Diaphragm and preparation method and application thereof

Abstract

The invention provides a diaphragm as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The preparation method of the diaphragm comprises the following steps: S1, mixing a diamine monomer and a dianhydride monomer according to a molar ratio of 0.9-1.1, adding an organic solvent, uniformly mixing, and reacting to obtain a polyamide acid solution; wherein the mass ratio of the diamine monomer to the organic solvent is (10-30): 100; s2, lithium chloride and lithium titanium aluminum phosphate are sequentially added into the polyamide acid solution and mixed to be uniform, a spinning solution is obtained, and the mass ratio of lithium chloride to lithium titanium aluminum phosphate to the polyamide acid solution is (0.5-3): (15-45): 100; s3, performing electrostatic spinning on the spinning solution to obtain an electrostatic spinning membrane; and S4, carrying out imidization treatment on the electrostatic spinning membrane to obtain the diaphragm. The diaphragm provided by the invention has excellent heat resistance, and is helpful for improving the cycle stability of the battery.

Description

Technical Field

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

[0002] Lithium-ion batteries, with their outstanding advantages of high energy density and long cycle life, have been widely used in electric vehicles, energy storage systems, and consumer electronics. However, as battery energy density continues to rise, safety concerns are becoming increasingly prominent. The separator, as one of the core components of a battery, directly affects the battery's thermal stability and electrochemical performance due to its heat resistance and ionic conductivity.

[0003] Currently, the separators used in commercial lithium-ion batteries are mainly based on polyolefin materials, commonly polyethylene (PE) and polypropylene (PP). While these materials possess good chemical stability and mechanical strength, their heat resistance is significantly insufficient—polyethylene has a melting point of approximately 130°C, and polypropylene only around 160°C. When the battery is exposed to high temperatures or subjected to thermal abuse, traditional polyolefin separators are prone to melting and shrinkage. This can lead to direct contact between the positive and negative electrodes, causing internal short circuits and, in severe cases, even thermal runaway. Furthermore, under high-temperature conditions, the pore structure of polyolefin separators is prone to collapse, hindering the normal transport of lithium ions within the battery and ultimately accelerating the rate of battery performance degradation.

[0004] To improve the thermal stability of membranes, researchers have explored various modification methods. For example, coating the membrane surface with ceramic particles such as alumina (Al2O3) and silica (SiO2) enhances its high-temperature resistance; or using high-temperature resistant polymers such as polyimide (PI) and aramid (PMIA) to directly replace traditional polyolefin materials in membrane preparation. However, these improvement methods often have significant limitations: ceramic coating processes may lead to increased membrane thickness and decreased flexibility, while the application of high-temperature resistant polymers significantly increases manufacturing costs. These issues limit the large-scale commercial application of modified membranes.

[0005] On the other hand, the ionic conductivity of the separator plays a crucial role in the rate performance and cycle stability of the battery. Traditional polyolefin separators are insulating materials, and their ion transport function relies on the ion channels formed after the electrolyte is fully wetted. However, the porosity of polyolefin separators is typically only 40% to 50%, and their surface has liquid-repellent properties. These two factors together limit the electrolyte permeation efficiency, ultimately resulting in a generally low ionic conductivity of the separator (usually below 1 mS / cm). Furthermore, under high-temperature environments, the electrolyte is prone to decomposition or volatilization, further weakening the ion transport efficiency and adversely affecting battery performance.

[0006] Current mainstream separator modification technologies mostly optimize only a single performance indicator, such as focusing solely on improving heat resistance or improving ionic conductivity, making it difficult to address the multi-dimensional needs of practical battery applications. In reality, commercially viable separators must simultaneously possess three core characteristics: high thermal stability, high ionic conductivity, and good mechanical properties. For example, while ceramic-coated separators effectively improve heat resistance, the coating may clog separator pores, leading to a decrease in ionic conductivity. Conversely, while high-porosity separators facilitate electrolyte wetting and ion transport, they often suffer from insufficient mechanical strength. Therefore, developing a novel separator structure that achieves excellent heat resistance, high ionic conductivity, and good processability has become a key challenge in overcoming the safety and overall performance bottlenecks of lithium-ion batteries. Summary of the Invention

[0007] Based on the technical problems existing in the background art, the present invention proposes a diaphragm, its preparation method and application.

[0008] The present invention provides a method for preparing a diaphragm, comprising the following steps:

[0009] S1. Mix the diamine monomer and dianhydride monomer at a molar ratio of 0.9 to 1.1, add an organic solvent and mix thoroughly, react to obtain a polyamic acid solution; wherein the mass ratio of diamine monomer to organic solvent is (10 to 30): 100.

[0010] S2. Add lithium chloride and lithium aluminum titanium phosphate to the polyamic acid solution in sequence and mix evenly to obtain the spinning solution. The mass ratio of lithium chloride, lithium aluminum titanium phosphate and polyamic acid solution is (0.5~3):(15~45):100.

[0011] S3. Obtain an electrospun membrane by electrospinning the spinning solution;

[0012] S4. The electrospun membrane is subjected to imidization treatment to obtain a diaphragm.

[0013] The membrane prepared by this invention has high porosity and is doped with ceramics of high ionic conductivity, exhibiting excellent ion transport and electrolyte wetting properties. In battery verification, it demonstrates excellent cycle and electrolyte retention performance. Furthermore, thanks to the high thermal stability of PI and the supporting effect provided by ceramic particle doping, this membrane possesses a high thermal breakage temperature and thermal shrinkage performance, making it highly valuable for applications in high-safety batteries.

[0014] Preferably, in S1, the reaction is carried out in an ice-water bath.

[0015] Preferably, in S1, the organic solvent is selected from one or more of N,N-dimethylformamide and N,N-dimethylacetamide.

[0016] Preferably, in S1, the diamine monomer is selected from one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, m-phenylenediamine, 2,2'-di(trifluoromethyl)diaminobiphenyl, and 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline].

[0017] Preferably, in S1, the dianhydride monomer is selected from one or more of pyromellitic dianhydride, biphenyl dianhydride, and diphenyl ether dianhydride.

[0018] The selection of organic solvents, diamine monomers, and dianhydride monomers helps to control the polymer to achieve the desired molecular weight, and obtain the desired structure and properties.

[0019] The amount of diamine monomer added within a certain range helps to control the solid content of the final product, resulting in optimal viscosity and spinning performance.

[0020] The molar ratio of dianhydride monomer to diamine monomer within a certain range helps to control the molecular weight, viscosity, and mechanical properties of the finished product and the spun film.

[0021] Preferably, in S2, the mass ratio of lithium chloride, lithium aluminum titanium phosphate, and polyamic acid solution is (0.5~3):(15~45):100.

[0022] The addition of lithium chloride within a certain range can help improve the conductivity of the spinning solution, increase spinning efficiency, and improve fiber uniformity.

[0023] Preferably, in S2, the D50 particle size of lithium aluminum titanium phosphate is 0.5~0.6μm.

[0024] Within a certain range, the amount and particle size of lithium aluminum titanium phosphate can fill the fiber framework to provide support, while also assisting lithium-ion transport and improving battery performance.

[0025] Preferably, in step S3, the thickness of the electrospun membrane is 3~10μm; the fiber diameter of the electrospun membrane is 100~500nm.

[0026] Within a certain range, the thickness of electrospun membranes helps to improve battery energy density and reduce lithium-ion transport resistance. Within a certain range, the fiber diameter of electrospun membranes helps to ensure membrane porosity and sufficient mechanical strength.

[0027] Preferably, in S3, the electrospinning parameters include: a liquid pushing rate of 0.8~1.0 mL / h, a receiving roller rotation speed of 200 rpm, a distance between the needle and the receiving device of 15~20 cm, and an applied voltage of 18~20 kV.

[0028] Controlling electrospinning parameters helps to efficiently and quickly prepare uniform spun films.

[0029] Preferably, in S4, the imidization treatment includes: first keeping the electrospun film at 80~100℃ for 30~40 min, then keeping it at 150~160℃ for 50~60 min, and finally keeping it at 180~190℃ for 90~120 min.

[0030] Using a stepped heating process during imidization helps to control the overall degree of imidization and ensure uniform imidization.

[0031] A diaphragm is prepared by the above-described preparation method.

[0032] A lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and the aforementioned separator.

[0033] Preferably, the cathode material is selected from one or more of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, and ternary lithium materials.

[0034] Preferably, the negative electrode material is selected from one or more of artificial graphite, natural graphite, and silicon carbide.

[0035] Preferably, the electrolyte comprises lithium hexafluorophosphate or lithium perchlorate.

[0036] The lithium-ion battery provided by this invention has excellent cycle stability.

[0037] The beneficial effects of this invention are as follows:

[0038] This invention presents an electrospun lithium-ion battery separator with high porosity, high conductivity, and high heat resistance and safety, prepared via electrospinning. This separator, constructed through electrospinning to create a network structure, possesses significantly higher porosity and mechanical strength than ordinary separators. Leveraging the excellent heat resistance of polyimide (PI), this separator exhibits superior heat resistance and dimensional stability. Simultaneously, the addition of LATP provides support and assists ion transport in the spun membrane, enhancing dimensional stability and ionic conductivity, thereby improving the overall battery performance.

[0039] This invention provides a polyimide (PI) fiber spinning doped separator and its preparation method. The PI spinning separator is prepared by electrospinning, and high-ionic-conductivity LATP ceramic is doped into the spinning solution to provide support and ionic conductivity to the separator. The lithium-ion battery separator prepared by electrospinning has a fibrous structure. The high heat resistance of PI fibers allows this separator to be used in high-heat-resistant and high-safety separators. The doping of LATP fills the pores of the fiber membrane, ensuring the separator shrinkage rate, while effectively improving the ionic conductivity of the separator, further enhancing the battery's fast-charging cycle performance. Attached Figure Description

[0040] Figure 1This is a schematic diagram of the structure of the polyimide (PI) fiber spinning doped membrane in Example 1 of the present invention.

[0041] Figure 2 These are electron microscope images of Embodiments 1 and 2 of the present invention.

[0042] Figure 3 This is a diagram showing the state of the diaphragm after the heat resistance test in Embodiment 1 of the present invention.

[0043] Figure 4 The graphs show a comparison of the battery cycle performance of Examples 1-2 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0044] The technical solution of the present invention will be described in detail through specific embodiments.

[0045] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0046] Example 1

[0047] A method for preparing a diaphragm includes the following steps:

[0048] S1. In an ice-water bath reactor, 100 mL of N,N-dimethylacetamide (DMAc) solvent was measured, and 15 g of 4,4'-diaminodiphenyl ether (ODA) monomer was added under continuous mechanical stirring until it was completely dissolved. Then, pyromellitic dianhydride (PMDA) was slowly added in three batches at a molar ratio of 0.98:1 to ODA, controlling the reaction temperature below 30°C. After all the dianhydride had been added, the reaction was continued for 6 hours to obtain a polyamic acid (PAA) solution.

[0049] S2. Add lithium chloride (LiCl) equivalent to 1.0% of the total mass of the above PAA solution and stir until completely dissolved. Then, add lithium aluminum titanium phosphate (LATP) ceramic particles with a particle size D50 of 0.53 μm equivalent to 20% of the total mass of the solution. Disperse them evenly in the solution by mechanical stirring and ultrasonic treatment (300W power, 1 hour) to obtain a uniform and stable PAA spinning solution.

[0050] S3. Inject the prepared PAA spinning solution into the electrospinning equipment; set the spinning parameters: the liquid pushing rate is 1.0 mL / h, the receiving roller speed is 200 rpm, the distance between the needle and the receiving device is 15 cm, the applied voltage is 18 kV, perform electrospinning, collect the fibers on the aluminum foil receiving plate, and finally prepare a PAA electrospinned fiber diaphragm with a thickness of about 9 μm and a fiber diameter of about 200 nm.

[0051] S4. Carefully peel the above-mentioned PAA electrospun fiber separator from the aluminum foil and fix it on a stainless steel frame to prevent shrinkage. Then place it in a programmable temperature-controlled oven for step-temperature imidization treatment: first, treat at 100°C for 30 minutes to remove residual solvent, then raise the temperature to 150°C for 60 minutes, and finally treat at 180°C for 90 minutes; after naturally cooling to room temperature, the final polyimide PI / LATP-doped composite separator is obtained.

[0052] Example 2

[0053] A method for preparing a diaphragm includes the following steps:

[0054] S1. In an ice-water bath reactor, 100 mL of N,N-dimethylacetamide (DMAc) solvent was measured, and 20 g of p-phenylenediamine (p-PDA) monomer was added under continuous mechanical stirring until it was completely dissolved. Subsequently, biphenyltetracarboxylic dianhydride (BPDA) was slowly added in three batches at a molar ratio of 1.02:1 to p-PDA, controlling the reaction temperature below 30°C. After all the dianhydride had been added, the reaction was continued for 8 hours to obtain a PAA solution.

[0055] S2. Add lithium chloride (LiCl) equivalent to 2.0% of the total mass of the above PAA solution and stir until completely dissolved. Then, add lithium aluminum titanium phosphate (LATP) ceramic particles with a particle size D50 of 0.53 μm equivalent to 35% of the total mass of the solution. Then, use a high-speed shear emulsifier (5000 rpm, 30 minutes) and ultrasonic treatment (400W power, 1.5 hours) to make it uniformly dispersed in the solution, and finally obtain a uniform and stable PAA spinning solution.

[0056] S3. Inject the prepared PAA spinning solution into the electrospinning equipment; set the spinning parameters: the liquid pushing rate is 0.8 mL / h, the receiving roller speed is 200 rpm, the distance between the needle and the receiving device is 20 cm, the applied voltage is 20 kV, and perform electrospinning. Collect the fibers on the aluminum foil receiving plate to finally prepare a PAA electrospinned fiber diaphragm with a thickness of about 9 μm and a fiber diameter of about 350 nm.

[0057] S4. Carefully peel the above-mentioned PAA electrospun fiber separator from the aluminum foil and fix it on a stainless steel frame to prevent shrinkage. Then place it in a programmable temperature oven for step-temperature imidization treatment: first treat at 80°C for 30 minutes to remove residual solvent, then heat to 150°C for 60 minutes, and finally treat at 190°C for 120 minutes; after naturally cooling to room temperature, the final polyimide PI / LATP doped composite separator is obtained.

[0058] Comparative Example 1

[0059] A 9μm commercial polyolefin (PE) membrane (Xingyuan Material 509c) was used as Comparative Example 1.

[0060] Comparative Example 2

[0061] A composite membrane was obtained by coating a 7μm commercial polyolefin PE membrane (Jinli New Energy SU07) with a 2μm alumina ceramic coating, as a comparative example 2.

[0062] Comparative Example 3

[0063] A composite membrane was obtained by coating a 7μm commercial polyolefin PE membrane (Jinli New Energy SU07) with a 2μm LATP coating, as a comparative example 3.

[0064] The above-mentioned diaphragm was subjected to performance tests, specifically including:

[0065] (1) Air permeability: The air permeability of the above-mentioned diaphragms was measured using a Wang Yan-style air permeability meter. The test results are shown in Table 1.

[0066] (2) Internal resistance test: The internal resistance of the battery assembled with the above-mentioned separator was tested using an electrochemical workstation. Test conditions: AC disturbance voltage 10mV, frequency range: 1MHz~10000Hz, and the test results are shown in Table 1.

[0067] (3) Heat resistance test: The above-mentioned diaphragm was cut into 100mm*100mm samples, and its longitudinal (MD) dimension a1 and transverse (TD) dimension b1 were measured. The marked samples were then placed in an oven at 200℃ for 1 hour, and the longitudinal dimension a2 and transverse dimension b2 were measured. The morphological photographs are shown below. Figure 3 As shown in Table 1, the longitudinal and transverse shrinkage rates of the modified diaphragm were calculated using the following formulas to evaluate its heat resistance.

[0068] TD shrinkage rate (%) = (b1-b2) / b1*100%;

[0069] TD shrinkage rate (%) = (b1-b2) / b1*100%.

[0070] (4) Surface morphology test: The diaphragms prepared in Examples 1-2 were observed under an optical microscope. The eyepiece magnification was set to 10× and the objective lens magnification to 20×. The surface morphology of the diaphragms was photographed, such as... Figure 2 The surface features are analyzed as shown.

[0071] (5) Liquid absorption and retention capacity test: The above-mentioned diaphragm and polyethylene diaphragm were cut into 100mm*500mm samples, and the mass was recorded as m1. They were soaked in 150mL of electrolyte for 1h, the surface electrolyte was wiped dry, and the mass was recorded as m2. The wiped diaphragm was placed in a fume hood and left to stand for 1h, and the mass was recorded as m3. The test results are shown in Table 1.

[0072] Liquid absorption rate = (m2-m1) / m1*100%;

[0073] Liquid retention rate = (m3-m1) / m1*100%;

[0074] Liquid retention efficiency = (m3-m1) / (m2-m1)*100%.

[0075] Table 1

[0076] Test Project unit Comparative Example 1 Comparative Example 2 Comparative Example 3 Example 1 Example 2 air permeability s / 100s 151 174 171 65 71 Internal resistance test <![CDATA[Ω·cm 2 ]]> 1.57 1.68 1.53 1.23 1.15 Heat shrinkage rate MD % 45.11 21.49 27.51 2.63 2.66 Heat shrinkage rate TD % 38.65 20.44 24.78 2.38 2.31 Liquid absorption rate % 43 89 71 122 122 Liquid retention rate % 39 79 63 101 107 Liquid retention efficiency % 25 26 25 78 91

[0077] As can be seen from the data in Table 1, the membrane prepared by this invention has low internal resistance and thermal shrinkage rate, and high liquid absorption rate, liquid retention rate, and liquid retention efficiency. The internal resistance of the PI / LATP-doped composite membrane prepared in Example 1 decreased by 0.34 Ω·cm compared to the ceramic membrane. 2 The thermal shrinkage rate is significantly improved compared to the base film and ceramic film. The lithium-ion battery separator prepared in Example 2 has similar morphology and performance to the lithium-ion battery separator prepared in Example 1, but the coarser fiber diameter and higher degree of imidization result in a lower areal density and improved mechanical properties compared to Example 1. The internal resistance change of the PI / LATP composite separator is basically consistent with that of Example 1, and the thermal shrinkage rate is significantly improved compared to the base film and the separator of Example 1.

[0078] The aforementioned separator, along with a positive electrode (lithium iron phosphate), a negative electrode (artificial graphite), and a ternary lithium battery electrolyte, were used to fabricate a battery using a winding process. The battery was subjected to 1000 charge-discharge cycles at 25±2℃ and 0.5C rate, and the change in battery capacity retention was recorded to evaluate the cycle stability of the modified separator. The test results are as follows: Figure 4 As shown, it can be seen that the capacity retention rate of the separator in Example 1 after 1000 cycles at room temperature reaches more than 96%, which shows excellent battery performance; the capacity retention rate of the separator in Example 2 after 1000 cycles at room temperature is similar to that in Comparative Example 2. The lithium-ion battery assembled with the separator provided by the present invention meets the requirements of the national standard GB / T31484-2015.

[0079] In summary, the separator provided by this invention has excellent heat resistance, which helps to improve the cycle stability of the battery.

[0080] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method of preparing a diaphragm, characterized in that, The method comprises the following steps: S1, mixing diamine monomers and dianhydride monomers according to a molar ratio of 0.9-1.1, adding an organic solvent, mixing uniformly, and reacting to obtain a polyamide acid solution; wherein the mass ratio of the diamine monomers to the organic solvent is (10-30):100; S2, adding lithium chloride and aluminum titanium lithium phosphate into the polyamide acid solution in sequence, mixing uniformly to obtain a spinning solution, wherein the mass ratio of the lithium chloride, the aluminum titanium lithium phosphate and the polyamide acid solution is (0.5-3):(15-45):100; S3, obtaining an electrostatic spinning film by electrospinning the spinning solution; S4, performing imidization treatment on the electrostatic spinning film to obtain a separator.

2. The production method according to claim 1, characterized by, In S1, the organic solvent is selected from one or more of N,N-dimethylformamide and N,N-dimethylacetamide; the diamine monomer is selected from one or more of 4,4'-diamino diphenyl ether, p-phenylenediamine, m-phenylenediamine, 2,2'-bis(trifluoromethyl)diaminobiphenyl and 4,4'-[1,4-phenylbis(oxy)]bis[3-(trifluoromethyl)aniline]; and the dianhydride monomer is selected from one or more of pyromellitic dianhydride, biphenyl tetracarboxylic dianhydride and diphenyl ether tetracarboxylic dianhydride.

3. The preparation method according to claim 1, characterized in that, In S2, the D50 particle size of the aluminum titanium lithium phosphate is 0.5-0.6 μm.

4. The method of claim 1, wherein, In S3, the thickness of the electrostatic spinning film is 3-10 μm; and the fiber diameter of the electrostatic spinning film is 100-500 nm.

5. The preparation method according to claim 1, characterized in that, In S3, the electrospinning parameters include: a liquid pushing rate of 0.8-1.0 mL / h, a receiving roller speed of 200 rpm, a distance between the needle and the receiving device of 15-20 cm, and an applied voltage of 18-20 kV.

6. The method of claim 1, wherein, In S4, the imidization treatment comprises: first, keeping the electrostatic spinning film at 80-100℃ for 30-40 min, then keeping it at 150-160℃ for 50-60 min, and finally keeping it at 180-190℃ for 90-120 min.

7. A diaphragm characterized by, The separator is prepared by the method of any one of claims 1-6.

8. A lithium-ion battery, characterized by The separator is prepared by the method of any one of claims 1-6. The separator is prepared by the method of any one of claims 1-6.

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