JP2007287954A - Electric double layer capacitor - Google Patents

Abstract

An object of the present invention is to improve the reliability of an electric double layer capacitor by improving the capacity of the polarizable electrode layer and preventing the deterioration of the reliability by the expansion of the polarizable electrode layer.
A pair of positive and negative electrodes in which a polarizable electrode layer 4 is formed on a current collector 3 made of a metal foil is disposed so that the polarizable electrode layer 4 is opposed to each other with a separator 5 interposed therebetween. In the electric double layer capacitor housed in the case 8 together with the electrolytic solution, the polarizable electrode layer 4 has a carbon crystallite in a hard carbon layer 4a which is a non-graphitizable carbon layer in which the carbon crystallite is anisotropically oriented. Is an electric double layer capacitor characterized by interposing a soft carbon layer 4b which is an easily graphitizable carbon layer having isotropic orientation.
[Selection] Figure 2

Description

  The present invention relates to an electric double layer capacitor used in various electronic devices, and more particularly to an electric double layer capacitor excellent in low resistance, low temperature characteristics and long-term reliability.

  In the conventional electric double layer capacitor, a pair of electrode bodies each having a polarizable electrode layer in which a polarizable electrode mainly composed of activated carbon is formed on the surface of a current collector such as an aluminum foil is provided. A capacitor element is configured by making polar electrode layers face each other with a separator interposed therebetween, and the capacitor element is configured to be impregnated with a driving electrolyte.

  The polarizable electrode layer is generally provided with an activated carbon electrode layer, and the activated carbon can be classified into easily graphitizable carbon that is easily graphitized and non-graphitizable carbon that is not easily graphitized.

  This graphitizable carbon has a microcrystalline structure portion similar to graphite and is called soft carbon. On the other hand, non-graphitizable carbon does not have a microcrystalline structure portion similar to graphite but has a turbostratic structure and is called hard carbon.

  Since this soft carbon is an electric double layer capacitor, ions in the driving electrolyte intercalate during voltage application and adsorb in the pores in such a way that it spreads between graphite-like layers. It is known that a high capacity is developed by inserting ions between layers.

  Therefore, in order to improve the capacity, soft carbon, which is graphitizable carbon in which carbon crystallites are isotropically oriented, is used for the polarizable electrode layer.

As prior art document information related to the invention of this application, for example, Non-Patent Document 1, Patent Document 1, and Patent Document 2 are known.
Edited by Carbon Society of Japan, “Introduction to New Carbon Materials” published by Realize Co., Ltd., published on August 10, 1998, 1st edition, 2nd edition (P8-12) Japanese Patent Laid-Open No. 2005-136397 JP 2002-151364 A

  In the above-mentioned conventional configuration, when the polarizable electrode layer made of soft carbon is configured as a capacitor, when charging and discharging, the electrolyte ions become a form that spreads the interlayer structure of carbon crystallites contained in the soft carbon, As a result, there was a problem that the soft carbon expanded and the polarizable electrode layer destroyed the separator.

  Accordingly, the present invention aims to solve such a conventional problem and to prevent the destruction of the separator due to the expansion of the polarizable electrode layer in order to make the electric double layer capacitor excellent in long-term reliability while improving the capacity. To do.

  In order to achieve this object, the present invention provides a polarizable electrode layer having an isotropic orientation of carbon crystallites in a hard carbon layer that is a non-graphitizable carbon layer in which carbon crystallites are anisotropically oriented. By making an electric double layer capacitor with a soft carbon layer that is a graphitizable carbon layer interposed therebetween, the separator is prevented from being broken due to expansion of the polarizable electrode layer.

  The electric double layer capacitor of the present invention can suppress expansion as a polarizable electrode layer by sandwiching hard carbon with low expansion at the time of charging / discharging from the top and bottom of soft carbon, so that the capacity can be improved and long-term reliability can be achieved. It is possible to provide an electric double layer capacitor having excellent properties.

  Embodiment 1 of the present invention will be described below with reference to the drawings.

(Embodiment)
FIG. 1 is a partially cutaway perspective view showing a configuration of an electric double layer capacitor in the present embodiment, and FIG. 2 is a cross-sectional view showing a polarizable electrode used in the electric double layer capacitor.

  In FIG. 1, reference numeral 1 denotes a capacitor element. The capacitor element 1 is a conductive agent (not shown) having conductivity such as carbon made of activated carbon and acetylene black on a current collector 3 made of aluminum foil to which lead wires 2 are connected. And a polarizable electrode (not shown) to which a water-soluble binder (not shown) having dispersibility and film-forming property is added is formed into a layered shape to form a polarizable electrode layer 4, and this polarizable electrode layer A pair of 4 is wound by interposing a separator 5 for preventing a short circuit therebetween.

  A pair of lead wire 2, current collector 3, polarizable electrode layer 4, and separator 5 are prepared so as to correspond to the positive and negative electrodes, respectively.

  As shown in FIG. 2, the polarizable electrode layer 4 on the current collector 3 made of aluminum foil or the like has a soft carbon layer 4b interposed in a hard carbon layer 4a. The hard carbon layer 4a is a non-graphitizable carbon layer in which carbon crystallites are anisotropically oriented, and the soft carbon layer 4b is a graphitizable carbon layer in which carbon crystallites are isotropically oriented.

  After the hard carbon layer 4a is applied on the current collector 3, the soft carbon layer 4b is applied, and the hard carbon layer 4a is applied again, so that the soft carbon layer 4b is formed in the hard carbon layer 4a. Intervene.

  Here, hard carbon is polyvinylidene chloride charcoal, charcoal, fruit shells such as coconut shell, phenol formaldehyde resin charcoal, etc., soft carbon is petroleum coke, coal coke, pitch coke, coal pitch coke, petroleum It refers to pitch coke and polyvinyl chloride charcoal.

  The capacitor element 1 is impregnated with a driving electrolyte (not shown), and a rubber sealing member 7 having a hole through which the lead wire 2 is inserted is fitted into the upper end portion of the capacitor element 1 as shown in FIG. The case 8 is configured to be stored in a bottomed cylindrical case 8 made of aluminum and the case 8 is sealed by compressing the sealing member 7 by drawing the opening of the case 8.

  Forming the polarizable electrode layer 4 with the soft carbon layer 4b interposed in the hard carbon layer 4a on the current collector 3 as described above is one of the technical features of the present invention. By doing so, the soft carbon layer 4b is sandwiched by the hard carbon layer 4a from above and below during charging and discharging, so that the expansion can be suppressed and the destruction of the separator 5 due to the expansion as the polarizable electrode layer 4 can be prevented. Is.

  At this time, by making the thickness of the soft carbon layer 4b smaller than that of the hard carbon layer 4a, the pressing force by the hard carbon layer 4a is further increased, and the effect of suppressing the expansion is also increased.

  In the embodiment of the present invention, the hard carbon layer 4a has one soft carbon layer 4b. However, this may be a plurality of layers, and the hard carbon layer 4a is coated on the current collector 3. Later, it can be formed by repeating the coating of the soft carbon layer 4b and the coating of the hard carbon layer 4a. By doing in this way, the capacity | capacitance can be improved further, preventing destruction of the separator 5 by the expansion as the polarizable electrode layer 4.

Example 1
In order to measure the expansion of the electrode in the electric double layer capacitor according to the embodiment of the present invention, a hard carbon layer 4a made of a phenol resin is formed on an aluminum foil as a current collector 3 as shown in FIG. A soft carbon layer 4b made of coal pitch coke was formed in the layer 4a. At this time, the polarizable electrode layer 4 was configured such that the thickness of the soft carbon layer 4b was smaller than the thickness of each of the hard carbon layers 4a.

Furthermore, the specific surface area of the phenol resin used for the hard carbon layer 4a is about 1800 to 2000 m ² / g and the activated carbon average particle diameter is 2.5 to 5.0 μm, and the specific surface area of the coal pitch coke used for the soft carbon layer 4b. Was about 600 to 800 m ² / g, and the activated carbon average particle size was 2.5 to 5.0 μm.

  Further, 6% by mass of acetylene black and 5.8% by mass of a water-soluble binder were added to the hard carbon layer 4a and the soft carbon layer 4b, respectively.

  This electric current collector 3 and polarizable electrode layer 4 are made into a pair of positive and negative, and an electric double layer capacitor is simulated so that this polarizable electrode layer 4 faces with a separator impregnated with a driving electrolyte solution interposed The thickness of the cell at the time of voltage application was measured using the laminate cell which comprised this.

  For voltage application, a voltage of 0 to 3.0 V was applied at 0.5 V intervals, held at each voltage for 5 minutes, and then the cell thickness at 2.5 V was measured with a micro gauge at five points, before voltage application. It was set as the electrode expansion coefficient by comparing with the thickness. The results are shown in (Table 1).

  Further, after winding the same structure as the above laminate cell, it is housed in a bottomed cylindrical case 8 together with the driving electrolyte as shown in FIG. Thus, a wound type electric double layer capacitor was produced.

  This electric double layer capacitor is manufactured to have a rated voltage of 2.0 V, a capacitance of 68 F, and a size of Ф18 mm × L50 mm. Using this, a constant current and constant voltage charge of 1.5 A and 2.5 V is performed, The discharge capacity and internal resistivity at 1.0 A discharge after charging were measured in a room temperature atmosphere at 25 ° C. The results are shown in (Table 1).

(Example 2)
A hard carbon layer 4a made of phenol resin is formed on the current collector 3, and two soft carbon layers 4b made of coal pitch coke are provided in the hard carbon layer 4a, with the hard carbon layer 4a interposed therebetween. Laminated. That is, the uppermost layer and the lowermost layer are the hard carbon layer 4a, and two layers of the soft carbon layer 4b and the hard carbon layer 4a are laminated therebetween.

  At this time, the polarizable electrode layer 4 is configured so that the total thickness of the soft carbon layer 4b is smaller than the respective thickness of the hard carbon layer 4a, and the current collecting is performed together with the total thickness of all the polarizable electrode layers 4. The thickness of the cell including the body 3 was made the same as in Example 1.

  Table 1 shows the results of measuring the thickness of the cell when a voltage was applied using a laminate cell with the same configuration as in Example 1 except for the above.

  Similarly, a wound electric double layer capacitor was prepared, and the discharge capacity and internal resistivity were measured (Table 1).

(Comparative example)
The polarizable electrode layer 4 is formed on the current collector 3 only by the hard carbon layer 4a made of phenol resin, and the thickness of the polarizable electrode layer 4 and the thickness of the cell are the same as those in the first and second embodiments. It was made to become.

  Similarly, Table 1 shows the results of measuring the electrode expansion coefficient during voltage application, and the discharge capacity and internal resistivity of a wound electric double layer capacitor using the same.

  As shown in (Table 1), compared to the comparative example, Example 1 and Example 2 resulted in improved capacity at room temperature and almost the same electrode expansion coefficient. This is because when the soft carbon layer 4b in Example 1 and Example 2 is used to improve the capacity, the expansion coefficient of the electrode has been greatly increased in the past. This is because the expansion of the soft carbon layer 4b is suppressed by sandwiching the hard carbon layer 4a from above and below the layer 4b.

  According to the embodiment of the present invention, the conductive agent such as acetylene black contained in the polarizable electrode layer 4 may be 8% by mass or less of the total material of the polarizable electrode layer 4, and the water-soluble binder is the same. It should just be made into 2-8 mass%. By doing so, the resistivity can be reduced and the low temperature characteristics can be greatly improved. In addition, this water-soluble binder has a more preferable range of 4-6 mass%.

  As described above, by optimizing the hard carbon layer 4a and the soft carbon layer 4b of the polarizable electrode layer 4, it is possible to suppress the deterioration of the capacity and the increase of the resistance.

  FIG. 3 is a graph comparing the expansion rates of hard carbon and soft carbon used in this embodiment when voltage is applied.

  As in Example 1 and Example 2, a voltage of 0 to 3.0 V was applied at 0.5 V intervals, held at each voltage for 5 minutes, and then the cell thickness at 2.5 V was measured with a micro gauge. Five points were measured and compared with the thickness before voltage application to determine the expansion rate.

  In FIG. 3, phenolic resin is used for hard carbon A, coconut shell is used for hard carbon B, coal pitch coke is used for soft carbon C, and petroleum coke is used for soft carbon D. The thickness before and after voltage application is as follows. The expansion rate compared is shown.

  As shown in FIG. 3, soft carbons C and D having a microcrystalline structure portion similar to graphite are characterized in that the coefficient of expansion rapidly increases when a voltage of 2.0 V or more is applied. The hard carbons A and B having a disordered layer structure that does not have a similar microcrystalline structure part are those whose expansion coefficient does not increase greatly even when a voltage of 2.0 V or higher is applied.

  In particular, according to the embodiment of the present invention, the expansion rate at 2.5 V charging is 0.5 V higher than 2.0 V, which is the rated voltage of hard carbon, is 2% or less, and 2. By using the one having an expansion rate of 6% or more at the time of 5V charge, the soft carbon is sandwiched by the hard carbon from the upper and lower sides at the time of charge and discharge, and the effect of suppressing the expansion as the polarizable electrode layer 4 is more strongly exhibited. It is something that can be done.

  In addition, according to the embodiment of the present invention, usable driving electrolyte is 4 in an organic solvent such as propylene carbonate, ethylene carbonate, and methyl ethyl carbonate, which are known to be used in electric double layer capacitors. An electrolytic solution in which a quaternary ammonium salt, a quaternary phosphonium salt, a quaternary imidazolium salt, or an amidine salt is dissolved can be used.

  As described above, according to the electric double layer capacitor of the present invention, the polarizable electrode layer can be prevented from breaking the separator due to expansion even when the capacity is increased by soft carbon. It is possible to improve the performance.

  As a result, it is useful for automobile systems that require high current and high reliability at low temperatures.

1 is a partially cutaway perspective view of an electric double layer capacitor according to an embodiment of the present invention. Sectional drawing of the collector and polarizable electrode layer of the electric double layer capacitor according to the embodiment of the present invention The graph which compared the expansion rate at the time of the voltage application of the hard carbon and soft carbon by embodiment of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Capacitor element 2 Lead wire 3 Current collector 4 Polarization electrode layer 4a Hard carbon layer 4b Soft carbon layer 5 Separator 7 Sealing member 8 Case

Claims (5)

A pair of positive and negative electrodes in which a polarizable electrode layer is formed on a current collector made of metal foil, a separator is interposed between the electrodes so that the polarizable electrode layers face each other, and this is stored in a case together with a driving electrolyte. In the electric double layer capacitor, the polarizable electrode layer is graphitizable in which the carbon crystallites are isotropically oriented in a hard carbon layer that is a non-graphitizable carbon layer in which the carbon crystallites are anisotropically oriented. Electric double layer capacitor characterized by interposing a soft carbon layer, which is a layer of carbon. The electric double layer capacitor according to claim 1, wherein a thickness of the soft carbon layer is smaller than a thickness of the hard carbon layer. The electric double layer capacitor according to claim 1, wherein a plurality of the soft carbon layers are provided. 2. The expansion rate of the non-graphitizable carbon at 2.5 V charging is 2% or less, and the expansion rate of the graphitizable carbon at 2.5 V charging is 6% or more. Electric double layer capacitor. 2. The electric double layer according to claim 1, wherein acetylene black is added to the polarizable electrode layer by 8% by mass or less as a conductive agent having conductivity and 2 to 8% by mass of a water-soluble binder having dispersibility and film formability. Multilayer capacitor.

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