JP2020092124A - Graphite-based porous carbon material for electrochemical capacitor electrode and method for producing the same, electrochemical capacitor electrode and electrochemical capacitor - Google Patents

Abstract

An object of the present invention is to provide a graphite-based porous carbon material for electrodes capable of increasing the capacity, a method for producing the same, an electrochemical capacitor electrode, and an electrochemical capacitor provided with the electrode. A graphite-based porous carbon material for an electrochemical capacitor electrode, which has a rhombohedral and hexagonal crystal structure, a rhombohedral content of 30% by weight or more, and a pore diameter of less than 2 nm. A carbon material having a pore volume of 0.01 to 0.50 cm 3 /g for micropores and a pore volume of 0.20 to 0.80 cm 3 /g for mesopores having a pore diameter of 2 to 50 nm. [Selection drawing] Fig. 5

Description

The present invention relates to a graphite-based porous carbon material for an electrochemical capacitor electrode, a method for producing the same, an electrochemical capacitor electrode and an electrochemical capacitor.

An electrochemical capacitor has a capacity that is caused by a non-Faraday reaction that does not involve the transfer of electrons between the electrode and the electrolyte (ions in the electrolytic solution) or transfer of electrons at the interface between the electrodes (positive electrode and negative electrode). It is an electricity storage device that utilizes the capacity developed due to the accompanying Faraday reaction. Compared with secondary batteries such as lithium-ion batteries, which are charged and discharged based on the Faraday reaction between the electrode and the electrolyte, they are characterized by high energy efficiency, rapid charge and discharge, long life, low reaction heat, and safety. is there. Due to such characteristics, the electrochemical capacitor is used as an auxiliary power source for hybrid vehicles and the like, a regenerative power storage device, an alternative device for secondary batteries, an energy buffer for solar power generation, etc. ..

Electrochemical capacitors are roughly classified into three types: electric double layer capacitors, hybrid capacitors, and pseudo-capacitance capacitors (redox capacitors). In the electric double layer capacitor, electrons are not exchanged between the electrodes and the ions in the electrolytic solution (non-Faraday reaction), and the charge and discharge are performed only by the physical desorption of the ions. When the ions are adsorbed on the surface of the electrode, an electric double layer is formed between the electrode and the electrolyte to store electricity. In order to increase the capacity, it is said that the larger the electrode surface area is, the more advantageous it is. Generally, activated carbon having a large specific surface area is used as an electrode material (active material).

In the hybrid capacitor, one of the electrodes, for example, the positive electrode, is charged only by physical adsorption of ions, and the other of the electrodes, for example, the negative electrode, is charged by the Faraday reaction, similarly to the electric double layer capacitor. In an electrode (for example, a positive electrode) that stores electricity only by physical adsorption of ions, a material (active carbon) similar to that of an electric double layer capacitor is used as an electrode material (active material), and in an electrode (for example, a negative electrode) where a Faraday reaction occurs, Intercalatable materials such as graphite and lithium titanate are mainly used. In a lithium-ion capacitor, which is a type of hybrid capacitor, activated carbon is used for the positive electrode, and a carbon material capable of intercalating lithium ions is used for the negative electrode.

As described above, in the electrochemical capacitor, it is common to use activated carbon as an electrode material (active material) that stores electricity only by physical adsorption of ions. However, activated carbon is a carbon material produced from coconut husks, etc., which has been activated by heating it in an atmosphere such as water vapor, and it contains a large amount of impurities and costly for activation treatment and its process control. There are problems such as. Therefore, in order to solve this problem, it has been proposed to use a graphite-based carbon material instead of activated carbon.

For example, in Patent Document 1, an electric double layer having a polarizable electrode, a separator, an organic solvent and a collecting electrode, and having a polarizable electrode composed of a positive electrode whose main composition is activated carbon and a negative electrode made of mechanically ground graphite. A capacitor is disclosed. The capacitor is made of a marketable material instead of a special material, and is equipped with a polarizable electrode that does not require a special manufacturing process such as activation treatment, and thus is compact and many It is said that it can store the amount of electricity.

Further, Patent Document 2 discloses a method for producing pulverized graphite, which comprises pulverizing graphite in a nitrogen atmosphere so that a new surface is formed. When used as a capacitor electrode, the capacitor is said to exhibit high initial discharge efficiency.

Further, Patent Document 3 discloses a composite conductive material in which a graphene-like graphite separated from a graphite-based carbon material and a conductive material are dispersed in a base material, and the graphite-based carbon material is a rhombohedral graphite. A layer (3R) and a hexagonal graphite layer (2H), and the rhombohedral graphite layer (3R) and the hexagonal graphite layer (2H) are expressed by an X-ray diffraction method: Rate (3R) )=P3/(P3+P4)×100 The rate Rate(3R) defined by 100 is 31% or more, and the composite conductive material is disclosed. Here, P3 is the peak intensity of the (101) plane of the rhombohedral graphite layer (3R) by the X-ray diffraction method, and P4 is the peak intensity of the hexagonal graphite layer (2H) of the (101) plane. Strength. According to this document, the use of such a graphite-based carbon material facilitates exfoliation into graphene, and a highly concentrated and highly dispersed graphene solution can be easily obtained.

JP, 2007-157954, A JP, 2010-126418, A International Publication No. 2016/002261

Thus, it has been proposed to use a graphite-based carbon material as an electrode material (active material) for the purpose of improving the performance of the electrochemical capacitor. On the other hand, capacitors are required to have even larger capacities in response to the demand for larger storage power and compact size. However, conventional graphite-based carbon materials have not been sufficient to increase the capacity of capacitors. Therefore, an object of the present invention is to provide a graphite-based carbon material for an electrochemical capacitor electrode, which can have a large capacity.

The present inventors have found that, in graphite-based carbon materials for electrochemical capacitor electrodes, the content ratio of rhombohedral crystals in the crystal structure and the pore volumes of micropores and mesopores contribute to increase the capacity of the capacitor. It is important that, and by using a planetary ball mill, by dry pulverizing graphite under predetermined conditions, it was found that a graphite-based porous carbon material capable of large capacity can be obtained, The present invention has been completed.

The present invention includes the following aspects (1) to (11). In the present specification, the expression "to" includes the numerical values at both ends thereof. That is, “X to Y” is synonymous with “X or more and Y or less”.

(1) A graphite-based porous carbon material for an electrochemical capacitor electrode, comprising:
It has a rhombohedral crystal structure and a hexagonal crystal structure, and the content ratio of the rhombohedral crystal is 30% by weight or more,
The pore volume of micropores smaller than a pore diameter 2nm is 0.01~0.50cm ³ / g, pore volume of mesopores having a pore diameter 2~50nm is 0.20~0.80cm ³ / g, a carbon material.

(2) The carbon material according to (1) above, wherein the rhombohedral crystal content is 50% by weight or more.

(3) The carbon material according to the above (1) or (2), which has a specific surface area of 400 to 800 m ² /g.

(4) The carbon material according to any one of (1) to (3), which has a combustion starting temperature of less than 500° C. in air.

(5) The method for producing a carbon material according to any one of (1) to (4) above,
A step of preparing raw graphite,
Dry pulverizing the raw material graphite in the atmosphere at a rotation speed of 500 rpm or more using a planetary ball mill.

(6) The method according to (5) above, wherein the dry pulverization is performed for 14 to 150 minutes.

(7) The method according to (5) or (6) above, wherein zirconia balls having a diameter of 1 to 10 mm are used as the grinding media of the planetary ball mill.

(8) The method according to any one of (5) to (7) above, which further comprises a wet dispersion step using the planetary ball mill after the dry pulverization step.

(9) An electrochemical capacitor electrode containing the carbon material according to any one of (1) to (4) above.

(10) The electrochemical capacitor electrode according to (9) above, wherein the initial electrode density is 0.8 g/cm ³ or more.

(11) An electrochemical capacitor provided with the electrode according to (9) or (10) above.

According to the present invention, there is provided a graphite-based porous carbon material for an electrochemical capacitor electrode capable of increasing the capacity and a method for producing the same. Further, according to the present invention, there are provided an electrochemical capacitor electrode capable of increasing the capacity and an electrochemical capacitor including the electrode.

It is a figure which shows the relationship between crushing time and the weight specific capacity of a carbon material. It is a figure which shows the relationship between crushing time and the area specific capacity of a carbon material. It is a figure which shows the relationship between a crushing time and the rhombohedral crystal content rate ( _xRh ) of a carbon material. It is a figure which shows the relationship between the grinding time and the combustion start temperature of a carbon material. It is a figure which shows the relationship between the rhombohedral crystal content rate ( _xRh ) of carbon material, and an area specific capacity. It is a figure which shows the relationship between the current density at the time of cell capacity measurement, and the area specific capacity of a carbon material. It is a figure which shows the relationship between the current density at the time of cell capacity measurement, and the capacity retention rate of a carbon material. It is a figure which shows the influence of the current response of a carbon material by the presence or absence of a wet dispersion process. It is a figure which shows the influence by the rotation speed of a planetary type ball mill.

Graphite-based porous carbon material The carbon material of the present invention is a graphite-based porous material. Pure graphite has a perfect crystalline structure and usually has no pores. In addition, activated carbon contains many amorphous parts. On the other hand, the carbon material of the present invention is porous so that its crystal structure is refined to the extent that the crystal structure of graphite is slightly left, and micropores and mesopores are present. Therefore, this carbon material is clearly distinguished from pure graphite and activated carbon in terms of crystal structure and pore structure. As described later, this graphite-based porous carbon material can be produced by dry pulverizing graphite under predetermined conditions using a planetary ball mill. In terms of the production method, the carbon material of the present invention is a material derived from graphite, but its crystal structure and pore structure are not the same as pure graphite.

The carbon material of the present invention has a rhombohedral crystal structure and a hexagonal crystal structure. Hexagonal and rhombohedral crystals are known as the crystal structure of graphite, but the general crystal structure is hexagonal. The hexagonal crystal has a crystal structure stacked in the order of ABABAB... And the rhombohedral crystal has a crystal structure stacked in the order of ABCABC.

Further, the carbon material of the present invention has a rhombohedral crystal content of 30% by weight or more. The area specific capacity is increased by increasing the content ratio of the rhombohedral crystal. Although the detailed mechanism is not certain, it is speculated that rhombohedral crystals have a higher ion adsorption density than hexagonal crystals. The rhombohedral crystal content is preferably 40% by weight or more, more preferably 50% by weight or more. The upper limit of the content ratio is not particularly limited, but is typically 70% by weight or less, and more typically 60% by weight or less.

Carbon material of the present invention, the pore volume of micropores smaller than a pore diameter 2nm is 0.01~0.50cm ³ / g, pore volume of mesopores having a pore diameter 2~50nm is 0.20~0.80cm ³ / It is g. When the pore volume of the micropores and the mesopores is within the above range, the capacitance of the capacitor is high. Although the detailed mechanism is not certain, it is thought that the electrolyte ions are efficiently adsorbed on the carbon material by controlling the pore volume of the micropores and the mesopores. That is, it is generally said that a larger electrode surface area is advantageous in order to increase the amount of ions adsorbed by forming the electric double layer. However, in reality, the mechanism for forming the electric double layer is complicated, and it is not enough that the surface area is simply large. It is considered that the fine structure of the electrode surface has an influence. In the case of a porous electrode material, it is desirable that the electrolyte ions can easily access the pores, and for that purpose, the pore diameter and pore volume of the electrode material are considered to be important factors. In the present invention, it is considered that electrolyte ions can easily access the pores by controlling the pore volume of the micropores and mesopores of the carbon material. The pore volume of micropores is preferably 0.10 to 0.30 cm ³ /g, and the pore volume of mesopores is preferably 0.40 to 0.80 cm ³ /g. In the present specification, the term "pores" refers to pores partially or wholly surrounded by a carbon material, one end of which may be an open pore, or both ends of which are open pores. May be

The carbon material preferably has a specific surface area of 400 m ² /g or more. By controlling the pore volume of the micropores and the mesopores and further increasing the specific surface area, the effect of increasing the capacity of the capacitor can be made more excellent. On the other hand, when the specific surface area is excessively high, the pore volume becomes too large and the electrode density may decrease. Therefore, the specific surface area is typically 2000 m ² /g or less, more typically 1200 m ² /g or less, and may be 800 m ² /g or less.

The carbon material preferably has a combustion start temperature of less than 500° C. in air. Pure graphite having high crystallinity has a combustion starting temperature of 600°C. On the other hand, in the carbon material of the present invention, the combustion start temperature is remarkably lowered by controlling the crystallinity, the content ratio of rhombohedral crystals, and the pore volume of micropores and mesopores. The combustion starting temperature is preferably 450°C or lower.

Method for producing graphite-based porous carbon material The method for producing a graphite-based porous carbon material of the present invention has a step of preparing graphite and a step of dry pulverization.

<Raw material graphite preparation process>
First, raw material graphite is prepared. Both natural graphite and artificial graphite can be used as the raw material graphite. As the natural graphite, known graphite such as flake graphite and earth graphite can be used. The grain size of the raw material graphite is not particularly limited, but for example, the crystal grain size (mode diameter) is 10 to 100 μm.

<Dry grinding process>
Next, the raw material graphite is dry-ground using a planetary ball mill at a rotation speed (rotation speed) of 500 rpm or more in the atmosphere. The planetary ball mill is a device for rotating and revolving a grinding container containing a sample and a grinding medium such as zirconia balls at a high speed, and a strong impact force is applied to the sample by a combination of the rotation and the revolution. As a result, the sample is strongly ground and a mechanochemical action is added.

In the present invention, by setting the rotation speed to 500 rpm or more, the content ratio of rhombohedral crystals in the crystal structure and the pore volumes of micropores and mesopores can be set within a predetermined range, and as a result, the capacity can be increased. It is possible to obtain a graphite-based porous carbon material that can be used. The number of rotations is preferably 550 rpm or more, more preferably 600 rpm or more. On the other hand, if the number of revolutions is excessively high, the amount of contamination increases, which may deteriorate the characteristics of the carbon material. Therefore, the number of rotations is preferably 900 rpm or less, more preferably 800 rpm or less.

Further, in the present invention, dry pulverization is performed in the atmosphere. By crushing in the atmosphere, expensive equipment for introducing an inert gas or the like becomes unnecessary. Therefore, the carbon material can be obtained by a simple method, and the manufacturing cost can be reduced. Moreover, by pulverizing in the atmosphere, a part of the oxygen-containing functional groups can be introduced, which has the effect of increasing the hydrophilicity of the carbon material.

If steel is used as the crushing container or crushing medium of the planetary ball mill, the amount of contamination such as iron may increase. Therefore, from the viewpoint of suppressing contamination during crushing, it is preferable to use zirconia-made crushing containers and crushing media. The grinding media are preferably zirconia balls with a diameter of 1-10 mm. Further, the ratio of the raw material graphite and the pulverizing medium is not particularly limited, but for example, the pulverizing medium is 10 to 80 with respect to the raw material graphite 1 in a weight ratio.

Dry crushing is preferably performed for 14 to 150 minutes. By setting the crushing time to 14 minutes or more, the content ratio of rhombohedral crystals in the crystal structure and the pore volumes of micropores and mesopores can be set within the predetermined ranges. On the other hand, if the pulverization is carried out for an excessively long time, the amount of contamination may increase. Therefore, the crushing time is preferably 150 minutes or less.

<Wet dispersion process>
If necessary, it may be wet-dispersed using the planetary ball mill after the dry grinding step. By wet dispersion, a carbon material having excellent current response can be obtained. Further, by wet-dispersing, the carbon material adhering to the grinding media and the inner wall surface of the grinding container can be efficiently collected, which has an effect of reducing the manufacturing cost.

Wet dispersion can be performed, for example, as follows. That is, after dry pulverization, a dispersion medium such as 10 to 20 cc of distilled water or IPA is added to the pulverized powder in the pulverization container. Next, the planetary ball mill is operated for 1 to 10 minutes to obtain a dispersion liquid. Then, the obtained dispersion liquid is filtered to obtain a wet-dispersed carbon material.

According to such a production method of the present invention, the content ratio of rhombohedral crystals in the crystal structure and the pore volume of micropores and mesopores are within a predetermined range, and a graphite-based porous carbon material capable of large capacity is obtained. Obtainable. Such a graphite-based porous carbon material and a method for producing the same have not heretofore been known. For example, Patent Documents 1 and 2 disclose that graphite is pulverized by a planetary ball mill to obtain an electrode material for electric double layer capacitors. These documents disclose rhombohedral carbon materials. It does not focus on the content ratio of crystals and the pore volume of micropores and mesopores. Further, these documents do not intend to increase the rotation speed of the planetary ball mill to 500 rpm or more. In particular, in Patent Document 2, the planetary ball mill treatment is performed at a rotation speed of 250 rpm in the example, and it is speculated that such a treatment at a low rotation speed can only provide a carbon material having a large capacity inferior effect. To be done.

In Patent Document 3, by setting the rate Rate (3R) of the rhombohedral graphite layer (3R) and the hexagonal graphite layer (2H) of the graphite-based carbon material to 31% or more, high concentration and high dispersion can be achieved. It is said that a graphene solution can be obtained, but it does not focus on the pore volume of micropores and mesopores, let alone, it does not disclose that the effect of increasing the capacity can be obtained. In fact, Patent Document 3 does not verify the effect of applying a graphite-based carbon material as an active material to an electrochemical capacitor in Examples.

On the other hand, in the production method of the present invention, using a planetary ball mill, a simple method of dry crushing graphite under predetermined conditions is a simple method of increasing the capacity, and a graphite-based porous material for electrochemical capacitor electrodes can be used. A carbon material can be obtained.

Electrode for Electrochemical Capacitor The electrode for electrochemical capacitor of the present invention can be produced by a known method using the above graphite-based porous carbon material as an active material. For example, a binder, a conductive material, and a solvent are added to a carbon material, and the mixture is kneaded to prepare a slurry, and the slurry is applied to the surface of a current collector and dried to prepare an electrode. The binder is not particularly limited and, for example, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), or the like can be used, and the solvent can be distilled water, an organic solvent, or the like. Further, carbon black such as acetylene black or Ketjen black can be used as the conductive material. The current collector is a member for taking out an electric current to the outside, and for example, an aluminum (Al) foil or the like is used. In order to obtain an electrode having a desired shape, the electrode may be punched out after applying the slurry. Further, in order to obtain a high-density electrode, pressing may be performed after applying the slurry. The pressing pressure is, for example, 100 to 500 MPa. The electrode has a thickness of, for example, 0.01 to 0.10 mm.

The electrode density for an electrochemical capacitor of the present invention can be increased by using the above graphite-based porous carbon material. Electrode density is preferably 0.8 g / cm ³ or more, more preferably 0.9~1.3g / cm ^3. Such an electrode for an electrochemical capacitor has a high density, and since the graphite-based porous carbon material used has a high area specific capacity, it greatly contributes to increasing the capacity of the capacitor.

Electrochemical Capacitor The electrochemical capacitor of the present invention is mainly composed of a positive electrode, a negative electrode and an electrolytic solution. The electrolytic solution is provided between the positive electrode and the negative electrode so as to impregnate the positive electrode and the negative electrode. Further, if necessary, a separator such as a papermaking sheet, a nonwoven fabric, or a microporous membrane may be provided between the positive electrode and the negative electrode. Further, other members such as spacers, washers and gaskets may be provided. A positive electrode, a negative electrode, an electrolytic solution, a separator and other members are enclosed in a cell case to form a capacitor. The shape of the capacitor is not particularly limited, and a known shape such as a coin type, a laminated type, a cylindrical type or a square type can be adopted. It is also possible to connect a plurality of capacitors to form a capacitor module. Such a capacitor module is suitable for use as an ultra-high capacity electrochemical capacitor used in automobiles and the like.

The electrochemical capacitor is typically an electric double layer capacitor. In that case, the above-mentioned electrode for electrochemical capacitors is used for both the positive electrode and/or the negative electrode. As the electrolytic solution, a known electrolytic solution such as an aqueous system, an organic system, or an ionic liquid can be used. The aqueous electrolytic solution includes, for example, an aqueous sulfuric acid solution, and the organic electrolytic solution includes, for example, a tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF ₄ /PC) solution.

The electrochemical capacitor may be a hybrid capacitor. In that case, the electrode for electrochemical capacitor is used for either one of the positive electrode and the negative electrode, for example, the positive electrode, and a known active material such as graphite or lithium titanate is collected for the other electrode, for example, the negative electrode. The one provided on the electric body is used. As the electrolytic solution, an aqueous electrolytic solution or an organic electrolytic solution can be used. The aqueous electrolytic solution includes, for example, a sulfuric acid aqueous solution, and the organic electrolytic solution includes, for example, a tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF ₄ /PC) solution or lithium hexafluorophosphate/ethylene carbonate/diethylene carbonate (LiPF). ₆ /EC/DEC) solution and the like.

The electrochemical capacitor of the present invention has the effect of increasing the capacity by using the above graphite-based porous carbon material for the electrode. Typically, the weight specific capacity is 7.0 to 15.0 F/g and the area specific capacity is 5.0 to 8.0 μF/cm ² when charged and discharged at a constant current of 0.1 A/g. is there.

The present invention will be more specifically described by the following examples.

Example 1 (comparison)
(1) Preparation of graphite-based porous carbon material <graphite preparation step>
Natural graphite having a crystal grain size of 42 μm and a specific surface area of 10 m ² /g or less was prepared as a raw material. The crystal grain size is the particle size (mode size) having the largest appearance ratio in the particle size distribution. The specific surface area was calculated by the BET method from the nitrogen adsorption isotherm.

<Dry grinding process>
Next, the raw material graphite was dry-pulverized to prepare a carbon material. First, 1 g of raw material graphite and 75 g of zirconia balls were sealed in air in a crushing container of a planetary ball mill. At that time, a zirconia ball having a diameter of 1 mm was used, and a zirconia container having an internal volume of 45 cc was used as a crushing container. Then, the rotation number of the planetary ball mill was set to 700 rpm, and pulverization was performed for 5 minutes to obtain a carbon material. The rotation speed is the revolution speed of the planetary ball mill (stand rotation speed), and the ratio of the revolution speed to the rotation speed (rotation revolution ratio) was fixed to 1:2.

(2) Production of electrode An electrode was produced using the obtained carbon material. First, 170 mg of a carbon material was weighed, carbon black (CB, Denka black), carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR) were added thereto, and distilled water was added dropwise and kneaded to prepare a slurry. At this time, the carbon material was 85% by weight, CB was 5% by weight, CMC was 5% by weight, and SBR was 5% by weight. Next, using a doctor blade, the obtained slurry was applied onto the Al current collector. Then, it was punched into a disk shape with a diameter of 10 mm, pressed at 20 kN, and vacuum dried at 120° C. overnight to produce a carbon electrode (film electrode) having a thickness of about 0.03 to 0.05 mm.

(3) Preparation of cell A bipolar electrode coin cell was prepared by using the obtained carbon electrode (film electrode) as a positive electrode and a negative electrode. First, a carbon electrode, a separator, a carbon electrode, a spacer, a washer, and a gasket were laminated in this order to form a laminate. A 1 mol/L tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF ₄ /PC) solution was prepared as an electrolytic solution. A 2032 type coin cell was produced by enclosing the laminated body together with an electrolytic solution in a cell container consisting of a case and a cap.

(4) Evaluation Various characteristics of the obtained carbon material, electrode and cell were evaluated as follows.

<Crystal grain size>
The crystal grain size of the carbon material was measured by a laser diffraction/scattering type particle size distribution measuring device. The crystal grain size is a particle size (mode size) having the largest appearance ratio in the particle size distribution.

<Specific surface area>
The specific surface area of the carbon material was calculated from the nitrogen adsorption isotherm by the BET method.

<Pore volume>
The pore volume of the micropores (pore diameter less than 2 nm) and mesopores (pore diameter 2 to 50 nm) of the carbon material is determined from the nitrogen adsorption isotherm by the quenching fixed density functional theory (QSDFT) method and the Barrett-Joyner-Halenda (BJH) method. Was measured by

<Crystal structure>
The rhombohedral crystal content ratio χ _Rh of the carbon material was determined by an X-ray diffraction method. Specifically, using the peak intensity of the (101) plane of the rhombohedral crystal (I _Rh ) and the peak intensity of the (101) plane of the hexagonal crystal (I _H ) in the X-ray diffraction pattern, the formula: χ _Rh =I _{It was determined by Rh} /(I _Rh +2/3×I _H ).

<Combustion start temperature>
The combustion starting temperature of the carbon material was measured by a thermogravimetric method in air.

<Electrode density>
The weight and thickness of the electrode were measured, and the electrode density was calculated.

<Capacity>
The capacity of the cell was measured, and the weight specific capacity and area specific capacity were calculated. When measuring the cell capacity, constant-current charging/discharging with a current density of 0.1 to 50 A/g was performed in a voltage range of 0 to 2.5 V. Then, the weight specific capacity (capacity per unit weight) and the area specific capacity (capacity per unit area) were calculated using the obtained capacity and the weight and specific surface area of the carbon material. The weight specific capacity is the capacity for both poles, and the area specific capacity is the capacity for a single pole.

Example 2 (comparison)
When the carbon material was produced, the carbon material, electrodes and cells were produced and evaluated in the same manner as in Example 1 except that the crushing time was 10 minutes.

Example 3
When the carbon material was produced, the carbon material, electrodes and cells were produced and evaluated in the same manner as in Example 1 except that the crushing time was 14 minutes.

Example 4
When producing the carbon material, the carbon material, the electrode and the cell were produced and evaluated in the same manner as in Example 1 except that the crushing time was 75 minutes.

Example 5
When producing the carbon material, the carbon material, the electrode and the cell were produced and evaluated in the same manner as in Example 1 except that the crushing time was 150 minutes.

Example 6 (comparison)
As the carbon material, commercially available activated carbon generally used as an electrode material for electric double layer capacitors was used. Except for this, the electrode and the cell were prepared and evaluated in the same manner as in Example 1.

Table 1 shows the evaluation results obtained in Examples 1 to 6. The weight specific capacity and area specific capacity shown in Table 1 are values when the current density at the time of measuring the cell capacity is 0.1 A/g.

In order to investigate the influence of the crushing time in more detail, the carbon material, the electrode and the cell were prepared and evaluated in the same manner as in Example 1 except that the crushing time was changed from 0 to 750 minutes. Here, the sample having a crushing time of 0 minutes is raw graphite. Further, the samples having the crushing times of 5, 10, 14, 75 and 150 minutes are the same as those in Examples 1 to 5 of Example 1.

Table 2 shows the obtained evaluation results. The weight specific capacity and area specific capacity shown in Table 2 are values when the current density at the time of measuring the cell capacity is 10 A/g. Further, based on the obtained results, the relationship between the pulverization time, the weight specific capacity of the carbon material, the area specific capacity, the rhombohedral crystal content ratio (χ _Rh ) and the combustion starting temperature was determined. These relationships are shown in FIGS. Further, FIG. 5 shows the relationship between the rhombohedral crystal content ratio (χ _Rh ) and the area specific capacity.

As shown in Table 1, in Examples 1 to 5, which are carbon materials using graphite, the pore volume of micropores and mesopores increased as the pulverization time during dry pulverization increased. Further, as shown in Tables 1 and 2 and FIGS. 1 to 4, the carbon material tends to increase the content ratio of rhombohedral crystals, the weight specific capacity and the area specific capacity as the pulverization time increases. In particular, the crushing time was increased to 14 minutes or more, which caused a sharp increase. Further, as can be seen from FIG. 5, there is a strong correlation between the rhombohedral crystal content rate and the area specific capacity, and when the rhombohedral crystal content rate is 30% by weight or more, the area specific capacity is 5 μF/cm ² or more. ..

Further, as shown in FIG. 4, the combustion start temperature of the carbon material decreased as the pulverization time increased. In particular, when the pulverization time was 14 minutes or more, the combustion start temperature was significantly lower than 500°C.

On the other hand, the electrode density was a constant value regardless of the grinding time. In Examples 3 to 5 (Examples), the electrode density was higher than that of Example 6 (Comparative Example) using activated carbon, and the area specific capacity was also higher than that of Example 6.

Regarding Example 5 and Example 6 of Example 1, the relationship between the current density and the area specific capacity and the capacity retention rate when the current density at the time of measuring the cell capacity was changed to 0.1 to 50 A/g is shown in FIGS. Show. Here, the capacity retention rate is the ratio of the area specific capacity of each current density to the area specific capacity when the current density is 0.1 A/g.

As shown in FIGS. 6 and 7, the area specific capacity of Example 5 (Example) was much higher than that of Example 6 (Comparative Example) regardless of the current density. This indicates that Example 5 has a smaller resistance and is excellent in high-speed response as compared with Example 6.

Example 7
At the time of dry pulverization, a carbon material, an electrode and a cell were prepared and evaluated in the same manner as in Example 1 except that zirconia balls having a diameter of 5 mm were used and the planetary ball mill was rotated at 500 rpm and pulverization was performed for 60 minutes. It was

Example 8
A carbon material, an electrode and a cell were prepared and evaluated in the same manner as in Example 7 except that a wet dispersion step was provided after dry pulverization. Here, the wet dispersion treatment was performed as follows.

<Wet dispersion process>
After dry grinding, 10 cc of distilled water was added to the ground powder in the planetary ball mill grinding container. After performing a planetary ball mill treatment at a rotation speed of 500 rpm for 1 minute, a 1/6 minute rest was performed. The ball mill treatment and the rest were repeated 10 times to obtain a dispersion liquid. The obtained dispersion liquid was filtered to obtain a carbon material.

In Examples 7 and 8, changes in the weight specific capacity when the current density during cell capacity measurement was changed to 0.1 to 50 A/g are shown in FIG.

As shown in FIG. 8, Example 8 in which the wet dispersion treatment was performed had a larger weight specific capacity than Example 7 in which the wet dispersion treatment was not performed, and the difference was larger as the current density was higher. From this, it can be seen that the wet dispersion treatment has excellent current response.

Example 9
A carbon material, an electrode and a cell were prepared and evaluated in the same manner as in Example 5 except that the rotation speed of the planetary ball mill during dry grinding was changed within the range of 500 to 900 rpm. The obtained results are shown in FIG.

As shown in FIG. 9, at a rotation speed of 700 rpm or less, the weight specific capacity increased as the rotation speed increased. On the other hand, at a rotation speed of 800 rpm or more, the weight specific capacity did not become so large. It is considered that this is because the rotation speed was excessively high and the contamination amount was increased. From this result, it can be seen that the rotation speed of 700 rpm is optimal.

Claims (11)

A graphite-based porous carbon material for an electrochemical capacitor electrode,
It has a rhombohedral crystal structure and a hexagonal crystal structure, and the content ratio of the rhombohedral crystal is 30% by weight or more,
The pore volume of micropores smaller than a pore diameter 2nm is 0.01~0.50cm ³ / g, pore volume of mesopores having a pore diameter 2~50nm is 0.20~0.80cm ³ / g, a carbon material. The carbon material according to claim 1, wherein the rhombohedral crystal content is 50% by weight or more. The carbon material according to claim 1, which has a specific surface area of 400 to 800 m ² /g. The carbon material according to any one of claims 1 to 3, which has a combustion start temperature of less than 500°C in air. A method for producing the graphite-based porous carbon material according to any one of claims 1 to 4,
A step of preparing raw graphite,
Dry pulverizing the raw material graphite in the atmosphere at a rotation speed of 500 rpm or more using a planetary ball mill. The method according to claim 5, wherein the dry milling is performed for 14 to 150 minutes. The method according to claim 5 or 6, wherein zirconia balls having a diameter of 1 to 10 mm are used as the grinding media of the planetary ball mill. The method according to claim 5, further comprising a wet dispersion step using the planetary ball mill after the dry pulverization step. An electrochemical capacitor electrode comprising the carbon material according to claim 1. The electrochemical capacitor electrode according to claim 9, wherein the initial electrode density is 0.8 g/cm ³ or more. An electrochemical capacitor comprising the electrode according to claim 9 or 10.