JPH1173963A - Composite material for lithium ion secondary battery negative electrode and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a composite material for a negative electrode, capable of manufacturing the negative electrode with low electrical resistance and having a large specific gravity. SOLUTION: Slurry, comprising filler graphite, a carbonizable binder, a solvent for dissolving the binder, is formed in a globular shape by a splay dry method, the globular material obtained is dried to form granules, the binder is made infusible or not made infusible, the granules are heated in an inert atmosphere at 900-1400 deg.C for a specified time period to carbonize the binder as the matrix carbon. Composite material which comprises the filler graphite and matrix carbon connecting the filler graphite, having a globular shape for a negative electrode of a lithium ion secondary battery is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art As electronic devices become smaller and lighter, batteries are required to have a higher energy density. From the viewpoint of resource saving, there is an urgent need to develop secondary batteries which can be repeatedly charged and discharged. In response to this demand, high energy density,
Lithium ion secondary batteries that are lightweight, small, and excellent in charge / discharge cycle characteristics have been proposed.

[0003] A lithium ion secondary battery has been developed to solve the problems of the lithium metal secondary battery having poor quick chargeability, short cycle life, and poor safety. In the case of a lithium metal secondary battery, metallic lithium is used for the negative electrode, whereas in the case of a lithium ion secondary battery, the above-mentioned problem is solved by using a carbon material for the negative electrode. .

That is, when charging is performed using a lithium compound as a positive electrode and a carbon material as a negative electrode, 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, at the time of discharge, lithium ions are dedoped from between layers of the carbon material, and the generated lithium ions are combined with the lithium compound of the positive electrode again. Thereby, a chargeable / dischargeable battery is formed.

[0005]

SUMMARY OF THE INVENTION Graphite, which is a carbon material,
In particular, when producing a negative electrode of a lithium ion secondary battery using natural graphite, there is a step of first forming a natural graphite film on a current collector using a binder. In this film forming process, since natural graphite is on a plate, the AB axis surface of graphite tends to be arranged in parallel to the current collector surface. As a result, in the obtained negative electrode, the C-axis direction of graphite is arranged in a direction perpendicular to the current collector surface and the positive electrode surface, that is, in a direction in which lithium ions and electrons flow.

Electric resistance (volume resistance) in the AB axis plane of graphite
Has an extremely small electric resistance of 4 to 7 × 10 ⁻⁵ Ωcm. However, the electrical resistance in the C-axis direction of natural graphite is 5 × 10
Both ^-3 Ωcm and 1-5 × 10 ^-1 Ωcm, the resistance value is larger than the electric resistance in the AB axis plane.

Therefore, despite the fact that natural graphite having a small electric resistance was originally used, the C-axis of graphite having a large electric resistance is arranged perpendicularly to the current collector surface and the positive electrode surface. The C-axis of graphite having a large electric resistance is arranged in the direction of conduction, which increases the internal resistance of the lithium ion secondary battery and causes the rate characteristics to deteriorate.

Further, in such an arrangement, during charging in which lithium ions are inserted between graphite layers, the thickness direction of the negative electrode expands only in a certain direction. At the time of discharge in which lithium ions inserted between the graphite layers are released, the negative electrode contracts only in the opposite direction. The repeated expansion and contraction of the negative electrode only in a certain direction due to charge and discharge causes a decrease in the charge and discharge capacity of the lithium ion secondary battery and a decrease in safety.

As a method of improving the discharge capacity, charge / discharge rate, cycle characteristics and safety of a lithium ion secondary battery using a conventional graphite-based negative electrode material, the present inventors can lower the electric resistance of the negative electrode. It was considered that the use of a carbon material for a negative electrode was extremely effective. The present inventors have conceived that the above problem can be solved by using a composite material of graphite and carbon as a carbon material capable of lowering the electric resistance of the negative electrode and making the shape spherical.

In particular, it has been found that by making the shape of the composite material spherical, it is possible to effectively contribute the inherently high conductivity of graphite in the composite material to the conduction direction of lithium ions. More specifically, the carbon material used as the negative electrode material of the lithium ion secondary battery is a composite material composed of graphite and carbon, and the shape of the composite material is spherical. The ratio in which the A and B surfaces are effectively arranged perpendicular to the current collector surface of the negative electrode and the positive electrode surface can be increased. As a result, the electric resistance of the negative electrode can be reduced, and the discharge capacity, charge / discharge speed,
Safety can be improved simultaneously with cycle characteristics.

Further, the present inventors have found that a composite material in which the surface of graphite is coated with carbon is most suitable. The present invention has been completed based on the above findings.

Accordingly, it is an object of the present invention to provide a composite material for a negative electrode of a lithium ion secondary battery, which can reduce the electric resistance of the negative electrode of the lithium ion secondary battery.

[0013]

According to the present invention, there is provided a lithium ion battery comprising: [1] a filler graphite and a matrix carbon for connecting the filler graphite, the shape of which is spherical. Composite material for secondary battery negative electrode,
And [2] filler graphite having a carbon coating layer on the surface,
A composite material for a negative electrode of a lithium ion secondary battery, comprising a matrix carbon for connecting filler graphite having the coating layer and having a spherical shape.

[3] The above [1] and [2] include that the filler graphite has an average particle size of 1 to 20 μm, and [4] the composite material has an average particle size of 5 to 40 μm. .

[0015] The present invention also provides: [5] a slurry comprising filler graphite, a binder capable of being carbonized, and a solvent dissolving the binder is formed into a spherical shape by a spray drying method and dried to form a granulated product. Obtaining and then infusibilizing the binder, or heat treating the granulated material without infusibilization at 900 to 1400 ° C. for a predetermined time under an inert atmosphere to carbonize the binder to form matrix carbon. A method for producing a composite material for a negative electrode of a lithium ion secondary battery, comprising filler graphite and matrix carbon connecting the filler graphite, wherein the composite material has a spherical shape. [6] Filler graphite and carbonization A slurry comprising a possible binder and a solvent that dissolves the binder is formed into a sphere by spray drying and dried to obtain a granulated product, and then the binder is made infusible or infusible. While the granulated material is subjected to heat treatment at 900 to 1400 ° C. for a predetermined time under an inert atmosphere without carbonization, the binder is carbonized into matrix carbon, or after the matrix carbon is formed,
Forming a carbon coating layer on the surface of filler graphite using a carbonaceous material, characterized by filler graphite having a carbon coating layer on the surface, and matrix carbon connecting the filler graphite having the coating layer And a method for producing a composite material for a negative electrode of a lithium ion secondary battery having a spherical shape.

[7] The above [5] and [6] include that the filler graphite has an average particle size of 1 to 20 μm, and [8] the composite material has an average particle size of 5 to 40 μm. .

Hereinafter, the present invention will be described in detail.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION A carbon material used for manufacturing a negative electrode of a lithium ion secondary battery of the present invention is a spherical graphite / carbon composite material comprising graphite as a filler and carbon as a matrix.

The shape of the graphite / carbon composite is spherical. The arrangement of the filler graphite particles inside the spherical composite material may be any arrangement such as mosaic, concentric, or radial.

The most preferred arrangement is a graphite having an arrangement structure in which the AB axis plane of the graphite is radially arranged from the center toward the outer surface, and the C axis of the graphite on the composite material surface is exposed to the spherical surface; A spherical composite material consisting of graphite and carbon having an array structure in which the AB axis surface of graphite is radially arranged from the center toward the outer surface and the C axis of graphite on the composite material surface is exposed to the spherical surface. This is a spherical composite material having a coating layer formed of carbon on the surface of graphite.

The filler graphite used in the present invention can be produced by using natural graphite or artificial graphite as a raw material and pulverizing the raw material. The raw material graphite can be pulverized as it is to obtain filler graphite, but it can also be made into exfoliated graphite by a known method and then pulverized to obtain filler graphite.

The method of pulverizing graphite includes pulverization by shear stress and impact pulverization. The filler graphite used in the present invention may be pulverized by any method.

When the particle size of the filler graphite is large, the initial discharge efficiency is high, but the discharge amount is low. When the particle size is small, the discharge amount is large, but the initial discharge efficiency tends to decrease. Therefore, the preferred average particle size of the filler graphite is 20 μm or less, more preferably 10 μm or less, and 1 μm or more.

In the present invention, the matrix carbon which forms the spherical composite material by connecting the filler graphite is obtained by carbonizing the binder used for bonding the filler graphite to each other as described later. It is. This binder is soluble in a solvent. Examples of the water-soluble binder include natural polymer compounds such as molasses and starch, and various water-soluble synthetic resins such as naphthalenesulfonic acid-formaldehyde condensate. Further, various types of coal-based and petroleum-based tars and pitches soluble in an organic solvent, and various synthetic resins can also be used.

The amount of the binder varies depending on the carbonization yield of the binder.
It is preferable to mix them in an amount of from 30 to 30% by weight. If the amount of matrix carbon is less than 5% by weight, the binder graphite is not sufficiently bonded, the bulk specific gravity of the composite material is low, and when incorporated as a negative electrode in a battery, there remains a problem in safety. When the compounding amount as a carbide exceeds 30% by weight, a composite material having a sufficiently large bulk density can be obtained, but the discharge capacity is reduced as a result of a decrease in the proportion of filler graphite.

The composite material obtained by the above method can be further coated with carbon by a method such as CVD, so that a more uniform coating layer can be formed. This is preferable in that respect.

In the case of a composite material in which the coating of graphite with matrix carbon is insufficient, the performance can be improved by further coating the carbon by a method such as CVD.

As the CVD method that can be used, various conventionally known methods can be used as they are.

A slurry comprising filler graphite, a binder and a solvent, or a slurry comprising filler graphite and a binder when the viscosity of the binder is sufficiently low, is granulated into spherical particles by a spray drying method and dried. Thus, a spherical granulated product composed of graphite and a binder can be obtained. Spherical particles can be produced by a method other than the spray-drying method.However, the spray-drying method can be used in consideration of a shape close to a true sphere, a small granulated particle size, and a desired graphite arrangement. Most preferred as a granulation method.

A spherical composite made of graphite and carbon can be obtained by heat-treating a spherical granulated product made of graphite and a binder. In this spherical graphite / carbon composite, the graphite arrangement inside the particles is It is determined at the granulation stage by spray drying. The relationship between the graphite arrangement and the spray drying conditions is not fully understood, but the graphite arrangement is concentric when the viscosity of the spray-dried liquid consisting of graphite / binder / solvent is high, and radial when the viscosity is low. Tend to be arranged in a mosaic at an intermediate viscosity.

The granules comprising graphite and binder after spray-drying may be obtained by subjecting the binder to infusibilization or by solidifying the binder without solidification. If so, heat treatment is performed in an inert atmosphere at 900 ° C. to 1400 ° C. to obtain a spherical composite material of graphite and carbon. When the heat treatment temperature is lower than 900 ° C., the initial discharge efficiency of the composite material is low. When the heat treatment temperature is higher than 1400 ° C., the charge / discharge amount is low, and it tends to be difficult to obtain a negative electrode material having preferable characteristics.

The arrangement state of the filler graphite on the surface and inside of the spherical composite of the present invention can be determined by observing the surface of the composite using a scanning microscope or observing the cross section of the composite using a polarizing microscope. It can be confirmed by the method.

The average particle size of the spherical composite material of graphite and binder produced in this way is 40 μm or less, preferably 30 μm to 10 μm. If the particle size of the filler graphite is small, the particle size of the obtained composite material can also be reduced, but if the average particle size of the filler graphite is less than 1 μm, the efficiency of the filler graphite itself decreases,
Since the efficiency of a composite material produced by using this tends to further decrease, the particle size of the composite material is preferably 5 μm or more.

The electric resistance perpendicular to the current collector surface and the positive electrode surface of the negative electrode for a lithium ion secondary battery manufactured using the composite material of the present invention having the above structure (that is, the conduction direction of lithium ions) is as follows: It is about 50 to 80% smaller than the electric resistance in the same direction of the conventional negative electrode manufactured using plate graphite as the negative electrode material. As a result, remarkable improvements in charge / discharge speed, high rate characteristics, suppression of heat generation during charge / discharge, suppression of expansion and contraction of graphite, and the like are remarkable.

In general, graphite fine particles which have been pulverized but not subjected to any treatment are bulky and have a bulk specific gravity of 0.05 to
It is about 0.25 g / cm ³ . Such bulky graphite fine particles can be used without any problem in a basic evaluation experiment as a negative electrode material requiring only a few tens of mg to a few g of a sample. However, in the actual battery production process, a slurry of graphite fine particles is formed using graphite fine particles and a solvent in which an adhesive is dissolved, and the graphite fine particle slurry is coated on a current collector metal, and then dried to form a coil. The negative electrode is manufactured by a winding method. In order to increase the charge / discharge amount of the lithium ion secondary battery, it is desirable to increase the amount of graphite fine particles coated on the current collector metal as much as possible. Therefore, it is desired to increase the bulk density of the negative electrode material in the slurry as much as possible.

However, when the graphite fine particles are bulky, most of the graphite fine particles may not even get wet with the solvent. In such a case, a graphite slurry having a high concentration cannot be prepared. Therefore, graphite fine particles having a large bulk density, that is, a small volume per unit weight, are desired for graphite fine particles as a negative electrode material for a lithium ion secondary battery.

In the composite material of graphite and carbon of the present invention, since carbon acts as a binder, the bulk density of the composite material of graphite and carbon is at least 0.3 g / cm ³ , usually 0.4 g / cm ^3. It is also a feature of the composite of the present invention that it has a large value of up to 1.2 g / cm ³ .

The method for preparing a negative electrode of a lithium ion secondary battery using the spherical composite material of graphite and carbon of the present invention is not particularly limited. For example, a binder (for example, polyvinylidene fluoride (PVDF)) is added to the composite material. ) Is dissolved in a solvent (for example, 1-methyl-2-pyrrolidone) and sufficiently kneaded to prepare a high-concentration graphite fine particle slurry having a graphite concentration of 60% by weight or more. This graphite fine particle slurry is coated on a current collector made of a metal foil to a thickness of 20 to 200 μm using a doctor blade. The graphite fine particle slurry on the metal foil is dried and adheres to the metal foil current collector. If necessary, pressure can be applied to increase the adhesion and make the thickness of the coating layer uniform.

As the binder, known materials such as various pitches and polytetrafluoroethylene can be used. Among them, polyvinylidene fluoride (PVD)
F) is optimal. The mixing ratio (weight ratio) of the composite material and the binder of the present invention is desirably 100: 2 to 100: 10.

The cathode material is not particularly limited, but those conventionally used as cathode materials can be used as they are. For example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like, and a mixture thereof are suitable. The powdery positive electrode material can be prepared by adding a conductive material, if necessary, sufficiently kneading the mixture with a solvent in which a binder is dissolved, and then molding the resultant with a current collector. These are known techniques.

The separator is not particularly limited, and a known material can be used.

As the non-aqueous solvent for the lithium ion secondary battery, a known aprotic solvent having a low dielectric constant that dissolves a lithium salt is preferable. Such solvents include, for example, ethylene carbonate, dimethyl carbonate, propylene carbonate, diethylene carbonate, acetonitrile, propionitrile, tetrahydrofuran, γ-butyrolactone, 2-methyltetrahydrofuran, 1,
3-dioxolan, 4-methyl-1,3-dioxolan, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, sulfolane, methylsulfolane, nitromethane, N, N-dimethylformamide, dimethylsulfoxide and the like. These solvents can be used alone or in combination of two or more.

As the lithium salt used as the electrolyte of the lithium ion secondary battery, LiCiO ₄ , LiAsF ₆ ,
LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiC
1, LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, and the like, and these salts are used alone or as a mixture of two or more kinds.

Each physical property value was measured by the following method.

Bulk density: The graphite particles were put into a 100 ml glass measuring cylinder and tapped. When the sample volume stopped changing, the sample volume was measured, and the value obtained by dividing the sample weight by the sample volume was defined as the bulk density. .

Crystal lattice constant C0 (002): X made by Toshiba
Using a line diffractometer XC-40H, the Cu-Kn line was made monochromatic with Ni, and measurement was performed by Gakushin method using high-purity silicon as a standard substance.

Average particle size: Measured using a laser diffraction particle size distribution analyzer SALD1100 manufactured by Shimadzu Corporation.

Scanning electron microscope observation: J made by JEOL Ltd.
Observation was performed using SM-5300.

Observation with a deflection microscope: The sample was embedded in a polyester resin, polished, and then observed using Orthorx Spor manufactured by Leitz.

Measurement of volume resistivity: A PVDF-methylpyrrolidone solution (5% by weight of PVDF vs. sample) was added to the sample to form a slurry, and the slurry was 2.5 cm in diameter by the doctor blade method.
² was applied to the SUS plate. After drying, the sample is sandwiched between SUS plates, and R6 manufactured by Advantest Co., Ltd. is applied under a pressure of 5 kg / cm ^2.
The resistance was measured with a 551 multimeter, and the volume resistivity was calculated.

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

[0052]

【Example】

Example 1 Chinese flaky graphite having an ash content of 0.1% by weight was pulverized by a CGS16 type jet mill manufactured by Mitsui Mining Co., Ltd.
The average particle size of the obtained filler graphite was 8.9 μm. The filler graphite had a crystal lattice constant Co (002) of 0.6708 nm and a bulk density of 0.187 g / cm ³ . After uniformly mixing 100 parts by weight of the filler graphite, 38 parts by weight of naphthalenesulfonic acid formaldehyde condensate, and 500 parts by weight of water, SD-25 manufactured by Mitsui Mining Co., Ltd.
Granulation and drying were performed using a disk atomizer with a mold spray dryer. The average particle size of the granules thus obtained was 25 μm.

The above granulated product was placed in a nitrogen gas atmosphere at 1200
C., and carbonized the naphthalenesulfonic acid formaldehyde condensate used as the binder to obtain a composite material comprising filler graphite and carbon. An electron micrograph of the obtained composite material is shown in FIG. 1, and a deflection micrograph is shown in FIG.

From FIG. 1, it is clear that the AB axis plane of the filler graphite is arranged almost radially from the center toward the outer surface, and FIG. 2 shows that the inside of the composite material has a mosaic shape in which the filler graphite is randomly arranged. is there.

The volume resistivity of the manufactured composite material was measured.
Table 1 shows the results.

[0056]

[Table 1] The basic properties of the negative electrode material of the lithium ion secondary battery were measured under the conditions shown in Table 2. Table 3 shows the results.

[0057]

[Table 2] EC: Ethylene carbonate DMC: Dimethyl carbonate In this case, intercalating lithium ions into graphite is referred to as charging.

[0058]

[Table 3] The current at the time of low current charge / discharge in Table 2 was 8 mA (3.2 mA /
The composite material was tested at a high rate under the conditions shown in Table 2 except that the composite material was cm ² ). Table 4 shows the results.

[0059]

[Table 4] Comparative Example 1 The volume resistivity of the negative electrode of a lithium ion secondary battery manufactured using the filler graphite used in Example 1 as it was as a negative electrode material was measured. Table 5 shows the results.

[0060]

[Table 5] The basic properties of the filler graphite as a negative electrode material of a lithium ion secondary battery were measured under the conditions shown in Table 2. Table 6 shows the results.

[0061]

[Table 6] The current at the time of constant current charge / discharge in Table 2 was 8 mA (3.2 mA /
The composite material of Example 1 was tested at a high rate under the conditions shown in Table 2 except that the cm ² ) was used. Table 7 shows the results.

[0062]

[Table 7] Comparing Tables 3 and 4 in Example 1, it can be seen that the high rate characteristics are good because no large decrease in the discharge amount is observed even when the current during charging and discharging is increased to 8 mA.

Even when the number of cycles reached 100, the discharge amount was slightly reduced in both Tables 3 and 4, indicating that the cycle characteristics were good.

As is evident from the results, the compounding of graphite with carbon slightly reduces the initial discharge capacity (first cycle number) of the negative electrode, but improves the high rate characteristics and the cycle characteristics. It is.

[0065]

As described above, the composite material of the present invention is constructed as described above. The electric resistance of the negative electrode for a lithium ion secondary battery manufactured using the composite material of the present invention is the 50 to 8 compared to the electric resistance of the negative electrode in the same direction
About 0% smaller. As a result, remarkable improvements in charge / discharge speed, high rate characteristics, suppression of heat generation during charge / discharge, suppression of expansion and contraction of graphite, and the like are remarkable. Further, since the composite material has a large specific gravity, it is suitable for manufacturing a negative electrode.

[Brief description of the drawings]

FIG. 1 is a drawing-substituted micrograph showing an example of the composite material of the present invention.

FIG. 2 is a deflection microscope photograph as a drawing showing an example of the composite material of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tadanori Tsunata 1-3-3 Hibiki-cho, Wakamatsu-ku, Kitakyushu-shi, Fukuoka In-house of Mitsui Mining Co., Ltd. (72) Inventor Koji Sakata 1 Hibiki-cho, Wakamatsu-ku, Kitakyushu-shi, Fukuoka 3 chome Mitsui Mining Co., Ltd.

Claims (8)

[Claims] 1. A composite material for a negative electrode of a lithium ion secondary battery, comprising a filler graphite and a matrix carbon connecting the filler graphite, wherein the composite material has a spherical shape. 2. A negative electrode for a lithium ion secondary battery comprising: filler graphite having a carbon coating layer on its surface; and matrix carbon connecting the filler graphite having the coating layer, and having a spherical shape. For composites. 3. The filler graphite has an average particle size of 1 to 20 μm.
The composite material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein 4. The composite material for a negative electrode of a lithium ion secondary battery according to claim 1, wherein the composite material has an average particle size of 5 to 40 μm. 5. A slurry comprising a filler graphite, a binder capable of being carbonized, and a solvent dissolving the binder is formed into a spherical shape by a spray dry method and dried to obtain a granulated product. Infusible, or, without infusi? Cation, the binder is carbonized into matrix carbon by performing a heat treatment at 900 to 1400 ° C. for a predetermined time under an inert atmosphere without infusing the granulated material. A method for producing a composite material for a negative electrode of a lithium ion secondary battery, comprising a filler graphite and matrix carbon connecting the filler graphite, wherein the composite material has a spherical shape. 6. A slurry comprising filler graphite, a binder capable of being carbonized, and a solvent dissolving the binder is formed into a spherical shape by a spray drying method and dried to obtain a granulated product. Infusible, or, without infusibilization, the granulated material was heat-treated at 900 to 1400 ° C. for a predetermined time under an inert atmosphere to carbonize the binder to form matrix carbon, or to form matrix carbon. After that, a carbon coating layer is formed on the surface of the filler graphite using a carbonaceous material, a filler graphite having a carbon coating layer on the surface, and a matrix carbon linking the filler graphite having the coating layer. A method for producing a composite material for a negative electrode of a lithium ion secondary battery, comprising a spherical shape. 7. The filler graphite has an average particle size of 1 to 20 μm.
The method for producing a composite material for a negative electrode of a lithium ion secondary battery according to claim 5, wherein 8. The method for producing a composite material for a negative electrode of a lithium ion secondary battery according to claim 5, wherein the composite material has an average particle size of 5 to 40 μm.