A silicon-carbon composite anode material, its preparation method and application

Abstract

The present invention belongs to the technical field of anode materials for lithium-ion batteries, and particularly relates to a silicon-carbon composite anode material, a preparation method thereof, and an application thereof. The silicon-carbon composite anode material has a core-shell structure. The silicon-carbon composite anode material includes amorphous carbon, porous carbon, and silicon nanosheets; the silicon nanosheets are core materials, the porous carbon and the amorphous carbon are shell materials, and there is a cavity between the core materials and the shell materials. The method can well solve the volume expansion problem of the silicon-carbon composite anode material, and at the same time can also improve the cycle performance and the first Coulomb efficiency of the silicon-carbon composite anode material.

Description

Technical Field

[0001] The present invention belongs to the technical field of anode materials for lithium-ion batteries, and particularly relates to a silicon-carbon composite anode material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium-ion batteries have a series of advantages such as high specific capacity, stable working voltage, good safety, and no memory effect, so they are widely used in many portable electronic instrument devices such as laptop computers, mobile phones, and instruments. With the rapid development of various electronic devices and electric vehicles, people's requirements for the energy density and cycle life of lithium-ion batteries are getting higher and higher. The anode material is an important part of the battery, and together with the cathode material, it determines the key performance of lithium-ion batteries such as cycle life, capacity, and safety, and has become the focus of research in various countries.

[0003] At present, the specific capacity of commercial graphite anode materials is low, only 372 mAh / g, which greatly limits the improvement of the overall capacity of lithium-ion batteries and can no longer meet the market demand. It is reported that the theoretical lithium storage capacity of silicon is as high as 4200 mAh / g, the lithium insertion platform is slightly higher than that of graphite, and the safety hazard is small, so it is an excellent substitute for graphite anode materials; however, silicon shows a volume change of up to 300% during charge and discharge, so it is extremely easy to cause problems such as pulverization of silicon particles, damage to the internal conductive network of the electrode, and poor conductivity. Summary of the Invention

[0004] Aiming at the problems such as serious volume expansion of silicon anode materials existing in the prior art, the present invention provides a method for preparing a silicon-carbon composite anode material by using montmorillonite and the prepared silicon-carbon composite anode material. The method can well solve the volume expansion problem of the silicon-carbon composite anode material, and at the same time improve the cycle performance and first Coulomb efficiency of the silicon-carbon composite anode material. The montmorillonite belongs to biomass renewable resources, is green, non-toxic, environmentally friendly, rich in resources and low in price, which helps to reduce the production cost of the silicon-carbon composite anode material.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A preparation method of a silicon-carbon composite anode material, the preparation method comprising the following steps:

[0007] (1) Mix montmorillonite and sodium alginate to prepare a suspension;

[0008] (2) Mix the suspension in step (1) with an aqueous solution of a zinc salt to carry out a cross-linking reaction to prepare a hydrogel;

[0009] (3) Dry the hydrogel obtained in step (2) and then calcine it to prepare two-dimensional montmorillonite nanosheets coated with porous carbon;

[0010] (4) Mix the two-dimensional montmorillonite nanosheets coated with porous carbon obtained in step (3) and magnesium powder under anhydrous and inert atmosphere, then place them in a reaction vessel to carry out a magnesiothermic reduction reaction to obtain an initial reaction product containing silicon nanosheets coated with porous carbon;

[0011] (5) Grind, wash with water and pickling the initial reaction product obtained in step (4) to prepare porous carbon@hollow cavity@silicon nanosheets;

[0012] (6) Carry out carbon coating treatment and sintering treatment on the porous carbon@hollow cavity@silicon nanosheets obtained in step (5) to prepare the silicon-carbon composite anode material.

[0013] According to an embodiment of the present invention, in step (1), the montmorillonite has a lamellar structure, the average thickness of the montmorillonite lamellae is 80 nm - 120 nm, such as 80 nm, 90 nm, 100 nm, 110 nm or 120 nm; the average length of the montmorillonite lamellae is 5 μm - 10 μm, such as 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.

[0014] According to an embodiment of the present invention, in step (1), the sources of the montmorillonite and sodium alginate are not particularly limited, and they can be obtained by methods known in the art.

[0015] According to an embodiment of the present invention, in step (1), the mass ratio of the montmorillonite to sodium alginate is 1:(3 - 5), for example, 1:3, 1:3.5, 1:4, 1:4.5 or 1:5.

[0016] According to an embodiment of the present invention, in step (1), the concentration of montmorillonite in the suspension is 0.1% - 0.3%, for example, 0.1%, 0.2% or 0.3%.

[0017] According to an embodiment of the present invention, in step (1), the following steps are included: add montmorillonite to water, stir and ultrasonicate, then add sodium alginate and stir until it is completely dissolved, and then let it stand at room temperature for 12 - 24 hours to obtain the suspension.

[0018] Preferably, add montmorillonite to deionized water, stir and ultrasonicate for 10 - 30 min, then add sodium alginate and stir at 60 - 100 °C until sodium alginate is completely dissolved, and then let it stand at room temperature for 12 - 24 hours to obtain a uniformly dispersed suspension.

[0019] Preferably, the mass ratio of the montmorillonite to water is (0.1 - 0.3):100, for example, 0.1:100, 0.2:100 or 0.3:100.

[0020] Preferably, after standing for 4-6 hours during the standing process, ultrasound for 1-2 hours, and then stand and ultrasound again.

[0021] According to an embodiment of the present invention, in step (2), the zinc salt is selected from at least one of zinc nitrate, zinc chloride, zinc sulfate, and zinc acetate.

[0022] According to an embodiment of the present invention, in step (2), the concentration of the aqueous solution of the zinc salt is 10-50 g / L, for example, 10 g / L, 20 g / L, 30 g / L, 40 g / L, or 50 g / L.

[0023] According to an embodiment of the present invention, in step (2), the mass ratio of the suspension to the aqueous solution of the zinc salt is (10-50):100, for example, 10:100, 20:100, 30:100, 40:100, or 50:100.

[0024] According to an embodiment of the present invention, in step (2), the temperature of the cross-linking reaction is room temperature, and the time of the cross-linking reaction is 16-28 hours, for example, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, or 28 hours.

[0025] According to an embodiment of the present invention, in step (2), after the cross-linking reaction, a post-treatment step is further included, and the post-treatment step includes washing. Exemplarily, the hydrogel is washed 2-5 times with deionized water, and the purpose of the washing is to remove the zinc ions adsorbed on the surface of the hydrogel to avoid affecting the performance of the silicon-carbon composite anode material.

[0026] According to an embodiment of the present invention, in step (2), the mixing method is to dropwise add the suspension of step (1) into the aqueous solution of the zinc salt.

[0027] According to an embodiment of the present invention, in step (2), the morphology of the hydrogel is a spherical hydrogel.

[0028] According to an embodiment of the present invention, in step (2), the diameter of the spherical hydrogel is 2 mm-5 mm. Further, the diameter of the spherical hydrogel can be adjusted by controlling the size of the suspension droplets.

[0029] According to an embodiment of the present invention, in step (3), the drying temperature is 50°C-100°C, for example, 50°C, 60°C, 70°C, 80°C, 90°C, or 100°C. The drying time is 8-24 hours, for example, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, or 24 hours.

[0030] According to an embodiment of the present invention, in step (3), the calcination includes a first calcination and a second calcination. The temperature of the first calcination is 500°C - 800°C (such as 500°C, 550°C, 600°C, 650°C, 700°C, 750°C or 800°C), and the time of the first calcination is 2 - 6 hours (such as 2 hours, 3 hours, 4 hours, 5 hours or 6 hours); the temperature of the second calcination is 900°C - 1100°C (such as 900°C, 950°C, 1000°C, 1050°C or 1100°C), and the time of the first calcination is 2 - 10 hours (such as 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours).

[0031] According to an embodiment of the present invention, in step (3), the calcination is carried out in a nitrogen atmosphere or an argon atmosphere.

[0032] According to an embodiment of the present invention, in step (3), during the first calcination, zinc oxide and carbon generated by the decomposition of zinc alginate exfoliate montmorillonite into two-dimensional montmorillonite nanosheets; during the second calcination, carbon reduces zinc oxide to generate zinc vapor which escapes from the reaction system, and two-dimensional montmorillonite nanosheets coated with porous carbon are obtained.

[0033] According to an embodiment of the present invention, in step (4), the weight ratio of the magnesium powder to the two-dimensional montmorillonite nanosheets coated with porous carbon is (0.8 - 1):1, for example, 0.8:1, 0.9:1 or 1:1.

[0034] According to an embodiment of the present invention, in step (4), the mixing is carried out, for example, under grinding conditions.

[0035] According to an embodiment of the present invention, in step (4), the magnesiothermic reduction reaction is carried out in an inert atmosphere. The inert atmosphere is an argon atmosphere.

[0036] According to an embodiment of the present invention, in step (4), the temperature of the magnesiothermic reduction reaction is 800°C - 1000°C (such as 800°C, 850°C, 900°C, 950°C or 1000°C), and the time of the magnesiothermic reduction reaction is 2 - 6 hours (such as 2 hours, 3 hours, 4 hours, 5 hours or 6 hours).

[0037] According to an embodiment of the present invention, in step (5), the water washing is to wash the ground reaction product with deionized water, and the purpose is to remove the water-soluble impurity components in the reaction product.

[0038] According to an embodiment of the present invention, in step (5), the pickling includes a first pickling and a second pickling; wherein, the purpose of the first pickling is to remove impurity components such as aluminum oxide and magnesium oxide in the reaction product; the purpose of the second pickling is to remove impurity components such as cristobalite in montmorillonite in the reaction product; the removal of the impurity components will form a cavity between the silicon nanosheets and the porous carbon, and the formation of this cavity is beneficial to further alleviating the volume expansion of silicon.

[0039] According to an embodiment of the present invention, in step (5), the acid used in the first pickling includes one or more of hydrochloric acid, sulfuric acid, and nitric acid, the pH of the acid solution used in the first pickling is 2 - 4, and the stirring time in the first pickling is 30 min - 10 h; the acid used in the second pickling is hydrofluoric acid, the mass percentage concentration of the acid solution used in the second pickling is 0.1% - 5%, and the stirring time in the second pickling is 30 min - 1 h.

[0040] According to an embodiment of the present invention, in step (5), after the pickling, post - treatment steps such as filtration and drying are also included.

[0041] According to an embodiment of the present invention, in step (6), the carbon coating treatment and the sintering treatment are methods known to those skilled in the art. Among them, the methods adopted for the carbon coating treatment include, but are not limited to, at least one of chemical vapor deposition, solid - phase mixing of carbon sources, or liquid - phase mixing of carbon sources.

[0042] Exemplarily, the method for the carbon coating treatment is to uniformly mix the porous carbon@cavity@silicon nanosheets in step (5) with pitch, so that the pitch powder is coated on the surface of the porous carbon@cavity@silicon nanosheets to obtain a mixture.

[0043] Exemplarily, the method for the sintering treatment is to perform a sintering treatment on the carbon - coated mixture, and after cooling, perform a dispersion and screening treatment to obtain the silicon - carbon composite negative electrode material.

[0044] The present invention also provides a silicon - carbon composite negative electrode material prepared by the above - mentioned method.

[0045] According to an embodiment of the present invention, the silicon - carbon composite negative electrode material has a core - shell structure.

[0046] According to an embodiment of the present invention, the silicon - carbon composite negative electrode material includes amorphous carbon, porous carbon, and silicon nanosheets; the silicon nanosheets are the core material, the porous carbon and the amorphous carbon are the shell materials, and there is a cavity between the core material and the shell materials.

[0047] Preferably, in the shell material, the amorphous carbon is located in the outermost layer and the porous carbon is located in the second outermost layer.

[0048] According to an embodiment of the present invention, the silicon-carbon composite negative electrode material includes amorphous carbon, porous carbon, and silicon nanosheets; the silicon nanosheets are the core material, the porous carbon and the amorphous carbon are the shell materials, the amorphous carbon is located in the outermost layer, and the porous carbon is located in the second outermost layer; and there is a cavity between the silicon nanosheets and the porous carbon.

[0049] According to an embodiment of the present invention, in the silicon-carbon composite negative electrode material, the mass percentage content of the porous carbon is 3-8% (such as 3%, 4%, 5%, 6%, 7%, or 8%).

[0050] According to an embodiment of the present invention, in the silicon-carbon composite negative electrode material, the mass percentage content of the amorphous carbon is 1-5% (such as 1%, 2%, 3%, 4%, or 5%).

[0051] According to an embodiment of the present invention, in the silicon-carbon composite negative electrode material, the mass percentage content of the silicon nanosheets is 87-96% (such as 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or 96%).

[0052] According to an embodiment of the present invention, in the silicon-carbon composite negative electrode material, the average thickness of the silicon nanosheets is 15-50 nm, for example, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm; the average length of the silicon nanosheets is 150-500 nm, for example, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, or 500 nm.

[0053] According to an embodiment of the present invention, the first discharge capacity of the silicon-carbon composite negative electrode material at a current density of 200 mA g -1 is ≥2200 mAh / g, and the first charge-discharge efficiency is ≥90.0%.

[0054] According to an embodiment of the present invention, the capacity retention rate of the silicon-carbon composite negative electrode material after 100 cycles at a current density of 200 mA g -1 is ≥80%.

[0055] The present invention also provides the use of the above-mentioned silicon-carbon composite negative electrode material in a lithium-ion battery, preferably as a negative electrode material for a lithium-ion battery.

[0056] Advantages of the present invention:

[0057] In the present invention, sodium ions on negatively charged sodium alginate and positive ions (Ca 2+)The exchange effect involves inserting sodium alginate into the interlayer of montmorillonite, and then weakening the interlayer force of montmorillonite through electrostatic repulsion to increase the interlayer spacing of montmorillonite platelets. Then, using the cations (Na + ) of sodium alginate and Zn 2+ a displacement cross-linking reaction occurs to obtain a hydrogel. After drying the hydrogel, it is calcined. When the first calcination is carried out at a temperature of 500 °C - 800 °C, the zinc oxide and carbon generated by the decomposition of zinc alginate exfoliate the montmorillonite into two-dimensional montmorillonite nanosheets. Then, the temperature is raised to 900 °C - 1100 °C for the second calcination. At this time, the generated carbon reduces the zinc oxide to generate zinc vapor and escapes from the reaction system, obtaining two-dimensional montmorillonite nanosheets coated with porous carbon. This process can solve the problem of difficult dispersion of silicon nanosheets. Then, a magnesiothermic reduction reaction is carried out. During this process, the two-dimensional montmorillonite nanosheets can be reduced to form a mixture composed of silicon nanosheets, alumina, magnesium oxide and other impurities. After removing impurity components such as alumina and magnesium oxide by using treatment processes such as water washing and acid washing, a cavity will be formed between the silicon nanosheets and the porous carbon, obtaining porous carbon@cavity@silicon nanosheets. The existence of the cavity can alleviate the volume expansion of silicon and solve the problem of low cycling performance of silicon nanosheets. Finally, amorphous carbon is coated on the surface of the porous carbon to improve the initial Coulomb efficiency of the silicon-carbon composite anode material. Detailed implementation manners

[0058] The technical solutions of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only illustrative of and explanatory of the present invention, and should not be construed as limiting the protection scope of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0059] Unless otherwise specified, the experimental methods used in the following embodiments are all conventional methods; unless otherwise specified, the reagents, materials, etc. used in the following embodiments can all be obtained from commercial channels.

[0060] Example 1

[0061] (1) Add 1 g of montmorillonite (average thickness of 80 nm, average length of 5 μm) to 1000 g of deionized water, stir and sonicate for 10 min, then add 3 g of sodium alginate and stir at 70 °C until completely dissolved, and then let it stand for 14 hours to obtain a uniformly dispersed suspension;

[0062] (2) Dropwise add 20 g of the suspension in step (1) into 90 g of an aqueous solution of Zn(NO3)2 . 6H2O (20 g / L), stir for a cross-linking reaction for 16 hours, then filter to obtain hydrogel balls with a diameter of 3 mm, and wash them 2 times with deionized water;

[0063] (3) The hydrogel spheres from step (2) are dried at 60 °C for 12 hours, then calcined at 600 °C for 6 hours in an argon atmosphere, and then further heated to 950 °C for 6 hours and cooled to obtain two-dimensional montmorillonite nanosheets coated with porous carbon;

[0064] (4) 0.8 g of the two-dimensional montmorillonite nanosheets coated with porous carbon from step (3) and 1 g of magnesium powder are mixed under anhydrous conditions with argon flowing through, and then ground to obtain a uniformly mixed powder; the powder is placed in a tubular furnace with argon flowing through and subjected to a magnesiothermic reduction reaction at 850 °C for 6 hours to obtain an initial reaction product containing silicon nanosheets coated with porous carbon;

[0065] (5) The initial product from step (4) is ground, dissolved in deionized water, pickled in a nitric acid solution with a pH of 4 for 2 h, and then pickled in a hydrofluoric acid solution with a mass percentage concentration of 2.0% for 40 min, filtered, and dried to obtain porous carbon@hollow@silicon nanosheets;

[0066] (6) 100 g of the porous carbon@hollow@silicon nanosheets from step (5) and 5 g of pitch are mixed and treated at 1000 °C for 4 hours under N2 protection, cooled to room temperature, and then dispersed, sieved, and demagnetized to obtain a silicon-carbon composite anode material.

[0067] The silicon-carbon composite anode material has a core-shell structure of amorphous carbon@porous carbon@hollow@silicon nanosheets, with the outermost layer being amorphous carbon, the second outermost layer being porous carbon, the core being silicon nanosheets, and there being a hollow cavity between the silicon nanosheets and the porous carbon. The average thickness of the silicon nanosheets in the silicon-carbon composite anode material is 34 nm, the average length is 200 nm, the mass ratio of the porous carbon is 4%, and the mass ratio of the amorphous carbon is 2%.

[0068] Example 2

[0069] (1) 2 g of montmorillonite (average thickness of 120 nm and average length of 10 μm) is added to 1000 g of deionized water, stirred and ultrasonically treated for 30 min, then 8 g of sodium alginate is added and stirred at 90 °C until completely dissolved, and then left to stand for 24 hours to obtain a uniformly dispersed suspension.

[0070] (2) 30 g of the suspension from step (1) is added dropwise to 100 g of an aqueous solution of Zn(NO3)2·6H2O (40 g / L), stirred for a crosslinking reaction for 24 hours, filtered to obtain hydrogel spheres with a diameter of 5 mm, and washed 4 times with deionized water;

[0071] (3) The hydrogel spheres from step (2) are dried at 70 °C for 20 hours, then calcined at 700 °C for 4 hours in an argon atmosphere, and then further heated to 1000 °C for 3 hours and cooled to obtain two-dimensional montmorillonite nanosheets coated with porous carbon;

[0072] (4) Mix 0.9 g of the two-dimensional montmorillonite nanosheets coated with porous carbon and 1 g of magnesium powder obtained in step (3) under anhydrous conditions and in an argon atmosphere, and then grind to obtain a uniformly mixed powder. Place the powder in a tubular furnace with argon flowing through it and carry out a magnesiothermic reduction reaction at 950 °C for 3 hours to obtain an initial reaction product containing silicon nanosheets coated with porous carbon.

[0073] (5) Grind the initial product obtained in step (4), dissolve it with deionized water, then carry out acid washing in a nitric acid solution with a pH of 3 for 2 h, and then carry out acid washing in a hydrofluoric acid solution with a mass percentage concentration of 4% for 30 min. Filter and dry to obtain porous carbon@hollow@silicon nanosheets.

[0074] (6) Mix 100 g of the porous carbon@hollow@silicon nanosheets obtained in step (5) and 10 g of pitch, and treat them at 1100 °C for 4 hours under N2 protection. After cooling to room temperature, carry out dispersion, screening, and demagnetization to obtain a silicon-carbon composite anode material.

[0075] The silicon-carbon composite anode material has a core-shell structure of amorphous carbon@porous carbon@hollow@silicon nanosheets. The outermost layer is amorphous carbon, the second outermost layer is porous carbon, the core is silicon nanosheets, and there is a hollow between the silicon nanosheets and the porous carbon. The average thickness of the silicon nanosheets in the silicon-carbon composite anode material is 25 nm, the average length is 350 nm, the mass ratio of the porous carbon is 6%, and the mass ratio of the amorphous carbon is 4%.

[0076] Example 3

[0077] (1) Add 1 g of montmorillonite (average thickness: 100 nm, average length: 8 μm) to 1000 g of deionized water, stir and sonicate for 20 min, then add 5 g of sodium alginate and stir at 80 °C until completely dissolved, and then let it stand for 20 hours to obtain a uniformly dispersed suspension.

[0078] (2) Dropwise add 30 g of the suspension obtained in step (1) into 100 g of an aqueous solution of Zn(NO3)2·6H2O (30 g / L), stir for a crosslinking reaction for 20 hours, then filter to obtain hydrogel spheres with a diameter of 4 mm, and wash them 3 times with deionized water.

[0079] (3) Dry the hydrogel spheres obtained in step (2) at 90 °C for 20 hours, then calcine them at 800 °C for 2 hours in an argon atmosphere, and then continue to heat up to 1050 °C for calcination for 3 hours, and then cool to obtain two-dimensional montmorillonite nanosheets coated with porous carbon.

[0080] (4) Mix 1 g of the two-dimensional montmorillonite nanosheets coated with porous carbon and 1 g of magnesium powder under anhydrous conditions with argon flowing, and then grind to obtain a uniformly mixed powder. Place the powder in a tubular furnace with argon flowing and carry out a magnesiothermic reduction reaction at 900 °C for 4 hours to obtain an initial reaction product containing silicon nanosheets coated with porous carbon.

[0081] (5) Grind the initial reaction product obtained in step (4), dissolve it with deionized water, then carry out pickling with a nitric acid solution at pH 4 for 2 h, and then carry out pickling in a hydrofluoric acid solution with a mass percentage concentration of 5.0% for 60 min, filter, and dry to obtain porous carbon@hollow@silicon nanosheets.

[0082] (6) Mix 100 g of the porous carbon@hollow@silicon nanosheets obtained in step (5) and 10 g of asphalt, and treat them at 1000 °C for 7 hours under N2 protection. After cooling to room temperature, carry out dispersion, screening, and demagnetization to obtain a silicon-carbon composite anode material.

[0083] The silicon-carbon composite anode material has a core-shell structure of amorphous carbon@porous carbon@hollow@silicon nanosheets. The outermost layer is amorphous carbon, the second outermost layer is porous carbon, the core is silicon nanosheets, and there is a hollow between the silicon nanosheets and the porous carbon. The average thickness of the silicon nanosheets in the silicon-carbon composite anode material is 20 nm, the average length is 280 nm, the mass fraction of the porous carbon is 7%, and the mass fraction of the amorphous carbon is 3%.

[0084] Electrochemical performance test:

[0085] Half-cell test method: Mix the silicon-carbon composite anode material prepared in the example, conductive carbon black (SP), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) evenly at a mass ratio of 95:1:1.5:2.5, coat it on a copper foil, and place the coated electrode in a vacuum drying oven at 120 °C for 12 hours. Carry out the assembly of a simulated battery in a Braun glove box under argon protection. The electrolyte is 1 M-LiPF6 + EC:DEC:DMC (volume ratio 1:1:1), and a lithium metal sheet is used as the counter electrode. Carry out the test of the simulated battery in a Neware battery test cabinet at 5 V and 10 mA. The charge-discharge voltage is 0.01 - 1.5 V, and the current density is 200 mA g -1 The first discharge capacity and the first Coulombic efficiency obtained from the test are listed in Table 1.

[0086] Full-cell test method: Use the silicon-carbon composite anode material prepared in the example as the anode, lithium cobaltate as the cathode, and 1 M-LiPF6 + EC:DEC:DMC (volume ratio 1:1:1) solution as the electrolyte to assemble a full cell. Carry out charge and discharge at room temperature at a current density of 200 mA g -1 The cycle performance obtained from the test is listed in Table 1.

[0087] Table 1 Electrochemical Performance Test Results

[0088]

[0089] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a silicon-carbon composite negative electrode material, the preparation method comprising the following steps: (1) mixing montmorillonite and sodium alginate to prepare a suspension; (2) mixing the suspension of step (1) with an aqueous solution of zinc salt to carry out a cross-linking reaction to prepare a hydrogel; (3) drying the hydrogel in step (2) and then calcining it to prepare porous carbon-coated two-dimensional montmorillonite nanosheets; (4) mixing the porous carbon-coated two-dimensional montmorillonite nanosheets and magnesium powder in step (3) under anhydrous and inert atmosphere and placing the mixture in a reaction vessel to perform a magnesium thermal reduction reaction to obtain an initial reaction product containing porous carbon-coated silicon nanosheets; (5) grinding, washing with water, and acid washing the initial reaction product of step (4) to prepare porous carbon@cavity@silicon nanosheets; (6) The porous carbon@cavity@silicon nanosheets of step (5) are subjected to carbon coating and sintering treatment to prepare the silicon-carbon composite negative electrode material.

2. The preparation method according to claim 1, wherein In step (1), the montmorillonite has a lamellar structure, the average thickness of the montmorillonite lamellar is 80nm-120nm; the average length of the montmorillonite lamellar is 5μm-10μm; And / or, in step (1), the mass ratio of montmorillonite to sodium alginate is 1:(3-5); And / or, in step (1), the concentration of montmorillonite in the suspension is 0.1%-0.3%.

3. The preparation method according to claim 1, wherein In step (2), the zinc salt is selected from at least one of zinc nitrate, zinc chloride, zinc sulfate and zinc acetate; And / or, in step (2), the concentration of the aqueous solution of the zinc salt is 10-50 g / L; And / or, in step (2), the mass ratio of the suspension to the aqueous solution of the zinc salt is (10-50):100; And / or, in step (2), the mixing method is to add the suspension of step (1) dropwise into the aqueous solution of zinc salt; And / or, in step (2), the hydrogel is in the form of a spherical hydrogel; And / or, in step (2), the diameter of the spherical hydrogel is 2 mm-5 mm.

4. The preparation method according to claim 1, wherein In step (3), the calcination includes a first calcination and a second calcination, the temperature of the first calcination is 500°C-800°C, and the time of the first calcination is 2-6 hours; the temperature of the second calcination is 900°C-1100°C, and the time of the first calcination is 2-10 hours.

5. The preparation method according to claim 1, wherein In step (4), the weight ratio of the magnesium powder to the porous carbon-coated two-dimensional montmorillonite nanosheets is (0.8-1):1; And / or, in step (4), the temperature of the magnesium thermal reduction reaction is 800° C.-1000° C., and the time of the magnesium thermal reduction reaction is 2-6 hours.

6. The preparation method according to any one of claims 1 to 5, wherein: In step (5), the pickling includes a first pickling and a second pickling; the acid used in the first pickling includes one or more of hydrochloric acid, sulfuric acid and nitric acid, the pH of the acid solution used in the first pickling is 2-4, and the stirring time of the first pickling is 30min-10h; the acid used in the second pickling is hydrofluoric acid, the mass percentage concentration of the acid solution used in the second pickling is 0.1%-5%, and the stirring time of the second pickling is 30min-1h.

7. A silicon-carbon composite negative electrode material prepared by the method according to any one of claims 1 to 6.

8. The silicon-carbon composite negative electrode material according to claim 7, wherein: The silicon-carbon composite negative electrode material comprises amorphous carbon, porous carbon and silicon nanosheets; the silicon nanosheets are core materials, the porous carbon and amorphous carbon are shell materials, and there is a cavity between the core material and the shell material.

9. The silicon-carbon composite negative electrode material according to claim 8, wherein: The silicon-carbon composite negative electrode material comprises amorphous carbon, porous carbon and silicon nanosheets; the silicon nanosheets are core materials, the porous carbon and amorphous carbon are shell materials, the amorphous carbon is located in the outermost layer, and the porous carbon is located in the second outermost layer; and there is a cavity between the silicon nanosheets and the porous carbon.

10. The silicon-carbon composite negative electrode material according to claim 8 or 9, wherein: In the silicon-carbon composite negative electrode material, the mass percentage of porous carbon is 3-8%; and / or the mass percentage of amorphous carbon is 1-5%; and / or the mass percentage of silicon nanosheets is 87-96%.

11. The silicon-carbon composite negative electrode material according to claim 8 or 9, wherein: In the silicon-carbon composite negative electrode material, the average thickness of the silicon nanosheets is 15-50 nm; the average length of the silicon nanosheets is 150-500 nm.

12. Use of the silicon-carbon composite negative electrode material according to any one of claims 7 to 11 in a lithium-ion battery.

13. The use according to claim 12, wherein The silicon-carbon composite negative electrode material is used as a negative electrode material for lithium-ion batteries.

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