Coated diaphragm containing biomass carbon material as well as preparation method and application of coated diaphragm

Abstract

The invention relates to the field of lithium battery diaphragms, and discloses a coated diaphragm containing a biomass carbon material and a preparation method and application thereof.The preparation method comprises the steps that straw is added into a chlorate solution to be soaked, and activated straw is obtained; calcining a part of the activated straws in an inert atmosphere to obtain biomass porous carbon; mixing the other part of activated straws with nano iron powder, and then calcining in an inert atmosphere to obtain biomass carbon nanotubes; biomass porous carbon and biomass carbon nanotubes are mixed according to the mass ratio of 1: (3-5), and a mixed biomass carbon material is obtained; mixing the mixed biomass carbon material with an adhesive, a dispersing agent and water to obtain coating slurry; and coating the surface of a base membrane with the film, and drying to obtain the coated diaphragm. Porous carbon and carbon nanotubes are prepared from straw and are applied to a coating material of the diaphragm, and the ion transmission efficiency and the mechanical property of the diaphragm can be remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery separators, and in particular to a coated separator comprising a biomass carbon material, and a preparation method and application thereof. Background Art

[0002] Plant straw, a common waste product from agricultural product processing, is commonly treated through on-site incineration or centralized processing. On-site incineration easily leads to random carbon dioxide emissions, while centralized processing, such as simple incineration or fertilizer production, simply achieves centralized emissions without realizing any additional value. Straw contains nitrogen and phosphorus elements, and if properly converted into biomass carbon, its utilization rate can be greatly improved, enhancing its value.

[0003] Currently, there is research on using biomass carbon in battery separators. For example, patent CN116864917A discloses a composite separator and its preparation method, comprising a separator substrate and a modified coating. The modified coating is disposed on the surface of the separator substrate and comprises a biomass carbon / metal oxide composite material; the biomass carbon / metal oxide composite material comprises porous biomass carbon and metal oxides. However, existing biomass carbon materials have uncontrollable pore structures and insufficient conductivity. Some materials also have excessively high micropore content. Using them in separators can hinder lithium ion transport and reduce battery rate performance. Summary of the Invention

[0004] The present invention aims to overcome the above-mentioned problems existing in the biomass carbon coated diaphragm in the prior art, and provides a coated diaphragm comprising biomass carbon material, and a preparation method and application thereof. Straw is made into porous carbon and carbon nanotubes, which are applied to the coating material of the diaphragm, which can significantly improve the ion transmission efficiency and mechanical properties of the diaphragm.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for preparing a coated membrane comprising a biomass carbon material, the steps comprising: (1) Washing, drying, and crushing the straw and then soaking it in a chloride solution to obtain activated straw; (2) calcining a portion of the activated straw under an inert atmosphere to obtain biomass porous carbon; (3) Mixing another portion of activated straw with nano-iron powder and then calcining it under an inert atmosphere to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:3 to 5 to obtain a mixed biomass carbon material; (5) mixing the mixed biomass carbon material with an adhesive, a dispersant, and water to obtain a coating slurry; (6) The coating slurry is coated on the surface of the base film, and the coated diaphragm is obtained after drying.

[0006] The present invention uses straw as raw material to prepare biomass porous carbon and biomass carbon nanotubes, which are used for coating particles in diaphragms, and can maximize the utilization of agricultural and sideline products. Straw contains N and P elements. The present invention crushes the straw and soaks it in a chloride solution. Subsequently, calcining in an inert atmosphere can obtain N and P-doped biomass porous carbon, which is used as coating particles. Because the introduction of nitrogen and phosphorus elements causes defects in the spatial structure of the carbon material, this defect has a promoting effect on the ion transfer inside the battery. At the same time, heteroatom doping can make the material have better wettability, can have more sufficient contact with the electrolyte, promote the free transmission of ions, and thus improve the ionic conductivity of the diaphragm. In addition, the present invention uses chloride as a hard template pore-forming agent, which can provide basic support when calcining, obtain holes with uniform pore size and uniform distribution, and can provide a good three-dimensional structure. This three-dimensional structure can allow the biomass porous carbon to maintain good stability when performing relevant charge and discharge tests, so that the collapse of the material at the microscopic level is reduced, providing basic mechanical stability. Porous carbon as a conductive material can effectively improve the conductivity of the composite separator and promote the lithium ion transport process inside the battery. The well-distributed uniform pores can better promote the transport of ions and shorten the ion transport path, which greatly promotes the improvement of the ion transport efficiency in the battery.

[0007] The present invention also combines another portion of activated straw with nano-iron powder and calcines it. The gases released during pyrolysis, catalyzed by the iron powder, form carbon nanotubes, significantly improving the electrochemical performance of carbon nanotubes compared to porous carbon. The resulting biomass porous carbon and biomass carbon nanotubes are mixed as coated particles. The carbon nanotubes form a long-range conductive network, while the porous carbon provides localized conductive points. The rigid skeleton of the biomass porous carbon and the flexible entanglement of the carbon nanotubes create a "rigid and flexible" structure, significantly improving the ionic conductivity of the separator and suppressing separator deformation during cycling, thereby enhancing the battery's rate performance and cycling stability.

[0008] Preferably, the chloride solution in step (1) is one or more of a cobalt chloride solution, a ferric chloride solution, a cupric chloride solution, and a nickel chloride solution; the concentration of the chloride solution is 0.5 to 2 mol / L; and the straw is soaked in the chloride solution for 12 to 24 hours. The activation time of the straw in the chloride solution should not be too long. Excessive soaking time will lead to excessive activation and the introduction of more ions, which will cause the carbon material to collapse during the calcination process. The structural collapse of the carbon material has a significant impact on the electrochemical properties of the carbon material. At the same time, due to the structural collapse, the porous structure will be significantly damaged, which will significantly affect the liquid absorption rate of the material.

[0009] Preferably, the calcination temperature in step (2) is 600-1200° C., and the calcination time is 3-5 h.

[0010] Preferably, the mass of the nano iron powder added in step (3) is 5-10% of the mass of the activated straw. The addition of iron powder can promote the nucleation and growth of carbon nanotubes to a certain extent. However, too high an iron powder content may cause defects in the structure of the carbon nanotubes. Because too much iron powder may make the reaction too intense, resulting in the deposition and arrangement of carbon atoms losing order, destroying the original regular tubular structure of the carbon nanotubes, and affecting their crystallinity and degree of graphitization. At the same time, during the biomass pyrolysis process, due to the presence of water, part of the iron powder will be oxidized to Fe3O4. Continuing to increase the iron powder content may increase the amount of impurities such as Fe3O4 generated. These impurities may adhere to the surface of the carbon nanotubes or embed into their structure, not only affecting the purity of the carbon nanotubes, but also having a negative impact on their physical and chemical properties, such as conductivity and mechanical properties. In addition, the increase in iron powder content may change the path and selectivity of the pyrolysis reaction. In addition to promoting the formation of carbon nanotubes, it may also trigger other side reactions, consume more carbon source, reduce the amount of carbon source used for carbon nanotube growth, and reduce the yield of carbon nanotubes, thereby affecting the performance of the membrane.

[0011] Preferably, the calcination temperature in step (3) is 700-900° C., and the calcination time is 3-5 h.

[0012] Preferably, in step (5), 15 to 25 parts of mixed biomass carbon material, 1 to 3 parts of adhesive, 10 to 15 parts of dispersant and 60 to 70 parts of water are mixed by weight to obtain a coating slurry.

[0013] Preferably, the thickness of the base film in step (6) is 12-20 μm, and the thickness of the coating after coating is 3-5 μm.

[0014] Preferably, the base film in step (6) is a polypropylene diaphragm.

[0015] In a second aspect, the present invention provides a coated diaphragm comprising a biomass carbon material prepared using the above preparation method.

[0016] In a third aspect, the present invention provides an application of a coated diaphragm comprising biomass carbon material prepared using the above preparation method in a lithium battery.

[0017] Therefore, the present invention has the following beneficial effects: (1) Using straw as raw material to prepare biomass porous carbon and biomass carbon nanotubes, which are used as coating particles in the separator, can maximize the utilization of agricultural by-products; (2) The porous carbon material prepared with straw as raw material has defects in its spatial structure due to the doping of heteroatoms. This defect promotes the ion transfer inside the battery. At the same time, the introduction of nitrogen and phosphorus elements improves the wettability of the porous carbon and the wettability with the electrolyte, which promotes the free transmission of ions. (3) The present invention mixes another portion of activated straw with nano iron powder and calcines the mixture. The gases released during the pyrolysis process can form carbon nanotubes under the catalysis of the iron powder. The electrochemical performance of the carbon nanotubes is greatly improved compared to that of porous carbon. (4) The present invention mixes the obtained biomass porous carbon and biomass carbon nanotubes as coating particles. The carbon nanotubes can construct a long-range conductive network, while the porous carbon provides local conductive points. The rigid skeleton of the biomass porous carbon and the flexible entanglement of the carbon nanotubes form a "rigid and flexible" structure, which can significantly improve the ionic conductivity of the diaphragm and inhibit the deformation of the diaphragm during the cycle, thereby improving the rate performance and cycle stability of the battery. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with specific embodiments.

[0019] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0020] Overall embodiment: A method for preparing a coated diaphragm containing biomass carbon material, comprising the steps of: (1) Washing, drying, and crushing the straw, and then soaking it in a ferric chloride solution to obtain activated straw; (2) calcining a portion of the activated straw under an inert atmosphere to obtain biomass porous carbon; (3) Mixing another portion of activated straw with nano-iron powder and then calcining it under an inert atmosphere to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:3 to 5 to obtain a mixed biomass carbon material; (5) mixing the mixed biomass carbon material with an adhesive, a dispersant, and water to obtain a coating slurry; (6) The coating slurry is coated on the surface of the base film, and the coated diaphragm is obtained after drying.

[0021] As a specific embodiment, the chloride salt solution in step (1) is one or more of cobalt chloride solution, ferric chloride solution, copper chloride solution, and nickel chloride solution; the concentration of the chloride salt solution is 0.5-2 mol / L; and the straw is soaked in the chloride salt solution for 12-24 hours.

[0022] As a specific embodiment, the calcination temperature in step (2) is 600-1200° C., and the calcination time is 3-5 hours.

[0023] As a specific implementation method, the mass of the nano iron powder added in step (3) is 5-10% of the mass of the activated straw.

[0024] As a specific embodiment, the calcination temperature in step (3) is 700-900° C., and the calcination time is 3-5 hours.

[0025] As a specific embodiment, in step (5), 15 to 25 parts of mixed biomass carbon material are mixed with 1 to 3 parts of adhesive, 10 to 15 parts of dispersant and 60 to 70 parts of water in parts by weight to obtain a coating slurry.

[0026] As a specific implementation manner, the adhesive in step (5) is selected from one or more of polytetrafluoroethylene, polyvinylidene fluoride, epoxy resin, polyimide, polyurethane, and polyvinyl acetal.

[0027] As a specific embodiment, the dispersant in step (5) is selected from one or more of triethylhexyl phosphate, sodium lauryl sulfate, methylpentanol, sodium carboxymethyl cellulose, polyacrylamide, guar gum, and fatty acid polyethylene glycol ester.

[0028] As a specific embodiment, the thickness of the base film in step (6) is 12-20 μm, and the thickness of the coating after coating is 3-5 μm.

[0029] As a specific embodiment, the base film in step (6) is a polypropylene diaphragm.

[0030] Example 1: A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 12 h to obtain activated straw; (2) Place two porcelain boats in a high-temperature tube furnace under argon protection. Place activated straw in the front porcelain boat and a mixture of activated straw and nano-iron powder in the rear porcelain boat. The mass of the nano-iron powder is 5% of the mass of the activated straw. Raise the temperature to 700°C at a rate of 5°C / min under argon protection and keep warm for 4 hours. (3) The product in the front porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass porous carbon; the product in the rear porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:4 to obtain a mixed biomass carbon material; (5) The mixed biomass carbon material was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (6) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0031] Example 2: A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L cobalt chloride solution for 24 h to obtain activated straw; (2) Place two porcelain boats in a high-temperature tube furnace under argon protection. Place activated straw in the front porcelain boat and a mixture of activated straw and nano-iron powder in the rear porcelain boat. The mass of the nano-iron powder is 10% of the mass of the activated straw. Raise the temperature to 700°C at a rate of 5°C / min under argon protection and keep warm for 4 hours. (3) The product in the front porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass porous carbon; the product in the rear porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:3 to obtain a mixed biomass carbon material; (5) The mixed biomass carbon material was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (6) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0032] Example 3: A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 24 h to obtain activated straw; (2) Place two porcelain boats in a high-temperature tube furnace under argon protection. Place activated straw in the front porcelain boat and a mixture of activated straw and nano-iron powder in the rear porcelain boat. The mass of the nano-iron powder is 10% of the mass of the activated straw. Raise the temperature to 700°C at a rate of 5°C / min under argon protection and keep warm for 4 hours. (3) The product in the front porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass porous carbon; the product in the rear porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:5 to obtain a mixed biomass carbon material; (5) The mixed biomass carbon material was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (6) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0033] Comparative Example 1 (activation time is too long): A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 36 h to obtain activated straw; (2) Place two porcelain boats in a high-temperature tube furnace under argon protection. Place activated straw in the front porcelain boat and a mixture of activated straw and nano-iron powder in the rear porcelain boat. The mass of the nano-iron powder is 5% of the mass of the activated straw. Raise the temperature to 700°C at a rate of 5°C / min under argon protection and keep warm for 4 hours. (3) The product in the front porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass porous carbon; the product in the rear porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:4 to obtain a mixed biomass carbon material; (5) The mixed biomass carbon material was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (6) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0034] Comparative Example 2 (too much iron powder added): A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 12 h to obtain activated straw; (2) Place two porcelain boats in a high-temperature tube furnace under argon protection. Place activated straw in the front porcelain boat and a mixture of activated straw and nano-iron powder in the rear porcelain boat. The mass of the nano-iron powder is 15% of the mass of the activated straw. Raise the temperature to 700°C at a rate of 5°C / min under argon protection and keep warm for 4 hours. (3) The product in the front porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass porous carbon; the product in the rear porcelain boat was washed five times with dilute sulfuric acid (10 wt%), deionized water and ethanol alternately, and then dried to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:4 to obtain a mixed biomass carbon material; (5) The mixed biomass carbon material was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (6) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0035] Comparative Example 3 (using only biomass porous carbon): A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 12 h to obtain activated straw; (2) Place a porcelain boat in a high-temperature tube furnace under argon protection, put the activated straw in the porcelain boat, and heat it to 700℃ at a rate of 5℃ / min under argon protection and keep it warm for 4h; (3) The product in the porcelain boat was washed alternately with dilute sulfuric acid (10 wt%), deionized water, and ethanol five times, and then dried to obtain biomass porous carbon; (4) The biomass porous carbon was mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose, and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (5) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0036] Comparative Example 4 (using only carbon nanotubes): A method for preparing a coated diaphragm containing biomass carbon material, comprising the following steps: (1) Wash, dry, and crush rice straw, then soak it in a 1 mol / L ferric chloride solution for 12 h to obtain activated straw; (2) Place a porcelain boat in a high-temperature tube furnace under argon protection, and put a mixture of activated straw and nano-iron powder in the porcelain boat. The mass of the nano-iron powder is 5% of the mass of the activated straw; under argon protection, heat it to 700℃ at a rate of 5℃ / min and keep it warm for 4h; (3) The product in the porcelain boat was washed five times alternately with dilute sulfuric acid (10 wt%), deionized water, and ethanol, and then dried to obtain biomass carbon nanotubes; (4) The biomass carbon nanotubes were mixed with epoxy resin adhesive (3M DP100NS), dispersant sodium carboxymethyl cellulose and deionized water in a mass ratio of 20:2:13:65 to obtain a coating slurry; (5) The coating slurry was coated on the surface of a 12 μm polypropylene diaphragm (Hefei Changyang New Energy Technology Co., Ltd.) with a coating thickness of 4 μm. The coated diaphragm was obtained after drying in an oven at 60°C for 10 min.

[0037] The properties of the coated separators prepared in the above examples and comparative examples were tested, and the results are shown in Table 1.

[0038] The test items and methods are as follows: (1) Tensile strength: The longitudinal and transverse tensile strength of the diaphragm were tested using a Xieqiang CTM universal testing machine. Five specimens were tested in each direction and the average value was calculated. (2) Puncture strength: The puncture strength of the diaphragm was tested using a Xieqiang CTM universal testing machine. Five specimens were tested and the average value was calculated. (3) Membrane rupture temperature: The membrane rupture temperature is measured by the resistance mutation method. The point where the resistance suddenly increases is the membrane rupture temperature. (4) Liquid absorption rate: The liquid absorption rate of the coated diaphragm is tested by weighing method. First, the mass of the diaphragm is recorded after it is completely dried. Then, the completely dried diaphragm is immersed in electrolyte for 24 hours. After the surface electrolyte is wiped dry, the weight of the diaphragm is recorded again. The difference between the two records is the liquid absorption rate of the diaphragm. Test 5 samples and calculate the average value. (5) Ionic conductivity: The bulk resistance was measured using the electrochemical impedance spectroscopy (EIS) mode of a VMP3B-10 electrochemical workstation (Bio-LogicScience Instruments) using a steel sheet / diaphragm / steel sheet assembly method. The perturbation voltage amplitude was 5 mV and the frequency was 10 mHz to 1 MHz. The relationship between bulk resistance and ionic conductivity is calculated as follows: ionic conductivity is equal to the ratio of the diaphragm thickness to the product of the diaphragm resistance and the effective contact area.

[0039] Table 1: Coated diaphragm performance test results

[0040] It can be seen from the data in Table 1 that the coated membranes prepared by the method of the present invention in Examples 1 to 3 have high tensile strength, puncture strength and membrane rupture temperature, as well as high liquid absorption rate and ionic conductivity.

[0041] In Comparative Example 1, the activation time of the straw in the ferric chloride solution is too long, and the membrane rupture temperature is not greatly affected. However, due to excessive activation, more ions are introduced, and the carbon material will experience structural collapse during the calcination process. The structural collapse of the carbon material has a great influence on the electrochemical properties of the carbon material. At the same time, due to the structural collapse, the porous structure will be significantly damaged, so that the liquid absorption rate of the diaphragm is significantly reduced compared with Example 1.

[0042] In Comparative Example 2, too much nano-iron powder was added during calcination, and the various properties of the diaphragm also decreased compared to those in Example 1. This is mainly because iron powder can promote the nucleation and growth of carbon nanotubes to a certain extent. However, too high an iron powder content may cause defects in the structure of the carbon nanotubes. Because too much iron powder may make the reaction too violent, the deposition and arrangement of carbon atoms lose order, destroying the originally regular tubular structure of the carbon nanotubes and affecting their crystallinity and degree of graphitization. At the same time, during the biomass pyrolysis process, due to the presence of water, part of the iron powder will be oxidized to Fe3O4. Continuing to increase the iron powder content may increase the generation of impurities such as Fe3O4; these impurities may adhere to the surface of the carbon nanotubes or embed into their structure, not only affecting the purity of the carbon nanotubes, but also having a negative impact on their physical and chemical properties, such as conductivity, mechanical properties, etc. In addition, an increase in the iron powder content may change the path and selectivity of the pyrolysis reaction. In addition to promoting the formation of carbon nanotubes, it may also trigger other side reactions, consume more carbon source, reduce the amount of carbon source used for carbon nanotube growth, and thus reduce the yield of carbon nanotubes.

[0043] In Comparative Example 3, only biomass porous carbon was used as the coating particles without adding biomass carbon nanotubes. Due to the lack of high conductivity of carbon nanotubes, the liquid absorption rate and ionic conductivity of the diaphragm decreased compared with those in Example 1.

[0044] In Comparative Example 4, using only biomass carbon nanotubes as coating particles without adding biomass porous carbon, the separator's puncture strength, liquid absorption rate, and ionic conductivity all decreased. This is primarily because the combined use of biomass carbon nanotubes and biomass porous carbon creates a continuous conductive network. The carbon nanotubes form a long-range conductive network, while the porous carbon provides localized conductive points, thereby optimizing lithium-ion transport. In contrast, using carbon nanotubes alone can lead to accumulation and localized breakage, which can degrade ion transport performance.

[0045] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A method for preparing a coated membrane comprising a biomass carbon material, characterized in that the steps include: (1) Washing, drying, and crushing the straw and then soaking it in a chloride solution to obtain activated straw; (2) calcining a portion of the activated straw under an inert atmosphere to obtain biomass porous carbon; (3) Mixing another portion of activated straw with nano-iron powder and then calcining it under an inert atmosphere to obtain biomass carbon nanotubes; (4) mixing the biomass porous carbon and the biomass carbon nanotubes in a mass ratio of 1:3 to 5 to obtain a mixed biomass carbon material; (5) mixing the mixed biomass carbon material with an adhesive, a dispersant, and water to obtain a coating slurry; (6) The coating slurry is coated on the surface of the base film, and the coated diaphragm is obtained after drying.

2. The method for preparing a coated diaphragm comprising biomass carbon material according to claim 1, wherein: The chloride solution in step (1) is one or more of cobalt chloride solution, ferric chloride solution, copper chloride solution, and nickel chloride solution; the concentration of the chloride solution is 0.5-2 mol / L; and the straw is soaked in the chloride solution for 12-24 hours.

3. The method for preparing a coated diaphragm comprising a biomass carbon material according to claim 1, wherein: The calcination temperature in step (2) is 600-1200° C., and the calcination time is 3-5 h.

4. The method for preparing a coated diaphragm comprising biomass carbon material according to claim 1, wherein: The mass of the nano iron powder added in step (3) is 5-10% of the mass of the activated straw.

5. The method for preparing a coated diaphragm comprising a biomass carbon material according to claim 1 or 4, wherein: The calcination temperature in step (3) is 700-900°C, and the calcination time is 3-5 hours.

6. The method for preparing a coated diaphragm comprising biomass carbon material according to claim 1, wherein: In step (5), 15 to 25 parts of the mixed biomass carbon material are mixed with 1 to 3 parts of an adhesive, 10 to 15 parts of a dispersant, and 60 to 70 parts of water in parts by weight to obtain a coating slurry.

7. The method for preparing a coated diaphragm comprising a biomass carbon material according to claim 1, wherein: The thickness of the base film in step (6) is 12-20 μm, and the thickness of the coating after coating is 3-5 μm.

8. The method for preparing a coated diaphragm comprising a biomass carbon material according to claim 1 or 7, wherein: The base film described in step (6) is a polypropylene diaphragm.

9. A coated diaphragm comprising a biomass carbon material, characterized in that: It is prepared using the preparation method according to any one of claims 1 to 8.

10. Use of a coated diaphragm comprising a biomass carbon material prepared by the preparation method according to any one of claims 1 to 8 or a coated diaphragm comprising a biomass carbon material according to claim 9 in a lithium battery.

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