Separator, secondary battery and electric device

Abstract

The present application relates to the technical field of batteries. Provided are a separator, a secondary battery and an electric device. The separator comprises a base film and a composite coating layer disposed on at least one surface of the base film. The composite coating layer comprises ceramic particles and polymer microspheres. Each polymer microsphere comprises a core, a second shell layer disposed on the surface of the core, and a first shell layer disposed on the surface of the second shell layer. The provision of the coating layer on the surface of the separator has a significant effect on the mechanical strength, flame retardancy, electrolyte wettability, ionic conductivity, and foreign particle resistance of the separator, thereby affecting the cycle performance, rate capability and safety performance of the secondary battery.

Description

A separator, a secondary battery and an electric device TECHNICAL FIELD

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

[0002] In the manufacturing process of a secondary battery, problems such as uneven coating of an electrode sheet, edge burrs and insufficient assembly process precision can easily cause mechanical damage to a separator; meanwhile, lithium dendrite growth can penetrate the separator to cause micro short circuits. These two types of defects not only cause capacity attenuation and cycle life reduction of the battery, but also can trigger safety hazards such as thermal runaway.

[0003] Therefore, the separator needs to have excellent properties, such as excellent puncture resistance and mechanical toughness, to resist physical damage caused by electrode sheet burrs, lithium dendrite penetration and assembly stress, so as to maintain the interface stability of the battery system and ensure safety.

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

[0005] Based on this, the purpose of the present application is to overcome the shortcomings of the prior art and provide a separator, a secondary battery and an electric device.

[0006] To achieve the above purpose, in a first aspect of the present application, a separator is provided, comprising a base film and a composite coating layer arranged on at least one surface of the base film; the composite coating layer comprises ceramic particles and polymer microspheres; the polymer microspheres comprise a core, a second shell layer arranged on the surface of the core, and a first shell layer arranged on the surface of the second shell layer; the glass transition temperature of the core is A, the glass transition temperature of the second shell layer is B, and the glass transition temperature of the first shell layer is C, satisfying A

[0007] In some embodiments, the separator satisfies at least one of the following (1) to (3):

[0008] (1) -50℃≤A≤-20℃;

[0009] (2) 150℃≤B≤250℃;

[0010] (3) 0℃≤C≤90℃.

[0011] In some embodiments, the average particle size of the polymer microspheres is 1.5 to 15 μm.

[0012] In some embodiments, in the polymer microspheres, the second shell layer comprises the following components in parts by mass: 20-80 parts of hard monomers and 5-25 parts of crosslinking monomers; the first shell layer comprises the following components in parts by mass: 10-50 parts of soft monomers and 1-15 parts of functional monomers; and the inner core comprises 20-50 parts of organic flame retardants.

[0013] In some embodiments, the diaphragm satisfies at least one of the following (a)-(f):

[0014] (a) The organic flame retardant comprises at least one of tri-methyl phenyl phosphate, triphenyl phosphate, tri-iso-propyl phenyl phosphate, tri-butyl phosphate, tri-octyl phosphate, and tolyl diphenyl phosphate; and the organic siloxane flame retardant comprises at least one of methyl silicone oil, phenyl silicone oil, and hydroxyl silicone oil.

[0015] (b) The soft monomer comprises at least one of butyl acrylate, ethyl acrylate, n-butyl acrylate, iso-octyl acrylate, lauryl acrylate, and lauryl methacrylate.

[0016] (c) The functional monomer comprises at least one of hydroxyethyl acrylate, acrylamide, methacrylic acid, vinyl triethoxy silane, and vinyl trimethoxy silane.

[0017] (d) The hard monomer comprises at least one of methyl methacrylate, ethyl methacrylate, and styrene.

[0018] (e) The crosslinking monomer comprises at least one of ethylene glycol dimethacrylate, allyl methacrylate, diacetone acrylamide and adipic acid dihydrazide, acetoacetoxyethyl methacrylate, and glycidyl methacrylate.

[0019] (f) The mass ratio of the inner core, the second shell layer, and the first shell layer is (2-5):(6-9):(1-4).

[0020] In some embodiments, the composite coating further comprises at least one of a mixture of polyvinylpyrrolidone and polyethyleneimine, a mixture of polyurea and vinyl trimethoxysilane, a mixture of polyaniline and polyvinyl alcohol, a mixture of tannic acid and polyacrylic acid, and a mixture of ureido pyrimidone and polyvinyl alcohol.

[0021] In some embodiments, the mass ratio of the two components in the binder is 1:2 to 2:1, exemplarily, the mass ratio of the polyvinylpyrrolidone and the polyethyleneimine is 1:2 to 2:1, the mass ratio of the polyurea and the vinyltrimethoxysilane is 1:2 to 2:1, the mass ratio of the polyaniline and the polyvinyl alcohol is 1:2 to 2:1, the mass ratio of the tannic acid and the polyacrylic acid is 1:2 to 2:1, and the mass ratio of the ureido pyrimidone and the polyvinyl alcohol is 1:2 to 2:1.

[0022] In some embodiments, the mass percentage of the binder in the composite coating is 1% to 15%.

[0023] In some embodiments, in the composite coating, at least one of the following (4) to (5) is satisfied:

[0024] (4) the mass percentage of the ceramic particles is 80% to 97%;

[0025] (5) the mass percentage of the polymer microspheres is 2% to 12%;

[0026] In some embodiments, the thickness of the composite coating is 1 to 8 μm.

[0027] In some embodiments, the liquid absorption rate of the separator is 100% to 800%, and the liquid retention rate is 80% to 800%.

[0028] In some embodiments, the puncture strength of the base film is ≥300 gf, the puncture strength of the separator is ≥500 gf, and the ratio of the puncture strength of the separator to the puncture strength of the base film is ≥1.67.

[0029] In the second aspect of the present application, a secondary battery is provided, comprising a positive electrode, a negative electrode, an electrolyte, and the separator described in the present application.

[0030] In the third aspect of the present application, an electric device is provided, comprising the secondary battery described in the present application.

[0031] The application provides a diaphragm, which comprises a base film and a composite coating arranged on at least one surface of the base film; the composite coating comprises ceramic particles and polymer microspheres; the polymer microspheres comprise a core, a second shell arranged on the surface of the core and a first shell arranged on the surface of the second shell. The application controls the glass transition temperature A of the core, the glass transition temperature B of the second shell and the glass transition temperature C of the first shell of the polymer microspheres to satisfy A BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a scanning electron microscope (SEM) image of the diaphragm prepared in Example 1. DETAILED DESCRIPTION

[0033] For the purpose of better illustrating the purpose, technical scheme and advantages of the application, the application will be further described below in combination with the drawings and specific examples, which aims to understand the content of the application in detail, rather than limiting the application. All other examples obtained by those skilled in the art without creative labor fall within the protection scope of the application. The experimental reagents and instruments designed in the examples and comparative examples are common reagents and instruments unless otherwise specified, which can be obtained from commercial channels. In the examples and comparative examples, the experimental methods used are conventional methods unless otherwise specified, and the raw materials used in parallel experiments are the same batch of raw materials unless otherwise specified.

[0034] The application provides a diaphragm, which comprises a base film and a composite coating arranged on at least one surface of the base film; the composite coating comprises ceramic particles and polymer microspheres; the polymer microspheres comprise a core, a second shell arranged on the surface of the core and a first shell arranged on the surface of the second shell.

[0035] The glass transition temperature Tg of the core is A, the glass transition temperature Tg of the second shell is B, and the glass transition temperature Tg of the first shell is C, which satisfies A

[0036] This application research has found that when the separator of this application is used, by forming a composite coating including ceramic particles and polymer microspheres on at least one surface of the base film, controlling the structure of the polymer microspheres, and controlling the glass transition temperature A of the core, the glass transition temperature B of the second shell layer, and the glass transition temperature C of the first shell layer in the polymer microspheres, so that A < C < B, and the glass transition temperature A of the core layer < 0°C, the mechanical strength, flame retardancy, and foreign matter resistance of the separator can be improved, significantly enhancing the safety performance of the secondary battery; at the same time, the above-mentioned technical features can also improve the wetting rate of the separator to the electrolyte and the ionic conductivity, thereby significantly improving the cycle performance and rate performance of the secondary battery.

[0037] In one embodiment, the glass transition temperature is obtained using a differential scanning calorimeter (DSC).

[0038] In one embodiment, the separator satisfies -50℃ ≤ A ≤ -20℃. For example, the glass transition temperature A of the core can be a range of -50℃, -45℃, -40℃, -35℃, -30℃, -25℃, -20℃, or any two of these values. When the glass transition temperature A of the core is within this range, the polymer can be kept in a highly elastic state at room temperature, absorbing mechanical stress through molecular chain segment movement, reducing the overall hardness of the polymer microspheres, improving the separator's flexibility, and making the microspheres more prone to deformation during shaping, generating excellent adhesion. This can reduce the relative displacement between the separator and the electrodes during charging and discharging, maintain battery structural stability, extend cycle life, and make the battery less susceptible to damage under mechanical stress, thus improving battery durability.

[0039] In one embodiment, the separator satisfies 150°C ≤ B ≤ 250°C. For example, the glass transition temperature B of the second shell layer can be a range of 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, or any two of these values. When the glass transition temperature B of the second shell layer is within this range, the second shell layer can melt during thermal runaway of the secondary battery, releasing the organic flame retardant therein, inhibiting the combustion of the secondary battery, and providing excellent thermal safety performance. Moreover, the high glass transition temperature of the second shell layer can reduce the swelling degree of the electrolyte, ensuring that the polymer microspheres do not swell due to the electrolyte during long-term cycling and storage, thereby maintaining their intact morphology.

[0040] In one embodiment, the diaphragm satisfies 0°C ≤ C ≤ 90°C. For example, the glass transition temperature C of the first shell layer can be a range of 0°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, or any two of these values. When the glass transition temperature C of the first shell layer is within this range, it ensures that the polymer particles have excellent adhesion and electrolyte wettability.

[0041] In one embodiment, the average particle size of the polymer microspheres is 1.5 to 15 μm, for example, it can be a range of 1.5 μm, 2 μm, 2.5 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 9 μm, 10 μm, 12 μm, 15 μm or any two of these values.

[0042] When the average particle size of the polymer microspheres is within this range, the polymer microspheres can be embedded in the ceramic layer to prevent them from falling off. During the shaping of the secondary battery, the protruding part plays a supporting and adhesive role, so that there is a certain gap between the separator and the electrode, providing a channel and place for electrolyte wetting, absorption and retention, which can improve the long-term charge and discharge cycle performance of the secondary battery.

[0043] In one embodiment, the polymer microspheres include, in the second shell, 20-80 parts by weight of hard monomers and 5-25 parts by weight of crosslinking monomers; in the first shell, 10-50 parts by weight of soft monomers and 1-15 parts by weight of functional monomers; and in the core, 20-50 parts by weight of organic flame retardant.

[0044] In one embodiment, the diaphragm satisfies at least one of the following (a) to (f):

[0045] (a) The organic flame retardant includes organophosphorus flame retardants and organosiloxane flame retardants, wherein the organophosphorus flame retardant includes at least one of tricresyl phosphate, triphenyl phosphate, triisopropylphenyl phosphate, tributyl phosphate, trioctyl phosphate, and tolyl diphenyl phosphate; and the organosiloxane flame retardant includes at least one of methyl silicone oil, phenyl silicone oil, and hydroxyl silicone oil.

[0046] (b) The soft monomer includes at least one of butyl acrylate, ethyl acrylate, n-butyl acrylate, isooctyl acrylate, lauryl acrylate, and lauryl methacrylate;

[0047] (c) The functional monomer includes at least one of hydroxyethyl acrylate, acrylamide, methacrylic acid, vinyltriethoxysilane acrylate, and vinyltrimethoxysilane;

[0048] (d) The hard monomer includes at least one of methyl methacrylate, ethyl methacrylate, and styrene;

[0049] (e) The crosslinking monomers include at least one of ethylene glycol dimethacrylate, allyl methacrylate, diacetone acrylamide and dihydrazine adipate, acetyl acetoxyethyl methacrylate, and glycidyl methacrylate.

[0050] (f) The mass ratio of the core, the second shell, and the first shell is (2-5):(6-9):(1-4).

[0051] This study found that the functional monomers added to the first shell layer can further improve the electrode adhesion, resulting in excellent processing performance for the secondary battery. The cross-linking monomers in the second shell layer can give the shell a high degree of cross-linking, inhibit electrolyte swelling, and stably maintain the polymer microsphere structure.

[0052] This application research found that when the components of the core, second shell, and first shell are selected within the aforementioned ranges, and the mass ratio of the core, second shell, and first shell is within the aforementioned range, the structure of the polymer microspheres can be better controlled. When the glass transition temperatures of the polymer microsphere core, second shell, and first shell are within a suitable range, the mechanical strength, flame retardancy, and foreign matter resistance of the separator can be improved, thereby enhancing the safety performance of the secondary battery. In addition, the above-mentioned technical features can also improve the wetting rate of the separator to the electrolyte and its ionic conductivity, thereby significantly improving the cycle performance and rate performance of the secondary battery.

[0053] In one embodiment, the composite coating further includes an adhesive comprising at least one of a mixture of polyvinylpyrrolidone (PVP) and polyethyleneimine (PEI), a mixture of polyurea (SPUA) and vinyltrimethoxysilane (VTMS), a mixture of polyaniline (PAN) and polyvinyl alcohol (PVA), a mixture of tannic acid and polyacrylic acid, and a mixture of ureidopyrimidinone (UPy) and polyvinyl alcohol (PVA).

[0054] In one embodiment, the mass ratio of the two components in the adhesive is 1:2 to 2:1. Exemplarily, the mass ratio of polyvinylpyrrolidone to polyethyleneimine is 1:2, 1:1, 2:1, or any two of these values; the mass ratio of polyurea to vinyltrimethoxysilane is 1:2, 1:1, 2:1, or any two of these values; the mass ratio of polyaniline to polyvinyl alcohol is 1:2, 1:1, 2:1, or any two of these values; the mass ratio of tannic acid to polyacrylic acid is 1:2, 1:1, 2:1, or any two of these values; and the mass ratio of ureidopyrimidinone to polyvinyl alcohol is 1:2, 1:1, 2:1, or any two of these values.

[0055] In one embodiment, the adhesive has a 1:1 mass ratio of the two components. Exemplarily, the mass ratio of polyvinylpyrrolidone to polyethyleneimine is 1:1, the mass ratio of polyurea to vinyltrimethoxysilane is 1:1, the mass ratio of polyaniline to polyvinyl alcohol is 1:1, the mass ratio of tannic acid to polyacrylic acid is 1:1, and the mass ratio of ureidopyrimidinone to polyvinyl alcohol is 1:1.

[0056] In one embodiment, the ceramic particles in the composite coating comprise alumina and / or boehmite.

[0057] In one embodiment, the composite coating further includes a dispersant and / or a wetting agent. The dispersant includes at least one of sodium polyacrylate, ammonium polyacrylate, styrene-maleic anhydride copolymer, and acrylate polymeric dispersants; the wetting agent includes at least one of polyether siloxane wetting agents and organosilicon wetting agents.

[0058] This application research found that composite coatings, including binders, have the following advantages: First, the multiple hydrogen bond network in the binder endows the polymer with greater mechanical strength and self-healing properties. Through the coordinated action of the internal multiple hydrogen bonds, it can provide excellent self-repair function for the secondary battery separator, repairing structural damage caused by foreign matter, electrode burrs, lithium dendrites, and other factors during processing, thereby improving the safety performance of the secondary battery. Second, the hydrogen bonds between the interpenetrating networks can adapt to stress intensity and the expansion force generated by the secondary battery during charging and discharging, thereby improving the cycle performance of the secondary battery. Third, the repair effect is intrinsic self-repair, which can continuously generate self-repair function, providing protection for the safety of the secondary battery throughout its entire life cycle.

[0059] In one embodiment, the ceramic particles in the composite coating comprise 80% to 97% by mass, for example, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 93%, 94%, 95%, 96%, 97%, or any two of these values. When the mass percentage of ceramic particles is within the above range, the separator can possess excellent heat resistance and electrolyte wettability, thereby improving the charge-discharge cycle life and safety performance of the secondary battery.

[0060] In one embodiment, the polymer microspheres in the composite coating comprise 2% to 12% by mass, for example, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, or any two of these values. The polymer microspheres' mass percentage within this range allows for a wetting gap between the separator and the electrode, providing excellent adhesion and flame retardant properties to the separator, while simultaneously ensuring the separator's heat resistance, further improving the cycle performance and safety of the secondary battery.

[0061] In one embodiment, the binder in the composite coating has a mass percentage content of 1% to 15%, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 13%, 14%, 15%, or any two of these values. When the mass percentage content of the binder is within the above range, the membrane can possess excellent self-healing properties and improve its liquid absorption and retention capacity without affecting its air permeability, significantly improving the cycle performance of the secondary battery, enhancing its resistance to foreign matter, and thus improving the safety performance of the secondary battery.

[0062] In one embodiment, the mass percentage of the dispersant in the composite coating is 0.5% to 2%, for example, it can be 0.5%, 0.8%, 1%, 1.5%, 1.8%, 2%, or any two of these values.

[0063] In one embodiment, the wetting agent in the composite coating has a mass percentage content of 0.1% to 0.3%, for example, it can be 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any two of these values.

[0064] This application research found that when the mass percentage of each component in the composite coating is within the above-mentioned range, it can improve the performance of the separator, ensure the cycle performance and rate performance of the secondary battery, and maintain the safety of the secondary battery.

[0065] In one embodiment, the thickness of the composite coating is 1–8 μm, for example, it can be a range of 1 μm, 2 μm, 2.5 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or any two of these values. A thickness within the above range can provide the separator with excellent heat resistance and electrolyte absorption and retention capabilities, thereby reducing the impedance of the secondary battery and improving its charge-discharge cycle performance.

[0066] In one embodiment, the liquid absorption rate of the diaphragm is 100% to 800%, for example, the liquid absorption rate of the diaphragm is 100%, 200%, 300%, 400%, 500%, 600%, 700%, 800%, or any two of these values.

[0067] In one embodiment, the liquid retention rate of the diaphragm is 80% to 800%, for example, the liquid retention rate is 80%, 100%, 200%, 300%, 400%, 500%, 600%, 700%, 800%, or any two of these values.

[0068] Studies have found that when the liquid absorption rate and liquid retention rate of the diaphragm are within the above-mentioned range, they can provide a sufficient lithium-ion transport environment for the secondary battery during the charging and discharging process, meet the requirements of rapid charging and discharging, and thus improve the fast charging performance and cycle performance of the secondary battery.

[0069] In one embodiment, the puncture strength of the base membrane is ≥300gf, the puncture strength of the diaphragm is ≥500gf, and the ratio of the puncture strength of the diaphragm to the puncture strength of the base membrane is ≥1.67.

[0070] For example, the puncture strength of the base membrane is a range of 300gf, 305gf, 310gf, 315gf, 320gf, 325gf or any two of these values.

[0071] For example, the puncture strength of the diaphragm is a range of 500gf, 520gf, 550gf, 600gf, 620gf, 650gf, 700gf, 720gf, 750gf or any two of these values.

[0072] For example, the ratio of the puncture strength of the diaphragm to the puncture strength of the base membrane is 1.67, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6 or any two of these values.

[0073] Research has found that the coating has a self-healing function, which can significantly enhance the puncture strength of the separator, thereby providing the separator with excellent resistance to foreign objects and improving the safety performance of secondary batteries.

[0074] In one embodiment, this application provides a method for preparing the diaphragm, comprising the following steps:

[0075] S1. Preparation of polymer microspheres:

[0076] (1) Preparation of the core: Stir the core layer flame retardant and deionized water for 25-35 minutes, while purging with nitrogen for protection, add the initiator, heat to 70-75℃ and react for 0.5-2 hours to obtain the core layer seed emulsion for later use.

[0077] (2) Preparation of the second shell: Take the core seed emulsion, add the second shell material and deionized water, heat to 80-85℃, add initiator, react for 2-4 hours to obtain core-shell polymer microsphere emulsion.

[0078] (3) Preparation of the first shell layer: The first shell layer material and initiator are added to the above core-shell polymer microsphere emulsion, and the reaction is carried out at 85-90°C for 2-4 hours to obtain the polymer microspheres.

[0079] S2. Preparation of the diaphragm: A mixed slurry prepared from ceramic particles, polymer microspheres, dispersant, wetting agent and deionized water is coated onto at least one side of the base membrane and dried to obtain the diaphragm.

[0080] In one embodiment, the initiator includes at least one selected from tert-butyl hydroperoxide, sodium bisulfite, sodium salt of an organic sulfinic acid derivative, vitamin C, sodium persulfate, ammonium persulfate, potassium persulfate, benzoyl peroxide, and azobisisobutyronitrile.

[0081] In one embodiment, this application provides a secondary battery, including a positive electrode, a negative electrode, an electrolyte, and the separator described in this application.

[0082] In one embodiment, the positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; the positive active material layer includes a positive active material. This application does not limit the positive active material; any known positive active material can be used.

[0083] In one embodiment, the negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector; the negative active material layer includes a negative active material. This application does not limit the negative active material; any known negative active material can be used.

[0084] In one embodiment, the electrolyte includes an organic solvent, a lithium salt, and additives.

[0085] In one embodiment, the organic solvent includes at least one of ethylene carbonate, propylene carbonate, and diethyl carbonate; the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium hexafluoroarsine phosphate; and the additive includes at least one of film-forming additives, conductive additives, and flame-retardant additives.

[0086] In one embodiment, the separator of the secondary battery is disposed between the positive and negative electrodes.

[0087] In one embodiment, this application provides an electrical device including the secondary battery described in this application.

[0088] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0089] Source of adhesive raw materials:

[0090] A mixture of polyvinylpyrrolidone and polyethyleneimine was prepared by mixing polyvinylpyrrolidone and polyethyleneimine at a mass ratio of 1:1. All raw materials were purchased from BASF.

[0091] A mixture of polyurea and vinyltrimethoxysilane was prepared by mixing polyurea and vinyltrimethoxysilane at a mass ratio of 1:1. All raw materials were purchased from BASF.

[0092] Example 1

[0093] Embodiment 1 of this application provides a separator and a secondary battery. The preparation method of the secondary battery includes the following steps:

[0094] (1) Preparation of the diaphragm

[0095] S1. Preparation of polymer microsphere core: 35 parts of organic flame retardant (trimethylbenzene phosphate) and 10 parts of deionized water were stirred for 30 min while nitrogen gas was introduced for protection. 0.3 parts of initiator (tert-butyl hydroperoxide) were added and heated to 75°C for 1 h to obtain core seed emulsion for later use.

[0096] S2. Preparation of the second shell of polymer microspheres: Take the core seed emulsion, add the second shell material (60 parts methyl methacrylate and 15 parts ethylene glycol dimethacrylate) and 50 parts deionized water, heat to 85℃, add 1.5 parts initiator (tert-butyl hydrogen peroxide), react for 3h to obtain the core-shell polymer microsphere emulsion.

[0097] S3. Preparation of the first shell of polymer microspheres: The first shell material (15 parts butyl acrylate and 5 parts acrylamide) and 0.6 parts initiator (tert-butyl hydrogen peroxide) were added to the above core-shell polymer microsphere emulsion, and the reaction was carried out at 90°C for 3 hours to obtain the polymer microspheres.

[0098] S4. Preparation of the diaphragm:

[0099] 7 wt.% of the above-mentioned organic polymer microspheres, 1 wt.% of dispersant (sodium polyacrylate), 0.2 wt.% of wetting agent (polyether siloxane), 5 wt.% of binder (polyvinylpyrrolidone-polyethyleneimine), 86.8 wt.% of boehmite and deionized water were mixed evenly and thoroughly stirred using a high-speed disperser to obtain an aqueous slurry.

[0100] The aqueous slurry is roller-coated onto one side of a polypropylene substrate, with a ceramic coating thickness of 3 μm. After drying, a diaphragm is obtained.

[0101] (2) Preparation of secondary batteries

[0102] S1. Preparation of the positive electrode sheet: The positive electrode active material (nickel-cobalt-manganese ternary material), conductive agent (SP), and binder (PVDF) are mixed and stirred in N-methylpyrrolidone (NMP) at a mass ratio of 95:3:2 to form a positive electrode slurry. The slurry is evenly coated on both sides of an aluminum foil, baked and dried, and the coated electrode sheet is cold-pressed, slit, and cut to obtain the positive electrode sheet.

[0103] S2. Preparation of the negative electrode sheet: The negative electrode active material graphite, conductive agent (SP), dispersant (carboxymethyl cellulose (CMC)), and binder (styrene-butadiene rubber (SBR)) are mixed in deionized water at a mass ratio of 93:2:3:2 to form a negative electrode slurry. The negative electrode slurry is coated on both sides of a copper foil, baked and dried, and the coated electrode sheet is cold-pressed, slit, and cut to obtain the negative electrode sheet.

[0104] S3. Preparation of electrolyte: Lithium hexafluorophosphate electrolyte is dissolved in a mixed solvent of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate (mass ratio of 1:2:1.5) to obtain lithium-ion battery electrolyte.

[0105] S4. Preparation of secondary battery: The prepared positive electrode, separator and negative electrode are wound, hot-pressed and shaped and the tabs are welded to obtain a bare secondary battery; the bare secondary battery is placed in an aluminum shell, the prepared electrolyte is injected, and after the processes of encapsulation, formation and capacity testing, a secondary battery is made.

[0106] Figure 1 shows the SEM image of the membrane prepared in Example 1. The polymer microspheres are regularly spherical and embedded in the ceramic layer, which meets the design requirements.

[0107] Examples 2 to 6

[0108] Embodiments 2 to 6 of this application provide a separator and a secondary battery, respectively. The difference between the preparation method of the secondary battery and that of Embodiment 1 lies in adjusting the glass transition temperature of the core, the second shell layer disposed on the surface of the core, and the first shell layer disposed on the surface of the second shell layer.

[0109] Specifically, by changing the types and amounts of monomers in the core, second shell, and first shell, the glass transition temperatures of the core, second shell, and first shell can be altered.

[0110] More specifically, the preparation of the polymer microspheres in Example 2 differs from that in Example 1 in that the core layer consists of 38 parts of tricresyl phosphate; the second shell consists of 70 parts of styrene and 10 parts of glycidyl methacrylate; and the first shell consists of 18 parts of isooctyl acrylate and 6 parts of methacrylic acid.

[0111] The difference between the preparation of polymer microspheres in Example 3 and Example 1 is that the core layer consists of 32 parts of triphenyl phosphate; the second shell consists of 55 parts of methyl methacrylate and 15 parts of ethylene glycol dimethacrylate; and the first shell consists of 23 parts of butyl acrylate and 5 parts of acrylamide.

[0112] The difference between the preparation of polymer microspheres in Example 4 and Example 1 is that the core layer consists of 41 parts of triphenyl phosphate; the second shell consists of 65 parts of methyl methacrylate and 20 parts of ethylene glycol dimethacrylate; and the first shell consists of 11 parts of butyl acrylate and 4 parts of acrylamide.

[0113] The difference between the preparation of polymer microspheres in Example 5 and Example 1 is that the core layer consists of 28 parts of triphenyl phosphate; the second shell layer consists of 58 parts of methyl methacrylate and 10 parts of ethylene glycol dimethacrylate; and the first shell layer consists of 20 parts of butyl acrylate and 10 parts of acrylamide.

[0114] The difference between the preparation of polymer microspheres in Example 6 and Example 1 is that the core layer consists of 44 parts of triphenyl phosphate; the second shell consists of 65 parts of methyl methacrylate and 22 parts of ethylene glycol dimethacrylate; and the first shell consists of 10 parts of butyl acrylate and 2 parts of acrylamide.

[0115] Examples 7 to 11

[0116] Examples 7-11 of this application provide a separator and a secondary battery, respectively. The difference between the preparation method of the secondary battery and that of Example 1 is that the average particle size of the polymer microspheres is adjusted.

[0117] Specifically, the average particle size of the polymer microspheres can be changed by altering the amount of initiator used.

[0118] Example 7

[0119] Example 7 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the total amount of initiator is 4.8 parts, wherein the weight ratio of initiator in the core layer, the second shell layer and the first shell layer (core layer: second shell layer: first shell layer = 0.3:1.5:0.6) remains unchanged.

[0120] Example 8

[0121] Example 8 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the total amount of initiator is 3.6 parts, wherein the weight ratio of initiator in the core layer, the second shell layer and the first shell layer (core layer: second shell layer: first shell layer = 0.3:1.5:0.6) remains unchanged.

[0122] Example 9

[0123] Example 9 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the total amount of initiator is 1.8 parts, wherein the weight ratio of initiator in the core layer, the second shell layer and the first shell layer (core layer: second shell layer: first shell layer = 0.3:1.5:0.6) remains unchanged.

[0124] Example 10

[0125] Example 10 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the total amount of initiator is 1.4 parts, wherein the weight ratio of initiator in the core layer, the second shell layer and the first shell layer (core layer: second shell layer: first shell layer = 0.3:1.5:0.6) remains unchanged.

[0126] Example 11

[0127] This application provides a membrane and a secondary battery in embodiment 11. The difference between the preparation method of the secondary battery and that in embodiment 1 is that the total amount of initiator is 1.0 part, wherein the weight ratio of initiator in the core layer, the second shell layer and the first shell layer (core layer: second shell layer: first shell layer = 0.3:1.5:0.6) remains unchanged.

[0128] Examples 12-14

[0129] Examples 12-14 of this application provide a separator and a secondary battery, respectively. The difference between the preparation method of the secondary battery and that of Example 1 is that the thickness of the composite coating is different during the preparation of the separator.

[0130] Example 15

[0131] Example 15 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the ceramic particles are alumina.

[0132] Example 16

[0133] Example 16 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the binder is polyurea-vinyltrimethoxysilane.

[0134] Example 17

[0135] Embodiment 17 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and that of Embodiment 1 is that the amount of binder used in the composite coating during the separator preparation process is 1 wt.%.

[0136] Example 18

[0137] This application provides a separator and a secondary battery in embodiment 18. The difference between the preparation method of the secondary battery and that in embodiment 1 is that the amount of binder used in the composite coating during the separator preparation process is 10 wt.%.

[0138] Example 19

[0139] This application provides a separator and a secondary battery in embodiment 19. The difference between the preparation method of the secondary battery and that in embodiment 1 is that the amount of binder used in the composite coating during the separator preparation process is 15 wt.%.

[0140] Examples 20-22

[0141] Examples 20-22 of this application provide a separator and a secondary battery, respectively. The difference between the preparation method of the secondary battery and that of Example 1 is that the mass percentage of the polymer microspheres in the composite coating is different.

[0142] Comparative Examples 1-2

[0143] Comparative Examples 1 and 2 of this application provide a separator and a secondary battery, respectively. The difference between the preparation method of the secondary battery and Example 1 lies in adjusting the glass transition temperatures of the core, the second shell layer disposed on the surface of the core, and the first shell layer disposed on the surface of the second shell layer. Specifically, the glass transition temperatures of the core, the second shell layer, and the first shell layer are changed by altering the type and amount of monomers in the core, the second shell layer, and the first shell layer.

[0144] The preparation of the polymer microspheres in Comparative Example 1 differs from that in Example 1 in that the core layer consists of 35 parts of triphenyl phosphate; the second shell consists of 48 parts of methyl methacrylate and 10 parts of ethylene glycol dimethacrylate; and the first shell consists of 5 parts of butyl acrylate and 4 parts of acrylamide.

[0145] The preparation of the polymer microspheres in Comparative Example 2 differs from that in Example 1 in that the core layer consists of 10 parts of triphenyl phosphate; the second shell consists of 65 parts of methyl methacrylate and 15 parts of ethylene glycol dimethacrylate; and the first shell consists of 15 parts of butyl acrylate and 5 parts of acrylamide.

[0146] Comparative Example 3

[0147] Comparative Example 3 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that it does not contain a first shell layer.

[0148] Comparative Example 4

[0149] Comparative Example 4 of this application provides a separator and a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that it does not contain a second shell layer.

[0150] The parameters and performance data of Examples 1-22 and Comparative Examples 1-4 are shown in Tables 1-3. The mass of the polymer microsphere core is Z, the mass of the first shell is Z1, and the mass of the second shell is Z2. The mass ratio of the polymer microsphere core, the second shell, and the first shell is Z:Z2:Z1.

[0151] The parameter testing methods in Tables 1-3 are as follows:

[0152] Diaphragm performance testing

[0153] (a) Contact angle test of the coating surface: The test was conducted using a contact angle measuring instrument. The electrolyte drop volume was set to 5 μl. The electrolyte was the electrolyte prepared in the above examples and comparative examples. Three parallel samples were taken from each group, and 15 points were measured for each sample. The average value was taken.

[0154] (II) Diaphragm Liquid Absorption: The diaphragm was placed in an electrolyte (1M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), where EC / EMC = 3:7 (mass ratio)) and soaked at room temperature for 8 hours. The initial weight of the diaphragm was m1 (average of 3 measurements). The mass of the dried sample was weighed m2 (average of 3 measurements). The surface liquid absorption rate was calculated (unit: g / m). 2 ), surface liquid absorption rate = (m2-m1) / S.

[0155] (III) Liquid retention capacity of the diaphragm: After absorbing liquid, the diaphragm is laid flat on the test bench and allowed to stand for 1 hour. The mass of the sample (m3) is weighed (the average value is taken from 3 measurements). The surface liquid retention rate is calculated (unit: g / m3). 2 ), Surface liquid retention rate = (m3-m1) / S.

[0156] (iv) Limiting Oxygen Index: The limiting oxygen index is tested using a limiting oxygen index tester. The specific steps are as follows: A diaphragm sample of a certain size (120mm × 6.5mm) is vertically clamped in a transparent combustion tube with a sample clamp, which contains an upward-flowing oxygen-nitrogen gas stream mixed in a certain proportion; the upper end of the sample is lit, and the subsequent combustion phenomenon is observed. The duration of combustion or the distance burned is recorded. If the combustion time of the sample exceeds 3 minutes or the flame front exceeds the 50mm mark, the oxygen concentration is reduced. If the combustion time of the sample is less than 3 minutes or the flame front does not reach the mark, the oxygen concentration is increased. This operation is repeated to gradually approach the specified value from both the upper and lower sides until the concentration difference between the two is less than 0.5%. The oxygen concentration at this point is the limiting oxygen index.

[0157] (V) Puncture strength test: Testing equipment: tensile tester; Testing method: Install the puncture clamp and puncture needle on the universal tensile tester. The diameter of the round-tipped needle is 1mm. Set the puncture speed to 50mm / min. Fix the sample flat on the clamp and start the equipment for testing.

[0158] (vi) Membrane conductivity: The ionic conductivity of the membrane is tested by assembling a symmetrical cell. Specifically, the membrane resistance Rs is measured by assembling a symmetrical cell, and the membrane ionic conductivity is calculated by referring to the following formula: σs=d / Rs·S, where d is the membrane thickness and S is the effective area of ​​the membrane.

[0159] Battery performance test

[0160] Using the above-mentioned separator, a soft-pack battery with a length and width of 60mm × 40mm and the same capacity (2.8Ah) was prepared, and the performance of the prepared soft-pack battery was evaluated.

[0161] (a) DC internal resistance (DCR): At 25℃, the secondary battery is fully charged to 4.2V at a rate of 1C, and then charged at a constant voltage (CV) until the current decays to 0.05C. Then it is discharged at a rate of 1C for 30min (50% state of charge (SOC)) and discharged at 3C for 10s (sampled at 0.1s intervals). The voltage drop ΔU and the current difference ΔI are recorded. Then DCR = ΔU / ΔI.

[0162] (ii) Cyclic test: Under conditions of 25℃ and 45℃ respectively, the battery is cycled to 80% capacity retention rate at a charge / discharge current of 1C / 1C from 0 to 100% SOC, and the number of cycles at this time is recorded.

[0163] (III) Secondary Battery Needle Penetration Test: Six secondary batteries were randomly selected for the secondary battery needle penetration test. A 3mm diameter steel needle was used, with a forward speed of 150mm / s, to completely penetrate the secondary battery. The test was held for 1 hour, and the secondary battery was observed to see if thermal runaway occurred. The number of secondary batteries that passed the test was recorded. A battery that did not catch fire or emit smoke was considered to have passed the test.

[0164] Table 1. Parameters related to the preparation of the diaphragms in the examples and comparative examples.

[0165] Table 2. Performance test results of the membranes prepared in the examples and comparative examples.

[0166] Table 3. Performance test results of the secondary batteries prepared in the examples and comparative examples.

[0167] As shown in the table above, the secondary battery prepared using the separator provided in this application exhibits excellent electrochemical performance, low impedance, excellent cycle performance, and superior safety performance. Specifically, it demonstrates a DCR ≤ 3.14 mΩ, ≥ 2400 cycle cycles at 25℃, ≥ 1600 cycle cycles at 45℃, and a secondary battery nail penetration test pass rate ≥ 80%.

[0168] The separator provided in this application can improve the lithium-ion transport performance of the battery during the charging and discharging process. When the secondary battery experiences thermal runaway, the shell melts and releases the core flame retardant, exhibiting excellent flame retardant effect and superior safety performance.

[0169] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A diaphragm, comprising a base membrane and a composite coating disposed on at least one surface of the base membrane; said composite coating comprising ceramic particles and polymer microspheres; The polymer microspheres include a core, a second shell disposed on the surface of the core, and a first shell disposed on the surface of the second shell. The glass transition temperature of the core is A, the glass transition temperature of the second shell is B, and the glass transition temperature of the first shell is C, satisfying A < C < B; the glass transition temperature of the core is A < 0℃.

2. The diaphragm as claimed in claim 1, wherein the diaphragm satisfies at least one of the following (1) to (3): (1)-50℃≤A≤-20℃； (2)150℃≤B≤250℃； (3)0℃≤C≤90℃。 3. The diaphragm according to claim 1 or 2, wherein the average particle size of the polymer microspheres is 1.5 to 15 μm.

4. The diaphragm as claimed in claim 1 or 2, wherein in the polymer microspheres, the second shell layer comprises the following components in parts by weight: 20 to 80 parts of hard monomer and 5 to 25 parts of crosslinking monomer; the first shell layer comprises the following components in parts by weight: 10 to 50 parts of soft monomer and 1 to 15 parts of functional monomer; and the core comprises 20 to 50 parts of organic flame retardant.

5. The diaphragm according to claim 4, wherein the average particle size of the polymer microspheres is 1.5 to 15 μm.

6. The diaphragm as claimed in claim 4, wherein the diaphragm satisfies at least one of the following (a) to (f): (a) The organic flame retardant includes organophosphorus flame retardants and organosiloxane flame retardants, wherein... The organophosphorus flame retardant includes at least one of tricresyl phosphate, triphenyl phosphate, triisopropylphenyl phosphate, tributyl phosphate, trioctyl phosphate, and toluene diphenyl phosphate; the organosiloxane flame retardant includes at least one of methyl silicone oil, phenyl silicone oil, and hydroxyl silicone oil. (b) The soft monomer includes at least one of butyl acrylate, ethyl acrylate, n-butyl acrylate, isooctyl acrylate, lauryl acrylate, and lauryl methacrylate; (c) The functional monomer includes at least one of hydroxyethyl acrylate, acrylamide, methacrylic acid, vinyltriethoxysilane acrylate, and vinyltrimethoxysilane; (d) The hard monomer includes at least one of methyl methacrylate, ethyl methacrylate, and styrene; (e) The crosslinking monomers include at least one of ethylene glycol dimethacrylate, allyl methacrylate, diacetone acrylamide and dihydrazine adipate, acetyl acetoxyethyl methacrylate, and glycidyl methacrylate. (f) The mass ratio of the core, the second shell, and the first shell is (2-5):(6-9):(1-4).

7. The diaphragm according to any one of claims 1-2 and 5-6, wherein the composite coating satisfies at least one of the following (4) to (5): (4) The mass percentage of the ceramic particles is 80-97%; (5) The polymer microspheres have a mass percentage content of 2-12%.

8. The diaphragm as claimed in claim 4, wherein the composite coating satisfies at least one of the following (4) to (5): (4) The mass percentage of the ceramic particles is 80-97%; (5) The polymer microspheres have a mass percentage content of 2-12%.

9. The diaphragm according to any one of claims 1, 2, 5-6 and 8, wherein the thickness of the composite coating is 1 to 8 μm.

10. The diaphragm according to any one of claims 1, 2, 5-6, and 8, wherein the liquid absorption rate of the diaphragm is 100% to 800% and the liquid retention rate is 80% to 800%.

11. The diaphragm as claimed in claim 9, wherein the liquid absorption rate of the diaphragm is 100% to 800%, and the liquid retention rate is 80% to 800%.

12. The diaphragm according to any one of claims 1, 2, 5-6, 8 and 11, wherein the puncture strength of the base membrane is ≥300 gf, the puncture strength of the diaphragm is ≥500 gf, and the ratio of the puncture strength of the diaphragm to the puncture strength of the base membrane is ≥1.

67.

13. The diaphragm of claim 10, wherein the puncture strength of the base membrane is ≥300 gf, the puncture strength of the diaphragm is ≥500 gf, and the ratio of the puncture strength of the diaphragm to the puncture strength of the base membrane is ≥1.

67.

14. A secondary battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator as described in any one of claims 1 to 13.

15. An electrical device comprising the secondary battery of claim 14.

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