JPH08273666A - Lithium ion secondary battery and negative electrode material thereof - Google Patents

Abstract

PURPOSE: To provide a lithium ion secondary battery which has large energy density and electric capacity according to a theoretical value and has the long cycle service life by specifying a crystal lattice constant, a thickness, a maximum particle length and the ratio of volume resistivity of a graphite particulate. CONSTITUTION: In an material for a lithium ion secondary battery molded by a graphite particulate, a flaky graphite particulate on which a crystal lattice constant Co(002) of a graphite crystal 002 surface of the graphite particulate is 0.670 to 0.673nm and a thickness of the graphite particulate is not more than 1μm and a maximum particle length is not more than 100μm and a value of the ratio of volume resistivity in the molding pressure vertical direction and the molding pressure parallel direction when the graphite particle is molded in 500kg/cm<2> is 1×10<-6> to 1×10<-4> , is provided. Therefore, a lithium ion secondary battery which has large capacity and has high electric potential and is excellent in a charge-discharge cycle characteristic can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium ion secondary battery having a large capacity, a high potential and excellent charge / discharge cycle characteristics, and a negative electrode material for the secondary battery.

[0002]

2. Description of the Related Art As electronic devices are made smaller and lighter, higher energy density of batteries is required, and from the viewpoint of resource saving, there is an urgent need to develop a secondary battery which can be repeatedly charged and discharged. To meet this demand, a lithium ion secondary battery having a high energy density, a light weight, a small size, and excellent charge / discharge cycle characteristics has been proposed. Lithium-ion secondary batteries were developed to solve the problems of lithium metal secondary batteries, such as poor chargeability, short cycle life, and poor safety. In contrast to the case where metallic lithium was used for the negative electrode, a carbon material is used for the negative electrode in the case of a lithium ion secondary battery to solve the above problems.

That is, when a lithium compound is used as a positive electrode and a carbon material is used as a negative electrode for charging, lithium ions are doped into the carbon material at the negative electrode to form a so-called carbon-lithium intercalation compound. On the other hand, during discharge, lithium ions are dedoped from the interlayer of the carbon material, and the lithium ions are recombined with the lithium compound of the positive electrode. As a result, a chargeable / dischargeable battery is formed.

When graphite, which is a carbon material, is used as a negative electrode material of a lithium ion secondary battery, theoretically a graphite-lithium ion intercalation compound is formed in which 6 carbon atoms are doped with 1 lithium atom. It At this time, the electric capacity per carbon weight is 372 mA · h / g. However, the electric capacity of the graphite negative electrode material used in commercially available lithium ion secondary batteries is 12
It is generally from 0 to 160 mA · h / g, which is only about 40% of the theoretical value.

[0005]

As a result of various investigations on carbon materials suitable for the negative electrode material of lithium-ion secondary batteries, the present inventors have found that carbon having a large lithium doping amount, a large discharge capacity and a long cycle life is used. It has been found that the material is highly crystalline and has a large external surface area.
That is, by using highly crystalline flaky graphite fine particles as a negative electrode material of a lithium ion secondary battery, a large energy density, which is the object of the present invention, and an electric capacity almost equal to the theoretical value are provided, and the cycle life is long. Further, they have found that a lithium ion secondary battery capable of rapid charge and discharge can be configured, and have completed the present invention. Therefore,
The purpose is to improve the carbon material used as the negative electrode, so that lithium ions are doped and dedoped at the low potential that graphite originally has, resulting in a large energy density and almost the theoretical value. It is an object of the present invention to provide a negative electrode material for a secondary battery capable of producing a lithium ion secondary battery having the above electric capacity and a long cycle life, and a lithium ion secondary battery incorporating the negative electrode material.

[0006]

In order to achieve the above object, the present invention provides a negative electrode material for a lithium-ion secondary battery formed by molding graphite fine particles, wherein the graphite fine particles have a crystal lattice constant C of the 002 plane of the graphite crystal. _{0 (002)} is 0.670-0.
673 nm, the thickness of the graphite fine particles is 1 μm or less, the maximum particle length is 100 μm or less, and the graphite fine particles are 500 μm or less.
The value of the ratio of the volume resistance in the direction perpendicular to the molding pressure and the volume resistance in the direction parallel to the molding pressure when molded at kg / cm ² is 1 × 10 ⁻⁶ to 1 × 10.
^-4 is a flaky graphite fine particle, which is a negative electrode material for lithium-ion secondary batteries, characterized in that the flaky graphite fine particle is obtained by ultrasonically crushing expanded graphite in a dispersion solvent. The flaky graphite fine particles are obtained by wet-grinding expansive graphite with a medium, and the flaky graphite fine particles are wet-grinding with expansive graphite with a pair of disk-shaped rotary grindstones. The flaky graphite fine particles are obtained by pulverizing expanded graphite in a dispersion solvent with ultrasonic waves, and flakes obtained by wet-grinding expanded graphite with a medium. Flake graphite fine particles obtained by grinding wet fine graphite fine particles or expanded graphite with a pair of rotary disc-shaped grindstones are further added to 2000 to 280.
Including those obtained by annealing at 0 ° C.

Further, the present invention is a lithium ion secondary battery incorporating the above negative electrode material.

[0008]

In order for the carbon material used as the negative electrode material of the lithium ion secondary battery to form a graphite-lithium intercalation compound having a theoretical value of C ₆ Li, it is desired that the crystallinity of graphite is extremely high. In addition, in order to perform quick charging, it is desirable that the carbon material be fine particles and have an outer surface area as large as possible. The carbon material for negative electrodes used in the present invention is flaky graphite fine particles, which are exfoliated and finely divided without impairing the high crystallinity of natural graphite. As a result, when the flaky graphite fine particles are molded and used for an electrode, it becomes possible to do the lithium ion doping almost as the theoretical value, and because of the large outer surface area, the lithium ion doping and the dope dope are smoothly performed. A large charge / discharge capacity and short-time charge are possible.

The present invention will be described in detail below.

The negative electrode material of the lithium ion secondary battery of the present invention
The carbon material used in the material is highly crystalline flaky graphite fine particles.
Is. Crystal lattice constant of 002 plane of the flaky graphite fine particles
C_{0 ( 002)}Is 0.670 nm to 0.673 nm
Is desirable. Crystal lattice constant C _{0 (002)}Is 0.673 nm
If it exceeds, the doping amount of lithium ions will be insufficient,
Not only can you not get enough charge,
Since the discharge rate (efficiency) becomes low, lithium ion
The electric capacity of the secondary battery tends to decrease.

The flaky graphite fine particles are graphite having extremely high crystallinity when judged from the crystal lattice constant. This is an extremely thin layered graphite structure obtained by exfoliating the flaky graphite fine particles along the AB plane of the expanded graphite having a weakened binding force by expanding in the C-axis direction of the graphite without impairing the crystallinity of the graphite. It is due to The thickness of the flaky graphite fine particles is 1 μm
Or less, preferably 0.1 μm or less, and the maximum particle length is 100 μm or less, preferably 50 μm or less.

As described above, the flaky graphite fine particles have a high aspect ratio. Assuming that the average particle size of the flaky graphite fine particles is 0.1 μm in thickness and the maximum length is 20 μm, the flaky graphite fine particles are approximately 14 times as large as a sphere having the same volume, and also approximately as large as a cube. It has 12 times the outer surface area. This large external surface area effectively contributes to the rapid progress of lithium ion doping and dedoping. Moreover, since the crystallinity is high, the doping of lithium ions is performed in stage 1, and lithium ions of approximately the theoretical value are introduced into the flaky graphite fine particles. Further, since the particles are fine particles, the outer surface area is increased, and the doping is completed quickly. From these facts, when the flaky graphite fine particles are molded and used as a negative electrode, a lithium ion secondary battery having a high electric potential and an electric capacity almost as the theoretical value and having a long cycle life and capable of rapid charge and discharge is obtained. A battery is obtained.

A feature of charge / discharge of a lithium ion secondary battery in which the flaky graphite is molded into a negative electrode is that a stepwise change in electric potential is recognized. This seems to correspond to the stages of doping (intercalation) and dedoping (disintercalation) of the thin flaky graphite particles with lithium ions.

The flaky graphite fine particles used in the present invention can be produced from expanded graphite. The method for producing expanded graphite is not particularly limited, but expanded graphite is, for example, a method of treating a graphite material such as natural flake graphite, Kish graphite, or highly crystalline pyrolytic graphite with a mixed acid of sulfuric acid and nitric acid. Or an intercalation compound of graphite-sulfuric acid obtained by electrochemically oxidizing graphite in sulfuric acid or an intercalation compound of graphite-organic substance such as graphite-tetrahydrofuran in an externally heated or internally heated furnace, and further by laser heating. It can be produced according to a known method such as rapid heat treatment for expansion by the above method.

The bulk density of the expanded graphite particles to be used varies depending on the manufacturing method of the expanded graphite, storage or transportation, handling method, etc., but it is easy to immerse the dispersion solvent, and preferable performance requirements of the obtained flaky graphite fine particles. From a viewpoint of 0.01 g /
cm ³ or less, more preferably 0.008 g / cm ³ or less.

The flaky graphite fine particles are obtained by exfoliating expanded graphite produced by such a method into fine particles. Examples of the method of forming fine particles include a method of crushing expanded graphite by using ultrasonic waves, a method of grinding by using a medium, and the like.

The method of crushing using ultrasonic waves is a method of immersing expanded graphite in a liquid and irradiating it with ultrasonic waves.
As the liquid, in addition to water, ketones such as acetone, alcohols such as methyl alcohol, ethyl alcohol, butyl alcohol, and paraffinic solvents such as hexane can be used, and among them, water or alcohols is preferable. The irradiation power and the irradiation amount are sufficient for crushing into the shape of the graphite fine particles specified in the present invention, and these should be appropriately determined in practice.

The method of grinding using media is to use a ball having high wear resistance such as steel balls or ceramic balls such as alumina, or rod-shaped steel or ceramics, and wet with a grinding machine such as a ball mill or a Henschel mixer. The crushed expanded graphite is.

The graphite fine particles obtained by crushing with ultrasonic waves are bulky and have difficulty in handling, but they can be used as the flaky graphite fine particles of the present invention by consolidating them. The thickness of the flaky graphite fine particles prepared by the ultrasonic method is usually 0.1 μm or less, and the feature of this method is that the flaky graphite fine particles having a large aspect ratio can be obtained.

Further, graphite fine particles obtained by a method of grinding expanded graphite using a medium are also preferable. If the grinding time is long, the crystallinity of graphite may be impaired. Even if the crystallinity is somewhat impaired, the graphite crystal lattice constant C _{0 (002)} is 0.673 nm or less, and Thickness is 1μm or less, maximum particle length is 100μ
If it is a flaky sheet having a size of m or less, it may be used. If the maximum particle length exceeds 100 μm, it is difficult to reduce the thickness of the graphite fine particles to 1 μm or less, and the aspect ratio of the graphite fine particles decreases. As a result, the outer surface area is reduced, and rapid charging / discharging or doping with a theoretical amount of lithium ions is difficult to perform, which makes it difficult to manufacture a lithium ion secondary battery having a large charge / discharge capacity.

The most preferable flaky graphite fine particles in the present invention are obtained by wet-grinding expanded graphite with a pair of rotary disc-shaped grindstones in a short time. According to this method, while maintaining the extremely high crystallinity of natural graphite, the thickness is 1 μm or less and the maximum particle length is 100 μm or less. It is possible to obtain fine flaky graphite fine particles having a high bulk density.

[0022] It is desirable bulk density of the flaky graphite particles used in the present invention is ^{0.02g / cm 3 ~0.6g / cm 3} , especially 0.2g / cm ³ ~0.6g / cm
³ is more preferable. Even if the flake graphite fine particles have a bulk density of less than 0.02 g / cm ³ , they cannot be used as a negative electrode material, but they are not only poor in handleability when preparing a negative electrode material, but also uniformly mixed with a binder or the like. Will be difficult. Further, if milling and grinding and mixing are carried out for a long time for uniform mixing, the crystallinity of graphite may be impaired. Therefore, it is desirable that the flake graphite fine particles are as compacted as possible and have a bulk density of 0.02 g / cm ³ or more, and more preferably 0.2 g / cm ³ or more. On the other hand, the flake graphite fine particles have a bulk density of 0.6 g.
/ Cm ³ is difficult, and the upper limit of the bulk density that can be industrially applied is 0.6 g / cm ³ .

The flaky graphite fine particles used in the present invention are preferably those produced by the above-mentioned method, but more preferably, a method of crushing expanded graphite using ultrasonic waves, a method of grinding using media, or a wet method. The flaky graphite fine particles obtained by grinding with a pair of rotary disk-shaped grindstones in a short time at 2000 ° C. to 2800 ° C. for about 0.1 to 10 hours are further annealed in an inert atmosphere for further crystallization. It is flake graphite fine particles with improved properties. If it is less than 2000 ° C, the annealing effect is not sufficient, and if it exceeds 2800 ° C, the effect of annealing does not change. Therefore, it is not necessary to treat at a temperature higher than the same temperature.

The flaky graphite fine particles obtained as described above have a large electrical anisotropy, and when the flaky graphite fine particles are pressure-molded, the AB plane of the graphite is oriented in the direction perpendicular to the molding pressure. Therefore, the molding pressure and the volume resistance in the vertical direction are significantly reduced. In the present invention, when molding is performed at a pressure of 500 kg / cm ² , the ratio of the molding pressure to the vertical volume resistance and the molding pressure to the parallel volume resistance is 1 × 10 ⁻⁶ to 10 ^6.
-4 is preferable. This anisotropy reflects not only the morphological characteristics of the flaky graphite fine particles but also the extremely high crystallinity of the flaky graphite fine particles, which reflects the remarkable anisotropy of electric resistance between the in-plane direction of the graphite crystal and the C-axis direction. Is a characteristic of flake graphite fine particles.

The method for preparing a negative electrode material for a lithium ion secondary battery using the above flaky graphite fine particles is not particularly limited. For example, a binder and a solvent are added to the flaky graphite fine particles and sufficiently kneaded, and then a metal mesh or the like is used. It can be used as a negative electrode by pressure bonding to the current collector. Known materials such as various pitches and polytetrafluoroethylene are used for the binder, and among them, polyvinylidene fluoride (PVDF) is most suitable. The compounding ratio (weight) of the graphite fine particles and the binder is preferably 100: 2 to 100: 10. Also, the crimping pressure is 500 to 10,000 kg
It is preferably f / cm ² .

Since graphite fine particles obtained by crushing using ultrasonic waves and graphite fine particles obtained by grinding with a pair of wet rotary disk-shaped grindstones in a short time have high compression moldability, a binder is used. In some cases, the negative electrode material can be obtained by pressure-bonding to the current collector. In this case, since no binder is used, high rate performance can be exhibited.

The above-mentioned negative electrode material of the present invention is used by being incorporated in a secondary battery as a negative electrode of a lithium ion secondary battery having a known structure using a positive electrode, a negative electrode and an electrolytic solution.

The positive electrode material is not particularly limited, but LiCo
O₂ , LiNiO₂ , LiMn₂ O _Four Lithium content such as
Oxides and the like are preferable. Powder positive electrode material is binder
In addition to the above, add conductive materials, solvents, etc., if necessary, and mix thoroughly.
After kneading, it can be prepared by molding together with a current collector. This
These are known techniques.

The separator is also not particularly limited, and known materials can be used.

As the non-aqueous solvent used in the present invention, a known aprotic solvent having a low dielectric constant and capable of dissolving a lithium salt is used. For example, ethylene carbonate, propylene carbonate, diethylene carbonate, acetonitrile, propionitrile, tetrahydrofuran, γ-
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, etc. The solvent may be used alone or in combination of two or more kinds.

The lithium salt used as the electrolyte is L
iClO ₄ , LiAsF ₅ , LiPF ₆ , LiBF ₄ ,
LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ S
There are O ₃ Li, CF ₃ SO ₃ Li and the like, and these salts may be used alone or as a mixture of two or more kinds.

Each physical property value was measured by the following methods.

Bulk Density: The bulk density is a value obtained by dividing the sample weight by the sample volume when the sample that has been crushed and dried is put into a 50 ml glass graduated cylinder and tapped, and when the volume does not change, the sample volume is measured. Was defined as the bulk density.

True specific gravity: Measured by the butanol substitution method.

Crystal lattice constant C ₍₀₀₂₎ , crystallite size L
_{C (002)} : Toshiba X-ray diffractometer XC-4OH, Cu
The −Kα line was monochromated with Ni, and the crystal lattice constant C ₍₀₀₂₎ and the crystallite size L _{C (002)} were measured by the Gakshin method using high-purity silicon as a standard substance.

Thickness: The sample was observed with a JSM-5300 scanning electron microscope manufactured by JEOL Ltd., and the thickness and maximum particle length of the sample were measured.

Average particle length and maximum particle length: An image observed by a JSM-5300 scanning electron microscope manufactured by JEOL Ltd. was analyzed and determined by an image analyzer Luzekus III U manufactured by Nireco.

[0038]

The present invention will be described in more detail with reference to the following examples. Example 1 Chinese flaky natural graphite was treated with a mixed acid of 9 parts by weight of sulfuric acid and 1 part by weight of nitric acid for 2 hours to obtain a graphite-sulfuric acid intercalation compound, which was washed with water, dried, and then placed in an electric furnace at 800 ° C. It is charged and subjected to heat expansion treatment to obtain a bulk density of 0.004 g / cm ³ ,
Expanded graphite having a particle length of 1 to 15 mm was obtained. This expanded graphite 3
After adding 97 parts by weight of water to the parts by weight and immersing the expanded graphite in the water, the expanded graphite was roughly pulverized for 1 minute with a 25S type cutter mixer manufactured by Aikosha Seisakusho Co., Ltd. to obtain an expanded graphite-water slurry. Subsequently, the expanded graphite-water slurry was ground under the following conditions using a MKZA-10-15 type micro grinder manufactured by Masuyuki Sangyo. Grinder: MKGS80, clearance between grinders: 60 μm, grinder rotation speed: 1200 rpm, residence time of expanded graphite-water slurry in the grinder: 20 seconds.

The crushed expanded graphite was dried at 110 ° C. for 1 hour to obtain flaky graphite fine particles.

The properties of the obtained flake graphite fine powder are as follows: bulk density: 0.25 g / cm ³ , true specific gravity: 2.20, crystal lattice constant C _{0 (002)} : 0.672 nm, crystallite size. L
_{C (002)} : 61 nm, thickness average: 0.15 μm, average particle length: 22 μm, maximum particle length: 80 μm, 500 kg / c
Volume resistance of the sample pressure-molded at m ² in the direction perpendicular to the pressing direction: 5.74 × 10 ⁻⁴ Ω · cm, volume resistance in the parallel direction:
The value was 1.60 × 10 Ω · cm, and the ratio of the volume resistance in the molding pressure vertical direction to the molding pressure parallel direction was 3.59 × 10 ⁻⁵ .

In order to study the performance of the flaky graphite fine particles as a negative electrode material for a lithium ion secondary battery, a non-aqueous solvent battery was prepared using flaky graphite as a positive electrode and metallic lithium as a negative electrode, and a charge / discharge test was conducted. The lithium ion doping (intercalation) and dedoping (disintercalation) capacities of the graphite electrode were measured.

A positive electrode using flaky graphite was prepared by the following method. To 40 parts by weight of flake graphite fine particles, 1 part by weight of ethylene propylene dimonomer as a binder and a small amount of dimethylformamide were added and mixed well to form a paste, and 1 tonf was placed on a circular stainless mesh (2.5 cm ² ).
After pressure molding at / cm ² , it was vacuum dried at 200 ° C for 2 hours to obtain a positive electrode.

Metallic lithium was used for the negative electrode. A mixed solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 2 was used as an electrolytic solvent, and Li PF ₆ was used as an electrolyte.
Was used and the concentration was set to 10 mol / 1. A porous polypropylene non-woven fabric is used as a separator, a glass fiber filter paper is impregnated with an electrolytic solution, and a coin-type cell is manufactured under an argon atmosphere.
A charging / discharging test was performed at 4 mA / cm ² .

The initial charge amount was 471 mA · h / g. The discharge was cut at 1.5 V and a cycle test was performed. The discharge amount was 363 mA · h / g in the first cycle, stable at 346 mA · h / g from the third cycle, and the efficiency (discharge amount / charge (Amount x 100%) was 98.9%. A 60-cycle test was conducted, but no decrease in the discharge amount was observed up to the 60th cycle. The charge / discharge curve at the third cycle is shown in FIG. (Example 2) 97 parts by weight of water was added to 3 parts by weight of the expanded graphite prepared in Example 1 and the expanded graphite was immersed in water.
2 ultrasonic waves with a frequency of 28-40 KHz (output 500 W)
After being left to act for a time, it was further treated by a wet vibration type ball mill for 1 hour for consolidation, and then dried at 110 ° C. for 1 hour to obtain flaky graphite fine particles.

The properties of the obtained flake graphite fine particles are as follows: bulk specific gravity: 0.23, true specific gravity: 2.25, crystal lattice constant C
_{0 (002)} 0.670 nm, crystallite size L _{C (002)} : 69
nm, average thickness: 0.07 μm, average particle length: 18 μ
m, maximum particle length: 56 μm, volume resistance in the direction perpendicular to the pressing direction of the sample pressure-molded at 500 kg / cm ² : 5.2
0 × 10 ⁻⁴ Ω · cm, volume resistance in parallel direction: 1.14 ×
The value of the volume resistance in the direction perpendicular to the molding pressure and the direction parallel to the molding pressure was 4.5 Ω × 10 ⁻⁵ .

Using the flaky graphite fine particles, Example 1
A positive electrode was prepared in the same manner as in. Furthermore, a coin-type cell was produced by the same method as in Example 1, and a charge / discharge test was conducted by the same method as in Example 1.

The initial charge amount was 405 mA · h / g. The discharge was cut at 1.5 V and a cycle test was performed. The discharge amount was 346 mA · h / g in the first cycle, stable at 354 mA · h / g from the third cycle, and the efficiency (discharge amount / charge (Amount x 100%) was 98.9%. The test was conducted for 60 cycles, but no decrease in the discharge amount was observed until the 60th cycle. (Example 3) The flaky graphite fine particles prepared in Example 1 were annealed by heat treatment at 2400 ° C for 120 minutes in an argon stream. The properties of the flaky graphite fine particles obtained by annealing are as follows: bulk specific gravity: 0.25, true specific gravity: 2.
25, crystal lattice constant C _{0 (002)} : 0.670 nm, crystal lattice size L _{C (002)} : 90 nm, average thickness: 0.15 μ
m, average particle length 22 μm, maximum particle length 80 μm, 500
The volume resistance in the direction perpendicular to the pressure direction of the sample pressure-molded at kg / cm ² is 3.83 × 10 ⁻⁴ Ω · cm, the volume resistance in the parallel direction is 1.77 × 10 Ω · cm, and the molding pressure is perpendicular. Value of the volume resistance in the direction parallel to the molding pressure is 2.16 × 10
It was ^-4 .

Using the flaky graphite fine particles, Example 1
A positive electrode was prepared in the same manner as in Example 1, a coin-type cell was prepared in the same manner as in Example 1, and a charge / discharge test was performed in the same manner as in Example 1.

The initial charge amount was 382 mA · h / g. The discharge was cut at 1.5 V and a cycle test was conducted. The discharge amount was 368 mA · h / g in the first cycle, was stable at 370 mA · h / g from the third cycle, and the efficiency (discharge amount / charge amount × 100%) was 98.9%. The test was conducted for 60 cycles, but no decrease in the discharge amount was observed until the 60th cycle. (Comparative Example 1) The natural graphite produced in China used in Example 1 was pulverized for 2 hours by a dry method using a vibration type ball mill manufactured by Chuo Kako. The physical properties of the obtained graphite fine powder were as follows.
Bulk density: 0.250 g / cm ³ , true specific gravity: 2.20, thickness: 2.50 μm, average particle length: 25 μm, maximum particle length: 150 μm, crystal lattice constant C _{0 (002)} : 0.672n
m, crystallite size L _{C (002)} : 91 nm, 500 kg /
cm ² in pressure molding the pressure direction and the parallel direction of the volume resistivity of the ^{sample: 9.6 × 10 -3 Ω · cm} , the vertical volume resistivity:
The value of the volume resistance was 7.1 × 10 ⁻¹ Ω · cm and the direction parallel to the molding pressure was 1.35 × 10 ⁻² .

Using this graphite fine powder, a negative electrode was prepared in the same manner as in Example 1, coin cells were prepared in the same manner as in Example 1, and charging / discharging was performed in the same manner as in Example 1. The test was conducted.

The initial charge amount was 290 mA · h / g. The discharge was cut at 1.5 V and a cycle test was conducted. The discharge amount was 115 mA · h / g in the first cycle and reached 164 mA · h / g, which was the maximum from the third cycle. Decreased, 122m at 60th cycle
It decreased to A · h / g.

From Comparative Example 1, even if the fine graphite powder has high crystallinity, if the powder thickness, maximum particle length, and volume resistance ratio in the direction parallel to the molding pressure and in the direction parallel to the molding pressure are large, the chargeability is high. It can be seen that the discharge capacity is low.

[0053]

Since the negative electrode material of the present invention is constructed as described above, it is possible to dope and dedope lithium ions at the original low potential of graphite, and thus has a higher energy density, and It is possible to manufacture a lithium-ion secondary battery that has an electric capacity that is almost the theoretical value and that has a long cycle life.

[Brief description of drawings]

FIG. 1 is a graph showing a charge / discharge curve at a third cycle in a charge / discharge test of a half cell for evaluating a negative electrode material using the negative electrode material of the present invention as a positive electrode and lithium as a negative electrode.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Sakata 2-1-1 Nihombashi Muromachi, Chuo-ku, Tokyo Mitsui Mining Co., Ltd.

Claims (6)

[Claims] 1. A negative electrode material for a lithium-ion secondary battery formed by molding graphite fine particles, wherein the graphite fine particles have a crystal lattice constant C _{0 (002) of} 002 plane of 0.670 to.
0.673 nm, the thickness of the graphite fine particles is 1 μm or less,
The maximum particle length is 100 μm or less, and the graphite fine particles are 5
The value of the ratio of the volume resistance between the molding pressure vertical direction and the molding pressure parallel direction when molded at 00 kg / cm ² is 1 × 10 ⁻⁶ to 1 ×.
A negative electrode material for a lithium-ion secondary battery, which is flake graphite fine particles having a size of 10 ⁻⁴ . 2. The negative electrode material according to claim 1, wherein the flaky graphite fine particles are obtained by pulverizing expanded graphite by ultrasonic waves in a dispersion solvent. 3. The negative electrode material according to claim 1, wherein the flaky graphite fine particles are obtained by wet-grinding expanded graphite using a medium. 4. The negative electrode material according to claim 1, wherein the flaky graphite fine particles are obtained by wet-grinding expanded graphite with a pair of disk-shaped rotary grindstones. 5. Flake graphite fine particles obtained by crushing expanded graphite with ultrasonic waves in a dispersion solvent by ultrasonic waves, flaky graphite fine particles obtained by wet-grinding expanded graphite with a medium, or Further 200 pieces of flaky graphite fine particles obtained by grinding expansive graphite with a pair of rotary disc-shaped grindstones
The negative electrode material according to claim 1, which is obtained by annealing at 0 to 2800 ° C. 6. A lithium ion secondary battery incorporating the negative electrode material according to any one of claims 1 to 5.