Gradient-pore aramid / polyimide nanofiber separator and method of making the same

Abstract

The application discloses a gradient-pore aramid / polyimide nanofiber diaphragm and a preparation method thereof, relates to the technical field of lithium ion battery diaphragms, and the diaphragm is an integrally formed structure, a gradient nanopore structure with a continuously decreasing pore size from the positive electrode side to the negative electrode side is formed in the thickness direction, ion guiding large pores with a size of 20-30 nm are formed near the positive electrode side, and ion homogenizing small pores with a size of 5-10 nm are formed near the negative electrode side. The preparation method adopts a bottom-up assembly strategy, the hydrogen bond network dynamics is controlled by constructing a concentration gradient of a quality supplier, gradient-pore directional growth is realized in one step, and the product is prepared through low-temperature polycondensation and phase separation. The diaphragm can significantly inhibit lithium dendrite growth, has zero size shrinkage after 300 DEG C heat treatment, has excellent high-temperature stability and electrochemical performance, and is suitable for high-performance lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, and more particularly to gradient pore size aramid / polyimide nanofiber separators and their preparation methods. Background Technology

[0002] Lithium-ion batteries, with their core advantages such as high operating voltage, high energy density, long cycle life, and environmental friendliness, have been widely used in numerous fields including consumer electronics, electric vehicles, energy storage power stations, aerospace, and medical equipment. Their comprehensive performance directly determines the user experience and safety reliability of related end products. The battery separator, as one of the four core components of a lithium-ion battery, is located between the positive and negative electrodes. It needs to provide channels for the rapid transport of lithium ions to ensure the battery's charge and discharge performance, while also preventing direct contact between the positive and negative electrodes to avoid internal short circuits. Furthermore, it must be compatible with the electrolyte and withstand the chemical and mechanical environments during battery charging and discharging. Its structure and performance directly determine the battery's rate performance, cycle life, safety performance, and adaptability to extreme environments.

[0003] Polyimide materials possess excellent high-temperature resistance, chemical corrosion resistance, flame retardancy, electrical insulation, and electrolyte wettability, while aramid nanofibers combine high strength, high modulus, high-temperature resistance, chemical corrosion resistance, and excellent barrier and electrical properties. Both are considered ideal matrix materials for preparing high-performance lithium-ion battery separators. However, existing aramid nanofiber or polyimide-based battery separators still suffer from several insurmountable technical drawbacks: First, the pore structure is difficult to precisely control. For example, the template method for preparing polyimide porous membranes disclosed in Chinese patent CN112309987A is difficult to completely remove the pore-forming agent, resulting in uneven membrane texture and decreased mechanical properties. Moreover, the removal of some pore-forming agents requires the use of highly corrosive reagents, increasing production costs and safety hazards. Although the electrospun aramid nanofiber membrane disclosed in Chinese patent CN109888472A has high porosity and good electrolyte wettability, the excessively high porosity will significantly reduce the mechanical strength of the membrane, posing a challenge to the large-scale assembly of batteries. At the same time, the pore size of the spun membrane is too large and unevenly distributed, which can easily cause battery self-discharge and reduce battery coulombic efficiency.

[0004] Secondly, the lithium dendrite suppression capability is insufficient. Existing membranes with homogeneous porous structures cannot achieve uniform control of lithium-ion transport. During the transport process, local concentration fluctuations of lithium ions are prone to occur, which exacerbates the uneven deposition of lithium ions on the negative electrode surface. The continuous growth of lithium dendrites can pierce the membrane, causing internal short circuits in the battery, and even leading to safety accidents such as thermal runaway, combustion, and explosion.

[0005] Third, extreme environmental adaptability and electrochemical performance cannot be improved synergistically. Existing commercial polyolefin separators are prone to severe thermal shrinkage under high temperature environments, while existing aramid / polyimide-based separators, although improving high-temperature stability, cannot simultaneously achieve high-rate performance, long cycle life, and wide temperature range adaptability, making it difficult to meet the extreme operating conditions required for lithium-ion batteries in fields such as new energy vehicles and aerospace.

[0006] In summary, developing a battery separator with precisely tunable pore structure, excellent lithium dendrite suppression, strong adaptability to extreme environments, and a combination of high-rate performance and long cycle life is a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the above-mentioned defects of the prior art and provide a gradient pore size aramid (polyimide) nanofiber membrane and its preparation method, thereby solving the technical problems of existing gradient pore size aramid (polyimide) nanofiber membranes, such as difficulty in precisely controlling the pore structure, weak lithium dendrite suppression ability, poor adaptability to extreme environments, and inability to synergistically improve high temperature stability and electrochemical performance.

[0008] To achieve the objectives of this invention, the invention is implemented through the following technical solutions: This invention provides a gradient-pore aramid / polyimide nanofiber separator. The matrix material of the separator is aramid nanofiber or polyimide nanofiber, or a combination of both. The separator is a one-piece molded structure with a gradient nanopore structure in the thickness direction where the pore size continuously decreases from the positive electrode side to the negative electrode side, rather than a stepped pore structure formed by multiple layers. Specifically, the separator forms large pores with a pore size of 20-30 nm on the side near the positive electrode of the lithium-ion battery, and these large pores constitute ion guiding channels for lithium ions. The separator also forms small pores with a pore size of 5-10 nm on the side near the negative electrode of the lithium-ion battery, and these small pores constitute ion distribution homogenizers for lithium ions.

[0009] Furthermore, the matrix material of the separator is aramid nanofiber or polyimide nanofiber, or both; the pore size of the separator near the positive electrode is 25~28nm, and the pore size near the negative electrode is 7~8nm.

[0010] This invention provides a method for preparing gradient-pore aramid / polyimide nanofiber membranes. Employing a bottom-up assembly strategy, it eliminates the need for traditional multilayer composite or post-processing pore-forming techniques, achieving directional growth of gradient channels in one step. The method specifically includes the following steps: Step 1: Prepare a raw material system containing a nanofiber precursor, a proton donor and a solvent, wherein the nanofiber precursor is an aramid nanofiber precursor or a polyimide nanofiber precursor, or both. Step 2: Disperse the nanofiber precursor in the solvent, and construct a concentration gradient of proton donor along the membrane thickness direction in the dispersion system by controlling the addition position and rate of the proton donor; Step 3: Under preset temperature and stirring conditions, the dispersion system undergoes a self-assembly reaction. The formation kinetics of the hydrogen bond network within the system are controlled by the proton donor concentration gradient to obtain a precursor membrane with a gradient nanoporous structure. Step 4: The precursor membrane is subjected to polycondensation reaction at 0~50℃ for 2~10h to obtain the polycondensation product; Step 5: Perform phase separation treatment on the polycondensation product to regulate the multi-level pore structure of the membrane; Step 6: The phase separation products are washed and dried to obtain the target gradient pore size nanofiber membrane.

[0011] The core mechanism of this invention is as follows: proton donor terephthalic acid oligomers can form hydrogen bonds with aramid / polyimide precursor molecules. The higher the concentration of proton donors in the system, the greater the cross-linking density of the hydrogen bond network, and the smaller the pore size formed by self-assembly. By constructing a proton donor concentration gradient that gradually decreases from the negative electrode side to the positive electrode side, the cross-linking density of the hydrogen bond network in the system can form a continuous gradient along the thickness direction, thereby guiding the directional growth of pores during self-assembly and forming a continuous gradient nanoporous structure with a large pore size on the positive electrode side and a small pore size on the negative electrode side in one step.

[0012] Furthermore, the proton donor in step one is a terephthalic acid oligomer, and the solvent is N-methylpyrrolidone or dimethylacetamide; the concentration gradient of the proton donor in step two is a gradual decrease in concentration along the direction from the side of the membrane near the negative electrode to the side near the positive electrode.

[0013] Furthermore, the temperature of the self-assembly reaction in step three is 20~40℃, the stirring speed is 200~500r / min, and the reaction time is 3~6h.

[0014] Furthermore, the polycondensation reaction described in step four is carried out under normal pressure without inert gas protection, at a reaction temperature of 30-40°C, and for a reaction time of 6-8 hours.

[0015] Furthermore, the phase separation process in step five is achieved by adding a non-solvent, changing the temperature, or changing the pressure; wherein when a non-solvent phase separation is used, the non-solvent is one or a mixture of two of ethanol and isopropanol, and the processing time is 2-3 hours.

[0016] Furthermore, the drying process described in step six is ​​vacuum drying at 60~70℃ for 10~12 hours.

[0017] The beneficial effects of this invention are as follows: 1. This invention achieves precise control of lithium-ion transport through a unique one-piece molded continuous gradient pore structure design, fundamentally suppressing the nucleation and growth of lithium dendrites. The separator of this invention forms continuous gradient channels in the thickness direction, with large pores on the positive electrode side and small pores on the negative electrode side. The large pores (20-30 nm) on the positive electrode side serve as ion guiding channels, significantly reducing lithium-ion transport resistance and enabling rapid lithium-ion passage, ensuring high-rate performance of the battery. The small pores (5-10 nm) on the negative electrode side serve as ion distribution homogenizers, uniformly dispersing the lithium-ion flow and significantly reducing lithium-ion concentration fluctuations. Simulation verification shows that the gradient structure of this invention can reduce the standard deviation of lithium-ion concentration fluctuations to 0.39 times that of traditional polyolefin separators, effectively avoiding localized enrichment of lithium ions on the negative electrode surface, significantly suppressing the nucleation and growth of lithium dendrites, and fundamentally improving the cycle life and safety performance of the battery.

[0018] 2. This invention pioneers a bottom-up assembly preparation method based on proton donor concentration gradient control, achieving precise and controllable preparation of membrane pore structures and overcoming the process bottlenecks of existing technologies. This invention abandons traditional multilayer composite and post-processing pore-forming techniques. By constructing a proton donor concentration gradient, it regulates the formation kinetics of hydrogen bond networks, guiding the pores to grow directionally along the membrane thickness direction, thus achieving the construction of gradient pore structures in one step. The process is simple and controllable, requiring no complex equipment and eliminating the problem of residual pore-forming agents. Furthermore, through a low-temperature polycondensation process, efficient synthesis can be achieved under normal pressure without inert gas protection, significantly reducing production costs and safety risks, making it suitable for large-scale industrial production.

[0019] 3. This invention achieves a synergistic improvement in the extreme environmental adaptability and electrochemical performance of the separator, with overall performance far exceeding that of existing commercial separators. On the one hand, thanks to the intrinsic high-temperature resistance of the aramid / polyimide material and the synergistic enhancement effect of the gradient structure, the separator of this invention exhibits zero dimensional shrinkage after heat treatment at 300℃ and a capacity retention rate of up to 95% after 100 cycles at 100℃, demonstrating excellent high-temperature dimensional stability and wide temperature range adaptability. This effectively avoids short-circuit accidents caused by separator thermal shrinkage under high-temperature conditions, significantly improving the safety and reliability of the battery under extreme operating conditions. On the other hand, the gradient pore structure of this invention balances the rapid transport and uniform distribution of lithium ions. The assembled LiFePO4 / / Li half-cell achieves a capacity retention rate of 85.3% after 300 cycles at room temperature under 5C high-rate conditions and 86.7% after 250 cycles at 55℃, achieving a balance between high safety, high-rate performance, and long cycle life, meeting the requirements for high-performance lithium-ion batteries. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the preparation process of the gradient pore size aramid / polyimide nanofiber separator of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] It should be noted that the technical means not described in detail in the following embodiments are all conventional means in the field, are not the key points of the invention, and will not be elaborated upon.

[0023] All raw materials used in the following embodiments of the present invention are commercially available conventional products. Test methods and battery assembly processes not described in detail are conventional techniques in the field and are not innovative points of the present invention, and will not be elaborated upon. The number-average molecular weight of the terephthalic acid oligomers used is 500-2000, and the mass concentration of the prepared solutions is 10%.

[0024] Example 1 like Figure 1 As shown, this embodiment provides a gradient pore size aramid nanofiber membrane, the preparation method of which specifically includes the following steps: Raw material preparation: Prepare aramid nanofiber precursor, proton donor terephthalic acid oligomer, and solvent N-methylpyrrolidone (NMP). Concentration gradient construction: 10g of aramid nanofiber precursor was dispersed in 100mL of NMP solvent. Under continuous stirring, terephthalic acid oligomer NMP solution was slowly added from the bottom of the container at a rate of 1mL / min, with a total addition of 20mL. This formed a proton donor concentration gradient in the system that gradually decreased from bottom to top (corresponding to the negative electrode side to the positive electrode side of the membrane). Gradient pore self-assembly: The system temperature was controlled at 25℃ and stirred continuously at 300r / min for 4h to complete the self-assembly reaction and obtain a precursor membrane with a gradient nanopore structure. Low-temperature polycondensation: The self-assembled precursor membrane was placed in an environment of 30°C, normal pressure, and no inert gas protection for polycondensation reaction for 6 hours to obtain the polycondensation product. Phase separation treatment: The polycondensation product was immersed in an ethanol solution for phase separation treatment and kept for 2 hours to adjust the multi-level pore structure of the membrane. Post-processing: The phase separation product was washed three times with deionized water and then dried in a vacuum drying oven at 60℃ for 12 hours to obtain the target gradient pore size aramid nanofiber membrane.

[0025] The performance of the membrane prepared in this embodiment was tested. The results showed that the average pore size of the membrane near the positive electrode was 25 nm and the average pore size near the negative electrode was 8 nm. The size shrinkage rate after heat treatment at 300℃ for 2 h was 0. After being assembled into a LiFePO4 / / Li half cell, the capacity retention rate was 85.3% after 300 cycles at room temperature at 5C, 86.7% after 250 cycles at 55℃, and 95% after 100 cycles at 100℃.

[0026] Example 2 This embodiment provides a gradient pore size polyimide nanofiber membrane, the preparation method of which specifically includes the following steps: Raw material preparation: Prepare polyimide fiber precursor, proton donor terephthalic acid oligomer, and solvent dimethylacetamide (DMAc). Concentration gradient construction: 15g of polyimide fiber precursor was dispersed in 120mL of DMAc solvent. By adding terephthalic acid oligomer DMAc solution at different heights in the container at different rates, with a total addition of 25mL, a proton donor concentration gradient with gradually decreasing concentration from bottom to top (corresponding to the negative electrode side to the positive electrode side of the membrane) was formed in the system. Gradient pore self-assembly: The system temperature was controlled at 30℃ and stirred continuously at 400r / min for 5h to complete the self-assembly reaction and obtain a precursor membrane with a gradient nanopore structure. Low-temperature polycondensation: The self-assembled precursor membrane was placed in an environment of 40°C, normal pressure, and no inert gas protection for polycondensation reaction for 8 hours to obtain the polycondensation product. Phase separation treatment: The polycondensation product was immersed in an isopropanol solution for phase separation treatment and kept for 3 hours to adjust the multi-level pore structure of the membrane. Post-processing: The phase separation product was washed four times with deionized water and then dried in a vacuum drying oven at 70°C for 10 hours to obtain the target gradient pore size polyimide nanofiber membrane.

[0027] The performance of the membrane prepared in this embodiment was tested. The results showed that the average pore size of the membrane near the positive electrode was 28 nm and the average pore size near the negative electrode was 7 nm. The size shrinkage rate after heat treatment at 300℃ for 2 h was 0. After being assembled into a LiFePO4 / / Li half cell, the capacity retention rate was 84.8% after 300 cycles at room temperature at 5C, 87.1% after 250 cycles at 55℃, and 94.7% after 100 cycles at 100℃.

[0028] Example 3 This embodiment provides a gradient pore size aramid / polyimide composite nanofiber membrane, the preparation method of which specifically includes the following steps: Raw material preparation: Aramid nanofiber precursor and polyimide fiber precursor are taken as mixed precursors in a mass ratio of 1:1. The proton donor is terephthalic acid oligomer. The solvent is a mixed solvent of NMP and DMAc in a volume ratio of 1:1. Concentration gradient construction: 12g of mixed precursor was dispersed in 110mL of mixed solvent. Under continuous stirring, terephthalic acid oligomer mixed solvent solution was slowly added from the bottom of the container at a rate of 1.2mL / min, with a total addition of 22mL. A proton donor concentration gradient with gradually decreasing concentration from bottom to top (corresponding to the negative electrode side to the positive electrode side of the membrane) was formed in the system. Gradient pore self-assembly: The system temperature was controlled at 35℃, and the mixture was stirred continuously at 350r / min for 4.5h to complete the self-assembly reaction and obtain a precursor membrane with a gradient nanopore structure. Low-temperature polycondensation: The self-assembled precursor membrane was placed in an environment of 35°C, normal pressure, and no inert gas protection for polycondensation reaction for 7 hours to obtain the polycondensation product. Phase separation treatment: The polycondensation product is immersed in a non-solvent mixture of equal volumes of ethanol and isopropanol for phase separation treatment, and kept for 2.5 hours to regulate the multi-level pore structure of the membrane. Post-processing: The phase separation product was washed three times with deionized water and then dried in a vacuum drying oven at 65℃ for 11 hours to obtain the target gradient pore size composite nanofiber membrane.

[0029] The performance of the membrane prepared in this embodiment was tested. The results showed that the average pore size of the membrane near the positive electrode was 26 nm and the average pore size near the negative electrode was 7.5 nm. The size shrinkage rate after heat treatment at 300℃ for 2 h was 0. After being assembled into a LiFePO4 / / Li half cell, the capacity retention rate was 85.1% after 300 cycles at room temperature at 5C, 86.9% after 250 cycles at 55℃, and 94.9% after 100 cycles at 100℃.

[0030] Example 4 This embodiment provides a gradient pore size aramid / polyimide composite nanofiber membrane, the preparation method of which specifically includes the following steps: Raw material preparation: Aramid nanofiber precursor and polyimide fiber precursor are taken as mixed precursors at a mass ratio of 2:1. The proton donor is terephthalic acid oligomer with a number average molecular weight of 500~2000 (the mass concentration of the prepared solution is 10%). The solvent is a mixed solvent of N-methylpyrrolidone (NMP) and dimethylacetamide (DMAc) at a volume ratio of 1:1. Concentration gradient construction: 12g of mixed precursor (8g of aramid nanofiber precursor and 4g of polyimide fiber precursor) was dispersed in 110mL of mixed solvent. Under continuous stirring, terephthalic acid oligomer mixed solvent solution was slowly added from the bottom of the container at a rate of 1.2mL / min, with a total addition of 22mL, forming a proton donor concentration gradient that gradually decreases from bottom to top (corresponding to the negative electrode side to the positive electrode side of the membrane). Gradient pore self-assembly: The system temperature was controlled at 35℃, and the mixture was stirred continuously at 350r / min for 4.5h to complete the self-assembly reaction and obtain a precursor membrane with a gradient nanopore structure. Low-temperature polycondensation: The self-assembled precursor membrane was placed in an environment of 35°C, normal pressure, and no inert gas protection for polycondensation reaction for 7 hours to obtain the polycondensation product. Phase separation treatment: The polycondensation product is immersed in a non-solvent mixture of equal volumes of ethanol and isopropanol for phase separation treatment, and kept for 2.5 hours to regulate the multi-level pore structure of the membrane. Post-processing: The phase separation product was washed three times with deionized water and then dried in a vacuum drying oven at 65℃ for 11 hours to obtain the target gradient pore size composite nanofiber membrane.

[0031] The performance of the membrane prepared in this embodiment was tested. The results showed that the average pore size of the membrane near the positive electrode was 27 nm and the average pore size near the negative electrode was 7.2 nm. The size shrinkage rate after heat treatment at 300℃ for 2 h was 0. After being assembled into a LiFePO4 / / Li half cell, the capacity retention rate was 84.9% after 300 cycles at room temperature at 5C, 86.5% after 250 cycles at 55℃, and 94.6% after 100 cycles at 100℃.

[0032] Test case This test example verifies the homogenization effect of the separator of the present invention on the lithium-ion concentration distribution through finite element simulation. All simulation processes and parameter settings comply with the conventional specifications in the field of lithium-ion battery separators. Those skilled in the art can repeat the simulation process based on this content, which fully supports the beneficial effects of the present invention.

[0033] Simulation software: COMSOL Multiphysics 6.1 finite element analysis software was used, and the simulation calculation was performed by coupling the "lithium-ion battery interface" and the "rare material transfer interface".

[0034] Model and sample parameter settings: The membrane model of this invention is a two-dimensional model that is completely identical to the membrane prepared in Example 1. The total thickness of the membrane is 25 μm. It is a one-piece molded continuous gradient nanoporous structure with a pore diameter of 25 nm near the positive electrode and 8 nm near the negative electrode, and a porosity of 45%. Control sample: Celgard2400, a commercially available polypropylene separator commonly used in the battery industry, was selected as the control. The total thickness of the separator was 25 μm, the homogeneous pore size was 20 nm, the porosity was 41%, and the remaining simulated boundary conditions were completely consistent with those of the separator of the present invention. Battery system model: Matching LiFePO4 / / Li half-cell system, with a positive electrode thickness of 50 μm and a negative electrode thickness of 40 μm. The electrolyte is a 1 mol / L LiPF6 / EC+DMC (volume ratio 1:1) system, and the lithium-ion diffusion coefficient is set to 1×10⁻¹. 0 m² / s, simulated ambient temperature 25℃.

[0035] Testing process: (1) Establish a two-dimensional axisymmetric cell model, import the structural parameters of the gradient pore size separator of the present invention and the control commercial polyolefin separator respectively, and divide the grid with an accuracy of 2μm to ensure the convergence of the simulation calculation; (2) Set charge and discharge boundary conditions: discharge at a constant current of 1C rate, discharge cutoff voltage of 2.5V, monitor the lithium ion concentration distribution at the negative electrode / separator interface in real time during the simulation, and continuously collect concentration data at 100 time nodes; (3) Perform statistical analysis on the collected lithium ion concentration data and calculate the standard deviation of concentration fluctuation (the smaller the standard deviation, the more uniform the lithium ion concentration distribution and the better the lithium dendrite suppression effect).

[0036] Raw data and results analysis: The gradient pore size membrane prepared in Example 1 of this invention has a standard deviation of 0.78 mol / m³ for lithium ion concentration fluctuation at the negative electrode / membrane interface; Compared with the commercial polyolefin separator Celgard2400, the standard deviation of lithium ion concentration fluctuation at the negative electrode / separator interface is 2.00 mol / m³. Calculations show that the standard deviation of lithium-ion concentration fluctuation in the membrane of this invention is only 0.39 times that of commercial polyolefin membranes, verifying that the gradient pore structure of this invention can significantly homogenize lithium-ion distribution and inhibit the nucleation and growth of lithium dendrites from the source, which is completely consistent with the beneficial effects of this invention.

[0037] Comparative Example 1 Except for the homogeneous pore structure, which does not have the integrally formed continuous gradient nanopore structure of the present invention, the raw materials, membrane thickness, post-processing technology, battery assembly and performance testing conditions of this comparative example are completely consistent with those of Example 1.

[0038] This comparative example uses a conventional electrospinning method to prepare a homogeneous aramid nanofiber membrane with an average pore size of 20 nm. Performance test results show that the membrane's dimensional shrinkage rate is 3.2% after heat treatment at 300℃ for 2 hours; after being assembled into a LiFePO4 / / Li half-cell, the capacity retention rate is 62.7% after 300 cycles at room temperature at 5C and 72.4% after 100 cycles at 100℃.

[0039] Comparative Example 2 Except for the homogeneous pore structure, which does not have the integrally formed continuous gradient nanopore structure of the present invention, the raw materials, membrane thickness, post-processing technology, battery assembly and performance testing conditions of this comparative example are completely the same as those of Example 2.

[0040] This comparative example uses a traditional template method to prepare a polyimide porous membrane with an average pore size of 15 nm. Performance test results show that the membrane's dimensional shrinkage rate is 5.8% after heat treatment at 300℃ for 2 hours; after being assembled into a LiFePO4 / / Li half-cell, the capacity retention rate is 56.2% after 300 cycles at room temperature at 5C rate and 65.8% after 100 cycles at 100℃.

[0041] The comparison between the examples and comparative examples clearly shows that the gradient pore size nanofiber membrane prepared by the present invention has significant advantages over the homogeneous membrane of the prior art in terms of high temperature dimensional stability, lithium dendrite suppression ability, rate performance and cycle life, achieving synergistic improvement of various performances and solving the technical problems that have long existed in the prior art.

[0042] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a gradient pore size aramid / polyimide nanofiber membrane, characterized in that, The matrix material of the separator is at least one of aramid nanofibers and polyimide nanofibers; the separator is a one-piece molded structure with a gradient nanopore structure in the thickness direction where the pore size continuously decreases from the positive electrode side to the negative electrode side, rather than a stepped pore size structure formed by multiple layers; wherein, the separator forms large channels with a pore size of 20~30nm on the side near the positive electrode of the lithium-ion battery, and the large channels constitute ion guiding channels for lithium ions; the separator forms small channels with a pore size of 5~10nm on the side near the negative electrode of the lithium-ion battery, and the small channels constitute ion distribution homogenizers for lithium ions. The preparation method includes the following steps: Step 1: Prepare a raw material system comprising a nanofiber precursor, a proton donor and a solvent, wherein the nanofiber precursor is at least one of aramid nanofiber precursor and polyimide nanofiber precursor. Step 2: Disperse the nanofiber precursor in the solvent, and construct a concentration gradient of proton donor along the membrane thickness direction in the dispersion system by controlling the addition position and rate of the proton donor; Step 3: Under preset temperature and stirring conditions, the dispersion system undergoes a self-assembly reaction. The formation kinetics of the hydrogen bond network within the system are controlled by the proton donor concentration gradient to obtain a precursor membrane with a gradient nanoporous structure. Step 4: The precursor membrane is subjected to polycondensation reaction at 0~50℃ for 2~10h to obtain the polycondensation product; Step 5: Perform phase separation treatment on the polycondensation product to regulate the multi-level pore structure of the membrane; Step 6: The phase separation products are washed and dried to obtain the target gradient pore size nanofiber membrane.

2. The method for preparing the gradient pore size aramid / polyimide nanofiber membrane according to claim 1, characterized in that, In step one, the proton donor is a terephthalic acid oligomer, and the solvent is N-methylpyrrolidone or dimethylacetamide; in step two, the concentration gradient of the proton donor gradually decreases along the direction from the side of the membrane closer to the negative electrode to the side closer to the positive electrode.

3. The method for preparing the gradient pore size aramid / polyimide nanofiber separator according to claim 1, characterized in that, The temperature of the self-assembly reaction in step three is 20~40℃, the stirring speed is 200~500r / min, and the reaction time is 3~6h.

4. The method for preparing the gradient pore size aramid / polyimide nanofiber separator according to claim 1, characterized in that, The polycondensation reaction described in step four is carried out under normal pressure without inert gas protection, at a reaction temperature of 30-40°C, and for a reaction time of 6-8 hours.

5. The method for preparing the gradient pore size aramid / polyimide nanofiber separator according to claim 1, characterized in that, The phase separation process described in step five is achieved by adding a non-solvent, changing the temperature, or changing the pressure; wherein when a non-solvent phase separation is used, the non-solvent is one or a mixture of two of ethanol and isopropanol, and the processing time is 2-3 hours.

6. The method for preparing the gradient pore size aramid / polyimide nanofiber separator according to claim 1, characterized in that, The drying process described in step six is ​​vacuum drying at 60~70℃ for 10~12 hours.

7. The method for preparing the gradient pore size aramid / polyimide nanofiber separator according to claim 1, characterized in that, The pore size of the diaphragm is 25-28 nm on the side near the positive electrode and 7-8 nm on the side near the negative electrode.

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