JPH10233205A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

(57) [Problem] To significantly improve battery capacity and cycle characteristics of a lithium ion secondary battery. SOLUTION: A flaky graphite powder formed by forming a flaky graphite powder having an average particle diameter of 1 to 50 μm and a specific surface area of 5 to 50 m ² / g in a thickness of 1 μm or less is used as a conductive substance in a positive electrode mixture. The addition amount is 0.5 to the positive electrode mixture.
9.5% by weight. Thereby, the conductive material in the positive electrode mixture exhibits excellent conductivity, and it is possible to carry electrons uniformly and effectively to the positive electrode active material. Therefore, the content of the positive electrode active material can be increased by reducing the content of the conductive material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a nonaqueous electrolyte secondary battery provided with a positive electrode mixture containing a positive electrode active material and a conductive material.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries and non-aqueous lithium ion secondary batteries, have a large discharge capacity, a high voltage and a high energy density. Expectations are high.

In this type of non-aqueous electrolyte secondary battery, a conductive material is widely added to a positive electrode mixture for the purpose of effectively supplying electrons to a positive electrode active material. Conventionally, as the conductive material, carbon black such as acetylene black (for example, a powder having a particle size of 0.1 μm or less and a specific surface area of 200 to 800 m ² / g) and graphite powder (for example, having a particle size of 3 to 100 μm and a specific Surface area 8 ~
30 m ² / g).

[0004]

However, since the conductivity and the moldability are not always sufficient, there is a limit in reducing the content of the conductive material and increasing the content of the positive electrode active material. There has been an inconvenience that it is difficult to significantly improve the battery capacity and cycle characteristics of the electrolyte secondary battery.

In view of the above circumstances, the present invention provides a non-aqueous electrolyte secondary battery which can significantly improve battery capacity and cycle characteristics by using a specific flaky graphite powder as a conductive substance in a positive electrode mixture. It is intended to provide a battery.

[0006]

The present inventors have investigated whether a graphite-based conductive material having excellent conductivity and formability can be used for a non-aqueous electrolyte secondary battery, and as a result, have found that a specific flaky graphite has been obtained. It has been found that the powder exhibits excellent performance, and that the performance is further improved by adjusting the mixing ratio, thereby completing the present invention.

That is, the present invention provides a nonaqueous electrolyte secondary battery provided with a positive electrode mixture containing a positive electrode active material and a conductive material, having an average particle size of 1 to 50 μm and a specific surface area of 5 to 50 μm.
An exfoliated graphite powder in which m ² / g of graphite powder is formed into a flake having a thickness of 1 μm or less is added as the conductive substance in a range of 0.5 to 9.5% by weight based on the positive electrode mixture. It is composed.

The reason why the thickness of the flaky graphite powder is limited to 1 μm or less is that if the thickness exceeds 1 μm, the conductivity and liquid absorption deteriorate, and electrons are uniformly and effectively applied to the positive electrode active material. This is because it cannot be transmitted. Further, the reason why the average particle size of the flaky graphite powder is limited to 1 to 50 μm is that when the average particle size exceeds 50 μm, the average particle size of the conductive material becomes larger than the particle size of the positive electrode active material. This is because electrons may not be able to be uniformly and effectively transported to the positive electrode active material dispersed throughout the mixture.

[0009]

Embodiments of the present invention will be described below.

[0010] A lithium ion secondary battery, which is a kind of non-aqueous electrolyte secondary battery according to the present invention, has an electrode body, and this electrode body comprises a positive electrode mixture composed of a metal oxide containing lithium, and a lithium mixture. And a negative electrode made of a carbonaceous substance capable of inserting and extracting a carbon dioxide through a separator made of polyethylene. An electrolytic solution is injected into the electrode body. The electrolytic solution is prepared by mixing LiPF ₆ as an electrolyte with a suitable non-aqueous solvent (for example, a solvent in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 1: 1).
ol / l.

The positive electrode mixture contains a positive electrode active material, and a conductive material is added to the positive electrode active material. This conductive material is a flaky graphite powder formed by forming a flaky graphite powder having an average particle diameter of 1 to 50 μm and a specific surface area of 5 to 50 m ² / g in a thickness of 1 μm or less. On the other hand, it is 0.5 to 9.5% by weight.

As a raw material of the flaky graphite powder, graphite powder having a crystallinity of Lc of 755 ° or more and Co of 3.35 ° or less (for example, powders of natural scale graphite, pyrolytic graphite, quiche graphite, etc.) ). Expanded graphite powder obtained by subjecting this graphite powder to H ₂ SO ₄ -GIC with an acid such as concentrated sulfuric acid or nitric acid and subjecting it to steps such as water washing, dehydration, drying and heat treatment can be used. Then, these graphite powders are subjected to a vapor adsorption treatment and a cooling and solidification treatment using a liquid such as benzene, alcohol, water or the like to be exfoliated, whereby flake graphite powders can be obtained. The method of exfoliating the graphite powder is not limited to the above method.

Since the lithium ion secondary battery according to the present invention has the above-described structure, the conductive material in the positive electrode mixture exhibits excellent conductivity and uniformly and effectively transfers electrons to the positive electrode active material. Becomes possible. Therefore, the content of the conductive material can be reduced and the content of the positive electrode active material can be increased, thereby significantly improving the battery capacity and cycle characteristics of the lithium ion secondary battery.

[0014]

Embodiments of the present invention will be described below. FIG. 1 is a graph showing a result of a charge / discharge cycle test of a lithium ion secondary battery, and FIG. 2 is a graph showing a measurement result of a battery capacity of the lithium ion secondary battery.

<Example 1> First, Li was used as a positive electrode active material.
Mn ₂ O ₄ , the above-mentioned flaky graphite powder as a conductive substance, and polyvinylidene fluoride (hereinafter referred to as PVDF) as a binder.
70: 86.95: 4.35 parts by weight in a mixture,
n-methyl-2-pyrrolidone (hereinafter referred to as NMP)
Was added to form a slurry. This slurry was applied to both sides of an aluminum foil and dried, rolled to a predetermined thickness, vacuum-dried at 200 ° C. for 5 hours, and then cut to a predetermined size to produce a sheet-shaped positive electrode. .

On the other hand, PVDF as a binder was mixed with coal-based pitch coke as a negative electrode active material, and NMP was added thereto to form a slurry. The slurry was applied to both surfaces of a copper foil and dried. This is rolled to a predetermined thickness,
After vacuum drying at 200 ° C. for 5 hours, the sheet was cut into a predetermined size to produce a sheet-shaped negative electrode.

Thereafter, these electrodes (positive electrode and negative electrode)
Were laminated via a film-like separator in a glove box in a dry air atmosphere, and this was spirally wound to obtain an electrode body.

Next, after inserting this electrode body into an appropriate case, 1 mol / l of LiPF ₆ as an electrolyte was added to a solvent in which propylene carbonate, ethylene carbonate and dimethyl carbonate were mixed at a ratio of 1: 1: 3. An electrolyte was injected to assemble a lithium ion secondary battery (Example 1).

The mixing ratio of the positive electrode of this lithium ion secondary battery is shown in the column of "Example 1" in Table 1.

[0020]

[Table 1]

Example 2 The mixing ratio of the positive electrode was 0.47:
The ratio was 94.79: 4.74 (see the column of “Example 2” in Table 1), and the other conditions were the same as in Example 1 to produce a lithium ion secondary battery (Example 2).

Example 3 The mixing ratio of the positive electrode was 0.38:
The ratio was 94.88: 4.74 (see the column of “Example 3” in Table 1), and the other conditions were the same as in Example 1 to produce a lithium ion secondary battery (Example 3).

Example 4 The mixing ratio of the positive electrode was 9.50:
86.20: 4.30 (see the column of “Example 4” in Table 1), and otherwise the procedure of Example 1 was repeated to prepare a lithium ion secondary battery (Example 4).

Example 5 The mixing ratio of the positive electrode was 0.28:
The ratio was 94.97: 4.75 (see the column of “Example 5” in Table 1), and the other conditions were the same as in Example 1 to produce a lithium ion secondary battery (Example 5).

Example 6 The mixing ratio of the positive electrode was 10.2
6: 85.47: 4.27 ("Example 6" in Table 1)
, A lithium ion secondary battery (Example 6) was produced in the same manner as in Example 1.

Example 7 The mixing ratio of the positive electrode was 4.55:
The lithium ion secondary battery (Example 7) was manufactured in the same manner as in Example 1 except that the ratio was 90.90: 4.55 (see the column of “Example 7” in Table 1).

Example 8 The mixing ratio of the positive electrode was 2.06:
93.48: 4.46 (see the column of “Example 8” in Table 1), and otherwise, a lithium ion secondary battery (Example 8) was produced in the same manner as in Example 1.

Example 9 The mixing ratio of the positive electrode was 7.14:
88.52: 4.34 (see the column of “Example 9” in Table 1), and otherwise the procedure of Example 1 was repeated to prepare a lithium ion secondary battery (Example 9).

<Comparative Example 1> A lithium ion secondary battery was prepared in the same manner as in Example 1 except that acetylene black was used as the conductive substance in the positive electrode (see the column of "Comparative Example 1" in Table 1). (Comparative Example 1) was produced.

<Charge / Discharge Cycle Test> First Embodiment
A charge / discharge cycle test was performed on the lithium ion secondary batteries of Comparative Examples 1 to 9 and Comparative Example 1. The results are shown in FIG. 1 as a graph (vertical axis: battery capacity, horizontal axis: number of cycles).

As is clear from FIG. 1, the lithium ion secondary batteries of Examples 5 and 6 have worse cycle characteristics than the lithium ion secondary batteries of Comparative Example 1, and the lithium ion secondary batteries of Examples 3 and 4 Shows cycle characteristics similar to those of the lithium ion secondary battery of Comparative Example 1, and
The cycle characteristics of the lithium ion secondary batteries of 2, 7 to 9 were better than those of the lithium ion secondary battery of Comparative Example 1.

Further, when the lithium ion secondary batteries of Examples 1 to 9 and Comparative Example 1 which were repeatedly charged and discharged were disassembled and observed, the lithium ion secondary batteries of Examples 1 to 9 showed that The generation of a substance considered to be a polymer disappeared. This is thought to be because side reactions were prevented.

<Measurement of Battery Capacity> In order to examine the effect of the mixing ratio of the flaky graphite powder on the battery capacity, the lithium ion secondary batteries of Examples 1 to 9 and Comparative Example 1 were charged and discharged for 350 cycles. The battery capacity at the time of repetition was measured. The results are shown in the graph of FIG. 2 (vertical axis: battery capacity, horizontal axis: compounding ratio of flaky graphite powder).

As is apparent from FIG. 2, focusing on the lithium ion secondary batteries of Examples 1 to 9, the battery capacity becomes maximum when the compounding ratio of the flaky graphite powder is about 4% by weight, and the flaky graphite The battery capacity tended to decrease as the mixing ratio of the powder shifted from this. Further, in comparison with the lithium ion secondary battery of Comparative Example 1, the lithium ion secondary batteries of Examples 1 to 4 and Examples 7 to 9, that is, the compounding ratio of the flaky graphite powder was 0.5 to 9.5. The lithium ion secondary battery in the range of weight% has a higher battery capacity than the lithium ion secondary battery of Comparative Example 1.

[0035]

As described above, according to the present invention, in a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery provided with a positive electrode mixture containing a positive electrode active material and a conductive material, Particle size 1-50 μm and specific surface area 5-50 m ²
/ G of graphite powder in the form of a flake having a thickness of 1 μm or less is added as the conductive material in the range of 0.5 to 9.5% by weight based on the positive electrode mixture. Therefore, by using the specific flaky graphite powder as a conductive material in the positive electrode mixture, the electron donating property to the positive electrode is improved, and electrons can be uniformly and effectively supplied to the positive electrode active material in the positive electrode mixture. In addition to carrying, side reactions during charging and discharging can be suppressed. As a result, the battery capacity of the nonaqueous electrolyte secondary battery is greatly increased, and at the same time, the cycle characteristics are greatly improved.

[Brief description of the drawings]

FIG. 1 is a graph showing a result of a charge / discharge cycle test of a lithium ion secondary battery.

FIG. 2 is a graph showing a measurement result of a battery capacity of a lithium ion secondary battery.

Claims (1)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a positive electrode mixture containing a positive electrode active material and a conductive material, wherein the average particle size is 1 to 50 μm and the specific surface area is 5 to 50 m ² / g.
Characterized in that flaky graphite powder formed in the form of flakes having a thickness of 1 μm or less was added as the conductive substance in the range of 0.5 to 9.5% by weight with respect to the positive electrode mixture. Non-aqueous electrolyte secondary battery.