Carbon nanotube-cobaltosic oxide-graphene composite electrode material for lithium ion battery and preparation method of carbon nanotube-cobaltosic oxide-graphene composite electrode material

Abstract

The invention relates to the field of lithium ion battery materials, and discloses a carbon nanotube-cobaltosic oxide-graphene composite electrode material for a lithium ion battery and a preparation method of the carbon nanotube-cobaltosic oxide-graphene composite electrode material. The composite electrode material has a four-stage composite structure, a conductive skeleton layer is a continuous network formed by assembling multi-walled carbon nanotubes, and an active layer is cobaltosic oxide nanosheets vertically grown on the surfaces of the carbon nanotubes. The buffer layer is porous carbon microspheres embedded in gaps of the cobaltosic oxide nanosheets and is bonded with oxygen atoms at the edges of the cobaltosic oxide nanosheets through surface hydroxyl groups, and the conductive network layer is a three-dimensional network formed by graphene nanosheets through pi-pi conjugation. The initial capacity of the composite electrode material reaches 950-1100 mAh / g, the capacity retention ratio is not lower than 89% after 1000 times of charging and discharging cycles under the current density of 1 C, the capacity retention ratio is not lower than 75% under the high rate of 20 C, and the composite electrode material has the advantages of high specific capacity, long cycle life, high rate performance, excellent energy storage performance and excellent charge transmission performance, and can be used as an electrode material of a lithium ion battery with high power density and high energy density.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries and its preparation method. Background Technology

[0002] As a core energy storage device in the current new energy field, the improvement of lithium-ion batteries in terms of energy density, cycle stability, and rate performance directly restricts the development of the driving range of new energy vehicles, the standby time of consumer electronics, and the operating efficiency of large-scale energy storage systems. Among lithium-ion battery electrode materials, cobalt tetroxide (Co3O4) has shown significant application potential in the field of anode materials due to its high theoretical lithium storage capacity of 890 mAh / g, becoming one of the key areas of focus for scientific research and industry in recent years. However, the practical application of Co3O4-based electrode materials still faces multiple core technological bottlenecks, which severely limit their performance and large-scale promotion.

[0003] First, Co3O4 is prone to volume expansion exceeding 200% during the electrochemical reaction of lithium-ion insertion / deintercalation. This drastic volume change leads to the pulverization of active particles, the collapse of electrode structures, and even the delamination of the electrode-current collector interface, ultimately causing rapid capacity decay. Although existing research has attempted to mitigate the volume expansion problem by designing nanostructures such as one-dimensional nanotubes and two-dimensional nanosheets, a single nanostructure cannot simultaneously ensure both active site exposure and structural stability. Even after long-term cycling, the problem of excessively rapid capacity decay remains unavoidable, failing to meet the long-life requirements of power batteries and energy storage batteries.

[0004] Secondly, Co3O4 has extremely low intrinsic conductivity, only about 10⁻⁶. -5 The current conductivity (S / cm) is insufficient to meet the requirements for efficient electron transport. To improve conductivity, existing technologies typically require the addition of large amounts of conductive additives such as carbon black and carbon nanotubes to the electrodes. However, excessive additives not only reduce the volumetric proportion of active materials (directly leading to a decrease in battery energy density), but also cause uneven dispersion of additives within the electrode, creating localized charge transport bottlenecks. Especially in high-rate charge-discharge scenarios (such as fast charging of new energy vehicles and peak power output of energy storage systems), insufficient electron transport efficiency leads to a sharp capacity decay, making it difficult to adapt to high-power applications.

[0005] Furthermore, existing composite processes for carbon materials (such as graphene and carbon nanotubes) and Co3O4 have significant drawbacks. Current mainstream composite methods mostly involve simple coating or physical mixing, which cannot achieve uniform dispersion of carbon materials and Co3O4 at the nanoscale. Moreover, the interfacial bonding between the two is weak, relying primarily on physical adsorption. During long-term charge-discharge cycles, this weak interfacial bonding is prone to separation, leading to the breakage of the conductive network, further exacerbating capacity decay and rate performance deterioration, and failing to form a stable cross-scale conductive system.

[0006] Furthermore, the pore structure design of traditional mesoporous Co3O4 materials has limitations. Their pore distribution is highly random, resulting in long diffusion paths for lithium ions within the electrode and a lack of synergistic matching with the conductive network. Even with a mesoporous structure, the lack of a cross-scale conductive network hinders the simultaneous improvement of lithium-ion and electron transport efficiency. For example, the capacity retention of conventional mesoporous Co3O4 materials at high rates (20C) is often below 40%, far from meeting the high-power energy storage requirements of emergency power supplies and vehicle start-stop systems.

[0007] The combination of these technical bottlenecks has made it difficult for Co3O4-based electrode materials to achieve a balance between energy density, cycle stability, and high-rate performance, severely limiting their large-scale application in high-performance lithium-ion batteries. Therefore, developing Co3O4-based composite electrode materials that can simultaneously address volume expansion suppression, conductivity improvement, and enhanced interface stability is currently a research hotspot in the lithium-ion battery field. Summary of the Invention

[0008] The purpose of this invention is to provide a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, its preparation method, and a lithium-ion battery. This composite electrode material has high specific capacity, long cycle life, and high rate performance, and has good energy storage and charge transport performance. It can be used as an electrode material for lithium-ion batteries with high power density, high energy density, and long cycle life, and can effectively solve the problems of volume expansion, poor conductivity, and insufficient interface stability of existing cobalt tetroxide-based electrode materials.

[0009] To achieve the above objectives, the present invention provides the following solution: A carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries is disclosed. This composite electrode material has a four-level composite structure, including a conductive framework layer, an active layer, a buffer layer, and a conductive network layer. The conductive framework layer is a continuous conductive network formed by carbon nanotubes; the active layer is cobalt tetroxide nanosheets grown vertically on the surface of the carbon nanotubes; the buffer layer is porous carbon microspheres embedded in the gaps between the cobalt tetroxide nanosheets; and the conductive network layer is a three-dimensional graphene network bonded to the surface of the material. The porous carbon microspheres are bonded to the oxygen atoms at the edge of the cobalt tetroxide nanosheets via surface hydroxyl groups, and the graphene three-dimensional network is formed by graphene nanosheets through π-π conjugation.

[0010] Furthermore, the conductive framework layer is assembled from multi-walled carbon nanotubes with a diameter of 10-50 nm, forming a continuous network with a thickness of 10-20 μm. The cobalt tetroxide nanosheets of the active layer grow perpendicular to the surface of the carbon nanotubes, with a thickness of 20~50nm and a lateral dimension of 100~300nm; The porous carbon microspheres in the buffer layer have a particle size of 200~500nm and are uniformly embedded in the gaps between the cobalt tetroxide nanosheets. The graphene nanosheets of the conductive network layer have a thickness of 1~2nm and a length and width of 200~600nm, and are bonded together to form a three-dimensional network.

[0011] The present invention also provides a method for preparing the above-mentioned carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, comprising the following steps: (1) A carbon nanotube / cobalt tetroxide nanosheet composite film was prepared by hydrothermal growth, including: Carbon nanotube membranes are immersed in a mixed solution of cobalt nitrate and urea, and hydrothermally reacted at 120-150°C for 6-10 hours. The reaction product is then calcined at 350-450°C for 2 hours at a heating rate of 5°C / min-10°C / min to obtain a carbon nanotube / cobalt tetroxide nanosheet composite membrane. The molar ratio of cobalt nitrate to urea is 1:5 to 1:8. (2) Porous carbon microspheres are embedded using a vacuum impregnation and filling method, including: After functionalizing porous carbon microspheres with concentrated nitric acid, they were dispersed in an ethanol solution to obtain a carbon microsphere dispersion with a concentration of 8~12 mg / mL. The composite membrane prepared in step (1) was immersed in the dispersion and filtered under a vacuum of -0.05 MPa for 10~15 min to allow the carbon microspheres to be embedded in the gaps between cobalt tetroxide nanosheets. (3) A three-dimensional graphene network was constructed using an electrophoretic bonding-reduction method, including: The composite membrane obtained in step (2) is used as the working electrode and immersed in a graphene oxide solution with a pH of 3-4 and a concentration of 6-9 mg / mL. Electrophoretic self-assembly is carried out for 8-12 min under the conditions of electrode spacing of 3-4 cm and constant voltage of 2.5-3.5 V. Then, it is reduced by hydrazine vapor at 80℃ for 20 min to form a three-dimensional graphene network, thus obtaining the composite electrode material.

[0012] Furthermore, in step (1), the carbon nanotube film has a thickness of 10~20 μm and a density of 0.8~1.2 g / cm³. 3 The solvent for the cobalt nitrate and urea mixed solution is an ethanol-water mixed solvent, and the volume ratio of ethanol to water is 1:1.

[0013] Further, in step (2), the functionalization treatment of the porous carbon microspheres includes: refluxing the porous carbon microspheres in 65% concentrated nitric acid for 2 hours to graft hydroxyl and carboxyl groups onto the surface of the carbon microspheres.

[0014] Furthermore, in step (3), the pH value of the graphene oxide solution is adjusted by hydrochloric acid; during electrophoretic self-assembly, a platinum sheet is used as the counter electrode.

[0015] The present invention also provides a lithium-ion battery electrode, comprising the aforementioned carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, or the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries prepared according to any one of claims 3-6.

[0016] The present invention also provides a lithium-ion battery, comprising: an electrode made of the above-mentioned carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, or the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries obtained according to the above preparation method.

[0017] According to specific embodiments provided by the present invention, the following technical effects are disclosed: The carbon nanotube-cobalt tetroxide-graphene composite electrode material and its preparation method provided by the present invention have a four-level structure of "support-activity-buffer-conductivity" consisting of a conductive framework layer, an active layer, a buffer layer, and a conductive network layer. A highly conductive multi-walled carbon nanotube film serves as the conductive framework, constructing a continuous conductive network with a thickness of 10-20 μm, providing a through-channel for electron transport and forming the core support unit in the four-level structure. Cobalt tetroxide nanosheets grown vertically on the surface of the carbon nanotubes increase the exposed area of ​​active sites and shorten the lithium-ion diffusion path, while avoiding the agglomeration problem of traditional nanoparticles, significantly improving the utilization rate of active materials. Porous carbon microspheres embedded in the gaps between the cobalt tetroxide nanosheets form chemical bonds with the oxygen atoms at the edge of the cobalt tetroxide nanosheets through surface hydroxyl groups. The physical confinement and chemical anchoring synergistically suppress the large volume expansion of cobalt tetroxide during charging and discharging, ensuring the integrity of the electrode structure. The three-dimensional graphene network bonded to the material surface forms a cross-scale conductive system with the one-dimensional conductive channels of carbon nanotubes, which significantly reduces the interfacial resistance and provides additional diffusion channels for lithium ions.

[0018] The aforementioned four-level structure overcomes the design limitations of traditional cobalt tetroxide-based materials through simple coating or physical mixing, achieving synergistic optimization of electron transport efficiency, structural stability, and ion diffusion rate. Specifically, the continuous carbon nanotube framework and graphene network address the issue of poor conductivity; the vertical cobalt tetroxide nanosheets and porous carbon microsphere buffer layer address the issues of volume expansion and insufficient active sites; and the multi-scale conductive network and ordered porous structure address the issue of poor rate performance. This multi-dimensional synergistic effect enables the material to possess high energy density, long cycle life, and high rate performance. The initial capacity of this composite electrode material can reach 950~1100 mAh / g, and the capacity retention rate is ≥89% after 1000 charge-discharge cycles at 1C current density, and ≥75% at a high rate of 20C. This significantly overcomes the technical bottlenecks of existing cobalt tetroxide-based electrode materials, such as volume expansion, poor conductivity, and insufficient interface stability, making it suitable for lithium-ion batteries with high power density, high energy density, and long cycle life.

[0019] On the other hand, the preparation method of the present invention uses a hydrothermal growth method to achieve the directional vertical growth of cobalt tetroxide nanosheets on the surface of carbon nanotube films, avoiding the problem of conductive network breakage caused by carbon nanotube powder agglomeration. The vacuum impregnation filling method ensures that porous carbon microspheres are uniformly embedded in the gaps between the cobalt tetroxide nanosheets, solving the defect of uneven dispersion in traditional physical mixing.

[0020] In summary, the carbon nanotube-cobalt tetroxide-graphene-based composite electrode material of this invention combines high power density, high energy density, and long cycle life. Its preparation method is efficient and controllable, and it has good application prospects in the field of lithium-ion batteries. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments or prior art of this specification, the drawings used in the description of the embodiments or prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a flowchart of the preparation method of the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries provided by the present invention; Figure 2 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / carbon microsphere film provided in Example 1 of the present invention; Figure 3 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / graphene-based film provided in Example 1 of this invention; Figure 4 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / graphene-based film provided in Example 1; Figure 5 This is the single-cycle specific capacity-voltage diagram provided in Embodiment 1 of the present invention; Figure 6 This is a 1C charge-discharge performance cycle diagram provided in Embodiment 1 of the present invention; Figure 7 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide nanosheet composite film provided in Example 2 of the present invention; Figure 8 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / carbon microsphere composite film provided in Example 2 of the present invention; Figure 9 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / graphene-based film provided in Example 2 of the present invention; Figure 10 This is a 0.1C constant current charge-discharge capacity diagram provided in Embodiment 2 of the present invention; Figure 11 This is a 1C charge-discharge performance cycle diagram provided in Embodiment 2 of the present invention; Figure 12 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide film provided in Example 3 of the present invention; Figure 13 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / carbon microsphere film provided in Example 3 of the present invention; Figure 14 This is a scanning electron microscope image of the carbon nanotube / cobalt tetroxide / graphene-based film provided in Example 3 of the present invention; Figure 15 This is a 0.1C constant current charge-discharge capacity diagram provided in Embodiment 3 of the present invention; Figure 16 This is a 1C charge-discharge performance cycle diagram provided in Embodiment 3 of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "comprising" and similar expressions used herein mean that the element or object preceding the word covers the element or object listed following the word and its equivalents, but do not exclude other elements or objects.

[0024] The purpose of this invention is to provide a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, its preparation method, and a lithium-ion battery. This composite electrode material has high specific capacity, long cycle life, and high rate performance, and has good energy storage and charge transport performance. It can be used as an electrode material for lithium-ion batteries with high power density, high energy density, and long cycle life, and can effectively solve the problems of volume expansion, poor conductivity, and insufficient interface stability of existing cobalt tetroxide-based electrode materials.

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] The carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries provided by this invention has a four-level composite structure consisting of a conductive framework layer, an active layer, a buffer layer, and a conductive network layer. The conductive framework layer is a continuous conductive network formed by carbon nanotubes. The active layer is cobalt tetroxide (Co3O4) nanosheets grown vertically on the surface of carbon nanotubes. The buffer layer is porous carbon microspheres embedded in the gaps between the Co3O4 nanosheets. The conductive network layer is a three-dimensional graphene network bonded to the surface of the material.

[0027] The microstructure of the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries is determined by both the raw materials and the manufacturing process. The composite electrode material is produced by directionally growing Co3O4 nanosheets on the surface of carbon nanotubes using a hydrothermal growth method, embedding porous carbon microspheres between the nanosheets using a vacuum impregnation method, and then constructing a three-dimensional graphene network via electrophoretic bonding-reduction, forming a "quaternary composite" structure. The bonding between the surface hydroxyl groups of the porous carbon microspheres and the oxygen atoms at the edge of the Co3O4 nanosheets, as well as the π-π conjugation between graphene and carbon nanotubes, synergistically enhance interfacial stability and conductivity.

[0028] In the conductive framework layer, the carbon nanotube film is assembled from multi-walled carbon nanotubes with a diameter of 10~50nm, forming a continuous network with a thickness of 10~20μm. This structure provides a through-hole electron transport channel for the material, while supporting the orderly distribution of the active layer and the buffer layer.

[0029] In the active layer, Co3O4 nanosheets grow perpendicular to the surface of carbon nanotubes, with a thickness of 20~50nm and a lateral dimension of 100~300nm. Their high specific surface area increases the exposure of active sites and shortens the lithium ion diffusion path. At the same time, the vertical growth morphology provides a space for the buffer layer to be filled.

[0030] In the buffer layer, the porous carbon microspheres have a particle size of 200~500nm and are uniformly embedded in the gaps between Co3O4 nanosheets. After being functionalized with concentrated nitric acid, the carbon microspheres are grafted with hydroxyl groups on their surface and achieve chemical anchoring by bonding with oxygen atoms at the edge of the Co3O4 nanosheets. At the same time, the physical confinement effect effectively suppresses the volume expansion of Co3O4 during charging and discharging.

[0031] In the conductive network layer, the three-dimensional graphene network is formed by graphene nanosheets with a thickness of 1~2nm and a length and width of 200~600nm through π-π conjugation; it synergistically constructs a cross-scale conductive network with carbon nanotubes (one-dimensional), which significantly reduces the interfacial resistance and provides a fast diffusion channel for lithium ions.

[0032] On a macroscopic level, the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries of this invention is in the form of a thin film, which can be attached to the surface of a substrate to form a thin film electrode, making it very suitable for manufacturing lithium-ion batteries.

[0033] The initial capacity of the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries is 950~1100mAh / g; after 1000 charge-discharge cycles at 1C current density, the capacity retention rate is ≥89%.

[0034] The present invention also provides a method for preparing the above-mentioned carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, comprising the following steps: (1) Preparation of carbon nanotube / Co3O4 nanosheet composite film by hydrothermal growth method: The carbon nanotube film is immersed in a mixed solution of cobalt nitrate and urea (molar ratio 1:5~1:8) and hydrothermally reacted at 120~150℃ for 6~10 hours. The reaction product is then calcined at 350~450℃ for 2 hours (heating rate 5℃ / min) to obtain the carbon nanotube / Co3O4 nanosheet composite film. The carbon nanotube film has a thickness of 10~20μm and a density of 0.8~1.2g / cm³. 3 The solvent for the mixed solution is an ethanol-water mixture (volume ratio 1:1). In this step, the carbon nanotube membrane needs to be a continuous film structure rather than a dispersed powder because powdered carbon nanotubes are prone to agglomeration, making it difficult to form a continuous conductive network after being combined with Co3O4, resulting in decreased conductivity.

[0035] (2) Vacuum impregnation and filling method for embedding porous carbon microspheres: The porous carbon microspheres were refluxed in 65% concentrated nitric acid for 2 hours for functionalization treatment (grafting hydroxyl and carboxyl groups on the surface), and dispersed in ethanol solution to obtain a carbon microsphere dispersion with a concentration of 8~12mg / mL; The composite membrane obtained in step (1) was immersed in the dispersion and filtered under a vacuum of -0.05MPa for 10~15min to embed the carbon microspheres into the gaps between Co3O4 nanosheets, thus obtaining a carbon nanotube / Co3O4 / carbon microsphere composite membrane.

[0036] (3) Electrophoretic bonding-reduction method to construct a three-dimensional graphene network: The composite film obtained in step (2) is used as the working electrode and immersed in a graphene oxide solution with pH=3~4 (concentration 6~9mg / mL, pH value is adjusted by hydrochloric acid); under the conditions of electrode spacing 3~4cm and constant voltage 2.5~3.5V, electrophoretic self-assembly is performed for 8~12min (with platinum sheet as counter electrode), and then reduced by hydrazine vapor at 80℃ for 20min to reduce graphene oxide to graphene and form a three-dimensional network, thus obtaining the composite electrode material.

[0037] To make the objectives, technical solutions, and advantages of one or more embodiments of this specification clearer, the technical solutions of one or more embodiments of this specification will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this specification, and not all of them. Based on the embodiments in this specification, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of one or more embodiments of this specification.

[0038] Example 1 Example 1 of this invention prepares a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries according to the following steps: (1) Preparation of carbon nanotube / Co3O4 nanosheet composite film Weigh out 0.004 mol of cobalt nitrate (Co(NO3)2·6H2O) and 0.032 mol of urea (CO(NH2)2), mix them, and dissolve them in an ethanol-water mixture (volume ratio 1:1) to a total volume of 60 mL. Stir magnetically for 30 minutes until completely dissolved to form a reaction solution. Then, transfer the reaction solution to a stainless steel reactor. Select a carbon nanotube membrane with a thickness of 11 μm and a density of 0.9 g / cm³. 3 Cut to 2×3 cm 2 A rectangular membrane was immersed in the reaction solution, and the reaction vessel was sealed and placed in an oven at 130°C for 8 hours. After the reaction, the composite membrane was removed, washed with deionized water and ethanol, and then calcined at 400°C for 2 hours at a heating rate of 5°C / min. It was then naturally cooled to room temperature to obtain a carbon nanotube / Co3O4 nanosheet composite membrane. The scanning electron microscope image of the obtained carbon nanotube / cobalt tetroxide membrane is shown below. Figure 2 As shown, from Figure 2 It can be seen that the size of cobalt tetroxide nanosheets is about 300 nm, which run through the carbon nanotubes and grow vertically on the surface of the carbon nanotubes; the carbon nanotubes form a continuous network.

[0039] (2) Vacuum impregnation and filling of carbon microspheres Carbon microspheres with a particle size of 300 nm were immersed in 65% concentrated nitric acid and refluxed for 2 hours to graft hydroxyl groups (-OH) onto the surface. After cooling, the microspheres were separated, washed with deionized water until neutral, and dried under vacuum at 60°C for 8 hours. The dried carbon microspheres were dispersed in ethanol to prepare a dispersion with a concentration of 10 mg / mL, and ultrasonically dispersed until uniform. The composite membrane obtained in step (1) was immersed in the dispersion and placed in a vacuum filtration device. It was filtered under -0.05 MPa for 10 minutes to allow the carbon microspheres to embed into the gaps between the Co3O4 nanosheets. Subsequently, it was dried under vacuum at 60°C for 4 hours to obtain a carbon nanotube / Co3O4 / carbon microsphere composite membrane. The obtained carbon nanotube / cobalt tetroxide / carbon microsphere composite membrane is shown below. Figure 3 As shown. Figure 3 As shown, carbon microspheres are distributed between cobalt tetroxide nanosheets, with a diameter of about 300 nm, and are in close contact with the edges of the nanosheets without obvious aggregation.

[0040] (3) Graphene self-assembly on the surface of carbon nanotube / cobalt tetroxide / carbon microsphere film Preparation of graphene oxide solution: Weigh graphene oxide powder, dissolve it in deionized water to prepare a solution with a concentration of 7 mg / mL, adjust the pH to 3.5 with hydrochloric acid, and ultrasonically disperse until uniform. Using the composite membrane obtained in step (2) as the working electrode and a platinum sheet as the counter electrode with an electrode spacing of 3 cm, immerse the composite membrane in the above graphene oxide solution, apply a constant voltage of 3.0 V, and pass the current for 10 minutes to allow graphene oxide to be deposited on the surface of the composite membrane through electrostatic adsorption. Subsequently, place the composite membrane in a hydrazine vapor environment and reduce it at 80℃ for 20 minutes to reduce graphene oxide to graphene, forming a π-π conjugated network, and obtain the finished product. The scanning electron microscope image of the obtained carbon nanotube / cobalt tetroxide / graphene-based membrane is shown below. Figure 4 As shown, from Figure 4 It can be seen that a layer of graphene nano-interconnected structure is formed on the surface, containing multiple graphene nanosheets. The graphene nanosheets are evenly distributed and interconnected. The thickness of the graphene nanosheets is 1-2 nm, and the length and width are 100-500 nm.

[0041] (4) Single-cycle specific capacity-voltage measurement The prepared carbon nanotube-cobalt tetroxide-graphene-based composite film was used as an electrode to form a button half-cell with a lithium sheet. The electrolyte solution was a 0.8 mol / L lithium hexafluorophosphate solution, and the solvent was ethylene carbonate:diethyl carbonate, with a volume ratio of 1:1.

[0042] The prepared electrode was tested as a lithium metal half-cell using a LANHE CT3002A battery tester. The single-cycle specific capacity-voltage diagram of the electrode was measured at a current density of 1.0C and a voltage range of 0.1~2.5V. Figure 5As shown, the electrode of Example 1 exhibits a high single-cycle specific capacity of 1020 mAh / g at a current density of 0.1C. For example... Figure 6 As shown, after 1000 cycles at a current density of 1C, the capacity retention of this electrode remains at 89.5%.

[0043] Example 2 Example 2 of this invention prepares a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries according to the following steps: (1) Preparation of carbon nanotube / Co3O4 nanosheet composite film Weigh out 0.004 mol of cobalt nitrate (Co(NO3)2·6H2O) and 0.024 mol of urea (CO(NH2)2), mix them, and dissolve them in an ethanol-water mixture (volume ratio 1:1) to a total volume of 30 mL. Stir magnetically for 30 minutes until completely dissolved, forming a reaction solution with a cobalt nitrate to urea molar ratio of 1:6. Then, transfer the reaction solution to a stainless steel reactor. A carbon nanotube membrane with a thickness of 15 μm and a density of 1.0 g / cm³ is selected. 3 The nanotubes were cut into 2×3 cm² rectangles, immersed in the reaction solution, and the reaction vessel was sealed and placed in an oven at 130℃ for 8 hours. After the reaction, the composite membrane was removed, washed with deionized water and ethanol, and then calcined at 400℃ for 2 hours at a heating rate of 5℃ / min. It was then naturally cooled to room temperature to obtain a carbon nanotube / Co3O4 nanosheet composite membrane. The scanning electron microscope image of the obtained carbon nanotube / Co3O4 nanosheet composite membrane is shown below. Figure 7 As shown, from Figure 7 It can be seen that the Co3O4 nanosheets are vertically distributed on the surface of the carbon nanotubes, with a thickness of about 30 nm and a lateral dimension of about 200 nm; the carbon nanotubes have a diameter of about 30 nm, forming a continuous network.

[0044] (2) Vacuum impregnation and filling of carbon microspheres Porous carbon microspheres with a particle size of 300 nm were immersed in 65% concentrated nitric acid and refluxed for 2 hours. Hydroxyl groups (-OH) and carboxyl groups (-COOH) were grafted onto the surface. After cooling, the microspheres were separated, washed with deionized water until neutral, and dried under vacuum at 60°C for 8 hours. The dried carbon microspheres were dispersed in ethanol to prepare a dispersion with a concentration of 10 mg / mL, and ultrasonically dispersed until uniform. The composite membrane obtained in step (1) was immersed in this dispersion and placed in a vacuum filtration device. Filtration was performed at -0.05 MPa for 12 minutes to embed the carbon microspheres into the gaps between the Co3O4 nanosheets. The membrane was then dried under vacuum at 60°C for 4 hours to obtain a carbon nanotube / Co3O4 / carbon microsphere composite membrane. The obtained carbon nanotube / Co3O4 / carbon microsphere composite membrane is shown below. Figure 8 As shown, carbon microspheres are distributed between cobalt tetroxide nanosheets, with a diameter of about 320 nm, and are in close contact with the edges of the nanosheets without obvious aggregation.

[0045] (3) Graphene self-assembly on the surface of carbon nanotube / cobalt tetroxide / carbon microsphere film Preparation of graphene oxide solution: Weigh graphene oxide powder, dissolve it in deionized water to prepare a solution with a concentration of 6 mg / mL, adjust the pH to 3.5 with hydrochloric acid, and ultrasonically disperse until uniform. Using the composite membrane obtained in step (2) as the working electrode and a platinum sheet as the counter electrode with an electrode spacing of 3.5 cm, immerse the above graphene oxide solution, apply a constant voltage of 2.5 V, and pass the current for 10 minutes to allow graphene oxide to be deposited on the surface of the composite membrane through electrostatic adsorption. Subsequently, place the composite membrane in a hydrazine vapor environment and reduce it at 80°C for 20 minutes to reduce graphene oxide to graphene, forming a π-π conjugated network, and obtain the finished product. The scanning electron microscope image of the obtained carbon nanotube / cobalt tetroxide / graphene-based membrane is shown below. Figure 9 As shown, from Figure 9 It can be seen that a layer of graphene nano-interconnected structure is formed on the surface, containing multiple graphene nanosheets. The graphene nanosheets are evenly distributed and interconnected. The thickness of the graphene nanosheets is 1~2nm, and the length and width are 250-400nm.

[0046] (4) Single-cycle specific capacity-voltage measurement The prepared carbon nanotube-cobalt tetroxide-graphene-based composite film was used as an electrode to form a button half-cell with a lithium sheet. The electrolyte solution was a 0.8 mol / L lithium hexafluorophosphate solution, and the solvent was ethylene carbonate:diethyl carbonate, with a volume ratio of 1:1.

[0047] The prepared electrode was tested as a lithium metal half-cell using a LANHE CT3002A battery tester. The single-cycle specific capacity-voltage diagram of the electrode was measured at a current density of 1.0C and a voltage range of 0.1~2.5V. Figure 10 As shown, the electrode of Example 2 has a single-cycle specific capacity of 1015 mAh / g at a current density of 0.1C. Figure 11 As shown, after 1000 cycles at a current density of 1C, the capacity retention of this electrode remains at 89.2%.

[0048] Example 3 Example 3 of this invention prepares a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries according to the following steps: (1) Preparation of carbon nanotube / Co3O4 nanosheet composite film Weigh out 0.003 mol of cobalt nitrate (Co(NO3)2·6H2O) and 0.015 mol of urea (CO(NH2)2), mix them, and dissolve them in an ethanol-water mixture (volume ratio 1:1) to a total volume of 30 mL. Stir magnetically for 30 minutes until completely dissolved, forming a reaction solution with a cobalt nitrate to urea molar ratio of 1:5. Then, transfer the reaction solution to a stainless steel reactor. Select a carbon nanotube membrane with a thickness of 10 μm and a density of 0.8 g / cm³. 3 Cut to 2×3 cm 2 A rectangular membrane was immersed in the reaction solution, and the reaction vessel was sealed and placed in an oven at 120°C for 6 hours. After the reaction, the composite membrane was removed, washed with deionized water and ethanol, and then calcined at 350°C for 2 hours at a heating rate of 5°C / min. It was then naturally cooled to room temperature to obtain a carbon nanotube / Co3O4 nanosheet composite membrane. The scanning electron microscope image of the obtained carbon nanotube / cobalt tetroxide membrane is shown below. Figure 12 As shown, from Figure 12 It can be seen that the cobalt tetroxide nanosheets are about 100 nm in size, grow uniformly on the surface of carbon nanotubes, and are tightly attached to the carbon nanotubes, which are about 10 nm in diameter, forming a dense network.

[0049] Scanning electron microscopy revealed that Co3O4 nanosheets grew uniformly on the surface of carbon nanotubes, with a thickness of approximately 20 nm and a lateral dimension of approximately 100 nm; the carbon nanotubes had a diameter of approximately 10 nm, forming a dense network.

[0050] (2) Vacuum impregnation and filling of carbon microspheres Porous carbon microspheres with a particle size of 200 nm were immersed in 65% concentrated nitric acid and refluxed for 2 hours. Hydroxyl (-OH) and carboxyl (-COOH) groups were grafted onto the surface. After cooling, the microspheres were separated, washed with deionized water until neutral, and dried under vacuum at 60°C for 8 hours. The dried carbon microspheres were dispersed in ethanol to prepare a dispersion with a concentration of 8 mg / mL, and ultrasonically dispersed until uniform. The composite membrane obtained in step (1) was immersed in the dispersion and placed in a vacuum filtration device. It was filtered under -0.05 MPa for 10 minutes to allow the carbon microspheres to embed into the gaps between the Co3O4 nanosheets. Then, it was dried under vacuum at 60°C for 4 hours to obtain a carbon nanotube / Co3O4 / carbon microsphere composite membrane. The scanning electron microscope image of the obtained carbon nanotube / Co3O4 / carbon microsphere composite membrane is shown below. Figure 13 As shown, from Figure 13 It can be seen that the carbon microspheres are uniformly embedded in the gaps between the cobalt tetroxide nanosheets, with a diameter of about 200 nm. They are tightly bonded to the edges of the nanosheets without agglomeration. The cobalt tetroxide nanosheets grow vertically on the surface of the carbon nanotubes, forming an ordered interstitial structure.

[0051] (3) Graphene self-assembly on the surface of carbon nanotube / cobalt tetroxide / carbon microsphere film Preparation of graphene oxide solution: Weigh graphene oxide powder, dissolve it in deionized water to prepare a solution with a concentration of 6 mg / mL, adjust the pH to 3 with hydrochloric acid, and ultrasonically disperse until uniform. Using the composite membrane obtained in step (2) as the working electrode and a platinum sheet as the counter electrode with an electrode spacing of 3 cm, immerse the composite membrane in the above graphene oxide solution, apply a constant voltage of 2.5 V, and pass the current for 8 minutes to allow graphene oxide to be deposited on the surface of the composite membrane through electrostatic adsorption. Subsequently, place the composite membrane in a hydrazine vapor environment and reduce it at 80°C for 20 minutes to reduce graphene oxide to graphene, forming a π-π conjugated network, and obtain the finished product. The scanning electron microscope image of the obtained carbon nanotube / cobalt tetroxide / graphene-based membrane is shown below. Figure 14 As shown, from Figure 14 It can be seen that a continuous three-dimensional graphene network structure is formed on the surface, containing multiple graphene nanosheets. The graphene nanosheets are evenly distributed and tightly bonded to carbon nanotubes. The thickness of the graphene nanosheets is 1~2nm, and the length and width are 200-300nm.

[0052] (4) Single-cycle specific capacity-voltage measurement The prepared carbon nanotube-cobalt tetroxide-graphene-based composite film was used as an electrode to form a button half-cell with a lithium sheet. The electrolyte solution was a 0.8 mol / L lithium hexafluorophosphate solution, and the solvent was ethylene carbonate:diethyl carbonate, with a volume ratio of 1:1.

[0053] The prepared electrode was tested as a lithium metal half-cell using a LANHE CT3002A battery tester. The single-cycle specific capacity-voltage diagram of the electrode was measured at a current density of 1.0C and a voltage range of 0.1~2.5V. Figure 15 As shown, from Figure 15 It can be seen that the electrode of Example 3 has a capacity of 950 mAh / g at 0.1C. Figure 16 After 1000 cycles at 1C, the capacity retention rate remained at 89.0%.

[0054] Examples 1-3 all demonstrate the following characteristics of carbon nanotube-cobalt tetroxide-graphene composite electrode materials for lithium-ion batteries: Structural integrity: Precisely constructing a four-level composite structure of "carbon nanotubes-cobalt tetroxide nanosheets-carbon microspheres-graphene"; Interface stability: Vertical growth of Co3O4 nanosheets, uniform embedding of carbon microspheres, and continuous coverage of graphene network;

[0055] High-performance synergy: initial capacity 950~1100 mAh / g, capacity retention ≥89% after 1000 cycles at 1C, and capacity retention ≥75% at 20C rate.

[0056] The foregoing has described specific embodiments of this specification. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than those shown in the embodiments and may still achieve the desired results.

[0057] The above description is merely an embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of the claims of this application.

Claims

1. A carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries, characterized in that, The composite electrode material has a four-level composite structure, including a conductive framework layer, an active layer, a buffer layer, and a conductive network layer. in The conductive framework layer is a continuous conductive network formed by carbon nanotubes; the active layer is cobalt tetroxide nanosheets grown vertically on the surface of the carbon nanotubes; the buffer layer is porous carbon microspheres embedded in the gaps between the cobalt tetroxide nanosheets; and the conductive network layer is a three-dimensional graphene network bonded to the surface of the material. The porous carbon microspheres are bonded to the oxygen atoms at the edge of the cobalt tetroxide nanosheets via surface hydroxyl groups, and the graphene three-dimensional network is formed by graphene nanosheets through π-π conjugation.

2. The carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries according to claim 1, characterized in that, The conductive framework layer is assembled from multi-walled carbon nanotubes with a diameter of 10-50 nm, forming a continuous network with a thickness of 10-20 μm. The cobalt tetroxide nanosheets of the active layer grow perpendicular to the surface of the carbon nanotubes, with a thickness of 20~50nm and a lateral dimension of 100~300nm; The porous carbon microspheres in the buffer layer have a particle size of 200~500nm and are uniformly embedded in the gaps between the cobalt tetroxide nanosheets. The graphene nanosheets of the conductive network layer have a thickness of 1~2nm and a length and width of 200~600nm, and are bonded together to form a three-dimensional network.

3. A method for preparing a carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries as described in any one of claims 1-2, characterized in that, Includes the following steps: (1) A carbon nanotube / cobalt tetroxide nanosheet composite film was prepared by hydrothermal growth, including: Carbon nanotube membranes are immersed in a mixed solution of cobalt nitrate and urea, and hydrothermally reacted at 120-150°C for 6-10 hours. The reaction product is then calcined at 350-450°C for 2 hours at a heating rate of 5°C / min-10°C / min to obtain a carbon nanotube / cobalt tetroxide nanosheet composite membrane. The molar ratio of cobalt nitrate to urea is 1:5 to 1:

8. (2) Porous carbon microspheres are embedded using a vacuum impregnation and filling method, including: After functionalizing porous carbon microspheres with concentrated nitric acid, they were dispersed in an ethanol solution to obtain a carbon microsphere dispersion with a concentration of 8~12 mg / mL. The composite membrane prepared in step (1) was immersed in the dispersion and filtered under a vacuum of -0.05 MPa for 10~15 min to allow the carbon microspheres to be embedded in the gaps between cobalt tetroxide nanosheets. (3) A three-dimensional graphene network was constructed using an electrophoretic bonding-reduction method, including: The composite membrane obtained in step (2) is used as the working electrode and immersed in a graphene oxide solution with a pH of 3-4 and a concentration of 6-9 mg / mL. Electrophoretic self-assembly is carried out for 8-12 min under the conditions of electrode spacing of 3-4 cm and constant voltage of 2.5-3.5 V. Then, it is reduced by hydrazine vapor at 80℃ for 20 min to form a three-dimensional graphene network, thus obtaining the composite electrode material.

4. The preparation method according to claim 3, characterized in that, In step (1), the carbon nanotube film has a thickness of 10~20 μm and a density of 0.8~1.2 g / cm³. 3 The solvent for the cobalt nitrate and urea mixed solution is an ethanol-water mixed solvent, and the volume ratio of ethanol to water is 1:

1.

5. The preparation method according to claim 3, characterized in that, In step (2), the functionalization treatment of the porous carbon microspheres includes: refluxing the porous carbon microspheres in 65% concentrated nitric acid for 2 hours to graft hydroxyl and carboxyl groups onto the surface of the carbon microspheres.

6. The preparation method according to claim 3, characterized in that, In step (3), the pH value of the graphene oxide solution is adjusted by hydrochloric acid; during electrophoretic self-assembly, a platinum sheet is used as the counter electrode.

7. A lithium-ion battery electrode, characterized in that, The material comprises the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries as described in any one of claims 1-2, or the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries prepared by the method described in any one of claims 3-6.

8. A lithium-ion battery, comprising: An electrode made from the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries as described in any one of claims 1-2, or from the carbon nanotube-cobalt tetroxide-graphene composite electrode material for lithium-ion batteries prepared by the preparation method described in any one of claims 3-6.

Images