Polyolefin separator for lithium-ion batteries and method for producing the same

Abstract

The application discloses a polyolefin diaphragm for lithium ion batteries and a preparation method thereof, and relates to the technical field of lithium ion batteries. The polyolefin diaphragm for lithium ion batteries comprises a polyolefin base film and a composite coating arranged on one side surface or both side surfaces of the polyolefin base film; the composite coating comprises the following raw materials in terms of dry base mass parts: 55-65 parts of lithium sulfonate functionalized conjugated microporous polymer@nanocellulose; 10-20 parts of polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheet; 15-25 parts of polyethylene oxide; and 3-8 parts of amino silane modified tannin acid nanoparticles. The polyolefin diaphragm for lithium ion batteries prepared by the application has excellent high-temperature resistance, liquid absorption performance and lithium dendrite inhibition performance, thereby improving the safety performance and cycle life of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a polyolefin separator for lithium-ion batteries and its preparation method. Background Technology

[0002] Since their commercialization in the 1990s, lithium-ion batteries (LIBs) have dominated the field of power batteries and energy storage devices due to their advantages such as high energy density, low self-discharge, and long cycle life. The separator is a key component of LIBs, located between the anode and cathode, acting as a barrier to prevent short circuits while allowing lithium ions to freely transport through the electrolyte within the membrane's pore structure. Although the separator is not the active material of the battery, its physicochemical properties, structure, and interactions with the electrodes and electrolyte play a crucial role in the battery's electrochemical performance. Polyolefin microporous membranes have been widely used in LIB separators due to their advantages such as low cost, high porosity, good mechanical properties, high chemical and electrochemical stability, and suitable thermal shut-off performance.

[0003] However, polyolefin materials are non-polar semi-crystalline polymers with three inherent defects, making them unsuitable for the development requirements of high-energy-density and high-safety lithium batteries. The most prominent shortcoming of polyolefin separators is insufficient thermal stability. PE and PP have melting points of around 130℃ and 160℃ respectively. When the battery's internal temperature rises sharply due to overcharging, short circuits, or other reasons, the separator undergoes large-scale thermal shrinkage or even melts, causing direct contact between the positive and negative electrodes and triggering thermal runaway. Secondly, polyolefin separators have poor electrolyte affinity. The non-polar surface of polyolefin materials results in poor wetting of polar carbonate electrolytes, leading to low liquid absorption and weak liquid retention capacity, increasing battery internal resistance and affecting rate performance and energy density. More importantly, under lithium metal anode or fast-charging graphite anode conditions, polyolefin separators cannot effectively control the uniform deposition of lithium ions. Their irregular pore structure and uneven local current density induce uncontrolled growth of lithium dendrites. These dendrites not only puncture the separator but also form "dead lithium," accelerating battery failure. Therefore, existing polyolefin separators for lithium-ion batteries suffer from poor electrolyte affinity, insufficient thermal dimensional stability, and weak lithium dendrite suppression capabilities, severely limiting the application of this technology. Summary of the Invention

[0004] The purpose of this invention is to provide a polyolefin separator for lithium-ion batteries and its preparation method, thereby solving the following technical problems: Existing polyolefin separators used in lithium-ion batteries suffer from poor electrolyte affinity, insufficient thermal dimensional stability, and weak lithium dendrite suppression capabilities.

[0005] The objective of this invention can be achieved through the following technical solutions: A polyolefin separator for lithium-ion batteries includes a polyolefin base film and a composite coating disposed on one or both surfaces of the polyolefin base film. The composite coating, by dry weight, comprises at least the following raw materials: Lithium sulfonate functionalized conjugated microporous polymer@nanocellulose 55-65 parts; polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets 10-20 parts; polyethylene oxide 15-25 parts; aminosilane modified tannic acid nanoparticles 3-8 parts.

[0006] As a further aspect of the present invention: the preparation method of the lithium sulfonate functionalized conjugated microporous polymer@nanocellulose includes at least the following steps: 1,3,5-triethynylbenzene and 1,4-diiodobenzene were dissolved in a mixed solvent of anhydrous toluene / anhydrous triethylamine. A composite catalyst of bis(triphenylphosphine)palladium dichloride and cuprous iodide was added. After degassing, the mixture was sealed under nitrogen protection and stirred at 70-90°C in the dark for 20-36 hours. The mixture was then washed, purified, and vacuum dried to obtain a conjugated microporous polymer. The conjugated microporous polymer was added to concentrated sulfuric acid, stirred and dispersed in an ice-water bath, heated to 50-60℃ and stirred for 4-6 hours, then cooled, precipitated, washed and vacuum dried to obtain sulfonated conjugated microporous polymer. The sulfonated conjugated microporous polymer was added to an aqueous lithium hydroxide solution, and after ion exchange, it was washed, vacuum dried and ground to obtain lithium sulfonate functionalized conjugated microporous polymer. The lithium sulfonate functionalized conjugated microporous polymer was dispersed in deionized water, then added dropwise to an aqueous dispersion of nanocellulose and stirred. After ultrasonic treatment in an ice-water bath, the mixture was dialyzed, concentrated by rotary evaporation under reduced pressure, and freeze-dried to obtain lithium sulfonate functionalized conjugated microporous polymer@nanocellulose.

[0007] As a further aspect of the present invention: the molar ratio of the 1,3,5-triethynylbenzene and the 1,4-diiodobenzene is 1:1-2, and the degree of sulfonation of the sulfonated conjugated microporous polymer is 0.6-0.7.

[0008] As a further aspect of the present invention: the concentration of the lithium hydroxide aqueous solution is 0.5-1.5 mol / L, and the mass ratio of the lithium sulfonate functionalized conjugated microporous polymer to the nanocellulose is 1:2-4.

[0009] As a further aspect of the present invention, the preparation method of the polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets includes at least the following preparation steps: Hexagonal boron nitride, sodium dodecyl sulfate, β-cyclodextrin and ammonia were mixed, ball-milled, centrifuged, washed and vacuum-dried to obtain hydroxylated boron nitride nanosheets. The hydroxylated boron nitride nanosheets were added to a water / ethanol mixed solution and ultrasonically dispersed. Hexadecyltrimethylammonium bromide was added, and the pH was adjusted to 10-11 with ammonia. Tetraethyl orthosilicate was added dropwise and stirred to react. After centrifugation and washing, the nanosheets were added to a sodium carbonate solution and etched by stirring at 70-90°C for 5-7 hours. After centrifugation and washing, the nanosheets were calcined at 500-600°C for 3-5 hours to obtain hollow mesoporous silica@hydroxylated boron nitride nanosheets. Hollow mesoporous silica@hydroxylated boron nitride nanosheets were added to Tris-HCl buffer, the pH was adjusted to 8-9, and the nanosheets were ultrasonically dispersed. Dopamine hydrochloride was added, and the mixture was stirred in the dark for 12-24 hours. After centrifugation, washing, and vacuum drying, polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets were obtained.

[0010] As a further aspect of the present invention: the mass ratio of the hydroxylated boron nitride nanosheets to the tetraethyl orthosilicate is 1:3-5, and the concentration of the sodium carbonate solution is 0.5-1 mol / L.

[0011] As a further aspect of the present invention: the mass ratio of the hollow mesoporous silica@hydroxylated boron nitride nanosheets to the dopamine hydrochloride is 1:0.05-0.2.

[0012] As a further aspect of the present invention: the aminosilane-modified tannic acid nanoparticles are obtained by reacting aminosilane and tannic acid, and the mass ratio of aminosilane to tannic acid is 0.5-2:1, wherein the aminosilane is at least one of γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane.

[0013] As a further aspect of the present invention: the thickness of the composite coating on one side is 2-10 μm, the thickness of the polyolefin base film is 9-25 μm, and the polyolefin base film is at least one of polypropylene membrane, polyethylene membrane, or polypropylene / polyethylene / polypropylene three-layer composite membrane.

[0014] A method for preparing a polyolefin separator for lithium-ion batteries as described in any one of the above claims, comprising at least the following preparation steps: Lithium sulfonate functionalized conjugated microporous polymer@cellulose nanocomposite particles and polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets were dispersed in deionized water, stirred and ultrasonically treated to obtain a mixed dispersion. The mixed dispersion was added to an aqueous solution of polyethylene oxide and stirred. Then, aminosilane-modified tannic acid nanoparticles were added and stirred again. After vacuum degassing, a slurry was obtained. The slurry is coated onto one or both sides of a polyolefin-based membrane, and after drying, a polyolefin separator for lithium-ion batteries is obtained. The beneficial effects of this invention are: This invention provides a polyolefin composite separator for lithium-ion batteries. It uses a polyolefin base film as the mechanical support substrate and forms a composite coating on the base film consisting of lithium sulfonate functionalized conjugated microporous polymer@cellulose nanoparticles, polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets, and polyethylene oxide and aminosilane-modified tannic acid nanoparticles. This coating can be widely applied to various systems such as lithium iron phosphate batteries, ternary lithium batteries, and lithium metal batteries. It effectively solves the technical defects of existing polyolefin separators, such as poor electrolyte affinity, insufficient thermal dimensional stability, and weak lithium dendrite suppression ability, while ensuring the bonding strength between the coating and the base film and the long-term cycle structural stability.

[0015] This invention prepares a lithium sulfonate-functionalized conjugated microporous polymer@cellulose nanoparticles, in which the lithium sulfonate-functionalized conjugated microporous polymer is in situ loaded onto cellulose nanoparticles. On the one hand, the fully conjugated framework of the conjugated microporous polymer has excellent electron delocalization ability, which can homogenize the interfacial electric field and physically eliminate the local current tips that induce lithium dendrite growth, thereby weakening the driving force for lithium dendrite growth at its source. After sulfonation and lithium modification, the lithium sulfonate groups introduced on the surface of the framework can efficiently promote lithium salt dissociation, improve the lithium ion transference number and ion conduction efficiency of the coating, and significantly enhance the electrolyte affinity of the coating. On the other hand, cellulose nanoparticles, as a continuous mechanical framework, anchor the conjugated microporous polymer particles through physical entanglement and hydrogen bonding, avoiding particle aggregation and migration, while improving the overall mechanical strength and puncture resistance of the coating, achieving a synergistic unity of ion conduction performance and mechanical support performance.

[0016] The polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in this invention achieve synergistic enhancement of thermal conductivity and electrolyte storage. The hydroxylated boron nitride nanosheets provide a continuous two-dimensional thermally conductive network, rapidly dispersing heat generated inside the battery and preventing localized heat accumulation. Simultaneously, they improve the high-temperature structural integrity of the coating and suppress membrane thermal shrinkage. The hollow mesoporous silica grown in situ on its surface possesses abundant mesoporous channels and hollow electrolyte storage space, significantly improving the electrolyte adsorption and storage capacity of the coating and reducing interfacial impedance. The surface polydopamine coating not only enhances the interfacial compatibility between fillers and reduces filler agglomeration but also further increases the surface polarity of the fillers, enhancing the capillary adsorption of the electrolyte.

[0017] This invention also introduces aminosilane-modified tannic acid nanoparticles as an interfacial bridging agent. The siloxane groups at one end of the molecular structure, after hydrolysis, can form stable covalent bonds with the hydroxyl groups on the surface of the inorganic filler. The tannic polyphenol structure at the other end can form multiple hydrogen bonds and hydrophobic interactions with the sulfonic acid groups, hydroxyl groups, and ether bonds in the organic matrix. This constructs a molecular-level bridging network between the inorganic filler and the organic matrix, significantly improving the interphase bonding force within the coating and preventing filler detachment, coating cracking, and phase separation during long-term cycling. It also enhances the adhesion between the coating and the low surface energy polyolefin film, improving the structural durability of the coating. Furthermore, tannic acid, a natural flame retardant, forms a multi-level synergistic thermal safety protection with the thermal conductivity of boron nitride and the heat-resistant framework of the polymer, greatly improving the intrinsic safety of the battery under extreme conditions. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1: The preparation method of lithium sulfonate functionalized conjugated microporous polymer@cellulose nanoparticles includes the following steps: 1.5 g (10 mmol) of 1,3,5-triethynylbenzene and 4.95 g (15 mmol) of 1,4-diiodobenzene were dissolved in 100 mL of anhydrous toluene / anhydrous triethylamine (1:1 v / v). A composite catalyst consisting of 0.21 g (0.3 mmol) of bis(triphenylphosphine)palladium dichloride and 0.114 g (0.6 mmol) of cuprous iodide was added, and the mixture was ultrasonically dispersed for 10 min. The mixture was then frozen in liquid nitrogen, and after three cycles of freezing-vacuuming-thawing to remove gas, it was sealed under nitrogen protection and reacted at 80 °C in the dark for 24 h. After the reaction was complete, the mixture was cooled to room temperature, filtered, and washed successively with anhydrous toluene, anhydrous methanol, and anhydrous acetone. The mixture was then transferred to a Soxhlet extractor and refluxed with tetrahydrofuran as the extractant for 48 h. After extraction, the mixture was soaked in 0.1 mol / L... The precipitate was collected by centrifugation in an aqueous solution of disodium EDTA, stirred and washed three times at 60°C for 6 hours each time. The precipitate was washed with deionized water until the filtrate was neutral, and then washed three times each with anhydrous methanol and anhydrous acetone. Finally, it was dried under vacuum at 100°C for 12 hours to obtain the conjugated microporous polymer. 5g of the above conjugated microporous polymer was slowly added to 50mL of 98% concentrated sulfuric acid. The mixture was stirred and dispersed in an ice-water bath for 30min. After the reaction was completed, the mixture was cooled to room temperature and poured into an ice-water mixture. The mixture was stirred vigorously and filtered. The precipitate was washed repeatedly with deionized water until the filtrate was neutral. The precipitate was then dried under vacuum at 80℃ for 12h to obtain the sulfonated conjugated microporous polymer. 4g of the above sulfonated conjugated microporous polymer was immersed in a 1mol / L lithium hydroxide aqueous solution. After ion exchange by magnetic stirring at room temperature for 18h, the solution was filtered and repeatedly washed with deionized water until the filtrate was neutral. The solution was then vacuum dried at 80℃ for 12h and ground through a 200-mesh sieve to obtain lithium sulfonate functionalized conjugated microporous polymer. 2g of the above lithium sulfonate functionalized conjugated microporous polymer was dispersed in 100mL of deionized water, and then added dropwise to 600g of an aqueous dispersion of nanocellulose (solid content 1wt%, diameter 50-100nm, carboxyl content 1.2mmol / g) and stirred. The mixture was then sonicated in an ice-water bath for 1h, dialyzed with deionized water for 24h (molecular weight cutoff 3500Da), with the water changed every 6 hours. The mixture was then concentrated under reduced pressure at 40℃ to a solid content of about 2wt%, and then freeze-dried for 48h to obtain lithium sulfonate functionalized conjugated microporous polymer@nanocellulose.

[0020] Example 2: The preparation method of polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets includes the following steps: 1g of hexagonal boron nitride, 1g of sodium dodecyl sulfate, 1g of β-cyclodextrin and 100mL of 28% ammonia water were added to an agate ball mill jar and mixed and ball milled. After ball milling, the mixture was centrifuged at 8000rpm for 5min, washed with deionized water and centrifuged again, and vacuum dried at 60℃ for 12h to obtain hydroxylated boron nitride nanosheets. 0.5 g of the above-mentioned hydroxylated boron nitride nanosheets were added to 250 mL of a water / ethanol (volume ratio 1:4) mixed solution and ultrasonically dispersed for 30 min. Then, 0.6 g of cetyltrimethylammonium bromide was added, and the pH was adjusted to 10.5 with ammonia. 2 g of tetraethyl orthosilicate was added dropwise and stirred for 6 h. After centrifugation at 8000 rpm for 5 min, the nanosheets were washed twice with deionized water and twice with anhydrous ethanol. The nanosheets were then dispersed in 150 mL of a 1 mol / L sodium carbonate aqueous solution and etched at 80 °C for 6 h. After etching, the nanosheets were centrifuged at 8000 rpm for 5 min, washed, and dried at 60 °C for 12 h. Finally, the nanosheets were placed in a muffle furnace and calcined at 550 °C for 4 h to obtain hollow mesoporous silica@hydroxylated boron nitride nanosheets. 0.5 g of the above hollow mesoporous silica@hydroxylated boron nitride nanosheets were added to 100 mL of Tris-HCl buffer, the pH was adjusted to 8.5, and the mixture was ultrasonically dispersed for 30 min. Then, 0.05 g of dopamine hydrochloride was added, and the mixture was stirred in the dark for 18 h. After centrifugation at 8000 rpm for 5 min, the mixture was washed and vacuum dried at 60 °C for 12 h to obtain polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets.

[0021] Example 3: The preparation method of aminosilane-modified tannic acid nanoparticles includes the following steps: Add 0.5g of tannic acid to 100mL of deionized water and stir magnetically for 10min until completely dissolved. While stirring, add 0.5g of γ-aminopropyltriethoxysilane and continue stirring at room temperature for 12h to obtain a dispersion of aminosilane-modified tannic acid nanoparticles.

[0022] Example 4: The preparation method of polyolefin separator for lithium-ion batteries includes the following steps: Based on dry weight, 55 parts by weight of the lithium sulfonate functionalized conjugated microporous polymer@cellulose nanocomposite particles prepared in Example 1 and 20 parts by weight of the polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2 were added to deionized water, magnetically stirred for 30 min, and then sonicated in an ice water bath for 30 min to obtain a mixed dispersion. Based on dry weight, the above mixed dispersion was added to 20 parts by weight of an aqueous solution of polyethylene oxide (2wt%) and stirred for 1 hour. Then, 5 parts by weight of the dispersion of aminosilane-modified tannic acid nanoparticles prepared in Example 3 were added and stirred for another 30 minutes. The mixture was then placed in a vacuum drying oven and vacuum degassed at room temperature for 30 minutes to obtain a slurry with a solid content of about 4.5wt%. The above slurry was uniformly coated onto one side of a corona-treated polyolefin-based membrane (20 μm, Celgard 2500) using a 100 μm wire rod at a speed of 10 cm / s. The membrane was dried at 60 °C for 15 min. The membrane was then flipped over, and the above operation was repeated to complete the coating on the other side. After vacuum drying at 60 °C for 12 h, a polyolefin membrane for lithium-ion batteries was obtained.

[0023] Example 5: The preparation method of polyolefin separator for lithium-ion batteries includes the following steps: Based on dry weight, 60 parts by weight of the lithium sulfonate functionalized conjugated microporous polymer@cellulose nanocomposite particles prepared in Example 1 and 15 parts by weight of the polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2 were added to deionized water, magnetically stirred for 30 min, and then sonicated in an ice water bath for 30 min to obtain a mixed dispersion. Based on dry weight, the above mixed dispersion was added to 20 parts by weight of an aqueous solution of polyethylene oxide (2wt%) and stirred for 1 hour. Then, 5 parts by weight of the dispersion of aminosilane-modified tannic acid nanoparticles prepared in Example 3 were added and stirred for another 30 minutes. The mixture was then placed in a vacuum drying oven and vacuum degassed at room temperature for 30 minutes to obtain a slurry with a solid content of about 4.5wt%. The above slurry was uniformly coated onto one side of a corona-treated polyolefin-based membrane (20 μm, Celgard 2500) using a 100 μm wire rod at a speed of 10 cm / s. The membrane was dried at 60 °C for 15 min. The membrane was then flipped over, and the above operation was repeated to complete the coating on the other side. After vacuum drying at 60 °C for 12 h, a polyolefin membrane for lithium-ion batteries was obtained.

[0024] Example 6: The preparation method of polyolefin separator for lithium-ion batteries includes the following steps: Based on dry weight, 65 parts by weight of the lithium sulfonate functionalized conjugated microporous polymer@cellulose nanocomposite particles prepared in Example 1 and 10 parts by weight of the polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2 were added to deionized water, magnetically stirred for 30 min, and then sonicated in an ice water bath for 30 min to obtain a mixed dispersion. Based on dry weight, the above mixed dispersion was added to 20 parts by weight of an aqueous solution of polyethylene oxide (2wt%) and stirred for 1 hour. Then, 5 parts by weight of the dispersion of aminosilane-modified tannic acid nanoparticles prepared in Example 3 were added and stirred for another 30 minutes. The mixture was then placed in a vacuum drying oven and vacuum degassed at room temperature for 30 minutes to obtain a slurry with a solid content of about 4.5wt%. The above slurry was uniformly coated onto one side of a corona-treated polyolefin-based membrane (20 μm, Celgard 2500) using a 100 μm wire rod at a speed of 10 cm / s. The membrane was dried at 60 °C for 15 min. The membrane was then flipped over, and the above operation was repeated to complete the coating on the other side. After vacuum drying at 60 °C for 12 h, a polyolefin membrane for lithium-ion batteries was obtained.

[0025] Comparative Example 1: The preparation method of conjugated microporous polymer@cellulose nanoparticles includes the following steps: 1.5 g (10 mmol) of 1,3,5-triethynylbenzene and 4.95 g (15 mmol) of 1,4-diiodobenzene were dissolved in 100 mL of anhydrous toluene / anhydrous triethylamine (volume ratio 1:1). A composite catalyst consisting of 0.21 g (0.3 mmol) of bis(triphenylphosphine)palladium dichloride and 0.114 g (0.6 mmol) of cuprous iodide was added. The mixture was ultrasonically dispersed for 10 min, frozen with liquid nitrogen, and degassed by three cycles of freezing-vacuuming-thawing. The mixture was then sealed under nitrogen protection and stirred at 80 °C in the dark for 24 h. After the reaction was completed, the mixture was cooled to room temperature, filtered, washed successively with anhydrous toluene, anhydrous methanol, and anhydrous acetone, and dried under vacuum at 100 °C for 12 h to obtain a conjugated microporous polymer. 2g of the above conjugated microporous polymer was dispersed in 100mL of deionized water, and then added dropwise to 600g of an aqueous dispersion of nanocellulose (solid content 1wt%, diameter 50-100nm, carboxyl content 1.2mmol / g) and stirred. The mixture was then sonicated in an ice-water bath for 1h, dialyzed with deionized water for 24h (molecular weight cutoff 3500Da), with the water changed every 6 hours. The mixture was then concentrated under reduced pressure at 40℃ to a solid content of about 2wt%, and then freeze-dried for 48h to obtain the conjugated microporous polymer@nanocellulose.

[0026] Compared with Example 4, Comparative Example 2 only replaced the lithium sulfonate functionalized conjugated microporous polymer@nanocellulose prepared in Example 1 with the same mass as the conjugated microporous polymer@nanocellulose prepared in Comparative Example 1. The remaining components and preparation methods were completely the same as those in Example 4.

[0027] Compared with Example 4, Comparative Example 3 only replaced the lithium sulfonate functionalized conjugated microporous polymer@nanocellulose prepared in Example 1 with the unmodified nanocellulose in Example 1. The remaining components and preparation methods were completely the same as those in Example 4.

[0028] Compared with Example 4, Comparative Example 4 only replaced the polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2 with the unmodified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2 by the same mass. The remaining components and preparation methods were completely consistent with Example 4.

[0029] Compared with Example 4, Comparative Example 5 only omits the polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets prepared in Example 2; the remaining components and preparation methods are completely consistent with Example 4.

[0030] Compared with Example 4, Comparative Example 6 only did not add the aminosilane-modified tannic acid nanoparticles prepared in Example 2; the other components and preparation methods were completely the same as in Example 4.

[0031] Compared with Example 4, Comparative Example 7 did not form a composite coating on the polyolefin-based film (20 μm, Celgard 2500) and served as a blank control.

[0032] Performance testing Battery assembly: The active material lithium iron phosphate, conductive carbon black (HCY-80, acetylene black) and binder (PVDF) were mixed in a mass ratio of 90:5:5 with NMP as solvent to form a well-dispersed slurry. The slurry was thoroughly ground in a mortar for 1 hour. The resulting slurry was then uniformly coated onto an aluminum foil with an etching thickness of 38 μm. The coated aluminum foil was then immediately dried in a blower at 80 °C for 2 hours. The foil was punched into small round pieces using an electrode cutter and then placed in a vacuum oven at 80 °C for 12 hours to obtain the electrode sheet. After weighing the electrode plates, they were placed in an environment filled with argon gas, humidity, and oxygen / water levels below 1 x 10⁻⁶. -6 In the glove box, CR2025 coin cell battery assemblies were assembled. The positive electrode shell, positive electrode sheet, separator prepared in Examples 4-6 and Comparative Examples 2-7, lithium negative electrode sheet, 0.5mm stainless steel gasket, spring sheet, and negative electrode shell were stacked in sequence. During the assembly process, 20μL of electrolyte was added to each side of the separator. The electrolyte was a mixed organic solution of 1mol / L LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC / DEC volume ratio of 1:1). After assembly, the batteries were sealed using an electric button cell sealing machine. The sealed batteries were placed in a 30°C constant temperature chamber and left to stand for 4 hours before testing.

[0033] Electrolyte contact angle (OCA) measurement: The contact angle between the diaphragm and the electrolyte was measured using a video optical contact angle meter (OCA20, Dataphysics, Germany). The diaphragm was flatly attached to the glass plate surface using double-sided tape. 2 μL of electrolyte was injected into the diaphragm surface each time, and the contact angle value at the instant the electrolyte contacted the diaphragm surface was recorded. The test results are shown in Table 1. Liquid uptake rate (EU) test: The diaphragm was immersed in the electrolyte at 30℃ for 30 min, and the mass of the diaphragm before and after immersion in the electrolyte was measured and calculated using the following formula: EU(%)=(W2-W1) / W1×100%; W1 and W2 represent the mass of the diaphragm before and after immersion in the electrolyte, respectively; the test results are shown in Table 1. Liquid retention rate (ER) test: The diaphragm was immersed in electrolyte at 30℃ for 30 min, and the mass of the diaphragm was measured every 10 min. The liquid retention rate of the diaphragm at different time points was calculated using the following formula: ER (%) = (W3 - W1) / (W2 - W1) × 100%; W1, W2, and W3 represent the masses of the diaphragm before, after, and after standing, respectively; the test results are shown in Table 1. Test of thermal dimensional stability: Place the diaphragm in an oven at 150°C for 30 minutes, and calculate the diaphragm area shrinkage rate T (%) using the following formula.

[0034] T = S1 / S0 × 100%; Where T is the thermal shrinkage rate of the diaphragm (%); S0 and S1 represent the area of ​​the diaphragm before and after heat treatment, respectively (mm²). 2 The test results are shown in Table 1. Table 1: Statistical Table of Diaphragm Performance Test Data for Examples 4-6 and Comparative Examples 2-7

[0035] Table 1 shows that the polyolefin separator for lithium-ion batteries prepared in this invention exhibits excellent electrolyte affinity and thermal dimensional stability. Comparison Example 2 shows that lithium sulfonate functionalization significantly improves the polarity and electrolyte affinity of the coating, while also enhancing its high-temperature structural stability. Comparison Example 3 shows that when pure nanocellulose is used as the coating substrate, the electrolyte wettability, liquid retention capacity, and thermal stability of the separator all deteriorate significantly. Comparison Example 4 shows that when using unmodified polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets, the separator contact angle increases, the liquid absorption and retention capacity decreases, and the thermal shrinkage rate slightly increases. Comparison Example 5 shows that without the addition of polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets, the separator's thermal shrinkage rate increases, and its liquid absorption and retention performance also significantly decreases. As shown in Comparative Example 6, without the addition of aminosilane-modified tannic acid nanoparticles, the electrolyte affinity and thermal stability of the membrane both declined to some extent. Comparative Example 7 served as a blank control, with the blank polypropylene membrane exhibiting the worst performance in all aspects.

[0036] Battery performance test Ionic conductivity testing: The ionic conductivity of the membrane sample was tested using electrochemical impedance spectroscopy (EIS) on an electrochemical workstation. An AC voltage of 10 mV was applied during the test, with a frequency range of 1-10. 6 The ionic conductivity (σ, mS / cm) is calculated using the following formula; σ = D / (S·RL); In the formula, D represents the sample thickness in cm; S represents the effective contact area between the electrode and the sample in cm².2 ; RL represents the bulk resistance of the sample, in Ω; the test results are shown in Table 2; Battery cycle performance test: The rate performance and charge-discharge cycle performance of the battery samples were tested using a battery tester in the voltage range of 2.5-4.5V. The stable cycle performance of the battery was tested by charging and discharging 50 times at a rate of 0.5C. The rate performance was determined by stable charging and discharging 5 times at 0.2, 0.5, 1, 2 and 3C. The test results are shown in Table 2. Table 2: Statistical Table of Lithium-ion Battery Performance Test Data for Examples 4-6 and Comparative Examples 2-7

[0037] Table 2 shows that the lithium-ion battery assembled with the separator prepared in this invention exhibits higher ionic conductivity, better cycle stability, and rate performance. Comparison Example 3 shows that when only a pure nanocellulose coating is used, the battery's ionic conductivity decreases, and both cycle and rate performance deteriorate significantly. Comparison Example 4 shows that when using inorganic fillers without polydopamine modification, the battery's ionic conductivity, cycle retention rate, and rate performance all decrease. Comparison Example 5 shows that without the addition of inorganic thermally conductive fillers, both battery cycle stability and rate performance decline. Comparison Example 6 shows that without the addition of aminosilane-modified tannic acid nanoparticles, the battery's cycle capacity retention rate and rate performance decrease significantly. Comparison Example 7 serves as a blank control; the battery using an unmodified polyolefin separator exhibits the worst performance across all categories.

[0038] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0039] The foregoing has provided a detailed description of one embodiment of the present invention, but this description is merely a preferred embodiment and should not be construed as limiting the scope of the invention. All equivalent variations and modifications made within the scope of the claims of this invention should still fall within the patent coverage of this invention.

Claims

1. A polyolefin separator for lithium-ion batteries, characterized in that, Includes a polyolefin-based film and a composite coating disposed on one or both surfaces of the polyolefin-based film; The composite coating, by dry weight, comprises at least the following raw materials: Lithium sulfonate functionalized conjugated microporous polymer@nanocellulose 55-65 parts; polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets 10-20 parts. 15-25 parts of polyethylene oxide; 3-8 parts of aminosilane-modified tannic acid nanoparticles.

2. The polyolefin separator for lithium-ion batteries according to claim 1, characterized in that, The preparation method of the lithium sulfonate functionalized conjugated microporous polymer@cellulose nanoparticles includes at least the following steps: 1,3,5-triethynylbenzene and 1,4-diiodobenzene were dissolved in a mixed solvent of anhydrous toluene / anhydrous triethylamine. A composite catalyst of bis(triphenylphosphine)palladium dichloride and cuprous iodide was added. After degassing, the mixture was sealed under nitrogen protection and stirred at 70-90°C in the dark for 20-36 hours. The mixture was then washed, purified, and vacuum dried to obtain a conjugated microporous polymer. The conjugated microporous polymer was added to concentrated sulfuric acid, stirred and dispersed in an ice-water bath, heated to 50-60℃ and stirred for 4-6 hours, then cooled, precipitated, washed and vacuum dried to obtain sulfonated conjugated microporous polymer. The sulfonated conjugated microporous polymer was added to an aqueous lithium hydroxide solution, and after ion exchange, it was washed, vacuum dried and ground to obtain lithium sulfonate functionalized conjugated microporous polymer. The lithium sulfonate functionalized conjugated microporous polymer was dispersed in deionized water, then added dropwise to an aqueous dispersion of nanocellulose and stirred. After ultrasonic treatment in an ice-water bath, the mixture was dialyzed, concentrated by rotary evaporation under reduced pressure, and freeze-dried to obtain lithium sulfonate functionalized conjugated microporous polymer@nanocellulose.

3. The polyolefin separator for lithium-ion batteries according to claim 2, characterized in that, The molar ratio of 1,3,5-triethynylbenzene to 1,4-diiodobenzene is 1:1-2, and the degree of sulfonation of the sulfonated conjugated microporous polymer is 0.6-0.

7.

4. The polyolefin separator for lithium-ion batteries according to claim 2, characterized in that, The concentration of the lithium hydroxide aqueous solution is 0.5-1.5 mol / L, and the mass ratio of the lithium sulfonate functionalized conjugated microporous polymer to the nanocellulose is 1:2-4.

5. A polyolefin separator for lithium-ion batteries according to claim 1, characterized in that, The preparation method of the polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets includes at least the following preparation steps: Hexagonal boron nitride, sodium dodecyl sulfate, β-cyclodextrin and ammonia were mixed, ball-milled, centrifuged, washed and vacuum-dried to obtain hydroxylated boron nitride nanosheets. The hydroxylated boron nitride nanosheets were added to a water / ethanol mixed solution and ultrasonically dispersed. Hexadecyltrimethylammonium bromide was added, and the pH was adjusted to 10-11 with ammonia. Tetraethyl orthosilicate was added dropwise and stirred to react. After centrifugation and washing, the nanosheets were added to a sodium carbonate solution and etched by stirring at 70-90°C for 5-7 hours. After centrifugation and washing, the nanosheets were calcined at 500-600°C for 3-5 hours to obtain hollow mesoporous silica@hydroxylated boron nitride nanosheets. Hollow mesoporous silica@hydroxylated boron nitride nanosheets were added to Tris-HCl buffer, the pH was adjusted to 8-9, and the nanosheets were ultrasonically dispersed. Dopamine hydrochloride was added, and the mixture was stirred in the dark for 12-24 hours. After centrifugation, washing, and vacuum drying, polydopamine-modified hollow mesoporous silica@hydroxylated boron nitride nanosheets were obtained.

6. A polyolefin separator for lithium-ion batteries according to claim 5, characterized in that, The mass ratio of the hydroxylated boron nitride nanosheets to the tetraethyl orthosilicate is 1:3-5, and the concentration of the sodium carbonate solution is 0.5-1 mol / L.

7. A polyolefin separator for lithium-ion batteries according to claim 5, characterized in that, The mass ratio of the hollow mesoporous silica@hydroxylated boron nitride nanosheets to the dopamine hydrochloride is 1:0.05-0.

2.

8. A polyolefin separator for lithium-ion batteries according to claim 1, characterized in that, The aminosilane-modified tannic acid nanoparticles are obtained by reacting aminosilane and tannic acid, and the mass ratio of aminosilane to tannic acid is 0.5-2:

1. The aminosilane is at least one of γ-aminopropyltriethoxysilane, γ-aminopropyltrimethoxysilane or N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane.

9. A polyolefin separator for lithium-ion batteries according to claim 1, characterized in that, The thickness of the composite coating on one side is 2-10 μm, the thickness of the polyolefin-based membrane is 9-25 μm, and the polyolefin-based membrane is at least one of polypropylene membrane, polyethylene membrane, or polypropylene / polyethylene / polypropylene three-layer composite membrane.

10. A method for preparing a polyolefin separator for lithium-ion batteries according to any one of claims 1-9, characterized in that, It includes at least the following preparation steps: Lithium sulfonate functionalized conjugated microporous polymer@cellulose nanocomposite particles and polydopamine modified hollow mesoporous silica@hydroxylated boron nitride nanosheets were dispersed in deionized water, stirred and ultrasonically treated to obtain a mixed dispersion. The mixed dispersion was added to an aqueous solution of polyethylene oxide and stirred. Then, aminosilane-modified tannic acid nanoparticles were added and stirred again. After vacuum degassing, a slurry was obtained. The slurry is coated onto one or both sides of a polyolefin-based membrane, and after drying, a polyolefin separator for lithium-ion batteries is obtained.

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