JP3436055B2 - Battery separator - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a battery separator which is a laminated porous film produced by a stretching method with improved puncture strength and longitudinal tearability. More specifically, the present invention relates to a battery separator in which the intermediate layer is a porous polyethylene film and both outer layers are porous polypropylene films, and a porous polyethylene film which is each constituent film of the laminated porous film. The present invention relates to a battery separator in which the puncture strength and the longitudinal tearability are further improved by making the porous polypropylene film and the porous polypropylene film have a predetermined thickness ratio, and more preferably a specific maximum pore size and a Gurley value.

[0002]

2. Description of the Related Art Conventionally, polyolefin-based porous films have been used as battery separators, but with the advance of technology in recent years, high-precision and high-performance separators are required for lithium batteries and the like. In place of the conventional single-layer porous film, a laminated porous film separator has attracted attention.

A separator is interposed in the battery to prevent short circuit between the positive and negative electrodes. In recent years, a non-aqueous electrolyte battery such as a lithium battery having high energy density, high electromotive force and little self-discharge, particularly a lithium secondary battery. Batteries have been developed and put into practical use. As a negative electrode of a lithium battery, for example, metallic lithium,
An alloy of lithium and another metal, an organic material having an ability to adsorb lithium ions such as carbon and graphite or an ability to occlude by intercalation, a conductive polymer material doped with lithium ions, etc. are known. Examples of the positive electrode include fluorinated graphite represented by (CF _x ) _n , MnO ₂ , V ₂ O ₅ , CuO, and Ag ₂ Cr.
Metal oxides such as O ₄ and TiO ₂ , sulfides, and chlorides are known.

As a non-aqueous electrolyte, ethylene carbonate is used.
, Propylene carbonate, γ-butyrolactone,
Acetonitrile, 1,2-dimethoxyethane, tetrahi
LiPF in organic solvents such as Drofran₆, LiBF_Four, L
iClO_Four, LiCF₃SO ₃Also dissolved electrolyte such as
Is used. However, lithium is particularly reactive
Therefore, if an abnormal current flows due to an external short circuit or incorrect connection, etc.
Battery temperature rises significantly, leading to an accident such as ignition.
Or thermal damage to equipment incorporating it.
I have a reminder. In order to avoid such a risk, conventional security
The following various porous films are used as a palletizer.
Is proposed.

A single layer porous film of a thermoplastic resin such as polyethylene or polypropylene (Japanese Patent Publication No. 46-401).
19, Japanese Patent Publication No. 55-32531, Japanese Patent Publication No. 5
9-37292, JP-A-60-23954, JP-A-2-75151, and US Pat. No. 3679.
No. 538, etc.). Porous film made of a polyethylene mixture having different molecular weights or a mixture of polyethylene and polypropylene (Japanese Patent Laid-Open No. 21559/1990, 33130/1993).
No. 6, etc.). Porous film using a thermoplastic resin or a non-woven fabric as a support (JP-A-3-245457, JP-A1-2583)
58, etc.). A laminated porous film in which a plurality of porous films of thermoplastic resins having different materials are laminated (JP-A-62-10857, JP-A-63-308866, JP-A-2-77).
108, JP-A-5-13062, and Japanese Patent Publication No. 3
-65776, JP-A-6-55629, JP-A-6-20671, JP-A-7-307146). The above-mentioned porous film is generally a stretching method in which an unstretched film is made porous by stretching, a filler that can be extracted, a filler, a plasticizer, and the like are extracted from the unstretched film with a solvent to extract the filler, the plasticizer, etc. It is manufactured by an extraction method that changes

The basic idea of using a single-layer or laminated porous film as a separator is to prevent short circuit between both electrodes,
While maintaining the battery voltage, etc., when the internal temperature of the battery rises above a certain temperature due to abnormal current, etc., the porous film is made non-porous, in other words, the pores are blocked and ions flow between the electrodes. To prevent this, increase the electric resistance and stop the battery function to prevent the danger of ignition due to excessive temperature rise and to ensure safety. The danger prevention function due to excessive temperature rise is an extremely important function as a battery separator, and is generally called non-porous or shutdown (abbreviated as SD).

In the battery separator, if the non-porous temperature is too low, there is a problem in practical use because the flow of ions is blocked by a slight temperature rise, and conversely, if it is too high, in a lithium battery or the like. Has a problem in terms of safety because it may cause a fire. It is generally recognized that a non-porous temperature of 110 to 160 ° C, preferably 120 to 150 ° C is suitable. In a battery that uses a porous film as a separator, when the temperature inside the battery rises above the heat-resistant temperature of the porous film, the film melts and breaks, the non-porous state is lost, and the ions are regenerated. It begins to flow and causes further temperature rise. Therefore, the battery separator is required to have appropriate porosity-free temperature and high heat resistance. Further, as the battery separator, in addition to the characteristics relating to the non-pore formation, low electric resistance, high tensile strength, high mechanical strength such as puncture strength, and small variations in thickness unevenness and electric resistance, etc. Is required.

[0008]

Various porous films have been proposed as mentioned above, but as a battery separator, a single layer porous film of polyolefin such as polypropylene has a non-porous temperature of 170. There is a problem that it is close to the melting point of lithium at around ° C or higher, and the single-layer porous film of polyethylene has a non-porous temperature of 135 °.
Although it is an appropriate temperature around C, the heat resistant temperature is 145 ° C.
In addition to the above, the piercing strength is low and there is a problem that holes are easily formed during the battery production process and during use.The polyolefin single-layer porous film is further improved in terms of safety and strength as a battery separator. There is room for

A porous film obtained by making a polyethylene mixture having different molecular weights porous has a heat resistant temperature of 150 ° C.
The degree is slightly higher than that of the single-layer porous film made of polyethylene. In addition, the sea-island structure porous film obtained by stretching a mixture of polyethylene and polypropylene to make it porous has an improved SD function at a heat-resistant temperature of about 180 ° C compared with the case of the polyethylene mixture, but its mechanical properties are not sufficient. It cannot be said that the formation of the sea-island structure in which the mixture is stretched to make it porous is likely to cause variations in quality, and there is a problem in reproducibility. Further, a porous film using a non-woven fabric or the like as a support has not only safety problems due to the non-woven fabric and the like, but also in terms of safety and the like, as in the case of the above-mentioned porous films such as polyethylene and polypropylene, high temperature In terms of reliability, it is necessary to improve.

Regarding a laminated porous film in which a plurality of porous films of thermoplastic resins of different materials are laminated and laminated, the film is preliminarily made porous by a stretching method, an extraction method or the like, and two kinds of materials and melting points are different. After manufacturing a porous film, it is laminated, stretched, pressed, manufactured by bonding with an adhesive, or by a method of making a laminated film thermo-compressed with a polyolefin having a different melting point by a stretching method or the like. Has been done.

These laminated porous films are obtained by combining two kinds of porous films having different melting points,
Provided is a battery separator having a non-porous temperature of about 135 ° C. and a heat resistant temperature of about 180 ° C. and excellent in safety. Further, by combining two types of porous films having different mechanical strengths, it is possible to provide a battery separator having high mechanical strength such as piercing strength, and basically, it has excellent characteristics as a battery separator. There is.

[0012] Further, the methods for making these laminated porous films porous are roughly classified into a stretching method (dry method) and an extraction method (wet method). The wet method is a method in which thermoplastic resins having different materials and melting points are used as fillers. It is a method of producing a laminated film by coextruding a resin composition containing a plasticizer and a plasticizer, and then extracting a filler and a plasticizer from the film to make the film porous, but in these methods, In order to obtain a laminated porous film having a fine and uniform pore size, it is necessary to mix and extract a filler and a plasticizer, which not only complicates the operation process, but also has a problem such as treatment of the extract. On the other hand, the stretching method is to co-extrude thermoplastic resins having different melting points, or stretch and permeate laminates obtained by separately extruding them, or to extrude and stretch thermoplastic resins having different melting points separately. It is manufactured by a method of laminating after making it porous. Since these stretching methods are dry processes that do not use any solvent at all, they are extremely simple and excellent in safety and are low-cost processes, and in addition, a porous membrane with a fine and uniform pore size can be obtained. It is superior to the wet method as a manufacturing method. Various types have been proposed as battery separators that use a laminated porous film that is porous-laminated by a stretching method having these excellent characteristics, but a porous polyethylene film is used as the intermediate layer and a porous film is used as both outer layers. By utilizing the difference in melting point and the difference in mechanical strength of polypropylene film, it is possible to satisfy the safety such as SD characteristics and the mechanical strength such as piercing strength required for battery separators. It has been put to practical use. However, the porous film made porous by the stretching method has a drawback that it is liable to be longitudinally split in the stretching direction when pierced by a sharp projection or the like, and its improvement has been pointed out.

[0013]

The present invention provides a battery separator in which the intermediate layer made porous by a stretching method is a porous polyethylene film and both outer layers are at least three layers made of polypropylene film. In, the elastic recovery rate at 100% elongation is 80 to 94
% Polypropylene film and elastic recovery at 50% elongation
Laminated with polyethylene film with a rate of 50-80%
The resulting laminated film is made porous by a stretching method
The present invention relates to a battery separator, which is a porous film and is a laminated porous film in which the thickness of the porous polyethylene film of the intermediate layer is larger than the average thickness of the porous polypropylene films of both outer layers. Also, the present invention
Is a porous polyethylene whose intermediate layer is made porous by the stretching method.
Ren film and both outer layers are polypropylene films
A laminated porous film comprising at least three layers
100% elongation in the manufacturing method of battery separator
Polypropylene fill with elastic recovery of 80-94%
Polyurethane with 50% to 80% elastic recovery at 50% elongation
A laminated film obtained by laminating a polyethylene film
After the low temperature drawing at the eggplant 20 ° C to plus 50 ° C, 70
It was drawn at a high temperature of 130 ° C and the intermediate layer of porous polyethylene fiber was drawn.
Porous polypropylene film with film thickness on both outer layers
Of a laminated porous film with a thickness greater than the average thickness of
The present invention relates to a method for manufacturing a battery separator having a feature. In the present invention, the maximum pore size of the laminated porous film is 0.14.
It is preferably smaller than μm. In the present invention, the Gurley value of the laminated porous film is 500 sec /
It is preferably larger than 100 cc.

The polyethylene and polypropylene used in the present invention may optionally contain additives such as a surfactant, an antioxidant, a plasticizer, a flame retardant and a coloring agent. Polypropylene is preferably one having high stereoregularity, and polyethylene is preferably high-density polyethylene, but medium-density polyethylene may be used. The laminated porous film which is the battery separator of the present invention comprises a porous polyethylene film as an intermediate layer and a porous polypropylene film as both outer layers. The laminated porous film can be produced by a known method (Japanese Patent Laid-Open No. 4-181).
651, JP 7-307146 A, JP 8-222197 A, etc.). As a specific example of the production method, a method of melt-extruding a polyethylene film and a polypropylene film and then stretched and porousized, a method of separately melt-extruding and laminated polyethylene film and polypropylene film, and a method of stretched and porous, a polyethylene film and There is a method in which the polypropylene film and the polypropylene film are separately melt-extruded, stretched and porous, and then laminated.
The battery separator of the present invention can be produced by any method. The melt extrusion method may be a melt extrusion molding method using a T die, an inflation method, or the like. For example, when a film is melt-molded by a T-die, it is generally performed at a temperature 20 to 60 ° C. higher than the melting temperature of each resin, and a draft ratio of 10 to 1000, preferably 100 to 500, and a take-up speed is particularly Although not limited, it is usually molded at 10 to 100 m / min. The melt extruded film is heat treated to enhance its crystallinity and its orientation.

The heat-treated polypropylene film is
The birefringence is 15 × 10 ^{−3 to} 21 × 10 ⁻³ , preferably 17 × 10 ^{−3 to} 20 × 10 ⁻³ , and the elastic recovery rate at 100% elongation is 80 to 94%, preferably 84 to 92. It is preferably in the range of%. The polyethylene film is
Its birefringence is 30 × 10 ^{−3 to} 48 × 10 ⁻³ , preferably 35 × 10 ^{−3 to} 45 × 10 ⁻³ , and the elastic recovery rate at 50% elongation is 50 to 80%, preferably 60 to 75. It is preferably in the range of%. When the birefringence is out of these ranges, it is not suitable because the porosity cannot be sufficiently obtained even by stretching, and even when the elastic recovery ratio is out of the above range, the degree of porosity becomes insufficient, and lamination after stretching The quality of the porous film tends to vary due to the influence on the pore size and pore size distribution, the porosity, the delamination strength, the mechanical strength and the like.

In the present invention, the birefringence is a value measured by using a polarizing microscope and a Berek compensator under crossed Nicols. The elastic recovery rate is calculated by the following equations (1) and (2). Formula (1) is for a polypropylene film, and formula (2) is for a polyethylene film. The polypropylene film has a temperature of 25 ° C,
Sample width 10 mm and length 50 mm at 65% relative humidity
Set in a tensile tester at 50 mm / min. At the speed of
Immediately after stretching to 00%, the one relaxed at the same speed was measured, and the polyethylene film was measured at 25 ° C, 6 ° C.
At a relative humidity of 5%, the sample was set to a tensile tester with a width of 15 mm and a length of 2 inches. At the speed of 50
After stretching to 100%, the stretched state was held for 1 minute, and then relaxed at the same speed.

[0017]

[Equation 1]

[0018]

[Equation 2]

The heat-treated film is laminated as it is or by thermocompression bonding and then stretched to make it porous. For example, in the case of stretching after being laminated, it is preferable that the stretching is performed at a low temperature and then at a high temperature. The low temperature drawing is usually carried out by the peripheral speed difference of the drawing rolls. Cold drawing temperature is -20
C. to plus 50.degree. C., particularly preferably 20 to 35.degree. If the stretching temperature is too low, the film is likely to break during the work, and if it is too high, the porosity is insufficient, which is not preferable. The draw ratio at low temperature is 5 to 200%, preferably 10
Is in the range of 100%. If the stretching ratio is too low, only the desired porosity is small, and if it is too high, the desired porosity and pore diameter cannot be obtained, so the above range is suitable. In the present invention, the low temperature draw ratio (E ₁ ) is according to the following formula (3). In the formula (3), L ₁ means the film size after the low temperature drawing, and L ₀ means the film size before the low temperature drawing.

(Formula 3) E ₁ = [(L ₁ −L ₀ ) / L ₀ ] × 100

The cold-stretched film is then hot-stretched. The high temperature drawing is usually carried out in a heated air circulating oven at a peripheral speed difference of a drawing roll. Hot drawing temperature is 7
0 to 130 ° C, particularly 100 to 128 ° C is preferable.
If it is out of this range, it is not suitable because it is not sufficiently porous. Further, the high temperature stretching is 40 to 100 ° from the temperature of the low temperature stretching.
It is preferable to carry out at a high temperature. The draw ratio for hot stretching is 1
The range is from 00 to 400%. If the draw ratio is too low,
The above range is preferable because the gas permeability is low, and if it is too high, the gas permeability becomes too high. In the present invention, the high temperature draw ratio (E ₂ ) is in accordance with the following formula (4). In the formula (4), L ₂ means the film size after high temperature drawing, and L ₁ means the film size after low temperature drawing.

(Formula 4) E ₁ = [(L ₂ −L ₁ ) / L ₁ ] × 100

In the present invention, after low-temperature stretching and high-temperature stretching, heat setting is performed at the high-temperature stretching temperature. In the heat setting, the length of the film after stretching is 10 to 50% in advance in order to prevent the film from shrinking in the stretching direction due to residual stress applied during stretching.
It is carried out by a method of heat shrinking to such an extent that it decreases, or a method of heating by controlling so that the dimension in the stretching direction does not change. By this heat setting, a separator having good dimensional stability and satisfying the intended problem can be obtained.

The laminated porous film of the battery separator manufactured in this manner has a porosity of 30 to 80%, preferably 35 to 60%, and a maximum pore diameter, although it varies somewhat depending on the selection of the above manufacturing conditions. 0.02 to 2 μm, preferably 0.05 to 0.14 μm, non-porous temperature is 130 to 14
The temperature is 0 ° C, and the non-porous maintenance upper limit temperature is 180 to 190 ° C. When the porosity is too low, the function when used as a battery separator is insufficient, and when it is too high, the mechanical strength becomes poor. If the maximum pore size is too small, the mobility of ions will deteriorate when used as a battery separator, and if the maximum pore size is too large, the ion migration will be too large, which is not suitable.

The gas permeation rate (Gurley value) of the battery separator made of a laminated porous film is 150 to 1500.
sec / 100cc, preferably 500-1000se
It is c / 100cc. When used as a battery separator, if the gas permeation rate is too slow, the flow of ions will be suppressed, and if it is too fast, the flow of ions will be too fast and the temperature rise upon failure will be increased, which is not suitable. The thickness of the battery separator film as a whole is 15 to 50 μm in view of mechanical strength, performance, and miniaturization, and further 20 to
40 μm is suitable.

The battery separator of the present invention is characterized in that the thickness of the intermediate layer porous polyethylene film constituting the laminated porous film is larger than the average thickness of both outer layer porous polypropylene films. As a method of obtaining the target thickness ratio of each layer, it can be easily achieved by adjusting the thickness of each layer to a predetermined ratio during melt extrusion molding. When manufacturing a laminated film by coextrusion, by adjusting each width of the multi-layer die, and when each film is melt-extruded separately and then laminated, the thickness of each film before lamination should be the above ratio. By adjusting the above, it is possible to obtain a laminated film having a target thickness ratio of each layer. Also, when the respective layer films are stretched and porous separately and then laminated, the respective porous films having the layer thickness ratios within a predetermined range may be laminated in the same manner. The laminated porous film in which the thickness ratio of each layer is within the range of the present invention improves the low puncture strength of the single-layer porous polyethylene film and at the same time improves the longitudinal tearability in the stretching direction, which is a drawback of the porous film by the stretching method. To be done. The battery separator is required to have high puncture strength in order to prevent the generation of minute defects (pinholes, etc.) during battery assembly and use, and generally 400 g or more, more preferably 450 g or more.
A puncture strength of g or more is required.

The puncture strength (g) is 1 for the sample film.
Fixed to a holder of 1.28 mmφ, tip shape 0.5
It was determined from the weight (g) when the sample was broken by piercing a needle with R and a diameter of 1 mmφ at a speed of 2 mm / sec.

The longitudinal tearability is evaluated by measuring the size of the major axis of the needle through hole and the length of the vertical tear (including the needle through hole) extending from the needle through hole as a starting point in the puncture strength measurement. It was evaluated by
The smaller the major diameter of the needle through hole and the shorter the length of vertical tearing, the better the vertical tearability.

A battery separator having such a high puncture strength can be obtained by disposing porous polyethylene in the intermediate layer and porous polypropylene excellent in mechanical strength in both outer layers thereof. However, such a laminated porous film is liable to be longitudinally split in the stretching direction starting from the through hole of the needle when measuring the puncture strength, and an improvement thereof has been demanded. It was found that if the thickness of the porous polyethylene film of the intermediate layer is larger than the average thickness of the porous polypropylene films of both outer layers, the length of vertical tearing in the stretching direction starting from the needle through hole can be shortened. The thickness ratio of the respective porous polypropylene films of both outer layers is not particularly limited as long as the average thickness thereof falls within the range of the present invention, but is generally about the same.

Further, it has been found that when the maximum pore size and Gurley value of the laminated porous film are within the specific ranges, the vertical tearability is further improved. Here, the maximum pore size is obtained from the maximum value of the pore distribution curve measured by a mercury porosimeter (manufactured by Yuasa Ionic Co., Ltd.), and the Gurley value is JIS P811.
It measured according to 7. When the maximum pore diameter of the laminated porous film is smaller than 0.14 μm, the vertical tearability of the laminated porous film is further improved. Further, it was revealed that when the Gurley value of the laminated porous film is larger than 500 sec / 100 cc, the vertical tearability of the laminated porous film is further improved. When a porous film is used as a battery separator, the maximum pore size is related to mechanical strength and ionic mobility, etc. Generally, increasing the maximum pore size lowers the mechanical strength, and decreasing the maximum pore size results in mechanical damage. Target strength becomes higher. The Gurley value is related to the maximum pore size, the porosity, the shape of the pores, etc. Generally, when the laminated porous film is used as a battery separator, the mechanical strength decreases as the Gurley value decreases and the Gurley value decreases. The higher the value, the higher the mechanical strength.

The maximum pore size and Gurley value can be easily controlled by changing the stretching ratio and stretching temperature in the stretching step, and the heat treatment temperature and time in the heat treatment step. For example, if the stretching ratio is increased in the stretching step, the maximum pore size increases and the Gurley value decreases, and if the stretching ratio decreases, the maximum pore size decreases and the Gurley value increases. further,
Gurley value of laminated porous film is 500 sec / 10
When it is larger than 0 cc, the Gurley value of the porous polyethylene film layer which determines the non-porous temperature is 30.
~ 800 sec / 100 cc, preferably 100-50
It is preferably 0 sec / 100 cc because it is excellent in SD characteristics.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION The laminated porous film of the battery separator of the present invention will be described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Example 1 A T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm was used, and the number average molecular weight was 70,000 and the weight average molecular weight was 48.
Polypropylene with 0000, melt index 3, and melting point of 166 ° C. (manufactured by Ube Industries, Ltd., Ube Polypro F10)
3EA) was melt extruded at 200 ° C. The discharge film was guided to a 90 ° C. cooling roll, cooled by blowing cold air of 25 ° C., and then taken off at 30 m / min.
The film thickness of the obtained unstretched polypropylene film is 9.5.
was μm. This unstretched polypropylene film is
In order to heat this, 3500m is applied to the 3-inch paper tube.
After being placed in a hot air circulation type oven (PS-222 type manufactured by Tabai Seisakusho Co., Ltd.) kept at 120 ° C. in a rolled state for 24 hours, it was taken out of the oven and cooled to room temperature. Birefringence of heat-treated unstretched polypropylene film is 2
0.1 × 10 ⁻³ (17.0 × 10 ⁻³ before heat treatment), 100
% Elastic recovery rate of 90.5% (75.3 before heat treatment)
%)Met.

Discharge width 1000 mm, discharge lip opening 2 m
m high density polyethylene (Mitsui Petrochemical Co., Ltd., Hi-Zex 2208J) with a density of 0.968, a melt index of 5.5 and a melting point of 132 ° C.
Melt extruded at 73 ° C. The discharge film was guided to a cooling roll at 115 ° C., cooled by blowing cold air at 25 ° C., and then taken off at 20 m / min. The film thickness of the obtained unstretched polyethylene film was 12 μm. This unstretched polyethylene film is heated to a temperature of 3500 m on a 3-inch paper tube in order to heat it.
Hot air circulation type oven maintained at ℃ (PS manufactured by Tabai Seisakusho)
(-222 type) and left for 24 hours, then removed from the oven and cooled to room temperature. The birefringence of the heat-treated unstretched polyethylene film is 40.5 × 10 ⁻³ (36.0 × 10 ⁻³ before heat treatment), and the elastic recovery rate at 50% elongation is 72.5% (before heat treatment 42 Was 0.0%).

Then, a three-layer laminated film having a sandwich structure in which both outer layers were polypropylene and the inner layer was polyethylene was produced as follows. The heat-treated unstretched polypropylene film and unstretched polyethylene film were unwound from three sets of roll rolls at an unwinding speed of 4.0 m / min, and introduced into a heating roll at a temperature of 1
Thermocompression bonding was performed at 34 ° C. and a linear pressure of 1.8 kg / cm, and then the film was guided to a cooling roll at 50 ° C. at the same speed and wound up. The speed at this time was 4.0 m / min, and the unwinding tension was 3 kg for polypropylene film and 0.9 for polyethylene film.
It was kg.

The three-layer laminated film was stretched at a low temperature of 20% between nip rolls maintained at 35 ° C. At this time, the distance between the rolls is 350 mm, and the roll speed on the supply side is 1.
It was 6 m / min. Then, the film was introduced into a hot air circulation oven heated to 126 ° C, drawn at a high temperature until the total amount of stretching reached 180% between the rollers by utilizing the difference in roll peripheral speed, and then 36% by a roll heated to 126 ° C. Relax, 2
After heat setting for 5 seconds, a laminated porous film was continuously obtained.

Film thickness of each layer of the obtained laminated porous film,
Table 1 shows the measurement results of the maximum pore diameter, the Gurley value, the Gurley value of the porous polyethylene film layer, the needle penetration force, the through hole major axis, and the longitudinal tearing length. The evaluation method was performed as follows. Porosity and maximum pore diameter Porosity and maximum pore diameter were determined from the maximum value of the pore distribution curve measured by a mercury porosimeter (manufactured by Yuasa Ionic Co., Ltd.). Specifically, a sample piece having an MD of 30 mm and a sample piece of TD of 300 mm was sampled, placed in a cell, and the porosity and the maximum pore size were determined from the amount of mercury and the pressure with respect to the pore size. Gurley value Measured according to JIS P8117. A B-type Gurley Densometer (manufactured by Toyo Seiki Co., Ltd.) was used as a measuring device. The sample piece is clamped in a circular hole having a diameter of 28.6 mm and an area of 645 mm ² . With the inner cylinder weight of 567 g, the air in the cylinder is passed outside the cylinder from the test circular hole. The air permeability (Gurley value) was measured by measuring the time taken for 100 cc of air to pass. Needle penetrating force A sample was fixed to a circular hole holder having a diameter of 11.28 mm and an area of 1 cm ² , and a needle having a tip shape of 0.5R and a diameter of 1 mmφ was lowered at a speed of 2 mm / sec and stabbed to measure a penetrating load.

Example 2 In Example 1, the unstretched polypropylene film had a thickness of 9.
2 μm, unstretched polyethylene film thickness is 12.7
A laminated porous film was obtained in the same manner as in Example 1 except that the thickness was μm. The measurement results of this laminated porous film are shown in Table 1.
Shown in.

Example 3 In Example 1, the unstretched polypropylene film had a thickness of 8.
0 μm, unstretched polyethylene film thickness is 15.0
A laminated porous film was obtained in the same manner as in Example 1 except that the thickness was μm. The measurement results of this laminated porous film are shown in Table 1.
Shown in.

Comparative Example 1 In Example 1, the unstretched polypropylene film had a thickness of 1
1.5μm, unstretched polyethylene film thickness 8μ
A laminated porous film was obtained in the same manner as in Example 1 except that m was m, and relaxation after high temperature stretching was 17%.
The measurement results of this laminated porous film are shown in Table 1.

[0041]

[Table 1]

[0042]

INDUSTRIAL APPLICABILITY The present invention relates to a battery separator in which the intermediate layer is a porous polyethylene film and both outer layers are porous polypropylene films. In the battery separator, porous polyethylene is a constituent film of the laminated porous film. By making the film and the porous polypropylene film have a predetermined thickness ratio, it is possible to provide a battery separator having improved puncture strength and vertical tearability.

Continuation of the front page (56) Reference JP-A-8-222197 (JP, A) JP-A-8-244152 (JP, A) JP-A-8-7871 (JP, A) JP-A-7-307146 (JP , A) Japanese Patent Laid-Open No. 3-219552 (JP, A) Actual Development 5-48209 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 2/16

Claims (4)

(57) [Claims] 1. A battery separator in which an intermediate layer made porous by a stretching method is a porous polyethylene film and both outer layers thereof are laminated porous films composed of at least three layers made of polypropylene film, and a 100% elongation is obtained.
Polypropylene film with an elastic recovery rate of 80 to 94% when stretched
Rum and 50% to 80% elastic recovery rate at 50% elongation
Laminated film obtained by laminating with ethylene film
It is a laminated porous film made porous by a stretching method.
The separator for a battery is a laminated porous film in which the thickness of the porous polyethylene film of the intermediate layer is larger than the average thickness of the porous polypropylene films of both outer layers. 2. The maximum pore diameter of the laminated porous film is 0.
The battery separator according to claim 1, which is smaller than 14 μm. 3. The Gurley value of the laminated porous film is 5
The battery separator according to claim 1, which is larger than 00 sec / 100 cc. 4. A large number of intermediate layers made porous by a stretching method.
Porous polyethylene film with both outer layers made of polypropylene
Laminated porous film consisting of at least 3 layers
In the method of manufacturing a battery separator for a film,
Polypro with elastic recovery of 80-94% at 100% elongation
50% to 8% elastic recovery at 50% elongation with pyrene film
Lamination obtained by laminating 0% polyethylene film
The film is stretched at a low temperature of -20 ° C to 50 ° C at a low temperature.
After that, it is stretched at a high temperature of 70 to 130 ° C. to obtain a porous layer of the intermediate layer.
The thickness of the polyethylene film is such that both outer layers are made of porous polypropylene.
Laminated porous film with thickness greater than average thickness
And a method for manufacturing a battery separator.