JP2007169126A - Porous material and electric double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous material which hardly expands when electricity is charged or discharged from the porous material, is low in resistance and the capacitance of which per volume can be increased and to provide an electric double layer capacitor using the porous material in a polarizable electrode. <P>SOLUTION: The porous material is a mixture of a porous material A obtained by carbonizing an easy-to-graphitize carbon material at <550°C and activating the carbonized material with another porous material B obtained by carbonizing the easy-to-graphitize carbon material at ≥550°C and activating the carbonized material. The average particle size of the porous material A is 5-20 μm and that of the porous material B is 1/20 to 1/2 of that of the porous material A. The mixing mass ratio (A/B) of the porous material A to the porous material B is 1-10. The porous material is used in the electric double layer capacitor as an electrode active material. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a porous material and an electric double layer capacitor using the porous material.

  An electric double layer capacitor is one of energy storage devices, and includes a pair of polarizable electrodes including a porous carbon material, a separator, an electrolyte solution, and the like.

  Such an electric double layer capacitor is a general energy storage device because the charging / discharging mechanism does not involve an electrochemical reaction and is based on simple adsorption / desorption of positive and negative ions of the electrolyte to and from the polarizable electrode interface. It has a feature not found in secondary batteries. That is, it has excellent instantaneous charge / discharge characteristics, stable charge / discharge characteristics over a wide temperature range, and low performance degradation due to repetition.

Since the capacitance of the electric double layer capacitor is proportional to the surface area of the polarizable electrode, conventionally, a porous material having a large specific surface area is used for the polarizable electrode, It has been investigated to obtain a double layer capacitor. As such a porous material, activated carbon (porous carbon material) having high conductivity, electrochemically relatively stable carbonaceous material, and having a large specific surface area is used. Activated carbon used for the polarizable electrode is a carbonaceous raw material such as coal, coal coke, coconut husk, wood powder, resin, oxidizing gas such as water vapor, air, oxygen, CO ₂ , or zinc chloride, hydroxide. An activation (porosification) treatment for forming pores with a chemical such as potassium is performed.

  Incidentally, recently developed electronic devices, electric vehicles, and the like are required to have low resistance, and there is an increasing demand for low resistance for electric double layer capacitors as energy devices used in these applications.

  To reduce resistance, use porous carbon materials obtained by activating a graphitizable carbon material (soft carbon-based carbon material) exhibiting high conductivity for the polarizable electrode of an electric double layer capacitor. Is preferred.

  It is said that the porous carbon material obtained by activating the graphitizable carbon material exhibits high conductivity because crystals are relatively developed.

  However, if a porous carbon material obtained by activating an easily graphitizable carbon material with relatively developed crystals is used as it is for a polarizable electrode of an electric double layer capacitor, the expansion of the electrode during charge / discharge is significant. The problem that the electric double layer capacitor expands has been pointed out.

  Several techniques have been shown as methods for suppressing the expansion of the porous carbon material that occurs during this charge / discharge.

  For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2002-83748) discloses an activation treatment for a mixture of a phenolic resin and a soft carbon-based carbon material that has a relatively low crystallinity and therefore has little expansion during charging and discharging. It is described that a porous carbon material in which expansion is suppressed can be obtained by applying.

Further, Patent Document 2 (Japanese Patent Application Laid-Open No. 2004-14762) discloses that expansion is achieved by controlling the crystal structure of the graphitizable carbon material obtained using coal tar pitch as a raw material and activating it under optimum conditions. It is described that a suppressed porous carbon material can be obtained.
JP 2002-83748 A JP 2004-14762 A

  However, in the method described in Patent Document 1, since the carbon material contains a phenol resin, the resistance is higher than that of the porous carbon material made of the soft carbon-based carbon material, and the packing density is higher than that of the soft carbon-based material. Since it is difficult to increase the volume capacity, there is a problem that the volume capacity is lowered.

  In addition, the method described in Patent Document 2 requires strict control of the primary QI content and softening point of the raw coal tar pitch, the atmosphere and temperature during carbonization, and the temperature during activation. There is a problem that practical application is difficult.

  Accordingly, the present invention provides a porous material that is small in expansion during charge and discharge and that can increase the capacitance per volume, and an electric double layer capacitor that uses this porous material for a polarizable electrode. For the purpose.

  The present inventors diligently studied on means for solving the above-described problems. During the examination, the present inventors have obtained a porous material obtained by subjecting the carbonaceous powder carbonized at a temperature of 550 ° C. or higher to the activation treatment without controlling the expansion of the electrode at the activation stage. The carbon material (porous material B) has a large expansion during charging / discharging, whereas the activated carbon material (porous material A) has a small degree of expansion when carbonized at a temperature of less than 550 ° C. I found out. In addition, the porous carbon material having a small expansion has a small filling density, and the volume capacity of the electric double layer capacitor is difficult to increase. On the other hand, the porous carbon material having a large expansion can improve the filling density. It has been found that this is effective in improving the volume capacity of the double layer capacitor.

  Therefore, as a result of further investigations, the present inventors have filled the voids in the powder of the porous carbon material (porous material A) having a small expansion with the porous carbon material (porous material B) having a large expansion. By adjusting the composition of the electrode, the porous carbon material having a large expansion (porous material B) only expands in a void having a degree of freedom, so that the expansion as a whole is remarkable during charging and discharging. It was found to be suppressed.

The present invention has been made based on the above findings and has the following characteristics.
[1] Porous material A obtained by carbonizing an easily graphitizable carbon material at a temperature of less than 550 ° C. and then activation treatment;
It is a mixture with a porous material B obtained by carbonizing an easily graphitizable carbon material at a temperature of 550 ° C. or higher and then performing an activation treatment.
The average particle size of the porous material A is 5 to 20 μm, the average particle size of the porous material B is 1/20 to 1/2 of the average particle size of the porous material A,
The mixing ratio of the porous material A and the porous material B is a mass ratio.
A / B = 1-10
A porous material characterized in that
[2] An electric double layer capacitor using the porous material according to [1] as an electrode active material.

  ADVANTAGE OF THE INVENTION According to this invention, the expansion | swelling at the time of charging / discharging is small, and the electric double layer capacitor which uses the porous material which can raise the electrostatic capacitance per volume, and this porous material for a polarizable electrode are provided. The

  Hereinafter, an example of an embodiment for carrying out the present invention will be described.

  In the following description, the expansion coefficient at the time of charging / discharging is indicated by a value obtained from the increase in the vertical direction with respect to the initial value of the electrode / separator / electrode laminated structure after assembling the electric double layer capacitor cell. This measuring method will be described in detail in Examples described later.

  The graphitizable carbon material used in the present invention is carbon that is easily graphitized by arranging the carbon structure in layers by heat-treating coal-based or petroleum-based pitch or coke in an inert atmosphere not containing oxygen. . Among these, it is preferable to use mesocarbon microspheres produced by carbonizing coal-based pitch in a liquid phase.

  The mesocarbon microspheres are spherical graphite precursors that easily form a graphite layer structure by heat treatment.

  Mesocarbon spherules are preferable because they have a relatively high crystallinity and can easily form a graphite-like layer structure, and can form a microstructure with a low resistance by a graphitization promoting treatment at a relatively low temperature. Moreover, the point that these are easy to obtain as a commercial item is also preferable.

  For example, mesocarbon microspheres can be produced by heating pitches such as coal-based tars or pitches thereof, petroleum-based heavy oils or pitches thereof to about 350 ° C. or more. When the pitches are heated, mesocarbon microspheres are generated in the pitches. The mesocarbon microspheres can be obtained by subjecting the pitches generated by the mesocarbon microspheres to a separation operation using a solvent such as quinoline or tar oil.

  In the present invention, the mesocarbon microspheres are carbonized as necessary to control the expansion rate during charging and to prevent fusion during activation. In this case, the temperature of the carbonization treatment can be arbitrarily selected from the range of about 450 to 850 ° C. according to the purpose.

  The carbonization temperature of the porous carbon material having a small expansion needs to be less than 550 ° C., and is preferably in the temperature range of 480 to 520 ° C.

  Moreover, the carbonization temperature of the porous carbon material having a large expansion needs to be 550 ° C. or higher, preferably 600 to 750 ° C., more preferably 650 to 750 ° C.

  Moreover, since the electrode thickness at the time of using for the electrode of an electric double layer capacitor is 150 micrometers or less normally, the average particle diameter of the porous carbon material used as a raw material is adjusted to 20 micrometers or less. At that time, the shape of the mesocarbon spherules is substantially retained after the carbonization treatment and activation treatment, and the finally obtained porous carbon material essentially corresponds to the average particle diameter of the porous carbon material used as a raw material. Average particle size.

As a method of adjusting the average particle size of the porous mesocarbon spherules finally obtained to 20 μm or less,
(1) A method of controlling the generation conditions of mesocarbon spherules to adjust the average particle size to 20 μm or less, and then performing carbonization and activation treatment. (2) Crushing mesocarbon spherules having an average particle size larger than 20 μm. , A method of performing carbonization and activation treatment after adjusting the average particle size to 20 μm or less by classification treatment, etc. (3) carbonizing and activating the mesocarbon microspheres having an average particle size larger than 20 μm, followed by pulverization treatment, A method of adjusting the average particle size to 20 μm or less by classification treatment or the like (4) A method in which two or more of the methods (1) to (3) are combined can be used.

  As the pulverization method used in the present invention, known methods such as a ball mill, a planetary ball mill, a hammer mill, a jet mill, an impeller mill, an atomizer, a pulverizer, and a jaw crusher can be used. A combination of these methods can also be used.

  As the classification method used in the present invention, a known method such as a wind classifier, a vibration sieve machine, a vibration sieve machine with an ultrasonic oscillator, or a low-tap sieve machine can be used. A combination of these methods can also be used.

  In the present invention, an alkali activation treatment is preferably performed as the porous (activation) of mesocarbon microsphere carbide. The activation may be performed separately for each carbide or after mixing the carbides.

  Porous formation by alkali activation of mesocarbon spherules can be carried out in the same manner as normal alkali activation treatment, but usually the mesocarbon spherules are placed in an inert gas atmosphere such as nitrogen gas or argon gas. It is performed by heating at 600 to 900 ° C. in the presence of an alkali metal compound. When the heating temperature is lower than 600 ° C., it is difficult to obtain a sufficient specific surface area, the volume specific resistance value becomes small, and it is difficult to achieve low resistance. On the other hand, heating at a high temperature exceeding 900 ° C. causes a problem of device corrosion due to an alkali metal compound. In addition, preferable heating temperature is 650-850 degreeC.

  The kind of the alkali metal compound is not particularly limited, and only one kind may be used, or a plurality may be used in combination. KOH, NaOH, CsOH and the like are preferably used.

  The amount of the alkali metal compound to be used varies depending on the desired specific surface area, but is usually about 0.5 to 4 times the mass ratio of the raw material mesocarbon microsphere carbide.

  The activation process is performed for about 0.5 to 10 hours. When the treatment time is less than 0.5 hours, it is difficult to obtain a sufficient specific surface area. On the other hand, when the treatment time exceeds 10 hours, overactivation is likely to occur. When it becomes excessively activated, the porosity is excessively increased, the specific surface area becomes larger than required in the present invention, and the density is lowered due to coalescence of pores.

  After the alkali activation treatment, the porous carbon material is usually obtained by neutralizing with a hydrochloric acid solution or the like and then washing with ion exchange water or the like.

By the above method, it is possible to obtain a porous carbon material having an expansion rate of 110% or less during charge / discharge and a capacitance target of 30 F / cm ³ or more. Further, such a porous carbon material is suitable as a polarizable electrode material for an electric double layer capacitor. If this is used for a polarizable electrode, the capacitance per volume is high and the expansion during charge / discharge is also high. A high-performance electric double layer capacitor with a low rate can be obtained.

  In the present invention, a polarizable electrode using the porous carbon material as described above and a high-performance electric double layer capacitor including the polarizable electrode are also provided.

  The characteristics of the porous carbon material of the present invention can be evaluated by the packing density when used for the polarizable electrode of the electric double layer capacitor, the expansion coefficient during charge and discharge, and the capacitance.

  The polarizable electrode of the electric double layer capacitor can be produced according to a general method except that the porous carbon material of the present invention is used as the carbon material. For example, a binder, a conductive agent, and the like are appropriately added to the porous carbon material as necessary, and are formed into a disk or a sheet to form an active material layer containing the porous carbon material.

  As the binder, polytetrafluoroethylene, polyvinylidene fluoride and the like can be usually used. The binder is usually used by adding about 0.1 to 20% by mass to the porous carbon material.

  As the conductive agent, carbon black or the like can be usually used. About 1-20 mass% is normally added and used for a electrically conductive agent with respect to a porous carbon material.

  The electrode may have a structure having an active material layer containing the porous carbon material on one side or both sides of the current collector layer. The current collector layer may be formed by pressure welding simultaneously with the formation of the active material layer from the mixture of the porous carbon material, the binder and the conductive agent, or may be formed in advance by a method such as compression molding. A current collector may be electrically connected to one side of the layer by a method such as aluminum spraying. Further, a current collector made of a metal foil such as aluminum or a metal net may be bonded to the active material layer with a conductive adhesive.

  When forming an active material layer containing a disk-like or sheet-like porous carbon material, it is preferable to use polytetrafluoroethylene or the like as the binder, and the porous carbon material, the binder, and optionally the conductive agent. A method of kneading at room temperature or under heating and molding at room temperature or under heating is preferably used.

  In order to form a thin active material layer having a thickness of up to about 200 μm on the current collector, a method of applying the slurryed active material with a doctor blade or the like is preferable. For example, when using polyvinylidene fluoride as a binder, dissolve it in an organic solvent such as N-methyl-2-pyrrolidone, and add a porous carbon material, and if necessary, a conductive agent to form a slurry. It can be carried out by coating uniformly on the current collector and drying. Further, for example, when styrene-butadiene rubber (SBR) is used as a binder, it is dispersed in water, and a porous carbon material, if necessary, a conductive agent and / or carboxymethyl cellulose (CMC). Is added to form a slurry, which is uniformly coated on the current collector and dried.

  In addition, after drying, the density of the active material layer can be increased by pressing at room temperature or under heating.

  The unit cell of an electric double layer capacitor generally uses a pair of polarizable electrodes obtained as described above, and faces each other through a liquid-permeable separator made of nonwoven fabric, paper, or other porous material as necessary. And formed by immersing in an electrolytic solution. The pair of polarizable electrodes may be the same as or different from each other. In using the electric double layer capacitor, the unit cell is used alone or a plurality of unit cells are connected in series and / or in parallel.

As the electrolytic solution of the electric double layer capacitor, either a non-aqueous solvent system or an aqueous system can be used. The nonaqueous solvent-based electrolytic solution is obtained by dissolving an electrolyte in an organic solvent. Examples of the electrolyte include (C ₂ H ₅ ) ₄ PBF ₄ , (C ₃ H ₇ ) ₄ PBF ₄ , and (C ₂ H ₅ ). ₄ NBF ₄ , (C ₂ H ₅ ) ₃ CH ₃ NBF ₄ , (C ₃ H ₇ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₄ PPF ₆ , (C ₂ H ₅ ) ₄ PCF ₃ SO ₃ , LiBF ₄ LiClO ₄ , LiCF ₃ SO _{3 and the} like can be used.

  As the organic solvent, for example, ethylene carbonate, propylene carbonate, γ-butyllactone, dimethyl sulfoxide, dimethylformamide, acetonitrile, tetrahydrofuran, dimethoxyethane and the like can be used. A mixture of two or more of these can also be used.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[Example 1]
<Manufacture of porous carbon material>
[Porous material A]: To mesocarbon microsphere carbide having an average particle size of 10 μm and a carbonization temperature of 500 ° C., 2.0 times (mass ratio) of KOH is added and mixed uniformly. And heated for 1 hour (activation treatment). After the activation treatment, the sample is washed with hydrochloric acid to neutralize the sample, washed with ion-exchanged water until the washing solution shows neutrality, and then dried to have a specific surface area of 1550 m ² / g and a packing density of 0.68 g / cm. ³ porous carbon materials were obtained.

[Porous material B]: 2.5 times the mass (mass ratio) of KOH was added to mesocarbon microsphere carbide having an average particle diameter of 2 μm and a carbonization temperature of 700 ° C., and uniformly mixed, and then 800 ° C. under a nitrogen stream. And heated for 1 hour (activation treatment). After the activation treatment, the sample is washed with hydrochloric acid to neutralize the sample, washed with ion-exchanged water until the washing solution shows neutrality, and then dried, with a specific surface area of 950 m ² / g and a packing density of 0.80 g / cm. ³ porous carbon materials were obtained.

  Each porous carbon material obtained above is mixed with porous material A: porous material B at a mass ratio of 7: 3 (A / B = 7/3), and the porous carbon material (mixed material) is mixed. It was adjusted.

<Preparation of polarizable electrode>
80 mg of the above porous carbon material (compounding material), 10 mg of carbon black and 5 mg of polytetrafluoroethylene (PTFE) were dry-mixed, and the aluminum mesh was used as a current collector to form a disk (disk) with a diameter of 13 mm and a press pressure of 148 GPa. Was pressure molded. Two of these were prepared and dried at 150 ° C. for 10 hours under reduced pressure (133.3 Pa). The packing density (unit: g / cm ³ ) of the electrode was calculated from the diameter, thickness and mass of the disk.

<Production of electric double layer capacitor>
Porous polyethylene (pore diameter 0.20 μm) is sandwiched between a pair of polarizable electrode plates prepared above in a glove box in which argon is controlled at a dew point of −80 ° C. or less. The cell was fabricated by incorporating it into a bipolar cell (expansion coefficient measurement type) and filling the electrolyte.

As the electrolytic solution, a solution obtained by dissolving tetraethylammonium tetrafluoroborate ((C ₂ H ₅ ) ₄ NBF ₄ ) in propylene carbonate at a concentration of 1M was used.

  About said porous carbon material (compounding material) and an electric double layer capacitor, the packing density, the electrostatic property of an electric double layer capacitor, and the expansion coefficient at the time of charging / discharging were evaluated. The evaluation results are shown in Table 1 below.

The average particle size, specific surface area, packing density of the porous material alone, electrostatic characteristics of the electric double layer capacitor and the evaluation method of the expansion coefficient during charging / discharging are shown below.
(A) Average particle diameter The average particle diameter of the mesocarbon microsphere material and the porous carbon material was measured using a particle size distribution measuring apparatus LMS-30 manufactured by Seishin Enterprise based on the laser diffraction method. The mesocarbon spherules and the porous carbon material were dispersed in ion-exchanged water using a cationic surfactant, made into a uniform slurry by ultrasonic treatment, and then used in the flow rate distribution measuring apparatus.

(B) Specific surface area (BET specific surface area)
The specific surface area of the porous carbon material was calculated by BET method based on the adsorption isotherm by N ₂ adsorption / desorption at 77K using ASAP2400 manufactured by micromeritis.

(C) Packing density of porous material alone Porous carbon material: Binder (PTFE: Mitsui-Dupont Fluorochemical-7J) = 80 mg: 1 mg is mixed and molded into a shape of 13 mmφ at a pressure of 294 MPa (3 ton / cm ² ). Then, after vacuum drying at 140 ° C. for 3 hours, the diameter, thickness and weight were measured and the packing density was calculated from the following formula (1).

Packing density (g / cm ³ ) = weight (g) / (circumference x (diameter (cm) / 2) ² x thickness (cm)) (1)
(D) Electrostatic characteristics of the electric double layer capacitor (capacitance)
The capacitance was calculated as follows.
First, a discharge curve (discharge voltage-discharge time) by a charge / discharge test is drawn. Here, in the charge / discharge test, charge / discharge is performed for 3 cycles at a current density of 0.5 mA / cm ² and a charge / discharge voltage of 0 to 2.4 V using a charge / discharge test apparatus (HJ1001SM8) manufactured by Hokuto Denko. did.
The discharge energy (total discharge energy (W · s) when time integration of discharge voltage x current) is obtained from the discharge curve at the third cycle, and the capacitance is calculated from the value of this discharge energy by the following equation (2). did.

Capacitance (F) = 2 × discharge energy (W · s) / (discharge start voltage (V)) ² (2)
The capacitance obtained above was divided by the mass of the porous carbon material constituting the polarizable electrode (positive electrode + negative electrode, unit: g) to obtain a capacitance per unit weight (F / g). In addition, a value obtained by multiplying the capacitance per unit weight by the electrode density (g / cm ³ ) of the polarizable electrode was defined as the capacitance per unit volume (F / cm ³ ).

(E) Expansion coefficient during charging / discharging FIG. 1 shows a schematic configuration of an expansion coefficient measuring apparatus. The expansion coefficient measuring device 1 includes a laser displacement measuring device 2 and a digital data processing device 3 shown in FIG. A bipolar cell (expansion coefficient measurement type) 4 manufactured by Hosen Co., Ltd. has a separator 6 sandwiched between two polarizable electrodes 5 and immersed in an electrolyte solution 7. The separator 6 is pressed from above. The bipolar cell 4 is incorporated in the expansion coefficient measuring apparatus 1 shown in FIG. 1, and the expansion coefficient measuring apparatus is operated simultaneously with the start of the charge / discharge test to measure the change in displacement with respect to the charge / discharge time.

The expansion coefficient was calculated as follows.
The displacement ΔT (mm) of the cell at the completion of charging in the third cycle in which charging / discharging is stabilized is measured. The expansion rate was calculated from the following equation (3), where T (mm) was the cell thickness before the start of charge / discharge.
Expansion rate (%) = (T + ΔT) / T × 100 (3).

[Example 2]
Similarly to Example 1 above, porous material A and porous material B were adjusted under the conditions shown in Table 1 (Example 2) below, and were blended and evaluated in the same manner as Example 1. The results are shown in Table 1 below. Shown in

[Examples 3 and 4]
As in Example 1 above, porous material A and porous material B were adjusted under the conditions shown in Table 1 (Examples 3 and 4) below, and blended and evaluated in the same manner as in Example 1. Table 1 shows.

[Example 5]
In the same manner as in Example 1, porous material A and porous material B were adjusted under the conditions shown in Table 1 (Example 5) below, and were blended and evaluated in the same manner as in Example 1. The results are shown in Table 1 below. Shown in

[Example 6]
In the same manner as in Example 1, the porous material A and the porous material B were adjusted under the conditions shown in Table 1 (Example 6), blended and evaluated in the same manner as in Example 1, and the results are shown in Table 1 below. Shown in

[Comparative Example 1]
The porous material A of Example 1 alone was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

[Comparative Example 2]
The porous material B of Example 1 alone was evaluated in the same manner as in Example 1, and the results are shown in Table 1.

[Comparative Example 3]
In Example 1, both the porous material A and the porous material B were blended and evaluated in the same manner as in Example 1 except that the average particle diameter was set to 5 μm, and the results are shown in Table 1.

[Comparative Example 4]
In Example 1, except that the mixing ratio of porous material A: porous material B was 1: 2, the mixing and evaluation were performed in the same manner as in Example 1, and the results are shown in Table 1.

  As shown in Table 1 above, it was confirmed that the method according to the present invention can produce an electric double layer capacitor having a small expansion during charging and discharging and a high capacitance per volume.

It is a figure which shows schematic structure of the expansion coefficient measuring apparatus which concerns on the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Expansion rate measuring device 2 Laser type displacement measuring device 3 Digital data processing device 4 Bipolar cell made by Hosen Co., Ltd.
5 Polarized electrode 6 Separator 7 Electrolyte 8 Sample holding block

Claims (2)

A porous material A obtained by carbonizing an easily graphitizable carbon material at a temperature of less than 550 ° C. and then activation treatment;
It is a mixture with a porous material B obtained by carbonizing an easily graphitizable carbon material at a temperature of 550 ° C. or higher and then performing an activation treatment.
The average particle size of the porous material A is 5 to 20 μm, the average particle size of the porous material B is 1/20 to 1/2 of the average particle size of the porous material A,
The mixing ratio of the porous material A and the porous material B is a mass ratio.
A / B = 1-10
A porous material characterized in that   An electric double layer capacitor using the porous material according to claim 1 as an electrode active material.

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