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

Abstract

This invention provides a composite separator, its preparation method, and a secondary battery. The composite separator includes a base membrane and a coating disposed on at least one side of the base membrane. The coating material comprises a triazine-based polymer, wherein the monomer structure of the triazine-based polymer includes a triazine ring, at least one halogenated hydrocarbon group, at least one alkenyl group, and an alkoxy aromatic group. The specific type of coating provided by this invention endows the composite separator with functions such as heat resistance, hydrophilicity, and rapid ion conduction, thereby further improving the overall performance of the secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a composite separator, its preparation method, and a secondary battery. Background Technology

[0002] With the rapid development of applications such as electric vehicles and renewable energy storage systems, the development of lithium-ion battery technology that combines high energy density and fast charging capability is becoming increasingly urgent. Traditional lithium-ion batteries use graphite as the anode material, but its energy density and charge / discharge speed are limited, and it is prone to problems such as lithium plating / dendration and slow diffusion kinetics under fast charging conditions. Compared with traditional graphite materials, silicon-based anode materials offer advantages such as high capacity (theoretical specific capacity up to 4200 mAh / g, 10 times that of graphite) and lower lithium insertion / extraction potential (~0.4V vs. Li / Li). + Graphite has a voltage difference of only ~0.05V compared to Li / Li. + Its advantages, such as [missing information], make it an ideal choice for anode materials.

[0003] In addition, as lithium-ion batteries move towards high-energy-density fast charging, the selection of separator materials is equally crucial. Currently, commercially available polyolefin separators and traditional coated separators exhibit the following problems, specifically:

[0004] (1) Insufficient thermal stability: Traditional polyolefin separators and coated separators usually undergo significant thermal shrinkage (shrinkage rate > 10%) at temperatures above 150°C, leading to short circuits between positive and negative electrodes. They are particularly difficult to adapt to the high-temperature environment during fast charging of silicon-based negative electrode materials (the internal temperature of the battery can reach above 100°C).

[0005] (2) Imbalance between mechanical strength and flame retardancy: The rigid coatings (such as Al2O3 coatings) used in the existing technology for coating the separator have a certain puncture resistance, but poor flame retardancy; while the flexible coatings (such as polyacrylic acid) used in the existing technology for coating the separator are easily torn by silicon expansion stress and are easy to burn at high temperatures, resulting in a high risk of battery thermal runaway.

[0006] (3) Contradiction between ion conduction and wettability: If the membrane has a low porosity, its ability to wet the electrolyte is insufficient, and at the same time, the interfacial impedance is high, and the lithium ion conduction efficiency is low when fast charging at high rates.

[0007] (4) Lack of thermal runaway protection: Traditional diaphragms do not have a thermal runaway warning mechanism, so they cannot suppress the spread of flames at high temperatures, resulting in insufficient battery safety.

[0008] Therefore, how to develop a separator that combines high ion conductivity, good structural stability, heat resistance, and interface stability to be suitable for high-performance lithium-ion batteries is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0009] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a composite separator, its preparation method and a secondary battery. The coating endows the composite separator with functions such as heat resistance, hydrophilicity and rapid ion conduction, thereby further improving the overall performance of the secondary battery.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] In a first aspect, the present invention provides a composite membrane comprising a base membrane and a coating disposed on at least one side of the base membrane, wherein the coating material comprises a triazine polymer, and the monomer structure of the triazine polymer comprises a triazine ring, at least one halogenated hydrocarbon group, at least one alkenyl group and an alkoxy aromatic group.

[0012] This invention significantly improves the thermal stability, mechanical strength, ionic conductivity, and electrolyte wetting properties of the composite separator by using a triazine-based polymer with a specific structure as a coating material. This results in a rechargeable battery that combines high energy density, fast charging capability, excellent cycle stability, and high-temperature safety performance, as detailed below:

[0013] (1) The 1,3,5-triazine rigid heterocyclic group in the triazine polymer structure can not only provide excellent thermal stability and suitable mechanical strength, but also promote rapid ion conduction and improve the rate performance of secondary batteries by weakly coordinating with metal ions through the lone pair electrons of nitrogen atoms.

[0014] (2) The alkoxy aromatic polar groups in the triazine polymer structure can form hydrogen bonds with the solvent in the electrolyte, thereby further improving the wetting performance of the membrane to the electrolyte.

[0015] (3) The halogenated hydrocarbon groups in the triazine polymer structure can release flame-retardant gases at high temperatures, thereby achieving thermal runaway early warning and flame suppression. At the same time, halogen atoms can serve as cross-linking nodes to further strengthen the coating structure.

[0016] Preferably, the molar ratio of the triazine ring, the haloalkyl group and the alkoxy aromatic group is 1:(1-2):1, more preferably 1:2:1, for example, it can be 1:1:1 or 1:2:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0017] In this invention, by adjusting the molar ratio of triazine ring, haloalkyl group, and alkoxy aromatic group to a suitable range, the resulting composite membrane exhibits optimal overall performance. If the number of haloalkyl groups is large, the resulting coating will have strong polarity, leading to a decrease in liquid absorption rate; if the number of alkoxy aromatic groups is small, the resulting coating will have insufficient wettability to the electrolyte.

[0018] Preferably, the monomer structure of the triazine-based polymer is shown in Formula 1:

[0019]

[0020] R1 and R2 are each independently selected from halogen atoms, and R3 is selected from alkoxyphenyl groups.

[0021] In this invention, the halogen atom exemplarily includes a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom.

[0022] In this invention, the alkoxy group includes, for example, a methoxy group or an ethoxy group.

[0023] This invention further optimizes the substitution positions of haloalkanes and their alkoxy aromatic groups to form a symmetrical structure, thereby avoiding uneven crosslinking caused by steric hindrance.

[0024] Preferably, the monomer of the triazine-based polymer is at least one of the following compounds:

[0025] Ⅰ(CAS No.: 42573-57-9).

[0026] II (CAS No.: 42880-07-9).

[0027] Ⅲ (CAS No.: 42880-08-0).

[0028] In this invention, the monomer structure of the triazine polymer preferably contains chlorine atoms, mainly for the following reasons:

[0029] Fluorine atoms: The bond energy of the CF bond is too high (485kJ / mol), making it difficult to break at high temperatures to release the flame-retardant gas in time, and it cannot undergo cross-linking reaction, resulting in increased brittleness of the coating.

[0030] Although the bond energy of the C-Br / CI bond is relatively low (285 / 228 kJ / mol), it is easily hydrolyzed and detached at room temperature, resulting in an unstable coating structure.

[0031] The chlorine atom: The C-Cl bond energy is moderate (339 kJ / mol), which allows it to release HCl flame-retardant gas promptly at high temperatures (>150℃) (achieving thermal runaway early warning), and it can also serve as a crosslinking site during polymerization (under high temperatures (>120℃) or alkaline conditions, the Cl atom can undergo nucleophilic substitution reactions with amino (-NH-) or hydroxyl (-OH, from the surface of inorganic oxides) in the polymer chain (e.g., Cl...). - (It leaves and forms CN or CO bonds) to achieve intermolecular cross-linking, strengthen the coating network structure, and thus balance the coating stability and flame retardancy.

[0032] Preferably, the weight-average molecular weight of the triazine polymer is 80,000 Da to 180,000 Da, for example, it can be 80,000 Da, 100,000 Da, 120,000 Da, 150,000 Da or 180,000 Da, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] In this invention, by controlling the weight-average molecular weight of the triazine-based polymer, the coating achieves both good processability and suitable mechanical strength, while ensuring the integrity of the membrane coating structure at high temperatures. If the weight-average molecular weight of the triazine-based polymer is low, the coating is difficult to form, has poor interfacial adhesion with the base film, and insufficient mechanical strength; if the weight-average molecular weight of the triazine-based polymer is high, the viscosity of the coating slurry increases, resulting in a decrease in coating uniformity and an increase in polymer brittleness.

[0034] Preferably, the material of the coating also includes inorganic oxides.

[0035] Preferably, in the coating, the triazine-based polymer coats the surface of the inorganic oxide.

[0036] In this invention, the inorganic oxide particles are embedded in the network formed by the triazine-based polymer, meaning the triazine-based polymer "encapsulates" the inorganic oxide particles, and the two have a synergistic effect, thereby forming a strong organic-inorganic interpenetrating network. The triazine-based polymer provides bonding and stress buffering effects, while the inorganic oxide particles provide rigid support and a thermal barrier effect, synergistically suppressing the high-temperature thermal shrinkage of the membrane and silicon expansion stress, thus further reducing the thermal shrinkage rate of the composite membrane at high temperatures.

[0037] Preferably, the inorganic oxide includes TiO2 and / or ZnO, and more preferably a combination of TiO2 and ZnO.

[0038] Preferably, the particle size D of the TiO2 is... 50The range is 8nm to 15nm, for example, it can be 8nm, 9nm, 10nm, 12nm or 15nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] Preferably, the particle size D of the ZnO is... 50 The range is 25nm to 45nm, for example, it can be 25nm, 28nm, 30nm, 32nm, 35nm, 38nm, 40nm, 42nm or 45nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0040] Preferably, the pore size of the ZnO is 2nm to 6nm, for example, it can be 2nm, 3nm, 4nm, 5nm or 6nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, when the inorganic oxide comprises a combination of TiO2 and ZnO, the mass percentage of TiO2 is 25% to 50% based on the total mass of the inorganic oxides being 100%. For example, it can be 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, or 50%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] Preferably, when the inorganic oxide comprises a combination of TiO2 and ZnO, the ZnO mass percentage is 50% to 75% based on the total mass of the inorganic oxides being 100%. For example, it can be 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, 72%, or 75%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0043] In this invention, by adjusting the contents of TiO2 and ZnO to a suitable range, the strength, interfacial stability, affinity for electrolyte, and liquid absorption rate of the composite membrane are synergistically enhanced. On the one hand, nano-TiO2 possesses high surface energy (100m² / 4 ... 2 ZnO ( / g) can enhance the hydrogen bonding between ZnO and the amino groups in the triazine polymer structure, and its semiconductor properties can modulate the interfacial potential. On the other hand, ZnO with a mesoporous structure (pore size 2nm~6nm) can increase the adsorption capacity of the membrane for electrolyte.

[0044] Preferably, when the inorganic oxide includes a combination of TiO2 and ZnO, the mass ratio of TiO2 to ZnO is 1:(1~2), for example, it can be 1:1, 1:1.2, 1:1.5, 1:1.8 or 1:2, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0045] In this invention, by further adjusting the mass ratio of nano-TiO2 and mesoporous ZnO to a suitable range, small-particle-size TiO2 fills the ZnO pores, forming a "rigid core-flexible shell" structure, thereby increasing the coating density and further improving the puncture resistance of the composite membrane.

[0046] Preferably, the coating material further includes a wetting agent, a dispersant, and a binder.

[0047] Preferably, the mass ratio of the triazine polymer, inorganic oxide, wetting agent, dispersant, and binder in the coating material is (20-35):(5-18):(0.02-0.08):(0.1-0.3):(2-6), for example, 20:18:0.02:0.3:2, 22:16:0.03:0.25:3, 25:14:0.04:0.2:4, 30:10:0.06:0.15:5, or 35:5:0.08:0.1:6, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0048] In this invention, the wetting agent can be, for example, at least one of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate, and its main function is to reduce the surface tension of the coating slurry and enhance the fluidity of the slurry.

[0049] In this invention, the dispersant can be, for example, at least one of silicate compounds, sodium polyacrylate, or sodium citrate, preferably a silicate compound. The above-mentioned dispersant can uniformly disperse inorganic oxide particles that are difficult to dissolve in liquids, while also preventing particle sedimentation and aggregation, thus forming an amphiphilic agent required for a stable suspension.

[0050] In this invention, the adhesive may be, for example, at least one of sodium carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose (HPMC), or polyvinyl alcohol (PVA).

[0051] Preferably, the thickness of a single layer of the coating is 1μm to 5μm, for example, it can be 1μm, 2μm, 3μm, 4μm or 5μm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0052] In this invention, by adjusting the thickness of a single layer of coating to a suitable range, the heat resistance, ion transport performance, electrolyte wetting performance, and air permeability of the composite separator are comprehensively improved. If a thinner coating is used, it is prone to uneven coverage, which not only provides insufficient buffering / protection against silicon anode expansion but also hinders the improvement of the composite separator's thermal stability and ion transport performance. If a thicker coating is used, the composite separator becomes thicker and its internal resistance increases, reducing the energy density and power density of the secondary battery. It may also lead to poor air permeability of the composite separator, thereby affecting electrolyte wetting and ion transport, and increasing production costs.

[0053] Preferably, the porosity of the composite membrane is 40% to 58%, for example, it can be 40%, 45%, 50%, 52%, 55% or 58%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0054] In this invention, by adjusting the porosity of the composite separator to a suitable range, the composite separator can have both good thermal stability and wettability while ensuring appropriate strength. If a composite separator with low porosity is used, the liquid absorption rate of the separator will be low, resulting in poor rate performance of the secondary battery. If a composite separator with high porosity is used, the mechanical strength of the separator will decrease, and the high-temperature thermal shrinkage rate will be large.

[0055] In this invention, the base film is exemplary to include at least one of polyethylene (PE) base film, polypropylene (PP) base film, or multilayer polyolefin base film (such as three-layer composite PP / PE / PP base film), preferably polyethylene base film.

[0056] In this invention, the thickness of the base film is 5μm to 13μm, for example, it can be 5μm, 8μm, 10μm, 12μm or 13μm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0057] In this invention, by adjusting the thickness of the base membrane to a suitable range, the mechanical strength of the composite separator and the electrochemical performance of the secondary battery are comprehensively improved. If a base membrane with a smaller thickness is used, the mechanical strength of the composite separator will decrease; if a base membrane with a larger thickness is used, the energy density of the secondary battery will decrease accordingly.

[0058] In a second aspect, the present invention provides a method for preparing the composite membrane as described in the first aspect, the method comprising the following steps:

[0059] The composite membrane is obtained by coating a coating slurry containing a triazine polymer onto at least one side of a base membrane;

[0060] The monomer structure of the triazine polymer includes a triazine ring, at least one halogenated hydrocarbon group, at least one alkenyl group, and an alkoxy aromatic group.

[0061] Preferably, the coating slurry further includes inorganic oxides.

[0062] Preferably, the coating slurry further includes a first solvent, a wetting agent, a dispersant, a second solvent, and a binder.

[0063] In this invention, the first solvent includes water.

[0064] In this invention, the second solvent includes isopropanol.

[0065] Preferably, the mass ratio of the triazine polymer, inorganic oxide, first solvent, wetting agent, dispersant, second solvent, and binder is (20-35):(5-18):(40-66):(0.02-0.08):(0.1-0.3):(2-5):(2-6), for example, 20:5:40:0.02:0.3:2:6, 25:8:45:0.04:0.25:3:5, 30:12:50:0.06:0.2:4:4, or 35:18:60:0.08:0.3:5:2, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0066] In this invention, the preparation method of the triazine polymer includes the following steps: mixing a triazine ring, a monomer containing at least one halogenated hydrocarbon group, a combination of at least one alkenyl and alkoxy aromatic groups, an initiator and an organic solvent, and then performing a polymerization reaction to obtain the triazine polymer.

[0067] Preferably, the polymerization reaction is carried out under an inert atmosphere, which exemplarily includes argon and / or nitrogen.

[0068] Preferably, the polymerization reaction temperature is 60℃ to 110℃, for example, it can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃ or 110℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0069] Preferably, the polymerization reaction time is 10h to 36h, for example, it can be 10h, 15h, 20h, 25h, 30h, 35h or 36h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0070] In this invention, the initiator exemplary includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, or benzoyl peroxide (BPO). Further, based on 100% of the total monomer mass, the mass percentage of the initiator is 0.2% to 1%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%, etc., and is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0071] In this invention, the organic solvents include, exemplarily, at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP), or N,N-dimethylformamide (DMF).

[0072] In this invention, the polymerization reaction further includes the following steps: adding the reaction solution after polymerization to the precipitation solvent to obtain a polymer precipitate, and then washing and drying the polymer precipitate to obtain the triazine polymer.

[0073] In this invention, the precipitation solvent includes, for example, at least one of propanol, isopropanol, or acetone.

[0074] In this invention, the method for preparing the coating slurry containing the triazine-based polymer includes the following steps:

[0075] First, the dispersant and the first solvent are stirred and mixed once, and then the inorganic oxide is added and stirred and mixed a second time to obtain a mixed solution. Then, the second solvent, binder, triazine polymer and wetting agent are added to the mixed solution and stirred and mixed a third time to obtain the coating slurry containing the triazine polymer.

[0076] In this invention, the rotation speed of the primary mixing process is 2000 r / min to 3100 r / min, for example, 2000 r / min, 2200 r / min, 2500 r / min, 2800 r / min or 3100 r / min, etc., the revolution speed is 20 r / min to 40 r / min, for example, 20 r / min, 30 r / min or 40 r / min, etc., and the mixing time is 10 min to 45 min, for example, 10 min, 20 min, 30 min, 40 min or 45 min, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0077] In this invention, the rotation speed of the secondary stirring and mixing is 2000 r / min to 3100 r / min, for example, it can be 2000 r / min, 2200 r / min, 2500 r / min, 2800 r / min or 3100 r / min, etc., the revolution speed is 20 r / min to 50 r / min, for example, it can be 20 r / min, 30 r / min, 40 r / min or 50 r / min, etc., and the stirring time is 10 min to 30 min, for example, it can be 10 min, 20 min or 30 min, etc., but it is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0078] In this invention, ultrasonication is performed simultaneously during the secondary stirring and mixing process. The frequency of the ultrasonication is 20kHz to 50kHz, for example, it can be 20kHz, 30kHz, 40kHz or 50kHz, etc., but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0079] In this invention, the three-stage stirring and mixing is carried out under vacuum oscillation.

[0080] In this invention, the rotation speed of the three-stage stirring and mixing is 1000 r / min to 3000 r / min, for example, it can be 1000 r / min, 1500 r / min, 2000 r / min, 2200 r / min, 2500 r / min, 2800 r / min or 3000 r / min, etc.; the revolution speed is 20 r / min to 40 r / min, for example, it can be 20 r / min, 30 r / min, 35 r / min or 40 r / min, etc.; the ultrasonic oscillation frequency is 5 kHz to 60 kHz, for example, it can be 5 kHz, 10 kHz, 20 kHz, 40 kHz or 60 kHz, etc.; and the stirring time is 15 min to 30 min, for example, it can be 15 min, 20 min or 30 min, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0081] In this invention, the above-mentioned stirring and blending is carried out using a planetary stirring device.

[0082] Preferably, the coating method includes microgravure printing coating. Compared with the traditional comma coating method, the microgravure printing coating method used in this invention can significantly improve the uniformity of coating thickness, thereby improving the overall performance of the secondary battery.

[0083] In this invention, the microgravure printing coating is a double-sided coating.

[0084] Preferably, the parameters for the microgravure printing coating include: anilox roller line count of 150 lines / cm to 200 lines / cm, coating speed of 12m / min to 32m / min, printing gap of 0.1mm to 0.3mm, and squeegee angle of 42° to 65°.

[0085] Specifically, the anilox roller line count can be, for example, 150 lines / cm, 160 lines / cm, 170 lines / cm, 180 lines / cm, 190 lines / cm, or 200 lines / cm, etc.; the coating speed can be, for example, 12m / min, 15m / min, 20m / min, 25m / min, 30m / min, or 32m / min, etc.; the printing gap can be, for example, 0.1mm, 0.15mm, 0.2mm, 0.25mm, or 0.3mm, etc.; the squeegee angle can be, for example, 42°, 45°, 50°, 55°, 60°, or 65°, etc., and is not limited to the listed values; other unlisted values ​​within this range are also applicable. In this invention, after the coating slurry containing the triazine polymer is coated onto at least one side of the base film, the process further includes drying and winding.

[0086] In this invention, the drying method is to traction the material into a drying device via a traction roller for drying.

[0087] Specifically, the drying temperature is 40℃ to 80℃, for example, it can be 40℃, 50℃, 60℃, 70℃ or 80℃, etc., and the drying time is 1min to 12min, for example, it can be 1min, 2min, 5min, 8min, 10min or 12min, etc., not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0088] In this invention, the winding tension is 10 N / m to 15 N / m to prevent the coating from cracking, and the wind speed is 15 m / s to 25 m / s to ensure that the solvent on the coating surface evaporates quickly and without flow marks.

[0089] Specifically, the winding tension is 10 N / m to 15 N / m, for example, it can be 10 N / m, 12 N / m, 14 N / m or 15 N / m, etc., and the wind speed is 15 m / s to 25 m / s, for example, it can be 15 m / s, 18 m / s, 20 m / s, 22 m / s or 25 m / s, etc., not limited to the listed values, other unlisted values ​​within this range are also applicable.

[0090] Thirdly, the present invention provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, the separator comprising a composite separator as described in the first aspect, or a composite separator prepared by the preparation method described in the second aspect.

[0091] Preferably, the active material of the negative electrode includes silicon-carbon material.

[0092] In this invention, the silicon mass percentage content in the silicon-carbon material is 35% to 50%, for example, it can be 35%, 40%, 45% or 50%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0093] Preferably, the active material of the positive electrode includes a ternary positive electrode material, and more preferably a high-nickel ternary positive electrode material.

[0094] Preferably, the electrolyte includes additives, which include any one or a combination of at least two of tris(4-nitrophenyl)phosphate, ethylene sulfate (DTD), or lithium bis(fluorosulfonyl)imide (LiFSI). This invention further synergistically reduces interfacial impedance by selecting specific types of additives for combined use, thereby better improving the rate performance of the secondary battery.

[0095] Preferably, the total content of the additive in the electrolyte of the secondary battery is 0.5wt% to 7.5wt%, for example, it can be 0.5wt%, 1wt%, 2wt%, 5wt%, 7wt% or 7.5wt%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0096] Specifically, in the electrolyte of the secondary battery, the mass percentage of tris(4-nitrophenyl)phosphate is 0.5wt% to 1.5wt%, preferably 1.0wt%, for example, it can be 0.5wt%, 0.6wt%, 0.8wt%, 1.0wt%, 1.2wt%, or 1.5wt%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0097] In the electrolyte of the secondary battery, the mass percentage of ethylene sulfate is 1wt% to 3wt%, for example, it can be 1wt%, 1.2wt%, 2wt%, 2.5wt% or 3wt%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0098] In the electrolyte of the secondary battery, the mass percentage of lithium bisfluorosulfonylimide is 1wt% to 3wt%, for example, it can be 1wt%, 1.2wt%, 2wt%, 2.5wt% or 3wt%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0099] Preferably, the electrolyte further includes a metal salt, preferably a lithium salt, which may include at least one of LiPF6, LiBF4, LiN(CF3SO2)2, LiBOB, LiDFOB, LiSO3F, LiClO4, or LiCF3SO3.

[0100] In the electrolyte, the concentration of the metal salt can be from 0.5 mol / L to 2 mol / L, for example, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0101] Preferably, the electrolyte further includes an organic solvent. The choice of the organic solvent is not particularly important and can be a conventional choice for secondary battery electrolytes, such as at least one of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, dipropyl carbonate, propylene carbonate, or γ-butyrolactone.

[0102] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0103] Compared with the prior art, the present invention has the following beneficial effects:

[0104] This invention provides a composite membrane that significantly improves the thermal stability, mechanical strength, ionic conductivity, and electrolyte wetting properties of the composite membrane by using a triazine-based polymer with a specific structure as a coating material. This results in a secondary battery with high energy density, fast charging capability, excellent cycle stability, and high-temperature safety performance, as detailed below:

[0105] (1) The rigid heterocycle of 1,3,5-triazine in the triazine polymer structure can not only provide excellent thermal stability and suitable mechanical strength, but also weakly coordinate with metal ions through the lone pair electrons of nitrogen atoms, thereby promoting rapid ion conduction and improving the rate performance of secondary batteries.

[0106] (2) The alkoxy aromatic polar groups in the triazine polymer structure can form hydrogen bonds with the solvent in the electrolyte, thereby further improving the wetting performance of the membrane to the electrolyte.

[0107] (3) The halogenated hydrocarbon groups in the triazine polymer structure can release flame-retardant gases at high temperatures, thereby achieving thermal runaway early warning and flame suppression. At the same time, halogen atoms can serve as cross-linking nodes to further strengthen the coating structure. Detailed Implementation

[0108] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0109] Example 1

[0110] This embodiment provides a composite separator, which includes a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material includes a triazine polymer, inorganic oxides, sodium hexametaphosphate, sodium silicate, and sodium carboxymethyl cellulose in a mass ratio of 25:11:0.05:0.2:4. The inorganic oxides include a combination of TiO2 and ZnO in a mass ratio of 1:1.5, and the TiO2 particle size D... 50 The particle size D of ZnO is 11 nm. 50 The pore size is 35nm, while the pore size of ZnO is 4nm.

[0111] The monomer of the triazine polymer is the compound shown in Formula I, and the weight-average molecular weight of the triazine polymer is 130,000 Da.

[0112]

[0113] The polyethylene base film has a thickness of 9 μm, the coating has a thickness of 3 μm, and the total thickness of the composite diaphragm is 12 μm, with a porosity of 49%.

[0114] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0115] (1) The compound shown in Formula I was added to tetrahydrofuran, and then azobisisobutyronitrile initiator was added (the mass percentage of azobisisobutyronitrile initiator was 0.6% based on the total mass of the compound shown in Formula I as 100%). The mixture was heated to 85°C under argon protection and reacted for 23 h to obtain a reaction solution. The reaction solution was added to propanol to obtain a polymer precipitate, which was then washed and dried to obtain a triazine polymer.

[0116] Sodium silicate and pure water are first stirred and mixed once, and then a combination of TiO2 and ZnO is added and stirred and mixed twice. During the above two stirring and mixing process, sonication is performed simultaneously to obtain a mixed solution. Isopropanol, sodium carboxymethyl cellulose, triazine polymer and sodium hexametaphosphate are added to the above mixed solution and stirred and mixed three times under vacuum to obtain a coating slurry containing triazine polymer.

[0117] The mass ratio of triazine polymer, inorganic oxide, water, sodium hexametaphosphate, sodium silicate, isopropanol, and sodium carboxymethyl cellulose was 25:11:53:0.05:0.2:3.5:4. The first mixing step involved a rotation speed of 2500 r / min, a revolution speed of 30 r / min, and a mixing time of 28 min. The second mixing step involved a rotation speed of 2500 r / min, a revolution speed of 35 r / min, and a mixing time of 20 min, with an ultrasonic frequency of 35 kHz. The third mixing step involved vacuum oscillation with a rotation speed of 2000 r / min, a revolution speed of 30 r / min, an ultrasonic oscillation frequency of 33 kHz, and a mixing time of 23 min.

[0118] (2) The coating slurry containing triazine polymer obtained in step (1) is coated on one side of the polyethylene film by microgravure printing double-sided coating. The parameters of microgravure printing coating include: anilox roller line count of 180 lines / cm, coating speed of 22m / min, printing gap of 0.2mm, and doctor blade angle of 54°. Then, it is pulled by traction roller to the drying equipment and dried at 60°C for 6min and then wound up. The winding tension is 12N / m and the wind speed is 20m / s to obtain the composite diaphragm.

[0119] Example 2

[0120] This embodiment provides a composite separator, which includes a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material includes a triazine polymer, inorganic oxides, sodium hexametaphosphate, sodium silicate, and sodium carboxymethyl cellulose in a mass ratio of 20:5:0.02:0.1:2. The inorganic oxides include a combination of TiO2 and ZnO in a mass ratio of 1:1, and the TiO2 particle size D... 50 The particle size D of ZnO is 8 nm. 50 The pore size is 25nm, while the pore size of ZnO is 2nm.

[0121] The monomer of the triazine polymer is the compound shown in Formula I, and the weight-average molecular weight of the triazine polymer is 80,000 Da.

[0122]

[0123] The polyethylene base film has a thickness of 5 μm, the coating has a thickness of 1 μm, and the total thickness of the composite diaphragm is 6 μm, with a porosity of 40%.

[0124] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0125] (1) The compound shown in Formula I was added to tetrahydrofuran, and then azobisisobutyronitrile initiator was added (the mass percentage of azobisisobutyronitrile initiator was 0.2% based on the total mass of the compound shown in Formula I as 100%). The mixture was heated to 60°C under argon protection and reacted for 36 h to obtain a reaction solution. The reaction solution was added to propanol to obtain a polymer precipitate, which was then washed and dried to obtain a triazine polymer.

[0126] Sodium silicate and pure water are first stirred and mixed once, and then a combination of TiO2 and ZnO is added and stirred and mixed twice. During the above two stirring and mixing process, sonication is performed simultaneously to obtain a mixed solution. Isopropanol, sodium carboxymethyl cellulose, triazine polymer and sodium hexametaphosphate are added to the above mixed solution and stirred and mixed three times under vacuum to obtain a coating slurry containing triazine polymer.

[0127] The mass ratio of triazine polymer, inorganic oxide, water, sodium hexametaphosphate, sodium silicate, isopropanol, and sodium carboxymethyl cellulose is 20:5:40:0.02:0.1:2:2. The first mixing process involves a rotation speed of 2000 r / min, a revolution speed of 20 r / min, and a mixing time of 45 min. The second mixing process involves a rotation speed of 2000 r / min, a revolution speed of 20 r / min, and a mixing time of 30 min, with an ultrasonic frequency of 20 kHz. The third mixing process involves vacuum oscillation with a rotation speed of 1000 r / min, a revolution speed of 20 r / min, an ultrasonic oscillation frequency of 5 kHz, and a mixing time of 30 min.

[0128] (2) The coating slurry containing triazine polymer obtained in step (1) is coated on one side of the polyethylene film by microgravure printing double-sided coating. The parameters of microgravure printing coating include: anilox roller line count of 150 lines / cm, coating speed of 12m / min, printing gap of 0.1mm, and doctor blade angle of 42°. Then, it is pulled by traction roller to the drying equipment and dried at 40°C for 12min and then wound up. The winding tension is 10N / m and the wind speed is 15m / s to obtain the composite diaphragm.

[0129] Example 3

[0130] This embodiment provides a composite separator, comprising a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material comprises a triazine polymer, inorganic oxides, sodium hexametaphosphate, sodium silicate, and sodium carboxymethyl cellulose in a mass ratio of 35:18:0.08:0.3:6. The inorganic oxides comprise a combination of TiO2 and ZnO in a mass ratio of 1:2, and the TiO2 particle size D... 50 The particle size D of ZnO is 15 nm. 50The pore size is 45nm, while the pore size of ZnO is 6nm.

[0131] The monomer of the triazine polymer is the compound shown in Formula I, and the weight-average molecular weight of the triazine polymer is 180,000 Da.

[0132]

[0133] The polyethylene base film has a thickness of 13 μm, the coating has a thickness of 5 μm, and the total thickness of the composite diaphragm is 18 μm, with a porosity of 58%.

[0134] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0135] (1) The compound shown in Formula I was added to tetrahydrofuran, and then azobisisobutyronitrile initiator was added (the mass percentage of azobisisobutyronitrile initiator was 0.8% based on the total mass of the compound shown in Formula I as 100%). The mixture was heated to 110°C under argon protection and reacted for 10 h to obtain a reaction solution. The reaction solution was added to propanol to obtain a polymer precipitate, which was then washed and dried to obtain a triazine polymer.

[0136] Sodium silicate and pure water are first stirred and mixed once, and then a combination of TiO2 and ZnO is added and stirred and mixed twice. During the above two stirring and mixing process, sonication is performed simultaneously to obtain a mixed solution. Isopropanol, sodium carboxymethyl cellulose, triazine polymer and sodium hexametaphosphate are added to the above mixed solution and stirred and mixed three times under vacuum to obtain a coating slurry containing triazine polymer.

[0137] The mass ratio of triazine polymer, inorganic oxide, water, sodium hexametaphosphate, sodium silicate, isopropanol, and sodium carboxymethyl cellulose is 35:18:66:0.08:0.3:5:6. The rotation speed of the first stirring and mixing is 3100 r / min, the revolution speed is 40 r / min, and the stirring time is 10 min. The rotation speed of the second stirring and mixing is 3100 r / min, the revolution speed is 50 r / min, the stirring time is 10 min, and the ultrasonic frequency is 50 kHz. The rotation speed of the third stirring and mixing is 3000 r / min, the revolution speed is 40 r / min, the ultrasonic oscillation frequency is 60 kHz, and the stirring time is 15 min.

[0138] (2) The coating slurry containing triazine polymer obtained in step (1) is coated onto one side of a polyethylene film using a microgravure double-sided coating method. The parameters for microgravure coating include: anilox roller line count of 200 lines / cm, coating speed of 32m / min, printing gap of 0.3mm, and squeegee angle of 65°. The film is then drawn by a traction roller into a drying device and dried at 80°C for 1 minute, followed by winding. The winding tension is 15N / m, and the air velocity is 25m / s, resulting in the composite diaphragm.

[0139] Example 4

[0140] The difference between this embodiment and Example 1 is that the compound shown in Formula I is replaced with an equal mass of the compound shown in Formula II:

[0141]

[0142]

[0143] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0144] Example 5

[0145] The difference between this embodiment and Example 1 is that the compound shown in Formula I is replaced with an equal mass of the compound shown in Formula A (CAS No.: 165954-13-2):

[0146]

[0147] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0148] Example 6

[0149] The difference between this embodiment and Example 1 is that the weight-average molecular weight of the triazine polymer is 50,000 Da. The change in the weight-average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. Everything else is the same as in Example 1.

[0150] Example 7

[0151] The difference between this embodiment and Example 1 is that the weight-average molecular weight of the triazine polymer is 220,000 Da. The change in the weight-average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. Everything else is the same as in Example 1.

[0152] Example 8

[0153] The difference between this embodiment and Embodiment 1 is that the mass ratio of TiO2 to ZnO is 1:0.5. The preparation process is the same as in Embodiment 1 except for adjusting the mass ratio of TiO2 to ZnO.

[0154] Example 9

[0155] The difference between this embodiment and Embodiment 1 is that the mass ratio of TiO2 to ZnO is 1:5. The preparation process is the same as in Embodiment 1 except for adjusting the mass ratio of TiO2 to ZnO.

[0156] Example 10

[0157] The difference between this embodiment and Embodiment 1 is that ZnO is replaced with an equal mass of TiO2. The preparation process is the same as in Embodiment 1 except that ZnO is replaced with an equal mass of TiO2.

[0158] Example 11

[0159] The difference between this embodiment and Embodiment 1 is that the pore size of ZnO is adjusted to 1 nm. The preparation process is the same as in Embodiment 1 except that the pore size of ZnO is adjusted to 1 nm.

[0160] Comparative Example 1

[0161] This comparative example only provides one polyethylene diaphragm with the same thickness as in Example 1.

[0162] Comparative Example 2

[0163] The difference between this comparative example and Example 1 is that the compound shown in Formula I is replaced with an equal mass of 2-[2-(3-methoxyphenyl)ethylene]phenol (CAS No.: 143212-74-2), with the structure shown below:

[0164]

[0165] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0166] Comparative Example 3

[0167] The difference between this comparative example and Example 1 is that the compound shown in Formula I is replaced with an equal mass of 2,4,6-tris(4-vinylphenyl)-1,3,5-triazine (CAS No.: 1445889-54-2), with the structure shown below:

[0168]

[0169] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0170] Comparative Example 4

[0171] The difference between this comparative example and Example 1 is that the compound shown in Formula I is replaced with an equal mass of 2-(allyloxy)-4,6-dichloro-1,3,5-triazine (CAS No.: 26650-76-0):

[0172]

[0173] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0174] The process of fabricating a lithium-ion battery using the separators provided in the above embodiments and comparative examples specifically includes the following steps:

[0175] (1) Preparation of positive electrode sheet

[0176] The ternary cathode active material NCM811(LiNi) 0.8 Co 0.1 Mn 0.1 O2), polyvinylidene fluoride binder (PVDF), conductive carbon black Super-P and single-walled carbon nanotubes (SWCNT) are mixed and stirred evenly in a mass ratio of 96:2:1.9:0.1 to obtain a positive electrode slurry. The positive electrode slurry is then coated onto aluminum foil through a coating process. After drying and cold pressing, a positive electrode sheet is obtained.

[0177] (2) Preparation of negative electrode sheet

[0178] Silicon-carbon anode material (50% silicon, the remainder carbon), conductive carbon black Super-P, SWCNT, polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) are mixed and stirred evenly in a mass ratio of 90:2:0.5:5:2.5 to obtain a negative electrode slurry. The solid content is controlled at 30%. The negative electrode slurry is then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, the negative electrode sheet is obtained.

[0179] (3) Selection of electrolyte

[0180] The electrolyte consists of a mixed solvent of EC, PC, DMC, DEC and FEC in a volume ratio of 15:20:25:30:10, 1 mol / L LiPF6 and 2 wt% LiFSI.

[0181] (4) Preparation of lithium-ion batteries

[0182] The positive electrode sheet, the separator obtained in the above embodiments and comparative examples, and the negative electrode sheet are stacked in sequence, with the separator between the positive and negative electrode sheets to provide isolation, and the polymer coating side facing the negative electrode sheet. Then, the cells are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping processes, a lithium-ion battery is obtained.

[0183] Performance testing

[0184] The lithium-ion batteries prepared in the above embodiments and comparative examples were subjected to performance testing on the LAND battery testing system of Wuhan Jinno Electronics Co., Ltd., with the charge and discharge voltage limited to 2.5V to 4.2V. The specific steps included are as follows:

[0185] (1) Initial Coulomb efficiency

[0186] At 25°C, the lithium-ion battery was charged at a constant current and constant voltage of 0.33C to 4.2V, allowed to stand for 10 minutes, and then discharged at a constant current of 0.33C to 2.5V, allowed to stand for 10 minutes. The initial coulombic efficiency of the lithium-ion battery was calculated.

[0187] Initial coulombic efficiency (%) = (total capacity of lithium-ion battery during initial discharge at 0.33C / total capacity of lithium-ion battery during initial charge at 0.33C) × 100%.

[0188] (2) Capacity retention rate after 1000 cycles at room temperature (1°C / 2°C)

[0189] At 25°C, the lithium-ion battery was charged at a 1C rate with constant current and constant voltage to 4.2V, with a cutoff current of 0.05C. After resting for 10 minutes, the lithium-ion battery was discharged at a 2C rate with constant current to 2.5V and then rested for 10 minutes. This constitutes one charge-discharge cycle. The lithium-ion battery was subjected to 1000 charge-discharge cycles using the above method. The capacity retention rate of the lithium-ion battery after 1000 charge-discharge cycles at 1C / 2C was calculated.

[0190] The capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0191] (3) Room temperature 6C rate performance - constant current charge ratio

[0192] At 25℃, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V, left to stand for 10 minutes, and then charged at a constant current and constant voltage rate of 6C to 4.2V with a cutoff current of 0.05C. After standing for 10 minutes, the constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charge ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charge ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0193] (4) Thermal shrinkage rate of diaphragm at 150℃ / 30min

[0194] Referring to the standard test method (GB / T 36363), the dimensional changes of the diaphragm sample in the free state are measured at the specified temperature (150℃) and time (30min), and the transverse (TD) and longitudinal (MD) thermal shrinkage rates are calculated.

[0195] The high-temperature furnace used for testing heat shrinkage rate has a temperature control accuracy of ±1℃ and an internal atmosphere of nitrogen inert gas; the sample clamp is a stainless steel frame to ensure that the sample shrinks freely without restraint; the measuring tool is a vernier caliper with an accuracy of 0.02mm; and graph paper is used to mark the initial dimensions.

[0196] Sample preparation:

[0197] Cut to size: 100mm × 100mm, 10mm away from the edge of the diaphragm;

[0198] Marking: Draw a cross on the sample surface and record the initial length L0 (accurate to 0.1 mm) in the transverse (TD) and longitudinal (MD) directions.

[0199] Test steps:

[0200] Pretreatment: The sample was placed in an environment of 23±2℃ and 50±5%RH for 24 hours.

[0201] High-temperature treatment:

[0202] Place the sample flat on the fixture and put it into the center of a high-temperature furnace preheated to 150°C, ensuring that the sample does not touch the furnace wall; after holding the temperature for 30 minutes, quickly remove the fixture and cool it at room temperature for 10 minutes.

[0203] Size measurement:

[0204] Measure the crosshair length L1 of the sample after cooling (measured separately in the TD and MD directions); test 3 parallel samples for each sample and take the average value.

[0205] Heat shrinkage rate (%) = (L0-L1) / L0×100%; record the transverse (TD) and longitudinal (MD) shrinkage rates respectively.

[0206] (5) Cell thermal runaway (ARC) test: The ARC adiabatic thermal runaway test is started. The test sample is heated from room temperature to 45±2℃ in the chamber. After being left for 90 minutes, the change in the battery temperature rise rate is detected. If the temperature rise exceeds 0.2℃ within 10 minutes (i.e., SHR>0.02℃ / min), it is considered that a self-exothermic reaction has occurred inside the battery. The adiabatic environment is maintained until the battery thermal runaway occurs. If the temperature rise does not exceed 0.2℃ within 10 minutes (i.e., SHR≤0.02℃ / min), the next step temperature rise test is continued. Each temperature step is 5℃. The steps are repeated on each step. The ARC test temperature range is 45℃~300℃. The self-generated heat start temperature is T1 (temperature rise rate SHR>0.02℃ / min), and the thermal runaway start temperature is T2 (temperature rise rate SHR>1℃ / min). Wherein, SHR is the self-generated heat temperature rise rate.

[0207] (6) Puncture intensity:

[0208] Referring to GB / T 36363-2018, a puncture instrument (needle diameter 1mm, puncture speed 10mm / min) was used, and the average value of 5 parallel samples was taken.

[0209] The test results are shown in Table 1:

[0210] Table 1

[0211]

[0212]

[0213] As can be seen from Table 1, compared with Comparative Example 1 without modification, the composite separators provided in Examples 1-4 of this invention significantly improve their thermal stability, mechanical strength, ion conductivity and electrolyte wetting performance by using triazine-based polymers with specific structures as coating materials. This results in the assembled lithium-ion batteries having high energy density, fast charging capability, excellent cycle stability and high-temperature safety performance.

[0214] Comparing Example 1 with Examples 6-7, it can be seen that the molecular weight of the triazine polymer has a certain influence on the overall performance of the composite separator. If the molecular weight of the triazine polymer is too low or too high, the electrochemical performance and safety performance of the assembled lithium-ion battery will decrease compared with Example 1.

[0215] Comparing Example 1 with Examples 8-10, it can be seen that the type and content of inorganic oxides in the coating affect the interfacial properties, electrolyte adsorption, thermal stability and puncture strength of the composite membrane.

[0216] Comparing Example 1 with Comparative Examples 2-4, it can be seen that the triazine ring, halogenated hydrocarbon group and alkoxy aromatic group in the triazine polymer structure have a synergistic effect, and none of them can be missing, which is beneficial to improving the overall performance of lithium-ion batteries.

[0217] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A composite diaphragm, characterized in that, The composite membrane includes a base membrane and a coating disposed on at least one side of the base membrane. The coating material includes a triazine-based polymer, and the monomer structure of the triazine-based polymer is shown in Formula 1. Formula 1; R1 and R2 are each independently selected from halogen atoms, and R3 is selected from alkoxyphenyl groups; The weight-average molecular weight of the triazine polymer is 80,000 Da to 180,000 Da.

2. The composite separator of claim 1, wherein The monomer of the triazine polymer is at least one of the following compounds: Ⅰ； Ⅱ； Ⅲ。 3. The composite separator of claim 1, wherein The coating material also includes inorganic oxides; In the coating, the triazine polymer is coated on the surface of the inorganic oxide.

4. The composite separator of claim 3, wherein The inorganic oxides include TiO2 and / or ZnO; The particle size D of the TiO2 50 is 8 nm to 15 nm; The particle size D of the ZnO is 25 nm to 45 nm. 50 25 nm to 45 nm; The pore size of the ZnO is 2nm~6nm.

5. The composite separator of claim 4, wherein The inorganic oxides include a combination of TiO2 and ZnO.

6. The composite separator of claim 5, wherein, When the inorganic oxide comprises a combination of TiO2 and ZnO, the mass percentage of TiO2 is 25% to 50%, based on the total mass of the inorganic oxides being 100%. When the inorganic oxide includes a combination of TiO2 and ZnO, the ZnO mass percentage is 50% to 75% based on the total mass of the inorganic oxide being 100%.

7. The composite separator of claim 6, wherein When the inorganic oxide includes a combination of TiO2 and ZnO, the mass ratio of TiO2 to ZnO is 1:(1~2).

8. The composite separator of claim 3, wherein The coating material also includes wetting agents, dispersants, and binders.

9. The composite separator of claim 8, wherein The mass ratio of triazine polymer, inorganic oxide, wetting agent, dispersant and binder in the coating material is (20~35):(5~18):(0.02~0.08):(0.1~0.3):(2~6); And / or, the thickness of a single layer of the coating is 1 μm to 5 μm; And / or, the porosity of the composite membrane is 40%~58%.

10. A method of preparing the composite separator according to any one of claims 1 to 9, characterized by, The method includes the following steps: The composite membrane is obtained by coating a coating slurry containing the triazine polymer onto at least one side of the base membrane.

11. The method according to claim 10, characterized in that, The coating slurry also includes inorganic oxides; And / or, the coating slurry further includes a first solvent, a wetting agent, a dispersant, a second solvent, and a binder.

12. The method of claim 11, wherein, The mass ratio of the triazine polymer, inorganic oxide, first solvent, wetting agent, dispersant, second solvent and binder is (20~35):(5~18):(40~66):(0.02~0.08):(0.1~0.3):(2~5):(2~6).

13. The method of claim 10, wherein, The coating method includes microgravure printing coating; The parameters for the microgravure printing coating include: anilox roller line count of 150 lines / cm to 200 lines / cm, coating speed of 12m / min to 32m / min, printing gap of 0.1mm to 0.3mm, and squeegee angle of 42° to 65°.

14. A secondary battery characterized by comprising: The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator includes a composite separator as described in any one of claims 1-9, or a composite separator prepared by any one of claims 10 to 13.

15. The secondary battery according to claim 14, characterized by The active material of the negative electrode includes silicon-carbon materials; And / or, the active material of the positive electrode includes a ternary positive electrode material; And / or, the electrolyte includes additives, the additives being any one or a combination of at least two of tris(4-nitrophenyl)phosphate, vinyl sulfate, or lithium difluorosulfonylimide.

16. The secondary battery according to claim 15, characterized by The total content of the additives in the electrolyte of the secondary battery is 0.5wt% to 7.5wt%.

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