Biomass-based hard carbon negative electrode material and preparation method thereof

Abstract

The invention discloses a preparation method of a biomass-based hard carbon negative electrode material. The preparation method comprises the following steps: (1) carrying out primary pore-forming by utilizing a pore-forming agent; (2) secondary pore forming is cyclically repeated through O2 and CO2; (3) pyrolyzing carbohydrate, and coating the surface of the secondary pore-forming pyrolytic carbon with the pyrolyzed carbohydrate to obtain a coating material; and (4) calcining the coating material to obtain the biomass-based hard carbon negative electrode material. The invention also provides the biomass-based hard carbon negative electrode material. According to the preparation method of the biomass-based hard carbon negative electrode material disclosed by the invention, after the pore-forming agent is adopted to form pores, oxygen and carbon dioxide are alternately introduced, and finally, the biomass-based hard carbon negative electrode material with high porosity and high slope capacity ratio is obtained through coating of carbohydrate; the hard carbon negative electrode material is high in capacity, large in pore volume, high in slope capacity ratio, proper in specific surface area and excellent in electrochemical performance.

Description

Technical Field

[0001] The invention belongs to the field of battery materials, and in particular relates to a negative electrode material and a preparation method thereof. Background Art

[0002] As a new generation of energy storage batteries, sodium-ion batteries have rapidly entered the energy storage market with the characteristics of high energy storage and fast charge and discharge. As an excellent high-energy storage battery, the negative electrode material is the key to limiting its electrochemical performance. Among the negative electrode materials of sodium-ion batteries, hard carbon has the best performance. Hard carbon presents a "short-range order and long-range disorder" structure, has a larger interlayer spacing than graphite, and is very suitable for the embedding and storage of sodium ions. At present, the precursors of hard carbon materials are mainly biomass materials. Biomass materials have the characteristics of low cost and high capacity. Pyrolytic carbon materials can be obtained by oxygen-free roasting at low temperature, and then hard carbon materials can be prepared by high-temperature sintering of pyrolytic carbon materials. However, the hard carbon materials prepared by conventional methods have relatively low material capacity, especially the low proportion of slope capacity, and even if the capacity of the material is increased, the specific surface area is too large, making it difficult to process and prepare into batteries. Summary of the invention

[0003] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology, and to provide a biomass-based hard carbon negative electrode material with high porosity and high slope capacity ratio and a preparation method thereof, so as to solve the technical problems of low capacity, small pore volume, low slope capacity ratio and large specific surface area of ​​biomass-based hard carbon negative electrode materials in the prior art.

[0004] In order to solve the above technical problems, the technical solution proposed by the present invention is: A method for preparing a biomass-based hard carbon negative electrode material comprises the following steps: (1) using a pore-forming agent to form pores in the pyrolytic carbon (obtained by oxygen-free roasting of biomass materials) to obtain primary pore-forming pyrolytic carbon; (2) placing the primary pore-forming pyrolytic carbon in an inert atmosphere and a first temperature for heat preservation, introducing O2 during the heat preservation process; the first temperature is 180-280°C; stopping the introduction of O2, then changing the first temperature to a second temperature, and heat preservation in an inert atmosphere, introducing CO2 during the heat preservation process; the second temperature is 780-990°C; repeating this step once or multiple times, cyclically introducing O2 and CO2 to treat the pyrolytic carbon to obtain secondary pore-forming pyrolytic carbon; (3) adding the secondary pore-forming pyrolytic carbon to the carbohydrate solution, stirring, placing in a hydrothermal reactor, using microwave heating to make the secondary pore-forming pyrolytic carbon and the carbohydrate in uniform contact, heating the carbohydrate to pyrolyze and then coat the surface of the secondary pore-forming pyrolytic carbon to obtain a coating material; (4) calcining the coating material to obtain the biomass-based hard carbon negative electrode material.

[0005] In the present invention, the biomass materials include bamboo, coconut shells, reeds, walnut shells and the like.

[0006] In the above preparation method, preferably, the secondary pore-forming pyrolytic carbon is subjected to a third pore-forming process using a pore-forming agent, and after the third pore-forming process, it is subjected to a carbon loading process and then added to the carbohydrate solution, wherein the first carbon loading process is performed by cracking and loading a hydrocarbon carbon source gas.

[0007] In the above preparation method, preferably, the pore-forming agent includes one or more of KOH, K2CO3, Na2CO3, and NaOH, the mass ratio of the pyrolytic carbon to the pore-forming agent is (95-99.9): (0.1-5), the pore-forming temperature is 800-1000°C, and the pore-forming time is 30-300min. The above pore-forming agent is used to perform deep pore formation by alkaline etching. If the process parameters are lower than the above process parameters, the deep pore formation effect is not obvious, and if the parameters are higher than the above parameters, the material will be over-activated, resulting in an increase in large pores.

[0008] In the above preparation method, preferably, during the first pore formation, the amount of pore forming agent and / or the pore forming temperature and / or the pore forming time are greater than those of the second pore formation. The first pore formation is the main pore formation process, and the third pore formation is the auxiliary pore formation process. The third pore formation mainly supplements the second pore formation. Therefore, the process conditions of the third pore formation can be lower than those of the first pore formation to avoid excessive pore size.

[0009] In the above preparation method, preferably, the hydrocarbon carbon source gas includes one or more of methane, ethane, acetylene, and propane, the flow rate of the hydrocarbon carbon source gas is 0.5-1.5L / min, the time of the hydrocarbon carbon source gas is 30-420min, and the temperature of the hydrocarbon carbon source gas cracking is 800-1000°C. During the first carbon loading, the carbon loading is achieved by cracking the hydrocarbon carbon source gas, which is used to repair the pores. When the hydrocarbon carbon source gas is cracked, if the gas flow rate and time are too small, the temperature is too low, and the carbon cracking effect is not obvious, if it is too high, the carbon coating layer will be too thick, and the distance from the closed pore to the surface of the material will be too large, and it will be difficult for sodium to enter the closed pore under the action of electric current.

[0010] In the present invention, a three-step pore-making process path is adopted. The first pore-making is used to make deep pores on the surface of pyrolytic carbon. The second pore-making is used to further make pores on the surface of pyrolytic carbon and in the macropores of the aforementioned first pore-making, so as to improve the porosity and increase more sodium ion adsorption sites. During the second pore-making, because the etching effect of carbon dioxide is different under the action of different sites, some sites may create pores with very small pore diameters, which cannot store sodium. Therefore, three deep pore-makings are performed after carbon dioxide etching, which can effectively enlarge these pores with small pore diameters to achieve sodium storage. After the three-step pore-making, the specific surface area of ​​the material is too large, and there are many macropores that cannot be repaired by subsequent carbohydrate solutions, and the pores are too large, and carbohydrates may enter the pores, causing the pores to be completely filled, resulting in the aforementioned pore-making being invalid. The present invention firstly processes the carbon load once, and repairs the excessive pores by the first step of vapor deposition, so that it is converted into pores that can be processed into closed pores by the back end, avoiding the phenomenon that the material has a low first coulomb efficiency due to the presence of excessive pores, and also avoiding the direct filling of macropores. Then the second carbon coating is to close most of the suitable open pores, so that they are all converted into closed pores, thereby further improving the material capacity. The progressive effect of the two-step coating realizes the adjustment of the pores of the previous pore formation, further improving the initial efficiency and capacity of the material.

[0011] In addition, the coating of carbon film will not cause the active sites brought by oxygen treatment to be covered. The interlayer spacing of carbon is sufficient for the entry of sodium ions, so it will not affect the binding of sodium ions with active sites. Moreover, for the pores caused by carbon dioxide etching, carbon atoms will not enter the pores during the carbon coating process, but will be coated at the pore openings to form closed pores, which is more conducive to the storage of sodium (pores with too large openings cannot achieve stable storage of sodium).

[0012] In the above preparation method, preferably, when O2 is introduced, the O2 gas flow rate is 0.5-1.5L / min, and the time for a single O2 introduction is 5-60min; when CO2 is introduced, the CO2 gas flow rate is 0.5-2.5L / min, and the time for a single O2 introduction is 10-360min. When the temperature is increased from the first temperature to the second temperature, the heating rate is 1-10℃ / min; the number of times step (2) is repeated is 2-5 times. If the oxygen and carbon dioxide gas flow rates are less than the above ranges, the effect is very low; if the oxygen introduction amount is too large, the material will catch fire and burn the material; and excessive carbon dioxide will cause excessive etching of the material, resulting in an excessively large specific surface area that cannot be reduced by subsequent coating, resulting in a decrease in the first coulombic efficiency of the material. Too many repetitions will also cause the problem of excessive specific surface area. The above oxygen and carbon dioxide introduction flow rates and the number of repeated introductions need to match the first pore making and the third pore making.

[0013] In the above preparation method, preferably, the carbohydrates in the carbohydrate solution include one or more of starch, glucose, maltose, and fructose, the mass concentration of the carbohydrates in the carbohydrate solution is 1-30%, the solvent in the carbohydrate solution is anhydrous ethanol, and the mass ratio of the carbohydrates to the secondary pore-forming pyrolytic carbon is 1: (4-99). When the carbohydrate solution is coated, the coating effect is not obvious if it is lower than the above process parameters, and the carbon coating is too thick if it is higher than the above parameters, which will make the distance from the closed pore to the material surface too large, and it is difficult for sodium to enter the closed pore under the action of electric current.

[0014] In the above preparation method, preferably, the power of microwave heating is 1-100 GHZ, the temperature of carbohydrate hydrothermal treatment pyrolysis is 120-200°C, and the time is 0.5-24h. The present invention adopts microwave heating, and the carbon coating effect is better.

[0015] In the above preparation method, preferably, the calcination temperature is 1100-1600°C and the calcination time is 0.5-3h. During calcination, if the calcination temperature is lower than 1100°C, the pore structure of the material is not easy to be closed-cell, and if the calcination temperature is higher than 1600°C, the interlayer spacing of the material is too low, which is not conducive to the storage of sodium.

[0016] As a general technical concept, the present invention also provides a biomass-based hard carbon negative electrode material prepared by the above-mentioned preparation method.

[0017] In the present invention, the method for preparing the biomass-based hard carbon negative electrode material specifically comprises the following steps: S1. Using a pore-forming agent to form pores in pyrolytic carbon to obtain primary pore-forming pyrolytic carbon.

[0018] S2. Place the once-pore-forming pyrolytic carbon in a rotary kiln, heat it to a first temperature at a certain heating rate under an inert atmosphere, ensure that O2 is introduced under the working state of the inert gas during the insulation stage, introduce more oxygen atom functional groups on the surface of the material, stop introducing O2 after a certain period of time, change to a second temperature, ensure that CO2 is introduced under the working state of the inert gas during the insulation stage, stop introducing O2 after a certain period of time, change the temperature again, and repeat the steps of introducing O2 and CO2 multiple times.

[0019] S3, using the pore-forming agent to perform pore-forming for the third time.

[0020] S4. Raise the temperature to a certain temperature, introduce a certain amount of hydrocarbon carbon source gas for a period of time, so that it is cracked and a layer of carbon film is attached to the surface of the material. S5, adding the carbohydrate solution into the reaction kettle, stirring, and then using microwave heating to make the two contact evenly, heating to pyrolyze the carbohydrate material and then coat it to obtain a coated material.

[0021] S6. Calcine the coating material obtained in step S5 to obtain a biomass-based hard carbon negative electrode material.

[0022] The inert gas introduced is one or more of nitrogen, helium, argon, neon, etc. The flow rate of the inert gas introduced is 1.0-4.5 L / min.

[0023] The preparation method of the biomass-based hard carbon negative electrode material of the present invention, during the secondary pore formation, oxygen and carbon dioxide are alternately introduced, while the porosity of the material is increased, oxygen-containing functional groups are further introduced into the material surface, and the alternating introduction of two gases effectively ensures the over-activation of the material by a single gas, and avoids the structural collapse of the material caused by over-activation. The etching effect of carbon dioxide can effectively improve the pore structure inside the material, increase the porosity of the material, and provide more sodium storage sites; but too much pore structure will reduce the kinetic properties of the material to a certain extent, and the pre-oxidation effect of oxygen can effectively introduce oxygen-containing functional groups on the surface of the material, provide more sodium ion adsorption sites on the surface of the material, greatly enhance the kinetic properties of the material, increase the slope capacity of the material during the charge and discharge process, and while alleviating the etching effect of carbon dioxide, further improve the sodium storage capacity of the material, effectively solving the problem of low slope capacity of biomass-based hard carbon materials in the industry. At the same time, during the process of introducing oxygen, oxygen will also produce a certain pore-forming effect, forming a synergistic effect with carbon dioxide etching.

[0024] For the oxygen introduction process, using inert gas as protection can effectively prevent the material from burning. When the temperature is below 180°C, oxygen cannot pre-oxidize the material. When the temperature is above 280°C, oxygen will cause local overoxidation of the material, resulting in collapse of the overall structure, which greatly reduces the material performance. For the carbon dioxide introduction process, compared with other gases, carbon dioxide can more easily enter the pores of the material due to its own carbon-oxygen bond structure. Under conditions below 780°C, the material is weakly etched by carbon dioxide. Under conditions above 990°C, the material will be over-etched and activated, forming a large number of large pores, resulting in a significant decrease in the electrochemical capacity performance of the material.

[0025] Pore ​​formation (tertiary pore formation) and pre-oxidation can effectively improve the sodium storage capacity and slope capacity of the material, but will cause the problem of excessive specific surface area of ​​the material, affecting the material processing performance in the battery homogenization process. In the present invention, hydrocarbon carbon source gas is first introduced, and the characteristics of hydrocarbon carbon source gas cracking under high temperature and catalyst are used to cover the surface of the material with a layer of carbon film, thereby preliminarily reducing the specific surface area of ​​the material to a certain extent, and then carbohydrates are used as coating agents, mixed with the precursor by stirring, and then microwave heating is used to achieve uniform heating of the material, promote full and uniform contact between the carbohydrates in the solution and the material, and achieve uniform coating of the precursor by carbohydrates, avoiding the phenomenon of uneven coating of pyrolytic carbon.

[0026] The specific principle of carbohydrate coating in the present invention is: carbohydrates are mainly composed of C, H, and O elements, and will gradually transform from a multi-branched long chain structure to a three-dimensional network chain structure under high pressure (closed reactor) at 120-200°C (most of the C-O, C=O, and C-H bonds will break), and trace amounts of H2O, CO2 and other gases will be produced. The final three-dimensional network chain structure is mainly composed of C-C and C=C bonds, containing a small amount of C-O, C=O, and C-H bonds. The three-dimensional network chain structure forms a thin film on the surface of the biomass-based hard carbon material to achieve the coating effect on the material (specifically composed of Figure 1 as shown).

[0027] Compared with the prior art, the advantages of the present invention are: The preparation method of the biomass-based hard carbon negative electrode material of the present invention is to use a pore-forming agent to form holes, then alternately introduce oxygen and carbon dioxide, and finally coat it with carbohydrates to obtain a biomass-based hard carbon negative electrode material with high porosity and high slope capacity ratio. The biomass-based hard carbon negative electrode material has high capacity, large pore volume, high slope capacity ratio, appropriate specific surface area, and excellent electrochemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 This is the TEM image of the biomass-based hard carbon material prepared in Example 1. DETAILED DESCRIPTION

[0030] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0031] Unless otherwise defined, all the professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0032] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0033] The pyrolytic carbon in the following examples and comparative examples is obtained by oxygen-free roasting of biomass materials, for example, by oxygen-free roasting of coconut shells.

[0034] Embodiment 1: A method for preparing a biomass-based hard carbon negative electrode material comprises the following steps: 600 g of pyrolytic carbon was mixed with 20 g of NaOH and then heated to 900° C. and kept warm for 300 min to perform primary pore formation to obtain primary pore-forming pyrolytic carbon.

[0035] The primary pore-forming pyrolytic carbon is placed in a rotary kiln, nitrogen is introduced at a rate of 2.0 L / min, and the temperature is raised to 200°C at a rate of 3°C / min. After the temperature is raised to 200°C, oxygen is introduced at a rate of 0.8 L / min for 25 minutes, and then the introduction of oxygen is stopped, and the temperature is raised to 800°C at a rate of 3°C / min. After the temperature is raised to 800°C, carbon dioxide is introduced at a rate of 1.5 L / min for 240 minutes, and then the introduction of carbon dioxide is stopped. After the furnace chamber is cooled to 200°C, oxygen is introduced at a rate of 0.8 L / min for 25 minutes. After repeating the above ventilation steps three times, secondary pore-forming pyrolytic carbon is obtained.

[0036] 400 g of secondary pore-forming pyrolytic carbon was mixed with 5 g of NaOH and then heated to 800° C. and kept warm for 100 min to perform tertiary pore-forming to obtain tertiary pore-forming pyrolytic carbon.

[0037] The tertiary pore-forming pyrolytic carbon was placed in a rotary kiln, the temperature of the rotary kiln was raised to 900°C, and methane gas was introduced at a rate of 1.0 L / min for 180 minutes. After cooling, a precursor containing more oxygen atom functional groups and preliminary coating was obtained.

[0038] 50 g of starch was added to 2000 mL of anhydrous ethanol and stirred to obtain a carbohydrate solution, which was then placed in a reactor.

[0039] 200 g of precursor carbon was added to a reactor containing a carbohydrate solution, stirred, and heated to 150° C. using a 50 GHZ microwave and kept warm for 20 hours to allow the carbohydrate to react and then coat the surface of the precursor to obtain a coated material.

[0040] The above-mentioned coating material was calcined at 1300℃ for 2h to obtain the biomass-based hard carbon negative electrode material (TEM image as shown in Figure 1 as shown).

[0041] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 4.58 m 2 / g, the total pore volume of the material is 0.065cm 3 / g.

[0042] The biomass-based hard carbon negative electrode material is made into a button cell according to the following steps: The biomass-based hard carbon negative electrode material, PVDF and acetylene black were mixed and ground in a mass ratio of 90:5:5. After grinding for 5 minutes, polyvinylidene fluoride binder was added and continued to grind for 5 minutes until it became thin and thick to obtain a slurry. The ground slurry was evenly coated on the surface of aluminum foil with a thickness of 100 μm, and sliced ​​after drying at 120 ° C to obtain a battery negative electrode sheet. In an oxygen-free and water-free glove box, the negative electrode battery shell, the above-mentioned negative electrode sheet, the diaphragm, the electrolyte, the sodium sheet (counter electrode), and the positive electrode battery shell were assembled in sequence, and after assembly, a small hydraulic press was used to compact the battery. After the battery was naturally placed for 6 hours, the electrochemical performance was tested using an electrochemical test system.

[0043] After testing, the first coulombic efficiency of the above battery was 90%, the first reversible specific capacity was 351.6 mAh / g, and the slope capacity accounted for 41.6%.

[0044] Embodiment 2: A method for preparing a biomass-based hard carbon negative electrode material comprises the following steps: 600 g of pyrolytic carbon was mixed with 15 g of KOH and then heated to 860° C. and kept warm for 230 min to perform a primary pore formation to obtain a primary pore-forming pyrolytic carbon.

[0045] The primary pore-forming pyrolytic carbon is placed in a rotary kiln, nitrogen is introduced at a rate of 2.0 L / min, and the temperature is raised to 190°C at a rate of 3°C / min. After the temperature is raised to 190°C, oxygen is introduced at a rate of 1.0 L / min for 45 minutes, and then the introduction of oxygen is stopped, and the temperature is raised to 850°C at a rate of 3°C / min. After the temperature is raised to 850°C, carbon dioxide is introduced at a rate of 1.8 L / min for 190 minutes, and then the introduction of carbon dioxide is stopped. After the furnace chamber is cooled to 190°C, oxygen is introduced at a rate of 1.0 L / min for 45 minutes. After repeating the above ventilation steps four times, secondary pore-forming pyrolytic carbon is obtained.

[0046] 400 g of secondary pore-forming pyrolytic carbon was mixed with 8 g of KOH and then heated to 810° C. and kept warm for 130 min to perform tertiary pore-forming to obtain tertiary pore-forming pyrolytic carbon.

[0047] The tertiary pore-forming pyrolytic carbon was placed in a rotary kiln, the temperature of the rotary kiln was raised to 950°C, and propane gas was introduced at a rate of 0.7L / min for 200min. After cooling, a precursor containing more oxygen atom functional groups and preliminary coating was obtained.

[0048] 40 g of glucose was added to 2000 mL of anhydrous ethanol, mixed, and stirred to obtain a carbohydrate solution, and the solution was placed in a reactor.

[0049] 200 g of the precursor was added into a reaction kettle containing a carbohydrate solution, and after stirring, the temperature was raised to 130° C. using a 50 GHZ microwave and kept warm for 16 hours to allow the carbohydrate to react and then be coated on the surface of the precursor to obtain a coated material.

[0050] The above-mentioned coating material is calcined at 1200° C. for 1.5 h to obtain a biomass-based hard carbon negative electrode material.

[0051] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 5.87 m 2 / g, the total pore volume of the material is 0.059cm 3 / g.

[0052] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 89%, the first reversible specific capacity was 348.8 mAh / g, and the slope capacity accounted for 40.3%.

[0053] Embodiment 3: A method for preparing a biomass-based hard carbon negative electrode material comprises the following steps: 600 g of pyrolytic carbon was mixed with 28 g of Na2CO3 and then heated to 840°C and kept warm for 170 min to perform primary pore formation to obtain primary pore-forming pyrolytic carbon.

[0054] The primary pore-forming pyrolytic carbon is placed in a rotary kiln, nitrogen is introduced at a rate of 2.0 L / min, and the temperature is raised to 250°C at a rate of 5°C / min. After the temperature is raised to 250°C, oxygen is introduced at a rate of 0.5 L / min for 38 minutes, and then the introduction of oxygen is stopped, and the temperature is raised to 920°C at a rate of 5°C / min. After the temperature is raised to 920°C, carbon dioxide is introduced at a rate of 2.0 L / min for 260 minutes, and then the introduction of carbon dioxide is stopped. After the furnace chamber is cooled to 250°C, oxygen is introduced at a rate of 0.5 L / min for 38 minutes. After repeating the above ventilation steps four times, secondary pore-forming pyrolytic carbon is obtained.

[0055] 400g of secondary pore-forming pyrolytic carbon was mixed with 11g of Na2CO3 and heated to 800°C for 130min to perform tertiary pore-forming to obtain tertiary pore-forming pyrolytic carbon.

[0056] The tertiary pore-forming pyrolytic carbon was placed in a rotary kiln, the temperature of the rotary kiln was raised to 850°C, and acetylene gas was introduced at a rate of 1.2 L / min for 80 minutes. After cooling, a precursor containing more oxygen atom functional groups and preliminary coating was obtained.

[0057] 40 g of fructose was added to 2000 mL of anhydrous ethanol, mixed, and stirred to obtain a carbohydrate solution, and the solution was placed in a reactor.

[0058] 500 g of the precursor was added into a reactor containing a carbohydrate solution, and after stirring, the temperature was raised to 130° C. using a 70 GHZ microwave and kept warm for 20 hours to allow the carbohydrate to react and then be coated on the surface of the precursor to obtain a coated material.

[0059] The above-mentioned coating material is calcined at 1400°C for 2 hours to obtain a biomass-based hard carbon negative electrode material.

[0060] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 4.68 m 2 / g, the total pore volume of the material is 0.063cm 3 / g.

[0061] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 89%, the first reversible specific capacity was 352.4 mAh / g, and the slope capacity accounted for 42.1%.

[0062] Embodiment 4: A method for preparing a biomass-based hard carbon negative electrode material comprises the following steps: 600 g of pyrolytic carbon was mixed with 20 g of K2CO3 and then heated to 980°C and kept warm for 230 min to perform a primary pore formation to obtain a primary pore-forming pyrolytic carbon.

[0063] The primary pore-forming pyrolytic carbon is placed in a rotary kiln, nitrogen is introduced at a rate of 2.0 L / min, and the temperature is raised to 260°C at a rate of 3°C / min. After the temperature is raised to 260°C, oxygen is introduced at a rate of 0.7 L / min for 16 minutes, and then the introduction of oxygen is stopped, and the temperature is raised to 820°C at a rate of 3°C / min. After the temperature is raised to 820°C, carbon dioxide is introduced at a rate of 1.7 L / min for 160 minutes, and then the introduction of carbon dioxide is stopped. After the furnace chamber is cooled to 260°C, oxygen is introduced at a rate of 0.7 L / min for 16 minutes. After repeating the above ventilation steps three times, secondary pore-forming pyrolytic carbon is obtained.

[0064] 400 g of secondary pore-forming pyrolytic carbon was mixed with 9 g of K2CO3 and heated to 870°C for 180 min to perform tertiary pore-forming to obtain tertiary pore-forming pyrolytic carbon.

[0065] The tertiary pore-forming pyrolytic carbon was placed in a rotary kiln, which was heated to 980°C, and then a bowl of gas was introduced at a rate of 0.7L / min for 30 minutes. After cooling, a precursor containing more oxygen atom functional groups and preliminary coating was obtained.

[0066] 30 g of maltose was added to 2000 mL of anhydrous ethanol, mixed, and stirred to obtain a carbohydrate solution, and the solution was placed in a reactor.

[0067] 300g of the precursor was added to a reactor containing a carbohydrate solution, stirred, and heated to 200°C using 80GHZ microwaves for 12 hours to allow the carbohydrates to react and then coat the surface of the precursor to obtain a coated material. The coated material was calcined at 1600°C for 3 hours to obtain a biomass-based hard carbon negative electrode material.

[0068] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 5.02 m 2 / g, the total pore volume of the material is 0.067cm 3 / g.

[0069] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 89%, the first reversible specific capacity was 358.6 mAh / g, and the slope capacity accounted for 38.6%.

[0070] Embodiment 5: A method for preparing a biomass-based hard carbon negative electrode material without hydrocarbon gas coating, compared with Example 1, the main difference is that no carbon coating (hydrocarbon gas pyrolysis) is performed, and other conditions are the same.

[0071] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 24.31 m 2 / g, the total pore volume of the material is 0.066cm 3 / g.

[0072] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 81%, the first reversible specific capacity was 342.1 mAh / g, and the slope capacity accounted for 39.9%.

[0073] Embodiment 6: A method for preparing a biomass-based hard carbon negative electrode material without using three-stage pore creation, compared with Example 1, the main difference is that no three-stage pore creation is used, and other conditions are the same.

[0074] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 4.25 m 2 / g, the total pore volume of the material is 0.048cm 3 / g.

[0075] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 86%, the first reversible specific capacity was 302.1 mAh / g, and the slope capacity accounted for 39.1%.

[0076] Embodiment 7: A method for preparing a biomass-based hard carbon negative electrode material without hydrocarbon gas coating and tertiary pore creation, compared with Example 1, the main difference is that it does not undergo primary carbon coating (hydrocarbon gas pyrolysis) and tertiary pore creation, and other conditions are the same.

[0077] The specific surface area of ​​the biomass-based hard carbon negative electrode material was determined to be 19.76 m 2 / g, the total pore volume of the material is 0.053cm 3 / g.

[0078] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 83%, the first reversible specific capacity was 316.9 mAh / g, and the slope capacity accounted for 38.7%.

[0079] Comparative Example 1: A method for preparing a biomass-based hard carbon negative electrode material without hydrocarbon gas and carbohydrate coating, compared with Example 1, the main difference is that it does not undergo primary carbon coating (hydrocarbon gas pyrolysis) and secondary carbon coating (carbohydrate pyrolysis), and other conditions are the same.

[0080] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 57.86 m 2 / g, the total pore volume of the material is 0.068cm 3 / g.

[0081] The above-mentioned biomass-based hard carbon negative electrode material is made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery is 72%, the first reversible specific capacity is 335.7 mAh / g, and the slope capacity accounts for 38.4%.

[0082] Comparative Example 2: A method for preparing a negative electrode material without using carbon dioxide pore formation and oxygen pre-oxidation treatment, compared with Example 1, the main difference is that no secondary pore formation is performed, and other conditions are the same.

[0083] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 3.98 m 2 / g, the total pore volume of the material is 0.038cm 3 / g.

[0084] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 87%, the first reversible specific capacity was 289.7 mAh / g, and the slope capacity accounted for 30.1%.

[0085] Comparative Example 3: A method for preparing a negative electrode material without using one-time pore creation, compared with Example 1, the main difference is that no one-time pore creation is used, and other conditions are the same.

[0086] The specific surface area of ​​the biomass-based hard carbon negative electrode material was 4.03 m 2 / g, the total pore volume of the material is 0.049cm 3 / g.

[0087] The above-mentioned biomass-based hard carbon negative electrode material was made into a button battery according to the steps in Example 1: After testing, the first coulombic efficiency of the above-mentioned battery was 87%, the first reversible specific capacity was 300.4 mAh / g, and the slope capacity accounted for 39.1%.

[0088] See Table 1 for a comparison of the electrochemical performance test results of the batteries of Examples 1 to 7 and Comparative Examples 1 to 3.

[0089] Table 1: Comparison of battery electrochemical performance test results

[0090] As can be seen from the above table, the electrochemical performance of Examples 5 to 7 is significantly reduced compared to Example 1. Comparative Examples 1 to 3 also show the same downward trend compared to Example 1.

Claims

1. A method for preparing a biomass-based hard carbon negative electrode material, characterized in that: The following steps are involved: (1) forming pores in the pyrolytic carbon using a pore-forming agent to obtain primary pore-forming pyrolytic carbon; (2) placing the primary pore-forming pyrolytic carbon in an inert atmosphere and a first temperature for heat preservation, introducing O2 during the heat preservation process; the first temperature is 180-280°C; stopping the introduction of O2, then changing the first temperature to a second temperature, and heat preservation in an inert atmosphere, introducing CO2 during the heat preservation process; the second temperature is 780-990°C; repeating this step once or multiple times, cyclically introducing O2 and CO2 to treat the pyrolytic carbon to obtain secondary pore-forming pyrolytic carbon; (3) adding the secondary pore-forming pyrolytic carbon to the carbohydrate solution, stirring, placing in a hydrothermal reactor, using microwave heating to make the secondary pore-forming pyrolytic carbon and the carbohydrate in uniform contact, heating the carbohydrate to pyrolyze and then coat the surface of the secondary pore-forming pyrolytic carbon to obtain a coating material; (4) calcining the coating material to obtain the biomass-based hard carbon negative electrode material.

2. The preparation method according to claim 1, characterized in that: The secondary pore-forming pyrolytic carbon is used as a pore-forming agent to form pores for the third time, and after the third pore-forming, it is subjected to a carbon loading and then added to the carbohydrate solution. The first carbon loading is performed by cracking and loading with hydrocarbon carbon source gas.

3. The preparation method according to claim 2, characterized in that: The pore-forming agent includes one or more of KOH, K2CO3, Na2CO3, and NaOH. The mass ratio of the pyrolytic carbon to the pore-forming agent is (95-99.9): (0.1-5). The pore-forming temperature is 800-1000°C and the pore-forming time is 30-300min.

4. The preparation method according to claim 3, characterized in that: During the primary pore formation, the amount of pore-forming agent used and / or the pore-forming temperature and / or the pore-forming time are greater than those of the secondary pore formation.

5. The preparation method according to claim 2, characterized in that: The hydrocarbon carbon source gas includes one or more of methane, ethane, acetylene, and propane. The flow rate of the hydrocarbon carbon source gas is 0.5-1.5 L / min. The introduction time of the hydrocarbon carbon source gas is 30-420 min. The cracking temperature of the hydrocarbon carbon source gas is 800-1000° C.

6. The preparation method according to claim 1, characterized in that: When O2 is introduced, the O2 gas flow rate is 0.5-1.5 L / min, and the single O2 introduction time is 5-60 min; when CO2 is introduced, the CO2 gas flow rate is 0.5-2.5 L / min, and the single O2 introduction time is 10-360 min. When the temperature is increased from the first temperature to the second temperature, the heating rate is 1-10°C / min; the number of times step (2) is repeated is 2-5 times.

7. The preparation method according to any one of claims 1 to 6, characterized in that The carbohydrates in the carbohydrate solution include one or more of starch, glucose, maltose, and fructose. The mass concentration of the carbohydrates in the carbohydrate solution is 1-30%. The solvent in the carbohydrate solution is anhydrous ethanol. The mass ratio of the carbohydrates to the secondary pore-forming pyrolytic carbon is 1: (4-99).

8. The preparation method according to any one of claims 1 to 6, characterized in that: The power of microwave heating is 1-100 GHZ, the temperature of carbohydrate hydrothermal treatment pyrolysis is 120-200° C., and the time is 0.5-24 h.

9. The preparation method according to any one of claims 1 to 6, characterized in that: The calcination temperature is 1100-1600° C., and the calcination time is 0.5-3 hours.

10. A biomass-based hard carbon negative electrode material prepared by the preparation method according to any one of claims 1 to 9.

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