A high-temperature resistant lithium battery separator based on cellulose nanofibers and its green preparation method

Abstract

This invention discloses a high-temperature resistant lithium battery separator based on cellulose nanofibers and its green preparation method, belonging to the field of lithium battery separator technology. It includes a CNF base film and a functionally graded composite layer constructed thereon, specifically comprising a dense trapping region, a smart response region, and a gradient transition region located between the dense trapping region and the smart response region. Through systematic integration and innovation of base film self-support, surface activation, gradient functional coating design, and controllable coating, this invention overcomes key technical bottlenecks of traditional coated separators, such as easy coating peeling, single function, insufficient thermomechanical stability, and lack of active protection under thermal abuse conditions. It successfully prepares a novel lithium battery separator that combines inherently high thermal stability, efficient dendrite suppression, and active thermal intervention response, providing an effective solution for improving the safety performance and cycle life of high-energy-density lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium battery separator technology, and in particular to a high-temperature resistant lithium battery separator based on cellulose nanofibers and its green preparation method. Background Technology

[0002] With the rapid development of electric vehicles and large-scale energy storage industries, unprecedentedly high standards have been set for the overall performance of lithium-ion batteries, especially their safety, reliability, and cycle life. As a key component inside the battery, the separator's performance directly determines the battery's safety margin and the stability of its electrochemical behavior.

[0003] Self-supporting separators based on cellulose nanofibrils (CNF) are considered ideal materials to replace traditional polyolefin separators due to their renewable raw materials, excellent thermal stability, and good electrolyte affinity. However, pure cellulose separators still face a key structural and functional bottleneck when moving towards high-safety and high-performance applications. Their homogeneous porous structure primarily provides basic physical isolation and ion transport functions, making it difficult to efficiently and firmly integrate advanced safety functions such as dendrite suppression and intelligent response without compromising their inherent excellent performance, thus hindering the construction of an integrated active protection system. Simply coating the surface of the cellulose-based membrane often results in weak interfacial bonding, easy peeling of the functional layer, or blockage of ion channels, causing its overall performance to fail to meet the stringent requirements of high-energy-density batteries.

[0004] Therefore, how to use cellulose nanofiber membranes as a platform and achieve robust integration and synergistic effect of multiple active safety functions through innovative structural design and process research is a core technical problem that urgently needs to be solved in the development of next-generation intrinsically safe lithium battery separators. Summary of the Invention

[0005] The purpose of this invention is to provide a high-temperature resistant lithium battery separator based on cellulose nanofibers and its green preparation method, in order to solve the key technical problem that existing cellulose-based separators have limited functionality and are difficult to integrate advanced safety mechanisms efficiently without sacrificing their inherent excellent performance. This invention strengthens the interfacial bonding through surface activation technology and integrates dense ion sieving and intelligent thermal response units into the cellulose-based membrane in an orderly manner through controllable preparation of gradient functional coatings. Thus, while maintaining its inherent high thermal stability and high ion conductivity, it successfully endows the separator with active dendrite suppression and thermal runaway intervention capabilities.

[0006] To achieve the above objectives, the present invention provides the following solution: A high-temperature resistant lithium battery separator based on cellulose nanofibers (CNFs) is characterized by comprising a CNF base film and a functionally graded composite layer constructed thereon. The composition and function of the functionally graded composite layer change gradiently from the side closer to the base film to the side farther away from the base film, specifically including: The dense trapping zone is formed by cross-linking and curing of slurry A, which includes metal-organic framework powder, cellulose nanocrystals grafted with poly(N-isopropylacrylamide), gelatin, and tannic acid. The intelligent response zone is formed by cross-linking and curing of slurry B, which contains temperature-sensitive microcapsules loaded with LiNO3, polyurethane urea self-healing polymer microspheres, polyN-isopropylacrylamide, and hydrophilic nano-silica. And the gradient transition region located between the dense capture region and the smart response region; In this process, the components of slurry A and slurry B diffuse into each other and form an interpenetrating network structure; the CNF-based membrane is composed of carboxylated cellulose nanofibers and sodium alginate cross-linked through a hydrogen bond network, and its surface is activated by gelatin molecules.

[0007] Furthermore, the aforementioned polyurethane urea self-healing polymer microspheres are microspheres with a dynamic hydrogen bond network inside. This dynamic hydrogen bond network can rearrange and rebuild at the point of damage caused by external force within the operating temperature range of the diaphragm, thereby achieving self-repair.

[0008] The key to the above-mentioned method for preparing lithium battery separators lies in the following specific aspects: S1. Preparation and activation of the base film: Carboxylated cellulose nanofibers and sodium alginate were mixed and dispersed in water, and CNF base film was obtained by casting and drying; then, it was impregnated and activated with gelatin solution to obtain surface activated CNF base film. S2. Preparation of functional slurry: Prepare high-viscosity slurry A containing metal-organic framework powder, cellulose nanocrystals grafted with poly(N-isopropylacrylamide), gelatin and tannic acid; prepare low-viscosity slurry B containing temperature-sensitive microcapsules loaded with LiNO3, polyurethane urea self-healing polymer microspheres, poly(N-isopropylacrylamide) and hydrophilic nano-silica. S3. Gradient coating and structure locking: Slurry A and slurry B are coated sequentially on the above surface-activated base film. After preliminary gelation induced by low temperature, the film is left to stand in a constant temperature and humidity environment to build a gradient interface. Then, it is immersed in an anhydrous ethanol coagulation bath for solvent replacement and structure locking. S4. Post-processing and performance integration: The structure-locked wet film is strengthened by green cross-linking and then subjected to low-temperature vacuum drying and inert atmosphere heat treatment to obtain the above-mentioned high-temperature resistant lithium battery separator.

[0009] Furthermore, in S1, the mass ratio of the above-mentioned carboxylated cellulose nanofibers to sodium alginate is 1:(0.05-0.15); the mass concentration of the gelatin solution is 0.1%-0.3%, and the impregnation time is 10s-30s; the thickness of the above-mentioned CNF base film is 20μm-50μm.

[0010] Furthermore, in S2, the preparation process of the above-mentioned cellulose nanocrystals grafted with poly(N-isopropylacrylamide) is as follows: 100 parts of cellulose nanocrystals are dispersed in 200-300 parts of deionized water, and 20-30 parts of N-isopropylacrylamide monomer and 1-2 parts of potassium persulfate initiator are added sequentially; the system is heated to 70-75°C and reacted for 4-6 hours; after the reaction is completed, the cellulose nanocrystals are obtained by centrifugation, washing, dialyzing and freeze-drying.

[0011] Furthermore, in S2, the viscosity of the above-mentioned slurry A at 25°C is 2000 mPa·s to 3500 mPa·s; and the viscosity of the above-mentioned slurry B at 25°C is 400 mPa·s to 800 mPa·s.

[0012] Specifically, in S2, the preparation process of the above-mentioned temperature-sensitive microcapsules loaded with LiNO3 is as follows: 20 parts of a LiNO3 solution with a mass concentration of 10% to 20% are added to 100 parts of a gelatin solution with a mass concentration of 2% to 4%, and then 100 parts of a gum arabic solution with a mass concentration of 2% to 4% are added. The mixture is stirred and the pH is adjusted to 3.8 to 4.2. The temperature is lowered to 5℃ to 10℃, and 5 to 15 parts of glutaraldehyde are added as a crosslinking agent. The mixture is reacted at 100 r / min to 200 r / min for 2 h to 4 h. After centrifugation, washing, and freeze-drying, the temperature-sensitive microcapsules loaded with LiNO3 are obtained.

[0013] Specifically, in S2, the preparation process of the above-mentioned polyurethane urea self-healing polymer microspheres is as follows: 100 parts of polyether diol are mixed with 20 to 35 parts of isophorone diisocyanate and reacted at 80°C to 85°C for 2 to 3 hours to obtain isocyanate-terminated polyurethane prepolymer; the temperature is lowered to 40°C to 60°C, and 5 to 15 parts of chain extender adipate dihydrazide and 100 to 200 parts of acetone are added; under high-speed shearing at 8000 r / min to 12000 r / min, the mixture is added dropwise to 300 parts of deionized water containing 3 to 8 parts of polyvinyl alcohol, and emulsified for 5 to 10 minutes to form a stable emulsion; the emulsion is transferred to a water bath at 70°C to 80°C and reacted at 300 to 500 r / min for 4 to 6 hours; after the reaction is completed, the mixture is centrifuged, washed, and vacuum dried to obtain polyurethane urea self-healing polymer microspheres.

[0014] Furthermore, in S3, the conditions for constructing the gradient interface are: in a closed environment with a temperature of 22℃~28℃ and a relative humidity of 85%~90%, the surface is left to stand horizontally for 5min~10min.

[0015] Furthermore, in S4, the aforementioned green crosslinking enhancement is achieved by treating the sample with a genipin aqueous solution at a mass concentration of 0.1% to 0.5% at 40°C to 50°C for 30 to 60 minutes.

[0016] The present invention discloses the following technical effects: This invention provides a high-temperature resistant lithium battery separator based on cellulose nanofibers and its preparation method. Through systematic integration and innovation of steps such as base film self-support, surface activation, gradient functional coating design, and controllable coating, it overcomes key technical bottlenecks of traditional coated separators, including easy peeling of coatings, limited functionality, insufficient thermomechanical stability, and lack of active protection under thermal abuse conditions. Specific technical effects are reflected in the following aspects: First, this invention constructs a self-supporting base membrane with inherently high thermal stability and a firmly integrated gradient functional coating. The self-supporting base membrane, constructed from a cellulose nanofiber / sodium alginate composite system, possesses a three-dimensional interconnected network structure, fundamentally overcoming the risk of high-temperature melting and shrinkage in traditional polyolefin membranes and endowing the membrane with excellent thermal dimensional stability. Through surface hydrogen bond network activation treatment, a strong bonding interface dominated by multiple hydrogen bonds is established between the base membrane and the functional coating, achieving high-strength coating adhesion. Furthermore, through viscosity gradient design and a controllable coating process, a gradient composite structure transitioning from a dense trapping layer to a smart response layer is formed, enabling the orderly synergy of functions such as mechanical support, ion sieving, dendrite suppression, and thermal triggering response, thereby improving the overall performance and reliability of the membrane.

[0017] Secondly, this invention achieves active safety protection for the separator under thermal abuse conditions. By introducing thermally triggered release microcapsules into the smart response layer, when the internal temperature of the battery abnormally rises, an interface stabilizer can be released directionally to repair and stabilize the electrode / electrolyte interface in situ, delaying the triggering of the thermal runaway chain reaction. This mechanism transforms the separator from a passive physical isolation layer into an active safety element with internal intervention capabilities, significantly improving battery safety under extreme conditions such as high temperatures.

[0018] Third, this invention effectively suppresses lithium dendrite growth and improves electrochemical stability through multi-scale structural design. The well-organized and porous metal-organic framework material in the dense trapping layer can efficiently sieve anions, increase the lithium-ion transference number, and promote uniform lithium-ion deposition; the interconnected hydrophilic channels formed by the functional coating and the base film ensure high ionic conductivity; the gradient interface structure helps guide the uniform distribution of lithium-ion flow, synergistically suppressing lithium dendrite penetration and extending battery cycle life. Simultaneously, the good compatibility of the composite coating with the electrolyte and its own chemical stability further enhance the membrane's oxidation resistance under high voltage.

[0019] Fourth, the preparation process of this invention is green, controllable, and scalable. The entire preparation process is mainly based on an aqueous system, employing industrially scalable unit operations such as coating, impregnation, and phase inversion, avoiding the large-scale use of organic solvents. By precisely controlling process parameters such as temperature, humidity, and time at each stage, the gradient structure and interface characteristics of the functional coating can be effectively controlled, ensuring the consistency and reproducibility of product performance and providing a clear and reliable path for technology transfer.

[0020] In summary, this invention, through the integration of innovative material systems, gradient structure design, and active safety mechanisms, successfully prepared a novel lithium battery separator that combines inherently high thermal stability, efficient dendrite suppression capabilities, and active thermal intervention response, providing an effective solution for improving the safety performance and cycle life of high-energy-density lithium batteries. Detailed Implementation

[0021] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0022] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0023] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0024] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0025] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0026] Example 1 This embodiment provides a high-temperature resistant lithium battery separator based on cellulose nanofibers, and the specific steps include: S1. Base film preparation and activation: S1.1 Preparation of cellulose nanofiber dispersion: Carboxylated cellulose nanofibers with a diameter of 50 nm and a length of 2 μm and sodium alginate were dispersed in deionized water at a mass ratio of 1:0.10 and subjected to high-speed shearing at 12000 r / min for 22 min to obtain a dispersion with a carboxylated cellulose nanofiber content of 1.0%.

[0027] S1.2 Preparation of CNF-based film: The dispersion was coated onto a flat polytetrafluoroethylene substrate, and the wet film thickness was controlled to be 400 μm. The film was left to stand for 45 minutes at a relative humidity of 75% and a temperature of 26°C to complete the initial shaping. The CNF base film was obtained by drying at 42℃ for 4.5 h and then at 65℃ for 3 h, and its thickness was measured to be 35 μm.

[0028] S1.3, Activation of hydrogen bond network on CNF base film surface: The CNF base film was immersed in a 0.2% gelatin aqueous solution at 28°C for 20 seconds. After removal, use a flexible scraper to remove excess droplets from the surface; Let it stand for 2.5 minutes in an environment with a relative humidity of 85%; The surface-activated CNF base film was obtained by drying at 45℃ for 6 min.

[0029] S2. Preparation of functional coating paste: S2.1 Preparation of dense trapping side functional slurry: 12 parts of the dispersion prepared in S1.1 and 50 parts of metal-organic framework powder ZIF-8 were dispersed at 2℃ and 2500r / min for 38min to obtain ZIF-8@CNF pre-hybrid slurry. 100 parts of cellulose nanocrystals were dispersed in 250 parts of deionized water. Under nitrogen protection and at 300 r / min, 25 parts of N-isopropylacrylamide monomer and 1.5 parts of potassium persulfate initiator were added sequentially. The system was heated to 72°C and reacted for 5 hours. After the reaction was completed, the mixture was centrifuged at 9000 r / min for 12 min, washed 4 times with deionized water, and dialyzed in a dialysis bag with a molecular weight cutoff of 10000 for 60 h. After being pre-frozen at -50℃ for 5 hours, the cellulose nanocrystals grafted with poly-N-isopropylacrylamide were obtained at -45℃ and vacuum degree 9 Pa for 30 hours, and were denoted as P-CNCs. Add 10 parts gelatin to 100 parts deionized water at 250 r / min and heat to 42℃ to dissolve; While maintaining stirring, add all of the ZIF-8@CNF pre-hybridized slurry, 4 parts of P-CNCs, and 1.0 part of tannic acid in sequence; The mixture was dispersed at 42℃ and 1800 r / min for 1.5 h, and then cooled to 25℃ to obtain a dense capture-side functional slurry, denoted as slurry A. Its viscosity was measured to be 2750 mPa·s.

[0030] S2.2 Preparation of intelligent response side slurry: At 350 r / min, 20 parts of 15% LiNO3 solution were added to 100 parts of 3% gelatin solution, followed by 100 parts of 3% gum arabic solution. The mixture was stirred and the pH was adjusted to 4.0. Cool to 8℃, add 10 parts of glutaraldehyde as a crosslinking agent, and react at 150 r / min for 3 h. Centrifuge at 4000 r / min for 8 min and wash with deionized water 4 times; After pre-freezing at -50℃ for 5 hours, the temperature-sensitive microcapsules loaded with LiNO3 were obtained by freeze-drying at -45℃ and a vacuum of 9Pa for 30 hours. Under nitrogen protection, 100 parts of vacuum-dehydrated polyether diol PPG-2000 were mixed with 28 parts of isophorone diisocyanate and reacted at 82°C for 2.5 h to obtain isocyanate-terminated polyurethane prepolymer. Cool to 50°C, then add 10 parts of chain extender adipic acid dihydrazide and 150 parts of acetone; Under high-speed shearing at 10000 r / min, it was added dropwise to 300 parts of deionized water containing 5 parts of polyvinyl alcohol and emulsified for 8 min to form a stable emulsion. The emulsion was transferred to a 75°C water bath and reacted at 400 rpm for 5 h. After the reaction was completed, the microspheres were centrifuged at 4000 r / min for 8 min, washed with deionized water 5 times, and vacuum dried at 45℃ and vacuum degree -0.09MPa for 18 h to obtain polyurethane urea self-healing polymer microspheres. At 250 r / min, add 5 parts gelatin powder and 5 parts poly-N-isopropylacrylamide powder to 100 parts deionized water, heat to 42°C and stir until completely dissolved, then cool to 15°C; While maintaining stirring, 25 parts of temperature-sensitive microcapsules loaded with LiNO3, 7 parts of polyurethane urea self-healing polymer microspheres, and 1.5 parts of hydrophilic nano silica were added sequentially. The stirring speed was increased to 700 r / min and dispersed for 30 min to obtain the intelligent response side slurry, which was denoted as slurry B. Its viscosity was measured to be 610 mPa·s.

[0031] S3, Gradient Coating and Structure Locking: S3.1 Precision coating of dense trapping slurry at the bottom layer: On the activated surface of the CNF base film, slurry A is coated, and the wet film coating thickness is controlled to be 40 μm. After coating, the film is transferred to 12℃ and left to stand for 3 min to complete the initial gelation.

[0032] S3.2, Overcoating and Gradient Construction of Surface Smart Response Slurry: Slurry B was coated onto the surface of slurry A after initial gelation, and the wet film coating thickness of slurry B was controlled to be 20 μm. After coating, it was left to stand horizontally for 8 min in a closed environment at 25℃ and 88% relative humidity.

[0033] S3.3 Solvent displacement and structure locking: The entire wet film was immersed in an anhydrous ethanol coagulation bath and soaked at 22°C for 90 seconds. After removal, it is immersed in anhydrous ethanol for a second wash for 45 seconds to completely replace the residual water inside the membrane.

[0034] S3.4, Green Crosslinking and Functional Enhancement: The diaphragm treated with S3.3 was immersed in a 0.3% (w / w) genipin aqueous solution and treated at 45°C for 45 min. After removal, rinse with deionized water and dry at 50°C for 15 minutes.

[0035] S4, Post-processing and Performance Integration The separator was vacuum dried at 55℃ and -0.09MPa for 3 hours to ensure complete removal of residual solvent and moisture; then it was heat-treated at 70℃ for 1.5 hours under nitrogen protection to obtain a high-temperature resistant lithium battery separator, which was designated as sample 1.

[0036] Example 2 S1. Base film preparation and activation: S1.1 Preparation of cellulose nanofiber dispersion: Carboxylated cellulose nanofibers with a diameter of 50 nm and a length of 3 μm and sodium alginate were dispersed in deionized water at a mass ratio of 1:0.05 and subjected to high-speed shearing at 15000 r / min for 15 min to obtain a dispersion with a carboxylated cellulose nanofiber content of 1.5%.

[0037] S1.2 Preparation of CNF-based film: The dispersion was coated onto a flat polytetrafluoroethylene substrate, and the wet film thickness was controlled to be 500 μm. The film was left to stand for 30 minutes at a relative humidity of 80% and a temperature of 28°C to complete the initial shaping. The CNF base film was obtained by drying at 45℃ for 4 hours and then at 70℃ for 2 hours, and its thickness was measured to be 50 μm.

[0038] S1.3, Activation of hydrogen bond network on CNF base film surface: The CNF base film was immersed in a 0.3% gelatin aqueous solution at 35°C for 10 seconds. After removal, use a flexible scraper to remove excess droplets from the surface; Let it stand for 2 minutes in an environment with a relative humidity of 90%; The surface-activated CNF base film was obtained by drying at 50℃ for 5 min.

[0039] S2. Preparation of functional coating paste: S2.1 Preparation of dense trapping side functional slurry: 15 parts of the dispersion prepared by S1.1 and 60 parts of metal-organic framework powder ZIF-8 were dispersed at 0℃ and 3000r / min for 30min to obtain ZIF-8@CNF pre-hybrid slurry. 100 parts of cellulose nanocrystals were dispersed in 300 parts of deionized water. Under nitrogen protection and at 400 r / min, 30 parts of N-isopropylacrylamide monomer and 2 parts of potassium persulfate initiator were added sequentially. The system was heated to 75°C and reacted for 4 hours. After the reaction was completed, the mixture was centrifuged at 10,000 r / min for 10 min, washed three times with deionized water, and then dialyzed in a dialysis bag with a molecular weight cutoff of 14,000 for 48 h. After pre-freezing at -50℃ for 4 hours, P-CNCs were obtained by freeze-drying at -40℃ and 10 Pa vacuum for 24 hours. At 300 r / min, add 12 parts gelatin to 100 parts of deionized water and heat to 45°C to dissolve; While maintaining stirring, add all of the ZIF-8@CNF pre-hybridized slurry, 5 parts of P-CNCs, and 1.5 parts of tannic acid in sequence; The mixture was dispersed at 45℃ and 2000 r / min for 1 h, then cooled to 30℃ to obtain slurry A, whose viscosity was measured to be 3500 mPa·s.

[0040] S2.2, Preparation of intelligent response side slurry: At 400 r / min, 20 parts of 20% LiNO3 solution were added to 100 parts of 4% gelatin solution, followed by 100 parts of 4% gum arabic solution. The mixture was stirred and the pH was adjusted to 3.8. Cool to 5℃, add 15 parts of glutaraldehyde as a crosslinking agent, and react at 200r / min for 2h. Centrifuge at 5000 r / min for 5 min and wash three times with deionized water; After pre-freezing at -50℃ for 4 hours, the temperature-sensitive microcapsules loaded with LiNO3 were obtained by freeze-drying at -40℃ and a vacuum of 10 Pa for 24 hours. Under nitrogen protection, 100 parts of PPG-2000 and 35 parts of isophorone diisocyanate were mixed and reacted at 85°C for 2 hours to obtain the prepolymer. Cool to 60°C, then add 15 parts of chain extender adipic acid dihydrazide and 200 parts of acetone; At 12000 r / min, it was added dropwise to 300 parts of deionized water containing 8 parts of polyvinyl alcohol and emulsified for 5 min to form an emulsion; The emulsion was transferred to an 80°C water bath and reacted at 500 rpm for 4 hours. After the reaction was completed, the microspheres were centrifuged at 5000 r / min for 5 min, washed three times with deionized water, and then vacuum dried at 50℃ and -0.08 MPa for 12 h to obtain polyurethane urea self-healing polymer microspheres. At 300 r / min, add 6 parts gelatin powder and 6 parts poly-N-isopropylacrylamide powder to 100 parts deionized water, heat to 45°C to dissolve, and cool to 20°C; While maintaining stirring, add 30 parts of temperature-sensitive microcapsules loaded with LiNO3, 10 parts of polyurethane urea self-healing polymer microspheres, and 2 parts of hydrophilic nano silica in sequence. Increase the stirring speed to 800 r / min and disperse for 20 min to obtain slurry B, whose viscosity is measured to be 800 mPa·s.

[0041] S3, Gradient Coating and Structure Locking: S3.1 Precision coating of dense trapping slurry at the bottom layer: On the activated surface of the CNF base film, slurry A is coated, and the wet film coating thickness is controlled to be 50 μm. After coating, the film is transferred to 15℃ and left to stand for 2 min to complete the initial gelation.

[0042] S3.2, Overcoating and Gradient Construction of Surface Smart Response Slurry: Slurry B was coated onto the surface of slurry A after initial gelation, and the wet film coating thickness of slurry B was controlled to be 25 μm. After coating, it was left to stand horizontally for 5 min in a closed environment at 28℃ and 90% relative humidity.

[0043] S3.3 Solvent displacement and structure locking: The entire wet film was immersed in an anhydrous ethanol coagulation bath and soaked at 25°C for 60 seconds. After removal, immerse in anhydrous ethanol for a second wash for 30 seconds.

[0044] S3.4, Green Crosslinking and Functional Enhancement: The separator treated with S3.3 was immersed in a 0.5% genipin aqueous solution and treated at 50°C for 30 min. After being removed, it was rinsed and dried at 60°C for 10 min to obtain a high-temperature resistant lithium battery separator, which was designated as sample 2.

[0045] Example 3 S1. Base film preparation and activation: S1.1 Preparation of cellulose nanofiber dispersion: Carboxylated cellulose nanofibers with a diameter of 50 nm and a length of 1 μm and sodium alginate were dispersed in deionized water at a mass ratio of 1:0.15 and subjected to high-speed shearing at 10000 r / min for 30 min to obtain a dispersion with a carboxylated cellulose nanofiber content of 0.5%.

[0046] S1.2 Preparation of CNF-based film: The dispersion was coated onto a flat polytetrafluoroethylene substrate, and the wet film thickness was controlled to be 300 μm. The film was left to stand for 60 minutes at a relative humidity of 70% and a temperature of 24°C to complete the initial shaping. The CNF base film was obtained by drying at 40℃ for 5 hours and then at 60℃ for 4 hours, and its thickness was measured to be 20 μm.

[0047] S1.3, Activation of hydrogen bond network on CNF base film surface: The CNF base film was immersed in a 0.1% gelatin aqueous solution at 20°C for 30 seconds. After removal, use a flexible scraper to remove excess droplets from the surface; Let it stand for 3 minutes in an environment with a relative humidity of 80%; The surface-activated CNF base film was obtained by drying at 40℃ for 8 min.

[0048] S2. Preparation of functional coating paste: S2.1 Preparation of dense trapping side functional slurry: Ten parts of the dispersion prepared by S1.1 and 40 parts of metal-organic framework powder ZIF-8 were dispersed at 5℃ and 2000r / min at high speed for 45min to obtain ZIF-8@CNF pre-hybrid slurry. 100 parts of cellulose nanocrystals were dispersed in 200 parts of deionized water. Under nitrogen protection and at 200 r / min, 20 parts of N-isopropylacrylamide monomer and 1 part of potassium persulfate initiator were added sequentially. The system was heated to 70°C and reacted for 6 hours. After the reaction was completed, the mixture was centrifuged at 8000 r / min for 15 min, washed 5 times with deionized water, and then dialyzed in a dialysis bag with a molecular weight cutoff of 8000 for 72 h. After pre-freezing at -50℃ for 6 hours, P-CNCs were obtained by freeze-drying at -50℃ and vacuum degree 8Pa for 36 hours. Add 8 parts gelatin to 100 parts deionized water at 200 r / min and heat to 40°C to dissolve; While maintaining stirring, add all of the ZIF-8@CNF pre-hybridized slurry, 3 parts of P-CNCs, and 0.5 parts of tannic acid in sequence; The mixture was dispersed at 40℃ and 1500 r / min for 2 h, then cooled to 20℃ to obtain slurry A, whose viscosity was measured to be 2000 mPa·s.

[0049] S2.2 Preparation of intelligent response side slurry: At 300 r / min, 20 parts of 10% LiNO3 solution were added to 100 parts of 2% gelatin solution, followed by 100 parts of 2% gum arabic solution. The mixture was stirred and the pH was adjusted to 4.2. Cool to 10℃, add 5 parts of glutaraldehyde as a crosslinking agent, and react at 100r / min for 4h. Centrifuge at 3000 r / min for 10 min and wash with deionized water 5 times; After pre-freezing at -50℃ for 6 hours, the temperature-sensitive microcapsules loaded with LiNO3 were obtained by freeze-drying at -50℃ and vacuum degree 8Pa for 36 hours. Under nitrogen protection, 100 parts of PPG-2000 and 20 parts of isophorone diisocyanate were mixed and reacted at 80°C for 3 hours to obtain the prepolymer. Cool to 40°C, then add 5 parts of chain extender adipic acid dihydrazide and 100 parts of acetone; At 8000 r / min, it was added dropwise to 300 parts of deionized water containing 3 parts of polyvinyl alcohol and emulsified for 10 min to form an emulsion; The emulsion was transferred to a 70°C water bath and reacted at 300 rpm for 6 hours. After the reaction was completed, the microspheres were centrifuged at 3000 r / min for 10 min, washed three times with deionized water, and then vacuum dried at 40℃ and -0.10 MPa for 24 h to obtain polyurethane urea self-healing polymer microspheres. At 200 r / min, add 4 parts gelatin powder and 4 parts poly-N-isopropylacrylamide powder to 100 parts deionized water, heat to 40°C to dissolve, and cool to 10°C; While maintaining stirring, add 20 parts of temperature-sensitive microcapsules loaded with LiNO3, 5 parts of polyurethane urea self-healing polymer microspheres, and 1 part of hydrophilic nano silica in sequence. Increase the stirring speed to 600 r / min and disperse for 40 min to obtain slurry B, whose viscosity is measured to be 400 mPa·s.

[0050] S3, Gradient Coating and Structure Locking: S3.1 Precision coating of dense trapping slurry at the bottom layer: On the activated surface of the CNF base film, slurry A is coated, and the wet film coating thickness is controlled to be 30 μm. After coating, the film is transferred to 10℃ and left to stand for 5 min to complete the initial gelation.

[0051] S3.2, Overcoating and Gradient Construction of Surface Smart Response Slurry: Slurry B was coated onto the surface of slurry A after initial gelation, and the wet film coating thickness of slurry B was controlled to be 15 μm. After coating, it was placed horizontally for 10 min in a closed environment at 22℃ and 85% relative humidity.

[0052] S3.3 Solvent displacement and structure locking: The entire wet film was immersed in an anhydrous ethanol coagulation bath and soaked at 20°C for 120 seconds. After removal, it is immersed in anhydrous ethanol for a second wash for 60 seconds.

[0053] S3.4, Green Crosslinking and Functional Enhancement: The diaphragm treated with S3.3 was immersed in a 0.1% genipin aqueous solution and treated at 40°C for 60 min. After removal, rinse and dry at 40℃ for 20 minutes.

[0054] S4, Post-processing and Performance Integration The diaphragm was vacuum dried at 50℃ and -0.10MPa for 4 hours. Under nitrogen protection, the high-temperature resistant lithium battery separator was obtained by heat treatment at 60°C for 2 hours, and it is designated as sample 3.

[0055] Comparative Example 1 This comparative example provides a comparative preparation method, the specific implementation of which is the same as in Example 1, except that: a common polyolefin membrane is used instead of the CNF base membrane, and the surface activation step is omitted. Specifically, it includes the following steps: S1. Preparation of the base film: A 25μm thick polypropylene diaphragm was selected as the substrate and used directly for subsequent coating without any surface treatment.

[0056] S2. Preparation of functional coating slurry: Same as step S2 in Example 1.

[0057] S3, Gradient Coating and Structure Locking: S3.1, Coating of the underlying dense trapping slurry: Slurry A is directly coated onto the surface of the polypropylene diaphragm, and the wet film coating thickness is controlled to be 40 μm. After coating, the membrane is transferred to 12℃ and left to stand for 3 min.

[0058] S3.2, Overcoating and subsequent treatment of surface intelligent response slurry: Same as step S3.2 in Example 1.

[0059] S3.3 Solvent replacement and structure locking: Same as step S3.3 in Example 1.

[0060] S3.4, Green Crosslinking and Functional Enhancement: Same as step S3.4 in Example 1.

[0061] S4. Post-processing and performance integration: Same as step S4 in Example 1, to obtain a control diaphragm, which is designated as control 1.

[0062] Comparative Example 2 This comparative example provides a comparative preparation method, the specific implementation of which is the same as in Example 1, except that the coating order of the two functional slurries is changed: the smart response side slurry B is coated first, and then the dense trapping side slurry A is coated. The specific steps include the following: S1. Base film preparation and activation: Same as step S1 in Example 1.

[0063] S2. Preparation of functional coating slurry: Same as step S2 in Example 1.

[0064] S3, Reverse Sequential Coating and Structure Locking: S3.1, Application of the underlying intelligent response slurry: On the surface-activated CNF base film, slurry B is first coated, and the wet film coating thickness is controlled to be 20 μm. After coating, the film is transferred to 12℃ and left to stand for 3 min to complete the initial gelation.

[0065] S3.2, Overcoating with a dense surface-capturing slurry: Slurry A is applied to the surface of slurry B, and the wet film coating thickness is controlled to be 40 μm. After coating, it is left to stand horizontally for 8 minutes in a closed environment at 25°C and 88% relative humidity.

[0066] S3.3 Solvent replacement and structure locking: Same as step S3.3 in Example 1.

[0067] S4. Post-processing and performance integration: Same as step S4 in Example 1, to obtain a control diaphragm, which is designated as control 2.

[0068] Comparative Example 3 This comparative example provides a comparative preparation method, the specific implementation of which is the same as in Example 1, except that the viscosities of slurry A and slurry B are adjusted to be within a similar range, specifically including the following steps: S1. Base film preparation and activation: Same as step S1 in Example 1.

[0069] S2. Preparation of functional coating slurry: S2.1 Preparation of Dense Capture Side Slurry A: The preparation method is the same as in Example 1 S2.1, but the components are adjusted. The amount of tannic acid added is reduced to 0.2 parts, and 5 parts of deionized water are added to the system for dilution, finally obtaining slurry A with a viscosity of 950 mPa·s.

[0070] S2.2 Preparation of intelligent response side slurry B: The preparation method is the same as in Example 1 S2.2, but the amount of hydrophilic nano silica added is increased to 3 parts, and finally a slurry B with a viscosity of 900 mPa·s is obtained.

[0071] S3, Gradient Coating and Structure Locking: Same as step S3 in Example 1, but because the two slurries have similar viscosities and no significant difference, the interface will mix disorderly when left to stand at low temperature, and a controlled gradient transition zone cannot be formed.

[0072] S4. Post-processing and performance integration: Same as step S4 in Example 1, to obtain a control diaphragm, which is designated as control 3.

[0073] Comparative Example 4 This comparative example provides a comparative preparation method, the specific implementation of which is the same as in Example 1, except that the temperature-sensitive microcapsules loaded with LiNO3 are replaced with an equal amount of a physical mixture of ordinary LiNO3 powder and inert polymer microspheres, specifically including the following steps: S1. Base film preparation and activation: Same as step S1 in Example 1.

[0074] S2. Preparation of functional coating slurry: S2.1 Preparation of dense capture-side functional slurry: Same as step S2.1 in Example 1.

[0075] S2.2 Preparation of intelligent response side slurry: At 250 r / min, add 5 parts gelatin powder and 5 parts polyN-isopropylacrylamide powder to 100 parts deionized water, heat to 42°C to dissolve, and cool to 15°C; While stirring, add 10 parts of ordinary LiNO3 powder, 15 parts of polystyrene microspheres with an average particle size of 5μm, 7 parts of polyurethane urea self-healing polymer microspheres, and 1.5 parts of hydrophilic nano silica in sequence. Increase the stirring speed to 700 r / min and disperse for 30 min to obtain control slurry B.

[0076] S3, Gradient Coating and Structure Locking: Same as step S3 in Example 1.

[0077] S4. Post-processing and performance integration: Same as step S4 in Example 1, to obtain a control diaphragm, which is designated as control 4.

[0078] Comparative Example 5 This comparative example provides a comparative preparation method, the specific implementation of which is the same as in Example 1, except that the constant temperature and humidity standing process in S3.2 is omitted, so that slurry A and slurry B are cured immediately after coating. Specifically, it includes the following steps: S1. Base film preparation and activation: Same as step S1 in Example 1.

[0079] S2. Preparation of functional coating slurry: Same as step S2 in Example 1.

[0080] S3, Gradient-free interface coating and structure locking: S3.1 Coating of the dense trapping slurry at the bottom layer: Same as step S3.1 in Example 1.

[0081] S3.2, Overcoating and Direct Curing of Surface Slurry: Slurry B is coated onto the surface of the initially gelled slurry A layer, and the wet film coating thickness is controlled to be 20 μm. After coating, without constant temperature and humidity standing, the wet film is immediately immersed in an anhydrous ethanol coagulation bath.

[0082] S3.3 Solvent replacement and structure locking: Same as step S3.3 in Example 1.

[0083] S3.4, Green Crosslinking and Functional Enhancement: Same as step S3.4 in Example 1.

[0084] S4. Post-processing and performance integration: Same as step S4 in Example 1, to obtain a control diaphragm, which is designated as control 5.

[0085] Analysis and Testing The samples 1-3 prepared in Examples 1-3 and the reference standards 1-5 prepared in Comparative Examples 1-5 were analyzed and tested as follows.

[0086] I. Analysis of Coating Adhesion and Thermal Stability The following tests were conducted, and the results are shown in Table 1: The peel strength between the coating and the base film was quantitatively tested using a 180° peel test. Thermomechanical analysis was used to test the thermal shrinkage rate of the diaphragm after heat treatment at 180°C for 30 min to evaluate its dimensional stability.

[0087] Table 1: Test results of membrane coating adhesion and thermal stability As shown in Table 1, samples 1-3 obtained using the green preparation method of this invention exhibit excellent coating adhesion and thermal stability. The peel strength of the coatings on all three samples is higher than 145 N / m, confirming that a tightly bonded interface layer, primarily composed of hydrogen bonds, was established between the CNF base film and the subsequent functional coating through gelatin surface activation, laying the foundation for the stability of the coating during long-term battery cycling. Simultaneously, the samples demonstrate excellent thermal stability, with thermal shrinkage rates below 1.1% at 180℃, fully reflecting the fundamental advantage of this invention's self-supporting CNF / sodium alginate base film. The extremely high thermal decomposition temperature and three-dimensional network structure of the cellulose material itself endow the separator with inherent thermal dimensional stability from the source, completely eliminating the risk of thermal shrinkage associated with traditional polyolefin separators.

[0088] It is important to note that the residual weight data at 600℃ in this invention primarily reflects the final inorganic residue after high-temperature carbonization of the organic / inorganic composite system. Its value depends on the relative content of functional fillers such as ZIF-8, rather than being a core characteristic of the material's intrinsic heat resistance. Therefore, although some comparative examples show similar values ​​to the samples of this invention in this indicator, this does not necessarily mean they possess the same level of thermal stability under actual use conditions. The thermal safety performance of this invention must be comprehensively and objectively evaluated in conjunction with its extremely low thermal shrinkage rate and performance in subsequent safety tests (as shown in Table 3).

[0089] In comparison, all comparative examples exhibited defects in different dimensions. Control 1, which used commercial PP membrane as the substrate, completely melted and severely shrank at 180°C, resulting in a relatively low residual weight. This was mainly due to the decomposition of organic matter in the base membrane, which thoroughly exposed the fundamental flaw of traditional polyolefin membranes in relying on the passive and unreliable safety mechanism of melt-closure.

[0090] Due to the failure of the functional gradient structure construction of the core of the present invention, the interlayer physical interlocking and interaction of control samples 2 and 3 were weakened, resulting in a significant decrease in peel strength. The increase in their thermal shrinkage rate also directly proves the key role of the ordered gradient structure in maintaining the overall dimensional stability.

[0091] Reference product 4, due to the loss of thermal triggering properties of the microcapsules, affected the integration and synergy between functional layers, resulting in its performance only approaching the optimal level in some indicators.

[0092] Reference product 5 did not achieve the optimal level of the present invention in terms of interfacial bonding strength and overall thermal shrinkage resistance because the interlayer interpenetrating network was not fully formed.

[0093] II. Analysis of Electrochemical Performance and Ion Transport Characteristics To evaluate the impact of the functional coating on the battery's kinetic performance, the various separators were assembled into CR2032 coin cells. The positive electrode was LiFePO4, the negative electrode was lithium foil, and the electrolyte was a 1 mol / L LiPF6 solution of ethylene carbonate / diethyl carbonate (volume ratio 1:1). The following tests were conducted, and the results are shown in Table 2: The ionic conductivity of the battery was tested using the AC impedance method. Li was measured using a combination of DC polarization method and AC impedance method. + Transference number is used to assess the selective transport capability of lithium ions. The coulombic efficiency of the battery during the first charge-discharge cycle at a 0.5C rate was tested. Linear sweep voltammetry was used to record the oxidation current up to 5.0 V to evaluate the membrane's ability to inhibit electrolyte oxidation and decomposition and its high-voltage stability.

[0094] Table 2: Results of Electrochemical Performance Tests of the Membrane Table 2 shows that the batteries prepared using samples 1–3 exhibit comprehensive and excellent electrochemical performance. Their high ionic conductivity is attributed to the inherent hydrophilicity and interconnected porous structure of the CNF-based film and gradient coating, ensuring rapid ion transport. Particularly noteworthy is the significantly enhanced Li-... + The high migration number is due to the effective sieving of anions by the regular channels of ZIF-8 in the dense trap region, which helps reduce battery polarization. The high first-cycle coulombic efficiency and oxidation onset potential exceeding 4.83V together demonstrate the good compatibility of this gradient composite coating with the electrolyte and its ability to withstand high-voltage conditions.

[0095] In contrast, the electrochemical performance of all batteries prepared with the reference standards was degraded. The battery prepared with reference standard 1 exhibited poor interfacial compatibility and the lowest Li⁺ migration number. Reference standards 2 and 3, due to the disruption of their functional gradient structures, failed to optimally utilize the ion sieving efficiency of ZIF-8, resulting in reduced Li⁺ migration numbers in their respective batteries. The battery prepared with reference standard 4 showed acceptable basic conductivity. The battery prepared with reference standard 5, due to its incomplete gradient interface, failed to reach the performance levels of the sample, confirming the importance of constructing gradient interpenetrating networks for synergistically improving electrochemical performance.

[0096] III. Battery Safety Performance and Long Cycle Performance Verification Each separator was used to assemble a 2.0 Ah pouch lithium battery (positive electrode: NCM811, negative electrode: silicon-carbon composite material) to evaluate its safety and cycle performance in a real battery. The following tests were conducted, and the results are shown in Table 3.

[0097] Thermal runaway safety test: Place a fully charged (4.35V) battery in a 150℃ oven for 30 minutes, record whether fire or explosion occurs, and measure the thermal runaway trigger time.

[0098] Dendrite penetration time: at a constant current density (0.5 mA / cm²) 2 Repeated deposition / stripping cycles were performed, and the time taken for the voltage curve to first show a sudden drop was recorded, indicating that lithium dendrite growth caused the separator to be punctured and an internal short circuit to occur.

[0099] Table 3: Battery Safety and Long Cycle Performance Test Results As shown in Table 3, the cellulose-based gradient composite separator of this invention exhibited excellent protective capabilities in both of the most stringent safety tests. In the 150°C hot box test, none of the batteries prepared from any of the samples experienced thermal runaway. This is primarily due to its inherently ultra-low thermal shrinkage rate, which ensures the physical integrity of the separator at high temperatures and fundamentally prevents large-area contact short circuits caused by shrinkage. Simultaneously, the temperature-sensitive microcapsules in the smart response layer played a crucial role; their targeted release of LiNO3 upon temperature increase facilitated in-situ repair and stabilization of the negative electrode interface, thereby delaying the triggering of the thermal runaway chain reaction. More notably, the batteries prepared from the samples all exhibited an ultra-long dendrite penetration time exceeding 1100 hours, directly demonstrating the superior effectiveness of the dense trapping layer and gradient structure in guiding uniform lithium-ion distribution and effectively suppressing rampant lithium dendrite growth and physical penetration.

[0100] In contrast, all batteries prepared from the control samples exhibited significant safety defects. The complete failure of control sample 1 demonstrates the fundamental vulnerability of traditional polyolefin separator systems to thermal abuse and dendrite challenges. Control samples 2, 3, and 5, due to disrupted gradient structures or incomplete interfacial bonding, showed a significant decrease in dendrite suppression and thermal stability, proving that the structural design of this invention is fundamental to its functional realization. While control sample 4 performed slightly better due to retaining some physical barrier function, its dendrite penetration time was still far shorter than that of this invention, and it exhibited severe bulging during hot-box testing, indicating a lack of intelligent response mechanisms for thermal triggering and targeted release, hindering timely and precise interfacial repair and resulting in limited overall protective effectiveness.

[0101] In summary, this invention has successfully created a novel lithium battery separator by constructing a unique structure with self-supporting cellulose nanofibers as the matrix and a gradient functional composite coating consisting of a ZIF-8 trapping layer, a P-CNCs temperature-sensitive layer, and a smart microcapsule response layer. This structure combines inherently high thermal stability, efficient dendrite suppression, and active thermal intervention response, comprehensively surpassing traditional separators and comparative separators with structural or functional defects. It provides an innovative and reliable solution to the problems of thermal runaway and internal short circuits in high-energy-density lithium batteries.

[0102] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing a cellulose nanofiber-based high-temperature-resistant lithium battery separator, characterized by, Specifically, it includes: S1. Preparation and activation of the base film: Carboxylated cellulose nanofibers and sodium alginate were mixed and dispersed in water, and CNF base film was obtained by casting and drying. Subsequently, the surface-activated CNF base film was obtained by impregnation and activation with a gelatin solution. S2. Preparation of functional slurry: Prepare high-viscosity slurry A containing metal-organic framework powder, cellulose nanocrystals grafted with poly(N-isopropylacrylamide), gelatin and tannic acid; prepare low-viscosity slurry B containing temperature-sensitive microcapsules loaded with LiNO3, polyurethane urea self-healing polymer microspheres, poly(N-isopropylacrylamide) and hydrophilic nano-silica. The preparation process of the cellulose nanocrystals grafted with poly(N-isopropylacrylamide) is as follows: 100 parts of cellulose nanocrystals are dispersed in 200-300 parts of deionized water, and 20-30 parts of N-isopropylacrylamide monomer and 1-2 parts of potassium persulfate initiator are added sequentially; the system is heated to 70℃-75℃ and reacted for 4-6 hours; after the reaction is completed, the cellulose nanocrystals are obtained by centrifugation, washing, dialyzing and freeze-drying. The preparation process of the temperature-sensitive microcapsules loaded with LiNO3 is as follows: 20 parts of a LiNO3 solution with a mass concentration of 10%–20% are added to 100 parts of a gelatin solution with a mass concentration of 2%–4%, and then 100 parts of a gum arabic solution with a mass concentration of 2%–4% are added. The mixture is stirred and the pH is adjusted to 3.8–4.

2. The temperature is lowered to 5℃–10℃, and 5 parts–15 parts of glutaraldehyde are added as a crosslinking agent. The mixture is reacted at 100 r / min–200 r / min for 2 h–4 h. After centrifugation, washing, and freeze-drying, the temperature-sensitive microcapsules loaded with LiNO3 are obtained. The polyurethane urea self-healing polymer microspheres are microspheres with a dynamic hydrogen bond network inside. The dynamic hydrogen bond network can rearrange and rebuild at the damage site caused by external force within the working temperature range of the high-temperature lithium battery separator, thereby achieving self-repair. S3. Gradient coating and structure locking: Slurry A and slurry B are sequentially coated on the surface-activated CNF base film. After preliminary gelation induced by low temperature, the film is left to stand in a constant temperature and humidity environment to build a gradient interface. Then, it is immersed in an anhydrous ethanol coagulation bath for solvent replacement and structure locking. In this process, a dense trapping region is formed by cross-linking and curing of slurry A, and a smart response region is formed by cross-linking and curing of slurry B. The components of slurry A and slurry B diffuse into each other and form an interpenetrating network structure, thus forming a gradient transition region between the dense trapping region and the smart response region. S4. Post-processing and performance integration: The structure-locked wet film is strengthened by green cross-linking and then subjected to low-temperature vacuum drying and inert atmosphere heat treatment to obtain the high-temperature resistant lithium battery separator.

2. The production method according to claim 1, characterized by, In S1, the mass ratio of carboxylated cellulose nanofibers to sodium alginate is 1:(0.05-0.15); the mass concentration of the gelatin solution is 0.1%-0.3%, and the impregnation time is 10s-30s; the thickness of the CNF base film is 20μm-50μm.

3. The preparation method according to claim 1, characterized in that, In S2, the viscosity of slurry A at 25°C is 2000 mPa·s to 3500 mPa·s; the viscosity of slurry B at 25°C is 400 mPa·s to 800 mPa·s.

4. The preparation method according to claim 1, characterized in that, In S2, the preparation process of the polyurethane urea self-healing polymer microspheres is as follows: 100 parts of polyether diol are mixed with 20 to 35 parts of isophorone diisocyanate and reacted at 80°C to 85°C for 2 to 3 hours to obtain isocyanate-terminated polyurethane prepolymer; the temperature is lowered to 40°C to 60°C, and 5 to 15 parts of chain extender adipate dihydrazide and 100 to 200 parts of acetone are added; under high-speed shearing at 8000 r / min to 12000 r / min, the mixture is added dropwise to 300 parts of deionized water containing 3 to 8 parts of polyvinyl alcohol, and emulsified for 5 to 10 minutes to form a stable emulsion; the emulsion is transferred to a water bath at 70°C to 80°C and reacted at 300 r / min to 500 r / min for 4 to 6 hours; after the reaction, the mixture is centrifuged, washed, and vacuum dried to obtain polyurethane urea self-healing polymer microspheres.

5. The preparation method according to claim 1, characterized in that, In S3, the conditions for constructing the gradient interface are: in a closed environment with a temperature of 22℃~28℃ and a relative humidity of 85%~90%, the surface is left to stand horizontally for 5min~10min.

6. The method of claim 1, wherein, In S4, the green crosslinking enhancement is achieved by treating the sample with a genipin aqueous solution at a mass concentration of 0.1% to 0.5% at 40°C to 50°C for 30 to 60 minutes.

Images