JPH0424848B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to polarizable electrodes used in electric double layer capacitors and batteries. Prior Art Electric double layer capacitors and batteries using carbon fibers as polarizable electrodes are already known. The method for manufacturing the fibers used in this carbon fiber polarizable electrode is as follows:
The raw material fibers are carbonized to obtain carbon fibers, and then the carbon fibers are activated at high temperatures using an appropriate activating gas to produce activated carbon fibers.The raw material fibers are directly carbonized and activated to produce raw materials in a single process. There is a method to obtain activated carbon fiber from. Conventionally, phenolic novolac fibers, cellulose fibers, polyacrylonitrile fibers, pitch fibers, etc. have been used as raw material fibers, and especially when phenolic novolac fibers are used, activated carbon fibers with a high specific surface area of 2000 to 2500 m ² /g are used. was obtained, and capacitors and batteries using this as a polarizable electrode were small and large in capacity. Problems to be Solved by the Invention On the polarizable electrodes of the electric double layer capacitor and battery as described above, electrode reactions occur as shown in equations (1) and (2), respectively, during charging. (C ₂ H ₅ ) ₄ ^NClO ₄ → (C ₂ H ₅ ) ₄ N ⁺ +ClO ^- ₄ (1) polarizes in this direction and forms an electric double layer on the surface. In a battery, an electric double layer is formed on the surface of the anode activated carbon depending on the amount of cathode lithium doping. At this time, for example, the capacitance C accumulated in the electric double layer capacitor is expressed by equation (3). C=∫ε/4πδdS (3) ε is the dielectric constant of the electrolyte, δ is the thickness of the electric double layer,
S is the surface area of activated carbon. The physical properties required of such a polarizable electrode are (1) a large specific surface area, and (2) a large electrical conductivity. The first specific surface area is as understood from equation (3), and the purpose is achieved by using activated carbon fibers. However, in the conventional techniques, obtaining the second electrode with high electrical conductivity tends to be inversely related to the first electrode with high specific surface area. Activated carbon fibers are made by activating carbon fibers, and as the carbon fibers, which have a high conductivity of about 4×10 ^-3 Ω・cm, are activated, the activation progresses as shown in Figure 3. Therefore, as the specific surface area increases, the electrical conductivity of the fibers decreases to 2500 m ² /g.
For activated carbon fibers with a specific surface area of 3×10 ² Ω・
I've become accustomed to cm. FIG. 4 shows an enlarged schematic diagram when activated carbon fibers are used as a component of a polarizable electrode. A current collector 22 made of a metal such as aluminum is formed on one side of a polarizable electrode 21 made of activated carbon fibers 20 entangled with each other by a method such as plasma spraying. Among these, the electric double layer capacitance formed on the surface of the activated carbon fiber layer 23 near the current collector 22 is extracted to the current collector 22 with little resistance loss. However, the activated carbon fiber layer 24 that exists in a portion away from the current collector 22
The electric double layer capacitance formed on the surface of the current collector 22
By the time the activated carbon fibers are taken out, the capacity decreases due to the resistance loss of the activated carbon fibers themselves, and as a result, the capacity extraction efficiency of the capacitor as a whole becomes low. FIG. 5 is an equivalent circuit of one electrode of the capacitor, and the electric double layer capacitance C ₂ existing in a portion away from the current collector 22 decreases due to resistance loss R ₂ up to the current collector 22. C ₁
is the electric double layer capacitance near the current collector 22, and R ₁
is the contact resistance between the activated carbon fiber and the collector electrode, which is extremely small compared to _R2 . In order to solve these drawbacks, attempts have been made to construct polarizable electrodes by mixing conductive substances such as metal fibers and acetylene black with activated carbon fibers, but the filling efficiency of the activated carbon fibers deteriorates. However, it has not been possible to obtain a satisfactory product due to the complicated manufacturing method and problems with high-temperature life characteristics. The reason why electrical conductivity decreases by activating carbon fibers is thought to be that activation causes porosity to progress from the surface of the fibers to the inside, resulting in a reduction in the actual fiber diameter. In view of the above, an object of the present invention is to provide a polarizable electrode with low resistance and high capacity. Means for Solving the Problems According to the present invention, a polarizable electrode is constructed using a carbon fiber whose interior is made of graphite and whose outer shell is porous and activated carbonized. Function The present invention uses carbon fibers whose interiors are highly conductive and whose outer shells are porous and activated carbonized, so that a polarizable electrode body with low resistance can be obtained by the intertwining of the carbon fibers. In such electrodes, the electric double layer capacitance formed on the surface of the fibers that is located away from the current collector is efficiently extracted to the current collector without being hindered by the resistance loss of the fibers themselves. In addition to extracting the electric double layer capacitance from the fiber surface near the electric body, a polarizable electrode with low resistance and large capacity as a whole is obtained. The carbon fiber used in the present invention can be obtained by activating highly conductive graphite fiber. In addition, since the resistance loss caused by the fiber itself when taking out the capacity is reduced, the electric double layer capacity formed on the inner surface of the pores having a fine pore diameter can also be effectively utilized. In other words, 1000% of highly conductive graphite fiber
Using carbon fiber activated to a specific surface area of about m ² /g, the phenolic novolac fiber can be reduced to 2000 m ² /g.
A polarizable electrode with approximately the same capacity as when using activated carbon fibers carbonized to a specific surface area of 1.5 g can be obtained.
Therefore, the former case is very efficient in terms of carbonization yield and strength. Examples Specific examples of the present invention are shown below. Example 1 Roving-shaped pitch graphite fibers are activated by applying water vapor at 950°C. The obtained activated carbonized graphite fibers are cut into chops, and a slurry of this and synthetic pulp mixed in a weight ratio of 80:20 is made into paper. A plasma sprayed aluminum layer with a thickness of 30 μm was formed on one side of the obtained paper with a basis weight of 100 g/m ² , and the paper was punched out to a diameter of 10 mm. Using this as two electrodes, a capacitor having the configuration shown in FIG. 2 is assembled.
The electrolytic solution used was one in which a tetraethylammonium perchlorate electrolyte was dissolved in a propylene carbonate solvent. The activated carbonized graphite fiber 1 obtained above has the structure shown in FIG. That is, the fiber is basically composed of a highly conductive fiber core portion 2 of graphite fiber and a fiber outer shell portion 3 made of porous activated carbon and having a high specific surface area. In FIG. 2, 10 and 11 are polarizable electrodes, 1
2 and 13 are current collectors formed on the back surfaces of these electrodes, 14 is a separator, 15 is a case, 16 is a sealing plate, and 17 is a gasket. Example 2 The chopped activated carbonized graphite fibers, synthetic pulp, and phenolic resin carbon fibers obtained in Example 1 were
Paper is made by mixing at a weight ratio of 30:10. The obtained paper with a basis weight of 100 g/m ² was used to assemble into a capacitor in the same manner as in Example 1. Example 3 The chop-shaped activated carbonized graphite fiber obtained in Example 1 was made into a felt shape. The obtained felt having a basis weight of 100 g/m ² was used to assemble a capacitor in the same manner as in Example 1. Example 4 Pitch-based graphite fibers are made into felt. The obtained felt having a basis weight of 100 g/m ² is treated with water vapor at 950° C. to form a felt-like activated carbonized graphite fiber. This is used to assemble into a capacitor in the same manner as in Example 1. Example 5 A plain-woven cloth with a basis weight of 100 g/m ² woven from pitch-based carbon fibers was graphitized and then activated carbonized by applying water vapor at 950°C. Thickness 30μ on one side of this cloth
A plasma sprayed layer of aluminum of m is formed and a capacitor is assembled in the same manner as in Example 1. Example 6 A 30 μm thick layer of nickel was formed by plasma spraying on one side of the paper obtained in Example 1.
A capacitor is assembled in the same manner as in Example 1. However, a 24% by weight aqueous solution of KOH is used as the electrolyte. Example 7 The paper with a basis weight of 100 g/m ² obtained in Example 1 with an aluminum plasma sprayed layer of μm thick formed on one side was used as an anode, a lithium cathode made of Li-doped Sn-Pb alloy, and propylene carbonate. A battery is assembled using an electrolyte containing lithium perchlorate. The structure of the battery is such that one polarizable electrode in the capacitor shown in FIG. 2 is replaced with the lithium electrode. Comparative Example 1 A capacitor was assembled in the same manner as in Example 1 using paper with a basis weight of 100 g/m ² obtained by cutting cellulose-based activated carbon fibers into chops. Comparative Example 2 The same capacitor as Comparative Example 1 was assembled, except that a 24% by weight aqueous solution of KOH was used instead of tetraethylammonium perchlorate as the electrolyte. Comparative Example 3 The same battery as in Example 7 was assembled except that the polarizable electrode of Comparative Example 1 was used as the anode. Capacities C _A and C _B when the above capacitors and batteries are discharged at 10 mA at 25°C and -25°C, (C _A
-C _B )/C _A · 100, △C, series resistance is shown in the following table.

[Table] In addition, in the examples, pitch carbon fiber was used as an example of the starting material for graphite fiber, but in addition to this, so-called graphitizable carbon (soft carbon)
Vinyl chloride-based carbon fibers, 3,5-dimethylphenol formaldehyde resin-based carbon fibers, etc., which are referred to as 3,5-dimethylphenol formaldehyde resins, can also be used. Further, although carbon fiber was given as an example in Example 2 as an additive to activated carbonized graphite fiber, graphite fiber, metal fiber, graphite powder, carbon powder, etc. are also effective. The graphite fiber before being activated carbon has a main diffraction peak in the lattice spacing of 3.3 to 3.4 Å in its X-ray diffraction, and has a low electric resistance, for example, 1×
10 ^-5 Ω·cm is preferable. Effects of the Invention As described above, the polarizable electrode of the present invention uses active carbonized graphite fibers whose core part is highly conductive and whose outer shell part is made of porous activated carbon. The resistance is low, and capacitance can be taken out in the thickness direction and lateral direction of the electrode with high efficiency. As a result, capacitors and batteries using this material are 50% to 100% more compact and have a higher capacity than capacitors and batteries using conventional activated carbon fibers.

[Brief explanation of drawings]

Figure 1 is an enlarged schematic diagram of activated carbonized graphite fiber used in the polarizable electrode of the present invention, Figure 2 is a vertical cross-sectional view showing an example of the structure of an electric double layer capacitor, and Figure 3 is its specific surface area due to activation of activated carbon. FIG. 4 is a schematic diagram showing the internal structure of a polarizable electrode using activated carbon fibers, and FIG. 5 is a diagram showing an equivalent circuit of the polarizable electrode. 1... Carbon fiber, 2... Graphite, 3... Porous activated carbon layer.

Claims (1)

[Scope of Claims] 1. A polarizable electrode made of graphite inside and porous activated carbonized carbon fiber alone or mainly composed of carbon fiber. 2. The polarizable electrode according to claim 1, wherein the carbon fiber is obtained by activating graphite fiber. 3. The component according to claim 1, wherein the polarizable electrode comprises the carbon fiber and one or more of natural pulp, synthetic pulp, graphite fiber, metal fiber, carbon fiber, graphite powder, and carbon powder. Polar electrode. 4. The polarizable electrode according to claim 2, wherein the carbon fiber is activated graphite fiber having a main diffraction X-ray peak at a lattice spacing of 3.3 to 3.4 Å. 5. The polarizable electrode according to claim 2, wherein the graphite fiber is obtained by graphitizing any one of pitch carbon fiber, vinyl chloride carbon fiber, and 3,5-dimethylphenol formaldehyde resin carbon fiber. .