Separator, secondary battery, electrochemical device, and electric device

Abstract

A separator comprises a base film and an organic coating disposed on a surface of the base film. The organic coating contains a first polymer and a second polymer. A melting point T of the first polymer satisfies 60°C ≤ T ≤ 100°C, and a swelling degree D of the second polymer after being placed in a solution at 60°C for 24 hours satisfies D ≤ 30%. The first polymer sustains an adhesion requirement in a charging and discharging cycle of a battery. When the battery heats up while charging or discharging, the first polymer is apt to soften and the adhesion decreases, opening an interface between the separator and a battery electrode sheet, thereby helping to reduce heat accumulation and improving safety performance of a heat case of a battery cell. The second polymer has a low swelling degree and can maintain an original physical appearance in the battery cell, thereby effectively alleviating the problem of melting and film forming of the first polymer having a low melting point in a hot pressing process of the battery cell, and improving cycle performance of the battery cell. Further provided are a secondary battery, an electrochemical device, and an electric device.

Description

Separator, secondary battery, electrochemical device and power-consuming device Technical Field

[0001] The present application relates to a separator, a secondary battery and an electrochemical device containing the separator, and an electrical device containing the secondary battery or the electrochemical device. Background Art

[0002] The rapid increase in battery cell charge rate and energy density is detrimental to improving the hot box window (the safe temperature at which the battery cell can withstand no fire, explosion, or smoke when placed in a hot box after charging). To improve the hot box window and enhance the safety performance of battery cells, two methods are generally used: adjusting the electrolyte solvent system; or using a low-adhesion polyvinylidene fluoride coating on the separator. However, adjusting the electrolyte system will deteriorate the cycle performance of the battery cell and risk deteriorating the low-temperature performance of the battery cell. Using a polyvinylidene fluoride coating with too low an adhesion strength can easily cause deformation of the battery cell, which will deteriorate the high-temperature storage performance of the battery cell.

[0003] Summary of the Invention

[0004] In view of this, the first aspect of the present application provides an isolation film, comprising:

[0005] basement membrane;

[0006] an organic coating layer disposed on a surface of the base film, the organic coating layer comprising a first polymer and a second polymer;

[0007] The melting point T of the first polymer satisfies 60°C≤T≤100°C;

[0008] The swelling degree D of the second polymer after being placed in an electrolyte at 60° C. for 24 hours is ≤30%. The mass ratio of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, propylene carbonate and propyl propionate in the electrolyte is 8.7:20:30:20:30.

[0009] In the isolation membrane provided in the first aspect, the first polymer can maintain the adhesion requirements during the battery charge and discharge cycle, and can also improve the high-temperature storage performance; when the battery is charged and discharged and generates heat, the low-melting-point first polymer is more likely to soften, and the adhesion is significantly reduced, so that the interface between the isolation membrane and the battery electrode (positive electrode) is opened, which helps to reduce heat accumulation and improve the safety performance of the battery cell hot box; the second polymer has a low swelling degree D and can maintain its original morphology in the battery cell, will not block the pores of the base membrane, and can effectively improve the problem of the low-melting-point first polymer melting and forming a film during the hot pressing process of the battery cell, thereby improving the cycle performance of the battery cell.

[0010] In combination with the first aspect, in some embodiments, in the organic coating, the weight percentage of the first polymer is greater than or equal to 40% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 50%.

[0011] It can be seen that the present application improves the hot box test window of the battery cell while ensuring the performance of the battery cell by rationally formulating the ratio of the first polymer to the second polymer. The higher the content of the first polymer, the better the high-temperature failure effect. However, if the content is too high, the isolation membrane is prone to pore blockage, resulting in a decrease in the dynamics of the battery cell and poor cycle performance. The higher the content of the second polymer, the better the cycle performance of the battery cell. However, if the content is too high, the adhesion of the isolation membrane at room temperature is too low, which can easily lead to poor high-temperature storage performance, the risk of deformation of the battery cell, and reduced cycle performance.

[0012] In combination with the first aspect, in some embodiments, in the organic coating, the weight percentage of the first polymer is greater than or equal to 60% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 30%.

[0013] In combination with the first aspect, in some embodiments, when the isolation membrane is applied to an electrochemical battery cell, the swelling degree D of the second polymer and the content H of the linear carboxylate in the electrolyte satisfy D=K*H+0.1, where 0.01≤K≤0.08.

[0014] In combination with the first aspect, in some embodiments, 10%≤H≤50%.

[0015] It can be seen that by adjusting the ratio of the first polymer to the second polymer and the linear carboxylate content of the electrolyte, the hot box test pass rate can be improved while ensuring the performance of the battery cell.

[0016] In combination with the first aspect, in some embodiments, the median particle size Dv50 of the first polymer satisfies 1.0 μm≤Dv50≤2 μm.

[0017] It can be seen that when the first polymer particle size is too large, the organic coating's adhesion is impaired and its thickness increases, resulting in a lower volumetric energy density of the battery cell. When the first polymer particle size is too small, it can cause pores in the separator to become blocked, leading to poor battery cycle performance. In this application, by setting an appropriate particle size range for the first polymer, the adhesion and thickness of the organic coating can be ensured.

[0018] In combination with the first aspect, in some embodiments, the melting point T of the first polymer satisfies 60°C≤T≤85°C.

[0019] In combination with the first aspect, in some embodiments, the second polymer is spherical or quasi-spherical, and the sphericity R of the second polymer satisfies 0.7≤R<1.

[0020] In combination with the first aspect, in some embodiments, the second polymer is spherical or quasi-spherical, and the sphericity R of the second polymer satisfies 0.8≤R<1.

[0021] It can be seen that the second polymer has a high sphericity, which can achieve better results with a smaller amount of the second polymer. The spherical or quasi-spherical second polymer has a low stacking density in the organic coating, which can increase the porosity of the organic coating and improve the cycle performance of the battery cell.

[0022] In combination with the first aspect, in some embodiments, the swelling degree D of the second polymer after being placed in an electrolyte at 85° C. for 24 hours satisfies 10%≤D≤50%.

[0023] It can be seen that by selecting a second polymer with a low swelling degree, the present application can maintain its original morphology well even when the isolation membrane is applied to the battery cell even if the temperature rises. Moreover, the second polymer has a higher melting point than the first polymer, for example, a melting point higher than 130°C, and can maintain its original morphology in the battery cell without clogging the pores of the base membrane. This can effectively improve the problem of the low-melting-point first polymer melting and forming a film during the hot pressing process of the battery cell, thereby improving the cycle performance of the battery cell. The selection of substances with different swelling degrees can be achieved by selecting different substance types, molecular weights, or different cross-linking degrees.

[0024] In combination with the first aspect, in some embodiments, the first polymer includes a graft copolymer, the monomer of the polymer body in the graft copolymer includes at least one of ethylene, propylene, vinylidene fluoride, acrylic acid, acrylate, styrene, acrylonitrile, maleic anhydride, vinyl chloride and allyl chloride, and the monomer used for grafting includes at least one of maleic anhydride, acrylate and acrylic acid; the weight percentage of the graft copolymer in the first polymer is greater than or equal to 3% and less than or equal to 10%.

[0025] In combination with the first aspect, in some embodiments, the second polymer includes a polymer of at least one monomer selected from the group consisting of vinylidene fluoride, hexafluoropropylene, propylene, vinyl chloride, styrene, butadiene, acrylate, and acrylic acid, and the second polymer is at least one of a homopolymer, a copolymer, and a blended polymer.

[0026] In a second aspect, the present application provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, and the above-mentioned isolation film arranged between the positive electrode sheet and the negative electrode sheet, wherein the organic coating is located between the positive electrode sheet and the base film.

[0027] In a third aspect, the present application provides an electrochemical device comprising a positive electrode sheet, a negative electrode sheet, and the above-mentioned isolation membrane arranged between the positive electrode sheet and the negative electrode sheet, wherein the organic coating is located between the positive electrode sheet and the base membrane.

[0028] In a fourth aspect, the present application provides an electrical device, the aforementioned secondary battery or the aforementioned electrochemical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG1 is a schematic structural diagram of an isolation membrane according to an embodiment of the present application.

[0030] FIG2 is a schematic diagram of the working mechanism of the isolation membrane according to one embodiment of the present application.

[0031] Main component symbols: Isolation film 10 Base film 11 Organic coating 13 Inorganic coating 15 Other organic coating 17 Positive electrode 20 Negative electrode 30 DETAILED DESCRIPTION

[0032] The embodiments of the present application are described below in conjunction with the drawings in the embodiments of the present application.

[0033] The separator is a critical component of a battery. It's a layer of diaphragm material placed between the positive and negative electrodes, directly impacting battery safety and cost. The separator's primary function is to separate the positive and negative electrodes, preventing electrons from freely passing through the battery while allowing ions in the electrolyte to pass freely.

[0034] This application provides a separator for a secondary battery that effectively increases the thermal window of a battery cell and enhances its safety. Referring to Figure 1 , separator 10 comprises a base film 11 and an organic coating 13 disposed on one surface of base film 11. Referring to Figure 2 , during use, the side of separator 10 with organic coating 13 faces the positive electrode plate 20. Organic coating 13 comprises a first polymer and a second polymer.

[0035] The present application has no particular restrictions on the type of base film 11, as long as the purpose of the present application can be achieved. The base film 11 can be a resin, glass fiber, inorganic material, etc. formed by a material that is stable to the electrolyte. In some embodiments, the base film 11 includes a porous sheet or a non-woven fabric-like material with excellent liquid retention. Examples of materials for the resin or glass fiber separator base film may include, but are not limited to, polyolefins, aromatic polyamides, polytetrafluoroethylene, polyethersulfone, etc. Taking polyolefins as an example, the material of the base film 11 can be selected from one or more of polypropylene and polyethylene.

[0036] The present application does not limit the thickness of the base film 11 as long as the purpose of the present application can be achieved. For example, the thickness of the base film 11 can be 5 to 50 μm, preferably 5 to 30 μm.

[0037] In the present application, the melting point T of the first polymer is relatively low, 60° C. ≤ T ≤ 100° C. Furthermore, in some embodiments, the melting point T of the first polymer satisfies 60° C. ≤ T ≤ 85° C.

[0038] The first polymer provides the viscosity of the organic coating 13. At a temperature below the melting point (e.g., room temperature), the first polymer has high bonding properties, which can maintain the bonding force requirements during the battery charge and discharge cycle, and can also improve the high-temperature storage performance of the battery. The present application selects a first polymer with a low melting point so that when the battery is charged and discharged and generates heat, for example, when the temperature rises to the melting point T or above, the first polymer with a low melting point is more likely to soften and produce fluidity, and the bonding force is significantly reduced, so that the interface between the separator and the positive electrode is opened, which helps to reduce heat accumulation and improve the safety performance of the battery cell hot box.

[0039] When the particle size of the first polymer is too large, the adhesion of the organic coating 13 will deteriorate and the thickness will increase, resulting in a lower volume energy density of the battery cell; if the particle size of the first polymer is too small, it will cause pores in the isolation membrane to be blocked, resulting in poor cycle performance of the battery. In the present application, the median particle size Dv50 of the first polymer satisfies 0.8μm<Dv50<2μm. In some embodiments, the median particle size Dv50 of the first polymer satisfies 1.0μm≤Dv50≤2μm. By setting the first polymer within an appropriate particle size range, the adhesion of the organic coating 13 and the appropriate thickness can be ensured.

[0040] When the separator 10 is used in a battery, the electrolyte penetrates the porous structure of the separator 10, and the organic coating is also immersed in the electrolyte. The second polymer has a swelling degree D of ≤30% after being placed in the electrolyte at 60°C for 24 hours. The electrolyte comprises lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, propylene carbonate, and propyl propionate in a mass ratio of 8.7:20:30:20:30.

[0041] Furthermore, in some embodiments, the swelling degree of the second polymer when placed in an electrolyte at 85°C for 24 hours is 10%≤D≤50%. In the present application, the swelling degree is defined as: when the polymer molecules adsorb solvent molecules to reach swelling equilibrium, the ratio of the volume after swelling to the volume before swelling minus 1. Generally, the swelling degree of a polymer is related to the polymer material composition, molecular weight, length of the molecular chain, and the degree of cross-linking of the molecular chain. The present application can ensure the swelling degree of the second polymer by selecting appropriate material composition, large molecular weight, long molecular chain or a large degree of cross-linking of the molecular chain.

[0042] By selecting a second polymer with high strength and low swelling, the separator 10 can maintain its original shape when applied to the battery cell. Furthermore, the second polymer has a higher melting point than the first polymer, for example, a melting point above 130°C, which allows it to maintain its original shape in the battery cell and prevent clogging the pores of the base film 11. This effectively improves the problem of the low-melting-point first polymer melting and forming a film during the hot pressing process of the battery cell, thereby improving the cycling performance of the battery cell.

[0043] The second polymer is spherical or quasi-spherical in the separator 10, and the sphericity R of the second polymer satisfies 0.7 ≤ R ≤ 1.0. Furthermore, in some embodiments, the sphericity R of the second polymer satisfies 0.8 ≤ R ≤ 1.0. The high sphericity of the second polymer allows for a smaller amount of the second polymer to be used in the organic coating 13 while achieving better results. Furthermore, the spherical or quasi-spherical second polymer has a lower packing density in the organic coating 13, which can increase the porosity of the organic coating and improve the cycling performance of the battery cell.

[0044] As the battery temperature rises, the first polymer is more likely to soften and lose its adhesiveness, while the second polymer, due to its relatively high melting point, does not soften and remains in its rigid granular form.

[0045] The higher the content of the first polymer, the better the high-temperature failure effect. However, if the content is too high, the separator 10 is prone to pore blockage, resulting in a decrease in the dynamics of the battery cell and poor cycle performance.

[0046] The higher the content of the second polymer, the better the cycle performance of the battery cell. However, if the content is too high, the adhesion of the isolation membrane at room temperature will be worse, which may easily lead to poor high-temperature storage performance and the risk of deformation during cycling.

[0047] Therefore, it is necessary to rationally formulate the ratio of the first polymer to the second polymer in order to improve the hot box test window of the battery cell while ensuring the performance of the battery cell. In the present application, in the organic coating 13, the weight percentage of the first polymer is greater than or equal to 40% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 50%. Furthermore, in some embodiments, in the organic coating 13, the weight percentage of the first polymer is greater than or equal to 60% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 30%.

[0048] In the present application, the weight percentage of the second polymer in the organic coating 13 is greater than or equal to 10%.

[0049] Furthermore, a higher content of the linear carboxylate in the electrolyte improves battery cell performance. However, excessively high levels can lead to excessive polymer swelling, clogging the separator pores and deteriorating battery cell performance. When the separator is used in a battery cell, the weight percentage of the linear carboxylate in the electrolyte is H, where 10% ≤ H ≤ 50%.

[0050] In the present application, the swelling degree D of the second polymer and the content H of the linear carboxylate in the electrolyte satisfy D=K*H+0.1, wherein 0.01≤K≤0.08.

[0051] In this way, by adjusting the ratio of the first polymer to the second polymer and the linear carboxylate content of the electrolyte, the upper limit window of the hot box test can be improved while ensuring the performance of the battery cell.

[0052] The first polymer has a Dv10 that satisfies 0.1 μm ≤ Dv10 ≤ 0.7 μm, and a Dv90 that satisfies 2 μm ≤ Dv90 ≤ 5 μm.

[0053] The second polymer has a Dv10 of 0.1 μm ≤ Dv10 ≤ 0.5 μm, a Dv50 of 0.5 μm ≤ Dv50 ≤ 2 μm, and a Dv90 of 2 μm ≤ Dv90 ≤ 5 μm. A ratio S of the Dv90 of the second polymer to the Dv90 of the first polymer satisfies 0.6 ≤ S ≤ 1.0.

[0054] In some embodiments, the first polymer comprises a polymer of at least one of ethylene, propylene, vinylidene fluoride, acrylic acid, acrylic ester, styrene, acrylonitrile, maleic anhydride, vinyl chloride, and allyl chloride. The first polymer is at least one of a homopolymer, a copolymer, and a polymer blend.

[0055] In some embodiments, the first polymer includes a graft copolymer, the monomer of the polymer body in the graft copolymer includes at least one of ethylene, propylene, vinylidene fluoride, acrylic acid, acrylate, styrene, acrylonitrile, maleic anhydride, vinyl chloride and allyl chloride, and the monomer used for grafting includes at least one of maleic anhydride, acrylate and acrylic acid.

[0056] In some embodiments, when the first polymer contains a graft copolymer, the weight percentage of the graft copolymer in the first polymer is greater than or equal to 3% and less than or equal to 10%.

[0057] In some embodiments, the second polymer includes a polymer of at least one monomer selected from vinylidene fluoride, hexafluoropropylene, propylene, vinyl chloride, styrene, butadiene, acrylate, and acrylic acid, and the second polymer is at least one selected from homopolymers, copolymers, and blended polymers.

[0058] In some embodiments, the average molecular weight of the first polymer is greater than or equal to 300,000 and less than or equal to 1,000,000. The average molecular weight of the second polymer is greater than or equal to 800,000 and less than or equal to 2,000,000.

[0059] It is understood that, in addition to the first and second polymers described above, the organic coating may optionally contain auxiliary components, with the weight percentage of the auxiliary components in the organic coating not exceeding 10%. The auxiliary components may include conventional thickeners and wetting agents. The thickener may be sodium carboxymethyl cellulose or lithium carboxymethyl cellulose, and its weight percentage in the organic coating may be 0.5% to 1.5%. Wetting agents include sulfonates and dimethyl silane, and their weight percentage in the organic coating may be 1% to 10%.

[0060] In some embodiments, the coating weight of the organic coating on the base film is greater than or equal to 0.5 mg / 5000 mm 2 And less than or equal to 3mg / 5000mm 2 The coating thickness of the organic coating is greater than or equal to 0.5 μm and less than or equal to 5 μm. Furthermore, in some embodiments, the coating weight of the organic coating on the base film is greater than or equal to 0.5 mg / 5000 mm 2 And less than or equal to 1.5mg / 5000mm 2 .

[0061] Optionally, in some embodiments, as shown in FIG1 , the isolation film 10 further includes an inorganic coating 15 disposed between the base film 11 and the organic coating 13. The inorganic coating 15 is used to improve the heat resistance and mechanical strength of the isolation film 10. The inorganic coating 15 includes a ceramic, and the ceramic includes at least one of aluminum oxide, boehmite, titanium dioxide, silicon dioxide, zirconium dioxide, tin dioxide, magnesium hydroxide, magnesium oxide, zinc oxide, barium sulfate, boron nitride, or aluminum nitride.

[0062] It is understood that, as shown in Figures 1 and 2, the separator 10 further includes an additional organic coating 17 provided on the other surface of the base film 11. The additional organic coating 17 and the aforementioned organic coating 13 are provided on opposite surfaces of the base film 11, that is, the additional organic coating 17 is provided on the surface of the base film 11 facing the negative electrode 30. The present application does not particularly limit the type of additional organic coating 17, as long as it can serve to bond the separator and the negative electrode. In some embodiments, the additional organic coating 17 mainly contains a binder and a thickener.

[0063] Please refer to Figure 2. The present application also provides an electrochemical device, comprising a positive electrode sheet 20, a negative electrode sheet 30, an electrolyte, and the above-mentioned separator 10 arranged between the positive electrode sheet 20 and the negative electrode sheet 30. The organic coating 13 is arranged on the surface of the base film 11 facing the positive electrode sheet 20, that is, it is located between the positive electrode sheet 20 and the base film 11 and can be used to bond the positive electrode sheet 20 and the separator 10. The electrochemical device can be a secondary battery. The electrolyte penetrates into the separator 10. As shown in Figure 2, the separator 10 does not appear to be bonded to the positive electrode sheet 20 and the negative electrode sheet 30. The positive electrode sheet, negative electrode sheet, and electrolyte of the electrochemical device can all be of types well known in the art.

[0064] [Positive electrode]

[0065] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two opposing surfaces along its thickness, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material.

[0066] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0067] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well known in the art. As an example, the positive electrode active material may include one or more of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05O2) and its modified compounds, etc. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, one or more of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0068] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0069] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0070] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0071] [Negative electrode]

[0072] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0073] As an example, the negative electrode current collector has two opposite surfaces in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0074] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0075] In some embodiments, the negative electrode active material may adopt the negative electrode active material for batteries that is well known in the art. As an example, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. The silicon-based material may be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0076] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may be selected from one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0077] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0078] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0079] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0080] [Electrolyte]

[0081] The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs, including electrolyte salts and solvents.

[0082] In some embodiments, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0083] In some embodiments, the solvent may be selected from one or more of ethylene carbonate, ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, propylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0084] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0085] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0086] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0087] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0088] The present application also provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and a separator arranged between the positive electrode sheet and the negative electrode sheet. Furthermore, the present application also provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, and a separator arranged between the positive electrode sheet and the negative electrode sheet.

[0089] The present application also provides an electrical device using the above-mentioned electrochemical device (secondary battery) as a power source, and the electrical device may be, but is not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery-powered vehicle, an electric car, a ship, a spacecraft, etc. Among them, the electric toy may include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, etc., and the spacecraft may include an airplane, a rocket, a space shuttle, and a spacecraft, etc.

[0090] The technical solutions of the embodiments of the present application are further described below through specific examples.

[0091] Inorganic coating preparation

[0092] Inorganic boehmite particles with a Dv50 of 1 μm and polyacrylate are mixed in a mass ratio of 90:10 and dissolved in deionized water to form an inorganic coating slurry with a solid content of 50%. The resulting inorganic coating slurry is then evenly coated on one side of a PE base film using a micro-concave coating method to obtain a heat-resistant layer, which is then dried in an oven.

[0093] The first group of examples of organic coating preparation

[0094] First Group of Examples An organic coating was prepared according to the parameters of the first polymer and the second polymer in each example in Table 1 below and auxiliary components (the thickener was sodium carboxymethyl cellulose, and the wetting agents were sulfonate and dimethyl silane). The first polymer, the second polymer, and the auxiliary components totaled 100 parts by weight. Deionized water was then added and stirred to adjust the slurry viscosity to 50 mPa·s and the solid content to 5%, thereby obtaining slurry A. The slurry A was evenly coated on the surface of the inorganic coating of the PE base film to obtain a first coating. The coating weight of the first coating was 0.2 g / m 2 , and complete drying in an oven.

[0095] Comparative Example Diaphragm Coating Preparation

[0096] According to the parameters of each comparative example in Table 1 below and adding auxiliary components (thickener is sodium carboxymethyl cellulose, wetting agent is sulfonate and dimethyl silane), the comparative organic coating was prepared. 1-4 only added the first polymer and auxiliary components, the first polymer and auxiliary components totaling 100 parts by weight; Comparative Examples 1-1 to Comparative Examples 1-3 added the first polymer, the second polymer and auxiliary components, the first polymer, the second polymer, and the auxiliary components totaling 100 parts by weight, and the swelling degree of the second polymer added in Comparative Examples 1-3 was greater than 30%; then deionized water was added and stirred to adjust the viscosity of the slurry to 50 mPa·s and the solid content to 5%, to obtain comparative slurry A. The comparative slurry A was evenly coated on the surface of the inorganic coating of the PE base film to obtain the first coating of the comparative example, and the coating weight of the first coating of the comparative example was 0.2 g / m 2 , and complete drying in an oven.

[0097] Preparation of high adhesion coating

[0098] 90g of a high molecular weight polymer binder was added to a blender, followed by 0.5g of sodium carboxymethyl cellulose, and the mixture was stirred until uniformly mixed. 8.5g of a wetting agent, dimethylsiloxane, was added, and deionized water was added and stirred. The viscosity of the slurry was adjusted to 40mPa·s and the solid content was 5%, thereby obtaining slurry B. The slurry B was evenly coated on the other surface of the PE base film to obtain a second coating layer. The coating weight of the second coating layer was 0.5g / m 2 , and complete drying in an oven.

[0099] Preparation of positive electrode

[0100] The positive electrode active materials, lithium cobalt oxide, acetylene black, and polyvinylidene fluoride (PVDF), were mixed in a mass ratio of 94:3:3. N-methylpyrrolidone (NMP) was then added as a solvent to form a slurry with a solid content of 75%, which was then stirred evenly. The slurry was evenly coated on one surface of a 12μm-thick aluminum foil, dried at 90°C, and cold-pressed to produce a positive electrode sheet with a 100μm-thick positive electrode active material layer. The above steps were then repeated on the other surface of the positive electrode sheet to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. The positive electrode sheet was cut into a size of 74mm x 867mm and the tabs were welded before use.

[0101] Preparation of negative electrode sheet

[0102] The negative electrode active materials, artificial graphite, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose, were mixed in a mass ratio of 96:1:1.5:1.5. Deionized water was then added as a solvent to form a slurry with a solid content of 70% by mass and stirred evenly. The slurry was evenly coated on one surface of an 8μm-thick copper foil, dried at 110°C, and cold-pressed to produce a single-sided negative electrode sheet with a 150μm-thick negative electrode active material layer. The above coating steps were then repeated on the other surface of the negative electrode sheet to produce a double-sided negative electrode sheet. The negative electrode sheet was cut into 74mm x 867mm dimensions and welded to the tabs for later use.

[0103] Preparation of electrolyte

[0104] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents of ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and propyl propionate (PP) are mixed in a mass ratio of 2:3:2:3, and then lithium hexafluorophosphate (LiPF6) is added to the non-aqueous organic solvent, dissolved and mixed evenly to obtain an electrolyte, wherein the ratio of LiPF6 to EC, DEC, PC, and PP is 8.7:20:30:20:30.

[0105] Preparation of lithium-ion batteries

[0106] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, with the side of the separator with the first coating in contact with the positive electrode sheet, and the side of the separator with the second coating in contact with the negative electrode sheet. The electrodes are then wound to form an electrode assembly. The electrode assembly is then packaged in an aluminum-plastic film bag, dehydrated at 80°C, and filled with the prepared electrolyte. The battery is then vacuum packaged, allowed to stand, formed, and shaped to produce a lithium-ion battery.

[0107] Adhesion test between isolation film and electrode plate

[0108] The national standard GB / T 2790-1995, that is, the 180° peel test standard is adopted to test the adhesion between the separator and the positive electrode sheet or the negative electrode sheet. The separator and the positive electrode sheet are cut into 54.2mm×72.5mm samples, and the separator and the positive electrode sheet are compounded and hot pressed using a hot press. The hot pressing conditions are: temperature 85°C, pressure 1Mpa, and hot pressing time 85s (seconds). The compounded sample is cut into 15mm×54.2mm strips, and the adhesion between the separator and the positive electrode sheet at room temperature (25°C) is tested according to the 180° peel test standard.

[0109] Hot Box Window Test

[0110] 1. Pre-treatment of battery cell full charging process

[0111] 1) The test temperature is 25°C;

[0112] 2) Sleep for 30 minutes;

[0113] 3) Charge at a constant current rate of 0.2C to 4.5V, and charge at a constant voltage of 4.5V to a cut-off current of 0.025C.

[0114] 2. Testing process:

[0115] ① Check the appearance and take photos before and after the test;

[0116] ②Thermosensitive line attachment position;

[0117] ③ Place the sample vertically in the box and heat it to 130℃ at a rate of 6℃ and maintain for 60 minutes;

[0118] ④Measurement frequency: Voltage and internal resistance measurements use 1KHz specifications, after pre-processing and testing;

[0119] ⑤Judgment criteria: no fire, no explosion, no smoke.

[0120] Cyclic performance test

[0121] Preparation before cycling: let the battery rest for 30 minutes at 25°C, then discharge it to 3V at a current density of 0.2C.

[0122] Let it stand for another 30 minutes, charge to 4.48V at a constant current of 0.5C, charge to 0.02C at a constant voltage; let it stand for 30 minutes; discharge to 3V at a current density of 0.2C, and finally let it stand for 30 minutes.

[0123] Cycle Process

[0124] Test process: Let it stand for 30 minutes at a test temperature of 25℃, then charge at a constant current of 12.4A to 4.22V, and charge at a constant voltage of 1.8C; then charge at a constant current of 1.8C to 4.3V, and charge at a constant voltage of 1.5C; charge at a constant current of 1.5C to 4.48V, and charge at a constant voltage of 1.2C; charge at a constant current of 1.2C to 4.53V, and charge at a constant voltage of 0.26C; let it stand for 5 minutes, discharge at a current density of 0.7C to 3.2V; let it stand for 5 minutes, and cycle 800 times from step 3 to step 9.

[0125] The high temperature cycle test was performed as above, except that the temperature was set to 45°C.

[0126] Swelling test method

[0127] Prepare the film: Disassemble the battery cell and remove the separator. Dissolve any separator greater than 10 cm in length and width in the organic solvent NMP at a solvent-to-solution ratio of 1:9. Place the beaker containing the suspension on a disperser and stir thoroughly for 1.5 hours. Let the stirred solution stand for 30 minutes to eliminate bubbles. Filter through a strainer to remove insoluble impurities and the base film. Pour this solution into the film preparation mold, removing any bubbles in advance. Place the mold in an oven at 60°C for 12 hours.

[0128] Cut the prepared film into strips weighing approximately 2g with scissors. Prepare two to three replicates for each film type. Weigh each strip, record the initial weight (M1), and place it in a small vial. Label the sample and add electrolyte to a depth 2-3cm above the film. Seal the vial and place it in an oven at 60°C. Test the swelling time for 24 hours. During the test, remove the film, wipe it clean with dust-free paper, and record the film weight (M2) at each time point.

[0129] Swelling degree = (M2-M1) / M1

[0130] Table 1

[0131] According to the test results in Table 1, the hot box test pass rates of the batteries of Examples 1-1 to 1-21 all reached 80% or more, which were significantly higher than the hot box test pass rates of the batteries of Comparative Examples 1-2 to 1-4. The capacity retention rates of the batteries of Examples 1-1 to 1-21 after 800 cycles of the normal temperature cycle test at 25°C / 4C all reached 80% or more. It can be seen that by adopting the organic coating containing the first polymer with a low melting point and the second polymer with a low swelling degree as described in the present application, the hot box test pass rate of the battery can be effectively improved while ensuring the normal temperature cycle performance of the battery.

[0132] Preparation of organic coating in the second embodiment

[0133] In the second group of examples, an organic coating was prepared according to the parameters of the first polymer and the second polymer in Table 2 below and auxiliary components (the thickener was sodium carboxymethyl cellulose, and the wetting agents were sulfonate and dimethyl silane). The first polymer was 65 parts by weight, the second polymer was 30 parts by weight, and the auxiliary components were 5 parts by weight. Deionized water was then added and stirred to adjust the slurry viscosity to 50 mPa·s and the solid content to 5%, thereby obtaining slurry A. The slurry A was evenly coated on the surface of the inorganic coating of the PE base film to obtain a first coating. The coating weight of the first coating was 0.2 g / m 2 The electrolyte solution is then dried in an oven. The proportion of cyclic carboxylic acid esters (such as EC) is increased accordingly to meet the requirements of the following linear carboxylic acid ester content control.

[0134] For other steps and parameters of the second embodiment, please refer to the steps and parameters of the first embodiment, and finally a lithium-ion battery is produced.

[0135] Table 2

[0136] The test results in Table 2 show that the swelling degree D of the second polymer and the linear carboxylate content H in the electrolyte satisfy the formula D = K*H + 0.1, where 0.01 ≤ K ≤ 0.08. By controlling the appropriate linear carboxylate content in the electrolyte and combining it with an appropriate swelling degree for the second polymer, the high-temperature cycling performance of the battery can be improved.

[0137] The third group of examples of organic coating preparation

[0138] In the third group of examples, an organic coating was prepared according to the parameters of the first polymer and the second polymer in Table 3 below and auxiliary components (the thickener was sodium carboxymethyl cellulose, and the wetting agents were sulfonate and dimethyl silane). The first polymer was 65 parts by weight, the second polymer was 30 parts by weight, and the auxiliary components were 5 parts by weight. Deionized water was then added and stirred to adjust the slurry viscosity to 50 mPa·s and the solid content to 5%, thereby obtaining slurry A. The slurry A was evenly coated on the surface of the inorganic coating of the PE base film to obtain a first coating. The coating weight of the first coating was 0.2 g / m 2 , and complete drying in an oven.

[0139] For other steps and parameters of the third group of embodiments, please refer to the steps and parameters of the first group of embodiments, and finally a lithium-ion battery is produced.

[0140] Table 3

[0141] According to the test results in Table 3, it can be seen that setting the second polymer to have a higher sphericity and a lower swelling degree (85° C.) is beneficial to improving the rate performance of the battery.

[0142] It should be noted that the above are only specific embodiments of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by any person skilled in the art within the technical scope disclosed in the present application should be included in the scope of protection of the present application; the embodiments of the present application and the features of the embodiments can be combined with each other unless there is a conflict. Therefore, the scope of protection of the present application shall be based on the scope of protection of the claims.

Claims

1. A separation film, characterized in that: include: Basement membrane; An organic coating layer disposed on a surface of the base film, the organic coating layer comprising a first polymer and a second polymer; The melting point T of the first polymer satisfies 60°C≤T≤100°C; The swelling degree D of the second polymer when placed in a solution at 60° C. for 24 hours is ≤30%. The mass ratio of lithium hexafluorophosphate, ethylene carbonate, diethyl carbonate, propylene carbonate and propyl propionate in the solution is 8.7:20:30:20:

30.

2. The isolation film according to claim 1, characterized in that: The median particle size Dv50 of the first polymer satisfies 0.5 μm<Dv50<2 μm.

3. The isolation film according to claim 1, characterized in that: In the organic coating, the weight percentage of the first polymer is greater than or equal to 40% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 50%.

4. The isolation film according to claim 1, characterized in that: In the organic coating, the weight percentage of the first polymer is greater than or equal to 60% and less than or equal to 95%, and the weight percentage of the second polymer is less than or equal to 30%.

5. The isolation film according to claim 1, characterized in that: When the isolation membrane is applied to an electrochemical battery cell, the swelling degree D of the second polymer and the content H of the linear carboxylic acid ester in the electrolyte satisfy D=K*H+0.1, wherein 0.01≤K≤0.

08.

6. The isolation film according to claim 5, characterized in that: 10％≤H≤50％。 7. The isolation film according to claim 1, characterized in that: The median particle size Dv50 of the first polymer satisfies 1.0 μm≤Dv50≤2 μm.

8. The isolation film according to claim 1, characterized in that: The melting point T of the first polymer satisfies 60°C≤T≤85°C.

9. The isolation film according to claim 1, characterized in that: The second polymer is spherical or quasi-spherical, and the sphericity R of the second polymer satisfies 0.7≤R<1.

10. The isolation film according to claim 1, characterized in that: The second polymer is spherical or quasi-spherical, and the sphericity R of the second polymer satisfies 0.8≤R<1.

11. The isolation film according to claim 1, characterized in that: The swelling degree D of the second polymer when placed in an electrolyte at 85° C. for 24 hours satisfies 10%≤D≤50%.

12. The isolation film according to claim 1, characterized in that: The first polymer includes a graft copolymer, the monomer of the polymer main body in the graft copolymer includes at least one of ethylene, propylene, vinylidene fluoride, acrylic acid, acrylate, styrene, acrylonitrile, maleic anhydride, vinyl chloride and allyl chloride, and the monomer used for grafting includes at least one of maleic anhydride, acrylate and acrylic acid; the weight percentage of the graft copolymer in the first polymer is greater than or equal to 3% and less than or equal to 10%.

13. The isolation film according to claim 1, characterized in that: The second polymer includes a polymer of at least one monomer selected from vinylidene fluoride, hexafluoropropylene, propylene, vinyl chloride, styrene, butadiene, acrylate, and acrylic acid, and the second polymer is at least one selected from homopolymers, copolymers, and blended polymers.

14. A secondary battery comprising a positive electrode sheet, a negative electrode sheet and a separator disposed between the positive electrode sheet and the negative electrode sheet, characterized in that: The separator is the separator according to any one of claims 1 to 13, wherein the organic coating is located between the positive electrode sheet and the base film.

15. An electrochemical device comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet, characterized in that: The separator is the separator according to any one of claims 1 to 13, wherein the organic coating is located between the positive electrode sheet and the base film.

16. An electrical device, characterized in that: The electrochemical device according to claim 15 or the secondary battery according to claim 14 is included.

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