JPH10134793A - Porous membrane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous membrane whose ion conduction is high when it is impregnated with electrolyte, which is easy to be impregnated with the eleptrolyte and ensures high safety when used for constructing a battery by applying cross-linking treatment to the porous membrane composed of a polymer including polyvinylidene fluoride or vinylidene fluoride. SOLUTION: A porous membrane is provided whose one surface layer is at least more dense than the other layer and which has a hole for the communication between both the layers. In consideration of ion conductionand cross- linking by electronic beam, 75wt.% of polyvinylidene fluoride is used and cross linked after the formation of porous thin film. In performing cross-linking an electronic beam or gamm rays are applied in order to prevent deterioration in performance caused by a minute quantity of water content as well as co- existing component. Accordingly, it is possible to provide the porous membrane whose ionic conduction is high when it is impregnated with electrolyte, which facilitates the impregnation of electrolyte at a high temp. and does not dissolve when a large current gnerates locally in a battery, which ensuring safety.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous membrane having excellent chemical resistance, and more particularly to a porous membrane used as a separator for preventing a short circuit between electrodes in a lithium ion battery or the like.

[0002]

2. Description of the Related Art Recently, miniaturization of mobile phones, personal computers, and the like,
High energy density batteries are required for weight reduction, and new batteries such as lithium ion batteries have been developed and industrialized to meet the demand. In such a high energy density battery, the interval between the positive electrode and the negative electrode is reduced as much as possible in order to increase the energy density per volume, and a porous polymer separator having a through hole is used to prevent a short circuit between the electrodes. I have. The electrolyte solution exists in a state of being impregnated in the separator.

The separator is required to have various functions for ensuring safety, in addition to the function of preventing a short circuit between electrodes. For example, when a flammable liquid is used in a battery having a high energy density such as a lithium ion battery, a function for preventing ignition is also required. That is,
When a local large current is generated in the battery for some reason, heat is generated, and the separator is exposed to high heat.However, in the case of a microporous film of polyolefin such as polyethylene which is widely used at present, Employs a mechanism (fuse effect) for blocking the conduction and blocking conduction by melting.

However, in the case of the microporous polyolefin membrane, since the material is non-ion-conductive and the porosity is set small in order to surely exhibit the fuse effect, the electrolyte solution is not used. The conductivity of the entire membrane when impregnated is extremely low, and is actually about one order of magnitude lower than the conductivity of the electrolyte solution itself. In addition, because of poor affinity with the electrolyte solution, it is extremely difficult to inject the liquid into the pores, and it was necessary to evacuate the system or use a surfactant. The method of evacuating the system is inefficient, complicates the manufacturing process, and the use of a surfactant may adversely affect the electrical properties.

It has also been proposed to use a fluoropolymer as a battery separator having excellent chemical resistance, weather resistance, etc. (JP-A-4-23941). In this case, by setting the pore diameter of the porous body in an appropriate range, it is possible to prevent performance deterioration and short circuit due to the generation of dendritic deposits called dendrites, and for this purpose, polyvinylidene fluoride or fluoride is used. It has been known that a porous body made of a vinylidene copolymer can be used.

[0006] Polyvinylidene fluoride-based polymers can obtain relatively high ionic conductivity, but when they are used in, for example, lithium ion batteries, they are easily dissolved in an electrolyte solution made of propylene carbonate or the like at high temperatures, and the Unlike the case, there was a risk of a short circuit due to a hole. In order to solve such problems, improvement of strength by crosslinking is considered to be an effective means, and US Pat. No. 5,429,891 discloses a separator for use as a gel electrolyte in which an electrolyte solution is swollen in a polymer. It has been proposed to use a crosslinked vinylidene fluoride-hexafluoropropylene copolymer film having no voids.

However, since the crosslinking method involves the coexistence of a crosslinking monomer, the resulting crosslinked polymer impairs the chemical stability of the original polyvinylidene fluoride.

[0008]

DISCLOSURE OF THE INVENTION The present invention provides a battery separator having high ionic conductivity when impregnated with an electrolyte solution, easy impregnation with the electrolyte solution, and high safety when constituting a battery. It is an object of the present invention to provide a porous membrane having a function for ensuring the following.

[0009]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on battery separators to solve the above-mentioned problems of the prior art, and as a result, have found that polyvinylidene fluoride or vinylidene fluoride having novel physical properties is obtained. The present inventors have found a porous film made of a copolymer containing the same, and have reached the present invention. That is,
The present invention is as follows. (1) A layer made of vinylidene fluoride resin, in which at least one surface layer is denser than other portions, and has a layer having a hole communicating between the one surface layer and the other surface layer. A porous membrane in which the layers are integrally continuous and cross-linked. (2) The porous membrane as described in 1 above, wherein the layer having communicating holes has a three-dimensional network structure. (3) The above-mentioned 1 having a void in the layer having a communicating hole.
Or 2 porous membranes. (4) The above 1, wherein the crosslinking treatment is electron beam or gamma ray irradiation.
2 or 3 porous membranes. (5) The porous membrane as described in any one of (1) to (4) above, wherein both surface layers are dense.

Hereinafter, the present invention will be described in detail. First, the porous membrane of the present invention is a porous membrane having at least one dense surface layer (hereinafter, also referred to as a dense layer) and having a layer having communication holes (hereinafter, also referred to as a porous layer). The average pore size of the dense layer on the surface of the porous membrane of the present invention is not necessarily limited because the range of suitability varies depending on the type of battery used, but it is necessary to suppress the generation of dendrites, for example, as in a lithium ion battery. In this case, the smaller the pore size, the better. On the other hand, in order to facilitate the impregnation of the electrolyte solution and reduce the resistance of the membrane when impregnated, it is preferable that the pore diameter is large.

The amount of water permeation (water permeability) of the porous membrane is determined on one side of the membrane.
If only the dense layer is present, the electrolyte may be
Water permeability may be zero,
It is not specified. On the other hand, there are dense layers on both surfaces of the film.
Electrolyte, the electrolyte leaks from the porous membrane when assembled into the battery.
More preferably, the water permeability is 20 l / m ^Two
・ More than hr ・ atm is suitable, 50 liters
/ M^TwoThose having more than -hr-atm are more preferable. 2
0 liter / m^Two・ Impregnation of electrolyte solution if less than hr ・ atm
Speed tends to be slow.

When the porous membrane is used as a separation membrane for filtration of a solution, it has a dense layer on the surface and an intermediate layer of the membrane having appropriate voids, so that it has sufficient water permeability and high separation. Performance can be realized, and stable separation performance can be maintained over a long period of time. The fact that the layer having the communicating holes has a three-dimensional network structure means that the void portions are three-dimensionally connected to each other, and there is no limitation on the shape of the void portions forming the communicating holes, and Any shape may be used as long as it does not control the impregnation and permeation speed of the liquid. Further, it is more preferable that the layer having the communication holes has voids, because the impregnation and permeation speed of the electrolytic solution are increased.

In the present invention, the void of the porous film means a macropore having a maximum major axis of the void of 10% or more of the film thickness. In the porous membrane of the present invention, that the dense layer on the surface and the layer having the communication hole are integrally continuous means that the structure is continuously changed. The thickness of the porous film of the present invention is preferably small because it affects the energy density per volume of the battery. However, if it is too thin, the strength is insufficient, and the workability when assembling the battery is reduced. Therefore, those having a range of 5 to 500 μm are suitable, and those having a range of 10 to 200 μm are suitable. More preferably, those having a thickness of 20 to 70 μm are suitable.

As the material of the porous membrane of the present invention, a polyvinylidene fluoride resin is used. Specifically, in addition to vinylidene fluoride homopolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride -Trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride -Trifluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, and the like. These can be used alone or as a mixture of these polymers, and can also be used as a mixture with a polymer containing no vinylidene fluoride.

Here, the vinylidene fluoride component is a copolymer,
Alternatively, when it is contained as a mixture, the vinylidene fluoride component is preferably at least 50% by weight. If it is less than 50% by weight, the ion conductivity of the separator becomes too low, and if it is crosslinked by an electron beam, it becomes difficult to crosslink if the amount of the vinylidene fluoride component is too small. preferable.

In producing the porous membrane of the present invention, the crosslinked structure may be introduced at any stage during polymerization, during the formation of the porous thin film, or after the formation of the porous thin film. Therefore, it is preferable to crosslink after forming the porous thin film. Examples of the crosslinking method include irradiation with radiation energy such as electron beam, γ-ray, X-ray, and ultraviolet ray, a method in which a radical initiator is contained and the reaction is carried out by irradiation with heat or radiation energy, and a reaction after alkali treatment (removing HF). A method of reacting a group or the like can be used. in this case,
A so-called crosslinkable monomer such as a di- or tri (meth) acrylate, a di- or triallyl compound, a di- or triglycidyl compound can also be allowed to coexist. Among these cross-linking methods, cross-linking by electron beam or γ-ray irradiation is preferred from the viewpoint that performance is not reduced by electrochemical side reaction due to coexisting components or hydrolysis due to a small amount of moisture. When the film thickness is small, crosslinking by electron beam irradiation is economical and more preferable.

When crosslinking by electron beam irradiation, the irradiation amount is preferably in the range of 5 to 100 Mrad,
More preferably, it is in the range of 8 to 50 Mrad. 5M
If it is less than rad, the effect of crosslinking is not sufficient, and
If d is exceeded, the polymer structure tends to collapse more than crosslinking. The formation of the crosslinked structure can be confirmed by the solubility in the solvent that dissolves the uncrosslinked polymer. That is, the crosslinked polyvinylidene fluoride polymer has a component that does not dissolve in the soluble solvent, and does not dissolve uniformly, so that the formation of the crosslinked structure can be determined. This soluble solvent is not limited because it varies depending on the type of polymer, but usually, ethylene carbonate, propylene carbonate, N-methylpyrrolidone, chloroform,
It can be determined by a solvent such as dichloromethane, dichloroethane, acetone, tetrahydrofuran, dimethylformamide, dimethylsulfoxide, and dimethylacetamide.
If the uncrosslinked polymer does not dissolve at room temperature, a temperature may be applied.

The method for producing the porous membrane of the present invention is not particularly limited. For example, a method for producing a microfilter or an ultrafilter can be used. For example, the method described in JP-A-3-215535 or the method disclosed in 61-
No. 38207, JP-A-54-1638.
The method described in Japanese Patent Publication No. 2 can be used. Specific examples of these include a melting method and a wet method. The melting method is to melt a polyvinylidene fluoride-based polymer together with a plasticizer or an inorganic powder, and then extrude it into a flat film to form a plasticizer or a plasticizer. It extracts and removes inorganic powder and the like. In addition, the wet method has a dense layer on the surface by dissolving a polyvinylidene fluoride polymer in a good solvent to form a film-forming solution, and extruding the solution into a solvent that solidifies the polymer in a thin film form. There is known a method for forming a thin film-like porous layer composed of continuous pores. A poor solvent may be added to the film forming solution as needed. As a film forming method, a sheet-like film having a dense layer on both sides can be manufactured by directly extruding a flat film into a coagulating solvent,
When a material cast on a substrate such as glass is poured into a solidifying solvent, a sheet-like film having a dense layer on only one side is formed.

When used as a battery separator, the porous membrane of the present invention has high ionic conductivity when impregnated with an electrolyte solution, is extremely easy to impregnate with a liquid, and has a high safety when constituting a battery. Because of its high cost, it can be applied not only to lithium-ion batteries but also to various batteries such as lead batteries, alkaline batteries, nickel-metal hydride batteries, and various electrochemical elements and devices such as photoelectrochemical devices and electrochemical sensors. Above is useful.

Further, when used as a separation membrane, it has a crosslinked structure, so that its mechanical strength is improved. In particular, it can be applied to filtration in a high-temperature liquid, improves pressure resistance, and can provide stable separation performance over a long period of time.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by way of examples. In addition, the measuring method is as follows. The measurement of ionic conductivity is performed by using an AC impedance method in which an electrochemical cell is constructed by sandwiching a porous membrane impregnated with an electrolyte solution between metal electrodes and applying an alternating current between the electrodes to measure the resistance component. Calculated from the real impedance intercept of the Cole plot. For the measurement of water permeability, the porous membrane punched to a diameter of 25 mm was immersed in ethanol for 15 minutes or more to perform a hydrophilic treatment,
Then, it is incorporated in a membrane filter holder having an effective area of 3.5 cm ² , filled with pure water at 25 ° C., and the amount of pure water permeated when a hydrostatic pressure of 1 atm is applied is measured. The thickness is measured by sandwiching the porous membrane dried at 60 ° C. for 4 hours or more in vacuum between two flat glass plates, measuring the thickness with a micrometer, and determining the thickness of the two glass plates from the value. I asked for it. The maximum major diameter of the void was determined from an SEM photograph of a cross section of the porous film. The porosity is measured by immersing the porous membrane in ethanol to perform a hydrophilization treatment, and then immersing the porous membrane in pure water at room temperature for 120 minutes or more to completely replace the inside of the voids with pure water and remove water on the membrane surface After wiping, the weight (A) of the film containing pure water in the gap is measured.
Subsequently, the porous film is dried in a vacuum at 60 ° C. for 4 hours or more, water in the voids is removed, and the weight (B) of the film alone is measured. It is calculated from these weights and the true specific gravity (d) of the material of the membrane.

Porosity (%) = (AB) / (B / d + A)
-B) x 100

[0023]

Example 1 17.2 parts by weight of vinylidene fluoride-hexafluoropropylene copolymer (# 2950 manufactured by Kureha Chemical), 70.5 parts by weight of dimethylacetamide as a solvent,
Polyethylene glycol (average molecular weight 20
0 (PEG200)) and 0.8 parts by weight of polyoxyethylene sorbitan monooleate as a poor solvent were mixed to form a uniform solution. This solution is cast on a glass plate at 25 ° C. with an applicator set at 250 μm, immediately immersed in water at 70 ° C., washed with water and alcohol, and then dried to form a film having a dense layer on the water contact side. 14
A 5 μm porous film was prepared. The porous film was irradiated with an electron beam (irradiation amount: 30 Mrad) to form a crosslinked film.

When the porous membrane was heated to 100 ° C. in a 1: 1 mixed solvent of ethylene carbonate and propylene carbonate, it was confirmed that the porous membrane had maintained its shape and had been crosslinked. The water permeability of the porous membrane is 176 liter / m ²
· Hr · atm. The porous film was immersed in a 1 mol / L solution of a 1: 1 mixed solvent of ethylene carbonate and propylene carbonate of LiBF ₄ as an electrolyte solution at room temperature for 10 minutes to obtain a transparent film impregnated with the solution.
At this time, it was observed that the electrolyte was impregnated from the porous layer side. The porous membrane was taken out and excess electrolyte not impregnated was wiped off. The porous membrane was sandwiched between stainless steel sheets (film thickness: 40 μm), and the impedance was measured (E & G, 389 type impedance meter). As a result, the ion conductivity at room temperature was 2.4 × 10 ^−3.
S / cm.

[0025]

Example 2 Vinylidene fluoride-hexafluoropropylene copolymer (Kynar RC9665A) 17.2
Parts by weight, 70.5 parts by weight of dimethylacetamide as a solvent, 11.5 parts by weight of polyethylene glycol (average molecular weight 200 (PEG200)) as a poor solvent, and 0.8 parts by weight of polyoxyethylene sorbitan monooleate as a poor solvent. Solution. At 25 ° C.,
It was cast on a glass plate with an applicator set to 100 μm, immediately immersed in water at 70 ° C., washed with water and alcohol, and then dried to form a 33 μm-thick porous film having a dense layer on the water contact surface side. .

Electron beam irradiation (irradiation amount: 15 Mra)
d) to prepare a crosslinked membrane. As in Example 1, when the porous membrane was heated to 100 ° C. in a mixed solvent of ethylene carbonate / propylene carbonate 1: 1, it was confirmed that the shape was maintained and that the film was crosslinked.
The water permeability of the porous membrane was 2 liter / m ² · hr · atm. The porosity of the porous membrane was 73%. In the cross section of the porous film, a large void having a maximum major axis corresponding to 60% of the film thickness was observed from the SEM photograph. The porous membrane was immersed in a 1 mol / l solution of a 1: 1 mixed solvent of LiBF ₄ of ethylene carbonate and propylene carbonate as an electrolyte solution at 100 ° C. for 60 minutes to obtain a transparent membrane impregnated with the solution. When the ionic conductivity of the porous membrane at room temperature was measured in the same manner as in Example 1, it was 1.9 × 10 ⁻³ S / cm.
Met.

[0027]

Example 3 Vinylidene fluoride-hexafluoropropylene copolymer (Kynar RC9665A) 22.5
Parts by weight and 77.5 parts by weight of dimethylacetamide as a solvent were mixed to form a uniform solution. This solution is cast on a glass plate at 25 ° C. with an applicator set to 100 μm, immediately immersed in water at 25 ° C., washed with water and alcohol, and dried, and has a dense layer on the water contact side. 2
A 4 μm porous membrane was prepared.

Electron beam irradiation (irradiation amount 30 Mra)
d) to obtain a crosslinked film. As in Example 1, when the porous membrane was heated to 100 ° C. in a mixed solvent of ethylene carbonate / propylene carbonate 1: 1, it was confirmed that the shape was maintained and that the film was crosslinked. The porous membrane did not permeate at a hydrostatic pressure of 1 atm, but became permeable when the surface layer was scraped by rubbing the dense surface on the water contact side with a razor blade. The porosity of the porous membrane was 53%. In the cross section of the porous film, small voids whose maximum major axis was 26% of the film thickness could be observed from the SEM photograph. The electrolyte was impregnated with the electrolyte in the same manner as in Example 2, and the ionic conductivity was measured to be 1.2 × 10 ⁻³ S / cm.

[0029]

Example 4 17.2 parts by weight of vinylidene fluoride-hexafluoropropylene copolymer (# 2950, manufactured by Kureha Chemical), 70.5 parts by weight of dimethylacetamide as a solvent,
Polyethylene glycol (average molecular weight 20
0 (PEG200)) and 0.8 parts by weight of polyoxyethylene sorbitan monooleate as a poor solvent were mixed to form a uniform solution. This solution was extruded into water at 70 ° C. from a T-die set to a thickness of 100 μm at 25 ° C., washed with water and alcohol, and dried to form a 38 μm-thick porous membrane having a dense skin layer on both sides of the membrane. Created.
The porous film was irradiated with an electron beam (irradiation amount: 30 Mrad) to form a crosslinked film.

When the porous membrane was heated to 100 ° C. in a 1: 1 mixed solvent of ethylene carbonate and propylene carbonate in the same manner as in Example 1, it was confirmed that the shape was maintained and that the film was crosslinked. The water permeability of the porous membrane was 50 l / m ² · hr · atm. The electrolyte was impregnated in the same manner as in Example 2, and the ionic conductivity at room temperature was measured. As a result, it was 1.0 × 10 ⁻³ S / cm.

[0031]

Comparative Example 1 A non-crosslinked porous film was prepared in the same manner as in Example 2 except that no electron beam irradiation was performed. 1 mol of a 1: 1 mixed solvent of ethylene carbonate / propylene carbonate of LiBF ₄ as an electrolyte solution
/ Liter solution at room temperature for 10 minutes to obtain a transparent film impregnated with the solution. As a result, the ionic conductivity at room temperature was 0.96 × 10 ⁻³ S / cm.

Further, when the porous membrane was heated to 100 ° C. in a 1: 1 mixed solvent of ethylene carbonate / propylene carbonate in the same manner as in Example 2, the porous membrane was completely dissolved and no membrane was obtained.

[0033]

When the porous membrane of the present invention is used as a battery separator, the ionic conductivity when impregnated with an electrolyte solution is high, and the impregnation of the electrolyte solution can be performed at a high temperature, which is extremely easy. Furthermore, even when a large current is locally generated in the battery, the porous membrane does not melt, so the safety is very high.Therefore, it is not limited to a lithium ion battery, and various batteries such as a lead battery, an alkaline battery, a nickel hydrogen battery, etc. It is industrially useful because it can be applied to various electrochemical elements and devices such as photoelectrochemical devices and electrochemical sensors.

Further, when used as a separation membrane, it has a crosslinked structure, so that its mechanical strength is improved. In particular, it can be applied to filtration in a high-temperature liquid, improves pressure resistance, and can provide stable separation performance over a long period of time.

Claims (5)

[Claims] 1. A layer comprising a vinylidene fluoride resin, wherein at least one surface layer is denser than another portion, and has a layer having a hole communicating between the one surface layer and the other surface layer, A porous membrane, wherein the layers are integrally continuous and cross-linked. 2. The porous membrane according to claim 1, wherein the layer having the communicating holes has a three-dimensional network structure. 3. The porous membrane according to claim 1, wherein the layer having the communicating holes has voids. 4. The porous membrane according to claim 1, wherein the crosslinking treatment is irradiation with an electron beam or γ-ray. 5. The porous membrane according to claim 1, wherein both surface layers are dense.