ion capacitor is a combination of the electrode group5, a positive current collecting member39, and a negative current collecting member45. As shown inFIGS. 1B and 2, the electrode group5is formed by winding a sheet-like positive electrode9and a sheet-like negative electrode11around a hollow cylindrical axial core7made of polypropylene via a first separator13and a second separator15. A lithium metal support member17including lithium metal is disposed in the electrode group5before doping as shown inFIG. 2. The positive electrode9is formed from two divided positive electrodes9A,9B. The first and second separators13,15may be a porous base material such as kraft paper.

The divided positive electrodes9A,9B forming the positive electrode9have the same structure as each other except for their lengths. As shown inFIGS. 3A and 3B, the divided positive electrodes9A,9B are formed by applying a positive active material mixture21to both surfaces of an aluminum foil (positive current collector)19, for example. Herein, the aluminum foil19includes an aluminum alloy foil. The positive active material mixture21may be a mixture of activated carbon, a binding agent such as an acrylic binder, and a dispersing agent made of carboxymethyl cellulose (CMC), for example. The aluminum foil19includes an applied portion23in which a multiplicity of through holes are formed and on which the positive active material mixture21is applied, and an unapplied portion25which is formed along the longitudinal direction of the applied portion23and in which through holes are not formed. The positive active material mixture21is applied to the applied portion23over a length less than the length of the applied portion23in the width direction. That is, the unapplied portion25of the aluminum foil19remains exposed along the layer of the positive active material mixture21.

The negative electrode11has a structure similar to that of the divided positive electrodes9A and9B shown inFIGS. 3A and 3B. That is, in the negative electrode11, a negative active material mixture29is applied to both surfaces of a copper foil (negative current collector)27. Herein, the copper foil includes not only a pure copper foil but also a copper alloy foil. The negative active material mixture29may be a mixture of amorphous carbon capable of occluding and releasing lithium ions, a binding agent made of polyvinylidene fluoride (PVDF), and a conductive assistance such as acetylene black, for example. The copper foil27includes an applied portion31in which a multiplicity of through holes are formed, and an unapplied portion33which is formed along the longitudinal direction of the applied portion31and in which through holes are not formed. The negative active material mixture29is applied to the applied portion31over a length less than the length of the applied portion31in the width direction. That is, the unapplied portion33of the copper foil27remains exposed along the layer of the negative active material mixture29.

The lithium metal support member17causes the negative active material (in the embodiment, amorphous carbon) of the negative electrode11to occlude (be doped with) lithium ions. As shown inFIGS. 4A and 4B, the lithium metal support member17includes lithium metal35having a thin plate shape and two copper foils (support members)37,37. The copper foils37,37may be obtained by cutting the same material as that for the copper foil forming the negative electrode11into predetermined dimensions. A multiplicity of through holes (not shown) are formed in the copper foils37,37. The lithium metal35is sandwiched between the two copper foils37,37to contact portions of the two copper foils37in which the multiplicity of through holes are formed.

As shown inFIG. 2, the electrode group5is formed by winding the positive electrode9(divided positive electrodes9A,9B) and the negative electrode11into a swirling shape in cross section about the axial core7via the two separators13,15such that the positive electrode9and the negative electrode11do not directly contact each other. The lithium metal support member17is disposed on the negative electrode11such that a layer in which the lithium metal support member17is wound is located in a radially middle region of the electrode group5. The positive electrode9and the negative electrode11are disposed such that their respective unapplied portions (25and33) project outside of the separators13,15in directions opposite to each other. A winding end portion of the electrode group5is fixed by affixing an adhesive tape across the winding end portion and the outer peripheral surface of the electrode group5to prevent unwinding of the electrode group5.

The positive current collecting member39is made of aluminum (including an aluminum alloy), and has a ring shape in which a circular hole41is formed in the center portion as shown inFIG. 5. As shown inFIG. 1B, the hole41has a diameter that allows the positive current collecting member39to be fitted with the upper end of the axial core7to prevent the positive current collecting member39from being displaced from the center of the electrode group5. The positive current collecting member39is welded to the unapplied portion25of the positive electrode9included in the electrode group5. Thus, as shown inFIG. 7, the positive current collecting member39is moved closer toward the electrode group5from above the side of the electrode group5where the unapplied portion25of the positive electrode9is located, so that the positive current collecting member39is placed on the unapplied portion25of the aluminum foil19of the positive electrode9. Then, the unapplied portion25and the positive current collecting member39are welded to each other by laser welding to be discussed later. For laser welding, the positive current collecting member39is provided with four grooves43that form recessed portions for welding that are convex toward the electrode group5and that are open in the direction opposite to the electrode group5. The grooves43are formed by pressing, and extend linearly radially from the imaginary center point of the positive current collecting member39. A positive terminal portion44A welded to the positive current collecting member39which is shown inFIG. 7is to be welded to a container lid55shown inFIG. 1B. During assembly, an insulating ring member made of rubber is mounted on the outer peripheral portion of the positive current collecting member39for electrical insulation as shown inFIG. 1B.

The negative current collecting member45is made of either nickel or a metal material obtained by plating copper with nickel. In the embodiment, the negative current collecting member45is made of a metal material obtained by plating copper with nickel. As shown inFIG. 6, the negative current collecting member45has a disk shape in which a circular dent47is formed in the center portion. The dent47is formed to receive the lower end of the axial core7. As shown inFIG. 7, the negative current collecting member45is moved closer toward the electrode group5from the side of the electrode group5where the unapplied portion33of the negative electrode11is located, so that the negative current collecting member45is placed on the unapplied portion33of the copper foil27. Then, the negative current collecting member45and the unapplied portion33of the copper foil27are welded to each other by laser welding. As with the positive current collecting member39, the negative current collecting member45is also provided with four grooves49that form recessed portions for welding that are convex toward the electrode group5and that are open in the direction opposite to the electrode group5. The grooves49are formed by pressing, and extend linearly radially from the imaginary center point of the negative current collecting member45.

<Welding between Electrode Group and Current Collecting Members>

Laser light is used to weld the unapplied portions25and33of the electrode group5and the current collecting members (positive current collecting member39and negative current collecting member45) to each other. In the embodiment, in particular, a direct-collecting semiconductor laser device (DDL, not shown) that continuously generates laser light is used as a laser welding device. The direct-collecting semiconductor laser device according to the embodiment uses laser diode light capable of high-efficiency oscillation. The laser light has a concentration of about one tenth that of YAG laser light and CO2laser light, has an elliptical beam shape, and enables welding by melting metal using heat conducted by the laser light. Use of such a direct-collecting semiconductor laser device enables reliable welding that produces little sputter due to scattered molten metal. Welding of the negative current collecting member45will be described as an example. The negative current collecting member45is locally melted by continuously applying laser light continuously generated by the direct-collecting semiconductor laser device along the grooves49of the negative current collecting member45from the outer peripheral side toward the center portion of the negative current collecting member45, to weld the unapplied portion33of the copper foil27of the negative electrode11and end portions of the support members37,37and the negative current collecting member45to each other with a molten metal. Performing laser welding using the direct-collecting semiconductor laser device as in the embodiment allows the negative current collecting member45to be efficiently melted, enables reliable welding, and reliably prevents an increase in resistance of the welded portion. Use of a fiber-guided semiconductor laser device in place of the direct-collecting semiconductor laser device also achieves good welding results.

FIGS. 8A and 8Bare a cross-sectional view before welding and a cross-sectional view after welding, respectively, showing the positive current collecting member39and the unapplied portion25of the aluminum foil19of the positive electrode9in cross section orthogonal to the groove43. In the state before welding shown inFIG. 8A, the positive current collector made of the aluminum foil19has been deformed by the tip of an angled convex thread formed by forming the groove43of the positive current collecting member39. In the state shown inFIG. 8Bin which welding is completed, a portion of the positive current collecting member39at the bottom portion of the groove43has been melted to weld the unapplied portion25of the aluminum foil19of the positive electrode9and the positive current collecting member39to each other with a molten metal.

The negative current collecting member45and the unapplied portion33of the negative electrode11are also welded to each other in the same manner. That is, the negative current collecting member45is melted to weld the unapplied portion33of the negative electrode11and the negative current collecting member45to each other with a molten metal. As discussed later, respective ends of the support members37,37forming the lithium metal support member17are also welded to the negative current collecting member45in the same manner.

FIG. 9Ais a cross-sectional view showing the region A ofFIG. 1Bas enlarged.FIG. 9Ashows a state in which the positive current collecting member39and the unapplied portion25of the positive electrode9are welded to each other such that the molten metal extends to the vicinity of the axial core7.FIG. 9Bshows the region B ofFIG. 1Bas enlarged.FIG. 9Bshows a state in which the positive current collecting member39and the unapplied portion25of the aluminum foil19are welded to each other in the vicinity of the wall surface of the container3. InFIGS. 9A and 9B, some members are not shown, and the number of layers in the electrode group5may be different from the actual number. In the embodiment, welding is performed while moving laser light from the container3side toward the center. As a result, a welding bead is formed to extend toward the axial core7as shown inFIG. 9Bwhen a molten metal51is hardened. Therefore, the molten metal51does not extend toward the container3beyond the outermost peripheral surface of the electrode group5. As a result, the hardened molten metal51does not contact the wall surface of the container3and cause a short circuit.

FIG. 10is a cross-sectional view showing the region C ofFIG. 1Bas enlarged.FIG. 10shows a state in which the negative current collecting member45and the unapplied portion33of the copper foil27are welded to each other. InFIG. 10, some members such as the axial core7and a molten metal53are not shown, and the number of layers in the electrode group5may be different from the actual number. In the embodiment, as is clear fromFIG. 10, not only the unapplied portion33of the copper foil27but also the support members37,37forming the lithium metal support member17are welded to the negative current collecting member45. The end portions of the support members37,37are configured such that the length of projection of the end portions of the support members37,37from the separators13,15is larger than the length of projection of the unapplied portion33from the separators13,15. This configuration allows more reliable welding between the negative current collecting member45and the support members37,37, and allows the lithium metal35to be reliably occluded without increasing the resistance value of the welded portion. Since the support members37,37are also welded, the remaining support members37,37can be prevented from slipping off after the lithium metal35is occluded.

Thus, in the embodiment, welding work can be performed for both the negative electrode side and the positive electrode side without changing the settings of a single direct-collecting semiconductor laser device, improving the production efficiency.

<Accommodation of Electrode Group into Container>

As shown inFIG. 11, the electrode group5to which the current collecting members have been welded, that is, the electrode group unit2for lithium ion capacitor, is housed in the container3. With the electrode group unit2for lithium ion capacitor housed in the container3, the dent47of the negative current collecting member45and the bottom portion of the container3are welded to each other by spot welding for electrical connection.

An insulating ring member63for electrical insulation between the positive current collecting member39and the container3is attached to the outer peripheral portion of the positive current collecting member39. A drawing process has been performed on a portion of the container3in the vicinity of the opening portion so that the electrode group unit2for lithium ion capacitor is fixed in the container3as shown inFIG. 1B.

The container lid55forming a positive electrode terminal is disposed above the positive current collecting member39. The container lid55includes a lid body57disposed on the positive current collecting member39, and a lid cap59combined with the lid body57. The lid body57is made of aluminum, and the lid cap59is made of steel plated with nicked as with the container3. The lid cap59includes an annular flat portion59aand a projected portion59bprojected from the center portion of the flat portion59a. The container lid55is formed by curling (crimping) the edge portion of the lid body57around the outer peripheral portion of the flat portion59aof the lid cap59. A void portion61is formed between the projected portion59bof the lid cap59and the lid body57.

A first end of the positive terminal portion44A, which is one of two positive terminal portions which are ribbon-like aluminum foils laminated over each other, is joined to the upper surface of the positive current collecting member39. A first end of the other positive terminal portion44B is welded to the outer bottom surface of the lid body57forming the container lid55. Second ends of the two positive terminal portions44A,44B are joined to each other. This allows the lid body57to be electrically connected to one of the electrodes (positive electrode9) of the electrode group5.

An annular stepped portion3ais formed in the container3which has been subjected to a drawing process as discussed above. The container lid55is disposed on the stepped portion3avia an insulating member65for electrical insulation between the container lid55and the container3. Then, an opening end portion3bis curled (crimpled) toward the container lid55. As a result, the container lid55is fixed between the opening end portion3bsubjected to a curling process and the stepped portion3avia the insulating member65. This allows the internal space of the capacitor1to be tightly sealed.

An amount of a non-aqueous electrolyte (not shown) that is enough to infiltrate the entire electrode group unit2for lithium ion capacitor is injected into the container3. The non-aqueous electrolyte may be a solution obtained by dissolving lithium phosphate hexafluoride (LiPF6) as lithium salt in a solvent obtained by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) at a volume ratio of 30:50:20, for example.

(Modification of Current Collecting Members)

FIGS. 12 and 13each show a modification of the current collecting member. InFIGS. 12 and 13, component parts that are the same as those in the embodiment shown inFIGS. 5 and 6are denoted by reference numerals obtained by adding 100 to the reference numerals affixed to their counterparts inFIGS. 5 and 6to omit their descriptions. In the current collecting member ofFIG. 12, grooves143are formed to be shorter than the diameter of a positive current collecting member139. Lug portions165are formed at the outer peripheral portion of the positive current collecting member139on extensions of the grooves143. The lug portions165hinder an electrode group105from moving radially. In addition, the lug portions165can prevent a molten metal from extending radially outward beyond the outermost layer of the electrode group105. Lug portions167of a negative current collecting member145have the same effect.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide an electrode group unit for capacitor in which current collecting members (positive current collecting member and negative current collecting member) and electrodes (positive electrode and negative electrode) are reliably welded to each other and which has a welded portion with a low resistance, and to provide a lithium ion capacitor with a low contact resistance.

REFERENCE SIGNS LIST