JPH04212265A - Polyethylene poromeric film for cell separator - Google Patents

Abstract

PURPOSE:To obtain a cell high in safety and reliability by keeping a non-porous condition, and concurrently and surely restraining chemical reaction within the cell even if temperature within the cell is increased due to external short circuiting and the like. CONSTITUTION:In a polymeric film made of the polyethylene having a viscosity average molecular weight of 160,000 to 2,000,000, a film of 50mu or less in thickness, and pores of 0.01 to 1.0mu in average diameter, and of 1mu or less in maximum diameter and of 50 to 80% in porosity, temperature is increased at a temperature increasing rate of 2 deg.C/min. from room temperature in a state where the length is kept constant with both the ends of the film fixed. A ratio of an evaluated maximum contraction stress (kg/cm<2>) to viscosity average molecular weight in the poromeric film is determined to be 7X0.00001 or less.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyethylene microporous membrane for battery separators, and more particularly to a polyethylene microporous 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 sources to be smaller, lighter, have higher output, and be safer. In particular, as accidents such as battery fires due to short circuits and the like are occurring, demands for safety are increasing.

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 have been proposed as seen in JP-A No. 1-258358, and for the purpose of further increasing safety, films made of polypropylene or polyethylene with micropores as seen in JP-A-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 is made of polyethylene having a viscosity average molecular weight of 160,000 to 2,000,000, a thickness of 50 μm or less, and an average pore diameter of 0.01 μ to 1.5 μm.
0μ, maximum pore diameter of 1μ or less, and porosity of 50% to 80%, the maximum shrinkage stress ( This is a polyethylene microporous membrane for a battery separator, characterized in that the ratio of the viscosity average molecular weight of the microporous membrane to the microporous membrane (kg/cm2) is 7×0.00001 or less.

[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. Furthermore, since the film needs to have a thickness that does not cause internal short circuits, a separator of 20 to 40 microns is preferable for small, lightweight batteries and high-output spiral-wound batteries.

[0008] The pores of the microporous membrane are naturally in a communicating state, and the average pore size must be 0.01μ to 1.0μ, preferably 0.01μ to 0.5μ, with good air permeability and 0.1μ to 0.5μ when considering the permeability of the electrolyte
is even 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. When 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 60 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 greater than 80%, the rate of occurrence of troubles such as internal short circuits increases.

The most important feature of the present invention is the maximum shrinkage stress (kg/cm2) evaluated by raising the temperature from room temperature at a heating rate of 2°C/min while keeping both ends fixed at a constant length and the microporous pores. The ratio of viscosity average molecular weight in the membrane is 7 x 0.000
01 or less, and this configuration greatly contributes to increasing the reliability of the battery separator. That is,
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 nonporous and suppresses chemical reactions inside the battery. The internal temperature does not drop suddenly, but tends to drop gradually. This indicates that the microporous membrane is kept at a temperature near its melting point for a long time and exposed to a pinched state. At this time, if the contraction force acting on the microporous membrane is not released sufficiently, defects may occur in the microporous membrane that has become non-porous.
Holes, tears, etc. occur, a regeneration chemical reaction begins, the internal temperature of the battery rises, the electrolyte gasifies, and there is a possibility of ignition or explosion, resulting in poor reliability as a battery separator. However, as disclosed in the present invention, the maximum shrinkage stress (kg/cm2) was evaluated by raising the temperature from room temperature at a heating rate of 2°C/min while keeping both ends fixed at a constant length.
and the viscosity average molecular weight of the microporous membrane is 7×0.0
By making it less than 0001, the contraction force acting on the microporous membrane at a temperature near the melting point is sufficiently released, and the nonporous membrane is less likely to have defects, holes, tears, etc., and has excellent reliability. Become something. Maximum shrinkage stress (kg/c
m2) and the viscosity average molecular weight of the microporous membrane is larger than 7 x 0.00001, the non-porous membrane tends to tear, resulting in poor high-temperature safety.

The viscosity average molecular weight must be 160,000 to 2,000,000, preferably 200,000 to 1,800,000. If the viscosity average molecular weight is 2,000,000 or more, the fluidity will be poor and it will be difficult to form a thin film, making it impractical, and if it is less than 160,000, the strength will be weak. In the present invention, polyethylene includes a crystalline homopolymer obtained by polymerizing ethylene or a copolymer of ethylene with 10 mol% or less of propylene, 1-butene, 4-methyl-1-pentene, or 1-hexene. can give.

[0013] Furthermore, there is no problem in mixing polyethylenes having different molecular weights to have a viscosity average molecular weight of 160,000 to 2,000,000. Further, in the present invention, a three-dimensional network structure (or sponge structure) in which holes are three-dimensionally intricate is more preferable as a structure, although the structure is not specified in any way.

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, and organic liquid have a mixed composition of 5 to 70% by volume, 10 to 55% by volume, and 20 to 75% by volume, respectively, and are mixed in a normal mixer such as a Henschel mixer, and then extruded. The kneaded product is kneaded using a melt kneading device such as a machine, and the resulting kneaded product is extruded to a size of 50 μm.
Mold to ~500μ thickness. Furthermore, the organic liquid is extracted from the molded product using an organic liquid solvent, and then the inorganic fine powder is extracted using an inorganic fine powder extraction solvent to obtain a porous membrane. Then, if necessary, the film is stretched to a predetermined thickness using a uniaxial or biaxial stretching machine to adjust the film thickness.

[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 Examples. Note that the measurement method and evaluation method in Examples are as follows. (1) Measurement using a film thickness dial gauge (minimum scale: 1μ). (2) Average pore size Evaluated by a 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 (decalin) The measurement temperature was 135° C., and the evaluation was performed using the following formula.

[η]=6.2×10−4 Mv0.7 (Chia
ng formula) (6) Maximum shrinkage stress Heat/stress/strain measuring device manufactured by Seiko Electronics Co., Ltd.
Using TMA/SS100, heating rate 2℃ under constant length
The temperature of the sample was raised from room temperature at a rate of 1/min.

[0017] The sample has a width of 3 mm, and the distance between the chucks is 10
Measured in mm. In addition, in calculating the stress, the cross-sectional area was calculated using the cross-sectional area at room temperature. (7) High temperature stability Heat/stress/strain measuring device manufactured by Seiko Electronics Industries, Ltd.
Using TMA/SS100, heating rate 2℃ under constant length
Evaluation was made based on the state of the sample when the temperature was raised from room temperature at a rate of /min. (8) Structural Observation After cooling with liquid nitrogen, it was fractured, and the fractured surface was observed with a scanning electron microscope (SEM).

[0018]

Example 1 22% by weight of finely divided silicic acid and 44% by weight of dioctyl phthalate were mixed in a Henschel mixer, 34% by weight of polyethylene resin was added, and the mixture was mixed again in the Henschel mixer. The mixture was transferred to a 30 m/mφ twin screw extruder for 45 minutes.
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 0 m/m.

The formed membrane was immersed in 1,1,1-trichloroethane for 10 minutes to extract dioctyl phthalate, and then dried, and further immersed in 25% caustic soda at 60°C for 60 minutes to extract finely divided silicic acid. was extracted and dried. Furthermore, the microporous membrane was stretched to a film thickness of 35 μm using a roll stretching machine heated to 114° C., and heat treated for 5 seconds in an atmosphere of 118° C. The obtained film had a three-dimensional network structure, and the properties of the film are shown in Table 1.

The maximum shrinkage stress of the obtained membrane was 12.6 kg.
/cm2, and the ratio of the maximum shrinkage stress (kg/cm2) to the viscosity average molecular weight of the microporous membrane is 6.3 x 0.000.
No. 01 did not cut even at temperatures higher than 160°C.

[0021]

[Example 2] 22% by weight of polyethylene resin, 2% of fine silicic acid
A membrane was obtained in the same manner as in Example 1 except that 3% by weight of dioctyl phthalate and 55% by weight of dioctyl phthalate were used. The obtained film had a three-dimensional network structure, and the properties of the film are shown in Table 1. The maximum shrinkage stress of the obtained membrane was 5.9 kg/cm2
The ratio of the maximum shrinkage stress (kg/cm2) to the viscosity average molecular weight of the microporous membrane was 1.0 x 0.00001, and the resulting membrane was tested for high temperature stability and was cut at 150°C.

[0022]

Example 3 25% by weight of polyethylene resin and 75% by weight of liquid paraffin were molded into a film having a thickness of 200 μm using the film manufacturing machine of Example 1 with a rolling mill attached. 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 form a membrane with a thickness of 30μ. Obtained. The obtained film had a three-dimensional network structure, and the properties of the film are shown in Table 1.

The maximum shrinkage stress of the obtained membrane was 10.9 kg.
/cm2, and the ratio of the maximum shrinkage stress (kg/cm2) to the viscosity average molecular weight of the microporous membrane is 0.7 x 0.000.
When the high temperature stability of the obtained film was tested in 01, 1
Cutting was performed at 50°C.

[0024]

[Comparative Example] A microporous polyethylene membrane (manufactured by Celgard, K-878) having a viscosity average molecular weight of 100,000 was used for evaluation. The properties of the membrane are shown in Table 1. Moreover, the structure of the membrane was a two-dimensional structure in which holes substantially perpendicular to the membrane surface were in communication. The maximum shrinkage stress of the membrane is 11.9 kg/cm2,
The ratio of the maximum shrinkage stress (kg/cm2) to the viscosity average molecular weight of the microporous membrane is 11.9 x 0.00001,
The resulting film was tested for high temperature stability and was cut at 139°C.

[0025]

[Table 1]

[0026]

Effects of the Invention According to the present invention, it is made of polyethylene with a viscosity average molecular weight of 160,000 to 2,000,000, has a thickness of 50 μm or less,
A microporous membrane with an average pore diameter of 0.01μ to 1.0μ, a maximum pore diameter of 1μ or less, and a porosity of 50% to 80% is heated from room temperature at a heating rate of 2°C/min while both ends are fixed and a constant length is maintained. The ratio of the maximum shrinkage stress (kg/cm2) evaluated by heating and the viscosity average molecular weight of the microporous membrane is 7 x 0.00.
By using a polyethylene microporous membrane with a molecular weight of 001 or less as a battery separator, the internal temperature of the battery will rise due to an external short circuit, etc., and reach a temperature near the melting point, and whether it will remain at that temperature or not. Even when the battery temperature gradually decreases, there are no defects, holes, or tears in the membrane; in other words, it remains non-porous, so chemical reactions inside the battery are reliably suppressed, making it highly safe. , and a highly reliable battery can be obtained.

Claims (1)

[Claims] Claim 1: Made of polyethylene with a viscosity average molecular weight of 160,000 to 2,000,000, a film thickness of 50 μm or less, and an average pore diameter of 0.0
1μ to 1.0μ, maximum pore diameter 1μ or less, porosity 50% to 8
The maximum shrinkage stress (kg/cm2) and the average viscosity of the microporous membrane were evaluated by raising the temperature from room temperature at a heating rate of 2°C/min while keeping both ends fixed and a constant length. A microporous polyethylene membrane for battery separators, characterized in that the ratio of molecular weights is 7×0.00001 or less.