JPH10269845A - Fixed liquid film conductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily provide a thin film having a large area, an excellent holding property of an electrolytic solution in a wide temperature range, long-term stability, and improved mechanical strength by fixing an aprotic electrolytic solution to a polyolefin fine porous film containing a graft polymer soluble in the aprotic electrolytic solution and an electronic conductive material. SOLUTION: A polymer soluble in an aprotic electrolytic solution is preferably radiation graft-polymerized to the surface of a fine porous film blended with an electronic conductive material to polyolefin such as ultrahigh-molecular weight polyethylene and the surface of fine holes. A solution solved with an electrolyte such as alkaline metal salt or alkaline earth metal salt in an aprotic high-dielectric constant solvent is fixed to the film by impregnating, coating, or spraying to obtain the prescribed relative dielectric constant. This fixed liquid film conductor has the porous film base skeleton of polyolefin, it suppresses excessive swelling and the evaporation of the electrolytic solution, the electronic conductivity is not reduced, and the safety at the time of an overcharge is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an immobilized liquid film conductor and a method for producing the same, and more particularly, to an immobilized liquid film conductor in which an ionic conductor is immobilized on a porous conductive film having high electron conductivity, and a method for manufacturing the same. It relates to a manufacturing method.

[0002]

2. Description of the Related Art A polymer material such as polyethylene, polypropylene, polyvinyl chloride, polysulfone, butadiene rubber, silicone rubber, ethylene-propylene-diene terpolymer, epoxy resin, etc. is mixed with an electronic conductive substance such as carbon black. Such conductors are widely known. And these conductors are antistatic materials,
It is used for electromagnetic wave shielding materials, conductive paints, adhesives, IC packaging materials, planar heating elements, planar switches, and the like.

Further, among such conductors, a thin film conductor (porous conductive film) which is porous but has high conductivity is extremely effective as an electrode or an electrode constituent material in a device using a solid polymer electrolyte or a liquid electrolyte. Can be used for That is, since it is porous, the contact interface between the electrode and the electrolyte can be increased in area, and thus, for example, using this, a lithium primary battery,
It is possible to manufacture a high-performance battery such as a lithium secondary battery.

As a development example of the above-mentioned thin film conductor, Ketjen black (Akzo black) is added to a plasticizer solution of polyethylene.
Chemic Co., Ltd.), and after forming and stretching a sheet, a porous thin film from which a plasticizer has been removed, and an electrolyte solution immobilized by utilizing the capillary condensation force, and a method for producing the same (Japanese Patent Laid-Open No. -87096). However, there is a problem in the electrolyte retention. On the other hand, recently, a copolymer of polyvinylidene fluoride and hexafluoropropylene has been
A technique has been proposed (US Pat. No. 5,296,318) in which a polymer gel impregnated with a carbonate-based solution in which n ₂ O ₄ and carbon black or petroleum coke and carbon black are mixed and a lithium salt is dissolved is used for a positive electrode or a negative electrode of a battery. However, there is a problem of electrolyte seepage due to gel shrinkage at a high temperature, and this is not a complete solution for electrolyte retention. Therefore, it is desired to develop a thin-film conductor which can be easily formed into a thin film and has a large area and has a stable ability to hold an electrolyte solution in a wide temperature range.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to easily form a thin film, to have a large area, and to have excellent aprotic electrolyte solution retention over a wide temperature range.
An object of the present invention is to provide an immobilized liquid film conductor having improved long-term stability and mechanical strength, and a method for producing the same.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to overcome the problems of the prior art and found that the solubility of the polymer graft-polymerized on the microporous polyolefin membrane containing an electronically conductive material was improved. Have found that the above object can be achieved by immobilizing the aprotic electrolyte on the membrane.

That is, the immobilized liquid film conductor of the present invention comprises:
An aprotic electrolyte solution is immobilized on a microporous polyolefin membrane containing a graft polymer having solubility in an aprotic electrolyte solution and an electron conductive material. Further, the method for producing an immobilized liquid membrane conductor of the present invention comprises graft-polymerizing a polymer having solubility in an aprotic electrolyte solution on the surface and pore surface of a polyolefin microporous film containing an electronic conductive material. , Which are impregnated with an aprotic electrolyte solution to be immobilized.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The immobilized liquid membrane conductor of the present invention has, as a main skeleton, a polyolefin microporous membrane containing an electronic conductive material, onto which a polymer having solubility in an aprotic electrolyte solution is graft-polymerized. And then impregnated with an aprotic electrolyte solution. The details of each configuration will be described below.

1. Polyolefin microporous membrane containing electronically conductive material a. Polyolefin Polyolefins include polyethylene, polypropylene, ethylene-propylene copolymer, polybutene-1,
Poly-4-methylpentene-1 and the like. Of these, polyethylene is preferred. As the polyethylene, those made of ultra-high molecular weight polyethylene, high density polyethylene, medium-low density polyethylene can be used, but those containing ultra-high molecular weight polyethylene or components thereof from the viewpoint of strength, safety, film forming property, etc. It is preferable to use The polyolefin has a weight average molecular weight of 5 × 10 ⁵ or more, preferably 1 × 10 ⁶ to 1 × 10 5.
^It is preferred that the ultrahigh molecular weight component ⁷ is contained in an amount of 1% by weight or more and the molecular weight distribution (weight average molecular weight / number average molecular weight) is 10 to 300. When the content of the ultrahigh molecular weight polyolefin component is less than 1% by weight, the portion which contributes to the improvement of the stretchability of the film is insufficient. On the other hand, the upper limit is not particularly limited. On the other hand, if the molecular weight distribution exceeds 300, breakage due to low molecular weight components occurs, and the strength of the entire thin film decreases, which is not preferable.

B. Electronic conductive material Examples of the electronic conductive material include various metals and semiconductors, oxide and sulfide electronic conductive materials, and carbon or graphite. These may be in any shape such as a particle shape, a fiber shape, a fibril shape, a whisker shape and the like. Particularly preferred are TiS ₃ , TiS ₂ ,
TiO ₂ , V ₂ O ₅ , NbSe ₃ , MnO ₂ , LiCoO ₂ ,
Battery positive electrode active materials such as LiNiO ₂ , LiMn ₂ O ₄ , PbO ₂ , NiOOH, petroleum coke, natural graphite,
Battery negative electrode active materials such as carbon fiber, Pb and Cd, acetylene black, Ketjen black (Akz
oChemic), carbon whiskers, graphite whiskers, graphite fibrils and the like.

C. The microporous polyolefin membrane containing the electronically conductive material is
It can be obtained by blending the above-mentioned electronic conductive material with polyolefin and forming a film. The compounding amount of the electronic conductive material is preferably 1 to 200% by weight, particularly preferably 5 to 100% by weight. If the amount is less than 1% by weight, it is difficult to obtain sufficient conductivity, and if it exceeds 200% by weight, it becomes difficult to obtain a film having practically sufficient strength. The film may be formed by the method described in JP-A-60-242035 or JP-A-3-64334. For example, it can be performed as follows. The polyolefin containing ultra-high molecular weight polyolefin is mixed with a solvent such as liquid paraffin for 10 to 50 minutes.
The weight percent is heated and dissolved to form a uniform solution, and the electronic conductive material is uniformly blended with the solution. After forming a sheet from this solution and cooling it into a gel-like sheet, the sheet is heated at a temperature equal to or lower than the melting point of the polyolefin plus 10 degrees, and stretched to an area magnification of 3 times or more. The solvent contained in the stretched film is extracted and removed with a volatile solvent such as methylene chloride, and then dried and heat-set. In addition, the polyolefin microporous membrane, if necessary, antioxidants, ultraviolet absorbers, lubricants, antiblocking agents, pigments, dyes, various additives such as inorganic fillers,
It can be added in a range that does not impair the purpose of the present invention.

D. Polyolefin microporous membrane containing electronic conductive material
It has a thickness of 1 to 1000 μm, preferably 5 to 500 μm. If the thickness is less than 1 μm, it is difficult to practically use from the viewpoint of mechanical strength and handling. On the other hand, 1000μ
If it exceeds m, the effective resistance becomes large and the volumetric efficiency as a conductor is disadvantageous. The porosity of the film is not limited, but is preferably 30 to 95%, more preferably 50 to 9%.
It is in the range of 0%. If the porosity is less than 30%, the immobilization of the aprotic electrolyte solution may be insufficient. On the other hand, if the porosity exceeds 95%, the mechanical strength of the membrane becomes small and the practicability is poor. . Further, the average pore size is preferably 1 μm or less. If it exceeds 1 μm, it becomes difficult to prevent the active material and the reaction product from diffusing when the amount of graft polymerization is small. The lower limit is not limited, but the average pore size is not more than 0.2.
If it is less than 005 μm, problems may occur in the uniformity and the polymerization rate in the graft polymerization by plasma. Further, the microporous polyolefin membrane containing the above-mentioned electron conductive material preferably has a breaking strength of 200 kg / cm ² or more. By setting the breaking strength to 200 kg / cm ² or more,
Deformation resistance against swelling when the aprotic electrolyte solution is dissolved in the graft polymer is sufficient.

2. Polymer graft-polymerized on the surface of microporous polyolefin membrane and the surface of pores containing an electronic conductive material a. Monomer Acrylic acid, methacrylic acid, acrylic acid ester, methacrylic acid ester, acrylamide, acrylonitrile, styrene and derivatives thereof are used as monomers constituting the polymer. Specifically, for example, acrylates include methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate,
Stearyl acrylate, ethyl decyl acrylate,
Ethyl hexadecyl acrylate, 2-ethoxyethyl acrylate, tetrahydrofurfuryl acrylate,
Trimethylolpropane triacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 1,4-butanediol diacrylate,
Acrylic monomers such as 1,6-hexanediol diacrylate are exemplified. Examples of methacrylates include methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, tridecyl methacrylate, stearyl methacrylate, methacrylic acid. Examples include methacrylic monomers such as cyclohexyl acid, benzyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, glycidyl methacrylate, and ethylene glycol dimethacrylate. One or more of these can be selected and used. If necessary, a crosslinkable monomer such as vinyl acrylate, vinyl methacrylate, divinylbenzene, and vinyl butyl acrylate can also be used. Among the above-mentioned monomers, it is preferable to use a monomer composed of acrylic acid, methacrylic acid, or an ester thereof, or an acrylic monomer composed of acrylamide or a derivative thereof.

B. Graft Polymerization Method As a method for graft-polymerizing the above-mentioned polymer onto the surface of the microporous polyolefin membrane containing the electron conductive material and the surface of the pores, a radiation graft polymerization method such as plasma or electron beam or γ-ray is used. A post-polymerization method in which radicals are generated in the polyolefin microporous membrane and then contacted with a selected monomer, or a simultaneous polymerization method in which radicals are generated in a state in contact with the monomers may be used. As a specific example of the plasma graft polymerization, a polyolefin porous membrane is usually subjected to a frequency of 10 to 3 in the presence of a gas such as argon, helium, nitrogen, or air at a pressure of 10 ^{-2 to} 10 mbar.
Plasma processing is performed at 0 MHz and output of 1 to 1000 W for 1 to 1000 seconds. Next, the selected monomer is
Polyolefin which has been subjected to plasma treatment in an inorganic or organic solvent containing 10% by volume and optionally 0.01 to 2% by volume of a crosslinking aid (the above-mentioned monomer and crosslinking aid are dissolved or suspended in this solvent) While immersing the microporous membrane and bubbling with nitrogen gas, argon gas, etc., 20 to 10
The graft polymerization reaction is performed at 0 ° C. for 1 to 60 minutes. In addition,
As the solvent, water, an alcohol such as methanol, an aqueous alcohol solution, or the like can be used. Further, as the electron beam graft polymerization method, the microporous polyolefin membrane described above,
Simultaneous irradiation method in which the selected monomer and the crosslinking aid coexist and simultaneously irradiate an electron beam, and after previously irradiating the polyolefin microporous membrane with an electron beam, the method is carried out in the presence of the crosslinking aid. There is a pre-irradiation method in which the monomer is reacted, but a method using the pre-irradiation method is preferable because homopolymerization of the selected monomer is suppressed.

In the pre-irradiation method, the microporous polyolefin membrane is irradiated with an electron beam having an acceleration voltage of preferably from 100 to 5000 KeV, more preferably from 200 to 800 KeV. Note that the electron beam irradiation can be performed in an air atmosphere.
The irradiation amount is suitably from 10 to 500 KGy, preferably from 50 to 200 KGy. If it is less than 10 KGy, the grafting of the selected monomer is insufficient,
On the other hand, if it exceeds 500 KGy, the microporous polyolefin membrane may deteriorate.

Next, the polyolefin microporous membrane irradiated with the electron beam is immersed in the selected monomer solution in the presence of the crosslinking aid to form a graft polymer.
By such graft polymerization, a graft polymer is selectively formed on the surface of the microporous polyolefin membrane, or a graft polymer is selectively formed on the inner surface of the pore, or on either the surface or the inner surface of the pore. Can also form a graft polymer. The homopolymer by-produced in the course of the graft polymerization may be left as it is, but it may be completely washed away using a solvent such as toluene, and only the graft polymer is removed from the surface of the polyolefin microporous membrane and its pores. It may be left on the inner surface.

C. Graft Polymerization Amount The amount of graft polymerization can be controlled by conditions such as the amount of radical generation or monomer concentration, the contact time, and the temperature. Graft polymerization amount (graft amount of polymer polymerized per unit area) is preferably 0.02~35mg / cm ^2, more preferably 0.03~30mg / cm ^2. Although it depends on the thickness of the microporous polyolefin membrane, it may be in the range of 0.1 to 1.0.
02mg / cm dissolution by electrolyte is less than ^2, the swelling effect is insufficient, preventing deformation of the polyolefin porous membrane, the effect of reduction in strength prevent insufficient exceeds 35 mg / cm ^2. With an increase in the amount of graft polymerization, the micropores of the polyolefin microporous membrane are gradually closed, the porosity is gradually lost, and finally, the membrane is substantially completely closed.

3. Immobilization of aprotic electrolyte solution on microporous polyolefin membrane containing electronic conductive material and graft polymer a. Electrolyte solution As an electrolyte of the aprotic electrolyte solution, an alkali metal salt, an alkaline earth metal salt is used, for example, LiF,
NaI, LiI, LiClO ₄ , LiAsF ₆ , LiPF
₆ , LiBF ₄ , LiCF ₃ SO ₃ , NaSCN and the like. Further, as the aprotic solvent for dissolving the electrolyte of the aprotic electrolyte solution, a solvent stable to an alkali metal, specifically, propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethoxyethane, acetonitrile, form An aprotic high dielectric constant solvent such as amide, tetrahydrofuran, diethyl ether or the like is used alone or in combination of two or more.

B. Immobilization method As a method of immobilizing an aprotic electrolyte solution on a polyolefin microporous membrane containing an electronic conductive material and a graft polymer to form an aprotic electrolyte thin film, impregnation, coating or spraying is used alone or in combination. can do. Also, immobilizing the electrolyte solution is
It may be before the battery is assembled, in the middle of battery assembly, or in the final battery assembly process. Among them, in view of the ease of handling during battery assembly, the prevention of wrinkles and the like, the adhesion to the positive and negative electrode surfaces, and the fact that the conventional battery assembly process can be applied as it is, the electrolyte may be used during the battery assembly process or the final battery assembly process. The method of immobilizing the solution is preferred.

4. The specific conductivity of the immobilized liquid film conductor The immobilized liquid film conductor of the present invention constituted by the above,
It has a specific conductivity of 10 ^-5 Scm ^-1 or more, preferably 10 ^-3 Scm ^-1 or more. If the specific conductivity is less than 10 ^-5 Scm ^-1 , the effective resistance becomes large and is not practical. For example, film thickness 1μ
m, the effective resistance is 1 μm / 10 ⁻⁵ Scm ⁻¹ , that is, 1
It becomes 0 Ωcm ² .

[0021]

The present invention will be described in more detail with reference to the following specific examples. In addition, the test method in an Example is as follows. (1) Film thickness: The cross section was measured by a scanning electron microscope. (2) Tensile breaking strength: The breaking strength of a 15 mm wide strip test piece was measured in accordance with ASTM D882. (3) Porosity: measured by a gravimetric method.

Example 1 Polyethylene (polyethylene 2 having a weight average molecular weight of 400,000)
5 parts by weight, 30 parts by weight of petroleum coke powder and 30 parts by weight of Ketjen black powder (trademark of Akzo Chemical Co.)
Mixture 10 containing 3 parts by weight and 70 parts by weight of liquid paraffin
0.37 parts by weight of an antioxidant was added to 0 parts by weight, and the mixture was heated and kneaded with a twin screw extruder. This was extruded from a die having a rectangular die and taken up with a chill roll to form a 1 mm thick sheet. This sheet is subjected to batch stretching using a batch type biaxial stretching machine.
Polyethylene containing an electron conductive material is stretched simultaneously at 20 ° C. by 5 × 5 times, washed with n-hexane for remaining liquid paraffin, dried at 120 ° C. while fixed to a metal frame, and heat-set. A microporous membrane was obtained. The obtained microporous polyethylene film (thickness: 30 μm, porosity: 38%, tensile strength: 1350 kg / cm ² ) containing the electron conductive material was irradiated with 0.1 mba, 10 W, and 60 seconds of plasma in an argon atmosphere. Graft polymerization was carried out by contacting an aqueous methyl acrylate solution (having a monomer concentration of 4% by volume and using water as a solvent) at 30 ° C. for 15 minutes. After the completion of the reaction, the resultant was washed with toluene and dried in a 50 ° C. oven to obtain a polyethylene film containing a graft-polymerized electronic conductive material. From the weight change before and after the polymerization, the amount of the graft polymer polymerized per unit area was 2.5 mg / cm ² . Further, the tensile breaking strength was 1240 kg / cm ² .

10 cm × 10 cm of the obtained polyethylene film containing the graft-polymerized electron conductive material.
The corner was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour to obtain an immobilized liquid film conductor. After removing the adhering liquid on the surface of the immobilized liquid film conductor,
The change of the weight with time was immediately measured, and the weight increase rate determined by extrapolation after 0 seconds was 125.7%. In addition, the time-dependent change of the weight starting from the weight extrapolated after 0 seconds in the state of being left in the air at 25 ° C. was measured. As a result, the weight loss rate after 1 hour was less than 0.5%. In addition, diameter 10
mm, and this is sandwiched between platinum black electrodes, and the frequency is 1k.
The electrical conductivity was measured with an alternating current of Hz, and the specific conductivity calculated from this value and the thickness and area of the immobilized liquid film conductor was 1.2 ×
It was 10 ^-1 Scm ^-1 .

Comparative Example 1 In Example 1, a microporous polyethylene film (thickness: 30 μm, containing an electron conductive material before the graft polymerization was performed)
1 with a porosity of 38% and a tensile strength of 1350 kg / cm ² )
A 0 cm × 10 cm square was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour to obtain an immobilized liquid membrane conductor. Immediately after removing the adhering liquid on the surface of the immobilized liquid film conductor, the time-dependent change in weight was measured, and extrapolated after 0 second, the weight increase rate was 81.5%.
%Met. Further, a change with time of the weight starting from the weight extrapolated after 0 seconds in a state of being left in the air at 25 ° C. was measured. As a result, the weight loss rate after 1 hour was 2.7%. Also, punched into a diameter of 10 mm, this was sandwiched between platinum black electrodes, the electrical resistance was measured with an alternating current of 1 kHz, and the specific conductivity calculated from this value and the thickness and area of the immobilized liquid film conductor was 4 × 10 ^-2 Scm ^-1 .

[0025]

The immobilized liquid membrane conductor of the present invention immobilizes the electrolyte solution by the solubility of the graft-polymerized polymer,
By suppressing the excessive swelling by the porous membrane base skeleton made of polyolefin, it is possible to stably hold the electrolyte solution in a wide temperature range and to keep the evaporation rate of the electrolyte solution extremely low, Good conductivity can be maintained over a wide temperature range. That is, safety in overcharging can be improved without significantly lowering the electronic conductivity. Further, the immobilized liquid film conductor has excellent mechanical strength due to a skeleton made of polyolefin, and can be applied with almost no change in a conventional battery manufacturing process. In addition, since this immobilized liquid film conductor has both ion and electron conductivity, the electrolyte,
It is particularly useful for electrodes using a liquid electrolyte, such as batteries, electrochromic devices, electric double layer capacitors, and liquid crystal devices. Since the ionic conductor in the immobilized liquid membrane conductor is continuous with the electrolyte between the electrodes and is in close contact with the porous conductive film over a wide area, it is effective as an electrode material for various cells and elements using the electrolyte. .

Claims (2)

[Claims] 1. An immobilized liquid membrane conductor in which an aprotic electrolyte solution is immobilized on a microporous polyolefin membrane containing a graft polymer having solubility in an aprotic electrolyte solution and an electron conductive material. 2. A polymer having solubility in an aprotic electrolyte solution is graft-polymerized on the surface of a microporous polyolefin membrane containing an electron conductive material and on the surface of pores, and this is impregnated with an aprotic electrolyte solution. For producing an immobilized liquid film conductor to be immobilized.