An aromatic polyamide porous membrane, a preparation method thereof, and a secondary battery comprising the same

Abstract

The present invention provides an aromatic polyamide porous membrane, a preparation method thereof, and a secondary battery comprising the same. The preparation method includes: mixing a first solvent, a second solvent and an aromatic polyamide to obtain a casting solution; the first solvent is a good solvent for the aromatic polyamide, the second solvent includes a poor solvent for the aromatic polyamide, and the boiling point of the first solvent < the boiling point of the second solvent; performing a film-forming treatment on the casting solution to obtain a liquid film; removing the first solvent in the liquid film to obtain a dry film; and removing the second solvent in the dry film to obtain the aromatic polyamide porous membrane. Through the design of the first solvent and the second solvent and their mutual compounding with a specific process, the obtained aromatic polyamide porous membrane has the advantages of large porosity and high ionic conductivity, and exhibits excellent mechanical properties and thermal stability; the preparation method is simple and environmentally friendly, the film-forming process is easy to adjust, the film structure is easy to control, and it is suitable for large-scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer membrane materials, and particularly relates to an aromatic polyamide porous membrane, a preparation method thereof, and a secondary battery comprising the same. Background Art

[0002] Facing the two major problems of energy and environment brought about by the rapid increase in population and the accelerating industrialization process, lithium-ion batteries, as a simple and efficient device with high energy storage and supply, play an important role in the development and popularization of new energy. A lithium-ion battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator wetted by the electrolyte between the two electrodes. Among them, the separator, also known as the "third electrode" of the lithium-ion battery, is an important component of the lithium-ion battery. The separator mainly has two functions during the charge and discharge process of the lithium-ion battery: one is to effectively avoid short circuits caused by direct contact between the positive and negative electrode materials of the battery, thereby ensuring the safety performance of the battery; the other is to provide a channel for the transmission of lithium ions so that they can pass freely. Since the characteristics and quality of the separator material play a decisive role in the battery rate performance, cycle life, and basic electrical performance, that is to say, the quality of the separator restricts the overall performance of the lithium-ion battery. Therefore, it is required that the separator itself is an insulator and has chemical stability, thermal stability, high porosity, and certain mechanical strength.

[0003] Microporous polyolefin separators mainly composed of polyethylene (PE) and polypropylene (PP) and composite membranes composed of these two materials are the current main commercial lithium-ion battery separators. This separator uses a single / double-sided thermal stretching orientation process to cause the polymer chain segments to undergo high orientation and plastic deformation at high temperatures, thereby forming a microporous structure. Although polyolefin microporous membranes have advantages such as low cost, good mechanical properties, excellent chemical stability, and electrochemical stability, due to their low surface energy, it is difficult for the electrolyte to fully wet the separator, resulting in low migration of lithium ions and poor ionic conductivity during the charge and discharge process, affecting the cycle life of the battery and limiting the improvement of the overall battery performance. In addition, the polyolefin separator has a low thermal deformation temperature. Under high-current high-rate charge and discharge conditions, due to the rapid increase in internal temperature, the separator will undergo serious thermal shrinkage deformation, leading to the occurrence of short circuits or explosion risks. Therefore, it is currently urgently needed to develop lithium-ion battery separators with excellent comprehensive performance.

[0004] The types of membrane materials and the preparation methods of membranes in novel diaphragms have a great influence on the performance of battery diaphragms and batteries. Aromatic polyamide materials have become one of the potential application materials for lithium-ion battery diaphragms due to their excellent mechanical properties, high temperature resistance, good chemical stability, dimensional stability, and thermal stability. CN111370625A discloses an aramid-coated lithium-ion battery diaphragm and its preparation method. The preparation method includes: dissolving aramid, a co-solvent, an oily auxiliary agent, and a pore-forming agent in a first solvent to obtain a uniformly mixed coating solution, then coating the coating solution on a diaphragm substrate, and immersing the film in a coagulation bath by phase inversion to obtain a lithium-ion battery diaphragm. Although this membrane has good thermal stability, the bonding force and firmness between the coating layer and the membrane matrix are poor, and the coating layer will affect the overall pores of the membrane. The performance of the membrane is mainly affected by the performance of the membrane matrix, and there are still problems of large thermal shrinkage deformation and low ionic conductivity. CN105723030A discloses a separator paper for an electrochemical cell, which contains at least 60% by weight of aromatic polyamide fibrils and at least 1% by weight of aromatic polyamide fibers. Although this separator paper provides a new direction for the application of aramid in battery diaphragms, as a battery diaphragm, the separator paper has problems of low tensile strength and large pore size, and it is difficult to meet the requirements of lithium-ion battery diaphragms. CN108666501A discloses a preparation method of a para-aramid polymer diaphragm for lithium-ion batteries prepared by electrospinning. However, the membrane prepared by electrospinning has a large pore size, poor mechanical strength, low production efficiency, and high cost, and is not suitable for large-scale applications. In addition, the prior art also discloses a porous membrane prepared by a non-solvent induced phase separation method. This method uses an organic solvent as a coagulation bath for film formation, and a large amount of organic solvent will be generated during the production process, bringing great environmental protection problems and waste liquid treatment pressure.

[0005] Therefore, it is an urgent problem to be solved in this field to develop a porous membrane with a simple and environmentally friendly preparation process, high porosity, high ionic conductivity, good thermal stability and mechanical properties, especially to meet the performance requirements of battery diaphragms. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide an aromatic polyamide porous membrane, a preparation method thereof, and a secondary battery containing the same. Through the design of a first solvent and a second solvent and their mutual combination with a specific process, the prepared aromatic polyamide porous membrane has the advantages of large porosity and high ionic conductivity, and exhibits excellent mechanical properties and excellent thermal stability. The preparation method is simple and environmentally friendly, the film-forming process is easy to adjust, the film structure is easy to control, and it is suitable for large-scale industrial production.

[0007] To achieve the purpose of this invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a method for preparing an aromatic polyamide porous membrane, the preparation method comprising:

[0009] Mixing a first solvent, a second solvent and an aromatic polyamide to obtain a casting solution; the first solvent is a good solvent for the aromatic polyamide, the second solvent includes a poor solvent for the aromatic polyamide, and the boiling point of the first solvent < the boiling point of the second solvent; subjecting the casting solution to a film-forming treatment to obtain a liquid film; removing the first solvent from the liquid film to obtain a dry film; removing the second solvent from the dry film to obtain the aromatic polyamide porous membrane.

[0010] The preparation method provided by the present invention uses aromatic polyamide to prepare a film. On the one hand, there are amide bonds in the aromatic polyamide molecule, and the amide bonds have strong hydrogen bond association ability. At the same time, there are large benzene rings on the aromatic polyamide molecule, and the molecular chain segments are difficult to undergo internal rotation, so that the probability of molecular chain folding is very small, which is conducive to the orderly arrangement of aromatic polyamide molecules, making the aromatic polyamide porous membrane have high mechanical strength and providing guarantee for the safety of the battery; on the other hand, aromatic polyamide is a material with high heat resistance, which can ensure that it is not prone to thermal deformation at high temperatures, providing a wider application range for the battery; in addition, aromatic polyamide has high affinity for the electrolyte. The prepared aromatic polyamide porous membrane as a battery separator has good wetting and liquid absorption and retention capabilities, and this excellent high wettability can extend the cycle life of the battery and greatly improve the fast charging performance of the battery; based on this, the aromatic polyamide porous membrane has great advantages for use as a separator in secondary batteries.

[0011] The present invention uses specific first solvent, second solvent and aromatic polyamide to prepare a casting solution. After forming a liquid film with it, a method of separately removing the first solvent and the second solvent in stages is used to form the aromatic polyamide porous membrane; the specific technical concept is as follows:

[0012] (1) The first solvent is a good solvent for the aromatic polyamide, ensuring excellent solubility of the aromatic polyamide; the second solvent includes a poor solvent for the aromatic polyamide, which has poor solubility for the aromatic polyamide. At the same time, the second solvent has good compatibility with the first solvent, ensuring that the second solvent can be evenly distributed in the casting solution and the liquid film. On the one hand, it avoids the precipitation of the aromatic polyamide, and on the other hand, it ensures that the aromatic polyamide porous membrane has a uniform porous structure and prevents the micropore diameters of the finally prepared aromatic polyamide porous membrane from being uneven.

[0013] (2) The boiling point of the second solvent > the boiling point of the first solvent; the second solvent has a pore-forming effect in the casting solution. To ensure that the second solvent functions as a pore-forming agent, during the process of removing the first solvent from the liquid film, while all or most of the first solvent is removed, the second solvent continues to remain in the dry film, that is, the dry film includes aromatic polyamide and the second solvent.

[0014] During the process of removing the first solvent from the liquid film, when the evaporation of the first solvent reaches a certain extent, the liquid film (casting solution) forms a thermodynamically unstable system, resulting in phase separation of the liquid film and forming a polymer-rich phase and a polymer-lean phase, where the polymer-lean phase contains a large amount of the second solvent. Since the boiling point of the second solvent is high and it is relatively difficult to evaporate compared to the first solvent, when the first solvent is completely evaporated and removed to form a dry film (solid film), there is still a large amount of the second solvent evenly distributed in the film; then after removing the second solvent, pores are formed at the positions of the second solvent in the film. Therefore, in the present invention, by designing suitable first and second solvents, the structure of the aromatic porous membrane can be regulated by simply changing the removal rate of the first solvent and the content of the second solvent, and an aromatic polyamide porous membrane with excellent performance can be prepared.

[0015] (3) In the present invention, the formation of the aromatic polyamide porous membrane mainly occurs during the process of removing the first solvent, and the process of forming the dry film is the process of forming; the process of removing the second solvent does not affect the formation of the membrane and only serves to remove the second solvent and form a porous structure. Therefore, compared with the traditional non-solvent induced phase separation method for preparing porous membranes, the preparation method of the present invention does not require the use of a large amount of organic solvents for coagulation bath forming and controlling the structure of the porous membrane. By simply changing the removal rate of the first solvent and the content of the second solvent, the microstructure of the aromatic polyamide porous membrane can be regulated, which has less impact on the environment, is more environmentally friendly, and does not bring the pressure of waste liquid treatment.

[0016] Therefore, in the preparation method provided by the present invention, through the design of the first and second solvents and their combination and mutual cooperation with specific process steps, an aromatic polyamide porous membrane with excellent performance can be obtained simply and quickly, making the obtained aromatic polyamide porous membrane have the advantages of large porosity and high ionic conductivity, and having excellent mechanical properties and thermal stability, which can fully meet the performance requirements of the separator of secondary batteries. The process of the preparation method is simple and environmentally friendly, the film-making process is easy to adjust, the film structure is easy to control, and it is suitable for large-scale industrial production.

[0017] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the purpose and beneficial effects of the present invention can be better achieved and realized.

[0018] Preferably, the boiling point of the first solvent is T 1 , and the boiling point of the second solvent is T 2 , T 2 -T 1 ≥30 °C. For example, the difference between the two can be 32 °C, 35 °C, 38 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C or 160 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0019] Preferably, the first solvent includes any one or a combination of at least two of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0020] As a preferred technical solution of the present invention, the first solvent has good solubility in aromatic polyamide and can prevent the presence of undissolved aromatic polyamide clusters in the casting solution. If there are undissolved aromatic polyamide clusters in the casting solution, then during the phase separation of the liquid film (casting solution), the undissolved aromatic polyamide clusters will aggregate into crystal nuclei, and the force between the crystal nuclei is relatively small, resulting in poor mechanical properties of the aromatic polyamide porous membrane. An appropriate amount of the first solvent with good solubility is beneficial to the preparation of an aromatic polyamide porous membrane with good mechanical properties.

[0021] Furthermore, in order to broaden the selection range of the second solvent and reduce the film-making time, on the premise of ensuring good solubility in aromatic polyamide, the lower the volatility of the first solvent, the better. Based on this, the first solvent is preferably N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0022] Preferably, the second solvent includes any one or a combination of at least two of triethyl phosphate, sulfolane, formamide, dibasic acid ester, diethylene glycol, glycerol, trimethyl phosphate, and N-methylpyrrolidone. Further preferably, the combination of triethyl phosphate, dibasic acid ester, glycerol and N-methylpyrrolidone, and the combination of formamide and N-methylpyrrolidone.

[0023] In the present invention, the dibasic acid ester is also called "MDBE" or "methyl ester of nylon acid", and the DuPont trade name is DBE.

[0024] The second solvent is the main factor for the existence of micropores in the aromatic polyamide porous membrane. Therefore, the selection of the second solvent is very important. First of all, the second solvent has good compatibility with the first solvent, enabling the second solvent to be evenly distributed in the casting solution and the liquid film, preventing the uneven pore size of the micropores in the finally prepared aromatic polyamide porous membrane, and also to ensure that the polymer does not precipitate. Secondly, the boiling point of the second solvent is higher than that of the first solvent to ensure that while removing the first solvent, the second solvent continues to remain in the dry film and plays the role of a pore-forming agent. More importantly, the second solvent includes a solvent with poor solubility in the aromatic polymer, so that during the process of removing the first solvent, phase separation occurs in the liquid film, forming a polymer lean phase containing a large amount of the second solvent, and then forming a porous structure after removing the second solvent. Triethyl phosphate, dibasic acid esters, glycerol, formamide, sulfolane, diethylene glycol, etc. have relatively poor solubility in aromatic polyamide and can be used alone as the second solvent, while N-methylpyrrolidone has good solubility in aromatic polyamide and cannot be used alone. It needs to be combined with a second solvent with even poorer solubility, such as glycerol, formamide, diethylene glycol, etc. Considering the above requirements, in the present invention, the second solvent is preferably a combination of triethyl phosphate, dibasic acid esters, glycerol and N-methylpyrrolidone, a combination of formamide and N-methylpyrrolidone, or a combination of diethylene glycol and N-methylpyrrolidone.

[0025] Preferably, the second solvent is a combination of glycerol and N-methylpyrrolidone, and the mass ratio of glycerol to N-methylpyrrolidone is 1:(0.5 - 4), for example, it can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, etc.

[0026] Preferably, the second solvent is a combination of formamide and N-methylpyrrolidone, and the mass ratio of formamide to N-methylpyrrolidone is 1:(0.5 - 4), for example, it can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, etc.

[0027] Preferably, the second solvent is a combination of diethylene glycol and N-methylpyrrolidone, and the mass ratio of diethylene glycol to N-methylpyrrolidone is 1:(0.5 - 4), for example, it can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, etc.

[0028] Preferably, the aromatic polyamide includes any one or a combination of at least two of poly(p-phenyleneterephthalamide), poly(m-phenylenediamine isophthalamide), poly(p-benzamide), and poly(p-phenylene sulfone terephthalamide), and further preferably poly(m-phenylenediamine isophthalamide) (meta-aramid).

[0029] As a preferred technical solution of the present invention, the aromatic polyamide is meta-aramid. Among them, adjacent amide bonds are prone to form hydrogen bonds with strong interactions. The amide groups in the molecular chain are connected to the phenyl groups in the meta-position, without a conjugation effect, the internal rotation potential energy is relatively low, the chain segment flexibility is relatively good, and the crystallinity is relatively low. Therefore, whether in the production or use process, the meta-aramid porous membrane is more suitable for use as a separator in secondary batteries (preferably lithium-ion batteries) compared with other aromatic polyamide porous membranes.

[0030] Preferably, the mass percentage content of the aromatic polyamide in the casting solution is 10%-20%. For example, it can be 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18% or 19%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0031] As a preferred technical solution of the present invention, the first solvent can not only dissolve the aromatic polyamide, but also adjust the viscosity of the casting solution.

[0032] Preferably, the mass percentage content of the first solvent in the casting solution is 50%-80%. For example, it can be 52%, 55%, 58%, 60%, 62%, 68%, 68%, 70%, 72%, 75% or 78%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0033] Preferably, the mass percentage content of the second solvent in the casting solution is 10%-30%. For example, it can be 12%, 15%, 18%, 20%, 22%, 25% or 28%, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0034] As a preferred technical solution of the present invention, the content of the second solvent has a decisive influence on the microporous structure of the aromatic polyamide porous membrane. The mass percentage content of the second solvent in the casting solution is 10%-30%. By changing the content of the second solvent in the casting solution, the aromatic polyamide porous membrane can be regulated. Specifically, when the mass percentage content of the second solvent in the casting solution is 10%-20%, the pore size of the prepared aromatic polyamide porous membrane is relatively small. The main reason is that as the first solvent in the liquid film (casting solution) evaporates, the thermodynamic state of the casting solution becomes unstable and phase separation occurs, forming a polymer-rich phase and a polymer-poor phase with less polymer. A large amount of the second solvent exists in the polymer-poor phase. The small droplets in the polymer-poor phase will gather together during the evaporation of the first solvent. Since the amount of the second solvent is relatively small and the viscosity of the casting solution is relatively large, the moving speed of the small droplets in the polymer-poor phase is relatively slow, and the small droplets in the polymer-poor phase are relatively few. Therefore, when the small droplets in the polymer-rich phase solidify, the droplet size formed by the aggregation of the small droplets in the polymer-poor phase is relatively small, and the micropores of the prepared aromatic polyamide porous membrane are relatively small. When the mass percentage content of the second solvent in the casting solution is 20%-30%, the pore size of the prepared aromatic polyamide porous membrane is relatively large and the porosity is relatively large. The main reason is that since the amount of the second solvent is relatively large and the viscosity of the casting solution is relatively small, the moving speed of the small droplets in the polymer-poor phase is relatively fast, and the small droplets in the polymer-poor phase are relatively many. Therefore, when the small droplets in the polymer-rich phase solidify, the droplet size formed by the aggregation of the small droplets in the polymer-poor phase is relatively large, and the micropores of the prepared aromatic polyamide porous membrane are relatively large. By controlling the content of the second solvent in the casting solution, the effective regulation of the microporous structure of the aromatic polyamide porous membrane can be achieved.

[0035] Preferably, the method for preparing the casting solution includes: providing an aromatic polyamide solution, which includes a combination of an aromatic polyamide and a first solvent; mixing the aromatic polyamide solution and a second solvent evenly to obtain the casting solution.

[0036] Preferably, the method for film formation treatment includes a doctor blade method, a roll coating method, an impregnation method, a suction filtration method or a casting method.

[0037] Preferably, the thickness of the liquid film is 80-350 μm, for example, it can be 90 μm, 100 μm, 120 μm, 150 μm, 180 μm, 200 μm, 220 μm, 250 μm, 280 μm, 300 μm, 320 μm or 340 μm, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the above range.

[0038] In the present invention, the thickness of the liquid film is the main factor affecting the thickness of the prepared aromatic polyamide porous membrane. When the aromatic polyamide porous membrane is relatively thin, the internal resistance of the aromatic polyamide porous membrane is relatively small, which makes the ion conduction speed increase. However, at the same time, the mechanical strength of the relatively thin aromatic polyamide porous membrane will decrease, resulting in certain safety hazards for the secondary battery. Therefore, in order to make the aromatic polyamide porous membrane have an appropriate thickness, balance the ion conduction performance and mechanical properties, and make the membrane as thin as possible under the condition of ensuring certain mechanical properties, the thickness of the liquid film is controlled at 80 - 350 μm. The prepared aromatic polyamide porous membrane is relatively thin and has excellent mechanical properties.

[0039] Preferably, the method for removing the first solvent from the liquid film includes drying (oven drying).

[0040] Preferably, the temperature of the drying is 30 - 120 °C, for example, it can be 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C or 110 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range. Further preferably, it is 45 - 90 °C, and even more preferably, it is 55 - 80 °C.

[0041] As a preferred technical solution of the present invention, by the method of drying (oven drying) to remove the first solvent, the drying temperature has an impact on the structure of the aromatic polyamide porous membrane. Too high or too low a temperature will cause a decrease in the surface porosity of the aromatic polyamide porous membrane. Specifically, when the drying temperature is relatively low, the evaporation rate of the first solvent is relatively slow, and coupled with the relatively low viscosity of the casting solution (liquid film), it leads to a relatively long solid-liquid phase separation time in the casting solution. The polymer in the casting solution has time to form crystal nuclei, and the crystal nuclei grow to form spherulites. The spherulites continue to grow until they come into contact with other spherulites, and the second solvent is dispersed in the gaps between the spherulites. Therefore, the finally prepared aromatic polyamide porous membrane is internally composed of aggregated aromatic polyamide spherulites, resulting in poor mechanical properties. When the drying temperature is relatively high, the first solvent evaporates quickly, the thermodynamic state of the casting solution is unstable, and the viscosity of the casting solution is relatively low, resulting in a very short liquid-liquid phase separation time in the casting solution, forming a polymer-rich phase and a polymer-poor phase with less polymer. The temperature of the casting solution is relatively high and the viscosity is low, and the small liquid droplets in the liquid phase quickly aggregate to form large droplets rich in the second solvent. Of course, due to the high temperature, part of the second solvent will also volatilize, resulting in a relatively small number of pores and a relatively large pore volume in the finally formed aromatic polyamide porous membrane. Such an aromatic polyamide porous membrane has good mechanical properties but relatively large internal resistance and low ion conductivity. Therefore, the present invention sets a preferred drying temperature to make the prepared aromatic polyamide porous membrane have a high porosity, excellent mechanical properties, and relatively large ion conductivity at the same time.

[0042] Preferably, the drying time is 0.2 - 2 h, for example, it can be 0.3 h, 0.5 h, 0.8 h, 1 h, 1.2 h, 1.5 h or 1.8 h, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0043] Preferably, the method for removing the second solvent from the dry film includes drying by heating or solvent replacement.

[0044] Preferably, the temperature for drying by heating is 120 - 170 °C, for example, it can be 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, 160 °C or 165 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range. Within this temperature range, on the one hand, the structure of the aromatic polyamide porous membrane can be ensured not to be damaged, and on the other hand, the second solvent can be ensured to be removed for its subsequent normal use.

[0045] Preferably, the time for drying by heating is 1 - 2 h, for example, it can be 1.1 h, 1.2 h, 1.3 h, 1.4 h, 1.5 h, 1.6 h, 1.7 h, 1.8 h or 1.9 h, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0046] Preferably, the solvent (replacement solvent) used for solvent replacement includes any one or a combination of at least two of water, ethanol, tert-butanol, and isopropanol, and water and / or ethanol are further preferred.

[0047] Preferably, the time for solvent replacement is 0.2 - 2 h, for example, it can be 0.3 h, 0.5 h, 0.8 h, 1 h, 1.2 h, 1.5 h or 1.8 h, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0048] Preferably, a drying step is further included after solvent replacement.

[0049] Preferably, the temperature for drying after solvent replacement is 40 - 60 °C, for example, it can be 42 °C, 45 °C, 48 °C, 50 °C, 52 °C, 55 °C or 58 °C, as well as the specific point values between the above point values. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0050] Preferably, the drying time after solvent replacement is 0.1 - 1 h, for example, it can be 0.2 h, 0.3 h, 0.4 h, 0.5 h, 0.6 h, 0.7 h, 0.8 h or 0.9 h, as well as the specific point values between the above point values. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the range.

[0051] When removing the second solvent by the method of solvent replacement, the dry film rich in the second solvent is placed in the replacement solvent. Since the second solvent is miscible with the replacement solvent and there is no second solvent in the replacement solvent, a diffusion phenomenon will occur. The second solvent in the dry film will diffuse into the replacement solvent, and the replacement solvent will enter the film. Because the replacement solvent is relatively easy to remove compared to the second solvent, this step is to prepare for easier film formation later. Drying after solvent replacement is to remove the replacement solvent so that the aromatic polyamide porous membrane can be used normally in the subsequent process.

[0052] Preferably, the preparation method includes the following steps:

[0053] (1) Mix the first solvent, the second solvent and aromatic polyamide to obtain a casting solution; the mass percentage content of aromatic polyamide in the casting solution is 10% - 20%, the mass percentage content of the first solvent is 50% - 80%, and the mass percentage content of the second solvent is 10% - 30%;

[0054] The first solvent is a good solvent for aromatic polyamide, including any one or a combination of at least two of N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, and N - methylpyrrolidone, and its boiling point is T 1 ;

[0055] The second solvent includes a poor solvent for aromatic polyamide, including any one or a combination of at least two of triethyl phosphate, sulfolane, formamide, diacid ester, diethylene glycol, glycerol, trimethyl phosphate, and N - methylpyrrolidone, and its boiling point is T 2 ; T 2 - T 1 ≥30 °C;

[0056] (2) Perform film - forming treatment on the casting solution by knife - coating, roll - coating, dipping, suction filtration or casting to obtain a liquid film with a thickness of 80 - 350 μm;

[0057] (3) Dry the liquid film at 45 - 90 °C for 0.2 - 2 h to remove the first solvent and obtain a dry film;

[0058] (4) Remove the second solvent in the dry film by Method A or Method B to obtain the aromatic polyamide porous membrane;

[0059] The method A includes: drying the dry film at 120 - 170 °C for 1 - 2 h to obtain the aromatic polyamide porous membrane;

[0060] The method B includes: after solvent replacement of the dry film for 0.2 - 2 h, drying at 40 - 60 °C for 0.1 - 1 h to obtain the aromatic polyamide porous membrane;

[0061] The solvent used for solvent replacement includes any one or a combination of at least two of water, ethanol, tert - butanol, and isopropanol.

[0062] In a second aspect, the present invention provides an aromatic polyamide porous membrane, which is prepared by the preparation method as described in the first aspect.

[0063] Preferably, the pore size of the aromatic polyamide porous membrane is 0.1 - 0.8 μm, more preferably 0.2 - 0.8 μm, and further preferably 0.24 - 0.65 μm.

[0064] Preferably, the porosity of the aromatic polyamide porous membrane is ≥45%, more preferably 45 - 65%, and further can be 47 - 60%.

[0065] Preferably, the thickness of the aromatic polyamide porous membrane is ≤30 μm, can be 19 - 30 μm, and further can be 22 - 28 μm.

[0066] Preferably, the air permeability of the aromatic polyamide porous membrane is ≤500 s / 100 cc, can be 155 - 500 s / 100 cc, more preferably 200 - 500 s / 100 cc, and further can be 350 - 450 s / 100 cc.

[0067] Preferably, the surface pore opening rate of the aromatic polyamide porous membrane is ≥30%, can be 30 - 55%, more preferably 35 - 55%, and further can be 40 - 50%.

[0068] Preferably, the electrolyte contact angle of the aromatic polyamide porous membrane is ≤35°, can be 20 - 35°, and further can be 23 - 30°.

[0069] Preferably, the liquid absorption rate of the aromatic polyamide porous membrane is ≥100%, can be 107 - 235%, more preferably 120 - 175%, and further can be 120 - 155%.

[0070] Preferably, the thermal shrinkage rate of the aromatic polyamide porous membrane treated at 150 °C for 1 h is ≤1%, can be 0.2 - 1%, and further can be 0.25 - 0.7%.

[0071] Preferably, the tensile strength of the aromatic polyamide porous membrane is ≥50 MPa, and it can be 50 - 70 MPa.

[0072] Preferably, the puncture strength of the aromatic polyamide porous membrane is ≥0.13 N / μm, and it can be 0.14 - 0.5 N / μm, and further can be 0.2 - 0.5 N / μm.

[0073] In a third aspect, the present invention provides an application of the aromatic polyamide porous membrane as described in the second aspect in a secondary battery.

[0074] Preferably, the aromatic polyamide porous membrane is applied as a separator in a secondary battery.

[0075] In a fourth aspect, the present invention provides a secondary battery, and the secondary battery includes the aromatic polyamide porous membrane as described in the second aspect.

[0076] Preferably, the secondary battery includes a lithium-ion battery, a sodium-ion battery, a lithium-sulfur battery, etc.

[0077] Preferably, the secondary battery is a lithium-ion battery.

[0078] Preferably, the lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, and the separator is the aromatic polyamide porous membrane as described in the second aspect.

[0079] Among them, the electrolyte is a liquid electrolyte (electrolyte solution) or a solid electrolyte.

[0080] Compared with the prior art, the present invention has the following beneficial effects:

[0081] (1) The present invention uses specific first and second solvents to prepare a casting solution, and then forms an aromatic polyamide porous membrane by a method of removing the first and second solvents in stages. During the process of removing the first solvent, the liquid film system is in a thermodynamically unstable state and undergoes phase separation, forming a rich phase with a higher polymer content and a poor phase with a lower polymer content. The first solvent is completely removed and the liquid film is solidified into a dry film, while the second solvent is stored in the dry film. By controlling the concentration of the second solvent, the microporous structure and quantity of the aromatic polyamide porous membrane can be controlled at the sub-micron scale, and an aromatic polyamide porous membrane with excellent performance can be prepared.

[0082] (2) The process steps of the preparation method provided by the present invention are simple, without the need to use a large amount of organic solvents, environmentally friendly, highly controllable, low in cost, and suitable for large-scale production.

[0083] (3) The pore size of the aromatic polyamide porous membrane provided by the present invention is 0.1 - 0.8 μm, the porosity is ≥ 45%, the thickness is ≤ 30 μm, the air permeability is ≤ 500 s / 100 cc, the surface pore opening rate is ≥ 30%, the electrolyte contact angle is ≤ 35°, the liquid absorption rate is ≥ 100%, the thermal shrinkage rate at 150 °C / 1 h is ≤ 1%, the tensile strength is ≥ 50 MPa, and the puncture strength is ≥ 0.13 N / μm. It has the characteristics of large porosity, good electrolyte wettability, excellent mechanical properties and thermal stability.

[0084] (4) The aromatic polyamide porous membrane has excellent electrochemical properties, mechanical properties, thermal stability and dimensional stability, is suitable as a separator for lithium-ion batteries, has a low interfacial impedance, and can effectively improve the cycle performance of lithium-ion batteries. Description of the Drawings

[0085] Figure 1 SEM image of the upper surface of the meta-aramid porous membrane provided in Example 1;

[0086] Figure 2 SEM image of the lower surface of the meta-aramid porous membrane provided in Example 1;

[0087] Figure 3 SEM cross-sectional image of the meta-aramid porous membrane provided in Example 1;

[0088] Figure 4 SEM cross-sectional image of the meta-aramid porous membrane provided in Example 17;

[0089] Figure 5 SEM cross-sectional image of the meta-aramid porous membrane provided in Example 18;

[0090] Figure 6 SEM image of the upper surface of the meta-aramid porous membrane provided in Example 21;

[0091] Figure 7 SEM cross-sectional image of the meta-aramid porous membrane provided in Comparative Example 1;

[0092] Figure 8 SEM cross-sectional image of the meta-aramid porous membrane provided in Comparative Example 2;

[0093] Figure 9 Process flow chart of the preparation method provided by the present invention. Detailed Embodiments

[0094] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0095] As used herein, the terms "comprising", "including", "having", "containing" or any other variation thereof are intended to cover non-exclusive inclusion. For example, a composition, step, method, article or apparatus comprising the recited elements is not necessarily limited to those elements, but may also include other elements not expressly listed or elements inherent to such composition, step, method, article or apparatus.

[0096] In the present invention, features defined with "first", "second" may explicitly or implicitly include one or more of such features, which are used to distinguish and describe features, without order or importance. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0097] The aromatic polyamide porous membrane of the present invention is suitable as a separator for secondary batteries, especially for lithium-ion battery separators. The following will detail the main performance and requirements of the separator, the reasons why the aromatic polyamide porous membrane meets the requirements of lithium-ion battery separators, and the performance testing methods of the aromatic polyamide porous membrane:

[0098] I. Pore size

[0099] Regarding the aromatic polyamide porous membrane in the embodiments of the present invention, in order to obtain a separator with a relatively small internal resistance and high safety, it is necessary to control the pore size of the aromatic polyamide porous membrane. The pore size of the lithium-ion battery separator must be at the sub-micron level. When the pore size of the lithium-ion battery separator is small, its internal resistance will be larger, the contact area with the electrolyte is smaller, and the lithium-ion permeability will decrease, resulting in a decline in the overall performance of the battery; when the pore size of the lithium-ion battery separator is large, although the ionic conductivity can be greatly improved, it will also cause self-discharge of the lithium-ion battery, resulting in energy loss of the battery. Moreover, the larger pore size of the separator will increase the risk of being pierced by lithium dendrites, causing safety problems such as short circuit and even explosion. Therefore, controlling the pore size of the aromatic polyamide porous membrane within a suitable range is crucial for its service performance.

[0100] The pore size of the aromatic polyamide porous membrane in the embodiments of the present invention is between 0.1 - 0.8 μm. Within the above pore size range, the aromatic polyamide porous membrane has both high ionic conductivity and high safety, and basically does not exhibit self-discharge phenomenon. In the embodiments of the present invention, the pore size is mainly controlled by controlling the content of the second solvent.

[0101] In the embodiment of the present invention, the pore size of the aromatic polyamide porous membrane is characterized by mercury intrusion porosimetry (refer to the method in GB / T 21650.1-2008). Mercury has no wetting effect on the aromatic polyamide porous membrane, so an external pressure needs to be applied to make mercury enter the pores of the diaphragm. Different external pressures are required for pores of different diameters, and the smaller the pore diameter, the greater the required external pressure. Therefore, by measuring the amount of mercury entering the pores of the diaphragm under different external pressures, the pore volume of pores with corresponding sizes can be known.

[0102] The principle of mercury intrusion porosimetry can be expressed by the Washburn equation: where r is the pore diameter of the capillary pore (m); σ is the surface tension of mercury (mN / m); θ is the contact angle between mercury and the capillary surface; p is the external pressure applied (mN / m 2 ).

[0103] II. Porosity

[0104] Porosity is defined as the ratio of the volume of micropores in the diaphragm to the total volume of the diaphragm. It is a very important evaluation criterion for lithium-ion battery diaphragms. It directly affects the transport speed of lithium ions in the diaphragm and the storage of electrolyte in the diaphragm. A diaphragm with high porosity can provide more storage sites for the electrolyte. The path for lithium ions to pass through the diaphragm becomes larger and wider, and the transmittance and transmission rate of lithium ions will increase, thereby improving the performance of lithium-ion batteries. Theoretically, parameters such as the gas permeability, liquid absorption capacity, and internal resistance of the diaphragm are all affected by it. The porosity of the aromatic polyamide porous membrane prepared in the present invention is above 45%, which is better than most current commercial diaphragms. The present invention controls the porosity of the aromatic polyamide porous membrane by regulating the content of the second solvent in the casting solution.

[0105] In the present invention, the porosity of the aromatic polyamide porous membrane is measured by the hexadecane absorption method (refer to the method in GB / T 33052-2016). First, cut three samples of the aromatic polyamide porous membrane to be measured into a size of 10 cm × 5 cm. Then, use a thickness gauge and an electronic balance to measure the thickness and weight of the samples respectively. Then, immerse the samples in hexadecane for 1 h and wipe the surface of the hexadecane with absorbent paper. Finally, weigh and calculate to obtain the porosity of the aromatic polyamide porous membrane.

[0106] The porosity calculation formula is: where X is the length of the sample (cm), Y is the width of the sample (cm), d is the thickness of the sample (cm), w 1 is the mass of the sample before absorbing hexadecane (g), w 2 is the mass of the sample after fully absorbing hexadecane (g), and ρ is the density of hexadecane (g / cm 3 ).

[0107] III. Thickness

[0108] Thickness is one of the basic parameters of lithium battery separators and has a great influence on the mechanical properties and lithium ion permeability of the separators. Generally speaking, the thicker the separator, the better its mechanical properties, and the risk of being pierced by lithium dendrites can be effectively reduced. However, the increase in thickness will make the permeation path of lithium ions in the separator more complex and tortuous, resulting in a significant decrease in the ion permeability and permeation rate. In addition, the increase in thickness will also occupy the space of the active materials in the lithium ion battery, resulting in a reduction in the battery capacity. Therefore, controlling the separator within a reasonable range is crucial for the performance of lithium ion batteries. The aromatic polyamide porous membrane of the present invention relies on the excellent mechanical properties of aromatic polyamide, so that the aromatic polyamide porous membrane can have a very small thickness (≤30 μm) under the condition of meeting the mechanical strength. The present invention controls the thickness of the aromatic polyamide porous membrane by regulating the thickness of the liquid membrane, and the thickness test refers to the method in GB / T 6672-2001.

[0109] IV. Gas permeability

[0110] Gas permeability refers to the time for gas to permeate through the separator under a certain pressure, which can indirectly reflect the lithium ion permeability. The better the gas permeability of the separator, the better its lithium ion permeability. The main factors affecting the gas permeability of the separator are the porosity and pore connectivity of the separator. The higher the porosity and the better the pore connectivity, the better the gas permeability of the separator. The aromatic polyamide porous membrane prepared by the present invention has a high porosity and good pore connectivity, so it has excellent gas permeability (≤500 s / 100 cc). The present invention mainly regulates the gas permeability of the aromatic polyamide porous membrane by regulating the thickness of the liquid membrane and the content of the second solvent, and the gas permeability test refers to the method in GB / T 1040.3-2006.

[0111] V. Surface opening ratio

[0112] In the present invention, the surface porosity is defined as the ratio of the area of the pores on the separator surface to the total surface area of the separator. The surface porosity determines the inlet area for lithium ions to enter the separator, and the size of the inlet area determines the rate at which lithium ions enter the separator. When the surface porosity is relatively large, the pore area on the separator surface is larger, and lithium ions can enter the interior of the separator at a faster rate, and the corresponding ionic conductivity will be larger. The separators prepared by the traditional non-solvent induced phase separation method are usually relatively dense on the surface and have a relatively small surface porosity. This is because in the process of preparing the porous membrane by the non-solvent induced phase separation method, the surface solvent rapidly exchanges with the non-solvent in the coagulation bath, and the exchange rate is much greater than the rate at which the solvent in the sublayer of the casting solution is transferred to the surface. Phase separation occurs instantaneously on the separator surface to form a dense layer. The aromatic polyamide porous membrane prepared by the method of the present invention has a relatively large surface porosity (≥30%). The main reason is that in the present invention, the solvent molecules in the liquid film cortex evaporate relatively fast, promoting liquid-liquid phase separation in the cortex to form a rich phase with more polymer and a lean phase with less polymer. As the first solvent is continuously removed, the rich phase is finally solidified, and the lean phase forms a pore structure. The lower surface of the aromatic polyamide porous membrane can also have excellent surface porosity because the liquid film cortex does not immediately form a dense layer. Therefore, the solvent on the lower surface can also be evaporated and removed at a relatively fast rate, promoting the rate of phase separation, and finally forming a lower surface with a relatively large surface porosity.

[0113] In the present invention, the following method is used to characterize the surface porosity. First, the aromatic polyamide porous membrane to be tested is put into liquid nitrogen for quenching and breaking, and then the aromatic polyamide porous membrane is fixed on the sample stage with conductive glue. The sample is sputter-coated with gold, and the cross-section of the sample is photographed and the picture is saved using a scanning electron microscope (SEM). All the pores in the above SEM picture are selected using Image J software, and the pore area and the total area in the SEM picture are obtained. Dividing the two can obtain the surface porosity of the aromatic polyamide porous membrane.

[0114] VI. Wettability

[0115] The wetting performance is mainly used to evaluate the wetting effect of the separator on the electrolyte and reveal the compatibility between the separator and the electrolyte. Good wetting property is beneficial to the affinity between the separator and the electrolyte. The larger the contact area between the separator and the electrolyte, the higher the transfer rate of lithium ions, and the charge-discharge efficiency and capacity of the battery are thus improved. The molecular chains of current commercial PP and PE separators are all polymerized from olefins, and there are few polar groups on the molecular chains. Therefore, their wetting properties for the electrolyte are very poor, and it is necessary to coat a liquid-philic material to increase their wetting properties. There are many polar groups on the molecular chains of aromatic polyamides. Therefore, the aromatic polyamide porous membrane has excellent wetting properties for the electrolyte (contact angle ≤ 35°). In the present invention, a contact angle tester is used to measure the contact angle between the aromatic polyamide porous membrane and the electrolyte (1.0 mol / L LiPF 6The contact angle of the electrolyte (the solvent is a mixed solvent of EC, EMC, and DMC with a mass ratio of 1:1:1) was measured, and the wetting degree of the separator to the electrolyte was evaluated by the size of the contact angle.

[0116] VII. Liquid absorption rate

[0117] The liquid absorption rate of the lithium-ion battery separator refers to the amount of electrolyte stored in the separator. To a certain extent, it controls the internal resistance of the separator. The larger the liquid absorption rate, the more electrolyte is stored, and the more and wider paths lithium ions can have to pass through the separator, resulting in a larger ionic conductivity. The liquid absorption rate of the lithium-ion battery separator is mainly affected by the affinity of the membrane material to the electrolyte and the porosity of the separator. Separators with excellent wettability and high porosity have a larger liquid absorption rate. Currently, there are no polar groups on the macromolecular chains of commercial PP and PE separators, resulting in poor wetting performance and low liquid absorption rate. In the present invention, since there are many polar groups on the macromolecular chains of aromatic polyamides, the wettability to the electrolyte is good. Coupled with the large porosity of the aromatic polyamide porous membrane prepared in the present invention, the aromatic polyamide porous membrane prepared in the present invention has a large liquid absorption rate (≥100%).

[0118] In the present invention, the liquid absorption rate of the aromatic polyamide porous membrane was measured by the weighing method. First, the weight of the aromatic polyamide porous membrane before soaking in the electrolyte was weighed. After it was immersed in the electrolyte (1.0 mol / L LiPF 6 electrolyte, the solvent is a mixed solvent of EC, EMC, and DMC with a mass ratio of 1:1:1) for a period of time, the weight of the aromatic polyamide porous membrane after soaking in the electrolyte was weighed. The liquid absorption rate of the separator was calculated by comparing the weight change of the aromatic polyamide porous membrane before and after immersion. The specific calculation formula is as follows: Among them, x represents the liquid absorption rate (%) of the separator, m represents the weight (g) of the membrane after soaking in the electrolyte, and m 0 represents the weight (g) of the membrane before soaking in the electrolyte.

[0119] VIII. Thermal stability

[0120] The thermal stability performance of the lithium-ion battery separator is an important indicator for evaluating the safety of lithium-ion batteries. During the charge and discharge process inside the battery, chemical reactions occur to generate heat, and external heat sources can also cause the temperature inside the battery to rise. Especially in extreme environments or when the battery is misused, the temperature inside the battery will rise sharply. If the temperature inside the battery is too high, the separator will shrink and lose its function of isolating the positive and negative electrodes, resulting in battery short circuit or even explosion. Therefore, the separator material must have good thermal properties to ensure the dimensional stability of the battery within a wide temperature range, and still be able to isolate the positive and negative electrodes of the battery even under extreme temperature conditions. The benzene ring and the high energy barrier of the C-N bond rotation in aromatic polyamides hinder the conformation of the aromatic polyamide molecular chain from becoming a fully extended chain. Coupled with the strong hydrogen bond interaction between its molecular chains, the heat resistance of aromatic polyamides is very strong, and the glass transition temperature is about 270 °C. Even when working continuously at 200 °C for 20,000 h, the strength can be maintained at 90% of the original. Thanks to the excellent thermal properties of aromatic polyamides, the thermal shrinkage rate of the aromatic polyamide porous membrane is ≤1% at 150 °C / 1 h.

[0121] The present invention tests the thermal shrinkage rate of the aromatic polyamide porous membrane as follows: Take a 10 cm × 10 cm sample of the aromatic polyamide porous membrane, lay the membrane flat on one piece of quantitative filter paper on the stainless steel plate in the middle of the forced-air constant temperature oven, and then press it with another piece of quantitative filter paper after completion. Close the oven door and start timing. Keep it at a temperature of 150 °C for 1 h. After the heating is completed, take out the membrane. After the separator returns to room temperature, measure the marked lengths in the longitudinal and transverse directions.

[0122] The calculation formula for the thermal shrinkage rate ΔS (%) is: where S 0 is the area of the membrane before heating (cm 2 ), and S is the area of the membrane after heating (cm 2 ).

[0123] IX. Tensile Strength

[0124] Tensile strength is a parameter that reflects the dimensional stability of the separator when subjected to external forces during use. Since lithium dendrites are formed during the use of lithium-ion batteries, a separator with high strength can effectively delay the penetration of lithium dendrites and ensure the safety performance of lithium-ion batteries. In addition, during the use of lithium-ion batteries, collisions, squeezes, impacts, etc. may occur, causing the battery to deform and exert a large force on the separator. Only a separator with high strength can resist this kind of damage and prevent direct contact between the positive and negative electrodes of the battery from triggering a safety accident. The main chain of aromatic polyamide is composed of aromatic rings and amide bonds. The aromatic ring structure has high rigidity, and the polymer chain forms a rod-like structure in an extended state. At the same time, the linear molecular chain makes the space utilization rate of aromatic polyamide high. Therefore, more polymers can be accommodated per unit volume, so the strength is relatively high. Therefore, the aromatic polyamide porous membrane of the present invention has a high tensile strength (≥50 MPa), and the test method for tensile strength refers to the method in GB / T 36363-2018.

[0125] X. Puncture Resistance Strength

[0126] Puncture resistance strength refers to the mass applied to a given needle-shaped object to pierce a given separator sample, which is used to characterize the ability of the separator to resist external pressure puncture. During the battery assembly and shaping process, the separator sandwiched between the positive and negative electrodes needs to withstand a large pressure to prevent the battery from short-circuiting. In addition, during the use of the battery, squeezing, collision, impact and other behaviors are inevitable. At this time, the separator also needs to have a certain puncture resistance strength to ensure the safety of the battery. Therefore, the lithium-ion battery separator must have a certain puncture resistance strength. Due to the presence of benzene rings, aromatic polyamide has a high internal rotation potential energy, so the molecular chain segments show a planar extended chain conformation, and there are many polar groups on the molecular chain. Therefore, the gaps between aromatic polyamides are small and the intermolecular forces are strong, resulting in the aromatic polyamide porous membrane having good puncture resistance strength (≥0.13 N / μm), and the test method for puncture resistance strength refers to the method in GB / T 36363-2018.

[0127] XI. Ionic Conductivity

[0128] The separator has two main uses during the charge and discharge process of a lithium-ion battery: one is to effectively avoid short circuits caused by direct contact between the positive and negative electrode materials of the battery, thereby ensuring the safety performance of the battery; the other is to provide a channel for the transmission of lithium ions so that they can pass freely. The ionic conductivity refers to the ionic flow ability of the separator / electrolyte system after being fully wetted by the electrolyte, and it is the most important index for evaluating the quality of the ability to provide a channel for the transmission of lithium ions. The ionic permeability of the separator is affected by factors such as porosity, pore size and pore size distribution, thickness, wettability, etc. The aromatic polyamide separator prepared in the present invention has high porosity, uniform pore size distribution, small thickness, good wettability, and high strength, so it can ensure that the separator has excellent ionic conductivity (≥0.7×10 -3 S / cm, which can be 0.77 - 1.15 mS / cm) while isolating the positive and negative electrodes of the lithium-ion battery.

[0129] The test of the ionic conductivity of the aromatic polyamide porous membrane in the present invention is as follows:

[0130] The membrane to be tested is immersed in an electrolyte with a volume ratio of 1:1:1 of lithium hexafluorophosphate (LiPF 6 ), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) with a concentration of 1.0 mol / L for 2 h. Then the electrolyte is injected into a resistance test mold, and the membrane sample is placed to test its AC impedance resistance. Taking the number of membrane layers as the abscissa and the membrane resistance as the ordinate to make a curve, and calculating the slope and linear fitting degree of the curve. When the linear fitting degree is greater than 0.99, the slope is taken as the membrane resistance value.

[0131] The calculation formula for ionic conductivity σ (S / cm) is: where d is the membrane thickness (μm); R is the membrane resistance value (Ω); S is the membrane area cut during the test (cm 2 ).

[0132] Twelve. Cycle performance

[0133] The completion of one charge-discharge cycle of a lithium-ion battery is called a cycle. The cycle performance is an important indicator to measure the service life of the battery. The cycle number, the first discharge capacity, and the retention capacity determine the cycle performance of the battery. The number of charge-discharge cycles of the battery is called the cycle number. When testing the charge-discharge performance of the battery for the first time, the discharge capacity obtained by the battery is called the first discharge capacity. The retention capacity refers to the discharge capacity that the battery still maintains after completing a certain number of cycles. During the charge-discharge process, the blockage and damage of the separator will cause incomplete charge-discharge, greatly reducing the capacity and service life of the battery. In the present invention, the aromatic polyamide porous membrane has a high ionic conductivity, a small interfacial impedance, a large porosity, and a liquid absorption rate, improving the lithium-ion migration ability, and the separator will not be easily blocked. Therefore, the battery exhibits more stable cycle performance. Under the 0.2C test condition, the first discharge specific capacity of the battery with the aromatic polyamide porous membrane is above 140 mAh / g, and after 100 cycles, the capacity retention rate of the battery with the aromatic polyamide porous membrane is above 80%.

[0134] In the specific implementation manner of the present invention, the assembly method of the lithium-ion battery is as follows:

[0135] The button-type lithium-ion battery is assembled in a glove box filled with argon, and assembled in the order of positive electrode case / positive electrode sheet / separator / negative electrode sheet / spacer / shell / negative electrode case. After picking up the positive electrode sheet with tweezers, 2-3 drops of electrolyte are sucked with a dropper and dropped on the positive electrode sheet. After picking up the separator, 2-3 drops of electrolyte are dropped on the separator, and the sheets are strictly aligned with each other. Among them, the positive electrode is a lithium iron phosphate single-sided electrode sheet with a thickness and diameter of 0.8 mm and 15 mm respectively, and the mass ratio of the active substance is 93%; the negative electrode sheet is a lithium sheet with a diameter and thickness of 15.6 mm and 0.45 mm respectively; the electrolyte composition is 1.0 mol / L of LiPF 6 electrolyte (the solvent is a mixed solvent of EC, EMC, and DMC with a mass ratio of 1:1:1); then the assembled battery is encapsulated with a sealing machine.

[0136] The cycle performance test method of the lithium-ion battery is as follows:

[0137] The cycle performance of the lithium-ion battery is tested by using a Blue Power battery test system (Blue Power Electronics, CT2001A, Wuhan). First, the above-assembled lithium-ion battery is left standing for 12 h and then undergoes 2 charge-discharge cycles at 0.1C on the Blue Power battery test system, and then undergoes a cycle test at 0.2C. The test voltage range is 2.5 - 4.2V, and the charge-discharge current = theoretical specific capacity × mass of active substance × 0.2C. Each separator sample is assembled into 4 button cells for testing, and the intermediate results are taken.

[0138] XII. Interfacial Impedance

[0139] During the charging and discharging process of a lithium-ion battery, lithium ions are deposited on the solid electrolyte interface (SEI) film formed between the electrode and the electrolyte. After a period of time, the SEI film will break, and lithium ions continue to deposit to form lithium dendrites. The lithium dendrites may pierce the separator, causing a short circuit in the battery and resulting in safety problems. If the interface compatibility between the separator and the electrode is good, the formation of lithium dendrites can be slowed down or inhibited. The interface resistance refers to the resistance generated during the electrochemical reaction process on the contact surface between the electrode and the electrolyte, which reflects the compatibility between the separator and the lithium metal electrode. In the present invention, the aromatic polyamide porous film has a low interface impedance (≤100Ω) because there are polar groups on the molecular chain of the aromatic polyamide, which has a good binding force with the electrolyte, is conducive to the migration of lithium ions at the battery separator interface, and improves the stability of the positive and negative electrodes.

[0140] The present invention uses the following method to test the interface impedance: First, assemble the battery in a glove box (the assembly method is the same as that for the cycle performance test). After placing it for 3 hours, use an electrochemical workstation to test the interface impedance using the AC impedance method. The scanning voltage range is 2.5 - 6V, and the scanning rate is 0.5mV / s.

[0141] The present invention will be described in more detail based on the following examples, but it cannot be said that the present invention is limited to these examples. In the following examples of the present invention, the aromatic polyamides used include: meta-aramid, purchased from Tayho Advanced Materials Co., Ltd.; para-aramid, purchased from Tayho Advanced Materials Co., Ltd.

[0142] Example 1

[0143] An aromatic polyamide porous film (meta-aramid porous film) and its preparation method. The process flow chart of the preparation method is as Figure 9 shown, and specifically includes the following steps:

[0144] (1) Preparation of the casting solution: Add triethyl phosphate to the meta-aramid solution (the solvent is N,N-dimethylacetamide), and stir until it is homogeneous and transparent to obtain the casting solution. The components of the casting solution are as follows in mass percentage: meta-aramid 15%, N,N-dimethylacetamide 70%, and triethyl phosphate 15%;

[0145] (2) Preparation of the liquid film: Ultrasonically degas the casting solution obtained in step (1), pour the completely degassed casting solution onto a glass film substrate, adjust the blade so that the thickness of the liquid film is 150μm, and move the blade uniformly to obtain a liquid film with a thickness of 150μm;

[0146] (3) Removal of the first solvent (N,N-dimethylacetamide): Place the liquid film and the substrate obtained in step (2) in a forced-air drying oven and dry at 60°C for 1 hour to remove N,N-dimethylacetamide and obtain a dry film;

[0147] (4) Removal of the second solvent (triethyl phosphate): The dry film obtained in step (3) was placed in deionized water for 2 h to displace the triethyl phosphate in the film, and finally placed in a forced-air drying oven and dried at 50 °C to remove the excess deionized water in the film. After drying, the meta-aramid porous membrane was obtained.

[0148] The microscopic morphology of the aromatic polyamide porous membrane was tested using a scanning electron microscope (Regulus 8100, Hitachi, Japan). The SEM images of the upper surface of the meta-aramid porous membrane provided in this example are as Figure 1 shown, the SEM images of the lower surface are as Figure 2 shown, and the SEM images of the cross-section are as Figure 3 shown; Combining the above three SEM images, it can be seen that the upper and lower surfaces of the meta-aramid porous membrane have a large surface porosity, and the pore diameters are relatively uniform. The cross-section is a uniform sponge structure. The interconnected structure between the surface and the interior can provide a transmission channel for the rapid migration of lithium ions. The connected and tortuous pore structure can effectively prevent the penetration of lithium dendrites, which is beneficial to avoiding internal short circuits. After testing, the porosity of the meta-aramid porous membrane was 59.3%. The separator with a high porosity can provide more storage sites for the electrolyte. The path for lithium ions to pass through the separator becomes larger and wider, and the transmittance and transmission rate of lithium ions will increase, thereby improving the performance of lithium-ion batteries.

[0149] Example 2

[0150] A meta-aramid porous membrane and its preparation method, the difference from Example 1 is only that the second solvent is a divalent acid ester, and the components of the casting solution by mass percentage are as follows: 15% meta-aramid, 70% N,N-dimethylacetamide, 15% divalent acid ester; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane was obtained.

[0151] Example 3

[0152] A meta-aramid porous membrane and its preparation method, the difference from Example 1 is only that the second solvent is a combination of N-methylpyrrolidone and glycerol, and the components of the casting solution by mass percentage are as follows: 15% meta-aramid, 70% N,N-dimethylacetamide, 10% N-methylpyrrolidone, 5% glycerol; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane was obtained.

[0153] Example 4

[0154] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the second solvent is a combination of N-methylpyrrolidone and formamide. The components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, N-methylpyrrolidone 10%, formamide 5%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0155] Example 5

[0156] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the second solvent is a combination of N-methylpyrrolidone and diethylene glycol. The components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, N-methylpyrrolidone 10%, diethylene glycol 5%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0157] Example 6

[0158] An aromatic polyamide porous membrane (para-aramid porous membrane) and its preparation method. The preparation method includes the following steps:

[0159] (1) Preparation of the casting solution: Add triethyl phosphate to the para-aramid solution (the solvent is N,N-dimethylacetamide), and stir until homogeneous and transparent to obtain the casting solution; the components of the casting solution by mass percentage are as follows: para-aramid 15%, N,N-dimethylacetamide 70%, triethyl phosphate 15%;

[0160] (2) Preparation of the liquid film: Ultrasonically degas the casting solution obtained in step (1), pour the completely degassed casting solution onto a glass film substrate, adjust the doctor blade so that the thickness of the liquid film is 150 μm, and move the doctor blade uniformly to obtain a liquid film with a thickness of 150 μm;

[0161] (3) Removal of the first solvent (N,N-dimethylacetamide): Put the liquid film and the substrate obtained in step (2) into a forced-air drying oven and dry at 60 °C for 1 h to remove N,N-dimethylacetamide and obtain a dry film;

[0162] (4) Removal of the second solvent (triethyl phosphate): Put the dry film obtained in step (3) into deionized water for 2 h to displace the triethyl phosphate in the film, and finally put it into a forced-air drying oven and dry at 50 °C to remove the excess deionized water in the film. After drying, the para-aramid porous membrane is obtained.

[0163] Example 7

[0164] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the mass percentage contents of the first solvent and the second solvent in the casting solution are different. The components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 60%, triethyl phosphate 25%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0165] Example 8

[0166] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that in step (2), the blade is adjusted so that the liquid film thickness is 250 μm, and the blade is moved uniformly to obtain a liquid film with a thickness of 250 μm; other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0167] Example 9

[0168] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that in step (3), the liquid film and the substrate are placed in a forced-air drying oven at 40 °C for 1 h to remove N,N-dimethylacetamide; other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0169] Example 10

[0170] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that in step (3), the liquid film and the substrate are placed in a forced-air drying oven at 80 °C for 1 h to remove N,N-dimethylacetamide; other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0171] Example 11

[0172] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the mass percentage contents of meta-aramid and the first solvent in the casting solution are different. The components of the casting solution by mass percentage are as follows: meta-aramid 12%, N,N-dimethylacetamide 73%, triethyl phosphate 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0173] Example 12

[0174] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the mass percentage contents of meta-aramid and the first solvent in the casting solution are different. The components of the casting solution by mass percentage are as follows: meta-aramid 17%, N,N-dimethylacetamide 68%, triethyl phosphate 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0175] Example 13

[0176] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the first solvent is N,N-dimethylformamide, and the components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylformamide 70%, triethyl phosphate 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0177] Example 14

[0178] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the first solvent is dimethyl sulfoxide, and the components of the casting solution by mass percentage are as follows: meta-aramid 15%, dimethyl sulfoxide 70%, triethyl phosphate 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0179] Example 15

[0180] A meta-aramid porous membrane and its preparation method. The difference from Example 1 is only that the first solvent is N-methylpyrrolidone and the second solvent is glycerol, and the components of the casting solution by mass percentage are as follows: meta-aramid 15%, N-methylpyrrolidone 70%, glycerol 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0181] Example 16

[0182] A meta-aramid porous membrane and its preparation method, and the preparation method includes the following steps:

[0183] (1) Preparation of the casting solution: Add triethyl phosphate to the meta-aramid solution (the solvent is N,N-dimethylacetamide), stir until homogeneous and transparent to obtain the casting solution; the components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, triethyl phosphate 15%;

[0184] (2) Preparation of the liquid film: Ultrasonically degas the casting solution obtained in step (1), and use the roll coating method to prepare the liquid film. Pour the completely degassed casting solution into the coating tray, and adjust the gap between the rollers so that the thickness of the liquid film is 150 μm, and a liquid film with a thickness of 150 μm can be obtained;

[0185] (3) Removal of the first solvent (N,N-dimethylacetamide): Place the liquid film obtained in step (2) and the substrate in a forced-air drying oven and dry at 60 °C for 1 h to remove N,N-dimethylacetamide to obtain a dry film;

[0186] (4) Removal of the second solvent (triethyl phosphate): The dry film obtained in step (3) was placed in deionized water for 2 h to displace the triethyl phosphate in the film, and finally placed in a forced-air drying oven and dried at 50 °C to remove the excess deionized water in the film. After drying, the meta-aramid porous film was obtained.

[0187] Example 17

[0188] A meta-aramid porous film and its preparation method, the difference from Example 1 is only that the method for removing the second solvent (triethyl phosphate) in step (4) is different: The dry film obtained in step (3) was continuously dried in a forced-air drying oven at 120 °C for 2 h to remove the second solvent triethyl phosphate in the film. After drying, the meta-aramid porous film was obtained.

[0189] The cross-sectional SEM image of the meta-aramid porous film provided in this example is as Figure 4 shown. Compared with the cross-sectional SEM image of the meta-aramid porous film in Example 1 as Figure 3 shown, there is no obvious difference between the two, indicating that whether the second solvent is removed by the heating drying method or the solvent displacement method, it does not bring obvious differences to the structure of the meta-aramid porous film, thus providing more choices for the preparation of the meta-aramid porous film.

[0190] Example 18

[0191] A meta-aramid porous film and its preparation method, the difference from Example 1 is only that the mass percentage contents of the first solvent and the second solvent in the casting solution are different. The components of the casting solution in terms of mass percentage are as follows: 15% meta-aramid, 50% N,N-dimethylacetamide, 35% triethyl phosphate; other steps and process parameters are the same as those in Example 1, and the meta-aramid porous film was obtained.

[0192] The cross-sectional SEM image of the meta-aramid porous film provided in this example is as Figure 5 shown. Compared with the cross-sectional SEM image of the meta-aramid porous film in Example 1 as Figure 3 shown, the meta-aramid porous film provided in Example 18 has a larger pore size and a higher porosity.

[0193] Example 19

[0194] A meta-aramid porous film and its preparation method, the difference from Example 1 is only that the mass percentage contents of the first solvent and the second solvent in the casting solution are different. The components of the casting solution in terms of mass percentage are as follows: 15% meta-aramid, 80% N,N-dimethylacetamide, 5% triethyl phosphate; other steps and process parameters are the same as those in Example 1, and the meta-aramid porous film was obtained.

[0195] Example 20

[0196] A meta-aramid porous membrane and its preparation method, the difference from Example 1 is only that the second solvent is sulfolane, and the components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, sulfolane 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0197] Example 21

[0198] A meta-aramid porous membrane and its preparation method, the difference from Example 1 is only that the second solvent is 1,2-propanediol, and the components of the casting solution by mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, 1,2-propanediol 15%; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0199] The SEM image of the upper surface of the meta-aramid porous membrane provided in this example is as Figure 6 shown. Compared with the SEM image of Figure 1 , the surface porosity of Figure 6 is significantly reduced. Because the boiling point of 1,2-propanediol is about 20°C different from that of N,N-dimethylacetamide, and the difference between the first solvent and the second solvent is less than 30°C, a large amount of the second solvent is also removed during the removal of the first solvent, and the content of the second solvent in the poor-phase small droplets during the phase separation of the casting solution is greatly reduced, and finally the porosity of the prepared aramid porous membrane is significantly reduced and the pore size becomes smaller.

[0200] Comparative Example 1

[0201] A meta-aramid porous membrane and its preparation method, the preparation method includes the following steps:

[0202] (1) Coat a meta-aramid solution (by mass percentage: meta-aramid 15%, N,N-dimethylacetamide 85%) on a glass membrane substrate, adjust the doctor blade so that the liquid film thickness is 150 μm, and move the doctor blade uniformly to obtain a liquid film with a thickness of 150 μm;

[0203] (2) Put the liquid film and the substrate obtained in step (1) into a forced-air drying oven and dry at 60°C for 1 h to remove N,N-dimethylacetamide, and thus obtain a meta-aramid porous membrane.

[0204] The cross-sectional SEM image of the meta-aramid porous membrane provided by Comparative Example 1 is as Figure 7 shown. Compared with the cross-sectional SEM image of the meta-aramid porous membrane in Example 1 shown in Figure 3 , the porosity of the meta-aramid porous membrane provided by Comparative Example 1 is significantly reduced.

[0205] Comparative Example 2

[0206] A meta-aramid porous membrane and a preparation method thereof. The preparation method includes the following steps:

[0207] (1) Preparation of the casting solution: Triethyl phosphate is added to the meta-aramid solution (with N,N-dimethylacetamide as the solvent), and stirred until homogeneous and transparent to obtain the casting solution. The components of the casting solution in terms of mass percentage are as follows: meta-aramid 15%, N,N-dimethylacetamide 70%, and triethyl phosphate 15%;

[0208] (2) Preparation of the liquid film: The casting solution obtained in step (1) is subjected to ultrasonic degassing. The completely degassed casting solution is poured onto a glass film substrate, and the blade is adjusted so that the thickness of the liquid film is 150 μm. The blade is moved uniformly to obtain a liquid film with a thickness of 150 μm;

[0209] (3) Synchronous removal of the first solvent and the second solvent: The liquid film and the substrate obtained in step (2) are placed in a blast drying oven and dried at 150 °C for 3 h to remove the first solvent N,N-dimethylacetamide and the second solvent triethyl phosphate in the film. After drying, the meta-aramid porous membrane is obtained.

[0210] The cross-sectional SEM image of the meta-aramid porous membrane of Comparative Example 2 is as Figure 8 shown. Two parts with different structures are formed in the cross-section: a dense part and a porous part. This is because at a relatively high temperature, the first solvent and the second solvent are simultaneously removed rapidly, and the solvent evaporates rapidly from the casting solution, driving the meta-aramid polymer to move upward simultaneously. With the aggregation of the meta-aramid polymer and the precipitation of the solvent, a dense layer is rapidly formed in the upper part of the meta-aramid, restricting the evaporation of the solvent, and finally forming a porous lower part.

[0211] The test results of the aromatic polyamide porous membranes provided in Examples 1-21 and Comparative Examples 1-2, and the performance test results of the lithium-ion batteries assembled with the aromatic polyamide porous membranes as separators are shown in Tables 1 and 2:

[0212] Table 1

[0213]

[0214]

[0215] Table 2

[0216]

[0217]

[0218] As can be seen from the data in Table 1 and Table 2, through the design of the first solvent and the second solvent and the combination of specific processes for separately removing the first solvent and the second solvent, the present invention can obtain an aromatic polyamide porous membrane with controllable microporous structure and number, and excellent electrochemical properties, mechanical properties, thermal stability and dimensional stability. Further, by setting the types and contents of the first solvent and the second solvent, regulating the liquid film thickness, and controlling the methods and process parameters for removing the first solvent and the second solvent, the structural characteristics and functional characteristics of the aromatic polyamide porous membrane can be refined and regulated.

[0219] Specifically, compared with Example 1, the thickness of the meta-aramid porous membrane in Example 8 increased significantly because the liquid film thickness in Example 8 was relatively large, and accordingly, the thickness of the obtained meta-aramid porous membrane was relatively large. The tensile strength of the 20-μm meta-aramid porous membrane in Example 1 was 56.84 MPa, and the tensile strength of the 29-μm separator in Example 8 increased to 69.92 MPa. This is because the increase in the film thickness increases the number of entanglement points of the meta-aramid polymer chains in the meta-aramid porous membrane, and the mechanical properties of the membrane are thus enhanced, which is beneficial to improving the safety of the lithium-ion battery during use. However, the increase in the thickness of the meta-aramid porous membrane will cause the path for lithium ions to pass through the membrane to become longer and more tortuous when it is used as a separator, so the ionic conductivity will decrease due to the increase in thickness, and the electrochemical properties of the thicker meta-aramid porous membrane are lower than those of the thinner meta-aramid porous membrane.

[0220] Compared with the meta-aramid porous membrane in Example 1, the micropores in the membrane in Example 7 are relatively large because the content of triethyl phosphate (the second solvent) in the casting solution in Example 7 is relatively large, resulting in a relatively large volume of the poor-phase small droplets formed during the phase separation of the casting solution. When triethyl phosphate is removed, relatively large pores appear in the separator. Similarly, when the content of triethyl phosphate in Example 19 is relatively small, the volume of the poor-phase small droplets formed during the phase separation of the casting solution is relatively small, and relatively large pores appear in the separator when triethyl phosphate is removed. The present invention can prepare aromatic polyamide porous membranes with different pore sizes by regulating the content of the second solvent to meet the needs of different environments.

[0221] Comparing the meta-aramid porous membranes of Comparative Examples 1, 11, and 12, it can be seen that as the meta-aramid content in the casting solution increases, the mechanical properties of the meta-aramid porous membrane also increase, but the electrochemical properties decrease. This is because the more meta-aramid polymer in the casting solution, the more viscous the casting solution, and the more difficult it is for the liquid-phase small droplets to aggregate during the phase separation process. However, after the second solvent is removed, the pore size and porosity of the porous membrane decrease. When it is used as a separator, the probability of lithium ions passing through the separator becomes smaller and the path becomes longer, resulting in a decrease in electrochemical properties. At the same time, an increase in the concentration of the polymer in the casting solution can promote closer entanglement between the macromolecular chains, thus enhancing the mechanical properties of the porous membrane.

[0222] In the meta-aramid porous membranes of Examples 1, 9, and 10, it can be seen that the drying temperature has a great influence on the structure of the porous membrane. When the drying temperature is relatively low, the evaporation rate of the first solvent is relatively slow, and coupled with the relatively low viscosity of the casting solution, it leads to a longer solid-liquid phase separation time in the casting solution. The polymer in the casting solution has time to form crystal nuclei, and the crystal nuclei grow to form spherulites. The spherulites continue to grow until they come into contact with other spherulites, and the second solvent is dispersed in the gaps between the spherulites. The separator is formed by the accumulation and bonding of spherulites, so the mechanical properties are poor. When the drying temperature is relatively high, the first solvent evaporates relatively fast, the thermodynamic state of the casting solution is unstable, and the viscosity of the casting solution is relatively low, resulting in rapid phase separation of the casting solution, forming a polymer-rich phase and a polymer-poor phase with less polymer. The temperature of the casting solution is high and the viscosity is low, and the liquid-phase small droplets quickly aggregate to form large droplets rich in the second solvent. Of course, due to the high temperature, part of the second solvent will also volatilize. The finally formed meta-aramid porous membrane has fewer micropores, a larger internal resistance, and a lower ionic conductivity. Therefore, as a preferred technical solution of the present invention, the drying temperature for removing the first solvent is 30-100 °C, more preferably 50-70 °C.

[0223] From the data in Table 1 and Table 2 and Figure 3 , Figure 4 it can be seen that compared with Example 1, the performance of the meta-aramid porous membrane of Example 17 has little difference, indicating that using the direct heating drying method or the solvent replacement method to remove the second solvent has no difference in the structure of the meta-aramid porous membrane, which provides more choices for the preparation of aromatic polyamide porous membranes.

[0224] From the data in Table 1 and Table 2 and Figure 3 , Figure 5It can be seen that compared with Example 1, the meta-aramid porous membrane of Example 18 has a higher porosity, higher ionic conductivity and battery capacity, but its mechanical properties are also poorer. Considering the safety of lithium-ion batteries, the mass percentage of the second solvent in the casting solution of the present invention is preferably 15-20%. The aromatic polyamide porous membrane prepared within this range not only has good mechanical properties but also excellent electrochemical properties.

[0225] From the data in Table 1 and Table 2 and Figure 1 、 Figure 6 it can be seen that in the casting solution of Example 21, 1,2-propanediol is used as the second solvent, and its boiling point is only about 20 °C higher than that of the first solvent (N,N-dimethylacetamide), resulting in a large amount of evaporation of the second solvent during the evaporation of the first solvent. The volume and number of poor-phase small droplets formed during the phase separation of the casting solution are greatly reduced, resulting in a smaller pore size, lower porosity and smaller surface pore opening of the prepared meta-aramid porous membrane. Therefore, in order to prepare a porous membrane with a high surface pore opening and high porosity, the boiling point of the second solvent should be at least 30 °C higher than that of the first solvent.

[0226] From the data in Table 1 and Table 2 and Figure 3 、 Figure 7 it can be seen that compared with Example 1, the porosity of the meta-aramid porous membrane of Comparative Example 1 is significantly smaller, and the other performance parameters also vary greatly. This is because at the same temperature, the first solvent is easier to evaporate than the second solvent, and the second solvent can always remain in the casting solution. After the first solvent evaporates for a period of time, the casting solution will undergo phase separation to form a polymer-rich phase and a polymer-poor phase. The second solvent will diffuse into the poor phase, resulting in an increase in the volume and number of the poor phase. When the second solvent is removed, pores are formed in the membrane. Therefore, the meta-aramid porous membranes of Examples 1-12 have a high porosity; when the casting solution contains only the first solvent, although the casting solution can still undergo phase separation, due to the lack of the second solvent, the poor-phase small droplets will be relatively small and the number will also be relatively small. Therefore, the porosity of the meta-aramid porous membrane of Comparative Example 1 is lower. Therefore, the present invention makes the prepared porous membrane have a higher porosity by adding a second solvent to the casting solution.

[0227] From the data in Table 1 and Table 2 and Figure 3 、 Figure 8It can be seen that the meta-aramid porous membrane prepared in Comparative Example 2 showed delamination, with a dense structure on the upper layer and a porous structure on the lower layer. This is because at a relatively high temperature, the precipitation of the meta-aramid polymer occurred on the upper surface due to the rapid evaporation of the first solvent, and the second solvent also evaporated rapidly due to the effect of high temperature. The first solvent and the second solvent could not achieve the effect of stepwise removal, and finally a dense part was formed rapidly. The formation of the dense part blocked the evaporation of the first solvent and the second solvent. Coupled with the high temperature promoting the movement of molecules and reducing the viscosity of the casting solution, the second solvent aggregated together and finally formed a porous structure. Due to the existence of the dense structure, the ionic conductivity of the meta-aramid porous membrane was lower than that of the meta-aramid porous membrane in Example 1. The interfacial impedance of the battery assembled with this aramid porous membrane was relatively large, and the battery capacity was relatively low. This shows that the meta-aramid porous membrane prepared by stepwise removing the first solvent and the second solvent separately is superior to the meta-aramid porous membrane prepared by the one-step solvent removal method.

[0228] In summary, in the preparation method of the present invention, the liquid film thickness, the content of the second solvent in the casting solution, the polymer concentration, and the temperature for evaporating the first solvent have a great influence on the membrane structure and performance. Considering factors such as cost and the difficulty of controlling process parameters, by controlling the differences in the above parameters, aromatic polyamide porous membranes applicable to different fields can be prepared. For example, for the separator of a mobile phone battery, a lower thickness and a higher ionic conductivity are required, and a thinner liquid film thickness, a higher content of the second solvent in the casting solution, etc. should be used to prepare the aromatic polyamide porous membrane; while for the battery separator of a new energy vehicle, a higher mechanical strength is required to improve safety, so it is required that the liquid film thickness during the preparation of the separator is relatively thick and the polymer concentration in the casting solution is relatively high.

[0229] The applicant declares that the present invention uses the above embodiments to illustrate the aromatic polyamide porous membrane and its preparation method and the secondary battery containing the same of the present invention, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing an aromatic polyamide porous membrane, characterized in that, the preparation method includes: mixing a first solvent, a second solvent and an aromatic polyamide to obtain a casting solution; the first solvent is a good solvent for the aromatic polyamide, the second solvent includes a poor solvent for the aromatic polyamide, and the boiling point of the first solvent < the boiling point of the second solvent; performing a film-forming treatment on the casting solution to obtain a liquid film; removing the first solvent in the liquid film to obtain a dry film; removing the second solvent in the dry film to obtain the aromatic polyamide porous membrane.

2. The preparation method according to claim 1, characterized in that, The boiling point of the first solvent is T 1 , and the boiling point of the second solvent is T 2 , T 2 - T 1 ≥ 30 °C.

3. The preparation method according to claim 1, characterized in that, The first solvent includes N,N - dimethylformamide, N,N - dimethylacetamide, dimethyl sulfoxide, N - N - methylpyrrolidone, or any combination of at least two of them.

4. The preparation method according to claim 1, characterized in that, The first solvent includes N,N -dimethylformamide and / or N,N -dimethylacetamide.

5. The preparation method according to claim 1, characterized in that, The second solvent includes any one or a combination of at least two of triethyl phosphate, sulfolane, formamide, dibasic acid ester, diethylene glycol, glycerol, trimethyl phosphate, N -methylpyrrolidone.

6. The preparation method according to claim 1, characterized in that, The second solvent includes a combination of triethyl phosphate, dicarboxylic acid ester, glycerol and N -methylpyrrolidone, a combination of formamide and N -methylpyrrolidone, a combination of diethylene glycol and N -methylpyrrolidone.

7. The preparation method according to claim 1, characterized in that, the aromatic polyamide includes any one or a combination of at least two of poly(p-phenyleneterephthalamide), poly(m-phenylenediamine isophthalamide), poly(p-benzamide), and poly(p-phenylene sulfone terephthalamide).

8. The preparation method according to claim 1, characterized in that, the aromatic polyamide includes poly(m-phenylenediamine isophthalamide).

9. The preparation method according to claim 1, characterized in that, the mass percentage content of the aromatic polyamide in the casting solution is 10% - 20%.

10. The preparation method according to claim 1, characterized in that, the mass percentage content of the first solvent in the casting solution is 50% - 80%.

11. The preparation method according to claim 1, characterized in that, the mass percentage content of the second solvent in the casting solution is 10% - 30%.

12. The preparation method according to claim 1, characterized in that, the preparation method of the casting solution includes: providing an aromatic polyamide solution, the aromatic polyamide solution includes a combination of an aromatic polyamide and a first solvent; mixing the aromatic polyamide solution and the second solvent evenly to obtain the casting solution.

13. The preparation method according to claim 1, characterized in that, the method of the film-forming treatment includes a doctor blade method, a roll coating method, an impregnation method, a suction filtration method or a casting method.

14. The preparation method according to claim 1, characterized in that, the thickness of the liquid film is 80 - 350 μm.

15. The preparation method according to claim 1, characterized in that, the method for removing the first solvent in the liquid film includes drying.

16. The preparation method according to claim 15, characterized in that, the temperature of the drying is 30 - 120 °C.

17. The preparation method according to claim 16, characterized in that, the temperature of the drying is 45 - 90 °C.

18. The preparation method according to claim 17, characterized in that, the temperature of the drying is 55 - 80 °C.

19. The preparation method according to claim 15, characterized in that, the time of the drying is 0.2 - 2 h.

20. The preparation method according to claim 1, wherein, the method for removing the second solvent from the dry film includes drying by heating or solvent replacement.

21. The preparation method according to claim 20, wherein, the temperature for drying by heating is 120 - 170 °C.

22. The preparation method according to claim 20, wherein, the time for drying by heating is 1 - 2 h.

23. The preparation method according to claim 20, wherein, the solvent used for solvent replacement includes any one or a combination of at least two of water, ethanol, tert-butanol, and isopropanol.

24. The preparation method according to claim 20, wherein, the time for solvent replacement is 0.2 - 2 h.

25. The preparation method according to claim 20, wherein, after solvent replacement, a drying step is further included.

26. The preparation method according to claim 25, wherein, the temperature for drying after solvent replacement is 40 - 60 °C.

27. The preparation method according to claim 25, wherein, the time for drying after solvent replacement is 0.1 - 1 h.

28. The preparation method according to claim 1, wherein, the preparation method includes the following steps: (1) Mix a first solvent, a second solvent, and an aromatic polyamide to obtain a casting solution; the mass percentage of the aromatic polyamide in the casting solution is 10% - 20%, the mass percentage of the first solvent is 50% - 80%, and the mass percentage of the second solvent is 10% - 30%; The first solvent is a good solvent for aromatic polyamide and includes N,N -dimethylformamide, N,N -dimethylacetamide, dimethyl sulfoxide, N -N-methylpyrrolidone, any one or a combination of at least two of them, and its boiling point is T 1 ; The second solvent includes a poor solvent for aromatic polyamide, including any one or a combination of at least two of triethyl phosphate, sulfolane, formamide, dibasic acid ester, diethylene glycol, glycerol, trimethyl phosphate, N N-methylpyrrolidone, and its boiling point is T 2 ; T 2 -T 1 ≥30 °C; (2) Perform film-forming treatment on the casting solution by a doctor blade method, a roll coating method, an impregnation method, a suction filtration method, or a casting method to obtain a liquid film with a thickness of 80 - 350 μm; (3) Dry the liquid film at 45 - 90 °C for 0.2 - 2 h to remove the first solvent and obtain a dry film; (4) Remove the second solvent from the dry film by Method A or Method B to obtain the aromatic polyamide porous membrane; Method A includes: drying the dry film at 120 - 170 °C for 1 - 2 h to obtain the aromatic polyamide porous membrane; Method B includes: after solvent replacement of the dry film for 0.2 - 2 h, drying at 40 - 60 °C for 0.1 - 1 h to obtain the aromatic polyamide porous membrane; the solvent used for solvent replacement includes any one or a combination of at least two of water, ethanol, tert-butanol, and isopropanol.

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