Aromatic polyamide porous membrane, method for producing the same, and use thereof

Abstract

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

Description

Technical Field

[0001] This invention belongs to the field of polymer membrane material technology, specifically relating to an aromatic polyamide porous membrane, its preparation method, and its application. Background Technology

[0002] Energy storage systems that convert chemical energy into electrical energy have become an important direction for the development of green new energy due to their advantages such as high energy density and conversion efficiency. Lithium-ion batteries are simple and efficient devices with high energy storage and supply capabilities. 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 lithium-ion batteries developed by the French company Saft can reach 4000 W / kg; they have long cycle life: the ICR 18650 lithium-ion battery can cycle 1000 times with a capacity retention rate of >85%; they have high energy conversion efficiency: the conversion efficiency can reach 96%; and they have high energy density: the specific energy can reach 180 Wh / kg. Therefore, lithium-ion batteries play an important role in the development and widespread utilization of new energy sources.

[0003] Lithium-ion batteries consist of positive and negative electrodes, an electrolyte, and a separator wetted by the electrolyte between the positive and negative electrodes. The separator, often referred to as the "third electrode," is a crucial component of the lithium-ion battery. It is a porous film with uniformly distributed micropores, positioned between the lithium positive and negative electrode materials, providing a channel for lithium-ion transport while preventing direct contact between the electrodes. Microporous polyolefin separators, primarily composed of polyethylene (PE), polypropylene (PP), and composites of these two materials, are currently the main commercial lithium-ion battery separators. However, polyolefin (PP, PE) separators have poor heat resistance; at high temperatures, they experience significant thermal shrinkage, failing to provide insulation and potentially causing short circuits, posing a significant safety hazard to lithium-ion batteries. Furthermore, the lack of activating groups on polyolefins makes them difficult to wet with the electrolyte, resulting in low ionic conductivity, poor cycle performance, and a tendency for lithium dendrites to puncture the separator and cause short circuits. Safety incidents caused by lithium-ion batteries have severely hampered the development of the lithium-ion battery industry. Preparing battery separators that combine heat resistance and hydrophilicity 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 have a significant impact on the performance of the battery separator and the battery itself. Aromatic polyamides, due to their excellent mechanical properties, high-temperature resistance, good chemical stability, dimensional stability, and thermal stability, are ideal materials for preparing lithium-ion battery separators. Currently, many researchers have prepared aromatic polyamides for lithium-ion battery separators. For example, CN115207559A uses a wet web-forming method to prepare an aramid porous membrane. Differentiated aramid short-cut fibers and differentiated aramid precipitated fibers are used as raw materials. These are mechanically pretreated to obtain an aramid fiber mixed dispersion. The dispersion is then wet-formed using an inclined web forming device to obtain an aramid porous membrane substrate. Finally, the aramid porous membrane substrate is treated by hot pressing to obtain the aramid porous membrane. Although this aramid porous membrane has excellent mechanical properties and high-temperature resistance, the large pore size makes it prone to self-discharge. Furthermore, the uniformity of the aramid porous membrane is difficult to control because the aramid short-cut fiber dispersion is difficult to achieve uniformity. CN111370625A discloses an aramid-coated lithium-ion battery separator and its preparation method. The preparation method includes: dissolving aramid, a co-solvent, an oily additive, and a pore-forming agent in a first solvent to obtain a uniformly mixed coating liquid; coating the coating liquid onto a separator substrate; and immersing the membrane in a coagulation bath using phase inversion to obtain the lithium-ion battery separator. Although this separator 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 porosity of the membrane. The membrane performance is mainly affected by the properties of the membrane substrate, and problems such as large thermal shrinkage deformation and low ionic conductivity still exist. CN113381122A uses a solvent-free phase separation method to prepare a meta-aramid porous membrane, controlling the membrane structure by controlling the solvent ratio in the coagulation bath. However, this method generates a large amount of organic solvent during production, causing significant environmental problems and wastewater treatment pressure. In addition, aromatic polyamide porous membranes have been prepared by electrospinning, but the membranes produced by electrospinning have large pore sizes, poor mechanical strength, low production efficiency, and high cost, making them unsuitable for large-scale applications.

[0005] To address the problems of poor thermal stability, poor wettability, complex preparation process, unsuitable pore structure, and low ionic conductivity in current commercial separators, it is urgent to solve the problem in this field to develop a porous membrane with a simple and environmentally friendly preparation process, high porosity, high ionic conductivity, good wettability, good thermal stability and mechanical properties, and especially to make it meet the performance requirements of battery separators. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide an aromatic polyamide porous membrane, its preparation method, and its application. Through the design of the preparation process, the obtained aromatic polyamide porous membrane has the characteristics of high porosity, good wettability, high ionic conductivity, good thermal stability, and good mechanical properties. Moreover, the preparation method is simple and environmentally friendly, the membrane preparation process is easy to adjust, and the membrane structure is easy to control, making it suitable for large-scale industrial production.

[0007] To achieve this objective, the present invention adopts the following technical solution:

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

[0009] Aromatic polyamide and solvent are mixed to obtain casting solution;

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

[0011] The surface of the liquid film is treated in a water mist environment to obtain a wet film.

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

[0013] This invention utilizes aromatic polyamides to fabricate membranes. Firstly, aromatic polyamide molecules contain amide bonds, which have strong hydrogen bonding capabilities. Simultaneously, the presence of benzene rings within the aromatic polyamide molecule makes internal rotation of the molecular chain segments difficult, resulting in a very low probability of molecular chain folding. This facilitates the orderly arrangement of aromatic polyamide molecules, giving the porous membrane high mechanical strength and ensuring battery safety. Secondly, aromatic polyamides are highly heat-resistant materials, ensuring minimal thermal deformation at high temperatures, thus expanding the application range for batteries. Furthermore, aromatic polyamides exhibit high affinity for electrolytes. The resulting porous membrane, used as a battery separator, possesses excellent wetting and liquid absorption / retention capabilities. This superior wettability extends battery cycle life and significantly improves fast-charging performance. Therefore, the aromatic polyamide porous membrane offers significant advantages for use as a separator in rechargeable batteries.

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

[0015] This invention reveals that when a liquid film is directly immersed in water, it rapidly undergoes liquid-liquid phase separation. The phase separation time is short, the solidification speed is fast, and crystallization continues even in the later stages of phase separation. This easily leads to a structure combining a dense skin layer and a finger-like macroporous sublayer. Lithium ions cannot pass through the dense surface layer, and the presence of the elongated pore sublayer results in poor mechanical properties. Therefore, such porous membranes are unsuitable for use as lithium-ion battery separators. When the liquid film is subjected to solvent evaporation at a certain temperature for an extended period, the phase separation time is long, the solidification speed is slow, and solid-liquid phase separation occurs. The resulting membranes mostly lack a dense skin layer and are composed of interconnected spherulitic particles with pores between them. These membranes have low porosity and are brittle, making it difficult for lithium ions to pass through. Similarly, such membranes are also unsuitable for use as lithium-ion battery separators.

[0016] In this invention, the surface of the liquid film is treated in a water mist environment. Water mist-induced phase separation is used to control the phase separation rate of the liquid film. A suitable phase separation rate is achieved, placing the liquid film's phase separation rate between the two film-forming methods mentioned above. The film-forming path involves simultaneous liquid-solid phase separation and liquid-liquid phase separation, resulting in a porous membrane with numerous and uniform openings on both the upper and lower surfaces, and a sublayer consisting of an interconnected sponge-like structure. The interconnected porous membrane facilitates the uniform passage of lithium ions, and the cross-linked sponge structure provides strong mechanical properties, which helps suppress the growth of lithium dendrites, thus meeting the requirements for a lithium-ion battery separator.

[0017] Therefore, the preparation method provided by this invention, through the water mist-induced phase separation design and its combination and synergy with specific process steps, can easily, efficiently, and quickly obtain a high-performance aromatic polyamide porous membrane. This membrane has high porosity on its upper and lower surfaces, an interconnected three-dimensional network structure inside, and advantages such as high porosity and high ionic conductivity. It also exhibits good electrolyte wettability and excellent mechanical properties and thermal stability, fully meeting the performance requirements of secondary battery separators. The preparation method is simple and environmentally friendly, the membrane fabrication process is easily adjustable, and the membrane structure is easily controlled, making it 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. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

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

[0020] As a preferred embodiment of the present invention, the aromatic polyamide is poly(m-phenylene isophthalamide) (meta-aramid), wherein adjacent amide bonds more readily form strong hydrogen bonds, the amide groups in the molecular chain are connected to the meta-phenyl groups, there is no conjugation effect, the internal rotational potential energy is low, the chain segment flexibility is relatively good, and the crystallinity is low. Therefore, whether in production or in use, meta-aramid porous membranes are more suitable for use as separators in secondary batteries (preferably lithium-ion batteries) compared to other aromatic polyamide porous membranes.

[0021] Preferably, the aromatic polyamide content in the casting solution is 5%-20% by mass, for example, it can be 6%, 8%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18% or 19%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0022] As a preferred embodiment of the present invention, the content of aromatic polyamide in the casting solution has a significant impact 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 controlled. When the content of aromatic polyamide in the casting solution is relatively low, water mist needs to react with the solvent for a long time before phase separation occurs. The phase separation rate is slow, and water mist can accumulate in the liquid membrane for a long time. After the large amount of accumulated water mist is removed, it forms the pores of the aromatic polyamide porous membrane, resulting in the aromatic polyamide porous membrane having high surface openness and porosity. When the aromatic polyamide content in the casting solution is relatively high, the solvent on the upper surface of the liquid film in contact with water mist is replaced by water mist, causing the aromatic polyamide to undergo phase separation more rapidly. This results in an increased polymer content and ultimately a lower porosity. Simultaneously, the viscosity increases significantly, preventing further water mist from entering the sublayers of the membrane. This prevents the sublayers from rapidly separating and forming elongated finger-like pores, and also reduces water mist accumulation in the liquid film. Therefore, the final aromatic polyamide porous membrane has a relatively small porosity and open area, but the smaller surface area and the more cross-linked sublayers significantly improve the mechanical strength of the polyamide porous membrane. By adjusting the aromatic polyamide content in the casting solution, aromatic polyamide porous membranes with different structures can be designed to meet the diverse requirements of different secondary battery separators.

[0023] In this invention, the solvent in the casting solution has good solubility for aromatic polyamides, preventing the presence of undissolved aromatic polyamide clusters in the casting solution while simultaneously dissolving a sufficient amount of aromatic polyamides to achieve significant control over the porous membrane structure of the aromatic polyamides. Furthermore, the solvent also regulates 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) does not precipitate in the casting solution.

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

[0025] Furthermore, to broaden the range of solid content while reducing film-forming time, the solvent exhibits excellent solubility for aromatic polyamides. Therefore, the solvent preferably comprises N,N-dimethylformamide and / or N,N-dimethylacetamide.

[0026] Preferably, the solvent content in the casting solution is 65%-90% by mass, for example, it can be 66%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 84%, 85% or 88%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

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

[0028] As a preferred embodiment of the present invention, a thickener is added to the casting solution. Its function is to increase the viscosity of the casting solution. When the amount of aromatic polyamide in the casting solution is low, the viscosity is low, making it prone to displacement on the film substrate, resulting in an uneven liquid film and potential defects. The final aromatic polyamide porous membrane will be uneven and defective. The introduction of the thickener can avoid these defects. The thickener needs to have good solubility with the solvent to ensure it does not precipitate in the casting solution and can be removed during solvent replacement.

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

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

[0031] Preferably, the mass percentage of 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%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

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

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

[0034] Preferably, the thickness of the liquid film is 50-300 μm, for example, it can 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 values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0035] In this 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, its internal resistance is relatively small, resulting in a higher ion conduction velocity. However, at the same time, the mechanical strength of the thinner aromatic polyamide porous membrane decreases, leading to certain safety hazards in the secondary battery. Therefore, in order to achieve a suitable thickness for the aromatic polyamide porous membrane, balancing ion conduction performance and mechanical properties, and ensuring certain mechanical properties while making the membrane as thin as possible, the thickness of the liquid film is controlled between 50-300 μm. This results in a thin aromatic polyamide porous membrane with excellent mechanical properties.

[0036] This invention involves treating the surface of the liquid film in a water mist environment. The water mist induces phase separation in the liquid film. Specifically, in a high water mist environment, the water mist displaces the solvent from the liquid film 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 space filled with uniform water mist) or direct water mist injection (water mist is directly injected from a mist outlet onto the liquid film in an open space), with direct water mist injection being preferred.

[0037] Preferably, the droplet size of the water mist in the water mist environment is 1-30μm, for example, it can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 12μm, 15μm, 16μm, 18μm, 20μm, 22μm, 25μm or 28μm, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0038] Preferably, the method for processing in the water mist environment includes: using a water mist generator to spray water mist onto the surface of the liquid film, so that the surface of the liquid film is placed in the water mist environment;

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

[0040] Preferably, the mist output of the water mist generator 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. The specific values ​​within the ranges of 4564 mL / h, 5000 mL / h, 5500 mL / h, 6000 mL / h, 6500 mL / h, 7000 mL / h, 7500 mL / h, 7567 mL / h, 8000 mL / h, 8500 mL / h, 9000 mL / h, or 9500 mL / h, as well as specific values ​​between the above values, are not exhaustively listed here for space limitations and for the sake of brevity.

[0041] Preferably, the distance between the mist outlet of the water mist generator and the surface of the liquid film is 1-50cm, for example, it can be 5cm, 8cm, 10cm, 14cm, 15cm, 20cm, 21cm, 22cm, 25cm, 30cm, 35cm, 40cm, 44cm or 45cm, as well as specific point values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific point values ​​included in the range.

[0042] Preferably, the diameter of the mist outlet of the water mist generator is 0.1-20cm, for example, it can be 0.5cm, 1cm, 2cm, 4cm, 5cm, 6cm, 8cm, 9cm, 10cm, 12cm, 13cm, 14cm, 15cm, 16cm, 18cm or 19cm, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

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

[0044] The enclosed water mist environment is a space filled with uniform water mist, preferably with a humidity of 60-100%, such as 65%, 70%, 75%, 80%, 85%, 90%, 95% or 98%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific 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 values ​​between the above values. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific values ​​included in the range.

[0046] In this invention, solvent replacement removes water, solvent, and thickener (if any) from the wet membrane. The replacement solvent used 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 membrane. Simultaneously, the replacement solvent itself is easily removable. When solvent replacement is used to remove solvent from the wet membrane, the solvent-rich wet membrane is placed in the replacement solvent. Due to the miscibility of the solvent and the replacement solvent, diffusion occurs, allowing the solvent in the wet membrane to diffuse into the replacement solvent, while the replacement solvent enters the membrane. Because the replacement solvent is relatively easier to remove than the solvent (from the casting solution), it is more beneficial for membrane fabrication.

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

[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 values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0049] Preferably, the drying temperature is 30-60℃, for example, it can be 32℃, 35℃, 38℃, 40℃, 42℃, 45℃, 48℃, 50℃, 52℃, 55℃ or 58℃, as well as specific values ​​between the above points. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0050] In this invention, drying after solvent displacement is to remove the displacement solvent. The drying temperature should not be too high, otherwise it will cause deformation of the aromatic porous membrane. The drying temperature is set to 30-60℃ to ensure rapid removal of the displacement 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 values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

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

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

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

[0055] The thickener includes any one or a combination of at least two of the following: glycerol, polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, glycerol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide.

[0056] The casting solution contains 5%-20% aromatic polyamide by mass, 65%-90% solvent by mass, and ≤15% thickener by mass.

[0057] (2) The casting solution is subjected to film-forming treatment to obtain a liquid film with a thickness of 50-300 μm;

[0058] (3) A water mist generator is used 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 is white and opaque, thus obtaining a wet film;

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

[0060] (4) The wet membrane is subjected to solvent replacement for 1-60 min, and then dried at 30-60℃ for 1-60 min to obtain the aromatic polyamide porous membrane;

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

[0062] In a second aspect, the present invention provides an aromatic polyamide porous membrane, which is prepared by the preparation method 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 values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific 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%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but 55-86% is further preferred.

[0065] Preferably, the thickness of the aromatic polyamide porous membrane is ≤35μm, for example, it can be 8μm, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, 28μm, 30μm, 32μm or 34μm, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, and 9-32μm is further preferred.

[0066] Preferably, the air permeability of the aromatic polyamide porous membrane is ≤200s / 100cc, for example, it can be 30s / 100cc, 40s / 100cc, 50s / 100cc, 60s / 100cc, 80s / 100cc, 100s / 100cc, 120s / 100cc, 140s / 100cc, 150s / 100cc, 160s / 100cc or 180s / 100cc, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific 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%, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range. More preferably, it is ≥40%, and more preferably, it is 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°, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, and it is further preferred to be ≤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 values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but it is further preferred to be ≥200%.

[0070] Preferably, the thermal shrinkage rate of the aromatic polyamide porous membrane after treatment 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 values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range.

[0071] Preferably, the tensile strength of the aromatic polyamide porous membrane is ≥7MPa, for example, it can be 8MPa, 9MPa, 10MPa, 12MPa, 14MPa, 15MPa, 16MPa, 18MPa, 20MPa, 22MPa, 25MPa, 28MPa, 30MPa or 32MPa, as well as specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, and it is further preferred to be ≥15MPa.

[0072] Preferably, the puncture resistance of the aromatic polyamide porous membrane is ≥0.12 N / μm, for example, it can be 0.13 N / μm, 0.14 N / μm, 0.15 N / μm, 0.2 N / μm, 0.25 N / μm, 0.3 N / μm, 0.35 N / μm, 0.4 N / μm, 0.45 N / μm, 0.5 N / μm, 0.55 N / μm, 0.6 N / μm, 0.65 N / μm or 0.7 N / μm, and specific values ​​between the above values. Due to space limitations and for the sake of brevity, the present invention will not exhaustively list the specific values ​​included in the range, but ≥0.15 N / μm is further preferred.

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

[0076] For example, the secondary battery includes lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, etc.

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

[0078] Preferably, the lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein 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 this invention, an aromatic polyamide porous membrane is prepared by water mist-induced phase separation. During the water mist-induced phase separation process, the liquid membrane system is in a thermodynamically unstable state and phase separation occurs, forming a rich phase with a high polymer content and a poor phase with a low polymer content. The rich phase ultimately forms the framework of the aromatic polyamide porous membrane, and the poor phase ultimately forms the pore structure of the aromatic polyamide porous membrane. Unlike traditional non-solvent-induced phase separation and solvent evaporation-induced phase separation, the water mist-induced phase separation rate of this invention is at an appropriate level, which can prepare a porous membrane with large surface porosity and high porosity. By simply controlling the content of aromatic polyamide in the casting solution, the structure of the final aromatic polyamide porous membrane can be controlled 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 0.1-0.5μm, a porosity of ≥50%, a thickness of ≤35μm, an air permeability of ≤200s / 100cc, a surface porosity of ≥30%, an electrolyte contact angle of ≤28°, a liquid absorption rate of ≥180%, a heat shrinkage rate of ≤0.1% at 150℃ / 1h, 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 performance, mechanical performance, thermal stability and dimensional stability, making it suitable as a separator for lithium-ion batteries. It has low interfacial impedance and can effectively improve the cycle performance, rate performance and interfacial stability of lithium-ion batteries. Attached Figure Description

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

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

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

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

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

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

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

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

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

[0094] Figure 10 SEM image of the upper surface of the meta-aramid porous membrane provided for Comparative Example 1;

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

[0096] Figure 12 The surface SEM image of the meta-aramid porous membrane provided for Comparative Example 3. Detailed Implementation

[0097] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0098] The terms “comprising,” “including,” “having,” “containing,” or any other variations thereof, as used herein, are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not limited to those elements and may also include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0099] The aromatic polyamide porous membrane provided by this invention is particularly suitable as a separator for secondary batteries, especially lithium-ion batteries. The following provides a detailed description of the main properties and requirements of the aromatic polyamide porous membrane and separator, the reasons why the aromatic polyamide porous membrane meets the requirements for lithium-ion battery separators, and the performance testing methods for the aromatic polyamide porous membrane:

[0100] I. Aperture

[0101] Regarding the aromatic polyamide porous membrane in the embodiments of the present invention, in order to obtain a separator 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 separator must be at the submicron level. When the pore size of the lithium-ion battery separator is small, its internal resistance will be large, the contact area with the electrolyte will be small, and the permeability of lithium ions will decrease, thus leading to a decrease in the overall performance of the battery. When the pore size of the lithium-ion battery separator is large, although it can improve ionic conductivity, it will also cause the lithium-ion battery to self-discharge, resulting in energy loss. Moreover, the larger pore size of the separator increases the risk of being punctured by lithium dendrites, thereby causing short circuits or even explosions. Therefore, controlling the pore size of the aromatic polyamide porous membrane within a suitable range is crucial to its performance.

[0102] The aromatic polyamide porous membrane provided by this invention has a pore size between 0.1 and 0.5 μm. Within this pore size range, the aromatic polyamide porous membrane exhibits both high ionic conductivity and high safety, and virtually no self-discharge. In this invention, the pore size of the aromatic polyamide porous membrane is mainly controlled by adjusting the content of aromatic polyamide in the casting solution and the preparation process.

[0103] In this embodiment of the invention, the pore size of the aromatic polyamide porous membrane is characterized using mercury intrusion porosimetry (referencing the method in standard GB / T21650.1-2008). Mercury has no wetting effect on aromatic polyamide porous membranes, so external pressure is required for mercury to enter the pores of the membrane. The required external pressure varies depending on the pore diameter; the smaller the pore diameter, the greater the required external pressure. Therefore, by measuring the amount of mercury entering the pores of the membrane under different external pressures, the pore volume of the corresponding pore size can be determined.

[0104] The principle of mercury porosimetry can be expressed by 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; and p is the applied external pressure (mN / m). 2 ).

[0105] II. Porosity

[0106] Porosity, defined as the ratio of the volume of micropores in a separator to the total volume of the separator, is a crucial evaluation criterion for lithium-ion battery separators. It directly affects the transport rate of lithium ions within the separator and the storage of electrolyte within it. Separators with high porosity provide more storage sites for the electrolyte, widening the path of lithium ions through the separator and increasing both the permeability and rate of lithium ion transmission, thereby improving the performance of the lithium-ion battery. Theoretically, parameters such as the separator's gas permeability, electrolyte absorption, and internal resistance are all affected by porosity. The aromatic polyamide porous membrane prepared in this invention has a porosity ≥50%, superior to most currently available commercial separators. This invention prepares aromatic polyamide porous membranes with different porosities by controlling the content of aromatic polyamide in the casting solution and the processing method.

[0107] In this invention, the porosity of aromatic polyamide porous membranes is determined using the hexadecane absorption method (referencing the method in standard GB / T 33052-2016). First, three 10cm × 5cm samples of the aromatic polyamide porous membrane to be tested are cut. Then, the thickness and weight of the samples are measured using a thickness gauge and an electronic balance, respectively. Next, the samples are immersed in hexadecane for 1 hour, and the surface hexadecane is wiped off with absorbent paper. Finally, the samples are weighed and the porosity of the aromatic polyamide porous membrane is calculated.

[0108] The formula for calculating porosity 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 hexadecane absorption (g), w2 is the mass of the sample after hexadecane absorption (g), and ρ is the density of hexadecane (g / cm³). 3 ).

[0109] III. Thickness

[0110] Thickness is one of the fundamental parameters of lithium-ion battery separators, significantly impacting their mechanical properties and lithium-ion permeability. Generally, a thicker separator provides better mechanical properties and effectively reduces the risk of puncture by lithium dendrites. However, increased thickness leads to a more complex and tortuous lithium-ion permeation path within the separator, resulting in a significant decrease in ion permeability and velocity. Furthermore, increased thickness also occupies space in the active material of the lithium-ion battery, reducing its capacity. Therefore, controlling the separator within a reasonable range is crucial for the performance of lithium-ion batteries. The aromatic polyamide porous membrane of this invention leverages the excellent mechanical properties of aromatic polyamides, enabling a very small thickness (≤35μm, preferably ≤25μm) while meeting mechanical strength requirements. This invention controls the thickness of the aromatic polyamide porous membrane by adjusting the thickness of the liquid film, with thickness testing referring to the method in standard GB / T 6672-2001.

[0111] IV. Breathability

[0112] Gas permeability refers to the time it takes for gas to pass through a membrane under a certain pressure, indirectly reflecting the permeability of lithium ions. Better gas permeability indicates better lithium ion permeability. The main factors affecting membrane permeability are porosity and pore connectivity; higher porosity and better pore connectivity result in better permeability. The aromatic polyamide porous membrane prepared in this invention has high porosity and good pore connectivity, thus exhibiting excellent gas permeability (≤200s / 100cc). This invention mainly regulates the gas permeability of the aromatic polyamide porous membrane by adjusting the thickness of the liquid membrane, the content of aromatic polyamide in the casting solution, and the preparation process. The gas permeability test refers to the method in standard GB / T 1040.3-2006.

[0113] V. Surface porosity

[0114] This invention defines surface porosity as the ratio of the area of ​​pores on the membrane surface to the total surface area of ​​the membrane. Surface porosity determines the inlet area for lithium ions to enter the membrane, and the size of the inlet area determines the speed at which lithium ions enter the membrane. When the surface porosity is large, the pore area on the membrane surface is larger, allowing lithium ions to enter the membrane interior at a faster rate, resulting in a higher ionic conductivity. Membranes prepared by traditional solvent-induced phase separation methods typically have a denser surface and a smaller surface porosity. This is because, during the preparation of porous membranes using solvent-induced phase separation methods, the surface solvent rapidly exchanges with the non-solvent in the coagulation bath. The exchange rate is much greater than the rate at which the solvent in the sublayer of the casting solution transfers to the surface, causing instantaneous phase separation on the membrane surface to form a dense layer. The aromatic polyamide porous membrane prepared by the method of this invention has a large surface porosity (≥30%, preferably ≥40%), mainly because: in this invention, solvent molecules in the liquid membrane skin layer exchange with water mist, promoting liquid-liquid phase separation in the skin layer, forming a polymer-rich phase and a polymer-poor phase. As the solvent is continuously exchanged, the polymer-rich phase eventually solidifies, and the poor phase forms a porous structure. The lower surface of the aromatic polyamide porous membrane also possesses excellent surface porosity because the liquid membrane skin layer does not immediately form a dense layer. Therefore, the solvent on the lower surface can also exchange with water mist at a relatively fast rate, promoting the phase separation speed and ultimately forming a lower surface with a large surface porosity.

[0115] In this invention, the surface porosity is characterized using the following method. First, the aromatic polyamide porous membrane to be tested is quenched in liquid nitrogen. Then, the aromatic polyamide porous membrane is fixed to a sample stage using conductive adhesive. The sample is then sputter-coated with gold, and the cross-section of the sample is photographed using a scanning electron microscope (SEM), and the image is saved. Using ImageJ software, all the pores in the SEM image are selected, and the pore area and total area in the SEM image are calculated. Dividing the two yields the surface porosity of the aromatic polyamide porous membrane.

[0116] VI. Wetting properties

[0117] Wetting performance is mainly used to evaluate the effect of the separator on the electrolyte wetting effect, revealing the compatibility between the separator and the electrolyte. Better wetting performance is beneficial to the affinity between the separator and the electrolyte; the larger the contact area between the separator and the electrolyte, the higher the lithium-ion transfer rate, thus improving the battery's charge / discharge efficiency and capacity. Currently, commercial PP and PE separators are polymerized from olefins, lacking polar groups in their molecular chains. Therefore, they have very poor wettability to electrolytes, requiring coating with hydrophilic materials to increase their wettability. Aromatic polyamide molecular chains possess more polar groups; therefore, the aromatic polyamide porous membrane exhibits excellent wettability to electrolytes (electrolyte contact angle ≤28°, preferably ≤20°). In this 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 being a mixed solvent of ethylene carbonate EC, ethyl methyl carbonate EMC, and dimethyl carbonate DMC in a mass ratio of 1:1:1). The size of the contact angle is used to measure the degree of wetting of the membrane with the electrolyte.

[0118] VII. Liquid Absorption Rate

[0119] The electrolyte uptake rate of a lithium-ion battery separator refers to the amount of electrolyte stored in the separator. It controls the internal resistance of the separator to a certain extent. The higher the uptake rate, the more electrolyte is stored, allowing lithium ions to have more and wider pathways to permeate the separator, resulting in higher ionic conductivity. The electrolyte uptake rate of a lithium-ion battery separator is mainly affected by the affinity of the membrane material for the electrolyte and the porosity of the separator. Separators with excellent wettability and high porosity have higher uptake rates. Currently, commercial PP and PE separators lack polar groups on their macromolecular chains, resulting in poor wettability and low uptake rates. In this invention, because aromatic polyamide macromolecular chains have more polar groups, they have better wettability for the electrolyte. Furthermore, the aromatic polyamide porous membrane prepared in this invention has a high porosity, thus exhibiting a high electrolyte uptake rate (≥180%, preferably ≥200%).

[0120] In this invention, the liquid absorption rate of the aromatic polyamide porous membrane is tested using a weighing method. First, the weight of the aromatic polyamide porous membrane before immersion in the electrolyte is measured. After immersion in the electrolyte for a period of time, the weight of the aromatic polyamide porous membrane after immersion is measured. The liquid absorption rate of the membrane is calculated by comparing the weight change of the aromatic polyamide porous membrane before and after immersion. The specific calculation formula is as follows: Where x represents the liquid absorption rate of the diaphragm (%), m represents the membrane weight (g) after soaking in electrolyte, and m0 represents the membrane weight (g) before soaking in electrolyte.

[0121] 8. Thermal stability

[0122] The thermal stability of the separator in lithium-ion batteries is a crucial indicator of battery safety. During charging and discharging, chemical reactions inside the battery generate heat, and external heat sources also contribute to increased internal temperature. Especially in extreme environments or under improper battery use, the internal temperature can rise rapidly. If the internal temperature becomes too high, the separator may shrink and lose its function of isolating the positive and negative electrodes, leading to a short circuit or even explosion. Therefore, the separator material must possess excellent thermal properties to ensure dimensional stability of the battery over a wide temperature range and maintain its function of isolating the positive and negative electrodes even under extreme temperature conditions. The high energy barrier of the benzene ring and CN-bond rotation in aromatic polyamides prevents the molecular chains from forming a fully extended chain conformation. Combined with the strong hydrogen bonding between the molecular chains, aromatic polyamides exhibit exceptional heat resistance, with a glass transition temperature of around 270℃. Even after continuous operation at 200℃ for 20,000 hours, their strength can be maintained at 90% of its original value. Thanks to the excellent thermal properties of aromatic polyamides, the aromatic polyamide porous membrane has a thermal shrinkage rate of ≤0.1% at 150℃ / 1h.

[0123] The present invention tests the thermal shrinkage rate of aromatic polyamide porous membranes 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 the blower-type constant temperature chamber, press it down with another quantitative filter paper, close the constant temperature chamber door, start the time, keep it at 150℃ for 1 hour, after the heating is completed, take out the membrane, wait for the membrane to return to room temperature, and measure the longitudinal and transverse mark lengths.

[0124] The formula for calculating the heat shrinkage rate ΔS (%) is: Where S0 is the area of ​​the membrane before heating (cm²) 2 S is the area of ​​the membrane after heating (cm²) 2 ).

[0125] 9. Tensile Strength

[0126] Tensile strength is a parameter reflecting the dimensional stability of a separator under external forces during use. Because lithium-ion batteries form lithium dendrites during use, a high-strength separator can effectively delay the penetration of these dendrites, ensuring the safety performance of the lithium-ion battery. Furthermore, lithium-ion batteries may experience collisions, compression, and impacts during use, causing deformation that exerts significant forces on the separator. Only a high-strength separator can withstand such damage and prevent direct contact between the positive and negative electrodes, thus preventing safety accidents. The main chain of aromatic polyamide consists of aromatic rings and amide bonds. The aromatic ring structure has high rigidity, and the polymer chains extend to form a rod-like structure. Simultaneously, the linear molecular chain structure of aromatic polyamide results in high space utilization, allowing for a large amount of polymer to be accommodated per unit volume, thus leading to higher strength. Therefore, the aromatic polyamide porous membrane of this invention possesses high tensile strength (≥7MPa, preferably ≥15MPa). The tensile strength test refers to the method in standard GB / T 36363-2018.

[0127] 10. Puncture resistance

[0128] Puncture resistance refers to the mass applied to a given needle-shaped object to puncture a given separator sample. It is used to characterize the separator's ability to resist puncture by external pressure. Since the negative electrode of a lithium-ion battery is composed of carbon materials, adhesives, etc., even after rolling, the surface of the negative electrode is still uneven due to the presence of granular materials. Therefore, during battery assembly and shaping, the separator sandwiched between the positive and negative electrodes needs to withstand significant pressure to prevent short circuits. Furthermore, batteries inevitably experience squeezing, collisions, and impacts during use; in these situations, the separator also needs a certain puncture resistance to ensure battery safety. Therefore, lithium-ion battery separators must possess a certain level of puncture resistance. Due to the presence of benzene rings, aromatic polyamides possess high internal rotation potential energy, resulting in a planar extended chain conformation. Furthermore, the presence of numerous polar groups on the molecular chains leads to smaller gaps and stronger interactions between aromatic polyamide molecules. Consequently, the porous aromatic polyamide membrane exhibits good puncture resistance (≥0.12 N / μm, preferably ≥0.15 N / μm). The puncture resistance test is performed according to the method in standard GB / T 36363-2018.

[0129] The following will describe in detail the aromatic polyamide porous membrane and its preparation method according to the present invention using several examples, but the aromatic polyamide porous membrane and its preparation method according to the present invention are not limited to these examples. In the following examples of the present invention, the materials for which preparation methods are not provided are all commercially available chemicals. 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 polyvinylpyrrolidone, both purchased from Kaimate (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 its preparation method, the preparation method comprising the following steps:

[0132] (1) Preparation of casting solution: Lithium chloride was added to meta-aramid solution (solvent is N,N-dimethylacetamide) and stirred until homogeneous and transparent to obtain casting solution; the content of each component of the casting solution is as follows: meta-aramid 10%, N,N-dimethylacetamide 85%, lithium chloride 5%;

[0133] (2) Preparation of liquid film: The casting liquid obtained in step (1) is subjected to ultrasonic degassing. The degassed casting liquid is poured onto the glass film substrate. The scraper is adjusted so that the liquid film thickness is 100μm. The scraper is moved at a uniform speed to obtain a liquid film with a thickness of 100μm.

[0134] (3) Water mist induced liquid film phase separation: Water mist is sprayed onto the surface of the liquid film obtained in step (2) using a water mist generator. The diameter of the mist outlet of the water mist generator is 1 cm, the distance between the mist outlet and the surface of the liquid film 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, thus 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, it is placed in a forced-air drying oven at 50°C to dry the membrane and remove excess deionized water. After drying, meta-aramid porous membrane is obtained.

[0136] Example 2

[0137] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the components in the casting solution are as follows: 5% meta-aramid, 85% N,N-dimethylacetamide, and 10% lithium chloride. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0138] Example 3

[0139] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 15% meta-aramid and 85% N,N-dimethylacetamide. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0140] Example 4

[0141] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 20% meta-aramid and 80% N,N-dimethylacetamide. All other steps and process parameters are the same as 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 its preparation method are disclosed. The only difference between this membrane and Example 1 is that the components in the casting solution are as follows: 10% para-aramid, 85% N-methylpyrrolidone, and 5% lithium chloride. All other steps and process parameters are the same as in Example 1, and a para-aramid porous membrane is obtained.

[0144] Example 6

[0145] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% formamide. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0146] Example 7

[0147] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% glycerol. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0148] Example 8

[0149] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% polyethylene oxide. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0150] Example 9

[0151] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylacetamide, and 5% polyvinylpyrrolidone. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0152] Example 10

[0153] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that in step (2), the scraper is adjusted so that the thickness of the liquid film is 150 μm. The other raw materials, steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0154] Example 11

[0155] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that in step (2), the scraper is adjusted so that the liquid film thickness is 200 μm. The other raw materials, steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0156] Example 12

[0157] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N,N-dimethylformamide, and 5% lithium chloride. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0158] Example 13

[0159] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that the components in the casting solution are as follows: 10% meta-aramid, 85% N-methylpyrrolidone, and 5% lithium chloride. All other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0160] Example 14

[0161] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that in step (2), the casting liquid is poured into the coating pan, and the gap between the rollers is adjusted so that the liquid film thickness is 100 μm, thus obtaining a liquid film with a thickness of 100 μm. Other raw materials, steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0162] Example 15

[0163] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this method and Example 1 is that ethanol is used as a displacement solvent in step (4) to remove N,N-dimethylacetamide and lithium chloride from the wet membrane. Other raw materials, steps, and process parameters are the same as in Example 1, resulting in a meta-aramid porous membrane.

[0164] Example 16

[0165] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that, in step (3), the diameter of the mist outlet of the water mist generator is 5 cm. All other raw materials, steps, and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0166] Example 17

[0167] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that, in step (3), the diameter of the mist outlet of the water mist generator is 0.1 cm. All other raw materials, steps, and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0168] Example 18

[0169] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that, in step (3), the mist output of the water mist generator is 1000 mL / h. All other raw materials, steps, and process parameters are the same as in Example 1, resulting in a meta-aramid porous membrane.

[0170] Example 19

[0171] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that, in step (3), the mist output of the water mist generator is 100 mL / h. All other raw materials, steps, and process parameters are the same as in Example 1, resulting in a meta-aramid porous membrane.

[0172] Example 20

[0173] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the distance between the mist outlet of the water mist generator and the surface of the liquid membrane in step (3) is 10 cm. All other raw materials, steps, and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0174] Example 21

[0175] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the distance between the mist outlet of the water mist generator and the surface of the liquid membrane in step (3) is 2 cm. All other raw materials, steps, and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0176] Example 22

[0177] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that in step (3), the distance between the mist outlet of the water mist generator and the surface of the liquid film is 10 cm, and the mist output is 1000 mL / h. All other raw materials, steps, and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0178] Example 23

[0179] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the diameter of the mist outlet of the water mist generator in step (3) is 5 cm, and the mist output is 100 mL / h. All other raw materials, steps, and process parameters are the same as in Example 1, resulting in a meta-aramid porous membrane.

[0180] Example 24

[0181] A meta-aramid porous membrane and its preparation method are disclosed. The only difference between this membrane and Example 1 is that the method for water mist-induced liquid membrane phase separation in step (3) is as follows: the liquid membrane is placed directly in a closed space filled with water mist at a humidity of 80%. Other steps and process parameters are the same as in Example 1, and a meta-aramid porous membrane is obtained.

[0182] Comparative Example 1

[0183] A meta-aramid porous membrane and its preparation method, 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 immersed in deionized water for 50 minutes to obtain an opaque wet film. The wet film is placed in a forced-air drying oven and dried at 50°C to remove excess deionized water from the film. After drying, meta-aramid porous membrane is obtained.

[0186] Comparative Example 2

[0187] A meta-aramid porous membrane and its preparation method, 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 obtained in step (2) and the substrate are placed in a forced-air drying oven at 60°C for 1 hour to remove N,N-dimethylacetamide. After drying, meta-aramid porous membrane is obtained.

[0190] Comparative Example 3

[0191] A meta-aramid porous membrane and its preparation method, 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) Vapor-induced liquid film phase separation: The liquid film obtained in step (2) and the substrate are placed in a 70% high humidity water vapor environment provided by a constant temperature and humidity chamber until the liquid film changes from transparent to 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, it is placed in a forced-air drying oven at 50°C to dry the membrane and remove excess deionized water. After drying, 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 microstructure 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 shown below. Figure 1 As shown, the SEM image of the lower surface is as follows. Figure 2 As shown, the cross-sectional SEM image is as follows. Figure 3 As shown in the three SEM images above, the meta-aramid porous membrane has a large surface porosity on both the upper and lower surfaces, with relatively uniform pore size and a uniform sponge-like cross-section. The interconnected structure between the surface and the interior provides a transport channel for the rapid migration of lithium ions, and the interconnected and tortuous pore structure effectively prevents the growth of lithium dendrites and avoids short circuits. Testing showed that the meta-aramid porous membrane of Example 1 has a porosity of 66.3%. The high porosity of the membrane provides more storage sites for the electrolyte, widening the path of lithium ions through the membrane, increasing the lithium ion permeability and permeation rate, thereby improving the performance of the lithium-ion battery.

[0200] The SEM image of the upper surface of the meta-aramid porous membrane provided in Example 2 is shown below. Figure 4As shown, compared to Example 1, the porous membrane in Example 2 has a larger surface porosity on its upper surface. This is because during the water mist-induced liquid-phase separation process in the casting liquid, a rich phase mainly composed of meta-aramid polymer and a poor phase containing solvent, thickener, and deionized water are formed. These two phases continuously cross-link together to form a wet membrane. When the solvent, deionized water, and thickener in the wet membrane are removed, the rich phase forms the framework structure of the meta-aramid porous membrane, while the poor phase containing 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 reduced meta-aramid content in the casting liquid results in a smaller proportion of the rich phase, mainly composed of meta-aramid, during phase separation. After the rich phase forms the meta-aramid framework structure, the porosity and surface porosity are relatively high. For lithium-ion battery separators, the porosity and surface porosity are important indicators for evaluating separator performance and directly affect the performance of lithium-ion batteries. The surface porosity determines the inlet area for lithium ions to enter the separator, and the size of the inlet area determines the speed at which lithium ions enter the separator. When the surface porosity is relatively large, there are more inlets on the separator surface that can enter the separator, allowing lithium ions to enter the separator interior at a faster speed, and the corresponding ionic conductivity will be greater. In Example 2, the surface porosity of the meta-aramid porous membrane reached 73.5%. Therefore, the meta-aramid porous membrane prepared using a casting solution with low polymer content has a relatively rich pore structure.

[0201] The SEM image of the upper surface of the meta-aramid porous membrane provided in Example 3 is shown below. Figure 5 As shown, the SEM image of the upper surface of the meta-aramid porous membrane provided in Example 4 is as follows. Figure 6As shown. In Examples 3 and 4, the surface porosity of the meta-aramid porous membrane decreased with increasing polymer content. This is because the reduced solvent content in the casting solution leads to a decrease in the proportion of depleted phases formed during phase separation, resulting in a less abundant pore structure in the final meta-aramid porous membrane compared to the low meta-aramid content membrane in Example 1. However, the increased meta-aramid content significantly improved the mechanical properties of the final meta-aramid porous membrane. This is because the increased meta-aramid density leads to a larger cross-linking area between polymers, thus greatly enhancing the mechanical properties of the meta-aramid porous membrane with increasing meta-aramid content in the casting solution. This enhanced mechanical property effectively inhibits lithium dendrite growth, preventing battery short circuits and safety accidents, thereby improving battery safety. Furthermore, lithium-ion batteries may experience collisions, compression, and impacts during use, causing deformation and significant forces on the separator. Only a high-strength separator can withstand such damage and prevent direct contact between the positive and negative electrodes, thus preventing safety accidents. The meta-aramid porous membrane in Example 4 exhibits a mechanical strength of 29.4 MPa, effectively improving battery safety. Therefore, the meta-aramid porous membrane prepared using a casting solution with high polymer content possesses strong strength.

[0202] As can be seen from Examples 16-23, the amount of water mist acting on the liquid film can be controlled by changing the water mist spray volume (mist output), the water mist action distance, and the mist outlet diameter, thereby altering the structure of the aromatic polyamide porous membrane. The greater the water mist spray volume, the larger the mist outlet diameter, and the closer the distance between the mist outlet and the liquid film, the greater the amount of water mist acting on the liquid film. When the amount of water mist is too large, some water mist may not have enough time to exchange with the solvent, thus agglomerating into water droplets, and consequently forming large defects in the membrane (such as…). Figure 7 As shown, this is a cross-sectional SEM image of the meta-aramid porous membrane provided in Example 18, which leads to a decrease in the mechanical properties of the membrane. When the water mist spray volume is smaller, the water mist outlet diameter is smaller, and the distance between the outlet and the liquid film is greater, the amount of water mist acting on the liquid film is also less. When the amount of water mist is less, the diffusion between the solvent and water mist in the liquid film is slower, the phase separation time is longer, the lean phase aggregates and grows, and after the solvent is removed, uniform macropores are formed. On the surface of the liquid film, due to the exchange between the solvent and water mist, the aromatic polyamide is gradually brought to the surface of the liquid film, eventually forming a relatively dense surface (such as...). Figure 8 As shown, this is a SEM image of the upper surface of the meta-aramid porous membrane provided in Example 21. By adjusting and optimizing the parameters of the water mist effect, this invention can prepare aromatic polyamide porous membranes with uniform surface openings, continuous internal structure, and excellent mechanical properties and strength. In Example 24, a porous membrane with excellent performance can also be prepared by placing the liquid membrane in a closed space with a high water mist environment.

[0203] Comparative Example 1 prepared a meta-aramid porous membrane using a traditional solvent-free phase separation method. The SEM image of its cross-section is shown below. Figure 9 As shown, the SEM image of the upper surface is as follows: Figure 10 As shown in the SEM images, the porous membrane prepared using the non-solvent phase separation method has a sublayer with large finger-like pores and a defective dense layer at the air-contact surface. When the liquid membrane is immersed in water, the solvent and water rapidly transfer mass at the interface between the liquid membrane and the coagulation bath, causing the non-solvent solubility in the liquid membrane to meet the requirements for liquid-liquid phase separation. Furthermore, the solvent in the sublayer continuously carries polymers and exchanges them with water, continuously increasing the polymer concentration in the skin layer, eventually forming a dense skin layer. Simultaneously, a large amount of water enters the sublayer structure, causing instantaneous liquid-liquid phase separation. A large amount of solvent and water exchange occurs, and the water- and solvent-depleted phase ultimately forms the finger-like pore structure. However, the meta-aramid porous membrane with a dense skin layer has a relatively low surface porosity, making it difficult for lithium ions to pass through, potentially causing the lithium-ion battery to malfunction. Conversely, the meta-aramid porous membrane with a sublayer containing finger-like pores has poor mechanical properties, and lithium dendrites can easily penetrate the separator, causing short circuits. Therefore, meta-aramid porous membranes prepared by traditional non-solvent phase separation have a relatively fast film formation rate, resulting in a dense skin layer and finger-like pore sublayer. Porous membranes with the above structure are not suitable for use as lithium-ion battery separators.

[0204] Comparative Example 2 prepared a meta-aramid porous membrane using evaporation-induced phase separation. A cross-sectional SEM image of this meta-aramid porous membrane is shown below. Figure 11 As shown, its cross-section is relatively dense and exhibits a spherulitic structure. This is because N,N-dimethylacetamide has a high boiling point and requires a long time to completely evaporate. During evaporation, solid-liquid phase separation occurs. The long phase separation time allows the polymer-rich phase to continuously grow, eventually connecting and contacting 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 porous structure hinders the normal passage of lithium ions, resulting in low ionic conductivity and specific capacity, and may even cause the lithium-ion battery to malfunction. Therefore, meta-aramid porous membranes prepared by evaporation-induced phase separation are also unsuitable as lithium-ion battery separators due to their low porosity caused by the slow phase separation rate.

[0205] The surface SEM image of the meta-aramid porous membrane provided in Comparative Example 3 is shown below. Figure 12As shown in Comparative Example 3, the amount of water vapor in a given space is relatively small, resulting in less water interaction and requiring a longer time for phase separation to occur. During this process, the water-rich, lean phase has time to grow, forming a larger lean phase. When the water and solvent in the lean phase are removed, larger pores are formed. In the skin layer of the liquid membrane, due to the slow exchange between solvent and water vapor, the meta-aramid polymer carried by the solvent to the skin layer slowly aggregates, eventually forming a relatively dense skin layer. In contrast, the porous membrane prepared by water mist-induced phase separation in this invention has a larger surface porosity, internal continuity, and excellent strength, making it more suitable as a lithium-ion battery separator.

[0206] Therefore, this invention employs water mist-induced phase separation, which makes the liquid film phase separation rate between that of non-solvent-induced phase separation and evaporation-induced phase separation, while simultaneously causing solid-liquid phase separation and liquid-liquid phase separation, forming a continuous and uniform three-dimensional porous sponge structure with a large surface porosity and porosity.

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

[0208] I. Ionic conductivity

[0209] Ionic conductivity refers to the ion flow capacity of the separator / electrolyte system after it has been fully wetted by the electrolyte. It is the most important indicator for evaluating the ability to provide a channel for lithium-ion transport. The higher the ionic conductivity, the lower the resistance to ion transport in the battery, which is beneficial for ion transport. This invention uses an aromatic polyamide porous membrane as the separator, and the method for assembling a lithium-ion battery and testing its ionic conductivity is as follows:

[0210] First, in a glove box, a blocked half-cell was assembled and sealed in the following order: positive electrode shell - stainless steel sheet - separator / electrolyte - stainless steel sheet - spring sheet - negative electrode shell. (The positive electrode is composed of lithium iron phosphate, conductive agent Super P, and binder polyvinylidene fluoride PVDF in a mass ratio of 8:1:1; the negative electrode is a lithium sheet purchased from Tianjin Zhongneng Lithium Industry Co., Ltd.; the electrolyte, LB5, was purchased from Taiyuan Lizhiyuan Technology Co., Ltd.; the lithium-ion batteries used in the following performance tests all use the above positive and negative electrodes and electrolytes.) After 12 hours, an electrochemical impedance spectroscopy (EIS) test was performed using an electrochemical workstation (Chenhua, CHI660E, Shanghai). This EIS is related to the solid diffusion process of lithium ions inside the separator. The intersection of the oblique line and the horizontal axis in the obtained EIS spectrum is the bulk impedance R of the lithium-ion battery separator. The frequency range was set to 0.1Hz-100kHz. The formula for calculating the ionic conductivity σ (S / cm) is: Where d is the film thickness (μm); R is the film resistance (Ω); and S is the film area cut during the experiment (cm²). 2 ).

[0211] The ionic conductivity of a separator is affected by factors such as porosity, pore size and pore size distribution, thickness, and wettability. The aromatic polyamide porous membrane provided by this invention has high porosity, uniform pore size distribution, small thickness, and good wettability. Therefore, it can ensure that the separator has excellent ionic conductivity while isolating the positive and negative electrodes of the lithium-ion battery. Calculations show that the ionic conductivity of the aromatic polyamide porous membrane in Example 1 is 0.84 mS / cm. -1 The higher ionic conductivity than that of the PP membrane indicates that lithium ions experience less resistance when passing through the aromatic polyamide porous membrane.

[0212] II. Cyclic Performance and Rate Performance

[0213] A single charge-discharge cycle of a lithium-ion battery is called a cycle. Cycle performance is a crucial indicator of battery lifespan, and the number of cycles, initial discharge capacity, and retained capacity determine the battery's cycle performance. The number of charge-discharge cycles is called the cycle life. The discharge capacity achieved during the initial charge-discharge performance test is called the initial discharge capacity. Retained capacity refers to the discharge capacity the battery retains after completing a certain number of cycles. This invention uses an aromatic polyamide porous membrane as the separator, and the method for assembling a lithium-ion battery and conducting cycle performance testing is as follows:

[0214] First, the button-type lithium-ion batteries were assembled in an argon-filled glove box in the following order: positive electrode shell / positive electrode plate / separator / negative electrode plate / gasket / shell / negative electrode shell. The assembled batteries were then sealed using a sealing machine and left to stand for 24 hours. The cycle performance of the lithium-ion batteries was tested using a Blue Electric Battery Testing System (Blue Electric Electronics, CT2001A, Wuhan). The assembled lithium-ion batteries were subjected to three charge-discharge cycles at 0.2C on the Blue Electric Battery Testing System, followed by cycle testing at 1C. The test voltage range was 2.8-3.85V, and the charge / discharge current was calculated as: theoretical specific capacity × active material mass × nC.

[0215] During charging and discharging, clogging or damage to the separator can lead to incomplete charging and discharging, significantly reducing battery capacity and lifespan. The aromatic polyamide porous membrane provided by this invention possesses high ionic conductivity, low interfacial impedance, high porosity, and high liquid absorption rate, enhancing lithium-ion migration ability. The separator is less prone to clogging, resulting in more stable battery cycle performance. Under 1C testing conditions, the aromatic polyamide porous membrane achieves a battery discharge specific capacity of 140.19 mAh / g. After 700 cycles, the discharge specific capacity remains at 127.55 mAh / g, corresponding to a capacity retention rate of 90.98%. In contrast, the PP separator's highest discharge specific capacity is only 126.4 mAh / g, with a capacity retention rate of 89.39% after 700 cycles. Furthermore, 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] Furthermore, this invention also tested the discharge specific capacity of batteries composed of PP separators and meta-aramid porous membranes at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, and 5C. At different rates, the discharge specific capacity of batteries using meta-aramid porous membranes as separators was higher than that of batteries using PP separators. In particular, at a high rate of 5C, the discharge specific capacity of batteries using aromatic polyamide porous membranes as separators could reach over 104.93 mAh / g, and when the rate returned to 0.1C, its discharge specific capacity could still reach 160.32 mAh / g. This indicates that the presence of aromatic polyamide porous membranes can endow the all-solid-state composite electrolyte with excellent structure and electrochemical stability, thereby giving the battery better electrochemical reversibility during cycling. Batteries using aromatic polyamide porous membranes as separators exhibit smooth and flat charge-discharge plateaus and high capacity at different rates from 0.1C to 5C, indicating that no side reactions occur during the cycling process of lithium-ion batteries assembled with aromatic polyamide porous membranes. This can be attributed to the high ionic conductivity and fast lithium-ion transport capability of aromatic polyamide porous membranes.

[0217] III. Constant charge-discharge performance of Li / Li symmetric batteries

[0218] To characterize the lithium-ion plating / stripping behavior and obtain the constant current charge-discharge performance of the aromatic polyamide porous membrane and PP separator, button-type lithium-ion batteries were assembled into Li / Li symmetric cells in an argon-filled glove box in the following order: positive electrode shell / lithium sheet / separator / lithium sheet / pad / shell / negative electrode shell. The assembled batteries were then sealed using a sealing machine and left to stand for 24 hours. The assembled Li / Li symmetric cells were then subjected to constant current charge-discharge tests on the AND-BT2013C battery testing system manufactured by Wuhan Landian Electronics Co., Ltd. The charge-discharge current density was 0.5 mA / cm². 2The charge / discharge time for each cycle is 1 hour, and the surface area of ​​the Li metal is fixed at 2 cm². 2 .

[0219] A battery consisting of a PP separator and an aromatic polyamide porous membrane operates at 0.5 mA / cm². 2 At current density, the battery composed of PP separator gradually increases its constant charge-discharge voltage value after 400 hours. This may be due to the significant increase in polarization voltage caused by the poor stability between the PP separator and the lithium metal interface. In contrast, the battery composed of aromatic polyamide porous membrane can operate stably for more than 700 hours at a lower voltage. This may be because the high and uniform pore structure in the aromatic polyamide porous membrane allows lithium ions to be deposited quickly and uniformly on the lithium metal surface. Moreover, the amide bonds on the aromatic polyamide can absorb anionic groups in the electrolyte, reducing the generation of side reactions, thereby enabling stable and long-term constant current charge-discharge.

[0220] In summary, this invention employs a water mist-induced phase separation method to prepare aromatic polyamide porous membranes. By simply controlling the content of aromatic polyamide porous membrane in the casting solution, porous membranes with different structures can be prepared to meet various application requirements. Compared to traditional non-solvent-induced phase separation and evaporation-induced phase separation, the water mist-induced phase separation rate of this invention falls between the two, allowing solid-liquid phase separation to occur simultaneously with liquid-liquid phase separation between the casting solution and the liquid membrane. The prepared aromatic polyamide porous membrane has high porosity on both the upper and lower surfaces and an internal interconnected three-dimensional network structure, exhibiting excellent mechanical properties and thermal stability. Thanks to its interconnected and uniform three-dimensional network structure, batteries composed of the aromatic polyamide porous membrane of this invention exhibit superior ionic conductivity, cycle performance, rate performance, and interface stability compared to batteries composed of PP membranes.

[0221] The applicant declares that this invention illustrates the aromatic polyamide porous membrane, its preparation method, and its application through the above embodiments. However, this invention is not limited to the above process steps, meaning that this invention does not necessarily rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of the raw materials used in this invention, additions of auxiliary components, and selection of specific methods all fall within the protection and disclosure scope of this invention.

Claims

1. A method for preparing an aromatic polyamide porous membrane for use as a separator material in secondary batteries, characterized in that, The preparation method includes the following steps: Aromatic polyamide and solvent are mixed to obtain casting solution; The casting solution is subjected to a film-forming treatment to obtain a liquid film; The surface of the liquid film is placed in a water mist environment for treatment until the film becomes white and opaque, thus obtaining a wet film. The wet membrane is subjected to solvent replacement and drying to obtain the aromatic polyamide porous membrane. The mass percentage of aromatic polyamide in the casting solution is 5%-20%; The thickness of the liquid film is 50-300 μm; The method for processing in the water mist environment includes: using a water mist generator to spray water mist onto the surface of the liquid film, so that the surface of the liquid film is placed in the water mist environment; The mist output of the water mist generator is 100-9000 mL / h, the distance between the mist outlet of the water mist generator and the surface of the liquid film is 5-10 cm, and the diameter of the mist outlet of the water mist generator is 0.1-4 cm. The treatment time in the water mist environment is 10-1000 s; The pore size of the aromatic polyamide porous membrane is 0.1-0.5 μm; The thickness of the aromatic polyamide porous membrane is ≤35 μm.

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

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

4. The preparation method according to claim 1, characterized in that, The solvent in the casting solution has a mass percentage of 65%-90%.

5. The preparation method according to claim 1, characterized in that, The casting solution also includes a thickener.

6. The preparation method according to claim 5, characterized in that, The thickener includes any one or a combination of at least two of the following: glycerol, polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide.

7. The preparation method according to claim 5, characterized in that, The mass percentage of thickener in the casting solution is ≤15%.

8. The preparation method according to claim 1, characterized in that, The film-forming methods include blade coating, roller coating, immersion coating, vacuum filtration, or casting.

9. The preparation method according to claim 1, characterized in that, The solvent used for solvent replacement includes any one or a combination of at least two of the following: water, ethanol, tert-butanol, and isopropanol.

10. The preparation method according to claim 1, characterized in that, The solvent replacement time is 1-60 min.

11. The preparation method according to claim 1, characterized in that, The drying temperature is 30-60℃.

12. The preparation method according to claim 1, characterized in that, The drying time is 1-60 min.

13. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (1) Mix aromatic polyamide, solvent and optionally thickener to obtain casting solution; The solvent in the casting solution includes any one or a combination of at least two of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone. The thickener includes any one or a combination of at least two of the following: glycerol, polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, diethylene glycol, formamide, propylene glycol, lithium chloride, calcium chloride, lithium bromide, and calcium bromide. The casting solution contains 5%-20% aromatic polyamide by mass, 65%-90% solvent by mass, and ≤15% thickener by mass. (2) The casting solution is subjected to film-forming treatment to obtain a liquid film with a thickness of 50-300 μm; (3) A water mist generator is used 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 is white and opaque, thus obtaining a wet film. The treatment time in the water mist environment is 10-1000 s. The mist output of the water mist generator is 100-9000 mL / h, the diameter of the mist outlet of the water mist generator is 0.1-4 cm, and the distance between the mist outlet of the water mist generator and the surface of the liquid film is 5-10 cm. (4) The wet membrane is subjected to solvent replacement for 1-60 min, and then dried at 30-60℃ for 1-60 min to obtain the aromatic polyamide porous membrane. The pore size of the aromatic polyamide porous membrane is 0.1-0.5 μm, and the thickness of the aromatic polyamide porous membrane is ≤35 μm. The solvent used for solvent replacement includes any one or a combination of at least two of the following: water, ethanol, tert-butanol, and isopropanol.

14. 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 1-13.

15. The aromatic polyamide porous membrane according to claim 14, characterized in that, The porosity of the aromatic polyamide porous membrane is ≥50%.

16. The aromatic polyamide porous membrane according to claim 14, characterized in that, The air permeability of the aromatic polyamide porous membrane is ≤200 s / 100 cc.

17. The aromatic polyamide porous membrane according to claim 14, characterized in that, The surface porosity of the aromatic polyamide porous membrane is ≥30%.

18. The aromatic polyamide porous membrane according to claim 17, characterized in that, The surface porosity of the aromatic polyamide porous membrane is ≥40%.

19. The aromatic polyamide porous membrane according to claim 14, characterized in that, The electrolyte contact angle of the aromatic polyamide porous membrane is ≤28°.

20. The aromatic polyamide porous membrane according to claim 19, characterized in that, The electrolyte contact angle of the aromatic polyamide porous membrane is ≤20°.

21. The aromatic polyamide porous membrane according to claim 14, characterized in that, The liquid absorption rate of the aromatic polyamide porous membrane is ≥180%.

22. The aromatic polyamide porous membrane according to claim 21, characterized in that, The liquid absorption rate of the aromatic polyamide porous membrane is ≥200%.

23. The aromatic polyamide porous membrane according to claim 14, characterized in that, The aromatic polyamide porous membrane exhibits a heat shrinkage rate of ≤0.1% after treatment at 150°C for 1 h.

24. The aromatic polyamide porous membrane according to claim 14, characterized in that, The tensile strength of the aromatic polyamide porous membrane is ≥15 MPa.

25. The aromatic polyamide porous membrane according to claim 14, characterized in that, The puncture resistance of the aromatic polyamide porous membrane is ≥0.15 N / μm.

26. The application of an aromatic polyamide porous membrane as described in any one of claims 14-25 in a secondary battery.

27. The application according to claim 26, characterized in that, The aromatic polyamide porous membrane is used as a separator material for secondary batteries.

28. A secondary battery, characterized in that, The secondary battery includes an aromatic polyamide porous membrane as described in any one of claims 14-25.

29. The secondary battery according to claim 28, characterized in that, The secondary battery is a lithium-ion battery.

30. The secondary battery according to claim 29, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the separator is an aromatic polyamide porous membrane as described in any one of claims 14-25.

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