Composite diaphragm, preparation method thereof and lithium ion battery

Abstract

This invention provides a composite separator, its preparation method, and a lithium-ion battery. The composite separator includes a base film and a polymer coating on at least one surface of the base film. The polymer coating comprises a polymer, and the monomers of the polymer include halosulfonyl groups, rigid groups containing at least one benzene ring, at least one carbon-carbon double bond, and a carboxyl group. The halosulfonyl groups include chlorosulfonyl groups and / or bromosulfonyl groups. The composite separator of this invention improves the mechanical properties, thermal stability, and ion conductivity of the composite separator by setting a specific polymer coating on the surface of the base film. The polymer monomers in the polymer coating contain halosulfonyl groups to enhance the mechanical strength of the separator, carboxyl groups to enhance the wettability of the electrolyte, and rigid groups to enhance the thermal stability of the separator. This improves the rate performance, fast charging performance, thermal stability, and safety performance of the lithium-ion battery.

Description

Technical Field

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

[0002] With the development of the new energy field, the requirements for long battery life and fast charging of lithium-ion batteries are becoming increasingly stringent. To achieve high specific energy fast charging, it is necessary to improve the energy density and power density of lithium-ion batteries. In this regard, existing technologies typically use silicon-based anodes with high theoretical specific capacity to replace graphite anodes. Since graphite anodes are approaching the theoretical upper limit of 372 mAh / g, while silicon-based anodes have a theoretical specific capacity as high as 4200 mAh / g, replacing graphite anodes with silicon-based anodes can significantly improve the energy density of lithium-ion batteries. Furthermore, silicon-based anodes can also avoid lithium plating on the surface during fast charging.

[0003] As lithium-ion batteries develop towards high-energy-density fast charging, commercial polyolefin separators and traditional coated separators that match silicon-based anodes have the following problems: (1) Insufficient thermal stability, with significant thermal shrinkage at high temperatures; (2) Imbalance between mechanical strength and flame retardancy: rigid coatings (such as Al2O3 coatings) in the separator are puncture resistant but have poor flame retardancy, while flexible coatings (such as polyacrylic acid) are easily torn by silicon expansion stress and are not resistant to high temperatures; (3) Contradiction between ion conduction and separator wettability, low porosity leads to insufficient electrolyte wetting, resulting in high interfacial impedance and low lithium-ion conduction efficiency during fast charging; (4) Existence of thermal runaway: traditional separators are not resistant to high temperatures and are prone to combustion runaway at high temperatures.

[0004] Based on the above research, there is a need to provide a composite separator that possesses excellent mechanical properties, thermal stability, and ion conductivity, and can be matched with fast charging systems. Summary of the Invention

[0005] The purpose of this invention is to provide a composite separator, its preparation method, and a lithium-ion battery. The composite separator is provided with a specific polymer coating on the surface of a base membrane. The polymer monomers in the polymer coating contain halosulfonyl groups to improve the mechanical strength of the separator, carboxyl groups to enhance the wettability of the electrolyte, and rigid groups to improve the thermal stability of the separator, thereby improving the mechanical properties, thermal stability, and ion conductivity of the composite separator.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a composite membrane comprising a base membrane and a polymer coating on at least one surface of the base membrane, the polymer coating comprising a polymer, wherein the monomers of the polymer comprise a halosulfonyl group, a rigid group containing at least one benzene ring, at least one carbon-carbon double bond and a carboxyl group, the halosulfonyl group comprising a chlorosulfonyl group and / or a bromosulfonyl group.

[0008] This invention employs a polymer coating comprising a specific polymer, applied to at least one side of a base membrane. The halosulfonyl groups in the polymer monomers exhibit high reactivity and can be used in cross-linking reactions, enabling the polymer coating to form a three-dimensional network structure and improving the mechanical properties of the membrane. The carboxyl groups in the polymer monomers enhance electrolyte wettability and promote ion transport, while the rigid groups containing at least one benzene ring in the polymer monomers improve the thermal stability of the membrane. Therefore, the polymer coating of this invention not only improves the mechanical properties, thermal stability, and ion conduction performance of the composite membrane, but also enhances the rate performance, fast-charging performance, thermal stability, and safety performance of lithium-ion batteries prepared using this composite membrane.

[0009] The halosulfonyl group described in this invention is preferably a chlorosulfonyl group, which has a lower bond energy and is more likely to undergo homolytic cleavage to generate free radicals and achieve low-temperature crosslinking. However, since the chemical bond energy in the fluorosulfonyl group is higher, it requires >200℃ to break, and the fluorosulfonyl group (-SO2F) is easily hydrolyzed to generate HF that corrodes the electrode, the halosulfonyl group in this invention cannot be a fluorosulfonyl group, and is preferably a chlorosulfonyl group.

[0010] Preferably, the monomers of the polymer include rigid segments and flexible segments, wherein the rigid segments include halosulfonyl groups and rigid groups containing at least one benzene ring, and the flexible segments include at least one carbon-carbon double bond and a carboxyl group.

[0011] Preferably, the molar ratio of the flexible segment to the rigid segment is (1-3):1, for example, it can be 1:1, 2:1 or 3:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0012] This invention can improve the overall performance of composite membranes by adjusting the molar ratio of flexible and rigid segments to balance various aspects of the membrane's properties.

[0013] Preferably, the monomer structure of the polymer is shown in Formula I):

[0014]

[0015] Wherein, R includes at least one benzene ring, X includes Cl or Br, and m:n is (1-3):1, for example, it can be 1:1, 2:1 or 3:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0016] Preferably, the benzene ring in R further includes a substituent, which includes alkyl and / or alkoxy groups, such as any one of methyl, ethyl, or alkoxy.

[0017] Preferably, R comprises phenyl or naphthyl.

[0018] Preferably, the monomers of the polymer include (CAS: 17641-30-4) (CAS: 610258-89-4) (CAS: 221466-52-0) or Any one or at least two of the following (CAS: 881384-48-1):

[0019] Preferably, the weight-average molecular weight of the polymer is 80,000 Da to 140,000 Da, for example, it can be 80,000 Da, 90,000 Da, 100,000 Da, 110,000 Da, 120,000 Da, 130,000 Da or 140,000 Da, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0020] The weight-average molecular weight of the polymer described in this invention affects the performance of the polymer coating. If the weight-average molecular weight of the polymer is too low, the film-forming properties and interfacial adhesion of the polymer coating will be insufficient, and the mechanical strength will be insufficient. If the molecular weight of the polymer is too high, the viscosity of the polymer coating slurry will increase, the coating uniformity will decrease, and the high molecular weight polymer will have higher crystallinity and higher melting temperature, but it will increase the brittleness of the polymer coating, thereby affecting the performance of the composite membrane.

[0021] Preferably, the thickness of the polymer 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.

[0022] If the thickness of the polymer coating described in this invention is too small, the polymer coating will not cover the base film surface evenly, and the buffering and protection for the expansion of the silicon anode will be insufficient, resulting in limited improvement in thermal stability. However, if the thickness of the polymer coating is too large, it will increase the total thickness and internal resistance of the composite separator, reduce the energy density and power density of the battery, and affect electrolyte wetting and ion transport, while also increasing costs.

[0023] Preferably, the polymer coating further includes inorganic materials.

[0024] Preferably, in the polymer coating, the polymer coats the surface of the inorganic material.

[0025] The polymer coating of the present invention also includes inorganic materials, which are located in the network formed by the polymer. That is, the polymer coats the surface of the inorganic materials to form a strong organic-inorganic network. The polymer provides adhesion and stress buffering, while the inorganic particles provide rigid support and thermal barrier, which work together to resist high-temperature thermal shrinkage and silicon expansion stress, thereby reducing the thermal shrinkage rate of the diaphragm.

[0026] Preferably, the inorganic material includes ZrO2.

[0027] The preferred inorganic material of this invention includes ZrO2. As an inorganic rigid particle, ZrO2 can form an 'organic-inorganic interpenetrating network' with polymers. Its high thermal conductivity can quickly disperse local heat inside the battery and synergistically reduce the thermal shrinkage rate of the separator.

[0028] Preferably, the particle size D50 of the inorganic material is 10nm to 30nm, for example, it can be 10nm, 15nm, 20nm, 25nm or 30nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] If the particle size D50 of the inorganic material of this invention is too large, the large particles will settle quickly in the coating slurry (Stokes' Law), which will result in the coating slurry being thinner at the top and thicker at the bottom, ultimately leading to an uneven polymer coating. If the particle size of the inorganic material is too small, the specific surface area of ​​the inorganic material will be too large, and the inorganic material will spontaneously agglomerate into micron-sized clumps.

[0030] Preferably, the polymer coating further includes a first wetting agent, a first dispersant, and a binder.

[0031] Preferably, the mass ratio of polymer, inorganic material, first wetting agent, first dispersant and binder in the polymer 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.

[0032] Preferably, the first wetting agent comprises any one or a combination of at least two of sodium hexametaphosphate, sodium tripolyphosphate, or sodium pyrophosphate.

[0033] Preferably, the first dispersant comprises any one or a combination of at least two of silicate compounds, sodium polyacrylate, or sodium citrate.

[0034] Preferably, the adhesive comprises any one or a combination of at least two of carboxymethyl cellulose, hydroxypropyl methyl cellulose, or polyvinyl alcohol.

[0035] Preferably, the porosity of the base membrane is 45% to 55%, for example, 45%, 50% or 55%, the air permeability is 80s / 100mL to 100s / 100mL, for example, 80s / 100mL, 85s / 100mL, 90s / 100mL, 95s / 100mL or 100s / 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.

[0036] 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.

[0037] Preferably, the base film comprises a PI-based film (polyimide-based film).

[0038] The base film described in this invention is preferably a PI base film, which has high temperature resistance (film 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 composite separator and improving the rate performance.

[0039] Preferably, the thickness of the composite membrane 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.

[0040] If the thickness of the composite membrane described in this invention is too small, the membrane is easily punctured; however, if the thickness of the composite membrane is too large, the ion transport distance increases and the rate capability decreases.

[0041] Preferably, the porosity of the composite membrane is 40% to 50%, for example, it can be 40%, 42%, 44%, 46%, 48% or 50%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] When the porosity of the composite membrane described in this invention is too large, the mechanical strength of the composite membrane decreases and the high-temperature shrinkage rate increases. If the porosity of the composite membrane is too low, the liquid absorption rate is low and the rate performance deteriorates.

[0043] Preferably, the composite diaphragm has a puncture strength >700gf, for example, 700gf, 720gf, 740gf, 760gf, 780gf, 800gf or 820gf, a TD heat shrinkage rate ≤4%, for example, 4%, 3.5%, 3%, 2.5%, 2%, 1.5% or 1%, and a MD heat shrinkage rate ≤4%, for example, 4%, 3.5%, 3%, 2.5%, 2%, 1.5% or 1%, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0044] In a second aspect, the present invention provides a method for preparing the composite separator as described in the first aspect, the method comprising the following steps:

[0045] The composite membrane is obtained by coating a polymer coating slurry onto at least one side of the base membrane;

[0046] The polymer coating slurry includes a polymer, wherein the monomers of the polymer include a halosulfonyl group, a rigid group containing at least one benzene ring, at least one carbon-carbon double bond and a carboxyl group, wherein the halosulfonyl group includes a chlorosulfonyl group and / or a bromosulfonyl group.

[0047] Preferably, the polymer coating slurry further includes inorganic materials.

[0048] Preferably, the polymer coating slurry further includes a first solvent, a first wetting agent (whose main function is to reduce surface tension and enhance slurry fluidity), a first dispersant, a second solvent, and a binder.

[0049] Preferably, the first solvent includes pure water.

[0050] Preferably, the second solvent comprises isopropanol.

[0051] Preferably, the method for preparing the polymer coating slurry includes the following steps:

[0052] First, the first dispersant and the first solvent are stirred and mixed once, then inorganic materials are added and stirred and mixed a second time to obtain a mixed solution. Then, the second solvent, binder, polymer and the first wetting agent are added to the mixed solution and stirred and mixed under vacuum to obtain the polymer coating slurry.

[0053] Preferably, ultrasonication is performed simultaneously during the secondary stirring and mixing process.

[0054] 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.

[0055] 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.

[0056] Preferably, the mass ratio of the polymer, 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.

[0057] Preferably, the coating method includes dot-coating.

[0058] This invention employs a dot-coating method combined with a high-porosity base membrane to prepare a composite 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.

[0059] 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.

[0060] Preferably, the coating process is followed by rewinding and slitting.

[0061] 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.

[0062] 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.

[0063] Preferably, the method for preparing the polymer includes the following steps:

[0064] The polymer monomers, initiator, and organic solvent are mixed and reacted. After the reaction, the resulting solution is added to the precipitation solvent to obtain the polymer.

[0065] In the monomer of the polymer described in this invention, the groups that undergo polymerization include carbon-carbon double bonds near the carboxyl group on the flexible chain, the halosulfonyl group can undergo esterification reaction with the carboxyl group in the monomer or the hydroxyl group on the substrate surface; and crosslinking also occurs at the chlorosulfonyl group.

[0066] Preferably, the reaction temperature is 60℃ to 120℃, for example, 60℃, 70℃, 80℃, 90℃, 100℃, 110℃ or 120℃, and the time is 10h to 32h, for example, 10h, 15h, 20h, 25h, 30h or 32h, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0067] Preferably, the reaction is carried out under an inert gas protection environment, such as argon protection.

[0068] 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).

[0069] 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.

[0070] Preferably, the amount of initiator added is 0.2% to 1% of the monomer mass of the polymer, 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.

[0071] Preferably, the precipitation solvent includes any one or a combination of at least two of propanol, isopropanol, or acetone.

[0072] Preferably, after the reaction, the resulting solution is added to the precipitation solvent, the polymer precipitates, and then the polymer is obtained after washing and drying.

[0073] Preferably, the method for preparing the base film includes the following steps:

[0074] 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.

[0075] 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.

[0076] Preferably, the third solvent comprises NMP (N-methylpyrrolidone).

[0077] Preferably, the second wetting agent comprises DIG 270.

[0078] 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.

[0079] Preferably, the pore-forming agent comprises polyethylene glycol.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] Thirdly, the present invention provides a lithium-ion battery, the lithium-ion battery comprising a composite separator as described in the first aspect, or a composite separator prepared by the preparation method described in the second aspect.

[0090] Preferably, the negative electrode material of the lithium-ion battery includes silicon-carbon material.

[0091] Preferably, the silicon content in the silicon-carbon material is 45% to 55%, for example, it can be 45%, 50% or 55%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0092] 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.

[0093] Preferably, the particle size D50 of the silicon carbide material is 3μm to 7μm, for example, it can be 3μm, 4μm, 5μm, 6μm or 7μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0094] 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.

[0095] 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).

[0096] The present invention also adds additives to the electrolyte, which can further reduce the interfacial impedance with the composite membrane and play a synergistic role.

[0097] 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.

[0098] Preferably, the electrolyte further includes a non-aqueous solvent and a lithium salt.

[0099] Preferably, the non-aqueous solvent includes EC (ethylene carbonate), PC (propylene carbonate), DMC (dimethyl carbonate), DEC (diethyl carbonate), and FEC (fluoroethylene carbonate).

[0100] 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.

[0101] Compared with the prior art, the present invention has the following beneficial effects:

[0102] This invention employs a polymer coating comprising a specific polymer, applied to at least one side of a base membrane. The halosulfonyl groups in the polymer monomers exhibit high reactivity and can be used in cross-linking reactions, enabling the polymer coating to form a three-dimensional network structure and improving the mechanical properties of the membrane. The carboxyl groups in the polymer monomers enhance electrolyte wettability and promote ion transport, while the rigid groups containing at least one benzene ring in the polymer monomers improve the thermal stability of the membrane. Therefore, the polymer coating of this invention not only improves the mechanical properties, thermal stability, and ion conduction performance of the composite membrane, but also enhances the rate performance, fast-charging performance, thermal stability, and safety performance of lithium-ion batteries prepared using this composite membrane. Detailed Implementation

[0103] 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.

[0104] Example 1

[0105] This embodiment provides a composite membrane, which includes a base membrane (specifically a PI base membrane) and a polymer coating on one side of the base membrane. The polymer coating includes a polymer, ZrO2, sodium hexametaphosphate, sodium polyacrylate and carboxymethyl cellulose in a mass ratio of 25:15:0.06:0.2:4, wherein the particle size D50 of ZrO2 is 20 nm.

[0106] The monomer of the polymer is (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid, with the structural formula:

[0107] The polymer has a molecular weight of 110,000 Da, and the polymer coating has a thickness of 2 μm.

[0108] The base membrane has a porosity of 50%, an air permeability of 90s / 100mL, a thickness of 7.5μm, and is a PI-based membrane.

[0109] The composite membrane has a thickness of 9.5 μm and a porosity of 45%.

[0110] The method for preparing the composite membrane includes the following steps:

[0111] (1) 300g of PI-based film resin and 60mL of NMP were ultrasonically mixed in a beaker for 80min, and then placed in a ball milling jar and ball milled at 35℃ and 700rpm for 8h. Then, 2g of wetting agent (specifically, DIG 270) and 4g of dispersant (specifically, triethylhexyl phosphate) were added to the ball milling jar in sequence, and ball milled at 600rpm for 4h. Then, 33g of pore-forming agent (specifically, polyethylene glycol) was added and ball milled for another 3h 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 soaking in a hot water bath at 110℃ three times to remove the pore-forming agent, thus obtaining the base film.

[0112] (2) Add (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid to DMF, then add an initiator (specifically benzoyl peroxide), the amount of initiator added is 0.6% of the mass of (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid, then heat to 100°C under argon protection and react for 22 h to obtain a polymer solution. Add the polymer solution to propanol to obtain a polymer precipitate, then wash and dry to obtain the polymer.

[0113] Sodium polyacrylate and pure water are first stirred and mixed once, then ZrO2 is added and stirred and mixed again. During the above two stirring and mixing process, ultrasonication is performed simultaneously to obtain a mixed solution. Isopropanol, carboxymethyl cellulose, polymer and sodium hexametaphosphate are added to the mixed solution and vacuum oscillation and stirring are performed to prepare a coating slurry.

[0114] The mass ratio of the polymer, ZrO2, pure water, sodium hexametaphosphate, sodium polyacrylate, isopropanol, and carboxymethyl cellulose is 25:15:50:0.06:0.2:2.5:4; 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 60 min; 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.

[0115] (3) The 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 60°C and the coating stretching speed difference is 5%. Then, it is rewound at a temperature of 80°C and a winding tension of 30N. Finally, it is slit with a winding tension of 10N and a contact pressure of 8N to obtain the composite diaphragm.

[0116] Example 2

[0117] This embodiment provides a composite membrane, which includes a base membrane (specifically a PI base membrane) and a polymer coating on one side of the base membrane. The polymer coating includes a polymer, ZrO2, sodium pyrophosphate, sodium citrate and polyvinyl alcohol in a mass ratio of 15:10:0.08:0.3:3, wherein the particle size D50 of ZrO2 is 10 nm.

[0118] The monomer of the polymer is (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid, with the structural formula:

[0119] The polymer has a molecular weight of 140,000 Da, and the polymer coating has a thickness of 3 μm.

[0120] The base membrane has a porosity of 55%, an air permeability of 100s / 100mL, a thickness of 6μm, and is a PI base membrane.

[0121] The composite membrane has a thickness of 9 μm and a porosity of 50%.

[0122] The method for preparing the composite membrane includes the following steps:

[0123] (1) 250g of PI-based film resin and 40mL 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 wetting agent (specifically, DIG 270) and 6g of dispersant (specifically, sodium dodecyl sulfate) were added to the ball milling jar in sequence, and ball milled at 700rpm for 2h. Then, 35g 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 soaking in a hot water bath at 120℃ three times to remove the pore-forming agent, thus obtaining the base film.

[0124] (2) Add (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid to DMF, and then add an initiator (specifically azobisisobutyronitrile). The amount of initiator added is 1% of the mass of (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid. Then heat at 120°C under argon protection for 32 hours to obtain a polymer solution. Add the polymer solution to isopropanol to obtain a polymer precipitate, and then wash and dry it to obtain the polymer.

[0125] Sodium citrate and pure water are first stirred and mixed once, then ZrO2 is added and stirred and mixed again. During the above two stirring and mixing process, ultrasound is performed simultaneously to obtain a mixed solution. Isopropanol, polyvinyl alcohol, polymer and sodium pyrophosphate are added to the mixed solution and vacuum oscillation and stirring are performed to prepare a coating slurry.

[0126] The mass ratio of the polymer, ZrO2, pure water, sodium pyrophosphate, sodium citrate, isopropanol, and polyvinyl alcohol is 15:10:40:0.08:0.3:4:3; 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; 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.

[0127] (3) The 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 5N and a contact pressure of 1N to obtain the composite diaphragm.

[0128] Example 3

[0129] This embodiment provides a composite membrane, which includes a base membrane (specifically a PI base membrane) and a polymer coating on one side of the base membrane. The polymer coating includes a polymer, ZrO2, sodium hexametaphosphate, sodium polyacrylate and carboxymethyl cellulose in a mass ratio of 30:23:0.02:0.1:7, wherein the particle size D50 of ZrO2 is 30 nm.

[0130] The monomer of the polymer is (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid, with the structural formula:

[0131] The polymer has a molecular weight of 80,000 Da, and the polymer coating has a thickness of 1 μm.

[0132] The base membrane has a porosity of 45%, an air permeability of 80s / 100mL, a thickness of 9μm, and is a PI base membrane.

[0133] The composite membrane has a thickness of 10 μm and a porosity of 40%.

[0134] The method for preparing the composite membrane includes the following steps:

[0135] (1) 350g of PI-based film resin and 90mL 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 wetting agent (specifically, DIG 270) and 3g of dispersant (specifically, triethylhexyl phosphate) were added to the ball milling jar in sequence, and ball milled at 500rpm for 5h. Then, 31g of pore-forming agent (specifically, polyethylene glycol) was added and ball milled for another 4h 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 soaking in a hot water bath at 100℃ three times to remove the pore-forming agent, thus obtaining the base film.

[0136] (2) Add (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid to DMF, then add an initiator (specifically benzoyl peroxide), the amount of initiator added is 0.2% of the mass of (E)-3-(4-(chlorosulfonyl)phenyl)acrylic acid, then heat to 60°C under argon protection and react for 15 h to obtain a polymer solution. Add the polymer solution to acetone to obtain a polymer precipitate, then wash and dry to obtain the polymer.

[0137] Sodium polyacrylate and pure water are first stirred and mixed once, then ZrO2 is added and stirred and mixed again. During the above two stirring and mixing process, ultrasonication is performed simultaneously to obtain a mixed solution. Isopropanol, carboxymethyl cellulose, polymer and sodium hexametaphosphate are added to the mixed solution and vacuum oscillation and stirring are performed to prepare a coating slurry.

[0138] The mass ratio of the polymer, ZrO2, pure water, sodium hexametaphosphate, sodium polyacrylate, isopropanol, and carboxymethyl cellulose is 30:23:66:0.02:0.1:1:7; 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; 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.

[0139] (3) The 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 16N to obtain the composite diaphragm.

[0140] Example 4

[0141] This embodiment provides a composite separator, wherein the composite separator, except that the monomer of the polymer is...

[0142] Except for the above, everything else is the same as in Example 1.

[0143] The preparation method of the composite membrane described in this embodiment is the same as that in Example 1, except that the monomer types are changed during polymer preparation.

[0144] Example 5

[0145] This embodiment provides a composite membrane, which is the same as that in Example 1 except that the weight-average molecular weight of the polymer is 70,000 Da.

[0146] The preparation method of the composite membrane described in this embodiment is the same as that in Example 1, except that the reaction time during polymer preparation is changed to adapt the weight-average molecular weight of the polymer.

[0147] Example 6

[0148] This embodiment provides a composite membrane, which is the same as that in Example 1 except that the weight-average molecular weight of the polymer is 150,000 Da.

[0149] The preparation method of the composite membrane described in this embodiment is the same as that in Example 1, except that the reaction time during polymer preparation is changed to adapt the weight-average molecular weight of the polymer.

[0150] Example 7

[0151] This embodiment provides a composite membrane, which is the same as that in Embodiment 1 except that the thickness of the polymer coating is 0.5 μm.

[0152] The preparation method of the composite diaphragm described in this embodiment is the same as that in Example 1, except for the change in the coating thickness of the polymer coating.

[0153] Example 8

[0154] This embodiment provides a composite membrane, which is the same as that in Embodiment 1 except that the thickness of the polymer coating is 4 μm.

[0155] The preparation method of the composite diaphragm described in this embodiment is the same as that in Example 1, except for the change in the coating thickness of the polymer coating.

[0156] Example 9

[0157] This embodiment provides a composite membrane, which is the same as that in Embodiment 1 except that the polymer coating does not contain ZrO2.

[0158] The preparation method of the composite diaphragm described in this embodiment is the same as that in Example 1, except that ZrO2 is not added to the coating slurry.

[0159] Comparative Example 1

[0160] This comparative example provides a diaphragm, which is the PI-based membrane from Example 1.

[0161] 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.

[0162] Comparative Example 2

[0163] This comparative example provides a composite separator, wherein the composite separator, except that the monomer of the polymer is...

[0164] Except for the above, everything else is the same as in Example 1.

[0165] The preparation method of the composite membrane described in this comparative example is the same as that in Example 1, except that the monomer types are changed during polymer preparation.

[0166] Comparative Example 3

[0167] This comparative example provides a composite separator, wherein the polymer monomer of the composite separator is phenylacrylic acid, and the structural formula is [structure not provided]. Except for (CAS: 140-10-3), everything else is the same as in Example 1.

[0168] The preparation method of the composite membrane described in this comparative example is the same as that in Example 1, except that the monomer types are changed during polymer preparation.

[0169] Comparative Example 4

[0170] This comparative example provides a composite separator, wherein the polymer monomer is p-vinylbenzenesulfonyl chloride, and the structural formula is [not specified]. Except for (CAS: 2633-67-2), everything else is the same as in Example 1.

[0171] The preparation method of the composite membrane described in this comparative example is the same as that in Example 1, except that the monomer types are changed during polymer preparation.

[0172] Comparative Example 5

[0173] This comparative example provides a composite membrane, which is the same as that in Example 1 except that the polymer is polyacrylic acid.

[0174] The preparation method of the composite membrane described in this comparative example is the same as that in Example 1, except that the monomer types are changed during polymer preparation.

[0175] The separators obtained in the above embodiments and comparative examples are used to prepare lithium-ion batteries. The preparation method includes the following steps:

[0176] (1) Preparation of positive electrode sheet

[0177] 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.

[0178] (2) Preparation of negative electrode sheet

[0179] Silicon-carbon anode material (50% silicon, the remainder carbon), conductive agent SP (Super-P conductive carbon black), SWCNT (single-walled carbon nanotubes), PAA (polyacrylic acid), and SBR (styrene-butadiene rubber) are 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 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, a negative electrode sheet is obtained.

[0180] (3) Selection of electrolyte

[0181] 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.

[0182] (4) Preparation of lithium-ion batteries

[0183] 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.

[0184] Performance testing

[0185] (1) Capacity retention rate after 1000 cycles at room temperature (1C / 2C)

[0186] 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.

[0187] 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.

[0188] (2) Room temperature 6C rate performance - constant current charge ratio

[0189] 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%.

[0190] (3) Thermal shrinkage rate of diaphragm at 180℃ / 30min

[0191] 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.

[0192] 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.

[0193] Sample preparation:

[0194] Cut to size: 100mm × 100mm, 10mm away from the edge of the diaphragm;

[0195] 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.

[0196] Test steps:

[0197] Pretreatment: The sample was placed in an environment of 23±2℃ and 50±5%RH for 24 hours.

[0198] High-temperature treatment:

[0199] 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.

[0200] Size measurement:

[0201] 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.

[0202] Heat shrinkage rate (%) = (L0-L1) / L0×100%; record the transverse (TD) and longitudinal (MD) shrinkage rates respectively.

[0203] (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.

[0204] (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.

[0205] (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 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.

[0206] The test results are shown in Table 1:

[0207] Table 1

[0208]

[0209] As can be seen from Table 1:

[0210] (1) As can be seen from Examples 1-9 and Comparative Example 1, the present invention can improve the puncture strength of the separator, reduce the thermal shrinkage rate, and improve the rate performance, thermal stability and safety of the battery by setting a specific polymer coating on the surface of the base film. As can be seen from Examples 1, 4 and Comparative Example 2, the halosulfonyl group of the present invention does not use fluorosulfonyl group, which has a higher bond energy and is easily hydrolyzed to generate HF to corrode the electrode, thereby reducing the puncture strength of the separator and the safety performance of the battery. As can be seen from Examples 1 and Comparative Example 3, the halosulfonyl group of the polymer monomer of the present invention can be cross-linked, which improves the mechanical properties of the separator. When the comparative example 3 does not contain this group, the performance of the separator and the battery decreases, especially the puncture strength of the separator decreases. As can be seen from Examples 1 and Comparative Example 4, when the polymer monomer of the present invention does not contain carboxyl group, the electrolyte wettability of the separator decreases, thereby reducing the rate performance and fast charging performance of the battery. As can be seen from Examples 1 and Comparative Example 5, when the polymer monomer of the present invention does not contain halosulfonyl group and phenyl group, the mechanical properties and thermal stability of the separator decrease, thereby reducing the safety performance of the battery.

[0211] (2) As can be seen from Examples 1 and 5-6, the weight-average molecular weight of the polymer of the present invention will affect the adhesion, mechanical strength and brittleness of the polymer coating, thereby affecting the thermal stability and mechanical strength of the composite separator, and affecting the safety and rate performance of the battery; As can be seen from Examples 1 and 7-8, the thickness of the polymer coating of the present invention will affect the performance of the polymer, thereby affecting the performance of the separator and the battery in many aspects; As can be seen from Examples 1 and 9, the present invention preferably includes inorganic materials in the polymer coating, the inorganic materials are embedded in the polymer network to form an organic-inorganic interpenetrating structure, and the polymer and inorganic materials can synergistically improve the performance of the separator and the battery.

[0212] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A composite diaphragm, characterized in that, The composite membrane includes a base membrane and a polymer coating on at least one surface of the base membrane, wherein the polymer coating includes a polymer; The monomer structure of the polymer is shown in Formula I): ； Formula I); Wherein, R includes a benzene ring, X includes Cl, and m:n is 1:

1.

2. The composite diaphragm according to claim 1, characterized in that, The benzene ring in R also includes substituents, which include alkyl and / or alkoxy groups.

3. The composite diaphragm according to claim 1 or 2, characterized in that, The monomers of the polymer include , , or Any one or at least two of them.

4. The composite diaphragm according to claim 1 or 2, characterized in that, The weight-average molecular weight of the polymer is 80,000 Da to 140,000 Da.

5. The composite diaphragm according to claim 1 or 2, characterized in that, The thickness of the polymer coating is 1μm to 3μm.

6. The composite diaphragm according to claim 1 or 2, characterized in that, The polymer coating also includes inorganic materials.

7. The composite diaphragm according to claim 6, characterized in that, In the polymer coating, the polymer is coated on the surface of the inorganic material.

8. The composite diaphragm according to claim 6, characterized in that, The inorganic material includes ZrO2.

9. The composite diaphragm according to claim 6, characterized in that, The particle size D50 of the inorganic material is 10nm~30nm.

10. The composite diaphragm according to claim 6, characterized in that, The polymer coating also includes a first wetting agent, a first dispersant, and a binder.

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

12. The composite 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 composite 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 composite 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 composite diaphragm according to claim 1 or 2, characterized in that, The base membrane has a porosity of 45%~55%, an air permeability of 80s / 100mL~100s / 100mL, and a thickness of 6μm~9μm.

16. The composite diaphragm according to claim 1 or 2, characterized in that, The base film includes a PI base film.

17. The composite diaphragm according to claim 1 or 2, characterized in that, The thickness of the composite diaphragm is 7μm~12μm.

18. The composite diaphragm according to claim 1 or 2, characterized in that, The porosity of the composite membrane is 40%~50%.

19. The composite diaphragm according to claim 1 or 2, characterized in that, The composite diaphragm has a puncture strength >700gf, a TD heat shrinkage rate ≤4%, and a MD heat shrinkage rate ≤4%.

20. A method for preparing a composite separator as described in any one of claims 1-19, characterized in that, The preparation method includes the following steps: The composite membrane is obtained by coating a polymer coating slurry onto at least one side of the base membrane; The polymer coating slurry includes a polymer, wherein the monomers of the polymer include a halosulfonyl group, a rigid group containing at least one benzene ring, at least one carbon-carbon double bond and a carboxyl group, wherein the halosulfonyl group includes a chlorosulfonyl group and / or a bromosulfonyl group.

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

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

23. The preparation method according to claim 22, characterized in that, The mass ratio of the polymer, 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).

24. The preparation method according to claim 20, characterized in that, The coating method includes dot-coating.

25. The preparation method according to claim 20, characterized in that, The coating temperature is 40℃~90℃, and the coating stretching speed difference is 0.1%~10%.

26. The preparation method according to claim 20, characterized in that, The coating process was followed by rewinding and slitting.

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

28. The preparation method according to claim 26, characterized in that, The winding and unwinding tension of the slitting process is 0.1N~20N, and the contact pressure is 0.01N~16N.

29. A lithium-ion battery, characterized in that, The lithium-ion battery includes the composite separator as described in any one of claims 1-19, or the composite separator prepared by the preparation method as described in any one of claims 20-28.

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

31. The lithium-ion battery according to claim 30, characterized in that, The silicon content in the silicon-carbon material is 45% to 55%.

32. The lithium-ion battery according to claim 30, characterized in that, The particle size D50 of the silicon-carbon material is 3μm~7μm.

33. The lithium-ion battery according to claim 29, 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.

34. The lithium-ion battery according to claim 33, 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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