Composite separator, method for manufacturing the same, and battery

Abstract

This invention relates to the field of battery technology, and more specifically, to a composite separator, its preparation method, and a battery. The composite separator of this invention includes a base film and a coating located on at least one surface of the base film. The coating comprises a composite material and a binder. The composite material includes carbon nanotubes and polyvinylpyrrolidone (PVP) coating the carbon nanotubes. In the composite separator of this invention, the carbon nanotubes in the composite material exhibit high electrical conductivity and good thermal stability, while the surface-coated PPV improves the dispersibility and wettability of the carbon nanotubes, thereby giving the resulting composite separator excellent thermal resistance, coating adhesion, electrolyte storage performance, and wetting properties.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically, to a composite separator, its preparation method, and a battery. Background Technology

[0002] With the rapid development of electric vehicles and mobile communication devices, the demand for lithium batteries is increasing. As a highly efficient and environmentally friendly energy storage device, the performance of lithium batteries directly affects the overall performance of the device. Lithium batteries are mainly composed of positive electrode materials, negative electrode materials, electrolyte, and separator. As a key component of lithium batteries, the separator's main function is to isolate the positive and negative electrode materials, prevent short circuits, and allow ions to pass through, completing the charging and discharging process.

[0003] Existing technology CN112646303B discloses a coating slurry for lithium battery separators and its preparation method. The slurry comprises an Al2O3-PMMA composite material, oxidized PVA, and additives. The Al2O3-PMMA composite material is a core-shell particle with high adhesion and high wetting efficiency. PMMA completely encapsulates the Al2O3 particles inside, forming spherical particles. Potassium persulfate is used to treat the PVA solution, causing partial chain breakage of the PVA molecules and introducing terminal carboxyl or aldehyde groups, as well as ketone groups into the molecular chains. The separator prepared by this method has certain wettability, but achieving complete Al2O3 encapsulation is difficult to control, and the PMMA shell is easily broken under hot pressing conditions, making the exposed Al2O3 easily absorb water.

[0004] Existing membrane coatings have unsatisfactory thermal stability and wettability; therefore, it is crucial to provide a composite membrane that combines both thermal stability and wettability.

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

[0006] One object of the present invention is to provide a composite diaphragm with excellent heat resistance, coating adhesion, liquid storage and wetting properties.

[0007] Another objective of this invention is to provide a method for preparing the composite diaphragm described above, which is simple and easy to implement, and the resulting composite diaphragm has good thermal stability and wettability.

[0008] Another object of the present invention is to provide a battery.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0010] A composite membrane includes a base membrane and a coating located on at least one side surface of the base membrane, the coating comprising a composite material and a binder, the composite material comprising carbon nanotubes and polyvinylpyrrolidone coating the carbon nanotubes.

[0011] In some embodiments, the mass ratio of the composite material to the binder is (50-70):(20-40).

[0012] In some embodiments, the carbon nanotubes have an average diameter of 40–60 mm and an average length of 1–10 μm.

[0013] In some embodiments, the polyvinylpyrrolidone has a coverage of 80% to 99% on the surface of the carbon nanotubes in the composite material.

[0014] In some embodiments, the adhesive comprises polymethyl methacrylate.

[0015] In some embodiments, the coating further includes a tackifier; the mass ratio of the adhesive to the tackifier is (20-40):(5-10).

[0016] In some embodiments, the coating further includes an additive, which includes at least one of a leveling agent, a defoamer, and a dispersant; the mass ratio of the composite material to the additive is (50-70):(1-5).

[0017] In some embodiments, the coating has a thickness of 0.5–5 μm and a porosity of 60%–90%.

[0018] In some embodiments, the base film includes one or more of the following laminated composites: PE film, PP film, PI film, PET film, and nonwoven membrane.

[0019] In some embodiments, the thickness of the base film is 5–10 μm.

[0020] In some embodiments, the porosity of the base membrane is 30% to 70%.

[0021] The method for preparing the composite membrane as described above includes the following steps:

[0022] A slurry is coated on at least one side of the base membrane to form a wet membrane. The slurry contains a composite material, a binder, and a solvent. The composite material is mainly obtained by mixing and hydrothermal reaction of a mixture system containing acid-modified carbon nanotubes, polyvinylpyrrolidone, and water. The wet membrane is dried to obtain a composite membrane.

[0023] In some embodiments, the mass ratio of the polyvinylpyrrolidone to the acid-modified carbon nanotubes is (200-400):(10-20).

[0024] In some embodiments, the mixture further comprises a surfactant and a dispersant, wherein the mass ratio of the acid-modified carbon nanotubes, the polyvinylpyrrolidone, the surfactant, and the dispersant is (200–400):(10–20):(0.5–5):(0.5–5).

[0025] In some embodiments, the solid content of the mixture is 20% to 30%.

[0026] In some embodiments, the mixing speed is 700–1500 rpm, and the mixing time is 10–60 min.

[0027] In some embodiments, the temperature of the hydrothermal reaction is 150–200°C, and the time of the hydrothermal reaction is 6–12 hours.

[0028] In some embodiments, the method further includes washing and drying the material obtained after the hydrothermal reaction.

[0029] In some embodiments, the preparation method of the acid-modified carbon nanotubes includes: placing carbon nanotubes in a mixed acid of sulfuric acid and nitric acid, wherein the mass ratio of sulfuric acid to nitric acid is (88-92):(7-12), stirring for 2-6 hours, then subjecting to ultrasonic treatment for 0.5-2 hours, and finally washing and drying to obtain acid-modified carbon nanotubes.

[0030] In some embodiments, the mass ratio of the composite material, the binder, and the solvent is (50-70):(20-40):(70-100).

[0031] In some embodiments, the slurry further includes a thickener, and the mass ratio of the composite material to the thickener is (50-70):(5-10).

[0032] In some embodiments, the slurry further includes an additive, which includes at least one of a leveling agent, a defoamer, and a dispersant; the mass ratio of the composite material to the additive is (50-70):(1-5).

[0033] A battery comprising the composite separator described above, or a composite separator prepared by the method described above.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] (1) In the composite membrane of the present invention, the carbon nanotubes in the composite material have high electrical conductivity and good thermal stability, and the polyvinylpyrrolidone coating on the surface can improve the dispersibility and wettability of the carbon nanotubes, thereby making the resulting composite membrane have excellent heat resistance, coating adhesion, liquid storage and wetting properties.

[0036] (2) The method for preparing the composite membrane of the present invention involves a composite material mainly prepared by mixing and hydrothermal reaction of a mixture containing acid-modified carbon nanotubes, polyvinylpyrrolidone (PVP), and water. The PPVP self-assembles into a tubular structure, which modifies the surface of the carbon nanotubes, forming a composite structure of PPVP-coated carbon nanotubes, exhibiting excellent thermal stability and wettability. A slurry containing the composite material, binder, and solvent is coated onto the surface of the base membrane, and after drying, the composite membrane is obtained. This method is simple and easy to implement, and the resulting composite membrane exhibits good thermal resistance, coating adhesion, liquid storage performance, and wettability.

[0037] (3) The battery of the present invention has excellent thermal stability and good safety performance. Attached Figure Description

[0038] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0039] Figure 1 The infrared spectrum of the composite material in Example 1 of this invention;

[0040] Figure 2 Here is a scanning electron microscope image of the composite material in Example 1 of the present invention;

[0041] Figure 3 This is a scanning electron microscope image of the pure carbon nanotubes in Example 1 of the present invention. Detailed Implementation

[0042] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0043] According to one aspect of the present invention, the present invention relates to a composite membrane comprising a base membrane and a coating located on at least one side surface of the base membrane, the coating comprising a composite material and a binder, the composite material comprising carbon nanotubes and polyvinylpyrrolidone (PVP) coating carbon nanotubes (CNTs).

[0044] In the composite membrane of the present invention, the carbon nanotubes in the composite material have high electrical conductivity and good thermal stability, and the polyvinylpyrrolidone coating on the surface can improve the dispersibility and wettability of the carbon nanotubes, thereby giving the obtained composite membrane excellent heat resistance, coating adhesion, liquid storage performance and wetting performance.

[0045] In some embodiments, the mass ratio of the composite material to the binder is (50–70):(20–40). In other embodiments, the mass ratio of the composite material to the binder includes, but is not limited to, 50:20, 55:25, 60:30, 60:40, 65:35, 70:40, or any range between the two. The appropriate mass ratio of the composite material and binder in this invention ensures that the resulting composite diaphragm exhibits superior thermal stability and wettability.

[0046] In some embodiments, the average diameter of the carbon nanotubes is 40–60 mm, including but not limited to 40 mm, 45 mm, 50 mm, 55 mm, 60 mm, or any value between these two ranges. The average length is 1–10 μm, including but not limited to 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value between these two ranges. The carbon nanotubes of the present invention have suitable length and diameter, thereby ensuring the electrical conductivity, thermal stability, and wettability of the composite material, and thus better improving the thermal stability and wettability of the composite membrane.

[0047] In some embodiments, the polyvinylpyrrolidone (PVP) in the composite material has a coverage rate of 80% to 99% on the surface of the carbon nanotubes, including but not limited to 80%, 82%, 85%, 88%, 90%, 92%, 95%, or 99%, or any value between these two ranges. The coverage rate is tested by selecting a specific area on the SEM image of the composite material and calculating the length ratio of the polymer-covered surface to the uncovered surface. In this invention, a suitable coverage rate of PPV in the carbon nanotube surface is more conducive to ensuring the thermal stability, wettability, and electrical conductivity of the composite material.

[0048] In some implementations, the binder includes polymethyl methacrylate (PMMA). Using PMMA as a binder can reduce process requirements and save production costs.

[0049] In some embodiments, the coating further includes a tackifier; the tackifier includes polyethylene oxide. The mass ratio of adhesive to tackifier is (20–40):(5–10), including but not limited to 20:5, 25:6, 30:7, 35:8, 40:10, or any range between the two. Using a suitable proportion of tackifier helps to ensure the bonding effect of the adhesive.

[0050] In some embodiments, the coating further includes an additive, which includes at least one selected from leveling agents, defoamers, and dispersants; the mass ratio of the composite material to the additive is (50–70):(1–5), including but not limited to 50:1, 55:2, 55:2.5, 60:3, 70:5, or any range between the two. In some embodiments, the defoamer includes polydimethylsiloxane. The leveling agent includes propylene glycol methyl ether acetate. The dispersant includes an organofluorine dispersant. In some embodiments, the mass ratio of the leveling agent, defoamer, and dispersant is (0.8–1.2):(0.8–1.2):(0.8–1.2), for example, 0.8:1:1, 1:1:1, 1.2:1:1, etc.

[0051] In some embodiments, the base film includes one or more laminated composites of PE film, PP film, PI film, PET film, and nonwoven membrane, such as PE / PP composite film, PI / PET composite film, and PP / PE / PP composite film. In some embodiments, the thickness of the base film is 5–10 μm, including but not limited to 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value between two values. In some embodiments, the porosity of the base film is 30%–70%, including but not limited to 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or any value between two values.

[0052] In some embodiments, the thickness of the coating is 0.5–5 μm, including but not limited to 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 4 μm, 5 μm, or any value between two of these. The porosity of the coating is 60%–90%, including but not limited to 60%, 65%, 70%, 75%, 80%, 85%, 90%, or any value between two of these. The coating of the present invention has suitable thickness and porosity to ensure the overall performance of the coating, giving the composite diaphragm better thermal stability and wettability.

[0053] According to another aspect of the present invention, the present invention relates to a method for preparing the composite diaphragm, comprising the following steps:

[0054] A slurry is coated on at least one side of the base membrane to form a wet membrane. The slurry contains a composite material, a binder, and a solvent. The composite material is mainly obtained by mixing and hydrothermal reaction of a mixture system containing acid-modified carbon nanotubes, polyvinylpyrrolidone, and water. The wet membrane is dried to obtain a composite membrane.

[0055] The method for preparing the composite membrane of this invention involves a composite material mainly prepared by mixing and hydrothermal reaction of a mixture containing acid-modified carbon nanotubes, polyvinylpyrrolidone (PVP), and water. PVP readily self-assembles into tubular structures, modifying the surface of the carbon nanotubes. The PVP then reassembles into a composite structure with PVP coating CNTs, exhibiting excellent thermal stability and wettability. A slurry containing the composite material, binder, and solvent is coated onto the surface of a base membrane, and after drying, the composite membrane is obtained. This method is simple and easy to implement, yielding a composite membrane with excellent thermal resistance, coating adhesion, liquid storage performance, and wetting properties.

[0056] In some embodiments, the preparation method of the acid-modified carbon nanotubes includes: placing carbon nanotubes in a mixed acid of sulfuric acid and nitric acid, wherein the mass ratio of sulfuric acid to nitric acid is (88-92):(7-12), including but not limited to 88:7, 90:10, 92:12, or any range between the two. The reaction is stirred for 2-6 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, etc., followed by ultrasonic treatment for 0.5-2 hours, for example, 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, etc., and then washed and dried to obtain acid-modified carbon nanotubes. Deionized water is used for washing, and the number of washing cycles is 5-8. After solid-liquid separation, drying is performed at a temperature of 60-90°C for 8-15 hours. Surface modification of carbon nanotubes is mainly because carbon nanotubes are prone to agglomeration, and direct application results in poor coating consistency. After surface modification, the agglomeration amount is relatively uniform because the surface has carboxyl groups, which exert a certain repulsive force when the agglomeration amount is large.

[0057] In some embodiments, the mass ratio of polyvinylpyrrolidone (PVP) to acid-modified carbon nanotubes is (200–400):(10–20), including but not limited to 200:10, 250:12, 280:15, 300:16, 350:18, and 400:20. A suitable ratio of PPVP to acid-modified carbon nanotubes ensures that PPVP forms a suitable coating on the surface of the acid-modified carbon nanotubes, resulting in a composite material with excellent thermal stability and wettability.

[0058] In some embodiments, the mixture further comprises a surfactant and a dispersant. The surfactant includes sodium dodecyl sulfonate, and the dispersant includes carboxymethyl cellulose. In some embodiments, the mass ratio of the acid-modified carbon nanotubes, the polyvinylpyrrolidone, the surfactant, and the dispersant is (200–400):(10–20):(0.5–5):(0.5–5), for example, 200:10:0.5:0.5, 250:12:1:1, 300:15:2:2, 350:16:3:4, 400:20:5:5, etc. A suitable mass ratio of acid-modified carbon nanotubes, polyvinylpyrrolidone, surfactant, and dispersant is more conducive to the hydrothermal reaction.

[0059] In some embodiments, the solid content of the mixture is 20% to 30%, including but not limited to 20%, 21%, 22%, 25%, 28% or 30%, or any value in between.

[0060] In some implementations, the solvent includes water, such as deionized water.

[0061] In some embodiments, the mixing speed is 700–1500 rpm, such as 700 rpm, 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, 1200 rpm, 1300 rpm, 1400 rpm, or 1500 rpm, or any range of both. In some embodiments, the mixing time is 10–60 min, such as 10 min, 20 min, 30 min, 40 min, 50 min, or 60 min, or any range between both. Suitable mixing conditions ensure thorough mixing of the materials, which is beneficial for subsequent hydrothermal reactions.

[0062] In some embodiments, the hydrothermal reaction temperature is 150–200°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C, or 200°C, or any range between the two. The hydrothermal reaction time is 6–12 hours, for example, 6 hours, 7 hours, 8 hours, 10 hours, or 12 hours, or any range between the two. Suitable hydrothermal reaction conditions are beneficial for the composite formation of acid-modified carbon nanotubes and polyvinylpyrrolidone.

[0063] In some embodiments, the process further includes washing and drying the material obtained after the hydrothermal reaction. Washing is performed sequentially with water and ethanol, 5 to 8 times, followed by filtration. Drying is carried out at 60 to 80°C for 8 to 15 hours.

[0064] In some embodiments, the mass ratio of the composite material, the binder, and the solvent is (50–70):(20–40):(70–100). In some embodiments, the mass ratio of the composite material, the binder, and the solvent is 50:20:70, 55:25:80, 60:30:80, 70:40:100, etc.

[0065] In some embodiments, the slurry further includes a tackifier, and the mass ratio of the adhesive to the tackifier is (20-40):(5-10), including but not limited to 20:5, 25:6, 30:7, 35:8, 40:10, or any range between the two. A suitable proportion of tackifier is used to ensure the bonding effect of the adhesive.

[0066] In some embodiments, the slurry further comprises additives, including at least one of leveling agents, defoamers, and dispersants; the mass ratio of the composite material to the additives is (50-70):(1-5), including but not limited to 50:1, 55:2, 55:2.5, 60:3, 70:5, or any range between the two. In some embodiments, the defoamer includes polydimethylsiloxane, which helps to suppress foam. The leveling agent includes propylene glycol methyl ether acetate, which helps to improve the leveling and uniformity of the slurry, improve the film-forming performance of the slurry coating, and make the coating smooth and uniform. The dispersant includes organic fluorine dispersants, which facilitate the uniform dispersion of materials in the slurry. Appropriate amounts of additives can effectively improve the physicochemical properties of the slurry, thereby improving the physicochemical properties of the coating.

[0067] In a preferred embodiment, the method for preparing the composite diaphragm includes the following steps:

[0068] (1) Place carbon nanotubes in a mixture of sulfuric acid and nitric acid, with a mass ratio of sulfuric acid to nitric acid of (88-92):(7-12), stir for 2-6 hours, then sonicate for 0.5-2 hours, wash 5-8 times, and dry at 60-90℃ for 8-15 hours to obtain acid-modified carbon nanotubes.

[0069] (2) A mixture of acid-modified carbon nanotubes, polyvinylpyrrolidone, surfactant, dispersant and solvent is mixed. The mass ratio of acid-modified carbon nanotubes, polyvinylpyrrolidone, surfactant and dispersant is (200-400):(10-20):(0.5-5):(0.5-5). The mixing speed is 700-1500 rpm and the mixing time is 10-60 min. Then it is transferred to a hydrothermal reactor for mechanical-hydrothermal reaction. The hydrothermal reaction temperature is 150-200℃ and the hydrothermal reaction time is 6-12 h. After the reaction, it is cooled to room temperature and then washed and dried. The washing is carried out with water and ethanol in sequence, and the number of washings is 5-8. Then it is filtered. The filtered solid material is dried at 60-80℃ for 8-15 h to obtain the composite material.

[0070] (3) Mix the composite material, PMMA, tackifier and additives (including leveling agent, defoamer and dispersant) into deionized water. The mass ratio of composite material, PMMA, tackifier, additive and water is (50~70):(20~40):(5~10):(1~5):(70~100). Stir thoroughly until the mixture is uniform to obtain the coating slurry.

[0071] (4) Coating slurry is applied to at least one side of the base film and dried to obtain a composite diaphragm.

[0072] In some implementations, the reaction of acid-modified carbon nanotubes is as follows:

[0073]

[0074] In some implementations, the composite material has the following structure:

[0075]

[0076] According to one aspect of the present invention, the present invention relates to a battery comprising the aforementioned composite separator, or a composite separator prepared by the method for preparing the aforementioned composite separator.

[0077] The battery of this invention has superior cycle performance, thermal stability, and safety performance.

[0078] The following explanation, in conjunction with specific embodiments, further clarifies the situation.

[0079] Example 1

[0080] A method for preparing a composite membrane includes the following steps:

[0081] (1) Carbon nanotubes with an average diameter of 50 mm and an average length of 6 μm, a 75% sulfuric acid solution, and a 60% nitric acid solution were mixed at a mass ratio of 3:90:7. The mixture was stirred at 150 rpm for 5 h at room temperature and then ultrasonically dispersed for 1 h. The mixture was then washed with deionized water in small amounts several times and filtered, repeated 6 times. The sample was then dried in a vacuum drying oven at 80 °C for 12 h to obtain acid-modified carbon nanotubes.

[0082] (2) PVP (PVPK25), acid-modified carbon nanotubes, sodium dodecyl sulfonate and carboxymethyl cellulose were mixed in a mass ratio of 320:15:3:3. A certain amount of deionized water was added, and the solid content was 25%. The mixture was stirred at 1000 rpm for 30 min in a planetary vacuum stirrer. The mixture was then transferred to a hydrothermal reactor. After assembly, it was placed in an oven at 165℃ and kept warm for 8 h. After the reaction, it was cooled to room temperature. The mixture in the reactor was washed with water and ethanol in sequence and filtered. This process was repeated 6 times. The sample was then placed in a vacuum drying oven at 70℃ and dried for 12 h. After drying, the composite material (PVP-CNTs) was obtained.

[0083] (3) Mix PVP-CNTs, PMMA, tackifier (polyoxyethylene) and additives, add them to deionized water. The additives are leveling agent, defoamer and dispersant in a mass ratio of 1:1:1. The mass ratio of PVP-CNTs, PMMA, tackifier, additives and water is 50:35:10:5:75. Stir thoroughly until the mixture is uniform to obtain the coating slurry.

[0084] (4) The coating slurry was coated on the surface of a 7μm PE base film using micro-gravure coating technology. The porosity of the PE base film was 41%, the coating thickness was 2μm, and after drying, a composite lithium-ion battery separator was obtained with a thickness of 9μm.

[0085] Example 2

[0086] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0087] The mass ratio of PVP-CNTs, PMMA, polyethylene oxide, and additives was adjusted to 70:17:10:3.

[0088] Example 3

[0089] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0090] The mass ratio of PVP-CNTs, PMMA, polyethylene oxide, and additives was adjusted to 60:30:8:2.

[0091] Example 4

[0092] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0093] In step (2), PVP (PVPK25), modified CNTs, sodium dodecyl sulfonate and carboxymethyl cellulose are mixed in a mass ratio of 260:14:2.5:2.5.

[0094] Example 5

[0095] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0096] In step (2), PVP (PVPK25), modified CNTs, sodium dodecyl sulfonate and carboxymethyl cellulose are mixed in a mass ratio of 380:17:3.8:4.

[0097] Example 6

[0098] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0099] In step (4), the coating slurry is coated on the surface of the 9μm composite base film using microgravure coating technology. The composite base film is a PP / PE / PP film with a porosity of 45% and a coating thickness of 3.5μm. After drying, a composite lithium-ion battery separator is obtained with a thickness of 12.5μm.

[0100] Example 7

[0101] A method for preparing a composite membrane includes the following steps:

[0102] (1) Carbon nanotubes with an average diameter of 60 mm and an average length of 8 μm, a 75% sulfuric acid solution, and a 60% nitric acid solution were mixed at a mass ratio of 3:90:7. The mixture was stirred at 2000 rpm for 3 h at room temperature and then ultrasonically dispersed for 0.5 h. The mixture was then washed with deionized water in small amounts several times and filtered, repeated 8 times. The sample was then dried in a vacuum drying oven at 90 °C for 10 h to obtain acid-modified carbon nanotubes.

[0103] (2) PVP (PVPK30), acid-modified carbon nanotubes, sodium dodecyl sulfonate and carboxymethyl cellulose were mixed in a mass ratio of 400:20:5:5. A certain amount of deionized water was added, and the solid content was 30%. The mixture was stirred at 1500 rpm for 20 min in a planetary vacuum stirrer. After that, it was transferred to a hydrothermal reactor. After assembly, it was placed in an oven at 200℃ and kept warm for 6 h. After that, it was cooled to room temperature. The mixture in the reactor was washed with water and ethanol in turn and filtered. This process was repeated 8 times. Then the sample was placed in a vacuum drying oven at 80℃ and dried for 8 h. After drying, the composite material (PVP-CNTs) was obtained.

[0104] (3) Mix PVP-CNTs, PMMA, tackifier (polyoxyethylene) and additives, add them to deionized water. The additives are leveling agent, defoamer and dispersant in a mass ratio of 1:1:1. The mass ratio of PVP-CNTs, PMMA, tackifier, additives and water is 70:40:10:5:100. Stir thoroughly until the mixture is uniform to obtain the coating slurry.

[0105] (4) The coating slurry was coated on the surface of a 7μm PP base film using micro-gravure coating technology. The porosity of the PP base film was 50%, the coating thickness was 2μm, and after drying, a composite lithium-ion battery separator was obtained with a thickness of 9μm.

[0106] Example 8

[0107] A method for preparing a composite membrane includes the following steps:

[0108] (1) Carbon nanotubes with an average diameter of 40 mm and an average length of 4 μm, a 75% sulfuric acid solution, and a 60% nitric acid solution were mixed at a mass ratio of 3:90:7. The mixture was stirred at 1000 rpm for 6 h at room temperature and then ultrasonically dispersed for 1.5 h. The mixture was then washed with deionized water in small amounts several times and filtered, repeated 6 times. The sample was then dried in a vacuum drying oven at 90 °C for 10 h to obtain acid-modified carbon nanotubes.

[0109] (2) PVP (PVPK30), acid-modified carbon nanotubes, sodium dodecyl sulfonate and carboxymethyl cellulose were mixed in a mass ratio of 200:10:0.5:0.5, a certain amount of deionized water was added, the solid content was 20%, and the mixture was stirred at 800 rpm for 40 min in a planetary vacuum stirrer. The mixture was then transferred to a hydrothermal reactor, assembled and placed in an oven at 150 ℃ for 10 h. After the reaction was completed, the mixture was cooled to room temperature. The mixture in the reactor was washed with water and ethanol in sequence and filtered. This process was repeated 6 times. The sample was then placed in a vacuum drying oven at 60 ℃ for 12 h. After drying, the composite material (PVP-CNTs) was obtained.

[0110] (3) Mix PVP-CNTs, PMMA, tackifier (polyoxyethylene) and additives, add them to deionized water. The additives are leveling agent, defoamer and dispersant in a mass ratio of 1:1:1. The mass ratio of PVP-CNTs, PMMA, tackifier, additives and water is 65:25:8:2:85. Stir thoroughly until the mixture is uniform to obtain the coating slurry.

[0111] (4) The coating slurry was coated on the surface of a 7μm PET base film using micro-gravure coating technology. The porosity of the PET base film was 55%, the coating thickness was 2μm, and after drying, a composite lithium-ion battery separator was obtained with a thickness of 9μm.

[0112] Comparative Example 1

[0113] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0114] Replace PVP-CNTs with ceramic.

[0115] Comparative Example 2

[0116] A method for preparing a composite diaphragm differs from that in Example 1 in that:

[0117] Replace PVP-CNTs with a mixture of ordinary PVP and CNTs.

[0118] Experimental Example

[0119] I. Spectral Analysis

[0120] The infrared spectrum of the composite material in Example 1 of this invention is as follows: Figure 1 As shown, the broad and strong absorption peaks appearing in the 3200–3600 cm⁻¹ region of the infrared spectrum are attributed to stretching vibration peaks caused by associated hydroxyl groups. The composite material PVP-CNTs exhibits a peak at 2926 cm⁻¹. -1 and 2850cm -1 The characteristic absorption peak of the typical methylene-CH2- carbon-hydrogen bond stretching vibration appeared at 1665 cm⁻¹, which can be attributed to the -CH2- in the PVP molecular chain; -1 An absorption peak for the carbonyl stretching vibration of PVP appeared at 1425 cm⁻¹; -1 A bending vibration absorption peak of methylene-CH2- was observed nearby. Pure CNTs did not exhibit these characteristic peaks, thus confirming the introduction of PVP onto the CNT surface.

[0121] The scanning electron microscope image of the composite material in Example 1 of this invention is shown below. Figure 2 As shown. A scanning electron microscope image of the pure carbon nanotubes in Example 1 is shown below. Figure 3 As shown, the surface of the carbon nanotubes is coated with PVP.

[0122] II. Performance Testing of Composite Separators

[0123] The performance of the composite membranes obtained in each embodiment and comparative example was tested as follows:

[0124] 1. Heat resistance test

[0125] Heat resistance was tested according to GB / T 12027-2004 "Test method for dimensional change rate of plastic films and sheets under heating".

[0126] 2. Adhesion test

[0127] Adhesion was tested according to GB / T 2792-2014 "Test method for peel strength of adhesive tape".

[0128] 3. Liquid storage performance test

[0129] Take a 10cm×10cm diaphragm and immerse it in a lithium salt electrolyte (EC, EMC and DEC in a mass ratio of 3:5:2, and LiPF6 at 1Mol / L) at 70℃ for 24h. Weigh the diaphragm before and after immersion. The liquid storage rate = mass after immersion / mass before immersion × 100%.

[0130] 4. Wettability test

[0131] Take a 10cm × 0.5cm diaphragm, clamp both ends and fix it to the steel frame at room temperature. Take 2ul of electrolyte (EC:PC:DEC = 1:1:1, volume ratio) and drop it at the center point. Record the electrolyte diffusion distance after 30s.

[0132] The test results are shown in Table 1.

[0133] Table 1 Performance test results of composite membrane

[0134]

[0135] As shown in Table 1, the composite diaphragm obtained by the method of the present invention has excellent heat resistance, coating adhesion, liquid storage and wetting properties; wherein, the thermal shrinkage rate of TD at 130℃ and 1h is less than or equal to 2.5%, and the thermal shrinkage rate of MD is less than or equal to 3%; the peel strength of the coating is greater than or equal to 60N / m, the liquid storage rate is greater than 140%, and the wettability is greater than 1cm.

[0136] The composite membranes of Comparative Examples 1 and 2 have relatively poor heat resistance, coating adhesion, liquid storage performance, and wetting performance.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite diaphragm, characterized in that, The coating includes a base film and a coating located on at least one side of the base film, the coating comprising a composite material and an adhesive, the composite material comprising carbon nanotubes and polyvinylpyrrolidone coating the carbon nanotubes; The carbon nanotubes are acid-modified carbon nanotubes; the acid-modified carbon nanotubes have carboxyl groups on their surface. The mass ratio of the polyvinylpyrrolidone to the acid-modified carbon nanotubes is (200~400):(10~20). The composite material has associated hydroxyl groups in its structure; The porosity of the coating is 60%~90%; The structure of the composite material is as follows: 。 2. The composite diaphragm according to claim 1, characterized in that, Includes at least one of the following features (1) to (4): (1) The mass ratio of the composite material to the adhesive is (50~70):(20~40); (2) The average diameter of the carbon nanotubes is 40~60nm and the average length is 1~10μm; (3) In the composite material, the polyvinylpyrrolidone has a coverage rate of 80% to 99% on the surface of the carbon nanotubes; (4) The adhesive includes polymethyl methacrylate.

3. The composite diaphragm according to claim 1, characterized in that, It includes at least one of the following features (1) to (2): (1) The coating further includes a tackifier; the mass ratio of the adhesive to the tackifier is (20~40):(5~10); (2) The coating further includes additives, including at least one of leveling agent, defoamer and dispersant; the mass ratio of the composite material and the additive is (50~70):(1~5).

4. The composite diaphragm according to claim 1, characterized in that, It includes at least one of the following features (1) to (4): (1) The thickness of the coating is 0.5~5μm; (2) The base film includes one or more of the following: PE film, PP film, PI film, PET film, and non-woven membrane; (3) The thickness of the base film is 5~10μm; (4) The porosity of the base membrane is 30%~70%.

5. The method for preparing the composite separator according to any one of claims 1 to 4, characterized in that, Includes the following steps: A slurry is coated on at least one side of the base membrane to form a wet membrane. The slurry contains a composite material, a binder, and a solvent. The composite material is mainly obtained by mixing and hydrothermal reaction of a mixture system containing acid-modified carbon nanotubes, polyvinylpyrrolidone, and water. The wet membrane is dried to obtain a composite membrane.

6. The method for preparing the composite diaphragm according to claim 5, characterized in that, The mixture also contains a surfactant and a dispersant, wherein the mass ratio of the polyvinylpyrrolidone, the acid-modified carbon nanotubes, the surfactant and the dispersant is (200~400):(10~20):(0.5~5):(0.5~5).

7. The method for preparing the composite diaphragm according to claim 5, characterized in that, It includes at least one of the following features (1) to (5): (1) The solid content of the mixture is 20%~30%; (2) The mixing speed is 700~1500 rpm, and the mixing time is 10~60 min; (3) The temperature of the hydrothermal reaction is 150~200℃, and the time of the hydrothermal reaction is 6~12h; (4) It also includes: washing and drying the material obtained after the hydrothermal reaction; (5) The preparation method of the acid-modified carbon nanotubes includes: placing the carbon nanotubes in a mixed acid of sulfuric acid and nitric acid, wherein the mass ratio of sulfuric acid to nitric acid is (88~92):(7~12), stirring for 2~6 hours, then ultrasonically treating for 0.5~2 hours, and then washing and drying to obtain acid-modified carbon nanotubes.

8. The method for preparing the composite diaphragm according to claim 5, characterized in that, The mass ratio of the composite material, the binder and the solvent is (50~70):(20~40):(70~100).

9. The method for preparing the composite diaphragm according to claim 5, characterized in that, It includes at least one of the following features (1) to (2): (1) The slurry also contains a thickener, and the mass ratio of the composite material to the thickener is (50~70):(5~10). (2) The slurry also contains additives, which include at least one of leveling agent, defoamer and dispersant; the mass ratio of the composite material to the additive is (50~70):(1~5).

10. A battery, characterized in that, The composite membrane comprising any one of claims 1 to 4, or the composite membrane prepared by any one of claims 5 to 9.

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