JPH06163023A - Battery separator - Google Patents

Abstract

PURPOSE:To improve battery productivity and battery performance by forming a battery separator of a microporous film composed of mixture of a specific quantity of polyethylene and a specific quantity of ethylene - propylene rubber. CONSTITUTION:Polyethylene 90-10weight% composed of extra-high molecular weight polyethylene and high molecular weight polyethylene and ethylene - propylene rubber 10-90weight% composed of random copolymer of ethylene and propylene, are mixed together, and polymer is formed. After mixture is formed by adding fine powder silicic acid to this polymer as inorganic fine powder and DOP as plasticizer, the inorganic fine powder and the plasticizer are extracted and removed, and it is also dried, and is drawn, and a battery separator composed of a microporous film is formed. Thereby, a low electric resistance battery separator which has well-balanced mechanical strength and is excellent in battery assembling charactristics and has excellent electrolyte impregnation characteristics and safety, can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator such as an alkaline battery / lithium primary / secondary battery or a lithium ion primary / secondary battery.

[0002]

2. Description of the Related Art Various microporous membranes have been proposed for battery separators. In particular, as a separator for a lithium battery, a microporous membrane made of polyethylene is desired from the viewpoint of safety as disclosed in JP-A-60-23954. JP-A-2-21559 discloses a viscosity average molecular weight of 1
A polyethylene separator comprising a mixture of polyethylene having a viscosity average molecular weight of 300,000 or less and polyethylene having a viscosity of, 000,000 or more is disclosed. Japanese Unexamined Patent Publication (Kokai) No. 3-64334 discloses a microporous membrane composed of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 700,000 or more and a polyolefin composition having a weight average molecular weight / number average molecular weight of 10 to 30.
JP-A-4-126352 discloses a viscosity average molecular weight of 1
A separator made of a mixture of polyethylene having a viscosity of, 000,000 or more and polyethylene and polypropylene having a viscosity average molecular weight of 300,000 or less is disclosed.

[0003]

DISCLOSURE OF THE INVENTION The present invention has an excellent balance of mechanical strength desired as a battery separator, a good battery assembling property, an excellent electrolyte impregnation property, a low electric resistance and a safety. The purpose is to provide a well-balanced separator. JP-A-2-21
As disclosed in Japanese Patent Laid-Open No. 4-126352, Japanese Patent No. 559 does not have sufficient resistance to mechanical stress.
A separator is disclosed in which polypropylene is added to a mixture of polyethylene. Although the addition of polypropylene to a mixture of polyethylene is recognized to improve the mechanical strength, in terms of safety, at least substantially the same safety (shutdown temperature) as that of a separator made of a mixture of polyethylene can be expected.

Further, Japanese Patent Laid-Open No. 3-64334 discloses that
Although a high-strength microporous membrane is disclosed, the pore size is too small and impregnation with the electrolytic solution is poor. A balanced battery separator as described above has not been obtained by conventional methods.

[0005]

Means for Solving the Problems The present inventor has found that it is essentially safer than polyethylene, a mixture of polyethylene, or a separator composed of a mixture of polyethylene and polypropylene, and has a good balance of mechanical strength and battery assemblability. Good and excellent in electrolyte impregnation, low electrical resistance, and also excellent in processability, which is preferable for industrial production.
A battery separator was found and the present invention was completed.

That is, 90 to 10% by weight of polyethylene
And a separator of ethylene-propylene rubber of 10 to 90% by weight, which is a battery separator using a microporous membrane. As polyethylene, ultra high molecular weight polyethylene and
Alternatively, it is preferable to use high molecular weight polyethylene.
As the ultrahigh molecular weight polyethylene and the high molecular weight polyethylene, a crystalline homopolymer obtained by polymerizing ethylene is preferable, and ethylene and 10 mol% or less of propylene, 1-butene, 4-methyl-1-pentene, 1-hexene and the like are used. May be a copolymer of Further, any of high density polyethylene, medium density polyethylene and low density polyethylene may be used, and high density polyethylene is particularly preferable.

The ultra-high molecular weight means a viscosity average molecular weight of 1,000,000 or more, and the high molecular weight means a viscosity average molecular weight of less than 1,000,000 and a critical molecular weight or more. Ultra-high molecular weight polyethylene is a preferable material for maintaining the balance of mechanical strength, and has an elastic modulus in the machine direction (hereinafter referred to as "MD") and M
It is useful for balancing the ratio of the D elastic modulus and the elastic modulus in the direction perpendicular to the machine direction (hereinafter referred to as "CMD").

The elastic modulus of MD is preferably 5000 kg / cm ² or more, and the ratio of the elastic modulus of MD to that of CMD is preferably 8 or more. More preferably, the elastic modulus of MD is 6000 k.
g / cm ² or more, and the ratio of the MD elastic modulus to the CMD elastic modulus is 9 or more. MD elastic modulus of 5000 kg /
When it is less than cm ² , the battery assembling property and the productivity are poor and the practicability is low. When the ratio between the MD elastic modulus and the CMD elastic modulus is less than 8, the short circuit failure rate during battery assembly increases.

When the viscosity average molecular weight of ultra high molecular weight polyethylene is less than 1,000,000, the MD elastic modulus is 5000 kg / c.
It is difficult to satisfy the condition of m ² and the ratio of the elastic modulus of MD to the elastic modulus of CMD of 8 or more. High molecular weight polyethylene is a preferable material from the viewpoint of molding processing, and if the viscosity average molecular weight exceeds 1,000,000, molding processing becomes difficult. If the molecular weight is below the limit, molding cannot be performed.

In the present invention, it is preferable to use both the ultrahigh molecular weight polyethylene and the high molecular weight polyethylene. It was also found that by using ethylene-propylene rubber, a balanced separator with improved safety and processability and without deterioration of mechanical properties can be obtained. Ethylene-propylene rubber refers to a random copolymer of ethylene and propylene, which is substantially amorphous (amorphous). For example, a solvent-soluble Ziegler catalyst and an organoaluminum compound are used to produce ethylene-propylene rubber. It can be obtained by a method of copolymerizing propylene or the like. A polyethylene-polypropylene random copolymer having mere crystallinity cannot be expected to improve safety and processability. Similarly, polypropylene cannot be expected to have improved safety. The propylene content of the ethylene-propylene rubber is 5 to 80% by weight, preferably 10 to 60% by weight, more preferably 15 to 30% by weight. Also,
The viscosity (Moonie viscosity) measured at a preheating time of 1 minute, a measurement time of 4 minutes and a temperature of 100 ° C. is 1 to 100 (ML _{1 + 4} 100).
C.), preferably 5 to 80 (ML _{1 + 4} 100 ° C.), more preferably 20 to 80 (ML _{1 + 4} 100 ° C.). If the weight average molecular weight exceeds 1,000,000, the moldability is deteriorated. Therefore, the weight average molecular weight is preferably 1,000,000 or less, and more preferably 8,000,000.
It is at most 0,000, more preferably at most 500,000.

In the above ethylene-propylene rubber,
If the amount is very small, a conjugated polyene monomer such as 5-
It may contain ethylidene-2-norbornene, dicyclopentadiene, 5-methyl-2.5-norbonadiene and the like. Further, in the mixture of polyethylene and ethylene-propylene rubber used in the present invention, a polymer such as propylene, 1-butene, 4-methyl-1-pentene or 1-hexene may be used if it is less than 20% by weight based on the total weight of the polymer. In the following, preferable characteristics of the microporous membrane of the present invention will be described.

The porosity is preferably 40% or more from the viewpoint of the impregnation property / impregnation amount of the electrolytic solution and the electric resistance inside the battery,
Further, it is preferably 45% or more, and when it is 85% or more, the mechanical strength is lowered, which causes a problem in battery assembly. Air permeability is 450 seconds / 100 cc or less, preferably 300 seconds /
100 cc or less, more preferably 250 seconds / 100 c
c or less. When the air permeability is 450 seconds / 100 cc or more, the electric resistance inside the battery becomes too high, which deteriorates the battery characteristics.

The dynamic friction coefficient of CMD is 0.6 or less, preferably 0.5 or less. For spiral type batteries, CMD
When the coefficient of kinetic friction exceeds 0.6, the electrode and the separator do not come off from the pin after the electrode and the separator are spirally wound on the pin in the battery assembling process, or the pin is difficult to come off, resulting in poor productivity. Bubble point in ethanol is 1 kg / cm ^{2 to} 10 kg / cm ² , preferably 2
^{kg / cm 2 ~9kg / cm 2} , more preferably 2k
g / cm ^{2 to} 7 kg / cm ² . When the bubble point in ethanol exceeds 10 kg / cm ² , the impregnation property of the electrolytic solution is poor and the productivity of the battery is poor. If the bubble point is less than 1 kg / cm ² , there is a concern about an increase in the short circuit failure rate during battery assembly.

The film thickness is 15 to 50 μm, preferably 20.
Is about 40 μm. If it is less than 15 μm, the short-circuit failure rate during battery assembly increases, and if it exceeds 50 μm, the battery performance deteriorates. In addition, the shutdown temperature is 1 from the viewpoint of safety.
It is 45 ° C or lower, preferably 140 ° C or lower.

The impregnating property of the electrolytic solution can be measured by the impregnating height when the separator is immersed in the electrolytic solution. When the impregnating height is 2 mm or more, and further 3 mm or more, the electrolytic solution is not used in the battery assembling process. It is preferable because the impregnation time is shortened and the battery productivity is excellent. The battery separator of the present invention is
For example, a polymer, an inorganic fine powder and a plasticizer are kneaded and melted by heating to form a sheet, and then the inorganic fine powder and the plasticizer are extracted and removed, respectively, and dried, and stretched only in a uniaxial direction or in a biaxial direction. Obtained.

Specifically, polyethylene, ethylene-propylene rubber, plasticizer and inorganic fine powder are mixed and molded,
It is manufactured by extracting and removing the plasticizer and the inorganic fine powder, drying, and further stretching. Examples of the inorganic fine powder include fine silicic acid, calcium silicate, calcium carbonate, fine powder talc, and the like, and fine silicic acid is particularly preferable.

The extraction solvent for the inorganic fine powder may be any solvent which dissolves the inorganic fine powder and does not dissolve the polymer. In the case of finely divided silicic acid, caustic soda is preferable. Examples of the plasticizer include DBP, DOP, DNP, DBS, TBP, liquid paraffin, etc., and DOP is particularly preferable. Solvents for the plasticizer include alcohols such as methanol and ethanol, ketones such as acetone and MEK, 1.1.1.
-General organic solvents such as chlorine-based hydrocarbons such as trichloroethane are used.

When ultra high molecular weight polyethylene and / or high molecular weight polyethylene is used as polyethylene,
It is desirable that the content of the ultra high molecular weight polyethylene having a viscosity average molecular weight of 1,000,000 or more is 40% by weight or more, preferably 45% by weight or more, and further 50% by weight or more based on the total weight of the polymer. When it is 40% by weight or less, it is difficult to obtain a separator having a well-balanced mechanical strength such as a high elastic modulus.

The content of high molecular weight polyethylene having a viscosity average molecular weight of less than 1,000,000 is preferably 5 based on the total weight of the polymer.
It is 0% by weight or less, more preferably 40% by weight or less. If it exceeds 50% by weight, the moldability is lowered and the safety is lowered. The content of ethylene-propylene rubber is 10 to 90% by weight of the total polymer weight, and 10 to 50% by weight is preferable from the viewpoint of improving the elastic modulus of the separator.

The amount of the plasticizer is 50 to 100% by weight, preferably 50, of the weight of the polymer / plasticizer / inorganic fine powder mixture.
~ 80% by weight. When the content is 50% by weight or less, an appropriate pore size (the bubble point in ethanol is 1 kg / cm ² to
10 kg / cm ² ) cannot be obtained. If it exceeds 100% by weight, molding processing becomes difficult. The amount of the inorganic fine powder is 5 to 35% by weight, preferably 15 to 25% by weight based on the weight of the mixture. Appropriate pore size below 5 wt% and above 35 wt% (bubble point in ethanol is 1 kg / cm
^{2 to} 10 kg / cm ² ) cannot be obtained.

As an example of the production method, a polymer, an inorganic fine powder, and a plasticizer are mixed in a predetermined mixing ratio by an ordinary mixer such as a Henschel mixer, and then a T-die or the like is used by a melt-kneading device such as an extruder. Then, it is molded into a sheet having a thickness of 80 μm to 200 μm, further, the plasticizer is extracted and removed from the molded product by using a solvent, and subsequently the inorganic fine powder is extracted with an extraction solvent for the inorganic fine powder, A method of obtaining a separator having a predetermined thickness by heating and stretching only in the uniaxial direction can be mentioned.

It can be used also as a separator for an alkaline battery by a known method before or after stretching, for example, by applying and impregnating a surfactant or hydrophilizing treatment such as corona treatment.

[0023]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to the following examples. The measuring method is shown below. (1) Film thickness Measured with a dial gauge having a minimum scale of 1 μm. (2) Porosity A sample with a size of 10 cm × 10 cm was cut out, and the weight of the sample when it was wet, the weight when it was absolutely dried, and the film thickness were measured and determined from the following formula.

Porosity = (pore volume / microporous membrane volume) × 1
00 (%) void volume = (wet weight (g) - absolute dry weight (g)) / water density (g / cm ³⁾ microporous membrane volume = 100 × thickness (cm) (3) Bubble Point ASTM The bubble point in ethanol was measured according to E-128-61. (4) Air Permeability According to JIS P-8117, a B-type Gurley type densometer manufactured by Toyo Seiki was used to measure the time required for the marking line scale 0 to 100 with a stopwatch. (5) Modulus of elasticity Using a Shimadzu model Autograph AG-A, a tensile test was conducted at a test piece size of 10 mm width x 100 mm length, a chuck distance of 50 mm, and a pulling speed of 200 mm / min. The rate was measured. The test piece is M
The size was cut out for each of D and CMD.

The cross-sectional area was calculated by (film thickness × film width measured in item (1)). (6) Ratio of MD Modulus to CMD Modulus Ratio of MD Modulus to CMD Modulus = MD Modulus / CMD Modulus. (7) Viscosity average molecular weight Using decalin, the viscosity was measured at a measurement temperature of 135 ° C. by an Ubbelose type viscometer, and the viscosity average molecular weight was determined by the Chang's formula. (8) Weight average molecular weight It was measured by GPC using Shodex Columns. (9) Friction coefficient Using a KES-FB4 type surface tester manufactured by Kato Tech Co., Ltd., an average friction coefficient was determined at a sample tension of 400 g. (10) Shutdown temperature 60m on a PTFE plate of 50mm x 50mm x 2mm thickness
An mx 60 mm test piece was clamped and fixed all around by clips, and left in a gear oven at a predetermined temperature for 10 minutes, then 25
The air permeability of the test piece that had been air-cooled to ° C was measured. The gear oven temperature at which the air permeability was 800 seconds / 100 cc or more was taken as the shutdown temperature. (11) Impregnation height CMD with a test piece of 5 mm in MD direction and 50 mm in CMD direction
Electrolyte solution (propylene carbonate 50 up to 25 mm in the direction
It was dipped in 50% by volume, 50% by volume of dimethoxyethane, and 1M of lithium perchlorate), and after 3 minutes, the height of the impregnated electrolytic solution from the surface of the solution was measured and defined as the impregnated height.

[0026]

Example 1 Ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000 (UH-900 manufactured by Asahi Kasei Corporation) 11
High-molecular-weight polyethylene having a weight-average molecular weight of 480,000 (HIZEX Million 030S manufactured by Mitsui Petrochemical Co., Ltd.) 8.8% by weight, ethylene-propylene rubber having a weight-average molecular weight of 200,000 (Nippon Synthetic Rubber Co., Ltd.) Company-made E
P01P) 2.2% by weight, fine powder silicic acid 21% by weight, DOP
57% by weight was mixed with a Henschel mixer, and the mixture was formed into a film having a thickness of 150 μm by a film manufacturing apparatus equipped with a φ30 mm twin-screw extruder equipped with a 450 mm wide T die.

The formed film was dipped in 1,1,1-trichloroethane for 10 minutes to extract DOP, washed with water and dried, and further dipped in 25% caustic soda at 60 ° C. for 60 minutes to obtain fine powder. After extracting the silicic acid, it was dried to obtain a microporous membrane. Further, the microporous film was stretched by a uniaxial roll stretching machine heated to 125 ° C. to have a thickness of 25 μm, and heat-treated in an atmosphere of 115 ° C. for 5 seconds,
It was used as a separator.

The characteristics of the obtained separator are shown in Table 1.

[0029]

Example 2 Example 1 was repeated except that ethylene-propylene rubber having a weight average molecular weight of 310,000 (EP07P manufactured by Nippon Synthetic Rubber Co., Ltd.) was used. The characteristics of the obtained separator are shown in Table 1.

[0030]

[Example 3] Ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000 (UH-900 manufactured by Asahi Kasei Corporation) 1
3.2 wt%, high molecular weight polyethylene having a viscosity average molecular weight of 150,000 (S-360 manufactured by Asahi Chemical Industry Co., Ltd.) 4.4 wt%, ethylene-propylene rubber having a weight average molecular weight of 200,000 (Nippon Synthetic Rubber Co., Ltd. ) EP01P) 4.4% by weight, fine powder silicic acid 21% by weight, DOP 57% by weight are mixed by a Henschel mixer, and the mixture is thickened by a film manufacturing apparatus equipped with a φ30 mm twin-screw extruder and a 450 mm wide T-die. It was formed into a film having a thickness of 150 μm.

The formed film was dipped in 1,1,1-trichloroethane for 10 minutes to extract DOP, washed with water, dried and further dipped in 25% caustic soda at 60 ° C. for 60 minutes to obtain fine powder. After extracting the silicic acid, it was dried to obtain a microporous membrane. Further, the microporous film was stretched by a uniaxial roll stretching machine heated to 125 ° C. to have a thickness of 25 μm, and heat-treated in an atmosphere of 115 ° C. for 5 seconds,
It was used as a separator.

The characteristics of the obtained separator are shown in Table 1.

[0033]

Comparative Example 1 Instead of ethylene-propylene rubber,
Polypropylene (E1100 manufactured by Asahi Kasei Corporation)
Was carried out in the same manner as in Example 1 except that was used.

[0034]

Comparative Example 2 Instead of ethylene-propylene rubber,
The procedure of Example 1 was repeated except that a crystalline polyethylene-polypropylene random copolymer (M3500 manufactured by Asahi Kasei Co., Ltd.) was used.

[0035]

[Comparative Example 3] Ultra high molecular weight polyethylene having a viscosity average molecular weight of 3,000,000 (UH-900 manufactured by Asahi Kasei Kogyo KK) 11
A high-molecular-weight polyethylene having a viscosity-average molecular weight of 480,000 (HIZEX Million 030S manufactured by Mitsui Petrochemical Co., Ltd.) was used as a polymer consisting of a mixture of polyethylene of 11% by weight, and the same procedure as in Example 1 was carried out.

The characteristics of the obtained separator are shown in Table 1.

[0037]

[Table 1]

[0038]

EFFECT OF THE INVENTION The separator of the present invention has a well-balanced mechanical strength, a good battery assembling property, an excellent electrolyte impregnation property, a low electric resistance, and a high safety balance. Is.

Claims (1)

[Claims] 1. A battery separator using a microporous membrane made of a mixture of 90 to 10% by weight of polyethylene and 10 to 90% by weight of ethylene-propylene rubber.