Preparation Method of Lithium-Ion Battery and Its Separator

Abstract

The present invention discloses a preparation method of a lithium-ion battery and its separator, relating to the technical field of battery preparation. The prepared polymethylsilane-zinc oxide nanocomposite particles have excellent hydrophobic properties, which can effectively reduce the water absorption of the separator, avoid excessive water content inside the lithium-ion battery caused by the water absorption of the separator, effectively save the baking time of the separator and the battery cell, reduce the time cost, avoid wrinkles and deformation of the separator caused by long-term baking, and reduce the potential safety risk of the lithium-ion battery. In addition, polymethylsilane will dissolve in the lithium-ion battery electrolyte during long-term charge and discharge processes, and can be used as a positive electrode protection additive to better passivate the surface of the positive electrode and reduce the interfacial resistance between the positive electrode material and the electrolyte, thereby improving the cycle life of the positive electrode.

Description

Technical Field

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

[0002] Lithium-ion batteries have become the main power sources for consumer electronics and electric vehicles, and are widely used in large-scale energy storage systems in smart grids. At the same time, the potential risks of lithium-ion batteries have received increasing attention, including low safety and cycle life. Polyolefin separators with a microporous structure are one of the key functional components of lithium-ion batteries, providing channels for lithium-ion transmission while preventing electronic short circuits between the positive and negative electrodes. Therefore, the composite modification of separators is an important technical approach for researching and developing high-performance and high-safety lithium-ion batteries. The patent with the application number CN202110874008.0 discloses "a multi-hydrogen bond cross-linked cellulose / carboxylated polyimide nanofiber composite separator and its preparation method and use", mixing a cellulose acetate solution and a polyamic acid solution, subjecting the obtained cellulose acetate / polyamic acid mixed spinning solution to electrospinning to obtain a cellulose acetate / polyamic acid composite membrane, then performing thermal imidization by stepwise heating to obtain a cellulose acetate / polyimide composite membrane, and finally placing it in a solution containing lithium hydroxide for alkali hydrolysis reaction, and obtaining the multi-hydrogen bond cross-linked cellulose / carboxylated polyimide nanofiber composite separator after cleaning and drying treatments. The separator prepared by this method has limited microporous structure, which is not conducive to the full absorption of the electrolyte and reduces the lithium-ion transmission performance.

[0003] The patent with the application number CN202110813656.5 discloses "a flexible linear high-temperature resistant polyimide aerogel battery separator, its preparation method and a lithium-ion battery", coating a polyimide solution on the surface of a substrate, and then performing aging, solvent exchange and supercritical fluid drying to obtain a polyimide aerogel battery separator, but the adhesion between the coating and the substrate is limited, and the operation cost is high, having certain technical limitations. Summary of the Invention

[0004] 1. Technical Problems to be Solved by the Invention

[0005] In view of the above technical problems, the present invention provides a preparation method of a lithium-ion battery and its separator, which increases the microporous structure and mechanical properties of the separator, and optimizes the mechanical stability and lithium-ion transmission performance of the separator.

[0006] 2. Technical Solutions

[0007] To solve the above problems, the technical solution provided by the present invention is: a preparation method of a lithium-ion battery separator, comprising the following steps:

[0008] (1) Add the dehydroxylated carboxylic acid ester and polymethylsilane to a tetrahydrofuran solvent at a mass ratio of (5 - 10):(70 - 90). Stir at 40 - 60 °C for 2 - 4 hours under an inert atmosphere. Then add a small amount of azobisisobutyronitrile (AIBN) as an initiator and stir at 40 - 60 °C for 7 - 10 hours. Heat the resulting mixture under vacuum at 90 - 110 °C to remove the excess solvent to obtain a crude product. After further purification of the crude product, carboxylic acid ester-grafted polymethylsilane is obtained;

[0009] (2) First, mechanically stir and mix zinc oxide nanoparticles and absolute ethanol at a mass ratio of (10 - 20):(80 - 150) at room temperature, and then perform ultrasonic stirring to obtain a mixed solution a; Add the carboxylic acid ester-grafted polymethylsilane prepared in step (1) to absolute ethanol, and the mass ratio of the carboxylic acid ester-grafted polymethylsilane to absolute ethanol is (15 - 30):(60 - 75). Mechanically stir and mix at room temperature to obtain a mixed solution b;

[0010] (3) Mix the mixed solution a and the mixed solution b prepared in step (2), and perform centrifugation on the mixed solution. Wash the obtained precipitate with absolute ethanol 2 - 4 times to remove the excess solvent and reactants, and poly(methylsilane)-zinc oxide nanocomposite particles are prepared;

[0011] (4) Dissolve polyimide and soybean protein fiber at a mass ratio of (60 - 80):(20 - 35) in a N-methylpyrrolidone solvent, with a total solute mass fraction of 10 - 30%. After stirring at room temperature, add the poly(methylsilane)-zinc oxide nanocomposite particles prepared in step (3), and continue to stir at room temperature for 3 - 5 hours to obtain a uniformly mixed system. Electrospinning treatment is performed on the obtained mixed system to obtain a separator with a thickness of 5 - 15 μm;

[0012] Optionally, the following steps are further included: Add oligosilsesquioxane to an ethyl acetate solvent and stir at room temperature to obtain a uniformly mixed system. Use spray drying to deposit oligosilsesquioxane on both surfaces of the separator prepared in step (4); Add poly(methylsilane)-zinc oxide nanocomposite particles to an ethyl acetate solvent and stir at room temperature to obtain a uniformly mixed system. Use spray drying to deposit poly(methylsilane)-zinc oxide nanocomposite particles on both surfaces of the separator with oligosilsesquioxane attached.

[0013] Optionally, in step (2), the mass ratio of the zinc oxide nanoparticles to the carboxylic acid ester-grafted polymethylsilane is (10 - 20):(2.0 - 5.0).

[0014] Optionally, the purification process in step (1) is as follows: Dissolve the dried crude product in n-hexane solvent, then add methanol to precipitate it, and dry the obtained precipitate in vacuum at 60 - 80 °C, repeat 1 - 2 times to finally obtain the carboxylic acid ester grafted polymethylsilane.

[0015] Optionally, the conditions for electrospinning in step (4) are: the voltage is 15 - 25 kV, and the spinning distance is 17 - 20 cm.

[0016] Optionally, the mass ratio of polyimide to polymethylsilane-zinc oxide nanocomposite particles in step (4) is (60 - 80):(3 - 6).

[0017] Optionally, the deposition thickness of the oligosilsesquioxane is 0.5 - 1.0 μm, and the deposition thickness of the polymethylsilane-zinc oxide nanocomposite particles is 1.0 - 2.0 μm.

[0018] Optionally, the mass ratio of carboxylic acid ester to azobisisobutyronitrile in step (1) is (5 - 10):(0.02 - 0.05).

[0019] Optionally, the speed of mechanical stirring in step (2) is 80 - 140 revolutions per minute, the mechanical stirring time is 30 - 60 minutes, and the ultrasonic stirring time is 45 - 60 minutes.

[0020] The present invention also discloses a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, and a separator prepared by the method for preparing a separator of a lithium-ion battery diaphragm described above.

[0021] 3. Beneficial effects

[0022] Adopting the technical solution provided by the present invention, compared with the prior art, it has the following beneficial effects:

[0023] (1) The method for preparing a separator of a lithium-ion battery proposed in the embodiment of the present application constructs a multi-microporous polyolefin substrate, and uniformly coats a composite coating composed of a binder and inorganic nanoparticles on the surface of the substrate, obtaining a separator of a lithium-ion battery with high safety and high performance, significantly improving the safety and cycle life of the lithium-ion battery.

[0024] (2) The preparation method of the lithium-ion battery separator proposed in the embodiments of the present application results in polymethylsilane-zinc oxide nanocomposite particles with excellent hydrophobic properties, which can effectively reduce the water absorption of the separator, avoid excessive moisture content inside the lithium-ion battery caused by the water absorption of the separator, effectively save the baking time of the separator and the battery core, reduce the time cost, avoid wrinkles and deformation of the separator caused by long-term baking, and reduce the potential safety risks of the lithium-ion battery. In addition, polymethylsilane will dissolve in the lithium-ion battery electrolyte during long-term charge and discharge processes, and can be used as a cathode protection additive to better passivate the cathode surface and reduce the interfacial resistance between the cathode material and the electrolyte, thereby improving the cathode cycle life. Detailed implementation manners

[0025] To further understand the content of the present invention, the present invention will be described in detail in combination with embodiments.

[0026] The following further elaborates on the present application in combination with embodiments. It can be understood that the specific embodiments described herein are only used to explain the related invention and do not limit the invention. The terms such as "first" and "second" in the present invention are set for the convenience of describing the technical solution of the present invention and have no specific limiting effect, and are all general references that do not limit the technical solution of the present invention. It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. In addition, the terms "first", "second", and "third" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. Unless otherwise clearly specified and limited, the terms "installation", "connection", and "coupling" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations. Multiple technical solutions in the same embodiment, as well as multiple technical solutions in different embodiments, can be arranged and combined to form new technical solutions without contradiction or conflict, and all fall within the scope of protection required by the present invention.

[0027] To achieve the above object, the present invention provides the following technical solution. A preparation method of a lithium-ion battery separator includes the following steps: (1) Add dehydroxylated carboxylic esters and polymethylsilane into a tetrahydrofuran solvent according to a mass ratio of (5 - 10):(70 - 90), stir at 40 - 60 °C for 2 - 4 hours under an inert atmosphere, then continue to add a small amount of azobisisobutyronitrile AIBN as an initiator, and the mass ratio of carboxylic esters to azobisisobutyronitrile is (5 - 10):(0.02 - 0.05). Then stir at 40 - 60 °C for 7 - 10 hours, and heat the obtained mixture at 90 - 110 °C under vacuum to remove the excess solvent to obtain a crude product. Under the action of the AIBN initiator, a hydrosilylation reaction occurs between carboxylic esters and polymethylsilane, introducing carboxylic ester groups into polymethylsilane. Here, polymethylsilane can be replaced by one of its derivatives, including a nitrided derivative, an aminated derivative, and a borated derivative; carboxylic esters can be replaced by one of other unsaturated esters such as ethylene carbonate;

[0028] Further purify the obtained crude product. The specific method is as follows: Dissolve all the dried solid substances in a n-hexane solvent, then add methanol to precipitate it, and dry the obtained precipitate at 60 - 80 °C under vacuum, repeat 1 - 2 times to finally obtain carboxylic ester-grafted polymethylsilane; Carboxylic ester-grafted polymethylsilane has a long carbon chain, many active sites, and branches that can play a space exclusion role, and has good dispersion performance;

[0029] (2) Mechanically stir and mix commercially purchased zinc oxide nanoparticles (average particle size of 10 - 30 nm) and absolute ethanol at room temperature according to a mass ratio of (10 - 20):(80 - 150), with a stirring speed of 80 - 140 revolutions per minute and a stirring time of 30 - 60 minutes, and then continue ultrasonic stirring for 45 - 60 minutes to obtain a mixed solution a; Add the carboxylic ester-grafted polymethylsilane prepared in step (1) into absolute ethanol, and the mass ratio of carboxylic ester-grafted polymethylsilane to absolute ethanol is (15 - 30):(60 - 75), mechanically stir and mix at room temperature, with a stirring speed of 80 - 140 revolutions per minute and a stirring time of 30 - 60 minutes to obtain a mixed solution b, where the mass ratio of zinc oxide nanoparticles to carboxylic ester-grafted polymethylsilane is (10 - 20):(2.0 - 5.0);

[0030] (3) Mix solution a and the solution, ultrasonically mix at 35 - 50 °C for 30 - 50 minutes, subject the mixture to centrifugation, wash the obtained precipitate with absolute ethanol 2 - 4 times to remove excess solvents and reactants, and prepare polymethylsilane-zinc oxide nanocomposite particles. Zinc oxide nanoparticles have good chemical and mechanical stability, heat resistance, and corrosion resistance, and have excellent lithium ion transport properties. They can be used as additives for lithium ion battery separators. However, due to their high specific surface area and high surface energy, their dispersibility is poor and they tend to agglomerate. After being compounded with polymethylsilane grafted with carboxylic esters, their surface energy can be greatly reduced, the interfacial compatibility with other polymer materials can be improved, the dispersibility of zinc oxide nanoparticles is significantly improved, which helps zinc oxide disperse better in the organic matrix, and also improves the chemical and thermal stability of zinc oxide.

[0031] The above zinc oxide nanoparticles can be replaced by one of nano-sized alumina, titanium dioxide, tin dioxide, or silica.

[0032] (4) Dissolve polyimide and soy protein fiber in a N-methylpyrrolidone solvent according to a mass ratio of (60 - 80):(20 - 35), with the total solute mass fraction being 10 - 30%, stir at a speed of 80 - 120 revolutions per minute at room temperature for 30 - 45 minutes, then add the polymethylsilane-zinc oxide nanocomposite particles prepared in step (3), and continue to stir at room temperature for 3 - 5 hours to obtain a uniformly mixed system, where the mass ratio of polyimide to polymethylsilane-zinc oxide nanocomposite particles is (60 - 80):(3 - 6). Electrospinning treatment is carried out on the obtained mixed system, and the specific conditions are: voltage is 15 - 25 kV, and the spinning distance is 17 - 20 cm, to obtain a separator with a thickness of 5 - 15 μm. Compared with the conventional polyolefin system, polyimide has good thermal stability, chemical stability, and outstanding mechanical properties, and its heat resistance temperature reaches above 200 °C. In addition, its polar functional groups increase the affinity and wettability with the electrolyte; soy protein fiber is rich in source and has good stability. After being mixed with polyimide, it can increase the microporous structure and mechanical properties of the separator; the polymethylsilane-zinc oxide nanocomposite particles have good interfacial compatibility with polyimide and soy protein fiber, which can further optimize the mechanical stability and lithium ion transport performance of the separator.

[0033] N-methylpyrrolidone NMP can be replaced by one of other conventional organic solvents, such as ethyl acetate, acetonitrile, benzene, toluene, etc.

[0034] (5) Add the oligosilsesquioxane into ethyl acetate solvent, and stir at a speed of 80 - 120 revolutions per minute for 2 - 4 hours at room temperature to obtain a homogeneous mixture system, where the mass fraction of the solute is 20 - 40%. Deposit the oligosilsesquioxane on the two side surfaces of the separator prepared in step (4) by spray drying method; add the polymethylsilane-zinc oxide nanocomposite particles into ethyl acetate solvent, and stir at a speed of 80 - 120 revolutions per minute for 2 - 4 hours at room temperature to obtain a homogeneous mixture system, where the mass fraction of the solute is 20 - 40%. Deposit the polymethylsilane-zinc oxide nanocomposite particles on the two side surfaces of the separator with oligosilsesquioxane attached by spray drying method. Among them, the deposition thickness of the oligosilsesquioxane is 0.5 - 1.0 μm, and the deposition thickness of the polymethylsilane-zinc oxide nanocomposite particles is 1.0 - 2.0 μm. Too high thickness will lead to an increase in internal resistance, causing unnecessary material waste, and too low thickness will result in an insignificant technical effect.

[0035] The oligosilsesquioxane has good adhesion, high temperature resistance and high pressure resistance (>5V). Spraying it on the surface of the separator can increase the adhesion and attachment properties of the polymethylsilane-zinc oxide nanocomposite particles to the separator matrix, which is beneficial to improving the stability of the separator. At the same time, the poly(silsesquioxane) can effectively prevent the polymethylsilane-zinc oxide nanocomposite particles from penetrating into the separator matrix, avoiding local particle aggregation and blockage, improving the wettability with the electrolyte, and reducing the battery internal resistance.

[0036] The polymethylsilane-zinc oxide nanocomposite particles have excellent hydrophobic properties, which can effectively reduce the water absorption of the separator, avoid excessive internal water content in the lithium-ion battery caused by separator water absorption, effectively save the baking time of the separator and the battery core, reduce the time cost, avoid the wrinkles and deformation of the separator caused by long-term baking, and reduce the potential safety risk of the lithium-ion battery. In addition, the polymethylsilane will dissolve in the lithium-ion battery electrolyte during long-term charge and discharge processes, and can be used as a positive electrode protection additive to better passivate the surface of the positive electrode and reduce the interfacial resistance between the positive electrode material and the electrolyte, improving the positive electrode cycle life.

[0037] The present invention also provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, and a separator prepared by the method for preparing a lithium-ion battery separator described above. The preparation method of the positive electrode sheet is as follows: The positive electrode active material, conductive agent, and binder are put into a high-energy vibration ball mill according to a mass ratio of (80-90):(3-6):(5-10), and ball milled at room temperature for 30-60 minutes. Then, the mixed powder is transferred into a mold and pressed into a positive electrode sheet under 100-300 standard atmospheric pressures. The thickness of the obtained positive electrode sheet is 50-250 μm. The active material is a layered nickel-cobalt-manganese ternary material; the conductive agent is one of carbon black, carbon nanotubes, conductive graphite, carbon nanofibers, or Ketjen black; the binder is one of polyvinylidene fluoride PVDF or polyurethane;

[0038] The preparation method of the negative electrode sheet is as follows: The negative electrode active material, conductive agent, and binder are put into a high-energy vibration ball mill according to a mass ratio of (80-90):(5-10):(5-10), and ball milled at room temperature for 30-60 minutes. Then, the mixed powder is transferred into a mold and pressed into a negative electrode sheet under 100-300 standard atmospheric pressures. The thickness of the obtained negative electrode sheet is 50-150 μm. The active material is artificial graphite; the conductive agent is one of carbon black, carbon nanotubes, conductive graphite, carbon nanofibers, or Ketjen black; the binder is one of polyvinylidene fluoride PVDF, polyacrylic acid, polyacrylonitrile, or styrene-butadiene rubber;

[0039] The above separator, positive electrode sheet, and negative electrode sheet are assembled into a full cell. The electrolyte is selected as a 1.0 M LiFP6 solution, and the solvent is a mixed solution of EC / EMC with a volume ratio of 3:7. In addition, a small amount of VC is added as an additive. After the battery assembly is completed, it is left standing at room temperature for 3-6 hours, and then continuously charged and discharged for 3 weeks at a rate of 0.1C within the range of 2.8-4.3V. The discharge capacity in the third week is taken as the nominal capacity of the battery. Then, at room temperature, it is cycled 300 times at 0.5C / 0.5C within the range of 2.8-4.3V, and the capacity retention rate of all batteries (=discharge capacity in the 300th week / discharge capacity in the first week) is recorded. The test results are shown in Table 2.

[0040] The safety tests of the lithium-ion battery include nail penetration, hot box, and overcharge tests. Except for the overcharge test, all the test batteries are charged at a constant current and constant voltage of 0.2C to 100% SOC before the experiment. In the nail penetration test, the diameter of the nail is 2.5 mm, and the penetration speed is 200 mm / min until it is completely penetrated. In the hot box experiment, the temperature is raised to 200°C at a rate of 5°C / min and then maintained for 60 minutes. In the overcharge test, first, the battery is discharged at a constant current of 0.2C to 0% SOC, and then charged at a constant current of 1C to 200% SOC and maintained for 60 minutes.

[0041] Example 1

[0042] 1. Preparation of Lithium-Ion Battery Separator:

[0043] (1) Add the dehydroxylated carboxylic acid ester and polymethylsilane into tetrahydrofuran solvent according to a mass ratio of 8:75, stir at 50 °C for 3 hours under an inert atmosphere, then add a small amount of azobisisobutyronitrile (AIBN) as an initiator. The mass ratio of the carboxylic acid ester to azobisisobutyronitrile is 8:0.04, and then stir at 50 °C for 9 hours. Heat the obtained mixture at 100 °C under vacuum to remove the excess solvent to obtain a crude product; dissolve all the dried solid substances in n-hexane solvent, then add methanol to precipitate it, and dry the obtained precipitate at 70 °C in vacuum, repeat 1 - 2 times to finally obtain the carboxylic acid ester grafted polymethylsilane;

[0044] (2) Mechanically stir and mix commercially purchased zinc oxide nanoparticles (average particle size of 17 nm) and absolute ethanol at room temperature according to a mass ratio of 16:92, with a stirring speed of 110 revolutions per minute and a stirring time of 45 minutes, then continue ultrasonic stirring for 50 minutes to obtain mixture a; then add the carboxylic acid ester grafted polymethylsilane prepared in step (1) into absolute ethanol, with a mass ratio of 18:66 between them, and mechanically stir and mix at room temperature, with a stirring speed of 110 revolutions per minute and a stirring time of 45 minutes to obtain mixture b, where the mass ratio of zinc oxide nanoparticles to the carboxylic acid ester grafted polymethylsilane is 13:3.5;

[0045] (3) Mix solution a and solution b, ultrasonically mix at 40 °C for 40 minutes, centrifuge the mixture, and wash the obtained precipitate with absolute ethanol 3 times to remove the excess solvent and reactants, thus obtaining the polymethylsilane-zinc oxide nanocomposite particles;

[0046] (4) Dissolve polyimide and soybean protein fiber according to a mass ratio of 70:22 in N-methylpyrrolidone (NMP), with a total solute mass fraction of 20%, stir at a speed of 100 revolutions per minute at room temperature for 33 minutes, then add the polymethylsilane-zinc oxide nanocomposite particles prepared in step (3), and continue to stir at room temperature for 4 hours to obtain a uniformly mixed system, where the mass ratio of polyimide to the polymethylsilane-zinc oxide nanocomposite particles is 70:5. Electrospinning treatment is carried out on the obtained mixed system, and the specific conditions are: voltage is 20 kV, spinning distance is 18 cm, and a separator with a thickness of 9 μm is obtained;

[0047] (5) Add the oligosilsesquioxane and polymethylsilane-zinc oxide nanocomposite particles into ethyl acetate solvent respectively. The mass fraction of the solute is 30%. Stir at a speed of 110 revolutions per minute for 3 hours at room temperature to obtain a uniformly mixed system for each. Then, use the spray drying method to uniformly deposit the oligosilsesquioxane and polymethylsilane-zinc oxide nanocomposite particles on the two side surfaces of the separator prepared in step c in sequence. The deposition thickness of the oligosilsesquioxane is 0.8 μm, and the deposition thickness of the polymethylsilane-zinc oxide nanocomposite particles is 1.5 μm.

[0048] 2. Preparation and performance evaluation of lithium-ion batteries:

[0049] Put the cathode active material, conductive agent, and binder into a high-energy vibration ball mill according to a mass ratio of 88:5:7. Ball mill at room temperature for 37 minutes. Transfer the mixed powder into a mold and press it into a cathode plate at 180 standard atmospheres. The thickness of the cathode plate is 110 μm. The active material is a layered nickel cobalt manganese ternary material; the conductive agent is carbon nanotubes, and the binder is polyvinylidene fluoride PVDF;

[0050] Put the anode active material, conductive agent, and binder into a high-energy vibration ball mill according to a mass ratio of 88:6:6. Ball mill at room temperature for 45 minutes. Transfer the mixed powder into a mold and press it into an anode plate at 150 standard atmospheres. The thickness of the anode plate is 85 μm. The active material is artificial graphite, the conductive agent is carbon nanofibers, and the binder is polyvinylidene fluoride PVDF;

[0051] Compose the above separator, cathode plate, and anode plate into a full cell. The electrolyte is 1.0 M LiFP6 solution, and the solvent is a mixed solution of EC / EMC with a volume ratio of 3:7. Additionally, add a small amount of VC as an additive. After the battery assembly is completed, let it stand at room temperature for 4 hours, then continuously charge and discharge at a rate of 0.1C within the range of 2.8 - 4.3V for 3 weeks. Take the discharge capacity of the third week as the nominal capacity of the battery. Then, at room temperature, cycle at 0.5C / 0.5C within the range of 2.8 - 4.3V for 300 weeks, and record the capacity retention rate of all batteries (= discharge capacity of the 300th week / discharge capacity of the first week).

[0052] The safety tests of lithium-ion batteries include nail penetration, thermal box, and overcharge tests. Except for the overcharge test, all the test batteries are charged at a constant current and constant voltage of 0.2C to 100% SOC before the experiment. In the nail penetration test, the diameter of the nail is 2.5 mm, and the piercing speed is 200 mm / min until it is completely pierced. The thermal box experiment is to heat up to 200°C at a speed of 5°C / min and then maintain it for 60 minutes. In the overcharge test, first discharge the battery at a constant current of 0.2C to 0% SOC, then charge it at a constant current of 1C to 200% SOC and maintain it for 60 minutes.

[0053] Example 2

[0054] Compared with Example 1, the preparation process of the lithium-ion battery separator in Example 2 is as follows:

[0055] (1) The dehydroxylated carboxylic acid ester and polymethylsilane are added to the tetrahydrofuran solvent at a mass ratio of 10:70, stirred at 60 °C for 2 hours under an inert atmosphere, and then a small amount of azobisisobutyronitrile AIBN is added as an initiator. The mass ratio of the carboxylic acid ester to azobisisobutyronitrile is 5:0.05, and then stirred at 60 °C for 10 hours. The obtained mixture is heated at 110 °C under vacuum to remove the excess solvent, and a crude product is obtained. All the dried solid substances are dissolved in the n-hexane solvent, and then methanol is added to precipitate it. The obtained precipitate is dried at 80 °C in vacuum, and repeated 2 times to finally obtain the carboxylic acid ester-grafted polymethylsilane;

[0056] (2) The commercially purchased zinc oxide nanoparticles (average particle size of 30 nm) and absolute ethanol are mechanically stirred and mixed at a mass ratio of 20:80 at room temperature. The stirring speed is 80 revolutions per minute, and the stirring time is 60 minutes. Then, ultrasonic stirring is continued for 60 minutes to obtain the mixed solution a. Next, the carboxylic acid ester-grafted polymethylsilane prepared in step (1) is added to absolute ethanol, and the mass ratio of the two is 15:75. Mechanical stirring and mixing are carried out at room temperature. The stirring speed is 140 revolutions per minute, and the stirring time is 30 minutes to obtain the mixed solution b, where the mass ratio of the zinc oxide nanoparticles to the carboxylic acid ester-grafted polymethylsilane is 10:2.0.

[0057] (3) The solution a and the solution are mixed, ultrasonically mixed at 35 °C for 30 minutes, and the mixed solution is centrifuged. The obtained precipitate is washed 2 times with absolute ethanol to remove the excess solvent and reactants, and the polymethylsilane-zinc oxide nanocomposite particles are prepared.

[0058] (4) Polyimide and soybean protein fiber are dissolved in N-methylpyrrolidone NMP at a mass ratio of 80:20, and the total solute mass fraction is 10%. Stirred at a speed of 80 revolutions per minute at room temperature for 30 minutes, and then the polymethylsilane-zinc oxide nanocomposite particles prepared in step (3) are added. Stirring is continued at room temperature for 3 hours to obtain a uniformly mixed system, where the mass ratio of polyimide to the polymethylsilane-zinc oxide nanocomposite particles is 80:3. The obtained mixed system is subjected to electrospinning treatment. The specific conditions are: voltage of 25 kV and spinning distance of 17 cm to obtain a separator with a thickness of 5 μm.

[0059] Step (5), the preparation of the lithium-ion battery and the performance evaluation method are the same as those in Example 1.

[0060] Example 3

[0061] Compared with Example 1, the preparation process of the lithium-ion battery separator in Example 3 is as follows:

[0062] (5) Add oligosilsesquioxane and polymethylsilane-zinc oxide nanocomposite particles into ethyl acetate solvent respectively. The mass fraction of the solute is 20%. Stir at a speed of 80 revolutions per minute at room temperature for 2 hours to obtain a uniformly mixed system for each. Then, use the spray drying method to uniformly deposit oligosilsesquioxane and polymethylsilane-zinc oxide nanocomposite particles on both surface sides of the separator prepared in step c in sequence. The deposition thicknesses are 1.0 μm and 2.0 μm respectively.

[0063] Steps (1), (2), (3), (4), the preparation of the lithium-ion battery and the performance evaluation method are the same as those in Example 1.

[0064] Example 4

[0065] Compared with Example 1, the preparation conditions of the lithium-ion battery separator in Example 4 are as follows:

[0066] The polymethylsilane in step (1) can be replaced by its nitrided derivative; the carboxylic acid esters in step (1) can be replaced by ethylene carbonate; the nano-zinc oxide particles in step (2) can be replaced by nano-sized alumina; the conductive agent in the positive electrode sheet during the preparation of the lithium-ion battery is conductive graphite, and the conductive agent in the negative electrode sheet is carbon black. Other conditions are the same as those in Example 1;

[0067] Comparative Example 1

[0068] Compared with Example 1, the polymethylsilane in the lithium-ion battery separator in Comparative Example 1 does not contain carboxylic acid ester groups, and other conditions are the same as those in Example 1;

[0069] Comparative Example 2

[0070] Compared with Example 1, the lithium-ion battery separator in Comparative Example 2 does not contain polymethylsilane, and other conditions are the same as those in Example 1;

[0071] Comparative Example 3

[0072] Compared with Example 1, the lithium-ion battery separator in Comparative Example 3 does not contain nano-zinc oxide particles, and only adds polymethylsilane grafted with carboxylic acid esters. Other conditions are the same as those in Example 1;

[0073] Comparative Example 4

[0074] Compared with Example 1, the lithium-ion battery separator in Comparative Example 4 uses pure nano-zinc oxide particles without being compounded with polymethylsilane. Other conditions are the same as those in Example 1;

[0075] Comparative Example 5

[0076] Compared with Example 1, the lithium-ion battery separator matrix in Comparative Example 5 does not contain soy protein fibers, and the remaining conditions are the same as those in Example 1;

[0077] Comparative Example 6

[0078] Compared with Example 1, the lithium-ion battery separator matrix in Comparative Example 6 does not contain polyimide, and the remaining conditions are the same as those in Example 1;

[0079] Comparative Example 7

[0080] Compared with Example 1, the lithium-ion battery separator matrix in Comparative Example 7 does not contain zinc oxide nanocomposite particles, and the remaining conditions are the same as those in Example 1;

[0081] Comparative Example 8

[0082] Compared with Example 1, the lithium-ion battery separator matrix in Comparative Example 8 is a conventional polyethylene, and the remaining conditions are the same as those in Example 1;

[0083] Comparative Example 9

[0084] Compared with Example 1, the lithium-ion battery separator in Comparative Example 9 is only coated with oligosilsesquioxane, and the remaining conditions are the same as those in Example 1;

[0085] Comparative Example 10

[0086] Compared with Example 1, the lithium-ion battery separator in Comparative Example 9 is only coated with polymethylsilane-zinc oxide nanocomposite particles, and the remaining conditions are the same as those in Example 1;

[0087] Comparative Example 11

[0088] Compared with Example 1, the thickness of the polymethylsilane-zinc oxide nanocomposite particles on the surface of the lithium-ion battery separator in Comparative Example 11 is 4 μm, and the remaining conditions are the same as those in Example 1;

[0089] Comparative Example 12

[0090] Compared with Example 1, the thickness of the oligosilsesquioxane coating on the surface of the lithium-ion battery separator in Comparative Example 12 is 3 μm, and the remaining conditions are the same as those in Example 1;

[0091] Comparative Example 13

[0092] Compared with Example 1, the lithium-ion battery separator in Comparative Example 13 uses a conventional commercial polyethylene and is not coated, and the remaining conditions are the same as those in Example 1;

[0093] The specific results are shown in Tables 1 and 2. In combination with Examples 1-4, the preparation method of the present invention was used to obtain a separator having excellent microporous structure and thermodynamic properties, including air permeability, porosity, thermal shrinkage, and tensile strength, ensuring sufficient lithium ion transmission channels and good thermodynamic stability. The prepared lithium ion battery has high capacity and cycle life. At the same time, safety performance including needle puncture, hot box, and overcharge was also significantly improved. Among them, Example 1 has the best technical effect. In combination with Example 1 and Comparative Examples 1-4, 7, 8, and 13, the addition of zinc oxide nanoparticles to the matrix or surface of the lithium ion battery separator can significantly enhance the heat resistance and mechanical stability of the separator, improve the structural and chemical stability of the separator during long-term cycling, and improve the safety of the lithium ion battery. The puncture and overcharge tests were passed (no fire or explosion). In the hot box test, the battery did not catch fire or explode until it was left for about 40 minutes. The safety time is much longer than that of Comparative Examples 8 and 13 using conventional polyolefin separators. Compared with pure zinc oxide nanoparticles, the addition of carboxylate-grafted polymethylsilane can significantly improve the surface energy of zinc oxide, improve the dispersibility of zinc oxide, avoid local agglomeration, and thus improve the technical effect. In combination with Example 1 and Comparative Examples 5 and 6, the addition of soybean protein fiber to the diaphragm matrix can optimize the porosity, tensile strength and thermal shrinkage properties of the diaphragm, which is beneficial to improving the safety and cycle life of lithium-ion batteries. In combination with Example 1 and Comparative Examples 9-12, the surface of the diaphragm is coated with oligomeric silsesquioxane and polymethylsilane-zinc oxide nanocomposite particles, which is beneficial to improving the stability of the diaphragm, preventing the polymethylsilane-zinc oxide nanocomposite particles from penetrating into the diaphragm matrix, and reducing the potential safety risks of lithium-ion batteries. In addition, polymethylsilane will dissolve in the lithium-ion battery electrolyte during long-term charging and discharging, and can be used as a positive electrode protection additive to improve the battery cycle life. In summary, it can be seen that the method proposed in the present invention can significantly improve the safety and electrochemical properties of lithium-ion battery diaphragms, and provide a technical reference for the development of high-performance lithium-ion power batteries.

[0094] Table 1 is a comparison of the electrochemical properties of the diaphragms prepared under different conditions:

[0095]

[0096] Table 1

[0097] Table 2 is a comparison of the safety performance of diaphragms prepared under different conditions:

[0098] Acupuncture Hot box (minutes) Overcharge Example 1 Passed 43 Passed Example 2 Passed 40 Passed Example 3 Passed 41 Passed Example 4 Passed 39 Passed Comparative Example 1 Failed 35 Passed Comparative Example 2 Failed 33 Passed Comparative Example 3 Failed 25 Failed Comparative Example 4 Failed 30 Passed Comparative Example 5 Failed 36 Passed Comparative Example 6 Failed 22 Failed Comparative Example 7 Failed 24 Failed Comparative Example 8 Failed 16 Failed Comparative Example 9 Failed 34 Passed Comparative Example 10 Failed 32 Passed Comparative Example 11 Passed 50 Passed Comparative Example 12 Passed 49 Passed Comparative Example 13 Failed 12 Failed

[0099] Table 2

[0100] The above has schematically described the present invention and its embodiments. Such description is not restrictive. Therefore, if those of ordinary skill in the art are inspired by it and, without departing from the gist of the present invention, creatively design structural forms and embodiments similar to the technical solution, they shall fall within the protection scope of the present invention.

Claims

1. A method for preparing a lithium ion battery separator, characterized in that: The steps include: (1) Adding dehydroxylated carboxylic acid esters and polymethylsilane in a mass ratio of (5-10): (70-90) to tetrahydrofuran solvent, stirring at 40-60°C for 2-4 hours under an inert atmosphere, adding a small amount of azobisisobutyronitrile (AIBN) as an initiator, and stirring at 40-60°C for 7-10 hours. The resulting mixture is heated at 90-110°C in vacuum to remove excess solvent to obtain a crude product, which is further purified to obtain carboxylic acid ester-grafted polymethylsilane; (2) Mechanically stirring and mixing zinc oxide nanoparticles and anhydrous ethanol at a mass ratio of (10-20): (80-150) at room temperature, and then ultrasonically stirring to obtain a mixed solution a; adding the carboxylate-grafted polymethylsilane prepared in step (1) to anhydrous ethanol, wherein the mass ratio of the carboxylate-grafted polymethylsilane to anhydrous ethanol is (15-30): (60-75), and mechanically stirring and mixing at room temperature to obtain a mixed solution b; (3) Mixing the mixed solution a prepared in step (2) with the mixed solution b, centrifuging the mixed solution, and washing the obtained precipitate with anhydrous ethanol 2-4 times to remove excess solvent and reactants, thereby preparing polymethylsilane-zinc oxide nanocomposite particles; (4) dissolving polyimide and soybean protein fiber in a methyl pyrrolidone solvent at a mass ratio of (60-80): (20-35) with a total solute mass fraction of 10-30%, adding the polymethylsilane-zinc oxide nanocomposite particles prepared in step (3) after stirring at room temperature, and continuing stirring at room temperature for 3-5 hours to obtain a uniformly mixed system, and electrospinning the obtained mixed system to obtain a diaphragm with a thickness of 5-15 μm; The method further comprises the following steps: adding oligomeric silsesquioxane to an ethyl acetate solvent, stirring at room temperature to obtain a uniform mixed system, and depositing the oligomeric silsesquioxane on both sides of the membrane prepared in step (4) by a spray drying method; adding polymethylsilane-zinc oxide nanocomposite particles to an ethyl acetate solvent, stirring at room temperature to obtain a uniform mixed system, and depositing the polymethylsilane-zinc oxide nanocomposite particles on both sides of the membrane to which the oligomeric silsesquioxane is attached by a spray drying method; The deposition thickness of the oligomeric silsesquioxane is 0.5-1.0 μm, and the deposition thickness of the polymethylsilane-zinc oxide nanocomposite particles is 1.0-2.0 μm.

2. The method for preparing a lithium ion battery separator according to claim 1, wherein In the step (2), the mass ratio of zinc oxide nanoparticles to carboxylate-grafted polymethylsilane is (10-20): (2.0-5.0).

3. The method for preparing a lithium ion battery separator according to claim 1, wherein: The purification step in step (1) is as follows: dissolving the dried crude product in n-hexane solvent, then adding methanol to precipitate it, drying the obtained precipitate at 60-80° C. in a vacuum, and repeating 1-2 times to finally obtain carboxylate-grafted polymethylsilane.

4. The method for preparing a lithium ion battery separator according to claim 1, wherein: The conditions for electrospinning in step (4) are: voltage of 15-25 kV, and spinning distance of 17-20 cm.

5. The method for preparing a lithium ion battery separator according to claim 1, wherein: The mass ratio of polyimide to polymethylsilane-zinc oxide nanocomposite particles in step (4) is (60-80):(3-6).

6. The method for preparing a lithium ion battery separator according to claim 1, wherein: The mass ratio of carboxylic acid esters to azobisisobutyronitrile in step (1) is (5-10): (0.02-0.05).

7. The method for preparing a lithium ion battery separator according to claim 1, wherein: In step (2), the mechanical stirring speed is 80-140 rpm, the mechanical stirring time is 30-60 minutes, and the ultrasonic stirring time is 45-60 minutes.

8. A lithium-ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet and a separator prepared by the method for preparing a lithium-ion battery separator according to any one of claims 1 to 7.

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