Preparation method of high-performance polyacrylonitrile-cellulose nanocrystalline composite battery diaphragm

Abstract

The invention discloses a preparation method of a high-performance polyacrylonitrile-cellulose nanocrystalline composite battery diaphragm, and belongs to the technical field of lithium ion battery diaphragms. The preparation method comprises the following steps: mixing cellulose nanocrystals, polyacrylonitrile and a solvent to obtain a spinning solution; and carrying out electrostatic spinning on the spinning solution to obtain the high-performance polyacrylonitrile-cellulose nanocrystalline composite battery diaphragm, the mass of the cellulose nanocrystal is 1-10% of the sum of the mass of the cellulose nanocrystal and the mass of the polyacrylonitrile; the cellulose nanocrystals are obtained by hydrolyzing sisal hemp cellulose through phosphoric acid. The polyacrylonitrile-cellulose nanocrystalline composite battery diaphragm disclosed by the invention has excellent thermal stability, mechanical property and electrolyte wettability, and has good cycle stability when being applied to a lithium ion battery. Meanwhile, the raw materials of the cellulose nanocrystals are wide in source, low in price and environment-friendly, and the production cost of the diaphragm is reduced by taking the cellulose nanocrystals as one of the raw materials.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery separators, and in particular to a method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator. Background Art

[0002] Lithium-ion batteries are widely used in portable electronics, electric vehicles, and grid-scale energy storage systems, powering our daily lives. Therefore, developing low-cost, eco-efficient, and environmentally friendly lithium-ion batteries is highly desirable for the future sustainability of large-scale renewable energy use. Separators, as a crucial component of batteries, significantly impact their safety, lifespan, and electrochemical performance, making them crucial for the sustainability of energy storage systems. Separators act as an electronic barrier between the positive and negative electrodes to prevent internal short circuits, requiring insulating properties, high mechanical strength, and electrochemical and thermal stability. Separators also provide pathways for lithium ion migration between the two electrodes, requiring good lithium ion conductivity in the liquid electrolyte. Separators are a crucial component of lithium-ion batteries and significantly influence their electrochemical performance. Commercial lithium-ion battery separators suffer from poor electrochemical performance, limiting their application in electric vehicles and energy storage systems. Polyolefin-based separators suffer from low porosity, high thermal shrinkage, and poor thermal stability, resulting in poor electrochemical performance. This issue can be addressed by using different polymer separator types and employing different separator manufacturing techniques. Electrospinning technology has received extensive attention in recent years and is helpful in designing new separator materials with high porosity, specific surface area, electrolyte absorption and ionic conductivity.

[0003] To address the issues with polyolefin separators, polymers such as polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyacrylonitrile, and polymethyl methacrylate have also been used in separators. Polyacrylonitrile has high heat resistance, and the cyano group (-CN) in its structure interacts with the carboxyl groups in the electrolyte components, thereby improving the compatibility of polyacrylonitrile with the electrolyte. Furthermore, polyacrylonitrile exhibits excellent spinnability in electrospinning technology.

[0004] In recent years, research has shown that membranes produced through electrospinning possess a three-dimensional structure, high porosity, and ionic conductivity, making them ideal for preparing lithium battery separators. Lithium-ion battery separator materials typically require a certain porosity to allow sufficient electrolyte to fill the membrane and ensure ion transport between the electrodes. However, excessive porosity can affect the battery's self-closing properties and significantly reduce the separator's mechanical strength. Commercially available polyolefin separators typically have a porosity of 40-50%. Fiber separators produced through electrospinning have a higher porosity, reaching 80% or even higher. Therefore, electrospun separators generally exhibit excellent ionic conductivity, significantly reducing battery impedance and improving battery performance. During the electrospinning process, because the jet solidifies within a very short time, the fibers experience minimal stretching during formation and exhibit low crystallinity, resulting in relatively poor mechanical properties. Furthermore, separators produced through electrospinning have larger pore sizes and uneven pore size distribution. These factors can make it easier for lithium dendrites to penetrate the separator, creating the risk of short-circuiting during battery operation. On the other hand, this can lead to unstable current distribution through the separator during battery operation, resulting in localized excessive current, which in turn affects battery performance. Solving these problems in preparing separators by electrospinning to achieve high-performance lithium-ion battery separators is a key area of ​​research for those skilled in the art. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator to solve the problems existing in the above-mentioned prior art.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] One of the technical solutions of the present invention is a method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator, comprising the following steps:

[0008] Mixing cellulose nanocrystals (abbreviated as CNCs), polyacrylonitrile (PAN) and a solvent to obtain a spinning solution; electrospinning the spinning solution to obtain the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (CNCs / PAN, abbreviated as PCP composite separator);

[0009] The mass of the cellulose nanocrystals is 1 to 10% of the sum of the mass of the cellulose nanocrystals and the polyacrylonitrile;

[0010] The cellulose nanocrystals are obtained by hydrolyzing sisal cellulose through phosphoric acid.

[0011] CNCs have high crystallinity and a high elastic modulus, effectively regulating the pore size of PAN-based membranes, making them smaller and more uniform, and increasing porosity. Their loading also increases the surface roughness of the PAN-based membrane. Furthermore, the polar -OH groups in the CNCs have better interfacial compatibility with the PAN matrix. Therefore, the addition of CNCs can increase the density of the separator, thereby improving its tensile strength. Applying separators with uniform and reduced pore size and increased surface roughness to lithium-ion batteries can improve the local shuttling of lithium ions (due to the increased roughness of the separator surface, it can absorb more electrolyte, and its absorption rate increases. More electrolyte absorption contains more lithium ions, making it easier to shuttle between the positive and negative electrodes), allowing them to be evenly deposited on the lithium sheet. Furthermore, the -OH groups present in the cellulose nanocrystals make the composite separator more compatible with the electrolyte, thereby giving the separator better wettability and making it easier to absorb more electrolyte. The composite membrane's advantages in pore size and wettability give it a high specific capacity and specific capacity retention when used in lithium-ion batteries. Furthermore, the physical and chemical properties of CNCs are relatively stable, and the accumulation of oxides produced by their thermal degradation on the polymer surface can hinder the diffusion of oxygen into the polymer matrix, acting as a thermal barrier. Therefore, the appropriate addition of CNCs can also improve the thermal stability of the composite membrane. In summary, the main reason for the improved performance of the composite membrane is that the addition of CNCs regulates the surface pore size and roughness of the membrane, resulting in excellent mechanical properties and electrochemical cycling stability.

[0012] Further, the solvent is N,N-dimethylformamide;

[0013] And / or, the ratio of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile to the volume of the solvent is 1.2 g:8 mL.

[0014] Furthermore, the mixing of cellulose nanocrystals, polyacrylonitrile and solvent to obtain a spinning solution includes: dispersing cellulose nanocrystals in a solvent, ultrasonicating for 1 to 2 hours to obtain a mixed solution; adding polyacrylonitrile to the mixed solution, stirring at room temperature for 12 hours to obtain a spinning solution.

[0015] Furthermore, the electrospinning parameters include: humidity of 28-35°, voltage of 17 kV, spray liquid flow rate of 0.0005-0.001 mL / min, receiving roller speed of 30-45 r / min, and a distance between the syringe needle tip and the receiving roller of 15-20 cm.

[0016] Furthermore, the preparation steps of the cellulose nanocrystals include: mixing sisal cellulose, sodium chlorite, acetic acid and water, heating and reacting once to obtain sisal cellulose microfibrils; mixing the sisal cellulose microfibrils and phosphoric acid, heating and reacting twice, standing after the reaction, and then centrifuging, neutralizing the centrifuged product with ammonia water and freeze-drying it to obtain the cellulose nanocrystals.

[0017] The present invention uses sisal cellulose extracted from sisal hemp as raw material to produce cellulose nanocrystals. Sisal hemp not only has a high cellulose content but is also widely available. The cellulose nanocrystals are obtained through phosphoric acid hydrolysis. The resulting cellulose nanocrystals are uniform in size and have more -OH groups exposed on the crystal surface.

[0018] Furthermore, the usage ratio of the sisal cellulose, sodium chlorite, acetic acid and water is 10 g:3.35 g:2.5 mL:350 mL;

[0019] and / or, the ratio of the sisal cellulose microfiber to the phosphoric acid is 5 g:100 mL;

[0020] and / or, the concentration of the phosphoric acid is 14.75 mol / L;

[0021] And / or, the temperature of the primary heating reaction is 70-75° C. and the time is 2.5-3 h;

[0022] And / or, the temperature of the secondary heating reaction is 50-60° C. and the time is 1.5-2 h;

[0023] And / or, the standing time is 12 to 24 hours;

[0024] And / or, the freeze-drying step includes: pre-freezing at -5°C for 12 hours, and then freeze-drying at -40°C for 32 hours.

[0025] Furthermore, after the primary heating reaction is completed, the steps of filtering, washing and drying are also included.

[0026] Furthermore, before the standing, the method further comprises the step of adding water to the reaction system after the secondary heating reaction is completed; the amount of water added is 3 times the volume of the phosphoric acid used in the secondary heating reaction.

[0027] Furthermore, the standing is performed at room temperature.

[0028] Furthermore, the preparation step of the sisal cellulose includes: mixing sisal fibers and an alkaline solution, and heating the mixture for reaction to obtain the sisal cellulose.

[0029] Furthermore, the alkaline solution is a KOH solution or a NaOH solution;

[0030] and / or, the concentration of the alkaline solution is 2 to 2.5 mol / L;

[0031] And / or, the usage ratio of the sisal fiber and the alkaline solution is 50g:700mL;

[0032] And / or, the heating reaction temperature is 160° C. and the time is 14 hours.

[0033] Furthermore, after the heating reaction is completed, the steps of filtering, washing and drying are also included.

[0034] Furthermore, before mixing the sisal fibers with the alkaline solution, the method further includes a pretreatment step of the sisal fibers, specifically, washing the sisal fibers with clean water, drying the fibers at 60° C., and then chopping the fibers for later use.

[0035] The second technical solution of the present invention: a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator prepared according to the above preparation method.

[0036] The third technical solution of the present invention: application of the above-mentioned high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator in the preparation of lithium-ion batteries.

[0037] Technical solution four of the present invention: A lithium-ion battery using the above-mentioned high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator as a separator.

[0038] The present invention discloses the following technical effects:

[0039] (1) The polyacrylonitrile-cellulose nanocrystal composite battery separator prepared by the electrospinning process of the present invention has high porosity, high liquid absorption rate and good wetting performance.

[0040] (2) The polyacrylonitrile-cellulose nanocrystal composite battery separator prepared by the electrospinning process of the present invention has good heat resistance and thermal stability.

[0041] (3) The present invention utilizes natural fibers as raw materials for preparing cellulose nanocrystals, which are widely available and inexpensive.

[0042] (4) The present invention prepares cellulose nanocrystals by phosphoric acid hydrolysis. The cellulose nanocrystals hydrolyzed by phosphoric acid have uniform particle size and good dispersibility in organic solution, and are ideal filling materials for composite diaphragms.

[0043] (5) Cellulose nanocrystals hydrolyzed by phosphoric acid have good compatibility with lithium iron phosphate, the positive electrode material of lithium batteries, and lithium-ion battery electrolyte (the main component is lithium hexafluorophosphate). BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0045] Figure 1 This is a scanning electron microscope image of the cellulose nanocrystals prepared in step (4) of Example 1.

[0046] Figure 2 These are scanning electron microscope images of the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5 and the pure polyacrylonitrile battery separator prepared in Comparative Example 1, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, (d) is Example 3, (e) is Example 4, and (f) is Example 5.

[0047] Figure 3 This is a scanning electron microscope image of the hydrochloric acid hydrolyzed cellulose prepared in Comparative Example 2.

[0048] Figure 4 These are infrared spectra of the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0049] Figure 5 These are thermogravimetric curves of a commercial PP battery separator, a PAN battery separator prepared in Comparative Example 1, and polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0050] Figure 6 These are the thermal dimensional stability test results of commercial PP battery separators, the PAN battery separator prepared in Comparative Example 1, and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0051] Figure 7 The stress-strain curves of the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5 are shown.

[0052] Figure 8 These are the contact angle test results of commercial PP battery separators, PAN battery separators prepared in Comparative Example 1, and polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, (d) is Example 3, (e) is Example 4, (f) is Example 5, and (g) is a PP separator.

[0053] Figure 9These are the porosity test results of commercial PP battery separators, the PAN battery separator prepared in Comparative Example 1, and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0054] Figure 10 These are the test results of liquid absorption rate of commercial PP battery separator, PAN battery separator prepared in Comparative Example 1, and polyacrylonitrile-cellulose nanocrystal composite battery separator prepared in Examples 1 to 5.

[0055] Figure 11 These are the average pore size test results of the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0056] Figure 12 These are the cycle performance test results of lithium-ion batteries prepared from the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5.

[0057] Figure 13 These are the rate performance test results of lithium-ion batteries prepared using the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separator prepared in Example 3. DETAILED DESCRIPTION

[0058] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0059] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0060] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0061] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0062] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0063] As a first aspect of the present invention, the present invention provides a method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator, comprising the following steps:

[0064] Mixing cellulose nanocrystals, polyacrylonitrile and a solvent to obtain a spinning solution; electrospinning the spinning solution to obtain the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator;

[0065] The mass of the cellulose nanocrystals is 1 to 10% of the sum of the mass of the cellulose nanocrystals and the polyacrylonitrile;

[0066] The cellulose nanocrystals are obtained by hydrolyzing sisal cellulose through phosphoric acid.

[0067] As a preferred embodiment of the present invention, the mass of the cellulose nanocrystals is 1%, 3%, 5%, 7% or 10% of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile.

[0068] As a preferred embodiment of the present invention, the mass of the cellulose nanocrystals is 5% of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile.

[0069] As an embodiment of the present invention, the preparation steps of the cellulose nanocrystals include: mixing sisal cellulose, sodium chlorite, acetic acid and water, heating and reacting once to obtain sisal cellulose microfibrils; mixing the sisal cellulose microfibrils and phosphoric acid, heating and reacting twice, standing after the reaction, and then centrifuging, neutralizing the centrifuged product with ammonia water and freeze-drying it to obtain the cellulose nanocrystals.

[0070] As an embodiment of the present invention, the preparation step of the sisal cellulose includes: mixing sisal fibers and an alkaline solution, and heating the mixture for reaction to obtain the sisal cellulose.

[0071] As a preferred embodiment of the present invention, before mixing the sisal fibers with the alkaline solution, a pretreatment step of the sisal fibers is further included, specifically: washing the sisal fibers with clean water, drying the fibers at 60° C., and then chopping the fibers for later use.

[0072] As a preferred embodiment of the present invention, the method for preparing the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator comprises the following specific steps:

[0073] (1) Pretreatment of sisal fiber: Wash the sisal fiber with clean water, dry it at 60℃, and then cut it into pieces for later use;

[0074] (2) Preparation of sisal cellulose: Dried and chopped sisal fibers were mixed with an alkaline solution (KOH solution or NaOH solution with a concentration of 2 to 2.5 mol / L) at a ratio of 50 g:700 mL, and heated at 160°C for 14 h. After the reaction was completed and cooled to room temperature, the reaction product was filtered, and the filter residue was repeatedly washed with deionized water until the filtrate remained neutral in color. The filter residue was dried at 60°C to a constant weight to obtain sisal cellulose.

[0075] (3) Preparation of sisal cellulose microfibers: Sisal cellulose, sodium chlorite, acetic acid, and deionized water were mixed in a ratio of 10 g:3.35 g:2.5 mL:350 mL, heated at 70-75 °C for 2.5-3 h, filtered, washed repeatedly until neutral, and dried at 60 °C to obtain sisal cellulose microfibers;

[0076] (4) Preparation of cellulose nanocrystals: Sisal cellulose microfibrils were mixed with phosphoric acid (concentration of 14.75 mol / L) at a ratio of 5 g:100 mL, and heated at 50-60°C for 1.5-2 h. After the reaction, deionized water was added (the amount of water added was 3 times the volume of the phosphoric acid used in the secondary heating reaction) and the mixture was allowed to stand at room temperature for 12-24 h. The suspension was centrifuged, and the centrifuged product was neutralized with ammonia water and centrifuged again. The solid matter was taken out and placed in a culture dish, first placed in a refrigerator at -5°C for pre-freezing for 12 h, and then placed in a freeze dryer and freeze-dried at -40°C for 32 h to obtain cellulose nanocrystals.

[0077] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: Cellulose nanocrystals are uniformly dispersed in N,N-dimethylformamide, and then ultrasonicated for 1 to 2 hours to obtain a mixed solution; polyacrylonitrile is added to the mixed solution after ultrasonication, and stirred at room temperature for 12 hours to form a spinning solution; the mass of the cellulose nanocrystals is 1 to 10% of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile; the volume ratio of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile to the N,N-dimethylformamide is 1.2 g:8 mL;

[0078] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, spray liquid flow rate of 0.0005-0.001 mL / min, receiving roller speed of 30-45 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15-20 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator was obtained.

[0079] As a preferred embodiment of the present invention, after the electrospinning is completed, the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator is also included in the steps of tearing off the aluminum foil and compacting it with a tablet press at 4 to 6 MPa to obtain a separator directly used for preparing lithium-ion batteries.

[0080] As a second aspect of the present invention, the present invention provides a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator prepared according to the above preparation method.

[0081] As a third aspect of the present invention, the present invention provides the use of the above-mentioned high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator in the preparation of lithium-ion batteries.

[0082] As a fourth aspect of the present invention, the present invention provides a lithium-ion battery using the above-mentioned high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator as a separator.

[0083] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0084] The raw materials used in the following examples, comparative examples and test examples are all common commercially available products, wherein the concentration of ammonia water is 25 wt%; the concentration of LiPF6 electrolyte is 1 mol / L, and the solvent is a mixed solvent obtained by mixing ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) in a mass ratio of 1:1:1; the model of commercial PP battery separator is Celgard2500, purchased from Guangdong Kelude New Energy Technology Co., Ltd.; sisal fiber is purchased from Guangxi Sisal Group Co., Ltd.

[0085] The room temperature involved in the following examples and comparative examples specifically refers to 20-30°C.

[0086] Example 1

[0087] A method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator, comprising the following steps:

[0088] (1) Pretreatment of sisal fiber: After washing the sisal fiber raw material with clean water, put it into a 60℃ oven to dry out the moisture, and finally cut it into pieces for later use.

[0089] (2) Preparation of sisal cellulose: 50 g of the dried and chopped sisal fiber obtained in step (1) was placed in a 1000 mL high-temperature reactor, and 700 mL of a 2.5 mol / L KOH solution was added. The high-temperature reactor was assembled, placed in an oven, and heated to 160° C. for 14 h. After the reaction was completed and cooled to room temperature, the reaction product was filtered, and the filter residue was repeatedly washed with deionized water until the filtrate remained neutral in color. The filter residue was dried in a vacuum drying oven at 60° C. to constant weight to obtain sisal cellulose.

[0090] (3) Preparation of Sisal Cellulose Microfibers: 10 g of the sisal cellulose prepared in step (2) was placed in a beaker. 3.35 g of sodium chlorite, 2.5 mL of acetic acid, and 350 mL of deionized water were uniformly mixed and poured into the beaker. The mixture was reacted in a 75°C water bath for 3 h. The reactant was filtered, washed repeatedly until neutral, and dried in a vacuum drying oven at 60°C to obtain the sisal cellulose microfibers.

[0091] (4) Preparation of cellulose nanocrystals: 5 g of sisal cellulose microfibers prepared in step (3) were placed in a beaker, 100 mL of 14.75 mol / L phosphoric acid was added to the beaker, and the beaker was placed in a 50°C water bath for reaction for 2 h. After the reaction, 300 mL of deionized water was added and allowed to stand at room temperature for 12 h. The suspension was centrifuged, and then the centrifuged product was neutralized with ammonia water (ammonia water was added dropwise to the centrifuged product for neutralization, and then washed with water until neutral), and finally centrifuged again. The solid matter was taken out and placed in a culture dish, first placed in a -5°C refrigerator for pre-freezing for 12 h, and then placed in a freeze dryer and freeze-dried at -40°C for 32 h to obtain cellulose nanocrystals (i.e., phosphoric acid-hydrolyzed cellulose nanocrystals, referred to as CNCs). The morphology was observed using a scanning electron microscope. Figure 1 shown.

[0092] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: 0.012 g of cellulose nanocrystals prepared in step (4) were uniformly dispersed in 8 mL of N,N-dimethylformamide, and then placed in an ultrasonic machine for 2 h. Finally, 1.188 g of polyacrylonitrile was added to the mixed solution after the ultrasonication was completed. The final mixed solution was stirred at room temperature for 12 h to form a spinning solution.

[0093] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: the voltage was 17 kV for the button-type half-cell, the flow rate of the spray liquid was 0.001 mL / min, the speed of the receiving roller was 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (abbreviated as PCP-1%, i.e., a polyacrylonitrile-cellulose nanocrystal composite separator with a CNCs content of 1 wt%) was obtained. The morphology was observed by scanning electron microscopy. Figure 2 shown.

[0094] Example 2

[0095] Steps (1) to (4) are the same as in Example 1.

[0096] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: 0.036 g of cellulose nanocrystals prepared in step (4) were uniformly dispersed in 8 mL of N,N-dimethylformamide, and then placed in an ultrasonic machine for 2 h. Finally, 1.164 g of polyacrylonitrile was added to the mixed solution after the ultrasonication was completed. The final mixed solution was stirred at room temperature for 12 h to form a spinning solution.

[0097] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, spray liquid flow rate of 0.001 mL / min, receiving roller speed of 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (abbreviated as PCP-3%, i.e., a polyacrylonitrile-cellulose nanocrystal composite separator with a CNCs content of 3 wt%) was obtained. Its morphology was observed by scanning electron microscopy. Figure 2 shown.

[0098] Example 3

[0099] Steps (1) to (4) are the same as in Example 1.

[0100] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: 0.06 g of cellulose nanocrystals prepared in step (4) were uniformly dispersed in 8 mL of N,N-dimethylformamide, and then placed in an ultrasonic machine for 2 h. Finally, 1.14 g of polyacrylonitrile was added to the mixed solution after the ultrasonication was completed. The final mixed solution was stirred at room temperature for 12 h to form a spinning solution.

[0101] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, spray liquid flow rate of 0.001 mL / min, receiving roller speed of 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (abbreviated as PCP-5%, i.e., a polyacrylonitrile-cellulose nanocrystal composite separator with a CNCs content of 5 wt%) was obtained. Its morphology was observed by scanning electron microscopy. Figure 2 shown.

[0102] Example 4

[0103] Steps (1) to (4) are the same as in Example 1.

[0104] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: 0.084 g of cellulose nanocrystals prepared in step (4) were uniformly dispersed in 8 mL of N,N-dimethylformamide, and then placed in an ultrasonic machine for 2 h. Finally, 1.116 g of polyacrylonitrile was added to the mixed solution after the ultrasonication was completed. The final mixed solution was stirred at room temperature for 12 h to form a spinning solution.

[0105] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, spray liquid flow rate of 0.001 mL / min, receiving roller speed of 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (abbreviated as PCP-7%, i.e., a polyacrylonitrile-cellulose nanocrystal composite separator with a CNCs content of 7 wt%) was obtained. Its morphology was observed by scanning electron microscopy. Figure 2 shown.

[0106] Example 5

[0107] Steps (1) to (4) are the same as in Example 1.

[0108] (5) Preparation of high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator: 0.12 g of cellulose nanocrystals prepared in step (4) were uniformly dispersed in 8 mL of N,N-dimethylformamide, and then placed in an ultrasonic machine for 2 h. Finally, 1.08 g of polyacrylonitrile was added to the mixed solution after the ultrasonication was completed. The final mixed solution was stirred at room temperature for 12 h to form a spinning solution.

[0109] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, jet flow rate of 0.001 mL / min, speed of the receiving roller of 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator (abbreviated as PCP-10%, i.e., a polyacrylonitrile-cellulose nanocrystal composite separator with a CNCs content of 10 wt%) was obtained. The morphology was observed by scanning electron microscopy. Figure 2 shown.

[0110] Comparative Example 1

[0111] Preparation of pure polyacrylonitrile (PAN) battery separator, the steps are as follows:

[0112] 1.08 g of polyacrylonitrile was evenly dispersed in 8 mL of N,N-dimethylformamide and stirred at room temperature for 12 h to form a spinning solution.

[0113] The obtained spinning solution was spun at a humidity of 28-35°. The parameters of the electrospinning machine were set as follows: voltage of 17 kV, spray liquid flow rate of 0.001 mL / min, receiving roller speed of 30 r / min, and the distance between the syringe needle tip and the receiving roller with aluminum foil was 15 cm. After 12 hours of electrospinning, a pure polyacrylonitrile battery separator (abbreviated as PAN) was obtained. Its morphology was observed by scanning electron microscopy. Figure 2 shown.

[0114] Comparative Example 2

[0115] The preparation steps of hydrochloric acid hydrolyzed cellulose are as follows:

[0116] (1) Pretreatment of sisal fiber: After washing the sisal cellulose raw material with clean water, put it into a 60°C oven to dry the moisture, and finally cut it into pieces for later use.

[0117] (2) Preparation of sisal cellulose: 50 g of the dried and chopped sisal fiber obtained in step (1) was placed in a 1000 mL high-temperature reactor, and 700 mL of a 2.5 mol / L KOH solution was added. The high-temperature reactor was assembled, placed in an oven, and heated to 160° C. for 14 h. After the reaction was completed and cooled to room temperature, the reaction product was filtered, and the filter residue was repeatedly washed with deionized water until the filtrate remained neutral in color. The filter residue was dried in a vacuum drying oven at 60° C. to constant weight to obtain sisal cellulose.

[0118] (3) Preparation of Sisal Cellulose Microfibers: 10 g of the sisal cellulose prepared in step (2) was placed in a beaker. 3.35 g of sodium chlorite, 2.5 mL of acetic acid, and 350 mL of deionized water were uniformly mixed and poured into the beaker. The mixture was reacted in a 75°C water bath for 3 h. The reactant was filtered, washed repeatedly until neutral, and dried in a vacuum drying oven at 60°C to obtain the sisal cellulose microfibers.

[0119] (4) Preparation of cellulose nanocrystals: Take 5 g of sisal cellulose microfibers prepared in step (3) and place them in a beaker. Add 100 mL of hydrochloric acid with a concentration of 14.75 mol / L to the beaker, and place the beaker in a 50°C water bath to react for 2 hours. After the reaction is completed, add 300 mL of deionized water and let it stand at room temperature for 12 hours. Centrifuge the suspension, and then neutralize the centrifuged product with ammonia water (add ammonia water drop by drop into the centrifuged product for neutralization, and wash with water until neutral), and finally centrifuge again. Take out the solid material and place it in a culture dish, first put it in a refrigerator at -5°C for pre-freezing for 12 hours, and then put it in a freeze dryer and freeze-dry at -40°C for 32 hours to obtain a solid. The morphology was observed using a scanning electron microscope. Figure 3 shown.

[0120] Test Example 1

[0121] Morphological characterization

[0122] Field emission scanning electron microscopy was used to characterize the cellulose nanocrystals and separators. Figure 1 This is a scanning electron microscope image of the cellulose nanocrystals prepared in step (4) of Example 1. It can be seen that the cellulose nanocrystals hydrolyzed with phosphoric acid have a uniform size of about 200 nm. Figure 2 The scanning electron microscope images of the polyacrylonitrile-cellulose nanocrystal composite battery separators prepared in Examples 1 to 5 and the pure polyacrylonitrile battery separator prepared in Comparative Example 1 are shown, wherein (a) is Comparative Example 1, (b) is Example 1, (c) is Example 2, (d) is Example 3, (e) is Example 4, and (f) is Example 5. Figure 2 It can be seen that the cellulose nanocrystals are dispersed between the silk threads or attached to the silk threads, making the surface of the membrane rougher. Figure 3 This is a scanning electron microscope image of the hydrochloric acid-hydrolyzed cellulose prepared in Comparative Example 2. It can be seen that the cellulose is only shortened in length, the amorphous region is not completely destroyed, and no cellulose nanocrystals are obtained.

[0123] Figure 4The infrared spectra of the PAN battery separator prepared in Comparative Example 1 and the polyacrylonitrile-cellulose nanocrystal composite battery separator prepared in Examples 1 to 5 (1% CNCs / PAN, 3% CNCs / PAN, 5% CNCs / PAN, 7% CNCs / PAN, and 10% CNCs / PAN represent PCP-1%, PCP-3%, PCP-5%, PCP-7%, and PCP-10%, respectively), wherein 1065.1 cm -1 The vibration peak corresponding to -OH can be seen from the figure. The -OH in the polyacrylonitrile-cellulose nanocrystal composite battery separator is obviously more than that in the PAN battery separator.

[0124] Test Example 2

[0125] Various performance tests of diaphragms

[0126] 1. Thermal stability test

[0127] (1) Thermogravimetric test

[0128] In order to evaluate the thermal stability of commercial PP battery separators, PAN battery separators prepared in comparative example 1, and polyacrylonitrile-cellulose nanocrystal composite battery separators (abbreviated as PCP composite separators) prepared in examples 1 to 5, thermal gravimetric tests were conducted. The test method is as follows: at room temperature, inert gas N2 is introduced as a protective gas and heated to 800°C at a heating rate of 10°C / min. The thermal stability of the separator is quantitatively evaluated by observing the onset decomposition temperature of the thermogravimetric curve. The test results are shown in Figure 2. Figure 5 As shown. Figure 5 It can be observed that the PP separator completely decomposes at 175°C, while the initial decomposition temperatures of the PAN and PCP composite separators are both above 317°C. Furthermore, the composite separators with 1% and 3% CNCs addition exhibit slightly higher initial temperatures compared to the original PAN separator, and the residual carbon yield increases from 45.8% to 60.9%. This is because the oxides produced by the thermal degradation of the CNCs accumulate on the polymer surface, hindering the diffusion of oxygen into the polymer matrix and acting as a thermal barrier. The composite separator with 5% CNCs has an initial temperature comparable to that of the original PAN separator. When the CNC addition reaches 7% and 10%, the thermal decomposition temperature of the PCP composite separator decreases slightly compared to the original PAN separator, but the difference is not significant. This is because excessive CNCs agglomerate, resulting in poor dispersion and reduced interaction between the CNCs and PAN molecular chains. These results indicate that the addition of a small amount of CNCs improves the thermal stability of PAN and increases its residual carbon yield.

[0129] (2) Thermal dimensional stability test

[0130] The commercial PP battery separator, the PAN battery separator prepared in Comparative Example 1, and the PCP composite separator prepared in Examples 1 to 5 were cut into circular pieces with a diameter of 16 mm. The different separators were then placed in a forced air drying oven at 120°C, 140°C, 160°C, and 180°C for 30 minutes each, and the dimensional changes of the separators were observed. The test results are shown in Figure 2. Figure 6 As shown. Figure 6 As can be seen, within the tested temperature range, the PAN and PCP composite separators exhibited no significant curling or deformation, with the exception of a color change after heat treatment at 180°C. However, as the temperature increased, the PP separator began to curl and soften at 120°C. As the temperature continued to rise, the separator continued to shrink until it completely lost its shape. These test results suggest that the excellent thermal dimensional stability is primarily due to the PAN substrate.

[0131] 2. Mechanical properties test

[0132] The PAN battery separator prepared in Comparative Example 1 and the PCP composite separator prepared in Examples 1 to 5 were cut into rectangular strips with a length of 5 cm and a width of 1 cm. The strips were subjected to a tensile test on an electronic universal tensile testing machine at a tensile speed of 5 mm / min. The tensile performance test results are shown in FIG. Figure 7 As shown. Figure 7 As can be seen, the mechanical tensile strength of the PCP composite membranes is significantly improved compared to the pure PAN membrane. This is due to the improved interfacial compatibility between the polar -OH groups in the CNCs and the PAN matrix, as well as the fact that the addition of CNCs reduces the pore size between the fibers and increases the density, thereby improving the tensile strength of the PCP composite membrane. The PCP-5% membrane has the highest elongation at break, reaching 11.75%, and also possesses a tensile strength of 21.69 MPa. The PAN membrane has an elongation at break of 5.75% and a tensile strength of only 10.21 MPa. This is due to the good dispersion of the small amount of CNCs added in the polymer matrix, which strengthens the interfacial adhesion between the CNCs and the polymer matrix. However, when the CNC addition level is increased to 10%, the elongation at break of the membrane decreases from 11.75% for the PCP-5% membrane to 1.385%, but the tensile strength still reaches 11.82 MPa, still higher than that of the PAN membrane. This is because the amount of CNCs added is too high and the viscosity of the spinning solution is high, which leads to unstable jet in the electrospinning process, large diameter of the split filaments, and entanglement between the fibers, which disperses the interaction force between them and makes them more prone to breakage.

[0133] 3. Electrolyte wettability test

[0134] (1) Contact angle test

[0135] By measuring the electrolyte contact angle of the diaphragm, the electrolyte wettability of the diaphragm can be intuitively evaluated. The commercial PP diaphragm, the PAN battery diaphragm prepared in Comparative Example 1, and the PCP composite diaphragm prepared in Examples 1 to 5 were cut into circular pieces with a diameter of 16 mm and tested on a contact angle instrument. The electrolyte was LiPF6 electrolyte. The contact angle test results are shown in Figure 2. Figure 8 As shown, (a) is comparative example 1, (b) is example 1, (c) is example 2, (d) is example 3, (e) is example 4, (f) is example 5, and (g) is a PP diaphragm. Figure 8 It can be observed that the electrolyte contact angle of the commercial PP membrane is 72.8°, and the electrolyte contact angle of the PAN and PCP composite membranes is 0°. This is because the membrane has good porosity and the polar group -OH of cellulose nanocrystals improves the affinity between the membrane and the electrolyte, thereby giving the membrane better wettability.

[0136] (2) Porosity test

[0137] Cut the membrane sample to be tested into a 16 mm diameter disc and soak it in n-butanol at room temperature for 1 hour. Then wipe the excess liquid from the surface with filter paper. Use an analytical balance to measure the mass of the sample before and after soaking. Calculate the porosity according to formula (1).

[0138]

[0139] Where W0 is the mass of the diaphragm before immersion, g; W1 is the mass of the diaphragm after immersion for 1 hour, g; ρ b is the density of n-butanol, g / cm 3 ; V0 is the apparent volume of the membrane before immersion, cm 3 ; P is the porosity of the diaphragm, %.

[0140] (3) Average pore size test

[0141] The pore size was measured by scanning electron microscope images. For each sample, 40 pores were randomly selected to calculate the pore size, and the average value was taken to calculate the average pore size.

[0142] (4) Liquid absorption test

[0143] The membrane samples to be tested were cut into 16 mm diameter discs and dried at 60°C for 12 h. The dried membranes were then immersed in LiPF6 electrolyte in a glove box for 2 h. The mass of the membranes before and after immersion in the electrolyte was measured using an analytical balance. The liquid absorption rate was calculated according to formula (2).

[0144]

[0145] Where W1 is the initial mass of the diaphragm, g; W2 is the mass of the diaphragm after absorbing liquid, g; ΔW is the liquid absorption rate, %.

[0146] The porosity test results of the commercial PP battery separator, the PAN battery separator prepared in Comparative Example 1, and the PCP composite separators prepared in Examples 1 to 5 are shown in FIG. Figure 9 As shown, the liquid absorption test results are as follows Figure 10 The average pore size test results of the PAN battery separator prepared in Comparative Example 1 and the PCP composite separator prepared in Examples 1 to 5 are shown in FIG. Figure 11 As shown. Figure 9-11 It can be seen that the porosity and liquid absorption rate of the PCP composite membrane with the addition of CNCs are higher than those of the pure PAN membrane, while the average pore size is smaller than that of the pure PAN membrane. Moreover, as the amount of cellulose nanocrystals added increases, the average pore size of the PCP composite membrane gradually decreases ( Figure 11 With the addition of CNCs, the porosity and liquid absorption rate of the PCP composite membrane first increased and then decreased. The best PCP-5% composite membrane had a porosity of 553.3%, while the commercial PP membrane had a porosity of only 251.35%. Figure 9 The liquid absorption rate of PCP-5% composite membrane reached the highest level of 84.8%, while that of PP membrane was the lowest, only 39.2%. Figure 10 The initial increase in the porosity of the PCP composite membrane is likely due to the CNCs being embedded within the fibers, which regulates the pore size between the fibers while increasing the number of pores, thereby increasing the membrane's porosity. Furthermore, since CNCs contain a large amount of -OH groups, this further promotes electrolyte absorption, resulting in better wettability and a higher liquid absorption rate for the composite membrane. However, increasing the amount of CNCs added further exacerbates the agglomeration of CNC particles, preventing them from being well dispersed within the PAN fibers, leading to a decrease in the membrane's porosity and liquid absorption rate.

[0147] 4. Electrochemical performance test

[0148] The diaphragm to be tested was cut into a circular lithium battery diaphragm with a diameter of 16 mm, and then the diaphragm was torn off from the aluminum foil. Finally, the diaphragm was compacted with a tablet press at 5 MPa and assembled into a button-type half-cell with lithium iron phosphate as the positive electrode material, lithium sheet as the negative electrode material, and LiPF6 as the electrolyte. The electrochemical performance of the button-type half-cell was tested. Specifically, the button-type half-cell was subjected to a 0.05 A·g -1 After activation with a current density of 0.2 A·g -1 The current density was used to perform 100 cycles of charge and discharge test. The cycle performance test results of the lithium ion battery prepared by the PAN battery separator prepared in Comparative Example 1 and the PCP-5% composite separator prepared in Examples 1 to 5 are shown in FIG. Figure 12As shown in FIG1 , the rate performance test results of the lithium ion battery prepared by the PAN battery separator prepared in Comparative Example 1 and the PCP composite separator prepared in Example 3 are as follows: Figure 13 shown. Figure 12 The results show that the specific capacity of all batteries will decay with the increase of cycle number. After 100 charge and discharge cycles, the best performance is the PCP-5% composite separator, with a specific capacity retention rate of 97.4% and a capacity of 147.46 mAh g -1 The final specific capacity retention rate of PAN is 62.9%, and the final specific capacity is 66.99 mAh g -1 The final specific capacity retention rates of PCP-1%, PCP-3%, PCP-7%, and PCP-10% were 71.1%, 90.8%, 92.6%, and 86.9%, respectively, and the final specific capacities were 87.81, 116.02, 134.98, and 127.83 mAh·g, respectively. -1 PAN is the membrane with the worst performance. The reason is that the pore size of the PAN membrane is larger than that of the PCP composite membrane, resulting in less electrolyte absorbed and fewer lithium ions that can shuttle freely. The reason why the PCP composite membrane has a higher specific capacity and specific capacity retention rate is that the added phosphoric acid-hydrolyzed cellulose nanocrystals (CNCs) contain a large number of -OH bonds, which are compatible with ester electrolytes and are easier to absorb more electrolyte. On the other hand, the addition of CNCs can adjust the pore size of the membrane, making the pore size uniform and smaller, and the lithium ions will improve the local shuttle situation, so that they are evenly deposited on the lithium sheet. Figure 13 The results show that when the discharge rate changes from 0.05C to 2C, the capacity retention rate of the PCP composite membrane is significantly higher than that of the PAN membrane. Even at a high discharge rate of 2C, the PCP-5% composite membrane with the addition of CNCs still has a capacity of 90.05 mAh g -1 The specific capacity of the battery with PAN separator is only 40.92 mAh g -1 When the charge-discharge rate returned to 0.05C, the battery charge specific capacity of the separators was close to their initial values, indicating that the capacity recovery behavior was reversible.

[0149] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator, characterized in that: The following steps are involved: Mixing cellulose nanocrystals, polyacrylonitrile and a solvent to obtain a spinning solution; electrospinning the spinning solution to obtain the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator; The mass of the cellulose nanocrystals is 1 to 10% of the sum of the mass of the cellulose nanocrystals and the polyacrylonitrile; The cellulose nanocrystals are obtained by hydrolyzing sisal cellulose through phosphoric acid.

2. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 1, characterized in that: The solvent is N,N-dimethylformamide; And / or, the ratio of the sum of the masses of the cellulose nanocrystals and the polyacrylonitrile to the volume of the solvent is 1.2 g:8 mL.

3. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 1, characterized in that: The electrospinning parameters include: humidity of 28-35°, voltage of 17 kV, jet flow rate of 0.0005-0.001 mL / min, rotation speed of receiving roller of 30-45 r / min, and distance between syringe needle tip and receiving roller of 15-20 cm.

4. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 1, characterized in that: The preparation steps of the cellulose nanocrystals include: mixing sisal cellulose, sodium chlorite, acetic acid and water, heating and reacting them once to obtain sisal cellulose microfibrils; mixing the sisal cellulose microfibrils and phosphoric acid, heating and reacting them twice, standing after the reaction, centrifuging, neutralizing the centrifuged product with ammonia water, and freeze-drying them to obtain the cellulose nanocrystals.

5. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 4, characterized in that: The usage ratio of the sisal cellulose, sodium chlorite, acetic acid and water is 10 g:3.35 g:2.5 mL:350 mL; and / or, the usage ratio of the sisal cellulose microfibers to the phosphoric acid is 5 g:100 mL; And / or, the concentration of the phosphoric acid is 14.75 mol / L; And / or, the temperature of the first heating reaction is 70-75° C. and the time is 2.5-3 h; And / or, the temperature of the secondary heating reaction is 50-60°C and the time is 1.5-2h; And / or, the standing time is 12 to 24 hours; And / or, the freeze-drying step includes: pre-freezing at -5°C for 12 hours, and then freeze-drying at -40°C for 32 hours.

6. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 4, characterized in that: The preparation steps of the sisal cellulose include: mixing sisal fibers and an alkali solution, and heating the mixture for reaction to obtain the sisal cellulose.

7. The method for preparing a high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator according to claim 6, characterized in that: The alkaline solution is a KOH solution or a NaOH solution; And / or, the concentration of the alkaline solution is 2.5 mol / L; And / or, the usage ratio of the sisal fiber to the alkaline solution is 50g:700mL; And / or, the heating reaction temperature is 160° C. and the time is 14 hours.

8. A high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator as claimed in claim 8 in the preparation of lithium-ion batteries.

10. A lithium ion battery, characterized in that: The high-performance polyacrylonitrile-cellulose nanocrystal composite battery separator described in claim 8 is used as the separator.

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