JP3406583B2 - Graphite-carbon composite material for negative electrode of lithium secondary battery, method for producing the same, and lithium secondary battery - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a graphite-carbon composite material for a negative electrode of a lithium secondary battery, which has large capacity, high potential, excellent charge / discharge cycle characteristics, and prevents decomposition of an electrolytic solution, and a method for producing the same. And a lithium secondary battery including the composite material.

[0002]

2. Description of the Related Art As electronic devices are made smaller and lighter, higher energy density of batteries is required. Also, from the viewpoint of resource saving, there is a demand for development of a high-performance secondary battery that can be repeatedly charged and discharged. A lithium secondary battery has been proposed and developed to meet such demands. The lithium secondary battery is classified into a lithium ion secondary battery, a lithium polymer secondary battery, an all solid lithium secondary battery, etc., depending on the type of electrolyte.

Among these secondary batteries, a lithium ion secondary battery comprises a lithium compound as a positive electrode and a carbon material as a negative electrode. When this battery is charged, the carbon material is doped with lithium ions in the negative electrode to form a so-called carbon-lithium intercalation compound.
On the other hand, during discharge, lithium ions are dedoped from the layers of the carbon material, and the lithium ions move to the positive electrode again and return to the lithium compound. With such a mechanism, the lithium ion secondary battery can be repeatedly charged and discharged.

As the negative electrode of the lithium ion secondary battery,
There are graphite-based negative electrodes and carbon-based negative electrodes. The discharge capacity of the carbon-based negative electrode is 600 mAh / g or more when discharging for a long time while ignoring practicality, but is about 250 to 300 mAh / g under practical use conditions. On the contrary,
The discharge capacity of the graphite-based negative electrode is excellent at 350 to 370 mAh / g even under practical use conditions.

The carbon-based negative electrode has a lower density of carbon material and a lower discharge voltage than the graphite-based negative electrode. Therefore, in order to manufacture a battery having a high energy density, it is preferable to employ the graphite-based negative electrode.

However, when a graphite-based negative electrode is used, the solvent constituting the electrolytic solution of the battery reacts with graphite, the Coulomb efficiency of the battery is lowered, and the solvent is decomposed to generate gas. When gas is generated in the sealed battery,
There is a risk that the internal pressure of the battery will rise and the battery will explode.

As the main solvent of the electrolytic solution used in the lithium ion secondary battery, there are many carbonates such as ethylene carbonate (hereinafter abbreviated as EC) and propylene carbonate (hereinafter abbreviated as PC). In these main solvents, LiPF ₆
Alternatively, an electrolyte such as LiBF ₄ is added and mixed to form an electrolytic solution. The reason why a solvent such as EC or PC is often used as a main solvent of an electrolytic solution is that these solvents have preferable solvent characteristics such as a high relative dielectric constant and a wide operating temperature range. Among them, PC is a solvent that can be used at low temperatures.

However, when the electrolytic solution containing PC and the graphite-based negative electrode are made to coexist in the battery, PC is decomposed to generate gas. Decomposition of PC is a phenomenon that is observed only when graphite is used for the negative electrode of the battery, and is not observed when a carbon-based negative electrode is used.

Conventionally, in order to solve this problem, a method of coating the surface of graphite particles with low crystalline carbon or amorphous carbon that does not decompose PC has been proposed. In contrast,
The present inventors have discovered that the above problems can be solved by uniformly coating the entire surface of graphite particles with crystalline carbon, and filed a patent application. (JP-A-200
0-106182). That is, when a graphite-carbon composite material in which the surface of graphite particles is uniformly and completely coated with crystalline carbon by using a chemical vapor deposition method is used as a negative electrode of a lithium ion secondary battery, this negative electrode ensures decomposition of PC and the like. Suppress to. This is because even if the coating layer formed is crystalline carbon, it is not graphite and does not decompose the solvent, and since the surface of the graphite is coated with the 002 surface of the crystalline carbon, the inside of the composite material particle is It is considered that the reason is that the solvent does not penetrate. Further, a battery obtained by using this negative electrode material has a high discharge capacity, can be charged at high speed, and has an electrode performance superior to that of a conventional negative electrode material coated with low crystalline carbon.

[0010]

However, the composite material in which graphite particles are coated with crystalline carbon or low crystalline carbon has a high mechanical strength of the coating layer, and therefore the energy density of the battery cannot be increased. A new problem occurred. When a small amount of binder is added to the graphite-carbon composite material and molded with a current collector to form an electrode, the density of the composite material in the molded body (abbreviated as electrode density) determines the energy density and charge / discharge speed of the battery. It becomes a factor to do. The higher the electrode density is, the higher the energy density is. However, in order to increase the charge / discharge rate, a void for holding the electrolytic solution is required inside the molded body. Therefore, the electrode density is 1.4 to 1.
Most preferably, it is 7 g / cm ³ . Although the graphite particles themselves are relatively soft particles, the composite material in which the surface of the graphite particles is coated with carbon is proportional to the coating amount regardless of whether the coating layer is low crystalline carbon or high crystalline carbon. The rigidity is increased. For this reason, the composite material particles are unlikely to be deformed to have a high density even when pressurized, and as a result, the electrode density is 1.2.
It decreases to ⁵ to 1.35 g / cm ³ . Therefore, the above-mentioned preferable electrode density cannot be achieved, and the characteristic of high energy density originally possessed by the graphite-based negative electrode cannot be sufficiently exhibited.

The electrode density increases as the coating carbon amount of the composite material decreases. However, in this case, the coating of the graphite particles becomes incomplete, so that the function of suppressing the decomposition of the solvent becomes incomplete, and as a result, the Coulomb efficiency decreases and it becomes difficult to secure the safety of the battery. On the contrary, if the amount of coated carbon is increased, the function of suppressing the decomposition of the solvent can be secured, but the energy density cannot be increased because the abundance ratio of the graphite particles decreases.

The inventors of the present invention have conducted extensive research on these points, and as a result of suppressing the decomposition of the solvent by the carbon coating layer, if the graphite particles are once oxidized before the chemical vapor deposition treatment for forming the coating layer on the graphite particles. It has been found that the effect is enhanced. That is, when graphite particles that have been subjected to oxidation are used as nuclei and a carbon coating layer is formed on the surface by chemical vapor deposition, the effect of suppressing the decomposition of the solvent is dramatically improved, and the amount of coated carbon is significantly increased compared to the conventional case. If the amount of coated carbon can be reduced, the rigidity of the obtained composite material will be reduced and the density of the composite material will be increased easily, which will allow the electrode density of the negative electrode material to be an optimum value and the same discharge. In the case of capacity, it was learned that the volume of the battery can be reduced and the size can be reduced.

The present invention has been completed based on the above findings, and its purpose is to provide a lithium battery which has a large capacity, a high potential, excellent charge / discharge cycle characteristics, and further prevents decomposition of the electrolytic solution. Carbon-graphite composite material for secondary battery negative electrode,
It is to provide a manufacturing method thereof and a lithium secondary battery using the composite material.

[0014]

The present invention for achieving the above object is as follows.

[1] A lithium secondary battery having a coating layer of crystalline carbon on the surface of graphite particles, characterized in that carbon is chemically vapor-deposited on oxidized graphite particles using a fluidized bed reactor. Manufacturing method of graphite-carbon composite material for negative electrode.

[2] Oxidation treatment is performed on graphite particles of 600-600.
The method for producing a graphite-carbon composite material for a lithium secondary battery negative electrode according to [1], which is an air oxidation treatment performed at a temperature of 800 ° C.

[3] The graphite-carbon for a lithium secondary battery negative electrode according to [1] or [2], wherein the oxidation treatment reduces the mass of the graphite particles by 0.1 to 20 mass% by oxidizing the graphite particles. Composite material manufacturing method.

[4] Chemical vapor deposition of carbon is 900-120.
The method for producing a graphite-carbon composite material for a negative electrode of a lithium secondary battery according to any one of [1] to [3], which is performed at 0 ° C.

[5] The graphite-carbon composite material for a negative electrode of a lithium secondary battery according to any one of [1] to [4], wherein a coating layer is formed on graphite particles by 2 to 10% by mass by chemical vapor deposition of carbon. Production method.

[6] A lithium secondary battery negative electrode comprising an oxidized graphite particle and a coating layer made of crystalline carbon coating the surface of the graphite particle, the proportion of the coating layer being 2 to 10% by mass. Graphite-carbon composite material for automobiles.

[7] The lithium secondary battery negative electrode according to [6], wherein the entire surface of the graphite particles is covered with crystalline carbon such that the surface of the graphite particles and the 002 surface of the crystalline carbon of the coating layer are parallel to each other. Graphite-carbon composite material for automobiles.

[8] The ⁷ Li-NMR spectrum of the composite material in which lithium ions are intercalated shows the absorption spectrum of lithium intercalated in graphite at the position of 40 to 50 ppm of the lithium chloride standard chemical shift, and 10 to 20 ppm. The graphite-carbon composite material for a negative electrode of a lithium secondary battery according to [6] or [7], which has a composite spectrum consisting of an absorption spectrum of lithium intercalated in crystalline carbon at the position.

[0023]

[9] The density of the molded body obtained by applying a pressure of 100 to 400 kg / cm ² is 1.40 to 1.
The graphite-carbon composite material for a negative electrode of a lithium secondary battery according to any one of [6] to [8], which has a weight of 70 g / cm ³ .

[10] The graphite particles are natural graphite [6] to

The graphite-carbon composite material for a lithium secondary battery negative electrode according to any one of [9].

[11] A lithium secondary battery formed using the graphite-carbon composite material for a lithium secondary battery negative electrode according to any one of [6] to [10].

[0026]

[Function] The mechanism that links the oxidation treatment with the effect of suppressing the decomposition of the solvent is not clear. In the present invention, the coating layer formed on the surface of the graphite particles by the chemical vapor deposition treatment is crystalline carbon, and the entire surface of the graphite particles is made of crystalline carbon.
It is covered with 02 faces. It is presumed that such a characteristic of the coating layer provides an excellent effect of suppressing decomposition of the electrolytic solution solvent. The oxidation treatment itself does not give the effect of suppressing the decomposition of the solvent. That is, simply by oxidizing the graphite particles, the effect of suppressing the decomposition of the solvent is not given to the graphite particles, and the effect of the oxidizing treatment is not exhibited. The effect of oxidation treatment appears when chemical vapor deposition treatment is performed on graphite particles that have been previously oxidized to form a coating layer on the surface, and the potential effect of dramatically improving the solvent decomposition inhibiting action of the coating layer. Is.

The potential effect imparted to the graphite particles by the oxidation treatment is exhibited by covering the entire surface of the graphite particles with the 002 surface of the crystalline carbon of the coating layer, as will be described later. As a result, the effect of sufficiently suppressing the decomposition of the solvent is exhibited with a small amount of coated carbon.

In the present invention, the coating layer is formed using a fluidized bed reactor in order to reduce the amount of coated carbon. In order to exert a sufficient electrolytic solution solvent decomposition suppressing effect with a small amount of coated carbon, the coating layer formed on the surface of the graphite particles is uniform, and the entire surface of the graphite particles is made so that there is no exposed portion of the surface of the graphite particles. Must be completely covered. Such a coating layer can be realized only by a chemical vapor deposition treatment method using a fluidized bed reactor, and is difficult with other methods.

Therefore, the present invention is characterized by exhibiting a synergistic effect by combining the oxidation treatment of graphite and the chemical vapor deposition treatment using a fluidized bed reactor.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION The graphite-carbon composite material for a negative electrode of a lithium secondary battery of the present invention is a graphite-carbon composite material composed of oxidized graphite particles and a carbon coating layer covering the surface thereof. The coating layer is crystalline carbon having a predetermined molecular orientation, which is formed by a chemical vapor deposition process.

Here, the "negative electrode material" of the lithium secondary battery
As, as well as carbon-based or graphite-based negative electrode material which is the main material, there are also auxiliary materials such as paste and conductive material, in the present invention, especially when referred to as "negative electrode material" of the lithium secondary battery, Unless otherwise noted, it refers to a carbon-based or graphite-based negative electrode material as the main material.

The raw material graphite for producing graphite particles may be natural graphite or artificial graphite, but the average interplanar spacing d ₀₀₂ is 0.336.
It is desirable that the crystalline material has a high crystallinity of nm or less.
It is desirable that the raw material graphite be pulverized to have a maximum particle size of 100 μm or less. The average particle diameter is preferably 5 to 30 μm, more preferably 10 to 20 μm. The graphite may be crushed by any known method such as impact crushing method and grinding method. The bulk density of the graphite particles after pulverization is preferably 0.7 g / cm ³ or more in consideration of easy handling, but usually it is difficult to set the bulk density to 1.3 g / cm ³ or more.

The crushed graphite particles have a bulk density of 0.7.
If it is less than g / cm ³ , handle it by compacting the graphite particles with a compactor, roller mill, disc mill, or vibration mill to a bulk density of 0.7 g / cm ³ or more. Can be facilitated.
This consolidation treatment may be performed before the oxidation treatment described below or after the oxidation treatment.

In the present invention, the graphite particles are oxidized. The oxidization treatment can be performed by a wet method or a dry method.

As the wet oxidation treatment method, a mixed acid of nitric acid and sulfuric acid is used at a temperature of room temperature to about 90 ° C. for 2 to 4 hours, or an alkali catalyst is added to hydrogen peroxide at room temperature to
Examples include a method of treating at a temperature of about 90 ° C. for 2 to 20 hours. Further, it can be treated with ozone water, iodine or an aqueous solution of hypochlorous acid. In the wet oxidation treatment method, it is necessary to sufficiently remove the oxidizing agent after the oxidation. A reactor equipped with a stirrer is preferable as the reactor used for the wet oxidation treatment.

The simplest dry oxidation method is an air oxidation method in which graphite particles are heated in air. The graphite is oxidized at 600 to 800 ° C., which is the oxidation start temperature of the graphite. Oxidative combustion starts at a temperature of around 650 ° C. for highly crystalline graphite and at around 600 ° C. for low crystalline graphite. In an atmosphere with sufficient oxygen for oxidation,
The reaction time at a reaction temperature of 650 ° C. is preferably 5 to 15 minutes. The mass reduction of graphite due to oxidation is preferably 0.1 to 20 mass%.

As another dry oxidation method, there is a method using ozone, chlorine, iodine, water vapor, nitrogen dioxide, or carbon dioxide as an oxidant.

Examples of the reactor used in the dry oxidation treatment method include a rotary kiln, a stirred fluidized bed reactor, a vibrating fluidized bed reactor and the like. Even when the mass loss after the oxidation treatment is small, as in the case where the mass loss is large, the obtained composite material sufficiently develops the effect of suppressing the decomposition of the electrolytic solution. ,
Alternatively, it is speculated that it may have an effect of fixing the active radicals of graphite by forming a lactone group.

As a method for forming a coating layer of crystalline carbon on the surface of the oxidized graphite particles, a chemical vapor deposition method using a fluidized bed reactor is excellent. By this method, the surfaces of the graphite particles can be completely covered with carbon, and crystalline carbon is vapor-deposited (hereinafter abbreviated as coated carbon or vapor-deposited carbon). Further, this method is simple and can efficiently perform a large amount of chemical vapor deposition treatment.

In the fluidized bed type reactor, the fluidizing gas supplied from the lower part of the reactor imparts buoyancy to the graphite particles to form a fluidized bed of graphite in which the particles violently and irregularly move. Fluidization can be performed only by the flow of gas, but it is also possible to use a method in which a stirrer is installed inside the reaction furnace to stir the inside of the fluidized bed, a method in which the reaction furnace is vibrated using a vibrator, etc. Thereby, a more stable fluidized bed can be obtained. The volume of the graphite particles in the fluidized bed is 1.2 to 1.6 times as large as the volume when the graphite particles are stationary. The bulk density when the graphite particles are fluidized is 0.1
˜0.5 g / cm ³ is desirable.

Since the fluidized bed has very good heat transfer, it is sufficient to heat the fluidized bed to a predetermined chemical vapor deposition treatment temperature by heating it from the outside of the reaction furnace with an electric heater or the like. However, the fluidizing gas may be preheated if necessary. Further, the chemical vapor deposition treatment can be carried out batchwise or continuously.
The chemical vapor deposition treatment temperature varies depending on the type of organic substance as a carbon source used in the treatment, but is usually 850 to 120.
0 degreeC is preferable and more preferable temperature is 900-1200.
C., and even more preferable temperature is 950 to 1150.degree. When the chemical vapor deposition treatment temperature is lower than 850 ° C., the deposition rate of pyrolytic carbon is low and the chemical vapor deposition treatment requires a long time, which is not preferable. The higher the chemical vapor deposition treatment temperature, the higher the conversion rate of organic matter to carbon. However, when the chemical vapor deposition temperature exceeds 1200 ° C., vapor-deposited carbon grows into fibrous or amorphous soot, and it becomes difficult to grow into a film. Therefore, in the present invention intended to form a uniform film, it is not preferable to carry out the chemical vapor deposition treatment at a temperature higher than 1200 ° C.

Preferred organic substances as carbon sources for chemical vapor deposition are benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, diphenyl,
Naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine,
Examples thereof include 1- to 3-ring aromatic hydrocarbons such as anthracene and phenanthrene, or derivatives thereof, or a mixture thereof. Among them, derivatives having one aromatic ring such as benzene, toluene, xylene, and styrene are preferable because they hardly generate tar during the chemical vapor deposition treatment. Also, gas gas oil obtained in the coal tar distillation step, creosote oil, anthracene oil, petroleum cracked oil, naphtha cracked tar oil,
Aliphatic hydrocarbons such as methane, ethane, propane, butane, pentane and hexane, and alcohols which are derivatives of the above aliphatic hydrocarbons can be used alone or as a mixture. Furthermore, organic substances having a double bond such as acetylene, ethylene, propylene, isopropylene, and butadiene can also be used.

In the chemical vapor deposition process, an organic substance as a carbon source is supplied to a fluidized bed reactor in the form of a mixed gas diluted with an inert gas.

Examples of the inert gas include nitrogen and argon. Nitrogen is particularly preferable because it is easily available and handled. The inert gas discharges oxygen and unreacted residual organic substances from the reaction system, and also plays an important role as a fluidizing gas that forms a fluidized bed of graphite particles.

The organic matter concentration in the mixed gas has a great influence on the crystallinity and molecular orientation of the coating layer formed. The preferred concentration of organic substances in the mixed gas is 2 to 50 mol%, more preferably 5 to 33 mol%. By adjusting the concentration within the above range, it is possible to easily form a vapor-deposited carbon coating layer having predetermined crystallinity and molecular orientation. When the concentration of the organic substance in the mixed gas is less than 2 mol%, the crystallinity of the coating layer is high, but the carbon deposition rate is low and the vapor deposition process requires a long time, which is not preferable. On the other hand, when the concentration of organic substances exceeds 50 mol%, the carbon deposition rate is high. However, the crystallinity of the obtained coating layer is lowered, and the vapor-deposited carbon does not form a film but grows in a fibrous or soot shape. Therefore, it is not preferable for producing the graphite-carbon composite material of the present invention for the purpose of forming a uniform coating layer on the surface of graphite particles.

The amount of the crystalline carbon coating layer coated on the surface of the graphite particles by the chemical vapor deposition treatment is preferably 2 to 10% by mass, more preferably 3 to 10% by mass based on the obtained graphite-carbon composite material. It is 7 mass%. When the amount of the coating layer is 2% by mass or more, a solvent decomposition inhibiting effect is exhibited. When the coating carbon amount exceeds 10% by mass, the rigidity of individual particles gradually increases and it becomes difficult to increase the density, and it becomes difficult to increase the electrode density even if roll pressing is performed when preparing the negative electrode. As a result, it becomes difficult to manufacture a battery having a high energy density.

Next, the results obtained by observing the above composite material with a transmission electron microscope will be described with reference to the drawings.

FIG. 1 shows an example of the composite material of the present invention. The negative electrode material 6 shown in this schematic cross-sectional view includes substantially flat graphite particles 2 and a coating layer 4 that uniformly coats the surface thereof. FIG. 2 is an enlarged view of a portion indicated by a boundary line A in FIG. When the molecular orientations of the graphite particles 2 and the coating layer 4 are examined in detail with reference to FIG. 2, the following findings are obtained.
As shown in FIG. 2, the substantially flat graphite particles 2 have the side surfaces 8 (110 surface) covered with the side surface coating layer 14, and the upper surface 10 (002 surface) covered with the upper surface coating layer 16. .

In FIG. 2, when the upper surface coating layer 16 is observed by irradiating the electron beam X of the transmission electron microscope from the upper side to the lower side, a clear lattice image of the carbon 002 plane cannot be observed. However, according to the electron diffraction method, carbon 0
02 side is observed. On the other hand, when the side surface coating layer 14 is observed by irradiating the electron beam Y of the transmission electron microscope from the upper side to the lower side, a clear lattice image of the carbon 110 surface can be observed. Therefore, it is theoretically concluded that the side surface coating layer 14 has a crystalline structure, and that the carbon 002 plane of the side surface coating layer 14 is parallel to the graphite particle surface (side surface 8).

This observation result shows that the graphite particle surface (upper surface 1
The carbon 002 surface of 0) is covered with the carbon 002 surface of the coating layer, and further the carbon 110 of the graphite particle surface (side surface 8).
Similarly, it means that the surface is coated with the carbon 002 surface of the coating layer. Therefore, it is understood that the composite material of the present invention has a structure in which the carbon 002 surface of the coating layer is parallel to the surface of the graphite particles and the carbon 002 surface of the coating layer covers the entire surface of the graphite particles.

Further, using the above-mentioned composite material in which lithium ions were intercalated as a sample, ⁷ Li-NM
The data obtained by measuring R can be used for structural evaluation of the graphite-carbon composite material. Specifically, a battery is composed of this graphite-carbon composite material and metallic lithium, and the lithium ion is intercalated into the composite material to obtain ⁷ Li-N.
Measure the MR. A compound having a composite spectrum composed of an absorption spectrum at a chemical shift of 40 to 50 ppm and an absorption spectrum at a chemical shift of 10 to 20 ppm based on lithium chloride (0 ppm) is preferable as a lithium secondary battery negative electrode material. . Where 40 ~
The absorption spectrum at the position of 50 ppm is an absorption spectrum derived from lithium ions intercalated in the highly crystalline graphite particles. The absorption spectrum at the position of 10 to 20 ppm is an absorption spectrum derived from lithium ions intercalated in the coating layer made of crystalline carbon. The existence of these two absorption spectra, and the position of their existence, and the absence of absorption spectra at 90 to 120 ppm characterize the composite material of the present invention, and in particular, the coating layer is crystalline carbon. Is important as an indication of.

On the other hand, when lithium ions are intercalated into a composite material in which graphite particles are coated with amorphous carbon, the ⁷ Li-NMR absorption spectrum shows 10-2.
Absorption spectrum of 0 ppm is not recognized, 90 to 120
An absorption spectrum is observed at the ppm position.

Furthermore, when lithium ions are intercalated in a composite material in which graphite particles are coated with low crystalline carbon, the ⁷ Li-NMR absorption spectrum thereof shows 10
Absorption spectra may be observed at the position of 20 ppm and at the position of 90 to 120 ppm, respectively.

The average spacing d ₀₀₂ of the carbon layers deposited on the surface of the graphite particles is preferably less than 0.34 nm, and is 0.33.
The average interplanar spacing d ₀₀₂ of the coated carbon layer is more preferably 0.3352 to 0.3369n.
m, and an even more preferable average surface spacing d ₀₀₂ is 0. 335
2 to 0.3359 nm. However, the average interplanar spacing of the carbon layer does not necessarily have to be in the above range.

The method for preparing a negative electrode for a lithium ion secondary battery using the graphite-carbon composite material of the present invention as a negative electrode material is not particularly limited, but a preferable example thereof will be shown below.

To the composite material, a binder (for example, PV
DF: Polyvinylidene fluoride) is dissolved in a solvent (for example, 1-methyl-2-pyrrolidone), and the mixture is sufficiently kneaded. By this operation, a high-concentration composite material slurry containing 40% or more composite material can be prepared. This composite material slurry is applied to a current collector made of metal foil (for example, copper foil) using a doctor blade or the like for 20 to 10 minutes.
Coat to a thickness of 0 μm. By drying this, the composite material particles adhere to the metal foil current collector. Further, pressure is applied by a roll press or the like to enhance the adhesion and the electrode density. Known materials such as various pitches, rubbers, and synthetic resins can be used for the binder. Among these, PVDF, carboxymethyl cellulose (CMC) and SBR latex rubber are most suitable.

Mixing ratio of composite material and binder (mass ratio)
Is preferably 100: 2 to 100: 20.

The electrode density is adjusted to 1.4 to 1.7 g / cm ³ . This density is 22 to 36 vol% in the electrode
A space corresponding to the porosity is created, and this porosity is the optimum space capacity for holding the electrolytic solution. In the graphite-carbon composite material, the ratio of the coating layer in the composite material is 10
It is possible to achieve this electrode density if it is less than or equal to mass%.

When the coating layer is formed by subjecting the graphite particles not subjected to the oxidation treatment to the chemical vapor deposition treatment, the coating layer is 10% by mass.
In the following cases, the effect of suppressing the decomposition of the electrolytic solution, which is the main purpose of forming the coating layer, becomes insufficient, the Coulombic efficiency decreases, and the electrical characteristics cannot be sufficiently satisfied. However, according to the method of the present invention for forming a coating layer on graphite particles that have been subjected to oxidation treatment, the effect of oxidation treatment significantly improves the effect of suppressing decomposition of the electrolytic solution, and the coating layer can be made 10 mass% or less. Become. As a result, the electrode density is 1.4 to 1.
It is possible to adjust to 7 g / cm ³ .

In a practical production line of a secondary battery, a roll press is frequently used to adjust the electrode density, and the composite material is pressed by the linear pressure of the roll. It can be done with pressure. That is, it can be considered that the pressing by the roll press corresponds to the pressing by the surface pressure of 100 to 400 kg / cm ² . Therefore, it is the most preferable condition for the graphite-carbon composite material that the electrode density is 1.4 to 1.7 g / cm ³ when pressed at a pressure of 100 to 400 kg / cm ² .

The positive electrode material is not particularly limited, but a lithium-containing compound such as LiCoO ₂ , Li NiO ₂ or LiMn ₂ O ₄ , which is known to those skilled in the art, or a mixture thereof is suitable. Powdered positive electrode material, if necessary, add a conductive agent,
It can be prepared by sufficiently kneading with a solvent in which a binder is dissolved and then molding together with a current collector. The separator is also not particularly limited, and known materials such as polypropylene and polyethylene can be used.

As the non-aqueous solvent which is the main solvent of the electrolytic solution,
A known aprotic solvent having a low dielectric constant that dissolves a lithium salt may be used.

For example, ethylene carbonate, dimethyl carbonate (hereinafter abbreviated as DMC), methyl ethyl carbonate (hereinafter abbreviated as MEC), propylene carbonate, diethylene carbonate, acetonitrile, propionitrile, tetrahydrofuran, r-butyrolactone, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, sulfolane, methylsulfolane, nitromethane, N, N-dimethylformamide, dimethylsulfoxide These solvents may be used alone, or two or more kinds of solvents may be mixed and used.

The lithium salt used as the electrolyte includes LiClO ₄ , LiAsF ₆ , LiPF ₆ , and LiB.
F ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃
SO ₃ Li, can be exemplified a CF ₃ SO ₃ Li and the like, can be used alone these salts, or two or more kinds.

A lithium polymer secondary battery may be formed by using a gel electrolyte obtained by gelling a non-aqueous solvent which is a main solvent of the above-mentioned electrolytic solution and an electrolyte, or a polymer electrolyte such as polyethylene oxide or polyacrylonitrile. it can.

Furthermore, an all-solid lithium secondary battery can be manufactured by using a solid electrolyte. The structure itself of these secondary batteries is known.

[0067]

EXAMPLES Each physical property value was measured by the following methods.

Bulk Density: The sample was placed in a 100 ml glass graduated cylinder and tapped. When the volume of the sample stopped changing, the sample volume was measured, and the value obtained by dividing the sample mass by the sample volume was defined as the bulk density.

Electrode density: PVDF1100 manufactured by Kureha Chemical Co., Ltd.
Was used as the binder, the solvent was NMP (1-methyl-2-pyrrolidone), and the graphite-carbon composite material was added to the 6.6 mass% binder solution and kneaded to prepare a slurry having a concentration of 40 mass%. The amount of binder after drying is 9.0% by mass.
Is. The obtained slurry is applied on a copper foil with a doctor blade to a thickness of about 200 μm and dried,
It was densified by applying a predetermined pressure using a uniaxial press. This was cut out to 2.5 cm ² , and the sample thickness (excluding the copper foil) was measured with a micrometer. The electrode density was obtained by dividing the sample mass by the area and the thickness.

Crystal lattice constant Co (002): Rigaku X
The Cu-Kα ray was converted into Ni by using the line diffractometer LINT111.
Monochromatization was carried out and high-purity silicon was used as a standard substance for measurement by Gakshin method.

Transmission Electron Microscope and Electron Beam Diffraction: A transmission electron microscope 2000FX manufactured by JEOL Ltd. was used to perform bright field image capturing and electron beam diffraction measurement.

⁷ Li Solid State NMR: A solid nuclear magnetic resonance apparatus DSX300wb manufactured by Bruker was equipped with a multi-nuclide wide probe head, and a lithium chloride aqueous solution was used as a standard for measurement.

Surface area: Using a high-precision automatic gas adsorption device BELSORB28 manufactured by Bell Japan, the nitrogen adsorption amount was measured at the liquid nitrogen temperature by the multipoint method, and the surface area was calculated by the BET method.

Amount of deposited carbon: A thermogravimetric analyzer TGA-50 manufactured by Shimadzu Corporation was used to measure the amount of mass reduction under an air stream, and the amount of mass reduction clearly different from the graphite component was taken as the amount of carbon.

Example 1 After natural Brazilian graphite was consolidated with a vibrating rod mill,
Sieving was performed at 53 μm, and the graphite particles having a bulk density of 0.800 g / cm ³ under the sieve were subjected to an air oxidation treatment.

A rotary kiln was used for the air oxidation treatment.
While sending 25 l / min of air per kg of graphite,
The temperature was 650 ° C. and the residence time was 5 minutes. The combustion loss due to oxidation was 2.1% by mass.

60 kg of oxidized graphite was charged into a fluidized bed reactor, the temperature inside the reactor was raised to 1000 ° C. while flowing nitrogen at 50 1 / min, and then nitrogen containing 40 mol% of toluene as a carbon source. 80 l / mi of gas
The chemical vapor deposition treatment was carried out for 25 minutes while supplying the liquid at n and maintaining the fluid state.

Using the 53 μm sieve sample of the obtained graphite-carbon composite material, a battery was constructed under the conditions of Table 1 and an evaluation test as a negative electrode material was conducted. The results are shown in Table 2.

The results of observing the obtained composite material with a transmission electron microscope and electron diffraction were as described above.

Example 2 Oxidation was carried out at an air flow rate of 50 l / min per 1 kg of graphite and a residence time of 12 minutes, and combustion loss due to oxidation was 1
The same operation as in Example 1 was performed except that the amount was 0.8% by mass.

In the same manner as in Example 1, a battery was constructed under the conditions shown in Table 1 and an evaluation test as a negative electrode material was conducted. The results are shown in Table 2.

Example 3 A battery was constructed under the conditions shown in Table 1 in the same manner as in Example 1 except that the chemical vapor deposition time was changed to 15 minutes, and an evaluation test as a negative electrode material was conducted. . The results are shown in Table 2.

Comparative Example 1 The 53 μm sieve sample of the graphite particles subjected to the consolidation treatment and the air oxidation treatment used in Example 1 was evaluated as a negative electrode material under the conditions of Table 1 without chemical vapor deposition treatment. The test was conducted. The results are shown in Table 3.

From this result, it is understood that the decomposition of the solvent cannot be suppressed only by the oxidation treatment.

Comparative Example 2 The graphite particles subjected to the consolidation treatment used in Example 1 were subjected to chemical vapor deposition under the same conditions as in Example 1 without subjecting them to air oxidation treatment.

An evaluation test of the obtained graphite-carbon composite material as a negative electrode material was conducted under the conditions shown in Table 1 with respect to a 53 μm sieved sample. The results are shown in Table 3.

From these results, it is found that the composite material not subjected to the oxidation treatment has a sufficient electrode density when the coating layer is small, but the decomposition inhibiting effect of the electrolytic solution is lowered because the coating layer is small. I understand.

Comparative Example 3 The consolidation-treated graphite particles used in Example 1 were subjected to a chemical vapor deposition treatment under the same conditions as in Example 1 without subjecting them to air oxidation treatment. However, the chemical vapor deposition treatment time was 120 minutes.

An evaluation test of the obtained graphite-carbon composite material as a negative electrode material was carried out on the 53 μm sieved sample under the conditions shown in Table 1. The results are shown in Table 3.

As a result, it can be seen that when the oxidation treatment is not carried out, when the amount of coated carbon is large, the effect of suppressing decomposition of the electrolytic solution is sufficiently exhibited, but the electrode density becomes low.

[0091]

[Table 1]

[0092]

[Table 2]

[0093]

[Table 3]

[0094]

In the graphite-carbon composite material of the present invention, the graphite particles are previously oxidized, so that the carbon content of the coating layer can be reduced without decomposing the electrolytic solution. As a result, the electrode density can be increased, and therefore, when a lithium secondary battery is manufactured by using this composite material as a negative electrode material, it has a large capacity, a high potential, excellent charge / discharge cycle characteristics, can be miniaturized, and can be electrolyzed. A battery in which the decomposition of the liquid is suppressed can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an example of the graphite-carbon composite material of the present invention.

FIG. 2 is a schematic enlarged view showing the inside of a boundary line A in FIG.

FIG. 3 is a ⁷ Li-NMR absorption spectrum of the composite material of the present invention of Example 1 in which lithium ions are intercalated.

[Explanation of symbols]

2 Graphite particles 4 coating layer 6 Graphite-carbon composite material 8 sides 10 Top surface 14 Side coating layer 16 Top coating layer X, Y electron beam

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Yoshinori Yasumoto Inventor Yoshinori Yasumoto 1-3 Hibyocho, Wakamatsu-ku, Kitakyushu, Mitsui Mining Co., Ltd. Research Institute (72) Yoichiro Hara 1 Hibiki-cho, Wakamatsu-ku, Kitakyushu, Fukuoka 3-chome, Mitsui Mining Co., Ltd. (56) References JP 10-12241 (JP, A) JP 2000-106182 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) H01M 4/36-4/62 H01M 4/02-4/04 H01M 10/40

Claims (10)

(57) [Claims] 1. A lithium secondary which forms a coating layer of 2 to 10% by mass of crystalline carbon on the surface of graphite particles by chemically vapor-depositing carbon on the oxidized graphite particles using a fluidized bed reactor. Manufacturing method of graphite-carbon composite material for battery negative electrode. 2. The oxidation treatment is performed on graphite particles by 600 to 800.
The method for producing a graphite-carbon composite material for a lithium secondary battery negative electrode according to claim 1, which is an air oxidation treatment performed at a temperature of ° C. 3. The graphite-carbon composite material for a lithium secondary battery negative electrode according to claim 1, wherein the oxidation treatment reduces the mass of the graphite particles by 0.1 to 20 mass% by oxidizing the graphite particles. Production method. 4. The method for producing a graphite-carbon composite material for a lithium secondary battery negative electrode according to claim 1, wherein the chemical vapor deposition of carbon is performed at 900 to 1200 ° C. 5. Oxidized graphite particles and a fluidized bed reactor
Forming the entire surface of the graphite particles by chemical vapor deposition using a
A graphite-carbon composite material for a negative electrode of a lithium secondary battery, comprising a coating layer made of crystalline carbon coated on the 002 surface of crystalline carbon, the proportion of the coating layer being 2 to 10% by mass. 6. The negative electrode for a lithium secondary battery according to claim 5, wherein the entire surface of the graphite particles is coated with crystalline carbon such that the surface of the graphite particles and the 002 plane of the crystalline carbon of the coating layer are parallel to each other. Graphite-carbon composite material. 7. The ⁷ Li-NMR spectrum of the composite material in which lithium ions are intercalated has an absorption spectrum of lithium intercalated in graphite at a position of 40 to 50 ppm of a lithium chloride standard chemical shift, and a 10 Li-NMR spectrum of 10 to 20 ppm. 7. The graphite-carbon composite material for a lithium secondary battery negative electrode according to claim 5, which has a composite spectrum composed of an absorption spectrum of lithium intercalated in crystalline carbon at a position. 8. The density of a molded body obtained by pressurizing at a pressure of 100 to 400 kg / cm ² is 1.40 to 1.70 g.
/ Cm ^{3 The} graphite-carbon composite material for a negative electrode of a lithium secondary battery according to any one of claims 5 to 7. 9. The graphite-carbon composite material for a negative electrode of a lithium secondary battery according to claim 5, wherein the graphite particles are natural graphite. 10. A lithium secondary battery formed by using the graphite-carbon composite material for a lithium secondary battery negative electrode according to claim 5.