Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
1998-10-13
2001-02-27
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S504000, C361S508000
Reexamination Certificate
active
06195251
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric double layer capacitor, particularly an electric double layer capacitor excellent in the operation reliability and an electrode assembly and a current collector therefor.
2. Discussion of Background
The electric double layer capacitor is based on a principle to store electric charge in an electric double layer formed at the interface between an electrode and an electrolyte. In order to improve the capacitance density of an electric double layer capacitor, it is common to employ for the electrode, a carbonaceous material such as activated carbon or carbon black having a high specific surface area or fine particles of a metal or conductive metal oxide. For efficient charging and discharging, the electrode is bonded to a current collector made of a foil or a layer having low resistance, such as a metal or graphite. As the current collector, it is common to employ a valve metal such as aluminum or a stainless steel such as SUS304 or SUS316L, which is electrochemically highly corrosion resistant.
As an electrolyte for the electric double layer capacitor, an organic electrolyte and an aqueous electrolyte are available. However, an attention has been drawn to an electric double layer capacitor employing an organic electrolyte, as the operating voltage is high, and the energy density in the charged state can be made high. In a case where an organic electrolyte is employed, if water is present in the cell of an electric double layer capacitor, the performance tends to deteriorate due to electrolysis of the water. Accordingly, the electrode is required to have water sufficiently removed, and it is common to apply drying treatment by heating under reduced pressure. As the electrode for an electric double layer capacitor, activated carbon is mainly used as the main component. However, active carbon is usually in a powder form. Accordingly, it is common to mix it with a binder containing a fluorine-containing resin such as polytetrafluoroethylene (hereinafter referred to as PTFE), followed by molding into an electrode of a sheet form, which is then electrically connected to a current collector via a conductive adhesive layer to form an electrode assembly. In order to improve the adhesion between the current collector and the electrode made of a carbonaceous material, JP-A-57-60828 or JP-A-57-84120 proposes an electrode assembly using a highly etched aluminum foil as a current collector. When such a highly etched aluminum foil is used as a current collector, and it is dipped in a slurry comprising a carbonaceous material such as active carbon, a binder and a solvent, or coated with such a slurry, followed by drying to obtain an electrode assembly, the adhesion will be improved over a case where a smooth aluminum foil or an aluminum foil having the surface roughened by e.g. sand blasting, is used. However, the obtained electrode assembly has poor strength and is likely to break in the process for producing the electrode assembly or in the process for preparing a capacitor by laminating the electrode assembly and a separator.
Further, it has been proposed to use an etched foil for an aluminum electrolytic capacitor, as the current collector for an electric double layer capacitor. However, the foil for an aluminum electrolytic capacitor is intended, as the basic purpose of the product, to have a strength sufficient to be rolled together with a separator and to provide a high capacitance. Whereas, the current collector for an electric double layer capacitor is required to have a high bonding strength with an electrode composed mainly of a carbonaceous material and to have a strength durable in various process steps for forming an electrode assembly by integrating it with the electrode, which is essentially different from the basic purpose of the foil for an aluminum electrolytic capacitor. Accordingly, even if the foil for an aluminum electrolytic capacitor is used for a current collector of an electric double layer capacitor, good properties as an electric double layer capacitor can hardly be obtained, and the productivity will also be poor.
SUMMARY OF THE INVENTION
In order to increase the power density and to secure the durability for charging and discharging cycles, it is important to increase the bond strength between the metal current collector and the electrode comprising a carbonaceous material and a binder and to reduce the electric resistance of the electrode assembly in an electrolyte, particularly when used under a severe condition, for example, in an application for an electric car or an engine-electric double layer capacitor hybrid car.
It is accordingly an object of the present invention to provide an electric double layer capacitor having a large capacitance and excellent durability for charging and discharging cycles, wherein the strength of the current collector is high, and the bond strength between the current collector and the electrode is high.
The present invention provides an electrode assembly for an electric double layer capacitor, comprising a current collector made of an aluminum foil, and an electrode comprising a carbonaceous material having a specific surface area of at least 500 m
2
/g and a binder, bonded on at least one side of the current collector, wherein said aluminum foil has a roughened layer having a thickness of from 0.5 to 5 &mgr;m on the surface on the side bonded to the electrode and has a breaking energy of at least 3 kg·mm with a dumbbell specimen of No. 1 form as stipulated in JIS K6301; and an electric double layer capacitor having such an electrode assembly.
Further, the present invention provides an electrode assembly for an electric double layer capacitor, comprising a current collector made of an aluminum foil, and an electrode comprising a carbonaceous material having a specific surface area of at least 500 m
2
/g and a binder, bonded on at least one side of the current collector, wherein said aluminum foil comprises a non-roughened portion having a thickness of from 8 to 50 &mgr;m and a roughened layer having a thickness of from 0.5 to 5 &mgr;m formed on one side or each side thereof; and an electric double layer capacitor having such an electrode assembly.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In this specification, one having an electrode comprising a carbonaceous material and a binder, integrated with a current collector, is referred to as an electrode assembly, and when used on a positive electrode side, the electrode assembly will be referred to as a positive electrode assembly, and when used on a negative electrode side, the electrode assembly will be referred to as a negative electrode assembly.
In the present invention, the thickness of the roughened layer on the surface of the aluminum foil is from 0.5 to 5 &mgr;m on average. If it is less than 0.5 &mgr;m, the bond strength between the electrode and the current collector tends to be poor. Particularly when the electrode is preliminarily formed into a sheet, followed by bonding to the current collector, the bond strength tends to be weak as the electrode sheet and the current collector will be bonded only at the surface of the current collector, whereby the bond strength tends to be poor. On the other hand, if it exceeds 5 &mgr;m, no further improvement in the bond strength tends to be observed, and the strength of the aluminum foil decreases as the thickness of the roughened layer increases. It is particularly preferably from 1 to 5 &mgr;m, more preferably from 2 to 4 &mgr;m.
In the present invention, in a case where an electrode is to be formed only on one side of the current collector, the surface-roughened layer may be formed only on the surface on one side of the aluminum foil which will be the bonding surface with the electrode. However, such a roughened layer may be formed on each side of the foil in order to form the surface-roughened layer continuously at a high speed and at a low cost on the aluminum foil.
In the present invention, the aluminum foil preferably ha
Higuchi Yoshiaki
Hiratsuka Kazuya
Ikeda Katsuji
Kawasato Takeshi
Suhara Manabu
Asahi Glass Company Ltd.
Kincaid Kristine
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thomas Eric
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