A high-heat-resistance high-breakdown-field lithium battery composite diaphragm and a preparation method thereof

Abstract

The application discloses a lithium battery composite diaphragm with high heat resistance and high breakdown field strength and a preparation method thereof, and belongs to the technical field of lithium battery composite diaphragms. The diaphragm comprises a flexible continuous glass fiber framework and a three-dimensional cross-linked organic polymer coating layer, the flexible continuous glass fiber framework is embedded in the three-dimensional cross-linked organic polymer coating layer, and the two are combined into one, thus avoiding the problems of inorganic coating layer falling off and powder falling of a traditional inorganic coated diaphragm, and making the composite diaphragm have the characteristics of high modulus and high porosity; the flexible continuous glass fiber framework and the three-dimensional cross-linked organic polymer coating layer both have high heat resistance, so that the heat resistance of the composite diaphragm is better; and the three-dimensional cross-linked organic functional layer has good electric insulation performance, and gives the composite diaphragm very high breakdown field strength.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery separator technology, specifically relating to a high heat resistance and high breakdown field strength lithium battery composite separator and its preparation method. Background Technology

[0002] A separator is an electrically insulating thin film with a microporous structure. Its function is to prevent short circuits between the positive and negative electrodes and to conduct ions, acting as a critical safety component in secondary batteries such as lithium-ion batteries. Commonly used lithium-ion battery separators are polyolefin separators such as polyethylene (PE) and polypropylene (PP). These separators have advantages such as excellent chemical stability, good tensile strength, and low cost, but they also have problems such as poor heat resistance, low modulus, and low breakdown field strength. Under the harsh operating conditions of large-scale energy storage and electric vehicles, especially at high temperatures and high voltages, the corresponding battery separators are prone to structural degradation and even failure, such as softening and shrinkage, melting, and electrical breakdown, severely restricting the reliability and safety of lithium-ion batteries.

[0003] To improve the thermal stability of diaphragms, a common strategy is to develop novel diaphragms using high-heat-resistant and electrically insulating polymers such as polyimide, meta-aramid, polyetheretherketone, and polybenzimidazole. However, the inherent processing difficulties and high costs of these heat-resistant polymers hinder their large-scale application. A second strategy involves developing coated diaphragms, which involve coating both sides of a commercial polyolefin diaphragm with inorganic particles such as alumina or boehmite, or polymers such as polyvinylidene fluoride, to form a composite diaphragm and improve its thermal stability. While this method improves the high-temperature resistance of the diaphragm to some extent, its maximum operating temperature remains below 150°C. Furthermore, it suffers from structural instability due to poor adhesion to the substrate, leading to coating powdering and deterioration of the breakdown field strength. Therefore, significant reliability and safety risks remain during use.

[0004] In summary, it is necessary to design and develop a novel lithium battery separator based on entirely new materials that exhibits excellent high-temperature and high-voltage electrochemical performance and is easy to prepare. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a high-heat-resistant, high-breakdown-field-strength lithium battery composite separator and its preparation method, thereby solving the problems of poor high-temperature resistance, voltage resistance, and structural stability in existing separator technologies.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A high heat-resistant and high breakdown field strength lithium battery composite separator, the separator comprising a flexible continuous glass fiber skeleton and a three-dimensional cross-linked organic polymer coating layer, wherein the flexible continuous glass fiber skeleton is embedded in the three-dimensional cross-linked organic polymer coating layer;

[0008] The flexible continuous glass fiber skeleton is one or both of flexible continuous nano glass fiber cloth and flexible alkali-free ultrafine glass fiber cloth.

[0009] The flexible continuous nanofiber cloth is amorphous SiO2.

[0010] A further improvement of the present invention is that:

[0011] Preferably, the flexible continuous nanofiber cloth is composed of multiple fibers with a fiber diameter of 100-400 nm and a thickness of 5-50 μm.

[0012] Preferably, the flexible continuous nanofiber cloth has an oriented distribution structure or a disordered nonwoven structure.

[0013] Preferably, the diameter of the fibers in the flexible alkali-free ultrafine glass fiber cloth is 0.1-8μm, and the thickness of the flexible alkali-free ultrafine glass fiber cloth is 5-50μm.

[0014] A method for preparing the above-mentioned high heat resistance and high breakdown field strength lithium battery composite separator includes the following steps:

[0015] Step 1: Select flexible continuous nanofiber cloth or flexible alkali-free ultrafine fiber cloth as the flexible continuous fiber skeleton;

[0016] Step 2: Mix the prepolymer, photoinitiator, and pore-forming agent to obtain the impregnation solution;

[0017] Step 3: Place the flexible continuous glass fiber skeleton on the substrate, and drop the impregnation liquid onto the flexible continuous glass fiber skeleton, so that the flexible continuous glass fiber skeleton is immersed in the impregnation liquid;

[0018] Step 4: Irradiate the impregnation liquid and the flexible continuous glass fiber skeleton with a light source. After the photovoltaic crosslinking and curing polymer in the impregnation liquid is cured, a three-dimensional crosslinked organic polymer coating layer is obtained. The flexible continuous glass fiber skeleton is embedded in the three-dimensional crosslinked organic polymer coating layer, which is a lithium battery composite separator with high heat resistance and high breakdown field strength.

[0019] Preferably, in step 1, the preparation method of the flexible continuous nanofiber cloth is as follows: tetraethyl silicate, dilute hydrochloric acid and ethanol are mixed and stirred to obtain solution A; N,N-dimethylformamide, dimethyl sulfoxide and polyvinylpyrrolidone are mixed and stirred to obtain solution B; solution A and solution B are mixed and stirred to obtain spinning solution; the spinning solution is electrospinned or solution air spinning to obtain precursor film; the precursor film is annealed in air to obtain flexible continuous nanofiber cloth.

[0020] Preferably, the annealing temperature is 650℃-850℃.

[0021] Preferably, during the electrospinning process, the structure obtained by the flat plate collector is a disordered nonwoven structure, while the structure obtained by the rotating roller is an oriented distribution structure.

[0022] Preferably, in step 5, a solvent is also added to the impregnation solution.

[0023] Preferably, the prepolymer is one or a combination of polyacrylate, epoxy resin, and polyurethane.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention discloses a high heat resistance and high breakdown field strength lithium battery composite separator. The separator includes a flexible continuous glass fiber skeleton and a light-irradiated cross-linked and cured polymer. The flexible continuous glass fiber skeleton is embedded in a three-dimensional cross-linked organic polymer coating layer, which is "flexible continuous glass fiber skeleton" + "three-dimensional cross-linked organic functional layer coating". This composite membrane has the following advantages: First, the flexible continuous glass fiber skeleton is a flexible material. Because the flexible continuous glass fiber skeleton is embedded in the photo-irradiated cross-linked and cured polymer, the two have good bonding, making the membrane itself a single unit. The entire membrane continuous glass fiber skeleton has the characteristics of high modulus and high porosity. Second, the three-dimensional cross-linked organic polymer coating layer is a photocurable material. Both the flexible continuous glass fiber skeleton and the three-dimensional cross-linked organic polymer coating layer have high heat resistance, which increases the heat resistance temperature of the composite membrane to nearly 350℃. Third, the three-dimensional cross-linked organic polymer coating layer has good electrical insulation properties, giving the composite membrane a very high breakdown field strength (>140kV / mm), which is a significant improvement over commercial polyolefin membranes (~50kV / mm). Fourth, the three-dimensional cross-linked organic polymer coating layer has the characteristics of electrochemical stability, adjustable pore size, good adhesion to electrodes, and rapid curing at room temperature, which makes the composite membrane have good electrochemical performance and excellent ease of preparation. The separator prepared by this invention can meet the requirements of high structural stability, high safety, high reliability, and excellent mechanical properties of lithium batteries. It also has good electrochemical performance and a simple preparation process, making it highly valuable for applications in large-scale energy storage and electric vehicles.

[0026] This invention also discloses a method for preparing a high-heat-resistant and high-breakdown field-strength lithium battery composite separator. The preparation method includes the following steps: (1) selecting flexible continuous nanofiber cloth or flexible alkali-free ultrafine fiber cloth as a flexible continuous fiber skeleton; (2) preparing a high-temperature and high-voltage resistant polymer prepolymer impregnation solution that can be cross-linked and cured by photoirradiation; (3) using the flexible continuous nanofiber cloth in step (1) as the separator skeleton, fully impregnating it with the impregnation solution in step (2), and then irradiating and curing it into a film to obtain a high-temperature and high-voltage resistant lithium battery composite separator. In the photocuring process, the small molecules in the prepolymer can undergo an in-situ bonding reaction with the hydroxyl groups on the surface of the flexible continuous fiber skeleton, resulting in a strong bonding force between the two. The separator finally produced can avoid the problems of inorganic coating peeling and powder shedding. Attached Figure Description

[0027] Figure 1 A scanning electron microscope image of a typical composite diaphragm;

[0028] Figure 2 A comparison of the heat resistance of a typical composite membrane and a commercial polyolefin membrane. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings:

[0030] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0031] In this article, unless otherwise specified, the terms “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of”. For example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a”.

[0032] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0033] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0034] This invention discloses a high heat resistance and high breakdown field strength lithium battery composite separator, which is composed of a flexible continuous glass fiber skeleton and a three-dimensional cross-linked organic polymer coating layer.

[0035] Preferably, the flexible continuous glass fiber skeleton is composed of one or both of flexible continuous nanofiber cloth and flexible alkali-free ultrafine glass fiber cloth.

[0036] The flexible continuous nanofiberglass cloth is an amorphous SiO2 obtained by electrospinning or solution gas spinning to form a film from a spinning solution, followed by annealing. The flexible alkali-free ultrafine fiberglass cloth, or simply alkali-free fiberglass cloth, is a flexible material made from alkali-free glass fiber yarn through a special process. The alkali-free glass fiber has extremely low alkali metal oxide content, and its chemical composition is mainly aluminum borosilicate. The flexible alkali-free ultrafine fiberglass cloth of this invention can be prepared in a laboratory or purchased directly.

[0037] Preferably, the flexible continuous nanofiber cloth has a diameter of 100-400 nm and a thickness of 5-50 μm, and its structure is an oriented distribution structure or a disordered nonwoven structure. The smaller the diameter of the flexible continuous nanofiber cloth, the better its macroscopic flexibility, and the higher its tensile strength and modulus, which is more conducive to the adjustment of membrane porosity. However, the preparation difficulty is greater. The fiber preparation process involved in this invention can produce fibers of about 100-400 nm in batches while ensuring flexibility.

[0038] Preferably, the flexible alkali-free ultrafine glass fiber cloth has a diameter of 0.1-8 micrometers, a thickness of 5-50 micrometers, and a positive interlaced structure.

[0039] The thickness of the aforementioned flexible continuous nanofiberglass cloth and flexible alkali-free ultrafine fiberglass cloth is 5-50 micrometers. The thinner the separator, the lower its tensile strength; the thicker it is, the higher its areal density. Too thick a separator is detrimental to the battery's energy density. Therefore, setting the thickness to 5-50 micrometers can basically ensure that the thickness and areal density of the separator meet the basic requirements of battery separators.

[0040] Preferably, the three-dimensional cross-linked organic polymer coating layer is composed of a light-irradiated cross-linked and cured polymer.

[0041] This invention also discloses a method for preparing a high-temperature resistant and high-voltage resistant lithium battery composite separator, comprising the following steps:

[0042] Step 1: Prepare flexible continuous nanofiber cloth as a flexible continuous fiberglass skeleton, or directly stack flexible alkali-free ultrafine fiberglass cloth as a flexible continuous fiberglass skeleton.

[0043] Step 2: Mix the light-irradiated crosslinking and curing polymer raw materials evenly to obtain an impregnation solution. The light-irradiated crosslinking and curing polymer raw materials consist of a prepolymer, a photoinitiator, and a pore-forming agent.

[0044] Step 3: Lay the flexible continuous nanofiber cloth or flexible alkali-free ultrafine fiber cloth stacked on the substrate, and drop the impregnation liquid obtained in step 2 onto the flexible continuous fiber skeleton, so that it completely wets the fiber skeleton.

[0045] Step 4: Irradiate it under a high-energy light source to solidify it, thus obtaining a high-temperature and high-voltage resistant lithium battery composite separator.

[0046] In the above process, a flexible continuous glass fiber skeleton is placed on a substrate. After the flexible continuous glass fiber skeleton is completely impregnated and wrapped with an impregnation liquid with a light-irradiated cross-linking and curing polymer as the main raw material, the light-irradiated cross-linking and curing polymer can react in situ and undergo photocuring under light conditions to form a lithium battery composite separator.

[0047] In some embodiments of the present invention, the flexible continuous nanofiber cloth in step 1 is obtained by electrospinning or solution gas spinning from a spinning solution, followed by air annealing at a temperature of 650℃-850℃. During annealing, the carrier polymer in the original spun fiber PVP-polysiloxane is removed, leaving only Si and O inorganic components. Simultaneously, the -Si-O-Si-inorganic network is reconstructed during annealing, forming amorphous SiO2. The annealing temperature and procedure are crucial to the flexibility and amorphous structure of the fiber. Annealing temperatures above 1000℃ may cause fiber crystallization, while annealing temperatures that are too low are detrimental to carrier polymer removal and -Si-O inorganic network reconstruction.

[0048] The annealing process of this invention is divided into three stages: the annealing temperature is gradually increased from low to high, and the temperature is slowly heated to a first set temperature (about 200°C) and held for a set time; the temperature is then increased from the first set temperature to a second set temperature (about 400°C) and held for a set time; and the temperature is then increased from the second set temperature to a third set temperature (about 600°C) and held for a set time. The first stage can remove residual unreacted TEOS, the second stage can remove the polymer carrier and cause fiber refinement (diameter reduction), and the third stage can completely transform the inorganic precursor into amorphous SiO2 fibers.

[0049] In one specific embodiment, the annealing stage is divided into three phases: 1) heating from 20°C to 180°C at a heating rate of 1°C / min, and holding at 180°C for 2 hours; 2) heating from 180°C to 325°C at a heating rate of 1°C / min, and holding at 325°C for 2 hours; 3) heating from 325°C to 750°C at a heating rate of 1°C / min, and holding at 750°C for 2 hours, ending the heating process. During this process, the holding at 180°C is to remove residual unreacted TEOS, the holding at approximately 325°C is to remove the polymer carrier and cause fiber refinement (diameter reduction), and the holding at approximately 750°C is to completely transform the inorganic precursor fibers into amorphous SiO2 fibers.

[0050] In one specific embodiment, electrospinning specifically includes the following steps:

[0051] Step 1.1, prepare the spinning solution;

[0052] Tetraethyl silicate (TEOS), dilute hydrochloric acid (HCl, prepared by adding three drops of concentrated hydrochloric acid to 25 ml of deionized water), and ethanol (EtOH) were mixed and stirred at room temperature for 12 h to allow TEOS to fully hydrolyze and obtain solution A. A measured amount of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) solvent were weighed out, and a certain amount of polyvinylpyrrolidone (PVP) was added to the above mixed solvent. The mixture was stirred thoroughly until PVP was completely dissolved to obtain solution B. Solution A was slowly added to solution B, and the mixture was stirred thoroughly for 1 h to obtain the spinning solution. The mass fractions of TEOS, PVP, HCl, EtOH, DMF, and DMSO were 13.3%, 9%, 5.3%, 2.9%, 53.5%, and 16%, respectively.

[0053] Step 1.2: Obtain the precursor membrane by electrospinning.

[0054] The spinning solution is loaded into a syringe, and spinning is performed through a flat plate collector or a rotating drum to obtain an electrospun precursor membrane. The electrospun precursor membrane obtained by electrospinning in this invention is a sheet-like material formed by web formation and bonding. Unlike conventional chopped yarns, the yarns in the precursor membrane prepared by this invention are long filaments. The fibers in the precursor membrane prepared by this invention are continuous fibers, which, after annealing, can yield continuous flexible glass fibers with an infinite aspect ratio.

[0055] The structure obtained through the plate collector is a disordered nonwoven structure, which refers to a structure in which the fiber arrangement is random, disordered, or disordered.

[0056] The electrospun precursor film obtained by rotating drum spinning has an oriented distribution structure, which refers to the specific alignment direction formed by the fibers during the preparation process. By controlling the electrospinning process parameters and the collection device, nanofibers with different orientation distributions can be obtained. These fibers can exhibit various orientation forms such as random arrangement, parallel arrangement, and cross arrangement. In the specific preparation process, a precursor film is first obtained by rotating the drum, and a precursor film is collected after a set time interval. The precursor films are then orthogonally stacked according to the set fiber orientation direction to obtain a precursor film with an orthogonal interwoven structure.

[0057] Step 1.3: Anneal the precursor membrane at a temperature of 650℃-850℃.

[0058] In some embodiments of the present invention, the prepolymer in step 2 is one or a combination of polyacrylate, epoxy resin, and polyurethane.

[0059] In some embodiments of the present invention, the photoinitiator in step 2 is a free radical initiator (such as 369, 819 and TPO, etc.), a cationic initiator (such as UV16976 and UV16922, etc.), or a macromolecular initiator (such as SR1130 and OmnipolTX), and the initiator ratio is 0.1-5wt%.

[0060] In some embodiments of the present invention, a solvent can also be added to the impregnation solution. When the viscosity of the impregnation solution is high, the solvent is added to adjust the viscosity.

[0061] In some embodiments of the present invention, the mass fraction of the prepolymer in step 2 is 30-80%, the mass fraction of the pore-forming agent is 30-80 wt%, and the mass fraction of the solvent is 10-50 wt%. The present invention does not impose any particular limitation on the types of solvent and pore-forming agent in step 2. However, it should be understood that the pore-forming agent, photoinitiator, and solvent should all be compatible with the prepolymer to ensure that the prepolymer can be successfully photocured. Since the flexible continuous glass fiber skeleton itself has a certain amount of porosity, introducing a pore-forming agent during the photocuring process can further adjust the pore size and number in the three-dimensional cross-linked organic polymer coating layer based on the pores of the flexible continuous glass fiber skeleton and the inorganic skeleton, making the pores uniform and adjustable, the pore distribution more uniform, meeting the set requirements, and the pores formed by the pore-forming agent are smaller and more uniform than those formed by the inorganic skeleton.

[0062] In some embodiments of the present invention, the substrate in step 3 is a rigid, high-transmittance, and high-flatness board, made of quartz glass, plexiglass, or high-transmittance resin board.

[0063] It should be understood that when the impregnation solution is added to the flexible continuous glass fiber skeleton, it is necessary to ensure that the impregnation solution can completely cover and penetrate the flexible continuous glass fiber skeleton so that all of the flexible continuous glass fiber skeleton is in the impregnation solution.

[0064] Preferably, the high-energy light source in step 4 is a high-pressure mercury lamp, an LED ultraviolet light source, or a xenon flash lamp.

[0065] The following description, in conjunction with specific embodiments, provides further details.

[0066] Example 1

[0067] Preparation of flexible continuous nanofiber nanofiber cloth: 7.24g tetraethyl orthosilicate (TEOS), 2.9g dilute hydrochloric acid (prepared by adding three drops of concentrated hydrochloric acid to 25ml deionized water), and 1.55g ethanol were mixed and stirred at room temperature for 12h to fully hydrolyze TEOS and obtain a TEOS solution. 29.18g DMF and 8.75g DMSO were mixed, and 4.9g PVP was added to the mixture. The mixture was stirred thoroughly until the PVP was completely dissolved to obtain a polymer template solution. The TEOS solution was slowly added to the polymer template solution, and the mixture was stirred thoroughly for 1h to obtain the spinning solution required for electrospinning. The spinning solution was loaded into a 20ml syringe for electrospinning. The spinning conditions were: voltage 15kV, distance between the plate collector and the needle 20cm, solution feed rate 1ml / h, temperature 25±3℃, and humidity 20±5%. Based on the electrospinning time, an electrospinning precursor film of the required thickness was prepared. The precursor membrane obtained by spinning was placed in a muffle furnace for pyrolysis. The heating program was as follows: 1) heating from 20℃ to 180℃ at a heating rate of 1℃ / min; 2) holding at 180℃ for 2h; 3) heating from 180℃ to 325℃ at a heating rate of 1℃ / min; 4) holding at 325℃ for 2h; 5) heating from 325℃ to 800℃ at a heating rate of 1℃ / min; 6) holding at 800℃ for 2h; 7) ending the heating process and cooling the membrane to room temperature with the furnace to obtain a flexible continuous electrospun glass fiber nanofiber nonwoven base membrane (EGNF).

[0068] Preparation of the composite membrane: Weigh appropriate amounts of 1,4-butanediol diacrylate (BD) and ethylene carbonate (EC, pore-forming agent), and mix them thoroughly at room temperature in a mass ratio of 4:6. Add 0.1 wt% of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone (photoinitiator 369) to the mixed solution, and heat and stir in an oil bath at 50°C until completely dissolved to obtain a photocurable impregnation solution. Take an EGNF base film of about 25 μm as the skeleton and lay it flat on a transparent polypropylene film (PP film). Drop the above photocurable solution onto the EGNF base film to completely wet the base film, and clamp it with two transparent glass plates to maintain flatness and uniform thickness. Then, use an LED ultraviolet curing instrument to irradiate and cure both sides for 4 minutes each to complete the curing. Peel the cured composite membrane off the PP film to obtain the composite membrane.

[0069] Example 2

[0070] Preparation of flexible continuous nanofiberglass cloth: 7.24g tetraethyl orthosilicate (TEOS), 2.9g dilute hydrochloric acid (prepared by adding three drops of concentrated hydrochloric acid to 25ml deionized water), and 1.55g ethanol were mixed and stirred at room temperature for 12h to fully hydrolyze TEOS and obtain a TEOS solution. 29.18g DMF and 8.75g DMSO were mixed, and 4.9g PVP was added to the mixture. The mixture was stirred thoroughly until the PVP was completely dissolved to obtain a polymer template solution. The TEOS solution was slowly added to the polymer template solution, and the mixture was stirred thoroughly for 1h to obtain the spinning solution required for electrospinning. The spinning solution was loaded into a 20ml syringe for electrospinning. The spinning conditions were: voltage 20kV, distance between the rotating drum collector and the needle tip 15cm, solution feed rate 0.5ml / h, drum speed 2000rpm / min, temperature 25±3℃, and humidity 20±5%. Precursor films were collected every 10 minutes during electrospinning and orthogonally stacked along the fiber orientation direction to obtain a precursor film with an orthogonal interwoven structure. The precursor film was then placed in a muffle furnace for pyrolysis. The heating program was as follows: 1) heating from 20℃ to 180℃ at a heating rate of 1℃ / min; 2) holding at 180℃ for 2 hours; 3) heating from 180℃ to 325℃ at a heating rate of 1℃ / min; 4) holding at 325℃ for 2 hours; 5) heating from 325℃ to 750℃ at a heating rate of 1℃ / min; 6) holding at 750℃ for 2 hours; 7) ending the heating process and cooling the film to room temperature with the furnace to obtain a flexible continuous electrospun glass fiber nanofiber nonwoven substrate film (EGNF).

[0071] Preparation of the composite membrane: Weigh appropriate amounts of S-06E epoxy resin, UV16976 photoinitiator, and ethyl acetate solvent, and mix them at a mass ratio of 96:4:50 for 30 minutes to obtain a uniform photocurable impregnation solution. Use an EGNF base film of approximately 30 μm as a skeleton, and lay it flat on a transparent polypropylene (PP) film. Drop the above photocurable solution onto the EGNF base film to ensure complete wetting, and clamp it with two transparent glass plates to maintain flatness and uniform thickness. Then, irradiate both sides with a high-pressure mercury lamp for 2 minutes each to complete the curing. Peel the cured composite membrane off the PP film to obtain the composite membrane.

[0072] Example 3

[0073] Preparation of flexible continuous nanofiber cloth: 14.48g tetraethyl orthosilicate (TEOS), 5.8g dilute hydrochloric acid (prepared by adding six drops of concentrated hydrochloric acid to 50ml deionized water), and 3.1g ethanol were mixed and stirred at room temperature for 12h to fully hydrolyze TEOS and obtain a TEOS solution. 58.36g DMF and 17.5g DMSO were mixed, and 9.8g PVP was added to the mixture. The mixture was stirred thoroughly until the PVP was completely dissolved to obtain a polymer template solution. The TEOS solution was slowly added to the polymer template solution, and the mixture was stirred thoroughly for 1h to obtain a spinning solution. The spinning solution was loaded into an air-blowing spinneret and spun at a rate of 5g / h, and precursor films of different thicknesses were prepared by controlling the spinning time. The precursor membrane obtained by spinning was placed in a muffle furnace for pyrolysis. The heating program was as follows: heating from 20°C to 325°C at a rate of 5°C / min and holding for 2 hours; heating from 325°C to 850°C at a rate of 5°C / min; holding at 850°C for 1.5 hours; and ending the heating process and cooling the furnace to room temperature to obtain a flexible continuous electrospun glass fiber nanofiber nonwoven base membrane (EGNF).

[0074] Preparation of the composite membrane: Polyurethane and anhydrous ethanol solvent were mixed at a mass ratio of 50:50 and magnetically stirred at room temperature in the dark for 10 min. Then, 1 wt% of trimethylbenzoyl-diphenylphosphine oxide (photoinitiator TPO) was added, and magnetic stirring was continued at room temperature in the dark for 1 h to obtain a uniform photocurable impregnation solution. An EGNF base film of about 40 μm was selected as the skeleton and laid flat on a surface-treated plexiglass plate. The above photocurable solution was impregnated on the base film skeleton, and then cured by irradiation with a high-pressure mercury lamp for 5 min. The membrane was then peeled off to obtain the final composite membrane.

[0075] Example 4

[0076] Weigh appropriate amounts of 1,4-butanediol diacrylate (BD) and ethylene carbonate (EC, pore-forming agent), and mix them thoroughly at room temperature in a mass ratio of 4:6. Add 0.1 wt% of 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone (photoinitiator 369) to the mixed solution, and heat and stir in an oil bath at 50°C until completely dissolved to obtain a photocurable impregnation solution. Take the approximately 20 μm EGNF base film obtained in Example 1 as a skeleton, and lay it on an approximately 25 μm thick ultrafine glass fiber cloth (GF, 3 μm in diameter, orthogonal structure). Then, lay the GF-EGNF composite base film formed by the two on a transparent resin plate, and drip the aforementioned photocurable solution onto the GF-EGNF composite base film to completely impregnate it. Use another transparent resin plate to hold it in place to maintain flatness and uniform thickness, and then use an ultraviolet curing instrument to irradiate both sides for 4 minutes each to complete the curing. The composite diaphragm is obtained by peeling the cured composite diaphragm off the resin plate.

[0077] Figure 1The image shown is a scanning electron microscope image of the microstructure of the typical composite membrane obtained in Example 4. It can be seen that the composite membrane is composed of nanofibers and a polymer layer coated on the surface, and there are nanoscale pore structures on the polymer. Figure 2 Table 1 compares the heat resistance of the composite separator with that of common commercial polyolefin separators. It shows that the polyolefin separator undergoes significant thermal shrinkage at around 150℃ and completely melts above 250℃, while the composite separator has a thermal stability temperature above 350℃, and its thermal weight loss above 550℃ is only 44%, with the remaining 56% being the inorganic component SiO2. According to literature, the modulus of this main component (>50 GPa) is significantly higher than that of the polyolefin separator (1-2 GPa). Table 2 shows that the breakdown field strength of the composite separator is greater than 140 kV / mm, significantly higher than that of common commercial polyolefin separators (~50 kV / mm). Table 3 shows that compared with polyolefin separators, the composite separator has comparable or superior electrochemical performance, such as higher ionic conductivity and lower battery cycle degradation rate.

[0078] Table 1 Comparison of DSC and TGA results between typical composite membranes and commercial polyolefin membranes

[0079]

[0080] Table 2 Breakdown field strength test data of typical composite diaphragms

[0081]

[0082] Table 3 Electrochemical performance of typical composite membranes and commercial polyolefin membranes

[0083]

[0084] *Note: Evaluation based on battery assembly using lithium iron phosphate cathode and lithium titanate anode, with an excess of anode.

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high heat resistant and high breakdown field strength lithium battery composite separator, characterized in that, The diaphragm comprises a flexible continuous glass fiber skeleton and a three-dimensional cross-linked organic polymer coating layer, and the flexible continuous glass fiber skeleton is embedded in the three-dimensional cross-linked organic polymer coating layer. The flexible continuous glass fiber skeleton is one or both of a flexible continuous nanometer glass fiber cloth and a flexible alkali-free superfine glass fiber cloth. The flexible continuous nanometer glass fiber cloth is amorphous SiO2. The preparation method of the lithium battery composite diaphragm comprises the following steps: Step 1: Select a flexible continuous nanometer glass fiber cloth or a flexible alkali-free superfine glass fiber cloth as a flexible continuous glass fiber skeleton. Step 2: Mix a prepolymer, a photoinitiator and a pore-forming agent to obtain an impregnating solution. Step 3: Place the flexible continuous glass fiber skeleton on a substrate, and drop the impregnating solution on the flexible continuous glass fiber skeleton, so that the flexible continuous glass fiber skeleton is soaked in the impregnating solution. Step 4: Irradiate the impregnating solution and the flexible continuous glass fiber skeleton by a light source, and after the photovoltaic cross-linked curing polymer in the impregnating solution is cured, a three-dimensional cross-linked organic polymer coating layer is obtained, and the three-dimensional cross-linked organic polymer coating layer has the flexible continuous glass fiber skeleton embedded therein, thereby obtaining a lithium battery composite diaphragm with high heat resistance and high breakdown field strength.

2. The high heat-resistant and high breakdown field strength lithium battery composite separator according to claim 1, characterized in that, The flexible continuous nanometer glass fiber cloth is composed of a plurality of fibers, and the fiber diameter is 100-400 nm, and the thickness of the flexible continuous nanometer glass fiber cloth is 5-50 microns.

3. The high heat-resistant and high breakdown field strength lithium battery composite separator of claim 2, wherein, The flexible continuous nanometer glass fiber cloth has an oriented distribution structure or an unordered non-woven structure.

4. The high heat resistant and high breakdown field strength lithium battery composite separator of claim 1, wherein, The diameter of the fiber in the flexible alkali-free superfine glass fiber cloth is 0.1-8 microns, and the thickness of the flexible alkali-free superfine glass fiber cloth is 5-50 microns.

5. A method for preparing a high-heat-resistant and high-breakdown-field-strength lithium battery composite separator as described in claim 1, characterized in that, The preparation method comprises the following steps: Step 1: Select a flexible continuous nanometer glass fiber cloth or a flexible alkali-free superfine glass fiber cloth as a flexible continuous glass fiber skeleton. Step 2: Mix a prepolymer, a photoinitiator and a pore-forming agent to obtain an impregnating solution. Step 3: Place the flexible continuous glass fiber skeleton on a substrate, and drop the impregnating solution on the flexible continuous glass fiber skeleton, so that the flexible continuous glass fiber skeleton is soaked in the impregnating solution. Step 4: Irradiate the impregnating solution and the flexible continuous glass fiber skeleton by a light source, and after the photovoltaic cross-linked curing polymer in the impregnating solution is cured, a three-dimensional cross-linked organic polymer coating layer is obtained, and the three-dimensional cross-linked organic polymer coating layer has the flexible continuous glass fiber skeleton embedded therein, thereby obtaining a lithium battery composite diaphragm with high heat resistance and high breakdown field strength.

6. The method of claim 5, wherein the high heat-resistant and high-breakdown- field composite separator for lithium batteries is prepared by the steps of: In step 1, the preparation method of the flexible continuous nanometer glass fiber cloth is as follows: mix tetraethyl silicate, dilute hydrochloric acid and ethanol to obtain solution A; mix N,N-dimethylformamide, dimethyl sulfoxide and polyvinylpyrrolidone, and mix and stir to obtain solution B; mix solution A and solution B, and stir to obtain a spinning solution. The spinning solution is obtained by electrospinning or solution gas spinning to obtain a precursor film, and the precursor film is subjected to air annealing to obtain the flexible continuous nanometer glass fiber cloth.

7. The method of claim 6, wherein the method is characterized by: The annealing temperature is 650-850 DEG C.

8. The method for preparing a high heat-resistant and high breakdown field strength lithium battery composite separator according to claim 6, characterized in that, In the electrospinning process, the unordered non-woven structure is obtained by a flat plate collector, and the oriented distribution structure is obtained by a rotating drum.

9. The method for preparing a high heat-resistant and high breakdown field strength lithium battery composite separator according to claim 5, characterized in that, In step 5, a solvent is further added to the impregnating solution.

10. The method for preparing a high-heat-resistant and high-breakdown-field-strength lithium battery composite separator according to claim 5, characterized in that, The prepolymer is one or a combination of more than one of polyacrylate, epoxy resin and polyurethane.

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