A silicon-carbon anode material with a hierarchical porous structure, its preparation method and application

Abstract

The present invention provides a silicon-carbon anode material with a hierarchical porous structure, a preparation method thereof, and an application thereof. The present invention uses a metal silicon alloy powder with a small particle size, and at the same time, with an ultra-high cooling rate, all the silicon in the alloy powder precipitates in the form of radially arranged nanofibers. Then, the metal silicon alloy is etched with an acid to obtain a porous microsphere framework composed of silicon nanofibers. Then, it is coated with carbon by chemical vapor deposition for the first time, and finally, it is coated with carbon by liquid-phase impregnation-carbonization for the second time. The prepared silicon-carbon anode material has a unique microstructure, with carbon coating on the surface of the silicon nanofibers inside the particles and overall carbon coating outside the particles, which can significantly improve the conductivity of the silicon-carbon anode material and improve the electrochemical performance. At the same time, the double carbon coating provides a buffer matrix for the expansion and contraction of silicon during charge and discharge, and can isolate the electrolyte from directly contacting with silicon, avoiding side reactions. The synergistic effect of the above multi-level structures ensures that the silicon-carbon anode material of the present invention has excellent reversible capacity and cycle stability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery preparation, and particularly relates to a silicon-carbon anode material with a hierarchical porous structure, a preparation method thereof, and an application thereof. Background Art

[0002] With the development of society and the rise of the electric vehicle field and the large-scale energy storage field, the energy density of existing lithium-ion batteries can no longer meet the daily needs. Limited by the material system, the existing commercial lithium-ion batteries have approached the limit of their energy density, 300 Wh / kg -1 , and it is necessary to improve the energy density level by developing anode and cathode materials with higher specific capacities.

[0003] At present, it is difficult to improve the specific capacity of cathode materials. The anode materials of lithium-ion batteries that are widely used commercially are carbon-based materials, such as natural graphite, hard carbon, artificial graphite, graphitized mesophase carbon microspheres, etc. However, now the carbon-based materials are also approaching their theoretical reversible capacity of 372 mAh / g -1 , and a more feasible method is to use a silicon-carbon anode material with a high specific capacity. Silicon has attracted extensive research due to its high theoretical specific capacity (4200 mAh / g -1 ), low working voltage, etc., and it is rich in reserves in the earth's crust, low in cost, and environmentally friendly. It is one of the most promising next-generation lithium-ion battery anode materials.

[0004] However, the electron conductivity and ion conductivity of silicon are relatively low, resulting in poor kinetic performance of its electrochemical reactions and poor cycle stability of ordinary pure silicon. Moreover, the phase change and volume expansion of silicon during the lithiation process will generate relatively large stress, resulting in electrode fracture and pulverization, increased resistance, and a sharp drop in cycle performance. At present, researchers improve the practicality of silicon anode materials through modification, such as nanosizing of silicon particles, structural design of silicon materials, preparation of composites of silicon and carbon materials, polymers, etc. In the silicon-carbon composite system, silicon particles serve as active substances to provide lithium storage capacity; carbon can not only buffer the volume change of the silicon anode during charge and discharge, but also improve the conductivity of the silicon-based material, and can also prevent the aggregation of silicon particles during charge and discharge cycles.

[0005] Patent CN107507972B uses silicon alloy powder as raw material. After pickling treatment to remove the other metals except silicon in the silicon alloy powder, porous silicon is obtained. The porous silicon is placed in a carbon precursor and subjected to carbon coating treatment to form a silicon-carbon composite material with a carbon coating layer. Then, through carbonization treatment, a silicon-carbon negative electrode material is obtained. The porous silicon core in the material is micron-sized. Although the carbon coating layer can play a certain protective role, with the progress of charge and discharge, the silicon inside the particles will still be pulverized, and the electrochemical performance deteriorates. Moreover, the internal micron-sized silicon has poor conductivity, which is not conducive to the electrochemical performance of the silicon-carbon negative electrode material. Patent CN105226285B reacts mechanically crushed silicon metal alloy debris with a liquid-phase pore-forming agent to remove the active metals, obtaining a porous silicon material. Then, it is washed with hydrofluoric acid solution to remove silicon oxide. Then, it is mixed with a polymer and ball-milled and calcined to obtain a porous silicon-carbon composite material. However, the aluminum and silicon in the aluminum-silicon alloy powder obtained by crushing the alloy ingot are unevenly distributed, and the porous silicon structure obtained after acid etching is thick, resulting in poor electrochemical performance.

[0006] Therefore, aiming at the problems of large silicon size, easy pulverization, and poor conductivity of the current silicon-carbon negative electrode material, it is particularly important to develop a silicon-carbon negative electrode material with a stable structure and excellent cycling performance. Summary of the Invention

[0007] The present invention provides a silicon-carbon negative electrode material with a hierarchical porous structure, its preparation method and application, aiming to solve the above problems existing in the background technology.

[0008] To achieve the above object, an embodiment of the present invention provides a preparation method of a silicon-carbon negative electrode material with a hierarchical porous structure, including the following steps:

[0009] S1: Using metal-silicon alloy powder as raw material, after pickling to remove the metals except silicon in the metal-silicon alloy powder, a water bath reaction is carried out to obtain porous silicon microspheres with a radial nanofiber structure.

[0010] S2: Pickle the porous microspheres with the radial nanofiber structure using hydrofluoric acid to remove SiO x , obtaining porous silicon microspheres without silicon oxide to improve the first charge-discharge efficiency of the material.

[0011] S3: Carry out a primary carbon coating reaction on the porous silicon microspheres without silicon oxide by chemical vapor deposition to obtain an intermediate of the silicon-carbon negative electrode material. This intermediate of the silicon-carbon negative electrode material is a primary coated silicon-carbon negative electrode material, and the coating layer is located on the surface of the nano-silicon fibers. The carbon coating layer obtained by chemical vapor deposition can evenly cover all the silicon fibers in the silicon microspheres, playing a good protective role in isolating silicon from direct contact with the electrolyte.

[0012] S4: Disperse the silicon-carbon anode material intermediate in an organic carbon source solution, stir and evaporate the solvent, and then pyrolyze at a high temperature to obtain a secondary-coated silicon-carbon anode material; the second carbon coating layer uniformly covers the surface of the silicon microspheres, further playing a protective role to prevent the silicon microspheres from breaking during charge and discharge.

[0013] The metal-silicon alloy powder particles are spherical or quasi-spherical, and the particle size is 0.1 - 5 μm.

[0014] In the chemical vapor deposition process, a carbon-containing gas and a carrier gas are introduced; the carbon-containing gas includes at least one of alkanes, alkenes, alkynes, benzene rings, carbon monoxide, and carbon dioxide.

[0015] The viscosity of the organic carbon source solution is 1 - 10000 mPa·s.

[0016] Among them, the thickness of the primary carbon coating layer is 5 - 100 nm; the thickness of the secondary carbon coating layer is 0.1 - 2 μm; there is a gap between the primary carbon coating layer and the secondary carbon coating layer; the volume of the gap accounts for 10% - 30% of the volume of the silicon-carbon anode material. That is to say, there is a gap between the second carbon coating layer and the first carbon coating layer; the volume of the gap accounts for 10 - 30% of the volume of the secondary-coated silicon-carbon microspheres, which can be used as a buffer space for the expansion and contraction of silicon, ensuring the cycle stability of the silicon-carbon anode material.

[0017] According to one aspect of the embodiments of the present invention, in step S1, the metal-silicon alloy is at least one of iron-silicon, manganese-silicon, aluminum-silicon, magnesium-silicon, zinc-silicon, calcium-silicon, lead-silicon, and nickel-silicon. The silicon content in the metal-silicon alloy is 5% - 50%, and the metal content in the metal-silicon alloy is 50 - 95%; the acid used for pickling is at least one of sulfuric acid, hydrochloric acid, nitric acid, acetic acid, and oxalic acid, and the concentration is 1 - 50 wt.%. The metal-silicon alloy powder is prepared by induction melting + rapid solidification technology, and the rapid solidification technology is at least one of water atomization, gas atomization, ultrasonic atomization, and rotary disk atomization. Due to the small particle size of the alloy powder and the cooling rate of the rapid solidification technology up to 10 7 ~10 8 K / s, silicon can be completely precipitated in the form of nanofiber structure.

[0018] According to one aspect of the embodiments of the present invention, in step S1, the silicon nanofibers in the porous silicon microspheres are radially distributed from the center. The diameter of the silicon nanofibers is 5 nm - 1 μm, and the length of the silicon nanofibers is 0.1 - 5 μm; the porosity of the porous silicon microspheres is 0.5 - 0.95, and the specific surface area is 10 - 1000 m 2 g -1 ; the water bath temperature is 25 - 90 °C, and the water bath reaction duration is 24 - 72 h.

[0019] According to one aspect of the embodiments of the present invention, in step S2, the concentration of hydrofluoric acid is 1 to 40 wt.%, the pickling time is 1 to 24 h, and the temperature is 25 to 60 °C.

[0020] According to one aspect of the embodiments of the present invention, in step S3, the temperature of chemical vapor deposition is 600 to 1000 °C.

[0021] According to one aspect of the embodiments of the present invention, in step S3, the carrier gas includes at least one of hydrogen, nitrogen, helium, neon, and argon.

[0022] According to one aspect of the embodiments of the present invention, in step S4, the organic carbon source solution includes an organic carbon source and a solvent, and the viscosity is 100 to 6000 mPa·s; the organic carbon source is at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenolic resin, xylenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene, and the concentration is 5 to 70 wt.%; the solvent is at least one of water, ethanol, and acetone.

[0023] Preferably, to ensure that the organic carbon source is only coated on the surface of the microspheres and does not enter the gaps between the silicon nanofibers, the mass fraction of the organic carbon source in the solvent is 15 to 50 wt%, and the viscosity of the formed solution is 100 to 6000 mPa·s; when the viscosity of the organic carbon source solution is 500 to 4000 mPa·s, the viscosity is moderate, the solution will not enter the pores between the carbon fibers at all, the pores are completely retained, and it is also easy to perform the stirring coating operation, so that the surface of the microspheres can be completely and uniformly coated, and the structure of the prepared material is optimal; when the viscosity is 100 to 500 mPa·s, the viscosity is slightly low, the pores between the silicon fibers will not be completely wetted by the organic carbon source solution, the space reserved for silicon expansion and contraction is reduced, and the structure of the prepared material is second; when the viscosity is 4000 to 6000 mPa·s, the viscosity is too high, although the solution will not enter the pores of the microspheres, it is difficult to completely coat the silicon microspheres, and the coating thickness is uneven, and the prepared material is the worst.

[0024] According to one aspect of the embodiments of the present invention, in step S4, the high-temperature pyrolysis is carried out under a protective atmosphere, the protective atmosphere is at least one of nitrogen, helium, and argon, and the temperature is 300 to 1200 °C.

[0025] Based on the general concept of an invention, the embodiments of the present invention provide a silicon-carbon anode material with a hierarchical porous structure obtained by the above preparation method.

[0026] The embodiments of the present invention also provide an application of a silicon-carbon anode material with a hierarchical porous structure obtained by the above preparation method in a lithium-ion battery.

[0027] The above solution of the present invention has the following beneficial effects:

[0028] (1) The present invention uses a small-particle metal silicon alloy powder, and at the same time, uses an ultra-high cooling rate to allow all silicon in the alloy powder to precipitate in the form of radial nanofibers. The metal silicon alloy is then acid-etched to obtain a porous microsphere framework composed of silicon nanofibers, which is then carbon-coated once by chemical vapor deposition, and finally carbon-coated twice by liquid phase impregnation-carbonization. The prepared silicon-carbon negative electrode material has a unique microstructure, as shown in the attached figure. Figure 1 As shown, the carbon coating on the surface of silicon nanofibers inside the particles and the overall carbon coating on the outside of the particles can significantly improve the conductivity of the silicon-carbon negative electrode material and improve the electrochemical performance; at the same time, the double carbon coating provides a buffer matrix for the expansion and contraction of silicon during charging and discharging, and can isolate the electrolyte from direct contact with silicon to avoid side reactions.

[0029] (2) The present invention uses small-particle metal-silicon alloy powder, and at the same time, uses an ultra-high cooling rate to make all the silicon in the alloy powder precipitate in the form of radial nanofibers, and then obtains a porous microsphere framework composed of silicon nanofibers by acid etching the metal-silicon alloy. The nano-scale silicon fibers and the pores between the fibers can effectively resist expansion and contraction during charging and discharging, ensuring that the silicon-carbon negative electrode material has excellent cycle stability. Compared with the existing large-particle metal-silicon alloy in which silicon exists in the form of flaky micron-sized primary silicon, only micron silicon particles or porous silicon with micron-sized walls can be obtained after etching. The size is too large and it is easy to pulverize during charging and discharging, and the performance of the obtained silicon-carbon negative electrode material is poor.

[0030] (3) Compared with the prior art in which the organic carbon source completely occupies the voids in the material, the present invention adopts a high-viscosity organic carbon source solution as a coating precursor for the first time, which is difficult to enter the voids between the silicon nanofibers and can only be coated on the surface of the porous silicon microspheres; therefore, there is a void between the second carbon coating layer and the first carbon coating layer, which reserves a buffer space for the expansion and contraction of the silicon nanofibers, so the second carbon coating layer will not be destroyed during the cycle, and the spherical structure of the material remains stable; the synergistic effect of the above multi-level structure ensures that the silicon-carbon negative electrode material of the present invention has excellent reversible capacity and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 Schematic diagram of the structure of a hierarchical porous silicon-carbon negative electrode material according to an embodiment of the present invention;

[0033] Figure 2It is the SEM image of the Al-Si alloy powder used in Example 1 and Comparative Example 1 of the present invention;

[0034] Figure 3 It is the SEM image of the microstructure after acid dissolution of the Al-Si alloy powder used in Example 1 of the present invention;

[0035] Figure 4 It is the SEM image of the microstructure after acid dissolution of the Al-Si alloy powder used in Comparative Example 1 of the present invention;

[0036] Figure 5 It is the cycle performance graph of the silicon-carbon anode materials prepared in Example 1 and Comparative Example 1 of the present invention. Detailed implementation manners

[0037] To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will be described in detail with reference to the accompanying drawings and specific embodiments.

[0038] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments, and are not intended to limit the protection scope of the present invention.

[0039] Unless otherwise specifically stated, various raw materials, reagents, instruments and equipment used in the present invention can be obtained through the market or can be prepared by existing methods.

[0040] The present invention provides a preparation method of a silicon-carbon anode material with a hierarchical porous structure for the existing problems, including the following steps:

[0041] S1: Using a metal-silicon alloy powder as a raw material, after pickling to remove metals other than silicon in the metal-silicon alloy powder, a water bath reaction is carried out to obtain a porous silicon microsphere with a radial nanofiber structure;

[0042] S2: Pickling the porous microsphere with the radial nanofiber structure with hydrofluoric acid to remove SiO x , to obtain a porous silicon microsphere without silicon oxide, so as to improve the first charge-discharge efficiency of the material;

[0043] S3: Carrying out a primary carbon coating reaction on the porous silicon microsphere without silicon oxide by chemical vapor deposition to obtain an intermediate of the silicon-carbon anode material; this intermediate of the silicon-carbon anode material is a primary coated silicon-carbon anode material, and this coating layer is located on the surface of the nanosilicon fibers; the carbon coating layer obtained by chemical vapor deposition can uniformly cover all the silicon fibers in the silicon microsphere, playing a good role in protecting the silicon from directly contacting the electrolyte;

[0044] S4: Disperse the silicon-carbon anode material intermediate in an organic carbon source solution, stir and evaporate the solvent, and then pyrolyze at high temperature to obtain a secondary-coated silicon-carbon anode material; the second carbon coating layer uniformly covers the surface of the silicon microspheres, further playing a protective role to prevent the silicon microspheres from breaking during charge and discharge;

[0045] The metal-silicon alloy powder particles are spherical or quasi-spherical, with a particle size of 0.1 - 5 μm;

[0046] In the chemical vapor deposition process, a carbon-containing gas and a carrier gas are introduced; the carbon-containing gas includes at least one of alkanes, alkenes, alkynes, benzene rings, carbon monoxide, and carbon dioxide;

[0047] The viscosity of the organic carbon source solution is 1 - 10000 mPa·s;

[0048] Among them, the thickness of the primary carbon coating layer is 5 - 100 nm; the thickness of the secondary carbon coating layer is 0.1 - 2 μm; there is a gap between the primary carbon coating layer and the secondary carbon coating layer; the volume of the gap accounts for 10% - 30% of the volume of the silicon-carbon anode material. That is to say, there is a gap between the second carbon coating layer and the first carbon coating layer; the volume of the gap accounts for 10 - 30% of the volume of the secondary-coated silicon-carbon microspheres, which can serve as a buffer space for silicon expansion and contraction, ensuring the cycle stability of the silicon-carbon anode material.

[0049] According to one aspect of the embodiments of the present invention, in step S1, the metal-silicon alloy is at least one of iron-silicon, manganese-silicon, aluminum-silicon, magnesium-silicon, zinc-silicon, calcium-silicon, lead-silicon, and nickel-silicon. The silicon content in the metal-silicon alloy is 5% - 50%, and the metal content in the metal-silicon alloy is 50 - 95%; the acid used for pickling is at least one of sulfuric acid, hydrochloric acid, nitric acid, acetic acid, and oxalic acid, with a concentration of 1 - 50 wt.%. The metal-silicon alloy powder is prepared by induction melting + rapid solidification technology, and the rapid solidification technology is at least one of water atomization, gas atomization, ultrasonic atomization, and rotary disk atomization. Due to the small particle size of the alloy powder and the high cooling rate of the improved rapid solidification technology up to 10 7 ~10 8 K / s, silicon can be completely precipitated in the form of nanofiber structures.

[0050] According to one aspect of the embodiments of the present invention, in step S1, the silicon nanofibers in the porous silicon microspheres are radially distributed from the center. The diameter of the silicon nanofibers is 5 nm - 1 μm, and the length of the silicon nanofibers is 0.1 - 5 μm; the porosity of the porous silicon microspheres is 0.5 - 0.95, and the specific surface area is 10 - 1000 m 2 g -1 ; the water bath temperature is 25 - 90 °C, and the water bath reaction duration is 24 - 72 h.

[0051] According to one aspect of the embodiments of the present invention, in step S2, the concentration of hydrofluoric acid is 1 to 40 wt.%, the pickling time is 1 to 24 h, and the temperature is 25 to 60 °C.

[0052] According to one aspect of the embodiments of the present invention, in step S3, the temperature of chemical vapor deposition is 600 to 1000 °C.

[0053] According to one aspect of the embodiments of the present invention, in step S3, the carrier gas includes at least one of hydrogen, nitrogen, helium, neon, and argon.

[0054] According to one aspect of the embodiments of the present invention, in step S4, the organic carbon source solution includes an organic carbon source and a solvent, and the viscosity is 100 to 6000 mPa·s; the organic carbon source is at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenolic resin, xylenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene, and the concentration is 5 to 70 wt.%; the solvent is at least one of water, ethanol, and acetone.

[0055] Preferably, to ensure that the organic carbon source is only coated on the surface of the microspheres and does not enter the gaps between the silicon nanofibers, the mass fraction of the organic carbon source in the solvent is 15 to 50 wt%, and the viscosity of the formed solution is 100 to 6000 mPa·s; when the viscosity of the organic carbon source solution is 500 to 4000 mPa·s, the viscosity is moderate, the solution will not enter the pores between the carbon fibers at all, the pores are completely retained, and it is also easy to perform the stirring and coating operation, so that the surface of the microspheres can be completely and uniformly coated, and the structure of the prepared material is the best; when the viscosity is 100 to 500 mPa·s, the viscosity is slightly low, the pores between the silicon fibers will not be completely infiltrated by the organic carbon source solution, the space reserved for silicon expansion and contraction is reduced, and the structure of the prepared material is second; when the viscosity is 4000 to 6000 mPa·s, the viscosity is too high, although the solution will not enter the pores of the microspheres, it is difficult to completely coat the silicon microspheres, and the coating thickness is uneven, and the structure of the prepared material is the worst.

[0056] According to one aspect of the embodiments of the present invention, in step S4, the high-temperature pyrolysis is carried out under a protective atmosphere, the protective atmosphere is at least one of nitrogen, helium, and argon, and the temperature is 300 to 1200 °C.

[0057] Based on the general concept of an invention, the embodiments of the present invention provide a silicon-carbon negative electrode material with a hierarchical porous structure obtained by the above preparation method.

[0058] The embodiments of the present invention also provide an application of a silicon-carbon negative electrode material with a hierarchical porous structure obtained by the above preparation method in a lithium-ion battery.

[0059] The following will be described in detail through specific examples

[0060] Example 1

[0061] A preparation method of a silicon-carbon anode material with a hierarchical porous structure, comprising the following steps:

[0062] (1) Using aluminum-silicon alloy powder as raw material, D 50 is 2 μm, the silicon content is 20 wt%, pickling with 20 wt% hydrochloric acid in excess by 1 time, reacting in a water bath at 40 °C for 24 h to obtain porous silicon microspheres with a radial nanofiber structure;

[0063] (2) Pickling the porous silicon microspheres obtained in (1) with a 5 wt% HF solution at a temperature of 40 °C for 12 h to remove SiO x , and obtaining porous silicon microspheres without silicon oxide to improve the initial charge-discharge efficiency of the material;

[0064] (3) Performing the first carbon coating treatment on the pickled porous silicon microspheres in (2) by chemical vapor deposition, using methane as the carbon source, nitrogen as the carrier gas, and a pyrolysis temperature of 800 °C; controlling the reaction time to obtain a primary carbon coating layer with a thickness of 10 nm;

[0065] (4) Dispersing the primary coated silicon-carbon anode material in (3) in an organic carbon source solution, stirring and evaporating the solvent, the organic carbon source is sucrose, the concentration is 30 wt%, water is the solvent, the solution viscosity is 600 mPa·s, and then performing high-temperature pyrolysis under a protective atmosphere of nitrogen, the pyrolysis temperature is 500 °C, controlling the sucrose dosage to obtain a secondary carbon coating layer with a thickness of 1 μm, and obtaining a silicon-carbon anode material with a secondary carbon coating.

[0066] The areal density of the silicon-carbon anode is 2 mg cm -2 , the electrolyte solute is LiPF 6 , the solvent is EC, DMC, DEC, and the volume ratio is 0.3:0.3:0.4, and the concentration of the solute is 1 mol L -1 . Perform half-cell tests, and perform charge-discharge tests between 0.01 and 1.5 V (perform the first three charge-discharge tests at a current density of 50 mAg -1 , and perform cyclic tests at a current density of 200 mA g -1 ), and the electrochemical performance is shown in Table 1.

[0067] Example 2

[0068] A silicon-carbon anode material with a hierarchical porous structure and its preparation method, comprising the following steps:

[0069] (1) Using iron-silicon alloy powder as raw material, D 50It is 3 μm, with a silicon content of 30 wt%, pickled with 40 wt% hydrochloric acid in an amount 1 time in excess, reacted in a water bath at 30 °C for 48 h to obtain porous silicon microspheres with a nanofiber structure;

[0070] (2) The porous silicon microspheres obtained in (1) are pickled in a second step with a 10 wt% HF solution at a temperature of 30 °C for a pickling time of 15 h to remove SiO x , obtaining porous silicon microspheres without silicon oxide and improving the initial charge-discharge efficiency of the material;

[0071] (3) The pickled porous silicon microspheres in (2) are subjected to a first carbon coating treatment by chemical vapor deposition, using acetylene as the carbon source and argon as the carrier gas, with a pyrolysis temperature of 900 °C; controlling the reaction time to obtain a primary carbon coating layer with a thickness of 20 nm;

[0072] (4) The primary-coated silicon-carbon negative electrode material in (3) is dispersed in an organic carbon source solution, the solvent is evaporated by stirring, the organic carbon source is glucose with a concentration of 40 wt%, water is the solvent, the solution viscosity is 800 mPa·s, and then it is pyrolyzed at a high temperature under an argon protective atmosphere at a pyrolysis temperature of 600 °C, controlling the amount of glucose used to obtain a secondary carbon coating layer with a thickness of 2 μm, obtaining a secondary carbon-coated silicon-carbon negative electrode material.

[0073] The areal density of the silicon-carbon negative electrode is 2 mg cm -2 , the electrolyte solute is LiPF 6 , the solvent is EC, DMC, DEC, with a volume ratio of 0.3:0.3:0.4, and the concentration of the solute is 1 mol L -1 . A half-cell test is carried out, and charge-discharge tests are carried out between 0.01 and 1.5 V (the first three charge-discharge tests are carried out at a current density of 50 mAg -1 , and cyclic tests are carried out at a current density of 200 mA g -1 ), and the electrochemical performance is shown in Table 2.

[0074] Example 3

[0075] A silicon-carbon negative electrode material with a hierarchical porous structure and a preparation method thereof, comprising the following steps:

[0076] (1) Using manganese-silicon alloy powder as the raw material, D 50 is 4 μm, with a silicon content of 40 wt%, pickled with 40 wt% hydrochloric acid in an amount 1 time in excess, reacted in a water bath at 50 °C for 36 h to obtain porous silicon microspheres with a nanofiber structure;

[0077] (2) The porous silicon microspheres obtained in (1) are pickled in a second step with a 5 wt% HF solution at a temperature of 45 °C for a pickling time of 9 h to remove SiO x, porous silicon microspheres without silicon oxide are obtained, improving the first charge-discharge efficiency of the material;

[0078] (3) The acid-washed porous silicon microspheres in (2) are subjected to the first carbon coating treatment by chemical vapor deposition, using methane as the carbon source, nitrogen as the carrier gas, and the pyrolysis temperature is 1000 °C; controlling the reaction time, a primary carbon coating layer with a thickness of 30 nm is obtained;

[0079] (4) The primary-coated silicon-carbon anode material in (3) is dispersed in an organic carbon source solution, the solvent is stirred and evaporated, the organic carbon source is dopamine, the concentration is 40 wt%, ethanol is the solvent, the solution viscosity is 1000 mPa·s, and then high-temperature pyrolysis is carried out under a nitrogen protective atmosphere, the pyrolysis temperature is 700 °C, controlling the amount of dopamine used, a secondary carbon coating layer with a thickness of 1 μm is obtained, and a silicon-carbon anode material with a secondary carbon coating is obtained.

[0080] The areal density of the silicon-carbon anode is 2 mg cm -2 , the electrolyte solute is LiPF 6 , the solvent is EC, DMC, DEC, and the volume ratio is 0.3:0.3:0.4, and the concentration of the solute is 1 mol L -1 . A half-cell test is carried out, and charge-discharge tests are carried out between 0.01 and 1.5 V (at a current density of 50 mAg -1 The first three charge-discharge tests are carried out, and the cycle test is carried out at a current density of 200 mA g -1 ), and the electrochemical performance is shown in Table 3.

[0081] Example 4

[0082] Adjust the viscosity of the organic carbon source solution in step (4) of Example 1 to 150 and 5000 mPa·s respectively, control other preparation conditions to be the same, prepare the material, assemble a half-cell for testing, and the testing mechanism is the same as that of Example 1. The electrochemical performance of the silicon-carbon anode materials prepared at the three viscosities is shown in Table 1.

[0083] Table 1 Performance comparison of silicon-carbon anode materials prepared from small-particle-size Al-Si alloys and large-particle-size Al-Si alloys

[0084]

[0085] As can be seen from Table 1, the material prepared at a viscosity of 600 mPa·s in Example 1 has the best performance. The viscosity is moderate, and the solution will not enter the pores between carbon fibers at all, completely retaining the pores, leaving sufficient space for the silicon volume effect, and a complete and uniform-thickness coating on the surface of the microspheres can be achieved. Therefore, the material has the highest reversible capacity and cycling performance. When the viscosity is 150 mPa·s, the viscosity is low, and the pores between silicon fibers will not be completely infiltrated by the organic carbon source solution, reducing the space reserved for the expansion and contraction of silicon. Stress accumulates during the cycling process, and the material will break, resulting in a decline in cycling performance. When the viscosity is 5000 mPa·s, the viscosity is too high, and it is difficult for the organic carbon source to completely coat the silicon microspheres, and the coating thickness is non-uniform. The mechanical strength of the material is weak, and it is difficult to maintain a stable structure during cycling. Therefore, the electrochemical performance of the material is also poor.

[0086] To highlight the beneficial effects of the present invention, the following comparative examples are set for the examples:

[0087] Comparative Example 1

[0088] Change step (1) in Example 1 to use a large-particle-size aluminum-silicon alloy, D 50 = 10 μm (see attached Figure 2 ), and the silicon content is 20 wt%; the porous silicon microspheres obtained after acid etching of this alloy particle are formed by the aggregation of a large number of micron-sized silicon particles (see attached Figure 4 ), while a silicon nanofiber framework is obtained after acid dissolution of the self-made aluminum-silicon alloy (see attached Figure 3 ). Control other experimental conditions to be the same, assemble a half-cell for testing, and the testing mechanism is the same as that in Example 1. The electrochemical performance of the prepared silicon-carbon negative electrode material is shown in Table 2.

[0089] Table 2 Performance comparison of silicon-carbon negative electrode materials prepared from small-particle-size and large-particle-size Al-Si alloys

[0090]

[0091]

[0092] As can be seen from Table 2, the silicon-carbon negative electrode material prepared from small-particle-size aluminum-silicon alloy powder has a more excellent internal structure. The silicon in the negative electrode particles is distributed in a radial fiber shape and is at the nanoscale. Although there will be expansion and contraction during charge and discharge, it is not easy to break; and the pores between the double carbon coatings provide a buffer space for the expansion and contraction of the silicon nanofibers in the particles, and the structure is more stable, which can effectively resist the silicon volume effect. Therefore, its electrochemical performance is more excellent.

[0093] Comparative Example 2

[0094] Modify step (3) in Example 2 without performing internal carbon coating by chemical vapor deposition; control other experimental conditions to be the same, and finally obtain a silicon-carbon anode material with a single surface carbon coating. Assemble a half-cell for testing, and the testing mechanism is the same as that in Example 2. The electrochemical performance of the prepared silicon-carbon anode material is shown in Table 3.

[0095] Table 3 Performance comparison of silicon-carbon anode materials without internal carbon coating and with secondary carbon coating

[0096]

[0097] As can be seen from Table 3, the performance of the silicon-carbon anode material with secondary carbon coating is more excellent. For the silicon-carbon anode material without internal carbon coating, due to the poor conductivity of the internal silicon fibers, compared with the silicon-carbon anode material with secondary carbon coating, although the cycle retention rate is similar, the overall capacity is low and the electrochemical performance has decreased.

[0098] Comparative Example 3

[0099] Modify the order of steps (3) and (4) in Example 3, control other experimental conditions to be the same, and finally only obtain a silicon-carbon anode material with two surface carbon coatings. Assemble a half-cell for testing, and the testing mechanism is the same as that in Example 3. The electrochemical performance of the prepared silicon-carbon anode material is shown in Table 4.

[0100] Table 4 Performance comparison of silicon-carbon anode materials prepared in Example 3 and Comparative Example 3

[0101]

[0102] As can be seen from Table 4, the performance of the silicon-carbon anode material after changing the order of the two carbon coating processes becomes worse. When the organic carbon source liquid-phase coating is carried out first, due to the too high viscosity of the solution, the organic carbon source is difficult to enter the gaps between the silicon fibers, and only the surface of the porous silicon microspheres can be coated. And for the subsequent chemical vapor deposition, due to the existing carbon coating layer on the surface, it also cannot enter the interior of the gaps. Therefore, there is no carbon coating on the surface of the internal silicon fibers and the conductivity is poor. So the electrochemical performance of the prepared silicon-carbon anode material is poor.

[0103] Comparative Example 4

[0104] To further highlight the importance of the viscosity of the organic carbon source solution in the second carbon coating, modify step (4) in Example 3. The concentration of the organic carbon source used for the second external carbon coating is reduced to 10 wt%, and the corresponding solution viscosity is reduced to 50 mPa·s; control other experimental conditions to be the same, and finally obtain a silicon-carbon anode material with secondary carbon coating. Assemble a half-cell for testing, and the testing mechanism is the same as that in Example 3. The electrochemical performance of the prepared silicon-carbon anode material is shown in Table 5.

[0105] Table 5 Performance Comparison of Silicon Carbon Anode Materials Coated with High-viscosity and Low-viscosity Organic Carbon Sources

[0106]

[0107] As can be seen from Table 5, the silicon carbon anode material coated twice with a high-concentration organic carbon source solution has more excellent performance. When the secondary coating is carried out with a lower-concentration organic carbon source solution, the voids between silicon nanofibers are completely infiltrated, and the silicon carbon anode material with secondary coating is obtained after pyrolysis. There is no reserved buffer space for silicon expansion and contraction inside the particles. During the cycling process, stress accumulates continuously, the particles are easily broken, and the spherical morphology cannot be maintained. The broken materials are easily lose electrical contact, affecting the capacity performance. Compared with the silicon carbon anode material coated twice with a high-concentration organic carbon source solution, although the performance of the silicon carbon anode material coated twice with a low-concentration organic carbon source solution is similar in the early stage of cycling, after the particles are gradually broken in the later stage, the reversible capacity and cycling retention rate deteriorate sharply.

[0108] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure, characterized in that: The steps include: S1: Using metal-silicon alloy powder as raw material, pickling with 1 times excess 20-40 wt% hydrochloric acid, reacting in a water bath at 30-50 °C for 24-48 h, to obtain porous silicon microspheres with radial nanofiber structure; the metal-silicon alloy powder is prepared by induction melting + rapid solidification technology; the rapid solidification technology is at least one of water atomization, gas atomization, ultrasonic atomization, and rotating disk atomization; the cooling rate of the rapid solidification technology is 10 7 ~10 8 K / s; using small-size metal silicon alloy powder and ultra-high cooling rate to make all silicon in the alloy powder precipitate in the form of radial nanofibers; S2: The porous microspheres having a radial nanofiber structure are washed with hydrofluoric acid to remove SiO x , to obtain porous silicon microspheres free of silicon oxide; S3: performing a carbon coating reaction on the porous silicon microspheres containing no silicon oxide by chemical vapor deposition to obtain a silicon-carbon negative electrode material intermediate; S4: dispersing the silicon-carbon negative electrode material intermediate in an organic carbon source solution, stirring to evaporate the solvent, and then pyrolyzing at high temperature to obtain a secondary coated silicon-carbon negative electrode material; The metal-silicon alloy powder particles are spherical or quasi-spherical, with a particle size of 0.1 to 5 μm; The chemical vapor deposition is conducted with carbon-containing gas and carrier gas; the carbon-containing gas comprises at least one of alkanes, alkenes, alkynes, and benzene rings; The viscosity of the organic carbon source solution is 500-4000 mPa·s; Among them, the thickness of the primary carbon coating layer is 5~100 nm; the thickness of the secondary carbon coating layer is 0.1~2 μm; there is a gap between the primary carbon coating layer and the secondary carbon coating layer; the volume of the gap accounts for 10%~30% of the volume of the silicon-carbon negative electrode material.

2. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: Step S1, the metal-silicon alloy is at least one of iron-silicon, manganese-silicon, aluminum-silicon, magnesium-silicon, zinc-silicon, calcium-silicon, lead-silicon, and nickel-silicon, the silicon content in the metal-silicon alloy is 5%~50%, and the metal content in the metal-silicon alloy is 50~95%.

3. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: Step S1, the silicon nanofibers in the porous silicon microspheres are radially distributed from the center, the diameter of the silicon nanofibers is 5 nm~1 μm, and the length of the silicon nanofibers is 0.1~5 μm; the porosity of the porous silicon microspheres is 0.5~0.95, and the specific surface area is 10~1000 m 2 g -1 .

4. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: In step S2, the concentration of hydrofluoric acid is 1-40 wt.%; the pickling time is 1-24 h, and the temperature is 25-60 °C.

5. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: In step S3, the temperature of chemical vapor deposition is 600-1000°C.

6. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: In step S3, the carrier gas includes at least one of hydrogen, nitrogen, helium, neon and argon.

7. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: Step S4, the organic carbon source solution includes an organic carbon source and a solvent; the organic carbon source is at least one of glucose, sucrose, citric acid, cellulose, dopamine salt, formaldehyde, phenolic resin, xylenol, resorcinol, polyacrylonitrile, polypyrrole, polyaniline, and polythiophene, with a concentration of 5 to 70 wt.%; the solvent is at least one of water, ethanol, and acetone.

8. The method for preparing a silicon-carbon negative electrode material with a hierarchical porous structure according to claim 1, characterized in that: Step S4, high temperature pyrolysis is carried out under a protective atmosphere, the protective atmosphere is at least one of nitrogen, helium, and argon, and the temperature is 300-1200°C.

9. A silicon-carbon negative electrode material with a hierarchical porous structure obtained by the preparation method according to any one of claims 1 to 8.

10. Use of a silicon-carbon negative electrode material with a hierarchical porous structure obtained by the preparation method according to any one of claims 1 to 8 in a lithium-ion battery.

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