JP2020001249A - Porous film, separator for secondary battery and secondary battery - Google Patents

Abstract

To provide a porous film high in adhesiveness with an electrode, having excellent battery, used for a separator for secondary battery.SOLUTION: There is provided a porous film by laminating a porous layer A on at least a single surface of a porous substrate, in which (1) gas permeability is 50 to 1000 sec/100 cm, (2) gas permeability after a heat treatment at 60°C for 30 min. is 1.05 times or more of that before the heat treatment, and (3) gas permeability after impregnation into a solvent constituted by at least one kind of dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate at 25°C for 24 hr. after the heat treatment of the (2) is conducted is 0.95 times or less of that before impregnation.SELECTED DRAWING: None

Description

  The present invention relates to a porous film, a separator for a secondary battery, and a secondary battery.

  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.

  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.

  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 attempts to achieve both ion permeability and adhesiveness with 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. Also, 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 the 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

  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.

  In view of the above problems, an object of the present invention is to provide a porous film having adhesiveness to an electrode and having excellent battery characteristics.

  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 prior arts such as Patent Documents 1 to 3, the adhesive layer has an adhesive property to the electrode active material by providing the adhesive layer. 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 is laminated on at least one surface of a porous substrate, and which satisfies the following (1) to (3).
(1) air permeability of the porous film is 50 sec / 100 cm ³ or more and 1000 sec / 100 cm ³ or less;
(2) The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment;
(3) After immersing the porous film subjected to the heat treatment of the above (2) in a solvent composed of at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and propylene carbonate at 25 ° C. for 24 hours. Air permeability is 0.95 times or less before immersion.

[2] The porous film according to [1], wherein the porous film after immersion in the solvent of the above (3) has an air permeability of 1000 sec / 100 cm ³ or less.

  [3] The porous film according to [1] or [2], 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. .

  [4] The porous film according to any one of [1] to [3], wherein the thickness of the porous layer A is 0.05 μm or more and 5 μm or less.

  [5] The porous film according to any one of [1] to [4], wherein the porous layer A contains inorganic particles.

  [6] The porous film according to any one of [1] to [5], wherein a porous layer B containing inorganic particles is laminated between the porous substrate and the porous layer A.

  [7] The porous film according to any one of [1] to [6], wherein the porous layer A is laminated on both surfaces of the porous substrate.

  [8] A separator for a secondary battery using the porous film according to any one of [1] to [7].

  [9] A secondary battery using the secondary battery separator according to [8].

According to the present invention, a porous film in which a porous layer A is laminated on at least one surface of a porous substrate, and a porous film satisfying the following (1) to (3) is obtained, By forming a porous film having the above-mentioned adhesiveness and excellent battery characteristics, it is possible to provide a porous film having an adhesiveness to an electrode and having excellent battery characteristics. By using the porous film of the present invention, a secondary battery having high productivity, high capacity, high output, and long life can be provided.
(1) air permeability of the porous film is 50 sec / 100 cm ³ or more and 1000 sec / 100 cm ³ or less;
(2) The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment;
(3) The air permeability after immersing the porous film subjected to the heat treatment of the above (2) in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours before the immersion 0.95 times or less.

A porous film in which a porous layer A is laminated on at least one surface of a porous base material, and having a porous film satisfying the following (1) to (3), has an adhesive property to an electrode. By forming a porous film having excellent battery characteristics, the porous film has an adhesive property to an electrode and has excellent battery characteristics.
(1). The air permeability of the porous film is 50 sec / 100 cm ³ or more, 1000 sec / 100 cm ³ or less,
(2). The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is 1.05 times or more the air permeability before heat treatment,
(3). The air permeability after immersing the porous film subjected to the heat treatment of (2) in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours is 0.1% before immersion. 95 times 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 50 sec / 100 cm ³ or more and 1000 sec / 100 cm ³ or less. Preferably of 80 sec / 100 cm ³ or more 800 sec / 100 cm ³ or less, more preferably 100 sec / 100 cm ³ or more 500 sec / 100 cm ^3, more preferably not more than 100 sec / 100 cm ³ or more 300 sec / 100 cm ^3. When the air permeability is 50 sec / 100 cm ³ or more, sufficient mechanical properties can be obtained. When the time is 1000 sec / 100 cm ³ or less, it is possible to prevent the impregnating property of the electrolytic solution from being reduced. If the air permeability is less than 50 sec / 100 cm ³ , sufficient mechanical properties may not be obtained. On the other hand, if it is more than 1000 sec / 100 cm ³ , the impregnation of the electrolytic solution may decrease.

  The air permeability after heat treatment of the porous film at 60 ° C. for 30 minutes is 1.05 times or more the air permeability before heat treatment. It is preferably 1.1 times or more, more preferably 1.2 times or more, further preferably 1.5 times or more, and most preferably 2.0 times or more. The increase in the air permeability due to the heat treatment indicates that the resin contained in the porous layer A is softened by heating and the film formation is promoted, and after the porous film is heat-treated at 60 ° C. for 30 minutes. When the air permeability is at least 1.05 times the air permeability before the heat treatment, the adhesiveness to the electrode is sufficient. If the ratio is less than 1.05 times, the adhesion to the electrode may not be sufficient.

Air permeability after immersing the heat-treated porous film in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours is 0.95 times or less that before immersion. is there. Preferably it is 0.9 times or less, more preferably 0.7 times or less, further preferably 0.5 times or less. When it is 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 is easily caused, sufficient ion mobility can be obtained, and battery characteristics can be improved. Can be improved. If it is more than 0.95 times, porosity due to immersion in the solvent is insufficient, sufficient ion mobility cannot be obtained, and battery characteristics may be deteriorated. Further, the air permeability of the porous film after being immersed in the above solvent is preferably 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 battery characteristics can be improved. When it is larger than 1000 sec / 100 cm ³ , sufficient ion mobility cannot be obtained, and battery characteristics may be deteriorated.

[Porous layer A]
(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 that forms a film of the organic resin itself by heat treatment. The term "film-forming resin" as used herein means a resin having a minimum film-forming temperature of 20 ° C to 100 ° C and a glass transition temperature of 15 ° C to 100 ° 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.

  It is preferable that the resin has a minimum film forming temperature of 15 ° C. or more and 100 ° C. or less and a glass transition temperature of 15 ° C. or more and 100 ° C. or less, since high adhesiveness to an electrode can be obtained. The step of bonding the electrode and the porous film is often performed by a hot pressing step, in which case the minimum film forming temperature is 15 ° C or higher and 100 ° C or lower, and the glass transition temperature is 15 ° C or higher and 100 ° C or lower. Part of the porous layer enters the gap between the active materials of the electrode by resin, heat, or press, and it is preferable because an anchor effect is exhibited to enable adhesion to the electrode. In the case of a resin having a minimum film forming temperature at a temperature higher than 100 ° C., sufficient adhesiveness to an electrode may not be obtained.

  As the organic resin constituting the porous layer of the present invention, acrylic resin, olefin resin such as polyethylene and polypropylene, styrene resin, cross-linked polystyrene, methyl methacrylate-styrene copolymer, polyimide, fluorine resin, melamine resin, phenol resin, Examples thereof include polyacrylonitrile, silicone resin, urethane resin, polycarbonate, carboxymethyl cellulose resin, and the like. One of these may be used alone, or a plurality of them 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 A may be used alone or as a mixture of two or more as necessary.

  The shape of the organic resin constituting the porous layer of the present invention is not particularly limited, but is preferably in the form of particles from the viewpoint of improving battery performance by making 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 a dense structure, and the battery characteristics can be improved. Further, when the thickness is 5 μm or less, the thickness of the entire porous film can be prevented from being increased, and the battery characteristics can be improved. If the average particle size is smaller than 0.01 μm, the porous structure may be dense and the battery characteristics may be reduced. On the other hand, when it is larger than 5 μm, the thickness of the entire porous film becomes large, and the battery characteristics may be reduced.

  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.

(Inorganic particles)
The porous layer A of the present invention 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.

  The average particle diameter 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. If it is smaller than 0.05 μm, the porous layer may be dense and the air permeability may be high. Further, since the pore diameter becomes small, the impregnation property of the electrolytic solution is reduced, which may affect the productivity. If it is larger than 5.0 μm, sufficient dimensional stability may not be obtained, and the thickness of the porous layer may increase, resulting in a decrease in battery characteristics.

  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.

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

  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 further 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 circuit due to foreign matter are sufficient.

(binder)
The porous layer A of the present invention 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, polyamideimide, polyimide, polyetherimide, 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 the porous film satisfies the following (1) to (3). By forming a porous film having an adhesive property with an electrode and having excellent battery characteristics, it is possible to obtain a porous film having an adhesive property with an electrode and having excellent battery properties by a method for producing the porous film. The method will be described below.
(1) air permeability of the porous film is 50 sec / 100 cm ³ or more and 1000 sec / 100 cm ³ or less;
(2) The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment;
(3) The air permeability after immersing the porous film subjected to the heat treatment of the above (2) in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours before the immersion 0.95 times or less.

  The organic resin constituting the porous layer A is dispersed at a predetermined concentration to prepare an aqueous dispersion coating liquid. 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, and dimethylformamide are exemplified. From the viewpoint of low environmental load, safety and economical viewpoint, an aqueous emulsion obtained by emulsifying an organic resin in water or a mixture of water and alcohol is preferable.

  Further, a binder, a film-forming auxiliary, 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 further preferably 2% by mass or more and 6% by mass based on the total amount of the coating solution. % Or less. When the coating liquid is applied to the porous base material, sufficient coating property can be obtained by setting the coating liquid to 0.1 mass or more, and when the coating liquid is applied to the porous substrate, the coating liquid becomes porous when the coating liquid is set to 10 mass% or less. Impregnation of the porous substrate can be prevented, and the productivity can be improved. When the amount is less than 0.1% by mass, sufficient film-forming properties may not be obtained. When the amount is more than 10% by mass, the coating liquid is The substrate may be impregnated and productivity may be reduced.

  As a method of dispersing the coating liquid, a known method may be used. Examples 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 liquid is applied on a porous substrate, dried, and a porous layer is laminated. As a coating method, a known method may be used. For example, 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, 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.

  The content of the organic resin A in the porous layer A is preferably from 1% by mass to 100% by mass, more preferably from 5% by mass to 100% by mass, based on 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 A in the porous layer A is 1% by mass or more, sufficient adhesiveness to an electrode is obtained. When the content of the organic resin A in the porous layer A is less than 1% by mass, sufficient adhesiveness to an electrode may not be obtained.

  The thickness of the porous layer A is preferably 0.05 μm or more and 5 μm or less. More preferably, it is 0.10 μm or more and 3 μm or less. More preferably, it is 0.2 μm or more and 1 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, a sufficient porous structure can be obtained, and battery characteristics can be improved. It is also advantageous in terms of cost. When the thickness of the porous layer A is smaller than 0.05 μm, sufficient adhesiveness to an electrode may not be obtained. On the other hand, if the thickness is more than 5 μm, a sufficient porous structure cannot be obtained, and battery characteristics may be deteriorated. In addition, there may be a disadvantage in cost.

[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.

  The amount 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% by mass or more, sufficient thermal dimensional stability can be obtained and short-circuiting due to foreign matter can be suppressed. If the amount of the inorganic particles contained in the porous layer B is less than 50% by mass, sufficient thermal dimensional stability and short-circuiting due to foreign matter may not be able to 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.

  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 or more, sufficient thermal dimensional stability can be obtained, and short-circuiting due to foreign matter can be suppressed. If the thickness of the porous layer B is less than 0.5 μm, sufficient thermal dimensional stability and short-circuiting due to foreign matter may not be able to 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. The distance was measured and observed and measured at each of five randomly extracted locations, and the average value was taken as the thickness of the porous B.

[Porous substrate]
In the present invention, the porous substrate refers to 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.

  Examples of the thermoplastic resin include a polyolefin-based resin, 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.

  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 more, 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 properties can be obtained. When the thickness of the porous substrate is larger than 50 μm, the internal resistance of the porous substrate may increase. If the thickness of the porous substrate is less than 3 μm, production becomes difficult, and sufficient mechanical characteristics may not be obtained.

  The thickness of the porous substrate can be measured by observing the cross section with a microscope. When the porous layer is laminated, the vertical distance between the interface between the porous substrate and the porous layer is measured as the thickness of the porous substrate. Observation and measurement were performed on each of the five randomly extracted locations, and the average value was defined as the thickness of the porous substrate.

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, when the air permeability is 50 seconds / 100 cm ³ or more and 500 seconds / 100 cm ³ or less, and the air permeability is 1,000 seconds / 100 cm ³ or less, sufficient ion mobility can be obtained and battery characteristics can be improved. it can. If it is 50 seconds / 100 cm ³ or more, sufficient mechanical properties can be obtained. If the air permeability is larger than 1,000 seconds / 100 cm ³ , sufficient ion mobility cannot be obtained, and the battery characteristics may be deteriorated.

[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 a negative electrode material composed of an active material and a binder resin laminated on a current collector. And a metal material such as Li, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). 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 described specifically 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) Initial air permeability One spot randomly extracted from each of three samples of 100 mm × 100 mm size was selected, and an Oken-type air permeability measuring device (EG01-5-1MR manufactured by Asahi Seiko Co., Ltd.) was used. It was measured according to JIS P 8117 (2009) and the average value was defined as the air permeability (sec / 100 cm ³ ).

(2) Rate of change in air permeability after heat treatment Three samples of 100 mm x 100 mm size were each heat-treated at 60 ° C for 30 minutes, and one sample was extracted at random to select one location. Using a degree measuring device (EG01-5-1MR, manufactured by Asahi Seiko Co., Ltd.), the measurement was performed in accordance with JIS P 8117 (2009), and the average value was defined as the air permeability (sec / 100 cm ³ ). Using the obtained air permeability and the initial air permeability, the air permeability change rate after the heat treatment was calculated from the following equation.
Air permeability change rate after heat treatment = air permeability rate after heat treatment / initial air permeability (3) Air permeability change rate after solvent immersion The sample after the heat treatment of the above (2) was subjected to dimethyl carbonate, ethyl methyl carbonate, The sample was immersed in a solvent composed of at least one of diethyl carbonate at 25 ° C. for 24 hours with the sample completely immersed. After that, after taking out the sample and drying, one place where each sample was extracted at random was selected, and 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.

Air permeability change rate after solvent immersion = Air permeability after solvent immersion / Air permeability after heat treatment (4) Film thickness of porous layer A A cross section of the sample is cut out with a microtome, and the cross section is subjected to electrolytic radiation scanning electron. Observed with a microscope (S-800, manufactured by Hitachi, Ltd., acceleration voltage: 26 kV), the highest point from the interface with the porous substrate was taken as the thickness. For one side, only one side, and for both sides, both sides The thickness was measured 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.

(5) Adhesiveness to Electrode The active material is Li (Ni _5/10 Mn _2/10 Co _3/10 ) O ₂ , the binder is vinylidene fluoride resin, and the conductive assistant is a positive electrode 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 ◎: The electrode and the porous film side peeled off with a strong force. ・ Adhesive strength ○: The electrode and the porous film peeled off with a relatively strong force. ・ Adhesive strength △: The electrode and the porous film peeled off with a weak force. Adhesive strength ×: The electrode and the porous film were peeled off by an extremely weak force.

(6) Battery Production The positive electrode sheet was composed of 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 out to 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.

(7) 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 defined as the capacity retention ratio. When the discharge capacity retention ratio was less than 55%, it was evaluated as x;

(8) 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. The case where the discharge capacity retention ratio was less than 60% was evaluated as x, the case of 60% or more and less than 70% as ○, and the case of 70% or more as ◎.

(Example 1)
Aqueous emulsion coating liquid in which film-forming particles whose main components are methacrylic acid / acrylate, the minimum film-forming temperature is 40 ° C., the glass transition temperature is 50 ° 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. About the obtained porous film, the initial air permeability, the rate of change in air permeability after heat treatment, the rate of change in air permeability after immersion in a solvent, the thickness of the porous layer A, the adhesion to the electrode, the discharge load characteristics and Table 1 shows the measurement results of 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 45 ° C and a glass transition temperature of 70 ° 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 60 ° C and a glass transition temperature of 85 ° C were used.

(Example 4)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the coating was performed so that the thickness of the porous layer A was 5.0 μm.

(Example 5)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the coating was performed so that the thickness of the porous layer A was 2.0 μm.

(Example 6)
A porous film of the present invention was obtained in the same manner as in Example 1, except that the coating was performed so that the thickness of the porous layer A was 0.05 μm.

(Example 7)
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 onto a polyethylene porous substrate (thickness: 7 μm, air permeability: 110 seconds / 100 cm ³ ) using a wire bar, and a solvent contained in a hot-air oven (dry setting temperature: 50 ° C.) Was dried until volatilized to form a porous layer B. Thereafter, the coating liquid prepared in Example 1 was applied onto the porous layer B, and dried in a hot air oven (drying setting temperature: 50 ° C.) until the contained solvent was volatilized. Thus, a porous film of the present invention was obtained.

(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 kind. In the same manner as in Example 1, a porous film was obtained.

(Comparative Example 2)
The coating liquid B used in Example 7 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)
A porous film was obtained in the same manner as in Example 1, except that film forming particles having a minimum film forming temperature of 5 ° C and a glass transition temperature of 10 ° C were used.

(Comparative Example 4)
A porous film 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 95 ° C were used.

From Table 1, Examples 1 to 7 are all porous films in which the porous layer A is laminated on at least one surface of the porous substrate, and satisfy the following (1) to (3). Since it is a film, sufficient electrode adhesion and good battery characteristics can be obtained.
(1) air permeability of the porous film is 50 sec / 100 cm ³ or more and 1000 sec / 100 cm ³ or less;
(2) The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment;
(3) The air permeability after immersing the porous film subjected to the heat treatment of the above (2) in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours before the immersion 0.95 times or less.

  On the other hand, in Comparative Example 1, the rate of change in air permeability after immersion in the solvent increased, and it is assumed that the porous film contained diethyl carbonate and swelled. Therefore, the permeability of the porous film deteriorates, and good battery characteristics cannot be obtained. In Comparative Example 2, the battery characteristics were good, but sufficient adhesion to the electrode was not obtained. In Comparative Example 3, since the initial air permeability is high, the air permeability after immersion in the solvent is high, and good battery characteristics cannot be obtained. In Comparative Example 4, since the rate of change in air permeability after the heat treatment was low, sufficient adhesion to the electrode could not be obtained.

Claims (9)

A porous film in which a porous layer A is laminated on at least one surface of a porous substrate, wherein the porous film satisfies the following (1) to (3).
(1) Air permeability of the porous film is 50 sec / 100 cm ³ or more, 1000 sec / 100 cm ³ or less,
(2) The air permeability of the porous film after heat treatment at 60 ° C. for 30 minutes is at least 1.05 times the air permeability before heat treatment;
(3) The air permeability after immersing the porous film subjected to the heat treatment of the above (2) in a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate at 25 ° C. for 24 hours before the immersion 0.95 times or less. The air permeability of the porous film after immersion in a solvent (3) is of 1,000 sec / 100 cm ³ or less, the porous film according to claim 1. 3. The porous film according to claim 1, 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. 4. 4. The porous film according to claim 1, wherein the thickness of the porous layer A is 0.05 μm or more and 5 μm or less. 5. The porous film according to any one of claims 1 to 4, wherein the porous layer A contains inorganic particles. The porous film according to any one of claims 1 to 5, 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 6, 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 comprising the secondary battery separator according to claim 8.