Capacitor

The present invention relates to a capacitor used in various fields such as a backup power supply and regeneration in various types of electronic appliances, hybrid electric vehicles, and fuel-cell electric vehicles, or for power storage.

Capacitors, which have high reliability in rapid charge and discharge, are used in various technical fields. A conventional electric double layer capacitor described in Patent Document 1 out of such capacitors includes positive and negative electrodes polarizable electrode layers mainly made of activated carbon. The electric double layer capacitor with aqueous electrolyte has a withstand voltage of 1.2V. The capacitor with organic electrolyte has a withstand voltage ranging from 2.5 to 3.3V.

Since energy of the capacitor is proportional to the square of its withstand voltage, the capacitor with the organic electrolyte (high withstand voltage) has higher energy than the capacitor with the aqueous electrolyte. Even the electric double layer capacitor with the organic electrolyte, however, has an energy density less than 1/10 of that of a secondary battery, such as a lead-acid battery. Capacitors are required to have larger energy density.

It is known that, to improve the durability of such capacitors, a separator made of resin, such as polyethylene and polypropylene, is used.

A conventional capacitor includes a separator having a sheet shape for isolation between the positive and negative electrodes. In view of manufacturing and strength, the separator necessarily has a thickness of approximately 25 μm, thus being prevented from having a small thickness and a large capacitance.

Patent Document 1: JP10-270293A

A capacitor includes a first collector made of metal foil, a first electrode layer placed on a surface of the first collector and mainly containing a carbonaceous material, a resin layer provided on the first electrode layer, a second electrode provided on the resin layer and mainly containing a carbonaceous material, a second collector provided on the second electrode layer and made of metal foil, a case accommodating the first collector, the first electrode layer, the resin layer, the second electrode, and the second collector therein, and an electrolyte accommodated in the case. The resin layer has a non-woven fabric form of fibers made of resin irregularly bonded to one another. The fibers of the resin layer are entangled with the first electrode layer.

This capacitor can be thin and small.

FIG. 1A is a sectional view of a capacitor element of a capacitor according to an exemplary embodiment of the present invention.

FIG. 1B is a sectional view of the capacitor according to the embodiment.

FIG. 2 is an enlarged sectional view of the capacitor element shown in FIG. 1A .

FIG. 3A shows evaluation results of the capacitor according to the embodiment.

FIG. 3B shows evaluation results of the capacitor according to the embodiment.

FIG. 4 is a perspective view of another capacitor according to the embodiment.

FIG. 5 is an enlarged sectional view of another capacitor element according to the embodiment.

FIG. 6A is an enlarged sectional view of still another capacitor element according to the embodiment.

FIG. 6B is an enlarged sectional view of a further capacitor element according to the embodiment.

FIG. 7 is a plan view of a capacitor element of a further capacitor according to the embodiment.

FIG. 8 is a sectional view of a further capacitor according to the embodiment.

3 Collector (First Collector, Second Collector)

3 D End of Collector

4 Electrode Layer (First Electrode Layer)

5 Resin Layer (First Resin Layer)

8 A Through-Hole

14 Electrode Layer (Second Electrode Layer)

15 Resin Layer (Second Resin Layer)

51 Electrolyte

52 Case

53 A Terminal (First Terminal)

53 B Terminal (Second Terminal)

FIG. 1A is a sectional view of capacitor element 1002 of electric double layer capacitor 1001 according to an exemplary embodiment of the present invention. FIG. 1B is a sectional view of electric double layer capacitor 1001 . FIG. 2 is an enlarged sectional view of capacitor element 1002 . As shown in FIGS. 1A and 2 , capacitor element 1002 includes positive electrode 1 and negative electrode 2 . Collector 3 made of metal foil, such as aluminum foil, has surface 3 A and surface 3 B opposite to surface 3 A. Each of positive electrode 1 and negative electrode 2 includes collector 3 , polarizable electrode layer 4 mainly containing activated carbon powder 4 A which is placed on surface 3 A and exposes end 3 D of collector 3 , polarizable electrode layer 14 mainly containing activated carbon powder 14 A which is provided placed on surface 3 B and exposes end 3 D of collector 3 , and resin layer 5 having a non-woven fabric structure bonded onto surface 4 D of polarizable electrode layer 4 . That is, resin layer 5 has fibers unevenly bonded to one another. Polarizable electrode layer 4 has surface 4 E situated on surface 3 A of collector 3 and surface 4 D opposite to surface 4 E. Polarizable electrode layer 14 has surface 14 D situated on surface 3 B of collector 3 and surface 14 E opposite to surface 14 D.

Positive electrode 1 and negative electrode 2 overlap each other such that ends 3 D of collectors 3 are directed alternately to directions 1002 A and 1002 B opposite to each other, providing capacitor unit 1003 . Plural capacitor units 1003 are stacked. End 3 D of the collector of positive electrode 1 is connected to collector plate 6 A. End 3 D of collector 3 of negative electrode 2 is connected to collector plate 6 B, thus providing capacitor element 1002 . As shown in FIG. 1B , electric double layer capacitor 1001 includes capacitor element 1002 , electrolyte 51 , and case 52 made of metal for accommodating capacitor element 1002 and electrolyte 51 therein.

A method of manufacturing electric double layer capacitors 1001 will be described below.

Aluminum foil having high purity with a thickness of approximately 22 μm is etched electrolytically in hydrochloric acid-based etching liquid to roughen a surface of the foil to produce collector 3 .

Phenolic resin-based, activated carbon powder 4 A and 14 A with an average particle diameter of 5 μm are prepared. Conductive agents 4 B and 14 B made of acetylene black with an average particle diameter of 0.05 μm are prepared. Binders 4 C and 14 C made of carboxymethylcellulose (CMC) and polytetrafluoroethylene (PTFE) are prepared. Dispersion medium containing which is mixture of methanol and water is prepared. 10 parts of weight of activated carbon powder 4 A ( 14 A), 2 parts of weight of conductive agent 4 B ( 14 B), 1 part of weight of binder 4 C ( 14 C) are mixed and kneaded with a kneading apparatus while the dispersion medium is added to them, thereby providing paste having a predetermined viscosity. The paste is applied onto surfaces 3 A and 3 B so as to expose portions 3 E and 3 F of surfaces 3 A and 3 B including end 3 D of collector 3 . Collector 3 having the paste applied thereto is dried in atmosphere at a temperature of 85° C. for 5 minutes, thereby forming polarizable electrode layers 4 and 14 on surfaces 3 A and 3 B of collector 3 , respectively.

Subsequently, resin layer 5 is bonded onto surface 4 D of polarizable electrode layer 4 by the following method. The resin may be thermoplastic resin. Melting thermoplastic resin, such as polybutylene terephthalate (PBT) is injected with high-temperature, high-speed airflow from a die with a cap having a lot of apertures therein. The injected resin has fine fibrous form and accumulates on surface 4 D of polarizable electrode layer 4 to form resin layer 5 . The accumulating resin having the fibrous form has a non-woven fabric structure, namely has fibers unevenly bonded to one another. The resin having the fine fibrous form accumulated on resin layer 5 is bonded so as to are entangled with asperities on surface 4 D of polarizable electrode layer 4 . Resin layer 5 has a thickness of about 10 μm. After resin layer 5 is formed, collector 3 with polarizable electrode layers 4 and 14 and resin layer 5 formed thereon is cut into positive electrode 1 and negative electrode 2 having a predetermined size.

Then, positive electrode 1 and negative electrode 2 are stacked such that end 3 D of collector 3 of positive electrode 1 and end 3 D of collector 3 of negative electrode 2 are directed alternately in directions 1002 A and 1002 B opposite to each other, thereby producing plural capacitor units 1003 . At this moment, resin layer 5 of positive electrode 1 is situated on surface 14 E of polarizable electrode layer 14 of negative electrode 2 . Next, plural capacitor units 1003 are stacked such that resin layer 5 of positive electrode 1 is situated on surface 14 E of polarizable electrode layer 14 of negative electrode 2 . After that, end 3 D of collector 3 of positive electrode 1 is connected to collector plate 6 A by laser welding, and end 3 D of collector 3 of negative electrode 2 is connected to collector plate 6 B by laser welding, thereby producing capacitor element 1002 .

After that, terminals 53 A and 53 B are joined to collector plates 6 A and 6 B, respectively. Then, capacitor element 1002 and electrolyte 51 are accommodated into case 52 made of metal, such as aluminum, through opening 52 A to impregnate capacitor element 1002 with electrolyte 51 . Electrolyte 51 may employ 4-ethyl ammonium 4-fluorinated boron dissolved in propylene carbonate.

Next, sealing rubber 54 is fit into opening 52 A of case 52 . Terminals 53 A and 53 B are exposed outside case 52 through through-holes 54 A and 54 B provided in sealing rubber 54 . After that, an outer circumference close to opening 52 A of case 52 is shrunk inward and circularly, and an open end of metal case 52 is curled to seal opening 52 A, thereby providing electric double layer capacitor 1001 .

In electric double layer capacitor 1001 according to the embodiment, components, particularly collector 3 , of each of positive electrode 1 and negative electrode 2 provides capacitor unit 1003 with mechanical strength. Collector plates 6 A and provide capacitor element 1002 with mechanical strength, and hence, allow the mechanical strength of resin layer 5 as a separator to be unnecessary, thus allowing resin layer 5 to only an insulating property.

In electric double layer capacitor 1001 , resin layer 5 is fixed tightly to polarizable electrode layer 4 by hydrogen bonding. Hence, resin layer 5 is made of material which can be bonded with polarizable electrode layer 4 by hydrogen bonding. Resin layer 5 is made of resin preferably containing halogen element in its chemical formula to cause the hydrogen bonding. Besides polybutylene terephthalate (PBT), resin layer 5 may be made of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyphenylene sulfide (PPS), aramid, polyimide, nylon, modified PP, modified PE, polyvinylidene fluoride (PVDF), or polytetrafluoroethylene (PTFE). These resins can be bonded to polarizable electrode layer 4 by hydrogen bonding with a large bonding strength. In order to have resistance to heat, resin layer 5 may be made more preferably of PBT, PET, PPS, aramid, polyimide, nylon, PVDF, or PTFE.

A conventional electric double layer capacitor including a separator that is a member separate from of polarizable electrode layers has a minimum thickness of 25 μm. Electric double layer capacitor 1001 according to the embodiment including resin layer 5 instead of a separator can have a minimum thickness less than 10 μm, and thus has a thin, small size, and a large capacitance.

Samples of Examples 1 to 9 and comparative examples 1 and 2 of electric double layer capacitor 1001 including polarizable electrode layers 4 and 14 with various thicknesses T 1 and resin layer 5 with various thicknesses T 2 were produced by the above method. Capacitor element 1002 of each sample had a cylindrical roll type with a diameter of 18 mm and a length of 50 mm. The samples were evaluated in the strength of bonding between resin layer 5 and polarizable electrode layer 4 , capacitances, and resistances, which are shown in FIGS. 3A and 3B . A constant voltage was applied to the samples by feeding a constant current so that the samples were fully charged and completely discharged in 1 minute. The capacitances and the resistances were measured while the samples were discharged with a constant current after the charging.

The strength of bonding between resin layer 5 and polarizable electrode layer 4 was evaluated according to “Adhesives. Test Method of Peel Bonding Strength Part 1; 90-degree peel” (JIS-K6854-1). In FIGS. 3A and 3B , a sample that represents destruction of resin layer 5 or peeling between polarizable electrode layer 4 and resin layer 5 is marked with “NG”; otherwise, “OK”.

The ratio R 1 of thickness T 2 of resin layer 5 to thickness T 1 of polarizable electrode layers 4 and 14 for Examples 1 to 9 ranges from 0.03 and 2.00. These samples did not represent the peeling between resin layer 5 and each of polarizable electrode layers 4 and 14 , and exhibited favorable capacitances and resistances. Particularly, samples of Examples 1, 2, 4, 5, and 8 includes resin layer 5 with a thickness of 10 μm which is smaller than a thickness of 20 μm of the separator of the conventional electric double layer capacitor.

The separator of the conventional electric double layer capacitor made mainly of cellulose has a thickness of 25 μm for mass production. In double layer capacitor 1001 according to the embodiment including fibriform resin layer 5 supported unitarily with polarizable electrode layer 4 , the thickness of resin layer 5 can be less than 20 μm.

In a sample of comparative example 1 having ratio R 1 of a large value, 2.50, a force of 17 N/m causes resin layer 5 to be peeled. If ratio R 1 is large, resin layer 5 itself has a small strength. Such resin layer 5 may be peeled off when a capacitor unit is rolled to produce a capacitor element, possibly short-circuiting between positive electrode 1 and negative electrode 2 . Resin layer 5 having fiber form adheres to polarizable electrode layer 4 by entangling with polarizable electrode layer 4 physically. As the ratio R 1 of thickness T 2 of resin layer 5 to thickness T 1 of polarizable electrode layer 4 ) increases, some of fibriform resin of resin layer 5 cannot entangle with polarizable electrode layer 4 and may divide resin layer 5 itself.

A sample of comparative example 2 having ratio R 1 of a small value, 0.013, has significantly a small capacitance and a large resistance. Resin layer 5 of this sample cannot retain a large amount of the electrolyte, accordingly prevents the capacitor from having a small capacitance and a large resistance.

In electric double layer capacitor 1001 according to the embodiment, fibriform resin of resin layer 5 entangles with polarizable electrode layers 4 and 14 physically to bond resin layer 5 to polarizable electrode layers 4 and 14 . Hydroxyl groups and carboxyl groups on surfaces 4 D and 14 E of polarizable electrode layers 4 and 14 are bonded mainly with hydroxyl groups and carboxyl groups on the surface of fibriform resin of resin layer 5 by hydrogen bonding to provide a chemical bonding force. Resin layer 5 is bonded securely with polarizable electrode layers 4 and 14 by the physical and chemical bonding forces.

FIG. 4 is a perspective view of another electric double layer capacitor 1004 according to the embodiment. In FIG. 4 , components identical to those of electric double layer capacitor 1001 shown in FIGS. 1A and 1B are denoted by the same reference numerals, and their description will be omitted. Electric double layer capacitor 1004 includes roll-type capacitor element 1005 produced by stacking and rolling positive electrode 1 and negative electrode 2 . Terminals 63 A and 63 B are connected to positive electrode 1 and negative electrode 2 , respectively. Capacitor element 1005 with terminals 63 A and 63 B connected thereto is accommodated in case 62 made of metal together with electrolyte 61 . Opening 2 of case 62 is sealed with sealing rubber 64 . Sealing rubber 64 has through-holes 64 A and 64 B formed therein. Terminals 63 A and 63 B are exposed outside case 62 through through-holes 64 A and 64 B. In electric double layer capacitor 1004 , resin layer 5 and polarizable electrode layers 4 , 14 of positive electrode 1 and negative electrode 2 are bonded securely to each other by the physical and chemical bonding forces, thus allowing roll-type capacitor element 1005 to be produced easily. Roll-type capacitor element 1005 can be produced at higher productivity at lower cost than stacked-type capacitor element 1002 , and can efficiently release heat generated upon rapid charging and discharging of eth capacitor with a large current. Hence, electric double layer capacitor 1004 including roll-type capacitor element 1005 can be used for devices including an industrial device, such as a hybrid vehicle, power shovel, and crane, with a large current passing the element,

Electric double layer capacitor 1001 according to the embodiment has a small resistance as well as a large capacitance.

Resin layer 5 of electric double layer capacitor 1001 does not include binder causing deterioration with time. The sample of Example 7 shown in FIG. 3B includes resin layer 5 with a thickness T 2 of 25 μm. That is, thickness T 2 of resin layer 5 of electric double layer capacitor 1001 can be 20 μm or larger. The sample of Example 7 has the initial capacitance and resistance substantially equal to those of the conventional capacitor including the separator, but is prevented from having a decrease of the capacitance with time and an increase of the resistance with time. The separator of the conventional capacitor contains binder to retain its shape. The binder decomposes when charging and discharging, adheres to the proximity of a polarizable electrode. The adhering binder decreases the capacitance of the capacitor and increases the resistance.

Resin layer 5 of electric double layer capacitor 1001 according to the embodiment which does not contain binder prevents the capacitance from decreasing with time due to the binder and prevents the resistance from increasing with time due to the binder.

FIG. 5 is an enlarged sectional view of another capacitor element 1005 according to the embodiment. In FIG. 5 , components identical to those of capacitor element 1002 shown in FIG. 2 are denoted by the same reference numerals, and their description will be omitted. In positive electrode 1 and negative electrode 2 of capacitor element 1002 shown in FIG. 2 , resin layer 5 is bonded to surface 4 D of polarizable electrode layer 4 . Positive electrode 1 and negative electrode 2 of capacitor element 1005 shown in FIG. 5 further include resin layer 15 bonded to surface 14 E of polarizable electrode layer 14 . Resin layer 15 has the same structure as that of resin layer 5 , and is bonded to surface 14 E of polarizable electrode layer 14 . Resin layers 5 and 15 overlap each other in capacitor element 1002 shown in FIGS. 1A and 1B . Resin layer 5 faces surface 3 A of collector 3 across polarizable electrode layer 4 . Resin layer 15 faces surface 3 B of collector 3 across polarizable electrode layer 14 .

In capacitor element 1005 shown in FIG. 5 , resin layers 5 and 15 may be made of resin with resistance to strong acid. The resistance to strong acid suppresses deterioration of positive electrode 1 . The resins with resistance to acid resistance may be PE, PP, PVDF, PET, PBT, PPS, aramid, modified PP, and modified PE. The resins can be preferably PP or modified PP, and more preferably PET, PBT, PPS, or aramid, which increases resistance to heat.

Here, modified PP or modified PE may be used as long as it has higher resistance to acid, alkali, and heat than regular PP or PE, and thus acid-modified PP or acid-modified PE is used. The acid modification includes carboxylic acid modification and maleic acid modification, and preferably carboxylic acid modification is used.

In capacitor element 1005 shown in FIG. 5 , resin layers 5 and 15 may be made of resin with resistance to strong alkali. The resistance to strong alkali prevents deterioration of negative electrode 2 . Resin with resistance to strong alkali may be cellulosic resin, PP, PBT, PPS, polyamide, or aramid, and may be preferably cellulosic resin, PP, PBT, or PPS.

In capacitor element 1005 shown in FIG. 5 , resin layer 5 may be made of resin with resistance to strong acid, and resin layer 15 has resistance to strong alkali. Resin layer 5 contacts resin layer 15 in capacitor unit 1003 and capacitor element 1002 . This structure provides resistance to both strong acid and strong alkali, thus allowing the electrode shown in FIG. 5 to be used for both positive electrode 1 and negative electrode 2 . Similarly, resin layer 15 may be made of resin with resistance to strong acid, and resin layer 5 may be made of resistance to strong alkali.

Alternatively, resin layers 5 and 15 may be made of material containing resin with resistance to strong acid and resin with resistance to strong alkali.

Instead, positive electrode 1 and negative electrode 2 may be heated at a temperature higher than the melting point of the resin forming resin layer 5 . Then, a portion of resin layer 5 melts and enters in polarizable electrode layer 4 from surface 4 D of polarizable electrode layer 4 , thereby further enhancing the bond strength between resin layer 5 and polarizable electrode layer 4 . The melting portion of resin layer 5 may contact and fusion-adhere to portion 3 E of collector 3 exposed from polarizable electrode layer 4 , and allows resin layer 5 to be bonded securely to collector 3 , accordingly allowing fixing resin layer 5 securely to polarizable electrode layer 4 .

In electricity double layer capacitor 1001 , terminal 53 A and terminal 53 B are joined to collector plate 6 A joined to positive electrode 1 and collector plate 6 B joined to negative electrode 2 , respectively. Terminals 53 A and 53 B may be directly joined to ends 3 D of collectors 3 of electrodes 1 and 2 , respectively.

FIG. 6A is an enlarged sectional view of further capacitor element 1006 according to the embodiment. In FIG. 6A , components identical to those of capacitor element 1005 shown in FIG. 5 are denoted by the same reference numerals, and their description will be omitted. In capacitor element 1006 , through-hole 8 A communicating with surfaces 3 A and 3 B of collector 3 is provided in portion 3 E of collector 3 which does not having polarizable electrode layers 4 and 14 thereon to be exposed from polarizable electrode layers 4 and 14 . Resin layer 5 bonded to surface 4 D of polarizable electrode layer 4 has portion 9 which extends through through-hole 8 A, which is bonded to polarizable electrode layer 14 , and which is connected to resin layer 15 . This structure increases the area of a portion of resin layer 5 contacts polarizable electrode layer 4 , accordingly fixing resin layers 5 and 15 to polarizable electrode layers 4 and 14 securely.

FIG. 6B is an enlarged sectional view of further capacitor element 1007 according to the embodiment. In FIG. 6B , components identical to those of capacitor element 1002 shown in FIG. 2 are denoted by the same reference numerals, and their description will be omitted. In capacitor element 1007 , through-hole 8 B communicating with surfaces 3 A and 3 B of collector 3 is provided in collector between polarizable electrode layers 4 and 14 . When spraying resin onto surface 4 D of polarizable electrode layer 4 so as to form resin layer 5 on polarizable electrode layer 4 , air is sucked through through-hole 8 B from surface 3 B of collector 3 . This operation causes resin forming resin layer 5 to enter into polarizable electrode layer 4 and to reach polarizable electrode layer 14 through through-hole 8 B. Thereby, fine, fibriform resin fiber 9 B made of the same material as that of resin layer 5 is formed, and resin fiber 9 B is placed inside polarizable electrode layer 4 to reach an inside of polarizable electrode layer 14 through through-hole 8 B. Resin fiber 9 B bonds resin layer 5 securely to surface 4 D of polarizable electrode layer 4 , thereby reliably preventing short-circuiting between positive electrode 1 and negative electrode 2 .

FIG. 7 is a plan view of capacitor element 1002 shown in FIG. 1A as seen from direction 1002 C. Portion 3 E of collector 3 of positive electrode 1 has two through-holes 12 A formed therein. Portion 3 E of collector 3 of negative electrode 2 has two through-holes 13 A formed therein. The distance between two through-holes 12 A is different from the distance between two through-holes 13 A.

Plural capacitor units 1003 each formed by stacking positive electrode 1 and negative electrode 2 are stacked to form capacitor element 1002 . Through-holes 12 A and 13 A are aligned to connect collectors 3 of positive electrode 1 and negative electrode 2 to collector plates 6 A and 6 B accurately by laser welding.

In capacitor element 1006 shown in FIG. 6A , through-hole 8 A formed in collector 3 may function as through-holes 12 A and 13 A for positioning. In capacitor element 1007 shown in FIG. 6B , through-hole 8 B formed in collector 3 may function as through-holes 12 A and 13 A for positioning.

Through-holes 12 A and 13 A have an elliptical shape, but may have a circular shape. Through-holes 12 A and 13 A having the elliptical shape can prevent rotation of positive electrode 1 and negative electrode 2 . The distance between two through-holes 12 A may be identical to or different from the distance between two through-holes 13 A. The different distances can prevent rotation of positive electrode 1 and negative electrode 2 .

FIG. 8 is a sectional view of further capacitor element 2001 according to the embodiment. In FIG. 8 , components identical to those of capacitor element 1002 shown in FIG. 1A are denoted by the same reference numerals, and their description will be omitted. Capacitor element 2001 includes plural capacitor elements 1002 ( 1004 , 1005 , 1006 , 1007 ) connected to one another. In capacitor element 2002 , plural capacitor elements 1002 ( 1004 , 1005 , 1006 , 1007 ) are connected serially, but may be connected by another manner, such as parallel connection.

Electric double layer capacitor 1001 according to the embodiment may be applicable to a capacitor including a collector, a carbonaceous material provided on a surface of the collector, and lithium ions stored in the carbonaceous material. This case requires a process for occluding lithium ions into the carbonaceous material (a pre-doping process). In this case, if a lithium source is provided directly on the surface of the polarizable electrode layer of the negative electrode in this process, a resin layer cannot be provided on the surface of the polarizable electrode layer of the negative electrode, but can be provided on the surface of the polarizable electrode layer of the positive electrode, providing the same effects.

In addition to lithium ions that are occluded and desorbed into a carbonaceous material, a capacitor using rare earthes (e.g. lanthanum ions) that are occluded and desorbed into alkali metal (e.g. sodium ions, potassium ions), alkali-earth metal, and fullerene provides the same effects.

In the case that the carbonaceous material has therein anions, not cations, occluded therein, a resin layer provided on an electrode layer of another electrode than the electrode performing occlusion and desorption provides the same effects, namely, completes the pre-doping process in a short time while decreasing the resistance and increasing the capacitance.

A capacitor according to the present invention can be thin and small, and is useful for a backup power supply and regeneration in hybrid electric vehicles and fuel-cell electric vehicle.