WO2020003805A1

WO2020003805A1 - Porous film, separator for secondary battery, and secondary battery

Info

Publication number: WO2020003805A1
Application number: PCT/JP2019/019847
Authority: WO
Inventors: 甲斐信康; 加門慶一; 佃明光
Original assignee: 東レ株式会社
Priority date: 2018-06-27
Filing date: 2019-05-20
Publication date: 2020-01-02
Also published as: KR20210021940A; CN111902965A; JPWO2020003805A1; CN111902965B; JP7331692B2

Abstract

[Problem] The purpose of the present invention is to provide a porous film that is highly adhesive with respect to an electrode and that has excellent battery properties. [Solution] Provided is a porous film in which a porous layer A which is adhesive with respect to an electrode is layered on at least one side of a porous substrate, wherein the air permeability of the porous film after being submerged in a solvent constituted of at least one carbonate from among dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25°C for 24 hours is not more than 0.95 times the air permeability prior to submergence, and the air permeability is 1,000 sec/100 cm³ or less.

Description

Porous film, separator for secondary battery and secondary battery

(4) The present invention relates to a porous film, a secondary battery separator, and a secondary battery having adhesiveness to an electrode and excellent battery characteristics.

Secondary batteries such as lithium-ion batteries are used in portable digital devices such as smartphones, tablets, mobile phones, laptops, digital cameras, digital video cameras, portable game machines, portable tools such as electric tools, electric motorcycles, and electric assist bicycles. It is widely used in equipment and automotive applications such as electric vehicles, hybrid vehicles and plug-in hybrid vehicles.

Lithium ion batteries generally have a secondary battery separator and an electrolyte interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. It has a configuration.

ポリ A polyolefin porous substrate is used as a secondary battery separator. The characteristics required for a secondary battery separator include the ability to contain an electrolyte in the porous structure and enable ion transfer, and the ability to close the porous structure by melting with heat when the lithium ion battery generates abnormal heat. In addition, there is a shutdown characteristic in which power generation is stopped by stopping ion movement.

Furthermore, in the manufacturing process of the secondary battery, the positive electrode, the separator, when transporting the laminated body of the negative electrode, in order to maintain the laminated body, or rolled positive electrode, separator, the laminated body of the negative electrode cylindrical, When inserting into a can, such as a square shape, insert the laminate by hot pressing, but in order to prevent the shape from collapsing at that time, or by hot pressing the laminate, more laminates In a can to increase the energy density, and in a laminate type, in order to prevent the shape from being deformed after being inserted into the exterior material, the adhesiveness between the separator and the electrode before impregnation with the electrolyte is It has been demanded.

一方で On the other hand, lithium-ion batteries are also required to have excellent battery characteristics such as high output and long life, and are required to exhibit good battery characteristics without deteriorating the battery characteristics.

In response to these requirements, Patent Literature 1 strives to achieve both ion permeability and adhesiveness to an electrode by laminating a heat-resistant porous layer containing a particulate organic binder and an inorganic filler. In Patent Literature 2, an adhesive layer formed on a heat-resistant layer is laminated to achieve both the adhesion to an electrode and the blocking resistance. Further, in Patent Document 3, when a force curve based on a pressing force is created using an atomic force microscope (AFM), a thermoplastic layer that defines the amount of deflection of a cantilever calculated from the force curve is laminated. It is said that the adhesiveness to electrodes and high-temperature storage characteristics are improved.

Japanese Patent No. 564378 Japanese Patent No. 6191597 JP 2017-147050 A

接着 As described above, the adhesiveness between the electrode and the separator is required by the hot pressing process in the manufacturing process of the secondary battery. In addition, excellent battery characteristics are also required, and it is necessary to achieve both adhesion and battery characteristics (discharge load characteristics, charge / discharge cycle characteristics).

目的 An object of the present invention is to provide a porous film, a separator for a secondary battery, and a secondary battery having adhesiveness to an electrode and excellent battery characteristics in view of the above problems.

Therefore, the present inventors have conducted intensive studies in order to provide a porous film having adhesiveness to an electrode and excellent battery characteristics. As a result, in the related arts such as Patent Documents 1 to 3, the adhesive layer has an adhesive property with the electrode active material by providing the adhesive layer, but the adhesive layer swells by hot pressing, and The present inventors have found that filling the voids of the substance and the separator lowers the porosity and lowers the ion transport rate, so that the battery characteristics are also reduced. In order to solve the above problems, the porous film of the present invention has the following configuration.
(1) A porous film in which a porous layer A having adhesiveness to an electrode is laminated on at least one surface of a porous substrate, and is composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. A porous film having an air permeability after immersion in a solvent of 25 ° C. for 24 hours of 0.95 times or less that before immersion and 1000 sec / 100 cm ³ or less.
(2) The porous film according to (1), wherein the rate of change of the root-mean-square roughness (Rq) before and after immersing the porous layer A in the solvent at 25 ° C. for 24 hours is 10% or more and 90% or less. .
(3) The porous film according to (1) or (2), wherein a root-mean-square roughness (Rq ₁ ) after immersion in the solvent at 25 ° C. for 24 hours is 20 nm or more and 80 nm or less.
(4) The porous film according to any one of (1) to (3), wherein the surface porosity of the porous layer A is 50% or less.
(5) The porous layer A according to any one of (1) to (4), wherein the porous layer A contains at least one resin selected from the group consisting of an acrylic resin, a styrene resin, a fluororesin, and an olefin resin. Porous film.
(6) The porous film according to any one of (1) to (5), wherein the thickness of the porous layer A is 0.05 μm or more and 5 μm or less.
(7) The porous film according to any one of (1) to (6), wherein the porous layer A contains inorganic particles.
(8) The porous film according to any one of (1) to (6), wherein a porous layer B containing inorganic particles is laminated between the porous substrate and the porous layer A.
(9) The porous film according to any one of (1) to (8), wherein the porous layer A is laminated on both surfaces of the porous substrate.
(10) A separator for a secondary battery using the porous film according to any one of (1) to (9).
(11) A secondary battery using the secondary battery separator according to (10).

According to the present invention, a porous film in which a porous layer A having an adhesive property to an electrode is laminated on at least one surface of a porous substrate, wherein at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate By forming a porous film having an air permeability of 0.95 times or less of that before immersion in a solvent composed of seeds at 25 ° C. for 24 hours and 1000 sec / 100 cm ³ or less, it is possible to form a porous film. A porous film having adhesive properties and excellent battery characteristics can be provided. By using the porous film of the present invention, it is possible to provide a secondary battery separator and a secondary battery with high productivity, high capacity, high output, and long life.

A porous film in which a porous layer A having an adhesive property to an electrode is laminated on at least one surface of a porous substrate, and a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate Is a porous film having an air permeability of 0.95 times or less that before immersion at 25 ° C. for 24 hours and 1000 sec / 100 cm ³ or less. Hereinafter, the present invention will be described in detail.

[Porous film]
(Air permeability)
The porous film of the present invention has an air permeability of 0.95 times or less after immersion in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours. Preferably it is 0.7 times or less, more preferably 0.5 times or less, and still more preferably 0.3 times or less. By making the content 0.95 times or less, porosity by immersion of a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate becomes sufficient, sufficient ion mobility is obtained, and battery characteristics are obtained. Can be prevented from decreasing. Further, the air permeability of the porous film after being immersed in the solvent is 1000 sec / 100 cm ³ or less. More preferably, it is 500 sec / 100 cm ³ or less, even more preferably, 300 sec / 100 cm ³ or less. When it is 1000 sec / 100 cm ³ or less, sufficient ion mobility can be obtained, and a decrease in battery characteristics can be prevented. When it is larger than 1000 sec / 100 cm ³ , sufficient ion mobility cannot be obtained, and battery characteristics may be deteriorated.

種類 The type of the solvent to be immersed is dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, which are chain carbonates constituting the non-aqueous electrolyte of the secondary battery. One type may be used alone, or two or more types may be combined according to the application. Furthermore, you may combine with cyclic carbonates, such as propylene carbonate, ethylene carbonate, and butylene carbonate. In this case, the total volume ratio of the chain carbonates of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate is preferably 20% or more. More preferably, it is at least 35%, more preferably at least 50%. When the volume ratio is 20% or more, both the solubility of the porous layer A and the battery characteristics can be achieved.

The air permeability before immersion in the solvent is preferably 100 sec / 100 cm ³ or more, more preferably 500 sec / 100 cm ³ or more, and even more preferably 1000 sec / 100 cm ³ or more. When the air permeability before dipping in the solvent is 100 sec / 100 cm ³ or more, the contact area between the portion having adhesiveness and the electrode is increased, so that it can be used in the secondary battery manufacturing process (before electrolyte injection). By the hot pressing step, sufficient adhesiveness to the electrode is obtained.

[Porous layer A]
(Porous structure)
The porous layer A has a porous structure. The porous structure refers to a structure having voids in the structure. The porous layer A preferably has a root-mean-square (Rq) change rate of 10% or more and 90% or less before and after immersion in the solvent at 25 ° C. for 24 hours. If the root-mean-square roughness (Rq) change rate before and after immersion in a solvent at 25 ° C. for 24 hours is 10% or more, sufficient ion mobility can be obtained and deterioration of battery characteristics can be prevented. On the other hand, when the rate of change is 90% or less, the porous layer A is not excessively dissolved in the solvent, the ion transport rate is prevented from lowering, and the battery characteristics can be improved. The rate of change of the root-mean-square (Rq) is defined as a ratio of the root-mean-square roughness before immersion in the solvent at 25 ° C. for 24 hours to Rq ₀ and the root-mean-square roughness after immersion as Rq ₁ 1- (Rq ₁ / _Rq 0)} defines × 100 (%) and. Since the root-mean-square (Rq) change rate is a numerical value indicating a structural change when immersed in the solvent at 25 ° C. for 24 hours, the degree of swelling and dissolution of the porous layer A can be represented. The root-mean-square (Rq) change rate is preferably from 20% to 80%, more preferably from 30% to 70%.

The root mean square roughness (Rq ₁ ) after immersion in the solvent at 25 ° C. for 24 hours is preferably 20 nm or more and 80 nm or less. The thickness is more preferably 30 nm or more and 70 nm or less, and further preferably 40 nm or more and 60 nm or less. When the thickness is 20 nm or more, the porous layer A will not be excessively dissolved in the solvent, the ion transport rate will not be reduced, and the battery characteristics can be improved. On the other hand, when the thickness is 80 nm or less, the surface structure of the porous layer A can be made uniform, and the battery characteristics can be improved. The root-mean-square roughness (Rq) is measured using a method described in Examples described later.

表面 The surface porosity of the porous layer A is preferably 50% or less. It is more preferably at most 40%, further preferably at most 30%. When the surface porosity of the porous layer A is 50% or less, a sufficient adhesiveness to the electrode may be obtained by increasing the contact area between the organic resin and the electrode. The surface porosity of the porous layer is determined using the following method. Ion coating is performed on the surface of the porous layer, and image data of the surface is obtained from a field emission scanning electron microscope (FE-SEM). Image analysis is performed from the obtained image data, and the area of the opening is calculated by subtracting the unopened portion from the entire image, and the surface opening ratio can be obtained.

膜厚 The thickness of the porous layer A is preferably 0.05 μm or more and 5 μm or less. More preferably, it is 0.2 μm or more and 3 μm or less. More preferably, it is 0.5 μm or more and 2 μm or less. In the case of a porous film having the porous layer A on one surface of the porous substrate, the film thickness of the porous layer A as referred to herein means the film thickness of the porous layer A. In the case of a porous film having a porous layer A, the total thickness of both porous layers A is referred to. When the thickness of the porous layer A is 0.05 μm or more, sufficient adhesiveness to the electrode can be obtained. When the thickness is 5 μm or less, the thickness of the porous layer A can be reduced, and the battery characteristics can be improved. It is also advantageous in terms of cost.

(Organic resin)
The porous layer A in the present invention contains an organic resin as a main component. The organic resin in the present invention is preferably a film forming resin for forming a film of the organic resin itself by heat treatment. The film-forming resin referred to herein is preferably a resin having a minimum film-forming temperature of −20 ° C. to 100 ° C. and a glass transition temperature of −30 ° C. to 100 ° C. More preferably, the minimum film forming temperature is 0 ° C. or more and 90 ° C. or less, and the glass transition temperature is 0 ° C. or more and 90 ° C. or less, and further preferably, the minimum film forming temperature is 30 ° C. or more and 80 ° C. or less, and the glass transition temperature is 15 ° C. Not less than 80 ° C. Here, the minimum film forming temperature is, for example, the minimum value at which a uniform film without cracks is formed when the resin emulsion is dried in accordance with the provisions of “JIS K6828-2 Determination of whitening temperature and minimum film forming temperature”. Temperature. In addition, the glass transition temperature refers to, for example, in the differential scanning calorimetry (DSC) according to the provisions of “JIS K7121: 2012 Plastic transition temperature measurement method”, when the temperature is raised first and then after the second temperature rise after cooling. The intersection of a straight line extending from the low temperature side base line to the high temperature side and a tangent drawn at a point where the gradient of the curve of the step change portion of the glass transition becomes maximum is defined as the glass transition temperature.

(4) A resin having a minimum film-forming temperature of -20 ° C to 100 ° C and a glass transition temperature of -30 ° C to 100 ° C is preferable because high adhesiveness to an electrode can be obtained. A hot pressing step is often used to bond the electrode and the porous film. At this time, the minimum film formation temperature is -20 ° C to 100 ° C, and the glass transition temperature is -30 ° C to 100 ° C. It is preferable that the resin is a resin because a part of the porous layer enters into the gap between the active materials of the electrode by heat or press, and an adhesion to the electrode is enabled by exhibiting an anchor effect. In the case of a resin having a minimum film formation temperature exceeding 100 ° C., sufficient adhesiveness to an electrode may not be obtained.

Examples of the organic resin constituting the porous layer A include olefin resins such as acrylic resin, polyethylene, and polypropylene, styrene resin, cross-linked polystyrene, methyl methacrylate-styrene copolymer, polyimide, fluorine resin, melamine resin, phenol resin, and polyacrylonitrile. , A silicone resin, a urethane resin, a polycarbonate, a carboxymethylcellulose resin, and the like. Of these, only one type may be used, or a plurality of types may be used in combination. Among these, from the viewpoint of adhesiveness to the electrode, for example, it is preferable to use an acrylic resin, an olefin resin, a styrene resin, a fluororesin, and a urethane resin, polyacrylonitrile, and an acrylic resin, a styrene resin, and a fluororesin are more preferable. . These organic resins may be used alone or as a mixture of two or more as necessary.

形状 The shape of the organic resin constituting the porous layer A is not particularly limited, but is preferably in the form of particles from the viewpoint of improving battery performance by making the resin porous. The shape of the particles is not particularly limited, and may be spherical, polygonal, flat, fibrous, or the like, but in the present invention, from the viewpoint of surface modification, dispersibility, and coatability, spherical is preferred. It is particularly preferable that the shape is closer to a true sphere. In the case of a particle shape, the average particle size of the particles is preferably 0.01 μm or more and 5 μm or less, more preferably 0.05 μm or more and 3 μm or less, and even more preferably 0.1 μm or more and 1 μm or less. When the average particle size is 0.01 μm or more, the porous structure can be prevented from becoming dense, and the air permeability can be reduced. In addition, since the pore diameter is increased, a decrease in the impregnation property of the electrolytic solution can be prevented, and the productivity can be improved. When the thickness is 5 μm or less, the thickness of the porous layer can be reduced, and the battery characteristics can be improved.

The average particle size of the particles is such that a square or rectangle with the smallest area completely surrounding the particles observed by microscopic observation of the surface of the porous layer is drawn, that is, the edges of the particles are in contact with the four sides of the square or rectangle. Draw a square or rectangle, and for a square, set the length of one side, and for a rectangle, set the length of the long side (major axis diameter) as the particle size. The diameter was measured, and the average value was defined as the average particle diameter.

有機 The content of the organic resin in the porous layer A is preferably 1% by mass or more and 100% by mass or less, more preferably 5% by mass or more and 100% by mass or less in 100% by mass of the entire porous layer. More preferably, the content is 10% by mass or more and 100% by mass or less. When the content of the organic resin in the porous layer A is 1% by mass or more, sufficient adhesiveness to an electrode can be obtained.

(Inorganic particles)
The porous layer A may contain inorganic particles. When the porous layer contains inorganic particles, thermal dimensional stability and suppression of short circuit due to foreign matter can be imparted.

Specifically, the inorganic particles include inorganic oxide particles such as aluminum oxide, boehmite, silica, titanium oxide, zirconium oxide, iron oxide, and magnesium oxide; inorganic nitride particles such as aluminum nitride and silicon nitride; calcium fluoride; Examples include hardly soluble ionic crystal particles such as barium chloride and barium sulfate. One type of these particles may be used, or two or more types may be mixed and used.

無機 The average particle size of the inorganic particles used is preferably 0.05 μm or more and 5.0 μm or less. More preferably, it is 0.10 μm or more and 3.0 μm or less, and further preferably 0.20 μm or more and 1.0 μm or less. When the thickness is 0.05 μm or more, the porous layer can be prevented from becoming dense, and the air permeability can be reduced. In addition, since the pore diameter is increased, a decrease in the impregnation property of the electrolytic solution can be prevented, and the productivity can be improved. When the thickness is 5.0 μm or less, sufficient dimensional stability can be obtained, the thickness of the porous layer can be reduced, and battery characteristics can be improved.

粒子 Examples of the shape of the particles used include spherical, plate-like, needle-like, rod-like, and elliptical shapes, and any shape may be used. Among them, a spherical shape is preferable from the viewpoint of surface modification, dispersibility, and coatability.

The average particle size of the particles is such that a square or rectangle with the smallest area completely surrounding the particles observed by microscopic observation of the surface of the porous layer is drawn, that is, the edges of the particles are in contact with the four sides of the square or rectangle. Draw a square or rectangle, and measure the particle size of 100 randomly selected particles as the length of one side for a square or the length of the long side (major axis diameter) for a rectangle The average value was defined as the average particle size.

(4) When inorganic particles are contained, the content relative to the entire porous layer A is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more. When the content of the inorganic particles with respect to the entire porous layer A is 50% by mass or more, thermal dimensional stability and suppression of short-circuiting due to foreign matter can be made sufficient.

(binder)
The porous layer A may contain a binder resin in order to bind the organic resin and the inorganic particles constituting the porous layer A. As the binder resin, a resin that is insoluble in the electrolyte of the battery and that is electrochemically stable within the range of use of the battery is preferable.

For example, polyamide, polyamide imide, polyimide, polyether imide, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polysulfone, polyketone, polyetherketone, polycarbonate, polyacetal, polyvinyl alcohol, polyethylene glycol , Cellulose ether, sodium alginate, acrylic acid, acrylamide, methacrylic acid and the like. These binder resins may be used alone or as a mixture of two or more as necessary.

(Formation of porous layer A)
The porous film of the present invention is a porous film in which a porous layer A is laminated on at least one surface of a porous substrate, and is a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. A porous film having an air permeability of not more than 0.95 times that before immersion at 25 ° C. for 24 hours and not more than 1000 sec / 100 cm ³ , thereby having adhesiveness to electrodes, and Although the porous film has excellent battery characteristics, a method for producing the porous layer A will be described below.

The aqueous dispersion coating liquid is prepared by dispersing the organic resin constituting the porous layer A to a predetermined concentration. The aqueous dispersion coating liquid is prepared by dispersing, suspending, or emulsifying an organic resin in a solvent. At least water is used as a solvent of the aqueous dispersion coating liquid, and a solvent other than water may be added. The solvent other than water is not particularly limited as long as the solvent does not dissolve the organic resin and can be dispersed, suspended or emulsified in a solid state. For example, organic solvents such as methanol, ethanol, 2-propanol, acetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, dimethylformamide and the like can be mentioned. From the viewpoint of low impact on the environment, safety and economy, an aqueous emulsion obtained by emulsifying an organic resin in water or a mixture of water and alcohol is preferable.

膜 Further, a film-forming aid, a dispersant, a thickener, a stabilizer, a defoaming agent, a leveling agent, and the like may be added to the coating liquid as needed. The film-forming aid is added to adjust the film-forming property of the organic resin and improve the adhesion to the porous substrate.Specifically, propylene glycol, diethylene glycol, ethylene glycol, butyl cellosolve acetate, butyl cellosolve, Cellosolve acetate, Texanol and the like. These film-forming aids may be used alone or in combination of two or more as necessary. The addition amount of the film-forming aid is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 8% by mass or less, and still more preferably 2% by mass or more and 6% by mass based on the total amount of the coating solution. % Or less. By setting the amount to 0.1% by mass or more, a sufficient film-forming property is obtained, and by setting the amount to 10% by mass or less, when the coating solution is applied to the porous base material, Impregnation of the substrate can be prevented, and productivity can be increased. If the amount is less than 0.1% by mass, a sufficient film-forming property may not be obtained. If the amount is more than 10% by mass, when the coating solution is applied to a porous substrate, the coating solution may be porous. The substrate may be impregnated, and the productivity may be reduced.

分散 Examples of the method of dispersing the coating liquid include a ball mill, a bead mill, a sand mill, a roll mill, a homogenizer, an ultrasonic homogenizer, a high-pressure homogenizer, an ultrasonic device, and a paint shaker. Dispersion may be performed stepwise by combining these plural mixing and dispersing machines.

Next, the obtained coating solution is applied on a porous substrate, dried, and a porous layer is laminated. Examples of the coating method include dip coating, gravure coating, slit die coating, knife coating, comma coating, kiss coating, roll coating, bar coating, spray coating, dip coating, spin coating, screen printing, inkjet printing, and pad printing. , Other types of printing, etc. are available. The coating method is not limited to these, and the coating method may be selected according to preferable conditions such as an organic resin to be used, a binder, a dispersant, a leveling agent, a solvent to be used, and a substrate. Further, in order to improve the coating property, for example, the porous substrate may be subjected to a surface treatment of a coating surface such as a corona treatment or a plasma treatment.

[Porous layer B]
In the porous film of the present invention, a porous layer B containing inorganic particles may be laminated between a porous substrate and a porous layer A. For the porous layer B, the same inorganic particles, binder, and other additives as those of the porous layer A may be used. Further, the porous layer B may be one side or both sides.

無機 The content of the inorganic particles contained in the porous layer B is 50% by mass or more, preferably 60% by mass or more, and more preferably 70% by mass or more. When the amount of the inorganic particles contained in the porous layer B is 50 mass% or more, sufficient thermal dimensional stability can be obtained, and short-circuiting due to foreign matter can be suppressed.

The method for laminating the porous layer B is not particularly limited, and a method of directly applying a coating liquid containing inorganic particles, a binder resin, other additives and a solvent on a porous substrate and removing the solvent; A method in which a porous substrate is immersed in a working solution, dip coding is performed, and then the solvent is removed.

When the porous layer B is laminated between the porous substrate and the porous layer A, the porous layer A may be laminated after the porous layer B is laminated on the porous substrate. After applying the coating liquid for the porous layer B, the coating liquid for the porous layer A may be further applied and dried for lamination. The porous layer B and the porous layer A may be simultaneously coated and laminated.

膜厚 The thickness of the porous layer B is 0.5 μm or more, preferably 1 μm or more, more preferably 2 μm or more. When the thickness of the porous layer B is 0.5 μm, sufficient thermal dimensional stability can be obtained, and short-circuiting due to foreign matter can be suppressed.

The thickness of the porous layer B is determined by observing the cross section with a microscope, and in the observation region, the vertical direction from the interface between the porous substrate and the porous layer B to the interface between the porous layer A and the porous layer B. Measured as distance.

[Porous substrate]
In the present invention, the porous substrate is a substrate having pores therein. In the present invention, examples of the porous substrate include a porous membrane having pores therein, a nonwoven fabric, and a porous membrane sheet made of a fibrous material. The material constituting the porous substrate is preferably made of a resin that is electrically insulating, electrically stable, and stable to an electrolyte. From the viewpoint of providing a shutdown function, the resin used is preferably a thermoplastic resin having a melting point of 200 ° C. or less. Here, the shutdown function is a function in which when the lithium-ion battery generates abnormal heat, the porous structure is closed by melting with heat, ion transfer is stopped, and power generation is stopped.

As the thermoplastic resin, for example, a polyolefin-based resin is exemplified, and the porous substrate is preferably a polyolefin-based porous substrate. Further, the polyolefin-based porous substrate is more preferably a polyolefin-based porous substrate having a melting point of 200 ° C. or less. Specific examples of the polyolefin-based resin include polyethylene, polypropylene, a copolymer thereof, and a mixture thereof. For example, a single-layer porous substrate containing 90% by mass or more of polyethylene, polyethylene and polypropylene And a multi-layered porous substrate composed of

As a method for producing a porous base material, a method of forming a sheet by forming a polyolefin-based resin and then stretching the sheet, or dissolving the polyolefin-based resin in a solvent such as liquid paraffin to form a sheet and extracting the solvent is used. And a method of making it porous.

厚み The thickness of the porous substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and 30 μm or less. When the thickness of the porous substrate is 50 μm or less, the internal resistance of the porous substrate can be reduced. Further, when the thickness of the porous substrate is 3 μm or more, the production becomes easy and sufficient mechanical characteristics can be obtained.

The air permeability of the porous substrate is preferably 50 seconds / 100 cm ³ or more and 1,000 seconds / 100 cm ³ or less. More preferably, it is 50 seconds / 100 cm ³ or more, and 500 seconds / 100 cm ³ or less. When the air permeability is 1,000 seconds / 100 cm ³ or less, sufficient ion mobility can be obtained, and battery characteristics can be improved. If it is 50 seconds / 100 cm ³ or more, sufficient mechanical properties can be obtained.

[Secondary battery]
The porous film of the present invention can be suitably used for a separator for a secondary battery such as a lithium ion battery. Lithium ion batteries have a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode in which a positive electrode active material is laminated on a positive electrode current collector and a negative electrode in which a negative electrode active material is laminated on a negative electrode current collector. I have.

The positive electrode is obtained by laminating a positive electrode material composed of an active material, a binder resin, and a conductive additive on a current collector. Examples of the active material include LiCoO ₂ , LiNiO ₂ , and Li (NiCoMn) O ₂ . Examples thereof include a lithium-containing transition metal oxide having a layered structure, a spinel-type manganese oxide such as LiMn ₂ O ₄ , and an iron-based compound such as LiFePO ₄ . As the binder resin, a resin having high oxidation resistance may be used. Specific examples include a fluorine resin, an acrylic resin, and a styrene-butadiene resin. Carbon materials such as carbon black and graphite are used as the conductive assistant. As the current collector, a metal foil is preferable, and particularly, aluminum is often used.

The negative electrode is formed by laminating a negative electrode material composed of an active material and a binder resin on a current collector. Examples of the active material include carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, and tin and silicon. And a metal material such as Li, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like. As the binder resin, a fluorine resin, an acrylic resin, a styrene-butadiene resin, or the like is used. As the current collector, a metal foil is suitable, and in particular, a copper foil is often used.

The electrolyte serves as a place for moving ions between the positive electrode and the negative electrode in the secondary battery, and has a configuration in which the electrolyte is dissolved in an organic solvent. As the _electrolyte, LiPF 6, LiBF _4, and the like LiClO ₄ and the like, solubility in organic solvents, LiPF ₆ is preferably used in view of ion conductivity. Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like. Two or more of these organic solvents may be used in combination.

As a method for manufacturing a secondary battery, first, an active material and a conductive auxiliary are dispersed in a binder solution to prepare a coating solution for an electrode, and the coating solution is applied on a current collector, and the solvent is dried. Thus, a positive electrode and a negative electrode are obtained. It is preferable that the thickness of the coating film after drying is 50 μm or more and 500 μm or less. A secondary battery separator is arranged between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, sealed in a packaging material such as an aluminum laminate film, and injected with an electrolytic solution. Install a safety valve and seal the exterior material. The secondary battery thus obtained has high adhesiveness to the electrode, has excellent battery characteristics, and can be manufactured at low cost.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto. The measuring method used in this example is shown below.

[Measuring method]
(1) One sample was randomly selected from three samples each having an initial air permeability of 100 mm × 100 mm, and was subjected to an Oken type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.). It was measured according to JIS P 8117 (2009) and the average value was defined as the air permeability (sec / 100 cm ³ ).

(2) Permeability change rate after solvent immersion Three samples of 100 mm × 100 mm size were immersed in 2 g of a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours. Thereafter, the sample was taken out and dried, and then one randomly extracted portion of each sample was selected, and the sample was selected using an Oken type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.). measured in accordance with JIS P 8117 (2009), and the average value was air permeability and (seconds / 100cm ^3). Using the obtained air permeability and the air permeability after the heat treatment, the air permeability change rate after immersion in the solvent was calculated from the following equation.
Permeability change rate after immersion in solvent = air permeability after immersion in solvent / initial air permeability (3) Mean square roughness (Rq)
The surface roughness of the porous layer A was measured using AFM (Dimension icon (manufactured since 2006)) manufactured by Bruker AXS Corporation. The measurement mode was ScanAsyst, and the measurement was performed using a 5.0 μm × 5.0 μm range as one field of view to measure the root mean square roughness (Rq ₀ ). Measurements were taken at 10 locations each randomly extracted from a 100 mm × 100 mm size sample, and the average value was adopted.

Next, the porous film was cut into 5 cm × 5 cm, and immersed in a 1 g solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25 ° C. for 24 hours. Thereafter, the porous film was taken out of the solvent, and the solvent was dried to obtain a porous film immersed in the solvent.

Thereafter, the measurement mode of the AFM was set to ScanAsyst, and the measurement was performed with the field of view of 5.0 μm × 5.0 μm as one field of view, and the root mean square roughness (Rq ₁ ) of the obtained porous layer A after immersion in the solvent was measured. Measurements were made at arbitrary 10 locations from a 100 mm × 100 mm size sample, and the average value was adopted. From the above Rq ₀ and Rq ₁ , the root-mean-square roughness (Rq) change rate = {1− (Rq ₁ / Rq ₀ )} × 100 (%) was calculated.

(4) Surface Porosity of Porous Layer A The surface of the porous film was subjected to ion coating using an ion coater to prepare a sample. The obtained sample was observed at an imaging voltage of 20,000 times with an accelerating voltage of 1.5 kV using a field emission scanning electron microscope (FE-SEM) S4800 manufactured by Hitachi High-Technologies Corporation to obtain image data (scale (Image of only the observation part without display of bar etc.) An image was cut out from the obtained image data so that only the porous layer remained, and HALCON Ver. Image analysis was performed using 10.0 to calculate the surface porosity. As an image analysis method, first, an 11-pixel average image A and a 3-pixel average image B were generated for a 256-tone monochrome image, and the area (Area_all) of the entire image B was calculated.

(4) Next, the image A was removed from the image B as a difference, an image C was generated, and a region D where luminance ≧ 10 was extracted. The extracted region D was divided into blocks, and a region E where the area ≧ 100 was extracted. For the area E, an area F is generated by performing a closing process using a circular element having a radius of 2.5 pixels, and an area G is obtained by performing an opening process using a rectangular element having a width of 1 × 5 pixels. Was removed. The fibril region was extracted by dividing the region G into blocks and extracting the region H where the area ≧ 500.

{Circle around (5)} In the image C, the region I where the image ≧ 5 was extracted, the region I was divided into blocks, and the region J where the area ≧ 300 was extracted. After opening the area J with a circular element having a radius of 1.5 pixels, an area K is generated by closing the area J with a circular element having a radius of 8.5 pixels. L was extracted. In the region L, an unopened region other than fibrils was extracted by generating a region M in which a dark portion having an area of ≧ 4,000 pixels was filled with a bright portion.

(4) Finally, the area of the unopened portion was obtained by generating the sum area N of the area H and the area M and calculating the area (Area_closed) of the sum area N. The surface porosity was calculated according to the following equation.

Surface porosity (%) = [(Area_all−Area_closed) / Area_all] × 100
By the above-mentioned method, it measured about each 10 places extracted at random in the same porous film, and made the average value into the surface porosity (%) of the sample.

(5) A cross section of the sample was cut out with a microtome of the thickness of the porous layer A, and the cross section was observed with an electrolytic emission scanning electron microscope (S-800, manufactured by Hitachi, Ltd., acceleration voltage 26 kV). The thickness was measured at the highest point from the interface with the substrate, only one side was measured for one side, and both sides were measured for both sides, and the total was defined as the thickness of the porous layer A. Five locations randomly extracted from a 100 mm × 100 mm size sample were measured and averaged.

(6) A positive electrode made of Li (Ni _5/10 Mn _2/10 Co _3/10 ) O ₂ , a binder of vinylidene fluoride resin, and a conductive assistant of acetylene black and graphite of 15 mm × 100 mm. The porous film is placed so that the active material and the porous layer are in contact with each other, hot-pressed with a hot roll press at 0.5 MPa, 80 ° C., 0.2 m / min, and manually peeled off using tweezers. The adhesive strength was evaluated in the following four stages. Similarly, the adhesive strength between the negative electrode and the porous film, which were made of graphite as the active material, vinylidene fluoride resin as the binder, and carbon black as the conductive assistant, was also measured, and each of the positive electrode and the negative electrode was evaluated.
-Adhesive strength A: The electrode and the porous film side peeled off with a strong force.-Adhesive strength B: The electrode and the porous film peeled off with a relatively strong force.-Adhesive strength C: The electrode and the porous film peeled off with a weak force. Adhesive strength D: The electrode and the porous film were peeled off by an extremely weak force.

(7) The battery-producing positive electrode sheet contains 92 parts by mass of Li (Ni _5/10 Mn _2/10 Co _3/10 ) O ₂ as a positive electrode active material and 2.5 parts by mass of acetylene black and graphite as positive electrode conduction aids. Each was prepared by applying, drying and rolling a positive electrode slurry obtained by dispersing 3 parts by mass of polyvinylidene fluoride as a positive electrode binder in N-methyl-2-pyrrolidone using a planetary mixer on an aluminum foil. (Coating weight: 9.5 mg / cm ² ).

This positive electrode sheet was cut into 40 mm × 40 mm. At this time, the current-collecting tab bonding portion without the active material layer was cut out so as to have a size of 5 mm × 5 mm outside the active material surface. An aluminum tab having a width of 5 mm and a thickness of 0.1 mm was ultrasonically welded to the tab bonding portion.

In the negative electrode sheet, 98 parts by weight of natural graphite as a negative electrode active material, 1 part by weight of carboxymethyl cellulose as a thickener, and 1 part by weight of a styrene-butadiene copolymer as a negative electrode binder were dispersed in water using a planetary mixer. The prepared negative electrode slurry was applied onto a copper foil, dried, and rolled to produce a coating (applied weight: 5.5 mg / cm ² ).

This negative electrode sheet was cut into 45 mm × 45 mm. At this time, the current-collecting tab bonding portion without the active material layer was cut out so as to have a size of 5 mm × 5 mm outside the active material surface. A copper tab of the same size as the positive electrode tab was ultrasonically welded to the tab bonding portion.

Next, the porous film was cut into 55 mm × 55 mm, and the positive electrode and the negative electrode were overlapped on both surfaces of the porous film so that the active material layer separated the porous film, so that all the positive electrode application portions faced the negative electrode application portion. It was arranged to obtain an electrode group. The positive electrode, the negative electrode, and the porous film were sandwiched between a single 90 mm × 200 mm aluminum laminated film, the long sides of the aluminum laminated film were folded, and the two long sides of the aluminum laminated film were heat-sealed to form a bag.

An electrolytic solution prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: diethyl carbonate = 1: 1 (volume ratio) to a concentration of 1 mol / liter was used. 1.5 g of the electrolytic solution was poured into the bag-shaped aluminum laminated film, and the short side of the aluminum laminated film was heat-sealed while impregnating under reduced pressure to obtain a laminated battery.

(8) Discharge load characteristics The discharge load characteristics were tested according to the following procedure, and evaluated by the discharge capacity retention ratio.

Using the above laminated battery, the discharge capacity when discharged at 0.5 C and the discharge capacity when discharged at 10 C at 25 ° C. were measured, and (discharge capacity at 10 C) / (at 0.5 C) The discharge capacity retention rate was calculated by (discharge capacity) × 100. Here, the charging condition was a constant current charge of 0.5 C and 4.3 V, and the discharge condition was a constant current discharge of 2.7 V. Five laminated batteries were manufactured, and the average of three measurement results excluding the results in which the discharge capacity retention ratio was maximum and minimum was determined as the capacity retention ratio. D indicates that the discharge capacity retention ratio was less than 50%, C indicates 50% or more and less than 55%, B indicates 55% or more and less than 60%, and A indicates 60% or more.

(9) Charge / discharge cycle characteristics The charge / discharge cycle characteristics were tested according to the following procedure, and evaluated by the discharge capacity retention ratio.
<1st to 300th cycles>
The charge and discharge were defined as one cycle, the charge conditions were 2C and a constant current charge of 4.3V, and the discharge conditions were a constant current discharge of 2C and 2.7V and charge and discharge were repeated 300 times at 25 ° C.
<Calculation of discharge capacity retention ratio>
The discharge capacity retention ratio was calculated by (discharge capacity at the 300th cycle) / (discharge capacity at the first cycle) × 100. Five laminated batteries were manufactured, and the average of three measurement results excluding the results in which the discharge capacity retention ratio was maximum and minimum was defined as the capacity retention ratio. D indicates that the discharge capacity retention ratio was less than 50%, C indicates 50% or more and less than 60%, B indicates 60% or more and less than 70%, and A indicates 70% or more.

(Example 1)
Aqueous emulsion coating liquid in which film-forming particles whose main components are methacrylic acid / acrylate, the minimum film forming temperature is 5 ° C., the glass transition temperature is 20 ° C., and the average particle size is 0.15 μm are dispersed by emulsion polymerization. Was adjusted. This coating solution was coated on both sides of a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and contained in a hot air oven (drying set temperature: 50 ° C.). The solvent was dried until the solvent volatilized to form a porous layer A, and a porous film of the present invention was obtained. Table 1 shows the production conditions of the porous film. About the obtained porous film, the initial air permeability, the rate of change in air permeability after immersion in a solvent (solvent: diethyl carbonate), the root-mean-square roughness (Rq ₁ ) after immersion in a solvent (solvent: diethyl carbonate), the root-mean-square Table 2 shows the measurement results of the roughness (Rq) change rate, the thickness of the porous layer A, the surface porosity of the porous layer A, the adhesion to the electrode, the discharge load characteristics, and the cycle characteristics.

(Example 2)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film forming particles having a minimum film forming temperature of 80 ° C and a glass transition temperature of 100 ° C were used.

(Example 3)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film forming particles having a minimum film forming temperature of 30 ° C and a glass transition temperature of 40 ° C were used.

(Example 4)
A coating liquid B was prepared by dispersing 95% by mass of alumina particles (average particle diameter: 0.4 μm) as inorganic particles and 5% by mass of an acrylic resin as a binder in water. This coating solution is applied to one side of a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and contained in a hot air oven (dry setting temperature: 50 ° C.). It dried until the solvent volatilized, and the porous layer B was formed. Thereafter, the coating solution prepared in Example 1 is applied onto the porous layer B and the side of the polyethylene porous substrate on which the porous layer B is not formed, and is applied in a hot air oven (setting temperature for drying: 50 ° C.). And dried until the contained solvent was volatilized to form a porous layer A to obtain a porous film of the present invention.

(Example 5)
A porous film of the present invention was obtained in the same manner as in Example 1, except that dimethyl carbonate was used as a solvent for immersion.

(Example 6)
A porous film of the present invention was obtained in the same manner as in Example 1, except that ethyl methyl carbonate was used as a solvent for immersion.

(Example 7)
A porous film of the present invention was obtained in the same manner as in Example 1, except that a solvent to be immersed was a mixed solution obtained by the following production method.

Preparation method of immersion solvent used: 1.0 mol of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in 1 kg of a mixed solvent of ethylene carbonate and diethyl carbonate of the present invention having a volume ratio of 1: 1 to prepare a mixed solution. did.

(Example 8)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film forming particles having a minimum film forming temperature of 40 ° C and a glass transition temperature of 50 ° C were used.

(Example 9)
A porous film of the present invention was obtained in the same manner as in Example 1 except that film forming particles having a minimum film forming temperature of 45 ° C and a glass transition temperature of 70 ° C were used.

(Example 10)
A porous film of the present invention was obtained in the same manner as in Example 1 except that film forming particles having a minimum film forming temperature of 60 ° C and a glass transition temperature of 85 ° C were used.

(Example 11)
A porous film of the present invention was obtained in the same manner as in Example 1, except that film forming particles having a minimum film forming temperature of 15 ° C and a glass transition temperature of 18 ° C were used.

(Example 12)
A porous film of the present invention was obtained in the same manner as in Example 8, except that the coating was performed so that the thickness of the porous layer A was 5.0 μm.

(Example 13)
A porous film of the present invention was obtained in the same manner as in Example 8, except that the coating was performed so that the thickness of the porous layer A was 2.0 μm.

(Example 14)
A porous film of the present invention was obtained in the same manner as in Example 8, except that the coating was performed so that the thickness of the porous layer A was 0.05 μm.

(Example 15)
A coating liquid B was prepared by dispersing 95% by mass of alumina particles (average particle diameter: 0.4 μm) as inorganic particles and 5% by mass of an acrylic resin as a binder in water. This coating liquid is applied to one side of a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and contained in a hot-air oven (dry setting temperature: 50 ° C.). The solvent was dried until the solvent volatilized to form a porous layer B. Thereafter, the coating liquid prepared in Example 8 is applied to the porous layer B and the side of the polyethylene porous substrate on which the porous layer B is not formed, and is applied in a hot air oven (setting temperature for drying: 50 ° C.). And dried until the contained solvent was volatilized to form a porous layer A to obtain a porous film of the present invention.

(Example 16)
A porous film of the present invention was obtained in the same manner as in Example 8, except that dimethyl carbonate was used as the solvent for immersion.

(Example 17)
A porous film of the present invention was obtained in the same manner as in Example 8, except that the solvent to be immersed was ethyl methyl carbonate.

(Example 18)
A porous film of the present invention was obtained in the same manner as in Example 8, except that the solvent to be immersed was a mixed solution obtained by the following production method.

(Example 19)
A porous film of the present invention was obtained in the same manner as in Example 15 except that barium sulfate (average particle size: 0.3 μm) was used as the inorganic particles.

(Example 20)
A porous film of the present invention was obtained in the same manner as in Example 15 except that boehmite (average particle size: 0.4 μm) was used as the inorganic particles.

(Example 21)
Film-forming particles having a minimum film-forming temperature of 40 ° C and a glass transition temperature of 50 ° C, and film-forming particles having a minimum film-forming temperature of 60 ° C and a glass transition temperature of 85 ° C and a film-forming temperature of a minimum of 80 ° C and glass transition A porous film of the present invention was obtained in the same manner as in Example 1, except that an aqueous emulsion coating liquid in which film-forming particles having a temperature of 75 ° C were dispersed at a mass ratio of 30/45/25 was used. Was.

(Example 22)
A coating liquid B was prepared by dispersing 95% by mass of alumina particles (average particle diameter: 0.4 μm) as inorganic particles and 5% by mass of an acrylic resin as a binder in water. This coating liquid is applied to one side of a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and contained in a hot-air oven (dry setting temperature: 50 ° C.). The solvent was dried until the solvent volatilized to form a porous layer B. Thereafter, the coating solution prepared in Example 21 was applied on the porous layer B and on the side of the polyethylene porous substrate on which the porous layer B was not formed, and was placed in a hot-air oven (setting temperature for drying: 50 ° C.). And dried until the contained solvent was volatilized to form a porous layer A to obtain a porous film of the present invention.

(Comparative Example 1)
The minimum film forming temperature is 35 ° C., the glass transition temperature is 45 ° C., except that dimethyl carbonate, ethyl methyl carbonate, and a solvent composed of at least one of diethyl carbonate and having high swelling properties with respect to a solvent formed of a solvent composed of at least one of diethyl carbonate. In the same manner as in Example 1, a porous film was obtained.

(Comparative Example 2)
The coating liquid B used in Example 4 was coated on a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and placed in a hot-air oven (dry setting temperature: 50 ° C.). Then, the mixture was dried until the contained solvent was volatilized to form a porous layer B, thereby obtaining a porous film.

(Comparative Example 3)
The same procedure as in Example 1 was performed except that the coating was performed so that the film thickness of the porous layer A was 2.0 μm using film forming particles having a minimum film forming temperature of 20 ° C. and a glass transition temperature of 30 ° C. Thus, a porous film of the present invention was obtained.

From Table 2, Examples 1 to 22 are all porous films in which a porous layer A having an adhesive property to an electrode is laminated on at least one surface of a porous substrate, and dimethyl carbonate, ethyl A porous film having an air permeability of not more than 0.95 times that before immersion in a solvent composed of at least one of methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours and not more than 1000 sec / 100 cm ³ , Therefore, sufficient electrode adhesion and good battery characteristics were obtained.

On the other hand, in Comparative Example 1, the rate of change in air permeability after immersion in the solvent was increased, and it is assumed that the porous film contained diethyl carbonate and swelled. Therefore, the permeability of the porous film deteriorated, and good battery characteristics could not be obtained. In Comparative Example 2, although the battery characteristics were good, sufficient adhesiveness to the electrode was not obtained because no adhesive component was contained. In Comparative Example 3, air permeability after immersion in the solvent was high, and good battery characteristics could not be obtained.

Claims

A porous film in which a porous layer A having an adhesive property to an electrode is laminated on at least one surface of a porous substrate, and a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate A porous film having an air permeability of not more than 0.95 times that before immersion at 25 ° C. for 24 hours and not more than 1000 sec / 100 cm 3 .
A porous film, wherein the porous layer A has a rate of change in root mean square (Rq) before and after immersion in the solvent at 25 ° C. for 24 hours of 10% or more and 90% or less.
The porous film according to claim 1 or 2, wherein a root-mean-square roughness (Rq 1 ) after immersion in the solvent at 25 ° C for 24 hours is 20 nm or more and 80 nm or less.
The porous film according to any one of claims 1 to 3, wherein the surface porosity of the porous layer A is 50% or less.
The porous film according to any one of claims 1 to 4, wherein the porous layer A contains at least one resin selected from the group consisting of an acrylic resin, a styrene resin, a fluororesin, and an olefin resin.
The porous film according to any one of claims 1 to 5, wherein the thickness of the porous layer A is 0.05 µm or more and 5 µm or less.
The porous film according to any one of claims 1 to 6, wherein the porous layer A contains inorganic particles.
The porous film according to any one of claims 1 to 7, wherein a porous layer (B) containing inorganic particles is laminated between the porous substrate and the porous layer (A).
The porous film according to any one of claims 1 to 8, wherein the porous layer (A) is laminated on both surfaces of the porous substrate.
A separator for a secondary battery, comprising the porous film according to claim 1.
A secondary battery using the secondary battery separator according to claim 10.