JPH1186630A - Get electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a good ionic conductivity and a satisfactory thermal stability, even at a high temperature by holding a polyvinylidene fluoride swollen with a nonaqueous electrolyte or a copolymer thereof by a cross-linked polymer formed by polymerizing a radiation polymerizable monomer having a bofunctoinal or more acryloyl group, and forming it into a solid. SOLUTION: A polyvinylidene fluoride which is superior in ionic conductivity is dispersed in a nonaqueous electrolyte in particle size. Although the polyvinylidene fluoride is insoluble in a general solvent used for lithium battery at ordinary temperature, or only a low molecule contained therein in an extremely trace amount is dissolved, the polyvinylidene fluoride is swollen when dispersed in such a solvent or electrolyte. The swollen polyvinylidene fluoride is held by a chemically cross-linked polymer and formed into a solid. Thus, the fusion by heating can be avoided, and a quick ion transport can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in an electrolyte used in a power storage technology.

[0002]

2. Description of the Related Art In recent years, with the development of the electronics field, miniaturization of electronic equipment has been remarkable. Especially mobile phones and PH
The demand for portable devices such as S and small personal computers has been remarkably increasing, and as these devices have become lighter and thinner, there has been a demand for a battery as a power supply to have not only high functionality but also small size and thin shape. Under such a background, a lithium battery that is small and can be expected to have a high capacity has attracted attention. Recently, lithium-ion secondary batteries have been used as power sources for mobile phones and the like, and research on lithium batteries has been actively conducted as a technology of interest in the information age.

[0003] Safety is considered important for lithium batteries, and separators, such as safety valves and PTC elements, are also designed to cut off current at high temperatures. On the other hand, non-combustibility of solvents has been the subject of recent research. Among them, a lithium battery using a solid polymer electrolyte has attracted attention as a next-generation lithium battery because a polymer having low flammability is used for the electrolyte. However, since operation at room temperature is essential for small electronic devices, a gel electrolyte which has relatively high ionic conductivity at room temperature and is solid in shape is promising.
At present, gel electrolytes using polyethylene oxide show an ion conductivity of 1.5 × 10 ⁻³ Scm ⁻¹ at room temperature by Aihara et al., J. Power Sources 65
(1997) 143-147. In previous studies, a gel electrolyte of polyethylene oxide having a degree of swelling equal to or less than the equilibrium swelling degree is completely different from a liquid electrolyte, and the conductivity is described by a relational expression such as VTF without following Arrhenius equation. Although details are under study, it may be necessary to consider the affinity between the polymer and ions in addition to the structural interaction of the gel electrolyte.

[0004]

In order to improve the ionic conductivity of the entire gel electrolyte, the gel electrolyte does not need ether oxygen as a donor as a molecular skeleton structure of the polymer. It can be said that a skeleton having little interaction is preferable. From that viewpoint, fluoropolymers are currently being reviewed. Regarding polyvinylidene fluoride, Tsuchida et al., Electrochem. Acta, 28 (198
Section 3) reports the study. However, since polyvinylidene fluoride is thermoreversible and becomes a solution at high temperatures, it has been difficult to use polyvinylidene fluoride by itself.

The present invention has been made in view of the above problems, and has as its object to provide a gel electrolyte having excellent ionic conductivity and good thermal stability.

[0006]

In order to solve the above-mentioned problems, in the present invention, polyvinylidene fluoride having excellent ion conductivity is dispersed in a non-aqueous electrolyte in a particle size to form a gel electrolyte. Polyvinylidene fluoride is N-methyl 2-
It is known that it is soluble in some solvents such as pyrrolidone, but at room temperature, common solvents used for lithium batteries are insoluble or only a small amount of small molecules contained in trace amounts are dissolved. is there. When dispersed in such a solvent or an electrolytic solution, polyvinylidene fluoride particles swell. In the present invention, this swollen state is simply referred to as a swollen gel electrolyte of polymer particles. Since this gel electrolyte is similar to a normal physical gel, it is melted by heating.
However, by covering this gel electrolyte with a matrix of a crosslinkable polymer, it becomes possible to avoid melting of the whole film. That is, by holding the gel electrolyte of polyvinylidene fluoride, which has a fast ion transfer, with a chemically cross-linked polymer, it is possible to maintain the physical stability as a membrane and at the same time, it is possible to perform a fast ion transfer.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, there is provided a gel electrolyte comprising a cross-linked polymer and polyvinylidene fluoride or a copolymer thereof swollen with a non-aqueous electrolyte, which is formed in a solid state. Further, the cross-linked polymer is a gel electrolyte formed by polymerizing a radiation-polymerizable monomer having a bifunctional or more acryloyl group,
The crosslinked polymer is a gel electrolyte formed by crosslinking a polyfunctional acrylate having a polyether main chain.

[0008]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited thereto.

(Invention 1) Polyvinylidene fluoride powder
5 g was dispersed in 4.0 g of 1 mol lithium tetrafluoroborate electrolyte solution of γ-butyrolactone. This has a molecular weight of 1100
0.5 g of the bifunctional acrylate of No. 0 was dissolved. This liquid was applied on a metal foil with a bar coater, and radical polymerization was performed by electron beam irradiation. The formed film was peeled from the metal foil and punched into a predetermined shape using a punch.

Comparative Example 1 1.0 g of polyethylene glycol diacrylate having a molecular weight of about 11,000 was dissolved in 4.0 g of 1 mol of lithium tetrafluoroborate γ-butyrolactone electrolytic solution, and this solution was applied on a metal foil with a bar coater.
Radical polymerization was performed by electron beam irradiation. The formed film was peeled from the metal foil and punched into a predetermined shape using a punch.

(Comparative Example 2) A dispersion of 1.0 g of the polyvinylidene fluoride powder used in the present invention 1 in 4.0 g of 1 mol of lithium tetrafluoroborate γ-butyrolactone electrolyte was placed in a cell for measuring liquid conductivity at 90 ° C. After heating for 30 minutes, it was confirmed that the mixture became uniformly transparent after 30 minutes, and the mixture was gradually cooled to room temperature and gelled in the cell.

The electrolytes of the present invention 1 and comparative examples 1 and 2 were fixed in a predetermined cell for measuring ionic conductivity. Each room temperature ~
The ionic conductivity was measured up to −20 ° C. Measurements were made using Solartron impedance analyzer model 12
Using an 86 interface and a model 1255, AC measurement was performed at 10000 Hz to 10 Hz. Each film was heated on a hot plate at 100 ° C. in order to examine the shape deformation due to heat. FIG. 1 shows an Arrhenius plot of the result of measuring the ionic conductivity. Comparative Example 2 showed the highest ion conductivity, and it was found that Invention 1 exhibited higher ion conductivity than Comparative Example 1. In Comparative Example 2, the absence of a chemically crosslinked polymer having strong interaction with a solvent and a solute is considered to be the cause of the high ionic conductivity. In the present invention, it is considered that the ionic conduction was higher than that of Comparative Example 1 due to the presence of a conduction path, that is, a conduction path of polyvinylidene fluoride, which was higher than that of the conventional polyether-based gel.

Next, a change in shape due to heating of each film on a hot plate at 100 ° C. was confirmed. The results are shown in Table 1.

[0014]

[Table 1]

With respect to Comparative Example 2, it was directly liquefied on a hot plate. With respect to the present invention 1, although the color of the film was changed from light milky white to transparent, the shape of the film was maintained. In Comparative Example 1, no change was observed. From the above, it is considered that Comparative Example 2 is not suitable when the use of the gel electrolyte membrane alone is considered. Comparing the thermally stable invention 1 with the comparative example 1, the invention 1 is superior in ionic conduction, so that the invention is thermally stable, and
It can be confirmed that high ionic conduction can be exhibited.

[0016]

The gel electrolyte of the present invention is thermally stable,
It has high ionic conductivity and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a view showing the results of measuring the ionic conductivity of the electrolyte of the present invention and Comparative Examples 1 and 2.

Claims (3)

[Claims] 1. A gel electrolyte comprising a cross-linked polymer and polyvinylidene fluoride or a copolymer thereof swollen with a non-aqueous electrolyte, formed in a solid state. 2. The gel electrolyte according to claim 1, wherein the crosslinked polymer is formed by polymerizing a radiation polymerizable monomer having an acryloyl group having two or more functional groups. 3. The gel electrolyte according to claim 1, wherein the crosslinked polymer is formed by crosslinking a polyfunctional acrylate having a polyether main chain.