Self-supporting composite separator with high thermal conductivity and preparation method and application thereof

Abstract

The application belongs to the technical field of secondary batteries, and specifically discloses a self-supporting composite diaphragm with high thermal conductivity and a preparation method and application thereof. The diaphragm comprises a polymer matrix with interpenetrating pores and high-thermal-conductivity fillers distributed in the polymer matrix. The interpenetrating pores formed inside constitute a three-dimensional porous framework. The high-thermal-conductivity fillers are distributed in the pores of the three-dimensional porous framework of the polymer matrix and are in mutual contact with the inside and / or surface of the three-dimensional porous framework in the in-plane direction. A continuous distribution of thermal conduction networks is formed through face-face stacking and / or edge-face lapping, so that the diaphragm forms a two-dimensional thermal conduction path in the in-plane direction. This structure significantly reduces the interface thermal resistance of the heat conduction process, endows the diaphragm with excellent active heat dissipation capacity, and can quickly and uniformly disperse the local heat accumulation generated in the battery charging and discharging process in the plane direction, thereby effectively preventing local failure induced by heat and greatly improving the thermal safety of high-specific-energy batteries.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a self-supporting composite separator with high thermal conductivity, its preparation method, and its application. Background Technology

[0002] With the rapid development of new energy vehicles and the large-scale energy storage market, the demand for high-energy-density batteries is becoming increasingly urgent. However, the increase in battery energy density is often accompanied by greater safety risks, with thermal runaway being the core of battery safety issues. As a key component between the positive and negative electrodes, the separator's performance directly affects battery safety and cycle life. Commercially available polyolefin separators, such as polyethylene (PE) and polypropylene (PP), while possessing good mechanical strength and electrochemical stability, have low melting points (PP approximately 165 ℃, PE approximately 135 ℃) and poor thermal stability. Under conditions of localized overheating inside the battery or high external temperatures, the polyolefin separator undergoes severe melting and shrinkage, leading to direct contact between the positive and negative electrodes and a short circuit, triggering thermal runaway or even fire and explosion. Furthermore, during fast charging or high-rate discharging, uneven deposition of the negative electrode metal can cause excessively high local current densities, generating significant heat. If this heat is not promptly dissipated or homogenized, it will accelerate dendrite growth, side reactions, and separator aging, creating a vicious cycle. Even more seriously, when the internal temperature of the battery reaches the melting point of the polyolefin separator (135-165 ℃), the separator will melt and shrink significantly, losing its electronic insulation properties. This directly leads to a large-area short circuit between the positive and negative contacts, instantly releasing enormous heat and rapidly triggering thermal runaway. This is a fundamental safety defect that current commercial separators cannot solve.

[0003] To improve the thermal stability of separators, researchers have explored various methods, such as coating the surface of polyolefin separators with inorganic ceramic particles like alumina (Al2O3) and silicon dioxide (SiO2). While these coatings improve the high-temperature resistance of the separator to some extent, their primary function is passive resistance to high temperatures. Their effectiveness in actively dissipating and homogenizing heat within the battery, and in preventing overall shrinkage of the separator substrate at high temperatures, is limited. Furthermore, if the coating is not dense enough or poorly bonded to the substrate, shrinkage remains a risk. Additionally, some inorganic fillers have poor compatibility with the electrolyte, potentially increasing interfacial impedance. Moreover, these coating methods primarily focus on creating a base film first, then coating it. There is a lack of innovation in directly and uniformly embedding thermally conductive materials into the bulk phase through a one-step wet process to form an independent, self-supporting composite separator.

[0004] Therefore, how to construct a high-efficiency heat conduction channel with low thermal resistance while maintaining the self-supporting mechanical strength and high porosity of the separator, and endow the separator with structural and chemical stability at extreme temperatures, has become a technical problem that urgently needs to be solved in the field of thermal management of secondary batteries. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned shortcomings of existing technologies by providing a self-supporting composite separator with high thermal conductivity, its preparation method, and its application. This invention overcomes the fatal flaws of existing commercial polyolefin separators, such as pore closure at high temperatures leading to obstructed ion transport and easy melting and shrinkage causing short circuits between the positive and negative electrodes. The invention provides a high thermal conductivity self-supporting battery separator based on a high thermal conductivity filler. This separator not only passively withstands high temperatures but also actively conducts heat to equalize the temperature. Most importantly, it maintains structural stability under high-temperature conditions, fundamentally avoiding pore closure and internal short circuits caused by separator shrinkage.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A first aspect of the present invention is to provide a self-supporting composite membrane with high thermal conductivity, comprising:

[0008] The polymer matrix has a three-dimensional porous framework formed by solvent-induced phase separation;

[0009] A high thermal conductivity filler is confined and distributed within the three-dimensional porous framework of the polymer matrix;

[0010] The high thermal conductivity filler is constructed by face-to-face stacking or edge-to-face overlapping inside and on the surface of the three-dimensional porous skeleton to form a two-dimensional thermal conductivity path that runs through the in-plane direction of the self-supporting composite membrane.

[0011] The two-dimensional thermal conduction pathway ensures that the dimensional change rate of the self-supporting composite membrane is less than 5% after being placed at 220°C for 30 minutes, and that the in-plane thermal conductivity of the self-supporting composite membrane is ≥85 W / (m·K) at 60°C.

[0012] Furthermore, the mass of the high thermal conductivity material is 5% to 60% of the total mass of the self-supporting composite diaphragm.

[0013] Furthermore, the overall thickness of the self-supporting composite membrane is 10~100μm, the porosity is 30%~60%, and the average pore size is 200nm~1μm.

[0014] Furthermore, the high thermal conductivity filler includes any one of boron nitride, aluminum nitride, and beryllium oxide, and its morphology is selected from zero-dimensional particles, one-dimensional fibers, and two-dimensional sheets.

[0015] Furthermore, the high thermal conductivity filler has a two-dimensional sheet structure with a lateral dimension of 100nm~10μm and a thickness of 1nm~100nm.

[0016] Furthermore, the polymer matrix is ​​selected from one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyimide (PI), polyethersulfone, polyacrylonitrile, and polyethylene oxide.

[0017] A second aspect of the present invention is to provide a method for preparing the self-supporting composite membrane, comprising:

[0018] Slurry preparation involves dispersing the polymer and / or highly thermally conductive filler in an organic solvent to form a slurry.

[0019] Film formation: The obtained slurry is formed into a film according to any of the following methods (A1)-A4):

[0020] A1) Casting-phase separation: A slurry is coated onto a substrate, and then immersed in a non-solvent containing the organic solvent for phase separation and curing to form a porous membrane;

[0021] A2) Electrospinning, which involves spinning slurry into fiber membranes using electrospinning equipment;

[0022] A3) Mold casting: The slurry is injected into the mold, and the initial film is formed by controlling the casting thickness and environmental conditions;

[0023] A4) Coating: Applying the slurry directly to the surface of a commercial substrate diaphragm;

[0024] The post-processing involves drying the resulting membrane and then roll-forming it to obtain the self-supporting composite diaphragm.

[0025] Furthermore, the organic solvent is selected from one or more of acetone, acetonitrile, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, or tetrahydrofuran.

[0026] A third aspect of the present invention is to provide a secondary battery that uses the above-mentioned self-supporting composite separator as an electronic insulation and thermal management component between the positive and negative electrodes.

[0027] Furthermore, the secondary battery includes a lithium-ion battery, a lithium metal battery, a sodium-ion battery, a sodium metal battery, or a lithium-sulfur battery.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] (1) The self-supporting composite separator provided by the present invention constructs an efficient and continuous two-dimensional thermal conduction path in the in-plane direction of the separator by face-to-face stacking or edge-to-face overlapping of high thermal conductivity fillers in the three-dimensional porous skeleton of the polymer matrix. This structure significantly reduces the interfacial thermal resistance in the heat conduction process, endows the separator with excellent active heat dissipation capability, and can quickly and uniformly disperse the local heat accumulation generated during battery charging and discharging along the planar direction, thereby effectively preventing thermally induced local failures and greatly improving the thermal safety of high-energy-density batteries.

[0030] (2) The self-supporting composite separator provided by the present invention has excellent high-temperature structural stability. Thanks to the extremely high melting point and chemical inertness of the high thermal conductivity material itself, as well as the stable composite structure formed by it with high-temperature resistant polymers (such as polyvinylidene fluoride, polyimide, polyethersulfone, polyacrylonitrile, etc.), the separator can maintain its basic structure and size (area shrinkage rate <5%) even after long-term treatment at 220 ℃ or even higher temperatures. This ensures the integrity of the physical barrier between the positive and negative electrodes of the battery, prevents the occurrence of internal short circuits, and completely solves the core thermal safety problem of short circuits caused by high-temperature melting and shrinkage of commercial PP / PE separators.

[0031] (3) The self-supporting composite membrane provided by the present invention can indirectly inhibit the growth of lithium dendrites in the process of efficient thermal management. The high thermal conductivity material itself does not react with the electrolyte or electrode material, and maintains the stability of the interface. At the same time, the controllable porous structure formed during the preparation process ensures that the electrolyte is fully wetted, provides sufficient ion transport channels, and takes into account both high ion conductivity and mechanical strength.

[0032] (4) The preparation method provided by this invention covers a variety of film-forming technologies, which can be flexibly selected according to performance requirements and production costs, and are easy to industrialize. The prepared separator is suitable for various systems such as lithium-ion, lithium metal, sodium batteries, and lithium-sulfur batteries.

[0033] (5) Through precise control of the microstructure, the present invention endows the separator with the ability to suppress thermal shrinkage at extremely high temperatures and the breakthrough heat dissipation efficiency, providing a brand-new solution to the thermal safety bottleneck of high-energy-density secondary batteries. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of the self-supporting composite diaphragm provided by the present invention;

[0035] Figure 2 This is a schematic diagram of the preparation process of the h-BN nanosheet self-supporting composite membrane in Example 1 of the present invention, illustrating the process of preparing a porous composite membrane and uniformly dispersing a highly thermally conductive filler.

[0036] Figure 3 These are optical and scanning electron microscope (SEM) images of Example 1 of the h-BN nanosheet self-supporting composite membrane of the present invention, wherein... Figure 1 In the image, A is an optical photograph, B is a surface SEM photograph, and C is a cross-sectional SEM photograph. The optical photograph shows the feasibility of the process, and the SEM photograph shows that the morphology and structure of the diaphragm are consistent with the design goals.

[0037] Figure 4 The optical photographs taken during the contact angle test show that the h-BN nanosheet self-supporting composite membrane of the present invention in Example 1 and the commercial PP membrane in Comparative Example 1 have better wetting performance.

[0038] Figure 5 The images show the size comparison between Example 1 of the h-BN nanosheet self-supporting composite membrane of the present invention and Comparative Example 1 of a commercial PP membrane, taken at initial room temperature; and the size comparison images taken after being gradually heated in an argon atmosphere and held at three temperature nodes of 120 ℃, 170 ℃ and 220 ℃ for 30 minutes each, respectively. The composite membrane shows dimensional stability under high temperature conditions.

[0039] Figure 6 The image shows a SEM image of the lithium metal surface of a coin cell assembled using the h-BN nanosheet self-supporting composite separator of the present invention after cycling. The lithium metal opposite the composite separator shows a flatter metal deposition.

[0040] Figure 7 To compare the cycling stability at high temperatures of coin cell Example 4, assembled using the h-BN nanosheet self-supporting composite separator of Example 1 of the present invention, and coin cell Example 2, assembled using a commercial PP separator (Celgard 2500) as Comparative Example 1, the following comparisons are made: Figure 7 In the diagram, A shows a panoramic optical photograph and a thickness measurement optical photograph of the two types of separators; B shows a 1C rate cycling diagram of the two types of coin cells at 60 ℃. Figure 7 C in the text is an excerpt of the charge-discharge curve from Example 4 in B. Figure 7 D in the figure is an excerpt of the charge-discharge curve of Comparative Example 2 in B. The coin cell assembled with composite separator shows better capacity retention in repeated charge-discharge cycles.

[0041] Figure 8 For Example 4, a 1 Ah soft-pack battery assembled using the h-BN nanosheet self-supporting composite separator of the present invention, and Comparative Example 2, a 1 Ah soft-pack battery assembled using the commercial PP separator, thermal images of the soft-pack batteries were taken at different cycling times when cycling at 5 C rate. The soft-pack battery assembled using the composite separator significantly alleviated the heat accumulation problem of the battery under high rate operation.

[0042] Figure 9 To compare the safety performance of a 1 Ah pouch cell (Example 4) assembled using the h-BN nanosheet self-supporting composite separator of Example 1 of the present invention with that of a 1 Ah pouch cell (Comparative Example 2) assembled using a commercial PP separator (Comparative Example 1), an accelerated calorimeter (ARC) was used to compare the batteries under thermal abuse conditions. Figure 9In the figures, A is an optical photograph of the pouch cell in Example 4 after ARC testing, B is an optical photograph of the pouch cell in Comparative Example 2 after ARC testing, and C is the ARC results using different separators. The ARC test results show that the pouch cell assembled with a composite separator improves the self-heating start temperature, thermal runaway initiation temperature, and maximum temperature of the battery. The boron nitride insulation layer inside the burned pouch cell was preserved, which is consistent with the design goal. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the specific embodiments and accompanying drawings are described in further detail below. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0044] Terminology Explanation:

[0045] In this invention, "self-supporting composite membrane" refers to a composite membrane structure that can maintain its overall shape stability and meet usage requirements by relying on its own structure without relying on an additional rigid support layer or external frame.

[0046] In this invention, "polymer matrix" refers to the continuous phase material constituting the membrane body, which has an interconnected porous structure inside.

[0047] In this invention, the "three-dimensional porous framework" refers to a continuous spatial network structure composed of the solid part formed by the polymer matrix and its internal interconnected pores. Macroscopically, this structure extends continuously in the in-plane and thickness directions inside the membrane.

[0048] In this invention, "high thermal conductivity material" refers to a material that has a high thermal conductivity and is electrically insulating.

[0049] In this invention, "thermal conductive network" refers to a continuous thermal conductive structure formed by multiple highly thermally conductive fillers in contact with each other in the in-mold direction.

[0050] In this invention, "through" means that the heat network extends continuously in the direction inward of the diaphragm surface and covers the effective area of ​​the diaphragm, rather than requiring that a completely continuous heat conduction path be formed at every local location of the diaphragm.

[0051] In this invention, "in-plane direction" refers to the direction parallel to the large surface of the diaphragm, that is, the plane formed by the X and Y axes, which includes the length and width directions of the diaphragm. "Out-of-plane direction" refers to the direction perpendicular to the diaphragm surface, that is, the Z-axis direction, which is usually the thickness direction of the diaphragm.

[0052] refer to Figure 1This invention provides a self-supporting composite membrane with high thermal conductivity, comprising a polymer matrix with interconnected pores and a high thermal conductivity filler distributed within the polymer matrix. The interconnected pores formed within the polymer matrix constitute a three-dimensional porous framework, which provides a continuous spatial support and confinement structure for the high thermal conductivity filler. This spatial confinement effect guides the high thermal conductivity filler to preferentially distribute along the in-plane direction within the membrane and suppresses its disordered accumulation in the thickness direction.

[0053] When the high thermal conductivity filler is preferably a material with a sheet-like or layered structure, under the aforementioned spatial confinement effect, adjacent fillers can easily form a continuous contact relationship through surface-to-surface stacking and / or edge-to-surface overlap, thereby providing a continuous path for heat conduction in the in-plane direction of the diaphragm. From a physical mechanism perspective, this ensures that the formation of the heat conduction network is inevitable rather than accidental.

[0054] Based on the above formation mechanism, by introducing a highly thermally conductive material into a polymer matrix with interconnected pores, the highly thermally conductive filler is distributed within the pores and / or on the surface of the three-dimensional porous framework, forming a stable contact with the porous framework. With the continuous distribution of the highly thermally conductive filler within the porous framework, the face-to-face and / or edge-to-face contacts between adjacent fillers gradually extend and interconnect in the in-plane direction of the membrane, thereby constructing a continuously distributed thermally conductive network within the membrane, resulting in a stable two-dimensional thermally conductive pathway in the in-plane direction of the self-supporting composite membrane.

[0055] It should be noted that this invention does not limit the specific method of introducing the high thermal conductivity filler into the three-dimensional porous framework. The high thermal conductivity filler can be introduced before the formation of the porous structure in the polymer matrix, or it can be introduced after the formation of the three-dimensional porous framework through filling, impregnation, or other methods. Similarly, the three-dimensional porous framework can be formed by phase separation, template casting, electrospinning, or other conventional methods that can form interconnected pores. Regardless of the method used, as long as an interconnected three-dimensional porous framework can be formed in the polymer matrix, and the high thermal conductivity filler is distributed in the three-dimensional porous framework, and a continuous contact relationship is formed in the in-plane direction of the membrane through surface-to-surface contact and / or edge-to-surface contact, the two-dimensional thermal conductivity pathway described in this invention can be achieved.

[0056] Furthermore, the formation of the two-dimensional thermally conductive pathway provided by this invention does not depend on precise control of the content, pore size, or specific arrangement of the high thermal conductivity filler. Even when there are variations in the filler distribution density or differences in the local structure of the porous framework, as long as the three-dimensional porous framework maintains an interconnected spatial structure and the high thermal conductivity filler is continuously distributed within the framework, the two-dimensional thermally conductive network can still form and function as a thermal conductor in the in-plane direction of the membrane. The two-dimensional thermally conductive network provided by this invention enables the self-supporting composite membrane to exhibit a dimensional change rate of <5% after being placed at 220°C for 30 minutes. The overall thickness of the self-supporting composite membrane is 10~100μm, the porosity is 30%~60%, and the average pore size is 200nm~1μm.

[0057] It should also be noted that the formation of the aforementioned thermally conductive network is not limited to specific types, sizes, or preparation methods of high thermal conductivity fillers, nor is it limited to specific pore sizes or molding processes of the three-dimensional porous framework. Any technical solution that can introduce high thermal conductivity fillers into a three-dimensional porous framework formed in a polymer matrix, and enable the high thermal conductivity fillers to form a continuous thermally conductive structure in the in-plane direction of the membrane through surface-to-surface contact and / or edge-to-surface contact, should fall within the protection scope of this invention.

[0058] The high thermal conductivity filler can be selected from materials with high thermal conductivity and electrical insulation, such as, but not limited to, boron nitride, aluminum nitride, beryllium oxide, etc., and its morphology includes zero-dimensional particles, one-dimensional fibers, two-dimensional sheets, etc. From the perspective of optimizing the formation of the thermally conductive network and maintaining the flexibility of the membrane, preferably, the high thermal conductivity filler is a two-dimensional sheet structure, such as hexagonal boron nitride (h-BN) nanosheets. In the embodiments of the present invention, hexagonal boron nitride is used as a high thermal conductivity filler to illustrate the structure and performance of the self-supporting composite membrane. However, it should be understood that the selection of hexagonal boron nitride in the embodiments is only to illustrate one way of implementing the technical solution of the present invention and does not constitute a substantial limitation on the type of high thermal conductivity filler.

[0059] The polymer matrix can be selected from one or more polymers with good electrochemical stability and film-forming ability, such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyimide (PI), polyethersulfone (PES), polyacrylonitrile (PAN), and polyethylene oxide (PEO).

[0060] To ensure the continuity of the thermal conductivity pathway, the high thermal conductivity filler has a sheet-like structure with a lateral dimension of 100 nm to 10 μm and a thickness of 1 nm to 100 nm. The overall thickness of the diaphragm is 1 to 100 μm, with a porosity of 32% to 80% and an average pore size of 85 nm to 270 nm. If the porosity is too low, the confined space is insufficient to induce filler orientation; if it is too high, the mechanical strength of the skeleton cannot support the continuous stacking of the filler, making it difficult to maintain the "self-supporting" characteristic. The diaphragm provided by this invention can exist in two main forms: one is as a self-supporting independent membrane, typically with a thickness of 5-100 μm; the other is as a modified coating, applied to one or both sides of a commercial substrate diaphragm (such as PE, PP, PE / PP, GF), with a coating thickness typically of 1-30 μm.

[0061] This invention also provides a method for preparing a self-supporting composite membrane, the key to which lies in the preparation of the slurry and the diverse film-forming processes. The steps include:

[0062] (1) Slurry preparation: The polymer and high thermal conductivity filler in a predetermined ratio are dispersed together in a suitable organic solvent, such as acetone, acetonitrile, N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc) or tetrahydrofuran (THF), and the mixture is made uniform by mechanical stirring and ultrasonic treatment to form a stable and uniform slurry.

[0063] (2) Film formation: Depending on the equipment and product requirements, one or more of the following film formation methods can be selected:

[0064] (2.1) Casting-phase separation method: The slurry is coated onto a flat substrate (such as a glass plate, aluminum foil, etc.), and then the substrate with the wet film is quickly immersed in a liquid (such as water, ethanol, methanol, etc.) that is a poor solvent for the slurry solvent but insoluble in the polymer and functional fillers. The solvent and non-solvent exchange causes the polymer to coagulate and precipitate, and the functional fillers are fixed to form a thin film with a porous structure.

[0065] (2.2) Electrospinning: The slurry is loaded into a syringe and spun under the action of a high-voltage electrostatic field. The fibers are deposited on the receiving device to form a non-woven fiber membrane. The fiber morphology can be controlled by adjusting parameters such as voltage, distance, and speed.

[0066] (2.3) Mold casting method: The slurry is injected into a mold (such as a polytetrafluoroethylene (PTFE) mold), and the thickness of the membrane layer is precisely controlled by controlling the solid content of the slurry and the casting thickness. Then, it is left to stand to allow the solvent to evaporate naturally, or it is gently heated to assist evaporation, so that the membrane is initially shaped and solidified, resulting in an independent composite membrane with complete structure and uniform thickness.

[0067] (2.4) Direct coating method: The slurry is directly coated onto the surface of the commercial substrate membrane by means of coating methods such as scraping, spraying or roller coating.

[0068] (3) Post-treatment: The formed wet film is dried to completely remove the solvent. For electrospun films or certain cast films, they can be further lightly pressed by a roller press to adjust the thickness and enhance the bonding between fibers.

[0069] This invention also provides a secondary battery that uses a self-supporting composite separator as an electronic insulation and thermal management component disposed between the positive and negative electrodes. During operation, the self-supporting composite separator, on the one hand, relies on the electrical insulation properties of the polymer matrix to achieve effective electronic isolation between the positive and negative electrodes, preventing internal short circuits; on the other hand, the two-dimensional thermally conductive path provides a low-thermal-resistance in-plane conduction path for the heat generated during charging, discharging, and rate changes, allowing heat to diffuse rapidly and be evenly distributed along the separator plane, thereby reducing the risk of localized hot spots and improving the overall thermal stability and safety of the battery.

[0070] Because the high thermal conductivity filler is distributed in the three-dimensional porous framework, the resulting thermally conductive network can maintain structural stability and thermal conductivity under repeated charging and discharging, volume changes and electrolyte wetting conditions, thus ensuring that the thermal management function can be realized and repeatedly used under actual battery operating conditions.

[0071] Furthermore, the three-dimensional porous structure of the self-supporting composite membrane provides continuous ion migration channels for the electrolyte, allowing lithium ions, sodium ions, or other current-carrying ions to migrate normally between the positive and negative electrodes, thus not affecting the basic electrochemical reaction mechanism of the secondary battery. By adjusting the porosity and pore size distribution of the polymer matrix, as well as the type, morphology, and filling method of the high thermal conductivity filler, the self-supporting composite membrane can simultaneously meet the requirements of different secondary battery systems for ionic conductivity, mechanical strength, and thermal management performance.

[0072] Therefore, the self-supporting composite separator provided by the present invention is not only structurally suitable as an isolation component between positive and negative electrodes in lithium-ion batteries, lithium metal batteries, sodium-ion batteries, sodium metal batteries, or lithium-sulfur batteries, but also functionally capable of stably achieving the synergistic effects of electronic insulation, ion conduction, and in-plane thermal management in the above-mentioned different battery systems.

[0073] This invention does not limit the specific positive and negative electrode material system, electrolyte type, or battery packaging form of the secondary battery. As long as the secondary battery uses the self-supporting composite separator provided by this invention as the isolation structure between the positive and negative electrodes, the same technical effect as this invention can be obtained.

[0074] The invention has now been generally described, and will be more readily understood by referring to the following embodiments, which are provided by way of example and not by way of limitation.

[0075] Example 1

[0076] BN / PES self-supporting composite membranes were prepared using a phase separation-coating method. The specific preparation process is as follows: Figure 2 As shown, the steps are as follows:

[0077] (1) Dissolve 0.5g PES in 9g DMF and stir until transparent.

[0078] (2) Add 0.5g h-BN nanosheets (average transverse size ~2 μm), stir and sonicate to form a uniform slurry.

[0079] (3) Use a scraper to evenly coat the slurry onto the surface of the glass substrate. In this state, the wet film thickness is about 80 μm.

[0080] (4) Immediately immerse the coated diaphragm in ethanol and let it stand for 5 minutes to allow the DMF to exchange with the ethanol and the BN / PES layer to solidify.

[0081] (5) Remove the diaphragm, gently rinse with ethanol, and then vacuum dry at 60 °C for 8 hours to obtain a porous BN / PES self-supporting composite diaphragm (e.g. Figure 3 As shown in A), the thickness is approximately 21 μm (as shown in Figure A). Figure 3 As shown in C). SEM results show (as shown in C). Figure 3 As shown in Figure B), by using an appropriate framework-to-boron nitride ratio, a thermally conductive pathway with overlapping nanosheets and uniform pore distribution was successfully synthesized on a polymer matrix framework. This membrane exhibits superwetting properties towards the electrolyte compared to PP membranes (e.g., ...). Figure 4 As shown, the contact angle is approximately 10.9°, compared to approximately 28.1° for the PP diaphragm.

[0082] (6) The heat transfer parameters of the composite membrane in the plane direction of the membrane were tested: thanks to the excellent heat transfer performance of h-BN nanosheet material, this excellent heat transfer performance was retained after the composite membrane was made. Compared with the traditional commercial PP membrane, its thermal diffusivity and thermal conductivity are significantly superior. The results are shown in Table 1.

[0083] (7) Thermal stability test of the diaphragm: Under argon atmosphere, the diaphragm was gradually heated to 220 ℃, and held at three temperature nodes of 120 ℃, 170 ℃, and 220 ℃ for 30 minutes each. After cooling to room temperature, its dimensions were measured. The results showed that its area and dimensions did not change significantly, and its appearance was smooth with no melting or breakage (e.g. Figure 5 (As shown). In contrast, commercial PP separators exhibit significant curling and melting shrinkage after being progressively heated to 170 °C and held for 30 minutes.

[0084] Example 2

[0085] BN / PVDF-HFP self-supporting composite membranes were prepared by electrospinning.

[0086] (1) Dissolve 0.8g PVDF-HFP in a mixed solvent of 10g NMP and acetone (mass ratio 7:3) and stir magnetically for 6 hours until completely dissolved.

[0087] (2) Add 0.2g h-BN nanosheets (average transverse size ~1 μm) to the above solution, stir mechanically for 2 hours, and then sonicate (power 300 W) for 1 hour to obtain a uniform spinning slurry.

[0088] (3) The slurry was injected into the electrospinning device with the following parameters: voltage 15 kV, needle-to-roll distance 18 cm, and feed speed 1.0 mL / h. The slurry was received with aluminum foil to obtain the nascent fiber membrane.

[0089] (4) The fiber membrane was dried in a vacuum oven at 80 °C for 12 hours, and then pressed under mild pressure by a roller press to obtain a self-supporting composite membrane with a thickness of about 35 μm.

[0090] Example 3

[0091] BN / PI self-supporting composite membranes were prepared by mold casting.

[0092] (1) 0.6g of PI prepolymer and 0.4g of h-BN nanosheets (average transverse size ~500 nm) were dispersed together in 9g of NMP and treated with high-speed stirring (800 rpm) and intermittent ultrasonication (400 W power) for 4 hours to form a high-viscosity, uniform casting slurry.

[0093] (2) Slowly inject the above slurry into a smooth PTFE mold coated with petroleum jelly, and control the depth of the slurry poured into the mold to be about 500 μm.

[0094] (3) Place the mold on a water platform and let it stand for 24 hours in a low humidity environment at 60 ℃ to allow the solvent to evaporate and the film to be initially shaped.

[0095] (4) Transfer the pre-shaped membrane along with the mold to a vacuum oven, raise the temperature to 180 °C and keep it for 4 hours to complete the imidization reaction of PI and completely remove the residual solvent.

[0096] (5) After cooling, peel it off from the PTFE mold to obtain a smooth and dense BN / PI self-supporting composite membrane with a total thickness of about 30 μm.

[0097] Example 4

[0098] Application of composite separators in lithium metal batteries.

[0099] In an argon-protected glove box, a 1Ah pouch cell with an electrode size of 7.5 × 6.5 cm and a CR2032 coin cell were assembled using a lithium metal sheet as the negative electrode, lithium iron phosphate (LFP) as the positive electrode, and the separator prepared in Example 1 as the separator. Charge-discharge cycle tests were then performed.

[0100] Comparative Example 1

[0101] A commercial PP (Celgard 2500) membrane was used. Its heat transfer parameters are shown in Table 1. Its wettability test showed a wettability angle of 28.1° (e.g., ...). Figure 4 (As shown). Its thermal stability test showed that at 220 °C, the commercial PP separator underwent severe shrinkage (e.g., Figure 5 (As shown).

[0102] Comparative Example 2

[0103] Using the same battery materials and assembly process as in Example 4, pouch and coin cells were assembled, the only difference being the use of the PP separator from Comparative Example 1. The coin cell cycle diagrams and charge-discharge curves showed faster degradation of high-temperature cycling performance (e.g., ...). Figure 7 (As shown in B, C, and D in the image). The thermal imaging of its pouch cell shows a more concentrated temperature distribution and a higher peak temperature (as shown in the image). Figure 8 (As shown). Its ARC results show a lower autogenous heat initiation temperature, thermal runaway initiation temperature, and maximum temperature.

[0104] The results of comparing the thermal diffusivity and thermal conductivity in the plane direction of the h-BN nanosheet self-supporting composite membrane of the present invention (Example 1) with those of the commercial PP membrane (Comparative Example 1) are summarized in Table 1.

[0105] Table 1. Comparison of thermal diffusivity and thermal conductivity.

[0106]

[0107] The h-BN composite membranes prepared by the three processes of phase separation-coating, electrospinning, and mold casting provided by the present invention have the following performance parameters: thickness, pore structure, ionic conductivity, and thermal conductivity under different process parameters and BN content. The results are summarized in Table 2.

[0108] Table 2. Summary of different processes.

[0109]

[0110] Note: The contents of the process parameters column represent the time for immersion in the non-solvent, the distance from the needle to the roller, and the settling time after casting for phase separation coating, electrospinning, and mold casting, respectively.

[0111] like Figure 6 The image shown is a SEM image of the lithium metal surface of a coin cell assembled using the h-BN nanosheet self-supporting composite separator of the present invention after cycling. After multiple cycles, the lithium metal anode surface opposite the composite separator shows a flatter deposition morphology, which helps the battery to operate for more cycles with high safety and high capacity.

[0112] like Figure 7 The image shows the charge-discharge cycle test results of the coin cell battery. Figure 7 In the image, A represents a panoramic optical photograph and a thickness measurement optical photograph of the two separators. This indicates that the battery retains a specific capacity of approximately 140 mAh / g after 300 cycles at 60 °C and 1 C rate (e.g., ...). Figure 7 As shown in B), the battery's charge-discharge curve exhibits a more stable voltage plateau and lower polarization (as shown in Figure B). Figure 7 As shown in C and D in the figure, it is significantly better than the battery comparison example 2 which uses a commercial PP separator.

[0113] In addition, the pouch battery was cycled at a high rate of 5C and thermal imaging was performed, with the following results: Figure 8 As shown, thanks to the excellent thermal conductivity of the boron nitride self-supporting composite separator in Example 1, its thermal imaging images show a lower peak temperature and a more uniform temperature field (compared to the pouch cell assembled with a PP separator in Comparative Example 2). Finally, ARC test results (as shown) Figure 9 As shown), after thermal failure, although the polymer matrix in the battery of Example 4 had burned out, the residual boron nitride still formed a thermally conductive and insulating protective layer (as shown). Figure 9 (As shown in A in the figure). Benefiting from the synergistic protective effect of the high thermal conductivity and high-temperature size stability of boron nitride nanosheets, as well as their good electrical insulation and chemical stability properties, their ARC results are as follows: Figure 9As shown in C, the higher and safer autogenous heating initiation temperature (T1), thermal runaway initiation temperature (T2), and maximum temperature (T3) compared to Comparative Example 2 are displayed.

[0114] As described above, h-BN nanosheets are uniformly dispersed in the polymer matrix and form physical cross-linking points. They do not undergo phase transitions or decomposition at high temperatures, effectively suppressing the thermal motion of polymer segments. Simultaneously, the interaction between h-BN and the polymer interface enhances the rigidity of the overall structure, thus maintaining the morphological integrity of the separator at high temperatures and preventing contact between the positive and negative electrodes due to thermal shrinkage. The h-BN nanosheets are oriented along the planar direction and overlap each other in the composite separator, forming a continuous heat conduction path. This network can rapidly diffuse the Joule heat or reaction heat generated at the electrode / electrolyte interface during charging and discharging along the planar direction, avoiding localized heat accumulation, thereby reducing the risk of thermal runaway and improving the battery's cycle stability at high rates.

[0115] Since the reaction kinetics of lithium metal deposition and dissolution are significantly affected by temperature, a uniform temperature field can reduce the non-uniformity of lithium ion deposition on the negative electrode surface and delay dendrite formation. h-BN itself does not undergo side reactions with the electrolyte or electrode materials, maintaining interface stability. At the same time, the controllable porous structure formed during the preparation process ensures full wetting of the electrolyte, provides sufficient ion transport channels, and balances high ion conductivity and mechanical strength.

[0116] For any points not covered above, existing technologies shall apply.

[0117] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A self-supporting composite diaphragm with high thermal conductivity, characterized in that, include: The polymer matrix has an internal interconnected porous structure, which constitutes a three-dimensional porous framework. A high thermal conductivity filler is distributed within the pores of the three-dimensional porous framework of the polymer matrix and is in contact with the three-dimensional porous framework. The high thermal conductivity filler is a filler with a sheet-like or layered structure, which contacts each other in the in-plane direction inside and / or on the surface of the three-dimensional porous skeleton, and forms a continuously distributed thermally conductive network through surface-to-surface stacking and / or edge-to-surface overlapping, so that the self-supporting composite membrane forms a through two-dimensional thermally conductive path in the in-plane direction. The self-supporting composite membrane has a porosity of 30%~60% and an average pore size of 200nm~1μm; At 60℃, the in-plane thermal conductivity of the self-supporting composite diaphragm is ≥85 W / (m·K); The two-dimensional thermal conductivity pathway ensures that the dimensional change rate of the self-supporting composite membrane is less than 5% after being placed at 220°C for 30 minutes. The high thermal conductivity filler is h-BN nanosheets; The polymer matrix is ​​selected from any one or more of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyimide, polyethersulfone, polyacrylonitrile, and polyethylene oxide. The self-supporting composite diaphragm contains 50% to 70% by mass of highly thermally conductive filler.

2. The self-supporting composite diaphragm according to claim 1, characterized in that, The mass of the high thermal conductivity material is 5% to 60% of the total mass of the self-supporting composite diaphragm.

3. The self-supporting composite diaphragm according to claim 1, characterized in that, The overall thickness of the self-supporting composite diaphragm is 10~100μm.

4. The self-supporting composite diaphragm according to any one of claims 1-3, characterized in that, The lateral dimensions of the h-BN nanosheets are 100 nm to 2 μm.

5. The self-supporting composite diaphragm according to claim 4, characterized in that, The thickness of the h-BN nanosheets is 1 nm to 100 nm.

6. A method for preparing a self-supporting composite diaphragm as described in any one of claims 1-5, characterized in that, include: Slurry preparation involves dispersing the polymer and / or h-BN nanosheets in an organic solvent to form a slurry. Film formation: The obtained slurry is formed into a film using a casting-phase separation method. The slurry is coated onto a substrate, and then immersed in a non-solvent form of the organic solvent for phase separation and curing to form a porous membrane; h-BN nanosheets account for 50%~70% of the mass fraction of the porous membrane. The post-processing involves drying the resulting membrane and then roll-forming it to obtain the self-supporting composite diaphragm.

7. The preparation method according to claim 6, characterized in that, The organic solvent is selected from one or more of acetone, acetonitrile, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, or tetrahydrofuran.

8. The preparation method according to claim 6, characterized in that, The phase separation and curing time is 1~15 min.

9. A secondary battery, characterized in that, The self-supporting composite diaphragm according to any one of claims 1-5 is used as an electronic insulation and thermal management component between the positive and negative electrodes.

10. The secondary battery according to claim 9, characterized in that, The secondary battery includes lithium-ion batteries, lithium metal batteries, sodium-ion batteries, sodium metal batteries, or lithium-sulfur batteries.

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