A method and product for preparing hard carbon anode materials using coconut shells

Abstract

This application discloses a method and product for preparing hard carbon anode materials using coconut shells, relating to the field of battery materials technology. The method includes the following steps: pre-treating and crushing coconut shells to obtain coconut shell powder; immersing the coconut shell powder in an activation solution, drying it, and then activating it at high temperature, followed by steam oxidation and pore expansion; carbonizing the activated coconut shell powder, and then surface-modifying it with graphene and nano-metal particles to obtain a hard carbon material; mixing the hard carbon material with a binder, conductive agent, and solvent to form a slurry, and then molding and annealing it to obtain the hard carbon anode material. This application utilizes coconut shells to prepare hard carbon anode materials, improving the specific surface area and lithium-ion transport and storage efficiency of the hard carbon anode material by modifying the process steps and parameters.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and more specifically, to a method and product for preparing hard carbon anode materials using coconut shells. Background Technology

[0002] With the urgent need for sustainable energy in modern society, lithium-ion batteries, as an important energy storage device, have been widely used in electric vehicles, portable electronic devices, and other fields. As a key component of lithium-ion batteries, the performance of the anode material directly affects important indicators such as energy density, cycle life, and charge / discharge rate. Traditional graphite anode materials, due to their limited theoretical specific capacity (approximately 372 mAh / g), are gradually failing to meet the ever-increasing demand for high energy density, making the development of new high-performance anode materials imperative.

[0003] Hard carbon materials have become a promising alternative to lithium-ion battery anode materials due to their high specific capacity, good cycle stability, and low potential plateau over a wide potential range. Hard carbon typically refers to amorphous carbon that is difficult to graphitize at high temperatures. Its unique disordered layer structure provides abundant lithium-ion storage sites, with a theoretical specific capacity as high as 700-1000 mAh / g, far exceeding that of graphite anodes. However, the relatively low electronic conductivity of hard carbon materials limits their charge / discharge rates and rate performance in practical applications.

[0004] Among the many raw materials for preparing hard carbon materials, biomass has attracted significant attention from researchers due to its wide availability, low cost, and environmental friendliness. Coconut shells, as a widely available agricultural waste, are rich in organic components such as cellulose, hemicellulose, and lignin, making them an ideal raw material for preparing hard carbon materials. Through a series of treatments, such as carbonization and activation, coconut shells can be transformed into hard carbon anode materials with specific structures and properties.

[0005] Early research on coconut shell-based hard carbon anode materials mainly focused on simple carbonization processes. Although materials with certain performance could be obtained, their specific surface area was small and their pore structure was imperfect, resulting in low lithium-ion transport and storage efficiency. Summary of the Invention

[0006] The purpose of this application is to provide a method and product for preparing hard carbon anode materials using coconut shells, thereby improving the specific surface area and lithium-ion transport and storage efficiency of the hard carbon anode materials by modifying the process steps and parameters.

[0007] To solve the above-mentioned technical problems, the technical solution adopted in this application is as follows:

[0008] On the one hand, this application provides a method for preparing hard carbon anode materials using coconut shells, comprising the following steps:

[0009] S1. Pre-treat and crush the coconut shells to obtain coconut shell powder;

[0010] S2. The coconut shell powder is immersed in the activation solution, dried and then activated at high temperature. Then, water vapor is introduced to oxidize and expand the pores.

[0011] S3. The activated coconut shell powder is carbonized and then modified with graphene and nano-metal particles to obtain hard carbon material.

[0012] S4. The above-mentioned hard carbon material is mixed with binder, conductive agent and solvent to form a slurry, and then subjected to molding and annealing treatment to obtain the hard carbon negative electrode material.

[0013] On the other hand, this application provides a hard carbon anode material prepared by the above method.

[0014] Compared with the prior art, the embodiments of this application have at least the following advantages or beneficial effects:

[0015] This application utilizes coconut shells to prepare hard carbon anode materials, and improves the specific surface area and lithium-ion transport and storage efficiency of the hard carbon anode materials by modifying the process steps and parameters.

[0016] This application uses a compound activator of citric acid and urea to activate coconut shell powder. Citric acid provides an acidic environment to hydrolyze hemicellulose and lignin in the coconut shell, forming initial pores. Urea decomposes at high temperature into ammonia and carbon dioxide, forming "bubble templates" in the carbon skeleton, expanding the pores and increasing surface roughness. After activation, the introduction of high-temperature steam can selectively oxidize and expand the pores, adjusting the pore size distribution.

[0017] This application not only employs chemical vapor deposition (CVD) for graphene coating but also introduces nano-metal particle doping. By ultrasonically dispersing nano-iron or nano-nickel particles and adsorbing them onto the surface of graphene-coated hard carbon material, and then heat-treating to form stable chemical bonds, the electron transport and lithium storage performance of the material are synergistically improved. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to specific embodiments.

[0020] A method for preparing hard carbon anode materials using coconut shells includes the following steps:

[0021] S1. Pre-treat and crush the coconut shells to obtain coconut shell powder;

[0022] S2. The coconut shell powder is immersed in the activation solution, dried and then activated at high temperature. Then, water vapor is introduced to oxidize and expand the pores.

[0023] S3. The activated coconut shell powder is carbonized and then modified with graphene and nano-metal particles to obtain hard carbon material.

[0024] S4. The above-mentioned hard carbon material is mixed with binder, conductive agent and solvent to form a slurry, and then subjected to molding and annealing treatment to obtain the hard carbon negative electrode material.

[0025] In some embodiments of this application, the pretreatment in step S1 specifically involves rinsing the coconut shells with running water and carefully selecting mature coconut shells that are hard and free of obvious defects. The selected coconut shells are then rinsed in running water to remove surface mud, impurities, and residual coconut meat. They are then dried at 100-120°C to a constant weight, ensuring that the moisture content of the coconut shells is below 5%. The crushing process involves first crushing the shells into small pieces with a particle size of 1-2 cm using a jaw crusher, and then further grinding them using a planetary ball mill with a ball-to-material ratio of 10:1 and a grinding time of 2-3 hours, until the material is ball-milled to a mesh size of 100-200 for subsequent processing.

[0026] In some embodiments of this application, the mass ratio of citric acid to urea in the activation solution in step S2 is 1:(0.5-1.2), the solvent is water, and the ratio of citric acid to water is 1g:(0.8-1.2)mL; the solid-liquid ratio of the coconut shell powder to the activation solution is 1g:(5-10)mL. Excessive citric acid concentration will lead to over-etching of the carbon skeleton and a decrease in mechanical strength, while insufficient concentration will result in insufficient activity.

[0027] Citric acid was prepared using analytical grade reagent with a concentration ≥99.5%, and urea was prepared using analytical grade reagent with a concentration ≥99%. The solvent was deionized water. The steps for preparing the activation solution were as follows: First, dissolve citric acid in deionized water to prepare an acidic solution. Then, add urea to the acidic solution and stir until completely dissolved to form a transparent mixture. The pH of the mixture was 1.5-2.5.

[0028] In some embodiments of this application, the above-mentioned impregnation step is specifically as follows: first, ultrasonic treatment at 40-60℃ for 1-2 hours with an ultrasonic frequency of 40kHz is performed to promote the penetration of the activation solution into the interior of the coconut shell; then, the shell is left to stand at room temperature for 6-12 hours to allow the activator to be fully adsorbed; the drying temperature is 60-80℃ and the drying time is 8-12 hours to remove moisture.

[0029] In some embodiments of this application, the above-mentioned high-temperature activation specifically involves heating to 600-700°C at a rate of 5-10°C / min in an inert atmosphere (nitrogen or argon) and holding at that temperature for 1-2 hours. Urea decomposes to produce gases (NH3, CO2), which work synergistically with citric acid to create pores. When the activation temperature is below 500°C, urea decomposition will be incomplete and the pore structure will not develop sufficiently. When the temperature is above 700°C, micropore collapse will occur, resulting in a decrease in specific surface area. The temperature of the water vapor introduced is 200-400°C, and the introduction time is 10-15 minutes. This selectively oxidizes and expands the pores, adjusting the pore size distribution.

[0030] This application uses a compound activator of citric acid and urea to activate coconut shell powder. Citric acid provides an acidic environment to hydrolyze hemicellulose and lignin in coconut shell, forming initial pores. Urea decomposes into ammonia and carbon dioxide at high temperature, forming a "bubble template" in the carbon skeleton, expanding the pores and increasing the surface roughness.

[0031] In some embodiments of this application, the carbonization in step S3 above includes primary carbonization and secondary carbonization. Specifically, primary carbonization involves heating to 500-600°C at a heating rate of 10-15°C / min under an argon protective atmosphere and carbonizing at a constant temperature for 1-2 hours. Primary carbonization mainly removes volatile substances from the coconut shell and initially forms a hard carbon structure. Secondary carbonization involves heating to 900-1000°C at a rate of 3-5°C / min on the temperature of primary carbonization and carbonizing at a constant temperature for 3-4 hours. Secondary carbonization can further optimize the graphitization degree of hard carbon and improve the conductivity and structural stability of the material.

[0032] In some embodiments of this application, the surface modification specifically involves: coating the carbonized coconut shell-based hard carbon material with graphene using chemical vapor deposition (CVD), with a mixture of methane and hydrogen as the carbon source, and reacting at 800-900°C and 10-20 Pa for 1-2 hours; then placing it in an ultrasonically dispersed nano-metal dispersion and ultrasonically treating it for 30-45 minutes to adsorb the nano-metal particles onto the material surface. Subsequently, drying it in a vacuum oven at 80-100°C, followed by heat treatment at 400-500°C for 1-2 hours, allows the nano-metal particles to form stable chemical bonds with the hard carbon material, further improving the material's electron transport and lithium storage performance.

[0033] In some embodiments of this application, the mass ratio of hard carbon material, binder, and conductive agent in step S4 is 85:10:5; the binder is polyvinylidene fluoride. The conductive agent is acetylene black; the solvent is N-methylpyrrolidone. The slurry is coated onto copper foil, and the coating thickness is controlled to be 80-100 μm using a doctor blade coating method. Then, it is dried in a vacuum oven at 120-150°C for 12-15 hours, and finally punched into circular electrode sheets with a diameter of 14-16 mm.

[0034] In some embodiments of this application, the annealing temperature is 300-400°C, and the annealing time is 2-3 hours. Annealing eliminates internal stress in the electrode sheet, further improving the crystallinity and stability of the material. The annealed electrode sheet is stored in a dry, oxygen-free environment for subsequent battery assembly and performance testing.

[0035] A hard carbon anode material is prepared using the method described above.

[0036] The features and performance of this application will be further described in detail below with reference to the embodiments.

[0037] Example 1

[0038] A method for preparing hard carbon anode materials using coconut shells includes the following steps:

[0039] S1. Select mature coconut shells that are hard and free of obvious defects. Rinse the selected coconut shells in running water to remove surface mud, impurities, and residual coconut meat. Then dry them in an oven at 110℃ until constant weight, ensuring the moisture content of the coconut shells is below 5%. Use a jaw crusher to initially crush the dried coconut shells into small pieces with a particle size of about 1-2 cm. Next, use a planetary ball mill for further grinding, with a ball-to-material ratio of 10:1 and a grinding time of 2.5 hours. Pass the powder through a 100-mesh sieve to obtain coconut shell powder.

[0040] S2. Dissolve 20g of citric acid in 20mL of deionized water to prepare an acidic solution. Then add 15g of urea to the acidic solution and stir until completely dissolved to form a transparent mixture, which is the activation solution. Place coconut shell powder in the activation solution at a solid-liquid ratio of 1g:8mL. First, sonicate at 50℃ for 1.5h at a sonic frequency of 40kHz. Then, let it stand at room temperature for 8h. Dry at 70℃ for 10h. Transfer the dried coconut shell powder to a tube furnace and heat it to 650℃ at a rate of 10℃ / min under a nitrogen protective atmosphere. Hold at this temperature for 1h. After activation, cool down to 300℃ and introduce steam for 15min. Finally, cool down to room temperature.

[0041] S3. The treated coconut shell powder is placed in a carbonization furnace and heated to 550℃ at a heating rate of 10℃ / min under an argon protective atmosphere. It is then carbonized at this temperature for 1.5 hours. The product after the first carbonization is placed back in the carbonization furnace and carbonized a second time at a higher temperature (950℃) at a rate of 5℃ / min for 3 hours. The material is then placed in a CVD reaction chamber, and a mixture of methane and hydrogen is introduced as the carbon source. The reaction is carried out at 850℃ and a pressure of 15Pa for 1 hour, allowing graphene to grow uniformly and coat the surface of the hard carbon particles. An appropriate amount of nano-iron (Fe) particles are selected and uniformly dispersed in an ethanol solution using ultrasonic dispersion. The graphene-coated hard carbon material is added to the above solution and ultrasonically treated for 30 minutes, allowing the nano-metal particles to adsorb onto the material surface. Subsequently, it is dried in a vacuum oven at 80℃ and then heat-treated at 450℃ for 1 hour. Stable chemical bonds are formed between the nano-metal particles and the hard carbon material, resulting in the hard carbon material.

[0042] S4. Mix the above-mentioned hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) at a mass ratio of 85:10:5. Add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent and stir to form a uniform slurry. Coat the slurry onto copper foil using a doctor blade coating method to control the coating thickness to 90 μm. Then dry it in a vacuum oven at 130°C for 12 h and finally punch it into a circular electrode sheet with a diameter of 15 mm. Place the formed electrode sheet in a vacuum environment and anneal it at 350°C for 2.5 h to obtain the final hard carbon negative electrode material.

[0043] Example 2

[0044] A method for preparing hard carbon anode materials using coconut shells includes the following steps:

[0045] S1. Select mature coconut shells that are hard and free of obvious defects. Rinse the selected coconut shells in running water to remove surface mud, impurities, and residual coconut meat. Then dry them in an oven at 115℃ until constant weight, ensuring the moisture content of the coconut shells is below 5%. Use a jaw crusher to initially crush the dried coconut shells into small pieces with a particle size of about 1-2 cm. Next, use a planetary ball mill for further grinding, with a ball-to-material ratio of 10:1 and a grinding time of 2 hours. Pass the powder through a 150-mesh sieve to obtain coconut shell powder.

[0046] S2. Dissolve 20g of citric acid in 20mL of deionized water to prepare an acidic solution. Then add 20g of urea to the acidic solution and stir until completely dissolved to form a transparent mixture, which is the activation solution. Place coconut shell powder in the activation solution at a solid-liquid ratio of 1g:10mL. First, sonicate at 50℃ for 1h at a sonic frequency of 40kHz. Then, let it stand at room temperature for 8h. Dry at 75℃ for 10h. Transfer the dried coconut shell powder to a tube furnace and heat it to 700℃ at a rate of 10℃ / min under a nitrogen protective atmosphere. Hold at this temperature for 1h. After activation, cool down to 250℃ and introduce steam for 10min. Finally, cool down to room temperature.

[0047] S3. The treated coconut shell powder is placed in a carbonization furnace and heated to 600℃ at a heating rate of 10℃ / min under an argon protective atmosphere. It is then carbonized at this temperature for 1 hour. The product after the first carbonization is placed back in the carbonization furnace and carbonized a second time at a higher temperature (980℃) at a rate of 5℃ / min for 3 hours. The material is then placed in a CVD reaction chamber, and a mixture of methane and hydrogen is introduced as the carbon source. The reaction is carried out at 900℃ and a pressure of 15Pa for 1 hour, allowing graphene to grow uniformly and coat the surface of the hard carbon particles. An appropriate amount of nano-iron (Fe) particles are selected and uniformly dispersed in an ethanol solution using ultrasonic dispersion. The graphene-coated hard carbon material is added to the above solution and ultrasonically treated for 30 minutes, allowing the nano-metal particles to adsorb onto the material surface. Subsequently, it is dried in a vacuum oven at 80℃ and then heat-treated at 400℃ for 1 hour. Stable chemical bonds are formed between the nano-metal particles and the hard carbon material, resulting in the hard carbon material.

[0048] S4. Mix the above-mentioned hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) at a mass ratio of 85:10:5 until homogeneous. Add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent and stir to form a homogeneous slurry. Coat the slurry onto copper foil using a doctor blade coating method to control the coating thickness to 90 μm. Then dry it in a vacuum oven at 120°C for 12 hours and finally punch it into a circular electrode sheet with a diameter of 15 mm. Place the formed electrode sheet in a vacuum environment and anneal it at 300°C for 3 hours to obtain the final hard carbon negative electrode material.

[0049] Example 3

[0050] A method for preparing hard carbon anode materials using coconut shells includes the following steps:

[0051] S1. Select mature coconut shells that are hard and free of obvious defects. Rinse the selected coconut shells in running water to remove surface mud, impurities, and residual coconut meat. Then dry them in an oven at 105℃ until constant weight, ensuring the moisture content of the coconut shells is below 5%. Use a jaw crusher to initially crush the dried coconut shells into small pieces with a particle size of about 1-2 cm. Next, use a planetary ball mill for further grinding, with a ball-to-material ratio of 10:1 and a grinding time of 3 hours. Pass the powder through a 200-mesh sieve to obtain coconut shell powder.

[0052] S2. Dissolve 20g of citric acid in 22mL of deionized water to prepare an acidic solution. Then add 15g of urea to the acidic solution and stir until completely dissolved to form a transparent mixture, which is the activation solution. Place coconut shell powder in the activation solution at a solid-liquid ratio of 1g:10mL. First, sonicate at 60℃ for 1h at a sonic frequency of 40kHz. Then, let it stand at room temperature for 8h. Dry at 70℃ for 12h. Transfer the dried coconut shell powder to a tube furnace and heat it to 600℃ at a rate of 10℃ / min under a nitrogen protective atmosphere. Hold at this temperature for 2h. After activation, cool down to 200℃ and introduce steam for 10min. Finally, cool down to room temperature.

[0053] S3. The treated coconut shell powder is placed in a carbonization furnace and heated to 550℃ at a heating rate of 10℃ / min under an argon protective atmosphere. It is then carbonized at this temperature for 1 hour. The product after the first carbonization is placed back in the carbonization furnace and carbonized a second time at a higher temperature (1000℃) at a rate of 5℃ / min for 3 hours. The material is then placed in a CVD reaction chamber, and a mixture of methane and hydrogen is introduced as the carbon source. The reaction is carried out at 900℃ and a pressure of 15Pa for 1 hour, allowing graphene to grow uniformly and coat the surface of the hard carbon particles. An appropriate amount of nano-nickel (Ni) particles are selected and uniformly dispersed in an ethanol solution using ultrasonic dispersion. The graphene-coated hard carbon material is added to the above solution and ultrasonically treated for 30 minutes, allowing the nano-metal particles to adsorb onto the material surface. Subsequently, it is dried in a vacuum oven at 80℃ and then heat-treated at 400℃ for 1.5 hours. Stable chemical bonds are formed between the nano-metal particles and the hard carbon material, resulting in the hard carbon material.

[0054] S4. Mix the above-mentioned hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) at a mass ratio of 85:10:5. Add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent and stir to form a uniform slurry. Coat the slurry onto copper foil using a doctor blade coating method to control the coating thickness to 90 μm. Then dry it in a vacuum oven at 150°C for 12 hours. Finally, punch it into a circular electrode sheet with a diameter of 15 mm. Place the formed electrode sheet in a vacuum environment and anneal it at 380°C for 2 hours to obtain the final hard carbon negative electrode material.

[0055] Experimental Example

[0056] 1. This experimental example explores the effect of activation treatment on the porosity of hard carbon materials.

[0057] The experimental group was the same as in Example 1; Control group 1 used urea solution alone as the activator; Control group 2 used potassium hydroxide as the conventional activator at a concentration of 2 mol / L; the high-temperature activation temperature in control group 3 was 500℃; the high-temperature activation temperature in control group 4 was 750℃; and no water vapor was introduced into control group 5 after activation.

[0058] The activated coconut shell powder was subjected to specific surface area and pore volume tests, and the results are as follows:

[0059] Experimental group: specific surface area of ​​1500 m² 2 / g, total pore volume is 0.8cm³ 3 / g, of which the micropore volume is 0.5cm³ 3 / g, mesoporous pore volume 0.3cm 3 / g.

[0060] Control group 1: Specific surface area is 1350 m² 2 / g, insufficient activation level, weakened penetration and reaction through coconut shell pores; total pore volume is 0.7cm³. 3 / g, of which the micropore volume is 0.4cm³ 3 / g, mesoporous pore volume 0.3cm 3 / g.

[0061] Control group 2: Specific surface area is 1200 m² 2 / g; Total pore volume is 0.7cm³ 3 / g, of which the micropore volume is 0.45cm³ 3 / g, mesoporous pore volume 0.25cm³ 3 / g.

[0062] Control group 3: Specific surface area is 1400 m² 2 / g, at a relatively low temperature, urea decomposition was incomplete, and the activation reaction was insufficient; the total pore volume was 0.75cm³. 3 / g, of which the micropore volume is 0.45cm³ 3 / g, mesoporous pore volume 0.3cm 3 / g.

[0063] Control group 4: Specific surface area was 1250 m² 2 / g, at excessively high temperatures, micropores collapse, resulting in a decrease in specific surface area; the total pore volume is 0.5cm³. 3 / g, of which the micropore volume is 0.25cm³ 3 / g, mesoporous pore volume 0.25cm³ 3 / g.

[0064] Control group 5: Specific surface area was 1420 m² 2 / g; Total pore volume is 0.75cm³ 3 / g, of which the micropore volume is 0.35cm³ 3 / g, mesoporous pore volume 0.4cm 3 / g.

[0065] 2. This experimental example explores the effect of surface modification on hard carbon anode materials.

[0066] The experimental group was the same as in Example 1; Control group 1 had no surface modification and was directly shaped and post-processed after carbonization; Control group 2 had only graphene coating and no nano-metal particle doping; Control group 3 had no graphene coating and only nano-metal particle doping modification. The remaining steps for Control groups 1-3 were the same as in Example 1.

[0067] The electrochemical performance of the hard carbon anode materials finally prepared in each experimental group was tested, and the results are as follows:

[0068] (I) Cyclic Voltammetry Test (Initial Scan, Scan Rate 0.1 mV / s)

[0069] Experimental group: initial charge specific capacity 1000mAh / g, initial discharge specific capacity 850mAh / g, coulombic efficiency 85%. Capacity retention rate after 100 cycles 80%.

[0070] Control group 1: The redox peak current density was lower, with the anodic peak current density being 0.2 mA / cm². 2 The cathode peak current density is 0.25 mA / cm². 2 The peak potential difference is relatively large, at 0.5V, indicating poor electrochemical reversibility.

[0071] Control group 2: Anode peak current density increased to 0.3 mA / cm² 2 The cathode peak current density is 0.35 mA / cm². 2 The peak potential difference was reduced to 0.4V, and the electrochemical reversibility was improved.

[0072] Control group 3: Anode peak current density is 0.28 mA / cm² 2 The cathode peak current density is 0.32 mA / cm². 2 The peak potential difference was 0.42V, which showed some improvement but was not as good as the graphene-only coating group.

[0073] (II) Constant current charge-discharge test (current density 100mA / g):

[0074] Experimental group: initial charge specific capacity 1000mAh / g, initial discharge specific capacity 850mAh / g, coulombic efficiency 85%. Capacity retention rate after 100 cycles 80%.

[0075] Control Group 1: Initial charge specific capacity 800mAh / g, initial discharge specific capacity 650mAh / g, coulombic efficiency 81%. After 100 cycles, capacity retention was 60%.

[0076] Control group 2: Initial charge specific capacity 900mAh / g, initial discharge specific capacity 780mAh / g, coulombic efficiency 87%. Capacity retention rate after 100 cycles 70%.

[0077] Compared to control group 3: the initial charge specific capacity was 850 mAh / g, the initial discharge specific capacity was 720 mAh / g, and the coulombic efficiency was 85%. After 100 cycles, the capacity retention rate was 65%.

[0078] (III) AC impedance test (frequency range 100kHz-0.01Hz):

[0079] Experimental group: Rct is the smallest, at 50Ω, and the total impedance is the lowest.

[0080] Control group 1: The charge transfer resistance (Rct) is 80Ω, and the total impedance is relatively high.

[0081] Compared to Group 2, Rct decreased to 60Ω, and the total impedance decreased.

[0082] Control group 3: Rct is 65Ω, and the total impedance is between control group 1 and control group 2.

[0083] The comparative experimental data above clearly show that the complete surface modification steps (graphene coating combined with nano-metal particle doping) have the best effect on improving the microstructure and electrochemical performance of coconut shell-based hard carbon anode materials.

[0084] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A method for preparing a hard carbon negative electrode material using coconut shell, characterized in that, The method comprises the following steps: S1, pretreating and crushing the coconut shell to obtain coconut shell powder; S2, immersing the coconut shell powder in an activation liquid, drying, high-temperature activation, and then introducing water vapor for oxidation and hole expansion; the mass ratio of citric acid to urea in the activation liquid is 1:(0.5-1.2); the high-temperature activation is specifically heating to 600-700℃ at a rate of 5-10℃ / min in an inert atmosphere, and holding for 1-2h; S3, carbonizing the activated coconut shell powder, and then performing surface modification by graphene and nano metal particles to obtain hard carbon material; S4, mixing the hard carbon material, a binder, a conductive agent, and a solvent to form a slurry, and then performing forming and annealing to obtain the hard carbon negative electrode material.

2. The method for preparing hard carbon negative material from coconut shell according to claim 1, characterized in that, The pretreatment in the S1 step is specifically water flushing and drying at 100-120℃ until constant weight; the crushing is first crushing into small pieces with a particle size of 1-2cm, and then ball milling to 100-200 meshes.

3. The method for preparing hard carbon negative material from coconut shell according to claim 1, characterized in that, The solvent of the activation liquid in the S2 step is water, and the ratio of citric acid to water is 1g:(0.8-1.2)mL; the solid-liquid ratio of the coconut shell powder to the activation liquid is 1g:(5-10)mL.

4. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The immersion step is specifically ultrasonic treatment at 40-60℃ for 1-2h, and then standing at room temperature for 6-12h; the drying temperature is 60-80℃, and the drying time is 8-12h.

5. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The temperature of the water vapor introduction is 200-400℃, and the introduction time is 10-15min.

6. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The carbonization in the S3 step includes primary carbonization and secondary carbonization; the primary carbonization is specifically heating to 500-600℃ at a rate of 10-15℃ / min under an argon protective atmosphere, and holding for 1-2h; the secondary carbonization is heating to 900-1000℃ at a rate of 3-5℃ / min based on the temperature of the primary carbonization, and holding for 3-4h.

7. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The surface modification is specifically graphene coating of the carbonized coconut shell-based hard carbon material by chemical vapor deposition, and then placing in an ultrasonically dispersed nano metal dispersion liquid and ultrasonic treatment for 30-45min.

8. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The mass ratio of the hard carbon material, the binder, and the conductive agent in the S4 step is 85:10:5; the binder is polyvinylidene fluoride; the conductive agent is acetylene black; and the solvent is N-methyl pyrrolidone.

9. The method for preparing hard carbon negative material from coconut shell according to claim 3, characterized in that, The annealing temperature is 300-400℃, and the annealing time is 2-3h.

10. A hard carbon negative electrode material, characterized in that, Prepared by the method in any one of claims 1-9.