A diaphragm and its preparation method and application

Abstract

The present invention discloses a diaphragm and a preparation method and application thereof, belonging to the technical field of battery diaphragms. The diaphragm comprises an aramid base film and a thermoplastic polyimide layer in-situ polymerized on at least one side of the aramid base film; the thermoplastic polyimide layer is formed by imidization of thermoplastic polyamic acid; the thermoplastic polyamic acid is polymerized by diamine monomers and aliphatic dibasic acid anhydrides; the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film does not exceed 80°C. The diaphragm can have both good strength and thermal stability. The preparation of the diaphragm comprises: coating a thermoplastic polyamic acid solution prepared from diamine monomers and aliphatic dibasic acid anhydrides on at least one side of the aramid base film, preparing the thermoplastic polyamic acid solution coating layer into a thermoplastic polyamic acid gel layer and performing imidization reaction to obtain a diaphragm. The method is simple to operate and easy to industrialize. The diaphragm can be used to prepare batteries with good performance.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separators, and in particular to a separator and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries have the advantages of high voltage, high specific energy, high power density, long cycle life, environmental friendliness, low self-discharge rate, stable discharge voltage, no memory effect and fast charge and discharge. They have become the representative of high-performance batteries and have been widely used in small household appliances such as laptops, smart phones, digital cameras, MP3s and power batteries.

[0003] As a key material that isolates the direct contact between the positive and negative electrodes, the separator loses its function under the condition of battery abuse. Specifically, external extrusion, overcharge, discharge and uncontrollable lithium dendrite growth can cause the separator to be punctured and ruptured, while the blockage of the separator pores and the shrinkage of the separator due to excessive internal temperature can cause safety problems such as combustion and explosion.

[0004] At present, the most widely used polyolefin microporous membranes in lithium-ion batteries, such as polyethylene (PE) and polypropylene (PP), have low melting points (e.g., PE is about 135°C, PP is about 165°C) and poor thermal stability. When the battery temperature is too high, the polyolefin microporous membrane will shrink thermally, and the mechanical properties will drop sharply, eventually leading to a short circuit in the battery. Therefore, there is an urgent need to develop new high-safety diaphragm materials.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] The object of the present invention is to provide a diaphragm and a preparation method and application thereof, so as to solve or improve the above-mentioned technical problems.

[0007] The present invention can be implemented like this:

[0008] In a first aspect, the present invention provides a diaphragm, which includes an aramid base film and a thermoplastic polyimide layer in-situ polymerized on at least one side of the aramid base film;

[0009] The thermoplastic polyimide layer is formed by imidization of thermoplastic polyamic acid; the thermoplastic polyamic acid is polymerized from diamine monomers and aliphatic dibasic acid anhydrides;

[0010] The difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film does not exceed 80°C.

[0011] In an optional embodiment, the diaphragm has at least one of the following features:

[0012] Feature 1: The total thickness of the diaphragm is 15μm~23μm;

[0013] Feature 2: In the diaphragm, the thickness of the thermoplastic polyimide layer disposed on the surface of either side of the aramid base film is 1 μm to 5 μm;

[0014] Feature 3: Aramid fiber models include 1313;

[0015] Feature 4: The difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film is 30°C to 80°C, and the glass transition temperature of the aramid base film is higher than the glass transition temperature of the thermoplastic polyimide layer;

[0016] Feature 5: The tensile strength of the diaphragm is not less than 140MPa;

[0017] Feature 6: The thermal shrinkage of the diaphragm after treatment at 250°C for 1 hour does not exceed 0.2%.

[0018] In an optional embodiment, the glass transition temperature of the thermoplastic polyimide layer is 200°C to 270°C; and the glass transition temperature of the aramid is 270°C to 280°C.

[0019] In a second aspect, the present invention provides a method for preparing a diaphragm as described in the aforementioned embodiment, comprising the following steps: coating a thermoplastic polyamic acid solution prepared from a diamine monomer and an aliphatic dibasic acid anhydride on at least one surface of an aramid base film, preparing a thermoplastic polyamic acid gel layer from the coating layer of the thermoplastic polyamic acid solution obtained after coating, and subjecting the thermoplastic polyamic acid in the thermoplastic polyamic acid gel layer to an imidization reaction to obtain a diaphragm.

[0020] In an alternative embodiment, the diamine monomer includes at least one of 2,2′-bis[4-(4-aminophenoxyphenyl)]propane, 4,4′-diaminodiphenyl ether, and p-phenylenediamine.

[0021] In an optional embodiment, the aliphatic dibasic acid anhydride includes at least one of 1,2,4,5-cyclohexanetetracarboxylic dianhydride, dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride, ethanetetracarboxylic dianhydride, butanetetracarboxylic dianhydride, cyclobutane dianhydride, methanetetraacetic dianhydride, cyclopentanetetracarboxylic dianhydride, tetrahydronaphthalene dianhydride and cis-1,2,3,4-cyclohexane dianhydride.

[0022] In an optional embodiment, the viscosity of the thermoplastic polyamic acid solution is 200 poise to 1000 poise;

[0023] And / or, the solid content of the thermoplastic polyamic acid solution is 10wt%~20wt%.

[0024] In an optional embodiment, preparing a thermoplastic polyamic acid gel layer includes: passing an air flow with a relative humidity of 70% to 90% through an aramid-based membrane having a thermoplastic polyamic acid solution coating layer to remove liquid in the thermoplastic polyamic acid solution coating layer to obtain a thermoplastic polyamic acid gel layer having a porous structure.

[0025] In an optional embodiment, the temperature of the imidization reaction is 200°C to 270°C.

[0026] In a third aspect, the present invention provides a battery, wherein the separator of the battery is the separator of the aforementioned embodiment.

[0027] The beneficial effects of the present invention include:

[0028] The thermoplastic polyamic acid in the present invention is polymerized from diamine monomers and aliphatic dibasic acid anhydrides. Aliphatic dibasic acid anhydrides refer to anhydride compounds whose anhydride rings are not on aromatic rings. They have lower glass transition temperatures and are more conducive to the in-situ polymerization reaction of the aramid-based polyimide layer, further improving the degree of cyclization and the dimensional stability of the diaphragm.

[0029] The imidization temperature of thermoplastic polyamic acid is relatively high (higher than 200°C), and conventional base films will melt or shrink severely under the imidization temperature of thermoplastic polyamic acid, and cannot be further prepared into qualified diaphragms. The present invention uses aramid as the base film and controls the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film to no more than 80°C. On the one hand, the aramid base film does not melt and does not shrink significantly under the imidization temperature of thermoplastic polyamic acid, and can effectively maintain the basic structure of the diaphragm; on the other hand, polyamic acid specifically uses thermoplastic polyamic acid glue in its preparation, which can greatly reduce the glass transition temperature of the polyimide film layer after imidization and thus reduce its imidization temperature; in addition, under high temperature conditions, it can ensure that there is no obvious thermal shrinkage size difference between the aramid base film and the thermoplastic polyimide layer, and effectively maintain the high temperature stability of the diaphragm.

[0030] The diaphragm provided by the present invention can have both good thermal stability and mechanical strength. The preparation method thereof is simple to operate and easy to industrialize. The diaphragm can be used to prepare batteries with good safety performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0032] Figure 1 This is a schematic diagram of the structure of the thermoplastic polyimide layer in the diaphragm provided in Example 1 of the present application;

[0033] Figure 2 This is a schematic diagram of the surface structure of the aramid-based membrane provided in Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0035] The following is a detailed description of the diaphragm provided by the present invention and its preparation method and application.

[0036] The invention provides a diaphragm, which comprises an aramid base film and a thermoplastic polyimide layer in-situ polymerized on at least one side surface of the aramid base film.

[0037] In some embodiments, the thermoplastic polyimide layer is provided on only one side of the aramid-based film. In other embodiments, the thermoplastic polyimide layer may be provided on both sides of the aramid-based film. When the thermoplastic polyimide layer is provided on both sides of the aramid-based film, the material of the thermoplastic polyimide layer provided on both sides is the same (that is, the glass transition temperature of the thermoplastic polyimide layer provided on both sides is the same), so that the heat shrinkage of each thermoplastic polyimide layer remains consistent.

[0038] In some optional embodiments, the total thickness of the diaphragm can be 15μm~23μm, such as 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm or 23μm, etc., or other values ​​within the range of 15μm~23μm. In some more typical embodiments, the total thickness of the diaphragm can be 17μm~20μm, such as 17μm, 18μm, 19μm or 20μm, etc., or any other value within the range of 17μm~20μm. In the above typical embodiments, the total thickness of the diaphragm is relatively thin, which effectively improves the safety performance of the battery without significantly reducing the energy density of the battery.

[0039] The thickness of the thermoplastic polyimide layer disposed on either side of the aramid base film may be 1 μm to 5 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, or other values ​​within the range of 1 μm to 5 μm.

[0040] The thickness of the aramid base film in the diaphragm is obtained by subtracting the total thickness of the thermoplastic polyimide layer from the total thickness of the diaphragm.

[0041] It should be noted that if the thickness of the thermoplastic polyimide layer is too thick, it is not conducive to the volatilization of the solvent inside the coating and the uniformity of the pores.

[0042] In the present invention, the thermoplastic polyimide layer is formed by imidization of thermoplastic polyamic acid; the thermoplastic polyamic acid is polymerized from diamine monomers and aliphatic dibasic acid anhydrides; and the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid-based film does not exceed 80°C.

[0043] In some optional embodiments, the type of aramid fiber may exemplarily include type 1313, and may also include type 1414 and the like.

[0044] The aliphatic dibasic acid anhydride in the present invention refers to an anhydride compound whose anhydride ring is not on the aromatic ring. It has a lower glass transition temperature and is more conducive to the in-situ polymerization reaction of the aramid-based polyimide layer, further improving the degree of cyclization and the dimensional stability of the diaphragm.

[0045] In some optional embodiments, the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film may be 80°C, 70°C, 60°C, 50°C, 40°C, 30°C, 20°C, 10°C or 5°C, etc., or other values ​​not exceeding 80°C (not 0). In some typical embodiments, the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film is 30°C to 80°C, such as 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, etc., or other values ​​within the range of 30°C to 80°C. Among them, the glass transition temperature of the aramid base film is higher than the glass transition temperature of the thermoplastic polyimide layer.

[0046] Exemplarily, the glass transition temperature of the thermoplastic polyimide layer can be 200°C to 270°C, such as 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C or 270°C, etc., or it can be other values ​​within the range of 200°C to 270°C.

[0047] The glass transition temperature of aramid may be 270°C to 280°C, such as 270°C, 272°C, 275°C, 278°C or 280°C, or other values ​​within the range of 270°C to 280°C.

[0048] It should be noted that the glass transition temperature of the polyimide layer can affect its imidization temperature. During the imidization process of polyamic acid, as the imidization process proceeds, the glass transition temperature of the polymer increases, and the polymer changes from a rubber state to a glass state. The reduction in the imidization rate is caused by the reduction in the activity of the macromolecules. The glass transition temperature of the polymer (copolymer of imide and amic acid) increases with the increase in the degree of imidization. When the polymer changes from a softened state to a solidified state, the imidization reaction stops. Therefore, the lower the glass transition temperature of the thermoplastic polyamic acid, the higher the molecular activity, and the lower the corresponding imidization temperature. The imidization temperature of non-thermoplastic polyamic acid is generally above 300°C.

[0049] As mentioned above, the imidization temperature of thermoplastic polyamic acid is relatively high (higher than 200°C), and conventional base films will melt or shrink severely under the imidization temperature of thermoplastic polyamic acid, and cannot be further prepared into qualified diaphragms. The present invention uses aramid as the base film and controls the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film to no more than 80°C. On the one hand, the aramid base film does not melt and does not shrink significantly under the imidization temperature of thermoplastic polyamic acid, and can effectively maintain the basic structure of the diaphragm; on the other hand, polyamic acid specifically uses thermoplastic polyamic acid glue in its preparation, which can greatly reduce the glass transition temperature of the polyimide film layer after imidization and thus reduce its imidization temperature; in addition, under high temperature conditions, it can ensure that there is no obvious thermal shrinkage size difference between the aramid base film and the thermoplastic polyimide layer, and effectively maintain the high temperature stability of the diaphragm.

[0050] In some optional embodiments, the tensile strength of the diaphragm is not less than 140 MPa. In some typical embodiments, the tensile strength of the diaphragm may be 144 MPa to 253 MPa, such as 144 MPa, 161 MPa, 177 MPa, 199 MPa, 214 MPa, 226 MPa, 238 MPa or 253 MPa, or other values ​​within the range of 144 MPa to 253 MPa.

[0051] In some optional embodiments, the thermal shrinkage of the separator after being treated at 250° C. for 1 hour does not exceed 0.2%. In some more typical embodiments, the thermal shrinkage of the separator after being treated at 250° C. for 1 hour may be 0.1% to 0.2%.

[0052] As mentioned above, the diaphragm provided by the present invention can have both good thermal stability and mechanical strength.

[0053] Correspondingly, the present invention also provides a method for preparing the above-mentioned diaphragm, which may include the following steps: coating a thermoplastic polyamic acid solution prepared from a diamine monomer and an aliphatic dibasic acid anhydride on at least one surface of an aramid base membrane, preparing a thermoplastic polyamic acid gel layer from the coating layer of the thermoplastic polyamic acid solution obtained after coating, and subjecting the thermoplastic polyamic acid in the thermoplastic polyamic acid gel layer to an imidization reaction to obtain a diaphragm.

[0054] In some optional embodiments, the diamine monomer may illustratively but not limitatively include at least one of 2,2′-bis[4-(4-aminophenoxyphenyl)]propane (BAPP), 4,4′-diaminodiphenyl ether (ODA) and p-phenylenediamine (PDA).

[0055] The aliphatic dibasic acid anhydride may illustratively but not limitatively include at least one of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA), dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride (HBPDA), ethanetetracarboxylic dianhydride (CPTA), butanetetracarboxylic dianhydride, cyclobutane dianhydride (CBDA), methanetetraacetic dianhydride, cyclopentanetetracarboxylic dianhydride, tetrahydronaphthalene dianhydride (THNA) and cis-1,2,3,4-cyclohexane dianhydride.

[0056] Thermoplastic polyimide is used because it has a low glass transition temperature and a low imidization temperature, which is beneficial to the dimensional stability, mechanical properties and thermodynamic properties of the diaphragm.

[0057] The polymerization of the above diamine monomer and aliphatic dibasic acid anhydride is a condensation polymerization process.

[0058] In some embodiments, the diamine monomer may be first dissolved in a solvent (such as a mixture of THF and methanol), and then an aliphatic dibasic acid anhydride may be added to perform polymerization to obtain a thermoplastic polyamic acid solution.

[0059] In some typical embodiments, the molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02. Since the polyamic acid synthesis uses a non-protonic polar solvent, such as a mixed solvent of tetrahydrofuran and methanol, the boiling point of the mixed solvent is relatively low, and the exchange with the water mist is relatively fast after the film is coated, which will not cause a large deformation and dissolution of the aramid base film. However, it is extremely difficult to completely remove water from the aprotic polar solvents used above (especially tetrahydrofuran). Therefore, when aprotic polar solvents are used as solvents for the synthesis of polyamic acid, the very small amount of water present in the aprotic polar solvent will hydrolyze part of the aliphatic dibasic acid anhydride and consume part of the aliphatic dibasic acid anhydride. Based on this, the amount of aliphatic dibasic acid anhydride is set to be slightly excessive compared to the amount of diamine monomer.

[0060] In some optional embodiments, the viscosity of the thermoplastic polyamic acid solution may be 200 poise to 1000 poise, such as 200 poise, 400 poise, 600 poise, 800 poise or 1000 poise, etc., or other values ​​within the range of 200 poise to 1000 poise.

[0061] The solid content of the thermoplastic polyamic acid solution may be 10 wt % to 20 wt %, such as 10 wt %, 15 wt %, 16 wt %, 17 wt %, 18 wt %, 19 wt % or 20 wt %, or may be other values ​​within the range of 10 wt % to 20 wt %.

[0062] The thermoplastic polyamic acid solution is kept warm at 0-10° C. (eg, 5° C.) For example, the thermoplastic polyamic acid solution can be evenly coated on the surface of the aramid base film by wire rod coating to obtain a thermoplastic polyamic acid solution coating layer.

[0063] In some optional embodiments, the coating speed of the thermoplastic polyamic acid solution does not exceed 100 m / min, such as 100 m / min, 90 m / min, 80 m / min, 70 m / min, 60 m / min, 50 m / min, 40 m / min, 30 m / min, 20 m / min or 10 m / min, etc.

[0064] In some optional embodiments, preparing a thermoplastic polyamic acid gel layer includes: passing an air flow with a relative humidity of 70% to 90% through an aramid-based membrane having a thermoplastic polyamic acid solution coating layer to remove liquid in the thermoplastic polyamic acid solution coating layer to obtain a thermoplastic polyamic acid gel layer having a porous structure.

[0065] The relative humidity of the air flow may be 70%, 75%, 80%, 85% or 90%, etc., or may be other values ​​within the range of 70% to 90%.

[0066] If the relative humidity of the air flow is too low, the exchange of solvent and non-solvent water vapor will be slow, causing the polyamic acid layer to shrink severely and form closed pores; if the relative humidity of the air flow is too high, the polyamic acid layer surface will become hard, which will increase the imidization temperature and be detrimental to the imidization process.

[0067] The circulation speed of the air flow can be 20m / min~60m / min, such as 20m / min, 25m / min, 30m / min, 35m / min, 40m / min, 45m / min, 50m / min, 55m / min or 60m / min, etc., or it can be other values ​​within the range of 20m / min~60m / min.

[0068] If the air flow rate is too slow, the exchange of solvent and non-solvent water vapor will be slow, which will cause the polyamic acid layer to shrink severely and close the pores; if the air flow rate is too fast, the polyamic acid layer surface will harden and increase the imidization temperature, which is not conducive to the subsequent imidization process.

[0069] In the above process, an air flow with a relative humidity of 70% to 90% flows at a speed of 20 m / min to 60 m / min at room temperature over the surface of the aramid base film coated with the thermoplastic polyamic acid solution, the solvent (a mixture of THF and methanol) in the thermoplastic polyamic acid solution begins to evaporate, and concentrated water droplets gather on the coating surface. The thermoplastic polyamic acid coating becomes opaque, and after the solvent and water evaporate, a thermoplastic polyamic acid gel layer with a porous structure is obtained.

[0070] In some optional embodiments, the pore size of the pores in the aramid-based membrane does not exceed 150 nm, and the pore size in the thermoplastic polyamic acid gel layer of the porous structure does not exceed 150 nm.

[0071] Furthermore, the thermoplastic polyamic acid gel layer and the aramid base film are subjected to imidization treatment together.

[0072] In some optional embodiments, the temperature of the imidization reaction can be 200° C. to 270° C., such as 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C. or 270° C. The specific temperature of the imidization reaction is set to enable the thermoplastic polyamic acid in the thermoplastic polyamic acid gel layer to undergo an imidization reaction. After the imidization reaction is completed, it can be cooled naturally.

[0073] In addition, the present invention also provides a battery, the separator of the battery is the separator, and the battery can have good safety performance in use.

[0074] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0075] Example 1

[0076] This embodiment provides a diaphragm, which includes an aramid base film and a thermoplastic polyimide layer in-situ polymerized on both side surfaces of the aramid base film.

[0077] The type of aramid in the aramid base film is 1313, the thickness of the aramid base film is 13μm, the thickness of the thermoplastic polyimide layer on each side of the aramid base film is 2μm, and the total thickness of the diaphragm is approximately 17μm. The glass transition temperature of the aramid base film is about 275℃, the glass transition temperature of the thermoplastic polyimide layer is about 250℃, and the difference in glass transition temperature between the thermoplastic polyimide layer and the aramid base film is about 20℃.

[0078] The preparation method of the diaphragm comprises the following steps:

[0079] S1: preparing a thermoplastic polyamic acid solution.

[0080] The temperature of the polymerization kettle was controlled not to exceed 5°C, 44.71kg of tetrahydrofuran (THF) and 11.18kg of methanol were added to the polymerization kettle for mixing. After the temperature was reached, 4.11kg of 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP) was added and dissolved for 1 hour. Then, 2.29kg of 1,2,4,5-cyclohexanetetracarboxylic dianhydride (HPMDA) was gradually added within 80 minutes. The mixture was stirred at 20°C ~ 25°C for 24 hours to obtain a thermoplastic polyamic acid solution with a solid content of 10wt%, a weight average molecular weight Mw = 155000 and a viscosity of 443 poise.

[0081] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0082] S2: preparing a thermoplastic polyamic acid solution coating layer.

[0083] The heat-insulating thermoplastic polyamic acid solution is uniformly coated on both sides of the aramid base film in a wire rod coating manner to obtain a thermoplastic polyamic acid solution coating layer.

[0084] The coating speed is 50 m / min, and the coating thickness is about 10 μm.

[0085] S3: preparing a thermoplastic polyamic acid gel layer.

[0086] An air flow with a relative humidity of 80% flows at a rate of 40 m / min over the surface of the aramid base membrane having a thermoplastic polyamic acid solution coating layer at room temperature. The solvent in the thermoplastic polyamic acid solution coating layer begins to evaporate, and concentrated water droplets gather on the surface of the thermoplastic polyamic acid solution coating layer. The thermoplastic polyamic acid solution coating layer becomes opaque. After the solvent and water evaporate, a thermoplastic polyamic acid gel layer with a porous structure is obtained.

[0087] S4: Imidization reaction.

[0088] The aramid-based membrane with the thermoplastic polyamic acid gel layer is subjected to imidization treatment to obtain a diaphragm.

[0089] The temperature of the imidization treatment is about 250° C. During the imidization treatment, the thickness of the thermoplastic polyimide layer on each side surface of the aramid-based film is controlled to be about 2 μm.

[0090] Example 2

[0091] The difference between this embodiment and embodiment 1 is that:

[0092] S1: Control the temperature of the polymerization kettle not to exceed 5°C, add 35.52kg of tetrahydrofuran (THF) and 8.88kg of methanol into the polymerization kettle for mixing. After the temperature is reached, add 2.0kg of 4,4'-diaminodiphenyl ether (ODA) and dissolve it for 1 hour. Then gradually add 3.11kg of dicyclohexyl-3,4,3',4'-tetracarboxylic dianhydride (HBPDA) within 80 minutes, stir at 20°C~25°C for 24 hours, and obtain a thermoplastic polyamic acid solution with a solid content of 10wt%, a weight average molecular weight Mw=146000, and a viscosity of 354 poise.

[0093] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0094] Example 3

[0095] The difference between this embodiment and embodiment 1 is that:

[0096] S1: Control the temperature of the polymerization kettle not to exceed 5°C, add 18.79kg of tetrahydrofuran (THF) and 4.7kg of methanol into the polymerization kettle for mixing, add 2.0kg of 4,4'-diaminodiphenyl ether (ODA) after the temperature is reached, dissolve for 1h, gradually add 2.14kg of cyclopentanetetracarboxylic dianhydride within 80min, stir at 20°C~25°C for 24h, and obtain a thermoplastic polyamic acid solution with a solid content of 15wt%, a weight average molecular weight Mw=123000, and a viscosity of 287 poise.

[0097] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0098] Example 4

[0099] The difference between this embodiment and embodiment 1 is that:

[0100] S1: The temperature of the polymerization kettle was controlled not to exceed 5°C, and 28.32kg of tetrahydrofuran (THF) and 7.08kg of methanol were added to the polymerization kettle for mixing. After the temperature was reached, 4.11kg of 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP) was added and dissolved for 1 hour. Then, 2.14kg of cyclopentanetetracarboxylic dianhydride was gradually added within 80 minutes, and stirred at 20°C~25°C for 24 hours to obtain a thermoplastic polyamic acid solution with a solid content of 15wt%, a weight average molecular weight Mw=118000, and a viscosity of 292 poise.

[0101] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0102] Example 5

[0103] The difference between this embodiment and embodiment 1 is that:

[0104] S1: The temperature of the polymerization kettle was controlled not to exceed 5°C, 32.48 kg of tetrahydrofuran (THF) and 8.12 kg of methanol were added to the polymerization kettle for mixing. After the temperature was reached, 4.11 kg of 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP) was added and dissolved for 1 hour. Then, 3.06 kg of tetrahydronaphthalene dianhydride (THNA) was gradually added within 80 minutes. The mixture was stirred at 20°C-25°C for 24 hours to obtain a thermoplastic polyamic acid solution with a solid content of 15 wt%, a weight average molecular weight Mw=125000 and a viscosity of 306 poise.

[0105] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0106] Example 6

[0107] The difference between this embodiment and embodiment 1 is that:

[0108] S1: Control the temperature of the polymerization kettle not to exceed 5°C, add 22.95kg of tetrahydrofuran (THF) and 5.74kg of methanol into the polymerization kettle for mixing. After the temperature is reached, add 2.0kg of 4,4'-diaminodiphenyl ether (ODA) and dissolve it for 1 hour. Then gradually add 3.06kg of tetrahydronaphthalene dianhydride (THNA) within 80 minutes, stir at 20°C~25°C for 24 hours, and obtain a thermoplastic polyamic acid solution with a solid content of 15wt%, a weight average molecular weight Mw=142000, and a viscosity of 372 poise.

[0109] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0110] Example 7

[0111] The difference between this embodiment and embodiment 1 is that in S1, the amount of solvent is adjusted under the condition that the reaction raw materials remain unchanged, so that the viscosity of the thermoplastic polyamic acid solution is 535 poise and the solid content of the thermoplastic polyamic acid solution is 15 wt %.

[0112] Example 8

[0113] The difference between this embodiment and embodiment 1 is that the thermoplastic polyimide layer is provided only on one surface of the aramid base film, the thickness of the aramid base film is 18 μm, the thickness of the thermoplastic polyimide layer is 2 μm, and the total thickness of the diaphragm is 20 μm.

[0114] Comparative Example 1

[0115] The difference between this comparative example and Example 1 is:

[0116] S1: Thermoplastic polyamic acid solution was synthesized by polymerization of aromatic dianhydride and aliphatic diamine. Specifically, the temperature of the polymerization kettle was controlled at 35°C, 9.57 kg of tetrahydrofuran (THF) and 2.39 kg of methanol were added to the polymerization kettle for mixing, 1.14 kg of cyclohexanediamine was added, and after dissolving for 1 hour, the temperature was lowered to room temperature and 1.32 kg of acetic acid was slowly added. After stirring for 30 minutes, 2.18 kg of pyromellitic dianhydride (PMDA) was added, and stirred at 20°C~25°C for 24 hours to obtain a thermoplastic polyamic acid solution with a solid content of 20wt%, a weight average molecular weight Mw=92000, and a viscosity of 286 poise.

[0117] The molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to about 1:1.02.

[0118] Comparative Example 2

[0119] The difference between this comparative example and Example 1 is that the molar ratio of the diamine monomer to the aliphatic dibasic acid anhydride is set to be approximately 1:1.

[0120] Comparative Example 3

[0121] This comparative example is an aramid-based film with a thickness of 17 μm.

[0122] Comparative Example 4

[0123] The difference between this comparative example and Example 1 is that the thickness of the thermoplastic polyimide layer disposed on each side surface of the aramid-based film is 0.5 μm.

[0124] Comparative Example 5

[0125] The difference between this comparative example and Example 1 is that the glass transition temperature of the thermoplastic polyimide layer used is 20° C. higher than that of the aramid-based film.

[0126] Comparative Example 6

[0127] The difference between this comparative example and Example 1 is that the glass transition temperature of the aramid-based film is 100° C. higher than the glass transition temperature of the thermoplastic polyimide layer.

[0128] Comparative Example 7

[0129] The difference between this comparative example and Example 1 is that the solid content of the thermoplastic polyamic acid solution is 5 wt % and the viscosity is 100 poise.

[0130] Comparative Example 8

[0131] The difference between this comparative example and Example 1 is that the solid content of the thermoplastic polyamic acid solution is 30 wt % and the viscosity is 1200 poise.

[0132] Test example

[0133] ①, taking Example 1 and Comparative Example 2 as examples, the structure of the prepared diaphragm was observed, and the results are as follows Figure 1 to Figure 2 shown.

[0134] Depend on Figure 1 It can be seen that the pore size distribution of the thermoplastic polyimide layer is smaller and more uniform than that of the aramid base layer.

[0135] Depend on Figure 2 It can be seen that the pore size of the aramid-based membrane is relatively large, and its surface pores are larger than the internal pores.

[0136] ②. The performance of the diaphragms prepared in Examples 1 to 8 and Comparative Examples 1 to 8 was tested, including tensile strength and thermal shrinkage. The results are shown in Table 1.

[0137] A. Tensile strength: Tensile strength is a parameter that reflects the dimensional stability of the diaphragm when subjected to external forces during use. If the tensile strength is insufficient, the diaphragm will not be easy to recover to its original size after deformation, which will cause a short circuit in the battery. This test is carried out in accordance with the provisions of GB / T1040.3-2006, using a type 2 specimen with a width of (15+0.1) mm, an initial distance between the clamps of the MTL (PC) tensile testing machine of (100+5) mm, and a test speed of (250+10) mm / min.

[0138] B. Thermal shrinkage rate: Since the diaphragm is prone to shrinkage and deformation at high temperatures, the thermal shrinkage rate can be used to characterize the dimensional stability of the diaphragm at high temperatures.

[0139] Specific test method: Use a steel ruler, blade or special tool with an accuracy of 0.5mm to cut a 100mm×100mm sample and mark the longitudinal and transverse directions of the film. Put the sample into a drying oven at a specific temperature and start timing. After a certain period of time, take out the sample and place it horizontally for 10 minutes. Measure the longitudinal and transverse dimensions of the sample respectively. Calculate according to the following formula: S=(L0-L) / L0×100%, where: S is the shrinkage rate; L0 is the length of the sample before heating, in millimeters (mm); L is the length of the sample after shrinkage, in millimeters (mm).

[0140] Table 1 Test results

[0141]

[0142] It can be seen from Table 1 that the diaphragms provided by Examples 1 to 7 of the present invention have better performance effects in terms of mechanical strength and thermal stability than those of the comparative examples.

[0143] The reaction raw materials used in Examples 2 to 6 are different from those in Example 1. From the results of Examples 1 to 6, it can be seen that when the reaction raw materials provided by the present invention are combined in different ways and reacted, the diaphragm can have better mechanical strength and thermal stability.

[0144] Combining the results of Example 1 and Example 7, it can be seen that when the reaction raw materials are the same, the viscosity and solid content of the thermoplastic polyamic acid solution can be changed by adjusting the conditions. As long as the viscosity and solid content of the thermoplastic polyamic acid solution are controlled within the range of 200 poise to 1000 poise and 10wt% to 20wt%, the corresponding diaphragms obtained can have good mechanical strength and thermal stability.

[0145] It can be seen from the results of Example 8 that when the thermoplastic polyimide layer is only provided on one side of the aramid base film, the tensile strength of the diaphragm is lower than that of the diaphragm with thermoplastic polyimide layers on both sides of the aramid base film, and the thermal stability is still good.

[0146] It can be seen from the results of Example 1 and Comparative Example 1 that when the thermoplastic polyimide layer is prepared using aromatic dianhydride and aliphatic diamine as raw materials, its mechanical strength and thermal stability are significantly lower than those of the diaphragm prepared using diamine monomer and aliphatic dibasic acid anhydride as raw materials, indicating that the choice of raw materials has a direct impact on the above-mentioned properties of the diaphragm.

[0147] It can be seen from the results of Example 1 and Comparative Example 2 that, under the premise of the same raw materials, when the molar ratio of the diamine monomer and the aliphatic dibasic acid anhydride is not properly set, the mechanical strength and thermal stability of the diaphragm will also be reduced.

[0148] It can be seen from the results of Example 1 and Comparative Example 3 that the strength of the diaphragm prepared in Example 1 is significantly increased compared with the pure aramid 1313 diaphragm; in terms of thermal shrinkage, the pure aramid 1313 diaphragm itself in Comparative Example 2 has higher heat resistance than the polyolefin diaphragm, and the diaphragm provided by the embodiment of the present invention has almost no obvious change in size after a heat treatment process of 250°C×1h, and its heat resistance is even better.

[0149] It can be seen from the results of Example 1 and Comparative Example 4 that when the thickness of the thermoplastic polyimide layer disposed on one side of the aramid-based membrane is relatively thin, the mechanical strength of the diaphragm is reduced and the thermal shrinkage rate is increased.

[0150] It can be seen from the results of Example 1 and Comparative Examples 5-6 that when the glass transition temperature relationship between the thermoplastic polyimide layer and the aramid base film is not set properly, the shrinkage properties of the aramid base film and the thermoplastic polyimide layer cannot match, and a qualified product cannot be obtained.

[0151] It can be seen from the results of Example 1 and Comparative Example 7 that when the solid content and viscosity of the thermoplastic polyamic acid solution are not set properly, the mechanical strength of the diaphragm will decrease and the thermal shrinkage rate will increase.

[0152] In summary, the diaphragm prepared by the present invention has good performance in terms of mechanical strength and thermal stability.

[0153] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A diaphragm, characterized in that: The diaphragm includes an aramid base film and a thermoplastic polyimide layer in-situ polymerized on at least one side surface of the aramid base film; The thermoplastic polyimide layer is formed by imidization of thermoplastic polyamic acid; the thermoplastic polyamic acid is polymerized from diamine monomers and aliphatic dibasic acid anhydrides; The difference in glass transition temperature between the thermoplastic polyimide layer and the aramid-based film is no more than 80° C., and the glass transition temperature of the aramid-based film is higher than the glass transition temperature of the thermoplastic polyimide layer.

2. The diaphragm according to claim 1, characterized in that The diaphragm has at least one of the following characteristics: Feature 1: The total thickness of the diaphragm is 15 μm to 23 μm; Feature 2: In the diaphragm, the thickness of the thermoplastic polyimide layer disposed on the surface of either side of the aramid base film is 1 μm to 5 μm; Feature 3: The aramid fiber type includes type 1313; Feature 4: The tensile strength of the diaphragm is not less than 140 MPa; Feature 5: The thermal shrinkage of the diaphragm after being treated at 250° C. for 1 hour does not exceed 0.2%.

3. The diaphragm according to claim 2, characterized in that The glass transition temperature of the thermoplastic polyimide layer is 200°C to 270°C; The glass transition temperature of the aramid fiber is 270°C to 280°C.

4. A method for preparing a diaphragm according to any one of claims 1 to 3, characterized in that: The following steps are involved: A thermoplastic polyamic acid solution prepared from a diamine monomer and an aliphatic dibasic acid anhydride is coated on at least one side of an aramid base film, a coating layer of the thermoplastic polyamic acid solution obtained after coating is prepared into a thermoplastic polyamic acid gel layer, and the thermoplastic polyamic acid in the thermoplastic polyamic acid gel layer is subjected to an imidization reaction to obtain a diaphragm.

5. The preparation method according to claim 4, characterized in that: The diamine monomer includes at least one of 2,2′-bis[4-(4-aminophenoxyphenyl)]propane, 4,4′-diaminodiphenyl ether and p-phenylenediamine.

6. The preparation method according to claim 4, characterized in that: The aliphatic dibasic acid anhydride includes at least one of 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride, dicyclohexyl-3,4,3',4'-tetracarboxylic acid dianhydride, ethanetetracarboxylic acid dianhydride, butanetetracarboxylic acid dianhydride, cyclobutane dianhydride, methanetetraacetic acid dianhydride, cyclopentanetetracarboxylic acid dianhydride, tetrahydronaphthalene dianhydride and cis-1,2,3,4-cyclohexane dianhydride.

7. The preparation method according to any one of claims 4 to 6, characterized in that The viscosity of the thermoplastic polyamic acid solution is 200 poise to 1000 poise; And / or, the solid content of the thermoplastic polyamic acid solution is 10wt%~20wt%.

8. The preparation method according to any one of claims 4 to 6, characterized in that The preparation of the thermoplastic polyamic acid gel layer includes: passing an air flow with a relative humidity of 70% to 90% through an aramid-based membrane having a thermoplastic polyamic acid solution coating layer to remove liquid in the thermoplastic polyamic acid solution coating layer to obtain a thermoplastic polyamic acid gel layer with a porous structure.

9. The preparation method according to any one of claims 4 to 6, characterized in that The temperature of the imidization reaction is 200°C~270°C.

10. A battery, characterized in that: The separator of the battery is the separator according to any one of claims 1 to 3.

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