Aromatic polyamide porous membrane as well as preparation method and application thereof

Abstract

The invention provides an aromatic polyamide porous membrane as well as a preparation method and application thereof, and the preparation method comprises the following steps: mixing aromatic polyamide and a solvent to obtain a membrane casting solution; performing membrane forming treatment on the membrane casting solution to obtain a liquid membrane; the surface of the liquid film is placed in a water mist environment to be treated, and a wet film is obtained; and carrying out solvent replacement and drying on the wet membrane to obtain the aromatic polyamide porous membrane. Through the design of water mist induced phase separation and the combination and mutual cooperation of the water mist induced phase separation and specific process steps, the aromatic polyamide porous membrane with excellent performance can be simply, efficiently and quickly obtained, and the aromatic polyamide porous membrane has the advantages of large porosity and high ionic conductivity, is good in electrolyte wettability, has excellent mechanical properties and thermal stability, and is suitable for industrial production. And the performance requirements of the secondary battery diaphragm can be fully met. The preparation method is simple and environment-friendly in process, the membrane preparation process is easy to adjust, the membrane structure is easy to control, and the preparation method is suitable for large-scale industrial production.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer membrane materials, and in particular relates to an aromatic polyamide porous membrane and a preparation method and application thereof. Background Art

[0002] Energy storage systems that convert chemical energy into electrical energy have become one of the important directions for the development of green new energy due to their advantages such as high energy density and conversion efficiency. Lithium-ion batteries are simple, efficient, and high-energy storage and supply devices. They have no memory effect and can be charged and discharged at any time. They can be charged and discharged at high rates. The power density of the ion battery developed by Saft in France can reach 4000W / kg. They have long cycle life: ICR 18650 ion batteries can cycle 1000 times, and the capacity retention rate is >85%. They have high energy conversion rates: the conversion rate can reach 96%; high energy density: the specific energy can reach 180W·h / kg. Therefore, lithium-ion batteries play an important role in the development and popularization of new energy.

[0003] Lithium-ion batteries are composed of positive and negative electrodes, electrolytes, and a diaphragm between the positive and negative electrodes that is soaked in electrolytes. Among them, the diaphragm is also called the "third electrode" of lithium-ion batteries and is an important component of lithium-ion batteries. The lithium-ion battery diaphragm is a porous film with uniformly distributed micropores. It is located between the lithium positive electrode material and the negative electrode material, and provides a channel for the transmission of lithium ions while avoiding direct contact between the positive and negative electrodes. Microporous polyolefin diaphragms mainly composed of polyethylene (PE) and polypropylene (PP) and composite films composed of these two materials are currently the main commercial lithium-ion battery diaphragms. Due to the poor heat resistance of polyolefin (PP, PE) diaphragms, at high temperatures, the diaphragms shrink severely, not only failing to play an insulating role, but may even become the culprit of short circuits, making lithium-ion batteries have great safety hazards. In addition, since there are no activated groups on polyolefins, it is difficult to be soaked by electrolytes, resulting in low ionic conductivity of the diaphragm, poor cycle performance, and easy generation of lithium dendrites that pierce the diaphragm and cause short circuits. Safety accidents caused by lithium-ion batteries have seriously restricted the development of the lithium-ion battery industry. The preparation of battery separators with both heat resistance and liquid affinity is an effective way to solve the safety problems of lithium-ion batteries.

[0004] The type of membrane material and the preparation method of the membrane in the new diaphragm have a great influence on the performance of the battery diaphragm and the battery. Aromatic polyamide materials are ideal materials for preparing lithium-ion battery diaphragms due to their excellent mechanical properties and high temperature resistance, good chemical stability, dimensional stability, and thermal stability. At present, many researchers have prepared aromatic polyamides for lithium-ion battery diaphragms. For example, CN115207559A uses a wet-laid method to prepare an aramid porous membrane, using differentiated aramid short fibers and differentiated aramid precipitated fibers as raw materials, mechanically pre-treating them to obtain an aramid fiber mixed dispersion, and then wet-papering the dispersion through an inclined mesh former to obtain an aramid porous membrane substrate, and finally using a hot pressing method to treat the aramid porous membrane substrate to obtain an aramid porous membrane; although the aramid porous membrane has excellent mechanical properties and high temperature resistance, the aramid porous membrane has a large pore size and is prone to self-discharge, and because the aramid short fiber dispersion is difficult to disperse evenly, the uniformity of the aramid porous membrane is relatively difficult to control. CN111370625A discloses an aramid coated lithium ion battery diaphragm and a preparation method thereof, the preparation method comprising: dissolving aramid, a cosolvent, an oily auxiliary agent and a pore-forming agent in a first solvent to obtain a uniformly mixed coating liquid, then applying the coating liquid to the diaphragm substrate, immersing the membrane in a coagulation bath by phase inversion, and obtaining a lithium ion battery diaphragm; although the diaphragm has good thermal stability, the bonding force and firmness between the coating layer and the membrane substrate are poor, and the coating layer affects the overall pores of the membrane, and the performance of the membrane is mainly affected by the performance of the membrane substrate, and there are still problems of large thermal shrinkage deformation and low ionic conductivity. CN113381122A uses a non-solvent induced phase separation method to prepare a meta-aramid porous membrane, and controls the membrane structure by controlling the ratio of the solvent in the coagulation bath. However, this method will produce a large amount of organic solvents during the production process, which brings great environmental problems and waste liquid treatment pressure. In addition, aromatic polyamide porous membranes have been prepared by electrospinning. However, the membranes produced by electrospinning have large pore sizes, poor mechanical strength, low production efficiency, and high costs, and are not suitable for large-scale applications.

[0005] In view of the problems of current commercial separators, such as poor thermal stability, poor wettability, complex preparation process, unsuitable pore structure, and low ionic conductivity, 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, large ionic conductivity, good wettability, good thermal stability and good mechanical properties, especially to make it meet the performance requirements of battery separators. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide an aromatic polyamide porous membrane and a preparation method and application thereof. Through the design of the preparation process, the obtained aromatic polyamide porous membrane has the characteristics of high porosity, good wettability, large ionic conductivity, good thermal stability and mechanical properties. The preparation method is simple and environmentally friendly, the membrane making process is easy to adjust, and the membrane structure is easy to control, which is suitable for large-scale industrial production.

[0007] In order to achieve the purpose of the 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 the following steps:

[0009] Mixing aromatic polyamide and solvent to obtain a casting solution;

[0010] The casting liquid is subjected to a film-forming treatment to obtain a liquid film;

[0011] placing the surface of the liquid film in a water mist environment for treatment to obtain a wet film;

[0012] The wet film is subjected to solvent replacement and drying to obtain the aromatic polyamide porous film.

[0013] The present invention adopts aromatic polyamide to prepare the membrane. On the one hand, there are amide bonds in the aromatic polyamide molecules, and the amide bonds have strong hydrogen bonding ability. At the same time, there are benzene rings in the aromatic polyamide molecules, and the molecular chain segments are difficult to rotate internally, which makes the probability of molecular chain folding very small, which is conducive to the orderly arrangement of aromatic polyamide molecules, so that the aromatic polyamide porous membrane has high mechanical strength, which provides a guarantee for the safety of the battery; on the other hand, aromatic polyamide is a highly heat-resistant material, which can ensure that it is not easy to produce thermal deformation at high temperatures, providing a wider range of applications for batteries; in addition, aromatic polyamide has a high affinity for electrolytes, and the prepared aromatic polyamide porous membrane has good wetting and liquid absorption and retention capabilities as a battery diaphragm, 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 in being used as a diaphragm for secondary batteries.

[0014] In the preparation method provided by the present invention, the casting liquid is subjected to film-forming treatment to form a liquid film, water mist is used to exchange the solvent in the liquid film, and the obtained wet film is subjected to solvent replacement and drying to obtain an aromatic polyamide porous membrane. The technical concept is as follows:

[0015] The present invention has found that when the liquid film is directly soaked in water, the liquid film will rapidly undergo liquid-liquid phase separation, the phase separation time is short, the solidification rate is fast, and crystallization still occurs in the later stage of phase separation, and it is easy to form a structure in which a dense cortex and a finger-like macroporous sublayer are combined, and lithium ions cannot pass through the dense layer on the surface, and the presence of the elongated pore sublayer causes the mechanical properties of the film to be poor, and the porous film formed in this way is not suitable for application in lithium ion battery separators. When the liquid film is at a certain temperature for a long time to evaporate the solvent, the phase separation time is long, the solidification rate is slow, and solid-liquid phase separation occurs. The film formed mostly does not have a dense cortex, but is composed of connected spherulite particles, and holes are formed between the spherulites. The film porosity at this time is low, and the brittleness is large, and lithium ions are difficult to pass through. The film formed in this way cannot be used as a lithium ion battery separator as well.

[0016] In the present invention, the surface of the liquid film is placed in a water mist environment for treatment, and the phase separation speed of the liquid film is regulated by water mist-induced phase separation. The phase separation speed is appropriate, so that the phase separation speed of the liquid film is between the above two film-forming methods. The film-forming path is that liquid-solid phase separation and liquid-liquid phase separation are carried out simultaneously, forming a porous film with multiple and uniform openings on the upper and lower surfaces, and the sublayer is a sponge-like structure that is interconnected. The interconnected porous film is conducive to the uniform passage of lithium ions, and the cross-linked sponge structure provides strong mechanical properties, which is conducive to inhibiting the growth of lithium dendrites and meets the requirements of being a lithium-ion battery separator.

[0017] Therefore, in the preparation method provided by the present invention, through the water mist induced phase separation design and its combination and mutual coordination with specific process steps, an aromatic polyamide porous membrane with excellent performance can be obtained simply, efficiently and quickly, and its upper and lower surfaces have a high degree of openness, and the interior is a three-dimensional network structure that is interconnected, with the advantages of large porosity and high ion conductivity, good electrolyte wettability, and excellent mechanical properties and thermal stability, which can fully meet the performance requirements of secondary battery separators. The preparation method is simple and environmentally friendly, the film-making process is easy to adjust, and the membrane structure is easy to control, which is suitable for large-scale industrial production.

[0018] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the objectives and beneficial effects of the present invention can be better achieved and realized.

[0019] Preferably, the aromatic polyamide includes any one of poly(p-phenylene terephthalamide) (para-aramid), poly(m-phenylene isophthalamide) (meta-aramid), poly(p-phenylene terephthalamide), and poly(phenylene sulfone terephthalamide), or a combination of at least two of them.

[0020] As a preferred technical solution of the present invention, the aromatic polyamide is polyisophthalamide (meta-aramid), in which adjacent amide bonds are more likely to form strong hydrogen bonds, the amide groups in the molecular chain are connected to the meta-phenylene groups, there is no conjugation effect, the internal rotation potential is low, the chain segment flexibility is relatively good, and the crystallinity is low. Therefore, whether in production or use, the meta-aramid porous membrane is more suitable for use in secondary battery (preferably lithium-ion battery) separators than other aromatic polyamide porous membranes.

[0021] Preferably, the mass percentage of aromatic polyamide in the casting solution is 5%-20%, for example, it can be 6%, 8%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18% or 19%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the said range.

[0022] As a preferred technical solution of the present invention, the content of aromatic polyamide in the casting solution has an important influence on the microporous structure of the aromatic polyamide porous membrane. The mass percentage of aromatic polyamide in the casting solution is 5%-20%. By changing the content of aromatic polyamide in the casting solution, the aromatic polyamide porous membrane can be regulated. When the content of aromatic polyamide in the casting solution is relatively low, the water mist needs to react with the solvent for a long time before phase separation occurs. The phase separation speed is slow, and the water mist can accumulate in large quantities in the liquid film for a long time. After the accumulated water mist is removed, the pores of the aromatic polyamide porous membrane are formed, resulting in the aromatic polyamide porous membrane having a higher surface opening and porosity. When the content of aromatic polyamide in the casting liquid is relatively high, the solvent on the upper surface of the liquid film in contact with the water mist is replaced by the water mist, causing the aromatic polyamide to phase separate faster, resulting in an increase in the polymer content and a lower final porosity; at the same time, its viscosity is greatly increased, preventing further water mist from entering the sublayer of the membrane, so that the sublayer will not phase separate quickly to form elongated finger-like holes, and at the same time reducing the accumulation of water mist in the liquid film, so the final aromatic polyamide porous membrane has a relatively small porosity and porosity, but at the same time, the surface with smaller openings and the sublayer with more polymer cross-linking greatly improve the mechanical strength of the polyamide porous membrane. By setting the content of aromatic polyamide in the casting liquid, aromatic polyamide porous membranes with different structures can be designed to meet the different needs of different secondary battery separators.

[0023] In the present invention, the solvent in the casting solution has good solubility for aromatic polyamide, can prevent the presence of undissolved aromatic polyamide clusters in the casting solution, and can dissolve a sufficient amount of aromatic polyamide to achieve a relatively large control of the structure of the aromatic polyamide porous membrane; at the same time, the solvent also plays a role in adjusting the viscosity of the casting solution. In addition, the solvent must have good compatibility with the thickener (if any) to ensure that the thickener (if any) will not precipitate in the casting solution.

[0024] Preferably, the solvent includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone, or a combination of at least two thereof.

[0025] Furthermore, in order to make the solid content range wider and reduce the film making time, the solvent has very good solubility for aromatic polyamide. Based on this, the solvent preferably includes N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0026] Preferably, the mass percentage of the solvent in the casting solution is 65%-90%, for example, it can be 66%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 84%, 85% or 88%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0027] Preferably, the casting solution also includes a thickener.

[0028] As a preferred technical solution of the present invention, a thickener is added to the casting liquid to increase the viscosity of the casting liquid. When there is less aromatic polyamide in the casting liquid, the viscosity of the casting liquid is low, and it is easy to cause displacement on the film substrate, resulting in uneven liquid film, and some defects may occur. The final aromatic polyamide porous membrane will be uneven and have defects; the introduction of the thickener can avoid the above defects. The thickener needs to have good solubility with the solvent to ensure that it will not precipitate in the casting liquid, and it must also ensure that it can be removed during the solvent replacement process.

[0029] Preferably, the thickener includes any one of glycerol, polyvinyl pyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, glycerol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide, or a combination of at least two of them, and more preferably any one of calcium chloride, glycerol, polyvinyl pyrrolidone, polyethylene glycol, and polyethylene oxide, or a combination of at least two of them.

[0030] Preferably, the polyethylene glycol is a high molecular weight polyethylene glycol.

[0031] Preferably, the mass percentage of the thickener in the casting solution is ≤15%, for example, it can be 0, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13% or 14%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0032] Preferably, the method for preparing the casting solution comprises: providing an aromatic polyamide solution, wherein the aromatic polyamide solution comprises a combination of aromatic polyamide and a solvent; and uniformly mixing the aromatic polyamide solution and an optional thickener to obtain the casting solution.

[0033] Preferably, the film-forming treatment method includes blade coating, roller coating, dipping, suction filtration or casting.

[0034] Preferably, the thickness of the liquid film is 50-300 μm, for example, it may be 60 μm, 80 μm, 100 μm, 120 μm, 150 μm, 180 μm, 200 μm, 220 μm, 250 μm or 280 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0035] 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 thin, the internal resistance of the aromatic polyamide porous membrane is relatively small, so that the ion conduction speed becomes larger, but at the same time, the mechanical strength of the thinner aromatic polyamide porous membrane will decrease, resulting in certain safety hazards in the secondary battery. Therefore, in order to make the aromatic polyamide porous membrane have a suitable 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 50-300μm, and the prepared aromatic polyamide porous membrane is thin and has excellent mechanical properties.

[0036] The present invention places the surface of the liquid film in a water mist environment for treatment, and the water mist induces phase separation of the liquid film. Specifically, in a high water mist environment, the solvent is replaced from the liquid film by water mist until the liquid film changes from transparent to a white opaque wet film. The treatment in the water mist environment (high water mist environment) includes two methods: a closed water mist environment (a certain space is filled with uniform water mist) or direct water mist injection (spraying water mist directly from the mist outlet onto the liquid film in an open space), and the method of direct water mist injection is preferred.

[0037] Preferably, the droplet particle size of the water mist in the water mist environment is 1-30 μm, for example, it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 15 μm, 16 μm, 18 μm, 20 μm, 22 μm, 25 μm or 28 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0038] Preferably, the method for treating in a water mist environment comprises: using a water mist generating device to spray water mist onto the surface of the liquid film so that the surface of the liquid film is placed in a water mist environment;

[0039] Alternatively, the surface of the liquid film is placed in a closed water mist environment.

[0040] Preferably, the mist output of the water mist generating device is 100-10000 mL / h, for example, it can be 157 mL / h, 200 mL / h, 222 mL / h, 343 mL / h, 500 mL / h, 800 mL / h, 1000 mL / h, 1345 mL / h, 1500 mL / h, 2000 mL / h, 2500 mL / h, 3000 mL / h, 3500 mL / h, 4000 mL / h, 4500 mL / h. mL / h, 4564mL / h, 5000mL / h, 5500mL / h, 6000mL / h, 6500mL / h, 7000mL / h, 7500mL / h, 7567mL / h, 8000mL / h, 8500mL / h, 9000mL / h or 9500mL / h, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the said range.

[0041] Preferably, the distance between the mist outlet of the water mist generating device and the surface of the liquid film is 1-50 cm, for example, it can be 5 cm, 8 cm, 10 cm, 14 cm, 15 cm, 20 cm, 21 cm, 22 cm, 25 cm, 30 cm, 35 cm, 40 cm, 44 cm or 45 cm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0042] Preferably, the diameter of the mist outlet of the water mist generating device is 0.1-20 cm, for example, it can be 0.5 cm, 1 cm, 2 cm, 4 cm, 5 cm, 6 cm, 8 cm, 9 cm, 10 cm, 12 cm, 13 cm, 14 cm, 15 cm, 16 cm, 18 cm or 19 cm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0043] Preferably, the water mist generating device comprises an ultrasonic water mist generating device.

[0044] The enclosed water mist environment is a space filled with uniform water mist, and the humidity is preferably 60-100%, for example, it can be 65%, 70%, 75%, 80%, 85%, 90%, 95% or 98%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0045] Preferably, the treatment time in the water mist environment is 10-1000s, for example, it can be 13s, 20s, 29s, 50s, 80s, 100s, 150s, 200s, 250s, 300s, 350s, 400s, 450s, 500s, 550s, 600s, 650s, 700s, 750s, 800s, 850s, 900s or 950s, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0046] In the present invention, the solvent replacement can remove water, solvent, thickener (if any) in the wet film; the replacement solvent used in the solvent replacement must not dissolve or affect the performance of the aromatic polyamide porous membrane, and must be miscible with the solvent and thickener in the casting solution to ensure complete removal of the solvent and thickener from the liquid film, while the replaced solvent itself is easy to remove. When the solvent replacement method is used to remove the solvent in the wet film, the solvent-rich wet film is placed in the replacement solvent. Since the solvent and the replacement solvent are miscible, diffusion occurs, and the solvent in the wet film diffuses into the replacement solvent, and the replacement solvent enters the film. Since the replacement solvent is easier to remove than the solvent (the solvent from the casting solution), it is more conducive to film making.

[0047] Preferably, the solvent used for the solvent replacement (replacement solvent) includes any one of water, ethanol, tert-butyl alcohol, isopropanol, or a combination of at least two thereof.

[0048] Preferably, the solvent replacement time is 1-60 min, for example, it can be 3 min, 5 min, 8 min, 10 min, 12 min, 15 min, 18 min, 20 min, 22 min, 25 min, 28 min, 30 min, 32 min, 35 min, 38 min, 40 min, 42 min, 45 min, 48 min, 50 min, 52 min, 55 min or 58 min, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0049] Preferably, the drying temperature is 30-60°C, for example, it can be 32°C, 35°C, 38°C, 40°C, 42°C, 45°C, 48°C, 50°C, 52°C, 55°C or 58°C, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0050] In the present invention, drying after solvent replacement is to remove the replacement solvent. The drying temperature cannot be too high, otherwise it will cause deformation of the aromatic porous membrane. The drying temperature is set to 30-60°C, which can ensure the rapid removal of the replacement solvent without deformation.

[0051] Preferably, the drying time is 1-60 min, for example, it can be 3 min, 5 min, 8 min, 10 min, 12 min, 15 min, 18 min, 20 min, 22 min, 25 min, 28 min, 30 min, 32 min, 35 min, 38 min, 40 min, 42 min, 45 min, 48 min, 50 min, 52 min, 55 min or 58 min, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0052] As a preferred technical solution of the present invention, the method for preparing the aromatic polyamide porous membrane comprises the following steps:

[0053] (1) mixing aromatic polyamide, a solvent and optionally a thickener to obtain a casting solution;

[0054] The solvent in the casting solution includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone, or a combination of at least two thereof;

[0055] The thickener includes any one of glycerol, polyvinyl pyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, glycerol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide, or a combination of at least two thereof;

[0056] The mass percentage of aromatic polyamide in the casting solution is 5%-20%, the mass percentage of solvent is 65%-90%, and the mass percentage of thickener is ≤15%;

[0057] (2) subjecting the casting liquid to a film-forming treatment to obtain a liquid film with a thickness of 50-300 μm;

[0058] (3) using a water mist generating device to spray water mist onto the surface of the liquid film, so that the surface of the liquid film is placed in a water mist environment and treated until the film becomes white and opaque, thereby obtaining a wet film;

[0059] The mist output of the water mist generating device is 100-10000 mL / h, the diameter of the mist outlet of the water mist generating device is 0.1-20 cm, and the distance between the mist outlet and the liquid film surface is 1-50 cm;

[0060] (4) subjecting the wet film to solvent replacement for 1-60 min, and then drying at 30-60° C. for 1-60 min to obtain the aromatic polyamide porous film;

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

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

[0063] Preferably, the pore size of the aromatic polyamide porous membrane is 0.1-0.5 μm, for example, it can be 0.15 μm, 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm, 0.4 μm or 0.45 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0064] Preferably, the porosity of the aromatic polyamide porous membrane is ≥50%, for example, it can be 52%, 55%, 60%, 65%, 70%, 75%, 80%, 85% or 88%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and 55-86% is further preferred.

[0065] Preferably, the thickness of the aromatic polyamide porous membrane is ≤35 μm, for example, it can be 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm, 30 μm, 32 μm or 34 μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and 9-32 μm is further preferred.

[0066] Preferably, the air permeability of the aromatic polyamide porous membrane is ≤200s / 100cc, for example, it may be 30s / 100cc, 40s / 100cc, 50s / 100cc, 60s / 100cc, 80s / 100cc, 100s / 100cc, 120s / 100cc, 140s / 100cc, 150s / 100cc, 160s / 100cc or 180s / 100cc, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and 38-195s / 100cc is further preferred.

[0067] Preferably, the surface porosity of the aromatic polyamide porous membrane is ≥30%, for example, it can be 32%, 35%, 38%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and it is further preferably ≥40%, and more preferably 40-80%.

[0068] Preferably, the electrolyte contact angle of the aromatic polyamide porous membrane is ≤28°, for example, it can be 5°, 6°, 8°, 10°, 12°, 15°, 18°, 20°, 22°, 25° or 27°, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and it is further preferably ≤20°.

[0069] Preferably, the liquid absorption rate of the aromatic polyamide porous membrane is ≥180%, for example, it can be 190%, 200%, 220%, 240%, 250%, 260%, 280%, 300%, 320%, 340%, 350% or 360%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and it is further preferably ≥200%.

[0070] Preferably, the thermal shrinkage rate of the aromatic polyamide porous membrane treated at 150°C for 1 hour is ≤0.1%, for example, it can be 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or 0.09%, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the said range.

[0071] Preferably, the tensile strength of the aromatic polyamide porous membrane is ≥7 MPa, for example, it may be 8 MPa, 9 MPa, 10 MPa, 12 MPa, 14 MPa, 15 MPa, 16 MPa, 18 MPa, 20 MPa, 22 MPa, 25 MPa, 28 MPa, 30 MPa or 32 MPa, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and it is further preferably ≥15 MPa.

[0072] Preferably, the puncture resistance of the aromatic polyamide porous membrane is ≥0.12N / μm, for example, it can be 0.13N / μm, 0.14N / μm, 0.15N / μm, 0.2N / μm, 0.25N / μm, 0.3N / μm, 0.35N / μm, 0.4N / μm, 0.45N / μm, 0.5N / μm, 0.55N / μm, 0.6N / μm, 0.65N / μm or 0.7N / μm, as well as specific point values ​​between the above point values. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range, and it is further preferred that it is ≥0.15N / μm.

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

[0074] Preferably, the aromatic polyamide porous membrane is used as a separator material for secondary batteries.

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

[0076] Exemplarily, the secondary battery includes a lithium ion battery, a sodium ion battery, a lithium sulfur battery, and the like.

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

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

[0079] Preferably, the electrolyte is a liquid electrolyte (electrolyte) or a solid electrolyte.

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

[0081] (1) In the preparation method provided by the present invention, water mist-induced phase separation is used to prepare an aromatic polyamide porous membrane. During the process of water mist-induced phase separation, the liquid membrane system is in a thermodynamically unstable state and phase separation occurs, forming a rich phase with a higher polymer content and a lean phase with a lower polymer content. The rich phase eventually forms the framework of the aromatic polyamide porous membrane, and the lean phase eventually forms the pore structure of the aromatic polyamide porous membrane. Different from the traditional non-solvent-induced phase separation and solvent evaporation-induced phase separation, the speed of the water mist-induced phase separation of the present invention is at an appropriate size, and a porous membrane with a large surface opening and a high porosity can be prepared. By simply controlling the content of aromatic polyamide in the casting liquid, the structure of the final aromatic polyamide porous membrane can be regulated to meet different application requirements.

[0082] (2) The preparation method provided by the present invention has simple process steps, does not require the use of a large amount of organic solvents, is environmentally friendly, highly efficient and controllable, has low cost, and is suitable for large-scale production.

[0083] (3) The aromatic polyamide porous membrane provided by the present invention has a pore size of 0.1-0.5 μm, a porosity of ≥50%, a thickness of ≤35 μm, an air permeability of ≤200s / 100cc, a surface opening rate of ≥30%, an electrolyte contact angle of ≤28°, a liquid absorption rate of ≥180%, a thermal shrinkage rate of 150°C / 1h of ≤0.1%, a tensile strength of ≥7MPa, and a puncture resistance of ≥0.12N / μm. It has the characteristics of large porosity, high ionic conductivity, 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 low interfacial impedance, and can effectively improve the cycle performance, rate performance and interface stability of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0088] Figure 4 This is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 2;

[0089] Figure 5 This is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 3;

[0090] Figure 6 This is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 4;

[0091] Figure 7 This is a cross-sectional SEM image of the meta-aramid porous membrane provided in Example 18;

[0092] Figure 8 This is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 21;

[0093] Fig. 9 A cross-sectional SEM image of the meta-aramid porous membrane provided in Comparative Example 1;

[0094] Fig.10 The upper surface SEM image of the meta-aramid porous membrane provided in Comparative Example 1;

[0095] Fig.11 A cross-sectional SEM image of the meta-aramid porous membrane provided in Comparative Example 2;

[0096] Fig.12 This is the surface SEM image of the meta-aramid porous membrane provided in Comparative Example 3. DETAILED DESCRIPTION

[0097] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0098] As used herein, the terms "comprises," "including," "having," "containing," or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, process, method, article, or apparatus that comprises the listed elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, process, method, article, or apparatus.

[0099] The aromatic polyamide porous membrane provided by the present invention is particularly suitable for use as a separator for secondary batteries, especially a separator for lithium-ion batteries. The following describes in detail the main properties and requirements of the aromatic polyamide porous membrane and separator, the reasons why the aromatic polyamide porous membrane meets the requirements of lithium-ion battery separators, and the performance test method of the aromatic polyamide porous membrane:

[0100] 1. Aperture

[0101] Regarding the aromatic polyamide porous membrane in the embodiment of the present invention, in order to obtain a diaphragm with low internal resistance and high safety, the pore size of the aromatic polyamide porous membrane must be controlled. The pore size of the lithium-ion battery diaphragm must be submicron level. When the pore size of the lithium-ion battery diaphragm is small, its internal resistance will be larger, the contact area with the electrolyte will be smaller, and the permeability of lithium ions will decrease, resulting in a decrease in the overall performance of the battery; when the pore size of the lithium-ion battery diaphragm is large, although it can improve ion conductivity, it will also cause the lithium-ion battery to self-discharge and cause battery energy loss. Not only that, the larger pore size of the diaphragm will increase the risk of being punctured by lithium dendrites, thereby causing safety issues such as short circuits or even explosions. Therefore, it is crucial to control the pore size of the aromatic polyamide porous membrane within a suitable range for its performance.

[0102] The pore size of the aromatic polyamide porous membrane provided by the present invention is between 0.1-0.5 μm. Within the above pore size range, the aromatic polyamide porous membrane has high ionic conductivity and high safety, and basically does not have self-discharge. In the implementation of the present invention, the pore size of the aromatic polyamide porous membrane is mainly controlled by controlling the content of aromatic polyamide in the casting solution and the preparation process.

[0103] In the embodiment of the present invention, the pore size of the aromatic polyamide porous membrane is characterized by mercury intrusion (reference standard GB / T21650.1-2008). Mercury has no wetting effect on the aromatic polyamide porous membrane, so external pressure needs to be applied to make the mercury enter the pores of the diaphragm. The external pressure required for pores of different diameters is different. The smaller the diameter of the pore, the greater the external pressure required. Therefore, by measuring the amount of mercury entering the pores of the diaphragm under different external pressures, the pore volume of the corresponding pore size can be known.

[0104] The principle of mercury porosimetry can be expressed as the Washburn equation: Where r is the capillary pore diameter (m); σ is the mercury surface tension (mN / m); θ is the contact angle between mercury and the capillary surface; p is the external pressure (mN / m 2 ).

[0105] 2. Porosity

[0106] 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 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. Diaphragms with high porosity can provide more storage sites for electrolytes, and the path of lithium ions through the diaphragm becomes larger and wider, and the permeability and permeability of lithium ions will increase, thereby improving the performance of lithium-ion batteries. In theory, the air permeability, liquid absorption, and internal resistance of the diaphragm are all affected by it. The porosity of the aromatic polyamide porous membrane prepared by the present invention is ≥50%, which is better than most current commercial diaphragms. The present invention prepares aromatic polyamide porous membranes of different porosities by regulating the content of aromatic polyamide in the casting solution and the process.

[0107] In the present invention, the porosity of the aromatic polyamide porous membrane is measured by the hexadecane absorption method (reference standard GB / T 33052-2016). First, the aromatic polyamide porous membrane to be measured is cut into three pieces of 10 cm × 5 cm in size, and then the thickness and weight of the sample are measured by a thickness gauge and an electronic balance, respectively. Then, the sample is immersed in hexadecane for 1 hour, and the hexadecane on the surface is wiped with oil-absorbing paper. Finally, the porosity of the aromatic polyamide porous membrane can be obtained by weighing and calculating.

[0108] 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), w1 is the mass of the sample before absorbing hexadecane (g), w2 is the mass of the sample after fully absorbing hexadecane (g), and ρ is the density of hexadecane (g / cm 3 ).

[0109] 3. Thickness

[0110] 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 separator. Generally speaking, the thicker the separator, the better the mechanical properties, which can effectively reduce the risk of being punctured by lithium dendrites. However, the increase in thickness will cause the lithium ion permeation path in the separator to become more complex and tortuous, and the ion permeability and permeation speed will be significantly reduced; in addition, the increase in thickness will also occupy the space of the active substances of the lithium ion battery, causing the battery capacity to decrease. Therefore, controlling the separator within a reasonable range is crucial to the performance of the lithium ion battery. The aromatic polyamide porous membrane of the present invention relies on the excellent mechanical properties of aromatic polyamide, so that the thickness of the aromatic polyamide porous membrane can be very small (≤35μm, preferably ≤25μm) while meeting the mechanical strength. The present invention controls the thickness of the aromatic polyamide porous membrane by regulating the thickness of the liquid film, and the thickness test refers to the method in the standard GB / T 6672-2001.

[0111] 4. Breathability

[0112] Permeability refers to the time it takes for a gas to pass through a diaphragm under a certain pressure, which can indirectly reflect the permeability of lithium ions. The better the permeability of the diaphragm, the better its lithium ion permeability. The main factors affecting the permeability of the diaphragm are the porosity and pore connectivity of the diaphragm. The higher the porosity and the better the pore connectivity, the better the permeability of the diaphragm. The aromatic polyamide porous membrane prepared by the present invention has high porosity and good pore connectivity, so it has excellent permeability (≤200s / 100cc). The present invention mainly regulates the permeability of the aromatic polyamide porous membrane by regulating the thickness of the liquid membrane, the content of aromatic polyamide in the casting liquid and the preparation process. The test reference standard for permeability is the method in GB / T 1040.3-2006.

[0113] 5. Surface opening rate

[0114] The present invention defines the surface porosity as the ratio of the area of ​​the pores on the surface of the diaphragm to the total surface area of ​​the diaphragm. The surface porosity determines the entrance area for lithium ions to enter the diaphragm, and the size of the entrance area determines the speed at which lithium ions enter the diaphragm. When the surface porosity is relatively large, the larger the pore area on the surface of the diaphragm, the faster the lithium ions enter the interior of the diaphragm, and the corresponding ionic conductivity will be greater. The diaphragms prepared by the traditional non-solvent induced phase separation method usually have a denser surface and 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 is rapidly exchanged with the non-solvent in the coagulation bath, and the exchange rate is much greater than the speed at which the sub-layer solvent of the casting liquid is transferred to the surface, and the diaphragm surface is instantly phase separated to form a dense layer. The aromatic polyamide porous membrane prepared by the method of the present invention has a large surface porosity (≥30%, preferably ≥40%), mainly because: in the present invention, the liquid film cortex solvent molecules are exchanged with water mist, which promotes the liquid-liquid phase separation of the cortex, forming a rich phase with more polymer and a poor phase with less polymer. As the solvent is continuously exchanged, the rich phase is finally solidified and the poor phase forms a pore structure. The lower surface of the aromatic polyamide porous membrane can also have an excellent surface porosity because the liquid film cortex does not immediately generate a dense layer, so the lower surface solvent can also exchange with the water mist at a faster speed, promoting the speed of phase separation, and finally forming a lower surface with a larger surface porosity.

[0115] In the present invention, the following method is used to characterize the surface porosity. First, the aromatic polyamide porous membrane to be tested is placed in liquid nitrogen for quenching, and then the aromatic polyamide porous membrane is fixed on the sample stage with conductive glue, the sample is sprayed with gold, and the cross section of the sample is photographed and saved using a scanning electron microscope SEM. All the holes in the above SEM picture are selected using Image J software, and the hole area and total area in the SEM picture are calculated. The surface porosity of the aromatic polyamide porous membrane can be obtained by dividing the two.

[0116] 6. Wettability

[0117] Wettability is mainly used to evaluate the wetting effect of the diaphragm on the electrolyte and reveal the compatibility of the diaphragm and the electrolyte. Better wettability is conducive to the affinity between the diaphragm and the electrolyte. The larger the contact area between the diaphragm and the electrolyte, the higher the transfer rate of lithium ions, and the battery's charge and discharge efficiency and capacity are improved. The molecular chains of current PP and PE commercial diaphragms are all made of olefin polymerization, and there is a lack of polar groups on the molecular chains. Therefore, their wettability to the electrolyte is very poor, and it is necessary to increase their wettability by coating with lyophilic materials. Aromatic polyamide has more polar groups on its molecular chain, so the aromatic polyamide porous membrane has excellent wettability to the electrolyte (electrolyte contact angle ≤28°, preferably ≤20°). In the present invention, a contact angle tester is used to test the contact angle between the aromatic polyamide porous membrane and the electrolyte (1.0 mol / L LiPF6 electrolyte, the solvent is a mixed solvent of ethylene carbonate EC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC in a mass ratio of 1:1:1), and the size of the contact angle is used to measure the wettability of the diaphragm to the electrolyte.

[0118] 7. Liquid absorption rate

[0119] The liquid absorption rate of lithium-ion battery diaphragm refers to the amount of electrolyte stored in the diaphragm, which controls the internal resistance of the diaphragm to a certain extent. The greater the liquid absorption rate, the more electrolyte is stored, the lithium ions can have more and wider paths to pass through the diaphragm, and the greater the ion conductivity. The liquid absorption rate of lithium-ion battery diaphragm is mainly affected by the affinity of the membrane material to the electrolyte and the porosity of the diaphragm. The diaphragm with excellent wettability and high porosity has a larger liquid absorption rate. At present, there are no polar groups on the macromolecular chains of PP and PE commercial diaphragms, resulting in poor wetting properties and low liquid absorption rate. In the present invention, since the aromatic polyamide macromolecular chain has more polar groups, it has better wettability to the electrolyte, and the porosity of the aromatic polyamide porous membrane prepared by the present invention is larger, so the aromatic polyamide porous membrane has a larger liquid absorption rate (≥180%, preferably ≥200%).

[0120] In the present invention, the weighing method is used to test the liquid absorption rate of the aromatic polyamide porous membrane. First, the weight of the aromatic polyamide porous membrane before being immersed in the electrolyte is weighed, and after it is immersed in the electrolyte for a period of time, the weight of the aromatic polyamide porous membrane after being immersed in the electrolyte is weighed, and the liquid absorption rate of the diaphragm is calculated by comparing the weight change of the aromatic polyamide porous membrane before and after immersion. The specific calculation formula is: Wherein, x represents the liquid absorption rate of the separator (%), m represents the weight of the membrane after being immersed in the electrolyte (g), and m0 represents the weight of the membrane without being immersed in the electrolyte (g).

[0121] 8. Thermal stability

[0122] The thermal stability of lithium-ion battery separators is an important indicator for evaluating the safety of lithium-ion batteries. Chemical reactions occur inside the battery during the charging and discharging process to generate heat, and heat sources outside the battery will also cause the temperature inside the battery to rise. Especially in extreme environments or when the battery is used incorrectly, the temperature inside the battery will rise sharply. If the temperature inside the battery is too high, the separator will shrink and lose the function of isolating the positive and negative electrodes, causing the battery to short-circuit or even explode. Therefore, the separator material must have good thermal properties to ensure the dimensional stability of the battery within a wide temperature range, and can still isolate the positive and negative electrodes of the battery even under extreme temperature conditions. The high energy barrier of the benzene ring and CN bond rotation in aromatic polyamides prevents the aromatic polyamide molecular chain from becoming a fully straight chain conformation. In addition, there is a strong hydrogen bond interaction force between its molecular chains, which makes aromatic polyamide very heat-resistant, with a glass transition temperature of about 270°C. Even if it works continuously for 20,000 hours at 200°C, the strength can be maintained at 90% of the original. Owing to the excellent thermal properties of aromatic polyamide, the thermal shrinkage rate of the aromatic polyamide porous film at 150° C. / 1 h is ≤0.1%.

[0123] The present invention tests the thermal shrinkage rate of aromatic polyamide porous membrane as follows: take a 10cm×10cm aromatic polyamide porous membrane sample, place the membrane flat on one of the quantitative filter papers on the stainless steel plate in the middle of a blast thermostatic box, press it with another quantitative filter paper, close the thermostatic box door, start counting the time, keep it at a temperature of 150°C for 1h, take out the membrane after the heating is completed, and measure the longitudinal and transverse marking lengths after the membrane returns to room temperature.

[0124] The calculation formula of thermal shrinkage ΔS (%) is: Where S0 is the area of ​​the film before heating (cm 2 ), S is the area of ​​the film after heating (cm 2 ).

[0125] 9. Tensile strength

[0126] Tensile strength is a parameter that reflects the dimensional stability of the diaphragm when it is subjected to external forces during use. Because lithium-ion batteries will form lithium dendrites during use, a high-strength diaphragm can effectively delay the penetration of lithium dendrites and ensure the safety performance of lithium-ion batteries. In addition, lithium-ion batteries may collide, squeeze, and hit during use, causing the battery to deform and exert a large force on the diaphragm. A high-strength diaphragm can resist such damage and prevent the direct contact between the positive and negative electrodes of the battery from causing safety accidents. 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 is in an extended state to form a rod-like structure. At the same time, the molecular chain of the linear structure makes the aromatic polyamide space utilization rate high, so the unit volume can accommodate more polymers and therefore the strength is higher. Therefore, the aromatic polyamide porous membrane of the present invention has a higher tensile strength (≥7MPa, preferably ≥15MPa). The test of tensile strength refers to the method in the standard GB / T 36363-2018.

[0127] 10. Puncture resistance

[0128] Puncture resistance refers to the mass applied to a given needle-shaped object to pierce a given diaphragm sample. It is used to characterize the ability of the diaphragm to resist puncture due to external pressure. Since the negative electrode of the lithium-ion battery is composed of carbon materials, adhesives, etc., even after rolling, the surface of the negative electrode is still rough due to the presence of granular materials. Therefore, during the battery assembly and shaping process, the diaphragm sandwiched by the positive and negative electrodes needs to withstand greater pressure to prevent the battery from short-circuiting. In addition, the battery will inevitably be squeezed, collided, impacted, etc. during use. At this time, the diaphragm also needs to have a certain puncture resistance to ensure the safety of the battery. Therefore, lithium-ion battery diaphragms must have a certain puncture resistance. Aromatic polyamide has a higher internal rotation potential energy due to the presence of benzene rings, so the molecular chain segments present a planar straight chain conformation, and the molecular chain contains more polar groups, so the gaps between aromatic polyamides are small and the interaction force is strong, resulting in the aromatic polyamide porous membrane having good puncture resistance (≥0.12N / μm, preferably ≥0.15N / μm). The test of puncture resistance refers to the method in the standard GB / T 36363-2018.

[0129] The following will take multiple embodiments as examples to describe in detail the aromatic polyamide porous membrane and the preparation method thereof of the present invention, but the aromatic polyamide porous membrane and the preparation method thereof of the present invention are not limited to these embodiments. In the following embodiments of the present invention, the materials for which the preparation method is not provided are all commercially available chemicals, and the aromatic polyamides used include: meta-aramid, purchased from Taihe New Materials Group Co., Ltd.; para-aramid, purchased from Taihe New Materials Group Co., Ltd.; polyethylene oxide and polyvinyl pyrrolidone are both purchased from Kaimart (Tianjin) Chemical Technology Co., Ltd. The water mist generating device used was purchased from Guangzhou Woguan Electric Co., Ltd.

[0130] Example 1

[0131] An aromatic polyamide porous membrane (meta-aramid porous membrane) and a preparation method thereof, the preparation method comprising the following steps:

[0132] (1) Preparation of casting solution: adding lithium chloride to a meta-aramid solution (solvent is N,N-dimethylacetamide), stirring until homogeneous and transparent, to obtain a casting solution; the contents of the components of the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% lithium chloride;

[0133] (2) Preparation of liquid film: ultrasonically degas the casting liquid obtained in step (1), pour the completely degassed casting liquid onto a glass film substrate, adjust the scraper so that the liquid film thickness is 100 μm, and move the scraper at a constant speed to obtain a liquid film with a thickness of 100 μm;

[0134] (3) Water mist-induced liquid film phase separation: a water mist generating device is used to spray water mist onto the surface of the liquid film obtained in step (2); the diameter of the mist outlet of the water mist generating device is 1 cm, the distance between the mist outlet and the liquid film surface is 5 cm, and the mist output is 350 mL / h. The surface of the liquid film is placed in a water mist environment until the film becomes white and opaque, thereby obtaining a wet film;

[0135] (4) Solvent replacement and drying: The wet membrane obtained in step (3) is placed in deionized water for 50 minutes to replace N,N-dimethylacetamide and lithium chloride in the wet membrane. Finally, the membrane is placed in a forced air drying oven at 50°C to remove excess deionized water in the membrane. After drying, a meta-aramid porous membrane is obtained.

[0136] Example 2

[0137] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 5% meta-aramid, 85% N,N-dimethylacetamide, and 10% lithium chloride; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0138] Example 3

[0139] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 15% meta-aramid and 85% N,N-dimethylacetamide; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0140] Example 4

[0141] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 20% meta-aramid and 80% N,N-dimethylacetamide; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0142] Example 5

[0143] An aromatic polyamide porous membrane (para-aramid porous membrane) and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% para-aramid, 85% N-methylpyrrolidone, and 5% lithium chloride; the other steps and process parameters are the same as those in Example 1, and a para-aramid porous membrane is obtained.

[0144] Example 6

[0145] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% formamide; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0146] Example 7

[0147] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% propylene glycol; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0148] Example 8

[0149] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% polyethylene oxide; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0150] Example 9

[0151] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% polyvinyl pyrrolidone; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0152] Example 10

[0153] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the scraper is adjusted in step (2) so that the thickness of the liquid film is 150 μm; other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0154] Embodiment 11

[0155] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that, in step (2), the scraper is adjusted so that the liquid film thickness is 200 μm; other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0156] Example 12

[0157] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylformamide, and 5% lithium chloride; other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0158] Example 13

[0159] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the components in the casting solution are as follows: 10% meta-aramid, 85% N-methylpyrrolidone, and 5% lithium chloride; the other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0160] Embodiment 14

[0161] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that in step (2), the casting liquid is poured into a coating tray, and the gap between the rollers is adjusted so that the liquid film thickness is 100 μm, thereby obtaining a liquid film with a thickness of 100 μm; other raw materials, steps and process parameters are the same as those in Example 1, thereby obtaining a meta-aramid porous membrane.

[0162] Embodiment 15

[0163] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that ethanol is used as a replacement solvent in step (4) to remove N,N-dimethylacetamide and lithium chloride in the wet membrane. Other raw materials, steps and process parameters are the same as those in Example 1 to obtain a meta-aramid porous membrane.

[0164] Example 16

[0165] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that, in step (3), the diameter of the mist outlet of the water mist generating device is 5 cm. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0166] Embodiment 17

[0167] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that, in step (3), the diameter of the mist outlet of the water mist generating device is 0.1 cm. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0168] Embodiment 18

[0169] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that, in step (3), the mist output of the water mist generating device is 1000 mL / h. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0170] Embodiment 19

[0171] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that, in step (3), the mist output of the water mist generating device is 100 mL / h. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0172] Embodiment 20

[0173] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the distance between the mist outlet of the water mist generating device and the surface of the liquid membrane in step (3) is 10 cm. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0174] Embodiment 21

[0175] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the distance between the mist outlet of the water mist generating device and the surface of the liquid membrane in step (3) is 2 cm. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0176] Embodiment 22

[0177] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the distance between the mist outlet of the water mist generating device and the liquid membrane surface in step (3) is 10 cm, and the mist output is 1000 mL / h. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0178] Embodiment 23

[0179] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that in step (3), the diameter of the mist outlet of the water mist generating device is 5 cm, and the mist output is 100 mL / h. Other raw materials, steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0180] Embodiment 24

[0181] A meta-aramid porous membrane and a preparation method thereof, which differs from Example 1 only in that the method of water mist-induced liquid membrane phase separation in step (3) is as follows: the liquid membrane is directly placed in a closed space filled with water mist, and the humidity is 80%. The other steps and process parameters are the same as those in Example 1, and a meta-aramid porous membrane is obtained.

[0182] Comparative Example 1

[0183] A meta-aramid porous membrane and a preparation method thereof, the preparation method comprising the following steps:

[0184] (1) and (2) are the same as in Example 1, and a liquid film with a thickness of 100 μm is obtained;

[0185] (3) Non-solvent induced liquid film phase separation: The liquid film obtained in step (2) and the substrate are immediately placed in deionized water and soaked for 50 minutes to obtain an opaque wet film. The wet film is placed in a forced air drying oven at 50°C to remove excess deionized water in the film. After drying, a meta-aramid porous membrane is obtained.

[0186] Comparative Example 2

[0187] A meta-aramid porous membrane and a preparation method thereof, the preparation method comprising the following steps:

[0188] (1) and (2) are the same as in Example 1, and a liquid film with a thickness of 100 μm is obtained;

[0189] (3) Evaporation-induced liquid film phase separation: The liquid film and substrate obtained in step (2) are placed in a forced air drying oven at 60° C. and dried for 1 h to remove N,N-dimethylacetamide. After drying, a meta-aramid porous membrane is obtained.

[0190] Comparative Example 3

[0191] A meta-aramid porous membrane and a preparation method thereof, the preparation method comprising the following steps:

[0192] (1) and (2) are the same as in Example 1, and a liquid film with a thickness of 100 μm is obtained;

[0193] (3) Steam-induced liquid film phase separation: placing the liquid film obtained in step (2) and the substrate in a 70% high humidity water vapor environment provided by a constant temperature and humidity chamber until the liquid film changes from a transparent to an opaque wet film;

[0194] (4) Solvent replacement and drying: The wet membrane obtained in step (3) is placed in deionized water for 50 minutes to replace N,N-dimethylacetamide and lithium chloride in the wet membrane. Finally, the membrane is placed in a forced air drying oven at 50°C to remove excess deionized water in the membrane. After drying, a meta-aramid porous membrane is obtained.

[0195] The test results of the aromatic polyamide porous membranes provided in Examples 1-24 and Comparative Examples 1-3 are shown in Table 1:

[0196] Table 1

[0197]

[0198]

[0199] The microscopic morphology of the aromatic polyamide porous membrane was tested using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Japan). The SEM image of the upper surface of the meta-aramid porous membrane provided in Example 1 is as follows: Figure 1 As shown, the SEM image of the lower surface is Figure 2 As shown, the cross-sectional SEM image is Figure 3 As shown; combined with 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 size is relatively uniform, the cross-section is a uniform sponge structure, and the structure of the surface and the interior interconnected can provide a transmission channel for the rapid migration of lithium ions. The interconnected tortuous pore structure can effectively prevent the growth of lithium dendrites and avoid short circuits. After testing, the porosity of the meta-aramid porous membrane of Example 1 is 66.3%. The high-porosity diaphragm can provide more storage sites for the electrolyte, the path of lithium ions through the diaphragm becomes larger and wider, and the permeability and permeability of lithium ions will increase, thereby improving the performance of lithium-ion batteries.

[0200] The SEM image of the upper surface of the meta-aramid porous membrane provided in Example 2 is as follows: Figure 4As shown, compared with Example 1, the upper surface of the porous membrane of Example 2 has a larger surface openness. This is because in the process of water mist-induced phase separation of the casting liquid, a rich phase mainly composed of meta-aramid polymer and a poor phase containing solvent, thickener, and deionized water are formed. The two are continuously cross-linked to form a wet film. When the solvent, deionized water, and thickener in the wet film are removed, the rich phase forms the skeleton structure of the meta-aramid porous membrane, and the poor phase with solvent, deionized water, and thickener becomes the pore structure of the meta-aramid porous membrane. Since the meta-aramid content in Example 2 is less than that in Example 1, the meta-aramid content in the casting liquid is reduced, resulting in a small proportion of the rich phase mainly composed of meta-aramid during the phase separation process. After the rich phase forms a meta-aramid framework structure, the porosity and surface openness are relatively high. For lithium-ion battery separators, the porosity and surface openness of the separator are important indicators for evaluating the performance of the separator, which directly affect the performance of the lithium-ion battery. The surface porosity determines the entrance area for lithium ions to enter the diaphragm, and the size of the entrance area determines the speed at which lithium ions enter the diaphragm. When the surface porosity is relatively large, the more entrances on the surface of the diaphragm that can enter the diaphragm, the faster the lithium ions enter the interior of the diaphragm, and the greater the corresponding ion conductivity. In Example 2, the surface porosity of the meta-aramid porous membrane reaches 73.5%. Therefore, the meta-aramid porous membrane prepared by using a casting solution with a low polymer content has a relatively rich pore structure.

[0201] The upper surface SEM image of the meta-aramid porous membrane provided in Example 3 is as follows: Figure 5 As shown, the upper surface SEM image of the meta-aramid porous membrane provided in Example 4 is as follows Figure 6As shown. In Examples 3 and 4, as the polymer content increases, the surface openness of the meta-aramid porous membrane also decreases. This is because the reduction in the solvent content in the casting solution leads to a reduction in the proportion of the lean phase formed during the phase separation process, and the pore structure of the finally formed meta-aramid porous membrane is not as rich as the porous membrane with a low meta-aramid content in Example 1. However, due to the increase in the content of meta-aramid, the mechanical properties of the finally formed meta-aramid porous membrane have been significantly improved. This is because the increase in the density of meta-aramid leads to an increase in the cross-linking area between polymers. Therefore, as the content of meta-aramid in the casting solution increases, the mechanical properties of the meta-aramid porous membrane have been greatly enhanced. The enhancement of mechanical properties can effectively inhibit the growth of lithium dendrites and prevent safety accidents caused by battery short circuits, thereby improving the safety of the battery. In addition, lithium-ion batteries may collide, squeeze, hit, etc. during use, causing the battery to deform and exert a large force on the diaphragm. High-strength diaphragms can resist such damage and prevent direct contact between the positive and negative electrodes of the battery and cause safety accidents. The mechanical strength of the meta-aramid porous membrane in Example 4 can reach 29.4 MPa, which can effectively improve the safety of the battery. Therefore, the meta-aramid porous membrane prepared by using a casting solution with a high polymer content has a strong strength.

[0202] In combination with Examples 16-23, it can be seen that the amount of water mist acting on the liquid film can be controlled by changing the amount of water mist sprayed (amount of mist output), the distance of water mist action, and the diameter of the mist outlet, thereby changing the structure of the aromatic polyamide porous membrane. When the amount of water mist sprayed is larger, the diameter of the mist outlet is larger, and the distance between the mist outlet and the liquid film is closer, the amount of water mist acting on the liquid film is also larger. When the amount of water mist acting is too large, some water mist may not have time to exchange with the solvent, and thus gather into water droplets, thereby forming large defects in the membrane (such as Figure 7 As shown, it is a cross-sectional SEM image of the meta-aramid porous membrane provided in Example 18), resulting in reduced mechanical properties of the membrane. When the amount of water mist sprayed is smaller, the diameter of the water mist outlet is smaller, and the distance between the outlet and the liquid film is farther, the amount of water mist acting on the liquid film is also less. When the amount of water mist acting is small, the diffusion between the solvent and the water mist in the liquid film is slow, the phase separation time is longer, the lean phase aggregates and grows, and uniform macropores are formed after the solvent is removed. Due to the exchange between the solvent and the water mist on the surface of the liquid film, the aromatic polyamide is gradually brought to the surface of the liquid film, and finally a relatively dense surface is formed (such as Figure 8 As shown, it is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 21). The present invention can prepare an aromatic polyamide porous membrane with uniform surface openings, continuous interior, excellent mechanical properties and strength by adjusting and optimizing the parameters of the water mist effect. In Example 24, when the liquid membrane is placed in a closed space with a high water mist environment, a porous membrane with excellent performance can also be prepared.

[0203] Comparative Example 1: A meta-aramid porous membrane was prepared by a conventional non-solvent induced phase separation method. The SEM image of the cross section is shown in FIG. Fig. 9 As shown, the SEM image of the upper surface is Fig.10 As shown in the figure, combined with the SEM image, it can be seen that the sublayer of the porous membrane prepared by the non-solvent phase separation method is a large finger-shaped pore, and the surface in contact with the air is a defective dense layer. When the liquid membrane is immersed in water, the solvent and water at the interface between the liquid membrane and the coagulation bath quickly transfer mass to each other, resulting in the non-solvent solubility in the liquid membrane reaching the requirements of liquid-liquid phase separation. In addition, the solvent in the sublayer continuously exchanges polymers with water, and the polymer concentration of the cortex continues to increase, eventually forming a dense cortex; at the same time, a large amount of water enters the structure of the sublayer, and the sublayer structure instantly undergoes liquid-liquid phase separation, a large amount of solvent exchanges with water, and a large amount of water and solvent constitute a poor phase that eventually forms a finger-shaped pore structure. However, the surface openness of the meta-aramid porous membrane with a dense cortex is relatively low, and it is difficult for lithium ions to pass through the porous membrane, which may cause the lithium-ion battery to fail to operate; while the meta-aramid porous membrane with a sublayer of a finger-shaped pore structure has poor mechanical properties, and lithium dendrites can easily pass through the diaphragm to cause a short circuit. Therefore, the meta-aramid porous membrane prepared by traditional non-solvent phase separation has a fast film formation speed, resulting in a dense cortex and finger-like pore sublayers in the meta-aramid porous membrane. The porous membrane with the above structure is not suitable for application in lithium-ion battery separators.

[0204] Comparative Example 2: A meta-aramid porous membrane was prepared by evaporation-induced phase separation. The cross-sectional SEM image of the meta-aramid porous membrane is shown in FIG. Fig.11 As shown, it can be seen that its cross section is relatively dense and has a spherulite structure. This is because N,N-dimethylacetamide has a high boiling point and takes a long time to evaporate completely. During the evaporation process, solid-liquid phase separation occurs. The phase separation time is long, and the rich phase containing a large amount of polymer can continue to grow. Finally, the rich phases are connected and contacted with each other to form a continuous rich phase structure. When the solvent is completely evaporated, the continuous rich phase solidifies to form a dense structure. The dense pore structure hinders the normal passage of lithium ions, resulting in lower ionic conductivity and specific capacity, and may even cause the lithium-ion battery to fail to operate normally. Therefore, the meta-aramid porous membrane prepared by evaporation-induced phase separation has a low porosity due to the slow phase separation speed, and is also not suitable as a lithium-ion battery separator.

[0205] The surface SEM image of the meta-aramid porous membrane provided in Comparative Example 3 is as follows: Fig.12As shown, in Comparative Example 3, due to the small amount of water vapor in a certain space, the amount of water action is small, and it takes a long time for phase separation to occur. In this process, the water-rich lean phase has time to grow and form a larger lean phase. When the water and solvent in the lean phase are removed, larger pores are formed. In the cortex of the liquid membrane, due to the slow exchange of solvent and water vapor, the meta-aramid polymer brought to the cortex by the solvent slowly aggregates and eventually forms a denser cortex. In contrast, the porous membrane prepared by water mist-induced phase separation in the present invention has a larger surface opening, is continuous inside, and has excellent strength, and is more suitable as a lithium-ion battery separator.

[0206] Therefore, the present invention adopts water mist to induce phase separation, so that the speed of liquid film phase separation is between non-solvent induced phase separation and evaporation induced phase separation, and solid-liquid phase separation and liquid-liquid phase separation occur simultaneously, forming a continuous and uniform three-dimensional porous sponge structure with a large surface opening and porosity.

[0207] In order to further verify the technical effect of the aromatic polyamide porous membrane prepared by water mist induced phase separation as a lithium ion battery separator, the present invention uses the meta-aramid porous membrane provided in Example 1 to assemble a lithium ion battery for electrochemical testing:

[0208] 1. Ionic conductivity

[0209] Ionic conductivity refers to the ion flow capacity of the diaphragm / electrolyte system after being fully wetted by the electrolyte. It is the most important indicator for evaluating the ability to provide a channel for the transmission of lithium ions. The higher the ionic conductivity, the smaller the ion transmission resistance in the battery, which is conducive to ion transmission. The present invention uses an aromatic polyamide porous membrane as a diaphragm, assembles a lithium-ion battery and performs an ionic conductivity test as follows:

[0210] First, in a glove box, a blocked half-cell was assembled and sealed in the order of positive electrode shell-stainless steel sheet-diaphragm / electrolyte-stainless steel sheet-shrapnel-negative electrode shell (wherein the positive electrode was composed of lithium iron phosphate, conductive agent Super P and binder polyvinylidene fluoride PVDF, with a mass ratio of 8:1:1; the negative electrode was a lithium sheet purchased from Tianjin Zhongneng Lithium Co., Ltd.; the electrolyte, LB5, was purchased from Taiyuan Lizhiyuan Technology Co., Ltd.; the lithium-ion batteries in the following performance tests all used the above positive and negative electrodes and electrolytes). After standing for 12 hours, an electrochemical AC impedance test was performed using an electrochemical workstation (Chenhua, CHI660E, Shanghai), which is related to the process of solid diffusion of lithium ions inside the diaphragm. The intersection of the oblique line and the horizontal axis in the obtained AC impedance spectrum is the bulk impedance R of the lithium-ion battery diaphragm. The frequency range was set to 0.1 Hz-100 kHz, and the calculation formula of the ionic conductivity σ (S / cm) was: Where, d is the film thickness (μm); R is the film resistance (Ω); S is the film area cut during the test (cm 2 ).

[0211] The ionic conductivity of the diaphragm is affected by factors such as porosity, pore size and pore size distribution, thickness, and wettability. The aromatic polyamide porous membrane provided by the present invention has high porosity, uniform pore size distribution, small thickness, and good wettability, so that the diaphragm can be ensured to have excellent ionic conductivity under the premise of isolating the positive and negative electrodes of the lithium ion battery. By conversion, it can be known that the ionic conductivity of the aromatic polyamide porous membrane of Example 1 is 0.84ms cm -1 , which is higher than the ionic conductivity of the PP separator, indicating that the resistance of lithium ions passing through the aromatic polyamide porous membrane is smaller.

[0212] 2. Cycle performance and rate performance

[0213] The completion of a charge and discharge of a lithium-ion battery is called a cycle. Cycle performance is an important indicator for measuring the service life of a battery. The number of cycles, the first discharge capacity, and the retention capacity determine the cycle performance of the battery. The number of times a battery is cyclically charged and discharged is called the number of cycles. When the battery is tested for the first charge and discharge performance, 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. The present invention uses an aromatic polyamide porous membrane as a diaphragm, and the method for assembling a lithium-ion battery and conducting a cycle performance test is as follows:

[0214] First, the button-type lithium-ion battery was assembled in an argon-filled glove box in the order of positive electrode shell / positive electrode sheet / diaphragm / negative electrode sheet / gasket / shell / negative electrode shell, and then the assembled battery was sealed with a sealing machine and left to stand for 24 hours. The cycle performance of the lithium-ion battery was tested using the Blue Electric Battery Test System (Blue Electric Electronics, CT2001A, Wuhan). The above-assembled lithium-ion battery was subjected to 3 cycles of charge and discharge at 0.2C on the Blue Electric Battery Test System, and then a cycle test was performed at 1C. The test voltage range is 2.8-3.85V, and the charge and discharge current = theoretical specific capacity × active material mass × n C.

[0215] During the charging and discharging process of the diaphragm, the clogging and damage of the diaphragm will lead to incomplete charging and discharging, greatly reducing the capacity and service life of the battery. The aromatic polyamide porous membrane provided by the present invention has high ionic conductivity, small interface impedance, large porosity and liquid absorption rate, improves the lithium ion migration ability, and the diaphragm will not be easily blocked, so the battery exhibits more stable cycle performance. Under 1C test conditions, the battery discharge specific capacity of the aromatic polyamide porous membrane can reach 140.19mAh / g. After 700 cycles, the battery discharge specific capacity of the aromatic polyamide porous membrane is 127.55mAh / g, and the corresponding capacity retention rate is 90.98%, while the battery discharge specific capacity of the PP diaphragm is only 126.4mAh / g at most, and the capacity retention rate after 700 cycles is 89.39%, and its coulombic efficiency is lower than that of the aromatic polyamide porous membrane, indicating that the battery assembled with the aromatic polyamide porous membrane exhibits stable cycle performance.

[0216] Moreover, the present invention also tests the discharge specific capacity of the battery composed of PP diaphragm and meta-aramid porous film at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C. The discharge specific capacity of the battery with meta-aramid porous film as the diaphragm at different rates is higher than that of the battery with PP diaphragm. In particular, the discharge specific capacity of the battery with aromatic polyamide porous film as the diaphragm at a high rate of 5C can reach more than 104.93 mAh / g, and when the rate returns to 0.1C, its discharge specific capacity can still reach 160.32 mAh / g, indicating that the presence of aromatic polyamide porous film can give the all-solid-state composite electrolyte excellent structure and electrochemical stability, thereby making the battery have better electrochemical reversibility during the cycle process. The batteries using aromatic polyamide porous membrane as separator have smooth and flat charge and discharge platforms and high capacity at different rates of 0.1C-5C, indicating that no side reactions occur during the cycle of lithium-ion batteries assembled with aromatic polyamide porous membrane, which can be attributed to the high ionic conductivity and fast lithium ion transport capability of aromatic polyamide porous membrane.

[0217] 3. Li / Li symmetric battery constant charge and discharge performance

[0218] In order to characterize the lithium ion plating / stripping behavior and obtain the constant current charge and discharge performance of the aromatic polyamide porous membrane and the PP diaphragm, the button-type lithium ion battery was assembled into a Li / Li symmetrical battery in the order of positive electrode shell / lithium sheet / diaphragm / lithium sheet / gasket / shell / negative electrode shell in an argon-filled glove box, and then the assembled battery was sealed with a sealing machine and left to stand for 24 hours. The assembled Li / Li symmetrical battery was placed on the AND-BT2013C battery testing system produced by Wuhan Blue Electric Electronics Co., Ltd. for constant current charge and discharge test. Among them, the charge and discharge current density is 0.5mA / cm 2The charge and discharge time of each cycle is 1 h, and the surface area of ​​Li metal is fixed at 2 cm 2 .

[0219] In the battery composed of PP separator and aromatic polyamide porous membrane, the 2 Under the current density, the voltage value of the battery composed of PP diaphragm gradually increases during the 400h constant charge and discharge, which may be due to the significant increase in polarization voltage caused by the poor stability between the PP diaphragm and the lithium metal interface. In comparison, the battery composed of aromatic polyamide porous membrane can operate stably for more than 700h at a lower voltage, which may be because the high and uniform pore structure in the aromatic polyamide porous membrane enables lithium ions to be quickly and evenly deposited on the lithium metal surface, and the amide bonds on the aromatic polyamide can absorb the anion groups in the electrolyte, reducing the occurrence of side reactions, thereby enabling stable constant current charge and discharge for a long time.

[0220] In summary, the present invention adopts a method of water mist induced phase separation to prepare an aromatic polyamide porous membrane. By simply controlling the content of the aromatic polyamide porous membrane in the casting liquid, porous membranes of different structures can be prepared to meet different application requirements. Compared with the traditional non-solvent induced phase separation and evaporation induced phase separation, the speed of the water mist induced phase separation of the present invention is between the two, so that the casting liquid and the liquid film undergo solid-liquid phase separation at the same time as the liquid-liquid phase separation. The prepared aromatic polyamide porous membrane has a high porosity on the upper and lower surfaces, and the interior is a three-dimensional network structure that is interconnected, showing excellent mechanical properties and thermal stability. Thanks to its interconnected uniform three-dimensional network structure, the battery composed of the aromatic polyamide porous membrane of the present invention has more excellent ionic conductivity, cycle performance, rate performance and interface stability than the battery composed of the PP separator.

[0221] The applicant declares that the present invention uses the above-mentioned embodiments to illustrate the aromatic polyamide porous membrane and its preparation method and application, but the present invention is not limited to the above-mentioned process steps, that is, it does not mean that the present invention must rely on the above-mentioned process steps to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of the raw materials selected by the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing an aromatic polyamide porous membrane, characterized in that: The preparation method comprises the following steps: Mixing aromatic polyamide and solvent to obtain a casting solution; The casting liquid is subjected to a film-forming treatment to obtain a liquid film; placing the surface of the liquid film in a water mist environment for treatment to obtain a wet film; The wet film is subjected to solvent replacement and drying to obtain the aromatic polyamide porous film.

2. The preparation method according to claim 1, characterized in that: The aromatic polyamide includes any one of poly(p-phenylene terephthalamide), poly(m-phenylene isophthalamide), poly(p-phenylene amide), and poly(phenylene sulfone terephthalamide), or a combination of at least two thereof; Preferably, the mass percentage of aromatic polyamide in the casting solution is 5%-20%; Preferably, the solvent includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone, or a combination of at least two thereof; Preferably, the mass percentage of the solvent in the casting solution is 65%-90%.

3. The preparation method according to claim 1 or 2, characterized in that: The casting solution also includes a thickener; Preferably, the thickener comprises any one of glycerol, polyvinyl pyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, glycerol, lithium chloride, calcium chloride, lithium bromide, calcium bromide, or a combination of at least two thereof; Preferably, the mass percentage of the thickener in the casting solution is ≤15%.

4. The preparation method according to any one of claims 1 to 3, characterized in that: The film forming treatment method includes a doctor blade coating method, a roller coating method, a dipping method, a suction filtration method or a casting method; Preferably, the thickness of the liquid film is 50-300 μm.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The method for treating in a water mist environment comprises: using a water mist generating device to spray water mist onto the surface of the liquid film, so that the surface of the liquid film is placed in a water mist environment; Or, placing the surface of the liquid film in a closed water mist environment; Preferably, the mist output of the water mist generating device is 100-10000 mL / h; Preferably, the distance between the mist outlet of the water mist generating device and the surface of the liquid film is 1-50 cm; Preferably, the diameter of the mist outlet of the water mist generating device is 0.1-20 cm; Preferably, the treatment time in the water mist environment is 10-1000s.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The solvent used for the solvent replacement includes any one of water, ethanol, tert-butyl alcohol, and isopropanol, or a combination of at least two thereof; Preferably, the solvent replacement time is 1-60min; Preferably, the drying temperature is 30-60°C; Preferably, the drying time is 1-60 min.

7. The preparation method according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: (1) mixing aromatic polyamide, a solvent and optionally a thickener to obtain a casting solution; The solvent in the casting solution includes any one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone, or a combination of at least two thereof; The thickener includes any one of glycerol, polyvinyl pyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, glycerol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide, or a combination of at least two thereof; The mass percentage of aromatic polyamide in the casting solution is 5%-20%, the mass percentage of solvent is 65%-90%, and the mass percentage of thickener is ≤15%; (2) subjecting the casting liquid to a film-forming treatment to obtain a liquid film with a thickness of 50-300 μm; (3) using a water mist generating device to spray water mist onto the surface of the liquid film, so that the surface of the liquid film is placed in a water mist environment and treated until the film becomes white and opaque, thereby obtaining a wet film; The mist output of the water mist generating device is 100-10000 mL / h, the diameter of the mist outlet of the water mist generating device is 0.1-20 cm, and the distance between the mist outlet and the liquid film surface is 1-50 cm; (4) subjecting the wet film to solvent replacement for 1-60 min, and then drying at 30-60° C. for 1-60 min to obtain the aromatic polyamide porous film; The solvent used for the solvent replacement includes any one of water, ethanol, tert-butyl alcohol, and isopropanol, or a combination of at least two of them.

8. An aromatic polyamide porous membrane, characterized in that: The aromatic polyamide porous membrane is prepared by the preparation method according to any one of claims 1 to 7; Preferably, the pore size of the aromatic polyamide porous membrane is 0.1-0.5 μm; Preferably, the porosity of the aromatic polyamide porous membrane is ≥50%; Preferably, the thickness of the aromatic polyamide porous membrane is ≤35 μm; Preferably, the air permeability of the aromatic polyamide porous membrane is ≤200s / 100cc; Preferably, the surface porosity of the aromatic polyamide porous membrane is ≥30%, more preferably ≥40%; Preferably, the electrolyte contact angle of the aromatic polyamide porous membrane is ≤28°, more preferably ≤20°; Preferably, the liquid absorption rate of the aromatic polyamide porous membrane is ≥180%, more preferably ≥200%; Preferably, the thermal shrinkage of the aromatic polyamide porous membrane treated at 150° C. for 1 hour is ≤0.1%; Preferably, the tensile strength of the aromatic polyamide porous membrane is ≥15MPa; Preferably, the puncture resistance of the aromatic polyamide porous membrane is ≥0.15 N / μm.

9. Use of the aromatic polyamide porous membrane as claimed in claim 8 in a secondary battery; Preferably, the aromatic polyamide porous membrane is used as a separator material for secondary batteries.

10. A secondary battery, characterized in that: The secondary battery comprises the aromatic polyamide porous membrane as claimed in claim 8; Preferably, the secondary battery is a lithium-ion battery; Preferably, the lithium-ion battery comprises a positive electrode, a negative electrode, an electrolyte and a separator, and the separator is the aromatic polyamide porous membrane according to claim 8.

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