Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-06-30
2002-01-22
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C361S505000, C361S508000, C361S518000, C428S209000
Reexamination Certificate
active
06341057
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a double layer capacitor, a method for the preparation thereof and the use thereof in a combined application with batteries.
BACKGROUND OF THE INVENTION
The development of capacitor technology based on the principle of energy storage in the electrochemical double layer formed at the interface between an ion-conducting phase, i.e. the electrolyte, and an electron-conducting phase, i.e, the electrode, has provided capacitors of extremely high capacitance. Such capacitors are usually referred to as double layer capacitors, supercapacitors, electrochemical capacitor or ultracapacitors.
The energy storage in such capacitors may involve electrochemical processes like in batteries, i.e. energy is stored chemically in the capacitor electrodes and the electrode reactions involve redox processes. One such system is the rubidium silver iodide low voltage electrochemical Hypercap capacitor from Technautics. Alternatively, the capacitor may be based entirely on the double layer energy storage principle. Such capacitors will below be referred to as double layer capacitors.
One important application of double layer capacitors is in hybrid combinations with batteries. Traditionally, double layer capacitors provide high power capabilities compared to batteries, e.g. during pulse applications a higher power is obtainable from the double layer capacitor than from the battery. Accordingly, the power capability of hybrid combinations is higher than for the battery alone, and further the stress on the battery is reduced due to more uniform load. A number of patents describes the use of such battery-capacitor hybrid systems, among those U.S. Pat. No. 5,587,250 and U.S. Pat. No. 5,670,266 to Motorola.
High capacitance and low impedance are the two main technical features providing the high power capabilities of double layer capacitors. The capacitance provides the energy for the load, whereas the low impedance allows good power accessibility in that the energy is available without too high internal losses.
In particular, as most pulsed loadings operate in the Hz-range from 1Hz to 1kHz, the capacitor impedance in this should be low, i.e. the main part of the energy stored in the capacitor should be available in this frequency range. A number of patents and patent applications describe approaches to double layer capacitors of low internal resistance.
European patent application EP 763,836 to Nisshinbo Industries discloses a polarisable electrode for use in a electric double layer capacitor, having a low internal resistance and a high capacitance, the carbon mixture of the electrode characterised by comprising fibrillated carbon. A conductive agent may be added to the carbon mixture to improve electrical conductivity. Such electrode has long life and can be charged and discharged at large electric currents.
European patent application EP 660,345 to Nisshinbo Industries discloses a polarisable electrode comprising a solid active carbon, characterised in that the electrode on the surface and/or inside has discontinuous portions free from said solid active carbon. A conductive agent such as graphite or carbon black may be added to the carbon mixture to improve electrical conductivity.
U.S. Pat. No. 5,682,288 to Japan Gore-Tex, Inc. discloses a planar layered electrode comprising acicular electrically conductive particles to provide low electrical resistance and which has high electric storage capacity.
U.S. Pat. No. 5,077,634 to Isuzu describes electrodes for double layer capacitors, which are compressed for reduction of internal resistance.
Low impedance double layer capacitors should display low intra-component impedance as well as low inter-component impedance. Whereas the above prior art has been focussed on low intra-component impedance in the electrode structures, the impedance of the interface to the current collectors is just as important. Electrodes of high resistance at the current collector-electrode interphase, e.g. due to poor adhesion and poor electrical contact, will suffer from poor power capability due to internal losses.
A number of patents describes double layer capacitors of elaborated electrode-current collector interface.
JP-06,084,700 to Matsushita discloses a conductor layer made of carbonaceous material such as graphite provided between the electrode and the collector, the binder being an acrylic acid-styrene copolymer and providing increased adhesion for the electrodes. JP-61,102,025 to Matsushita discloses an electric double layer capacitor of polarising electrodes from activated carbon fiber felt, the electrode having a conductive layer on one side. The conductive layer may be aluminium, titanium, nickel or tantalum, stainless steel or a conductive paint containing carbon particles. The electrolyte of said capacitor is a solution of (NEt
4
) ClO
4
in propylene carbonate. Upon repeated charging and discharging, the capacitor has stable capacitance.
JP-60,050,914 to NEC discloses a double layer capacitor having a reduced internal resistance, the capacitor having electrodes covered by conductive layers consisting of a polymer such as polypropylene and a conductor such as carbon black or carbon fibers.
U.S. Pat. No. 5,150,283 to Matsushita describes electrodes for a double layer capacitor composed of electrically conductive substrates, coated with a mixture of activated carbon with a water-soluble material-based binding agent. Such capacitor has low internal resistance, withstanding high voltages.
U.S. Pat. No. 5,115,378 to Isuzu describes an electrode for a double layer capacitor formed from a porous sintered body of joined active carbon particles, conductivity being provided during sintering. Such capacitor has reduced internal resistance as low contact resistance between electrode body and current collector is obtained.
U.S. Pat. No. 5,099,398 to Isuzu describes an electrode for a double layer capacitor applied on an electrically conductive film, the surface of which is dissolved in a solvent and the dissolved portions of said current collector being present in the pores of the electrode bodies. Such capacitor has low internal resistance due to intimate contact at the molecular level.
Accordingly, several approaches to low internal resistance capacitors exist, including introducting of conductive coatings at the interface between the electrode and the current collector. Still, however, the is a need for a conductive coating which is stable under the severe conductions existing at this interface. This conductive coating should provide good adhesion to the current collector and to the electrode, and it should maintain its mechanical integrity upon handling such as folding and winding. Further, the coating should be stable at highly oxidative potentials at the positive electrode, and at highly reductive potentials at the negative electrode as well as being stable against corrosive electrolytes.
OBJECT OF THE INVENTION
Therefore, it is the objective of the present invention to provide a double layer capacitor with a conductive coating, which is mechanically, chemically, and eletrochemically stable during manufacture and operation. Such coating comprises a conductive agent, a binder and optionally auxiliary materials. Auxiliary materials may be materials used entirely during the processing of the conductive coating and which are removed from the final coating, or they be materials which facilitate the processing and which remain in the final coating.
BRIEF DESCRIPTION OF THE INVENTION
From a comprehensive study of a high number of conductive coatings it has now been found, that those coatings based on binders of the melamine resin type fulfil the above objectives.
Surprisingly it was found, that the melamine resin binders provide long term reduction stability at potentials as low as +1.OV vs. Li/Li
+
. Whereas the considerable stability of melamine resins against oxidation is well known from their long term stability under ambient conditions, their high stability against reduction is surprising considering their chemi
Beck Hans Christian
Nissen Ole Stig
Schou Michael
Danionics A/S
Darby & Darby
Ha Nguyen T
Reichard Dean A.
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