Composite diaphragm, preparation method thereof and secondary battery

Abstract

The invention provides a composite diaphragm, a preparation method thereof and a secondary battery. The composite diaphragm comprises a base membrane and a coating arranged on at least one side of the base membrane, the material of the coating comprises a piperidinyl polymer, and the monomer structure of the piperidinyl polymer comprises a piperidine ring, at least one ester group, at least one alkenyl group and a group capable of forming hydrogen-bond interaction. According to the present invention, the coating of the specific type is arranged on the base film, such that the electrolyte wettability and the heat resistance required by the diaphragm can be met, the swelling rate of the diaphragm can be reduced, and the bonding strength between the diaphragm and the negative electrode plate can be improved so as to comprehensively improve the high-temperature cycle and the storage 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 growth of the global electric vehicle market and the increasing demand for fast-charging battery technology, the development of high-performance lithium-ion batteries with fast-charging capabilities has become an inevitable trend. Among these, silicon (Si)-based anode materials stand out due to their ultra-high theoretical specific capacity (4200 mAh g / g). -1 With its low lithium intercalation potential (0.4V vs. Li+ / Li), it is widely regarded as one of the ideal materials for high energy density lithium-ion batteries.

[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] (5) Insufficient bonding between the separator and the electrode: The currently used coated separator is usually bonded to the battery electrode under high temperature and high pressure to improve the hardness of the battery and protect it. However, in the application process, there is still a problem that the bonding strength of the coating is small, resulting in poor bonding performance between it and the substrate, and it cannot play the role of hardening the battery and protecting the battery.

[0009] Therefore, how to develop a separator that combines high ion conductivity, good structural stability, heat resistance, and adhesion properties 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

[0010] To address the shortcomings of existing technologies, the present invention aims to provide a composite separator, its preparation method, and a secondary battery. By applying a specific type of coating to the base membrane, the present invention not only satisfies the electrolyte wettability and heat resistance requirements of the separator, but also reduces the separator swelling rate and improves the bonding strength between the separator and the negative electrode, thereby comprehensively enhancing the high-temperature cycling and storage performance of the secondary battery.

[0011] To achieve this objective, the present invention adopts the following technical solution:

[0012] 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 piperidinyl polymer, and the monomer structure of the piperidinyl polymer comprises a piperidinyl ring, at least one ester group, at least one alkenyl group and a group capable of forming hydrogen bonds.

[0013] The piperidine-based polymer provided by this invention, as a coating material, has the following core advantages: (1) The piperidine heterocycle contained in the polymer structure can provide a rigid skeleton and good thermal stability, and can also weakly coordinate with metal ions through the lone pair electrons of nitrogen atoms, thereby promoting the dissociation of metal salts and their ion conduction rate; (2) The piperidine-based polymer utilizes polar groups that can form hydrogen bonds with the solvent in the electrolyte to form hydrogen bonds, which significantly enhances the wettability of the membrane and the absorption rate of the electrolyte, while providing more transport channels for ion transport. In addition, the above-mentioned polar groups can also form hydrogen bonds with the hydroxyl groups on the surface of silicon-based negative electrode materials, thereby improving the bonding force between the composite membrane and the negative electrode sheet; (3) The ester groups in the polymer structure improve the structural stability and reliability of the composite membrane under high temperature environment.

[0014] In summary, this invention, by controlling the composition of the membrane coating material and the types of its active groups, enables its application in secondary batteries to comprehensively improve fast charging performance, high-temperature performance, and safety.

[0015] Preferably, the molar ratio of the piperidine ring, the ester group, and the group capable of forming hydrogen bonds is (0.9-1.1):(0.9-1.1):(0.9-1.1), and more preferably 1:1:1.

[0016] In this invention, by adjusting the molar ratio of piperidine rings, ester groups, and groups capable of forming hydrogen bonds to a suitable range, the resulting composite membrane exhibits optimal overall performance. If the number of groups capable of forming hydrogen bonds is large, the resulting coating is more polar, leading to a decrease in the liquid absorption rate of the composite membrane; conversely, if the number of groups capable of forming hydrogen bonds is small, the adhesive strength of the coating decreases. If the number of ester groups is small, the swelling ratio of the resulting coating is large, and the mechanical strength is low; if the number of ester groups is large, the brittleness of the composite membrane increases.

[0017] Preferably, the ester group is a tert-butyl ester group. The tert-butyl ester group is preferred in this invention because its steric hindrance effect can suppress excessive swelling of the coating and further increase the glass transition temperature of the polymer to adapt to high-temperature environments.

[0018] Preferably, the groups capable of forming hydrogen bonds include hydroxyl and / or carboxyl groups.

[0019] Preferably, the monomer structure of the piperidinyl polymer is shown in Formula 1:

[0020]

[0021] R1 and R2 are each independently selected from C2-C5 alkenyl or hydroxylated C1-C4 alkyl groups, and R1 is different from R2, and R3 is selected from tert-butyl.

[0022] In this invention, the C2-C5 alkenyl groups typically include any one of vinyl, n-propenyl, isopropenyl, or isobutylene; the C1-C4 alkyl groups typically include any one of methyl, ethyl, n-propyl, isopropyl, n-butyl, or isobutyl.

[0023] Preferably, the monomer of the piperidinyl polymer is at least one of the following compounds:

[0024] Ⅰ(CAS No.: 236406-37-4).

[0025] II (CAS No.: 2413866-79-0).

[0026] Ⅲ (CAS No.: 2376766-26-4).

[0027] Preferably, the weight-average molecular weight of the piperidinyl polymer is 110,000 Da to 250,000 Da, for example, 110,000 Da, 120,000 Da, 150,000 Da, 180,000 Da, 200,000 Da, 220,000 Da or 250,000 Da, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] In this invention, by controlling the weight-average molecular weight of the piperidine polymer, the coating achieves both good film-forming properties and suitable mechanical strength, while also improving the adhesion between the diaphragm coating and the base film. If the weight-average molecular weight of the piperidine polymer is low, the coating is difficult to form, resulting in poor interfacial adhesion with the base film and insufficient mechanical strength; if the weight-average molecular weight of the piperidine polymer is high, the viscosity of the coating slurry increases, causing a decrease in coating uniformity and an increase in polymer brittleness.

[0029] Preferably, the material of the coating also includes inorganic oxides.

[0030] Preferably, in the coating, the piperidine-based polymer coats the surface of the inorganic oxide.

[0031] In this invention, the inorganic oxide particles are embedded in the network formed by the piperidinyl polymer, that is, the piperidinyl polymer "encapsulates" the inorganic oxide particles, and the two have a synergistic effect to form a strong organic-inorganic interpenetrating network. The piperidinyl polymer provides bonding and stress buffering effects, while the inorganic oxide particles provide rigid support and thermal barrier effects, synergistically suppressing the high-temperature thermal shrinkage of the membrane and silicon expansion stress, thereby further reducing the thermal shrinkage rate of the composite membrane at high temperatures.

[0032] Preferably, the inorganic oxide comprises boehmite and / or TiO2, and more preferably a combination of boehmite and TiO2.

[0033] Preferably, the grain size D of the boehmite is... 50 The particle size is 5nm to 15nm, for example, 5nm, 8nm, 9nm, 10nm, 12nm, or 15nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable. This invention achieves superior dispersion performance by controlling the particle size of boehmite to have a smaller particle size than alumina.

[0034] Preferably, the particle size D of the TiO2 is... 50The range is 20nm to 45nm, for example, it can be 20nm, 22nm, 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.

[0035] Preferably, the pore size of the TiO2 is 3nm to 7nm, for example, it can be 3nm, 4nm, 5nm, 6nm or 7nm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0036] Preferably, the mesoporous content of the TiO2 is greater than 80%, for example, it can be 82%, 85%, 88% or 90%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0037] In this invention, the mesopore volume of the TiO2 is 1.0 cm³. 3 / g~1.3cm 3 / g (close to SiO2), for example, it can be 1.0cm 3 / g, 1.1cm 3 / g, 1.2cm 3 / g or 1.3cm 3 / g, etc., are not limited to the listed values; other unlisted values ​​within this range also apply.

[0038] In this invention, by controlling the particle size and pore structure parameters of TiO2, a "macroporous framework + microporous filling" structure is formed, thereby improving the ion transport performance and electrolyte adsorption capacity of the composite membrane, and further reducing interfacial polarization.

[0039] In this invention, the TiO2 is preferably anatase TiO2.

[0040] Preferably, when the inorganic oxide comprises a combination of boehmite and TiO2, the mass percentage of boehmite is 20% to 35% based on the total mass of the inorganic oxide as 100%, for example, it can be 20%, 22%, 25%, 28%, 30%, 32% or 35%, 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 boehmite and TiO2, the mass percentage of TiO2 is 65% to 80% based on the total mass of the inorganic oxide as 100%, for example, it can be 65%, 68%, 70%, 72%, 75%, 78% or 80%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] In this invention, by adjusting the content of boehmite and TiO2 to a suitable range, the bonding ability between inorganic oxides and piperidine polymers is comprehensively enhanced, as well as the thermal stability, mechanical strength, ion transport performance, and liquid absorption of the composite membrane. On one hand, the decomposition temperature of boehmite is >300℃, which can improve the high-temperature resistance of the composite membrane. Furthermore, the hydroxyl groups on its surface can further combine with groups in the piperidine polymer structure that can form hydrogen bonds, thereby improving the interfacial bonding strength. In addition, the amphoteric properties of boehmite itself can adsorb HF from electrolyte decomposition, further improving the stability of the electrolyte. On the other hand, TiO2, with its mesoporous structure (pore size 3nm~7nm), not only increases the membrane's adsorption capacity for electrolyte but also promotes metal ion transport and reduces interfacial polarization.

[0043] Preferably, when the inorganic oxide comprises a combination of boehmite and TiO2, the mass ratio of boehmite to TiO2 is 1:(2-3), for example, it can be 1:2, 1:2.2, 1:2.5, 1:2.8 or 1:3, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] In this invention, by further adjusting the mass ratio of nano-boehmite to mesoporous TiO2 to a suitable range, small-diameter boehmite fills the mesoporous pores of TiO2, thereby further improving the elastic modulus and puncture resistance of the coating, while further reducing the thermal shrinkage rate of the composite membrane.

[0045] Preferably, the coating material further includes wetting agents, dispersants, and binders.

[0046] Preferably, the mass ratio of piperidinyl 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.

[0047] 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.

[0048] 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.

[0049] 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).

[0050] 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.

[0051] 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.

[0052] Preferably, the porosity of the composite membrane is 42% to 55%, for example, it can be 42%, 45%, 50%, 52% or 55%, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0053] 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 accordingly, and the high-temperature thermal shrinkage rate will be large.

[0054] 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.

[0055] In this invention, the thickness of the base film is 6μm to 13μm, for example, it can be 6μm, 8μm, 10μm, 12μm or 13μm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0056] 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.

[0057] 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:

[0058] The composite membrane is obtained by coating a coating slurry containing a piperidine polymer onto at least one side of a base membrane;

[0059] The monomer structure of the piperidinyl polymer includes a piperidinium ring, at least one ester group, at least one alkenyl group, and a group capable of forming hydrogen bonds.

[0060] Preferably, the coating slurry also includes inorganic oxides.

[0061] Preferably, the coating slurry further includes a first solvent, a wetting agent, a dispersant, a second solvent, and a binder.

[0062] In this invention, the first solvent includes water.

[0063] In this invention, the second solvent includes isopropanol.

[0064] Preferably, the mass ratio of the piperidinyl 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.

[0065] In this invention, the preparation method of the piperidinyl polymer includes the following steps: mixing a monomer comprising a combination of a piperidinyl ring, at least one ester group, at least one alkenyl group and a group capable of forming hydrogen bonds, an initiator and an organic solvent, and then performing a polymerization reaction to obtain the piperidinyl polymer.

[0066] Preferably, the polymerization reaction is carried out under an inert atmosphere, which exemplarily includes argon and / or nitrogen.

[0067] Preferably, the polymerization reaction temperature is 60℃ to 124℃, for example, it can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 120℃ or 124℃, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0068] Preferably, the polymerization reaction time is 15h to 32h, for example, 15h, 20h, 25h, 30h or 32h, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0069] 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 initiator's mass percentage is 0.2% to 0.8%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, or 0.8%, etc., and is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0070] In this invention, the organic solvents include, by example, at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP), or N,N-dimethylformamide (DMF).

[0071] 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 piperidinyl polymer.

[0072] In this invention, the precipitation solvent includes, for example, at least one of propanol, isopropanol, or acetone.

[0073] In this invention, the method for preparing the coating slurry containing piperidine-based polymers includes the following steps:

[0074] 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, piperidine polymer and wetting agent are added to the mixed solution and stirred and mixed a third time to obtain the coating slurry containing the piperidine polymer.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] In this invention, the three-stage stirring and mixing is carried out under vacuum oscillation.

[0079] 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.

[0080] In this invention, the above-mentioned stirring and blending is carried out using a planetary stirring device.

[0081] 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.

[0082] In this invention, the microgravure printing coating is a double-sided coating.

[0083] 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°.

[0084] 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; the coating speed can be, for example, 12m / min, 15m / min, 20m / min, 25m / min, 30m / min, or 32m / min; the printing gap can be, for example, 0.1mm, 0.15mm, 0.2mm, 0.25mm, or 0.3mm; and the squeegee angle can be, for example, 42°, 45°, 50°, 55°, 60°, or 65°, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0085] In this invention, after the coating slurry containing piperidine polymer is applied to 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 90℃, for example, it can be 40℃, 50℃, 60℃, 70℃, 80℃ or 90℃, 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 40% to 55%, for example, it can be 40%, 45%, 50% or 55%, 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, by using a piperidine-based polymer as a coating material, possesses the following core advantages: (1) The piperidine heterocycles contained in the polymer structure provide a rigid framework and good thermal stability, and can also weakly coordinate with metal ions through the lone pair electrons of nitrogen atoms, thereby promoting the dissociation of metal salts and their ion conduction rate; (2) The piperidine-based polymer utilizes polar groups capable of forming hydrogen bonds to form hydrogen bonds with the solvent in the electrolyte, significantly enhancing the wettability of the membrane and the electrolyte absorption rate, while providing more transport channels for ion transport. In addition, the aforementioned polar groups can also form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based negative electrode material, thereby improving the bonding force between the composite membrane and the negative electrode sheet; (3) The ester groups in the polymer structure improve the structural stability and reliability of the composite membrane under high-temperature conditions.

[0105] In summary, this invention, by controlling the composition of the membrane coating material and the types of its active groups, enables its application in secondary batteries to comprehensively improve fast charging performance, high-temperature performance, and safety. Detailed Implementation

[0106] 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.

[0107] Example 1

[0108] This embodiment provides a composite membrane, comprising a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material comprises a piperidinyl 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 comprise a combination of boehmite and TiO2 in a mass ratio of 1:2.5, and the boehmite has a particle size D. 50 The particle size D of TiO2 is 10 nm. 50 It has a diameter of 33nm, a pore size of 5nm, and a mesoporosity of 85%.

[0109] The monomer of the piperidinyl polymer is the compound shown in Formula I, and the weight-average molecular weight of the piperidinyl polymer is 180,000 Da.

[0110]

[0111] The polyethylene base film has a thickness of 9.5 μm, the coating has a thickness of 3 μm, and the total thickness of the composite diaphragm is 12.5 μm, with a porosity of 49%.

[0112] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0113] (1) The compound shown in Formula I was added to tetrahydrofuran, and then azobisisobutyronitrile initiator was added (the mass percentage of azobisisobutyronitrile initiator was 0.5% based on the total mass of the compound shown in Formula I as 100%). The mixture was heated to 92°C under argon protection and reacted for 24 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 piperidinyl polymer.

[0114] Sodium silicate and pure water are first stirred and mixed once, and then a combination of boehmite and TiO2 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, piperidine 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 piperidine polymer.

[0115] The mass ratio of piperidinyl 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 process 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 process 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 process 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.

[0116] (2) The coating slurry containing piperidine 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 65°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.

[0117] Example 2

[0118] This embodiment provides a composite membrane, comprising a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material comprises a piperidinyl 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 comprise a combination of boehmite and TiO2 in a mass ratio of 1:2, and the boehmite has a particle size D. 50 The particle size D of TiO2 is 5 nm. 50It has a wavelength of 20 nm, a pore size of 3 nm, and a mesoporosity of 81%.

[0119] The monomer of the piperidinyl polymer is the compound shown in Formula I, and the weight-average molecular weight of the piperidinyl polymer is 110,000 Da.

[0120]

[0121] The polyethylene base film has a thickness of 6 μm, the coating has a thickness of 1 μm, and the total thickness of the composite diaphragm is 7 μm, with a porosity of 42%.

[0122] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0123] (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 32 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 piperidinyl polymer.

[0124] Sodium silicate and pure water are first stirred and mixed once, and then a combination of boehmite and TiO2 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, piperidine 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 piperidine polymer.

[0125] The mass ratio of piperidinyl 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.

[0126] (2) The coating slurry containing piperidine 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.

[0127] Example 3

[0128] This embodiment provides a composite membrane, comprising a polyethylene base membrane and a coating disposed on one side of the polyethylene base membrane. The coating material comprises a piperidinyl 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 boehmite and TiO2 in a mass ratio of 1:3, and the boehmite has a particle size D. 50 The particle size D of TiO2 is 15 nm. 50 It has a wavelength of 45 nm, a pore size of 7 nm, and a mesoporosity of 88%.

[0129] The monomer of the piperidinyl polymer is the compound shown in Formula I, and the weight-average molecular weight of the piperidinyl polymer is 250,000 Da.

[0130]

[0131] 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 55%.

[0132] This embodiment also provides a method for preparing the above-mentioned composite membrane, which includes the following steps:

[0133] (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 124°C under argon protection and reacted for 15 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 piperidinyl polymer.

[0134] Sodium silicate and pure water are first stirred and mixed once, and then a combination of boehmite and TiO2 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, piperidine 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 piperidine polymer.

[0135] The mass ratio of piperidinyl 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 blend 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 blend 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 blend 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.

[0136] (2) The coating slurry containing piperidine 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 200 lines / cm, coating speed of 32m / min, printing gap of 0.3mm, and doctor blade angle of 65°. Then, it is pulled by traction roller to the drying equipment and dried at 90°C for 1min and then wound up. The winding tension is 15N / m and the wind speed is 25m / s to obtain the composite diaphragm.

[0137] Example 4

[0138] 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 III:

[0139]

[0140] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0141] Example 5

[0142] The difference between this embodiment and Embodiment 1 is that the mass ratio of boehmite to TiO2 is 1:1. The preparation process is the same as in Embodiment 1 except for adjusting the mass ratio of boehmite to TiO2.

[0143] Example 6

[0144] The difference between this embodiment and Embodiment 1 is that the mass ratio of boehmite to TiO2 is 1:5. The preparation process is the same as in Embodiment 1 except for adjusting the mass ratio of boehmite to TiO2.

[0145] Example 7

[0146] The difference between this embodiment and Example 1 is that the weight-average molecular weight of the piperidinyl polymer is 55,000 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.

[0147] Example 8

[0148] The difference between this embodiment and Example 1 is that the weight-average molecular weight of the piperidinyl polymer is 300,000 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.

[0149] Example 9

[0150] The difference between this embodiment and Embodiment 1 is that TiO2 is replaced with an equal mass of boehmite. The preparation process is the same as in Embodiment 1 except that TiO2 is replaced with an equal mass of boehmite.

[0151] Example 10

[0152] The difference between this embodiment and Embodiment 1 is that the pore size of TiO2 is adjusted to 1 nm. The preparation process is the same as in Embodiment 1 except that the pore size of TiO2 is adjusted to 1 nm.

[0153] Comparative Example 1

[0154] This comparative example only provides one polyethylene diaphragm with the same thickness as in Example 1.

[0155] Comparative Example 2

[0156] The difference between this comparative example and Example 1 is that the compound shown in Formula I is replaced with an equal mass of tert-butyl 4-allylpiperidine-1-carboxylate (CAS No.: 206446-47-1), with the structure shown below:

[0157]

[0158] The preparation process is the same as in Example 1, except for the replacement of the type of polymerizable monomer.

[0159] Comparative Example 3

[0160] The difference between this comparative example and Example 1 is that the compound shown in Formula I is replaced with an equal mass of ethyl 4-allyl-4-piperidinecarboxylate hydrochloride (CAS No.: 1186663-51-3), as shown below:

[0161]

[0162] The preparation process is the same as in Example 1, except that the type of polymerizable monomer is replaced.

[0163] The membranes provided in the above embodiments and comparative examples are used to prepare lithium-ion batteries, specifically including the following steps:

[0164] (1) Preparation of positive electrode sheet

[0165] 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.

[0166] (2) Preparation of negative electrode sheet

[0167] 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.

[0168] (3) Selection of electrolyte

[0169] 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.

[0170] (4) Preparation of lithium-ion batteries

[0171] 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.

[0172] Performance testing

[0173] 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:

[0174] (1) Initial Coulomb efficiency

[0175] 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.

[0176] 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%.

[0177] (2) Capacity retention rate after 1000 cycles at room temperature (1°C / 2°C)

[0178] 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.

[0179] 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.

[0180] (3) Room temperature 6C rate performance - constant current charge ratio

[0181] 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%.

[0182] (4) Thermal shrinkage rate of diaphragm at 150℃ / 30min

[0183] 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.

[0184] 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.

[0185] Sample preparation:

[0186] Cut to size: 100mm × 100mm, 10mm away from the edge of the diaphragm;

[0187] 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.

[0188] Test steps:

[0189] Pretreatment: The sample was placed in an environment of 23±2℃ and 50±5%RH for 24 hours.

[0190] High-temperature treatment:

[0191] 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.

[0192] Size measurement:

[0193] 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.

[0194] Heat shrinkage rate (%) = (L0-L1) / L0×100%; record the transverse (TD) and longitudinal (MD) shrinkage rates respectively.

[0195] (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.

[0196] (6) Bond strength:

[0197] Sample preparation: The composite separator was cut into 2cm×10cm pieces, and the silicon-carbon negative electrode sheet (silicon content 40%) was cut into the same size; the two were hot-pressed at 80℃ and 0.5MPa pressure for 30min to form a "negative electrode-separator" composite structure.

[0198] Test parameters:

[0199] Tensile testing machine model: Instron 5967;

[0200] Peeling angle: 90°, stretching speed: 50mm / min;

[0201] Record the average tensile force (N) during the peeling process and calculate the bond strength:

[0202] Bond strength (N / m) = average tensile force / sample width (0.02m).

[0203] Parallel test: 5 samples were tested in each group and the average value was taken. The higher the bonding strength, the stronger the bonding force between the composite diaphragm and the negative electrode.

[0204] Table 1

[0205]

[0206]

[0207]

[0208] As shown in Table 1, compared to Comparative Example 1 without modification, the composite separators provided in Examples 1-4 of this invention, due to the use of piperidine-based polymers with specific structures as coating materials, possess excellent thermal stability, ion conductivity, and wettability. Furthermore, the specific types of polar groups in the piperidine-based polymer can form hydrogen bonds with the hydroxyl groups on the surface of the silicon-based anode material, thereby enhancing the bonding force between the composite separator and the anode plate. This results in the assembled lithium-ion battery exhibiting high initial coulombic efficiency, fast charging capability, excellent cycle stability, and high-temperature safety performance.

[0209] Comparing Example 1 with Examples 5-6 and Example 9, it can be seen that the type and content of inorganic oxides in the coating affect the thermal stability, mechanical strength, ion transport performance and liquid absorption of the composite membrane.

[0210] Comparing Example 1 with Examples 7-8, it can be seen that the molecular weight of the piperidine polymer has a certain influence on the overall performance of the composite separator. If the molecular weight of the piperidine polymer is too low or too high, the strength and adhesion of the composite separator will decrease, and the overall performance of the assembled lithium-ion battery will decrease compared with Example 1.

[0211] Comparing Example 1 with Comparative Examples 2-3, it can be seen that the piperidine ring, ester group and hydrogen-bonding group in the piperidine 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.

[0212] 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 separator, characterized by, The composite separator comprises a base film and a coating layer provided on at least one side of the base film, and the material of the coating layer comprises a piperidyl polymer, the monomer structure of the piperidyl polymer comprising a piperidine ring, at least one ester group, at least one alkenyl group and a hydrogen bond forming group.

2. The composite separator of claim 1, wherein The molar ratio of the piperidine ring, the ester group and the hydrogen bond forming group is (0.9-1.1):(0.9-1.1):(0.9-1.1), preferably 1:1:1; Preferably, the ester group is a tert-butyl ester group. Preferably, the hydrogen bond forming group comprises a hydroxyl group and / or a carboxyl group.

3. The composite separator according to claim 1 or 2, characterized in that, The monomer structure of the piperidyl polymer is shown in Formula 1: wherein R1, R2 are each independently selected from C2-C5 alkenyl or hydroxyl-substituted C1-C4 alkyl, and R1 and R2 are different, and R3 is selected from tert-butyl.

4. The composite separator of claim 3, wherein The monomer of the piperidyl polymer is at least one of the following compounds:

5. The composite separator according to any one of claims 1 to 4, wherein The weight average molecular weight of the piperidyl polymer is 110000 Da-250000 Da.

6. The composite separator according to any one of claims 1 to 5, wherein The material of the coating layer further comprises an inorganic oxide; Preferably, in the coating layer, the piperidyl polymer is coated on the surface of the inorganic oxide; Preferably, the inorganic oxide comprises boehmite and / or TiO2, preferably a combination of boehmite and TiO2; Preferably, the boehmite has a particle size D 50 from 5 nm to 15 nm; Preferably, the TiO2 has a particle size D 50 from 20 nm to 45 nm; Preferably, the pore size of the TiO2 is 3 nm-7 nm; Preferably, the mesoporous rate of the TiO2 is greater than 80%; Preferably, when the inorganic oxide comprises a combination of boehmite and TiO2, the mass percentage of the boehmite is 20%-35% based on the total mass of the inorganic oxide; Preferably, when the inorganic oxide comprises a combination of boehmite and TiO2, the mass percentage of the TiO2 is 65%-80% based on the total mass of the inorganic oxide; Preferably, when the inorganic oxide comprises a combination of boehmite and TiO2, the mass ratio of the boehmite to TiO2 is 1:(2-3); Preferably, the material of the coating layer further comprises a wetting agent, a dispersant and a binder; Preferably, the mass ratio of the piperidyl polymer, the inorganic oxide, the wetting agent, the dispersant and the binder in the material of the coating layer is (20-35):(5-18):(0.02-0.08):(0.1-0.3):(2-6); Preferably, the single layer thickness of the coating layer is 1 μm-5 μm; Preferably, the porosity of the composite separator is 42%-55%.

7. A method of preparing the composite separator according to any one of claims 1 to 6, characterized by, The method comprises the following steps: coating a coating layer slurry comprising a piperidyl polymer on at least one side of a base film to obtain the composite separator; wherein the monomer structure of the piperidyl polymer comprises a piperidine ring, at least one ester group, at least one alkenyl group and a hydrogen bond forming group.

8. The method of claim 7, wherein, The coating layer slurry further comprises an inorganic oxide; Preferably, the coating layer slurry further comprises a first solvent, a wetting agent, a dispersant, a second solvent and a binder; Preferably, the mass ratio of the piperidyl 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); Preferably, the coating method comprises microgravure coating; Preferably, the parameters of the microgravure coating comprise a wire number of 150-200 lines / cm, a coating speed of 12-32 m / min, a printing gap of 0.1-0.3 mm, and a doctor blade angle of 42-65°.

9. A secondary battery characterized by comprising: The secondary battery comprises a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator comprises the composite separator according to any one of claims 1-6 or the composite separator prepared by the preparation method of claims 7 or 8.

10. The secondary battery according to claim 9, characterized by The active material of the negative electrode comprises a silicon-carbon material; Preferably, the active material of the positive electrode comprises a ternary positive electrode material; Preferably, the electrolyte comprises an additive, wherein the additive comprises any one or a combination of at least two of tris(4-nitrophenyl)phosphate, vinyl sulfate, or lithium bisfluorosulfonylimide; Preferably, the total content of the additive in the electrolyte of the secondary battery is 0.5-7.5 wt%.

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