A modified separator and its preparation method in lithium-ion batteries

Abstract

This invention provides a modified separator, its preparation method, and a lithium-ion battery. The modified separator includes a base film and a modified coating on at least one surface of the base film. The modified coating includes a copolymer, and the monomers of the copolymer include a first monomer and a second monomer. The first monomer includes an alkenyl group, a polar-substituted phenyl group, a nitrogen-containing heterocycle, and a thiol group. The second monomer includes an alkenyl group, a bromine-substituted group, a nitrogen-containing heterocycle, and a thiol group. The modified separator of this invention is obtained by setting a modified coating on the surface of the base film. The copolymer in the modified coating is obtained by copolymerization of specific monomers. The nitrogen-containing heterocycle in the monomer can significantly improve the high-temperature resistance of the separator. The flame retardancy of the bromine-substituted group and the reduction stability of the thiol group synergistically improve the safety of the separator. The polar-substituted phenyl group can enhance the affinity of the separator for the electrolyte, thereby achieving improvements in thermal stability, wettability, and safety.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a modified separator, its preparation method, and lithium-ion batteries. Background Technology

[0002] The separator in a lithium-ion battery plays a crucial role in isolating the positive and negative electrodes and preventing short circuits. Its safety performance largely determines the overall safety of the battery. Currently, commonly used separator materials are mostly polyolefin materials, such as PP and PE separators. These materials have high porosity and high strength, but they also have low liquid absorption, poor thermal stability, and are prone to thermal runaway.

[0003] Furthermore, as the driving range of electric vehicles continues to increase, range anxiety is alleviated, but charging anxiety becomes prominent, making the development of fast charging a trend. Therefore, it is necessary to improve the fast charging performance of lithium-ion batteries. Silicon-based anode materials have low lithium insertion / extraction potentials and high theoretical specific capacity, and can avoid lithium plating on the surface during fast charging, making them suitable for fast charging systems. However, traditional separators have low liquid absorption, poor thermal stability, and poor safety performance, making them unsuitable for fast charging systems.

[0004] Existing technologies improve the performance of traditional separators by applying a coating to their surface. For example, CN106519742A discloses a flame-retardant ceramic-modified slurry and a lithium-ion battery separator coated with the slurry. The preparation method of the flame-retardant ceramic-modified slurry involves: pretreating the surface of inorganic particles and a flame retardant using a silane coupling agent to form a flame-retardant ceramic-modified powder; then stirring the powder evenly in a solvent, ball milling it, and sequentially adding a binder and a thickener while stirring at high speed to form the flame-retardant ceramic-modified slurry. The flame-retardant ceramic-modified slurry is then coated onto a polyolefin microporous membrane that has undergone low-temperature plasma surface modification treatment to form a lithium-ion battery separator. However, the above modification exhibits significant thermal shrinkage and weakened separator strength at high temperatures, and its resistance to silicon expansion stress is poor, making it unsuitable for fast-charging systems.

[0005] Based on the above research, there is a need to provide a modified diaphragm that can improve various properties such as thermal stability, wettability, and safety. Summary of the Invention

[0006] The purpose of this invention is to provide a modified separator, its preparation method, and a lithium-ion battery. The modified separator is obtained by setting a modified coating on the surface of a base membrane. The copolymer in the modified coating is obtained by copolymerization of a specific monomer. The nitrogen-containing heterocycle in the monomer can significantly improve the high-temperature resistance of the separator. The flame retardancy of the bromine substituent and the reduction stability of the thiol group synergistically improve the safety of the separator. The polar group-substituted phenyl group can enhance the affinity of the separator for the electrolyte, thereby achieving improvements in thermal stability, wettability, and safety.

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

[0008] In a first aspect, the present invention provides a modified membrane comprising a base membrane and a modified coating on at least one surface of the base membrane, the modified coating comprising a copolymer, wherein the monomers of the copolymer comprise a first monomer and a second monomer, the first monomer comprising an alkenyl group, a polar-substituted phenyl group, a nitrogen-containing heterocycle, and a thiol group, and the second monomer comprising an alkenyl group, a bromine-substituted group, a nitrogen-containing heterocycle, and a thiol group.

[0009] The modified coating of this invention comprises a copolymer obtained by copolymerizing two monomers. The nitrogen-containing heterocycles in the first and second monomers have high bond energy, thermal stability, and chemical corrosion resistance, which can significantly improve the high-temperature resistance of the modified separator and inhibit high-temperature shrinkage. The thiol groups have reduction stability. The reduction stability of the thiol groups and the flame retardancy of the bromine substituents synergistically improve the thermal safety of the modified separator, solving the problem of incompatibility between traditional flame retardants and electrolytes. Furthermore, the first monomer also includes phenyl groups substituted with polar groups. The polar groups can enhance the affinity between the separator and the electrolyte and accelerate ion transport. The phenyl groups can improve mechanical strength. Therefore, this invention achieves a triple improvement in the thermal stability, wettability, and safety of the modified separator by setting a modified coating including specific copolymers. It can be matched with fast charging systems and improve the fast charging performance, rate performance, thermal stability, and safety performance of the battery.

[0010] The flame-retardant mechanism of the bromine-substituent in this invention is that its low bond energy allows it to break and release Br·, which captures H· / HO· free radicals and undergoes the following reactions: Br· + H· → HBr (endothermic), HBr + HO· → Br· + H2O (cycle blocking), thus achieving gas-phase flame retardancy. In contrast, C-Cl has a high bond energy and requires a higher temperature to decompose, resulting in low flame-retardant efficiency. CF has an even higher bond energy and is difficult to decompose, so neither can achieve flame retardancy.

[0011] Preferably, in the polar group-substituted phenyl group, the polar group includes alkoxy or hydroxyl groups.

[0012] Preferably, the second monomer further includes a furan ring.

[0013] The second monomer of the present invention also includes a furan ring, whose conjugated structure with a nitrogen-containing heterocycle can enhance the mechanical strength of the membrane.

[0014] Preferably, the general structural formula of the first monomer is shown in Formula I):

[0015]

[0016] Wherein, R1 is selected from phenyl groups substituted with polar groups (e.g., phenyl groups substituted with alkoxy or phenyl groups substituted with hydroxy), R2 and R4 are independently selected from hydrogen, substituted or unsubstituted alkyl groups (e.g., any one of hydrogen, methyl, ethyl or isopropyl), and R3 is selected from substituted or unsubstituted alkyl groups (e.g., any one of methyl, ethyl or isopropyl).

[0017] Preferably, the general structural formula of the second monomer is shown in Formula II):

[0018]

[0019] R5 is selected from a furan ring, R6 and R8 are each independently selected from hydrogen, substituted or unsubstituted alkyl groups (e.g., any one of hydrogen, methyl, ethyl or isopropyl), and R7 is selected from substituted or unsubstituted alkyl groups (e.g., any one of methyl, ethyl or isopropyl).

[0020] For example, the first monomer includes (4-Allyl-5-(2-methoxyphenyl)-4H-1,2,4-triazol-3-thiol, CAS: 23195-30-4) (CAS: 88614-18-0) or Any one or at least two of the following (CAS: 80570-90-7):

[0021] For example, the second monomer includes (CAS: 522597-53-1).

[0022] Preferably, in the monomers of the copolymer, the molar ratio of the first monomer to the second monomer is (1-3):(2-5), for example, it can be 1:2, 1.5:3, 2:4, 2.5:4.5 or 3:5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0023] The molar ratio of the first monomer and the second monomer in this invention affects the overall performance of the modified separator. If there is too much of the first monomer and too little of the second monomer, there will be too few bromine substituents and furan groups, which will affect the flame retardant performance and mechanical properties, etc. If there is too little of the first monomer and too much of the second monomer, there will be too few phenyl groups substituted with polar groups, which will affect the affinity between the separator and the electrolyte, and affect the rate performance and fast charging performance of the battery, etc.

[0024] Preferably, the weight-average molecular weight of the copolymer is 90,000 Da to 180,000 Da, for example, it can be 90,000 Da, 100,000 Da, 110,000 Da, 120,000 Da, 130,000 Da, 140,000 Da, 150,000 Da, 160,000 Da, 170,000 Da or 180,000 Da, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] When the weight-average molecular weight of the copolymer described in this invention is too low, the film-forming properties and interfacial adhesion of the modified coating are insufficient, and the mechanical strength of the modified coating is insufficient. However, if the molecular weight of the copolymer is too high, the viscosity of the coating slurry during the preparation process will increase, the uniformity of the coating will decrease, and the brittleness of the modified coating will increase, thereby affecting the performance of the modified diaphragm.

[0026] Preferably, the thickness of the modified coating is 1μm to 3μm, for example, it can be 1μm, 1.5μm, 2μm, 2.5μm or 3μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] If the thickness of the modified coating in this invention is too small, the modified coating will not effectively improve the performance of the base film, and will not provide sufficient buffering and protection for the expansion of the negative electrode, resulting in limited performance improvement. However, if the thickness of the modified coating is too large, it will increase the total thickness and internal resistance of the modified separator, affecting electrolyte wetting and ion transport, reducing the battery energy density and power density, and increasing costs.

[0028] Preferably, the modified coating further includes inorganic materials.

[0029] Preferably, in the modified coating, the copolymer coats the surface of the inorganic material.

[0030] In the modified coating of this invention, the copolymer and inorganic materials work synergistically. The inorganic materials are embedded in the network of the copolymer, and the copolymer encapsulates the inorganic materials. The copolymer provides adhesion and stress buffering, while the inorganic particles provide rigid support and thermal barrier. Together, they resist high-temperature thermal shrinkage and silicon expansion stress, further reducing the thermal shrinkage rate of the modified membrane.

[0031] Preferably, the inorganic material comprises alkyl-modified hollow SiO2 nanospheres, and more preferably methyl-modified hollow SiO2 nanospheres.

[0032] This invention uses alkyl-modified hollow SiO2 nanospheres as inorganic materials, such as methyl-modified hollow SiO2 nanospheres (MHS), whose surface methyl-substituted hydroxyl groups can increase the contact angle of inorganic materials and reduce water absorption, thereby improving the stability of the modified membrane.

[0033] Preferably, the particle size D50 of the inorganic material is 15nm to 38nm, for example, it can be 15nm, 20nm, 25nm, 30nm, 35nm or 38nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] If the particle size D50 of the inorganic material in this invention is too large, the sedimentation rate of the inorganic material in the coating slurry will be too fast (Stokes' Law), which will result in the coating slurry being thinner at the top and thicker at the bottom, ultimately leading to uneven modified coating. If the particle size of the inorganic material is too small, the specific surface area of ​​the inorganic material will be too large, which will cause the inorganic material to spontaneously agglomerate into micron-sized clumps, affecting the uniformity of the distribution of the inorganic material in the modified coating.

[0035] Preferably, the modified coating further includes a first wetting agent, a first dispersant, and a binder.

[0036] Preferably, the mass ratio of the copolymer, inorganic material, first wetting agent, first dispersant and binder in the modified coating is (15-30):(10-23):(0.02-0.08):(0.1-0.3):(3-7), for example, it can be 15:23:0.02:0.3:7, 20:15:0.04:0.15:4, 25:20:0.06:0.2:6 or 30:10:0.08:0.1:3, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0037] Preferably, the first wetting agent comprises any one or a combination of at least two of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate.

[0038] Preferably, the first dispersant comprises any one or a combination of at least two of silicate compounds, sodium polyacrylate, or sodium citrate;

[0039] Preferably, the adhesive comprises any one or a combination of at least two of carboxymethyl cellulose, hydroxypropyl methyl cellulose, or polyvinyl alcohol.

[0040] Preferably, the porosity of the base membrane is 45% to 55%, for example, 45%, 50% or 55%, the air permeability is 85s / 100mL to 108s / 100mL, for example, 85s / 100mL, 90s / 100mL, 95s / 100mL, 100s / 100mL, 105s / 100mL or 108s / 100mL, and the thickness is 6μm to 9μm, for example, 6μm, 7μm, 8μm or 9μm, but not limited to the listed values, other unlisted values ​​within the range are also applicable.

[0041] If the thickness of the base film in this invention is too low, it will result in low mechanical strength; if the thickness of the base film is too high, it will result in a decrease in the energy density of the battery.

[0042] Preferably, the base film comprises a PI-based film (polyimide-based film).

[0043] The base film described in this invention is preferably a PI base film, which has high temperature resistance (breakage temperature up to 430°C), which can improve battery safety and increase the pass rate of needle penetration and hot box safety tests. In addition, the nitrogen-containing polar groups of PI can form hydrogen bonds with the electrolyte solvent (EC or other solvents), thereby improving the wettability of the modified separator and improving the rate performance.

[0044] Preferably, the thickness of the modified diaphragm is 7μm to 12μm, for example, it can be 7μm, 8μm, 9μm, 10μm, 11μm or 12μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0045] If the thickness of the modified membrane described in this invention is too small, the membrane is easily punctured; however, if the thickness of the modified membrane is too large, the ion transport distance increases and the rate performance decreases.

[0046] Preferably, the porosity of the modified membrane is 38% to 48%, for example, it can be 38%, 40%, 42%, 44%, 46% or 48%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] When the porosity of the modified diaphragm described in this invention is too large, the mechanical strength decreases and the high-temperature shrinkage rate increases. If the porosity of the modified diaphragm is too low, the liquid absorption rate is low and the rate performance deteriorates.

[0048] In a second aspect, the present invention provides a method for preparing the modified membrane as described in the first aspect, the method comprising the following steps:

[0049] The modified diaphragm is obtained by coating a modified coating slurry onto at least one side of the base membrane;

[0050] The modified coating slurry includes a copolymer, the monomers of which include a first monomer and a second monomer. The first monomer includes an alkenyl group, a polar substituted phenyl group, a nitrogen-containing heterocycle, and a thiol group. The second monomer includes an alkenyl group, a bromine substituent, a nitrogen-containing heterocycle, and a thiol group.

[0051] Preferably, the modified coating slurry further includes inorganic materials.

[0052] Preferably, the modified coating slurry further includes a first solvent, a first wetting agent, a first dispersant, a second solvent, and a binder.

[0053] Preferably, the first solvent includes pure water.

[0054] Preferably, the second solvent comprises isopropanol.

[0055] Preferably, the mass ratio of the copolymer, inorganic material, first solvent, first wetting agent, first dispersant, second solvent, and binder is (15-30):(10-23):(40-66):(0.02-0.08):(0.1-0.3):(1-4):(3-7), for example, it can be 15:23:40:0.02:0.3:1:7, 20:15:50:0.04:0.15:2:4, 25:20:60:0.06:0.2:3:6 or 30:10:66:0.08:0.1:4:3, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0056] Preferably, the preparation method of the modified coating slurry includes the following steps:

[0057] First, the first dispersant and the first solvent are stirred and mixed once, and then inorganic materials are added and stirred and mixed a second time to obtain a mixed solution. Then, the second solvent, binder, copolymer and first wetting agent are added to the mixed solution and stirred and mixed under vacuum to obtain the modified coating slurry.

[0058] Preferably, ultrasonication is performed simultaneously during the secondary stirring and mixing process.

[0059] Preferably, the rotation speed of the primary mixing process is 1000 r / min to 2000 r / min, for example, 1000 r / min, 1500 r / min or 2000 r / min; the revolution speed is 40 r / min to 60 r / min, for example, 40 r / min, 50 r / min or 60 r / min; and the mixing time is 46 min to 70 min, for example, 46 min, 50 min, 60 min or 70 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] Preferably, the rotation speed of the secondary mixing is 3200 r / min to 3800 r / min, for example, 3200 r / min, 3300 r / min, 3600 r / min or 3800 r / min, the revolution speed is 10 r / min to 30 r / min, for example, 10 r / min, 20 r / min or 30 r / min, and the mixing time is 30 min to 50 min, for example, 30 min, 40 min or 50 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0061] Preferably, the coating method includes dot-coating.

[0062] This invention employs a dot-coating method combined with a high-porosity base membrane to prepare a modified separator with high air permeability and high porosity, which in turn results in lower impedance, thus greatly improving the lithium-ion transport rate and enhancing the rate performance of the battery.

[0063] Preferably, the coating temperature is 40℃ to 90℃, for example, 40℃, 50℃, 60℃, 70℃, 80℃ or 90℃, and the coating stretching rate difference is 0.1% to 10%, for example, 0.1%, 1%, 3%, 5%, 7%, 9% or 10%, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0064] Preferably, the coating process is followed by rewinding and slitting.

[0065] Preferably, the rewinding temperature is 60℃~110℃, for example, it can be 60℃, 80℃, 100℃ or 110℃, and the winding tension is 0.1N~50N, for example, it can be 0.1N, 1N, 10N, 20N, 30N, 40N or 50N, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] Preferably, the unwinding and rewinding tension of the slitting is 0.1N to 20N, for example, it can be 0.1N, 1N, 5N, 10N, 15N or 20N, and the contact pressure is 0.01N to 16N, for example, it can be 0.01N, 0.1N, 1N, 5N, 10N or 16N, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0067] Preferably, the method for preparing the copolymer includes the following steps:

[0068] The first monomer, the second monomer, the initiator, and the organic solvent are mixed and reacted. After the reaction, the resulting solution is added to the precipitation solvent to obtain the polymer.

[0069] In this invention, copolymerization occurs at the carbon-carbon double bonds connected to R3 and R7 in the first and second monomers.

[0070] Preferably, the molar ratio of the first monomer to the second monomer is (1-3):(2-5), for example, it can be 1:2, 1.5:3, 2:4, 2.5:4.5 or 3:5, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0071] Preferably, the reaction temperature is 60℃ to 120℃, for example, 60℃, 70℃, 80℃, 90℃, 100℃, 110℃ or 120℃, and the time is 15h to 36h, for example, 15h, 20h, 25h, 30h, 35h or 36h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0072] Preferably, the reaction is carried out under an inert gas protection environment, such as argon protection.

[0073] Preferably, the organic solvent includes any one or a combination of at least two of benzene, THF (tetrahydrofuran), NMP (N-methylpyrrolidone), or DMF (N,N-dimethylformamide).

[0074] Preferably, the initiator includes an azo initiator and / or a peroxide initiator, wherein the azo initiator includes azobisisobutyronitrile and / or azobisisoheptanenitrile, and the peroxide initiator includes benzoyl peroxide.

[0075] Preferably, the amount of the initiator added is 0.2% to 1% of the total mass of the first monomer and the second monomer, for example, it can be 0.2%, 0.4%, 0.6%, 0.8% or 1%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0076] Preferably, the precipitation solvent includes any one or a combination of at least two of propanol, isopropanol, or acetone.

[0077] Preferably, after the reaction, the resulting solution is added to the precipitation solvent, the copolymer precipitates, and then the copolymer is obtained after washing and drying.

[0078] Preferably, the method for preparing the base film includes the following steps:

[0079] The base film is obtained by mixing, casting, drying, and removing the pore-forming agent, along with the third solvent, second wetting agent, second dispersant, and pore-forming agent.

[0080] Preferably, the solid-liquid ratio of the base film material and the third solvent is (250-350)g:(40-90)mL, wherein (250-350)g can be, for example, 250g, 300g or 350g, and (40-90)mL can be, for example, 40mL, 60mL, 80mL or 90mL, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0081] Preferably, the third solvent comprises NMP (N-methylpyrrolidone).

[0082] Preferably, the second wetting agent comprises DIG 270.

[0083] Preferably, the second dispersant comprises any one or a combination of at least two of the following: triethylhexylphosphate, sodium dodecyl sulfate, methylpentanol, cellulose derivatives, polyacrylamide, glucon, or fatty acid polyethylene glycol esters.

[0084] Preferably, the pore-forming agent comprises polyethylene glycol.

[0085] Preferably, the method of removing the pore-forming agent includes immersion and washing in hot water at a temperature of 100°C to 120°C, for example, 100°C, 110°C or 120°C, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0086] Preferably, mixing the base film material, the third solvent, the second wetting agent, the second dispersant, and the pore-forming agent includes: mixing the base film material and the third solvent, ultrasonicating, performing a first ball milling, then adding the second wetting agent and the second dispersant for a second ball milling, and then adding the pore-forming agent and continuing ball milling to obtain a slurry.

[0087] Preferably, the duration of the mixed ultrasound is 60 min to 90 min, for example, it can be 60 min, 70 min, 80 min or 90 min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0088] Preferably, the temperature of the first ball mill is 25℃ to 45℃, for example, 25℃, 35℃ or 45℃, the time is 6h to 9h, for example, 6h, 7h, 8h or 9h, and the speed is 500rpm to 900rpm, for example, 500rpm, 600rpm, 700rpm, 800rpm or 900rpm, but not limited to the listed values, other unlisted values ​​within the range are also applicable.

[0089] Preferably, the amount of the second wetting agent added is 1g to 3g, for example, it can be 1g, 2g or 3g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0090] Preferably, the amount of the second dispersant added is 3g to 6g, for example, it can be 3g, 4g, 5g or 6g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0091] Preferably, the speed of the second ball mill is 500 rpm to 700 rpm, for example, 500 rpm, 600 rpm or 700 rpm, and the time is 2h to 5h, for example, 2h, 3h, 4h or 5h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0092] Preferably, the amount of pore-forming agent added is 31g to 35g, for example, 31g, 33g or 35g, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0093] Preferably, the ball milling time is 2h to 4h, for example, it can be 2h, 3h or 4h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0094] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the modified separator as described in the first aspect, or the modified separator prepared by the preparation method described in the second aspect.

[0095] Preferably, the negative electrode material of the lithium-ion battery includes silicon-carbon material.

[0096] Preferably, the silicon content in the silicon-carbon material is 42% to 52%, for example, it can be 42%, 45%, 50% or 52%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0097] In the silicon-carbon material of this invention, if the silicon content is too high, the expansion will be large; if the silicon content is too low, the volumetric energy will be reduced and the rate performance will be poor.

[0098] Preferably, the particle size D50 of the silicon carbide material is 5μm to 10μm, for example, it can be 5μm, 7μm, 9μm or 10μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0099] If the particle size D50 of the silicon-carbon material of this invention is too large, the rate performance will be reduced; if the particle size D50 is too small, the silicon-carbon material will be difficult to disperse.

[0100] Preferably, the electrolyte of the lithium-ion battery includes additives, which include any one or a combination of at least two of DTD (ethylene sulfate), LiPO2F2 (lithium difluorophosphate) or TMSP (tris(trimethylsilane)phosphate).

[0101] The present invention also adds additives to the electrolyte, which can further reduce the interfacial impedance with the modified diaphragm and play a synergistic role.

[0102] Preferably, the content of the additive in the electrolyte of the lithium-ion battery is 0.2wt% to 0.8wt%, for example, it can be 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt% or 0.8wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0103] Preferably, the electrolyte further includes a non-aqueous solvent and a lithium salt.

[0104] Preferably, the non-aqueous solvent includes EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), and FEC (fluoroethylene carbonate).

[0105] Preferably, the concentration of lithium salt in the electrolyte is 0.9 mol / L to 1.3 mol / L, for example, it can be 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, 1.2 mol / L or 1.3 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0106] Compared with the prior art, the present invention has the following beneficial effects:

[0107] The modified coating of this invention comprises a copolymer obtained by copolymerizing two monomers. The nitrogen-containing heterocycles in the first and second monomers have high bond energy, thermal stability, and chemical corrosion resistance, which can significantly improve the high-temperature resistance of the modified separator and inhibit high-temperature shrinkage. The thiol groups have reduction stability. The reduction stability of the thiol groups and the flame retardancy of the bromine substituents synergistically improve the thermal safety of the modified separator, solving the problem of incompatibility between traditional flame retardants and electrolytes. Furthermore, the phenyl groups substituted with polar groups in the first monomer can enhance the affinity between the separator and the electrolyte and accelerate ion transport. Therefore, this invention achieves a triple improvement in the thermal stability, wettability, and safety of the modified separator by setting a modified coating including specific copolymers. It can be matched with fast charging systems and improve the fast charging performance, rate performance, thermal stability, and safety performance of batteries. 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 modified diaphragm, which includes a base film (specifically a PI base film) and a modified coating on one side surface of the base film. The modified coating includes a copolymer in a mass ratio of 26:17:0.05:0.2:5, an inorganic material (specifically methyl-modified hollow SiO2 nanospheres, purchased from Ningbo Jinlei Nanomaterials Technology Co., Ltd.), a first wetting agent (specifically sodium tripolyphosphate), a first dispersant (specifically sodium polyacrylate), and a binder (specifically polyvinyl alcohol), wherein the particle size D50 of the inorganic material is 25 nm.

[0111] The copolymer comprises a first monomer and a second monomer in a molar ratio of 2:3.5, wherein the first monomer is... The second monomer is

[0112] The copolymer has a weight-average molecular weight of 130,000 Da; the modified coating has a thickness of 2 μm.

[0113] The base membrane has a porosity of 48%, an air permeability of 95 s / 100 mL, and a thickness of 7 μm.

[0114] The modified membrane has a thickness of 9 μm and a porosity of 42%.

[0115] The method for preparing the modified diaphragm includes the following steps:

[0116] (1) 300g of PI-based film resin and 65mL of NMP were ultrasonically mixed in a beaker for 75min, and then placed in a ball milling jar and ball milled at 35℃ and 750rpm for 7.5h. Then, 1.5g of second wetting agent (specifically, DIG 270) and 5g of second dispersant (specifically, sodium dodecyl sulfate) were added to the ball milling jar in sequence, and ball milled at 600rpm for 4h. Then, 32g of pore-forming agent (specifically, polyethylene glycol) was added and ball milled for another 3.5h to obtain a slurry. After casting and film formation using a casting coating machine, the film was dried in a drying zone and then washed away by three hot water immersions in a 110℃ hot water bath to remove the pore-forming agent, thus obtaining the base film.

[0117] (2) The first monomer and the second monomer are added to DMF, and then an initiator (specifically azobisisobutyronitrile) is added. The amount of initiator added is 0.5% of the total mass of the first monomer and the second monomer. Then, the mixture is heated to 90°C under argon protection and reacted for 26 hours to obtain a polymer solution. The polymer solution is added to isopropanol to obtain a polymer precipitate, which is then washed and dried to obtain the copolymer.

[0118] The first dispersant and water are stirred and mixed once, and then inorganic materials are added and stirred and mixed a second time to obtain a mixed solution. Isopropanol, binder, copolymer and first wetting agent are added to the mixed solution and stirred and mixed under vacuum to obtain a modified coating slurry.

[0119] The mass ratio of the copolymer, inorganic material, water, first wetting agent, first dispersant, isopropanol, and binder is 26:17:55:0.05:0.2:2.5:5; the rotation speed of the first stirring and mixing is 1500 r / min, the revolution speed is 50 r / min, and the stirring time is 55 min; ultrasonication is performed simultaneously during the second stirring and mixing process; the rotation speed of the second stirring and mixing is 3500 r / min, the revolution speed is 20 r / min, and the stirring time is 40 min.

[0120] (3) The modified coating slurry described in step (2) is applied to one side of the base film described in step (1) by dot coating. The coating temperature is 70°C and the coating stretching speed difference is 5%. Then, it is rewound at a temperature of 75°C and a winding tension of 25N. Finally, it is slit with a winding tension of 10N and a contact pressure of 7.5N to obtain the modified diaphragm.

[0121] Example 2

[0122] This embodiment provides a modified diaphragm, which includes a base film (specifically a PI base film) and a modified coating on one side surface of the base film. The modified coating includes a copolymer in a mass ratio of 15:10:0.08:0.3:3, an inorganic material (specifically methyl-modified hollow SiO2 nanospheres, purchased from Ningbo Jinlei Nanomaterials Technology Co., Ltd.), a first wetting agent (specifically sodium hexametaphosphate), a first dispersant (specifically sodium citrate), and a binder (specifically carboxymethyl cellulose), wherein the particle size D50 of the inorganic material is 38 nm.

[0123] The copolymer comprises a first monomer and a second monomer in a molar ratio of 1:2, wherein the first monomer is... The second monomer is

[0124] The copolymer has a weight-average molecular weight of 180,000 Da; the modified coating has a thickness of 1 μm.

[0125] The base membrane has a porosity of 55%, an air permeability of 108 s / 100 mL, and a thickness of 6 μm.

[0126] The modified membrane has a thickness of 7 μm and a porosity of 48%.

[0127] The method for preparing the modified diaphragm includes the following steps:

[0128] (1) 250g of PI-based film resin and 40mL of NMP were ultrasonically mixed in a beaker for 90min, and then placed in a ball milling jar and ball milled at 25℃ and 900rpm for 6h. Then, 3g of second wetting agent (specifically, DIG 270) and 3g of second dispersant (specifically, triethylhexyl phosphate) were added to the ball milling jar in sequence, and ball milled at 500rpm for 5h. Then, 35g of pore-forming agent (specifically, polyethylene glycol) was added and ball milled for another 4h to obtain a slurry. After casting and film formation by a casting coating machine, the film was dried in a drying zone and then washed away by hot water immersion in a 100℃ hot water bath three times to remove the pore-forming agent, thus obtaining the base film.

[0129] (2) The first monomer and the second monomer are added to DMF, and then an initiator (specifically benzoyl peroxide) is added. The amount of initiator added is 1% of the total mass of the first monomer and the second monomer. Then, the mixture is heated to 120°C under argon protection and reacted for 36 hours to obtain a polymer solution. The polymer solution is added to propanol to obtain a polymer precipitate. Then, the precipitate is washed and dried to obtain the copolymer.

[0130] The first dispersant and water are stirred and mixed once, and then inorganic materials are added and stirred and mixed a second time to obtain a mixed solution. Isopropanol, binder, copolymer and first wetting agent are added to the mixed solution and stirred and mixed under vacuum to obtain a modified coating slurry.

[0131] The mass ratio of the copolymer, inorganic material, water, first wetting agent, first dispersant, isopropanol, and binder is 15:10:40:0.08:0.3:4:3; the rotation speed of the first stirring and mixing is 1000 r / min, the revolution speed is 60 r / min, and the stirring time is 70 min; ultrasonication is performed simultaneously during the second stirring and mixing process; the rotation speed of the second stirring and mixing is 3200 r / min, the revolution speed is 30 r / min, and the stirring time is 50 min.

[0132] (3) The modified coating slurry described in step (2) is applied to one side of the base film described in step (1) by dot coating. The coating temperature is 40°C and the coating stretching speed difference is 10%. Then, it is rewound at a temperature of 60°C and a winding tension of 50N. Finally, it is slit with a winding tension of 20N and a contact pressure of 5N to obtain the modified diaphragm.

[0133] Example 3

[0134] This embodiment provides a modified diaphragm, which includes a base film (specifically a PI base film) and a modified coating on one side surface of the base film. The modified coating includes a copolymer in a mass ratio of 30:23:0.02:0.1:7, an inorganic material (specifically methyl-modified hollow SiO2 nanospheres, purchased from Ningbo Jinlei Nanomaterials Technology Co., Ltd.), a first wetting agent (specifically sodium tripolyphosphate), a first dispersant (specifically sodium polyacrylate), and a binder (specifically carboxymethyl cellulose), wherein the particle size D50 of the inorganic material is 15 nm.

[0135] The copolymer comprises a first monomer and a second monomer in a molar ratio of 3:5, wherein the first monomer is... The second monomer is

[0136] The copolymer has a weight-average molecular weight of 90,000 Da; the modified coating has a thickness of 3 μm.

[0137] The base membrane has a porosity of 45%, an air permeability of 85s / 100mL, and a thickness of 9μm.

[0138] The modified membrane has a thickness of 12 μm and a porosity of 38%.

[0139] The method for preparing the modified diaphragm includes the following steps:

[0140] (1) 350g of PI-based film resin and 90mL of NMP were ultrasonically mixed in a beaker for 60min, and then placed in a ball milling jar and ball milled at 45℃ and 500rpm for 9h. Then, 1g of second wetting agent (specifically, DIG 270) and 6g of second dispersant (specifically, sodium dodecyl sulfate) were added to the ball milling jar in sequence, and ball milled at 700rpm for 2h. Then, 31g of pore-forming agent (specifically, polyethylene glycol) was added and ball milled for another 2h to obtain a slurry. After casting and film formation using a casting coating machine, the film was dried in a drying zone and then washed away by three hot water immersions in a 120℃ hot water bath to remove the pore-forming agent, thus obtaining the base film.

[0141] (2) The first monomer and the second monomer are added to DMF, and then an initiator (specifically azobisisobutyronitrile) is added. The amount of initiator added is 0.2% of the total mass of the first monomer and the second monomer. The mixture is then heated to 60°C under argon protection and reacted for 15 hours to obtain a polymer solution. The polymer solution is added to propanol, isopropanol and acetone to obtain a polymer precipitate. The precipitate is then washed and dried to obtain the copolymer.

[0142] The first dispersant and water are stirred and mixed once, and then inorganic materials are added and stirred and mixed a second time to obtain a mixed solution. Isopropanol, binder, copolymer and first wetting agent are added to the mixed solution and stirred and mixed under vacuum to obtain a modified coating slurry.

[0143] The mass ratio of the copolymer, inorganic material, water, first wetting agent, first dispersant, isopropanol, and binder is 30:23:66:0.02:0.1:1:7; the rotation speed of the first stirring and mixing is 2000 r / min, the revolution speed is 40 r / min, and the stirring time is 46 min; ultrasonication is performed simultaneously during the second stirring and mixing process; the rotation speed of the second stirring and mixing is 3800 r / min, the revolution speed is 10 r / min, and the stirring time is 30 min.

[0144] (3) The modified coating slurry described in step (2) is applied to one side of the base film described in step (1) by dot coating. The coating temperature is 90°C and the coating stretching speed difference is 1%. Then, it is rewound at a temperature of 110°C and a winding tension of 10N. Finally, it is slit with a winding tension of 3N and a contact pressure of 16N to obtain the modified diaphragm.

[0145] Example 4

[0146] This embodiment provides a modified separator, wherein the modified separator, except that the first monomer is... Except for the above, everything else is the same as in Example 1.

[0147] The preparation method of the modified membrane is the same as that in Example 1, except for the change in the type of the first monomer.

[0148] Example 5

[0149] This embodiment provides a modified membrane, which is the same as that in Embodiment 1 except that the molar ratio of the first monomer and the second monomer is 1:6.

[0150] The preparation method of the modified membrane is the same as that in Example 1, except for the change in the molar ratio of the first monomer and the second monomer.

[0151] Example 6

[0152] This embodiment provides a modified membrane, which is the same as that in Embodiment 1 except that the molar ratio of the first monomer to the second monomer is 4:1.

[0153] The preparation method of the modified membrane is the same as that in Example 1, except for the change in the molar ratio of the first monomer and the second monomer.

[0154] Example 7

[0155] This embodiment provides a modified membrane, which is the same as that in Example 1 except that the weight average molecular weight of the copolymer is 80,000 Da.

[0156] The preparation method of the modified diaphragm is the same as that in Example 1, except for the change in reaction time during copolymer preparation.

[0157] Example 8

[0158] This embodiment provides a modified membrane, which is the same as that in Example 1 except that the weight average molecular weight of the copolymer is 190,000 Da.

[0159] The preparation method of the modified diaphragm is the same as that in Example 1, except for the change in reaction time during copolymer preparation.

[0160] Example 9

[0161] This embodiment provides a modified diaphragm, which is the same as that in Embodiment 1 except that the thickness of the modified coating is 0.5 μm.

[0162] The preparation method of the modified diaphragm is the same as that in Example 1, except for the change in the coating thickness of the modified coating.

[0163] Example 10

[0164] This embodiment provides a modified diaphragm, which is the same as that in Embodiment 1 except that the thickness of the modified coating is 4 μm.

[0165] The preparation method of the modified diaphragm is the same as that in Example 1, except for the change in the coating thickness of the modified coating.

[0166] Example 11

[0167] This embodiment provides a modified diaphragm, which is the same as that in Embodiment 1 except that the modified coating does not contain inorganic materials.

[0168] The preparation method of the modified diaphragm is the same as that in Example 1, except that no inorganic materials are added during the preparation of the modified coating.

[0169] Comparative Example 1

[0170] This comparative example provides a diaphragm, which is the PI-based membrane from Example 1.

[0171] The preparation method of the diaphragm described in this comparative example is the same as the preparation method of the PI-based membrane in Example 1.

[0172] Comparative Example 2

[0173] This comparative example provides a modified diaphragm, which is identical to that in Example 1 except that the copolymer monomer in the modified coating is only the first monomer.

[0174] The preparation method of the modified diaphragm is the same as that in Example 1, except that no second monomer is added during the preparation of the copolymer.

[0175] Comparative Example 3

[0176] This comparative example provides a modified diaphragm, which is identical to that in Example 1 except that the copolymer monomer in the modified coating is only the second monomer.

[0177] The preparation method of the modified diaphragm is the same as that in Example 1, except that the first monomer is not added during the preparation of the copolymer.

[0178] The separators obtained in the above embodiments and comparative examples are used to prepare lithium-ion batteries. The preparation method includes the following steps:

[0179] (1) Preparation of positive electrode sheet

[0180] The ternary material NCM811(LiNi) 0.8 Co 0.1 Mn 0.1 O2, PVDF (polyvinylidene fluoride), SP (super-P conductive carbon black), and SWCNT (single-walled carbon nanotubes) are mixed and stirred evenly in a mass ratio of 94:3:2.9:0.1 to obtain a positive electrode slurry. The positive electrode slurry is then coated onto aluminum foil through a coating process, and after drying and cold pressing, a positive electrode sheet is obtained.

[0181] (2) Preparation of negative electrode sheet

[0182] Silicon-carbon anode material (48% silicon content, the remainder carbon), conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), PAA (polyacrylic acid), and SBR (styrene-butadiene rubber) were mixed and stirred evenly in a mass ratio of 92:2:0.5:3:2.5 to obtain a negative electrode slurry. The solid content was controlled at 30%. The negative electrode slurry was then coated onto a copper foil current collector through a coating process. After vacuum drying and cold pressing, a negative electrode sheet was obtained.

[0183] (3) Selection of electrolyte

[0184] The electrolyte consists of EC, PC, DMC, DEC and FEC in a volume ratio of 15:20:25:30:10, 1 mol / L LiPF6 and 0.5 wt% DTD.

[0185] (4) Preparation of lithium-ion batteries

[0186] 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.

[0187] Performance testing

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

[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] (2) 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] (3) Thermal shrinkage rate of diaphragm at 180℃ / 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 (180℃) 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 inert gas atmosphere; the sample clamp is a stainless steel frame to ensure that the sample shrinks freely without constraint; 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 180°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] (4) 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] (5) Cell heating chamber test 150℃ & 30min: After the single cell is fully charged (at 25℃, the lithium-ion battery after capacity division is charged with constant current and constant voltage to 4.2V at a rate of 1C, and the cut-off current is 0.05C), it is placed in a temperature chamber and heated from room temperature to 150±2℃ at a rate of 5℃ / min. After maintaining this temperature for 30min, heating is stopped and observed for 1h to see if an explosion or fire occurs.

[0208] (6) Puncture strength: Using a hardened stainless steel puncture needle (needle tip spherical radius 0.5 mm, diameter 1.0 mm), the needle penetrates a 50 mm diameter circular diaphragm sample at a speed of 50 mm / min on an electronic universal testing machine. The maximum force value (unit: gf) is recorded. The test temperature is 23±2℃ and the humidity is 50±10%. The average value of 5 tests is taken.

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

[0210] Table 1

[0211]

[0212]

[0213] As can be seen from Table 1:

[0214] As shown in Examples 1-11 and Comparative Example 1, the modified separator of the present invention has higher mechanical strength and lower thermal shrinkage rate compared with the unmodified base film. The lithium-ion battery prepared using this modified separator has excellent rate performance, fast charging performance, thermal stability, and safety performance. As shown in Examples 1 and Comparative Examples 2-3, the present invention uses two monomers in synergy to simultaneously improve the thermal stability, wettability, and safety of the modified separator. If only one monomer is used, the overall performance of the modified separator and the battery cannot be guaranteed. As shown in Examples 1 and Examples 5-6 It is known that the preferred molar ratio of the first monomer and the second monomer in this invention is within a specific range, which can enhance the synergistic effect between the two monomers, thereby improving the overall performance of the modified separator and the battery. As can be seen from Examples 1 and 7-10, the molecular weight of the copolymer and the thickness of the modified coating in this invention will affect the performance of the copolymer and the electrochemical performance of the modified separator and the battery. As can be seen from Examples 1 and 11, this invention preferably also adds inorganic materials to the modified coating, which can form an organic-inorganic interpenetrating structure in the modified coating, thereby improving the performance of the modified separator and the battery.

[0215] 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 modified separator, characterized by, The modified membrane includes a base membrane and a modified coating on at least one side surface of the base membrane. The modified coating includes a copolymer. The monomers of the copolymer include a first monomer and a second monomer. The first monomer includes an alkenyl group, a polar group-substituted phenyl group, a nitrogen-containing heterocycle, and a thiol group. The second monomer includes an alkenyl group, a bromine-substituted group, a nitrogen-containing heterocycle, and a thiol group. The general structural formula of the first monomer is shown in Formula I): ； Formula I); Wherein, R1 is selected from phenyl groups substituted with polar groups, R2 and R4 are independently selected from hydrogen, substituted or unsubstituted alkyl groups, and R3 is selected from substituted or unsubstituted alkyl groups; among the phenyl groups substituted with polar groups, the polar groups include alkoxy or hydroxyl groups; The general structural formula of the second monomer is shown in Formula II): ； Formula II); R5 is selected from a furan ring, R6 and R8 are independently selected from hydrogen, substituted or unsubstituted alkyl groups, and R7 is selected from substituted or unsubstituted alkyl groups.

2. The modified separator according to claim 1, characterized by, In the monomers of the copolymer, the molar ratio of the first monomer to the second monomer is (1~3):(2~5).

3. The modified separator of claim 1, wherein The copolymer has a weight-average molecular weight of 90,000 Da to 180,000 Da.

4. The modified separator of claim 1, wherein The thickness of the modified coating is 1μm to 3μm.

5. The modified separator of claim 1, wherein The modified coating also includes inorganic materials.

6. The modified separator of claim 5, wherein, In the modified coating, the copolymer coats the surface of the inorganic material.

7. The modified diaphragm according to claim 5, characterized in that, The inorganic material includes alkyl-modified hollow SiO2 nanospheres.

8. The modified diaphragm according to claim 7, characterized in that, The inorganic material is a methyl-modified hollow SiO2 nanosphere.

9. The modified diaphragm according to claim 5, characterized in that, The particle size D50 of the inorganic material is 15nm~38nm.

10. The modified diaphragm according to claim 1, characterized in that, The modified coating also includes a first wetting agent, a first dispersant, and a binder.

11. The modified diaphragm according to claim 10, characterized in that, The mass ratio of copolymer, inorganic material, first wetting agent, first dispersant and binder in the modified coating is (15~30):(10~23):(0.02~0.08):(0.1~0.3):(3~7).

12. The modified diaphragm according to claim 10, characterized in that, The first wetting agent includes any one or a combination of at least two of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate.

13. The modified diaphragm according to claim 10, characterized in that, The first dispersant includes any one or a combination of at least two of silicate compounds, sodium polyacrylate, or sodium citrate.

14. The modified diaphragm according to claim 10, characterized in that, The adhesive includes any one or a combination of at least two of carboxymethyl cellulose, hydroxypropyl methyl cellulose, or polyvinyl alcohol.

15. The modified diaphragm according to claim 1, characterized in that, The base membrane has a porosity of 45%~55%, an air permeability of 85s / 100mL~108s / 100mL, and a thickness of 6μm~9μm.

16. The modified diaphragm according to claim 1, characterized in that, The base film includes a PI base film.

17. The modified diaphragm according to claim 1, characterized in that, The thickness of the modified diaphragm is 7μm~12μm.

18. The modified diaphragm according to claim 1, characterized in that, The porosity of the modified diaphragm is 38%~48%.

19. A method for preparing the modified diaphragm as described in any one of claims 1-18, characterized in that, The preparation method includes the following steps: The modified diaphragm is obtained by coating a modified coating slurry onto at least one side of the base membrane; The modified coating slurry includes a copolymer, the monomers of which include a first monomer and a second monomer. The first monomer includes an alkenyl group, a polar substituted phenyl group, a nitrogen-containing heterocycle, and a thiol group. The second monomer includes an alkenyl group, a bromine substituent, a nitrogen-containing heterocycle, and a thiol group.

20. The preparation method according to claim 19, characterized in that, The modified coating slurry also includes inorganic materials.

21. The preparation method according to claim 19, characterized in that, The modified coating slurry also includes a first solvent, a first wetting agent, a first dispersant, a second solvent, and a binder.

22. The preparation method according to claim 21, characterized in that, The mass ratio of the copolymer, inorganic material, first solvent, first wetting agent, first dispersant, second solvent and binder is (15~30):(10~23):(40~66):(0.02~0.08):(0.1~0.3):(1~4):(3~7).

23. The preparation method according to claim 19, characterized in that, The coating method includes dot-coating.

24. The preparation method according to claim 19, characterized in that, The coating temperature is 40℃~90℃, and the coating stretching speed difference is 0.1%~10%.

25. The preparation method according to claim 19, characterized in that, The coating process was followed by rewinding and slitting.

26. The preparation method according to claim 25, characterized in that, The rewinding temperature is 60℃~110℃, and the winding and unwinding tension is 0.1N~50N.

27. The preparation method according to claim 25, characterized in that, The winding and unwinding tension of the slitting process is 0.1N~20N, and the contact pressure is 0.01N~16N.

28. A lithium-ion battery, characterized in that, The lithium-ion battery includes the modified separator as described in any one of claims 1-18, or the modified separator prepared by the preparation method described in any one of claims 19-27.

29. The lithium-ion battery according to claim 28, characterized in that, The negative electrode material of the lithium-ion battery includes silicon-carbon material.

30. The lithium-ion battery according to claim 29, characterized in that, The silicon content in the silicon-carbon material is 42% to 52%.

31. The lithium-ion battery according to claim 29, characterized in that, The particle size D50 of the silicon carbide material is 5μm~10μm.

32. The lithium-ion battery according to claim 28, characterized in that, The electrolyte of the lithium-ion battery includes additives, which include any one or a combination of at least two of DTD, LiPO2F2, or TMSP.

33. The lithium-ion battery according to claim 32, characterized in that, The content of the additive in the electrolyte of the lithium-ion battery is 0.2wt%~0.8wt%.

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