Battery separator, method for manufacturing the same, and secondary battery

Abstract

This application provides a battery separator, a method for preparing the same, and a secondary battery. The battery separator includes a substrate layer, an aramid coating, and a ceramic coating. Along the thickness direction of the battery separator, the aramid coating is located between the substrate layer and the ceramic coating. The aramid coating includes aramid fibers and first ceramic particles. The ceramic coating includes second ceramic particles. The first and second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide, or boehmite. The median particle size of the first ceramic particles is D1 nm, and the specific surface area of ​​the first ceramic particles is S1 m². 2 / g, the median particle size of the second ceramic particles is D2 nm, and the specific surface area of ​​the second ceramic particles is S2 m. 2 / g, satisfying: 0.8≤K≤3.0. The battery separator of this application has a lower thermal shrinkage rate, good flexibility, and a high membrane rupture temperature.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a battery separator, its preparation method, and a secondary battery. Background Technology

[0002] With the rapid development of electric vehicles, large-scale energy storage, and other fields, the market has placed unprecedented demands on the energy density, power output, and safety and reliability of secondary batteries (such as lithium-ion batteries). As one of the key internal components of a battery, the performance of the separator directly affects the overall performance of the battery. An ideal separator needs to have good ion conductivity (high porosity, suitable air permeability) at room temperature, while effectively closing pores to cut off current when the battery experiences abnormal temperature rise, and possessing excellent thermal dimensional stability (low thermal shrinkage rate) and mechanical strength to prevent thermal runaway caused by contact between the positive and negative electrodes.

[0003] Currently, commercially available lithium-ion battery separator substrates primarily use polyolefin microporous membranes such as polyethylene (PE) or polypropylene (PP). While these materials offer good electrochemical stability and cost advantages, their melting points are generally low (e.g., PE's melting point is approximately 135°C), making them prone to melting and severe shrinkage at high temperatures, posing a major risk of internal short circuits in batteries. To compensate for the thermal defects of polyolefin separators, surface coating modification techniques can be employed. However, traditional ceramic coatings are brittle, affecting flexibility and causing the coating to crack and peel off easily during battery winding or bending. This not only weakens the thermal protection effect but may also introduce particulate impurities, further exacerbating the short circuit risk. Furthermore, other existing coated separators are still prone to cracking at high temperatures, with relatively low rupture temperatures.

[0004] Therefore, developing a technical solution that can enable battery separators to have both lower thermal shrinkage rate, good flexibility and high rupture temperature in order to improve the overall performance of separators has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this application is to provide a battery separator, its preparation method, and a secondary battery, to reduce the thermal shrinkage rate of the battery separator, increase its rupture temperature and flexibility, thereby improving the safety performance of the secondary battery. The specific technical solution is as follows:

[0006] A first aspect of this application provides a battery separator, comprising a substrate layer and an aramid coating and a ceramic coating disposed on at least one surface of the substrate layer, wherein the aramid coating is located between the substrate layer and the ceramic coating along the thickness direction of the battery separator; the aramid coating comprises aramid fibers and first ceramic particles; the ceramic coating comprises second ceramic particles, wherein the first ceramic particles and the second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide, or boehmite; the median particle size of the first ceramic particles is D1 nm, and the specific surface area of ​​the first ceramic particles is S1 m². 2 / g, the median particle size of the second ceramic particles is D2 nm, and the specific surface area of ​​the second ceramic particles is S2 m. 2 / g, satisfying: , 0.8≤K≤3.0.

[0007] In one embodiment of this application, 30≤D1≤500, 250≤D2≤550, 5≤S1≤120, and 5≤S2≤20.

[0008] In one embodiment of this application, the areal density of the ceramic coating is m1 g / m 2 The areal density of the aramid coating is m² g / m². 2 The ceramic coating has a thickness of μ1 μm, and the aramid coating has a thickness of μ2 μm, satisfying the following: , 1≤η≤4.

[0009] In one embodiment of this application, 0.8≤m1 / μ1≤3, 0.37≤m2 / μ2≤2.5, 0.5≤μ1≤4, and 0.5≤μ2≤4.

[0010] In one embodiment of this application, the areal density of the substrate layer is m³ g / m³. 2 The thickness of the substrate layer is μ3 μm, 0.2≤m3 / μ3≤1, 3≤μ3≤16.

[0011] In one embodiment of this application, the aramid fiber is selected from at least one of para-aramid fiber, meta-aramid fiber or bio-based modified aramid fiber, the glass transition temperature Tg1 of the aramid fiber is ≥180℃, and the thermal decomposition temperature Td of the aramid fiber is ≥400℃.

[0012] In one embodiment of this application, based on the mass of the aramid coating, the mass percentage of aramid fiber is 2% to 80%, and the mass percentage of the first ceramic particles is 20% to 98%.

[0013] In one embodiment of this application, the ceramic coating further includes a first binder and a second binder, wherein the glass transition temperature Tg2 of the first binder is ≤50℃ and the glass transition temperature Tg3 of the second binder is ≥160℃.

[0014] In one embodiment of this application, the first adhesive is selected from polyacrylate adhesives, and the second adhesive is selected from at least one of polyacrylic acid adhesives or polyacrylamide adhesives; the polyacrylamide adhesive is selected from at least one of anionic polyacrylamide adhesives, cationic polyacrylamide adhesives, and nonionic polyacrylamide adhesives, and the polyacrylate adhesive is selected from at least one of waterborne styrene-acrylic emulsion polyacrylate adhesives, waterborne pure acrylic emulsion polyacrylate adhesives, and waterborne silicone-acrylic emulsion polyacrylate adhesives.

[0015] In one embodiment of this application, based on the quality of the ceramic coating, the mass percentage of the second ceramic particles is 88% to 98%, the mass percentage of the first binder is 0.5% to 5%, and the mass percentage of the second binder is 1% to 5%.

[0016] The second aspect of this application provides a method for preparing the battery separator of the first aspect of this application, which includes the following steps: providing an aramid raw solution, the aramid raw solution comprising aramid fibers; mixing and dispersing the aramid raw solution and first ceramic particles uniformly to obtain an aramid coating slurry with a solid content of 1.5 wt% to 45 wt%; providing a ceramic coating slurry, the ceramic coating slurry comprising second ceramic particles, the ceramic coating slurry having a solid content of 15 wt% to 50 wt%; coating the aramid coating slurry onto at least one surface of a substrate layer to form an aramid coating; then coating the ceramic coating slurry onto the surface of the aramid coating away from the substrate layer, and drying to obtain a battery separator.

[0017] In one embodiment of this application, the aramid solution further includes an auxiliary agent and an organic solvent. The auxiliary agent is selected from at least one of calcium chloride, potassium hydroxide, lithium chloride, or pyridine, and the organic solvent is selected from at least one of N-methylpyrrolidone or dimethylacetamide. Based on the mass of the aramid solution, the mass percentage of aramid fiber is 0.5% to 9%, the mass percentage of the auxiliary agent is 0.5% to 8%, and the mass percentage of the organic solvent is 83% to 99%.

[0018] In one embodiment of this application, the solvent of the aramid raw material is an organic solvent selected from at least one of N-methylpyrrolidone or dimethylacetamide. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is subjected to a coagulation bath, water washing, and drying to form an aramid coating.

[0019] In one embodiment of this application, the solvent of the aramid raw material is water. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is dried to form an aramid coating.

[0020] The third aspect of this application provides a secondary battery, which includes the battery separator provided in the first aspect of this application, or the battery separator prepared according to the preparation method provided in the second aspect of this application.

[0021] The beneficial effects of this application are:

[0022] This application provides a battery separator, a method for preparing the same, and a secondary battery. The battery separator includes a substrate layer and an aramid coating and a ceramic coating disposed on at least one surface of the substrate layer. Along the thickness direction of the battery separator, the aramid coating is located between the substrate layer and the ceramic coating. The aramid coating includes aramid fibers and first ceramic particles. The ceramic coating includes second ceramic particles. The first and second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide, or boehmite. The median particle size of the first ceramic particles is D1 nm, and the specific surface area of ​​the first ceramic particles is S1 m². 2 / g, the median particle size of the second ceramic particles is D2 nm, and the specific surface area of ​​the second ceramic particles is S2 m. 2 / g, satisfying: 0.8≤K≤3.0. This application designs the structure of the battery separator so that the K value satisfies the above relationship. The first ceramic particles and the second ceramic particles both have suitable median particle size and specific surface area, and they are matched with each other. The hard ceramic coating can act as a "skeleton" to resist heat shrinkage, while the flexible aramid coating can act as a "buffer layer" and "reinforcing mesh" to resist impact and tearing. The two work together to make the battery separator have a lower heat shrinkage rate, good flexibility, and high membrane rupture temperature, as well as good thermal stability and mechanical reliability.

[0023] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these accompanying drawings.

[0025] Figure 1 This is a schematic diagram of the battery separator along the thickness direction according to one embodiment of this application;

[0026] Figure 2 This is a scanning electron microscope image along the thickness direction of a battery separator according to one embodiment of this application.

[0027] Reference numerals: 10 battery separator, 11 ceramic coating, 12 aramid coating, 13 substrate layer. Detailed Implementation

[0028] The technical solutions of this application will be clearly and completely described below with reference to the embodiments and accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0029] A first aspect of this application provides a battery separator, comprising a substrate layer and an aramid coating and a ceramic coating disposed on at least one surface of the substrate layer, wherein the aramid coating is located between the substrate layer and the ceramic coating along the thickness direction of the battery separator; the aramid coating comprises aramid fibers and first ceramic particles; the ceramic coating comprises second ceramic particles, wherein the first ceramic particles and the second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide, or boehmite; the median particle size of the first ceramic particles is D1 nm, and the specific surface area of ​​the first ceramic particles is S1 m². 2 / g, the median particle size of the second ceramic particles is D2 nm, and the specific surface area of ​​the second ceramic particles is S2 m. 2 / g, satisfying: 0.8 ≤ K ≤ 3.0. For example, the value of K can be 0.8, 1.1, 1.4, 1.7, 2.0, 2.2, 2.5, 2.8, 3.0 or a range of any two of these values.

[0030] The inventors of this application have found that, in order to compensate for the thermal defects of polyolefin separators, surface coating modification technology can be used. It is mainly divided into the following two technical routes, but each has its own shortcomings: (1) Single ceramic coating separator: The ceramic coating is brittle, and the coating composed of ceramic particles and polymer binder has poor flexibility. It is easy to crack, pulverize and fall off when the battery is wound or subjected to force. It is also easy to crack and break after high temperature heat treatment. At the same time, the ceramic coating has insufficient bonding force with the inert polyolefin base film, and the interface bonding is weak, which affects the long-term reliability; (2) Single aramid or heat-resistant polymer coating separator: Although aramid polymers do not melt at high temperatures, their linear molecular structure will still produce a certain degree of orientation relaxation when heated, which makes it difficult for the overall thermal shrinkage rate of the separator to meet the extremely high requirements.

[0031] This application designs the battery separator structure as a composite structure of a substrate layer, an aramid coating, and a ceramic coating. The median particle sizes D1 and D2 of the first ceramic particles and the specific surface areas S1 and S2 of the second ceramic particles are controlled to ensure that the value of K satisfies 0.8 ≤ K ≤ 3.0. The substrate layer provides basic mechanical strength and electronic insulation. The aramid coating exhibits excellent heat resistance, electrolyte wettability, and intrinsic flame retardancy. It can form a flexible network structure that combines with the substrate layer through physical forces, enhancing the mechanical strength of the substrate layer and increasing the membrane breakage temperature. The ceramic coating improves thermal stability and reduces thermal shrinkage. The rigid ceramic coating acts as a "skeleton" to resist heat shrinkage, while the flexible aramid coating serves as a "buffer layer" and "reinforcing mesh" to resist impact and tearing. The coatings work together, and the "flexible network + particle support" formed by the aramid coating is precisely connected to the substrate layer and the pores of the ceramic coating. This can also buffer interfacial stress, reduce the risk of separation between layers and coating cracking, and enable the ceramic coating to form a continuous heat-resistant barrier. This improves the flexibility and rupture temperature of the battery separator, reduces the heat shrinkage rate, and allows the battery separator to simultaneously ensure thermal stability and mechanical reliability.

[0032] Simultaneously, by adjusting the median particle size D1 and D2, and the specific surface area S1 and S2 of the first and second ceramic particles respectively, the value of K is made to satisfy 0.8≤K≤3.0. Both the first and second ceramic particles have suitable median particle size and specific surface area, and they are matched with each other. The first ceramic particles can be uniformly dispersed in the network formed by aramid fibers, forming an interlocking structure of "aramid fiber-first ceramic particle". This ensures the flexibility of the aramid coating and enhances its supporting role. It also makes the second ceramic particles densely packed, forming a continuous and dense heat-resistant barrier with a porosity of 35%-45%, and the pores are mainly interconnected pores. At the same time, due to the matching of the size and activity of the ceramic particles at the interface between the aramid coating and the ceramic coating, the aramid coating and the ceramic coating can form a "gradual transition" with strong bonding force, peel force ≥5N / 25mm, and no rigid abrupt change. This results in the battery separator having a lower thermal shrinkage rate, good flexibility, and a higher membrane breaking temperature. Specifically, it exhibits good thermal stability, with a heat shrinkage rate of ≤3% at 280℃, a film breaking temperature of ≥250℃, no cracking at high temperatures, and no breakage after stretching; it also demonstrates good mechanical strength and flexibility, with a puncture strength of ≥280gf, improved tear resistance, and no coating peeling during winding. Furthermore, it features smooth ion transport, an air permeability of 218-350s / 100cc, sufficient electrolyte wetting, and low ion transport resistance.

[0033] When K < 0.8, the first ceramic particles have a larger particle size and lower activity, while the second ceramic particles have a smaller particle size and higher activity. The first ceramic particles in the aramid coating have a problem with an excessively large median particle size D1 or an excessively high specific surface area S1, making it difficult to uniformly embed into the aramid fiber network. This easily leads to localized agglomeration, disrupting the continuity of the flexible network and causing cracks in the coating. Consequently, the mechanical strength and flexibility of the battery separator deteriorate, and the puncture strength decreases, for example, to < 250 gf. Simultaneously, the second ceramic particles in the ceramic coating have a small median particle size D2 and a small surface area S2. These fine particles easily agglomerate to form "secondary particles," which, while achieving dense packing, cause pore closure and blockage. The ion transport channels and the air permeability of the battery separator >280s / 100cc lead to a decrease in the charging and discharging efficiency of the secondary battery. It also causes uneven heat conduction in the battery separator, resulting in a thermal shrinkage rate of >5% at 280℃. Aggregates in the ceramic coating can also become stress concentration points, making the battery separator brittle and reducing its flexibility. In addition, the large size difference between the ceramic particles in the aramid coating and the ceramic coating, for example, when the median particle size of the first ceramic particle in the aramid coating is 600nm and the median particle size of the second ceramic particle in the ceramic coating is 50nm, will form a "rigidity abrupt change". This makes the battery separator prone to peeling at the interface when winding or puncturing, affecting the performance of the battery separator.

[0034] When K > 3.0, the median particle size D1 or specific surface area S1 of the first ceramic particles in the aramid coating is small, meaning the first ceramic particles are ultrafine particles. They are prone to agglomeration and blockage of the aramid network pores, leading to densification of the aramid coating and resulting in an air permeability > 300s / 100cc, which affects the cycle performance of the secondary battery. At the same time, the median particle size D2 or specific surface area S2 of the second ceramic particles in the ceramic coating is large, meaning the second ceramic particles are coarser and loosely packed, resulting in a porosity of > 50% for the ceramic coating. This makes it easy for heat to penetrate the ceramic coating, failing to form an effective heat-resistant barrier to reduce thermal shrinkage. The thermal shrinkage rate in the MD direction at 200℃ is > 30%. In addition, the excessive porosity of the ceramic coating leads to excessively low air permeability (e.g., < 180s / 100cc). The mismatch in porosity between coatings and the imbalance in air permeability affect the cycle performance of the secondary battery. Furthermore, the ceramic coating has fewer bonding points between the second ceramic particles and the binder, making the ceramic coating prone to powdering and detachment, reducing the pass rate of the puncture test and affecting the flexibility of the battery separator. Furthermore, due to the dense aramid coating and the loose ceramic coating, uneven stress distribution is likely to occur at the interface between the two, and delamination is likely to occur at high temperatures due to differences in thermal expansion, which affects the performance of the battery separator.

[0035] Therefore, this application combines the macroscopic "ceramic layer + aramid layer" laminated structure with the potential synergistic effect of the two at the microscopic level, overcoming the inherent defects of single-coated separators, including the high thermal shrinkage rate of the substrate layer at high temperatures, the brittleness and easy breakage of the ceramic coating leading to structural failure, the low membrane breakage temperature of existing coated separators, and the problem that single-coated separators cannot simultaneously meet the requirements of thermal stability, mechanical strength and flexibility, resulting in insufficient comprehensive performance. This leads to a battery separator that simultaneously possesses good flexibility, high membrane breakage temperature and low thermal shrinkage rate, enabling the battery separator to simultaneously take into account thermal stability and mechanical reliability, and can improve the safety performance of secondary batteries when used in batteries.

[0036] For ease of understanding, in this application, the thickness direction of the battery separator is defined as Y. It should be understood that the above definition of direction is for the purpose of conveniently describing this application. Figure 1 As shown, the battery separator 10 includes a substrate layer 13 and an aramid coating 12 and a ceramic coating 11 disposed on one surface of the substrate layer. Along the thickness Y direction of the battery separator, the aramid coating 12 is located between the substrate layer 13 and the ceramic coating 11.

[0037] from Figure 2 As can be seen, the battery separator includes a substrate layer 13 and an aramid coating 12 and a ceramic coating 11 disposed on one surface of the substrate layer 13. Along the thickness direction of the battery separator, the aramid coating 12 is located between the substrate layer 13 and the ceramic coating 11, and the ceramic coating 11 is disposed on the other surface of the substrate layer 13.

[0038] In one embodiment of this application, 30≤D1≤500, 250≤D2≤550, 5≤S1≤120, and 5≤S2≤20. For example, the value of D1 can be 30, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, or a range of any two of these values; the value of D2 can be 250, 300, 350, 400, 450, 500, 550, or a range of any two of these values; the value of S1 can be 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 120, or a range of any two of these values; and the value of S2 can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 18, 20, or a range of any two of these values. By adjusting D1 and S1 within the aforementioned range, it is beneficial to uniformly disperse the first ceramic particles in the aramid fiber network, forming an interlocking structure of "aramid fiber-first ceramic particle," which ensures both the flexibility of the aramid coating and enhances its supporting function. By adjusting D2 and S2 within the aforementioned range, it is beneficial to form a dense packing of the second ceramic particles, thereby forming a continuous and dense heat-resistant barrier. Furthermore, due to the size and activity matching of the ceramic particles at the interface between the aramid coating and the ceramic coating, the aramid coating and the ceramic coating can form a "gradual transition," resulting in strong adhesion, a peel force ≥5N / 25mm, and no abrupt changes in rigidity. This, in turn, helps the battery separator to possess both a lower thermal shrinkage rate and good flexibility, as well as a high membrane rupture temperature.

[0039] In one embodiment of this application, the areal density of the ceramic coating is m1 g / m 2 The areal density of the aramid coating is m2g / m 2 The ceramic coating has a thickness of μ1 μm, and the aramid coating has a thickness of μ2 μm, satisfying the following: The value of η is 1 ≤ η ≤ 4. For example, the value of η can be 1, 1.5, 2, 2.5, 3, 3.5, 4, or any range of two of these values. This application controls the thickness and areal density of the aramid coating and the ceramic coating to ensure that η satisfies 1 ≤ η ≤ 4. The aramid coating is densely packed and not loose, which can embed the first ceramic particles to form a support network of "aramid fiber - first ceramic particle" and provide channels for ion conduction. The ceramic coating is densely packed and not closed, forming a continuous heat-resistant barrier, and the pores are connected to the aramid coating, so as not to block the ion transport path. The aramid coating and the ceramic coating form a structural synergy effect and their interfaces are compatible. The porous structure of the aramid coating can accommodate the slight shrinkage of the ceramic coating. The dense structure of the ceramic coating provides rigid support for the aramid coating, which helps reduce the risk of delamination and cracking of the battery separator. Within this range, the battery separator has excellent comprehensive performance, which can simultaneously improve its thermal shrinkage rate, flexibility, membrane breakage temperature and ion conductivity. The battery separator has a thermal shrinkage rate of ≤3% at 280℃, a membrane breakage temperature of ≥260℃, no cracking or breakage at high temperature, a puncture strength of ≥300gf, a tear resistance improvement of 30%, no coating peeling during winding or stacking, and an air permeability of 210s / 100cc to 240s / 100cc.

[0040] When η < 1, the ceramic coating is not dense enough to form an effective heat-resistant barrier. Simultaneously, the aramid coating is too dense, resulting in insufficient porosity and hindering ion conduction, leading to increased resistance. This causes the ceramic coating's thermal shrinkage inhibition to fail, and the battery separator's thermal shrinkage rate exceeds 10% above 200℃. Furthermore, impaired ion conduction and excessive air permeability cause a decrease in the secondary battery's charge / discharge rate. When η > 4, the ceramic coating is too dense, increasing the number of closed pores and hindering ion conduction in the battery separator. Air permeability exceeds 320s / 100cc, increasing secondary battery polarization and shortening cycle life. The excessive density of the ceramic coating also causes localized heat accumulation, easily leading to "hot spots" at high temperatures, causing localized fracture of the battery separator and reducing thermal stability. Simultaneously, the aramid coating is too loose, failing to provide effective support, resulting in poor rigidity, high shrinkage upon heating, and low rupture temperature. The looseness of the aramid coating also reduces its overall toughness, making it prone to fracture during puncture.

[0041] In one embodiment of this application, 0.8≤m1 / μ1≤3, 0.37≤m2 / μ2≤2.5, 0.5≤μ1≤4, and 0.5≤μ2≤4. For example, the value of m1 / μ1 can be 0.8, 1.0, 1.2, 1.4, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.3, 2.6, 3, or a range of any two values; the value of m2 / μ2 can be 0.37, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.3, 2.5, or a range of any two values; the value of μ1 can be 0.5, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.5, 3, 3.5, 4, or a range of any two values; the value of μ2 can be 0.5, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.5, 4, or a range of any two values. By controlling the areal density and thickness of the aramid coating and the ceramic coating within the above range, the aramid coating can form a "flexible network + particle support", and the ceramic coating is dense and non-closed, which can form a continuous heat-resistant barrier. Furthermore, the two form a structural synergistic effect. The porous structure of the aramid coating can accommodate the slight shrinkage / expansion of the ceramic coating, and the dense structure of the ceramic coating provides rigid support for the aramid coating. This helps to reduce the risk of battery separator delamination and cracking, thereby enabling the battery separator to have a lower thermal shrinkage rate, good flexibility, and a high membrane rupture temperature.

[0042] In one embodiment of this application, the areal density of the substrate layer is m³ g / m³. 2 The thickness of the substrate layer is μ3 μm, with 0.2 ≤ m3 / μ3 ≤ 1 and 3 ≤ μ3 ≤ 16. For example, the value of m3 / μ3 can be 0.2, 0.3, 0.45, 0.50, 0.55, 0.60, 0.65, 0.7, 0.8, 0.9, 1, or any two of these values; the value of μ3 can be 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or any two of these values. By controlling the areal density and thickness of the substrate layer within the above ranges, the substrate layer can provide unobstructed initial ion channels, providing support to stabilize the battery separator and prevent it from becoming loose. This results in a lower thermal shrinkage rate, better flexibility, and a higher membrane rupture temperature for the battery separator.

[0043] In this application, the substrate layer is a polyolefin substrate layer. This application does not specifically limit the type of polyolefin, as long as it achieves the purpose of this application. In one embodiment, the polyolefin is selected from at least one of polyethylene, polypropylene, polybutene, and poly4-methylpentene. The weight-average molecular weight of the polyolefin is 100,000 to 7,000,000. For example, the weight-average molecular weight of the polyolefin can be 100,000, 300,000, 500,000, 700,000, 1,000,000, 1,200,000, 1,500,000, 1,700,000, 2,000,000, 2,200,000, 2,400,000, 2,600,000, 2,800,000, 3,000,000, 3,200,000, 3,400,000, 3,600,000, 3,800,000, 4,000,000, 4,200,000, 4,400,000, 4,600,000, 4,800,000, 5,500,000, 6,000,000, 6,500,000, 7,000,000, or a range consisting of any two of these values.

[0044] In one embodiment of this application, the aramid fiber is selected from at least one of para-aramid fiber, meta-aramid fiber, or bio-based modified aramid fiber. The glass transition temperature (Tg1) of the aramid fiber is ≥180℃, and the thermal decomposition temperature (Td) of the aramid fiber is ≥400℃. The para-aramid fiber is selected from at least one of aramid type I, aramid type II, or aramid type III. In this application, the glass transition temperature (Tg1) of the aramid fiber is the critical temperature at which its molecular chains transition from a "rigid solid state" to a "flexible relaxed state." When the ambient temperature is <Tg1, the aramid fiber molecular chains are tightly arranged and structurally stable, maintaining a flexible network morphology and providing stable support for the ceramic coating while buffering interfacial stress. When the ambient temperature is ≥Tg1, the aramid fiber molecular chains relax and curl, the flexible network collapses, and it cannot support the ceramic coating. The ceramic coating is prone to cracking and peeling, ultimately leading to film rupture. Therefore, the glass transition temperature (Tg1) of aramid fibers determines the "upper limit of heat resistance" of the battery separator. The higher the Tg1, the wider the temperature range at which the aramid fiber molecular chains are stable, the higher the temperature the battery separator can withstand, and the higher the membrane breakage temperature. Therefore, this application controls the glass transition temperature (Tg1) of the aramid fibers within the aforementioned range. Even if the ambient temperature reaches 180°C, the aramid fibers remain in a rigid solid state, the interface structure between the ceramic coating and the substrate layer remains intact, and the battery separator will not break. Breakage may only occur when the ambient temperature approaches or exceeds Tg1 and persists for a certain period, thereby increasing the membrane breakage temperature. Furthermore, in this application, the thermal decomposition temperature of the aramid fibers meets the aforementioned range, which helps the aramid coating maintain structural stability and integrity at high temperatures, maintain good ion conductivity, and improve the thermal safety performance of the battery separator. This results in a battery separator that combines lower thermal shrinkage, good flexibility, and a high membrane breakage temperature.

[0045] In one embodiment of this application, based on the mass of the aramid coating, the mass percentage of aramid fiber is 2% to 80%, and the mass percentage of the first ceramic particles is 20% to 98%. For example, the mass percentage of aramid fiber can be 2%, 10%, 20%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or a range consisting of any two of these values; the mass percentage of the first ceramic particles can be 20%, 30%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, 93%, 98%, or a range consisting of any two of these values. This application controls the mass percentage of aramid fiber and first ceramic particles within the above-mentioned range. The aramid fiber can form a flexible network structure, and the first ceramic particles can fill the network structure to optimize the pore structure. Through the synergistic effect of the two, the mechanical strength and membrane breaking temperature of the coating are balanced, and the risk of ion transport channels being blocked due to excessive aramid coating is reduced. This is beneficial for the battery separator to have both lower thermal shrinkage rate, good flexibility and high membrane breaking temperature.

[0046] In one embodiment of this application, the aramid coating may further include other substances, including dispersants, binders, thickeners, and wetting agents. The mass percentage of these other substances can range from 0% to 5%, depending on the mass of the aramid coating. The dispersant may be acrylic-modified polyurethane, the binder may be polyacrylic acid, the thickener may be sodium carboxymethyl cellulose, and the wetting agent may be silicone.

[0047] In one embodiment of this application, the ceramic coating further includes a first binder and a second binder. The glass transition temperature (Tg2) of the first binder is ≤50℃, and the glass transition temperature (Tg3) of the second binder is ≥160℃. By controlling the glass transition temperatures of the first and second binders within the above-mentioned ranges, this application achieves a relatively lower glass transition temperature for the first binder, which is beneficial for improving the peel strength and elasticity of the ceramic coating and reducing shrinkage stress at high temperatures. Conversely, the relatively higher glass transition temperature of the second binder is beneficial for improving the heat resistance of the battery separator. This results in a battery separator that combines lower thermal shrinkage, good flexibility, and a high rupture temperature.

[0048] In one embodiment of this application, the first binder is selected from polyacrylate binders, and the second binder is selected from at least one of polyacrylic acid binders or polyacrylamide binders; the polyacrylamide binder is selected from at least one of anionic polyacrylamide adhesives, cationic polyacrylamide adhesives, and nonionic polyacrylamide adhesives, and the polyacrylate binder is selected from at least one of waterborne styrene-acrylic emulsion polyacrylate adhesives, waterborne pure acrylic emulsion polyacrylate adhesives, and waterborne silicone-acrylic emulsion polyacrylate adhesives. The selection of the first and second binders from the above-mentioned substances is beneficial for the battery separator to possess both lower thermal shrinkage, good flexibility, and high membrane rupture temperature. In this application, there is no particular limitation on the manufacturer or brand of the first and second binders, as long as the purpose of this application can be achieved. For example, the glass transition temperature Tg2 of the first binder and the glass transition temperature Tg3 of the second binder are within the range of this application.

[0049] In one embodiment of this application, based on the quality of the ceramic coating, the mass percentage of the second ceramic particles is 88% to 98%, the mass percentage of the first binder is 0.5% to 5%, and the mass percentage of the second binder is 1% to 5%. For example, the mass percentage of the second ceramic particles can be 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or any range of two such values; the mass percentage of the first binder can be 0.5%, 1%, 2%, 3%, 4%, 5%, or any range of two such values; and the mass percentage of the second binder can be 1%, 2%, 3%, 4%, 5%, or any range of two such values. This application, by controlling the mass percentage content of the second ceramic particles, the first binder, and the second binder within the above-mentioned range, can increase the packing density of the second ceramic particles, improve the thermal stability of the ceramic coating, and at the same time improve the porosity of the ceramic coating, further improve the electrochemical behavior of the ceramic coating, increase ion transfer channels, and thus improve the ionic conductivity of the battery separator. This is beneficial for the battery separator to have both lower thermal shrinkage rate, good flexibility, and high membrane rupture temperature.

[0050] In one embodiment of this application, the ceramic coating further includes a dispersant, a thickener, and a wetting agent. The dispersant is selected from at least one of sodium polyacrylate and polyacrylamide, the thickener is selected from at least one of sodium carboxymethyl cellulose and lithium carboxymethyl cellulose, and the wetting agent is selected from at least one of polyether siloxane, polyether-modified siloxane, and acetylenic diol. Based on the mass of the ceramic coating, the mass percentage of the dispersant is 0.01% to 1%, the mass percentage of the thickener is 0.01% to 1%, and the mass percentage of the wetting agent is 0.01% to 1%. For example, the mass percentage of the dispersant can be 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any range of two values ​​thereof; the mass percentage of the thickener can be 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any range of two values ​​thereof; the mass percentage of the wetting agent can be 0.01%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any range of two values ​​thereof. In this application, there are no particular limitations on the manufacturer or brand of the above-mentioned dispersant, thickener, and wetting agent, as long as the purpose of this application can be achieved.

[0051] In one embodiment of this application, the battery separator has a thermal shrinkage rate of ≤3% at 280°C and a membrane breakage temperature of ≥260°C. With the thermal shrinkage rate and membrane breakage temperature of the battery separator within the above range, the coating does not peel or crack at high temperatures, which is beneficial for the battery separator to possess both a lower thermal shrinkage rate, good flexibility, and a high membrane breakage temperature.

[0052] In one embodiment of this application, the air permeability of the battery separator is 218~350s / 100cc. Air permeability within this range is beneficial for sufficient electrolyte wetting and results in low ion transport resistance.

[0053] In one embodiment of this application, the puncture strength of the battery separator is ≥280gf.

[0054] In one embodiment of this application, the cross-sectional pore area ratio of the aramid coating along its thickness direction is 15% to 50%. For example, the cross-sectional pore area ratio of the aramid coating can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range of any two of these values. When the cross-sectional pore area ratio of the aramid coating is within the above range, the air permeability and heat resistance of the diaphragm product can be balanced.

[0055] The second aspect of this application provides a method for preparing the battery separator of the first aspect of this application, comprising the following steps: providing an aramid raw solution, the aramid raw solution comprising aramid fibers; mixing and dispersing the aramid raw solution and first ceramic particles uniformly to obtain an aramid coating slurry with a solid content of 1.5 wt% to 45 wt%; providing a ceramic coating slurry, the ceramic coating slurry comprising second ceramic particles, the ceramic coating slurry having a solid content of 15 wt% to 50 wt%; coating the aramid coating slurry onto at least one surface of a substrate layer to form an aramid coating; then coating the ceramic coating slurry onto the surface of the aramid coating away from the substrate layer, and drying to obtain a battery separator. The method for preparing the battery separator provided by this application, by coating an aramid coating onto the surface of a substrate layer and then coating a ceramic coating onto the surface of the aramid coating, can obtain a battery separator with low thermal shrinkage, high membrane breakage temperature, and good flexibility. Furthermore, the preparation process is simple, and its application in secondary batteries can improve their safety performance.

[0056] For example, the solid content of aramid coating slurry can be 1.5wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, or any range of two of these values; the solid content of ceramic coating slurry can be 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, or any range of two of these values. The dispersant, thickener, binder, and wetting agent materials in this application inherently contain a certain amount of water; the solid content is the ratio of the mass of the solid components in the slurry to the total mass of the liquid and solid components.

[0057] In one embodiment of this application, the aramid solution further includes an auxiliary agent and an organic solvent. The auxiliary agent is selected from at least one of calcium chloride, potassium hydroxide, lithium chloride, or pyridine, and the organic solvent is selected from at least one of N-methylpyrrolidone or dimethylacetamide. Based on the mass of the aramid solution, the mass percentage of aramid fiber is 0.5% to 9%, the mass percentage of the auxiliary agent is 0.5% to 8%, and the mass percentage of the organic solvent is 83% to 99%. For example, the mass percentage of aramid fiber can be 0.5%, 1.0%, 1.5%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or any two of these values; the mass percentage of auxiliaries can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any two of these values; the mass percentage of organic solvents can be 83%, 85%, 89.8%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 97.2%, 99%, or any two of these values. Having the mass percentages of aramid fiber, auxiliaries, and organic solvents within these ranges is beneficial for the complete dissolution of the aramid fiber and the formation of a stable network structure in the coagulation bath.

[0058] In this application, the aramid solution can be prepared or purchased directly, as long as the performance of the aramid solution meets the requirements. For example, the Tg1 and Td of the aramid fiber in the aramid solution are within the scope of this application.

[0059] In one embodiment of this application, the solvent of the aramid raw material is an organic solvent selected from at least one of N-methylpyrrolidone or dimethylacetamide. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is subjected to a coagulation bath, water washing, and drying to form an aramid coating.

[0060] In some embodiments, after coating the aramid slurry, a coagulation bath is applied, followed by washing and drying. The coagulation bath involves immersion in a coagulation solvent with a concentration of 40 wt% to 70 wt% for 0.1 min to 1 min. The coagulation solvent is an organic solvent and water, with the organic solvent selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylacetamide (DMAF). The concentration of the coagulation bath is the same as the concentration of the organic solvent. This application does not particularly limit the washing treatment, as long as it achieves the purpose of this application. For example, the water temperature can be from 10°C to 30°C, and the number of washes can be from 1 to 8. This application does not particularly limit the drying treatment, as long as it achieves the purpose of this application. For example, the drying temperature can be from 50°C to 90°C, and the drying time can be from 0.1 min to 10 min.

[0061] In one embodiment of this application, the solvent of the aramid raw material is water. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is dried to form an aramid coating.

[0062] In some implementations, the glass transition temperature (Tg1) of aramid fibers can be controlled by adjusting the molecular chain structure. For example, when other conditions remain constant, an increase in the proportion of rigid segments in the aramid fiber leads to a higher Tg1 value, while a decrease in the proportion of rigid segments leads to a lower Tg1 value. Similarly, when other conditions remain constant, an increase in the molecular weight or crosslinking density of the aramid fiber leads to a higher Tg1 value, while a decrease in the molecular weight or crosslinking density leads to a lower Tg1 value.

[0063] In some implementations, the thermal decomposition temperature (Td) of aramid fibers can be controlled by adjusting the molecular chain structure or polymerization process parameters. For example, when other conditions remain constant, increasing the proportion of rigid aromatic ring structures or introducing heterocyclic structures in aramid fibers increases the Td value; decreasing the proportion of rigid aromatic ring structures or reducing heterocyclic structures decreases the Td value. Similarly, when other conditions remain constant, increasing the degree of polymerization or crystallinity of aramid fibers increases the Td value; decreasing the degree of polymerization or crystallinity decreases the Td value.

[0064] In some implementations, Tg1 and Td can be controlled by adjusting the mass ratio of p-phenylenediamine to terephthaloyl chloride added during the preparation of the para-aramid solution. For example, when other conditions remain unchanged, the smaller the mass ratio of p-phenylenediamine to terephthaloyl chloride, i.e., the higher the content of terephthaloyl chloride, the larger Tg1 and Td will be; conversely, the larger the mass ratio of p-phenylenediamine to terephthaloyl chloride, i.e., the lower the content of terephthaloyl chloride, the smaller Tg1 and Td will be.

[0065] In some implementations, Tg1 and Td can be controlled by adjusting the proportion of ODA (diaminodiphenyl ether) in the diamine during the preparation of ODA-modified para-aramid fibers. For example, when other conditions remain unchanged, the higher the proportion of ODA in the diamine, the lower Tg1 and Td; the lower the proportion of ODA in the diamine, the higher Tg1 and Td.

[0066] In this application, the thickness of the coating can be controlled by adjusting the areal density of the coating. For example, when other conditions remain unchanged, increasing the areal density of the coating increases the coating thickness; decreasing the areal density of the coating decreases the coating thickness.

[0067] In this application, the median particle size D1 of the first ceramic particles and the median particle size D2 of the second ceramic particles can be controlled by adjusting the ball milling time. For example, when other conditions remain unchanged, extending the ball milling time reduces the median particle size; shortening the ball milling time reduces the median particle size.

[0068] In this application, the specific surface area S1 of the first ceramic particles and the specific surface area S2 of the second ceramic particles can be controlled by adjusting the ball milling time. For example, when other conditions remain unchanged, extending the ball milling time increases the specific surface area, while shortening the ball milling time decreases the specific surface area.

[0069] In some implementations, the glass transition temperatures Tg2 and Tg3 of the first and second binders can be controlled by adjusting their molecular chain structures or polymerization process parameters, respectively. For example, when other conditions remain constant, an increase in the proportion of rigid segments (such as aromatic rings, heterocycles, amide bonds, etc.) in the molecular chain leads to higher Tg2 and Tg3 values; a decrease in the proportion of rigid segments leads to lower Tg2 and Tg3 values. Similarly, when other conditions remain constant, an increase in molecular weight or crosslinking density leads to higher Tg2 and Tg3 values; a decrease in molecular weight or crosslinking density leads to lower Tg2 and Tg3 values.

[0070] In this application, the aramid fiber, the first ceramic particle, the second ceramic particle, the first binder, and the second binder can be purchased. The aramid fiber with the required Tg1 and Td, the first ceramic particle with the required median particle size and specific surface area, the second ceramic particle with the required median particle size and specific surface area, the first binder with the required Tg2, and the second binder with the required Tg3 can be selected as needed.

[0071] In some implementation schemes, testing is conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries". Five square samples of the battery separator are cut along the TD direction using a 10cm×10cm mold for testing. If the TD direction is less than 10cm, a 10cm sample is taken along the MD direction, in which case the sample is not square. The four corners and the center point of the sample are measured using a Mahr thickness gauge (C1202). The average value of these five points is taken as the thickness value of a single sample, and the total thickness of the battery separator is obtained. The surface ceramic coating is scraped off, and the thickness of the remaining sample is measured using a thickness gauge. The thickness of the ceramic coating is obtained by subtracting the thickness of the remaining sample from the total thickness of the battery separator. The aramid coating is scraped off, and the thickness of the aramid coating can be obtained in the same way. After scraping off the ceramic coating and the aramid coating, the thickness of the remaining substrate layer is measured. The average value of the five samples is taken as the thickness of the ceramic coating, the thickness of the aramid coating, and the thickness of the substrate layer.

[0072] In some implementation schemes, the median particle size of the first ceramic particle and the median particle size of the second ceramic particle can be tested according to the national standard GB / T19077-2024 "Laser Diffraction Method for Particle Size Distribution". Using a laser particle size analyzer (Dandong Baite, BT-9300ST), the first and second ceramic particles are ultrasonically dispersed for 3 minutes with a circulation pump speed of 1600 rpm. The particle sizes of the first and second ceramic particles are then measured, and the values ​​of the 50% particle size obtained are the median particle size D1 of the first ceramic particle and the median particle size D2 of the second ceramic particle. The first and second ceramic particles to be tested can be directly tested at the raw material end, or the second ceramic particles can be obtained by gentle agitation using an ultrasonic disperser (Ningbo Xinzhi SCIENTZ-750F). The first ceramic particles can also be obtained from an aramid coating; for example, a battery separator with the ceramic coating completely removed can be placed in a muffle furnace and the high-molecular organic matter (e.g., substrate layer, aramid fiber) can be completely burned off at a high temperature of 600°C to obtain the first ceramic particles.

[0073] A third aspect of this application provides a secondary battery comprising the battery separator provided in the first aspect of this application, or a battery separator prepared according to the preparation method provided in the second aspect of this application. The battery separator provided in this application exhibits good thermal stability and mechanical reliability, thereby providing the secondary battery with excellent performance, such as safety performance.

[0074] The secondary battery of this application also includes a positive electrode, a negative electrode, and an electrolyte. This application does not impose any particular limitations on the positive electrode, negative electrode, and electrolyte; those skilled in the art can choose according to actual needs, as long as the purpose of this application can be achieved.

[0075] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium-ion secondary batteries (lithium-ion batteries), lithium metal secondary batteries, sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0076] Example

[0077] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0078] Glass transition temperature test

[0079] The battery separator was immersed in deionized water and gently agitated using an ultrasonic disperser (Ningbo SCIENTZ-750F) to remove the ceramic coating and the first ceramic particles in the aramid coating. Scanning electron microscopy (SEM) was used to determine if the ceramic coating and the first ceramic particles in the aramid coating had been completely removed. After removal, the aramid coating was scraped off with a knife and dried in a vacuum oven at 60°C for later use, yielding an aramid fiber sample.

[0080] Aramid fiber samples were taken using a differential scanning calorimeter (DSC, Netzsch, DSC3500, Germany). The temperature range was 25-600℃, and the glass transition temperature Tg1 of the aramid fiber was obtained by heating at a rate of 10K / min under a nitrogen atmosphere.

[0081] Thermal decomposition temperature test

[0082] The battery separator was immersed in deionized water and gently agitated using an ultrasonic disperser (Ningbo SCIENTZ-750F) to remove the ceramic coating and the first ceramic particles in the aramid coating. Scanning electron microscopy (SEM) was used to determine if the ceramic coating and the first ceramic particles in the aramid coating had been completely removed. After removal, the aramid coating was scraped off with a knife and dried in a vacuum oven at 60°C for later use, yielding an aramid fiber sample.

[0083] Aramid fiber samples were taken using a thermogravimetric analyzer (TGA, TA Instruments, Q600, USA). The heating rate was 10 K / min under a nitrogen atmosphere, and the temperature range was 25-800℃. The thermal decomposition temperature Td of the aramid fiber was then measured.

[0084] Particle size testing

[0085] The particle size of the battery separator was measured by observing its cross-section along the thickness direction using scanning electron microscopy (SEM). The battery separator was processed using a diamond cutter or ion polishing to prepare a flat MD-ZD (length-thickness direction) cross-section, which was then observed using SEM. Image processing tools were used to binarize the cross-sectional SEM images, accurately distinguishing the first ceramic particles in the aramid coating, the second ceramic particles in the ceramic coating, and the substrate layer. The image processing tools automatically identified each independent cross-section and calculated the area of ​​the first and second ceramic particles, then converted them into equivalent circle diameters. All the collected equivalent circle diameters were substituted into a data table, and a volume / quantity weighted conversion was performed to calculate the Dv50 of the first and second ceramic particles. Dv50 was used as the median particle size. The number of first and second ceramic particles was counted as 4000 each.

[0086] Specific surface area test

[0087] Obtaining the second ceramic particles in the ceramic coating of the battery separator: 1m 2 The battery separator was immersed in a 5000mL beaker, and 2000mL of deionized water was added. The ceramic coating in the battery separator was removed by gentle agitation using an ultrasonic disperser (Ningbo SCIENTZ-750F). Through precipitation, washing and drying, second ceramic particles that can be used for specific surface area testing were obtained again.

[0088] Obtaining the first ceramic particles in the aramid coating: The battery separator with the ceramic coating completely removed is placed in a muffle furnace and the high molecular organic matter (such as the substrate layer and aramid fiber) is completely burned off at a high temperature of 600°C to obtain the first ceramic particles.

[0089] The specific surface area (S1) of the first ceramic particle and the specific surface area (S2) of the second ceramic particle were measured using a fully automated rapid specific surface area and porosity analyzer (Beijing Bio-Tech Electronic Technology Co., Ltd., SSA-4000). The specific surface area of ​​the ceramic particle samples was calculated using nitrogen adsorption-desorption isotherms. Before testing, the samples underwent degassing pretreatment to obtain stable adsorption curves, thereby ensuring the accuracy and repeatability of the data.

[0090] Thickness test

[0091] Five square battery separator samples were cut along the TD direction using a 10cm×10cm mold for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the MD direction, in which case the sample was not square. The thicknesses of the substrate layer, aramid coating, and ceramic coating were measured using scanning electron microscopy images of the battery separator cross-section. Five test points were taken at each of the substrate layer, aramid coating, and ceramic coating of the test sample for thickness measurement, and the average value of these five points was taken as the thickness value of each coating for a single test sample. The average value of the five test samples was taken as the thickness of the substrate layer, the aramid coating, and the ceramic coating.

[0092] Areal density test

[0093] Referring to GB / T 451.2-2002 "Determination of Basis Weight of Paper and Paperboard" (Paper Surface Density Test), five square samples of battery separator were cut along the transverse (TD) direction using a 10cm×10cm mold for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the longitudinal (MD) direction; in this case, the sample was not square. The total mass of each of the five samples was weighed using an analytical balance (Mettler-Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02) with a measurement accuracy of 0.0001g, and the average mass was taken as the total mass of the battery separator. Similarly, the ceramic coating on the sample surface was scraped off (by means of scraping with a knife or peeling off by adhesive), weighed, and the average mass was taken as the mass of the remaining sample. The mass of the ceramic coating was obtained by subtracting the mass of the remaining sample from the total mass of the sample. The mass of the aramid coating could be obtained similarly. After scraping off both the ceramic and aramid coatings, they were weighed, and the average mass was taken as the mass of the remaining substrate layer. The areal density of the substrate layer is calculated as follows: substrate layer sample mass / (substrate layer sample length × substrate layer sample width); ceramic coating is calculated as follows: ceramic coating mass / (battery separator sample length × battery separator sample width); aramid coating is calculated as follows: aramid coating mass / (battery separator sample length × battery separator sample width). The average value of five samples is taken as the areal density of the substrate layer, ceramic coating, and aramid coating. The longitudinal direction refers to the length of the battery separator, and the transverse direction refers to the width of the battery separator.

[0094] Heat shrinkage rate test

[0095] The test was conducted according to the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries". A 150mm × 150mm battery separator was cut, and a 100mm mark was made on it in both the length (MD direction) and width (TD direction) directions using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C). The battery separator was then heated at 200℃ for 1 hour and 280℃ for 1 hour in an electric heating drying oven to test its thermal shrinkage rate. The length and width values ​​of the battery separator after heating, measured by the fully automatic image measuring projector, were recorded as L1 mm and L2 mm, respectively. Three measurements were taken, and the average value was recorded. The thermal shrinkage rate in the MD direction was (100-L1) / 100 × 100%; the thermal shrinkage rate in the TD direction was (100-L2) / 100 × 100%.

[0096] Breathability test

[0097] The air permeability test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries". A 600mm × 100mm sample of the substrate layer and a sample of the coated battery separator were cut. An O-type air permeability meter (ASAHI Corporation, EG01-55-1MR) was used. The test time was 3 seconds. The air permeability of the substrate layer and the battery separator was measured at arbitrary positions at 100mm intervals along a 600mm axis. The average value of these five measurements was recorded as the air permeability of the substrate layer and the battery separator.

[0098] Membrane rupture temperature test

[0099] A battery separator measuring 80mm × 4mm was cut and subjected to thermomechanical analysis (TMA) using a Netzsch TMA 402 F1 / F3 Hyperion® analyzer (Germany). The test conditions were: nitrogen atmosphere, gas flow rate of 100mL / min, tensile force of 0.03N, heating rate of 5℃ / min, and termination temperature of 400℃. Temperature was plotted on the x-axis, and deformation on the y-axis. As temperature increased, the deformation of the battery separator changed. When the battery separator ruptured, the deformation on the y-axis of the TMA curve reached its maximum value; the temperature corresponding to this point on the x-axis is the rupture temperature of the battery separator.

[0100] Flexibility test

[0101] A 15cm × 15cm battery separator was cut and heat-treated at 280℃ for 1 hour using an electric heating drying oven. The heat-treated separator was then observed under a high-resolution scanning electron microscope to determine if cracking had occurred. If no cracks were found, a tensile test was performed. Samples measuring 1.5cm × 15cm were cut from the heat-treated separator, both transversely and longitudinally. Using a tensile testing machine, the distance between the clamps was set to (120±5) mm. After fixing the sample, a force of 1 MPa was applied and the sample was stretched at 50 mm / min for 5 seconds. The absence of cracking and fracture after stretching after heat treatment indicates good flexibility.

[0102] Aramid coating internal pore morphology test

[0103] The battery separator was treated with a diamond cutter or ion polishing to prepare a smooth MD-ZD (length-thickness direction) cross-section, which was then observed using a scanning electron microscope (SEM).

[0104] The cross-section of the aramid coating was observed using a scanning electron microscope (SEM). Five photos with a magnification of 10,000 were taken at different locations. The outline of the holes was drawn with a pen. The area Si of each hole on the surface was calculated using image processing software. Then, the diameter d of each hole (equivalent diameter, i.e., the diameter of a circle with the same area as the hole) was calculated according to formula (1).

[0105] (1),

[0106] Average aperture dn: Calculate the average aperture of each measured aperture according to formula (2).

[0107] (2),

[0108] Where ∑d is the sum of the diameters d of all holes, and n is the number of holes.

[0109] Pore ​​size distribution SD: First calculate the volume average pore size dv according to formula (3-1), then calculate the pore size distribution SD according to formula (3-2).

[0110] (3-1)

[0111] (3-2)

[0112] Where, ∑d 4 The sum of the fourth power of the aperture diameter d; ∑d 3 It is the sum of the cube of the aperture diameter d.

[0113] Hole area percentage S%: The percentage of the surface hole area to the total surface area. Specifically calculated using formula (4):

[0114] (4),

[0115] Wherein, ∑Sm is the sum of the areas observed by the SEM (i.e., the sum of the areas of the photographed regions).

[0116] Example 1-1

[0117] <Preparation of Aramid Coating Slurry>

[0118] 84 kg of NMP solvent (Henan Maiqi Chemical Co., Ltd., purity ≥99.5%) and 3.82 kg of anhydrous calcium chloride (Xilong Scientific Co., Ltd., AR) were added to a 200 kg reactor. The mixture was heated to 80 °C under a nitrogen atmosphere and stirred for 4 h to completely dissolve the calcium chloride (a portion of CaCl2 was reserved for dilution preparation to ensure that the final system contained approximately 2% CaCl2). Subsequently, 2.392 kg of p-phenylenediamine (Inner Mongolia Kailidi Chemical Co., Ltd., purity >99.5%) and 0.266 kg of ODA (diaminodiphenyl ether, purity >99.5%, accounting for 10% of the total mass of the diamine) were added, and stirring continued until both amine monomers were completely dissolved, resulting in a uniform and transparent diamine / NMP / CaCl2 mixed solution.

[0119] Using chilled water, the temperature of the mixed solution was lowered to 10±0.5℃. While maintaining low temperature stirring, 4.860 kg of terephthaloyl chloride (Ningxia Hui Autonomous Region Fenghua Biotechnology Co., Ltd., purity >99.5%) was slowly added in batches. The dropping rate was controlled to avoid excessive local reaction. After the addition was completed, low temperature stirring was continued for 2 hours to obtain an aromatic type II para-aramid polymer solution with 10% ODA introduced. The viscosity of the slurry at this time was about 200 mPa·s.

[0120] Calcium oxide (Tianjin Xintaiyi Technology Co., Ltd., analytical grade) was added to the polymerization solution, and the pH of the solution was adjusted to pH=6 by slow stirring. After the neutralization reaction was complete, it was further diluted with 4.64% NMP / CaCl2 solution (the amount of CaCl2 in the diluent was made up to make up to about 2% of the total CaCl2 in the system). Finally, a para-aramid dope solution with a mass percentage of 2% was obtained, in which the mass percentage of the auxiliary agent calcium chloride was about 2% and the mass percentage of the organic solvent NMP was about 96%. The rotational viscosity of the material was retested and it was stable at about 300 mPa·s, and the specific logarithmic viscosity was 1.6 g / dL.

[0121] 294 parts of ODA-modified para-aramid fiber stock solution and 5.88 parts of first ceramic particles Al2O3 were added sequentially to a beaker and dispersed at high speed at 8000 rpm for 1 hour. The high-speed dispersion process was carried out in an ice bath, and then dispersed uniformly under a high-pressure homogenizer at a pressure of 500 bar. During the homogenization and dispersion process, circulating cooling water was used to cool the mixture, resulting in an aramid coating slurry with a solid content of 3.92 wt%. The Tg1 and Td of the aramid fiber, the median particle size and specific surface area of ​​the first ceramic particles are shown in Table 1.

[0122] <Preparation of Ceramic Coating Slurry>

[0123] Add 44.5 parts of second ceramic particles (Al2O3), 0.5 parts of sodium polyacrylate dispersant, and 45 parts of water to a beaker. After grinding and dispersion, an alumina dispersion is obtained. Take 192.43 parts of the alumina dispersion and add 123.67 parts of water and 0.8 parts of thickener sodium carboxymethyl cellulose. Disperse in a planetary mixer at 1200 rpm rotation and 30 rpm revolution for 60 min. After grinding once at 500 rpm, add 7.143 parts of second binder polyacrylic acid (Tg3 as shown in Table 2, solid content 21 wt%), 6.5 parts of first binder waterborne pure acrylic emulsion type polyacrylate adhesive (Tg2 as shown in Table 2, solid content 40 wt%), and 0.9 parts of polyether siloxane wetting agent. Stir for 60 min to obtain a ceramic coating slurry with a solid content of 30.2 wt%. The median particle size and specific surface area of ​​the second ceramic particles are shown in Table 2.

[0124] <Preparation of Battery Separator>

[0125] A 9µm thick polyethylene substrate layer (manufactured by Sinoma Lithium Membrane Co., Ltd.) was coated with an aramid coating slurry on one side surface of the substrate layer using a doctor blade coating method. The substrate was then immersed in a 60wt% NMP / water coagulation bath solvent at room temperature for 0.5 min, followed by washing (20℃, 5 washes) and drying (70℃, 5 min) to obtain a 2µm thick aramid coating. A ceramic coating slurry was then applied to the surface of the aramid coating away from the substrate layer using a gravure roller and dried to obtain the battery separator, where the ceramic coating thickness was 2µm. The m1 / μ1, m2 / μ2, and m3 / μ3 ratios are shown in Tables 1 and 2.

[0126] Examples 1-2 to Examples 1-7

[0127] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1. Among them, Tg1 and Td1 are adjusted by controlling the proportion of ODA in the diamine as shown in Table 1.

[0128] Examples 1-8

[0129] Except for the preparation method used in <Preparation of Aramid Coating Slurry>, the rest is the same as in Example 1-1.

[0130] <Preparation of Aramid Coating Slurry>

[0131] Under nitrogen protection, 180 kg of solvent N,N-dimethylacetamide (DMAc, Anyang Jiukong, purity ≥99.5%) was added to a 200 L prepolymer reactor, followed by 8.8 kg of m-phenylenediamine (MPD, Zhejiang Annofang Amine Chemical Co., Ltd., purity >99.5%, molar amount 0.081 mol). Mechanical stirring was started (speed 200 r / min), and stirring was carried out at room temperature for 30 min to ensure that the m-phenylenediamine was completely dissolved, resulting in a uniform and transparent MPD / DMAc solution. The solution temperature was then lowered to 10±0.5℃ using chilled water. While maintaining low temperature and stirring, 13.1 kg of isophthaloyl chloride (IPC, Yantai Yuxiang Fine Chemical Co., Ltd., purity >99.5%, molar amount 0.064 mol, accounting for 60% of the total isophthaloyl chloride) was slowly added over a dropping time of 40 min. After the addition was complete, stirring was continued at low temperature for 1.5 h to obtain a viscous prepolymer (solid content approximately 10%). All of the prepolymer was transferred to a 500L polycondensation reactor, maintaining a nitrogen atmosphere and temperature of 10±0.5℃. Stirring continued (180 r / min), and the remaining 8.7 kg of isophthaloyl chloride (IPC, purity >99.5%, molar amount 0.043 mol, accounting for 40% of the total) was slowly added over a dropping time of 30 min. Throughout the polymerization process, the molar ratio of isophthaloyl chloride (IPC) to m-phenylenediamine (MPD) was strictly controlled at 1.02:1 (a slight excess of IPC ensured complete MPD reaction and prevented low molecular weight). After the addition was complete, the temperature was gradually increased to 25°C and stirred at a constant temperature for 3 hours until the viscosity of the reaction system reached approximately 600 mPa·s, yielding a high molecular weight meta-aramid polymer (poly(m-phenylenediamine isophthaloyl)). The polymer in the polycondensation reactor was pumped into a 300 L neutralization reactor, with stirring maintained (150 r / min) and the temperature controlled at 25-30°C. Simultaneously, 4.2 kg of neutralizing agent calcium hydroxide (analytical grade) and 20 kg of DMAc were prepared into a uniform suspension (calcium hydroxide concentration approximately 17.4%). This neutralizing agent suspension was gradually added to the neutralization reactor over a 1-hour period, with continuous stirring to ensure complete neutralization. During the neutralization process, the pH value of the system was monitored, and the pH value of the system was finally adjusted to 6. After the neutralization reaction was completed, the mixture was stirred for another 30 minutes to obtain a uniform and stable meta-aramid fiber solution (meta-aramid fiber content of about 8%, DMAc solvent content of about 91.5%, and neutralization product calcium chloride content of about 0.5%).

[0132] 50 parts of aramid fiber stock solution (rotational viscosity of 200 mPa·s), 4 parts of first ceramic particles Al2O3, and 46 parts of DMAC solvent were added sequentially to a beaker. The mixture was dispersed at high speed at 8000 rpm for 1 hour with an ice bath during the high-speed dispersion process. The mixture was then dispersed uniformly at 500 bar using a high-pressure homogenizer. During the homogenization and dispersion process, circulating cooling water was used to cool the mixture, resulting in an aramid coating slurry with a solid content of 8 wt%.

[0133] Examples 1-9

[0134] Except for the preparation methods used in <Preparation of Aramid Coating Slurry> and <Preparation of Battery Separator>, the rest are the same as in Examples 1-1.

[0135] <Preparation of Aramid Coating Slurry>

[0136] 40 parts of a bio-based aramid nanofiber dispersion with a solid content of 5% and 0.8 parts of an acrylic acid-modified polyurethane dispersant (solid content of 45%) were added to 87 parts of water and dispersed evenly to obtain the first slurry. The temperature of the first slurry was controlled at around 15°C during this process. 66.12 parts of alumina were added to the first slurry and dispersed and ground evenly to obtain the second slurry. 12.6 parts of a polyacrylic acid heat-resistant binder (thermal decomposition temperature of 300°C, solid content of 21%) were added to the second slurry and dispersed evenly to obtain the third slurry. 2.5 parts of a thickener sodium carboxymethyl cellulose solution (CMC, solid content of 4%) and 0.4 parts of an organosilicon wetting agent (brand name: TEGO SURTEN202E) were added to the third slurry and dispersed evenly to obtain an aramid coating slurry with a solid content of approximately 34 wt%.

[0137] The dispersant and polyacrylic acid heat-resistant binder mentioned above are both water-based suspensions, and the solvent for the sodium hydroxymethyl cellulose solution mentioned above is water; in the above bio-based aramid nanofiber dispersion, the solvent is water, the number-average molecular weight of the bio-based aramid nanofibers is approximately 17,000, the average diameter of the bio-based aramid nanofibers is 10 nm, and their structural formula is:

[0138] <Preparation of Battery Separator>

[0139] A 9µm thick polyethylene substrate layer (manufactured by Sinoma Lithium Membrane Co., Ltd.) was used. An aramid coating slurry was applied to one surface of the substrate layer and dried to form an aramid coating. Then, a ceramic coating slurry was applied to the surface of the aramid coating away from the substrate layer using a gravure roller and dried to obtain a battery separator, wherein the thickness of the ceramic coating was 2µm. m1 / μ1 is the same as in Example 1-1, and m2 / μ2 and m3 / μ3 are shown in Table 1.

[0140] Examples 1-10 to Examples 1-14

[0141] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0142] Examples 1-15 to Examples 1-24

[0143] Except for adjusting the corresponding preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0144] Comparative Example 1

[0145] Except for the preparation method of the battery separator, the rest is the same as in Example 1-1.

[0146] <Preparation of Battery Separator>

[0147] A 9µm thick polyethylene substrate layer (manufactured by Sinoma Lithium Membrane Co., Ltd.) was coated with an aramid coating slurry on one side surface of the substrate layer using a doctor blade coating method. The substrate was then immersed in a 60wt% NMP / water coagulation bath solvent at room temperature for 0.5 min. Following washing (20℃, 5 washes) and drying (70℃, 5 min), a 2µm thick aramid coating was obtained. The m² / μ² and m³ / μ³ ratios are shown in Table 1.

[0148] Comparative Examples 2 to 3

[0149] Except for adjusting the median particle size D1 and specific surface area S1 of the first ceramic particles according to Table 1, the rest is the same as in Example 1-1.

[0150] Comparative Example 4

[0151] Except for the preparation method of the battery separator, the rest is the same as in Example 1-1.

[0152] <Preparation of Battery Separator>

[0153] A 9µm thick polyethylene substrate layer (manufactured by Sinoma Lithium Membrane Co., Ltd.) was coated with a ceramic coating slurry on one side of the substrate layer using a gravure roller and dried to obtain a battery separator, wherein the thickness of the ceramic coating was 2µm. m1 / μ1 is shown in Table 2, and m3 / μ3 is the same as in Example 1-1.

[0154] Comparative Example 5

[0155] Except for adjusting the median particle size D2 and specific surface area S2 of the second ceramic particles according to Table 2, the rest is the same as in Example 1-1.

[0156] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0157] Table 1

[0158] Table 2

[0159] Table 3

[0160] Note: " / " in Table 3 indicates that it does not contain or cannot be tested.

[0161] As can be seen from Examples 1-1 to 1-24 and Comparative Examples 1 to 5, this application, by sequentially coating at least one surface of the substrate layer with an aramid coating and a ceramic coating, and by controlling the physical properties of the aramid coating and the ceramic coating within the scope of this application, enables the battery separator to simultaneously possess lower thermal shrinkage and air permeability, good flexibility, and high rupture temperature. However, the battery separator of Comparative Example 1 did not have a ceramic coating, the K values ​​of Comparative Examples 2, 3, and 5 exceeded the scope of this application, and Comparative Example 4, without an aramid coating, had a lower rupture temperature, higher thermal shrinkage and air permeability. This indicates that the battery separators of Comparative Examples 1 to 5 cannot simultaneously possess lower thermal shrinkage, good flexibility, and high rupture temperature.

[0162] As can be seen from Examples 1-1 to 1-24, the following factors affect the thermal shrinkage rate, flexibility, and rupture temperature of the battery separator: m1 / μ1, m2 / μ2, μ1, μ2, m3 / μ3, μ3, η, the type of aramid fiber, glass transition temperature Tg1, thermal decomposition temperature Td, and mass percentage; the type, particle size, specific surface area, and mass percentage of the first and second ceramic particles; and the type, glass transition temperature, and mass percentage of the first and second binders. By controlling the above-mentioned influencing factors within the scope of this application, the battery separator can simultaneously possess lower thermal shrinkage rate and air permeability, good flexibility, and high rupture temperature.

[0163] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0164] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0165] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A battery separator, characterized by, The battery separator includes a substrate layer and an aramid coating and a ceramic coating disposed on at least one surface of the substrate layer, wherein the aramid coating is located between the substrate layer and the ceramic coating along the thickness direction of the battery separator; The aramid coating comprises aramid fibers and first ceramic particles; the ceramic coating comprises second ceramic particles, wherein the first ceramic particles and the second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide or boehmite; The first ceramic particles have a median particle diameter of D1 nm, and a specific surface area of S1 m2 / g 2 The second ceramic particles have a median particle diameter of D2 nm, and a specific surface area of S2 m2 / g 2 / g, and satisfy: 0.8 ≤ K ≤ 3.

0.

2. The battery separator of claim 1, wherein, 30≤D1≤500, 250≤D2≤550, 5≤S1≤120, 5≤S2≤20.

3. The battery separator of claim 1, wherein, The areal density of the ceramic coating is m1 g / m 2 The areal density of the aramid coating is m² g / m³. 2 The ceramic coating has a thickness of μ1 μm, and the aramid coating has a thickness of μ2 μm, satisfying the following: , 1≤η≤4.

4. The battery separator of claim 3, wherein, 0.8≤m1 / μ1≤3, 0.37≤m2 / μ2≤2.5, 0.5≤μ1≤4, 0.5≤μ2≤4.

5. The battery separator of claim 1, wherein, The areal density of the substrate layer is m³ g / m³ 2 The thickness of the substrate layer is μ3 μm, 0.2≤m3 / μ3≤1, 3≤μ3≤16.

6. The battery separator of claim 1, wherein, The aramid fiber is selected from at least one of para-aramid fiber, meta-aramid fiber, or bio-based modified aramid fiber, wherein the glass transition temperature Tg1 of the aramid fiber is ≥180℃ and the thermal decomposition temperature Td of the aramid fiber is ≥400℃.

7. The battery separator of claim 1, wherein, Based on the quality of the aramid coating, the aramid fiber has a mass percentage content of 2% to 80%, and the first ceramic particle has a mass percentage content of 20% to 98%.

8. The battery separator of claim 1, wherein, The ceramic coating further includes a first binder and a second binder, wherein the glass transition temperature Tg2 of the first binder is ≤50℃ and the glass transition temperature Tg3 of the second binder is ≥160℃.

9. The battery separator of claim 8, wherein, The first adhesive is selected from polyacrylate adhesives, and the second adhesive is selected from at least one of polyacrylic acid adhesives or polyacrylamide adhesives; The polyacrylamide adhesive is selected from at least one of anionic polyacrylamide adhesives, cationic polyacrylamide adhesives, and nonionic polyacrylamide adhesives, and the polyacrylate adhesive is selected from at least one of waterborne styrene-acrylic emulsion polyacrylate adhesives, waterborne pure acrylic emulsion polyacrylate adhesives, and waterborne silicone-acrylic emulsion polyacrylate adhesives.

10. The battery separator of claim 8, wherein, Based on the quality of the ceramic coating, the second ceramic particles have a mass percentage content of 88% to 98%, the first binder has a mass percentage content of 0.5% to 5%, and the second binder has a mass percentage content of 1% to 5%.

11. A method of producing a battery separator as claimed in any one of claims 1 to 10, characterised in that, Includes the following steps: An aramid solution is provided, the aramid solution comprising aramid fibers, and the aramid solution and first ceramic particles are mixed and dispersed evenly to obtain an aramid coating slurry with a solid content of 1.5wt% to 45wt%. A ceramic coating slurry is provided, the ceramic coating slurry comprising second ceramic particles, the ceramic coating slurry having a solid content of 15 wt% to 50 wt%; The aramid coating paste is applied to at least one surface of the substrate layer to form the aramid coating. The ceramic coating slurry is then applied to the surface of the aramid coating away from the substrate layer, and dried to obtain the battery separator.

12. The method of claim 11, wherein, The aramid solution also includes an auxiliary agent and an organic solvent. The auxiliary agent is selected from at least one of calcium chloride, potassium hydroxide, lithium chloride, or pyridine, and the organic solvent is selected from at least one of N-methylpyrrolidone or dimethylacetamide. Based on the mass of the aramid solution, the aramid fiber has a mass percentage content of 0.5% to 9%, the auxiliaries have a mass percentage content of 0.5% to 8%, and the organic solvent has a mass percentage content of 83% to 99%.

13. The preparation method according to claim 11, characterized in that, The solvent of the aramid raw solution is an organic solvent selected from at least one of N-methylpyrrolidone or dimethylacetamide. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is subjected to coagulation bath, water washing, and drying to form the aramid coating.

14. The method of claim 11, wherein, The solvent of the aramid raw solution is water. After the aramid coating slurry is coated on at least one surface of the substrate layer, it is dried to form the aramid coating.

15. A secondary battery characterized by comprising: The secondary battery includes the battery separator according to any one of claims 1 to 10, or the battery separator prepared by the preparation method according to any one of claims 11 to 14.

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