High peel strength high heat resistant battery separator, method of making and secondary battery

Abstract

The application relates to the technical field of battery separator membranes, and more particularly to a high-peeling-strength high-heat-resistance battery separator membrane, a preparation method thereof and a secondary battery. The battery separator membrane comprises a porous base material and a self-assembled coating arranged on at least one surface of the porous base material, the self-assembled coating comprises self-assembled fibers and heat-resistant particles, the heat-resistant particles are filled in a porous network formed by the self-assembled fibers, and the self-assembled fibers are mainly formed by self-assembly of a plurality of nanofibers. The self-assembled fibers formed by self-assembly of a plurality of nanofibers form a skeleton structure filled with heat-resistant particles, the mechanical strength and the thermal stability of the self-assembled fibers are improved, the porous network structure formed by the self-assembled fibers enables the coating to have better bonding force with the porous base material, the self-assembled fibers and the heat-resistant particles also have better bonding force, the peeling strength of the coating is improved, the stability of the size of the separator membrane is effectively ensured, the heat resistance of the separator membrane is improved, and the problems of coating falling off or powdering are solved.

Description

Technical Field

[0001] This application relates to the field of battery separator technology, and more specifically to a high peel strength and high heat resistance battery separator, its preparation method, and a secondary battery. Background Technology

[0002] Sodium-ion batteries mainly consist of a positive electrode, an electrolyte, a negative electrode, and a separator. The separator is known as the third "electrode material" in the battery, which shows that it is a key component of sodium-ion batteries. The functions of the separator in sodium-ion batteries include: (1) separating the positive and negative electrodes to prevent the active materials of the positive and negative electrodes from coming into contact with each other and to prevent short circuits inside the battery; (2) maintaining sufficient electrolyte and serving as a transport channel for sodium ions during charging and discharging, while the separator itself does not participate in any battery reaction.

[0003] Currently, commonly used sodium-ion battery separators are polyolefin separators or ceramic-coated separators. Compared to polyolefin separators, ceramic-coated separators can improve the heat resistance and electrolyte wetting properties of the separator to a certain extent. However, after prolonged use, the ceramic coating may peel off and shed powder.

[0004] Therefore, improving the peel strength and heat resistance of the diaphragm is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This application provides a high peel strength and high heat resistance battery separator, its preparation method and secondary battery, the coating of which is not easy to shed powder or fall off, and can improve the peel strength and heat resistance of the separator.

[0006] This application provides a high peel strength and high heat resistance battery separator, comprising a porous substrate and a self-assembled coating disposed on at least one surface of the porous substrate. The self-assembled coating comprises self-assembled fibers and heat-resistant particles, wherein the heat-resistant particles fill the porous network formed by the self-assembled fibers, and the self-assembled fibers are mainly formed by the self-assembly polymerization of multiple nanofibers.

[0007] In an optional embodiment, the diameter of the self-assembled fiber ranges from 100nm to 1000nm.

[0008] In an optional embodiment, the nanofibers satisfy at least one of the conditions in (1)-(3):

[0009] (1) The nanofibers are heat-resistant nanofibers, and the nanofibers include at least one of nanocellulose, aramid nanofibers and polyimide nanofibers;

[0010] (2) The diameter of the nanofibers ranges from 3 nm to 25 nm;

[0011] (3) The length of the nanofiber is in the range of 100nm-1000nm.

[0012] In an optional embodiment, the nanofibers are heat-resistant nanofibers with hydrophobic surface modification;

[0013] Optionally, the heat-resistant nanofibers are made of cellulose nanofibers.

[0014] In an optional embodiment, the heat-resistant particles satisfy at least one of the conditions in (1)-(2):

[0015] (1) The heat-resistant particles include inorganic particles, which include at least one of aluminum oxide, magnesium oxide, silicon dioxide, calcium oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, magnesium sulfate, calcium sulfate, barium sulfate, calcium carbonate, boehmite, aluminum nitride and titanium nitride.

[0016] (2) The particle size D50 of the heat-resistant particles ranges from 100nm to 1000nm.

[0017] In an optional embodiment, the self-assembling coating satisfies at least one of the conditions in (1)-(6):

[0018] (1) In the self-assembly coating, the mass ratio of the self-assembly fiber to the heat-resistant particles is 1:3-1:15;

[0019] (2) In the self-assembly coating, the mass percentage of the self-assembly fibers is 5%-25%;

[0020] (3) In the self-assembly coating, the mass percentage of the heat-resistant particles is 65%-85%;

[0021] (4) The self-assembly coating also includes a binder and a dispersant;

[0022] Wherein, the adhesive has a mass percentage of 5%-15%, and / or the dispersant has a mass percentage of 0.5%-2%;

[0023] (5) The self-assembly coating is prepared by spraying, roller coating, dot coating or dip coating;

[0024] (6) The thickness of the self-assembled coating ranges from 0.5μm to 8μm.

[0025] In an optional embodiment, the battery separator satisfies at least one of the conditions in (1)-(5):

[0026] (1) The air permeability increase of the coating of the battery separator is ≤100s / 100mL;

[0027] (2) The MD direction thermal shrinkage rate of the battery separator after being treated at 180°C for 1 hour is ≤5%;

[0028] (3) The porous substrate includes at least one of polyethylene film, polypropylene film, polyimide film, polyvinylidene fluoride film, polyvinylidene fluoride-hexafluoropropylene film, polyamide film and polyethylene terephthalate film;

[0029] (4) The thickness of the porous substrate ranges from 5 μm to 25 μm;

[0030] (5) The porosity of the porous substrate ranges from 30% to 65%.

[0031] This application provides a method for preparing a high peel strength and high heat resistance battery separator, wherein the battery separator is as described above, and the steps include:

[0032] A self-assembled coating slurry is prepared by mixing nanofibers and heat-resistant particles according to a certain ratio, wherein multiple nanofibers are polymerized and assembled to form self-assembled fibers;

[0033] A self-assembly coating slurry is applied to at least one surface of a porous substrate and dried to obtain a battery separator.

[0034] The nanofibers are made of surface-hydrophobically modified cellulose nanofibers, and their preparation steps include:

[0035] Adjust the pH of the aqueous dispersion of nanofibers to 8-9;

[0036] Add buffer and plant polyphenols to the aqueous dispersion after pH adjustment, stir and mix evenly, then add an ethanol solution of long-chain alkylamine, and continue mechanical stirring under a 45°C water bath to obtain a mixed solution.

[0037] The mixed solution was centrifuged and washed with ethanol and deionized water to finally obtain the surface-hydrophobic modified nanocellulose.

[0038] This application provides a secondary battery, including the battery separator as described above, or the battery separator prepared by the method described above.

[0039] The battery separator according to the above embodiments includes a porous substrate and a self-assembled coating disposed on at least one surface of the porous substrate. The self-assembled coating includes self-assembled fibers and heat-resistant particles. The heat-resistant particles are filled in a porous network formed by the self-assembled fibers, which are mainly formed by the self-assembly and polymerization of multiple nanofibers. The porous mesh structure formed by the polymerization of multiple nanofibers serves as a skeleton structure for the heat-resistant particles, improving the mechanical strength and thermal stability of the self-assembled fibers. Simultaneously, the porous network structure composed of self-assembled fibers provides better adhesion between the coating and the porous substrate, and also better adhesion between the self-assembled fibers and the heat-resistant particles. This improves the peel strength of the coating, effectively ensuring the dimensional stability of the separator, and further enhancing the heat resistance of the separator, thereby solving the problem of coating peeling or powder shedding. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the battery separator structure in one embodiment;

[0041] Figure 2 This is a flowchart illustrating the preparation process of the battery separator in one embodiment.

[0042] Wherein: 100, self-assembly coating; 110, first self-assembly coating; 120, second self-assembly coating; 200, porous substrate. Detailed Implementation

[0043] The present application will now be described in further detail with reference to specific embodiments and accompanying drawings. Similar elements in different embodiments are referred to by related similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the present application. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, certain operations related to the present application are not shown or described in the specification. This is to avoid obscuring the core parts of the present application with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0044] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments, and the operational steps involved in each embodiment can also be rearranged or adjusted in a manner that is obvious to those skilled in the art. Therefore, the specification and drawings are only for clearly describing a particular embodiment and do not imply that they represent the necessary components and / or order.

[0045] The serial numbers assigned to components in this document, such as "first" and "second," are used only to distinguish the described objects and have no sequential or technical meaning. The terms "connection" and "linkage" used in this application, unless otherwise specified, include both direct and indirect connections (linkages).

[0046] The "separator" referred to in this application is an important component of a secondary battery cell. During electrolysis, the separator is a thin film used to separate the positive and negative electrodes, preventing direct reaction and energy loss. The performance of the separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting the battery's capacity, cycle life, and safety performance. A high-performance separator plays a crucial role in improving the overall performance of the battery.

[0047] A rechargeable battery is a battery that can be recharged after being discharged, allowing the active materials to be reactivated and reused. These batteries typically utilize the reversibility of chemical reactions; that is, once a chemical reaction converts into electrical energy, the electrical energy can be used to repair the chemical system, and then the chemical reaction can be converted back into electrical energy. Common rechargeable batteries include, but are not limited to, nickel-metal hydride batteries, nickel-cadmium batteries, lead-acid (or lead-acid) batteries, lithium-ion batteries, polymer lithium-ion batteries, and sodium-ion batteries.

[0048] This application discloses a high peel strength and high heat resistance battery separator, which can be applied to secondary batteries, mainly sodium-ion batteries. It includes a porous substrate 200 and a self-assembled coating 100 disposed on at least one surface of the porous substrate 200. The self-assembled coating 100 includes self-assembled fibers and heat-resistant particles. The heat-resistant particles fill the porous network formed by the self-assembled fibers, which are mainly formed by the self-assembly and polymerization of multiple nanofibers. In some specific embodiments, such as... Figure 1 The porous substrate 200 shown has self-assembly coatings 100 on both surfaces, including a first self-assembly coating 110 and a second self-assembly coating 120. In specific implementations, since the two surfaces of the separator correspond to the positive or negative electrode in the battery, the first self-assembly coating 110 and the second self-assembly coating 120 are respectively connected to the positive or negative electrode. The characteristics of the first self-assembly coating 110 and the second self-assembly coating 120 can be slightly different. For example, the thickness and mass ratio of the components of the first self-assembly coating 110 and the second self-assembly coating 120 can be adjusted as needed. Of course, in other embodiments, the self-assembly coating 100 can also be provided on one of the surfaces of the porous substrate 200.

[0049] This application creatively utilizes the polymerization of multiple nanofibers to form self-assembled fibers, and then uses these self-assembled fibers to form a porous mesh structure as a skeleton structure for filling heat-resistant particles. The polymerization of multiple nanofibers can improve the mechanical strength of the self-assembled fibers themselves. Furthermore, since multiple nanofibers are combined through intermolecular forces (mainly hydrogen bonds) to form self-assembled fibers, compared to dispersed nanofibers, the mechanical strength and thermal stability of the self-assembled fibers can be further improved. Simultaneously, the porous network structure composed of self-assembled fibers provides better adhesion between the coating and the porous substrate 200, and also better adhesion between the self-assembled fibers and the heat-resistant particles, improving the peel strength of the coating, effectively ensuring the dimensional stability of the diaphragm, and thus further enhancing the heat resistance of the diaphragm.

[0050] In some embodiments, the porous substrate 200 is at least one of polyethylene membrane, polypropylene membrane, polyimide membrane, polyvinylidene fluoride membrane, polyvinylidene fluoride-hexafluoropropylene membrane, polyamide membrane, and polyethylene terephthalate membrane. The material preparation process is mature, making the preparation cost controllable. Simultaneously, the porous substrate 200 material has good processing performance and, with a small thickness, also possesses high strength and heat resistance, which helps the separator to achieve both low thickness and high heat resistance, thus contributing to high energy density, good processing performance, and high reliability in the secondary battery.

[0051] In some embodiments, the thickness of the porous substrate 200 is 5 μm to 25 μm. For example, the thickness of the porous substrate 200 is 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 15 μm, 20 μm or 25 μm.

[0052] In some embodiments, the porosity of the porous substrate 200 is 30%-65%. For example, the porosity of the porous substrate 200 is 30%, 35%, 40%, 45%, 50%, 55%, 60%, or 65%. The porosity of the porous substrate 200 has a meaning known in the art and can be measured using methods known in the art. For example, it can be tested with reference to GB / T 36363-2018.

[0053] In some embodiments, the diameter of the self-assembled fibers ranges from 100 nm to 1000 nm. The diameter of the self-assembled fibers reaches the micrometer level and can be observed under a scanning electron microscope at 10,000-20,000x magnification on the surface of the diaphragm coating. Compared to dispersed nanofibers, the aforementioned micrometer-sized self-assembled fibers can improve the overall mechanical strength of the coating and enhance the bonding between the heat-resistant particles and the self-assembled fibers, thereby improving the peel strength of the coating.

[0054] In some embodiments, the nanofibers are heat-resistant nanofibers, including at least one of cellulose nanofibers, aramid nanofibers, and polyimide nanofibers. Heat-resistant nanofibers possess good heat resistance, and the self-assembled fibers they form serve as the skeletal structure in the self-assembled coating 100 of the diaphragm, thereby improving the heat resistance of the diaphragm. Compared to plant fibers, the nanofibers used in this application have a larger specific surface area and abundant surface functional groups, making them easier to disperse into aqueous slurries (with better processing performance), and exhibiting certain adhesive properties. The coating formed with inorganic fillers shows better stability. Simultaneously, the nanofibers have better high-temperature resistance, capable of withstanding temperatures above 200°C, further enhancing the heat resistance of the diaphragm.

[0055] Specifically, cellulose nanofibers include, but are not limited to, cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and bacterial cellulose (BC). Aramid nanofibers can be selected from meta-aramid nanofibers and para-aramid nanofibers.

[0056] In some embodiments, the diameter of the nanofibers ranges from 3 nm to 25 nm. For example, the diameter of the nanofibers is 3 nm, 4 nm, 5 nm, 10 nm, 15 nm, 20 nm, or 25 nm. Optionally, the diameter of the nanofibers ranges from 4 nm to 10 nm. By adjusting the average diameter of the nanocellulose within the above range, it is beneficial for the nanocellulose to polymerize into self-assembled fibers with higher strength. It is also beneficial for the self-assembled fibers formed by polymerization to overlap and form a three-dimensional porous mesh structure. Furthermore, it is beneficial for the formed three-dimensional porous mesh structure to overlap with the granular heat-resistant particles to form an integrated coating, thereby improving the heat resistance of the separator and thus improving the performance of the secondary battery.

[0057] In some embodiments, the length of the nanofibers ranges from 100 nm to 1000 nm. For example, the length of the nanofibers is 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. Optionally, the diameter of the nanofibers ranges from 100 nm to 500 nm. By adjusting the average length of the nanocellulose within the above range, it is beneficial for the nanocellulose to polymerize into self-assembled fibers with higher strength. It is also beneficial for the self-assembled fibers formed by polymerization to overlap and form a three-dimensional porous mesh structure. Furthermore, it is beneficial for the formed three-dimensional porous mesh structure to overlap with the granular heat-resistant particles to form an integrated coating, thereby improving the heat resistance of the separator and thus improving the performance of the secondary battery. By adjusting the average length of the nanocellulose within the above range, it is also beneficial for the coating slurry to have a suitable viscosity, which is beneficial for coating application and also helps to improve the uniformity and consistency of the coating.

[0058] In some embodiments, to further enhance the performance of the diaphragm, the nanofibers are surface-modified heat-resistant nanofibers. Specifically, by depositing hydrophobic modifiers on the surface of the nanofibers, aggregation between fibers can be promoted, allowing the fibers to self-assemble into an externally hydrophobic self-assembled fiber structure through intermolecular forces (mainly hydrogen bonding). This improves the peel strength and thermal dimensional stability of the diaphragm coating and reduces the moisture content of the diaphragm. The hydrophobic modifiers include plant polyphenols and long-chain alkylamines.

[0059] In some embodiments, the heat-resistant particles comprise inorganic particles, including at least one selected from alumina, magnesium oxide, silicon dioxide, calcium oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, magnesium sulfate, calcium sulfate, barium sulfate, calcium carbonate, boehmite, aluminum nitride, and titanium nitride. The heat-resistant particles exhibit good thermal stability. The porous network formed by self-assembled fibers serves as a framework, filling the voids in the self-assembled fibers, resulting in good coating uniformity and further enhancing the heat resistance of the membrane.

[0060] In some embodiments, the particle size D50 of the heat-resistant particles ranges from 100 nm to 1000 nm. For example, the particle size D50 of the heat-resistant particles is 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, or 1000 nm. Optionally, the particle size D50 of the heat-resistant particles ranges from 300 nm to 600 nm. By controlling the average particle size of the heat-resistant particles within the above range, the porous structure of the membrane can be effectively preserved, providing channels for ion transport. At the same time, the selection of heat-resistant particles with this particle size can improve the strength of the coating, thereby improving the overall mechanical strength of the membrane.

[0061] In some embodiments, the mass ratio of self-assembled fibers to heat-resistant particles in the self-assembled coating 100 is 1:3 to 1:15. For example, the mass ratio of self-assembled fibers to heat-resistant particles is 1:3, 1:5, 1:10, 1:12, or 1:15. Controlling the mass ratio of self-assembled fibers to heat-resistant particles within the above range is beneficial for improving the mechanical strength of the coating, as well as its adhesion and peel strength, thereby improving the stability of the diaphragm structure. It also facilitates the application of the coating slurry, resulting in good coating consistency and uniformity.

[0062] In some embodiments, the self-assembled fibers in the self-assembly coating 100 comprise 5%-25% by mass. For example, the self-assembled fibers may comprise 5%, 10%, 15%, 20%, or 25% by mass.

[0063] In some embodiments, the heat-resistant particles in the self-assembly coating 100 comprise 65%-85% by mass. For example, the heat-resistant particles may comprise 65%, 70%, 75%, 80%, or 85% by mass.

[0064] In some embodiments, the self-assembly coating 100 further includes a binder and a dispersant. The binder enhances the bonding force between the components, which is beneficial for improving the mechanical strength and peel strength of the coating. The dispersant improves the uniformity of the coating slurry, thereby enhancing the consistency of the coating.

[0065] In some embodiments, the adhesive is present in a mass percentage of 5%-15%. For example, the adhesive mass percentage may be 5%, 6%, 7%, 8%, 9%, 10%, 12%, or 15%.

[0066] In some embodiments, the mass percentage of the dispersant is 0.5%-2%. For example, the mass percentage of the dispersant is 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, or 2%.

[0067] In some embodiments, the self-assembling coating 100 is prepared by spraying, roller coating, dotting or dipping.

[0068] In some embodiments, the thickness of the self-assembled coating 100 is 0.5 μm-8 μm. For example, the thickness of the self-assembled coating 100 is 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, or 8 μm. Optionally, the thickness of the self-assembled coating 100 is 0.5 μm-4 μm. Controlling the thickness of the self-assembled coating 100 effectively improves the strength and heat resistance of the separator, and also allows for the design of a minimum separator thickness, which is beneficial for achieving high energy density in secondary batteries.

[0069] In some embodiments, the coating permeability increase of the battery separator is ≤100s / 100mL. Controlling the coating permeability increase within this range ensures the separator has good ion transport performance, which is beneficial for improving the performance of the secondary battery.

[0070] In some embodiments, the MD-direction thermal shrinkage rate of the battery separator after treatment at 180°C for 1 hour is ≤5%. A thermal shrinkage rate within this range results in good thermal stability and excellent heat resistance of the separator.

[0071] Please see Figure 2 This application also provides a method for preparing a battery separator, the steps of which include:

[0072] S101. Mix nanofibers and heat-resistant particles according to the specified ratio to prepare a self-assembled coating slurry, wherein multiple nanofibers polymerize and assemble to form self-assembled fibers.

[0073] Specifically, after mixing nanofibers and heat-resistant particles, a self-assembled coating slurry can be formed. Multiple nanofibers can eventually assemble into self-assembled fibers with a porous mesh structure. Heat-resistant particles fill the porous mesh of the self-assembled fibers. The self-assembled fibers formed by the polymerization of multiple nanofibers have micron-scale dimensions, which can effectively improve the mechanical strength and heat resistance of the membrane.

[0074] S102. Apply a self-assembly coating slurry to at least one surface of the porous substrate 200 and dry it to obtain a battery separator.

[0075] Specifically, a uniform coating slurry is prepared by mixing self-assembled fibers and heat-resistant particles according to a specified ratio. Then, a coating is uniformly coated on at least one surface of a porous substrate 200 using a preparation process such as spraying, roller coating, dot coating, or dip coating. After baking, a battery separator with high peel strength and high heat resistance is obtained.

[0076] In some embodiments, prior to step S101, the nanofibers undergo hydrophobic modification treatment to obtain hydrophobically modified nanofibers. Specific steps include: adjusting the pH of the aqueous dispersion of the nanofibers to 8-9; adding a buffer and plant polyphenols to the pH-adjusted aqueous dispersion, stirring and mixing thoroughly, and then adding an ethanol solution of a long-chain alkylamine. The mixture is then mechanically stirred in a 45°C water bath to obtain a mixed solution; the mixed solution is washed by centrifugation with ethanol and deionized water to finally obtain surface-hydrophobically modified nanocellulose. Specifically, the long-chain alkylamine can be hexadecylamine or octadecylamine. The plant polyphenols include one or more of gallic acid, methyl gallate, ethyl gallate, propyl gallate, and protocatechuic acid. The mass fraction of the long-chain alkylamine in the ethanol solution is 1%-3%.

[0077] For example, taking cellulose nanofibers (CNFs) as an example, the pH of 100g of a 3%-8% (w / w) CNFs aqueous dispersion was adjusted to 8-9 using NaOH solution. 0.4g of tris(hydroxymethyl)aminomethane (Tris) was added as a buffer, followed by 0.4g-2g of plant polyphenols. The mixture was mechanically stirred at 400 rpm for 8 hours at room temperature. A 1%-3% (w / w) ethanol solution of octadecylamine (ODA) was prepared and mixed with the above solution. The mixture was then mechanically stirred for another 8 hours in a 45°C water bath. The resulting product was washed sequentially by centrifugation with ethanol and deionized water to obtain hydrophobically modified CNFs. The hydrophobicity of the CNFs surface after modification with plant polyphenols and long-chain alkylamines increased, and this modification promoted the aggregation and adhesion between nanofibers, which is beneficial for the assembly of nanofibers into self-assembled fibers with hydrophobic surface groups.

[0078] Among them, plant polyphenols include at least one of gallic acid (GA), methyl gallate (MG), ethyl gallate (EG), propyl gallate (PG), and protocatechuic acid (PCA).

[0079] This application also provides a secondary battery, including the battery separator described above, or the battery separator prepared by the preparation method described above.

[0080] The technical solution of this application will be further described below through specific embodiments.

[0081] Example 1

[0082] Preparation of hydrophobically modified nanocellulose: The pH of 100g of an aqueous dispersion of CNFs (diameter 4nm-10nm, length 100nm-500nm) with a solid content of 4% was adjusted to 8-9 with 0.1% NaOH aqueous solution. 0.4g of Tris was added as a buffer, followed by 0.4g of propyl gallate. The mixture was mechanically stirred at 400r / min at room temperature for 8h. A 100g solution of 2% ODA or ethanol was prepared and mixed with the above solution. The mixture was then mechanically stirred for another 8h in a 45℃ water bath. The resulting product was washed sequentially by centrifugation with ethanol and deionized water to obtain the hydrophobically modified CNFs. The hydrophobically modified CNFs were dispersed in an ethanol / water mixed solvent (ethanol to water mass ratio 1:2) to prepare a hydrophobically modified CNFs dispersion with a solid content of 4%.

[0083] Coating slurry preparation: Mix 75g of alumina particles (D50 of 400nm), 1g of dispersant and 24g of deionized water. Then, according to the mass ratio of CNFs to alumina of 1:15, add hydrophobic modified CNFs dispersion with a solid content of 4% and mix. Stir until uniform. Then add 10g of binder and continue stirring until uniform to obtain the coating slurry.

[0084] Separator preparation: The above coating slurry was double-sided roller coated onto a polypropylene porous membrane (thickness 12μm, porosity 40%), and dried to obtain the separator of this embodiment.

[0085] Example 2, Example 3

[0086] The difference between Examples 2 and 3 and Example 1 is that the mass ratio of CNFs to alumina is different.

[0087] Example 4 and Example 5

[0088] The difference between Examples 4 and 5 and Example 2 is that the particle size D50 of the alumina particles is different.

[0089] Example 6

[0090] The difference between this embodiment and Embodiment 2 is that modified CNFs were not used.

[0091] In this embodiment, the preparation process of the coating slurry is as follows: 75g of alumina particles (D50 = 400nm), 1g of dispersant, and 24g of deionized water are mixed and stirred. NaOH aqueous solution is added to adjust the pH to 8-9 to prepare an alumina dispersion. Then, according to a CNFs to alumina mass ratio of 1:5, an aqueous dispersion of CNFs (diameter 4nm-10nm, length 100nm-500nm) with a solid content of 4% is mixed with the above alumina dispersion and stirred evenly. Then, 10g of binder is added, and stirring is continued until evenly mixed to obtain the coating slurry.

[0092] Comparative Example 1

[0093] The difference between this comparative example and Example 1 is that the coating slurry does not contain CNFs.

[0094] In this comparative example, the preparation process of the coating slurry is as follows: 75g of alumina particles (D50=400nm), 1g of dispersant and 130g of deionized water are mixed and stirred, then 10g of binder is added and stirred evenly to obtain the coating slurry.

[0095] Comparative Example 2

[0096] The difference between this comparative example and Example 6 is that the preparation steps of the coating slurry are different.

[0097] In this comparative example, the preparation process of the coating slurry is as follows: according to the mass ratio of nanocellulose, alumina particles (D50=400nm), dispersant and deionized water of 5g:75g:1g:24g, a CNFs aqueous dispersion with a solid content of 4% (diameter 4nm-10nm, length 100nm-500nm), alumina particles, dispersant and deionized water are mixed and stirred evenly, and then 10g of binder is added and stirred evenly to obtain the coating slurry.

[0098] The composition and preparation process of the battery separators in Examples 1 to 6 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0099] Table 1

[0100]

[0101] The battery separators of the above embodiments and comparative examples were tested. The test methods are as follows:

[0102] Coating-based air permeability enhancement: A 100mm × 100mm membrane sample was cut and tested using a Gurley 4110N air permeability tester with a 100mL test gas mode. The time it took for all the test gas to pass through the sample with the membrane was recorded as the air permeability value. The coating-based air permeability enhancement is the air permeability value of the coated membrane minus the air permeability value of the uncoated membrane (i.e., pure porous substrate 200).

[0103] Heat shrinkage: The test method shall be conducted in accordance with the requirements of GB / T12027-2004. The following steps are included: Cut a diaphragm to a size of 15*15cm; mark the longitudinal and transverse directions on the diaphragm surface; measure the longitudinal and transverse lengths of the sample using a ruler; measure the longitudinal length of the sample using a ruler; lay the sample flat in a fixture, then place it in an oven at 180℃ for 1 hour; after heating, remove the sample, allow it to return to room temperature, and measure the longitudinal mark length again. Calculate the shrinkage rate according to the following formula, and finally take the average of several samples as the heat shrinkage rate.

[0104]

[0105] ΔL: Thermal shrinkage rate in the longitudinal direction of the sample, expressed as a percentage; L0: Length of the sample in the longitudinal direction before heating, in millimeters (mm); L: Length of the sample in the longitudinal direction after heating, in millimeters (mm).

[0106] Coating peel strength: 3M tape was applied to the coating surface, and the peel strength was tested. The test method was in accordance with national standard GB / T 2790-1995. After the test, the test data was processed according to the provisions of national standard GB / T 17200-1998 "Processing of Adhesion Peel Test Data".

[0107] Moisture content: tested using a Karl Fischer moisture analyzer.

[0108] The test results are shown in Table 2 below.

[0109] Table 2

[0110]

[0111]

[0112] Comparative Example 1 did not add nanofibers, while Comparative Example 2 prepared a coating by directly mixing nanofibers and heat-resistant particles. The coating of Comparative Example 2 showed a fine fibrous structure with fiber diameters less than 100 nm. The coatings of Examples 1 to 6, however, showed a coarse fibrous structure with fiber diameters between 100 nm and 1000 nm. As can be seen from the data in Table 2, compared to Comparative Example 2, the coatings of Examples 1 to 6 formed self-assembled fibers, exhibiting better heat resistance and coating peel strength.

[0113] Examples 1 to 5 used hydrophobically modified CNFs. Compared to Example 6, which used unmodified CNFs, the diameter of the self-assembled fibers in the coating was slightly increased. This is because surface hydrophobic modification can promote the aggregation between fibers. Simultaneously, the use of hydrophobically modified CNFs in the coating can reduce the water absorption of nanocellulose and decrease the moisture content of the membrane.

[0114] The above examples illustrate this application only to aid understanding and are not intended to limit its scope. Those skilled in the art to which this application pertains can make various simple deductions, modifications, or substitutions based on the ideas presented.

Claims

1. A high peel strength high heat resistant battery separator, characterized by, The battery separator includes a porous substrate and a self-assembled coating layer arranged on at least one surface of the porous substrate, the self-assembled coating layer is coated by a self-assembled coating slurry prepared by mixing nanofibers and heat-resistant particles in a certain proportion, a plurality of the nanofibers are polymerized and assembled to form a self-assembled fiber, the heat-resistant particles are filled in a porous network formed by the self-assembled fiber, the nanofiber is a surface hydrophobic modified heat-resistant nanofiber, and a preparation step of the surface hydrophobic modified heat-resistant nanofiber is adjusting a pH value of an aqueous phase dispersion of nanocellulose to 8-9, and adding a hydrophobic modifier into the aqueous phase dispersion after the pH value is adjusted to mix the surface hydrophobic modified heat-resistant nanofiber.

2. The high peel strength high heat resistant battery separator of claim 1, wherein, The diameter of the self-assembled fiber ranges from 100 nm to 1000 nm.

3. The high peel strength high heat resistant battery separator of claim 1, wherein, The nanofiber satisfies at least one of conditions (1)-(3): (1) the nanofiber is a heat-resistant nanofiber, and the nanofiber includes at least one of nanocellulose, aramid nanofiber and polyimide nanofiber; (2) the diameter of the nanofiber ranges from 3 nm to 25 nm; (3) the length of the nanofiber ranges from 100 nm to 1000 nm.

4. The high peel strength high heat resistant battery separator of claim 1, wherein, The heat-resistant nanofiber is nanocellulose.

5. The high peel strength high heat resistant battery separator according to any one of claims 1 to 4, characterized by, The heat-resistant particle satisfies at least one of conditions (1)-(2): (1) the heat-resistant particle includes inorganic particles, and the inorganic particles include at least one of alumina, magnesia, silica, calcium oxide, titanium oxide, aluminum hydroxide, magnesium hydroxide, magnesium sulfate, calcium sulfate, barium sulfate, calcium carbonate, boehmite, aluminum nitride and titanium nitride; (2) the particle size D50 of the heat-resistant particle ranges from 100 nm to 1000 nm.

6. The battery separator of any one of claims 1 to 4, wherein, The self-assembled coating layer satisfies at least one of conditions (1)-(6): (1) in the self-assembled coating layer, the mass ratio of the self-assembled fiber to the heat-resistant particle is 1:3-1:15; (2) in the self-assembled coating layer, the mass percentage of the self-assembled fiber is 5%-25%; (3) in the self-assembled coating layer, the mass percentage of the heat-resistant particle is 65%-85%; (4) the self-assembled coating layer further includes a binder and a dispersant; wherein, the mass percentage of the binder is 5%-15%, and / or the mass percentage of the dispersant is 0.5%-2%; (5) the self-assembled coating layer is prepared by spraying, rolling, spotting or dipping; (6) the thickness of the self-assembled coating layer ranges from 0.5 μm to 8 μm.

7. The high peel strength high heat resistant battery separator according to any one of claims 1 to 4, characterized by, The battery separator satisfies at least one of conditions (1)-(5): (1) the coating layer of the battery separator has a gas permeability value of ≤100 s / 100 mL; (2) the MD direction heat shrinkage rate of the battery separator treated at 180℃ for 1 h is ≤5%; (3) the porous substrate includes at least one of polyethylene film, polypropylene film, polyimide film, polyvinylidene fluoride film, polyvinylidene fluoride-hexafluoropropylene film, polyamide film and polyethylene terephthalate film; (4) the thickness of the porous substrate ranges from 5 μm to 25 μm; (5) the porosity of the porous substrate ranges from 30% to 65%.

8. A method for producing a high peel strength high heat resistant battery separator, the battery separator being the battery separator according to any one of claims 1 to 7, characterized by, The steps include: The nano-fibers and heat-resistant particles are mixed according to a ratio to prepare a self-assembled coating slurry, wherein a plurality of the nano-fibers are polymerized and assembled to form self-assembled fibers; The self-assembled coating slurry is coated on at least one surface of the porous substrate and dried to obtain a battery separator.

9. The method for preparing a high peel strength and high heat resistance battery separator according to claim 8, characterized in that, The nano-fibers are surface hydrophobic modified nano-cellulose, and the preparation steps include: The aqueous dispersion of nano-cellulose is adjusted to a pH value of 8-9; A buffer and a plant polyphenol are added to the aqueous dispersion after the pH value is adjusted, stirred and mixed uniformly, and an ethanol solution of long-chain alkyl amine is added, and the mixed solution is continuously mechanically stirred under the condition of a 45℃ water bath to obtain a mixed solution; The mixed solution is centrifugally washed with ethanol and deionized water to finally obtain the surface hydrophobic modified nano-cellulose.

10. A secondary battery characterized by comprising: The battery separator includes the battery separator as claimed in any one of claims 1-7, or the battery separator prepared by the preparation method as claimed in claim 8 or 9.

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