JPH10283838A - Aprotic type electrolytic thin film and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolytic thin film which is easy to be formed into a thin film with a wide surface area and has excellent aprotic electrolytic solvent retaining property in a wide temperature range by fixing a aprotic type electrolytic solution in a polyolefin type finely porous film containing a wax having affinity to an aprotic electrolytic solution. SOLUTION: This electrolytic thin film is preferably produced by employing a method carried out by forming a wax coating on a polyolefin type finely porous film and then fixing the electrolytic solution or a method carried out by forming a finely porous film from a polyolefin composition containing a wax and fixing the electrolytic solution. The polyolefin type finely porous film is produced by thermally dissolving a polyolefin composition in a solvent such as decalin, molding the resultant solution by die extrusion, cooling the molded product to give a gel product, extracting the solvent by a volatile solvent, drying the resultant product, and thermally setting the product. The porosity of the obtained film is preferably within a range of 50-90%. The ion conductivity can easily be controlled based on the use purposes by selectively using different types of waxes.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aprotic electrolyte thin film and a method for producing the same, and more particularly to an aprotic electrolyte thin film in which an aprotic electrolyte solution is immobilized on a polyolefin microporous membrane and a method for producing the same.

[0002]

2. Description of the Related Art Electrolyte thin films are fields requiring low membrane resistance and excellent mechanical strength, such as fuel cells, salt electrolysis, primary batteries, secondary batteries, separation membranes for facilitated transport, electrochromic devices, and sensors. Widely available to. Among them, it can be used as a solid polymer electrolyte such as a lithium secondary battery. Polymer solid electrolyte-based lithium secondary batteries prevent lithium metal dendrite formation and solve battery short-circuit damage and ignition problems.There is no liquid leakage compared to solution-based secondary batteries. Development is desired because it enables a large area.

A polymer electrolyte in which a lithium salt such as LiClO ₄ is dissolved in a polyether such as polyethylene oxide or polypropylene oxide, a polyester, a polyimide or a polyether derivative has been developed, but the ionic conductivity is 10 ⁻⁵ to 10 ^{−. 3} Scm ^-1 is not exhibited unless it is sufficiently higher than room temperature. In order to reduce the effective resistance, as an example of thinning, a liquid ionic conductor is immobilized by utilizing capillary condensation in fine pores of 0.1 μm or less of a solid polymer porous thin film of 50 μm or less. Method (JP-A-1-158)
No. 051), but the problem of operating temperature has not been fundamentally solved.

Further, as a gel polymer in which a polymer matrix is impregnated with a salt and solvent solution similar to that of a conventional liquid type lithium battery, a technique using a cross-linked polyalkylene oxide as an electrolyte (US Pat.
3,748), and a technique (US Pat. No. 4,830,939) in which polyarylate is gelled and used as an electrolyte has been proposed. Recently, a technique (USP 5,296,318) using a polymer gel obtained by impregnating a carbonate-based solution obtained by dissolving a lithium salt in a copolymer of polyvinylidene fluoride and hexafluoropropylene as an electrolyte has been proposed.
Although promising, there is a problem of oozing out of the electrolytic solution due to gel shrinkage at a high temperature, and there is a problem in solvent retention.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems, is easy to form a thin film and has a large area, has excellent aprotic electrolyte solvent retention over a wide temperature range, and has a long-term stability. An object of the present invention is to provide an aprotic electrolyte thin film having improved properties and mechanical strength, and a method for producing the same.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies to overcome the above-mentioned problems of the prior art, and have found that the affinity of wax contained in a microporous polyolefin membrane makes it possible to obtain an aprotic electrolyte. It has been found that the above object can be achieved by immobilizing on a membrane.

That is, the present invention is an aprotic electrolyte thin film in which an aprotic electrolyte solution is immobilized on a microporous polyolefin membrane containing wax having an affinity for the aprotic electrolyte solution. Further, the first method for producing an aprotic electrolyte thin film of the present invention comprises coating a polyolefin microporous membrane with a wax having an affinity for the aprotic electrolyte solution, and impregnating this with an aprotic electrolyte solution. It is to be fixed. Further, the second method for producing an aprotic electrolyte thin film of the present invention comprises forming a microporous polyolefin membrane from a polyolefin composition containing a wax having an affinity for an aprotic electrolyte solution, The solution is impregnated and immobilized.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The aprotic electrolyte thin film of the present invention is constituted by immobilizing an aprotic electrolyte solution on a microporous polyolefin membrane containing wax having an affinity for the aprotic electrolyte solution. You. The details will be described below.

1. Raw materials 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. Waxes As waxes having an affinity for aprotic electrolyte solutions, natural waxes such as vegetable waxes, animal waxes, mineral waxes, petroleum waxes, synthetic hydrocarbons,
Synthetic waxes such as modified waxes, hydrogenated waxes, fatty acids, acid amides, esters and ketones can be used. Among them, the ester [RCOO (CH ₂ CH ₂ ) _n CO
OR ′], a synthetic wax such as an ester wax represented by [RCOOR ′], a monohydric alcohol fatty acid ester represented by [RCOOOR ′], [RCOOCH ₂ CH (OH) CH ₂ OH]
A glycerin fatty acid ester represented by the formula [RCOOCH
₂ CH ₂ OH], an ethylene glycol ester represented by the formula:
Fatty oil-based synthetic waxes such as sorbitan fatty acid esters are expected to not only have an affinity for aprotic electrolyte solutions, but also form a kind of complex with lithium ions to promote ionic conduction.

C. 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.

2. Production Method The method for producing the aprotic electrolyte thin film of the present invention is described in A. A method of coating a microporous polyolefin membrane with wax to fix an electrolyte solution, There are two methods, a method of forming a microporous membrane from a wax-containing polyolefin composition and fixing an electrolyte solution.

A. Method for coating a polyolefin microporous membrane with wax to fix an electrolyte solution Aa. Production method of microporous membrane The production of polyolefin microporous membrane is described in JP-A-60-242.
035 and JP-A-3-64334. For example, it can be performed as follows. First, a solution is prepared by heating and dissolving the polyolefin composition in a solvent. The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin composition, and may be the same as those described in JP-A-60-242035, etc., but volatile solvents such as decalin and xylene may also be used. Can be used. The heat dissolution is performed while stirring at a temperature at which the polyolefin composition is completely dissolved in the solvent. The temperature varies depending on the polymer and solvent used, but is preferably in the range of 140 to 250 ° C. The concentration of the polyolefin composition solution is 10 to 50% by weight, preferably 10 to 40% by weight. Next, the heated solution of the polyolefin composition is extruded from a die and molded. As the die, a sheet die having a rectangular base shape is usually used, but a double cylindrical hollow fiber die, an inflation die, or the like can also be used. When a sheet die is used, the die gap is usually 0.1 to 5 mm, and is heated to 140 to 250 ° C during extrusion molding. The extrusion speed at this time is usually 20 to 30 cm / min to 5 to 10 m / min.

The solution thus extruded from the die is formed into a gel by cooling. The cooling rate is preferably at least 50 ° C./min up to the gelation temperature or lower. Next, this gel-like molded product is stretched. The stretching is performed at a predetermined magnification by heating the gel-like molded product and using a usual tenter method, a roll method, an inflation method, a rolling method or a combination of these methods. Biaxial stretching is preferred, and either longitudinal or transverse simultaneous stretching or sequential stretching may be used, but simultaneous biaxial stretching is particularly preferred. The stretching temperature is the melting point of polyethylene of the polyolefin composition + 10 ° C.
Hereinafter, the range is preferably from the crystal dispersion temperature to less than the crystal melting point. For example, 90-140 ° C, more preferably 1
It is in the range of 00 to 130 ° C. Further, 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 may contain various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an antiblocking agent, a pigment, a dye, and an inorganic filler, if necessary, in a range that does not impair the object of the present invention. Can be added.

The above-mentioned microporous polyolefin membrane has the following properties:
It has a thickness of 000 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, if it exceeds 1000 μm, the effective resistance becomes large and the volume efficiency as an aprotic electrolyte thin film becomes disadvantageous. The porosity of the film is not limited, but is in the range of 30 to 95%, more preferably 50 to 90%. Porosity of 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 film is low, and the film is not practical.

Ab. Wax Coating Method Wax is diluted with a diluting solvent and coated on the above-mentioned microporous polyolefin membrane. As the coating method, impregnation, application, spraying, or the like can be used alone or in combination. In addition, as a diluting solvent,
Hydrocarbons such as pentane, hexane, heptane, and toluene; chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride;
Examples thereof include fluorinated hydrocarbons such as ethane trichloride, ethers such as diethyl ether and dioxane, and alcohols such as methanol and ethanol.

Ac. Method of fixing electrolyte solution As a method of fixing an aprotic electrolyte solution to a wax-containing microporous polyolefin membrane to form an aprotic electrolyte thin film, impregnation, coating or spraying can be used alone or in combination. . In addition, the electrolyte solution may be fixed before being incorporated into the battery, during the battery assembly process, or at 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. The content of the electrolyte solution is 70 to 350% by weight, preferably 80 to 250% by weight of the wax-containing microporous polyolefin membrane. When the retention is less than 70% by weight, the interface with the electrolyte solution is reduced, and the application as a battery, a capacitor, and an electrochromic element is restricted from the viewpoint of practicality. On the other hand, 35
If it exceeds 0% by weight, the mechanical strength of the film becomes insufficient.

B. Method for preparing a microporous membrane from a wax-containing polyolefin composition and immobilizing an electrolyte solution Ba-a. Composition ratio of polyolefin composition The composition ratio of the ultra-high molecular weight polyolefin alone or the polyolefin composition with the polyolefin and the wax is such that the wax is 1 to 50% by weight, preferably 3 to 30% by weight based on the whole resin composition. is there. If it is less than 1% by weight, the effect of immobilizing the solvent in the electrolyte solution cannot be expected. on the other hand,
If it exceeds 50% by weight, the mechanical strength is significantly reduced.

B-b. Method for Producing Microporous Membrane The method for producing a polyolefin microporous film from a wax-containing polyolefin composition can be performed in the same manner as in the method for producing a polyolefin composition (A-a). B-c. Method for Immobilizing Electrolyte Solution The method for immobilizing the electrolyte solution can be performed in the same manner as the method described in Ac.

[0020]

EXAMPLES The present invention will be described in more detail with reference to the following specific examples, but the present invention is not particularly limited to these 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) Porosity: measured by a gravimetric method. (3) Tensile breaking strength: measured according to ASTM D882. (4) Average pore size: measured by Omnisoap 360.

Example 1 A microporous polyethylene membrane (weight average molecular weight 1.1 million, molecular weight distribution 20, film thickness 25 μm, average pore size 0.03 μm, porosity 43.5%, tensile strength 1049 kg / cm ² ) was treated with methylene chloride. Stearine octadecyl diluted to 15% by weight was applied and left to dry at room temperature for 24 hours to obtain a composite film having a stearin octadecyl loading of 49% by weight.

A small amount of a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. was dropped into a 10 × 10 mm square polyethylene microporous membrane filled with stearin octadecyl, and left in an airtight container for 1 hour to obtain a swelling ratio ( An aprotic electrolyte thin film having a weight increase rate of 98% was obtained. The obtained aprotic electrolyte thin film was punched out to a diameter of 10 mm, this was sandwiched between platinum black electrodes, the electric resistance was measured with an alternating current at a frequency of 1 kHz, and the ionic conductivity calculated from this value and the thickness and area of the thin film was 5 × 10 ^-3 Scm ^-1 .

Example 2 5 parts by weight of stearin octadecyl, 5 parts by weight of ultra high molecular weight polyethylene (weight average molecular weight 2 × 10 ⁶ ), 20 parts by weight of high density polyethylene (weight average molecular weight 4 × 10 ⁵ ) and liquid paraffin (64 cSt / (40 ° C.) 0.1 part by weight of 2,6-di-t-butyl-p-cresol and 100 parts by weight of a mixture comprising 70 parts by weight of tetrakis [methylene-3- (3,5-di-t-butyl) 0.24 parts by weight of [-4-hydroxyphenyl) propionate] methane was added as an antioxidant, and the mixture was heated and kneaded with a twin-screw extruder. This was extruded from a die having a rectangular die, and was taken out with a chill roll adjusted to a temperature of 30 ° C. to obtain a sheet having a thickness of 0.5 mm. This sheet was processed using a batch type biaxial stretching machine for 110 hours.
The film was simultaneously biaxially stretched 5 × 5 times at 5 ° C., and the remaining liquid paraffin was washed with n-hexane and then fixed to a metal frame.
After drying at 5 ° C. and heat setting, a microporous polyethylene membrane containing stearin octadecyl was obtained.

A 10 × 10 mm square polyethylene microporous membrane containing stearin octadecyl was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour, and an aprotic electrolyte having a swelling rate (weight increase rate) of 125% was used. A thin film was obtained. The obtained aprotic electrolyte thin film was punched out to a diameter of 10 mm, this was sandwiched between platinum black electrodes, the electric resistance was measured with an alternating current of 1 kHz, and the ionic conductivity calculated from this value and the thickness and area of the thin film was 1 × 10 ^-2
It was Scm ^-1 .

Example 3 Ester wax (VPCSnew, manufactured by Hoechst) 5
Parts by weight, ultra high molecular weight polyethylene (weight average molecular weight 2 ×
10 ⁶ ) 5 parts by weight, 20 parts by weight of high-density polyethylene (weight average molecular weight 4 × 10 ⁵ ) and liquid paraffin (64c
(St / 40 ° C.) 0.1 part of 2,6-di-tert-butyl-p-cresol was added to 100 parts by weight of a mixture comprising 70 parts by weight.
25 parts by weight and 0.25 parts by weight of tetrakis [methylene-3- (3,5-di-tert-butyl-4-hydroxyphenyl) propionate] methane as an antioxidant were added.
The mixture was heated and kneaded with a screw extruder. This was extruded from a die having a rectangular die, and was taken out with a chill roll adjusted to a temperature of 30 ° C. to obtain a sheet having a thickness of 0.5 mm. This sheet was simultaneously biaxially stretched 5 × 5 times at 110 ° C. using a batch-type biaxial stretching machine, and the remaining liquid paraffin was washed with n-hexane and dried at 115 ° C. while being fixed to a metal frame. Heat setting was performed to obtain a microporous polyethylene membrane containing an ester wax.

A 10 × 10 mm square polyethylene microporous membrane containing this ester wax is mixed with 1 mol of Li at 25 ° C.
It was immersed in a propylene carbonate solution containing PF ₆ for 1 hour to obtain a non-proton electrolyte thin film having a swelling ratio (weight increase ratio) of 105%. The obtained aprotic electrolyte thin film was formed with a diameter of 10
mm, and this is sandwiched between platinum black electrodes.
The ionic conductivity calculated from the electric resistance value measured with an alternating current of Hz and the thickness and the area of the thin film was 9 × 10 ⁻³ Scm.
It was ^-1 .

Comparative Example 1 In Example 1, 10 × 10 mm of the microporous polyethylene membrane before coating with stearin octadecyl was used.
The corner was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour, and the swelling ratio (weight increase ratio)
A 72.5% aprotic electrolyte thin film was obtained. The obtained aprotic electrolyte thin film was punched into a diameter of 10 mm, sandwiched between platinum black electrodes, and measured for electrical resistance with an alternating current at a frequency of 1 kHz. The ion was calculated from this value and the thickness and area of the aprotic electrolyte thin film. Conductivity is 3 × 10 ^-3 Scm
It was ^-1 .

[0028]

The aprotic electrolyte thin film of the present invention immobilizes the electrolyte solution by the affinity of the introduced wax,
By suppressing the excessive swelling by the porous membrane substrate skeleton made of polyolefin, the electrolyte solution can be stably held in a wide temperature range, and the evaporation rate of the electrolyte solution solvent can be kept extremely low. In addition, by properly using the type of wax, the ionic conductivity can be easily controlled according to the purpose of use. That is,
The safety in overcharging can be improved without significantly lowering the ionic conductivity. Further, the aprotic electrolyte thin film has excellent mechanical strength due to the skeleton made of polyolefin, and can be applied with almost no change in the conventional battery manufacturing process. In particular, when an ultra-high molecular weight polyethylene component is used as the polyolefin, the aprotic electrolyte thin film has excellent mechanical strength and durability, and a primary battery, a secondary battery, a capacitor using an aprotic electrolyte, and especially a lithium primary battery. And a lithium secondary battery.

Claims (3)

[Claims] 1. An aprotic electrolyte thin film comprising an aprotic electrolyte solution immobilized on a polyolefin microporous membrane containing a wax having an affinity for an aprotic electrolyte solution. 2. A method for producing an aprotic electrolyte thin film in which a microporous polyolefin membrane is coated with a wax having an affinity for an aprotic electrolyte solution, and impregnated with the wax and immobilized with the aprotic electrolyte solution. 3. An aprotic electrolyte thin film formed by forming a polyolefin microporous membrane from a polyolefin composition containing a wax having an affinity for an aprotic electrolyte solution and impregnating the polyolefin microporous membrane with the aprotic electrolyte solution. Manufacturing method.