JP2003100292A - Carbon material for negative electrode and manufacturing method thereof, and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon material for a negative electrode which is basically composed of artificial graphite and natural graphite and is applied to a lithium ion secondary battery of high capacity and high efficiency, and to provide the lithium ion secondary battery using the carbon material for the negative electrode. SOLUTION: At least either of the artificial graphite or natural graphite is mixed with a pitch in a weight ratio of 95/5 to 50/50 to form a mixture, which is then subjected to a heat treatment at a temperature of 600 to 800 deg.C under an inert atmosphere, thus obtaining a carbon material for the negative electrode. The carbon material is used for the negative electrode of the lithium ion secondary battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon material for a negative electrode used in a lithium ion secondary battery and the like, a method for producing the same, and a lithium ion secondary battery using the carbon material for a negative electrode. More specifically, mobile phones, notebook P
The present invention relates to a carbon material, a method for producing the same, and a lithium ion secondary battery using the carbon material, which are particularly preferably used as a power source for small portable devices such as C and information devices.

[0002]

_2. Description of the Related Art A lithium-ion secondary battery is a non-aqueous secondary battery that uses a carbon material such as graphite for its negative electrode and a lithium composite oxide such as LiCoO ₂ or LiNiO _{2 for} its positive electrode. The market is rapidly expanding in the fields of small mobile devices such as mobile phones and notebook PCs, and information devices by taking advantage of the features of.

In these fields, there is a continuous demand for higher capacity batteries due to the need for higher functionality and smaller size and lighter weight of equipment. For this reason, research is being carried out to improve the capacity and efficiency of the negative electrode graphite material.

In this flow, JP-A-4-368778, JP-A-4-370662, JP-A-5-94838, and JP-A-5-1
In Japanese Patent No. 21066, Japanese Patent Laid-Open No. 9-213328, and the like, a carbon material in which the surface of graphite particles is coated with amorphous carbon is proposed. These surface-modified graphite-based materials suppress decomposition of the electrolytic solution, and are effective for increasing battery capacity and improving cycle characteristics.

According to the technique described in JP-A-4-368778, a technique for forming a carbon coating layer by thermal decomposition of hydrocarbon gas is disclosed in JP-A-4-370662 and JP-A-5-94838. Japanese Patent Laid-Open No. 5-121066 discloses a technique of mixing a liquid phase organic compound and graphite particles and firing the mixture.
Japanese Patent Publication No. 14 describes a method of adding an aromatic organic solvent to a liquid phase organic compound and graphite particles, adjusting the viscosity, mixing and heating, and then firing. The technique of coating the surface of these graphites with amorphous carbon is generally performed at a high temperature exceeding 800 ° C.

[0006]

The graphite-based carbon material obtained by coating the surface of the graphite-based material with the carbon material is made of inexpensive natural graphite or artificial graphite as a raw material, and has a theoretical capacity of 372 mAh / g. It is a material that is useful as a negative electrode for lithium-ion secondary batteries because it can obtain near capacity with high efficiency (85% or more). However, with the recent demand for higher capacity batteries, these carbon materials are required to have higher capacities. Further, in the future, when it is applied to a large-sized lithium ion secondary battery, a material that is easier to manufacture and that is low in cost is required.

An object of the present invention is to provide a negative electrode carbon material which is suitable for a high capacity and high efficiency lithium ion secondary battery based on artificial graphite and natural graphite, and a lithium ion secondary battery using the same. It is in providing. A further object of the present invention is to provide a method for producing a carbon material for a lithium ion secondary battery, which is based on artificial graphite or natural graphite and can be manufactured at low cost and is easy to produce.

[0008]

The carbon material for a negative electrode of the present invention is prepared by mixing artificial graphite, natural graphite or a mixture thereof with pitch at 95/5 to 50/50 (weight ratio), and under an inert atmosphere. 6
It is obtained by heat treatment at a temperature of 00 ° C to 800 ° C.
Further, the method for producing a carbon material for a negative electrode of the present invention, artificial graphite, natural graphite or a mixture thereof, after solid phase mixing of solid pitch at room temperature, under an inert atmosphere, 600 ℃ or more 800
This is a method of heat treatment at a temperature of ℃ or less.

The lithium ion secondary battery of the present invention uses the above carbon material for a negative electrode as a negative electrode.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon material for a negative electrode of the present invention is a mixture of artificial graphite, natural graphite or a mixture thereof and pitch at a weight ratio of 95/5 to 50/50, and 600 ° C. or more in an inert atmosphere 80
It can be obtained by heat treatment at 0 ° C or lower. This carbon material has a capacity exceeding the theoretical capacity of graphite of 372 mAh / g and 85%.
Has an efficiency exceeding. Further, the carbon material of the present invention has a small specific surface area and is stable to an electrolytic solution.
Since it is based on inexpensive artificial graphite and natural graphite and has a low heat treatment temperature of 600 ° C to 800 ° C, it is a more superior material in terms of cost.

In the present invention, the artificial graphite and natural graphite used as the raw material are not particularly limited, but for example, graphitized MCMB
Those having a larger capacity and lower cost than other graphite carbon materials such as (mesocarbon microbeads) are particularly preferably used.

The average particle size of the artificial graphite and natural graphite as the raw materials is preferably 1 to 50 μm, more preferably 5 to 35 μm.
m. When the average particle size exceeds the upper limit, it is necessary to pulverize for electrode processing after heat treatment, and the pulverization may cause active points in the graphite particles, which may reduce the efficiency. On the other hand, when the amount is less than the lower limit, it is difficult to process the electrode, and there is a possibility that problems may occur in the electrode strength and the like.

The pitch used in the present invention is not particularly limited, and examples thereof include coal pitch, petroleum pitch, naphthalene pitch, and the like. Petroleum pitch is preferable, and petroleum isotropic pitch is more preferable. preferable.

These pitches are mixed with artificial graphite, natural graphite or a mixture thereof and heat treated. The mixing method is not particularly limited. For example, (1) solid powdery pitch and artificial graphite, natural graphite or a mixture thereof are mixed, (2) liquid pitch and artificial pitch by heating or the like. Examples thereof include a method of mixing graphite, natural graphite or a mixture thereof, or (3) a method of mixing pitch dissolved in a solvent with artificial graphite, natural graphite or a mixture thereof. Of the above-exemplified methods, artificial graphite such as (1) above,
A method in which natural graphite or a mixture thereof and solid pitch are solid-phase mixed at room temperature is preferable because secondary aggregation after heat treatment is small and the manufacturing operation is easy.

When artificial graphite, natural graphite or a mixture thereof and solid pitch are solid-phase mixed at room temperature, the average particle size of the pitch is preferably 1 to 50 μm, more preferably 5 to 35 μm. When the average particle size exceeds the upper limit, it is necessary to pulverize for electrode processing after heat treatment, and the pulverization may cause active points in the graphite particles, which may reduce the efficiency. Further, if it is less than the lower limit, it may be difficult to process the electrode and a problem may occur in the electrode strength and the like, depending on the pitch mixing ratio.

In the present invention, the mixing ratio of artificial graphite, natural graphite or a mixture thereof and pitch is 95% by weight.
/ 5 to 50/50, more preferably 90/10 to 60/40. The above mixing ratio is appropriately determined within the above range depending on the composition of pitch, properties such as softening point, properties such as specific surface area of artificial graphite and natural graphite, properties such as average particle size, and target capacity and efficiency. If the mixing amount of pitch is less than the above range, sufficient capacity cannot be obtained, and if it exceeds the above range,
It is not preferable because the efficiency of the obtained material decreases.

In the present invention, the heat treatment of pitch with artificial graphite, natural graphite or a mixture thereof is performed at 600 ° C. or higher in an inert gas stream such as nitrogen or argon, in an inert gas closed state, in an inert atmosphere such as a vacuum state. The temperature is 800 ° C or lower, preferably 600 ° C or higher and 700 ° C or lower. When the heat treatment temperature exceeds the above range, the capacity decreases, and when the heat treatment temperature falls below the above range, the capacity or efficiency decreases, which is not preferable.

The heating rate in the heat treatment is not particularly limited, but is appropriately selected from the range of, for example, about 1 ° C./hr to 300 ° C./hr. The heat treatment time is not particularly limited, but is preferably about 3 hours to 100 hours. As described above, the heat treatment of the present invention is sufficiently lower than the treatment temperature (the temperature exceeding 800 ° C.) of the conventional coating carbon material, so that the facility cost and the manufacturing cost are low.

The carbon material obtained by heat treatment of pitch and artificial graphite, natural graphite or mixtures thereof in the present invention has a specific surface area by the BET method, 0.1 m ² / g or more 10 m ² /
It is preferably not more than g, more preferably 0.1 m ² /
g or more and 5 m ² / g or less, more preferably 0.1 m ² / g or more 3
m ² / g or less.

The specific surface area of the carbon material can be appropriately controlled according to the properties such as the softening point of the pitch used, the specific surface area of artificial graphite and natural graphite, the heat treatment temperature, etc., and the specific surface area is 10 m ² / g. When it exceeds, the efficiency of the obtained carbon material tends to decrease.

The carbon material of the present invention can be used for a negative electrode for a lithium ion secondary battery. The lithium ion secondary battery using the carbon material of the present invention as the negative electrode will be described below.

A lithium ion secondary battery using the carbon material of the present invention as a negative electrode comprises a positive electrode, a negative electrode using the carbon material of the present invention, and a non-aqueous electrolyte in which a lithium salt is dissolved. The positive electrode material is not particularly limited as long as it is a lithium-based positive electrode material, and lithium composite cobalt oxide, lithium composite nickel oxide, lithium composite manganese oxide, or a mixture thereof, and further, a metal element different from these composite oxides. It is possible to use a system in which one or more of the above are added.

The positive electrode or the negative electrode is a known binder,
After mixing with a conductive agent or the like, an active material layer is formed on the current collector. The binder is not particularly limited, and fluorine-based polymers such as polytetrafluoroethylene and polyvinylidene fluoride, polyolefin-based polymers such as polyethylene and polypropylene, and synthetic rubbers can be used.

As a method for forming the electrode, a paste in which an active material and a binder are mixed is prepared, and an active material layer is formed on a current collector by a doctor blade, a bar coater or the like, or Examples thereof include a method in which a mixture with a binder is put in a molding machine or the like and a molded body is obtained by pressing or the like. Here, examples of the current collector include known foils of metal such as copper, nickel, stainless steel, aluminum, and titanium, meshes, and porous bodies.

As the non-aqueous electrolyte in which the lithium salt is dissolved, a known non-aqueous electrolyte containing a lithium salt can be used, which is appropriately determined depending on the use conditions such as the positive electrode material, the negative electrode material and the charging voltage. More specifically, LiPF ₆ , L
A lithium salt such as iBF ₄ or LiClO _{4 is} mixed with propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, dimethoxyethane, γ-butyl lactone, methyl acetate, methyl formate, or a mixed solvent of two or more of these. Examples thereof include those dissolved in the organic solvent.

The concentration of the electrolytic solution (electrolyte) is not particularly limited, but generally 0.5 to 2 mol / l is practical, and the electrolytic solution naturally has a moisture content. Is 10
It is preferable to use one having a content of 0 ppm or less. The non-aqueous electrolyte used in the present specification means a concept including a non-aqueous electrolyte solution and an organic electrolyte solution, and also a concept including a gel or solid electrolyte.

Further, as the separator for holding the electrolytic solution, a known electrically insulating synthetic resin fiber,
Examples include non-woven fabrics or woven fabrics such as glass fibers and natural fibers, and powder compacts such as alumina. Among these, non-woven fabrics made of synthetic resins such as polyethylene and polypropylene are preferable in terms of quality stability.

Some of these synthetic resin non-woven fabrics have a function of separating the positive electrode and the negative electrode by melting the separator by heat when the battery abnormally heats up.
From the viewpoint of safety, these can also be preferably used.

The thickness of the separator is not particularly limited,
It is only necessary to be able to hold the required amount of electrolyte and to prevent a short circuit between the positive electrode and the negative electrode, usually 0.01 mm
It is about 1 mm, preferably about 0.02 mm to 0.05 mm.

[0030]

The present invention will be further described below with reference to examples, but the present invention is not limited to these examples.

<Measurement of Average Particle Size> Laser particle size distribution measuring device SALD-2000J (manufactured by Shimadzu Corporation) was used for measurement. The sample was ultrasonically dispersed in water using a dispersant and then measured.

<Measurement of Specific Surface Area> Specific Surface Area Measuring Device NOVA-1
It measured using 200 (made by Yuasa Ionics). As a pretreatment, the sample was vacuum dried at 200 ° C for 20 minutes, the nitrogen isotherm was measured at 77K, and the specific surface area was determined by the BET method from the obtained adsorption isotherm.

<Electrochemical characteristics> (1) Electrode trial production PVDF in N-methyl-2-pyrrolidone (hereinafter referred to as NMP) solution (concentration 6% by weight) 18.3 g and carbon material 10 g in a mortar 10
After mixing for about 1 minute, about 0.5 g of NMP was further added, and after mixing for 30 minutes, an electrode slurry was obtained. The electrode slurry was applied to a copper foil having a thickness of 14 μm, dried and pressed to obtain an electrode having a thickness of 100 μm. 1.5 cm square (amount of active material: approx. 1
0 mg / cm ² ) was used. (2) Creation of evaluation cell The evaluation cell (3 pole type) is 1.5 cm square (active material amount: about 10 mg / cm ² ).
The electrode cut into 2 was used as a working electrode, and metallic lithium foil was used as a counter electrode and a reference electrode. The electrolyte was a solution prepared by dissolving 1 mol / l LiPF ₆ in a solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 7, and the assembly was performed in a dry box. (3) Measurement of capacity and efficiency Charge is constant current-constant voltage method (current: 50mA / g, voltage 1mV vs. L
i / Li ⁺ , 12 hours), constant discharge (2.0V cutoff)
And was carried out at 25 ° C. The capacity was evaluated by the discharge capacity, and the efficiency was calculated by [discharge capacity / charge capacity] × 100 (%).

Example 1 Natural graphite has a specific surface area of 4.1 m ²
/ g, average particle size 27.2 μm, pitch has a softening point of 265
A petroleum-based isotropic pitch at ℃ was used. After 30 parts by weight of pitch powder and 90 parts by weight of natural graphite are solid-phase mixed at room temperature, they are placed in a ceramic dish and placed in a small cylindrical electric furnace (core tube inner diameter 10
0 mm) was used for heat treatment. Temperature rise from room temperature to 635 ° C
The temperature was maintained at 635 ° C. for 4 hours, then naturally cooled to 60 ° C., and taken out of the furnace. The heat treatment is performed in a nitrogen atmosphere at normal pressure, and the nitrogen flow rate is 0.5 l.
/ min. The carbon material for negative electrode obtained by the heat treatment was passed through a sieve having an opening of 45 μm to prepare an evaluation electrode. Average particle size (μm), specific surface area of carbon material for negative electrode (m ² / g), capacity (m
Ah / g) and efficiency (%) are shown in Table 1.

Example 2 A carbon material for a negative electrode was obtained in the same manner as in Example 1 except that a coal-based isotropic pitch having a softening point of 80 ° C. was used. An evaluation electrode was prepared using the obtained carbon material for a negative electrode. Table 1 shows the average particle size, the specific surface area of the carbon material for the negative electrode, the capacity, and the efficiency.

Example 3 The same operation as in Example 1 was carried out except that petroleum pitch having a softening point of 265 ° C. was used as the pitch and the mixing ratio was 10 parts by weight of pitch powder and 90 parts by weight of natural graphite. A carbon material for negative electrode was obtained. An evaluation electrode was prepared using the obtained carbon material for a negative electrode. Table 1 shows the average particle size, the specific surface area of the carbon material for the negative electrode, the capacity, and the efficiency.

Example 4 The same operation as in Example 1 was carried out except that petroleum pitch having a softening point of 265 ° C. was used as the pitch and the mixing ratio was 40 parts by weight of pitch powder and 60 parts by weight of natural graphite. A carbon material for negative electrode was obtained. An evaluation electrode was prepared using the obtained carbon material for a negative electrode. Table 1 shows the average particle size, the specific surface area of the carbon material for the negative electrode, the capacity, and the efficiency.

[Comparative Example 1] A carbon material for comparison was obtained in the same manner as in Example 1 except that only the natural graphite used in Example 1 was used. An evaluation electrode was prepared using the obtained carbon material. Table 1 shows the average particle size, the specific surface area of the carbon material, the capacity, and the efficiency.

Comparative Example 2 A carbon material for comparison was obtained in the same manner as in Example 1 except that petroleum pitch having a softening point of 265 ° C. was used as the pitch and the heat treatment temperature was 1000 ° C. An evaluation electrode was prepared using the obtained carbon material. Table 1 shows the average particle size, the specific surface area of the carbon material, the capacity, and the efficiency.

[Comparative Example 3] Graphitized MCM manufactured by Osaka Gas Co., Ltd.
The same operation as in Example 1 was carried out except that B was used to obtain a carbon material for comparison. An evaluation electrode was prepared using the obtained carbon material for comparison. Table 1 shows the average particle size, the specific surface area of the carbon material, the capacity, and the efficiency.

[0041]

[Table 1]

According to Examples 1 to 4, graphite and pitch are 95/5.
Carbon materials obtained by mixing at ~ 50/50 (weight ratio) and heat-treating at 600 ℃ or more and 800 ℃ or less in an inert atmosphere can achieve 88% or more efficiency at a capacity of 380mAh / g or more. I understand. Further, it is clear that when the carbon material for the negative electrode is only graphite, or when the heat treatment temperature deviates from a predetermined temperature, the theoretical capacity of graphite cannot exceed 372 mAh / g.

[0043]

As described above, the carbon material for a negative electrode of the present invention is prepared by mixing at least one of artificial graphite and natural graphite with pitch at a weight ratio of 95/5 to 50/50, and under an inert atmosphere. The structure is obtained by heat treatment at 600 ° C or higher and 800 ° C or lower.

Therefore, it is possible to provide a carbon material for a negative electrode which is suitable for a high capacity and high efficiency lithium ion secondary battery based on artificial graphite or natural graphite, and a lithium ion secondary battery using the same. Produce an effect.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Koichi Morita             Osaka Prefecture Osaka City Torishima Island 5-11-151 Osaka Gas             Within Chemical Co., Ltd. F-term (reference) 4G046 CA00 CA07 CB08 CC02 CC03                 5H029 AJ03 AJ14 AK03 AL07 AL08                       AM03 AM04 AM05 AM07 CJ02                       CJ08 CJ28 HJ01 HJ07 HJ14                 5H050 AA08 AA19 BA17 CA07 CA08                       CA09 CB08 CB09 EA10 GA02                       GA10 GA27 HA01 HA07 HA14

Claims (7)

[Claims] 1. At least one of artificial graphite and natural graphite and pitch are mixed in a weight ratio of 95/5 to 50/50, and heat treated at 600 ° C. or higher and 800 ° C. or lower in an inert atmosphere. The obtained carbon material for negative electrodes. 2. At least one of artificial graphite and natural graphite and pitch are mixed in a weight ratio of 95/5 to 50/50, and heat-treated at 600 ° C. or more and 700 ° C. or less in an inert atmosphere. The obtained carbon material for negative electrodes. 3. The mixing ratio of at least one of artificial graphite and natural graphite and pitch is 90/10 as a weight ratio.
The carbon material for a negative electrode according to claim 1 or 2, wherein the carbon material is 60/40. 4. The carbon material for a negative electrode according to claim 1, which has a specific surface area of 0.1 m ² / g or more and 3 m ² / g or less. 5. An artificial graphite and / or a natural graphite, and a pitch which is solid at room temperature are solid-phase mixed and then heat-treated at 600 ° C. or more and 800 ° C. or less in an inert atmosphere. A method for producing a carbon material for a negative electrode. 6. At least one of artificial graphite and natural graphite, and a pitch which is solid at room temperature are solid-phase mixed, and then heat-treated at 600 ° C. or more and 700 ° C. or less in an inert atmosphere. A method for producing a carbon material for a negative electrode. 7. A lithium ion secondary battery comprising the negative electrode carbon material according to claim 1 for a negative electrode.