Composite coating separator, method for preparing the same, and secondary battery

Abstract

The application discloses a kind of composite coating diaphragm and its preparation method and secondary battery, the composite coating diaphragm includes base film and the composite functional coating being combined on the surface of base film, the composite functional coating includes the first functional coating being relatively close to base film and the second functional coating being relatively far from base film, the porosity of first functional coating is less than the porosity of second functional coating, and raw material component all include cis-1,4-polybutadiene and polyethylene glycol segment are double grafted with modified nanocellulose and gas phase aluminum oxide.By constructing the diaphragm coating with gradient pore structure, and combining advanced material modification technology, so that the composite coating diaphragm set high thermal stability, excellent electrolyte wettability and low temperature adaptability in one, still can keep stable structure and interface characteristics under extreme temperature working condition, so as to have good scalability and industrial application prospect, be applicable to the development and application demand of high-performance lithium ion battery and next-generation energy storage device.

Description

Technical Field

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

[0002] With the widespread application of lithium-ion batteries in new energy vehicles, consumer electronics, and large-scale energy storage, the continuous improvement of battery performance has placed higher demands on key components—separator materials. As a core component for achieving physical isolation between the positive and negative electrodes and ensuring efficient lithium-ion conduction, the separator not only needs to possess excellent mechanical strength and electrochemical stability, but also needs to maintain structural integrity and interfacial compatibility under extreme temperature conditions. In recent years, to address the safety challenges brought about by the increasing energy density of batteries and the diversification of usage environments, composite separators based on coating modification have become a key research and industrial development direction.

[0003] In existing technologies, coating the surface of a polyolefin-based membrane with inorganic nanoparticles (such as Al2O3 and SiO2) to form a ceramic coating membrane has become a mainstream approach to improve thermal stability. This type of coating can effectively suppress the thermal shrinkage behavior of the base membrane at high temperatures, controlling the thermal shrinkage rate of the membrane at 180°C to within the range of 3% to 5%, thereby reducing the risk of internal short circuits caused by thermal runaway. However, due to the significant interfacial polarity difference between the inorganic particles and the organic polymer base membrane, their bonding force is weak, with typical peel strength values ​​generally below 1.8 N / 25 mm. Under long-term cycling conditions, the coating is prone to cracking or peeling, affecting battery life and reliability.

[0004] On the other hand, to improve electrolyte wetting performance and promote ion transport, some technologies employ organic polymer grafting strategies. For example, introducing hydrophilic segments such as polyethylene glycol (PEG) into the surface of polypropylene (PP) base membranes can reduce the contact angle to 35-45°, significantly improving wettability. However, the introduction of such flexible organic structures often sacrifices the material's heat resistance, resulting in a thermal shrinkage rate of over 5% at 180°C for the modified membrane, making it difficult to meet the safety requirements of high-temperature applications.

[0005] For low-temperature applications, adding antifreeze (such as fluoroethylene carbonate) to the electrolyte can lower the freezing point of the system, but it cannot solve the problem of mechanical property degradation caused by the increased rigidity of the membrane material under low-temperature conditions.

[0006] In addition, the preparation of composite slurries generally suffers from problems such as inorganic particle agglomeration, uneven dispersion and sedimentation, which further restricts the large-scale production and consistency control of high-performance coated diaphragms.

[0007] In summary, current separator modification technologies are limited by the interplay of material properties, making it difficult to achieve synergistic optimization of high heat resistance, high electrolyte wettability, and low-temperature adaptability, resulting in significant performance contradictions and technical bottlenecks. Therefore, overcoming these bottlenecks in synergistic material properties and developing composite coated separators that combine high heat resistance, high wettability, and freeze resistance while meeting industrialization needs has become one of the key technical challenges hindering the development of high-performance lithium-ion batteries.

[0008] In view of this, the present invention is proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a composite coated separator, its preparation method, and a secondary battery to improve the above-mentioned technical problems.

[0010] This invention is implemented as follows:

[0011] In a first aspect, the present invention provides a composite coated membrane, comprising a base membrane and a composite functional coating laminated on the surface of the base membrane. The composite functional coating comprises a first functional coating relatively close to the base membrane and a second functional coating relatively far from the base membrane. The porosity of the first functional coating is less than that of the second functional coating. The raw material components of both the first functional coating and the second functional coating include modified nanocellulose and fumed alumina with double grafting of cis-1,4-polybutadiene and polyethylene glycol segments.

[0012] In an optional embodiment, the porosity of the first functional coating is 30% to 38%, and the porosity of the second functional coating is 40% to 55%.

[0013] Preferably, the porosity of the first functional coating is 30%~35%, and the porosity of the second functional coating is 45%~55%.

[0014] Preferably, the pore size of the first functional coating is 50nm~100nm, and the pore size of the second functional coating is 150nm~200nm.

[0015] In an optional embodiment, the raw material components of the first functional coating and the second functional coating are the same, and the mass ratio of the modified nanocellulose to the fumed alumina is 1:(1.8~2.2).

[0016] And / or, the modified nanocellulose has a cis-1,4-polybutadiene grafting rate of 25%~35% and a polyethylene glycol segment grafting rate of 15%~28%;

[0017] And / or, the particle size of the fumed alumina is 80nm~120nm.

[0018] In an optional embodiment, the thickness of the composite functional coating is 1.8 μm to 2.2 μm;

[0019] And / or, the thickness of the base film is 5 μm to 20 μm;

[0020] And / or, the base film is selected from any one of PE base film, PP base film, or PP / PE / PP base film.

[0021] In an optional embodiment, a transition adhesive layer is provided between the composite functional coating and the base film. The transition adhesive layer is a polydopamine transition layer. The polydopamine transition layer forms hydrogen bonds and coordination bonds with the groups on the surface of the base film. The polydopamine transition layer forms covalent bonds with the composite functional coating through amino groups.

[0022] Preferably, the thickness of the polydopamine transition layer is 0.25 μm to 0.27 μm.

[0023] In an optional embodiment, the raw material components of both the first functional coating and the second functional coating further include at least one of the following components:

[0024] The adhesive is preferably selected from at least one of acrylates, polyacrylamide, polyacrylic acid, epoxy resins and acrylonitrile.

[0025] The dispersant is preferably selected from at least one of lecithin, chitosan, gum arabic, pectin, gelatin, sodium caseinate, and xanthan gum;

[0026] The antifreeze agent is preferably a compound of fluoroethylene carbonate and ethylene glycol dimethacrylate in a mass ratio of (2~3):1;

[0027] Crosslinking agent, preferably N,N"-methylenebisacrylamide.

[0028] Secondly, the present invention provides a method for preparing a composite coated membrane as described in any of the foregoing embodiments, comprising: pouring a composite functional coating slurry containing the modified nanocellulose and the fumed alumina into a coating tank; placing the base film on the unwinding roller of a coating machine; performing gradient coating through a dual coating head; adjusting the coating gap and setting the bottom coating speed to 15 m / min to 18 m / min and the surface coating speed to 25 m / min to 28 m / min to ensure continuous coating of the two wet films; and then curing and heat setting.

[0029] In an optional embodiment, the composite functional coating slurry comprises, by weight, 24-26 parts modified nanocellulose, 45-55 parts fumed alumina, 3-5 parts binder, 8-12 parts antifreeze agent, 0.3-0.5 parts dispersant and 1-3 parts crosslinking agent;

[0030] And / or, the solvent of the composite functional coating slurry is water, and the solid content is 20wt%~25wt%;

[0031] And / or, the preparation steps of the composite functional coating slurry include: adding each raw material component to water and then performing segmented ultrasonic treatment, wherein the segmented ultrasonic treatment includes first treating under ultrasonic power of 380W~420W for at least 8 minutes, and then treating under ultrasonic power of 680W~720W for at least 18 minutes. Preferably, the slurry temperature is controlled to be ≤30℃ during the ultrasonic treatment.

[0032] In an optional embodiment, the coating gap is adjusted to 0.08mm~0.12mm for the bottom layer coating and 0.08mm~0.12mm for the top layer coating, and the interval between the two wet film coatings is ≤30s.

[0033] And / or, curing and heat setting include pre-curing at 78~82℃ for 10~12 min, then irradiating with a 365nm UV lamp with a power of 80~100W for 5~6 min, and finally heat setting at 110~120℃ for 20~25 min;

[0034] And / or, the base film is pretreated with plasma before use. The pretreatment conditions are: power 380W~420W, Ar gas flow rate 15~25sccm, time 25~30s, and the O element content on the surface of the base film after pretreatment is ≥3.0% and the contact angle is ≤65°.

[0035] And / or, when the composite coated diaphragm has a transitional adhesive layer, the base film is activated and then treated in a dopamine hydrochloride solution before coating the composite functional coating slurry to form the transitional adhesive layer.

[0036] Thirdly, the present invention provides a secondary battery comprising a composite coated separator as described in any of the foregoing embodiments.

[0037] This invention offers the following advantages: By constructing a membrane coating with a gradient pore structure and combining it with advanced material modification technology, a composite coating membrane with multiple integrated functional properties has been successfully developed. The functional layers of this membrane employ an innovative gradient pore structure design: the bottom layer uses a low-porosity structure to ensure mechanical strength, while the surface layer uses a high-porosity structure to significantly enhance electrolyte wetting performance and permeation efficiency, thereby optimizing the battery's interfacial transport performance. In terms of the functional material system design, a combination of double-grafted modified nanocellulose and fumed alumina is used. Specifically, the nanocellulose undergoes synergistic grafting modification by simultaneously introducing cis-1,4-polybutadiene (PBD) and polyethylene glycol (PEG) segments, achieving a synergistic improvement in both antifreeze and wettability. This not only effectively inhibits the excessive aggregation of interhydroxyl hydrogen bonds at low temperatures, endowing the material with excellent low-temperature flexibility and structural stability, but also significantly enhances its affinity for the electrolyte and improves ion transport kinetics. The introduction of fumed alumina further enhances the heat resistance of the composite coating, effectively suppressing material shrinkage and thermal runaway risks under high-temperature conditions. Therefore, the developed composite coating membrane integrates high thermal stability, excellent electrolyte wettability, and low-temperature adaptability, maintaining stable structural and interfacial properties even under extreme temperature conditions. Simultaneously, the addition of a bio-based dispersant combined with segmented ultrasonication improves slurry storage stability. This preparation process possesses good scalability and industrial application prospects, suitable for the development and application needs of high-performance lithium-ion batteries and next-generation energy storage devices. Attached Figure Description

[0038] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a schematic diagram of the structure of the composite coating membrane provided in an embodiment of the present invention.

[0040] Icons: 1-Base film; 2-Transition adhesive layer; 3-Composite functional coating; 31-Modified nanocellulose; 32-Vacuum-phase alumina. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0042] Some embodiments of the present invention provide a composite coated membrane, which includes a base membrane and a composite functional coating laminated on the surface of the base membrane. The composite functional coating includes a first functional coating relatively close to the base membrane and a second functional coating relatively far from the base membrane. The porosity of the first functional coating is less than that of the second functional coating. The raw material components of both the first and second functional coatings include modified nanocellulose and fumed alumina with double grafting of cis-1,4-polybutadiene and polyethylene glycol segments.

[0043] Nanocellulose, a novel functional material derived from biomass, holds significant promise for applications in lithium-ion battery separators due to its excellent mechanical properties, good electrolyte compatibility, and outstanding environmental friendliness. Thermal analysis shows that the initial thermal decomposition temperature of nanocellulose can reach over 270℃, exhibiting significantly better thermal stability than traditional polyolefin-based separator materials. From a surface chemical perspective, the abundant hydroxyl functional groups in nanocellulose can form strong hydrogen bonds with polar electrolytes; however, pure nanocellulose tends to form rigid hydrogen bond clusters at low temperatures, hindering ion migration. Fumed alumina, a high-performance nanomaterial, possesses advantages such as high specific surface area, excellent thermal stability, and chemical inertness. Although fumed alumina coating technology can effectively improve the heat resistance of separators, existing technologies have not yet solved the key issues of slurry stability and adaptability to large-scale production. In the above embodiments, nanocellulose and vaporized alumina are creatively used together as functional components of the functional coating. Nanocellulose is synergistically modified by introducing cis-1,4-polybutadiene (PBD) and polyethylene glycol (PEG) segments, achieving a synergistic improvement in both the material's antifreeze and wettability properties. Specifically, PBD grafting effectively inhibits the excessive aggregation of interhydroxyl hydrogen bonds at low temperatures, endowing the material with excellent low-temperature flexibility and structural stability. PEG segment grafting significantly enhances its affinity for electrolytes and improves ion transport kinetics. The introduction of vaporized alumina further enhances the heat resistance of the composite coating, effectively suppressing the risk of material shrinkage and thermal runaway under high-temperature conditions.

[0044] In addition, the functional layers of the membrane adopt an innovative gradient pore structure design: the bottom layer adopts a low porosity structure to ensure mechanical strength, while the surface layer adopts a high porosity structure to significantly enhance electrolyte wetting performance and permeation efficiency, thereby optimizing the interfacial transport performance of the battery.

[0045] Therefore, the composite coated separator developed based on the above approach possesses excellent thermal stability, electrolyte wettability, and low-temperature adaptability, maintaining stable structural characteristics and interfacial properties even under extreme temperature conditions. Crucially, this fabrication process exhibits good compatibility with existing coating production lines, demonstrating significant scalability advantages and providing a reliable technical path for industrial applications. This innovative achievement effectively meets the development needs and application requirements of high-performance lithium-ion batteries and next-generation energy storage devices.

[0046] Specifically, in some embodiments, the porosity of the first functional coating is 30% to 38%, such as 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, or 38%, preferably 30% to 35%. The porosity of the second functional coating is 40% to 55%, such as 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, or 55%, preferably 45% to 55%.

[0047] Based on the specific porosity design of the first and second functional coatings, the first functional coating not only meets the basic requirements of ion transport channels but also has the following advantages: First, it significantly enhances the mechanical strength of the separator, effectively resisting the mechanical stress generated by the volume expansion of the electrodes during charging and discharging, thereby avoiding the risk of short circuits caused by separator puncture; second, it optimizes the contact state between the separator and the electrode interface, reducing interface impedance; third, it selectively blocks active material particles detached from the electrodes from entering the separator pores, preventing pore blockage from adversely affecting ion conduction efficiency. The second functional coating improves battery performance through the following mechanisms: First, its special structure significantly improves the adsorption capacity and wetting rate of the electrolyte, forming a highly efficient continuous ion transport network, effectively reducing ion migration resistance, thereby improving the rate performance of the battery; second, the high-porosity structure can store more electrolyte, alleviating the problem of electrolyte loss during cycling and extending the battery cycle life; in addition, the abundant pore structure also provides a larger contact interface for functional components, ensuring full contact with the electrolyte and electrodes, thereby more effectively exerting specific functional characteristics.

[0048] Furthermore, based on the above porosity values, in some embodiments, the pore size of the first functional coating is 50nm~100nm, and the pore size of the second functional coating is 150nm~200nm.

[0049] In some embodiments, the raw material composition of the first functional coating and the second functional coating is the same, which facilitates process control. Of course, in some other embodiments, the composition of the first functional coating and the second functional coating may be slightly different as needed.

[0050] In some methods, the mass ratio of modified nanocellulose to fumed alumina is 1:(1.8~2.2), such as 1:1.8, 1:1.9, 1:2, 1:2.1, or 1:2.2. The selection of these mass ratios achieves an optimal balance between the adhesive strength, structural stability, and ion conductivity of the functional coating, while also considering process operability and cost control.

[0051] In some embodiments, to achieve a better synergistic effect between antifreeze and wettability, the cis-1,4-polybutadiene grafting rate of the modified nanocellulose is 25% to 35%, for example, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, or 35%; the polyethylene glycol segment grafting rate is 15% to 28%, for example, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, or 28%. For example, the number average molecular weight of the polyethylene glycol is 2000.

[0052] In some embodiments, in order to achieve better uniform dispersion and enhanced heat resistance of fumed alumina, the particle size of fumed alumina can be selected as 80nm~120nm.

[0053] Furthermore, in some embodiments, the thickness of the base film is 5μm~20μm, and the thickness of the composite functional coating is 1.8μm~2.2μm. The base film (5μm~20μm) serves as the main body of the separator, providing core mechanical support and microporous ion channels. The composite coating (1.8μm~2.2μm) forms a continuous functional layer with an ultra-thin thickness, which not only plays a role in high temperature resistance, suppressing lithium dendrites, and enhancing adhesion, but also does not significantly increase the ion conduction path length, thus avoiding a decrease in rate performance. The combination of the two achieves a highly efficient synergy of "main body support + ultra-thin functional enhancement". In addition, the above thickness design avoids excessive rigidity and reduced flexibility of the separator due to excessive coating thickness, and also meets the requirements for the gradient structure formation of the coating with "low porosity at the bottom layer and high porosity at the surface layer". At the same time, the ultra-thin coating can form a tight interfacial adhesion with the base film, reducing the risk of coating detachment caused by electrode volume expansion during charging and discharging.

[0054] For reference, the base film can be selected from any one of PE base film, PP base film, or PP / PE / PP base film.

[0055] Furthermore, some embodiments of the present invention provide a composite coated membrane with a three-layer structure. A transition adhesive layer is disposed between the composite functional coating and the base membrane. The transition adhesive layer is a polydopamine transition layer. The polydopamine transition layer forms hydrogen bonds and coordination bonds with the groups on the surface of the base membrane, and the polydopamine transition layer forms covalent bonds with the composite functional layer through amino groups. The composite functional coating contains double-grafted modified nanocellulose and fumed alumina. The double bonding effect is achieved through the catechol groups in the polydopamine molecular structure: on the one hand, it forms hydrogen bonds with the substrate surface, and on the other hand, it forms covalent bonds with the upper coating. This synergistic mechanism significantly enhances the interfacial bonding strength, thereby effectively solving the technical problem of easy detachment of traditional coatings.

[0056] In order to achieve better bonding performance without affecting the membrane performance, in some embodiments, the thickness of the polydopamine transition layer is 0.25μm~0.27μm, for example 0.25μm, 0.26μm or 0.27μm.

[0057] In some embodiments, the raw material components of both the first functional coating and the second functional coating further include at least one of a binder, a dispersant, an antifreeze agent, and a crosslinking agent.

[0058] The addition of binders anchors nanocellulose and functional fillers, strengthens the adhesion between the coating and the base film, prevents coating detachment and pulverization during battery cycling, and ensures structural stability. The addition of dispersants improves the dispersibility of nanocellulose and fillers, prevents particle agglomeration, ensures uniform coating pores, and maintains unobstructed ion conduction channels. Antifreeze agents further enhance the coating's flexibility and electrolyte wettability at low temperatures, preventing coating cracking at low temperatures and ensuring the battery's low-temperature rate performance. Crosslinking agents promote the formation of crosslinked networks between nanocellulose molecules, enhancing the coating's mechanical strength, resistance to electrolyte swelling, and high-temperature resistance, thus reducing the risk of battery thermal runaway.

[0059] For example, the adhesive is preferably selected from at least one of acrylates, polyacrylamide, polyacrylic acid, epoxy resin and acrylonitrile; the dispersant is preferably selected from at least one of lecithin, chitosan, gum arabic, pectin, gelatin, sodium caseinate and xanthan gum; the antifreeze agent is preferably a mixture of fluoroethylene carbonate and ethylene glycol dimethacrylate in a mass ratio of (2~3):1; and the crosslinking agent is preferably N,N"-methylenebisacrylamide.

[0060] Some embodiments of the present invention also provide a method for preparing a composite coated diaphragm as described in any of the foregoing embodiments, comprising: pouring a composite functional coating slurry containing modified nanocellulose and fumed alumina into a coating tank, placing a base film on the unwinding roller of a coating machine, performing gradient coating through a dual coating head, adjusting the coating gap and setting the bottom layer coating speed to 15 m / min to 18 m / min and the surface layer coating speed to 25 m / min to 28 m / min to ensure continuous coating of the two wet films, followed by curing and heat setting.

[0061] In some embodiments, the preparation method of the composite coated diaphragm specifically includes the following steps:

[0062] S1, Base film pretreatment

[0063] Specifically, the pretreatment is plasma pretreatment, and the pretreatment conditions are: power 380W~420W, Ar gas flow rate 15~25sccm, time 25~30s, and the O element content on the base film surface after pretreatment is ≥3.0%, and the contact angle is ≤65°.

[0064] Pretreatment of the base film creates tiny pits and grooves on its surface, increasing surface roughness and specific surface area. This provides more physical anchoring points for the coating slurry, significantly reducing the risk of coating peeling. Furthermore, plasma can break the non-polar CC and CH bonds on the base film surface, introducing polar functional groups such as hydroxyl (-OH) and carboxyl (-COOH), transforming it from hydrophobic to hydrophilic. This enhances the chemical bonding (hydrogen bonds, covalent bonds) with polar coating components such as nanocellulose and ceramic fillers, improving interfacial compatibility.

[0065] S2, Preparation of transition adhesive layer

[0066] Specifically, the pretreated base film is placed in a dopamine hydrochloride solution to form a transitional adhesive layer.

[0067] In some embodiments, a dopamine hydrochloride solution is first prepared (e.g., a 10 mmol / L Tris-HCl buffer solution, pH=8.5±0.1, adjusted with NaOH solution); the base membrane is completely immersed in the above solution, placed in a constant temperature water bath, and magnetically stirred for reaction; the base membrane is removed, rinsed with deionized water to remove unreacted dopamine; and then dried in a forced-air drying oven.

[0068] S3, Preparation of modified nanocellulose

[0069] Specifically, nanocellulose was added to deionized water and ultrasonically dispersed to form a homogeneous suspension. Cis-1,4-polybutadiene (PBD) monomer and cerium ammonium nitrate (CAN) were added to the suspension, and N2 (99.999% purity) was introduced to remove oxygen. The mixture was placed in an oil bath and mechanically stirred. After the reaction was complete, hydroquinone (polymerization inhibitor) was added to terminate the reaction. Then, silane coupling agent KH570 was added, and the reaction continued after heating. Polyethylene glycol (PEG) modified with methacrylic anhydride and ammonium persulfate were added, nitrogen was introduced to remove oxygen, and the mixture was heated again and mechanically stirred. The reaction solution was centrifuged to remove ungrafted monomers, and the precipitate was washed with ethanol (analytical grade) and dried in a freeze dryer.

[0070] It should be noted that step S3 can be performed before or simultaneously with steps S1-S2, and there is no restriction on the order of these steps.

[0071] S3, Preparation and Dispersion Control of Composite Slurry

[0072] Specifically, the raw materials are weighed, and by weight, the composite functional coating slurry includes 24-26 parts of modified nanocellulose, 45-55 parts of fumed alumina, 3-5 parts of binder, 8-12 parts of antifreeze agent, 0.3-0.5 parts of dispersant and 1-3 parts of crosslinking agent.

[0073] After adding all raw material components to water, adjust the solid content to 20wt%~25wt%; then perform segmented ultrasonic treatment: first, treat under ultrasonic power of 380W~420W for at least 8 minutes to break down large agglomerates, and then treat under ultrasonic power of 680W~720W for at least 18 minutes to refine particle size. During ultrasonic treatment, control the slurry temperature ≤30℃ to avoid the volatilization of additives due to ultrasonic heat.

[0074] By adding a bio-based dispersant and using segmented ultrasonication, the slurry storage stability is ≥72h and the agglomerate particle size is ≤200nm.

[0075] S5. Preparation of composite functional coatings

[0076] Specifically, the modified base film is placed on the unwinding roller of a coating machine, and the composite slurry is poured into the coating tank. The coating gap is adjusted to 0.08mm~0.12mm for the bottom layer coating (i.e., the gap with the base film surface) and 0.08mm~0.12mm for the top layer coating (i.e., the gap with the bottom wet film). The interval between the two wet film coatings is ≤30s to avoid the bottom layer drying affecting the bonding, thus ensuring continuous coating of the two wet films. Subsequently, it is pre-cured at 78~82℃ for 10~12min to remove 80% of the solvent and avoid bubbling during subsequent curing. Then, it is irradiated with a 365nm UV lamp with a power of 80~100W for 5~6min, and the MBA crosslinking agent initiates the polymerization of double bonds to form a three-dimensional network structure. Finally, it is heat-set at 110~120℃ for 20~25min to further promote the crosslinking reaction and improve the density of the coating.

[0077] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0078] Example 1

[0079] This embodiment provides a composite coated diaphragm, the diaphragm structure of which is as follows: Figure 1 As shown, it includes a base film 1, a transition adhesive layer 2, and a composite functional coating 3, wherein the composite functional coating contains double-grafted modified nanocellulose 31 and fumed alumina 32. Its preparation method includes the following steps:

[0080] S1, Base film pretreatment

[0081] The base film was placed in a vacuum plasma treatment chamber, and Ar gas (purity 99.999%) was introduced at a flow rate of 20 sccm to maintain a chamber pressure of 10 Pa. The power was set to 400 W and the treatment time to 25 s. After treatment, the film was immediately transferred to a drying oven (humidity ≤30%) for later use.

[0082] S2, Preparation of transition adhesive layer

[0083] Prepare a 2 g / L dopamine hydrochloride solution (solvent: 10 mmol / L Tris-HCl buffer, pH=8.5, adjusted with NaOH solution); completely immerse the activated base membrane in the above solution, place it in a constant temperature water bath (30℃), and react magnetically (150 rpm) for 3 h; remove the base membrane and rinse with deionized water (resistivity ≥18.2 MΩ). Rinse three times (1 min each time) to remove unreacted dopamine; dry in a forced-air drying oven (80℃) for 2 h to obtain a 0.25±0.02μm adhesive layer.

[0084] S3, Preparation of modified nanocellulose

[0085] 10g of nanocellulose was added to 90 mL of deionized water and ultrasonically dispersed (300W, 10 min) to a concentration of 10wt% to form a homogeneous suspension. 8g of cis-1,4-polybutadiene (PBD) monomer and 0.5g of cerium ammonium nitrate (CAN) were added to the suspension, and N2 (99.999% purity) was introduced to remove oxygen for 30 min. The mixture was then placed in an oil bath (60℃) and mechanically stirred (300 rpm) for 4 h. After the reaction was complete, 0.1g of hydroquinone (polymerization inhibitor) was added to terminate the reaction. Finally, 4g of silane coupling agent KH57 was added. 0. Heat to 50℃ and continue the reaction for 2.5h; add 24.4g of methacrylic anhydride modified polyethylene glycol (PEG) and 0.05g of ammonium persulfate, purge with nitrogen (flow rate 20sccm) to remove oxygen for 15min, heat to 55℃, and mechanically stir (speed 250rpm) for 3h; centrifuge the reaction solution (8000pm, 20min) to remove ungrafted monomers, wash the precipitate 3 times with ethanol (analytical grade), and place it in a freeze dryer (-50℃, vacuum degree ≤10Pa) to dry for 48h.

[0086] S4. Preparation and Dispersion Control of Composite Slurry

[0087] Weigh out (by weight) 25 parts of double-grafted modified nanocellulose, 45 parts of fumed alumina, 4 parts of polyacrylamide binder, 8 parts of antifreeze agent (fluoroethylene carbonate: ethylene glycol dimethacrylate = 2:1), 0.3 parts of dispersant lecithin, and 1 part of crosslinking agent N,N'-methylenebisacrylamide (MBA); add deionized water to adjust the solid content to 20wt%, and place in an ultrasonic disperser; perform segmented ultrasonic treatment: first stage 400W, 10min; second stage 700W, 20min, during which an ice-water bath is used to control the slurry temperature ≤30℃.

[0088] S5, Composite Coating Preparation

[0089] The modified base film was placed on the unwinding roller of the coating machine, and the composite slurry was poured into the coating tank. The coating gap was adjusted (0.1 mm for the bottom layer and 0.1 mm for the top layer). The bottom layer coating speed was set to 15 m / min and the top layer coating speed to 25 m / min to ensure continuous coating of the two wet films (interval time ≤ 30 s to avoid the bottom layer drying affecting the bonding). Then it was placed in a forced-air drying oven (80℃) for pre-curing for 12 min. Then it was irradiated with a 365 nm ultraviolet lamp (power 80 W) for 6 min. Finally, it was placed in a hot air oven (110℃) for heat setting for 25 min.

[0090] Example 2

[0091] This embodiment provides a composite coated membrane, whose overall membrane structure is the same as in Embodiment 1, and whose preparation method includes the following steps:

[0092] S1, Same as Example 1;

[0093] S2, Same as Example 1;

[0094] S3, Preparation of modified nanocellulose

[0095] 10 g of nanocellulose was added to 90 mL of deionized water and ultrasonically dispersed (400 W, 20 min) to a concentration of 10 wt%, forming a homogeneous suspension. 12 g of PBD monomer and 0.75 g of CAN were added to the suspension, and N2 (99.999% purity) was introduced for deoxygenation for 30 min. The mixture was placed in an oil bath (60℃) and mechanically stirred (300 rpm) for 5 h. After the reaction was complete, 0.1 g of hydroquinone was added to terminate the reaction. Then, 8 g of KH570 was added, the temperature was raised to 50 ± 0.5℃, and the reaction continued for 2.5 h. 65.6 g of methacrylic anhydride-modified PEG and 0.07 g of ammonium persulfate were added, and nitrogen gas (20 sccm flow rate) was introduced for deoxygenation for 15 min. The temperature was raised to 55℃, and the mixture was mechanically stirred (250 rpm). The reaction mixture was centrifuged at 10000 rpm for 30 min to remove ungrafted monomers. The precipitate was washed 6 times with ethanol (analytical grade) and then dried in a freeze dryer (-50℃, vacuum ≤10Pa) for 56 h.

[0096] S4. Preparation and Dispersion Control of Composite Slurry

[0097] Weigh out (by weight) 25 parts of double-grafted modified nanocellulose, 45 parts of fumed alumina, 4 parts of polyacrylamide binder, 15 parts of antifreeze agent (fluoroethylene carbonate: ethylene glycol dimethacrylate = 3:1), 0.3 parts of dispersant lecithin, and 1 part of crosslinking agent N,N'-methylenebisacrylamide (MBA); add deionized water to adjust the solid content to 20wt%, and place in an ultrasonic disperser; perform segmented ultrasonic treatment: first stage 400 W, 10 min; second stage 700 W, 20 min, during which an ice-water bath is used to control the slurry temperature ≤30℃.

[0098] S5. Preparation of composite coating film: Adjust the UV curing parameters (100W, 6min) and oven temperature (120℃, 25min), and the remaining steps are the same as in Example 1.

[0099] Example 3

[0100] This embodiment provides a composite coated membrane, whose overall membrane structure is the same as in Embodiment 1, and whose preparation method includes the following steps:

[0101] S1, Same as Example 1;

[0102] S2, Same as Example 1;

[0103] S3, Preparation of modified nanocellulose

[0104] 10 g of nanocellulose was added to 90 mL of deionized water and ultrasonically dispersed (350 W, 15 min) to a concentration of 10 wt%, forming a homogeneous suspension. 10 g of PBD monomer and 0.625 g of CAN were added to the suspension, and N2 (99.999% purity) was introduced for deoxygenation for 30 min. The mixture was placed in an oil bath (60℃) and mechanically stirred (300 rpm) for 4.5 h. After the reaction, 0.1 g of hydroquinone (polymerization inhibitor) was added to terminate the reaction. Then, 6 g of KH570 was added, the temperature was raised to 50℃, and the reaction continued for 2.5 h. 49.2 g of methacrylic anhydride-modified PEG and 0.05 g of ammonium persulfate were added, and nitrogen gas (flow rate 20 sccm) was introduced for deoxygenation for 15 min. The temperature was raised to 55℃, and mechanical stirring (250 rpm) was continued. The reaction mixture was centrifuged at 8000 rpm for 20 min to remove ungrafted monomers. The precipitate was washed five times with ethanol (analytical grade) and then dried in a freeze dryer (-50℃, vacuum ≤10 Pa) for 48 h.

[0105] S4. Preparation and Dispersion Control of Composite Slurry

[0106] Weigh out (by weight) 25 parts of double-grafted modified nanocellulose, 50 parts of fumed alumina, 4 parts of polyacrylamide binder, 10 parts of antifreeze agent (fluoroethylene carbonate: ethylene glycol dimethacrylate = 2.5:1), 0.3 parts of dispersant lecithin, and 1 part of crosslinking agent N,N'-methylenebisacrylamide (MBA); add deionized water to adjust the solid content to 20 wt%, and place in an ultrasonic disperser; perform segmented ultrasonic treatment: first stage 400W, 10min; second stage 700W, 20min, during which an ice-water bath is used to control the slurry temperature ≤30℃.

[0107] S5, Composite Coating Preparation

[0108] The PDA-modified base film was placed on the unwinding roller of the coating machine, and the composite slurry was poured into the coating tank. The coating gap was adjusted (0.1 mm for the bottom layer and 0.1 mm for the top layer). The bottom layer coating speed was set to 15 m / min and the top layer coating speed to 25 m / min to ensure continuous coating of the two wet films (interval time ≤ 30 s). Then, it was placed in a forced-air drying oven (80℃) for pre-curing for 10 min. Then, it was irradiated with a 365nm ultraviolet lamp (100W power) for 5 min. Finally, it was placed in a hot air oven (120℃) for heat setting for 20 min.

[0109] Example 4

[0110] This embodiment provides a composite coated membrane, whose overall membrane structure is the same as in Embodiment 1, and whose preparation method includes the following steps:

[0111] Only the preparation and dispersibility control of the S4 composite slurry were changed; the remaining steps and parameters were the same as in Example 1.

[0112] S4. Preparation and Dispersion Control of Composite Slurry

[0113] Weigh out (by weight) 25 parts of double-grafted modified nanocellulose, 55 parts of fumed alumina, 4 parts of polyacrylamide binder, 12 parts of antifreeze agent (fluoroethylene carbonate: ethylene glycol dimethacrylate = 3:1), 0.5 parts of dispersant chitosan, and 1 part of crosslinking agent N,N'-methylenebisacrylamide (MBA); add deionized water to adjust the solid content to 25 wt%, and place in an ultrasonic disperser; perform segmented ultrasonic treatment: first stage 400W, 10 min; second stage 800W, 20 min, during which an ice-water bath is used to control the slurry temperature ≤30℃.

[0114] Comparative Example 1

[0115] The only change was to step S2 in the preparation of modified nanocellulose; the parameters of the remaining steps were the same as in Example 1.

[0116] S2, Preparation of modified nanocellulose

[0117] 10g of nanocellulose was added to 90mL of deionized water and ultrasonically dispersed (300W, 10min) to a concentration of 10wt% to form a homogeneous suspension. 8g of KH570 was added to the suspension, and the temperature was raised to 50℃, and the reaction was continued for 2.5h. 65.6g of methacrylic anhydride-modified PEG and 0.07g of ammonium persulfate were added, and nitrogen gas (flow rate 20sccm) was introduced to remove oxygen for 15min. The temperature was raised to 55℃, and the mixture was mechanically stirred (250rpm) for 4h. The reaction solution was centrifuged (8000rpm, 20min) to remove ungrafted monomers. The precipitate was washed three times with ethanol (analytical grade) and dried in a freeze dryer (-50℃, vacuum degree ≤10Pa) for 48h.

[0118] Comparative Example 2

[0119] This comparative example features a polydopamine-free transition adhesive layer design and does not include nanocellulose in the raw materials. The remaining steps are the same as in Example 1.

[0120] Comparative Example 3

[0121] This method does not add nanocellulose to the raw materials, and the remaining steps are the same as in Example 1.

[0122] Taking Example 1 as an example, by adjusting only the velocity difference between the bottom layer and the surface layer of the gradient coating and the wet film thickness, three sets of examples and two sets of comparative examples were designed to explore the influence of process parameters on pore gradient and performance.

[0123] Example 5

[0124] Compared with Example 1, only the bottom coating speed was changed to 18 m / min and the top coating speed was changed to 22 m / min, while the parameters of the other steps remained unchanged.

[0125] Example 6

[0126] Compared with Example 1, only the bottom coating speed was changed to 12m / min and the top coating speed was changed to 30m / min, while the other step parameters remained unchanged.

[0127] Comparative Example 4

[0128] Compared with Example 1, only the bottom coating speed and the top coating speed were changed to 22 m / min, while the parameters of the other steps remained unchanged.

[0129] Comparative Example 5

[0130] Compared with Example 1, only the bottom coating speed was changed to 10 m / min and the top coating speed to 35 m / min, while the other step parameters remained unchanged.

[0131] The membranes prepared in Examples 1-6 and Comparative Examples 1-5 were subjected to performance tests. The test items are as follows, and the test results are shown in Tables 1 and 2.

[0132] 1. Heat shrinkage rate: Cut a 120mm × 120mm diaphragm sample. Lightly draw markings (L0 = 100mm) along the MD (longitudinal) and TD (transverse) directions of the diaphragm on the sample surface. Place the sample between two sheets of A4 paper in a 180℃ oven for 30 minutes. After heating, remove the sample and allow it to cool to room temperature until stable. Measure the final spacing L1 between the markings. Calculation: Heat shrinkage rate = (L0 - L1) / L0 × 100%. Calculate the values ​​in the MD and TD directions respectively, and take the average of three tests.

[0133] 2. Tensile strength: Cut 15mm wide strips of specimen along the MD (longitudinal) and TD (transverse) directions of the diaphragm. Before testing, measure the specimen thickness and record it on the software. Then fix the specimen on the tensile testing machine fixture, ensuring that the specimen axis is consistent with the tensile direction. Adjust the fixture spacing to 100±5mm, set the test speed to 250mm / min, and click Start Test until the specimen breaks. The equipment will automatically record the relevant data and take the average value of three tests.

[0134] 3. Puncture strength: Take an undamaged and wrinkle-free diaphragm sample with a length and width of 50mm×50mm. Fix the diaphragm flat between the upper and lower clamps. Start the equipment to allow the puncture needle to puncture vertically until the sample breaks. The equipment automatically records the relevant data and takes the average value of three calculations.

[0135] 4. Peel Strength: Fix the diaphragm sample, attach it to one side of the ceramic coating with peeling tape, and cut it into a strip 19mm wide and 100mm long. Fix the sample in the upper and lower clamps of the testing machine, and run the universal tensile testing machine until the sample is completely peeled off. The tensile strength is the peel strength. Calculation: Peel strength (N / m) = Average peel force (N) ÷ Sample width (m), take the average of three calculations.

[0136] 5. Electrolyte Contact Angle and Complete Spreading Time: The diaphragm sample is fixed on the sample stage of the contact angle tester. Then, the electrolyte is suspended and dropped onto the sample surface. The image is captured by a high-resolution camera, and the angle is automatically measured to obtain the contact angle. The time when the contact angle is ≤5° is recorded as the complete spreading time of the electrolyte on the diaphragm sample.

[0137] 6. Ionic Conductivity: Cut the separator sample into 19mm round pieces. Place the positive electrode piece in the center of the positive electrode shell, then place the cut separator sample on top. After thoroughly wetting the sample with electrolyte by adding drops of electrolyte using a pipette, place the negative electrode piece, gasket, spring contact, and negative electrode shell in sequence, and then encapsulate it into a button cell. Use an electrochemical workstation to set the test parameters (frequency range 1Hz-10 Hz). 6 (Hz, AC signal amplitude 10mV), scan to obtain the Nyquist plot (impedance spectrum). Read the diaphragm resistance R from the Nyquist plot. b Substituting into the formula: σ=L / (R) b (σ is the ionic conductivity, S / cm; L is the membrane thickness, cm; S is the effective electrode area, cm²) 2 ).

[0138] 7. Coating cracking rate after cycling: The diaphragm after 200 cycles of 1C charge-discharge was cut into standard samples of 2cm × 2cm and fixed on a glass slide with conductive adhesive. Using an optical microscope (low magnification for positioning) and a scanning electron microscope (SEM, high magnification for detail observation), at least 5 non-overlapping areas (each area not less than 0.1mm²) were randomly selected for photographing. The cracked area S1 was manually outlined or automatically identified using ImageJ. The total coating area S0 of each observed area was calculated and calculated using the formula "single-area cracking rate = (S1 / S0) × 100%". The final cracking rate was the average of the 5 areas.

[0139] 8. Coating peeling rate after cycling: Weigh the coated diaphragm before cycling using an electronic balance (m0) (subtracting the mass of the uncoated substrate diaphragm m0). 基Next, gently wipe the surface powder off the diaphragm after circulation with anhydrous ethanol, dry it to constant weight, and weigh it (m1). Finally, calculate the shedding rate using the formula: "Shedding rate = ((m0 - m1) / ... 基 )-(m1-m 基 )) / (m0-m 基 Calculate using ")×100%", repeat the test 3 times and take the average value.

[0140] 8. Particle size test: The composite slurry was tested using an Omec particle size analyzer under the following conditions: refractive index 1.52.

[0141] 9. Diaphragm porosity test: The porosity of the bottom layer and the top layer of the gradient coating was tested by combining image analysis and immersion method. (1) Image analysis method: SEM+EDS was used for positioning, and the effective area of ​​the bottom layer (20%~50% coating thickness on the interface) and the top layer (0~30% coating thickness below the outer surface) was determined by element distribution; 5 fields of view were selected for each area to collect images, and the pore area ratio was analyzed by software such as ImageJ. Surface / bottom layer porosity = ((surface / bottom layer) pore area / (surface / bottom layer) total field area) × 100%, and the average value of 5 fields of view was taken. (2) Immersion method: The initial mass m0 of the sample was weighed, and it was immersed in anhydrous ethanol in a vacuum immersion device. After vacuuming, it was soaked; the surface was wiped dry, and the mass m1 after immersion was weighed; the error was deducted by a blank uncoated base film, and the open pore porosity was calculated according to the ethanol density and compared with the results of the image method. Porosity = (Sample liquid absorption mass (m1-m0) - Blank uncoated base film liquid absorption mass m2) / (Anhydrous ethanol density × Test area volume) × 100%. Where the test area volume = Sample planar area × Corresponding effective coating thickness. Effective coating thickness: Underlayer effective thickness = Total underlayer thickness × (50%-20%), Surface layer effective thickness = Total surface layer thickness × 30%.

[0142] Table 1

[0143]

[0144] Based on the performance data comparison and analysis in Table 1, the following conclusions can be drawn:

[0145] 1. Thermal stability advantage: As shown in Table 1, the thermal shrinkage rate of Example 3 at 180°C is only 1.8% in the MD direction and 2.0% in the TD direction, which is about 78% lower than that of the pure base film and about 48% lower than that of the single fumed alumina coating (Comparative Example 3). This significant improvement is attributed to the synergistic effect of the high rigidity skeleton of fumed alumina and the three-dimensional network structure formed by UV crosslinking, which effectively suppresses the shrinkage behavior of the base film under high temperature conditions.

[0146] 2. Breakthrough in interfacial bonding: The peel strength of the coating in Example 3 reached 4.1 N / 25 mm, which is 173% higher than the solution without a transition layer (Comparative Example 2) and 135% higher than the single Al2O3 coating (Comparative Example 3). This performance improvement is mainly due to the dual bonding mechanism of "hydrogen bond-covalent bond" in the polydopamine transition adhesive layer, which significantly enhances the interfacial bonding strength.

[0147] 3. Improved wetting efficiency: The electrolyte in Example 3 spread completely in 0.07 s, which is 98.6% shorter than that of the pure base film and 68% shorter than that of the solution without a transition layer (Comparative Example 2). This improvement is due to the synergistic effect of the hydrophilic groups introduced by PEG grafting modification and the surface activity of polydopamine, which enables rapid wetting of the electrolyte.

[0148] 4. Low-temperature ion conductivity: The ion conductivity of Example 2 at -20°C was 0.82 mS / cm, significantly higher than the 0.15 mS / cm of the base membrane, representing a 141.2% improvement over the single grafting scheme (Comparative Example 1). This performance improvement can be attributed to the antifreeze flexibility properties imparted to the membrane by the cis-1,4-polybutadiene grafting in the system. By disrupting the low-temperature hydrogen bond cluster structure, it effectively reduces ion transport resistance, thereby significantly improving the ion conductivity under low-temperature conditions.

[0149] 5. Mechanical property optimization: The puncture strength of Examples 2 and 3 is better than that of Example 1. This is mainly due to the increase in PBD content in the double-grafted modified nanocellulose system, which lowers the glass transition temperature of the material and enhances the flexibility of the molecular chain segments, thereby improving the mechanical properties of the diaphragm.

[0150] 6. Industrial-scale slurry stability: Examples 1-4 all showed significant improvements in slurry storage stability compared to Comparative Example 2. By employing a bio-based dispersant combined with a segmented ultrasonic treatment process, 72-hour storage stability of the slurry was achieved, with agglomerate particle size controlled within 200 nm. In particular, in Example 4, by increasing the amount of bio-based dispersant, the slurry storage stability was extended to 96 hours, while maintaining the agglomerate particle size within 200 nm.

[0151] 7. Long-term cycling stability: After 60 cycles (200 tests), no coating cracking or peeling was observed in Examples 1-4 and Comparative Example 1, while the coating cracking and peeling rate of Comparative Example 2 reached 20%. This comparative result fully confirms that the "transitional bonding layer + gradient pores + double-grafted modified nanocellulose-vapor phase alumina composite coating" structure proposed in this invention has excellent long-term stability.

[0152] Table 2

[0153]

[0154] Based on the data shown in Table 2, the following analysis results can be obtained:

[0155] 1. Heat resistance performance analysis: By constructing a dense underlying structure, Example 1 effectively suppressed the transmission of high-temperature stress, and its thermal shrinkage rate at 180℃ was significantly reduced by 38% compared with Comparative Example 4. In contrast, Comparative Example 5 exhibited the worst thermal stability due to the presence of coating delamination.

[0156] 2. Mechanical Properties: Thanks to the special structural design of "dense bottom layer and loose surface layer", the tensile strength of Example 1 is 25.4% higher than that of Comparative Example 4, with the dense bottom layer providing good mechanical support. In contrast, Comparative Example 5, which suffered from coating delamination due to improper speed gradient control during preparation, exhibited significantly deteriorated mechanical properties.

[0157] 3. Wetting properties and ion conduction characteristics: The data in Table 2 show that the high porosity structure of the surface layer of Example 1 provides an efficient channel for ion transport, resulting in a 44.7% increase in its conductivity at 25°C compared to Comparative Example 4. However, Example 5, due to its poor pore gradient design, exhibits poor performance in both conductivity and electrolyte spreading speed.

[0158] 4. Cyclic Stability Assessment: After 200 cycles, Example 1 exhibited excellent cyclic stability, with a cracking rate of 0% and a shedding rate of only 0.3%, representing improvements of 100% and 83.3% respectively compared to Comparative Example 4. This advantage mainly stems from its gradient structure design: 1) The moderate "dense-loose" gradient effectively buffered the electrolyte swelling stress; 2) Compared to Example 5, which had an insignificant pore gradient, it had better stress buffering capacity; 3) Compared to Example 6, which suffered from a decrease in interfacial bonding due to an excessive gradient, its interfacial stability was more outstanding.

[0159] In summary, the embodiments of this invention, through the design of a "transitional adhesive layer + gradient pores + double-grafted modified nanocellulose-fumed alumina composite coating" structure, address the interfacial bonding problem with the polydopamine adhesive layer, achieve synergistic effects of antifreeze and wettability through the introduction of double-grafted modified nanocellulose and antifreeze agents, and ensure the heat resistance of the membrane. The prepared composite coated membrane possesses the following excellent properties:

[0160] 1. Breakthrough in antifreeze performance: Ionic conductivity reaches 0.61~0.82 mS / cm at -20℃, which is more than 3 times higher than that of traditional coatings;

[0161] 2. High heat resistance and stability: The heat shrinkage rate at 180℃ is only 1.8%~2.2%, far exceeding the industry standard (heat shrinkage rate ≤5%).

[0162] 3. Ultra-fast wetting: Electrolyte contact angle ≤20°, wetting time <0.1s.

[0163] 4. Excellent mechanical properties: puncture strength ≥780MPa, peel strength ≥3N / 25mm, meeting the stringent requirements of power batteries.

[0164] 5. Industrial-scale slurry optimization: Slurry storage stability ≥72h, agglomerate particle size ≤200nm.

[0165] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite coated diaphragm, characterized in that, It includes a base film and a composite functional coating laminated on the surface of the base film. The composite functional coating includes a first functional coating relatively close to the base film and a second functional coating relatively far from the base film. The porosity of the first functional coating is less than that of the second functional coating. The raw material components of the first functional coating and the second functional coating both include modified nanocellulose and fumed alumina with double grafting of cis-1,4-polybutadiene and polyethylene glycol segments. The preparation steps of the modified nanocellulose include: adding nanocellulose to deionized water and dispersing it to form a uniform suspension; Add cis-1,4-polybutadiene monomer and cerium ammonium nitrate to the above suspension, and purge with nitrogen to remove oxygen; place in an oil bath and mechanically stir the reaction. After the reaction is complete, add hydroquinone to terminate the reaction; then add silane coupling agent, heat and continue the reaction; add methacrylic anhydride-modified polyethylene glycol and ammonium persulfate, purge with nitrogen to remove oxygen, heat and mechanically stir the reaction again; centrifuge the reaction solution to remove ungrafted monomers, wash the precipitate with ethanol and then dry.

2. The composite coated diaphragm according to claim 1, characterized in that, The porosity of the first functional coating is 30%~38%, and the porosity of the second functional coating is 40%~55%.

3. The composite coated diaphragm according to claim 2, characterized in that, The porosity of the first functional coating is 30%~35%, and the porosity of the second functional coating is 45%~55%.

4. The composite coated diaphragm according to claim 2, characterized in that, The pore size of the first functional coating is 50nm~100nm, and the pore size of the second functional coating is 150nm~200nm.

5. The composite coated diaphragm according to claim 1, characterized in that, The first functional coating and the second functional coating have the same raw material composition, and the mass ratio of the modified nanocellulose to the fumed alumina is 1:(1.8~2.2). And / or, the modified nanocellulose has a cis-1,4-polybutadiene grafting rate of 25%~35% and a polyethylene glycol segment grafting rate of 15%~28%; And / or, the particle size of the fumed alumina is 80 nm to 120 nm.

6. The composite coated diaphragm according to claim 1, characterized in that, The thickness of the composite functional coating is 1.8 μm to 2.2 μm; And / or, the thickness of the base film is 5 μm to 20 μm; And / or, the base film is selected from any one of PE base film, PP base film, or PP / PE / PP base film.

7. The composite coated diaphragm according to any one of claims 1 to 6, characterized in that, A transition adhesive layer is provided between the composite functional coating and the base film. The transition adhesive layer is a polydopamine transition layer. The polydopamine transition layer forms hydrogen bonds and coordination bonds with the groups on the surface of the base film. The polydopamine transition layer forms covalent bonds with the composite functional coating through amino groups.

8. The composite coated diaphragm according to claim 7, characterized in that, The thickness of the polydopamine transition layer is 0.25 μm to 0.27 μm.

9. The composite coated diaphragm according to any one of claims 1 to 6, characterized in that, The raw material components of both the first functional coating and the second functional coating further include at least one of the following components: An adhesive, wherein the adhesive is selected from at least one of acrylates, polyacrylamide, polyacrylic acid, epoxy resin and acrylonitrile; The dispersant is selected from at least one of lecithin, chitosan, gum arabic, pectin, gelatin, sodium caseinate, and xanthan gum; The antifreeze agent is a compound of fluoroethylene carbonate and ethylene glycol dimethacrylate in a mass ratio of (2~3):1; The crosslinking agent is N,N'-methylenebisacrylamide.

10. A method for preparing a composite coated diaphragm as described in any one of claims 1 to 9, characterized in that, It includes: The composite functional coating slurry containing the modified nanocellulose and the fumed alumina is poured into the coating tank. The base film is placed on the unwinding roller of the coating machine and coated by gradient coating through dual coating heads. The coating gap is adjusted and the bottom coating speed is set to 15m / min~18m / min and the surface coating speed is set to 25m / min~28m / min to ensure continuous coating of the two wet films. Then, it is cured and heat-set.

11. The method for preparing the composite coated diaphragm according to claim 10, characterized in that, By weight, the composite functional coating slurry comprises 24-26 parts modified nanocellulose, 45-55 parts fumed alumina, 3-5 parts binder, 8-12 parts antifreeze agent, 0.3-0.5 parts dispersant and 1-3 parts crosslinking agent; And / or, the solvent of the composite functional coating slurry is water, and the solid content is 20wt%~25wt%; And / or, the preparation steps of the composite functional coating slurry include: adding each raw material component to water and then performing segmented ultrasonic treatment, wherein the segmented ultrasonic treatment includes first treating under ultrasonic power of 380W~420W for at least 8 minutes, and then treating under ultrasonic power of 680W~720W for at least 18 minutes, and controlling the slurry temperature ≤30℃ during the ultrasonic treatment.

12. The method for preparing the composite coated diaphragm according to claim 10 or 11, characterized in that, The coating gap is adjusted to 0.08mm~0.12mm for the bottom layer coating and 0.08mm~0.12mm for the top layer coating. The interval between the two wet film coatings is ≤30s. And / or, curing and heat setting include pre-curing at 78~82℃ for 10~12 min, then irradiating with a 365nm UV lamp with a power of 80~100W for 5~6 min, and finally heat setting at 110~120℃ for 20~25 min; And / or, the base film is pretreated with plasma before use. The pretreatment conditions are: power 380W~420W, Ar gas flow rate 15~25sccm, time 25~30s, and the oxygen content on the surface of the base film after pretreatment is ≥3.0% and the contact angle is ≤65°. And / or, when the composite coated diaphragm has a transitional adhesive layer, the base film is activated and then treated in a dopamine hydrochloride solution before coating the composite functional coating slurry to form the transitional adhesive layer.

13. A secondary battery, characterized in that, It includes the composite coated diaphragm as described in any one of claims 1 to 9.

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