A composite separator for lithium-ion batteries and a lithium-ion battery

Abstract

This invention relates to the field of lithium-ion battery technology, and more particularly to a composite separator for lithium-ion batteries and a lithium-ion battery. The composite separator includes a base film and a ceramic coating, with the ceramic coating applied to one or both sides of the base film. The ceramic coating is composed of an adhesive, ceramic particles, a dispersant, a surfactant, and a thickener. The adhesive is a water-soluble modified polyacrylamide copolymer copolymerized from acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, with a weight ratio of (50-98):(1-30):(1-20) of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers. The composite separator of this invention has high heat resistance, high electrolyte wetting performance, and low moisture content, meeting the requirements of high-performance lithium-ion batteries for high-performance separators.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a composite separator for lithium-ion batteries and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles, electrochemical energy storage, and electronic digital products due to their advantages such as high energy density, long cycle life, good rate performance, and low self-discharge. The rapid development of new energy vehicles has placed higher demands on battery energy density and safety performance. Battery manufacturers commonly increase energy density by improving single-cell capacity, reducing module and packaging materials, and improving space utilization, such as with CTP and CTC technologies. However, increasing single-cell capacity leads to more prominent thermal safety issues, making battery safety more critical and requiring higher dimensional stability of the separator at high temperatures. Furthermore, the increased battery size makes it difficult for the electrolyte to fully wet the battery, affecting internal resistance and ionic conductivity. Poor localized electrolyte wetting can lead to lithium plating, reducing battery capacity, and in severe cases, puncturing the separator. Related technologies often use ceramic-coated separators to improve the separator's heat resistance and wettability. The main components of the ceramic coating are a mixture of polyacrylate or polyacrylic acid adhesives and ceramic particles with a D50 of 0.7-0.9 μm. Polyacrylate adhesives have low glass transition temperatures and large high-temperature deformation of the coating, making it difficult to meet the heat resistance requirements of battery separators. While PAA adhesives can further improve heat resistance, the separator has a very high moisture content, and batteries are very sensitive to moisture, especially high-nickel ternary cathode materials used in high-end, high-energy-density batteries, where high moisture content will cause a significant degradation in battery performance. Although ceramic particles with D50 of 0.7-0.9μm can improve the heat resistance of the separator, their heat resistance and wettability are not as good as those of ceramic particles with smaller particle sizes.

[0003] Therefore, it is necessary to provide a composite separator for lithium-ion batteries that has good high-temperature dimensional stability, as well as high electrolyte wettability and low moisture content. Summary of the Invention

[0004] This invention provides a composite separator for lithium-ion batteries and a lithium-ion battery, addressing the aforementioned technical problems of existing composite separators for lithium-ion batteries. The composite separator of this invention features high heat resistance, high wettability, and low moisture content, thus improving the safety, electrolyte wettability, and moisture issues present in high-energy-density, high-capacity, and large-volume lithium-ion batteries.

[0005] According to a first aspect of the present invention, the present invention provides a composite separator for lithium-ion batteries, the composite separator comprising a base film and a ceramic coating, the ceramic coating being coated on one or both sides of the base film; the ceramic coating comprising an adhesive, ceramic particles, a dispersant, a surfactant, and a thickener;

[0006] The adhesive is a water-soluble modified polyacrylamide copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, the acrylic monomers, and the water-soluble acrylate monomers is (50-98):(1-30):(1-20).

[0007] In the above-described scheme, the composite separator for lithium-ion batteries of the present invention includes a base film and a ceramic coating. The ceramic coating is applied to one or both sides of the base film to improve the heat resistance and wettability of the base film. The ceramic coating is composed of an adhesive, ceramic particles, a dispersant, a surfactant, and a thickener. The adhesive is a water-soluble modified polyacrylamide copolymer copolymerized from acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers. All acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers are non-toxic monomers, serving as polymerizing monomers to form the adhesive. This ensures that the composite separator for lithium-ion batteries provided by the present invention is safe and reliable during preparation and use, reducing adverse impacts on the environment and users. Meanwhile, acrylamide monomers increase the cohesive energy density of the adhesive itself and its chemical interaction with ceramic particles, thereby increasing the bonding strength of the ceramic coating. Acrylic monomers improve the dispersibility of the adhesive in the ceramic coating emulsion, enhance the stability of the ceramic coating, reduce agglomerates, and thus make the adhesive more uniformly distributed in the prepared ceramic coating. Acrylic monomers can also interact with ceramic particles adsorbed with dispersants, enhancing the interaction between ceramic particles and dispersants, thereby further improving the uniformity and bonding strength of the ceramic coating. Water-soluble acrylate monomers can lower the glass transition temperature of the adhesive and improve the flexibility of the ceramic coating. Furthermore, this invention limits the weight ratio of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers to a reasonable range, enabling the prepared adhesive to simultaneously possess the advantages of good bonding performance, good heat resistance, good electrolyte wetting, and low moisture content, meeting the requirements of high-performance lithium-ion batteries for high-performance separators.

[0008] Furthermore, the water-soluble modified polyacrylamide copolymer has a weight-average molecular weight of 2w-40w and a glass transition temperature of 150℃-220℃.

[0009] In the above scheme, by limiting the weight-average molecular weight and glass transition temperature of the water-soluble modified polyacrylamide copolymer to a reasonable range, the prepared adhesive has superior bonding performance, heat resistance, electrolyte wetting performance, and lower moisture content, better meeting the requirements of high-performance lithium-ion batteries for high-performance separators. Furthermore, if the weight-average molecular weight of the water-soluble modified polyacrylamide copolymer is below 2w, the molecular chain segments of the adhesive are too short, resulting in limited support for the separator at high temperatures; if the molecular weight exceeds 40w, the viscosity of the adhesive is too high, easily causing poor leveling and appearance defects after coating with ceramic slurry. A glass transition temperature below 150 degrees Celsius offers limited improvement to the high-temperature dimensional stability of the separator; when the glass transition temperature is above 220 degrees Celsius, the adhesive is too hard and brittle, lacking adhesion to ceramic particles and the base film layer, easily resulting in powder shedding after coating, and failing to possess the most basic bonding function of an adhesive.

[0010] Furthermore, the acrylamide monomer is one or more of acrylamide, methacrylamide, N,N-dimethylacrylamide, N-hydroxymethylacrylamide, N,N-methylenebisacrylamide, N-(2-hydroxyethyl)acrylamide, and N-(2-hydroxypropyl)acrylamide;

[0011] And / or, the acrylic monomer is one or more of acrylic acid, methacrylic acid, itaconic acid, and aconitic acid;

[0012] And / or, the water-soluble acrylate monomer is one or more of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and 3-hydroxypropyl methacrylate.

[0013] In the above scheme, by making reasonable selections of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, the prepared adhesive can have better bonding performance, heat resistance, electrolyte wetting performance, and lower moisture content, which can better meet the requirements of high-performance lithium-ion batteries for high-performance separators.

[0014] Furthermore, the ceramic particles are composed of a mixture of particles with two sizes: D50 of 0.2-0.4 μm and 0.7-0.9 μm, respectively; wherein the ceramic particles with D50 of 0.2-0.4 μm account for 50-80% of the total weight of the ceramic particles, and the ceramic particles with D50 of 0.7-0.9 μm account for 20-50% of the total weight of the ceramic particles.

[0015] In the above-described scheme, this invention employs a mixture of ceramic particles of different sizes to avoid the defects associated with using particles of a single size. Combined with a water-soluble modified polyacrylamide copolymer, the resulting composite separator exhibits advantages such as good heat resistance, low moisture content, and good electrolyte wetting. Using only small ceramic particles would result in a very high moisture content in the composite separator; using only large ceramic particles would degrade the heat resistance and electrolyte wetting performance. The mixture of both large and small ceramic particles simultaneously achieves the advantages of good heat resistance, good wetting, and low moisture content. Furthermore, by limiting the proportion of ceramic particles of different sizes within a more reasonable range, the resulting composite separator exhibits even superior performance.

[0016] Furthermore, the particle size distribution of the ceramic particles satisfies 1≤(D90-D10) / D50≤2.

[0017] In the above scheme, if (D90-D10) / D50>2, it indicates that the ceramic particle size distribution is very wide, the heat resistance of the composite membrane will be worse, the moisture content will be higher, and the presence of large ceramic particles will worsen the processing performance; if (D90-D10) / D50<1, it indicates that the ceramic particle size distribution is narrow. Although the performance meets the requirements, the production cost of ceramic particles is high and it is not suitable for large-scale commercialization.

[0018] Furthermore, the ceramic particles comprise one or more of the following: alumina, boehmite, barium titanate, magnesium oxide, silicon oxide, and magnesium hydroxide.

[0019] In the above scheme, the overall performance of the composite membrane can be further improved by selecting appropriate types of ceramic particles.

[0020] Further, by weight, the ceramic coating consists of 3-6 parts of adhesive, 90-96 parts of ceramic particles, 0.1-4 parts of dispersant, 0.05-0.5 parts of surfactant, and 0.05-3 parts of thickener; and / or, the thickness of the ceramic coating is 1-4 μm.

[0021] In the above scheme, by limiting the amount of each raw material in the ceramic coating to a reasonable range, the overall performance of the composite membrane can be further improved. Furthermore, by limiting the thickness of the ceramic coating to a reasonable range, the overall performance of the composite membrane can be further improved.

[0022] Furthermore, the dispersant is sodium polyacrylate or ammonium polyacrylate;

[0023] And / or, the surfactant is an acetylenic diol copolymer or a polyether siloxane copolymer;

[0024] And / or, the thickener is sodium carboxymethyl cellulose or polyvinyl alcohol.

[0025] In the above scheme, by limiting the types of dispersants, surfactants and thickeners, the raw materials can achieve better synergistic effects, resulting in better adhesive performance and further improving the overall performance of the composite membrane.

[0026] Further, the base membrane is a polyethylene porous film, a polypropylene porous film, or a polypropylene / polyethylene / polypropylene composite porous film; and / or, the thickness of the base membrane is 3-20 μm; and / or, the porosity of the base membrane is 20-80%.

[0027] In the above scheme, by reasonably limiting the type, thickness and porosity of the base membrane, the overall performance of the composite membrane can be better improved, so that the composite membrane has good high-temperature dimensional stability while having high electrolyte wettability and low moisture content.

[0028] Furthermore, after baking at 180°C for 30 minutes, the composite membrane exhibits a thermal shrinkage rate of less than 3% in both its longitudinal and transverse directions; the electrolyte wetting area of ​​the composite membrane is >100 mm². 2 The moisture content of the composite separator is <1000ppm. Such a composite separator can meet the requirements of high-performance lithium-ion batteries for high-performance separators.

[0029] According to a second aspect of the present invention, the present invention also provides a lithium-ion battery, including a battery assembly, the battery assembly including a battery separator, characterized in that the battery separator is the composite separator described above.

[0030] The present invention provides a composite separator for lithium-ion batteries, which uses a water-soluble modified polyacrylamide copolymer as a ceramic coating adhesive, giving it the advantages of good heat resistance, good electrolyte wetting, and low moisture content, thus meeting the requirements of high-performance lithium-ion batteries for high-performance separators.

[0031] The present invention provides a composite separator for lithium-ion batteries, which uses a mixture of ceramic particles of different sizes and a water-soluble modified polyacrylamide copolymer. The resulting composite separator has the advantages of good heat resistance, low moisture content, and good electrolyte wetting. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0033] Figure 1This is a schematic diagram of composite membrane sample preparation in electrolyte wetting test for embodiments and comparative examples of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0035] Example 1

[0036] This embodiment provides a composite separator for lithium-ion batteries, comprising a polyethylene porous membrane and ceramic coatings coated on both sides of the polyethylene porous membrane. The polyethylene porous membrane has a thickness of 10 μm and a porosity of 46%. The ceramic coating on one side has a thickness of 1.5 μm, and the sum of its thickness and that of the ceramic coating on the other side is 3 μm. The ceramic coating is composed of 4 parts of adhesive, 93.6 parts of ceramic particles, 2 parts of dispersant, 0.2 parts of surfactant, and 0.2 parts of thickener.

[0037] The adhesive contained in the ceramic coating is a water-soluble modified polyacrylamide copolymer with a weight-average molecular weight of 10w and a glass transition temperature of 191℃.

[0038] Specifically, the water-soluble modified polyacrylamide copolymer is a copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers is 80:15:5. Specifically, the acrylamide monomer is acrylamide, the acrylic monomer is methacrylic acid, and the water-soluble acrylate monomer is 2-hydroxyethyl acrylate.

[0039] The ceramic coating contains alumina particles, which are composed of a mixture of particles with two sizes: D50 of 0.2-0.4μm and 0.7-0.9μm. The alumina particles with D50 of 0.2-0.4μm account for 60% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.5; the alumina particles with D50 of 0.7-0.9μm account for 40% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.6.

[0040] The dispersant in the ceramic coating is ammonium polyacrylate.

[0041] The surfactant contained in the ceramic coating is an acetylenic diol copolymer.

[0042] The thickener contained in the ceramic coating is polyvinyl alcohol.

[0043] The composite separator is prepared as follows: High-purity water and a dispersant are added to a mixing tank and stirred for 5 minutes. Ceramic particles are then added and stirred for 30 minutes. A thickener is added and stirred for another 30 minutes. The mixture is then ground once using a sand mill. An adhesive is added and stirred for 30 minutes. A surfactant is then added and stirred for another 30 minutes to obtain a ceramic slurry for the composite separator. The ceramic slurry is transferred to a porous polyethylene membrane using a microgravure coating method. After drying, a composite separator for lithium-ion batteries with good heat resistance, low moisture content, and good electrolyte wetting is obtained.

[0044] Example 2

[0045] This embodiment provides a composite separator for lithium-ion batteries, comprising a polyethylene porous membrane and ceramic coatings applied to both sides of the polyethylene porous membrane. The polyethylene porous membrane has a thickness of 9 μm and a porosity of 48%. The ceramic coating on one side has a thickness of 1.5 μm, and the sum of the thicknesses of the ceramic coating on one side and the ceramic coating on the other side is 3 μm. The ceramic coating is composed of 3.5 parts of adhesive, 94.6 parts of ceramic particles, 1.5 parts of dispersant, 0.2 parts of surfactant, and 0.2 parts of thickener.

[0046] The adhesive contained in the ceramic coating is a water-soluble modified polyacrylamide copolymer with a weight-average molecular weight of 8w and a glass transition temperature of 187℃.

[0047] Specifically, the water-soluble modified polyacrylamide copolymer is a copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers is 85:10:5. Specifically, the acrylamide monomer is acrylamide, the acrylic monomer is itaconic acid, and the water-soluble acrylate monomer is 2-hydroxyethyl acrylate.

[0048] The ceramic coating contains boehmite particles, which are composed of two sizes of particles with D50 of 0.2-0.4μm and 0.7-0.9μm, respectively. Among them, the weight of boehmite particles with D50 of 0.2-0.4μm accounts for 50% of the total weight of ceramic particles, (D90-D10) / D50=1.6; the weight of boehmite particles with D50 of 0.7-0.9μm accounts for 50% of the total weight of ceramic particles, (D90-D10) / D50=1.8.

[0049] The dispersant in the ceramic coating is sodium polyacrylate.

[0050] The surfactant contained in the ceramic coating is a polyether siloxane copolymer.

[0051] The thickener contained in the ceramic coating is sodium carboxymethyl cellulose.

[0052] Example 3

[0053] This embodiment provides a composite separator for lithium-ion batteries, comprising a polyethylene porous membrane and ceramic coatings applied to both sides of the polyethylene porous membrane. The polyethylene porous membrane has a thickness of 16 μm and a porosity of 50%. The ceramic coating on one side has a thickness of 2.0 μm, and the sum of its thickness and that of the ceramic coating on the other side is 4 μm. The ceramic coating is composed of 4.5 parts of adhesive, 92.6 parts of ceramic particles, 2.5 parts of dispersant, 0.2 parts of surfactant, and 0.5 parts of thickener.

[0054] The adhesive contained in the ceramic coating is a water-soluble modified polyacrylamide copolymer with a weight-average molecular weight of 30w and a glass transition temperature of 206℃.

[0055] Specifically, the water-soluble modified polyacrylamide copolymer is a copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers is 90:7:3. The acrylamide monomer is methacrylamide, the acrylic monomer is methacrylic acid, and the water-soluble acrylate monomer is 2-hydroxyethyl methacrylate.

[0056] The ceramic coating contains alumina particles, which are composed of a mixture of particles with two sizes: D50 of 0.2-0.4μm and 0.7-0.9μm. The alumina particles with D50 of 0.2-0.4μm account for 80% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.3; the alumina particles with D50 of 0.7-0.9μm account for 20% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.5.

[0057] The dispersant in the ceramic coating is ammonium polyacrylate.

[0058] The surfactant contained in the ceramic coating is an acetylenic diol copolymer.

[0059] The thickener contained in the ceramic coating is polyvinyl alcohol.

[0060] Example 4

[0061] This embodiment provides a composite separator for lithium-ion batteries, comprising a polyethylene porous membrane and ceramic coatings applied to both sides of the polyethylene porous membrane. The polyethylene porous membrane has a thickness of 12 μm and a porosity of 46%. The ceramic coating on one side has a thickness of 1.7 μm, and the sum of its thickness and that of the ceramic coating on the other side is 3.4 μm. The ceramic coating consists of 4 parts of adhesive, 93.6 parts of ceramic particles, 2 parts of dispersant, 0.2 parts of surfactant, and 0.2 parts of thickener.

[0062] The adhesive contained in the ceramic coating is a water-soluble modified polyacrylamide copolymer with a weight-average molecular weight of 15w and a glass transition temperature of 194℃.

[0063] Specifically, the water-soluble modified polyacrylamide copolymer is a copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers is 65:25:10. The acrylamide monomer is methacrylamide, the acrylic monomer is itaconic acid, and the water-soluble acrylate monomer is 2-hydroxyethyl acrylate.

[0064] The ceramic coating contains alumina particles, which are composed of a mixture of particles with two sizes: D50 of 0.2-0.4μm and 0.7-0.9μm. The alumina particles with D50 of 0.2-0.4μm account for 70% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.5; the alumina particles with D50 of 0.7-0.9μm account for 30% of the total weight of the ceramic particles, (D90-D10) / D50 = 1.6.

[0065] The dispersants, surfactants, and thickeners contained in the ceramic coating are the same as in Example 1.

[0066] Example 5

[0067] This embodiment provides a composite separator for lithium-ion batteries, comprising a porous polyethylene membrane and ceramic coatings applied to both sides of the porous polyethylene membrane. The porous polyethylene membrane has a thickness of 7 μm and a porosity of 43%. The ceramic coating on one side has a thickness of 1.2 μm, and the sum of its thickness and that of the ceramic coating on the other side is 2.4 μm. The ceramic coating is composed of 4 parts adhesive, 93.6 parts ceramic particles, 2 parts dispersant, 0.2 parts surfactant, and 0.2 parts thickener.

[0068] The adhesive contained in the ceramic coating is a water-soluble modified polyacrylamide copolymer with a weight-average molecular weight of 6w and a glass transition temperature of 167℃.

[0069] Specifically, the water-soluble modified polyacrylamide copolymer is a copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers is 72:20:8. Specifically, the acrylamide monomer is acrylamide, the acrylic monomer is acrylic acid, and the water-soluble acrylate monomer is 2-hydroxyethyl acrylate.

[0070] The ceramic coating contains boehmite particles, which are composed of two sizes of particles with D50 of 0.2-0.4μm and 0.7-0.9μm, respectively. Among them, the weight of boehmite particles with D50 of 0.2-0.4μm accounts for 60% of the total weight of ceramic particles, (D90-D10) / D50=1.2; the weight of boehmite particles with D50 of 0.7-0.9μm accounts for 40% of the total weight of ceramic particles, (D90-D10) / D50=1.4.

[0071] The dispersants, surfactants, and thickeners contained in the ceramic coating are the same as in Example 1.

[0072] Example 6

[0073] This embodiment provides a composite separator for lithium-ion batteries. The difference from Embodiment 1 is that the weight ratio of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers in the adhesive is different, specifically 50:30:20. The weight average molecular weight of the adhesive is 22w, and the glass transition temperature is 152°C.

[0074] Example 7

[0075] This embodiment provides a composite separator for lithium-ion batteries. The difference from Embodiment 1 is that the weight ratio of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers in the adhesive is different, specifically 98:1:1. The weight average molecular weight of the adhesive is 4w, and the glass transition temperature is 183°C.

[0076] Comparative Example 1

[0077] The difference from Example 1 is that the ceramic coating in this comparative example contains an adhesive of polyacrylate polymer with a weight-average molecular weight of 20w and a glass transition temperature of 8°C.

[0078] Comparative Example 2

[0079] The difference from Example 1 is that the adhesive contained in the ceramic coating of this comparative example is a polyacrylic polymer with a weight-average molecular weight of 4w and a glass transition temperature of 155°C.

[0080] Comparative Example 3

[0081] The difference from Example 1 is that the ceramic particles in the ceramic coating of this comparative example are all composed of alumina particles with a D50 between 0.7 and 0.9 μm.

[0082] Comparative Example 4

[0083] The difference from Example 1 is that the ceramic particles in the ceramic coating of this comparative example are all composed of alumina particles with a D50 between 0.2 and 0.4 μm.

[0084] The heat shrinkage, electrolyte wetting area, and moisture content of the composite membranes obtained in Examples 1-7 and Comparative Examples 1-4 were measured. The test methods for heat shrinkage rate, electrolyte wetting area, and moisture content in Table 1 are as follows:

[0085] Heat shrinkage rate: Cut the composite diaphragm into A4 size, with the longer direction as MD and the shorter direction as TD. Draw a 100mm×100mm frame in the middle. Place 11 A4 sheets of paper on the top and bottom of the diaphragm. Place it in an oven at 180℃ for 30 minutes. After taking it out, measure the size of the drawn frame and record it as L. Heat shrinkage rate = (100-L)%.

[0086] Electrolyte wetting area: Cut the composite diaphragm into 50mm × 50mm pieces, according to... Figure 1 As shown in the sample preparation diagram, 2 μL of electrolyte (the solvent is composed of EC:EMC:DMC in a volume ratio of 1:1:1, and the electrolyte is LiPF6 with a concentration of 1 mol / L) is taken with a microsyringe and dropped vertically onto the sample surface. The size of the electrolyte diffusion area is measured after 5 minutes using an industrial camera. The obtained data is the area wetted by the electrolyte.

[0087] Moisture content test: Under an environment with a dew point of less than -40℃, take 1-1.5g of composite membrane to prepare a sample, bake at 120℃ for 5min, and test the moisture content of the composite membrane using the Karl Fischer method.

[0088] The results are shown in Table 1 below.

[0089] Table 1

[0090]

[0091] As shown in Table 1 above, the composite separators for lithium-ion batteries in the embodiments of the present invention all use water-soluble modified polyacrylamide copolymer as adhesive, and are mixed with two ceramic particles of different D50 sizes. The resulting composite separators, after heat treatment at 180°C for 0.5 hours, exhibit thermal shrinkage of <3% and electrolyte wetting area >100 mm². 2All composite membranes with a moisture content <1000ppm exhibit high heat resistance, high wettability, and low moisture content. Comparative Example 1, using polyacrylate polymers as adhesives, resulted in composite membranes with poor heat resistance and small electrolyte wetting area. Comparative Example 2, using polyacrylic acid polymers as adhesives, resulted in composite membranes with very high moisture content. The monomers used to prepare the adhesive of this invention are mainly acrylamide monomers. Acrylamide monomers have high Tg and good heat resistance, and the amide groups are less hydrophilic than carboxyl groups, resulting in composite membranes with low moisture content. Comparative Example 3 used only alumina ceramic particles with a D50 between 0.7-0.9μm, which, although low in moisture, exhibited poor high-temperature dimensional stability and large thermal shrinkage. Comparative Example 4 used only alumina ceramic particles with a D50 between 0.2-0.4μm, resulting in composite membranes with excellent heat resistance and electrolyte wettability, but a moisture content exceeding 1000ppm.

[0092] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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 of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite separator for lithium-ion batteries, characterized in that, The composite membrane includes a base membrane and a ceramic coating, wherein the ceramic coating is applied to one or both sides of the base membrane; the ceramic coating is composed of an adhesive, ceramic particles, a dispersant, a surfactant, and a thickener. The adhesive is a water-soluble modified polyacrylamide copolymer copolymer composed of acrylamide monomers, acrylic monomers, and water-soluble acrylate monomers, wherein the weight ratio of the acrylamide monomers, the acrylic monomers, and the water-soluble acrylate monomers is (50-98):(1-30):(1-20). The water-soluble modified polyacrylamide copolymer has a weight-average molecular weight of 2w-40w and a glass transition temperature of 150℃-220℃. The ceramic particles are composed of a mixture of particles with two sizes: D50 of 0.2-0.4 μm and 0.7-0.9 μm, respectively; wherein the ceramic particles with D50 of 0.2-0.4 μm account for 50-80% of the total weight of the ceramic particles, and the ceramic particles with D50 of 0.7-0.9 μm account for 20-50% of the total weight of the ceramic particles. The particle size distribution of the ceramic particles satisfies 1≤(D90-D10) / D50≤2.

2. The composite diaphragm according to claim 1, characterized in that, The acrylamide monomer is one or more of acrylamide, methacrylamide, N,N-dimethylacrylamide, N-hydroxymethylacrylamide, N,N-methylenebisacrylamide, N-(2-hydroxyethyl)acrylamide, and N-(2-hydroxypropyl)acrylamide; And / or, the acrylic monomer is one or more of acrylic acid, methacrylic acid, itaconic acid, and aconitic acid; And / or, the water-soluble acrylate monomer is one or more of 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and 3-hydroxypropyl methacrylate.

3. The composite diaphragm according to claim 1, characterized in that, The ceramic particles contain one or more of the following: alumina, boehmite, barium titanate, magnesium oxide, silicon oxide, and magnesium hydroxide.

4. The composite diaphragm according to claim 1, characterized in that, By weight, the ceramic coating comprises 3-6 parts of adhesive, 90-96 parts of ceramic particles, 0.1-4 parts of dispersant, 0.05-0.5 parts of surfactant, and 0.05-3 parts of thickener; and / or, the thickness of the ceramic coating is 1-4 μm.

5. The composite diaphragm according to claim 1, characterized in that, The dispersant is sodium polyacrylate or ammonium polyacrylate; And / or, the surfactant is an acetylenic diol copolymer or a polyether siloxane copolymer; And / or, the thickener is sodium carboxymethyl cellulose or polyvinyl alcohol.

6. The composite diaphragm according to claim 1, characterized in that, The base membrane is a polyethylene porous film, a polypropylene porous film, or a polypropylene / polyethylene / polypropylene composite porous film; and / or, the thickness of the base membrane is 3-20 μm; and / or, the porosity of the base membrane is 20-80%.

7. The composite diaphragm according to any one of claims 1-6, characterized in that, After being baked at 180°C for 30 minutes, the composite membrane exhibits a thermal shrinkage rate of less than 3% in both its longitudinal and transverse directions; the electrolyte wetting area of ​​the composite membrane is greater than 100 mm². 2 The moisture content of the composite membrane is <1000ppm; the method for testing the electrolyte wetting area is to cut the composite membrane into 50mm×50mm pieces, take 2μL of electrolyte using a microsyringe, the solvent is composed of EC:EMC:DMC in a volume ratio of 1:1:1, the electrolyte is LiPF6 with a concentration of 1mol / L, drop it vertically onto the sample surface, and take a picture with an industrial camera to measure the size of the electrolyte diffusion area after 5 minutes. The obtained data is the electrolyte wetting area. The moisture content test was conducted by taking 1-1.5g of composite membrane sample in an environment with a dew point of less than -40℃, baking at 120℃ for 5 minutes, and then testing the moisture content of the composite membrane using the Karl Fischer method.

8. A lithium-ion battery, characterized in that, The battery assembly includes a battery separator, wherein the battery separator is a composite separator as described in any one of claims 1-6.

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