Graphene / carbon nanotube composite material as well as preparation method and application thereof

Abstract

The invention belongs to the field of carbon materials, and provides a graphene / carbon nanotube composite material and a preparation method and application thereof. The method for preparing the graphene / carbon nanotube composite material comprises the following steps: uniformly mixing graphene oxide, soluble metal salt, an organic nitrogen source and water to obtain a suspension; carrying out hydrothermal reaction on the suspension to obtain reduced graphene oxide hydrogel; dipping the reduced graphene oxide hydrogel in a phosphoric acid solution, and then performing first drying to obtain reduced graphene oxide aerogel; and feeding the reduced graphene oxide aerogel into a fluidized bed, carrying out gradient heating treatment in the presence of mixed gas, and carrying out washing and secondary drying on the obtained product to obtain the graphene / carbon nanotube composite material. The method provided by the invention can improve the conductivity of the graphene / carbon nanotube composite material.

Description

Technical Field

[0001] This application belongs to the field of carbon materials. Specifically, it provides a graphene / carbon nanotube composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Graphene has been applied to a certain extent in lithium-ion batteries due to its high electrical conductivity, large specific surface area, good chemical stability and other characteristics. However, during the use of graphene, it is prone to agglomeration and stacking, which will greatly reduce the effective specific surface area of graphene, making it difficult to exert its excellent characteristics. At the same time, due to the two-dimensional sheet structure of graphene, when working with a large current, it will cause an increase in the diffusion impedance of Li + and a decrease in the rate performance of the battery. Carbon nanotubes can be regarded as one-dimensional tubular nanomaterials formed by curling graphene sheets. Carbon nanotubes not only have excellent electrical conductivity, but also have a fibrous structure, which is beneficial to form an effective conductive network in the electrode.

[0003] Combining graphene and carbon nanotubes is expected to play the advantages of both to improve the electrochemical performance of lithium batteries. The traditional method is to mechanically mix graphene and carbon nanotubes evenly to prepare a slurry, and then through solid-liquid separation and drying, a composite material of graphene and carbon nanotubes is obtained. However, in this simple mechanical mixing, the carbon nanotubes and graphene only rely on physical latching, resulting in too high contact resistance and unable to effectively improve the electrical conductivity. Related technologies also mention that a carbon source is vapor-deposited on the surface of graphene, and carbon nanotubes are formed on its surface after graphene adsorbs the carbon source. However, the ability of graphene to adsorb the carbon source is limited, which may lead to uneven distribution of the carbon nanotubes formed by surface deposition and stacking phenomena. Summary of the Invention

[0004] Aiming at the above problems existing in the prior art, the purpose of this application is to provide a graphene / carbon nanotube composite material, a preparation method thereof, and an application thereof. The method of this application can improve the uniformity of the distribution of carbon nanotubes on the surface of graphene and improve the electrical conductivity of the graphene / carbon nanotube composite material.

[0005] The first aspect of this application provides a method for preparing a graphene / carbon nanotube composite material, including:

[0006] S1: Mix graphene oxide, a soluble metal salt, an organic nitrogen source and water evenly to obtain a suspension, wherein the metal salt includes at least one of nickel salt, iron salt and cobalt salt;

[0007] S2: Carry out a hydrothermal reaction on the suspension to obtain a reduced graphene oxide hydrogel;

[0008] S3: Impregnate the reduced graphene oxide hydrogel with a phosphoric acid solution, and then carry out a first drying to obtain a reduced graphene oxide aerogel;

[0009] S4: Feed the reduced graphene oxide aerogel into a fluidized bed, and perform a gradient temperature increase treatment in the presence of a mixed gas. The obtained product is washed and secondarily dried to obtain the graphene / carbon nanotube composite material; wherein, the mixed gas includes an inert gas and hydrogen, and the gradient temperature increase treatment includes stage I, stage II, and stage III carried out in sequence,

[0010] Stage I: Feed the mixed gas into the fluidized bed, heat up to the first temperature T 1 , and keep it at the first temperature T 1 for 1 - 3 h to form metal-nitrogen-phosphorus co-doped graphene;

[0011] Stage II: Heat up to the second temperature T 2 , then feed a carbon source gas, and keep it at the second temperature T 2 for 0.5 - 2 h to form carbon nanotubes on the surface of the metal-nitrogen-phosphorus co-doped graphene;

[0012] Stage III: Stop feeding the carbon source gas, then heat up to the third temperature T 3 , and keep it at the third temperature T 3 for 1 - 3 h;

[0013] T 1 、T 2 and T 3 satisfy the relational expression: 100°C ≤ T 2 -T 1 ≤ 300°C, 150°C ≤ T 3 -T 2 ≤ 300°C, and T 1 is 400 - 600°C.

[0014] In the method provided by this application, metal ions are anchored through the electrostatic action of oxygen-containing functional groups on the surface of graphene oxide, which can fix the growth sites of carbon nanotubes; a three-stage gradient temperature increase is adopted to achieve sufficient pore formation of graphene, promote the effective doping of N and P, increase defect sites, and promote the fixation and uniform distribution of carbon nanotubes on the surface of graphene in the presence of metal particles (formed after the reduction of metal ions by hydrogen), thereby improving the electrical conductivity of the graphene / carbon nanotube composite material.

[0015] In some embodiments of this application, the sheet diameter of the graphene oxide is 1 - 20 μm, the thickness is 1 - 5 nm, and the oxygen content is 30% - 60%.

[0016] In some embodiments of this application, step S1 includes the following processes:

[0017] Disperse graphene oxide in water to prepare a graphene oxide dispersion;

[0018] Add the soluble metal salt and the organic nitrogen source to the graphene oxide dispersion liquid and mix evenly to obtain the suspension.

[0019] Further, the concentration of the graphene oxide dispersion liquid is 1-20 mg / mL.

[0020] In some embodiments of the present application, the organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea, and melamine.

[0021] In some embodiments of the present application, the mass ratio of the organic nitrogen source to the graphene oxide is (1-5):1.

[0022] In some embodiments of the present application, the iron salt is selected from one or more of ferric chloride, ferric nitrate, and ferric sulfate.

[0023] In some embodiments of the present application, the nickel salt is selected from one or more of nickel chloride, nickel nitrate, and nickel sulfate.

[0024] In some embodiments of the present application, the cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate, and cobalt sulfate.

[0025] In some embodiments of the present application, the mass ratio of the soluble metal salt to the graphene oxide is (2-7):1.

[0026] In some embodiments of the present application, the temperature of the hydrothermal reaction is 150-250 °C, and the reaction time is 6-24 h.

[0027] In some embodiments of the present application, the concentration of the phosphoric acid solution is 0.05-1 mol / L.

[0028] Further, the impregnation time is 12-24 h. In this way, while ensuring a relatively high doping amount of phosphorus atoms, the collapse of the graphene structure caused by too long impregnation time can be minimized.

[0029] In some embodiments of the present application, the first drying method is freeze-drying, and the freeze-drying time is 15-30 h.

[0030] In some embodiments of the present application, in the mixed gas, the volume ratio of the inert gas to hydrogen is (1-5):1.

[0031] In some embodiments of the present application, the inert gas is argon.

[0032] In some embodiments of the present application, in Stage I, Stage II, and Stage III, the flow rate of the mixed gas is 1 - 5 L / min relative to every 10 g of the reduced graphene oxide aerogel.

[0033] In some embodiments of the present application, the carbon source gas is selected from at least one of methane, ethane, ethylene, and acetylene.

[0034] In some embodiments of the present application, in Stage II, the flow rate of the carbon source gas is 0.5 - 2 L / min relative to every 10 g of the reduced graphene oxide aerogel.

[0035] In some embodiments of the present application, the second temperature T 2 is 700 - 850 °C.

[0036] In some embodiments of the present application, the third temperature T 3 is 900 - 1100 °C.

[0037] The second aspect of the present application provides a graphene / carbon nanotube composite material prepared by the method described in the first aspect of the present application.

[0038] The third aspect of the present application provides the use of the graphene / carbon nanotube composite material described in the second aspect of the present application in a lithium-ion battery.

[0039] The additional aspects and advantages of the present application will be partially given in the following description, partially become apparent from the following description, or be understood through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, where:

[0041] Figure 1 is a schematic flow chart of preparing a graphene / carbon nanotube composite material according to an embodiment;

[0042] Figure 2 is a schematic structural diagram of the prepared graphene / carbon nanotube composite material according to an embodiment;

[0043] Figure 3 is a comparison chart of the impedance in the application of the graphene / carbon nanotube composite materials of Example 1 and Comparative Examples 1 - 3 in a lithium-ion battery;

[0044] Figure 4 is a comparison chart of the rate performance in the application of the graphene / carbon nanotube composite materials of Example 1 and Comparative Examples 1 - 3 in a lithium-ion battery. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The embodiments of the present application will be described in detail below. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as a limitation to the present application.

[0046] The "range" disclosed in the present application is defined in the form of a lower limit and / or an upper limit. A given range is defined by selecting a lower limit and / or an upper limit. The range defined in this way can include the end values or not include the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range not explicitly recorded, and any lower limit can be combined with other lower limits to form a range not explicitly recorded. Similarly, any upper limit can be combined with any other upper limit to form a range not explicitly recorded. In addition, each separately disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recorded.

[0047] If there is no special description, all the embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution, and such a technical solution should be considered to be included in the disclosure of the present application.

[0048] In the present application, the terms "first" and "second" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features.

[0049] In a first aspect, the present application provides a method for preparing a graphene / carbon nanotube composite material, and the method includes the following steps S1 to S4.

[0050] S1: Mix graphene oxide, a soluble metal salt, an organic nitrogen source and water evenly to obtain a suspension.

[0051] S2: Perform a hydrothermal reaction on the suspension to obtain a reduced graphene oxide hydrogel.

[0052] S3: Immerse the reduced graphene oxide hydrogel with a phosphoric acid solution and then perform a first drying to obtain a reduced graphene oxide aerogel.

[0053] S4: Feed the reduced graphene oxide aerogel into a fluidized bed and perform a gradient temperature increase treatment in the presence of a mixed gas (including an inert gas and hydrogen), and the obtained product is washed and subjected to a second drying to obtain a graphene / carbon nanotube composite material.

[0054] According to the present application, in-situ fixation of metal ions and an organic nitrogen source on graphene oxide can be achieved through step S1.

[0055] In the present application, the flake diameter of the graphene oxide can be 1 to 20 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, etc.; the thickness of the graphene oxide can be 1 to 5 nm, for example, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, etc.; the oxygen content of the graphene oxide can be 30% to 60%, for example, 30%, 35%, 40%, 42%, 45%, 47%, 50%, 60%, etc.

[0056] In the present application, the graphene oxide can be obtained commercially, or can be prepared by methods well known in the art, such as by the Hummers method using various graphites as raw materials.

[0057] In the present application, the soluble metal salt refers to a metal salt that can be dissolved in water, specifically including at least one of nickel salt, iron salt and cobalt salt. In the gradient heating of step S4, the soluble metal salt is reduced to metal particles by hydrogen to play a catalytic role.

[0058] As some preferred examples, the nickel salt is selected from one or more of nickel chloride, nickel nitrate and nickel sulfate.

[0059] As some preferred examples, the iron salt is selected from one or more of ferric chloride, ferric nitrate and ferric sulfate.

[0060] As some preferred examples, the cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate and cobalt sulfate.

[0061] In the present application, the mass ratio of the soluble metal salt to the graphene oxide can generally be (1-10):1, such as 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc. In order to promote the formation of carbon nanotubes while avoiding the agglomeration of metal particles caused by excessive metal salt, preferably, the mass ratio of the soluble metal salt to the graphene oxide is (2-7):1.

[0062] In the present application, the organic nitrogen source can be selected from various organic substances containing one or more amino groups, and the amino groups are cross-linked with oxygen-containing functional groups in graphene oxide to fix the N element sites and hinder graphene aggregation.

[0063] In some embodiments, the organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea and melamine.

[0064] In some embodiments, the mass ratio of the organic nitrogen source to the graphene oxide may be (1-5):1, for example, 1:1, 2:1, 2.5:1, 3:1, 4:1, 5:1, etc.

[0065] In some embodiments, step S1 includes the following processes:

[0066] Disperse graphene oxide in water to obtain a graphene oxide dispersion;

[0067] Add the soluble metal salt and the organic nitrogen source to the graphene oxide dispersion and mix evenly to obtain the suspension.

[0068] Furthermore, the concentration of the graphene oxide dispersion can be 1-20 mg / mL, such as 1 mg / mL, 2 mg / mL, 3 mg / mL, 4 mg / mL, 5 mg / mL, 6 mg / mL, 8 mg / mL, 10 mg / mL, 12 mg / mL, 15 mg / mL, 20 mg / mL, etc.

[0069] This application has no special requirements for the mixing method, as long as the soluble metal salt and the organic nitrogen source can be evenly distributed in the graphene oxide dispersion respectively. The mixing methods include, but are not limited to, ultrasonic and magnetic stirring.

[0070] According to this application, during the hydrothermal reaction of step S2, the graphene oxide (GO) in the suspension forms hydrogen bonds with water by using the oxygen-containing functional groups (such as hydroxyl and carboxyl) on its surface, enabling GO to self-assemble into a gel structure in water. Moreover, the amino group provided by the organic nitrogen source has reducibility and can partially reduce GO to obtain reduced graphene oxide hydrogel. The hydrothermal reaction can be carried out in a high-pressure reactor.

[0071] In some embodiments, the temperature of the hydrothermal reaction can be 150-250 °C, such as 150 °C, 160 °C, 165 °C, 170 °C, 172 °C, 175 °C, 180 °C, 190 °C, 193 °C, 200 °C, 210 °C, 220 °C, 235 °C, 240 °C, 250 °C, etc.; the time of the hydrothermal reaction can be 6-24 h, such as 6 h, 8 h, 9.5 h, 10 h, 12 h, 15 h, 16 h, 17 h, 18 h, 20 h, 22 h, 24 h, etc. Preferably, the temperature of the hydrothermal reaction is 160-200 °C, and the reaction time is 10-24 h.

[0072] According to this application, in step S3, impregnating the reduced graphene oxide hydrogel with phosphoric acid solution can promote the uniform distribution of the phosphorus source in the hydrogel. Furthermore, the diameter of the P atom is relatively large, and co-doping with N can further increase the layer spacing, avoid graphene stacking, and increase the defect sites.

[0073] In some embodiments, the concentration of the phosphoric acid solution can be 0.05 - 1 mol / L, such as 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.5 mol / L, 0.7 mol / L, 1 mol / L, etc.

[0074] In some embodiments, the impregnation time is 12 - 24 h, such as 12 h, 13 h, 14 h, 14.5 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 22 h, 23 h, 24 h, etc.

[0075] In step S3, in order to remove impurities such as free ammonia and metal ions, it is preferred that step S3 further includes: washing the reduced graphene oxide hydrogel. For example, soaking the reduced graphene oxide hydrogel in water for 12 - 24 h, then washing with water until neutral, and then performing phosphoric acid solution impregnation.

[0076] In step S3, the first drying aims to quickly remove the moisture in the hydrogel to form an aerogel. Preferably, the first drying method is freeze-drying, and the freeze-drying time can be 15 - 30 h, such as 15 h, 17 h, 19 h, 20 h, 22 h, 23 h, 24 h, 25 h, 26 h, 27 h, 28 h, 30 h, etc.

[0077] In some embodiments, in step S3, the particle size of the prepared reduced graphene oxide aerogel is 2 - 10 μm. The aerogel with this particle size is subjected to a temperature increase treatment under the action of a subsequent fluidized bed, which can further increase the contact area between reduced graphene oxide (rGO) and the gas-phase carbon source and improve the adsorption capacity of rGO for the gas-phase carbon source.

[0078] According to the present application, in step S4, the gradient temperature increase treatment is carried out in a mixed gas. The mixed gas contains an inert gas and hydrogen. The inert gas can be selected from various gases that do not participate in the reaction, such as argon, nitrogen, etc., and preferably argon. In some embodiments, in the mixed gas, the volume ratio of the inert gas to hydrogen is (1 - 5)∶1, such as 1∶1, 2∶1, 3∶1, 4∶1, 5∶1, etc.

[0079] In step S4, the gradient temperature increase treatment includes stages I, II, and III carried out in sequence.

[0080] Stage I: Introduce the mixed gas into the fluidized bed and heat up to the first temperature T 1 (400 - 600 °C), and at the first temperature T 1Keep warm at a lower temperature for 1 - 3 h to form metal-nitrogen-phosphorus co-doped graphene. In this stage, the reduced graphene oxide aerogel is calcined at a low temperature in the mixed gas. On the one hand, metal ions are reduced to metal nanoparticles under the action of hydrogen, and on the other hand, small molecules in the aerogel (such as components like NH 3 and H 2 O, etc.) slowly overflow to achieve pore formation and doping of heteroatoms (N and P).

[0081] As some specific examples, the first temperature T 1 can be 400 °C, 420 °C, 450 °C, 480 °C, 500 °C, 520 °C, 530 °C, 550 °C, 560 °C, 580 °C, 600 °C, etc. The heat preservation time at the first temperature T 1 can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc.

[0082] In stage I, it is preferable to heat up the fluidized bed to the first temperature T 1 at a heating rate of 1 - 10 °C / min (such as 5 °C / min, 10 °C / min).

[0083] Stage II: Heat up to the second temperature T 2 (100 °C ≤ T 2 - T 1 ≤ 300 °C), then continuously introduce the carbon source gas, and at the same time keep warm at the second temperature T 2 for 0.5 - 2 h to form carbon nanotubes on the metal-nitrogen-phosphorus co-doped graphene. In this stage, in the presence of the mixed gas, the introduced carbon source gas can enable the carbon nanotubes to grow in-situ on the metal sites of the graphene.

[0084] In some embodiments, the second temperature T 2 is 700 - 850 °C, such as 700 °C, 720 °C, 750 °C, 770 °C, 800 °C, 810 °C, 815 °C, 830 °C, 850 °C, etc.

[0085] In stage II, it is preferable to heat up the fluidized bed to the second temperature T 2 at a heating rate of 1 - 10 °C / min (such as 5 °C / min, 10 °C / min).

[0086] In stage II, the carbon source gas can be selected from various organic carbon source gases. According to some embodiments, the carbon source gas is selected from at least one of methane, ethane, ethylene, and acetylene.

[0087] In some embodiments, in Stage II, relative to every 10 g of the reduced graphene oxide aerogel, the flow rate of the carbon source gas may be 0.5 - 2 L / min, such as 0.5 L / min, 0.7 L / min, 1 L / min, 1.2 L / min, 1.5 L / min, 1.6 L / min, 2 L / min, etc.

[0088] As some examples, the heat preservation time at the second temperature T 2 may be 1 h, 1.5 h, 2 h, etc.

[0089] Stage III: Stop introducing the carbon source gas, and then heat up to the third temperature T 3 (150 °C ≤ T 3 - T 2 ≤ 300 °C), and keep warm at the third temperature T 3 for 1 - 3 h. Through the high-temperature treatment in this stage, the oxygen-containing functional groups and defects in the graphene can be reduced, the graphitization degree of the graphene and carbon nanotubes can be improved, and their orderliness and crystallinity can be increased, further improving the conductivity of the composite material.

[0090] In some embodiments, the third temperature T 3 is 900 - 1100 °C, such as 900 °C, 920 °C, 950 °C, 960 °C, 965 °C, 970 °C, 1000 °C, 1050 °C, 1100 °C, etc.

[0091] In Stage III, it is preferable to heat up the fluidized bed to the third temperature T 3 at a heating rate of 1 - 5 °C / min (such as 1 °C / min, 5 °C / min).

[0092] As some examples, the heat preservation time at the third temperature T 2 may be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, etc.

[0093] In some embodiments, in Stage I, Stage II and Stage III, relative to every 10 g of the reduced graphene oxide aerogel, the flow rate of the mixed gas may be 1 - 5 L / min, such as 1 L / min, 2 L / min, 3 L / min, 4 L / min, 5 L / min, etc.

[0094] Step S4 further includes: washing and second drying the product after the gradient heating treatment. Among them, the washing method may include: first pickling the product and then washing it with water until it is neutral. For example, a hydrochloric acid or sulfuric acid solution with a concentration of 0.5 - 1 mol / L may be used for the pickling.

[0095] In some embodiments, the second drying method is vacuum drying. The temperature of vacuum drying can be, for example, 50 - 90°C, and the drying time can be 15 - 30 h.

[0096] According to some specific embodiments, such as Figure 1 As shown, the process for preparing the graphene / carbon nanotube composite material includes:

[0097] Disperse graphene oxide in water to obtain a graphene oxide dispersion;

[0098] Disperse the graphene oxide dispersion, soluble metal salt and organic nitrogen source uniformly, and obtain a reduced graphene oxide hydrogel through hydrothermal reaction;

[0099] Impregnate the reduced graphene oxide hydrogel with phosphoric acid solution, and then obtain a reduced graphene oxide aerogel through freeze-drying;

[0100] Perform gradient temperature increase treatment on the reduced graphene oxide aerogel, and the obtained product is subjected to pickling, water washing and vacuum drying to obtain a graphene / carbon nanotube composite material.

[0101] In the method of the present application, during the gradient temperature increase treatment of the reduced graphene oxide aerogel, the organic nitrogen source and phosphoric acid on the graphene release gases upon heating. In this way, heteroatom doping and pore formation can be achieved in one step, reducing graphene agglomeration and increasing its specific surface area. In addition, by using a fluidized bed, the contact area between graphene and the gas-phase carbon source can be increased, reducing the non-uniformity of carbon nanotube deposition. Moreover, the metal ions in-situ fixed on the graphene are reduced to metal nanoparticles during the gradient temperature increase treatment, which can act as a catalyst to promote the in-situ growth of carbon nanotubes on the graphene, solving the problem of poor contact caused by physical mixing.

[0102] Therefore, in a second aspect, the present application provides a graphene / carbon nanotube composite material prepared by the method described in the first aspect of the present application. According to some embodiments, the schematic structural diagram of the graphene / carbon nanotube composite material is as Figure 2 shown. In the composite material, the two-dimensional planar structure of graphene (shown as the black plane), and the carbon nanotubes grow along the one-dimensional radial direction in the pores of the graphene sheets (the longitudinal line part).

[0103] In a third aspect, the present application provides the use of the graphene / carbon nanotube composite material described in the second aspect of the present application in a lithium-ion battery.

[0104] The graphene / carbon nanotube composite material of the present application can be used as a conductive agent in a lithium-ion battery, for example, as a conductive agent for the negative electrode material. When forming a negative electrode film layer with the graphene / carbon nanotube composite material, negative electrode material, binder and other components, it can present "point-line-plane" contact with the negative electrode material, better promoting the electron transfer process.

[0105] The embodiments of the present application are described below. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as a limitation to the present application.

[0106] In the following examples and comparative examples, the argon-hydrogen mixed gas refers to a mixed gas composed of argon and hydrogen, and the volume ratio of argon to hydrogen is 2:1. In addition, unless otherwise specified, during the three-stage gradient heating process, the argon-hydrogen mixed gas is continuously introduced, and in stage II, the carbon source gas is introduced.

[0107] Example 1

[0108] Graphene oxide (GO, sheet diameter 2 μm, thickness 3 nm, oxygen content 40%) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL;

[0109] Nickel nitrate was added to the dispersion A1, and ultrasonic dispersion was carried out for 60 min, then stirring was carried out at 300 rpm for 30 min, and then glycine was added, and stirring was continued for 30 min to obtain a suspension; among them, the mass ratio of nickel nitrate, glycine to GO in the dispersion A1 was 5:4:1;

[0110] After transferring the suspension into a high-pressure reactor, it was reacted at 180 °C for 12 h to obtain a reduced graphene oxide (rGO) hydrogel B1;

[0111] The hydrogel B1 was soaked and washed with water until neutral, then immersed in a 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for treatment for 24 h to obtain an rGO aerogel C1;

[0112] 10 g of the aerogel C1 was transferred into a fluidized bed, the argon-hydrogen mixed gas was introduced and gradient heating was carried out. The temperature in the first stage: heated to 500 °C at a rate of 5 °C / min and held for 2 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; the temperature in the second stage: heated to 800 °C at a rate of 5 °C / min, then the carbon source (acetylene) was introduced and held for 1 h, the flow rate of the argon-hydrogen mixed gas was 3 L / min, and the flow rate of the carbon source was 1 L / min; the temperature in the third stage: heated to 1000 °C at a rate of 2 °C / min and held for 2 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; finally, it was cooled to room temperature to obtain a dry powder D1;

[0113] The dry powder D1 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, then washed with water until neutral, and dried in vacuum at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-1.

[0114] Example 2

[0115] Disperse graphene oxide (GO, with a sheet diameter of 5 μm, a thickness of 3 nm, and an oxygen content of 40%) in water to obtain a GO dispersion A2 with a concentration of 5 mg / mL;

[0116] Add ferric chloride to the GO dispersion A2, ultrasonically disperse for 40 min, then stir at 300 rpm for 30 min, and then add glycine and continue stirring for 30 min to obtain a suspension; wherein, the mass ratio of ferric chloride, glycine to GO in the dispersion A2 is 4:4:1;

[0117] Transfer the suspension to a high-pressure reactor and react at 180 °C for 12 h to obtain a reduced graphene oxide (rGO) hydrogel B2;

[0118] Soak the hydrogel B2 and wash it with water until neutral, then immerse it in a 0.2 mol / L phosphoric acid solution for 14 h, and then transfer it to a freeze dryer for 24 h to obtain an rGO aerogel C2;

[0119] Transfer 10 g of the aerogel C2 to a fluidized bed, introduce an argon-hydrogen mixed gas and carry out gradient heating. First-stage temperature: heat up to 600 °C at a rate of 10 °C / min and hold for 1.5 h, with an argon-hydrogen mixed gas flow rate of 4 L / min; Second-stage temperature: heat up to 850 °C at a rate of 5 °C / min, then introduce a carbon source (methane) and hold for 1 h, with an argon-hydrogen mixed gas flow rate of 4 L / min and a carbon source flow rate of 1 L / min; Third-stage temperature: heat up to 950 °C at a rate of 2 °C / min and hold for 2 h, with an argon-hydrogen mixed gas flow rate of 4 L / min; Finally, cool down to room temperature to obtain a dry powder D2;

[0120] Place the dry powder D2 in a 1 mol / L hydrochloric acid solution, stir at 300 rpm for 60 min, then wash with water until neutral, and dry in vacuum at 80 °C for 24 h to obtain a graphene / carbon nanotube composite, denoted as G / C-2.

[0121] Example 3

[0122] Disperse graphene oxide (GO, with a sheet diameter of 2 μm, a thickness of 3 nm, and an oxygen content of 50%) in water to obtain a GO dispersion A3 with a concentration of 2 mg / mL;

[0123] Add nickel nitrate to the dispersion A3, ultrasonically disperse for 60 min, then stir at 300 rpm for 30 min, and then add glycine and continue stirring for 30 min to obtain a suspension; wherein, the mass ratio of nickel nitrate, glycine to GO in the dispersion A3 is 5:5:1;

[0124] Transfer the suspension to a high-pressure reactor and react at 160 °C for 18 h to obtain a reduced graphene oxide (rGO) hydrogel B3;

[0125] The hydrogel B3 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred to a freeze dryer for 24 h to obtain the rGO aerogel C3;

[0126] 10 g of the aerogel C3 was transferred to a fluidized bed, and an argon-hydrogen mixed gas was introduced and heated at a gradient. The first-stage temperature: heated to 550 °C at a rate of 5 °C / min and held for 2 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; the second-stage temperature: heated to 750 °C at a rate of 5 °C / min, then carbon source (acetylene) was introduced and held for 1.5 h, the flow rate of the argon-hydrogen mixed gas was 3 L / min, and the flow rate of the carbon source was 0.5 L / min; the third-stage temperature: heated to 1050 °C at a rate of 2 °C / min and held for 1.5 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; finally, it was cooled to room temperature to obtain the dry powder D3;

[0127] The dry powder D3 was placed in 1.5 mol / L sulfuric acid solution, stirred at 350 rpm for 60 min, then washed with water until neutral, and dried in vacuum at 80 °C for 24 h to obtain the graphene / carbon nanotube composite material, denoted as G / C-3.

[0128] Example 4

[0129] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that glycine was replaced by melamine, and the mass ratio of melamine to graphene oxide was 3:1. The prepared graphene / carbon nanotube composite material was denoted as G / C-4.

[0130] Example 5

[0131] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that glycine was replaced by thiourea, and the mass ratio of thiourea to graphene oxide was 5:1. The prepared graphene / carbon nanotube composite material was denoted as G / C-5.

[0132] Example 6

[0133] The graphene / carbon nanotube composite material was prepared according to the method of Example 1, except that nickel nitrate was replaced by cobalt nitrate, and the mass ratio of cobalt nitrate to graphene oxide was 2:1. The prepared graphene / carbon nanotube composite material was denoted as G / C-6.

[0134] Comparative Example 1

[0135] Graphene oxide (GO, sheet diameter 2 μm, thickness 3 nm, oxygen content 40%) was dispersed in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL;

[0136] Nickel nitrate was added to dispersion A1, and ultrasonic dispersion was carried out for 60 min, followed by stirring at 300 rpm for 30 min to obtain a suspension; wherein, the mass ratio of nickel nitrate to GO in dispersion A1 was 5:1;

[0137] After transferring the suspension into a high-pressure reactor, it was reacted at 180 °C for 18 h to obtain reduced graphene oxide (rGO) hydrogel B-11;

[0138] The hydrogel B-11 was soaked and washed with water until neutral, and then transferred into a freeze dryer for treatment for 24 h to obtain rGO aerogel C-11;

[0139] 10 g of aerogel C-11 was transferred into a fluidized bed, and argon-hydrogen mixed gas was introduced and gradient heating was carried out. The first-stage temperature: heated to 500 °C at a rate of 5 °C / min and held for 2 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; the second-stage temperature: heated to 800 °C at a rate of 5 °C / min, then carbon source (acetylene) was introduced and held for 1 h, the flow rate of the argon-hydrogen mixed gas was 3 L / min, and the flow rate of the carbon source was 1 L / min; the third-stage temperature: heated to 1000 °C at a rate of 2 °C / min and held for 2 h, and the flow rate of the argon-hydrogen mixed gas was 3 L / min; finally, it was cooled to room temperature to obtain dry powder D-11;

[0140] The dry powder D-11 was placed in a 1 mol / L sulfuric acid solution, stirred at 300 rpm for 60 min, then washed with water until neutral, and dried in vacuum at 80 °C for 24 h to obtain a graphene / carbon nanotube composite, denoted as G / C-d1.

[0141] Comparative Example 2

[0142] Graphene oxide (GO, sheet diameter 2 μm, thickness 3 nm, oxygen content 40%) was dispersed in water to prepare GO dispersion A1 with a concentration of 5 mg / mL;

[0143] Nickel nitrate was added to dispersion A1, ultrasonic dispersion was carried out for 60 min, then stirred at 300 rpm for 30 min, and then glycine was added, and stirring was continued for 30 min to obtain a suspension; wherein, the mass ratio of nickel nitrate, glycine to GO in dispersion A1 was 5:4:1;

[0144] After transferring the suspension into a high-pressure reactor, it was reacted at 180 °C for 12 h to obtain reduced graphene oxide (rGO) hydrogel B1;

[0145] The hydrogel B1 was soaked and washed with water until neutral, then immersed in 0.1 mol / L phosphoric acid solution for 18 h, and then transferred into a freeze dryer for treatment for 24 h to obtain rGO aerogel C1;

[0146] Transfer 10 g of aerogel C1 into a rotary furnace, introduce an argon-hydrogen mixture gas, and perform gradient heating. The temperature in the first stage: heat up to 500 °C at a rate of 5 °C / min, hold for 2 h, and the flow rate of the argon-hydrogen mixture gas is 3 L / min. The temperature in the second stage: heat up to 800 °C at a rate of 5 °C / min, then introduce a carbon source (acetylene) and hold for 1 h, the flow rate of the argon-hydrogen mixture gas is 3 L / min, and the flow rate of the carbon source is 1 L / min. The temperature in the third stage: heat up to 1000 °C at a rate of 2 °C / min, hold for 2 h, and the flow rate of the argon-hydrogen mixture gas is 3 L / min. Finally, cool down to room temperature to obtain dry powder D-12;

[0147] Place the dry powder D-12 in a 1 mol / L sulfuric acid solution, stir at 300 rpm for 60 min, then wash with water until neutral, and dry in vacuum at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-d2.

[0148] Comparative Example 3

[0149] Disperse graphene oxide (GO, sheet diameter 2 μm, thickness 3 nm, oxygen content 40%) in water to prepare a GO dispersion A1 with a concentration of 5 mg / mL;

[0150] Add nickel nitrate to the dispersion A1, ultrasonically disperse for 60 min, then stir at 300 rpm for 30 min, and then add glycine and continue to stir for 30 min to obtain a suspension; among them, the mass ratio of nickel nitrate, glycine to GO in the dispersion A1 is 5:4:1;

[0151] Transfer the suspension into a high-pressure reactor and react at 180 °C for 12 h to obtain a reduced graphene oxide (rGO) hydrogel B1;

[0152] Soak the hydrogel B1 and wash with water until neutral, then immerse it in a 0.1 mol / L phosphoric acid solution for 18 h, and then transfer it to a freeze dryer for treatment for 24 h to obtain rGO aerogel C1;

[0153] Transfer 10 g of aerogel C1 into a fluidized bed, introduce an argon-hydrogen mixture gas and perform the first-stage temperature: heat up to 800 °C at a rate of 5 °C / min, then introduce a carbon source (acetylene) for 1 h, and hold at 800 °C for a total of 5 h, and the flow rate of the argon-hydrogen mixture gas is 3 L / min; finally, cool down to room temperature to obtain dry powder D-13;

[0154] Place the dry powder D-13 in a 1 mol / L sulfuric acid solution, stir at 300 rpm for 60 min, then wash with water until neutral, and dry in vacuum at 80 °C for 24 h to obtain a graphene / carbon nanotube composite material, denoted as G / C-d3.

[0155] Test Example

[0156] The test examples are used to illustrate the application performance of the graphene / carbon nanotube composites G / C-1 to G / C-6 and G / C-d1 to G / C-d3 prepared in the above examples and comparative examples.

[0157] 1. Resistivity test: The resistivity of the powder was measured using a GM-I type multifunctional powder resistivity automatic measuring instrument, and the test results are shown in Table 1.

[0158] Table 1

[0159] Serial number Material number Resistivity (mΩ·cm) Example 1 G / C-1 0.05 Example 2 G / C-2 0.03 Example 3 G / C-3 0.08 Example 4 G / C-4 0.06 Example 5 G / C-5 0.03 Example 6 G / C-6 0.04 Comparative example 1 G / C-d1 0.12 Comparative example 2 G / C-d2 0.23 Comparative example 3 G / C-d3 0.18

[0160] Combined with Table 1, it can be seen that when comparing Examples 1-6 with Comparative Examples 1-3, the resistivity of the graphene / carbon nanotube composites prepared in Examples 1-6 is lower, indicating higher conductivity.

[0161] When comparing Example 1 with Comparative Examples 1-3 respectively, it can be seen that in Example 1, N and P co-doping is used to change the electronic structure of graphene, enhancing the electronic conductivity and increasing the defect sites, thereby improving the electrochemical activity; the fluidized bed ensures sufficient contact between the gas-phase carbon source and the material to ensure the uniform growth of carbon nanotubes; the three-stage temperature rise ensures sufficient pore formation, doping, and the uniformity of carbon nanotube growth, thus enhancing the overall conductivity.

[0162] 2. Electrical performance test

[0163] G / C-1 to G / C-6 and G / C-d1 to G / C-d3 were respectively used as conductive agents to make button cells for performance testing.

[0164] The graphene / carbon nanotube composite, dispersant (CMC), and water were mixed, and after being ultrasonically dispersed with a rod for 1 h, a conductive paste with a solid content of the conductive agent of 3% and a dispersant concentration of 1 wt% was prepared.

[0165] The silicon-carbon negative electrode material (as the active substance), binder (model LA133), and conductive paste were mixed according to the mass ratio of active substance∶binder∶conductive agent = 95.5∶4∶0.5, using water as the solvent to make a negative electrode paste. The paste was coated on copper foil, the counter electrode was a lithium sheet, and together with the electrolyte (1M LiPF 6 electrolyte, solvent composition: ethylene carbonate (EC) and dimethyl carbonate (DMC), v / v = 50∶50), and a PE porous separator were made into a 2032-type button cell for evaluation.

[0166] (1) Impedance test

[0167] The EIS of the battery was obtained using an electrochemical workstation, with a test frequency range of 0.01 - 1000000 Hz and a scanning voltage of 10 mV. The results are as Figure 3 shown.

[0168] (2) Rate performance test

[0169] At 25 °C, the battery is charged with a low current, the charging mode is CC-CV, and it is cycled for 3 times of charge and discharge, with the voltage range of 2.8 - 4.25 V; keep constant current charging at 0.3C, and discharge at 1C, 2C, and 3C constant current respectively to obtain the capacity retention rate.

[0170] The results are shown in Table 2 and Figure 4 as follows.

[0171] Table 2

[0172] Conductive agent 0.3C 1C 2C 3C G / C-1 100％ 94.60％ 85.45％ 70.76％ G / C-2 100％ 95.12％ 86.77％ 72.13％ G / C-3 100％ 93.76％ 84.56％ 70.02％ G / C-4 100％ 94.32％ 85.12％ 70.21％ G / C-5 100％ 95.21％ 86.44％ 71.92％ G / C-6 100％ 94.86％ 85.68％ 70.95％ G / C-d1 100％ 92.12％ 80.06％ 65.44％ G / C-d2 100％ 89.34％ 74.45％ 53.12％ G / C-d3 100％ 91.15％ 78.22％ 60.45％

[0173] Figure 3 It is the impedance diagram of the battery. The abscissa Z' represents the real part of the impedance, and the ordinate Z” represents the negative of the imaginary part. It can be seen from the figure that compared with Comparative Examples 1 - 3, the lithium battery prepared with the graphene / carbon nanotube composite material (conductive agent G / C-1) of Example 1 shows low resistance and has high conductivity.

[0174] Table 2 and Figure 4 show the rate effect. Combining Table 2 and Figure 4 it can be known that compared with Comparative Examples 1 - 3, the graphene / carbon nanotube composite materials prepared in Examples 1 - 6 as conductive agents maintain a high capacity retention rate at different rates, and can effectively improve the rate performance of lithium batteries.

[0175] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should all be covered within the scope of the claims and the description of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed in the text, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for preparing a graphene / carbon nanotube composite material, characterized in that: include: S1: mixing graphene oxide, a soluble metal salt, an organic nitrogen source and water to obtain a suspension, wherein the soluble metal salt comprises at least one of a nickel salt, an iron salt and a cobalt salt; S2: subjecting the suspension to a hydrothermal reaction to obtain a reduced graphene oxide hydrogel; S3: impregnating the reduced graphene oxide hydrogel with a phosphoric acid solution, and then performing a first drying to obtain a reduced graphene oxide aerogel; S4: feeding the reduced graphene oxide aerogel into a fluidized bed, and performing a gradient temperature increase treatment in the presence of a mixed gas, and washing and second drying the obtained product to obtain a graphene / carbon nanotube composite material; wherein the mixed gas contains an inert gas and hydrogen; the gradient temperature increase treatment includes stage I, stage II and stage III performed in sequence, Stage I: introducing the mixed gas into the fluidized bed, raising the temperature to a first temperature T1, and maintaining the temperature at the first temperature T1 for 1 to 3 hours to form metal-nitrogen-phosphorus co-doped graphene; Stage II: heating to a second temperature T2, introducing a carbon source gas, and maintaining the temperature at the second temperature T2 for 0.5 to 2 hours to form carbon nanotubes on the surface of the metal-nitrogen-phosphorus co-doped graphene; Stage III: stop introducing the carbon source gas, raise the temperature to the third temperature T3, and keep it at the third temperature T3 for 1 to 3 hours; T1, T2 and T3 satisfy the relationship: 100°C≤T2-T1≤300°C, 150°C≤T3-T2≤300°C, and T1 is 400-600°C.

2. The method according to claim 1, characterized in that The sheet diameter of the graphene oxide is 1-20 μm, the thickness is 1-5 nm, and the oxygen content is 30%-60%. Preferably, step S1 includes the following process: dispersing graphene oxide in water to prepare a graphene oxide dispersion; Adding the soluble metal salt and the organic nitrogen source to the graphene oxide dispersion and mixing them uniformly to obtain the suspension; Preferably, the concentration of the graphene oxide dispersion is 1 to 20 mg / mL.

3. The method according to claim 1 or 2, characterized in that: The organic nitrogen source is selected from at least one of urea, glycine, dicyandiamide, thiourea and melamine; Preferably, the mass ratio of the organic nitrogen source to the graphene oxide is (1-5):

1.

4. The method according to any one of claims 1 to 3, characterized in that: The iron salt is selected from one or more of ferric chloride, ferric nitrate and ferric sulfate; Preferably, the nickel salt is selected from one or more of nickel chloride, nickel nitrate and nickel sulfate; Preferably, the cobalt salt is selected from one or more of cobalt chloride, cobalt nitrate and cobalt sulfate; Preferably, the mass ratio of the soluble metal salt to the graphene oxide is (2-7):

1.

5. The method according to any one of claims 1 to 4, characterized in that: The temperature of the hydrothermal reaction is 150-250° C., and the reaction time is 6-24 hours.

6. The method according to any one of claims 1 to 5, characterized in that: The concentration of the phosphoric acid solution is 0.05-1 mol / L; Preferably, the immersion time is 12 to 24 hours; Preferably, the first drying method is freeze drying, and the freeze drying time is 15 to 30 hours.

7. The method according to any one of claims 1 to 6, characterized in that: In the mixed gas, the volume ratio of the inert gas to the hydrogen is (1-5):1; Preferably, the inert gas is argon; Preferably, in stage I, stage II and stage III, the flow rate of the mixed gas is 1 to 5 L / min per 10 g of the reduced graphene oxide aerogel.

8. The method according to any one of claims 1 to 7, characterized in that: The carbon source gas is selected from at least one of methane, ethane, ethylene and acetylene; Preferably, in stage II, the flow rate of the carbon source gas is 0.5 to 2 L / min per 10 g of the reduced graphene oxide aerogel; Preferably, the second temperature T2 is 700-850°C; Preferably, the third temperature T3 is 900-1100°C.

9. A graphene / carbon nanotube composite material prepared by the method according to any one of claims 1 to 8.

10. Use of the graphene / carbon nanotube composite material according to claim 9 in lithium ion batteries.

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