Porous structure

Abstract

Provided is a porous structure containing a polyimide resin, and that has a porosity of 20% or more. The porous structure may be a nonwoven fabric formed from nanofibers containing a polyimide resin, and can be suitably used as a separator for a battery such as a lithium-ion battery.

Description

porous structure

[0001] One embodiment of the present invention relates to a porous structure comprising a polyimide resin.

[0002] Porous structures are structures with multiple pores inside and on their surfaces, and are used in a variety of fields due to their diverse functions. Generally, porous structures are required to have high porosity, excellent air permeability, thinness, and sufficient mechanical strength.

[0003] For example, in the field of energy devices, porous structures can be used as separators. In the field of lithium secondary batteries, porous structures made of polypropylene (PP) or the like have been widely used as separators, and improvements to meet various needs have been studied. For example, Patent Document 1 discloses a polypropylene (PP) porous film provided with an inorganic coating or ceramic coating to improve the mechanical strength and heat resistance when used as a separator.

[0004] However, many of the PP porous films disclosed for use as separators have a multilayer structure including a PP substrate and a porous layer. Therefore, it is difficult to meet the demand for thinner PP porous films with conventional PP porous film configurations. Furthermore, in order to obtain excellent battery characteristics, the separator is desired to have excellent electrolyte wettability. However, the PP substrate, which significantly affects the physical properties of the PP porous film, does not have fully satisfactory electrolyte wettability, leaving room for improvement.

[0005] Special table 2015-503201 publication

[0006] On the other hand, polyimide resins are used in a variety of fields due to their excellent heat resistance, mechanical strength, chemical resistance, and the like. In the field of energy devices, with the recent trend toward higher battery capacities, separators with high heat resistance and high strength are desired, and the development of separators using polyimide resins is anticipated. However, it is generally not easy to manufacture porous structures such as porous films and nonwoven fabrics that contain polyimide resins as their main component. Therefore, there is a need for porous structures that contain polyimide resins and have properties that make them suitable for use in applications such as separators.

[0007] In view of the above circumstances, one embodiment of the present invention provides a porous structure that contains a polyimide resin and has excellent wettability with an electrolyte solution.

[0008] The present invention includes the following embodiments. However, the present invention is not limited to the following embodiments and includes various embodiments: <1> A porous structure containing a polyimide resin and having a porosity of 20% or more.

[0009] <2> The porous structure according to the above <1>, which has a contact angle with water of 90° or more.

[0010] <3> The porous structure according to <1> or <2> above, which is used as a battery separator.

[0011] <4> The porous structure according to any one of the above <1> to <3>, wherein the contact angle of the electrolyte is 10° or less.

[0012] <5> The porous structure according to any one of <1> to <4> above, which is used as a separator for a lithium ion secondary battery.

[0013] <6> The porous structure according to any one of <1> to <5> above, which is a nonwoven fabric formed from nanofibers containing a polyimide resin.

[0014] <7> The porous structure according to <6> above, wherein the nanofibers have an average diameter of 1 to 1,000 nm. The disclosure of the present application is related to the subject matter described in Japanese Patent Application No. 2024-012849, filed on January 31, 2024, the entire disclosure of which is incorporated herein by reference.

[0015] According to an embodiment of the present invention, it is possible to provide a porous structure, such as a porous film or nonwoven fabric, which contains a polyimide resin and has excellent wettability.

[0016] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments and includes various forms.

[0017] In addition, numerical ranges indicated using "to" in this disclosure mean ranges that include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the numerical ranges described in stages in this disclosure, the upper limit or lower limit of a certain numerical range may be replaced with the upper limit or lower limit of another numerical range. The upper limit or lower limit of a numerical range described in this disclosure may be replaced with a value shown in the examples. A numerical value may be selected from each of the upper limit and lower limit numerical values ​​described in stages in this disclosure to form a stepped numerical range. The upper limit and lower limit numerical values ​​described in this disclosure may be replaced with a value shown in the examples.

[0018] Each component described in the present disclosure may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified.

[0019] In the present disclosure, each structure in a polymer (resin) may contain multiple types of corresponding structures. When multiple types of structures corresponding to each structure exist in a resin, the content or amount of each structure means the total content or amount of the multiple types of structures present in the resin, unless otherwise specified.

[0020] <1> Porous Structure One embodiment of the present invention relates to a porous structure containing a polyimide resin and having a porosity of 20% or more. Generally, as the porosity of a porous structure increases, properties such as air permeability, flexibility, and elongation tend to improve, but mechanical strength tends to decrease. In contrast, the porous structure of this embodiment is formed using a polyimide resin, making it easy to balance the porosity and mechanical strength.

[0021] The porous structure may further contain additives such as inorganic fillers, as necessary. However, from the viewpoint of fully exhibiting the functions of the polyimide resin, the content of the polyimide resin is preferably 50% by mass or more, based on the total mass of the porous structure. The content of the polyimide resin may more preferably be 60% by mass or more, even more preferably 70% by mass or more, and even more preferably 80% by mass or more. In one embodiment, the content may be 90% by mass or more, or may be 100% by mass. A combination of multiple polyimide resins may be used to form the porous structure, and a terminal-treated polyimide resin may be included.

[0022] From the viewpoint of fully utilizing the functions provided by the pores, it is preferable to increase the number of pores in the porous structure to increase the specific surface area. In one embodiment, the porosity of the porous structure is preferably at least 20%. When the porosity of a polyimide resin porous structure is 20% or more, for example, when used as a battery separator, excellent wettability with an electrolyte solution tends to be easily obtained. The average pore diameter of the pores may be preferably 0.1 to 20 μm, more preferably 0.1 to 10 μm, and even more preferably 0.1 to 5 μm. In the porous structure, it is preferable that a plurality of pores are connected from the inside to the surface.

[0023] In one embodiment, the porosity of the porous structure may be more preferably 30% or more, even more preferably 40% or more, and particularly preferably 50% or more. On the other hand, from the viewpoint of maintaining the handleability and mechanical strength of the porous structure, the porosity may be preferably 90% or less, more preferably 85% or less, and even more preferably 80% or less.

[0024] Here, the porosity refers to a value calculated from the following formula (1): Formula (1) Porosity (%) = 100 - {[Basis weight (g / m ２ )×100] / [resin density (g / cm ３ ) / thickness (μm)]}

[0025] In one embodiment, the porosity of the porous structure may be 20 to 90%, and the porous structure is preferably in the form of a porous film or nonwoven fabric containing a polyimide resin. The porous structure containing a polyimide resin may have either a single-layer structure or a multi-layer structure including a layer containing a polyimide resin. A single-layer structure is preferred from the viewpoint of easily obtaining the properties of the polyimide resin.

[0026] For example, when producing a porous film, a resin solution is applied to a substrate, and the coating is dried and / or cured to form a film on the substrate. This method requires peeling the film from the substrate. Therefore, if the film thickness is too small, it tends to be difficult to handle. Furthermore, if the film thickness is too large, it tends to be difficult to obtain a uniform film. From this perspective, although not particularly limited, the thickness of the porous film may be preferably 10 to 200 μm, more preferably 10 to 100 μm, and even more preferably 10 to 50 μm. On the other hand, in the case of nonwoven fabrics, spinning methods can be applied and they are easy to handle, making them easy to thin. For example, the thickness of the nonwoven fabric can be adjusted to a thickness of 5 to 50 μm.

[0027] As described below, the porous film and nonwoven fabric can be produced according to known methods using a resin solution containing at least a polyimide resin (polyimide resin precursor) and a solvent. While not particularly limited, the porous structure is preferably a nonwoven fabric from the viewpoint of easily obtaining a high porosity and a uniform surface structure. In one embodiment, when a porous film containing a polyimide resin is used as a separator, the porosity may be preferably 20 to 60%, more preferably 20 to 40%, and even more preferably 20 to 30%. When the porosity of the porous film is adjusted to the above range, it tends to be easier to obtain good wettability with the electrolyte solution.

[0028] The wettability of the porous structure can be evaluated from the contact angle between the surface of the porous structure and a liquid. For example, when the porous structure is used as a separator for a lithium ion secondary battery, it is preferable that the porous structure has excellent wettability with the electrolyte. A PP porous film made of polypropylene (PP), a typical separator, may not have sufficient wettability with the electrolyte. From the viewpoint of improving the wettability of the electrolyte in a typical PP porous film, it is preferable that the contact angle of the electrolyte with the porous structure is smaller than the contact angle of the electrolyte with the PP porous film. From this viewpoint, the electrolyte contact angle of the porous structure of this embodiment may be, for example, 45° or less, more preferably 30° or less, and even more preferably 20° or less.

[0029] In one embodiment, since the porous structure can sufficiently retain the electrolyte, the electrolyte contact angle with the porous structure may be preferably 20° or less, more preferably 15° or less, and even more preferably 10° or less. In one embodiment, the electrolyte contact angle with the porous structure may be 0°. When the electrolyte contact angle is 0°, the electrolyte permeates the porous structure and is retained inside. When the porous structure has a porosity of at least 20%, an electrolyte contact angle within the above range can be easily obtained.

[0030] In the above embodiment, the electrolyte solution may have the composition of an electrolyte solution normally used in lithium ion secondary batteries. The electrolyte solution typically includes a non-aqueous solvent and an electrolyte, and the non-aqueous solvent may be a mixed solvent containing two or more solvents. In one embodiment, the non-aqueous solvent may be a mixed solvent containing two or more solvents selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). For example, in the mixed solvent, the mixing ratio (volume ratio) of EC:EMC:DEC may be 3:5:5. An electrolyte that can be used to constitute the electrolyte solution is, for example, lithium hexafluorophosphate (LiPF ６), and the amount of electrolyte used in 1 L of the electrolyte solution may be approximately 1 mole. The electrolyte solution may further contain additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as needed. The content of the additive may be 0.1 to 10 mass % based on the total mass of the electrolyte solution.

[0031] In one embodiment, the water contact angle of the porous structure may be preferably 80° or more, more preferably 90° or more, and even more preferably 100° or more. On the other hand, the water contact angle may be preferably 130° or less, more preferably 120° or less, and even more preferably 110° or less. Since the main body of the porous structure of this embodiment is made of a polyimide resin, a water contact angle within the above range can be easily obtained.

[0032] When the water contact angle of the porous structure is adjusted to fall within the above range, excellent water repellency can be easily achieved. Therefore, for example, the porous structure can be suitably used in applications such as separation membranes (separation filters) such as oil-water separation membranes and gas-liquid separation membranes, and excellent separation functions can be easily obtained. The porous structure can also be used as a fuel cell separator. In this case, the water repellency of the surface of the porous structure makes it easy to discharge water generated during fuel cell operation.

[0033] In the above embodiment, the electrolyte contact angle and water contact angle are values ​​measured by the θ / 2 method. A commercially available contact angle measuring device can be used for the measurement. Specific measurement methods and measurement conditions are as described in the examples below.

[0034] The porous structure of this embodiment can be suitably used as a separator for a lithium battery because excellent wettability can be easily obtained. From this perspective, one embodiment of the present invention is a lithium secondary battery comprising an anode, a cathode, a separator, and an electrolyte, and using the porous structure of the above embodiment as the separator. The porous structure of this embodiment has various functions such as separation, adsorption, heat insulation, and sound absorption. Therefore, the use of the porous structure of the above embodiment is not limited to separators for batteries such as lithium secondary batteries, and it can also be used in various applications. For example, it can be suitably used in various applications such as separation filters, adsorption materials, and heat insulation materials.

[0035] <2> Resin Solution Containing Polyimide Resin Precursor Polyimide resins can typically be produced by reacting a diamine with a tetracarboxylic dianhydride to form a polyimide resin precursor (polyamic acid), and then heat-treating the polyamic acid to perform imidization. Because polyimide resins have poor solubility, it is difficult to prepare a polyimide resin solution for producing a porous structure. Therefore, one embodiment of the present invention may be a resin solution containing a polyimide resin precursor (polyamic acid) and an organic solvent. Because polyamic acid can form a polyimide resin by heat treatment, the resin solution can be used to produce a porous structure containing a polyimide resin. The polyimide resin precursor will be specifically described below.

[0036] (Polyimide Resin Precursor) The polyimide resin precursor contains a polyamic acid, and a portion of the polyamic acid may be imidized. The polyamic acid can be produced according to a known method. For example, it can be obtained by reacting a diamine with a tetracarboxylic dianhydride. The diamine and tetracarboxylic dianhydride usable as raw materials may be either aliphatic or aromatic compounds, and multiple types of each may be combined as desired. From the viewpoint of heat resistance, the polyimide resin is preferably an aromatic polyimide resin. Therefore, aromatic diamines and aromatic tetracarboxylic dianhydrides can be suitably used.

[0037] Specific examples of aromatic diamines include p-phenylenediamine, m-phenylenediamine, 1,4-diaminonaphthalene, 1,5-diaminonaphthalene, 1,8-diaminonaphthalene, 2,6-diaminonaphthalene, 2,7-diaminonaphthalene, 2,6-diaminoanthracene, 2,7-diaminoanthracene, 1,8-diaminoanthracene, 2,4-diaminotoluene, 2,5-diamino(m-xylene), 2,5-diaminopyridine, 2,6-diaminopyridine, 3,5-diaminopyridine, and 2,4-diaminotoluene. Benzidine, 3,3'-diaminobiphenyl, 3,3'-dichlorobenzidine, 3,3'-dimethylbenzidine, 3,3'-dimethoxybenzidine, 2,2'-diaminobenzophenone, 4,4'-diaminobenzophenone, 3,3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, 3,4'-diaminodiphenyl sulfonate 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl sulfide, 3,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl sulfide, 4,4'-diaminodiphenyl thioether, 4,4'-diamino-3,3',5,5'-tetramethyldiphenyl ether, 4,4'-diamino-3,3',5,5'-tetraethyldiphenyl ether, 4,4'-diamino-3,3',5,5'-tetramethyldiphenylmethane, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(3-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 2,6-bis(3-aminophenoxy)pyridine, 1,4-bis(3-aminophenylsulfonyl)benzene, 1,4-bis(4-aminophenylsulfonyl)benzene, 1,4-bis(3-aminophenylthioether)benzene, 1,4-bis(4-aminophenylthioether)benzene, 4,4'-bis(3-aminophenoxy)diphenylsulfone, 4,4'-bis(4-aminophenoxy)diphenyl sulfone, bis(4-aminophenyl)amine bis(4-aminophenyl)-N-methylamine bis(4-aminophenyl)-N-phenylamine bis(4-aminophenyl)phosphine oxide, 1,1-bis(3-aminophenyl)ethane, 1,1-bis(4-aminophenyl)ethane, 2,2-bis(3-aminophenyl)propane, 2,2-bis(4-aminophenyl)propane, 2,2-bis(4-amino-3,5-dimethylphenyl)propane, 4,4'-bis(4- bis[4-(4-aminophenoxy)phenyl]biphenyl, bis[4-(3-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]ether, bis[4-(4-aminophenoxy)phenyl]methane, bis[3-methyl-4-(4-aminophenoxy)phenyl]methane, bis[3-chloro-4-(4-aminophenoxy)phenyl]methane, bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]methane, 1,1-bis[4-(4-aminophenoxy)phenyl] 1,1-bis[3-methyl-4-(4-aminophenoxy)phenyl]ethane, 1,1-bis[3-chloro-4-(4-aminophenoxy)phenyl]ethane, 1,1-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]ethane, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, 2,2-bis[3-methyl-4-(4-aminophenoxy)phenyl]propane, 2,2-bis[3-chloro-4-(4-aminophenoxy)phenyl]propane, 2,2-bis[3, 5-dimethyl-4-(4-aminophenoxy)phenyl]propane, 2,2-bis[4-(4-aminophenoxy)phenyl]butane, 2,2-bis[3-methyl-4-(4-aminophenoxy)phenyl]butane, 2,2-bis[3,5-dimethyl-4-(4-aminophenoxy)phenyl]butane, 2,2-bis[3,5-dibromo-4-(4-aminophenoxy)phenyl]butane, 1,1,1,3,3,3-hexafluoro-2,2-bis(4-aminophenyl)propane, 1,1,1,3,3,3-hexafluoro-2,2-bis[3-methyl-4-(4-aminophenoxy)phenyl]propane, etc. If necessary, an aromatic diamine and an aliphatic diamine may be used in combination.

[0038] Specific examples of aromatic tetracarboxylic dianhydrides include pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, 2,3,5,6-pyridinetetracarboxylic dianhydride, 2,3,4,5-thiophenetetracarboxylic dianhydride, 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 2,3',3,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 2,3,3',4 '-biphenyltetracarboxylic dianhydride, 3,3',4,4'-p-terphenyltetracarboxylic dianhydride, 2,2',3,3'-p-terphenyltetracarboxylic dianhydride, 2,3,3',4'-p-terphenyltetracarboxylic dianhydride, 1,2,4,5-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 1,2,6,7-naphthalenetetracarboxylic dianhydride, 1,4,5,8-naphthalenetetracarboxylic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride hydrate, 2,3,6,7-anthracenetetracarboxylic dianhydride, 1,2,5,6-anthracenetetracarboxylic dianhydride, 1,2,6,7-phenanthrenetetracarboxylic dianhydride, 1,2,7,8-phenanthrenetetracarboxylic dianhydride, 1,2,9,10-phenanthrenetetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, 2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2,7-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 2, Examples of the dianhydride include 3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride, 1,4,5,8-tetrachloronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 3-(carboxymethyl)-1,2,4-cyclopentanetricarboxylic acid 1,4:2,3-dianhydride, and 3,3',4,4'-diphenylsulfonetetracarboxylic dianhydride.If necessary, an aromatic tetracarboxylic dianhydride and an aliphatic tetracarboxylic dianhydride may be used in combination.

[0039] The ratio of the diamine to the tetracarboxylic dianhydride may be adjusted as appropriate. For example, from the viewpoint of the molecular weight and degree of crosslinking of the resulting polyamic acid, the amount of diamine is preferably 0.8 to 1.2 mol, more preferably 0.85 to 1.15 mol, and particularly preferably 0.90 to 1.10 mol per 1.0 mol of the total amount of the acid dianhydride.

[0040] In one embodiment, the polyamic acid may be treated with an end-capping agent, such as phthalic anhydride, hexahydrophthalic anhydride, or aniline.

[0041] The number-average molecular weight of the polyimide resin precursor (polyamic acid) constituting the resin solution is preferably 20,000 or more from the viewpoint of ensuring mechanical strength during the production of a porous structure. On the other hand, the number-average molecular weight is preferably 500,000 or less from the viewpoint of ensuring solubility in organic solvents. In one embodiment, the number-average molecular weight is preferably 400,000 or less, more preferably 300,000 or less, and even more preferably 200,000 or less, so that the viscosity can be easily adjusted to a level suitable for the spinning process during the production of a nonwoven fabric.

[0042] The number-average molecular weight of the polyamic acid can be controlled within the above-mentioned preferred range by sampling during resin synthesis, measuring the sample by gel permeation chromatography (GPC) using a calibration curve of standard polystyrene, and continuing synthesis until the target number-average molecular weight is reached. The GPC measurement conditions will be specifically explained in the examples below.

[0043] In one embodiment, the number-average molecular weight of the polyamic acid may be preferably in the range of 20,000 to 200,000, more preferably in the range of 50,000 to 200,000, and even more preferably in the range of 50,000 to 150,000. Using a polyimide resin having a number-average molecular weight within the above range facilitates achieving a good balance between heat resistance and solubility. As a result, it becomes possible to easily produce a nonwoven fabric formed from nanofibers by spinning using an electrospinning method. The average diameter of the nanofibers may be 1 to 1,000 nm. The average diameter may be preferably 10 to 800 nm, more preferably 50 to 600 nm.

[0044] (Method for Producing Polyimide Precursor) The polyimide resin precursor can be produced according to a known method. In one embodiment, the method for producing the polyimide resin precursor includes a polymerization step of reacting a diamine with a tetracarboxylic dianhydride in an organic solvent. In a preferred embodiment, the organic solvent includes dimethylacetamide. Compounds that can be used as raw materials are as described above. When producing a polyamic acid with capped ends, in addition to the polymerization step, a step of treating with a terminal capping agent may be further included. The polymerization step and the treatment step may be performed separately, or both steps may be performed simultaneously.

[0045] The organic solvent used in the polymerization step may be one or more polar solvents selected from N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, γ-butyrolactone, dimethyl sulfoxide, 1,3-dimethyl-2-imidazolidine, dimethylacetamide, dimethylformamide, N-acetylmorpholine, and the like. In one embodiment, dimethylacetamide or a mixed solvent containing dimethylacetamide and another solvent is preferably used as the polymerization solvent (synthesis solvent). In this case, the polymerization solution obtained in the polymerization step can be used directly as a polyimide resin solution for producing a porous structure such as a nonwoven fabric. Dimethylacetamide is preferably used both as a synthesis solvent and as a dilution solvent, as described below. When dimethylacetamide is used, excellent drying properties are obtained.

[0046] The amount of polymerization solvent used in the polymerization step is not particularly limited and can be adjusted as appropriate. In one embodiment, from the viewpoint of the solubility of the polyimide resin precursor, it is preferable to use 50 to 1,000 parts by mass of polymerization solvent per 100 parts by mass of the total amount of diamine and tetracarboxylic dianhydride. The amount of polymerization solvent used may more preferably be 200 to 1,000 parts by mass, and even more preferably 300 to 800 parts by mass. The reaction temperature is not particularly limited, and is generally preferably 0 to 180°C. The polymerization reaction is preferably carried out under an atmosphere such as nitrogen to reduce the influence of moisture in the air.

[0047] (Method for producing a resin solution containing a polyimide resin precursor) A resin solution containing the polyimide resin precursor of this embodiment (hereinafter also referred to as a resin solution) can be produced by mixing the polyimide resin precursor (polyamic acid) of the above embodiment with an organic solvent. Dimethylacetamide is preferably used as the organic solvent. In particular, a resin solution containing dimethylacetamide can be suitably used to produce a nonwoven fabric.

[0048] Alternatively, the resin solution can be produced directly using the reaction solution obtained by the above-described method for producing a polyimide resin precursor. In this case, a diluent solvent may be added as necessary. Therefore, in one embodiment, the method for producing the resin solution may include a polymerization step of reacting a diamine with a tetracarboxylic dianhydride in a solvent containing dimethylacetamide. In another embodiment, the production method includes a polymerization step of reacting a diamine with a tetracarboxylic dianhydride in a solvent, and a step of adding a diluent solvent to the resulting reaction solution or polyimide resin precursor. In this embodiment, the solvent and / or diluent solvent used in the polymerization step preferably contains at least dimethylacetamide.

[0049] The content of the polyimide resin precursor in the resin solution of the above embodiment can be set appropriately and is not particularly limited. In a preferred embodiment, from the viewpoint of balance with other components, the content of the polyamic acid in the resin solution is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more. On the other hand, the content is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 30% by mass or less. By adjusting the content of the polyamic acid in the resin solution within the above range, it becomes easy to obtain fibers having sufficient strength during spinning, and it is possible to suppress a decrease in fluidity and maintain good workability.

[0050] <Organic Solvent> The organic solvent constituting the resin solution containing a polyimide precursor is not particularly limited as long as it can dissolve the polyimide resin precursor. In one embodiment, the organic solvent includes dimethylacetamide. The resin solution containing a polyimide resin precursor may include a solvent other than dimethylacetamide as long as the effect of the present invention is not reduced.

[0051] Other solvents that can be used include water and one or more polar solvents selected from N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, γ-butyrolactone, dimethyl sulfoxide, 1,3-dimethyl-2-imidazolidine, dimethylacetamide, dimethylformamide, and N-acetylmorpholine. Furthermore, co-solvents may be optionally used, including ether compounds such as anisole, diethyl ether, and ethylene glycol; ketone compounds such as acetophenone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanenone, and cyclopentanone; aromatic hydrocarbon solvents such as xylene and toluene; and alcohols such as ethanol and 2-propanol.

[0052] When a solvent other than dimethylacetamide is used as the solvent to prepare a mixed solvent, the content of dimethylacetamide in the mixed solvent is preferably 50% by mass or more, and more preferably 80% by mass or more, in order to fully exhibit the effects of the preferred embodiment.

[0053] The resin solution may further contain components other than the polyimide resin precursor and the organic solvent, as needed. For example, the resin solution may contain a filler, as needed, to improve the water resistance of the porous structure, such as a nonwoven fabric. The type of filler can be selected depending on the application of the porous structure, taking into account its water resistance and chemical resistance. Specific examples of fillers include metal powder, metal oxides (aluminum oxide, zinc oxide, tin oxide, titanium oxide, etc.), glass beads, glass flakes, glass particles, ceramics, silicon carbide, silicon oxide, calcium fluoride, carbon black, graphite, mica, and barium sulfate.

[0054] The resin solution containing the polyimide resin precursor may further contain a surfactant, if necessary. Resin solutions containing a surfactant can be suitably used to produce nonwoven fabrics. The surfactant is not particularly limited, but it is preferable that the resin solution is uniformly mixed and does not undergo layer or phase separation until the fibers are dried after spinning, and that no significant residue remains when the fibers are assembled into a nonwoven fabric. The surfactant content is not particularly limited, but may be preferably 0.01 to 10% by mass, more preferably 0.5 to 5% by mass, based on the total mass of the resin solution. Adjusting the surfactant content within the above range can maintain a uniformly mixed state of the resin solution and prevent defects from occurring during the production of nonwoven fabrics. In some embodiments, the viscosity of the resin solution containing the polyimide resin precursor may be preferably 1 to 16 Pa·s, more preferably 2 to 14 Pa·s, and even more preferably 3 to 12 Pa·s. Adjusting the viscosity of the resin solution within the above range facilitates the production of nonwoven fabrics using the electrospinning method described below. The viscosity is a value measured at 25°C using a Brookfield viscometer.

[0055] <3> Method for Producing Porous Structure The porous structure may be in the form of a porous film or nonwoven fabric, and can be produced according to a known method using a resin solution containing at least a polyimide resin precursor and an organic solvent. Heating during the production of the porous structure promotes imidization of the polyimide resin precursor to form a polyimide resin. Although not particularly limited, the imidization rate in the porous structure obtained after heating may be preferably 50% or more, more preferably 80% or more, and even more preferably 90% or more. The imidization rate can be easily adjusted by changing the heating temperature and heating time.

[0056] Porous films can be produced according to known production methods. For example, a coating film is formed on a substrate using a resin solution containing a polyimide resin precursor (first component), a solvent capable of dissolving the polyimide resin precursor (second component), and a third component such as a poor solvent or a resin filler. The coating film is then heated to promote imidization, resulting in a film containing a polyimide resin. In such a production method, pores are formed by a heat treatment that sufficiently removes the second and third components. In some embodiments, the process may be carried out in two stages using different heating conditions. For example, the first stage is preferably carried out at a temperature at which the solvents for the second and third components can evaporate. The second stage is then preferably carried out at a temperature at which the resin filler in the third component can be burned off.

[0057] Nonwoven fabrics can also be produced according to known production methods. In one embodiment, nonwoven fabrics can be produced by spinning a resin solution containing a polyimide resin precursor and an organic solvent capable of dissolving the polyimide resin precursor, and then heating and molding the resulting fiber aggregate into a predetermined shape to partially fuse (bond) the fibers together. The fibers can be bonded together not only by heating, but also by spraying a solution containing a binder resin onto an aggregate of polyimide resin fibers. Examples of binder resins that can be used include polyethersulfone resin (PES), polyamide resin, epoxy compounds, isocyanate compounds, and melamine compounds, either alone or in combination. In some embodiments, an additional heat treatment may be performed. In some embodiments, the heat treatment can be performed at a temperature preferably in the range of 100 to 350°C, more preferably 250 to 350°C, and even more preferably 300 to 350°C. Such heat treatment facilitates the removal of residual solvent and tends to increase the strength of the nonwoven fabric. In particular, heat treatment in a temperature range exceeding 300°C tends to facilitate obtaining higher strength through ring closure, crosslinking, and packing.

[0058] In another embodiment, the nonwoven fabric can be suitably produced by electrospinning. Electrospinning is a method in which a positive high voltage is applied to a resin solution, and the resin solution is sprayed onto a grounded or negatively charged surface, resulting in fiberization. This method can produce fibers with a small fiber diameter and excellent structural uniformity. Furthermore, since the fibers can be formed in a two-dimensionally spread state on the target plate, a nonwoven fabric can be obtained without further processing the fibers after spinning. Nonwoven fabrics can be produced by known methods using electrospinning. In some embodiments, an additional heat treatment may be performed, as described above. Additional heat treatment tends to facilitate increasing the strength of the nonwoven fabric. Therefore, when additional heat treatment is performed, a nonwoven fabric with the desired electrolyte contact angle and water contact angle and high strength can be easily obtained. Because nonwoven fabrics produced by electrospinning are composed of fine fibers, it is easy to achieve a balance between porosity and mechanical strength. In one embodiment, a nonwoven fabric formed from nanofibers spun by electrospinning has a high porosity and good wettability with an electrolyte, and therefore can be suitably used as a separator for a lithium ion secondary battery.

[0059] The embodiments of the present invention will be described below with reference to examples, but the embodiments of the present invention are not limited to the following examples.

[0060] 1. Preparation Examples of Polyamic Acid Solutions (Preparation Example 1) 33.5 g of 4,4'-diaminodiphenyl ether and 270.9 g of dimethylacetamide were placed in a flask equipped with a thermometer, a stirrer, and a condenser. Next, 36.5 g of pyromellitic dianhydride was added to the flask while stirring in a dry nitrogen stream. Stirring was continued for 4 hours, after which the reaction was stopped, yielding Polyamic Acid Solution 1 (Resin Solution 1). The number-average molecular weight of the polyamic acid, measured as described below, was 75,000.

[0061] (Number Average Molecular Weight) The number average molecular weight (Mn) was measured by gel permeation chromatography (GPC) and converted using a calibration curve of standard polystyrene. The calibration curve was approximated by a cubic equation using a set of five standard polystyrene samples ("TSK Standard POLYSTYRENE", manufactured by Tosoh Corporation). The GPC conditions are as follows: GPC apparatus: High-speed GPC apparatus HLC-8320GPC (manufactured by Tosoh Corporation) Detector: Ultraviolet absorption detector UV-8320 (manufactured by Tosoh Corporation) Column: Gelpack GL-S300MDT-5 (total of 2 columns) (manufactured by Showa Denko Materials Co., Ltd.) Eluent: THF / DMF = 1 / 1 (volume ratio) + LiBr (0.06 mol / L) + H ３ P.O. ４ (0.06 mol / L) Flow rate: 1 mL / min Column size: 8 mm ID x 300 mm Sample concentration: 5 mg / 1 mL Injection volume: 5 μL Measurement temperature: 40°C

[0062] Preparation Example 2: 129.2 g of 4,4'-diaminodiphenyl ether and 1,194.8 g of N-methylpyrrolidone were placed in a flask equipped with a thermometer, a stirrer, and a condenser. Next, 140.8 g of pyromellitic dianhydride was added while stirring in a dry nitrogen stream. Stirring was continued for 4 hours, after which the reaction was stopped, yielding a polyamic acid solution (resin solution 2). The number-average molecular weight of the polyamic acid, measured in the same manner as in Preparation Example 1, was 65,000.

[0063] 2. Production of Porous Structure Using the resin solution prepared above, a porous structure containing a polyimide resin was produced as described below. (Example 1) PI-1 Nanofiber Nonwoven Fabric Using the resin solution 1 (20% solids, 40 Pa s viscosity) obtained in Preparation Example 1, fibers were formed using a nanofiber electrospinning device ("NANON-04" manufactured by MEC Co., Ltd.) under the following spinning conditions to obtain a nonwoven fabric. Note that the viscosity of the above solution was measured at 25°C using a Brookfield viscometer. The same applies to the viscosity of the solutions described below. Flow rate: 1.0 ml / hr, distance between electrodes: 150 mm, voltage: 20 kV. The fiber diameters of multiple fibers were measured from SEM (scanning electron microscope) images of the nonwoven fabric, and the average value was calculated. The average fiber diameter was 258 nm.

[0064] (Example 2) PI-2 Porous Film A mixed solution was obtained by adding acrylic particles in a blending amount of 40% by mass to the resin solution 2 obtained in Preparation Example 2 and mixing. This mixed solution was applied to a substrate for release to form a coating film, and the coating film was pre-dried at 80°C in air. Next, the pre-dried coating film was heated to 370°C to thermally decompose the acrylic particles, obtaining a polyimide porous structure with a film thickness of 30 μm. The film was peeled from the glass substrate and used as a porous film for various evaluations. Note that when the surface of the film peeled from the glass substrate as described above was observed using an SEM (scanning electron microscope) image, multiple pores with opening diameters (pore diameters) of 1 to 5 μm were confirmed.

[0065] (Reference Example) PI-2 Nonwoven Fabric An attempt was made to produce a nonwoven fabric under the same conditions as in Example 1 using the resin solution 2 (solid content 20%, viscosity 20 Pa s) obtained in Preparation Example 2, but the resin solution turned into droplets during spinning, and no nonwoven fabric was obtained.

[0066] (Comparative Example 1) PP Porous Film A porous film made of polypropylene (PP) was used for various evaluations.

[0067] 3. Evaluation of Porous Structures The properties of the porous structures obtained in Examples 1 and 2 and Comparative Example 1 were evaluated according to the methods described below. The results are shown in Table 1.

[0068] (Contact Angle Measurement) The contact angle between the porous structure (nonwoven fabric or porous film) and the liquid was measured using a contact angle meter CA-X manufactured by Kyowa Interface Science Co., Ltd. In this specification, the contact angle refers to the angle between the liquid and the surface of the porous structure measured 10 seconds after a single drop of liquid (approximately 30 to 50 μL) was dropped onto the surface of the porous structure using a syringe, and the angle is the average value of multiple measurements. In measuring the contact angle of the electrolyte, a solution having the same composition as an electrolyte solution commonly used in lithium ion secondary batteries was used. Specifically, an electrolyte solution containing a non-aqueous solution, an electrolyte, and an additive was prepared and used as follows. (Preparation of Electrolyte Solution) A non-aqueous solvent was mixed with ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) in a ratio of 3:5:5 (volume ratio), and lithium hexafluorophosphate (LiPF ) was added as an electrolyte. ６ ) was added in an amount of 1 mol / kg, and further vinylene carbonate (VC) was added as an additive in an amount of 1 mass %, and these were mixed to obtain an electrolyte solution.

[0069] (Porosity) The film thickness and mass of the porous structure cut to a predetermined size were measured. The mass per unit area (weighed value) was calculated from the measured values, and the porosity of the porous structure was calculated using the following formula. In the formula, "thickness" means the film thickness obtained by the above measurement. Porosity (%) = 100 - {[basis weight (g / m ２ )×100] / [resin density (g / cm ３ ) / thickness (μm)]}

[0070]

[0071] As shown in Table 1, according to one embodiment of the present invention, a porous structure can be constructed using a polyimide resin. It can be seen that the porous structures according to the present invention (Examples 1 and 2) have a smaller electrolyte contact angle than a PP porous film (Comparative Example 1), which is a typical separator for lithium-ion secondary batteries. In particular, the electrolyte contact angle of the nonwoven fabric shown in Example 1 is 0°, indicating that the nonwoven fabric can absorb and retain the electrolyte. Therefore, according to the present invention, it is possible to provide a porous structure that is significantly better in wettability with respect to the electrolyte than a typical conventional PP porous film and can be suitably used as a separator.

[0072] 4. Lithium-ion secondary battery In order to more specifically evaluate the performance of the porous structure of this embodiment, a lithium-ion secondary battery was fabricated using the nanofiber nonwoven fabric obtained in Example 1, and its function as a separator was evaluated. Details are as follows. <Fabrication of lithium secondary battery> A lithium secondary battery was fabricated as follows. (Fabrication of separator) The nanofiber nonwoven fabric obtained in Example 1 above was heat-treated at 300°C for 30 minutes and used as a separator. The electrolyte contact angle and water contact angle of the nanofiber nonwoven fabric after the heat treatment were measured according to the method described above, and were found to be almost the same.

[0073] (Preparation of Negative Electrode) A silicon-based active material (Si alloy) and a carbon-based active material (graphite, Hitachi Chemical Co., Ltd. "SMGYM2") were used as the negative electrode active material. The negative electrode active material and a polyamideimide binder were blended to a capacity density of 600 mAh / kg. Specifically, the mass ratio of the solid content (carbon-based active material: silicon-based active material: polyamideimide) was mixed to a ratio of 105:45:25, and NMP was added to adjust the viscosity to obtain a slurry-like negative electrode mixture. The negative electrode mixture obtained as described above was applied substantially evenly and homogeneously to one side of a current collector (metal foil (Cu, 10 μm)). The coating was then dried and compression-molded by pressing to obtain a current collector / negative electrode mixture layer laminate. The laminate was then heated at 270 ° C. for 30 minutes to harden the negative electrode mixture layer. In this way, a negative electrode was obtained.

[0074] (Secondary Battery Fabrication) A cathode composite containing NCM111 (nickel-cobalt manganese oxide, manufactured by BASF Toda Battery Materials, LLC) was prepared as the cathode active material. A cathode composite layer was formed using the cathode composite on one side of a current collector (metal foil (Al, 15 μm)), yielding a cathode (current collector / cathode composite layer). This cathode, the negative electrode obtained as described above, and a separator (heat-treated nanofiber nonwoven fabric) were placed in a cell. A secondary battery was fabricated by injecting an electrolyte into the cell. The electrolyte used was a solution containing ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (1 vol / 2 vol) and vinylene carbonate (VC) (1 wt%). As a control, the PP porous film obtained in Comparative Example 1 was used as a separator. A secondary battery was fabricated in the same manner as described above.

[0075] <Evaluation of Lithium Secondary Battery> The capacity retention rate of the secondary battery obtained as described above was measured according to the following method.

[0076] (Measurement of Discharge Capacity Retention Rate) The obtained secondary battery was subjected to a charge-discharge cycle test at a temperature of 25°C using the following method. First, the battery was charged at a current of 0.5 C until the voltage reached 4.2 V (CC charging). Next, the battery was charged at a voltage of 4.2 V until the current reached 0.01 C (CV charging). After leaving the battery for 30 minutes, the battery was discharged at a current of 0.5 C until the voltage reached 3.23 V (CC discharging). A series of CC charging, CV charging, and CC discharging operations constituted one cycle, and 100 cycles were performed. Next, the sum of the time-integrated values ​​of the current during the CC charging and CV charging in the first cycle was defined as the charge capacity (mAh) in the first cycle, and the time-integrated value of the current during the CC discharging in the first cycle was determined as the discharge capacity (mAh) in the first cycle. Similarly, the discharge capacity (mAh) in the 100th cycle was determined, and the discharge capacity retention rate of the battery in the 100th cycle was calculated according to the following (Equation 2). (Equation 2): Discharge capacity retention rate (%) at 100th cycle = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) × 100 Furthermore, for each of the discharge capacity retention rates (%) obtained above, a relative value was calculated, with the discharge capacity retention rate at 100th cycle of the control (battery using the PP porous film obtained in Comparative Example 1 as a separator) being set at 100. The results are shown in Table 2.

[0077]

[0078] As is clear from the results shown in Table 2, when the porous structure (nonwoven fabric) of this embodiment is used as a separator, it is possible to improve battery performance compared to a conventional representative separator (comparative example).

Claims

1. A porous structure containing polyimide resin and having a porosity of 20% or more.

2. The porous structure according to claim 1, which has a water contact angle of 90° or more.

3. The porous structure according to claim 1, which is used as a separator for a battery.

4. The porous structure according to claim 1, wherein the contact angle of the electrolyte is 10° or less.

5. The porous structure according to claim 4, which is used as a separator for a lithium ion secondary battery.

6. The porous structure according to claim 1, which is a nonwoven fabric formed from nanofibers containing a polyimide resin.

7. The porous structure according to claim 6, wherein the nanofibers have an average diameter of 1 to 1,000 nm.

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