Graphene / silicon@hard carbon@carbon nanotube composite negative electrode material and preparation method and application thereof

Abstract

The application provides a graphene / silicon@hard carbon@carbon nanotube composite negative electrode material and a preparation method and application thereof. The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a three-dimensional conductive network structure, wherein silicon@hard carbon@carbon nanotubes are uniformly interpenetrated between graphene layers. The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material can effectively relieve the volume expansion of silicon material. The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material comprises flexible graphene and high-strength carbon nanotubes, and the crosslinked network structure formed by the two can significantly enhance the electrical conductivity of the composite negative electrode material. The lithium ion battery comprising the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has better cycle performance and rate performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of negative electrode materials for lithium ion batteries, and particularly relates to a graphene / silicon@hard carbon@carbon nanotube composite negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium ion batteries have a series of advantages such as high specific capacity, stable working voltage, good safety, no memory effect, etc., and are widely used in notebook computers, mobile phones, instruments and meters and many other portable electronic instruments and equipment. With the rapid development of various electronic devices and electric vehicles, people's requirements for the energy and cycle life of lithium ion batteries are becoming higher and higher. The negative electrode material is an important component in the battery, which together with the positive electrode material determines the key performances of the lithium ion battery such as cycle life, capacity and safety, and has become the focus of research in various countries.

[0003] At present, the specific capacity of commercialized graphite-based negative electrode material is low, only 372 mAh / g, which limits the improvement of the overall capacity of lithium ion batteries and cannot meet the market demand. It is reported that the theoretical lithium storage capacity of silicon is as high as 4200 mAh / g, the lithium intercalation platform is slightly higher than that of graphite, and the safety hidden danger is small, which is an excellent substitute for graphite-based negative electrode material; however, silicon shows a volume change of up to 300% during charging and discharging, thus easily leading to the pulverization of silicon particles, the destruction of the internal conductive network of the electrode, and the occurrence of problems such as poor conductivity.

[0004] In order to solve a series of problems caused by the volume expansion of silicon, those skilled in the art modify it by various methods, including nanocrystallization, alloying, porosity and dispersion in various network systems, etc. These methods can improve the performance of the battery to some extent, but there are still many problems such as poor cycle performance, excessive expansion, low tap density, etc. Therefore, how to more effectively alleviate the volume expansion of silicon particles, ensure the cycle stability of the battery, and obtain a silicon-carbon negative electrode material with good cycle performance and rate performance is still a technical hotspot to be solved in the current lithium ion battery field. SUMMARY

[0005] In order to improve the deficiencies of the prior art, the present application provides a graphene / silicon@hard carbon@carbon nanotube composite negative electrode material and a preparation method and application thereof. The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material can effectively alleviate the volume expansion of silicon material. The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material includes flexible graphene and high-strength carbon nanotubes, and the cross-linked network structure formed by the two can significantly enhance the electrical conductivity of the composite negative electrode material. The lithium ion battery composed of the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has good cycle performance and rate performance.

[0006] Specifically, the present application provides the following technical solutions:

[0007] A method for preparing a graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the method comprising the following steps:

[0008] (1) Kneading nano-silicon, organic carbon and a transition metal salt to obtain a kneaded material as a precursor A;

[0009] (2) Passing carbon source gas and hydrogen into the precursor A of step (1) to perform heat treatment, and then purifying with an acid to obtain silicon@hard carbon@carbon nanotube;

[0010] (3) Grafting a silane coupling agent to the surface of the silicon@hard carbon@carbon nanotube of step (2) to obtain a precursor B;

[0011] (4) Mixing the precursor B of step (3) with graphene oxide to react to prepare the silicon@hard carbon@carbon nanotube / graphene composite negative electrode material.

[0012] According to an embodiment of the present application, in step (1), the average particle size of the nano-silicon is 10-50 nm, for example, 10 nm, 20 nm, 30 nm, 40 nm or 50 nm; and the purity of the nano-silicon is 99.9% or more.

[0013] According to an embodiment of the present application, in step (1), the organic carbon is at least one of petroleum-based resin (for example, petroleum-based resin C5 and / or petroleum-based resin C9), phenol resin, epoxy resin, starch, glucose and cellulose, etc.

[0014] According to an embodiment of the present application, in step (1), the carbon residue value of the organic carbon is 3-10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

[0015] According to an embodiment of the present application, in step (1), the transition metal is at least one of iron, cobalt, nickel and chromium; and the salt is at least one of nitrate, chloride, sulfate, acetate and oxalate.

[0016] According to an embodiment of the present application, in step (1), the transition metal salt is at least one of ferric nitrate, ferric chloride, ferric sulfate, ferric acetate, ferric oxalate, cobalt nitrate, cobalt chloride, cobalt sulfate, cobalt acetate, cobalt oxalate, nickel nitrate, nickel chloride, nickel sulfate, nickel acetate, nickel oxalate, chromium nitrate, chromium chloride, chromium sulfate, chromium acetate and chromium oxalate.

[0017] According to an embodiment of the present application, in step (1), the mass ratio of the organic carbon to the nano-silicon is (3-10):100, for example, 3:100, 4:100, 5:100, 6:100, 7:100, 8:100, 9:100 or 10:100.

[0018] According to an embodiment of the present application, in step (1), the mass ratio of the nanosilicon to the transition metal salt is 100:(1-5), for example, 100:1, 100:2, 100:3, 100:4 or 100:5.

[0019] According to an embodiment of the present application, in step (1), the kneading, for example, comprises: placing the nanosilicon, the organic carbon and the transition metal salt in a kneader, controlling the kneading temperature to be 60-100℃, the rotating speed to be 50-500r / min, and the kneading time to be 1-300min, to obtain a kneaded material.

[0020] According to an embodiment of the present application, in step (1), the kneading is carried out under a nitrogen atmosphere.

[0021] According to an embodiment of the present application, in step (1), the precursor A is a mixed system of nanosilicon, organic carbon and transition metal salt, and further, the organic carbon with low carbon residue value can adhere the transition metal salt to the surface of the nanosilicon as a binder.

[0022] According to an embodiment of the present application, in step (2), the temperature of the heat treatment is 500℃-800℃, for example, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃; and the time of the heat treatment is 5-10 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours.

[0023] According to an embodiment of the present application, in step (2), during the heat treatment, the organic carbon is carbonized at high temperature to form hard carbon, and since the organic carbon has a low carbon residue value, the hard carbon formed after the carbonization of the organic carbon has a low content, which can effectively avoid the secondary agglomeration of the nanosilicon, and the presence of the hard carbon can tightly combine the transition metal salt and the nanosilicon together. Further, the transition metal salt is reduced to form transition metal under the hydrogen condition, and the transition metal can catalyze the in-situ generation of carbon nanotubes on the surface of the hard carbon, and the in-situ formed carbon nanotubes are coated on the surface of the hard carbon, which can greatly improve the conductivity of the nanosilicon.

[0024] According to an embodiment of the present application, in step (2), the silicon@hard carbon@carbon nanotube comprises hard carbon, nanosilicon and carbon nanotube; the hard carbon is coated on the surface of the nanosilicon, and the carbon nanotube is coated on the surface of the hard carbon.

[0025] According to an embodiment of the present application, in step (2), in the silicon@hard carbon@carbon nanotube, the mass ratio of the nanosilicon to the hard carbon is 6:(0.02-0.045); and the mass ratio of the nanosilicon to the carbon nanotube is 6:(1.8-2.2).

[0026] According to an embodiment of the present application, in step (2), the flow rate of the carbon source gas is 1-3 L / min·g of precursor A, that is, 1-3 L of carbon source gas is introduced into 1 g of precursor A per minute.

[0027] According to an embodiment of the present application, in step (2), the volume flow rate ratio of the carbon source gas and hydrogen is 1:3-15, for example, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:10, 1:12, 1:13, 1:14 or 1:15.

[0028] According to an embodiment of the present application, in step (2), the carbon source gas is selected from at least one of acetylene, ethylene, methane, ethane, propane and n-butane.

[0029] According to an embodiment of the present application, in step (2), the precursor A of step (1) is introduced with the carbon source gas, inert gas and hydrogen, and then subjected to heat treatment. Preferably, at room temperature, the precursor A of step (1) is introduced with the inert gas and hydrogen, and then subjected to heat treatment at a temperature of 500-800 ℃, and then the carbon source gas is introduced at this temperature for 5-10 hours, and then the introduction of the carbon source gas and hydrogen is stopped, and then the inert gas is stopped after cooling to room temperature.

[0030] According to an embodiment of the present application, step (2) specifically comprises the following steps:

[0031] The inert gas and hydrogen are introduced into the precursor A of step (1), and then the carbon source gas is introduced after being heated to a temperature of 500-800 ℃, and then the temperature is kept for 5-10 hours, and then the introduction of the carbon source gas and hydrogen is stopped, and then the composite is obtained by cooling to room temperature;

[0032] The composite cooled to room temperature is immersed in an acid solution for purification, and then subjected to dehydration and drying treatment to obtain silicon@hard carbon@carbon nanotube.

[0033] The inert gas is selected from at least one of nitrogen, argon and the like.

[0034] The volume ratio of the inert gas and hydrogen is (1-3):1.

[0035] The acid solution is one or more of nitric acid solution, hydrochloric acid solution and sulfuric acid solution; the pH of the acid solution is 3-5. The purification time is 0.5-6 h.

[0036] According to an embodiment of the present application, in step (2), the acid purification can remove the transition metal generated in situ during the heat treatment.

[0037] According to an embodiment of the present application, in step (3), the silane coupling agent is selected from a silane coupling agent containing an amino group, for example, 3-aminopropyl triethoxysilane (APTES).

[0038] According to the embodiment of the present application, the step (3) specifically comprises the following steps:

[0039] Disperse the silicon@hard carbon@carbon nanotube into an organic solvent to obtain a dispersion liquid a;

[0040] Add the silane coupling agent into the hydroalcoholic solution to obtain a dispersion liquid b;

[0041] Mix the dispersion liquid a and the dispersion liquid b, and react to obtain the silane coupling agent grafted silicon@hard carbon@carbon nanotube, i.e. the precursor B.

[0042] The organic solvent is xylene.

[0043] The mass-volume ratio of the silicon@hard carbon@carbon nanotube and the organic solvent is 1g:(100-200ml).

[0044] The mass-volume ratio of the silane coupling agent and the hydroalcoholic solution is 1g:(400-1000ml).

[0045] The mass ratio of the silane coupling agent and the silicon@hard carbon@carbon nanotube is 1:(20-40).

[0046] The hydroalcoholic solution is a mixed solution of ethanol and water, and the volume ratio of the ethanol and the water is 7-9:1-3, for example, 7:3, 8:2 or 9:1.

[0047] The reaction time is 4-8 hours, and the reaction temperature is room temperature.

[0048] After the reaction, the post-processing steps such as suction filtration, anhydrous ethanol washing and vacuum drying are further included.

[0049] According to the embodiment of the present application, in the step (3), the precursor B is a composite of the silicon@hard carbon@carbon nanotube surface grafted silane coupling agent, and the introduction of the silane coupling agent can make the surface of the silicon@hard carbon@carbon nanotube composite positively charged.

[0050] According to the embodiment of the present application, in the step (4), the reaction temperature is 700-900℃, such as 700℃, 750℃, 800℃, 850℃ or 900℃; and the reaction time is 2-6 hours, such as 2 hours, 3 hours, 4 hours, 5 hours or 6 hours.

[0051] According to the embodiment of the present application, in the step (4), the mass ratio of the precursor B and the graphene oxide is (7.8-8.3):(2.8-3.2).

[0052] According to an embodiment of the present application, in step (4), the reaction is carried out under a protective atmosphere of nitrogen and hydrogen. Preferably, the volume ratio of nitrogen to hydrogen is (1-3):1.

[0053] The present application also provides a graphene / silicon@hard carbon@carbon nanotube composite negative electrode material prepared by the above method.

[0054] According to an embodiment of the present application, the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a three-dimensional conductive network structure, wherein the silicon@hard carbon@carbon nanotube is uniformly interpenetrated between the graphene layers, and the carbon nanotube and the graphene form a three-dimensional conductive network of rigidity and flexibility.

[0055] According to an embodiment of the present application, the silicon@hard carbon@carbon nanotube comprises hard carbon, nanosilicon and carbon nanotube; the hard carbon is coated on the surface of the nanosilicon, and the carbon nanotube is coated on the surface of the hard carbon.

[0056] According to an embodiment of the present application, in the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass fraction of graphene is 25-30%, such as 25%, 26%, 27%, 28%, 29% or 30%.

[0057] According to an embodiment of the present application, in the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass ratio of silicon to hard carbon is 6:(0.02-0.045).

[0058] According to an embodiment of the present application, in the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass ratio of silicon to carbon nanotube is 6:(1.8-2.2).

[0059] According to the present application, the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a first discharge capacity ≥2000 mAh / g and a first charge-discharge efficiency ≥70.0% at a current density of 100 mA g -1

[0060] According to the present application, the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a capacity retention rate ≥80% after 100 cycles at a current density of 100 mA g -1

[0061] The present application also provides the use of the above graphene / silicon@hard carbon@carbon nanotube composite negative electrode material in lithium ion batteries, preferably as a lithium ion battery negative electrode material.

[0062] The present application has the following beneficial effects:

[0063] ​​The present application firstly adheres transition metal salt on the surface of nano-silicon by selecting organic carbon with suitable residual carbon value and further adjusting the mass ratio of organic carbon and nano-silicon, and the carbonized organic carbon after heat treatment generates low content of hard carbon, which can effectively avoid the secondary agglomeration of nano-silicon, and the existence of hard carbon can tightly combine the transition metal salt and nano-silicon together. Further, the transition metal salt is reduced to form transition metal under hydrogen condition, and the transition metal can catalyze the in-situ generation of carbon nanotube on the surface of hard carbon from carbon source gas, and the in-situ formed carbon nanotube is coated on the surface of hard carbon material, which can greatly improve the conductivity of nano-silicon. Then the silane coupling agent is grafted to the surface of silicon@hard carbon@carbon nanotube particles to obtain silicon@hard carbon@carbon nanotube with amino group at the end. Finally, the silicon@hard carbon@carbon nanotube / graphene composite material with three-dimensional conductive framework matrix is obtained by electrostatic self-assembly and reaction with negatively charged graphene oxide.

[0064] The silicon@hard carbon@carbon nanotube / graphene composite material has the following advantages: on the one hand, the carbon nanotube has high conductivity, which can improve the conductivity of nano-silicon; on the other hand, the high strength can effectively alleviate the stacking phenomenon between graphene layers, avoid the large increase of graphene resistivity in the vertical interlayer direction caused by graphene stacking, hinder the transmission of Li + , and affect the rate performance of the composite negative electrode material; on the other hand, the bidirectional limited structure of flexible graphene and high-strength carbon nanotube is more conducive to improving the structural stability of the composite negative electrode material, avoiding the falling of nano-silicon from graphene, so that the composite negative electrode material has good cycle performance. In summary, the cross-linked network structure formed by graphene and carbon nanotube enhances the electrical conductivity of the composite negative electrode material, which is conducive to the embedding of lithium ions from multiple directions, and significantly improves the rate performance and cycle performance of the composite negative electrode material. DETAILED DESCRIPTION

[0065] The preparation method of the present application will be further described in detail below in combination with specific examples. It should be understood that the following examples are only illustratively described and explained, and should not be interpreted as limiting the scope of protection of the present application. Any technology realized based on the above description of the present application is covered within the scope of protection intended by the present application.

[0066] The experimental methods used in the following examples are conventional methods unless otherwise specified; the reagents, materials, etc. used in the following examples can be obtained from commercial channels unless otherwise specified.

[0067] Example 1

[0068] (1) The nano-silicon (D 5010 g of the silicon@hard carbon@carbon nanotube was dispersed in 1200 ml of xylene and ultrasonically vibrated for 8 h. The above dispersion liquid was transferred into a flask. 0.3 g of APTES was added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 and hydrolyzed for 50 min. The hydrolysis liquid was added to the flask, and the reaction was stirred at room temperature for 10 h. After filtration and washing with anhydrous ethanol twice, vacuum drying at 50°C was performed to obtain the precursor B.

[0069] (2) 10 g of the silicon@hard carbon@carbon nanotube was dispersed in 1200 ml of xylene and ultrasonically vibrated for 8 h. The above dispersion liquid was transferred into a flask. 0.3 g of APTES was added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 and hydrolyzed for 50 min. The hydrolysis liquid was added to the flask, and the reaction was stirred at room temperature for 10 h. After filtration and washing with anhydrous ethanol twice, vacuum drying at 50°C was performed to obtain the precursor B;

[0070] (3) 7.85 g of the precursor B of step (2) was mixed with 2.8 g of graphene oxide, and high-temperature treatment was performed at 800°C for 3 h to obtain the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material.

[0071] The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a three-dimensional conductive network structure, in which the silicon@hard carbon@carbon nanotube is uniformly interpenetrated between the graphene layers, and the carbon nanotube and the graphene construct a three-dimensional conductive network with rigidity and flexibility. In the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass ratio of graphene is 26.2%; the mass ratio of silicon and hard carbon is 6:0.04, and the mass ratio of silicon and carbon nanotube is 6:(1.82).

[0072] Example 2

[0073] (1) 10 g of nano-silicon (D 50 10 g of the silicon@hard carbon@carbon nanotube was dispersed in 1200 ml of xylene and ultrasonically vibrated for 8 h. The above dispersion liquid was transferred into a flask. 0.3 g of APTES was added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 and hydrolyzed for 50 min. The hydrolysis liquid was added to the flask, and the reaction was stirred at room temperature for 10 h. After filtration and washing with anhydrous ethanol twice, vacuum drying at 50°C was performed to obtain the precursor B.

[0074] (2) 10 g of the silicon@hard carbon@carbon nanotube was dispersed in 1500 ml of dimethylbenzene and ultrasonically vibrated for 10 h. The above dispersion liquid was moved into a flask. 0.4 g of APTES was measured and added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 to hydrolyze for 50 min. The hydrolysis liquid was added to the flask, and the reaction was stirred at room temperature for 12 h. Filtration, washing with anhydrous ethanol twice, and vacuum drying at 50°C gave the precursor B;

[0075] (3) 8 g of the precursor B of step (2) was mixed with 3 g of graphene oxide, and high-temperature treatment was performed at 900°C for 4 h to obtain the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material.

[0076] The graphene / silicon@hard carbon@carbon nanotube composite negative electrode material has a three-dimensional conductive network structure, in which the silicon@hard carbon@carbon nanotube is uniformly interpenetrated between the graphene layers, and the carbon nanotube and graphene construct a three-dimensional conductive network of rigidity and flexibility. In the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass ratio of graphene is 27.3%; the mass ratio of silicon and hard carbon is 6:0.034, and the mass ratio of silicon and carbon nanotube is 6:1.96.

[0077] Example 3

[0078] (1) Nanosilicon (D 50 The obtained kneaded material was precursor A after cooling. 10 g of the precursor A was placed in a rotary furnace, nitrogen gas with a flow rate of 120 L / min and hydrogen gas with a flow rate of 80 L / min were introduced, the temperature was raised to 800°C, then ethane was introduced at a flow rate of 20 L / min for 6.0 h, the introduction of ethane and hydrogen was stopped, and the composite was cooled to room temperature. Then the composite was immersed in a nitric acid solution with a pH of 4 for reaction for 2 h, and then dehydrated and dried in sequence to obtain silicon@hard carbon@carbon nanotube;

[0079] (2) 10 g of the silicon@hard carbon@carbon nanotube was dispersed in 1200 ml of dimethylbenzene and ultrasonically vibrated for 12 h. The above dispersion liquid was moved into a flask. 0.45 g of APTES was measured and added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 to hydrolyze for 50 min. The hydrolysis liquid was added to the flask, and the reaction was stirred at room temperature for 15 h. Filtration, washing with anhydrous ethanol twice, and vacuum drying at 50°C gave the precursor B;

[0080] (3) Mix 8g of precursor B from step (2) with 3.2g of graphene oxide and perform high-temperature treatment at 800℃ for 5 hours to obtain the graphene / silicon@hard carbon@carbon nanotube composite anode material.

[0081] The graphene / silicon@hard carbon@carbon nanotube composite anode material has a three-dimensional conductive network structure, in which silicon@hard carbon@carbon nanotubes are uniformly interspersed between graphene layers, and carbon nanotubes and graphene construct a rigid-flexible three-dimensional conductive network. In the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass percentage of graphene is 28.5%; the mass ratio of silicon to hard carbon is 6:0.03, and the mass ratio of silicon to carbon nanotubes is 6:1.96.

[0082] Comparative Example 1

[0083] (1) Nano-silicon (D 50 A mixture of silicon@hard carbon@carbon nanotubes was prepared by adding 40 nm phenolic resin (with a residual carbon value of 8%) and nickel nitrate in a mass ratio of 100:7:3 into a kneader. The kneading speed was adjusted to 120 r / min, and the mixture was stirred at 80 °C under a nitrogen atmosphere for 1 hour. The resulting mixture was cooled and became precursor A. 10 g of precursor A was placed in a rotary kneader, and nitrogen gas at a flow rate of 105 L / min and hydrogen gas at a flow rate of 55 L / min were introduced. The temperature was raised to 700 °C, and then acetylene was introduced at a flow rate of 15 L / min for 3.0 h. The acetylene and hydrogen gas were then stopped, and the mixture was cooled to room temperature to obtain a composite. The composite was then soaked in a nitric acid solution with a pH of 5 and reacted for 2 h. After that, it was dehydrated and dried to obtain silicon@hard carbon@carbon nanotubes.

[0084] The silicon@hard carbon@carbon nanotube composite anode material does not possess a three-dimensional conductive network structure. This is because the anode material does not contain graphene, making it impossible to construct a three-dimensional conductive network. In the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass percentage of graphene is 0%; the mass ratio of silicon to hard carbon is 6:0.034, and the mass ratio of silicon to carbon nanotubes is 6:1.96.

[0085] Comparative Example 2

[0086] (1) Nano-silicon (D 50 40nm), phenolic resin (carbon residue value of 8%) and nickel nitrate were added to a kneader at a mass ratio of 100:7:3. The speed was adjusted to 120 r / min, and the mixture was mixed at 80°C under a nitrogen atmosphere for 1 hour. The resulting mixture was cooled and became precursor A. 10g of precursor A was placed in a rotary kneader, and nitrogen gas at a flow rate of 105 L / min and hydrogen gas at a flow rate of 55 L / min were introduced. The temperature was raised to 700°C and maintained for 3.0 h. The hydrogen gas was then stopped, and the mixture was cooled to room temperature to obtain a composite. The composite was then soaked in a nitric acid solution with a pH of 5 and reacted for 2 h. After dehydration and drying, silicon@hard carbon was obtained.

[0087] (2) 10 g of silicon@hard carbon was dispersed in 1500 ml of xylene and ultrasonically vibrated for 10 h, and the above dispersion liquid was transferred into a flask. 0.4 g of APTES was measured and added to a solution of 200 ml of ethanol and water in a volume ratio of 9:1 and hydrolyzed for 50 min. The hydrolyzed liquid was added to the flask, and the reaction was stirred at room temperature for 12 h, and then filtered and washed with anhydrous ethanol twice and dried at 50°C under vacuum to obtain the precursor B;

[0088] (3) 8 g of the precursor B of step (2) was mixed with 3 g of graphene oxide, and high-temperature treatment was carried out at 900°C for 4 h to obtain a graphene / silicon@hard carbon composite negative electrode material.

[0089] The graphene / silicon@hard carbon composite negative electrode material has a three-dimensional conductive network structure, in which silicon@hard carbon is interpenetrated between graphene layers, but the strength of the three-dimensional conductive network structure is poor. This is because the negative electrode material does not contain carbon nanotubes, and cannot be constructed into a three-dimensional conductive network with rigidity and flexibility. In the graphene / silicon@hard carbon@carbon nanotube composite negative electrode material, the mass ratio of graphene is 27.3%; the mass ratio of silicon and hard carbon is 6:0.034.

[0090] Electrochemical performance test:

[0091] Half-cell test method: The composite negative electrode material prepared in the examples and comparative examples: conductive carbon black (SP): carboxymethyl cellulose (CMC): styrene-butadiene rubber (SBR) = 95:1:1.5:2.5 (mass ratio) was mixed uniformly and coated on a copper foil, and the coated electrode was dried in a vacuum drying oven at 120°C for 12 h. Simulated battery assembly was carried out in an argon-protected Braun glove box, the electrolyte was 1M-LiPF6+EC:DEC:DMC (volume ratio 1:1:1), and lithium metal was used as the counter electrode. Simulated battery test was carried out in a 5V, 10mA new Wei battery test cabinet, the charge and discharge voltage was 0.01-1.5V, the charge and discharge rate was 0.1C, and the 0.1C initial charge capacity and 0.1C initial efficiency obtained by test were listed in Table 1.

[0092] Full-cell test method: the composite negative electrode material prepared in the examples and comparative examples was used as the negative electrode, lithium cobaltate was used as the positive electrode, and 1M-LiPF6+EC:DEC:DMC (volume ratio 1:1:1) solution was used as the electrolyte to assemble a full cell. The charge and discharge at room temperature was carried out at a rate of 0.1C, and the voltage range was 3.0-4.2V. The cycle performance obtained by test was listed in Table 1. Rate performance: after the current density was sequentially subjected to 0.1, 0.2, 0.5, 1, 2 and 5 A g -1 , the current density was again reduced to 0.1 A g -1 , and the capacity retention efficiency was tested.

[0093] Table 1 Electrochemical performance test results

[0094]

[0095] The above describes the embodiments of the present application. However, the present application is not limited to the above-described embodiments. Any modification, equivalent replacement, improvement, and the like made within the spirit and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A method for preparing a graphene / silicon@hard carbon@carbon nanotube composite anode material, the method comprising the following steps: (1) Nano-silicon, organic carbon and transition metal salt are mixed and kneaded to obtain a mixture as precursor A; (2) Carbon source gas and hydrogen are introduced into the precursor A in step (1) and heat treatment is performed. Then, it is purified with acid to obtain silicon@hard carbon@carbon nanotubes. (3) Graft the silane coupling agent onto the surface of the silicon@hard carbon@carbon nanotubes in step (2) to obtain precursor B; (4) The precursor B from step (3) is mixed with graphene oxide and reacted to prepare the silicon@hard carbon@carbon nanotube / graphene composite anode material. The silicon@hard carbon@carbon nanotube includes hard carbon, nano-silicon, and carbon nanotube; the hard carbon is coated on the surface of the nano-silicon, and the carbon nanotube is coated on the surface of the hard carbon; in the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass ratio of silicon to hard carbon is 6:(0.02-0.045).

2. The composite negative electrode material according to claim 1, wherein, In step (1), the organic carbon is at least one of petroleum-based resin, phenolic resin, epoxy resin, starch, glucose and cellulose; And / or, in step (1), the residual carbon value of the organic carbon is 3-10%; And / or, in step (1), the transition metal is selected from at least one of iron, cobalt, nickel and chromium; the salt is selected from at least one of nitrate, chloride, sulfate, acetate and oxalate.

3. The composite negative electrode material according to claim 1, wherein, In step (1), the mass ratio of organic carbon to nano-silicon is (3-10):100; And / or, in step (1), the mass ratio of the nano-silicon to the transition metal salt is 100:(1-5).

4. The composite negative electrode material according to claim 1, wherein, In step (2), the temperature of the heat treatment is 500℃-800℃; the time of the heat treatment is 5-10 hours. And / or, in step (2), the flow rate of the carbon source gas is 1-3 L / min·g precursor A, that is, 1-3 L of carbon source gas is introduced into 1 g precursor A per minute. And / or, in step (2), the volume flow ratio of the carbon source gas and hydrogen is 1:3-15; And / or, in step (2), the carbon source gas is selected from at least one of acetylene, ethylene, methane, ethane, propane and n-butane.

5. The composite negative electrode material according to claim 4, wherein, Step (2) includes the following steps: at room temperature, inert gas and hydrogen are introduced into precursor A from step (1), the temperature is raised to 500℃~800℃ and then carbon source gas is introduced, the temperature is maintained at this temperature for 5-10 hours, the introduction of carbon source gas and hydrogen is stopped, and after cooling to room temperature, the introduction of inert gas is stopped.

6. The composite negative electrode material according to any one of claims 1-5, wherein, In step (3), the silane coupling agent is selected from 3-aminopropyltriethoxysilane (APTES).

7. The composite negative electrode material according to claim 1, wherein, Step (3) specifically includes the following steps: Silicon@hard carbon@carbon nanotubes were dispersed in an organic solvent to obtain dispersion a; A silane coupling agent was added to an aqueous alcohol solution to obtain dispersion b; Dispersions a and b were mixed and reacted to obtain silicon@hard carbon@carbon nanotubes grafted with silane coupling agent, i.e., precursor B.

8. The composite negative electrode material according to claim 7, wherein, The mass-to-volume ratio of silicon@hard carbon@carbon nanotubes to organic solvent is 1 g:(100-200 ml); and / or, the mass-to-volume ratio of silane coupling agent to aqueous alcohol solution is 1 g:(400-1000 ml); and / or, the mass ratio of silane coupling agent to silicon@hard carbon@carbon nanotubes is 1:(20-40).

9. The composite negative electrode material according to any one of claims 1-5, wherein, In step (4), the reaction temperature is 700℃-900℃; the reaction time is 2-6 hours. And / or, in step (4), the mass ratio of the precursor B to graphene oxide is (7.8-8.3):(2.8-3.2).

10. A graphene / silicon@hard carbon@carbon nanotube composite anode material prepared by the method according to any one of claims 1-9.

11. The graphene / silicon@hard carbon@carbon nanotube composite anode material according to claim 8, wherein, The graphene / silicon@hard carbon@carbon nanotube composite anode material has a three-dimensional conductive network structure, wherein silicon@hard carbon@carbon nanotubes are uniformly interspersed between graphene layers. And / or, in the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass percentage of graphene is 25-30%; And / or, in the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass ratio of silicon to hard carbon is 6:(0.02-0.045); And / or, in the graphene / silicon@hard carbon@carbon nanotube composite anode material, the mass ratio of silicon to carbon nanotubes is 6:(1.8-2.2).

12. Use of the graphene / silicon@hard carbon@carbon nanotube composite anode material according to claim 10 or 11 in lithium-ion batteries.

13. The use according to claim 12, wherein the graphene / silicon@hard carbon@carbon nanotube composite anode material is used as a lithium-ion battery anode material.

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