Multilayer polyimide porous membrane

Abstract

This invention provides a polymer porous membrane that enables the supply of energy storage devices with excellent resistance to processes that involve external expansion and contraction, as well as to restraining members that suppress expansion and contraction. [Solution] A multilayer polyimide porous film having surface layers formed on both sides of an intermediate layer, characterized in that the rate of change in the Gahl value when 5 MPa is applied in the film thickness direction is less than 5%. Preferably, this polyimide porous film has a layer structure of three or more layers. Furthermore, the rate of change in the Gahl value when 15 MPa is applied in the film thickness direction is less than 10%. Additionally, the ionic conductivity measured using a standard electrolyte is 2 mS / cm or higher. Furthermore, the compressive modulus is 50 MPa or higher.

Description

Technical Field

[0001] The present invention relates to a polyimide porous film having a multilayer structure.

Background Art

[0002] In recent years, power storage devices such as lithium-ion batteries and capacitors have become widespread. Among them, technologies such as lithium-ion batteries using graphite as the negative electrode, lithium-ion capacitors, and lithium metal batteries using metallic lithium as the negative electrode have developed significantly. These power storage devices have a structure in which a positive electrode, a separator, and a negative electrode are laminated in order and filled with an electrolytic solution, and the positive electrode and the negative electrode are insulated by the separator. As these negative electrodes, for example, metallic lithium, an alloy of lithium and other metals, carbon, graphite, etc. are often used.

[0003] In particular, although the use of a polyimide porous film having relatively high heat resistance as an insulating separator has been studied, since it takes time and effort to produce a polyimide porous film compared to a polyolefin porous film, improvement in productivity has become an issue. In contrast, Patent Document 1 discloses a method for producing a polyimide porous film that is simpler and more productive than conventional methods by using resin particles.

Prior Art Documents

Patent Documents

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, compared with polyolefin porous films, the strength of polyimide porous films is still not sufficient, and further improvement is required. In particular, there was a need to further improve the mechanical strength in the film thickness direction. This invention was made in view of these circumstances, and aims to improve the mechanical properties of polyimide porous films in the film thickness direction compared to conventional methods. [Means for solving the problem]

[0005] The polyimide porous membrane of the present invention has the following configurations [1] to [3] in order to achieve the above-mentioned objectives.

[0006] [1] A multilayer polyimide porous film having interconnected pores formed by removing fine particles, with surface layers layered on both sides of the intermediate layer, characterized in that the rate of change in the Gaarle value when 5 MPa is applied in the film thickness direction is less than 5%. In particular, it is desirable to keep the rate of change in the Gaarle value to less than 10% when 15 MPa is applied in the film thickness direction.

[0007] [2] Furthermore, it is desirable that the ionic conductivity of the polyimide porous membrane of the present invention be 1.5 mS / cm or higher, measured while pressurized to 25 MPa in the film thickness direction after immersion in an electrolyte solution prepared by mixing 1 M LiPF6 with an organic solvent prepared by mixing ethylene carbonate and diethyl carbonate in a 1:1 volume ratio. The compressive modulus is preferably 50 MPa or higher, more preferably 60 MPa or higher, and even more preferably 70 MPa or higher.

[0008] [3] Furthermore, it is desirable that the polyimide porous membrane of the present invention contains at least one of biphenyltetracarboxylic acid dianhydride and oxydiphthalic acid dianhydride as the carboxylic acid dianhydride. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a polyimide porous film that can maintain pore size while increasing mechanical strength in the film thickness direction, thereby suppressing an increase in air permeability (Gare value). [Brief explanation of the drawing]

[0010] [Figure 1] This graph shows the tensile strength of the examples and comparative examples. [Figure 2] This chart shows the relationship between porosity, tensile strength, and Gure value. [Figure 3] This graph shows the relationship between ionic conductivity and activation energy. [Figure 4] This graph shows the change in ionic conductivity under pressurized conditions. [Figure 5] This graph shows the change in membrane resistance under pressurized conditions. [Figure 6] This graph shows the percentage change in the Gale value before and after pressurization. [Figure 7] This graph shows the compressive modulus before and after pressurization. [Modes for carrying out the invention]

[0011] The polyimide porous membrane of the present invention will be described below. The polyimide porous membrane of the present invention, composed of a copolymer of a carboxylic acid and a diamine, may be used as a separator for energy storage devices such as lithium-ion batteries, lithium metal batteries, or capacitors, or it may be used for other applications; there are no limitations on its use.

[0012] When the polyimide porous membrane of the present invention is used as a separator for energy storage devices, the porosity by gravimetric method is preferably 45 to 75%, and the Gahl value (air permeability) is preferably 300 seconds or less. If the porosity is 45% or more, for example, with a film thickness of 20 μm, the Gahl value can be made 300 seconds / 100 cc or less, providing a separator that is preferable not only from the viewpoint of the amount of Li reaction described later, but also from the viewpoint of the Gahl value. Increasing the porosity can reduce the Gahl value, but the higher the porosity, such as above 70%, the weaker the film strength of the polyimide porous membrane becomes. Therefore, it is difficult to assemble batteries using polyimide porous membranes with porosity above 80% because the film may be damaged during actual battery manufacturing.

[0013] The thickness of the polyimide porous film is preferably about 4 μm to 50 μm, for example. When the thickness of the polyimide porous film is too thick, the ionic conductivity tends to decrease. It is also related to the short-circuit resistance. The thinner the film thickness, the more likely the insulation cannot be maintained. Also, if the polyimide porous film is too thin, the film strength decreases. Further, the maximum stress of the polyimide porous film is preferably 25 N / mm 2 or more.

[0014] In the production of the polyimide porous film of the present invention, for example, known methods such as a method of imidizing a polyamic acid by chemical imidization or heating can be used, and it can be produced in the following steps: A step of preparing a slurry by mixing fine particles for pore formation into a polyimide precursor solution containing a polyamic acid (so-called polyimide varnish) made from a carboxylic anhydride and an organic amine compound, A step of forming the prepared slurry into a thin film on a support to form a non-porous base film, A step of peeling the base film from the support, A step of firing the base film to make it porous. Note that the removal of the fine particles for pore formation and the imidization of the base film can be carried out simultaneously and in parallel, but it is also possible to remove the fine particles from the base film to form pores and then carry out imidization.

[0015] [Slurry preparation] The polyimide precursor solution contains a polyamic acid made from a carboxylic anhydride and an organic amine compound. A slurry can be prepared by using a polyamic acid or a mixture of a polyamic acid and a solvent as the polyimide precursor solution and mixing fine particles for opening into this. Note that the polyimide precursor solution may be a solution obtained by polymerizing a tetracarboxylic dianhydride and a diamine in the presence of an organic solvent, or a solution obtained by dissolving a polyamic acid in an organic solvent, either is acceptable.

[0016] A film-forming slurry is prepared by mixing a polyimide precursor solution, pore-opening microparticles, a dispersant, and a solvent. This slurry can then be formed into a thin film to produce a non-porous raw material. A dispersant is not essential and should be used as needed. The material of the pore-forming microparticles should be insoluble in the organic solvent used in the polyimide precursor solution and selectively removable after film formation. For example, examples of nanoparticles composed of inorganic materials include silica (silicon dioxide), titanium dioxide, and metal oxides such as alumina (Al2O3). Examples of fine particles composed of organic materials include polyolefins such as polypropylene and polyethylene, polystyrene (PS), acrylic resins (e.g., methyl methacrylate, isobutyl methacrylate, polymethyl methacrylate (PMMA), etc.), polyurethane resin (PUR), melamine resin (MF), urea resin (UF), phenolic resin (PF), epoxy resin, cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl acetate (PVAc), ABS resin, AS resin, polyacrylonitrile (PAN), polycarbonate (PC), polyamide (PA), polyethylene terephthalate (PET), polyester, polyether, and other organic polymers (hereinafter referred to as resin fine particles).

[0017] There are no restrictions on the shape of the microparticles, but a spherical or nearly spherical shape is preferable. Ideally, a shape close to a perfect sphere is preferable, but a nearly spherical shape, such as an ellipse with some distortion or fine irregularities, is also acceptable. Furthermore, fine particles that are close to a perfect sphere and have a small particle size distribution index (small variation in particle size) are preferred. By using fine particles that meet these conditions, the variation in pore size formed in the polyimide porous film can be reduced. This allows lithium to be moved more uniformly on the separator when the polyimide porous membrane is used as a separator in an energy storage device. The particle size of the fine particles should preferably be 800 nm or less. This allows the pore diameter of the porous membrane obtained by removing the fine particles to be 800 nm or less. A more preferable particle size is 600 nm or less, and even more preferably 500 nm or less.

[0018] A dispersant may be added to the polyimide precursor solution to uniformly disperse the fine particles. By adding a dispersant, the polyamic acid and fine particles can be mixed more uniformly, and the fine particles can be distributed more uniformly in the raw material described later. By uniformly distributing the fine particles, the distribution of pores in the porous polyimide film obtained by removing the fine particles can be made more uniform.

[0019] <Carboxylic acid anhydrides and polyamic acids> As the polyamic acid, one obtained by polymerizing a carboxylic acid dianhydride and a diamine can be used. There are no particular restrictions on the amount of carboxylic acid dianhydride and diamine used, but it is preferable to use 0.50 to 1.50 moles of diamine per mole of carboxylic acid dianhydride, more preferably 0.60 to 1.30 moles, and particularly preferably 0.70 to 1.20 moles of diamine. The carboxylic acid dianhydride should be appropriately selected from those conventionally used as raw materials for the synthesis of polyamic acids. The carboxylic acid dianhydride may be either an aromatic tetracarboxylic acid dianhydride or an aliphatic tetracarboxylic acid dianhydride, but from the viewpoint of the heat resistance of the resulting polyimide resin, it is preferable to use an aromatic tetracarboxylic acid dianhydride. Two or more carboxylic acid dianhydrides may be used in combination.

[0020] Suitable specific examples of aromatic tetracarboxylic dianhydrides include pyromellitic dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, oxydiphthalic acid dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride. Examples include dianhydrides of rubonates, bis(3,4-dicarboxyphenyl) ether dianhydrides, bis(2,3-dicarboxyphenyl) ether dianhydrides, benzophenonetetracarboxylic dianhydrides, 1,2,5,6-naphthalenetetracarboxylic dianhydrides, 1,4,5,8-naphthalenetetracarboxylic dianhydrides, 2,3,6,7-naphthalenetetracarboxylic dianhydrides, 1,2,3,4-benzenetetracarboxylic dianhydrides, diphthalic dianhydrides, 3,4,9,10-perylenetetracarboxylic dianhydrides, 2,3,6,7-anthracenetetracarboxylic dianhydrides, 1,2,7,8-phenanthrenetetracarboxylic dianhydrides, 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydrides, and 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydrides. Examples of aliphatic tetracarboxylic dianhydrides include ethylenetetracarboxylic dianhydride, butanetetracarboxylic dianhydride, cyclopentanetetracarboxylic dianhydride, cyclohexanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, and 1,2,3,4-cyclohexanetetracarboxylic dianhydride. It is particularly preferable that the carboxylic acid dianhydride include at least one of biphenyltetracarboxylic dianhydride and oxydiphthalic acid dianhydride.

[0021] <Organic amine compounds> The diamine, which is an organic amine compound, can be appropriately selected from diamines that have been conventionally used as raw materials for the synthesis of polyamic acids. The diamine may be an aromatic diamine or an aliphatic diamine, and can be appropriately selected in consideration of the properties of the target polyimide resin, but aromatic diamines are preferred. In addition, two or more types of diamines may be used in combination.

[0022] Examples of aromatic diamines include phenylenediamine and its derivatives, diaminobiphenyl compounds and their derivatives, diaminodiphenyl compounds and their derivatives, diaminotriphenyl compounds and their derivatives, diaminonaphthalene and its derivatives, aminophenylaminoindan and its derivatives, diaminotetraphenyl compounds and their derivatives, diaminohexaphenyl compounds and their derivatives, and cardo-type full orangeamine derivatives.

[0023] Based on the number of benzene rings, specific examples of diamines are as follows: 1) to 4) 1) Benzenediamine with one benzene ring Examples include 2,4-diaminotoluene, phenylenediamine, and 2,6-diaminotoluene.

[0024] 2) Two diamines with a benzene ring 3,3'-dimethyl-4,4'-diaminobiphenyl, 2,2'-dimethyl-4,4'-diaminobiphenyl, 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, 3,3'-dimethyl-4,4'-diaminodiphenylmethane, 3,3'-dicarboxy-4,4'-diaminodiphenylmethane, 3,3',5,5'-tetramethyl-4,4'-diaminodiphenylmethane, bis(4-aminophenyl) sulfide, 4,4'-Diaminobenzanilide, Dimethylbenzidine, 3,3'-Dimethoxybenzidine, 2,2'-Dimethoxybenzidine, 3,3'-Diaminodiphenyl ether, 3,4'-Diaminodiphenyl ether, 4,4'-Diaminodiphenyl ether, 3,3'-Diaminodiphenyl sulfide, Oxydianiline, 3,4'-Diaminodiphenyl sulfide, 4,4'-Diaminodiphenyl sulfide, 3,3'-Diaminodi Phenyl sulfone, 3,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminobenzophenone, 3,3'-diamino-4,4'-dichlorobenzophenone, 3,3'-diamino-4,4'-dimethoxybenzophenone, 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 2,2-bis(3-aminophenyl)propane, 2, 2-bis(4-aminophenyl)propane, 2,2-bis(3-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 2,2-bis(4-aminophenyl)-1,1,1,3,3,3-hexafluoropropane, 3,3'-diaminodiphenyl sulfoxide, 3,4'-diaminodiphenyl sulfoxide, 4,4'-diamino-3,3'-dimethyldiphenylmethane, 4,4'-diaminodiphenyl sulfoxide, etc.

[0025] 3) Three diamines with a benzene ring Examples include 1,3-bis(3-aminophenyl)benzene, 1,3-bis(4-aminophenyl)benzene, 1,4-bis(3-aminophenyl)benzene, 1,4-bis(4-aminophenyl)benzene, and 1,3-bis(4-aminophenoxy)benzene.

[0026] 4) Benzene nucleus four diamines 3,3'-bis(3-aminophenoxy)biphenyl, bis[3-(3-aminophenoxy)phenyl]methane, bis[3-(4-aminophenoxy)phenyl]methane, 9,9-bis(4-aminophenyl)fluorene, bis[4-(3-aminophenoxy)phenyl]methane, bis[4-(4-aminophenoxy)phenyl]methane, 2,2-bis[3-(3-aminophenoxy)phenyl]propane, 2,2-bis[3-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(3-aminophenoxy)phenyl]propane, 2,2-bis[3-(3-aminophenoxy)phenyl]-1,1,1,3,3,3-hexafluoropropane, etc.

[0027] <Organic solvents> The reaction between tetracarboxylic dianhydride and diamine is generally carried out in an organic solvent. The organic solvent used in the reaction between tetracarboxylic dianhydride and diamine is not particularly limited as long as it can dissolve the tetracarboxylic dianhydride and diamine and does not react with them. The organic solvent may be used alone or in mixtures of two or more, and should be selected as appropriate. Examples of organic solvents used in the reaction between tetracarboxylic dianhydrides and diamines include N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-diethylacetamide, N,N-dimethylformamide, N,N-diethylformamide, N-methylcaprolactam, β-propiolactone, γ-butyrolactone, dimethyl sulfoxide, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, dioxane, phenolic solvents, cyclic ketones such as cyclopentanone and cyclohexanone, aromatics such as toluene and xylene, and alicyclic compounds such as cyclopentane and cyclohexane. These organic solvents can be used individually or in mixtures of two or more. There are no particular restrictions on the amount of organic solvent used, but it is desirable that the polyamic acid content of the resulting product be 5 to 50% by mass.

[0028] The organic solvent used in the polyimide precursor solution can be any solvent that dissolves the polyamic acid or polyimide resin being used without dissolving the fine particles. In the polyimide precursor solution, the content of organic solvents is preferably 50 to 95% by mass, and more preferably 60 to 85% by mass. The solid content in the polyimide precursor solution is preferably 5 to 50% by mass, and more preferably 15 to 40% by mass. In addition to the components mentioned above, the polyimide precursor solution may contain, as appropriate, antistatic agents, flame retardants, chemical imidizing agents, condensing agents, release agents, surface modifiers, dimensional stabilizers, etc., for purposes such as antistatic properties, flame retardancy, low-temperature firing, mold release properties, coatability, low hygroscopicity, low coefficient of thermal expansion, and reduced thermal shrinkage.

[0029] The more specific manufacturing procedure for polyimide porous films is as follows (1) to (6). However, when using inorganic particles such as silica to create pores in the raw material, it is necessary to immerse the raw material in a solvent to remove the inorganic particles, making it difficult to perform the porosization process and the calcination process simultaneously. In this case, it is desirable to perform the porosization process before the calcination process. On the other hand, when using resin particles to create porosity in the raw material, it becomes possible to perform the porosity creation process and the firing process simultaneously, making the manufacturing process simpler and more compact. The procedure for manufacturing polyimide porous membranes is as follows: (1) A step of preparing a polyimide precursor solution having polyamic acid synthesized from carboxylic acid anhydrides and organic amine compounds; (2) A step of preparing a slurry by mixing a polyimide precursor solution with fine particles; (3) A process of forming a non-porous base material by coating a slurry onto a support; (4) A peeling step in which the raw material formed in the raw material formation step is peeled off from the support; (5) A porosizing process to make the raw material peeled from the support by the peeling process porous; and (6) A firing process in which the raw material is fired.

[0030] [Preparation of polyimide precursor solution] As mentioned above, polyamic acids can be obtained by polymerizing carboxylic acid dianhydrides and organic amine compounds. Polyamic acids can be imidized by applying heat (thermal imidization) or by chemically imidizing them (chemical imidization), which causes the carboxylic acid portion to cyclize and become polyimide. The imidization rate should preferably be about 80% or more, more preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more, but this can also be adjusted according to the desired physical properties.

[0031] As described above, the present invention is a porous polyimide film having numerous interconnected pores formed by removing fine particles and composed of a copolymer of a carboxylic acid and a diamine, characterized in that the carboxylic acid is composed of BPDA in a molar ratio of more than 70% to ODPA and less than 30%. The ratio of BPDA to ODPA is not limited to more than 70% to less than 30% of BPDA, but for example, 80% or more to 20% or less of BPDA is preferred, more preferably 87% or more to less than 13% of BPDA and even more preferably 85% or more to 15% or less of ODPA. As described above, it is desirable that the carboxylic acid contains both biphenyltetracarboxylic acid and oxydiphthalic anhydride, but it may also contain at least one selected from benzophenonetetracarboxylic dianhydride and 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride. Furthermore, when it is desired to improve the initial Coulomb efficiency in lithium-ion batteries or lithium metal batteries by using the porous membrane of the present invention as a separator, polyimide is preferable as it has a lower property of adsorbing or reacting with Li.

[0032] Regarding the diamine, among o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine, m-phenylenediamine and p-phenylenediamine are preferred as phenylenediamine, and p-phenylenediamine is even more preferred. Diaminodiphenylmethanes include 3,3'-diaminodiphenylmethane, 3,4'-diaminodiphenylmethane, and 4,4'-diaminodiphenylmethane, but 4,4'-diaminodiphenylmethane is preferred. In general, the diamine is preferably at least one selected from p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diamino-3,3'-dimethyldiphenylmethane, o-dianisidine, 9,9-bis(4-aminophenyl)fluorene, etc. The diamine may be a single type or a mixture of two or more types.

[0033] As described above, organic polar solvents can be used as solvents for polymerizing polyamic acids. Preferred organic polar solvents include, for example, tetramethylurea, phenol, N-methyl-2-pyrrolidone (NMP), pyridine, N,N-dimethylacetamide (DMAc), N,N-dimethylformamide, p-chlorophenol, o-chlorophenol, dimethyl sulfoxide, and cresol.

[0034] Other conditions for preparing polyamic acids include, for example, reacting tetracarboxylic dianhydride and diamine in approximately equimolar amounts (nearly equimolar amounts) under temperature conditions preferably of about 80°C or lower, more preferably 70°C or lower, even more preferably 0 to 65°C, and particularly preferably 10 to 60°C. The reaction time is preferably about 0.1 hours or more, more preferably 0.2 to 72 hours, and even more preferably 0.5 to 60 hours to produce polyamic acid. When preparing the polyimide precursor solution, components to adjust the molecular weight can also be added to the reaction solution. The prepared polyimide precursor solution consists, for example, of 5-50% by mass of polyamic acid and 50-95% by mass of an organic polar solvent. If the polyamic acid content is less than 5% by mass, the film strength of the porous polyimide film decreases, and if it exceeds 50% by mass, the viscosity of the porous polyimide film becomes too high, reducing its handling properties.

[0035] [Slurry preparation] A slurry is prepared by mixing the polyimide precursor solution and fine particles as described above. Organic solvents for adjusting the concentration, as well as additives already mentioned, such as antistatic agents, low-temperature firing agents, release agents, coatable agents, low hygroscopic agents, low thermal expansion agents, and chemical imidizing agents, may be added to the slurry as appropriate. The slurry consists of 5-90% by mass of polyimide precursor solution, 2-40% of fine particles for pore opening, 0-95% by mass of organic solvent for concentration adjustment, and, although not particularly limited, preferably 0-20% by mass of additives. These are mixed using a stirring device. For stirring, it is preferable to use a rotational and revolutionary stirrer such as "Awatori Rentaro" (manufactured by Shinky Co., Ltd.).

[0036] The viscosity of the slurry solution should be appropriately determined according to the characteristics of the die and coater machine used for coating. For example, from the viewpoint of ease of coating and film strength, a viscosity of approximately 0.1 to 1000 Pa·s, preferably 0.5 to 300 Pa·s, and more preferably 1 to 250 Pa·s, can be used with various dies and coater machines.

[0037] [Original fabric forming process] The prepared slurry can be coated onto a support to form a raw material. There are no particular restrictions on the coating method; for example, it can be coated onto a support such as a glass plate, stainless steel plate, PET (polyethylene terephthalate) film, or PEN (polyethylene naphthalate) film using a slurry blade or T-die. This allows for the formation of a raw material in which the slurry is spread in layers on the support. Furthermore, as a support, an endless mechanism such as a metal belt or a resin film may be used. The support can be anything that is not affected, or is less affected, by the prepared slurry. For endless mechanisms, a metal such as stainless steel may be used, and for resin films, a resin such as PET or polytetrafluoroethylene may be used. When forming a multilayer polyimide porous film, a feed block type or multi-manifold type die may be used as the T-die. The present invention may also be applied to a polyimide porous film having a multilayer structure. By applying the present invention to, for example, the surface layer or the intermediate layer of the multilayer structure, it becomes possible to provide a multilayer polyimide porous film that achieves a higher level of physical properties such as maximum stress, elongation, and elastic modulus than conventional films.

[0038] [Peeling process] After forming the raw material on the support and before the firing process, the raw material is peeled off the support at an appropriate time. There are no particular restrictions on the length of the raw material, but from the viewpoint of productivity, it is preferable to have a long roll (e.g., 5m or more), more preferably 10m or more, and even more preferably 20m or more. There are no particular restrictions on the upper limit of the length of the raw material, but for example, it is 4000m or less, and typically 1000m or less makes it easier to handle the raw material. It is preferable to properly dry the raw material to make it easier to peel off the support.

[0039] [Porous process] By removing fine particles from the raw material peeled from the support using an appropriate method, a porous polyimide film having spherical or substantially spherical pores can be manufactured. Methods for removing fine particles include dissolving them with a solvent, acid, or alkali, or removing them by calcination, but the choice depends on the fine particles used. Considering productivity and cost, resin particles are preferred as the fine particles for forming the pores. When using inorganic particles such as silica as porous particles, they can be removed by dissolving them in contact with an acid or alkali. When the fine particles are resin particles, they can be dissolved and removed using an organic solvent that does not dissolve the polyimide film but is soluble in the resin particles. Examples of such organic solvents include ethers such as tetrahydrofuran; aromatics such as toluene; ketones such as acetone; and esters such as ethyl acetate. Among these, ethers such as tetrahydrofuran are preferred, and tetrahydrofuran is even more preferred. In the case of resin fine particles, pores can be formed by decomposing and removing the resin fine particles by heating them to a temperature above the thermal decomposition temperature of the resin fine particles but below the thermal decomposition temperature of the polyimide resin.

[0040] [Firing process] The obtained raw material or porous raw material can be subjected to imidization (hereinafter referred to as thermal imidization) by applying heat, thereby forming a polyimide porous film. The shrinkage rate of the polyimide porous film after thermal imidization should be suppressed to 5% or less in the mechanical direction (MD direction) and also to 5% or less in the width direction (TD direction). There are no particular restrictions on the means or methods for suppressing the shrinkage rate, but it is preferable to reduce the tension applied to the raw material or the film after perforation. The temperature conditions can be appropriately set, for example, in the range of 250 to 500°C, for 1 to 300 minutes, preferably 5 to 240 minutes, and more preferably 10 to 120 minutes. To increase productivity, it is desirable to keep the heating and firing time as short as possible. In this thermal imidation treatment, it is desirable that the heating rate in the temperature range of 200°C or higher be 20°C / min or more, preferably 30°C / min or more. By heating at the above heating rate in the temperature range of 100 to 250°C, where the imidation reaction occurs significantly, a porous polyimide film of the present invention can be obtained with significantly improved surface aperture ratio and pore size.

[0041] The firing temperature varies depending on the type of polyamic acid and the desired degree of imidization, but 120 to 500°C is preferred, and 150 to 500°C is more preferred. When firing, the drying process and the firing process may be separated, but they may also be carried out without strict separation. For example, when firing at 360°C, there are methods such as continuously raising the temperature from room temperature to 360°C and then firing at 360°C for several tens of minutes, or gradually raising the temperature from room temperature to 360°C and then firing at 360°C for several tens of minutes. The most suitable procedure should be selected as appropriate. The raw material should be peeled off the support at an appropriate timing during the temperature increase from room temperature.

[0042] The fired polyimide porous film is preferably wound onto a core with a diameter of, for example, 2.5 cm (1 inch) to 25 cm (10 inches). The core diameter is preferably 5 cm (2 inches) to 10 cm (4 inches). There are no particular restrictions on the material of the core, but examples include paper, metal such as stainless steel, and hard plastics such as ABS, PP, PE, PVC, PET, FRP, and Bakelite.

[0043] When the above firing process also serves to remove resin particles, if the organic material constituting the resin particles decomposes at a lower temperature than polyimide, then only the resin particles can be eliminated without causing thermal damage to the polyimide. For this reason, the decomposition temperature of the resin fine particles is preferably, for example, between 120°C and 500°C.

[0044] The polyimide porous film produced by the above procedure has multiple spherical or nearly spherical pores formed inside, with at least some of the pores communicating with each other. The pore diameter on the surface and inside the polyimide porous film can be controlled by appropriately selecting or adjusting the type and size of the fine particles used when manufacturing the film. In the polyimide porous membrane of the present invention, it is preferable that the pores have little variation in pore size and a more uniform distribution. As an indicator of the small variation in pore size and distribution of pores within the film, a method can be used in which the Gahl value of the film is measured at several points in different locations and the variation in the values ​​at that time is evaluated. For any 10 points of the polyimide porous membrane, the air permeability (the number of seconds it takes for 100 mL of air to pass through the film) can be measured by the Gahl method in accordance with JIS P 8117, and the average value and standard deviation of the air permeability can be determined.

[0045] Films with pores that exhibit small variations in pore size and distribution can be manufactured by using fine particles with a high sphericity and a small particle size distribution index during film production, or by adjusting the viscosity of the polyimide precursor solution containing the fine particles and polyamic acid or polyimide resin to an appropriate viscosity that allows for uniform coating. The air permeability measured by the Gaarle method using the above method is preferably 400 seconds or less, more preferably 300 seconds or less, and even more preferably 250 seconds or less. It is believed that such air permeability allows lithium and lithium ions to be moved with lower resistance.

[0046] The thickness of the polyimide porous film of the present invention is preferably 2 μm to 100 μm, more preferably 3 μm to 80 μm, and even more preferably 4 μm to 50 μm, when the film is used as a separator for batteries. The thickness can be determined by measuring the thickness at multiple points using a micrometer or the like and averaging the result.

[0047] The above describes the method of fabricating a single-layer polyimide porous film; below, we will describe the procedure for fabricating a multilayer polyimide porous film by co-extrusion. The basic manufacturing procedure is the same as for the single-layer structure described above, but the dies used are different. The manufacturing procedure for multilayer polyimide porous membranes is as follows: A first slurry preparation step involves mixing pore-opening fine particles with a polyimide precursor solution to prepare a first slurry; A second slurry preparation step for preparing a second slurry different from the first slurry; A process of supplying the first slurry and the second slurry to a die for co-extrusion molding; A process in which a layer composed of a first slurry and a layer composed of a second slurry are continuously extruded from the discharge port of a co-extrusion die onto a smooth support as an integrated thin film (multilayer raw material formation process); A step of drying the multilayer thin film on the support and peeling the multilayer thin film off the support (peeling step); and A process of heating a multilayer thin film to remove fine particles and create a porous structure (porosity creation process). The multilayer thin film before pore opening is also called a multilayer raw material.

[0048] In the multilayer polyimide porous membrane of the present invention, the temperature of the polyamic acid solution when it is extruded from the multilayer extrusion die toward the support is 0 to 150°C, with a particularly preferred temperature of 5 to 100°C, and even more preferably 10 to 60°C. The polyamic acid solution extruded in film form is dried to some extent in order to peel it away from the support, and this drying temperature can be appropriately determined within the range of 50 to 200°C, particularly 60 to 180°C. The film-like polyamic acid peeled from the support is preferably subjected to a heat treatment at a temperature of 300-500°C for approximately 1-80 minutes, particularly 2-60 minutes, as this allows for the imidization of the polyamic acid and the removal of resin particles.

[0049] <Multilayer fabric formation process> In the multilayer substrate formation process, a multilayer die is used to layer a slurry onto a support. In this co-extrusion method, multiple types of slurries are simultaneously extruded onto a support to form a multilayered, non-porous raw material in one go. During this process, the slurries come into direct contact with each other without drying, resulting in strong adhesion between the layers of the multilayered raw material. This not only reduces the number of steps involved, but also shortens the formation time of the multi-layer base material compared to sequential coating methods. For example, when using additives such as dehydrating agents, the risk of them accumulating between layers can be reduced compared to sequential coating methods.

[0050] As mentioned above, there are no restrictions on the material of the support from which the slurry is discharged; any smooth material that does not melt with the slurry is acceptable. Preferably, a multilayer base material with excellent release properties is preferred. Examples include a metal belt (endless mechanism) such as stainless steel, a resin film such as PET, or a rotating drum, but the system is not limited to these, and any suitable material may be used as appropriate.

[0051] From a manufacturing efficiency standpoint, a rotating drum or an endless mechanism is preferable as the support structure, but this should also be selected as appropriate. With an endless mechanism, multilayer raw materials can be continuously formed by rotational drive, eliminating the need for disposable resin films, which leads to a reduction in manufacturing costs. Furthermore, when firing the multilayered raw material, the width of the edges of the multilayered raw material may be fixed using fixing means such as tenter pins or chucks.

[0052] The present invention provides a configuration that allows multiple types of polyimide-based varnishes to be simultaneously extruded onto a support, and one means of achieving this is to use a multilayer co-extrusion die. Compared to the method of laminating the second layer after the first layer (sequential coating method), the co-extrusion method, which forms multiple liquid films simultaneously, can improve the adhesion between layers and reduce the formation of interlayer interfaces, thus preventing problems such as delamination.

[0053] <Peeling process> In the peeling process, the multilayer base material is peeled off from the support as a non-porous film that stands independently. There are no restrictions on the conditions for peeling the multilayer substrate from the support; any conditions that allow for good peeling of the multilayer substrate from the support, such as partially evaporating the organic solvent from the multilayer substrate or imidizing a portion of the polyamic acid, are acceptable. The temperature for peeling the multilayer substrate from the support is preferably between 50°C and 200°C, and may be appropriately set between 60°C and 180°C. By setting the temperature for peeling the multilayer substrate from the support to within the range of 60-200°C, the multilayer substrate can be made more gel-like, making it easier to peel from the support. There are no particular restrictions on the heating time, and it should be determined appropriately considering the film thickness and productivity.

[0054] <Porous process> In the porosity formation process, the multilayer raw material peeled from the support is fired to promote imidation while simultaneously burning off resin particles and residual solvent, thereby forming a multilayer polyimide porous film. This firing process simultaneously promotes the evaporation of solvent components contained in the multilayer raw material, accelerates the imidation reaction of polyamic acid, and removes resin particles. This results in a multilayer polyimide porous film in which multiple polyimide porous layers are laminated.

[0055] There are no particular restrictions on the heating method used in the firing process for creating porosity; any method that can effectively heat the multilayer base material peeled from the support is acceptable. For example, a method of heating the entire multilayer base material by blowing hot air at 100°C or higher from the top surface (one side), the bottom surface (the other side), or both sides, or a method of irradiating the base material with far-infrared rays is suitable. The heating temperature is preferably between 200°C and 600°C, but it should be determined appropriately in consideration of the composition of the polyamic acid. The temperature should be increased gradually. Rapidly increasing the heating temperature can easily lead to uneven firing of the multilayer base material, potentially affecting the smoothness of the film surface. Conversely, if the heating temperature is too low or the heating time is too short, the resin particles may not be sufficiently burned away, resulting in insufficient porosity.

[0056] [Applications of polyimide porous membranes] The polyimide porous membrane of the present invention can be used as a separator in energy storage devices such as nickel-cadmium batteries, nickel-metal hydride batteries, lithium-ion primary / secondary batteries using graphite as the negative electrode, and lithium-metal primary / secondary batteries using metallic lithium as the negative electrode. Among these, it is preferable to use it as a separator for lithium-ion secondary batteries or lithium-metal secondary batteries. The polyimide porous membrane of the present invention may also be used as an electrolyte membrane for fuel cells or as a substrate for electronic circuits.

[0057] The energy storage device using the polyimide porous membrane of the present invention as a separator includes a negative electrode, a positive electrode, and an electrolyte, in addition to the separator. The energy storage device can take various forms, such as wound type (cylindrical or rectangular in appearance), laminate type, and coin type. An electrode structure in which the positive electrode, separator, and negative electrode are stacked in order is housed in an outer casing, and the electrolyte is poured into this outer casing to fabricate the energy storage device.

[0058] The negative electrode of a lithium-ion battery has a structure in which a negative electrode mixture, consisting of, for example, a negative electrode active material, a conductive additive, and a binder, is molded on a current collector. As the negative electrode active material, any material capable of electrochemically doping lithium can be used. Examples of such active materials include carbon materials, silicon, aluminum, tin, and Wood's alloys. In the case of lithium metal batteries, as is well known, metallic lithium is used as the negative electrode.

[0059] Examples of conductive additives for the negative electrode include carbon materials such as acetylene black and Ketjenblack. The binder consists of an organic polymer, such as polyvinylidene fluoride and carboxymethylcellulose. Copper foil, stainless steel foil, nickel foil, etc., can be used for the current collector.

[0060] Furthermore, the positive electrode can have a structure in which a positive electrode mixture consisting of a positive electrode active material, a conductive additive, and a binder is molded on a current collector. For example, as the positive electrode active material, nickel hydroxide can be used in the case of nickel-cadmium batteries, and nickel hydroxide or nickel oxyhydroxide can be used in the case of nickel-metal hydride batteries. In the case of lithium-ion secondary batteries, lithium-containing transition metal oxides can be used as the positive electrode active material, specifically LiCoO2, LiNiO2, LiMn0.5Ni0.5O2, LiCo1 / 3Ni1 / 3Mn1 / 3O2, LiMn2O4, LiFePO4, LiCo0.5Ni0.5O2, LiAl0.25Ni0.75O2, etc. Conductive additives can be carbon materials such as acetylene black and Ketjenblack.

[0061] The electrolyte for lithium-ion batteries and lithium metal batteries is typically a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent, but an aqueous electrolyte may be used depending on the desired properties. Examples of lithium salts include LiPF6, LiBF4, LiClO4, and LiFSI. Examples of non-aqueous solvents include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, and vinylene carbonate, which may be used alone or in combination with additives.

[0062] Examples of outer packaging materials include metal cans or aluminum laminate packs. While batteries can take various shapes such as rectangular, cylindrical, or coin-shaped, the separator of the present invention can be suitably applied to any of these shapes. As an example, the following describes the manufacturing procedures for laminated batteries, cylindrical batteries, and coin batteries. [Laminated lithium-ion secondary batteries and lithium metal secondary batteries] [Positive electrode, negative electrode] Commercially available products can be used as the positive and negative electrodes. For example, the positive electrode active material is a mixture of Li1,1Mn1,9O4 having a spinel structure and lithium nickel cobalt manganese oxide (Ni / Li molar ratio 0.7), polyvinylidene fluoride can be used as a binder, and carbon black powder as a conductive additive.

[0063] For the negative electrode, graphite or similar materials can be used as the negative electrode active material, and the porosity and pore size of the positive electrode active material layer and the negative electrode active material layer should be adjusted as appropriate. The active material layer on one side of the current collector of the positive and negative electrodes is peeled off and cut to a size of, for example, 29 mm x 40 mm for use. Alternatively, a metallic lithium layer can be formed to a predetermined thickness and used as the negative electrode instead of graphite. In this case, an improvement in energy density can be expected.

[0064] An aluminum positive electrode tab is welded to the positive electrode current collector of the positive electrode, and a copper negative electrode tab (negative electrode current collector plate) is welded to the negative electrode current collector of the negative electrode. The positive electrode active material layer of the positive electrode and the negative electrode active material layer, with these tabs welded to them, are placed facing each other, and a separator is placed between them to create a single plate-shaped electrode structure.

[0065] The electrode structure described above is sandwiched between an outer casing made of laminate film with an aluminum layer (for example, a rectangular shape of 60mm x 60mm), and three of the four sides of the rectangular shape are heat-sealed to form the outer casing. The electrolyte is injected into this outer casing using a vacuum impregnation device (e.g., TOSPACK V-307GII; manufactured by Tosei Electric Co., Ltd.), and the remaining side is vacuum-sealed by thermocompression to create the cell. After that, the injected electrolyte is left to stand for a predetermined time, for example at room temperature, until it has sufficiently impregnated the pores of the electrode structure.

[0066] [Wound-type lithium-ion secondary battery / lithium metal battery] A wound lithium-ion secondary battery / lithium metal secondary battery (hereinafter referred to as a lithium secondary battery) has a configuration in which, for example, the electrode structure is housed together with a non-aqueous electrolyte in a cylindrical casing. The electrode structure in a wound lithium secondary battery is made by preparing a strip-shaped positive electrode, a negative electrode, and two separators, and winding them in layers. This electrode structure is housed in a cylindrical casing, and the electrolyte is poured into the casing and sealed to create a wound battery.

[0067] In wound lithium secondary batteries, for example, a positive electrode is used that consists of a long sheet-shaped positive electrode current collector and a positive electrode composite layer containing positive electrode active material and provided on the positive electrode current collector. A negative electrode can also be used that consists of a long sheet-shaped negative electrode current collector and a negative electrode composite layer containing negative electrode active material and provided on the negative electrode current collector. The separator, like the positive and negative electrodes, is formed in the form of a long sheet, and this separator is wound up with the positive and negative electrodes interposed between them.

[0068] The exterior comprises a bottomed cylindrical case body and a lid that closes the opening of the case body. The lid and the case body are made of, for example, metal and are insulated from each other. The lid is electrically connected to the positive current collector, and the case body is electrically connected to the negative current collector. The lid may also serve as the positive terminal, and the case body as the negative terminal.

[0069] Lithium-ion batteries can be charged and discharged at temperatures ranging from -10 to 80°C, for example. To prevent internal pressure buildup within the battery, measures such as installing a safety valve in the battery cover or making cuts in the battery case or the gasket fitted to the case can be employed. Additionally, a current cutoff mechanism that senses the internal pressure of the battery and cuts off the current can be installed in the cover to prevent overcharging. [Examples]

[0070] The present invention will be described in more detail below with reference to examples, but the scope of the present invention is not limited to these examples. [film thickness] The measurement was taken using a contact-type thickness gauge (manufactured by Peacock).

[0071] [Gale value (air permeability)] A test specimen measuring 80 mm in width in the MD direction was taken from the manufactured microporous membrane. Measurements were taken at three points: the center and both ends (50 mm inward from the end face) using a Type B Gahl densometer (Kumagai Riki Kogyo Co., Ltd., Model No. 2060) in accordance with JIS P8117. The average value of the three points was calculated as the Gahl value.

[0072] [Porosity (gravimetric method)] Twenty porous membranes measuring 3.5 x 4.5 cm were punched out, and a porous membrane with a total area of ​​315 cm² was prepared. The film thickness and weight were then measured. For the measurement, the true density of the porous membrane was measured using a true density analyzer (BELPycno: manufactured by Microtrac-Bel). A measurement cell with a size of 3.5 cc was used, and the porosity was calculated using the following formula, with the obtained true density being D, the area of ​​the porous membrane being S, the film thickness being d, and the weight being w: Porosity=(1-(w / (S×d×D)×100 (1)

[0073] [Maximum stress (Tensile Strength)] Test specimens conforming to the JIS K-6251-6 sample shape were subjected to tensile stress testing at a tensile speed of 1 mm / min and a chuck distance of 50 mm under a measurement environment of 25°C (room temperature). The highest tensile stress value (MPa) before the film broke was defined as the maximum stress. A Shimadzu Autograph AGS-50NX tensile testing machine was used.

[0074] [modulus of elasticity] Test specimens conforming to the JIS K-6251-6 sample shape were subjected to tensile testing at 25°C (room temperature) with a tensile speed of 1 mm / min and a chuck distance of 50 mm. The stress-strain curve was then analyzed, and the gradient of the initial rise was determined. A Shimadzu Autograph AGS-50NX tensile testing machine was used.

[0075] [stretch] A test specimen conforming to the sample shape of JIS K-6251-6 was subjected to tensile testing at a measurement environment of 25°C (room temperature) with a tensile speed of 1 mm / min and a chuck distance of 50 mm. The elongation was defined as the breaking strain when the film ruptured. A Shimadzu Autograph AGS-50NX tensile testing machine was used.

[0076] [Fracture energy] Using a Shimadzu Autograph AGS-50NX, a test specimen conforming to the JIS K-6251-6 sample shape was measured at a tensile speed of 1 mm / min and a chuck distance of 50 mm under a measurement environment of 25°C (room temperature). The "force-displacement curve" data calculated during the tensile test using Shimadzu Autograph software TRAPEZIUM LITE X was obtained. The integral value of this data until the film broke was defined as the breaking energy [J], and the breaking energy per unit thickness [J / mm] was calculated using the following formula: Energy per unit film thickness [J / mm] = Energy [J] / Film thickness [mm]

[0077] [film resistance] The sample was cut to a diameter of φ18, washed, and air-dried. Aluminum foil was cut to a diameter of φ16 and lead tabs were attached. Two pieces of this aluminum foil were prepared, and the cut sample was sandwiched between them so that the aluminum foil would not short-circuit. The sample was impregnated with 1M LiPF6 electrolyte, EC:DEC (1:1; volume ratio). This was then sealed so that the tabs were outside the aluminum laminate. This cell was placed in a constant temperature bath at 25°C, and the resistance of the cell was measured using the AC impedance method at an amplitude of 14.1mV and a frequency range of 1MHz to 10kHz. The film resistance (Ω·cm2) was determined based on the measured resistance value of the cell.

[0078] [curvature rate] The curve ratio was calculated using the above-mentioned film resistance based on the following formula.

[0079]

number

[0080] [Ionic conductivity] The ionic conductivity was calculated using the film resistance described above, based on the following formula.

[0081]

number

[0082] The activation energy was calculated according to a known procedure. The results are shown in Table 2.

[0083] [Compression modulus] Multiple 15mm x 25mm separator samples were taken from the manufactured microporous membrane and stacked to create a stacked sample with a thickness of 0.4mm ± 2.5%. A 3mm diameter metal cylinder was pressed against the stacked sample, and a stress-strain curve in the compression direction was prepared using, for example, a Shimadzu AGSX-100N with a 100N load cell and a chuck cross head speed of 0.1mm / min. The compressive modulus was calculated from the slope of the portion of the stress-strain curve where the slope became constant. Here, stress is defined as the compressive load (N) per unit area (mm2) = compressive stress (N / mm2), and its unit is MPa. For example, the stress when a load of 100N is applied to a metal cylinder with a diameter of 3mm is 100N / (1.5mm × 1.5mm × π) ≈ 14.1MPa. "Strain" is the value obtained by dividing the displacement caused by deformation when compressive stress is applied by the initial thickness (0.4 mm), and it has no units. For example, if the test deforms the material from an initial thickness of 0.4 mm to 0.398 mm, the displacement is 0.02 mm, and the strain is 0.02 mm / 0.4 mm = 0.05.

[0084] [Measurement under pressurized conditions] Cells prepared using the method described in paragraph 0076 were sandwiched between glass epoxy plates and set in a screw press. An NPA System SPR-5 screw press was used. The load was adjusted so that a predetermined pressure was applied to the cells using the press. The resistance of the cells under load was measured at room temperature using the AC impedance method. The measurement conditions were an amplitude of 14.1 mV and a frequency range of 1 MHz to 10 kHz. Subsequently, the load was continuously increased without releasing the pressure from the cell, and the cell resistance was measured under each pressure condition. Based on the measured cell resistance values, the film resistance (Ω·cm²) under each pressure condition was determined. [Examples]

[0085] Example 1 [Preparation of polyamic acid] 16 g of N,N-dimethylacetamide was mixed with 2.3215 g of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 0.6119 g of 4,4'-oxydiphthalic acid anhydride, and 1.0666 g of p-phenylenediamine. The mixture was stirred in a separable flask for 12 hours to obtain polyamic acid. A slurry was prepared by adding 300 nm-particle-sized polymethyl methacrylate particles to the polyamic acid to achieve a theoretical porosity of 50%. This slurry was extruded onto a support from a single-layer T-die, and the particles were removed by heat treatment to produce a polyimide porous film. The obtained single-layer polyimide porous film had a thickness of 18 μm, a porosity of 56%, and an air permeability (Gare value) of 48 seconds / 100 cc. The measured film properties are summarized in Table 1. The properties of Examples 2 and 3 and Comparative Example 1 described below are also summarized in Table 1.

[0086] Example 2 A slurry was prepared by adding 300 nm-particle-sized polymethyl methacrylate particles to the polyamic acid prepared in Example 1 to achieve a theoretical porosity of 72%. This slurry was extruded onto a support from a single-layer T-die, and the particles were removed by heat treatment to produce a polyimide porous film. The obtained single-layer polyimide porous film had a thickness of 21 μm, a porosity of 74%, and an air permeability of 13 seconds / 100 cc.

[0087] Example 3 A first slurry was prepared by adding polymethyl methacrylate particles with a particle size of 300 nm to the polyamic acid prepared in Example 1. A second slurry was then prepared by adding a different amount of polymethyl methacrylate particles to the polyamic acid compared to the first slurry. These first and second slurries were extruded onto a support via a three-layer die to form a three-layer structure with the first slurry as the intermediate layer and the second slurry as the surface layer. The particles were then removed by heat treatment to produce a three-layer polyimide porous film. The theoretical porosity of the surface layer was set to 72%, and the theoretical porosity of the intermediate layer to 50%, with a layer ratio of 1:3:1 for the surface layer:intermediate layer:surface layer. The resulting three-layer polyimide porous film had a thickness of 24 μm, a porosity of 65%, and an air permeability of 20 seconds / 100 cc.

[0088] Comparative Example 1 A polyolefin-based separator was fabricated by a dry process using a known method. The obtained polyolefin-based separator had a film thickness of 36 μm, a porosity of 43%, and an air permeability (Gare value) of 692 seconds / 100 cc.

[0089] Figure 1 shows the tensile strengths of the examples and comparative examples measured using the method described above, and Figure 2 shows the relationship between porosity and tensile strength, and between porosity and Gahl value. Ionic conductivity and activation energy are shown in Figure 3 and Table 2. Figures 4 and 5 show the ionic conductivity and membrane resistance measured under pressurized conditions. Figure 6 and Table 3 show the rate of change in the Gahl value before and after pressurization, and Figure 7 shows the measured compressive modulus.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

Claims

1. A multilayer polyimide porous film having interconnected pores formed by removing fine particles, with surface layers layered on both sides of the intermediate layer, A multilayer polyimide porous film in which the variation rate of the Gaurle value when 5 MPa is applied in the film thickness direction is less than 5%.

2. A multilayer polyimide porous film in which the variation rate of the Gaurle value when 15 MPa is applied in the film thickness direction is less than 10%.

3. The multilayer polyimide porous membrane according to claim 1, wherein the ionic conductivity measured while pressurizing the membrane to 25 MPa in the film thickness direction after immersion in an electrolyte solution prepared by mixing 1 M LiPF6 with an organic solvent prepared by mixing ethylene carbonate and diethyl carbonate in a 1:1 volume ratio is 1.5 mS / cm or more.

4. A multilayer polyimide porous film according to any one of claims 1 to 3, wherein the compressive modulus is 50 MPa or more.

5. A multilayer polyimide porous membrane according to claim 1, having a three-layer structure with surface layers on both sides of the intermediate layer.

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