CN118020207A

CN118020207A - Porous film, separator for secondary battery, and secondary battery

Info

Publication number: CN118020207A
Application number: CN202280064213.3A
Authority: CN
Inventors: 加门庆一; 甲斐信康; 佐佐木崚; 大桥纯平; 佃明光; 今津直树
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2021-09-30
Filing date: 2022-08-30
Publication date: 2024-05-10
Also published as: WO2023053821A1

Abstract

A porous film comprising a porous substrate and a porous layer comprising inorganic particles and an organic resin A, which is located on at least one surface of the porous substrate, wherein the porous film satisfies 0.1-0.8 in terms of dry adhesion [ gamma ] (N/m) measured under condition a (70 ℃/5MPa/7 sec) and wet adhesion [ delta ] (N/m) measured under condition b (condition a+electrolyte injection step+60 ℃/17 hours of standing). A porous film having improved yield and electrolyte injection properties at the time of battery production and initial charge/discharge is provided at low cost.

Description

Porous film, separator for secondary battery, and secondary battery

Technical Field

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

Background

Secondary batteries such as lithium ion batteries are widely used in automotive applications such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, smart phones, tablets, mobile phones, notebook personal computers, digital cameras, digital video cameras, portable digital devices such as portable game machines, electric tools, electric bicycles, electric power assisted bicycles, and the like.

In general, 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.

As the separator for secondary batteries, a polyolefin porous substrate was used. As the characteristics required for the separator for a secondary battery, there are given characteristics that an electrolyte is contained in a porous structure and ion migration is possible; and a shutdown characteristic in which, when the lithium ion battery abnormally generates heat, the porous structure is sealed by melting due to heat, thereby stopping ion migration and stopping discharge.

The volumetric battery characteristics are also required to exhibit good battery characteristics without deteriorating the high output characteristics.

Further, in order to increase the energy density of the lithium ion battery, the battery shape is replaced from a winding type to a stacked type. In the case of lamination, in the manufacturing process of a secondary battery using an electrode laminate in which a positive electrode, a separator, and a negative electrode are laminated, the laminate structure shifts when the electrode laminate is conveyed, and therefore deterioration of the battery manufacturing yield becomes a problem. In addition, after the electrolyte is injected into the battery, the electrode expands and contracts due to the initial charge/discharge process, and the laminate structure shifts to form a gap, thereby causing a capacity decrease during the initial charge/discharge process and still reducing the yield.

In order to solve the above problems, in patent document 1, an adhesive layer formed on a heat-resistant layer is laminated, whereby the transfer property is improved by the appearance of the adhesion (dry adhesion) between a separator and an electrode before the electrolyte is impregnated. In patent document 2, a porous layer containing a poly-1, 1-difluoroethylene resin as a main component, which has adhesiveness (wet adhesiveness) to an electrode in a state of being impregnated with an electrolyte, is laminated on a porous substrate made of polyolefin, whereby a gap is less likely to occur between the electrode and a separator at the time of charge and discharge.

In patent documents 3 and 4, both dry adhesion and wet adhesion are exhibited, so that improvement in yield at the time of battery production and yield at the time of initial charge and discharge is studied.

Prior art literature

Patent literature

Patent document 1: japanese patent No. 6191597

Patent document 2: japanese patent application laid-open No. 2012-221741

Patent document 3: international publication No. 2013/151144

Patent document 4: international publication No. 2016/098684

Disclosure of Invention

Problems to be solved by the invention

However, in patent document 1, when the electrolyte is impregnated after the dry adhesion is exhibited, the acrylic resin may be swelled or dissolved by the electrolyte, and the separator may be easily peeled off from the electrode. In this case, even if the separator and the electrode are dry-bonded, a gap is formed between the separator and the electrode in the battery in a state immersed in the electrolyte, and therefore the inter-electrode distance is not constant at the time of initial charge and discharge, and capacity is reduced at the time of initial discharge, and improvement in yield is not achieved. In patent document 2, since dry adhesion is not mentioned, the electrode and the separator are offset at the time of battery production, and thus the yield is not improved yet.

In the methods described in patent documents 3 and 4, when the dry adhesion and the wet adhesion are strong, the electrolyte may not easily penetrate into the electrode and the separator when the electrolyte is injected, and the battery characteristics such as the rate characteristics and the lifetime characteristics may be degraded. On the other hand, in the case where both the dry adhesion and the wet adhesion are weak, the yield at the time of battery production and at the time of initial charge and discharge is not easily improved.

As described above, the improvement of the performance and the cost reduction of the secondary battery are both achieved by improving the yield and the electrolyte injection property both at the time of manufacturing the battery and at the time of initial charge and discharge. In view of the above problems, an object of the present invention is to provide a separator for a secondary battery that improves the yield and electrolyte injection properties at the time of battery production and initial charge/discharge at low cost.

Means for solving the problems

Accordingly, the present inventors have found that the battery manufacturing yield and the electrolyte injection performance are excellent by injecting an electrolyte solution after bonding an electrode and a porous film used as a separator by hot pressing the electrode and the porous film (dry adhesion) before injecting the electrolyte solution and then injecting the electrolyte solution so that the adhesion (wet adhesion) between the electrode and the porous film in the electrolyte solution is in a suitable range.

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

(1) A porous film comprising a porous substrate and a porous layer comprising inorganic particles and an organic resin A, which is located on at least one surface of the porous substrate, wherein the porous film satisfies 0.1-0.8 in terms of dry adhesion [ gamma ] (N/m) measured under condition a (70 ℃/5MPa/7 sec) and wet adhesion [ delta ] (N/m) measured under condition b (condition a+electrolyte injection step+60 ℃/17 hours standing).

(2) According to the porous film of (1), the volume content α (volume%) of the inorganic particles and the occupancy β (area%) of the inorganic particles in the surface portion of the porous layer satisfy β/α < 1 when the volume of all the constituent components of the porous layer is 100 volume%.

(3) The porous film according to (1) or (2), wherein the organic resin A is an organic resin particle.

(4) The porous film according to any one of (1) to (3), wherein the organic resin A is a polymer obtained by polymerizing at least 1 monomer selected from the group consisting of fluorine-containing (meth) acrylate monomers, unsaturated carboxylic acid monomers, (meth) acrylate monomers, styrene monomers, olefin monomers, diene monomers, and amide monomers.

(5) The porous film according to any one of (1) to (4), wherein the organic resin A is a copolymer of a polymer obtained by polymerizing a fluorine-containing (meth) acrylate monomer and a polymer obtained by polymerizing a (meth) acrylate monomer having a hydroxyl group.

(6) According to the porous film of (5), the ratio of the (meth) acrylate monomer having a hydroxyl group is 5.0 mass% or less, based on 100 mass% of the total constituent monomer components of the organic resin a.

(7) The porous film according to any one of (4) to (6), wherein at least 1 monomer among the monomers used as a raw material of the polymer contained in the organic resin A is a monomer having a glass transition temperature of-100 ℃ to 0 ℃ inclusive, which is a polymer obtained by polymerizing only the monomer.

(8) According to the porous film of (7), when the total constituent monomer components of the organic resin are 100 mass%, the glass transition temperature of the polymer obtained by polymerizing only the monomer is not less than-100 ℃ and not more than 0 ℃ and the proportion (. Epsilon.) of the monomer is less than 7.0 mass%.

(9) The porous film according to any one of (4) to (8), wherein the proportion of the fluorine-containing (meth) acrylate monomer is more than 20% by mass and 60% by mass or less based on 100% by mass of the total constituent monomer components of the organic resin A.

(10) The porous membrane according to any one of (4) to (9), wherein the number of fluorine atoms contained in one molecule of the fluorine-containing (meth) acrylate monomer is 3 or more and 13 or less.

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

(12) The porous film according to any one of (1) to (11), wherein the porous layer has a film thickness of 2 μm or more and 8 μm or less.

(13) A separator for a secondary battery, comprising the porous film according to any one of (1) to (12).

(14) A secondary battery having the separator for a secondary battery according to (13).

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to provide a porous film that can improve the yield and electrolyte injection properties at the time of battery production and initial charge/discharge at low cost.

Detailed Description

The porous film is characterized by comprising a porous substrate and a porous layer comprising inorganic particles and an organic resin A, which is positioned on at least one surface of the porous substrate, wherein the dry adhesion gamma (N/m) measured under the condition a (70 ℃/5MPa/7 seconds) and the wet adhesion delta (N/m) measured under the condition b (the condition a+electrolyte injection step+60 ℃/17 hours standing) satisfy 0.1-delta/gamma-0.8.

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

The porous film of the present invention has a dry adhesiveness of γ (N/m) measured under condition a (70 ℃ C./5 MPa/7 seconds) and a wet adhesiveness of δ (N/m) measured under condition b (condition a+electrolyte injection step+60 ℃ C./17 hours stationary) of 0.1.ltoreq.δ/γ.ltoreq.0.8. The condition a, described in detail below, refers to a step of dry-bonding an electrode (positive electrode and negative electrode) and a porous film by performing hot pressing under conditions of 70 ℃/5MPa/7 seconds after each of 1 sheet of the electrode and the porous film is stacked. The condition b is described in detail below, and means a step of wet-bonding an electrode and a porous film by vacuum-sealing the electrode and the porous film which have been dry-bonded under the condition a with an aluminum laminate film, then pouring a predetermined amount of electrolyte, and then allowing the resulting solution to stand at 60 ℃/17 hours. Delta/gamma refers to the rate of change of dry adhesion versus wet adhesion. By setting the lower limit value of δ/γ to 0.1 or more, formation of gaps due to displacement of the electrode and the porous film during initial charge and discharge is suppressed, and thus the yield during initial charge and discharge can be improved. Further, by setting the upper limit value to 0.8 or less, the adhesiveness to the electrode can be appropriately adjusted, and the electrolyte injection property becomes good, so that deterioration of the battery characteristics can be suppressed. The delta/gamma is more preferably 0.35 to 0.55. When the amount is within the above range, the yield at the time of initial charge and discharge is improved and the electrolyte solution injection property is further improved. The method for setting the conditions a and b to the above ranges is not particularly limited, and may be appropriately adjusted by the type of the electrode to be used and the type of the electrolyte. For example, if an electrode having a small porosity is used as the electrode, the electrolyte is less likely to enter between the active materials of the electrode, and therefore the organic resin a is not in contact with the electrolyte and is less likely to swell. Thus, δ becomes large, and hence δ/γ becomes large. On the other hand, if an electrode having a small surface roughness is used as the electrode, the contact area between the electrode and the porous layer tends to increase during hot pressing, and γ increases, so δ/γ decreases. In addition, if an electrolyte having a high LiPF ₆ concentration of a solute is used as the electrolyte, the viscosity of the electrolyte increases, and the electrolyte is less likely to enter between active materials of the electrode, so that the organic resin a is not in contact with the electrolyte and is less likely to swell. Thus, δ becomes large, and hence δ/γ becomes large. On the other hand, if the amount of the electrolyte is made large, the contact area with the porous layer increases, and the organic resin a tends to swell, and δ becomes smaller, so δ/γ becomes smaller.

[ Porous layer ]

The porous layer in the present invention contains inorganic particles and an organic resin a. The porous layer preferably has a volume content α (volume%) of inorganic particles and an occupancy β (area%) of inorganic particles in a surface portion of the porous layer satisfying β/α < 1, when the total constituent components of the porous layer are 100 volume%. The ratio β/α being smaller than 1 indicates that the occupancy rate of the inorganic particles in the surface portion of the porous layer is lower than the content rate of the inorganic particles in the entire porous layer, that is, indicates that the organic resin a is biased to the surface portion of the porous layer. Since the organic resin a is biased to the surface portion of the porous layer, the surface portion is present in a large amount, and thus the organic resin a exhibits sufficient dry adhesion and wet adhesion to the electrode.

The β/α is more preferably 0.5 or less, and still more preferably 0.3 or less. The lower limit of β/α is not particularly limited, but is preferably 0.01 or more.

The method for forming the porous layer having β/α in the above range is not particularly limited. The coating composition may be formed through a 2-stage coating process using 2 kinds of coating liquids, and more preferably, the coating composition may be formed through a 1-stage coating process using 1 kind of coating liquid. If the coating can be formed by a 1-stage coating process, the cost can be reduced by reducing the number of coating steps. In the case of the coating step of 1 stage, β/α can be set to a predetermined range by appropriately adjusting the surface free energy of the organic resin a, the viscosity of the coating liquid, the solid content concentration, and the drying temperature, for example. The solid content concentration of the coating liquid is preferably 5% or more and 40% or less. When the coating stability is within a predetermined range, the surface bias of the organic resin a during coating and drying can be combined, and the value of β becomes smaller, so that β/α becomes smaller. The solution viscosity of the coating liquid is preferably 5 mPas or more and 50 mPas or less. When the dispersion of the coating liquid is within a predetermined range, both the dispersibility of the coating liquid and the surface bias of the organic resin a during coating and drying can be achieved. By adjusting the solid content concentration of the coating liquid to be low and the viscosity to be small in the above-described preferable range, the value of β can be adjusted so as to bias the organic resin particles on the surface layer, and thus β/α becomes small. The drying temperature is preferably 40 ℃ or higher and 100 ℃ or lower. By doing so, the surface bias of the organic resin a caused by the layer separation at the time of drying is thereby liable to occur. Since the value of β becomes smaller, β/α tends to be smaller, and in the case of less than 40 ℃, the solvent of the coating liquid is not dried. On the other hand, when the temperature is higher than 100 ℃, the amount of heat generated during drying increases, and the shape of the particles cannot be maintained, so that the surface bias of the organic resin particles does not occur, and thus the result is β/α=0. Therefore, by adjusting β/α to a predetermined range, both good adhesion to the electrode and cost reduction due to an increase in coating and drying speeds can be achieved.

The volume content α of the inorganic particles is preferably 30% by volume or more and 80% by volume or less, more preferably 40% by volume or more and 70% by volume or less, and still more preferably 50% by volume or more and 60% by volume or less, based on 100% by volume of the total constituent components of the porous layer. The volume content β of the inorganic particles is 30% by volume or more, whereby sufficient thermal dimensional stability is obtained. Further, the content of the organic resin particles is 80% by volume or less, whereby the dry adhesion and wet adhesion to the electrode are improved. The volume content α of the inorganic particles to the whole porous layer can be calculated by the measurement method described in the examples.

The occupancy rate β of the inorganic particles in the surface portion of the porous layer is preferably greater than 0%. More preferably 1% or more, and still more preferably 5% or more. By having β larger than 0%, the cost can be reduced by reducing the number of coating steps by the 1-stage coating process or by reducing the cost of the coating material by the reduction of the raw material. The upper limit is not particularly limited, but is preferably less than 50%. More preferably 30% or less, and still more preferably 20% or less. When the content is less than 50%, the adhesion to the electrode becomes good. The term β=0 means that no inorganic particles are present on the surface of the porous layer, and all inorganic particles are present in the porous layer. The occupancy β of the inorganic particles in the surface portion of the porous layer can be calculated by the measurement method described in the examples. The surface portion of the porous layer is the outer surface of the porous layer and a deep surface layer affecting adhesion to the electrode, and is shown by an image obtained by SEM-EDX described later.

The surface free energy of the porous layer is preferably 10 to 80mN/m, more preferably 15 to 70mN/m, and still more preferably 20 to 60 mN/m. When the coating stability of the porous layer is 10mN/m or more, the coating stability of the porous layer is improved. Further, by being 80mN/m or less, surface bias due to layer separation of the organic resin A is liable to occur, and β/α is liable to be controlled.

The porous layer preferably has a glass transition temperature of 20 ℃ or higher and less than 80 ℃. The lower limit is more preferably 30℃or higher, and still more preferably 40℃or higher. The upper limit is more preferably 70℃or lower, and still more preferably 60℃or lower. When the glass transition temperature is 20 ℃ or higher, swelling in the electrolyte is suppressed, and wet adhesion, rate characteristics, and lifetime characteristics are improved. In addition, by being less than 80 ℃, the dry adhesion to the electrode is further improved. In order to bring the glass transition temperature into an appropriate range, it may be appropriately selected from a specific monomer group.

(1) Organic resin A

The organic resin a improves adhesion to the electrode. The resin constituting the organic resin a is preferably a resin having adhesion to the electrode, and the ion permeability and the rate characteristics are improved by biasing the organic resin a on the surface layer of the porous film. In addition, by making the surface free energy of the organic resin a low, β/α can be made low.

The organic resin a is preferably a polymer obtained by polymerizing at least 1 monomer selected from the group consisting of fluorine-containing (meth) acrylate monomers, unsaturated carboxylic acid monomers, (meth) acrylate monomers, styrene monomers, olefin monomers, diene monomers, and amide monomers. Among these, it is particularly desirable that the organic resin a has a mixture of a polymer obtained by polymerizing only a fluorine-containing (meth) acrylate monomer with other polymer or a copolymer of a fluorine-containing (meth) acrylate monomer with other polymer. Thus, the organic resin a can be biased to the surface side by making the surface free energy of the organic resin a low, and the adhesion between the porous layer and the electrode can be improved. In the present specification, "(meth) acrylate" means acrylate and/or methacrylate. The polymer obtained by polymerizing a fluorine-containing (meth) acrylate monomer contains a repeating unit obtained by polymerizing a fluorine-containing (meth) acrylate.

As the fluorine-containing (meth) acrylate monomer, examples thereof include 2, 2-trifluoroethyl (meth) acrylate, 2, 3-pentafluoropropyl (meth) acrylate, 2- (perfluorobutyl) ethyl (meth) acrylate, 3- (perfluorobutyl) -2-hydroxypropyl (meth) acrylate, 2- (perfluorohexyl) ethyl (meth) acrylate, 3-perfluorohexyl-2-hydroxypropyl (meth) acrylate, 3- (perfluoro-3-methylbutyl) -2-hydroxypropyl (meth) acrylate 1H, 3H-tetrafluoropropyl (meth) acrylate, 1H, 5H-octafluoropentyl (meth) acrylate, 1H, 7H-dodecafluoroheptyl (meth) acrylate, 1H-1- (trifluoromethyl) trifluoroethyl (meth) acrylate, and 1H, 3H-hexafluorobutyl (meth) acrylate, 1, 2-tetrafluoro-1- (trifluoromethyl) ethyl (meth) acrylate, 2- (perfluorooctyl) ethyl (meth) acrylate, and the like. The fluorine-containing (meth) acrylate monomer may be used alone in 1 kind, or may be used in combination in an arbitrary ratio of 2 or more kinds.

The proportion of the fluorine-containing (meth) acrylate monomer used in the organic resin a is preferably more than 20 mass%, more preferably 22 mass% or more, still more preferably 25 mass% or more, and still more preferably 30 mass% or more, based on 100 mass% of the total constituent monomer components of the organic resin a. Further, it is preferably 60% by mass or less, more preferably 50% by mass or less, further preferably 45% by mass or less, still more preferably 40% by mass or less, and most preferably 35% by mass or less. By setting the range as described above, the surface layer is easily surface-offset, and sufficient adhesion to the electrode is obtained.

Whether or not the fluorine-containing (meth) acrylate monomer is contained in the organic resin a can be measured by a known method, and the ratio of the fluorine-containing (meth) acrylate monomer in the organic resin a can be further measured. For example, first, the porous layer is separated from the porous film using an organic solvent such as water or alcohol, and the organic solvent such as water or alcohol is sufficiently dried to obtain the constituent components contained in the porous layer. An organic solvent for dissolving the organic resin component is added to the obtained constituent component to dissolve only the organic resin component. Then, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. The obtained organic resin component can be used and calculated from the signal intensity representing the fluorine-containing (meth) acrylate monomer by nuclear magnetic resonance (¹H-NMR、¹⁹F-NMR、¹³ C-NMR), infrared absorption spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), fluorescent X-ray analysis (EDX), elemental analysis, thermal cracking gas chromatograph mass spectrometer (thermal cracking GC/MS) and the like. In particular, after confirming the presence or absence of the fluorine-containing (meth) acrylate monomer by thermal cracking GC/MS, the ratio of the fluorine-containing (meth) acrylate monomer to be used can be determined by ¹³ C-NMR (filling the solid NMR sample tube with the organic resin component and an appropriate amount of solvent (deuterated chloroform), and measuring by DD/MAS method after standing overnight.

The number of fluorine atoms contained in one molecule of the fluorine-containing (meth) acrylate monomer is preferably 3 or more and 13 or less. More preferably 3 to 11, still more preferably 3 to 9. When the amount is within the above range, the surface free energy of the organic resin a can be reduced, and the coating property can be achieved. When the number of fluorine atoms is 3 or more, the surface free energy of the organic resin a is sufficiently reduced, and the adhesion to the electrode is sufficient. In addition, when the number of fluorine atoms is 13 or less, the coating property on the porous substrate is ensured, and the productivity is improved.

The number of fluorine atoms contained in one molecule of the fluorine-containing (meth) acrylate monomer can be measured by a known method. For example, first, the porous layer is separated from the porous film using an organic solvent such as water or alcohol, and the organic solvent such as water or alcohol is sufficiently dried to obtain the constituent components contained in the porous layer. An organic solvent for dissolving the organic resin component is added to the obtained constituent component to dissolve only the organic resin component, and the organic resin component is separated from the inorganic particles. Then, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. The obtained organic resin component can be used and calculated from the signal intensity representing the fluorine-containing (meth) acrylate monomer by nuclear magnetic resonance (¹H-NMR、¹⁹F-NMR、¹³ C-NMR), infrared absorption spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), fluorescent X-ray analysis (EDX), elemental analysis, thermal cracking gas chromatograph mass spectrometer (thermal cracking GC/MS) and the like. Among these, thermal cracking GC/MS is useful, in particular.

Examples of the unsaturated carboxylic acid monomer include acrylic acid, methacrylic acid, and crotonic acid. The unsaturated carboxylic acid monomer may be used alone or in combination of 2 or more kinds in any ratio.

Examples of the (meth) acrylic acid ester monomer include acrylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate, cyclohexyl acrylate, hydroxyethyl acrylate, benzyl acrylate, isobornyl acrylate, dicyclopentanyl acrylate, dicyclopentenyl acrylate, hydroxymethyl acrylate, 2-hydroxyethyl acrylate, 3-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 5-hydroxypentanyl acrylate, 6-hydroxyhexyl acrylate, 7-hydroxyheptyl acrylate, and 8-hydroxyoctyl acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, t-butyl cyclohexyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, hydroxyethyl methacrylate, benzyl methacrylate, isobornyl methacrylate, dicyclohexyl methacrylate, dicyclopentenyl methacrylate, hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl methacrylate, 5-hydroxypentanyl methacrylate, 6-hydroxyhexyl methacrylate, 7-hydroxyheptyl methacrylate, 8-hydroxyoctyl methacrylate and the like. The (meth) acrylic acid ester monomer may be used alone in 1 kind, or may be used in combination in an arbitrary ratio of 2 or more kinds.

Among the above (meth) acrylate monomers, a (meth) acrylate monomer having a hydroxyl group is preferably used. The glass transition temperature of the organic resin a can be adjusted by using a (meth) acrylate monomer having a hydroxyl group, so that the dry adhesion to an electrode is excellent. The (meth) acrylate monomer having a hydroxyl group may be used alone or in combination of 1 or more than 2 kinds in any ratio. Particular preference is given to hydroxyethyl acrylate (HEA), 4-hydroxybutyl acrylate (4-HBA), 2-hydroxypropyl acrylate (2-HPA).

Examples of the styrene monomer include styrene, α -methylstyrene, p-methylstyrene, t-butylstyrene, chlorostyrene, chloromethylstyrene, and hydroxymethylstyrene. Examples of the olefin monomer include ethylene and propylene. Examples of the diene monomer include butadiene and isoprene. The amide monomer includes acrylamide and the like. Of these, 1 kind may be used alone, or 2 or more kinds may be used in combination at an arbitrary ratio.

Olefins system

The organic resin a preferably contains a copolymer of a polymer obtained by polymerizing a (meth) acrylate monomer containing fluorine and a polymer obtained by polymerizing a (meth) acrylate monomer having a hydroxyl group.

When the total constituent monomer components of the organic resin a are set to 100 mass%, the proportion of the (meth) acrylate monomer having a hydroxyl group is preferably 5.0 mass% or less. The content is preferably 3.0 mass% or less, more preferably 2.0 mass% or less, still more preferably 1.5 mass% or less, and still more preferably 1.0 mass% or less. When the ratio is 5.0 mass% or less, sufficient dry adhesion to the electrode is obtained, and further, when swelling in the electrolyte is suppressed, sufficient wet adhesion to the electrode is obtained, whereby the inter-electrode distance becomes constant, and a decrease in capacity at the time of initial charge and discharge can be suppressed, so that the yield at the time of initial charge and discharge is improved.

The proportion of the (meth) acrylate monomer having a hydroxyl group can be measured by 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 constituent components contained in the porous layer. An organic solvent for dissolving the organic resin component is added to the obtained constituent component to dissolve only the organic resin component. Then, the organic solvent is dried from the solution in which the organic resin component is dissolved, and only the organic resin component is extracted. The presence or absence of a (meth) acrylate monomer having a hydroxyl group can be confirmed by thermal cracking GC/MS using the obtained organic resin component, and then the proportion of the (meth) acrylate monomer having a hydroxyl group can be determined by ¹³ C-NMR (solid NMR sample tube is filled with the above organic resin component and an appropriate amount of solvent (deuterated chloroform), and then the mixture is allowed to stand overnight and then measured by DD/MAS method).

In the case of obtaining a copolymer obtained by polymerizing a fluorine-containing (meth) acrylate monomer and a (meth) acrylate monomer having a hydroxyl group, it is preferable to further polymerize a monomer having 2 or more reactive groups per 1 molecule. By using a monomer having 2 or more reactive groups per 1 molecule, polymer particles which are excellent in electrolyte resistance, in which swelling in an electrolyte is suppressed, and which are excellent in dry adhesion to an electrode and wet adhesion to an electrode in an electrolyte can be obtained. The term "polymer formed from a monomer having 2 or more reactive groups per 1 molecule" refers to a polymer or copolymer obtained by polymerization using a monomer having 2 or more reactive groups per 1 molecule.

As the polymer formed from a monomer having 2 or more reactive groups per 1 molecule, a monomer that forms a crosslinked structure upon polymerization can be used. As the monomer having 2 or more reactive groups per 1 molecule, for example, a (meth) acrylate monomer having 2 or more reactive groups per 1 molecule is preferably used, and alkylene glycol di (meth) acrylate, and urethane di (meth) acrylate are more preferably used.

The polymer contained in the organic resin a is preferably a polymer obtained by polymerizing at least 1 monomer among monomers used as a raw material of the polymer, using only monomers having a glass transition temperature of-100 ℃ or higher and 0 ℃ or lower, which are polymers obtained by polymerizing the monomers. The glass transition temperature is more preferably in the range of-70℃to-10℃and still more preferably in the range of-50℃to-20 ℃. The glass transition temperature is defined in JIS K7121: 2012, mid-point glass transition temperature as measured by Differential Scanning Calorimeter (DSC). The intermediate point glass transition temperature is a temperature at a point where a straight line equidistant from the straight line extending from each base line in the longitudinal axis direction intersects with a curve of the stepwise change portion of the glass transition.

The polymer obtained by polymerizing only the monomer preferably has a glass transition temperature of-100 ℃ or higher and a monomer ratio epsilon of 0 ℃ or lower of less than 7.0 mass%. The proportion is more preferably less than 5.0 mass%, still more preferably less than 2.0 mass%, and most preferably less than 1.0 mass%. When the amount is less than 7.0% by mass, softening of the organic resin a is less likely to occur, and swelling of the organic resin a in the electrolyte can be suppressed. If the ratio of monomers having a glass transition temperature of-100 ℃ or higher and 0 ℃ or lower is small, the rate of decrease in wet adhesion after dry adhesion tends to be suppressed, and δ/γ tends to be large, which is advantageous in improving wet adhesion.

The porous layer of the porous film of the present invention may contain, in addition to the organic resin a having adhesiveness to the electrode, other organic resins imparting different functions. That is, the porous layer may contain at least 2 kinds of organic resin particles. As the organic resin imparting different functions, an emulsion binder may be used in order to bond the organic resin and the inorganic particles to each other, or in order to have a binder function of bonding the organic resin and the porous substrate to each other. The adhesive function improves the adhesion, and thus the adhesion to the electrode can be further improved. The organic resin is preferably one that is electrochemically stable in the range of use of the battery.

The organic resin a preferably has a particle shape. The substance having a particle shape promotes the bias of the organic resin a to the surface of the porous layer. The organic resin a includes, in addition to a substance having a particle shape, a substance which is partially formed into a film and fused with peripheral particles and a binder. The shape is not particularly limited, and may be any of spherical, polygonal, flat, fibrous, and the like.

When the organic resin a is selected to have a particle shape, the average particle diameter of the particles is preferably 100nm or more and 500nm or less. The lower limit is more preferably 120nm or more, still more preferably 150nm or more, and most preferably 170nm or more. The upper limit is more preferably 400nm or less, still more preferably 300nm or less, and most preferably 250nm or less. When the average particle diameter is 100nm or more, the porous structure is formed, and the rate characteristics and lifetime characteristics are further improved. Further, by setting the thickness of the porous layer to 500nm or less, the rate characteristics and lifetime characteristics can be further improved.

(2) Inorganic particles

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

Specific examples of the inorganic particles include inorganic oxide particles such as alumina, boehmite, silica, titania, zirconia, iron oxide, and magnesia, inorganic nitride particles such as aluminum nitride and silicon nitride, and insoluble ion crystal particles such as calcium fluoride, barium fluoride, and barium sulfate. Among the inorganic particles, alumina having an effect of increasing strength, boehmite and barium sulfate having an effect of reducing wear of components in the step of dispersing the organic resin particles and the inorganic particles are particularly preferable. Further, 1 kind of these inorganic particles may be used, or 2 or more kinds may be mixed and used.

Examples of the shape of the inorganic particles to be used include spherical, plate-like, needle-like, rod-like, and oval, and any shape can be used. Among them, spherical shapes are preferable from the viewpoints of surface modification, dispersibility, and coatability.

(3) Adhesive agent

The porous layer of the porous film of the present invention may contain a binder in order to adhere the organic resin a and the inorganic particles described above to each other and to adhere the organic resin a and the inorganic particles to the porous substrate. As the binder, a resin that is electrochemically stable in the range of use of the battery is preferable. The binder may be an organic solvent-soluble binder, a water-soluble binder, an emulsion binder, or the like, and may be used alone or in combination.

In the case of using an organic solvent-soluble binder and a water-soluble binder, the viscosity of the binder itself is preferably 10,000mpa·s or less at a concentration of 15 mass%. More preferably 8,000 mPas or less, and still more preferably 5,000 mPas or less. When the viscosity is set to 10,000mpa·s or less at a concentration of 15 mass%, the increase in viscosity of the coating agent can be suppressed, and the organic resin a is biased to the surface, thereby improving the dry adhesion and wet adhesion to the electrode.

In the case of using the emulsion adhesive, the dispersant may be water, an alcohol solvent such as ethanol, or a ketone solvent such as acetone, which is an organic solvent, but a water-dispersible substance is preferable in terms of handling property and miscibility with other components. The particle size of the emulsion binder is 30 to 1,000nm, preferably 50 to 500nm, more preferably 70 to 400nm, and even more preferably 80 to 300nm. By setting the particle size of the emulsion binder to 30nm or more, the increase in air permeability can be suppressed, and the battery characteristics can be improved. Further, the thickness of the porous layer is 1,000nm or less, whereby sufficient adhesion between the porous layer and the porous substrate is obtained.

Examples of the resin used for the binder include resins such as polyamide, polyamideimide, polyimide, polyetherimide, poly-1, 1-difluoroethylene, 1-difluoroethylene-hexafluoropropylene copolymer, polytetrafluoroethylene, polysulfone, polyketone, polyetherketone, polycarbonate, polyacetal, polyvinyl alcohol, polyethylene glycol, cellulose ether, acrylic resin, polyethylene, polypropylene, polystyrene, and urethane. Among them, the use of an acrylic resin is particularly preferable because stronger adhesion is obtained by interaction with the organic resin a. In addition, the use of poly-1, 1-difluoroethylene or a 1, 1-difluoroethylene-hexafluoropropylene copolymer (hereinafter, sometimes referred to as "poly-1, 1-difluoroethylene resin") is particularly preferable because the adhesion to an electrode in an electrolyte is further improved. These resins may be used in an amount of 1 or 2 or more kinds may be used in combination as required.

The content of 1, 1-difluoroethylene in the poly-1, 1-difluoroethylene resin is preferably 80% by mass or more and less than 100% by mass of the components constituting the resin. More preferably 85 mass% or more and still more preferably 99 mass% or less. Further preferably, the content is not less than 90% by mass, and not more than 98% by mass. If the content of 1, 1-difluoroethylene is less than 80 mass%, sufficient mechanical strength cannot be obtained, and the strength is weak although the adhesion to an electrode is exhibited, so that the peeling may be easily performed. In addition, when the content of 1, 1-difluoroethylene is 100 mass%, the electrolyte resistance is lowered, and thus sufficient adhesiveness may not be obtained.

The amount of the water-soluble binder to be added is preferably 0.5 mass% or more based on the total amount of the organic resin a and the inorganic particles. More preferably 1% by mass or more, and still more preferably 1.5% by mass or more. Further, it is preferably 10 mass% or less. More preferably 8 mass% or less, and still more preferably 6 mass% or less. By setting the amount of the water-soluble binder to 0.5 mass% or more, sufficient adhesion between the porous layer and the porous substrate is obtained. Further, by being 10 mass% or less, the increase in air permeability can be suppressed, and the battery characteristics can be improved.

The amount of the emulsion binder to be added is preferably 1 mass% or more relative to the total amount of the organic resin a and the inorganic particles. More preferably 5 mass% or more, still more preferably 7.5 mass% or more, and most preferably 10 mass% or more. The content is preferably 30% by mass or less, more preferably 25% by mass or less, and still more preferably 20% by mass or less. By setting the amount of the emulsion-based binder to 1 mass% or more, sufficient adhesion between the porous layer and the porous substrate is obtained. Further, by being 30 mass% or less, the increase in air permeability can be suppressed, and the battery characteristics can be improved. In particular, by being 7.5 mass% or more and 20 mass% or less, not only adhesion between the organic resin a and the inorganic particles and adhesion between these particles to the substrate are promoted, but also interaction with the organic resin a is exhibited, and dry adhesion and wet adhesion to the electrode are improved.

(4) Formation of porous layer

The method of forming the porous layer will be described below.

The porous layer may be formed by a 2-stage coating process or may be formed by a 1-stage coating process. In the case of forming the coating layer by the 2-stage coating process, the coating layer containing inorganic particles is formed in the 1 st stage, and the coating layer containing the organic resin a is formed in the 2 nd stage. The coating liquid containing the organic resin a is on the surface layer, so that adhesion to the electrode can be easily ensured. In this case, the porous layer containing the organic resin a can be thinned, or the amount of the organic resin a added to be locally present on the surface can be reduced, so that the cost can be reduced.

The 2-stage coating step is a method of forming a porous film by adjusting a coating liquid composed of inorganic particles and a solvent, coating the coating liquid on a porous substrate, drying the solvent of the coating liquid, then adjusting a coating liquid composed of an organic resin a and a solvent, coating the coating liquid on the inorganic particle coating layer, and drying the solvent of the coating liquid. As the coating method, spray coating can be used.

The 1-stage coating step is a method of preparing a coating liquid composed of an organic resin a, inorganic particles and a solvent, coating the coating liquid on a porous substrate, and drying the solvent of the coating liquid to form a porous layer, thereby obtaining a porous film.

The coating method is not particularly limited, but the latter can achieve cost reduction by reducing the number of coating times. Therefore, a method for forming a porous layer using the latter will be described below.

(I) First, the organic resin a is dispersed at a predetermined concentration to prepare a coating liquid. The coating liquid is prepared by dispersing, suspending, or emulsifying the organic resin a in a solvent. The solvent of the aqueous dispersion coating liquid is not particularly limited as long as it is a solvent capable of dispersing, suspending or emulsifying the organic resin a in a solid state. Examples thereof include organic solvents such as methanol, ethanol, 2-propanol, acetone, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, N-methyl pyrrolidone, dimethylacetamide, dimethylformamide, and dimethylformamide. From the viewpoints of reduction of environmental load, safety and economy, an aqueous emulsion obtained by emulsifying an organic resin in water or a mixed solution of water and alcohol is preferable. In the case of using water, a solvent other than water may be further added.

The solid content concentration of the coating liquid is preferably 5% or more and 40% or less. By setting the coating stability to a predetermined range, both coating stability and surface bias during coating and drying can be achieved. The solution viscosity of the coating liquid is preferably 5 mPas or more and 50 mPas or less. When the dispersion is within a predetermined range, both the dispersibility of the coating liquid and the surface bias during coating and drying can be achieved. The concentration of the solid content of the coating liquid is adjusted to be low and the viscosity is adjusted to be small in the above-described preferable range, whereby the organic resin particles can be adjusted so as to be biased to the outer layer.

In addition, if necessary, a film forming auxiliary agent, a dispersant, a thickener, a stabilizer, a defoaming agent, a leveling agent, and the like may be added to the coating liquid.

Examples of the dispersion method of the coating liquid include a ball mill, a bead mill, a sand mill, a roller mill, a homogenizer, an ultrasonic homogenizer, a high pressure homogenizer, an ultrasonic device, and a paint shaker. Among them, by selecting a dispersion method (bead mill, sand mill) in which the pressure applied to the inorganic particles and the organic resin a at the time of dispersion is high, the dispersibility is improved, and the surface bias of the organic resin a is more likely to occur. These plural mixer-dispersers may be combined to disperse stepwise.

(Ii) Next, the obtained coating liquid was coated on a porous substrate. As the coating method, for example, dip coating (i.e., コ -to-roll), gravure coating, slot die coating, doctor blade coating, comma coating, kiss coating, roll coating, bar coating, spray coating, dip coating (コ -to-roll coating), spin coating, screen printing, ink jet printing, pad printing, other types of printing, and the like can be used. The coating method is not limited to these, and may be selected according to preferable conditions of the organic resin, binder, dispersant, leveling agent, solvent used, substrate, and the like used.

(Iii) Then, the solvent of the coating liquid is dried to form a porous layer. The drying temperature is preferably 40 ℃ to 100 ℃. Thus, the drying of the porous layer is uniform, the film thickness of the porous layer is uniform, and the adhesion to the electrode is improved. At less than 40 ℃, the solvent of the coating liquid is not dried. On the other hand, when the temperature is higher than 100 ℃, the heat at the time of drying increases, and the organic resin a forms a film, so that the surface bias of the organic resin a does not occur, and thus β/α=0. Therefore, the dry adhesion to the electrode, the wet adhesion to the electrode, and the coating property can be both improved by the predetermined range, and the cost can be reduced by the improvement of the drying speed.

The thickness of the porous layer is preferably 2.0 μm or more. More preferably more than 3.0. Mu.m, still more preferably 4.0. Mu.m. In addition, it is preferably 8.0 μm or less. More preferably 7.0 μm or less, and still more preferably 6.0 μm or less. Here, the film thickness of the porous layer refers to the film thickness of the porous layer in the case of a porous film having a porous layer on one surface of the porous base material, and refers to the sum of the film thicknesses of the two porous layers in the case of a porous film having a porous layer on both surfaces of the porous base material. By setting the film thickness of the porous layer to 2.0 μm or more, sufficient thermal dimensional stability and adhesion to the electrode are obtained. Further, the porous structure is formed by 8.0 μm or less, whereby the battery characteristics are improved. In addition, it sometimes becomes advantageous in terms of cost.

[ Porous substrate ]

In the present invention, the porous base material has a structure in which micropores are formed in the inside and the micropores are connected from one surface to the other surface. Examples of the porous substrate include a microporous film, a nonwoven fabric, and a porous membrane sheet made of a fibrous material. The material constituting the porous substrate is preferably composed of a resin which is electrically insulating, electrically stable, and stable in the electrolyte. In addition, from the viewpoint of imparting a shutdown function, the resin used is preferably a thermoplastic resin having a melting point of 200 ℃ or less. The shutdown function here is a function of closing the porous structure by heat and melting when the lithium ion battery abnormally generates heat, stopping ion migration and stopping discharge.

The thermoplastic resin may be, for example, a polyolefin resin, and the porous substrate is preferably a polyolefin porous substrate. The polyolefin porous substrate is more preferably a polyolefin porous substrate having a melting point of 200 ℃ or less. The polyolefin resin includes, for example, polyethylene, polypropylene, an ethylene-propylene copolymer, and a mixture thereof, and examples thereof include a porous substrate having a single layer containing 90 mass% or more of polyethylene, a porous substrate having a plurality of layers made of polyethylene and polypropylene, and the like.

Next, a method for producing the porous substrate will be described. The method for producing the porous substrate is not particularly limited, and a method for producing a polyolefin microporous membrane will be described below.

(A) First, a plasticizer is added to a resin composition composed of the polyolefin in a twin-screw extruder, and the resin composition is melt kneaded to prepare a resin solution.

The polyolefin resin composition is composed of a polyolefin resin, and may be composed of a single type or a mixture of 2 or more types of polyolefin resins. Examples of the polyolefin resin include polyethylene and polypropylene, but are not limited thereto. The polyethylene resin may be a single composition of ultra-high molecular weight polyethylene, high density polyethylene, or medium density polyethylene, or may be a mixture of different molecular weights. It is preferable to use a mixture of 2 or more polyethylenes selected from the group consisting of ultra-high molecular weight polyethylene, high density polyethylene, medium density polyethylene and low density polyethylene, and particularly preferable is a mixture of ultra-high molecular weight polyethylene (a) having a Mw of 1×10 ⁶ or more and polyethylene (B) having a Mw of 1×10 ⁴ or more and less than 9×10 ⁴, and more preferable is an ultra-high molecular weight polyethylene having a Mw of 1×10 ⁶ or more. Regarding the mixing ratio of (a) and (B), the total of (a) and (B) is set to 100 mass%, and the content of (a) is preferably 50 mass% or more.

The plasticizer is preferably a liquid at room temperature in order to enable stretching at a higher magnification. Examples of the solvent include aliphatic solvents such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin. The mixing ratio of the polyolefin resin composition to the plasticizer is preferably such that the content of the polyolefin resin composition is 10% by mass or more and 50% by weight or less.

(B) Next, the resin solution is fed from the extruder to the die, and the gel-like sheet is molded by extruding and cooling the resin solution sheet.

(C) Then, the gel-like sheet was stretched. The gel sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation method, or a combination thereof.

(D) Then, the polyolefin microporous membrane can be obtained by drying by removing the plasticizer using a washing solvent.

(E) In addition to the above steps, stretching (also referred to as dry re-stretching) may be performed after the drying step. The second stretching may be performed by a tenter method or the like in the same manner as the stretching described above while heating the polyolefin microporous film. The ratio of the re-stretching is preferably 1.01 to 2.0 times in the case of uniaxial stretching, and is preferably 1.01 to 2.0 times in the case of biaxial stretching.

The thickness of the porous substrate is preferably 3 μm or more and 50 μm or less, more preferably 5 μm or more and further 30 μm or less. The thickness of the porous substrate is 50 μm or less, whereby an increase in the internal resistance of the porous substrate can be suppressed. In addition, the thickness of the porous base material is 3 μm or more, whereby the porous base material can be produced, and sufficient mechanical properties can be obtained.

The thickness of the porous substrate may be measured by microscopic observation of a cross section. When porous layers are laminated, the vertical distance between the interfaces of the porous substrate and the porous layers is measured as the thickness of the porous substrate. 5 pieces of the sample were cut out in a size of 100mm×100mm, and the center of the sample was observed and measured for each of the 5 pieces, and the average value was defined as the thickness of the porous substrate.

The porous substrate preferably has an air permeability of 50 seconds/100 cc or more and 1,000 seconds/100 cc or less. More preferably 50 seconds/100 cc or more and 500 seconds/100 cc or less. By setting the air permeability to 50 seconds/100 cc or more, sufficient mechanical properties can be obtained. Further, the battery characteristics are improved by setting the ion mobility to 1,000 seconds/100 cc or less, thereby obtaining sufficient ion mobility.

Secondary battery

The porous film of the present invention can be suitably used as a separator for secondary batteries such as lithium ion batteries. The 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 material in which a positive electrode material composed of an active material, a binder resin, and a conductive additive is laminated on a current collector, and examples of the active material include a lithium-containing transition metal oxide having a layered structure such as LiCoO ₂、LiNiO₂、Li(NiCoMn)O₂, a spinel-type manganese oxide such as LiMn ₂O₄, and an iron-based compound such as LiFePO ₄. As the binder resin, a resin having high oxidation resistance may be used. Specifically, a fluororesin, an acrylic resin, a styrene-butadiene resin, or the like can be cited. As the conductive auxiliary agent, a carbon material such as carbon black or graphite can be used. As the current collector, a metal foil is suitable, and in particular, aluminum foil is often used.

The negative electrode is a substance in which a negative electrode material composed of an active material and a binder resin is laminated on a current collector, and examples of the active material include carbon materials such as artificial graphite, natural graphite, hard carbon, and soft carbon, lithium alloy materials such as tin and silicon, metal materials such as Li, and lithium titanate (Li ₄Ti₅O₁₂). As the binder resin, a fluororesin, an acrylic resin, a styrene-butadiene resin, or the like can be used. As the current collector, a metal foil is suitable, and in particular, a copper foil is often used.

The electrolyte solution is a solution in which an electrolyte is dissolved by an organic solvent, and serves as a place for ion migration between the positive electrode and the negative electrode in the secondary battery. Examples of the electrolyte include LiPF ₆、LiBF₄、LiClO₄ and LiTFSI, and LiPF ₆ is preferably used from the viewpoints of solubility in an organic solvent and ion conductivity. Examples of the organic solvent include ethylene carbonate, propylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethylmethyl carbonate, and 2 or more of these organic solvents may be used in combination.

As a method for producing a secondary battery, a coating liquid for an electrode is prepared by dispersing an active material and a conductive auxiliary agent in a solution of a binder resin, the coating liquid is applied to a current collector, and a solvent is dried to obtain a positive electrode and a negative electrode, respectively. The film thickness of the dried coating film is preferably 50 μm or more and 500 μm or less. A secondary battery separator is disposed between the positive electrode and the negative electrode so as to be in contact with the active material layers of the respective electrodes, and the secondary battery separator is sealed in an exterior material such as an aluminum laminate film, and after the electrolyte is injected, a negative electrode lead and a safety valve are provided to seal the exterior material. The secondary battery electrode obtained in this way has high adhesion to the secondary battery separator, excellent rate characteristics and life characteristics, and can be manufactured at low cost.

Examples

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited thereto. The assay used in this example is shown below. In the following description, "%" and "parts" represent "% by mass" and "parts by mass", respectively.

[ Measurement method ]

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

The porous layer was extracted from a sample cut out of the porous film to 10cm×10cm using 40g of water, and the water was sufficiently dried to obtain constituent components contained in the porous layer. When water is not sufficiently removed, an organic solvent such as alcohol may be used. After the mass of the total amount of the constituent components was measured, the constituent components were burned at a high temperature to such an extent that the components of the organic resin a were melted/decomposed, and the mass of only the inorganic particles was measured. The content of inorganic particles in the porous layer was calculated as mass% from the equation of (mass of inorganic particles/mass of the total amount of constituent components) ×100. The content of the organic resin component a in the porous layer was calculated as mass% from the equation ((mass of the entire amount of the constituent components-mass of the inorganic particles)/(mass of the entire amount of the constituent components)) ×100. Next, the specific gravity of the inorganic particle component and the organic resin a component was measured by a densitometer. The volume content of the inorganic particles in the porous layer was calculated from the mass content (mass%) of the inorganic particle component and the organic resin a component obtained previously and the specific gravity of the inorganic particle component and the organic resin a component as volume%. The above measurement was performed on 5 samples, and the measured values were averaged.

(2) Occupancy beta (occupancy beta) of inorganic particles in the surface portion of the porous layer

The surface of the porous layer on which the Pt/Pd 30 seconds porous film was deposited was measured by SEM-EDX (HITACHI SE8200,8230) at a magnification of 10,000 times and an acceleration voltage of 5.0kV, and the inorganic element of the inorganic particles was analyzed. Using image analysis software (toyo, SPIP6.0.10), the elemental symbol display, magnification display, scale, and acceleration voltage display in the EDX image were masked and removed, and then the detection method was performed in the "particle/Kong Jiexi" mode so that the threshold value was 130nm, and the area ratio was set to be the occupancy β of the inorganic particles. The above measurement was performed on 5 samples, and the measured values were averaged.

(3) Surface free energy of porous layer

The surface free energy (mN/m) of the porous layer was measured by the Young-Dupre equation by measuring the contact angle of each of water, ethylene glycol, diiodomethane and formamide on the surface of a sample cut out of the porous film to 100mm X100 mm. The above measurement was performed on 5 samples, and the measured values were averaged.

(4) Film thickness of porous layer

A section of a sample was cut out of a center portion of the sample having a thickness of 100mm X100 mm from the porous film, and the section was observed at a magnification of 10,000 times by a field emission scanning electron microscope (S-800, manufactured by Hitachi Ltd.) to measure a distance from an interface with a porous substrate to a highest position on a surface. In the case of one surface, only one surface was measured, and in the case of both surfaces, both surfaces were measured, and the total was set as the film thickness of the porous layer. In the interface between the porous substrate and the porous layer, the area where the inorganic particles are not observed in the region where the inorganic particles are present is defined as the interface. The above measurement was performed on 5 samples, and the measured values were averaged.

(5) Glass transition temperature of porous film

In accordance with "JIS K7121: 2012, a method for measuring the temperature by shift temperature (a method for measuring the transition temperature of plastics), "is a method for measuring the intermediate glass transition temperature of a porous film by Differential Scanning Calorimetry (DSC). The intermediate point glass transition temperature is a temperature at a point where a straight line equidistant from the straight line extending from each base line in the longitudinal axis direction intersects with a curve of the stepwise change portion of the glass transition. The above measurement was performed on 3 samples, and the measured values were averaged.

(6) Heat shrinkage (thermal dimensional stability)

For 3 samples cut out of the porous film at 100mm×100mm, the midpoint of one side and the midpoint of the opposite side of each sample were marked, the length between the midpoints was measured, and then, the sample was subjected to heat treatment in a hot-air oven at 150 ℃ for 30 minutes without tension. The length between the midpoints of the samples after heat treatment and the positions before heat treatment was measured, and the heat shrinkage was calculated from the following equation.

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

The calculation was performed at 2 points from 1 sample at the same time, and the average value of all the values was set as the heat shrinkage (thermal dimensional stability), and the evaluation criteria of the thermal dimensional stability were as follows.

And (3) excellent: the heat shrinkage is less than 5%.

Excellent: the heat shrinkage is 5% or more and less than 10%.

Good: the heat shrinkage is 10% or more and less than 20%.

Qualified: the heat shrinkage is 20% or more and less than 40%.

Disqualification: the heat shrinkage rate is more than 40%.

(7) Dry adhesion gamma to electrode

The active material used was natural graphite, the binder used was 1, 1-difluoroethylene resin (hereinafter, PVDF resin), the conductive additive used was carbon black (hereinafter, CB), and the composition was made of natural graphite: PVDF resin: cb=98: 1:1 (mass%) and a negative electrode (width 20 mm. Times. Length 70 mm) having a coating amount of 6.0mg/cm ² and a density of 1.45g/cm ³. The porous film (width 25 mm. Times. Length 80 mm) was placed so that the electrode and the bottom of the end portion of the porous film in the longitudinal direction were aligned and overlapped, and the active material was placed in contact with the porous layer, and hot-pressed under the condition a (70 ℃ C./5 MPa/7 seconds) to bond the electrode and the porous film to each other, thereby producing a test piece. Next, the negative electrode side of the obtained test piece was adhered to an acrylic plate having a thickness of 2 mm. Then, the porous film was peeled at 180℃under the condition of a peeling speed of 300 mm/min, and an average value of 30mm to 60mm in the longitudinal direction was calculated. 5 test pieces were prepared, and the average of the measurement results was defined as the dry adhesion γ to the electrode.

(8) Wet adhesion delta to electrode

The active material used natural graphite, the binder used PVDF resin, the conductive aid used CB, and the composition was made of natural graphite: PVDF resin: cb=98: 1:1 (mass%) and a negative electrode (width 20 mm. Times. Length 70 mm) having a coating amount of 6.0mg/cm ² and a density of 1.45g/cm ³. The porous film (width 25 mm. Times. Length 80 mm) was set so that the electrode and the end portion of the porous film in the longitudinal direction were overlapped, and the active material was in contact with the porous layer, and the electrode and the porous film were bonded by hot pressing under the condition a (70 ℃ C./5 MPa/7 seconds), to prepare a test piece. The test piece was set in an aluminum laminate film in a bag shape by closing 3 pieces, and after the electrolyte injection step (i.e., 1g of electrolyte was infiltrated from the porous film side of the test piece), the remaining 1 side of the aluminum laminate film was sealed by a vacuum sealer. Here, the electrolyte used was a solution prepared by mixing ethylene carbonate: diethyl carbonate=1: 1 (volume ratio), liPF ₆ as a solute was dissolved in a mixed solvent so that the concentration became 1 mol/liter. Next, the aluminum laminate film after sealing the test piece was stored at 60℃for 17 hours under a standing condition. The test piece was taken out from the aluminum laminate film, the electrolyte solution on the surface of the test piece was wiped off, and the negative electrode side of the test piece was stuck to an acrylic plate having a thickness of 2 mm. Then, the porous film was peeled at 180℃under the condition of a peeling speed of 300 mm/min, and an average value of 30mm to 60mm in the longitudinal direction was calculated. 5 test pieces were prepared, and the average of the measurement results was set to be the wet adhesion δ to the electrode.

(9)δ/γ

The change rate δ/γ of the dry adhesion and the wet adhesion was calculated by dividing the δ value by the γ value using the dry adhesion γ (N/m) to the electrode and the wet adhesion δ (N/m) to the electrode obtained in (7) and (8).

(10) Yield-1 (yield at the time of battery production)

The positive electrode sheet was produced by applying a positive electrode slurry, which was obtained by dispersing 1.0 part by mass of Li (Ni _5/10Mn_2/10Co_3/10)O₂) as a positive electrode active material, acetylene black (hereinafter, AB) and graphite (hereinafter, gr) as positive electrode conductive additives, and 2 parts by mass of PVDF resin as a positive electrode binder, in N-methyl-2-pyrrolidone using a planetary mixer, on both sides of an aluminum foil, drying, and rolling (coating order: 10.0mg/cm ², 34% porosity) the positive electrode sheet was cut out to 70mm×70mm, at this time, a tab bond for current collection without an active material layer was cut out so as to be 5mm×5mm in size outside the active material face, an aluminum tab bond having a width of 5mm and a thickness of 0.1mm was subjected to ultrasonic welding, a negative electrode sheet was produced by cutting out 98 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder in water, and a negative electrode slurry obtained by dispersing in water using a planetary mixer, and applying, drying and rolling both sides on a copper foil (application mesh: 6.0mg/cm ², 20% porosity), the negative electrode sheet was cut out to be 75mm×75mm, at this time, a tab bond for current collector without an active material layer was cut out so as to be 5mm×5mm in size outside the active material face, a porous tab bond was produced by cutting out a copper tab bond with a porous film having the same size as the positive electrode sheet, and a porous tab bond was produced in order of 80mm, a porous membrane was cut out, and a porous membrane was produced in order of 80mm, the positive electrode 8 sheets, the negative electrode 8 sheets, and the porous film 9 sheets were arranged so that the positive electrode coating portions were all opposed to the negative electrode coating portions, and so that the positive electrode sheet and the negative electrode sheet were completely covered with the porous film, thereby obtaining an electrode group. Next, a test piece was produced by bonding the electrode to the porous film by hot pressing under the condition a (70 ℃/5MPa/7 seconds). Then, the hot-pressed test piece was held by forceps, and the test piece was dropped from a height of 50cm, and the degree of peeling of the positive electrode, porous film, and negative electrode occurring at this time was evaluated on the following 5-scale. 5 test pieces were prepared, and the average of the measurement results was set to be the yield-1. The evaluation criteria for the yield-1 are as follows.

And (3) excellent: after hot pressing, the positive electrode, the negative electrode and the porous film are all bonded, and peeling does not occur.

Excellent: after hot pressing, the positive electrode, the negative electrode, and a part of the porous film (total of 1 sheet or more and 7 sheets or less) are scattered.

Good: after hot pressing, the positive electrode, the negative electrode, and a part of the porous film (total of 8 sheets or more and 14 sheets or less) are scattered.

Qualified: after hot pressing, the positive electrode, the negative electrode, and a part of the porous film (total of 15 sheets or more and 20 sheets or less) are scattered.

Disqualification: after hot pressing, the positive electrode, the negative electrode, and a part of the porous film (total of 21 sheets or more and 25 sheets or less) are scattered.

(11) Yield-2 (yield at initial charge and discharge)

The positive electrode sheet was produced by applying 96 parts by mass of Li (Ni _5/10Mn_2/10Co_3/10)O₂ (hereinafter, NCM 523) as a positive electrode active material, 1.0 part by mass of each of AB and Gr as positive electrode conductive additives, and 2 parts by mass of PVDF resin as a positive electrode binder to a positive electrode slurry dispersed in N-methyl-2-pyrrolidone using a planetary mixer, and drying and rolling both sides of an aluminum foil (coating order: A positive electrode sheet was produced by cutting a positive electrode sheet into a size of 70mm by 70mm, in which case a tab-bonding portion for current collection without an active material layer was cut out so as to be 5mm by 5mm on the outer side of the active material surface, and an aluminum tab-and tab-bonding portion having a width of 5mm and a thickness of 0.1mm was subjected to ultrasonic welding, and a positive electrode sheet was produced by cutting a positive electrode sheet into a size of 75mm by 1 part by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder in water using a planetary mixer, and applying, drying and rolling both sides of a copper foil (coating mesh: 6.0mg/cm ², density: 1.45g/cm ³), and a positive electrode sheet for current collection without an active material layer was cut out so as to be 75mm by 75mm, and a positive electrode sheet for current collection was cut out so as to be 5mm by 5mm on the outer side of the active material surface, and a positive electrode sheet was cut out into a porous sheet having a size of 80mm, and a positive electrode sheet for current collector sheet was cut out of the same order as that a porous sheet was cut out of a porous sheet formed by dispersing a porous sheet film, and a porous sheet was coated in the same order as that the positive electrode sheet was cut out, and a porous sheet was made by 80mm, the positive electrode sheet 8, the negative electrode sheet 8, and the porous film 9 were arranged so that the positive electrode sheet and the negative electrode sheet were completely covered with the porous film, thereby obtaining an electrode group. The electrode assembly was formed into a pouch by sandwiching the positive electrode/porous film/negative electrode between 1 sheet of 90mm×200mm aluminum laminate film, folding the long side of the aluminum laminate film, and thermally fusing the long side 2 of the aluminum laminate film. Next, hot pressing was performed under the condition a (70 ℃/5MPa/7 seconds), and after bonding the electrode to the porous film, ethylene carbonate was added: methylethyl carbonate=3: 7 (volume ratio), and 6g of an electrolyte prepared by dissolving LiPF ₆ as a solute in a mixed solvent so that the concentration becomes 1 mol/liter. Then kept at 60℃for 17 hours.

As initial charge and discharge, 0.1C charge×0.1C discharge was repeated 2 times, 0.2C charge×0.2C discharge was repeated 2 times, and 0.5C charge×0.5C discharge was repeated 10 times at 25 ℃. The initial capacity reduction ratio was calculated from the discharge capacity of 0.5C charge x 0.5C discharge at the 1 st time and the discharge capacity of 10 th time, and (discharge capacity of 10 th time)/(discharge capacity of 1 st time) ×100, and was used as an index of yield at the time of initial charge and discharge, and evaluated on the following 5-scale. 5 samples were prepared, and the average of the measurement results was set to yield-2. The evaluation criteria for the yield-2 are as follows.

And (3) excellent: the discharge capacity maintenance rate is 99% to 100%.

Excellent: the discharge capacity maintenance rate is 98% or more and less than 99%.

Good: the discharge capacity maintenance rate is 97% or more and less than 96%.

Qualified: the discharge capacity maintenance rate is 95% or more and less than 96%.

Disqualification: the discharge capacity maintenance rate is less than 95%.

(12) Electrolyte injection

A negative electrode slurry was prepared by dispersing 98 parts by mass of natural graphite as a negative electrode active material, 1 part by mass of carboxymethyl cellulose as a thickener, and 1 part by mass of a styrene-butadiene copolymer as a negative electrode binder in water using a planetary mixer, and applying, drying, and rolling both sides of a copper foil (coating order: 6.0mg/cm ², density: 1.45g/cm ³). The negative electrode 5 sheet was cut out to 75mm×75mm. The porous film 6 was cut out to 80mm×80mm. Then, the negative electrode and the porous film were alternately arranged so that the active material layer and the porous layer were in contact, and hot-pressed at 70 ℃/5MPa/7 seconds, to prepare a laminate in which the negative electrode and the porous film were bonded. Then, after sandwiching the laminate between 1 sheet of 90mm×200mm aluminum laminate film, ethylene carbonate was added: methylethyl carbonate=3: 7 (volume ratio), and 6g of an electrolyte prepared by dissolving LiPF ₆ as a solute in a mixed solvent so that the concentration becomes 1 mol/liter. After the electrolyte was impregnated for 30 minutes, the laminate was taken out, the porous film (1 st sheet), the negative electrode (1 st sheet), the porous film (2 nd sheet) and the negative electrode (2 nd sheet) were peeled off in this order from the laminate, and the porous film (4 th sheet) and the negative electrode (4 th sheet) were taken out in a state of being stacked. Since the brightness of the porous film is easily known by overlapping the negative electrode, it is possible to confirm which degree of the electrolytic solution is impregnated. The degree of impregnation of the porous membrane with the electrolyte (described in detail below) was evaluated on a 5-scale, and the porous membrane was set to be electrolyte-injected. 5 laminates were produced, and the average of 3 measurement results from which the largest and smallest impregnation areas were removed was set as the electrolyte injection property.

The impregnation area measurement method is as follows. First, an image was taken of the porous film and the negative electrode taken out so that the porous film entirely entered from above the sample. Then, the image analysis software ((a) by the cross, SPIP) was used to analyze the target cell as follows. After the impregnation boundary of the electrolyte solution was enhanced by using a general-purpose insert (PrugIn to BoundaryDropper), the area impregnated with the electrolyte solution contained in the visual field was calculated by automatic calculation of the threshold value without the end of the image pattern, and then the impregnation of the electrolyte solution was calculated by (the area impregnated with the electrolyte solution)/(the area per 1 sheet of porous film: 6400mm ²). Times.100 (%).

The area×100/(the area of the carbon element contained in the entire areas of the outer layer, the intermediate layer, and the inner layer) of the carbon element contained in the area of the outer layer was calculated, and the same measurement was performed on 5 samples, and the average value was defined as the content α (%) of the organic resin. The evaluation criteria for the electrolyte injection properties are as follows.

And (3) excellent: the porous membrane is impregnated with the electrolyte solution at 90% or more.

Excellent: the porous membrane is impregnated with an electrolyte solution in an amount of 75% to less than 90%.

Good: the porous membrane is impregnated with an electrolyte solution in an amount of 60% to less than 75%.

Qualified: 45% or more and less than 60% of the porous membrane is impregnated with the electrolyte.

Disqualification: less than 45% of the electrolyte solution in the porous membrane is impregnated.

(13) Battery fabrication

The positive electrode sheet was cut out to 40mm×40mm as described in (10) and (11). At this time, the tab adhesive portion for current collection without the active material layer is cut out so as to be 5mm×5mm in size outside the active material surface. An aluminum tab having a width of 5mm and a thickness of 0.1mm was ultrasonically welded to the tab adhesive portion. The negative electrode sheet was cut out to 45mm×45mm in the same manner as described in (10) and (11). At this time, the tab adhesive portion for current collection without the active material layer is cut out so as to be 5mm×5mm in size outside the active material surface. The copper tab and tab adhesive portion having the same size as the positive electrode tab were subjected to ultrasonic welding. Next, the porous film was cut out to 55mm×55mm, and the positive electrode and the negative electrode were stacked on both sides of the porous film so as to separate the porous film from each other, and the positive electrode coating portions were disposed so that the whole positive electrode coating portions were opposed to the negative electrode coating portions, thereby obtaining an electrode group. Then, hot pressing was performed at 70℃under 5MPa/10 seconds to bond the positive electrode/porous film/negative electrode. The positive electrode, porous film and negative electrode were sandwiched between 1 sheet of 90mm×200mm aluminum laminate film, and the long sides of the aluminum laminate film were folded and the long sides 2 of the aluminum laminate film were heat-fused to form a pouch. The method comprises the following steps of: methylethyl carbonate=3: 7 (volume ratio), liPF ₆ as a solute was dissolved in a mixed solvent so that the concentration became 1 mol/liter. 1.5g of an electrolyte was poured into the aluminum laminate film in the form of a bag, and the short side portion of the aluminum laminate film was thermally fused while impregnating the film under reduced pressure. Then, the mixture was allowed to stand at 60℃for 17 hours, and then subjected to initial charge/discharge and final charge/discharge to prepare a laminate battery.

(14) Multiplying power characteristics

For the rate characteristics, the following procedure was used for the test, and the discharge capacity maintenance rate was used for the evaluation. Using the above-described laminate battery, the discharge capacity at 25 ℃ when the discharge was performed at 0.5C and the discharge capacity at 10C were measured, and the discharge capacity maintenance rate was calculated as (discharge capacity at 10C)/(discharge capacity at 0.5C) ×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. 5 laminated batteries were produced, and the average of 3 measurement results obtained by removing the results having the maximum and minimum discharge capacity maintenance rates was set as the discharge capacity maintenance rate. The evaluation criteria were that the discharge capacity maintenance rate was set to be less than 40%, that 45% or more and less than 50% were good, that 50% or more and less than 55% were excellent, and that 55% or more were excellent.

(15) Life characteristics

The life characteristics were tested by the following procedure, and the discharge capacity maintenance rate was evaluated.

Cycle 1-300

The charge and discharge were set to 1 cycle, the charge conditions were set to constant current charge of 2C and 4.3V, the discharge conditions were set to constant current discharge of 2C and 2.7V, and the charge and discharge were repeated 300 times at 25 ℃.

Calculation of discharge capacity maintenance Rate

The discharge capacity maintenance rate was calculated as (discharge capacity at 300 th cycle)/(discharge capacity at 1 st cycle) ×100. 5 laminated batteries were produced, and the average of 3 measurement results obtained by removing the results having the maximum and minimum discharge capacity maintenance rates was set as the discharge capacity maintenance rate. The evaluation criteria for lifetime characteristics are as follows.

And (3) excellent: the discharge capacity maintenance rate is 70% or more.

Excellent: the discharge capacity maintenance rate is 60% or more and less than 70%.

Good: the discharge capacity maintenance rate is 50% or more and less than 60%.

Disqualification: the discharge capacity maintenance rate is less than 50%.

Example 1

Dispersion A

120 Parts of ion-exchanged water and 1 part of a so-called sleeve SR-1025 (emulsifier manufactured by the company of Amyda) were added to the reactor, and stirring was started. 0.4 part of 2,2' -azobis (2- (2-imidazolin-2-yl) propane) (manufactured by Wako pure chemical industries, ltd.) was added under a nitrogen atmosphere, and a monomer mixture composed of 35 parts of 2, 2-trifluoroethyl methacrylate (3 FMA), 63 parts of benzyl acrylate (BZA), 1.0 part of 2-hydroxyethyl acrylate (HEA), 1.0 part of Alkylene Glycol Dimethacrylate (AGDMA) (Tg= -34 ℃ C. Of a polymer formed only from a monomer), 9 parts of 3-trifluoroethyl methacrylate (manufactured by Mida. Co., ltd.) and 115 parts of ion-exchanged water was continuously dropped at 60 ℃ C. For 2 hours, and after the end of the drop, a polymerization treatment was carried out for 4 hours to produce a dispersion liquid A containing organic resin particles A (particle diameter 180nm, glass transition temperature: 49 ℃ C.) formed from a copolymer.

Dispersion B

Alumina particles (alumina) having an average particle diameter of 0.5 μm were used as inorganic particles, and water in the same amount as the inorganic particles and carboxymethyl cellulose as a dispersant in an amount of 1 mass% relative to the inorganic particles were added as a solvent, and then dispersed by a bead mill to prepare a dispersion liquid B.

Coating liquid

The dispersion a, the dispersion B, and the acrylic emulsion binder were dispersed in water so that the volume content α of the inorganic particles contained in the porous layer became 55 vol%, the volume content of the organic resin particles a became 25 vol%, the volume content of the acrylic emulsion binder became 20 vol%, and the solid content concentration of the coating liquid became 20 mass%, and mixed by a stirrer. The viscosity of the obtained coating liquid was 15 mPas. The obtained coating liquid was coated on both sides of a polyethylene porous substrate (thickness: 9 μm, air permeability: 70 seconds/100 cc) using a bar #10, and dried in a hot air oven (drying set temperature: 60 ℃) for 1 minute, and the solvent contained therein was volatilized to form a porous layer, thereby obtaining a porous film. Table 1 shows the types of monomers included in the organic resins a used in examples 1 to 29 and comparative examples 1 to 6, the glass transition temperature (Tg) of the polymer formed only from each monomer, the content (mass%) of each monomer included in the organic resin a, epsilon (mass%), the morphology and particle diameter of the organic resin, the polymerization stability, and the type and volume content α of the inorganic particles. Table 2 shows the measurement results of β, β/α, surface free energy, glass transition temperature (c), and film thickness of the porous layers obtained in examples 1 to 29 and comparative examples 1 to 6, and thermal dimensional stability, dry adhesion γ, wet adhesion δ, δ/γ, yield, electrolyte solution injection property, rate characteristics, and life characteristics of the porous films and batteries using the porous films.

In the measurement of wet adhesion δ, yield-2, electrolyte solution pouring property, rate characteristics, and life characteristics, ethylene carbonate was used as an electrolyte solution: diethyl carbonate=1: 1 (volume ratio), liPF ₆ as a solute was dissolved in a mixed solvent so that the concentration became 1 mol/liter.

Example 2

A porous film was obtained in the same manner as in example 1 except that the composition of the organic resin a was changed to the composition shown in table 1.

Example 3

A porous film was obtained in the same manner as in example 1, except that benzyl acrylate (BZA) was used as cyclohexyl acrylate (CHA) and 2-hydroxyethyl acrylate (HEA) was used as 4-hydroxybutyl acrylate (4-HBA) as the composition of the organic resin a.

Example 4

Example 5

Example 6

Example 7

Example 8

Example 9

Example 10

Example 11

Example 12

A porous film was obtained in the same manner as in example 1, except that the composition of the organic resin a was changed to Alkylene Glycol Dimethacrylate (AGDMA) (tg= -34 ℃) of the polymer formed from only the monomer, and Alkylene Glycol Dimethacrylate (AGDMA) (tg= -25 ℃) of the polymer formed from only the monomer.

Example 13

A porous film was obtained in the same manner as in example 1, except that the composition of the organic resin a was changed to Alkylene Glycol Dimethacrylate (AGDMA) (tg= -34 ℃) of the polymer formed from only the monomer, and Alkylene Glycol Dimethacrylate (AGDMA) (tg= -15 ℃) of the polymer formed from only the monomer.

Example 14

Example 15

Example 16

Example 17

Example 18

Example 19

Example 20

Example 21

Example 22

Example 23

Example 24

A porous film was obtained in the same manner as in example 1 except that the film thickness of the porous layer was set to the film thickness shown in table 2.

Example 25

Example 26

A porous film was obtained in the same manner as in example 1, except that the volume content α of the inorganic particles included in the porous layer was 30% by volume, the volume content of the organic resin particles a was 50% by volume, and the volume content of the acrylic emulsion binder was 20% by volume.

Example 27

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 80% by volume, the volume content of the organic resin particles a was 5% by volume, and the volume content of the acrylic emulsion binder was 15% by volume.

Example 28

A porous film was obtained in the same manner as in example 1, except that 1h,5 h-octafluoropentylacrylate (8 FA) was used instead of 3FMA as the composition of the organic resin a.

Example 29

A porous film was obtained in the same manner as in example 1, except that 2- (perfluorohexyl) ethyl acrylate (13 FA) was used instead of 3FMA as the composition of the organic resin a.

Comparative example 1

Comparative example 2

Comparative example 3

Comparative example 4

A porous film was obtained in the same manner as in example 1, except that the composition of the organic resin a was changed to the composition shown in table 1 by using hydroxyethyl methacrylate (HEMA) instead of 4 HBA.

Comparative example 5

A porous film was produced in the same manner as in example 1. Then, organic resin particles made of a polymer formed of 100 parts of 1, 1-difluoroethylene monomer were dispersed in water so that the solid content concentration of the coating liquid became 10 mass%, and mixed by a stirrer. The viscosity of the obtained coating liquid was 5 mPas. The obtained coating liquid was coated on both sides of the porous film using a bar #10, and dried in a hot air oven (drying set temperature 60 ℃) for 1 minute, and the solvent contained therein was volatilized to obtain a porous film.

Comparative example 6

A porous film was obtained in the same manner as in example 1, except that the composition shown in table 1 was changed without using the organic resin particles a.

TABLE 1

TABLE 2

According to tables 1 and 2, examples 1 to 29 each were a porous film having a porous substrate and a porous layer containing inorganic particles and an organic resin A on at least one surface of the porous substrate, and were excellent in yield and electrolyte liquid injection properties and also in rate characteristics and life characteristics, in which dry adhesiveness γ (N/m) measured under condition a (70 ℃/5MPa/7 seconds) and wet adhesiveness δ (N/m) measured under condition b (condition a+electrolyte liquid injection step+60 ℃/17 hours of standing) were 0.1. Ltoreq.δ/γ.ltoreq.0.8.

On the other hand, comparative examples 1 to 4 have a δ/γ of less than 0.1, and therefore have poor yield at the time of initial charge and discharge. In comparative example 5, since the porous layer containing the inorganic particles and the organic resin a was coated with the poly-1, 1-difluoroethylene particles having high wet adhesion, the δ/γ was more than 0.8, and thus the electrolyte injection property was poor. Since comparative example 6 does not contain organic resin particles, the surface bias of the organic resin particles does not occur, and thus β/α=1, and sufficient adhesion to the electrode is not obtained.

Industrial applicability

The porous film of the present invention can be suitably used as a separator for secondary batteries such as lithium ion batteries.

Claims

1. A porous film comprising a porous substrate and a porous layer comprising inorganic particles and an organic resin A, which is located on at least one surface of the porous substrate, wherein the dry adhesiveness [ gamma ] of the porous film measured under condition a and the wet adhesiveness [ delta ] of the porous film measured under condition b satisfy 0.1 [ delta ]. Gamma ]. Ltoreq.0.8, wherein the condition a is 70 ℃/5MPa/7 seconds, the unit of [ gamma ] is N/m, the condition b is the condition a+electrolyte injection step+60 ℃/17 hours of standing, and the unit of [ delta ] is N/m.

2. The porous film according to claim 1, wherein the volume content α of the inorganic particles and the occupancy β of the inorganic particles in the surface portion of the porous layer satisfy β/α < 1, the unit of α is volume% and the unit of β is area% when the volume of all the constituent components of the porous layer is 100 volume%.

3. The porous film according to claim 1 or 2, wherein the organic resin a is an organic resin particle.

4. The porous film according to any one of claims 1 to 3, wherein the organic resin a is a polymer obtained by polymerizing at least 1 monomer selected from the group consisting of fluorine-containing (meth) acrylate monomers, unsaturated carboxylic acid monomers, (meth) acrylate monomers, styrene monomers, olefin monomers, diene monomers, and amide monomers.

5. The porous film according to any one of claims 1 to 4, wherein the organic resin a has a copolymer of a polymer obtained by polymerization using a (meth) acrylate monomer containing fluorine and a polymer obtained by polymerization using a (meth) acrylate monomer having a hydroxyl group.

6. The porous film according to claim 5, wherein the proportion of the (meth) acrylate monomer having a hydroxyl group in the organic resin a is 5.0 mass% or less, based on 100 mass% of the total constituent monomer components of the organic resin a.

7. The porous film according to any one of claims 4 to 6, wherein at least 1 monomer among the monomers used as a raw material of the polymer contained in the organic resin a is a monomer having a glass transition temperature of-100 ℃ or higher and 0 ℃ or lower, which is a polymer obtained by polymerizing only the monomer.

8. The porous film according to claim 7, wherein the proportion epsilon of the monomer having a glass transition temperature of-100 ℃ or higher and 0 ℃ or lower is less than 7.0 mass% based on the polymer obtained by polymerizing the monomer alone.

9. The porous film according to any one of claims 4 to 8, wherein the proportion of the fluorine-containing (meth) acrylate monomer is more than 20 mass% and 60 mass% or less, based on 100 mass% of the total constituent monomer components of the organic resin a.

10. The porous membrane according to any one of claims 4 to 9, wherein the number of fluorine atoms contained in one molecule of the fluorine-containing (meth) acrylate monomer is 3 or more and 13 or less.

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

12. The porous film according to any one of claims 1 to 11, wherein the porous layer has a film thickness of 2 μm or more and 8 μm or less.

13. A separator for a secondary battery, having the porous film according to any one of claims 1 to 12.

14. A secondary battery having the separator for a secondary battery according to claim 13.