Coating composition for battery separator, composite separator, secondary battery

Abstract

The application provides a coating composition for a battery separator, a composite separator and a secondary battery, wherein the coating composition comprises: first polymer particles, second polymer particles, inorganic fillers and a binder, wherein the melting point of the first polymer particles is greater than or equal to 90 DEG C to less than or equal to 120 DEG C, and the melting point of the second polymer particles is greater than 120 DEG C to less than or equal to 150 DEG C; the total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 5% to 45%; and the mass ratio of the first polymer particles to the second polymer particles is 1:0.2 to 5. According to the application, the coating composition can be used to prepare a coating on the surface of a base film to obtain a composite separator, and the composite separator has a low heat closure temperature and heat shrinkage rate, and can significantly improve the safety performance of a battery.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a coating composition for battery separators, a composite separator, and a secondary battery. Background Technology

[0002] Lithium-ion batteries are widely used due to their high specific energy, small size, and light weight. However, lithium-ion batteries also have safety hazards. The frequent reports of battery explosions and other disasters have raised questions about the safety performance of lithium-ion batteries. The battery separator is an important part of ensuring battery safety.

[0003] Currently, commercially available batteries primarily use polyolefin membrane materials with microporous structures, such as single-layer or multi-layer membranes of polyethylene and polypropylene. Due to the inherent characteristics of the polymer, although polyolefin membranes can provide sufficient mechanical strength and chemical stability at room temperature, they exhibit significant thermal shrinkage under high-temperature conditions, leading to short circuits at the positive and negative electrode contacts and causing thermal runaway.

[0004] Therefore, it is necessary to improve the thermal performance of the battery separator in order to enhance battery safety. Summary of the Invention

[0005] This application provides coating compositions for battery separators, composite separators, and secondary batteries, aiming to reduce the thermal closure temperature and thermal shrinkage rate of battery separators to improve battery safety performance.

[0006] In a first aspect, this application provides a coating composition for a battery separator, comprising: first polymer particles, second polymer particles, inorganic filler, and binder.

[0007] Wherein, the melting point of the first polymer particle is greater than or equal to 90°C and less than or equal to 120°C, and the melting point of the second polymer particle is greater than 120°C and less than or equal to 150°C.

[0008] The total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 5% to 45%;

[0009] The mass ratio of the first polymer particle to the second polymer particle is 1:0.2 to 5.

[0010] According to this application, the above-mentioned coating composition can be used to prepare a coating on the base film surface to obtain a composite separator. The composite separator has a low thermal shut-off temperature and thermal shrinkage rate. When applied to a battery, it can play a thermal shutdown role at a lower temperature, thereby preventing thermal runaway caused by further temperature increases. At the same time, the low thermal shrinkage rate can ensure that the positive and negative electrodes of the battery do not short-circuit at higher temperatures, thereby significantly improving the safety performance of the battery.

[0011] In some embodiments of this application, the particle size Dv50 of the first polymer particles is 2–8 μm; and / or

[0012] The particle size Dv50 of the second polymer particles is 0.1–4 μm; and / or

[0013] The particle size Dv50 of the inorganic filler is 0.1–4 μm.

[0014] In some embodiments of this application, the first polymer particles comprise one or more of Fischer-Tropsch wax, polyethylene wax, and ethylene-vinyl acetate copolymer; and / or

[0015] The second polymer particles comprise one or more of polyethylene, polymethyl methacrylate, and polypropylene; and / or

[0016] The inorganic filler includes one or more of magnesium hydroxide, boehmite, and alumina; and / or

[0017] The binder includes one or more of the following: polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, and polyvinylpyrrolidone.

[0018] In some embodiments of this application, the inorganic filler is in the shape of a sheet and / or a sphere.

[0019] In some embodiments of this application, the total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 9% to 38%; and / or

[0020] The inorganic filler constitutes 57% to 85.5% of the coating composition by mass; and / or

[0021] The adhesive constitutes 4% to 10% of the coating composition by mass.

[0022] Secondly, this application provides a composite separator, comprising: a base membrane; and

[0023] A coating formed by a coating composition according to any embodiment of the first aspect is disposed on at least one surface of the base film.

[0024] In some embodiments of this application, the areal density of the coating is 2–6 g / m³. 2 .

[0025] In some embodiments of this application, the thermal closure temperature of the composite diaphragm is 100–130°C; and / or

[0026] The composite diaphragm, when kept at 130°C for 10 minutes, has an air permeability value >1000s / 100cc, and the air permeability value growth rate is 1000% to 10000%.

[0027] In some embodiments of this application, the composite diaphragm, after being kept at 130°C for 10 minutes, exhibits a thermal shrinkage rate of <3% in both the transverse and machine directions; and / or

[0028] The composite diaphragm, after being kept at 130°C for 1 hour, exhibits a thermal shrinkage rate of <5% in both the transverse and machine directions.

[0029] Thirdly, this application provides a secondary battery, comprising: a positive electrode, a negative electrode, an electrolyte, and a composite separator according to any embodiment of the second aspect. Attached Figure Description

[0030] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0031] Figure 1 This is a schematic diagram of a composite diaphragm in one embodiment of this application.

[0032] Figure 2 This is a schematic diagram of a composite diaphragm in one embodiment of this application.

[0033] In the figure, 1-first polymer particle, 2-second polymer particle, 3-inorganic filler, 4-base film.

[0034] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0035] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0036] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0037] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0038] It should be noted that, in the context of this application, unless otherwise specified, any description of the melting point of the polymer refers to crystalline polymers with a fixed melting point, which can be detected by existing methods and instruments, such as differential scanning calorimetry (DSC).

[0039] As described in the background section, the battery separator is a crucial component for ensuring battery safety. It plays a vital role in conducting ions, isolating the positive and negative electrodes, and preventing short circuits. Currently, commercially available battery separators are primarily made of polyolefin materials with microporous structures. While these polyolefin materials provide sufficient mechanical strength and chemical stability at room temperature, they experience severe thermal shrinkage at high temperatures. This can lead to direct contact and short circuits between the positive and negative electrodes, causing thermal runaway and potentially resulting in serious safety incidents.

[0040] To address these issues, existing technologies include coatings, such as ceramic coatings, on the surface of polyolefin separators. The resulting composite separator exhibits low thermal shrinkage at high temperatures, effectively resolving the short-circuit problem at the positive and negative electrode contacts. However, this does not affect the thermal shut-off temperature of the polyolefin separator. The separator still only melts when the temperature reaches its melting point (>130°C), causing the micropores to disappear and blocking lithium-ion conduction, thus achieving its thermal shut-off effect. Therefore, the thermal shut-off temperature of the composite separator must reach the melting point of the separator, preventing timely thermal shut-off and reducing battery safety.

[0041] To address the aforementioned problems, this application provides a coating composition for battery separators, which can be used to prepare a coating on a base film surface to obtain a composite separator. This composite separator has a lower thermal closure temperature and thermal shrinkage rate, thereby improving the safety performance of the battery. The technical solution of this application is described in detail below.

[0042] In a first aspect, this application provides a coating composition for a battery separator, comprising: first polymer particles, second polymer particles, inorganic filler, and binder.

[0043] The first polymer particle has a melting point of 90°C or higher to 120°C or lower, and the second polymer particle has a melting point of 120°C or higher to 150°C or lower.

[0044] The total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 5% to 45%.

[0045] The mass ratio of the first polymer particle to the second polymer particle is 1:0.2 to 5.

[0046] According to this application, the coating composition can be used to prepare a coating on the surface of a base film to obtain a composite diaphragm, as shown in the schematic diagram of the obtained composite diaphragm. Figure 1 As shown, the coating consists of first polymer particles 1, second polymer particles 2, inorganic filler 3, and binder (binder is not shown in the figure). Since the coating is a porous structure obtained by the accumulation of particles and fillers, it does not affect the liquid phase transport of ions.

[0047] In the coating, the first polymer particles have a lower melting point, while the second polymer has a higher melting point. When the temperature is between the melting points of the first and second polymer particles, the first polymer particles melt and collapse, blocking ion channels. The second polymer particles expand upon heating but do not melt, effectively reducing the pore size of the coating. The combined effect of these two particles effectively blocks the ion channels of the composite separator, resulting in a lower thermal shutdown temperature. This allows for thermal shutdown at lower temperatures, improving battery safety. Furthermore, when the temperature exceeds the melting point of the second polymer, it melts and collapses, further blocking ion channels and achieving a second shutdown effect, further enhancing battery safety.

[0048] The inorganic filler and the high-melting-point second polymer particles in the coating form a skeleton, which enables the composite separator to have a low thermal shrinkage rate. This avoids thermal runaway caused by direct contact between the positive and negative electrodes in the battery at high temperatures, thus improving the battery's safety performance.

[0049] Specifically, the melting point of the first polymer particles is greater than or equal to 90°C and less than or equal to 120°C. If the temperature is too low, the thermal closure temperature of the composite separator may be too low, affecting the high-temperature cycle performance of the battery. If the temperature is too high, the thermal closure temperature of the composite separator cannot be effectively reduced. For example, the melting point of the first polymer particles can be 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, or any of the above values. The melting point of the second polymer particles is greater than 120°C and less than or equal to 150°C. If the temperature is too low, the thermal shrinkage rate of the composite separator may increase, affecting the safety performance of the battery. If the temperature is too high, the thermal expansion effect may not be able to achieve a lower thermal closure temperature in conjunction with the first polymer particles. For example, the melting point of the second polymer particles can be 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, or any of the above values.

[0050] According to this application, the total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 5% to 45%. This is because if the total mass fraction of the two polymer particles in the coating composition is too low, the thermal shut-off temperature of the composite separator will be too high, making it impossible to achieve the thermal shut-off effect at a lower temperature. However, if the total mass fraction is too high, it will affect the thermal shrinkage performance of the composite separator. Therefore, when the total mass fraction is controlled between 5% and 45%, the resulting composite separator has both a low thermal shut-off temperature and a low thermal shrinkage rate, which can effectively improve the safety performance of the battery. For example, the total mass fraction of the first polymer particles and the second polymer particles in the coating composition can be 5%, 10%, 15%, 20%, 35%, 40%, 45%, or any of the above values.

[0051] In this application, the mass ratio of the first polymer particles to the second polymer particles is defined as 1:0.2 to 5. As mentioned above, the melting and collapse of the first polymer particles in the coating, combined with the thermal expansion of the second polymer particles, gives the composite membrane a lower thermal closure temperature. At the same time, the second polymer, together with the inorganic filler, affects the thermal shrinkage performance of the composite membrane. Therefore, if there are too many first polymer particles, it will affect the thermal shrinkage performance of the composite membrane; if there are too many second polymer particles, it may increase the thermal closure temperature of the composite membrane. Thus, the mass ratio of the first polymer particles to the second polymer particles can be controlled within the range of 1:0.2 to 5. For example, the mass ratio can be 1:0.2, 1:0.4, 1:0.6, 1:0.9, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5, 1:4, 1:4.5, 1:5, or any of the above values.

[0052] It is understood that this application does not further limit the mass fraction of inorganic fillers and binders. After determining the content of the first polymer particles and the second polymer particles in the coating composition, those skilled in the art can adjust the content of inorganic fillers and binders according to actual needs. The role of the binder is to bond the components together and to stably bond the coating to the base film. Its type and content can be selected according to actual conditions. It is understood that, in addition to the above-mentioned components, those skilled in the art can add small amounts of other known functional additives to the coating composition as needed.

[0053] In some embodiments of this application, the particle size Dv50 of the first polymer particles is 2–8 μm; and / or

[0054] The particle size Dv50 of the second polymer particles is 0.1–4 μm; and / or

[0055] The particle size Dv50 of the inorganic filler is 0.1–4 μm.

[0056] In some of the above embodiments, the particle sizes of the first polymer particles, the second polymer particles, and the inorganic filler are further defined. The particle size Dv50 of the first polymer particles can be 2–8 μm, which is larger than that of the second polymer particles and the inorganic filler. Figure 1 As shown, the larger particle size can allow the first polymer particles to penetrate the coating. During the cell manufacturing process, due to the need for hot pressing, the low melting point of the first polymer particles can effectively bond the positive and / or negative electrodes of the battery, which helps to bond the composite separator to the positive and negative electrodes of the battery, thereby improving the cell rigidity, slowing down the cell deformation caused by battery cycling, and thus improving the cycle life of the battery. For example, the particle size Dv50 of the first polymer particles can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, or any of the above values, preferably 4 to 8μm.

[0057] The particle size Dv50 of the second polymer particles and inorganic filler can be 0.1–4 μm. As the skeleton of the coating, the smaller the particle size, the smaller the thermal shrinkage rate of the composite membrane. However, if the particle size is too small, it may lead to a decrease in the porosity of the coating, affecting the ion transport rate. If the particle size is too large, it may also cause the coating to easily shed slag, which is not conducive to the stability of the coating. Therefore, it is necessary to control the particle size Dv50 of the second polymer particles and inorganic filler within the range of 0.1–4 μm. For example, the particle size Dv50 of the second polymer particles and inorganic filler can be 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, or any of the above values, preferably 0.1–2 μm.

[0058] It should be noted that when the particle sizes of the first polymer particles, the second polymer particles, and the inorganic filler all meet the above-mentioned ranges, the configuration of different particle sizes can make the coating have a suitable porosity and the resulting coating more stable, ensuring the smooth flow of ion transport channels and improving the performance of the battery.

[0059] Furthermore, the dispersion of the first polymer particles (Dv90-Dv10) / Dv50 is 0.1 to 4, for example, it can be 0.1, 0.2, 0.4, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, or any of the above values, preferably 0.1 to 2.

[0060] The first polymer particles have a relatively large particle size, and their dispersion can be controlled within the range of 0.1 to 4. If the dispersion is too large, it may cause the coating to be uneven and prone to flaking, which is not conducive to the stability of the coating. If the dispersion is too low, it will lead to an increase in cost. Therefore, the dispersion of the first polymer particles can be controlled within the above range.

[0061] It should be noted that in this application, Dv10, Dv50, and Dv90 have meanings known in the art. Dv10 indicates that 10% of the particles in the sample's volumetric particle size distribution are smaller than this value, and Dv50 indicates that 50% of the particles in the sample's volumetric particle size distribution are smaller than this value. The above parameters can be determined according to methods and instruments known in the art. For example, they can be determined using a laser particle size analyzer (e.g., Malvern Mastersizer 2000E) according to GB / T19077-2016 Particle Size Distribution Laser Diffraction Method.

[0062] In some embodiments of this application, the inorganic filler is in the shape of a sheet and / or a sphere.

[0063] In some of the above embodiments, when the inorganic filler is in sheet form, a schematic diagram of the composite diaphragm prepared using the coating composition is shown below. Figure 1 As shown, the coating using sheet-like inorganic fillers has a low packing density, resulting in high porosity, better air permeability, and a small increase in air permeability, thus ensuring the ion transport rate. A schematic diagram of the composite membrane prepared using this coating composition when the inorganic filler is spherical is shown below. Figure 2 As shown, the coating with spherical inorganic filler has a high packing density. Although the air permeability is slightly lower than that with sheet-like inorganic filler, it has better heat resistance and the resulting composite membrane has a lower thermal shrinkage rate.

[0064] It is also understood that, based on the above characteristics, those skilled in the art can select a certain proportion of spherical inorganic filler and sheet-like inorganic filler as inorganic filler according to actual needs to obtain a composite membrane with suitable performance.

[0065] In some embodiments of this application, the first polymer includes one or more of Fischer-Tropsch wax, polyethylene wax, and ethylene-vinyl acetate copolymer; and / or

[0066] The second polymer includes one or more of polyethylene, polymethyl methacrylate, and polypropylene; and / or

[0067] Inorganic fillers include one or more of magnesium hydroxide, boehmite, and alumina; and / or

[0068] The binder includes one or more of the following: polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, and polyvinylpyrrolidone.

[0069] In the above embodiments, several types of first polymer particles, second polymer particles, inorganic fillers, and binders are specifically listed, and those skilled in the art can select according to actual needs. It is understood that the first and second polymer particles only need to be polymers that meet the melting point conditions mentioned above, and are not limited to the polymers listed above; the inorganic filler only needs to meet the conditions mentioned above and be stable and not react with the components in the electrolyte, and is not limited to the materials listed above; the binder is also only an example, and those skilled in the art can choose any one or more binders that can be applied to batteries in the prior art.

[0070] In some embodiments of this application, the first polymer particles and the second polymer particles constitute 9% to 38% of the total mass fraction of the coating composition; and / or

[0071] The inorganic filler constitutes 57% to 85.5% of the coating composition by mass; and / or

[0072] The adhesive accounts for 4% to 10% of the mass fraction of the coating composition.

[0073] In some of the above embodiments, the total mass fraction of the first polymer particles and the second polymer particles in the coating composition, as well as the mass fraction of the inorganic filler and binder, are further defined. Under these conditions, the resulting composite separator has a lower thermal closure temperature and thermal shrinkage rate, which can further improve the safety performance of the battery.

[0074] Secondly, this application provides a composite separator, comprising: a base membrane; and

[0075] A coating formed by a coating composition according to any embodiment of the first aspect, disposed on at least one surface of a base film.

[0076] According to this application, the surface of the composite diaphragm has a coating formed according to the coating composition of any embodiment of the first aspect, and therefore the composite diaphragm may have the beneficial effects of any embodiment of the first aspect.

[0077] It is also understood that this application does not limit the material of the base membrane. Commonly used membranes in the art can be used as the base membrane, such as polyolefin membranes or polyolefin multilayer composite membranes, which are lower in cost. More specifically, such as polyethylene membranes, polypropylene membranes, polypropylene-polyethylene-polypropylene composite membranes, etc.

[0078] In some embodiments of this application, the areal density of the coating is 2–6 g / m³. 2 .

[0079] In some of the above embodiments, the areal density of the coating is further limited. If the areal density of the coating is too low, it may lead to an increase in the thermal closure temperature and thermal shrinkage rate of the composite separator, resulting in poor safety performance. On the other hand, if the areal density of the coating is too high, it will increase costs and also affect ion transport efficiency and battery kinetic performance. Therefore, the areal density of the coating can be controlled between 2 and 6 g / m³. 2 Within this range, the resulting composite membrane exhibits a low thermal closure temperature and thermal shrinkage rate, without affecting ion transport efficiency.

[0080] In some embodiments of this application, the thickness of the base film is <16 μm; and / or

[0081] The pore size of the base membrane is 0.01–0.1 μm; and / or

[0082] The porosity of the base membrane is 30%–40%.

[0083] In some of the above embodiments, the thickness, pore size, and porosity of the base membrane are further limited. Since a coating needs to be prepared on the base membrane, the thickness of the base membrane should not be too thick in order not to affect the ion transport efficiency, and can be less than 16 μm. If the pore size and porosity are too small, it will affect the ion transport efficiency. If they are too large, it will increase the thermal closure temperature. Therefore, the pore size can be controlled at 0.01 to 0.1 μm and the porosity can be controlled at 30% to 40%, so the composite membrane has better overall performance.

[0084] In some embodiments of this application, the thermal closure temperature of the composite diaphragm is 100–130°C; and / or

[0085] The composite diaphragm, after being kept at 130℃ for 10 minutes, has an air permeability value >1000s / 100cc, and the air permeability value growth rate is 1000%~10000%.

[0086] In some of the above embodiments, the thermal shut-off temperature of the composite separator was found to be 100–130°C, which is significantly lower than the thermal shut-off temperature of polyolefin separators commonly used in the prior art. This allows it to perform thermal shutdown at lower temperatures, preventing further temperature increases in the battery and improving battery safety. It should be noted that the thermal shut-off temperature of the separator has a meaning known in the art and can be detected using existing methods and instruments. For example, the thermal shut-off temperature can be obtained by using the resistance mutation method based on the impedance-temperature change curve of the tested separator.

[0087] Furthermore, to verify the thermal shutdown effect of the composite membrane, after holding the composite membrane at 130℃ for 10 minutes, the air permeability value of the composite membrane was >1000s / 100cc, and the air permeability value growth rate was 1000%~10000%. It can be understood that the higher the air permeability value of the composite membrane, the worse its air permeability, i.e., the smaller the membrane porosity and the fewer the corresponding ion transport channels, thus resulting in a more significant thermal shutdown effect. The above results demonstrate that the composite membrane can rapidly exert its thermal shutdown effect at high temperatures.

[0088] Similarly, it is understandable that air permeability value and air permeability growth rate have meanings known in the art. Air permeability value can be tested according to existing methods and instruments. The air permeability growth rate is calculated as (air permeability value of the diaphragm after insulation - air permeability value of the diaphragm before insulation) / air permeability value of the diaphragm before insulation × 100%.

[0089] In some embodiments of this application, the composite diaphragm, after being kept at 130°C for 10 minutes, exhibits a thermal shrinkage rate of <3% in both the transverse (TD) and machine direction (MD) directions; and / or

[0090] The composite diaphragm, after being kept at 130℃ for 1 hour, exhibits a thermal shrinkage rate of less than 5% in both the transverse and machine directions.

[0091] In some of the above embodiments, after testing, the composite separator, after being kept at 130°C for 10 minutes, showed a thermal shrinkage rate of less than 3% in both the transverse and machine directions. This indicates that when the composite separator performs its thermal shutdown function, its own thermal shrinkage rate is very low, and it can still effectively isolate the positive and negative electrodes, prevent the positive and negative electrodes from directly contacting and short-circuiting, and further improve the safety performance of the battery.

[0092] Furthermore, after being kept at 130℃ for 1 hour, the thermal shrinkage rate of the composite separator in both the transverse and machine directions was less than 5%. The composite separator can maintain its low thermal shrinkage rate for a long time under high temperature conditions, indicating that the composite separator has excellent thermal shrinkage performance, which can improve the safety performance of the battery.

[0093] It should be noted that the heat shrinkage rates in the transverse direction (TD) and machine direction (MD) have meanings known in the art and can be detected and calculated using existing methods and instruments. For example, the heat shrinkage rate of TD = (length of diaphragm TD before insulation - length of diaphragm TD after insulation) / length of diaphragm TD before insulation × 100%, and the heat shrinkage rate of MD = (length of diaphragm MD before insulation - length of diaphragm MD after insulation) / length of diaphragm MD before insulation × 100%.

[0094] Thirdly, this application provides a secondary battery, comprising: a positive electrode, a negative electrode, an electrolyte, and a composite separator according to any embodiment of the second aspect.

[0095] According to this application, since the secondary battery uses a composite separator according to any embodiment of the second aspect, it has the beneficial effects of any embodiment of the second aspect.

[0096] It should be noted that this application does not limit the types of positive electrode plates, negative electrode plates, and electrolytes. Using commonly used positive electrode plates, negative electrode plates, and electrolytes in the art will have the above-mentioned beneficial effects.

[0097] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0098] Example 1

[0099] Preparation of composite membrane:

[0100] The first polymer particles, the second polymer particles, the inorganic filler, the binder (polyacrylate) and deionized water were mixed, and 0.2 wt% of the wetting agent (alkylphenol polyoxyethylene ether) was added and stirred thoroughly to prepare a uniform slurry with a solid content of 30%.

[0101] The above slurry was uniformly roller-coated onto a 9μm PE membrane with a porosity of 38%, and then thoroughly dried in an oven at 60℃. The surface density of the single-sided coating after drying was 2.88 g / m². 2 .

[0102] The types, melting points, and particle sizes of the first polymer particles and the second polymer particles, as well as the types and particle sizes of the inorganic fillers, are shown in Table 1. The mass fractions of the first polymer particles, the second polymer particles, and the inorganic fillers in the coating are also shown in Table 1.

[0103] Example 2

[0104] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that the coating surface density is 3.66 g / m³. 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0105] Example 3

[0106] Preparation of the composite diaphragm: The preparation method is similar to that in Example 1, except that the coating surface density is 4.16 g / m³. 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0107] Example 4

[0108] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that the coating surface density is 5.12 g / m³. 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0109] Example 5

[0110] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that: the base membrane is a 7μm PE membrane with a porosity of 38%; the coating surface density is 2.12 g / m². 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0111] Example 6

[0112] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that the coating surface density is 2.52 g / m³. 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0113] Example 7

[0114] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that: the base membrane is a 7μm PE membrane with a porosity of 38%; the coating surface density is 2.78 g / m². 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0115] Example 8

[0116] Preparation of the composite diaphragm: The preparation method is similar to that in Example 1, except that the coating surface density is 2.53 g / m³. 2 The first polymer particles, the second polymer particles, and the inorganic filler in the coating are different, and the mass fraction of each component in the coating is different, as shown in Table 1.

[0117] Comparative Example 1

[0118] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that the coating surface density is 2.92 g / m³. 2 The first polymer particles and inorganic fillers in the coating are different, and the second polymer particles are not present. The mass fractions of each component in the coating are different, as shown in Table 1.

[0119] Comparative Example 2

[0120] Preparation of the composite membrane: The preparation method is similar to that in Example 1, except that the coating surface density is 1.73 g / m³. 2 The first polymer particles and the second polymer particles in the coating are different, and there is no inorganic filler. The mass fraction of each component in the coating is different, as shown in Table 1.

[0121] Table 1

[0122]

[0123]

[0124] Note: " / " indicates that the ingredient or parameter is not present.

[0125] Test section The composite membranes obtained in the above embodiments and comparative examples were subjected to the following tests.

[0126] Bond strength test First, the adhesive separator and the positive electrode were hot-pressed together, and then the peel strength between the separator and the electrode was tested. The positive electrode coating consisted of LiCoO2: conductive agent (Carbon ECP): adhesive (PVDF 761) in a ratio of 15:4:4. The hot-pressing conditions were 1 MPa, 95℃, and 5 min. The results are shown in Table 2.

[0127] TD and MD heat shrinkage rate test Cut the diaphragm into 120mm*120mm test samples. Use a steel ruler (0.5mm accuracy) to draw 100mm*100mm perpendicular lines along the TD / MD direction in the middle of the sample. Place the sample between A4 paper and a 130℃ forced-air oven. After maintaining the temperature for a certain period, remove the sample and measure the length of the marked lines. Calculate the heat shrinkage rate. Heat shrinkage rate = (Length of marked lines before insulation - Length of marked lines after insulation) / Length of marked lines before insulation × 100%.

[0128] Thermal closure temperature test The thermal closure temperature of the diaphragm was tested using the resistance catastrophe method: A 60mm*60mm diaphragm was immersed in an electrolyte solution (1mol / L LiPF6, EC:DEC:DMC = 1:1:1) for 1 hour. The diaphragm was then placed in a test fixture between two electrodes and a metal plate. The fixture containing the diaphragm sample was placed in a programmable heating chamber for heating at a rate of 7℃ / min for temperatures <110℃ and 3℃ / min for temperatures between 110℃ and 200℃. The fixture impedance was collected throughout the process, and an impedance-temperature curve was plotted. The thermal closure temperature was determined when the impedance reached 1000Ω. The results are shown in Table 3.

[0129] Breathability test The diaphragm is placed in an air permeability tester, and the time required for 100 mL of air to pass through a 1 square inch diaphragm under a pressure of 1.22 kPa is measured. This time is the air permeability value of the diaphragm. The instrument used is the Asahi Seiko diaphragm air permeability tester EG01-55-1MR.

[0130] Table 2

[0131]

[0132] Table 2 shows the results of the thermal shrinkage rate of the composite separator after being kept at 130°C for 1 hour and 15 minutes. It can be seen that the separator obtained in the examples has low thermal shrinkage performance, which can effectively improve the safety performance of the battery. In Comparative Example 1, the coating does not contain the second polymer particles with a higher melting point, and compared to Example 1, which uses the first polymer particles with a lower melting point, its thermal shrinkage performance is poorer. Comparative Example 2 has the worst thermal shrinkage performance because it does not contain inorganic fillers.

[0133] By comparing Examples 1 and 8, it can be seen that the composite membrane obtained by spherical inorganic fillers has a lower thermal shrinkage rate than that obtained by sheet-like inorganic fillers.

[0134] According to the results of the bonding strength, the melting point, mass fraction and particle size of the first polymer particles in the coating are related to the bonding strength between them and the electrode. Generally speaking, the lower the melting point, the higher the mass fraction and the larger the particle size, the better the bonding strength.

[0135] Table 3

[0136]

[0137] Table 3 shows that, based on the increase in air permeability of the coating and the air permeability of the composite membrane, the composite coating obtained by preparing the coating on the PE membrane will reduce the air permeability of the membrane, but the change is not significant, thus having no significant impact on ion transport. Furthermore, comparing Examples 1 and 8 shows that, compared to spherical inorganic fillers, the sheet-like inorganic filler results in a smaller increase in air permeability and better air permeability.

[0138] Based on the air permeability and thermal shut-off temperature of the separator after 10 minutes of heat preservation at different temperatures, it can be seen that all embodiments and comparative examples have reduced thermal shut-off temperatures and good thermal shutdown effects. However, according to the results in Table 2, Comparative Example 1 and Comparative Example 2 have very poor thermal shrinkage performance. At higher temperatures, severe thermal shrinkage may lead to short circuits between the positive and negative electrodes, causing safety hazards. The composite separators obtained in the embodiments of this application have both low thermal shrinkage rates and low thermal shut-off temperatures, thereby effectively improving the safety performance of the battery.

[0139] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A coating composition for a battery separator, characterized in that, include: The composition comprises first polymer particles, second polymer particles, inorganic fillers, and binders; the particle size Dv50 of the first polymer particles is 2~8μm, the particle size Dv50 of the second polymer particles is 0.1~1.5μm, and the particle size Dv50 of the inorganic fillers is 0.1~4μm. Wherein, the melting point of the first polymer particle is greater than or equal to 90°C and less than or equal to 120°C, and the melting point of the second polymer particle is greater than 120°C and less than or equal to 150°C. The total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 5% to 45%. The mass ratio of the first polymer particle to the second polymer particle is 1:0.2~5.

2. The coating composition according to claim 1, characterized in that, The first polymer particles comprise one or more of Fischer-Tropsch wax, polyethylene wax, and ethylene-vinyl acetate copolymer; and / or The second polymer particles include one or more of polyethylene, polymethyl methacrylate, and polypropylene.

3. The coating composition according to claim 1 or 2, characterized in that, The inorganic filler includes one or more of magnesium hydroxide, boehmite, and alumina; and / or The binder includes one or more of the following: polyacrylate, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, and polyvinylpyrrolidone.

4. The coating composition according to claim 1, characterized in that, The inorganic filler is in the shape of plates and / or spheres.

5. The coating composition according to claim 1, characterized in that, The total mass fraction of the first polymer particles and the second polymer particles in the coating composition is 9% to 38%; and / or The inorganic filler has a mass fraction of 57% to 85.5% in the coating composition; and / or The adhesive has a mass fraction of 4% to 10% in the coating composition.

6. A composite diaphragm, characterized in that, include: Base film; and A coating formed by the coating composition according to any one of claims 1 to 5 is disposed on at least one surface of the base film.

7. The composite diaphragm according to claim 6, characterized in that, The areal density of the coating is 2~6 g / m³. 2 .

8. The composite diaphragm according to claim 6 or 7, characterized in that, The composite diaphragm has a thermal closure temperature of 100~130℃; and / or The composite membrane, when kept at 130°C for 10 minutes, has an air permeability value >1000s / 100cc, and the air permeability value growth rate is 1000%~10000%.

9. The composite diaphragm according to claim 8, characterized in that, The composite diaphragm, after being kept at 130°C for 10 minutes, exhibits a thermal shrinkage rate of <3% in both the transverse and machine directions; and / or The composite diaphragm, when kept at 130°C for 1 hour, exhibits a thermal shrinkage rate of <5% in both the transverse and machine directions.

10. A secondary battery, characterized in that, include: Positive electrode, negative electrode, electrolyte, and composite separator according to any one of claims 6 to 9.

Images