A lithium-sulfur battery positive electrode material and its preparation method and application

Abstract

The present invention is a lithium-sulfur battery positive electrode material and its preparation method and application. The present invention first uses graphene oxide and a compound containing cobalt ions as raw materials to obtain a cobalt complex-coated reduced graphene precursor by reflux reaction; then adopts a CVD method to obtain a cobalt carbide-modified reduced graphene-carbon nanotube composite material, and finally uses a melt-diffusion method to uniformly distribute sulfur nanoparticles on the surface of the cobalt carbide-modified graphene-carbon nanotube composite material to obtain a cobalt carbide-modified graphene-carbon nanotube / sulfur (Co2C@RGO-CNT / S) composite material. The composite material of the present invention has the advantages of large specific surface area, porous structure, and high conductivity as a lithium-sulfur battery positive electrode material. Applying it to a lithium-sulfur battery can significantly improve the cycle performance and rate performance of the positive electrode material, so as to effectively solve the problem of too fast capacity decay occurring during the electrode reaction. The present invention is simple and easy to implement and can be applied to engineering production.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-sulfur batteries, and in particular relates to a lithium-sulfur battery positive electrode material and a preparation method and application thereof, and specifically relates to a Co2C@RGO-CNT / S composite material and a preparation method and application thereof. Background Art

[0002] The theoretical specific energy of lithium-sulfur batteries is 2600Whkg -1 , is the current theoretical specific energy of lithium-ion batteries (500Whkg -1 ), and is therefore recognized as the most promising next-generation lithium secondary battery. It is expected to be widely used in portable electronics, electric vehicles, aerospace vehicles, and power grid transmission, thus playing a huge role in the development of today's society, economy, and technology, and has great application prospects.

[0003] However, at room temperature, elemental sulfur has low conductivity and generates soluble polysulfides during the charge and discharge process of lithium-sulfur batteries, which in turn causes a variety of side reactions and changes in battery volume. This results in low utilization of the active material in the cathode of lithium-sulfur batteries, poor rate performance, and a short cycle life. This restricts the application and development of sulfur as a positive electrode material for lithium secondary batteries. Therefore, how to improve the utilization of the active material in the cathode of lithium-sulfur batteries and the cycle life will become a key development direction for lithium-sulfur batteries in the future.

[0004] Graphene is a new type of two-dimensional nanomaterial whose nanosheets are composed of sp 2 The two-dimensional material composed of a single atomic layer of hybrid carbon atoms is currently the thinnest and hardest nanomaterial in the world. Its strength is as high as 1.01Tpa, 100 times that of structural steel, while its density is only 1 / 5 of that of structural steel. Graphene also has a thermal conductivity of 5300W / m·K, higher than carbon nanotubes and diamonds, and its electron mobility at room temperature exceeds 200,000cm 2 / V·S, higher than carbon nanotubes or silicon crystals. Its resistivity is only about 1Ω·m, lower than copper or silver, making it the world's lowest resistivity material. Carbon nanotubes, as one-dimensional nanomaterials, possess excellent electrical conductivity. Graphene / carbon nanotube hybrid materials organically combine graphene and carbon nanotubes through covalent bonds, preventing graphene stacking while forming a three-dimensional conductive network, significantly improving the composite's electrical conductivity.

[0005] Graphene / carbon nanotube hybrids have very high electrical conductivity. Their application as cathode materials for lithium-sulfur batteries can address the problem of sulfur's lack of electrical conductivity, thereby improving the conductivity of the cathode material. Furthermore, due to their excellent toughness and strength, using graphene as the backbone of lithium-sulfur battery cathode materials can effectively address the volume change issue. However, graphene / carbon nanotube hybrid carbon materials are inherently non-polar, and therefore have limited contribution to polysulfide immobilization. Therefore, how to improve polysulfide immobilization and effectively address the problem of sulfur's lack of electrical conductivity, thereby improving the conductivity of the cathode material and reducing the volume change issue, remains an urgent technical challenge in the field. Summary of the Invention

[0006] In order to improve the above technical problems, the present invention provides a Co2C@RGO-CNT / S composite material, which includes an RGO-CNT hybrid material and Co2C particles and S particles loaded on the RGO-CNT hybrid material.

[0007] According to an embodiment of the present invention, in the Co2C@RGO-CNT / S composite material, the mass fraction of Co2C particles is 1 to 20%, exemplified by 1%, 5%, 10%, 15%, and 20%.

[0008] According to an embodiment of the present invention, in the Co2C@RGO-CNT / S composite material, the mass fraction of S particles is 50-95%, exemplified by 50%, 60%, 70%, 80%, 90%, and 95%.

[0009] According to an embodiment of the present invention, the Co2C@RGO-CNT / S composite material has an "anemone"-like structure with an outer size of 5 to 10 μm, exemplified by 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and 10 μm.

[0010] According to an embodiment of the present invention, in the Co2C@RGO-CNT / S composite material, the length of the carbon nanotubes is 0.2 to 10.0 μm, exemplified by 0.2 μm, 1.0 μm, 2.0 μm, 4.0 μm, 6.0 μm, 8.0 μm, and 10.0 μm; the diameter of the carbon nanotubes is 10 to 100 nm, exemplified by 10 nm, 20 nm, 40 nm, 60 nm, 80 nm, and 100 nm.

[0011] According to an embodiment of the present invention, in the Co2C@RGO-CNT / S composite material, the diameter of the Co2C particles is 1 to 20 nm, exemplified by 1 nm, 5 nm, 10 nm, 15 nm, and 20 nm.

[0012] According to an embodiment of the present invention, in the Co2C@RGO-CNT / S composite material, the diameter of the sulfur particles is 5-50 nm, exemplified by 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, and 50 nm.

[0013] The present invention also provides a preparation method of the above-mentioned Co2C@RGO-CNT / S composite material, comprising using a cobalt ion-containing compound and graphene oxide as raw materials, depositing the cobalt ion-containing complex on the surface of the graphene oxide by a reflux reaction, then growing carbon nanotubes on the surface of the reduced graphene oxide by a CVD method, and after annealing, generating Co2C particles on the surface of the reduced graphene oxide to obtain a Co2C@RGO-CNT composite material, and then attaching S particles to the surface of the Co2C@RGO-CNT composite material by a melt diffusion method to finally form a Co2C@RGO-CNT / S composite material.

[0014] According to an embodiment of the present invention, the mixing mass ratio of the graphene oxide and the compound containing cobalt ions is 1:1 to 1:10, exemplified by 1:1, 1:2, 1:5, 1:8, and 1:10.

[0015] Preferably, the compound containing cobalt ions may be at least one of cobalt acetate, cobalt hydroxide, cobalt chloride, cobalt sulfate, and cobalt nitrate, preferably cobalt acetate.

[0016] Preferably, the outer dimensions of the graphene oxide are 5 to 10 μm, exemplified by 5 μm, 8 μm, and 10 μm. Furthermore, the specific surface area of the graphene oxide is 300 to 1600 m 2 / g, 300m 2 / g、500m 2 / g、800m 2 / g、1000m 2 / g、1200m 2 / g、1500m 2 / g、1600m 2 / g.

[0017] According to an embodiment of the present invention, the graphene oxide and the compound containing cobalt ions are reacted in an ethylene glycol solvent system. For example, the graphene oxide and the compound containing cobalt ions are first dispersed in ethylene glycol to obtain a mixed dispersion of graphene oxide and cobalt acetate.

[0018] Preferably, the usage ratio of the graphene oxide to ethylene glycol (mg:mL) is 20:1 to 1:1, exemplified by 1 mg:1 mL, 5 mg:1 mL, 10 mg:1 mL, and 20 mg:1 mL.

[0019] According to one embodiment of the present invention, the process further comprises subjecting the mixed dispersion to ultrasonic dispersion, for example, the ultrasonic dispersion is dispersed for 20 to 40 minutes, exemplified by 20 minutes, 30 minutes, or 40 minutes.

[0020] According to one embodiment of the present invention, the method further includes heating and stirring the mixed dispersion to produce a reduced graphene oxide precursor coated with a compound containing cobalt ions. Preferably, the heating temperature is 150-200°C, exemplified by 150°C, 165°C, 180°C, and 200°C. Furthermore, the heating time is 30-180 minutes, exemplified by 30 minutes, 60 minutes, 90 minutes, 120 minutes, 150 minutes, and 180 minutes. According to an embodiment of the present invention, the preparation method further includes the steps of filtering, washing, and drying the reduced graphene oxide powder coated with the compound containing cobalt ions after the mixed dispersion has cooled. For example, the drying temperature is 50-70°C, exemplified by 50°C, 60°C, and 70°C; and the drying time is 12-24 hours, exemplified by 12 hours, 15 hours, 18 hours, and 24 hours.

[0021] According to an embodiment of the present invention, the CVD method is performed in a hydrogen / argon mixed atmosphere. For example, the hydrogen / argon mixing ratio is 1:10 to 1:20, exemplified by 1:10, 1:15, and 1:20.

[0022] According to an embodiment of the present invention, a carbon source gas is further introduced into the CVD method to generate carbon nanotubes on the surface of reduced graphene oxide to prepare the RGO-CNT hybrid material. For example, the carbon source gas can be one of methane, ethylene, propylene or acetylene. Furthermore, the flow rate of the carbon source gas is 100 to 200 cc min -1 , for example 100ccmin -1 、150cc min -1 、200cc min -1 .

[0023] According to an embodiment of the present invention, the generation of carbon nanotubes and Co2C particles on the surface of reduced graphene oxide requires a two-stage heating process. Preferably, a carbon source gas is introduced during the first heating stage, and the introduction of the carbon source gas is stopped after the first heating reaction is completed.

[0024] Preferably, in a two-stage heating process:

[0025] The temperature of the first stage heating is 600-900°C, exemplified by 600°C, 700°C, 800°C, and 900°C;

[0026] The first stage heating time is 30 to 90 minutes, exemplified by 30 minutes, 60 minutes, and 90 minutes;

[0027] The gas flow rate in the first stage is 100-200cc min -1 , for example 100cc min -1 、150cc min -1 、200cc min -1 .

[0028] Preferably, in the two-stage heating process, the temperature of the second stage heating is 200-300°C, exemplified by 200°C, 250°C, and 300°C;

[0029] The second stage heating time is 5 to 10 hours, exemplified by 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, and 10 hours;

[0030] The gas flow rate in the second stage is 100-200cc min -1 , for example 100cc min -1 、150cc min -1 、200cc min -1 .

[0031] In the present invention, the second stage heating process is the annealing process.

[0032] According to an embodiment of the present invention, the process of attaching S particles to the surface of the Co2C@RGO-CNT composite material by the melt diffusion method includes: mixing the above-mentioned Co2C@RGO-CNT composite material with sublimated sulfur powder, and preparing the Co2C@RGO-CNT / S composite material by heating.

[0033] Preferably, the mass ratio of the Co2C@RGO-CNT composite material to sulfur powder is 1:1 to 1:9; exemplary ratios are 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, and 1:9.

[0034] Preferably, the heating temperature is 150-165°C, exemplified by 150°C, 160°C, and 165°C; further, the heating time is 12-24h, exemplified by 12h, 15h, 18h, and 24h.

[0035] According to an embodiment of the present invention, the preparation method of the Co2C@RGO-CNT / S composite material comprises the following steps:

[0036] (1) dispersing graphene oxide and a compound containing cobalt ions in ethylene glycol, and ultrasonically dispersing the mixture to obtain a mixed dispersion of graphene oxide and the cobalt-containing compound;

[0037] (2) stirring and heating the mixed dispersion to obtain a reduced graphene oxide dispersion coated with a cobalt ion-containing complex;

[0038] (3) After the dispersion is cooled, it is filtered and washed, and then the reduced graphene oxide powder coated with the cobalt ion complex is dried to serve as a precursor;

[0039] (4) placing the reduced graphene oxide precursor powder coated with the cobalt ion complex prepared in step (3) into a CVD furnace, introducing a carbon source gas, and heating in two stages in a hydrogen / argon mixed atmosphere to prepare a Co2C@RGO-CNT composite material;

[0040] (5) Mix the Co2C@RGO-CNT composite material with sublimated sulfur powder in a glove box and place it in a sealed container;

[0041] (6) The sealed container is heated in a drying oven to obtain the Co2C@RGO-CNT / S composite material.

[0042] The present invention also provides application of the Co2C@RGO-CNT / S composite material in a lithium secondary battery.

[0043] According to an embodiment of the present invention, the application of the Co2C@RGO-CNT / S composite material in a lithium-sulfur battery is preferably used as a positive electrode material for a lithium-sulfur battery.

[0044] The present invention also provides a positive electrode sheet containing the above-mentioned Co2C@RGO-CNT / S composite material.

[0045] The present invention also provides a lithium-sulfur battery, which contains the above-mentioned Co2C@RGO-CNT / S composite material and / or a positive electrode sheet.

[0046] According to an embodiment of the present invention, the positive electrode sheet of the lithium-sulfur battery optionally further includes a conductive agent and / or a binder.

[0047] Preferably, the conductive agent is at least one of conductive carbon black microspheres, carbon nanotubes, graphene, etc.

[0048] Preferably, the adhesive is polyvinylidene fluoride (PVDF).

[0049] According to an embodiment of the present invention, the mass ratio of the Co2C@RGO-CNT / S composite material to the conductive agent and the adhesive is (1-8):(1-5):1, exemplified by 8:1:1, 1:1:1, 1:2:1, 1:3:1, 1:4:1, and 1:5:1.

[0050] Beneficial effects of the present invention:

[0051] Metal carbides are polar materials with high conductivity. They have a strong chemical reaction with polysulfides and can effectively adsorb polysulfides. At the same time, metal carbides have a catalytic effect on the electrochemical reaction of lithium-sulfur batteries, thereby increasing the electrochemical reaction rate. Therefore, Co2C@RGO-CNT composite materials have become an ideal skeleton for lithium-sulfur battery positive electrode materials. Based on this:

[0052] (1) The present invention provides a method for preparing a Co2C@RGO-CNT composite material, wherein a cobalt complex is first deposited on the surface of graphene oxide by a reflux reaction, carbon nanotubes are then grown on the surface of reduced graphene oxide by a CVD method, and Co2C particles are generated on the surface of reduced graphene oxide, ultimately forming a Co2C@RGO-CNT / S composite material to solve the many problems existing in the positive electrode materials of lithium-sulfur batteries. The electrochemical performance test results of lithium-sulfur batteries using Co2C@RGO-CNT / S as the positive electrode material show that the Co2C@RGO-CNT / S electrode exhibits high specific capacity and excellent cycle performance. At a rate of 0.1C, its initial capacity is as high as 1386.3mAh·g -1 , the capacity retention rate after 100 cycles is as high as 94.1%.

[0053] (2) The method of the present invention achieves uniform distribution of Co2C and S particles on the surface of the carbon material. At the same time, the process of the present invention is simple and easy to achieve the preparation of large quantities of Co2C@RGO-CNT / S composite materials, which is conducive to industrial application.

[0054] (3) The Co2C@RGO-CNT / S composite material prepared by the present invention can be used as a positive electrode material for lithium-sulfur batteries, which can play the following advantages: First, the high electrical conductivity of the graphene-carbon nanotube hybrid material is conducive to improving the conductivity of the positive electrode material of the lithium-sulfur battery; second, the high strength and high toughness of the graphene-carbon nanotube hybrid material can effectively regulate the volume change of polysulfides during the electrode reaction; finally, Co2C can effectively fix polysulfides during the electrode reaction to increase the electrochemical reaction rate of polysulfides, thereby improving the cycle life of the battery. The present invention uses the Co2C@RGO-CNT / S composite material as a positive electrode material for lithium-sulfur batteries to effectively improve the electrochemical performance of lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Figure 1 This is a scanning electron microscope photograph of the Co2C@RGO-CNT composite material prepared in Example 3.

[0056] Figure 2 This is a scanning electron microscope photograph of the Co2C@RGO-CNT / S composite material prepared in Example 3. DETAILED DESCRIPTION

[0057] The technical solutions of the present invention will be described in further detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are encompassed within the scope of protection that the present invention is intended to protect.

[0058] Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by known methods.

[0059] In the following examples of the present invention, the carbon source gas is introduced only during the first stage of heating, and the introduction of the carbon source gas is stopped after the first stage of heating reaction is completed.

[0060] In the following examples of the present invention, button cell charge and discharge tests were performed on a CT2001A battery test system from Wuhan Landian Electronics Co., Ltd. The test temperature was 25°C, the charge and discharge cutoff voltage range was 1.7-2.8V, and the charge and discharge current was selected according to actual needs.

[0061] Example 1

[0062] A method for preparing a Co2C@RGO-CNT / S composite material comprises the following steps:

[0063] Step 1: Disperse graphene oxide and cobalt acetate in ethylene glycol in a mass ratio of 1:1, with the amount ratio of graphene oxide to ethylene glycol being 5 mg:1 mL, and ultrasonically disperse for 30 min to obtain a mixed dispersion of graphene oxide and cobalt acetate;

[0064] Step 2: Transfer the mixed dispersion into a round-bottom beaker, heat at 150° C. for 30 min with magnetic stirring to obtain a black cobalt acetate-coated reduced graphene oxide dispersion;

[0065] Step 3: After the dispersion is cooled, vacuum filtration is used, and after repeated washing with ethylene glycol and anhydrous ethanol, the cobalt acetate-coated reduced graphene oxide powder is placed in a vacuum drying furnace and dried at a temperature of 60° C. for 12 hours.

[0066] Step 4: Place 500 mg of the precursor powder obtained in step 3 into a CVD furnace, use acetylene gas as the carbon source, and the gas flow rate is 100 cc min -1 , and heated in two stages in a hydrogen / argon (volume ratio of 1:20) mixed atmosphere: the first stage heating temperature was 600 ° C, the heating time was 30 min, and the gas flow rate was 100 cc min -1 The second stage heating temperature is 200℃, the heating time is 10h, and the gas flow rate is 100cc min-1 , and obtain Co2C@RGO-CNT composite material;

[0067] Step 5: In a glove box, the Co2C@RGO-CNT composite material prepared in step 4 was mixed with sublimated sulfur powder (purchased from Sinopharm Group) and placed in a reactor with a mass ratio of the composite material to the sulfur powder of 1:1.

[0068] Step 6: Place the reactor in a drying oven and heat it at 150°C for 12 hours to obtain the Co2C@RGO-CNT / S composite material.

[0069] The electrode preparation process is as follows:

[0070] (1) The Co2C@RGO-CNT / S composite material, conductive agent (HS100) and binder (PVDF) prepared in this example were weighed in a ratio of 8:1:1, and NMP was used as a solvent to prepare a positive electrode slurry, and magnetic stirring was performed to ensure that the substances in the slurry were evenly mixed;

[0071] (2) The positive electrode slurry was evenly coated on the carbon-coated aluminum foil current collector using a doctor blade method, and then dried under an infrared lamp, and then transferred to a vacuum drying oven at 60°C for 12 hours to remove residual moisture in the positive electrode material;

[0072] (3) Cut the positive electrode sheet into discs with a diameter of 14 mm and weigh the positive electrode sheet.

[0073] The lithium-sulfur battery assembly process is as follows: The lithium-sulfur battery assembly is carried out in a glove box with a moisture content of less than 1ppm and an oxygen content of less than 1ppm. A 2016-type button cell shell is used, with the above-mentioned positive electrode sheet, a 14mm diameter lithium metal sheet and a 16mm diameter lithium metal sheet. 2325 separator as a battery component. The electrolyte composition is 1.0M LiTFSI + DOL:DME = 1:1 Vol% + 1.0% LiNO3. The above components are stacked together and the appropriate amount of electrolyte is added using a pipette, controlling the electrolyte / sulfur ratio to maintain at 12μl mg -1 about.

[0074] The electrochemical performance of the lithium-sulfur battery using the Co2C@RGO-CNT / S prepared in this example as the positive electrode material shows that the Co2C@RGO-CNT / S electrode exhibits high specific capacity and excellent cycling performance. At a rate of 0.1C, its initial capacity is as high as 1065.8 mAh g -1 , the capacity retention rate after 100 cycles is as high as 91.3%.

[0075] Example 2

[0076] A method for preparing a Co2C@RGO-CNT / S composite material comprises the following steps:

[0077] Step 1, graphene oxide and cobalt acetate are dispersed in ethylene glycol in a mass ratio of 1:10, with the amount ratio of graphene oxide to ethylene glycol being 20 mg:1 mL, and ultrasonically dispersed for 30 min to obtain a mixed dispersion of graphene oxide and cobalt acetate;

[0078] Step 2: Transfer the mixed dispersion into a round-bottom beaker, heat at 200° C. for 180 min with magnetic stirring to obtain a cobalt acetate-coated reduced graphene oxide dispersion;

[0079] Step 3: After the dispersion is cooled, vacuum filtration is used, and after repeated washing with ethylene glycol and anhydrous ethanol, the cobalt acetate-coated reduced graphene powder is placed in a vacuum drying furnace and dried at a temperature of 60° C. for 12 hours.

[0080] Step 4: Place 500 mg of the precursor powder obtained in step 3 into a CVD furnace, using acetylene gas as the carbon source at a gas flow rate of 200 cc min -1 , and heated in two stages in a hydrogen / argon (volume ratio of 1:10) mixed atmosphere: the first stage heating temperature was 900 ° C, the heating time was 90 min, and the gas flow rate was 200 cc min -1 The second stage heating temperature is 300℃, the heating time is 8h, and the gas flow rate is 200cc min -1 , and obtain Co2C@RGO-CNT composite material;

[0081] Step 5: In a glove box, the Co2C@RGO-CNT composite material prepared in step 4 was mixed with sublimed sulfur powder and placed in a reactor, with a mass ratio of the composite material to sulfur powder of 1:9;

[0082] Step 6: Place the reactor in a drying oven and heat it at 165°C for 24 hours to obtain the Co2C@RGO-CNT / S composite material.

[0083] The electrochemical performance of the lithium-sulfur battery (assembly process of the lithium-sulfur battery is the same as that of Example 1) using the Co2C@RGO-CNT / S prepared in this example as the positive electrode material shows that the Co2C@RGO-CNT / S electrode exhibits high specific capacity and excellent cycling performance. At a rate of 0.1C, its initial capacity is as high as 1289.3 mAh·g -1 , the capacity retention rate after 100 cycles is as high as 92.8%.

[0084] Example 3

[0085] A method for preparing a Co2C@RGO-CNT / S composite material comprises the following steps:

[0086] Step 1: Disperse graphene oxide and cobalt acetate in ethylene glycol in a mass ratio of 1:5, with the amount ratio of graphene oxide to ethylene glycol being 10 mg:1 mL, and ultrasonically disperse for 30 min to obtain a mixed dispersion of graphene oxide and cobalt acetate;

[0087] Step 2: Transfer the mixed dispersion into a round-bottom beaker, heat at 180° C. for 60 min with magnetic stirring to obtain a cobalt acetate-coated reduced graphene oxide dispersion;

[0088] Step 3: After the dispersion is cooled, vacuum filtration is used, and after repeated washing with ethylene glycol and anhydrous ethanol, the cobalt acetate-coated reduced graphene oxide powder is placed in a vacuum drying furnace and dried at a temperature of 60° C. for 12 hours.

[0089] Step 4: Place 500 mg of the precursor powder obtained in step 3 into a CVD furnace, use acetylene gas as the carbon source, and the gas flow rate is 150 cc min -1 , and heated in two stages in a hydrogen / argon mixed atmosphere (1:20): the first stage heating temperature was 700 °C, the heating time was 30 min, and the gas flow rate was 150 cc min -1 The second stage heating temperature is 250℃, the heating time is 5h, and the gas flow rate is 150cc min -1 , and obtain Co2C@RGO-CNT composite material;

[0090] Step 5: In a glove box, the Co2C@RGO-CNT composite material prepared in step 4 was mixed with sublimed sulfur powder and placed in a reactor, with a mass ratio of the composite material to sulfur powder of 1:6;

[0091] Step 6: Place the reactor in a drying oven and heat it at 155°C for 24 hours to obtain the Co2C@RGO-CNT / S composite material.

[0092] Figure 1 This is a scanning electron microscope photograph of the Co2C@RGO-CNT composite material prepared in this example.

[0093] Figure 2 This is a scanning electron microscope photograph of the Co2C@RGO-CNT / S composite material prepared in this example.

[0094] The electrochemical performance of the lithium-sulfur battery (assembly process of the lithium-sulfur battery is the same as that of Example 1) using the Co2C@RGO-CNT / S prepared in this example as the positive electrode material shows that the Co2C@RGO-CNT / S electrode exhibits high specific capacity and excellent cycling performance. At a rate of 0.1C, its initial capacity is as high as 1386.3 mAh·g -1 , the capacity retention rate after 100 cycles is as high as 94.1%.

[0095] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A method for preparing a Co2C@RGO-CNT / S composite material, characterized in that: The preparation method comprises the following steps: (1) dispersing graphene oxide and a compound containing cobalt ions in ethylene glycol, and ultrasonically dispersing the mixture to obtain a mixed dispersion of the graphene oxide and the compound containing cobalt ions; The mixing mass ratio of the graphene oxide and the compound containing cobalt ions is 1:1 to 1:10; the compound containing cobalt ions is cobalt acetate; (2) stirring and heating the mixed dispersion to obtain a dispersion of reduced graphene oxide coated with a complex containing cobalt ions; the heating temperature is 150-200 C, the heating time is 30 to 180 minutes; (3) After the dispersion is cooled, it is filtered and washed, and then the reduced graphene powder coated with the cobalt ion complex is dried to obtain a precursor; (4) placing the reduced graphene oxide precursor powder coated with the cobalt ion complex prepared in step (3) into a CVD furnace, introducing a carbon source gas, and heating in two stages in a hydrogen / argon mixed atmosphere to prepare a Co2C@RGO-CNT composite material; In a two-stage heating process: The first stage heating temperature is 600~900 C, the first stage heating time is 30~90min, and the first stage gas flow rate is 100~200 cc min -1 ; The second stage heating temperature is 200~300 C, the second stage heating time is 5~10h, and the second stage gas flow rate is 100~200 cc min -1 ; (5) Mixing the Co2C@RGO-CNT composite material with sublimated sulfur powder in a glove box and placing the mixture into a sealed container; the mass ratio of the Co2C@RGO-CNT composite material to sulfur powder is 1:1 to 1:9; (6) Heating the sealed container in a drying oven to obtain the Co2C@RGO-CNT / S composite material; the heating temperature is 150~165 C, the heating time is 12 to 24 hours; The composite material includes an RGO-CNT hybrid material and Co2C particles and S particles loaded on the RGO-CNT hybrid material.

2. The method for preparing the Co2C@RGO-CNT / S composite material according to claim 1, wherein: In the Co2C@RGO-CNT / S composite material, the mass fraction of Co2C particles is 1-20%; and / or, in the Co2C@RGO-CNT / S composite material, the mass fraction of S particles is 50-95%; And / or, the Co2C@RGO-CNT / S composite material has an "anemone"-like structure with an outer dimension of 5 to 10 μm; and / or, in the Co2C@RGO-CNT / S composite material, the length of the carbon nanotubes is 0.2-10.0 μm, and the diameter of the carbon nanotubes is 10-100 nm; and / or, in the Co2C@RGO-CNT / S composite material, the diameter of the Co2C particles is 1-20 nm; And / or, in the Co2C@RGO-CNT / S composite material, the diameter of the sulfur particles is 5-50 nm.

3. The preparation method according to claim 1, wherein The process is carried out in a hydrogen / argon mixed atmosphere, wherein the hydrogen / argon mixing ratio is 1:10 to 1:

20.

4. The preparation method according to claim 1, wherein The carbon source gas is one of methane, ethylene, propylene or acetylene.

5. The preparation method according to claim 1, wherein The flow rate of the carbon source gas is 100-200 cc / min -1 .

6. Use of the Co2C@RGO-CNT / S composite material prepared by the preparation method according to any one of claims 1 to 5 in lithium secondary batteries.

7. The use according to claim 6, wherein the Co2C@RGO-CNT / S composite material is used in a lithium-sulfur battery.

8. The use according to claim 7, wherein the Co2C@RGO-CNT / S composite material is used as a positive electrode material for lithium-sulfur batteries.

9. A positive electrode sheet, characterized in that: The composite material contains the Co2C@RGO-CNT / S composite material prepared by the preparation method according to any one of claims 1 to 5.

10. A lithium-sulfur battery, characterized in that: It contains the Co2C@RGO-CNT / S composite material prepared by the preparation method according to any one of claims 1 to 5 and / or the positive electrode sheet according to claim 9.

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