JP2001135356A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery having good contact with a collector, excellent cyclic characteristics, and easy handling of an electrode plate during coating wherein a scaly natural graphite is used as a negative electrode for a non- aqueous electrolyte secondary battery. SOLUTION: A scaly natural graphite is used for a negative electrode active substance for a nonaqueous electrolyte secondary battery, wherein the scaly natural graphite is processed into spherical.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery using graphite as a negative electrode active material.

[0002]

2. Description of the Related Art In recent years, lithium ion secondary batteries, which have been actively studied, have been put to practical use mainly in fields requiring small and lightweight batteries. Until now, the effectiveness of lithium secondary batteries has been predicted for a long time, but there are many problems to be solved before they can be put to practical use. In particular, since it was discovered that lithium cobalt oxide (LiCoO ₂ ) was effective as a positive electrode material for a secondary battery, the development of a negative electrode active material became a major issue.

When lithium metal is used as a negative electrode active material,
As charge and discharge are repeated, lithium locally grows in a dendritic manner, resulting in a problem that the capacity is reduced and the separator breaks through to cause an internal short circuit. Therefore, use of a lithium alloy instead of lithium metal was considered, but there were difficulties in cycle characteristics and energy density.

[0004] At present, a secondary battery using a carbon material as a negative electrode active material and using a reaction in which lithium ions are inserted into and desorbed from carbon as a negative electrode reaction has been put to practical use.

[0005] Carbon materials are roughly divided into those obtained by heat-treating graphite and amorphous carbon, so-called non-graphitizable carbon materials. Graphite is classified into so-called artificial graphite obtained by heat-treating natural graphite and graphitizable carbon. Non-graphitizable carbon is carbon that does not undergo graphitization even after heat treatment at a high temperature without rearrangement of the carbon layer surface. On the other hand, graphitizable carbon is carbon that is graphitized when heat-treated at a high temperature.

Among these carbon materials, natural graphite is cheaper than artificial graphite, abundant on the earth, has high crystallinity, and has been used as a negative electrode active material for nonaqueous electrolyte secondary batteries. In this case, the theoretical capacity of graphite is 372 mAh /
It shows a discharge capacity close to g.

[0007]

In a battery using scaly natural graphite as a negative electrode active material, generally, a carbon material is coated on a metal foil as a current collector and pressed to be used. I have. However, when flaky natural graphite is used as a negative electrode of a non-aqueous electrolyte secondary battery, handling during electrode coating is not easy, and the c-axis direction of the flaky natural graphite is perpendicular to the current collector foil during electrode pressing. The active material falls off due to the poor adhesion to the current collector and the expansion and contraction of the active material due to the lithium insertion / desorption reaction into the flaky natural graphite. There is a problem of causing cycle deterioration.

[0008]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to use spherical shaped flake natural graphite as a negative electrode active material of a non-aqueous electrolyte secondary battery. Features. Preferably, the content of the flaky natural graphite processed into a sphere with respect to the total weight of the negative electrode active material is 20 to 80 parts by weight. Further, the active material other than the flaky natural graphite processed into a spherical shape in the negative electrode active material is a flaky natural graphite which is not processed into a spherical shape.

Further, in the present invention, preferably, BET of flaky natural graphite processed into a spherical shape as a negative electrode active material is used.
The specific surface area is 3.5 m ² / g or less, and the average particle size is 10 to 3
0 μm.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Natural graphite is a hexagonal or hexagonal plate-like flat crystal having high crystallinity, and has a shape such as flaky, granular, flaky, massive, or earth-like. In this method, flaky natural graphite processed into a spherical shape is used as a negative electrode active material of a nonaqueous electrolyte secondary battery.

As a method of processing the flaky natural graphite into a sphere, a method of pulverizing the flaky natural graphite and sieving only the spherical particles, or a method of pressing the flaky natural graphite into a pellet and pulverizing it And a method of collecting a large number of flaky natural graphite into a spherical mass.

Here, the term “scale-shaped natural graphite processed into a sphere” does not necessarily mean only a perfect sphere, but also includes a shape close to a sphere and may have irregularities on the surface.

The content of the flaky natural graphite processed into a sphere with respect to the total weight of the negative electrode active material may be 100 parts by weight, but by setting the content to 20 to 80 parts by weight, the current collector of the negative electrode mixture can be used. Is improved, and a battery having excellent cycle characteristics can be obtained. In this case, the effect can be obtained even more by using a flaky natural graphite which is not processed into a spherical shape from an active material other than the flaky natural graphite processed into a spherical shape in the negative electrode active material.

Further, in the present invention, the spherical shaped flake natural graphite as the negative electrode active material has a BET specific surface area of 3.5 m ² / g or less and an average particle size of 10 to 30 μm. The adhesiveness between the active material and the active material and the current collector in the agent is improved. Thereby, the discharge capacity can be increased, and a non-aqueous electrolyte secondary battery with a higher energy density can be obtained.

By using flaky natural graphite processed into a sphere as the negative electrode active material of the nonaqueous electrolyte battery, the adhesion between the negative electrode active material and the negative electrode mixture and the current collector is improved. As a result, the electron conductivity between the scaly natural graphite particles and the scaly natural graphite and the current collector is maintained even after repeated charging and discharging,
The cycle characteristics of the battery are improved.

As the positive electrode active material of the non-aqueous electrolyte secondary battery according to the present invention, lithium or lithium ions are occluded.
Although the compounds of any type if releasing compounds can also be used, especially LixMO ₂ (however, M represents one or more transition metals) and Li x Mn ₂ O ₄ compound mainly alone, or two or more mixed It is preferable to use them. Furthermore, from these discharge compounds, it is apparent that Co, Ni and M
It is more preferable to use a transition metal selected from the group consisting of n.

As the solvent for the non-aqueous electrolyte, ethylene carbonate, propylene carbonate, butylene carbonate, trifluoropropylene carbonate, γ-butyrolactone, sulfolane, 1,2-dimethoxyethane,
1,2-diethoxyethane, tetrahydrofuran, 2-
Non-aqueous solvents such as methyl tetrahydrofuran, 3-methyl-1,3-dioxone, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl propyl carbonate, etc. , Alone or a mixed solvent thereof can be used.

The non-aqueous electrolyte is prepared by adding a supporting salt to these non-aqueous solvents.
Dissolve and use. As the supporting salt, LiClO_Four,
LiAsF₆, LiPF₆, LiBF_Four, LiAsF₆, L
iCF_ThreeCO_Two, LiCF_ThreeSO_Three, LiCF_ThreeCF_TwoS
O_Three, LiCF_ThreeCF_TwoCF_TwoSO_Three, LiN (CF_ThreeS
O_Two)_Two, LiN (CF_TwoCF_ThreeSO_Two)_Two, LiN (CF_Three
CO) _TwoAnd LiN (CF_TwoCF_ThreeCO)_TwoSalt if
Alternatively, a mixture of these can be used.

Further, instead of such a liquid electrolyte, a solid ionic conductive polymer electrolyte can be used. When the polymer electrolyte membrane is made of polyethylene oxide, polyacrylonitrile, polyethylene glycol, or a modified product thereof, it is lightweight and flexible, and is advantageous when used for a wound electrode plate. Furthermore, the ion conductive polymer electrolyte membrane and the organic electrolyte can be used in combination. In addition, as the electrolyte, other than the polymer electrolyte, any known inorganic solid electrolyte or a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound by an organic binder, any of which can be used. .

The non-aqueous electrolyte secondary battery according to the present invention usually comprises a positive electrode, a negative electrode and a combination of a separator and a non-aqueous electrolyte. The separator is a porous polyvinyl chloride membrane. Such a porous polymer membrane or a lithium ion or ionic conductive polymer electrolyte membrane can be used alone or in combination. Further, the shape of the battery can be various shapes such as a cylinder, a square, a coin, a button, and a paper.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to preferred embodiments, but it goes without saying that the present invention is not limited to the following embodiments unless it exceeds the gist of the present invention.

A prismatic nonaqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active material and a carbon material as a negative electrode active material was manufactured. FIG. 1 is a diagram showing a cross-sectional structure of a prismatic nonaqueous electrolyte secondary battery. In FIG. 1, 1 is a prismatic nonaqueous electrolyte secondary battery, 2 is a wound electrode group, 3 is a positive electrode, 4 is a negative electrode, 5 is a separator, 6 is a battery case, 7 is a battery cover, 8 is a safety valve, 9 Denotes a positive electrode terminal, and 10 denotes a positive electrode lead wire.

The wound electrode group 2 is housed in a battery case 6, a safety valve 8 is provided in the battery case 6, and the battery lid 7 and the battery case 6 are sealed by laser welding. The positive electrode terminal 9 is connected to the positive electrode 3 via the positive electrode lead 10 and the negative electrode 4
Is connected to the inner wall of the battery case 6 by contact.

The positive electrode was manufactured as follows. 90 parts by weight of LiCoO ₂ as an active material, 5 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of polyvinylidene fluoride as a binder were mixed to form a positive electrode mixture, and N-methyl-2-pyrrolidone was added. A paste was produced by dispersing. This paste was uniformly applied to an aluminum current collector having a thickness of 20 μm, dried, and then compression molded by a roll press to produce a positive electrode.

The negative electrode mixture was prepared by mixing spherical natural flake graphite and non-spherical flake graphite at the composition ratios shown in Table 1.

[0026]

[Table 1]

90 parts by weight of these mixed carbon materials,
A paste was produced by mixing 10 parts by weight of polyvinylidene fluoride as a binder to prepare a negative electrode mixture and dispersing the mixture in N-methyl-2-pyrrolidone.

The negative electrode paste was uniformly applied to a copper foil having a thickness of 15 μm, dried at 100 ° C. for 5 hours, and then compression molded by a roll press to produce a negative electrode.

As the separator, a microporous polyethylene film having a thickness of about 25 μm was used.

As the electrolyte, ethylene carbonate (E
A non-aqueous electrolyte obtained by dissolving 1.0 M of LiPF ₆ in a 1: 1 mixed solvent (volume ratio) of C) and diethyl carbonate (DEC) was used. As described above, seven types of batteries of Examples 1 to 5 and Comparative Examples 1 and 2 shown in the table were produced.

This lithium secondary battery was heated at 25 ° C. for 1 hour.
The battery was charged at a constant current and a constant voltage up to 4.1 V with a current of C for 3 hours to obtain a fully charged state. Subsequently, the current of 1C is 2.75.
Discharged to V. This is taken as one cycle, for a total of 300
The transition of the discharge capacity was examined by cycling.

FIG. 2 shows the cycle test results of the seven types of batteries of Examples 1 to 5 and Comparative Examples 1 and 2. From FIG.
The cycle characteristics of the non-aqueous electrolyte secondary batteries (Examples 1, 2, and 3) using the negative electrode in which the content of the flaky natural graphite processed into a spherical shape in the negative electrode active material is 20, 50, and 80 parts by weight are extremely high. Good and the capacity retention after 300 cycles is 80%
The above is shown. Here, the capacity retention after 300 cycles was the ratio of the discharge capacity at the 300th cycle to the discharge capacity at the first cycle.

The cycle characteristics of the non-aqueous electrolyte secondary batteries (Examples 4 and 5) using the negative electrode in which the content of the spherically shaped flaky natural graphite in the negative electrode active material is 90 parts by weight and 100 parts by weight are as follows. The content of the spherically shaped flaky natural graphite in the negative electrode active material was slightly higher than that of the nonaqueous electrolyte secondary battery used in the negative electrode (Examples 1, 2, and 3) in which the content of the flaky natural graphite was 20, 50, and 80 parts by weight. Poor, but still good.

A non-aqueous electrolyte secondary battery using only non-spherical flake natural graphite as the negative electrode active material (Comparative Example 1), or the content of spherically processed flake natural graphite in the negative electrode active material The cycle characteristics of the nonaqueous electrolyte secondary battery (Comparative Example 2) used for the negative electrode, which was 10 parts by weight, were poor.

In the non-aqueous electrolyte secondary batteries of Comparative Examples 1 and 2, all or most of the negative electrode active material is flaky natural graphite that is not processed into a spherical shape, and has poor adhesion to the current collector. As a result, when charging and discharging are repeated, it is considered that the active material falls off due to the expansion and contraction of the active material due to the lithium insertion / desorption reaction into the flaky natural graphite that is not processed into a spherical shape, and the cycle characteristics are degraded. .

For the above reasons, it is more preferable that the negative electrode active material contains 20 to 80 parts by weight of flaky natural graphite processed into a spherical shape.

Next, B of the flaky natural graphite processed into a spherical shape
FIG. 3 shows the relationship between the ET specific surface area, the average particle size, and the adhesion.
Shown in In addition, all the average particle diameters were measured by the laser diffraction method. The adhesion was evaluated by a cross-cut peel test. The cross-cut peeling test is a test in which 100 squares of 1 mm square are prepared by making cuts in 1 cm squares at 1 mm intervals in a vertical and horizontal direction, a tape is adhered, peeled off, and the peeled state is observed.
In the present peeling test, after the tape was peeled off, the copper foil as the current collector had a visible mass of 30% or less.
% Is judged as defective.

When the BET specific surface area was 3.5 m ² / g or less, the adhesion between the active materials and between the active material and the current collector was good, and peeling hardly occurred. On the other hand, BE
Those having a T specific surface area of more than 3.5 m ² / g were peeled off and defective. This is due to the large specific surface area,
This is because the binder does not sufficiently adhere the active materials and the active material and the current collector.

The average particle size is 10 μm or more and 30 μm
In the following, the peeling state was good, but those having an average particle size of less than 10 μm were peeled off and defective. This is because the binder cannot sufficiently adhere the active materials and the active material to the current collector because the average particle size is small. In addition, those having an average particle size of more than 30 μm were peeled off and were defective. This is due to the presence of abnormally large particles.

From the above, it is preferable that the BET specific surface area of the flaky natural graphite processed into a spherical shape is 3.5 m ² / g or less, and the average particle size is 10 to 30 μm.

In the above Examples and Comparative Examples, a case was described in which flaky natural graphite which was not processed into a spherical shape was used as an active material other than the spherically shaped flaky natural graphite in the negative electrode active material. In addition to flaky natural graphite that is not processed into a spherical shape, non-flaky natural graphite, artificial graphite,
The present invention is effective even when other carbon materials are used.

[0042]

In the nonaqueous electrolyte secondary battery according to the present invention, flaky natural graphite processed into a spherical shape is used as a negative electrode active material. By processing into a spherical shape, handling during electrode fabrication becomes easy. That is, since the specific surface area of the active material is small, the amount of the solvent used for producing the negative electrode mixture is small, the application to the current collector is easy, and the electrode is less likely to be peeled off. As a result, the electron conductivity between the scaly natural graphite particles and between the scaly natural graphite and the current collector is maintained even after repeated charge and discharge, and the cycle characteristics of the battery are improved. Therefore, the industrial value of the present invention is extremely high.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cross-sectional structure of a prismatic battery of an example and a comparative example of the present invention.

FIG. 2 is a diagram showing the results of a life test of the batteries of Examples and Comparative Examples of the present invention when the composition of the negative electrode active material was changed.

FIG. 3 is a diagram showing the relationship between the BET specific surface area, the average particle size, and the adhesion of the flaky graphite of the present invention processed into a sphere.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Non-aqueous electrolyte secondary battery 2 Electrode group 3 Positive electrode plate 4 Negative electrode plate 5 Separator 6 Case 7 Lid 8 Safety valve 9 Positive electrode terminal 10 Positive electrode lead

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H003 AA04 AA08 BA03 BA04 BB02 BC00 BD02 BD04 BD05 5H014 AA02 BB06 EE08 HH01 HH06 5H029 AJ05 AJ14 AK03 AL06 AL07 AM02 AM03 AM04 AM05 AM07 AM16 BJ02 BJ03 H01

Claims (4)

[Claims] 1. A non-aqueous electrolyte secondary battery using flaky natural graphite processed into a sphere as a negative electrode active material. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the flaky natural graphite processed into a sphere with respect to the total weight of the negative electrode active material is 20 to 80 parts by weight. 3. The non-aqueous electrolyte secondary according to claim 1, wherein the active material other than the spherically processed flaky natural graphite in the negative electrode active material is a flaky natural graphite that is not spherically processed. battery. 4. A BET specific surface area of a flaky natural graphite processed into a sphere as a negative electrode active material is 3.5 m ² / g or less,
4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the average particle diameter is 10 to 30 [mu] m.