Composite coating for lithium ion battery separator and method of making, battery separator and battery

Abstract

The application discloses a composite coating for lithium ion battery diaphragm, a preparation method of the composite coating, a battery diaphragm and a battery. The composite coating comprises fumed alumina ceramic particles and fiber materials, and the mass ratio of the fiber materials to the fumed alumina ceramic particles is 1:10-1:5; the fiber materials form a three-dimensional network structure in the coating, and the fumed alumina ceramic particles are at least partially attached to the network. The application builds additional lithium ion transmission channels through the special nano-porous aggregate structure of the fumed alumina, effectively compensates for the decrease in porosity caused by fiber filling, and at the same time, the fiber network restrains the displacement of the ceramic particles at high temperature and inhibits the expansion of micro-cracks, so that the high-temperature dimensional stability and lithium ion permeability of the diaphragm are considered. The application also provides a method for preparing the composite coating by using a step-by-step pulp preparation process, and a lithium ion battery diaphragm and a battery comprising the composite coating.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, specifically a composite coating for lithium-ion battery separators, its preparation method, battery separators, and batteries. Background Technology

[0002] Currently, polyolefin microporous membranes are widely used as lithium-ion battery separator substrates due to their low cost and good mechanical properties. However, their poor heat resistance makes them prone to severe thermal shrinkage at high temperatures (such as 150°C), with shrinkage rates exceeding 20%. This can lead to direct contact between the positive and negative electrodes, causing internal short circuits and posing serious safety hazards. To improve their thermal stability, the industry commonly uses an alumina ceramic coating on the base membrane surface. However, traditional ceramic coatings are simply formed by the stacking of alumina particles, with the particles bonded by physical contact and a small amount of binder. This results in a brittle coating with weak adhesion to the base membrane, making it prone to microcracks at high temperatures. Consequently, the thermal shrinkage rate remains high (usually much higher than 5%), and the coating is at risk of peeling off, affecting the long-term cycle stability of the battery.

[0003] In recent years, researchers have attempted to composite fibrous materials such as nanocellulose with inorganic ceramic particles, utilizing the three-dimensional network structure formed by the fibers to constrain the relative displacement of the ceramic particles, aiming to suppress the generation of high-temperature microcracks. Simultaneously, the hydrophilic properties of the fibers can improve the wettability of the separator to the electrolyte. However, existing technologies still face two major challenges: First, nanocellulose, with a diameter of only a few nanometers to tens of nanometers, has a large specific surface area and high surface energy, making it prone to agglomeration in alumina slurry, hindering the achievement of uniform nanoscale dispersion, and a mature industrial solution is still lacking. Second, while fiber incorporation can improve the structural stability of the coating, its filling of the voids between ceramic particles significantly blocks lithium-ion transport channels, leading to a sharp increase in separator permeability, severely degrading the battery's rate performance and cycle life. How to maintain a low increase in permeability while improving heat resistance and structural stability has become a bottleneck restricting the industrialization of this technology. Summary of the Invention

[0004] In view of this, the present invention provides a composite coating for lithium-ion battery separators, a method for preparing the same, a battery separator, and a battery. By replacing conventional alumina powder with fumed alumina and introducing fiber materials into the coating, the fibers form a three-dimensional network structure in the coating to constrain the high-temperature displacement of ceramic particles. At the same time, the nanoscale size of the fumed alumina particles and the optimized particle stacking method provide more transport channels for lithium ions, thereby significantly reducing ion transport resistance and ultimately achieving a significant improvement in permeability. This balances the high-temperature dimensional stability of the separator with lithium-ion permeability.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention discloses a composite coating for lithium-ion battery separators, the composite coating comprising fumed alumina ceramic particles and fiber material, wherein the mass ratio of the fiber material to the fumed alumina ceramic particles is 1:10 to 1:5; wherein the fiber material forms a three-dimensional network structure in the composite coating, and the fumed alumina ceramic particles are at least partially attached to the three-dimensional network structure. The fumed alumina is predominantly γ-crystalline.

[0006] As a further aspect of the present invention, the thickness of the composite coating is 1 to 5 μm.

[0007] As a further aspect of the present invention: the fiber material is nanocellulose, selected from at least one of microfibrillated cellulose, bacterial cellulose and carboxylated nanocellulose, wherein the average diameter of the nanocellulose is 5-50 nm and the aspect ratio is 100-1000.

[0008] As a further aspect of the present invention: the specific surface area of ​​the fumed alumina ceramic particles is 50–200 m². 2 / g, with an average particle size of 10–50 nm.

[0009] As a further aspect of the present invention: the raw materials for the composite coating further include a dispersant, a wetting agent, a thickener, and a binder; wherein: The dispersant is selected from at least one of ammonium polyacrylate, sodium polyacrylate, and polyvinylpyrrolidone; The wetting agent is selected from at least one of polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, or polyoxyethylene fatty acid ester; The adhesive is selected from at least one of polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

[0010] Secondly, the present invention discloses a method for preparing the composite coating as described above, comprising the following steps: S1. Mix fumed alumina ceramic particles, dispersant and a portion of solvent, and grind them to obtain an alumina dispersion. S2. Mix the fiber material with another portion of the solvent, and disperse it by shearing to obtain a fiber dispersion; S3. Mix the alumina dispersion with the fiber dispersion, and add binder, thickener and wetting agent, stir evenly to obtain a composite slurry with a solid content of 30% to 45%. S4. The composite slurry is coated onto the surface of the porous base membrane, and then dried to solidify the composite slurry into shape, thus obtaining the final product.

[0011] As a further embodiment of the present invention: by weight, the fumed alumina ceramic particles are 30-50 parts, the fiber material is 1-10 parts, the dispersant is 0.1-2 parts, the binder is 1-10 parts, the wetting agent is 0.01-1 parts, the thickener is 2-4 parts, and the solvent is 30-60 parts.

[0012] As a further aspect of the present invention: in step S2, the shear dispersion includes high-speed shear dispersion and low-speed grinding dispersion; wherein, the high-speed shear dispersion has a rotation speed of 800-1200 rpm and a time of 20-30 min; the low-speed grinding dispersion has a rotation speed of 200-300 rpm.

[0013] Thirdly, the present invention discloses a lithium-ion battery separator, comprising a porous base membrane and a composite coating as described above disposed on at least one surface of the porous base membrane; the porous base membrane is a polyolefin microporous membrane, selected from at least one of polyethylene membrane, polypropylene membrane, or polyethylene / polypropylene composite membrane.

[0014] Fourthly, the present invention discloses a lithium-ion battery comprising the lithium-ion battery separator as described above.

[0015] Invention Mechanism: In this invention, the fumed alumina ceramic particles differ from conventional alumina; their nanoparticles aggregate into three-dimensional porous aggregates with a nanostructure. This structure itself acts like a sponge, creating abundant pores within the coating and providing numerous channels for ion transport, forming a three-dimensional ion transport network that spans the entire coating thickness, effectively compensating for the porosity reduction caused by the filling of fibrous materials. Simultaneously, nanofibers uniformly dispersed in the interparticle gaps can construct a continuous three-dimensional network with extremely low addition amounts. This network weaves the discrete alumina particles into a unified structure through the interfacial bonding force between the fibers and the particle surface, as well as the physical cross-linking between the fibers, effectively constraining particle displacement at high temperatures and inhibiting microcrack propagation. The synergistic structure of fumed alumina creating channels and nanofibers building the framework allows for the simultaneous achievement of two traditionally mutually restrictive properties: low thermal shrinkage and high ion conductivity.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention addresses the contradiction between fiber agglomeration and deteriorated permeability at the material source by employing a high-speed shearing followed by low-speed grinding process on the fiber material and constructing a lithium-ion transport network in fumed alumina. On one hand, the fibers can form a continuous three-dimensional network at a low proportion, effectively constraining the high-temperature displacement of ceramic particles and significantly suppressing coating thermal shrinkage. On the other hand, the stacking of porous aggregates in fumed alumina provides additional lithium-ion transport paths, enabling the coating to maintain low ion transport resistance while enhancing structural stability.

[0017] This invention employs a stepwise pulping strategy, first preparing alumina dispersion and fiber dispersion separately, and then mixing them. The fiber dispersion undergoes shear dispersion treatment to achieve uniform dissociation at the nanoscale, while the alumina is ground separately to prevent rigid particles from damaging the fiber structure during dispersion. This process not only ensures the long-term stability and coating uniformity of the slurry but also avoids mutual interference between fibers and alumina during the dispersion stage through the stepwise pulping sequence. This allows the structural advantages of the formulation design to be fully transformed into coating performance, providing a reliable technical path for large-scale production. Attached Figure Description

[0018] Figure 1 Photographs showing the wettability test of the composite coating slurry obtained in Example 1 of this invention; Figure 2 This is a photograph of the wettability test of the composite coating slurry obtained in Comparative Example 1 of the present invention. Detailed Implementation

[0019] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of the present invention.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0021] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; unless otherwise specified, the parts in the following embodiments refer to parts by weight.

[0022] Example 1 40 parts deionized water, 0.4 parts dispersant (ammonium polyacrylate, Dow Chemical), and 36 parts fumed alumina ceramic powder (average particle size 50 nm, specific surface area 80 m²) were mixed. 2 / g of Evonik AEROXIDE® and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel. First, the mixture was pre-dispersed by high-speed shearing at 1000 rpm for 20 minutes, and then ground and refined twice at 1000 rpm for 40 minutes to obtain an alumina dispersion.

[0023] Take another 10 parts of deionized water and 5 parts of nanocellulose (microfibrillated cellulose MFC, average diameter 10nm, aspect ratio 300, Guilin Qihong) and mix them. First, perform high-speed shearing pre-dispersion at 1000rpm for 20 minutes, and then perform low-speed grinding and fine dispersion twice at 250rpm for 40 minutes to achieve uniform distribution of nanocellulose at the nanoscale in the liquid phase, thus obtaining a fiber dispersion.

[0024] The alumina dispersion and fiber dispersion were stirred and mixed evenly. Then, 5.5 parts of binder (styrene-butadiene rubber latex, Guangdong Rongdong) and 0.1 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0025] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 80 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0026] Example 2 42 parts deionized water (as the first solvent), 0.4 parts dispersant (sodium polyacrylate, BYK Chemical), and 40 parts fumed alumina ceramic powder (average particle size 20 nm, specific surface area 100 m²) were mixed. 2 / g of Evonik AEROXIDE® and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel and dispersed at 1000 rpm for 20 minutes. After being mixed evenly, the mixture was transferred to a grinder and ground twice at 1200 rpm to obtain an alumina dispersion.

[0027] Take another 8 parts of deionized water (as the second part of the solvent) and mix it with 4 parts of nanocellulose (bacterial cellulose BC, Guilin Qihong, average diameter 8nm, aspect ratio 500). First, perform high-speed shearing pre-dispersion at 800rpm for 20 minutes, and then perform low-speed grinding and fine dispersion twice at 200rpm (for 40 minutes) to obtain the fiber dispersion.

[0028] The two dispersions were stirred and mixed evenly. Then, 2.5 parts of binder (polyacrylate emulsion, Hunan Gaorui) and 0.1 parts of wetting agent (polyoxyethylene alkylphenol ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0029] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 85 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0030] Example 3 38 parts of deionized water (as the first solvent), 0.5 parts of dispersant (polyvinylpyrrolidone, Dow Chemical), 38 parts of fumed alumina ceramic powder (average particle size 15nm, specific surface area 150m² / g, Zhonghang Nano ZH-Alum80), and 2 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel and dispersed at 1100 rpm for 20 minutes. After mixing evenly, the mixture was transferred to a grinder and ground twice at 1200 rpm (for 40 minutes) to obtain an alumina dispersion.

[0031] Take another 12 parts of deionized water (as the second solvent) and mix it with 4.5 parts of nanocellulose (bacterial cellulose BC, Guilin Qihong, average diameter 18nm, aspect ratio 200). First, perform high-speed shearing pre-dispersion at 1000rpm for 20 minutes, and then perform low-speed grinding and fine dispersion twice at 300rpm (for 40 minutes) to obtain a fiber dispersion.

[0032] The two dispersions were stirred and mixed evenly. Then, 5 parts of binder (polyacrylate emulsion from Hunan Gaorui and styrene rubber emulsion from Guangdong Rongdong, compounded at a mass ratio of 3:7) and 0.1 parts of wetting agent (polyoxyethylene fatty acid ester, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0033] The obtained composite slurry was coated onto the surface of a polypropylene base film (16 μm thick) using a micro-grooved roller coating method at a coating speed of 100 m / min and dried at 85 °C to obtain a lithium-ion battery separator with a composite coating.

[0034] Example 4 45 parts deionized water (as the first solvent), 0.3 parts dispersant (ammonium polyacrylate, Dow Chemical), and 30 parts fumed alumina ceramic powder (average particle size 10 nm, specific surface area 200 m²) were mixed. 2 / g, Zhonghang Nano ZH-Alum100) and 4 parts of thickener (CMC2020, Guilin Qihong, solid content of 4%) were sequentially added into the dispersion vessel and dispersed at 950 rpm for 20 minutes. After mixing evenly, the mixture was transferred to a grinder and ground twice at 1200 rpm (for 40 minutes) to obtain an alumina dispersion.

[0035] Take another 6 parts of deionized water (as the second part of the solvent) and mix with 3 parts of nanocellulose (carboxylated nanocellulose CNF, Guilin Qihong, average diameter 12nm, aspect ratio 400). First, perform high-speed shearing pre-dispersion at 1100rpm for 20 minutes, and then perform low-speed grinding and fine dispersion twice at 220rpm (for 40 minutes) to obtain the fiber dispersion.

[0036] The two dispersions were stirred and mixed evenly. Then, 11.5 parts of binder (a mixture of styrene-butadiene rubber latex from Guangdong Rongdong and polyacrylate latex from Hunan Gaorui at a mass ratio of 1:2) and 0.2 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0037] The obtained composite slurry was coated onto the surface of a polyethylene / polypropylene composite base film (9 μm thick) using a micro-grooved roller coating method at a coating speed of 85 m / min and dried at 75 °C to obtain a lithium-ion battery separator with a composite coating.

[0038] Comparative Example 1 40 parts of deionized water, 0.4 parts of dispersant (ammonium polyacrylate, Dow Chemical), 36 parts of ordinary alumina (Tianma, TM-LA-T) and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel. First, high-speed shearing pre-dispersion was performed at 1000 rpm for 20 minutes, and then grinding and fine dispersion were performed twice at 1200 rpm for 40 minutes to obtain an alumina dispersion.

[0039] Take another 10 parts of deionized water and 5 parts of nanocellulose (microfibrillated cellulose MFC, average diameter 10nm, aspect ratio 300, Guilin Qihong) and mix them. First, perform high-speed shearing pre-dispersion at 1000rpm for 20 minutes, and then perform low-speed grinding and fine dispersion twice at 250rpm (for a total of 40 minutes) to make the nanocellulose achieve uniform distribution at the nanoscale in the liquid phase, and obtain the fiber dispersion.

[0040] The alumina dispersion and fiber dispersion were stirred and mixed evenly. Then, 5.5 parts of binder (styrene-butadiene rubber latex, Guangdong Rongdong) and 0.1 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0041] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 80 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0042] Comparative Example 2 40 parts deionized water, 0.4 parts dispersant (ammonium polyacrylate, Dow Chemical), and 36 parts fumed alumina ceramic powder (average particle size 50 nm, specific surface area 80 m²) were mixed. 2 / g of Evonik AEROXIDE® and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel and dispersed at 1000 rpm for 30 minutes to obtain a uniform alumina dispersion.

[0043] Take another 10 parts of deionized water and 5 parts of nanocellulose (microfibrillated cellulose MFC, average diameter 10nm, aspect ratio 300, Guilin Qihong) and mix them. Perform high-speed shear pre-dispersion at 1000rpm for 30 minutes to achieve uniform distribution of nanocellulose at the nanoscale in the liquid phase, and obtain fiber dispersion.

[0044] The alumina dispersion and fiber dispersion were stirred and mixed evenly. Then, 5.5 parts of binder (styrene-butadiene rubber latex, Guangdong Rongdong) and 0.1 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0045] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 80 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0046] Comparative Example 3 40 parts deionized water, 0.4 parts dispersant (ammonium polyacrylate, Dow Chemical), and 36 parts fumed alumina ceramic powder (average particle size 50 nm, specific surface area 80 m²) were mixed. 2 / g of Evonik AEROXIDE® and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel and dispersed at 1000 rpm for 20 minutes. After mixing evenly, the mixture was transferred to a grinder and ground twice at 200 rpm (for a total of 40 minutes) to obtain a uniform alumina dispersion.

[0047] Take another 10 parts of deionized water and 2 parts of nanocellulose (microfibrillated cellulose MFC, average diameter 10nm, aspect ratio 300, Guilin Qihong) and mix them. First, pre-disperse the nanocellulose in the mixture at 1000rpm using high-speed shearing, and then grind and refine it twice at 250rpm (for a total of 40 minutes) to achieve uniform nanoscale distribution of nanocellulose in the liquid phase, thus obtaining a fiber dispersion.

[0048] The alumina dispersion and fiber dispersion were stirred and mixed evenly. Then, 5.5 parts of binder (styrene-butadiene rubber latex, Guangdong Rongdong) and 0.1 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0049] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 80 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0050] Comparative Example 4 40 parts deionized water, 0.4 parts dispersant (ammonium polyacrylate, Dow Chemical), and 36 parts fumed alumina ceramic powder (average particle size 50 nm, specific surface area 80 m²) were mixed. 2 / g of Evonik AEROXIDE® and 3 parts of thickener (CMC2020, Guilin Qihong, solid content 4%) were sequentially added to a dispersion vessel and dispersed at 1000 rpm for 20 minutes. After mixing evenly, the mixture was transferred to a grinder and ground twice at 200 rpm (for a total of 40 minutes) to obtain a uniform alumina dispersion.

[0051] Take another 10 parts of deionized water and 12 parts of nanocellulose (microfibrillated cellulose MFC, average diameter 10nm, aspect ratio 300, Guilin Qihong) and mix them. First, perform high-speed shearing pre-dispersion at 1000rpm, and then perform low-speed grinding and fine dispersion at 250rpm to make the nanocellulose achieve uniform distribution at the nanoscale in the liquid phase, thus obtaining a fiber dispersion.

[0052] The alumina dispersion and fiber dispersion were stirred and mixed evenly. Then, 5.5 parts of binder (styrene-butadiene rubber latex, Guangdong Rongdong) and 0.1 parts of wetting agent (polyoxyethylene alkyl ether, BYK Chemical) were added, and stirring was continued until fully mixed. The resulting slurry was filtered through a 300-mesh filter bag to obtain the finished composite coating slurry.

[0053] The obtained composite slurry was coated onto the surface of a polyethylene film (12 μm thick) using a micro-grooved roller coating method at a coating speed of 90 m / min and dried at 80 °C to obtain a lithium-ion battery separator with a composite coating thickness of 3.0 μm.

[0054] Test case The performance of the diaphragm samples obtained in each embodiment and comparative example was tested below. All tests were conducted under environmental conditions of 23±2℃ and 50±5% relative humidity. Before testing, each diaphragm sample was allowed to stand for 24 hours under the same environmental conditions to eliminate internal stress. The specific test items and methods are as follows: (1) Coating thickness: Using a Marl thickness gauge (accuracy 0.01μm), 10 measurement points were evenly selected on the base film (without slurry coating) and the base film strip after slurry coating to test the thickness of the coating film and the base film and take the average value to obtain the coating thickness.

[0055] (2) Coating surface density: The mass difference of the diaphragm before and after coating was weighed using an analytical balance with an accuracy of 0.01 mg. The coating surface density was calculated by dividing the mass difference by the coating area. The test standard refers to GB / T 451.2-2002.

[0056] (3) Air permeability enhancement: The time required for 100 mL of air to permeate through the diaphragm was measured using a Gurley air permeability meter (model 4110N). The air permeability values ​​of the base membrane before coating and the diaphragm after coating were measured separately. Air permeability enhancement = air permeability value after coating Air permeability value before coating, unit: s / 100mL. Test standard: JIS P8117, test pressure: 1.22 kPa, test area: 6.45 cm². 2 The increase in air permeability per micrometer of coating is calculated using the formula "increase in air permeability ÷ coating thickness".

[0057] (4) Heat shrinkage rate at 150℃: Cut the diaphragm sample into 100mm×100mm squares, and mark the initial distance L0 (accurate to 0.1mm) in the MD (longitudinal) and TD (transverse) directions respectively. Place the sample in a 150℃ constant temperature oven for 60min, then remove it and cool it to room temperature before measuring the final distance L1 between the marks. The heat shrinkage rate is calculated according to the formula "(L0) = L1 + L2 + L3 + L4 + L5 + L1 + L2 ... The thermal shrinkage rates in the MD and TD directions are calculated as follows: L1) / L0×100%”. The test standard is in accordance with GB / T 12027-2004.

[0058] (5) Peel strength: A universal tensile testing machine (model Instron 5967) was used. The diaphragm sample was fixed to the steel plate with double-sided tape, and the coating was peeled off at a rate of 50 mm / min along the 180° direction. The peel force was recorded, and the peel strength (unit N / m) was obtained by dividing the peel force by the sample width. The test standard refers to GB / T 2790-1995.

[0059] (6) Wetting properties: 4 μL of lithium-ion battery electrolyte (1 M LiPF6 dissolved in EC / EMC / DMC = 1:1:1 volume ratio) was pipetted and dropped vertically onto the center of the separator coating surface. After standing for 60 seconds, the droplet spread image was taken and the droplet spread area was calculated using image analysis software. The average value of 5 different test positions for each sample was taken as the wettability result.

[0060] (7) Liquid retention rate: Cut a 100mm×100mm coated film sample, weigh it using a precision electronic balance and take the mass as L0. Then immerse the sample completely in the lithium-ion battery electrolyte for 1 hour. After taking it out, sandwich it between ten pieces of dust-free paper (five on top and five on the bottom) and hold it with glass plates for 30 seconds to absorb the residual electrolyte on the surface. Then weigh the sample and take the mass as L1. Liquid retention rate = (L1-L0) / L0.

[0061] (8) Puncture strength: The maximum force required to puncture the diaphragm sample using a 1mm diameter round-tipped needle (0.5mm spherical radius of the needle tip) at a speed of 100±5mm / min. After flattening the membrane surface, place it on a circular fixture and press down the fixture to fix it. When measuring a single-sided ceramic membrane, the coated side should face upwards. Measure multiple times (N≥3) and record the readings. Sampling and testing environment: temperature 25℃±5℃, humidity ≤50%RH) The test results are shown in Table 1.

[0062] Table 1

[0063] Figure 1 This is a photograph of the wettability test in Example 1. Figure 1 The white substrate represents a lithium-ion battery separator coated with the composite coating of this invention (coating facing upwards). The gray circular area represents the spread of 4 μL of electrolyte on the separator surface after 1 minute. A 1 cm scale bar is attached to the lower right corner of the image. As shown, the electrolyte droplets spread evenly and in a regular circular pattern on the coating surface, with clear and smooth diffusion boundaries, and no obvious burrs or uneven wetting. Image analysis software calculations show that its wetting area is significantly larger than that of Comparative Example 1 (e.g., ...). Figure 2 As shown, in ordinary alumina-coated diaphragms, uneven wetting occurs in localized areas. The regular and large-area spreading morphology indicates that the hydrophilic properties of nanocellulose and the capillary effect of fumed alumina in the composite coating of this invention work synergistically to significantly improve the diaphragm's affinity for the electrolyte and its wetting rate.

[0064] Although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0065] Therefore, the above description is only a preferred embodiment of this application and is not intended to limit the scope of this application; that is, all equivalent modifications made in accordance with the scope of the claims of this application shall be within the protection scope of the claims of this application.

Claims

1. A composite coating for lithium-ion battery separators, characterized in that: The composite coating comprises fumed alumina ceramic particles and fiber material, wherein the mass ratio of the fiber material to the fumed alumina ceramic particles is 1:10 to 1:5; wherein the fiber material forms a three-dimensional network structure in the composite coating, and the fumed alumina ceramic particles are at least partially attached to the three-dimensional network structure.

2. The composite coating according to claim 1, characterized in that, The thickness of the composite coating is 1–5 μm.

3. The composite coating according to claim 1, characterized in that, The fiber material is nanocellulose, selected from at least one of microfibrillated cellulose, bacterial cellulose and carboxylated nanocellulose, wherein the average diameter of the nanocellulose is 5-50 nm and the aspect ratio is 100-1000.

4. The composite coating according to claim 1, characterized in that, The specific surface area of ​​the fumed alumina ceramic particles is 50–200 m². 2 / g, with an average particle size of 10–50 nm.

5. The composite coating according to claim 1, characterized in that, The raw materials for the composite coating also include dispersants, wetting agents, thickeners, and binders; The dispersant is selected from at least one of ammonium polyacrylate, sodium polyacrylate, and polyvinylpyrrolidone; The wetting agent is selected from at least one of polyoxyethylene alkyl ether, polyoxyethylene alkylphenol ether, or polyoxyethylene fatty acid ester; The adhesive is selected from at least one of polyacrylate, styrene-butadiene rubber, and polyvinylidene fluoride.

6. The method for preparing the composite coating according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Mix fumed alumina ceramic particles, dispersant and a portion of solvent, and grind them to obtain an alumina dispersion. S2. Mix the fiber material with another portion of the solvent, and disperse it by shearing to obtain a fiber dispersion; S3. Mix the alumina dispersion with the fiber dispersion, and add binder, thickener and wetting agent, stir evenly to obtain a composite slurry with a solid content of 30% to 45%. S4. The composite slurry is coated onto the surface of the porous base membrane, and then dried to solidify the composite slurry into shape, thus obtaining the final product.

7. The preparation method according to claim 6, characterized in that, By weight, the fumed alumina ceramic particles are 30-50 parts, the fiber material is 1-10 parts, the dispersant is 0.1-2 parts, the binder is 1-10 parts, the wetting agent is 0.01-1 parts, the thickener is 2-4 parts, and the solvent is 30-60 parts.

8. The preparation method according to claim 6, characterized in that, In step S2, the shear dispersion includes high-speed shear dispersion and low-speed grinding dispersion; wherein, the high-speed shear dispersion has a rotation speed of 800-1200 rpm and a time of 20-30 min; and the low-speed grinding dispersion has a rotation speed of 200-300 rpm.

9. A lithium-ion battery separator, characterized in that, It includes a porous base membrane and a composite coating as described in any one of claims 1-5 disposed on at least one surface of the porous base membrane; the porous base membrane is a polyolefin microporous membrane, selected from at least one of polyethylene membrane, polypropylene membrane or polyethylene / polypropylene composite membrane.

10. A lithium-ion battery, characterized in that, Includes the lithium-ion battery separator as described in claim 9.

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