A method for preparing a waterborne polyurethane composite swelling film

Abstract

This invention relates to the field of waterborne polyurethane membrane preparation technology, specifically to a method for preparing a waterborne polyurethane composite swelling membrane, comprising the following steps: mesoporous silica and graphene oxide undergo a condensation reaction under the action of a coupling agent to form a GO-MSN composite material; the GO-MSN composite material is dispersed in deionized water and then mixed and stirred with a mixed matrix formed by a thermoplastic polyurethane dispersion and a waterborne polyurethane emulsion to prepare a composite casting solution; after coating, gradient drying, swelling treatment, washing, and drying again, a battery separator is obtained; the battery separator prepared by this invention introduces a composite of polyether-type thermoplastic polyurethane dispersion and polyether-type waterborne polyurethane emulsion, and supplements it with GO-MSN nanocomposite material as a reinforcing phase, constructing a synergistic reinforcing structure of flexible matrix-rigid nanoframework; the tensile strength of the prepared separator reaches 24.9 MPa, the puncture resistance reaches 1.2 N / μm, and the thermal stability and ionic conductivity of the separator are greatly improved.

Description

Technical Field

[0001] This invention relates to the field of waterborne polyurethane membrane preparation technology, specifically to a method for preparing a waterborne polyurethane composite swelling membrane. Background Technology

[0002] Lithium-ion battery separators are insulating polymer films with microporous structures, typically 5-25 μm thick. The main type used is polyolefin separators. Currently, commercially available lithium-ion battery polyolefin separators are mainly made from polypropylene (PP), polyethylene (PE), and other polyolefin materials through dry or wet processes. Their main function is to separate the positive and negative electrodes to prevent short circuits and to enable lithium-ion transport through the microporous structure.

[0003] The hydrophobic surface of polyolefin separators leads to poor wettability and retention of liquid electrolytes. Polyolefin separators also have poor thermal stability and electrochemical performance. Currently, modified coatings are often constructed on the separator surface to improve separator performance. Among them, the most widely used method is coating, which includes inorganic coating, organic coating, and organic-inorganic composite coating. Currently, commonly used oxides in inorganic coating include alumina, magnesium hydroxide, silicon dioxide, zirconium oxide, titanium dioxide, and boehmite. The high heat resistance and mechanical strength of inorganic particles can improve the mechanical strength and thermal stability of the separator. Related research is relatively mature and has been applied to commercial batteries. However, the accumulation of inorganic particles can also lead to problems such as decreased electrolyte adsorption and low ionic conductivity.

[0004] Waterborne polyurethane (WPU) materials have been widely used in military, civilian, sports, and aerospace fields due to their green and environmentally friendly properties, excellent thermal stability, ease of modification, and non-flammability. They have become one of the most commonly used synthetic polymer materials today. Waterborne polyurethane (WPU) has potential application value in the field of battery separators due to its environmentally friendly film formation, good flexibility, and strong adhesion. Some literature studies have used waterborne polyurethane in the production of battery separators.

[0005] Due to its inherently insufficient mechanical properties, waterborne polyurethane exhibits low puncture resistance in the dry state, and its mechanical properties further deteriorate after swelling treatment. This makes it susceptible to puncture by electrode burrs during battery assembly and membrane damage during cyclic charging and discharging, leading to battery capacity decay and shortened lifespan. Secondly, waterborne polyurethane has poor thermal stability. When the battery overheats, the membrane is prone to shrinkage or even rupture, causing short circuits between the positive and negative electrodes and posing a risk of thermal runaway. Furthermore, as waterborne polyurethane is an insulating polymer with poor molecular chain regularity and no continuous ion transport channels, waterborne film formation tends to create a dense structure, resulting in obstructed ion transport channels, low ionic conductivity, high internal resistance of the battery membrane, increased heat generation during battery charging and discharging, poor battery charge / discharge rate performance, and significant voltage drop at high rates.

[0006] Thermoplastic polyurethane (TPU) is a thermoplastic polymer with both soft and hard segments. This structural design gives TPU adjustable hardness, high elasticity, abrasion resistance, oil resistance, solvent resistance, and puncture resistance. TPU also has applications in battery separator fabrication, specifically through blending TPU with polyacrylonitrile (PAN) and then passing the resulting fiber membrane through… 60 Co-γ-ray modification was used to prepare a novel lithium-ion battery separator. By using a blending method, TPU and PAN formed a uniform continuous phase, which improved the overall mechanical properties of the composite separator. However, there is no research on the use of thermoplastic polyurethane dispersions and waterborne polyurethane as composite matrices. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a method for preparing battery separators using waterborne polyurethane composite thermoplastic polyurethane, which solves the following problems existing in traditional methods: 1. The mechanical properties of battery separators prepared with waterborne polyurethane are insufficient, affecting battery life; 2. Waterborne polyurethane has poor thermal stability, and the battery separator is prone to thermal runaway risk when overheated; 3. Waterborne polyurethane tends to form a dense structure during film formation, resulting in low ionic conductivity and significant battery voltage drop.

[0008] To address the aforementioned problems, this invention provides a method for preparing an aqueous polyurethane composite swelling film, comprising the following steps: S1, preparation of modified slurry: adding modified nanoparticles to deionized water under low-speed stirring, then switching to high-speed stirring and continuously stirring for 30-60 minutes to form a modified slurry; the mass-to-volume ratio of modified nanoparticles to deionized water is 1g:(60-80)mL, and the modified nanoparticles are GO–MSN composite materials, obtained by condensation reaction of graphene oxide and mesoporous silica under the action of a coupling agent.

[0009] S2. Resin matrix mixing: The polyether-type waterborne polyurethane emulsion and the polyether-type thermoplastic polyurethane dispersion are mixed by stirring at a mass ratio of (70-90):(10-30), and ultrasonic-assisted treatment is performed at a frequency of 20-30kHz and a power of 100-150W to obtain the mixed matrix.

[0010] S3. Preparation of composite casting solution: Mix the mixed matrix and modified slurry, and add four additives in sequence: dispersant, defoamer, pore-forming agent and crosslinking agent. Continue stirring while adding the additives to obtain the composite casting solution. The mass ratio of the mixed matrix, modified slurry, dispersant, defoamer, pore-forming agent and crosslinking agent is: 100%: (3%-10%): (0.5%-2.0%): (0.1%-0.5%): (5%-10%): (1%-3%).

[0011] S4. Coating and film formation: Fix the PP base film on the worktable of the coating machine, and apply the composite casting liquid onto the PP base film using the doctor blade coating method. Transfer the coated substrate to the drying oven. The composite casting liquid is applied in a thin coating and multi-layer stacking manner, with a single coating amount of 0.5-1g / m² and 2-4 stacking times.

[0012] S5. Gradient drying and shaping: In the first stage, dry at a low temperature of 40-50℃ for 30-60 minutes. In the second stage, raise the temperature to 60-80℃ and dry for 2-4 hours. Then peel off the dried composite film.

[0013] S6. Modified swelling treatment: The peeled composite membrane is completely immersed in the swelling solution and subjected to ultrasonic-assisted treatment at a frequency of 20-40kHz and a power of 200-300W for 30-60 minutes.

[0014] S7. Post-treatment: The swollen composite membrane is washed with deionized water until neutral, and then placed in a drying oven for gradient drying. The temperature of the gradient drying is the same as in step S5 to obtain the desired waterborne polyurethane composite swollen membrane.

[0015] Preferably, in step S1, the initial low-speed stirring speed of the deionized water is 300-500 rpm, and the high-speed stirring speed after all the modified nanopowder is added is 2000-4000 rpm.

[0016] Preferably, the modified nanopowder preparation process in step S1 is as follows: S11, mesoporous silica pretreatment: mesoporous silica is dispersed in anhydrous ethanol, ultrasonically dispersed at a frequency of 30kHz and a power of 300-500W for 30-60min, then concentrated hydrochloric acid is added, the temperature is raised to 50-60℃, and refluxed and stirred for 2-4h; then filtered, washed 3 times with anhydrous ethanol, and vacuum dried at 60-80℃ for 12h for later use; wherein the mass-volume ratio of mesoporous silica, anhydrous ethanol and concentrated hydrochloric acid is 1g:(30-50)mL:(1.5-2.5)mL.

[0017] Preparation of S12 and NH2-MSN: Anhydrous toluene was added to the pretreated mesoporous silica, and the mixture was ultrasonically dispersed at a frequency of 30 kHz and a power of 200-400 W for 15 min. Intermittent ultrasonication was performed with a 2 min pause after every 5 min of ultrasonication. Under nitrogen protection, 0.2-0.5 parts by weight of aminosilane coupling agent were added to the mesoporous silica. The temperature was raised to 80-110℃, and the mixture was refluxed and stirred for 8-12 h. The mixture was cooled to room temperature, filtered, washed 3 times with anhydrous toluene, and vacuum dried at 60℃ for 12 h to obtain amino-functionalized mesoporous silica (NH2-MSN). The mass-to-volume ratio of mesoporous silica to anhydrous toluene was 1 g: (20-40) mL.

[0018] S13. Preparation of GO aqueous dispersion: Graphene oxide was dispersed in deionized water and ultrasonically dispersed for 30-60 min at a frequency of 30 kHz and a power of 300-500 W. The dispersion was prepared by using an intermittent mode of ultrasonication for 5 min followed by a 2 min pause. Hydroquinone was added and stirred for 1 h to obtain a pretreated GO aqueous dispersion. The mass-volume ratio of graphene oxide, hydroquinone, and deionized water was 1 g: (0.1-0.15) g: (200-500) mL. The mass ratio of graphene oxide to mesoporous silica was 1: (3-4).

[0019] S14. Production of modified nanopowders: The pretreated GO aqueous dispersion is mixed with NH2-MSN, the pH is adjusted to 4-5, the temperature is raised to 40-50℃, stirred for 12-24h, filtered, washed 4 times with deionized water and anhydrous ethanol, and dried under vacuum at 60℃ for 12h to obtain modified nanopowders.

[0020] Preferably, step S2 is as follows: A polyether-type thermoplastic polyurethane dispersion (polyether-type TPUD) is stirred at 30-40℃ at a speed of 200-250 rpm for 10 minutes. Half the total amount of compatibilizer is added to the polyether-type thermoplastic polyurethane dispersion, and stirring is continued for another 10 minutes to obtain a modified TPU dispersion. The modified TPU dispersion is then added dropwise to a polyether-type aqueous polyurethane emulsion (polyether-type APUE) being stirred at high speed, and the remaining half of the compatibilizer is added to the polyether-type aqueous polyurethane emulsion. Ultrasonic-assisted mixing is used while stirring continuously to finally obtain a mixed matrix. The dropping rate of the TPU dispersion is 3-5 mL / min, the ultrasonic power is 100-150 W, and the ultrasonic time is 20-30 minutes.

[0021] Preferably, the defoamer is a polyether-modified siloxane, and the pore-forming agent is one or more of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and ammonium bicarbonate.

[0022] Preferably, the compatibilizer is one or more of PEO-PU-PEO triblock copolymer, carboxyl-modified acrylate copolymer emulsion, and propylene glycol methyl ether. The total amount of compatibilizer added is 3%-6% of the solid content of the mixed matrix. The compatibilizer needs to be prepared into a solution with deionized water before being added.

[0023] Preferably, the dispersant is one or more of polyethylene glycol, polyvinylpyrrolidone, Tween 80, and sodium dodecyl sulfate.

[0024] Preferably, when the modified slurry is poured into the mixing matrix at a rate of 10-12 mL / min, the mixing matrix is ​​stirred at a speed of 200-300 rpm. After all the modified slurry is added, it is stirred at 400-600 rpm for 10-15 min. When adding the additives, the speed is reduced to 200 rpm, and stirring is continued for 10 min after each additive is added.

[0025] Preferably, the crosslinking agent is one or more of polyethylene glycol diglycidyl ether (CAS No.: 2224-15-9), modified polyether epoxy crosslinking agent, and 4-butanediol diglycidyl ether (CAS No.: 2425-79-8).

[0026] Preferably, the swelling solution is prepared by mixing deionized water and anhydrous ethanol in a ratio of (3-4):1. The swelling solution also contains 0.2%-0.5% polyethylene glycol, 0.5%-1.0% aminosilane coupling agent, and 5%-8% inorganic salt relative to the total mass of the swelling solution. The polyethylene glycol is first dissolved in anhydrous ethanol, then mixed with deionized water, and then the pre-hydrolyzed aminosilane coupling agent and inorganic salt are added. The diaphragm is ultrasonicated at a temperature of 25-35°C for 30-60 minutes. The inorganic salt can be sodium chloride or potassium chloride.

[0027] This invention has at least one of the following technical effects: First, by introducing a polyether-type thermoplastic polyurethane dispersion and a polyether-type aqueous polyurethane emulsion composite, and supplementing it with GO-MSN nanocomposite material as a reinforcing phase, a synergistic reinforcing structure of flexible matrix-rigid nanoskeleton is constructed. The tensile strength of the resulting separator reaches 24.9 MPa, and the puncture resistance reaches 1.2 N / μm, which is about 30% higher than the mechanical properties of existing separators on the market. This reduces the risk of separator puncture due to electrode burrs during battery assembly and effectively inhibits separator structure deterioration during cycling, thus extending battery life.

[0028] 2. Both GO and MSN are high thermal stability materials. The GO-MSN composite material is connected by covalent bonds to build a continuous three-dimensional network structure in the membrane, which significantly improves the heat distortion temperature and mechanical strength of the membrane. The thermal stability of the membrane can reach more than 372℃ and the thermal shrinkage rate is less than 2.5%.

[0029] Third, by constructing ion channels through the mesoporous and lamellar structures of the GO-MSN composite material itself, the membrane forms a porous structure with high porosity and high connectivity, which greatly improves the ionic conductivity, helps the electrolyte to fully wet, significantly reduces the internal resistance of the battery, and alleviates the voltage drop phenomenon.

[0030] IV. By adjusting parameters such as the WPU / TPU ratio, GO-MSN addition amount, and swelling solution composition, the porosity, mechanical strength, and electrochemical performance of the separator can be controlled within a wide range to meet the performance requirements of the separator for different application scenarios such as power batteries, energy storage batteries, and flexible batteries. Attached Figure Description

[0031] Figure 1 This is a scanning electron microscope image of the modified nanopowder prepared in Example 1 of this invention.

[0032] Figure 2 This is a sample image of the waterborne polyurethane composite swelling film prepared in Example 1 of the present invention. Detailed Implementation

[0033] The present invention will now be described in detail through specific embodiments. However, these illustrative embodiments are for purposes and uses only to illustrate the invention and do not constitute any limitation on the actual scope of protection of the invention, nor are they intended to limit the scope of protection of the invention to these embodiments. All equivalent transformations or simple substitutions made based on the substantive content of this application should fall within the scope of protection of this application. For parameter ranges not mentioned, intermediate values ​​are selected. Furthermore, for mass percentages or weight percentages not explicitly stated or mentioned, they generally refer to the final concentration after addition.

[0034] The singular forms “for,” “or,” “a,” “any,” and “the” used in this application are intended to include the plural forms unless the context clearly indicates otherwise.

[0035] Example 1.

[0036] S1. Preparation of modified slurry: S11. Pretreatment of mesoporous silica: Take 9g of mesoporous silica and disperse it in 270mL of anhydrous ethanol. Ultrasonic treatment is carried out at a frequency of 30kHz and a power of 300W for 30min. After every 5min of ultrasonic treatment, pause for 2min. Add 13.5mL of concentrated hydrochloric acid with a mass fraction of 37%, heat to 50℃, and reflux and stir for 2h to remove the residual template agent on the surface of mesoporous silica and activate the silanol groups. Filter, wash 3 times with anhydrous ethanol, and vacuum dry at 60℃ for 12h to obtain pretreated mesoporous silica.

[0037] Preparation of S12 and NH2-MSN: 270 mL of anhydrous toluene was added to the pretreated mesoporous silica, and the mixture was ultrasonically dispersed at a frequency of 30 kHz and a power of 200 W for 15 min. After every 5 min of ultrasonication, the mixture was paused for 2 min, and 1.8 g of γ-aminopropyltriethoxysilane (APTES) was added. Under nitrogen protection, the mixture was heated to 80 °C and refluxed for 8 h to allow the ethoxy group of γ-aminopropyltriethoxysilane to undergo a condensation reaction with the silanol group (Si–OH) on the surface of the mesoporous silica (MSN) to graft amino groups. After cooling to room temperature, the mixture was filtered, washed three times with anhydrous toluene, and dried under vacuum at 60 °C for 12 h to obtain amino-functionalized mesoporous silica (NH2-MSN).

[0038] S13, Preparation of GO aqueous dispersion: 3g of graphene oxide was dispersed in 400mL of deionized water and ultrasonicated at a frequency of 30kHz and a power of 300W for 30min. After every 5min of ultrasonication, the mixture was paused for 2min to obtain the GO aqueous dispersion. 0.3g of hydroquinone was added to the GO aqueous dispersion and stirred for 1h to activate the carboxyl groups on the GO surface, thus obtaining the pretreated GO aqueous dispersion.

[0039] S14. Production of modified nanopowder: The pretreated GO aqueous dispersion and NH2-MSN were mixed, and the pH of the system was adjusted to 4 with dilute hydrochloric acid. The temperature was raised to 40℃ and magnetically stirred at 200 rpm for 12 h to allow the activated carboxyl groups (–COOH) on the GO surface to undergo an amidation reaction with the –NH2 on the NH2-MSN surface. The hydroxyl groups in the carboxyl groups were replaced by amino groups to form covalent bonds. The mixture was filtered and washed four times with deionized water and anhydrous ethanol, and then vacuum dried at 60℃ for 12 h to obtain the GO–MSN composite material, i.e., the modified nanopowder.

[0040] Place 180mL of deionized water in a mixing cup, and add 3g of modified nanoparticles while magnetically stirring at 400rpm. After all the modified nanoparticles have been added, continue stirring at 3000rpm for 30-60min to form a modified slurry.

[0041] S2. Resin matrix mixing: Take 28.6g of waterborne polyether-type TPUD dispersion, heat it in a water bath at 35℃, and stir it at 200rpm for 10min. Heating and stirring can reduce the viscosity of the polyether-type TPUD dispersion, making it easier to mix with the polyether-type waterborne polyurethane emulsion in the next step. Add a compatibilizer containing 3g of PEO-PU-PEO triblock copolymer to the polyether-type TPUD dispersion that is being continuously stirred, and continue stirring for 10min to obtain the modified TPU dispersion.

[0042] Take 257.1g of polyether-type WPU emulsion, stir at high speed of 3000rpm, add modified TPU dispersion dropwise to polyether-type WPU emulsion at 3mL / min, add compatibilizer solution containing 3g of PEO-PU-PEO triblock copolymer to polyether-type WPU emulsion, sonicate at 20kHz for 30min, ultrasonic power 150W, to obtain mixed matrix.

[0043] S3. Preparation of casting solution: Place the mixed matrix in a stirring container and stir the mixed matrix at 200 rpm. Slowly pour the modified slurry into the mixed matrix at a rate of 10 mL / min. After all the modified slurry has been poured in, increase the stirring speed to 400 rpm and continue stirring for 10 min.

[0044] Reduce the speed to 200 rpm, and add 0.5 mL of polyethylene glycol, 0.1 mL of polyether-modified siloxane, 5 g of polyvinylpyrrolidone, and 1 mL of polyethylene glycol diglycidyl ether to the mixture in sequence. After each additive is added, continue stirring for 10 min to finally obtain the composite casting solution.

[0045] S4. Coating and film formation: Fix the PP base film on the worktable of the coating machine, and apply the composite casting liquid onto the PP base film using the doctor blade coating method. Transfer the coated substrate to the drying oven. The doctor blade coating method is an existing technology and will not be described in detail.

[0046] Among them, the composite casting solution adopts a thin coating and multi-layer stacking method, with a single coating amount of 0.5g / m² and a stacking number of 4 times. While ensuring the thickness of the diaphragm, it improves the pore connectivity and enhances the ionic conductivity.

[0047] S5. Gradient drying and shaping: In the first stage, the film is dried at a low temperature of 40℃ for 30 minutes. In the second stage, the temperature is raised to 60℃ and dried for 2 hours. The dried composite film is then peeled off.

[0048] S6. Modified swelling treatment: Dissolve 1.6g of polyethylene glycol in 600mL of anhydrous ethanol, then mix with 200mL of deionized water, and then add 4g of γ-aminopropyltriethoxysilane and 40g of sodium chloride to form a swelling solution. Immerse the diaphragm in the swelling solution at 25℃ and sonicate at 20kHz and 200W for 30min.

[0049] S7. Post-treatment: The swollen composite membrane is washed with deionized water until neutral, and then placed in a drying oven for gradient drying. The temperature of the gradient drying is the same as in step S5 to obtain the desired waterborne polyurethane composite swollen membrane.

[0050] The modified nanopowder prepared in Example 1 was subjected to electron microscopy scanning, and the electron micrograph observed at 500 nm is shown below. Figure 1As shown; meanwhile, a sample image of the waterborne polyurethane composite swelling film prepared in this embodiment is shown in the figure. Figure 2 As shown.

[0051] Example 2-15.

[0052] Examples 2-15 refer to the process flow and experimental methods of Example 1, with some parameters adjusted, as detailed in Tables 1-6. Parameter selection covers all endpoint and intermediate values ​​in the claims to ensure correct mass ratios and consistency between the total mass and the sum of the individual mass fractions.

[0053] Table 1. Process parameters for step S1 in Examples 1-9

[0054]

[0055] Table 2 Process parameters for step S2 in Examples 1-9

[0056]

[0057] Table 3 Process parameters for steps S3-S6 in Examples 1-9

[0058]

[0059] Table 4 Process parameters for step S1 in Examples 10-15

[0060]

[0061] Table 5. Process parameters for step S2 in Examples 10-15

[0062]

[0063] Table 6. Process parameters for steps S3-S6 in Examples 10-15

[0064]

[0065] Comparative example.

[0066] Comparative Examples 1-15 refer to the process flow of Example 1, but adjust some key components or parameters, and demonstrate disadvantages through omission, replacement or exceeding the range, as shown in Tables 7 and 4.

[0067] Table 7 Process parameters for step S1 in Comparative Examples 1-9

[0068]

[0069] Table 8. Process parameters for step S2 in Comparative Examples 1-9

[0070]

[0071] Table 9. Process parameters for steps S3-S6 of Comparative Examples 1-9

[0072]

[0073] Table 10 Process parameters for step S1 in Comparative Examples 10-15

[0074]

[0075] Table 11 Process parameters for step S2 in Comparative Examples 10-15

[0076]

[0077] Table 12 Process parameters for steps S3-S6 of Comparative Examples 10-15

[0078]

[0079] The above examples and comparative examples illustrate the preparation process and key process parameters of the swelling film. Examples 1-15 cover the endpoint values ​​of all parameters in the claims, ensuring the accuracy of the mass ratio. Comparative examples 1-15 verify the necessity of each component and parameter by replacing key components (graphene oxide, mesoporous silica), omitting key components (polyether thermoplastic polyurethane dispersion, modified nanoparticles), omitting key processing steps (swelling treatment, ultrasonic-assisted treatment), and changing the addition ratio of key components.

[0080] Table 13 Performance test values ​​for Examples 1-15

[0081]

[0082] As shown in Table 13, when the proportion of polyether-type thermoplastic polyurethane dispersion replacing polyether-type aqueous polyurethane emulsion is 20%, and the addition ratio of modified nanoparticles is 7% of the solid content of the mixed matrix, the performance of the prepared battery separator is within the optimal range. As the proportions of mesoporous silica (MSN) and graphene oxide (GO) gradually increase, the tensile strength, puncture resistance, and interfacial impedance of the battery separator decrease slightly, but the thermal shrinkage rate, ionic conductivity, porosity, and thermal stability increase slightly. This is because the mesoporous structure of MSN is prone to adsorption and aggregation, and the sheet-like structure of GO is prone to stacking. When the proportion of MSN added increases, the modified nanoparticles are more prone to local agglomeration, leading to stress concentration points within the mixed matrix and a slight decrease in mechanical properties. However, increasing the proportion of MSN introduces additional micropores into the mixed matrix, resulting in a slight increase in ionic porosity. This increased porosity leads to an increase in electrolyte retention. The mesoporous structure of MSN and the lamellar structure of GO provide more microchannels for ion transport, thereby increasing ionic conductivity. Consequently, the ion transport channels at the interface are more abundant, migration resistance is reduced, and interfacial impedance decreases slightly.

[0083] Table 14 Performance test values ​​for Comparative Examples 1-15

[0084]

[0085] As shown in Table 14, when no inorganic nanoparticles are added or only one type of nanoparticle is added, such as nano-alumina, mesoporous silica, and graphene oxide, the properties of the resulting separators are all worse than those of the separators in this application. When only nano-alumina is added, the ionic conductivity of the battery separators decreases significantly (the ionic conductivity of Example 1 is 4.5 × 10⁻⁶). -3 S / cm, Comparative Example 1 is 2.5 × 10 -3 S / cm, Example 7 is 6.8 × 10 -3 S / cm, Comparative Example 7 is 3×10 -3 (S / cm); When only graphene oxide is added, although the ionic conductivity is slightly improved compared to Comparative Example 1, the lack of mesoporous silica provides a discontinuous ion transport channel. When only mesoporous silica is added, the membrane can achieve basic ion transport and thermal stability through the mesoporous structure and has a cost advantage. However, due to the lack of an organic-inorganic synergistic reinforcement structure, the mechanical performance and cycle stability cannot meet the requirements of mid-to-high-end lithium batteries.

[0086] This indicates that using GO–MSN composite material can improve the performance of battery separators compared to single nanopowders, especially in terms of ionic conductivity and interfacial impedance. GO–MSN composite material forms a synergistic structure of ion transport network and pore carrier through two core raw materials, which single nanopowders cannot simultaneously satisfy the continuity of ion transport pathways and migration efficiency.

[0087] When only GO–MSN composite material is added without thermoplastic polyurethane, the membrane performance also drops significantly. Waterborne polyurethane alone cannot provide higher mechanical strength (the tensile strength of Comparative Example 6 is the lowest). Thermoplastic polyurethane has a block structure of soft-segment-hard-segment. The soft segments of polyurethane can encapsulate GO–MSN composite material to prevent its aggregation. At the same time, the hard segments form hydrogen bonds with the oxygen-containing groups on the GO surface, enhancing the interfacial bonding force between the organic and inorganic phases, forming physical crosslinking points, and improving the tensile / puncture resistance of the membrane.

[0088] In addition, the pore-forming agent added to the composite casting solution needs to be selectively dissolved or induce phase separation through swelling treatment to build a porous structure. Without swelling treatment (Comparative Example 12), the pore-forming agent cannot play a full role, the internal structure of the membrane is dense, the porosity is greatly reduced, resulting in a significant decrease in ionic conductivity. At the same time, the electrolyte wettability is poor, and the interfacial impedance increases sharply.

[0089] When only deionized water is used as the swelling solution (Comparative Example 10), the polarity of the swelling solution is unbalanced, the pore-forming agent is not fully dissolved, the porosity decreases, and the corresponding ionic conductivity decreases. Polyethylene glycol acts as a pore regulator in the swelling solution. When polyethylene glycol is absent (Comparative Example 13), the pores formed by swelling are prone to collapse, the porosity decreases, and the affinity between the membrane and the electrolyte is reduced, resulting in increased interfacial impedance. APTES acts as an interfacial modifier, enhancing the interfacial bonding force between the organic and inorganic phases. The absence of APTES leads to a decrease in the tensile strength and puncture resistance of the membrane (Comparative Example 14). Inorganic salts act as pore-forming aids in the swelling solution, promoting polymer phase separation through the salting-out effect, expanding pore size and increasing porosity. When inorganic salts are absent (Comparative Example 15), the porosity decreases. Inorganic salt ions can moderately modify the surface charge of the pores, reducing the resistance to lithium ion migration. The ionic conductivity decreases slightly after their absence.

[0090] Ultrasound-assisted swelling is mainly used to save swelling time; under the premise of achieving the same swelling effect, ultrasound-assisted swelling takes less time.

[0091] In summary, the embodiments of this application use thermoplastic polyurethane dispersion and aqueous polyurethane emulsion as a mixed matrix, and modify the mixed matrix with GO-MSN composite material. The battery separator prepared by ultrasonic-assisted swelling has better performance than the battery separator prepared by the comparative example using only aqueous polyurethane emulsion as the matrix, or only using a single inorganic nanopowder to modify the mixed matrix, or not using swelling measures. In particular, when the proportion of polyether-type thermoplastic polyurethane dispersion replacing polyether-type aqueous polyurethane emulsion is 20%, and the addition ratio of modified nanopowder is 7% of the solid content of the mixed matrix, the performance of the prepared battery separator is in the optimal range.

[0092] The above results demonstrate and describe the basic principles and main features of this application, as well as its advantages.

[0093] Those skilled in the art should understand that this application is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of this application. Various changes and modifications can be made to this application without departing from the spirit and scope thereof, and all such changes and modifications fall within the scope of this application as claimed. The scope of protection of this application is defined by the equivalents of the appended claims.

Claims

1. A method for preparing an aqueous polyurethane composite swollen film, characterized by, Includes the following steps: S1. Preparation of modified slurry: The modified nanopowder is added to deionized water under low-speed stirring, and then stirred continuously for 30-60 minutes under high-speed stirring to form a modified slurry; the modified nanopowder is a GO-MSN composite material, which is obtained by condensation reaction of graphene oxide and mesoporous silica under the action of a coupling agent. S2. Resin matrix mixing: The polyether-type waterborne polyurethane emulsion and the polyether-type thermoplastic polyurethane dispersion are mixed by stirring at a mass ratio of (70-90):(10-30) and then subjected to ultrasonic-assisted treatment to obtain a mixed matrix. S3. Preparation of composite casting solution: The mixed matrix and modified slurry are stirred and mixed, and the dispersant, defoamer, pore-forming agent and crosslinking agent are added in sequence while stirring to obtain the composite casting solution; the mass ratio of mixed matrix, modified slurry, dispersant, defoamer, pore-forming agent and crosslinking agent is: 100% : (3%-10%) : (0.5%-2.0%) : (0.1%-0.5%) : (5%-10%) : (1%-3%). S4. Coating and film formation: Fix the base film on the worktable of the coating machine, and apply the composite casting liquid onto the base film using the doctor blade coating method. Transfer the coated substrate to the drying oven. S5. Gradient drying and shaping: In the first stage, dry at a low temperature of 40-50℃ for 30-60 minutes. In the second stage, raise the temperature to 60-80℃ and dry for 2-4 hours. Peel off the dried composite film. S6. Modified swelling treatment: The peeled composite membrane is completely immersed in the swelling solution and treated with ultrasonic assistance for 30-60 minutes. S7. Post-treatment: The swollen composite membrane is washed with deionized water until neutral, and then placed in a drying oven for gradient drying to obtain the desired waterborne polyurethane composite swollen membrane.

2. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: In step S1, the initial low-speed stirring speed of the deionized water is 300-500 rpm, and the high-speed stirring speed after all the modified nanopowder is added is 2000-4000 rpm; wherein the mass-volume ratio of the modified nanopowder to the deionized water is 1 g: (60-80) ml.

3. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: The modified nanopowder preparation process in step S1 is as follows: S11. Pretreatment of mesoporous silica: Mesoporous silica is dispersed in anhydrous ethanol and ultrasonically dispersed at 300-500W for 30-60 min. Then concentrated hydrochloric acid is added, the temperature is raised to 50-60℃, and the mixture is refluxed and stirred for 2-4 h. Then the mixture is filtered, washed three times with anhydrous ethanol, and vacuum dried at 60-80℃ for 12 h for later use. The mass-volume ratio of mesoporous silica, anhydrous ethanol and concentrated hydrochloric acid is 1 g: (30-50) ml: (1.5-2.5) ml. Preparation of S12 and NH2-MSN: Anhydrous toluene was added to the pretreated mesoporous silica, and the mixture was ultrasonically dispersed at 200-400W for 15 min. Under nitrogen protection, 0.2-0.5 parts by weight of aminosilane coupling agent were added to the mesoporous silica, the temperature was raised to 80-110℃, and the mixture was refluxed and stirred for 8-12 h. After cooling to room temperature, the mixture was filtered, washed three times with anhydrous toluene, and dried under vacuum at 60℃ for 12 h to obtain amino-functionalized mesoporous silica, abbreviated as NH2-MSN; the mass-volume ratio of mesoporous silica to anhydrous toluene was 1 g: (20-40) ml. S13. Preparation of GO aqueous dispersion: Graphene oxide is dispersed in deionized water and ultrasonically dispersed at 300-500W for 30-60 min to obtain GO aqueous dispersion. Hydroquinone is added and stirred for 1 h to obtain pretreated GO aqueous dispersion. The mass-volume ratio of graphene oxide, hydroquinone, and deionized water is 1 g: (0.1-0.15) g: (200-500) ml; the mass ratio of graphene oxide to mesoporous silica is 1: (3-4). S14. Production of modified nanopowders: The pretreated GO aqueous dispersion is mixed with NH2-MSN, the pH is adjusted to 4-5, the temperature is raised to 40-50℃, stirred for 12-24h, filtered, washed 4 times with deionized water and anhydrous ethanol, and dried under vacuum at 60℃ for 12h to obtain modified nanopowders.

4. The method for preparing an aqueous polyurethane composite swelling film according to claim 2, characterized in that: The specific process of step S2 is as follows: Stir the polyether-type thermoplastic polyurethane dispersion at 30-40℃, with a stirring speed of 200-250 rpm and a stirring time of 10 min. Add half of the total amount of compatibilizer to the polyether-type thermoplastic polyurethane dispersion and continue stirring for 10 min to obtain the modified TPU dispersion. Add the modified TPU dispersion dropwise to the polyether-type aqueous polyurethane emulsion being stirred at high speed, and add the remaining half of the compatibilizer to the polyether-type aqueous polyurethane emulsion. While stirring continuously, use ultrasound-assisted mixing to finally obtain the mixed matrix. The dropping rate of the TPU dispersion is 3-5 ml / min, the ultrasonic power is 100-150 W, and the ultrasonic time is 20-30 min.

5. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: The defoamer is a polyether-modified siloxane, and the pore-forming agent is one or more of polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and ammonium bicarbonate.

6. The method for preparing an aqueous polyurethane composite swelling film according to claim 4, characterized in that: The compatibilizer is one or more of PEO-PU-PEO triblock copolymer, carboxyl-modified acrylate copolymer emulsion, and propylene glycol methyl ether, and the total amount of compatibilizer added is 3%-6% of the solid content of the mixed matrix.

7. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: The dispersant is one or more of polyethylene glycol, polyvinylpyrrolidone, Tween 80, and sodium dodecyl sulfate.

8. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: When the modified slurry is poured into the mixing matrix at a rate of 10-12 ml / min, the mixing matrix is ​​stirred at a speed of 200-300 rpm. After all the modified slurry is added, it is stirred at 400-600 rpm for 10-15 min. When adding the additives, the speed is reduced to 200 rpm, and stirring is continued for 10 min after each additive is added.

9. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: The crosslinking agent is one or more of polyethylene glycol diglycidyl ether, modified polyether-type epoxy crosslinking agent, and 4-butanediol diglycidyl ether.

10. The method for preparing an aqueous polyurethane composite swelling film according to claim 1, characterized in that: The swelling solution is prepared by mixing deionized water and anhydrous ethanol in a ratio of (3-4):

1. The swelling solution also contains 0.2%-0.5% polyethylene glycol, 0.5%-1.0% aminosilane coupling agent, and 5%-8% inorganic salt, relative to the total mass of the swelling solution. The polyethylene glycol is first dissolved in anhydrous ethanol, then mixed with deionized water, followed by the addition of pre-hydrolyzed aminosilane coupling agent and inorganic salt. The diaphragm is then sonicated at 25-35°C for 30-60 minutes.

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