JP2009211946A - Porous film for battery separator, and battery equipped with the film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a porous film for a battery separator of low resistance, and a battery which is equipped with the film and has superior output characteristics. <P>SOLUTION: This is the porous film for the battery separator, having a minute hole structure that penetrates the front and rear faces, and in which the minute hole structure includes the respective hollow hole parts of (A) and (B), and includes: (A) a plate-like hollow hole part arranged substantially in parallel with the front and the rear-face direction of the film; and (B) a hollow hole part, having a volume larger than the volume of the plate-like hollow hole part. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a porous film for a battery separator and a battery including the film.

In recent years, with the development of electric and electronic devices, various types of primary batteries and secondary batteries have been developed, and the improvement in performance such as energy density and output density is remarkable. In particular, secondary batteries are widely used for communication devices such as mobile phones, notebook computers, electric tools, and the like, and more recently, application to power sources for electric vehicles and hybrid vehicles is also being studied.
The separator, which is one of the battery constituent members, is installed between the positive and negative electrodes in the battery, and it is desired to have a low resistance that allows the electrolyte to permeate well while completely preventing a short circuit between both electrodes. For example, a single layer or multilayer porous film made of a thermoplastic resin as described in Patent Documents 1 to 5 is used.
Japanese Patent Publication No. 50-2176 Japanese Patent Laid-Open No. 52-32976 Japanese Patent Laid-Open No. 10-110052 JP 2001-6739 A Japanese Patent Laid-Open No. 2002-25531

  However, recently, batteries with higher energy density and output density, especially secondary batteries, have been demanded due to the effects of higher performance and multi-functionality of electrical and electronic devices. Expectations for lithium-ion secondary batteries, which are batteries, are great.

  In view of the above circumstances, an object of the present invention is to provide a low-resistance porous film for a battery separator and a battery having the film and having excellent output characteristics.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a porous film having two types of specific pores as a microporous structure is used as a battery separator, compared with an existing separator. The inventors have found that the resistance is particularly low, and found that a battery including the porous film has particularly excellent output characteristics, and completed the present invention.

That is, the present invention is as follows.
[1]
A porous film for a battery separator having a fine pore structure penetrating the front and back surfaces,
The fine pore structure has the following hole portions (A) and (B):
(A): Plate-like hole portion arranged substantially parallel to the front and back surfaces of the film (B): Porous battery separator including a hole portion having a volume larger than the volume of the plate-like hole portion the film.
[2]
The porous film for battery separators according to the above [1], wherein the maximum pore diameter is 0.5 μm or more and 20 μm or less.
[3]
A battery comprising a positive electrode, a negative electrode, an electrolytic solution, and a separator interposed between the positive electrode and the negative electrode,
A battery, wherein the separator is the porous film for battery separator according to the above [1] or [2].
[4]
A method for producing a porous film for a battery separator, including the following steps:
(A) a step of obtaining an original film by mixing and extruding an inorganic filler and a thermoplastic resin;
(B) A step of annealing the raw film and then making it porous by stretching and heat-treating.
[5]
The method for producing a porous film for a battery separator according to the above [4], wherein the draw ratio in the mixed melt extrusion molding is 50 or more.
[6]
The method for producing a porous film for a battery separator according to the above [4] or [5], wherein the annealing temperature in the annealing is from a melting point of −80 ° C. to a melting point of −5 ° C. of the thermoplastic resin.

According to the present invention, it is possible to provide a porous film for a battery separator that has a particularly low resistance compared to existing separators.
In addition, by using the porous film of the present invention as a separator, it is possible to provide a battery having particularly excellent output characteristics as compared with existing batteries.

  Hereinafter, the best mode for carrying out the present invention (hereinafter referred to as the present embodiment) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by changing variously within the range of the summary.

[Porous film for battery separator]
The battery separator porous film of the present embodiment (hereinafter also simply referred to as a porous film) has a microporous structure penetrating the front and back surfaces, and the microporous structure is the following (A) and (B) From each hole part, (A): The plate-shaped hole part arrange | positioned substantially parallel to the front and back direction of a film, (B): The hole part provided with the volume larger than the said plate-shaped hole part volume, Become.

  FIG. 1 is an image (magnification 30000 times) obtained by observing a cross section of the porous film of the present embodiment with a scanning electron microscope (SEM). In the film, (A): the front and back directions of the film are substantially the same. It can be seen that there are two types of hole portions: plate-like hole portions arranged in parallel and (B): a hole portion having a volume larger than the volume of the plate-like hole portion. . 2 and 3 are images obtained by observing the surface of the porous film of the present embodiment with an SEM (the magnification is 5000 times and the magnification is 30000 times, respectively), and the pores (B ) Can be confirmed.

  The porous film of the present embodiment has two types of specific pores, so that the internal holes are connected with a high probability in the film thickness direction. The internal resistance can be suppressed lower than that of the produced porous film, and as a result, a battery having excellent output characteristics can be obtained.

  Examples of the thermoplastic resin used as the material for the porous film include polyethylene, polypropylene, poly-4-methyl-1-pentene, polyphenylene ether, polyamide, polyimide, polyacetal, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, Examples thereof include polyether ether ketone, which may be a single resin or a mixture of two or more. A known additive may be added to these thermoplastic resins as appropriate for the purpose of preventing oxidation.

  Usually, the method for making a porous film is roughly classified into a wet method and a dry method. In the wet method, a thermoplastic resin composition mixed with a filler and a plasticizer is extruded into a film shape, and then the filler and the plasticizer are extracted from the film to make it porous. On the other hand, in the dry method, a method of controlling the crystal structure of the thermoplastic resin in the process of melt-extrusion of the thermoplastic resin, and then performing porosity by generation and growth of crazes between lamellar crystals accompanying stretching (stretching method) and A method of extruding a resin / filler interface to obtain a porous film after melt extrusion molding a material in which a filler is dispersed in a thermoplastic resin to obtain an original film, and then stretching the original film (interface peeling) There are two main types.

  As the forming method of “(A): plate-like hole portions arranged substantially parallel to the front and back surface directions” of the present embodiment, the above-described stretching method can be employed. The hole portion (A) opened by the stretching method has an unstretched plate-like plane group formed substantially parallel to the film front and back direction and a space between the plate-like plane groups substantially parallel to the film stretching direction. Formed by stretch-oriented thin fibril groups arranged and connected between plate-like planes. Here, the unstretched plate-like plane group is a lamellar crystal region, and the thin fibril group is an amorphous region that connects the lamella crystals.

  On the other hand, as the formation method of “(B): hole portion having a volume larger than the plate-like hole portion volume” in the present embodiment, the above-described interface peeling method can be employed. In the interfacial peeling method, the resin / inorganic filler interface is peeled by stretching a raw film containing an inorganic substance as a filler to form a void (B).

  The inorganic filler contained in the porous film is not particularly limited as long as the battery reaction is not inhibited. For example, barium sulfate, silicic anhydride, titanium oxide (titania), aluminum oxide (alumina), potassium titanate, magnesium oxide Boron oxide, mica, talc, kaolinite, montmorillonite, mica, calcium carbonate, potassium titanate, magnesium titanate, barium titanate, tin oxide, zinc oxide, zirconia, etc., among others, barium sulfate, anhydrous silica Acid, titania, alumina, calcium carbonate and zirconia are preferred. An inorganic filler may use only 1 type or may use 2 or more types together. Moreover, as long as it does not inhibit a battery reaction, surface treatment etc. can also be given to an inorganic filler by a well-known method.

  The average particle diameter of the inorganic filler is preferably 10 μm or less, more preferably 5 μm or less, and even more preferably 3 μm or less. The lower limit of the average particle diameter is preferably 0.01 μm or more, more preferably 0.05 μm or more, and further preferably 0.1 μm or more. When the average particle size of the inorganic filler is increased, the mechanical strength tends to be weakened, and when the average particle size is decreased, the filler tends to aggregate and easily generate flaws. When used as a battery separator, a porous film having particularly low resistance can be obtained. Here, the average particle diameter is a value (JIS Z 8825-1) measured using a laser diffraction method.

  The shape of the inorganic filler is not particularly limited and may be spherical, fibrous, plate-like, other amorphous shapes, or a mixture thereof.

  The mixing ratio of the inorganic filler is preferably 1.5% by volume or more and 20% by volume or less, more preferably 1.5% by volume or more and 10% by volume or less with respect to the porous film excluding the pores. If the mixing ratio is small, it tends to be difficult to reduce the resistance when used for a battery separator. If the mixing ratio is large, there is a tendency that fluffing occurs or the film strength decreases, but the mixing ratio is in the above range. Further, it is possible to obtain a porous film having particularly low resistance while suppressing the occurrence rate of defects in the film and the occurrence rate of defects such as film tearing during film processing and battery production. Here, the mixing ratio of the inorganic filler can be expressed by the following formula 1.

[Mixing ratio (volume%)]
= [Amount of inorganic filler (m ³ )]
/ ([Inorganic filler amount (m ³ )] + [thermoplastic resin amount (m ³ )]) × 100
... (Formula 1)

  The thickness of the porous film of the present embodiment is preferably 5 μm or more and 50 μm or less. When the thickness of the porous film is reduced, the strength tends to decrease and it is likely to break, and when the thickness is increased, the resistance when used for a battery separator tends to increase. In particular, when the battery is manufactured, the film is hardly broken, and when used as a battery separator, the resistance can be suppressed particularly low.

  The porosity of the porous film is preferably 30% or more and 80% or less, more preferably 35% or more and 65% or less. If the porosity is small, the resistance tends to be high when used for a battery separator. If the porosity is large, the strength tends to decrease and the glass tends to break, but if the porosity is in the above range, the stretching process or the battery In particular, the film is hardly broken at the time of production, and when used as a battery separator, the resistance can be suppressed particularly low. Here, the porosity of the porous film can be expressed by the following formula 2.

[Porosity (%)]
= (1- [Membrane mass (kg)] / [Resin and filler mixture density (kg / m ³ )] / [Membrane volume (m ³ )]) × 100
... (Formula 2) (where
[Density of resin and filler mixture (kg / m ³ )]
= ([Resin ratio (volume%)] × [resin specific gravity (kg / m ³ )] + [filler ratio (volume%)] × [filler specific gravity (kg / m ³ )]) / 100)

  Furthermore, the maximum pore diameter of the porous film is preferably 0.5 μm or more and 20 μm or less, more preferably 0.5 μm or more and 10 μm or less, and further preferably 0.5 μm or more and 5 μm or less. When the maximum pore size is increased, the strength of the film tends to be weakened. When the maximum pore size is decreased, the resistance tends to be increased when used as a battery separator. Sometimes the film is not easily torn and the resistance can be kept particularly low when used as a battery separator. Here, the maximum pore diameter refers to the maximum pore diameter in each image by observing the surface of the porous film and the cross-section cut in parallel with the MD direction with an electron microscope. However, if the shape of the hole differs between the major axis and the minor axis, the major axis is selected as the pore diameter.

  The porous film of the present embodiment may be used as a single layer or may be used after being laminated. In the case of a laminated product, at least one of the layers may be substantially the porous film layer of the present embodiment. As a lamination method, after melt coextrusion molding so as to form a desired layer, a method of obtaining a laminated porous film by stretching and forming a porous layer, or laminating and forming a laminated porous film after separately producing the desired layers, respectively. A method for obtaining a film can be used.

[Method for producing porous film for battery separator]
The method for producing a porous film for a battery separator of the present embodiment includes (a) a step of obtaining an original film by mixing and extruding an inorganic filler and a thermoplastic resin, and (b) after annealing the original film, And a step of performing heat treatment by making it porous by stretching.

[Step (a)]
The step (a) is a step of obtaining a raw film by mixing and extruding the thermoplastic resin and inorganic filler listed above.

  In this step, the inorganic filler and the thermoplastic resin may be mixed and dispersed in advance and supplied to the extruder for melt extrusion molding, or the thermoplastic resin and the inorganic filler may be separately supplied to the extruder. It may be melt extruded. As a method of mixing and dispersing in advance, a known method can be used. For example, a method of obtaining pellets by melt kneading using a single screw extruder, a twin screw extruder, a mixing roll, etc., a Henschel mixer, a tumbler The method of performing air blending etc. is mentioned. For the melt extrusion molding, a known uniaxial or biaxial extruder can be used, and a known die used for film production, such as a T die or a circular die, can also be used.

  In this step, the take-up speed is adjusted so that the draw ratio during molding is preferably 50 or more, more preferably 200 or more, and even more preferably 300 or more. When the draw ratio is in the above range, the lamella crystals of the extruded raw film can be particularly highly oriented, and as a result, when the film is stretched and made porous, the pores tend to be highly connected. .

  Furthermore, in order to fix the oriented lamellar crystals, the molten film may be quenched and fixed immediately after being extruded from the die. As the rapid cooling method, a known method such as a method of spraying a cooling medium directly on the molten film or a method of indirectly cooling using a roll cooled by a refrigerant or the like can be used.

[Step (b)]
Step (b) is a step in which the raw film obtained in step (a) is annealed and then made porous by stretching and heat-treated.

  The raw film obtained as described above can be annealed in order to enhance crystal orientation. The annealing temperature of the used thermoplastic resin is preferably a melting point of −80 ° C. or higher and a melting point of −5 ° C. or lower. If the annealing temperature is too high, the raw film may be dissolved and the crystal structure itself may be broken.If the annealing temperature is too low, the effect of increasing the orientation tends to be low. The orientation can be increased without breaking the crystal structure.

  The stretching method at the time of stretching may be a uniaxial stretching method or a biaxial stretching method, and the biaxial stretching method may be a simultaneous biaxial stretching method or a sequential biaxial stretching method. The stretching temperature is preferably not lower than the glass transition temperature and not higher than the melting point of the thermoplastic resin to be used, and more preferably not lower than the glass transition temperature and not higher than the melting point−10 ° C. When the film is stretched in this temperature range, the original film can be stretched without breaking, and a porous structure in which the pores are highly connected tends to be obtained. Furthermore, the film may be stretched once up to a desired stretch ratio, or may be stretched in multiple times, and the stretching temperature can be changed for each.

  Furthermore, heat treatment can be performed on the porous film made porous by the stretching. It is necessary to perform the heat treatment temperature in a temperature range where the shape does not substantially change such as the melting point or softening point of the thermoplastic resin used.

[battery]
In the present embodiment, a battery having excellent output characteristics can be provided by using the porous film obtained above as a separator. The battery in this embodiment is manufactured in a cylindrical shape, a rectangular shape, a coin shape, or the like by a known method. Although constituent members other than the separator constituting the battery are not particularly limited, the following can be exemplified.

As the positive electrode material (positive electrode active material), a lithium-containing metal compound such as a lithium-containing metal oxide, sulfide or chloride is used. As the lithium-containing metal oxide, for example, a lithium composite oxide of at least one metal selected from the group consisting of cobalt, manganese, nickel, chromium, iron, and vanadium and lithium is used. Examples of such a lithium composite oxide include LiCoO ₂ , LiMn ₂ O ₄ , and LiNiO ₂ .

  The positive electrode is prepared by kneading the above positive electrode material with a conductive agent such as acetylene black or carbon black and a binder such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF) to form a positive electrode mixture. The material is applied to an aluminum foil or a stainless steel lath plate as a current collector, dried, press-molded, and then vacuum-heated at a temperature of about 50 ° C. to 250 ° C. for about 2 hours.

  As the negative electrode (negative electrode active material), carbon capable of occluding and releasing lithium or a carbon material containing graphite, for example, carbon materials such as coke, natural graphite and artificial graphite, and composite tin oxide are used. In particular, it is preferable to use a carbon material having a graphite-type crystal structure in which the lattice spacing (002) (d002) is 0.335 to 0.340 nm. The powdery carbon material is kneaded with a binder such as ethylene propylene diene terpolymer (EPDM), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and used as a negative electrode mixture.

The electrolyte used is an electrolyte dissolved in an organic solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran, etc. Is done. Examples of the electrolyte include LiPF ₆ , LiBF ₄ , LiClO ₄ , CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, LiC (SO ₂ CF ₃ ) _{3 and the} like. Is mentioned. These electrolytes may be used alone or in combination of two or more. These electrolytes are used after being dissolved in the organic solvent at a concentration of usually 0.1 to 3 M / L, preferably 0.5 to 1.5 M / L.

Although the manufacturing method of the lithium battery using the said structural member is not specifically limited, For example, a cylindrical battery can be manufactured with the following methods. 80% by mass of LiCoO ₂ (positive electrode active material), 10% by mass of acetylene black (conductive agent) and 10% by mass of polyvinylidene fluoride (binder) are mixed, and this is mixed with 1-methyl-2-pyrrolidone. A mixture obtained by adding a solvent is applied onto an aluminum foil, dried, pressure-molded, and heat-treated to prepare a positive electrode. Graphite (negative electrode active material) is mixed in 90% by mass and polyvinylidene fluoride (binder) in a proportion of 10% by mass, 1-methyl-2-pyrrolidone solvent is added thereto, and the resulting mixture is placed on a copper foil. The negative electrode is prepared by coating, drying, pressure molding, and heat treatment. And the said positive electrode, a negative electrode, and the separator of this invention are wound cylindrically, The said nonaqueous electrolyte solution is inject | poured, and a cylindrical lithium secondary battery (diameter 18mm, height 65mm) can be produced.

  Examples and comparative examples that specifically describe the present embodiment will be exemplified below, but the present embodiment is not limited to the following examples unless it exceeds the gist. The evaluation methods for various characteristic values are as follows.

[Evaluation methods]
(1) Observation of porous structure: The surface of the sample was observed with a scanning electron microscope (S4700, manufactured by Hitachi, Ltd.). The presence or absence of the following holes (A) was confirmed at a magnification of 30000 times, and the presence or absence of the following holes (B) was confirmed at a magnification of 5000 times. When the hole was confirmed, “Yes” was indicated. When the hole was not confirmed, “No” was indicated.
(A): Plate-like hole portion arranged substantially parallel to the front and back surfaces of the film (B): Hole portion having a volume larger than the plate-like hole portion volume (2) Film thickness: Dial gauge ( Measurement was performed using PEACOCK No. 25 (trademark) manufactured by Ozaki Seisakusho.
(3) Porosity: A 10 cm square sample was taken and calculated from its volume and mass using (Equation 2) described above.
(4) Maximum pore diameter: The surface and cross section of the sample were arbitrarily photographed with a scanning electron microscope (S4700, manufactured by Hitachi, Ltd.), respectively (magnification 5000 times (field size approximately 25 μm × approximately 17 μm) When the hole diameter is small and no hole can be confirmed, the maximum hole diameter is defined as the diameter of the largest hole diameter in each photograph. However, in the case where the shape of the hole is an ellipse, an ellipse, a layer, or the like, and the major axis and the minor axis are different, the major axis side is defined as the pore diameter.
(5) Puncture strength: Using a “KES-G5 Handy Compression Tester” (trademark) manufactured by Kato Tech, a puncture test is performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. (N) was measured. The evaluation was performed according to the following criteria.
○: 2N or more ×: Less than 2N (6) Battery output characteristics: A porous film obtained by the method shown in the following Examples and Comparative Examples was used as a separator, LiCoO ₂ was used as a positive electrode active material, graphite, and acetylene black. Conductive agent and fluororubber as binder were mixed with LiCoO2: graphite: acetylene black: fluororubber = 88: 7.5: 2.5: 2 in a weight ratio, applied to aluminum foil as dimethylformamide paste and dried. A sheet obtained by mixing the sheet with a positive electrode and a mass ratio of needle coke: fluororubber = 95: 5 as a dimethylformamide paste and drying on a copper foil is used as a negative electrode, and propylene carbonate and butyrolactone are used as an electrolyte. The solution prepared with 1.0M lithium borofluoride in the solvent mixed in 1. To produce a lithium ion secondary battery to have.
The 1C discharge capacity and 5C discharge capacity up to 3V of the discharge end voltage of this battery were measured, and 5C capacity / 1C capacity was defined as the output characteristic value. The output characteristics were evaluated according to the following criteria.
○: 0.7 or more Δ: 0.5 or more and less than 0.7 ×: less than 0.5

Example 1
Polypropylene (trade name: F113G (manufactured by Prime Polymer Co., Ltd.), density 0.90, weight average molecular weight 300,000) 97.5% by volume, alumina filler (trade name: advanced alumina AA03 (manufactured by Sumitomo Chemical Co., Ltd.)) average 2.5 volume% (particle diameter 0.3 μm) was charged into a twin screw extruder through a feeder. The extruder used had an aperture of 25 mm and L / D = 48, the raw material was kneaded at a molding temperature of 220 ° C. and an extrusion rotational speed of 100 rpm. Thereafter, the molten resin was extruded from a T-die having a lip width of 6 mm, and was taken up with a cast roll whose temperature was adjusted to 95 ° C. while blowing air at 25 ° C. and cooling with air to obtain a porous raw film. At this time, the draw ratio was 300.
The obtained raw film was annealed in a 130 ° C. environment for 1 hour, and then uniaxially stretched in a film running direction in a 30 ° C. environment so as to change the dimension of the original film by + 50%. The film was uniaxially stretched so as to change the dimension by + 150% with respect to the original dimension in the film running direction in an environment of ° C. Then, it heat-processed on the conditions for 2 minutes in 150 degreeC environment, and produced the porous film.

(Example 2)
Polypropylene was changed to polyethylene (trade name: S160S (Asahi Kasei Chemicals Co., Ltd.), density 0.95, weight average molecular weight 200,000), and the annealing conditions of the raw film were changed to 110 ° C for 1 hour and the heat treatment conditions A porous film was produced in the same manner as in Example 1 except that the temperature was 130 ° C. for 2 minutes.

(Example 3)
A porous film was produced in the same manner as in Example 1 except that the average particle size of the alumina filler was changed from 0.3 μm to 2.5 μm (trade name: Advanced Alumina AA3, manufactured by Sumitomo Chemical Co., Ltd.).

(Comparative Example 1)
Polypropylene (trade name: F113G (manufactured by Prime Polymer Co., Ltd.), density 0.90, weight average molecular weight 300,000) was charged into a twin screw extruder through a feeder. The extruder used had an aperture of 25 mm and L / D = 48, the raw material was kneaded at a molding temperature of 220 ° C. and an extrusion rotational speed of 100 rpm. Thereafter, the molten resin was extruded from a T-die having a lip width of 6 mm, and was taken up with a cast roll whose temperature was adjusted to 95 ° C. while blowing air at 25 ° C. and cooling with air to obtain a porous raw film. At this time, the draw ratio was 300.
The obtained original film was annealed in a 130 ° C. environment for 1 hour, and then uniaxially stretched in a 30 ° C. environment so as to change the dimension of the original film by + 50% in the film running direction. The film was uniaxially stretched so as to change the dimension by + 150% with respect to the original dimension in the film running direction in an environment of ° C. Then, it heat-processed on the conditions for 2 minutes in 150 degreeC environment, and produced the porous film.

(Comparative Example 2)
Polypropylene was changed to polyethylene (trade name: S160S (manufactured by Asahi Kasei Chemicals Corporation), density 0.95, weight average molecular weight 200,000), and the annealing condition of the raw film was 110 ° C. for 1 hour and the heat treatment condition was 130. A porous film was produced in the same manner as in Comparative Example 1 except that the temperature was set at 2 ° C. for 2 minutes.

(Comparative Example 3)
97.5% by volume of polypropylene (density 0.90, weight average molecular weight 300,000) and 2.5% by volume of alumina filler (trade name: Advanced Alumina AA03 (Sumitomo Chemical Co., Ltd.) average particle size 0.3 μm) It put into the twin screw extruder through a feeder. The extruder used had an aperture of 25 mm and L / D = 48, the raw material was kneaded at a molding temperature of 220 ° C. and an extrusion rotational speed of 100 rpm. Thereafter, the molten resin was extruded from a T-die having a lip width of 0.5 mm and taken up with a cast roll whose temperature was adjusted to 95 ° C. to obtain a porous raw film. At this time, the draw ratio was 25.
The obtained raw film was annealed in a 130 ° C. environment for 1 hour, and then uniaxially stretched in a film running direction in a 30 ° C. environment so as to change the dimension of the original film by + 50%. The film was uniaxially stretched so as to change the dimension by + 150% with respect to the original dimension in the film running direction in an environment of ° C. Then, it heat-processed on the conditions for 2 minutes in 150 degreeC environment, and produced the porous film.

(Comparative Example 4)
A porous film was produced in the same manner as in Example 1 except that the annealing treatment was not performed.
Table 1 summarizes the raw material ratio, molding conditions, physical properties, and the like in each Example and Comparative Example.

As is clear from the results in Table 1, the porous films of the present embodiment (Examples 1 to 3) have two types of specific pores, and the resistance is particularly low. The battery produced in this way was excellent in output characteristics.
In contrast, the porous films of Comparative Examples 1 and 2 have no pores (B), and the porous films of Comparative Examples 3 and 4 have no pores (A). Was not sufficiently low, and the output characteristics of a battery produced using the same were inferior.

FIG. 1 is an image obtained by observing a cross section of the porous film of the present embodiment with a scanning electron microscope (magnification 30000 times). FIG. 2 is an image obtained by observing the surface of the porous film of the present embodiment with a scanning electron microscope (magnification 5000 times). FIG. 3 is an image obtained by observing the surface of the porous film of the present embodiment with a scanning electron microscope (magnification 30000 times).

Claims (6)

A porous film for a battery separator having a fine pore structure penetrating the front and back surfaces,
The fine pore structure has the following hole portions (A) and (B):
(A): Plate-like hole portion arranged substantially parallel to the front and back surfaces of the film (B): Porous battery separator including a hole portion having a volume larger than the volume of the plate-like hole portion the film.   The porous film for battery separators according to claim 1, wherein the maximum pore diameter is 0.5 μm or more and 20 μm or less. A battery comprising a positive electrode, a negative electrode, an electrolytic solution, and a separator interposed between the positive electrode and the negative electrode,
A battery, wherein the separator is the porous film for a battery separator according to claim 1. A method for producing a porous film for a battery separator, including the following steps:
(A) a step of obtaining an original film by mixing and extruding an inorganic filler and a thermoplastic resin;
(B) A step of annealing the raw film and then making it porous by stretching and heat-treating.   The manufacturing method of the porous film for battery separators of Claim 4 whose draw ratio in the case of the said mixing melt extrusion molding is 50 or more.   The manufacturing method of the porous film for battery separators of Claim 4 or 5 whose annealing temperature in the said annealing is melting | fusing point -80 degreeC or more and the melting point -5 degreeC or less of the said thermoplastic resin.