A double-layer coated silicon-based negative electrode material and a preparation method thereof

Abstract

The application provides a double-layer coated silicon-based negative electrode material and a preparation method thereof, the negative electrode material comprises a core body, a carbon coating layer and a carbon wrapping layer from inside to outside; the core body is uniformly arranged silicon and silicate, the carbon coating layer is dense carbon with an amorphous structure, and the carbon wrapping layer is a porous network structure formed by carbon nanotubes interlaced with each other; the proportions of silicon, silicate and carbon in the negative electrode material are 40-80%, 10-57% and 1-10% respectively. The double-layer coated silicon-based negative electrode material prepared by the application can effectively slow down expansion, improve the conductivity of the material, ensure high-rate performance in the application process of the battery, and effectively reduce the addition of single-walled carbon nanotubes in the slurry preparation process, thereby further reducing the preparation cost of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery materials, and in particular relates to a double-layer coated silicon-based negative electrode material and a preparation method thereof. Background Art

[0002] The current market demand for high-capacity and high-rate lithium-ion batteries is growing. Traditional graphite, due to its relatively low capacity (372mAh / g), cannot meet the capacity requirements of cylindrical batteries of 3Ah and above (represented by 18650). Silicon negative electrodes have a higher capacity (theoretical gram capacity of 4200mAh / g), and when paired with high-nickel positive electrode materials, the battery capacity can reach over 3.4Ah. However, the large volume expansion of silicon during charging and discharging, as well as the continuous generation and consumption of the SEI film during cycling, lead to poor cycling performance of the battery, ultimately hindering its industrialization process.

[0003] The current strategies adopted to address the above problems are: silicon nano-sizing, carbon coating, and construction of low-expansion hybrid structures. Silicon nano-sizing is mainly prepared by vapor deposition and physical grinding. Although the vapor phase method is expensive, the prepared nano-silicon grains are smaller in size, which is more conducive to reducing the impact of volume expansion. Carbon coating is mainly solid-phase and vapor-phase coating, among which vapor phase coating can effectively coat the matrix material completely, but the loss rate is high and the cost is high. Low-expansion composite structures mainly include porous silicon materials and core-shell silicon-based composite material structures. Among them, the core-shell composite structure has become the mainstream of current applications due to its higher material density. Although the above-mentioned methods can alleviate the problems in the application process of silicon negative electrodes to a certain extent, the large volume expansion after lithium insertion, poor conductivity, and structural instability during lithium insertion and deintercalation still cannot be effectively solved. Therefore, the development of a low-expansion, high-initial-efficiency, low-cost silicon-based negative electrode material is of great significance for the large-scale application of silicon negative electrode materials. Summary of the Invention

[0004] In order to overcome the technical problems in the prior art of large volume expansion, poor conductivity and structural instability of silicon-based negative electrode materials after lithium insertion and deintercalation, the present invention provides a double-layer coated silicon-based negative electrode material and a preparation method thereof. The prepared double-layer coated silicon-based negative electrode material can effectively slow down the expansion, improve the conductivity of the material, and ensure high rate performance during battery application. At the same time, the high conductivity can effectively reduce the addition of single-walled carbon nanotubes in the back-end pulping process, further reducing the preparation cost of the battery.

[0005] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0006] The first aspect of the present invention provides a double-layer coated silicon-based negative electrode material, which comprises a core body, a coating carbon layer and a wrapping carbon layer from the inside to the outside;

[0007] The core body is uniformly arranged silicon and silicate, the coating carbon layer is dense carbon with an amorphous structure, and the coating carbon layer is a porous network structure formed by interlaced carbon nanotubes;

[0008] The proportions of silicon, silicate and carbon in the negative electrode material are 40-80%, 10-57% and 1-10% respectively.

[0009] The silicate in the core body of the present invention can provide a buffer space for the expansion of nano-silicon; the dense carbon layer with an amorphous structure is used to increase the conductivity of the material while providing expansion stress space for the particles; the outer layer is provided with a coating carbon layer, and the coating carbon layer is formed by carbon nanotubes interlaced with each other, and the coating carbon layer is formed into a "cage structure", which can maintain structural stability while slowing down expansion and improving the conductivity of the material; in addition, there are extended linear carbon nanotubes and flaky graphene structures on the outer surface of the coating carbon layer, which can provide stress resistance expansion during the process of lithium insertion and removal of the particles, further reducing the overall expansion of the material.

[0010] As an optional embodiment, in the double-layer coated silicon-based negative electrode material provided by the present invention, the silicon is nano-silicon, which is one of crystalline silicon and amorphous silicon, and the size of the crystalline silicon is 0.1-4 nm.

[0011] As an optional embodiment, in the double-layer coated silicon-based negative electrode material provided by the present invention, the particle size of the negative electrode material D50 is 4-15 um.

[0012] In the present invention, the nano-silicon is configured to be crystalline silicon or amorphous silicon, and has a size of 0.1-4 nm, so as to provide high capacity while reducing the overall volume expansion of the nano-silicon.

[0013] As an optional embodiment, in the double-layer coated silicon-based negative electrode material provided by the present invention, the coated carbon layer has a graphite-like structure, and the thickness of the coated carbon layer is 3-30 nm.

[0014] The coated carbon layer in the present invention is a dense carbon layer, specifically a graphite-like layer, which can enhance the electrical conductivity and the density of the coated carbon layer.

[0015] As an optional embodiment, in the double-layer coated silicon-based negative electrode material provided by the present invention, the carbon layer thickness of the coated carbon layer is 3-30nm, the outer surface of the coated carbon layer contains flaky graphene, and the diameter of the graphene sheet is 20-100nm.

[0016] The outer surface of the carbon coating layer in the present invention is a flaky graphene structure, which can provide stress resistance and expansion resistance for the particles during lithium insertion and extraction, thereby further reducing the overall expansion of the material.

[0017] As an optional embodiment, in the double-layer coated silicon-based negative electrode material provided by the present invention, the silicate is one or more of Li2Si2O5, Li2SiO3, Na4SiO4, Mg2SiO4, Na2MgSiO4, K4SiO4, and K2MgSiO4.

[0018] A second aspect of the present invention provides a method for preparing a double-layer coated silicon-based negative electrode material, comprising the following steps:

[0019] S1, SiO x The precursor material is crushed to obtain material A;

[0020] S2, uniformly dispersing the material A, carbon source and inorganic salt obtained in step S1 and then performing liquid phase coating to obtain material B;

[0021] S3, depolymerizing the material B obtained in step S2 and uniformly mixing it with the catalyst to obtain material C;

[0022] S4. Place the material C obtained in step S3 in a vapor deposition device for carbon coating. The reaction conditions are 700-900°C and the reaction time is 3-5h. During the deposition process, argon is used as a protective gas and the carbon source is an organic carbon source. After coating, a double-layer coated silicon-based negative electrode material is obtained.

[0023] The function of step S2 in the present invention is to obtain the core body and liquid-phase coat a dense carbon layer with an amorphous structure, thereby providing a carbon growth site for the next gas-phase coating. At the same time, liquid-phase coating can also improve the utilization rate of the front-end material; the function of step S3 is to form a uniform mixture of material B and the catalyst; in step S4, a vapor deposition method is adopted, and under the action of the catalyst, a carbon source is introduced into the growth site to further grow linear carbon nanotubes, so that they form an interlaced porous network-like coated carbon layer structure. At the same time, some carbon nanotubes will extend out of the coated carbon layer, and at the same time, a flaky graphene structure is also grown on the outside of the coated carbon layer, providing stress resistance expansion for the particles during lithium deintercalation and further reducing the overall expansion of the material.

[0024] As an optional embodiment, in the preparation method provided by the present invention, SiO x The precursor material is gas-phase amorphous non-crystalline, with D50=0.01-10 mm and x=0.1-1.2.

[0025] As an optional embodiment, in the preparation method provided by the present invention, D50 of material A in step S1 is 3-10 mm.

[0026] As an optional embodiment, in the preparation method provided by the present invention, the crushing equipment in step S1 is one or a combination of air flow mill, vertical cone mill, universal grinder, mechanical mill, impact mill.

[0027] As an optional embodiment, in the preparation method provided by the present invention, the mass ratio of material A, carbon source and inorganic salt in step S2 is (0.1-10): (0.01-10): (0.01-10).

[0028] As an optional embodiment, in the preparation method provided by the present invention, the carbon source in step S2 is one or more of sucrose, glucose, starch, polyethylene, polypropylene alcohol, polypyrrole, polypropylene ester, polyacrylic acid, polyacrylamide, polypropylene pyrrolidone, phenolic resin, etc.

[0029] As an optional embodiment, in the preparation method provided by the present invention, the inorganic salt in step S2 is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, lithium acetate, lithium bromide, methyl lithium, phenyl lithium, tert-butyl lithium, sodium carbonate, and potassium carbonate.

[0030] As an optional embodiment, in the preparation method provided by the present invention, the mass ratio of material B to the catalyst in step S3 is (0.1-10): (0.001-0.1).

[0031] As an optional embodiment, in the preparation method provided by the present invention, the catalyst in step S3 is one or more of ferrocene, ferric oxalate, melamine, titanium dioxide, copper oxide, zirconium oxide, zinc oxide, and magnesium oxide.

[0032] As an optional embodiment, in the preparation method provided by the present invention, the organic carbon source in step S4 is one or more of methane, ethane, ethylene, acetylene, propylene, propane, butadiene, and benzene.

[0033] As an optional embodiment, in the preparation method provided by the present invention, the vapor deposition device in step S4 is one of a rotary kiln, a microwave vapor deposition furnace, a fluidized bed, an atomic layer deposition furnace, and a vacuum deposition furnace.

[0034] As an optional embodiment, in the preparation method provided by the present invention, the molar ratio of the organic carbon source to argon in step S4 is (0.1-10): (0.01-1).

[0035] A third aspect of the present invention provides a lithium-ion battery comprising the above-mentioned double-layer coated silicon-based negative electrode material.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The double-layer coated silicon-based negative electrode material prepared by the present invention consists of a core body, a coating carbon layer and a wrapping carbon layer. The middle layer is a dense carbon layer of an amorphous structure prepared by liquid phase coating, which is used to increase the conductivity of the material while providing expansion stress space for the particles. The outer layer is provided with a wrapping carbon layer, and the wrapping carbon layer is formed by carbon nanotubes interlaced with each other, forming a "cage structure" of the wrapping carbon layer. Its function is to maintain structural stability while slowing down expansion and improving the conductivity of the material. At the same time, there are extended linear carbon nanotubes and flaky graphene structures on the outer surface of the wrapping carbon layer, which can provide stress resistance expansion for the particles during lithium insertion and removal, further reduce the overall expansion of the material, and provide a rapid electron migration channel for the negative electrode material.

[0038] (2) The synthesis method of the present invention uses two methods, liquid phase coating and gas phase coating, in sequence. Liquid phase coating makes the formed coated carbon layer denser, more conductive, and more uniform. At the same time, the use of liquid phase coating can also improve the utilization rate of the front-end material and provide growth sites for the back-end gas phase deposition. Gas phase deposition coating can make carbon nanotubes deposited on the coated carbon layer in a directional manner by adjusting the process parameters and catalysts to obtain a coated carbon layer. There are also extended carbon nanotubes or grown sheet-like graphene on the outer surface of the coated carbon layer, which can also replace the carbon nanotubes added in the battery manufacturing process. In addition, the use of double-layer carbon coating can greatly improve the utilization rate of crushed fine powder and reduce production costs. The synthesis method is simple, convenient and effective, suitable for large-scale production of silicon negative electrodes. At the same time, the high conductivity can effectively reduce the addition of single-walled carbon nanotubes in the back-end pulping process, further reducing costs.

[0039] (3) The double-layer coated silicon-based negative electrode material prepared by the present invention has a compressive strength of more than 89 MPa, an initial efficiency of more than 87%, a reversible capacity of more than 1350 mAh / g, and an expansion rate of less than 18%. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0041] Figure 1 Schematic diagram of the structure of the double-layer coated silicon-based negative electrode material prepared by the present invention;

[0042] Figure 2 This is an SEM image of the double-layer coated silicon-based negative electrode material prepared in Example 1;

[0043] Figure 3 for Figure 2 Magnified image of;

[0044] Figure 4 for Figure 3 Magnified image of;

[0045] Figure 5 This is the XRD pattern of the double-layer coated silicon-based negative electrode material prepared in Example 1;

[0046] Figure 6 TEM image of the double-layer coated silicon-based negative electrode material prepared in Example 1;

[0047] Figure 7 is a SEM image of the negative electrode material prepared in Comparative Example 1;

[0048] Figure 8 This is the SEM image of the silicon-based negative electrode material prepared in Comparative Example 4.

[0049] Reference numerals:

[0050] 1. Nano-silicon; 2. Silicate; 3. Coated carbon layer; 4. Coated carbon layer. DETAILED DESCRIPTION

[0051] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0052] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0053] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0054] 1. Structure of double-layer coated silicon-based anode material

[0055] The negative electrode material in this application includes a core body, a coating carbon layer 3 and a coating carbon layer 4 from the inside out, wherein the core body is uniformly arranged nano-silicon 1 and silicate 2, the coating carbon layer 3 is dense carbon with an amorphous structure, and the outer surface of the coating carbon layer 4 contains linear carbon nanotubes or sheet-like graphene. The structural diagram is shown in FIG. Figure 1 shown.

[0056] The silicate 2 in the core body is evenly distributed around the nano-silicon, which can provide a buffer space for the expansion of the nano-silicon; the amorphous structure of the coated carbon layer 3 is coated on the outside of the core body, which increases the conductivity of the material and provides expansion stress space for the particles. The outer layer is provided with a coated carbon layer 4, which is a porous network structure formed by the interlaced carbon nanotubes, forming a "cage structure" of the coated carbon layer 4. Its function is to maintain structural stability while slowing down the expansion and improving the conductivity of the material. At the same time, there are also extended linear carbon nanotubes and flaky graphene structures on the outer surface of the coated carbon layer 4, such as Figure 2-4 As shown, it can provide stress resistance expansion for particles during lithium insertion and extraction, further reducing the overall expansion of the material, and at the same time provide a rapid electron migration channel for the negative electrode material.

[0057] The proportions of silicon, silicate and carbon in the negative electrode material are 40-80%, 10-57% and 1-10% respectively, and the particle size of the negative electrode material is D50=4-15um.

[0058] Specifically, the silicon is nano-silicon, which is crystalline silicon or amorphous silicon, and the size of the crystalline silicon is 0.1-4 nm.

[0059] Specifically, the coating carbon layer 3 is a graphite-like structure, and the thickness of the carbon layer is 3-30 nm.

[0060] Specifically, the thickness of the carbon layer of the coating carbon layer 4 is 3-30 nm, the diameter of the carbon nanotube is 10-50 nm, and the diameter of the graphene sheet is 20-100 nm.

[0061] Specifically, the silicate 2 is one or more of Li2Si2O5, Li2SiO3, Na4SiO4, Mg2SiO4, Na2MgSiO4, K4SiO4, and K2MgSiO4.

[0062] 2. Preparation of negative electrode materials.

[0063] Example 1:

[0064] (1) Weigh 5000 g of silicon oxide particles with a D50 of 3 mm and crush them using a pneumatic crusher; the equipment feeding frequency is 10 Hz and the torque is adjusted to 30-35% to obtain a precursor material with a D50 of 13 μm.

[0065] (2) Disperse 10 g of sucrose and 100 g of lithium carbonate into 2000 g of aqueous solution, and stir with an electric stirrer at 1500 rpm for 2 h to disperse uniformly; then gradually add 500 g of precursor material and continue stirring for 5 h; place the dispersed material in a forced air drying oven at 110 ° C for 15 h to dry.

[0066] (3) 500 g of the dried material was placed in a universal grinder at 500 rpm for 20 min to disaggregate and disperse the material, and 5 g of ferrocene was added to the dried material and stirred in a VC mixer at 500 rpm for 1 h to mix uniformly.

[0067] (4) 500 g of the mixed material was coated with carbon by vapor deposition at 900 °C for 4 h. During the deposition process, argon was used as the protective gas, methane was used as the carbon source, the volume ratio of the carrier gas and the carbon source was 1:1, and the flow rate of the mixed gas was 1 L / min. After the reaction was completed, a double-layer coated material was obtained.

[0068] Example 2

[0069] (1) Weigh 5000 g of silicon dioxide particles with a D50 of 10 mm and crush them using a mechanical mill with a feeding frequency of 5 Hz and a crushing frequency of 60 Hz to obtain a precursor material with a D50 of 8 μm.

[0070] (2) Disperse 30 g of polyacrylamide and 100 g of sodium hydroxide into 1000 g of aqueous solution, and stir with an electric stirrer at 1500 rpm for 1 h to disperse uniformly; then gradually add 500 g of precursor material and continue stirring for 2 h; place the dispersed material in a forced air drying oven at 110 ° C and dry it for 15 h.

[0071] (3) 500 g of the dried material was placed in a depolymerizer, the crushing frequency was set to 30 Hz, and the time was 20 min for depolymerization and dispersion. 10 g of melamine was added to the dried material and mixed uniformly using a VC mixer at 500 rpm for 1 h.

[0072] (4) 500 g of the mixed material was coated with carbon by vapor deposition at 800 °C for 4 h. During the deposition process, nitrogen was used as the protective gas, acetylene was used as the carbon source, the volume ratio of the carrier gas to the carbon source was 12:1, and the flow rate of the mixed gas was 1 L / min. After the reaction was completed, a double-layer coated material was obtained.

[0073] Example 3

[0074] (1) Weigh 5000 g of silicon dioxide particles with a D50 of 8 mm and use a ball mill as a crushing device. Adjust the ball mill speed to 2000 rpm and crush for 2 h to obtain a precursor material with a D50 of 3 μm.

[0075] (2) Disperse 70 g of polyvinyl pyrrolidone and 200 g of lithium hydroxide into 2000 g of isopropanol solution, and disperse uniformly by stirring at 1500 rpm for 1 h; then gradually add 500 g of precursor material and continue stirring at 2000 rpm for 2 h; place the dispersed material in a vacuum drying oven at 100 ° C for 12 h for drying.

[0076] (3) 500 g of the dried material was placed in a wheel mill at 1000 rpm for 30 min to disaggregate and disperse the material, and 5 g of ferric oxalate was added to the dried material and stirred in a VC mixer at 1000 rpm for 1 h to mix uniformly.

[0077] (4) 500 g of the mixed material was coated with carbon by vapor deposition at 700 °C for 4 h. During the deposition process, argon was used as the protective gas, a mixture of methane and acetylene in a molar ratio of 1:1 was used as the carbon source, the volume ratio of the carrier gas to the carbon source was 1:2, and the flow rate of the mixed gas was 3 L / min. After the reaction was completed, a double-layer coated material was obtained.

[0078] Comparative Example 1

[0079] (1) Weigh 5000 g of silicon oxide particles with a D50 of 3 mm, adjust the feeding frequency of the air crusher to 10 Hz, the torque to 30-35%, and the classifier frequency to 190 Hz to obtain a precursor material with a D50 of 4 μm and a diameter of 1.5, with a yield of 75%.

[0080] (2) Disperse 10 g of lithium carbonate into 200 g of aqueous solution and stir with an electric stirrer at 1500 rpm for 2 h to disperse uniformly; then gradually add 500 g of precursor material and continue stirring for 5 h; place the dispersed material in a forced air drying oven at 110 ° C for 15 h to dry.

[0081] (3) 500 g of the depolymerized material was coated with carbon by deposition at 900 °C in a vapor deposition device for 4 h. During the deposition process, argon was used as the protective gas, methane gas was used as the carbon source, the volume ratio of the carrier gas to the carbon source was 1:1, and the mixed gas flow rate was 2 L / min. After the reaction was completed, the coated material was obtained.

[0082] Comparative Example 2

[0083] (1) Weigh 5000 g of silicon dioxide particles with a D50 of 3 mm, adjust the feeding frequency of the air crusher to 10 Hz, the torque to 30-35%, and the classifier frequency to 185 Hz to obtain a precursor material with a D50 of 8 μm and a diameter of 2, with a yield of 85%.

[0084] (2) 500 g of precursor material was taken and coated with carbon by deposition at 900 ° C for 4 hours in a vapor deposition device. During the deposition process, argon was used as the protective gas, methane gas was used as the carbon source, the volume ratio of the carrier gas and the carbon source was 1:1, and the mixed gas flow rate was 2 L / min; after the reaction was completed, the coated material was obtained.

[0085] Comparative Example 3

[0086] (1) Weigh 5000 g of silicon dioxide particles with a D50 of 8 mm and use an impact mill as a crushing device. Adjust the impact mill crushing frequency to 25 Hz and the classification frequency to 58 Hz to obtain a precursor material with a D50 of 10 μm.

[0087] (2) Disperse 50 g of polyethylene into 2000 g of aqueous solution and stir the mixture with an electric stirrer at 1000 rpm for 4 h until uniform dispersion is achieved; then gradually add 500 g of the precursor material and continue stirring at 2000 rpm for 2 h; place the dispersed material in a vacuum drying oven at 110°C for 10 h for drying.

[0088] (3) 500 g of the dried material was placed in a depolymerizer with a crushing frequency of 50 Hz and a classification frequency of 60 Hz. The material was depolymerized and dispersed for 30 minutes. 5 g of melamine was added to the dried material and stirred in a VC mixer at 1000 rpm for 1 hour to mix uniformly.

[0089] (4) 500 g of the mixed material was coated with carbon by vapor deposition at 1000 °C for 6 h. During the deposition process, nitrogen was used as the protective gas, a mixture of methane and acetylene in a molar ratio of 1:1 was used as the carbon source, the volume ratio of the carrier gas to the carbon source was 1:2, and the flow rate of the mixed gas was 3 L / min. After the reaction was completed, a double-layer coated material was obtained.

[0090] Comparative Example 4

[0091] (1) Weigh 5000 g of silicon oxide particles with a D50 of 3 mm and crush them using a pneumatic crusher; the equipment feeding frequency is 10 Hz and the torque is adjusted to 30-35% to obtain a precursor material with a D50 of 13 μm.

[0092] (2) Disperse 10 g of sucrose and 100 g of lithium carbonate into 2000 g of aqueous solution, and stir with an electric stirrer at 1500 rpm for 2 h to disperse uniformly; then gradually add 500 g of precursor material and continue stirring for 5 h; place the dispersed material in a forced air drying oven at 110 ° C for 15 h to dry.

[0093] (3) 500 g of the dried material was placed in a universal grinder at 500 rpm for 20 min to depolymerize and disperse the material, and then carbon was deposited in a vapor deposition device at 900 °C for 4 h. During the deposition process, argon was used as the protective gas, methane was used as the carbon source, the volume ratio of the carrier gas to the carbon source was 1:1, and the mixed gas flow rate was 1 L / min. After the reaction was completed, a double-layer coated material was obtained.

[0094] 3. Performance Testing

[0095] (1) Physical indicator testing

[0096] The double-layer coated negative electrode materials prepared in Examples 1-3 and the materials prepared in Comparative Examples 1-3 were tested for particle size, carbon content, silicon content, and silicate content. The test results are shown in Table 1 below.

[0097] Table 1: Particle size and content detection

[0098] serial number Particle size (D50) Carbon content (%) Silicon content (%) Silicate content (%) Example 1 15um 1 40.8 56.7 Example 2 9um 5 78.0 16.8 Example 3 4um 10 49.2 40.0 Comparative Example 1 5um 3 63.1 32.6 Comparative Example 2 4.5um 2 77.6 0 Comparative Example 3 12um 10 59.2 0 Comparative Example 4 14um 1 43.3 54.2

[0099] Note: In Comparative Examples 2-3, except for the silicon content, the other elements are mainly O content.

[0100] As can be seen from Table 1, in the double-layer carbon-coated negative electrode material prepared by the present invention, D50 is in the range of 4-15 μm, and the measured silicate, silicon and carbon are also within the protection scope of the present invention, ensuring the electrochemical performance.

[0101] (2) Electron microscopy

[0102] The double-layer carbon-coated negative electrode material prepared in Example 1 and the negative electrode material prepared in Comparative Example 1 were subjected to scanning electron microscopy (SEM) testing, and the test results are as follows. Figure 2 and 3 It can be seen that the outer carbon layer of the double-layer carbon-coated negative electrode material prepared in Example 1 has carbon nanotubes with a tube diameter of about 30 nm and graphene sheet-like carbon with a sheet diameter of about 50 nm. Figure 4 for Figure 3 A further enlarged view of Figure 4 It can be seen that the coating carbon layer is formed by the interlacing of carbon nanotubes, forming a coating layer with a certain gap. The gap is the inner coating carbon layer of the double-layer carbon-coated negative electrode material, and the arrow indicates the sheet-like graphene structure. Figure 6 A double carbon layer can be observed in the PDMS film, and the thickness of the coating carbon layer is 28.3 nm, which is within the range of 3-3 nm in the present invention. Figure 7 The negative electrode material prepared in Comparative Example 1 is a negative electrode material prepared without liquid phase coating. Figure 7 It can be seen that the surface of the negative electrode material that has not been liquid-coated is relatively rough because there is no dense internal carbon layer. Figure 8 The negative electrode material prepared in Comparative Example 4 is different from that in Example 1. The carbon nanotubes prepared therefrom are not intertwined to form a porous network structure, and no flaky graphene structure is generated on the outside of the carbon nanotubes. The figure shows that a layer of disordered carbon nanotubes or carbon fibers is deposited on the surface of the particles, making it difficult to achieve the growth of wrapped carbon tubes similar to that in Example 1.

[0103] Figure 5 This is the XRD characterization diagram of the double-layer carbon-coated negative electrode material prepared in Example 1. From the diagram, it can be calculated that the size of the nano-silicon grains is 2.6 nm, the silicon content is 40.8%, and the content of silicate is 56.7%.

[0104] (3) Electrical performance test

[0105] The negative electrode materials prepared in Examples 1-3 and Comparative Examples 1-4 were subjected to a buckling performance test: silicon-based negative electrode material: CMC: SBR = 7:2:1 were slurried, the solid content of the slurry was controlled at 45%, and the slurry was coated on a copper foil current collector to prepare a negative electrode sheet, and the compaction density of the sheet was controlled at 1.3-1.5 g / cm 3 A 2032 coin-shaped cell was assembled using a lithium metal sheet as the counter electrode and a 1 mol / L LiPF6 / EC+DMC electrolyte. The cell was tested using the LAND battery test system with constant current charge and discharge at 0.1C over a voltage range of 0.001-1.5V. Rate performance was tested using 18650 cylindrical cells with a capacity of 3.2Ah. The electrode expansion rate at full charge was measured, as well as the capacity retention at 1C / 8C. The test results are shown in Table 2 below.

[0106] Table 2: Characterization parameters of silicon-based anode

[0107]

[0108] As can be seen from Table 2, after the negative electrode materials prepared by Examples 1-3 and Comparative Examples 1-4 were subjected to the buckle performance test, there were certain differences in the ionic compressive strength and expansion rate. The negative electrode materials in Examples 1-3 were double-layer carbon coated, and the expansion rate was less than 18%, while the negative electrode materials in Comparative Example 1-2 were single-layer coated, and the expansion rate was above 21%, indicating that the first-layer coated carbon layer not only served as a conductive layer, but also provided anti-expansion stress for the particles, which is an effect that cannot be achieved by relying on a single coating layer alone. Compared with Comparative Example 2, the button batteries prepared in Examples 1-3 not only have a high first efficiency, a low expansion rate, but also a higher high-rate discharge capacity retention rate, indicating that the double-layer coating and the silicate structure can provide the particles with expansion space and strong stress resistance, ultimately achieving the effect of reducing the expansion of the battery pole piece; at the same time, the double-layer coating layer can still maintain a discharge capacity of more than 94% at a high rate, indicating that the conductivity of the particles under the double-layer coating is better exerted, and the rate performance is more excellent. Compared to Comparative Example 3, which directly liquid- and vapor-coated the precursor material, Example 1 achieved an initial efficiency of 91.2%, far exceeding 76.2%. This indicates that the production of silicates significantly improves the material's reversible capacity. While sacrificing some capacity, the expansion rate and rate performance are effectively improved, ensuring the excellent performance of the battery. Compared to Comparative Example 4, Example 1 exhibits a lower expansion rate and better rate performance, demonstrating that the carbon nanotube coating significantly limits expansion and effectively enhances the rapid migration of lithium ions.

[0109] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for preparing a double-layer coated silicon-based negative electrode material, characterized in that: The following steps are involved: S1, SiO x The precursor material is crushed to obtain material A; S2, uniformly dispersing the material A, carbon source and inorganic salt obtained in step S1 and then performing liquid phase coating to obtain material B; S3, depolymerizing the material B obtained in step S2 and uniformly mixing it with the catalyst to obtain material C; S4, placing the material C obtained in step S3 in a vapor deposition device for carbon coating, the reaction conditions are 700-900° C., the reaction time is 3-5 hours, argon is used as a protective gas during the deposition process, and the carbon source is an organic carbon source, and after coating, a double-layer coated silicon-based negative electrode material is obtained; The negative electrode material comprises, from the inside out, a core body, a coating carbon layer, and a wrapping carbon layer; the core body is uniformly arranged silicon and silicate, the coating carbon layer is dense carbon with an amorphous structure, and the wrapping carbon layer is a porous network structure formed by interlaced carbon nanotubes; the proportions of silicon, silicate, and carbon in the negative electrode material are 40-80%, 10-57%, and 1-10%, respectively; The outer surface of the carbon coating layer also has extended linear carbon nanotubes, and the outer surface of the carbon coating layer contains flaky graphene.

2. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: The silicon is nano-silicon, which is crystalline silicon or amorphous silicon. The size of the crystalline silicon is 0.1-4 nm, and the particle size D50 of the negative electrode material is 4-15 μm.

3. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: The coating carbon layer has a graphite-like structure, and the thickness of the coating carbon layer is 3-30 nm.

4. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: The carbon layer thickness of the coated carbon layer is 3-30 nm, and the sheet diameter of the graphene is 20-100 nm.

5. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: The silicate is one or more of Li2Si2O5, Li2SiO3, Na4SiO4, Mg2SiO4, Na2MgSiO4, K4SiO4, and K2MgSiO4.

6. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: SiO in step S1 x The precursor material is gas phase amorphous non-crystalline, D50=0.01-10mm, x=0.1-1.

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7. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: In step S2, the mass ratio of material A, carbon source and inorganic salt is (0.1-10): (0.01-10): (0.01-10); The carbon source is one or more of sucrose, glucose, starch, polyethylene, polypropylene alcohol, polypyrrole, polypropylene ester, polyacrylic acid, polyacrylamide, polypropylene pyrrolidone, phenolic resin and the like.

8. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: In step S3, the mass ratio of material B to catalyst is (0.1-10): (0.001-0.1); The catalyst is one or more of ferrocene, ferric oxalate, melamine, titanium dioxide, copper oxide, zirconium oxide, zinc oxide, and magnesium oxide.

9. The method for preparing a double-layer coated silicon-based negative electrode material according to claim 1, characterized in that: In step S4, the organic carbon source is one or more of methane, ethane, ethylene, acetylene, propylene, propane, butadiene, and benzene.

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