Ultra-thin heat-resistant composite lithium ion battery separator, preparation method thereof and battery

Abstract

The application relates to the technical field of batteries, and discloses an ultrathin heat-resistant composite lithium ion battery diaphragm, a preparation method thereof and a battery, which comprises a base film and a composite coating arranged on at least one surface of the base film; the composite coating comprises aluminum oxide, and carboxylated cellulose nanowhiskers for promoting the uniform dispersion of the aluminum oxide and connecting the aluminum oxide to form a continuous liquidophilic porous structure; the particle size of the aluminum oxide is 200-500 nm. Through the technical scheme, the problems of the related art, such as the poor combination of the coating and the base film, the poor heat resistance, mechanical strength and ion conductivity of the battery diaphragm, and the weak liquid retention and liquid holding capacity of the battery diaphragm for electrolyte, are solved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to an ultrathin heat-resistant composite lithium-ion battery separator, its preparation method, and the battery thereof. Background Technology

[0002] The ultrathin separator of lithium-ion battery is located between the positive and negative electrodes. It not only serves as an electronic insulator but also undertakes the functions of electrolyte holding and lithium-ion transport. Its performance directly affects the safety, rate performance and cycle stability of the battery. Most of the existing commercial ultrathin separators use polyolefin microporous membranes. Although they have good mechanical strength and chemical stability, they have the following shortcomings: (1) Polyolefin microporous membranes generally have a low melting point, usually below 150°C. Under high temperature or abuse conditions, they are prone to thermal shrinkage or even melting and collapse, which leads to the closure of the pores of the ultrathin separator and the obstruction of ion transport. In severe cases, it can cause the separator to fail and bring safety hazards; (2) Polyolefin microporous membranes have low surface polarity, poor electrolyte wettability and limited liquid absorption and retention capacity, resulting in large ion migration resistance and high interfacial impedance, which restricts the structural stability and reliability of the ultrathin separator.

[0003] To improve battery separator performance, existing technologies typically employ a slurry containing inorganic particles such as alumina to coat the surface, forming a coating to enhance the separator's heat resistance, wettability, and interfacial stability. However, while this can improve the separator's thermal stability to some extent, the interfacial adhesion between the coating and the base film is limited. During thermal stress, winding stress, and charge-discharge cycles, problems such as particle shedding, powdering, localized pulverization, and coating cracking easily occur. This not only weakens the separator's structural stability but may also clog some micropores, increasing ion transport resistance. Furthermore, inorganic particles are prone to agglomeration in the slurry system, leading to insufficient coating dispersion and uniformity. This results in discontinuous local pore structures and uneven ion flux distribution, causing locally high current densities, which is detrimental to uniform lithium-ion deposition and increases the risk of dendrite growth and internal short circuits, ultimately affecting the battery separator's electrochemical performance.

[0004] To further address the aforementioned issues, it is necessary to propose an ultrathin heat-resistant composite lithium-ion battery separator and its preparation method, which improves the electrochemical performance of the ultrathin battery separator and the peel strength of its coating while enhancing its heat resistance. Summary of the Invention

[0005] This invention proposes an ultra-thin heat-resistant composite lithium-ion battery separator, its preparation method, and the battery thereof, in order to solve or alleviate at least one of the above-mentioned problems.

[0006] The technical solution of the present invention is as follows: This invention proposes an ultrathin, heat-resistant composite lithium-ion battery separator, comprising a base film and a composite coating disposed on at least one surface of the base film; the composite coating includes... Alumina is uniformly dispersed in the composite coating. Carboxylated cellulose nanocrystals are used to promote the uniform dispersion of the alumina and to connect the alumina to form a continuous hydrophilic porous structure. The alumina has a particle size of 200~500nm.

[0007] Preferably, the carboxylated cellulose nanocrystals have a diameter of 15-40 nm and a length of 100-400 nm.

[0008] Preferably, the mass ratio of the composite coated alumina to carboxylated cellulose nanocrystals is 0.25 to 4:1.

[0009] Preferably, the thickness of the composite coating is 2~3μm.

[0010] Preferably, a photocurable coating is further included between the composite coating and the base film, the photocurable coating comprising inorganic particles and a photoinitiator.

[0011] Preferably, the thickness of the photocurable coating is 0.5~2μm, and the thickness ratio of the photocurable coating to the composite coating is 1:3~3:1.

[0012] Preferably, the base membrane includes one of the following: a polyolefin microporous membrane, an ozone-activated polyolefin microporous membrane, a UV-activated polyolefin microporous membrane, and a polyolefin microporous membrane that has been synergistically activated by UV and ozone.

[0013] Preferably, the thickness of the base film is 7~9μm and the porosity is 35%~55%.

[0014] The present invention also proposes a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator, comprising the following steps: depositing a coating on at least one surface of a base film to obtain an ultrathin heat-resistant composite lithium-ion battery separator.

[0015] The present invention also proposes a battery comprising the ultra-thin heat-resistant composite lithium-ion battery separator or the ultra-thin heat-resistant composite lithium-ion battery separator prepared by the preparation method described above.

[0016] The beneficial effects of this invention are as follows: In this invention, the composite coating contains carboxylated cellulose nanofibers and alumina with an average particle size of 200-500 nm. Through the synergistic effect of these two components, the bonding strength between the composite coating and the base film is improved, thereby enhancing the heat resistance, mechanical strength, and ionic conductivity of the ultrathin battery separator. Furthermore, the wetting, adsorption, and retention capabilities of the ultrathin battery separator for the electrolyte are also improved. In existing technologies, coatings containing only a single inorganic particle are prone to agglomeration, and their bonding strength with the base film is limited, thus affecting the overall performance of the ultrathin battery separator.

[0017] In this invention, the surface of carboxylated cellulose nanofibers contains abundant polar functional groups such as carboxyl groups. On the one hand, this can inhibit alumina agglomeration, improve the rheological properties and coating uniformity of the composite coating material, connect the dispersed alumina, and form a composite support framework of organic fiber and inorganic alumina particles in synergy. This creates a continuous hydrophilic porous structure, allowing the electrolyte to be continuously distributed in the composite coating, ensuring the smooth and uniform lithium-ion transport pathway, improving ionic conductivity, and enhancing the overall adhesion strength and thermal stability of the composite coating. On the other hand, the abundant polar functional groups on the surface of carboxylated cellulose nanofibers, and the high specific surface area of ​​alumina with an average particle size of 200~500nm, together enable the membrane to have excellent rapid wetting and adsorption retention capabilities for the electrolyte. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 The image shown is an electron microscope image of the composite coating slurry in Example 1, magnified at 15,000x. Detailed Implementation

[0020] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. In the absence of conflict, the following embodiments and features in the embodiments can be combined with each other.

[0021] Existing commercial ultrathin battery separators mostly use polyolefin microporous membranes. While these membranes possess good mechanical strength and chemical stability, their low surface polarity, poor electrolyte wettability, and limited liquid absorption and retention capacity result in significant ion migration resistance and high interfacial impedance, making it difficult to meet the application requirements of high-rate and high-safety lithium-ion batteries. Furthermore, the strong chemical inertness of polyolefin microporous membranes limits their adhesion to functional coatings, restricting the structural stability and reliability of the coated separator. To improve separator performance, current technologies typically employ inorganic particles such as alumina for surface coating to enhance heat resistance, wettability, and interfacial stability. However, single inorganic particle coatings still suffer from problems such as particle agglomeration, poor slurry dispersion stability, insufficient coating pore continuity, poor uniformity, and limited adhesion to the base film. During processing and use, these coatings are prone to sedimentation, powdering, delamination, or microcracks, affecting the coating processability and electrochemical performance of ultrathin battery separators.

[0022] Based on the aforementioned problems, this invention optimizes and improves the components in the composite coating by selecting a combination of carboxylated cellulose nanofibers and alumina with a specific particle size to enhance the performance of the ultrathin battery separator. To further improve the performance of the ultrathin battery separator, a photocurable coating is added to both the composite coating and the base film. After photocuring, the base film surface forms a cured layer with high surface energy and reactivity, thereby improving the wettability and load-bearing capacity of the subsequent composite coating, enhancing the interfacial bonding between the coating and the base film, and providing a foundation for more stable adhesion of the composite coating, thus improving the overall performance of the ultrathin battery separator.

[0023] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will be described through embodiments.

[0024] A specific embodiment of the first aspect of the present invention provides an ultra-thin heat-resistant composite lithium-ion battery separator. Includes a base film and a composite coating disposed on at least one surface of the base film; the composite coating includes Alumina, uniformly dispersed in the coating. Carboxylated cellulose nanocrystals are used to promote uniform dispersion of alumina and connect alumina to form a continuous hydrophilic porous structure. The particle size of alumina is 200~500nm, for example, it can be any point value in 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm and any range between any two point values.

[0025] In one embodiment of the present invention, the diameter of the carboxylated cellulose nanocrystals is 15-40 nm, for example, any point value or the range between any two point values ​​from 15 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 25 nm, 26 nm, 28 nm, 30 nm, 32 nm, 34 nm, 35 nm, 36 nm, 38 nm, and 40 nm, and the length is 100-400 nm, for example, any point value or the range between any two point values ​​from 100 nm, 140 nm, 150 nm, 160 nm, 200 nm, 240 nm, 250 nm, 260 nm, 300 nm, 340 nm, 350 nm, 360 nm, 380 nm, and 400 nm.

[0026] In one embodiment of the present invention, the mass ratio of alumina to carboxylated cellulose nanocrystals is 0.25 to 4:1, for example, any point value of 0.25:1, 0.6:1, 0.7:1, 1.0:1, 1.5:1, 2:1, 3:1, 4:1 and any range between any two point values.

[0027] In one embodiment of the present invention, the composite coating comprises the following raw materials in parts by weight: 30 parts solvent, 10-40 parts alumina, 10-40 parts carboxylated cellulose nanofibers, 8.5-9.5 parts binder, 0.1-0.5 parts dispersant, and 8-10 parts thickener.

[0028] In one embodiment of the present invention, the dispersant is one or more of polyvinylpyrrolidone, sodium polyacrylate, polyethylene glycol, and ammonium salt of acrylic acid copolymer, preferably polyvinylpyrrolidone. Polyvinylpyrrolidone molecules contain a lactam structure, which can be adsorbed onto the surface of alumina particles and improve the particle dispersion stability through steric hindrance.

[0029] In one embodiment of the present invention, the thickener is at least one of polyacrylic acid thickeners, cellulose thickeners, amide thickeners, and natural polymer thickeners. Cellulose thickeners may be, for example, at least one of carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose.

[0030] In one embodiment of the present invention, the thickness of the composite coating is 2~3 μm.

[0031] In one embodiment of the present invention, the thickness of the base film is 7~9 μm, for example, it can be any point value or any range between any two point values ​​from 7.0 μm, 7.2 μm, 7.4 μm, 7.5 μm, 7.6 μm, 7.8 μm, 8.0 μm, 8.2 μm, 8.4 μm, 8.5 μm, 8.6 μm, 8.8 μm, and 9.0 μm, and the porosity is 35%~55%, for example, it can be any point value or any range between any two point values ​​from 35%, 36%, 40%, 44%, 45%, 50%, 54%, and 55%.

[0032] In one embodiment of the present invention, the base membrane includes one of the following: a polyolefin microporous membrane, an ozone-activated polyolefin microporous membrane, a UV-activated polyolefin microporous membrane, and a polyolefin microporous membrane that has been synergistically activated by UV and ozone.

[0033] In one embodiment of the present invention, the ozone concentration of the ozone-activated polyolefin microporous membrane and the ozone concentration of the ozone-activated polyolefin microporous membrane are independently 10~200 mg / L, and the ozone activation time is independently 10~30 min.

[0034] In one embodiment of the present invention, the UV-activated polyolefin microporous membrane and the polyolefin microporous membrane assisted by UV and ozone activation treatment each have an independent UV activation intensity of 5~300mW / cm². 2 The UV activation treatment time is 10~30 min for each individual.

[0035] The surface of the polyolefin microporous membrane, after UV and ozone activation, exhibits increased oxygen-containing polar groups, resulting in improved surface energy and wettability. Due to the presence of polar sites such as hydroxyl groups on the surface of the ultrafine alumina in the composite coating, and the presence of hydroxyl and carboxyl groups on the surface of the carboxylated cellulose nanofibers, both can form hydrogen bonds, dipole interactions, or interfacial bridging with the activated base membrane surface. The carboxylated cellulose nanofibers can also form a nano-network structure between the ultrafine alumina particles, improving the dispersion stability of the alumina particles and enhancing the continuity and flexibility of the composite coating. Therefore, UV and ozone activation treatments can synergistically enhance the ultrafine alumina / carboxylated cellulose nanofiber composite coating, improving the adhesion between the coating and the base membrane, reducing the risk of coating peeling, cracking, and powdering, while simultaneously improving the membrane's electrolyte wettability, electrolyte retention capacity, thermal dimensional stability, and overall electrochemical performance.

[0036] A specific embodiment of the second aspect of the present invention provides a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator, used to prepare the ultrathin heat-resistant composite lithium-ion battery separator provided in the specific embodiment of the first aspect of the present invention, comprising the following steps: A composite coating is applied to at least one surface of the base film to obtain an ultrathin heat-resistant composite lithium-ion battery separator.

[0037] A specific embodiment of the third aspect of the present invention provides a battery, including the ultrathin heat-resistant composite lithium-ion battery separator provided by the specific embodiment of the first aspect of the present invention or the ultrathin heat-resistant composite lithium-ion battery separator prepared by the preparation method provided by the specific embodiment of the second aspect of the present invention.

[0038] A specific embodiment of the fourth aspect of the present invention provides an ultrathin heat-resistant composite lithium-ion battery separator, comprising a base film, a photocurable coating disposed on at least one surface of the base film, and a composite coating disposed on the photocurable coating. Photocurable coatings include inorganic particles and photoinitiators; Composite coatings include Alumina, uniformly dispersed in the coating. Carboxylated cellulose nanocrystals are used to promote uniform dispersion of alumina and connect alumina to form a continuous hydrophilic porous structure. The particle size of alumina is 200~500nm.

[0039] In one embodiment of the present invention, the carboxylated cellulose nanocrystals have a diameter of 15-40 nm and a length of 100-400 nm.

[0040] In one embodiment of the present invention, the mass ratio of alumina to carboxylated cellulose nanocrystals is 0.25 to 4:1.

[0041] In one embodiment of the present invention, the composite coating comprises the following raw materials in parts by weight: 30 parts solvent, 10-40 parts alumina, 10-40 parts carboxylated cellulose nanofibers, 8.5-9.5 parts first binder, 0.1-0.5 parts first dispersant, and 8-10 parts first thickener; The photocurable coating comprises the following raw materials in parts by weight: 20-60 parts inorganic particles, 1-5 parts photoinitiator, 0.5-5 parts dispersant, 0.5-5 parts thickener, 5-20 parts binder, 20-60 parts alcohol solvent, and 10-40 parts water.

[0042] In one embodiment of the present invention, the first dispersant and the second dispersant are each independently one or more of polyvinylpyrrolidone, sodium polyacrylate, polyethylene glycol, and ammonium salt of acrylic acid copolymer, preferably polyvinylpyrrolidone. The polyvinylpyrrolidone molecule contains a lactam structure, which can be adsorbed onto the surface of alumina particles and improve the particle dispersion stability through steric hindrance.

[0043] In one embodiment of the present invention, the first thickener and the second thickener are each independently at least one selected from polyacrylic acid thickeners, cellulose thickeners, amide thickeners, and natural polymer thickeners. Cellulose thickeners may, for example, be at least one selected from carboxymethyl cellulose, sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methyl cellulose. In the photocurable coating of the present invention, the inorganic particles are mainly used to improve the heat resistance, mechanical strength, and dimensional stability of the diaphragm. The second binder is used to enhance the bonding strength within the coating and between the coating and the base film. The photoinitiator is used to initiate a free radical reaction under ultraviolet light irradiation. The second dispersant and the second thickener are respectively used to improve the dispersion state of nano-alumina and the slurry application performance.

[0044] In this invention, by performing photocuring treatment on the base film, a cured layer with high surface energy and good interfacial activity can be formed on the surface of the base film, thereby improving the wettability, load-bearing capacity and interfacial bonding ability of the base film to the composite slurry layer, and providing a foundation for the stable adhesion of the composite coating. In one embodiment of the present invention, the first adhesive and the second adhesive are each independently one of an acrylate polymer, a polyacrylate, or a polyurethane acrylate oligomer, preferably a polyurethane acrylate oligomer, which is a pale yellow to colorless transparent viscous liquid with a solid content ≥95% and a viscosity of 2000~8000 mPa·s, for example, any point value or range between any two points from 2000 mPa·s, 2500 mPa·s, 3000 mPa·s, 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, 5500 mPa·s, 6000 mPa·s, 6500 mPa·s, 7000 mPa·s, 7500 mPa·s, to 8000 mPa·s, and a density of 1.05~1.20 g / cm³. 3 For example, it could be 1.05 g / cm³. 3 1.06 g / cm 3 1.08g / cm 3 1.1g / cm 3 1.14 g / cm 3 1.15g / cm 3 1.16 g / cm 3 1.18 g / cm 3 1.2g / cm 3 The value of any point in the array and the range between any two point values.

[0045] In one embodiment of the present invention, the photoinitiator includes one or more of benzoin and its derivatives, benzoyl groups, alkyl benzophenones, acyl phosphorus oxides, benzophenones, and thioxanthones; acyl phosphorus oxides may be, for example, TPO photoinitiators; benzophenones may be, for example, BP photoinitiators.

[0046] In one embodiment of the present invention, the base membrane includes one of the following: a polyolefin microporous membrane, an ozone-activated polyolefin microporous membrane, a UV-activated polyolefin microporous membrane, and a polyolefin microporous membrane that has been synergistically activated by UV and ozone.

[0047] In the photocurable coating, inorganic particles provide an inorganic heat-resistant skeleton, and the binder forms an organic cross-linked network. Both are anchored to the surface of the base film. UV activation, ozone activation, or synergistic activation of UV and ozone improves the surface energy of the base film. The inorganic particles in the photocurable layer have a high specific surface area and surface polar sites, while the carboxylated cellulose nanocrystals in the composite coating are rich in carboxyl and hydroxyl groups. Through a multi-level interface enhancement mechanism of base film activation-binder pre-coating of inorganic particles-photocurable cross-linking-nanofiber network bridging, the shrinkage constraint on the base film can be continuously applied, thereby significantly reducing the thermal shrinkage rate of the separator and improving the heat shrinkage resistance, mechanical strength, peel strength, electrolyte absorption and retention capacity, and ion conduction performance of the ultrathin battery separator.

[0048] In one embodiment of the present invention, the ozone concentration of the ozone-activated polyolefin microporous membrane and the ozone concentration of the ozone-activated polyolefin microporous membrane are independently 100~200 mg / L, and the ozone activation time is independently 10~30 min.

[0049] In one embodiment of the present invention, the UV-activated polyolefin microporous membrane and the polyolefin microporous membrane assisted by UV and ozone activation treatment each have an independent UV activation intensity of 100~200mW / cm². 2 The UV activation treatment time is 10~30 min for each individual.

[0050] In one embodiment of the present invention, the inorganic particles include alumina, the average particle size of which is 200~500nm, for example, any point value among 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, and 500nm, and the range between any two point values.

[0051] In one embodiment of the present invention, the thickness of the photocurable coating is 0.5~2μm, for example, it can be any point value or any range between two points from 0.5μm, 0.6μm, 1μm, 1.2μm, 1.5μm, 1.6μm, 2μm. Within this range, a continuous and stable protective layer can be formed to avoid clogging the micropores of the base film, maintain air permeability and pore connectivity, thereby reducing interfacial impedance and improving lithium ion migration efficiency and membrane ionic conductivity without sacrificing mechanical and heat resistance properties. The thickness ratio of the photocurable coating to the composite coating is 1:3 to 3:1.

[0052] A specific embodiment of the fifth aspect of the present invention provides a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator, used to prepare the ultrathin heat-resistant composite lithium-ion battery separator provided in the specific embodiment of the fourth aspect of the present invention, comprising the following steps: A photocurable coating is applied to at least one surface of a base film, and a composite coating is applied to the photocurable coating to obtain an ultrathin heat-resistant composite lithium-ion battery separator.

[0053] In one embodiment of the present invention, a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: coating a photocurable coating slurry onto one surface of a base film and then curing it to form a photocurable coating; mixing the raw materials of the composite coating and coating them onto the surface of the photocurable coating, drying them to form a composite coating, thereby obtaining an ultrathin heat-resistant composite lithium-ion battery separator.

[0054] In one embodiment of the present invention, a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: coating the upper and lower surfaces of a base film with a photocurable coating slurry and then curing it to form a photocurable coating I and a photocurable coating II, respectively; mixing the raw materials of the composite coating and coating them onto the surfaces of the photocurable coating I and the photocurable coating II, and drying them to form composite coating I and composite coating II, thereby obtaining an ultrathin heat-resistant composite lithium-ion battery separator.

[0055] In one embodiment of the present invention, any one of the following polyolefin microporous membranes—ozone-activated, UV-activated, and UV-and-ozone synergistically activated—is pre-dried at 40-60°C for 5-20 minutes before activation treatment; wherein, 40-60°C can be any value from 40°C, 45°C, 50°C, 55°C, and 60°C, or any range between any two values; 5-20 minutes can be any value from 5 minutes, 6 minutes, 8 minutes, 10 minutes, 12 minutes, 14 minutes, 15 minutes, 16 minutes, 18 minutes, and 20 minutes, or any range between any two values. Pre-drying the polyolefin microporous membrane removes moisture and volatile impurities from the surface and pores of the membrane, enhancing the effect of subsequent activation treatment of the membrane surface and improving the surface energy of the base film and the adhesion of the coating.

[0056] In one embodiment of the invention, the coating includes either roller coating or dot coating.

[0057] In one embodiment of the present invention, the photocurable coating slurry is cured by photocuring followed by thermal curing, and the irradiance of the photocuring is 100 mW / cm². 2 The photocuring time is 60 seconds; the heat curing temperature is 75~85℃, for example, any point value or any range between any two points from 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, 81℃, 82℃, 83℃, 84℃, 85℃; the heat curing time is 15~25 minutes, for example, any point value or any range between any two points from 15 minutes, 16 minutes, 18 minutes, 20 minutes, 22 minutes, 24 minutes, 25 minutes; the relative humidity for heat curing is 32%~36%, for example, any point value or any range between any two points from 32%, 33%, 34%, 35%, 36%. During the photocuring process, free radicals are generated after absorbing ultraviolet light energy, which further initiates free radical coupling reactions in the adhesive, thickener, hydroxyl groups on the surface of nano-alumina particles, and the surface-activated polyolefin porous membrane. Through this reaction, the organic binder, nano-alumina particles and polyethylene film in the coating form a three-dimensional cross-linked network structure that is interpenetrating and firmly bonded. After photocuring, further thermal curing can promote the further drying and structural stability of the photocured coating. After UV curing and thermal curing, the photocured coating can form a continuous, uniform and stably adhered photocured coating on the surface of the base film.

[0058] A specific embodiment of the sixth aspect of the present invention provides a battery, including the ultrathin heat-resistant composite lithium-ion battery separator provided in the specific embodiment of the fourth aspect of the present invention or the ultrathin heat-resistant composite lithium-ion battery separator prepared by the preparation method provided in the specific embodiment of the fifth aspect of the present invention.

[0059] A specific embodiment of the seventh aspect of the present invention provides an ultrathin heat-resistant composite lithium-ion battery separator, comprising a base film and a composite coating disposed on at least one surface of the base film. Composite coatings include Alumina, uniformly dispersed in the coating. Carboxylated cellulose nanocrystals are used to promote uniform dispersion of alumina and connect alumina to form a continuous hydrophilic porous structure. The particle size of alumina is 200~500nm. Between the composite coating and the base film, there is also a photocurable coating, which includes the following raw materials: 20-60 parts inorganic particles, 1-5 parts photoinitiator, 0.5-5 parts dispersant, 0.5-5 parts thickener, 5-20 parts binder, 20-60 parts alcohol solvent, and 10-40 parts water; In the preparation of the photocurable coating, inorganic particles and a portion of the binder are first dispersed in a mixed solution of a portion of alcohol solvent and a portion of water. After vacuum treatment, pretreated inorganic particles are obtained. A photoinitiator is added to the remaining portion of alcohol solvent and dispersed to obtain a first solution. The pretreated inorganic particles and other remaining components are added to the remaining portion of water and dispersed through emulsification to obtain a second solution. The first and second solutions are mixed to obtain a photocurable coating slurry. The photocurable coating slurry is coated onto at least one surface of a base film and photocured to obtain a photocurable coating.

[0060] In one embodiment of the present invention, the mass of a portion of the alcohol solvent is 30-70% of the total mass of the alcohol solvent; The mass of some water is 30-70% of the total mass of water.

[0061] In this invention, by using vacuum treatment, the air adsorbed between the inorganic particles and on their surface can be gradually discharged. The negative pressure effect promotes the full wetting and coating of the inorganic particle surface by some of the adhesive. As a result, the adhesive is not only distributed on the outside of the inorganic particles, but also forms a uniform coating and bridging structure on the particle surface. This enhances the anchoring effect between the inorganic particles, adhesive and base film in the subsequent coating, thereby further improving the peel strength of the coating membrane.

[0062] In one embodiment of the present invention, the carboxylated cellulose nanocrystals have a diameter of 15-40 nm and a length of 100-400 nm.

[0063] In one embodiment of the present invention, the mass ratio of alumina to carboxylated cellulose nanocrystals is 0.25 to 4:1.

[0064] In one embodiment of the present invention, the composite coating comprises the following raw materials in parts by weight: 30 parts solvent, 10-40 parts alumina, 10-40 parts carboxylated cellulose nanofibers, 8.5-9.5 parts first binder, 0.1-0.5 parts first dispersant, and 8-10 parts first thickener; The photocurable coating comprises the following raw materials in parts by weight: parts inorganic particles, parts photoinitiator, parts second dispersant, parts second thickener, parts second binder, alcohol solvent, and water. In one embodiment of the invention, the inorganic particles comprise alumina, wherein the average particle size of the alumina is 200-500 nm.

[0065] In one embodiment of the present invention, the base membrane includes one of the following: a polyolefin microporous membrane, an ozone-activated polyolefin microporous membrane, a UV-activated polyolefin microporous membrane, and a polyolefin microporous membrane that has been synergistically activated by UV and ozone.

[0066] In one embodiment of the present invention, the thickness of the composite coating is 2~3 μm.

[0067] In one embodiment of the present invention, the thickness of the photocurable coating is 0.5~2μm, and the thickness ratio of the photocurable coating to the composite coating is 1:3~3:1.

[0068] In one embodiment of the present invention, the thickness of the base film is 7~9μm and the porosity is 35%~55%.

[0069] The eighth aspect of the present invention provides a method for preparing an ultrathin heat-resistant composite lithium-ion battery separator, used to prepare the ultrathin heat-resistant composite lithium-ion battery separator provided in the seventh aspect of the present invention, comprising the following steps: Inorganic particles and a portion of the binder are dispersed in a mixed solution of a portion of alcohol solvent and a portion of water. After vacuum treatment, pretreated inorganic particles are obtained. A photoinitiator is added to the remaining portion of the alcohol solvent and dispersed to obtain a first solution. The pretreated inorganic particles and other remaining components are added to the remaining portion of water and dispersed through emulsification to obtain a second solution. The first and second solutions are mixed to obtain a photocurable coating slurry. The photocurable coating slurry is coated onto at least one surface of a base film and photocured to form a photocurable coating. The raw materials for the composite coating are mixed and coated onto the surface of a photocurable coating, then dried to form a composite coating, resulting in an ultrathin heat-resistant composite lithium-ion battery separator. A specific embodiment of the ninth aspect of the present invention provides a battery comprising the ultrathin heat-resistant composite lithium-ion battery separator provided in the specific embodiment of the seventh aspect of the present invention or the ultrathin heat-resistant composite lithium-ion battery separator prepared by the preparation method provided in the specific embodiment of the eighth aspect of the present invention.

[0070] The present invention will now be described in detail with reference to preferred embodiments and comparative examples. The preferred embodiments of the invention described below can be modified in various ways, and therefore the scope of the invention should not be construed as limited to the preferred embodiments described in detail below. Preferred embodiments are provided to help those skilled in the art to more readily understand the invention.

[0071] In the following embodiments and comparative examples: Acrylic copolymer solution: Hubei Jiufenglong Chemical Co., Ltd., the weight average molecular weight of the acrylate copolymer solution is 20,000 to 500,000 g / mol; Acrylic acid copolymer ammonium salt: Model SN-330A, Shanghai Snow New Materials Co., Ltd.; Carboxymethyl cellulose: The viscosity of a 5wt% carboxymethyl cellulose aqueous solution is 800-1200 mPa·s, model CMC-LV, Hebei Yezhiyuan Chemical Co., Ltd. The polyurethane acrylate oligomer is an aliphatic polyurethane acrylate oligomer: model 6112-100, Changxing Chemical Materials Co., Ltd. Polyvinylpyrrolidone: Model PVP-K30; Cellulose nanocrystals are derived from cellulose nanocrystal dispersion, model QH-19, produced by Guilin Qihong Technology Co., Ltd. Carboxylated cellulose nanofibers are derived from a carboxylated cellulose nanofiber dispersion. The carboxylated cellulose nanofiber dispersion has the following characteristics: solid content 11%, pH value 6.0~9.0, fiber diameter 15~40nm, length 100~400nm, and carboxylation degree 0.5~2.0 mmol / g.

[0072] Example 1 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanofibers were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform and non-agglomerated slurry. Subsequently, 9.5 parts acrylate copolymer solution and 10 parts carboxymethyl cellulose were added to the slurry and mixed at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Then, 0.3 mm diameter zirconia beads were used for circulating milling in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm (its electron micrograph is shown in the figure). Figure 1 As shown in the figure), the composite coating slurry was coated onto one surface of the ozone-activated polyethylene microporous membrane using a roller coating method. After baking at 60°C for 10 min, a composite coating with a thickness of 3 μm was formed, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane. The ozone-activated polyethylene microporous membrane had a thickness of 9 μm and a porosity of 55%. Before ozone activation, the polyethylene microporous membrane was pre-dried at 40°C for 20 min. The ozone concentration during ozone activation was 10 mg / L, and the ozone activation time was 30 min.

[0073] Example 2 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.1 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 20 parts alumina (average particle size of 200 nm) and 30 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. Subsequently, 8.5 parts acrylate copolymer solution and 8 parts carboxymethyl cellulose were added to the slurry and mixed at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Afterwards, 0.3 mm diameter zirconia beads were used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto the upper and lower surfaces of the ozone-activated polyethylene microporous membrane using a roller coating method. After baking at 60°C for 10 min, composite coating I and composite coating II with a thickness of 2 μm were formed, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane. The ozone-activated polyethylene microporous membrane had a thickness of 7 μm and a porosity of 35%. Before ozone activation treatment, the polyethylene microporous membrane was pre-dried at 60°C for 5 min. The ozone concentration during ozone activation treatment was 200 mg / L, and the ozone activation treatment time was 10 min.

[0074] Example 3 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 30 parts alumina (average particle size of 200 nm) and 20 parts carboxylated cellulose nanofibers were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. Subsequently, 9.5 parts of water were added to the slurry. A solution of acrylate copolymer and 10 parts of carboxymethyl cellulose were mixed for 10 minutes at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz. Then, 0.3 mm diameter zirconia beads were used to circulate and grind the mixture in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. The membrane was then baked at 60 °C for 10 minutes to form a composite coating with a thickness of 3 μm, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane.

[0075] Example 4 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts acrylic acid copolymer ammonium salt were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 20 parts alumina (average particle size of 200 nm) and 30 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. Subsequently, 9.5 parts of water were added to the slurry. A solution of acrylate copolymer and 10 parts of carboxymethyl cellulose were mixed for 10 minutes at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz. Then, 0.3 mm diameter zirconia beads were used to circulate and grind the mixture in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. The membrane was then baked at 60 °C for 10 minutes to form a composite coating with a thickness of 3 μm, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane.

[0076] Example 5 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 10 parts alumina (average particle size of 200 nm) and 40 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform and non-agglomerated slurry. Subsequently, 9... Five parts of acrylate copolymer solution and ten parts of carboxymethyl cellulose were mixed at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Then, 0.3 mm diameter zirconia beads were used to circulate and grind the mixture in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was coated onto one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. The membrane was then baked at 60 °C for 10 min to form a composite coating with a thickness of 3 μm, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane.

[0077] Example 6 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. Subsequently, 9.5 parts acrylate copolymer solution and 10 parts carboxymethyl cellulose were added to the slurry and dispersed at a revolution speed of 40 r / min. After mixing at a speed of 20kHz ultrasonic frequency for 10 minutes, 0.3mm diameter zirconia beads were used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with D50 of 0.26μm and D90 of 0.83μm. The composite coating slurry was then coated onto one surface of a UV-activated polyethylene microporous membrane using a roller coating method. After baking at 60℃ for 10 minutes, a composite coating with a thickness of 3μm was formed, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane. The UV-activated polyethylene microporous membrane had a thickness of 9μm and a porosity of 55%. It was pre-dried at 40℃ for 20 minutes before UV activation treatment. The irradiation intensity of the UV activation treatment was 100mW / cm². 2 The UV activation treatment time is 30 minutes.

[0078] Example 7 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. Subsequently, 9.5 parts acrylate copolymer solution and 10 parts carboxymethyl cellulose were added to the slurry and mixed at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz. After 10 minutes, zirconia beads with a diameter of 0.3 mm were used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then applied to one surface of a polyethylene microporous membrane that had undergone UV and ozone synergistic activation treatment using a roller coating method. The membrane was then baked at 60°C for 10 minutes to form a 3 μm thick composite coating, resulting in an ultra-thin heat-resistant composite lithium-ion battery membrane. The polyethylene microporous membrane, after UV and ozone synergistic activation treatment, had a thickness of 9 μm and a porosity of 55%. Before activation treatment, the polyethylene microporous membrane was pre-dried at 40°C for 20 minutes. During synergistic activation treatment, it was first activated by UV and then by ozone. The irradiation intensity of the UV activation treatment was 100 mW / cm². 2 The UV activation treatment time was 30 min, the ozone concentration during ozone activation treatment was 10 mg / L, and the ozone activation treatment time was 30 min.

[0079] Example 8 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: S1. Mix 20 parts alumina (average particle size 200 nm), 1 part TPO photoinitiator, 0.5 parts polyvinylpyrrolidone, 0.5 parts carboxymethyl cellulose, 0.5 parts acrylate copolymer solution, 20 parts isobutanol, and 10 parts water to obtain a photocurable coating slurry. Apply the photocurable coating slurry to one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. Then, apply the slurry at 100 mW / cm². 2 Irradiation, photocuring for 60 seconds followed by heat curing at 75°C for 25 minutes, to form a photocured coating with a thickness of 1μm; S2. 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated solution. The slurry was then mixed with 9.5 parts of acrylate copolymer solution and 10 parts of carboxymethyl cellulose at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Zirconia beads with a diameter of 0.3 mm were then used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto a photocurable coating using a roller coating method and baked at 60℃ for 10 min to form a composite coating with a thickness of 3 μm, resulting in an ultrathin heat-resistant composite lithium-ion battery separator.

[0080] Example 9 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: S1. Mix 60 parts alumina (average particle size 200 nm), 5 parts TPO photoinitiator, 5 parts polyvinylpyrrolidone, 5 parts carboxymethyl cellulose, 5 parts acrylate copolymer solution, 60 parts isobutanol, and 40 parts water to obtain a photocurable coating slurry. Apply the photocurable coating slurry to one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. Then, apply the slurry at 100 mW / cm². 2 Irradiation, photocuring for 60 seconds followed by heat curing at 75°C for 25 minutes, to form a photocured coating with a thickness of 1μm; S2. 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated solution. The slurry was then mixed with 9.5 parts of acrylate copolymer solution and 10 parts of carboxymethyl cellulose at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Zirconia beads with a diameter of 0.3 mm were then used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto a photocurable coating using a roller coating method and baked at 60℃ for 10 min to form a composite coating with a thickness of 3 μm, resulting in an ultrathin heat-resistant composite lithium-ion battery separator.

[0081] Example 10 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: S1. Mix 60 parts alumina (average particle size 200 nm), 5 parts TPO photoinitiator, 5 parts polyvinylpyrrolidone, 5 parts carboxymethyl cellulose, 5 parts acrylate copolymer solution, 60 parts isobutanol, and 40 parts water to obtain a photocurable coating slurry. Apply the photocurable coating slurry to one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. Then, apply the slurry at 100 mW / cm². 2 Irradiation, photocuring for 60 seconds followed by heat curing at 75°C for 25 minutes, to form a photocured coating with a thickness of 2μm; S2. 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated solution. The slurry was then mixed with 9.5 parts of acrylate copolymer solution and 10 parts of carboxymethyl cellulose at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Zirconia beads with a diameter of 0.3 mm were then used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto a photocurable coating using a roller coating method and baked at 60℃ for 10 min to form a composite coating with a thickness of 2 μm, resulting in an ultrathin heat-resistant composite lithium-ion battery separator.

[0082] Example 11 S1. Mix 60 parts alumina (average particle size 200 nm), 5 parts TPO photoinitiator, 5 parts polyvinylpyrrolidone, 5 parts carboxymethyl cellulose, 5 parts acrylate copolymer solution, 60 parts isobutanol, and 40 parts water to obtain a photocurable coating slurry. Apply the photocurable coating slurry to one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method. Then, apply the slurry at 100 mW / cm². 2 Irradiation, photocuring for 60 seconds, followed by heat curing at 75°C for 25 minutes, to form a photocured coating with a thickness of 1.5 μm; S2. Mix 30 parts water and 0.5 parts ammonium salt of acrylic copolymer in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then add 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanofibers, and disperse in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated slurry. The solution was then mixed with 9.5 parts of acrylate copolymer solution and 10 parts of carboxymethyl cellulose at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Zirconia beads with a diameter of 0.3 mm were then used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto a photocurable coating using a roller coating method and baked at 60℃ for 10 min to form a composite coating with a thickness of 2.5 μm, resulting in an ultrathin heat-resistant composite lithium-ion battery separator.

[0083] Example 12 A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator includes the following steps: S1. Weigh out 60 parts by weight of alumina (average particle size of 200nm), 5 parts of TPO photoinitiator, 5 parts of polyvinylpyrrolidone, 5 parts of carboxymethyl cellulose, 5 parts of acrylate copolymer solution, 60 parts of isobutanol, and 40 parts of water for preparation. Alumina, a portion of binder, a portion of alcohol solvent, and a portion of water were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min. After vacuum treatment, pretreated alumina was obtained. The photoinitiator and the remaining alcohol solvent were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain the first solution. Add pretreated alumina, dispersant and thickener to the remaining water, and disperse for 10 min in a dual planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz to obtain the second solution. The first and second solutions were mixed and dispersed for 20 minutes in a dual planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz to obtain a photocurable coating slurry. This photocurable coating slurry was then coated onto one surface of an ozone-activated polyethylene microporous membrane (same as in Example 1) using a roller coating method, and subjected to an ultrasonic frequency of 100 mW / cm². 2 Irradiation, light curing for 60 seconds followed by heat curing at 75°C for 25 minutes, forming a light-cured coating with a thickness of 1 μm; the mass of the alcohol solvent is 70% of the total mass of the alcohol solvent, and the mass of the water is 70% of the total mass of the water; the ozone-activated polyethylene microporous membrane has a thickness of 9 μm and a porosity of 35%; the ozone concentration during ozone activation treatment is 10 mg / L, and the ozone activation treatment time is 30 minutes.

[0084] S2. 30 parts water and 0.5 parts ammonium salt of acrylic copolymer were mixed in a double planetary mixer at a rotation speed of 1000 r / min and a revolution speed of 40 r / min for 10 min to obtain a uniformly dispersed solution. Then, 40 parts alumina (average particle size of 200 nm) and 10 parts carboxylated cellulose nanocrystals were added and dispersed in a double planetary mixer with ultrasonic oscillation function at a rotation speed of 2000 r / min, a revolution speed of 40 r / min, and an ultrasonic frequency of 20 kHz for 30 min to obtain a uniform, non-agglomerated solution. The slurry was then mixed with 9.5 parts of acrylate copolymer solution and 10 parts of carboxymethyl cellulose at a revolution speed of 40 r / min and an ultrasonic frequency of 20 kHz for 10 min. Zirconia beads with a diameter of 0.3 mm were then used for circulating grinding in a nano-sand mill to obtain a composite coating slurry with a D50 of 0.26 μm and a D90 of 0.83 μm. The composite coating slurry was then coated onto a photocurable coating using a roller coating method and baked at 60℃ for 10 min to form a composite coating with a thickness of 3 μm, resulting in an ultrathin heat-resistant composite lithium-ion battery separator.

[0085] Comparative Example 1 This comparative example is the same as Example 1, except that the average particle size of alumina in the raw material of the composite coating is 600 nm.

[0086] Comparative Example 2 This comparative example is the same as Example 1, except that the carboxylated cellulose nanocrystals are replaced with cellulose nanocrystals.

[0087] Comparative Example 3 This comparative example is the same as Example 1, except that the alumina in the composite coating raw material is replaced with boehmite with an average particle size of 200 nm.

[0088] Comparative Example 4 This comparative example is the same as Example 1, except that the alumina in the raw material of the composite coating is replaced with porous alumina with an average particle size of 200 nm.

[0089] Comparative Example 5 This comparative example is the same as Example 1, except that the raw materials for the composite coating contain 50 parts of alumina and do not contain carboxylated cellulose nanofibers.

[0090] The following performance tests were performed on the battery separators of Examples 1-12 and Comparative Examples 1-5: (1) Air permeability enhancement: The air permeability tester EG01-55-1MR was used for testing. The sample size was 50mm × width. The measurement mode was 500 and the test unit was JIS. The time required for 100mL of nitrogen to pass through the diaphragm was recorded. Air permeability increase = air permeability of composite lithium-ion battery separator - air permeability of polyethylene microporous separator.

[0091] (2) Tensile strength: The tensile strength was tested according to the test method in GB / T 36363-2018 "Polyolefin separator for lithium-ion batteries". The average value of 20 sets of separator test data was taken.

[0092] (3) Heat shrinkage: The heat shrinkage performance of the separator sample at 150℃ for 1h was tested according to the test method specified in GB / T 36363-2018 "Polyolefin separator for lithium-ion batteries".

[0093] (4) Peel strength: Cut the diaphragm into test samples of 3cm×15cm size, attach 3M release tape (2.6cm×15cm) to the center of the coating surface of the diaphragm, and roll the test sample with a 2kg pressure roller (roll forward 4 times to ensure that the entire test sample is rolled in place, and roll in the same direction, without rolling back and forth), tear off 5cm of one end of the 3M release tape, and clamp the 3M release tape and the diaphragm straight in the upper and lower clamps of the electronic tensile testing machine. The test speed is 100mm / min, and the peel strength is obtained. The formula for calculating the peel strength is: peel strength (N / m) = average force (N) / width of release tape (m) × 100%.

[0094] (5) Liquid absorption rate and liquid retention rate: Cut the diaphragm into 50mm×50mm samples, and weigh the samples before testing and record the weight as m1; Liquid absorption rate test procedure: The weighed diaphragm is immersed in the electrolyte (1.0 mol / L LiPF6 solution, the solvent is composed of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 1:1:1) at 25℃ for 30 min, then removed and placed on industrial wiping paper. After wiping the free electrolyte with industrial wiping paper, the weight is recorded as m2. Liquid retention rate test procedure: After weighing the sample and placing it at 25℃ for 1 hour, the liquid absorption rate is recorded as m3; Liquid absorption rate (%) = (m2-m1) / m1×100; Liquid retention rate (%) = (m3-m1) / m1×100; (6) Ionic conductivity: Ionic conductivity was tested in accordance with GB / T 36363-2018 "Polyolefin separators for lithium-ion batteries", where the temperature was 40℃ and the relative humidity was 50%. (7) Adhesion of hot-pressed electrode sheets (positive / negative): The electrode can be either a positive electrode or a negative electrode. When the electrode is a positive electrode, it exhibits positive electrode adhesion strength; when the electrode is a negative electrode, it exhibits negative electrode adhesion strength. Positive electrode adhesion strength: (The following appears to be a separate, unrelated metric:) When the diaphragm is cut to a size of 25... 150mm, positive electrode plate size is 25 150mm; Adjust the hot press temperature to 80℃ and the pressure to 1000KG. Preheat the diaphragm and positive electrode sheet for 5 seconds using the hot press. Test the bonding strength of the cold-pressed positive electrode sheet using an electronic tensile testing machine. Peel the diaphragm and positive electrode sheet until the tensile distance of the electronic tensile testing machine is 50mm. The speed of the electronic tensile testing machine is 100mm / min, and the peel angle is 180°. The bonding strength of the positive electrode sheet = peel force divided by the tensile distance of the electronic tensile testing machine. Peel force: the average value of the force collected by the electronic tensile testing machine during the peeling process of the diaphragm and positive electrode sheet. The bonding strength test of the hot-pressed positive electrode sheet is calculated based on the data between 10~40mm, that is, the bonding strength of the positive electrode sheet = peel force between 10~40mm divided by 30mm. The positive electrode sheet is a ternary lithium nickel cobalt aluminum oxide with the chemical formula LiNi. 0.8 Co 0.15 Al 0.05 The O2 electrode is a carbon-based graphite electrode (containing 91% carbon).

[0095] The test results are shown in Tables 1 and 2 below.

[0096] Table 1. Basic performance of battery separators

[0097] Table 2 Core performance of battery separators

[0098] Compared with Example 1, Comparative Example 1 changed the particle size of alumina in the composite coating, Comparative Example 2 replaced carboxylated cellulose nanofibers with cellulose nanofibers, Comparative Example 3 replaced alumina with boehmite of equal particle size, Comparative Example 4 replaced alumina with porous alumina of equal particle size, and Comparative Example 5 did not contain carboxylated cellulose nanofibers. As a result, the tensile strength of the battery separator in Comparative Examples 1 to 5 was lower than that in Example 1, and the heat resistance, peel strength and electrochemical performance were also lower than those in Example 1. This shows that the composite coating in this invention can improve the tensile strength, heat resistance, peel strength and electrochemical performance of the battery separator.

[0099] The above are merely preferred embodiments of the present invention and are 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. An ultra-thin heat-resistant composite lithium-ion battery separator, characterized in that, It includes a base film and a composite coating disposed on at least one surface of the base film; the composite coating includes Alumina is uniformly dispersed in the composite coating. Carboxylated cellulose nanocrystals are used to promote the uniform dispersion of the alumina and to connect the alumina to form a continuous hydrophilic porous structure. The alumina has a particle size of 200~500nm.

2. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 1, characterized in that, The carboxylated cellulose nanocrystals have a diameter of 15-40 nm and a length of 100-400 nm.

3. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 1, characterized in that, The mass ratio of alumina to carboxylated cellulose nanocrystals is 0.25 to 4:

1.

4. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 1, characterized in that, The thickness of the composite coating is 2~3μm.

5. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 1, characterized in that, The composite coating and the base film also include a photocurable coating, which comprises inorganic particles and a photoinitiator.

6. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 5, characterized in that, The thickness of the photocurable coating is 0.5~2μm, and the thickness ratio of the photocurable coating to the composite coating is 1:3~3:

1.

7. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 1, characterized in that, The base membrane includes one of the following: a polyolefin microporous membrane, an ozone-activated polyolefin microporous membrane, a UV-activated polyolefin microporous membrane, and a polyolefin microporous membrane that has been synergistically activated by UV and ozone.

8. The ultra-thin heat-resistant composite lithium-ion battery separator according to claim 4, characterized in that, The thickness of the base film is 7~9μm and the porosity is 35%~55%.

9. A method for preparing an ultrathin heat-resistant composite lithium-ion battery separator, used to prepare the ultrathin heat-resistant composite lithium-ion battery separator according to any one of claims 1 to 4, characterized in that, Includes the following steps: A composite coating is applied to at least one surface of the base film to obtain an ultrathin heat-resistant composite lithium-ion battery separator.

10. A battery, characterized in that, The ultra-thin heat-resistant composite lithium-ion battery separator includes the ultra-thin heat-resistant composite lithium-ion battery separator according to any one of claims 1 to 8 or the ultra-thin heat-resistant composite lithium-ion battery separator prepared by the preparation method according to claim 9.

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