JP2008181950A - Electrode material for electric double-layer capacitor, production process and electric double-layer capacitor therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode material for an electric double-layer capacitor, exhibiting high output characteristics in a low-temperature region, and to provide its production process and the electric double-layer capacitor. <P>SOLUTION: In the process for producing the electrode material of an electric double-layer capacitor by performing alkali activation of a mixture of a carbonaceous material and an alkali compound, the temperature region in a system for performing alkali activation is 600°C-900°C, and the inside of the system for performing alkali activation is under inert atmosphere having a pressure of 0.11 MPa or higher. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electrode material for an electric double layer capacitor, a manufacturing method thereof, and an electric double layer capacitor.

  The electric double layer capacitor is configured by facing two pairs of electrodes in a liquid electrolyte via a separator. Charging / discharging is performed by applying a voltage to the cell so that ions in the electrolytic solution are electrically adsorbed / desorbed on the electrode surface. In this way, since the electric double layer capacitor does not involve a chemical reaction between the electrode and the electrolyte, compared with other secondary batteries such as lithium ion secondary battery and nickel metal hydride battery, high output, high life, Furthermore, it is characterized by high safety.

  The electric double layer capacitor has been put to practical use as a backup power source for semiconductor memories, and then has been used for road signs, lighting, etc. combined with a solar cell with an increase in capacity. In recent years, electric double layer capacitors that are attracting attention are in-vehicle power supplies and instantaneous power outages. In particular, in automotive applications, demands for reliability, life, and output characteristics of power sources are increasing along with electronic control and hybridization of automobiles, and electric double layer capacitors that are excellent in these characteristics are attracting attention.

  Activated carbon having a high specific surface area is used for the electrode material of the electric double layer capacitor. Activated carbon is porous carbon composed of nano-sized pores. Since electric double layer formation and ion migration occur in the pores, improvement of activated carbon as an electrode material has been actively studied for the purpose of improving the characteristics of the electric double layer capacitor.

  Electric double layer capacitors for on-vehicle power supplies are required to have high output characteristics, particularly high output at low temperatures. In general, in order to increase the output of an electric double layer capacitor from an electrode material, (1) reduce the electric resistance inherent to the electrode material, (2) reduce the contact resistance between the electrode materials, (3) For example, the diffusion resistance of the electrolytic solution in the activated carbon pores may be reduced. In particular, in the low temperature range, the diffusion resistance of ions in the pores of (3) is the main factor of the resistance component, and many attempts to improve output characteristics by controlling the pore diameter of activated carbon have been reported ( For example, see Patent Documents 1 and 2).

  By the way, as a method for producing activated carbon from ancient times, a gas activation method using steam or the like has been used. However, in recent years, an alkali activation method capable of obtaining a high capacity as an electrode material for an electric double layer capacitor has attracted attention. For example, reports have been made on the production of high-capacity activated carbon by alkali activation of mesophase pitch carbon fibers. (For example, refer to Patent Document 3). On the other hand, as a report example of alkali activation for higher output, in addition to developing mesopores by performing alkali activation after steam activation, trying to increase output of electrode material, activation temperature, activation time, alkali compound addition amount Examples of reports that attempt to increase the output by controlling the pore diameter of the electrode material from the manufacturing conditions such as the above (see, for example, Patent Documents 2 and 4).

JP 2002-33249 A JP 2001-118753 A JP-A-10-121336 JP-A-8-119614

  In order to output the electrode material at a high output, it is important to control the pore diameter. In the conventional report, alkali activation is performed after steam activation, or the production conditions are studied by controlling the precursor species, the activation temperature, the activation time, the alkali compound species, and the mixed amount thereof. However, these production methods are costly and require adjustment of a number of production conditions, resulting in problems such as complicated processes.

  The inventors have found that the pore diameter can be easily controlled by controlling the pressure during alkali activation, and the output of the electrode material can be increased. The present invention provides an electrode material for an electric double layer capacitor having high output characteristics in a low temperature region, a method for producing the electrode material, and an electric double layer capacitor.

In addition to the precursor species, the activation temperature, the activation time, the alkali compound species, and the mixed amount thereof, which have been conventionally studied as production conditions for alkali activation in the method for producing an electrode material for an electric double layer capacitor by alkali activation, The pore diameter of the electrode material can be easily controlled by controlling the atmosphere pressure of alkali activation, and the electrode material that can be alkali activated at 0.11 MPa or more has high output characteristics as an electrode material for an electric double layer capacitor. I found it.
Specifically, the items described in [1] to [3] below are characterized.
[1] In the method for producing an electrode material for an electric double layer capacitor obtained by alkali-activating a mixture containing a carbonaceous substance and an alkali compound, the temperature range in the system for carrying out alkali activation is 600 ° C. to 900 ° C. A method for producing an electrode material for an electric double layer capacitor, wherein the system in which the activation is performed is in an inert atmosphere with a pressure of 0.11 MPa or more.
[2] An electrode material for an electric double layer capacitor produced by the method for producing an electrode material for an electric double layer capacitor according to [1].
[3] An electric double layer capacitor using the electrode material for an electric double layer capacitor according to [2].

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the electrode material for electrical double layer capacitors which has a high output characteristic in a low temperature range, its manufacturing method, and an electrical double layer capacitor.

Hereinafter, the present invention will be described in detail.
In the method for producing an electrode material for an electric double layer capacitor of the present invention, a mixture containing a carbonaceous material and an alkali compound is subjected to an inert atmosphere at a pressure of 0.11 MPa or more in a system in a temperature range of 600 ° C. to 900 ° C. It is characterized by performing alkali activation. Hereinafter, activation represents alkali activation. By activating under an inert atmosphere of 0.11 MPa or more, the pore diameter can be expanded as compared with the electrode material activated at less than 0.11 MPa. The pressure in the system is preferably 0.11 to 0.30 MPa, more preferably 0.12 to 0.25 MPa, and even more preferably 0.12 to 0.20 MPa. Excessive pressurization is not preferable because there is a concern that the risk is increased in consideration of the fact that highly reactive alkali metals are generated in the furnace (in the system).

  The temperature at which the pressurization is performed is preferably close to the temperature range where carbon is activated. Specifically, it is 600-900 degreeC, 650-900 degreeC is preferable and 700-900 degreeC is more preferable. When pressurization is performed at a temperature lower than 600 ° C., the dehydration reaction of the mixed alkali compound is inhibited, and the activation reaction in a high temperature range is affected. Further, a temperature exceeding 900 ° C. is not preferable because corrosion of the furnace body and the container due to alkali becomes remarkable.

  The method of pressurization is not particularly limited as long as the pressure in the system for activation is 0.11 MPa or more, but it is preferable to control the internal pressure according to the design and structure of the furnace because it can be easily and reliably controlled. For example, it is possible to control the internal pressure by inserting a pressure sensor in an atmosphere furnace designed to withstand pressure and controlling the opening and closing of the exhaust valve. Further, during alkali activation, highly reactive metal alkali is generated and scattered in the furnace or piping. For this reason, it is necessary to take sufficient safety measures such as providing a backflow prevention valve to prevent outside air from being introduced into the furnace. In addition, it is necessary to design the furnace with sufficient consideration so that the piping and valves are not blocked by the scattered alkali.

  Examples of the carbonaceous material used in the present invention include carbides obtained by subjecting pitch, coke, polyvinyl chloride, phenol resin, furan resin, polyvinylidene chloride, etc. to a heat treatment in an inert atmosphere and carbonizing.

  It is preferable to carbonize the cured and pulverized material. The carbonization is usually preferably performed in an inert atmosphere such as nitrogen, argon, helium, vacuum, etc. in the range of 500 to 1000 ° C., more preferably 600 to 900 ° C., and 700 to 800 ° C. Further preferred. When the carbonization temperature is less than 500 ° C., the obtained electrode material tends to have poor output characteristics because of low crystallinity. When the carbonization temperature exceeds 1000 ° C., a large amount of alkali compound is required at the time of activation, and the improvement in crystallinity tends not to greatly contribute to the output characteristics. The obtained carbide is preferably further pulverized to the target particle size. Examples of the pulverizer include a pin mill, a jet mill, a ball mill, and a bead mill. These may be performed alone or in combination of two or more methods.

  Moreover, there is no restriction | limiting in particular about the alkali compound used in this invention, For example, potassium hydroxide, sodium hydroxide, potassium carbonate etc. are mentioned. Of these, potassium hydroxide is preferable because an electrode material having a high specific surface area can be obtained.

  The alkali activation may be performed as follows. Carbide (carbonaceous material) and alkali compound are mixed using a mixer such as a planetary mixer. This mixture is put into a Ni container and treated in a range of 600 to 900 ° C. under an inert atmosphere such as nitrogen, argon or helium at a pressure of 0.11 MPa or more for 0.5 to 3 hours.

  After pressure activation, it is preferable to dissolve and extract the metal impurities mixed from the alkali compound or Ni container with an acid. Although this method is not particularly limited, it is preferable to purify the electrode material as much as possible because metal impurities affect the life characteristics and self-discharge properties of the electric double layer capacitor. As an example of a method for increasing the purity, for example, there is a method in which a metal mixture is dissolved by stirring the activated mixture in 4 wt% hydrochloric acid while heating to 80 ° C. or higher. Thereafter, the acid solution is filtered, and the above process is repeated again 5 times using the same concentration of hydrochloric acid. Subsequently, the same process as described above is performed 5 times or more using pure water, and hydrochloric acid adhering to the electrode material is removed, whereby a high-purity electrode material is obtained.

  The purified electrode material is preferably further heat-treated in an inert atmosphere in order to reduce surface functional groups. The heat treatment temperature is preferably 500 to 1000 ° C, more preferably 600 to 900 ° C, and further preferably 700 to 800 ° C. When the temperature is lower than 500 ° C., the surface functional groups tend not to be sufficiently reduced, and the life characteristics tend to be lowered. On the other hand, when the heat treatment temperature exceeds 1000 ° C., the specific surface area and pore volume tend to decrease, and the capacitance and output characteristics tend to decrease.

  The electrode material for electric double layer capacitors obtained by the present invention is suitable as an electrode material for electric double layer capacitors that require high output characteristics and capacitance at low temperatures. Although there is no restriction | limiting in particular about the structure of the electrical double layer capacitor which uses the electrode material of this invention, a manufacturing method, For example, an electrical double layer capacitor is producible as follows.

  The electrode is prepared by mixing and dispersing the electrode material for an electric double layer capacitor of the present invention, a binder, various additives, and the like together with a solvent with a stirrer, a kneader, or the like. An electrode can be produced by applying this to a current collector. Further, it is possible to produce an electrode by forming a paste-like paint into a sheet shape, a pellet shape, or the like and integrating it with a current collector.

  Although there is no limitation in particular as said binder, fluorinated resin, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and styrene butadiene rubber (SBR) Or the like. These binders are usually used in the form of powder dissolved or dispersed in a solvent, but it is also possible to use the powder as it is without using a solvent. The electrode is preferably mixed with a conductive additive. As the conductive assistant, graphite, carbon black such as acetylene black, ketjen black, or the like can be used.

  About the said electrical power collector, the strip | belt-shaped thing which made aluminum, nickel, stainless steel etc. into foil shape, perforated foil shape, mesh shape, etc. can be used, for example. A porous material such as porous metal (foamed metal) or carbon paper can also be used.

  The method for applying the electrode slurry is not particularly limited. For example, a metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, doctor blade method, gravure coating method, screen printing method, etc. A well-known method is mentioned. After the application, a rolling process using a flat plate press, a calendar roll, or the like is performed as necessary. Further, the integration of the electrode material formed into a sheet shape or a pellet shape and the current collector can be performed by a known method such as a roll press.

  The electric double layer capacitor of the present invention can be manufactured by injecting an electrolytic solution by usually disposing electrodes using the electrode material of the present invention through a separator. In addition, it is preferable to carry out in an inert atmosphere using a glove box etc. so that moisture mixing may not occur at the time of capacitor cell production. The electrode is preferably used after sufficiently removing the adsorbed moisture.

The electrolytic solution can be broadly classified into an aqueous type and an organic type, but it is preferable to use an organic electrolytic solution from the viewpoint that it can be used at a high voltage. No particular limitation is imposed on the type of electrolyte and the electrolyte, for _{example, (C 2 H 5) 4} NBF 4, (C 2 H 5) 3 CH 3 NBF 4 quaternary ammonium salts such as, 1-ethyl-3 It is possible to use a room temperature molten salt such as methylimidazolium BF ₄ or a lithium-containing salt such as LiBF ₄ dissolved in an organic solvent such as propylene carbonate. As the anion species of the electrolyte, in addition to BF ₄ , ClO ₄ ⁻ , PF ₄ ⁻ , AsF ₆ ^{− and the} like can be used. As the organic solvent for dissolving the electrolyte, γ-butyrolactone, acetonitrile, sulfolane and the like can be used in addition to PC (propylene carbonate). About the structure of these electrolyte solutions, it is preferable to select suitably from use conditions. As said separator, the nonwoven fabric, cloth, microporous film, paper, etc. which have polyolefins, such as polyethylene and a polypropylene, as a main component can be used.

  The structure of the electric double layer capacitor of the present invention is not particularly limited. Usually, the positive and negative electrodes and the separator are wound in a flat spiral shape to form a wound electrode plate group, or these are laminated in a flat plate shape to form a laminated electrode. In general, a plate group is used, and the electrode plate group is enclosed in an exterior body. The electric double layer capacitor of the present invention is usually used as a coin type, multilayer type, cylindrical cell or the like. In addition to the electric double layer capacitor, the electrode material for electric double layer capacitor of the present invention can be applied to electrode materials for various power storage devices, electrochemical elements and the like.

The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to the following examples.
(Example 1)
500 g of phenol formaldehyde resin was weighed and ground and mixed with 50 g of hexamine. The mixture was melt-mixed with a vat coated with PTFE (polytetrafluoroethylene) on a hot plate to obtain a semi-cured product of a phenol resin. The obtained phenol resin semi-cured product was after-cured at 180 ° C. for 2 hours with a hot air dryer to obtain a cured resin product. The obtained cured resin is pulverized to about 100 μm with a cutter mill, heated in a nitrogen stream in an atmosphere baking furnace at a flow rate of 300 ml / min from room temperature (25 ° C.) to 700 ° C. and held for 2 hours to hold a phenol resin. Carbide (carbonaceous material) was produced. The obtained carbide (carbonaceous material) is pulverized to an average particle size of 4 μm, mixed with 2.5 times the amount of potassium hydroxide relative to the weight of the carbide, and this mixture is pressurized in an atmosphere control furnace. In a nitrogen stream, the temperature was raised from room temperature (25 ° C.) to 450 ° C. at a flow rate of 5 L / min, held for 5 hours, then heated to 800 ° C. and held for 2 hours for alkali activation. At this time, the pressure in the furnace was controlled to 0.1 MPa from room temperature (25 ° C.) to 600 ° C., and 0.15 MPa in the temperature range from 600 ° C. to 900 ° C. Further, after holding at 800 ° C. for 2 hours, the pressure was 0.1 MPa during cooling.

  When the temperature returned to room temperature (25 ° C.), a sample was taken out, metal impurities were removed by the method described above, and this was dried at 120 ° C. for 40 hours to obtain activated carbon. The obtained activated carbon was heated from room temperature (25 ° C.) to 800 ° C. in a nitrogen atmosphere and then heat treated for 1 hour to obtain an electrode material. The obtained electrode material was measured for the BET specific surface area, pore volume, average pore size, average particle size, and surface functional group amount by the following methods. In addition, the capacitance and output characteristics were also evaluated by producing an electrode cell by the following method. The results are shown in Table 1.

(BET specific surface area, pore volume, average pore diameter)
The BET specific surface area, pore volume, and average pore diameter in the present invention can be measured by N ₂ gas adsorption measurement. The average pore diameter can be evaluated from the BET specific surface area and the pore volume using the following formula. In the present invention, ASAP-2010 manufactured by Shimadzu Corporation was used as the N ₂ gas adsorption measuring apparatus.
Average pore diameter D (nm) = 4 V / S (V: pore volume, S: BET specific surface area)

(Surface functional group content measurement method)
1 g of electrode material and 100 ml of 0.1 mol / L sodium hydroxide are placed in a volumetric flask and stirred at 25 ° C. for 20 hours. After filtration of the mixture, 50 ml of the filtrate is accurately weighed with a whole pipette and placed in a volumetric flask. As an indicator, 3 drops of methyl orange are appropriately applied to the filtrate, and this aqueous solution is back titrated with 0.1 mol / L hydrochloric acid. The surface functional group amount of the electrode material can be evaluated from the following formula.
Electrode material surface functional group amount (mmol / g) = (50-hydrochloric acid dropping amount (ml)) × 0.1 × 2

(Average particle size)
The average particle diameter in the present invention is obtained by measuring using a laser diffraction particle size measuring apparatus (SALD-3000J manufactured by Shimadzu Corporation). In addition, the average particle diameter is a 50% value of the cumulative particle size distribution based on the volume standard.

(Electrode cell manufacturing method)
Electrode material and conductive additive (HS100 manufactured by Denki Kagaku Kogyo Co., Ltd.) and carboxymethylcellulose (DN-10L manufactured by Daicel Chemical Industries, Ltd.) 2 wt% aqueous solution, 60 wt% PTFE aqueous dispersion (Daikin Kogyo Co., Ltd. M-390) ) Was mixed at a ratio of 100: 10: 200: 5, and water was added to prepare a slurry, which was then applied to an aluminum etching foil (film pressure 20 μm manufactured by Hosen) so as to have an electrode thickness of 70 μm. The coated electrode was dried in a dryer at 80 ° C. for 5 hours and 120 ° C. for 3 hours, and then the electrode was punched into a circular shape having a diameter of 16 mm to produce an electrode. Prepare two of these electrodes for the negative electrode for the positive electrode and vacuum dry them at 120 ° C for 3 hours using a vacuum separator with a paper separator (TF40 manufactured by Nippon Kogyo Paper Industries Co., Ltd.), a SUS coin cell upper and lower lid, and an aluminum spacer. went. After drying, a coin-type capacitor cell was produced in an argon-substituted glove box. In the cell, two electrodes were opposed to each other through a separator, and then an aluminum spacer was inserted to fill the space in the cell. After adding about 0.03 ml of the electrolytic solution, the coin cell was sealed after performing a reduced pressure impregnation treatment in a side box at a reduced pressure of 10 torr or less for 10 minutes. The produced cell was connected to a charge / discharge tester (TOSCAT manufactured by Toyo System Co., Ltd.), and a charge / discharge test was performed. In addition, although 1.5 mol / L propylene carbonate solution (manufactured by Guangei Chemical Industry Co., Ltd.) of ethyl methyl imidazolium tetrafluoroborate was used as the electrolytic solution, the effect of the present invention is particularly limited to the type and concentration of the electrolytic solution. Not done.

(Electrode property evaluation method)
The capacitor cell prepared above was left in a thermostatic chamber at a predetermined temperature for 3 hours or more, and then subjected to a charge / discharge test under the following conditions to evaluate electrode characteristics at 25 ° C. (room temperature) and −30 ° C. (low temperature). went. The results are shown in Table 1.
・ Charging conditions: constant current / constant voltage ・ Charging current: 2 mA
・ Charging voltage: 2V
・ Charging time: 2 hours ・ Discharging conditions: constant voltage ・ Discharging current: 2 mA
・ Discharge voltage: 0V
The capacitance is 1.7 V (voltage: V1, time: T1 (seconds)) to 1.3 V (voltage: V2, time: T2 (seconds)) of the discharge curve obtained in the charge / discharge test described above. Calculated from the slope (see the following formula).
Capacitance (F / g) = (T2-T1) × 0.002 / (V1-V2) / G
(G: active material amount (g))

The output characteristics were evaluated using the DC resistance value as an index. For the DC resistance value, an approximate straight line was drawn for the curve from 10 seconds to 40 seconds of the discharge curve, and the difference between the intercept value and the full charge voltage value was defined as a voltage drop ΔV, and the resistance value was obtained by the following equation.
Resistance value (Ω) = ΔV / 0.002

(Example 2)
It carried out similarly to Example 1 except having implemented alkali activation by 0.12 MPa in the temperature range of 600 degreeC or more and 900 degrees C or less.

(Comparative Example 1)
It carried out similarly to Example 1 except having implemented alkali activation at 0.10 MPa in the temperature range of 600 degreeC or more and 900 degrees C or less.

(Comparative Example 2)
The same procedure as in Comparative Example 1 was performed except that the amount of potassium hydroxide mixed was 2.7 times the carbide weight.

  As shown in Table 1, Examples 1-2 in which the mixture was alkali-activated in a temperature range of 600 ° C. or higher and 900 ° C. or lower in an inert atmosphere with a pressure of 0.11 MPa or higher were obtained at −30 ° C. (low temperature It can be seen that the resistance value at) is lower than those of Comparative Examples 1 and 2. Moreover, in Examples 1-2, the average pore diameter of an electrode material is 1.74 nm or more, and the pressure at the time of alkali activation is controlled, such as larger than the average pore diameter of the electrode material of Comparative Examples 1-2, 1.72 nm. Thus, it was found that the average pore diameter can be controlled. According to the present invention, an electrode material for an electric double layer capacitor and an electric double layer capacitor having high output characteristics in a low temperature range can be obtained.

Claims (3)

  In the method for producing an electrode material for an electric double layer capacitor obtained by activating a mixture containing a carbonaceous material and an alkali compound, the temperature range in the system for activating the alkali is 600 ° C. to 900 ° C., and the alkali activation is performed. The manufacturing method of the electrode material for electric double layer capacitors whose inside is the inert atmosphere of the pressure of 0.11 Mpa or more.   The electrode material for electric double layer capacitors produced by the manufacturing method of the electrode material for electric double layer capacitors of Claim 1.   An electric double layer capacitor using the electrode material for an electric double layer capacitor according to claim 2.