JP3497531B2 - Microporous membrane coating - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous membrane covering suitable for a separator for a capacitor, a separator for a battery and the like. As a battery separator, a sheet-like microporous membrane made of a polyolefin resin, a solid electrolyte separator made of a polymer organic substance or a polymer organic substance containing an alkali metal salt, or the like is used. [0003] A conventional separator made of a polyolefin-based resin is required to have a high strength when assembling a battery or the like, and sacrifices electrical characteristics, for example, electric resistance, for the high strength. Had become. Further, the solid electrolyte separator does not have such strength as to be able to assemble a spiral-wound battery or the like. [0004] Therefore, it is desired to provide a microporous membrane having sufficient strength and maintaining excellent properties such as the electrical properties of a conventional separator made of a polyolefin resin. The inventor of the present invention has found that a microporous membrane-coated body in which a microporous membrane is coated with a high molecular organic material containing an alkali metal salt can achieve the object. Heading, the present invention has been completed. In the present invention, a high molecular organic substance containing an alkali metal salt is coated on a microporous membrane by an immersion method, and the coated microporous membrane has an air permeability of 1500 sec / 1.
00 cc or less, the maximum pore size is 0.01 μm to 2.0 μm,
A microporous membrane covering having a porosity of 10% to 80%. The resin constituting the microporous membrane used in the present invention is not particularly limited, but a polyolefin resin is preferable. The polyolefin resin is polyethylene,
It refers to polypropylene, ethylene-1-butene copolymer, ethylene-1-hexene copolymer, ethylene-1-octene copolymer and the like. [0007] Among the polyolefin resins, polyethylene resins are most preferred, and polypropylene and the like may be mixed. Polyethylene resin is low density polyethylene,
Either medium-density polyethylene or high-density polyethylene may be used, and high-density polyethylene is particularly preferable, or a mixture thereof. Although the molecular weight of the polyolefin resin is not particularly limited, the viscosity average molecular weight is preferably 200,000 or more from the viewpoint of strength, and the viscosity average molecular weight is preferably 5,000,000 or less from the viewpoint of processability, more preferably 250,000 or more, and 3,000,000 or more. Less than,
Most preferably, it is 300,000 or more and 2,000,000 or less. The microporous membrane used in the present invention is, for example, a mixture of the above-mentioned polyolefin resin and inorganic fine powder and an organic liquid, melt kneading, extrusion molding, extraction of the inorganic fine powder and the organic liquid, drying and stretching. It is obtained by doing. Examples of the organic liquid include phthalic acid esters such as dioctyl phthalate and di-n-butyl phthalate, liquid paraffin, and mineral oils such as process oils. Dioctyl phthalate and liquid paraffin are preferred.
Further, the inorganic fine powder includes finely divided silica, calcium silicate,
Examples thereof include calcium carbonate and fine talc, and fine silica is preferable. In order to extract the organic liquid, any solvent may be used as long as it is a good solvent for the organic liquid without deteriorating the polyolefin resin or the fluorine resin. Alcohols such as methanol, ethanol and isopropanol, and ketones such as acetone and methyl ethyl ketone. And organic solvents such as hydrocarbons such as 1,1,1-trichloroethane and dichloromethane. In order to extract the inorganic fine powder, any material can be used as long as the inorganic fine powder can be eluted without deteriorating the polyolefin resin or the fluorine resin. In the case of fine silica, caustic soda is desirable. The stretching may be uniaxial stretching or, if necessary, biaxial stretching, and the stretching method is not limited. In biaxial stretching, either simultaneous biaxial stretching or sequential biaxial stretching may be used. In the present invention, the high molecular organic substances used include polyethylene oxide, propylene oxide,
Polyvinylidene fluoride (PVdF); and the high molecular weight organic material contains an alkali metal salt such as lithium or sodium. Among them, polyethylene oxide and polyvinylidene fluoride are preferable as handling organic materials from the viewpoint of handling, and polyethylene oxide is particularly preferable. The molecular weight of the high molecular weight organic substance is not particularly limited, but the viscosity average molecular weight is preferably 4,000,000 or less from the viewpoint of coating. If the coating amount is too large, the air permeability increases, so the coating amount is set within the range of the air permeability of the present invention. Alkali metal salt concentration, when applying the microporous membrane coating of the present invention,
It is determined as appropriate. The microporous membrane-coated body of the present invention refers to a microporous membrane coated with the above-mentioned high-molecular organic material containing an alkali metal salt, at least on both surfaces of the microporous membrane, but without closing the micropores. In order to obtain such a microporous membrane-coated body, for example, polyethylene oxide was dissolved in an organic solvent such as methylene chloride, and propylene carbonate and a lithium salt were further added to the solution and the microporous membrane was immersed in the solution. Thereafter, a method of vacuum drying and the like can be mentioned. The method of coating may be any known method as long as the air permeability, porosity and pore size characteristic of the present invention are not impaired, but the dipping method is particularly preferred. The thus-obtained microporous membrane-coated body is coated with a high-molecular-weight organic material containing an alkali metal salt. However, there is no problem even if the above-mentioned solvent such as propylene carbonate is contained. The air permeability of the microporous membrane coating of the present invention is 1500 sec / 100 c
c or less, preferably 1000 sec / 100 cc, more preferably 500 sec / 100 cc or less, and most preferably 300 sec / 100 cc or less. If the air permeability exceeds 1500 sec / 100 cc, the electrical properties are poor. The air permeability here is a value obtained in accordance with JIS-P 8117. The larger the value, the lower the gas permeability. The maximum pore size is 0.01 to 2.0 μm, preferably 0.05 to 1.0 μm, and more preferably 0.05 to 0.5 μm. Maximum pore size is 0.01
mu m Not Mitsurude has poor impregnation of the electrolytic solution, the problem of internal short circuit is concerned exceeds 2.0 .mu.m. Since the organic electrolyte needs to be impregnated into the microporous membrane coating from the viewpoint of electrical properties, the porosity is 10% to 80%, preferably 20% to
80%, more preferably 30% to 80%. If the porosity is less than 10%, the electrical properties are poor, and if it exceeds 80%, the mechanical strength of the microporous membrane coating becomes low. When the microporous membrane-coated body of the present invention is used as a separator, the film thickness is not limited, but from the viewpoint of improving the capacity of the battery, 10 to 50 μm, preferably 15 to 40 μm, and more preferably 20 to 35 μm. It is. When the film thickness is 10 μm or less, short-circuit failure of the battery may increase, and when the film thickness is 50 μm or more, the battery capacity may be insufficient. When used as a separator, the modulus of elasticity in the longitudinal direction (generally, the machine direction) of the separator is 2,000 kg / cm ² or more, preferably 3,000 kg / cm ² or more, and more preferably, from the viewpoint of battery assemblability. 35
Desirably, it is not less than 00 kg / cm ² . In addition, a desired function of the separator for an organic electrolyte battery is a shutdown function. This function is used when the internal temperature of the battery rises due to external short circuit, etc., and when the temperature reaches an appropriate temperature, the separator melts and the pores of the separator are closed, so that conduction between the positive electrode and the negative electrode is almost eliminated, It refers to a function that can prevent a further rise in temperature. In this application, the temperature when the impedance reaches 10 ⁴ Ω is referred to as the shutdown temperature,
^The temperature range that holds ⁴ Ω or more is defined as the shutdown temperature range. The wider the shutdown temperature range, the safer the separator. The shutdown temperature and the shutdown temperature range of the microporous membrane coating of the present invention preferably satisfy the following ranges. The shutdown temperature is 150 ° C. or lower, preferably 145 ° C. or lower, more preferably 140 ° C.
It is below ° C. The shutdown temperature range is 20
° C or higher, preferably 30 ° C or higher, more preferably 40 ° C or higher.
° C or higher. EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not particularly limited to the examples. The measuring method and the evaluation method in this example are as follows. (1) A dial gauge having a minimum thickness scale of 1 μm was used. (2) Porosity The porosity was calculated from the following equation. Porosity = pore volume ÷ membrane total volume × 100 pore volume = membrane total volume−membrane weight ÷ resin density (3) Modulus of elasticity According to ASTM-D-882, an Instron type tensile tester is used. It was measured. (4) The maximum pore size was calculated from the bubble point in ethanol according to ASTM-E-128-61. (5) The intrinsic viscosity {η} at a measurement temperature of 135 ° C. was measured using decalin as a solvent having a viscosity average molecular weight of the polyethylene resin, and calculated by the following equation. {Η} = 6.2 × 10 ^-4 Mv ^0.7 (6) The intrinsic viscosity {η} at a measurement temperature of 35 ° C. is measured using water as a solvent having a viscosity average molecular weight of polyethylene oxide, and is calculated from the following equation. did. {Η} = 6.4 × 10 ⁻⁵ Mv ^0.82 (7) Electric resistance Measured with an apparatus shown in FIG. 1 at 25 ° C. and 1 kHz with an AG-4311 type LCR meter manufactured by Ando Electric Co., Ltd. In this measurement, 1 mol / 1 of lithium perchlorate was dissolved in an equal volume solution of propylene carbonate and dimethoxyethane as an electrolytic solution. The electrode was a platinum black electrode with an electrode plate area of 0.785 cm ² . (8) Shutdown Temperature and Shutdown Temperature Region In the measuring device shown in FIG. 2, (A), (B),
(C) is put in an oven, and 25 ° C. to 195 ° C. 2
The impedance is continuously measured while heating at a heating rate of ° C./min. The shutdown temperature was a temperature at which the impedance reached 10 ⁴ Ω, and the shutdown temperature region was a temperature range in which the impedance was maintained at 10 ⁴ Ω or more. In this measurement, 1 m of lithium tetrafluoroborate was added to propylene carbonate as the electrolytic solution.
ol / dm ³ was used. Further, the impedance was measured at an electrode area of 1.5 cm ² and 1 kHz using a nickel foil electrode. The measurement was performed using AG-431 manufactured by Ando Electric Co., Ltd.
A type 1 LCR meter was used. (9) Air permeability Measured according to JIS-P-8117. Example 1 20% by weight of high-density polyethylene having a viscosity-average molecular weight of 480,000, 30% by weight of high-density polyethylene having a viscosity-average molecular weight of 210,000, 38% by weight of dioctyl phthalate, and 12% by weight of finely divided silica are mixed with a Henschel mixer. The mixture was mixed, and the mixture was supplied to a film forming apparatus equipped with a 650 mm wide T die in a 35 mm twin screw extruder to obtain a molded product. The molded article is immersed in 1,1,1-trichloroethane, dried, then immersed in caustic soda, washed with water and dried to extract and remove dioctyl phthalate and finely divided silica, thereby obtaining an 80 μm thick raw membrane. Got. Furthermore, two sheets of this raw film were laminated and stretched 4.8 times in the machine direction at a temperature of 125 ° C. and then 2.0 times in the width direction with a tenter at a temperature of 125 ° C. to obtain a microporous film. . To 1 g of polyethylene oxide having a viscosity average molecular weight of 300,000 dissolved in 150 cc of methylene chloride, 100 cc of propylene carbonate was added, and 0.5 g of lithium tetrafluoroborate was further dissolved. After the microporous membrane was immersed in this solution for 3 minutes, it was dried in a vacuum at 60 ° C. for 5 hours. Table 1 shows the characteristics of the microporous membrane coated body obtained as described above. Comparative Example 1 Table 1 shows the characteristics of the microporous membrane before coating in Example 1.
It was shown to. COMPARATIVE EXAMPLE 2 Using the raw film of Example 1, two sheets were laminated and rolled in a uniaxial stretching machine at a temperature of 125 ° C., 4.8 times in the machine direction, and then 1.8 in the width direction with a tenter. The film was stretched twice to obtain a microporous film. Table 1 shows the characteristics of the microporous membrane. [Table 1] The microporous membrane-coated body of the present invention has a high mechanical strength and a well-balanced property in which excellent properties such as electrical properties are maintained, so that it is very effective as a separator. It is a typical thing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an assembly for measuring electric resistance according to the present invention. FIG. 2 is a schematic diagram of an impedance measuring apparatus for measuring a shutdown temperature and a shutdown temperature region according to the present invention. [Description of Signs] 1 Electrode 2 Teflon packing 3 Microporous membrane or microporous membrane coating 4 Teflon packing 5 with outer diameter 2 cm, inner diameter 1 cm, thickness 1 mm Electrodes 6A, 6B Nickel foil electrodes 7A, 7B 10 μm thick Glass Plate 8 Microporous membrane or microporous membrane coating impregnated with electrolyte 9 Case 10 Thermocouple 11 Teflon tape for masking 12 Recording device 13 Impedance measurement device

Claims (1)

(57) [Claim 1] A high molecular organic material containing an alkali metal salt is coated on a microporous membrane by an immersion method, and the air permeability of the coated microporous membrane is 1500 sec / 100 cc or less. Having a maximum pore size of 0.01 μm to 2.0 μm and a porosity of 10% to 8
0%, characterized in that it is coated with a microporous membrane.