High-safety composite diaphragm and preparation method thereof

Abstract

The invention relates to the technical field of battery composite diaphragms, in particular to a high-safety composite diaphragm and a preparation method thereof. The composite diaphragm comprises a porous polymer base membrane and a composite coating coated on the surface of the porous polymer base membrane, wherein the coating is prepared from the following raw materials: a flame retardant, halloysite nanotubes, a lithium titanium aluminum phosphate solid electrolyte filler and a coupling agent. The halloysite nanotube is used as a nano container to load the flame retardant, can slowly release the flame retardant under the action of high temperature or mechanical force, and cooperates with the lithium titanium aluminum phosphate to improve the ionic conductivity and thermal stability. The preparation method comprises the steps of base film forming, flame retardant loading, powder surface modification, slurry coating and the like. The diaphragm has high ionic conductivity, excellent thermal stability and efficient flame retardant property, and can effectively prevent thermal runaway of the battery and improve the safety of the battery.

Description

High-safety composite membrane and its preparation method Technical Field

[0001] This invention relates to the field of battery composite separator technology, specifically to a high-safety composite separator and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles, energy storage systems, and portable electronic devices, the requirements for energy density, cycle life, and safety of lithium-ion batteries are increasing. As one of the core components of lithium-ion batteries, the battery composite separator plays a crucial role in isolating the positive and negative electrodes, allowing lithium ions to pass freely, and preventing short circuits between the positive and negative electrodes. However, traditional polyolefin composite separators (polyethylene PE, polypropylene PP) are prone to thermal shrinkage at high temperatures, leading to internal short circuits, thermal runaway, and affecting the safety performance of the battery cell.

[0003] To improve the safety of composite separators, methods such as surface coating and the introduction of flame-retardant additives are widely used. Among these, alumina (Al2O3) and silicon dioxide (SiO2) are widely used for surface coating due to their good thermal and mechanical stability. However, Al2O3 and SiO2 also have problems such as easy agglomeration, high brittleness, and insufficient bonding with polyolefin substrates. During long-term cycling, they may actually reduce the mechanical strength of the composite separator, affecting its long-term stability. Directly introducing flame retardants into the composite separator substrate can lead to serious problems such as decreased mechanical properties, low flame-retardant efficiency, and processing difficulties. Using surface coating with flame-retardant layers (such as alumina, polyphosphate, and polyimide) can preserve the excellent mechanical and electrochemical properties of the substrate. However, flame-retardant coatings are usually insulating materials or have a certain degree of density, which increases the ion transport resistance between the electrolyte and the electrodes, leading to increased internal resistance, reduced ionic conductivity, and affecting the battery's rate performance and charge / discharge efficiency.

[0004] In recent years, solid electrolyte materials such as lithium titanium aluminum phosphate (Li 1+x Al x Ti 2-x (PO4)3 (LATP) is considered an ideal functional filler for membranes due to its high ionic conductivity, excellent thermal stability, and wide electrochemical window, which can improve the thermal safety of membranes without sacrificing ion conduction. Meanwhile, halloysite nanotubes (HNTs), a natural nanomaterial, show potential in the field of composite material reinforcement and flame retardancy due to their unique hollow tubular structure, high specific surface area, and intrinsic flame-retardant properties.

[0005] However, current research mostly focuses on the functional modification of single components, and there is no systematic synergistic composite of high-ionic conductors LATP with HNTs to construct a smart flame-retardant separator system with dual triggering release mechanisms of thermal and mechanical response. Therefore, developing a novel composite separator that can maintain high ionic conductivity while possessing excellent thermal stability, mechanical strength, and efficient and long-lasting flame-retardant properties has become a key technological direction for improving the intrinsic safety of lithium-ion batteries. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention aims to provide a high-safety composite diaphragm with high ionic conductivity, excellent mechanical strength, good thermal stability and flame retardant properties, and a method for preparing the same.

[0007] To achieve the above objectives, the present invention provides a high-safety composite membrane, comprising a porous polymer base membrane and a composite coating coated on the surface of the porous polymer base membrane, wherein the raw material components of the composite coating include flame retardants, nano-containers, solid electrolyte fillers, and coupling agents.

[0008] As a preferred embodiment of the present invention, the raw material components of the composite coating have at least one of the following characteristics a1) to a5): a1) the flame retardant is a phosphorus-based flame retardant, including at least one of ammonium polyphosphate and phosphate; a2) the nanocontainer includes halloysite nanotubes (HNTs) with an aspect ratio of 10 to 30; a3) the solid electrolyte filler includes lithium aluminum titanium phosphate (LATP) with a particle size of 50 to 300 nm; a4) the coupling agent includes at least one of silane coupling agents (such as KH-550, KH-570) and titanate coupling agents, used to improve the interfacial bonding between inorganic particles and the polymer matrix; a5) the mass fraction of each raw material component in the porous polymer base film is as follows: flame retardant 2 to 10 wt%, nanocontainer 3 to 15 wt%, solid electrolyte filler 5 to 25 wt%, and coupling agent 0.1 to 1 wt%.

[0009] As a preferred embodiment of the present invention, the porous polymer-based membrane is a polyolefin-based membrane, selected from at least one of polyethylene (PE) and polypropylene (PP).

[0010] As a preferred embodiment of the present invention, the porous polymer base film has a thickness of 5-9 μm, the composite coating has a thickness of 3-10 μm, and the total thickness of the composite diaphragm is 10-20 μm.

[0011] As a preferred embodiment of the present invention, the porous polymer-based membrane has a porosity of 35-60% and an average pore size of 0.1-1.5 μm.

[0012] Secondly, the present invention provides a method for preparing the high-safety composite membrane, comprising the following steps: preparing a porous polymer base membrane; dispersing a nanocontainer in a solvent, adding a flame retardant, and subjecting it to ultrasonic or heat treatment to obtain a flame retardant-loaded nanocontainer; uniformly dispersing a solid electrolyte filler and a flame retardant-loaded nanocontainer in a solvent, adding a coupling agent for surface modification to obtain a composite powder; mixing the composite powder with a solvent to form a slurry, coating it on the surface of the porous polymer base membrane, and drying it to form a composite coating to obtain the composite membrane.

[0013] As a preferred embodiment of the present invention, the porous polymer-based film is manufactured by melt blending, casting, biaxial stretching, and heat setting processes; the biaxial stretching process is as follows: first, longitudinal stretching with a stretching ratio of 1.5 to 3.0, then transverse stretching with a stretching ratio of 2.0 to 4.0, and the stretching temperature is 110 to 140°C; and / or, the heat setting process is as follows: the heat setting temperature is 130 to 150°C, and the duration is 5 to 10 minutes.

[0014] As a preferred embodiment of the present invention, in the ultrasonic or heating treatment process, the ultrasonic frequency is 20~40kHz, the ultrasonic time is 30~60min, the heating reflux temperature is 80~100℃, and the heating reflux time is 3~5h.

[0015] As a preferred embodiment of the present invention, the process for obtaining the composite powder has at least one of the following features b1) to b3): b1) the mass ratio of the solid electrolyte filler to the nanocontainer loaded with flame retardant is 1:1 to 1:3; b2) the dispersion is performed by ultrasonic dispersion, the ultrasonic frequency is 20 to 30 kHz, and the ultrasonic time is 1 to 2 h; b3) the reaction temperature for surface modification is 60 to 80 °C, and the reaction time is 2 to 4 h.

[0016] As a preferred embodiment of the present invention, the solvent includes at least one of water, ethanol, and N,N-dimethylformamide.

[0017] Compared with existing technologies, this invention achieves a synergistic improvement in the safety, electrochemical performance, and structural stability of battery separators by constructing a composite coating on the surface of a porous polymer-based membrane, consisting of halloysite nanotubes (HNTs) loaded with flame retardants, lithium aluminum titanium phosphate (LATP) solid electrolyte, and coupling agents. Specific beneficial effects include: 1. Excellent thermal stability: This invention relies on the high heat resistance of LATP and the tubular structure of HNTs to collaboratively construct a three-dimensional support network. Combined with the mechanical substrate of the polyolefin-based membrane, this significantly suppresses the thermal shrinkage behavior of the separator under high-temperature conditions, blocking the risk of short circuits between the positive and negative electrodes from the structural root, and effectively avoiding battery thermal runaway. Simultaneously, the intrinsic flame retardancy of HNTs and the dense protection of the composite coating further enhance the high-temperature resistance limit of the separator, solving the core problem of traditional polyolefin separators' susceptibility to high-temperature failure.

[0018] 2. Highly efficient flame retardant performance: This invention uses HNTs as a nano-container to load phosphorus-based flame retardants, which avoids the agglomeration problem caused by the direct addition of traditional flame retardants. It also significantly improves the flame retardant efficiency through a dual synergistic mechanism of gas-phase flame retardancy (slow release of flame retardant) + solid-phase flame retardancy (HNTs forming a heat-insulating carbon layer). The flame retardant is confined inside the nanotubes and slowly released at high temperatures, which can prolong the flame retardant time. Furthermore, through the multiple response characteristics of HNTs to thermal and mechanical stimuli (the HNT tube wall structure loosens at high temperatures, releasing the flame retardant; tearing or shearing forces destroy the HNTs structure, releasing the flame retardant, as shown in Figure 4), the slow release of the flame retardant is achieved, prolonging the flame retardant effect.

[0019] 3. Excellent ionic conductivity: This invention selects LATP with a particle size of 50~300nm as a solid electrolyte filler. Its excellent lithium-ion conductivity complements the 35~60% porosity and 0.1~1.5μm average pore size of the porous base membrane, constructing a continuous and efficient ion transport channel. At the same time, the thickness of the composite coating is controlled at 3~10μm (total membrane thickness 10~20μm), avoiding the problem of increased ion transport resistance caused by traditional insulating flame-retardant coatings, significantly reducing the battery internal resistance, improving rate performance and charge-discharge efficiency, and achieving a synergistic unity of high safety and high electrochemical performance.

[0020] 4. Excellent interfacial compatibility: This invention modifies the surface of LATP and HNTs loaded with flame retardants using a silane / titanium ester coupling agent, precisely improving the interfacial bonding force between inorganic particles and the polyolefin base film. This solves the problems of weak bonding force between traditional coatings and substrates and easy detachment during long-term cycling, while also improving the dispersion uniformity of the material and avoiding agglomeration and interfacial defects. At the same time, the tubular structure of HNTs can enhance the mechanical toughness of the coating. Combined with the mechanical strength imparted by the biaxial stretching process of the base film, this ensures the structural integrity of the separator during long-term charge-discharge cycles and extends the battery life.

[0021] 5. The preparation process of this invention is highly compatible with existing membrane production processes. The solvents used are environmentally friendly reagents such as water and ethanol, and the proportions of each component can be flexibly adjusted, making it suitable for large-scale production. The prepared composite membrane has multiple advantages such as high safety, high ionic conductivity, and strong mechanical stability, and can be widely adapted to lithium-ion batteries in new energy vehicles, energy storage systems, portable electronic devices, and other fields, with a wide range of application scenarios. Attached Figure Description

[0022] Figure 1 is a schematic diagram of halloysite nanotubes supporting flame retardants.

[0023] Figure 2 is a schematic diagram of the surface modification of nanocontainers with solid electrolyte filler and flame retardant loading.

[0024] Figure 3 is a schematic diagram of the high-safety composite diaphragm of the present invention.

[0025] Figure 4 is a schematic diagram of the principle of mechanically and thermally responsive slow-release flame retardants.

[0026] Figure reference numerals: 1-Porous polymer base film; 2-Composite coating. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0028] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0029] Example 1 This example provides a method for preparing a high-safety composite diaphragm, including the following steps: Base membrane forming: 100g of PE is extruded by a twin-screw extruder, cast, and stretched in two steps (longitudinal stretching ratio 1.8, transverse stretching ratio 2.5) at a stretching temperature of 120°C. After stretching, heat setting is performed at a heat setting temperature of 140°C for 8 minutes to obtain a porous polymer base membrane 1 with a thickness of 9μm, a porosity of 38%, and an average pore size of 0.6μm.

[0030] HNTs loaded with flame retardant: 8g of HNTs (length-to-diameter ratio 20) were dispersed in ethanol, 5g of ammonium polyphosphate was added, and the mixture was sonicated at a frequency of 25kHz for 50min. The mixture was then centrifuged at 8000rpm for 15min at room temperature, washed 2-3 times with deionized water, and freeze-dried at -10℃ for 24h to obtain HNTs loaded with flame retardant, denoted as HNTs-R, as shown in Figure 1.

[0031] Pretreatment of composite coating material: 10g LATP powder (particle size 150nm) and 10g HNTs-R were added to 100mL ethanol and dispersed by ultrasonication for 1.5 hours at an ultrasonic frequency of 20kHz. Then, silane coupling agent KH-550 was added and reacted in a 60℃ water bath for 3 hours. After centrifugation at 6000rpm for 10min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 24h to obtain surface-modified LATP / HNTs-R composite powder, as shown in Figure 2.

[0032] Composite coating: 10g of surface-modified LATP / HNTs-R composite powder was dispersed in 100mL of ethanol to prepare a slurry. The LATP / HNTs-R slurry was uniformly sprayed onto the porous polymer base membrane using a spraying process. After drying, LATP / HNTs-R composite coating 2 was formed with a thickness of 8μm, thus obtaining a high-safety composite membrane, as shown in Figure 3.

[0033] Example 2 This example provides a method for preparing a high-safety composite diaphragm, including the following steps: Base membrane forming: 100g of PE is extruded by a twin-screw extruder, cast, and stretched in two steps (longitudinal stretching ratio 1.6, transverse stretching ratio 3.0) at a stretching temperature of 130℃. After stretching, heat setting is performed at a heat setting temperature of 140℃ for 10 minutes to obtain a porous polymer base membrane 1 with a thickness of 9μm, a porosity of 37%, and an average pore size of 0.7μm.

[0034] HNTs loaded with flame retardant: 6g of HNTs (length-to-diameter ratio 22) were dispersed in ethanol, 5g of ammonium polyphosphate was added, and the mixture was sonicated at 30kHz for 30min. The mixture was then centrifuged at 9000rpm for 10min at room temperature, washed 2-3 times with deionized water, and freeze-dried at -10℃ for 24h to obtain HNTs loaded with flame retardant, denoted as HNTs-R, as shown in Figure 1.

[0035] Pretreatment of composite coating material: 8g LATP powder (particle size 150nm) and 12g HNTs-R were added to 100mL ethanol and dispersed by ultrasonication for 1.5 hours at an ultrasonic frequency of 20kHz. Then, silane coupling agent KH-550 was added and reacted in a 60℃ water bath for 2.5 hours. After centrifugation at 7000rpm for 10min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 24h to obtain surface-modified LATP / HNTs-R composite powder, as shown in Figure 2.

[0036] Composite coating: 15g of surface-modified LATP / HNTs-R composite powder was dispersed in 120mL of ethanol to prepare a slurry. The LATP / HNTs-R slurry was uniformly sprayed onto the porous polymer base membrane using a spraying process. After drying, LATP / HNTs-R composite coating 2 was formed with a thickness of 9μm, thus obtaining a high-safety composite membrane, as shown in Figure 3.

[0037] Example 3 This example provides a method for preparing a high-safety composite diaphragm, including the following steps: Base membrane forming: 100g of PE is extruded by a twin-screw extruder, cast, and stretched in two steps (longitudinal stretching ratio 2.6, transverse stretching ratio 2.4) at a stretching temperature of 120℃. After stretching, heat setting is performed at a heat setting temperature of 135℃ for 10 minutes to obtain a porous polymer base membrane 1 with a thickness of 7μm, a porosity of 40%, and an average pore size of 1.3μm.

[0038] HNTs loaded with flame retardant: 10g of HNTs (length-to-diameter ratio 15) were dispersed in ethanol, 8g of ammonium polyphosphate was added, and the mixture was sonicated at a frequency of 20kHz for 60min. After centrifugation at 9000rpm for 10min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 24h to obtain HNTs loaded with flame retardant, denoted as HNTs-R, as shown in Figure 1.

[0039] Pretreatment of composite coating material: 10g LATP powder (particle size 100nm) and 12g HNTs-R were added to 100mL N,N-dimethylformamide and dispersed by ultrasonication for 2 hours at an ultrasonic frequency of 20kHz. Then, silane coupling agent KH-550 was added and reacted in a water bath at 65℃ for 3 hours. After centrifugation at 7000rpm for 8min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 36h to obtain surface-modified LATP / HNTs-R composite powder, as shown in Figure 2.

[0040] Composite coating: 10g of surface-modified LATP / HNTs-R composite powder was dispersed in 100mL of ethanol to prepare a slurry. The LATP / HNTs-R slurry was uniformly sprayed onto the porous polymer base membrane using a spraying process. After drying, LATP / HNTs-R composite coating 2 was formed with a thickness of 10μm, thus obtaining a high-safety composite membrane, as shown in Figure 3.

[0041] Example 4 This example provides a method for preparing a high-safety composite diaphragm, including the following steps: Base membrane forming: 100g of PP is extruded by a twin-screw extruder, cast, and stretched in two steps (longitudinal stretch ratio 2.9, transverse stretch ratio 3.5) at a stretching temperature of 120℃. After stretching, it is heat-set at a temperature of 140℃ for 10 minutes to obtain a porous polymer base membrane 1 with a thickness of 7μm, a porosity of 40%, and an average pore size of 1.4μm.

[0042] HNTs loaded with flame retardant: 10g of HNTs (length-to-diameter ratio 20) were dispersed in ethanol, 8g of ammonium polyphosphate was added, and the mixture was sonicated at a frequency of 25kHz for 60min. After centrifugation at 8000rpm for 12min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 24h to obtain HNTs loaded with flame retardant, denoted as HNTs-R, as shown in Figure 1.

[0043] Pretreatment of composite coating material: 10g LATP powder (particle size 100nm) and 12g HNTs-R were added to 100mL N,N-dimethylformamide and dispersed by ultrasonication for 2 hours at an ultrasonic frequency of 25kHz. Then, titanate coupling agent was added and reacted in a 60℃ water bath for 3 hours. After centrifugation at 8000rpm for 10min, the mixture was washed 2-3 times with deionized water and freeze-dried at -10℃ for 36h to obtain surface-modified LATP / HNTs-R composite powder, as shown in Figure 2.

[0044] Composite coating: 8g of surface-modified LATP / HNTs-R composite powder was dispersed in 100mL of ethanol to prepare a slurry. The LATP / HNTs-R slurry was uniformly sprayed onto the porous polymer base membrane using a spraying process. After drying, LATP / HNTs-R composite coating 2 was formed with a thickness of 7μm, thus obtaining a high-safety composite membrane, as shown in Figure 3.

[0045] Comparative Example 1 is basically the same as Example 1, except that no flame retardant is added.

[0046] Comparative Example 2 is basically the same as Example 1, except that HNTs are not added.

[0047] Comparative Example 3 is basically the same as Example 1, except that no coupling agent is added.

[0048] Comparative Example 4 is basically the same as Example 1, except that the thickness of the composite coating is 20 μm.

[0049] Comparative Example 5 is basically the same as Example 1, except that the thickness of the composite coating is 0.5 μm.

[0050] Comparative Example 6 uses a traditional polyolefin composite membrane with a base membrane thickness of 9 μm and a 3 μm alumina (D) coating on one side. 50 =0.6μm), single-sided spraying of 2μm PVDF (0.5g / m²) 2 ).

[0051] In the experimental examples, the materials prepared in Examples 1-4 and Comparative Examples 1-6 were tested or assembled into lithium-ion coin cells for electrochemical performance determination. The specific testing methods are as follows: 1. Ionic conductivity testing method: AC impedance method. The separator was assembled into a symmetrical cell. Test conditions: 25°C, frequency range 10 Hz. 6 Hz~10 -2 Hz, amplitude 10mV, calculation formula: Where: σ: ionic conductivity (S / cm); L: membrane thickness (cm); R: bulk resistance obtained from impedance spectroscopy (Ω); A: effective electrode area (cm²). 2 ).

[0052] 2. Heat Shrinkage Rate: After folding the diaphragm flat, cut a 10cm*10cm sample using a mold along the transverse direction. Coat the diaphragm with a film, fold it in half along the longitudinal direction, and cut a sample larger than 10cm*10cm. Use a steel ruler to measure 10cm*10cm cross lines and mark the transverse (TD) and longitudinal (MD) of the diaphragm. Turn on the oven power, set the temperature, and after the temperature stabilizes, flatten the sample between A4 paper, cover the top and bottom of the A4 paper with 8mm thick glass plates, and place it in the oven and start timing. After the required time is reached, remove the sample and allow it to return to room temperature. Then, use a film ruler or a fully automatic image measuring projector to measure the transverse and longitudinal lengths again and calculate the transverse and longitudinal shrinkage rates.

[0053] 3. Puncture strength is tested using a puncture tester. Puncture needle diameter: Ф=1.0mm; puncture speed: v=200mm / min; place the sample to be tested on the sample stage, ensuring that the sample is in the center and the membrane surface is flat and wrinkle-free. During measurement, the needle slowly descends, automatically returns to its original position after puncturing the diaphragm, and the system automatically records the test value.

[0054] 4. Tensile Strength and Elongation at Break: Cut 15mm wide samples from each roll of diaphragm along both the longitudinal and transverse directions. Using a tensile testing machine, place the cut longitudinal and transverse samples onto the upper and lower clamps of the machine, ensuring the samples are flat, wrinkle-free, vertical, and without tilting. Set the sample width and thickness (input the base film thickness during testing), with a distance of 100mm between the upper and lower clamps. For transverse widths less than 100mm, the distance between the upper and lower clamps should be 40mm. The testing speed should be 200mm / min. The system automatically records the test values.

[0055] 5. Vertical combustion test method for flame retardant performance: Fix the diaphragm sample vertically, ignite its lower end with a flame, and record the flame self-extinguishing time, whether it drips, and whether it ignites the degreased cotton below.

[0056] Tables 1 and 2 below show the button cell test data prepared in Examples 1-4 and Comparative Examples 1-6.

[0057] Table 1 Table 2 Based on the experimental data from Examples 1-4, Comparative Examples 1-6, and Tables 1 and 2, the following conclusions can be drawn: the high-safety composite membrane of the present invention exhibits excellent overall performance. Regarding ionic conductivity, the ionic conductivity of all embodiments is between 4.9 × 10⁻⁶. -4 ~6.3×10 -4 The S / cm ratio is higher than that of conventional coated separators, which is beneficial for high-rate charge and discharge of batteries. It exhibits significant thermal stability; after heating at 120℃ for 1 hour, the longitudinal and transverse thermal shrinkage rates of the separator are both less than 1%, far superior to traditional polyolefin separators, effectively suppressing dimensional changes and short-circuit risks at high temperatures. It demonstrates outstanding flame retardant performance, with a self-extinguishing time of no more than 3 seconds, indicating a rapid response of the flame-retardant system. In terms of mechanical strength, the puncture strength is between 486 and 581 gf, and the longitudinal tensile strength is 2834–3523 kgf / cm². 2 The horizontal range is 1911–2234 kgf / cm². 2 The elongation at break also remained at a reasonable level, indicating that the diaphragm has good resistance to mechanical damage and toughness.

[0058] Each component plays a crucial and irreplaceable role in the membrane. The flame retardant (phosphorus-based) provides gas-phase flame retardancy, significantly shortening the self-extinguishing time; in Comparative Example 1, lacking the flame retardant, the self-extinguishing time is extended to over 5 seconds, and the thermal shrinkage rate increases. Halloysite nanotubes (HNTs), as nano-containers supporting the flame retardant, achieve a sustained-release effect, while their tubular structure enhances thermal stability and mechanical strength; in Comparative Example 2, the absence of HNTs leads to an increased thermal shrinkage rate of 2.3% and a decrease in puncture strength, confirming the importance of HNTs. Lithium aluminum titanium phosphate (LATP) solid electrolyte filler improves ionic conductivity and synergistically constructs a three-dimensional network with HNTs, further stabilizing the membrane structure. Coupling agents (such as KH-550) improve the interfacial bonding between inorganic particles and the polymer-based membrane, enhancing coating adhesion; in Comparative Example 3, the absence of coupling agents significantly reduces tensile strength and elongation at break, indicating poorer interfacial bonding. The coating thickness needs to be precisely controlled. In the examples, the coating thickness is 3 to 10 micrometers (total thickness 10 to 20 micrometers) and the performance is good. In Comparative Example 4, the coating is too thick (20 micrometers) and the ionic conductivity decreases. In Comparative Example 5, the coating is too thin (0.5 micrometers) and the thermal shrinkage rate increases and the mechanical strength is insufficient.

[0059] Significant synergistic effects exist among the components. The combination of HNTs and flame retardants achieves a dual mechanism of slow-release flame retardancy and solid-phase thermal insulation, improving flame retardant efficiency and durability. HNTs and LATP synergistically construct a three-dimensional thermally stable network, maintaining high ionic conductivity while inhibiting thermal shrinkage. The coupling agent enhances the interfacial bonding between inorganic particles and the base membrane through surface modification, preventing coating detachment and improving cycle stability. The biaxially stretched base membrane combined with the composite coating provides basic mechanical strength, while the coating enhances thermal stability and flame retardancy, resulting in excellent mechanical and thermal safety performance of the diaphragm.

[0060] In summary, this high-safety composite separator achieves an excellent balance between ionic conductivity, thermal stability, flame retardancy, and mechanical strength through multi-component synergistic design and precise process control. It is suitable for lithium-ion battery fields with high safety requirements, such as new energy vehicles and energy storage systems, and has broad prospects for industrial application.

[0061] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-safety composite diaphragm, characterized in that, It includes a porous polymer base membrane and a composite coating applied to the surface of the porous polymer base membrane. The raw material components of the composite coating include flame retardants, nanocontainers, solid electrolyte fillers, and coupling agents.

2. The high-safety composite diaphragm according to claim 1, characterized in that, The raw material components of the composite coating have at least one of the following characteristics: a1) to a5) a1) the flame retardant is a phosphorus-based flame retardant, including at least one of ammonium polyphosphate and phosphate; a2) the nanocontainer includes halloysite nanotubes with an aspect ratio of 10 to 30; a3) the solid electrolyte filler includes lithium aluminum titanium phosphate with a particle size of 50 to 300 nm; a4) the coupling agent includes at least one of silane coupling agent and titanate coupling agent; a5) the mass fraction of each raw material component in the porous polymer base film is as follows: flame retardant 2 to 10 wt%, nanocontainer 3 to 15 wt%, solid electrolyte filler 5 to 25 wt%, and coupling agent 0.1 to 1 wt%.

3. The high-safety composite diaphragm according to claim 1, characterized in that, The porous polymer-based membrane is a polyolefin-based membrane, selected from at least one of polyethylene and polypropylene.

4. The high-safety composite diaphragm according to claim 1, characterized in that, The porous polymer base membrane has a thickness of 5–9 μm, the composite coating has a thickness of 3–10 μm, and the total thickness of the composite diaphragm is 10–20 μm.

5. The high-safety composite diaphragm according to claim 1, characterized in that, The porous polymer-based membrane has a porosity of 35-60% and an average pore size of 0.1-1.5 μm.

6. A method for preparing a high-safety composite separator as described in any one of claims 1 to 5, characterized in that, Includes the following steps: A porous polymer-based membrane is prepared; a nanocontainer is dispersed in a solvent, a flame retardant is added, and the nanocontainer is subjected to ultrasonic or heat treatment to obtain a flame retardant-loaded nanocontainer; a solid electrolyte filler and a flame retardant-loaded nanocontainer are uniformly dispersed in a solvent, and a coupling agent is added for surface modification to obtain a composite powder; the composite powder is mixed with a solvent to form a slurry, which is coated on the surface of the porous polymer-based membrane and dried to form a composite coating, thus obtaining the composite diaphragm.

7. The preparation method according to claim 6, characterized in that, The porous polymer-based film is made by melt blending, casting, biaxial stretching and heat setting processes; the biaxial stretching process is as follows: first longitudinal stretching with a stretch ratio of 1.5 to 3.0, then transverse stretching with a stretch ratio of 2.0 to 4.0, and the stretching temperature is 110 to 140°C; and / or, the heat setting process is as follows: heat setting temperature is 130 to 150°C, and the duration is 5 to 10 minutes.

8. The preparation method according to claim 6, characterized in that, In the ultrasonic or heat treatment process, the ultrasonic frequency is 20~40kHz, the ultrasonic time is 30~60min, the heating reflux temperature is 80~100℃, and the heating reflux time is 3~5h.

9. The preparation method according to claim 6, characterized in that, The process for obtaining the composite powder has at least one of the following characteristics b1) to b3): b1) the mass ratio of the solid electrolyte filler to the nanocontainer loaded with flame retardant is 1:1 to 1:3; b2) the dispersion is performed by ultrasonic dispersion, with an ultrasonic frequency of 20 to 30 kHz and an ultrasonic time of 1 to 2 h; b3) the reaction temperature for surface modification is 60 to 80 °C and the reaction time is 2 to 4 h.

10. The preparation method according to claim 6, characterized in that, The solvent includes at least one of water, ethanol, and N,N-dimethylformamide.

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