JP2000082470A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the deterioration of capacity without causing deterioration, even if a battery is used in an on-vehicle environment by using a binder formed of a copolymer of PVDF and HFP wherein the ratio of PVDF to HFP is in a specific range. SOLUTION: A binder in which the ratio of PVDF to HFP is 80-95:20-5 is used. When the ratio of initial capacity to the capacity after heating is evaluated as a capacity recovery rate, the capacity recovery rate is improved as compared with PVDF alone, when the mixing ratio of PVDF to HFP is in the range of 80-95:20-5. The initial peeling strength of an electrode agent according to a tape peel test is improved as compared with PVDF alone as well. Because gaps between particles inside a positive electrode agent and a negative electrode agent can be prevented from growing by improving the adhesion of the binder so that the internal resistance of a battery can be restrained from rising, the deterioration of capacity can be reduced. When this copolymer is used, baking temperature can be lowered as compared with the case that PVDF is used alone, and baking cost can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a secondary battery, particularly a lithium ion secondary battery in which a binder used for an electrode is improved.

[0002]

2. Description of the Related Art Conventionally, PVDF has been used as a binder for binding active materials constituting electrodes of a lithium ion secondary battery or for binding an active material and a current collector.

[0003] Also, JP-A-4-95363 discloses that
In order to improve the binding property of PVDF (vinylidene fluoride), it has been proposed to use a copolymer of PVDF and HFP (hexafluoropropylene).

[0004]

However, the above conventional P
In the case where a binder made of VDF is used, there is a problem that the battery capacity is lowered because the binder is exposed to a high temperature for a long period of time, especially in a vehicle-mounted use. This is because the positive electrode material and the negative electrode material of the lithium ion secondary battery are deteriorated by prolonged exposure to a high temperature, and P
It is considered that VDF also deteriorated and its adhesive strength was reduced. When the adhesive strength decreases, the particles inside the positive electrode agent and the negative electrode agent separate from each other or from the current collector, a gap is formed between them, and the contact resistance increases, so that the internal resistance of the battery increases and the battery capacity decreases. It is.

[0005] When the above-mentioned copolymer of PVDF and HFP is used, rubber elasticity is imparted to the binder to improve the binding property. PVD because of the high temperature environment
There is an optimal range for the ratio between F and HFP. Therefore,
It does not mean that any copolymer of PVDF and HFP can be used.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide a lithium ion battery that does not deteriorate even when used in an on-vehicle environment and can suppress a decrease in capacity.
Another object is to provide a battery.

[0007]

In order to achieve the above object, the present invention relates to a lithium ion secondary battery, comprising:
Consisting of a copolymer of DF and HFP, PVDF: HF
It is characterized in that a binder having P = 80 to 95:20 to 5 is used.

[0008]

Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

[0009] The present inventors have studied binders that do not lose their adhesive strength even after prolonged exposure to high temperatures, and have found that PVDF and H
A copolymer in which the blending ratio with FP was optimized was found.

FIG. 1 shows the results of a high-temperature deterioration test when the mixing ratio of PVDF and HFP is variously changed. Here, in the high-temperature deterioration test, the initial capacity of a lithium ion secondary battery configured by forming an electrode using each binder, impregnating the electrode with the electrolyte, and arranging the electrode facing each other with a separator made of polyethylene or the like interposed therebetween. The capacity is measured after heating at 60 ° C. for 50 hours, and the ratio between the initial capacity and the capacity after heating is evaluated as a capacity recovery rate. As can be seen from FIG. 1, PVD of the copolymer used as a binder
The compounding ratio of F to HFP is PVDF: HFP = 80 to
In the range of 95:20 to 5, the capacity recovery rate is improved as compared with PVDF alone.

FIG. 2 shows the test results of the initial peel strength of the electrode material. In this test, the blend ratio of PVDF and HFP of the copolymer used as a binder was changed,
The initial peel strength of the electrode agent is measured by a tape peel test. As can be seen from FIG. 2, as in the case of FIG. 1, in the range of PVDF: HFP = 80 to 95:20 to 5, the initial peel strength is higher than that of PVDF alone.

In FIG. 1, the capacity recovery rate in the high-temperature deterioration test is higher than that of the PVDF alone,
By adding HFP to VDF, the initial adhesive strength is improved as shown in FIG.
It is considered that the adhesive strength is higher than that of F alone. By improving the adhesive force of the binder, it is possible to prevent a gap from being generated between particles inside the positive electrode agent and the negative electrode agent, and it is possible to suppress an increase in the internal resistance of the battery. it can. However, PVDF / HFP =
When the blending ratio of PVDF becomes higher than 95/5,
Since the characteristics are almost the same as PVDF alone, PVD
It is considered that the upper limit of the compounding ratio of F is 95%.

FIG. 3 shows the relationship between the blending ratio of PVDF in the copolymer used as a binder and the melting point of the copolymer. As shown in FIG.
Is less than 80%, the melting point of the copolymer becomes 1
It will be below 00 ° C. Therefore, lithium ion 2
It cannot be used for in-vehicle applications where the secondary battery is exposed to high temperatures for long periods. Also, during the electrode manufacturing process, NMP which is a solvent is used.
There is a step of drying the electrode at about 80 ° C. in order to remove the like, but when the melting point of the copolymer used as the binder becomes low, the binder dissolves in this drying step, and the electrode having a uniform film thickness is formed. Cannot be manufactured. For this reason, PV
It is considered that the lower limit of the blending ratio of DF is 80%.

From FIG. 1, FIG. 2 and FIG. 3 described above, it can be seen that PVDF and HFP in the copolymer used as a binder are used.
Is blended with PVDF: HFP = 80-95: 20.
A range of ~ 5 is considered optimal. The melting point of the copolymer at this time is 110 to 140 ° C.

FIG. 4 shows that PVDF: HFP = 87.5:
Using a copolymer having a compounding ratio of 12.5 as a binder,
The result of the high temperature degradation test when the electrode firing temperature is changed is shown. In FIG. 4, the vertical axis indicates 60 with respect to the initial capacity.
The capacity recovery rate after heating at 50 ° C. for 50 hours is shown. This electrode baking is performed to fix the active material applied on the current collector together with the binder to the current collector. As shown in FIG. 4, when the electrode firing temperature is 140 ° C. to 160 ° C., the capacity recovery rate is considered to exceed 60%, and firing in this temperature range is considered suitable. Since the sintering temperature when PVDF is used alone as a binder is about 175 ° C., when the copolymer according to the present invention is used,
The firing temperature can be reduced, and the cost for firing can be reduced.

[0016]

As described above, according to the present invention,
By using a copolymer of PVDF and HFP as a binder and optimizing the compounding ratio, the adhesiveness of the binder can be improved, and an increase in the resistance of the electrode can be suppressed. Therefore, an increase in the internal resistance of the battery can be prevented, and a decrease in capacity can be suppressed.

[Brief description of the drawings]

FIG. 1: PVD of a copolymer used as a binder
It is a figure showing the result of a high temperature degradation test at the time of changing the compounding ratio of F and HFP.

FIG. 2. PVD of a copolymer used as a binder
It is a figure showing the result of initial peel strength of an electrode agent at the time of changing the compounding ratio of F and HFP.

FIG. 3. PVD of a copolymer used as a binder
It is a figure which shows the change of the melting point of a binder at the time of changing the compounding ratio of F and HFP.

FIG. 4 is a diagram showing a relationship between an electrode firing temperature and a capacity recovery rate.

Claims (1)

[Claims] 1. A lithium ion secondary battery comprising a binder comprising a copolymer of PVDF and HFP, wherein PVDF: HFP = 80-95: 20-5.