Preparation method of heteroatom synergistically doped porous carbon and silicon carbon negative electrode material

Abstract

The invention discloses a preparation method of a heteroatom synergistically doped porous carbon and silicon carbon negative electrode material, and belongs to the technical field of lithium ion battery negative electrode materials. According to the preparation method, the biomass material is subjected to etherification, phosphorus and sulfur co-doping, carbonization and activation to prepare the porous carbon with high pore volume, high micropore proportion and high specific surface area, and then the porous carbon is subjected to silicon deposition and carbon coating to prepare the silicon-carbon negative electrode material, so that the interface stability of the silicon-carbon material and the cycling stability of the lithium ion battery are effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of anode materials for lithium-ion batteries, and particularly relates to a preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material. Background Art

[0002] Silicon-carbon anode materials are one of the core directions for next-generation high-energy-density lithium-ion batteries. The theoretical specific capacity of silicon (about 4200 mAh / g) is much higher than that of traditional graphite (372 mAh / g). However, during the charge and discharge process, its volume expansion rate is as high as 300% - 400%, resulting in electrode pulverization and rapid capacity decay. Porous carbon, as a carrier or composite matrix for silicon, mainly plays the following roles: 1. The high specific surface area and pore structure of porous carbon can provide a buffer space for the volume expansion of silicon; 2. The carbon matrix improves the overall conductivity of the electrode and makes up for the low conductivity defect of silicon; 3. The combination of porous carbon and silicon can inhibit the agglomeration of silicon particles. Therefore, the preparation technology of porous carbon is the key technical path for silicon-carbon anode materials.

[0003] However, the high porosity of porous carbon may reduce the conductivity of the overall material, and it is necessary to improve the conductivity through graphitization treatment or doping. Secondly, the physical / chemical bonding between silicon and carbon is not firm, which easily leads to debonding, and it is necessary to introduce chemical bonding such as Si - O - C to increase its interface stability. Finally, the batch stability of porous carbon is also a technical difficulty for its large-scale application. Summary of the Invention

[0004] Aiming at the above problems, the present invention provides a preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material. By etherifying, doping (sulfur doping and phosphorus doping), carbonizing, and then activating the biomass material, porous carbon with high pore volume, high micropore ratio, and high specific surface area is prepared. Then, through silicon deposition and carbon coating, a silicon-carbon anode material is obtained, effectively improving the interface stability of the silicon-carbon material and the cycle stability of the lithium-ion battery.

[0005] To achieve the above object, the present invention specifically adopts the following technical solutions: The present invention provides a preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, including the following steps: Step 1: Mix soluble starch with an etherifying agent for etherification. After the etherification is completed, add phosphorus and sulfur dopants, and stir evenly for standby; Step 2: Spray-dry the material obtained in Step 1 to obtain a carbon material precursor. Subsequently, under a protective gas, heat up for carbonization, and crush to obtain a powder for standby; Step 3: Add the powder obtained in Step 2 to a fluidized bed, heat up while introducing a protective gas, and then introduce an activation gas for activation. After the activation is completed, porous carbon is obtained; Step 4: Add the porous carbon obtained in Step 3 to a fluidized bed. While raising the temperature, introduce a protective gas, then introduce silane gas for silicon deposition, and then introduce carbon source gas for secondary coating to obtain a porous silicon-carbon anode material.

[0006] Further, the etherifying agent in Step 1 is at least one of ethylene oxide, propylene oxide, and chloroacetic acid; the mass ratio of soluble starch to the etherifying agent is (9 - 9.5):(0.5 - 1).

[0007] Further, in Step 1, when performing the etherification, sodium hydroxide needs to be added to control the pH of the reaction system to 10 - 12, the etherification temperature is 40 - 80°C, and the time is 2 - 6 h.

[0008] Further, the phosphorus and sulfur doping agents in Step 1 are sodium phosphate and sodium sulfate respectively, and their doping amounts are both 1% - 3% of the mass of soluble starch.

[0009] Further, the conditions for spray drying in Step 2 are: the temperature is 70 - 90°C, the solution feed flow rate is 4 - 8 L / min, and the atomization pressure is 0.4 - 0.8 MPa.

[0010] Further, the specific process of raising the temperature for carbonization in Step 2 is: first raise the temperature at a rate of 1 - 5°C / min to 400 - 520°C and hold for 1 - 3 h, then continue to raise the temperature at a rate of 1 - 5°C / min to 850 - 1000°C and hold for 1 - 4 h.

[0011] Further, the activation gas in Step 3 is carbon dioxide, the activation temperature is 800 - 950°C, and the activation time is 4 - 8 h.

[0012] Further, the silane gas in Step 4 is at least one of monosilane and disilane, and its feeding flow rate is 3 - 5 L / min; the temperature for silicon deposition is 450 - 550°C, and the time is 4 - 8 h.

[0013] Further, the carbon source gas in Step 4 is acetylene, and its feeding flow rate is 3 - 5 L / min; the time for secondary coating is 1 - 2 h.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the present invention, phosphorus and sulfur elements are doped simultaneously. On the one hand, introducing the S element expands the carbon layer spacing to form a fast diffusion channel, and at the same time, introducing trace S - O functional groups increases the interface stability and is more conducive to electrolyte wetting; on the other hand, introducing the P element can introduce empty orbitals, form P - π conjugation to improve conductivity, and Li - P coordination increases interface adsorption to improve the electrical properties of the material; the synergistic doping of phosphorus and sulfur can significantly enhance the cycle stability; 2. Etherify the starch to prevent a series of problems such as the destruction of its structure and low carbon yield caused by the release of a large amount of volatile products during the pyrolysis process of the starch; 3. Activate the carbon precursor using a fluidized bed. Since the materials in the fluidized bed are in a fluidized state, the gas-solid contact efficiency, heat transfer, and penetrability of the activator in the fluidized bed are all superior to those in a rotary kiln, and the material consistency is good. Description of the Drawings

[0015] Figure 1 SEM image of the porous silicon-carbon anode material prepared in Example 2 of the present invention.

[0016] Figure 2 Isothermal adsorption and desorption curve of the porous silicon-carbon anode material prepared in Example 2 of the present invention.

[0017] Figure 3 BJH desorption mesopore size differential distribution curve of the porous silicon-carbon anode material prepared in Example 2 of the present invention. Detailed Embodiments

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

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field of the present invention. The terms used in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0020] Example 1 A preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, comprising: 1. Add soluble starch and ethylene oxide in a mass ratio of 9.2:0.8 to water to obtain a solution, add sodium hydroxide to adjust the pH to 11, and carry out etherification in a reaction kettle. The etherification temperature is 50 °C and lasts for 4 h. After the etherification is completed, add sodium phosphate and sodium sulfate and stir at room temperature for 4 h (the doping amounts of sodium phosphate and sodium sulfate are both 2% of the mass of the soluble starch), and set aside.

[0021] 2. Add a spray desiccant to the material obtained from etherification, and carry out spray drying at 80 °C, a solution feed flow rate of 6 L / min, and an atomization pressure of 0.6 MPa to obtain a carbon material precursor. The obtained carbon material precursor is placed in an atmosphere furnace. Under a nitrogen atmosphere, first heat it at a heating rate of 2 °C / min to 460 °C and hold for 2 h, then continue to heat it at a heating rate of 2 °C / min to 900 °C and hold for 3 h for carbonization. After carbonization is completed, use a mechanical mill or a jet mill to crush it to D 50 = 12 μm to obtain a powder.

[0022] 3. Add the crushed powder to a fluidized bed, heat it at a heating rate of 5 °C / min to 850 °C, and pass nitrogen during the heating process, with a nitrogen flow rate of 15 L / min; after reaching the temperature, hold for 30 min, then reduce the nitrogen flow rate to 5 L / min, and introduce the activation gas steam for activation, with a steam flow rate of 15 L / min and an activation time of 6 h. After activation is completed, a porous carbon is prepared. The porous carbon is subjected to pickling and water washing to remove impurities, and washed until neutral, and then classified and sieved for standby.

[0023] 4. Place the prepared porous carbon in a fluidized bed, introduce nitrogen and heat it at a heating rate of 5 °C / min to 520 °C. After reaching the temperature, hold for 30 min and perform pipeline purging for 20 min. Then introduce silane (flow rate 4 L / min) for silicon deposition (time 4.5 h). After completion, introduce acetylene gas (flow rate 4 L / min) and maintain for 1.5 h for secondary coating. After cooling, discharge to obtain a porous silicon-carbon anode material.

[0024] Example 2 A preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, comprising: 1. Add soluble starch and ethylene oxide with a mass ratio of 9.2:0.8 to water to obtain a solution, add sodium hydroxide to adjust the pH to 11, and carry out etherification in a reaction kettle. The etherification temperature is 50 °C and lasts for 4 h. After the etherification is completed, add sodium phosphate and sodium sulfate and stir at room temperature for 4 h (the doping amounts of sodium phosphate and sodium sulfate are both 2% of the mass of soluble starch), and set aside.

[0025] 2. Add a spray desiccant to the material obtained from etherification, and carry out spray drying at 80 °C, a solution feed flow rate of 6 L / min, and an atomization pressure of 0.6 MPa to obtain a carbon material precursor. The obtained carbon material precursor is placed in an atmosphere furnace. Under a nitrogen atmosphere, first heat it at a heating rate of 2 °C / min to 460 °C and hold for 2 h, then continue to heat it at a heating rate of 2 °C / min to 900 °C and hold for 3 h for carbonization. After carbonization is completed, use a mechanical mill or a jet mill to crush it to D 50 = 12 μm to obtain a powder.

[0026] 3. Add the powdered materials obtained by crushing into a fluidized bed, heat it to 900 °C at a heating rate of 5 °C / min, and pass nitrogen during the heating process. The nitrogen flow rate is 15 L / min. After the temperature reaches, keep it warm for 30 min, then reduce the nitrogen flow rate to 5 L / min, and introduce the activation gas carbon dioxide for activation. The carbon dioxide flow rate is 15 L / min, and the activation time is 6 h. After activation, porous carbon is prepared. Carry out acid washing and water washing on the porous carbon to remove impurities, and wash it until it is neutral, then classify and screen it for standby.

[0027] 4. Place the prepared porous carbon in a fluidized bed, pass nitrogen and heat it to 520 °C at a heating rate of 5 °C / min. After the temperature reaches, keep it warm for 30 min and conduct pipeline purging for 20 min. Then introduce silane (flow rate 4 L / min) for silicon deposition (time 4.5 h). After completion, introduce acetylene gas (flow rate 4 L / min) and keep it for 1.5 h for secondary coating. After cooling, discharge the material to obtain the porous silicon-carbon anode material.

[0028] Example 3 A preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, comprising: 1. Add soluble starch and ethylene oxide with a mass ratio of 9:1 into water to obtain a solution, add sodium hydroxide to adjust the pH to 11, and carry out etherification in a reaction kettle. The etherification temperature is 50 °C and it lasts for 4 h. After the etherification is completed, add sodium phosphate and sodium sulfate and stir at room temperature for 4 h (the doping amounts of sodium phosphate and sodium sulfate are both 3% of the mass of soluble starch), and keep it for standby.

[0029] 2. Add a spray dryer to the material obtained by etherification, and carry out spray drying at 80 °C, a solution feed flow rate of 6 L / min, and an atomization pressure of 0.6 MPa to obtain a carbon material precursor. Place the obtained carbon material precursor in an atmosphere furnace. Under a nitrogen atmosphere, first heat it to 460 °C at a heating rate of 2 °C / min and keep it warm for 2 h, then continue to heat it to 900 °C at a heating rate of 2 °C / min and keep it warm for 3 h for carbonization. After the carbonization is completed, use a mechanical mill or a jet mill to crush it to D 50 = 12 μm to obtain powdered materials.

[0030] 3. Add the powdered materials obtained by crushing into a fluidized bed, heat it to 900 °C at a heating rate of 5 °C / min, and pass nitrogen during the heating process. The nitrogen flow rate is 15 L / min. After the temperature reaches, keep it warm for 30 min, then reduce the nitrogen flow rate to 5 L / min, and introduce the activation gas carbon dioxide for activation. The carbon dioxide flow rate is 15 L / min, and the activation time is 6 h. After activation, porous carbon is prepared. Carry out acid washing and water washing on the porous carbon to remove impurities, and wash it until it is neutral, then classify and screen it for standby.

[0031] 4. Place the prepared porous carbon in a fluidized bed, introduce nitrogen, and heat it to 520 °C at a heating rate of 5 °C / min. After the temperature reaches, keep it warm for 30 min, and conduct pipeline purging for 20 min. Then introduce silane (flow rate 3 L / min) for silicon deposition (time 4.5 h). After completion, introduce acetylene gas (flow rate 3 L / min) and keep it for 1.5 h for secondary coating. After cooling, discharge to obtain the porous silicon-carbon anode material.

[0032] Example 4 A preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, comprising: 1. Add soluble starch and ethylene oxide in a mass ratio of 9:1 to water to obtain a solution, add sodium hydroxide to adjust the pH to 11, and conduct etherification in a reaction kettle. The etherification temperature is 50 °C and lasts for 4 h. After the etherification is completed, add sodium phosphate and sodium sulfate and stir at room temperature for 4 h (the doping amounts of sodium phosphate and sodium sulfate are both 1% of the mass of soluble starch), and set aside.

[0033] 2. Add a spray dryer to the material obtained by etherification, and conduct spray drying at 80 °C, a solution feed flow rate of 6 L / min, and an atomization pressure of 0.6 MPa to obtain a carbon material precursor. The obtained carbon material precursor is placed in an atmosphere furnace. Under a nitrogen atmosphere, first heat it to 460 °C at a heating rate of 2 °C / min and keep it warm for 2 h, and then continue to heat it to 900 °C at a heating rate of 2 °C / min and keep it warm for 3 h for carbonization. After the carbonization is completed, use a mechanical mill or a jet mill to crush it to D 50 = 12 μm to obtain a powder.

[0034] 3. Add the crushed powder to a fluidized bed, heat it to 900 °C at a heating rate of 5 °C / min, and introduce nitrogen during the heating process. The nitrogen flow rate is 15 L / min; after the temperature reaches, keep it warm for 30 min, and then reduce the nitrogen flow rate to 5 L / min, introduce the activation gas carbon dioxide for activation, the carbon dioxide flow rate is 15 L / min, and the activation time is 6 h. After the activation is completed, obtain porous carbon. Conduct acid washing and water washing to remove impurities from the porous carbon, and wash it until neutral, and then classify and screen it, and set aside.

[0035] 4. Place the prepared porous carbon in a fluidized bed, introduce nitrogen, and heat it to 520 °C at a heating rate of 5 °C / min. After the temperature reaches, keep it warm for 30 min, and conduct pipeline purging for 20 min. Then introduce silane (flow rate 3 L / min) for silicon deposition (time 4.5 h). After completion, introduce acetylene gas (flow rate 3 L / min) and keep it for 1.5 h for secondary coating. After cooling, discharge to obtain the porous silicon-carbon anode material.

[0036] Comparative Example 1 Referring to Example 2, the difference is that phosphorus and sulfur dopants are not added.

[0037] Comparative Example 2 Referring to Example 2, the difference is that only sodium phosphate is added and sodium sulfate is not added.

[0038] Comparative Example 3 Referring to Example 2, the difference is that only sodium sulfate is added and sodium phosphate is not added.

[0039] Comparative Example 4 Referring to Example 2, the difference is that the activation is carried out in a rotary kiln, and the flow rate of the activation gas carbon dioxide is changed to 10 L / min.

[0040] Test Example A physical adsorption instrument was used to analyze the pore structure and pore size distribution of the samples prepared above. Using liquid nitrogen or liquid argon as the adsorbate, adsorption-desorption experiments were carried out. The specific surface area was calculated by the single-point method, and the pore size distribution data was obtained through the density functional theory model. The specific surface area and pore volume data of the porous silicon-carbon samples prepared in Examples 1-4 and Comparative Examples 1-4 are shown in Table 1.

[0041] Table 1 Specific surface area and pore volume of porous silicon-carbon samples

[0042] As can be seen from Table 1, Example 2 has the best comprehensive physical indexes of specific surface area, pore volume, and microporosity; considering the examples and comparative examples comprehensively, it can be seen that using water as the activation gas can obtain porous carbon with a larger pore volume, but there are significant differences in the specific surface area and microporosity compared with carbon dioxide activation; due to the relatively large atomic radius of S, S-containing gases may be generated during the activation process, so S doping tends to form mesopores or macropores, and appropriate S doping can increase the pore volume; P doping can increase the reaction activity of carbon, promote activation, and appropriate doping can increase the specific surface area and the proportion of micropores of porous carbon.

[0043] The half-cell test method is to use the prepared silicon-carbon negative electrode material as the negative electrode active material to prepare a slurry. The slurry ratio is active material: CNTs (including dispersant): CMC: SBR = 89%: 4.5%: 1.5%: 5%. Then the slurry is coated on a copper foil and vacuum dried for 12 h to make a negative electrode sheet. The electrolyte is commercially purchased, the separator is a PE membrane, and the lithium sheet is the counter electrode. A half-cell is assembled in a glove box. A constant current charge-discharge experiment is carried out on a LAND battery test system. Unless otherwise specified, the following data are all charged and discharged at 1C, and the charge-discharge voltage is limited to 0.005-1.5 V. A computer-controlled charge-discharge cabinet is used for data acquisition and control.

[0044] Table 2 Coin cell test results

[0045] As can be seen from Table 2, considering the first efficiency and cycle stability comprehensively, the electrochemical performance of Example 2 is the best; for Example 1, Example 3 and Comparative Example 3, due to the low microporosity ratio and specific surface area, the silicon deposition effect is affected, resulting in the incomplete utilization of their capacities; comprehensively, excessive doping of S element will lead to an increase in side reactions, showing a low first efficiency, and appropriate doping of S and P is beneficial to improving the cycle stability of the battery.

[0046] Figure 1 SEM image of the porous silicon-carbon anode material prepared in Example 2; from Figure 1 it can be seen that after etherification and spray drying, starch maintains its good natural spherical morphology, which is beneficial to improving its processing performance in subsequent applications. Figure 2 Isothermal adsorption-desorption curve of the porous silicon-carbon anode material prepared in Example 2. From Figure 2 it can be seen that this curve belongs to Type I isotherm. The adsorption amount increases rapidly at relatively low pressures and reaches saturation after a certain pressure, which reflects the filling phenomenon of micropores. Figure 3 BJH desorption mesopore size differential distribution curve of the porous silicon-carbon anode material prepared in Example 2. Combining Figure 3 it can be seen that the prepared porous silicon-carbon mainly consists of micropores and a small amount of mesopores, meeting the design expectations and being able to meet the application requirements at the silicon-carbon anode material end.

[0047] The above-described embodiments merely represent several preferred embodiments of the present invention. The description is relatively specific and detailed, but it does not limit the present invention. It should be noted that for those skilled in the art, the present invention may have various changes and modifications. Any modifications, equivalent replacements, improvements, etc. made within the concept and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material, characterized in that, The method includes the following steps: Step 1: Mix soluble starch with an etherifying agent, carry out etherification, and after the etherification is completed, add phosphorus and sulfur dopants, stir evenly and set aside; Step 2: Spray-dry the material obtained in Step 1 to obtain a carbon material precursor, and then under a protective gas, heat up for carbonization, crush to obtain a powder and set aside; Step 3: Add the powder obtained in Step 2 to a fluidized bed, heat up while introducing a protective gas, and then introduce an activation gas for activation. After the activation is completed, porous carbon is obtained; Step 4: Add the porous carbon obtained in Step 3 to a fluidized bed, heat up while introducing a protective gas, then introduce silane gas for silicon deposition, and then introduce a carbon source gas for secondary coating to obtain a silicon-carbon negative electrode material.

2. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, wherein, The etherifying agent in Step 1 is at least one of ethylene oxide, propylene oxide, and chloroacetic acid; the mass ratio of the soluble starch to the etherifying agent is (9 - 9.5):(0.5 - 1).

3. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that For the etherification in Step 1, sodium hydroxide needs to be added to control the pH of the reaction system to 10 - 12, the etherification temperature is 40 - 80 °C, and the time is 2 - 6 h.

4. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, wherein The phosphorus and sulfur dopants in Step 1 are sodium phosphate and sodium sulfate respectively, and their doping amounts are both 1% - 3% of the mass of the soluble starch.

5. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that, The conditions for the spray drying in Step 2 are: the temperature is 70 - 90 °C, the solution feed flow rate is 4 - 8 L / min, and the atomization pressure is 0.4 - 0.8 MPa.

6. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that, The specific process of heating up for carbonization in Step 2 is: first heat up at a heating rate of 1 - 5 °C / min to 400 - 520 °C and keep it warm for 1 - 3 h, and then continue to heat up at a heating rate of 1 - 5 °C / min to 850 - 1000 °C and keep it warm for 1 - 4 h.

7. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that, The activation gas in Step 3 is carbon dioxide, the activation temperature is 800 - 950 °C, and the activation time is 4 - 8 h.

8. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that, The silane gas in Step 4 is at least one of monosilane and disilane, and its flow rate is 3 - 5 L / min; the temperature of the silicon deposition is 450 - 550 °C, and the time is 4 - 8 h.

9. The preparation method of a heteroatom co-doped porous carbon and silicon-carbon anode material according to claim 1, characterized in that, The carbon source gas in Step 4 is acetylene, and its flow rate is 3 - 5 L / min; the time for the secondary coating is 1 - 2 h.

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