JP3581774B2 - Aprotic electrolyte thin film and method for producing the same - Google Patents

Description

[0001]
TECHNICAL 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]
[Prior art]
The electrolyte thin film can be widely used in fields requiring low membrane resistance and excellent mechanical strength, such as fuel cells, salt electrolysis, primary batteries, secondary batteries, separators for facilitated transport, electrochromic devices, and sensors. Among them, it can be used as a solid polymer electrolyte such as a lithium secondary battery.
Polymer solid electrolyte type 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 type secondary batteries. Development is desired because it enables a large area.
[0003]
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, and 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 using capillary condensation in fine pores of 0.1 μm or less of a solid polymer porous thin film of 50 μm or less. Although there is a method (Japanese Patent Laid-Open No. 1-158051), the problem of operating temperature has not been fundamentally solved.
[0004]
Furthermore, 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. No. 4,303,748) and polyarylate are used. A technique (US Pat. No. 4,830,939) for gelling and using as an electrolyte has been proposed. Recently, a technique 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 (US Pat. No. 5,296,318) has been proposed and is promising. However, 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]
[Problems to be solved by the invention]
The present invention solves the above-mentioned problems, facilitates thinning, large area, etc., and has excellent aprotic electrolyte solvent retention over a wide temperature range, and has improved long-term stability and mechanical strength. An object of the present invention is to provide a protic electrolyte thin film and a method for producing the same.
[0006]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to overcome the problems of the prior art, and found that the aprotic electrolyte solution was immobilized on the membrane due to the affinity of the organopolysiloxane contained in the polyolefin microporous membrane. It has been found that the above object can be achieved by the above.
[0007]
That is, the present invention is an aprotic electrolyte thin film in which an aprotic electrolyte solution is immobilized on a polyolefin microporous film containing an organopolysiloxane having an affinity for the aprotic electrolyte solution.
In the first method for producing an aprotic electrolyte thin film of the present invention, a polyolefin microporous membrane is coated with an organopolysiloxane having an affinity for the aprotic electrolyte solution, and impregnated with the aprotic electrolyte solution. And immobilize it.
Further, in the second method for producing an aprotic electrolyte thin film of the present invention, a gel-like molded article comprising a polyolefin composition is filled with an organopolysiloxane having an affinity for an aprotic electrolyte solution to form a film. Is impregnated with an aprotic electrolyte solution to be 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 an organopolysiloxane having an affinity for the aprotic electrolyte solution. The details will be described below.
[0009]
1. Raw material a. Polyolefin Examples of the polyolefin include polyethylene, polypropylene, ethylene-propylene copolymer, polybutene-1 and poly-4-methylpentene-1. Of these, polyethylene is preferred. As this polyethylene, those composed of ultra-high-molecular-weight polyethylene, high-density polyethylene, and 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 properties, etc. It is preferable to use
Further, the polyolefin contains an ultrahigh molecular weight component having a weight average molecular weight of 5 × 10 ⁵ or more, preferably 1 × 10 ⁶ to 1 × 10 ⁷ , at least 1% by weight, and has a molecular weight distribution (weight average molecular weight / number average molecular weight). ) Is preferably from 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.
[0010]
b. Organopolysiloxane The organopolysiloxane used in the present invention is a polymer having at least one silicon-carbon bond per silicon and having a silicon-oxygen bond (→ Si—O—Si ←) as a repeating unit. Compound. For example, commercially available silicone oils and silicone rubbers having a polymerization degree of 10 or more, preferably 10 to 10000 can be used. If the degree of polymerization is less than 10, volatile dissipation is not negligible and is not preferred. On the other hand, if the degree of polymerization greatly exceeds 100, the viscosity becomes high and it becomes difficult to uniformly fill the gel-like molded product. Therefore, dilution with a volatile solvent is preferred.
[0011]
Specific examples of the organopolysiloxane include polydimethylsiloxane in which an organic group is a methyl group, and hydrogen, a vinyl group, a hydroxyl group, an amino group, a carboxyl group, an epoxy group, and a methacryloxy group at the end or inside of the polysiloxane chain. , A modified polysiloxane having a mercapto group, a long-chain alkyl group, a phenyl group, chlorine or fluorine bonded thereto, and further having a non-polysiloxane portion in the main chain, for example, an alkylene oxide-modified polysiloxane, a silicone-modified copolymer, an alkoxysilane Modified polymers and the like can be mentioned. Polymethylvinylsiloxane, polymethylphenylvinylsiloxane, polymethylfluorovinylsiloxane, or polydimethylsiloxane with a hydroxyl-terminated end, such as a modified polysiloxane having a vinyl or hydroxyl group bonded to the inside or end of the polysiloxane chain, may also be used. it _{is, - (CH 2 CH 2 O} ) n modified polysiloxane or with an ether-based group in the side chain, such as -R - _{[(CH 2) m COO (CH} 2) n ] ester such as -R The modified polysiloxane having a group is expected to form not only an affinity with an aprotic electrolyte solution but also a kind of complex with lithium ions to promote ionic conduction.
[0012]
Volatile solvents for diluting organopolysiloxanes with high degrees of polymerization include hydrocarbons such as pentane, hexane, heptane, and toluene, chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, and ethane trifluoride ethane. And ethers such as diethyl ether and dioxane, and alcohols such as methanol and ethanol.
The above-mentioned organopolysiloxane or its diluted solution may contain, if necessary, a crosslinking agent for the organopolysiloxane, for example, an organic peroxide, an organosilicon compound having three or more functional groups, an alkyl orthosilicate, a metal catalyst. Alternatively, a small amount of a reinforcing material such as synthetic silica or other additives may be blended.
[0013]
c. As the electrolyte of the electrolytic solution aprotic electrolytic solution, alkali metal salts, alkaline earth metal salts are 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.
[0014]
2. Production Method The method for producing the aprotic electrolyte thin film of the present invention is described in A. B. a method of coating an organopolysiloxane on a polyolefin microporous membrane and immobilizing an electrolyte solution; There are two production methods: a method in which a gel-like molded product made of a polyolefin composition is filled with an organopolysiloxane to form a film, and an electrolyte solution is fixed.
[0015]
A. Method of coating organopolysiloxane on microporous polyolefin membrane and fixing electrolyte solution Aa. Production of Microporous Membrane A microporous polyolefin membrane may be produced by the method described in JP-A-60-242035 or 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 that described in JP-A-60-242035, but may also be a volatile solvent such as decalin or xylene. 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 and the like can also be used. The die gap when a sheet die is used 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.
[0016]
The solution extruded from the die in this manner is formed into a gel by cooling. The cooling rate is preferably at least 50 ° C./min at least up to the gelling temperature.
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 equal to or lower than the melting point of polyethylene of the polyolefin composition + 10 ° C, preferably in the range from the crystal dispersion temperature to less than the crystal melting point. For example, it is in the range of 90 to 140C, more preferably 100 to 130C.
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 film 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.
[0017]
The above-mentioned microporous polyolefin membrane 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, 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%. When the porosity is less than 30%, the immobilization of the aprotic electrolyte solution may be insufficient. On the other hand, when the porosity exceeds 95%, the mechanical strength of the membrane becomes small and the practicability is poor. .
[0018]
Ab. Coating method of organopolysiloxane As a coating method of organopolysiloxane, a solution of an organopolysiloxane dissolved in an organic solvent which is a non-solvent for the above-mentioned microporous polyolefin membrane is sprayed, applied or coated on the polyolefin microporous membrane. In a liquid, and other coating operations. More preferably, the film obtained by evaporating and solidifying the organic solvent is further crosslinked. The thickness of the coating is preferably 1 to 1000 μm as a composite film.
[0019]
Ac. Method of fixing electrolyte solution As a method of fixing an aprotic electrolyte solution to a microporous polyolefin membrane containing an organopolysiloxane to form an aprotic electrolyte thin film, impregnation, coating or spraying, etc., may be used alone or in combination. Can be. 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, from the viewpoints of ease of handling at the time of battery assembly, prevention of wrinkles and the like, adhesion to the positive and negative electrode plate surfaces, and the fact that the conventional battery assembling process can be applied as it is, the electrolyte may be used during the battery assembling process or the battery assembling final 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 microporous polyolefin membrane containing organopolysiloxane. 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, if it exceeds 350% by weight, the mechanical strength of the film becomes insufficient.
[0020]
B. Method of filling organopolysiloxane into a gel-like molded product comprising a polyolefin composition to form a film, and fixing an electrolyte solution Ba-a. Preparation of gel-like molded article A gel-like sheet comprising the polyolefin composition is prepared in the same manner as the gel-like sheet before the stretching step in the method for producing a microporous membrane described in A-a. That is, a gel-like sheet can be obtained by heating and dissolving an ultrahigh molecular weight polyolefin in a solvent, forming the solution into a sheet, and cooling the sheet.
[0021]
B-b. In order to fill the organopolysiloxane and fill the organopolysiloxane, the solvent in the gel sheet is first removed. Examples of the removal method include removal of the solvent by evaporation of the gel-like sheet by heating, removal by compression, extraction and removal of the solvent by a volatile solvent, and removal of the solvent while maintaining the gel-like sheet network by freeze-drying. However, in order to remove the solvent without significantly changing the structure of the gel-like sheet, extraction with a volatile solvent is preferable. Further, it is preferable to remove the solvent in the gel-like sheet to 1% by weight or less.
[0022]
The filling of the organopolysiloxane is performed by dipping or coating the gel-like sheet subjected to the desolvation treatment in an organopolysiloxane liquid or a dilute solution thereof in the presence or absence of a volatile solvent for the desolvation treatment. Done by Further, even in the case of an untreated gel-like sheet that has not been subjected to a solvent removal treatment, it can be filled by press-fitting an organopolysiloxane liquid or a diluted solution thereof. However, it is more preferable to immerse the gel-like sheet in the presence of the volatile solvent after the desolvation treatment, because it can be easily filled.
[0023]
The dilution concentration of the diluted organopolysiloxane solution depends on the amount of the organopolysiloxane filled in the gel sheet, but is preferably at least 0.05% by weight or more, particularly preferably 0.5% by weight or more. If the concentration is less than 0.05% by weight, the amount of the organopolysiloxane filled in the gel-like sheet is insufficient, and pinholes are likely to occur during stretching. Further, the solution viscosity is preferably less than 500 cSt at 25 ° C., particularly preferably less than 100 cSt in order to uniformly fill the entire gel sheet.
The filling amount of the organopolysiloxane in the gel sheet is preferably from 5 to 90% by weight, particularly preferably from 10 to 80% by weight.
Next, the obtained organopolysiloxane-containing gel-like sheet is stretched. The stretching method can be performed in the same manner as the stretching step of the gel-like molded product described in A-a.
[0024]
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.
[0025]
【Example】
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 the 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.
[0026]
Example 1
A polyethylene microporous 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 ) diluted to 15% by weight with xylene 2 A liquid type room temperature curing silicone rubber compound [KE103RTV (trade name), manufactured by Shin-Etsu Chemical Co., Ltd.] was applied and left at room temperature for 24 hours to obtain a cured organopolysiloxane filled film having a filling amount of 53% by weight.
[0027]
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 of the polyethylene microporous membrane filled with the organopolysiloxane, and left for 1 hour in a closed container to obtain a swelling ratio (weight increase). (Proportion) of 105% 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 5 × 10 ⁻³ Scm ⁻¹ .
[0028]
Example 2
100 parts by weight of a mixture composed of 3 parts by weight of ultrahigh molecular weight polyethylene (weight average molecular weight 2 × 10 ⁶ ), 14 parts by weight of high density polyethylene (weight average molecular weight 4 × 10 ⁵ ) and 83 parts by weight of liquid paraffin (64 cSt / 40 ° C.) To 0.1 part by weight of 2,6-di-t-butyl-p-cresol and 0.25 part of tetrakis [methylene-3- (3,5-di-t-butyl-4-hydroxyphenyl) propionate] methane A part by weight 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 taken up by a chill roll adjusted to a temperature of 30 ° C. to obtain a sheet having a thickness of 2 mm. The sheet was immersed in a large amount of methylene chloride for 60 minutes, and immediately without drying the sheet, 20% by weight of methyl vinyl silicone oil (manufactured by Nippon Unicar, trade name: NUC silicone gum stock W-9613) was used. After being transferred into a methylene solution and immersed for 60 minutes, methylene chloride was dried and removed in a state where the four sides were fixed to obtain a sheet with a methyl vinyl silicone oil filling of 49% by weight.
[0029]
This methyl vinyl silicone oil-filled sheet was simultaneously biaxially stretched 5 × 5 times at 125 ° C. using a batch type biaxial stretching machine. When the infrared absorption spectrum of the obtained film was measured, no vinyl group was recognized, and no elution of methylvinylsilicone oil by methylene chloride extraction was observed.
The polyethylene film filled with this organopolysiloxane was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour to obtain an aprotic electrolyte thin film having a thickness of 22 μm and a swelling ratio (weight increase ratio) of 126%. .
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 7 × It was 10 ⁻³ Scm ⁻¹ .
[0030]
Example 3
In Example 2, a sheet having a filling amount of 45% by weight was obtained using dimethyl silicone oil (trade name: NUC silicone oil L-45, manufactured by Nippon Unicar) instead of methyl vinyl silicone oil. Stretched film was formed and irradiated with 5 Mrad of γ-ray from a Co-60 radiation source to obtain a polyethylene film filled with organopolysiloxane. No dimethyl silicone oil was eluted by methylene chloride extraction of the obtained membrane.
A 10 × 10 mm square polyethylene film filled with this organopolysiloxane is immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour, and a non-proton electrolyte having a thickness of 23 μm and a swelling ratio (weight increase ratio) of 105% is 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 at a frequency of 1 kHz, and the ionic conductivity calculated from this value and the thickness and area of the thin film was 6 × It was 10 ⁻³ Scm ⁻¹ .
[0031]
Comparative Example 1
In Example 1, a 10 × 10 mm square of the microporous polyethylene membrane before being filled with the organopolysiloxane was immersed in a propylene carbonate solution containing 1 mol of LiPF ₆ at 25 ° C. for 1 hour, and a swelling rate (weight increase rate) was obtained. A 72.5% aprotic electrolyte thin film was obtained.
The obtained aprotic electrolyte thin film was punched out to a diameter of 10 mm, sandwiched between platinum black electrodes, measured for electrical resistance with an alternating current at a frequency of 1 kHz, and ion-calculated from this value and the thickness and area of the aprotic electrolyte thin film. The conductivity was 3 × 10 ⁻³ Scm ⁻¹ .
[0032]
【The invention's effect】
The aprotic electrolyte thin film of the present invention is stable over a wide temperature range by immobilizing the electrolyte solution by the affinity of the introduced organopolysiloxane and suppressing its excessive swelling by the porous membrane base skeleton made of polyolefin. And the evaporation rate of the electrolyte solvent can be kept extremely low.
In addition, the ionic conductivity can be easily controlled according to the purpose of use by properly selecting the type of the organopolysiloxane. That is, safety in overcharging can be improved without significantly lowering ionic conductivity.
Further, the aprotic electrolyte thin film 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 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, particularly a lithium primary battery. And a lithium secondary battery.

Claims (6)

An aprotic electrolyte thin film comprising an aprotic electrolyte solution immobilized on a polyolefin microporous membrane containing an organopolysiloxane having an affinity for an aprotic electrolyte solution. A method for producing an aprotic electrolyte thin film comprising coating a polyolefin microporous membrane with an organopolysiloxane having an affinity for an aprotic electrolyte solution, impregnating the coating with an aprotic electrolyte solution, and immobilizing the resultant. An aprotic electrolyte thin film in which a gel-like molded product composed of a polyolefin composition is filled with an organopolysiloxane having an affinity for an aprotic electrolyte solution to form a film, which is then impregnated with the aprotic electrolyte solution and immobilized. Manufacturing method. The gel-like molded product is obtained by heating and dissolving a polyolefin in a solvent, shaping the solution into a sheet, and cooling it, and removing the solvent in the gel-like sheet before filling with the organopolysiloxane. The method for producing an aprotic electrolyte thin film according to claim 3. The method for producing an aprotic electrolyte thin film according to claim 3 or 4, wherein the organopolysiloxane-containing gel-like sheet obtained by filling the organopolysiloxane is stretched. A lithium battery using the aprotic electrolyte thin film according to claim 1.