A modified preparation method for mesophase carbon microsphere-based silicon-carbon composite electrode material

Abstract

The present invention provides a modified preparation method for a mesophase carbon microsphere-based silicon-carbon composite electrode material. First, mesophase carbon microspheres are used as the core and an oxygen-containing organosilicon compound is used as the silicon source. The two are compounded by a hydrothermal reaction, and then reduced by a one-step magnesium thermal reaction to obtain silicon-coated mesophase carbon microspheres. Calcium carbonate is then used as a sacrificial layer to introduce voids into the material. The outermost layer is coated with a layer of amorphous carbon as an outer shell, which effectively constrains the excessive expansion of silicon. The "carbon-void-silicon-carbon" four-level "yolk-eggshell" type silicon-carbon composite electrode material obtained by the present invention has the advantages of good conductivity, high reversible specific capacity, good rate performance, and stable cycle performance. The first-cycle coulomb efficiency is above 70%, the reversible specific capacity is above 570mAh / g at a current density of 0.1C, and the specific capacity is above 540mAh / g at a high current density of 10C.

Description

Technical Field

[0001] The invention belongs to the field of lithium ion battery electrode materials and relates to a modification and preparation method of a mesophase carbon microsphere-based silicon-carbon composite electrode material. Background Art

[0002] Lithium-ion batteries are highly favored for their excellent performance, such as high capacity, high voltage, high cycle stability, high energy density and environmental friendliness. In recent years, lithium-ion batteries have been increasingly used in electric vehicles, but the energy density of the power batteries of new energy electric vehicles is currently low, which sometimes makes it difficult to meet people's living needs. Si has a very high theoretical lithium storage capacity (4200mAh / g), and has the characteristics of low lithium insertion potential and high natural abundance. It has become the preferred material for high-energy-density lithium-ion battery negative electrodes. However, Si also has obvious disadvantages. When used directly as a negative electrode material for lithium-ion batteries, it is accompanied by a large volume effect (>300%), which leads to severe collapse of the active silicon particle structure and repeated regeneration of the solid electrolyte interface (SEI) film. In order to improve the shortcomings of Si-based materials in the process of using them as lithium battery negative electrode materials, researchers have proposed to composite Si with other materials, such as carbon materials, to complement the shortcomings of both. However, most of the research on silicon-carbon composites focuses on the structure of silicon particles, such as constructing core-shell structures, hollow spheres, three-dimensional (3D) porous structures, and yolk-shell structures. In fact, studying the role of carbon components in the structural design of silicon-carbon composites is also crucial.

[0003] Mesophase carbon microspheres have a unique lamellar structure and good physicochemical properties, so they become precursors for the preparation of various carbon materials. As a type of graphite material, mesophase carbon microspheres can be combined with Si to not only improve the cyclic stability and conductivity of silicon, but also enhance the specific capacity of the mesophase carbon microspheres. However, the properties of active silicon particles and their requirements for carbon components, especially microstructure, are different from those of traditional graphite materials. The type of pitch and pyrolysis temperature have a great influence on the microstructure of soft carbon compounds, especially the unique chaotic layer arrangement of the carbon layer inside the mesophase carbon microspheres, which will be a potential way to optimize the electrochemical properties of active silicon. The present invention innovatively utilizes mesophase carbon microspheres as a carbon source, and utilizes its chaotic layer structure to achieve a stable bond with silicon, constructing a "carbon-silicon-void-carbon" four-level "yolk-eggshell" type silicon-carbon composite material, and preparing a silicon-carbon composite electrode material with good performance. Summary of the Invention

[0004] In order to overcome the shortcomings of the existing technology, the present invention provides a modification and preparation method of mesophase carbon microsphere-based silicon-carbon composite electrode material. First, mesophase carbon microspheres and oxygen-containing organic silicon compounds are used to prepare a silicon-carbon composite material as the core through several steps of inorganic acid activation, hydrothermal reaction, magnesium thermal reduction and high-temperature carbonization. Then, calcium carbonate and an amorphous carbon layer are wrapped in sequence on the outer layer. Finally, hydrochloric acid etching is performed to obtain a "carbon-void-silicon-carbon" quaternary structure "yolk-eggshell" type silicon-carbon composite lithium ion battery negative electrode material, which has the advantages of good conductivity, good rate performance and stable cycle performance.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A modification and preparation method of mesophase carbon microsphere-based silicon-carbon composite electrode material comprises the following specific steps:

[0007] Step 1: Using mesophase carbon microspheres as raw materials, immersing them in a 3-8 mol / L inorganic strong acid solution and stirring at 250-350° C. for 24 hours to obtain activated mesophase carbon microspheres; the inorganic strong acid includes nitric acid, sulfuric acid, or a mixed acid of the two with a volume ratio of 1:1;

[0008] Step 2: Ultrasonic dispersion of the activated mesophase carbon microspheres in a certain amount of deionized water, followed by dropwise addition of an oxygen-containing organosilicon compound, and continued ultrasonication until completely dispersed; the dispersion was transferred to a hydrothermal reactor, reacted at 160-200° C. for 6-15 hours, the hydrothermal reactant was filtered, and the precipitate was washed three times with ethanol and deionized water, respectively, and dried in an oven to obtain a dried product;

[0009] Step 3: placing the dried product in a tube furnace and pre-oxidizing it in an air atmosphere at 320°C for 2-4 hours; mixing the pre-oxidized product with magnesium powder and placing it in a stainless steel reactor, and performing a magnesium thermal reaction at 600°C under an argon atmosphere for 4-8 hours; washing the magnesium thermal reaction product with dilute hydrochloric acid, ethanol, and deionized water respectively until neutral to obtain a treated magnesium thermal reaction product;

[0010] Step 4: Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, followed by sequential addition of sodium carbonate and calcium chloride, and vigorous stirring to obtain a white precipitate uniformly coated with calcium carbonate; the white precipitate is then added to a solution of isotropic asphalt, stirred for 24 hours, and filtered, and the precipitate is washed with deionized water and dried; the dried precipitate is placed in a tube furnace, pre-oxidized at 320°C in an air atmosphere for 2-4 hours, and carbonized at 800-1200°C in an argon atmosphere for 2 hours to obtain a carbonized product;

[0011] Step 5: Place the carbonized product in 0.1 mol / L hydrochloric acid, wash thoroughly to remove calcium carbonate precipitate, then wash with deionized water until neutral, and dry to obtain a "carbon-void-silicon-carbon" quaternary core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material.

[0012] In the present invention, the oxygen-containing organosilicon compound is selected from one of triisopropylsilyl trifluoromethanesulfonate, 3-tert-butyldimethylsilyl-2-propyn-1-ol or (1,1-dimethylethyl)dimethylsilyl trifluoromethanesulfonate.

[0013] In the present invention, the silicon content in the product electrode material is controlled by the addition amount of the oxygen-containing organosilicon compound in the hydrothermal synthesis raw material. The more the oxygen-containing organosilicon compound is added, the higher the silicon content in the final product. Preferably, in steps 1 and 2, the amount ratio of the mesophase carbon microspheres, the inorganic strong acid solution and the oxygen-containing organosilicon compound is 1g:200mL:(0.1-0.5)g; the amount ratio of the dried product obtained in step 2, magnesium powder, sodium carbonate and calcium chloride is 1g:0.1g:(1-1.5)g:(1.2-1.8)g; and the amount ratio of the white precipitate to the isotropic asphalt in step 4 is 1g:(1-1.5)g.

[0014] In steps 3 and 4 of the present invention, the heating rate of the air atmosphere pre-oxidation process is 1°C / min.

[0015] In the present invention, the isotropic asphalt is a coal-based coated asphalt, petroleum-based coated asphalt or high-temperature coal asphalt with a softening point of 100-200°C. The method for preparing the solution of the isotropic asphalt is to dissolve the isotropic asphalt in a solvent, and the solvent is selected from one or more of tetrahydrofuran, toluene, N-methylpyrrolidone, and pyridine; the ratio of the solvent to the isotropic asphalt is 100mL: (1-1.5)g.

[0016] In the present invention, in the "carbon-void-silicon-carbon" four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material, the silicon layer thickness is 0.1-0.2 μm, the void structure thickness is 0.3-1 μm, and the amorphous carbon layer thickness is 0.5-1 μm.

[0017] The present invention also provides a mesophase carbon microsphere-based silicon-carbon composite electrode material prepared by the above method. The electrode material has a four-level core-shell structure of "carbon-void-silicon-carbon", a first-cycle coulombic efficiency of more than 70%, a reversible specific capacity of more than 570 mAh / g at a current density of 0.1C, and a specific capacity of more than 540 mAh / g at a high current density of 10C.

[0018] The present invention uses mesophase carbon microspheres as a carbon substrate with a wide range of sources. The mesophase carbon microspheres are a graphite-like material with a relatively disordered carbon layer arrangement, which is beneficial to the transfer of lithium ions during the charge and discharge process. The disordered layer structure of the carbon layer arrangement provides a large number of active sites for silicon to connect on its surface after activation. Moreover, as a carbon substrate with a wide range of sources, it effectively buffers the volume expansion of silicon-based materials. The hydrothermal reaction process of the oxygen-containing organic silicon compound allows silicon to be uniformly deposited on the surface of the carbon substrate, promoting a close combination of the two. Calcium carbonate is used as a sacrificial layer. Compared with the commonly used SiO2 sacrificial layer, this method avoids the use of hydrofluoric acid as an etchant, reducing the safety risk in the preparation process. Calcium carbonate is also easy to etch, leaving enough space for the volume expansion of the inner silicon. The outermost coating carbon layer further constrains the expansion of silicon due to the volume change of silicon during the charge and discharge process, resulting in a "carbon-void-silicon-carbon" four-level "yolk-eggshell" type composite silicon-carbon electrode material. By appropriately increasing or decreasing the amount of sodium carbonate, calcium chloride and isotropic asphalt according to the amount of oxygen-containing organic silicon compound, the thickness of the void layer and the coated carbon layer can be increased or decreased simultaneously with the increase or decrease of the silicon layer thickness, thereby ensuring that the performance of the prepared silicon-carbon composite electrode material reaches the optimal level, and ultimately preparing an electrode material with high reversible cycle specific capacity and high coulombic efficiency.

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

[0020] 1. The carbon substrate in the composite material is mesophase carbon microspheres prepared from heavy oil. The internal carbon layer arrangement presents a disordered layer structure, which has excellent rate performance. This feature is utilized to achieve a significant increase in the number of active sites, which is conducive to the stable connection of silicon on the surface and has good cycle performance.

[0021] 2. Using calcium carbonate as a sacrificial layer avoids the use of hydrofluoric acid when using SiO2 as a sacrificial layer, reducing safety risks during the preparation process. Calcium carbonate is easy to etch, and the void layer after etching reserves sufficient space for the volume expansion of the inner silicon layer.

[0022] 3. The carbon precursor is activated by an inorganic strong acid solution, which increases the oxygen-containing functional groups on the mesophase carbon microspheres. At the same time, an oxygen-containing organosilicon compound is used as a silicon source. The interaction between the surface functional groups and the organosilicon promotes the combination of the two. The reduction is then carried out through a one-step magnesium thermal reaction. Compared with mechanical mixing, the combination of silicon and carbon is tighter.

[0023] 4. Using isotropic asphalt to wrap a layer of amorphous carbon on the outermost layer of the material effectively constrains the volume expansion of silicon during the charging and discharging process and improves the cycle stability of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1Schematic diagram of the structure of the prepared four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material.

[0025] Figure 2 This is a polarized photo of the mesocarbon microspheres used in the present invention. DETAILED DESCRIPTION

[0026] The following describes the invention in conjunction with embodiments.

[0027] Example 1:

[0028] This embodiment provides a method for modifying and preparing a mesocarbon microsphere-based silicon-carbon composite electrode material, comprising the following specific steps:

[0029] (1) 1 g of mesophase carbon microspheres was added to 200 mL of 8 mol / L sulfuric acid solution and stirred at 250 °C for 24 h to obtain activated mesophase carbon microspheres;

[0030] (2) The activated mesocarbon microspheres were dispersed in a certain amount of deionized water and ultrasonicated for 1 h. Then, 20 mL of 0.01 g / mL triisopropylsilyl trifluoromethanesulfonate ethanol solution was added dropwise and ultrasonicated for 1.5 h until completely dispersed. The dispersion was transferred to a hydrothermal reactor and reacted at 160 ° C for 8 h. The product of the hydrothermal reaction was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and dried in an oven at 80 ° C for 24 h to obtain a dried product.

[0031] (3) 1 g of the dried product was placed in a tube furnace and pre-oxidized in an air atmosphere at 320°C for 2 h; the pre-oxidized product was mixed with 0.1 g of magnesium powder and placed in a stainless steel reactor for a magnesium thermal reaction at 600°C under an argon atmosphere for 6 h; the magnesium thermal reaction product was washed with 0.5 mol / L dilute hydrochloric acid, ethanol, and deionized water to neutrality, and vacuum dried at 100°C for 24 h to obtain a treated magnesium thermal reaction product; wherein the heating rate of the pre-oxidation process was 1°C / min;

[0032] (4) Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, and sequentially adding 1.4 g of sodium carbonate and 1.6 g of calcium chloride, stirring vigorously to obtain a white precipitate uniformly coated with calcium carbonate; completely dissolving 1.4 g of high-temperature coal tar pitch in 100 mL of toluene to obtain a solution of isotropic asphalt, adding 1 g of white precipitate to the solution, stirring for 24 h, and then filtering, washing the precipitate with deionized water and drying; placing the dried precipitate in a tubular furnace, pre-oxidizing at 320 ° C in an air atmosphere for 2 h, and carbonizing at 1000 ° C in an argon atmosphere for 2 h to obtain a carbonized product; wherein, the heating rate of the pre-oxidation process is 1 ° C / min;

[0033] (5) The carbonized product was placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitate, then washed with deionized water until neutral, and dried to obtain a "carbon-void-silicon-carbon" four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material A; wherein the thickness of the silicon layer is 0.1 μm, the thickness of the void structure is 0.5 μm, and the thickness of the amorphous carbon layer is 0.7 μm.

[0034] Example 2:

[0035] This embodiment provides a method for modifying and preparing a mesocarbon microsphere-based silicon-carbon composite electrode material, comprising the following specific steps:

[0036] (1) 1 g of mesophase carbon microspheres was added to 200 mL of 5 mol / L sulfuric acid solution and stirred at 200 °C for 24 h to obtain activated mesophase carbon microspheres;

[0037] (2) The activated mesophase carbon microspheres were dispersed in a certain amount of deionized water and ultrasonicated for 1 hour. Then, 30 mL of 0.01 g / mL triisopropylsilyl trifluoromethanesulfonate ethanol solution was added dropwise and ultrasonicated for 1.5 hours until completely dispersed. The dispersion was transferred to a hydrothermal reactor and reacted at 160°C for 8 hours. The product of the hydrothermal reaction was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and dried in an oven at 80°C for 24 hours to obtain a dried product.

[0038] (3) 1 g of the dried product was placed in a tube furnace and pre-oxidized in an air atmosphere at 320°C for 2.5 h; the pre-oxidized product was mixed with 0.1 g of magnesium powder and placed in a stainless steel reactor for a magnesium thermal reaction at 600°C under an argon atmosphere for 6 h; the magnesium thermal reaction product was washed with 0.5 mol / L dilute hydrochloric acid, ethanol, and deionized water to neutrality, and vacuum dried at 100°C for 24 h to obtain a treated magnesium thermal reaction product; wherein the heating rate of the pre-oxidation process was 1°C / min;

[0039] (4) Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, and sequentially adding 1.4 g of sodium carbonate and 1.6 g of calcium chloride, stirring vigorously to obtain a white precipitate uniformly coated with calcium carbonate; completely dissolving 1.4 g of high-temperature coal tar pitch in 100 mL of toluene to obtain a solution of isotropic asphalt, adding 1 g of white precipitate to the solution, stirring for 24 h, and then filtering, washing the precipitate with deionized water and drying; placing the dried precipitate in a tubular furnace, pre-oxidizing at 320 ° C in an air atmosphere for 2 h, and carbonizing at 1000 ° C in an argon atmosphere for 2 h to obtain a carbonized product; wherein, the heating rate of the pre-oxidation process is 1 ° C / min;

[0040] (5) The carbonized product was placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitate, then washed with deionized water until neutral, and dried to obtain a "carbon-void-silicon-carbon" four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material B; wherein the thickness of the silicon layer is 0.18 μm, the thickness of the void structure is 0.8 μm, and the thickness of the amorphous carbon layer is 0.8 μm.

[0041] Example 3:

[0042] This embodiment provides a method for modifying and preparing a mesocarbon microsphere-based silicon-carbon composite electrode material, comprising the following specific steps:

[0043] (1) 1 g of mesophase carbon microspheres was added to 200 mL of 8 mol / L nitric acid solution and stirred at 200 °C for 24 h to obtain activated mesophase carbon microspheres;

[0044] (2) The activated mesophase carbon microspheres were dispersed in a certain amount of deionized water, ultrasonicated for 1 hour, and then 30 mL of 0.01 g / mL 3-tert-butyldimethylsilyl-2-propyn-1-ol ethanol solution was added dropwise, and ultrasonicated for 1.5 hours until completely dispersed; the dispersion was transferred to a hydrothermal reactor and reacted at 160°C for 12 hours; the hydrothermal reaction product was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and placed in an oven at 80°C for 24 hours to obtain a dried product;

[0045] (3) 1 g of the dried product was placed in a tube furnace and pre-oxidized in an air atmosphere at 320°C for 2 h; the pre-oxidized product was mixed with 0.1 g of magnesium powder and placed in a stainless steel reactor for a magnesium thermal reaction at 600°C under an argon atmosphere for 6 h; the magnesium thermal reaction product was washed with 0.5 mol / L dilute hydrochloric acid, ethanol, and deionized water to neutrality, and vacuum dried at 100°C for 24 h to obtain a treated magnesium thermal reaction product; wherein the heating rate of the pre-oxidation process was 1°C / min;

[0046] (4) Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, and sequentially adding 1.5 g of sodium carbonate and 1.8 g of calcium chloride, stirring vigorously to obtain a white precipitate uniformly coated with calcium carbonate; completely dissolving 1.5 g of high-temperature coal tar pitch in 100 mL of toluene to obtain a solution of isotropic asphalt, adding 1 g of white precipitate to the solution, stirring for 24 h, and then filtering, washing the precipitate with deionized water and drying; placing the dried precipitate in a tubular furnace, pre-oxidizing at 320 ° C in an air atmosphere for 2 h, and carbonizing at 1000 ° C in an argon atmosphere for 2 h to obtain a carbonized product; wherein, the heating rate of the pre-oxidation process is 1 ° C / min;

[0047] (5) The carbonized product is placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitate, then washed with deionized water until neutral, and dried to obtain a "carbon-void-silicon-carbon" four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material C; wherein the thickness of the silicon layer is 0.2 μm, the thickness of the void structure is 1 μm, and the thickness of the amorphous carbon layer is 1 μm.

[0048] Example 4:

[0049] (1) 1 g of mesophase carbon microspheres was added to 200 mL of 5 mol / L nitric acid solution and stirred at 200 °C for 24 h to obtain activated mesophase carbon microspheres;

[0050] (2) The activated mesophase carbon microspheres were dispersed in a certain amount of deionized water and ultrasonicated for 1 hour. Then, 40 mL of 0.01 g / mL triisopropylsilyl trifluoromethanesulfonate ethanol solution was added dropwise and ultrasonicated for 1.5 hours until completely dispersed. The dispersion was transferred to a hydrothermal reactor and reacted at 160°C for 14 hours. The product of the hydrothermal reaction was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and dried in an oven at 80°C for 24 hours to obtain a dried product.

[0051] (3) 1 g of the dried product was placed in a tube furnace and pre-oxidized at 320°C in an air atmosphere for 2 h; the pre-oxidized product was mixed with 0.1 g of magnesium powder and placed in a stainless steel reactor for magnesium thermal reaction at 600°C in an argon atmosphere for 6 h; the magnesium thermal reaction product was washed with 0.5 mol / L dilute hydrochloric acid, ethanol and deionized water to neutrality, and vacuum dried at 100°C for 24 h to obtain the treated magnesium thermal reaction product; wherein the heating rate of the pre-oxidation process was 1°C / min;

[0052] (4) Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, and sequentially adding 1.5 g of sodium carbonate and 1.8 g of calcium chloride, stirring vigorously to obtain a white precipitate uniformly coated with calcium carbonate; completely dissolving 1.5 g of high-temperature coal tar pitch in 100 mL of toluene to obtain a solution of isotropic asphalt, adding 1 g of white precipitate to the solution, stirring for 24 h, and then filtering, washing the precipitate with deionized water and drying; placing the dried precipitate in a tubular furnace, pre-oxidizing at 320 ° C in an air atmosphere for 2 h, and carbonizing at 1000 ° C in an argon atmosphere for 2 h to obtain a carbonized product; wherein, the heating rate of the pre-oxidation process is 1 ° C / min;

[0053] (5) The carbonized product was placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitate, and then washed with deionized water until neutral. After drying, a "carbon-void-silicon-carbon" four-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material D was obtained; wherein the thickness of the silicon layer was 0.2 μm, the thickness of the void structure was 1 μm, and the thickness of the amorphous carbon layer was 1 μm.

[0054] Example 5:

[0055] This embodiment provides a method for modifying and preparing a mesocarbon microsphere-based silicon-carbon composite electrode material, comprising the following specific steps:

[0056] (1) 1 g of mesophase carbon microspheres was added to 200 mL of 5 mol / L nitric acid solution and stirred at 200 °C for 24 h to obtain activated mesophase carbon microspheres;

[0057] (2) The activated mesophase carbon microspheres were dispersed in a certain amount of deionized water and ultrasonicated for 1 hour. Then, 30 mL of 0.01 g / mL triisopropylsilyl trifluoromethanesulfonate ethanol solution was added dropwise and ultrasonicated for 1.5 hours until completely dispersed. The dispersion was transferred to a hydrothermal reactor and reacted at 160°C for 12 hours. The product of the hydrothermal reaction was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and dried in an oven at 80°C for 24 hours to obtain a dried product.

[0058] (3) 1 g of the dried product was placed in a tube furnace and pre-oxidized at 320°C in an air atmosphere for 2 h; the pre-oxidized product was mixed with 0.1 g of magnesium powder and placed in a stainless steel reactor for magnesium thermal reaction at 600°C in an argon atmosphere for 6 h; the magnesium thermal reaction product was washed with 0.5 mol / L dilute hydrochloric acid, ethanol and deionized water to neutrality, and vacuum dried at 100°C for 24 h to obtain the treated magnesium thermal reaction product; wherein the heating rate of the pre-oxidation process was 1°C / min;

[0059] (4) 1.2 g of high-temperature coal tar pitch was completely dissolved in 100 mL of toluene to obtain a homologous asphalt solution. 0.8 g of the magnesium thermal reaction product was added to the solution, stirred for 24 h, filtered, washed with deionized water, and dried.

[0060] (5) The dried precipitate was placed in a tube furnace, pre-oxidized at 320°C in an air atmosphere for 2 h, and carbonized at 1000°C in an argon atmosphere for 2 h; wherein the heating rate during the pre-oxidation process was 1°C / min; and a "carbon-silicon-carbon" three-level core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material E was obtained; wherein the thickness of the silicon layer was 0.2 μm, and the thickness of the amorphous carbon layer was 1 μm.

[0061] Example 6:

[0062] This embodiment provides a modification and preparation method of a petroleum coke-based silicon-carbon composite electrode material. The raw material petroleum coke used is Grade I petroleum coke that complies with GB / T 24533-2019, and its technical indicators are shown in Table 1.

[0063] Table 1 Main technical indicators of Grade I petroleum coke

[0064]

[0065] The specific steps include:

[0066] (1) Crush petroleum coke, sieve through a 1000-mesh sieve, take 1 g of petroleum coke particles and add them to 200 mL of 5 mol / L nitric acid solution, activate and stir at 200°C for 24 h to obtain activated petroleum coke particles;

[0067] (2) The activated petroleum coke particles were dispersed in deionized water and ultrasonicated for 1 h. Then, 30 mL of a 0.01 g / mL ethanol solution of 3-tert-butyldimethylsilyl-2-propyn-1-ol was added dropwise and ultrasonic dispersion was continued for 1.5 h until the particles were completely dispersed. The dispersion was transferred to a hydrothermal reactor and reacted at 160 ° C for 8 h. The product of the hydrothermal reaction was filtered, and the precipitate was washed three times with ethanol and deionized water respectively, and dried in an oven at 80 ° C for 24 h to obtain a dried product.

[0068] (3) 1 g of the dried product was mixed with 0.1 g of magnesium powder in a stainless steel reactor and reacted at 600°C for 6 h under an argon atmosphere. The product was then washed with 0.5 mol / L dilute hydrochloric acid, ethanol, and deionized water until neutral; and vacuum dried at 100°C for 24 h to obtain a magnesium thermal reaction product.

[0069] (4) Ultrasonic dispersion of the product of the magnesium thermal reaction in deionized water, followed by the addition of 1.5 g of sodium carbonate and 1.8 g of calcium chloride, and vigorous stirring to obtain a white precipitate uniformly coated with calcium carbonate; 1.5 g of high-temperature coal tar was completely dissolved in 100 mL of toluene to obtain a solution of isotropic asphalt, 1 g of white precipitate was added to the solution, and the mixture was stirred for 24 h and then filtered. The precipitate was washed with deionized water and dried; the dried precipitate was placed in a tubular furnace, pre-oxidized at 320 ° C in an air atmosphere for 2 h, and carbonized at 1000 ° C in an argon atmosphere for 2 h to obtain a carbonized product; wherein the heating rate during the pre-oxidation process was 1 ° C / min;

[0070] (5) The carbonized product was placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitate, and then washed with deionized water until neutral. After drying, a "carbon-void-silicon-carbon" four-level core-shell type petroleum coke-based silicon-carbon composite electrode material F was obtained, wherein the thickness of the silicon layer was 0.2 μm, the thickness of the void structure was 1 μm, and the thickness of the amorphous carbon layer was 1 μm.

[0071] Example 7: Battery Performance Test

[0072] 1. Electrode preparation: The electrode materials prepared in Examples 1-6 were mixed with acetylene black and PVDF in a mass ratio of 8:1:1, ground with NMP (N-methylpyrrolidone) as a solvent to form a uniform slurry, and coated on a copper foil. The mixture was vacuum dried at 90°C for 24 hours and rolled to obtain an electrode sheet.

[0073] 2. Battery Performance Testing: The resulting electrode sheets were cut into 12mm diameter pieces for battery assembly. The assembly process was carried out in an argon-filled glove box, and the water and oxygen content was less than 0.01ppm. The battery used a CR2032 button cell, with a metal lithium sheet as the counter electrode, a polypropylene film as the separator, and a 1M lithium hexafluorophosphate (the solvent was a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, with 5% fluoroethylene carbonate added) as the electrolyte. The assembled button cell was subjected to 0.1C charge-discharge cycle performance testing and 10C high-current discharge performance testing at 25°C in the voltage range of 0.05V-2.2V. The measured battery first-cycle specific capacity (mAh / g), first-cycle charge-discharge efficiency (%), and reversible cycle specific capacity (mAh / g, 500 cycles) were recorded. The silicon content of each electrode material is also given. The results are shown in Table 2.

[0074] Table 2 Comparison of electrode material properties

[0075]

[0076] It can be seen from the data in the table that compared with the existing graphite negative electrode (360mAh / g), the silicon-carbon composite electrode materials A, B, C and D prepared by the method described in Examples 1-4 of the present invention all have higher charge and discharge cycle performance (all above 580mAh / g). By comparison, it can be found that the first-cycle coulomb efficiency of the battery increases with the increase of the silicon content of the composite material, but the specific capacity shows a trend of first increasing and then decreasing with the increase of silicon content. This is because the increase in the proportion of silicon, the greater the capacity that silicon can provide. However, due to the huge volume expansion of the silicon material, too high a silicon content will lead to a decay of the specific capacity. The final experimental results show that 8.0% is the optimal silicon content.

[0077] The electrode material E prepared in Example 5 does not introduce a void structure, and the reversible cycle specific capacity is significantly lower than that of Examples 1-4, indicating that the void structure is well formed in the electrode materials A, B, C and D, which effectively alleviates the volume expansion of silicon inside the composite material.

[0078] In Example 6, using Grade I petroleum coke compliant with GB / T 24533-2019 as the raw material, the first-cycle Coulombic efficiency and reversible cycle specific capacity were lower than those of Examples 1-4, indicating that the internal chaotic layer structure of the mesocarbon microspheres facilitates the free intercalation and deintercalation of lithium ions during high-current charge and discharge, resulting in excellent rate performance. Furthermore, by controlling the amount of added sodium carbonate and calcium chloride, different thicknesses of the interstitial layer can be prepared, which can be flexibly adjusted according to different silicon loadings.

Claims

1. A modification and preparation method of mesophase carbon microsphere-based silicon-carbon composite electrode material, characterized in that: The specific steps include: Step 1: Using mesophase carbon microspheres as raw materials, immersing them in a 3-8 mol / L inorganic strong acid solution and stirring at 250-350°C for 24 hours to obtain activated mesophase carbon microspheres; Step 2: Ultrasonic dispersion of the activated mesophase carbon microspheres in a certain amount of deionized water, followed by dropwise addition of an oxygen-containing organosilicon compound, and continued ultrasonication until completely dispersed; the dispersion was transferred to a hydrothermal reactor, reacted at 160-200° C. for 6-15 hours, the hydrothermal reactant was filtered, and the precipitate was washed three times with ethanol and deionized water, respectively, and dried in an oven to obtain a dried product; Step 3: placing the dried product in a tube furnace and pre-oxidizing it in an air atmosphere at 320°C for 2-4 hours; mixing the pre-oxidized product with magnesium powder and placing it in a stainless steel reactor, and performing a magnesium thermal reaction at 600°C under an argon atmosphere for 4-8 hours; washing the magnesium thermal reaction product with dilute hydrochloric acid, ethanol, and deionized water respectively until neutral to obtain a treated magnesium thermal reaction product; Step 4: Ultrasonic dispersion of the treated magnesium thermal reaction product in deionized water, followed by sequential addition of sodium carbonate and calcium chloride, and vigorous stirring to obtain a white precipitate uniformly coated with calcium carbonate; the white precipitate is then added to a solution of isotropic asphalt, stirred for 24 hours, and filtered, and the precipitate is washed with deionized water and dried; the dried precipitate is placed in a tube furnace, pre-oxidized at 320°C in an air atmosphere for 2-4 hours, and carbonized at 800-1200°C in an argon atmosphere for 2 hours to obtain a carbonized product; Step 5: The carbonized product is placed in 0.1 mol / L hydrochloric acid, washed thoroughly to remove calcium carbonate precipitates, then washed with deionized water until neutral, and dried to obtain a "carbon-void-silicon-carbon" quaternary core-shell mesophase carbon microsphere-based silicon-carbon composite electrode material; The mesophase carbon microsphere-based silicon-carbon composite electrode material has a four-level core-shell structure of "carbon-void-silicon-carbon", with a silicon layer thickness of 0.1-0.2 μm, a void structure thickness of 0.3-1 μm, and an amorphous carbon layer thickness of 0.5-1 μm.

2. The modification preparation method of a mesocarbon microsphere-based silicon-carbon composite electrode material according to claim 1, characterized in that: The inorganic strong acid includes nitric acid, sulfuric acid or a mixed acid of the two in a volume ratio of 1:1; the oxygen-containing organic silicon compound is selected from one of triisopropylsilyl trifluoromethanesulfonate, 3-tert-butyldimethylsilyl-2-propyn-1-ol or (1,1-dimethylethyl)dimethylsilyl trifluoromethanesulfonate.

3. The modification preparation method of a mesocarbon microsphere-based silicon-carbon composite electrode material according to claim 1, characterized in that: The ratio of mesophase carbon microspheres, inorganic strong acid solution and oxygen-containing organosilicon compound is 1 g: 200 mL: (0.1-0.5) g; The dried product obtained in step 2, magnesium powder, sodium carbonate and calcium chloride are used in a ratio of 1 g: 0.1 g: (1-1.5) g: (1.2-1.8) g; In step 4, the ratio of white precipitate to isotropic asphalt is 1 g:(1-1.5) g.

4. The modification preparation method of a mesocarbon microsphere-based silicon-carbon composite electrode material according to claim 1, characterized in that: The isotropic asphalt is coal-based coated asphalt, petroleum-based coated asphalt or high-temperature coal asphalt with a softening point of 100-200°C. The solution preparation method of the isotropic asphalt is to dissolve the isotropic asphalt in a solvent, and the solvent is selected from one or more of tetrahydrofuran, toluene, N-methylpyrrolidone, and pyridine; the ratio of the solvent to the isotropic asphalt is 100mL: (1-1.5)g.

5. The modification preparation method of a mesocarbon microsphere-based silicon-carbon composite electrode material according to claim 1, characterized in that: The heating rate of the air atmosphere pre-oxidation process is 1℃ / min.

6. A mesophase carbon microsphere-based silicon-carbon composite electrode material, characterized in that: The modified preparation method according to any one of claims 1 to 5 is used for preparation, and the first-cycle coulombic efficiency is above 70%, the reversible specific capacity is above 570 mAh / g at a current density of 0.1 C, and the specific capacity is above 540 mAh / g at a high current density of 10 C.

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