Coaxial electrostatic spinning flame-retardant coating diaphragm and preparation method thereof

Abstract

The invention relates to the technical field of battery diaphragms, in particular to a flame-retardant diaphragm prepared by coaxial electrostatic spinning and a preparation method of the flame-retardant diaphragm. According to the coaxial electrostatic spinning diaphragm with the core-shell structure with the flame-retardant modified polyimide as the core and the flame retardant and the ceramic mixed coating as the shell, the diaphragm is endowed with excellent high temperature resistance and flame retardance, meanwhile, the ionic conductivity of the diaphragm is improved, and the safety of the lithium ion battery is further improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator technology, specifically to a flame-retardant coated separator prepared by coaxial electrospinning and its preparation method. Background Technology

[0002] With the widespread application of lithium-ion batteries, driven by the high-performance demands of various industries, the energy density and cycle performance of lithium-ion batteries are continuously improving. However, when lithium-ion batteries are subjected to external forces such as impacts or punctures, or when improper use causes short circuits or overcharging, thermal runaway can occur, leading to fires or explosions and other safety hazards. In recent years, frequent lithium battery charging fires have raised concerns about lithium battery safety. Traditional commercially available separators are polyolefin separators, which exhibit significant thermal shrinkage at high temperatures, easily causing short circuits between the positive and negative electrodes, leading to explosions. CN114883743A discloses the preparation of an inorganic flame-retardant separator with a stable network structure and high mechanical properties through electrospinning, but does not specify the separator's flame retardancy. To address this issue, rigid ceramic materials are coated onto polyolefin separators, significantly improving thermal shrinkage. However, when the battery experiences thermal runaway, the ceramic separator can still melt and burn. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, the present invention aims to provide a coaxial electrospun flame-retardant coated separator and its preparation method, which aims to overcome the problem of separators burning when heated in the prior art. The present invention provides a coaxial electrospun flame-retardant coated separator with a core-shell structure of flame-retardant modified polyimide as the core and flame retardant / ceramic mixed coating as the shell, which endows the separator with excellent high temperature resistance and flame retardancy, while improving the ionic conductivity of the separator and further enhancing the safety of lithium-ion batteries.

[0004] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A coaxial electrospun flame-retardant coated diaphragm, wherein the diaphragm adopts a core-shell structure, wherein the core-shell structure is composed of a flame-retardant modified polyimide as the core layer and a flame retardant, ceramic, and polyacrylonitrile mixed coating as the shell layer.

[0005] Preferably, the ratio of the wall thickness of the flame-retardant diaphragm core to the wall thickness of the shell prepared by coaxial electrospinning is 1:0.5-2. The mixture of flame retardant, ceramic, and polyacrylonitrile forms a composite coating.

[0006] Preferably, the flame retardant is polyphosphazene microspheres; the ceramic is one or more of boehmite, alumina, titanium dioxide, silicon dioxide, antimony dioxide, and zirconium dioxide.

[0007] Preferably, the mass ratio of the ceramic, flame retardant and polyacrylonitrile is 1:0.1~0.5:5~10, and the solid content is 5-25%.

[0008] Preferably, the thickness of the diaphragm is 1~5 μm, and more preferably, the thickness of the diaphragm is 2~3 μm.

[0009] A method for preparing a coaxial electrospun flame-retardant coated diaphragm is provided, comprising the following steps: Step (1): Dissolve the modified flame-retardant silicone oil and polyimide in dimethylformamide in a certain proportion, stir and mix evenly to obtain the core spinning solution; Step (2): Dissolve the flame retardant, ceramics, and polyacrylonitrile in dimethylformamide in a certain proportion, stir and mix evenly to obtain the shell spinning solution; Step (3): Using the spinning solutions obtained in steps (1) and (2) as electrospinning solutions, an electrospinned diaphragm is prepared by coaxial electrospinning; Step (4): Vacuum dry the electrospun diaphragm prepared in step (3), and then hot press it; Step (5): Place the electrospun separator prepared in step (4) in an oven to dry, and obtain a coaxial electrospun lithium-ion battery separator.

[0010] Preferably, in step (1), the mass ratio of polyimide to flame-retardant silicone oil in the core spinning solution is 1:0.1-0.5, and the solid content is 5-25%.

[0011] Preferably, in step (2), the flame retardant selected in the shell spinning solution is polyphosphazene microspheres, and the ceramic is boehmite, alumina, titanium dioxide, silicon dioxide, antimony dioxide, and zirconium dioxide. The mass ratio of ceramic, flame retardant, and polyacrylonitrile is 1:(0.1~0.5):(5~10), with a solid content of 5~25%.

[0012] Preferably, in step (3), the process parameters for coaxial electrospinning are set as follows: the core spinning solution flow rate is 0.1~0.6 mL / h, the shell spinning solution flow rate is 0.2~1 mL / h, the voltage is 15~30 kV, the rotation speed is 200-300 rpm, and the distance between the transmitter and the receiving substrate is 12~28 cm.

[0013] Preferably, the hot pressing conditions in step (4) are: hot pressing for 10 to 30 minutes at 100 to 200 ℃ and 2 to 5 MPa.

[0014] Preferably, the drying conditions in step (5) are: drying at 80~150 ℃ for 1~2 h.

[0015] Preferably, the modified flame-retardant silicone oil is prepared by adding amino silicone oil and triethylamine to anhydrous dichloromethane. Under an ice-water bath and nitrogen atmosphere, a mixture of phosphoryl chloride and dichloromethane is slowly added dropwise. After the mixture is completely added, the reaction continues in the ice-water bath, followed by a reaction at room temperature. After the reaction is completed, the byproduct triethylamine salt is removed by filtration, and the solvent dichloromethane is removed. The crude product is redissolved in dichloromethane and repeatedly washed with deionized water to remove the white crystals in the liquid, resulting in a light yellow transparent oily modified flame-retardant silicone oil.

[0016] In this invention, if there is a conflict between the Chinese name and the structural formula of a compound, the structural formula shall prevail, unless the structural formula is obviously incorrect.

[0017] The beneficial effects of this invention are as follows: (1) The present invention prepares lithium-ion battery separators by coaxial electrospinning. The core layer is made of flame-retardant modified silicone oil and polyimide. The flame-retardant silicone oil and polyimide have stable structures and good heat resistance, which avoids severe thermal shrinkage of the separator after heating. The shell layer is made of flame retardant, ceramic materials and polyacrylonitrile. The flame retardant and ceramic can quickly establish a flame-retardant heat insulation layer after the battery thermal runaway, and the overall flame-retardant heat resistance efficiency of the separator is improved.

[0018] (2) In this invention, the polyphosphazene flame retardant and ceramic material can enhance the electrolyte wettability of the polyimide core layer, improve the liquid absorption and retention of the diaphragm, and thus improve the ionic conductivity.

[0019] (3) The coaxial electrospun lithium-ion battery separator prepared by the present invention has better flame retardancy than traditional polyolefin separators and ceramic separators, can self-extinguish in a short time, and can achieve a lower maximum heat release rate and total heat release. Detailed Implementation

[0020] The present invention is illustrated below with reference to examples, but is not intended to limit the invention. Any simple substitutions or modifications made to the present invention by those skilled in the art are within the scope of the technical solutions protected by this invention.

[0021] Flame-retardant silicone oil was first prepared as follows: 2-2.5 mol of amino silicone oil (Anhui Aiyota Silicone Oil Co., Ltd., CAS: 2469-55-8) and 2-2.5 mol of triethylamine were added to a four-necked flask containing 250 mL of anhydrous dichloromethane. Under an ice-water bath and nitrogen atmosphere, a mixture of 1 mol of phosphoric acid chloride and 250 mL of dichloromethane was slowly added dropwise. After the addition was complete, the reaction was continued in the ice-water bath for 3 h, followed by a reaction at room temperature for 24 h. After the reaction was complete, the byproduct triethylamine salt was removed by filtration, and the solvent dichloromethane was removed by rotary evaporation. The resulting crude product was redissolved in dichloromethane, and repeatedly washed with deionized water to remove white crystals. After vacuum drying at 100℃ for 12 h, a pale yellow, transparent, oily modified amino silicone oil was obtained. This modified amino silicone oil is the flame-retardant silicone oil.

[0022] Example 1: 5 g of polyimide and 2 g of flame-retardant silicone oil were dissolved in 40 g of dimethylformamide and stirred for 4 h to mix evenly, thus obtaining the core spinning solution. 1 g of alumina, 0.4 g of flame retardant and 8 g of polyacrylonitrile were dissolved in 40 g of dimethylformamide and stirred for 6 h to mix evenly to obtain a shell spinning solution. Take 6 mL of the core spinning solution from step (1) and slowly inject it into a 10 mL syringe. Then take 6 mL of the shell spinning solution from step (2) and slowly inject it into a 10 mL syringe. Connect the two spinning solutions to the core and shell layers of the coaxial spinning needle, respectively. Set the injection rate of the core spinning solution to 0.2 mL / h, the injection rate of the shell spinning solution to 0.4 mL / h, the rotation speed of the receiving device to 250 rpm, the spinning distance between the transmitter and the receiving substrate to 16 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a coaxial electrospun membrane with flame-retardant polyimide as the core and polyphosphazene / alumina / polyacrylonitrile as the shell is obtained on the receiving device. The electrospun diaphragm prepared in step (3) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 120℃ and 5MPa. (5) Finally, place the electrospun diaphragm prepared in step (4) in an oven and dry it at 100 °C for 1 h to obtain a coaxial electrospun flame-retardant coated diaphragm.

[0023] Example 2: 2 g of polyimide and 1 g of flame-retardant silicone oil were dissolved in 30 g of dimethylformamide and stirred for 4 h to mix evenly to obtain a core spinning solution. 0.5 g of alumina, 0.25 g of flame retardant and 3 g of polyacrylonitrile were dissolved in 37.5 g of dimethylformamide and stirred for 6 h to mix evenly to obtain a shell spinning solution. Take 6 mL of the core spinning solution from step (1) and slowly inject it into a 10 mL syringe. Then take 6 mL of the shell spinning solution from step (2) and slowly inject it into a 10 mL syringe. Connect the two spinning solutions to the core and shell layers of the coaxial spinning needle, respectively. Set the injection rate of the core spinning solution to 0.2 mL / h, the injection rate of the shell spinning solution to 0.4 mL / h, the rotation speed of the receiving device to 250 rpm, the spinning distance between the transmitter and the receiving substrate to 16 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a coaxial electrospun membrane with flame-retardant polyimide as the core and polyphosphazene / alumina / polyacrylonitrile as the shell is obtained on the receiving device. The electrospun diaphragm prepared in step (3) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 120℃ and 5MPa. Finally, the electrospun diaphragm prepared in step (4) was placed in an oven and dried at 100 °C for 1 h to obtain a coaxial electrospun flame-retardant coated diaphragm.

[0024] Example 3: 5 g of polyimide and 1 g of flame-retardant silicone oil were dissolved in 30 g of dimethylformamide and stirred for 4 h to mix evenly to obtain a core spinning solution. 1 g of alumina, 0.4 g of flame retardant and 8 g of polyacrylonitrile were dissolved in 40 g of dimethylformamide and stirred for 6 h to mix evenly to obtain a shell spinning solution. Take 6 mL of the core spinning solution from step (1) and slowly inject it into a 10 mL syringe. Then take 6 mL of the shell spinning solution from step (2) and slowly inject it into a 10 mL syringe. Connect the two spinning solutions to the core and shell layers of the coaxial spinning needle, respectively. Set the injection rate of the core spinning solution to 0.4 mL / h, the injection rate of the shell spinning solution to 0.7 mL / h, the rotation speed of the receiving device to 250 rpm, the spinning distance between the transmitter and the receiving substrate to 20 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a coaxial electrospun membrane with flame-retardant polyimide as the core and polyphosphazene / alumina / polyacrylonitrile as the shell is obtained on the receiving device. The electrospun diaphragm prepared in step (3) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 150℃ and 5MPa. Finally, the electrospun diaphragm prepared in step (4) was placed in an oven and dried at 120 °C for 1 h to obtain a coaxial electrospun flame-retardant coated diaphragm.

[0025] Comparative Example 1 Dissolve 5 g of polyimide and 1 g of flame-retardant silicone oil in 40 g of dimethylformamide and stir for 4 h to mix evenly to obtain a spinning solution. Take 6 mL of the spinning solution from step (1) and slowly inject it into a 10 mL syringe. Set the injection rate of the spinning solution to 0.4 mL / h, the rotation speed of the receiving device to 250 rpm, the spinning distance between the transmitter and the receiving substrate to 16 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a flame-retardant polyimide membrane prepared by electrospinning is obtained on the receiving device. The electrospun diaphragm prepared in step (2) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 120℃ and 5MPa. (4) Finally, place the electrospun diaphragm prepared in step (3) in an oven and dry it at 100 °C for 1 h to obtain an electrospun flame-retardant coated diaphragm.

[0026] Comparative Example 2 1 g of alumina, 0.4 g of flame retardant and 8 g of polyacrylonitrile were dissolved in 40 g of dimethylformamide and stirred for 6 h to mix evenly to obtain a spinning solution. Take 6 mL of the shell spinning solution from step (2) and slowly inject it into a 10 mL syringe. Set the injection rate of the spinning solution to 0.4 mL / h, the rotation speed of the receiving device to 250 rpm, the distance between the transmitter and the receiving substrate to a spinning distance of 16 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a lithium-ion separator prepared by electrospinning is obtained on the receiving device. The electrospun diaphragm prepared in step (2) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 120℃ and 5MPa. (4) Finally, place the electrospun diaphragm prepared in step (3) in an oven and dry it at 100 °C for 1 h to obtain an electrospun flame-retardant coated diaphragm.

[0027] Comparative Example 3 5 g of polyimide, 1 g of flame-retardant silicone oil, 1 g of alumina, 0.4 g of flame retardant and 8 g of polyacrylonitrile were dissolved in 80 g of dimethylformamide and stirred for 4 h to mix evenly to obtain a spinning solution. Take 6 mL of the spinning solution from step (1) and slowly inject it into a 10 mL syringe. Set the injection rate of the spinning solution to 0.4 mL / h, the rotation speed of the receiving device to 250 rpm, the distance between the transmitter and the receiving substrate to a spinning distance of 16 cm, and the electrostatic voltage to 20 kV. After the solvent dimethylformamide evaporates, a lithium-ion membrane prepared by electrospinning is obtained on the receiving device. The electrospun diaphragm prepared in step (3) was first vacuum dried (-0.1MPa, 100℃, 4h), and then hot-pressed for 30 min under experimental conditions of 120℃ and 5MPa. (4) Finally, place the electrospun diaphragm prepared in step (3) in an oven and dry it at 100 °C for 1 h to obtain an electrospun flame-retardant coated diaphragm.

[0028] Performance testing 1. Air permeability test of flame-retardant coated diaphragm: The air permeability of the diaphragm should be measured according to GB / T 36363; 2. Heat shrinkage rate test of flame-retardant coated diaphragm: Cut the diaphragm into 200mm×100mm size and sandwich it between two glass plates. Place it in an oven and heat it at 130℃ for 1 hour. After the heating is completed, take out the diaphragm, measure the diaphragm size, take the average value of multiple measurements, and calculate the thermal shrinkage rate in the MD and TD directions. Record the data in the table below. 3. Limiting oxygen index test of flame-retardant coated diaphragm: Cut the diaphragm to 200mm × 20mm and wrap it around the specified rod according to standard ASTM D 4804. After wrapping, secure the end of the sample roll with adhesive tape. Then, pull the rod out of the rolled film and cut off 20cm from the top of the roll. Draw markings at 10mm and 60mm from the ignition point of the sample. Vertically install the sample in the center of the combustion tube of the oxygen indexer, with the top of the sample 100mm below the top opening of the combustion tube. Adjust the gas mixer and flow meter to mix the oxygen and nitrogen gases until the oxygen concentration reaches the set value (i.e., the sample remains non-combustible at this oxygen concentration). Flow the mixture through the combustion tube at a rate of 40mm / s. After flushing the combustion tube with the mixed gas for 30s, lower the igniter to apply a visible flame to the top surface of the sample, approximately 6mm from the vertical plane. Continuously apply the flame for 30s, checking the combustion interruption status of the sample every 5s until the vertical plane is in steady-state combustion or the visible burning portion reaches the upper mark of the support frame. The sample is then considered ignited. Record the oxygen concentration at this point and calculate the oxygen concentration used. The data is expressed as a volume fraction.

[0029] 4. Liquid retention rate Cut diaphragm samples: 50mm × 50mm (or 100cm) 2(Uniform area, avoid edge burrs), 3-5 parallel samples per group Dry in a vacuum oven at 105℃±2℃ for 2 hours, then remove and cool to room temperature in a desiccator. Weigh M0 (accurate to 0.1mg). The electrolyte used in the test battery (e.g., LiPF6 / EC+DMC+EMC, 1mol / L) was used, and the entire process was carried out in a glove box with a dew point ≤-40℃. Completely immerse the diaphragm in the electrolyte and let it stand for 30 minutes to 2 hours. After removing it, gently press the surface with lint-free paper (or hang it for 30 seconds) to remove free electrolyte from the surface. Quickly weigh M1 (accurate to 0.1 mg to avoid electrolyte evaporation). Liquid retention rate = (M1 - M0) / M0 * 100%.

[0030] 5. Ionic conductivity reference standard GB / T 36363-2018 6. Peak heat release rate and total heat release rate were tested using a micro calorimeter. A sample of about 2 mg was taken, and the temperature range was set to 30-800 degrees Celsius with a heating rate of 10 degrees Celsius / s.

[0031] The membranes obtained in the above embodiments and comparative examples were subjected to performance tests, and the results are shown in the table below: Examples 1-3, despite their relatively thin membrane thickness, maintain good heat shrinkage resistance while exhibiting a high limiting oxygen index, low peak heat release rate, and low total heat release, demonstrating the excellent flame retardancy of the coaxial electrospun flame-retardant coated membrane. High liquid retention and high ionic conductivity prove the good wettability of the coaxial electrospun flame-retardant coated membrane. The membranes in Comparative Examples 1 and 2 are not coaxial electrospun membranes, and their properties are inferior to those of the examples. Currently, membranes are trending towards thinner and more efficient designs, while Comparative Example 3 demonstrates that membranes prepared using non-coaxial electrospinning, even with the same raw material ratio, only achieve good heat shrinkage resistance and flame retardancy when the membrane is thicker. Therefore, in summary, the coaxial electrospun flame-retardant coated membranes of Examples 1-3 are more advantageous.

[0032] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A coaxial electrospun flame-retardant coated diaphragm, characterized in that, The diaphragm adopts a core-shell structure, which consists of a core layer of flame-retardant modified polyimide and a shell layer of flame retardant, ceramic, and polyacrylonitrile mixed coating.

2. The diaphragm according to claim 1, characterized in that, The flame retardant is polyphosphazene microspheres; the ceramic is one or more of boehmite, alumina, titanium dioxide, silicon dioxide, antimony dioxide, and zirconium dioxide.

3. The diaphragm according to claim 1, characterized in that: The mass ratio of the ceramic, flame retardant and polyacrylonitrile is 1:0.1~0.5:5~10.

4. The diaphragm according to claim 1, characterized in that, The thickness of the diaphragm is 1~5 μm.

5. A method for preparing a coaxial electrospun flame-retardant coated diaphragm, characterized in that, Includes the following steps: Step (1): Dissolve the modified flame-retardant silicone oil and polyimide in dimethylformamide in a certain proportion, stir and mix evenly to obtain the core spinning solution; Step (2): Dissolve the flame retardant, ceramics, and polyacrylonitrile in dimethylformamide in a certain proportion, stir and mix evenly to obtain the shell spinning solution; Step (3): Using the spinning solutions obtained in steps (1) and (2) as electrospinning solutions, an electrospinned diaphragm is prepared by coaxial electrospinning; Step (4): Vacuum dry the electrospun diaphragm prepared in step (3), and then hot press it; Step (5): Place the electrospun separator prepared in step (4) in an oven to dry, and obtain a coaxial electrospun lithium-ion battery separator.

6. The preparation method according to claim 5, characterized in that, In step (1), the mass ratio of polyimide to flame-retardant silicone oil in the core spinning solution is 1:0.1-0.5, and the solid content is 5-25%. In step (2), the solid content is 5-25%.

7. The preparation method according to claim 5, characterized in that, In step (3), the process parameters for coaxial electrospinning are set as follows: the core spinning solution flow rate is 0.1~0.6 mL / h, the shell spinning solution flow rate is 0.2~1 mL / h, the voltage is 15~30 kV, the rotation speed is 200-300 rpm, and the distance between the transmitter and the receiving substrate is 12~28 cm.

8. The preparation method according to claim 5, characterized in that, The hot pressing conditions described in step (4) are: hot pressing for 10 to 30 minutes at 100~200 ℃ and 2-5 MPa.

9. The preparation method according to claim 5, characterized in that, The drying conditions described in step (5) are: drying at 80~150℃ for 1~2 h.

10. The preparation method according to claim 5, characterized in that, The modified flame-retardant silicone oil is prepared as follows: amino silicone oil and triethylamine are added to anhydrous dichloromethane. Under an ice-water bath and nitrogen atmosphere, a mixture of phosphoryl chloride and dichloromethane is slowly added dropwise. After the mixture is completely added, the reaction continues in the ice-water bath, followed by a reaction at room temperature. After the reaction is completed, the byproduct triethylamine salt is removed by filtration, and the solvent dichloromethane is removed. The crude product is redissolved in dichloromethane and repeatedly washed with deionized water to remove the white crystals in the liquid, resulting in a light yellow transparent oily modified flame-retardant silicone oil.

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