Composite separator, method for manufacturing the same, and secondary battery

Abstract

The application discloses a kind of composite diaphragm and its preparation method and secondary battery, the composite diaphragm includes base film;Polymer coating is arranged in at least one surface of the base film, the polymer coating includes the crosslinked polymer with network structure formed by in-situ polymerization of first monomer and second monomer;Wherein, the first monomer is at least including carbon-carbon double bond and ester group functional group monomer;The second monomer is at least including carbon-carbon double bond and carbonyl functional group monomer.Compared with prior art, the composite diaphragm of the application is applied in lithium ion battery, when battery overheating or short circuit and the like abnormal condition, polymer coating forms dense crosslinked polymer network, overcome the defect that traditional polyolefin base diaphragm is prone to break membrane under high temperature, effectively inhibit the occurrence of battery thermal runaway, also can stop the electrochemical reaction of battery, significantly improve the safety of battery.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a composite separator, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, due to their high energy density, long cycle life, and environmental friendliness, have been widely used in various electronic devices and electric vehicles. Against the backdrop of national efforts to promote new energy development and rising living standards, the market's demands for battery range and safety are becoming increasingly stringent. However, as the industry as a whole pursues higher energy density and individual battery capacity, the cell technology has transitioned from low-capacity, low-energy-density to high-capacity, high-energy-density cells, and the chemical system has shifted from low-specific-capacity graphite to high-specific-capacity silicon-graphite systems, thereby achieving high energy density. However, battery safety issues, especially thermal runaway, have become a major challenge for high-energy-density battery systems.

[0003] Thermal runaway in lithium-ion batteries is typically caused by various forms of abuse. Among these, gas crosstalk between the positive and negative electrodes is a key factor leading to intense internal heating. When the temperature rises above the phase transition point of the positive and negative electrode materials, active oxygen and reducing gases diffuse into each other, generating a large amount of heat and triggering thermal runaway.

[0004] As one of the four main materials of lithium-ion batteries, the separator plays the role of isolating the positive and negative electrodes from contact and conducting lithium ions. However, traditional separators are prone to rupture at high temperatures and cannot prevent gas crosstalk between the positive and negative electrodes, thus suppressing the occurrence of thermal runaway in lithium-ion batteries.

[0005] Given the aforementioned defects of current diaphragms, it is indeed necessary to provide a technical solution to address these problems. Summary of the Invention

[0006] The purpose of this invention is to provide a composite separator that can suppress the occurrence of battery thermal runaway and significantly improve battery safety.

[0007] To achieve this objective, the present invention provides the following solution:

[0008] A composite membrane, comprising:

[0009] Base film;

[0010] A polymer coating is disposed on at least one surface of the base film, the polymer coating comprising a cross-linked polymer with a network structure formed by in-situ polymerization of a first monomer and a second monomer;

[0011] Wherein, the first monomer is a monomer that includes at least a carbon-carbon double bond and an ester functional group; the second monomer is a monomer that includes at least a carbon-carbon double bond and a carbonyl functional group.

[0012] Preferably, the first monomer is selected from at least one of the following monomers: methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, amyl acrylate, amyl methacrylate, n-octyl acrylate, n-octyl methacrylate, isooctyl acrylate, isooctyl methacrylate, isononyl acrylate, and isononyl methacrylate.

[0013] Preferably, the molecular weight of the second monomer is 100 to 300; the second monomer is at least one of acrylic anhydride, methacrylic anhydride, phthalic anhydride, maleic anhydride, and succinic anhydride.

[0014] Preferably, the mass ratio of the first monomer to the second monomer is (7-12):(0.5-1.5).

[0015] Preferably, the deep eutectic solvent comprises a cyano compound and a lithium salt, wherein the cyano compound is selected from at least one of succinic anionyl nitrile, adiponitrile, acetonitrile, propionitrile, butyric anionyl nitrile, acrylonitrile, methacrylonitrile, and styrene acetonitrile; and the lithium salt is selected from at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonic acid)imide, and lithium tri(trifluoromethanesulfonic acid)methyl; and the mass ratio of the cyano compound to the lithium salt is (15-25):(12-17).

[0016] Preferably, the base membrane is a composite membrane formed from one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalate; the porosity of the base membrane is 20-50%, and the air permeability of the base membrane is 30-400 sec / 100cc.

[0017] Preferably, the thickness ratio of the base film to the polymer coating is (3-20):(1-5); the thickness of the base film is 3-20 μm; and the thickness of the polymer coating is 1-5 μm.

[0018] Preferably, the initiator is at least one selected from azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate.

[0019] This invention also provides a method for preparing a composite membrane, comprising the following steps:

[0020] Step 1: Mix the cyano compound and lithium salt to prepare a deep eutectic solvent;

[0021] Step 2: Add the first monomer and the second monomer to the deep eutectic solvent, stir for 20-40 minutes until homogeneous, and obtain a mixed solution; add 0.4-0.6 wt% of initiator, stir for 10-20 minutes until homogeneous, and obtain a polymer coating slurry;

[0022] Step 2: Coat at least one surface of the base membrane with the polymer coating slurry and dry it at 55-65°C to obtain a composite membrane.

[0023] Preferably, in step one, the solid content of the polymer coating slurry is 50% to 70%.

[0024] The present invention also provides a secondary battery, comprising a positive electrode, a negative electrode, an electrolyte, a battery casing, and the aforementioned composite separator.

[0025] Compared to existing technologies, the advantages of this invention are as follows: When the composite separator of this invention is applied in a lithium-ion battery, in the event of abnormal conditions such as overheating or short circuit, an in-situ cross-linking polymerization reaction occurs in the polymer coating, forming a dense cross-linked polymer network. This cross-linked polymer network is resistant to high temperatures and has a high membrane rupture temperature, overcoming the defect that traditional polyolefin-based separators are prone to rupture at high temperatures. It can also adsorb active gases generated by overheated cells, blocking gas crosstalk between the positive and negative electrodes and effectively suppressing the occurrence of battery thermal runaway. The abundant polar groups in the cross-linked polymer network can also effectively chelate lithium ions, slow down the diffusion of lithium ions, thereby stopping the electrochemical reaction of the battery and significantly improving battery safety. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a composite membrane structure according to an embodiment of the present invention;

[0027] Figure 2 The figures show the thermal runaway test results of lithium-ion batteries in Example 1 and Comparative Example 9.

[0028] Among them, 1. base film; 2. polymer coating. Detailed Implementation

[0029] To make the technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] According to a first aspect of the present invention, a composite separator is provided, comprising:

[0031] Base film 1;

[0032] A polymer coating 2 is disposed on at least one surface of the base film 1, the polymer coating 2 comprising a polymer with a network structure formed by in-situ polymerization of a first monomer and a second monomer;

[0033] Wherein, the first monomer is a monomer that includes at least a carbon-carbon double bond and an ester functional group; the second monomer is a monomer that includes at least a carbon-carbon double bond and a carbonyl functional group.

[0034] When the composite separator is used in lithium-ion batteries, in case of abnormal conditions such as overheating or short circuit, the polymer coating 2 undergoes a cross-linking polymerization reaction under high temperature conditions. This cross-linked polymer network is resistant to high temperatures and has a high membrane rupture temperature, overcoming the defect that traditional polyolefin-based separators are prone to rupture at high temperatures. It can also adsorb active gases generated by overheated cells, blocking gas crosstalk between the positive and negative electrodes and effectively suppressing the occurrence of battery thermal runaway. The abundant polar groups (ester groups: -COO-) in the cross-linked polymer network can also effectively chelate lithium ions, slow down the diffusion of lithium ions, thereby stopping the electrochemical reaction of the battery and significantly improving battery safety.

[0035] In one embodiment of the present invention, the mass ratio of the first monomer to the second monomer is (7-12):(0.5-1.5); for example, it can be 7:1, 7.5:1, 8:1, 8.5:1, 9:1, 9.5:1, 10:1, 10.5:1, 11:1, 11.5:1, 12:1, 7:0.5, 7.5:0.5, 8:0.5, 8.5:0.5 The mass ratios of the first and second monomers should be within this range. If there is too much of the first monomer, it will be difficult to achieve the in-situ thermally induced crosslinking chemical reaction of the polymer; if there is too little of the first monomer, it will be difficult to form a polymer coating.

[0036] In one embodiment of the present invention, the first monomer is selected from at least one of the following monomers: methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, amyl acrylate, amyl methacrylate, n-octyl acrylate, n-octyl methacrylate, isooctyl acrylate, isooctyl methacrylate, isononyl acrylate, and isononyl methacrylate. More preferably, the first monomer is methyl methacrylate with a molecular weight of 110.116, which is a highly active monomer with a wide electrochemical window and good compatibility with high-voltage metal oxide cathode materials. Acrylic anhydride is an organic compound containing a large number of active unsaturated double bonds and is a key component for realizing the in-situ polymer thermally induced crosslinking chemical reaction.

[0037] In one embodiment of the present invention, the molecular weight of the second monomer is 100 to 300, for example, 100, 150, 200, 250, or 300; when the monomer is used for polymerization, low molecular weight monomers are often used to facilitate polymerization, while high molecular weight monomers are not easy to polymerize; the second monomer is at least one of acrylic anhydride, phthalic anhydride, maleic anhydride, and succinic anhydride.

[0038] In one embodiment of the present invention, the base membrane 1 is a composite membrane formed from one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalate; the porosity of the base membrane 1 is 20-50%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, or 50%; the polyolefin base membrane 1 itself has good thermal stability, and an appropriate porosity helps to further improve its thermal stability. Under abnormal conditions such as battery overheating or short circuit, the electrolyte in the pores can absorb some heat and slow down the rate of temperature rise, thereby reducing the risk of thermal runaway of the battery.

[0039] The air permeability of the base film 1 is 30-400 sec / 100cc. The polyolefin base film 1 with good air permeability can quickly release heat through the air permeable channel when the battery overheats, thereby reducing the internal temperature of the battery. This helps to prevent thermal runaway and fire risks caused by overheating of the battery.

[0040] In one embodiment of the present invention, the thickness ratio of the base film 1 to the polymer coating 2 is (3-20):(1-5); the thickness of the base film 1 is 3-20 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or 20 μm; the polyolefin base film 1 with a moderate thickness can reduce the ineffective space inside the battery, so that more active materials can be encapsulated in the battery, thereby improving the energy density of the battery, and can also reduce the mechanical stress of the battery during charging and discharging, thereby reducing the risk of separator damage.

[0041] The thickness of the polymer coating 2 is 1 to 5 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm. The polymer coating 2 should not be too thick or too thin. If it is too thick, it will affect the energy density of the battery. If it is too thin, the polymer coating 2 will not be able to suppress the occurrence of thermal runaway of lithium-ion batteries and improve the safety performance of lithium-ion batteries.

[0042] In one embodiment of the present invention, the initiator is at least one selected from azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate.

[0043] In one embodiment of the present invention, the polymer coating 2 further includes a deep eutectic solvent comprising a cyano compound and a lithium salt, wherein the cyano compound is selected from at least one of succinic anionyl nitrile, adiponitrile, acetonitrile, propionitrile, butyric anionyl nitrile, acrylonitrile, methacrylonitrile, and styrene acetonitrile; and the lithium salt is selected from at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonic acid)imide, and lithium tri(trifluoromethanesulfonic acid)methyl; and the mass ratio of the cyano compound to the lithium salt is (15-25):(12-1) 7), for example, can be 15:15, 16:15, 17:15, 18:15, 19:15, 20:15, 21:15, 22:15, 23:15, 24:15, 25:15, 15:12, 16:12, 17:12, 18:12, 19:12, 20:12, 21:12, 22:12, 23:12, 24:12, 25:12, 15:17, 16:17, 17:17, 18:17, 19:17, 20:17, 21:17, 22:17, 23:17, 24:17, 25:17. The eutectic solvent, composed of nitrile compound solvent and lithium salt, has thermal and electrochemical stability. Adding this eutectic solvent to the coating of the composite membrane improves the thermal and electrochemical stability of the composite membrane; it can also provide a reaction environment for the first and second monomers, preparing for subsequent suppression of thermal runaway.

[0044] In one embodiment of the present invention, the lithium salt includes at least one selected from lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonic acid)imide, and lithium tri(trifluoromethanesulfonic acid)methyl.

[0045] According to a second aspect of the present invention, a method for preparing a composite separator is also provided, comprising the following steps:

[0046] Step 1: Mix the cyano compound and lithium salt to prepare a deep eutectic solvent;

[0047] Step 2: Add the first monomer and the second monomer to the eutectic solvent, stir for 20-40 minutes until homogeneous, and obtain a mixed solution; add 0.4-0.6 wt% of initiator, stir for 10-20 minutes until homogeneous, and obtain polymer coating 2 slurry;

[0048] Step 3: Apply the polymer coating slurry 2 to at least one surface of the base membrane 1 and dry it at 55-65°C to obtain a composite membrane.

[0049] In one embodiment of the present invention, in step one, the solid content of the polymer coating slurry 2 is 50% to 70%, for example, it can be 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, or 70%.

[0050] In a third aspect of the invention, a secondary battery is also provided, comprising a positive electrode, a negative electrode, an electrolyte, a battery casing, and the aforementioned composite separator.

[0051] The positive electrode includes a positive current collector and a positive electrode material layer coated on at least one surface of the positive current collector. The positive electrode active material layer can be, but is not limited to, layered oxide positive electrode materials, spinel structure positive electrode materials, olivine structure positive electrode materials, and polyanionic positive electrode materials. The layered oxide positive electrode materials include lithium nickel oxide (such as lithium nickel oxide), lithium cobalt oxide (such as lithium cobalt oxide LiCoO2), nickel cobalt manganese oxide (such as NCM), and nickel cobalt aluminum oxide (such as NCA). The spinel structure positive electrode materials include lithium manganese oxide (LiMn2O4). The olivine structure positive electrode materials include lithium iron phosphate (LiFePO4). The polyanionic positive electrode materials include phosphates and sulfates.

[0052] The positive electrode current collector can be any material suitable for use as a positive electrode current collector in lithium-ion batteries, such as aluminum foil, copper foil, nickel foil, stainless steel, or carbon materials. The negative electrode includes a negative electrode current collector and a negative electrode active material layer coated on at least one surface of the negative electrode current collector. The negative electrode active material layer can be one or more of the following, including but not limited to graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, or other metals that can form alloys with lithium.

[0053] The graphite can be selected from one or more of artificial graphite, natural graphite, and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is typically a structure or component that collects current. The negative electrode current collector can be any material suitable for use as a negative electrode current collector in lithium-ion batteries, for example, it can be, but is not limited to, metal foil, and more specifically, it can be, but is not limited to, copper foil.

[0054] The secondary battery also includes an electrolyte, which comprises an organic solvent, an electrolyte lithium salt, and additives. The electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-resistant electrolytes; or it can be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; or it can be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additives include, but are not limited to, at least one of film-forming additives, conductive additives, flame-retardant additives, overcharge-resistant additives, additives for controlling the H2O and HF content in the electrolyte, additives for improving low-temperature performance, and multifunctional additives.

[0055] The present invention will be further described below through specific embodiments.

[0056] Example 1

[0057] Preparation of composite membrane:

[0058] Step 1: Mix succinate and lithium bis(trifluoromethanesulfonyl)imide at a mass ratio of 20:15 to obtain a deep eutectic solvent;

[0059] Step 2: Add methyl methacrylate and acrylic anhydride monomer at a mass ratio of 9:1, stir for 30 minutes until homogeneous, and obtain a mixed solution; add 0.5 wt% azobisisobutyronitrile, stir for 15 minutes until homogeneous, and obtain polymer coating 2 slurry;

[0060] Step 2: Coat one surface of the base membrane 1 with the polymer coating slurry 2 and dry it to obtain a composite membrane; wherein the base membrane 1 has a thickness of 10 μm and the polymer coating 2 has a thickness of 2 μm.

[0061] The fabrication of lithium-ion batteries:

[0062] The negative electrode, positive electrode, and the aforementioned separator are interleaved and wound together, then encapsulated with an aluminum-plastic film to form a battery cell. This cell is placed in a battery casing, an electrolyte is added, and then the casing is sealed to obtain a lithium-ion battery. The active material of the negative electrode is graphite; the active material of the positive electrode is lithium iron phosphate; the electrolyte salt is lithium hexafluorophosphate, and the solvent is ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate prepared in a mass ratio of 1:2:1, with an electrolyte concentration of 1 mol / L.

[0063] Table 1 shows the data for Examples 1-10 and Comparative Examples 1-8. The rest are the same as in Example 1, and will not be repeated here.

[0064] Table 1

[0065]

[0066]

[0067] Comparative Example 9

[0068] Unlike Example 1, the polymer coating 2 is replaced with an alumina ceramic coating, wherein the alumina ceramic coating has a thickness of 10 μm.

[0069] Everything else is the same as in Example 1, and will not be repeated here.

[0070] Comparative Example 10

[0071] Unlike Example 1, polymer coating 2 does not use a deep eutectic solvent, but instead uses succinate as the solvent.

[0072] Everything else is the same as in Example 1, and will not be repeated here.

[0073] Comparative Example 11

[0074] Unlike Example 1, N-methylpyrrolidone is used as a solvent in polymer coating 2.

[0075] Everything else is the same as in Example 1, and will not be repeated here.

[0076] Comparative Example 12

[0077] Unlike Example 1, the polymer coating 2 uses N-methylpyrrolidone and lithium bis(trifluoromethanesulfonyl)imide mixed at a mass ratio of 20:15 as a solvent.

[0078] Everything else is the same as in Example 1, and will not be repeated here.

[0079] Performance testing:

[0080] The membrane rupture temperature of the membranes of Examples 1-10 and Comparative Examples 1-12 was tested, and the test results are shown in Table 2.

[0081] The lithium-ion batteries of Examples 1-10 and Comparative Examples 1-12 were subjected to thermal runaway temperature tests, and the test results are shown in Table 2.

[0082] The lithium-ion batteries of Example 1 and Comparative Example 9 were subjected to thermal runaway temperature tests. The test results are shown in [Figure Number]. Figure 1 .

[0083] Table 2

[0084]

[0085]

[0086] As shown in Table 2 and Figure 2 The thermal runaway test data shown indicates that the battery in Comparative Example 9 experienced severe thermal runaway when the temperature rose to 150°C, while the composite separators in Examples 1-10 did not exhibit thermal runaway even at a high temperature of 250°C. This demonstrates that when the composite separator of the present invention is applied to a lithium-ion battery, when the battery overheats, the polymer coating 2 undergoes an in-situ polymerization reaction under high-temperature conditions, forming a dense cross-linked polymer network. This cross-linked polymer network can adsorb active gases, block gas crosstalk between the positive and negative electrodes, and effectively suppress the occurrence of battery thermal runaway. The abundant polar groups in the cross-linked polymer network can also effectively chelate lithium ions, slow down the diffusion of lithium ions, thereby stopping the electrochemical reaction of the battery and significantly improving battery safety.

[0087] A comparison of the experimental data of Example 1 and Comparative Examples 1-2 shows that the data of Example 1 is better than that of Comparative Examples 1-2, indicating that if the mass ratio of the first monomer and the second monomer is too large or too small, it will be difficult to achieve the in-situ polymer thermally induced crosslinking chemical reaction or to form a polymer coating.

[0088] A comparison of the experimental data of Example 1 and Comparative Examples 3-4 shows that the data of Example 1 is better than that of Comparative Examples 3-4. This indicates that when only the first monomer or only the second monomer is present, the polymer coating 2 cannot undergo in-situ polymerization, resulting in poor coating performance and no effect on improving the safety performance of lithium-ion batteries.

[0089] Comparing the experimental data of Example 1 and Comparative Examples 5-6, it can be seen that if the thickness of the polymer coating 2 is too thick or too thin, although it can optimize the thermal runaway data of the composite separator to a certain extent, if the thickness of the polymer coating 2 is too thick, it will still affect the thermal runaway data and the energy density of the battery; while if it is too thin, the polymer coating 2 will not achieve the purpose of suppressing the occurrence of thermal runaway of lithium-ion battery and improving the safety performance of lithium-ion battery.

[0090] Comparing the experimental data of Example 1 and Comparative Examples 7-8, it can be seen that if the mass ratio of the initiator is too high or too low, although it can optimize the thermal runaway data of the composite separator to a certain extent, too much initiator will waste initiator material, increase costs, and also have a negative impact on thermal runaway data; while too little initiator will result in an incomplete polymer coating 2, leading to poor thermal runaway data and failing to achieve the goal of improving the safety performance of lithium-ion batteries.

[0091] Comparing the experimental data of Example 1 and Comparative Examples 10-12, it can be seen that when the polymer coating 2 does not have a deep eutectic solvent, the first monomer and the second monomer cannot achieve in-situ polymer thermally induced crosslinking chemical reaction, resulting in poor coating performance. Consequently, it cannot suppress the occurrence of thermal runaway of lithium-ion batteries, nor can it improve the safety performance of lithium-ion batteries.

[0092] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.

Claims

1. A composite diaphragm, characterized in that, include: Base film; A polymer coating is disposed on at least one surface of the base film, the polymer coating comprising a cross-linked polymer with a network structure formed by in-situ polymerization of a first monomer and a second monomer; Wherein, the first monomer is a monomer that includes at least a carbon-carbon double bond and an ester functional group; the second monomer is a monomer that includes at least a carbon-carbon double bond and a carbonyl functional group; It also includes a deep eutectic solvent, which includes cyano compounds and lithium salts.

2. The composite diaphragm according to claim 1, characterized in that: The first monomer is selected from at least one of the following monomers: methyl methacrylate, ethyl methacrylate, methyl acrylate, ethyl acrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, amyl acrylate, amyl methacrylate, n-octyl acrylate, n-octyl methacrylate, isooctyl acrylate, isooctyl methacrylate, isononyl acrylate, and isononyl methacrylate.

3. The composite diaphragm according to claim 1, characterized in that, The molecular weight of the second monomer is 100 to 300; the second monomer is at least one of acrylic anhydride, methacrylic anhydride, phthalic anhydride, maleic anhydride, and succinic anhydride.

4. The composite diaphragm according to claim 1, characterized in that, The mass ratio of the first monomer to the second monomer is (7-12):(0.5-1.5).

5. The composite diaphragm according to claim 1, characterized in that, The cyano compound is selected from at least one of succinic anion, adiponitrile, acetonitrile, propionitrile, butyric acid nitrile, acrylonitrile, methacrylonitrile, and phenylacetonitrile; the lithium salt is selected from at least one of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonic acid)imine, and lithium tri(trifluoromethanesulfonic acid)methyl; the mass ratio of the cyano compound to the lithium salt is (15-25):(12-17).

6. The composite diaphragm according to claim 1 or 5, characterized in that, It also includes an initiator, which is at least one of azobisisobutyronitrile, azobisisoheptanenitrile, and dimethyl azobisisobutyrate.

7. The composite diaphragm according to claim 1, characterized in that, The base membrane is a composite membrane formed from one or more of polyethylene, polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylonitrile, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene ether, polyphenylene sulfide, and polyethylene naphthalate; the porosity of the base membrane is 20-50%, and the air permeability of the base membrane is 30-400 sec / 100cc.

8. The composite diaphragm according to claim 1, characterized in that, The thickness ratio of the base film to the polymer coating is (3-20):(1-5); the thickness of the base film is 3-20 μm; and the thickness of the polymer coating is 1-5 μm.

9. A method for preparing a composite diaphragm, characterized in that, Includes the following steps: 1) Prepare a deep eutectic solvent by mixing cyano compounds and lithium salts; 2) Add the first monomer and the second monomer to the eutectic solvent and stir until homogeneous to obtain a mixed solution; add 0.4-0.6 wt% of initiator to the mixed solution and stir until homogeneous to obtain a polymer coating slurry; 3) The polymer coating slurry is coated on at least one surface of the base film and dried at 55-65°C to obtain the diaphragm.

10. A secondary battery, comprising a positive electrode, a negative electrode, and a separator spaced between the positive and negative electrode, characterized in that, The diaphragm is the diaphragm according to any one of claims 1-8 or the diaphragm prepared by the method of claim 9.

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