A PVDF / APP nanosphere flame retardant composite film and its preparation method and application

Abstract

The present invention proposes a PVDF / APP nano-microsphere flame-retardant composite film and its preparation method and application. The preparation of the PVDF / APP nano-microsphere flame-retardant composite film includes: dissolving PVDF powder and APP powder in DMF solvent to obtain a casting liquid; spreading the casting liquid on the surface of a coagulation bath solution to obtain a wet PVDF / APP composite film; transferring it to a flat inorganic glass substrate and spreading it flat; then dripping the coagulation bath solution; flattening and wrapping the glass plate; then performing a drying and heat preservation treatment; finally taking out the PVDF / APP composite film clamped between the two glass substrates, soaking and washing, and naturally air-drying to obtain a PVDF / APP nano-microsphere composite film. The composite film provided by the present invention and its application in lithium battery separators can achieve a good flame retardant effect by controlling the addition amount of the flame retardant to less than 10%. Compared with the 40%-50% addition amount usually required in the prior art, the present invention not only significantly reduces the production cost, but also makes the composite film more lightweight.

Description

Technical Field

[0001] The present invention relates to the technical field of battery separator materials, and in particular to a PVDF / APP nano-microsphere flame-retardant composite film and a preparation method and application thereof. Background Art

[0002] With the rise of electric vehicles and renewable energy storage systems, the demand for lithium-ion batteries has increased significantly. Battery safety has become an important consideration in design and application, so the development of highly safe and flame-retardant battery separators is particularly important. Battery separators are an important component of lithium-ion batteries, providing ion conduction, mechanical support, and thermal management. Lithium-ion batteries may experience thermal runaway under high temperature, overcharge, or physical damage, causing the temperature inside the battery to rise sharply. This can lead to the decomposition of the electrolyte and the reaction of electrode materials, triggering more serious chemical reactions, leading to combustion or even explosion. Therefore, battery separators with excellent flame-retardant properties can delay or even prevent the occurrence and further spread of fire after the battery experiences thermal runaway.

[0003] Currently, coating and composite materials are two important techniques for preparing flame-retardant battery separators. Coating technology involves uniformly mixing a flame retardant with a binder to form a coating material. The coating material is then applied to the surface of a commercial polyethylene (PE) or polypropylene (PP) separator by blade coating or spraying to create a flame-retardant separator material. Although coating technology can improve the flame retardancy of separators, it is affected by the stability and bulk density of the coating itself, leading to problems such as decreased ion conductivity, increased thickness, increased electrolyte viscosity, and increased side reactions, ultimately resulting in poor battery performance and reduced energy density. Composite material technology involves compounding flame retardants with polymer substrates through methods such as melt extrusion and phase inversion to achieve excellent flame retardant properties. However, this technology is limited by the type of battery separator material or the compounding method. Composite material technology often suffers from defects such as poor dispersion of the flame retardant and the large amount of added flame retardant, resulting in increased costs and difficulty in large-scale industrial production.

[0004] Ammonium polyphosphate (APP) is a common phosphorus-based inorganic flame retardant. Its notable characteristics include strong hygroscopicity, easy precipitation, and hydrolysis at certain temperatures and in the presence of moisture. Therefore, researchers often consider surface modification and protection to prepare high-performance composite flame retardants. For example, patent CN111349355A uses a silane coupling agent to surface-modify APP, followed by grafting hexagonal boron nitride onto the APP surface to improve its thermal stability and water resistance. Patent CN112011093A also designs a multilayer core-shell composite material in which chitosan and phytic acid are adsorbed on the surface of APP particles through oppositely charged electrostatic flocculation, reducing the hydrophilicity of the APP. However, due to the micron-sized bulk particles of APP, the high flame retardancy of APP-filled composite battery separators requires a high APP loading, which can damage the separator's pore structure and increase its internal resistance, severely limiting the battery's electrochemical performance.

[0005] Therefore, it is urgent to develop a new battery separator to effectively utilize the flame retardant properties of APP and reduce the need for high APP addition. Summary of the Invention

[0006] In view of this, the present invention proposes a PVDF / APP nano-microsphere flame-retardant composite film capable of balancing the relationship between APP addition amount and battery performance, as well as a preparation method and application thereof.

[0007] In a first aspect, the present invention provides a PVDF / APP nanosphere flame retardant composite film, wherein the PVDF / APP nanosphere flame retardant composite film comprises polymer PVDF and flame retardant APP, wherein the flame retardant APP accounts for at most 10% of the mass of the polymer PVDF.

[0008] Furthermore, the average pore size of the PVDF / APP nanosphere flame retardant composite membrane is 50-250 nm, and the size of the flame retardant APP is 100-200 nm.

[0009] In a second aspect, the present invention provides a method for preparing the above-mentioned PVDF / APP nanosphere flame retardant composite film, the method comprising the following steps:

[0010] S1. Dissolve PVDF powder and APP powder in N,N-dimethylformamide (DMF) solvent and stir at room temperature to obtain a casting solution;

[0011] S2, spreading the casting solution of step S1 on the surface of the coagulation bath solution to obtain a wet PVDF / APP composite membrane;

[0012] S3, transferring the PVDF / APP composite film from step S2 onto a flat inorganic glass substrate and laying it flat;

[0013] S4, adding the coagulation bath solution of step S2 dropwise onto the surface of the PVDF / APP composite membrane treated in step S3 to keep the PVDF / APP composite membrane in a wet state;

[0014] S5. Take another flat inorganic glass substrate and completely fit it on the surface of the PVDF / APP composite film treated in step S4, and fully wrap the upper and lower glass substrates and the PVDF / APP composite film sandwiched in the middle with plastic wrap;

[0015] S6, drying and heat-insulating the wrapped article in step S5;

[0016] S7, opening the package after the treatment in step S6, taking out the PVDF / APP composite membrane sandwiched between the two glass substrates, soaking and washing it three times with pure water, and naturally air-drying it to obtain a PVDF / APP nanosphere composite membrane.

[0017] Because ammonium polyphosphate undergoes hydrolysis at certain temperatures and in the presence of moisture, hydrolysis is often avoided in flame retardant and related fields to prevent product performance from being affected. However, the present application fully utilizes this characteristic of ammonium polyphosphate's easy absorption and hydrolysis. When preparing the composite film of the present application based on the film preparation method of invention patent CN118126373A or invention patent CN116751382A, the ammonium polyphosphate is fully exposed to certain temperature and water conditions to promote its full hydrolysis, reducing the ammonium polyphosphate particle size from the initial 20-30μm to 100-200nm and forming a regular spherical structure. This significantly improves its dispersibility and compatibility with the polymer (PVDF), and reduces the addition amount to below 20%, significantly improving the flame retardant efficiency and reducing its impact on the film's own structure. As a result, the composite film can be used as a flame-retardant battery separator with excellent performance in the battery field.

[0018] Furthermore, in step S1, the mass ratio of the PVDF powder to the DMF solvent is 1:(6.2-6.3).

[0019] Furthermore, in step S1, the stirring time is 45 to 48 hours.

[0020] Furthermore, in step S2, the coagulation bath solution is prepared by mixing DMF solvent and water; in terms of volume percentage, the volume ratio of the DMF solvent to the water is 60-70:40-30.

[0021] Furthermore, in step S4, the amount of the coagulation bath solution added is 2 Add 18~24ml to the membrane surface.

[0022] By adopting this technical solution, ammonium polyphosphate (APP) can be fully hydrolyzed in a wet state to form regular nanospheres, which helps maintain the porous structure of the film and avoid cracks and uneven structure. In addition, the formation of nanospheres further improves the dispersion and flame retardant efficiency of the flame retardant. Its uniform distribution in the polyvinylidene fluoride (PVDF) matrix also enhances the mechanical properties of the film.

[0023] Furthermore, in step S6, the drying and heat preservation temperature is 50-60° C. and the time is 18-20 hours.

[0024] By employing this technical solution, the film can be solidified, preventing deformation and shrinkage caused by prolonged exposure to air, thereby forming a stable structure. Furthermore, drying and heat preservation help evaporate moisture from the film, resulting in a dense and uniform film structure and improving its mechanical strength and toughness. Furthermore, after drying and heat preservation, the film's physical properties are further stabilized.

[0025] Furthermore, in step S7, the natural air drying is performed for 60 to 90 minutes.

[0026] Furthermore, the invention also includes the application of the PVDF / APP nanosphere flame retardant composite film in lithium batteries.

[0027] By adopting the above technical solution, the PVDF / APP nanosphere flame-retardant composite film of the present invention, when used as a separator material for lithium batteries, exhibits a high specific surface area and can be evenly dispersed within the PVDF matrix, thereby significantly improving flame retardancy. When a battery experiences thermal runaway, the nanospheres rapidly decompose to form acidic substances such as polyphosphoric acid and metaphosphoric acid, which promote the carbonization of PVDF, forming a dense carbonized layer that isolates combustibles and oxygen, delaying or even preventing combustion. Furthermore, the formation of the nanospheres does not disrupt the porous structure of the membrane, thereby enabling the battery separator material to maintain good air permeability and ion conductivity.

[0028] In the present invention, the peak heat release rate of the lithium battery equipped with the PVDF / APP nanosphere flame retardant composite film of the present invention is reduced by no more than 45%, and the total heat release is reduced by no more than 26%.

[0029] By adopting the above technical solutions, the safety performance of lithium batteries in the event of thermal runaway is significantly improved, the risk of combustion is reduced, the battery life is extended, and the reliability of thermal management and use is improved.

[0030] The PVDF / APP nanosphere flame retardant composite film provided by the present invention has the following advantages over the prior art:

[0031] (1) The composite film provided by the present invention and its application in lithium battery separators can achieve a good flame retardant effect by controlling the addition amount of the flame retardant to below 10%. Compared with the conventional art which usually requires an addition amount of 40%-50%, the present invention not only significantly reduces the production cost but also makes the composite film lighter.

[0032] (2) In existing battery separators, the phosphorus-based flame retardant particles are no less than 0.6 μm in size and are mostly irregular block structures, which can easily damage the film structure and reduce porosity and air permeability. Compared to the existing technology, the flame retardant in the composite membrane of the present invention has a size of 100-200 nm and is uniformly spherical in shape, resulting in a higher specific surface area and better compatibility.

[0033] (3) The peak heat release rate of the soft-pack battery assembled with the flame-retardant diaphragm of the prior art is reduced by no more than 30%, and the total heat release is no more than 20%. The peak heat release rate of the soft-pack battery assembled with the composite membrane involved in the present invention is reduced by up to 45%, and the total heat release is reduced by up to 26%. Compared with the flame-retardant diaphragm of the prior art, the composite membrane involved in the present invention has better flame retardant properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 Transmission electron micrographs of the hydrolysis process of ammonium polyphosphate in the coagulation bath solution at different times according to Example 1 of the present invention;

[0036] In the figure, a is a transmission electron micrograph of ammonium polyphosphate without any treatment, b is a transmission electron micrograph of ammonium polyphosphate after hydrolysis for 10 hours, c is a transmission electron micrograph of ammonium polyphosphate after hydrolysis for 15 hours, and d is a transmission electron micrograph of ammonium polyphosphate after hydrolysis for 20 hours;

[0037] Figure 2 This is a scanning electron microscope (SEM) image of the PVDF / APP nanosphere flame retardant composite film prepared in Example 1 of the present invention;

[0038] In the figure, a is a surface electron microscope scanning SEM image of the PVDF / APP nano-microsphere flame retardant composite film prepared in Example 1, and b is a cross-sectional electron microscope scanning SEM image of the PVDF / APP nano-microsphere flame retardant composite film prepared in Example 1;

[0039] Figure 3This is a combustion photo of the PVDF / APP nanosphere flame retardant composite film prepared in Example 1 of the present invention;

[0040] Figure 4 This is a scanning electron microscope (SEM) image of the PVDF / APP nanosphere flame retardant composite film prepared in Example 1 of the present invention;

[0041] In the figure, a is a SEM image of the PVDF / APP nanosphere flame retardant composite film at the time of initial combustion, and b is a SEM image of the PVDF / APP nanosphere flame retardant composite film after combustion;

[0042] Figure 5 This is a combustion photograph of the PVDF / APP composite membrane prepared in Comparative Example 1 of the present invention;

[0043] Figure 6 This is a scanning electron microscope (SEM) image of the PVDF / APP composite membrane prepared in Comparative Example 1 of the present invention;

[0044] In the figure, a is a SEM image of the PVDF / APP composite membrane at the time of initial combustion, and b is a SEM image of the PVDF / APP composite membrane after combustion;

[0045] Figure 7 This is a graph of the heat release rate of the soft-pack battery assembled from Comparative Example 1 and Example 1 during combustion;

[0046] In the figure, a is a heat release rate diagram of the soft-pack battery assembled with the PVDF / APP composite film prepared in Comparative Example 1 during combustion, and b is a heat release rate diagram of the soft-pack battery assembled with the PVDF / APP nanosphere flame-retardant composite film prepared in Example 1 during combustion;

[0047] Figure 8 This is a graph showing the total heat release during combustion of the soft-pack battery assembled from Comparative Example 1 and Example 1 of the present invention;

[0048] In the figure, a is a graph showing the total heat release of a soft-pack battery assembled with the PVDF / APP composite film prepared in Comparative Example 1 during combustion, and b is a graph showing the total heat release of a soft-pack battery assembled with the PVDF / APP nanosphere flame-retardant composite film prepared in Example 1 during combustion;

[0049] Figure 9 Photos of heat resistance experiments of the PVDF / APP nanosphere composite film prepared in Example 1 of the present invention and a commercial PP film;

[0050] Figure 10 This is a graph showing the cycle performance of a button cell assembled with a PVDF / APP composite membrane prepared in a comparative example of the present invention at room temperature;

[0051] Figure 11This is a cycle performance diagram of a button battery assembled with the PVDF / APP nanosphere flame retardant composite film prepared in Example 1 of the present invention at room temperature. DETAILED DESCRIPTION

[0052] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0053] The present invention will be further described below with reference to specific examples, but the scope of protection of the present invention is not limited by the following examples. Unless otherwise specified, the materials mainly involved in the examples and comparative examples are conventional commercial products.

[0054] Ammonium polyphosphate 20~30μm CAS Maclean Company.

[0055] Example 1

[0056] The PVDF / APP nanosphere flame-retardant composite film of this embodiment is prepared by the following steps, including:

[0057] S1 13.5g of PVDF powder and 2.7g of APP raw material powder were put into 83.8g of DMF solvent and stirred at room temperature for 48 hours to obtain a casting solution;

[0058] S2. The casting solution of step S1 is spread and solidified on the surface of the coagulation bath solution to obtain a wet PVDF / APP composite membrane; wherein the volume ratio of DMF solvent to H2O in the coagulation bath solution is 65:35;

[0059] S3. The wet PVDF / APP composite film obtained in step S2 is transferred to a flat inorganic glass substrate and fully tiled;

[0060] S4. The surface of the wet PVDF / APP composite membrane fully spread in step S3 is per 100 cm 2 20 mL of the coagulation bath solution in step S2 was added dropwise to the membrane surface to wet the PVDF / APP composite membrane;

[0061] S5. Take another flat inorganic glass substrate used in step S3, completely fit the surface of the PVDF / APP composite film covered in the fully tiled wet state in step S4, and fully wrap the upper and lower glass substrates and the composite film sandwiched in the middle with plastic wrap;

[0062] S6. The package in step S5 is placed in a blast drying oven and kept at 60°C for 20 hours;

[0063] S7. Open the package in step S6, take out the wet PVDF / APP composite membrane sandwiched between the glass substrates, soak and wash it three times with pure water, and then air-dry it at room temperature for 1 hour to obtain the final PVDF / APP nanosphere composite membrane.

[0064] The hydrolysis process of APP in this example at different times in the coagulation bath solution is as follows: Figure 1 As shown in the figure, it can be seen that with the extension of time, the APP raw material gradually changes from micron-sized blocks to nano-sized microspheres.

[0065] The PVDF / APP nanosphere composite membrane prepared in this example was tested by scanning electron microscopy (SEM).

[0066] The test results are as follows Figure 2 As shown. Figure 2 It can be seen that APP nanospheres are evenly distributed on the surface and inside of the composite membrane.

[0067] The PVDF / APP nanosphere composite membrane prepared in this example was cut into strips of 8mm*2mm in size and ignited with a flame to observe the combustion of the composite membrane. The results are as follows: Figure 3 shown.

[0068] Depend on Figure 3 It can be seen that thanks to the good dispersibility and compatibility of the nano-microsphere ammonium polyphosphate, the PVDF / APP nano-microsphere composite membrane of this embodiment exhibits good self-extinguishing property and good dimensional stability.

[0069] The PVDF / APP nanosphere composite membrane prepared in this example was subjected to scanning electron microscopy during initial combustion and after combustion. Figure 4 shown.

[0070] Depend on Figure 4 It can be seen that: during the initial combustion, APP is in the form of nano-scale microspheres and is evenly distributed on the surface of the composite membrane, forming a dense structure. After combustion, the APP nano-scale microspheres on the surface of the composite membrane disappear, and the surface becomes smooth, further improving the density. As can be seen from the illustration, after combustion, the composite membrane structure remains intact.

[0071] Comparative Example 1

[0072] The method for preparing the PVDF / APP composite membrane in this comparative example comprises the following steps:

[0073] S1 13.5g of PVDF powder and 2.7g of APP raw material powder were put into 83.8g of DMF solvent and stirred at room temperature for 48 hours to obtain a casting solution;

[0074] S2. The casting solution of step S1 is spread and solidified on the surface of the coagulation bath solution, and then dried to obtain a PVDF / APP composite membrane; wherein the volume ratio of DMF solvent to H2O in the coagulation bath solution is 65:35.

[0075] The PVDF / APP composite membrane prepared in this comparative example was cut into strips of 8mm*2mm and ignited with a flame to observe the combustion of the composite membrane. The results are as follows: Figure 5 shown.

[0076] Depend on Figure 5 It can be seen that due to the large size and irregular shape of ammonium polyphosphate, the composite films in Comparative Example 1 showed obvious combustion when exposed to flames, showing poor flame retardant properties.

[0077] The PVDF / APP composite membrane prepared in this comparative example was subjected to scanning electron microscopy during initial combustion and after combustion. The results are as follows: Figure 6 shown.

[0078] Depend on Figure 6 It can be seen that the PVDF / APP composite membrane obtained by direct drying without the treatment of steps S3 to S7 of the present invention has APP micron-sized blocks that are not evenly distributed during initial combustion. After combustion, a loose, porous and incomplete structure is formed, that is, it turns into powder.

[0079] The PVDF / APP nanosphere flame retardant composite film prepared in Example 1 and the PVDF / APP composite film prepared in Comparative Example 1 were further used as lithium battery soft pack battery separator materials to test their heat release rate and total heat release. The test method specifically includes: using the PVDF / APP composite film to assemble a 100mm*100mm soft pack battery standard sample, placing the soft pack battery standard sample in a cone calorimeter model NK8115 of Dongguan Nayu Testing Equipment Co., Ltd., setting the power to 35kW and the oxygen content to 20%, collecting the heat release rate and total heat release of the sample during the test, and the test results are as follows: Figures 7 and 8 shown.

[0080] Depend on Figure 6 We can see that compared with Comparative Example 1, the heat release rate of the soft-pack battery combustion test of the composite membrane prepared in Example 1 is lower, and its peak heat release rate is reduced by 45%, while the peak heat release rate of Comparative Example 1 is only reduced by 30%, indicating that the hydrolyzed ammonium polyphosphate composite membrane has better flame retardant effect.

[0081] Depend on Figure 7We can see that compared with Comparative Example 1, the total heat release of the soft-pack battery combustion test of the composite diaphragm prepared in Example 1 is lower, and its total heat release is reduced by 26%, while the total heat release of Comparative Example 1 is reduced by less than 20%, indicating that the hydrolyzed ammonium polyphosphate composite diaphragm has better flame retardant effect.

[0082] It can be seen from this that the PVDF / APP nano-microsphere flame-retardant composite film prepared by the present invention has good self-extinguishing properties, and compared with the membrane material not treated in steps S3 to S7, the peak heat release rate decrease and the total heat release decrease in the soft-pack battery combustion test assembled with the composite film prepared by the present invention are increased by 50% and 30% respectively. Overall, the PVDF / APP nano-microsphere flame-retardant composite film prepared by the present invention has better flame retardant properties.

[0083] The PVDF / APP nanosphere flame retardant composite film prepared in Example 1 was subjected to heat resistance test with a commercially available PP film (Celgard 2500, Suzhou Sinero Technology Co., Ltd.). The test results are shown in Figure 2. Figure 9 shown.

[0084] Depend on Figure 9 It can be seen that the heat resistance of the composite film prepared in the embodiment of the present invention is significantly better than that of the commercial PP film.

[0085] The PVDF / APP composite film prepared in Comparative Example 1 and the PVDF / APP nanosphere flame-retardant composite film prepared in Example 1 were assembled into a lithium battery Li||LFP button cell separator material. The charge-discharge cycle performance of the button cell at room temperature was tested using a LANHE test system (model CT3002A) of Wuhan LANHE Electronics Co., Ltd. The test results are shown in Figure 2. Figure 10 ~11.

[0086] Figure 10 This is a graph showing the cycling performance of a button cell assembled with the PVDF / APP composite membrane prepared in Comparative Example 1 at room temperature. As can be seen from the graph, the battery capacity retention rate fluctuates significantly around 550 cycles, and the coulombic efficiency decreases significantly around 300 cycles.

[0087] Figure 11 This is a graph showing the cycling performance of a button cell assembled with the PVDF / APP nanosphere flame-retardant composite film prepared in Example 1 at room temperature. The graph shows that after 750 cycles, the battery's capacity retention rate is 98%, and its coulombic efficiency is approximately 99%.

[0088] Combine Figures 10-11It can be concluded that the cycle stability of the battery assembled with the PVDF / APP nanosphere composite membrane prepared by the present invention can be improved by more than 130% (number of stable cycle cycles); and the battery capacity retention rate assembled with the PVDF / APP nanosphere composite membrane prepared by the present invention is improved by more than 100% when a high amount of flame retardant is added to the composite membrane; therefore, the PVDF / APP nanosphere composite membrane prepared by the present invention has better electrochemical properties.

[0089] In summary, the present invention successfully hydrolyzes ammonium polyphosphate from large, irregular blocks into small, regularly shaped spheres through a hydrolysis reaction, thereby improving its dispersibility and compatibility with the polymer matrix. The spheres are evenly dispersed in the PVDF porous membrane to prepare a composite flame-retardant diaphragm. The composite diaphragm is then assembled into soft-pack batteries and button batteries, and combustion tests and electrochemical performance tests are performed, respectively, successfully verifying that the diaphragm has both excellent flame retardant properties and good electrochemical properties.

[0090] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A PVDF / APP nanosphere flame retardant composite film, characterized in that: The PVDF / APP nanosphere flame retardant composite film comprises polymer PVDF and flame retardant APP, wherein the flame retardant APP accounts for at most 10% of the mass of the polymer PVDF; The method for preparing the PVDF / APP nanosphere flame retardant composite film comprises the following steps: S1. Dissolve PVDF powder and APP powder in DMF solvent and stir at room temperature to obtain a casting solution; S2, spreading the casting solution of step S1 on the surface of the coagulation bath solution to obtain a wet PVDF / APP composite membrane; S3, transferring the PVDF / APP composite film from step S2 onto a flat inorganic glass substrate and laying it flat; S4, adding the coagulation bath solution of step S2 dropwise onto the surface of the PVDF / APP composite membrane treated in step S3; S5. Take another flat inorganic glass substrate and completely fit it on the surface of the PVDF / APP composite film treated in step S4, and fully wrap the upper and lower glass substrates and the PVDF / APP composite film sandwiched in the middle with plastic wrap; S6, drying and heat-insulating the wrapped article in step S5; S7, opening the package after the treatment in step S6, taking out the PVDF / APP composite membrane sandwiched between the two glass substrates, soaking and washing it three times with pure water, and naturally air-drying it to obtain a PVDF / APP nanosphere composite membrane.

2. The PVDF / APP nanosphere flame retardant composite film according to claim 1, characterized in that: The average pore size of the PVDF / APP nanosphere flame retardant composite membrane is 50-250 nm, and the size of the flame retardant APP is 100-200 nm.

3. A method for preparing the PVDF / APP nanosphere flame retardant composite film according to claim 1 or 2, characterized in that: The method comprises the following steps: S1. Dissolve PVDF powder and APP powder in DMF solvent and stir at room temperature to obtain a casting solution; S2, spreading the casting solution of step S1 on the surface of the coagulation bath solution to obtain a wet PVDF / APP composite membrane; S3, transferring the PVDF / APP composite film from step S2 onto a flat inorganic glass substrate and laying it flat; S4, adding the coagulation bath solution of step S2 dropwise onto the surface of the PVDF / APP composite membrane treated in step S3; S5. Take another flat inorganic glass substrate and completely fit it on the surface of the PVDF / APP composite film treated in step S4, and fully wrap the upper and lower glass substrates and the PVDF / APP composite film sandwiched in the middle with plastic wrap; S6, drying and heat-insulating the wrapped article in step S5; S7, opening the package after the treatment in step S6, taking out the PVDF / APP composite membrane sandwiched between the two glass substrates, soaking and washing it three times with pure water, and naturally air-drying it to obtain a PVDF / APP nanosphere composite membrane.

4. The method according to claim 3, wherein In step S1, the mass ratio of the PVDF powder to the DMF solvent is 1:(6.2-6.3).

5. The method according to claim 4, wherein The stirring time is 45 to 48 hours.

6. The method according to claim 3, wherein In step S2, the coagulation bath solution is prepared by mixing DMF solvent and water; in terms of volume percentage, the volume ratio of the DMF solvent to the water is 60-70:40-30.

7. The method according to claim 3, wherein In step S4, the amount of the coagulation bath solution added is 2 Add 18~24ml to the membrane surface.

8. The method according to claim 3, wherein In step S6, the drying and heat preservation temperature is 50-60° C. and the time is 18-20 hours.

9. The method according to claim 3, wherein In step S7, the natural air drying is carried out for 60 to 90 minutes.

10. Use of the PVDF / APP nanosphere flame retardant composite film according to claim 1 or 2 in lithium batteries.

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