Electrospinning preparation method for fiber separator for batteries with excellent high temperature resistance and flame retardancy

Abstract

The present invention discloses a method for preparing a fiber separator for batteries with excellent high-temperature resistance and flame retardant properties through electrospinning. The method comprises adding polyacrylonitrile, boric acid, melamine, and exfoliated boron nitride nanosheets to N,N-dimethylformamide to obtain a polymer spinning solution with good dispersibility, and then preparing the fiber separator using electrospinning technology. The preparation conditions of the present invention are simple and mild. Melamine and boron nitride nanosheets improve the high-temperature resistance and tensile strength of the separator, and boric acid effectively improves the flame retardant properties of the separator. The obtained fiber separator has a thickness of 70 to 90 μm, an interconnected pore structure, high tensile strength and low area thermal shrinkage, higher ionic conductivity and electrolyte absorption, and relatively low interfacial resistance. A lithium-ion battery assembled with the separator achieved a high stable capacity retention rate at 0.5C during 400 cycles, and has the potential for application in energy storage devices such as lithium-ion secondary batteries with high power, high-temperature resistance, and excellent flame retardant properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional material application and battery diaphragm preparation, and specifically relates to an electrospinning preparation method for fiber diaphragms for batteries with excellent high-temperature resistance and flame retardant properties. Background Art

[0002] In recent decades, polymer processing technology has continued to advance to meet the performance requirements of various applications, and various high-performance polymer materials have developed rapidly. Since the origin of synthetic polymers, the development and preparation of polymer fibers has always attracted great attention from researchers, and their applications range from textiles, biomedical materials to energy storage device membranes. Electrospinning technology is a mature method for preparing fiber materials and can be used to develop continuous fibers with diameters ranging from tens of microns to hundreds of nanometers. High-temperature resistance and flame retardancy are important performance characteristics of polymer materials. In situ modification of polymer materials with nanomaterials is an effective means to improve the performance of functional polymer materials.

[0003] Nanomaterials are an important area of ​​research for the development of functional materials. Their large surface area and high surface activity contribute to their unique physicochemical properties. However, due to their large surface area and high number of active sites, nanomaterials often exhibit significant aggregation, significantly reducing their ability to fully exploit their physicochemical properties. This, for example, limits the performance improvement of lithium-ion battery (LIB) separators using nanomaterials. In recent years, one-dimensional nanofibers have garnered significant research attention due to their small pore size, high porosity, and direct application in LIB separators. One-dimensional nanomaterials with small pore size and high porosity can be directly fabricated into LIB fiber separators through various methods, including meltblowing, wet-laid lamination, vacuum filtration, and electrospinning. Electrospinning is an effective method for preparing fiber films. Using different polymer solutions, a variety of functional fiber films with controllable composition and unique microstructures can be prepared, offering particular advantages in improving the performance of LIB fiber separators. However, the production of nanofiber films using electrospinning technology faces two key technical bottlenecks and challenges. First, the variety of natural polymers used to prepare nanofibers by electrospinning is very limited, resulting in an insufficient diversity of structures and properties in the prepared fiber films. This has resulted in the preparation of fiber films being in the experimental application stage, and there are major problems with industrial production. Second, the performance of organic / inorganic composite nanofiber films prepared by electrospinning is not only related to the structure of the nanoparticles, but also to the aggregation mode and synergistic properties of the nanoparticles, the structural properties of the polymer matrix, the interface structure properties between the particles and the matrix, and the processing and composite technology. Therefore, how to prepare high-performance, multifunctional nanofiber films through electrospinning technology is very challenging.

[0004] Research results have shown that porous polyacrylonitrile (PAN) films are a type of fiber separator with excellent chemical and thermal stability. The acrylonitrile monomer in PAN can interact with lithium ions, enhancing compatibility with the electrolyte and improving ionic conductivity. At the same time, the mechanical properties and thermal stability of PAN can be significantly enhanced by partially oxidizing and cyclizing polyacrylonitrile (oxy-PAN). However, when PAN materials are exposed to heat sources for a long time, they will degrade as ammonia, acrylonitrile, and other nitriles are released, which seriously affects the performance of PAN materials. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a fiber separator for batteries with excellent high-temperature resistance and flame retardancy and uniform thickness by using an electrospinning method.

[0006] To achieve the above-mentioned purpose, the technical solution adopted by the present invention consists of the following steps:

[0007] Step 1: Adding boron nitride solid to isopropyl alcohol, and then obtaining an exfoliated boron nitride nanosheet dispersion by probe sonication and high-speed centrifugation;

[0008] Step 2: Add polyacrylonitrile, boric acid, and melamine to N,N-dimethylformamide and stir until completely dissolved to obtain a polymer solution; then add the exfoliated boron nitride nanosheet dispersion to the polymer solution and stir evenly to obtain a uniformly dispersed polymer spinning solution;

[0009] Step 3: Place the polymer spinning solution in a plastic syringe connected to a stainless steel needle to prepare a fiber membrane under electrospinning conditions;

[0010] Step 4: Separate the fiber membrane prepared in step 3 from the collector, and then dry the fiber membrane to remove the residual solvent therein.

[0011] In the above step 1, the amount of boron nitride solid added to the isopropanol is preferably 10-30 mg / mL, the power of the probe ultrasound is 250-400 W, the ultrasound time is 5-7 hours, the high-speed centrifugation speed is 6000-10000 rpm, and the centrifugation time is 20-30 minutes.

[0012] In the above step 2, the mass ratio of the polyacrylonitrile, boric acid and melamine is preferably 8-10:1-3:0.8-1, wherein the concentration of melamine in the polymer solution is 0.05-0.1 mg / L; the mass ratio of the total mass of polyacrylonitrile, boric acid and melamine in the polymer solution to the exfoliated boron nitride nanosheet dispersion is 2-4:1.

[0013] In the above step 2, the exfoliated boron nitride nanosheet dispersion is preferably added to the polymer solution, and stirred at 30-60° C. for 8-12 hours to obtain a uniformly dispersed polymer spinning solution.

[0014] In the above step 3, the polymer spinning solution is preferably placed in a plastic syringe connected to a stainless steel needle, and electrospinning is performed using a syringe equipped with a high-voltage power supply at a feed rate of 0.07 to 0.2 mL / hour and a voltage of 12 to 15 kV. The distance between the syringe tip and the drum collector is 10 to 15 cm, and the relative humidity is set to less than 40%.

[0015] In the above step 4, the fiber membrane prepared in step 3 is preferably separated from the collector, and then the fiber membrane is placed in a vacuum drying oven and dried at 80-100°C for 8-12 hours to remove the residual solvent in the fiber membrane. The thickness of the obtained fiber membrane after drying is fixed at 70-90 μm.

[0016] The beneficial effects of the present invention are as follows:

[0017] 1. The present invention adds exfoliated boron nitride nanosheets with excellent thermal stability, high-strength tensile properties and good impedance properties to the spinning solution of the fiber membrane prepared by electrospinning technology using boric acid, melamine and polyacrylonitrile as raw materials. Melamine and boron nitride nanosheets improve the high-temperature resistance and tensile resistance of the membrane, and boric acid effectively improves the flame retardant properties of the membrane. The four work synergistically to obtain a fiber membrane for batteries with excellent high-temperature resistance and flame retardant properties and uniform thickness.

[0018] 2. The reaction conditions of the present invention are simple and mild, the method cost is low, and the preparation technology is simple and convenient. By adjusting the reaction time, the amount of boric acid / melamine / boron nitride nanosheets added, the ultrasonic power and time, and the centrifugal speed and time, the performance of the fiber diaphragm for batteries with excellent high temperature resistance and flame retardancy and uniform thickness can be optimized.

[0019] 3. The fiber diaphragm obtained by the present invention has a thickness of 70 to 90 μm and an interconnected pore structure. It exhibits high tensile strength and low area thermal shrinkage, higher ionic conductivity and electrolyte absorption, and relatively low interfacial resistance. The lithium-ion battery assembled with this diaphragm achieved a high and stable capacity retention rate at 0.5C during 400 cycles, and has application potential in energy storage devices such as lithium-ion secondary batteries with high power, high temperature resistance and excellent flame retardancy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an optical photograph of the dispersion of the exfoliated boron nitride nanosheets in Example 1.

[0021] Figure 21. The transmission image (a) and lattice fringe pattern (b) of the exfoliated boron nitride nanosheets in Example 1.

[0022] Figure 3 These are field emission scanning electron microscope photos of fiber membranes obtained with different mass ratios of boric acid, melamine and boron nitride nanosheets.

[0023] Figure 4 These are photos of the wetting properties of commercial polypropylene battery separators PP, pure polyacrylonitrile PAN, and fiber separators PB3, PB3N1, PB3N1-BN, and PB3N1BN.

[0024] Figure 5 These are photos of the thermal stability properties of commercial polypropylene battery separators PP, pure polyacrylonitrile PAN, and fiber separators PB3, PB3N1, PB3N1-BN, and PB3N1BN.

[0025] Figure 6 These are the TGA curves of commercial polypropylene battery separator PP, pure polyacrylonitrile PAN, and fiber separators PB3N1, PB3N1-BN, and PB3N1BN.

[0026] Figure 7 These are the X-ray diffraction patterns of commercial polypropylene battery separator PP, pure polyacrylonitrile PAN, and fiber separators PB3N1, PB3N1-BN, and PB3N1BN.

[0027] Figure 8 These are the mechanical property curves of commercial polypropylene battery separator PP, pure polyacrylonitrile PAN, and fiber separators PB3N1, PB3N1-BN, and PB3N1BN.

[0028] Figure 9 Comparison of electrochemical impedance spectroscopy between the fiber separator PB3N1BN prepared in Example 1 and the commercial polypropylene battery separator PP.

[0029] Figure 10 This is a performance comparison of the charge and discharge curves (0.5C) of the fiber separator PB3N1BN prepared in Example 1 and pure polyacrylonitrile PAN.

[0030] Figure 11 The electrochemical cycle performance of the fiber membrane PB3N1BN prepared in Example 1 and the commercial polypropylene battery membrane PP are compared.

[0031] Figure 12 The figure compares the electrochemical rate performance of the fiber membrane PB3N1BN prepared in Example 1 and the commercial polypropylene membrane PP. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the protection scope of the present invention is not limited to these examples.

[0033] In order to determine the technical solution of the present invention, the inventors conducted a large number of laboratory research experiments, the specific experiments are as follows:

[0034] 1. Effect of boric acid and its addition amount on the performance of fiber diaphragms

[0035] Step 1: Add 3g of boron nitride to 200mL of isopropanol and stir rapidly for 1h until it is completely dispersed to obtain a boron nitride precursor dispersion. The boron nitride precursor dispersion is ultrasonically treated with a probe for 6h at an ultrasonic power of 300W. The dispersion is then centrifuged at 8000r / min for 30min, and the supernatant is collected to obtain an exfoliated boron nitride nanosheet dispersion. The exfoliated boron nitride nanosheet dispersion and its nanosheets are characterized using optical photographs, X-ray diffractometers, and transmission electron microscopy. The results are shown in Figure 2. Figures 1-2 From the optical photograph of the exfoliated boron nitride nanosheet dispersion, it can be seen that the exfoliated boron nitride nanosheet dispersion has an obvious Tyndall effect, showing that the colloidal dispersion system composed of boron nitride nanosheets ( Figure 1 ). Transmission electron microscopy images show that the exfoliated boron nitride nanosheets have a thin flake morphology, and the lattice fringes of the high-resolution transmission electron microscopy images correspond to the (002) crystal plane ( Figure 2 ), which is consistent with the XRD results.

[0036] Step 2: Boric acid and 1g of polyacrylonitrile were added to 15mL of N,N-dimethylformamide and stirred to obtain a polymer solution. 0.5g of the exfoliated boron nitride nanosheet dispersion was then added to the resulting polymer solution and stirred at 60°C for 12 hours to obtain a uniformly dispersed polymer spinning solution. The boric acid addition amounts were 0.1g, 0.2g, 0.3g, and 0.6g, respectively.

[0037] Step 3: The polymer spinning solution was placed in a 10 mL plastic syringe connected to a stainless steel needle and electrospun using a syringe equipped with a high-voltage power supply at a feed rate of 0.2 mL / h and a voltage of 14 kV to prepare a fiber membrane. The distance between the syringe tip and the drum collector was 12 cm, and the relative humidity was set to less than 40%.

[0038] Step 4: The fiber membrane prepared in Step 3 was separated from the collector and then dried in a vacuum drying oven at 80°C for 10 hours to remove any residual solvent. The membrane thickness was fixed at 70-90 μm to obtain fiber membranes with different boric acid contents. In the fiber membrane, polyacrylonitrile is abbreviated as P, boric acid is abbreviated as B, and boron nitride nanosheets are abbreviated as BN. The resulting fiber membranes were named PB1BN, PB2BN, PB3BN, and PB6BN, respectively, based on their composition. A sample containing 0.3 g of boric acid but without exfoliated boron nitride nanosheet dispersion (BNNS) was also used as a control (denoted as PB3).

[0039] 2. Effect of melamine and its addition amount on the performance of fiber diaphragms

[0040] In step 2 of Experiment 1 above, boric acid was omitted, but 0.1, 0.2, and 0.3 g of melamine were added, respectively. Under otherwise identical preparation conditions, electrospinning was used to produce fiber membranes containing varying amounts of melamine, without the addition of boric acid. Melamine is abbreviated as N, and the resulting fiber membranes were named PN1BN, PN2BN, and PN3BN, respectively, based on their composition.

[0041] 3. Effect of boric acid and melamine addition on fiber diaphragm performance

[0042] In step 2 of the above experiment 1, 0.2, 0.3, and 0.6 g of boric acid and 0.1 and 0.2 g of melamine were added respectively. Under the same other preparation conditions, fiber membranes with different boric acid and melamine contents were prepared by electrospinning. According to the composition, the obtained fiber membranes were named PB2N1BN, PB2N2BN, PB3N1BN, PB3N2BN, PB6N1BN, and PB6N2BN. At the same time, a sample with 0.3 g of boric acid and 0.1 g of melamine but no exfoliated boron nitride nanosheet dispersion was added (denoted as PB3N1) and a sample with 0.3 g of boric acid, 0.1 g of melamine, and 0.5 g of boron nitride powder was added (denoted as PB3N1-BN) as a control sample.

[0043] The performance of the fiber membranes obtained by different raw material ratios in the above experiments 1 to 3 was compared with that of commercial PP and pure polyacrylonitrile (PAN) membranes. Figures 3 to 9 .

[0044] from Figure 3The scanning electron microscope image shows that when only polyacrylonitrile, boric acid and boron nitride nanosheets were added to the polymer spinning solution in experiment 1, more droplets of polymer appeared on the diaphragm with the increase of the amount of boric acid added, indicating that the amount of boric acid should not be too large; when only polyacrylonitrile, melamine and boron nitride nanosheets were added to the polymer spinning solution in experiment 2, as the amount of melamine added increased, it can be seen that the spherical droplets of the spinning diaphragm became more obvious, which was more uneven than the above experiment 1; when polyacrylonitrile, boric acid, melamine and boron nitride nanosheets were added to the polymer spinning solution in experiment 3 at the same time, it can be found that the spherical droplets at the diaphragm were significantly reduced and the spinning was more uniform, indicating that the mass ratio of boric acid, melamine and boron nitride nanosheets has an important influence on the uniformity of the electrospun diaphragm.

[0045] In summary, based on the morphological uniformity, it can be seen that PB1BN, PB2BN, PB3BN, and PB6BN are essentially unspinnable due to the dense droplet-like aggregates on their surfaces. Similarly, PN1BN, PN2BN, and PN3BN cannot be spun due to the lack of boric acid, resulting in blocky accumulation. PB2N1BN, PB2N2BN, PB3N2BN, PB6N1BN, and PB6N2BN have a small amount of droplet-like aggregates, but they also meet the spinning requirements. PB3, PB3N1, PB3N1-BN, and PB3N1BN have essentially no droplet-like aggregates and spin uniformly.

[0046] from Figure 4 The wetting angle characterization results show that the fiber membranes obtained by electrospinning different proportions of boric acid, melamine and boron nitride nanosheets have different electrolyte absorption rates. The degree of excellence in electrolyte absorption greatly determines the ionic conductivity performance of the membrane. The smaller the wetting angle, the more it is. This is mainly due to the addition of BN NNs increasing the pores of the separator and enhancing the affinity between the membrane and the electrolyte by forming a porous structure. In addition, the enhanced wettability of PB3N1BN can also be attributed to the reduction in surface energy, which gradually reduces the contact angle. The higher the porosity, the more electrolyte can be retained, thereby providing more channels for lithium ions, proving that the membrane has stronger electrolyte absorption. From Figure 5 The thermal stability of the fiber separator shows that different ratios of boric acid, melamine, and boron nitride nanosheets significantly affect the thermal shrinkage of the fiber separator. Boric acid improves the separator's heat resistance and shortens its melting time. Melamine, as a nitrogen-containing flame retardant, enhances flame retardancy. Thermal shrinkage can affect the electrochemical properties of the separator. These experimental data demonstrate that PB3N1BN exhibits excellent flame retardancy, thermal stability, and ionic conductivity.

[0047] from Figure 6TGA characterization results show that different ratios of boric acid, melamine, and boron nitride nanosheets significantly affect the high-temperature resistance and flame retardancy of the electrospun fiber membranes. Boric acid and melamine together form a boron-nitrogen synergistic mechanism, and the combustion process forms BOC, BON, and BOB structures, which have improved antioxidant stability. Electrospun membranes incorporating a boron nitride nanosheet dispersion exhibited improved thermal stability compared to those without the addition of boron nitride nanosheets, demonstrating the excellent flame retardancy of boron nitride nanosheets.

[0048] from Figure 7 XRD patterns show that the uniformity of the electrospun fiber membranes increases with the addition ratio of boric acid and melamine. The diffraction peak is lowest when the membrane contains boric acid, melamine, and boron nitride nanosheets, indicating that the boron and nitrogen elements in boric acid and melamine can better integrate and integrate, resulting in more uniform membrane spinning. Lower diffraction peaks indicate a more uniform distribution of elements within the fiber membrane, with PB3N1BN achieving the best results.

[0049] from Figure 8 Mechanical property characterization results reveal significant differences in the mechanical properties of fiber separators electrospun using different mass ratios of boric acid and melamine. Boric acid crosslinking reduces the spacing between the layered separation aerogels, while the introduction of melamine forms polymer fibrils connecting the layers. Due to the strong chemical and hydrogen bonds between PAN and BN NS, crosslinking occurs between the two. Consequently, the formation of a crosslinked network significantly improves the mechanical properties of the composite fiber separator. Better uniformity correlates with improved mechanical properties, with the PB3N1BN separator exhibiting the most superior mechanical properties.

[0050] Example 1

[0051] Step 1: Add 3g of boron nitride to 200mL of isopropanol and rapidly stir for 1 hour until completely dispersed to obtain a boron nitride precursor dispersion. The boron nitride precursor dispersion is then ultrasonicated for 6 hours at 300W. The dispersion is then centrifuged at 8000 rpm for 30 minutes, and the supernatant is collected to obtain a dispersion of exfoliated boron nitride nanosheets.

[0052] Step 2: Add 0.3g of boric acid, 0.1g of melamine, and 1g of polyacrylonitrile to 15mL of N,N-dimethylformamide and stir to obtain a polymer solution. Then, add 0.5g of the exfoliated boron nitride nanosheet dispersion to the resulting polymer solution and stir at 60°C for 12 hours to obtain a uniformly dispersed polymer spinning solution.

[0053] Step 3: The polymer spinning solution was placed in a 10 mL plastic syringe connected to a stainless steel needle and electrospun using a syringe equipped with a high-voltage power supply at a feed rate of 0.2 mL / h and a voltage of 14 kV to prepare a fiber membrane. The distance between the syringe tip and the drum collector was 12 cm, and the relative humidity was set to less than 40%.

[0054] Step 4: The fiber membrane prepared in step 3 was separated from the collector and then dried in a vacuum drying oven at 80°C for 10 hours to remove the residual solvent in the fiber membrane. The thickness of the membrane was fixed at 70-90 μm. The electrochemical impedance spectroscopy results of the fiber membrane obtained in this example (denoted as PB3N1BN) were compared with those of commercial polypropylene battery membrane (PP). Figure 9 It can be seen that the ionic conductivity of the obtained fiber membrane is far superior to that of the commercial polypropylene membrane. Figure 10 It can be seen that the discharge capacity and voltage difference of PB3N1BN after 200 cycles at a current density of 0.5C are superior to those of pure PAN. Figure 11 、 Figure 12 From the comparison of the electrochemical cycle performance and rate performance, it can be seen that the fiber membrane obtained in this embodiment has a smaller voltage difference and a more stable capacity retention rate after 200 cycles, which proves that the electrochemical performance of the membrane is far superior to that of the commercial polypropylene membrane.

Claims

1. A method for preparing a fiber separator for batteries with excellent high temperature resistance and flame retardancy through electrospinning, characterized in that It consists of the following steps: Step 1: Adding boron nitride solid to isopropanol, and then obtaining an exfoliated boron nitride nanosheet dispersion by probe sonication and high-speed centrifugation; the amount of boron nitride solid added to the isopropanol is 10-30 mg / mL, the power of the probe sonication is 250-400 W, the sonication time is 5-7 hours, and the high-speed centrifugation speed is 6000-10000 rpm for 20-30 minutes; Step 2: adding polyacrylonitrile, boric acid, and melamine to N,N-dimethylformamide and stirring until completely dissolved to obtain a polymer solution; then adding the exfoliated boron nitride nanosheet dispersion to the polymer solution and stirring evenly to obtain a uniformly dispersed polymer spinning solution; the mass ratio of the polyacrylonitrile, boric acid, and melamine is 8-10:1-3:0.8-1, wherein the concentration of melamine in the polymer solution is 0.05-0.1 mg / L; the mass ratio of the total mass of polyacrylonitrile, boric acid, and melamine in the polymer solution to the exfoliated boron nitride nanosheet dispersion is 2-4:1; Step 3: Place the polymer spinning solution in a plastic syringe connected to a stainless steel needle to prepare a fiber membrane under electrospinning conditions; Step 4: Separate the fiber membrane prepared in step 3 from the collector, and then dry the fiber membrane to remove the residual solvent therein.

2. The electrospinning method for preparing a fiber separator for batteries with excellent high temperature resistance and flame retardancy according to claim 1, characterized in that: In step 2, the exfoliated boron nitride nanosheet dispersion is added to the polymer solution, and the mixture is stirred at 30-60° C. for 8-12 hours to obtain a uniformly dispersed polymer spinning solution.

3. The electrospinning method for preparing a fiber separator for batteries with excellent high temperature resistance and flame retardancy according to claim 1, characterized in that: In step 3, the polymer spinning solution is placed in a plastic syringe connected to a stainless steel needle and electrospun using a syringe equipped with a high-voltage power supply at a feed rate of 0.07 to 0.2 mL / hour and a voltage of 12 to 15 kV. The distance between the syringe tip and the drum collector is 10 to 15 cm, and the relative humidity is set to less than 40%.

4. The electrospinning method for preparing a fiber separator for batteries with excellent high temperature resistance and flame retardancy according to claim 1, characterized in that: In step 4, the fiber membrane prepared in step 3 is separated from the collector, and then the fiber membrane is placed in a vacuum drying oven and dried at 80-100°C for 8-12 hours to remove the residual solvent in the fiber membrane. After drying, the thickness of the obtained fiber membrane is fixed at 70-90 µm.

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