Diaphragm, method for manufacturing the same, secondary battery, and electric device

Abstract

The application provides a diaphragm and a preparation method thereof, a secondary battery and an electric device. The diaphragm comprises a base film and an aramid coating layer coated on at least one side surface of the base film, wherein the aramid coating layer is a porous structure coating layer, the pore diameter of the surface pores of the aramid coating layer is a first pore diameter, the pore diameter D50 of the internal pores of the aramid coating layer is a second pore diameter, the D50 of the first pore diameter is 0.8-1.5 times the D50 of the second pore diameter, and both the first pore diameter and the second pore diameter are less than 1 mu m. The heat shrinkage rate of the diaphragm at 150 DEG C is controlled within 10%, and the film breaking temperature is above 200 DEG C.

Description

Technical Field

[0001] This application relates to the field of batteries, specifically to a separator and its preparation method, a secondary battery, and an electrical device. Background Technology

[0002] Rechargeable lithium batteries, as electrochemical energy storage devices, are widely used in portable smart devices, electric vehicles, and many other fields. With industry development, the demand for high-energy-density and high-safety batteries is increasing. The separator, as a crucial component of rechargeable batteries, provides a pathway for ion transport, isolates the positive and negative electrodes, and ensures battery safety. As battery safety issues become increasingly prominent, the industry's performance requirements for separators are also gradually increasing. Currently, most separators are polyolefin separators. To improve separator safety, aramid or ceramic coatings are typically applied to the surface of the polyolefin separator. However, in practical applications, it has been found that the high-temperature resistance of separators, regardless of whether they are coated with aramid or ceramic, cannot be guaranteed. Their thermal shrinkage at around 130℃ can usually be controlled below 3.5%, but when the temperature rises to 150℃, the thermal shrinkage performance of the separator decreases significantly, with the thermal shrinkage rate usually exceeding 10%. Furthermore, the rupture temperature of this type of separator is generally below 200℃. To improve the high-temperature resistance of diaphragms, existing research has used various coating methods to enhance the shrinkage resistance of diaphragms at 150°C and increase the rupture temperature. However, multiple coating methods can significantly affect the thickness, quality, process complexity, and production efficiency of the diaphragm. Therefore, the key to effectively improving diaphragm safety and achieving commercialization lies in achieving high-temperature resistance of the diaphragm on the basis of a single-layer coating, controlling its thermal shrinkage rate to within 10% at 150°C, and increasing its rupture temperature to above 200°C. Summary of the Invention

[0003] This application provides a separator and its preparation method, a secondary battery and an electrical device, to improve the high temperature resistance of the separator, so that its thermal shrinkage rate at 150°C is controlled within 10%, and its membrane rupture temperature is increased to above 200°C.

[0004] In a first aspect, this application provides a diaphragm comprising a base membrane and an aramid coating coated on at least one side of the base membrane, wherein the aramid coating is a porous structure coating, the pore diameter of the surface pores of the aramid coating is a first pore diameter, the pore diameter of the internal pores of the aramid coating is a second pore diameter, the D50 of the first pore diameter is 0.8-1.5 times the D50 of the second pore diameter, and both the first pore diameter and the second pore diameter are less than 1 μm.

[0005] The diaphragm of this application has an aramid coating on at least one side surface of the base membrane. The ratio of the first pore size (D50) to the second pore size (D50) of the aramid coating is 0.8-1.5, ensuring that the pore sizes throughout the aramid coating are relatively close; that is, the pore sizes of the surface pores and the internal pores of the aramid coating are controlled within a similar range. By controlling the pore sizes of the surface pores and the internal pores within the aramid coating within the aforementioned range, uniform three-dimensional pores are formed inside the aramid coating. This allows for the formation of more arched structures within the aramid coating, improving the flexural strength of the diaphragm, thereby enhancing its thermal shrinkage performance, reducing the thermal shrinkage rate of the diaphragm at 150°C, and increasing the diaphragm rupture temperature.

[0006] Furthermore, in existing separators with aramid coatings, due to limitations in the manufacturing process, the pore sizes vary significantly, with a large difference between the surface pore size and the internal pore size. Typically, the surface pore size is smaller, hindering the transport of active ions and thus impeding fast-charging performance. Conversely, when the surface pore size meets the requirements for active ion transport, the internal pore size of the aramid coating tends to be too large, potentially exacerbating self-discharge in the secondary battery. In contrast, the separator of this application maintains the pore size D50 of the surface pores and the internal pore size D50 within the ratio specified in this application. This ensures greater uniformity of pore size throughout the aramid coating, facilitating active ion transport and thus improving fast-charging performance without aggravating self-discharge.

[0007] In one possible implementation, the ratio of the surface pore diameter D75 to the surface pore diameter D25 of the aramid coating is less than or equal to 2, preferably 1.8, and more preferably 1.5; the ratio of the internal pore diameter D75 to the internal pore diameter D25 of the aramid coating is less than or equal to 2, preferably 1.8, and more preferably 1.5. Here, pore diameter D75 indicates that 75% of the pores are smaller than this diameter, and pore diameter D25 indicates that 25% of the pores are smaller than this diameter. Through extensive experimental statistics, this application has shown that when the ratio of the surface pore diameter D75 to the surface pore diameter D25 is controlled within 2, the pore diameter distribution of the surface pores in the aramid coating becomes more uniform. Similarly, controlling the ratio of the internal pore diameter D75 to the internal pore diameter D25 within 2 results in a higher uniformity of the pore diameter distribution of the internal pores in the aramid coating. This is more conducive to improving the heat resistance and mechanical properties of the diaphragm. The closer the D75 and D25 values ​​are, the better the pore size uniformity throughout the membrane, which can significantly improve the membrane's heat shrinkage resistance and rupture temperature, as well as give the membrane better air permeability and the ability to transport active ions. When used in secondary batteries, this can reduce the self-discharge of the secondary battery and improve its charging performance.

[0008] In one possible implementation, the D50 of both the first and second pore sizes is 50-350 nm. By controlling the D50 of the first and second pore sizes within the range of 50-350 nm, a uniformly distributed nanoscale pore structure can be formed in the aramid coating. This facilitates the passage of ions and avoids stress concentration in the aramid coating, thereby improving the heat shrinkage resistance of the diaphragm without increasing its internal resistance.

[0009] In one possible implementation, the pore density of the aramid coating surface and any cross-section is 8,000,000-1,300,000 pores / mm². 2 This indicates that the pores of the aramid coating can be evenly distributed on the surface and inside of the aramid coating. The more uniform distribution of pores on any cross section of the surface and inside the aramid coating can further improve the high temperature resistance of the diaphragm, reduce the thermal shrinkage rate, and improve the mechanical properties of the diaphragm, as well as increase the diaphragm rupture temperature.

[0010] In one possible implementation, the intrinsic viscosity of the aramid in the aramid coating is 1.1-1.8 dL / g. A higher intrinsic viscosity indicates a larger molecular weight. The molecular weight of the aramid affects its pore-forming effect; a larger molecular weight results in faster phase separation. However, excessively high molecular weight leads to difficulty in pore formation, causing a dense layer to easily form on the surface, reducing the membrane's permeability and affecting internal resistance. Conversely, excessively low molecular weight fails to achieve ideal mechanical strength. Therefore, controlling the intrinsic viscosity of the aramid within the range of 1.1-1.8 dL / g allows the obtained aramid coating to maintain a certain mechanical strength while also possessing pores of a specific size, thus ensuring both good permeability and low internal resistance.

[0011] In one possible implementation, the aramid coating has a thickness of 1-4 μm. By coating with a micron-sized aramid coating, the overall thickness of the membrane can be kept low, avoiding an increase in membrane mass caused by an excessively thick aramid coating.

[0012] In one possible implementation, the aramid coating contains inorganic particles, with the inorganic particles accounting for 50%-90% of the weight of the aramid coating, preferably 50%-85%, and more preferably 50%-80%. Adding inorganic particles increases the density of the aramid coating, thereby further improving the thermal shrinkage performance of the diaphragm and maintaining a high rupture temperature. In another possible implementation, the particle size D50 of the inorganic particles is ≤600nm, preferably ≤500nm, and more preferably 100-400nm. Controlling the particle size D50 of the inorganic particles to within 600nm helps the aramid coating form pores with a diameter less than 1μm. Simultaneously, the smaller particle size helps improve the thermal shrinkage resistance of the diaphragm. Furthermore, adding inorganic particles effectively improves the electrostatic effect in the aramid coating, reduces the volume difference of the aramid coating before and after drying, and improves the edge curling problem of the diaphragm.

[0013] Secondly, this application also provides a method for preparing the above-mentioned diaphragm, the method comprising:

[0014] A base membrane coated with aramid slurry undergoes multi-stage extraction followed by drying to obtain a diaphragm. The multi-stage extraction includes at least a first, second, and third extraction. The mass concentration of the good solvent in the first extraction is 35-58%, in the second extraction it is 20-35%, and in the third extraction it is ≤2.5%. In the first extraction, the aramid slurry is not fully cured. By controlling the mass concentration of the good solvent in the first extraction within the range of 35-55%, a uniform pore structure can be formed in the thickness direction of the aramid coating. In the second extraction, the aramid slurry is fully cured. By controlling the mass concentration of the good solvent in the second extraction within the range of 20-35%, both macropores and a dense coating in the aramid coating can be avoided. By controlling the mass concentration of the good solvent in the third extraction below 2.5%, residual good solvent on the coated diaphragm can be removed.

[0015] In a preferred embodiment, the mass concentration of the good solvent in the first extraction stage is 35-55%, more preferably 35-53%; the mass concentration of the good solvent in the second extraction stage is 20-33%, more preferably 20-30%. By further optimizing the concentrations of the good solvent in the first and second extraction stages, the uniformity of the pore size in the aramid coating can be further improved.

[0016] In one optional implementation, multi-stage extraction includes: placing a base film coated with aramid slurry in a multi-stage extraction tank for extraction; wherein the multi-stage extraction tank includes at least a first extraction tank, a second extraction tank, and a third extraction tank arranged sequentially, and in the multi-stage extraction, the base film coated with aramid slurry is sequentially extracted through the first extraction tank, the second extraction tank, and the third extraction tank. The first extraction tank is also called a coagulation bath, in which the aramid slurry coagulates. Along the direction of movement of the base film within the first extraction tank, the concentration of the good solvent in the first extraction tank gradually decreases, and the difference between the highest and lowest concentration values ​​of the good solvent in the first extraction tank is 5-15 wt%, preferably 6-13 wt%, more preferably 7-12 wt%. Exemplarily, along the direction of movement of the base film within the first extraction tank, the concentration of the good solvent in the first extraction tank exhibits a gradient distribution, and the concentration gradually decreases. By setting a gradually decreasing concentration of a good solvent in the first extraction tank, and controlling the difference between the highest and lowest concentrations of the good solvent in the first extraction tank within the range of 5-15 wt%, the diffusion rates of the good and bad solvents can be kept consistent over time in the thickness direction of the aramid coating. This helps to form pores with high pore size consistency in the aramid coating and makes the pore distribution more uniform. While improving the thermal shrinkage and rupture temperature of the diaphragm, it can significantly improve the gas permeability increment of the diaphragm.

[0017] In one alternative implementation, the extraction time for both the first and second extraction stages is 5-15 seconds.

[0018] In this application, the data in the various possible implementations, such as the first aperture, the second aperture, the thickness of the aramid coating, the particle size of the inorganic particles, and the amount added, should all be understood as being within the range defined in this application, provided that the values ​​are within the engineering measurement error range.

[0019] Thirdly, this application provides a secondary battery, which includes a positive electrode, a negative electrode, and a separator between the positive and negative electrodes, wherein the separator may be the separator of the first aspect of this application.

[0020] The secondary battery provided in this application, because it includes the separator of the first aspect of this application, can achieve better safety when the separator of this application has high heat resistance. At the same time, because the separator of this application has a high air permeability value, the secondary battery of this application can also achieve high fast charging performance.

[0021] Fourthly, this application provides an electrical device that includes the secondary battery described in the third aspect of this application.

[0022] The electrical equipment includes, but is not limited to, electronic devices, electric vehicles, and power storage systems. Based on the good safety and fast-charging performance of the secondary batteries in the various possible embodiments of this application, electrical equipment using the secondary batteries of this application as a driving power source can also achieve the same effects. Attached Figure Description

[0023] Figure 1 This is a schematic cross-sectional view of the diaphragm along the thickness direction according to an embodiment of this application.

[0024] Figure 2 This is a surface SEM image of the diaphragm in Embodiment 1 of this application;

[0025] Figure 3 This is a SEM image of a cross-section of the diaphragm in Example 1 along its thickness direction;

[0026] Figure 4 This is a surface pore size distribution diagram of the diaphragm in Example 1;

[0027] Figure 5 This is a diagram showing the internal pore size distribution at a cross-section of the diaphragm in Example 1;

[0028] Figure 6 This is a test graph showing the membrane rupture temperature of the diaphragm in Example 1.

[0029] Reference numerals: 11-base film; 12-aramid coating; 121-surface pore; 122-internal pore. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the application will now be described in further detail with reference to the accompanying drawings.

[0031] For ease of understanding, the relevant terms used in this application will be explained below.

[0032] Secondary battery: A device that uses the difference in potential between two electrodes to generate a potential difference, thereby allowing electrons to flow and generating an electric current. This device can convert chemical energy into electrical energy.

[0033] Separator: A medium used to separate the positive and negative electrodes in a battery cell, preventing direct contact and short circuits. The basic characteristics of a separator are porosity (providing channels for ion transport) and electronic insulation (preventing leakage).

[0034] Heat shrinkage rate: It represents the rate of dimensional change of the diaphragm in the longitudinal (machine direction, MD, i.e., along the long side of the diaphragm) and transverse (transverse direction, TD, which is perpendicular to MD, i.e., along the short side of the diaphragm) directions before and after heating.

[0035] Membrane rupture temperature: The temperature at which the separator melts to a certain extent and ruptures, causing a partial or complete short circuit in the secondary battery.

[0036] D50: Also known as median aperture, it is the aperture that corresponds to a cumulative aperture distribution percentage of 50%. Its physical meaning is that apertures smaller than it account for 50%, and apertures larger than it also account for 50%.

[0037] D75: The aperture corresponding to a cumulative aperture distribution percentage of 75%. Its physical meaning is that apertures smaller than it account for 75% and apertures larger than it account for 25%.

[0038] D25: The aperture corresponding to a cumulative aperture distribution percentage of 25%. Its physical meaning is that apertures smaller than it account for 25% and apertures larger than it account for 75%.

[0039] Currently, secondary batteries, such as lithium-ion batteries, commonly use polyolefin separators. These separators have a melting point range of only 130–165°C, which is insufficient to guarantee the safe operation of high-power lithium batteries. To improve the safety performance of the separator, aramid or ceramic coatings are typically used to modify the polyolefin separator. Ceramic coating significantly improves the heat shrinkage resistance of the separator at certain temperatures, but at higher temperatures, the ceramic coating loses its mechanical strength and thus its protective ability. Aramid coatings have a high decomposition temperature and can maintain strength at higher temperatures, thus possessing a high burst temperature. However, aramid coatings have poor heat shrinkage resistance, requiring the addition of inorganic particles to increase the coating's density and improve the separator's heat shrinkage resistance. However, adding inorganic particles can easily cause the aramid coating to lose mechanical strength, making it unable to maintain the corresponding burst temperature. Therefore, current single-coating separators cannot achieve a combination of low shrinkage and high burst temperature.

[0040] To address the aforementioned technical problems, this application provides a diaphragm. Figure 1 This is a schematic diagram of a cross-sectional structure of a diaphragm along its thickness direction, as shown below. Figure 1 As shown, the diaphragm includes a base membrane 11 and an aramid coating 12 coated on at least one side of the base membrane 11. Figure 1 In the diaphragm shown, one surface of the base membrane 11 is coated with an aramid coating 12. It is understood that the aramid coating 12 can be disposed on one surface of the base membrane 11 alone, or simultaneously on both surfaces of the base membrane 11. The following refers to... Figure 1 The diaphragm with only one layer of aramid coating 12 is explained, but the diaphragm with aramid coating 12 on both sides of the base film 11 is also within the scope of protection of this application.

[0041] Reference Figure 1The base membrane 11 has a microporous membrane structure, which provides porosity and insulation to facilitate ion transport and maintain insulation between the positive and negative electrodes of the secondary battery. The base membrane 11 can be a polyolefin film with a thickness of 4-20 μm, providing the separator with basic tensile strength and puncture resistance.

[0042] Continue to refer to Figure 1 The aramid coating 12 is a porous coating with a thickness of, for example, 1-4 μm. The pore diameter of the surface pores 121 of the aramid coating 12 is a first pore diameter, and the pore diameter of the internal pores 122 of the aramid coating 12 is a second pore diameter. The surface pores 121 of the aramid coating 12 are pores that can be observed from any surface of the aramid coating. The surface of the aramid coating 12 can be a side surface parallel to the base film 11 and away from the base film 11, or it can be a side surface of the aramid coating 12, i.e., a surface perpendicular to the base film 11. The internal pores 122 of the aramid coating 12 are pores corresponding to any cross-section of the aramid coating 12. Any cross-section of the aramid coating 12 can be any cross-section parallel to the thickness direction of the aramid coating 12, a cross-section perpendicular to the thickness direction of the aramid coating 12, or an inclined cross-section, i.e., a cross-section obtained by cutting the aramid coating 12 in any direction. The aperture of the hole corresponding to this cross section is the aperture of the internal hole 122 of the aramid coating 12.

[0043] In the aramid coating 12 of this application embodiment, the D50 of the first pore size is 0.8-1.5 times, preferably 0.8-1.4 times, and more preferably 0.8-1.3 times, of the D50 of the second pore size. Both the first and second pore sizes are less than 1 μm, preferably less than 800 nm, and more preferably less than 600 nm. For example, both the D50 of the first and second pore sizes are 50-350 nm. The ratio of the D50 of the first and second pore sizes can be, for example, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5. The D50 of the first aperture and the D50 of the second aperture can be, for example, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 120nm, 150nm, 200nm, 220nm, 250nm, 250nm, 300nm, 320nm or 350nm.

[0044] The ratio of the surface pore diameter D50 to the internal pore diameter D50 corresponding to any cross-section is limited to a range of 0.8-1.5. This allows the surface and internal pores of the aramid coating 12 to be closer in size and have a narrower pore size distribution. This creates uniform three-dimensional pores within the aramid coating 12, facilitating smoother ion transport and improving the fast-charging capability of the secondary battery. Pores within the aforementioned range can also form more arched structures within the aramid coating 12, increasing the flexural strength of the separator, thereby improving its thermal shrinkage performance, reducing the thermal shrinkage rate at 150°C, and increasing the membrane rupture temperature.

[0045] In one alternative embodiment, the ratio of the aperture D75 of the surface holes of the aramid coating 12 to its aperture D25 is less than or equal to 2; the ratio of the aperture D75 to the aperture D25 of the internal holes corresponding to any cross-section of the aramid coating 12 is less than or equal to 2. This range of ratios allows for more uniform apertures throughout the aramid coating.

[0046] In one optional embodiment, the intrinsic viscosity of the aramid in the aramid coating 12 is 1.1-1.8 dL / g. Exemplarily, the intrinsic viscosity of the aramid may be, for example, 1.1 dL / g, 1.2 dL / g, 1.3 dL / g, 1.4 dL / g, 1.5 dL / g, 1.6 dL / g, 1.7 dL / g, or 1.8 dL / g. The aramid may be at least one of meta-aramid, para-aramid, or other heterocyclic aramids or modified aramids.

[0047] In one possible implementation, the pore density of the aramid coating surface and any cross-section is 8,000,000-1,300,000 pores / mm². 2 For example, it could be 8,000,000 pieces / mm 2 9,000,000 pieces / mm 2 10,000,000 pieces / mm 2 11,000,000 pieces / mm 2 12,000,000 pieces / mm 2 Or 13,000,000 pieces / mm 2 .

[0048] In an optional embodiment, the aramid coating 12 contains inorganic particles, the inorganic particles accounting for 50%-90% by weight, preferably 70%-90%. Exemplarily, the weight percentage of inorganic particles in the aramid coating 12 can be, for example, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, or 90%. The aramid coating 12, excluding the inorganic particles in the aforementioned weight proportions, consists of aramid fiber. The particle size D50 of the inorganic particles can be less than or equal to 600 nm. The inorganic particles can be, for example, at least one selected from SrTiO3, SnO2, Mg(OH)2, MgO, Al(OH)3, Al2O3, SiO2, BaSO4, or TiO2.

[0049] Adding inorganic particles in the aforementioned weight ratio to the aramid coating 12 can improve its density, thereby enhancing the membrane's heat shrinkage resistance. However, excessively high inorganic particle content can cause the aramid coating 12 to lose mechanical strength, making it unable to maintain the corresponding membrane breakage temperature at high temperatures. Therefore, adding 50%-90% inorganic particles by weight can improve the membrane's mechanical strength while maintaining a high membrane breakage temperature for the aramid coating 12. Furthermore, the particle size of the inorganic particles in the aramid coating 12 cannot be too large. Excessively large inorganic particles are detrimental to reducing heat shrinkage and may reduce the mechanical strength of the aramid coating 12, preventing the achievement of the designed membrane breakage temperature. Therefore, adding 50%-90% inorganic particles by weight to the aramid coating 12, with a particle size D50 ≤ 600 nm, can effectively improve the mechanical strength of the aramid coating 12 while simultaneously enhancing the membrane's heat shrinkage resistance and membrane breakage temperature. For example, the diaphragm in this application embodiment has a heat shrinkage of ≤10% at 150°C, a rupture temperature of ≥200°C, and an air permeability of ≤200s / 100ml.

[0050] The structure and composition of the diaphragm have been introduced above. The specific preparation method of the diaphragm will be explained below.

[0051] The method for preparing the diaphragm provided in this application includes the following steps: a base membrane coated with aramid slurry is subjected to multi-stage extraction, followed by drying, to obtain the diaphragm of this application. The multi-stage extraction can be coagulation bath extraction.

[0052] The aramid slurry may include aramid resin, inorganic particles, and one or more good solvents. In preparing the aramid slurry, the aramid resin and inorganic particles can be simultaneously dissolved in a good solvent and dispersed evenly to obtain the aramid slurry. Alternatively, a solution of the aramid resin and a dispersion of the inorganic particles can be prepared separately, and then the aramid slurry can be mixed to obtain the aramid slurry. The good solvent may be, for example, at least one of N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylformamide (DMF).

[0053] It is understood that aramid slurries may include, in addition to aramid resin and inorganic particles, co-solvents, pore-forming agents, and unsuitable solvents for aramid resin. The co-solvent or pore-forming agent is selected from one or a combination of at least two of lithium chloride, magnesium chloride, calcium chloride, calcium carbonate, and calcium chloride. Unsuitable solvents may be selected from one or a combination of at least another of water, methanol, ethanol, propanol, acetone, ethyl acetate, dichloromethane, and petroleum ether.

[0054] As an example, in one embodiment of this application, the aramid slurry contains 2-10% aramid resin by weight, 0-30% inorganic particles by weight, 0-8% co-solvent by weight, 0-5% poor solvent by weight, and 60-85% good solvent by weight.

[0055] The aramid slurry can be coated on at least one surface of the base film. The coating method can be one of gravure roller coating, wire rod coating, extrusion coating, or dip coating.

[0056] A base film coated with an aramid coating can be sequentially immersed in multi-stage extraction tanks of different concentrations for multi-stage extraction and curing to form an aramid coating with the desired pore size on the base film surface. The multi-stage extraction tanks include at least a first extraction tank, a second extraction tank, and a third extraction tank arranged sequentially. In the multi-stage extraction, the base film coated with the aramid slurry is sequentially extracted and cured in the first, second, and third extraction tanks. The extraction in the first extraction tank is the first stage extraction, the extraction in the second extraction tank is the second stage extraction, and the extraction in the third extraction tank is the third stage extraction. The mass concentration of the good solvent in the first extraction tank is 35-55%, the mass concentration of the good solvent in the second extraction tank is 20-35%, and the mass concentration of the good solvent in the third extraction tank is ≤2.5%. The good solvent can be, for example, at least one of NMP, DMAC, or DMF. The specific selection of the good solvent can be combined with the type of co-solvent or pore-forming agent, and is not specifically limited here. Water can be the unsuitable solvent in the multi-stage extraction tanks. The extraction time for both the first and second stages of extraction can be 5-15 seconds.

[0057] It should be noted that the multi-stage extraction tank used in the above-mentioned multi-stage extraction process includes at least three extraction tanks, namely a first extraction tank, a second extraction tank, and a third extraction tank. In addition to these three extraction tanks, the multi-stage extraction tank may also include other extraction tanks. When other extraction tanks are set, the added extraction tanks can be set between the second and third extraction tanks, as long as the concentration of the good solvent in the last extraction tank, i.e., the third extraction tank, meets the range defined in the embodiments of this application.

[0058] In one embodiment of this application, along the moving direction of the base membrane within the first extraction tank, the concentration of the good solvent in the first extraction tank gradually decreases, and the difference between the highest and lowest concentration values ​​of the good solvent in the first extraction tank is 5-15 wt%. The preparation method of this embodiment, by controlling the concentration gradient of the good solvent in the first extraction tank, can make the pore size of the pores formed in the resulting aramid coating more consistent in the thickness direction of the aramid coating, thereby effectively improving the thermal shrinkage performance and rupture temperature of the diaphragm while achieving a small increase in air permeability.

[0059] It should be noted that the concentration of the good solvent in the first extraction tank can be made to have a stepped distribution by the following method:

[0060] 1) For example, baffles or other components can be installed in the direction of movement of the base film to slow down the diffusion efficiency of undesirable solvents in the extraction tank;

[0061] 2) A small amount of good solvent can be continuously added to the base membrane at the water inlet position in the first extraction tank;

[0062] 3) When replenishing the first extraction tank with a poor solvent, such as water, the amount of poor solvent replenished can be distributed in a gradient.

[0063] It is understood that the above methods are merely illustrative examples, and any method that allows for a gradient change of the good solvent within the first extraction tank should be understood as being within the scope of this application.

[0064] In one embodiment of this application, after multi-stage extraction, drying can be performed using an oven. The oven can be a three-stage oven, with temperatures of 50°C, 55°C, and 60°C for each stage.

[0065] The preparation method of the diaphragm has been explained above. The performance of the diaphragm of this application will be further described in detail below with reference to specific embodiments and comparative examples.

[0066] Example 1

[0067] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0068] Step S11: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0069] Step S12: Add 2.0 kg of alumina with a particle size of 0.2 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0070] Step S13: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0071] Step S14: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film enters a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 40-50%, with the DMAC concentration at the base film entry point being 50% and the DMAC concentration at the exit point being 40%. The DMAC content in the second extraction tank is 30%, and the DMAC content in the third extraction tank is ≤2.5%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0072] Step S15: Then, the membrane is dried using a three-stage drying oven with temperatures of 50℃, 55℃, and 60℃ for each stage. After drying, a 10μm membrane is obtained.

[0073] Small-diameter inorganic particles, which facilitate pore formation, are selected, and the proportion of inorganic particles is increased. Combined with two extraction tanks using high-concentration solvents, the aramid precipitation process achieves uniform pore formation, with similar pore sizes on the surface and inside. In the first extraction tank, with an average DMAC concentration of 45%, the aramid slurry coating is not fully cured, allowing sufficient liquid-liquid phase separation time, which helps to form a uniform gradient of pore sizes along the thickness direction. In the second extraction tank, the aramid slurry coating is fully cured, and the 30% solvent concentration is moderate, preventing the formation of large pores or a dense coating. The uniform pore distribution in the aramid coating avoids the formation of defects and helps maintain high mechanical strength of the aramid coating even with high inorganic particle content, while also improving the film breaking temperature and heat shrinkage resistance.

[0074] Example 2

[0075] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0076] Step S21: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0077] Step S22: Add 2.0 kg of alumina with a particle size of 0.2 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0078] Step S23: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0079] Step S24: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film enters a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 40-50%, with the DMAC concentration at the base film entry point being 50% and the solvent concentration at the exit point being 40%. The DMAC content in the second extraction tank is 20%, and the DMAC content in the third extraction tank is ≤2.5%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0080] Step S25: Then, the membrane is dried using a three-stage drying oven with temperatures of 50°C, 55°C, and 60°C for each stage. After drying, a 10μm membrane is obtained.

[0081] Example 3

[0082] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0083] Step S31: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0084] Step S32: Add 1.6 kg of alumina with a particle size of 0.2 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0085] Step S33: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0086] Step S34: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film enters a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 40-50%, the good solvent concentration at the base film entry point is 50%, and the DMAC concentration at the exit point is 40%. The DMAC content in the second extraction tank is 20%, and the DMAC content in the third extraction tank is ≤2.5%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0087] Step S35: Then, the membrane is dried using a three-stage drying oven with temperatures of 50℃, 55℃, and 60℃ for each stage. After drying, a 10μm membrane is obtained.

[0088] Example 4

[0089] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0090] Step S41: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0091] Step S42: Add 2.0 kg of alumina with a particle size of 0.4 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0092] Step S43: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0093] Step S44: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film enters a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 40-50%, with the DMAC concentration at the base film entry point being 50% and the DMAC concentration at the exit point being 40%. The DMAC content in the second extraction tank is 20%, and the DMAC content in the third extraction tank is ≤2.5%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0094] Step S45: Then, the membrane is dried using a three-stage drying oven with temperatures of 50°C, 55°C, and 60°C for each stage. After drying, a 10μm membrane is obtained.

[0095] Comparative Example 1

[0096] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0097] Step S101: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0098] Step S102: Add 2 kg of alumina with a particle size of 0.2 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0099] Step S103: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0100] Step S104: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film is placed in a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 20%, the DMAC content in the second extraction tank is 5%, and the DMAC content in the third extraction tank is ≤2.5%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0101] Step S105: Then, the membrane is dried using a three-stage drying oven with temperatures of 50℃, 55℃, and 60℃ for each stage. After drying, a 10μm membrane is obtained.

[0102] Comparative Example 2

[0103] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0104] Step S201: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0105] Step S202: Add 2.0 kg of alumina with a particle size of 0.4 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0106] Step S203: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0107] Step S204: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film is placed in a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 60%, the DMAC content in the second extraction tank is 50%, and the DMAC content in the third extraction tank is 40%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0108] Step S205: After extraction and solidification, the membrane is washed with water in the fourth and fifth extraction tanks. The concentration of good solvent in the fourth and fifth extraction tanks is 2%. Then, it is dried using a three-stage oven with temperatures of 50℃, 55℃, and 60℃ for each stage. After drying, a 10μm diaphragm is obtained.

[0109] Comparative Example 3

[0110] This embodiment is a diaphragm, and the preparation process of the diaphragm includes the following steps:

[0111] Step S301: Add 0.4 kg of pore-forming agent calcium chloride to 6 kg of DMAC and stir thoroughly to dissolve. Then add 0.6 kg of meta-aramid resin powder and stir in a constant temperature water bath at 40°C for 1 hour until the aramid is completely dissolved to obtain an aramid solution.

[0112] Step S302: Add 2.0 kg of alumina with a particle size of 0.4 μm to 9 kg of DMAC, and grind and disperse it thoroughly to obtain an alumina dispersion.

[0113] Step S303: Mix the aramid solution and the alumina dispersion to prepare the aramid slurry;

[0114] Step S304: Use a gravure roller to coat the aramid slurry onto both sides of a 7μm PE base film at a coating speed of 30m / min. After coating, the film enters a mixture of DMAC and water for multi-stage extraction and curing. The DMAC content in the first extraction tank is 60%, the DMAC content in the second extraction tank is 20%, and the DMAC content in the third extraction tank is 2%. The time for the diaphragm to pass through each extraction tank is 8 seconds.

[0115] Step S305: Then, the membrane is dried using a three-stage drying oven with temperatures of 50℃, 55℃, and 60℃ for each stage. After drying, a 10μm membrane is obtained.

[0116] The dimensions and performance parameters of the diaphragms in each embodiment and comparative example were tested. Test items included the thickness of the base membrane, the thickness of the aramid coating, the first pore size, the second pore size, the maximum surface pore size, the maximum cross-sectional pore size, heat shrinkage at 150℃, membrane rupture temperature, and air permeability. The specific testing procedures for each item are as follows:

[0117] 1) Measurement of the thickness of the base film and the aramid coating.

[0118] A. Sampling: For diaphragms with a width of <200mm, determine a point every 40mm±5mm along the longitudinal direction, with no less than 10 test points. The number of test points can be determined according to the width of the diaphragm. The measurement starting point should be no less than 20mm from the edge.

[0119] For diaphragms with a width ≥ 200 mm, a point is determined every 80 mm ± 5 mm in the transverse direction, with a minimum of 10 test points. The number of test points can be determined according to the width of the diaphragm. The starting point of the measurement should be at least 20 mm away from the edge.

[0120] B. Testing: Each test point is tested using a thickness gauge at a temperature of 23±2℃. The diameter of the measuring surface is between 2.5mm and 10mm, and the load applied to the sample by the measuring surface should be between 0.5N and 1.0N.

[0121] C. Data processing: Measure the thickness at each test point and take the arithmetic mean.

[0122] 2) Measurement of D50 of the first aperture, D50 of the second aperture, maximum surface aperture, and maximum cross-sectional aperture.

[0123] A. Divide a roll of diaphragm into three equal parts along the TD and MD directions to form a nine-square grid. Randomly select and cut a 1cm*1cm sample from each grid, for a total of 9 samples.

[0124] B. Use SEM to take pictures of the cross-section and surface of each sample, and select the pictures at a magnification of 20K for later use;

[0125] C. Import the SEM images into ImageJ software;

[0126] D. Set the corresponding scale ratio in Image J according to the SEM image scale;

[0127] E. For the cross-section and surface electron microscopy of each sample, a rectangular area of ​​6.0μm*1.5μm was selected, and the pore size was marked in sequence. When marking, all obvious pores were marked as much as possible. Pores smaller than 20nm that could not be effectively identified were not recorded.

[0128] F. Electron microscopy revealed unevenness on the surface of the aramid coating. When multiple pores were superimposed in the same location along the thickness gradient direction at the undulating areas, the outermost pore was selected for recording. For irregular pores, the maximum diameter was used as the recording value. If multiple pores were connected together and the length exceeded 500 nm, they were characterized separately.

[0129] G. The surface and cross-sectional pore diameters of the 9 samples are summed to create a number pore diameter distribution map of the surface and cross-section; D50 is the pore diameter size when the cumulative number of all pore diameters of the 9 samples is 50%, and the D50 of the first pore diameter and the second pore diameter are obtained respectively.

[0130] 3) Heat shrinkage at 150℃

[0131] A. Sampling: Randomly cut 6 samples within the full width of the diaphragm. The specific sampling for each sample may include: cutting 100mm along the MD direction of the diaphragm; when the TD direction of the diaphragm is greater than 100mm, the length of the test sample in the TD direction can be 100mm; when the TD direction of the microporous membrane is less than 100mm, the length of the test sample in the TD direction can be based on the actual length.

[0132] B. Testing: Mark the longitudinal and transverse dimensions of the sample, measure and record the longitudinal and transverse dimensions of each sample; place the sample flat in the paper sleeve layer, ensuring the sample is free from folds, wrinkles, and adhesions; place the paper sleeve containing the sample (e.g., 10 layers) flat in the middle of the constant temperature oven (door opening time not exceeding 3 seconds); heat the sample to 150°C using the electric heating constant temperature oven for 1 hour; remove the sample and cool it to room temperature, then measure the longitudinal and transverse lengths.

[0133] C. Data Processing:

[0134] Calculate the thermal shrinkage rate of each sample:

[0135] T = (L0 - L) / L0 × 100%,

[0136] Where T is the heat shrinkage rate of the sample (%), L0 is the size of the sample before heating (mm), and L is the size of the sample after heating (mm). The arithmetic mean of the heat shrinkage rates of the samples is calculated. The heat shrinkage rates in Table 1 of this application are the average shrinkage rates in both the TD and MD directions.

[0137] 4) Film breaking temperature

[0138] The test was conducted using a TMA device. A diaphragm sample with a length of 8 mm and a width of 4 mm was cut along the TD / MD direction. A constant load of 0.02 N was applied along the length direction, and the temperature was programmed to rise at 5 °C / min. The amount of deformation of the diaphragm was recorded. When the amount of deformation reached its maximum, it meant that the diaphragm had broken. This point was recorded as the rupture point, and the corresponding temperature was the rupture temperature in the TD / MD direction. In this application, the rupture temperature is the rupture temperature in the MD direction.

[0139] 5) Breathability

[0140] The procedure is as follows, according to section 6.5.4 of GB / T 36363-2018: Cut three membrane pieces longitudinally at 150mm intervals from the membrane roll. If the membrane width is ≥100mm, the sample size is 100mm × 100mm; if the membrane width is <100mm, the sample size is 100mm × membrane width. Place the membrane in the test head of an air permeability meter with a suitable test range for air permeability testing, and take the average of the three test results as the air permeability of the membrane.

[0141] The test results for each of the above test items are listed in Table 1. For ease of understanding, please refer to... Figures 2 to 6 Understand the relevant test data. Figure 2 This is a surface SEM image of the diaphragm in Example 1. Figure 3 This is a SEM image of a cross-section along the thickness direction of the diaphragm in Example 1. Figure 4 This is a surface pore size distribution diagram of the diaphragm in Example 1. Figure 5 This is a diagram showing the internal pore size distribution at a cross-section of the diaphragm in Example 1. Figure 6 This is a test graph showing the membrane rupture temperature of the diaphragm in Example 1.

[0142] like Figures 2 to 5 As shown, the diaphragm of Example 1 has surface pores and internal pores with essentially the same pore size, and their median pore size D50 is also essentially the same. This indicates that the diaphragm obtained using the preparation method of this application has a uniform pore size distribution and consistent pore size throughout the aramid coating. Furthermore, from Figure 6 It can be seen that the diaphragm in Example 1 has a high rupture temperature. Specific data can be found in Table 1.

[0143] Table 1

[0144]

[0145] Comparative data from Examples 1 and 2 in Table 1 show that changing the concentration of the good solvent in the second extraction tank increases the pore size of the aramid coating, thus affecting the membrane rupture temperature. Comparative data from Examples 2 and 3 show that changing the proportion of inorganic particles in the aramid coating decreases the membrane's heat shrinkage resistance. Comparative data from Examples 2 and 4 show that increasing the particle size of the inorganic particles also affects the membrane's heat shrinkage resistance.

[0146] As can be seen from the comparative data of Example 1 and Comparative Examples 1-2 in Table 1, when the concentrations of the good solvent in the first, second, and third extraction tanks are all outside the range specified in this application, the performance of the membranes obtained using the extraction processes of Comparative Examples 1 and 2 is significantly lower than that of the membrane in Example 1. In the membrane obtained in Comparative Example 1, the difference between the surface pore size and the internal pore size at the cross-section is large, resulting in a lower membrane rupture temperature and a much higher thermal shrinkage rate at 150°C compared to Example 1.

[0147] In summary, by selecting a suitable multi-stage extraction process, this application can obtain an aramid coating with uniform pore size and distribution. Specifically, during the membrane preparation process, smaller particle size of the inorganic particles in the aramid slurry is more conducive to achieving uniform pore distribution. Furthermore, increasing the inorganic particle content can relatively reduce the aramid content. During the extraction and curing process, the aramid coating exhibits minimal volume change, which helps reduce pore size and contributes to the formation of pores with uniform pore size.

[0148] Furthermore, in aramid slurries, the higher the molecular weight of the aramid, the faster the precipitation rate, which is more conducive to the formation of large pores. If the molecular weight is too low, the necessary mechanical strength cannot be achieved. Therefore, the molecular weight of the aramid must be controlled within a suitable range. When the molecular weight of the aramid is high, an appropriate amount of co-solvent can be added to aid dissolution. During the extraction process, the co-solvent can usually be washed away with water. By selecting a suitable co-solvent or pore-forming agent, and choosing an appropriate concentration of additives, it is beneficial to improve the pore size and uniformity in the aramid coating.

[0149] The diaphragm of this application embodiment can be prepared by selecting appropriate aramid resin and inorganic particles, adjusting the ratio of aramid resin, inorganic particles and additives, and selecting appropriate multi-stage extraction process conditions to prepare an aramid coating with a surface pore size and internal pore size that meets the requirements of this application, thereby achieving the purpose of improving the heat shrinkage resistance and membrane rupture temperature of the diaphragm.

[0150] According to the same inventive purpose, this application also provides a secondary battery, which may include a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the separator is disposed between the positive electrode and the negative electrode, and the electrolyte fills the space between the positive electrode and the negative electrode and wets the separator. The separator may be the separator provided in this application.

[0151] The secondary battery provided in this application, due to the inclusion of the separator of this application, can achieve better safety when the separator of this application has high heat resistance. At the same time, since the separator of this application has a high air permeability value, the secondary battery of this application can also achieve high fast charging performance.

[0152] It is understood that the secondary battery of this application can be a lithium-ion battery, sodium-ion battery, potassium-ion battery, magnesium-ion battery, or calcium-ion battery, etc., and there is no limitation on the specific type of secondary battery, as long as it can be assembled into a secondary battery using the separator of this application. Furthermore, the specific types of positive and negative electrodes can be selected according to the specific type of secondary battery. Taking a lithium-ion battery as an example, the positive electrode active material in the positive electrode includes, but is not limited to, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide; the negative electrode active material in the negative electrode includes, but is not limited to, graphite, activated carbon, or graphene. The specific selection can be based on the performance of the secondary battery.

[0153] For the same inventive purpose, this application also provides an electrical device that includes the secondary battery of this application. The secondary battery provides power to the electrical device to drive its normal operation.

[0154] The electrical equipment includes, but is not limited to, electronic devices, electric vehicles, and power storage systems. Based on the good safety and fast-charging performance of the secondary batteries in the various possible embodiments of this application, electrical equipment using the secondary batteries of this application as a driving power source can also achieve the same effects.

[0155] The terminology used in the above embodiments is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. As used in the specification and appended claims of this application, the singular expressions “a,” “an,” “the,” “the,” “the,” and “this” are intended to also include expressions such as “one or more,” unless the context clearly indicates otherwise.

[0156] References to "one embodiment" or "some embodiments" as described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "in one embodiment," "in some embodiments," "in other embodiments," "in still other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized. The terms "comprising," "including," "having," and variations thereof mean "including but not limited to," unless otherwise specifically emphasized.

[0157] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A diaphragm, characterized in that, The coating includes a base film and an aramid coating applied to at least one side of the base film. The aramid coating is a porous coating. The pore diameter of the surface pores of the aramid coating is a first pore diameter, and the pore diameter of the internal pores of the aramid coating is a second pore diameter. The D50 of the first pore diameter is 0.8-1.5 times the D50 of the second pore diameter, and both the first pore diameter and the second pore diameter are less than 1 μm. The aramid coating contains inorganic particles, and the inorganic particles account for 50%-90% of the weight of the aramid coating. The pore density of the aramid coating on its surface and at any cross-section is 8,000,000-1,3,000,000 pores / mm². 2 ; The ratio of D75 to D25 of the first aperture is less than or equal to 2; the ratio of D75 to D25 of the second aperture is less than or equal to 2.

2. The diaphragm according to claim 1, characterized in that, The D50 of both the first aperture and the second aperture is 50-350 nm.

3. The diaphragm according to claim 1 or 2, characterized in that, The thickness of the aramid coating is 1-4 μm.

4. The diaphragm according to claim 1 or 2, characterized in that, The particle size D50 of the inorganic particles is ≤600nm.

5. A method for preparing a diaphragm as described in any one of claims 1-4, characterized in that, include: The base membrane coated with aramid slurry is subjected to multi-stage extraction and then dried to obtain the diaphragm; wherein the multi-stage extraction includes at least a first stage extraction, a second stage extraction and a third stage extraction, the mass concentration of the good solvent in the first stage extraction is 35-58%, the mass concentration of the good solvent in the second stage extraction is 20-35%, and the mass concentration of the good solvent in the third stage extraction is ≤2.5%.

6. The preparation method according to claim 5, characterized in that, The multi-stage extraction includes: placing a base film coated with aramid slurry in a multi-stage extraction tank for extraction; wherein... The multi-stage extraction tank includes at least a first extraction tank, a second extraction tank, and a third extraction tank arranged in sequence. In the multi-stage extraction, the base film coated with aramid slurry is extracted sequentially through the first extraction tank, the second extraction tank, and the third extraction tank. Along the direction of movement of the base film in the first extraction tank, the mass concentration of the good solvent in the first extraction tank gradually decreases, and the difference between the highest and lowest concentration values ​​of the good solvent in the first extraction tank is 5-15 wt%.

7. The preparation method according to claim 5 or 6, characterized in that, The extraction time for the first extraction stage and the extraction time for the second extraction stage are both 5-15 seconds.

8. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator as described in any one of claims 1-4 disposed between the positive electrode and the negative electrode.

9. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 8.

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