JP7197089B2 - GRAPHITE-BASED POROUS CARBON MATERIAL FOR ELECTROCHEMICAL CAPACITOR ELECTRODE, METHOD FOR MANUFACTURING SAME, ELECTROCHEMICAL CAPACITOR ELECTRODE, AND ELECTROCHEMICAL CAPACITOR - Google Patents

Description

TECHNICAL FIELD 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.

Electrochemical capacitors are characterized by the capacity or electron transfer between the electrodes (positive electrode and negative electrode) due to non-faradaic reactions that do not involve the transfer of electrons between the electrodes and the electrolyte (ions in the electrolyte). It is an electricity storage device that utilizes the capacity that is generated due to the accompanying Faraday reaction. Compared to secondary batteries such as lithium-ion batteries, which are charged and discharged based on the Faraday reaction between the electrode and the electrolyte, it has features such as high energy efficiency, rapid charging and discharging, long life, low reaction heat and safety. be. Due to these characteristics, electrochemical capacitors have been used as auxiliary power sources for hybrid vehicles, regenerative power storage devices, alternative devices for secondary batteries, energy buffers for photovoltaic power generation, etc., and have attracted particular attention in recent years. .

Electrochemical capacitors are roughly classified into three types: electric double layer capacitors, hybrid capacitors, and pseudocapacitance capacitors (redox capacitors). In the electric double layer capacitor, no transfer of electrons occurs between the electrodes and ions in the electrolytic solution (non-Faraday reaction), and charging and discharging are performed only by physical desorption of ions. When ions are adsorbed on the electrode surface, an electric double layer is formed between the electrode and the electrolyte, and electricity is stored. It is said that the larger the surface area of the electrode, the more advantageous it is to increase the capacity, and activated carbon, which has a large specific surface area, is generally used as the 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, similarly to the electric double layer capacitor, and the other electrode, for example, the negative electrode is charged by the Faraday reaction. In the electrode (e.g., positive electrode) that stores electricity only by physical adsorption of ions, the same material (activated carbon) as that of the electric double layer capacitor is used as the electrode material (active material). Materials that can be intercalated, such as graphite and lithium titanate, are mainly used. A lithium ion capacitor, which is a type of hybrid capacitor, uses activated carbon for the positive electrode and a carbon material capable of intercalating lithium ions for the negative electrode.

Thus, in electrochemical capacitors, activated carbon is generally used as an electrode material (active material) that stores electricity only by physical adsorption of ions. However, activated carbon is produced by heating and activating carbon materials such as coconut shells in an atmosphere such as water vapor, and contains many impurities. etc. Therefore, in order to solve this problem, it has been proposed to use a graphite-based carbon material instead of activated carbon.

For example, Patent Document 1 discloses an electric double layer comprising a polarizable electrode, a separator, an organic solvent and a collecting electrode, and having a polarizable electrode composed of a positive electrode mainly composed of activated carbon and a negative electrode mainly composed of mechanically pulverized graphite. A capacitor is disclosed, which is made of marketable materials rather than special materials, has polarizable electrodes that do not require special manufacturing processes such as activation treatment, is compact and can be used in many applications. It is said to be able to store electricity.

In addition, Patent Document 2 discloses a method for producing pulverized graphite, which is characterized by pulverizing graphite in a nitrogen atmosphere so as to form a new surface. As capacitor electrodes, capacitors are said to exhibit high initial discharge efficiency.

Furthermore, Patent Document 3 discloses a composite conductive material in which at least graphene-like graphite exfoliated from a graphite-based carbon material and a conductive material are dispersed in a base material, wherein the graphite-based carbon material is rhombohedral graphite. It has a layer (3R) and a hexagonal graphite layer (2H), and the formula: Rate (3R )=P3/(P3+P4)×100, the composite conductive material is characterized in that the ratio Rate(3R) is 31% or more. Here, P3 is the peak intensity of the (101) plane of the rhombohedral graphite layer (3R) according to the X-ray diffraction method, and P4 is the peak of the (101) plane according to the X-ray diffraction method of the hexagonal graphite layer (2H). strength. According to this document, by using such a graphite-based carbon material, graphene can be easily exfoliated, and a highly concentrated and highly dispersed graphene solution can be easily obtained.

JP 2007-157954 A JP 2010-126418 A WO2016/002261

Thus, the use of graphite-based carbon materials as electrode materials (active materials) has been proposed for the purpose of improving the performance of electrochemical capacitors. On the other hand, capacitors are desired to have a larger capacity in response to the demand for larger stored power and compact size. However, conventional graphite-based carbon materials are not sufficient for increasing the capacity of capacitors. Accordingly, an object of the present invention is to provide a graphite-based carbon material for an electrochemical capacitor electrode capable of increasing the capacity.

The present inventors have recently found that, in graphite-based carbon materials for electrochemical capacitor electrodes, the content ratio of rhombohedral crystals in the crystal structure and the pore volume of micropores and mesopores are and that it is possible to obtain a graphite-based porous carbon material capable of increasing the capacity by dry pulverizing graphite under predetermined conditions using a planetary ball mill. I completed the present invention.

The present invention includes the following aspects (1) to (11). In this specification, the expression "-" includes both numerical values. 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,
It has a rhombohedral crystal structure and a hexagonal crystal structure, and the rhombohedral crystal content is 30% by weight or more,
A carbon material, wherein micropores with a pore diameter of less than 2 nm have a pore volume of 0.01 to 0.50 cm ³ /g, and mesopores with a pore diameter of 2 to 50 nm have a pore volume of 0.20 to 0.80 cm ³ /g.

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

(3) The carbon material of (1) or (2) above, 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) above, which has a combustion initiation temperature of less than 500° C. in air.

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

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

(7) The method of (5) or (6) above, wherein zirconia balls with 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, further comprising a step of wet dispersion 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 of (9) above, which has an initial electrode density of 0.8 g/cm ³ or more.

(11) An electrochemical capacitor comprising the electrode of (9) or (10) above.

ADVANTAGE OF THE INVENTION According to this invention, the graphite-type porous carbon material for electrochemical capacitor electrodes which can be increased in capacity, and its manufacturing method are provided. Further, according to the present invention, an electrochemical capacitor electrode capable of increasing the capacity and an electrochemical capacitor provided with the electrode are provided.

FIG. 4 is a diagram showing the relationship between the pulverization time and the weight specific capacity of the carbon material; FIG. 4 is a diagram showing the relationship between the pulverization time and the area-specific capacity of the carbon material. FIG. 4 is a diagram showing the relationship between the pulverization time and the rhombohedral crystal content ratio (χ _Rh ) of the carbon material. FIG. 4 is a diagram showing the relationship between the pulverization time and the combustion start temperature of the carbon material; FIG. 4 is a diagram showing the relationship between the rhombohedral crystal content ratio (χ _Rh ) of the carbon material and the area specific capacity. FIG. 4 is a diagram showing the relationship between the current density and the area-specific capacity of the carbon material when measuring the cell capacity. FIG. 4 is a diagram showing the relationship between the current density and the capacity retention rate of a carbon material during cell capacity measurement. FIG. 5 is a diagram showing the influence of current response of a carbon material with or without wet dispersion treatment. It is a figure which shows the influence by the rotation speed of a planetary 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 no pores. In addition, activated carbon contains many amorphous parts. On the other hand, the carbon material of the present invention is a porous material having micropores and mesopores, while its crystal structure is refined to such an extent that the crystal structure of graphite remains slightly. Therefore, this carbon material is clearly distinguished from pure graphite and activated carbon in terms of crystal structure and pore structure. As will be described later, this graphite-based porous carbon material can be produced by dry pulverizing graphite under predetermined conditions using a planetary ball mill. Based on 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 those of pure graphite.

The carbon material of the present invention has rhombohedral and hexagonal crystal structures. Hexagonal crystal and rhombohedral crystal are known as the crystal structure of graphite, but the general crystal structure is hexagonal crystal. The hexagonal crystal has a crystal structure in which the layers are stacked in the order of ABABAB, and the rhombohedral crystal has a crystal structure in which the layers are 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. By increasing the content of rhombohedral crystals, the area-specific capacity is increased. Although the detailed mechanism is not certain, it is speculated that the rhombohedral crystal has a higher ion adsorption density than the hexagonal crystal. The content of rhombohedral crystals is preferably 40% by weight or more, more preferably 50% by weight or more. The upper limit of the content is not particularly limited, but is typically 70% by weight or less, more typically 60% by weight or less.

In the carbon material of the present invention, the pore volume of micropores with a pore diameter of less than 2 nm is 0.01 to 0.50 cm ³ /g, and the pore volume of mesopores with a pore diameter of 2 to 50 nm is 0.20 to 0.80 cm ³ /g. is g. When the pore volume of micropores and mesopores is within the above range, the capacitor capacity is high. Although the detailed mechanism is not certain, we believe that controlling the pore volume of micropores and mesopores may enable electrolyte ions to be efficiently adsorbed on the carbon material. That is, in general, it is said that the larger the electrode surface area, the more advantageous it is to increase the amount of ion adsorption due to the formation of an electric double layer. However, in reality, the mechanism of electric double layer formation is complicated, and simply increasing the surface area is not sufficient. It is believed that the fine structure of the electrode surface has an effect. In the case of a porous electrode material, it is desirable that electrolyte ions can easily access the inside of the pores, and for this reason, 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 inside of the pores by controlling the pore volume of the micropores and mesopores of the carbon material. The pore volume of micropores is preferably between 0.10 and 0.30 cm ³ /g, and the pore volume of mesopores is preferably between 0.40 and 0.80 cm ³ /g. In this specification, the pore refers to a pore partially or entirely surrounded by a carbon material, and may be an open pore with one end communicating with the outside, or an open pore with both ends communicating with the outside. may be

The carbon material preferably has a specific surface area of 400 m ² /g or more. By controlling the pore volume of micropores and mesopores and further increasing the specific surface area, the effect of increasing the capacity of the capacitor can be enhanced. On the other hand, if the specific surface area is excessively high, the pore volume may become too large and the electrode density may decrease. Accordingly, 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 initiation temperature of less than 500°C in air. Pure graphite with high crystallinity has a combustion initiation temperature of 600°C. In contrast, in the carbon material of the present invention, the combustion initiation temperature is remarkably lowered by controlling the crystallinity, rhombohedral content, and pore volume of micropores and mesopores. The combustion initiation temperature is preferably 450° C. or less.

Method for Producing Graphite-Based Porous Carbon Material The method for producing a graphite-based porous carbon material of the present invention comprises the steps of preparing graphite and dry-pulverizing it.

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

<Dry pulverization process>
Next, using a planetary ball mill, the raw material graphite is dry pulverized in the atmosphere at a rotational speed (rotational speed) of 500 rpm or more. A planetary ball mill is a device that rotates and revolves at high speed a grinding container containing a sample and grinding media such as zirconia balls. As a result, the sample is strongly pulverized 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 volume of micropores and mesopores can be set within a predetermined range, and as a result, the capacity can be increased. It becomes possible to obtain a graphite-based porous carbon material capable of. The rotation speed is preferably 550 rpm or more, more preferably 600 rpm or more. On the other hand, if the rotation speed is excessively high, the amount of contamination increases and there is a risk of deterioration of the characteristics of the carbon material. Therefore, the rotation speed is preferably 900 rpm or less, more preferably 800 rpm or less.

Also, in the present invention, dry pulverization is performed in the air. Pulverization in the atmosphere eliminates the need for expensive equipment for introducing inert gas or the like. Therefore, the carbon material can be obtained by a simple method, and there is an effect of reducing the manufacturing cost. In addition, pulverization in air can introduce some oxygen-containing functional groups, which has the effect of increasing the hydrophilicity of the carbon material.

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

Dry milling is preferably carried out for 14 to 150 minutes. By setting the pulverization time to 14 minutes or more, the content of rhombohedral crystals in the crystal structure and the pore volume of micropores and mesopores can be within a predetermined range. On the other hand, pulverization for an excessively long time may increase the amount of contamination. Therefore, the grinding time is preferably 150 minutes or less.

<Wet dispersion process>
If necessary, wet dispersion may be performed using the planetary ball mill after the dry pulverization step. A carbon material having excellent current response can be obtained by wet dispersion. In addition, the wet dispersion makes it possible to efficiently recover the carbon material adhering to the grinding media and the inner wall surface of the grinding vessel, which has the effect of reducing the manufacturing cost.

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

By such a production method of the present invention, a graphite-based porous carbon material having a content ratio of rhombohedral crystals in the crystal structure and a pore volume of micropores and mesopores within a predetermined range and capable of increasing the capacity is produced. Obtainable. Such a graphite-based porous carbon material and a method for producing the same are not conventionally known. For example, Patent Document 1 and Patent Document 2 disclose that graphite is pulverized in a planetary ball mill to obtain an electrode material for an electric double layer capacitor. It does not focus on the content of crystals or the pore volume of micropores or mesopores. Also, these documents do not intend to increase the rotational speed of the planetary ball mill above 500 rpm. In particular, in Patent Document 2, the planetary ball mill treatment is performed at a rotation speed of 250 rpm in the examples, and it is speculated that such a low rotation speed treatment can only obtain a carbon material that is inferior in the effect of increasing the capacity. be done.

In Patent Document 3, the ratio Rate (3R) between the rhombohedral graphite layer (3R) and the hexagonal graphite layer (2H) of the graphite-based carbon material is set to 31% or more to achieve high concentration and high dispersion. However, it does not focus on the pore volume of micropores and mesopores, much less does it disclose the effect of increasing the capacity. 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 the examples.

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

Electrochemical Capacitor Electrode The electrochemical capacitor electrode of the present invention can be produced by a known method using the 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, 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) or styrene-butadiene rubber (SBR) can be used, and the solvent can be distilled water, an organic solvent, or the like. Furthermore, carbon black such as acetylene black and ketjen black can be used as the conductive material. A current collector is a member for extracting current to the outside, and for example, an aluminum (Al) foil or the like is used. After applying the slurry, the electrodes may be punched to obtain electrodes of desired shape. Moreover, in order to obtain a high-density electrode, pressing may be performed after applying the slurry. The pressing pressure is, for example, 100-500 MPa. The electrode has a thickness of, for example, 0.01 to 0.10 mm.

The electrode density of the electrochemical capacitor electrode of the present invention can be increased by using the graphite-based porous carbon material. The electrode density is preferably 0.8 g/cm ³ or more, more preferably 0.9 to 1.3 g/cm ³ . Such an electrochemical capacitor electrode has a high density and the graphite-based porous carbon material used has a high area-to-area capacity, and thus 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. If necessary, a separator such as a sheet made of paper, a nonwoven fabric, or a microporous film may be provided between the positive electrode and the negative electrode. Additionally, 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 known shapes such as coin shape, laminate shape, cylindrical shape, and square shape can be adopted. It is also possible to configure a capacitor module by connecting a plurality of capacitors. Such a capacitor module is suitable for ultra-large-capacity electrochemical capacitors used in automobiles and the like.

Electrochemical capacitors are typically electric double layer capacitors. In that case, the electrochemical capacitor electrode is used for both or either one of the positive electrode and the negative electrode. As the electrolytic solution, known electrolytic solutions such as aqueous, organic, and ionic liquids can be used. Examples of the aqueous electrolytic solution include an aqueous solution of sulfuric acid, and examples of the organic electrolytic solution include a tetraethylammonium tetrafluoroborate/propylene carbonate (TEABF ₄ /PC) solution.

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

The electrochemical capacitor of the present invention has an effect of increasing the capacity by using the above graphite-based porous carbon material for the electrodes. Typically, when charged and discharged at a constant current density of 0.1 A/g, 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 . be.

The invention is further illustrated 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 grain size is the grain size (mode diameter) with the largest appearance ratio in the grain size distribution. The specific surface area was calculated from the nitrogen adsorption isotherm by the BET method.

<Dry pulverization 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 enclosed in a pulverization container of a planetary ball mill in air. At that time, a zirconia ball having a diameter of 1 mm and a zirconia container having an internal volume of 45 cc were used as the crushing container. After that, the rotation speed of the planetary ball mill was set to 700 rpm, and pulverization was performed for 5 minutes to obtain a carbon material. The number of rotations is the number of rotations of the planetary ball mill (the number of rotations of the pedestal), and the ratio of the number of revolutions to the number of rotations (rotation/revolution ratio) is fixed at 1:2.

(2) Production of Electrode An electrode was produced using the obtained carbon material. First, 170 mg of 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, the resulting slurry was applied onto an Al current collector using a doctor blade. After that, 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 prepare a carbon electrode (film electrode) with a thickness of about 0.03 to 0.05 mm.

(3) Production of cell Using the obtained carbon electrode (film electrode) as a positive electrode and a negative electrode, a bipolar coin cell was produced. 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 sealing the laminate together with an electrolytic solution in a cell container consisting of a case and a cap.

(4) Evaluation Various properties 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 with a laser diffraction/scattering particle size distribution analyzer. The grain size is the grain size (mode diameter) with the largest appearance ratio in the grain 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 volumes of micropores (pore diameter of less than 2 nm) and mesopores (pore diameter of 2 to 50 nm) of carbon materials are obtained from the nitrogen adsorption isotherm by the quench fixed density functional theory (QSDFT) method and the Barrett-Joyner-Halenda (BJH) method. Each was measured by

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

<Combustion start temperature>
Combustion initiation temperatures of carbon materials were measured by thermogravimetry in air.

<Electrode density>
The weight and thickness of the electrodes were measured to calculate the electrode density.

<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 charge/discharge was performed at a current density of 0.1 to 50 A/g in a voltage range of 0 to 2.5V. Thereafter, using the obtained capacity, weight and specific surface area of the carbon material, weight specific capacity (capacity per unit weight) and area specific capacity (capacity per unit area) were calculated. The weight specific capacity is the capacity per both electrodes, and the area specific capacity is the capacity per single electrode.

Example 2 (Comparison)
A carbon material, an electrode and a cell were produced and evaluated in the same manner as in Example 1, except that the pulverization time was set to 10 minutes when producing the carbon material.

Example 3
A carbon material, an electrode and a cell were produced and evaluated in the same manner as in Example 1, except that the pulverization time was changed to 14 minutes when producing the carbon material.

Example 4
A carbon material, an electrode and a cell were produced and evaluated in the same manner as in Example 1, except that the pulverization time was changed to 75 minutes when producing the carbon material.

example 5
A carbon material, an electrode and a cell were produced and evaluated in the same manner as in Example 1, except that the pulverization time was set to 150 minutes when producing the carbon material.

Example 6 (Comparison)
As the carbon material, commercially available activated carbon generally used as an electrode material for electric double layer capacitors was used. Other than that, it was carried out in the same manner as in Example 1, and an electrode and a cell were produced and evaluated.

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 is 0.1 A/g when measuring the cell capacity.

In order to examine the effect of pulverization time in more detail, carbon materials, electrodes and cells were produced and evaluated in the same manner as in Example 1, except that the pulverization time was changed between 0 and 750 minutes. Here, the sample with a pulverization time of 0 minutes is raw material graphite. The samples with grinding times of 5, 10, 14, 75 and 150 minutes are the same as Examples 1-5 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 is 10 A/g when the cell capacity is measured. Also, based on the obtained results, the relationship between the pulverization time, weight specific capacity, area specific capacity, rhombohedral crystal content ratio (χ _Rh ) and combustion initiation temperature of the carbon material was determined. These relationships are shown in FIGS. 1-4. Further, FIG. 5 shows the relationship between the rhombohedral 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 volumes of micropores and mesopores increased as the pulverization time during dry pulverization increased. In addition, as shown in Tables 1 and 2 and FIGS. 1 to 4, the carbon material tends to increase the rhombohedral crystal content, weight specific capacity, and area specific capacity as the pulverization time increases. In particular, it rapidly increased when the pulverization time was 14 minutes or longer. Moreover, as can be seen from FIG. 5, there is a strong correlation between the rhombohedral crystal content ratio and the area specific capacity, and when the rhombohedral crystal content ratio is 30% by weight or more, the area specific capacity is 5 μF/cm ² or more. .

Furthermore, as shown in FIG. 4, the combustion initiation temperature of the carbon material decreased as the pulverization time increased. In particular, when the pulverization time was 14 minutes or more, the combustion initiation temperature was significantly lowered to less than 500°C.

On the other hand, the electrode density was almost constant regardless of the pulverization time. Examples 3 to 5 (working examples) had higher electrode densities than example 6 (comparative example) using activated carbon, and also had higher area specific capacities than example 6.

For Examples 5 and 6 of Example 1, the relationship between the current density, the area specific capacity, and the capacity retention when changing the current density at the time of measuring the cell capacity from 0.1 to 50 A / g is shown in FIGS. show. Here, the capacity retention rate is the ratio of the area-specific capacity at each current density to the area-specific capacity at a current density of 0.1 A/g.

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

Example 7
A carbon material, an electrode, and a cell were produced and evaluated in the same manner as in Example 1, except that zirconia balls with a diameter of 5 mm were used during dry pulverization, the number of revolutions of the planetary ball mill was set to 500 rpm, and pulverization was performed for 60 minutes. rice field.

Example 8
A carbon material, an electrode and a cell were produced 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 vessel. A planetary ball mill treatment was performed at a rotation speed of 500 rpm for 1 minute, and then rested for 1/6 minute. The ball mill treatment and rest were repeated 10 times to obtain a dispersion. The resulting dispersion was filtered to obtain a carbon material.

FIG. 8 shows changes in the weight specific capacity when changing the current density from 0.1 to 50 A/g when measuring the cell capacity in Examples 7 and 8.

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 increased as the current density increased. From this, it can be seen that the current response is excellent when the wet dispersion treatment is performed.

Example 9
Carbon materials, electrodes and cells were produced and evaluated in the same manner as in Example 5, except that the rotation speed of the planetary ball mill during dry pulverization was changed within the range of 500 to 900 rpm. The results obtained are shown in FIG.

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

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 rhombohedral crystal content is 30% by weight or more,
A carbon material, wherein micropores with a pore diameter of less than 2 nm have a pore volume of 0.01 to 0.50 cm ³ /g, and mesopores with a pore diameter of 2 to 50 nm have a pore volume of 0.20 to 0.80 cm ³ /g. 2. The carbon material according to claim 1, wherein the content of rhombohedral crystals is 50% by weight or more. 3. 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 initiation temperature of less than 500°C in air. A method for producing a graphite-based porous carbon material according to any one of claims 1 to 4,
A step of preparing raw material graphite;
and dry pulverizing the raw material graphite using a planetary ball mill at a rotational speed of 500 rpm or more in the atmosphere. A method according to claim 5, wherein said dry milling is carried out for 14-150 minutes. The method according to claim 5 or 6, wherein zirconia balls with a diameter of 1 to 10 mm are used as grinding media for the planetary ball mill. The method according to any one of claims 5 to 7, 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 any one of claims 1-4. 10. The electrochemical capacitor electrode according to claim 9, having an initial electrode density of 0.8 g/cm< ³ > or higher. An electrochemical capacitor comprising the electrode according to claim 9 or 10.

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