JPH04212264A - Polyethylene poromeric film for cell separator - Google Patents

Abstract

PURPOSE:To provide a safe and reliable cell separator composed of a single poromeric film by defining a viscosity average molecular weight, and a crystallization degree of a polyethylene, and concurrently defining an average pore diameter, a maximum pore diameter and a range of porosity of a poromeric film. CONSTITUTION:A polyethylene which is 160,000 to 2,000,000 and 60 to 90% in viscosity average molecular weight and in crystallization degree, respectively, is used. And a poromeric film is 50mu or less in thickness, 0.01 to 1.0mu in average pore diameter, 1mu or less in maximum diameter and 50 to 80% in porosity. In this constitution, the film is prevented from failing, making a dent, being damaged and so on, so that a non-porous condition is thereby kept on, even if temperature within a cell is increased almost up to a melting point due to external short circuiting and the like so as to be kept on as it is, or so as to be gradually lowered, the chemical reaction within the cell is surely restrained, and the cell high in safety and reliability is thereby provided.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microporous polyethylene membrane for battery separators, and more particularly to a microporous polyethylene membrane for non-aqueous electrolyte battery separators.

0002

BACKGROUND OF THE INVENTION In recent years, with the spread of small electronic devices, there have been extremely high expectations for the batteries that serve as their power source to be smaller, lighter, higher output, and safer. In particular, safety requirements are increasing as accidents such as battery fires due to external short circuits and the like are occurring.

Various types of battery separators are known, such as those disclosed in Japanese Patent Application Laid-Open No. 60-23954.
Films made of polypropylene or polyethylene with micropores as seen in Japanese Patent Application Laid-open No. 1-258358 have been proposed. , a separator has been proposed in which a microporous membrane made of a low melting point resin with a melting point of 120° C. or less is laminated together.

0004

[Problems to be Solved by the Invention] However, JP-A No. 60-23954 does not mention maintaining safety at temperatures above the melting point, and there are doubts about its use as a highly reliable battery separator. there were. In addition, as shown in JP-A-1-258358, in a separator made of two layers of a microporous membrane and a microporous membrane or nonwoven fabric, the thickness of the separator becomes thicker, and the same volume of the separator increases. Since the amount of active material in the battery is reduced, there is a problem in that the demand for a compact battery with high energy density cannot be met.

[0005] As a result of conducting research with the aim of developing a safer and more reliable battery separator using a single microporous membrane, the present inventor succeeded in developing a battery separator composed of a microporous membrane based on a new concept. and completed the present invention.

[0006]

[Means for Solving the Problems] In order to achieve the above object, the present invention has the following configuration. That is, the present invention has a viscosity average molecular weight of 160,000 to 2,000,000 and a crystallinity of 60.
% to 90% polyethylene, film thickness 50μ or less,
This is a polyethylene microporous membrane for battery separators, characterized by having an average pore diameter of 0.01 μm to 1.0 μm, a maximum pore diameter of 1 μm or less, and a porosity of 50% to 80%.

[0007] The thickness of the microporous membrane in the present invention is 50 μm.
It is as follows. When the film thickness is thicker than 50 μm, the separator occupies at least 10% of the internal volume of the battery, which hinders miniaturization and higher energy storage of the battery. Further, since the film needs to have a thickness that does not cause internal short circuits, a separator of 20 to 40 μm is preferable for small and lightweight batteries and high-output spiral-wound batteries.

[0008] The pores of the microporous membrane are, of course, in a communicating state, and it is essential that the average pore diameter is 0.01μ to 1.0μ, preferably 0.01μ to 0.5μ, and has good air permeability. When considering the permeability of the electrolytic solution, 0.1 to 0.5μ is more preferable. If the average pore size is less than 0.01μ, the permeability of the electrolytic solution is significantly reduced, and if it is larger than 1.0μ, short circuits are likely to occur.

[0009] Furthermore, the maximum pore diameter must be so small that it can become clogged during heating and melting, and must not cause an internal short circuit, and must be 1 μm or less.
Preferably it is 0.7μ or less. If the maximum pore diameter is larger than 1 μm, internal short circuits are likely to occur. The porosity as referred to in the present invention is expressed by the following formula.

Porosity = (pore volume / microporous membrane volume) × 100 pore volume = microporous membrane volume - (dry weight / resin density) The porosity of the microporous membrane of the present invention must be 50 to 80%. However, it is preferably 55 to 80%. If the porosity is less than 50%, a membrane with excellent permeability such as ion permeability cannot be obtained. Furthermore, if the porosity is 80% or more, the rate of occurrence of troubles such as internal short circuits increases.

The most important component of the present invention is polyethylene with a viscosity average molecular weight of 160,000 to 2,000,000 and a crystallinity of 60% to 90%, preferably a viscosity average molecular weight of 200,000 to 180,000.
Polyethylene with a crystallinity of 60% to 85% is used, and this configuration greatly contributes to increasing the reliability of the battery separator. In other words, in a battery separator, when the internal temperature of the battery rises due to an external short circuit and reaches a temperature near the melting point of the microporous membrane material, the microporous membrane becomes non-porous and the chemical reaction inside the battery is suppressed. , the temperature inside the battery does not drop suddenly, but tends to drop gradually. This indicates that the microporous membrane is kept at a temperature close to its melting point for a long time and exposed to a pinched state. At this time, if the material of the microporous membrane is easily deformed, defects, holes, tears, etc. will occur in the non-porous membrane, and the chemical reaction will start again, raising the internal temperature of the battery and causing the electrolyte to gasify. , there is a possibility of ignition or explosion, resulting in poor reliability as a battery separator. However, as disclosed in the present invention, the viscosity average molecular weight is 160,000 to 2,000,000,
By using polyethylene with a crystallinity of 60% to 90%, it becomes difficult to deform at temperatures near the melting point. However, when the molecular weight becomes too high, that is, when the viscosity average molecular weight exceeds 2 million, the fluidity becomes poor and it becomes difficult to form a thin film, making it impractical. Moreover, when the degree of crystallinity is 90% or more, it becomes easy to break.

Polyethylene as referred to in the present invention is a crystalline homopolymer obtained by polymerizing ethylene, or ethylene and 10 mol% or less of propylene, 1-butene, 4-butene, etc.
Examples include copolymers with methyl-1-pentene and 1-hexene. Furthermore, there is no problem in mixing polyethylenes having different molecular weights to obtain a viscosity average molecular weight of 160,000 to 2,000,000.

Further, in the present invention, a three-dimensional network structure (or a sponge structure) in which holes are three-dimensionally intricate is more preferable as a structure, although there is no restriction in terms of structure. The microporous membrane of the present invention can be obtained by mixing polyethylene with a solvent, a plasticizer, an inorganic fine powder, etc., molding, extracting and drying, and further stretching.

For example, polyethylene resin, inorganic fine powder,
The mixed composition of the organic liquid is 5 to 70% by volume and 10% by volume, respectively.
~55% by volume and 20 to 75% by volume, mixed in a normal mixer such as a Henschel mixer, then kneaded in a melt kneading device such as an extruder, and the resulting kneaded product is extruded to a size of 50 to 500 μm. Form to a thickness of . Furthermore, extracting the organic liquid from the molded product using an organic liquid solvent,
Subsequently, the inorganic fine powder is extracted using an extraction solvent for inorganic fine powder to obtain a porous membrane. Then, if necessary, up to the specified thickness,
The film thickness is adjusted by stretching with a uniaxial or biaxial stretching machine.

[0015]

[Examples] The present invention will be explained in more detail with reference to examples below, but the present invention is not particularly limited to the following examples. Note that the measurement method and evaluation method in Examples are as follows. (1) Film Measured using a thickness dial gauge (minimum scale: 1μ). (2) Average pore diameter Evaluated by half dry method based on ASTM F-316-70. (3) Maximum pore diameter Calculated from the bubble point in ethanol in accordance with ASTM E-128-61. (4) Porosity Porosity = (pore volume / microporous membrane volume) × 100 pore volume = microporous membrane volume - (dry weight / resin density) (5) Viscosity average molecular weight (Mv) Solvent (Delican) The measurement temperature was 135° C., and the evaluation was performed using the following formula.

[η]=6.2×10−4Mv0.7 (Chiang
(Formula) (6) Crystallinity Differential Scanning Calorimeter DSC2 manufactured by Seiko Electronics Industry Co., Ltd.
Approximately 1 mg of the sample was melted under a nitrogen stream using Model 10, and then measured from room temperature at a heating rate of 10° C./min, and evaluated based on the endothermic peak accompanying melting. (7) Melting point Differential scanning calorimeter DSC2 manufactured by Seiko Electronics Industry Co., Ltd.
Approximately 1 mg of the sample was heated using a Type 10 under a nitrogen stream at a heating rate of 1.
The endothermic peak temperature measured from room temperature at 0°C/min was defined as the melting point. (8) High temperature stability Seiko Electronics Industry Co., Ltd., heat/stress/strain measuring device T
Using MA/SS100, heating rate 2℃/m under constant length
Evaluation was made based on the state of the sample when the temperature was raised from room temperature. (9) Structural Observation A sample cooled with liquid nitrogen was fractured, and the fractured surface was observed using a SEM.

[0017]

[Example 1] 22% by weight of finely divided silicic acid and dioctyl phthalate
44% by weight of a polyethylene resin having a viscosity average molecular weight of 300,000 and a crystallinity of 65% was added thereto and mixed again in a Henschel mixer.

[0018] The mixture was transferred to a 30 m/mφ twin screw extruder for 4
It was molded into a film with a thickness of 95 μm using a film manufacturing apparatus equipped with a T-die having a width of 50 m/m. The formed membrane is 1,1
, immersed in 1-trichloroethane for 10 minutes to extract dioctyl phthalate, dried, and further heated at 25°C at 60°C.
% caustic soda for 60 minutes to extract fine powder silicic acid, and then dried.

Further, the microporous membrane was stretched using a roll stretching machine heated to 114°C, and then stretched for 50 minutes in an atmosphere of 118°C.
Heat treatment was performed for seconds. The obtained film has a three-dimensional network structure, and the properties of the film are as shown in the table. When the high temperature stability of the obtained film was tested, it did not break even at temperatures higher than the melting point of 135°C.

[0020]

[Example 2] A film was produced in the same manner as in Example 1, except that 22% by weight of polyethylene resin with a viscosity average molecular weight of 600,000 and a crystallinity of 60%, 23% by weight of fine silicic acid, and 55% by weight of dioctyl phthalate were used. I got it. The obtained film has a three-dimensional network structure, and the properties of the film are as shown in the table.

[0021] When the high temperature stability of the obtained film was tested, it was cut at 150°C, which is 13°C higher than the melting point of 137°C.

[0022]

Example 3 A membrane was obtained in the same manner as in Example 1, except that a polyethylene resin having a viscosity average molecular weight of 200,000 and a crystallinity of 65% was used. The properties of the obtained membrane are shown in the table. When the high temperature stability of the resulting film was tested, it had a melting point of 134°C.
It did not cut even at higher temperatures.

[0023]

[Example 4] Viscosity average molecular weight 1.5 million, crystallinity 70%
After mixing in a Henschel mixer using 25% by weight of polyethylene resin and 75% by weight of liquid paraffin instead of dioctyl phthalate, a rolling mill was attached to the film manufacturing machine of Example 1 to form a film with a thickness of 500μ. Molded.

The formed membrane was stretched using a biaxial stretching machine at a temperature of 140°C, and further immersed in 1,1,1-trichloroethane for 10 minutes to extract liquid paraffin, and then dried to obtain a membrane. Ta. The obtained film has a three-dimensional network structure, and the properties of the film are shown in the table. When the high temperature stability of the obtained film was tested, it was cut at 150°C, which is 10°C higher than the melting point of 140°C.

[0025]

[Comparative Example 1] A microporous polyethylene membrane (manufactured by Celguard, K-878) having a viscosity average molecular weight of 100,000 was used for evaluation. The properties of the membrane are shown in the table. When the high temperature stability of the film was tested, it was only 4°C below the melting point of 135°C, 13
Cut at 9°C.

[0026]

[Comparative Example 2] Viscosity average molecular weight 1.5 million, crystallinity 55%
A membrane was obtained in the same manner as in Example 4, except that the polyethylene resin was used. The properties of the obtained membrane are shown in the table. When the high temperature stability of the obtained film was tested, it had a melting point of 138
It was cut at 142°C, which was only 4°C above 30°C.

[0027]

[Table 1]

[0028]

According to the present invention, the film is made of polyethylene with a viscosity average molecular weight of 160,000 to 2,000,000, a crystallinity of 60% to 90%, a film thickness of 50 μm or less, and an average pore diameter of 0.01 μ to 1.0 μm.
By using a polyethylene microporous membrane with a maximum pore diameter of 1μ or less and a porosity of 50% to 80% as a battery separator, the internal temperature of the battery will rise due to external short circuits and reach a temperature near the melting point. Even if the temperature is maintained at that temperature or the temperature gradually decreases, the membrane does not develop defects, holes, tears, etc., in other words, it remains non-porous, so the battery Internal chemical reactions are reliably suppressed, resulting in a highly safe and reliable battery.

Claims (1)

[Claims] Claim 1: Made of polyethylene with a viscosity average molecular weight of 160,000 to 2,000,000 and a crystallinity of 60% to 90%, with a film thickness of 5
0 μ or less, average pore size 0.01 μ to 1.0 μ, maximum pore size 1
A microporous polyethylene membrane for battery separators, characterized in that it has a porosity of 50% to 80%.