JP2001126732A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preparing a positive electrode plate of a lithium secondary cell, and to provide a cell restraining to a very low extent the deterioration of or charging capacity or a loading property in the use of a repeat charging-discharging cycle. SOLUTION: A lithium secondary cell comprises a positive electrode formed by applying a paste for an electrode to a collector by an adhesive with a molecular weight of a fluorine resin of 1,000,000 to 400,000 and drying it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to an improvement in a positive electrode plate thereof.

[0002]

2. Description of the Related Art Conventionally, in manufacturing an electrode plate of a lithium secondary battery, a paste in which an active material, a conductive agent, a binder, and a thickener are uniformly mixed and dispersed is applied to a current collector and dried. , Rolling and the like to obtain a predetermined density. By controlling the temperature at the time of mixing as disclosed in JP-A-8-273661, the aggregation of the paste is suppressed, or the blending ratio is regulated as disclosed in JP-A-9-82364. By doing so, the dispersibility of the paste was improved.

[0003]

However, according to this production method, the mixing degree of the active material, the conductive agent, the binder and the thickener in the paste state at the time of mixing is improved, but the shearing of the stirring blades is improved. Due to the force, a part of the binder became fibrous to form aggregates, and separation and sedimentation occurred during paste storage. In addition, the binding properties of the active materials after application and drying and the adhesiveness of the current collector are poor, and during storage at high temperatures or during repeated charge / discharge use, the active material is peeled off from the current collector, falling off,
There is a problem that the discharge capacity is reduced and the load characteristics are deteriorated.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems by kneading and dispersing a positive electrode active material mainly composed of a lithium-containing transition metal composite oxide, a binder, and a thickener. A paste obtained by applying the paste obtained on the current collector to a current collector and drying the same, and using a fluororesin having a molecular weight of 1,000,000 to 4,000,000 as the binder. Battery.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, in the step of producing a paste for a positive electrode by kneading and dispersing a positive electrode active material, a binder and a thickener, the binder has a molecular weight of 1,000,000 or more.
In this method, the positive electrode paste is applied to a current collector, dried, and then rolled to a predetermined density to be used for a positive electrode plate.

A battery using this electrode plate can minimize the deterioration of the charge / discharge capacity and the deterioration of the load characteristics in repeated use of charge / discharge.

[0007] As the fluororesin, polytetrafluoroethylene resin (PTFE), fluoroalkoxy resin (PF)
A), fluorinated ethylene propylene resin (FEP) and the like. Any of these may be used as long as it is dispersed in water to form a fluororesin dispersion.

[0008]

Embodiments of the present invention will be described below. 50 parts by weight of LiCoO ₂ powder as a positive electrode active material, 1.5 parts by weight of acetylene black as a conductive agent, and 30 parts by weight of an aqueous solution prepared by dissolving 1 part by weight of carboxymethyl cellulose as a thickener in 99 parts by weight of water. Then, an aqueous solution obtained by mixing and dispersing was obtained. As a binder, 3 parts by weight of a 50 parts by weight aqueous solution of PTFE having a molecular weight of 3,000,000 of the present invention were mixed with the above aqueous solution and mixed and dispersed to obtain a positive electrode paste.

Next, this positive electrode paste is coated with a thickness of 20 μm.
The thickness of one side is coated to 180 μm using a die coater on an aluminum foil, and then dried, and then the positive electrode plate is heated at a heat treatment temperature of PTFE of 200 to 350 ° C. to adhere the current collector to the positive electrode mixture layer. Improve the quality. After this, the thickness 0.18
mm, and cut to obtain a positive electrode plate. One embodiment of the lithium secondary battery of the present invention is a cylindrical lithium secondary battery as shown in FIG. 1, which includes an electrode plate group using the negative electrode plate 6 obtained by the method for producing a negative electrode paste of the present invention. , Electrolyte and
It consists of a battery case that houses these. The electrode plate group includes a sheet-shaped positive electrode plate 5, a sheet-shaped negative electrode plate 6, a sheet-shaped separator 7 that insulates between the positive electrode plate 5 and the negative electrode plate 6,
It comprises a positive electrode lead 3, a negative electrode lead 9, an upper insulating plate 4, and a lower insulating plate 10.

The positive electrode plate 5 is formed by coating both sides of an aluminum foil. The separator 7 is a porous polypropylene film, which is stacked and spirally wound, and is tightly housed in a cylindrical battery case.

A method for manufacturing the negative electrode plate 6 will be described. First, 50 parts by weight of flaky graphite powder, 5 parts by weight of styrene butadiene rubber as a binder, and 23 parts by weight of an aqueous solution dissolved in 99 parts by weight of water with respect to 1 part by weight of carboxymethyl cellulose as a thickener were mixed and dispersed. Thus, a paste for a negative electrode was obtained. The obtained negative electrode paste was applied to both surfaces of a negative electrode current collector made of copper foil having a thickness of 40 μm using a die coater, dried, rolled to a thickness of 0.2 mm, and cut to form a sheet-shaped negative electrode plate 6. did.

The electrolytic solution contains 30 vol% of ethylene carbonate.
Diethyl carbonate 50 vol% and methyl propionate 20 v
1 mol / liter of LiPF _{6 in} the mixture with
Consists of a substance dissolved at a concentration of This electrolytic solution is accommodated in a battery case and impregnated in the positive electrode active material layer and the negative electrode active material layer. In the battery reaction, the Li ion between the positive electrode plate 5 and the negative electrode plate 6 passes through micropores of a porous separator. Responsible for the move.

The battery case includes a case body 8 obtained by deep drawing of a stainless steel sheet having resistance to organic electrolyte, and a sealing plate 1.
And an insulating gasket 2 which insulates between the case body 8 and a gas case.

The above battery was used as the battery of the present invention.

(Comparative Example 1) P having a binder having a molecular weight of 800,000
A positive electrode plate prepared by the same adjustment procedure as in the example except that TFE was used was used as a comparative electrode plate 1, and a battery using this was used as a comparative battery 1.

(Comparative Example 2) A comparative electrode plate 2 prepared by the same adjustment procedure as in the example except that PTFE having a molecular weight of 7,000,000 was used as a binder, and a battery using the same was used as a comparative battery 2 And

On the other hand, the positive electrodes of the batteries 1 and 2 in comparison with the battery of the present invention were evaluated as shown below. The number of agglomerates and pinholes present on the surface of the positive electrode plate of 1000 cm ² was visually counted and shown in Table 1.

[0018]

[Table 1]

As shown in Table 1, the electrode plate of the present invention was able to prevent agglomerates and pinholes. (Table 1)
Shows an example in which the molecular weight is 3,000,000, but the molecular weight is 100
The same effect was obtained when the value was in the range of 10,000 to 4,000,000.

When the molecular weight exceeds 4,000,000, fibrous formation of PTFE proceeds due to shearing action of blades or the like during stirring, and agglomerates in the paste are generated. When the molecular weight is less than 1,000,000, the fiber length of PTFE is short and fiberization is insufficient, so that the positive electrode active material peeled off the current collector and pinholes were generated.

FIG. 2 shows the results of the cycle life characteristics of the batteries 1 and 2 in comparison with the battery of the present invention.

As a condition of the cycle life test, charging is 5
The operation was performed at a constant current of 00 mA, and when the voltage reached 4.1 V, the operation was started.
Switching to 1 V constant voltage charging was performed, and charging was performed for a total of 2 hours. The discharge was performed at 20 ° C. and 720 mA. When the discharge potential reached 3.0 V, the discharge was terminated and the next charge was started.

FIG. 2 shows that the battery of the present invention has less capacity deterioration and superior cycle characteristics to the comparative battery even after repeated charging and discharging.

This is because the battery of the present invention has improved dispersibility with the positive electrode active material and the binder, and has improved adhesion with the current collector. It is considered that it became difficult.

These batteries were charged at 60 ° C. 20
After several days at normal temperature, charge and discharge several times
Discharging was performed at 20 mA to determine the capacity until the voltage reached 3.0 V, and the ratio to the capacity before storage was shown in (Table 2).

[0026]

[Table 2]

As shown in (Table 2), it was found that the battery of the present invention showed less capacity deterioration even at high temperature storage.

[0028]

As described above, according to the present invention, the deterioration of the charge / discharge capacity and the deterioration of the load characteristics can be suppressed to a very small value in the use of repeated charge / discharge. Further, even under severe conditions such as being left at a high temperature for a long period of time, deterioration of the charge / discharge capacity and deterioration of the load characteristics can be suppressed.

Further, since the adhesion between the paste coating film and the current collector on the electrode plate is also improved, the coating film does not fall off from the current collector during the battery assembling process, and the workability during manufacturing is improved. Can be.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a lithium secondary battery of the present invention.

FIG. 2 is a diagram showing cycle life characteristics of a battery of the present invention and a battery in comparison with the battery of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sealing plate 2 Insulating gasket 3 Positive electrode lead 4 Upper insulating plate 5 Positive electrode plate 6 Negative electrode plate 7 Separator 8 Case body 9 Negative electrode lead 10 Lower insulating plate

 ──────────────────────────────────────────────────の Continuing from the front page (72) Hiroshi Matsuno 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. F term (reference) 5H003 AA03 AA04 BA01 BA03 BB05 BB11 BC01 BD00 5H014 AA02 AA06 BB01 BB06 BB08 HH00 5H029 AJ04 AJ05 AK03 AL12 AM03 AM05 AM07 BJ02 BJ14 CJ02 CJ08 CJ22 DJ16 EJ12

Claims (1)

[Claims] 1. A paste obtained by kneading and dispersing a positive electrode active material mainly composed of a lithium-containing transition metal composite oxide, a conductive agent, a thickener and a binder, and applying the paste to a current collector. A lithium secondary battery using an electrode plate obtained by drying the above, wherein the binder has a molecular weight of 1,000,000 or more 4
A lithium secondary battery using a fluororesin of 100,000 or less.