KR20240007116A - Porous films, separators for secondary batteries and secondary batteries - Google Patents

Abstract

The object of the present invention is to provide a porous film with excellent adhesion to electrodes, electrolyte injection property, thermal dimensional stability, and low resistance, comprising a porous substrate and inorganic particles and an organic resin on at least one side of the porous substrate. A porous film having a porous layer containing particles, wherein the porous substrate has an arithmetic mean height (Sa) of 0.01 µm or more and less than 0.09 µm on a 2200 µm square on the surface on the side in contact with the porous layer, and all the components of the porous layer When the volume is set to 100 volume%, the relationship between the volume content of inorganic particles α (volume %) and the occupancy rate β (area %) of the inorganic particles on the surface of the porous layer is a porous film that satisfies β/α<1. Do it for the original purpose.

Description

Porous films, separators for secondary batteries and secondary batteries

The present invention relates to porous films, separators for secondary batteries, and secondary batteries.

Secondary batteries such as lithium-ion batteries are used in automobiles such as electric cars, hybrid cars, and plug-in hybrid cars, as well as portable digital devices such as smartphones, tablets, mobile phones, laptops, digital cameras, digital video cameras, and portable game consoles, and power tools. , It is widely used in electric bicycles, electric assist bicycles, etc.

Lithium ion batteries generally have a configuration in which a separator for a secondary battery 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.

A polyolefin-based porous substrate is used as a separator for secondary batteries. The characteristics required for a separator for secondary batteries include the ability to contain an electrolyte solution in the porous structure and enable ion movement, and when the lithium ion battery generates abnormal heat, the porous structure is closed by melting with heat, thereby stopping ion movement. One example is the shutdown characteristic that stops discharge.

Additionally, in order to improve the energy density of lithium ion batteries, the battery type is being replaced from a wound type to a stacked type. In the case of the stacked type, in the manufacturing process of a secondary battery using an electrode stack of a positive electrode, a separator, and a negative electrode, to maintain the stack structure when transporting the electrode stack, or to shape the electrode stack into a cylindrical or square shape. To prevent the shape from collapsing when inserted into a can, or to increase energy density by putting more electrode laminates into the can, or to prevent the shape from deforming after insertion into the exterior material in the case of a laminated battery. To achieve this, adhesion (dry adhesion) between the separator and the electrode before impregnating the electrolyte solution is required. Therefore, in the above process, heat pressing may be performed on the electrode laminate. On the other hand, it is necessary to prevent impregnation of the electrolyte solution into the electrode laminate after heat pressing, that is, deterioration of the electrolyte solution injectability. In addition, lithium ion batteries are also required to have good battery characteristics such as adhesion to electrodes in an electrolyte-impregnated state (wet adhesion), high output, and long service life.

In response to the requirement for dry adhesion, Patent Document 1 attempts to achieve dry adhesion with an electrode by laminating an adhesive layer formed on a heat-resistant layer. In Patent Document 2, the dry adhesion to the electrode is improved by satisfying a specific relationship between the particle diameter of the particle-shaped polymer and the particle diameter of the inorganic particle. Additionally, it is known to improve the impregnability of the electrolyte solution by roughening the outermost surface of the separator (Patent Document 3). In addition, by developing both dry adhesiveness and wet adhesiveness, improvement in yield during battery production and initial charging/discharging is being studied (Patent Document 4).

International Publication No. 2013/151144 International Publication No. 2018/034094 International Publication No. 2014/021293 International Publication No. 2016/098684

As described above, the heat press process in the secondary battery manufacturing process requires both dry adhesion of the electrode and separator and electrolyte injection properties. In addition, with the increase in the size of batteries, there is a demand for larger electrode laminates and additional higher capacities, and there is a need to improve wet adhesion and electrolyte injection properties in addition to dry adhesion to electrodes. Additionally, as demand for stacked batteries increases, productivity is also required to be improved, and heat press time must be shortened to improve adhesion. In order to achieve both adhesion to the electrode and electrolyte injectability, it is being considered to appropriately adjust the amount of organic resin particles responsible for adhesion to the electrode. However, if the amount of organic resin is increased, electrolyte injectability decreases, and the battery resistance decreases. There is a problem that rate characteristics and battery life deteriorate due to increase. On the other hand, if the amount of organic resin is reduced, there is also a problem that the adhesion to the electrode decreases.

In view of the above problems, an object of the present invention is to provide a porous film that has excellent adhesion to electrodes (dry adhesion, wet adhesion), electrolyte injectability, thermal dimensional stability, and low resistance.

Accordingly, the present inventors focused on the adhesion to the electrode by high-pressure heat press to shorten the heat press time, and as a result of repeated studies, the arithmetic mean height of the porous substrate was set to a certain height and organic resin particles were placed in the surface layer. By localizing the inorganic particles and separating the layers, it was found that dry adhesion to the electrode and electrolyte injection property were excellent. Additionally, it was found that by appropriately selecting the composition of the organic resin particles, in addition to the above-mentioned characteristics, wet adhesiveness was also excellent.

In order to solve the above problems, the porous film of the present invention has the following structure.

(1) A porous film having a porous base material and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous base material, wherein the porous base material has an area of 2200 μm on all sides of the surface on the side in contact with the porous layer. When the arithmetic mean height (Sa) in the porous layer is 0.01 ㎛ or more and less than 0.09 ㎛, and the volume of all components of the porous layer is 100 volume %, the volume content α (volume %) of the inorganic particles and the A porous film in which the occupancy rate β (area%) of inorganic particles satisfies β/α<1.

(2) The porous film according to (1), wherein the arithmetic mean height (Sa) of the surface on the side having the porous layer in all directions of 2200 μm is 0.005 μm or more and less than 0.085 μm.

(3) The porous film according to (1) or (2), wherein the volume content α of the inorganic particles is 30 volume% or more and 80 volume% or less when the volume of all components of the porous layer is 100 volume%.

(4) The porous film according to any one of (1) to (3), wherein the occupancy rate β of the inorganic particles in the porous surface portion is greater than 0%.

(5) The porous film according to any one of (1) to (4), wherein the porous substrate has an arithmetic mean roughness (Ra) of 10 nm or more and less than 80 nm in a 12 nm square on the surface on the side in contact with the porous layer.

(6) The porous film according to any one of (1) to (5), wherein the porous substrate has a protruding ridge height (Spk) of 0.01 μm or more and less than 0.12 μm on a 2200 μm square of the surface on the side in contact with the porous layer.

(7) The porous film according to any one of (1) to (6), wherein the surface free energy of the porous layer is 10 mN/m or more and 80 mN/m or less.

(8) The porous film according to any one of (1) to (7), wherein the porous substrate is a polyolefin microporous film.

(9) The porous film according to any one of (1) to (8), wherein the inorganic particles are particles composed of at least one member selected from the group consisting of inorganic hydroxides, inorganic oxides and inorganic sulfur oxides.

(10) The organic resin particles are fluorine-containing (meth)acrylate monomer, unsaturated carboxylic acid monomer, (meth)acrylic acid ester monomer, styrene monomer, olefin monomer, diene monomer, acrylamide monomer, and vinylidene fluoride. The porous film according to any one of (1) to (9), which has a polymer polymerized using at least one monomer selected from the group consisting of monomers.

(11) With respect to the organic resin particles, when the total monomer component of the organic resin particles is 100% by mass, the proportion of fluorine-containing (meth)acrylate monomer is 20% by mass or more and 80% by mass or less. Porous film.

(12) The porous film according to (11), wherein the organic resin particles contain a polymer polymerized only with a fluorine-containing (meth)acrylate monomer.

(13) The porous film according to any one of (10) to (12), wherein the number of fluorine atoms contained in one molecule of the fluorine-containing (meth)acrylate monomer is 3 to 13.

(14) The porous film according to (12) or (13), wherein the organic resin particles further contain a polymer or copolymer polymerized using a (meth)acrylate monomer having a hydroxyl group.

(15) With respect to the organic resin particles, when the total monomer component of the organic resin particles is 100% by mass, the proportion of (meth)acrylate monomers having a hydroxyl group is greater than 0% by mass and 7.0% by mass or less (14) The porous film described in .

(16) With respect to the polymer contained in the organic resin particles, at least one monomer among the monomers that is the raw material of the polymer is a monomer whose glass transition temperature of the polymer is -100°C or higher and 0°C or lower when polymerized with only that monomer. The porous film according to any one of (10) to (15), characterized in that.

(17) The monomer having a glass transition temperature of -100°C or higher and 0°C or lower when polymerized with only that monomer is greater than 0% by mass and is 10.0% by mass when the total monomer component of the organic resin particles is 100% by mass. % or less. The porous film described in (16).

(18) The porous film according to any one of (1) to (17), wherein the porous layer contains at least two types of organic resin particles.

(19) The porous film according to any one of (1) to (18), wherein the organic resin particles have an average particle diameter of 100 nm or more and 500 nm or less.

(20) The porous film according to any one of (1) to (19), wherein the porous substrate has a film thickness of 3 μm or more and 15 μm or less.

(21) The porous film according to any one of (1) to (20), wherein the porous layer has a film thickness of 2 μm or more and 8 μm or less.

(22) A separator for secondary batteries using the porous film according to any one of (1) to (21).

(23) A secondary battery formed using the separator for secondary batteries described in (22).

(24) A porous film having a porous substrate and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous substrate,

The porous substrate is made of a polyolefin microporous film having an arithmetic mean height (Sa) of 0.01 µm or more and less than 0.09 µm on a surface of 2200 µm in all directions on the side in contact with the porous layer,

The inorganic particles are particles composed of at least one type selected from the group consisting of inorganic hydroxides, inorganic oxides and inorganic sulfur oxides,

The organic resin particles are composed of a fluorine-containing (meth)acrylate monomer, an unsaturated carboxylic acid monomer, a (meth)acrylic acid ester monomer, a styrene-based monomer, an olefin-based monomer, a diene-based monomer, an acrylamide-based monomer, and a vinylidene fluoride monomer. It is a polymer polymerized using at least one monomer selected from the group consisting of,

When the volume of all components of the porous layer is 100 volume%, the volume content α of inorganic particles is 30 volume% or more and 80 volume% or less,

A battery separator in which the relationship between the volume content α of the inorganic particles and the occupancy rate β of the inorganic particles in the surface portion of the porous layer satisfies β>0 and β/α<1.

According to the present invention, it is possible to provide a porous film with excellent adhesion to electrodes (dry adhesion, wet adhesion), electrolyte injectability, thermal dimensional stability, and low resistance. Especially when subjected to high-pressure heat pressing, excellent dry adhesive properties can be achieved. In addition, by using the porous film as a battery separator, it is possible to improve the yield of the battery manufacturing process and provide a secondary battery with excellent battery life and rate characteristics.

The porous film of the present invention is a porous film having a porous substrate and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous substrate, wherein the porous substrate is formed on all sides of the surface on the side in contact with the porous layer. When the arithmetic mean height (Sa) at 2200 ㎛ is 0.01 ㎛ or more and less than 0.09 ㎛, and the volume of all components of the porous layer is 100 volume %, the volume content α (volume %) of inorganic particles and the surface of the porous layer It is characterized in that the occupancy rate β (area%) of the inorganic particles in the portion satisfies β/α<1.

Hereinafter, the porous film of the present invention will be described in detail.

The porous film of the present invention has an arithmetic mean height (Sa) of 0.01 µm or more and less than 0.09 µm in a 2200 µm square on the surface of the porous substrate on the side in contact with the porous layer. By setting the lower limit of the arithmetic mean height (Sa) to 0.01 μm or more, the adhesion to the electrode can be further improved. In addition, by setting the upper limit of the arithmetic mean height (Sa) to less than 0.09 ㎛, it becomes easier for the electrolyte solution to impregnate the electrode laminate where the electrode and the porous film are bonded, the electrolyte injection property is improved, and the rate when used as a battery separator is improved. Characteristics and battery life are improved.

Until now, in order to improve dry adhesion to electrodes, smoothing the adhesive layer provided on the outermost surface of the separator has been studied. Additionally, recently, in order to improve the dry adhesive yield of the separator and the electrode before impregnating the electrolyte solution, the press pressure in the bonding process (heat press) with the electrode is significantly increased, thereby significantly shortening the bonding process time between the separator and the electrode. It is being considered. In that case, in order to improve dry adhesion, the outermost surface of the separator needs to be deeply pressed by the electrode to follow the irregularities of the active material present in the electrode. However, if the outermost surface is smooth, it cannot follow the irregularities, so the electrode It was found that the dry adhesion with the adhesive decreased conversely. On the other hand, it was found that if the outermost surface is too rough, it becomes easy to follow the irregularities of the active material, and the dry adhesion to the electrode becomes higher than necessary, resulting in a decrease in the electrolyte solution pouring ability. Therefore, by smoothing the outermost surface of the separator within a certain range, it becomes possible to adjust the tracking of the irregularities of the active material, and both dry adhesion to the electrode and electrolyte injection property can be achieved.

In addition, in order to improve dry adhesion and wet adhesion to the electrode, increasing the content of organic resin particles responsible for adhesion to the electrode is being considered. However, it becomes difficult for the electrolyte to impregnate into the electrode when injecting the electrolyte, which leads to the battery There were cases where productivity decreased and membrane resistance worsened due to inhibition of ion transport. Therefore, there was a problem in achieving both dry adhesion and wet adhesion to the electrode and maintaining low resistance of the film.

The porous film of the present invention achieves both dry adhesion to electrodes and electrolyte injection properties by setting the arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with the porous layer in all directions of 2200 μm to 0.01 μm or more and less than 0.09 μm. I found out what I could do. By setting the arithmetic mean height (Sa) to the above range, the arithmetic mean height of the surface of the porous layer is also reduced, and during the dry bonding process (heat pressing) with the electrode, the uneven followability to the active material present in the electrode can be adjusted, This indicates both dry adhesion to the electrode and electrolyte injection properties. Dry adhesion to the electrode can be improved by suppressing the cost without increasing the amount of organic resin particles, and membrane resistance can be lowered by improving the electrode liquid injectability. Therefore, a battery using the porous film of the present invention as a battery separator can achieve both rate characteristics and battery life.

Here, the arithmetic mean height (Sa) on a 2200㎛ square of the surface of the porous substrate on the side in contact with the porous layer is the type of resin of the porous substrate, the type of plasticizer, the presence or absence of cooling after sheet formation, the stretching method, the stretching ratio, the stretching temperature, By adjusting various manufacturing conditions such as dry re-stretching after stretching, it becomes possible to achieve a predetermined range.

The porous layer in the present invention contains inorganic particles and organic resin particles. In the porous layer, the volume content α (volume %) of inorganic particles when the volume of all components of the porous layer is 100 volume% and the occupancy rate β (area %) of the inorganic particles on the surface of the porous layer are β/ Satisfies α<1. The fact that β/α is less than 1 indicates that the occupancy rate of inorganic particles in the surface portion of the porous layer is lower than the content rate of inorganic particles in the entire porous layer, that is, it indicates that the organic resin particles are localized in the surface portion of the porous layer. . The organic resin particles are unevenly distributed in the surface portion of the porous layer, and the surface portion exhibits sufficient adhesion to the electrode because many organic resin particles exist.

Meanwhile, the volume content α of the inorganic particles and the occupancy rate β of the inorganic particles described above are determined by the method described in the Example section.

In addition, the porous film of the present invention uses a porous substrate whose arithmetic mean height (Sa) at 2200 μm of the surface of the porous substrate on the side in contact with the porous layer is 0.01 μm or more and less than 0.09 μm, and the porous layer is a porous layer. When the total constituents are set to 100% by volume, the volume content α (volume%) of the inorganic particles and the occupancy rate β (area%) of the inorganic particles on the surface of the porous layer satisfy β/α<1, especially When high-pressure heat pressing is performed, high dry adhesion to electrodes, wet adhesion, and electrolyte injection properties are obtained. β/α is more preferably 0.5 or less, and even more preferably 0.3 or less. The lower limit of β/α is not particularly limited, but is preferably 0.01 or more.

The lower limit of β/α is not particularly limited, but is preferably 0.01 or more.

In order to form a porous layer with β/α within the above range, it may be formed through a two-step application process using two types of coating solutions, or it may be formed through a one-step application process using one type of coating solution. desirable. If it can be formed in a one-step application process, it becomes possible to reduce costs by reducing the number of applications. In order to use it as a one-step application process, it becomes possible to set β/α within a predetermined range by, for example, appropriately adjusting the surface free energy of the organic resin particles, the viscosity of the coating liquid, the solid content concentration, and the drying temperature. Details will be described later.

The volume content α of the inorganic particles is preferably 30 volume% or more and 80 volume% or less, more preferably 40 volume% or more and 70 volume% or less, when the volume of all components of the porous layer is 100 volume%, More preferably, it is 50 volume% or more and 60 volume% or less. By setting the volume content β of the inorganic particles to 30% by volume or more, sufficient thermal dimensional stability can be obtained. Moreover, by setting it to 80 volume% or less, the content of organic resin particles becomes sufficient, and dry adhesion and wet adhesion with an electrode improve.

It is preferable that the occupancy rate β of the inorganic particles in the surface portion of the porous layer is greater than 0%. More preferably, it is 1% or more, and even more preferably, it is 5% or more. When β is greater than 0%, cost reduction is possible by reducing the number of applications through a one-step application process or reducing the cost of the coating material by reducing the raw materials. The upper limit is not particularly limited, but is preferably less than 50%. More preferably, it is less than 30%, and even more preferably, it is less than 20%. By being less than 50%, adhesion to the electrode becomes good. β=0 indicates that no inorganic particles exist on the surface of the porous layer and that all of them exist in the porous material. On the other hand, the surface portion of the porous layer is a surface layer at a depth that affects the outer surface of the porous layer and the adhesiveness with the electrode, and is represented by an image obtained using SEM-EDX described later.

[Porous layer]

The porous film of the present invention has a porous layer containing inorganic particles and organic resin particles formed on at least one side of a porous substrate, which will be described in detail later.

The lower limit of the film thickness of the porous layer is preferably 2 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more. Additionally, the upper limit of the film thickness of the porous layer is preferably 8 μm or less, preferably 7 μm or less, and more preferably 6 μm or less. The film thickness of the porous layer here refers to the total thickness of the porous layers formed on the porous substrate. By setting the film thickness of the porous layer to 2 μm or more, sufficient thermal dimensional stability and uneven distribution of the organic resin easily occur, thereby ensuring adhesion to the electrode. Additionally, by setting it to 8 μm or less, a porous structure is obtained, and rate characteristics and battery life improve. Additionally, it becomes advantageous in terms of cost.

The surface free energy of the porous layer is preferably 10 mN/m or more and 80 mN/m or less, more preferably 15 mN/m or more and 70 mN/m or less, and even more preferably 20 mN/m or more and 60 mN/m or less. By setting it to 10 mN/m or more, the coating stability of the porous layer improves. Also, by setting it to 80 mN/m or less, surface unevenness due to layer separation of organic resin particles is likely to occur, making it easier to control β/α.

In addition, the porous layer preferably has a glass transition temperature of 20°C or more and less than 80°C. The lower limit is more preferably 30°C or higher, and even more preferably 40°C or higher. Moreover, the upper limit is more preferably 70°C or lower, and even more preferably 60°C or lower. By setting the glass transition temperature to 20°C or higher, swelling in the electrolyte solution is suppressed, and wet adhesion, rate characteristics, and battery life become good. Also, by setting it below 80°C, dry adhesion to the electrode is further improved. In order to keep the glass transition temperature in an appropriate range, it can be appropriately selected from a specific monomer group.

(1) Organic resin particles

Organic resin particles improve adhesion to electrodes. The resin constituting the organic resin particles is preferably a resin that has adhesiveness to electrodes, and by distributing the organic resin particles on the surface layer of the porous film, ion permeability is improved and rate characteristics are improved. Additionally, β/α can be lowered by lowering the surface free energy of the organic resin particles.

Organic resin particles include a group consisting of fluorine-containing (meth)acrylate monomers, unsaturated carboxylic acid monomers, (meth)acrylic acid ester monomers, styrene-based monomers, olefin-based monomers, diene-based monomers, acrylamide-based monomers, and vinylidene fluoride monomers. It is preferred to have a polymer polymerized using at least one monomer selected from: Among these, it is particularly preferable that the organic resin particles have a mixture of a polymer polymerized only with a fluorine-containing (meth)acrylate monomer and another polymer, or a copolymer polymerized with a fluorine-containing (meth)acrylate monomer. Thereby, by lowering the surface free energy of the organic resin particles, the organic resin particles can be distributed on the surface side, and the adhesion to the electrode of the porous layer can be improved. In addition, in this specification, “(meth)acrylate” means both “acrylate” and “methacrylate.”

The polymer polymerized using a fluorine-containing (meth)acrylate monomer contains a repeating unit obtained by polymerizing the fluorine-containing (meth)acrylate.

Examples of fluorine-containing (meth)acrylate monomers include 2,2,2-trifluoroethyl (meth)acrylate, 2,2,3,3,3-pentafluoropropyl (meth)acrylate, and 2-(perfluoropropyl)acrylate. Lobutyl)ethyl(meth)acrylate, 3-(perfluorobutyl)-2-hydroxypropyl(meth)acrylate, 2-(perfluorohexyl)ethyl(meth)acrylate, 3-perfluoro Hexyl-2-hydroxypropyl (meth)acrylate, 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, etc. can be mentioned. One type of fluorine-containing (meth)acrylate monomer may be used individually, or two or more types may be used in combination at an arbitrary ratio.

The proportion of fluorine-containing (meth)acrylate monomer used in the organic resin particles is preferably greater than 20% by mass, more preferably 22% by mass or more, based on 100% by mass of the total monomer components of the organic resin particles. , more preferably 25 mass% or more, and even more preferably 30 mass% or more. Moreover, 80 mass% or less is preferable, more preferably 60 mass% or less, further preferably 50 mass% or less, even more preferably 40 mass% or less, and most preferably 35 mass% or less. By setting it within the above range, the organic resin particles become easier to distribute in the surface layer, and sufficient adhesion to the electrode is obtained.

Whether or not a fluorine-containing (meth)acrylate monomer is used in the organic resin particles can be measured, and further, the ratio of the fluorine-containing (meth)acrylate monomer used in the organic resin particles can be measured using a known method. For example, first, the porous layer is separated from the porous film using an organic solvent such as water and alcohol, and the organic solvent such as water and alcohol is sufficiently dried to obtain the components contained in the porous layer. An organic solvent that dissolves the organic resin component is added to the obtained components to dissolve only the organic resin component. Subsequently, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. Using the obtained organic resin component, nuclear magnetic resonance ( ¹ H-NMR, ¹⁹ F-NMR, ¹³ C-NMR), infrared absorption spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and fluorescence X-ray analysis (EDX) ), and elemental analysis, pyrolysis gas chromatography mass spectrometry (pyrolysis GC/MS), etc. can be used to calculate the intensity of the signal derived from the fluorine-containing (meth)acrylate monomer. In particular, after confirming whether fluorine-containing (meth)acrylate monomer was used by thermal decomposition GC/MS, ¹³ C-NMR (solid NMR) sample tube was filled with the organic resin component and an appropriate amount of solvent (deuterated chloroform) and left to stand overnight. The ratio of the fluorine-containing (meth)acrylate monomer used can be determined by later measurement using the DD/MAS method.

Additionally, the number of fluorine atoms contained in one molecule of the fluorine-containing (meth)acrylate monomer is preferably 3 to 13. The number of fluorine atoms is more preferably 3 to 11, and even more preferably 3 to 9. By setting it within the above range, the surface free energy of the organic resin particles can be reduced and coatability can be achieved at the same time. When the number of fluorine atoms is 3 or more, the surface free energy of the organic resin particles is sufficiently reduced, and the adhesion to the electrode becomes sufficient. Additionally, when the number of fluorine atoms is 13 or less, coatability to a porous substrate is ensured and productivity improves.

On the other hand, the number of fluorine atoms in fluorine-containing (meth)acrylate can be measured using a known method. For example, first, the porous layer is separated from the porous film using an organic solvent such as water and alcohol, and the organic solvent such as water and alcohol is sufficiently dried to obtain the components contained in the porous layer. An organic solvent that dissolves the organic resin component is added to the obtained components to dissolve only the organic resin component and separate it from the inorganic particles. Subsequently, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. Using the obtained organic resin component, nuclear magnetic resonance ( ¹ H-NMR, ¹⁹ F-NMR, ¹³ C-NMR), infrared absorption spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and fluorescence X-ray analysis (EDX) ), and elemental analysis, pyrolysis gas chromatography mass spectrometry (pyrolysis GC/MS), etc. can be used to calculate the intensity of the signal derived from the fluorine-containing (meth)acrylate monomer. Among these, pyrolysis GC/MS is particularly useful.

Examples of unsaturated carboxylic acid monomers include acrylic acid, methacrylic acid, and crotonic acid. One type of unsaturated carboxylic acid monomer may be used individually, or two or more types may be used in combination at an arbitrary ratio.

(Meth)acrylic acid ester monomers include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butylacrylate, isobutylacrylate, t-butylacrylate, pentyl acrylate, and hexyl acrylate. , heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate, cyclohexyl acrylate, hydroxyethyl acrylate. Latex, benzyl acrylate, isobornyl acrylate, dicyclopentanyl acrylate, dicyclopentenyl acrylate, hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxide Acrylic acid esters such as hydroxybutyl acrylate, 5-hydroxypentyl acrylate, 6-hydroxyhexyl acrylate, 7-hydroxyheptisyl acrylate, and 8-hydroxyoctylacrylate; Methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, t-Butylcyclohexyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate Crylate, n-tetradecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, hydroxyethyl methacrylate, benzyl methacrylate, isobornyl methacrylate, dicyclopentanyl methacrylate, DC Clopentenyl methacrylate, hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, 5-hydroxypentyl methacrylate, and methacrylic acid esters such as 6-hydroxyhexyl methacrylate, 7-hydroxyhepticyl methacrylate, and 8-hydroxyoctyl methacrylate. One type of (meth)acrylic acid ester monomer may be used individually, or two or more types may be used in combination in an arbitrary ratio.

Among the above (meth)acrylic acid ester monomers, it is preferable to use a (meth)acrylate monomer having a hydroxyl group. By using a (meth)acrylate monomer having a hydroxyl group, the glass transition temperature of the organic resin particles can be adjusted to ensure excellent dry adhesion to the electrode. The (meth)acrylate monomer having a hydroxyl group may be used individually, or two or more types may be used in combination at any ratio. In particular, hydroxyethyl acrylate (HEA), 4-hydroxybutylacrylate (4-HBA), and 2-hydroxypropylacrylate (2-HPA) are preferred.

Examples of styrene-based monomers include styrene, α-methylstyrene, paramethylstyrene, t-butylstyrene, chlorostyrene, chloromethylstyrene, and hydroxymethylstyrene. Examples of olefinic monomers include ethylene and propylene. Examples of diene monomers include butadiene and isoprene.

Examples of the acrylamide-based monomer include acrylamide. Among these, one type may be used individually, and two or more types may be used in combination at an arbitrary ratio.

The vinylidene fluoride monomer may be a homopolymer using vinylidene fluoride monomer alone or a copolymer with other monomers. Monomers copolymerizable with vinylidene fluoride include, for example, tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, trichloroethylene, vinyl fluoride, (meth)acrylic acid, methyl (meth)acrylate, and ethyl (meth)acrylate. (meth)acrylic acid ester, vinyl acetate, vinyl chloride, and acrylonitrile. These may be used individually, or two or more types may be used in combination at any ratio.

The organic resin particles are a mixture containing a polymer polymerized using a fluorine-containing (meth)acrylate monomer and a polymer polymerized using a (meth)acrylate monomer having a hydroxyl group, or a fluorine-containing (meth)acrylate monomer and a hydroxyl group. It is preferable that a monomer mixture containing a (meth)acrylate monomer having is used to contain a polymerized copolymer.

The content of the polymer or copolymer polymerized using a (meth)acrylate monomer having a hydroxyl group contained in the organic resin particles is greater than 0% by mass, and is 7.0% by mass, when the total components of the organic resin particles are 100% by mass. % or less is preferable. More preferably, it is 0.5 mass% or more and 5.0 mass% or less, and even more preferably, it is 1.0 mass% or more and 3.0 mass% or less. When this content is greater than 0% by mass, the polymerization stability of the organic resin particles improves. Also, by setting it to 7.0% by mass or less, sufficient dry adhesion to the electrode can be obtained. In addition, by suppressing the swelling in the electrolyte solution, sufficient wet adhesion with the electrode is obtained, so that the distance between electrodes becomes constant, and a decrease in capacity during initial charging and discharging can be suppressed, thereby improving the yield during initial charging and discharging. It improves.

Additionally, the content of the polymer or copolymer polymerized using a (meth)acrylate monomer having a hydroxyl group contained in the organic resin particles can be measured using a known method. For example, first, the porous layer is separated from the porous film using an organic solvent such as water and alcohol, and the organic solvent such as water and alcohol is sufficiently dried to obtain the components contained in the porous layer. An organic solvent that dissolves the organic resin component is added to the obtained components to dissolve only the organic resin component. Subsequently, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. Using the obtained organic resin component, the presence or absence of a (meth)acrylate monomer having a hydroxyl group ^was confirmed by thermal decomposition GC/MS, and then the organic resin component and an appropriate amount of solvent (in The content of the copolymer polymerized using a (meth)acrylate monomer having a hydroxyl group can be determined by filling with chloroform and measuring by DD/MAS method after standing overnight.

When obtaining a copolymer polymerized using a fluorine-containing (meth)acrylate monomer and a (meth)acrylate monomer having a hydroxyl group, it is advisable to further polymerize using a monomer having two or more reactive groups per molecule. desirable. By using a monomer having two or more reactive groups per molecule, polymer particles with excellent electrolyte resistance that suppresses swelling in the electrolyte solution and excellent dry adhesion to electrodes and wet adhesion to electrodes in electrolyte solution can be obtained. You can.

As the monomer having two or more reactive groups per molecule, for example, it is preferable to use a (meth)acrylate monomer having two or more reactive groups per molecule, such as alkylene glycol di(meth)acrylate, and urethane di. It is more preferable to use (meth)acrylate.

The polymer contained in the organic resin particles is polymerized using at least one monomer among the monomers that are the raw materials of the polymer, and the glass transition temperature of the polymer when polymerized with only that monomer is -100°C or higher and 0°C or lower. It is desirable. The range of this glass transition temperature is more preferably -70°C or higher and -10°C or lower, and even more preferably -50°C or higher and -20°C. Here, the glass transition temperature is the midpoint glass transition temperature measured by differential scanning calorimetry (DSC) according to JIS K7121:2012. The midpoint glass transition temperature is the temperature at the point where a straight line equidistant from the straight line extending from each baseline and the curve of the step-shaped change portion of the glass transition intersect.

The proportion of monomers having a glass transition temperature of -100°C or higher and 0°C or lower when polymerized with only the above-mentioned monomers is greater than 0% by mass and is 10.0% by mass when the total components of the organic resin particles are 100% by mass. It is preferable that it is below. This ratio is more preferably 1 mass% or more and 7.0 mass% or less, and even more preferably 3.0 mass% or more and 5.0 mass% or less. By making it greater than 0% by mass, the organic resin particles become flexible and the adhesiveness becomes good. On the other hand, by setting it to less than 10.0% by mass, softening of the organic resin particles becomes difficult to occur, thereby suppressing the swelling property of the organic resin particles into the electrolyte solution. If the proportion of monomers whose glass transition temperature is -100°C or higher and 0°C or lower when polymerized with only those monomers is reduced, the rate of decline in wet adhesion after dry adhesion tends to be suppressed and is advantageous in terms of improvement in wet adhesion. .

The porous layer of the porous film of the present invention may contain organic resin particles that provide different functions in addition to organic resin particles that have adhesiveness to the electrode. That is, the porous layer may contain at least two types of organic resin particles. Organic resin particles imparting different functions may be used as an emulsion binder to bring organic resin particles into close contact with each other or between organic resin particles and inorganic particles, or to provide a binder function to bring organic resin particles into close contact with a porous substrate. Adhesion is improved by the binder function, and adhesion to the electrode can be further improved. As organic resin particles, resins that are electrochemically stable within the range of battery use are preferable.

In this specification, organic resin particles include those that have a particle shape and those that are partially formed into a film and fused to surrounding particles and a binder. The shape is not particularly limited and may be spherical, polygonal, flat, fibrous, etc.

The average particle diameter of the organic resin particles is preferably 100 nm or more. More preferably 120 nm or more, further preferably 150 nm or more, and most preferably 170 nm or more. Additionally, 500 nm or less is preferable, more preferably 400 nm or less, further preferably 300 nm or less, and most preferably 250 nm or less. By setting the average particle diameter to 100 nm or more, a porous structure is obtained, and rate characteristics and battery life become better. Additionally, by setting it to 500 nm or less, the film thickness of the porous layer becomes appropriate, and rate characteristics and battery life can be further improved.

The average particle diameter of organic resin particles can be measured using the following method. Using an electrolytic radial scanning electron microscope (S-3400N manufactured by Hitachi Seisakusho Co., Ltd.), an image (Image 1) of the surface of the porous layer was captured at a magnification of 30,000 times, and elements contained only in the inorganic particles were captured in the same field of view. Obtain the target EDX image (image 2). The image size 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. Particles that do not contain elements found only in inorganic particles in Image 2 are called organic resin particles. When they cannot be distinguished with the naked eye, those containing the fluorine element are called organic resin particles through elemental analysis. Next, for the organic resin particles, a circumscribed rectangle (including a square) with the minimum area for each organic resin is drawn on Image 1, and the length of the long side (one side in the case of a square) is defined as that of the particle. As the particle size, each particle size was measured for all organic resin particles visible in Image 1, and the arithmetic mean value was taken as the average particle size. However, if 50 organic resin particles are not observed in image 1, multiple images are captured, and imaging is performed until the total number of organic resin particles included in the multiple images reaches 50. did. The organic resin particles were measured, and the arithmetic mean value was taken as the average particle diameter.

(2) Inorganic particles

The porous layer of the porous film of the present invention contains inorganic particles. When the porous layer contains inorganic particles, thermal dimensional stability and short-circuiting due to foreign substances can be suppressed.

Specific examples of 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, barium fluoride, and barium sulfate. and poorly soluble ionic crystal particles. The inorganic particles are preferably particles composed of at least one selected from the group consisting of inorganic hydroxides, inorganic oxides, and inorganic sulfur oxides, and among them, aluminum oxide, which is effective in increasing strength, and dispersion of organic resin particles and inorganic particles. Boehmite and barium sulfate, which are effective in reducing wear of parts, are particularly preferred. Additionally, these inorganic particles may be used alone or in combination of two or more types.

The shape of the inorganic particles to be used includes spherical shape, plate shape, needle shape, rod shape, elliptical shape, etc., and may be of any shape. Among these, spherical shapes are preferable from the viewpoints of surface modification, dispersibility, and coatability.

(3) Binder

The porous layer of the porous film of the present invention may contain a binder in order to bring the organic resin particles and inorganic particles described above into close contact with each other and to bring the organic resin particles and inorganic particles into close contact with the porous substrate. As the binder, a resin that is electrochemically stable within the battery usage range is preferable. In addition, binders include binders soluble in organic solvents, water-soluble binders, and emulsion binders, and may be used alone or in combination.

When using a binder soluble in an organic solvent or a water-soluble binder, the preferred viscosity of the binder itself is preferably 10000 mPa·s or less when the concentration is 15% by mass. More preferably, it is 8000 mPa·s or less, and even more preferably, it is 5000 mPa·s or less. By setting the concentration to 15% by mass and the viscosity to 10000 mPa·s or less, an increase in the viscosity of the coating agent can be suppressed, and the organic resin particles are distributed on the surface, thereby improving dry adhesion and wet adhesion to the electrode.

In addition, when using an emulsion binder, the dispersant may be water or an organic solvent, such as an alcohol-based solvent such as ethanol or a ketone-based solvent such as acetone. However, water dispersions are used in terms of ease of handling and miscibility with other components. It is desirable in The particle size of the emulsion binder is 30 to 1000 nm, preferably 50 to 500 nm, more preferably 70 to 400 nm, and even more preferably 80 to 300 nm. By setting the particle size of the emulsion binder to 30 nm or more, an increase in air permeability can be suppressed and battery characteristics become good. Additionally, by setting it to 1000 nm or less, sufficient adhesion between the porous layer and the porous substrate can be obtained.

Resins that can be used in the binder include, for example, polyamide, polyamidoimide, polyimide, polyetherimide, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polysulfone, Resins such as polyketone, polyether ketone, polycarbonate, polyacetal, polyvinyl alcohol, polyethylene glycol, cellulose ether, acrylic resin, polyethylene, polypropylene, polystyrene, and urethane can be mentioned. Among these, the use of an acrylic resin is particularly preferable because stronger adhesion is obtained through interaction with the organic resin particles. In addition, by using polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer (hereinafter sometimes referred to as “polyvinylidene fluoride-based resin”), the adhesion to the electrode in the electrolyte solution is further improved. Particularly desirable. These resins may be used singly or in combination of two or more types as needed.

In addition, the vinylidene fluoride content of the polyvinylidene fluoride-based resin is preferably 80% by mass or more and less than 100% by mass of the components constituting the resin. More preferably, it is 85 mass% or more and is 99 mass% or less. More preferably, it is 90 mass% or more and is further preferably 98 mass% or less. If the vinylidene fluoride content is less than 80% by mass, sufficient mechanical strength is not obtained, and although adhesiveness to the electrode is developed, the strength is weak, so peeling may become easy. Additionally, when the vinylidene fluoride content is 100% by mass, the electrolyte resistance decreases, so there are cases where sufficient adhesiveness cannot be obtained.

When using a water-soluble binder, the content in the porous layer is preferably 0.5% by mass or more relative to the total amount of organic resin particles and inorganic particles. More preferably, it is 1 mass% or more, and even more preferably, it is 1.5 mass% or more. Additionally, 10% by mass or less is preferable. More preferably, it is 8 mass% or less, and even more preferably, it is 6 mass% or less. By setting the water-soluble binder content to 0.5% by mass or more, sufficient adhesion between the porous layer and the porous substrate can be obtained. Additionally, by setting it to 10% by mass or less, an increase in air permeability can be suppressed, and battery characteristics become good.

When using an emulsion binder, the content in the porous layer is preferably 1% by mass or more relative to the total amount of organic resin particles and inorganic particles. More preferably 5 mass% or more, further preferably 7.5 mass% or more, and most preferably 10 mass% or more. Moreover, 30 mass% or less is preferable, more preferably 25 mass% or less, and even more preferably 20 mass% or less. By setting the content of the emulsion-based binder to 1% by mass or more, sufficient adhesion between the porous layer and the porous substrate can be obtained. Additionally, by setting it to 30% by mass or less, an increase in air permeability can be suppressed, and battery characteristics become good. In particular, by setting it to 7.5% by mass or more and 20% by mass or less, not only does it promote adhesion of the organic resin particles and inorganic particles and the adhesion of these particles to the substrate, but also interacts with the organic resin particles, showing dry adhesion to the electrode and wet adhesion. Adhesion is also improved.

(4) Formation of porous layer

The method of forming the porous layer is explained below.

The porous layer may be formed through a two-step application process using two types of coating liquids, or may be formed through a one-step application process using one type of coating liquid.

The two-step application process refers to adjusting a coating solution made of inorganic particles and a solvent, applying it on a porous substrate, drying the solvent of the water-based dispersed coating solution, and then adjusting the coating solution made of organic resin particles and a solvent. This is a method of obtaining a porous film by applying this onto the inorganic particle coating layer and drying the solvent of the coating solution to form a porous layer. As an application method, spray coating may be used.

The first-step application process is a method of preparing a coating solution consisting of organic resin particles, inorganic particles, and a solvent, applying it on a porous substrate, drying the solvent in the coating solution to form a porous layer, and obtaining a porous film.

There is no particular limitation on the coating method, but for the latter, cost reduction is possible by reducing the number of coating applications. Accordingly, the method of forming a porous layer by the latter method will be described below.

(i) First, a coating solution is prepared by dispersing organic resin particles at a predetermined concentration. Aqueous dispersion coating liquids are prepared by dispersing, suspending, or emulsifying organic resin particles in a solvent. The solvent for the coating solution is not necessarily limited, but is not particularly limited as long as it is a solvent that can disperse, suspend, or emulsify the organic resin particles 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. there is. From the viewpoint of low environmental load, safety, and economics, an aqueous emulsion obtained by emulsifying an organic resin in water or a mixture of water and alcohol is preferable. When using water, you may add additional solvents other than water.

It is preferable that the solid content concentration of the coating liquid is 5% or more and 40% or less. By setting it within a predetermined range, both coating stability and surface unevenness during coating and drying can be achieved. Additionally, the solution viscosity of the coating liquid is preferably 5 mPa·s or more and 50 mPa·s or less. By setting it within a predetermined range, it is possible to achieve both dispersibility of the coating liquid and surface unevenness during coating and drying. By adjusting the solid content concentration of the coating solution to be low and the viscosity to be within the above preferable range, the organic resin particles can be adjusted to be distributed unevenly in the outer layer.

Additionally, film forming aids, dispersants, thickeners, stabilizers, anti-foaming agents, leveling agents, etc. may be added to the coating liquid as needed.

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. Among them, by selecting a dispersion method (bead mill or sand mill) in which the pressure applied to the inorganic particles and organic resin particles is high during dispersion, dispersibility is improved and surface unevenness of the organic resin particles becomes more likely to occur. These plurality of Dispersion may be carried out step by step by combining a mixing and dispersing machine.

(ii) Next, the obtained coating solution is applied onto the porous substrate. 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, pat printing, Other types of printing, etc. are available. It is not limited to these, and the coating method may be selected according to the desired conditions such as the organic resin used, binder, dispersant, leveling agent, solvent used, and base material.

(iii) Afterwards, the solvent in the coating solution is dried to form a porous layer. The drying temperature is preferably 40°C or higher and 100°C or lower. By doing so, the drying of the porous layer becomes uniform, and the arithmetic mean height (Sa) in a 2200 ㎛ square of the surface of the porous film on the side in contact with the porous layer is equal to the arithmetic mean height (Sa) in a 2200 ㎛ square on the surface of the porous substrate on the side in contact with the porous layer. It becomes easier to be influenced by the arithmetic mean height (Sa), and adhesion to the electrode improves. When the temperature is below 40°C, the solvent in the coating liquid does not dry. On the other hand, when it is higher than 100°C, the amount of heat during drying increases, the shape of the particles is not maintained and the film is formed, and the surface unevenness of the organic resin particles does not occur, so β/α = 0. Therefore, it becomes possible to achieve both good adhesion to electrodes by setting it within a predetermined range and low cost by improving coating and drying speeds.

[Porous substrate]

In the present invention, the porous substrate has micropores inside and has a structure in which these micropores are connected from one side to the other side. Examples of porous substrates include microporous membranes, nonwoven fabrics, and porous membrane sheets made of fibrous materials. Arithmetic average height (Sa) in 2200 ㎛ square and arithmetic average roughness in 12 nm square of the surface of the porous substrate on the side in contact with the porous layer, based on the viewpoint and manufacturing method that is electrically insulating, electrically stable, and stable in electrolyte solutions. (Ra), it is preferable that the porous substrate is a polyolefin microporous film from the viewpoint of ease of adjustment of the height (Spk) of the protruding ridge in a square of 2200 μm. That is, it is preferable that it is a porous membrane made of polyolefin. Resins constituting the polyolefin microporous membrane include polyethylene, polypropylene, ethylene-propylene copolymer, and mixtures of these. The polyolefin microporous membrane may be a single membrane or a laminated membrane having a plurality of layers. Examples include a single membrane containing 90% by mass or more of polyethylene, a laminated membrane made of polyethylene and polypropylene, etc.

The lower limit of the arithmetic mean roughness (Ra) in 12 nm in all directions on the surface of the porous substrate on the side in contact with the porous layer is preferably 10 nm or more and less than 80 nm. By setting the arithmetic mean roughness (Ra) to 10 nm or more, the transportability of the porous base material when forming the porous layer is improved, so that uniform layer formation is possible and the arithmetic mean roughness of the porous film is also improved, Since fine irregularities exist in a local area of 12 nm in all directions, it becomes easy to follow the irregularities of the active material in the electrode, and dry adhesion and wet adhesion with the electrode can be further improved. In addition, by setting the arithmetic mean roughness (Ra) to less than 80 nm, electrolyte injection properties are improved, and when used as a battery separator, good rate characteristics and battery life can be exhibited. The upper limit of the arithmetic mean roughness (Ra) is preferably 60 nm or less, and more preferably 40 nm or less.

The height (Spk) of the protruding ridges on the surface of the porous substrate on the side in contact with the porous layer in a square of 2200 ㎛ is preferably 0.01 ㎛ or more and less than 0.12 ㎛, and the lower limit is more preferably 0.03 ㎛ or more, and is 0.05 ㎛ or more. It is more desirable. The upper limit of the height of the protruding ridge (Spk) is more preferably less than 0.12 μm, and even more preferably 0.10 μm or less. In addition, by setting the height (Spk) of the protruding ridge to 0.01 ㎛ or more, the transportability of the porous substrate when forming the porous layer is improved, and uniform layer formation becomes possible, resulting in uneven distribution of the organic resin particles to the surface layer. tends to occur, improving dry adhesion and wet adhesion to the electrode. On the other hand, by setting the height (Spk) of the protruding ridge to less than 0.12 μm, electrolyte injection properties are improved, and good rate characteristics and battery life are achieved.

The thickness of the porous substrate is preferably 3 μm or more and 15 μm or less, the lower limit is more preferably 5 μm or more, and the upper limit is more preferably 12 μm or less.

Next, the manufacturing method of the porous substrate is explained. Here, the manufacturing method of the porous substrate is not particularly limited, but the manufacturing method of the polyolefin microporous membrane is explained below.

(a) First, a plasticizer is added to the resin composition made of the above-described polyolefin in a twin-screw extruder, and melt-kneaded to adjust the resin solution.

The polyolefin resin composition is composed of polyolefin resin, and may be a single composition or a mixture of two or more types of polyolefin resin. Polyolefin resins include, but are not limited to, polyethylene and polypropylene. As the polyethylene resin, ultra-high molecular weight polyethylene, high-density polyethylene, and medium-density polyethylene low-density may be used as a single composition, or a mixture of different molecular weights may be used. It is preferable to use a mixture of two or more types of polyethylene selected from the group consisting of ultra-high molecular weight polyethylene, high-density polyethylene, medium-density polyethylene, and low-density polyethylene. In particular, ultra-high molecular weight polyethylene (A) with Mw of 1 × 10 ⁶ or more and Mw of 1 A mixture composed of polyethylene (B) having a mass of 10 ⁴ or more and less than 9 10 ⁴ is preferred, and a mixture containing ultra-high molecular weight polyethylene with a Mw of 1 10 ⁶ or more is more preferable. The ratio of ultra-high molecular weight polyethylene (A) and polyethylene (B) is preferably less than 50 mass%, with the total of ultra-high molecular weight polyethylene (A) and polyethylene (B) being 100% by mass. .

In order to enable stretching at a relatively high magnification, the plasticizer is preferably liquid at room temperature. Specific examples include aliphatic hydrocarbons such as nonane, decane, decaline, paraxylene, undecane, dodecane, and liquid paraffin. The mixing ratio of the polyolefin resin composition and the plasticizer is preferably such that the content of the polyolefin resin composition is 50% by mass or more and 90% by weight or less.

By providing the polyolefin resin composition, the content of the plasticizer, and the cooling process, the arithmetic mean height (Sa) or protruding peak height (Spk) in a square of 2200 ㎛ on the surface of the porous substrate on the side in contact with the porous layer, and the height of the protruding ridge (Spk) in a square of 12 nm It becomes possible to adjust the arithmetic mean roughness (Ra) to be small, and it becomes easy to keep it within a predetermined range.

(b) Next, the resin solution is fed from the extruder to the die, and the gel-like sheet is formed by extruding and cooling into a sheet shape.

(c) Afterwards, the gel sheet is stretched. The gel sheet is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof. The stretched surface magnification is preferably 4 to 100 times, more preferably 12 to 64 times, and especially preferably 25 to 49 times. As the draw ratio is increased, the arithmetic mean height (Sa), the height of protruding ridges (Spk), and the arithmetic mean roughness (Ra) in a 12 nm square of the porous substrate can be reduced.

(d) Afterwards, the plasticizer is removed using a washing solvent and dried to obtain a polyolefin microporous film.

(e) In addition to the above steps, stretching (also called dry re-stretching) may be performed after the drying step. The second stretching can be performed by a tenter method or the like, similar to the stretching described above, while heating the polyolefin microporous membrane. The magnification of re-stretching is preferably 1.01 to 2.0 times in the case of uniaxial stretching, and preferably 1.01 to 2.0 times in the case of biaxial stretching. The larger the re-drawing ratio, the smaller the arithmetic mean height (Sa), the height of protruding ridges (Spk), and the arithmetic mean roughness (Ra) at 12 nm in all directions on the surface of the porous substrate on the side in contact with the porous layer. You can.

[Porous film]

The porous film of the present invention preferably has an arithmetic mean height (Sa) of 0.005 µm or more and less than 0.085 µm in an area of 2200 µm in all directions on the surface on the side where the porous layer is formed. More preferably, it is 0.060㎛ or less. By setting it within the above range, it becomes possible to adjust the followability to the unevenness of the active material present in the electrode during the bonding process (heat pressing) with the electrode described above, and the electrolyte solution pouring property can be further improved. For porous films, the arithmetic mean height (Sa) on a 2200 ㎛ square on the surface on the side where the porous layer is formed is greatly influenced by the arithmetic mean height (Sa) on a 2,200 ㎛ square on the surface of the porous substrate on the side where the porous layer is formed. There is a tendency for the arithmetic mean height (Sa) of the surface on the side in contact with the porous layer of the porous substrate to be within a predetermined range by appropriately selecting the arithmetic mean height (Sa) within the above-mentioned preferable range.

[Secondary battery]

The porous film of the present invention can be suitably used in a separator for secondary batteries such as lithium ion batteries. A lithium ion battery has a structure in which a separator for a secondary battery 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.

The positive electrode is a positive electrode material composed of an active material, a binder resin, and a conductive additive laminated on a current collector, and the active material is a lithium-containing transition metal oxide with a layered structure such as LiCoO ₂ , LiNiO ₂ , Li(NiCoMn)O ₂ , LiMn ₂ Spinel-type manganese oxides such as O ₄ and iron-based compounds such as LiFePO ₄ are included. As the binder resin, a resin with high oxidation resistance may be used. Specifically, fluorine resin, acrylic resin, styrene-butadiene resin, etc. can be mentioned. Carbon materials such as carbon black and graphite are used as conductive additives. As a current collector, metal foil is preferable, and aluminum foil is especially often used.

The negative electrode consists of an active material and a binder resin laminated on a current collector. The active material includes carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, lithium alloy materials such as tin and silicon, and metallic lithium. Metal materials, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), etc. can be mentioned. As the binder resin, fluorine resin, acrylic resin, styrene-butadiene resin, etc. are used. As a current collector, metal foil is preferable, and copper foil is especially often used.

The electrolyte solution serves as a field for moving ions between the positive electrode and the negative electrode in a secondary battery, and is composed of an electrolyte dissolved in an organic solvent. Examples of electrolytes include LiPF ₆ , LiBF ₄ , and LiClO ₄ , but LiPF ₆ is preferably used from the viewpoints of solubility in organic solvents and ionic conductivity. Examples of organic solvents include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate, and two or more types of these organic solvents may be mixed and used.

As a method of manufacturing a secondary battery, first, an active material and a conductive additive are dispersed in a binder resin solution to prepare a coating liquid for electrodes, this coating liquid is applied onto a current collector, and the solvent is dried to obtain a positive electrode and a negative electrode, respectively. The film thickness of the coated film after drying is preferably 50 μm or more and 500 μm or less. A separator for a secondary battery is placed between the obtained positive electrode and the negative electrode so as to be in contact with the active material layer of each electrode, encapsulated in an exterior material such as an aluminum laminated film, and after injecting the electrolyte solution, a negative electrode lead and a safety valve are installed, and the exterior material is sealed. do. The secondary battery obtained in this way has high adhesion between the electrode and the separator for secondary batteries, has excellent rate characteristics and battery life, and can be manufactured at low cost.

Example

Hereinafter, the present invention will be described in detail by way of examples, but the present invention is not limited thereto. In addition, in the following description, “%” and “part” represent “% by mass” and “part by mass”, respectively.

Hereinafter, the present invention will be specifically described by way of examples, but the present invention is not limited thereto in any way.

[measurement method]

(1) Arithmetic average height (Sa) and protruding peak height (Spk) in 2200㎛ on all sides of the surface of the porous substrate on the side in contact with the porous layer

The porous layer was removed from a sample cut into 5 cm x 5 cm pieces from the porous film using 20 g of water, and the water was sufficiently dried to obtain the desired porous substrate. Additionally, when sufficient separation cannot be achieved with water, an organic solvent such as alcohol is used and the residue is thoroughly dried after separation. Next, the obtained porous substrate was attached to a metal frame measuring 6 cm on all sides on the external side and 3 cm on all sides on the inner side so that there were no wrinkles. Using a scanning white interference microscope VS-1540 manufactured by Hitachi High-Tech Science, surface roughness was measured under the following conditions, and three-dimensional surface roughness parameters (Sa, Spk) were calculated based on ISO 25178. When a porous layer was formed on both sides of the porous substrate, measurements were performed on both surfaces of the porous substrate under the same conditions at 4 points on each side, a total of 8 points, and the measured values were averaged. In addition, when a porous layer was formed on one side of the porous substrate, measurements were made at 4 points on the surface of the porous substrate on the side in contact with the porous layer side under the same conditions, and the measured values were averaged.

·Objective lens: 5x

· Tube lens: 0.5 times

·Wavelength filter: 530white

·Camera: High resolution

·Measurement mode: Wave

·Cutoff: None.

(2) Volume content α of inorganic particles in the porous layer (volume content α)

The porous layer was extracted from a sample cut into 10 cm x 10 cm from the porous film using 40 g of water, and the water was sufficiently dried to obtain the components of the porous layer. Additionally, when sufficient separation cannot be achieved with water, an organic solvent such as alcohol may be used. After measuring the mass of the entire amount of the obtained components, the components were burned at a high temperature enough to melt and decompose the organic resin component to separate the organic resin component and the inorganic component, and then the mass of only the inorganic particles was measured. The content of inorganic particles in the porous layer was calculated in mass% from the equation (mass of inorganic particles/mass of total amount of constituents) x 100. Additionally, the content of the organic resin component in the porous layer was calculated in mass% using the equation ((mass of the total amount of constituents - mass of inorganic particles)/(mass of the total amount of constituents)) x 100. Then, the specific gravity of each component was measured with a hydrometer. The volume content of the inorganic particles in the porous layer was calculated in volume % from the mass content (mass %) of the inorganic particles and the organic resin component obtained first and the density of the inorganic particles and the organic resin component. The above measurement was performed on five samples, and the measured values were averaged.

(3) Occupancy rate β of inorganic particles on the surface of the porous layer (occupancy rate β)

The surface of the porous layer of the sample in which Pt/Pd was deposited on the porous film for 30 seconds was measured using SEM-EDX (Hitachi SE8200) at a magnification of 10,000 times and an acceleration voltage of 5.0 kV, and the inorganic elements of the inorganic particles were analyzed. Using image analysis software (Toyo Technica, SPIP 6.0.10), mask and exclude the element symbol display, magnification display, scale bar, and acceleration voltage display portion in the EDX image, then use the “Particle/hole analysis” mode as a detection method. The threshold level of the threshold was set to 130 nm, and the area ratio (%), which is a parameter expressing the area of the part of the inorganic particle corresponding to the inorganic element in percentage with respect to the area of the image, was set as the occupancy rate β of the inorganic particle. The above measurement was performed on five samples, and the measured values were averaged.

(4) Arithmetic average height (Sa) in 2200㎛ on all sides of the porous film surface on the side where the porous layer is formed

The sample was cut into 5cm x 5cm pieces from the porous film and attached to a metal frame measuring 6cm square on the outside and 3cm square on the inside to prevent wrinkles. Using a scanning white interference microscope VS-1540 manufactured by Hitachi High-Tech Science, surface roughness was measured under the following conditions, and the three-dimensional surface roughness parameter (Sa) was calculated based on ISO 25178. Under the same conditions, measurements were made on both surfaces of the porous film at 4 points on each side, for a total of 8 points, and the measured values were averaged. In addition, when the porous layer was installed on one side, the values measured at 4 points on the surface on the side where the porous layer was formed were averaged.

·Objective lens: 5x

· Tube lens: 0.5 times

·Wavelength filter: 530white

·Camera: High resolution

·Measurement mode: Wave

·Cutoff: None.

(5) Arithmetic average roughness (Ra) in 12 nm in all directions of the surface of the porous substrate on the side in contact with the porous layer

The porous layer was removed from a 12 cm x 12 cm sample cut from the porous film using 50 g of water, and the water was sufficiently dried to obtain the desired porous substrate. Additionally, when sufficient separation cannot be achieved with water, an organic solvent such as alcohol is used, and the solvent is sufficiently dried after separation. Using a scanning probe microscope (SPA-500 manufactured by Seiko Instruments), the porous substrate was fixed to the sample table with carbon tape, SI-DF40 was used as the cantilever, the measurement area was 12 ㎛ square, the amplitude attenuation rate was -0.25, The arithmetic mean roughness (Ra) was measured in DFM mode with a scanning frequency of 0.5 Hz. When a porous layer was formed on both sides of the porous substrate, measurements were performed on both surfaces of the porous substrate under the same conditions at 3 points on each side, a total of 6 points, and the measured values were averaged. In addition, when a porous layer was formed on one side of the porous substrate, three-point measurements were performed on the surface on the porous layer side of the porous substrate under the same conditions, and the measured values were arithmetic averaged.

(6) Surface free energy of porous layer

The surface of a sample cut to 100 mm Measured. The above measurement was performed on five samples, and the measured values were arithmetic averaged.

(7) Film thickness of porous layer

A cross-section of the sample was cut out on a microtome from the center of the sample cut out to 100 mm It was observed at a magnification of , and the distance from the interface with the porous substrate to the highest point on the surface was measured. In the case of a single side, only one side was measured, and in the case of a double side, both sides were measured, and the sum of the measurements was taken as the film thickness of the porous layer. Additionally, with respect to the interface between the porous substrate and the porous layer, in the area where inorganic particles exist, the place where inorganic particles were no longer identified was defined as the interface. The above measurement was performed on five samples, and the measured values were arithmetic averaged.

(8) Thickness of porous film

The film thickness at the center of the sample cut out to 100 mm x 100 mm from the porous film was measured using a contact thickness meter (Lightmatic, manufactured by Mitutoyo Co., Ltd.). The above measurement was performed on five samples, and the measured values were arithmetic averaged.

(9) Glass transition temperature of porous film

The midpoint glass transition temperature of the porous film was measured by differential scanning calorimetry (DSC) according to the provisions of “JIS K7121:2012 Method for measuring transition temperature of plastics.” The midpoint glass transition temperature was set as the temperature at the point where a straight line equidistant from the straight line extending from each baseline and the curve of the stepwise change portion of the glass transition intersect. The above measurement was performed on three samples, and the measured values were arithmetic averaged.

(10) Peel strength of porous substrate and porous layer

Samples cut into 25 mm wide and 100 mm long pieces from the porous film were attached to an acrylic plate with a thickness of 2 cm. Next, a tape (manufactured by 3M, product number 810-3-15) cut to 18 mm wide and 150 mm long was attached to the sample by roll pressing at 0.5 MPa, 25°C, and 0.2 m/min. After that, the tape was peeled at 180 degrees under the condition of a peeling speed of 300 mm/min, and the average value from 30 mm to 70 mm in the longitudinal direction was taken as the peeling strength.

· Peeling strength is “excellent”: The porous base material and the porous layer were peeled off with stronger force.

· Peeling strength is “excellent”: The porous base material and the porous layer were peeled off with strong force.

· Peeling strength is “good”: The porous base material and the porous layer were peeled off with a somewhat strong force.

· Peeling strength is “possible”: The porous base material and the porous layer were peeled off with a weak force.

· Peeling strength is “bad”: The porous base material and the porous layer were peeled off with very weak force.

Additionally, the magnitude of force required for peeling is “very good” > “excellent” > “good” > “possible” > “poor.”

(11) Thermal shrinkage (thermal dimensional stability)

The midpoint of one side and the midpoint of the opposite side of each sample were marked on three samples cut to 100 mm x 100 mm from the porous film, the length between the midpoints was measured, and heat treatment was performed for 30 minutes under no force in an oven at 150°C. For the sample after heat treatment, the length between the midpoints of the same location as before heat treatment was measured, the heat shrinkage rate was calculated from the equation below, and the heat shrinkage rate was calculated in the following 5 levels (less than 5% as "very good", 5% to 10% as "very good") The evaluation was performed as "Excellent", 10% to less than 20% as "Good", 20% to less than 40% as "Possible", and 40% or more as "Poor."

Heat shrinkage rate (%) = [(length between midpoints before heat treatment - length between midpoints after heat treatment)/(length between midpoints before heat treatment)] x 100.

(12) Dry adhesion with electrodes

The active material is Li(Ni _5/10 Mn _2/10 Co _3/10 )O ₂ , the binder is vinylidene fluoride resin, the conductive additive is acetylene black, a graphite positive electrode of 20 mm × 50 mm and a porous film of 25 mm × 55 mm are used. The layers were placed in contact, and heat pressing was performed for 10 seconds at a pressure of 6 MPa in a heating environment of 75°C to adhere the electrode and the porous film. Next, it was manually peeled using tweezers, and the adhesive strength was evaluated in the following five steps. Similarly, the adhesive strength of the negative electrode and the porous film, where the active material is graphite, the binder is vinylidene fluoride resin, and the conductive additive is carbon black, were measured, and the average adhesive strength combining the respective evaluation results of the positive electrode and negative electrode was determined as the adhesive strength. .

· Adhesion strength is “excellent”: the electrode and the porous film were separated with stronger force.

· Adhesion strength is “excellent”: The electrode and the porous film were peeled off with strong force.

· Adhesion strength is “good”: The electrode and the porous film were peeled off with a somewhat strong force.

· Adhesive strength is “possible”: The electrode and the porous film were peeled off with a weak force.

· Adhesive strength is “bad”: The electrode and the porous film peeled off with very weak force.

On the other hand, the magnitude of force required for peeling is "very good" > "excellent" > "good" > "possible" > "poor."

(13) Wet adhesion to electrodes

A negative electrode (width 20 mm x length 70 mm) containing graphite as the active material, vinylidene fluoride resin as the binder, and carbon black as the conductive additive was used as the electrode. A porous film (width 25 mm A test piece was produced by heat pressing for seconds) to bond the electrode and the porous film. The test piece was placed in an aluminum laminate film that was made into a bag by closing three pieces, and after the electrolyte injection process (1 g of the electrolyte solution was infiltrated from the porous film side of the test piece), the remaining side of the aluminum laminate film was sealed using a vacuum sealer. did. Here, the electrolyte solution was 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. Next, the aluminum laminate film after encapsulating this test piece was stored under standing conditions for 17 hours in an environment at 60°C. A test piece was taken out from the aluminum laminate film, and the electrolyte solution on the surface of the test piece was wiped off. After that, it was peeled off manually using tweezers, and the adhesive strength was evaluated in the following five steps.

· Adhesion strength is “excellent”: the electrode and the porous film were separated with stronger force.

· Adhesion strength is “excellent”: The electrode and the porous film were peeled off with strong force.

· Adhesion strength is “good”: The electrode and the porous film were peeled off with a somewhat strong force.

· Adhesive strength is “possible”: The electrode and the porous film were peeled off with a weak force.

· Adhesive strength is “bad”: The electrode and the porous film peeled off with very weak force.

On the other hand, the magnitude of force required for peeling is "very good" > "excellent" > "good" > "possible" > "poor."

(14) Electrolyte injectability

Five negative electrode sheets containing graphite as the active material, vinylidene fluoride resin as the binder, and carbon black as the conductive additive are cut into 8 cm x 8 cm. Also, cut 6 sheets of porous film into 8.5cm x 8.5cm. After that, the negative electrode and the porous film were placed alternately so that the active material layer and the porous layer were in contact, and heat pressing was performed at 75°C/6 MPa/10 seconds to produce a laminate in which the negative electrode and the porous film were adhered. After that, the above _- described laminate was sandwiched between 1 sheet of 90 mm It was dissolved as much as possible, and 6 g of the prepared electrolyte solution was added. After impregnating the electrolyte solution for 10 minutes, the laminate is taken out, and the porous film (1st sheet), the negative electrode (1st sheet), and the porous film (2nd sheet) are peeled from the laminate in that order, along with the negative electrode (2nd sheet), 4 Each porous film was taken out. The extent to which the porous film was impregnated with the electrolyte solution (electrolyte infusion ability) was evaluated in the following five steps.

· Electrolyte solution injectability was “excellent”: More than 90% of the porous film was impregnated with electrolyte solution.

· Electrolyte solution injectability was “excellent”: 75% or more but less than 90% of the porous film was impregnated with electrolyte solution.

· Electrolyte solution injectability was “good”: 60% or more but less than 75% of the porous film was impregnated with electrolyte solution.

· Electrolyte solution injectability was “possible”: 45% or more but less than 60% of the porous film was impregnated with electrolyte solution.

· Electrolyte solution injectability was “impossible”: less than 45% of the porous film was impregnated with electrolyte solution.

(15) Battery production

The positive electrode sheet contains 96 parts by mass of Li(Ni _5/10 Mn _2/10 Co _3/10 )O ₂ as a positive electrode active material, 1.0 parts by mass of acetylene black and graphite as a positive electrode conductive additive, and polyvinylidene fluoride 2 as a positive electrode binder. A positive electrode slurry in which a mass portion was dispersed in N-methyl-2-pyrrolidone using a planetary mixer was produced by applying, drying, and rolling onto aluminum foil (weight per unit area of application: 10.0 mg/cm ² ). This positive electrode sheet was cut into 40 mm x 40 mm. At this time, a tab adhesive portion for current collection without an active material layer was cut to a size of 5 mm x 5 mm on the outside of the active material surface. An aluminum tab with a width of 5 mm and a thickness of 0.1 mm was ultrasonic welded to the tab bonding portion. The negative electrode sheet contained 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 styrene-butadiene copolymer as a negative electrode binder, and a negative electrode slurry dispersed in water using a planetary mixer. It was produced by applying, drying, and rolling on the surface (weight per unit area of application: 6.6 mg/cm ² ). This negative electrode sheet was cut into 45 mm x 45 mm. At this time, a tab adhesive portion for current collection without an active material layer was cut to a size of 5 mm x 5 mm on the outside of the active material surface. A positive electrode tab and a copper tab of the same size were ultrasonic welded to the tab adhesive portion. Next, the porous film is cut to 55 mm x 55 mm, the positive electrode and the negative electrode are overlapped on both sides of the porous film with the active material layer sandwiching the porous film, and the positive electrode application portions are arranged so that all negative electrode application portions face each other to form an electrode group. got it After that, heat pressing was performed for 10 seconds at a pressure of 6 MPa in a heating environment of 75°C to bond the positive electrode, porous film, and negative electrode. The above positive electrode, porous film, and negative electrode were sandwiched between one sheet of 90 mm x 200 mm aluminum laminate film, 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 shape. LiPF ₆ as a solute was dissolved in a mixed solvent of ethylene carbonate:ethylmethyl carbonate = 3:7 (volume ratio) at a concentration of 1 mol/liter, and the prepared electrolyte solution was used. 1.5 g of electrolyte solution was injected into the aluminum laminate film in the shape of a bag, and the short sides of the aluminum laminate film were heat-sealed while impregnated under reduced pressure to obtain a laminated type battery.

(16) Discharge load characteristics

The discharge load characteristics were tested in the following order and evaluated by the discharge capacity maintenance rate. Using the above laminated battery, the discharge capacity when discharged at 0.5C at 25°C and the discharge capacity when discharged at 10C at 25°C were measured, (discharge capacity at 7C)/(0.5C) The discharge capacity maintenance rate (%) was calculated using the formula: (discharge capacity in) × 100. Here, the charging conditions were constant current charging at 0.5C and 4.3V, and the discharging conditions were constant current discharging at 2.7V. Five laminated batteries were manufactured, and the average of the three measurement results, excluding the results with the maximum and minimum discharge capacity maintenance rates, was taken as the capacity retention rate. A discharge capacity retention rate of less than 40% was considered “bad,” a discharge capacity retention rate of 45% to less than 50% was “good,” 50% to less than 55% was “excellent,” and 55% or more was “very good.”

(17) Life characteristics

The lifespan characteristics were tested in the following order and evaluated by the discharge capacity maintenance rate.

<1st to 300th cycle>

Charging and discharging were performed as one cycle, charging conditions were constant current charging of 2C and 4.3V, and discharging conditions were constant current discharging of 2C and 2.7V, and charging and discharging were repeated 300 times at 25°C.

<Calculation of discharge capacity maintenance rate>

The discharge capacity maintenance rate (%) was calculated using the formula (discharge capacity at 300th cycle)/(discharge capacity at 1st cycle) x 100. Five laminated batteries were manufactured, and the average of the three measurement results, excluding the results with the maximum and minimum discharge capacity maintenance rates, was taken as the capacity retention rate. If the discharge capacity retention rate is less than 50%, the life characteristic is “poor,” if it is 50% or more and less than 60%, the life characteristic is “good,” if it is 60% or more but less than 70%, the life characteristic is “excellent,” and if it is 70% or more, the life characteristic is “good.” He said it was “very excellent.”

(Example 1)

Dispersion A

120 parts of ion-exchanged water and 1 part of Adecaria Soph SR-1025 (emulsifier manufactured by Adeka Co., Ltd.) were added to the reactor, and stirring was started. To this, 0.4 parts of 2,2'-azobis(2-(2-imidazolin-2-yl)propane) (manufactured by Wako Pure Chemical Industries, Ltd.) was added under a nitrogen atmosphere, and 2,2,2-tri Fluoroethyl methacrylate (3FM) 24 parts, cyclohexyl acrylate (CHA) 56 parts, hydroxyethyl methacrylate (HEMA) 12 parts, alkylene glycol dimethacrylate (AGDMA) 9 parts, Adecaria A monomer mixture consisting of 8 parts of Soap SR-1025 (emulsifier, manufactured by Adeka Chemical Co., Ltd.) and 115 parts of ion-exchanged water was continuously added dropwise at 70°C over 1.5 hours, and polymerization was performed over 5 hours after completion of the dropwise addition. Dispersion A containing organic resin particles (polymer A) made of a copolymer (particle diameter: 190 nm, glass transition temperature: 42°C) was prepared.

Dispersion Z

Alumina particles (aluminum oxide) with an average particle diameter of 0.5 ㎛ were used as inorganic particles, the same amount of water as the inorganic particles as a solvent, and carboxymethyl cellulose as a dispersant were added in an amount of 1% by mass based on the inorganic particles, and then dispersed using a bead mill to form a dispersion solution. Z was prepared.

Coating amount

Dispersion A and Dispersion Z were dispersed in water and mixed with a stirrer so that the volume content α of the inorganic particles contained in the porous layer was 50% by volume and the solid content concentration was 22% by mass. The viscosity of the obtained coating liquid was 13 mPa·s.

The obtained coating solution was spread on a polyolefin microporous film (arithmetic average height (Sa) 0.06 μm, protruding ridge height (Spk) 0.06 μm, arithmetic average roughness (Ra) 40 nm, thickness 9 μm) as a porous substrate using a #10 wire bar. It was coated on both sides, dried for 1 minute in a hot air oven (drying temperature set at 45°C), and the contained solvent was volatilized to form a porous layer, thereby obtaining a porous film.

Table 1 shows the arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with the porous layer used in Examples 1 to 30, the type of inorganic particles, the volume content α, the type of organic resin particles, and coating conditions. Table 2 shows β/α of the porous films obtained in Examples 1 to 30, occupancy rate β of inorganic particles in the porous surface layer, arithmetic mean height (Sa) of the surface of the porous film on the side having the porous layer, surface free energy, and porous film. The glass transition temperature (°C) and film thickness of the porous layer are shown. Table 3 shows the measurement results of thermal dimensional stability, peel strength, dry adhesion to electrodes, electrolyte injectability, rate characteristics, and life characteristics for the porous films obtained in Examples 1 to 30 and batteries using them.

(Example 2)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.04 μm.

(Example 3)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.08 μm.

(Example 4)

A porous film of the present invention was obtained in the same manner as in Example 1, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.01 μm.

(Example 5)

A porous film was obtained in the same manner as in Example 1, except that the volume content α of the inorganic particles contained in the porous layer was changed to 30% by volume.

(Example 6)

A porous film was obtained in the same manner as in Example 1, except that the volume content α of the inorganic particles contained in the porous layer was changed to 80% by volume.

(Example 7)

A porous film was obtained in the same manner as in Example 1, except that the solid content concentration of the coating liquid was changed to 10 mass% and the viscosity was changed to 10 mPa·s.

(Example 8)

A porous film was obtained in the same manner as in Example 1, except that the solid content concentration of the coating liquid was changed to 40% by mass and the viscosity was changed to 40mPa·s.

(Example 9)

Dispersion A and Dispersion Z were adjusted as in Example 1.

Dispersion B

Dispersion B consisting of a polymer (polymer B) (particle diameter 140 nm) consisting of methyl acrylate as an acrylic acid ester monomer was prepared. Afterwards, dispersion A, dispersion Z, and dispersion B were mixed into a porous layer with a volume content α of inorganic particles of 50% by volume, a content of organic resin particles (polymer A) of 35% by volume, and a content of organic resin particles (polymer B). It was dispersed in water and mixed with a stirrer so that the content was 15% by volume and the solid content concentration was 22% by mass. The viscosity of the obtained coating liquid was 13 mPa·s. Examples except that the organic resin particles were made of a two-type mixture containing 90% by mass of polymer A made of fluorine-containing methacrylate monomer and 10% by mass of polymer B (average particle diameter 130nm) made of acrylic acid ester monomer. A porous film was obtained in the same manner as in 1.

(Example 10)

Polymer C was prepared in the same manner as dispersion A in Example 1, except that 2,2,2-trifluoroethyl methacrylate (3FM) was replaced with 1H,1H,5H-octafluoropentylacrylate (8FA). Dispersion C was prepared. After that, a porous film was obtained in the same manner as in Example 1.

(Example 11)

As monomers for composing polymer D forming organic resin particles, 2,2,2-trifluoroethyl methacrylate (3FM) 24 parts, cyclohexyl acrylate (CHA) 56 parts, hydroxyethyl methacrylate Instead of 12 parts of (HEMA) and 9 parts of alkylene glycol dimethacrylate (AGDMA), 100 parts of ethyl methacrylate was used as a methacrylic acid ester monomer, and was prepared in the same manner as dispersion A in Example 1, containing polymer D. Dispersion D was prepared. After that, a porous film was obtained in the same manner as in Example 1.

(Example 12)

A porous film was obtained in the same manner as in Example 11, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.08 μm.

(Example 13)

A porous film was obtained in the same manner as in Example 11, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.01 μm.

(Example 14)

As monomers for composing polymer E forming organic resin particles, 2,2,2-trifluoroethyl methacrylate (3FM) 24 parts, cyclohexyl acrylate (CHA) 56 parts, hydroxyethyl methacrylate Dispersion E containing polymer E was prepared in the same manner as dispersion A in Example 1, except that 100 parts of vinylidene fluoride monomer was used instead of 12 parts of (HEMA) and 9 parts of alkylene glycol dimethacrylate (AGDMA). . After that, a porous film was obtained in the same manner as in Example 1.

(Example 15)

A porous film was obtained in the same manner as in Example 1 except that the average particle diameter of the organic resin particles was set to 75 nm.

(Example 16)

A porous film was obtained in the same manner as in Example 1, except that the average particle diameter of the organic resin particles was 100 nm.

(Example 17)

A porous film was obtained in the same manner as in Example 1 except that the average particle diameter of the organic resin particles was 500 nm.

(Example 18)

A porous film was obtained in the same manner as in Example 1, except that the average particle diameter of the organic resin particles was 700 nm.

(Example 19)

A porous film was obtained in the same manner as in Example 1 except that the inorganic particles were barium sulfate particles.

(Example 20)

A porous film was obtained in the same manner as in Example 1, except that the inorganic particles were boehmite particles.

(Example 21)

A porous film was obtained in the same manner as in Example 1 except that the film thickness of the porous layer was 5 μm.

(Example 22)

A porous film was obtained in the same manner as in Example 1, except that the film thickness of the porous layer was 8 μm.

(Example 23)

A porous film was prepared in the same manner as in Example 1, except that the production conditions for dispersion A in Example 1 were 20 parts of 3FM, and the amounts of CHA, HEMA, and AGDMA were adjusted so that the glass transition temperature was 20°C to obtain polymer F. got it

(Example 24)

A porous film was prepared in the same manner as in Example 1, except that the production conditions of dispersion A in Example 1 were 80 parts of 3FM, and the amounts of CHA, HEMA, and AGDMA were adjusted so that the glass transition temperature was 20°C to obtain polymer G. got it

(Example 25)

A porous film was obtained in the same manner as in Example 1, except that the drying temperature of the coating liquid was set to 75°C.

(Example 26)

A porous film was obtained in the same manner as in Example 1, except that the drying temperature of the coating liquid was set to 100°C.

(Example 27)

As a porous substrate, the same polyolefin microporous film as in Example 1 was used, and dispersion Z was applied on both sides of the polyolefin microporous film using a #9 wire bar, and dried in a hot air oven (drying temperature set at 50°C) for 1 minute. , the contained solvent volatilized to form the first porous layer. After that, dispersion A was dispersed in water so that the solid content concentration was 6% by mass, and mixed with a stirrer. The viscosity of the obtained coating liquid was 7 mPa·s. The coating liquid is applied on both sides of the first coating layer using a #1.5 wire bar, dried in a hot air oven (drying temperature set at 50°C) for 1 minute, and the solvent contained therein volatilizes to form a second porous layer. A porous film was obtained in the same manner as in Example 6 except that this was done. By using the solid content concentration and wire bar as mentioned above, the occupancy rate β of the inorganic particles in the surface portion of the porous layer was 20%.

(Example 28)

A porous film was obtained in the same manner as in Example 27 except that Polymer D was used.

(Example 29)

In the same manner as in Example 28, except that Polymer H obtained by selecting a preferred methacrylic acid ester monomer (butyl methacrylate) was used to construct the polymer forming the organic resin particles so that the glass transition temperature of the porous layer was 20°C. Thus, a porous film was obtained.

(Example 30)

Select an acrylic acid ester monomer (50 parts methyl acrylate, 50 parts butyl acrylate) containing a preferred fluorine-containing acrylate monomer to form the polymer forming the organic resin particles so that the glass transition temperature of the porous layer is 80°C. A porous film was obtained in the same manner as in Example 1 except that the copolymer I obtained as described above was used.

The polymers in the table mean the following.

Polymer A: Polymer polymerized using fluorine-containing methacrylate monomer (proportion of fluorine-containing methacrylate monomer to total monomers used: 30% by mass)

Polymer B: Polymer polymerized using acrylic acid ester monomer

Polymer C: Polymer polymerized using fluorine-containing acrylate monomers

Polymer D: Polymer polymerized using methacrylic acid ester monomer

Polymer E: Polymer polymerized using vinylidene fluoride monomer

Polymer F: Polymer polymerized using fluorine-containing methacrylate monomer (proportion of fluorine-containing methacrylate monomer to total monomers used: 20% by mass)

Polymer G: Polymer polymerized using fluorine-containing methacrylate monomer (proportion of fluorine-containing methacrylate monomer to total monomers used: 80% by mass)

Polymer H: Polymer polymerized using methacrylic acid ester monomer

Polymer I: Copolymer polymerized using acrylic acid ester monomer

(Example 31)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean roughness (Ra) of the polyolefin microporous film was changed to 5 nm.

(Example 32)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean roughness (Ra) of the polyolefin microporous film was changed to 10 nm.

(Example 33)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean roughness (Ra) of the polyolefin microporous film was changed to 50 nm.

(Example 34)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean roughness (Ra) of the polyolefin microporous film was changed to 70 nm.

(Example 35)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean roughness (Ra) of the polyolefin microporous film was changed to 100 nm.

(Example 36)

A porous film was obtained in the same manner as in Example 1, except that the height (Spk) of the protruding ridge of the polyolefin microporous membrane was changed to 0.01 μm.

(Example 37)

A porous film was obtained in the same manner as in Example 1 except that the height (Spk) of the protruding ridge of the polyolefin microporous membrane was changed to 0.04 μm.

(Example 38)

A porous film was obtained in the same manner as in Example 1, except that the height (Spk) of the protruding ridge of the polyolefin microporous membrane was changed to 0.09 μm.

(Example 39)

A porous film was obtained in the same manner as in Example 1, except that the height (Spk) of the protruding ridge of the polyolefin microporous membrane was changed to 0.15 μm.

The arithmetic mean height (Sa), arithmetic mean roughness (Ra) or protruding ridge height (Spk) of the porous substrate used in Examples 31 to 39, the peeling strength for the porous film obtained in Examples 33 to 41 and the battery using the same, The measurement results of dry adhesion to the electrode, rate characteristics, and life characteristics are shown in Tables 4 and 5.

(Example 40)

Dispersion J containing polymer J was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 41)

Dispersion K containing polymer K was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 42)

Dispersion L containing polymer L was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 43)

Dispersion M containing polymer M was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 44)

Dispersion N containing polymer N was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 45)

Dispersion O containing polymer O was prepared in the same manner as dispersion A in Example 1, except that the composition of the organic resin particles was changed as shown in Table 6. After that, a porous film was obtained in the same manner as in Example 1.

(Example 46)

A porous film of the present invention was obtained in the same manner as in Example 42, except that the volume content α of the inorganic particles contained in the porous layer was changed to 30% by volume.

(Example 47)

A porous film of the present invention was obtained in the same manner as in Example 42, except that the volume content α of the inorganic particles contained in the porous layer was changed to 80% by volume.

(Example 48)

A porous film of the present invention was obtained in the same manner as in Example 42, except that the porous substrate was changed to a polyolefin microporous film with an arithmetic mean height (Sa) of 0.08 μm.

(Example 49)

A porous film of the present invention was obtained in the same manner as in Example 1, except that the porous substrate was changed to a polyolefin microporous film with an arithmetic mean height (Sa) of 0.01 μm.

Table 6 shows the arithmetic mean height (Sa) of the porous substrate used in Examples 40 to 49, the type of monomer used for polymerization of the polymer constituting the organic resin particles, and the glass transition of the polymer polymerized using only the monomer for each monomer. Temperature (℃), usage ratio (%) of each monomer, and usage ratio of monomers whose glass transition temperature of the polymer polymerized with only those monomers is -100℃ or higher and 0℃ or lower (indicated as “γ” in Table 6) ).

Table 7 shows the types of inorganic particles used in Examples 40 to 49, the volume content α, and β/α of the obtained porous film, the occupancy rate β of the inorganic particles in the porous surface layer, and the arithmetic mean of the porous film surface on the side where the porous layer is formed. Height (Sa), surface free energy, glass transition temperature (°C), thermal dimensional stability, peel strength, dry adhesion to electrodes, wet adhesion to electrodes, and rate characteristics and life characteristics of batteries using the above porous film. represents.

(Comparative Example 1)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.09 μm.

(Comparative Example 2)

A porous film was obtained in the same manner as in Example 1, except that the solid content concentration of the coating liquid was changed to 50 mass% and the viscosity was changed to 80 mPa·s.

(Comparative Example 3)

A porous film was obtained in the same manner as in Example 1, except that the drying temperature of the coating liquid was set to 110°C.

(Comparative Example 4)

A porous film was obtained in the same manner as in Example 1, except that the organic resin particles A were not used and the composition shown in Table 8 was changed.

(Comparative Example 5)

A porous film was obtained in the same manner as in Example 1 except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.15 μm.

(Comparative Example 6)

A porous film was obtained in the same manner as in Example 1, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.008 μm.

(Comparative Example 7)

A porous film was obtained in the same manner as in Example 42, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.008 μm.

(Comparative Example 8)

A porous film was obtained in the same manner as in Example 42, except that the arithmetic mean height (Sa) of the polyolefin microporous film was changed to 0.15 μm.

Table 8 shows the arithmetic mean height (Sa) of the porous substrate used in Comparative Examples 1 to 8, the type of monomer used for polymerization of the polymer constituting the organic resin particles, and the glass transition of the polymer polymerized using only the monomer for each monomer. Temperature (℃), usage ratio (%) of each monomer, and usage ratio of monomers whose glass transition temperature of the polymer polymerized with only those monomers is -100℃ or higher and 0℃ or lower (indicated as “γ” in Table 8) ). Table 9 shows the types of inorganic particles used in Comparative Examples 1 to 8, the volume content α, and β/α of the obtained porous film, the occupancy rate β of the inorganic particles in the porous surface layer, and the arithmetic mean of the surface of the porous film on the side where the porous layer is formed. Height (Sa), surface free energy, glass transition temperature (°C), and film thickness of the porous layer are shown. Table 10 shows the measurement results of thermal dimensional stability, peel strength, dry adhesion to electrodes, wet adhesion to electrodes, rate characteristics, and life characteristics for the porous films obtained in Comparative Examples 1 to 8 and batteries using them.

(Reference Example 1)

For the porous film obtained in Example 1, the heat press conditions for adhesion to the electrode were set to 75°C in a heating environment and a pressure of 2 MPa for 10 seconds to determine dry adhesion to the electrode, electrolyte injection ability, rate characteristics, and life characteristics. was measured. The measurement results obtained in Reference Examples 1 and 2 are shown in Table 11.

(Reference Example 2)

For the porous film obtained in Example 4, the heat press conditions for adhesion to electrodes were 75°C in a heating environment and a pressure of 2 MPa for 10 seconds to determine dry adhesion to electrodes, electrolyte injection properties, rate characteristics, and life characteristics. was measured.

From Tables 1 to 7, Examples 1 to 49 are all porous films having a porous substrate and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous substrate, wherein the porous substrate is the porous film. The arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with the layer in all directions of 2200 ㎛ is 0.01 ㎛ or more and less than 0.09 ㎛, and the volume of the total constituents of the porous layer is 100% by volume. The volume content of inorganic particles It is a porous film with α (volume %) and β (mass %) occupancy rate of inorganic particles on the surface of the porous layer being β/α<1, has excellent dry adhesion to electrodes, and has good rate characteristics and battery life. do. In addition, Examples 40 to 44 and 46 to 49 have excellent wet adhesion to electrodes by appropriately selecting the composition of the organic resin particles.

On the other hand, in Comparative Examples 1, 5, and 8, the arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with the porous layer was 0.09 μm or more, so the electrolyte solution injection property was poor. In Comparative Example 2, since the viscosity and solid content concentration of the coating solution were outside the desirable range, surface localization of the organic resin particles was inhibited and β/α = 1, so sufficient adhesion to the electrode was not obtained. In Comparative Example 3, since the drying temperature of the coating solution exceeded 100°C, the organic resin particles did not maintain their particle shape during the drying process and formed a film, thereby inhibiting the surface localization of the organic resin particles, β/α = 1 As a result, sufficient adhesion to the electrode is not obtained. Since Comparative Example 4 does not contain organic resin particles, surface unevenness of the organic resin particles does not occur, and thus β/α = 1, and sufficient adhesion to the electrode is not obtained. In Comparative Examples 6 and 7, the arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with the porous layer was less than 0.01 μm, so the adhesion to the electrode was poor.

In addition, Reference Examples 1 and 2 were different from Examples 1 and 4 when the adhesion between the porous film and the electrode obtained in Examples 1 and 4 was changed from 6MPa to 2MPa under heat press conditions, and the porous layer This shows an example in which the dry adhesion to the electrode increases and the electrolyte solution injectability decreases as the arithmetic mean height (Sa) of the surface of the porous substrate on the side in contact with 2200 μm in all directions decreases.

Claims (24)

A porous film having a porous substrate and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous substrate, wherein the porous substrate has an arithmetic area of 2200 μm on all sides of the surface on the side in contact with the porous layer. When the average height (Sa) is 0.01 ㎛ or more and less than 0.09 ㎛, and the volume of the total components of the porous layer is 100 volume %, the volume content α (volume %) of the inorganic particles and the inorganic particles on the surface of the porous layer are A porous film in which the relationship between occupancy β (area%) satisfies β/α<1. According to claim 1,
The porous film has an arithmetic mean height (Sa) of 0.005 µm or more and less than 0.085 µm in a 2200 µm square on the surface of the porous film. The method of claim 1 or 2,
A porous film in which the volume content α of the inorganic particles is 30 volume% or more and 80 volume% or less when the volume of all components of the porous layer is 100 volume%. The method according to any one of claims 1 to 3,
A porous film in which the occupancy rate β of inorganic particles in the surface portion of the porous layer is greater than 0%. The method according to any one of claims 1 to 4,
The porous substrate is a porous film in which the arithmetic mean roughness (Ra) of the surface on the side in contact with the porous layer is 10 nm or more and less than 80 nm in 12 nm in all directions. The method according to any one of claims 1 to 5,
The porous substrate is a porous film having a protruding ridge height (Spk) of 0.01 ㎛ or more and less than 0.12 ㎛ on a 2200 ㎛ square of the surface on the side in contact with the porous layer. The method according to any one of claims 1 to 6,
A porous film in which the surface free energy of the porous layer is 10 mN/m or more and 80 mN/m or less. The method according to any one of claims 1 to 7,
A porous film wherein the porous substrate is a polyolefin microporous film. The method according to any one of claims 1 to 8,
A porous film wherein the inorganic particles are particles composed of at least one member selected from the group consisting of inorganic hydroxides, inorganic oxides and inorganic sulfur oxides. The method according to any one of claims 1 to 9,
The organic resin particles are composed of a fluorine-containing (meth)acrylate monomer, an unsaturated carboxylic acid monomer, a (meth)acrylic acid ester monomer, a styrene-based monomer, an olefin-based monomer, a diene-based monomer, an acrylamide-based monomer, and a vinylidene fluoride monomer. A porous film having a polymer polymerized using at least one monomer selected from the group. According to claim 10,
A porous film in which the proportion of fluorine-containing (meth)acrylate monomer is 20% by mass or more and 80% by mass or less when the total monomer component of the organic resin particles is 100% by mass with respect to the organic resin particles. According to claim 10,
A porous film in which the organic resin particles include a polymer polymerized only with a fluorine-containing (meth)acrylate monomer. The method according to any one of claims 10 to 12,
A porous film in which the number of fluorine atoms contained in one molecule of the above-mentioned fluorine-containing (meth)acrylate monomer is 3 to 13. The method of claim 12 or 13,
A porous film in which the organic resin particles further include a polymer or copolymer polymerized using a (meth)acrylate monomer having a hydroxyl group. According to claim 14,
A porous film in which the proportion of (meth)acrylate monomers having a hydroxyl group is greater than 0 mass% and 7.0 mass% or less when the total monomer components of the organic resin particles are 100 mass% with respect to the organic resin particles. The method according to any one of claims 10 to 15,
With respect to the polymer contained in the organic resin particles, at least one monomer among the monomers that are the raw materials of the polymer is a monomer whose glass transition temperature of the polymer is -100°C or higher and 0°C or lower when polymerized with only that monomer. Characterized by porous film. According to claim 16,
The monomer having a glass transition temperature of -100°C or higher and 0°C or lower when polymerized using only the above-mentioned monomer has a porosity greater than 0% by mass and less than or equal to 10.0% by mass when the total monomer component of the organic resin particle is 100% by mass. film. The method according to any one of claims 1 to 17,
A porous film in which the porous layer contains at least two types of organic resin particles. The method according to any one of claims 1 to 18,
A porous film wherein the organic resin particles have an average particle diameter of 100 nm or more and 500 nm or less. The method according to any one of claims 1 to 19,
A porous film wherein the porous substrate has a film thickness of 3 μm or more and 15 μm or less. The method according to any one of claims 1 to 20,
A porous film wherein the porous layer has a film thickness of 2 μm or more and 8 μm or less. A separator for secondary batteries using the porous film according to any one of claims 1 to 21. A secondary battery formed using the separator for secondary batteries according to claim 22. A porous film having a porous substrate and a porous layer containing inorganic particles and organic resin particles on at least one side of the porous substrate,
The porous substrate is made of a polyolefin microporous film having an arithmetic mean height (Sa) of 0.01 µm or more and less than 0.09 µm on a surface of 2200 µm in all directions on the side in contact with the porous layer,
The inorganic particles are particles composed of at least one type selected from the group consisting of inorganic hydroxides, inorganic oxides and inorganic sulfur oxides,
The organic resin particles are a group consisting of a fluorine-containing (meth)acrylate monomer, an unsaturated carboxylic acid monomer, a (meth)acrylic acid ester monomer, a styrene-based monomer, an olefin-based monomer, a diene-based monomer, an amide-based monomer, and a vinylidene fluoride monomer. It is a polymer polymerized using at least one monomer selected from,
When the volume of all components of the porous layer is 100 volume%, the volume content α of inorganic particles is 30 volume% or more and 80 volume% or less,
A battery separator in which the relationship between the volume content α (volume%) of the inorganic particles and the occupancy rate β (area%) of the inorganic particles in the surface portion of the porous layer satisfies β>0 and β/α<1.