JP5011561B2 - Electrode material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode material having high capacity and low impedance characteristics, its manufacturing method and an electrochemical element using it. <P>SOLUTION: As for the electrode material composed by carrying polyfluorene or its derivative on the surface of a monolayer carbon nano tube of the diameter of 2 to 5 nm and the length of 0.3 to 3 mm, adhesiveness of the monolayer carbon nano tube and polyfluorene or its derivative is superior, resistance of the electrode material is reduced, voltage reduction due to IR drop in discharging is small, and furthermore, since oxidation reduction potential of the negative electrode is low and oxidation reduction potential of the positive electrode is high, this has high voltage characteristics, and furthermore, the high capacity and the low impedance characteristics. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to an electrode material and an electrochemical element such as a secondary battery and a capacitor using the electrode material, and more particularly to an electrode material having excellent output and cycle characteristics, and having high capacity and low impedance characteristics.

  In recent years, due to environmental problems on the earth, there are increasing expectations for electric vehicles and hybrid vehicles in place of engine-driven gasoline vehicles and diesel vehicles. In these electric vehicles and hybrid vehicles, an electrochemical element having high energy density and high output density characteristics is used as a power source for driving the motor. Such electrochemical elements include secondary batteries and electric double layer capacitors.

  Secondary batteries include lead batteries, nickel / cadmium batteries, nickel metal hydride batteries, or proton batteries. Since these secondary batteries use an acidic or alkaline aqueous electrolyte having high ion conductivity, they have excellent output characteristics that a large current can be obtained during charging and discharging, but the electrolysis voltage of water is low. Since it is 1.23V, a voltage higher than that cannot be obtained. As a power source for an electric vehicle, a high voltage of about 200 V is necessary, so that many batteries have to be connected in series, which is disadvantageous for reducing the size and weight of the power source.

As a high voltage type secondary battery, a lithium ion secondary battery using an organic electrolyte is known. Since this lithium ion secondary battery uses an organic solvent having a high decomposition voltage as the electrolyte solvent, if the lithium ion having the lowest potential is used as a charge involved in the charge / discharge reaction, it exhibits a potential of 3 V or more. Lithium ion secondary batteries mainly use carbon that absorbs and releases lithium ions as a negative electrode and lithium cobaltate (LiCoO ₂ ) as a positive electrode. As the electrolytic solution, a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) dissolved in a solvent such as ethylene carbonate or propylene carbonate is used. Such a lithium ion secondary battery has an average operating voltage of 3.6V.

  However, this lithium ion secondary battery is excellent as a power source because of its high voltage and high energy density. It is disadvantageous for power sources for electric vehicles that require instantaneous current. Therefore, there is an attempt to use a polythiophene derivative for the positive electrode in order to improve the charge / discharge characteristics at a high voltage, but the operating voltage is 4.0V. (Patent Document 1)

  In addition, the electric double layer capacitor uses a polarizable electrode such as activated carbon as positive and negative electrodes, and an electrolytic solution obtained by dissolving quaternary onium salt of boron tetrafluoride or phosphorus hexafluoride in an organic solvent such as propylene carbonate. . In such an electric double layer capacitor, the electric double layer generated at the interface between the electrode surface and the electrolyte has a capacitance, and there is no reaction involving ions like a battery. Little capacity degradation due to discharge cycle. However, the energy density due to the double layer capacity is lower than that of the battery, and it is significantly insufficient as a power source for electric vehicles. On the other hand, there is an attempt to use polypyrrole for the positive electrode for the purpose of increasing the capacity, but the operating voltage is 2.6V. (Patent Document 2)

Thus, an electrochemical capacitor using a conductive polymer or metal oxide as an electrode material having high energy density and high output characteristics has been developed. This electrochemical capacitor uses an anion and a cation in the electrolyte solution as the charge storage mechanism and is excellent in both energy density and output characteristics. In particular, electrochemical capacitors using conductive polymers such as polyaniline, polypyrrole, polyacene, and polythiophene derivatives must be p-doped or n-doped with anions or cations in non-aqueous electrolytes. To charge and discharge. Since the doping potential is low on the negative electrode side and high on the positive electrode side, a high voltage characteristic of 2.5 V or more can be obtained. (Patent Document 3)
JP 2003-297362 A JP-A-6-104141 JP 2000-315527 A

  However, in power supply applications such as electric vehicles, in addition to high voltage, there is a constant demand for downsizing and high cycle characteristics. Therefore, there is a strong demand for high capacity and low impedance. Accordingly, an object of the present invention is to provide an electrode material having high voltage characteristics, high capacity, and low impedance characteristics.

  In order to solve the above-mentioned problems, the present invention has studied the conductive polymer as an electrode material and the production method thereof. It has been found that an electrochemical element having high capacity and low impedance characteristics can be obtained by using an electrode material carrying fluorene or a derivative thereof. The electrode material prepared as described above has a small diameter when polyfluorene or a derivative thereof is supported on the surface of a single-walled carbon nanotube having a diameter of 2 to 5 nm and a length of 0.3 to 3 mm. Increased specific surface area of carbon increases capacity, and long single-walled carbon nanotubes reduce resistance by reducing the number of contact points between carbon nanotubes. By using this electrode, high capacity and low impedance characteristics can be achieved. An electrochemical device having the following can be provided.

As a derivative of polyfluorene, the substituent at the 9-position of fluorene is an alkyl group, a carboxyl group, a nitro group, a cyano group, an alkyl cyano group, a phenyl group (-Ph), a halogen atom (-X), -CX ₃ It is preferable to use a halogenated phenyl group, an alkylphenyl group, or an alkylhalogenated phenyl group.

Then, single-walled carbon nanotubes having a diameter of 2 to 5 nm and a length of 0.3 to 3 mm are dipped in a solution obtained by dissolving doped polyfluorene or a derivative thereof in a basic solvent, and the solvent is evaporated to obtain polyfluorene or When an electrode in which a film of the derivative is formed on the single-walled carbon nanotube is used, a thin and uniform electrode film can be formed, and the discharge voltage can be kept high by reducing the resistance of the electrode. Capacitance and low impedance characteristics can be obtained.

    As described above, by using an electrode material in which polyfluorene or a derivative thereof is supported on the surface of a single-walled carbon nanotube having a diameter of 2 to 5 nm and a length of 0.3 to 3 mm, a low redox potential of a polyfluorene negative electrode is obtained. In addition to the high voltage characteristics due to the high oxidation-reduction potential of the positive electrode, an electrochemical element having high capacity and low impedance characteristics can be obtained.

    The polyfluorene or a derivative thereof of the present invention can be obtained by polymerizing a monomer of fluorene or a derivative thereof by chemical polymerization or electrolytic polymerization. Then, polyfluorene obtained by electrolytic polymerization or chemical polymerization or a derivative thereof is dissolved in a basic solvent such as chloroform, tetrahydrofuran, N-methylpyrrolidone, etc., and this solution has a diameter of 2 to 5 nm and a length of 0.3 to 3 mm. A single-walled carbon nanotube is immersed and pulled up, and the solvent is evaporated to form an electrode. At this time, a thin and uniform layer of polyfluorene or a derivative thereof can be formed, and in addition to reducing the resistance of the electrode material due to good adhesion between the polymer and the carbon material, the resistance of the layer is also reduced, so The IR drop is further reduced, and high capacity and low impedance characteristics can be obtained.

  Single-walled carbon nanotubes with a diameter of 2 to 5 nm and a length of 0.3 to 3 mm used in the present invention use a chemical vapor reaction (CVD) method to control the amount of water during the growth of single-walled carbon nanotubes. Obtained by promoting sustained activity.

    The polyfluorene or its derivative thus polymerized is in an oxidized state by doping with an anion in the polymerization solution, and this is used as the positive electrode. This positive electrode causes a discharge reaction and a reduction reaction by undoping an anion. Then, this polyfluorene derivative is electrically or chemically reduced and doped with a cation to be used as a negative electrode. This negative electrode causes a discharge reaction and an oxidation reaction by dedoping cations. Alternatively, the polymer after polymerization may be reduced to a neutral state to form a bipolar electrode, and the negative electrode may be reduced and the positive electrode oxidized by a charging reaction.

  Further, a self-doped positive electrode can be obtained by reacting fluorene or a derivative thereof with an anion that can be covalently bonded to fluorene such as an alkyl sulfonic acid or an alkyl phosphonic acid and polymerizing it. This positive electrode causes a discharge reaction and an oxidation reaction by doping with a cation in the electrolytic solution. Alternatively, polyfluorene or a derivative thereof can be reacted with a cation that can be covalently bonded to fluorene such as tertiary ammonium and polymerized to form a self-doped negative electrode. The negative electrode undergoes a discharge reaction and an oxidation reaction by doping with an anion in the electrolytic solution.

Here, the polyfluorene derivative is preferably a derivative having a substituent at the 9-position of fluorene. This is because the electron conductivity is not lowered, and the anion and cation doping and dedoping reactions are accelerated by this substituent and the output characteristics are improved. In addition, as a substituent, an alkyl group, a carboxyl group, a nitro group, a cyano group, an alkyl cyano group, a phenyl group (-Ph), a halogen atom (-X), -CX ₃ , a halogenated phenyl group, an alkylphenyl group, It is preferable to use an alkyl halogenated phenyl group. In addition, fluorene having no substituent at the 9-position has high reactivity at the 9-position hydrogen, and when a reduction potential is applied, the proton is eliminated, and the reduction potential of this proton is higher than that of fluorene, so that the reduction reaction of fluorene does not easily occur. It is difficult to use fluorene as a negative electrode.

    Among these polyfluorene derivatives, when the 9-position substituent is a substituent having an alkyl group or a phenyl group, the anion, cation doping, and dedoping reactions are further accelerated and the output characteristics are improved. This is preferable. Examples of the former include 9,9-dimethylfluorene and 9,9-dioctylfluorene, and examples of the latter include 9-methyl-9-phenylfluorene, 9-methyl-9-benzylfluorene, benzalfluorene, and benzhydrylidinefluorene. be able to. In particular, when used as a negative electrode doped or dedoped with a large molecular cation or a self-doped positive electrode, an alkyl group of n = 1 to 8 is preferable.

The present invention will be described more specifically with reference to the following examples.

After dissolving the fluorene derivative in acetonitrile, a sufficient amount of iron (III) chloride was dissolved and stirred for 72 hours to proceed the polymerization. Next, the reaction solution was filtered under reduced pressure, and the filtrate was vacuum dried with 60 C 2 ^O for about 12 hours to obtain a crude product. This crude product was dissolved in chloroform to be saturated, and methanol was added to reprecipitate the product. The reprecipitate was again filtered under reduced pressure, and the filtrate was vacuum-dried with 60 C 2 ^O for about 12 hours to obtain a purified polyfluorene derivative.

In 1 L of THF solvent, 1.2 g of the resulting polyfluorene derivative was dissolved to prepare a polyfluorene derivative solution. Single-walled carbon nanotubes (SWNT) having a diameter of 3.5 nm and a length of 2 mm were immersed in this solution and pulled up, and the solvent was evaporated to form an electrode. This electrode was joined to an aluminum current collector, and a polyfluorene / SWNT film was formed on the current collector to give an example. In addition, carbon fiber was used instead of SWNT, and a current collector / polyfluorene / carbon fiber film was formed in the same manner as a comparative example. Using these electrodes as positive electrodes, platinum plates as negative electrodes, and Ag-Ag ion electrodes as reference electrodes, capacity density and internal resistance were measured.

As described above, the capacity density and internal resistance of the electrochemical device of the present invention were better than those of the comparative examples.

Claims (3)

An electrode material for a positive electrode or a negative electrode of an electrochemical device, in which polyfluorene or a derivative thereof is supported on the surface of a single-walled carbon nanotube having a diameter of 2 to 5 nm and a length of 0.3 to 3 mm. The 9-position substituent of the polyfluorene derivative is an alkyl group, a carboxyl group, a nitro group, a cyano group, an alkyl cyano group, a phenyl group (—Ph), a halogen atom (—X), —CX ₃ , a halogenated phenyl group, 2. The electrode material according to claim 1, which is an alkylphenyl group or an alkylhalogenated phenyl group. A single-walled carbon nanotube having a diameter of 2 to 5 nm and a length of 0.3 to 3 mm is dipped in a solution obtained by dissolving doped polyfluorene or a derivative thereof in a basic solvent, and the solvent is evaporated to obtain polyfluorene or a derivative thereof. The electrode material according to claim 1, wherein the film is formed on the single-walled carbon nanotube.