A method for improving heat resistance and flame retardancy of a composite fiber membrane

Abstract

The application relates to a method for improving the heat resistance and flame resistance of a composite fiber membrane and belongs to the field of battery diaphragm materials. The application aims to solve the problem that the existing lithium ion battery diaphragm cannot meet the requirement of high energy density while realizing good heat resistance and flame resistance. The method comprises the following steps: S1, preparation of boehmite dispersion liquid; S2, preparation of a spinning liquid; S3, preparation of a PEI-PVDF coaxial fiber membrane; and S4, preparation of a PPB composite fiber membrane. The application is used for improving the heat resistance and flame resistance of the composite fiber membrane.

Description

Technical Field

[0001] This invention belongs to the field of battery separator materials. Background Technology

[0002] Lithium-ion batteries have always been the preferred choice for green and environmentally friendly batteries, and are widely used in power systems, electronic products, energy storage systems, and aerospace. With the continuous development of battery technology, people's requirements for battery performance have further increased. Currently, lithium-ion battery separators mainly face problems such as poor electrolyte affinity and low thermal dimensional stability, which pose safety hazards and lead to frequent accidents.

[0003] Lithium-ion battery safety incidents are mainly caused by thermal runaway triggered by short circuits. Their normal operating temperature is room temperature, with a maximum operating temperature of 55℃~60℃, at which point the components remain stable. When some batteries are abused, causing the battery temperature to rise to or near the melting point of the separator matrix material (PE and PP melting points are 135℃ and 170℃ respectively), the separator shrinks, causing the cathode and anode to come into contact, resulting in a short circuit and thermal runaway. During thermal runaway, the battery rapidly releases a large amount of heat in a short time, causing the electrolyte to vaporize, expanding the battery casing, and even igniting and exploding. Ensuring high battery safety while meeting high energy density requirements has become a primary technical challenge. Previous coating modifications only covered the film surface, providing only surface flame retardancy. The film still shrinks due to the poor heat resistance of the polymer matrix, failing to fully exert its flame-retardant effect when exposed to flame. Summary of the Invention

[0004] This invention aims to address the problem that existing lithium-ion battery separators cannot achieve good heat resistance and flame retardancy while meeting high energy density requirements, and thus provides a method to improve the heat resistance and flame retardancy of composite fiber membranes.

[0005] A method for improving the heat resistance and flame retardancy of composite fiber membranes comprises the following steps:

[0006] I. Preparation of Boehmite Dispersion:

[0007] Boehmite particles, deionized water and adhesive are mixed evenly to obtain a mixed solution. Then, a surfactant is added and mixed evenly. Finally, defoaming is performed to obtain a boehmite dispersion.

[0008] The volume ratio of the deionized water to the mass ratio of the boehmite particles is 1 mL:(0.3-0.5) g; the volume ratio of the deionized water to the mass ratio of the adhesive is 1 mL:(0.01-0.02) g; and the mass ratio of the mixed solution to the surfactant is 1:(0.0005-0.0015).

[0009] II. Preparation of spinning solution:

[0010] Prepare core spinning solution and shell spinning solution; the mass percentage of polyetherimide in the core spinning solution is 18% to 24%; the mass percentage of polyvinylidene fluoride in the shell spinning solution is 15% to 21%.

[0011] III. Preparation of PEI-PVDF coaxial fiber membrane:

[0012] PEI-PVDF fiber membranes were prepared by coaxial electrospinning using core-shell spinning solution and shell-shell spinning solution.

[0013] IV. Preparation of PPB composite fiber membrane:

[0014] At room temperature, PEI-PVDF fiber membrane is immersed in boehmite dispersion for 5 to 30 minutes and then dried to complete the method of improving the heat resistance and flame retardancy of composite fiber membrane.

[0015] The beneficial effects of this invention are:

[0016] This invention prepares a heat-resistant and flame-retardant dual-functional composite fiber membrane. By using a PEI-PVDF core-shell structure fiber membrane as a substrate and impregnating it with a BM dispersion containing a surfactant to simulate the in-situ growth effect, a novel polymer film is obtained. This film is expected to be used as a separator material in the field of lithium-ion batteries.

[0017] Unlike previous coating modifications, this invention reduces the surface tension of the coating solution by adding a surfactant, allowing the solution to quickly penetrate the surface and enter the interior. The coating adheres uniformly to the surface of individual fibers, forming a good individual fiber coating. The film prepared by this method exhibits improved heat resistance, flame retardancy, mechanical properties, and thermal conductivity, and batteries assembled using this film demonstrate excellent battery cycle performance. PEI-PVDF-BM heat-resistant and flame-retardant dual-functional composite fiber membrane (designated PPB).

[0018] (1) The polymer composite film prepared does not burn or deform when placed in a flame for 3 seconds; (2) The film shrinkage is less than 3.0% after being heat-treated in a 300℃ oven for 10 minutes; (3) The electrolyte quickly wets the film; (4) The saturated liquid absorption rate reaches 113.6wt%; (5) The performance of the assembled battery is comparable to that of commercial separators.

[0019] This invention relates to a method for improving the heat resistance and flame retardancy of composite fiber membranes.

[0020] Instruction manual illustrations

[0021] Figure 1 This is a SEM image of the PEI-PVDF fiber membrane surface prepared in step three of Example 1;

[0022] Figure 2 The images show SEM images of the PPB fiber membrane surface. a) is a comparative experiment of PPB fiber membrane impregnated with no SDS boehmite dispersion, and b) is Example 1 of PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion.

[0023] Figure 3 The images are SEM images of cross-sections of PPB fiber membranes. a) is a comparative experiment of PPB fiber membrane impregnated with no SDS boehmite dispersion, b) is Example 1 of PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion, c) is a magnified view of the surface of a), d) is a magnified view of the center of a), and e) is a magnified view of the center of b).

[0024] Figure 4 This is a comparison graph of the surface tension of boehmite dispersions (without adhesive) with different SDS contents in Example 1 and comparative experiments;

[0025] Figure 5 The images show the contact angles of boehmite dispersions (without adhesive) with different SDS contents on the surface of the PEI-PVDF fiber membrane prepared in step three of Example 1 at different times. a) is the comparative experiment without SDS boehmite dispersion, and b) is Example 1 with 0.1% SDS boehmite dispersion.

[0026] Figure 6 This describes the mechanism by which the surfactant SDS wets the diaphragm of the boehmite dispersion in Example 1.

[0027] Figure 7 The images are infrared spectra. a is the PEI-PVDF fiber membrane prepared in step three of Example 1; b is the PPB fiber membrane impregnated with SDS boehmite dispersion in the comparative experiment; c is the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1.

[0028] Figure 8 A comparison chart of saturated liquid absorption rate, liquid retention rate, and porosity between the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1.

[0029] Figure 9 Images and curves showing the contact angles of the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 with the electrolyte, a) 0s, b) 0.1s, c) 0.2s, d) 0.5s, e) 1s, f) 2s, g) 3s;

[0030] Figure 10The following are comparison diagrams showing the spread area of ​​the electrolyte on the film: a) Celgard 2325 commercial membrane without electrolyte added; b) Celgard 2325 commercial membrane with electrolyte added for 5 seconds; c) PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 without electrolyte added; d) PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 with electrolyte added for 5 seconds.

[0031] Figure 11 The temperature of the PPB fiber membrane on a 100°C hot plate changes over time. 1 is the PEI-PVDF fiber membrane prepared in step 3 of Example 1, and 2 is the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. a is 0s, b is 0.5s, c is 1s, and d is 2s.

[0032] Figure 12 Optical photographs of Celgard 2325 commercial membrane, PVDF membrane, PEI-PVDF fiber membrane prepared in step three of Example 1, PPB fiber membrane impregnated with SDS boehmite dispersion in comparative experiment, and PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 after heat treatment at 25°C, 130°C, 210°C, 250°C and 300°C for 10 min.

[0033] Figure 13 The image shows a SEM image of a PPB fiber membrane impregnated with a 0.1% SDS boehmite dispersion in Example 1 after treatment at 300°C for 10 min.

[0034] Figure 14 The images show PPB fiber membranes before and after flame combustion. a) is a Celgard 2325 commercial membrane, b) is a PPB fiber membrane impregnated with SDS boehmite dispersion in the comparative experiment, and c) is a PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. A is before flame combustion, B is after 3 seconds of combustion, and C is after combustion.

[0035] Figure 15 SEM image of the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 after combustion;

[0036] Figure 16 The figures are stress-strain curves. 1 is the PEI-PVDF fiber membrane prepared in step 3 of Example 1. 2 is the PPB fiber membrane impregnated with SDS boehmite dispersion in the comparative experiment. 3 is the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1.

[0037] Figure 17The discharge capacity of the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 after 37 cycles. Detailed Implementation

[0038] Specific Implementation Method 1: This implementation method provides a method for improving the heat resistance and flame retardancy of composite fiber membranes, which is carried out according to the following steps:

[0039] I. Preparation of Boehmite Dispersion:

[0040] Boehmite particles, deionized water and adhesive are mixed evenly to obtain a mixed solution. Then, a surfactant is added and mixed evenly. Finally, defoaming is performed to obtain a boehmite dispersion.

[0041] The volume ratio of the deionized water to the mass ratio of the boehmite particles is 1 mL:(0.3-0.5) g; the volume ratio of the deionized water to the mass ratio of the adhesive is 1 mL:(0.01-0.02) g; and the mass ratio of the mixed solution to the surfactant is 1:(0.0005-0.0015).

[0042] II. Preparation of spinning solution:

[0043] Prepare core spinning solution and shell spinning solution; the mass percentage of polyetherimide in the core spinning solution is 18% to 24%; the mass percentage of polyvinylidene fluoride in the shell spinning solution is 15% to 21%.

[0044] III. Preparation of PEI-PVDF coaxial fiber membrane:

[0045] PEI-PVDF fiber membranes were prepared by coaxial electrospinning using core-shell spinning solution and shell-shell spinning solution.

[0046] IV. Preparation of PPB composite fiber membrane:

[0047] At room temperature, PEI-PVDF fiber membrane is immersed in boehmite dispersion for 5 to 30 minutes and then dried to complete the method of improving the heat resistance and flame retardancy of composite fiber membrane.

[0048] Preparing lithium-ion battery separators with good heat resistance and flame retardancy is one of the effective means to improve battery safety. Flame retardancy is the guarantee of battery safety, and heat resistance is the basis for flame retardancy to function. Only by ensuring sufficient high-temperature dimensional stability of the separator can it be guaranteed that the separator will not shrink during combustion. Flame retardant materials can effectively play their flame retardant role. The two complement each other and are indispensable.

[0049] This specific embodiment utilizes coaxial electrospinning technology to prepare a polyetherimide-polyvinylidene fluoride (PEI-PVDF) core-shell coaxial fiber membrane as the matrix membrane. Micron-sized boehmite (BM) is selected as the inorganic flame-retardant modifying material, and adhesives and surfactants are added to impregnate and coat the coaxial film for modification, achieving good modification results. This method is simple and fast, and the resulting film exhibits excellent comprehensive performance.

[0050] The composite film prepared in this specific embodiment uses a PEI-PVDF core-shell structured fiber membrane as the substrate. The substrate is immersed in a flame-retardant powder dispersion. Under the action of surfactant, the surface tension of the dispersion decreases, allowing it to quickly penetrate into the film. Combined with the action of adhesive, this achieves good coating of individual electrospun fibers by the flame-retardant powder. Boehmite (BM) particles have a large number of hydroxyl groups (-OH) on their surface, which can form hydrogen bonds with the adhesive, such as the highly polar cyano (-CN) and ester (-COO) groups in the polar adhesive LA133. At the same time, the cyano and ester groups also form hydrogen bonds with the PVDF layer of the coaxial fiber, improving the adhesion between the BM powder and the fiber. However, on the other hand, the BM dispersion itself has strong cohesive force, while the PVDF fiber surface has low surface energy. Therefore, during the dip-coating process, the BM layer can only accumulate on the surface of the fiber membrane, forming a coating on the surface, resulting in limited modification effect. This method involves adding a surfactant to the BM dispersion, which reduces the surface tension and alters the interfacial properties of the dispersion. This allows the surfactant to rapidly penetrate into the gaps between fibers during immersion, and as the water evaporates, the BM layer coats each individual fiber. The coating shell formed by the flame-retardant powder effectively protects the internal polymer fibers when heated, ensuring the basic thermal stability of the composite film and guaranteeing the effective performance of the flame-retardant powder. The film does not ignite or shrink under a 3-second burning test, demonstrating excellent flame-retardant properties.

[0051] The core-shell coaxial fiber membrane uses PEI as the core layer and PVDF as the shell layer, constructed via coaxial electrospinning. PEI provides a heat-resistant framework, offering both heat resistance and mechanical support to ensure the membrane's heat resistance. PVDF provides electrolyte affinity, absorbing and retaining the electrolyte. Boehmite (BM) dispersion, aided by surfactants, penetrates the fiber membrane. After drying, an adhesive uniformly bonds it to the coaxial fiber surface and fills the fiber gaps, providing support. Boehmite particles possess excellent thermal conductivity, rapidly dissipating heat and protecting the internal fiber structure. Furthermore, at high temperatures, they decompose into solid and gas phases, absorbing heat and preventing further combustion of the battery.

[0052] The composite membrane exhibits such excellent flame retardancy and heat resistance for the following reasons: Boehmite (BM) decomposes at around 400℃, absorbing a large amount of heat and forming a dense alumina layer (Al2O3) while releasing water vapor. The formation of water vapor lowers the temperature and dilutes the flammable decomposition products and oxygen in the air, preventing further combustion and achieving a good flame-retardant effect (as shown in Reaction 1). Simultaneously, the dense alumina layer formed after BM decomposition not only isolates oxygen and heat but also conducts heat away. Furthermore, the BM filling the fiber gaps provides support, ensuring the dimensional stability of the membrane at high temperatures.

[0053]

[0054] The beneficial effects of this embodiment are:

[0055] This embodiment prepares a heat-resistant and flame-retardant dual-functional composite fiber membrane. By using a PEI-PVDF core-shell structure fiber membrane as a substrate, and impregnating it with a BM dispersion containing a surfactant to simulate the in-situ growth effect, a novel polymer film is obtained. This film is expected to be used as a separator material in the field of lithium-ion batteries.

[0056] Unlike previous coating modifications, this embodiment adds a surfactant to the coating solution to reduce the surface tension of the coating solution, allowing the coating solution to quickly penetrate the surface and enter the interior. The coating adheres uniformly to the surface of individual fibers, forming a good single-fiber coating. The film prepared by this method has improved heat resistance, flame retardancy, mechanical properties, and thermal conductivity, and the battery assembled using this film has good battery cycle performance. PEI-PVDF-BM heat-resistant and flame-retardant dual-functional composite fiber membrane (named PPB).

[0057] (1) The polymer composite film prepared does not burn or deform when placed in a flame for 3 seconds; (2) The film shrinkage is less than 3.0% after being heat-treated in a 300℃ oven for 10 minutes; (3) The electrolyte quickly wets the film; (4) The saturated liquid absorption rate reaches 113.6wt%; (5) The performance of the assembled battery is comparable to that of commercial separators.

[0058] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the boehmite particles mentioned in step one are obtained by drying at a temperature of 50℃~100℃ for 24h~72h, and the particle size of the boehmite particles is 0.5μm~3μm. Everything else is the same as in Specific Implementation Method One.

[0059] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the adhesive mentioned in step one is a styrene-butadiene rubber latex adhesive, a carboxymethyl cellulose adhesive, or a water-based adhesive with polyacrylic acid, polyacrylonitrile, or polyacrylate as the main components. Everything else is the same as in Specific Implementation Method One or Two.

[0060] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the surfactant mentioned in step one is sodium dodecyl sulfate, sodium perfluoroalkyl sulfonate, sodium ethoxylated alkyl sulfate, sodium dodecyl benzenesulfonate, or sodium perfluorononenoxybenzenesulfonate. Everything else is the same as in Specific Implementation Methods One to Three.

[0061] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the defoaming process described in step one specifically involves removing excess air bubbles by vacuuming for 1 to 4 hours at room temperature. Everything else is the same as in Specific Implementation Methods One to Four.

[0062] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the preparation of the core spinning solution in step two is carried out as follows: Polyetherimide and N-methylpyrrolidone are heated and stirred at a temperature of 70℃ to 90℃ for 4 to 8 hours to obtain a pale yellow core spinning solution. Everything else is the same as in Specific Implementation Methods One to Five.

[0063] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the preparation of the shell spinning solution in step two is carried out according to the following steps: Polyvinylidene fluoride and N,N-dimethylformamide are heated and stirred for 4 to 8 hours at a temperature of 50℃ to 70℃ to obtain a transparent shell spinning solution. Everything else is the same as in Specific Implementation Methods One to Six.

[0064] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: the preparation of PEI-PVDF fiber membranes using coaxial electrospinning with core and shell spinning solutions in step three is carried out according to the following steps: coaxial electrospinning is performed under the following conditions: core layer advance rate of 0.2 mL / h to 0.6 mL / h, shell layer advance rate of 0.4 mL / h to 1.2 mL / h, coaxial needle inner-outer diameter ratio of 1:(1.5 to 2.5), spinneret-receiving distance of 15 cm to 25 cm, spinning temperature of 25°C to 35°C, humidity of 10% to 20%, and voltage of 13 KV to 17 KV. Finally, the film is removed and dried at 50°C to 70°C for 6 to 12 hours to obtain the PEI-PVDF fiber membrane. The rest is the same as in Specific Implementation Methods One to Seven.

[0065] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: during the coaxial electrospinning process, the electrospun film is collected on a reciprocating roller collector. The rotation speed of the roller collector is 90 r / min to 150 r / min, and the reciprocating cycle is 20 times / min to 40 times / min. Everything else is the same as in Specific Implementation Methods One to Eight.

[0066] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that the drying in step four is specifically carried out as follows: First, scrape off the excess boehmite dispersion from the surface, then dry in a forced-air drying oven at a temperature of 50℃ to 70℃ for 5 min to 20 min, and then vacuum dry in a vacuum drying oven at a temperature of 50℃ to 70℃ for 24 h to 72 h. Everything else is the same as in Specific Implementation Methods One to Nine.

[0067] The beneficial effects of the present invention are verified using the following embodiments:

[0068] Example 1:

[0069] A method for improving the heat resistance and flame retardancy of composite fiber membranes comprises the following steps:

[0070] I. Preparation of Boehmite Dispersion:

[0071] 2.9 g boehmite particles, 7 mL deionized water and 0.1 g adhesive LA133 were stirred at room temperature for 2 h to obtain a mixed solution. Then 0.01 g sodium dodecyl sulfate (SDS) was added and stirred at room temperature for 1 h. Finally, the solution was vacuumed at room temperature for 2 h to remove excess air bubbles, and a boehmite dispersion containing 0.1% SDS was obtained.

[0072] II. Preparation of spinning solution:

[0073] Polyetherimide and N-methylpyrrolidone were heated and stirred at 80°C for 6 hours to obtain a core spinning solution; the mass percentage of polyetherimide in the core spinning solution was 22%.

[0074] Polyvinylidene fluoride and N,N-dimethylformamide were heated and stirred for 6 hours at a temperature of 60°C to obtain a shell spinning solution; the mass percentage of polyvinylidene fluoride in the shell spinning solution was 18%.

[0075] III. Preparation of PEI-PVDF coaxial fiber membrane:

[0076] Using core spinning solution and shell spinning solution, coaxial electrospinning was carried out for 6 hours under the following conditions: core advance rate of 0.5 mL / h, shell advance rate of 1.0 mL / h, coaxial needle inner and outer diameter ratio of 1:2, spinneret and receiving distance of 20 cm, spinning temperature of 25℃, humidity of 15%, and voltage of 15 kV. Finally, the film was removed and dried at 60℃ for 72 hours to obtain PEI-PVDF fiber membrane.

[0077] During coaxial electrospinning, the electrospun film is collected on a reciprocating roller collector with a rotation speed of 120 r / min and a reciprocating cycle of 30 times / min.

[0078] IV. Preparation of PPB composite fiber membrane:

[0079] At room temperature, a 6cm×6cm PEI-PVDF fiber membrane is immersed in 10mL of boehmite dispersion for 10min. After removal, excess boehmite dispersion is scraped off the surface, and then dried in a forced-air drying oven at 60℃ for 10min. Finally, it is vacuum dried in a vacuum drying oven at 60℃ for 12h. This completes the method for improving the heat resistance and flame retardancy of the composite fiber membrane, and a PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion is obtained.

[0080] The boehmite particles mentioned in step one are obtained by drying at 60°C for 72 hours, and the particle size of the boehmite particles is 1 μm.

[0081] In this embodiment, the polyetherimide (PEI) has a molecular weight of 600,000; the polyvinylidene fluoride (PVDF) has a molecular weight of 300,000; and the adhesive LA133 is an aqueous dispersion of acrylonitrile multi-component copolymer with model number MA-EN-BI-OG produced by Dongguan Kelude Innovation Technology Co., Ltd.

[0082] Comparative Experiment: This comparative experiment differs from Example 1 in that the addition of sodium dodecyl sulfate (SDS) in step one is omitted, resulting in an SDS-free boehmite dispersion; step four yields a PPB fiber membrane impregnated with the SDS-free boehmite dispersion. Everything else is the same as in Example 1.

[0083] Figure 1 The image shows the SEM image of the PEI-PVDF fiber membrane prepared in step three of Example 1. As can be seen from the image, the microstructure of the PEI-PVDF core-shell coaxial composite fiber membrane is smooth, uniform in size, without bead formation, and the fiber diameter is about 1 μm. The overall structure presents a three-dimensional spatial network structure.

[0084] Figure 2The images show SEM images of the PPB fiber membrane surface. a) shows the PPB fiber membrane impregnated with SDS-free boehmite dispersion in the comparative experiment, and b) shows the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. As can be seen from image a), the boehmite coating without surfactant SDS forms a uniform and dense coating layer on the substrate membrane, and no fiber structure can be observed on the surface. In image b), the boehmite coating containing 0.1% SDS can penetrate the fiber surface and seep into the interior, partially adhering to the fiber surface and filling the fiber gaps.

[0085] To further investigate the permeation of the boehmite coating within the fiber membrane, scanning electron microscopy was used to observe the cross-section of the film. PPB fiber membranes cut to 0.5 cm × 2 cm were immersed in liquid nitrogen for 30 seconds. After the samples were fully frozen, they were subjected to brittle fracture by holding both ends with tweezers. The cross-section was then fixed to the side of an aluminum sample stage with conductive adhesive, and the microstructure of the cross-section was characterized after gold sputtering. The results showed… Figure 3 middle.

[0086] Figure 3 The images are SEM images of cross-sections of PPB fiber membranes. a) is a comparative experiment of PPB fiber membrane impregnated with no SDS boehmite dispersion, b) is Example 1 of PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion, c) is a magnified view of the surface of a), d) is a magnified view of the center of a), and e) is a magnified view of the center of b). Figure 3 a) shows the cross-sectional morphology of a PPB fiber membrane impregnated with a boehmite dispersion without surfactant. Magnified surface image c) shows the coating solution drying into a smooth shell adhering to the surface of the coaxial fiber membrane, with localized fractures observed. The coating thickness is approximately 10.9 μm. Magnified center image d) shows only the fiber structure, with no BM particles present, indicating that the boehmite is distributed on the outer surface of the fiber membrane and has not penetrated into its interior. b) shows the cross-section of a PPB fiber membrane impregnated with a boehmite dispersion containing 0.1% SDS. After adding the surfactant, no dense boehmite coating was observed on the fiber membrane surface; the coating solution penetrated into the fiber membrane. Magnified center image e) shows BM particles uniformly adhered to the surface of individual fibers, indicating that under the combined action of the surfactant and adhesive LA133, the BM particles adhered to the fiber surface, forming a good coating on the individual fibers. This structure is expected to improve the heat resistance and flame retardant properties of the membrane.

[0087] To analyze the role of the surfactant, 0.1g of adhesive in the mixed solution of Example 1 and Comparative Experiment Step 1 was replaced with an equal amount of deionized water. Specifically, in Example 1 and the Comparative Experiment, 2.9g of boehmite particles and 7.1mL of deionized water were stirred at room temperature for 2 hours to obtain a boehmite aqueous solution. In Example 1, 0.01g of sodium dodecyl sulfate (SDS) was then added and stirred at room temperature for 1 hour to obtain a boehmite dispersion containing 0.1% SDS. In the Comparative Experiment, the addition of sodium dodecyl sulfate (SDS) in Step 1 was omitted, resulting in a boehmite dispersion without SDS. The surface tension and contact angle of both solutions were then tested, as follows: Figure 4 and Figure 5 The surface tension of boehmite dispersions with different surfactant additions was tested using the five-point pendant drop method. The results showed... Figure 4 middle.

[0088] Figure 4 This is a comparison of the surface tension of boehmite dispersions (without adhesive) with different SDS contents in Example 1 and comparative experiments. It can be seen that the surface tension of the boehmite aqueous solution without SDS is 82.77 mN / m, similar to that of its solvent, water. When 0.1% SDS is added, the surface tension of the boehmite solution rapidly decreases to 50.81 mN / m. This is because the surfactant molecules are oriented on the solution surface. This arrangement creates a layer of hydrophobic groups on the surface of the boehmite solution, reducing the surface free energy. Therefore, the boehmite solution doped with the surfactant SDS has a lower surface tension.

[0089] Figure 5 These are optical photographs of the contact angles of boehmite dispersions (without adhesive) with different SDS contents on the surface of the PEI-PVDF fiber membrane prepared in step three of Example 1 at different times. a) is a comparative experiment without SDS boehmite dispersion, and b) is Example 1 with 0.1% SDS boehmite dispersion. Figure 5 It can be seen that the boehmite dispersion without SDS has a larger contact angle than the boehmite dispersion containing 0.1% SDS due to its excessive surface tension. Furthermore, because the tension is too high, capillary action cannot be initiated between the fibers, preventing the dispersion droplets from penetrating the membrane, and the contact angle remains essentially unchanged over time. The boehmite dispersion containing 0.1% SDS has a tension below the threshold required to induce capillary action; therefore, the coaxial fiber membrane can completely absorb the dispersion within 3 minutes.

[0090] Figure 6This illustrates the mechanism by which the surfactant SDS wets the diaphragm with boehmite dispersion in Example 1. As shown in the figure, whether capillary action occurs in the coating solution is the result of the combined effect of adhesion and surface tension. The adhesion force F' of the boehmite solution to the PVDF fiber layer causes the solution to spread into the fiber interior, forming a concave surface. However, the surface tension F of the solution creates the opposite force, tending to pull the liquid surface upwards. Without SDS, the surface tension is too high, and tension plays a dominant role. Furthermore, the PVDF in the coaxial fiber shell is highly hydrophobic, preventing the coating solution from penetrating the fiber interior and allowing it to disperse only on the fiber surface. After adding SDS, the surface tension of the boehmite solution rapidly decreases. At this point, adhesion becomes dominant, pulling the solution into the fiber pores and inducing capillary action, allowing the boehmite particles to fully penetrate the fiber interior.

[0091] To characterize the chemical structure of the diaphragm, Fourier transform infrared spectroscopy was used to analyze the chemical composition of different groups of diaphragm membranes. Figure 7 The images are infrared spectra. a) is the PEI-PVDF fiber membrane prepared in step three of Example 1; b) is the PPB fiber membrane impregnated with no SDS boehmite dispersion in the comparative experiment; c) is the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. The curve at 1077.5 cm⁻¹ is shown in curve c. -1 The peak at 3276.5 cm⁻¹ represents the Al-OH bending vibration absorption peak in boehmite. -1 With 3086.5cm -1 The characteristic peaks at 1721.2 cm⁻¹ are (Al)OH and (Al)-H of boehmite, respectively. -1 The peak at this point represents the absorption peak of the C=O double bond in the core layer of polyetherimide. For the PPB film without added surfactant (curve b), the intensity of this absorption peak is lower than that of the film with added surfactant (curve c). The reason for this is that in the PPB film without added surfactant, boehmite particles cover the film in the form of a dense layer, which hinders some infrared light from passing through the membrane. In contrast, in the film with added surfactant, boehmite particles permeate into the fiber gaps, and the C=O double bond in the polyetherimide produces a stronger absorption peak.

[0092] Saturated liquid absorption rate, liquid retention rate, and porosity are key factors affecting the electrical performance and cycle life of batteries; liquid absorption rate, liquid retention rate, and porosity were tested using an electrolyte (1.0M LiPF6 in EC:DMC = 1:1 vol%). Figure 8 This is a comparison chart of the saturated liquid absorption rate, liquid retention rate, and porosity of the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion from Example 1. Figure 8As shown, the saturated liquid absorption rate, liquid retention rate, and porosity of the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion were 113.6%, 84.3%, and 53%, respectively, all higher than the 72.5%, 78.9%, and 27% of the Celgard 2325 commercial membrane. This is because the special three-dimensional network structure of the coaxial electrospun PEI / PVDF fiber membrane itself has a large number of pores, which creates space for electrolyte wetting. In addition, the shell substrate PVDF has polar groups, which have good affinity with the electrolyte. Furthermore, the amorphous regions of PVDF will form a gel structure when immersed in electrolyte, which is beneficial to stabilizing the electrolyte. The PPB membrane with added SDS has a large number of BM particles distributed in the gaps, and the electrolyte penetrates into these pores and has a strong affinity for them, which enhances the ability of the composite fiber membrane to retain electrolyte. The -CN (cyano) polar group of the binder LA133 exhibits electrolyte-friendly properties. Therefore, the PPB composite fiber membrane with added SDS exhibits high saturation liquid absorption rate, liquid retention rate and porosity.

[0093] The affinity of the separator for the electrolyte directly affects the performance of the battery. The contact angle of different separators with electrolyte (1.0M LiPF6 in EC:DMC = 1:1 vol%) was measured over time using a contact angle meter. The results are shown in Figure 9. Figure 9 Images and curves showing the contact angles of the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 with the electrolyte are shown. a) is 0 s, b) is 0.1 s, c) is 0.2 s, d) is 0.5 s, e) is 1 s, f) is 2 s, and g) is 3 s. Figure 9 It can be seen that the instantaneous initial contact angle of the electrolyte droplet added to the commercial membrane was 61.2°, which changed to 60.8° at 1s, 59.9° at 2s, and 57.6° at 3s. This indicates that the commercial membrane has poor affinity with the electrolyte, and the electrolyte cannot quickly penetrate into the membrane after being added. The instantaneous initial contact angle of the PPB composite fiber membrane was 45.1°, which rapidly decreased to 17.3° at 3s and partially penetrated into the membrane. This indicates that the PPB composite fiber membrane exhibits excellent electrolyte affinity. The rapid completion of the entire electrolyte wetting process within 3s is due to the fact that the composite fiber membrane is mainly composed of PVDF, adhesive LA133, and BM flame-retardant particles. PVDF has good electrolyte affinity, and the cyano groups of LA133 and the hydroxyl groups on the AlOOH surface both show good affinity with carbonate electrolytes. In addition, the electrolyte can be easily stored in various gaps in the membrane, which is beneficial to electrolyte wetting.

[0094] To visually demonstrate the degree of wetting of the membrane by the electrolyte (1.0 M LiPF6 in EC: DMC = 1:1 vol%), the spreading area of ​​the electrolyte on the thin film was measured, and the optical photographs show that... Figure 10middle. Figure 10 The figures show a comparison of the electrolyte spreading area on the membranes: a) Celgard 2325 commercial membrane without electrolyte added; b) Celgard 2325 commercial membrane with electrolyte added for 5 seconds; c) PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 without electrolyte added; d) PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 with electrolyte added for 5 seconds. Figure 10 It can be seen that when the electrolyte is added to the commercial diaphragm for 5 seconds, the electrolyte spreading area is 0.13 cm². 2 When the electrolyte is dropped onto the PPB composite fiber membrane, the electrolyte spreading area can reach 0.78 cm² after 5 seconds. 2 Furthermore, the electrolyte is completely immersed in the interior of the membrane. This is due to the good affinity between the flame-retardant BM particles and PVDF materials in the membrane material and the electrolyte. In addition, due to the unique porous structure of the fiber membrane prepared by electrospinning, capillary absorption occurs, which can better absorb the electrolyte and spread the electrolyte quickly throughout the entire membrane. The PPB composite fiber membrane exhibits excellent electrolyte wettability.

[0095] To characterize the thermal conductivity of the diaphragms, two types of diaphragms were placed on a 100°C constant-temperature hot stage, and the temperature change of the diaphragms over time was observed using an infrared thermal imaging camera. The change process is shown in the diagram. Figure 11 middle. Figure 11 The temperature change of the PPB fiber membrane on a 100°C hot plate over time is shown in Figure 1. Figure 2 shows the PEI-PVDF fiber membrane prepared in step three of Example 1, and Figure 3 shows the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. a represents 0 s, b represents 0.5 s, c represents 1 s, and d represents 2 s. Figure 11 The changes in temperature over time between the coaxial fiber membrane with BM particles and the coaxial film without BM, as observed by an infrared thermal imaging instrument, are shown. At 0 s, at room temperature, both films exhibit a uniform purple hue. At 0.5 s, the original coaxial membrane reaches 42 °C, while the edge of the SDS-added PPB film reaches 50 °C. At 1 s, the original coaxial membrane reaches 73 °C, while the average temperature of the SDS-added PPB film reaches 81 °C. At 2 s, 70% of the surface of the SDS-containing PPB film reaches 98 °C, while the surface temperature of the coaxial membrane is only 90 °C, indicating that BM particles significantly improve thermal conductivity. Lithium-ion battery separators with high thermal conductivity can effectively transfer excess heat generated during battery operation to the outside environment, reducing the possibility of thermal runaway due to heat accumulation and greatly improving battery safety. Experimental results show that boehmite, as a high thermal conductivity ceramic, can significantly improve the thermal conductivity of polymer battery separators.

[0096] To observe the thermo-dimensional stability of the diaphragm under high-temperature conditions, the diaphragm was heat-treated using a gradual heating method. The results showed... Figure 12 middle. Figure 12 Optical photographs of Celgard 2325 commercial membrane, PVDF membrane, PEI-PVDF fiber membrane prepared in step three of Example 1, PPB fiber membrane impregnated with SDS boehmite dispersion in comparative experiment, and PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 after heat treatment at 25°C, 130°C, 210°C, 250°C, and 300°C for 10 min; as shown. Figure 12 As shown, the Celgard 2325 commercial membrane exhibited significant shrinkage after heat treatment at 130℃ for 10 minutes. The PVDF-PEI coaxial composite fiber membrane substrate, due to the thermodynamic support provided by PEI in the core layer, showed a shrinkage rate of only 2.7% compared to the pure PVDF membrane, which reached 76.4% after heat treatment at 210℃ for 10 minutes. The PPB film with trace amounts of SDS showed no shrinkage after heating from 25℃ to 250℃ for 10 minutes. At 300℃ for 10 minutes, the composite fiber membrane showed slight shrinkage, with a shrinkage rate of only 2.2%. In contrast, the PPB film without SDS, because the coating solution could not penetrate the fibers to provide support and heat conduction, showed significant shrinkage at 250℃, exceeding 50%, compared to the PVDF-PEI coaxial membrane. At 300℃, the PPB composite fiber membrane exhibited noticeable yellowing, due to the oxidation of the adhesive LA133 at high temperatures, which turned yellow and caused the membrane to yellow. Because the core layer PEI material in the composite fiber membrane matrix contains ether structures and aromatic amine functional groups, it exhibits high-temperature resistance and good dimensional stability. As a support for the membrane, it enhances the membrane's heat resistance. BM can act as a reinforcing agent in the polymer, improving the composite material's flame retardancy, heat distortion temperature, and thermal conductivity. Under the action of surfactants, BM inorganic particles mimic fiber growth and adhere to the fibers, forming an inorganic skeleton structure that further enhances the heat resistance of the composite fiber membrane. Furthermore, the BM filling the fiber gaps also provides support at high temperatures. Therefore, even at 300℃, the PPB film with added SDS retains 97.8% of its initial membrane area, greatly ensuring the thermal dimensional stability of the composite fiber membrane.

[0097] Figure 13 The image shows a SEM image of a PPB fiber membrane impregnated with a 0.1% SDS boehmite dispersion, as described in Example 1, after treatment at 300°C for 10 min. Figure 13As shown, the microstructure of the PPB film with added 0.1% wtSDS after treatment at 300℃ for 10 min was observed using scanning electron microscopy. It can be seen that the coaxial fiber shell PVDF melted, while the core layer PEI maintained its intact structure. BM particles remained attached to the fiber surface and filled the fiber gaps under high temperature conditions, playing a skeletal support role and keeping the membrane relatively intact in size.

[0098] Figure 14 Images show PPB fiber membranes before and after flame combustion. a) is a Celgard 2325 commercial membrane; b) is a PPB fiber membrane impregnated with SDS-free boehmite dispersion in a comparative experiment; c) is a PPB fiber membrane impregnated with 0.1% SDS-containing boehmite dispersion in Example 1. A is before flame combustion, B is after 3 seconds of combustion, and C is after combustion. Figure 14 As shown, when Celgard 2325 commercial membranes are burned with a flame, the membranes burn and shrink rapidly. When PPB films without added SDS are continuously burned with a flame, the flame retardancy improves compared to the commercial membranes, but significant dimensional shrinkage still occurs. When PPB films with 0.1% SDS are continuously burned with a flame, the membrane surface turns black and the edges glow red during combustion. The composite fiber membrane accumulates a large amount of heat but does not ignite, and no significant dimensional shrinkage occurs, demonstrating its excellent flame retardancy.

[0099] The microstructure of the composite film after flame scorching was observed using scanning electron microscopy. Figure 15 This is a SEM image of the PPB fiber membrane impregnated with a 0.1% SDS boehmite dispersion from Example 1 after combustion; as shown in the image, from Figure 15 As can be seen, the composite membrane fibers melt and collapse under continuous high-temperature flame combustion, leaving empty fiber channels in the surrounding inorganic particles. The remaining particles are alumina particles from the decomposition of BM, which densely cover the diaphragm surface. This covering film protects the internal structure of the fiber membrane, not only inhibiting the combustion reaction but also providing support, maintaining the integrity of the diaphragm shape, and giving the diaphragm excellent flame retardancy.

[0100] Mechanical properties are fundamental to ensuring safe battery operation. A universal electronic tensile testing machine was used to characterize the mechanical properties of the battery separator. PPB separators with different SDS contents and coaxial original separators were cut into 1cm × 10cm rectangular standard samples, with 1cm indentation on each side, and placed on the universal mechanical testing machine fixture. Tensile mode was selected, and the tensile rate was adjusted to 5mm / min. Each sample was measured five times, and the stress-strain curves were finally obtained. The results show... Figure 16 middle. Figure 16The figures show stress-strain curves. 1 represents the PEI-PVDF fiber membrane prepared in step three of Example 1; 2 represents the PPB fiber membrane impregnated with SDS-free boehmite dispersion in the comparative experiment; and 3 represents the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1. The figures show that the maximum tensile strength of the original coaxial membrane and the SDS-free PPB film are 2.90 MPa and 3.01 MPa, respectively. The elongation at break is 7.8% and 9.1%, respectively. With the addition of 0.1% SDS, the coating solution fully penetrates the fiber membrane. After drying, the BM particles provide support between the fibers, and the binder LA133 connects the randomly distributed fibers together. This not only increases the stress to 4.27 MPa but also significantly improves the elongation at break compared to the original membrane, reaching 42.3%. The slope of the curve also indicates that the modulus of the PPB film with 0.1% SDS is improved. In electrospun fiber membranes, the fibers are randomly distributed and interwoven, resulting in very weak inter-fiber forces and thus low stress and elongation at break. When 0.1% SDS is added, boehmite particles and the binder LA133 penetrate the fiber gaps. Under the influence of polar groups, these particles connect the different fibers, strengthening the inter-fiber forces and thus comprehensively improving the film's modulus, tensile strength, and elongation at break.

[0101] Stable electrochemical performance and good cycle rate are fundamental to ensuring the long-term stable use of batteries. Charge-discharge tests were conducted on button batteries assembled with different separators, and the discharge specific capacity was calculated. Specific test results are as follows: Figure 17 As shown. Figure 17 The discharge capacity of the Celgard 2325 commercial membrane and the PPB fiber membrane impregnated with 0.1% SDS boehmite dispersion in Example 1 after 37 cycles; from Figure 17 It can be seen that the discharge specific capacity of the two membranes is similar. The discharge specific capacity fluctuation of the PPB film with added SDS is slightly larger than that of the commercial membrane. After 37 cycles, the discharge capacity decay rate of the PPB film containing 0.1% SDS is 95.23%, which is higher than that of the commercial membrane (94.42%). This indicates that the PPB film containing 0.1% SDS has better cycle performance. This is because PVDF has better electrolyte absorption, higher membrane porosity, and the SDS content has the least impact on the electrolyte at this point.

Claims

1. A method for improving the heat resistance and flame retardancy of composite fiber membranes, characterized in that... It is done in the following steps: I. Preparation of Boehmite Dispersion: Boehmite particles, deionized water and adhesive are mixed evenly to obtain a mixed solution. Then, a surfactant is added and mixed evenly. Finally, defoaming is performed to obtain a boehmite dispersion. The volume ratio of the deionized water to the mass ratio of the boehmite particles is 1 mL:(0.3-0.5) g; the volume ratio of the deionized water to the mass ratio of the adhesive is 1 mL:(0.01-0.02) g; and the mass ratio of the mixed solution to the surfactant is 1:(0.0005-0.0015). The boehmite particles are specifically obtained by drying at a temperature of 50℃ to 100℃ for 24h to 72h, and the particle size of the boehmite particles is 1μm to 3μm. The surfactant is sodium dodecyl sulfate; II. Preparation of spinning solution: Prepare core spinning solution and shell spinning solution; the mass percentage of polyetherimide in the core spinning solution is 18% to 24%; the mass percentage of polyvinylidene fluoride in the shell spinning solution is 15% to 21%. III. Preparation of PEI-PVDF coaxial fiber membrane: Coaxial electrospinning was performed under the following conditions: core layer advance rate of 0.5 mL / h to 0.6 mL / h, shell layer advance rate of 1.0 mL / h to 1.2 mL / h, coaxial needle inner and outer diameter ratio of 1:(1.5 to 2.5), spinneret and receiving spinning distance of 20 cm, spinning temperature of 25℃ to 35℃, humidity of 15% to 20%, and voltage of 15kV to 17kV. Finally, the film was removed and dried at 50℃ to 70℃ for 6h to 12h to obtain PEI-PVDF fiber membrane. IV. Preparation of PPB composite fiber membrane: At room temperature, PEI-PVDF fiber membrane is immersed in boehmite dispersion for 5 min to 30 min and then dried to complete the method of improving the heat resistance and flame retardancy of composite fiber membrane. The drying process is carried out in the following steps: first, scrape off the excess boehmite dispersion from the surface, then dry in a forced-air drying oven at a temperature of 50℃~70℃ for 5min~20min, and then vacuum dry in a vacuum drying oven at a temperature of 50℃~70℃ for 24h~72h.

2. The method for improving the heat resistance and flame retardancy of composite fiber membranes according to claim 1, characterized in that... The adhesives mentioned in step one are styrene-butadiene rubber latex adhesives, carboxymethyl cellulose adhesives, and water-based adhesives with polyacrylic acid, polyacrylonitrile, or polyacrylate as the main components.

3. The method for improving the heat resistance and flame retardancy of composite fiber membranes according to claim 1, characterized in that... The defoaming process described in step one specifically involves removing excess air bubbles by vacuuming at room temperature for 1 to 4 hours.

4. The method for improving the heat resistance and flame retardancy of composite fiber membranes according to claim 1, characterized in that... The preparation of the core spinning solution in step two is carried out in the following steps: under the condition of 70℃~90℃, polyetherimide and N-methylpyrrolidone are heated and stirred for 4h~8h to obtain a light yellow core spinning solution.

5. The method for improving the heat resistance and flame retardancy of composite fiber membranes according to claim 1, characterized in that... The preparation of the shell spinning solution in step two is carried out in the following steps: under the condition of 50℃~70℃, polyvinylidene fluoride and N,N-dimethylformamide are heated and stirred for 4h~8h to obtain a transparent shell spinning solution.

6. The method for improving the heat resistance and flame retardancy of composite fiber membranes according to claim 1, characterized in that... During coaxial electrospinning, the electrospun film is collected on a reciprocating roller collector with a rotation speed of 90 r / min to 150 r / min and a reciprocating cycle of 20 times / min to 40 times / min.

Images