Polyamide diaphragm, preparation method thereof and secondary battery

Abstract

The invention provides a polyamide diaphragm, a preparation method thereof and a secondary battery, and relates to the technical field of lithium ion battery diaphragm materials. The preparation method of the polyamide diaphragm comprises the following steps: S1, uniformly stirring a solvent, a diamine monomer and a diacyl chloride monomer to obtain a polyamide solution; s2, adding a cosolvent, a defoaming agent and a pore-forming agent into the polyamide solution, heating and stirring to obtain film-forming slurry, and coating a substrate with the film-forming slurry to obtain a wet film; s3, performing heat treatment on the wet film to obtain a heat treatment film; and S4, the heat treatment film is cleaned and dried, and the film is obtained. The polyamide diaphragm provided by the invention has excellent heat resistance, and the electrochemical performance and the safety performance of the battery can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery separator materials, and particularly to a polyamide separator, a preparation method thereof, and a secondary battery. Background Art

[0002] Currently, the development of batteries tends to reduce the electrolyte in the batteries and develop towards condensed batteries, semi-solid batteries or quasi-solid batteries. Among them, the cathode materials also develop towards high-nickel ternary (nickel cobalt manganese NCM) materials with higher energy density, from the 5-series (nickel accounting for 50%) to the 7-series (nickel accounting for 70%), 8-series (nickel accounting for 80%) and even 9-series (nickel accounting for 90%). The safety boundary temperature at which thermal runaway is likely to occur is also gradually decreasing. The safety boundary temperature of the high-nickel ternary cathode material has been lower than 180°C. Conventional polyolefin-based separators can no longer meet the requirements of such batteries, and there is an urgent need to design and develop lithium battery separators with higher heat resistance temperatures.

[0003] The existing preparation methods of aromatic polyamide materials are mainly non-solvent phase inversion methods, but their process steps are complex and there are many influencing parameters. At present, they are not yet mature, resulting in relatively large pore sizes of the prepared separators and excessive differences between the inner and outer pore sizes, which are likely to cause short circuits when installed in batteries. Summary of the Invention

[0004] Based on the technical problems existing in the background art, the present invention proposes a polyamide separator, a preparation method thereof, and a secondary battery.

[0005] A preparation method of a polyamide separator proposed by the present invention includes the following steps:

[0006] S1. Stir a solvent, a diamine monomer, and a diacyl chloride monomer evenly to obtain a polyamide solution;

[0007] S2. Add a co-solvent, an antifoaming agent, and a pore-forming agent to the polyamide solution, heat and stir to obtain a film-forming slurry, and coat the film-forming slurry on a substrate to obtain a wet film;

[0008] S3. Heat-treat the wet film to obtain a heat-treated film;

[0009] S4. Wash and dry the heat-treated film to obtain the product.

[0010] The preparation method of the present invention has a simple preparation process and can be continuously completed from synthesis to film formation. The prepared polyamide separator has excellent heat insulation performance.

[0011] Preferably, in the step S1, the solvent is selected from one or more of DMAc (dimethylacetamide), NMP (N-methylpyrrolidone), DMF (N,N-dimethylformamide), DMSO (dimethyl sulfoxide), and THF (tetrahydrofuran).

[0012] The selection of the solvent helps to synthesize a polyamide solution.

[0013] Preferably, in the above S1, the diamine monomer is selected from one or more of m-phenylenediamine, p-phenylenediamine, 1,4-cyclohexanediamine, naphthalene-2,6-diamine, 3-trifluoromethyl-m-phenylenediamine, 4,4'-di-sec-butylaminodiphenylmethane, 2,3,5,6-trifluoromethyl-p-phenylenediamine, 2,5-trifluoromethyl-p-phenylenediamine, 2,4,6-trifluoromethyl-m-phenylenediamine, 4,4-diaminodiphenyl sulfone, p-diaminodiphenyl ether, 3,3'-dimethoxy-[1,1'-biphenyl]-4,4'-diamine, 4-benzoyl-o-phenylenediamine.

[0014] Selecting different diamine monomers for combination enables the copolymerization of a small amount of diamine monomers with rigid structures in the molecular chain, which is beneficial to enhancing the heat resistance of the polyamide.

[0015] More preferably, the diamine monomer includes m-phenylenediamine and a diamine monomer with a rigid structure.

[0016] More preferably, the molar ratio of m-phenylenediamine to the diamine monomer with a rigid structure is 8:2 to 10:0.

[0017] More preferably, the diamine monomer with a rigid structure is selected from one or more of naphthalene-2,6-diamine, p-diaminodiphenyl ether, 4,4-diaminodiphenyl sulfone, 4-benzoyl-o-phenylenediamine.

[0018] Preferably, in the above S1, the diacyl chloride monomer is selected from one or more of 2,5-furandicarbonyl chloride, isophthaloyl chloride, terephthaloyl chloride, naphthalene-2,6-dicarbonyl chloride, naphthalene-2,3-dicarbonyl chloride, 1,4-cyclohexanedicarbonyl chloride, 1,3-cyclohexanedicarbonyl chloride, [1,1'-biphenyl]-4,4'-dicarbonyl chloride.

[0019] Selecting the diacyl chloride monomer as a monomer containing a benzene ring, cycloalkane or furyl group enables the polyamide material to have excellent heat resistance.

[0020] Preferably, in the above S1, the molar ratio of the diacyl chloride monomer to the diamine monomer is 1.01:1 to 1.1:1.

[0021] Preferably, in the above S1, after adding the diamine monomer, the temperature is controlled at -5 to 5°C.

[0022] Preferably, in the above S1, after adding the diacyl chloride monomer, the temperature is controlled at 20 to 40°C.

[0023] Preferably, in the above S1, the stirring speed is 200 to 800 rpm.

[0024] Preferably, in the step S1, the polyamide solution has a glass transition temperature of greater than 280 °C, a thermal decomposition temperature of greater than 350 °C, and a solid content of 5% to 20%.

[0025] Preferably, in the step S2, the co-solvent is selected from one or more of lithium chloride, calcium chloride, and sodium chloride.

[0026] The co-solvent can dissolve in the solvent and can bind to the hydrogen bonds in the polyamide, breaking the intermolecular hydrogen bonds and improving the solubility.

[0027] Preferably, in the step S2, the defoamer is selected from one or more of diethylhexanol, n-octanol, isooctanol, and isoamyl alcohol.

[0028] Preferably, in the step S2, the pore former is selected from one or more of polyhydroxyalkanoates (PHA), poly(β-hydroxybutyrate) (PHB), and poly(ε-caprolactone) (PCL).

[0029] The function of the pore former is to utilize the difference in the thermal decomposition temperatures of the polymers to form pores in the polyamide separator. The formed pores are uniform and have appropriate pore sizes, with a small difference in pore sizes between the surface and the interior, avoiding problems such as uneven three-dimensional pore structures that are prone to cause battery short circuits and limited improvement in thermal performance.

[0030] Preferably, in the step S2, the thermal decomposition temperature of the pore former is not greater than 200 °C, and the molecular weight is 2000 to 10000.

[0031] PCL is soluble in DMAc, and the degradation temperature is about 200 °C. At this time, the polyamide has no change. Using the difference in thermal decomposition temperatures, pores are formed in the polyamide separator.

[0032] Preferably, in the step S2, the mass of the co-solvent is 1% to 5% of the mass of the polyamide solution; the mass of the defoamer is 0% to 0.5% of the mass of the polyamide solution; the mass of the pore former is 0% to 15% of the mass of the polyamide solution.

[0033] Controlling the masses of the co-solvent, defoamer, and pore former within a certain range helps to form a film-forming slurry with moderate viscosity.

[0034] Preferably, in the step S2, the heating temperature is 30 to 70 °C.

[0035] Preferably, in the step S2, the stirring speed is 200 to 800 rpm, and the stirring time is 1 to 4 h.

[0036] Preferably, in the above S2, the substrate is selected from inorganic glass or organic polymer film; the inorganic glass is float glass, and the organic polymer film is selected from one or more of polyethylene terephthalate (PET), polycarbonate (PC), polymethyl methacrylate (PMMA), and polyimide (PI).

[0037] Preferably, in the above S2, the wet film thickness is 20 - 60 μm, more preferably 30 - 50 μm.

[0038] A wet film thickness within a certain range helps to form a film with sufficient strength and also helps the solvent to volatilize quickly.

[0039] Preferably, in the above S3, the heat treatment includes first treating at 50 - 80 °C for 2 - 6 h, and then treating at 180 - 220 °C for 2 - 5 h.

[0040] More preferably, during the heat treatment process, an oxygen-containing gas is continuously introduced.

[0041] More preferably, the oxygen-containing gas includes oxygen or air.

[0042] During the heat treatment process, the pore-forming agent is completely degraded into volatile small molecules, which diffuse with the flowing gas and leave microscopic pores on the polyamide film, completing the diaphragm pore-forming process.

[0043] Preferably, in the above S4, the cleaning solvent is an alcohol solvent, and the alcohol solvent is selected from one or more of methanol, ethanol, and butanol.

[0044] The polyamide film after high-temperature treatment is cleaned with the cleaning solvent to remove the residual hydrochloric acid and the small molecules remaining after thermal decomposition.

[0045] Preferably, in the above S4, the drying temperature is 40 - 60 °C, and the drying time is 1 - 2 h.

[0046] The present invention also provides a polyamide diaphragm prepared by the above preparation method.

[0047] A secondary battery includes a positive electrode sheet, a negative electrode sheet, and the above polyamide diaphragm.

[0048] Preferably, the positive electrode sheet includes a ternary NCM811 (nickel: cobalt: manganese = 8:1:1) material.

[0049] Preferably, the negative electrode sheet includes artificial graphite.

[0050] The beneficial effects of the present invention are as follows:

[0051] The present invention provides a polyamide separator mainly used for lithium-ion battery products, which can significantly increase the thermal safety temperature of the separator. First, diamine monomers and diacyl chloride monomers with relatively low prices and excellent performance are screened, and aromatic polyamides with excellent thermal properties are prepared by polycondensation at low temperature. Subsequently, a new method of high-temperature pyrolysis pore formation is adopted. Utilizing the difference in thermal stability between polymers, polyamide separators are prepared by high-temperature pyrolysis pore formation. This preparation process is simple, can be continuously completed from synthesis to film formation, has uniform pore formation and appropriate pore size, and has a small difference in pore size between the surface and the interior. The thermal properties of the separator are very excellent, greatly increasing the thermal safety boundary temperature of the polyamide film, and solving the problems of expensive heat-resistant lithium-ion battery separator membranes in the past, long preparation process flow with many parameters, large membrane pore size, uneven three-dimensional pore structure prone to cause battery short circuit, and limited improvement in thermal properties. The polyamide separator provided by the present invention has an extremely low thermal shrinkage rate at high temperature, can improve the safety performance of the battery under extreme thermal conditions, and can increase the surface wettability, shorten the wetting time, and effectively improve the battery capacity.

[0052] The present invention synthesizes a polyamide material containing aromatic rings and furan rings with extremely excellent heat resistance, and copolymerizes a small amount of diamine monomers with large molecular chain rigidity in the molecular chain. In addition, there are a large number of hydrogen bonds between the molecular chains, and its melting point and decomposition temperature are greater than 350 °C, and the heat resistance is extremely excellent. In addition, compared with traditional methods such as non-solvent phase inversion method and thermally induced phase separation method, the present invention utilizes the difference in the thermal decomposition temperature of polymers to complete the pore formation of the polyamide film, which is a new method for pore formation of polyamide separators.

[0053] When synthesizing polyamide, the present invention copolymerizes a small amount of rigid structure monomers to enhance the heat resistance of polyamide; when preparing the polyamide separator, the present invention selects a bio-based polymer with low molecular weight and low thermal decomposition temperature as the pore former, and utilizes the difference in thermal stability between the pore former and polyamide to complete the pore formation of the separator. The polyamide separator is assembled with a high-nickel ternary cathode and a graphite anode to form a soft-pack battery, which shows great advantages in terms of initial efficiency, cycle stability, and high-temperature thermal safety. The process route of the present invention is simple, and combined with the continuously decreasing raw material cost of polyamide, the present invention has great application advantages. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a schematic flow chart of the preparation method of the polyamide separator proposed by the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0055] The technical solutions of the present invention are described in detail through specific embodiments.

[0056] The materials, reagents, etc. used in the following examples and comparative examples can be obtained from commercial channels without special instructions.

[0057] Example 1

[0058] A preparation method of a polyamide separator, comprising the following steps:

[0059] S1. Add m-phenylenediamine monomer and naphthalene-2,6-diamine monomer into a reaction kettle containing dimethylacetamide (DMAc) according to a molar ratio of 9:1. Control the temperature of the reaction kettle at 0 °C, and then drop 2,5-furandicarbonyl chloride into the reaction kettle. The molar ratio of the diacyl chloride monomer to the diamine monomer is 1.05:1. Raise the temperature of the reaction kettle to 40 °C, and the stirring rate of the stirring paddle is 500 rpm. React for 3 h to obtain a polyamide solution with a solid content of 15%. Copolymerizing a small amount of naphthalene-2,6-diamine monomer with a rigid structure can improve the heat resistance of the polyamide;

[0060] S2. Add 2% of LiCl cosolvent and 0.1% of n-octanol defoamer to the polyamide solution, and stir evenly at 50 °C to obtain a film-forming slurry. The stirring rate is 500 rpm, and the stirring duration is 2 h. The viscosity of the film-forming slurry is 15420 mPa·s. Use a coater to coat the film-forming slurry onto a polycarbonate (PC) film to obtain a wet film with a thickness of 30 μm;

[0061] S3. Transfer the wet film to an ordinary drying oven, set the temperature at 70 °C, and dry for 2 h. The solvent content of the dried film is about 10%. Transfer the film to a high-temperature oven, set the temperature at 210 °C, and dry for 3 h. Continuously introduce oxygen into the oven during the drying process to obtain a heat-treated film;

[0062] S4. Wash the heat-treated film with ethanol, and then place it in a blast drying oven to dry. The drying temperature is 50 °C, and the drying time is 2 h. The polyamide separator is prepared, and the thickness of the separator is 13 μm.

[0063] Example 2

[0064] A preparation method of a polyamide separator, comprising the following steps:

[0065] S1. Add m-phenylenediamine monomer and naphthalene-2,6-diamine monomer into a reaction kettle containing dimethylacetamide (DMAc) according to a molar ratio of 9:1. Control the temperature of the reaction kettle at 0 °C, and then drop 2,5-furandicarbonyl chloride into the reaction kettle. The molar ratio of the diacyl chloride monomer to the diamine monomer is 1.05:1. Raise the temperature of the reaction kettle to 40 °C, and the stirring rate of the stirring paddle is 500 rpm. React for 3 h to obtain a polyamide solution with a solid content of 15%. Copolymerizing a small amount of naphthalene-2,6-diamine monomer with a rigid structure can improve the heat resistance of the polyamide; When synthesizing the diamine monomer and the dianhydride monomer, a large amount of HCl will be generated. The polyamide solution is not neutralized with alkali after synthesis, and the polyamide solution is strongly acidic;

[0066] S2. Add 2% LiCl cosolvent and 0.1% n-octanol defoamer to the polyamide solution, continuously stir at 50 °C, slowly add a PCL pore former with a content of 2% and a molecular weight of 5000, stir evenly to obtain a film-forming slurry, with a stirring rate of 500 rpm and a stirring duration of 2 h. The viscosity of the film-forming slurry is 16340 mPa·s; use a coater to coat the film-forming slurry onto a polycarbonate (PC) film to obtain a wet film with a thickness of 30 μm;

[0067] S3. Transfer the wet film to a common drying oven, set the temperature at 70 °C, and dry for 2 h. The solvent content of the dried film is about 10%. Then transfer the film to a high-temperature oven, set the temperature at 210 °C, and dry for 3 h. During the drying process, continuously introduce oxygen-containing gas into the oven to promote the complete thermal degradation of PCL under high temperature and strong acidic conditions (pH = 1.8) to complete the pore-forming process, obtaining a heat-treated film;

[0068] S4. Wash the heat-treated film with ethanol, then place it in a forced-air oven to dry, with a drying temperature of 50 °C and a drying time of 2 h. The preparation of the polyamide separator is completed, and the thickness of the separator is 12 μm.

[0069] Example 3

[0070] A method for preparing a polyamide separator, comprising the following steps:

[0071] S1. Add m-phenylenediamine monomer and naphthalene-2,6-diamine monomer to a reaction kettle containing dimethylacetamide (DMAc) according to a molar ratio of 9:1. Control the temperature of the reaction kettle at 0 °C, and then drop 2,5-furandicarbonyl chloride into the reaction kettle. The molar ratio of the diacyl chloride monomer to the diamine monomer is 1.05:1. Raise the temperature of the reaction kettle to 40 °C, with a stirring rate of 500 rpm for the stirring paddle, and react for 3 h to obtain a polyamide solution with a solid content of 15%; copolymerizing a small amount of rigid-structured naphthalene-2,6-diamine monomer can improve the heat resistance of the polyamide;

[0072] S2. Add 2.5% LiCl cosolvent and 0.2% n-octanol defoamer to the polyamide solution, continuously stir at 50 °C, slowly add a PCL pore former with a content of 5% and a molecular weight of 5000, stir evenly to obtain a film-forming slurry, with a stirring rate of 500 rpm and a stirring duration of 2 h. The viscosity of the film-forming slurry is 18470 mPa·s; use a coater to coat the film-forming slurry onto a polycarbonate (PC) film to obtain a wet film with a thickness of 30 μm;

[0073] S3. Transfer the wet film to a common drying oven, set the temperature at 70 °C, and dry for 2 h. After drying, the solvent content of the film is about 10%. Then transfer the film to a high-temperature oven, set the temperature at 210 °C, and dry for 3 h. During the drying process, continuously introduce oxygen-containing gas into the oven to promote the complete thermal degradation of PCL under high temperature and strong acidic conditions (pH = 1.7) to complete the pore-forming process, and obtain a heat-treated film;

[0074] S4. Wash the heat-treated film with ethanol, and then place it in a forced-air oven for drying. The drying temperature is 50 °C and the drying time is 2 h. The polyamide separator is prepared, and the separator thickness is 13 μm.

[0075] Example 4

[0076] A method for preparing a polyamide separator, comprising the following steps:

[0077] S1. Add m-phenylenediamine monomer and naphthalene-2,6-diamine monomer to a reaction kettle containing dimethylacetamide (DMAc) according to a molar ratio of 9:1. Control the temperature of the reaction kettle at 0 °C, and then drop 2,5-furandicarbonyl chloride into the reaction kettle. The molar ratio of the diacyl chloride monomer to the diamine monomer is 1.05:1. Raise the temperature of the reaction kettle to 40 °C, and the stirring rate of the stirrer is 500 rpm. React for 3 h to obtain a polyamide solution with a solid content of 15%. Copolymerizing a small amount of rigid naphthalene-2,6-diamine monomer can improve the heat resistance of the polyamide;

[0078] S2. Add 2.5% of LiCl cosolvent and 0.2% of n-octanol defoamer to the polyamide solution, continuously stir at 50 °C, and slowly add 8% content and 5000 molecular weight PCL pore-forming agent. Stir evenly to obtain a film-forming slurry. The stirring rate is 500 rpm and the stirring time is 2 h. The viscosity of the film-forming slurry is 21500 mPa·s. Use a coater to coat the film-forming slurry on a polycarbonate (PC) film to obtain a wet film, and the thickness of the wet film is 30 μm;

[0079] S3. Transfer the wet film to a common drying oven, set the temperature at 70 °C, and dry for 2 h. After drying, the solvent content of the film is about 10%. Then transfer the film to a high-temperature oven, set the temperature at 210 °C, and dry for 3 h. During the drying process, continuously introduce oxygen-containing gas into the oven to promote the complete thermal degradation of PCL under high temperature and strong acidic conditions (pH = 2.1) to complete the pore-forming process, and obtain a heat-treated film;

[0080] S4. Wash the heat-treated film with ethanol, and then place it in a forced-air oven for drying. The drying temperature is 50 °C and the drying time is 2 h. The polyamide separator is prepared, and the separator thickness is 13 μm.

[0081] Example 5

[0082] A preparation method of a polyamide separator, comprising the following steps:

[0083] S1. Add m-phenylenediamine monomer and naphthalene-2,6-diamine monomer into a reaction kettle containing dimethylacetamide (DMAc) according to a molar ratio of 9:1. Control the temperature of the reaction kettle at 0 °C, and then drop 2,5-furandicarbonyl chloride into the reaction kettle. The molar ratio of the diacyl chloride monomer to the diamine monomer is 1.05:1. Raise the temperature of the reaction kettle to 40 °C, and the stirring rate of the stirring paddle is 500 rpm. React for 3 h to obtain a polyamide solution with a solid content of 15%. Copolymerizing a small amount of rigid-structured naphthalene-2,6-diamine monomer can improve the heat resistance of the polyamide;

[0084] S2. Add 3% of LiCl cosolvent and 0.3% of n-octanol defoamer to the polyamide solution. Stir continuously at 50 °C, and slowly add a PCL pore former with a content of 15% and a molecular weight of 5000. Stir evenly to obtain a film-forming slurry. The stirring rate is 500 rpm, and the stirring duration is 2 h. The viscosity of the film-forming slurry is 33120 mPa·s. Use a coater to coat the film-forming slurry onto a polycarbonate (PC) film to obtain a wet film with a thickness of 30 μm;

[0085] S3. Transfer the wet film to a common drying oven, set the temperature at 70 °C, and dry for 2 h. The solvent content of the dried film is about 10%. Transfer the film to a high-temperature oven, set the temperature at 210 °C, and the drying time is 3 h. Continuously introduce oxygen-containing gas into the oven during the drying process to promote the complete thermal degradation of PCL under high temperature and strong acidic conditions (pH = 2.6) to complete the pore-forming process and obtain a heat-treated film;

[0086] S4. Wash the heat-treated film with ethanol, and then place it in a forced-air oven for drying. The drying temperature is 50 °C, and the drying time is 2 h. The polyamide separator is prepared, and the thickness of the separator is 13 μm.

[0087] Comparative Example 1

[0088] Commercial wet-process biaxially stretched polyethylene separator, with a thickness of the polyethylene separator of 9 μm.

[0089] Comparative Example 2

[0090] Dry-process stretched polypropylene separator, with a thickness of the polypropylene separator of 14 μm.

[0091] The formulation of the film-forming slurry in the above solution is shown in Table 1.

[0092] Table 1

[0093]

[0094] Test the basic physical properties and high-temperature thermal shrinkage of the above separators, and the test results are shown in Table 2.

[0095] Table 2

[0096]

[0097] Assemble the above diaphragm into a secondary battery: Prepare a positive electrode paste using 95% ternary NCM811 (nickel: cobalt: manganese = 8:1:1) material, 2% conductive agent carbon nanotubes, 3% binder PVDF (polyvinylidene fluoride), and NMP as a solvent. After that, through coating, drying, and rolling steps, prepare a positive electrode sheet; Prepare a negative electrode paste using 95% artificial graphite, 2% conductive agent carbon nanotubes, 3% binder SBR (styrene-butadiene copolymer), and deionized water as a solvent. After that, through coating, drying, and rolling steps, prepare a negative electrode sheet; Use a soft-pack battery stacker to stack the positive electrode sheet, diaphragm, and negative electrode sheet in sequence to assemble a 10Ah soft-pack secondary battery. And conduct 1C charge-discharge cycle tests and full-charge temperature rise thermal runaway tests on the soft-pack secondary battery. The test results are shown in Table 3.

[0098] Table 3

[0099]

[0100] Test the contact angle of the above diaphragm. The electrolyte composition for the test is: EC:EMC:DMC = 1:1:1, and the lithium salt concentration is 1 mol / L. The test results are shown in Table 4.

[0101] Table 4

[0102] Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Contact Angle / ° 12 12 12 12 11 54 46

[0103] Test the pore size and porosity of the above diaphragm. The test results are shown in Table 5.

[0104] Table 5

[0105] Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Comparative Example 2 Average Pore Diameter / nm / 18 25.3 47.5 697 38.4 36.8 Maximum Pore Diameter / nm / 27.4 41.5 89.5 2023 42.3 43.2 Porosity / % 0 17 26 51 68 42 35

[0106] As can be seen from Tables 2 to 5, in the polyamide separator prepared by the present invention, when the proportion of the pore-forming agent PCL reaches more than 5%, the air permeability of the polyamide separator will be significantly improved. When the PCL content is greater than 8%, the air permeability of the separator is less than 300 s / 100 cc, showing relatively excellent air permeability. When the PCL content reaches 15%, the air permeability of the separator is only 5 s / 100 cc. The polyamide separators prepared in Examples 1 to 5 all have good thermal shrinkage properties, with a shrinkage rate of less than 3% at 250 °C (in Example 5, the PCL content is too high and the pore size is large, resulting in slightly larger thermal shrinkage). Moreover, the polyamide separator prepared by the present invention has a smaller contact angle, which can increase the surface wettability, shorten the wetting time, and effectively improve the battery capacity. Through the electrical performance test and safety test of the soft-pack secondary battery, it is found that the polyamide separator prepared in Example 4 has the most excellent performance, with a relatively high initial efficiency, the best capacity retention rate after 1C hundred-cycle, and can pass the 200 °C thermal safety test. Compared with the conventional base film, it has excellent safety and very excellent electrochemical performance. The polyamide separator of the present invention has obvious performance advantages.

[0107] In summary, the polyamide separator provided by the present invention has excellent heat resistance and can effectively improve the electrochemical performance and safety performance of the battery.

[0108] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, shall be covered by the protection scope of the present invention.

Claims

1. A preparation method of a polyamide separator, characterized in that, It includes the following steps: S1. Stir a solvent, a diamine monomer, and a diacyl chloride monomer evenly to obtain a polyamide solution; S2. Add a co-solvent, an antifoaming agent, and a pore-forming agent to the polyamide solution, heat and stir to obtain a film-forming slurry, and coat the film-forming slurry on a substrate to obtain a wet film; S3. Heat-treat the wet film to obtain a heat-treated film; S4. Wash and dry the heat-treated film to obtain the product.

2. The preparation method according to claim 1, characterized in that, In S2, the pore-forming agent is selected from one or more of polyhydroxyalkanoates, poly(β-hydroxybutyrate), and poly(ε-caprolactone), and the molecular weight of the pore-forming agent is 2,000 to 10,000.

3. The preparation method according to claim 1, characterized in that In S1, the diamine monomer is selected from one or more of m-phenylenediamine, p-phenylenediamine, 1,4-cyclohexanediamine, naphthalene-2,6-diamine, 3-trifluoromethyl-m-phenylenediamine, 4,4'-di-sec-butylaminodiphenylmethane, 2,3,5,6-tetrafluoromethyl-p-phenylenediamine, 2,5-tetrafluoromethyl-p-phenylenediamine, 2,4,6-tetrafluoromethyl-m-phenylenediamine, 4,4-diaminobenzophenone, p-diaminodiphenyl ether, 3,3'-dimethoxy-[1,1'-biphenyl]-4,4'-diamine, and 4-benzoyl-o-phenylenediamine; the diacyl chloride monomer is selected from one or more of 2,5-furandicarbonyl chloride, isophthaloyl chloride, terephthaloyl chloride, naphthalene-2,6-dicarbonyl chloride, naphthalene-2,3-dicarbonyl chloride, 1,4-cyclohexanedicarbonyl chloride, 1,3-cyclohexanedicarbonyl chloride, and [1,1'-biphenyl]-4,4'-dicarbonyl chloride; the solvent is selected from one or more of dimethylacetamide, N-methylpyrrolidone, N,N-dimethylformamide, dimethyl sulfoxide, and tetrahydrofuran.

4. The preparation method according to claim 1, characterized in that, In S2, the co-solvent is selected from one or more of lithium chloride, calcium chloride, and sodium chloride; the antifoaming agent is selected from one or more of diethylhexanol, n-octanol, isooctanol, and isoamyl alcohol.

5. The preparation method according to claim 1, characterized in that, In S2, the mass of the co-solvent is 1% to 5% of the mass of the polyamide solution; the mass of the antifoaming agent is 0% to 0.5% of the mass of the polyamide solution; the mass of the pore-forming agent is 0% to 15% of the mass of the polyamide solution.

6. The preparation method according to claim 1, characterized in that, In S2, the thickness of the wet film is 20 to 60 μm.

7. The preparation method according to claim 1, characterized in that, In S3, the heat treatment includes first treating at 50 to 80 °C for 2 to 6 h, and then treating at 180 to 220 °C for 2 to 5 h; an oxygen-containing gas is continuously introduced during the heat treatment process, and the oxygen-containing gas includes oxygen or air.

8. The preparation method according to claim 1, characterized in that, In S4, the solvent for washing is an alcohol solvent, and the alcohol solvent is selected from one or more of methanol, ethanol, and butanol.

9. A polyamide separator, characterized in that, Prepared by the preparation method according to any one of claims 1 to 8.

10. A secondary battery, characterized in that, It includes a positive electrode sheet, a negative electrode sheet, and the polyamide separator according to claim 9 or the polyamide separator prepared by the preparation method according to any one of claims 1 to 8.

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