JP2008257883A - 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 a manufacturing method of a carbon material for the electrode of an electrochemical element having a high capacity, high charge discharge stability, and coping with high output (high current density) charging/discharging. <P>SOLUTION: The carbon material for the electrode is obtained by mixing prescribed graphitizing carbon and a prescribed alkali metal carbonate in a ratio of 1:1 to 1:4, and heat-treating at 600-1500°C in an inert gas atmosphere. As the alkali metal, potassium, sodium, or lithium is used. The electrode for the electrochemical element is manufactured with the carbon material obtained, in this way. <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, carbon material obtained by baking crystalline cellulose at 1,800 ° C. under nitrogen gas flow (see Patent Document 2), coal pitch or petroleum pitch was graphitized at 2,500 ° C. or higher in an inert atmosphere. (See Patent Document 3), advanced graphitized material processed at a high temperature exceeding 2,000 ° C., and the like, although there is a decrease in capacity as compared with lithium metal and lithium alloy, cycle stability Something has 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 carbon material obtained from graphitizable carbon has a narrow surface spacing, 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, and the apparent capacity. Is presumed to decrease.

  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 and manufacturing method thereof)
The carbon material for an electrode according to the present invention is produced using a graphitizable carbon material. As the starting material for the graphitizable carbon, petroleum coke, coal coke, petroleum pitch (tar), coal pitch (tar), mesophase carbon, polyvinyl chloride, polyimide and the like are used. In addition, these starting materials may be used alone or in a mixture of a plurality of types.

  The graphitizable carbon raw material is heat-treated and carbonized to obtain graphitizable carbon. This carbonization treatment is suitably performed in an inert gas atmosphere at a temperature range of about 500 ° C to 1000 ° C. As the inert gas, nitrogen gas and rare gases such as argon gas and helium gas are preferably used.

  Next, the graphitizable carbon obtained as described above is preferably pulverized so as to have a predetermined particle size. In the case where the graphitizable carbon is obtained in the form of a powder, the pulverization treatment is not always necessary. By this pulverization treatment, the reaction of the partial oxidation treatment in the next step can be made uniform and the treatment time can be shortened. In addition, the well-known various methods can be used for the grinding | pulverization process here regardless of a dry type and wet type.

  Then, the graphitizable carbon and an alkali metal carbonate (alkali metal carbonate) are mixed and subjected to heat treatment. The mixing ratio of graphitizable carbon and alkali metal carbonate is preferably 1: 1 to 1: 4.

  When the mixing ratio of graphitized carbon and alkali metal carbonate is less than 1: 1, the amount of alkali metal that enters between carbon layers decreases, so that the resulting carbon material is a carbon material with a wide interplanar spacing and normal carbon. It is thought that materials will be mixed. For this reason, in the electrode produced with this carbon material, the output characteristic in the case of charging / discharging falls. On the other hand, even when the ratio exceeds 1: 4, there is no change in the characteristics of the obtained carbon material, but the cleaning time for obtaining the carbon material becomes long, so that the work efficiency is deteriorated.

  Moreover, as an alkali metal, potassium, sodium, lithium, rubidium, and cesium can be used, but potassium, sodium, and lithium are particularly preferable. This is because lithium, sodium, and potassium have small atomic radii and can easily enter between carbon layers.

  Further, the heat treatment performed after mixing graphitizable carbon and alkali metal carbonate is performed at a temperature equal to or higher than the temperature at which alkali metal vapor is generated in an inert gas atmosphere. The temperature range of such heat treatment is preferably in the range of 600 ° C to 1500 ° C. If it is less than 600 degreeC, an alkali metal vapor | steam will not generate | occur | produce and the effect processed with an alkali metal will not be acquired. On the other hand, if the temperature exceeds 1500 ° C., the oxidation reaction of carbon proceeds, and carbon is lost as a gas, so that the capacity decreases, which is not preferable.

  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 above steps is graphitized and an alkali metal enters between the graphite layers so that the layers are widely formed. And since an alkali metal is removed by subsequent washing | cleaning, the carbon material which consists of graphite 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.

  ADVANTAGE OF THE INVENTION According to this invention, the carbon material for electrodes of the electrochemical element which is excellent in cycle stability with high capacity | capacitance, and can respond also to charging / discharging of high output (high current density), its manufacturing method, and the electrode for electrochemical elements are provided. can do.

Example 1
Mesocarbon microbean powder, which is graphitizable carbon, and potassium carbonate powder were mixed at a weight ratio of 4: 1. This mixture powder was heat-treated at 900 ° C. for 3 hours in an argon gas atmosphere. Thereafter, the resulting treated product was washed with pure water, further preliminarily dried at 70 ° C. for 4 hours, and then dried at 120 ° C. for 12 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.

(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)
When the specific surface area and charge transfer resistance of these two electrodes were measured, the results shown in Table 1 were obtained. The charge transfer resistance was calculated by AC impedance measurement. 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.

  The charge transfer resistance is an index serving as a standard for the charge / discharge characteristics of the capacitor, and indicates that the smaller the charge transfer resistance, the faster the charge / discharge can be performed. The value of Example 1 was superior to that of Conventional Example 1 using non-graphitizable carbon, which is considered to have better charge / discharge characteristics than graphitized carbon.

Claims (5)

  It is obtained by mixing a predetermined graphitizable carbon and a predetermined alkali metal carbonate so that the mixing ratio is 1: 1 to 1: 4 and heat-treating in a range of 600 to 1500 ° C. Carbon material for electrodes of electrochemical devices.   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.   A predetermined graphitizable carbon and a predetermined alkali metal carbonate are mixed so as to have a mixing ratio of 1: 1 to 1: 4, and heat-treated in an inert gas atmosphere in a range of 600 to 1500 ° C. A method for producing a carbon material for an electrode of an electrochemical element.   The method for producing a carbon material for an electrode of an electrochemical element according to claim 3, wherein the alkali metal is any one of potassium, sodium, and lithium.   An electrode for an electrochemical device comprising the carbon material for an electrode according to claim 1 or 2.