JP7327044B2 - Porous film, secondary battery separator and secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a porous film, a separator for a secondary battery, and a secondary battery which have adhesiveness to an electrode and are excellent in blocking resistance and battery characteristics.

Secondary batteries such as lithium-ion batteries are widely used in portable digital devices such as smartphones, tablets, mobile phones, laptops, digital cameras, digital video cameras, and portable game consoles, as well as in portable devices such as power tools, electric motorcycles, and power-assisted 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. have a configuration.

Polyolefin-based porous substrates are used as secondary battery separators. The characteristics required for secondary battery separators are the ability to contain electrolyte in the porous structure and enable ion movement, and the ability to close the porous structure by melting when the lithium ion battery overheats. It has a shutdown characteristic that stops the power generation by stopping the movement of ions.
Furthermore, in the manufacturing process of the secondary battery, in order to maintain the laminate when transporting the laminate in which the positive electrode, the separator, and the negative electrode are laminated, or the wound laminate of the positive electrode, the separator, and the negative electrode is cylindrically shaped, When inserting into a rectangular can, etc., the laminate is hot-pressed before insertion. is placed in a can to increase the energy density, and in the laminate type, to prevent the shape from deforming 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, there is also a demand for anti-blocking properties that prevent blocking from occurring between the films after the porous film has been wound into a roll and stored, thereby preventing the above-mentioned adhesiveness from being impaired.

In response to these demands, in Patent Document 1, a heat-resistant porous layer containing a particulate organic binder and an inorganic filler is laminated to have adhesiveness with an electrode. In Patent Document 2, two different kinds of particulate polymers are laminated to achieve both adhesion to electrodes and blocking resistance. Further, in Patent Document 3, a particulate polymer laminates an adhesive layer having a (meth)acrylic acid ester monomer unit, an ethylenically unsaturated carboxylic acid monomer unit, and a crosslinkable monomer unit. By doing so, we are trying to achieve both anti-blocking properties.

Japanese Patent No. 5647378 JP 2015-41603 A Japanese Patent No. 6191597

As described above, the adhesion between the electrode and the separator is required by the heat press process in the manufacturing process of the secondary battery. Blocking resistance is also required, and it is necessary to achieve both adhesion and battery characteristics. SUMMARY OF THE INVENTION 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 blocking resistance.

Therefore, the present inventors have made extensive studies in order to provide a porous film that has adhesiveness to electrodes and excellent blocking resistance and battery characteristics. As a result, in the prior arts such as Patent Documents 1 to 3, by providing an adhesive layer, the adhesive layer has adhesiveness to the electrode active material. The present inventors have found that filling the voids in the material or the separator reduces the porosity and the ion transport rate, thereby degrading the battery characteristics, leading to the present invention.

In order to solve the above problems, the porous film of the present invention has the following configuration.
(1) A porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate, wherein the first porous layer contains an organic resin. A porous film, wherein the glass transition temperature of the first porous layer is 40° C. or higher and 85° C. or lower, and the surface elastic modulus of the first porous layer is 1 GPa or higher and 3 GPa or lower.
(2) The porous film according to (1), wherein the first porous layer contains at least one organic resin selected from the group consisting of acrylic resins and styrene resins.
(3) The porous film according to (2), wherein the acrylic resin contains a polymer having fluorine-containing (meth)acrylate monomer units.
(4) The weight reduction of the first porous layer after immersion in a solvent composed of at least one selected from ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate at 25°C for 24 hours is 0. 0.97 times or less, the porous film according to any one of (1) to (3).
(5) The porous film according to any one of (1) to (4), wherein the porous film has an air permeability of 50 sec/100 cm ³ or more and 1,000 sec/100 cm ³ or less.
(6) The porous film according to any one of (1) to (5), wherein the thickness of the first porous layer 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 first porous layer is laminated on both sides of the porous substrate.
(8) Any one of (1) to (7), wherein a second porous layer containing inorganic particles is laminated between the porous substrate and the first porous layer. porous film.
(9) A secondary battery separator comprising the porous film according to any one of (1) to (8).
(10) A secondary battery comprising the secondary battery separator according to (9).

According to the present invention, there is provided a porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate, wherein the first porous layer is an organic By forming a porous film containing a resin, the first porous layer having a glass transition temperature of 40° C. or higher and 85° C. or lower and having a surface elastic modulus of 1 GPa or higher and 3 GPa or lower, It is possible to provide a porous film having adhesiveness to electrodes and excellent anti-blocking properties and battery properties. By using the porous film of the present invention, it is possible to provide a highly productive secondary battery separator and secondary battery.

A porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate, wherein the first porous layer contains an organic resin, and a first The porous film of (1) has a glass transition temperature of 40° C. or higher and 85° C. or lower, and a surface elastic modulus of the first porous layer of 1 GPa or higher and 3 GPa or lower.
The present invention will be described in detail below.

[First porous layer]
(organic resin)
The first porous layer in this embodiment contains an organic resin.
The organic resin in this embodiment is a film-forming resin that forms a film of the organic resin itself by heat treatment. The film-forming resin referred to here is a resin having a minimum film-forming temperature of 30° C. or higher and 70° C. or lower. Here, the minimum film-forming temperature is the lowest temperature at which a crack-free uniform film is formed when the resin emulsion is dried, for example, according to the provisions of "JIS K6828-2 Determination of whitening temperature and minimum film-forming temperature". Let temperature be
Also, the first porous layer has a glass transition temperature of 40° C. or higher and 85° C. or lower. The temperature is preferably 45°C or higher and 80°C or lower, more preferably 50°C or higher and 75°C or lower.
Furthermore, the surface elastic modulus of the first porous layer is 1 GPa or more and 3 GPa or less. It is preferably 1.2 GPa or more and 2.8 GPa or less, more preferably 1.5 GPa or more and 2.5 GPa or less.

When the glass transition temperature of the first porous layer is 40° C. or higher and 85° C. or lower, and the surface elastic modulus of the first porous layer is 1 GPa or higher and 3 GPa or lower, high adhesion to the electrode and blocking suppression effect can be obtained. be done.
A hot press step is often used for the step of adhering the electrode and the porous film. If the resin has a surface elastic modulus of 1 GPa or more and 3 GPa or less, a part of the porous layer enters the gap between the active materials of the electrode by heat or press, and exhibits an anchor effect, thereby improving adhesion with the electrode. It becomes possible. When the glass transition temperature of the first porous layer is higher than 85° C. and the surface elastic modulus of the first porous layer is higher than 3 GPa, sufficient adhesion to the electrode may not be obtained. When the glass transition temperature of the first porous layer is lower than 40°C and the surface elastic modulus of the first porous layer is lower than 1 GPa, after the porous film is wound into a roll and stored, blocking may occur. The surface elastic modulus is measured using the method described in Examples below.

Organic resins constituting the first porous layer of the present embodiment include acrylic resins, olefin resins such as polyethylene and polypropylene, styrene resins, crosslinked polystyrene, methyl methacrylate-styrene copolymers, polyimides, melamine resins, and phenol resins. , polyacrylonitrile, silicone resin, urethane resin, polycarbonate, carboxymethyl cellulose resin, etc., and these may be used alone or in combination. Among these, for example, acrylic resin, olefin resin, styrene resin, fluororesin, urethane resin, and polyacrylonitrile are preferably used, and acrylic resin and styrene resin are more preferable, from the viewpoint of adhesion to the electrode.

The acrylic resin preferably contains a polymer having a monomer unit selected from the monomer unit group A. Monomers constituting the monomer unit group A include fluorine-containing (meth)acrylate monomer units, unsaturated carboxylic acid monomer units, acrylic acid ester monomer units, and methacrylic acid ester monomer units. , styrene-based monomer units, olefin-based monomer units, diene-based monomer units, amide-based monomer units, and the like. Among these, a fluorine-containing (meth)acrylate monomer unit is particularly preferred from the viewpoint of improving battery characteristics by reducing the moisture content of the porous film.

Fluorine-containing (meth)acrylate monomers include 2,2,2-trifluoroethyl (meth)acrylate, 2,2,3,3,3-pentafluoropropyl (meth)acrylate, 2-(perfluorobutyl ) ethyl (meth) acrylate, 3-(perfluorobutyl)-2-hydroxypropyl (meth) acrylate, 2-(perfluorohexyl) ethyl (meth) acrylate, 3-perfluorohexyl-2-hydroxypropyl (meth) acrylates, 3-(perfluoro-3-methylbutyl)-2-hydroxypropyl (meth)acrylate, 1H,1H,3H-tetrafluoropropyl (meth)acrylate, 1H,1H,5H-octafluoropentyl (meth)acrylate, 1H,1H,7H-dodecafluoroheptyl (meth)acrylate, 1H-1-(trifluoromethyl)trifluoroethyl (meth)acrylate, 1H,1H,3H-hexafluorobutyl (meth)acrylate, 1,2,2 , 2-tetrafluoro-1-(trifluoromethyl)ethyl (meth)acrylate, 2-(perfluorooctyl)ethyl (meth)acrylate and the like. One fluorine-containing (meth)acrylate monomer may be used alone, or two or more may be used in combination at an arbitrary ratio.

In the present invention, "(meth)acrylate" means acrylate and/or methacrylate. A fluorine-containing (meth)acrylate monomer unit is a repeating unit obtained by polymerizing a fluorine-containing (meth)acrylate monomer.

Examples of unsaturated carboxylic acid monomer units include acrylic acid, methacrylic acid, and crotonic acid. Acrylic acid ester monomer units include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2 -ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate, cyclohexyl acrylate, hydroxyethyl acrylate, benzyl acrylate, isobornyl acrylate, dicyclopentanyl acrylate, dicyclopentenyl acrylate, etc. be done. Methacrylate ester monomer units include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, t-butylcyclohexyl methacrylate, and pentyl methacrylate. , hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, hydroxyethyl methacrylate, benzyl methacrylate, isobornyl methacrylate, dicyclo pentenyl methacrylate, dicyclopentenyl methacrylate and the like.

Or styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, chlorostyrene, chloromethylstyrene, hydroxymethylstyrene, etc. for the purpose of increasing chemical resistance to the chain carbonate that constitutes the non-aqueous electrolyte of the secondary battery. styrene monomers, olefin monomers such as ethylene and propylene, diene monomers such as butadiene and isoprene, and amide monomers such as acrylamide.

These organic resins may be used singly or in combination of two or more if necessary, and two or more kinds of particles are used to form a core-shell structure comprising a core and a shell covering the outer surface of the core. may have Moreover, the shell part may be in a state of partially covering the outer surface of the core part. Thereby, it becomes possible to give functionality to each of the core portion and the shell portion. In other words, by optimizing the core portion and the shell portion, it is possible to control the surface elastic modulus while maintaining the glass transition temperature.

Although the shape of the organic resin constituting the first porous layer of the present embodiment is not particularly limited, it is preferably in the form of particles from the viewpoint of improving battery performance by making the layer porous. The shape of the particles is not particularly limited, and may be any of spherical, polygonal, flat, fibrous, etc., and may be partially formed into a film and fused with surrounding particles and binder. include. In the present embodiment, the shape of the particles is preferably spherical from the viewpoints of surface modification properties, dispersibility, and coatability, and more preferably a shape close to a perfect sphere. When the shape of the organic resin is particles, 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 still more preferably 0.1 μm or more and 1 μm or less. If the average particle size is less than 0.01 μm, the porous structure may become dense and the battery characteristics may deteriorate. Moreover, when the average particle diameter is larger than 5 μm, the thickness of the entire porous film may be increased, and the battery characteristics may be deteriorated.

In addition, the average particle size of the particles as used herein is a value obtained by measuring using the following method. Using a field emission scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.), the surface of the porous layer was observed at a magnification of 30,000. The image size at that time is 4.0 μm×3.0 μm. The number of pixels is 1,280 pixels×1,024 pixels, and the size of one pixel is 3.1 nm×2.9 nm. Next, in one particle on the obtained image, at least three sides are in contact with each other, and a quadrangle composed of the three sides is drawn. The diameter of each particle on the image was measured, and the arithmetic average value was taken as the average particle diameter. If 50 particles were not observed in the captured image, multiple images were captured and the particles were measured so that the total of all particles contained in the multiple images was 50 or more. , and the arithmetic mean thereof is taken as the average particle size.

(Porous structure)
The first porous layer of this embodiment has a porous structure. A porous structure refers to a structure having voids within the structure.
The porous structure in the first porous layer of the present embodiment preferably has a basis weight reduction of 0.97 times or less after being immersed in a solvent described later at 25° C. for 24 hours. The basis weight reduction amount is defined as W ₁ /W ₀ (fold) where W ₀ is the initial basis weight before immersion in a solvent described later at 25° C. for 24 hours, and W ₁ is the basis weight after solvent immersion. Amount of weight reduction: W ₁ /W ₀ (times) is a numerical value indicating a structural change when immersed in a solvent described later at 25 ° C. for 24 hours, so it can represent the degree of swelling and dissolution of the first porous layer. can.
In the present embodiment, it is preferable that the weight reduction amount is 0.97 times or less. The basis weight reduction amount is more preferably 0.95 times or less, and still more preferably 0.90 times or less. When the weight loss after immersion in the solvent described later at 25° C. for 24 hours is 0.97 times or less, sufficient ion mobility can be obtained and deterioration of battery characteristics can be prevented.

Examples of the type of solvent to be immersed include ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate, which are chain carbonates that constitute the non-aqueous electrolyte of the secondary battery. One of these may be used alone, or two or more may be used in combination. Furthermore, it may be combined with a cyclic carbonate such as propylene carbonate, ethylene carbonate, or butylene carbonate. In that case, the volume ratio of chain carbonates such as ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate is preferably 20% by volume or more. More preferably 35% by volume or more, more preferably 50% by volume or more. When the volume ratio is 20% or more, both the solubility of the first porous layer and the battery characteristics can be achieved. The basis weight reduction amount is measured using the method described in Examples described later.

(Formation of first porous layer)
The porous film of the present embodiment is a porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate, wherein the first porous layer is a porous film containing an organic resin, a first porous layer having a glass transition temperature of 40° C. to 85° C., and a surface elastic modulus of 1 GPa or more and 3 GPa or less. The first porous layer is preferably laminated on both sides of the porous substrate.

A method for manufacturing the first porous layer will be described below.
A water-based dispersion coating liquid is prepared by dispersing the organic resin constituting the first porous layer at a predetermined concentration. A water-based dispersion coating liquid is prepared by dispersing, suspending, or emulsifying an organic resin in a solvent. At least water is used as a solvent for the water-based dispersion coating liquid, and a solvent other than water may be added. The solvent other than water is not particularly limited as long as it does not dissolve the organic resin and can disperse, suspend or emulsify the organic resin in a solid state. Examples include organic solvents such as methanol, ethanol, 2-propanol, acetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, N-methylpyrrolidone, dimethylacetamide, dimethylformamide and dimethylformamide. From the viewpoints of low environmental load, safety and economy, water-based emulsions in which an organic resin is emulsified in water or a mixture of water and alcohol are preferred.

Moreover, you may add a film-forming aid, a dispersing agent, a thickener, a stabilizer, an antifoaming agent, a leveling agent, etc. to a coating liquid as needed. A 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 singly or in combination of two or more if necessary. The amount of film-forming aid added 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 with respect to the total amount of the coating liquid. % or less. By making it 0.1% by mass or more, sufficient film-forming properties can be obtained, and by making it 10% by mass or less, when the coating liquid is applied to a porous substrate, the porousness of the coating liquid can be reduced. Impregnation of the base material can be prevented, and productivity can be improved. If it is less than 0.1% by mass, sufficient film-forming properties may not be obtained, and if it is more than 10% by mass, the coating liquid becomes porous when applied to a porous substrate. The base material is impregnated, and productivity may decrease.

Examples of methods for dispersing the coating liquid include ball mills, bead mills, sand mills, roll mills, homogenizers, ultrasonic homogenizers, high-pressure homogenizers, ultrasonic devices, and paint shakers. A plurality of these mixing and dispersing machines may be combined for stepwise dispersion.
Next, the obtained coating liquid is applied onto the porous substrate and dried to laminate a porous layer. Coating methods include, 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, 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 the organic resin, binder, dispersant, leveling agent, solvent to be used, and base material. In order to improve coatability, for example, the porous substrate may be subjected to surface treatment such as corona treatment or plasma treatment.

The content of the organic resin in the first porous layer 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, based on 100% by mass of the entire porous layer. . More preferably, it is 10% by mass or more and 100% by mass or less. If the content of the organic resin in the first porous layer is 1% by mass or more, sufficient adhesiveness to the electrode can be obtained.

The film thickness of the first porous layer 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. The film thickness of the first porous layer as used herein refers to the film thickness of the first porous layer in the case of a porous film having the first porous layer on one side of the porous substrate, In the case of a porous film having a first porous layer on both sides of a porous substrate, it means the total thickness of both first porous layers. When the film thickness of the first porous layer is 0.05 μm or more, sufficient adhesiveness to the electrode can be obtained. Further, 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.

[Second porous layer]
In the porous film of the present embodiment, a second porous layer containing inorganic particles may be laminated between the porous substrate and the first porous layer. The second porous layer of this embodiment has a porous structure. The second porous layer is a heat-resistant layer containing a binder resin and inorganic particles.
The second porous layer of the present embodiment preferably contains a binder resin in order to bind the fillers forming the second porous layer. As such a 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 preferred.
Examples of binder resins include polyamide, polyamideimide, polyimide, polyetherimide, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polysulfone, polyketone, polyetherketone, polycarbonate, polyacetal, Examples include resins such as polyvinyl alcohol, polyethylene glycol, cellulose ether, sodium alginate, acrylic acid, acrylamide, and methacrylic acid. These binder resins may be used singly or in combination of two or more if necessary.

The second porous layer of this embodiment may contain inorganic particles. When the porous layer contains inorganic particles, it is possible to impart thermal dimensional stability and suppress short circuits caused by foreign matter.
Specific examples of 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; Poorly soluble ionic crystal particles such as barium chloride and barium sulfate may be used. 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. It is more preferably 0.10 μm or more and 3.0 μm or less, and still more 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 lowered. In addition, since the pore diameter is increased, it is possible to prevent impregnation of the electrolytic solution from being deteriorated and improve productivity. When the thickness is 5.0 μm or less, sufficient dimensional stability can be obtained, and the film thickness of the porous layer can be reduced, thereby improving battery characteristics.

The shape of the particles used may be spherical, plate-like, needle-like, rod-like, elliptical, etc. Any shape may be used. Among them, a spherical shape is preferable from the viewpoint of surface modification properties, dispersibility, and coatability.
The average particle diameter of particles is a value obtained by measuring using the following method. Using a field emission scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.), the surface of the porous layer was observed at a magnification of 30,000. The image size at that time is 4.0 μm×3.0 μm. The number of pixels is 1,280 pixels×1,024 pixels, and the size of one pixel is 3.1 nm×2.9 nm. Next, in one particle on the obtained image, at least three sides are in contact with each other, and a quadrangle composed of the three sides is drawn. The diameter of each particle on the image was measured, and the arithmetic average value was taken as the average particle diameter. If 50 particles are not observed in the captured image, multiple images are captured, and the inorganic particles are measured so that the total of all particles contained in the multiple images is 50 or more. and the arithmetic average value thereof is taken as the average particle size.

When inorganic particles are contained, the content is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more with respect to the entire second porous layer. When the content of the inorganic particles in the entire porous layer A is 50% by mass or more, sufficient thermal dimensional stability and suppression of short circuits due to foreign matter can be obtained. If the content of the inorganic particles in the entire second porous layer is less than 50% by mass, the thermal dimensional stability and suppression of short circuits due to foreign matter may not be sufficient.

When laminating the second porous layer between the porous substrate and the first porous layer, after laminating the second porous layer on the porous substrate, the first porous layer is laminated. Alternatively, after applying the coating liquid for the second porous layer, the coating liquid for the first porous layer may be further applied and dried to laminate. Alternatively, the second porous layer and the first porous layer may be simultaneously coated on the porous base material by multilayer die coating or the like and laminated.
The film thickness of the second porous layer is preferably 0.5 μm or more, more preferably 1 μm or more, and still more preferably 2 μm or more. When the film thickness of the second porous layer is 0.5 μm or more, sufficient thermal dimensional stability can be obtained, and short circuits due to foreign matter can be suppressed. If the film thickness of the second porous layer is less than 0.5 μm, sufficient thermal dimensional stability and suppression of short circuits due to foreign matter may not be achieved.
The film thickness of the second porous layer is determined by observing the cross section with a microscope, and measuring the thickness of the first porous layer and the second porous layer from the interface between the porous base material and the second porous layer within the observation area. Measured as the vertical distance to the interface with the matrix.

[Porous substrate]
In this embodiment, the porous substrate is a substrate having pores inside. In the present embodiment, examples of the porous substrate include a porous membrane having pores inside, a nonwoven fabric, and a porous membrane sheet made of a fibrous material. The material constituting the porous substrate is preferably composed of a resin that is electrically insulating, electrically stable, and stable in the electrolytic solution. From the viewpoint of imparting a shutdown function, a thermoplastic resin having a melting point of 200° C. or less is preferable. The shutdown function here is a function that, when the lithium-ion battery overheats, closes the porous structure by melting with heat, stops the movement of ions, and stops power generation.

Examples of thermoplastic resins include polyolefin-based resins, 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 lower. Specific examples of polyolefin-based resins include polyethylene, polypropylene, copolymers thereof, and mixtures thereof. Examples include single-layer porous substrates containing 90% by mass or more of polyethylene, polyethylene and polypropylene. A multi-layered porous substrate consisting of

As a method for producing a porous base material, a method of forming a polyolefin resin into a sheet and then stretching it to make it porous, or a method of dissolving the polyolefin resin in a solvent such as liquid paraffin to form a sheet and then extracting the solvent. 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 less, the internal resistance of the porous substrate can be lowered. In addition, by setting the thickness of the porous substrate to 3 μm or more, the production is facilitated and sufficient mechanical properties are obtained.
The air permeability of the porous substrate is preferably 50 sec/100 cm ³ or more and 1,000 sec/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. Sufficient mechanical properties are obtained when it is 50 seconds/100 cm ³ or more.

[Porous film]
The porous film of this embodiment is a porous film having the above-described porous layer on at least one side of a porous substrate. The porous layer is preferably sufficiently porous to have ion permeability, and the air permeability of the porous film is preferably 50 sec/100 cm ³ or more and 1,000 sec/100 cm ³ or less. preferable. More preferably, it is 50 seconds/100 cm ³ or more and 500 seconds/100 cm ³ or less. More preferably, it is 50 seconds/100 cm ³ or more and 300 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. Sufficient mechanical properties are obtained when it is 50 seconds/100 cm ³ or more.

[Secondary battery]
The porous film of the present embodiment can be suitably used as a separator for secondary batteries such as lithium ion batteries. A lithium-ion battery has 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. there is

The positive electrode is obtained by laminating a positive electrode material _composed of an active material, _a binder resin _, and a conductive aid on a current collector. Lithium-containing transition metal oxides having a layered structure, spinel-type manganese oxides such as LiMn ₂ O ₄ , and iron-based compounds such as LiFePO ₄ can be used. A resin having high oxidation resistance may be used as the binder resin. Specific examples include fluorine resins, acrylic resins, styrene-butadiene resins, and the like. Carbon materials such as carbon black and graphite are used as conductive aids. As the current collector, a metal foil is suitable, and aluminum is often used in particular.
The negative electrode is obtained by laminating a negative electrode material composed of an active material and a binder resin on a current collector, and the active material includes carbon materials such as artificial graphite, natural graphite, hard carbon and soft carbon, tin and silicon. , metal materials such as Li, and lithium titanate (Li ₄ Ti ₅ O ₁₂ ). A fluorine resin, an acrylic resin, a styrene-butadiene resin, or the like is used as the binder resin. As the current collector, metal foil is suitable, and copper foil is often used in particular.

The electrolytic solution serves as a field for transferring ions between the positive electrode and the negative electrode in the secondary battery, and is made by dissolving the electrolyte in an organic solvent. Examples of electrolytes include LiPF ₆ , LiBF ₄ , and LiClO ₄ , and LiPF ₆ is preferably used from the viewpoint of solubility in organic solvents and ion conductivity. Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, etc. Two or more of these organic solvents may be used in combination.

As a method for producing a secondary battery, first, an active material and a conductive agent are dispersed in a binder solution to prepare a coating liquid for an electrode. A positive electrode and a negative electrode are obtained by drying. The film thickness of the coating film after drying is preferably 50 μm or more and 500 μm or less. A secondary battery separator is placed between the obtained positive electrode and negative electrode so as to be in contact with the active material layer of each electrode, enclosed in an outer packaging material such as an aluminum laminate film, and after injecting an electrolytic solution, the negative electrode lead and the 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.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited by these examples. The measurement methods used in this example are shown below.
Incidentally, Examples 1 to 11 shall be read as Reference Examples 1 to 11.

[Measuring method]
(1) Glass transition temperature The glass transition temperature in the present embodiment is measured using a DSC (Differential Scanning Calorimeter, Epolead Service Co., Ltd., DSC6220) in accordance with JIS K 7121 (method for measuring transition temperature of plastics). It was measured. The measured value of the glass transition temperature of the film-forming resin can be regarded as the glass transition temperature (° C.) of the first porous layer.

(2) Surface elastic modulus The surface elastic modulus of the porous film was measured using AFM (Dimension icon (manufactured from 2006)) manufactured by Bruker AXS. The measurement mode was set to Quantitative Nanomechanical Mapping, and a probe suitable for the measured value of the surface elastic modulus (Tap525 for elastic modulus from 1 GPa to 20 GPa, RTESPA for elastic modulus from 200 MPa to 2 GPa) was used. After setting the probe spring constant and probe tip curvature using a sample with a known elastic modulus, the surface elastic modulus (GPa) of the first porous layer was measured at normal temperature over a measurement range of 100 nm. Measurements were taken at 10 locations randomly selected from a sample of 100 mm x 100 mm size, and the average value was adopted.

(3) Weight Reduction Amount The mass of each of three porous film samples of 100 mm×100 mm size was measured. The average value was converted into the mass per unit area to obtain the initial basis weight: _W0 . Next, the sample was immersed in 2 g of a solvent composed of at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate in an atmosphere of 25° C. for 24 hours. The samples were then removed and dried before weighing each sample. The average value was converted into mass per unit area to obtain the basis weight after solvent immersion: _W1 . After that, the basis weight reduction amount: W ₁ /W ₀ (times) was calculated.

(4) Air permeability Select one point randomly extracted from each of the three samples of 100 mm × 100 mm size of the porous film, Oken type air permeability measuring device (EG01-5- 1 MR) was used to measure according to JIS P 8117 (2009), and the average value was taken as the air permeability (sec/100 cc).

(5) Film thickness of the first porous layer A sample cross section of the porous film is cut out with a microtome, and the cross section is observed with a field emission scanning electron microscope (S-800 manufactured by Hitachi, Ltd., acceleration voltage 26 kV). Observe, the highest point from the interface with the porous substrate is the thickness, and if it is one-sided, measure only one side, if it is double-sided, measure both sides, and the total is the thickness of the first porous layer (μm). and Five locations randomly selected from a 100 mm x 100 mm size sample were measured and averaged.

(6) Adhesive strength with electrodes
Li(Ni _5/10 Mn _2/10 Co _3/10 )O ₂ as the active material, vinylidene fluoride resin as the binder, acetylene black as the conductive aid, and a positive electrode of 15 mm×100 mm and a porous film of graphite as the active material. Installed so that the porous layers are in contact, hot press at 0.5 MPa, 80 ° C., 0.2 m / min with a hot roll press, manually peeled using tweezers, adhesive strength in the following four stages was evaluated. Similarly, the adhesive strength between the negative electrode and the porous film, in which the active material is graphite, the binder is vinylidene fluoride resin, and the conductive aid is carbon black, was also measured, and the positive electrode and the negative electrode were each evaluated to obtain the adhesive strength.
- Adhesion strength ⊚: The electrode and the porous film side were separated by a strong force.
- Adhesion strength ◯: The electrode and the porous film were separated with a slightly strong force.
• Adhesion strength Δ: The electrode and the porous film were peeled off with a weak force.
- Adhesion strength ×: The electrode and the porous film were separated with an extremely weak force.

(7) Blocking resistance Cut out two samples of 50 mm length and 50 mm width from the porous film, overlap the first porous layer surface of one of the two test pieces and the back surface of the other opposite side, and A stainless steel plate measuring 25 mm long and 25 mm wide was placed on the central portion and adhered to the plate under a load of 0.5 kN at a temperature of 45° C. for 3 weeks.
• Blocking resistance ⊚: The porous film was peeled off with an extremely weak force.
• Blocking resistance ◯: The porous film was peeled off with a weak force.
• Blocking resistance Δ: The porous film was peeled off with a slightly strong force.
• Blocking resistance ×: The porous film was peeled off with a strong force.

(8) Battery production The positive electrode sheet contains 92 parts by mass of Li(Ni ₅ /10Mn ₂ /10Co ₃ /10) O ₂ as a positive electrode active material, 2.5 parts by mass of acetylene black and graphite as positive electrode conductive aids, A positive electrode slurry in which 3 parts by mass of polyvinylidene fluoride as a positive electrode binder is dispersed in N-methyl-2-pyrrolidone using a planetary mixer is coated on an aluminum foil, dried, and rolled (coated basis weight: 9.5 mg/cm ² ).
This positive electrode sheet was cut into a size of 40 mm×40 mm. At this time, a current-collecting tab bonding portion without an active material layer was cut out to 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.
For the negative electrode sheet, 98 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder are dispersed in water using a planetary mixer. The resulting negative electrode slurry was coated on a copper foil, dried, and rolled to prepare a negative electrode slurry (coating basis weight: 5.5 mg/cm ² ).
This negative electrode sheet was cut into a size of 45 mm×45 mm. At this time, a current-collecting tab bonding portion without an active material layer was cut out to a size of 5 mm×5 mm outside the active material surface. A copper tab having the same size as the positive electrode tab was ultrasonically welded to the tab bonding portion.

Next, the porous film was cut into a size of 55 mm×55 mm, and the positive electrode and the negative electrode were superimposed on both sides of the porous film so that the active material layer separated the porous film, and the positive electrode coated portion was entirely opposed to the negative electrode coated portion. Arranged to obtain an electrode group. The positive electrode/negative electrode/porous film was sandwiched between a sheet of aluminum laminate film of 90 mm×200 mm, the long sides of the aluminum laminate film were folded, and the two long sides of the aluminum laminate 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) at a concentration of 1 mol/liter was used. 1.5 g of the electrolytic solution was poured into a bag-shaped aluminum laminate film, and the short sides of the aluminum laminate film were heat-sealed while being impregnated under reduced pressure to obtain a laminate type battery.

(9) Charge-Discharge Cycle Characteristics Charge-discharge cycle characteristics were tested according to the following procedure, and evaluated by the discharge capacity retention rate.
<1st to 300th cycles>
Charging and discharging were set as one cycle, charging conditions were constant current charging at 2C and 4.3V, and discharging conditions were constant current discharging at 2C and 2.7V, and charging and discharging were repeated 300 times at 25°C.
<Calculation of discharge capacity maintenance rate>
The discharge capacity retention rate was calculated by {(discharge capacity at 300th cycle)/(discharge capacity at 1st cycle)}×100. Five laminate-type batteries were produced, and the average of the measurement results of the three batteries after removing the maximum and minimum discharge capacity retention rates was taken as the capacity retention rate. When the discharge capacity retention rate was less than 60%, it was rated as x;

(Example 1)
A water-based emulsion coating liquid in which a film-forming resin having a minimum film-forming temperature of 35°C, a glass transition temperature of 40°C, and an average particle diameter of 0.15 µm is dispersed by emulsion polymerization as a main component of methacrylic acid/acrylic acid ester. was prepared. 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 placed in a hot air oven (drying temperature set 50° C.). The porous film of the present invention was obtained by drying until the solvent was volatilized to form a first porous layer. Regarding the obtained porous film, the surface elastic modulus of the first porous layer, the amount of weight loss of the first porous layer, the air permeability, the film thickness of the first porous layer, the adhesion to the electrode, Blocking resistance and cycling properties were measured. Diethyl carbonate was used as an immersion solvent when measuring the weight reduction of the first porous layer. Table 1 shows the measurement results. Table 1 also shows the measurement results for the following examples and comparative examples.

(Example 2)
The porous film of the present embodiment was prepared in the same manner as in Example 1, except that the main component was methacrylic acid/acrylic acid ester, and a film-forming resin having a minimum film-forming temperature of 40°C and a glass transition temperature of 50°C was used. got

(Example 3)
The porous film of the present embodiment was prepared in the same manner as in Example 1, except that the main component was methacrylic acid/acrylic acid ester, and a film-forming resin having a minimum film-forming temperature of 60°C and a glass transition temperature of 70°C was used. got

(Example 4)
It has a core-shell structure, the main component of the core is methacrylic acid/acrylic acid ester, the minimum film-forming temperature is 70 ° C., the glass transition temperature is 85 ° C., the average particle diameter is 0.10 μm, and the main component of the shell is In the same manner as in Example 1, except that a film-forming resin having a methacrylic acid/acrylate ester, a minimum film-forming temperature of 5° C., a glass transition temperature of 20° C., and an average particle size of 0.05 μm was used. A morphological porous film was obtained.

(Example 5)
A porous film of the present embodiment was obtained in the same manner as in Example 2, except that the first porous layer was coated so as to have a thickness of 5.0 μm.

(Example 7)
The main component is methacrylic acid/acrylic acid ester, the minimum film-forming temperature is 35°C, and the glass transition temperature is 45°C. A film-forming resin having a high

(Example 8)
A coating liquid B was prepared by dispersing 95% by mass of alumina particles (average particle size 0.4 μm) as inorganic particles and 5% by mass of acrylic resin as a binder in water. This coating solution was applied onto a polyethylene porous substrate (thickness 7 μm, air permeability 110 sec/100 cm ³ ) using a wire bar, and placed in a hot air oven (drying temperature set 50° C.). It was dried until the solvent was volatilized to form a second porous layer. After that, the coating liquid prepared in Example 1 was applied onto the second porous layer and dried in a hot air oven (drying temperature set at 50°C) until the contained solvent was volatilized. A porous layer was formed to obtain the porous film of the present embodiment.

(Example 9)
A porous film of the present embodiment was obtained in the same manner as in Example 1, except that dimethyl carbonate was used as an immersion solvent when measuring the basis weight reduction of the first porous layer.

(Example 10)
A porous film of the present embodiment was obtained in the same manner as in Example 1, except that ethyl methyl carbonate was used as an immersion solvent when measuring the basis weight reduction of the first porous layer.

(Example 11)
Example 1 except that a mixed solvent of ethylene carbonate and diethyl carbonate (ethylene carbonate/diethyl carbonate = 50% by volume/50% by volume) was used as an immersion solvent when measuring the basis weight reduction of the first porous layer. A porous film of the present embodiment was obtained in the same manner as above.

(Example 12)
300 parts of ion-exchanged water and 0.2 parts of sodium lauryl sulfate were charged into a reactor, and stirring was started. To this, 0.5 parts of ammonium persulfate was added at 70° C. under a nitrogen atmosphere, and 48 parts of 2,2,2-trifluoroethyl methacrylate, 25 parts of cyclohexyl methacrylate, 25 parts of cyclohexyl acrylate, 2 parts of hydroxyethyl methacrylate, and lauryl sulfate were added. A monomer mixture consisting of 2 parts of sodium and 50 parts of ion-exchanged water was continuously added dropwise over 4 hours, and after completion of the dropwise addition, polymerization was carried out for 3 hours, resulting in a minimum film-forming temperature of 55°C and a glass transition temperature of 60°C. A porous film of the present embodiment was obtained in the same manner as in Example 1, except that an aqueous emulsion coating solution in which an acrylic resin having an average particle size of 0.20 μm was dispersed was prepared.

(Example 13)
48 parts of 2,2,2-trifluoroethyl methacrylate, 21 parts of cyclohexyl methacrylate, 22 parts of cyclohexyl acrylate, 7 parts of urethane acrylate, 2 parts of hydroxyethyl methacrylate, 2 parts of sodium lauryl sulfate, and 50 parts of deionized water. The mixture was continuously added dropwise over 4 hours, and polymerized for 3 hours after completion of the dropwise addition. An acrylic resin having a minimum film-forming temperature of 45°C, a glass transition temperature of 50°C, and an average particle size of 0.20 µm was dispersed. A porous film of the present embodiment was obtained in the same manner as in Example 1, except that a water-based emulsion coating liquid was prepared.

(Comparative example 1)
A porous film was obtained in the same manner as in Example 1, except that the main component was methacrylic acid/acrylic acid ester, and a film-forming resin having a minimum film-forming temperature of 5°C and a glass transition temperature of 15°C was used.

(Comparative example 2)
A porous film was obtained in the same manner as in Example 1, except that the main component was methacrylic acid/acrylic acid ester, and a film-forming resin having a minimum film-forming temperature of 70°C and a glass transition temperature of 85°C was used.

(Comparative Example 3)
A porous film was obtained in the same manner as in Example 1, except that the main component was methacrylic acid/acrylic acid ester, and a film-forming resin having a minimum film-forming temperature of 85°C and a glass transition temperature of 95°C was used.

(Comparative Example 4)
A porous film was obtained in the same manner as in Example 1, except that the main component was polyvinylidene fluoride, and a film-forming resin having a minimum film-forming temperature of 75°C and a glass transition temperature of 75°C was used.

From Table 1, each of Examples 1 to 5 and 7 to 13 is a porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate. wherein the first porous layer contains an organic resin, the glass transition temperature of the first porous layer is 40° C. or higher and 85° C. or lower, and the surface elastic modulus of the first porous layer is 1 GPa or higher and 3 GPa. Sufficient electrode adhesion, good anti-blocking properties and battery properties can be obtained because it is a porous film having the following properties.
In particular, in Examples 1 to 5, 7 and 8, the decrease in basis weight of the first porous layer after being immersed in diethyl carbonate for 24 hours at 25° C. was 0.97 times or less, so even better battery characteristics were obtained. .
In Examples 9 to 11, even if the type of immersion solvent used for measuring the amount of weight reduction is dimethyl carbonate, ethyl methyl carbonate, or a mixed solvent, the amount of weight reduction is 0.97 times or less, and is similarly excellent. battery characteristics can be obtained.
On the other hand, in Comparative Example 1, the glass transition temperature of the first porous layer is 15° C. and the surface elastic modulus of the first porous layer is 0.6 GPa. did. Further, in Comparative Example 2, since the surface elastic modulus of the first porous layer was 3.2 GPa, the first porous layer was difficult to deform and the adhesion to the electrode deteriorated. In Comparative Examples 3 and 4, both the glass transition temperature of the first porous layer and the surface elastic modulus of the first porous layer are within the specified ranges of the present invention. Indicates that the battery characteristics are incompatible.

Claims (8)

A porous film obtained by laminating a first porous layer having adhesiveness to an electrode on at least one side of a porous substrate, wherein the first porous layer contains an acrylic resin , and the acrylic The resin contains a polymer having a fluorine-containing (meth)acrylate monomer unit, the glass transition temperature of the first porous layer is 40° C. or higher and 85° C. or lower, and the surface elasticity of the first porous layer is A porous film having a modulus of 1 GPa or more and 3 GPa or less. After immersion in a solvent composed of at least one selected from ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate at 25° C. for 24 hours, the weight reduction of the first porous layer is 1.3 times. 2. The porous film of claim 1 , wherein: 3. The porous film according to claim 1, wherein the porous film has an air permeability of 50 sec/100 cm ³ or more and 1,000 sec/100 cm ³ or less. The porous film according to any one of claims 1 to 3 , wherein the thickness of the first porous layer 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 said first porous layer is laminated on both sides of said porous substrate. The porous material according to any one of claims 1 to 5 , wherein a second porous layer containing inorganic particles is further laminated between the porous substrate and the first porous layer. film. A secondary battery separator comprising the porous film according to claim 1 . A secondary battery comprising the secondary battery separator according to claim 7 .