Alumina-carbon double-layer coated silicon composite material, and preparation method and application thereof

Abstract

The application provides an alumina-carbon double-layer coated silicon composite material and a preparation method and application thereof, and relates to the technical field of lithium ion battery negative electrode materials.The application provides an alumina-carbon double-layer coated silicon composite material, which comprises nanosilicon, an amorphous carbon layer coated on the surface of the nanosilicon, and an alumina layer coated on the surface of the amorphous carbon layer.Through the double-layer coating of the carbon layer and the alumina layer, the problems of volume expansion of the silicon negative electrode, poor conductivity and instability of the SEI film are solved, the electrochemical reaction activity of the battery is increased, and the long cycle stability of the negative electrode material is significantly improved.The data of the embodiment show that the alumina-carbon double-layer coated silicon composite negative electrode material prepared by the application has a first circle discharge specific capacity of 2251.45 mAh / g at 0.1 C, and the reversible discharge specific capacity can reach 719.23 mAh / g after 1000 cycles at 0.5 C.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery anode material technology, specifically to an alumina-carbon bilayer coated silicon composite material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries (LIBs) are widely used in electronic devices and electric vehicles due to their high energy density and long cycle life. However, the capacity of traditional graphite anode materials (372 mAh / g) can no longer meet the ever-increasing energy demand. Silicon has an ultra-high theoretical specific capacity (3579 mAh / g), far exceeding that of graphite anodes, and is therefore considered the ideal anode material for next-generation high-energy-density batteries.

[0003] Silicon-based anodes face several challenges in practical applications, primarily including volume expansion, low electronic conductivity, and an unstable solid electrolyte interphase (SEI) membrane. During charge and discharge, silicon undergoes volume expansion exceeding 300%, leading to material fragmentation and capacity decay. Simultaneously, silicon has poor conductivity, and the SEI membrane continuously ruptures and regenerates during charge and discharge as silicon expands, which also affects the battery's long-cycle performance.

[0004] Currently, the main technology for modifying silicon-based materials is to coat silicon with various carbon materials. However, the rigidity of carbon materials is insufficient to limit the volume expansion of silicon materials, which leads to the continuous damage and growth of the SEI film and reduces cycle stability. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide an alumina-carbon bilayer coated silicon composite material, its preparation method and application. This invention solves the problems of volume expansion, poor conductivity and SEI film instability of silicon anode by double coating with carbon layer and alumina layer, increases the electrochemical reaction activity of battery and significantly improves the long cycle stability of anode material.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides an alumina-carbon bilayer coated silicon composite material (Si@C@Al2O3 or silicon@carbon@alumina composite material), comprising nano-silicon, an amorphous carbon layer coated on the surface of the nano-silicon, and an alumina layer coated on the surface of the amorphous carbon layer; the mass ratio of the amorphous carbon layer to the alumina layer is 25~35:3~6.

[0007] Preferably, the particle size of the nano-silicon is 200~500nm.

[0008] This invention provides a method for preparing the alumina-carbon bilayer coated silicon composite material described above, comprising the following steps: Nano-silicon was mixed with an organic carbon source precursor and then subjected to hydrothermal reaction and first calcination carbonization to obtain a silicon-carbon composite material. The silicon-carbon composite material is mixed with aluminum alkoxide and subjected to hydrolysis and a second calcination to obtain the alumina-carbon bilayer coated silicon composite material.

[0009] Preferably, the organic carbon source precursor includes an organic carbon source, a weak acid, and water; the mass ratio of the nano-silicon to the organic carbon source in the organic carbon source precursor is 1:2~6.

[0010] Preferably, the aluminum alkoxide includes one or more of aluminum isopropoxide, aluminum ethoxide, and aluminum tert-butoxide; the mass ratio of the silicon-carbon composite material to the aluminum alkoxide is 1:0.1~0.6.

[0011] Preferably, the hydrothermal reaction temperature is 180~220℃ and the reaction time is 8~12h.

[0012] Preferably, the temperature of the first calcination and carbonization is 500~800℃, and the holding time is 1~3h.

[0013] Preferably, the hydrolysis reaction is initiated by adding water dropwise to the reaction system, the water dropwise acceleration rate is 10~15s / drop, and the ratio of silicon-carbon composite material to water is 0.5~1.5g:200~400μL.

[0014] Preferably, the second calcination temperature is 400~500℃, and the holding time is 1~3h.

[0015] This invention provides the application of the alumina-carbon bilayer coated silicon composite material described in the above technical solution or the alumina-carbon bilayer coated silicon composite material prepared by the above preparation method as a negative electrode material in lithium-ion batteries.

[0016] The present invention provides an alumina-carbon bilayer coated silicon composite material, comprising nano-silicon, an amorphous carbon layer coated on the surface of the nano-silicon, and an alumina layer coated on the surface of the amorphous carbon layer.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention constructs a nano-silicon-carbon-alumina bilayer coating structure. The inner amorphous carbon layer effectively isolates the nano-silicon from direct contact with the electrolyte, while also possessing a certain degree of flexibility to buffer the volume expansion of silicon during lithium insertion / extraction. The outer alumina layer has inert properties, further preventing electrolyte erosion and inhibiting continuous interfacial reactions. Furthermore, the alumina layer provides efficient lithium-ion diffusion channels, enhancing lithium-ion transport kinetics and thus increasing the electrochemical reactivity of the battery. The synergistic effect of the dual interfaces improves structural integrity, thereby significantly enhancing the long-cycle stability of the material. Data from the embodiments show that the alumina-carbon bilayer coated silicon composite material prepared in this invention has a first-cycle discharge specific capacity of 2251.45 mAh / g at 0.1C, and a reversible discharge specific capacity of 719.23 mAh / g after 1000 cycles at 0.5C.

[0018] The present invention also provides a method for preparing the alumina-carbon bilayer coated silicon composite material described in the above technical solution. By controlling the amount of organic carbon source and aluminum alkoxide, the present invention can achieve stable adjustment of the coating thickness, with high repeatability and strong controllability, so that the nano-silicon particles are uniformly coated by the amorphous carbon layer and the alumina layer in sequence to form a stable core-shell structure system.

[0019] Furthermore, the entire preparation process of this invention uses conventional ball milling, hydrothermal reaction and tube furnace calcination and other mature equipment. The process path is clear, the operating conditions are mild, it is easy to scale up production, and it has good feasibility for industrial application. Attached Figure Description

[0020] Figure 1 This is a flowchart illustrating the preparation process of an alumina-carbon bilayer coated silicon composite material. Figure 2 This is a scanning electron microscope image of the alumina-carbon bilayer coated silicon composite material in Example 1; Figure 3 The constant current charge-discharge curve of Si@C@Al2O3 at 0.1C in Example 1 is shown. Figure 4 This is a cycle diagram of Si@C@Al2O3 in Example 1 after 1000 cycles at 0.5C. Detailed Implementation

[0021] The present invention provides an alumina-carbon bilayer coated silicon composite material, comprising nano-silicon, an amorphous carbon layer coated on the surface of the nano-silicon, and an alumina layer coated on the surface of the amorphous carbon layer; the mass ratio of the amorphous carbon layer to the alumina layer is 25~35:3~6.

[0022] In this invention, the particle size of the nano-silicon is preferably 200~500nm, specifically 300nm, 350nm, 400nm or 450nm.

[0023] In this invention, the thickness of both the amorphous carbon layer and the alumina layer is preferably at the nanometer level, and the thickness of the amorphous carbon layer is greater than the thickness of the alumina layer.

[0024] The present invention also provides a method for preparing the alumina-carbon bilayer coated silicon composite material, comprising the following steps: Nano-silicon was mixed with an organic carbon source precursor and then subjected to hydrothermal reaction and first calcination carbonization to obtain a silicon-carbon composite material. The silicon-carbon composite material is mixed with aluminum alkoxide and subjected to hydrolysis and a second calcination to obtain the alumina-carbon bilayer coated silicon composite material.

[0025] Unless otherwise specified, the materials and equipment used in this invention are all commercially available products in the field.

[0026] This invention mixes nano-silicon with an organic carbon source precursor, and then sequentially undergoes hydrothermal reaction and a first calcination carbonization to obtain a silicon-carbon composite material (silicon@carbon composite material).

[0027] In this invention, the nano-silicon is preferably obtained by ball milling silicon powder, and the silicon powder preferably includes flake, needle, or spherical shapes. More preferably, the silicon powder is industrial wire-cut silicon, commercial micron-sized silicon, or SiO2. x One or more of the following, wherein the particle size of the commercial micron-sized silicon is preferably 1~10μm, and the SiO x The particle size is preferably in the nanometer to micrometer range.

[0028] In this invention, industrial wire-cut silicon wafer scrap is preferably used as the silicon powder. The particle size of silicon is reduced by ball milling, which has the advantages of low cost, simple operation, environmental friendliness, and the ability to achieve large-scale industrial production.

[0029] In this invention, the silicon powder is preferably placed in a ball mill jar and milled using a planetary ball mill. The ball milling is preferably performed using zirconia grinding beads. The revolution speed of the planetary ball mill is preferably 400-700 r / min, specifically 500 r / min, 550 r / min, 600 r / min, or 650 r / min. The transmission ratio of the planetary ball mill is preferably 0.5-0.8, specifically 0.6 or 0.7. The milling time is preferably 8-12 hours, specifically 9 hours, 10 hours, or 11 hours. The milling is preferably carried out in an atmospheric pressure air atmosphere.

[0030] In this invention, the organic carbon source precursor preferably includes an organic carbon source, a weak acid, and water. The organic carbon source precursor is preferably used in the form of a precursor solution. In this invention, the organic carbon source is preferably mixed with the weak acid, deionized water is added, and the mixture is magnetically stirred to ensure complete dissolution, thereby obtaining the precursor solution.

[0031] In this invention, the preferred mass ratio of the organic carbon source to the weak acid is 4-8:1, specifically 5:1, 6:1, or 7:1; the preferred mass ratio of the organic carbon source to the volume ratio of deionized water is 1-8g:50-250mL, specifically 2g:100mL, 3g:100mL, 4g:100mL, or 7g:200mL; the preferred magnetic stirring time is 3-6 hours, specifically 4 hours or 5 hours; the preferred organic carbon source includes one or more of sucrose, glucose, lactose, and starch; the preferred weak acid includes one or more of oxalic acid, citric acid, and acetic acid with a mass fraction of 36%; the preferred ratio of the organic carbon source to acetic acid is 1-8g:8-16mL; the preferred concentration of the weak acid in the organic carbon source precursor is 0.1mol / L-0.4mol / L; the weak acid provides an acidic environment to promote the hydrolysis of the organic carbon source.

[0032] In this invention, nano-silicon is preferably mixed with the precursor solution under stirring conditions, and the stirring time is preferably 2 to 4 hours. This invention does not have any special limitations on the stirring method and conditions, and any stirring method known to those skilled in the art can be used.

[0033] In this invention, the precursor solution preferably also includes a dispersant, which preferably includes hexadecyltrimethylammonium bromide or sodium dodecyl sulfate. The dispersant is used to disperse nano-silicon and prevent agglomeration.

[0034] In this invention, the mass ratio of nano-silicon to organic carbon source is preferably 1:2 to 6, specifically 1:3, 1:4 or 1:5; the mass ratio of nano-silicon to dispersant is preferably 1:1.5 to 2, specifically 1:1.6, 1:1.7 or 1:1.8.

[0035] In this invention, the hydrothermal reaction is preferably carried out in a homogeneous reactor, with multiple reaction vessels rotating within a constant temperature chamber to achieve a more uniform hydrothermal reaction. The rotation speed is preferably 20-35 r / min, specifically 25 r / min, 28 r / min, or 30 r / min. The temperature of the hydrothermal reaction is preferably 180-220℃, specifically 190℃, 200℃, or 210℃. The reaction time of the hydrothermal reaction is preferably 8-12 h, specifically 9 h, 10 h, or 11 h.

[0036] The product (silicon@hydrothermal carbon) obtained after the hydrothermal reaction is preferably subjected to centrifugation, drying and the first calcination carbonization in sequence to obtain the silicon-carbon composite material.

[0037] In this invention, the centrifugation speed is preferably 8000~11000 r / min, specifically 8500 r / min, 9000 r / min or 10000 r / min; the centrifugation time is preferably 10~15 min, specifically 11 min, 12 min, 13 min or 14 min; the number of centrifugations is preferably 3~5 times; the drying temperature is preferably 60~80℃, specifically 65℃, 70℃ or 75℃; and the drying time is preferably 8~12 h, specifically 9 h, 10 h or 11 h.

[0038] In this invention, the temperature of the first calcination carbonization (annealing or pyrolysis) is preferably 500~800℃, specifically 600℃, 650℃, 700℃ or 750℃, and the holding time is preferably 1~3h, specifically 1.5h, 2h or 2.5h.

[0039] The present invention preferably raises the temperature from room temperature to the temperature of the first calcination and carbonization at a heating rate of 4~6℃ / min.

[0040] In this invention, the first calcination and carbonization is preferably carried out in a tube furnace filled with nitrogen, and the flow rate of the nitrogen is preferably 120~240 sccm.

[0041] After the first calcination and carbonization is completed, the resulting product is preferably cooled in the furnace and then ground into powder with a mortar and pestle without sieving to obtain the silicon-carbon composite material.

[0042] In this invention, the organic carbon source is hydrolyzed into sucrose in a high-temperature and high-pressure environment (homogeneous reactor), and then dehydrated and polymerized to finally coat the silicon surface to form a layer of hydrothermal carbon. The hydrothermal carbon is carbonized by a first calcination under a high-temperature inert atmosphere, and the carbon atoms rearrange to transform into an amorphous carbon layer.

[0043] After obtaining the silicon-carbon composite material, the present invention mixes the silicon-carbon composite material with aluminum alkoxide, and then subjectes it to a hydrolysis reaction and a second calcination to obtain the alumina-carbon bilayer coated silicon composite material.

[0044] In this invention, the aluminum alkoxide preferably includes one or more of aluminum isopropoxide, aluminum ethoxide, and aluminum tert-butoxide; the mass ratio of the silicon-carbon composite material to the aluminum alkoxide is preferably 1:0.1 to 0.6, specifically 1:0.2, 1:0.3, 1:0.4, or 1:0.5.

[0045] In this invention, the aluminum alkoxide is preferably used in the form of an aluminum alkoxide solution, and the solvent of the aluminum alkoxide solution is preferably ethanol, more preferably anhydrous ethanol. The mass ratio of the aluminum isopropoxide to the volume ratio of anhydrous ethanol is preferably 0.1~0.6g:30~60mL, specifically 0.2g:40mL, 0.3g:40mL, 0.4g:40mL or 0.5g:50mL.

[0046] In this invention, the silicon-carbon composite material is preferably dispersed in an aluminum alkoxide solution. The dispersion temperature is preferably 30°C, and the dispersion is preferably performed by ultrasonic treatment. The frequency of the ultrasonic treatment is preferably 20~40KHz, and the time is preferably 15~30min.

[0047] In this invention, the hydrolysis reaction is preferably initiated by adding water dropwise to the reaction system. The water drop rate is preferably 10-15 s / drop, specifically 11 s / drop, 12 s / drop, 13 s / drop, or 14 s / drop. The ratio of silicon-carbon composite material to water is preferably 0.5-1.5 g: 200-400 μL, specifically 1 g: 300 μL or 1.2 g: 300 mL.

[0048] The hydrolysis reaction is preferably carried out under stirring conditions, and the stirring time is preferably 6 to 8 hours, specifically 6.5 hours, 7 hours or 7.5 hours.

[0049] After the hydrolysis reaction is completed, centrifugation, drying, and the second calcination (annealing) are preferably performed sequentially. The centrifugation speed is preferably 8000~11000 r / min, specifically 8500 r / min, 9000 r / min, or 10000 r / min. The centrifugation time is preferably 10~15 min, specifically 11 min, 12 min, 13 min, or 14 min. The number of centrifugations is preferably 1~2 times. The drying temperature is preferably 60~80℃, specifically 65℃, 70℃, or 75℃. The drying time is preferably 6~8 h.

[0050] In this invention, the second calcination temperature is preferably 400~500℃, specifically 430℃, 450℃ or 480℃, and the holding time is preferably 1~3h, specifically 1.5h, 2h or 2.5h.

[0051] The present invention preferably raises the temperature from room temperature to the second calcination temperature at a heating rate of 4~6℃ / min.

[0052] In this invention, the second calcination is preferably carried out in a tube furnace filled with nitrogen, and the flow rate of the nitrogen is preferably 120~240 sccm.

[0053] After the second calcination is completed, the resulting product is preferably cooled in the furnace to obtain the alumina-carbon bilayer coated silicon composite material.

[0054] In this invention, the aluminum alkoxide forms an Al-O-Al structure through a nucleophilic substitution reaction and a condensation reaction of water molecules. During the second calcination process, water molecules in the structure are removed at high temperature to obtain Al2O3, which forms the aluminum oxide layer.

[0055] This invention provides the application of the alumina-carbon bilayer coated silicon composite material described in the above technical solution or the alumina-carbon bilayer coated silicon composite material prepared by the above preparation method as a negative electrode material in lithium-ion batteries.

[0056] In this invention, the negative electrode material preferably further includes a binder and a conductive agent.

[0057] In this invention, the preferred mass ratio of the alumina-carbon bilayer coated silicon composite material, the binder, and the conductive agent is 80:10:10.

[0058] In this invention, the adhesive preferably comprises lithium polyacrylate (PAA-Li).

[0059] In this invention, the conductive agent preferably includes conductive carbon black (Super-P).

[0060] In this invention, the alumina-carbon double-layer coated silicon composite material, binder, conductive agent, and deionized water are mixed to obtain a slurry. The slurry is then coated onto a copper foil, followed by vacuum drying and rolling to obtain a negative electrode sheet. This invention does not have a specific limitation on the amount of water used, as long as a slurry is obtained.

[0061] The present invention also provides a battery, including a counter electrode, the negative electrode sheet, a separator, and an electrolyte.

[0062] In this invention, the counter electrode preferably comprises a lithium metal sheet.

[0063] In this invention, the diaphragm preferably comprises a polypropylene microporous membrane.

[0064] In this invention, the electrolyte preferably comprises a solvent and a solute, the solute is preferably a lithium hexafluorophosphate solution (LiPF6), the concentration of the lithium hexafluorophosphate solution is preferably 1 mol / L, the solvent is preferably a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC), and the volume ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is preferably 1:1:1.

[0065] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0066] The cetyltrimethylammonium bromide used in the examples was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number H811117-500g, with a purity of 99%; aluminum isopropoxide was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number A800381-100g, with a purity of 99.99%; and oxalic acid was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., catalog number O815177-100g, with a purity of 99%.

[0067] Example 1 Figure 1 The flowchart for the preparation of the alumina-carbon bilayer coated silicon composite material in Example 1 includes the following steps: Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0068] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 3g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0069] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred. The mass ratio of nano-silicon, hexadecyltrimethylammonium bromide, and organic carbon source was 1:2:3, the mass of nano-silicon was 1g, and the stirring time was 2h. Then, a hydrothermal reaction was carried out in a homogeneous reactor at a temperature of 200℃, a rotation speed of 27r / min, and a reaction time of 10h. Then, centrifugation was performed 5 times at a rotation speed of 10000r / min for 10min. Subsequently, it was dried in a vacuum drying oven at 60℃ for 12h. Finally, the first calcination carbonization was carried out in a tube furnace filled with nitrogen. The temperature was increased to 700℃ at a heating rate of 5℃ / min and held for 2h. The nitrogen flow rate was 160sccm, and then the furnace was cooled to obtain silicon@carbon composite material.

[0070] Step 4: Preparation of silicon@carbon@alumina composite material: 0.2g of aluminum isopropoxide was dissolved in 40mL of ethanol and stirred for 3h. Then, 1g of silicon-carbon composite material was ultrasonically dispersed in the ethanol solution of aluminum isopropoxide. 200μL of deionized water was added dropwise at a rate of 10s / drop, and stirring was continued for 6h. Then, the mixture was centrifuged twice at 10000r / min for 10min. The mixture was then dried in a vacuum drying oven at 60℃ for 12h. Finally, a second calcination was carried out in a tube furnace filled with nitrogen. The temperature was increased to 450℃ at a heating rate of 5℃ / min and held for 2h. The nitrogen flow rate was 160sccm. The mixture was then cooled with the furnace to obtain silicon@carbon@alumina composite material with a mass ratio of amorphous carbon layer to alumina layer of 6:1, denoted as Si@C@Al2O3.

[0071] Example 2 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0072] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 3g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0073] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred. The mass ratio of nano-silicon, hexadecyltrimethylammonium bromide, and organic carbon source was 1:2:3, the mass of nano-silicon was 1g, and the stirring time was 2h. Then, a hydrothermal reaction was carried out in a homogeneous reactor at a temperature of 200℃, a rotation speed of 27r / min, and a reaction time of 10h. Then, centrifugation was performed 5 times at a rotation speed of 10000r / min for 10min. Subsequently, it was dried in a vacuum drying oven at 60℃ for 12h. Finally, the first calcination carbonization was carried out in a tube furnace filled with nitrogen. The temperature was increased to 700℃ at a heating rate of 5℃ / min and held for 2h. The nitrogen flow rate was 160sccm, and then the furnace was cooled to obtain silicon@carbon composite material.

[0074] Step 4: Preparation of silicon@carbon@alumina composite material: Dissolve 0.1g of aluminum isopropoxide in 40mL of ethanol and stir for 3h. Then, disperse 1g of silicon@carbon composite material in the ethanol solution of aluminum isopropoxide by ultrasonication. Add 200μL of deionized water dropwise at a rate of 10s / drop, and continue stirring for 6h. Then, centrifuge twice at 10000r / min for 10min. Then, dry in a vacuum drying oven at 60℃ for 12h. Finally, perform a second calcination in a tube furnace filled with nitrogen, heating to 450℃ at a heating rate of 5℃ / min and holding for 2h, with a nitrogen flow rate of 160sccm. Then, cool with the furnace to obtain silicon@carbon@alumina composite material with a mass ratio of amorphous carbon layer to alumina layer of 11:1.

[0075] Example 3 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0076] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 3g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0077] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred. The mass ratio of nano-silicon, hexadecyltrimethylammonium bromide, and organic carbon source was 1:2:3, the mass of nano-silicon was 1g, and the stirring time was 2h. Then, a hydrothermal reaction was carried out in a homogeneous reactor at a temperature of 200℃, a rotation speed of 27r / min, and a reaction time of 10h. Then, centrifugation was performed 5 times at a rotation speed of 10000r / min for 10min. Subsequently, it was dried in a vacuum drying oven at 60℃ for 12h. Finally, the first calcination carbonization was carried out in a tube furnace filled with nitrogen. The temperature was increased to 700℃ at a heating rate of 5℃ / min and held for 2h. The nitrogen flow rate was 160sccm, and then the furnace was cooled to obtain silicon@carbon composite material.

[0078] Step 4: Preparation of silicon@carbon@alumina composite material: Dissolve 0.3g of aluminum isopropoxide in 40mL of ethanol and stir for 3h. Then, disperse 1g of silicon@carbon composite material in the ethanol solution of aluminum isopropoxide by ultrasonication. Add 200μL of deionized water dropwise at a rate of 10s / drop, and continue stirring for 6h. Then, centrifuge twice at 10000r / min for 10min. Then, dry in a vacuum drying oven at 60℃ for 12h. Finally, perform a second calcination in a tube furnace filled with nitrogen, heating to 450℃ at a heating rate of 5℃ / min and holding for 2h, with a nitrogen flow rate of 160sccm. Then, cool with the furnace to obtain silicon@carbon@alumina composite material with a mass ratio of amorphous carbon layer to alumina layer of 4:1.

[0079] Example 4 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0080] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 2g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0081] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred at a mass ratio of 1:2:2, with 1 g of nano-silicon and 2 h of stirring time. Then, a hydrothermal reaction was carried out in a homogeneous reactor at 200℃ and 27 r / min for 10 h. This was followed by five centrifugations at 10000 r / min for 10 min each. The mixture was then dried in a vacuum drying oven at 60℃ for 12 h. Finally, a first calcination carbonization was carried out in a nitrogen-filled tube furnace, heated to 700℃ at a heating rate of 5℃ / min, held for 2 h, and with a nitrogen flow rate of 160 sccm. The mixture was then cooled in the furnace to obtain the silicon@carbon composite material.

[0082] Step 4: Preparation of silicon@carbon@alumina composite material: Dissolve 0.2g of aluminum isopropoxide in 40mL of ethanol and stir for 3h. Then, disperse 1g of silicon@carbon composite material in the ethanol solution of aluminum isopropoxide by ultrasonication. Add 200μL of deionized water dropwise at a rate of 10s / drop, and continue stirring for 6h. Then, centrifuge twice at 10000r / min for 10min. Then, dry in a vacuum drying oven at 60℃ for 12h. Finally, perform a second calcination in a tube furnace filled with nitrogen, heating to 450℃ at a heating rate of 5℃ / min and holding for 2h, with a nitrogen flow rate of 160sccm. Then, cool with the furnace to obtain silicon@carbon@alumina composite material with a mass ratio of amorphous carbon layer to alumina layer of 4:1.

[0083] Example 5 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0084] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 4g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0085] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred at a mass ratio of 1:2:4, with 1 g of nano-silicon and 2 h of stirring time. Then, a hydrothermal reaction was carried out in a homogeneous reactor at 200℃ and 27 r / min for 10 h. This was followed by five centrifugations at 10000 r / min for 10 min each. The mixture was then dried in a vacuum drying oven at 60℃ for 12 h. Finally, a first calcination carbonization was carried out in a nitrogen-filled tube furnace, heated to 700℃ at a heating rate of 5℃ / min, held for 2 h, and with a nitrogen flow rate of 160 sccm. The mixture was then cooled in the furnace to obtain the silicon@carbon composite material.

[0086] Step 4: Preparation of silicon@carbon@alumina composite material: Dissolve 0.2g of aluminum isopropoxide in 40mL of ethanol and stir for 3h. Then, disperse 1g of silicon@carbon composite material in the ethanol solution of aluminum isopropoxide by ultrasonication. Add deionized water dropwise at a rate of 10s / drop and continue stirring for 6h. Then, centrifuge twice at 10000r / min for 10min. Then, dry in a vacuum drying oven at 60℃ for 12h. Finally, perform a second calcination in a tube furnace filled with nitrogen, heating to 450℃ at a heating rate of 5℃ / min and holding for 2h. The nitrogen flow rate is 160sccm. Then, cool with the furnace to obtain silicon@carbon@alumina composite material with a mass ratio of amorphous carbon layer to alumina layer of 8:1.

[0087] Comparative Example 1 Preparation of nano-silicon: Industrial wire-cut silicon was placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads were selected. The speed of the planetary ball mill was 450 r / min, the transmission ratio was 0.5, and the ball milling time was 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0088] Comparative Example 2 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0089] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 3g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0090] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred at a mass ratio of 1:2:3 (nano-silicon, hexadecyltrimethylammonium bromide, and organic carbon source), with 1 g of nano-silicon and a stirring time of 2 h. Then, a hydrothermal reaction was carried out in a homogeneous reactor at 200 °C, 27 r / min, and 10 h. The mixture was then centrifuged five times at 10000 r / min for 10 min each time. Subsequently, it was dried in a vacuum drying oven at 60 °C for 12 h. Finally, it was calcined in a nitrogen-filled tube furnace, heated to 700 °C at a heating rate of 5 °C / min, held for 2 h, and cooled with the furnace to obtain the silicon@carbon composite material.

[0091] Comparative Example 3 Step 1: Preparation of nano-silicon: Industrial wire-cut silicon is placed in a ball mill jar and ball-milled using a planetary ball mill. Zirconia grinding beads are selected, the ball mill speed is 450 r / min, the transmission ratio is 0.5, and the ball milling time is 10 h to obtain nano-silicon with a particle size of 200~500 nm.

[0092] Step 2: Preparation of precursor solution: Mix sucrose and oxalic acid in a beaker at a mass ratio of 4:1. Add 3g of organic carbon source and 100mL of deionized water. Stir magnetically for 3 hours to ensure complete dissolution and obtain a homogeneous precursor solution.

[0093] Step 3: Preparation of silicon@carbon composite material: Nano-silicon, hexadecyltrimethylammonium bromide, and precursor solution were mixed and stirred. The mass ratio of nano-silicon, hexadecyltrimethylammonium bromide, and organic carbon source was 1:2:3, the mass of nano-silicon was 1g, and the stirring time was 2h. Then, a hydrothermal reaction was carried out in a homogeneous reactor at a temperature of 200℃, a rotation speed of 27r / min, and a reaction time of 10h. Then, centrifugation was performed 5 times at a rotation speed of 10000r / min for 10min. Subsequently, it was dried in a vacuum drying oven at 60℃ for 12h. Finally, the first calcination carbonization was carried out in a tube furnace filled with nitrogen. The temperature was increased to 700℃ at a heating rate of 5℃ / min and held for 2h. The nitrogen flow rate was 160sccm, and then the furnace was cooled to obtain silicon@carbon composite material.

[0094] Step 4: Preparation of titanium dioxide-carbon bilayer coated silicon composite material: Mix 5 mL of 80% titanium isopropoxide solution with 40 mL of ethanol and stir for 3 h. Then, disperse 1 g of silicon@carbon composite material in the above solution by ultrasonication. Add deionized water dropwise at a rate of 10 s / drop and continue stirring for 6 h. Then, centrifuge twice at a speed of 10000 r / min for 10 min. Then, dry in a vacuum drying oven at 60 °C for 12 h. Finally, perform a second calcination in a tube furnace filled with nitrogen, heating to 450 °C at a heating rate of 5 °C / min and holding for 2 h. The nitrogen flow rate is 160 sccm. Then, cool with the furnace to obtain titanium dioxide-carbon bilayer coated silicon composite material.

[0095] Test conditions: The materials prepared in the comparative example and the embodiment were used as negative electrode materials, and mixed with binder PAA-Li lithium polyacrylate and conductive agent (Super-P) conductive carbon black at a mass ratio of 80:10:10. Deionized water was added as a solvent to form a slurry, which was then coated on copper foil with a loading of 1 mg·cm³. -2The negative electrode was prepared by vacuum drying and rolling. A lithium metal sheet was used as the counter electrode. An electrolyte solution of 1 mol / L LiPF6 lithium hexafluorophosphate three-component mixed solvent (EC:DMC:EMC = 1:1:1, v / v) was used. A polypropylene microporous membrane was used as the separator. The CR2032 coin cell was assembled in an inert gas-filled glove box. Charge-discharge tests of the coin cells were conducted on a battery testing system from Shenzhen Xinwei. Constant current charge-discharge was performed at room temperature, with the charge-discharge voltage limited to 0.01~2V. The results are shown in Table 1.

[0096] Table 1 Performance parameters of samples from Examples 1-5 and Comparative Examples 1-3

[0097] As shown in Table 1, the alumina-carbon bilayer coated silicon composite material prepared in Example 1 has a first-cycle discharge specific capacity of 2251.45 mAh / g, a first-cycle charge specific capacity of 1814.04 mAh / g, and a first-cycle coulombic efficiency of 80.57%. After 1000 cycles at 0.5C (1C=3579 mA / g), the reversible discharge specific capacity reaches as high as 719.23 mAh / g, which is significantly better than Comparative Example 1 (pure nano-silicon, 1.24 mAh / g), Comparative Example 2 (single-layer carbon coating, 182.35 mAh / g), and Comparative Example 3 (titanium dioxide coating, 121.74 mAh / g). This invention produces a significant synergistic effect through the dual coating of amorphous carbon layer and alumina layer. The carbon layer buffers the volume expansion of silicon during lithium insertion / extraction, and the outer alumina layer further blocks electrolyte erosion and inhibits continuous interfacial reaction. Moreover, the preparation process is simple and the raw materials used are low in cost.

[0098] Figure 2 This is a scanning electron microscope (SEM) image of the alumina-carbon bilayer coated silicon composite material in Example 1. Figure 2 It can be seen that the sample surface is rough, and the coating layer is complete and continuous without any exposed areas, indicating that the carbon layer and alumina layer have been uniformly covered on the silicon surface.

[0099] Figure 3 The constant current charge-discharge curve of Si@C@Al2O3 in Example 1 at 0.1C shows that the first discharge specific capacity at 0.1C is 2251.45mAh / g, the first charge specific capacity is 1814.04mAh / g, and the first coulombic efficiency is 80.57%.

[0100] Figure 4 The cycling graph of Si@C@Al2O3 in Example 1 after 1000 cycles at 0.5C shows that the reversible discharge specific capacity is as high as 719.23mAh / g after 1000 cycles at 0.5C.

[0101] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. An alumina-carbon bilayer coated silicon composite material, characterized in that, It includes nano-silicon, an amorphous carbon layer covering the surface of the nano-silicon, and an alumina layer covering the surface of the amorphous carbon layer; the mass ratio of the amorphous carbon layer to the alumina layer is 25~35:3~6.

2. The alumina-carbon bilayer coated silicon composite material according to claim 1, characterized in that, The particle size of the nano-silicon is 200~500nm.

3. The method for preparing the alumina-carbon bilayer coated silicon composite material according to claim 1 or 2, characterized in that, Includes the following steps: Nano-silicon was mixed with an organic carbon source precursor and then subjected to hydrothermal reaction and first calcination carbonization to obtain a silicon-carbon composite material. The silicon-carbon composite material is mixed with aluminum alkoxide and subjected to hydrolysis and a second calcination to obtain the alumina-carbon bilayer coated silicon composite material.

4. The preparation method according to claim 3, characterized in that, The organic carbon source precursor includes an organic carbon source, a weak acid, and water; the mass ratio of the nano-silicon to the organic carbon source in the organic carbon source precursor is 1:2~6.

5. The preparation method according to claim 3 or 4, characterized in that, The aluminum alkoxide includes one or more of aluminum isopropoxide, aluminum ethoxide, and aluminum tert-butoxide; the mass ratio of the silicon-carbon composite material to the aluminum alkoxide is 1:0.1~0.

6.

6. The preparation method according to claim 3, characterized in that, The hydrothermal reaction is carried out at a temperature of 180~220℃ for 8~12 hours.

7. The preparation method according to claim 3, characterized in that, The temperature of the first calcination and carbonization is 500~800℃, and the holding time is 1~3h.

8. The preparation method according to claim 3, characterized in that, The hydrolysis reaction is initiated by adding water dropwise to the reaction system at a rate of 10-15 s / drop, and the ratio of silicon-carbon composite material to water is 0.5-1.5 g: 200-400 μL.

9. The preparation method according to claim 3, characterized in that, The second calcination temperature is 400~500℃, and the holding time is 1~3h.

10. The application of the alumina-carbon bilayer coated silicon composite material according to claim 1 or 2, or the alumina-carbon bilayer coated silicon composite material prepared by any one of claims 3 to 9, as a negative electrode material in lithium-ion batteries.

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