Battery separator and its preparation method, secondary battery

Abstract

This invention provides a battery separator, its preparation method, and a secondary battery. The battery separator is formed by multiple core-shell type fibers interwoven together, with cross-linking nodes formed at the cross-contact areas of the fibers. The core-shell type fiber includes a core and a shell covering the core. The core includes polyimide; the shell and the cross-linking nodes both include polyvinylidene fluoride (PVDF) and cross-linking components. The technical solution of this invention constructs a core-shell fiber structure composed of a polyimide core and a PVDF shell containing cross-linking components, and forms stable fusion-cross-linking composite nodes at the fiber cross-sections of the shell. This achieves the technical effect of simultaneously improving the mechanical strength, thermal dimensional stability, and interfacial stability of the separator while maintaining high porosity and a connected pore network structure.

Description

Technical Field

[0001] This invention relates to the field of secondary batteries, and more specifically, to a battery separator, its preparation method, and a secondary battery. Background Technology

[0002] Among the four key materials of lithium-ion batteries (positive electrode, negative electrode, electrolyte, and separator), the separator not only acts as a physical barrier to isolate the positive and negative electrodes and prevent internal short circuits, but also serves as a channel for lithium ion transport between the positive and negative electrodes. Therefore, the separator must possess electronic insulation, good ionic conductivity, excellent mechanical strength, and outstanding thermal dimensional stability. As lithium-ion batteries rapidly develop towards higher energy density, higher power density, and higher safety, traditional polyolefin microporous separators (such as polyethylene PE or polypropylene PP separators) are gradually becoming unable to meet the stringent safety requirements of high-energy-density power batteries and large-scale energy storage systems due to their inherent defects such as poor heat resistance, susceptibility to thermal shrinkage or even melting and closure at high temperatures leading to internal short circuits.

[0003] To overcome the heat resistance bottleneck of polyolefin membranes, existing technologies mainly adopt two improvement paths: one is to construct composite coated membranes by introducing inorganic ceramic particles (such as alumina and silica); the other is to develop novel membrane materials based on heat-resistant polymers. Among these, electrospinning technology is considered a highly promising membrane preparation process because it can produce nanofiber network structures with high porosity, high specific surface area, and uniform pore size distribution.

[0004] However, existing electrospun fiber separators still face many challenges in practical applications. First, the fiber membranes formed by electrospinning mainly rely on the physical entanglement and van der Waals forces between fibers, resulting in relatively low mechanical strength. They are unable to withstand the puncture stress during battery assembly and the volume expansion during charge and discharge. To improve mechanical strength, existing technologies typically employ hot-pressing. However, during hot pressing, relying solely on the physical melting and bonding of thermoplastic polymers often leads to the overall collapse of the fiber network, destroying the originally interconnected three-dimensional porous structure, significantly reducing porosity, and thus hindering rapid ion transport and increasing battery internal resistance. Second, for core-shell fiber membranes containing fusible components, the nodes formed by simple hot pressing are merely physical welds. These nodes are prone to loosening or softening again under long-term cycling or high-temperature environments, resulting in insufficient separator structural stability. Furthermore, to compensate for the insufficient mechanical properties, multilayer composite separators are commonly constructed by coating the separator surface with an inorganic ceramic layer. While this strategy improves the heat resistance and mechanical strength of the separator to some extent, the multilayer structure introduces additional interfaces, increasing the risk of interfacial delamination. Furthermore, the inorganic ceramic layer is brittle and prone to cracking during battery cycling, leading to increased interfacial impedance. More importantly, multilayer composite separators are typically thicker, and the ceramic coating occupies valuable pore volume, reducing the separator's porosity and electrolyte retention rate, thus limiting the battery's energy density.

[0005] Therefore, how to simultaneously improve the mechanical strength, thermal dimensional stability, and interface stability of the diaphragm without destroying the continuous morphology of the fiber matrix and the high-porosity interconnected pore network is a technical problem that needs to be solved in this field. Summary of the Invention

[0006] The main objective of this invention is to provide a battery separator and its preparation method, as well as a secondary battery, to solve the problem that existing battery separators are difficult to balance mechanical properties, thermal stability, and electrical properties.

[0007] To achieve the above objectives, a first aspect of the present invention provides a battery separator formed by interweaving multiple core-shell type fibers, with cross-linking nodes formed at the cross-contact areas of the core-shell type fibers. The core-shell type fibers include a core and a shell covering the core. The core includes polyimide. The shell and cross-linking nodes both include polyvinylidene fluoride (PVDF) and cross-linking components. The battery separator exhibits a unique fiber interweaving morphology. The polyimide core layer of each fiber provides a continuous, heat-resistant skeleton, giving the separator excellent thermal dimensional stability, maintaining structural integrity at high temperatures, and exhibiting extremely low thermal shrinkage. The PVDF shell layer encapsulates the core layer, providing good electrolyte wettability and improving interfacial compatibility with the electrodes. Simultaneously, the fiber cross-points form fusion-cross-linking composite nodes, transforming the originally loose point contacts into stable chemical connections, significantly improving the mechanical strength of the separator, enabling it to withstand mechanical stress during battery assembly and volume changes during charging and discharging, preventing internal short circuits.

[0008] Furthermore, the crosslinking component contains polymerizable unsaturated double bonds in its molecular structure; preferably, the crosslinking component is an acrylate compound; more preferably, the crosslinking component is polyethylene glycol diacrylate. This results in crosslinking nodes that possess both high strength and superior ion transport performance and high porosity retention.

[0009] Furthermore, based on the weight of polyvinylidene fluoride (PVDF) as 100%, the content of crosslinking components in the shell and crosslinking nodes is independently 1 wt.% to 20 wt.%. This preferred range enables the formation of a sufficiently strong covalent bond network at the fiber contact points to fix the nodes, while also more effectively maintaining the appropriate flexibility of the shell matrix, achieving a better balance between mechanical reinforcement and ion transport performance.

[0010] Furthermore, in the battery separator, the weight ratio of polyimide to polyvinylidene fluoride is (50:50) to (90:10); and / or, in the core-shell type fiber, the shell thickness is 30 nm to 300 nm, and the core diameter is 0.3 μm to 3 μm. The above-mentioned preferred weight ratio range allows the cross-linking nodes to more effectively fix the fibers while exhibiting good interfacial compatibility and ion transport capabilities. The above-mentioned preferred range regarding dimensions ensures that the fibers possess sufficient mechanical strength to serve as a supporting framework, while also having a suitable specific surface area, thereby achieving higher strength, porosity, and better interfacial contact.

[0011] Furthermore, the battery separator has a porosity of 60%–90%; and / or a thickness of 8 μm–20 μm; and / or a tensile strength of 20 MPa–40 MPa; and / or a puncture strength of 5 N–10 N; and / or a thermal shrinkage rate of 0.1%–5% after being subjected to a heat treatment at 200±5℃ for 30±2 min; and / or an interfacial resistance of 40 Ω–90 Ω. In other words, the resulting battery separator possesses high mechanical strength, high thermal stability, high porosity, good interfacial stability, and low internal resistance, thus solving the technical challenge of balancing heat resistance, mechanical strength, and porosity in traditional separators.

[0012] A second aspect of the present invention provides a method for preparing the above-mentioned battery separator, comprising: step S1, preparing a core spinning solution containing polyimide; preparing a shell spinning solution containing polyvinylidene fluoride and a crosslinking component; the crosslinking component being a thermal crosslinking component and / or a photocrosslinking component; step S2, using the core spinning solution as the inner liquid and the shell spinning solution as the outer liquid, coaxial electrospinning to obtain a fiber membrane containing fiber filaments; step S3, hot-pressing the fiber membrane to form a local initial fusion structure at the cross-sections of the fiber filaments, thereby obtaining a hot-pressed membrane; step S4, crosslinking the hot-pressed membrane to form crosslinking nodes at the local initial fusion structures, thereby obtaining the battery separator. Compared with the multilayer composite separators using polyolefin-based membranes, ceramic coatings, and independent PVDF adhesive layers in the prior art, the above-mentioned preparation method of the present invention, through "local fusion + crosslinking fixation," synergistically constructs an internal reinforcing node network, enabling stable connections to be formed in the fiber cross-section areas, rather than overall hot-melt compaction, thereby enhancing mechanical properties while maintaining a high-porosity interconnected pore structure.

[0013] Further, in step S1, the mass fraction of polyimide in the core spinning solution is 10 wt.% to 20 wt.%; and / or, the mass fraction of polyvinylidene fluoride (PVDF) in the shell spinning solution is 5 wt.% to 15 wt.%; and / or, in the shell spinning solution, based on 100% of PVDF weight, the content of the crosslinking component is 1 wt.% to 20 wt.%. The PVDF concentration within the above preferred range is sufficient to form a continuous coating layer on the fiber surface, providing sufficient reaction sites for the crosslinking reaction, and also helps to form finer fibers, increase the specific surface area, and improve the electrolyte adsorption rate. The preferred content of the crosslinking component enables the formation of a more stable three-dimensional network structure at the fiber crossover points, thereby significantly improving the tensile strength and puncture resistance of the final membrane.

[0014] Furthermore, the shell spinning solution also includes a thermal initiator and / or a photoinitiator at a mass fraction of 1 wt.% to 3 wt.%; and / or, the thermal initiator is an organic peroxide initiator or an azo initiator, and the photoinitiator is selected from one or more of photoinitiator 1173, photoinitiator 184, photoinitiator 2959, and photoinitiator 819. A preferred initiator content can drive the rapid polymerization of the crosslinking components, forming denser crosslinking nodes.

[0015] Further, in step S2, the applied voltage for coaxial electrospinning is 18±2kV, the receiving distance is 15±2cm, and the roller collection speed is 200±20rpm; the inner needle diameter used in coaxial electrospinning is 0.5±0.1 mm, the outer needle diameter is 1.2±0.2 mm, the inner liquid flow rate is 0.6±0.1mL / h, and the outer liquid flow rate is 0.8±0.1mL / h; coaxial electrospinning is carried out at a temperature of 25±2℃ and a relative humidity of 40±5%RH. These preferred parameters enable the final prepared membrane to achieve more significant improvements in porosity, thickness, and mechanical properties, while also improving the consistency of fiber morphology between batches.

[0016] Furthermore, the hot-pressing temperature is 80℃~140℃, the hot-pressing pressure is 1MPa~5MPa, and the hot-pressing time is 5s~60s. This allows the PVDF shell to fully contact and fuse with the shells of adjacent fibers, forming an initial fusion structure. This provides a tighter interface for subsequent crosslinking, resulting in a membrane with superior mechanical strength and thermal dimensional stability.

[0017] Furthermore, the crosslinking treatment satisfies at least one of the following conditions: (1) irradiation with ultraviolet light at a wavelength of 200 nm to 400 nm for 100 s to 200 s; (2) heat treatment at 80 °C to 180 °C for 5 min to 60 min. Both of the above crosslinking methods can form covalently bonded composite nodes at the fiber cross-sections, thereby significantly enhancing the mechanical strength of the obtained membrane. At the same time, they can also effectively limit the swelling of the shell polymer in the electrolyte, further improving its stability.

[0018] A third aspect of the present invention provides a secondary battery, including a separator, which is the battery separator described above; or, the separator is prepared by the method for preparing the battery separator described above. The resulting secondary battery exhibits excellent safety. The high thermal dimensional stability of the separator prevents internal short circuits at high temperatures, and its high mechanical strength resists dendrite growth and mechanical stress, thereby significantly reducing the risk of thermal runaway of the battery.

[0019] By applying the technical solution of this invention, a core-shell fiber structure composed of a polyimide core layer and a polyvinylidene fluoride shell layer containing crosslinking components is constructed, and a stable fusion-crosslinking composite node is formed at the fiber intersection position of the shell layer. This achieves the technical effect of simultaneously improving the mechanical strength, thermal dimensional stability and interface stability of the membrane while maintaining high porosity and interconnected pore network structure. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0021] As described in the background section, existing battery separators suffer from difficulties in simultaneously achieving mechanical properties, thermal stability, and electrical performance. To address these technical problems, a first aspect of the present invention provides a battery separator formed by interweaving multiple core-shell type fibers, with cross-linking nodes formed at the cross-contact areas of the core-shell type fibers. The core-shell type fibers include a core and a shell covering the core, the core comprising polyimide; the shell and the cross-linking nodes each comprise polyvinylidene fluoride and a cross-linking component.

[0022] In the aforementioned battery separator, the polyimide (PI) core serves as a continuous, high-modulus skeleton, providing rigid support for the entire three-dimensional fiber network. This ensures that the fiber matrix does not melt or collapse under external stress or high-temperature conditions, thus maintaining the high porosity and interconnected ion transport channels within the separator, guaranteeing electrolyte wetting and rapid lithium-ion migration. The shell encapsulating the PI core is composed of polyvinylidene fluoride (PVDF) and cross-linking components. PVDF provides good matrix compatibility and a certain degree of flexibility, while the cross-linking components form stable cross-linking nodes at the fiber cross-contact areas. This transforms the fiber nodes, originally bound only by physical contact or van der Waals forces, into rigid nodes connected by covalent bonds. This cross-linked composite structure not only significantly improves the inter-fiber bonding force, giving the separator excellent tensile strength and puncture resistance, but also, because the cross-linking reaction is limited to the contact points, avoids large-area chemical cross-linking of the fiber matrix, thereby protecting the microporous structure within the fibers. Furthermore, the presence of the cross-linked network restricts the swelling behavior of the PVDF shell in the electrolyte, enhancing the dimensional and interfacial stability of the separator during long-term cycling.

[0023] For the crosslinking component, it is preferable that its molecular structure contains polymerizable unsaturated double bonds. Under the action of thermal or photoinitiators, these bonds open and form active free radicals, which then connect with the double bonds of other molecules to form a covalent network, thereby endowing the crosslinking nodes with extremely high mechanical strength and thermal stability. Further preferred crosslinking components are acrylates, which have higher double bond activity, faster reaction rates, and acrylate polymers possess good flexibility and chemical resistance. In several typical embodiments, polyethylene glycol diacrylate is more preferably the crosslinking component. PEGDA, as a bifunctional crosslinking agent, combines rigid crosslinking points with flexible polyethylene glycol segments, enabling the formation of an efficient network structure at fiber intersections. Its ether bonds (-O-) are hydrophilic, which can improve the affinity and wettability of the shell to polar electrolytes, accelerate ion transport, and reduce interfacial impedance. Simultaneously, the flexible segments of PEGDA also help alleviate stress concentration in the crosslinking network under stress, reducing the possibility of brittle fracture at the nodes. During the formation of the aforementioned diaphragm, PEGDA can rapidly solidify at the fiber contact surface, more firmly connecting the fibers. This results in cross-linked nodes that possess both high strength and superior ion transport performance and high porosity retention.

[0024] To better balance the crosslinking density and the mechanical flexibility of the fiber, it is preferable that the content of the crosslinking component in the shell and the crosslinking nodes is independently 1 wt.% to 20 wt.%, based on 100% of the weight of polyvinylidene fluoride. This preferred range enables the formation of a sufficiently strong covalent bond network at the fiber contact points to fix the nodes, while also more effectively maintaining the appropriate flexibility of the shell matrix, achieving a better balance between mechanical reinforcement and ion transport performance.

[0025] In the battery separator, the weight ratio of polyimide to polyvinylidene fluoride is preferably (50:50) to (90:10), more preferably (60:40) to (80:20), thereby enabling the PI core layer to provide sufficient rigid support to maintain the pore structure, while the PVDF shell layer can also provide sufficient reaction sites and bonding interfaces, so that the crosslinking nodes can not only fix the fibers more effectively, but also exhibit good interfacial compatibility and ion transport capabilities.

[0026] In several preferred embodiments, the shell thickness of the core-shell fiber is 30 nm to 300 nm, and the diameter of the core is 0.3 μm to 3 μm (more preferably 0.5 μm to 2 μm). These preferred dimensional ranges ensure that the fiber possesses sufficient mechanical strength to serve as a supporting framework, while also having a suitable specific surface area, thereby achieving higher strength, porosity, and better interfacial contact.

[0027] Furthermore, to better balance the forming stability during spinning and the controllability of subsequent hot pressing, it is preferable that the number average molecular weight of polyimide is 50,000~200,000; and / or that the number average molecular weight of polyvinylidene fluoride is 100,000~500,000. This results in higher chain entanglement density in PI, leading to the formation of a smooth, continuous, heat-resistant, rigid skeleton; simultaneously, it promotes a more suitable melt viscosity of PVDF during hot pressing, ultimately maximizing the mechanical strength of the resulting membrane without excessively sacrificing ion transport channels.

[0028] In several preferred embodiments, the porosity of the battery separator is 60%~90% (more preferably 70%~80%); and / or, the thickness of the battery separator is 8μm~20μm (more preferably 15±2μm); and / or, the tensile strength of the battery separator is 20MPa~40MPa (more preferably 25MPa~30MPa); and / or, the puncture strength of the battery separator is 5N~10N (more preferably 6N~8N); and / or, the battery separator is subjected to a heat treatment at 200±5°C for 30±2min, and its thermal shrinkage rate is 0.1%~5% (more preferably 2.05~3.5%); and / or, the interfacial resistance of the battery separator is 40Ω~90Ω (more preferably 60Ω~70Ω). In other words, the resulting battery separator can simultaneously possess high mechanical strength, high thermal stability, high porosity, good interfacial stability, and low internal resistance, thus solving the technical challenge of traditional separators in achieving a balance between heat resistance, mechanical strength, and porosity.

[0029] A second aspect of the present invention provides a method for preparing the above-mentioned battery separator, comprising: step S1, preparing a core spinning solution containing polyimide (PI); preparing a shell spinning solution containing polyvinylidene fluoride (PVDF) and a crosslinking component; the crosslinking component being a thermal crosslinking component and / or a photocrosslinking component; step S2, using the core spinning solution as the inner liquid and the shell spinning solution as the outer liquid, coaxial electrospinning to obtain a fiber membrane containing fiber filaments; step S3, hot-pressing the fiber membrane to form a local initial fusion structure at the cross-sections of the fiber filaments, thereby obtaining a hot-pressed membrane; step S4, crosslinking the hot-pressed membrane to form crosslinking nodes at the local initial fusion structures, thereby obtaining a battery separator.

[0030] The above preparation method utilizes the differences in core-shell structure and the controllability of crosslinking reaction during spinning to construct high-strength fusion-crosslinking composite nodes at fiber contact points. This solves the problems of low mechanical strength and poor thermal stability in traditional electrospun diaphragms, while overcoming the defects of pore structure collapse caused by traditional hot pressing processes. Specifically: the PI core layer polyimide has extremely high glass transition temperature and thermal decomposition temperature, and is chemically stable. Under subsequent hot pressing and crosslinking temperatures, PI maintains solid rigidity, serving as a heat-resistant skeleton; while PVDF is thermoplastic, softening or melting at high temperatures, exhibiting fluidity. During spinning, due to the high viscosity and high heat resistance of the inner PI layer and the lower melting point of the outer PVDF containing crosslinking agents, phase separation occurs during solvent evaporation, forming a stable concentric circular cross-sectional structure. During subsequent hot pressing, basically only the surface layer in fiber contact participates in fusion, while the inner core layer is almost unaffected, thus achieving a structural state of localized fusion rather than overall melting. Through hot pressing, the PVDF molecular chains in the shell gain mobility and become viscous. The PVDF shells of adjacent fibers physically contact each other at the intersection points, diffusing and entangled to form initial physical weld points. However, the PI core layer provides rigid support, preventing the fiber body from collapsing or fusing entirely; only the contact points deform and bond. Subsequently, through cross-linking treatment, free radicals initiate chain growth and cross-linking reactions in the welded areas where fibers intersect, forming a rigid three-dimensional covalent network. The cross-linked nodes and the rigid PI core layer work synergistically to further endow the diaphragm with higher tensile strength and puncture resistance.

[0031] In summary, compared with existing multilayer composite separators using polyolefin-based membranes, ceramic coatings, and independent PVDF adhesive layers, the preparation method of this invention, through "partial fusion + cross-linking fixation," synergistically constructs an internal reinforcing node network, enabling stable connections at fiber cross-sections, rather than overall thermal melting and compaction. This approach enhances mechanical properties while maintaining a high-porosity interconnected pore structure. The beneficial effects of the resulting battery separator include:

[0032] Firstly, node stability is enhanced: local fusion-crosslinking composite nodes are formed in the fiber cross-section area. After the nodes are crosslinked and fixed, they are not easy to soften again, thereby improving the tensile strength, puncture resistance and structural stability of the diaphragm during cycling.

[0033] Secondly, maintaining the interconnected pore structure: local node reinforcement mainly occurs at fiber intersections, and the fiber body maintains a continuous fiber morphology, avoiding the overall collapse caused by traditional hot pressing, thereby maintaining high porosity and interconnected pore network;

[0034] Third, integrated structure: the shell directly undertakes the interfacial bonding function, without the need for additional independent PVDF adhesive layer and ceramic layer, reducing the number of interfaces and reducing the risk of interfacial peeling, while achieving membrane thinning.

[0035] Fourth, improved thermal dimensional stability: The polyimide core layer forms a continuous heat-resistant skeleton, enabling the diaphragm to maintain its structural integrity under high temperature conditions;

[0036] Fifth, reduced interfacial impedance: During the hot pressing process, the shell layer forms a fusion interface with the PVDF binder in the electrode, and the interface structure is fixed by cross-linking, thereby improving the uniformity of interfacial contact and cycle stability.

[0037] Sixth, improved resistance to electrolyte swelling: the cross-linked structure restricts the swelling behavior of the shell in the electrolyte, thereby improving the dimensional stability and interfacial stability during long-term cycling.

[0038] In step S1, to promote the formation of a continuous fiber skeleton with sufficient mechanical strength and further improve the stability of the electrospinning process, it is preferable that the mass fraction of polyimide in the core spinning solution is 10 wt.% to 20 wt.%. In the shell spinning solution, the mass fraction of polyvinylidene fluoride (PVDF) is preferably 5 wt.% to 15 wt.%; and, based on 100% of the weight of PVDF, the content of the crosslinking component is preferably 1 wt.% to 20 wt.%, more preferably 5 wt.% to 10 wt.%. The PVDF concentration within the above preferred range is sufficient to form a continuous coating layer on the fiber surface, providing sufficient reaction sites for the crosslinking reaction, and also helps to form finer fibers, increase the specific surface area, and improve the electrolyte adsorption rate. The preferred content of the crosslinking component enables the formation of a more stable three-dimensional network structure at the fiber intersections, thereby significantly improving the tensile strength and puncture resistance of the final membrane.

[0039] In practical applications, the preferred shell spinning solution also includes a thermal initiator and / or a photoinitiator with a mass fraction of 1 wt.% to 3 wt.%, to facilitate the rapid polymerization of the crosslinking components and form denser crosslinking nodes. Specifically, the thermal initiator can be selected from one or more of organic peroxide initiators or azo initiators, and the photoinitiator can be selected from one or more of photoinitiator 1173, photoinitiator 184, photoinitiator 2959, and photoinitiator 819.

[0040] In step S2, the applied voltage for coaxial electrospinning is preferably 18±2 kV, the receiving distance is 15±2 cm, and the roller collection speed is 200±20 rpm, thereby optimizing the electric field force and hydrodynamics to obtain core-shell fibers with more uniform diameter and more stable structure. To ensure the core and shell layers maintain a stable concentric structure during extrusion and reduce core layer misalignment or shell layer breakage, the coaxial electrospinning preferably uses an inner needle diameter of 0.5±0.1 mm, an outer needle diameter of 1.2±0.2 mm, an inner liquid flow rate of 0.6±0.1 mL / h, and an outer liquid flow rate of 0.8±0.1 mL / h. Furthermore, to achieve more stable solvent evaporation and reduce porosity, defects, and electrostatic accumulation, the coaxial electrospinning is preferably carried out at a temperature of 25±2℃ and a relative humidity of 40±5%RH. The aforementioned optimized parameters enable the final membrane to achieve more significant improvements in porosity, thickness, and mechanical properties, while also enhancing the consistency of fiber morphology between batches.

[0041] Furthermore, the preferred hot-pressing temperature is 80℃~140℃, the hot-pressing pressure is 1MPa~5MPa, and the hot-pressing time is 5s~60s (more preferably, the hot-pressing temperature is 90℃~120℃, the hot-pressing pressure is 1MPa~3MPa, and the hot-pressing time is 10s~30s). Within the above-mentioned hot-pressing parameter range, the PVDF shell is in a viscoelastic state between its glass transition temperature and melting point, and the molecular chain segments have sufficient mobility to deform and diffuse under pressure, thereby forming physical bonds at the fiber intersections. Simultaneously, this temperature is much lower than the softening temperature of the PI core layer, thus ensuring the core layer remains rigid and supports the fiber network. Furthermore, this allows the PVDF shell to fully contact and fuse with the shells of adjacent fibers, forming an initial fusion structure, providing a tighter interface for subsequent cross-linking, and ultimately resulting in a membrane with superior mechanical strength and thermo-dimensional stability.

[0042] In several typical embodiments, the crosslinking treatment preferably satisfies at least one of the following conditions: (1) irradiation with ultraviolet light of wavelength 200nm~400nm for 100s~200s (more preferably irradiation with ultraviolet light of wavelength 365±5nm for 120±10s); (2) heat treatment at 80℃~180℃ for 5min~60min. In the above preferred embodiments, ultraviolet light of suitable wavelength can efficiently excite the photoinitiator to generate free radicals, initiating the polymerization reaction of the crosslinked components in the shell. Thus, the crosslinking reaction can proceed rapidly and uniformly in the fusion region of fiber intersections, forming a denser three-dimensional crosslinked network. The above thermal crosslinking conditions can achieve rapid crosslinking reaction, reduce potential thermal damage to the PVDF matrix or PI core layer, and also promote the full completion of the crosslinking reaction, forming more stable chemical nodes. Both of the above crosslinking methods can form covalently bonded composite nodes at fiber intersections, thereby significantly enhancing the mechanical strength of the obtained membrane, and can also effectively limit the swelling of the shell polymer in the electrolyte, further improving its stability.

[0043] A third aspect of the present invention provides a secondary battery, including a separator, which is the battery separator described above; or, the separator is prepared by the method for preparing the battery separator described above. The resulting secondary battery exhibits excellent safety. The high thermal dimensional stability of the separator prevents internal short circuits at high temperatures, and its high mechanical strength resists dendrite growth and mechanical stress, thereby significantly reducing the risk of thermal runaway of the battery.

[0044] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in this application.

[0045] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0046] Example 1

[0047] A method for preparing a battery separator:

[0048] (1) Weigh polyimide (PI, number average molecular weight of 100,000) and add it to N,N-dimethylacetamide (DMAc) to prepare a core spinning solution with a mass fraction of 15 wt.%. Stir magnetically at 60°C for 8 h and then degas at room temperature. Weigh polyvinylidene fluoride (PVDF, number average molecular weight of 200,000) and add it to N,N-dimethylformamide (DMF) to prepare a shell spinning solution with a mass fraction of 10 wt.%. At the same time, add polyethylene glycol diacrylate (PEGDA, added at 8% of the mass of PVDF) and 2 wt% photoinitiator 1173 to the shell solution and stir at 50°C for 6 h to form a uniform transparent solution.

[0049] (2) Spinning was carried out using a coaxial electrospinning device with an inner needle diameter of 0.5 mm and an outer needle diameter of 1.2 mm. The applied voltage was 18 kV, the receiving distance was 15 cm, the ambient temperature was 25℃, the relative humidity was 40%, the core layer flow rate was 0.6 mL / h, the shell layer flow rate was 0.8 mL / h, and the roller collection speed was 200 rpm. Continuous collection formed a core-shell structure fiber membrane with a thickness of about 15 μm.

[0050] (3) The obtained fiber membrane was hot-pressed at 100℃ and 2 MPa for 20 s to form a local initial fusion structure at the fiber intersection. Then, it was photocrosslinked by irradiation with 365 nm ultraviolet light for 120 s to form a local fusion-crosslink composite node in the fusion area.

[0051] SEM results showed that the obtained fiber matrix maintained a continuous fiber morphology, with discrete reinforcing nodes only forming at the intersections, and the membrane as a whole maintained a connected pore structure. In the obtained fiber membrane, the shell thickness of each fiber was 200 nm, and the core diameter was 1 μm; the total mass ratio of polyimide to polyvinylidene fluoride was 70:30. In both the shell and the crosslinking nodes, the crosslinking component content was 8 wt.% (based on 100% polyvinylidene fluoride weight).

[0052] Example 2

[0053] The only difference between this embodiment and Embodiment 1 is that the amount of PEGDA added in step (1) is changed so that the amount added accounts for 1% of the mass of PVDF. At this time, in the shell and crosslinking nodes, the content of the crosslinking component is 1 wt., based on the weight of polyvinylidene fluoride as 100%.

[0054] Example 3

[0055] The only difference between this embodiment and Example 1 is that the amount of PEGDA added in step (1) is changed so that the amount added accounts for 20% of the mass of PVDF. At this time, in the shell and crosslinking nodes, the content of the crosslinking component is 20 wt., based on the weight of polyvinylidene fluoride as 100%.

[0056] Example 4

[0057] The only difference between this embodiment and Example 1 is that in step (1), an equal amount of trimethylolpropane triacrylate (TMPTA) is used instead of PEGDA as the crosslinking component.

[0058] Example 5

[0059] The only difference between this embodiment and embodiment 1 is that the hot pressing conditions in step (3) are changed to: hot pressing for 60 s at 80°C and 5MPa.

[0060] Example 6

[0061] The only difference between this embodiment and embodiment 1 is that the hot pressing conditions in step (3) are changed to: hot pressing for 5s at 140°C and 1MPa.

[0062] Example 7

[0063] The only difference between this embodiment and embodiment 1 is that the crosslinking conditions in step (3) are changed to: irradiation with 200nm ultraviolet light for 200 s.

[0064] Example 8

[0065] The only difference between this embodiment and embodiment 1 is that the crosslinking conditions in step (3) are changed to: irradiation with 400nm ultraviolet light for 100 s.

[0066] Example 9

[0067] The only difference between this embodiment and embodiment 1 is that the crosslinking conditions in step (3) are changed to: heat preservation at 80°C for 60 min.

[0068] Example 10

[0069] The only difference between this embodiment and embodiment 1 is that the crosslinking conditions in step (3) are changed to: heat treatment at 180°C for 5 minutes.

[0070] Comparative Example 1

[0071] A method for preparing a battery separator:

[0072] Polyimide resin was weighed and added to DMAc to prepare a single-component spinning solution with a mass fraction of 18%. The solution was stirred at room temperature for 12 h until completely dissolved and degassed. Spinning was performed using a single-nozzle electrospinning device with an applied voltage of 18 kV, a receiving distance of 18 cm, a flow rate of 0.8 mL / h, and a roller speed of 300 rpm. Nanofiber membranes were continuously collected at 25℃ and 40% humidity, with a thickness controlled at 15±1 μm. After vacuum drying at 80℃ for 12 h, the membranes were directly used for testing without hot pressing or crosslinking treatment, and the fibers maintained a natural point contact state.

[0073] Comparative Example 2

[0074] A method for preparing a battery separator:

[0075] Polyimide and polyvinylidene fluoride were weighed and added to a DMAc / DMF mixed solvent at a mass ratio of 7:3 to prepare a blend spinning solution with a total solids content of 18%. The solution was stirred at room temperature for 12 h to form a homogeneous solution. Spinning was performed using a single-nozzle electrospinning device with an applied voltage of 18 kV, a receiving distance of 18 cm, a flow rate of 0.8 mL / h, and a roller speed of 300 rpm. A blend fiber membrane with a thickness of approximately 15 μm was collected and vacuum dried at 80 °C for 12 h, followed by hot pressing at 100 °C and 3 MPa for 20 s. Because PVDF is dispersed throughout the fiber, the fiber softened and compacted during hot pressing, causing local pore structures to collapse and preventing the formation of local fusion-crosslinking nodes.

[0076] Comparative Example 3

[0077] A method for preparing a battery separator:

[0078] Polyimide core spinning solution and polyvinylidene fluoride shell spinning solution were prepared separately. PI core / PVDF shell structured fiber membranes were prepared using coaxial electrospinning. No crosslinkable components were added to the shell layer. The spinning conditions were the same as in Example 1. The resulting fiber membranes were hot-pressed at 100°C and 2 MPa for 20 s and then directly cooled without further crosslinking treatment. SEM observation showed localized thermoplastic adhesion at the fiber cross-sections, but the node structure exhibited some relaxation after immersion in the electrolyte, and some areas showed compaction of the pore structure during hot pressing.

[0079] Comparative Example 4

[0080] A method for preparing a battery separator:

[0081] A commercially available ceramic-coated polyolefin membrane manufactured by Celgard was selected as a comparison sample. The base membrane was approximately 12 μm thick, with double-sided alumina ceramic coatings, totaling approximately 18 μm in thickness and a porosity of approximately 45%. It was used directly for subsequent performance testing without any further processing.

[0082] For the separator samples obtained in each embodiment and comparative example: tensile strength was tested using an electronic universal testing machine, and the test method was performed according to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets"; puncture strength was tested using a separator puncture strength tester, and the test method was performed according to GB / T 36363—2018 "Polyolefin separators for lithium-ion batteries", using a stainless steel needle of specified diameter to puncture the separator vertically at a constant speed, and recording the maximum puncture force; porosity was tested using the n-butanol absorption method, and the test method was performed according to GB / T 36363—2018 "Polyolefin separators for lithium-ion batteries", and the porosity was calculated based on the mass change of the separator before and after liquid absorption and the material density; heat shrinkage rate was determined after heat treatment at 200℃ for 30 min, and liquid absorption rate was tested using the electrolyte immersion method, in which the separator was immersed in 1 mol / L LiPF6 / EC:DMC (volume ratio 1:1) electrolyte at 25±2℃ for 2 minutes. h, after removing the residual liquid from the surface, weigh it and calculate the liquid absorption rate according to the formula (W1-W0) / W0×100%, where W0 is the mass before soaking and W1 is the mass after soaking; the interfacial impedance is tested by AC impedance method.

[0083] For the battery samples: CR2032 coin cells were assembled using the LiFePO4 / graphite system for electrochemical performance testing. The electrolyte used was a 1 M LiPF6 EC / DMC (volume ratio 1:1) system.

[0084] The test results are shown in Table 1.

[0085] Table 1

[0086]

[0087] As can be seen from the above description, the embodiments of the present invention have constructed a core-shell structure fiber composed of a polyimide core layer and a polyvinylidene fluoride shell layer containing crosslinking components, and formed stable fusion-crosslinking composite nodes at the fiber intersections of the shell layer, thereby achieving the technical effect of simultaneously improving the mechanical strength, thermal dimensional stability and interface stability of the diaphragm while maintaining high porosity and interconnected pore network structure.

[0088] Specifically, in each embodiment:

[0089] Comparing Examples 2 and 3 with Example 1, it can be seen that by optimizing the amount of crosslinking components, a more stable three-dimensional network structure can be formed at the fiber intersections, thereby significantly improving the tensile strength and puncture resistance of the final membrane.

[0090] Comparing Example 4 with Example 1, it can be seen that by optimizing the types of crosslinking components, the resulting crosslinking nodes can have both high strength and better ion transport performance and high porosity retention.

[0091] Comparing Examples 5 and 6 with Example 1, it can be seen that by optimizing the hot-pressing conditions, the PVDF shell can fully contact and fuse with the shells of adjacent fibers to form an initial fusion structure, providing a tighter interface for subsequent crosslinking, and ultimately enabling the diaphragm to have better mechanical strength and thermal dimensional stability.

[0092] Comparing Examples 7 to 10 with Example 1, it can be seen that by optimizing the crosslinking treatment conditions, the mechanical strength of the obtained membrane can be significantly enhanced, and the swelling of the shell polymer in the electrolyte can be effectively limited, thereby further improving its stability.

[0093] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0094] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A battery separator, characterized in that, The battery separator is formed by interweaving multiple core-shell type fibers, and cross-linking nodes are formed in the cross-contact areas of the core-shell type fibers; The core-shell type fiber includes a core and a shell covering the core, wherein the core includes polyimide; The shell and the crosslinking nodes both include polyvinylidene fluoride and crosslinking components.

2. The battery separator according to claim 1, characterized in that, The crosslinking component contains polymerizable unsaturated double bonds in its molecular structure; preferably, the crosslinking component is an acrylate compound; more preferably, the crosslinking component is polyethylene glycol diacrylate; and / or, Based on the weight of the polyvinylidene fluoride as 100%, the content of the crosslinking component in the shell layer and the crosslinking node is independently 1 wt.% to 20 wt.%.

3. The battery separator according to claim 1 or 2, characterized in that, In the battery separator, the weight ratio of the polyimide to the polyvinylidene fluoride is (50:50) to (90:10); and / or, In the core-shell type fiber, the thickness of the shell layer is 30nm~300nm, and the diameter of the core is 0.3μm~3μm.

4. The battery separator according to any one of claims 1 to 3, characterized in that, The porosity of the battery separator is 60%~90%; and / or, The thickness of the battery separator is 8μm~20μm; and / or, The tensile strength of the battery separator is 20 MPa to 40 MPa; and / or, The puncture strength of the battery separator is 5N~10N; and / or, The battery separator is subjected to a heat treatment at 200±5℃ for 30±2 min, and its thermal shrinkage rate is 0.1%~5%; and / or, The interfacial impedance of the battery separator is 40Ω~90Ω.

5. A method for preparing a battery separator according to any one of claims 1 to 4, characterized in that, include: Step S1: Prepare a core spinning solution containing polyimide; prepare a shell spinning solution containing polyvinylidene fluoride and crosslinking components. The crosslinking component is a thermal crosslinking component and / or a photocrosslinking component; Step S2: Using the core spinning solution as the inner liquid and the shell spinning solution as the outer liquid, a fiber membrane containing fiber filaments is obtained by coaxial electrospinning. Step S3: The fiber membrane is subjected to hot pressing treatment to form a local initial fusion structure at the intersection of the fiber filaments, thereby obtaining a hot-pressed membrane; Step S4: The hot-pressed film is cross-linked to form cross-linked nodes in the local initial fusion structure, thereby obtaining the battery separator.

6. The method for preparing the battery separator according to claim 5, characterized in that, In step S1 In the core spinning solution, the mass fraction of the polyimide is 10 wt.%~20 wt.%; and / or, In the shell spinning solution, the mass fraction of polyvinylidene fluoride is 5 wt.%~15 wt.%; and / or, In the shell spinning solution, based on the weight of the polyvinylidene fluoride as 100%, the content of the crosslinking component is 1 wt.% to 20 wt.%.

7. The method for preparing the battery separator according to claim 6, characterized in that, The shell spinning solution also includes a thermal initiator and / or a photoinitiator with a mass fraction of 1 wt.% to 3 wt.%; The thermal initiator is one or more of organic peroxide initiators or azo initiators, and the photoinitiator is selected from one or more of photoinitiator 1173, photoinitiator 184, photoinitiator 2959 and photoinitiator 819.

8. The method for preparing the battery separator according to any one of claims 5 to 7, characterized in that, In step S2 The applied voltage for the coaxial electrospinning is 18±2kV, the receiving distance is 15±2cm, and the roller collection speed is 200±20rpm. The coaxial electrospinning uses an inner needle diameter of 0.5±0.1 mm, an outer needle diameter of 1.2±0.2 mm, an inner liquid flow rate of 0.6±0.1 mL / h, and an outer liquid flow rate of 0.8±0.1 mL / h. The coaxial electrospinning was carried out at a temperature of 25±2℃ and a relative humidity of 40±5%RH.

9. The method for preparing the battery separator according to any one of claims 5 to 8, characterized in that, The hot pressing temperature is 80℃~140℃, the hot pressing pressure is 1MPa~5MPa, and the hot pressing time is 5s~60s.

10. The method for preparing the battery separator according to any one of claims 5 to 9, characterized in that, The crosslinking treatment satisfies at least one of the following conditions: (1) Irradiate with ultraviolet light with a wavelength of 200nm~400nm for 100s~200s; (2) Keep warm at 80℃~180℃ for 5min~60min.

11. A secondary battery, comprising a separator, characterized in that, The separator is the battery separator according to any one of claims 1 to 4; or, the separator is prepared by the method for preparing the battery separator according to any one of claims 5 to 10.

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