Capacitor

This application is a U.S. national stage application of PCT International Application PCT/JP2006/324523, filed Dec. 8, 2006.

The present invention relates to a capacitor used for various types of electronic devices, regeneration for a hybrid car or a fuel-cell electric vehicle, storing electric power, or the like.

FIG. 13 is a sectional view illustrating the structure of a conventional capacitor. Capacitor element 20 is contained in case 22 , which is made of aluminum and has a closed-end cylinder-shape, along with an electrolyte (not shown). The upper and lower end surfaces of capacitor element 20 have positive and negative electrodes extracted therefrom. Metal plate 21 is connected to one of the electrodes of capacitor element 20 . One end of rod-shaped core material 24 disposed at the central portion of capacitor element 20 has flat-shaped terminal 24 A for external connection. Core material 24 and metal plate 21 are electrically connected with each other. Meanwhile, the inner bottom surface of case 22 is connected to the other electrode of capacitor element 20 . Flat-shaped terminal 25 for external connection is joined to the outer surface of case 22 . The inner bottom surface of case 22 is provided with projection 22 A. Insulating member 26 insulates core material 24 from case 22 .

Insulative sealing plate 23 seals the opening of case 22 . Sealing plate 23 is provided with hole 32 through which core material 24 pierces and hole 33 through which an electrolyte is injected. O-ring 29 is incorporated into sealing plate 23 around hole 32 to seal the space between core material 24 and sealing plate 23 . Block 27 made of a rubber-like elastic insulating member is combined with cap 28 , and blocks hole 33 , thereby forming a pressure-regulating valve. Sealing rubber 30 disposed at the top peripheral surface of sealing plate 23 is compressed by forming curled part 22 B that is the opening end of case 22 curled, to seal the space between case 22 and sealing plate 23 . In a capacitor thus structured, case 22 functions as a collecting terminal. Such a capacitor is disclosed in Patent document 1, for example.

However, to decrease in size and to increase in capacitance of the above-described conventional capacitor, sealing plate 23 decreases in size as the external diameter of case 22 decreases. Consequently, it is extremely difficult to provide hole 33 and a pressure-regulating valve (for allowing an internal gas to escape outward when the pressure inside the capacitor rises to a given value or higher) on shrunk sealing plate 23 from the viewpoint of space. Concretely, a capacitor with such a structure becomes difficult to produce if the external diameter of case 22 is 20 mm or smaller.

Patent document 1: Japanese Patent Unexamined Publication No. 2000-315632

The present invention is a capacitor where a terminal plate sealing the opening of the case with a small external diameter is provided with a hole through which an electrolyte is injected, and the hole is reliably sealed. The capacitor of the present invention has a capacitor element, a tubular metallic case, an electrolyte, and a terminal plate. The capacitor element has a first electrode, a second electrode, and a separator interposed therebetween. The case contains the capacitor element, and the inner bottom surface of the case has the first electrode joined thereto. The terminal plate is joined to the second electrode to seal the opening of the case. At least one of the terminal plate and case is provided with a through hole, into which a hermetic plug made of a rubber-like elastic body and having an external diameter larger than the diameter of the through hole is inserted. As described above, in a capacitor according to the present invention, a hole for injecting an electrolyte provided at the terminal plate can be reliably sealed even when the capacitor is miniaturized.

FIG. 1A is a top view of a capacitor according to a first exemplary embodiment of the present invention.

FIG. 1B is a front sectional view of the capacitor shown in FIG. 1A .

FIG. 1C is a bottom view of the capacitor shown in FIG. 1A .

FIG. 1D is an enlarged sectional view of the capacitor element in FIG. 1B .

FIG. 2A is a top view of a hermetic plug used for the capacitor shown in FIG. 1A .

FIG. 2B is a front sectional view of the hermetic plug shown in FIG. 2A .

FIG. 3A is a top view of another capacitor according to the first embodiment of the present invention.

FIG. 3B is a front sectional view of the capacitor shown in FIG. 3A .

FIG. 3C is a bottom view of the capacitor shown in FIG. 3A .

FIG. 4A is a top view of another hermetic plug used for the capacitor shown in FIG. 1A .

FIG. 4B is a front sectional view of the hermetic plug shown in FIG. 4A .

FIG. 5A is a top view of still another hermetic plug used for the capacitor shown in FIG. 1A .

FIG. 5B is a front sectional view of the hermetic plug shown in FIG. 5A .

FIG. 6A is a top view of yet another hermetic plug used for the capacitor shown in FIG. 1A .

FIG. 6B is a front sectional view of the hermetic plug shown in FIG. 6A .

FIG. 7A is a front sectional view of further another hermetic plug used for the capacitor shown in FIG. 1A .

FIG. 7B is a bottom view of the hermetic plug shown in FIG. 7A .

FIG. 8A is a sectional view of the substantial part of the hermetic plug shown in FIG. 7A , in an attached state.

FIG. 8B is a sectional view of the substantial part of the hermetic plug shown in FIG. 7A , in an operating state.

FIG. 9 is a sectional view of the substantial part of still further another hermetic plug used for the capacitor shown in FIG. 1A , in an attached state.

FIG. 10A is a top view of a capacitor according to a second exemplary embodiment of the present invention.

FIG. 10B is a front sectional view of the capacitor shown in FIG. 10A .

FIG. 10C is a bottom view of the capacitor shown in FIG. 10A .

FIG. 11 is a sectional view of the substantial part of a hermetic plug used for a capacitor according to a third exemplary embodiment of the present invention, in an attached state.

FIG. 12 is a sectional view of the substantial part of another hermetic plug used for a capacitor according to the third embodiment of the present invention, in an attached state.

FIG. 13 is a sectional view illustrating a structure of a conventional capacitor.

1 Capacitor element

2 Case

2 A, 2 D, 3 A, 12 C Recess

2 B Drawn part

2 C Curled part

2 E, 3 B, 12 E Through hole (hole)

3 , 11 , 12 Terminal plate

4 Insulating member

5 Sealing rubber

6 , 7 , 8 , 9 , 10 , 13 , 34 , 36 Hermetic plug (plug)

6 A, 6 B, 7 A, 7 B, 8 A, 8 B, 9 A, 9 B, 10 A, 10 B, 34 A, 34 B, 36 A, 36 B Flange

6 C, 7 C, 8 C Tapered part

6 D, 7 D, 8 D round part

7 E Straight part

8 E, 36 D Recess

6 E, 9 C, 10 C, 36 C Attaching part

9 D Slit

10 D Groove

10 E Appressed part

12 A Screw

12 B Projection

14 Pressure-regulating valve

15 First electrode

16 Second electrode

15 A First current collector

16 A Second current collector

15 B, 16 B Polarizable electrode layer

18 Separator

19 Electrolyte

20 Capacitor element

21 Metal plate

22 Case

22 A Projection

22 B Curled part

23 Sealing plate

24 Core material

24 A, 25 Terminal

26 Insulating member

27 Block

28 Cap

29 O-ring

30 Sealing rubber

31 Substrate

32 , 33 Hole

35 Insulating layer

37 Pin

37 A Through hole

First Exemplary Embodiment

FIGS. 1A through 1C are a top view, a front sectional view, and a bottom view, respectively of a capacitor according to the first exemplary embodiment of the present invention. FIG. 1D is an enlarged sectional view of the capacitor element in FIG. 1B . FIGS. 2A , 2 B are a top view and a front sectional view of a hermetic plug used for this capacitor.

Capacitor element 1 is formed of first electrode 15 , second electrode 16 , and separator 18 . First electrode 15 has polarizable electrode layer 15 B formed on first current collector 15 A made of aluminum foil. Second electrode 16 as well has polarizable electrode layer 16 B formed on second current collector 16 A made of aluminum foil. Then, first electrode 15 and second electrode 16 are arranged so as to be displaced in the reverse direction, and rolled up with separator 18 interposed therebetween. Both upper and lower end surfaces of capacitor element 1 have first current collector 15 A and second current collector 16 A projecting therefrom, respectively.

Capacitor element 1 is contained in case 2 , which is made of aluminum and a closed-end cylinder-shape, along with electrolyte 19 . The bottom surface of case 2 is provided with recess 2 A. First electrode 15 exposed on one end surface of capacitor element 1 is pressed to the inner surface of recess 2 A and joined by such as laser welding (e.g. emitting laser light from the outer bottom surface of case 2 ). The inner surface of recess 2 A and first electrode 15 of capacitor element 1 are thus connected with each other mechanically and electrically.

Terminal plate 3 blocks the opening of case 2 . Terminal plate 3 is provided with recess 3 A. Second electrode 16 exposed on the other end surface of capacitor element 1 is pressed to the bottom surface of recess 3 A and joined by such as laser welding (e.g. emitting laser light from the outer surface of terminal plate 3 ). The bottom surface of recess 3 A and second electrode 16 of capacitor element 1 are thus connected with each other mechanically and electrically.

Ring-like insulating member 4 made of insulating material is disposed on the upper part of the inner peripheral surface of drawn part 2 B (formed by caulking the proximity of the opening end of case 2 ). Terminal plate 3 is disposed on insulating member 4 . Sealing rubber 5 is disposed on the top peripheral surface of terminal plate 3 and compressed by forming curled part 2 C that is the opening end of case 2 curled, to seal the space between insulate case 2 and terminal plate 3 while insulating from each other.

After an electrolyte (not shown) is injected into case 2 through hole 3 B for electrolyte injection provided at terminal plate 3 , hermetic plug (hereinafter referred to as “plug”) 6 is press-fitted so as to block hole 3 B. Plug 6 is made of a rubber-like elastic body such as butyl rubber (IIR) or ethylene-propylene terpolymer (EPT). The external diameter of plug 6 is larger than the diameter of hole 3 B. In this structure, terminal plate 3 can be provided with hole 3 B even when the capacitor is miniaturized. Further, such dimensional relations of plug 6 and hole 3 B enable plug 6 to reliably seal hole 3 B.

An insulating tape (not shown) is attached to second electrode 16 of capacitor element 1 and its proximity, thereby preventing a short circuit between the inner peripheral surface of case 2 and second electrode 16 . Here, an insulating tape can be omitted by applying an insulating coat to the relevant part of the inner peripheral surface of case 2 .

The beginning and end of plug 6 in the direction for inserting (i.e. vertical direction in FIG. 2 ) are preferably provided with first flange 6 A and second flange 6 B (hereinafter both referred to as “flange”), respectively. Flanges 6 A, 6 B prevent plug 6 from being unpluged easily or from being buried in the inside of the capacitor. The end peripheral surface of flange 6 A, at the side inserted into the inside of the capacitor, is provided with tapered part 6 C. Further, the periphery of flanges 6 A, 6 B, at the side inserted, is provided with round part 6 D. In at least one of such structures, plug 6 is preferably easy to be press-fitted into hole 3 B and is resistant to damage when being press-fitted. Suitably, the angle of tapered part 6 C is 95° to 175°; the curvature radius of round part 6 D is 0.1 mm or larger; and the maximum thickness of flanges 6 A, 6 B is 0.2 mm or larger.

The external diameter of the smallest part of plug 6 , to be inserted into hole 3 B, is preferably not smaller than 106% and smaller than 170% of the diameter of hole 3 B. A smaller external diameter than this value decreases the sealing performance, thereby possibly causing leakage. On the other hand, a too large one possibly causes local breakage.

To prevent leakage while securing the sealing performance of plug 6 , a stress occurring on the peripheral surface of plug 6 tightly contacting hole 3 B is preferably 1.0 MPa or more. A stress smaller than this decreases the sealing performance. To examine a preferable shape and stress performance of plug 6 , ten pieces of plugs 6 with different external diameters are produced. The evaluation result is shown in table 1.

In table 1, the leakage of an electrolyte is examined by varying compression ratio by changing the external diameter of plug 6 press-fitting into hole 3 B with its diameter of 1. The leakage is evaluated while a gas pressure of 0.6 MPa is applied to plug 6 . Linear structural analysis and experiments are performed for plug 6 made of IIR or EPT with a Young's modulus of approximately 5.0 MPa, a Poisson's ratio of 0.48, a breaking elongation of not less than 200% to not more than 300%. In the leakage evaluation in table 1, “C” indicates leakage occurs in 5 or more samples, “B” indicates leakage occurs in 1 to 4 samples, and “A” indicates leakage does not occur in all samples.

From table 1, for a compression ratio of plug 6 of 1.05% or smaller, leakage is assumed to occur due to a small reactive force by plug 6 against the inner peripheral surface of hole 3 B. For a compression ratio exceeding 170%, leakage occurs as well. For a compression ratio of 170%, the maximum strain measured by linear structural analysis is 1.54 mm per 1 mm, as shown in table 1. Such a large strain causes the rate of elongation of plug 6 to be 254% locally. Thus, the leakage is assumed to be caused as a result that local breakage occurs in plug 6 , and thus a leakage path is generated due to a decrease in the reactive force on the inner peripheral surface of hole 3 B and breakage of the material. From these results, in order for plug 6 to seal hole 3 B, the external diameter of the smallest part of plug 6 which is to be fitted into hole 3 B is preferably not smaller than 106% and smaller than 170% of the diameter of hole 3 B.

At a compression ratio of plug 6 of 1.05%, the stress occurring on the outer peripheral surface of plug 6 measured by linear structural analysis is 0.89 MPa. At a compression ratio of plug 6 of 1.06%, the stress ranges between 1.00 MPa and 1.03 MPa, with some variation, where no leakage occurs in any case. From these results, the stress occurring on the outer peripheral surface of plug 6 needs to be at least 1.0 MPa, which does not depend on material of plug 6 . Meanwhile, the upper limit of the stress depends on the strength of material of plug 6 . From all of these, as the sealing performance against leakage, the stress occurring on the peripheral surface of plug 6 tightly contacting hole 3 B is preferably 1.0 MPa or more.

As shown in FIGS. 3A through 3C , hole 2 E for electrolyte injection may be provided at the base of case 2 , not in terminal plate 3 , to be sealed with plug 6 . Such a structure brings the same advantage as those in FIGS. 1A through 1C . Further, both hole 3 B and hole 2 E may be provided to be sealed with plug 6 . In this case, injecting an electrolyte becomes easy, which is the same for other shapes of hermetic plugs described hereinafter.

Next, a description is made for the shape of a hermetic plug with its workability improved when press-fitting into hole 3 B. FIGS. 4A and 4B are a plan view and a front sectional view illustrating another example of a hermetic plug. Hermetic plug (hereinafter referred to as “plug”) 7 , as well as plug 6 , is provided with flanges 7 A, 7 B, tapered part 7 C, round part 7 D, and straight part 7 E between the end surface at the flange 7 A side and tapered part 7 C. Straight part 7 E extends in the direction for inserting plug 7 from flange 7 A and is thinner than attaching part 7 F provided between flanges 7 A and 7 B. Straight part 7 E thus provided facilitates plug 7 to be press-fitted into hole 3 B.

FIGS. 5A , 5 B are as well a plan view and a front sectional view illustrating still another example of a hermetic plug. Hermetic plug (hereinafter referred to as “plug”) 8 , as well as plug 6 , is provided with flanges 7 A, 7 B, tapered part 7 C, round part 7 D, and recess 8 E on the end surface at the flange 8 A side. That is, recess 8 E is provided on the end surface in the direction for inserting plug 8 . With this structure, the end to be inserted is subject to deformation, thereby facilitating plug 8 to be inserted. Particularly, recess 8 E deeper than round part 8 D of flange 8 A decreases a stress at the flange 8 A side, thereby facilitating plug 8 to be press-fitted into hole 3 B.

Next, a description is made for an example where a hermetic plug has a pressure-regulating function. FIGS. 6A , 6 B are a top view and a front sectional view of such a hermetic plug. The bottom and top ends of hermetic plug (hereinafter referred to as “plug”) 9 are provided with flanges 9 A, 9 B, respectively, and the vertically central part of plug 9 has attaching part 9 C, which is inserted into hole 3 B to be cramped and formed circularly.

In addition, the central part is provided with slit 9 D vertically communicating. In a state of plug 9 press-fitted into hole 3 B, the entire plug 9 is compressed toward the center. Consequently, slit 9 D is sealed, thereby preventing an electrolyte injected into case 2 from leaking.

When the pressure within the capacitor thus formed rises to a given value or higher, the increased pressure presses and expands slit 9 D. Slit 9 D is pressed and expanded with a pressure of 0.15 MPa to 1.0 MPa, for example. Consequently, the inside of case 2 communicates partially with the outside, from which the internal gas is discharged outward, thereby decreasing the pressure inside case 2 . When the pressure inside case 2 decreases to a given value or lower, slit 9 D provided at plug 9 is again sealed in close contact, and plug 9 seals so as to prevent an electrolyte injected into case 2 from leaking.

Plug 9 thus seals hole 3 B and regulates the pressure inside case 2 . Consequently, even a miniaturized capacitor without a space for a pressure-regulating valve can be reliable with a pressure-regulating function.

Both ends of slit 9 D are preferably provided with tapered parts. The tapered parts do not contact the hole closely, and thus a gas inside case 2 is reliably discharged outward. The periphery of flange 9 A at the bottom end is preferably provided with a tapered part, which facilitates plug 9 to be press-fitted into hole 3 B.

Next, a description is made for another example where the hermetic plug has a pressure-regulating function. FIGS. 7A , 7 B are a front sectional view and bottom view of such a hermetic plug. FIGS. 8A , 8 B are sectional views of the substantial parts of the hermetic plug in an attached and operating state, respectively.

The bottom and top ends of hermetic plug (hereinafter referred to as “plug”) 10 are provided with flanges 10 A, 10 B, respectively, and the vertically central part of plug 10 has attaching part 10 C, which is inserted into hole 3 B to be cramped and formed circularly. Attaching part 10 C is formed with a length approximately twice the thickness of terminal plate 11 , and the lower part of attaching part 10 C, excluding the part tightly contacting terminal plate 11 , is provided with groove 10 D vertically cut into a part of the peripheral surface. Appressed part 10 E, where plug 10 tightly contacts the terminal plate, is formed between groove 10 D and flange 10 B. In other words, plug 10 is provided with groove 10 D extending beyond the thickness of terminal plate 3 , from the beginning toward the end in the direction for inserting plug 10 , and appressed part 10 E between groove 10 D and the end in the direction for inserting plug 10 .

A description is made for the operation of plug 10 thus formed. First, as shown in FIG. 8A , plug 10 is press-fitted into hole 3 B in terminal plate 3 , and flange 10 B is in close contact with terminal plate 3 , in a usual state. Consequently, plug 10 reliably seals hole 3 B. As shown in FIG. 8B , a rise of the pressure inside case 2 causes plug 10 to be pressed up to a position where flange 10 A tightly contacts terminal plate 11 , resulting in groove 10 D provided at the lower part of attaching part 10 C to be exposed at the front surface of terminal plate 11 . Consequently, the inside of case 2 communicates with the outside, allowing a gas inside case 2 to escape outward to decrease the pressure. That is, plug 10 moves in the direction opposite to insertion when the pressure inside case 2 rises to a given value or higher, allowing a gas inside case 2 to escape outward through groove 10 D. Here, the user can press plug 10 pressed up into the hole (the state shown in FIG. 8A ) to seal case 2 with plug 10 again.

Plug 10 thus seals hole 3 B and regulates the pressure inside case 2 . Consequently, even a miniaturized capacitor without a space for a pressure-regulating valve can be reliable with a pressure-regulating function.

Here, the periphery of flange 10 A at the bottom end is preferably provided with a tapered part, which facilitates press-fitting plug 10 into hole 3 B.

Next, a description is made for another example of a hermetic plug formed of two members. FIG. 9 is a sectional view of the substantial part of such a hermetic plug, in an attached state. The bottom and top ends of hermetic plug (hereinafter referred to as “plug”) 36 are provided with flanges 36 A, 36 B, respectively, and the vertically central part of plug 36 has attaching part 36 C, which is inserted into hole 3 B to be cramped and formed circularly. The central part is provided with recess 36 D outside the insertion direction, where pin 37 is inserted into recess 36 D. Plug 36 is made of elastic material such as rubber as well as the other hermetic plugs, and pin 37 is made of material such as metal or resin, harder than plug 36 .

To attach plug 36 and pin 37 in this way, plug 36 is inserted into hole 3 B first. In this case, with recess 36 D provided, plug 36 is subject to deformation, thereby facilitating hole 3 B to be attached. Then, pin 37 is inserted into recess 36 D. This seals recess 36 D with pin 37 , and thus plug 36 becomes resistant to deformation. As a result, plug 36 can seal hole 3 B in the same way as in FIG. 2B . Using plug 36 and pin 37 in this way improves the attaching workability compared to a hermetic plug without a recess or hollow provided inside, such as plug 6 .

Here, the central part of pin 37 is preferably provided with hole 37 A piercing in the direction for inserting pin 37 . When inserting pin 37 into recess 36 D, air inside recess 36 D escapes through hole 37 A. This prevents pin 37 from lifting toward the insertion side due to compressed air after pin 37 is inserted into recess 36 D.

There are no hollows between flanges of plugs 7 , 8 , 10 shown in FIGS. 4B , 5 B, 7 A as well, and thus the external diameter at these parts is preferably not smaller than 106% and smaller than 170% of the diameter of hole 3 B. In plug 36 shown in FIG. 9 , because recess 36 D is sealed with pin 37 , the external diameter of attaching part 36 C is preferably not smaller than 106% and smaller than 170% of the diameter of hole 3 B, as well as in plug 6 . A stress occurring on the peripheral surface of plugs 7 , 8 , 10 , 36 is preferably 1.0 MPa or more when it is appressed to hole 3 B. Even in a case where there is a hollow (slit 9 D) between the flanges, as in plug 9 shown in FIG. 6B , the external diameter of attaching part 9 C (excluding the thickness of slit 9 D) is preferably not smaller than 106% and smaller than 170% of the diameter of hole 3 B. A stress occurring on the peripheral surface of attaching part 9 C is preferably 1.0 MPa or more when it is appressed to hole 3 B.

Second Exemplary Embodiment

FIGS. 10A through 10C are a top view, a front sectional view, and a bottom view, respectively, of a capacitor according to the second exemplary embodiment of the present invention. In the capacitor according to the embodiment, a hermetic plug with a pressure-regulating function, which is described using FIGS. 6A through 8B is not used, but one of the hermetic plugs shown in FIGS. 2B , 4 B, 5 B, 9 is used, and a pressure-regulating valve is provided at the terminal plate. Components other than these are the same as the first embodiment, and thus a same component is given the same mark to omit its detailed description. Only the differences are described hereinafter using the related drawings.

Terminal plate 12 has screw 12 A for external connection. Hermetic plug (hereinafter referred to as “plug”) 13 is press-fitted into hole 12 E for electrolyte injection provided at terminal plate 12 . Pressure-regulating valve 14 is attached to terminal plate 12 . Pressure-regulating valve 14 is formed by sealing hole 12 F provided at terminal plate 12 with a gas-permeable sheet and a rubber valve, and by fixing them with a cap.

The inner surface of terminal plate 12 is provided with projection 12 B. The outer surface of terminal plate 12 is provided with recess 12 C and with projection 12 D for antirotation. The bottom surface of case 2 is provided with recess 2 D.

In the capacitor thus formed, the pressure inside case 2 of a given value or higher causes pressure-regulating valve 14 provided at terminal plate 12 to open, allowing the inside gas to escape outward. The operating pressure of pressure-regulating valve 14 is set to 0.15 MPa to 1.0 MPa, for example. Consequently, plug 13 is not necessary to have a pressure-regulating function, thereby allowing one of plugs 6 , 7 , 8 , or plug 36 and pin 37 with a simple structure according to the first embodiment to be used for plug 13 . Accordingly, when terminal plate 12 has a space for attaching plug 13 and pressure-regulating valve 14 , both can be attached simultaneously.

Third Exemplary Embodiment

FIG. 11 is a sectional view of the substantial part of a hermetic plug used for a capacitor according to the third embodiment of the present invention, in an attached state. This embodiment is different from the first embodiment in that the inner peripheral surface of hole 3 B provided at terminal plate 3 and part of the front and back surfaces linking to the inner peripheral surface are provided with insulating layer 35 . Otherwise the basic structure is the same as the first embodiment described using FIGS. 1A through 2B . Insulating layer 35 is formed with material not dissolving by an electrolyte, such as polypropylene resin. Hereinafter, advantages of insulating layer 35 are described referring to FIG. 1B as well.

One electrode of capacitor element 1 is joined to terminal plate 3 , and the other is joined to case 2 . If the negative electrode of capacitor element 1 is joined to terminal plate 3 , using the capacitor in a high-temperature, high-humidity environment facilitates the electrolyte to permeate into the interface between terminal plate 3 and hermetic plug (hereinafter referred to as “plug”) 6 , thereby possibly causing the electrolyte to leak outward.

This phenomenon is supposed to occur in the following way. That is, moisture contained in the electrolyte electrochemically reacts with aluminum composing terminal plate 3 (negative electrode) near plug 6 to generate hydroxide ions according to formula (1). The hydroxide ions react with positive ions in the electrolyte to cause the electrolyte to show alkalinity. The electrolyte thus alkalified passes through the interface between terminal plate 3 and plug 6 to contact the entire plug 6 . This alkali breaks the chemically cross-linked structure of plug 6 to deteriorate plug 6 . This decreases the sealing performance of plug 6 to cause the electrolyte to leak outward.

A slight gap in the interface between terminal plate 3 and plug 6 can allow the electrolyte showing alkalinity to permeate thereinto due to an interfacial tension, thereby possibly causing leakage. For this reason, terminal plate 3 and plug 6 need to tightly contact each other to eliminate leakage of the electrolyte.

Meanwhile, when the positive electrode of capacitor element 1 is joined to terminal plate 3 , for example, tetraethylammonium fluoroborate (N(C 2 H 5 ) 4 BF 4 ) as the solute of an electrolyte is used, tetrafluoroborate anion (BF 4 − ) (i.e. negative ions in the electrolyte) approaches the sealed part of case 2 , and hydronium ions (H 3 O + ) are generated in the electrolyte with the reaction shown by formula (3) through formula (2). With this reaction, the electrolyte near terminal plate 3 shows acidity. When the electrolyte showing acidity passes through the interface between terminal plate 3 and plug 6 and contacts terminal plate 3 , the electrolyte corrodes the surface of terminal plate 3 , thereby decreasing the sealing performance of plug 6 .

BF 4 − +H 2 O⇄BF 3 (OH) − +HF  (2)

HF+H 2 O→H 3 O + +F −   (3)

However, in this embodiment, the aluminum forming terminal plate 3 is covered with insulating layer 35 , which as a result an electrochemical reaction does not occur even when the negative electrode of capacitor element 1 is joined to terminal plate 3 . Consequently, hydroxide ions are not generated or the electrolyte does not show alkalinity, which as a result suppresses a decrease of the sealing performance of plug 6 . In this case, flanges 6 A and 6 B can also seal hole 3 B by closely contacting terminal plate 3 .

Insulating layer 35 may be formed of, other than polypropylene resin, resin or the like made of a mixture of IIR, EPT or styrene-butadiene rubber, and alicyclic petroleum resin, aliphatic petroleum resin or terpene resin. In other words, any material may be used as long as it is not dissolved by an electrolyte and is insulative.

Insulating layer 35 is preferably formed of water-repellent material such as polypropylene resin. Even when an electrolyte showing acidity passes through the interface between terminal plate 3 and plug 6 , insulating layer 35 formed with such material prevents the electrolyte from contacting terminal plate 3 in a case where the positive electrode of capacitor element 1 is joined to terminal plate 3 . This suppresses corrosion of the surface of terminal plate 3 at the interface between terminal plate 3 and plug 6 , which as a result suppresses a decrease of the sealing performance of plug 6 .

In this embodiment, the description is made for the case where sealing is made by inserting plug 6 into hole 3 B. However, in a case where sealing is made using such as a pressure-regulating valve or explosion-proof valve, instead of plug 6 , the same advantage is achieved by forming insulating layer 35 at the inner peripheral surface of hole 3 B and part of the front and back surfaces of terminal plate 3 linking to the inner peripheral surface.

As described in the first embodiment using FIGS. 4A through 4C , hole 2 E for electrolyte injection may be provided in the bottom of case 2 . In this case, it is adequate if insulating layer 35 is provided on the inner peripheral surface of hole 2 E and part of the front and back surfaces of case 2 linking to the inner peripheral surface.

If the hermetic plug can seal hole 3 B by tightly contacting the flange to terminal plate 3 through insulating layer 35 , the hermetic plug can have an explosion-proof function as well. FIG. 12 is a sectional view illustrating the structure of such hermetic plug 34 . Hermetic plug 34 is mounted to hole 3 B, which is provided at terminal plate 3 and whose inner surface is covered with insulating layer 35 . The central portion of hermetic plug 34 is provided with hollow 34 C at the side inward of the capacitor, in an attached state. The top end of hollow 34 C is sealed with thin-walled member 34 D. Hermetic plug 34 seals hole 3 B by tightly contacting flanges 34 A, 34 B to terminal plate 3 through insulating layer 35 .

If the inner pressure of the capacitor rises to a given value in this state, the pressure breaks thin-walled member 34 D to cause the gas in the capacitor to be discharged outward, with which hermetic plug 34 fulfils an explosion-proof function.

Hereinbefore, the description is made for an electric double-layer capacitor with capacitor element 1 formed of a pair of electrodes, where polarizable electrode layers are formed on both surfaces of a pair of current collectors, and a separator. However, the present invention is not limited to this structure, but is applicable to another type of capacitor with an electrolyte such as an aluminum electrolytic capacitor.

A capacitor according to the present invention can be provided with a hole for injecting an electrolyte and a pressure-regulating function at the terminal plate, which is useful for a hybrid car, for example.