JP2003187870A - Polymer electrolyte and nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polymer electrolyte which has a good electrolyte solution holding property and excellent high temperature characteristics, and to provide a nonaqueous electrolyte secondary battery. <P>SOLUTION: A copolymer, which is formed by copolymerizing a mixture including a vinylidene fluoride monomer, a tetrafluoroethylene monomer, and a hexafluoropropylene monomer, is used as a polymer material composing the polymer electrolyte. The copolymer is characterized in that the ratio of the tetrafluoroethylene unit is not less than 5 wt.% and below 30 wt.%. Alternatively, the copolymer is characterized in that the ratio of the hexafluoropropylene unit is not more than 10 wt.%. As a result, the polymer electrolyte, which has the good electrolyte solution holding property and the excellent high temperature characteristics, and the nonaqueous electrolyte secondary battery can be provided. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a polymer electrolyte,
And a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art In recent years, non-aqueous electrolyte secondary batteries (hereinafter sometimes simply referred to as "batteries") have the purpose of reducing the amount of free electrolyte and improving battery safety. Therefore, a polymer battery using a polymer electrolyte instead of the conventional liquid electrolyte is attracting attention. This polymer electrolyte has a constitution in which a polymer material having a solid or gel-like network structure is made compatible with and holds an electrolytic solution having ion conductivity.

Examples of the polymer material used for such a polymer electrolyte include vinylidene fluoride-hexafluoropropylene copolymer. This one is
Since it easily contains an electrolytic solution as a plasticizer and easily forms a gel, it is preferably used as a polymer material for a polymer electrolyte.

[0004]

However, in a battery in which such a copolymer is applied to a polymer electrolyte,
When left in a high temperature environment for a long time, the copolymer may not be able to maintain its shape and may be dissolved in the electrolytic solution, increasing the viscosity of the electrolytic solution and causing a decrease in electrical conductivity. Therefore, sufficient high temperature characteristics cannot be obtained, and improvement has been demanded.

In particular, since the polymer electrolyte has a solid structure, it has a problem that its electric conductivity is lower than that of the liquid electrolyte. As a remedy for this problem, it is common to increase the copolymerization ratio of hexafluoropropylene in the copolymer to increase the amount of electrolyte retained. Thereby, the plasticity of the polymer electrolyte is increased, so that the conductivity of lithium ions can be improved and the electrical conductivity can be improved.
However, in such a structure, the mechanical strength of the copolymer becomes low, so that the problem of deterioration in a high temperature environment becomes remarkable.

The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to have a good electrolyte retention property and
It is intended to provide a polymer electrolyte having excellent high temperature characteristics and a non-aqueous electrolyte secondary battery battery.

[0007]

Means for Solving the Problems The present inventor has found that a polymer electrolyte having good electrolyte retention and excellent high temperature characteristics,
In addition, as a result of intensive research to provide a non-aqueous electrolyte secondary battery, it was found that a copolymer formed by copolymerizing monomers including vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene can be used. It was found to be effective.

Further, it has been found that by optimizing the composition of the copolymer, it is possible to provide a battery having a high electrolyte retaining property and excellent high-temperature characteristics, and completed the present invention. The present invention has been made based on such novel findings.

That is, the present invention is a polymer electrolyte for a non-aqueous electrolyte secondary battery in which a polymer material holds an electrolytic solution, and the polymer electrolyte holds a non-aqueous electrolyte secondary battery. A polymer electrolyte for use, wherein the polymer material is a copolymer formed by copolymerizing a monomer containing vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene, and vinylidene fluoride The ratio of tetrafluoroethylene units to the total of units, tetrafluoroethylene units, and hexafluoropropylene units is 5% by weight or more and 30% by weight.
It is characterized by being less than.

The non-aqueous electrolyte secondary battery of the present invention is a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a polymer electrolyte in which a polymer material holds an electrolytic solution,
The polymer material is formed by copolymerizing a monomer containing vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene, and contains vinylidene fluoride units, tetrafluoroethylene units, and hexafluoropropylene units. The copolymer is characterized in that the ratio of tetrafluoroethylene units to the total is 5% by weight or more and less than 30% by weight.

The polymer material used in the present invention includes:
A copolymer obtained by copolymerizing three kinds of monomers of vinylidene fluoride, tetrafluoroethylene and hexafluoropropylene can be preferably used. As described above, vinylidene fluoride-hexafluoropropylene copolymer is easily dissolved at high temperature, but it is considered that the dissolution can be suppressed by copolymerizing tetrafluoroethylene with this and adjusting the plasticity of the copolymer. To be Further, in addition to these three kinds of monomers, other monomers may be copolymerized within a range that does not impair the effects of the present invention.

In particular, the proportion of tetrafluoroethylene units in the copolymer should be 5% by weight or more and less than 30% by weight based on the total of vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units. Is preferred. This is because if it is 30% by weight or more, the compatibility with the electrolytic solution becomes poor, and a sufficient amount of the electrolytic solution cannot be retained. On the other hand, if it is less than 5% by weight, the copolymer is likely to dissolve when left at high temperature.

The proportion of hexafluoroethylene units in the copolymer is preferably 10% by weight or less based on the total of vinylidene fluoride units, tetrafluoroethylene units and hexafluoropropylene units. This is because if it exceeds 10% by weight, the crystallinity of the copolymer is lowered and the copolymer is easily dissolved when left at high temperature.

The method of forming a polymer electrolyte using such a copolymer is not particularly limited as long as it is a method usually applied to the production of non-aqueous electrolyte secondary batteries. Specifically, for example, the positive electrode plate and the negative electrode plate may be impregnated with the copolymer dispersed in an appropriate solvent, and the electrolytic solution may be retained after a solvent extraction step using purified water or the like. .
Alternatively, the separator may be impregnated with or coated with a dispersion of the copolymer in a suitable solvent and dried, and then the electrolytic solution may be held therein. As a separator,
What is normally used as a separator of a non-aqueous electrolyte secondary battery may be used, and for example, a polypropylene microporous membrane or the like can be used. Further, the electrolytic solution may be held in a film-shaped copolymer without using a separator.

The solvent used in the electrolytic solution of the present invention is not particularly limited as long as it is a solvent usually used in electrolytic solutions for non-aqueous electrolyte secondary batteries, and examples thereof include ethylene carbonate, diethyl carbonate, dimethyl carbonate, propylene carbonate, γ-butyrolactone, dimethyl sulfoxide, tetrahydrofuran, dimethoxyethane, dimethylacedamide and the like can be used. These solvents may be used alone or in combination of two or more.

The electrolyte salt contained in the electrolytic solution is not particularly limited as long as it is an electrolyte salt normally used in non-aqueous electrolyte secondary batteries, and examples thereof include LiClO ₄ and LiB.
_{F 4, LiPF 6, LiB (} C 6 H 5) 4, LiN (S
O ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiOS
O ₂ CF ₃ or the like can be used. Further, the electrolytic solution may contain additives such as an antioxidant, a flame retardant, a radical scavenger, and a surfactant.

[0017]

EFFECTS OF THE INVENTION According to the present invention, it is possible to provide a polymer electrolyte and a non-aqueous electrolyte secondary battery which have good electrolyte retention and excellent high temperature characteristics.

[0018]

EXAMPLES The present invention will be described in detail below with reference to examples.

<Example 1> 1. Preparation of Lithium Ion Secondary Battery 1) Preparation of Polymer Material An autoclave made of SUS316 having a capacity of 10 L equipped with a stirrer was evacuated and diethyl malonate 1.0 g, ammonium fluorooctanoate 2.0 g, and disodium hydrogen phosphate 10 were prepared. 0.0 g and ion-exchanged water 4500 g were introduced. Next, a mixed gas of 54 g of vinylidene fluoride, 21 g of tetrafluoroethylene, and 32 g of hexafluoropropylene was injected under a gauge pressure of 2.5 MPa using a compressor. Then, the autoclave was heated to 80 ° C., and 4.0 g of ammonium peroxosulfate was introduced by a lightweight pump to start the polymerization reaction.

After initiation of the polymerization reaction, vinylidene fluoride 325
g, tetrafluoroethylene 61 g, and hexafluoropropylene 47 g were mixed and added over 2 hours. After the addition was completed, the autoclave was cooled to room temperature. Then, the residual gas was purged, and the emulsion was taken out from the autoclave and dropped into a 1 wt% calcium chloride aqueous solution with stirring. After completion of the dropping, the coagulated product was filtered off and ultrapure water (ionic conductivity of 1.0 μS / cm at 20 ° C.)
The mixture was stirred, washed with water, filtered, and dried. Thus, white powdery polyvinylidene fluoride-tetrafluoroethylene-hexafluoropropylene (P (VdF-
A TFE-HFP)) copolymer was obtained.

Copolymer composition of the obtained copolymer ( ¹⁹ F-
(By NMR) was 82% by weight vinylidene fluoride, 15% by weight tetrafluoroethylene, 3% by weight hexafluoropropylene.

2) Preparation of Positive Electrode Lithium cobalt oxide was used as a positive electrode active material, and vinylidene fluoride as a binder and acetylene black as a conductive agent were added to the positive electrode active material at a weight ratio of 91: 5: 4. The mixture was mixed, and N-methylpyrrolidone was added to prepare a positive electrode mixture paste. This paste was evenly applied to both sides of a current collector made of an aluminum foil having a thickness of 20 μm, dried,
Pressed. In this way, a strip-shaped positive electrode sheet having the positive electrode active material layer was produced. A positive electrode lead was welded to one end of this positive electrode sheet.

3) Preparation of Negative Electrode Graphite was used as a negative electrode active material, and vinylidene fluoride as a binder was added to the graphite in a weight ratio of 92: 8.
And mixed with N-methylpyrrolidone to prepare a negative electrode mixture paste. This paste is 10μm thick
The current collector made of copper foil was evenly applied on both sides, dried, pressed, and then cut. In this way, a strip-shaped negative electrode sheet provided with the negative electrode active material layer was produced. A negative electrode lead was welded to one end of this negative electrode sheet.

4) Preparation of Electrolyte Solution Ethylene carbonate and dimethyl carbonate were mixed in a volume ratio of 3: 7 to prepare a non-aqueous solvent. LiPF ₆ as an electrolyte salt was added to the non-aqueous solvent in an amount of 1.
An electrolyte solution was prepared by adding at a concentration of 2 mol / l.

5) Impregnation of positive electrode sheet and negative electrode sheet with polymer material The P (VdF-TFE-HFP) copolymer prepared in 1) above was dissolved in N-methylpyrrolidone to a concentration of 10%. Thus, a copolymer solution was prepared. The positive electrode sheet and the negative electrode sheet prepared in 2) and 3) above were immersed in this copolymer solution to sufficiently impregnate the solution. Next, both of these sheets were immersed in water to replace the solvent N-methylpyrrolidone with water. Then, both sheets were dried to prepare a positive electrode sheet and a negative electrode sheet holding a polymer material having a network structure.

6) Preparation of Battery A laminated film in which a polyethylene terephthalate film, an aluminum foil, an adhesive layer, a first modified polyolefin layer and a second modified polyolefin layer are laminated in this order,
A bag-shaped battery case was prepared by folding back the second modified polyolefin layer side as the inside and welding the bottom side and the side side.

The positive electrode sheet and the negative electrode sheet holding the polymer material produced in the above 5) were laminated and then wound to produce a power generating element. Then, this power generating element was housed in a battery case.

The electrolytic solution prepared in the above 4) was poured into the battery case, and the electrolytic solution was held in the polymer material to obtain a gel polymer electrolyte. Then, the opening of the battery case was sealed by thermocompression bonding to complete the battery. The nominal capacity of the manufactured battery was 600 mAh.

2. Charge / Discharge Test and Leave Test The battery prepared by the above method was charged at a constant current of 600 mA (1 C) to 4.2 V in a temperature atmosphere of 25 ° C. and then charged at a constant voltage of 4.2 V for 2.5 hours. I went. Then, discharge at a constant current of 600 mA to 3.0 V,
The discharge capacity (hereinafter referred to as 1C discharge capacity) was measured. Next, about this battery, in a temperature atmosphere of 25 ° C., 60
After charging to 4.2 V with a constant current of 0 mA, charging was performed for 2.5 hours at a constant voltage of 4.2 V. Then 1200mA
Discharge was performed to 3.0 V with a constant current of (2C), and the discharge capacity (hereinafter, referred to as 2C discharge capacity) was measured. Furthermore, about this battery, in a temperature atmosphere of 25 ° C., 600 mA
After being charged to 4.2V with a constant current of No. 2, the battery was charged with a constant voltage of 4.2V for 2.5 hours. After charging, this battery was left at 80 ° C. for 48 hours. 600m about this battery after leaving
A constant current of A was discharged up to 3.0 V, the discharge capacity (hereinafter, referred to as discharge capacity after standing) was measured, and the capacity retention rate was obtained. The capacity retention rate is indicated by the ratio of the discharge capacity after standing to the 1C discharge capacity.

Example 2 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 75% by weight of vinylidene fluoride, 20% by weight of tetrafluoroethylene, and 5% by weight of hexafluoropropylene. % Of the copolymer was used to assemble a battery as in Example 1. This battery was tested in the same manner as in Example 1.

Example 3 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 65% by weight of vinylidene fluoride, 25% by weight of tetrafluoroethylene, and 5% by weight of hexafluoropropylene. % Of the copolymer was used to assemble a battery as in Example 1. This battery was tested in the same manner as in Example 1.

Example 4 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 87% by weight of vinylidene fluoride, 12% by weight of tetrafluoroethylene, and 1% by weight of hexafluoropropylene. % Of the copolymer was used to assemble a battery as in Example 1. This battery was tested in the same manner as in Example 1.

Example 5 A copolymer having a copolymerization composition (by ¹⁹ F-NMR) of 90% by weight of vinylidene fluoride and 10% by weight of tetrafluoroethylene was used instead of the polymer material of Example 1. Used and assembled a battery as in Example 1. This battery was tested in the same manner as in Example 1.

Comparative Example 1 A battery was assembled in the same manner as in Example 1 except that polyvinylidene fluoride was used instead of the polymer material of Example 1. This battery was tested in the same manner as in Example 1.

Comparative Example 2 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 96% by weight of vinylidene fluoride and 4% by weight of tetrafluoroethylene.
A battery was assembled in the same manner as in Example 1 using the copolymer This battery was tested in the same manner as in Example 1.

Comparative Example 3 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 83% by weight of vinylidene fluoride, 5% by weight of tetrafluoroethylene, and 12% by weight of hexafluoropropylene. % Of the copolymer was used to assemble a battery as in Example 1. This battery was tested in the same manner as in Example 1.

Comparative Example 4 Instead of the polymer material of Example 1, the copolymer composition (by ¹⁹ F-NMR) was 60% by weight vinylidene fluoride, 35% by weight tetrafluoroethylene, 5% by weight hexafluoropropylene. % Of the copolymer was used to assemble a battery as in Example 1. This battery was tested in the same manner as in Example 1.

<Results and Discussion> For each Example and Comparative Example, Table 1 shows the copolymer composition of the polymer material, and Table 2 shows 1
The C discharge capacity, the 2C discharge capacity, the discharge capacity after standing and the capacity retention rate are shown. The batteries of Comparative Example 1, Comparative Example 2, and Comparative Example 4 were not subjected to the high temperature storage test because the 2C discharge capacity was significantly reduced.

[0039]

[Table 1]

[0040]

[Table 2]

From Table 1 and Table 2, Examples 1 to 5 are shown.
Then, in all the batteries, the 2C discharge capacity was 90% or more of the 1C discharge capacity. The capacity retention rate is 93%
And showed excellent high temperature characteristics. On the contrary,
In the batteries of Comparative Example 1, Comparative Example 2, and Comparative Example 4, the 2C discharge capacity was reduced to about 60% of the 1C discharge capacity, and the high rate discharge characteristics were inferior. In these batteries, the content of the hexafluoropropylene unit is low or the content of the tetrafluoroethylene unit is high, so that the compatibility between the copolymer and the electrolytic solution is poor, and as a result, the high rate discharge characteristics are low. It is considered to have decreased. Further, in the battery of Comparative Example 3, the 2C discharge capacity was maintained at about 90% of the 1C discharge capacity, but the capacity retention rate in the high temperature storage test was lowered to 74%. With this battery,
It is considered that because the content of hexafluoropropylene unit is large or the content of tetrafluoroethylene unit is small, the plasticity of the polymer electrolyte is large and the copolymer is dissolved by standing at high temperature.

The technical scope of the present invention is not limited to the above-described embodiments, but extends to an equivalent range.

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Claims (3)

[Claims] 1. A polymer electrolyte for a non-aqueous electrolyte secondary battery, wherein a polymer material holds an electrolytic solution, wherein the polymer material contains vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene. It is a copolymer formed by copolymerizing a monomer, and the ratio of tetrafluoroethylene units to the total of vinylidene fluoride units, tetrafluoroethylene units, and hexafluoropropylene units is 5% by weight.
A polymer electrolyte characterized by being at least 30% by weight. 2. The copolymer, wherein the ratio of hexafluoropropylene units to the total of vinylidene fluoride units, tetrafluoroethylene units, and hexafluoropropylene units is 10% by weight or less. The polymer electrolyte according to 1. 3. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a polymer electrolyte in which a polymer material holds an electrolytic solution, wherein the polymer material is vinylidene fluoride or tetrafluoro It is formed by copolymerizing a monomer containing ethylene and hexafluoropropylene, and the ratio of tetrafluoroethylene units to the total of vinylidene fluoride units, tetrafluoroethylene units, and hexafluoropropylene units is 5% by weight. A non-aqueous electrolyte secondary battery, characterized in that it is a copolymer whose content is at least 30% by weight.