A preparation method for heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials

Abstract

The present invention discloses a method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, belonging to the technical field of lithium-ion battery negative electrode materials. The method prepares porous carbon with high pore volume, high micropore ratio, and high specific surface area by etherifying biomass materials, co-doping with phosphorus and sulfur, carbonizing, and then activating the materials. The porous carbon is then subjected to silicon deposition and carbon coating to produce the silicon-carbon negative electrode material, effectively improving the interfacial stability of the silicon-carbon material and the cycling stability of the lithium-ion battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery negative electrode materials, and in particular relates to a method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials. Background Art

[0002] Silicon-carbon anode materials are one of the core areas of focus for next-generation high-energy-density lithium-ion batteries. Silicon's theoretical specific capacity (approximately 4200 mAh / g) is far higher than that of traditional graphite (372 mAh / g), but its volume expansion during charge and discharge is as high as 300%–400%, leading to electrode pulverization and rapid capacity decay. Porous carbon, as a carrier or composite matrix for silicon, primarily serves the following functions: 1. The high surface area and pore structure of porous carbon provide a buffer for silicon's volume expansion; 2. The carbon matrix enhances the overall conductivity of the electrode, compensating for silicon's low conductivity; and 3. The composite of porous carbon and silicon inhibits the aggregation of silicon particles. Therefore, porous carbon preparation technology is a key technological path for silicon-carbon anode materials.

[0003] However, the high porosity of porous carbon may reduce the conductivity of the overall material, and the conductivity needs to be improved through graphitization or doping. Secondly, the physical / chemical bonding between silicon and carbon is not strong, which can easily lead to debonding, and chemical bonding such as Si-OC needs to be introduced to increase its interface stability. Finally, the batch stability of porous carbon is also a technical difficulty that leads to its large-scale application. Summary of the Invention

[0004] In response to the above problems, the present invention provides a method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials. The method comprises etherifying, doping (sulfur doping and phosphorus doping), carbonizing, and then activating the biomass material to prepare porous carbon with high pore volume, high micropore ratio, and high specific surface area. The silicon-carbon negative electrode material is then prepared by silicon deposition and carbon coating, which effectively improves the interface stability of the silicon-carbon material and the cycle stability of the lithium-ion battery.

[0005] To achieve the above-mentioned purpose, the present invention specifically adopts the following technical solutions:

[0006] The present invention provides a method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, comprising the following steps:

[0007] Step 1: taking soluble starch and mixing it with an etherifying agent to carry out etherification, adding phosphorus and sulfur dopants after the etherification is completed, stirring evenly and setting aside;

[0008] Step 2: spray-drying the material obtained in step 1 to obtain a carbon material precursor, and then heating it under protective gas to carbonize it, and crushing it to obtain a powder for later use;

[0009] Step 3: Add the powder obtained in step 2 into the fluidized bed, increase the temperature while introducing protective gas, and then introduce activation gas for activation, and obtain porous carbon after activation;

[0010] Step 4: Add the porous carbon obtained in step 3 into the fluidized bed, increase the temperature while introducing protective gas, then introduce silane gas for silicon deposition, and then introduce carbon source gas for secondary coating to obtain a porous silicon-carbon negative electrode material.

[0011] Furthermore, the etherifying agent in step 1 is at least one of ethylene oxide, propylene oxide, and chloroacetic acid; and the mass ratio of the soluble starch to the etherifying agent is (9-9.5):(0.5-1).

[0012] Furthermore, the etherification in step 1 further requires the addition of sodium hydroxide to control the pH of the reaction system to 10-12, the etherification temperature to 40-80° C., and the time to 2-6 h.

[0013] Furthermore, the phosphorus and sulfur dopants in step 1 are sodium phosphate and sodium sulfate, respectively, and the doping amounts thereof are 1%-3% of the mass of the soluble starch.

[0014] Furthermore, the spray drying conditions in step 2 are: temperature of 70-90° C., solution feed flow rate of 4-8 L / min, and atomization pressure of 0.4-0.8 MPa.

[0015] Furthermore, the carbonization step 2 is specifically as follows: firstly heating to 400-520°C at a heating rate of 1-5°C / min and keeping warm for 1-3 hours, then heating to 850-1000°C at a heating rate of 1-5°C / min and keeping warm for 1-4 hours.

[0016] Furthermore, in step 3, the activation gas is carbon dioxide, the activation temperature is 800-950° C., and the activation time is 4-8 h.

[0017] Furthermore, 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.

[0018] Furthermore, the carbon source gas in step 4 is acetylene, and its flow rate is 3-5 L / min; the secondary coating time is 1-2 h.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] 1. The present invention simultaneously dopes phosphorus and sulfur. On the one hand, the introduction of S expands the distance between carbon layers to form a fast diffusion channel, while simultaneously introducing a trace amount of SO functional groups to increase interface stability and facilitate electrolyte wetting. On the other hand, the introduction of P introduces an empty orbital, forming a P-π conjugation to improve conductivity, and a Li-P coordination to increase interfacial adsorption, thereby improving the electrical properties of the material. The synergistic doping of phosphorus and sulfur can significantly enhance cycle stability.

[0021] 2. Etherify starch to prevent the release of a large amount of volatile products during the pyrolysis process, which may lead to structural damage, low carbon yield and other problems;

[0022] 3. Use a fluidized bed to activate the carbon precursor. Since the material in the fluidized bed is in a fluidized state, its gas-solid contact efficiency, heat transfer, and activator permeability are better than activation in a rotary furnace, and the material consistency is good. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is an SEM image of the porous silicon-carbon negative electrode material prepared in Example 2 of the present invention.

[0024] Figure 2 This is the isothermal adsorption-desorption curve of the porous silicon-carbon negative electrode material prepared in Example 2 of the present invention.

[0025] Figure 3 This is the BJH desorption mesopore size differential distribution curve of the porous silicon-carbon negative electrode material prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0026] To make the objects, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be described clearly and completely below in conjunction with the examples. Where specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or the conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art in the art. The terms used in the specification 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" as used herein includes any and all combinations of one or more of the associated listed items.

[0028] Example 1

[0029] A method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, comprising:

[0030] 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. Etherification is carried out in a reactor at 50°C for 4 hours. After etherification, sodium phosphate and sodium sulfate are added and stirred at room temperature for 4 hours (the amount of sodium phosphate and sodium sulfate is 2% of the mass of the soluble starch). Set aside.

[0031] 2. Add a spray dryer to the etherified material and spray dry it 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 and, under a nitrogen atmosphere, first heated to 460°C at a heating rate of 2°C / min and kept warm for 2 h, then continued to heat to 900°C at a heating rate of 2°C / min and kept warm for 3 h for carbonization. After carbonization, use a mechanical mill or air flow mill to crush it to D 50 =12 μm to obtain powder.

[0032] 3. The crushed powder was added to a fluidized bed and heated to 850°C at a rate of 5°C / min. Nitrogen was introduced during the heating process at a rate of 15 L / min. After reaching the desired temperature, the mixture was kept warm for 30 minutes. The nitrogen rate was then reduced to 5 L / min, and water vapor, an activating gas, was introduced for activation at a rate of 15 L / min for 6 hours. After activation, porous carbon was obtained. The porous carbon was acid-washed and washed with water to remove impurities until neutral, then graded and sieved for later use.

[0033] 4. The prepared porous carbon was placed in a fluidized bed, and nitrogen was introduced while heating to 520°C at a heating rate of 5°C / min. After reaching the temperature, it was kept warm for 30 minutes and the pipeline was purged for 20 minutes. Subsequently, monosilane (flow rate 4 L / min) was introduced for silicon deposition (time 4.5 hours). After the end, acetylene gas (flow rate 4 L / min) was introduced and maintained for 1.5 hours for secondary coating. After cooling, the material was discharged to obtain a porous silicon-carbon negative electrode material.

[0034] Example 2

[0035] A method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, comprising:

[0036] 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. Etherification is carried out in a reactor at 50°C for 4 hours. After etherification, sodium phosphate and sodium sulfate are added and stirred at room temperature for 4 hours (the amount of sodium phosphate and sodium sulfate is 2% of the mass of the soluble starch). Set aside.

[0037] 2. Add a spray dryer to the etherified material and spray dry it 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 and, under a nitrogen atmosphere, first heated to 460°C at a heating rate of 2°C / min and kept warm for 2 h, then continued to heat to 900°C at a heating rate of 2°C / min and kept warm for 3 h for carbonization. After carbonization, use a mechanical mill or air flow mill to crush it to D 50 =12 μm to obtain powder.

[0038] 3. The crushed powder was added to a fluidized bed and heated to 900°C at a heating rate of 5°C / min. Nitrogen was introduced during the heating process at a nitrogen flow rate of 15 L / min. After reaching the temperature, the mixture was kept warm for 30 minutes. The nitrogen flow rate was then reduced to 5 L / min, and carbon dioxide was introduced as an activation gas for activation at a carbon dioxide flow rate of 15 L / min for 6 hours. After activation, porous carbon was obtained. The porous carbon was acid-washed and washed with water to remove impurities until it was neutral, then graded and sieved for later use.

[0039] 4. The prepared porous carbon was placed in a fluidized bed, and nitrogen was introduced while heating to 520°C at a heating rate of 5°C / min. After reaching the temperature, it was kept warm for 30 minutes and the pipeline was purged for 20 minutes. Subsequently, monosilane (flow rate 4 L / min) was introduced for silicon deposition (time 4.5 hours). After the end, acetylene gas (flow rate 4 L / min) was introduced and maintained for 1.5 hours for secondary coating. After cooling, the material was discharged to obtain a porous silicon-carbon negative electrode material.

[0040] Example 3

[0041] A method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, comprising:

[0042] 1. Add soluble starch and ethylene oxide in a 9:1 mass ratio to water to obtain a solution. Add sodium hydroxide to adjust the pH to 11. Etherify in a reactor at 50°C for 4 hours. After etherification, add sodium phosphate and sodium sulfate (each in an amount of 3% by mass of the soluble starch) and stir at room temperature for 4 hours. Set aside.

[0043] 2. Add a spray dryer to the etherified material and spray dry it 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 and, under a nitrogen atmosphere, first heated to 460°C at a heating rate of 2°C / min and kept warm for 2 h, then continued to heat to 900°C at a heating rate of 2°C / min and kept warm for 3 h for carbonization. After carbonization, use a mechanical mill or air flow mill to crush it to D 50=12 μm to obtain powder.

[0044] 3. The crushed powder was added to a fluidized bed and heated to 900°C at a heating rate of 5°C / min. Nitrogen was introduced during the heating process at a nitrogen flow rate of 15 L / min. After reaching the temperature, the mixture was kept warm for 30 minutes. The nitrogen flow rate was then reduced to 5 L / min, and carbon dioxide was introduced as an activation gas for activation at a carbon dioxide flow rate of 15 L / min for 6 hours. After activation, porous carbon was obtained. The porous carbon was acid-washed and washed with water to remove impurities until it was neutral, then graded and sieved for later use.

[0045] 4. The prepared porous carbon was placed in a fluidized bed, and nitrogen was introduced while heating to 520°C at a heating rate of 5°C / min. After reaching the temperature, it was kept warm for 30 minutes and the pipeline was purged for 20 minutes. Subsequently, monosilane (flow rate 3 L / min) was introduced for silicon deposition (time 4.5 hours). After the end, acetylene gas (flow rate 3 L / min) was introduced and maintained for 1.5 hours for secondary coating. After cooling, the material was discharged to obtain a porous silicon-carbon negative electrode material.

[0046] Example 4

[0047] A method for preparing heteroatom-co-doped porous carbon and silicon-carbon negative electrode materials, comprising:

[0048] 1. Add soluble starch and ethylene oxide in a 9:1 mass ratio to water to obtain a solution. Add sodium hydroxide to adjust the pH to 11. Etherify in a reactor at 50°C for 4 hours. After etherification, add sodium phosphate and sodium sulfate (each containing 1% of the mass of the soluble starch) and stir at room temperature for 4 hours. Set aside.

[0049] 2. Add a spray dryer to the etherified material and spray dry it 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 and, under a nitrogen atmosphere, first heated to 460°C at a heating rate of 2°C / min and kept warm for 2 h, then continued to heat to 900°C at a heating rate of 2°C / min and kept warm for 3 h for carbonization. After carbonization, use a mechanical mill or air flow mill to crush it to D 50 =12 μm to obtain powder.

[0050] 3. The crushed powder was added to a fluidized bed and heated to 900°C at a heating rate of 5°C / min. Nitrogen was introduced during the heating process at a nitrogen flow rate of 15 L / min. After reaching the temperature, the mixture was kept warm for 30 minutes. The nitrogen flow rate was then reduced to 5 L / min, and carbon dioxide was introduced as an activation gas for activation at a carbon dioxide flow rate of 15 L / min for 6 hours. After activation, porous carbon was obtained. The porous carbon was acid-washed and washed with water to remove impurities until it was neutral, then graded and sieved for later use.

[0051] 4. The prepared porous carbon was placed in a fluidized bed, and nitrogen was introduced while heating to 520°C at a heating rate of 5°C / min. After reaching the temperature, it was kept warm for 30 minutes and the pipeline was purged for 20 minutes. Subsequently, monosilane (flow rate 3 L / min) was introduced for silicon deposition (time 4.5 hours). After the end, acetylene gas (flow rate 3 L / min) was introduced and maintained for 1.5 hours for secondary coating. After cooling, the material was discharged to obtain a porous silicon-carbon negative electrode material.

[0052] Comparative Example 1

[0053] Refer to Example 2, except that no phosphorus or sulfur dopants are added.

[0054] Comparative Example 2

[0055] Refer to Example 2, except that only sodium phosphate is added without sodium sulfate.

[0056] Comparative Example 3

[0057] Refer to Example 2, except that only sodium sulfate is added, and no sodium phosphate is added.

[0058] Comparative Example 4

[0059] Refer to Example 2, except 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.

[0060] Test example

[0061] The pore structure and pore size distribution of the samples prepared above were analyzed using a physical adsorption instrument. Adsorption-desorption experiments were conducted using liquid nitrogen or liquid argon as the adsorbent. The specific surface area was calculated using a single-point method, and the pore size distribution data were obtained using a 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.

[0062] Table 1 Specific surface area and pore volume of porous silicon carbon samples

[0063]

[0064] It can be seen from Table 1 that Example 2 has the best comprehensive physical indicators of specific surface area, pore volume and microporosity; from the combined examples and comparative examples, it can be seen that water as activation gas can obtain porous carbon with a larger pore volume, but the specific surface area and microporosity are significantly different from those of carbon dioxide activation; S has a larger atomic radius and may produce S-containing gas during the activation process, so S doping tends to generate mesopores or macropores, and an appropriate amount of S doping can increase the pore volume; P doping can increase the reaction activity of carbon and promote activation. An appropriate amount of doping can increase the specific surface area and micropore ratio of porous carbon.

[0065] The half-cell testing method uses the prepared silicon-carbon anode material as the anode active material to form a slurry. The slurry ratio is 89% active material: CNTs (including dispersant): CMC: SBR = 4.5%: 1.5%: 5%. The slurry is then coated onto copper foil and vacuum-dried for 12 hours to form the anode sheet. The electrolyte is commercially available, the separator is a PE film, and the lithium sheet serves as the counter electrode. The half-cell is assembled in a glove box. Constant current charge and discharge experiments are conducted on a LAND battery test system. Unless otherwise specified, the following data are based on 1C charge and discharge. The charge and discharge voltage is limited to 0.005-1.5 V. Data acquisition and control are performed using a computer-controlled charge and discharge cabinet.

[0066] Table 2 Power-off test results

[0067]

[0068] As can be seen from Table 2, from the comprehensive perspective of first efficiency and cycle stability, Example 2 has the best electrochemical performance; Examples 1 and 3 and Comparative Example 3 have low microporosity and specific surface area, which affects the silicon deposition effect, resulting in their capacity not being fully utilized; Overall, excessive S element doping will lead to an increase in its side reactions, showing low first efficiency, and appropriate doping of S and P is beneficial to improving the battery cycle stability.

[0069] Figure 1 The SEM image of the porous silicon-carbon negative electrode material prepared in Example 2; Figure 1 From the above, it can be seen that starch maintains its good natural spherical morphology after etherification and spray drying, which is beneficial to improving its processing performance in subsequent applications. Figure 2 The isothermal adsorption-desorption curve of the porous silicon-carbon negative electrode material prepared in Example 2 is Figure 2 It can be seen that the curve belongs to a type I isotherm. The adsorption amount increases rapidly at relatively low pressure, and the adsorption is saturated after reaching a certain pressure, which reflects the filling phenomenon of micropores. Figure 3 The BJH desorption mesopore size differential distribution curve of the porous silicon-carbon negative electrode material prepared in Example 2 is shown in FIG. Figure 3 It can be seen that the prepared porous silicon carbon is mainly composed of micropores, with a small amount of mesopores, which is in line with the design expectations and can meet the application requirements of silicon carbon negative electrode materials.

[0070] The embodiments described above merely represent several preferred embodiments of the present invention. While the descriptions are relatively specific and detailed, they are not intended to limit the present invention. It should be noted that those skilled in the art will readily appreciate that the present invention is susceptible to various variations and modifications. Any modifications, equivalent substitutions, or improvements within the scope of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A method for preparing heteroatom-co-doped silicon-carbon negative electrode material, characterized in that: The method includes the following steps: Step 1: taking soluble starch and mixing it with an etherifying agent to carry out etherification, adding phosphorus and sulfur dopants after the etherification is completed, stirring evenly and setting aside; The etherifying agent is at least one of ethylene oxide, propylene oxide, and chloroacetic acid, and the mass ratio of the soluble starch to the etherifying agent is (9-9.5):(0.5-1); the phosphorus and sulfur dopants are sodium phosphate and sodium sulfate, respectively, and the doping amount thereof is 1%-3% of the mass of the soluble starch; Step 2: spray-drying the material obtained in step 1 to obtain a carbon material precursor, and then heating it under protective gas to carbonize it, and crushing it to obtain a powder for later use; Step 3: Add the powder obtained in step 2 to the fluidized bed, increase the temperature while introducing protective gas, and then introduce activation gas for activation, and obtain porous carbon after activation; Step 4: Add the porous carbon obtained in step 3 into the fluidized bed, increase the temperature while introducing protective gas, then introduce silane gas for silicon deposition, and then introduce carbon source gas for secondary coating to obtain a silicon-carbon negative electrode material.

2. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode material according to claim 1, characterized in that: In the etherification step 1, sodium hydroxide is further 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 hours.

3. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode material according to claim 1, characterized in that: The spray drying conditions in step 2 are: temperature of 70-90° C., feed flow rate of 4-8 L / min, and atomization pressure of 0.4-0.8 MPa.

4. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode material according to claim 1, characterized in that: The carbonization step in step 2 is as follows: firstly, the temperature is increased to 400-520°C at a heating rate of 1-5°C / min and kept at that temperature for 1-3 hours, and then the temperature is further increased to 850-1000°C at a heating rate of 1-5°C / min and kept at that temperature for 1-4 hours.

5. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode material according to claim 1, characterized in that: In step 3, the activation gas is carbon dioxide, the activation temperature is 800-950°C, and the activation time is 4-8 hours.

6. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode material according to claim 1, characterized in that: In step 4, the silane gas is at least one of monosilane and disilane, and the flow rate is 3-5 L / min; the temperature of the silicon deposition is 450-550° C., and the time is 4-8 h.

7. The method for preparing a heteroatom-co-doped silicon-carbon negative electrode 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 secondary coating time is 1-2 h.

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