Coating diaphragm, preparation method thereof and lithium ion battery

Abstract

The invention provides a coated diaphragm, a preparation method thereof and a lithium ion battery, the coated diaphragm comprises a base membrane and a coating layer arranged on the surface of one side or two sides of the base membrane, and the material of the coating layer comprises chitosan grafted cellulose; chitosan grafted cellulose is selected as a material of a coating layer, and chitosan which is similar to the cellulose in structure and has a steric effect is utilized to perform grafting modification on the cellulose, so that the dispersion stability and uniformity of the cellulose in water are effectively improved, and the problem that the cellulose is tightly and disorderly stacked in a wet forming process is avoided; according to the preparation method, a chitosan grafted cellulose coating layer is formed, so that the formed chitosan grafted cellulose coating layer has a uniform porous structure, and the finally obtained coating diaphragm has excellent heat resistance, air permeability and electrolyte wettability, is suitable for being used as a lithium ion battery diaphragm, and is beneficial to improving the electrical properties of a lithium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of membrane technology, specifically relating to a coated membrane, its preparation method, and a lithium-ion battery. Background Technology

[0002] Currently, the mainstream lithium battery separators on the market are polyolefin separators, mainly including two categories: dry-process polypropylene (PP) separators and wet-process polyethylene (PE) separators. Among them, dry-process PP separators are characterized by excellent unidirectional mechanical properties, excellent heat resistance, and low manufacturing cost, but they are prone to tearing and being punctured by lithium dendrites. Wet-process PE membranes are characterized by excellent mechanical properties, resistance to lithium dendrite puncture, and uniform pore size distribution, but they have poor heat resistance and unstable electrochemical performance.

[0003] To address the aforementioned issues, the traditional solution is to modify the separator, with surface coating being the most suitable modification method. However, the inorganic material coating obtained by surface coating also brings a series of defects. For example, conventional ceramic coatings increase the thickness of the separator, thereby increasing the internal resistance of the battery and indirectly reducing the battery capacity. On the other hand, reducing the coating thickness has very limited improvement in heat resistance. While existing pure fiber coatings have unique advantages in heat resistance and thinness due to their crystalline material properties, their abundant hydrogen bonds on the surface make them difficult to disperse uniformly in water. During wet molding, cellulose fibers tend to stack tightly and disorderly, which is not conducive to the construction of a uniform porous structure inside the cellulose fibers, thus affecting the porosity of the cellulose-coated separator and further impacting the electrochemical performance of the battery.

[0004] Therefore, how to improve the dispersibility of cellulose in water and obtain coated membranes with excellent heat resistance and high porosity remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a coated separator, its preparation method, and a lithium-ion battery. By selecting chitosan-grafted cellulose as the material for the separator coating layer, the coating layer has a uniform porous structure, thereby enabling the coated separator to possess excellent heat resistance, air permeability, and electrolyte wettability.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a coated diaphragm, the coated diaphragm comprising a base film and a coating layer disposed on one or both sides of the base film;

[0008] The coating material includes chitosan grafted with cellulose.

[0009] The coated separator provided by this invention includes a base membrane and a coating layer disposed on one or both sides of the base membrane. The material of the coating layer is defined as chitosan-grafted cellulose. By selecting chitosan-grafted cellulose as the material of the coating layer, and utilizing chitosan, which has a similar structure to cellulose and has steric hindrance effect, to graft and modify cellulose, the dispersion stability and uniformity of cellulose in water are effectively improved. This effectively avoids the problem of its dense and disordered stacking during wet molding, so that the formed chitosan-grafted cellulose coating layer has a uniform porous structure. As a result, the final coated separator has excellent heat resistance, air permeability and electrolyte wettability, making it suitable as a lithium-ion battery separator and beneficial to improving the electrical performance of lithium-ion batteries.

[0010] Preferably, the cellulose in the chitosan-grafted cellulose includes at least one of fibrous nanocrystals, nanocellulose, or bacterial cellulose, and more preferably fibrous nanocrystals.

[0011] Among them, the raw materials for fiber nanocrystals include cotton, animal fibers, or bacteria, and are generally prepared by acid hydrolysis. They have the characteristics of high crystallinity, abundant surface hydroxyl groups, and easy modification. The raw materials for nanocellulose include wood, cotton, flax, or wheat straw, and are generally prepared by TEMPO-catalyzed oxidation or high-pressure homogenization. They have the characteristics of highly elastic network structure, large aspect ratio, and excellent mechanical properties. The raw materials for bacterial cellulose include low molecular weight sugars or alcohols, and are generally synthesized by bacteria. They have the characteristics of large aspect ratio, excellent heat resistance, and excellent mechanical properties.

[0012] In this invention, there are no special requirements for the raw materials (cellulose and chitosan) of the chitosan-grafted cellulose; conventional materials can be selected. However, in order to ensure that the coated diaphragm obtained by this invention has the best heat resistance, air permeability, and electrolyte wettability, the parameters of cellulose and chitosan in the chitosan-grafted cellulose are preferred as follows:

[0013] Preferably, the D of the fiber nanocrystals 50 A diameter of 10–50 nm (e.g., a range of 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, or any combination thereof), L 50The length is 100–300 nm (e.g., 100, 150, 200, 250, 300 nm or any combination thereof). With a fixed diameter, increasing the length of the fiber nanocrystals increases the aspect ratio, leading to a larger specific surface area and consequently, a higher viscosity of the prepared coating slurry. If the fiber nanocrystals are too long, the resulting coating slurry will have excessively high viscosity, poor flow properties, and severely affect the coating process. Furthermore, excessively long fiber nanocrystals can cause the rod-shaped cellulose nanocrystals to become completely entangled, reducing the porosity of the coating layer, decreasing the porosity of the coated membrane, and worsening its permeability, thus affecting lithium-ion transport and ultimately reducing the electrical performance of lithium ions. Therefore, L is preferred. 50 Fiber nanocrystals with a length of 100–300 nm.

[0014] Preferably, the D of the nanocellulose 50 A diameter of 10–80 nm (e.g., a range of 10 nm, 20 nm, 40 nm, 60 nm, 80 nm, or any combination thereof), L 50 The length is 300–1000 nm (e.g., a range of 300 nm, 500 nm, 700 nm, 900 nm, 1000 nm, or any combination thereof); similarly, if the L of the nanocellulose... 50 Excessive length can also reduce the porosity of the coated separator, decrease its permeability, and consequently reduce the electrical performance of the lithium-ion battery.

[0015] Preferably, the bacterial cellulose has a D 50 A diameter of 5–10 nm (e.g., a range of 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any combination thereof), L 50 The length is 500–5000 nm (e.g., a range of 500 nm, 1000 nm, 2000 nm, 3000 nm, 4000 nm, 5000 nm, or any combination thereof); similarly, if the L of the bacterial cellulose… 50 Excessive length can also reduce the porosity of the coated separator, decrease its permeability, and consequently reduce the electrical performance of the lithium-ion battery.

[0016] Preferably, the chitosan in the chitosan-grafted cellulose has the molecular formula (C8H12H2O). 13NO5)n, where n is an integer from 1 to 300 (e.g., 1, 50, 100, 150, 200, 250, 300 or any combination thereof), more preferably an integer from 1 to 100; where n is the degree of polymerization, the larger n is, the higher the molecular weight of chitosan. As the molecular weight of chitosan increases, the entanglement of chitosan grafted with cellulose increases, causing a certain degree of blockage to the pores of the coating layer, resulting in a decrease in the air permeability and electrolyte wettability of the final coated membrane.

[0017] In this invention, there are no special requirements for the thickness of the coating layer and the base film, as well as the type of the base film. However, in order to ensure that the resulting coated diaphragm has the best air permeability, electrolyte wettability, and heat resistance, while also being thinner and lighter, the following preferred options are made for the thickness of the coating layer and the base film, as well as the type of the base film:

[0018] Preferably, the thickness of the coating layer is 1 to 1.5 μm. The thickness of the coating layer directly affects the thinness, air permeability and electrolyte wettability of the coated separator. If the coating layer is too thin, the thermal shrinkage and wettability of the resulting coated separator will be poor. If the coating layer is too thick, the volume of the resulting coated separator will be large, the capacity of the battery assembled later will be smaller, and the charge and discharge cycle capability of the battery will also be affected.

[0019] Preferably, the thickness of the base film is 5 to 8 μm, for example, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm or any combination thereof.

[0020] Preferably, the base film comprises a polyolefin base film.

[0021] Preferably, the material of the polyolefin-based film includes any one or a combination of at least two of the homopolymers or copolymers of ethylene, propylene, 1-butene, pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, and 1-decene.

[0022] In a second aspect, the present invention provides a method for preparing a coated diaphragm as described in the first aspect, the method comprising: coating a chitosan-grafted cellulose slurry onto one or both sides of a base membrane, and drying the diaphragm to obtain the coated diaphragm.

[0023] In the preparation method provided by this invention, there are no particular limitations on the coating method, rate, and temperature. Conventional coating methods and rates can be selected. However, to achieve the best coating effect and highest coating efficiency, and considering the viscosity range of the chitosan-grafted cellulose slurry, the following preferred methods, rates, and temperatures for coating are:

[0024] Preferably, the coating method is micro-grooved roller coating.

[0025] Preferably, the coating rate is 60 to 110 m / min, for example, 60 m / min, 70 m / min, 80 m / min, 90 m / min, 100 m / min, 110 m / min or any combination thereof; if the coating rate is too fast, problems such as missed coating and uneven surface density will occur, while if the coating rate is too slow, the coating efficiency will be affected.

[0026] Preferably, the coating temperature is 60-70°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C or any combination thereof; if the coating temperature is too high, the edges of the resulting coated membrane will warp to varying degrees, and if the coating temperature is too low, the surface of the resulting coated membrane may not dry properly.

[0027] Preferably, the chitosan-grafted cellulose slurry is obtained by uniformly mixing chitosan-grafted cellulose, an aqueous binder, a wetting and dispersing agent, and water.

[0028] First, this invention does not impose any particular limitation on the specific mixing method described above, as long as the mixture is homogeneous. However, to maximize mixing efficiency and uniformity, the following mixing method is preferred: Chitosan-grafted cellulose and water are premixed at 1200–1500 rpm (e.g., 1200 rpm, 1300 rpm, 1400 rpm, 1500 rpm, or any combination thereof) for 1–2 hours (e.g., 1 hour, 1.3 hours, 1.6 hours, 1.9 hours, 2 hours, or any combination thereof). Then, an aqueous binder is added at 1000–1200 rpm (e.g., 1050 rpm, 11…). Mix at a speed of 00 rpm, 1150 rpm, 1200 rpm or any combination thereof for 1 to 2 hours (e.g., 1 hour, 1.3 hours, 1.6 hours, 1.9 hours, 2 hours or any combination thereof), and finally add a wetting and dispersing agent and mix at a speed of 500 to 800 rpm (e.g., 500 rpm, 550 rpm, 600 rpm, 650 rpm, 700 rpm, 750 rpm, 800 rpm or any combination thereof) for 0.5 to 1 hour (e.g., 0.5 hours, 0.7 hours, 0.9 hours, 1 hour or any combination thereof) to complete the mixing.

[0029] Secondly, the present invention does not impose any special restrictions on the solid content of the chitosan-grafted cellulose slurry, as long as it meets the coating process requirements. However, in order to make the coating layer more uniform and the coating effect better, the solid content of the chitosan-grafted cellulose slurry is preferably 18-20%, for example, 18%, 18.5%, 19%, 19.5%, 20%, or any combination thereof.

[0030] Furthermore, the present invention does not impose any particular limitations on the preparation method of the chitosan-grafted cellulose. However, in order to obtain the best grafting effect of the chitosan-grafted cellulose, the following preparation method is preferred, which includes the following steps:

[0031] (1) Cellulose is oxidized by an oxidizing agent to obtain oxidized cellulose containing aldehyde groups;

[0032] (2) The oxidized cellulose containing aldehyde groups obtained in step (1) is subjected to a grafting reaction with chitosan to obtain the chitosan-grafted cellulose.

[0033] It is important to note that because cellulose contains hydroxyl groups on its surface, it can be oxidized to aldehyde groups by oxidizing agents. At the same time, it can break the hydrogen bonds in cellulose, forming smaller molecular chain segments. This allows the hydroxyl functional groups on the cellulose surface to escape the hydrogen bond-restricted region, making it easier for them to interact with lithium salts and lithium ions, thereby promoting lithium ion migration. In addition, after cellulose is oxidized with sodium periodate, oxidized cellulose containing aldehyde groups can be obtained. Furthermore, the aldehyde groups and the amino groups on chitosan can be condensed through a Schiff base reaction to obtain the chitosan-grafted cellulose.

[0034] Preferably, the oxidant includes sodium periodate or 2,2,6,6-tetramethylpiperidine-nitrogen oxides.

[0035] Preferably, the reaction temperature of the oxidation treatment is 40–60°C (e.g., 40°C, 42°C, 44°C, 46°C, 48°C, 50°C, 52°C, 54°C, 56°C, 58°C, 60°C, or any combination thereof). When the oxidation treatment temperature is below 40°C, the reaction will be incomplete, resulting in the hydroxyl groups on the surface of cellulose not being completely converted into aldehyde groups. When the oxidation treatment temperature is above 60°C, the surface of the nanocellulose will turn yellow because the high temperature will cause changes in the chemical structure of the nanocellulose, making it more susceptible to light absorption, thereby affecting its optical properties.

[0036] Preferably, the reaction time of the oxidation treatment is ≥2h (e.g., 2h, 2.2h, 2.4h, 2.6h, 2.8h, 3h or any combination thereof), more preferably 2-3h; when the reaction time of the oxidation treatment is less than 2h, the reaction will be incomplete, resulting in the hydroxyl groups on the surface of cellulose not being completely converted into aldehyde functional groups; when the reaction time of the oxidation treatment is greater than 3h, manufacturing capacity may be wasted, resulting in unnecessary energy consumption and increased production costs.

[0037] Preferably, to improve the oxidation effect of cellulose, the oxidation treatment further includes a pretreatment step of the cellulose. The pretreatment method includes: rinsing the cellulose with water 2-4 times (to remove residual medium and impurities until no residual impurities are visible on the cellulose surface); then drying at a temperature of 50-70°C (e.g., 50°C, 55°C, 60°C, 65°C, 70°C, or any combination thereof) for 3-4 hours (e.g., 3 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4 hours, or any combination thereof); and then immersing the dried cellulose in a solution with a concentration of 0.1-0.3 mol / L (e.g., 0.1 mol / L, 0.15 mg / L). The cellulose is boiled completely in a sodium hydroxide solution with a concentration of 0.3 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, or any combination thereof for 60–90 min (e.g., 60 min, 65 min, 70 min, 75 min, 80 min, 85 min, 90 min, or any combination thereof), then neutralized with an acetic acid solution with a concentration of 0.3–0.5 mol / L (e.g., 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, or any combination thereof), and finally washed with distilled water until its pH reaches neutral to obtain the pretreated cellulose.

[0038] Preferably, to improve the oxidation effect of cellulose, the oxidation treatment method specifically includes: adding cellulose to water to prepare a cellulose dispersion with a mass percentage of 4-6% (e.g., 4%, 4.5%, 5%, 5.5%, 6%, or any combination thereof), dispersing it at a rotation speed of 3000-5000 rpm (e.g., 3000 rpm, 3500 rpm, 4000 rpm, 4500 rpm, 5000 rpm, or any combination thereof) for 1-2 hours (e.g., 1 hour, 1.5 hours, 2 hours, or any combination thereof), then adding sodium periodate to carry out the oxidation reaction. When the reaction solution turns dark yellow, it indicates that the oxidation reaction is basically completed. After the oxidation reaction is completed, the solution is centrifuged and washed 2-4 times, and then dried to obtain oxidized cellulose containing aldehyde groups, which needs to be stored in the dark.

[0039] Preferably, the mass ratio of the oxidant to cellulose is at least 5:1, for example, 5.5:1, 6:1, 6.5:1, 7:1, 7.5:1, 8:1, 8.5:1, or any combination thereof, and more preferably (5-8):1. When the amount of oxidant is insufficient (the mass ratio to cellulose is less than 5:1), the oxidation rate will slow down, the number of aldehyde groups generated will be insufficient, which will affect the subsequent grafting reaction (Schiff base reaction), resulting in incomplete grafting and ultimately affecting the air permeability of the coated membrane. When the amount of oxidant is excessive (the mass ratio to cellulose is greater than 8:1), the oxidation rate is extremely high, but the pH is alkaline, and the oxidation process will lead to an increase in side reactions (white flocculent substances will precipitate), and the oxidation effect will decrease significantly. This will cause cellulose to easily decompose in an alkaline environment, reducing the yield of oxidized cellulose.

[0040] Preferably, the grafting reaction temperature is 60–70°C, for example, 60°C, 62°C, 64°C, 66°C, 68°C, 70°C, or any combination thereof. When the grafting reaction temperature is higher than 70°C, the reaction solution is prone to gelation, ultimately causing macromolecules to entangle together, which greatly reduces the free radical activity on the cellulose molecular chains and decreases the grafting rate. When the grafting reaction temperature is lower than 60°C, the grafting reaction cannot occur (Schiff base reaction).

[0041] Preferably, the grafting reaction lasts for ≥2 hours, more preferably 2 to 3 hours, for example, 2 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, 3 hours, or any combination thereof. When the grafting reaction time is less than 2 hours, the grafting reaction will be incomplete and the grafting rate will be reduced. When the grafting reaction time is longer than 3 hours, manufacturing capacity may be wasted, resulting in unnecessary energy consumption and increased production costs.

[0042] Preferably, in order to improve the grafting effect of chitosan-grafted cellulose, the grafting reaction method specifically includes: dispersing chitosan in an acetic acid solution to obtain a transparent acetic acid solution, then immersing oxidized cellulose containing aldehyde groups in the above transparent acetic acid solution for grafting reaction, and washing to remove chitosan and acetic acid to obtain the chitosan-grafted cellulose (existing in the form of a dispersion, with water as the solvent).

[0043] Preferably, in order to make the subsequently obtained chitosan-grafted cellulose slurry more suitable for coating, the D of the chitosan-grafted cellulose is limited. 50 The particle size is 0.2–0.8 μm, for example, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, or 0.8 μm, or any combination thereof; when chitosan is grafted with cellulose D 50When the particle size is greater than 0.8 μm, the viscosity of the chitosan-grafted cellulose slurry will be too high, resulting in poor flowability and making it unsuitable for machine coating. While reducing the solids content of the chitosan-grafted cellulose slurry can solve the viscosity and flowability issues, it will significantly reduce process efficiency and easily lead to problems such as missed coating and uneven coating. 50 When the particle size is less than 0.2μm, the particle size is too small and the specific surface area is too large, which easily leads to the problem of internal aggregation of particles. This causes defects such as black spots and bright spots on the film surface to easily appear in the subsequent coating process of the chitosan grafted cellulose slurry.

[0044] Finally, this invention does not impose any special restrictions on the type and amount of the aqueous binder and wetting and dispersing agent in the chitosan-grafted cellulose slurry; conventional materials and amounts in the art can be selected. However, in order to obtain a coated diaphragm with the best heat resistance, air permeability, and electrolyte wettability, the following preferred types and amounts of the aqueous binder and wetting and dispersing agent are specified:

[0045] Preferably, the water-based binder in the chitosan-grafted cellulose slurry has a mass percentage of 3-5%, for example, 3%, 3.5%, 4%, 4.5%, 5%, or any combination thereof. When the water-based binder content in the chitosan-grafted cellulose slurry is less than 3%, powder shedding will occur during the later coating process, resulting in poor peel strength of the coating. In severe cases, the coating may even peel off, greatly affecting the heat resistance and wetting properties of the coated diaphragm. When the water-based binder content in the chitosan-grafted cellulose slurry is greater than 5%, the viscosity of the chitosan-grafted cellulose slurry will be too high, and the leveling properties will be poor, which will have an adverse effect on the coating process, ultimately leading to abnormal phenomena such as uneven surface density and missed coating in the coated diaphragm.

[0046] Preferably, the water-based adhesive includes at least one of water-based acrylic, polyacrylate, or polyacrylamide.

[0047] Preferably, the wetting and dispersing agent in the chitosan-grafted cellulose slurry has a mass percentage content of 0.1% to 0.3%, for example, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any combination thereof.

[0048] Preferably, the wetting and dispersing agent includes at least one of anionic sulfate surfactants, alkylphenol polyoxyethylene ethers, or polyethylene glycol.

[0049] Preferably, the solid content of the chitosan-grafted cellulose slurry is 18-20%, for example, 18%, 18.5%, 19%, 19.5%, 20%, or any combination thereof. If the solid content of the chitosan-grafted cellulose slurry is too high, the viscosity of the slurry will be too high, the fluidity will be poor, and it will not be suitable for machine coating. If the solid content of the chitosan-grafted cellulose slurry is too low, the viscosity of the slurry will be too low, the flow rate will be too fast, and problems such as missed coating and uneven coating will easily occur during the coating process.

[0050] Thirdly, the present invention provides an application of the coated separator as described in the first aspect as a lithium battery separator.

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] The coated separator provided by this invention includes a base membrane and a coating layer disposed on one or both sides of the base membrane, wherein the material of the coating layer includes chitosan-grafted cellulose. By selecting chitosan-grafted cellulose as the material of the coating layer, and utilizing chitosan, which has a similar structure to cellulose and has a steric hindrance effect, to graft and modify cellulose, the dispersion stability and uniformity of cellulose in water are effectively improved. This avoids the problem of dense and disordered stacking of cellulose during wet molding, resulting in a chitosan-grafted cellulose coating layer with a uniform porous structure. Consequently, the final coated separator has excellent heat resistance, air permeability, and electrolyte wettability, making it suitable as a lithium-ion battery separator and beneficial for improving the electrical performance of lithium-ion batteries. Attached Figure Description

[0053] Figure 1 Infrared spectra of chitosan and fibrous nanocrystals before and after grafting;

[0054] Figure 2 The images show the XRD patterns before and after grafting with fiber nanocrystals.

[0055] Figure 3 This is a scanning electron microscope image of the surface of the chitosan-grafted fiber nanocrystal coating layer in Example 1;

[0056] Figure 4 This is a scanning electron microscope image of the surface of the chitosan-grafted fiber nanocrystal coating layer in Example 2;

[0057] Figure 5 This is a scanning electron microscope image of the surface of the chitosan-grafted fiber nanocrystal coating layer in Example 3;

[0058] Figure 6 This is a scanning electron microscope image of the surface of the fiber nanocrystal coating in Comparative Example 1;

[0059] Figure 7This is a cross-sectional scanning electron microscope image of the chitosan-grafted fiber nanocrystal coating layer in Example 1;

[0060] Figure 8 This is a cross-sectional scanning electron microscope image of the chitosan-grafted fiber nanocrystal coating layer in Example 2;

[0061] Figure 9 This is a cross-sectional scanning electron microscope image of the chitosan-grafted fiber nanocrystal coating layer in Example 3;

[0062] Figure 10 This is a cross-sectional scanning electron microscope image of the fiber nanocrystal coating in Comparative Example 1. Detailed Implementation

[0063] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0064] Example 1

[0065] A coated diaphragm includes a base film and a coating layer coated on one side surface of the base film;

[0066] The base film is a polyethylene film with a thickness of 7 μm;

[0067] The coating layer is a 1μm thick chitosan-grafted fiber nanocrystal coating layer;

[0068] The method for preparing the coated diaphragm provided in this embodiment includes the following steps:

[0069] (1) Take fiber nanocrystals (L 50 The length is 200nm, D 50 The nanocrystals (20 nm in diameter and 10 aspect ratio) were rinsed three times with water and dried at 60 °C for 3.5 h. The dried nanocrystals were then boiled in 100 mL of 0.2 mol / L sodium hydroxide solution for 70 min, neutralized with 100 mL of 0.4 mol / L acetic acid solution, and finally washed with distilled water until the pH reached neutral to obtain the pretreated nanocrystals.

[0070] (2) Add water to the pretreated fiber nanocrystals obtained in step (1) to prepare a fiber nanocrystal dispersion with a mass percentage of 5%. Disperse the dispersion at 4000 rpm for 1.5 h using a high-speed stirrer. Take 500 mL of the fiber nanocrystal dispersion and add it to 834 mL of sodium periodate (NaIO4) with a concentration of 0.7 mol / L. React at 50 °C for 2 h. After washing by centrifugation three times, dry at 70 °C to obtain oxidized fiber nanocrystals containing aldehyde groups.

[0071] (3) Add 10g of chitosan (molecular formula (C8H) 13 NO5)n (where n is an integer from 1 to 50) powder is dispersed in 100 mL of acetic acid solution with a mass percentage of 2%, and stirred at 65°C for 1 h to prepare a transparent chitosan solution with a solid content of 10%. The aldehyde-containing oxidized fiber nanocrystals obtained in step (2) are immersed in the above transparent chitosan solution and reacted at 65°C for 2.5 h. Then, excess chitosan is removed by washing with 2% acetic acid solution, and excess acetic acid is removed by washing with deionized water to obtain a chitosan grafted fiber nanocrystal dispersion with a solid content of 20%.

[0072] (4) The chitosan grafted fiber nanocrystal dispersion obtained in step (3) and water were premixed at 1500 rpm for 1.5 h. Waterborne acrylic acid (Hunan Gaorui New Materials Co., Ltd., brand name 8816, added at 4 wt% of the final slurry) was added and mixed at 1200 rpm for 1.5 h. Finally, a wetting and dispersing agent (Hunan Gaorui New Materials Co., Ltd., brand name 8823, added at 0.2 wt% of the final slurry) was added and mixed at 800 rpm for 0.5 h to obtain a solid content of 19.5%, a viscosity of 205 mPa·s, and a D0.05. 50 Chitosan-grafted fiber nanocrystal slurry with a particle size of 0.25 μm;

[0073] (5) Using a micro-concave roller coating method, the chitosan grafted fiber nanocrystal slurry obtained in step (4) is coated onto a polyethylene film at a coating temperature of 65°C and a coating rate of 110 m / min. After drying, the coated diaphragm is obtained.

[0074] Example 2

[0075] A coated diaphragm includes a base film and a coating layer coated on one side surface of the base film;

[0076] The base film is a polyethylene film with a thickness of 7 μm;

[0077] The coating layer is a 1μm thick chitosan-grafted fiber nanocrystal coating layer;

[0078] The method for preparing the coated diaphragm provided in this embodiment includes the following steps:

[0079] (1) Take fiber nanocrystals (L 50 The length is 100nm, D 50The nanocrystals (20 nm in diameter and aspect ratio 5) were rinsed with water four times and dried at 60 °C for 4 h. The dried nanocrystals were then boiled in 100 mL of 0.3 mol / L sodium hydroxide solution for 90 min, neutralized with 100 mL of 0.5 mol / L acetic acid solution, and finally washed with distilled water until the pH reached neutral to obtain the pretreated nanocrystals.

[0080] (2) Add the pretreated fiber nanocrystals obtained in step (1) to water to prepare a fiber nanocrystal dispersion with a mass percentage of 6%. Disperse the dispersion at 5000 rpm for 2 hours using a high-speed stirrer. Take 500 mL of the fiber nanocrystal dispersion and add it to 1000 mL of sodium periodate (NaIO4) with a concentration of 0.7 mol / L. React at 60°C for 3 hours. After washing by centrifugation 4 times, dry at 85°C to obtain oxidized fiber nanocrystals containing aldehyde groups.

[0081] (3) Add 10g of chitosan (molecular formula (C8H) 13 NO5)n, where n is an integer from 50 to 100, powder is dispersed in 100 mL of acetic acid solution with a mass percentage of 2%, and stirred at 70°C for 1 h to prepare a transparent chitosan solution with a solid content of 10%. The aldehyde-containing oxidized fiber nanocrystals obtained in step (2) are immersed in the above transparent chitosan solution and reacted at 70°C for 3 h. Then, excess chitosan is removed by washing with 2% acetic acid solution, and excess acetic acid is removed by washing with deionized water to obtain a chitosan-grafted fiber nanocrystal dispersion with a solid content of 20.0%.

[0082] (4) The chitosan grafted fiber nanocrystal dispersion obtained in step (3) and water were premixed at 1500 rpm for 2 h. Water-based acrylic acid (Hunan Gaorui New Materials Co., Ltd., brand name 8816, added at 5 wt% of the final slurry) was added and mixed at 1200 rpm for 2 h. Finally, a wetting and dispersing agent (Hunan Gaorui New Materials Co., Ltd., brand name 8823, added at 0.3 wt% of the final slurry) was added and mixed at 800 rpm for 0.5 h to obtain a solid content of 19.8%, a viscosity of 302 mPa·s, and a D0.05. 50 Chitosan-grafted fiber nanocrystal slurry with a particle size of 0.33 μm;

[0083] (5) Using a micro-concave roller coating method, the chitosan grafted fiber nanocrystal slurry obtained in step (4) is coated onto a polyethylene film at a coating temperature of 70°C and a coating rate of 90 m / min. After drying, the coated diaphragm is obtained.

[0084] Example 3

[0085] A coated diaphragm includes a base film and a coating layer coated on one side surface of the base film;

[0086] The base film is a polyethylene film with a thickness of 7 μm;

[0087] The coating layer is a chitosan-grafted fiber nanocrystal coating layer with a thickness of 1 μm;

[0088] The method for preparing the coated diaphragm provided in this embodiment includes the following steps:

[0089] (1) Take fiber nanocrystals (L 50 The length is 300nm, D 50 The nanocrystals (20 nm in diameter and 15 aspect ratio) were rinsed twice with water and dried at 60 °C for 3 h. The dried nanocrystals were then boiled in 100 mL of 0.1 mol / L sodium hydroxide solution for 60 min, neutralized with 100 mL of 0.3 mol / L acetic acid solution, and finally washed with distilled water until the pH reached neutral to obtain the pretreated nanocrystals.

[0090] (2) Add water to the pretreated fiber nanocrystals obtained in step (1) to prepare a fiber nanocrystal dispersion with a mass percentage of 4%. Disperse the dispersion at 3000 rpm for 1 h using a high-speed stirrer. Take 500 mL of the fiber nanocrystal dispersion and add it to 668 mL of sodium periodate (NaIO4) with a concentration of 0.7 mol / L. React at 40 °C for 2 h. After washing by centrifugation twice, dry at 60 °C to obtain oxidized fiber nanocrystals containing aldehyde groups.

[0091] (3) Add 10g of chitosan (molecular formula (C8H) 13 NO5)n, where n is an integer from 100 to 300) powder is dispersed in 100 mL of acetic acid solution with a mass percentage of 2%, and stirred at 60°C for 1 h to prepare a transparent chitosan solution with a solid content of 10%. The aldehyde-containing oxidized fiber nanocrystals obtained in step (2) are immersed in the above transparent chitosan solution and reacted at 60°C for 2 h. Then, excess chitosan is removed by washing with 2% acetic acid solution, and excess acetic acid is removed by washing with deionized water to obtain a chitosan grafted fiber nanocrystal dispersion with a solid content of 20%.

[0092] (4) The chitosan grafted fiber nanocrystal dispersion obtained in step (3) and water were premixed at 1200 rpm for 1 h. Waterborne acrylic acid (Hunan Gaorui New Materials Co., Ltd., brand name 8816, added at 3 wt% of the final slurry) was added and mixed at 1000 rpm for 1 h. Finally, a wetting and dispersing agent (Hunan Gaorui New Materials Co., Ltd., brand name 8823, added at 0.1 wt% of the final slurry) was added and mixed at 500 rpm for 0.5 h to obtain a solid content of 19.2%, a viscosity of 500 mPa·s, and a D0.05. 50 Chitosan-grafted fiber nanocrystal slurry with a particle size of 0.42 μm;

[0093] (5) Using a micro-concave roller coating method, the chitosan grafted fiber nanocrystal slurry obtained in step (4) is coated onto a polyethylene film at a coating temperature of 60°C and a coating rate of 60 m / min. After drying, the coated diaphragm is obtained.

[0094] Example 4

[0095] A coated diaphragm, differing from Example 1 in that it uses nanocellulose (D... 50 With a diameter of 50nm, L 50 The fiber nanocrystals were replaced with those having a length of 500 nm and an aspect ratio of 10. All other structures, materials, parameters and preparation methods were the same as in Example 1.

[0096] Example 5

[0097] A coated diaphragm, differing from Example 1 in that it uses bacterial cellulose (D... 50 The diameter is 8nm, L 50 The fiber nanocrystals (800 nm in length and 100 aspect ratio) were replaced, while the other structures, materials, parameters and preparation methods were the same as in Example 1.

[0098] Example 6

[0099] A coated diaphragm, differing from Example 1 in that the L-shaped fiber nanocrystals... 50 The length is 400nm, D 50 The diameter is 20 nm, the aspect ratio is 20, and the other structures, materials, parameters and preparation methods are the same as those in Example 1.

[0100] Example 7

[0101] A coated diaphragm differs from Example 1 in that the degree of polymerization of chitosan, n, is greater than 300, while the other structures, materials, parameters, and preparation methods are the same as in Example 1.

[0102] Example 8

[0103] A coated diaphragm differs from Example 1 in that the coating thickness is 1.5 μm, while the other structures, materials, parameters, and preparation methods are the same as in Example 1.

[0104] Example 9

[0105] A coated diaphragm differs from Example 1 in that the thickness of the base film is 5 μm, while the other structures, materials, parameters, and preparation methods are the same as in Example 1.

[0106] Example 10

[0107] A coated diaphragm differs from Example 1 in that the mass ratio of fiber nanocrystals to sodium periodate in step (2) is 1:8, while the other structures, materials, parameters and preparation methods are the same as in Example 1.

[0108] Example 11

[0109] A coated diaphragm differs from Example 1 in that the mass ratio of fiber nanocrystals to sodium periodate in step (2) is 1:4, while the other structures, materials, parameters and preparation methods are the same as in Example 1.

[0110] Comparative Example 1

[0111] A coated diaphragm includes a base film and a coating layer coated on one side surface of the base film;

[0112] The base film is a polyethylene film with a thickness of 7 μm;

[0113] The coating layer is a 1μm thick fiber nanocrystal coating layer;

[0114] The method for preparing the coated diaphragm provided in this embodiment includes the following steps:

[0115] (1) Take fiber nanocrystals (CNC, L) 50 The length is 200nm, D 50 The nanocrystals (20 nm in diameter and 10 aspect ratio) were rinsed three times with water and dried at 60 °C for 3.5 h. The dried nanocrystals were then boiled in 100 mL of 0.2 mol / L sodium hydroxide solution for 70 min, neutralized with 100 mL of 0.4 mol / L acetic acid solution, and finally washed with distilled water until the pH reached neutral to obtain the pretreated nanocrystals.

[0116] (2) The pretreated fiber nanocrystals obtained in step (1) and water were premixed at 1500 rpm for 1.5 h. Water-based acrylic acid (Hunan Gaorui New Materials Co., Ltd., brand name 8816, added at 4 wt% of the final slurry) was added and mixed at 1200 rpm for 1.5 h. Finally, a wetting and dispersing agent (Hunan Gaorui New Materials Co., Ltd., brand name 8823, added at 0.2 wt% of the final slurry) was added and mixed at 800 rpm for 0.5 h to obtain a solid content of 19.5%, a viscosity of 689 mPa·s, and a D0.05. 50 Fiber nanocrystalline slurry with a particle size of 0.55μm;

[0117] (3) The fiber nanocrystal slurry obtained in step (2) is coated onto a polyethylene film using a micro-concave roller coating method. The coating temperature is 65°C and the coating rate is 110 m / min. After drying, the coated diaphragm is obtained.

[0118] Comparative Example 2

[0119] A polyethylene film with a thickness of 7 μm.

[0120] Comparative Example 3

[0121] A coated diaphragm includes a base film and a coating layer coated on one side surface of the base film;

[0122] The base film is a polyethylene film with a thickness of 7 μm;

[0123] The coating layer is a boehmite coating layer with a thickness of 1 μm;

[0124] The method for preparing the boehmite-coated diaphragm provided in this comparative example includes the following steps:

[0125] (1) First, prepare the boehmite dispersion (Shandong Guoci-HP-070, D) 50 The boehmite slurry with a particle size of 0.5 μm and a solid content of 20% was premixed with water for 1 hour at a speed of 1200 rpm. Water-based acrylic acid (Hunan Gaorui New Materials Co., Ltd., grade 8816, added at 4 wt% of the final slurry) was added and mixed at 1200 rpm for 1.5 hours. Finally, wetting and dispersing agent (Hunan Gaorui New Materials Co., Ltd., grade 8823, added at 0.2 wt% of the final slurry) was added and mixed at 800 rpm for 0.5 hours to obtain a boehmite slurry with a solid content of 50%.

[0126] (2) The boehmite slurry obtained in step (2) is coated onto a polyethylene film using a micro-concave roller coating method. The coating temperature is 65°C and the coating rate is 110 m / min. After drying, the boehmite-coated diaphragm is obtained.

[0127] Structural characterization:

[0128] (1) Fourier transform infrared spectroscopy was used to identify the functional groups of chitosan-grafted fiber nanocrystals in Example 1, chitosan-grafted fiber nanocrystals in Example 2, chitosan-grafted fiber nanocrystals in Example 3, aldehyde-containing oxidized fiber nanocrystals, fiber nanocrystals, and chitosan, respectively, to determine whether the grafting was successful (Schiff base reaction occurred). The infrared spectra of chitosan and fiber nanocrystals before and after grafting were obtained as follows: Figure 1 As shown;

[0129] from Figure 1 It can be seen that the OH stretching, CH stretching, COC stretching, and chitosan ring stretching in the fiber nanocrystals are reflected at 3380 cm⁻¹. -1 2893cm -1 1067cm -1 and 897cm -1 In the absorption band, the characteristic absorption peak corresponding to the formation of aldehyde groups in oxidized fiber nanocrystals is at 1738 cm⁻¹. -1 When formed in the middle, and further grafted into chitosan grafted fiber nanocrystals, at 1726 cm⁻¹ -1 and 1572cm -1 Two characteristic peaks were formed at the point, which correspond to C=N and CN bonds respectively. This indicates that after the fiber nanocrystals are oxidized by sodium periodate, they condense with chitosan through Schiff base reaction to form new bonds, thereby forming chitosan-grafted fiber nanocrystals.

[0130] (2) The structures of chitosan-grafted fiber nanocrystals in Example 1, Example 2, and Example 3 were tested using X-ray diffraction. The XRD patterns of the fiber nanocrystals before and after grafting are shown below. Figure 2 As shown;

[0131] from Figure 2 It can be seen that at 2θ, 14.6°, 16.9° and 22.6° represent the (1~10), (110) and (200) crystal plane peaks, respectively, which represent the typical crystal structure of fiber nanocrystals. Although the width and intensity of the three characteristic peaks are decreasing after grafting, they still have characteristic peaks compared with the original fiber nanocrystals, indicating that sodium periodate oxidation does not oxidize the crystallographic features of fiber nanocrystals.

[0132] (3) The surfaces of the chitosan-grafted fiber nanocrystal coating layer in Example 1, the chitosan-grafted fiber nanocrystal coating layer in Example 2, the chitosan-grafted fiber nanocrystal coating layer in Example 3, and the fiber nanocrystal coating layer in Comparative Example 1 were observed using a scanning electron microscope. The surface scanning electron microscope image of the chitosan-grafted fiber nanocrystal coating layer provided in Example 1 is shown below. Figure 3As shown, the surface scanning electron microscope image of the chitosan grafted fiber nanocrystal coating layer provided in Example 2 is as follows. Figure 4 As shown, the surface scanning electron microscope image of the chitosan grafted fiber nanocrystal coating layer provided in Example 3 is as follows. Figure 5 As shown, the surface scanning electron microscope image of the fiber nanocrystal coating provided in Comparative Example 1 is as follows. Figure 6 As shown;

[0133] from Figures 3-6 It can be seen that the pure fiber nanocrystal coating has a dense network structure. Figure 6 Due to hydrogen bonding, the fibers are tightly packed and exhibit significant flocculation. When chitosan is grafted onto the fiber nanocrystals, the chitosan-grafted fiber nanocrystal structure indicates a certain porosity. Figures 3-5 This is because the addition of chitosan increases the spacing between the fibers of the fibrous nanocrystals, creating a steric hindrance effect, causing the morphology to become curled and generating more voids; simultaneously, from Figure 5 It can be seen that as the degree of polymerization of chitosan increases, the entanglement effect of chitosan increases, leading to flocculation. That is, chitosan indirectly blocks the gaps between the fibers of the fibrous nanocrystals, thus reducing the porosity.

[0134] (4) The cross-sections of the chitosan grafted fiber nanocrystal coating layer in Example 1, the chitosan grafted fiber nanocrystal coating layer in Example 2, the chitosan grafted fiber nanocrystal coating layer in Example 3, and the fiber nanocrystal coating layer in Comparative Example 1 were observed using a scanning electron microscope. The cross-sectional scanning electron microscope image of the chitosan grafted fiber nanocrystal coating layer provided in Example 1 is shown below. Figure 7 As shown, the cross-sectional scanning electron microscope image of the chitosan grafted fiber nanocrystal coating layer provided in Example 2 is as follows. Figure 8 As shown, the cross-sectional scanning electron microscope image of the chitosan grafted fiber nanocrystal coating layer provided in Example 3 is as follows. Figure 9 As shown, the cross-sectional scanning electron microscope image of the fiber nanocrystal coating provided in Comparative Example 1 is as follows. Figure 10 As shown;

[0135] from Figures 7-10 It can be seen that the cross-section of the pure fiber nanocrystal coating provided in Comparative Example 1 is relatively compact (with a relatively high packing density), while the cross-sectional structure of the chitosan-grafted fiber nanocrystal coating provided in Examples 1-3 is relatively loose. Due to the good dispersibility and grafting properties of chitosan, the fiber nanocrystals are uniformly mixed with chitosan of different molecular weights, with almost no flocculation and excellent pore structure. Further observation... Figure 9The cross-section is significantly thinner and the structure is less loose, further verifying that the increased degree of chitosan polymerization leads to an increased entanglement effect of chitosan.

[0136] Performance testing:

[0137] (1) Air permeability value and air permeability increment: Tested according to the method provided in GB / T36363-2018;

[0138] (2) Porosity: Tested according to the method provided in GB / T36363-2018;

[0139] (3) Heat shrinkage rate: The test was conducted at 150℃ and 180℃ for 1 hour, respectively, according to the method provided in GB / T12027-2004;

[0140] (4) Electrolyte contact angle: The method provided in GB / T 30447 is used to measure the wettability of the electrolyte.

[0141] The diaphragms provided in Examples 1-11 and Comparative Examples 1-3 were tested using the above test methods, and the test results are shown in Table 1:

[0142] Table 1

[0143]

[0144]

[0145] According to the data in Table 1:

[0146] The coated diaphragms provided in Examples 1-11 have an air permeability of 180-240 s / 100 mL, an air permeability increment of 20-80 s / 100 mL, a porosity of 38-48%, and a thermal shrinkage rate of only 1.0-2.0% in the MD direction and only 0.8-2.0% in the TD direction after heat treatment at 150℃. After heat treatment at 180℃, the thermal shrinkage rate in the MD direction is 1.5-3.3%, and the thermal shrinkage rate in the TD direction is 1.6-3.0%. The contact angle is 12.8-18.2°. This indicates that the coated diaphragms provided in Examples 1-11 possess excellent heat resistance, air permeability, electrolyte wettability, and thinness. Furthermore, by further optimizing the raw materials and preparation methods for chitosan grafted fiber nanocrystals, the heat resistance, air permeability, electrolyte wettability, and thinness of the coated diaphragms provided in Examples 1-5 and 8-10 can be optimized.

[0147] Specifically, compared to Example 1, the L-shaped fiber nanocrystals used in Example 6... 50Excessive length leads to a decrease in the porosity of the coated membrane, increasing the air permeability to 220s / 100mL, resulting in decreased air permeability. Simultaneously, the thermal shrinkage rates in both the MD and TD directions increase after heat treatment, leading to slightly poorer heat resistance. In Example 7, the excessive polymerization of chitosan resulted in increased entanglement of chitosan-grafted cellulose, causing some blockage of the pores in the coating layer. Consequently, the porosity of the resulting coated membrane decreased, while the air permeability remained high. Furthermore, the increased thermal shrinkage rates in both the MD and TD directions after heat treatment indicated a slight decrease in heat resistance. In Example 11, the low mass ratio of sodium periodate to fiber nanocrystals slowed the oxidation rate, resulting in insufficient aldehyde groups and consequently, a decrease in the porosity of the final coated membrane, a high air permeability, and a slight decrease in air permeability.

[0148] Compared with Example 1, the coated diaphragm provided in Comparative Example 1 has a high permeability of 540s / 100mL due to the coating material being only fiber nanocrystals; the polyethylene membrane provided in Comparative Example 2 has poor heat resistance and cannot withstand high-temperature treatment at 150°C; the boehmite coated diaphragm provided in Comparative Example 3 also has poor heat resistance. Although it did not melt after heat treatment at 150°C and 180°C, the thermal shrinkage rates in both the TD and MD directions were very large, and the contact angle was too high, indicating poor electrolyte wettability.

[0149] The applicant declares that this invention illustrates a coated separator, its preparation method, and a lithium-ion battery through the above embodiments. However, this invention is not limited to the above process steps, meaning that this invention does not necessarily rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of the raw materials used in this invention, additions of auxiliary components, and selection of specific methods all fall within the protection and disclosure scope of this invention.

Claims

1. A coated diaphragm, characterized in that, The coated diaphragm includes a base film and a coating layer disposed on one or both sides of the base film; The coating material includes chitosan grafted with cellulose.

2. The coated diaphragm according to claim 1, characterized in that, The cellulose in the chitosan-grafted cellulose includes at least one of fibrous nanocrystals, nanocellulose or bacterial cellulose, preferably fibrous nanocrystals. Preferably, the D of the fiber nanocrystals 50 Diameter of 10–50 nm, L 50 The length is 100–300 nm; Preferably, the D of the nanocellulose 50 Diameter of 10–80 nm, L 50 The length is 300–1000 nm; Preferably, the bacterial cellulose has a D 50 Diameter of 5–10 nm, L 50 The length is 500–5000 nm; Preferably, the chitosan in the chitosan-grafted cellulose has the molecular formula (C8H12H2O). 13 NO5)n, where n is an integer from 1 to 300, and more preferably an integer from 1 to 100; Preferably, the thickness of the coating layer is 1 to 1.5 μm.

3. The coated diaphragm according to claim 1 or 2, characterized in that, The thickness of the base film is 5–8 μm; Preferably, the base film comprises a polyolefin base film; Preferably, the material of the polyolefin-based film includes any one or a combination of at least two of the homopolymers or copolymers of ethylene, propylene, 1-butene, pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, and 1-decene.

4. A method for preparing a coated diaphragm as described in any one of claims 1 to 3, characterized in that, The preparation method includes: coating a chitosan-grafted cellulose slurry onto one or both sides of a base membrane, drying it, and obtaining the coated membrane.

5. The preparation method according to claim 4, characterized in that, The coating rate is 60–110 m / min; Preferably, the coating temperature is 60–70°C.

6. The preparation method according to claim 4 or 5, characterized in that, The chitosan-grafted cellulose slurry is obtained by uniformly mixing chitosan-grafted cellulose, an aqueous binder, a wetting and dispersing agent, and water.

7. The preparation method according to claim 6, characterized in that, The method for preparing the chitosan-grafted cellulose includes the following steps: (1) Cellulose is oxidized by an oxidizing agent to obtain oxidized cellulose containing aldehyde groups; (2) The oxidized cellulose containing aldehyde groups obtained in step (1) is subjected to a grafting reaction with chitosan to obtain the chitosan-grafted cellulose. Preferably, the oxidant includes sodium periodate or 2,2,6,6-tetramethylpiperidine-nitrogen oxides; Preferably, the reaction temperature of the oxidation treatment is 40–60°C; Preferably, the oxidation treatment time is ≥2 hours, more preferably 2 to 3 hours; Preferably, the mass ratio of the oxidant to cellulose is at least 5:1, more preferably (5-8):1; Preferably, the grafting reaction temperature is 60–70°C; Preferably, the grafting reaction time is ≥2 hours, more preferably 2 to 3 hours; Preferably, the chitosan grafted with cellulose has a D 50 The particle size is 0.2–0.8 μm.

8. The preparation method according to any one of claims 6 or 7, characterized in that, The water-based binder in the chitosan-grafted cellulose slurry has a mass percentage content of 3-5%. Preferably, the wetting and dispersing agent in the chitosan-grafted cellulose slurry has a mass percentage content of 0.1% to 0.3%.

9. The preparation method according to any one of claims 4 to 8, characterized in that, The solid content of the chitosan-grafted cellulose slurry is 18-20%.

10. A lithium-ion battery, characterized in that, The lithium-ion battery includes the coated separator as described in any one of claims 1 to 3 or the coated separator prepared by the preparation method as described in any one of claims 4 to 9.

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