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 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 aramid fibers are selected from at least one of aramid I-type para-aramid fibers, aramid II-type para-aramid fibers, or aramid III-type para-aramid fibers, and the glass transition temperature Tg1 of the aramid fibers is ≥250℃. The ceramic coating includes second ceramic particles, and 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 battery separator provided by 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] The first aspect of this application provides a battery separator, which 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 aramid fibers are selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers, or aramid type III para-aramid fibers, and the glass transition temperature Tg1 of the aramid fibers is ≥250℃. The ceramic coating includes second ceramic particles. 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.

[0007] 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 areal density of the substrate layer is m3 g / m 2 The ceramic coating has a thickness of μ1 μm, the aramid coating has a thickness of μ2 μm, and the substrate layer has a thickness of μ3 μm. , 1.0≤η≤2.0.

[0008] In one embodiment of this application, 1.4≤m1 / μ1≤2.5, 0.4≤m2 / μ2≤1.2, 0.3≤m3 / μ3≤0.8, 0.5≤μ1≤3.5, 0.5≤μ2≤3.5, and 5≤μ3≤16.

[0009] In one embodiment of this application, the thermal decomposition temperature Td of the aramid fiber is ≥520℃.

[0010] In one embodiment of this application, based on the quality of the aramid coating, the mass percentage of aramid fiber is 20% to 70%, and the mass percentage of the first ceramic particle is 30% to 80%.

[0011] In one embodiment of this application, the particle size Dv50 of the first ceramic particle is 50 nm to 110 nm, and the particle size Dv50 of the second ceramic particle is 200 nm to 600 nm.

[0012] 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℃.

[0013] 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.

[0014] 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%.

[0015] In one embodiment of this application, the cross-sectional area of ​​the aramid coating along the thickness direction accounts for 15% to 45%.

[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, the aramid fibers being selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers or aramid type III para-aramid fibers, the glass transition temperature Tg1 of the aramid fibers being ≥250℃; mixing and dispersing the aramid raw solution and first ceramic particles uniformly to obtain an aramid coating slurry with a solid content of 2.5wt% to 8wt%; providing a ceramic coating slurry, the ceramic coating slurry comprising second ceramic particles, the ceramic coating slurry having a solid content of 15wt% to 50wt%; coating the aramid coating slurry onto at least one surface of a substrate layer, subjecting it to a coagulation bath, washing with water, and drying 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 it 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 1% to 3%, the mass percentage of the auxiliary agent is 1% to 8%, and the mass percentage of the organic solvent is 89% to 98%.

[0018] 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.

[0019] The beneficial effects of this application are:

[0020] This application provides a battery separator, its preparation method, 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 aramid fibers are selected from at least one of aramid I-type para-aramid fibers, aramid II-type para-aramid fibers, or aramid III-type para-aramid fibers, and the glass transition temperature Tg1 of the aramid fibers is ≥250℃. The ceramic coating includes second ceramic particles, and the first and second ceramic particles are each independently selected from at least one of barium sulfate, alumina, silicon dioxide, titanium dioxide, or boehmite. This application designs the battery separator structure by using a rigid ceramic coating as a "skeleton" to resist heat shrinkage, while the flexible aramid coating acts as a "buffer layer" and "reinforcing mesh" to resist impact and tearing. The two work synergistically to give the battery separator a lower heat shrinkage rate, good flexibility, and a high rupture temperature, while also possessing good thermal stability and mechanical reliability.

[0021] 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

[0022] 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.

[0023] Figure 1 This is a schematic diagram of the battery separator along the thickness direction according to one embodiment of this application;

[0024] Figure 2 This is a scanning electron microscope image along the thickness direction of a battery separator according to one embodiment of this application.

[0025] Reference numerals: 10 battery separator, 11 ceramic coating, 12 aramid coating, 13 substrate layer. Detailed Implementation

[0026] 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.

[0027] The first aspect of this application provides a battery separator, which 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 aramid fibers are selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers, or aramid type III para-aramid fibers, and the glass transition temperature Tg1 of the aramid fibers is ≥250℃. The ceramic coating includes second ceramic particles. 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.

[0028] 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.

[0029] This application designs the battery separator structure as a composite structure of a substrate layer, an aramid coating, and a ceramic coating, and controls the Tg1 value within the aforementioned range. The substrate layer provides basic mechanical strength and electronic insulation; the aramid coating exhibits excellent heat resistance, electrolyte wettability, and intrinsic flame retardancy, forming a flexible network structure that enhances the mechanical strength of the substrate layer and increases the separator's breakage temperature; the ceramic coating improves thermal stability and reduces thermal shrinkage. The rigid ceramic coating acts as a "skeleton" to resist thermal shrinkage, while the flexible aramid coating serves as a "buffer layer" and "reinforcing mesh" for impact and tear resistance. The coatings work synergistically, and the "flexible network + particle support" formed by the aramid coating precisely connects with the pores of the substrate layer and the ceramic coating, buffering interfacial stress and reducing the risk of separation between layers and coating cracking. This allows the ceramic coating to form a continuous heat-resistant barrier, thereby improving the battery separator's flexibility and breakage temperature, reducing the thermal shrinkage rate, and enabling the battery separator to simultaneously achieve both thermal stability and mechanical reliability.

[0030] Meanwhile, the core cause of membrane rupture in this application lies in the fact that the aramid coating first loses its structural stability, leading to the detachment or cracking of the ceramic coating, which in turn exposes the substrate layer and causes melting and rupture. 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 packed and structurally stable, maintaining a flexible network morphology that provides stable support for the ceramic coating and buffers interfacial stress. When the ambient temperature is ≥Tg1, the aramid fiber molecular chains relax and curl, the flexible network collapses, and it can no longer support the ceramic coating. The ceramic coating is prone to cracking and detachment, ultimately leading to membrane rupture. Therefore, the Tg1 of the aramid fiber 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 rupture temperature. Therefore, by controlling the glass transition temperature Tg1 of the aramid fiber within the above range, even if the ambient temperature reaches 250°C (the initial temperature of extreme thermal runaway of the battery), the aramid fiber remains in a rigid solid state, the interface structure between the ceramic coating and the substrate layer is intact, and the battery separator will not break. Only when the ambient temperature approaches or exceeds Tg1 and continues for a certain period of time will the membrane breakage occur, thereby increasing the membrane breakage temperature of the battery separator.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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 areal density of the substrate layer is m3 g / m 2 The ceramic coating has a thickness of μ1 μm, the aramid coating has a thickness of μ2 μm, and the substrate layer has a thickness of μ3 μm. 1.0 ≤ η ≤ 2.0. For example, the value of η can be 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 1.95, 2.0, or a range of any two of these values. The inventors of this application have discovered that the thickness and areal density of the substrate layer, the aramid coating, and the ceramic coating determine the thermal shrinkage rate, flexibility, and rupture temperature of the battery separator, as well as its permeability to the electrolyte. This application controls the thickness and areal density of the substrate layer, aramid coating, and ceramic coating to ensure that η satisfies 1.0≤η≤2.0. The substrate layer, ceramic coating, and aramid coating are well-matched, with the ceramic coating exhibiting moderate density. This provides sufficient mechanical constraint to reduce melting and shrinkage of the substrate layer at high temperatures while retaining adequate open pores to ensure sufficient electrolyte wetting and efficient lithium-ion transport. Simultaneously, the aramid coating effectively buffers the thermal expansion difference between the ceramic layer and the substrate layer to reduce the risk of interfacial shear stress accumulation, without clogging the membrane pores and affecting air permeability and ion conductivity. Within this range, the battery separator exhibits excellent overall performance, simultaneously improving its thermal shrinkage rate, flexibility, and rupture temperature. This results in a thermal shrinkage rate ≤3.5% at 280℃, air permeability ≤330s / 100cc, electrolyte wetting speed ≥15mm / min, and a strong bond between the coating and the substrate layer, with no cracking or peeling at high temperatures. Applying the aforementioned battery separator to secondary batteries can improve the electrolyte wetting rate and provide unobstructed initial ion channels, thereby improving the charge and discharge efficiency of secondary batteries and enhancing their safety.

[0035] When η < 1.0, the ceramic coating is not dense enough. In addition, the substrate layer has a low melting point and is prone to melting and severe shrinkage at high temperatures. Its self-support is weak, so when the substrate layer shrinks at high temperatures, the ceramic coating cannot effectively restrain it, resulting in cracking or falling off, and ultimately causing the heat barrier to fail. At the same time, the aramid coating is too dense, which blocks the pore structure of the separator, making it difficult for the electrolyte to wet and hindering lithium ion transport. This results in a 280°C thermal shrinkage rate of the battery separator > 4%, an air permeability > 360s / 100cc, and a decrease in battery charge and discharge efficiency. When η > 2.0, the ceramic coating is too dense, resulting in excessively compact particle packing. The open pores originally used to store electrolyte are transformed into closed pores, and the connected ion transport channels are broken, hindering the penetration and wetting of electrolyte. At the same time, the aramid coating structure is too loose and cannot effectively buffer the difference in thermal expansion coefficients between the ceramic coating and the substrate layer. Under temperature changes or long-term cycling, the shear stress generated at the interface can easily cause the aramid coating to peel off from the surface of the substrate layer, resulting in interfacial delamination failure. This leads to an electrolyte wetting rate of <15mm / min for the battery separator, and the coating is prone to peeling off at high temperatures, causing the battery separator to break.

[0036] In one embodiment of this application, 1.4≤m1 / μ1≤2.5, 0.4≤m2 / μ2≤1.2, 0.3≤m3 / μ3≤0.8, 0.5≤μ1≤3.5, 0.5≤μ2≤3.5, and 5≤μ3≤16. For example, the value of m1 / μ1 can be 1.4, 1.6, 1.64, 1.68, 1.72, 1.76, 1.80, 1.84, 1.87, 1.9, 2.2, 2.5, or a range of any two values; the value of m2 / μ2 can be 0.4, 0.55, 0.6, 0.64, 0.68, 0.72, 0.76, 0.80, 0.82, 0.84, 0.86, 0.88, 0.9, 1.0, 1.2, or a range of any two values; the value of m3 / μ3 can be 0.3, 0.45, 0.49, 0.53, 0. The values ​​of μ1 can be 0.5, 1, 1.2, 1.4, 1.6, 1.8, 2, 3, 3.5, or any two of these values; the values ​​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, or any two of these values; the values ​​of μ3 can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, or any two of these values. The substrate layer provides unobstructed initial ion channels, acting as a support to stabilize the battery separator and prevent it from becoming loose. The aramid coating forms a "flexible network + particle support," precisely connecting with the pores of the substrate layer and ceramic coating, while also buffering interfacial stress and reducing the risk of delamination and cracking of the battery separator. The dense yet non-sealed ceramic coating forms a continuous heat-resistant barrier. This application achieves a synergistic effect by controlling the areal density and thickness of the substrate layer, aramid coating, and ceramic coating within the aforementioned ranges. This results in a battery separator with a lower thermal shrinkage rate, good flexibility, and a high rupture temperature, thus giving the battery separator both good thermal stability and mechanical reliability.

[0037] 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.

[0038] In one embodiment of this application, the thermal decomposition temperature Td of the aramid fiber is ≥520℃. The thermal decomposition temperature of the aramid fiber in this application meets the above range, which is beneficial for maintaining the structural stability and integrity of the aramid coating at high temperatures, preserving good ion conductivity, and improving the thermal safety performance of the battery separator. This, in turn, helps the battery separator to possess both lower thermal shrinkage, good flexibility, and a high membrane rupture temperature.

[0039] In one embodiment of this application, based on the mass of the aramid coating, the mass percentage of aramid fiber is 20% to 70%, and the mass percentage of the first ceramic particles is 30% to 80%. For example, the mass percentage of aramid fiber can be 20%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, or any two of these values; the mass percentage of the first ceramic particles can be 30%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 80%, or any two of these values. By controlling the mass percentages of aramid fiber and the first ceramic particles within the above ranges, this application allows the aramid fiber to form a flexible network structure, and the first ceramic particles to fill the network structure to optimize the pore structure. Through their synergistic effect, the mechanical strength and rupture temperature of the coating are balanced, reducing the risk of ion transport channels being blocked due to an overly dense aramid coating. This helps the battery separator to have both a lower thermal shrinkage rate, good flexibility, and a high rupture temperature.

[0040] In one embodiment of this application, the particle size Dv50 of the first ceramic particles is 50 nm to 110 nm, and the particle size Dv50 of the second ceramic particles is 200 nm to 600 nm. In this application, Dv50 represents the particle size of 50% of the particles in the volumetric particle size distribution of the material that is smaller than this value. For example, the particle size Dv50 of the first ceramic particles can be 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 110 nm, or a range of any two of these values; the particle size Dv50 of the second ceramic particles can be 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, or a range of any two of these values. This application, by controlling the particle size Dv50 of the first ceramic particles within the aforementioned range, facilitates an increase in the surface roughness of the aramid coating. Simultaneously, it regulates the pore structure of the aramid coating, reducing the risk of ion transport channels being blocked due to excessive density of the aramid coating. This results in a battery separator that combines lower thermal shrinkage, good flexibility, and a high rupture temperature. Controlling the particle size Dv50 of the second ceramic particles within the aforementioned range increases the particle packing density, thereby improving the thermal stability of the ceramic coating. It also helps to maintain the porosity of the ceramic coating within a suitable range, increasing ion transfer channels and thus improving the ionic conductivity of the battery separator.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] In one embodiment of this application, the battery separator has a thermal shrinkage rate of ≤3.5% 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.

[0046] In one embodiment of this application, the air permeability of the battery separator is 208~330s / 100cc, and the electrolyte wetting rate is ≥15mm / min. The air permeability of the battery separator and the electrolyte wetting rate are within the above ranges, which is beneficial for ion transport.

[0047] In one embodiment of this application, the peel strength of the coating is ≥20MPa. When the peel strength of the coating is within the above range, the separator can maintain structural stability during the winding / stacking process, without stress concentration failure, which is beneficial to improving the mechanical properties of the battery separator.

[0048] In one embodiment of this application, the cross-sectional pore area ratio of the aramid coating along its thickness direction is 15% to 45%. For example, the cross-sectional pore area ratio of the aramid coating can be 15%, 20%, 25%, 30%, 35%, 40%, 45%, or a range consisting 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.

[0049] 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, the aramid fibers being selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers or aramid type III para-aramid fibers, the glass transition temperature Tg1 of the aramid fibers being ≥250℃; mixing and dispersing the aramid raw solution and first ceramic particles uniformly to obtain an aramid coating slurry with a solid content of 2.5wt% to 8wt%; providing a ceramic coating slurry, the ceramic coating slurry comprising second ceramic particles, the ceramic coating slurry having a solid content of 15wt% to 50wt%; coating the aramid coating slurry onto at least one surface of a substrate layer, subjecting it to a coagulation bath, washing with water, and drying 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 it to obtain a battery separator. The battery separator preparation method provided in this application, by coating an aramid coating on the surface of a substrate layer and then coating a ceramic coating on the surface of the aramid coating, can obtain a battery separator with low thermal shrinkage, high membrane breakage temperature, and good flexibility. Moreover, the preparation process is simple and can improve the safety performance of secondary batteries.

[0050] For example, the solid content of aramid coating slurry can be 2.5wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, 5.5wt%, 6wt%, 6.5wt%, 7wt%, 7.5wt%, 8wt%, or any combination of two of these values; the solid content of ceramic coating slurry can be 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 50wt%, or any combination 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.

[0051] In this application, 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 40wt% to 70wt% for 0.1 min to 1 min. The coagulation solvent is an organic solvent and water. The organic solvent is selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAC), and N,N-dimethylacetamide (DMAF), and the concentration of the coagulation bath is the same as the concentration of the organic solvent. This application does not particularly limit the washing process, 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 also does not particularly limit the drying process, 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.

[0052] 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 1% to 3%, the mass percentage of the auxiliary agent is 1% to 8%, and the mass percentage of the organic solvent is 89% to 98%. For example, the mass percentage of aramid fiber can be 1%, 1.2%, 1.4%, 1.6%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.5%, 3%, or any two of these values; the mass percentage of the auxiliary agent can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or any two of these values; and the mass percentage of the organic solvent can be 89%, 89.8%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 97.2%, 98%, or any two of these values. The mass percentages of aramid fibers, auxiliaries, and organic solvents within the above-mentioned ranges are conducive to the full dissolution of aramid fibers and the formation of a stable network structure in the coagulation bath.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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 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.

[0057] 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.

[0058] In this application, the particle size Dv50 of the first ceramic particles and the particle size Dv50 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 decreases Dv50, while shortening the ball milling time increases Dv50.

[0059] 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.

[0060] 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 Dv50, the second ceramic particle with the required Dv50, the first binder with the required Tg2, and the second binder with the required Tg3 can be selected as needed.

[0061] 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.

[0062] In some implementation schemes, the Dv50 of the first ceramic particle and the Dv50 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 measured, and the values ​​of the 50% particle size obtained are the Dv50 of the first and second ceramic particles. 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 (such as the substrate layer or aramid fiber) can be completely burned off at a high temperature of 600°C to obtain the first ceramic particles.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] Example

[0067] 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.

[0068] Test methods and equipment:

[0069] Glass transition temperature test

[0070] 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.

[0071] 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.

[0072] Thermal decomposition temperature test

[0073] 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.

[0074] 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.

[0075] Dv50 Test

[0076] 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. The number of first and second ceramic particles was counted as 4000 each.

[0077] Thickness test

[0078] 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.

[0079] Areal density test

[0080] 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.

[0081] Heat shrinkage rate test

[0082] 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%.

[0083] Breathability test

[0084] 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.

[0085] Membrane rupture temperature test

[0086] 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.

[0087] Flexibility test

[0088] 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.

[0089] Aramid coating internal pore morphology test

[0090] 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).

[0091] 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).

[0092] (1),

[0093] Average aperture dn: Calculate the average aperture of each measured aperture according to formula (2).

[0094] (2),

[0095] Where ∑d is the sum of the diameters d of all holes, and n is the number of holes.

[0096] 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).

[0097] (3-1)

[0098] (3-2)

[0099] 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.

[0100] Hole area percentage S%: The percentage of the surface hole area to the total surface area. Specifically calculated using formula (4):

[0101] (4),

[0102] Wherein, ∑Sm is the sum of the areas observed by the SEM (i.e., the sum of the areas of the photographed regions).

[0103] Example 1-1

[0104] <Preparation of Aramid Coating Slurry>

[0105] 84 kg of NMP solvent (Henan Maiqi Chemical Co., Ltd., purity ≥99.5%) and 5.45 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. Then, 2.658 kg of p-phenylenediamine (Inner Mongolia Kailidi Chemical Co., Ltd., purity >99.5%) was added, and stirring continued until the solid was completely dissolved. The solution temperature was lowered to 10 ± 0.5 °C using chilled water. 4.860 kg of terephthaloyl chloride (Ningxia Fenghua Biotechnology Co., Ltd., purity >99.5%) was added slowly in multiple batches, and the mixture was stirred at a constant temperature for 4 h to obtain an aromatic type II para-aramid polymerization solution. Calcium oxide (Tianjin Xintaiyi Technology Co., Ltd., analytical grade) was added to adjust the pH of the adhesive solution to 6. The solution was further diluted with 4.64% NMP / CaCl2 solution to obtain a para-aramid type II solution with a mass percentage of 2%, calcium chloride as an auxiliary agent with a mass percentage of 5%, and NMP as an organic solvent with a mass percentage of 93%. The rotational viscosity of the tested material was 13000 mPa·s, and the specific logarithmic viscosity was 1.6 g / dL.

[0106] 294 parts of 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, followed by uniform dispersion in a high-pressure homogenizer at 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 and the particle size Dv50 of the first ceramic particles are shown in Table 1.

[0107] <Preparation of Ceramic Coating Slurry>

[0108] 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 particle size Dv50 of the second ceramic particles is shown in Table 2.

[0109] <Preparation of Battery Separator>

[0110] 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.

[0111] Examples 1-2 to Examples 1-3

[0112] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0113] Examples 1-4

[0114] Except for adjusting the Tg1 and Td of the aramid fiber according to Table 1, the rest is the same as in Example 1-1. Specifically, the Tg1 and Td are adjusted as shown in Table 1 by controlling the mass ratio of p-phenylenediamine to terephthaloyl chloride.

[0115] Examples 1-5

[0116] Except for the preparation method used in <Preparation of Aramid Coating Slurry>, the rest is the same as in Example 1-1.

[0117] <Preparation of Aramid Coating Slurry>

[0118] In a 200 kg reactor, 83.5 kg of NMP solvent (Henan Maiqi Chemical Co., Ltd., purity ≥99.5%) and 4.2 kg of anhydrous calcium chloride (Xilong Scientific Co., Ltd., AR) were added. The mixture was heated to 80 °C under a nitrogen atmosphere and stirred for 4 h to completely dissolve the calcium chloride. Subsequently, 2.68 kg of p-aminobenzoic acid (Inner Mongolia Kailidi Chemical Co., Ltd., purity >99.5%) was added, and stirring continued until the solid was completely dissolved. The solution temperature was lowered to 15 ± 0.5 °C using chilled water, and 0.85 kg of pyridine (Ningxia Fenghua Biotechnology Co., Ltd., purity >99.5%) was added as a catalyst. The mixture was stirred at a constant temperature for 3.5 h to obtain an aromatic type I para-aramid polymerization solution. Calcium oxide (Tianjin Xintaiyi Technology Co., Ltd., analytical grade) was added to adjust the pH of the adhesive solution to 6. The solution was further diluted with 4.64% NMP / CaCl2 solution to obtain a 2% (w / w) para-aramid aramid stock solution of type I, with 5% (w / w) of calcium chloride and pyridine as auxiliaries and 93% (w / w) of NMP as the organic solvent. The rotational viscosity of the tested material was 12500 mPa·s, and the specific logarithmic viscosity was 1.48 g / dL.

[0119] 294 parts of 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, followed by uniform dispersion in a high-pressure homogenizer at 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 and the particle size Dv50 of the first ceramic particles are shown in Table 1.

[0120] Examples 1-6

[0121] Except for the preparation method used in <Preparation of Aramid Coating Slurry>, the rest is the same as in Example 1-1.

[0122] <Preparation of Aramid Coating Slurry>

[0123] 82.8 kg of NMP solvent (Henan Maiqi Chemical Co., Ltd., purity ≥99.5%) and 5.5 kg of anhydrous calcium chloride (Xilong Scientific Co., Ltd., AR) were added to a 200 kg reactor. The mixture was heated to 82 °C under a nitrogen atmosphere and stirred for 4.5 h to completely dissolve the calcium chloride. Subsequently, 2.42 kg of p-phenylenediamine (Inner Mongolia Kailidi Chemical Co., Ltd., purity >99.5%) and 0.38 kg of 5(6)-amino-2-(4-aminophenyl)benzimidazole (M3, Ningxia Fenghua Biotechnology Co., Ltd., purity >99.5%) were added, and stirring continued until the solid was completely dissolved. The solution temperature was lowered to 8 ± 0.5 °C using chilled water, and 4.85 kg of terephthaloyl chloride (Ningxia Fenghua Biotechnology Co., Ltd., purity >99.5%) was added slowly in multiple batches and stirred at a constant temperature for 4 h to obtain the aromatic type III para-aramid polymerization solution. Calcium oxide (Tianjin Xintaiyi Technology Co., Ltd., analytical grade) was added to adjust the pH of the adhesive solution to 6. The solution was further diluted with 4.64% NMP / CaCl2 solution to obtain a 2% (w / w) para-aramid aramid stock solution of type III, with 5% (w / w) calcium chloride as an auxiliary agent and 93% (w / w) NMP as an organic solvent. The rotational viscosity of the tested material was 13200 mPa·s, and the specific logarithmic viscosity was 1.72 g / dL.

[0124] 294 parts of 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, followed by uniform dispersion in a high-pressure homogenizer at 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 and the particle size Dv50 of the first ceramic particles are shown in Table 1.

[0125] Examples 1-7

[0126] Except for adjusting the Tg1 and Td of the aramid fiber according to Table 1, the rest is the same as in Example 1-1. Specifically, the Tg1 and Td are adjusted as shown in Table 1 by controlling the mass ratio of p-phenylenediamine to terephthaloyl chloride.

[0127] Examples 1-8 to Examples 1-14

[0128] Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0129] Examples 1-15 to Examples 1-23

[0130] Except for adjusting the corresponding preparation parameters according to Table 2, the rest is the same as in Examples 1-1.

[0131] Comparative Example 1

[0132] Except for the preparation method of the battery separator, the rest is the same as in Example 1-1.

[0133] <Preparation of Battery Separator>

[0134] 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.

[0135] Comparative Example 2

[0136] Except for adjusting the Tg1 and Td of the aramid fiber according to Table 1, the rest is the same as in Example 1-1. Specifically, the Tg1 and Td are adjusted as shown in Table 1 by controlling the mass ratio of p-phenylenediamine to terephthaloyl chloride.

[0137] Comparative Example 3

[0138] Except for the preparation method of the battery separator, the rest is the same as in Example 1-1.

[0139] <Preparation of Battery Separator>

[0140] 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.

[0141] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Tables 1 to 3.

[0142] Table 1

[0143] Table 2

[0144] Table 3

[0145] Note: " / " in Table 3 indicates that it does not contain or cannot be tested.

[0146] As can be seen from Examples 1-1 to 1-23 and Comparative Examples 1 to 3, this application, by sequentially coating an aramid coating and a ceramic coating on at least one surface of the substrate layer, and by controlling the physical properties of the substrate layer, the aramid coating, and the ceramic coating within the scope of this application, enables the battery separator to simultaneously possess a lower thermal shrinkage rate, good flexibility, and a high rupture temperature. However, the battery separator of Comparative Example 1 did not have a ceramic coating, the glass transition temperature (Tg1) of the aramid fiber in Comparative Example 2 was outside the scope of this application, and Comparative Example 3 did not have an aramid coating; its battery separator had a lower rupture temperature and a higher thermal shrinkage rate, or the thermal shrinkage rate could not be measured due to poor heat resistance. Furthermore, the battery separator of Comparative Example 3 exhibited cracking and breakage, indicating that the battery separators of Comparative Examples 1 to 3 could not simultaneously possess a lower thermal shrinkage rate, good flexibility, and a high rupture temperature.

[0147] As can be seen from Examples 1-1 to 1-23, the areal density and thickness of the substrate layer, aramid coating, and ceramic coating; the range of values ​​for η; the type, glass transition temperature Tg1, thermal decomposition temperature Td, and mass percentage of the aramid fiber; the type, particle size Dv50, 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 affect the thermal shrinkage rate, flexibility, and rupture temperature of the battery separator. By controlling the above-mentioned influencing factors within the scope of this application, the battery separator can simultaneously possess a lower thermal shrinkage rate, good flexibility, and a high rupture temperature.

[0148] 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.

[0149] 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.

[0150] 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, 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, wherein the aramid fibers are selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers, or aramid type III para-aramid fibers, and the glass transition temperature Tg1 of the aramid fibers is ≥250℃. The ceramic coating includes 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.

2. The battery separator of claim 1, wherein, a face density of the ceramic coating is ml g / m 2 a face density of the aramid coating is m2 g / m 2 a face density of the substrate layer is m3 g / m 2 a thickness of the ceramic coating is pi pm, a thickness of the aramid coating is p2 pm, a thickness of the substrate layer is p3 pm, 1.0 < η < 2.

0.

3. The battery separator of claim 2, wherein, 1.4≤m1 / μ1≤2.5, 0.4≤m2 / μ2≤1.2, 0.3≤m3 / μ3≤0.8, 0.5≤μ1≤3.5, 0.5≤μ2≤3.5, 5≤μ3≤16.

4. The battery separator of claim 1, wherein, The thermal decomposition temperature Td of the aramid fiber is ≥520℃.

5. The battery separator of claim 1, wherein, Based on the quality of the aramid coating, the aramid fiber has a mass percentage content of 20% to 70%, and the first ceramic particle has a mass percentage content of 30% to 80%.

6. The battery separator according to claim 1, wherein, The particle size Dv50 of the first ceramic particle is 50nm to 110nm, and the particle size Dv50 of the second ceramic particle is 200nm to 600nm.

7. 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℃.

8. The battery separator of claim 7, 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.

9. The battery separator of claim 7 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%.

10. The battery separator of claim 1, wherein, The cross-sectional area of ​​the aramid coating along its thickness direction accounts for 15% to 45%.

11. A method for preparing a battery separator as described in any one of claims 1 to 10, comprising the following steps: An aramid solution is provided, the aramid solution comprising aramid fibers, the aramid fibers being selected from at least one of aramid type I para-aramid fibers, aramid type II para-aramid fibers, or aramid type III para-aramid fibers, wherein the glass transition temperature Tg1 of the aramid fibers is ≥250℃; the aramid solution and first ceramic particles are mixed and dispersed uniformly to obtain an aramid coating slurry with a solid content of 2.5wt% to 8wt%; 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 slurry is applied to at least one surface of the substrate layer, and then subjected to a coagulation bath, water washing, and drying 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 making according to 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 1% to 3%, the auxiliaries have a mass percentage content of 1% to 8%, and the organic solvent has a mass percentage content of 89% to 98%.

13. A secondary battery comprising the battery separator according to any one of claims 1 to 10, or the battery separator prepared by the preparation method according to claim 11 or 12.

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