Method for preparing hard carbon negative electrode material from coconut shells and product

Abstract

The invention provides a method for preparing a hard carbon negative electrode material by using coconut shells and a product, and relates to the technical field of battery materials. The method comprises the following steps: pretreating and crushing coconut shells to obtain coconut shell powder; the method comprises the following steps: soaking coconut shell powder in an activating solution, drying, activating at high temperature, and introducing water vapor for oxidizing and chambering; carrying out carbonization treatment on the activated coconut shell powder, and then carrying out surface modification through graphene and nano metal particles to obtain a hard carbon material; and mixing the hard carbon material with a binder, a conductive agent and a solvent to form slurry, and carrying out molding and annealing treatment to obtain the hard carbon negative electrode material. The hard carbon negative electrode material is prepared from coconut shells, and the specific surface area and the lithium ion transmission and storage efficiency of the hard carbon negative electrode material are improved by improving the process steps and parameters.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, and specifically, to a method and product for preparing a hard carbon anode material using coconut shells. Background Art

[0002] With the urgent demand for sustainable energy in modern society, lithium-ion batteries, as an important energy storage device, have been widely used in fields such as electric vehicles and portable electronic devices. As a key component of lithium-ion batteries, the performance of the anode material directly affects important indicators such as the energy density, cycle life, and charge-discharge rate of the battery. Due to the limited theoretical specific capacity (about 372 mAh / g) of traditional graphite anode materials, it has gradually become difficult to meet the growing demand for high energy density, and the development of new high-performance anode materials is imminent.

[0003] Hard carbon materials have become potential alternatives for lithium-ion battery anode materials due to their high specific capacity, good cycle stability, and low potential plateau in a wide potential range. Hard carbon generally refers to amorphous carbon that is difficult to graphitize at high temperatures. Its unique turbostratic structure can provide abundant lithium-ion storage sites, and the theoretical specific capacity can be as high as 700 - 1000 mAh / g, far higher than that of graphite anodes. However, the relatively low electronic conductivity of hard carbon materials limits their charge-discharge rate and rate performance in practical applications.

[0004] Among the raw materials for preparing hard carbon materials, biomass has received great attention from researchers due to its wide source, low price, and environmental friendliness. As a large amount of agricultural waste, coconut shells are rich in organic components such as cellulose, hemicellulose, and lignin, and are ideal raw materials for preparing hard carbon materials. By performing a series of treatments on coconut shells, such as carbonization and activation, they 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 properties could be obtained, the specific surface area of the materials was small and the pore structure was imperfect, resulting in low lithium-ion transport and storage efficiency. Summary of the Invention

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

[0007] To solve the above technical problems, the technical solution adopted in the present application is:

[0008] On the one hand, the present application provides a method for preparing a hard carbon anode material using coconut shells, including the following steps:

[0009] S1. Pretreat and crush coconut shells to obtain coconut shell powder;

[0010] S2. Immerse the coconut shell powder in an activation solution, dry it, then perform high-temperature activation, and then introduce steam for oxidative pore expansion;

[0011] S3. Carbonize the activated coconut shell powder, and then obtain a hard carbon material after surface modification with graphene and nano-metal particles;

[0012] S4. Mix the above hard carbon material with a binder, a conductive agent, and a solvent to form a slurry, and obtain the hard carbon negative electrode material after shaping and annealing treatments.

[0013] On the other hand, the present application provides a hard carbon negative electrode material prepared by the above method.

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

[0015] The present application uses coconut shells to prepare a hard carbon negative electrode material, and improves the specific surface area and the lithium ion transmission and storage efficiency of the hard carbon negative electrode material by improving the process steps and parameters therein.

[0016] The present application uses an activator prepared by compounding citric acid and urea to activate the coconut shell powder. Among them, citric acid provides an acidic environment to hydrolyze hemicellulose and lignin in the coconut shell to form initial pores. Urea decomposes into ammonia and carbon dioxide at high temperature to form a "bubble template" in the carbon skeleton, expanding the pores and increasing the surface roughness. After activation, introducing high-temperature steam can perform selective oxidative pore expansion on the voids to adjust the pore size distribution.

[0017] The present application not only uses the chemical vapor deposition (CVD) method for graphene coating, but also introduces nano-metal particle doping. By ultrasonically dispersing nano-iron or nano-nickel particles and adsorbing them on the surface of the graphene-coated hard carbon material, and performing heat treatment to form stable chemical bonding, the electron transport and lithium storage performance of the material are synergistically improved. Detailed Embodiments

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0019] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application will be described in detail below with reference to specific embodiments.

[0020] A method for preparing a hard carbon anode material using coconut shells, comprising the following steps:

[0021] S1. Pretreat and crush the coconut shells to obtain coconut shell powder;

[0022] S2. Immerse the coconut shell powder in an activation solution, dry it, then perform high-temperature activation, and then introduce water vapor for oxidation and pore expansion;

[0023] S3. Carbonize the activated coconut shell powder, and then obtain the hard carbon material after surface modification with graphene and nano metal particles;

[0024] S4. Mix the above-mentioned hard carbon material with a binder, a conductive agent, and a solvent to form a slurry, and obtain the hard carbon anode material after forming and annealing treatments.

[0025] In some embodiments of the present application, the pretreatment in the above S1 step is specifically to rinse the coconut shells with running water and carefully select mature coconut shells with hard texture and no obvious defects. Place the selected coconut shells in running clean water for rinsing to remove substances such as sediment, impurities, and residual coconut meat attached to the surface; and dry them to constant weight at 100-120 °C to ensure that the moisture content of the coconut shells is less than 5%; the crushing is first carried out using a jaw crusher to crush them into small pieces with a particle size of 1-2 cm, and then further ground using a planetary ball mill, with the ball-to-material ratio set to 10:1, the grinding time is 2-3 h, and the ball milling is carried out to 100-200 mesh for subsequent treatment.

[0026] In some embodiments of the present application, the mass ratio of citric acid to urea in the activation solution in the above S2 step is 1:(0.5-1.2), the solvent is water, and the ratio of citric acid to water is 1 g:(0.8-1.2) mL; the solid-liquid ratio of the coconut shell powder to the activation solution is 1 g:(5-10) mL. If the concentration of citric acid is too high, it will cause excessive etching of the carbon skeleton and a decrease in mechanical strength, and if the concentration is too low, the activity will be insufficient.

[0027] Citric acid uses an analytical pure reagent with a concentration ≥99.5%, urea uses an analytical pure reagent with a concentration ≥99%, and the solvent is deionized water. The steps for preparing the activation solution are: first dissolve citric acid in deionized water to form an acidic solution, and then add urea to the acidic solution and stir until completely dissolved to form a transparent mixture. The pH of the mixture is 1.5-2.5.

[0028] In some embodiments of the present application, the above impregnation step is specifically: first perform ultrasonic treatment at 40-60 °C for 1-2 h, with an ultrasonic frequency of 40 kHz, to promote the penetration of the activation solution into the interior of the coconut shells; then stand at room temperature for 6-12 h to allow the activator to be fully adsorbed; the drying temperature is 60-80 °C, and the drying time is 8-12 h to remove moisture.

[0029] In some embodiments of the present application, the above-mentioned high-temperature activation is specifically carried out by heating to 600-700 °C at a rate of 5-10 °C / min in an inert atmosphere (nitrogen or argon), holding for 1-2 h. Urea decomposes to produce gases (NH3, CO2), which cooperate with citric acid to form pores; when the activation temperature is lower than 500 °C, incomplete decomposition of urea will occur, and the pore structure will not develop sufficiently. When the temperature is higher than 700 °C, micropores will collapse and the specific surface area will decrease; the temperature of the introduced water vapor is 200-400 °C, and the introduction time is 10-15 min for selective oxidation to expand pores and adjust the pore size distribution.

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

[0031] In some embodiments of the present application, the carbonization in the above S3 step includes primary carbonization and secondary carbonization. The primary carbonization is specifically carried out by heating to 500-600 °C at a heating rate of 10-15 °C / min under an argon protection atmosphere and holding for 1-2 h for carbonization; the primary step is mainly to remove volatile substances in the coconut shell and initially form a hard carbon structure; the secondary carbonization is to raise the temperature to 900-1000 °C at a rate of 3-5 °C / min based on the temperature of the primary carbonization and hold for 3-4 h for carbonization. The secondary carbonization can further optimize the graphitization degree of the hard carbon and improve the conductivity and structural stability of the material.

[0032] In some embodiments of the present application, the above surface modification is specifically as follows: Chemical vapor deposition is used to coat the carbonized coconut shell-based hard carbon material with graphene. The carbon source is a mixed gas of methane and hydrogen, and the coating conditions are to react for 1-2 h under the conditions of 800-900 °C and 10-20 Pa; then it is placed in a nano-metal dispersion after ultrasonic dispersion and ultrasonically treated for 30-45 min to adsorb nano-metal particles on the material surface. Subsequently, it is dried in a vacuum oven at 80-100 °C and then heat-treated at 400-500 °C for 1-2 h to form a stable chemical bond between the nano-metal particles and the hard carbon material, further improving the electron transport performance and lithium storage performance of the material.

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

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

[0035] A hard carbon negative electrode material is prepared by the above method.

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

[0037] Example 1

[0038] A method for preparing a hard carbon negative electrode material using coconut shells includes the following steps:

[0039] S1. Select mature coconut shells with hard texture and no obvious defects. Place the selected coconut shells in flowing clean water for rinsing to remove substances such as sediment, impurities, and residual coconut meat attached to the surface. Then dry them in an oven at 110 °C until constant weight is achieved, ensuring that the water content of the coconut shells is less than 5%. Use a jaw crusher to preliminarily crush the dried coconut shells into small pieces with a particle size of about 1 - 2 cm. Then, use a planetary ball mill for further grinding. Set the ball-to-material ratio to 10:1, the grinding time to 2.5 h, and pass through a 100-mesh sieve to obtain coconut shell powder.

[0040] S2. Dissolve 20 g of citric acid in 20 mL of deionized water to form an acidic solution. Then add 15 g of urea to the acidic solution and stir until completely dissolved to form a transparent mixed solution, which is the activation solution. Place the coconut shell powder in the activation solution according to a solid-liquid ratio of 1 g:8 mL. First, perform ultrasonic treatment at 50 °C for 1.5 h, with an ultrasonic frequency of 40 kHz. Then let it stand at room temperature for 8 h. The drying temperature is 70 °C, and the drying time is 10 h. Transfer the dried coconut shell powder to a tube furnace. Under a nitrogen protection atmosphere, heat it to 650 °C at a rate of 10 °C / min and hold for 1 h. After activation, cool it to 300 °C, introduce water vapor for 15 min, and finally cool it to room temperature.

[0041] S3. Put the processed coconut shell powder into a carbonization furnace. Under the protection of argon atmosphere, heat it to 550 °C at a heating rate of 10 °C / min, keep it at a constant temperature for carbonization for 1.5 h. The product after the first carbonization is placed in the carbonization furnace again and heated to a higher temperature (950 °C) at a rate of 5 °C / min for secondary carbonization, and the constant temperature time is 3 h. Place the material in a CVD reaction chamber, introduce a mixed gas of methane and hydrogen as a carbon source, and react for 1 h under the conditions of 850 °C and a pressure of 15 Pa to uniformly grow and coat graphene on the surface of the hard carbon particles; Select an appropriate amount of nano iron (Fe) particles and uniformly disperse them in an ethanol solution by ultrasonic dispersion; Add the hard carbon material coated with graphene to the above solution and perform ultrasonic treatment for 30 min to adsorb the nano metal particles on the surface of the material. Subsequently, dry it in a vacuum oven at 80 °C and then perform heat treatment at 450 °C for 1 h to form a stable chemical bond between the nano metal particles and the hard carbon material, obtaining the hard carbon material;

[0042] S4. Mix the above hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) in a mass ratio of 85:10:5, add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent, and stir to make a uniform slurry. Coat the slurry on a copper foil, use 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 perform annealing treatment at 350 °C for 2.5 h to obtain the final hard carbon negative electrode material.

[0043] Example 2

[0044] A method for preparing a hard carbon negative electrode material using coconut shells, comprising the following steps:

[0045] S1. Select mature coconut shells with hard texture and no obvious defects. Place the selected coconut shells in flowing clean water for rinsing to remove the attached sediment, impurities, and residual coconut meat and other substances on the surface. Subsequently, dry them in an oven at 115 °C to constant weight to ensure that the water content of the coconut shells is less than 5%; Use a jaw crusher to preliminarily crush the dried coconut shells into small pieces with a particle size of about 1 - 2 cm. Then, use a planetary ball mill for further grinding, set the ball-to-material ratio to 10:1, grind for 2 h, and pass through a 150-mesh sieve to obtain coconut shell powder;

[0046] S2. Dissolve 20 g of citric acid in 20 mL of deionized water to prepare an acidic solution. Then add 20 g of urea to the acidic solution and stir until completely dissolved to form a transparent mixed solution, which is the activation solution. Place the coconut shell powder in the activation solution according to a solid-liquid ratio of 1 g:10 mL. First, perform ultrasonic treatment at 50 °C for 1 h with an ultrasonic frequency of 40 kHz. Then let it stand at room temperature for 8 h. The drying temperature is 75 °C and the drying time is 10 h. Transfer the dried coconut shell powder to a tubular furnace. Under a nitrogen protection atmosphere, heat it to 700 °C at a rate of 10 °C / min and hold for 1 h. After activation, cool it to 250 °C, introduce water vapor for 10 min, and finally cool it to room temperature.

[0047] S3. Put the treated coconut shell powder into a carbonization furnace. Under an argon protection atmosphere, heat it to 600 °C at a heating rate of 10 °C / min and carry out constant-temperature carbonization for 1 h. The product after the first carbonization is placed in the carbonization furnace again and heated to a higher temperature (980 °C) at a rate of 5 °C / min for secondary carbonization, and the constant-temperature time is 3 h. Place the material in a CVD reaction chamber, introduce a mixed gas of methane and hydrogen as a carbon source, and react at 900 °C and a pressure of 15 Pa for 1 h to uniformly grow and coat graphene on the surface of the hard carbon particles. Select an appropriate amount of nano-iron (Fe) particles and uniformly disperse them in an ethanol solution by ultrasonic dispersion. Add the hard carbon material coated with graphene to the above solution and perform ultrasonic treatment for 30 min to adsorb the nano-metal particles on the material surface. Subsequently, dry it in a vacuum oven at 80 °C and then perform heat treatment at 400 °C for 1 h to form a stable chemical bond between the nano-metal particles and the hard carbon material, obtaining the hard carbon material.

[0048] S4. Mix the above hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) in a mass ratio of 85:10:5, add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent, and stir to make a uniform slurry. Coat the slurry on a copper foil, control the coating thickness to 90 μm by the doctor blade coating method, and then dry it in a vacuum oven at 120 °C for 12 h. Finally, punch it into circular electrode sheets with a diameter of 15 mm. Place the formed electrode sheets in a vacuum environment and perform annealing treatment at 300 °C for 3 h to obtain the final hard carbon negative electrode material.

[0049] Example 3

[0050] A method for preparing a hard carbon negative electrode material using coconut shells, comprising the following steps:

[0051] S1. Select mature coconut shells with hard texture and no obvious defects. Place the selected coconut shells in flowing clean water for rinsing to remove the attached sediment, impurities, and residual coconut meat and other substances on the surface. Then, dry them in an oven at 105 °C until constant weight to ensure that the water content of the coconut shells is less than 5%. Use a jaw crusher to preliminarily 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. Set the ball-to-material ratio to 10:1, the grinding time to 3 h, and pass through a 200-mesh sieve to obtain coconut shell powder;

[0052] S2. Dissolve 20 g of citric acid in 22 mL of deionized water to prepare an acidic solution. Then, add 15 g of urea to the acidic solution and stir until completely dissolved to form a transparent mixed solution, which is the activation solution. Place the coconut shell powder in the activation solution according to a solid-to-liquid ratio of 1 g:10 mL. First, perform ultrasonic treatment at 60 °C for 1 h, and the ultrasonic frequency is 40 kHz. Then, let it stand at room temperature for 8 h. The drying temperature is 70 °C, and the drying time is 12 h. Transfer the dried coconut shell powder to a tubular furnace. Under a nitrogen protection atmosphere, heat it to 600 °C at a rate of 10 °C / min and hold for 2 h. After activation, cool it to 200 °C, introduce water vapor, and the introduction time is 10 min. Finally, cool it to room temperature;

[0053] S3. Put the treated coconut shell powder into a carbonization furnace. Under an argon protection atmosphere, heat it to 550 °C at a heating rate of 10 °C / min and carry out constant-temperature carbonization for 1 h. The product after the first carbonization is placed in the carbonization furnace again and heated to a higher temperature (1000 °C) at a rate of 5 °C / min for secondary carbonization, and the constant-temperature time is 3 h. Place the material in a CVD reaction chamber, introduce a mixed gas of methane and hydrogen as the carbon source, and react at 900 °C and a pressure of 15 Pa for 1 h to uniformly grow and coat graphene on the surface of the hard carbon particles. Select an appropriate amount of nano-nickel (Ni) particles and uniformly disperse them in an ethanol solution by ultrasonic dispersion. Add the hard carbon material coated with graphene to the above solution and perform ultrasonic treatment for 30 min to make the nano-metal particles adsorb on the material surface. Then, dry it in a vacuum oven at 80 °C and then perform heat treatment at 400 °C for 1.5 h to form a stable chemical bond between the nano-metal particles and the hard carbon material to obtain the hard carbon material;

[0054] S4. Mix the above hard carbon material with an appropriate amount of binder (polyvinylidene fluoride) and conductive agent (such as acetylene black) in a mass ratio of 85:10:5, add an appropriate amount of N-methylpyrrolidone (NMP) as a solvent, and stir to make a uniform slurry. Coat the slurry on a copper foil, use a doctor blade coating method to control the coating thickness to 90 μm, and then dry it in a vacuum oven at 150 °C for 12 h. Finally, punch it into a circular electrode sheet with a diameter of 15 mm. Place the formed electrode sheet in a vacuum environment and perform annealing treatment at 380 °C for 2 h 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 pores of hard carbon materials.

[0057] The experimental group is the same as Example 1; in Control Group 1, urea solution alone is used as the activator; in Control Group 2, conventional activator potassium hydroxide with a concentration of 2 mol / L is used; in Control Group 3, the high-temperature activation temperature is 500 °C; in Control Group 4, the high-temperature activation temperature is 750 °C; in Control Group 5, no steam is introduced after activation.

[0058] The specific surface area and pore volume of the activated coconut shell powder were tested, and the results are as follows:

[0059] Experimental group: The specific surface area is 1500 m 2 / g, and the total pore volume is 0.8 cm 3 / g, among which the micropore volume is 0.5 cm 3 / g, and the mesopore volume is 0.3 cm 3 / g.

[0060] Control Group 1: The specific surface area is 1350 m 2 / g, the activation degree is insufficient, and the penetration and reaction degree of the coconut shell pores are weakened; the total pore volume is 0.7 cm 3 / g, among which the micropore volume is 0.4 cm 3 / g, and the mesopore volume is 0.3 cm 3 / g.

[0061] Control Group 2: The specific surface area is 1200 m 2 / g; the total pore volume is 0.7 cm 3 / g, among which the micropore volume is 0.45 cm 3 / g, and the mesopore volume is 0.25 cm 3 / g.

[0062] Control Group 3: The specific surface area is 1400 m 2 / g, the temperature is relatively low, the urea decomposition is incomplete, and the activation reaction degree is insufficient; the total pore volume is 0.75 cm 3 / g, among which the micropore volume is 0.45 cm 3 / g, and the mesopore volume is 0.3 cm 3 / g.

[0063] Control Group 4: The specific surface area is 1250 m 2 / g, the temperature is too high, the micropores collapse, and the specific surface area decreases; the total pore volume is 0.5 cm 3 / g, among which the micropore volume is 0.25 cm 3 / g, and the mesopore volume is 0.25 cm 3 / g.

[0064] Control group 5: Specific surface area is 1420 m 2 / g; Total pore volume is 0.75 cm 3 / g, among which the micropore volume is 0.35 cm 3 / g, and the mesopore volume is 0.4 cm 3 / g.

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

[0066] The experimental groups are the same as in Example 1; Control group 1 has no surface modification and directly undergoes shaping and post-treatment after carbonization; Control group 2 is only coated with graphene and has no doping of nano metal particles; Control group 3 has no graphene coating and is only doped and modified with nano metal particles. The remaining steps of Control groups 1-3 are the same as in Example 1.

[0067] The prepared hard carbon anode materials of each experimental group are subjected to electrochemical performance tests, and the results are as follows:

[0068] (I). Cyclic voltammetry test (first scan, scan rate 0.1 mV / s)

[0069] Experimental group: The first charge specific capacity is 1000 mAh / g, the first discharge specific capacity is 850 mAh / g, and the Coulomb efficiency is 85%. The capacity retention rate after 100 cycles is 80%.

[0070] Control group 1: The redox peak current density is relatively low. The anodic peak current density is 0.2 mA / cm 2 , and the cathodic peak current density is 0.25 mA / cm 2 , and the peak potential difference is relatively large, being 0.5 V, indicating poor electrochemical reversibility.

[0071] Control group 2: The anodic peak current density increases to 0.3 mA / cm 2 , and the cathodic peak current density is 0.35 mA / cm 2 , and the peak potential difference shrinks to 0.4 V, and the electrochemical reversibility is somewhat improved.

[0072] Control group 3: The anodic peak current density is 0.28 mA / cm 2 , and the cathodic peak current density is 0.32 mA / cm 2 , and the peak potential difference is 0.42 V. There is a certain improvement but it is not as good as the group only coated with graphene.

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

[0074] Experimental group: The specific charge capacity for the first charge is 1000 mAh / g, the specific discharge capacity for the first discharge is 850 mAh / g, and the Coulombic efficiency is 85%. The capacity retention rate after 100 cycles is 80%.

[0075] Control Group 1: The specific charge capacity for the first charge is 800 mAh / g, the specific discharge capacity for the first discharge is 650 mAh / g, and the Coulombic efficiency is 81%. After 100 cycles, the capacity retention rate is 60%.

[0076] Control Group 2: The specific charge capacity for the first charge is 900 mAh / g, the specific discharge capacity for the first discharge is 780 mAh / g, and the Coulombic efficiency is 87%. The capacity retention rate after 100 cycles is 70%.

[0077] Control Group 3: The specific charge capacity for the first charge is 850 mAh / g, the specific discharge capacity for the first discharge is 720 mAh / g, and the Coulombic efficiency is 85%. The capacity retention rate after 100 cycles is 65%.

[0078] (3) AC impedance test (frequency range: 100 kHz - 0.01 Hz):

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

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

[0081] Control Group 2: The Rct is reduced to 60 Ω, and the total impedance decreases somewhat.

[0082] Control Group 3: The Rct is 65 Ω, and the total impedance is between that of Control Group 1 and Control Group 2.

[0083] It can be clearly seen from the above comparative experimental data 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 the hard carbon anode material based on coconut shell.

[0084] The embodiments described above are some, but not all, of the embodiments of the present application. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application claimed, but merely represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts fall within the scope of protection of the present application.

Claims

1. A method for preparing hard carbon negative electrode material using coconut shell, characterized in that: The following steps are involved: S1, pre-treating and crushing coconut shell to obtain coconut shell powder; S2, the coconut shell powder is placed in the activation solution for dipping, after drying, high temperature activation is performed, and then water vapor is introduced for oxidation pore expansion; S3, carbonizing the activated coconut shell powder, and then modifying the surface of the coconut shell powder with graphene and nano-metal particles to obtain a hard carbon material; S4, mixing the hard carbon material with a binder, a conductive agent and a solvent into a slurry, and obtaining the hard carbon negative electrode material after molding and annealing.

2. The method for preparing hard carbon negative electrode material using coconut shell according to claim 1, characterized in that: The pretreatment in step S1 specifically includes washing the coconut shell with running water and drying it at 100-120° C. to constant weight; the crushing includes first crushing it into small pieces with a particle size of 1-2 cm, and then ball milling it to 100-200 mesh.

3. The method for preparing hard carbon negative electrode material using coconut shell according to claim 1, characterized in that: 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.

4. The method for preparing hard carbon negative electrode material using coconut shell according to claim 3, characterized in that: The impregnation step is specifically as follows: firstly ultrasonically treat at 40-60° C. for 1-2 hours, and then stand at room temperature for 6-12 hours; the drying temperature is 60-80° C., and the drying time is 8-12 hours.

5. The method for preparing hard carbon negative electrode material using coconut shell according to claim 3, characterized in that: The high temperature activation is specifically to raise the temperature to 600-700°C at a rate of 5-10°C / min in an inert atmosphere and keep the temperature for 1-2h; the temperature of the water vapor introduced is 200-400°C and the introduction time is 10-15min.

6. The method for preparing hard carbon negative electrode material using coconut shell according to claim 3, characterized in that: The carbonization in step S3 includes primary carbonization and secondary carbonization, wherein the primary carbonization is specifically 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; the secondary carbonization is heating to 900-1000°C at a rate of 3-5°C / min on the temperature of the primary carbonization, and carbonizing at a constant temperature for 3-4 hours.

7. The method for preparing hard carbon negative electrode material using coconut shell according to claim 3, characterized in that: The surface modification specifically comprises: coating the carbonized coconut shell-based hard carbon material with graphene by chemical vapor deposition, and then placing the carbonized coconut shell-based hard carbon material in a nano-metal dispersion after ultrasonic dispersion, and ultrasonically treating the nano-metal dispersion for 30-45 minutes.

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

9. The method for preparing hard carbon negative electrode material using coconut shell according to claim 3, characterized in that: The annealing temperature is 300-400° C., and the annealing time is 2-3 hours.

10. A hard carbon negative electrode material, characterized in that: The method is prepared by any one of claims 1 to 9.