Electric double-layer capacitor and carbon material therefor

Details Electric double-layer capacitor and carbon material therefor Electric double-layer capacitor and carbon material therefor

2001-07-02

2004-04-13

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Double layer electrolytic capacitor

C361S503000, C361S504000, C361S508000, C361S516000, C029S025030, C429S329000, C429S341000

Reexamination Certificate

active

06721168

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric double-layer capacitor using a certain carbon material and, more particularly, to a carbon acting as an active material used in polarized electrodes that are employed in an electric double-layer capacitor. The invention relates also to a method of fabricating the aforementioned carbon, a method of selecting the constituent materials of an electrolyte solution for use in an electric double-layer capacitor, a method of fabricating such an electric double-layer capacitor, and its usage.
2. Description of the Related Art
Electric double-layer capacitors (also known as supercapacitors, ultracapacitors, pseudocapacitors, hybrid capacitors, or electrochemical capacitors) capable of being charged and discharged with large currents are promising as electric power storage devices that are frequently charged and discharged such as batteries for electric vehicles, auxiliary power supplies for solar batteries, and auxiliary power supplies for wind generators. Therefore, there is a demand for an electric double-layer capacitor having a high energy density, being capable of being quickly charged and discharged, and having excellent durability (see, for example, “Fourth Symposium On State-Of-The-Art Techniques Regarding EV/HEV: Present Situation Of Capacitor Technologies And Problems To Be Solved”, Executive Committee On “International Symposium On State-Of-The-Art Techniques Regarding Batteries For Electric Vehicles”, Nov. 8, 1999).
In an electric double-layer capacitor, a pair of polarized electrodes are located opposite to each other via a separator within an electrolyte solution to form positive and negative electrodes. In principle, electric charge is stored in an electric double-layer formed at the interface between each polarized electrode and the electrolyte solution. Accordingly, only activated carbon having a large specific surface area has been used in the past because it has been considered that the capacitance of the electric double-layer capacitor is roughly proportional to the surface area of the polarized electrodes.
In other words, a substance having a maximum surface area per unit weight has been selected as the material of the polarized electrodes because the electric double-layer capacitor is formed at the interface between a conductive material in solid phase and an electrolyte solution. In practice, the aforementioned “per unit weight” should be read as “per unit volume”, since the space consisting of thin holes between the carbon particles forming the electrode and inside the carbon particles is filled with the electrolyte solution and the weight of the electrolyte solution is added.
Such activated carbon is fabricated by carbonizing a carbonaceous material at a temperature lower than 500° C. and then activating the material. The activating operation is carried out, for example, by heating the material to 600° C. to 1000° C. within an atmosphere of water vapor, carbon dioxide, or the like or mixing zinc chloride, potassium hydroxide, or the like into the material and heating the mixture within an inert atmosphere. Micropores are created by the activating process, thus increasing the specific surface area of the activated carbon. Specific surface areas measured by a BET gas absorption measurement method are generally about 1,000 to 2,000 m
2
/g.
On the other hand, it has already been proposed to provide a novel polarized electrode made of a carbon having a specific surface area of about 300 to 400 m
2
/g and an interplanar spacing (inter-layer distance) (d
002
) of 0.365 to 0.385 nm in order to obtain an electric double-layer capacitor having a large capacitance (Japanese patent laid-open No. 11-317333). In particular, an easily graphitizable carbon containing a large amount of crystallites of a multilayer graphite structure having a well developed carbon mesh surface structure is prepared. This carbon is dry distilled at 700 C. to 850 C. to remove the volatile components. The remaining material is thermally treated together with KOH at 800 C. Using this carbon, an improvement of about 40% in capacitance over the electric double-layer capacitor using the prior art activated carbon has been accomplished.
However, the electric double-layer capacitor proposed by the present applicant as mentioned above suffers from some difficulties. That is, the capacitance decreases during repeated use. This involves generation of gas. Also, the internal resistance increases. In addition, the mechanism of capacitance production is not understood.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an electric double-layer capacitor which is free of the foregoing problems and has improved energy density and power density.
We have considered the capacitance generation mechanism of the above-described carbon fabricated by the aforementioned method from an easily graphitizable carbon in which crystallites of the multilayer graphite structure described above have developed. That is, a large capacitance is created despite the fact that the surface area is small. We have analyzed the cause of generation of gas when the capacitor is operated at a voltage of 3.0 V or higher and the cause of deterioration of the characteristics.
Thus, it is a more specific object of the invention to provide an electric double-layer capacitor which has an improved capacitance per unit volume, improved repeated use durability at an operating voltage of 3.0 V or higher, a decreased internal resistance, and an enhanced power density, and which can be quickly charged and discharged.
It is important to improve the stability of the capacitance value and the usable voltage when improving the energy density. It is important that the response speed is improved and the internal resistance is reduced when enhancing the power density.
In summary, the present invention provides a novel electric double-layer capacitor which uses a carbon material entirely different in characteristics from those of the prior art activated carbon and which produces a capacitance by a mechanism entirely different from the capacitance generation mechanism of an electric double-layer capacitor using the prior art activated carbon.
We have found that an improved capacitance is obtained by preparing an easily graphitizable carbon in which crystallites of multilayer graphite have developed, dry distilling the carbon at 700° C. to 850° C., treating the resulting carbon together with a caustic alkali such as KOH at 800° C. to 900° C., removing the remaining alkali with heated water vapor, and using the obtained carbon. Furthermore, we have discussed the large capacitance generation mechanism of the electric double-layer capacitor using this carbon. Additionally, we have found various factors such as polarized electrodes used for the capacitor (i.e., the carbon electrodes) and solvents to impart higher energy density (given in Wh/Kg) and higher power density (given in W/Kg) to the electric double-layer capacitor. In this way, the present invention has been completed.
That is, the electric double-layer capacitor in accordance with the present invention has an organic electrolyte in which polarized electrodes are immersed. A carbon is used as a substance for activating the polarized electrodes and contains graphite-like crystallites of carbon. This carbon is a nonporous carbon having an interplanar spacing d
002
of greater than 0.360 nm. Unlike the prior art activated carbon, at the beginning of assembly of a capacitor, the nonporous carbon has substantially no interface forming an electric double layer. When the applied voltage exceeds a certain threshold voltage during initial charging, ions of the electrolyte intrude into the carbon structure, together with the solvent. This phenomenon is known as solvent co-intercalation. This is the first time that an interface creating an electric double layer is formed. Subsequently, this interface is maintained by the hysteresis effect. The electric double-layer capacitor functions

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