JPH11158304A - Porous film, separator for battery by using the same, and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the practical and safe subject film having specific physical properties, excellent in shutdown characteristics, and capable of forming a thin film by forming a composition containing a low molecular weight polyethylene and a high melting point polymer. SOLUTION: This film has η5 Ω.cm<2> membrane resistance measured in an organic electrolyte liquid at room temperature and >=100 Ω.cm<2> membrane resistance measured in the organic electrolyte at room temperature after sealing the film for one sec under a substantial seal pressure of 5 kg/cm<2> within the temperature range of 100-130 deg.C. Preferably, the air permeability of the film is <=400 sec/100 cc. Preferably, this film is obtained by forming a film composed of a composition comprising (A) 10-80 wt.% low molecular weight polyethylene having >=50% crystallinity and 500-10,000 weight average molecular weight, and (B) 90-20 wt.% polymer having a melting point >=5 deg.C higher than that of the component A (a high melting point polymer), and subjecting the film to orientation to form the film into the porous one.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a porous film, a battery separator and a battery using the porous film.

[0002]

2. Description of the Related Art In recent years, as electronic devices have become cordless and portable, lithium batteries having high energy density, high electromotive force and low self-discharge have attracted attention as power sources for driving them. A battery usually has a separator interposed between its positive and negative electrodes to prevent a short circuit between the electrodes. As such a separator, a porous film having a large number of micropores is used in order to secure ion permeability between the positive and negative electrodes.

A lithium battery or a lithium ion battery uses an organic solvent as an electrolytic solution. Therefore, if an abnormal current flows due to an external short circuit or erroneous connection, the internal temperature rises significantly and the internal temperature rises. There is concern that thermal damage may occur to equipment. Therefore, if the temperature of the separator rises due to an abnormal current, it is made nonporous at a predetermined temperature to increase its electrical resistance, cut off the battery reaction, and prevent excessive temperature rise to ensure safety. A function to secure it is required. Such a function is generally called a shutdown (Shut-down) function, and is an important function for a separator for a non-aqueous electrolyte battery such as a lithium battery.

Accordingly, the present inventors have proposed a porous film which is excellent in shutdown characteristics and suitable for use as a battery separator capable of being made into a thin film, which contains polyethylene and polypropylene as essential components and has a thickness in the thickness direction of the film. A porous film having a changed content was previously proposed (JP-A-7-216118).

[0005]

However, in recent years, with the demand for higher capacity and higher energy density of batteries, the shutdown start temperature is set to a lower temperature than in the past, and It is desired to improve safety.

Therefore, as disclosed in Japanese Patent Publication No. 7-70309 and Japanese Patent Application Laid-Open No. 7-130349, an attempt is made to lower the shutdown start temperature by coating or adhering a low molecular weight polyethylene on the surface of a porous film. Has been made. However, since such a porous film is coated or adhered with low-molecular-weight polyethylene, there are problems such as an increase in film thickness and an increase in initial film resistance.
On the other hand, JP-A-8-20659 proposes a porous film obtained by kneading low-molecular-weight polyethylene wax and polyethylene. However, due to the low content of low molecular weight polyethylene wax, there was still a point to be improved in shutdown characteristics.

The present invention solves the above-mentioned conventional problems by providing excellent shutdown characteristics, practical and safe,
It is an object of the present invention to provide a porous film that can be thinned and that is inexpensive to manufacture, a battery separator and a battery using the porous film.

[0008]

In order to achieve the above object, a porous film of the present invention has a film resistance of 5 Ω · cm ² or less measured in an organic electrolyte at room temperature,
The actual sealing pressure is 5 kg / cm in the temperature range of 0 to 130 ° C.
² , wherein the film resistance measured in an organic electrolytic solution at room temperature after sealing for 1 second is 100 Ω · cm ² or more. Here, the film resistance R (Ω · cm ² ) is an electrical resistance measured in the thickness direction of the porous film in a state where the porous film having an area S (cm ² ) is immersed in the organic electrolyte solution, is Rb. (Ω), R = (Rb−Ra) × S, where Ra (Ω) is the electric resistance of the organic electrolyte solution similarly measured without the porous film.

Such a porous film exhibits a sufficiently low film resistance at room temperature, and at a suitable temperature (usually 10 ° C.).
(0 to 130 ° C.) to rapidly make the membrane nonporous and increase the membrane resistance. Therefore, by using such a porous film, a battery separator having excellent shutdown characteristics can be obtained.

[0010] The porous film preferably has an air permeability of 400 seconds / 100 cc or less. However, the air permeability is represented by a Gurley value. Although the characteristics of the battery (for example, battery capacity) deteriorate under low-temperature conditions, the use of a battery separator having excellent air permeability can reduce the deterioration of battery characteristics at low temperatures.
Therefore, according to such a porous film, a battery separator having not only excellent shutdown characteristics but also characteristics capable of improving low-temperature characteristics of a battery can be obtained.

[0011] The porosity of the porous film is preferably 45% or more.

The porous film has a crystallinity of 50% or more and a weight average molecular weight of 500 to 500.
The content of the low molecular weight polyethylene in the range of 10,000 is 10 to 80% by weight, and the content of the polymer whose melting point is 5 ° C. or more higher than the melting point of the low molecular weight polyethylene is 90 to 20% by weight. preferable. Furthermore,
The content of the low molecular weight polyethylene is 15 to 70% by weight.
And the content of the polymer is preferably 85 to 30% by weight. With such a composition, a porous film having the above-described physical properties can be obtained more reliably.

In the porous film, it is preferable that the polymer has a weight average molecular weight of 300,000 or more and a weight average molecular weight / number average molecular weight of 20 or more. In a conventional porous film, the pore size tends to increase in order to improve the air permeability, and when used in a battery as a battery separator, it is difficult to improve both the shutdown characteristics and the low-temperature characteristics of the battery. there were. However, according to this preferred example of the present invention, by including the specific polymer as described above, a porous film having high air permeability can be obtained without excessively increasing the pore diameter. Therefore,
More reliably, it has excellent shutdown characteristics, and
A separator for a non-aqueous electrolyte battery having excellent low-temperature characteristics can be obtained.

In the porous film, polyethylene or a mixture of polyethylene and polypropylene can be suitably used as the polymer.

Another embodiment of the porous film of the present invention is a multi-layer porous film including at least one layer containing the porous film according to any one of claims 1 to 5. Even with such a porous film, a battery separator having excellent shutdown characteristics can be obtained.

The separator for a non-aqueous electrolyte battery according to the present invention comprises:
The porous film of the present invention is used as a constituent material.
Such a non-aqueous electrolyte battery separator has excellent shutdown characteristics, so that a non-aqueous electrolyte battery excellent in safety can be obtained.

The non-aqueous electrolyte battery of the present invention is a non-aqueous electrolyte battery in which the electrolyte is a non-aqueous electrolyte and the positive electrode and the negative electrode are separated by a battery separator. The porous film of the invention is used.
According to such a non-aqueous electrolyte battery, since a porous film having excellent shutdown characteristics is used as a separator, safety is excellent.

[0018]

Film resistance of the porous film of the embodiment of the present invention is, 5 [Omega · cm ² or less, preferably 3 [Omega] · cm ² or less. If the membrane resistance exceeds 5 Ω · cm ² , sufficient ion permeability cannot be secured when used as a battery separator.

The membrane resistance of the porous film of the present invention is such that the substantial sealing pressure is 5 k in a temperature range of 100 to 130 ° C.
By applying a heat and pressure treatment at 1 g / cm ² for 1 second, the pressure increases to 100 Ω · cm ² or more. That is, the film resistance after the treatment is at least 20 times the film resistance before the treatment. The film resistance after the heating and pressurizing treatment is 100Ω · c.
If it is less than m ² , sufficient shutdown characteristics cannot be realized when used as a battery separator.

The air permeability of the porous film of the present invention is preferably 400 seconds / 100 cc or less. If the air permeability exceeds 400 seconds / 100 cc, it becomes difficult to improve the low-temperature characteristics of the battery when used as a battery separator. The lower limit is not particularly limited, but is preferably 10 seconds / 100 cc or more in order to secure sufficient film strength. The porosity of the present invention is preferably 45% or more.

The porous film of the present invention has the above-mentioned specific physical properties, and is preferably a low-molecular-weight polyethylene and a polymer having a melting point higher than that of the low-molecular-weight polyethylene (hereinafter referred to as “high-melting polymer”). ).

The low molecular weight polyethylene has a weight average molecular weight of 500 to 10,000, preferably 1,000 to 50.
00 range. If the weight-average molecular weight is less than 500, it may be difficult to form pores, and if the weight-average molecular weight exceeds 10,000, the melting point becomes high. Therefore, it is difficult to set a suitable shutdown start temperature for the porous film. This is because The melting point of the low molecular weight polyethylene is preferably in the range of 100 to 130 ° C.

The crystallinity of the low molecular weight polyethylene is 50% or more, preferably 55% or more. If the crystallinity is less than 50%, it is difficult to make the film porous, and it is difficult to increase the content to the above-described range.

The high melting point polymer has a melting point at least 5 ° C. higher than the melting point of the low molecular weight polyethylene;
Preferably, the melting point of the low molecular weight polyethylene is 10 ° C or less.
It is a polymer whose temperature is higher than ℃. The high melting point polymer is not particularly limited as long as it is a thermoplastic polymer having crystallinity. For example, high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, polybutene-1,4- Polyolefins such as methylpentene-1 and copolymers thereof, polyacetals, polymethylene sulfides, polyethylene sulfides, polyphenylene sulfides, polyamides (eg, nylon 6, nylon 66), polyesters (eg, polyethylene terephthalate) and the like can be used. Particularly preferred high melting polymers are polyethylene and polypropylene. Further, two or more kinds of polymers can be used in combination as the high melting point polymer.

The high melting point polymer preferably has a weight average molecular weight of 300,000 or more. Weight average molecular weight of 3
If the molecular weight is less than 10,000, the pore size may be excessively increased when trying to improve the air permeability of the porous film. The upper limit of the weight average molecular weight is not particularly limited as long as the mixture with the low molecular weight polyethylene can be formed into a film, but, for example, 500,000 when the high melting point polymer is a high density polyethylene. When the high melting point polymer is polypropylene, it is preferably less than 1,000,000. The high melting point polymer has a weight average molecular weight (Mw) to number average molecular weight (Mn) ratio (Mw / Mn) of 20.
It is preferable that it is above. If the Mw / Mn is less than 20, the pore size may be excessively increased when trying to improve the air permeability of the porous film.

The porous film of the present invention may contain one or more additives, if necessary, in addition to the low-molecular-weight polyethylene and the high-melting-point polymer. Examples of the additives include a surfactant, an antioxidant, a plasticizer, a flame retardant, and a colorant. The content of the additive is not particularly limited as long as the properties of the porous film are not hindered.

Another form of the porous film of the present invention is a porous film having a multilayer structure, and preferably has at least one layer containing the low-molecular-weight polyethylene and the high-melting polymer as described above. Including. In the porous film having the multilayer structure, the number of layers and the laminated structure are not particularly limited. For example, a porous film containing the low-molecular-weight polyethylene and the high-melting-point polymer, a structure in which a plurality of porous films having different contents of the low-molecular-weight polyethylene are laminated,
A structure in which a porous film containing the low molecular weight polyethylene and the high melting point polymer and a porous film containing the high melting point polymer as a main component is laminated.

Next, an example of the method for producing the porous film of the present invention will be described. The porous film of the present invention can be produced by forming a film made of a composition containing the low-molecular-weight polyethylene and the high-melting-point polymer, and making this film stretched and porous. Further, the porous film having a multilayer structure can be produced by forming a film having a multilayer structure including a layer containing the low-molecular-weight polyethylene and the high-melting polymer, and making the film stretched and porous.

First, the low-molecular-weight polyethylene and the high-melting-point polymer are kneaded by a twin-screw extruder and pelletized. Next, the mixed resin is formed into a film by a single screw extruder. In some cases, a gear pump may be attached to the tip of the twin-screw extruder to form a film.

When producing a porous film having a multilayer structure, a multilayer film including at least one layer made of the above-mentioned mixed resin is formed. Examples of the method of forming a multilayer film include, for example, a method of simultaneously extruding each layer by a multilayer extruder, a method of forming a film of the mixed resin, and then extruding a resin constituting another layer on the film (or
A method of extruding the mixed resin onto the film after forming a resin film constituting another layer), a method of forming each layer separately into a film, and heat-sealing these films. .

The film can be subjected to a heat treatment if necessary. This heat treatment is performed for the purpose of improving the crystallinity of the film and the like.

The method of heat treatment is not particularly limited. For example, a method of contacting a film with a heated roll or a metal plate, a method of heating a film in air or an inert gas, and a method of heating a film on a core For example, a method of heating in a gaseous phase or with a medium in a state of being wound into a roll shape. When the film is brought into contact with a heated roll or metal plate, or when the film is heated in air or an inert gas, the heat treatment may be performed with the film sandwiched between carrier films. When the film is wound in a roll on a core and heated in a gas phase or a medium, the film is separated from the film when the film is wound on a core to prevent blocking. The mold sheets can be overlapped. The carrier film and the release sheet are not particularly limited as long as they have a melting point higher than the heat treatment temperature and can be separated from the film after the heat treatment.

The temperature and time of the heat treatment are appropriately set according to the heat treatment method and the like.
The temperature is about 65 ° C. and the time is about 2 seconds to 50 hours. Also,
The heat treatment methods and heat treatment temperatures may be combined.

The heat-treated film is made porous by stretching. The stretching method is not particularly limited,
As described below, it is preferable to apply a two-stage stretching method in which stretching is performed at a high temperature after stretching at a low temperature.

First, the film is stretched in a uniaxial direction at a low temperature (hereinafter referred to as "low temperature stretching"). The stretching temperature is usually -20 to 60C. If the temperature is lower than −20 ° C., the film may be broken during stretching, and if the temperature is higher than 60 ° C., it is difficult to make the film porous. The low-temperature stretching can be performed by a conventional roll-type stretching or tenter-type stretching.

The stretching ratio in the low-temperature stretching is not particularly limited, but is usually 20 to 400%, preferably 3 to 400%.
0% to 200%. The stretching ratio (M ₁ ) is a value calculated by the following equation (Equation 1). In the following formula (Equation 1), L ₀ is the film dimensions before cold drawing, L ₁ is the film dimension after cold drawing.

[0037]

M ₁ (%) = (L ₁ −L ₀ ) / L ₀ × 100

Next, the porous film that has been stretched at a low temperature is stretched in a uniaxial or biaxial direction at a high temperature (hereinafter, referred to as “high-temperature stretching”). The stretching temperature is usually in the range of 60 ° C. to the melting point of the high melting point polymer. The high-temperature stretching can be carried out by conventional roll-type stretching, tenter-type stretching or the like, similarly to the low-temperature stretching.

The stretching ratio of the high-temperature stretching is not particularly limited, but is usually about 10 to 500%. This stretching ratio (M ₂ ) is a value calculated by the following equation (Equation 2). In Equation _2, L ₂ is the film dimension after high-temperature stretching, and L ₁ is the film dimension after low-temperature stretching (ie,
Film dimensions before hot stretching).

[0040]

## EQU2 ## M ₂ (%) = (L ₂ −L ₁ ) / L ₀ × 100

A porous film is obtained by the two-stage stretching method as described above. This porous film is preferably subjected to a heat setting treatment in order to reduce the stress remaining in the porous film and improve the dimensional stability of the porous film. As a method of heat setting treatment, a method of contacting the porous film with a heated roll, a method of heating the porous film in air or an inert gas, and a method of winding the porous film into a roll on a core body A method of heating in a gaseous phase or a medium in a state of being heated can be employed. The processing temperature is usually in the range from the stretching temperature to the melting point of the high melting point polymer. In the heat setting treatment, when a method of contacting the porous film with a heated roll or a method of heating the porous film in air or an inert gas is employed, the size of the porous film is reduced by 5 to 40%. It can be implemented to the extent that it does. Further, when heating in a gas phase or a medium in a state where the porous film is wound on a core in a roll shape, the heating can be performed in a state where the porous film is wound with a fixed length. Further, the heat setting method and the heat setting temperature may be performed by combining the methods and the temperatures described above.

The porous film of the present invention is suitable for use as a separator for a non-aqueous electrolyte battery. Further, the porous film of the present invention can be applied to a wide range of applications such as a separator for a non-aqueous electrolyte battery, a separation membrane, a breathable film for construction, and a breathable film for clothing.

The separator for a non-aqueous electrolyte battery of the present invention comprises:
The porous film of the present invention as described above is used as a constituent material. The separator may include a support such as a nonwoven fabric in addition to the porous film of the present invention. This separator has excellent shutdown characteristics, and preferably, the resistance value at a specific temperature in the range of 100 to 130 ° C. rapidly increases to several tens to several thousands times the resistance value at room temperature. In addition, the shape can be maintained at a high heat-resistant temperature, preferably up to a temperature 25 ° C. higher than the shutdown start temperature.

The nonaqueous electrolyte battery of the present invention uses the above-described porous film of the present invention as a constituent material. Hereinafter, an example of the configuration of the nonaqueous electrolyte battery will be described.

First, a strip-shaped positive electrode and a strip-shaped negative electrode are laminated and wound via a separator using the porous film of the present invention as a constituent material to form an electrode body. The battery is obtained by storing this electrode body together with the electrolytic solution in a battery can and appropriately disposing other necessary members.

As the positive electrode active material of this battery, (CF)
fluorinated graphite, LiCoO ₂ , LiNi represented by _n
O ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , CuO, Ag ₂ Cr
Metal oxides such as O ₄ and sulfides such as TiS and CuS are used. Particularly, high voltage, high energy density is obtained, and the cycle characteristics are excellent, so that LiCoO ₂ , Li
NiO ₂ is preferably used. On the other hand, as the negative electrode active material, a lithium ion, a lithium alloy, a carbon material such as carbon or graphite having the ability to occlude or adsorb lithium ions, or a conductive polymer such as polyacetylene or polypyrrole doped with lithium ions is used. In view of the fact that the energy density per unit volume is high, a carbon material is preferably used. These positive and
Each of the negative electrode active materials is formed using a current collector such as a stainless steel net as a core material to form a belt-shaped positive electrode and a negative electrode.

As the electrolyte, ethylene carbonate, propylene carbonate, acetonitrile, γ-
Butyrolactone, 1,2-dimethoxyethane, a single solvent such as tetrahydrofuran or a mixed solvent of two or more,
An organic electrolyte in which LiClO ₄ , LiPF ₆ , LiAsF ₆ or the like is dissolved as an electrolyte can be used.

[0048]

The present invention will be described more specifically below with reference to examples and comparative examples. The measuring methods in the following examples and comparative examples are as follows.

(1) Weight average molecular weight Gel permeation chromatography (Waters
By GPC-150C) and 135 ° C. using o-dichlorobenzene as a solvent. The column used was Shodex KF-80M (manufactured by Showa Denko KK), and a data processing system manufactured by TRC was used for data processing. The molecular weight was calculated based on polystyrene.

(2) Density The density was measured according to ASTM D1505. The unit is (g
/ Cm ³ ).

(3) Crystallinity The crystallinity was determined by differential scanning calorimetry (DSC).
After heating the sample to 180 ° C. at 10 ° C./min,
The temperature was lowered to 0 ° C. at 10 ° C./min. Again, the heat of fusion when the temperature was raised to 180 ° C. at 10 ° C./min was determined, and the crystallinity was determined by the following equation (Equation 3). In addition, the measurement of the heat of fusion was performed using DSC3100 manufactured by Mac Science.

[0052]

## EQU3 ## Crystallinity (%) = heat of fusion obtained by measurement / heat of fusion of polyethylene with 100% crystallinity (237 mJ /
mg) x 100

(4) Initial Membrane Resistance The porous film was fixed to a cell for measuring electric resistance. With the porous film immersed in the electrolytic solution, an LCR meter (Hewlett Packard (Hewlet Packard)) connected to the cell was used.
The electric resistance at 10 KHz was measured according to 4262A) manufactured by Packard). In addition, as the electrolytic solution, propylene carbonate and dimethoxyethane were mixed by the same weight,
A solution prepared by dissolving lithium perchlorate to a concentration of 1 mol / liter was used. Further, as a blank, the electric resistance of only the electrolytic solution was measured in the same manner as above except that the porous film was not immersed in the electrolytic solution. Based on the measurement result, the film resistance R (Ω · cm ² ) was calculated by the following equation (Equation 4). However, R in equation (4)
a is the electric resistance (Ω) of the electrolytic solution (liquid temperature 20 ° C.), Rb is the electric resistance (Ω) measured with the porous film immersed in the electrolytic solution, and S is the area (cm ² ) of the porous film. It is.

[0054]

## EQU4 ## R (Ω · cm ² ) = (Rb−Ra) × S

Since the electric resistance measuring cell used here has a slight leakage current, the electric resistance of the non-porous film can be measured only up to about 1000 Ω · cm ² .

(5) Evaluation of Shutdown Characteristics The porous film was cut into a square having a side of about 50 mm, and this was cut into a jig (30 mm × 30 m on a metal SUS plate).
m with a double-sided tape on a wooden board cover)
Then, a sample was prepared with four sides fixed so as not to cause wrinkles. This sample was heated at a temperature of 1 with a heat sealer.
Heat and pressure treatment was performed at 30 ° C. and a surface pressure of 5 kg / cm ² for 1 second. After the heating and pressurizing treatment, the porous film was peeled from the jig and cut into an appropriate size, and the film resistance was measured by the same operation as the above-described method for measuring the initial film resistance.

(6) Air permeability (Gurley value) According to JIS K 8117, 10 cc of air has an area of 6
The time required for permeation through a 42 mm ² porous film was measured, and the measured value was multiplied by 10 to obtain a value. For the measurement,
Gurley type densometer No. manufactured by Yasuda Seiki Seisakusho 32
3-Auto was used.

(7) Evaluation of Low-Temperature Characteristics of Battery
Precharge / discharge was performed 5 times at a current density of m ² . next,
After charging at a current density of 3.2 mA / cm ² , the battery was
Put in a 10 ° C. incubator and 3.2 m after 1 hour
Discharge was performed at a current density of A / cm ^{2, and} the amount of electricity (that is, battery capacity) that could be taken out by the end of discharge was measured. The charging was switched to constant voltage charging when the voltage reached 4.1 V, and the charging time was 5 hours for preliminary charging and discharging and 1 hour for normal charging. The discharge is 3.0 V
It was decided to stop when it became. The time from the end of the charge to the start of the discharge and the time from the end of the discharge to the start of the charge were both 1 hour.

(Example 1) Crystallinity 86%, molecular weight 20
A mixture of 20% by weight of a low-molecular-weight polyethylene having a molecular weight of 00% and 80% by weight of a high-density polyethylene having a weight-average molecular weight of 350,000 and a density of 0.963 g / cm ³ was cooled at a die temperature of 200 ° C. using a T-die extruder. At a temperature of 80 ° C., a film having a thickness of about 35 μm was formed. In this film,
Heat treatment was performed in a dryer at 110 ° C. for 24 hours. After the heat treatment, the film is stretched at a low temperature by a roll stretching machine so that the stretching ratio becomes 50%.
%. Then, after stretching, temperature 1
By shrinking by 10% in the stretching direction at 10 ° C. (based on the dimensions after high-temperature stretching), a film thickness of 27 μm was obtained.
A white porous film having a Gurley value of 500 seconds / 100 cc was obtained. Table 1 shows the properties of the obtained porous film.

(Example 2) Crystallinity 57%, molecular weight 10
A porous film was produced in the same manner as in Example 1 except that the low-molecular-weight polyethylene of No. 00 was used. Table 1 shows the properties of the porous film.

Example 3 Mixing of 40% by weight of low molecular weight polyethylene having a crystallinity of 86% and a weight average molecular weight of 2000, and 60% by weight of a high density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ^3. With the resin layer as the outer layer,
A three-layer film having a layer of 100% by weight of high-density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ³ was extruded by a multilayer extruder. In the subsequent steps, heat treatment, low-temperature stretching, high-temperature stretching, and heat setting were sequentially performed in the same manner as in Example 1 to obtain a white porous film having a thickness of 27 μm. Table 1 shows the properties of the porous film.

(Example 4) Crystallinity 86%, molecular weight 20
40 low-molecular-weight polyethylene of 40% by weight and
350,000 particles, density 0.963g / cm^Three High density polyet
20% by weight of styrene, weight average molecular weight of 350,000, density of 0.
91g / cm^Three Wood with 40% by weight of polypropylene
Fat layer as outer layer, weight average molecular weight 350,000, density 0.96
3g / cm ^Three 100% by weight layer of high density polyethylene
Extrude a three-layer film as an inner layer using a multilayer extruder
Formed. The subsequent steps are heat treatment and low temperature rolling as in Example 1.
Stretching, high-temperature stretching and heat setting are sequentially performed to obtain a thickness of 27 μm.
m white porous film was obtained. This porous film
Are shown in Table 1.

Comparative Example 1 A film made of 100% by weight of high-density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ³ was formed. In the subsequent steps, heat treatment, low-temperature stretching, high-temperature stretching, and heat setting were sequentially performed in the same manner as in Example 1 to obtain a 27-μm-thick white porous film. Table 1 shows the properties of the porous film.

Comparative Example 2 As in Example 2 of JP-A-8-20659, the crystallinity was 86% and the molecular weight was 2,000.
A mixture of 8 parts by weight of low molecular weight polyethylene and 100 parts by weight of a high-density polyethylene having a melt index of 1.1 and a density of 0.954 g / cm ^{3 was} molded using a T-die extruder at a die temperature of 170 ° C. and a chill roll temperature. 80 ℃
To form a film having a draft ratio of 100 and a thickness of about 35 μm. This film was placed in a dryer at 110 ° C. for 24 hours and subjected to heat treatment. After heat treatment, with a roll stretching machine,
Cold stretching was performed so that the stretching ratio became 50%, and further high-temperature stretching was performed at 110 ° C. in the same direction so that the stretching ratio became 110%. After stretching, the porous film is heat-set at 115 ° C. for 5 minutes while maintaining the stretched state, and has a Gurley value of 500.
A white porous film of 100 s / 100 cc was obtained. Table 1 shows the properties of the obtained porous film.

Comparative Example 3 Crystallinity 47%, Molecular Weight 20
Example 1 except that the low molecular weight polyethylene of No. 00 was used.
A porous film was produced in the same manner as described above. Table 1 shows the properties of the porous film.

[0066]

[Table 1]

As shown in Table 1, the porous film of the example of the present invention has a membrane resistance of 5 Ω · cm ² or less measured in an organic electrolyte at room temperature, and has a sufficient ion permeability. I have. After sealing for 1 second at a substantial sealing pressure of 5 kg / cm ^{2 in} a temperature range of 100 to 130 ° C., the film resistance measured in an organic electrolytic solution at room temperature is 100 Ω · cm.
^{It was 2} or more, and it was confirmed that the shutdown characteristics were excellent.

Example 5 Crystallinity 86%, Molecular Weight 20
20 low-molecular-weight polyethylene having a weight average molecular weight of 370,000, Mw / Mn = 22, and a density of 0.958 g / c.
A mixture with 80% by weight of m ³ high-density polyethylene was melt-kneaded by a twin-screw extruder and then pelletized. A three-layer film having the mixed resin layer as an outer layer and a 100% by weight layer of high-density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ³ as an inner layer was subjected to a die temperature of 200 by a multilayer extruder. And a cooling roll temperature of 80 ° C. The thickness of this film was about 37 μm. Next, the film was subjected to a heat treatment in a dryer at 110 ° C. for 24 hours. The heat-treated film is stretched at a low temperature by a roll stretching machine so that the stretching ratio becomes 60%.
The film was stretched at 10 ° C. in the same direction at a high temperature so that the stretching ratio became 170%. After stretching, at a temperature of 115 ° C.,
The film was shrunk by 5% (based on the dimensions after high-temperature stretching) to obtain a white porous film having a thickness of 27 μm. Table 2 shows the properties of the porous film.

Further, using the obtained porous film as a separator, a coin-type battery was produced in the following manner. Lithium cobaltate as a positive electrode active material, carbon powder as a conductive agent, and polyvinylidene fluoride as a binder were added to n-methyl-2-pyrrolidone to prepare a slurry, and the slurry was 25 μm thick. Was applied to one side of an aluminum foil. 100 ℃ of this aluminum foil
After pressing with a roll press and pressing with a punch,
A positive electrode was prepared by punching out a piece having a size of 6 mm. Next, a slurry was prepared by adding graphite as a negative electrode active material and polyvinylidene fluoride as a binder to n-methyl-2-pyrrolidone, and applying this slurry to one surface of a copper foil having a thickness of 25 μm. did. This copper foil was dried at 100 ° C., pressed by a roll press, and then punched out to a size of 15 mm in diameter with a punch to produce a negative electrode. The prepared positive electrode and negative electrode were laminated via a porous film, and stored in a battery can together with an electrolytic solution to obtain a coin-type battery. As the electrolytic solution, a solution prepared by dissolving lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate to a concentration of 1 M was used. Table 2 shows the results of evaluating the low-temperature characteristics of the obtained batteries.

(Example 6) A film before stretching was applied at 95 ° C.
A white porous film having a thickness of 25 μm was obtained in the same manner as in Example 1 except that the heat treatment was performed in a dryer for 24 hours. Table 2 shows the properties of the porous film. Using this porous film, a coin-type battery was produced in the same manner as in Example 5, and its low-temperature characteristics were evaluated. Table 2 shows the results.

Comparative Example 4 Crystallinity 86%, Molecular Weight 20
20 weight% of low molecular weight polyethylene having a weight average molecular weight of 150,000, Mw / Mn = 20, density of 0.959 g / c
A mixture with 80% by weight of m ³ high-density polyethylene was melt-kneaded by a twin-screw extruder and then pelletized. This mixed resin layer was used as an outer layer, and a film having a three-layer structure in which a layer of 100% by weight of high-density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ³ was used as an inner layer. And a cooling roll temperature of 80 ° C. The thickness of this film was about 37 μm. In the subsequent steps, heat treatment, low-temperature stretching, high-temperature stretching, and heat setting were sequentially performed in the same manner as in Example 1 to obtain a white porous film having a thickness of 27 μm. Table 2 shows the properties of the porous film. Using this porous film, a coin-type battery was produced in the same manner as in Example 5, and its low-temperature characteristics were evaluated. Table 2 shows the results.

Comparative Example 5 Crystallinity 86%, Molecular Weight 20
20 weight% of low molecular weight polyethylene having a weight average molecular weight of 350,000, Mw / Mn = 15, and a density of 0.960 g / c.
A mixture with 80% by weight of m ³ high-density polyethylene was melt-kneaded by a twin-screw extruder and then pelletized. This mixed resin layer was used as an outer layer, and a film having a three-layer structure in which a layer of 100% by weight of high-density polyethylene having a weight average molecular weight of 350,000 and a density of 0.963 g / cm ³ was used as an inner layer. And a cooling roll temperature of 80 ° C. The thickness of this film was about 37 μm. This film was subjected to a heat treatment in a drier at 95 ° C. for 24 hours. The film after the heat treatment was sequentially subjected to low-temperature stretching, high-temperature stretching and heat setting in the same manner as in Example 1,
A white porous film having a thickness of 27 μm was obtained. Table 2 shows the properties of the porous film. Using this porous film, a coin-type battery was produced in the same manner as in Example 5, and its low-temperature characteristics were evaluated. Table 2 shows the results.

[0073]

[Table 2]

As shown in Table 2, the porous films of Examples of the present invention have a film resistance of 5 Ω · cm ² or less measured in an organic electrolyte at room temperature and a temperature range of 100 to 130 ° C. After sealing for 1 second at a substantial sealing pressure of 5 kg / cm ² , the membrane resistance measured in an organic electrolyte at room temperature is 1
Since it was 00 Ω · cm ² or more, it was confirmed that the shutdown characteristics were excellent. Furthermore, the air permeability represented by the Gurley value, which is a preferred embodiment of the present invention, is 400 seconds /
According to the porous film of 100 cc or less, it was confirmed that the low-temperature characteristics of a battery using the porous film as a separator could be improved.

[0075]

As described above, the porous film of the present invention has a film resistance of 5 Ω measured in an organic electrolyte at room temperature.
· Cm ² or less and then, and, 100 [Omega membrane resistance measured in an organic electrolytic solution at room temperature after 1 second seal substantially sealing pressure as 5Kg / cm ² in a temperature range of 100 to 130 ° C.
By setting the cm ² or more, it is possible to provide a porous film which is excellent in shutdown characteristics, practical and safe, can be thinned, and is inexpensive to manufacture, and a battery separator and a battery using the same. .

Continued on the front page (51) Int.Cl. ⁶ Identification code FI C08L 23/10 C08L 23/10 (72) Inventor Satoshi Ishizaki 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation

Claims (8)

[Claims] 1. A film having a film resistance of 5 Ω · cm ² or less measured in an organic electrolytic solution at room temperature and a sealing pressure of 5 kg / cm ^{2 in} a temperature range of 100 to 130 ° C. for 1 second, followed by room temperature Film resistance measured in organic electrolyte of 1
A porous film having a resistivity of at least 00 Ω · cm ² . 2. The porous film according to claim 1, which has an air permeability of 400 seconds / 100 cc or less. 3. The content of low molecular weight polyethylene having a crystallinity of 50% or more and a weight average molecular weight in the range of 500 to 10,000 is 10 to 80% by weight, and the melting point of said low molecular weight polyethylene is The polymer content of at least 5 ° C higher than the melting point is 90 to 20% by weight.
Or the porous film according to 2. 4. The content of the low molecular weight polyethylene is 1
The porous film according to claim 3, wherein the content is 5 to 70% by weight and the content of the polymer is 85 to 30% by weight. 5. The polymer of claim 3, wherein the polymer is polyethylene or a mixture of polyethylene and polypropylene.
Or the porous film according to 4. 6. A porous film comprising at least one layer containing the porous film according to claim 1. 7. A battery separator comprising the porous film according to claim 1 as a constituent material. 8. A non-aqueous electrolyte battery in which the electrolyte is a non-aqueous electrolyte, and the positive electrode and the negative electrode are separated by a battery separator. A nonaqueous electrolyte battery using the porous film described in the above.