Coating slurry, coating diaphragm, diaphragm preparation method and battery

Abstract

The invention relates to coating slurry, a coating diaphragm, a preparation method and a battery. The coating slurry comprises the following main components: a solvent and adhesive polymer resin, and the weight ratio of the adhesive polymer resin in the slurry is 5-20wt%; the adhesive polymer resin comprises a PVDF (polyvinylidene fluoride)-based adhesive resin polymer, a combination of polyimide-based adhesive resins or a combination of at least two polyimide-based adhesive resins; wherein the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer or the mass ratio of the first polyimide-based adhesive resin to the second polyimide-based adhesive resin is (50-85): (15-50). The diaphragm has excellent heat resistance, high breakdown resistance and excellent bonding performance, and the high safety performance and the cycling stability performance of the subsequently obtained lithium ion battery are greatly improved.

Description

Coating slurry, coating diaphragm, diaphragm preparation method and battery Technical Field

[0001] The present invention relates to the technical field of battery separators, and in particular to a coating slurry used for coating a battery separator, a separator coated with the slurry, a method for preparing the separator, and a battery. Background Art

[0002] The lithium battery separator is one of the four core components of a lithium-ion battery. It separates the positive and negative electrodes, allows lithium ions to pass through, and insulates against electrons. The performance of the separator directly impacts the performance of the lithium-ion battery and is a key technology limiting its development.

[0003] Existing coated separators for lithium-ion batteries, regardless of whether the substrate is PP, PE or PP / PE / PP composite film, or double-sided or single-sided coated ceramic film, have a breakdown voltage in the range of 1.0 to 2.0 kV. It is difficult to further improve its breakdown voltage performance. As a result, the assembled lithium-ion batteries have poor needle puncture resistance and generally find it difficult to pass the needle puncture resistance test.

[0004] Therefore, how to effectively improve the high pressure resistance and needle puncture resistance of the diaphragm has become a technical problem that the industry urgently needs to solve.

[0005] Summary of the Invention

[0006] The present invention provides a coating slurry, a coating diaphragm, a diaphragm preparation method and a battery, so as to improve the high pressure resistance and acupuncture resistance of the battery diaphragm.

[0007] According to a first aspect of the present invention, a coating slurry is provided, the main components of which include a solvent and an adhesive polymer resin, wherein the weight ratio of the adhesive polymer resin in the slurry is 5-20wt%; wherein the adhesive polymer resin includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, and the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50;

[0008] Alternatively, the adhesive polymer resin comprises a combination of at least two polyimide adhesive resins; wherein the at least two polyimide adhesive resins comprise a first polyimide adhesive resin and a second polyimide adhesive resin, and the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin is 50-85:15-50.

[0009] In some embodiments, the glass transition temperature of the first polyimide-based adhesive resin is greater than 200°C, and the glass transition temperature of the second polyimide-based adhesive resin is greater than 140°C.

[0010] In some embodiments, the solvent is an organic solvent, the polyimide adhesive resin is a modified polyimide, and the modified polyimide is soluble in the organic solvent.

[0011] In some embodiments, when the adhesive polymer resin includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, the polyimide-based adhesive resin is selected from at least one of the following polymer resins: ether anhydride polyimide, fluoroanhydride polyimide, ketone anhydride polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, and polyetherimide;

[0012] In the case where the adhesive polymer resin includes a combination of at least two polyimide adhesive resins, the at least two polyimide adhesive resins are selected from at least two of the following polymer resins: ether anhydride polyimide, fluoroanhydride polyimide, ketone anhydride polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, and polyetherimide.

[0013] In some embodiments, a coating pore-enlarging agent is further included, and the weight ratio of the coating pore-enlarging agent in the slurry is 0.05-5wt%.

[0014] In some embodiments, the coating pore expanding agent is one or a combination of water, ethanol, polyethylene glycol, methanol, etc.

[0015] In some embodiments, ceramic particles are further included, and the weight ratio of the ceramic particles in the slurry is greater than 0 wt % and less than or equal to 5 wt %.

[0016] In some embodiments, the ceramic particles are any one of aluminum oxide, boehmite, silicon oxide, magnesium oxide, titanium oxide, zirconium oxide, or a combination thereof.

[0017] In some embodiments, the weight ratio of the solvent in the slurry is 70-95 wt %.

[0018] In some embodiments, the solvent is one of acetone, N-methylpyrrolidone, DMAc, or a combination thereof.

[0019] In some embodiments, the substrate is any one of a PP film, a PE film, a PP / PE / PP composite film, a double-sided or single-sided ceramic coating film, and a double-sided or single-sided nanofiber coating film.

[0020] According to a second aspect of the present invention, a coated diaphragm is provided, comprising a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate; wherein the polymer layer contains at least two functional groups selected from imide groups, aromatic heterocycles, and aromatic rings, and at least one functional group selected from ester groups, ether bonds, thioether bonds, ketone groups, amide groups, phenolic hydroxy ethers, methyl groups, methylene groups, trifluoromethyl groups, and CF bonds.

[0021] According to the second aspect of the present invention, a coated diaphragm is provided, comprising a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate, and the polymer layer is formed by coating the coating slurry provided by the first aspect of the present invention.

[0022] According to the second aspect of the present invention, a coated diaphragm is also provided, comprising a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate; the coated diaphragm has a breakdown voltage of 2.1 kV to 5.0 kV and a breakdown strength greater than 4.5 kV / mil.

[0023] In some embodiments, the surface layer of the polymer layer has a porous structure.

[0024] In some embodiments, ceramic particles are further included, and the ceramic particles are embedded in the polymer layer to form an island structure.

[0025] In some embodiments, the size of the island structure is 0.5 μm to 2 μm.

[0026] In some embodiments, the pore structure is a honeycomb-shaped, dense pore structure.

[0027] According to a third aspect of the present invention, there is provided a method for preparing a coated diaphragm, comprising the following steps:

[0028] providing a substrate;

[0029] Preparing a slurry: adding 5-20 wt% of an adhesive polymer resin to a solvent and fully dissolving it to obtain a coating slurry; wherein the adhesive polymer resin comprises a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, and the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50; or the adhesive polymer resin comprises a combination of at least two polyimide-based adhesive resins, the at least two polyimide-based adhesive resins comprising a first polyimide-based adhesive resin and a second polyimide-based adhesive resin, and the mass ratio of the first polyimide-based adhesive resin to the second polyimide-based adhesive resin is 50-85:15-50;

[0030] Coating: coating the coating slurry prepared above on at least one surface of the substrate to form a polymer layer on the substrate.

[0031] According to a fourth aspect of the present invention, a battery is provided, comprising a coated separator, a positive electrode, a negative electrode and an electrolyte, wherein the coated separator is the coated separator provided in the second aspect of the present invention.

[0032] Compared with the prior art, the present invention has at least the following technical effects:

[0033] The coating slurry and coated diaphragm provided by the present invention are prepared by setting the weight ratio of the adhesive polymer resin in the slurry to 5-20wt%; and setting the adhesive polymer resin to include a combination of a PVDF-based adhesive resin polymer and a polyimide adhesive resin, the mass ratio of the polyimide adhesive resin to the PVDF-based adhesive resin polymer being 50-85:15-50; or the adhesive polymer resin includes a combination of at least two polyimide adhesive resins; wherein the at least two polyimide adhesive resins include a first polyimide adhesive resin and a second polyimide adhesive resin, the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin being 50-85:15-50; thereby, the obtained diaphragm has excellent heat resistance, high breakdown resistance and electrode sheet interface adhesion performance, greatly improving the high safety performance and cycle stability performance of the subsequently obtained lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0035] FIG1 is a comparison of the discharge capacity of a PI-coated diaphragm and a CCS diaphragm at -40 degrees according to an embodiment of the present invention;

[0036] 2A-2B are SEM morphology images of the surface of a coated diaphragm provided in one embodiment of the present invention;

[0037] FIG3 is a schematic flow chart of a method for preparing a coated diaphragm according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0039] Before filing this application, the applicant conducted a series of research and experiments on existing diaphragms:

[0040] Existing coated separators for lithium-ion batteries, regardless of whether the substrate is PP, PE, or PP / PE / PP composite film, or double-sided or single-sided ceramic film, or high-heat-resistant nanofiber coating film, generally have a low breakdown voltage, basically in the range of 1.0 to 2.0 kV / mm. It is difficult to further improve its breakdown voltage performance, thereby hindering the further improvement of the safety performance of the assembled lithium-ion batteries, and it is generally difficult to pass the needle penetration test.

[0041] In view of this, the present application provides a coating slurry, a coating diaphragm, a diaphragm preparation method and a battery, aiming to improve the high pressure resistance and needle puncture resistance of the battery diaphragm.

[0042] The following specific embodiments are used to describe the technical solution of the present invention in detail. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described in detail in some embodiments.

[0043] The present invention provides a coating slurry, the main components of which include a solvent and an adhesive polymer resin. The weight ratio of the adhesive polymer resin in the slurry is 5-20wt%. The adhesive polymer resin includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, and the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50.

[0044] Alternatively, the adhesive polymer resin comprises a combination of at least two polyimide adhesive resins; wherein the at least two polyimide adhesive resins comprise a first polyimide adhesive resin and a second polyimide adhesive resin, and the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin is 50-85:15-50.

[0045] Specifically, the weight ratio of the adhesive polymer resin in the slurry can be, for example, 5wt%, 10wt%, 15wt%, or 20wt%. As a further preferred embodiment, the weight ratio of the adhesive polymer resin in the slurry is 5-15wt%. This is because coating is relatively easy when the weight of the adhesive polymer resin in the slurry is within this range. If the weight ratio exceeds 15%, the viscosity of the slurry is relatively high, which requires certain coating processes.

[0046] Wherein, the mass ratio of the polyimide adhesive resin to the PVDF-based adhesive resin polymer can be, for example, 50:15, 50:20, 50:25, 50:30, 60:15, 60:20, 60:25, 60:30, 60:35, 60:40, 60:45, 60:50, 70:15, 70:20, 70:25, 70:30, 70:35, 70:40, 70:45, 70:50, 80:15, 80:20, 80:25, 80:30, 80:35, 80:40, 80:45, 80:50, 85:15, 85:20, 85:25, 85:30, 85:35, 85:40, 85:45, 85:50.

[0047] The mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin can be, for example, 50:15, 50:20, 50:25, 50:30, 60:15, 60:20, 60:25, 60:30, 60:35, 60:40, 60:45, 60:50, 70:15, 70:20, 70:25, 70:30, 70:35, 70:40, 70:45, 70:50, 80:15, 80:20, 80:25, 80:30, 80:35, 80:40, 80:45, 80:50, 85:15, 85:20, 85:25, 85:30, 85:35, 85:40, 85:45, or 85:50.

[0048] Of course, it should be appreciated that the present invention is not limited to this, and the mass ratio of the polyimide adhesive resin to the PVDF-based adhesive resin polymer and / or the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin can also be other values, as long as the mass ratio of the polyimide adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50 or the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin is 50-85:15-50.

[0049] The glass transition temperature of the first polyimide adhesive resin is greater than 200°C, and the glass transition temperature of the second polyimide adhesive resin is greater than 140°C. The glass transition temperature of the first polyimide adhesive resin is greater than 200°C, which can provide sufficient heat resistance and safety for the coating. The glass transition temperature of the second polyimide adhesive resin does not need to be too high, and can be greater than 140°C. This has the advantages of ensuring a certain degree of heat resistance for the separator and providing stronger bonding performance and adhesion to the substrate.

[0050] As an example, the solvent is an organic solvent, the polyimide adhesive resin is a modified polyimide, and the modified polyimide is soluble in the organic solvent. It is understood that the modified polyimide can be prepared by grafting functional groups onto polyimide, and of course, other modification methods can also be used to achieve solubility in organic solvents.

[0051] In some embodiments, when the adhesive polymer resin includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, the polyimide-based adhesive resin is selected from at least one of the following polymer resins: ether anhydride polyimide, fluoroanhydride polyimide, ketone anhydride polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, and polyetherimide;

[0052] In the case where the adhesive polymer resin comprises a combination of at least two polyimide adhesive resins, the at least two polyimide adhesive resins are selected from at least two of the following polymer resins: ether anhydride polyimide, fluoroanhydride polyimide, ketone anhydride polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, and polyetherimide. Of course, it should be appreciated that these polymer resins are merely examples, and the present invention is not limited thereto. Those skilled in the art may also select other polymer resins to achieve the purpose of the present invention.

[0053] In some embodiments, a coating pore expander is further included, and the weight ratio of the coating pore expander in the slurry is 0.05-5wt%. As an example, the weight ratio of the coating pore expander in the slurry can be, for example, 0.05wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, or the like, or a range between any two values. Of course, it should be appreciated that the present invention is not limited thereto, and the weight ratio of the coating pore expander in the slurry can also be other values, as long as it is within 0.05-5wt%.

[0054] In some embodiments, the coating pore expander is one of water, ethanol, polyethylene glycol, methanol, etc., or a combination thereof. Of course, it should be appreciated that the present invention is not limited thereto, and the coating pore expander may also be other materials. By adding the coating pore expander to the slurry, a honeycomb-shaped, dense pore structure is formed on the coating surface of the diaphragm prepared using the slurry, so that the coating ensures a certain degree of air permeability, adsorbs sufficient electrolyte, provides sufficient ion channels, and reduces interfacial impedance; at the same time, it provides higher electronic insulation and increases breakdown resistance.

[0055] In some embodiments, ceramic particles are further included, and the weight ratio of the ceramic particles in the slurry is greater than 0wt% and less than or equal to 5wt%. As an example, the weight ratio of the ceramic particles in the slurry can be, for example, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, or a range between any two values. Of course, it should be appreciated that these values ​​are merely examples and the present invention is not limited thereto. The weight ratio of the ceramic particles in the slurry can also be other values, as long as it is greater than 0wt% and less than or equal to 5wt%.

[0056] As an example, the ceramic particles are any one of aluminum oxide, boehmite, silicon oxide, magnesium oxide, titanium oxide, zirconium oxide, or a combination thereof. Of course, it should be appreciated that the present invention is not limited thereto, and the ceramic particles may also be other materials. By adding ceramic particles to the slurry, the diaphragm prepared using the slurry has granular ceramics distributed on its coating surface, and the granular ceramics are embedded in the polymer layer to form an isolated island structure, and the diameter of the island structure is 0.5 μm to 2 μm. Among them, the island structure is not completely a regular cone, and the diameter here refers to the average value of the line connecting the center to the edge of the projection of the island structure on the substrate surface. These isolated island structures can increase the porosity of the coating, improve the permeability, and at the same time increase the electrolyte absorption rate, increase the electrolyte wettability, and reduce the internal interface impedance of the lithium-ion battery; due to the special island structure, it can also increase the coating interface embedded in the surface gap of the electrode sheet, increase the interface adhesion, and improve the battery cycle stability.

[0057] In some embodiments, the weight ratio of the solvent in the slurry is 70-95 wt %. For example, the weight ratio of the solvent in the slurry can be 70 wt %, 75 wt %, 80 wt %, 85 wt %, 90 wt %, 95 wt %, or any range between two values. Of course, it should be understood that these values ​​are merely examples and the present invention is not limited thereto. The weight ratio of the solvent in the slurry can also be other values ​​as long as it is between 70-95 wt %.

[0058] For example, the solvent is one of acetone, N-methylpyrrolidone, DMAc, or a combination thereof. Of course, it should be appreciated that the present invention is not limited thereto, and the solvent may also be other materials as long as they can dissolve the adhesive polymer resin.

[0059] In some embodiments, the substrate is any one of PP, PE, PP / PE / PP composite film, double-sided or single-sided ceramic film, and double-sided or single-sided nanofiber film. Of course, it should be understood that the present invention is not limited to these, and other substrates are also within the scope of protection of the present invention.

[0060] In addition, an embodiment of the present invention further provides a coated diaphragm, comprising a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate; wherein the polymer layer comprises at least two functional groups selected from imide groups, aromatic heterocycles, and aromatic rings, and at least one functional group selected from ester groups, ether bonds, thioether bonds, ketone groups, amide groups, phenolic hydroxy ethers, methyl groups, methylene groups, trifluoromethyl groups, and CF bonds. Specifically, if the adhesive polymer resin in the coating slurry comprises a combination of at least two polyimide adhesive resins, then the polymer layer comprises at least two functional groups selected from imide groups, aromatic heterocycles, and aromatic rings, and at least one functional group selected from ester groups, ether bonds, thioether bonds, ketone groups, amide groups, phenolic hydroxy ethers, methyl groups, methylene groups, trifluoromethyl groups, and CF bonds. If the adhesive polymer resin in the coating slurry includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, the polymer layer is mainly composed of polyimide, and the functional groups mainly reflect the functional groups in the polyimide molecule (for example, any one of an imide group, an aromatic heterocycle, and an aromatic ring). At the same time, the trifluoromethyl and CF bond functional groups in PVDF will be present.

[0061] In addition, an embodiment of the present invention further provides a coated diaphragm, comprising a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate, and the polymer layer is formed by coating the coating slurry provided above in the present invention.

[0062] In particular, referring to Figure 1 , the high-voltage, puncture-resistant lithium-ion battery separator provided by the present invention has a low-temperature 1C discharge capacity of greater than 90% for lithium-ion batteries, which is higher than that of conventional ceramic-coated composite separators. Applying the coating slurry provided by the present invention to a separator based on a double-sided or single-sided ceramic film, a double-sided or single-sided nanofiber film, PP or PE, or a PP / PE / PP composite film will impart excellent heat resistance, high breakdown resistance, and electrode sheet adhesion to the lithium-ion battery separator, greatly enhancing the high safety performance and cycle stability of the lithium-ion battery. This makes the lithium-ion battery of the present invention particularly suitable for applications such as EVs and energy storage.

[0063] In some embodiments, the surface layer of the polymer layer has a pore structure. As an example, the pore structure is a honeycomb-shaped, dense pore structure. Here, dense means that the pore size is nanometer-sized and dense coating structure reflected in the SEM electron microscope image, as shown in Figure 2A. Because the surface layer of the polymer layer has a pore structure, the coated diaphragm of the present invention can ensure a certain degree of air permeability, absorb sufficient electrolyte, provide sufficient ion channels, and reduce interfacial impedance; at the same time, it provides higher electronic insulation and increases breakdown resistance.

[0064] In some embodiments, ceramic particles are also included, and the ceramic particles are embedded in the polymer layer in an island structure, as shown in Figure 2B. As an example, the size of the island structure is 0.5μm to 2μm. These isolated island structures can increase the porosity of the polymer layer and improve the air permeability; at the same time, they increase the electrolyte absorption rate, increase the electrolyte wettability, and reduce the internal interface impedance of the lithium-ion battery; due to the special island structure, it can also increase the coating interface embedded in the surface gap of the electrode sheet, increase the interface adhesion, and improve the battery cycle stability.

[0065] Please refer to FIG3 . As shown in FIG3 , an embodiment of the present invention further provides a method for preparing a coated diaphragm, comprising the following steps:

[0066] S1: providing a substrate;

[0067] The substrate may be, for example, a PP film, a PE film, a PP / PE / PP composite film, a double-sided or single-sided ceramic coating film, a double-sided or single-sided nanofiber coating film, or the like.

[0068] S2: Preparing a slurry: adding 5-20 wt % of an adhesive polymer resin to a solvent and fully dissolving it to obtain a coating slurry; wherein the adhesive polymer resin comprises a combination of a PVDF-based adhesive resin polymer and a polyimide adhesive resin, and the mass ratio of the polyimide adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50; or the adhesive polymer resin comprises a combination of at least two polyimide adhesive resins, the at least two polyimide adhesive resins comprising a first polyimide adhesive resin and a second polyimide adhesive resin, and the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin is 50-85:15-50;

[0069] S3: coating: coating the coating slurry prepared above on at least one surface of the substrate to form a polymer layer on the substrate.

[0070] The slurry can be coated on one or both sides of the substrate. The coating process can specifically be a NIPS process. In one example, for example, a porous gel-ceramic hybrid coating can be prepared by using an immersion precipitation phase inversion method, using an organic solvent such as NMP, acetone, or DMAc as a solvent and a polymer as a solute, through simultaneous double-sided micro-gravure coating, using solidified gel phase separation to form pores, combined with a water washing and drying process.

[0071] Because the diaphragm prepared in this way has a polymer layer containing an adhesive polymer resin, it has properties such as high voltage resistance, insulation, flame retardancy, and adhesion, thereby providing lithium-ion battery diaphragms with excellent breakdown voltage performance, with a breakdown voltage in the range of 2.1kV to 5.0kV and a breakdown strength greater than 4.5kV / mil; at the same time, it has excellent interface adhesion performance with the electrode sheet; lithium-ion batteries assembled with this diaphragm have excellent puncture resistance and high breakdown voltage performance.

[0072] In addition, an embodiment of the present invention further provides a battery, comprising a coated separator, a positive electrode, a negative electrode and an electrolyte, wherein the coated separator is the coated separator provided in the aforementioned embodiment of the present invention.

[0073] The product performance of some embodiments of the present invention will be analyzed through experiments below.

[0074] Example 1

[0075] S1: providing a substrate, wherein both sides of the substrate are coated with a ceramic layer; for convenience, the substrate is referred to as a ceramic film;

[0076] S2: Prepare slurry: add 9wt% of adhesive polymer resin, 2.5wt% of coating pore expander, and 2wt% of ceramic particles into a solvent, and after they are fully dissolved, obtain a coating slurry; wherein, the adhesive polymer resin includes a first polymer resin and a second polymer resin, specifically the first polymer resin is polyetherimide, the second polymer resin is polyamide-imide, and the ratio of the first polymer resin to the second polymer resin is 60:40; the solvent is acetone; the coating pore expander is water; and the ceramic particles are aluminum oxide.

[0077] S3: coating: the coating slurry prepared above is coated on the ceramic membrane by micro-gravure on both sides simultaneously, and the porous gel-ceramic hybrid coating is prepared by phase separation of solidified gel, combined with water washing and drying process; the corresponding coated diaphragm is obtained.

[0078] Based on Example 1, by varying the ratio of the first polymer resin to the second polymer resin, the ratio of the first polymer resin to the second polymer resin, the ratio of the polymer to the slurry, the substrate, the coating thickness, the thickness of the separator, and the ratio of the ceramic particles to the slurry, Examples 2 to 17 and Comparative Examples 1 to 3 were produced, respectively. The differences between the Examples and Comparative Examples are detailed in Table 1. The coating thickness in Table 1 refers to the double-sided coating thickness, i.e., the double-sided coating thickness obtained by simultaneously or sequentially micro-gravure-coating the above-prepared coating slurry onto the ceramic membrane.

[0079] Please refer to Table 1, which shows the properties of the corresponding diaphragms obtained in Examples 1-17 and Comparative Examples 1-3.

[0080] The adhesion test method involves forming a standard test strip of the coating film sample to be tested (coating film (25mm*180mm), electrode (20mm*150mm)). Using a hot press, the coated surface of the diaphragm and the electrode are bonded together under pressure and at 60°C. A clamp is then used to clamp the electrode and the other end of the sample, and a tensile force is applied until the tensile machine travels to the end. The tensile machine parameters are: positioning displacement: 150mm, test speed: 300mm / min.

[0081] The specific test method of heat shrinkage rate at 130℃*60min is as follows: First, cut the membrane into 50mm*50mm test samples along the horizontal and vertical directions of the membrane; second, sandwich the cut samples between A4 papers, with 5 A4 papers on the top and 5 on the bottom; then, set the oven temperature to 130℃, wait until the temperature rises to the set temperature, and fully preheat to ensure that the internal temperature reaches the set temperature stably; then quickly put the prepared sample sandwiched between the A4 papers into the oven together with the A4 papers. In the oven, the sample should be placed in the middle of the upper layer of the oven. The lower layer cannot be placed there. Close the oven door immediately. After closing the oven door, set the test time to 60 minutes according to the test requirements. After the time is up, remove the sample film from the oven, wait for it to cool to room temperature, gently flatten the film, measure the distance between the horizontal / vertical diaphragm edges, record the data and calculate the thermal shrinkage rate: Thermal shrinkage rate calculation formula: Thermal shrinkage rate (%) = (initial mark length - mark length after heating) / initial mark length × 100%.

[0082] The membrane rupture temperature is obtained through TMA testing. The membrane rupture temperature of TMA testing refers to tensioning the diaphragm with a certain force (simulating the state inside the battery) and then gradually raising the temperature to rupture.

[0083] Breakdown voltage test:

[0084] Instrument used: Hipot tester, which consists of a high-voltage boost circuit (which can adjust the test voltage required for the output), a leakage current detection circuit (which can set the alarm current) and an indicating instrument (which directly reads the output voltage and leakage current value <or breakdown alarm current value>).

[0085] Working principle: During the test, when the object under test is under the specified test voltage for the specified time, the instrument automatically cuts off the output voltage; once breakdown occurs, that is, the leakage current exceeds the set alarm current, the instrument will sound an alarm, and the alarm value is the breakdown value.

[0086] Test method: Take an A4-sized sample of the film to be tested and spread it flat on a clean, flat surface of tin foil or aluminum foil; set the upper limit output voltage to 6KV, the output current to 1mA, and the rise time to 30S on the instrument; gently place the copper conductor column on the film sample, click the instrument START button, and the instrument begins testing; during the current and voltage rising process, once a breakdown occurs, that is, the leakage current exceeds the set alarm current, the instrument will sound an alarm, and the alarm value is the breakdown value; evenly select 10 different points on the film sample surface and repeat the test 10 times, and the average value is the breakdown voltage value described in this patent.

[0087] The dielectric strength is: breakdown voltage value * 25.4 / thickness of the film sample; expressed as the voltage resistance performance per unit thickness.

[0088] The permeability increment is the difference between the air permeability of the coated membrane and the air permeability of the substrate, representing the increased permeability of the membrane due to the coating. Refer to GB / T36363-2018 for polyolefin separators for lithium-ion batteries for air permeability testing.

[0089] It can be seen from Table 1 that the present application greatly improves the overall breakdown voltage resistance and adhesion of the diaphragm. Examples 1 to 7, Example 8, Example 9, Example 10 to 12, and Example 13 to 15 are respectively coating the slurry on different substrates such as ceramic coating film, UV cross-linking coating film, nanofiber coating film, PE base film, and PP base film. It can be seen that no matter what substrate is used, the diaphragm coated by the present application can greatly improve the breakdown resistance while taking into account high adhesion and lower permeability increment compared to the diaphragm of the comparative example. Among them, Examples 1 to 7 are prepared by mixing polymers of different proportions and coating them on the surface of the ceramic coated diaphragm substrate. It can be seen that when the first polymer resin is selected as polyetherimide and the ratio thereof to the second polymer resin is 80:20 (Example 3), the coated diaphragm has excellent breakdown resistance, dielectric strength and adhesion, and the air permeability increase is also low, and the overall performance is better. This is because the first polymer polyetherimide has good dielectric strength and breakdown resistance, and when the proportion is high, it can provide the diaphragm with better insulation and breakdown voltage resistance; the ratio of the first polymer and the second polymer is coordinated with the proportion of the polymer in the slurry to provide a better macroporous structure of the coating, thereby increasing the air permeability.

[0090] Example 8 uses a UV cross-linked coated diaphragm as the substrate for coating. Compared with Example 3, Example 8 has a thinner coating thickness, but its dielectric strength and breakdown voltage are comparable to those of Example 3. It also has better adhesion properties and high temperature resistance. This is because the cross-linked network structure formed by the UV coated diaphragm substrate coating adhesive makes it difficult for the polyimide coating to penetrate into the bottom layer. Instead, a chemical bond is formed at the interface between the UV coated diaphragm substrate and the polyimide coating, and a denser coating structure is formed, which increases the coating adhesion effect while improving the voltage resistance.

[0091] Example 9 is an example of using a nanofiber coated diaphragm as a base material for coating. Compared with Example 3, Example 9 has comparable dielectric strength and breakdown voltage, and also has better adhesion properties and high temperature resistance. This is due to the superior heat resistance of the nanofiber coated diaphragm itself and the excellent voltage resistance of the polyimide coating. Example 10 is an example of using a PE base film as a base material for coating. Compared with conventional PE base films, the film rupture temperature and breakdown resistance are greatly improved (the conventional PE base film has a film rupture temperature of generally 145°C and a breakdown voltage of 1.4KV). Compared with Example 11 in which the second polymer resin uses PVDF-HFP copolymer, Example 10 has a higher film rupture temperature and breakdown resistance, which is due to the higher heat resistance and insulation properties of the polyimide resin itself.

[0092] Example 13 uses a PP base film as the base material for coating, and the heat resistance is greatly improved compared with Example 10 which uses a PE base film for coating. Compared with Comparative Examples 2 and 3, Example 13 shows that the proportion of the first polymer resin and the second polymer resin in the slurry is very critical. If the coating thickness is lower than 5%, it is difficult to control and the interfacial adhesion of the diaphragm to the electrode sheet and the adhesion to the substrate cannot be guaranteed, and there is a risk of the coating falling off; when the proportion is too high, such as more than 20%, the coating thickness is difficult to control, and the coating process cannot be smoothly implemented. The porosity of the coating is very low, resulting in an excessively large air permeability increment. It can be understood that the coating thickness is difficult to control means that the overall thickness uniformity of the coating is poor during the coating process. For example, the coating thickness in some areas of Comparative Example 3 may be lower than 3μm, and higher than 5μm in some areas. Comparing Example 13 with Example 16 and Example 17, it can be seen that the diaphragm formed by not coating the polymer slurry with ceramic particles has improved breakdown performance, heat resistance and adhesion compared with conventional diaphragms, but its air permeability increase is large, which may affect the ion permeability of the diaphragm. When the proportion of ceramic particles exceeds 5%, it will lead to difficulty in controlling the coating thickness and air permeability increase, thereby significantly reducing the adhesion performance of the diaphragm.

[0093] Example 15 illustrates an implementation using 0% pore expander. Comparison with the other examples and comparative examples demonstrates that the addition of a pore expander increases the coating porosity, providing ample ion channels and reducing the overall impedance of the lithium-ion battery. Without the pore expander, the impedance increases, and while the breakdown voltage slightly improves, the denser the pores, the greater the permeability, resulting in a reduced cell cycle life, lower ionic conductivity, and lower energy density, making the battery unsatisfactory for product use.

[0094] In this patent, the first polymer resin selected is a polyimide resin with excellent dielectric and insulating properties as the main material. The material itself has good dielectric, insulating, and heat resistance, providing the diaphragm with high-voltage electrical breakdown performance. The second polymer resin material, while ensuring a certain degree of heat resistance, provides the diaphragm with high adhesion performance, good adhesion to the substrate, and excellent film-forming properties to ensure process feasibility. The flexible selection of substrates with different characteristics can meet customers' different needs for heat resistance, film rupture temperature, and breakdown voltage, ultimately achieving differentiated product design and further improving the overall safety performance of the diaphragm.

[0095] In addition, an embodiment of the present invention also provides a coated diaphragm, including a substrate and a polymer layer, wherein the polymer layer is coated on one or both sides of the substrate; the coated diaphragm has a breakdown voltage of 2.1kV to 5.0kV and a breakdown strength greater than 4.5kV / mil.

[0096] The PI coated diaphragm of Example 2 of the present invention and the CCS coated diaphragm with the same substrate as Example 2 were assembled into a lithium cobalt oxide positive electrode cylindrical battery. The comparative test results are shown in Table 2 below:

[0097] Table 2

[0098] It should be noted that the voltage in the table above refers to the voltage applied during the test, the temperature refers to the cell temperature, and the resistance refers to the internal resistance of the assembled cell. The tests show that the CCS-coated separator exhibited strong ignition and combustion during the test when the cell was punctured under a certain voltage, resulting in a failure. However, the PI-coated separator of Example 2 exhibited no obvious ignition or combustion during the test when the cell was punctured under a certain voltage, resulting in a pass.

[0099] Throughout this specification, references to terms such as "one embodiment," "an example," "a specific implementation," or "an example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A coating slurry, characterized in that: The main components include a solvent and an adhesive polymer resin, wherein the weight ratio of the adhesive polymer resin in the slurry is 5-20wt%; wherein the adhesive polymer resin includes a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, and the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50; Or the adhesive polymer resin includes a combination of at least two polyimide adhesive resins; wherein the at least two polyimide adhesive resins include a first polyimide adhesive resin and a second polyimide adhesive resin, and the mass ratio of the first polyimide adhesive resin to the second polyimide adhesive resin is 50-85:15-50.

2. The coating slurry according to claim 1, characterized in that: The glass transition temperature of the first polyimide adhesive resin is higher than 200°C, and the glass transition temperature of the second polyimide adhesive resin is higher than 140°C.

3. The coating slurry according to claim 1, characterized in that: The solvent is an organic solvent, the polyimide-based adhesive resin is a modified polyimide, and the modified polyimide can be dissolved in the organic solvent.

4. The coating slurry according to claim 3, characterized in that: In the case where the adhesive polymer resin comprises a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, the polyimide-based adhesive resin is selected from at least one of the following polymer resins: ether anhydride-type polyimide, fluorine anhydride-type polyimide, ketone anhydride-type polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, polyetherimide; In the case where the adhesive polymer resin includes a combination of at least two polyimide adhesive resins, the at least two polyimide adhesive resins are selected from at least two of the following polymer resins: ether anhydride polyimide, fluoroanhydride polyimide, ketone anhydride polyimide, bismaleimide, alkenyl-terminated polyimide, polyesterimide, polyamide-imide, and polyetherimide.

5. The coating slurry according to any one of claims 1 to 4, characterized in that: The slurry also includes a coating pore expander, wherein the weight ratio of the coating pore expander in the slurry is 0.05-5wt%.

6. The coating slurry according to claim 5, characterized in that: The coating pore-enlarging agent is one of water, ethanol, polyethylene glycol, methanol, etc. or a combination thereof.

7. The coating slurry according to claim 6, characterized in that: It also includes ceramic particles, the weight ratio of the ceramic particles in the slurry is greater than 0wt% and less than or equal to 5wt%. Preferably, the ceramic particles are any one of aluminum oxide, boehmite, silicon oxide, magnesium oxide, titanium oxide, zirconium oxide, or a combination thereof.

8. The coating slurry according to claim 7, characterized in that: The weight ratio of the solvent in the slurry is 70-95wt%, and the preferred solvent is acetone, N-methylpyrrolidone, DMAc or a combination thereof.

9. A coated diaphragm, characterized in that: It includes a substrate and a polymer layer, wherein the polymer layer is coated on one side or both sides of the substrate; wherein the polymer layer contains at least two functional groups selected from imide groups, aromatic heterocycles, and aromatic rings, and at least one functional group selected from ester groups, ether bonds, thioether bonds, ketone groups, amide groups, phenolic hydroxy ethers, methyl groups, methylene groups, trifluoromethyl groups, and CF bonds.

10. A coated diaphragm, characterized in that: The invention comprises a substrate and a polymer layer, wherein the polymer layer is coated on one side or both sides of the substrate, and the polymer layer is formed by coating the coating slurry according to any one of claims 1 to 8.

11. A coated diaphragm, characterized in that: It comprises a substrate and a polymer layer, wherein the polymer layer is coated on one side or both sides of the substrate; the coated diaphragm has a breakdown voltage of 2.1 kV to 5.0 kV and a breakdown strength of more than 4.5 kV / mil.

12. The coated diaphragm according to any one of claims 9 to 11, characterized in that: The surface layer of the polymer layer has a pore structure. Preferably, the pore structure is a honeycomb-shaped, dense pore structure.

13. The coated membrane according to claim 12, characterized in that It also includes ceramic particles, which are embedded in the polymer layer to form an island structure. Preferably, the diameter of the island structure is 0.5 μm to 2 μm.

14. A method for preparing a coated diaphragm according to any one of claims 11 to 13, characterized in that: The following steps are involved: Providing a substrate; Prepare slurry: add 5-20wt% of adhesive polymer resin into solvent and dissolve it completely. A coating slurry is obtained; wherein the adhesive polymer resin comprises a combination of a PVDF-based adhesive resin polymer and a polyimide-based adhesive resin, and the mass ratio of the polyimide-based adhesive resin to the PVDF-based adhesive resin polymer is 50-85:15-50; or the adhesive polymer resin comprises a combination of at least two polyimide-based adhesive resins, the at least two polyimide-based adhesive resins comprise a first polyimide-based adhesive resin and a second polyimide-based adhesive resin, and the mass ratio of the first polyimide-based adhesive resin to the second polyimide-based adhesive resin is 50-85:15-50; Coating: coating the coating slurry prepared above on at least one surface of the substrate to form a polymer layer on the substrate.

15. A battery, characterized in that: The invention comprises a coated separator, a positive electrode, a negative electrode and an electrolyte, wherein the coated separator is the coated separator according to any one of claims 9 to 14.

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