JP2005235918A - Electric double-layer capacitor and carbon material therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric double layer capacitor having high capacitance and small internal resistance, and to provide a carbon material used for a polarized material thereof. <P>SOLUTION: The carbon material 21 is used for an electrode material of the electric double layer capacitor 1 wherein a sulfur content is 0.2 weight % or below, or the size L<SB>a</SB>of a crystallite that is calculated from an equation L<SB>a</SB>=Kλ/β cosθ by the use of a half value breadth β(radian) of a diffracted ray measured by an X-ray diffracted method about 110 surfaces, a diffracted angle θ thereof, a wavelength λ of an X-ray used therefor, and a constant K=1.84 is within a range of 1nm to 3nm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a carbon material for an electric double layer capacitor and an electric double layer capacitor using the same for a polarizable electrode.

  The electric double layer capacitor has a structure in which a pair of polarizable electrodes are arranged facing each other and a nonaqueous electrolyte solution is interposed between them. Electric double layer capacitors are used as energy devices and have features such as short charging time, fast discharge and large instantaneous energy extraction, long cycle life, environmental friendliness and maintenance free. doing.

  In this electric double layer capacitor, energy is taken in and out by utilizing the adsorption / desorption phenomenon of ions to / from the interface electric double layer formed at the interface between the polarizable electrode and the electrolyte. Therefore, from the viewpoint of capacitance, it is considered advantageous that the surface area per unit weight of the polarizable electrode is large.

For these reasons, conventionally, as a material for a polarizable electrode, activated carbon whose specific surface area is increased by optimizing activation conditions, for example, a specific surface area obtained by using a BET adsorption isotherm (hereinafter, simply referred to as a specific surface area) Activated carbon having a specific surface area of 1000 m ² / g or more was used. However, the capacitance that can be achieved by increasing the specific surface area of the activated carbon is reaching its limit.

  In order to realize a higher capacitance, it has been proposed to use a carbon material in which the lattice spacing of the 002 plane is controlled as the material of the polarizable electrode.

For example, in Patent Document 1 below, non-porous carbon having a microcrystalline carbon similar to graphite and having a lattice plane spacing of 002 planes of 0.360 nm to 0.380 nm is used as a material for a polarizable electrode. It is described. Although this non-porous carbon has a specific surface area as small as 270 m ² / g or less, it can realize a relatively high capacitance when used as an electrode material of an electric double layer capacitor. Reference 1 describes that electrolyte ions intercalate with a solvent between layers of microcrystalline carbon in relation to the reason why a high capacitance is obtained when this carbon material is used. ing.

As described above, when a carbon material having a wide lattice spacing on the 002 plane is used as a material for the polarizable electrode, a relatively high capacitance can be realized. However, the electric double layer capacitor is required to have a larger capacitance. In addition, the present inventors have found that an electric double layer capacitor using a carbon material having a wide 002 plane lattice spacing as a material for a polarizable electrode has a higher internal capacity than an ordinary electric double layer capacitor using activated carbon. We find that resistance is high.
JP 2002-25867 A

  An object of the present invention is to provide an electric double layer capacitor having a large capacitance and a small internal resistance, and a carbon material used for the polarizable material.

  According to a first aspect of the present invention, there is provided a carbon material used as an electrode material for an electric double layer capacitor, wherein the carbon content is 0.2 wt% or less.

According to the second aspect of the present invention, a carbon material used as an electrode material of an electric double layer capacitor, which has a half-value width β (radian) of a diffraction line measured by an X-ray diffraction method on the 110 plane, and a diffraction angle thereof. using the theta, and the wavelength λ of X-rays used to it, and a constant K = 1.84, equation: L _a = Kλ / crystallite calculated from βcosθ size L _a is 1nm to 3nm range A carbon material is provided that is characterized by being within.

  According to the third aspect of the present invention, it comprises a pair of polarizable electrodes opposed to each other and a non-aqueous electrolyte solution interposed therebetween, wherein at least one of the pair of polarizable electrodes is the first side or the second. An electric double layer capacitor characterized by containing the carbon material according to the side surface is provided.

  According to the present invention, an electric double layer capacitor having a large capacitance and a small internal resistance and a carbon material used for the polarizable material are provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same referential mark is attached | subjected to the same or similar component, and the overlapping description is abbreviate | omitted.

FIG. 1 is a cross-sectional view schematically showing an electric double layer capacitor according to an aspect of the present invention.
The electric double layer capacitor 1 includes a pair of polarizable electrodes 2a and 2b facing each other. A separator 3 is interposed between the polarizable electrodes 2a and 2b. Current collectors 4a and 4b are provided on the outer surfaces of the polarizable electrodes 2a and 2b, respectively. The polarizable electrodes 2a and 2b, the separator 3, and the current collectors 4a and 4b are enclosed in a container (not shown) together with the nonaqueous electrolyte solution 5.

  The electric double layer capacitor 1 is further provided with a pair of external extraction electrodes (not shown) for use in charging and discharging. One end of each of the external extraction electrodes is connected to the current collectors 4a and 4b, and the other end is located outside the container.

  The polarizable electrodes 2a and 2b contain a powdery carbon material 21 as an electrode material. The polarizable electrodes 2 a and 2 b are porous bodies formed by aggregating these carbon materials 21 and impregnated with the non-aqueous electrolyte solution 5. As the carbon material 21 of at least one of the polarizable electrodes 2a, 2b, typically both of them, for example, one produced by the following method can be used.

First, as a raw material, a sulfur content C _S prepares a petroleum or coal coke less than 0.5 wt%. Incidentally, conventionally, in the production of an electrode material of an electric double layer capacitor, a sulfur content C _S is using much higher coke and pitch than 0.5 wt%. Therefore, when using petroleum or coal-based coke containing sulfur at a relatively high concentration as in the past, the sulfur content C of petroleum or coal-based coke is obtained by, for example, hydrotreating and desulfurizing. _{Reduce S} to less than 0.5 wt%. Alternatively, there is a relatively expensive, sulfur content C _S purchases petroleum or coal-based coke less than 0.5 wt%, which is used as a raw material of the carbon material.

This sulfur content C _S can be measured, for example, by the following method.
First, the sample is burned to generate an oxidizing gas, and a certain amount of the oxidizing gas is collected. Next, the sulfur oxide gas in the collected oxidizing gas is absorbed by the starch aqueous solution. Next, a predetermined amount of potassium iodide is added to the aqueous solution, and the absorbance is measured. The sulfur content C _S in the sample is determined by referring to this absorbance with a calibration curve. This calibration curve can be obtained by, for example, preparing a plurality of mixtures having different sulfur concentrations by mixing high-purity graphite and sulfur, and performing the same measurement as above except that these mixtures are used as samples. obtain.

Next, petroleum or coal-based coke having a sulfur content C _S of less than 0.5% by weight is carbonized in an inert atmosphere. This carbonization is carried out so that the resulting product has the following physical properties:
d ₀₀₂ = 0.34 nm to 0.35 nm
L _a = 0.5 nm to 3.0 nm
L _c = 1.0 nm to 3.0 nm
Here, d ₀₀₂ indicates the lattice spacing of the 002 plane. This lattice plane distance _d002 can be measured using, for example, an X-ray diffraction method.

L _a and L _c indicate the size and thickness of the crystallite, respectively. Size L _a crystallite has a half width beta (radian) and the diffraction angle θ of the diffraction line for the 110 plane obtained by X-ray diffraction method, the wavelength λ and (nm) of the X-rays used to it, the constant Calculated from the equation: L _a = Kλ / βcos θ using K = 1.84. On the other hand, the thickness L _{c of the} crystallite is determined by the half-value width β (radian) and diffraction angle θ of the diffraction line about the 002 plane obtained by the X-ray diffraction method, and the wavelength λ (nm) of the X-ray used therefor. And the constant K = 0.9 and is calculated from the equation: L _c = Kλ / βcos θ.

Physical properties of the products obtained by the carbonization treatment, i.e. the size L _a and the thickness L _c of the lattice plane of a 002 plane spacing d ₀₀₂ and crystallite, the condition of the carbonization treatment and physical and chemical properties of the material It changes according to. Typically, the carbonization temperature is in the range of 600 ° C. to 900 ° C. and the time is in the range of 0.5 hours to 4 hours.

  Next, activation treatment using a caustic alkali such as potassium hydroxide is performed on the carbonized material. Next, excess alkaline components and the like are removed by subjecting this material to, for example, pressure filtration washing. This cleaning is performed until the alkali component in the carbon material is 10,000 ppm or less. Thereafter, if necessary, surface treatment such as hydrogenation is performed on the above material. Further, this material is pulverized so that the average particle diameter becomes, for example, about 1 μm to 50 μm. As described above, the carbon material 21 used for the polarizable electrodes 2a and 2b is obtained.

  The steps from the activation treatment to pulverization are performed so that the final product has the following physical properties. In addition, SSA has shown the specific surface area obtained using a BET adsorption isotherm.

SSA ≦ 500m ² / g
d ₀₀₂ = 0.355 nm to 0.385 nm
L _a = 1.0 nm to 3.0 nm
L _c = 1.0 nm to 2.0 nm
C _S ≦ 0.2% by weight
Physical and chemical properties of the final product, i.e., the specific surface area SSA, lattice planes of a 002 plane spacing d ₀₀₂ of crystallite size L _a and the thickness L _c, as well as the sulfur content C _S, is after carbonization It varies depending on the physical and chemical properties of the material, the conditions for the activation treatment, the conditions for the surface treatment performed as necessary, and the like. As for the activation treatment, typically, the amount of caustic added is in the range of 1.5 to 2.5 times (weight ratio) with respect to the carbonized material, and the temperature of the heat treatment performed in an inert atmosphere. Is in the range of 700 ° C. to 900 ° C. and the time is in the range of 1 hour to 10 hours.

  In addition to the carbon material 21, the polarizable electrodes 2a and 2b can further contain a conductive auxiliary agent such as carbon black and a binder such as polytetrafluoroethylene (PTFE).

  The separator 3 is made of an ion-permeable dielectric and prevents a short circuit between the polarizable electrodes 2a and 2b. As the separator 3, for example, a microporous separator made of polypropylene, polyethylene, or the like can be used.

  As a material for the current collectors 4a and 4b, for example, a metal such as aluminum or an alloy can be used.

The nonaqueous electrolyte solution 5 is an aprotic solution containing an electrolyte and an organic solvent. As this electrolyte, for example, an ion that generates a cation such as a tetraalkylammonium ion and an anion such as a tetrafluoroborate ion, a hexafluorophosphate ion, or a perchlorate ion by ionization can be used. Examples of tetraalkylammonium ions include Me ₄ N ⁺ , Et _n Me _{4 -n} N ⁺ , Et ₄ N ⁺ , and n-Bu ₄ N ⁺ . Here, “Me” represents a methyl group, “Et” represents an ethyl group, and “Bu” represents a butyl group.

  As an organic solvent of the nonaqueous electrolyte solution 5, for example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, diethoxyethane, γ-butyllactone, acetonitrile, a mixture thereof, and the like can be used. For example, propionitrile, ethylene carbonate, dimethyl sulfoxide, nitromethane, a mixture thereof and the like can be further added to the organic solvent.

  Examples of the present invention will be described below.

(Example 1)
In this example, the electric double layer capacitor 1 of FIG. 1 was produced by the following method.
First, carbonization treatment was performed on a commercially available petroleum coke having a sulfur content C _S of 0.4% by weight, and then the average particle size was adjusted to about 100 μm by pulverizing it. Specifically, the petroleum-based coke powder was heat-treated at 750 ° C. for 1 hour in a N ₂ stream. As a result, the sulfur content C _{S of the} coke was reduced to 0.25% by weight.

  When various measurements were performed on the product obtained by this carbonization treatment, the following results were obtained.

d ₀₀₂ = 0.342nm
L _a = 1.40 nm
L _c = 2.33 nm
C _S = 0.25% by weight
Next, the carbonized material was subjected to activation treatment. Specifically, twice the weight ratio of potassium hydroxide was added to the carbonized material, and this mixture was subjected to heat treatment at 800 ° C. for 8 hours in an inert atmosphere.

  Next, this material was washed with a pressure filter or the like to remove excess alkali components. This washing was performed until the alkali component in the carbon material became 10,000 ppm or less.

  Thereafter, the previous material was subjected to hydrogenation treatment. Specifically, this petroleum coke powder was heat-treated at 700 ° C. for 4 hours in a hydrogen stream.

  Further, this material was pulverized so that the average particle size was about 12 μm. As described above, the carbon material 21 used for the polarizable electrodes 2a and 2b was obtained.

  When various measurements were performed on the carbon material 21 thus obtained, the following results were obtained.

SSA = 150m ² / g
d ₀₀₂ = 0.357nm
L _a = 2.39 nm
L _c = 1.23 nm
C _S = 0.08% by weight
Next, 80 parts by weight of the carbon material 21, 10 parts by weight of carbon black, and a PTFE solution containing 10 parts by weight of PTFE were sufficiently kneaded. Next, this mixture was rolled into a sheet having a thickness of 0.1 mm, and cut into a circular shape having an area of 2 cm ² .

Thereafter, the polarizable electrodes 2a and 2b thus obtained were impregnated with a propylene carbonate solution containing Et ₄ N · BF ₄ at a concentration of 1 mol / L as the non-aqueous electrolyte solution 5. This nonaqueous electrolyte solution 5 was also included in the microporous separator 3.

  Subsequently, a current collector 4a made of aluminum, a polarizable electrode 2a, a separator 3, a polarizable electrode 2b, and a current collector 4b made of aluminum were sequentially laminated, and this laminate was sealed in a container (not shown). As described above, the coin-type electric double layer capacitor 1 was completed.

(Example 2)
In this example, the electric double layer capacitor 1 of FIG. 1 was produced by the following method.
First, carbon oil was subjected to carbonization treatment on a commercially available petroleum coke having a sulfur content C _S of 0.4% by weight, and then the average particle size was adjusted to about 100 μm by grinding. Specifically, the petroleum coke powder was heat-treated at 750 ° C. for 2 hours in a hydrogen stream. As a result, the sulfur content C _{S of the} coke was reduced to 0.12% by weight.

  When various measurements were performed on the product obtained by this carbonization treatment, the following results were obtained.

d ₀₀₂ = 0.340nm
L _a = 1.41 nm
L _c = 2.61 nm
C _S = 0.12% by weight
Next, the carbonized material was subjected to activation treatment. Specifically, twice the weight ratio of potassium hydroxide was added to the carbonized material, and this mixture was subjected to heat treatment at 800 ° C. for 4 hours in an inert atmosphere.

  Next, this material was washed with a pressure filter or the like to remove excess alkali components. This washing was performed until the alkali component in the carbon material became 10,000 ppm or less.

  Thereafter, the previous material was subjected to hydrogenation treatment. Specifically, this petroleum coke powder was heat-treated at 700 ° C. for 4 hours in a hydrogen stream.

  Further, this material was pulverized so that the average particle size was about 12 μm. As described above, the carbon material 21 used for the polarizable electrodes 2a and 2b was obtained.

  When various measurements were performed on the carbon material 21 thus obtained, the following results were obtained.

SSA = 80m ² / g
d ₀₀₂ = 0.359nm
L _a = 1.79 nm
L _c = 1.12 nm
C _S = 0.02% by weight
Thereafter, the electric double layer capacitor 1 was completed by the same method as described in Example 1 except that this carbon material 21 was used.

(Comparative Example 1)
In this example, an electric double layer capacitor was prepared by the following method.
First, a commercial petroleum coke having a sulfur content C _S of 1.05% by weight was carbonized, and then pulverized to adjust the average particle size to about 100 μm. Specifically, petroleum coke powder was subjected to carbonization treatment at 750 ° C. for 1 hour in a nitrogen stream. When various measurements were performed on the product obtained by this carbonization treatment, the following results were obtained.

d ₀₀₂ = 0.344nm
L _a = 1.28 nm
L _c = 1.63 nm
C _S = 1.01 wt%
Next, the carbonized material was subjected to activation treatment. Specifically, twice the weight ratio of potassium hydroxide was added to the carbonized material, and this mixture was subjected to heat treatment at 800 ° C. for 6 hours in an inert atmosphere.

  Next, this material was washed using a pressure filter or the like to remove excess alkali components. This washing was performed until the alkali component in the carbon material became 10,000 ppm or less.

  Thereafter, the previous material was subjected to hydrogenation treatment. Specifically, this petroleum coke powder was heat-treated at 700 ° C. for 4 hours in a hydrogen stream.

  Further, this material was pulverized so that the average particle size was about 12 μm. As described above, a carbon material used for the polarizable electrode was obtained.

  When various measurements were performed on the carbon material thus obtained, the following results were obtained.

SSA = 20m ² / g
d ₀₀₂ = 0.361 nm
L _a = 0.75 nm
L _c = 1.23 nm
C _S = 0.24% by weight
Thereafter, an electric double layer capacitor was completed by the same method as described in Example 1 except that this carbon material was used.

(Comparative Example 2)
In this example, an electric double layer capacitor was prepared by the following method.
First, carbonization treatment was performed on a commercially available petroleum coke having a sulfur content C _S of 5.8% by weight, and then this was pulverized to adjust the average particle size to about 100 μm. Specifically, petroleum coke powder was subjected to carbonization treatment at 750 ° C. for 1 hour in a nitrogen stream. When various measurements were performed on the product obtained by this carbonization treatment, the following results were obtained.

d ₀₀₂ = 0.342nm
L _a = 2.08 nm
L _c = 1.33 nm
C _S = 4.1% by weight
Next, the carbonized material was subjected to activation treatment. Specifically, twice the weight ratio of potassium hydroxide was added to the carbonized material, and this mixture was subjected to heat treatment at 800 ° C. for 2 hours in an inert atmosphere.

  Next, this material was washed using a pressure filter or the like to remove excess alkali components. This washing was performed until the alkali component in the carbon material became 10,000 ppm or less.

  Further, this material was pulverized so that the average particle size was about 12 μm. As described above, a carbon material used for the polarizable electrode was obtained.

  When various measurements were performed on the carbon material thus obtained, the following results were obtained.

SSA = 350m ² / g
d ₀₀₂ = 0.364 nm
L _a = 3.36 nm
L _c = 0.91 nm
C _S = 0.43 wt%
Thereafter, an electric double layer capacitor was completed by the same method as described in Example 1 except that this carbon material was used.

For each of the electric double layer capacitors according to Examples 1 and 2 and Comparative Examples 1 and 2 obtained as described above, first, the charging voltage was sequentially increased to 2V, 2.5V, 3V, 3.5V, and 4V. The capacitance was measured for each charging pressure. Next, the charge pressure was decreased sequentially to 4V, 3.5V, 3V, 2.5V, and 2V, and the capacitance was measured for each charge pressure. Here, the charge / discharge current was ² mA / cm ² .

  FIG. 2 is a graph showing the relationship between the charging pressure and the capacitance. In the figure, the horizontal axis indicates the charging pressure, and the vertical axis indicates the capacitance per unit volume of the electric double layer capacitor. FIG. 2 shows only data obtained for the electric double layer capacitor according to Example 1 and Comparative Example 1.

  As shown in FIG. 2, the capacitance of these electric double layer capacitors was a very small value immediately after manufacture, but increased with an increase in charging pressure. This capacitance was decreased by lowering the charging pressure, but did not return to the initial state.

Next, the initial performance of the electric double layer capacitor subjected to the previous test was evaluated. Specifically, with respect to the electric double layer capacitors according to Examples 1 and 2 and Comparative Examples 1 and 2 with a charge pressure of 3.0 V and a charge / discharge current of ² mA / cm ² , the capacitance per unit volume and the internal capacity Resistance was measured. The results are shown in the following table together with the physical and chemical properties of the carbon material used as the electrode material.

3 and 4 are graphs showing the capacitance and internal resistance of the electric double layer capacitors according to Examples 1 and 2 and Comparative Examples 1 and 2. FIG. In FIG. 3, the horizontal axis indicates the sulfur content C _S of the carbon material used as the electrode material, and the vertical axis indicates the capacitance per unit volume and the internal resistance of the electric double layer capacitor. On the other hand, in FIG. 4, the horizontal axis represents the magnitude of L _a crystallite of the carbon material used as an electrode material, and the vertical axis represents the capacitance and the internal resistance per unit volume of the electric double layer capacitor.

As shown in FIGS. 3 and 4, an electric double layer capacitance and the internal resistance of the capacitor, the the size L _a sulfur content C _S and the crystallite of the carbon material used as an electrode material, see a correlation It was. That is, as shown in FIG. 3, the sulfur content C _S of the carbon material used as an electrode material of 0.2 wt% or less, particularly 0.1 wt% or less, when a sulfur content C _S is 0.2 Compared to the case of exceeding%, the capacitance was larger and the internal resistance was smaller. Further, as shown in FIG. 4, the range size L _a crystallite of 1nm to 3nm carbon material used as an electrode material, particularly 1.5nm to within the range of 2.5 nm, when the crystallite size L _a is compared with the case where more than the case and 3nm is less than 1nm, the capacitance is larger, the internal resistance was smaller.

1 is a cross-sectional view schematically showing an electric double layer capacitor according to one embodiment of the present invention. The graph which shows the relationship between a charging pressure and an electrostatic capacitance. The graph which shows the electrostatic capacitance and internal resistance of the electric double layer capacitor which concern on Examples 1 and 2 and Comparative Examples 1 and 2. FIG. The graph which shows the electrostatic capacitance and internal resistance of the electric double layer capacitor which concern on Examples 1 and 2 and Comparative Examples 1 and 2. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Electric double layer capacitor, 2a ... Polarizable electrode, 2b ... Polarizable electrode, 3 ... Separator, 4a ... Current collector, 4b ... Current collector, 5 ... Non-aqueous electrolyte solution, 21 ... Carbon material.

Claims (7)

  A carbon material used as an electrode material for an electric double layer capacitor, wherein the sulfur content is 0.2% by weight or less. Using the half-value width β (radian) and diffraction angle θ of the diffraction line for the 110 plane obtained by the X-ray diffraction method, the wavelength λ (nm) of the X-ray used therefor, and a constant K = 1.84 The carbon material according to claim 1, wherein the crystallite size L _a calculated from the equation: L _a = Kλ / βcos θ is in the range of 1 nm to 3 nm. A carbon material used as an electrode material of an electric double layer capacitor, which is a half-value width β (radian) of a diffraction line measured by an X-ray diffraction method on the 110 plane, its diffraction angle θ, and the X-ray used Carbon having a crystallite size L _a calculated from the equation: L _a = Kλ / βcos θ using a wavelength λ and a constant K = 1.84. material. The carbon material according to any one of claims 1 to 3, wherein a specific surface area is 500 m ² / g or less.   The carbon material according to any one of claims 1 to 4, wherein a lattice plane spacing of the 002 plane is in a range of 0.355 nm to 0.385 nm.   The carbon material according to any one of claims 1 to 5, wherein a raw material of the carbon material is petroleum or coal-based coke and has a sulfur content of 0.5 wt% or less.   A pair of polarizable electrodes facing each other and a nonaqueous electrolyte solution interposed therebetween are provided, at least one of the pair of polarizable electrodes according to any one of claims 1 to 6. An electric double layer capacitor comprising a carbon material.

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