JP5604227B2 - Method for producing activated carbon for capacitor and activated carbon - Google Patents

Description

The present invention relates to a method for producing a lithium ion capacitor positive electrode or activated carbon for an electric double layer capacitor electrode, which exhibits high capacitance, excellent rate characteristics, and cycle characteristics in charge and discharge in an electrode, and has low resistance, and activated carbon.

At the interface where two different phases contact, such as a solid electrode and an electrolyte solution, positive and negative charges are arranged and distributed over a very short distance. When the electrode is positively charged, anions are arranged on the solution side. A layer generated by this charge arrangement is an electric double layer.
The capacity that appears at the electrode interface along with the generation of the electric double layer is referred to as an electric double layer capacity, and an energy storage device using such a principle is an electric double layer capacitor.
Electric double layer capacitors have a mechanism in which charging / discharging does not involve chemical reaction, that is, physical adsorption / desorption of ions, so that the characteristic deterioration in repeated use is very small, the operating temperature range is wide, and fast charge / discharge characteristics It has many features such as excellent performance and is widely used as a backup power source for small electronic devices such as computers, and is expected to be used as a combined power source for electric vehicles and the like in the future.

The capacitance C of the electric double layer capacitor is expressed by the following equation.
C = ∫ε / (4πδ) ・ dS
(2)
Where ε is the dielectric constant of the electrolyte, δ is the distance from the electrode interface to the center of the electrolyte ion, and S is the surface area of the electrode interface

Accordingly, when a large surface area electrode is used as the polarizable electrode used in the capacitor, the capacitance of the capacitor increases.
For this reason, activated carbon and activated carbon fiber having a large specific surface area and conductivity are often used as electrode materials for capacitors.

In addition, a hybrid capacitor has been studied with the aim of improving the energy density while maintaining the advantages of the electric double layer capacitor particularly in connection with the increase in size of the capacitor.
A hybrid capacitor uses an activated carbon electrode with a large specific surface area that stores charge by non-Faraday reaction without oxidation / reduction on one of the positive electrode and negative electrode, and the other electrode is used in a secondary battery or redox capacitor. -Conductive polymers, oxides, and carbon materials, which are redox materials that store charges by a Faraday reaction with reduction, are used. Since the specific capacity of the Faraday electrode is larger than that of the non-Faradic electrode, the hybrid capacitor has a higher capacitance than the electric double layer capacitor. In detail, the cell capacitance C _EDLC of an electric double layer capacitor (EDLC) is 1 / C _EDLC = 1 / C _a + 1 / C _c
(C _a: negative electrode capacity, C _c: the positive electrode capacity) is represented by, when the positive and negative electrode capacity is approximately equal _{(C a = C c = C} ) is the cell capacitance C _EDLC = C / 2. On the other hand, for example, in the case of a hybrid capacitor (HC) using an activated carbon electrode for the positive electrode, the negative electrode capacity can be made sufficiently larger than the positive electrode capacity (C _c << C _a ), so the cell capacity C _{HC of the} capacitor is 1 / C _HC = 1 / C _a + 1 / C _c = 1 / C _c . As described above, when C _C = C, the cell capacity can be expressed very roughly as C _HC = C, so that the cell capacity can be improved as compared with the electric double layer capacitor.
Moreover, the charge / discharge depth of the Faradic electrode is shallow, and it can be a secondary power supply system having characteristics such as long life, high output, and high reliability similar to those of the electric double layer capacitor. Among the hybrid capacitors, a system using activated carbon for the positive electrode, a carbon material that can be doped or dedoped with lithium ions for the negative electrode, and an organic electrolyte containing a lithium ion salt for the electrolytic solution is called a lithium ion capacitor.
Currently, activated carbon is mainly produced by activating coal, coconut shell, phenol resin, etc. in an oxidizing gas (water vapor, carbon dioxide, air, etc.) atmosphere, but the activated carbon structure is optical. Is isotropic, the conductivity is insufficient. When the conductivity is low, the internal resistance of the capacitor increases, resulting in a decrease in capacitance.

According to Randin, J and Yeager, E (J. Electroan. Chm., 36, 257 (1972)), when the activated carbon surface is formed by an edge surface on the graphite layer surface, the electrostatic capacity of the activated carbon The capacity is greatly improved. However, in the case of normal activated carbon having optical isotropy, it is difficult to control the arrangement of the edge surface and the basal surface of the graphite layer surface, and therefore it is not expected to improve the capacitance by this method. In the production of an electric double layer capacitor, a binder (binder) and a conductive material for ensuring conductivity are usually added to an electrode material, and an electrode is formed together with a current collector. As the conductive material, carbon black, natural graphite powder, ruthenium oxide or the like is used alone or in combination. For example, Patent Document 1 (Japanese Patent Laid-Open No. 10-4037) describes an electrode for an electric double layer capacitor in which activated carbon, a binder, and a specific amount of carbon black are mixed. Usually, it is about 1 to 50%. However, if a large amount of conductive material is used, the content of extra components other than the electrode material in the electrode increases, so that the capacitance per weight decreases.
In addition, carbon black and natural graphite powder used as a conductive material have a large oil absorption and a low bulk density. Therefore, when an electrode is produced using them, a large amount of binder is required, and the electrode density is further reduced. End up. As a result, the capacitance per volume decreases.
In general, according to the equation (2), the capacitance of the capacitor increases with the surface area of the electrode. Therefore, as the electrode material, the above-mentioned powder having a large specific surface area or fibrous activated carbon is used. However, the actual condition is that the capacitance as high as expected from the size of the specific surface area of the electrode material used is not obtained. Rather, in order to increase the expressed capacitance, there is an idea that it is effective to use activated carbon having selectively a suitable pore distribution including mesopores as an electrode material. It has not been put into practical use as an electrode material.
Therefore, in order to solve such problems, the applicant previously developed activated carbon obtained by mixing, firing, pulverizing and activating carbon black and a carbon precursor resin as disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2006-229099). . This activated carbon achieved high electrostatic capacity and excellent rapid charge / discharge characteristics.
However, when a resin such as thermosetting phenol is used as the carbon precursor, the liquid resin has a high viscosity and tends to generate heat during mixing. If the resin is cured by this heat generation, the carbon black surface is uniformly coated. It cannot be granulated. Also, when the liquid resin is diluted with a solvent and mixed with carbon black, the mixture becomes a paste, which requires removal and recovery of the solvent, which increases costs in terms of capital investment and work efficiency.
Further, since the resin charcoal selectively generates micropores at the time of activation, many pores that do not contribute to the capacitance are generated, and the capacitance per volume is reduced.

Japanese Patent Laid-Open No. 10-4037 JP 2006-229099 A

  In view of the above problems, the present invention is not highly viscous like a thermosetting phenol resin on a carbon precursor, is uniformly coated on the surface of carbon black, and micropores are not selectively generated during activation. The present invention provides a method for producing activated carbon that exhibits a high capacitance as an electrode material for an ion capacitor positive electrode and an electric double layer capacitor.

Carbon black and carbon precursor pitch are heated and mixed at a weight ratio of 1: 0.1 to 1: 3, fired in an oxidizing gas atmosphere at 600 to 1000 ° C., and then activated in an oxidizing atmosphere at 700 to 1000 ° C. This solves the points raised as problems.
When a capacitor is formed, if a surface activated carbon containing (selectively) a large number of micropores or mesopores is used as an electrode material, a high capacitance is developed. It is thought.
Generally, carbon black has many mesopores, and activated carbon has many micropores. Therefore, when the carbon precursor containing carbon black is carbonized by firing and activated, a necessary amount of holes are formed in the capacitor by a relatively light activation treatment.
In addition, since carbon black having conductivity is contained inside the activated carbon, the activated carbon (electrode material) according to the present invention does not need to add a conductive material at the time of electrode preparation, and is superior to conventional activated carbon in that respect. ing.

The activated carbon obtained by the above production method has an average particle diameter D50 of 1 to 20 μm, Dtop of 80 μm or less, a specific surface area of nitrogen gas adsorption (BET) method of 100 m ² g ⁻¹ or more, and a micropore specific surface area of 800 m ² g ^-1 or less, micropore volume is 0.4
It is desirable for the activated carbon for lithium ion capacitors and electric double layer capacitors to be not more than cm ³ g ⁻¹ .

Carbon black can be appropriately selected from commercially available furnace black, acetylene black, lamp black, and other carbon blacks, and the carbon black may be used alone, or the pore size distribution of the activated carbon to be prepared In consideration, they may be mixed and used at an arbitrary ratio.

As the pitch, a general binder pitch or impregnation pitch can be used. Either coal-based or petroleum-based may be used, and the softening point is about 70 to 250 ° C, preferably about 80 to 150 ° C, and more preferably about 80 to 120 ° C.
If the softening point is too low, the handling is inconvenient or the residual coal rate is low, which causes high costs, which is not preferable. On the other hand, if the softening point is too high, it is unsuitable for processing with a general heating kneader, and special equipment must be used, which is not suitable for mass production. Moreover, since the pitch price becomes high, it is not preferable in terms of cost.

The mixing ratio of carbon black and pitch is 1 by weight.
The range of 0.1 to 1: 3 is desirable.
If the pitch ratio is too large, the heat mixing of the carbon black and the pitch can be performed well, but in some places, lumps are formed, or large lumps are formed in the mixing chamber, which causes a problem that the sample after heating and mixing is difficult to handle. On the other hand, if the pitch ratio is too small, the problem that the carbon black and the pitch are not well heated and mixed, that is, the coating and bonding of the carbon black with the pitch becomes unstable, and the desired state cannot be obtained.

The method and conditions for the heat mixing of carbon black and pitch are not particularly limited as long as the pitch is uniformly coated on the carbon black surface.
An apparatus used for heating and mixing is not particularly limited, but a biaxial kneader or the like is preferable.

Next, the heated mixture is fired at 600 to 1000 ° C. in an inert gas atmosphere.

Finally, the activated carbon for a capacitor of the present invention is obtained by activation treatment at 700 to 1000 ° C. in an oxidizing gas atmosphere.
As the oxidizing gas, steam, carbon dioxide, air, or the like can be used, but steam activation is preferable in view of control of activation reaction and cost.

The activated carbon obtained by the above production method has the following characteristics.
Since the activated carbon of the present invention uses carbon black as a raw material together with pitch, it has high conductivity.
Also, on the surface of the activated carbon, there are almost no macropores that do not contribute to the increase in capacitance as electrode materials for capacitors, and mesopores and micropores that are effective in improving capacitance are present at an appropriate ratio. Activated carbon can be used as a high capacitance electrode material.

The activated carbon of the present invention is a granulated body in which carbon black is coated and bonded with low crystalline carbon, and has a specific surface area of 100 m ² by nitrogen gas adsorption (BET) method.
g- ¹ or more. If the specific surface area is too small, the number of sites where electrolyte ions can be adsorbed and desorbed when charging / discharging with the activated carbon electrode is reduced, and the capacitance becomes small.

When the activated carbon of the present invention is used as an electrode material for a capacitor, it is desirable that the average particle diameter D50 is 1 to 20 μm and the Dtop is 80 μm or less.
If D50 and Dtop deviate from this range, the surface state of the activated carbon changes, so that the dispersibility of activated carbon in water and organic solvents and the wettability with the electrolyte decrease, and aggregates are generated during electrode preparation. Or the adhesion between the activated carbon electrode and the metal current collector is lowered, and as a result, the charge / discharge characteristics are deteriorated.

Binders (binders) used to form an electrode using the activated carbon of the present invention include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), and the like.
For the electrolyte, a tetraalkylammonium salt having excellent stability, for example, (CH ₃ ) ₄ NBF ₄ , (C ₂ H ₅ ) ₄ NBF ₄ , (C ₃ H ₇ ) ₄ NBF ₄ , (CH ₃ ) ₄ NPF ₆ , (C ₂ H ₅ ) ₄ NPF ₆ , (C ₃ H ₇ ) ₄ NPF _{6 and the} like can be used.
Since the activated carbon of the present invention is characterized in that mesopores and micropores are appropriately mixed, non-aqueous ethylene carbonate, propylene carbonate, r-butyl lactone, acetonitrile, dimethylformamide and the like are used as the solvent of the electrolytic solution. Can be used.

Since the activated carbon of the present invention uses carbon black together with pitch as a raw material, it has high conductivity. Therefore, as a capacitor electrode material, it is not necessary to add a conductive material at the time of electrode preparation. Even in the case, high conductivity is exhibited by using a very small amount of conductive material.
For this reason, when used as an electrode material, even if a small amount of conductive material is added or even when no conductive material is used, sufficient conductivity can be exhibited, eliminating the problems caused by using a large amount of conductive material as before. can do. That is, the capacitance per weight is not reduced, the binder is not used excessively, and the capacitance per volume is not lowered, and as a result, the capacitance can be improved.
When conventional activated carbon is used as an electrode material, the addition of a large amount of conductive material and the use of an extra binder can reduce the capacitance per unit weight and volume, which is evaluated by constant current charging / discharging of the electrode. However, when the present invention is used, such a problem can be solved, that is, the capacitance per weight and per volume can be improved.
Further, the pores formed on the activated carbon surface, which are evaluated by N ₂ adsorption / desorption, are mesopores and micropores necessary for the expression of capacitance, and furthermore, the presence of these in an appropriate ratio allows Capacitance per weight and volume evaluated by constant current charge / discharge can be improved.
By these things, the activated carbon of this invention can express a high electrostatic capacitance as a positive electrode material for lithium ion capacitors, and an electrode material for electric double layer capacitors.
The pitch used as the carbon precursor is less expensive than the phenol resin conventionally used as the carbon precursor and has a merit that the activated carbon can be obtained at low cost because the residual carbon ratio after firing is high.
The activated carbon of the present invention is less likely to generate macropores that do not contribute to the capacitance than activated carbon prepared using a phenol resin, so that there are few wasted pores and the capacitance per area can be increased.

The activated carbon pore structure such as specific surface area was evaluated by adsorption and desorption of N ₂ gas. As the apparatus, an automatic specific surface area / pore distribution measuring apparatus Tristar 3000 manufactured by Micromeritics was used.
The specific surface area was determined by a BET multipoint method in which the amount of adsorbed gas obtained from the adsorption isotherm was evaluated as a monolayer and the surface area was calculated.
P / V (P ₀ -P) = (1 / V _m C) + {(C-1) / V _m C (P / P ₀ )} ... (1)
S = kV _m ……………………………………… (2)
P ₀ : Saturated vapor pressure
P: adsorption equilibrium pressure
V: Adsorption amount at adsorption equilibrium pressure P
V _m : Monolayer adsorption amount
C: Parameters related to heat of adsorption
S: Specific surface area
k: Nitrogen single molecule occupation area 0.162 nm ²
The average pore diameter is the Dubinin-Astakhov formula
log (V) = log (V ₀ )-(RT / βE ₀ ) ^N・ [log (P ₀ / P)] ^N …… (3)
V: Adsorption volume at equilibrium pressure (cm ³
g ^-1 STP)
V ₀ : Pore volume (cm ³ g ^-1
STP)
P ₀ : Saturation pressure of gas at temperature T (mmHg)
P: Equilibrium pressure (mmHg)
T: Analysis tank temperature (K)
R: Gas constant (0.0083144 kJ K ^-1
mol ^-1 )
E ₀ : Characteristic energy (kJ mol ^-1 )
N: Astakhov power number optimized value or specified value β: Obtained from the fitting constant between analysis gas and P ₀ gas.
Average pore diameter D _MEAN / Å = 2 [(10 ³ nm ³ / Å ³ ) / βE ₀ ] ^1/3 / Γ [(3N + 1) / 3N] …… (4)
Here, Γ is a region exceeding 0 ≦ x ≦ 1, and is calculated by a polynomial approximation as follows.
Γ (x + 1) = 1 + b ₁ x + b ₂ x ² + b ₃ x ³ + b ₄ x ⁴ + b ₅ x ⁵ + b ₆ x ⁶ + b ₇ x ⁷ + b ₈ x ⁸ + ε ( x)
| ε (x) | ≦ 3 × 10 ^-7

The particle size distribution is measured by LASER manufactured by Seishin Corporation.
Using a Micron Sizer-30, the measurement was performed in a state where the sample was ultrasonically dispersed using a small amount of a surfactant with water as a dispersion medium.

In the electrochemical charge / discharge test, 2-3 parts by weight of carboxymethylcellulose (CMC) as a dispersant and 1-6 parts by weight of styrene butadiene rubber (SBR) as a binder are combined with 100 parts by weight of activated carbon. A water-based slurry was prepared, applied to an Al foil to a thickness of 100 μm using a doctor blade, dried at 110 ° C., roll-pressed, and punched to φ12 to form an electrode. The electrode after pressing had a thickness of 60 to 80 μm.
Next, a bipolar coin-type cell in which a cellulosic porous membrane was sandwiched between two electrode sheets was assembled in a glove box in an argon gas atmosphere.
As the electrolytic solution, propylene carbonate (concentration 1M) in which tetraethylammonium tetrafluoroborate as an electrolyte was dissolved was used, and constant current charge / discharge was performed at room temperature.

The pore distribution map of the activated carbon of an Example and a comparative example.

Next, the present invention will be described in detail with reference to the following examples.
Table 1 shows the preparation conditions of the following examples and comparative examples, Table 2 shows the powder characteristics of the obtained activated carbon, Table 3 shows the capacitance per surface area, and Table 4 shows the production yield. FIG. 1 shows the pore size distribution of the activated carbon obtained.

Commercially available carbon black (specific surface area by BET method: 260 m ² g ⁻¹ , average pore diameter: 8.6 nm) and binder pitch having a softening point of 110 ° C. were heated and mixed at a weight ratio of 1: 0.5. After firing this at 750 ° C. in a nitrogen atmosphere, using a batch type rotary kiln furnace,
Activation treatment was performed under the conditions of 1 hour at ° C.
The obtained activated carbon had an average particle diameter D50 of 10.3 μm and Dtop of 54.6 μm. N ₂ adsorption-desorption N ₂ BET specific surface area of activated carbon is calculated from the adsorption-desorption isotherm 435 m ² g ^-1 as measured by, micropore specific surface area of 329 m ² g ^-1, micropore volume 0.132
cm ³ g ^-1 .

The carbon black used in Example 1 and the binder pitch were heated and mixed at a weight ratio of 1: 0.25. This was fired at 750 ° C. in a nitrogen atmosphere, and then activated using a batch-type rotary kiln furnace in a water vapor atmosphere (the amount of water vapor was half that of Example 1) at 900 ° C. for 1 hour. The average particle diameter D50 of the activated carbon was 12.7 μm, and Dtop was 62.2 μm. BET specific surface area of the activated carbon, which is calculated from the N ₂ adsorption-desorption isotherms measured by N ₂ adsorption-desorption is 701 m ²
g ⁻¹ , the micropore specific surface area was 436 m ² g ⁻¹ , and the micropore volume was 0.178 cm ³ g ⁻¹ .

  2 parts by weight of CMC and 1 part by weight of SBR were added to 100 parts by weight of the activated carbon of Examples 1 and 2, and distilled water was added to prepare a slurry. It is coated on a 14 μm thick aluminum foil by the doctor blade method, dried, roll-pressed to make a punched electrode of φ12 size, and a cellulosic system between two electrode sheets in a glove box in an argon gas atmosphere A bipolar coin-type cell with a porous membrane sandwiched was assembled, and a constant current charge / discharge was performed at room temperature using propylene carbonate (concentration: 1M) in which tetraethylammonium tetrafluoroborate was dissolved as an electrolyte, and the capacitance was measured. . The current value of charge / discharge was set to the same value for charge and discharge.

(Comparative Example 1)
Commercially available resin-based activated carbon (average particle size D50: 5.7 μm, Dtop:
19.7 μm, BET specific surface area: 3056 m ² g ⁻¹ , micropore specific surface area: 2416 m ² g ⁻¹ , micropore volume: 0.966 cm ³ g ⁻¹ ) were used as they were.
A slurry was prepared by adding 2 parts by weight of CMC and 2 parts by weight of SBR to 100 parts by weight of this activated carbon. In addition, the conductive auxiliary material is not added.

This was coated on a 30 μm thick aluminum foil by the doctor blade method, dried, punched out to a size of φ12, and pressed at 1 t / cm ² to obtain an electrode.
Next, a bipolar coin type cell in which a cellulosic porous film was sandwiched between two electrode sheets was assembled in a glove box in an argon gas atmosphere.
As the electrolyte, propylene carbonate (concentration 1M) in which tetraethylammonium tetrafluoroborate as an electrolyte was dissolved was charged and discharged at a constant current at room temperature, and the capacitance was measured. The current value of charge / discharge was set to the same value for charge and discharge.

(Comparative Example 2) YP
Commercially available coconut shell activated carbon (average particle size D50: 4.9 μm, Dtop:
16.6 μm, BET specific surface area: 1628 m ² g ⁻¹ , micropore specific surface area: 1278 m ² g ⁻¹ , micropore volume: 0.509 cm ³ g ⁻¹ ) were used as they were.
A charge / discharge test was performed in the same procedure as in Example 1, and the capacitance was measured.

(Comparative Example 3) # 9
The carbon black used in Example 1 and the binder pitch were heated and mixed with a kneader at a weight ratio of 1: 0.08. During heating, the sample flew around and scattered around, and there were many physical losses of materials, workability and yield of heating and mixing were poor, and the properties were not much different from the raw material carbon black, so activation was stopped.
If the proportion of pitch is too small in the heat mixing of carbon black and pitch, material loss is undesirably large.

(Comparative Example 4) ED1
Commercially available carbon black (specific surface area by BET method: 461 m ² g ⁻¹ ) and liquid phenol resin were mixed at a weight ratio of 1: 1, dried and cured, and then pulverized to an average particle size of 3.5 μm. This was heat-treated and fired at 700 ° C. in a nitrogen atmosphere, and then activated using a small rotary kiln in a steam atmosphere at 900 ° C. for 1 hour. The obtained activated carbon had an average particle diameter D50 of 3.3 μm and a Dtop of 16.9 μm. Specific surface area of activated carbon by BET method is 836 m ² g ^-1 , micropore specific surface area is 536 m ² / g ^-1 , and micropore volume is 0.218
cm ³ g ⁻¹ and the average pore diameter was 2.3 nm.

To 100 parts by weight of this activated carbon, 2 parts by weight of CMC and 2 parts by weight of SBR were added, and distilled water was added to obtain a slurry.
This was applied to a 30 μm thick aluminum foil by the doctor blade method, dried, punched out to a size of φ12, pressed at 1 t / cm ² to form an electrode, and a charge / discharge test was performed in the same procedure as in Example 1. Capacitance was measured.

As shown in FIG. 1, Comparative Examples 1, 2, and 4 selectively have many small pores (micropores) having a diameter of 5 nm or less. The activated carbons of Examples 1 and 2 are as shown in Table 3. This indicates that the capacitance per surface area is high, there are no useless pores that do not contribute to the capacitance, and a high capacitance per volume is obtained.
The total yield at the time of preparation of activated carbon shown in Table 4 is higher than that of Comparative Example 4 in which the present invention uses resin charcoal, and activated carbon can be produced at low cost.

(Table 1)

(Table 2)

(Table 3)

(Table 4)

Claims (5)

Carbon black and pitch, which is a carbon precursor, are heated and mixed at a weight ratio of 1: 0.1 to 1: 3, fired at 600 to 1000 ° C. in an inert gas atmosphere, and then 700 to 1000 in an oxidizing gas atmosphere. A method for producing activated carbon for capacitor electrodes activated at ℃. 2. The method for producing activated carbon for capacitor electrodes according to claim 1, wherein the oxidizing gas atmosphere is a water vapor atmosphere. A capacitor having a micropore specific surface area of 500 m ² g ^-1 or less and a micropore volume of 0.2 cm ³ g ^-1 or less by a nitrogen gas adsorption (BET) method, wherein the activated carbon is produced by the production method according to claim 1 or 2. Activated carbon for electrodes. 3. An activated carbon for a capacitor electrode, which is an activated carbon produced by the production method according to claim 1, wherein the activated carbon is a granulated body having a specific surface area of 300 m ² g ⁻¹ or more by a nitrogen gas adsorption (BET) method. 3. An activated carbon for a capacitor electrode manufactured by the manufacturing method according to claim 1, wherein the average particle diameter D ₅₀ is 1 to 20 μm and the D _top is 80 μm or less.

Images