Porous structure

Abstract

Provided is a porous structure containing a polyamide-imide resin having a number average molecular weight of 20,000 or more. The porous structure may be a nonwoven fabric formed from nanofibers containing a polyamide-imide resin, and can be suitably used as a separator for a battery such as a lithium-ion battery.

Description

porous structure

[0001] An embodiment of the present invention relates to a porous structure comprising a polyamideimide 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, polyamide-imide resins are known to have excellent heat resistance, mechanical strength, chemical resistance, and the like, and are used in a variety of fields. 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 polyamide-imide resins is anticipated. However, it is generally not easy to manufacture porous structures such as porous films and nonwoven fabrics that contain polyamide-imide resins as their main component. Therefore, there is a need for porous structures that contain polyamide-imide 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 containing a polyamide-imide resin and having excellent wettability.

[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 comprising a polyamide-imide resin having a number average molecular weight of 20,000 or more.

[0009] <2> The porous structure according to <1> above, wherein the polyamideimide resin contains a structure (A1) represented by the following formula: In the formula, each R is independently an alkyl group having 1 to 3 carbon atoms, and * represents a bonding site to another structure.

[0010] <3> The porous structure according to <1> or <2> above, wherein the polyamideimide resin contains a structure (A2) represented by the following formula: In the formula, X is —CH ２ - or -O-, and "*" represents a bonding site with another structure.

[0011] <4> The porous structure according to <3>, wherein the polyamideimide resin contains the structure (A1) and the structure (A2), and the ratio of the structure (A1) / the structure (A2) in the polyamideimide resin is 1 / 9 to 9 / 1.

[0012] <5> The porous structure according to any one of the above <1> to <4>, which has a contact angle with water of 90° or more.

[0013] <6> The porous structure according to any one of <1> to <5> above, which is used as a battery separator.

[0014] <7> The porous structure according to any one of the above <1> to <6>, which has a contact angle with an electrolyte of 10° or less.

[0015] <8> The porous structure according to any one of <1> to <7> above, which is used as a separator for a lithium ion secondary battery.

[0016] <9> The porous structure according to any one of <1> to <8> above, which is a nonwoven fabric formed from nanofibers containing the polyamideimide resin.

[0017] <10> The porous structure according to <9> 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. 2024-013123, filed on January 31, 2024, the entire disclosure of which is incorporated herein by reference.

[0018] 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 polyamide-imide resin and has excellent wettability.

[0019] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments and includes various forms.

[0020] 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.

[0021] 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.

[0022] In the present disclosure, each structure in a polymer may contain multiple types of corresponding structures. When multiple types of structures corresponding to each structure exist in a polymer, the content or amount of each structure means the total content or amount of the multiple types of structures present in the polymer, unless otherwise specified.

[0023] <1> Porous Structure One embodiment of the present invention relates to a porous structure containing a polyamide-imide resin having a number-average molecular weight of 20,000 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 constructed using a polyamide-imide resin having a number-average molecular weight of 20,000 or more, making it easy to balance porosity and mechanical strength.

[0024] The porous structure may further contain additives such as inorganic fillers, as necessary. However, from the viewpoint of fully exhibiting the functions of the polyamideimide resin, the content of the polyamideimide resin is preferably 50% by mass or more, based on the total mass of the porous structure. The content of the polyamideimide 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 even be 100% by mass. A combination of multiple polyamideimide resins may be used to form the porous structure, and a polyamideimide resin with blocked terminals may be included.

[0025] From the viewpoint of fully utilizing the functions of the pores in the porous structure, it is preferable to increase the number of pores 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 polyamide-imide 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 achieved. The average 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.

[0026] 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. 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)]}

[0027] 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 a nonwoven fabric. The porous structure containing a polyamide-imide resin may have either a single-layer structure or a multi-layer structure including a layer containing a polyamide-imide resin. A single-layer structure is preferred from the viewpoint of easily obtaining the properties of the polyimide resin.

[0028] As described below, porous films or nonwoven fabrics can be produced according to known methods using a resin solution containing at least a polyamideimide resin and a solvent. 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. Although not particularly limited, a nonwoven fabric is preferred as the porous structure from the viewpoints of ease of thinning, high porosity, and the ease of obtaining a uniform surface structure.

[0029] 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 a typical PP porous film with the electrolyte, it is preferable that the electrolyte contact angle of the porous structure is smaller than the electrolyte contact angle of 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.

[0030] In one embodiment, since the porous structure can sufficiently retain the electrolyte, the electrolyte contact angle of 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 of 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.

[0031] In the above embodiment, the electrolyte solution may have the composition of an electrolyte solution commonly used in lithium ion secondary batteries. The electrolyte solution typically contains a non-aqueous solvent and an electrolyte salt, 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 3:5:2. An electrolyte salt that can be used to constitute the electrolyte is, for example, lithium hexafluorophosphate (LiPF ６ ), and the amount of electrolyte salt 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.

[0032] In one embodiment, the water contact angle of the porous structure may be preferably 90° or more, more preferably 95° 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 porous structure of this embodiment is made of a polyamide-imide resin, a water contact angle within the above range can be easily obtained.

[0033] 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 separator for a fuel cell. In this case, the water repellency of the surface of the porous structure makes it easy to discharge water generated during operation of the fuel cell.

[0034] 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.

[0035] 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 battery separators for lithium batteries and the like, and it can also be used in various other applications. For example, it can be suitably used in applications such as separation filters, adsorption materials, and heat insulation materials.

[0036] <2> Polyamide-imide Resin Solution One embodiment of the present invention relates to a polyamide-imide resin solution containing a polyamide-imide resin having a number-average molecular weight of 20,000 or more and an organic solvent. This polyamide-imide resin solution can be suitably used to produce the porous structure of the above embodiment. In particular, the polyamide-imide resin solution (hereinafter also referred to as a resin composition) of the above embodiment can be suitably used in the production of nonwoven fabrics.

[0037] <Polyamide-imide resin> A polyamide-imide resin is a resin obtained by reacting a diisocyanate (or diamine) with a tribasic acid anhydride or tribasic acid halide as an acid component, and has an amide bond and an imide bond in the molecule. From the viewpoint of heat resistance, the polyamide-imide resin is preferably an aromatic polyamide-imide resin. Here, the diisocyanate (diamine) and the acid component usable as raw materials may each be a combination of two or more types. It is preferable that both the diisocyanate (diamine) and the acid component are aromatic compounds.

[0038] (Diisocyanate, diamine) The diisocyanate is not particularly limited, but examples thereof include 4,4'-diphenylmethane diisocyanate, xylylene diisocyanate, 3,3'-diphenylmethane diisocyanate, 3,3'-dimethoxybiphenyl-4,4'-diisocyanate, paraphenylene diisocyanate, hexamethylene diisocyanate, tolylene diisocyanate, naphthalene diisocyanate, isophorone diisocyanate, 4,4'-diisocyanate-3,3'-dimethylbiphenyl, etc. Among these, aromatic diisocyanates can be preferably used.

[0039] In one embodiment, the polyamideimide resin preferably has a structure represented by the following formula (A1) in the molecule, and more preferably has a structure represented by the following formula (A1-1):

[0040]

[0041] In the formula, R is independently an alkyl group having 1 to 3 carbon atoms. In one embodiment, R may be an alkyl group having 1 carbon atom (methyl group). "*" represents a bonding site with another structure. In addition to the R, the hydrogen atoms in the two benzene rings constituting the biphenyl structure may be further substituted with a substituent R1 other than an alkyl group. Examples of the substituent R1 include a hydroxyl group, a carboxylic acid, an amine group, a halogen group, and a cyano group. In one embodiment, from the viewpoint of enhancing the hydrophilicity of the porous structure, it is preferable that the substituent R1 has a hydrophilic group such as a hydroxyl group or a carboxylic acid. In the structure represented by the above formula (A1) or the above formula (A1-1), the number of substituents R1 may be 1 to 6, preferably 1 to 4, and more preferably 1 or 2.

[0042] As a diisocyanate into which the above structure (A1-1) can be introduced, for example, 4,4'-diisocyanate-3,3'-dimethylbiphenyl can be suitably used.

[0043] In one embodiment, the polyamide-imidimide resin constituting the porous structure preferably has a structure (A2) represented by the following formula, and more preferably has a structure (A2-1) represented by the following formula:

[0044] In the formula, X is —CH ２ - or -O-, ２ - is preferred. "*" represents a bonding site with another structure. The hydrogen atoms in the two aromatic rings may be optionally substituted with a substituent R2. The substituent R2 may be, for example, an alkyl group having 1 to 3 carbon atoms, a hydroxyl group, a carboxylic acid, an amine group, a halogen group, or a cyano group. In the structure represented by the above formula (A2) or (A2-1), the number of substituents R2 may be 1 to 8, preferably 1 to 4, and more preferably 1 or 2.

[0045] As a diisocyanate compound into which the above structure (A2-1) can be introduced, for example, 4,4'-diphenylmethane diisocyanate and 4,4'-diphenylether diisocyanate can be suitably used.

[0046] When a polyamide-imide resin has a biphenyl structure such as the structure (A1), its rigidity tends to be easily improved. On the other hand, if the rigidity of the polyamide-imide resin is too high, the porous structure tends to become brittle, making it difficult to obtain sufficient mechanical strength. For this reason, in one embodiment, the polyamide-imide resin preferably has the structure (A1) and the structure (A2). Although not particularly limited, the ratio of the structure (A1) to the structure (A2) in the polyamide-imide resin may be preferably 1 / 9 to 9 / 1, more preferably 3 / 7 to 7 / 3, and even more preferably 3 / 7 to 6 / 4.

[0047] From the above viewpoint, it is preferable to adjust the ratio of the compound having Structure (A1) to the compound having Structure (A2) (Structure (A1) / Structure (A2)) in the diisocyanate (diamine) used as a raw material within the above range. In one embodiment, a diamine may be used in addition to a diisocyanate as a raw material for the polyamide-imide resin. In some embodiments, a diamine having a structure other than Structure (A1) and Structure (A2) may also be used. Specific examples of diamines include, but are not limited to, 4,4'-diaminodiphenylsulfone, 3,3'-diaminodiphenylsulfone, xylylenediamine, phenylenediamine, and isophoronediamine.

[0048] (Acid Component) The tribasic acid anhydride is not particularly limited, but preferably an aromatic tribasic acid anhydride is used, and among them, trimellitic anhydride is preferred. The tribasic acid halide is also not particularly limited, but preferably a tribasic acid chloride, more preferably an aromatic tribasic acid chloride, such as trimellitic anhydride chloride (trimellitic anhydride chloride), etc. From the viewpoint of reducing the burden on the environment, it is preferred to use trimellitic anhydride.

[0049] In addition to the above-mentioned tribasic acid anhydrides (or tribasic acid halides), saturated or unsaturated polybasic acids such as dicarboxylic acids and tetracarboxylic dianhydrides can be used as acid components, provided that they do not impair the properties of the polyamide-imide resin. Examples of dicarboxylic acids include, but are not limited to, terephthalic acid, isophthalic acid, adipic acid, and sebacic acid. Examples of tetracarboxylic dianhydrides include, but are not limited to, pyromellitic dianhydride, benzophenone tetracarboxylic dianhydride, and biphenyl tetracarboxylic dianhydride. These may be used alone or in any combination of two or more. The total amount of carboxylic acids other than tribasic acids (dicarboxylic acids and tetracarboxylic acids) is preferably in the range of 0 to 50 mol% of the total carboxylic acids, in order to maintain the properties of the polyamide-imide resin. That is, the total amount may be preferably 50 mol% or less, more preferably 30 mol% or less, and even more preferably 10 mol% or less. In some embodiments, the total amount of carboxylic acids other than tribasic acids may be 0 mol%.

[0050] The ratio of diisocyanate (and diamine) to acid component (total amount of tribasic acid anhydride or tribasic acid anhydride halide, and optionally used dicarboxylic acid and tetracarboxylic dianhydride) may be adjusted as appropriate. For example, from the viewpoint of the molecular weight and degree of crosslinking of the resulting polyamide-imide resin, the number of moles of diisocyanate compound (and diamine compound) is preferably 0.8 to 1.1 moles per 1.0 mole of total acid component. The number of moles is more preferably 0.95 to 1.08 moles, and even more preferably 1.0 to 1.08 moles.

[0051] In one embodiment, the polyamideimide resin preferably has a structure (I) represented by the following formula:

[0052] In the formula, Ar represents a divalent organic group obtained by removing two isocyanate groups (amino groups) from a diisocyanate (diamine). Ar is preferably at least one of the above structures (A1) and (A2). Ar is more preferably at least one of the above structures (A1-1) and (A2-1).

[0053] In one embodiment, the polyamideimide resin may be a blocked polyamideimide resin in which the terminal isocyanate group is treated with a blocking agent (terminal blocking agent). Examples of usable terminal blocking agents include alcohols, oximes, and lactams. When a blocked polyamide resin is used in a polyamideimide resin composition, decomposition due to hydrolysis is suppressed and stability over time is improved, making it easier to achieve excellent heat resistance.

[0054] The number average molecular weight of the polyamideimide resin 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 200,000 or less from the viewpoint of ensuring solubility in organic solvents. In one embodiment, the number average molecular weight is preferably 100,000 or less, more preferably 80,000 or less, and even more preferably 60,000 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.

[0055] The number average molecular weight of the polyamide-imide resin can be controlled within the above-mentioned preferred range by sampling the resin during synthesis, measuring it 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 described in the Examples below.

[0056] In one embodiment, the number-average molecular weight of the polyamideimide resin may be preferably in the range of 20,000 to 65,000, more preferably in the range of 20,000 to 55,000, and even more preferably in the range of 20,000 to 50,000. Using a polyamideimide 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 nonwoven fabrics 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.

[0057] (Method for producing polyamideimide resin) Polyamideimide resin can be produced according to a known method. In one embodiment, the method for producing polyamideimide resin includes a polymerization step of reacting a diisocyanate (or diamine) with a tribasic acid anhydride and / or a tribasic acid halide 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 blocked polyamideimide resin, in addition to the polymerization step, a step of blocking the terminal isocyanate groups of the polyamideimide resin with a blocking agent such as alcohol may be further included. The polymerization step and the blocking step may be performed separately, or both steps may be performed simultaneously.

[0058] 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, etc. 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 polyamideimide resin solution for producing a porous structure such as a nonwoven fabric. Dimethylacetamide is preferably used both as the synthesis solvent and as the dilution solvent described below.

[0059] 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 resin solubility, 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 diisocyanate (and diamine) and acid component. The amount of polymerization solvent used may more preferably be 100 to 700 parts by mass, and even more preferably 100 to 500 parts by mass. The reaction temperature is not particularly limited, and is generally preferably 80 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.

[0060] In the polymerization process, pyridine, triethylamine, inorganic acetic acid, 1,8-diazabicyclo[5.4.0]-7-undecene (DBU), and other common reaction catalysts may be used to increase the molecular weight of the resin.

[0061] When synthesizing a blocked polyamideimide resin, the blocking step may be carried out simultaneously with the polymerization step by reacting a blocking agent during resin synthesis, or the blocking agent may be reacted with the resin after the polymerization step. In the former case, the blocking agent may be added to the polymerization solvent. The amount of the terminal blocking agent used in blocking is preferably 1.0 to 10.0 parts by mass, and more preferably 2.5 to 5.0 parts by mass, based on 100 parts by mass of the total amount of diisocyanates used in resin production, from the viewpoint of storage stability of the resulting resin composition.

[0062] (Method for Producing Polyamide-Imide Resin Composition) The polyamide-imide resin composition of this embodiment (hereinafter also referred to as resin solution) can be produced by mixing a polyamide-imide resin having a number-average molecular weight of 20,000 or more with an organic solvent. It is preferable to use dimethylacetamide as the organic solvent. Alternatively, a resin composition can be produced by directly using the reaction solution containing the polyamide-imide resin obtained by the above-described method for producing a polyamide-imide resin. In this case, a dilution solvent may be added as necessary. Therefore, in one embodiment, the method for producing a polyamide-imide resin composition may include a polymerization step of reacting a diisocyanate with a tribasic acid anhydride and / or a tribasic acid halide in a solvent containing dimethylacetamide.

[0063] In another embodiment, the production method includes a polymerization step of reacting a diisocyanate compound with a tribasic acid anhydride and / or a tribasic acid halide in a solvent containing the diisocyanate compound, and a step of adding a diluent solvent to the resulting reaction solution or resin. In this embodiment, the solvent and / or the diluent solvent used in the polymerization step preferably contains at least dimethylacetamide.

[0064] The amount of polyamideimide resin in the resin composition is not particularly limited and may be adjusted as appropriate. In one embodiment, from the viewpoint of balance with other components, the content of polyamideimide resin is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, based on the total mass of the resin composition. 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 amount of polyamideimide resin in the resin composition within the above range, for example, it becomes easy to obtain fibers having sufficient strength during spinning to produce a nonwoven fabric, and it is also possible to suppress a decrease in fluidity and maintain good workability.

[0065] <Organic Solvent> The organic solvent constituting the polyamideimide resin composition is not particularly limited as long as it can dissolve the polyamideimide resin. In one embodiment, the organic solvent preferably contains dimethylacetamide. The polyamideimide resin composition may contain a solvent other than dimethylacetamide as long as the effect of the present invention is not reduced.

[0066] 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, as a co-solvent, any of the following may be used: 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.

[0067] When a solvent other than dimethylacetamide is used as the organic 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.

[0068] The polyamide-imide resin composition may further contain components other than the polyamide-imide resin and the organic solvent, as necessary. For example, the polyamide-imide resin composition may contain a filler, as necessary, to improve the water resistance of a 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.

[0069] The polyamideimide resin composition may further contain a surfactant, if necessary. Resin compositions containing a surfactant can be suitably used to produce nonwoven fabrics. The surfactant is not particularly limited, but it is preferable that the resin composition 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 is preferably 0.01 to 10% by mass, more preferably 0.5 to 5% by mass, based on the total mass of the resin composition. Adjusting the surfactant content within the above range can maintain a uniformly mixed state of the resin composition and prevent defects from occurring during the production of nonwoven fabrics. In some embodiments, the viscosity of the polyamideimide resin composition 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 polyamideimide resin composition within the above range facilitates the production of nonwoven fabrics by the electrospinning method described below. The viscosity is a value measured at 25°C using a Brookfield viscometer.

[0070] <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 known methods using a polyamideimide resin composition (resin solution) containing at least a polyamideimide resin and an organic solvent. For example, a porous film can be produced by forming a coating film using a resin solution containing a polyamideimide resin (first component), a solvent capable of dissolving the polyamideimide resin (second component), and a third component such as a poor solvent or a resin filler, and then heating the coating film. In such a production method, pores are formed by removing the second and third components during heating. 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 (third component) can be burned off.

[0071] Meanwhile, 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 polyamideimide resin and an organic solvent capable of dissolving the polyamideimide resin, and then molding the resulting fiber aggregate into a predetermined shape under heat to partially fuse (bond) the fibers together. Welding of the fibers together is not limited to heating, but can also be achieved, for example, by spraying a solution containing a binder resin onto an aggregate of fibers made of polyamideimide resin. Examples of binder resins that can be used include polyethersulfone resin (PES), polyimide resin (PI), polyamide resin, epoxy compound, isocyanate compound, and melamine compound, either alone or in combination. While the organic solvent can be removed by heating during spinning, 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 a heat treatment tends to facilitate removal of residual solvent and increase the strength of the nonwoven fabric, particularly in a temperature range exceeding 250°C, which tends to facilitate obtaining higher strength through higher molecular weight and crosslinking.

[0072] 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.

[0073] 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.

[0074] 1. Preparation Examples of Polyamide-Imide Resins (Resin Solutions) (Preparation Example 1) Preparation of PAI-1 153.7 g of trimellitic anhydride, 141.5 g of 4,4'-diphenylmethane diisocyanate, 64.1 g of 4,4'-diisocyanato-3,3'-dimethylbiphenyl, and 496.2 g of dimethylacetamide were placed in a flask equipped with a thermometer, a stirrer, and a condenser. While stirring in a dry nitrogen stream, the temperature was gradually increased to 90°C over 1 hour, and heating was continued for 2 hours while maintaining the temperature at 90°C. Thereafter, while paying attention to the rapid foaming of carbon dioxide gas generated by the reaction, the temperature was gradually increased to 140°C. After heating for 4 hours from the start of heating, the reaction was stopped, and a polyamide-imide resin (PAI-1) solution was obtained. The number average molecular weight of the polyamide-imide resin (PAI-1), measured as described below, was 25,000.

[0075] (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

[0076] (Preparation Example 2) Preparation of PAI-2 67.2 g of trimellitic anhydride, 35.7 g of 4,4'-diphenylmethane diisocyanate, 56.6 g of 4,4'-diisocyanato-3,3'-dimethylbiphenyl, and 296.4 g of dimethylacetamide were placed in a flask equipped with a thermometer, a stirrer, and a condenser. While stirring in a dry nitrogen stream, the temperature was gradually raised to 90°C over 1 hour, and the temperature was maintained at 90°C for 2 hours. Thereafter, the temperature was gradually raised to 140°C, while taking care to avoid the rapid foaming of carbon dioxide gas generated by the reaction. After heating for 8 hours from the start of heating, the reaction was stopped, and a polyamideimide resin (PAI-2) solution was obtained. The number average molecular weight of the polyamideimide resin (PAI-2), measured in the same manner as in Preparation Example 1, was 35,000.

[0077] (Preparation Example 3) Preparation of PAI-3 56.7 g of trimellitic anhydride, 15.0 g of 4,4'-diphenylmethane diisocyanate, 65.0 g of 4,4'-diisocyanato-3,3'-dimethylbiphenyl, and 488.1 g of dimethylacetamide were placed in a flask equipped with a thermometer, a stirrer, and a condenser. While stirring in a dry nitrogen stream, the temperature was gradually raised to 90°C over 1 hour, and heating was continued for 2 hours while maintaining the temperature at 90°C. Thereafter, while paying attention to the sudden foaming of carbon dioxide gas generated by the reaction, the temperature was gradually raised to 140°C. After heating for 6 hours from the start of heating, the reaction was stopped, and a polyamideimide resin (PAI-3) solution was obtained. The number average molecular weight of the polyamideimide resin (PAI-3), measured in the same manner as in Preparation Example 1, was 45,000.

[0078] 2. Production of Porous Structure (Example 1) Production of PAI-1 Nanofiber Nonwoven Fabric Using the PAI-1 solution (solid content 30%, viscosity 11.4 Pa s) obtained in Preparation Example 1, fibers were formed under the following spinning conditions using a nanofiber electrospinning device ("NANON-04" manufactured by MEC Co., Ltd.) to obtain a nonwoven fabric. 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 images of the nonwoven fabric, and the average value was calculated. The average fiber diameter was 40 nm.

[0079] (Example 2) Production of PAI-2 nanofiber nonwoven fabric Using the PAI-2 solution (solid content 20%, viscosity 4.1 Pa s) obtained in Preparation Example 2, fibers were formed into a nonwoven fabric using a nanofiber electrospinning device ("NANON-04" manufactured by MEC Co., Ltd.) under the following spinning conditions: flow rate: 1.0 ml / hr, distance between electrodes: 150 mm, voltage: 20 kV. The fiber diameters of multiple fibers were measured from SEM images of the nonwoven fabric, and the average value was calculated. The average fiber diameter was 168 nm.

[0080] (Example 3) Production of PAI-3 Porous Film 20 g of the PAI-3 solution (20% solids) obtained in Preparation Example 3 and 20 g of tetraethylene glycol dimethyl ether were weighed and mixed to obtain a resin solution. This resin solution was applied to a glass substrate to form a coating film, which was then heated at 80°C for 30 minutes to obtain a dried coating film. This dried coating film was further heated at 270°C for 30 minutes to obtain a film with a film thickness of 20 μm. The film was peeled from the glass substrate and used as a porous film for various evaluations. When the surface of the film peeled from the glass substrate as described above was observed using STM images, multiple pores with opening diameters of 1 to 5 μm were confirmed.

[0081] (Comparative Example 1) PP Porous Film A porous film made of polypropylene (PP) was used for various evaluations.

[0082] 3. Evaluation of Porous Structures The properties of the porous structures of Examples 1 to 3 and Comparative Example 1 were evaluated according to the methods described below. The results are shown in Table 1.

[0083] (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 solution, 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 prepared by mixing ethylene carbonate (EC):ethyl methyl carbonate (EMC):diethyl carbonate (DEC) in a ratio of 3:5:2 (volume ratio), and lithium hexafluorophosphate (LiPF ) was added as an electrolyte salt. ６ ) was added in an amount of 1 mol / L, 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.

[0084] (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)]}

[0085]

[0086] As shown in Table 1, according to one embodiment of the present invention, a porous structure with high porosity can be constructed using a polyamide-imide resin having a number-average molecular weight of 20,000 or more. It can be seen that the porous structures according to the present invention (Examples 1 to 3) have extremely small electrolyte contact angles compared to 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 Examples 1 and 2 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 superior in wettability to electrolytes compared to a typical conventional PP porous film and is suitable for use as a separator.

[0087] 4. Lithium-ion secondary battery In order to more specifically evaluate the performance of the porous structure of this embodiment, lithium-ion secondary batteries were fabricated using the nanofiber nonwoven fabrics obtained in Examples 1 and 2, and their function as separators was evaluated. Details are as follows. <Fabrication of lithium secondary battery> Lithium secondary batteries were fabricated as follows. (Fabrication of separator) The nanofiber nonwoven fabrics obtained in Examples 1 and 2 were each subjected to a heat treatment at 300°C for 30 minutes and used as separators. 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.

[0088] (Preparation of Negative Electrode) A silicon-based active material (Si alloy) and a carbon-based active material (graphite, Hitachi Chemical Co., Ltd.'s "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.

[0089] (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 the 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 the separator. A secondary battery was fabricated in the same manner as described above.

[0090] <Evaluation of Lithium Secondary Battery> The capacity retention rate of the secondary battery obtained as described above was measured according to the following method.

[0091] (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.

[0092]

[0093] As is clear from the results shown in Table 2, when the porous structure (nonwoven fabric) of the present embodiment is used as a separator, it is possible to improve battery performance in comparison with a conventional representative separator (Comparative Example 1).

Claims

1. A porous structure comprising a polyamide-imide resin having a number average molecular weight of 20,000 or more.

2. The porous structure according to claim 1, wherein the polyamide-imide resin contains a structure (A1) represented by the following formula: [In the formula, each R is independently an alkyl group having 1 to 3 carbon atoms, and * represents a bonding site to another structure.] 3. The porous structure according to claim 1 or 2, wherein the polyamide-imide resin contains a structure (A2) represented by the following formula: wherein X is —CH ２ - or -O-, and * represents a bonding site with another structure.] 4. The porous structure according to claim 3, wherein the polyamide-imide resin contains the structure (A1) and the structure (A2), and the ratio of the structure (A1) / the structure (A2) in the polyamide-imide resin is 1 / 9 to 9 / 1.

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

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

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

8. The porous structure according to claim 7, which is used as a separator for a lithium ion battery.

9. The porous structure according to claim 1, which is a nonwoven fabric formed from nanofibers containing the polyamideimide resin.

10. The porous structure according to claim 9, wherein the average diameter of the nanofibers is 1 to 1,000 nm.

Images