JPH0211007B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a small-sized, large-capacity wet type electric double layer capacitor. 1. Structure of conventional example and its problems FIG. 1 shows an example of the structure of a conventional electric double layer capacitor. It has a structure in which an activated carbon fiber cloth is used as the polarizable electrode body 1, and a metal layer of aluminum, titanium, etc., or a conductive resin layer is formed as the conductive electrode 2. After stacking these with a separator 3 in between and injecting an electrolytic solution, the positive and negative electrodes were insulated with a gasket 4 and housed in a coin-shaped case 5, which was then sealed. Here, the metal conductive electrode 2 is formed by a plasma spraying method or an arc spraying method, or if a conductive resin is used, a conductive resin containing mainly carbon as conductive particles is formed by a screen printing method, a spray method, or a dip method. Formed by any of the laws. When a conductive resin is used, the internal impedance becomes larger than when a metal layer is used, making the capacitor unsuitable for strong discharge applications. Conventionally, this type of capacitor uses (1) an aqueous electrolyte and (2) a non-aqueous electrolyte. The non-aqueous, ie, organic electrolyte of (2) has a lower electrical conductivity than an aqueous electrolyte, but has a higher withstand voltage. Solvents include propylene carbonate, γ-butyl lactone, N
Using -N-dimethylformamide, acetonitrile, etc., a perchlorate of tetraalkylammonium such as tetraethylammonium perchlorate, a hexafluorophosphate or borofluoride salt of tetraalkylammonium, and also lithium, sodium , using solutes such as potassium perchlorate. In particular, electric double layer capacitors currently in practical use use activated carbon as polarizable electrodes. When activated carbon is used as a polarizable electrode, the capacitor characteristics are largely influenced by the following three items. Specific surface area Pore diameter Pore volume Also, the capacity of the electric double layer capacitor is expressed by the formula. η=d/4πδφ... η: Charge per unit area d: Dielectric constant of medium δ: Average distance between solid surface and electrolyte ions φ: Double layer potential Therefore, the amount of charge accumulated per single cell is When Q is the double layer forming area and S is the double layer forming area, Q can be expressed by the formula. Q=d/4πδS·φ... Therefore, the larger the double layer formation area of the electric double layer capacitor, the larger the amount of charge accumulated. Therefore, the larger the specific surface area, the better. However, in order to increase the specific surface area, it is necessary to proceed with activation, which has the disadvantage that the mechanical strength is significantly reduced. Generally, the larger the specific surface area, the larger the pore volume. However, no matter how large the specific surface area is, if the pore diameter is small as shown in FIG. 2, a double layer cannot be efficiently formed. In the figure, 6 is a polarizable electrode, 6a is a pore,
6b is an electrolyte ion. According to the theory of capillary condensation, in order to efficiently form a double layer, the pore diameter 8 needs to be at least four times the electrolyte ion diameter 7, as shown in FIG. In particular, at low temperatures below 0°C, the viscosity of organic solvents increases and aqueous electrolytes begin to freeze, which not only greatly reduces the mobility of electrolyte ions and makes it difficult to form a double layer.
Once formed, the double layer cannot be easily discharged. For the above-mentioned reasons, conventional electric double layer capacitors that use activated carbon fibers as polarizable electrodes and organic electrolytes as electrolytes have very large specific surface areas of 2000 m ² /g for both positive and negative electrodes. The pore diameter is also 2~
Activated carbon fibers, which are mostly present at 4 nm, have been used.
However, activated carbon fibers having such great characteristics have the disadvantage that the activation yield is very low at about 20%. OBJECT OF THE INVENTION The object of the present invention is to obtain an electric double layer capacitor in which the efficiency of forming an electric double layer per unit volume is improved. Structure of the Invention In order to achieve this object, the present invention is such that the specific surface area of the polarizable electrode body on the positive electrode side is smaller than the specific surface area of the polarizable electrode body on the negative electrode side. Description of Examples Before describing specific examples, an electric double layer formed of activated carbon, carbon, or graphite and electrolyte ions used in the positive electrode side and the negative electrode side of the present invention will be described. The effects of the present invention are more remarkable when an organic electrolyte is used than an aqueous electrolyte, and when activated carbon fibers are used rather than activated carbon particles. The reason for this is explained below. When an electrolyte such as tetraethylammonium perchlorate or lithium perchlorate is dissolved in an organic solvent such as propylene carbonate or γ-butyllactone, the perchlorate ion (ClO ^- ) has an ionic radius of 2.36A and becomes a lithium ion. Although (Li ⁺ ) has a small ionic radius of 0.6A,
In general, aprotic polar solvents solvate cations more strongly than anions, so the ionic diameter including solvation is conversely larger for cations than for anions. FIG. 4 schematically shows the state. 9 is a solvated anion, 10 is a solvated cation, 11 is a positive electrode activated carbon fiber, 1
1' is a negative electrode activated carbon fiber, and 6a is a pore. When carbonization is activated from the same starting material, the products shown in Table 1 are produced. Therefore, in order to create a coin-shaped capacitor, such a thing is punched out with the same area, so to form a double layer with the positive electrode, that is, the anion, stage (2) in Table 1 is used. , the activation yield is good, the resistance is low, and the strength is also strong, making it the best. However, on the negative electrode side, there are activated carbon fibers that have undergone sufficient activation as shown in stage (3) and have a large specific surface area and a pore diameter large enough to allow solvated cations to penetrate into the pores. Must be used.

[Table] Figure 5 schematically shows the pores of the granular activated carbon 12 and the activated carbon fibers 11. As can be seen from this figure, activated carbon particles have micropores 14 within macropores 13, so electrolyte can penetrate more easily than activated carbon fibers, which have micropores 14 directly. So, Tadashi,
It is considered that when activated carbon particles are used for both the negative electrode and the negative electrode, the effects of the present invention are not noticeable. As described above, the present invention is very effective with polarizable electrode bodies such as activated carbon fibers whose pore size can be easily controlled and with organic electrolyte systems that strongly solvate cations. In other words, it is necessary to use a material that is capable of penetrating into pores even with strongly solvated cations in the negative polarizable electrode and forming a double layer. However, the positive polarizable electrode should have a higher carbonization activation rate, stronger strength, and lower electrical resistance than the negative electrode to which anions can penetrate, and when combined with the negative polarizable electrode body, the best A double layer can be formed efficiently, raw materials can be used effectively, and productivity can be improved. Example 1 Phenol-based, acrylonitrile-based, and rayon-based fibers were carbonized and carbonized, respectively, to obtain carbon fibers and activated carbon fibers having the characteristics shown in Table 2. For the current collector, an aluminum layer of approximately 300 μm was formed by plasma spraying. Coin-shaped capacitors with different polarizable electrode bodies for positive and negative polarities as shown in FIG. 6 were manufactured using the combinations shown in Tables 2 to 3, and their various characteristics are also shown in the same table. In Figure 6,
15 is a positive electrode case, 16 is a positive polarizable electrode having a current collector (aluminum) 16a, 17 is a negative electrode case, and 18 is a current collector (aluminum) 18a.
19 is a separator,
20 is a gasket. The electrolytic solution used was an organic electrolytic solution in which 1 mole of electrochemically stable tetraethylammonium perchlorate was dissolved in a 1:1 mixed solvent of propylene carbonate and γ-butyl lactone. Each electrode was punched into a circular shape with a diameter of 14 mm.

【table】

【table】

[Table] With the electrode combination in Table 3 of carbon fibers and activated carbon fibers with the characteristics in Table 2, No. 1 in Table 3,
2, 3, 4, 8, 9, 10, 11, 12, 13, 15, 17,
All No. 18 exhibit good capacitor characteristics. That is, a low impedance capacitor with sufficiently large capacity and good low-temperature characteristics can be obtained without using sufficiently activated positive and negative electrodes like No. 7. In Nos. 5 and 6, solvated cations cannot easily penetrate into the negative electrode side, resulting in extremely poor low-temperature characteristics. Example 2 As a negative polarizable electrode, the specific surface area is 1400 m ² /
g, 80% or more of activated carbon particles with a pore size of 20 to 100 A, pressed onto a titanium net with 1 part of Polyflon added as a binder to 100 parts of activated carbon particles, and the positive electrode was A capacitor shown in FIG. 6 was constructed using the activated carbon fiber of d. Table 4 shows the capacitor characteristics. From Table 4, it can be seen that this example also exhibits good characteristics.

[Table] Example 3 The large capacitors shown in FIGS. 7 and 8 were manufactured using the combinations shown in Table 3, No. 4. It measures 10cm in height and 5cm in width. In the figure, 21 is a positive electrode lead, 22 is a positive electrode, 23 is a separator, 24 is a negative electrode lead, 25 is a negative electrode, 26 is a polyethylene laminate resin, and Figure 8 is a cross section of Figure 7 taken along the a-a' line. It is a diagram. The fifth characteristic of this capacitor is
Shown in the table. In this example, the electrolyte contains propylene carbonate and γ-butyllactone in a ratio of 1:1.
A mixed solvent in which 1 mole of lithium perchlorate was dissolved was used.

[Table] Effects of the Invention As described above, the present invention uses electrodes that can efficiently form an electric double layer with anions and cations as the positive electrode and the negative polarizable electrode, respectively, so the present invention has a low impedance, small size and large capacity. An electric double layer capacitor can be obtained.

[Brief explanation of drawings]

FIG. 1 is a half-sectional front view showing an example of a conventional electric double layer capacitor; FIGS. 2 to 5 are schematic diagrams showing the states of pores of a polarizable electrode and electrolyte ions;
FIG. 6 is a sectional view showing an electric double layer capacitor according to one embodiment of the present invention, FIG. 7 is a plan view showing an electric double layer capacitor according to another embodiment of the present invention, and FIG. 8 is a of FIG. FIG. 3 is a cross-sectional view taken along the -a′ line. 16... Positive polarizable electrode, 18... Negative polarizable electrode, 22... Positive electrode, 25... Negative electrode.

Claims (1)

[Claims] 1. An electric double layer formed at the interface between the polarizable electrode body and the electrolyte, and the specific surface area of the polarizable electrode body on the positive electrode side is smaller than the specific surface area of the polarizable electrode body on the negative electrode side. An electric double layer capacitor featuring: 2 The pore diameter of the polarizable electrode body on the positive electrode side is 10 to 30A.
2. The electric double layer capacitor according to claim 1, wherein the pore diameter of the polarizable electrode body on the negative electrode side is 70% or more in the range of 20 to 40A. 3. The electric double layer capacitor according to claim 1, wherein activated carbon or carbon in the form of fibers, paper, felt, or porous is used as the polarizable electrode body. 4. The electric double layer capacitor according to claim 1, wherein an organic electrolyte is used as the electrolyte.