JP2008257888A - Carbon material for electrode of electrochemical element, manufacturing method therefor, and electrode for electrochemical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the manufacturing method of a carbon material for the electrode of an electrochemical element having high capacity, high cycle stability, and realizing high output (high current density) charging/discharging. <P>SOLUTION: The carbon material is manufactured in a process that forms a complex of a prescribed condensed polycyclic aromatic compound and a prescribed alkali metal in an ether solvent, a process that obtains a precursor of the carbon material by drying the alkali metal complex solution obtained, and then a process heat treating at 600-1,000°C. As the alkali metal, potassium, sodium, or lithium is used. As the condensed polycyclic aromatic compound, naphthalene, acenaphthylene, pyrene, anthracene, phenanthrene, or either one or two or more of these derivatives are used. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrode material for an electrochemical element such as a secondary battery or a capacitor. More specifically, the present invention relates to an electrode carbon material for an electrochemical element having excellent output and cycle characteristics and high voltage characteristics, a method for producing the same, and an electric The present invention relates to a chemical element electrode.

  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.

  Examples of the secondary battery include a lead battery, a nickel / cadmium battery, a nickel metal hydride battery, and a proton battery. 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. This lithium ion secondary battery mainly uses 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 solution obtained by dissolving a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) 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.

  Furthermore, an electric double layer capacitor in which an electrode facing each other with a separator in between and an electrolytic solution are contained in a container, the positive electrode is a polarizable electrode mainly composed of activated carbon, and the negative electrode is occluded in a state of ionizing lithium. There is known an electric double layer capacitor that is an electrode mainly composed of a carbonaceous material in which lithium is occluded in advance by a chemical method or an electrochemical method in a carbon material that can be detached, and the electrolytic solution is a non-aqueous electrolytic solution. (Patent Document 1).

  The electric capacity per unit weight of such a carbon material is determined by the doping amount of lithium. Therefore, in order to increase the charge / discharge capacity of the battery, it is desirable to increase the doping amount of the carbon material to lithium as much as possible ( Theoretically, the ratio of one Li atom to six carbon atoms is the upper limit).

  Moreover, the carbon material used for the negative electrode is classified into graphitizable carbon and non-graphitizable carbon when classified by crystal structure. The characteristic of graphitizable carbon is that it is excellent in flatness of the discharge potential, but it is known that the capacity decreases extremely when the charge / discharge current density is increased. Therefore, the application is limited to applications having a relatively low current density such as an electric double layer capacitor for memory backup and a secondary battery.

  On the other hand, the characteristic of non-graphitizable carbon is that it can be charged and discharged at a higher current density than easily graphitized carbon, although it is inferior in flatness of the discharge potential. However, even when this non-graphitizable carbon is used, it is not sufficient for applications requiring a large current, such as an electric vehicle.

  Thus, both graphitizable carbon and non-graphitizable carbon have features and drawbacks. Attempts have been made to improve these characteristics, and various patent applications have been filed to date. For example, a carbon material obtained by firing crystalline cellulose at 1,800 ° C. under a nitrogen gas flow (see Patent Document 2), a coal pitch or a petroleum pitch graphitized at 2,500 ° C. or higher in an inert atmosphere (Refer to Patent Document 3), advanced graphitized material processed at a high temperature exceeding 2,000 ° C. is used, and although there is a decrease in capacity compared to metallic lithium and lithium alloy, it has cycle stability Things have been obtained.

  The negative electrode made of such a carbon material has a sufficiently mild reaction with water even in a charged state, that is, in a state where lithium is occluded in carbon, compared to metallic lithium or lithium alloy, and formation of dendrite accompanying charging / discharging. It is an excellent one with almost no signs.

As the classification method graphitizable carbon and non-graphitizable carbon, mainly plane spacing and C-axis direction of the d ₂₀₀ by X-ray diffraction method, a direction of the crystallite size, lamination by laser Raman spectral analysis A method of classifying by the ratio of structure and turbulent structure is used. These two evaluation methods are effective for carbon that has been sufficiently carbonized (firing temperature of 1,500 ° C. or higher).
Japanese Patent Laid-Open No. 8-1007048 Japanese Patent Laid-Open No. 3-176963 Japanese Patent Laid-Open No. 2-82466

  However, even the negative electrodes shown in Patent Documents 1 to 3 as described above have a problem that sufficient cycle stability is not obtained in charge and discharge at a high current density. This is because the surface spacing of the carbon material obtained from graphitizable carbon is narrow, so it is difficult for lithium to enter between the surfaces of the graphite. Therefore, rapid charge and discharge cannot follow the insertion and extraction of lithium, reducing the capacity. It is estimated that it will.

  The present invention has been proposed to solve the above-described problems of the prior art, and its purpose is to provide high capacity, excellent cycle stability, and high output (high current density) charge / discharge. Is to provide a carbon material for an electrode for an electrochemical element, a method for producing the same, and an electrode for an electrochemical element.

  In order to solve the above-mentioned problems, the present inventors have made extensive studies on carbon materials used for negative electrodes such as lithium ion batteries and electric double layer capacitors, and methods for producing the same. As a result, the present invention has been completed. It is.

(Carbon material for electrodes)
As a starting material for the electrode carbon material according to the present invention, a condensed polycyclic aromatic compound is used. The condensed polycyclic aromatic is not particularly limited, and various types can be used. For example, naphthalene, acenaphthylene, pyrene, anthracene, phenanthrene and the like are preferable because they are relatively inexpensive and available in large quantities.

  In addition, derivatives of condensed polycyclic aromatic compounds in which a functional group is added to naphthalene, acenaphthylene, pyrene, anthracene, phenanthrene, or the like can also be used. As this functional group, an electron donating group such as a hydrocarbon group such as a methyl group, an ethyl group, a vinyl group, or a phenyl group is preferable because it stabilizes the lithium complex. Moreover, these condensed polycyclic aromatic compounds and derivatives thereof may be used alone or in combination of two or more.

(Method for producing carbon material for electrode)
Said condensed polycyclic aromatic compound and alkali metal are mixed in an organic solvent of ethers. By this mixing, the condensed polycyclic aromatic compound is considered to form a complex as shown in FIG. By drying the solution in which this complex is formed, ether is removed and a precursor powder of the carbon material is obtained. Subsequently, the carbon material for an electrode is obtained by heat-treating the carbon material precursor powder in an inert gas atmosphere such as nitrogen in a temperature range of 600 to 1000 ° C. for 12 hours or more.

  When the temperature is lower than 600 ° C., impurities are increased in the obtained electrode carbon material. On the other hand, when the temperature exceeds 1000 ° C., the capacity of the electrochemical element is reduced in the electrode using the obtained electrode carbon material.

  In addition, as an alkali metal, lithium, sodium, potassium, rubidium, cesium etc. can be used, for example, Among these, lithium, sodium, and potassium are preferable. This is because lithium, sodium, and potassium have small atomic radii and can easily enter between carbon layers.

  As described above, the carbon material that has been heat-treated in the presence of an alkali metal has an unnecessary alkali metal compound attached to the carbon material, and thus needs to be removed. Therefore, an alcoholic solvent such as methanol or ethanol, distilled water, or the like is used to dissolve an alkali metal compound that is unnecessarily adhered to the carbon material, and the carbon material is washed and filtered. Thus, the carbon material used for the negative electrode of an electrochemical element is obtained.

  It is presumed that the carbon material produced by the process as described above is formed so that the alkali metal enters between the carbon layers and the layers are widely formed. And since an alkali metal is removed by subsequent washing | cleaning, the carbon material with a wide interlayer is obtained.

(Electrodes for electrochemical devices)
The electrode was produced as follows. A binder such as polyvinylidene fluoride was dissolved in an organic solvent such as N-methylpyrrolidone, and the above-described carbon material was mixed with this solution to prepare a slurry. This slurry is applied to a current collector made of a strip-shaped copper foil with a uniform thickness and dried to form an electrode. Such an electrode can be applied as a negative electrode of a lithium ion secondary battery or an electric double layer capacitor.

  Hereinafter, a lithium secondary battery using the electrode of the present invention as a negative electrode will be described. In the lithium secondary battery, for example, an electrode group is housed in a bottomed cylindrical case made of stainless steel. The electrode group has a structure in which a strip obtained by laminating a positive electrode, a separator, and a negative electrode in this order is wound in a spiral shape so that the negative electrode is located outside.

  The positive electrode is produced, for example, by mixing an active material with a conductive agent and a binder in a suitable solvent to form a slurry, and applying the slurry to a current collector and drying to form a thin plate.

The positive electrode active material is preferably a metal compound mainly composed of at least one metal selected from the group consisting of cobalt, nickel, manganese, vanadium, titanium, molybdenum, and iron, and containing lithium. As the metal compound, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), and lithium manganese oxide (LiMn ₂ O ₄ , LiMnO ₂ ) are preferable.

  Examples of the conductive agent include acetylene black, carbon black, and graphite. As the binder, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDE), styrene-butadiene rubber (SBR), or the like can be used. As the current collector, for example, an aluminum foil, a stainless steel foil, a nickel foil or the like having a thickness of 10 to 40 μm is preferably used. Moreover, the separator is formed from the synthetic resin nonwoven fabric, the polyethylene porous film, and the polypropylene porous film, for example.

  In the case, an electrolytic solution is accommodated together with the electrode group, and the opening is sealed to constitute a lithium secondary battery.

Examples of the electrolytic solution include ethylene carbonate, propylene carbonate, butylene carbonate, diethyl carbonate, γ-butyrolactone, sulfolane, acetonitrile, benzonitrile, 1,2-dimethoxyethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, Examples of at least one non-aqueous solvent selected from 2-methyltetrahydrofuran include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium borofluoride (LiBF ₄ ), and arsenic hexafluoride. An electrolytic solution in which a lithium salt such as lithium (LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), lithium bistrifluoromethylsulfonylimide [LiN (CF ₃ SO ₂ ) ₂ ] is dissolved is used.

  The electric double layer capacitor can have the same configuration as the above-described lithium secondary battery except that an activated carbon electrode is used for the positive electrode.

  According to the present invention, even when a starting material that can be easily graphitized carbon is used, it is possible to obtain a carbon material that has a wide interplanar spacing of carbon and that can be rapidly charged and discharged compared to ordinary graphitizable carbon. .

  Thus, it is possible to provide a carbon material for an electrode for an electrochemical element, a method for producing the same, and an electrode for an electrochemical element, which can have high capacity, excellent cycle stability, and can be used for charge / discharge of high output (high current density). it can.

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
Acenaphthylene (0.5 mol / liter) was dissolved in 200 ml of THF (tetrahydrofuran), and then lithium metal (0.3 mol / liter) was added and mixed. Thereafter, the obtained mixed solution was dried at room temperature for 24 hours to obtain a carbon material precursor powder in which lithium and acenaphthylene were mixed. This precursor powder was heat-treated in a nitrogen atmosphere at a temperature of 600 ° C. for 3 hours. Thereafter, the treated product was washed with pure water and further dried at 70 ° C. for 24 hours to obtain a carbon material.

  An electrode was produced using this carbon material. The electrode was produced as follows. 10 parts by weight of a binder made of polyvinylidene fluoride was dissolved in 90 parts by weight of N-methylpyrrolidone, and the above-mentioned carbon material (100 parts by weight) was mixed with this solution to prepare a slurry. This slurry was applied to a current collector made of a strip-shaped copper foil with a thickness of 100 μm, a width of 10 mm, and a length of 10 mm, and dried to form an electrode.

(Example 2)
A carbon material was obtained in the same process as in Example 1 except that the heat treatment temperature in Example 1 was 800 ° C. An electrode was produced using this carbon material. The method for manufacturing the electrode is the same as in Example 1.

(Comparative Example 1)
A carbon material was obtained in the same manner as in Example 1 except that the heat treatment temperature in Example 1 was set to 400 ° C. An electrode was produced using this carbon material. The method for manufacturing the electrode is the same as in Example 1.

(Conventional example 1)
Non-graphitizable carbon (manufactured by Kureha Co., Ltd .: Carbotron P) was used as a carbon material for producing the electrode. An electrode was produced using this carbon material. The method for manufacturing the electrode is the same as in Example 1.

(Measurement result)
These carbon materials were subjected to XRD measurement to calculate the interplanar spacing, and the charge transfer resistance of the electrodes prepared using the respective carbon materials was measured. The results are shown in Table 1. The charge transfer resistance was calculated by AC impedance measurement. The impedance measurement was performed under conditions of a frequency range of 20 KHz-10 mHz and an amplitude of 10 mV. The result obtained by the impedance measurement is displayed in a complex plane, and the diameter of the semicircular portion appearing at that time is defined as the charge transfer resistance.

  As can be seen from Table 1, the interplanar spacings of Example 1, Example 2 and Comparative Example 1 all showed substantially the same values as in Conventional Example 1 using non-graphitizable carbon.

  Further, the charge transfer resistance is an index serving as a standard for the charge / discharge characteristics of the capacitor, and it is shown that rapid charge / discharge is possible as the charge transfer resistance is small. The values of Examples 1 and 2 were superior to those of Conventional Example 1 using non-graphitizable carbon, which is considered to have better charge / discharge characteristics than graphitizable carbon.

  In Table 1, the charge transfer resistance of Comparative Example 1 was not measured because an impurity peak was observed as a carbon material, as shown in FIGS. 2 and 3 described later, and no electrode was produced.

  Moreover, when the relationship between the heat treatment temperature and the XRD measurement was seen, good results were obtained in Example 1 and Example 2, as shown in FIG. On the other hand, in Comparative Example 1 where the heat treatment temperature was 400 ° C., since a peak that seemed to be an impurity was detected, a detailed examination revealed that impurities were contained as shown in FIG.

  Thus, in the carbon material containing impurities, when an electrode is produced, unexpected behavior due to the impurities may occur. Therefore, Comparative Example 1 was not evaluated as an electrode. From these results, it was found that the heat treatment needs to be performed at a temperature of 600 ° C. or higher.

The schematic diagram which shows the complex formed by mixing a condensed polycyclic aromatic compound and an alkali metal in the organic solvent of ethers. The figure which shows the relationship between heat processing temperature and a XRD measurement pattern. The figure which shows the mixing state of an impurity about the comparative example 1. FIG.

Claims (7)

  A precursor of a carbon material for an electrode obtained by complexing a predetermined condensed polycyclic aromatic and a predetermined alkali metal in an ether solvent and drying the obtained alkali metal complex solution, in a temperature range of 600 to 1000 ° C. A carbon material for an electrode of an electrochemical element obtained by heat treatment.   The carbon material for an electrode of an electrochemical element according to claim 1, wherein the alkali metal is any one of potassium, sodium, and lithium.   The electrode of an electrochemical element according to claim 1 or 2, wherein the condensed polycyclic aromatic is naphthalene, acenaphthylene, pyrene, anthracene, phenanthrene, or a derivative thereof, or two or more thereof. Carbon material.   A step of complexing a predetermined condensed polycyclic aromatic and a predetermined alkali metal in an ether solvent, a step of drying the obtained alkali metal complex solution to obtain a precursor of a carbon material for an electrode, and A method for producing an electrode carbon material for an electrochemical element, comprising a step of heat-treating the obtained precursor of the electrode carbon material in a temperature range of 600 to 1000 ° C.   The method for producing a carbon material for an electrode of an electrochemical element according to claim 4, wherein the alkali metal is any one of potassium, sodium, and lithium.   The electrode of an electrochemical device according to claim 4 or 5, wherein the condensed polycyclic aromatic is naphthalene, acenaphthylene, pyrene, anthracene, phenanthrene, or a derivative thereof, or two or more thereof. Carbon material manufacturing method.   An electrode for an electrochemical element comprising the carbon material for an electrode according to any one of claims 1 to 3.