JP2000007316A - Solid active carbon and electric double layer capacitor using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a solid active carbon capable of sufficiently taking out the electrostatic capacity predicted from the ratio of the active carbon to various organic resins, having a practically high electrostatic capacity and a high mechanical strength, excellent in durability and applicable to various kinds of uses, especially a solid active carbon suitable as an electrode material of various cells or batteries for auxiliary power supplies or a polarizing electrode of an electric double layer capacitor and to provide the electric double layer capacitor having a high capacity ratio of the electrostatic capacity at the time of low-current discharge to that of high-current discharge and excellent in durability. SOLUTION: This solid active carbon has 0.35-0.70 cc/g pore volume of pores having <=10 Å pore diameter and the pore volume of pores having 10-40 Å pore diameter accounting for 40-65% of the total pore volume in measurements of the pore distribution determined from an argon adsorption isotherm according to a Horvath- Kawazoe(HK) method. The electric double layer capacitor has >=40% capacity ratio of the electrostatic capacity at the time of low-cutrent discharge to that at the time of high-crrent density when measuring the electrostatic capacity by using the solid active carbon as a polarizing electrode and a nonaqueous electrolyte.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode material used for various large-capacity capacitors such as electric double-layer capacitors and lithium ion batteries, backup power supplies, vehicle power supplies, auxiliary power supplies, and the like. For solid activated carbon that can be generally widely used as a porous activated carbon sintered body used as a filter medium for gas adsorbents, water purification, food purification, wastewater purification, and the like, and an electric double-layer capacitor using the same, In particular, the present invention relates to an electric double layer capacitor using solid activated carbon and a non-aqueous electrolytic solution and using the solid activated carbon as a polarizable electrode, which can realize a power storage system having a large capacitance and a large energy density.

[0002]

2. Description of the Related Art In recent years, application of activated carbon to various application fields focusing on its characteristics has been studied. In particular, activated carbon has been used as a porous carbon material for electrode materials of small and large-capacity capacitors and various batteries. It has been taken up and variously studied.

[0003] Such a polarizable electrode mainly composed of carbonaceous material such as activated carbon has an electrolyte contained between the polarizable electrodes, and is compared with a conventional capacitor using an electric double layer formed at the interface between the two. It is often used for electric double-layer capacitors with a capacitance several thousand times per unit volume. Such electric double-layer capacitors have the functions of both capacitors and batteries, so they can be used to drive small memory backup power supplies and automobiles. The demand is rapidly growing with the development of the electronics field, such as application to an auxiliary power supply such as a large-capacity motor mounted as a power source.

[0004] Initially, porous solid activated carbon which is generally widely used is used as the electrode material. Examples of such solid activated carbon include carbonaceous materials such as activated carbon and carbon black. A sheet formed by kneading an organic resin such as a fluoropolymer and forming the sheet by a known forming means such as a roll forming method has been used.

[0005] However, as the electrode material of the above-mentioned capacitor or battery, in order to satisfy the performance of particularly high capacitance and low internal resistance, and to satisfy the recent demand for miniaturization of electronic components, it is necessary to use a predetermined material. Minimizing the volume relative to the capacity, and having a porous structure that takes into account the impregnating properties of the electrolytic solution, so that cracks and breakage are unlikely to occur, as well as being superior in durability and mechanical properties for general use, etc. Was being requested.

In particular, the capacitance of an electric double layer capacitor is governed by the surface area of the polarizable electrode on which the electric double layer is formed, the capacitance per unit volume, the resistance of the electrode, and the like.
There is a close relationship with the size of the ions in the electrolyte forming the electric double layer, and it is considered that the specific surface area of the micropores determines the capacitance.

[0007] Therefore, in order to increase the capacitance per unit volume of the electrode, it has been required that the porous activated carbon has a large specific surface area without impairing the durability and mechanical strength.

Therefore, in order to satisfy the above requirements, activated carbon fine particles and carbon fibers are mixed and sintered under pressure, or a mixture of activated carbon powder and powdery phenol resin is injection-molded and heat-treated. Pulp fiber or the like is added to the activated carbon fiber to form a paper, a prepreg sheet containing activated carbon powder and cellulose fiber as main components is pressed and calcined, or a mixture of activated carbon powder and a phenol resin is formed into a film on a substrate. Heat-treating in a oxidizing atmosphere, or heating and curing a mixture of a spherical carbide obtained by carbonizing a curable spherical phenolic resin and a heat-reactive spherical phenolic resin, heat-treating in an inert atmosphere, and then performing an activation treatment. In addition, solid activated carbon produced by various production methods has been proposed (Japanese Patent Application Laid-Open No. Hei 8-
JP-A-119614, JP-A-6-69075, JP-A-5-129157, JP-A-3-201516
Reference).

[0009]

However, when the solid activated carbon is used as a polarizable electrode of an electric double layer capacitor, the non-aqueous electrolyte used as an electrolyte of the electric double layer capacitor is more than an aqueous electrolyte. Withstand voltage is high, and theoretically a non-aqueous electrolyte can obtain several times higher energy density, and although it is advantageous for miniaturization, an electric double layer capacitor using the non-aqueous electrolyte has a high internal resistance. Therefore, for example, 3 mA / cm
Discharge with a low current of about ² and measure its capacitance.
It shows a capacitance of about 0 F / g, but when discharged at a high current of about 30 mA / cm ^{2, the} capacitance decreases due to a voltage drop, and the capacitance at the time of low current discharge is lower than the capacitance at the time of high current discharge. In some cases, the capacitance ratio of the capacitance was about 35%.

Therefore, such solid activated carbon is suitable for use in electrodes of electric double layer capacitors, but is used for various secondary batteries such as auxiliary power sources for large-capacity motors and the like that require rapid charging and discharging. It cannot be applied to applications that require a capacitance at the time of high current discharge, such as an electrode material to be used, and has a problem that the application is limited and cannot be adopted as a multipurpose solid activated carbon.

[0011]

SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned problem. The object of the present invention is to sufficiently extract the capacitance expected from the mixing ratio of activated carbon and various organic resins, and to realize a practical and high-capacity device. Solid activated carbon that has a capacitance and high mechanical strength and is durable and can be used in various applications.In particular, solid activated carbon suitable as an electrode material for various batteries for auxiliary power sources and a polarizable electrode for electric double layer capacitors. An electric power having a highly durable polarizable electrode having a large shape of activated carbon and having a large capacity ratio of a capacitance at a low current discharge to a capacitance at a high current discharge using the solid activated carbon as a polarizable electrode. The object is to obtain a double-layer capacitor.

[0012]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on the above-mentioned problems, and have found that various activated carbon materials having a large specific surface area as long as durability and mechanical strength are not impaired are obtained. By molding a mixture comprising a resin, carbonizing and unifying the molded body in a non-oxidizing atmosphere, further activating in the air, and controlling the pore distribution of the solid activated carbon, particularly, an electric double layer capacitor Even if a non-aqueous electrolyte is used as the electrolyte, the capacitance ratio at the time of low current discharge to the capacitance at the time of high current discharge is increased while maintaining the capacitance at the time of low current discharge high, and It has been found that a solid activated carbon having practical capacitance and excellent mechanical strength and durability can be obtained.

That is, the solid activated carbon of the present invention is Horv
In a pore distribution measurement obtained from an argon adsorption isotherm by an ath-Kawazoe method (hereinafter, referred to as an HK method), a ratio of a volume occupied by pores having a diameter of 10 ° or less is 0.35 to 0.70 cc / g. And its diameter is 10
The volume of pores showing 〜40 ° is 40-65% of the total pore volume
Which is characterized by

Further, in the electric double layer capacitor of the present invention, the solid activated carbon is used as a polarizable electrode, and a non-aqueous electrolytic solution is interposed between the polarizable electrodes, and the capacitance is measured by the following constant current discharging method. when measured, i.e., the volume ratio of the capacitance shown at low current discharge of 3mA / cm ² with respect to the capacitance shown during a high current discharge of 30 mA / cm ² is characterized in that not less than 40% is there.

[0015]

According to the solid activated carbon of the present invention, the ratio of the volume occupied by pores having a diameter of 10 ° or less in the pore distribution determined from the argon adsorption isotherm by the HK method is 0.35 to 0.7.
0 cc / g and the pore volume with a pore diameter of 10 to 40 ° is 40 to 65% of the total pore volume, so that the mechanical strength and the durability are balanced with the internal resistance and the capacitance. Versatile solid activated carbon excellent in heat resistance.

The electric double layer capacitor using the solid activated carbon as a polarizable electrode has a capacitance of 30 mA / cm when its capacitance is measured by a constant current discharge method using a non-aqueous electrolyte.
3mA / cm to the electric capacitance shown when the ^second high-current discharge
^Since the capacitance ratio of the capacitance shown at the time of low-current discharge of ² is 40% or more, there are many portions where lithium perchlorate ions or triethylmethylammonium ions in the non-aqueous electrolyte are trapped, and The pore size is 1
0 ° means that the size of an ion such as the perchlorate ion or triethylmethylammonium ion is about 5
Å, it is of an appropriate size for efficient trapping, and in addition, there are many 10-40 ° pores that function as a path for transporting the electrolyte to pores of 10 ° or less. The liquid spreads to every corner of the electrode, and the internal resistance in the electrode is reduced and the voltage drop is reduced.

As a result, the capacitance at the time of low-current discharge is large, and the ratio of the capacitance at the time of low-current discharge to the time of high-current discharge is also large. Thus, a small electric double layer capacitor having excellent characteristics can be obtained.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The solid activated carbon of the present invention and an electric double layer capacitor using the same will be described in detail below.
In the solid activated carbon of the present invention, the volume occupied by pores having a diameter of 10 ° or less in the pore distribution determined from the argon adsorption isotherm by the HK method is 0.35 to 0.70 cc / g, and 10 to 40 °. Is characterized in that the volume of the pores having a diameter of 40 to 65% of the total pore volume.

In the present invention, when the volume occupied by pores having a diameter of 10 ° or less in the pore distribution determined from the argon adsorption isotherm by the HK method is less than 0.35 cc / g, the solid activated carbon has Not only the number of pores is reduced and the formed electric double layer is reduced, but also when used as an adsorbent, the adsorbed portion is reduced and the desired effect cannot be obtained, which is not suitable.

On the other hand, if the volume occupied by the pores having a diameter of 10 ° or less exceeds 0.70 cc / g, the mechanical strength is extremely poor due to the large number of pores in the solid activated carbon, and handling becomes impossible. The pore volume is specified to be 0.35 to 0.70 cc / g, so that even if it can be applied to a filter medium, the pore volume is specified to be 0.35 to 0.70 cc / g. Maintaining and considering the wide application to other fields in terms of high adsorptivity and product life, its pore volume is 0.40 to 0.60 cc / g.
Is particularly desirable.

When the volume of the pores having a diameter of 10 to 40 ° in the pore distribution is less than 40% of the total pore volume, the volume of the pores of the solid activated carbon is small. It is limited to applications such as types of gas adsorbents, cannot be applied as multipurpose solid activated carbon, and has little filtration effect when applied to water purification, food purification, wastewater purification,
If the module becomes large and exceeds 65%, the inside becomes extremely porous due to the presence of a large number of large pores in the solid activated carbon, the strength is extremely reduced, and the durability is extremely poor.

Therefore, the solid activated carbon of the present invention has a pore volume of 0.35 to 0.70 c having a pore diameter of 10 ° or less in the pore distribution determined from the argon adsorption isotherm by the HK method.
Within the range of c / g, it is necessary that the pore volume having a pore diameter of 10 to 40 ° is 40 to 65% of the total pore volume.

In the electric double layer capacitor of the present invention, the volume occupied by pores having a diameter of 10 ° or less in the pore distribution determined from the argon adsorption isotherm by the HK method is 0.35 to 0.70 cc / g. And 10-40１０〜
A solid activated carbon in which the volume of pores having a diameter of 40 to 65% of the total pore volume is used as a polarizable electrode, and a non-aqueous electrolyte is used as an electrolyte. When measuring the capacitance by the constant current discharge method,
3 for the capacitance at the time of high current discharge of 30 mA / cm ²
The capacitance ratio of the capacitance at the time of low current discharge of mA / cm ² is 40.
% Or more, it approaches 100% as the internal resistance decreases.

In the present invention, 1% in the pore distribution of the solid activated carbon obtained from the argon adsorption isotherm by the HK method.
The ratio of the volume occupied by the pore diameter of 0 ° or less is 0.35 cc /
When the amount is less than g, the portion that traps perchloric lithium ions or triethylmethylammonium ions in the non-aqueous electrolyte decreases, so that the electric double layer formed at the interface between the electrode surface and the electrolyte decreases. From this, the capacitance at the time of low current discharge is less than 20 F / g, and the capacitance ratio to the capacitance at the time of high current discharge is 40%.
Therefore, it cannot be used as a material for a polarizable electrode of an electric double layer capacitor requiring a high capacitance.

If the ratio of the volume occupied by the pore diameter of 10 ° or less exceeds 0.70 cc / g, the mechanical strength becomes extremely weak at a bending strength of less than 200 gf / mm ² , making handling extremely difficult. Measurement of the capacitance becomes impossible.

On the other hand, even when the pore volume of the pore diameter of 10 ° or less is in the range of 0.35 to 0.70 cc / g,
In the pore distribution obtained from the argon adsorption isotherm by the K method, the pore volume having a pore diameter of 10 to 40 ° is 4% of the total pore volume.
When it is less than 0%, even if the capacitance at the time of the low current discharge is high, the capacitance at the time of high current discharge is low, so that it cannot be applied to electrode materials as various batteries for auxiliary power supply.

Similarly, when the pore volume of the pore diameter of 10 to 40 ° exceeds 65% of the total pore volume, the solid activated carbon has a large number of pores having a large diameter. Degradation occurs.

Therefore, the pore volume of the pore diameter of 10 ° or less in the pore distribution of the solid activated carbon determined from the argon adsorption isotherm by the HK method is in the range of 0.35 to 0.70 cc / g. It is necessary that the pore volume of the pore diameter of Å40 ° is 40 to 65% of the total pore volume, and from the viewpoint that a stable current can be taken out over a wide range, it is in the range of 45 to 56%. Most desirable.

In the case of the solid activated carbon as described above, even if the solid activated carbon itself is immersed in a non-aqueous electrolyte as a polarizable electrode of an electric double layer capacitor, the solid activated carbon itself is eroded by the non-aqueous electrolyte and the strength is deteriorated. Also, the large specific surface area means that many electric double layers are formed at the interface between the electrolyte and the electrode, so that the solid activated carbon is used as the polarizable electrode, and the non-aqueous electrolyte is used as the electrolyte. When a double-layer capacitor is used, an optimum capacitor can be obtained.

In the solid activated carbon of the present invention, the activated carbon powder and activated carbon fiber as raw materials are not particularly limited, and may be any of coconut shell type, coal type, wood type, etc. In terms of point, coconut shell system is most desirable.

The activated carbon powder as the raw material may have its specific surface area selected according to the desired capacitance. In particular, when it is used for a polarizable electrode for an electric double layer capacitor, from the viewpoint of the capacitance. Has a specific surface area of 1500 to
2500 m ² / g is preferable, and when activated carbon fiber is used, the fiber diameter is 6 to 18 μm from the viewpoint of mixing with a binder and smoothness of the surface, and from the viewpoint of capacitance, Those having a specific surface area of 1000 to 2500 m ² / g are preferred.

The carbonizable resin added as a binder can be any known organic resin.
Although not particularly limited, for example, phenol, Teflon, coal tar, polyvinyl butyral (PV
B), polyvinyl acetal such as polyvinyl formal (PVFM), vinyl acetate, etc., and polyvinyl butyral (PVB) is particularly preferable from the viewpoint of moldability or strength of the obtained solid activated carbon.

Next, an example of the method for producing the solid activated carbon of the present invention will be described. First, with respect to 100 parts by weight of activated carbon powder and / or activated carbon fiber, at least one kind of a known carbonizable resin such as phenol or Teflon, coal tar, polyvinyl butyral (PVB) is mixed at a ratio of 20 to 200 parts by weight, After preparing a material for molding by preparing a slurry or granules from the mixture, a known molding method such as a doctor blade method, a tape molding method, a pressure molding method, a roll molding method, or a combination method thereof is used. Prior to the carbonization heat treatment, the molded body formed by the method is subjected to an aging treatment at a temperature of 150 to 300 ° C. in the air.

This aging treatment can maintain the strength of the solid activated carbon at a high level with a small amount of the carbonizable resin, makes it difficult to close the pores of the activated carbon itself, reduces its surface area, and maximizes the properties of the activated carbon. It is intended to draw out to the limit.

Next, a carbonizing heat treatment is carried out in a non-oxidizing atmosphere to carbonize the carbonized resin in the molded article, and the activated carbon powder and / or activated carbon fiber and the carbide of the carbonized resin are calcined and integrated to form solid activated carbon. Get.

The carbonization heat treatment temperature is set at 800 to 1200 ° C. in order to sufficiently promote the carbonization of the carbonizable resin and to promote the sintering of the neck portion of the activated carbon powder or the activated carbon fiber to maintain sufficient strength. Temperature is preferred, especially 8
Temperatures between 00 and 1000 ° C. are optimal.

In the carbonization treatment, the temperature may be increased,
If the carbonization time is lengthened, the strength is improved, but the total pore volume is reduced and the capacitance at the time of low current discharge is reduced, so from the balance between the strength and the capacitance according to the application, it is preferable. It is important to select carbonization conditions so as to have a fine pore distribution.

Further, by activating the solid activated carbon subjected to the carbonization heat treatment in the non-oxidizing atmosphere in the air,
Although it is possible to obtain solid activated carbon with controlled pores, the activation treatment decomposes the undecomposed binder to restore the pores and reacts with the carbon of the activated carbon to form new pores. In this case, the processing temperature is preferably in the range of 400 to 550 ° C., and the temperature of 400 to 450 ° C. is optimal.

In the activation treatment, if the treatment temperature is increased or the treatment time is lengthened, the pore volume increases and the capacitance is improved, but the strength is reduced. It is important to select the activation treatment conditions so as to have a suitable pore distribution in consideration of the capacitance.

The solid activated carbon of the present invention can be formed into an activated carbon substrate by forming it into a sheet by a doctor blade method, a calender roll method, or the like, formed into a block by various press molding methods, or by an extrusion molding method. Various shapes can be obtained by making the shape of a rod or a tube, or by combining them.

Further, after laminating a plurality of the sheet-shaped molded bodies, it is possible to perform a heat treatment in a non-oxidizing atmosphere.
By laminating a plurality of sheet-like molded bodies and bonding them by thermocompression bonding or with an adhesive liquid or an adhesive, it becomes possible to offset the warping directions of each other and reduce the occurrence of warpage during heat treatment. .

[0042]

EXAMPLES The solid activated carbon of the present invention and an electric double layer capacitor using the same were evaluated as follows. First,
BET value ^{1000m 2 / g, 1500m 2 /} g, and 2000 m ² / for three kinds of coconut shell activated carbon powder 100 parts by weight of g, fast mixing and stirring were blended 20 parts by weight 100 parts by weight of mesophase the PVB The resulting powder was sieved with a 40-mesh sieve to prepare a raw material for molding.

Next, the obtained molding material is press-molded,
Alternatively, after forming a flat molded body by roll molding, the molded body is kept in the atmosphere at a temperature of 200 ° C. for 48 hours to perform an aging treatment, and then in a vacuum, under the carbonization heat treatment conditions shown in Table 1. After performing carbonization treatment and carbonizing PVB and mesophase to produce a composite of activated carbon and carbon, the composite was subjected to activation treatment under the activation treatment conditions shown in Table 1, and was 70 mm long, 50 mm wide and 1 mm thick. An activated carbon substrate for evaluation was prepared.

[0044]

[Table 1]

With respect to the activated carbon substrate for evaluation thus obtained, ASAP 201 manufactured by Shimadzu-Micromeritics was used.
The pore distribution was measured from the argon adsorption isotherm by the HK method using a 0M type, and the ratio of the pore volume having a pore diameter of 10 ° or less was determined from the weight of the activated carbon substrate sample.

FIG. 1 shows a pore distribution obtained from an argon adsorption isotherm of Sample No. 2, which is a typical solid activated carbon of the present invention.

Further, the activated carbon substrate for evaluation was used as a polarizable electrode of an electric double layer capacitor, and in a non-aqueous electrolyte containing 1 mol of LiClO ₄ per liter of propylene carbonate at a voltage of 2.5 V. After charging for 30 minutes, 3
mA / cm ^{2 and} a constant current discharge method of 30 mA / cm ² determined the capacitance per unit electrode weight (F / g), 30mA
3 mA / c with respect to the capacitance at the time of high current discharge of / cm ²
The capacitance ratio of capacitance at the time of low current discharge of m ² was calculated.

On the other hand, using the activated carbon substrate for evaluation,
The three-point bending strength was measured according to the IS-R-1601 standard.

[0049]

[Table 2]

As is clear from Table 2, Sample Nos. 1, 7, 12, and 16, which are outside the scope of the present invention, are all 1
A pore volume of 0 ° or less exceeds 0.70 cc / g;
Low current discharge of 3mA / cm ² also can not measure the capacitance in any case of 30 mA / cm ² at high current discharge, and flexural strength 190 g / mm ² lower than the lower, likewise Sample No. 33, In 34, 36, 41, 43, 45,
A pore volume of 10 ° or less is less than 0.35 cc / g;
In addition, the pore volume having a diameter of 10 to 40 ° is less than 40%, the capacitance at the time of low current discharge is as low as less than 20 F / g, and the capacitance at the time of low current discharge is smaller than the capacitance at the time of high current discharge. The capacitance ratio of the capacitance is less than 40%, and the sample Nos. 30, 38, 42, and 44 also have a pore volume having a diameter of 10 to 40 ° of less than 40%, which is smaller than the capacitance at the time of the high current discharge. The capacitance ratio of the capacitance at the time of low current discharge is also less than 40%, and any of them is unsuitable as a versatile solid activated carbon including a polarizable electrode of an electric double layer capacitor.

On the other hand, in the present invention, the capacitance at the time of low current discharge exceeds 20 F / g, the capacitance ratio to the capacitance at the time of high current discharge is 40.9% or more, and the bending strength is low. 291 g / mm ² or more.

It should be noted that the present invention is not limited to the above-described embodiment, and any shape and form may be used as long as the solid activated carbon satisfies the gist of the present invention and an electric double layer capacitor using the same. Needless to say.

[0053]

As described above in detail, according to the solid activated carbon of the present invention and the electric double layer capacitor using the same,
When applied to various gas adsorbents that require mechanical strength and durability and filter media for water supply, food refining, wastewater purification, etc., it can be applied to various uses such as showing a selective filtration effect. When a non-aqueous electrolyte is used as the polarizable electrode and the non-aqueous electrolyte is used as the electrolyte, the surface area in contact with the electrolyte increases and the number of electric paths through which the electric charge passes increases. Therefore, the capacitance at the time of low current discharge is large, and the capacitance ratio to the capacitance at the time of high current discharge is large, so that the volume can be reduced with respect to the capacitance. It is suitable as an electrode material for various types of batteries, and it is possible to obtain a small-sized electric double-layer capacitor with a simple structure having a practical capacitance, high efficiency, and excellent durability.

[Brief description of the drawings]

FIG. 1 is a pore distribution diagram of a typical solid activated carbon of the present invention obtained from an argon adsorption isotherm by the HK method.

Claims (2)

[Claims] In a pore distribution obtained from an argon adsorption isotherm, the volume of pores having a diameter of 10 ° or less is 0.35 to 0.5.
Solid activated carbon characterized in that the volume of pores having a size of 70 cc / g and having a diameter of 10 to 40 ° occupies 40 to 65% of the total pore volume. 2. When the solid activated carbon according to claim 1 is used as a polarizable electrode and the capacitance is measured by a constant current discharge method using a non-aqueous electrolytic solution, a high current discharge of 30 mA / cm ² is obtained. An electric double layer capacitor, wherein a capacitance ratio of a capacitance at a low current discharge of 3 mA / cm ² to a capacitance is 40% or more.