Diaphragm, secondary battery, and electric device

Abstract

The invention discloses a diaphragm, a secondary battery and an electric device, and belongs to the technical field of batteries, the diaphragm comprises a polyimide film, the polyimide film contains a modified element, the modified element comprises an S element, and at least part of the S element exists in the form of-S-S-. The polyimide film comprises an S element, at least part of the S element exists in the form of-S-S-(disulfide bond), when the battery is short-circuited or locally overheated to cause the temperature of the diaphragm to be higher than or equal to 120 DEG C, the disulfide bond is broken by short circuit or high temperature to form free sulfur atoms and sulfydryl, and after the temperature is reduced to about 80 DEG C, the sulfur atom and sulfydryl are separated from each other. Sulfur atoms are recombined into disulfide bonds through diffusion to repair cracks or defects, and the disulfide bonds can trigger a fracture-recombination process through heat or an electric field to drive a macromolecular chain to slide and crosslink, so that the cracks are repaired, the thermal stability of the diaphragm is effectively improved, the thermal shrinkage of the diaphragm is reduced, and the cycling stability of the secondary battery is effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a separator, a secondary battery and an electric device. BACKGROUND

[0002] Secondary batteries are widely used in electric vehicles, energy storage systems and other fields due to their high energy density and long cycle life.

[0003] As a key component of secondary batteries, the performance of the separator directly affects the safety, charge-discharge performance and life of the battery. The traditional polyethylene (PE) and polypropylene (PP) separators have low melting points (PE: 130℃, PP: 160℃) and are prone to melting and shrinking at high temperatures, losing their insulation function.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provide a separator, a secondary battery and an electric device. The present application can effectively improve the thermal stability of the separator, reduce the thermal shrinkage of the separator, and effectively improve the cycle stability of the secondary battery.

[0006] To achieve the above-mentioned purpose, the first aspect of the present application provides a separator comprising a polyimide film, wherein the polyimide film contains a modified element, and the modified element comprises S element, and at least part of the S element exists in the form of -S-S-.

[0007] As an embodiment of the present application, a first coating layer is arranged on at least one surface of the polyimide film, and the first coating layer comprises a hydrophobic material.

[0008] As an embodiment of the present application, a second coating layer is further arranged on the surface of the first coating layer, and the second coating layer comprises a quantum dot material.

[0009] As an embodiment of the present application, the hydrophobic material comprises at least one of boron nitride and polytetrafluoroethylene; and / or the quantum dot material comprises at least one of zinc oxide quantum dots, titanium dioxide quantum dots and silicon dioxide quantum dots.

[0010] As an embodiment of the present application, the mass percentage content of S element in the polyimide film is 1.2-1.8%.

[0011] As an embodiment of the present application, the mass percentage content of Si element in the polyimide film is 0.3-0.5%.

[0012] As an embodiment of the present application, the porosity of the polyimide film is 40-60%.

[0013] As an embodiment of the present application, the first coating has a contact angle of ≥150 degrees with deionized water.

[0014] As an embodiment of the present application, the quantum dot material has a particle size Dv50 of 5-10 nm.

[0015] A second aspect of the present application provides a secondary battery comprising the above-mentioned separator.

[0016] A third aspect of the present application provides an electric device comprising the above-mentioned secondary battery as a power supply for the electric device.

[0017] The polyimide film described in the present application contains S elements, at least part of which exist in the form of -S-S- (disulfide bond). When the temperature of the separator rises to ≥120℃ due to short circuit or local overheating of the battery, the disulfide bond is broken by the short circuit or high temperature to form free sulfur atoms and mercapto groups (-SH). After the temperature drops to about 80℃, the sulfur atoms recombine into disulfide bonds by diffusion, repairing cracks or defects. When the separator is mechanically damaged (such as puncture, extrusion), microcracks are generated by the sliding of fibers in the polyimide film, and the disulfide bond can be triggered to break and recombine by heat or electric field, driving the sliding and crosslinking of polymer chains, thereby repairing the cracks and improving the structural stability of the separator, slowing down the performance degradation of the secondary battery, effectively improving the thermal stability of the separator, reducing the thermal shrinkage of the separator, and effectively improving the cycle stability of the secondary battery. DETAILED DESCRIPTION

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some, but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0019] In the present application, the technical features described in an open manner include both the closed technical solutions consisting of the listed features and the open technical solutions containing the listed features.

[0020] In the present application, as long as there is no special description, the numerical range is considered to be continuous and includes the minimum value and the maximum value of the range and every value between the minimum value and the maximum value. Further, when the range refers to an integer, every integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0021] The inventors of the present application have found that the diaphragm, as a key component of the secondary battery, still faces the following problems: 1. Mechanical damage risk: the diaphragm is prone to short circuit caused by puncture, extrusion during battery assembly or use, and even thermal runaway; 2. Lack of self-repairing ability: existing diaphragms cannot actively repair internal micro-defects (such as diaphragm defects caused by electrode particle shedding or dendrite growth), and the performance significantly decreases after long-term use; 3. Electrolyte affinity problem: hydrophobic diaphragm easily leads to poor electrolyte wettability, affecting ion conduction efficiency.

[0022] In view of the above problems, the prior art introduces ceramic coatings (such as aluminum oxide, silicon dioxide) on polyolefin diaphragms or uses aramid (poly-para-phenyleneterephthalamide) diaphragms to improve high temperature resistance, but there are problems such as high cost and complex process. In addition, current self-repairing diaphragms mostly rely on physical phase change (such as shape memory polymers) or external stimuli (such as light, magnetic field), which are difficult to meet the dynamic needs of battery operating environment.

[0023] Therefore, based on the above problems, the embodiments of the present application provide a diaphragm, which comprises a polyimide film containing a modified element, and the modified element comprises an S element, and at least part of the S element exists in the form of -S-S-.

[0024] The polyimide film described in the present application contains S elements, and at least part of the S elements exist in the form of -S-S- (disulfide bond). When the battery short-circuit or local overheating causes the temperature of the diaphragm to rise to ≥120℃, the short-circuit or high temperature causes the disulfide bond to break, forming free sulfur atoms and mercapto groups (-SH). When the temperature drops to about 80℃, the sulfur atoms recombine into disulfide bonds by diffusion, repairing cracks or defects. When the diaphragm is mechanically damaged (such as puncture, extrusion), micro-cracks are generated by the sliding of fibers of the polyimide film. The disulfide bond can be triggered to break and recombine by heat or electric field, driving the sliding and crosslinking of the polymer chain, thereby repairing the cracks, improving the structural stability of the diaphragm, slowing down the performance degradation rate of the secondary battery, effectively improving the thermal stability of the diaphragm, reducing the thermal shrinkage of the diaphragm, and effectively improving the cycle stability of the secondary battery.

[0025] In some embodiments, a first coating layer is provided on at least one surface of the polyimide film, and the first coating layer comprises a hydrophobic material. By providing a first coating layer on at least one surface of the polyimide film, the first coating layer contains a hydrophobic material, so that the first coating layer has excellent hydrophobic effect, which can effectively inhibit the contact of excess electrolyte with the negative electrode, reduce the occurrence of side reactions, and on the other hand, has excellent interfacial bonding force with the polyimide film, significantly improving the thermal safety, ion transmission efficiency and cycle life of the diaphragm.

[0026] In some embodiments, the first coating surface is further provided with a second coating, and the second coating comprises quantum dot material. The second coating with quantum dot material can catalyze the decomposition of HF generated by the electrolyte, reduce the corrosion risk of the separator, accelerate the formation of disulfide bonds, optimize porosity and surface charge, improve the wettability of the electrolyte, protect the polyimide film and the first coating from corrosion, and improve the self-repair efficiency, ion transport performance, mechanical strength, and thermal stability of the separator.

[0027] In the present application, quantum dot material refers to a semiconductor material (such as zinc oxide, titanium oxide, silicon oxide, etc.) whose size is reduced to the nanometer level, and the movement of electrons and holes is limited in a very small space, resulting in the energy level changing from a continuous energy band to discrete quantum energy levels.

[0028] In some embodiments, the hydrophobic material comprises at least one of boron nitride and polytetrafluoroethylene. Boron nitride and polytetrafluoroethylene have excellent hydrophobic properties and thermal stability, and are used as hydrophobic materials to improve the mechanical strength and thermal stability of the separator.

[0029] In some embodiments, the quantum dot material comprises at least one of zinc oxide quantum dots, titanium dioxide quantum dots, and silicon dioxide quantum dots.

[0030] In some embodiments, the mass percentage of S element in the polyimide film is 1.2-1.8%, for example, it can be 1.2%, 1.4%, 1.5%, 1.6%, 1.8%, or a range formed by any two of the above values. By controlling the content of S element in this range, the mechanical properties and self-repair efficiency can be effectively improved.

[0031] In some embodiments, the mass percentage of Si element in the polyimide film is 0.3-0.5%, for example, it can be 0.3%, 0.4%, 0.5%, or a range formed by any two of the above values. By controlling the mass percentage of Si element in the polyimide film in this range, the density and distribution of siloxane bonds (-Si-O-) in the polyimide film can be adjusted, and the interface bonding force, self-repair efficiency, and electrochemical performance can be synergistically optimized.

[0032] In some embodiments, the polyimide film contains silane, and the mass percentage of silane in the polyimide film is 1-2%, for example, it can be 1%, 1.2%, 1.5%, 1.6%, 1.8%, 2%, or a range formed by any two of the above values.

[0033] The mass percentage content of Si and S elements in the present application can be obtained by XPS (X-ray photoelectron spectroscopy) detection. The mass percentage content of disulfide bond can be obtained by redox titration detection.

[0034] In some embodiments, the porosity of the polyimide film is 40-60%, for example, 40%, 42%, 45%, 48%, 50%, 52%, 54%, 55%, 56%, 60%, or a range defined by any two of the above values. By controlling the porosity of the polyimide film in this range, the wettability of the electrolyte to the separator can be improved, the lithium ion insertion and extraction can be promoted, and the structural stability of the separator can be improved.

[0035] The test method for the porosity of the polyimide film is as follows: The polyimide film is dried (60°C, 2h) to remove moisture, and cut into a raw piece with a diameter of 25mm. A mercury injection instrument is used to inject 10mL of mercury volume. The test is performed under a vacuum degree <10 -3 Pa, and the pressure range is 0.003-360MPa.

[0036] In some embodiments, the contact angle of the first coating layer to deionized water is ≥150°.

[0037] In some embodiments, the contact angle of the first coating layer to deionized water is 150°-157°, for example, 150°, 151°, 152°, 153°, 154°, 155°, 156°, 157°, or a range defined by any two of the above values. The contact angle is measured by a contact angle measuring instrument.

[0038] In some embodiments, the particle size Dv50 of the quantum dot material is 5-10nm, for example, 5nm, 6nm, 7nm, 8nm, 9nm, 10nm, or a range defined by any two of the above values. By controlling the particle size Dv50 of the quantum dot material in this range, the agglomeration of the quantum dot material can be more effectively avoided, the continuity of the coating layer can be effectively improved, the active sites can be increased, the catalytic efficiency can be improved, and the ion transmission efficiency can be effectively improved.

[0039] In some embodiments, the thickness of the polyimide film is 10-15μm, for example, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or a range defined by any two of the above values.

[0040] In some embodiments, the thickness of the first coating layer is 10-50nm, for example, 10nm, 15nm, 20nm, 30nm, 40nm, 50nm, or a range defined by any two of the above values.

[0041] In some embodiments, the second coating layer has a thickness of 5-10 nm, for example, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or a range defined by any two of the aforementioned values.

[0042] The polyimide film is prepared by the following method:

[0043] The polyamide acid (PAA) is prepared into a slurry with a solvent, and the slurry is spun by a spinning machine to obtain a PI fiber film (polyimide fiber film).

[0044] The PI fiber film is immersed in a mercapto silane coupling agent solution, ultrasonically treated, and dried to obtain a precursor.

[0045] The precursor is immersed in a mixture of ethylene glycol diglycidyl ether and mercaptoacetic acid, reacted, and dried to obtain a polyimide film.

[0046] In some embodiments, the solid content of the slurry is 10-20%, for example, 10%, 12%, 14%, 15%, 16%, 18%, 20%, or a range defined by any two of the aforementioned values.

[0047] In some embodiments, the solvent includes at least one of N,N-dimethylacetamide, N-methylpyrrolidone, ethanol, acetone, propanol, and methanol.

[0048] In some embodiments, the voltage for spinning is 15-20 kv, and the receiving distance is 15-20 cm.

[0049] In some embodiments, the mercapto silane coupling agent includes at least one of 3-mercaptopropyl trimethoxysilane and γ-mercaptopropyl triethoxysilane.

[0050] In some embodiments, the ultrasonic treatment time is 20-50 min, for example, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, or a range defined by any two of the aforementioned values.

[0051] In some embodiments, the molar ratio of ethylene glycol diglycidyl ether to mercaptoacetic acid is 1:(0.5-2).

[0052] In some embodiments, the reaction temperature is 60-90℃, and the reaction time is 2-6 h.

[0053] In some embodiments, the method for forming the first coating layer is:

[0054] The hydrophobic material is dispersed in a solvent to form a slurry, the slurry is applied to the surface of the polyimide film, and dried to form a first coating layer.

[0055] In some embodiments, the solvent includes at least one of ethanol, methanol, propanol, water, N,N-dimethylacetamide, N-methylpyrrolidone, and acetone.

[0056] In some embodiments, the solid content of the slurry is 1 to 5%.

[0057] In some embodiments, the method of forming the second coating layer is:

[0058] The quantum dot material is dispersed in a solvent, applied to the surface of the first coating layer, and dried to form a second coating layer.

[0059] An embodiment of the present application provides a secondary battery including the above-described separator.

[0060] In some embodiments, the secondary battery further includes a positive electrode tab including a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including a positive electrode active material.

[0061] In some embodiments, the positive electrode active material can include lithium-containing phosphates, lithium transition metal oxides, and modified compounds thereof. However, the present application is not limited to these materials or substances, and other conventional materials or substances that can be used as positive electrode active materials for secondary batteries can also be used. These positive electrode active materials can be used alone or in combination with two or more. Non-limiting examples of the lithium transition metal oxide can include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof.

[0062] In some embodiments, the type of the positive electrode current collector is not particularly limited, and can be a material known to be suitable for use as a positive electrode current collector.

[0063] In some embodiments, the positive electrode current collector includes metal materials such as aluminum, stainless steel, a nickel plating layer, titanium, tantalum, and carbon materials such as carbon cloth and carbon paper.

[0064] In some embodiments, the form of the positive electrode current collector is not particularly limited. When the positive electrode current collector is a metal material, the form of the positive electrode current collector can be a metal foil, a metal cylinder, a metal roll, a metal sheet, a metal foil, a metal sheet mesh, a punched metal, a foamed metal, or the like. When the positive electrode current collector is a carbon material, the form of the positive electrode current collector can include, but is not limited to, a carbon sheet, a carbon film, a carbon cylinder, or the like.

[0065] In some embodiments, the positive electrode active material layer further includes a conductive agent and a binder.

[0066] In some embodiments, the secondary battery further includes a negative electrode tab including a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material.

[0067] In the present application, the negative electrode current collector is not particularly limited as long as the object of the present application can be achieved, and for example, can be a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, or a composite current collector, etc.

[0068] In some embodiments, the negative electrode active material includes at least one of graphite, a silicon-based material, meso-carbon microbeads (MCMB), hard carbon, soft carbon, a Li-Sn alloy, a Li-Sn-O alloy, Sn, SnO, SnO2, a spinel-structured lithium titanate Li4Ti5O12, a Li-Al alloy, and metallic lithium, etc. 12

[0069] In some embodiments, the negative electrode active material layer further includes a conductive agent and a binder.

[0070] In some embodiments, the kind of the conductive agent mentioned in the present application is not limited, and a known conductive agent can be used.

[0071] In some embodiments, the conductive agent includes at least one of acetylene black, needle coke, carbon nanotubes, graphene, etc.

[0072] In some embodiments, the kind of the binder mentioned in the present application is not limited, and a known binder can be used.

[0073] In some embodiments, the binder mentioned includes at least one of polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, cellulose nitrate, styrene butadiene rubber, nitrile rubber, fluoro rubber, isoprene rubber, polybutadiene rubber, ethylene-propylene rubber, styrene-butadiene-styrene block copolymer or its hydrogenated product, ethylene-propylene-diene terpolymer, styrene-ethylene-butadiene-ethylene copolymer, styrene-isoprene-styrene block copolymer, syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyvinylidene fluoride, polytetrafluoroethylene, fluorinated polyvinylidene fluoride, polytetrafluoroethylene-ethylene copolymer, etc.

[0074] In some embodiments, the secondary battery can include an outer package.

[0075] ​In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0076] The shape of the secondary battery according to the present application is not particularly limited, and can be cylindrical, square, or other shapes.

[0077] An embodiment of the present application provides an electric device comprising the secondary battery described above as a power supply of the electric device.

[0078] Exemplarily, the electric device described above can include a mobile device (such as a mobile phone, a notebook computer, or the like), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, or the like), an electric train, a ship and a satellite, an energy storage system, or the like, but is not limited thereto.

[0079] The present application is further described below with specific examples:

[0080] Example 1

[0081] A method for preparing a secondary battery, comprising the following steps:

[0082] (1) Preparation of a separator

[0083] S1, uniformly mix N,N-dimethylacetamide and water in a volume ratio of 8:2 to obtain a mixed solvent, disperse polyamide acid (molecular weight 12000 g / mol) in the mixed solvent, and prepare a slurry with a solid content of 15 wt%, use a high-voltage electrospinning machine (voltage 15 kV, receiving distance 16 cm) to spin the slurry, and use an aluminum foil rotating drum with a speed of 80 rpm to obtain a PI fiber membrane;

[0084] S2, immerse the PI fiber membrane in a 3-mercaptopropyltrimethoxysilane solution with a mass concentration of 2 wt%, ultrasonic treatment for 30 min, and bake at 60°C for 2 h to obtain a precursor;

[0085] S3, immerse the precursor in an EGDE (ethylene glycol diglycidyl ether)-TGA (mercaptoacetic acid) mixed solution (molar ratio 1:1, pH = 7.5) and react at 80°C for 4 h, then take out and vacuum dry at 150°C for 6 h to obtain a polyimide membrane.

[0086] (2) Preparation of the positive electrode sheet: the positive electrode active material LiCoO2, the conductive agent carbon nanotube (CNT), the conductive agent carbon black (Super P), and the binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 97.5:0.5:0.9:1.1 in a proper amount of solvent NMP to form a uniform positive electrode slurry; the positive electrode slurry was uniformly coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, the positive electrode sheet was obtained. The area density of the positive electrode film layer was 10 mg / cm2. 2 .

[0087] (3) Preparation of the negative electrode sheet: the artificial graphite material, the binder styrene-butadiene rubber (SBR), the thickening agent sodium carboxymethyl cellulose (CMC-Na), and the conductive agent carbon black (Super P) were mixed in a weight ratio of 96.2:1.8:1.2:0.8 in a proper amount of solvent deionized water to form a uniform negative electrode slurry; the negative electrode slurry was uniformly coated on the surface of the negative electrode current collector copper foil, and after drying and cold pressing, the negative electrode sheet was obtained. The area density of the negative electrode film layer was 5 mg / cm2. 2 .

[0088] (4) Preparation of the electrolyte: at room temperature, in an argon-filled glove box (H2O <1 ppm, O2 <1 ppm), ethylene carbonate (EC), methyl ethyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed uniformly in a mass ratio of 1:1:1 to obtain a mixed solution, and lithium salt LiPF6 was dissolved in the mixed solution to obtain an electrolyte. The concentration of the lithium salt was 1.0 mol / L.

[0089] (5) The positive electrode sheet, the separator, and the negative electrode sheet were stacked in order with the separator between the positive and negative electrode sheets, and after winding, heat pressing, and shaping, the tab was welded to obtain a bare cell. The bare cell was packaged in an outer packaging aluminum plastic film, placed in an oven at 100±5℃ for 48 h, injected with the above electrolyte, and then allowed to stand, form, and be divided to obtain a secondary battery.

[0090] The parameters of the separator are shown in Table 1.

[0091] Examples 2-7

[0092] Examples 2-7 are different from Example 1 in that the concentration of the 3-mercaptopropyl trimethoxysilane solution of S2 is changed to change the mass percentage of sulfur in the polyimide film.

[0093] Examples 8-12

[0094] Examples 8-12 are different from Example 1 in that the voltage and receiving distance of the electrospinning machine of S1 are changed to change the porosity of the polyimide film.

[0095] Examples 13-14

[0096] Examples 13-14 differ from Example 1 in that the thickness of the polyimide film is different, as shown in Table 1.

[0097] Example 15

[0098] Example 15 differs from Example 1 in that the preparation method of the separator is different.

[0099] The preparation method of the separator of the present example is as follows:

[0100] Preparation of the separator

[0101] S1, uniformly mix N,N-dimethylacetamide and water in a volume ratio of 8:2 to obtain a mixed solvent, disperse polyamic acid (molecular weight 12000 g / mol) in the mixed solvent to prepare a slurry with a solid content of 15 wt%, and use a high-voltage electrospinning machine (voltage 15 kV, receiving distance 16 cm) to spin the slurry, using an aluminum foil rotating drum at a speed of 80 rpm to obtain a PI fiber membrane;

[0102] S2, immerse the PI fiber membrane in a 3-mercaptopropyltrimethoxysilane solution with a mass concentration of 2 wt%, ultrasonic treatment for 30 min, and baking at 60°C for 2 h to obtain a precursor;

[0103] S3, immerse the precursor in an EGDE (ethylene glycol diglycidyl ether)-TGA (mercaptoacetic acid) mixture (molar ratio 1:1, pH = 7.5) and react at 80°C for 4 h, then take it out and vacuum dry at 150°C for 6 h to obtain a polyimide film.

[0104] S4, disperse boron nitride powder in an ethanol solution to form a boron nitride slurry with a solid content of 3 wt%;

[0105] Immerse the polyimide film in the boron nitride slurry (dip coating) at a coating speed of 50 mm / min, and dry at 80°C for 1 h to form a first coating layer.

[0106] The specific parameters of the separator of the present example are shown in Table 1.

[0107] Examples 16-17

[0108] Examples 16-17 differ from Example 15 in that the concentration of boron nitride in the ethanol solution is changed to adjust the contact angle of the first coating layer.

[0109] Examples 18-19

[0110] Examples 18-19 differ from Example 15 in that the thickness of the first coating layer is different, as shown in Table 1.

[0111] Example 20

[0112] Example 20 differs from Example 1 in that the preparation method of the separator is different.

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

[0114] Preparation of the separator

[0115] S1, uniformly mix N,N-dimethylacetamide and water in a volume ratio of 8:2 to obtain a mixed solvent, disperse polyamide acid (molecular weight 12000 g / mol) in the mixed solvent, and prepare a slurry with a solid content of 15 wt%, use a high-voltage electrospinning machine (voltage 15 kV, receiving distance 16 cm) to spin the slurry, and use an aluminum foil rotating drum with a speed of 80 rpm to obtain PI fibers;

[0116] S2, immerse the PI fibers in a 3-mercaptopropyltrimethoxysilane solution with a mass concentration of 2 wt%, ultrasonic treat for 30 min, and bake at 60°C for 2 h to obtain a precursor;

[0117] S3, immerse the precursor in an EGDE (ethylene glycol diglycidyl ether)-TGA (mercaptoacetic acid) mixture (molar ratio 1:1, pH = 7.5) and react at 80°C for 4 h, then take it out and vacuum dry at 150°C for 6 h to obtain a polyimide film.

[0118] S4, disperse boron nitride powder in an ethanol solution to form a boron nitride slurry with a solid content of 3 wt%;

[0119] Immerse the polyimide film in the boron nitride slurry (dip coating) at a coating speed of 50 mm / min, and dry at 80°C for 1 h to form a first coating layer.

[0120] S5, hydrothermal synthesis: dissolve zinc nitrate (Zn(NO3)2·6H2O) and hexamethylenetetramine (HMT) in deionized water in a ratio of 1:1, react at 90°C for 6 h, centrifuge at 8000 rpm for 10 min, and then ball mill after drying to obtain ZnO quantum dots with a particle size Dv50 of 5 nm;

[0121] S6, slurry preparation: disperse the ZnO quantum dots in NMP (concentration 0.5 wt%);

[0122] S7, uniformly coat the first coating layer surface by using a doctor blade method; vacuum dry at 120°C for 4 h to form a second coating layer.

[0123] The specific parameters of the separator of this example are shown in Table 1.

[0124] Examples 21-23

[0125] Examples 21-23 differ from Example 20 in that the particle size Dv50 of the ZnO quantum dots is adjusted by changing the ball milling parameters in step S5.

[0126] Example 24

[0127] Example 24 differs from Example 1 in that the thickness of the second coating layer is different, as shown in Table 1.

[0128] Comparative Example 1

[0129] Comparative Example 1 differs from Example 1 in that the polyimide film of Comparative Example 1 does not contain disulfide bonds.

[0130] The method for preparing the separator of Comparative Example 1 is as follows:

[0131] S1, uniformly mix N,N-dimethylacetamide and water in a volume ratio of 8:2 to obtain a mixed solvent, disperse polyamic acid (molecular weight 12000 g / mol) in the mixed solvent to prepare a slurry with a solid content of 15 wt%, and use a high-voltage electrospinning machine (voltage 15 kV, receiving distance 16 cm) to spin the slurry, using an aluminum foil rotating drum at a speed of 80 rpm to obtain a PI fiber membrane, and using the PI fiber membrane as the separator.

[0132] Table 1

[0133]

[0134]

[0135] Performance test

[0136] 1) Thermal shrinkage:

[0137] 1. Sample preparation:

[0138] The separators of the examples and comparative examples were cut into rectangular pieces (size 20 mm x 5 mm, without wrinkles);

[0139] The initial length (L0) and thickness (d0) were measured using a vernier caliper, and the average of three measurements was recorded.

[0140] 2. Instrument settings:

[0141] A thermal mechanical analyzer (TA Instruments Q400) was used, the sample was installed and fixed;

[0142] The test parameters were set: heating rate 5°C / min (to avoid thermal stress mutation), temperature range 25-200°C (covering the use temperature range of the film material), atmosphere nitrogen (to prevent oxidation), and the length L1 and thickness d1 of the separator after thermal shrinkage were recorded, and the thermal shrinkage rate of the separator was calculated.

[0143] 2) Ion conductivity:

[0144] 1. Sample preparation:

[0145] The membrane of the examples and comparative examples was cut into a circular shape (10 mm in diameter), and the surface was cleaned with DMF (N,N-dimethylformamide) to remove residual solvent; the sample was dried in a vacuum drying oven at 60°C for 2 h to ensure that no moisture remained.

[0146] 2. Cell assembly (simulated cell):

[0147] A symmetric cell was prepared: the positive and negative electrodes were both lithium metal sheets (12 mm in diameter, 0.5 mm in thickness), and a separator was placed in between; an electrolyte (the solvent system of the electrolyte was ethylene carbonate and dimethyl carbonate at a mass ratio of 1:1, containing 1.0 mol / L of lithium salt LiPF6) was injected, and the cell was sealed and left to stand for 2 h (to ensure that the interface was stable).

[0148] 3. EIS test:

[0149] An electrochemical workstation (Autolab PGSTAT302N) was used, and the parameters were set as follows:

[0150] Frequency range: 10-2~ 10 5 Hz (covering low-frequency ion migration and high-frequency double-layer response);

[0151] Disturbance voltage: 5 mV (small amplitude to avoid polarization);

[0152] Temperature: 25°C (controlled by a thermostat); the ion conductivity of the separator was obtained from the workstation.

[0153] 3) Closed pore temperature:

[0154] 1. Sample preparation:

[0155] The separator of the examples and comparative examples was fixed on a glass slide;

[0156] The initial pore morphology was observed using a polarizing microscope (crossed polarizers); the bright areas were open pores.

[0157] 2. Hot stage heating:

[0158] The glass slide was placed on a hot stage (Linkam THMS600) of a polarizing microscope, and the temperature was set to increase at a rate of 5°C / min; the change in the pores was observed in real time: the transition from open pores (bright) to closed pores (dull).

[0159] 3. Closed pore temperature was recorded:

[0160] When more than 50% of the pores changed from bright to dull, the temperature at this time was recorded as the closed pore temperature.

[0161] 4) Cycle life:

[0162] Cycle conditions:

[0163] Charge: constant current-constant voltage (CC-CV), current density 0.5 mA / cm 2 (0.5C), cut-off voltage 4.2 V;

[0164] Discharge: constant current (CC), current density 0.5 mA / cm 2 , cut-off voltage 3.0 V;

[0165] Capacity retention rate calculation:

[0166] Initial capacity (C0): discharge capacity of the 1st cycle;

[0167] Cycle to the capacity reaches 80% C0, stop testing, record the number of cycles at this time.

[0168] Table 2

[0169]

[0170]

[0171] As can be seen from Table 2, the polyimide film described in the present application contains S elements, at least part of the S elements exist in the form of -S-S- (disulfide bond), when the battery short circuit or local overheating causes the temperature of the separator to rise to ≥ 120℃, the short circuit or high temperature makes the disulfide bond break, forming free sulfur atoms and mercapto groups (-SH), after the temperature drops to about 80℃, the sulfur atoms recombine into disulfide bonds by diffusion, repairing cracks or defects, when the separator is mechanically damaged (such as puncture, extrusion), the fiber slip of the polyimide film produces microcracks, the disulfide bond can trigger the breakage-recombination process by heat or electric field, driving the polymer chain slip and crosslinking, thereby repairing the cracks, improving the structural stability of the separator, slowing down the performance degradation rate of the secondary battery, effectively improving the thermal stability of the separator, reducing the thermal shrinkage of the separator, and effectively improving the cycle stability of the secondary battery.

[0172] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. A diaphragm, characterized in that, The invention includes a polyimide film containing a modifying element, the modifying element including an S element, at least a portion of which is present in the form of -SS-.

2. The diaphragm according to claim 1, characterized in that, The polyimide film has a first coating on at least one surface, the first coating comprising a hydrophobic material.

3. The diaphragm according to claim 2, characterized in that, The surface of the first coating is further provided with a second coating, which contains quantum dot material.

4. The diaphragm according to claim 3, characterized in that, The hydrophobic material includes at least one of boron nitride and polytetrafluoroethylene; and / or the quantum dot material includes at least one of zinc oxide quantum dots, titanium dioxide quantum dots, and silicon dioxide quantum dots.

5. The diaphragm according to claim 1, characterized in that, The mass percentage of sulfur in the polyimide film is 1.2% to 1.8%.

6. The diaphragm according to claim 1, characterized in that, The polyimide film also contains Si element, and the mass percentage of Si element in the polyimide film is 0.3-0.5%.

7. The diaphragm according to claim 1, characterized in that, The porosity of the polyimide membrane is 40-60%.

8. The diaphragm according to claim 3, characterized in that, The first coating has a contact angle ≥150° with deionized water; and / or The particle size Dv50 of the quantum dot material is 5–10 nm.

9. A secondary battery, characterized in that, Includes the diaphragm as described in any one of claims 1 to 8.

10. An electrical device, characterized in that, Includes the secondary battery as described in claim 9, wherein the secondary battery serves as the power supply for the electrical device.

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