Battery diaphragm, preparation method thereof and secondary battery

Abstract

The invention discloses a battery diaphragm, a preparation method thereof and a secondary battery, and belongs to the technical field of batteries. The battery diaphragm comprises a base membrane and a heat-resistant layer stacked on the surface of at least one side of the base membrane, the heat-resistant fibers in the heat-resistant layer are selected from polyimide fibers, polyethylene glycol terephthalate fibers, polyamide-imide fibers, polyether-ether-ketone fibers, cellulose nanofibers, poly-p-phenylene terephthamide fibers, poly-m-phenylene isophthamide fibers, polyphenylene sulfide fibers and polyacrylonitrile fibers, and the heat-resistant fibers in the heat-resistant layer are selected from polyimide fibers, polyethylene glycol terephthalate fibers, polyamide-imide fibers, polyether-ether-ketone fibers, cellulose nanofibers, poly-p-phenylene terephthamide fibers, poly-m-phenylene isophthamide fibers, polyphenylene sulfide fibers and polyacrylonitrile fibers. The fiber is at least one of polyvinyl alcohol fiber and poly (p-phenylene benzobisoxazole) fiber; the thickness H1 of the heat-resistant layer and the thickness H2 of the base film satisfy 0.02 < = H2 / H1 < = 10; the porosity (X) of the base film and the porosity (Y) of the heat-resistant layer satisfy 20% < = X + Y < = 90% and 20% < = X < = 60%. The battery diaphragm can be baked at the temperature of 100 DEG C or above, and has relatively high puncture strength.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a battery separator and a preparation method thereof, and a secondary battery. Background Art

[0002] A lithium-ion battery includes a shell, an electrolyte contained inside the shell, positive and negative electrodes, and a separator. Before injecting the electrolyte, the battery needs to be baked to remove moisture from the positive and negative electrodes and the separator.

[0003] Separators in related technologies are typically made of polyolefin materials, such as polyethylene or polypropylene. These materials have poor heat resistance and cannot be baked at temperatures above 100°C. Low-temperature baking (less than 100°C) requires a long baking time, impacting production efficiency and making it difficult to dry out moisture. Furthermore, polyolefin separators have low puncture strength, impacting battery safety and cycle performance. Summary of the Invention

[0004] The embodiments of the present application provide a battery separator and a method for preparing the same, as well as a secondary battery. The battery separator supports baking at temperatures above 100°C and has high puncture strength, thereby improving the safety and cycle performance of the battery. The technical solution is as follows:

[0005] In one aspect, a battery separator is provided, comprising: a base film and a heat-resistant layer laminated on at least one surface of the base film;

[0006] The heat-resistant layer comprises heat-resistant fibers, and the heat-resistant fibers are selected from at least one of polyimide fibers, polyethylene terephthalate fibers, polyamide-imide fibers, polyetheretherketone fibers, cellulose nanofibers, poly(p-phenylene terephthalamide) fibers, poly(m-phenylene isophthalamide) fibers, polyphenylene sulfide fibers, polyacrylonitrile fibers, polyvinyl alcohol fibers, and poly(p-phenylene benzobisoxazole) fibers;

[0007] The thickness of the heat-resistant layer is H1, the thickness of the base film is H2, and H1 and H2 satisfy: 0.02≤H2 / H1≤10;

[0008] The porosity of the base film is X, the porosity of the heat-resistant layer is Y, and X and Y satisfy the following: 20%≤X+Y≤90% and 20%≤X≤60%.

[0009] In a possible implementation, the length of the heat-resistant fiber is D1, the diameter of the heat-resistant fiber is D2, and D1 and D2 satisfy: 0.1≤D1 / D2≤200000.

[0010] In another possible implementation, the heat-resistant layer further includes: inorganic particles and a first binder;

[0011] The weight of the heat-resistant fiber is 5 to 80 parts, the weight of the inorganic particles is 10 to 95 parts, and the weight of the first binder is 0.5 to 20 parts.

[0012] In another possible implementation, the diameter of the heat-resistant fiber is D2, the particle size D50 of the inorganic particles is D3, the thickness of the heat-resistant layer is H1, and D2, D3 and H1 satisfy: H1 / 10<D2+D3<13*H1.

[0013] In another possible implementation, the inorganic particles are selected from at least one of boehmite, alumina, and silica;

[0014] The first binder is selected from at least one of polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

[0015] In another possible implementation, the battery separator further includes: an adhesive layer, arranged between the base film and the heat-resistant layer, for bonding the heat-resistant layer to the base film;

[0016] The adhesive layer includes a second adhesive, and the second adhesive is selected from at least one of polyvinylidene fluoride, polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

[0017] In another possible implementation, the average pore diameter of the heat-resistant layer is 10 nm to 100 nm, and the pore tortuosity is ≤1.5.

[0018] In another aspect, a method for preparing a battery separator is provided. The battery separator is as described in any one of the above items, and the preparation method comprises:

[0019] preparing a slurry corresponding to the heat-resistant layer;

[0020] The heat-resistant layer is bonded to at least one surface of a base film through the slurry to obtain the battery separator.

[0021] In a possible implementation, the slurry corresponding to the heat-resistant layer includes the following components in parts by weight: 5 to 80 parts of heat-resistant fiber, 10 to 95 parts of inorganic particles, 0.5 to 20 parts of a first binder, and 70 to 90 parts of a first solvent;

[0022] The method of bonding the heat-resistant layer to at least one surface of a base film by using the slurry to obtain the battery separator comprises:

[0023] The slurry is applied to at least one side of the surface of the base film and dried, so that the slurry forms the heat-resistant layer on at least one side of the surface of the base film, thereby obtaining the battery separator.

[0024] In another possible implementation, the slurry corresponding to the heat-resistant layer includes the following components in parts by weight: 30 to 80 parts of heat-resistant fiber and 70 to 90 parts of the second solvent;

[0025] The method of bonding the heat-resistant layer to at least one surface of a base film by using the slurry to obtain the battery separator comprises:

[0026] The slurry is prepared into a heat-resistant layer by adopting an electrostatic spinning process;

[0027] The heat-resistant layer is bonded to at least one surface of the base film by a second adhesive and subjected to a thermal composite treatment to obtain the battery separator.

[0028] On the other hand, a secondary battery is provided, comprising: a shell, an electrolyte contained inside the shell, a negative electrode sheet, a positive electrode sheet and a battery separator, wherein the negative electrode sheet and the positive electrode sheet are separated by the battery separator, and the battery separator is as described in any one of the above items.

[0029] The embodiment of the present application provides a battery separator having a heat-resistant layer arranged on the surface of the base film. The heat-resistant layer includes heat-resistant fibers. The heat-resistant fibers have good heat resistance, which is beneficial to improving the heat resistance of the battery separator, allowing the battery separator to support baking at temperatures above 100°C and quickly and thoroughly remove moisture. In addition, the heat-resistant fibers are interwoven into a three-dimensional mesh structure, which is beneficial to improving the puncture strength of the battery separator, giving the battery separator excellent resistance to foreign matter, significantly reducing the Hi-pot (High Potential Test) short-circuit rate caused by foreign matter puncture during the battery assembly production process, thereby improving the safety and cycle performance of the battery. DETAILED DESCRIPTION

[0030] In order to make the technical solutions and advantages of the present application clearer, the implementation methods of the present application are described in further detail below.

[0031] In one aspect, an embodiment of the present application provides a battery separator, comprising: a base film and a heat-resistant layer stacked on at least one surface of the base film.

[0032] The heat-resistant layer includes heat-resistant fibers, and the heat-resistant fibers are selected from at least one of polyimide (PI) fibers, polyethylene terephthalate (PET) fibers, polyamide-imide (PAI) fibers, polyetheretherketone (PEEK) fibers, cellulose nanofibers, poly(p-phenylene terephthalamide) fibers (para-aramid), poly(m-phenylene isophthalamide) fibers (meta-aramid), polyphenylene sulfide (PPS) fibers, polyacrylonitrile (PAN) fibers, polyvinyl alcohol (PVA) fibers, and poly(p-phenylene benzobisoxazole) (PBO) fibers.

[0033] The thickness of the heat-resistant layer is H1, the thickness of the base film is H2, and H1 and H2 satisfy: 0.02≤H2 / H1≤10.

[0034] The porosity of the base film is X, the porosity of the heat-resistant layer is Y, and X and Y satisfy: 20%≤X+Y≤90% and 20%≤X≤60%.

[0035] The present invention provides a battery separator having a heat-resistant layer disposed on the surface of a base film. The heat-resistant layer includes heat-resistant fibers. The heat-resistant fibers have excellent heat resistance, which helps improve the heat resistance of the battery separator, allowing the battery separator to withstand baking temperatures above 100°C and quickly and thoroughly remove moisture. Furthermore, the heat-resistant fibers are interwoven into a three-dimensional mesh structure, which helps improve the puncture strength of the battery separator, giving the battery separator excellent resistance to foreign matter. This significantly reduces the Hi-pot short-circuit rate caused by foreign matter puncture during the battery assembly process, thereby improving the safety and cycle performance of the battery.

[0036] As described above, the heat-resistant fiber provided in the embodiments of the present application is selected from at least one of PI fiber, PET fiber, PAI fiber, PEEK fiber, cellulose nanofiber, para-aramid, meta-aramid, PPS fiber, PAN fiber, PVA fiber, and PBO fiber. Furthermore, the heat-resistant fiber is selected from at least one of PI fiber, para-aramid, meta-aramid, and cellulose nanofiber.

[0037] PI fibers have a decomposition temperature of approximately 500°C and above, demonstrating excellent heat resistance and chemical stability. PET fibers have a melting point of approximately 250°C and exhibit good heat resistance. PAI fibers have a glass transition temperature of approximately 280°C to 290°C and a decomposition temperature of approximately 300°C and above, exhibiting excellent heat resistance. PEEK fibers have a melting point of approximately 340°C and exhibit good heat resistance. The decomposition temperature of cellulose nanofibers depends on their source and processing, but is generally between 200°C and 300°C, demonstrating excellent heat resistance. Both para-aramid and meta-aramid fibers exhibit excellent high-temperature resistance. Para-aramid has a glass transition temperature of above 300°C and a thermal decomposition temperature of up to 560°C. Meta-aramid has a glass transition temperature of 270°C and exhibits no significant decomposition or carbonization below 350°C. PPS fibers exhibit excellent heat resistance and mechanical properties, with an initial thermal degradation temperature of 542°C and a maximum decomposition rate of 586°C. The decomposition temperatures of PAN fiber and PVA fiber are both above 200°C, and the decomposition temperature of PBO fiber is around 650°C, which shows excellent heat resistance.

[0038] Therefore, using the above materials as heat-resistant fibers can significantly improve the heat resistance of the battery separator, so that the battery separator will not be closed at ≥200℃. The baking temperature of the secondary battery prepared with the battery separator can be increased from 60℃~105℃ to 200℃ before liquid injection. On the one hand, this can ignore the moisture problem in the battery assembly production process, and use a baking temperature of 200℃ in the baking stage to completely remove moisture; on the other hand, baking at a high temperature of 200℃ can greatly shorten the moisture baking time, improve production efficiency and reduce energy consumption.

[0039] In addition, the above-mentioned material as a three-dimensional network structure woven into heat-resistant fibers is beneficial to improving the puncture strength of the battery separator, so that the battery separator has excellent resistance to foreign matter, and the puncture strength is ≥5.0N / m. The secondary battery prepared with this battery separator can greatly reduce the Hi-pot short circuit rate caused by foreign matter puncture during the battery assembly production process.

[0040] In a possible implementation, the thickness H1 of the heat-resistant layer is 0.5 μm to 30 μm, and the thickness H2 of the base film is 3 μm to 25 μm.

[0041] Combined with H1 and H2 mentioned above, H1 and H2 satisfy: 0.02≤H2 / H1≤10.

[0042] For example, the thickness H1 of the heat-resistant layer may be 0.5 μm, 1 μm, 2 μm, 5 μm, 8 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, etc., the thickness H2 of the base film may be 3 μm, 5 μm, 8 μm, 10 μm, 15 μm, 18 μm, 20 μm, 25 μm, etc., and H2 / H1 may be 0.02, 0.03, 0.05, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. Furthermore, 0.02≤H2 / H1≤1 or 1≤H2 / H1≤10.

[0043] In the embodiment of the present application, when 0.02≤H2 / H1≤1, it means that the thickness of the base film is less than or equal to the thickness of the heat-resistant layer, the thicker heat-resistant layer improves the heat resistance and puncture strength of the battery separator through the heat-resistant fiber, and the thinner base film relies on the heat resistance of the thicker heat-resistant layer to inhibit high-temperature shrinkage and prevent the base film from melting and deforming. When 1≤H2 / H1≤10, it means that the thickness of the base film is greater than or equal to the thickness of the heat-resistant layer, the thicker base film bears the main mechanical stress, the high ductility of the base film absorbs the puncture energy, and prevents the heat-resistant layer from brittle cracking. At the same time, the thinner heat-resistant layer provides local anti-puncture support through the three-dimensional network structure woven by the heat-resistant fibers. In summary, H2 / H1 can be used to adjust the mechanical division of labor between the base film and the heat-resistant layer to balance the overall heat resistance and puncture strength of the battery separator.

[0044] As described above, X and Y satisfy: 20%≤X+Y≤90% and 20%≤X≤60%.

[0045] For example, X+Y may be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, etc., and X may be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, etc. Correspondingly, if 0≤Y≤70%, Y may be 0, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, etc.

[0046] In the embodiment of the present application, 20% ≤ X + Y ≤ 90%. Therefore, when the porosity of the base membrane is greater than that of the heat-resistant layer, the high-porosity base membrane can improve the wettability of the electrolyte, and the low-porosity heat-resistant layer can compensate for thermal stability through the heat-resistant fibers. When the porosity of the heat-resistant layer is greater than that of the base membrane, the high-porosity heat-resistant layer can absorb the electrolyte to form a thermal buffer layer, while the low-porosity base membrane maintains mechanical integrity, preventing the mechanical strength of the battery separator from decreasing.

[0047] In a possible implementation, the length D1 of the heat-resistant fiber is 0.5 μm to 1000 μm, and the diameter D2 of the heat-resistant fiber is 0.005 μm to 5.0 μm.

[0048] In combination with D1 and D2 described above, D1 and D2 satisfy: 0.1≤D1 / D2≤200000.

[0049] For example, the length D1 of the heat-resistant fiber can be 0.5 μm, 1 μm, 5 μm, 10 μm, 20 μm, 50 μm, 100 μm, 200 μm, 300 μm, 500 μm, 800 μm, 900 μm, 1000 μm, etc., and the diameter D2 of the heat-resistant fiber can be 0.0005 μm, 0.001 μm, 0.002 μm, 0.005 μm, 0.01 μm, 0.05 μm. , 0.1 μm, 0.5 μm, 1.0 μm, 2.0 μm, 5.0 μm, etc., and D1 / D2 can be 0.1, 0.5, 1, 5, 50, 100, 200, 300, 500, 800, 1000, 2000, 5000, 8000, 10000, 20000, 50000, 80000, 100000, 150000, 180000, 200000, etc. Further, 0.1≤D1 / D2≤1000 or 1000≤D1 / D2≤200000.

[0050] In the embodiment of the present application, when 0.1≤D1 / D2≤1000, it indicates that the heat-resistant fiber is a low aspect ratio fiber. The low aspect ratio fiber is easier to disperse evenly, reducing the concentration of thermal expansion stress inside the coating, thereby improving the heat resistance of the battery separator. In addition, the low aspect ratio fiber can also enhance the puncture strength of the heat-resistant layer through the short fiber bridging mechanism, preventing the coating from being delaminated and peeling during the puncture process, thereby improving the puncture strength of the battery separator. When 1000≤D1 / D2≤200000, it indicates that the heat-resistant fiber is a high aspect ratio fiber. The high aspect ratio fiber forms a denser three-dimensional network structure, which can effectively block the heat conduction path, thereby improving the heat resistance of the battery separator. In addition, the high aspect ratio fiber can form a more effective support structure, effectively resisting puncture impact, thereby improving the puncture strength of the battery separator.

[0051] In the embodiments of the present application, a heat-resistant layer slurry can be applied to the surface of a base film to form a battery separator. Accordingly, the heat-resistant layer further comprises inorganic particles and a first binder; the heat-resistant fiber comprises 5 to 80 parts by weight, the inorganic particles comprise 10 to 95 parts by weight, and the first binder comprises 0.5 to 20 parts by weight.

[0052] For example, the weight ratio of the heat-resistant fiber can be 5 parts, 8 parts, 10 parts, 20 parts, 30 parts, 40 parts, 50 parts, 60 parts, 70 parts, 80 parts, etc., and the weight ratio of the inorganic particles can be 10 parts, 20 parts, 30 parts, 40 parts, 50 parts, 60 parts, 70 parts, 80 parts, 90 parts, 95 parts, etc. The weight ratio of the first binder can be 0.5 parts, 1 part, 2 parts, 3 parts, 4 parts, 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, 12 parts, 15 parts, 18 parts, 20 parts, etc.

[0053] In the embodiments of the present application, both the heat-resistant fibers and inorganic particles have excellent heat resistance. Therefore, a higher content of heat-resistant fibers and inorganic particles helps improve the heat resistance of the battery separator, allowing the battery separator to withstand baking temperatures above 100°C and quickly and thoroughly remove moisture. Furthermore, the high content of heat-resistant fibers interwoven into a three-dimensional network structure and the high hardness of the inorganic particles synergistically improve the puncture strength of the battery separator, giving the battery separator excellent resistance to foreign matter, significantly reducing the Hi-pot short circuit rate caused by foreign matter puncture during battery assembly production, thereby improving battery safety and cycle performance. When the weight percentage of the first binder is less than 0.5 parts, it is not possible to effectively and firmly bond the heat-resistant fibers to the inorganic particles. When the weight percentage of the first binder is greater than 20 parts, it increases costs. Therefore, limiting the weight percentage of the first binder to 0.5 to 20 parts can effectively and firmly bond the heat-resistant fibers to the inorganic particles, improve the bonding strength between the base film and the heat-resistant layer, and reduce costs.

[0054] In a possible implementation, the particle size D50 of the inorganic particles is D3, and D3 is 0.1 μm to 1.5 μm.

[0055] Combining D2, D3 and H1 above, D2, D3 and H1 satisfy: H1 / 10<D2+D3<13*H1.

[0056] Illustratively, D30 can be 0.1μm, 0.2μm, 0.5μm, 0.8μm, 1.0μm, 1.2μm, 1.5μm, etc., and D2+D3 can be H1 / 9, H1 / 8, H1 / 7, H1 / 6, H1 / 5, H1 / 4, H1 / 3, H1 / 2, H1, 2H1, 3H1, 4H1, 5H1, 6H1, 7H1, 8H1, 9H1, 10H1, 11H1, 12H1, etc.

[0057] In the embodiment of the present application, when D2+D313*H1, it means that the diameter of the heat-resistant fiber and / or the particle size of the inorganic particles are large, which easily leads to long fibers and / or large particles penetrating the surface of the heat-resistant layer and destroying the base membrane support structure. Therefore, limiting H1 / 10<D2+D3<13*H1 can form a stable skeleton through the mechanical interlocking between the heat-resistant fibers and the inorganic particles, avoiding the loose structure of the heat-resistant layer, and can also avoid long fibers and / or large particles penetrating the surface of the heat-resistant layer and destroying the base membrane support structure, thereby improving the stability of the battery separator.<h1>

[0058] In a possible implementation, the inorganic particles are selected from at least one of boehmite, alumina, and silica; and the first binder is an acrylic binder selected from at least one of polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

[0059] Boehmite has a melting point of approximately 1900°C, alumina has a melting point of approximately 2000°C, and silica has a melting point of approximately 1700°C. Boehmite, alumina, and silica all have high hardness. Therefore, using these materials as inorganic particles can significantly improve the heat resistance and puncture strength of battery separators.

[0060] Polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate contain polar groups. Based on their polar groups, the heat-resistant fibers can be effectively and firmly combined with the inorganic particles, thereby improving the bonding strength between the base film and the heat-resistant layer.

[0061] In the embodiments of the present application, the heat-resistant layer can be first formed into a heat-resistant layer by first forming the heat-resistant layer from the slurry, then a second adhesive can be applied to the surface of the base film, and finally the heat-resistant layer and the base film can be bonded together using the second adhesive to form a battery separator. Accordingly, the battery separator further includes an adhesive layer disposed between the base film and the heat-resistant layer for bonding the heat-resistant layer to the base film.

[0062] The adhesive layer includes a second adhesive, and the second adhesive is selected from at least one of polyvinylidene fluoride, polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

[0063] In the embodiment of the present application, the above-mentioned material is used as the second adhesive, and the heat-resistant layer can be firmly bonded to the base film through intermolecular forces, thereby improving the bonding strength between the base film and the heat-resistant layer.

[0064] In a possible implementation, the thickness of the adhesive layer is 0.1 μm to 2 μm.

[0065] For example, the thickness of the adhesive layer may be 0.1 μm, 0.2 μm, 0.5 μm, 0.8 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, etc.

[0066] In the embodiment of the present application, when the thickness of the adhesive layer is too thin (<0.1 μm), the bonding strength between the heat-resistant layer and the base film may be insufficient, and stratification may easily occur during the charge and discharge cycle. When the thickness of the adhesive layer is too thick (>2 μm), the overall brittleness of the diaphragm may increase. Therefore, limiting the thickness of the adhesive layer to between 0.1 μm and 2 μm can not only improve the bonding strength between the heat-resistant layer and the base film, but also achieve effective stress transfer between the heat-resistant layer and the base film, thereby improving the puncture strength of the diaphragm.

[0067] In a possible implementation, the average pore diameter of the heat-resistant layer is 10 nm to 100 nm, and the pore tortuosity is ≤1.5.

[0068] For example, the average pore size of the heat-resistant layer can be 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, etc. Further, the average pore size of the heat-resistant layer can be 10 to 50 nm or 50 to 100 nm. The pore tortuosity can be 1.5, 1.4, 1.3, 1.2, 1.1, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, etc.

[0069] In the embodiments of the present application, when the average pore diameter of the heat-resistant layer is 10 nm to 50 nm, it indicates that the average pore diameter of the heat-resistant layer is relatively small, which can improve the electrolyte adsorption capacity by increasing the specific surface area, enhance the wettability of the battery separator, and reduce lithium ion concentration polarization. When the average pore diameter of the heat-resistant layer is 50 nm to 100 nm, it indicates that the average pore diameter of the heat-resistant layer is relatively large, which can reduce ion transmission resistance and accelerate the charge and discharge rate.

[0070] In addition, the pore tortuosity is ≤1.5, indicating that the pores are close to straight lines, which can reduce the resistance to ion migration and improve ionic conductivity. Moreover, the low tortuosity can effectively suppress the risk of lithium dendrite penetration.

[0071] In a possible implementation, the base film is selected from at least one of polyethylene (PE) and polypropylene (PP).

[0072] When the base film is selected from PE and PP, the base film may be a co-extruded base film formed of PP / PE / PP.

[0073] In a possible implementation, the thickness H2 of the base film is 3 μm to 25 μm.

[0074] For example, the thickness of the base film may be 3 μm, 5 μm, 7 μm, 9 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, or the like.

[0075] In the embodiment of the present application, the thickness H2 of the base film is between 3 μm and 25 μm, which can effectively reduce the total thickness of the diaphragm, provide space for improving the energy density of the battery, and effectively improve the puncture strength of the diaphragm.

[0076] For any of the aforementioned battery separators, the closed-cell temperature of the separator tested using the TMA (thermomechanical analysis) method is ≥ 200°C. This indicates that the separator has high heat resistance and can be baked at 200°C, reducing the moisture control requirements during the battery assembly production process, and even eliminating the need for ambient moisture control, thereby reducing production costs.

[0077] On the other hand, an embodiment of the present application further provides a method for preparing a battery separator, wherein the battery separator is as described above, and the preparation method comprises:

[0078] A slurry corresponding to the heat-resistant layer is prepared; the heat-resistant layer is bonded to at least one surface of the base film through the slurry to obtain a battery separator.

[0079] In one possible implementation, the slurry corresponding to the heat-resistant layer includes the following components by weight: 5 to 80 parts heat-resistant fiber, 10 to 95 parts inorganic particles, 0.5 to 20 parts first binder, and 70 to 90 parts first solvent. The heat-resistant fiber, inorganic particles, first binder, and first solvent are uniformly mixed to form a slurry having a solids content of 10% to 35%. The slurry is applied to at least one side of the base film and dried to form a heat-resistant layer on at least one side of the base film, thereby obtaining a battery separator.

[0080] Illustratively, the weight proportion of the first solvent can be 70 parts, 75 parts, 80 parts, 82 parts, 85 parts, 90 parts, etc., and the solid content of the slurry can be 10%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 35%, etc.

[0081] In this implementation, the slurry can be evenly coated on at least one side of the base film by roller coating.

[0082] The first solvent can be set and changed as needed, and is not specifically limited thereto. For example, the first solvent can be an organic solvent such as water, N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), dimethylacetamide (DMAc), acetone, ethyl acetate, toluene, xylene, or butanone.

[0083] In another possible implementation, the slurry corresponding to the heat-resistant layer includes the following components by weight: 30 to 80 parts heat-resistant fiber and 70 to 90 parts second solvent. The heat-resistant fiber is dissolved in the second solvent to obtain a slurry with a solid content of 5% to 30%. The slurry is prepared into a heat-resistant layer using an electrospinning process. The heat-resistant layer is bonded to at least one side of the base film using a second binder and subjected to a thermal composite treatment to obtain a battery separator.

[0084] For example, the weight proportion of the heat-resistant fiber can be 30 parts, 40 parts, 50 parts, 60 parts, 70 parts, 80 parts, etc., the weight proportion of the second solvent can be 70 parts, 72 parts, 75 parts, 80 parts, 82 parts, 85 parts, 90 parts, etc., and the solid content of the slurry can be 5%, 10%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, etc.

[0085] In this implementation, the second adhesive slurry can be first coated on the surface of the base film, and then the heat-resistant layer can be bonded to at least one side surface of the base film, and then the heat-resistant layer and the base film can be thermally composited through a thermal composite process to obtain a battery separator.

[0086] The second binder slurry includes 20 to 90 parts by weight of the second binder and 50 to 98 parts by weight of the third solvent. The second binder slurry is obtained by uniformly mixing the second binder and the third solvent. The solid content of the second binder slurry is 5% to 35%.

[0087] Exemplarily, the weight proportions of the second binder can be 20 parts, 30 parts, 40 parts, 50 parts, 60 parts, 70 parts, 80 parts, 85 parts, 90 parts, etc., the weight proportions of the third solvent can be 50 parts, 60 parts, 70 parts, 80 parts, 85 parts, 90 parts, 95 parts, 98 parts, etc., and the solid content of the second binder slurry can be 5%, 10%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 35%, etc.

[0088] The conditions of the electrospinning process are: voltage of 10 kV to 30 kV, and receiving distance of 3 cm to 50 cm.

[0089] The conditions of the thermal composite process are as follows: the thermal composite temperature is 40° C. to 80° C., and the thermal composite pressure is 0.1 tons to 10 tons.

[0090] For example, the voltage of the electrospinning process can be 10kV, 15kV, 18kV, 20kV, 22kV, 25kV, 28kV, 30kV, etc., and the receiving distance can be 3cm, 5cm, 10cm, 20cm, 25cm, 30cm, 35cm, 40cm, 50cm, etc.

[0091] The thermal composite temperature of the thermal composite process can be 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, etc., and the thermal composite pressure can be 0.1 ton, 0.5 ton, 1 ton, 2 tons, 5 tons, 8 tons, 9 tons, 10 tons, etc.

[0092] In the embodiments of the present application, the interface matching between the heat-resistant layer and the base membrane can be optimized by controlling the conditions of the electrospinning process, and the interface bonding strength between the heat-resistant layer and the base membrane can be enhanced by controlling the conditions of the thermal composite process, thereby improving the performance of the diaphragm.

[0093] In some examples, an inorganic particle slurry may be first coated on the surface of the base film. After drying, the inorganic particles form an inorganic particle layer on the surface of the base film. A second adhesive slurry may then be coated on the surface of the inorganic particle layer. The heat-resistant layer may then be bonded to at least one side of the base film through the second adhesive slurry. The heat-resistant layer may then be thermally composited with the base film through a thermal composite process to obtain a battery separator.

[0094] The second solvent may be the same as or different from the first solvent, and there is no specific limitation on this. In the embodiment of the present application, the second solvent may also be selected from one or more organic solvents such as water, NMP, DMF, DMAc, acetone, ethyl acetate, toluene, xylene, and butanone.

[0095] The third solvent may be the same as or different from the first solvent, and is not specifically limited thereto. In the embodiments of the present application, the third solvent may also be selected from one or more organic solvents such as water, NMP, DMF, DMAc, acetone, ethyl acetate, toluene, xylene, and butanone.

[0096] It can be seen that the present application can adopt different methods to prepare battery separators according to actual needs.

[0097] On the other hand, an embodiment of the present application also provides a secondary battery, which includes: a shell, an electrolyte contained inside the shell, a negative electrode plate, a positive electrode plate and a battery separator, wherein the negative electrode plate and the positive electrode plate are separated by a battery separator, and the battery separator is as described above.

[0098] The secondary battery provided in the embodiments of the present application has all the advantages of a battery separator. In addition, the secondary battery can be a lithium-ion battery or other types of batteries, such as a sodium-ion battery, without specific limitation.

[0099] The technical solution of this application will be described in detail below through specific embodiments.

[0100] In the following specific examples, operations involving conditions not specified were performed according to conventional conditions or conditions recommended by the manufacturer.

[0101] Example 1

[0102] Example 1 provides a battery separator comprising: a PE base film; and a heat-resistant layer bonded to one surface of the base film via an adhesive layer. The PE base film has a thickness of 7 μm and a porosity of 40%. The heat-resistant layer comprises PI fibers, each 150 μm long and 1.0 μm in diameter. The heat-resistant layer has a thickness of 10 μm and a porosity of 50%. The adhesive layer comprises PVDF and has a thickness of 0.2 μm.

[0103] The preparation method of the battery separator is as follows:

[0104] PI fibers with a length of 150 μm and a diameter of 1.0 μm were dissolved in DMF to obtain a slurry with a solid content of 20%. PVDF was dissolved in NMP to obtain a second binder slurry with a solid content of 18%. The slurry was made into a heat-resistant layer using an electrospinning process, and then a layer of the second binder slurry was coated on the surface of the base film as a bonding layer. The heat-resistant layer was bonded to the surface of the base film through the bonding layer, and then the heat-resistant layer and the base film were thermally composited using a thermal composite process to obtain a battery separator. Among them, the voltage of the electrospinning process was 20 kV, the receiving distance was 10 cm, the thermal composite temperature of the thermal composite process was 60 ° C, and the thermal composite pressure was 3 tons.

[0105] Example 2

[0106] Example 2 provides a battery separator, the structure of which can be referred to Example 1, except that the base film is a PP base film with a thickness of 16 μm and a porosity of 50%; the heat-resistant layer includes PET fibers, the length of the PET fibers is 300 μm, the diameter is 2.3 μm, the thickness of the heat-resistant layer is 8 μm, and the porosity is 30%; the bonding layer includes polyethyl acrylate, and the thickness of the bonding layer is 0.5 μm.

[0107] The preparation method of the battery separator can be referred to Example 1, except that PET fibers are dissolved in NMP to obtain a slurry with a solid content of 23%, and polyethyl acrylate is dissolved in NMP to obtain a second binder slurry with a solid content of 15%.

[0108] Example 3

[0109] Example 3 provides a battery separator, the structure of which can be referred to Example 1, except that the base film is a PP base film with a thickness of 10 μm and a porosity of 30%; the heat-resistant layer includes cellulose nanofibers with a length of 80 μm and a diameter of 0.8 μm, the heat-resistant layer has a thickness of 12 μm and a porosity of 40%; the bonding layer includes polymethyl acrylate, and the bonding layer has a thickness of 1 μm.

[0110] The preparation method of the battery separator can be referred to Example 1, except that the cellulose nanofibers are dissolved in DMAc to obtain a slurry with a solid content of 25%, and polymethyl acrylate is dissolved in DMAc to obtain a second binder slurry with a solid content of 20%.

[0111] Example 4

[0112] Example 4 provides a battery separator comprising a PE base film and a heat-resistant layer laminated on one surface of the base film. The PE base film has a thickness of 7 μm and a porosity of 40%. The heat-resistant layer comprises 10 parts para-aramid, 90 parts boehmite, and 15 parts polymethyl methacrylate. The para-aramid has a length of 180 μm and a diameter of 0.8 μm. The boehmite has a D50 of 0.8 μm. The heat-resistant layer has a thickness of 12 μm and a porosity of 50%.

[0113] The preparation method of the battery separator is as follows:

[0114] Para-aramid, boehmite, polymethyl methacrylate and NMP are mixed evenly to obtain a slurry with a solid content of 25%. The slurry is evenly coated on one surface of the base film by roller coating, and dried to obtain a battery separator.

[0115] Example 5

[0116] Example 5 provides a battery separator. The structure of this battery separator is similar to that of Example 4, except that the base film is a PP base film with a thickness of 14 μm and a porosity of 30%. The heat-resistant layer comprises 50 parts of meta-aramid, 40 parts of aluminum oxide, and 10 parts of polyethyl acrylate. The meta-aramid is 200 μm long and 1.2 μm in diameter, and the aluminum oxide has a D50 of 0.8 μm. The heat-resistant layer is 10 μm thick and has a porosity of 20%.

[0117] The preparation method of the battery separator can be referred to Example 4, except that the meta-aramid, alumina, polyethyl acrylate and water are uniformly mixed to obtain a slurry with a solid content of 25%.

[0118] Example 6

[0119] Example 6 provides a battery separator. The structure of this battery separator is similar to that of Example 4, except that the base film is a PP / PE / PP co-extruded base film with a thickness of 16 μm and a porosity of 20%. The heat-resistant layer comprises 80 parts of PEEK fiber, 15 parts of silica, and 5 parts of polymethyl acrylate. The PEEK fiber is 430 μm long and 1.1 μm in diameter. The silica has a D50 of 0.8 μm. The heat-resistant layer is 10 μm thick and has a porosity of 60%.

[0120] The preparation method of the battery separator can be referred to Example 4, except that the PEEK fiber, silica, polymethyl acrylate and DMAc are uniformly mixed to obtain a slurry with a solid content of 25%.

[0121] The battery separators prepared in Examples 1 to 6 were subjected to closed-cell temperature tests, thermal shrinkage tests, and puncture strength tests. The test items are as follows:

[0122] Closed-cell temperature test method:

[0123] Cut the diaphragm into long strips, such as 15 mm long and 5 mm wide, and ensure that the edges are smooth and free of burrs.

[0124] The specimen was fixed vertically in the TMA fixture and a constant load (≤0.1N) was applied.

[0125] Set the heating program: starting temperature is 25℃, heating rate is 5℃ / min, and ending temperature is 300℃.

[0126] Record the curve of the sample length changing with temperature, and determine the intersection of the tangent lines when the length begins to decrease (i.e. the starting point of shrinkage) on the curve. The temperature corresponding to this intersection is the closed-cell temperature.

[0127] Thermal shrinkage test method:

[0128] The core steps include: sampling → initial measurement → high temperature treatment → cooling measurement → calculation of thermal shrinkage.

[0129] Sampling: Specifically, uniform rectangular battery separator samples with a size of 100mm×100mm are cut from the separator roll, and 3 sheets are cut as a set of parallel samples; and the number axis (XY axis) is drawn on the surface of each of the 3 battery separator samples as the measurement reference, and a marking line L is determined on the reference.

[0130] Initial measurement: Specifically, the coordinates of the marking line L are measured using a laser micrometer (with an accuracy of 0.01); the coordinates are measured twice in the X direction and twice in the Y direction, and the average value is taken to finally determine the coordinates of the marking line as (X0, Y0).

[0131] High temperature treatment: Specifically, the target temperature of the hot air circulation oven (oven temperature deviation ≤ ± 2°C) is set to 130°C, and the three battery separators of the group are placed in the oven at the same time for 1 hour.

[0132] Cooling measurement: After turning off the power to the oven, the diaphragm group was moved into a dryer and cooled to room temperature for 30 minutes. The coordinates of the marking line L were then measured using a laser micrometer (with an accuracy of 0.01). The coordinates were measured twice in the X and Y directions respectively, and the average values ​​were taken to finally determine the coordinates of the marking line as (X1, Y1).

[0133] Among them, the transverse thermal shrinkage rate is the length change of the battery separator in the X direction, and the calculation formula is:

[0134]

[0135] The longitudinal thermal shrinkage rate is the length change of the battery separator in the Y direction, and the calculation formula is:

[0136]

[0137] Puncture strength test method:

[0138] Environmental settings: Set the environmental chamber temperature to normal temperature, for example, 25°C, and keep the temperature stable.

[0139] Sample installation: Place the diaphragm flat on the test platform of the puncture strength tester to ensure that the sample is firmly fixed and will not move during the test.

[0140] Initial measurement: Measure the average thickness c of the battery separator for subsequent calculation of puncture strength.

[0141] Puncture test: Start the puncture strength tester and make the puncture probe puncture the battery diaphragm at the specified speed and force. The puncture strength tester automatically records the force value change during the puncture process until the puncture probe completely penetrates the diaphragm, and records the maximum puncture force value F at this time.

[0142] Calculation of puncture strength P: Repeat the above steps for the battery separator. According to the measured average thickness c of the battery separator and the maximum puncture force value F, calculate the puncture strength according to the puncture strength calculation formula P = F / c. Take the average value of multiple measurements to obtain P.

[0143] The test results of Examples 1 to 6 are shown in Table 1.

[0144] Table 1

[0145]

[0146] It can be seen from Table 1 that the diaphragm provided in the present application has a relatively high closed-cell temperature, both greater than 200° C., and has relatively small transverse and longitudinal thermal shrinkage rates, while also having relatively high puncture strength.

[0147] The above description is only for the purpose of facilitating those skilled in the art to understand the technical solution of this application and is not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included in the scope of protection of this application.

Claims

1. A battery separator, characterized in that: The battery separator comprises: a base film and a heat-resistant layer stacked on at least one side of the base film; The heat-resistant layer comprises heat-resistant fibers, and the heat-resistant fibers are selected from at least one of polyimide fibers, polyethylene terephthalate fibers, polyamide-imide fibers, polyetheretherketone fibers, cellulose nanofibers, poly(p-phenylene terephthalamide) fibers, poly(m-phenylene isophthalamide) fibers, polyphenylene sulfide fibers, polyacrylonitrile fibers, polyvinyl alcohol fibers, and poly(p-phenylene benzobisoxazole) fibers; The thickness of the heat-resistant layer is H1, the thickness of the base film is H2, and H1 and H2 satisfy: 0.02≤H2 / H1≤10; The porosity of the base film is X, the porosity of the heat-resistant layer is Y, and X and Y satisfy the following: 20%≤X+Y≤90% and 20%≤X≤60%.

2. The battery separator according to claim 1, characterized in that The length of the heat-resistant fiber is D1, the diameter of the heat-resistant fiber is D2, and D1 and D2 satisfy: 0.1≤D1 / D2≤200000.

3. The battery separator according to claim 1, characterized in that The heat-resistant layer further comprises: inorganic particles and a first binder; The weight of the heat-resistant fiber is 5 to 80 parts, the weight of the inorganic particles is 10 to 95 parts, and the weight of the first binder is 0.5 to 20 parts.

4. The battery separator according to claim 3, characterized in that The diameter of the heat-resistant fiber is D2, the particle size D50 of the inorganic particles is D3, the thickness of the heat-resistant layer is H1, and D2, D3 and H1 satisfy: H1 / 10<D2+D3<13*H1.

5. The battery separator according to claim 4, characterized in that: The inorganic particles are selected from at least one of boehmite, alumina, and silica; The first binder is selected from at least one of polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

6. The battery separator according to claim 1, characterized in that The battery separator further includes: an adhesive layer disposed between the base film and the heat-resistant layer, for bonding the heat-resistant layer to the base film; The adhesive layer includes a second adhesive, and the second adhesive is selected from at least one of polyvinylidene fluoride, polymethyl methacrylate, polymethyl acrylate, and polyethyl acrylate.

7. The battery separator according to any one of claims 1 to 6, characterized in that The average pore diameter of the heat-resistant layer is 10 nm to 100 nm, and the pore tortuosity is ≤1.

5.

8. A method for preparing a battery separator, characterized in that: The battery separator according to any one of claims 1 to 7, wherein the preparation method comprises: preparing a slurry corresponding to the heat-resistant layer; The heat-resistant layer is bonded to at least one surface of a base film through the slurry to obtain the battery separator.

9. The preparation method according to claim 8, characterized in that The slurry corresponding to the heat-resistant layer includes the following components in parts by weight: 5 to 80 parts of heat-resistant fibers, 10 to 95 parts of inorganic particles, 0.5 to 20 parts of a first binder, and 70 to 90 parts of a first solvent; The method of bonding the heat-resistant layer to at least one surface of a base film by using the slurry to obtain the battery separator comprises: The slurry is applied to at least one side of the surface of the base film and dried, so that the slurry forms the heat-resistant layer on at least one side of the surface of the base film, thereby obtaining the battery separator.

10. The preparation method according to claim 8, characterized in that The slurry corresponding to the heat-resistant layer includes the following components in parts by weight: 30 to 80 parts of heat-resistant fiber and 70 to 90 parts of the second solvent; The method of bonding the heat-resistant layer to at least one surface of a base film by using the slurry to obtain the battery separator comprises: The slurry is prepared into a heat-resistant layer by adopting an electrostatic spinning process; The heat-resistant layer is bonded to at least one surface of the base film by a second adhesive and subjected to a thermal composite treatment to obtain the battery separator.

11. A secondary battery, characterized in that: The secondary battery comprises: a shell, an electrolyte contained in the shell, a negative electrode sheet, a positive electrode sheet and a battery separator, wherein the negative electrode sheet and the positive electrode sheet are separated by the battery separator, and the battery separator is as described in any one of claims 1 to 7.

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