MOF-template-based self-grown nitrogen-doped carbon nanotube composites

Abstract

This invention provides a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template. The composite material uses a cobalt salt as the metal center and an organic compound as the ligand to synthesize a metal-organic framework template, followed by the autocatalytic generation of nitrogen-doped carbon nanotubes under an inert atmosphere. The preparation method includes: first, preparing a MOF template using a cobalt salt and organic ligand; then, introducing a nickel source onto the template surface via a hydrothermal reaction to obtain a composite precursor; finally, using dicyandiamine as both a nitrogen and carbon source, stepwise calcining the composite precursor to allow in-situ growth of MOF-template-derived carbon nanotubes, simultaneously forming a Ni / Co2N alloy to obtain the composite material. The composite material of this invention possesses a stable structure and excellent catalytic activity. Using it as a cathode catalyst in lithium-air batteries can effectively reduce polarization during the charge-discharge reaction process, improve the battery's rate capability and cycle performance, and the preparation process is relatively simple, showing promising application prospects.

Description

Technical Field

[0001] This invention relates to the field of battery cathode material technology, specifically to a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template. Background Technology

[0002] Lithium-oxygen batteries, due to their extremely high theoretical energy density (approximately 3500 Wh / kg), are considered one of the important development directions for next-generation high-specific-energy storage systems. However, their practical application is severely limited by the slow kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the cathode side, resulting in the ineffective decomposition of the discharge product Li₂O₂, high battery overpotential, and poor cycle stability. To improve reaction kinetics, researchers have developed various cathode catalyst materials, among which nitrogen-doped carbon nanotubes (NCNTs) have attracted widespread attention due to their high specific surface area, excellent conductivity, and electrocatalytic stability.

[0003] Chinese patent CN104953135A discloses a nitrogen-doped carbon nanotube-supported cobalt-based electrocatalytic material (Co / N-CNTs), prepared by one-step calcination of a Co-MOF precursor, with uniformly dispersed metallic cobalt or cobalt oxide nanoparticles inside. However, there are significant shortcomings when applying this type of material to the cathode of lithium-oxygen batteries: First, the chemical stability of cobalt-based catalysts is challenged in the strong oxidizing environment of lithium-oxygen batteries, and cobalt will become deactivated due to irreversible oxidation during long-term cycling, while cobalt oxide has limited catalytic ability for the oxygen evolution reaction (OER); second, the metal particles are mostly simply embedded inside the carbon nanotubes, with weak interfacial bonding, and are prone to detachment and aggregation during long-term cycling, leading to structural collapse and performance degradation; in addition, this material is mainly designed for the oxygen reduction reaction (ORR) of the battery and lacks bifunctional activity for efficiently catalyzing the decomposition of Li2O2 in the OER, making it difficult to meet the comprehensive requirements of lithium-oxygen batteries for cathode catalysts.

[0004] Chinese patent CN110085453B discloses a method for preparing a core-shell Ni-Co LDH@Ni-MnLDH supported on carbon nanotube foam. This technology involves first hydrothermally growing Ni-Co layered double hydroxide (LDH) nanosheets on nitrogen-doped carbon nanotube foam, and then chemically depositing Ni-Mn LDH to form a core-shell structure. The resulting composite material exhibits high specific capacitance and good cycle stability in supercapacitors. However, the LDH material itself has poor conductivity, and electron transport between the active material and the conductive framework is still limited. Furthermore, the active layer and the substrate are two-phase separated heterogeneous structures, lacking chemical bonding and having weak interfacial adhesion. Under the volume changes and strong oxidation environment of lithium-oxygen battery charging and discharging, the core-shell structure is easily peeled off, leading to poor long-term cycle stability. Finally, the catalytic activity of the LDH material mainly comes from the redox reaction of transition metals, which is suitable for charge storage in supercapacitors, but it lacks the design of active sites for efficient catalytic ORR / OER bifunctional reactions, especially its insufficient ability to decompose the discharge product Li2O2.

[0005] Although the aforementioned existing technologies have explored electrocatalysts from the perspectives of single-metal nitrogen-doped carbon nanotubes and bimetallic core-shell structures, they still suffer from common problems such as insufficient catalytic activity, weak interfacial bonding between active components and carbon-based supports, and poor structural stability of materials under harsh conditions. In addition, existing preparation methods often require external nitrogen sources or multi-step processes, making it difficult to achieve uniform doping of active components and integrated construction of structures.

[0006] Therefore, how to prepare bifunctional catalysts with high catalytic activity, good conductivity and structural stability through simple processes to reduce the overpotential of lithium-oxygen batteries and extend their cycle life remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a self-grown nitrogen-doped carbon nanotube composite material based on MOF template, which solves the technical problem that "existing catalytic materials have single catalytic active sites and weak interfacial bonding between active components and carbon matrix in lithium-oxygen battery cathode applications, resulting in poor long-term cycle stability".

[0008] To achieve the above objectives, the present invention is implemented using the following technical solution: In a first aspect, the present invention provides a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template, denoted as Ni / Co2N@NCNTs. This material is prepared by self-catalyzing carbon nanotubes to produce nitrogen-doped carbon nanotubes with metal encapsulation. The composite material uses a cobalt salt as the metal center and 1,3,5-benzenetricarboxylic acid (BTC) as the organic ligand to synthesize a Co-BTC metal-organic framework (MOF) template. A composite precursor is constructed by introducing a nickel source through hydrothermal treatment, followed by high-temperature pyrolysis and self-catalytic growth. Its formation mechanism utilizes the self-catalytic effect of Co to induce in-situ growth of carbon nanotubes, while converting the nickel-cobalt bimetallic component into a Ni / Co2N alloy and encapsulating it on the outer surface of the carbon nanotubes to form a stable metal alloy-encapsulated carbon nanotube structure. This material provides high-density active sites through the synergistic catalytic effect of the outer Ni / Co2N bimetallic nitride, constructs efficient electron transport channels through the inner carbon nanotubes, and achieves structural stability through the tight bonding between the metal alloy and the carbon nanotubes.

[0009] Secondly, the present invention provides a method for preparing a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template, comprising the following steps: (1) First, dissolve the cobalt salt in a solvent, which is a 1:1 volume mixture of deionized water and ethanol. Add polyvinylpyrrolidone to obtain the first solution. Then, dissolve 1,3,5-benzenetricarboxylic acid in a mixture of deionized water and ethanol to obtain the second solution. Add the second solution dropwise to the first solution. After aging, collect the precipitate by centrifugation and washing, and dry it to obtain the MOF template of Co-BTC. (2) The above MOF template is mixed with nickel salt, ammonium fluoride and urea and subjected to hydrothermal reaction to form Co-MOF / Ni(OH)2 composite precursor; (3) Place the composite precursor obtained in step (2) into the first quartz boat and place it on the gas outlet side of the tube furnace. Weigh dicyandiamine and place it into the second quartz boat and place it on the gas inlet side of the tube furnace. Calcinate in stages under an inert atmosphere to obtain Ni / Co2N@NCNTs composite material.

[0010] Specifically, the cobalt salt is at least one of cobalt acetate, cobalt nitrate, and cobalt sulfate, preferably cobalt acetate.

[0011] Specifically, the organic ligand is 1,3,5-benzenetricarboxylic acid, which is synthesized with cobalt salt in the presence of the dispersant polyvinylpyrrolidone to form a metal-organic framework template for Co-BTC.

[0012] Specifically, the nickel salt is one of NiCl2•6H2O, Ni(NO3)2·6H2O and NiSO4·6H2O, preferably NiCl2•6H2O.

[0013] Specifically, the mass ratio of the cobalt salt to the organic ligand is 1:(1~10), and the mass ratio of the cobalt salt to polyvinylpyrrolidone is 1:(10~30); the mass ratio of the template, nickel salt, ammonium fluoride, and urea is 1:(1~5):(0.5~5):(1~10); the hydrothermal reaction temperature is 160~200℃, and the hydrothermal reaction time is 10~12h.

[0014] Specifically, the inert atmosphere is either argon or nitrogen; the segmented heating calcination involves first heating at 10℃ / min to 380~430℃ and holding for 1.5~2.5h, then heating at 10℃ / min to 760~850℃ and holding for 3~4h.

[0015] Thirdly, the present invention provides an application of a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template in a cathode catalyst for lithium-oxygen batteries.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are: (1) This invention provides a composite material with a Ni / Co2N@NCNTs structure, which is generated by autocatalysis using a MOF template with high specific surface area and high porosity. By self-catalyzing carbon nanotubes, a nitrogen-doped carbon nanotube material with metal encapsulation is prepared. The Ni / Co2N metal alloy regulates the electronic structure through synergistic effect, optimizes the adsorption energy of oxygen intermediates (LiO2, Li2O2), and significantly improves the bifunctional catalytic activity of ORR and OER.

[0017] (2) The composite material of the present invention can significantly reduce the overpotential of lithium-oxygen battery, greatly improve the discharge specific capacity and extend the cycle life. The three-dimensional conductive network constructed by carbon nanotubes ensures the rapid transport of electrons and reduces the charge transfer resistance. The tight combination of metal alloy and carbon nanotubes effectively anchors the active components and prevents them from agglomerating and falling off during long cycles. At the same time, the synergistic effect of nitrogen-doped carbon nanotubes and metal alloys promotes the reversible generation and decomposition of discharge product Li2O2.

[0018] (3) The preparation process of the present invention is simple and does not require multiple complex operations. It has good universality and adjustability, and can achieve excellent catalytic performance under different process parameters, and has broad prospects for industrial application. Attached Figure Description

[0019] Figure 1 This is a scanning electron microscope (SEM) image of the Ni / Co2N@NCNTs composite material prepared in Example 2.

[0020] Figure 2 The diagram shows the overpotential results of the Ni / Co2N@NCNTs composite material prepared in Example 2.

[0021] Figure 3 The image shows the battery cycle performance of the Ni / Co2N@NCNTs composite material prepared in Example 2.

[0022] Figure 4 The diagram shows the overpotential results of the Ni@NCNTs material prepared in Comparative Example 1.

[0023] Figure 5 The image shows the battery cycle performance of the Ni@NCNTs material prepared in Comparative Example 1.

[0024] Figure 6 The overpotential results are shown in Figure 2 for the Co2N@NCNTs material prepared in Comparative Example 2.

[0025] Figure 7 The image shows the battery cycle performance of the Co2N@NCNTs material prepared in Comparative Example 2. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] This invention provides a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template. The core concept is as follows: using cobalt salt as the metal center and 1,3,5-benzenetricarboxylic acid as the organic ligand, under the morphology regulation and dispersion effect of polyvinylpyrrolidone, a Co-BTC metal-organic framework (MOF) template with a regular pore structure and high specific surface area is formed through coordination self-assembly, laying the structural foundation for subsequent nickel source loading, catalytic carbon nanotube growth, and bimetallic nitride formation. Then, using Co-BTC as the template, a nickel source is uniformly loaded onto the template surface and within the pores through a hydrothermal reaction, and in situ converted to Ni(OH)2, forming a Co-MOF / Ni(OH)2 composite precursor. Using cobalt in the composite precursor as a catalyst, the carbon source (derived from the decomposition of the MOF organic ligand and dicyandiamine) is induced to undergo in-situ graphitization growth to form nitrogen-doped carbon nanotubes. Simultaneously, the nickel-cobalt bimetal is converted into a Ni / Co2N alloy under a nitrogen atmosphere and encapsulated on the outer surface of the carbon nanotubes.

[0028] This invention also provides a method for preparing a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template, comprising the following steps: (1) Dissolve cobalt salt in a mixture of deionized water and ethanol, add polyvinylpyrrolidone under stirring to obtain a first solution; dissolve 1,3,5-benzenetricarboxylic acid in a mixture of deionized water and ethanol to obtain a second solution, add the second solution dropwise to the first solution, stir for 1~3h, age at 25~30℃ for 20~24h, centrifuge, wash, collect the precipitate, dry, and obtain Co-BTC; (2) Dissolve 0.1–0.5 g of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 0.1–0.5 g of nickel salt, 0.1–0.5 g of ammonium fluoride and 0.5–1 g of urea while stirring. Stir for 20–40 min, transfer to a reaction vessel, and hydrothermally react at 160–200 °C for 10–12 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, and dry to obtain the composite precursor. In the uniform alkaline environment provided by the slow hydrolysis of urea, Ni 2+ Ni(OH)2 is precipitated in situ on the surface and within the pores of the Co-BTC template, while the main structure of the template is preserved, thus constructing a composite precursor in which Ni(OH)2 is uniformly loaded onto Co-BTC. (3) Weigh the composite precursor and place it in the first quartz boat, and weigh dicyandiamine and place it in the second quartz boat. The mass ratio of the composite precursor to dicyandiamine is 1:(15~28). Place the second quartz boat on the gas inlet side of the tube furnace and the first quartz boat on the gas outlet side of the tube furnace. Calcinate in stages under a nitrogen atmosphere. During the staged heating process, the cobalt catalytic carbon source (derived from the decomposition of MOF organic ligands and dicyandiamine) in the composite precursor is graphitized in situ to form nitrogen-doped carbon nanotubes. At the same time, the nickel-cobalt bimetal is converted into Ni / Co2N alloy under the nitrogen-rich atmosphere generated by the decomposition of dicyandiamine. By designing the airflow with dicyandiamine on the gas inlet side and the composite precursor on the gas outlet side, uniform doping of the nitrogen source and moderate nitriding of the metal are achieved, and finally, the Ni / Co2N@NCNTs composite material is obtained.

[0029] In this application, the room temperature is 18-30°C; the deionized water and ethanol are mixed as a solvent in a volume ratio of 1:1; and the dropping rate of the second solution is 2 mL / min.

[0030] As one of the preferred embodiments of the present invention, the cobalt salt is one of cobalt acetate, cobalt nitrate and cobalt sulfate, and is more preferably cobalt acetate.

[0031] As one of the preferred embodiments of the present invention, the mass ratio of the cobalt salt to the organic ligand is 1:(1~10), and the mass ratio of the cobalt salt to polyvinylpyrrolidone is 1:(10~30).

[0032] As one of the preferred embodiments of the present invention, the nickel salt is NiCl2•6H2O.

[0033] As one of the preferred embodiments of the present invention, the mass ratio of Co-BTC to nickel salt is 1:(1~5); the mass ratio of Co-BTC to ammonium fluoride is 1:(0.5~5); and the mass ratio of Co-BTC to urea is 1:(1~10).

[0034] As one of the preferred embodiments of the present invention, the temperature of the hydrothermal reaction is 160-200°C and the reaction time is 10-12 hours.

[0035] As one of the preferred embodiments of the present invention, the inert atmosphere is either argon or nitrogen.

[0036] As one of the preferred embodiments of the present invention, the mass ratio of the composite precursor to dicyandiamine is 1:(15~28).

[0037] As one of the preferred embodiments of the present invention, the segmented heating and calcination involves first heating at 10°C / min to 380~430°C and holding at that temperature for 1.5~2.5h, then heating at 10°C / min to 760~850°C and holding at that temperature for 3~4h.

[0038] The technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0039] Example 1; (1) Dissolve 100 mg of cobalt salt in a mixture of 20 mL of deionized water and 20 mL of ethanol, and add 1 g of polyvinylpyrrolidone while stirring to obtain the first solution; dissolve 500 mg of 1,3,5-benzenetricarboxylic acid in a mixture of 20 mL of deionized water and 20 mL of ethanol to obtain the second solution, add the second solution dropwise to the first solution, stir for 1 h, age at 25 °C for 20 h, centrifuge, wash and dry to obtain Co-BTC.

[0040] (2) Dissolve 200 mg of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 500 mg of NiCl2•6H2O, 100 mg of ammonium fluoride and 500 mg of urea while stirring. Stir for 20 min, transfer to a reaction vessel, and hydrothermally react at 160 °C for 10 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, dry, and obtain the composite precursor. (3) Weigh the composite precursor and put it into the first quartz boat, weigh the dicyandiamine and put it into the second quartz boat. The mass ratio of the composite precursor to dicyandiamine is 1:15. Place the second quartz boat on the gas inlet side of the tube furnace and the first quartz boat on the gas outlet side of the tube furnace. Calcinate in a nitrogen atmosphere. First, heat the furnace to 380°C at 10°C / min and hold for 1.5h. Then heat the furnace to 760°C at 10°C / min and hold for 3h. Cool the furnace to room temperature to obtain the Ni / Co2N@NCNTs composite material.

[0041] Example 2; (1) Dissolve 392 mg of cobalt acetate in a mixture of 20 mL of deionized water and 20 mL of ethanol, and add 2.4 g of polyvinylpyrrolidone under stirring to obtain the first solution; dissolve 0.72 g of 1,3,5-benzenetricarboxylic acid in a mixture of 20 mL of deionized water and 20 mL of ethanol to obtain the second solution, add the second solution dropwise to the first solution, stir for 2 h, age at 28 °C for 22 h, centrifuge, wash and dry to obtain Co-BTC; (2) Dissolve 300 mg of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 485 mg of NiCl2•6H2O, 232 mg of ammonium fluoride and 546 mg of urea while stirring. Stir for 20-40 min, transfer to a reaction vessel, and hydrothermally react at 180 °C for 11 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, dry, and obtain the composite precursor. (3) Weigh the composite precursor and put it into the first quartz boat, weigh the dicyandiamine and put it into the second quartz boat. The mass ratio of the composite precursor to dicyandiamine is 1:20. Place the second quartz boat on the gas inlet side of the tube furnace and the first quartz boat on the gas outlet side of the tube furnace. Calcinate in a nitrogen atmosphere. First, heat the furnace to 400℃ at 10℃ / min and hold for 2h. Then heat the furnace to 800℃ at 10℃ / min and hold for 3.5h. Cool the furnace to room temperature to obtain the Ni / Co2N@NCNTs composite material.

[0042] Example 3; (1) Dissolve 500 mg of cobalt salt in a mixture of 20 mL of deionized water and 20 mL of ethanol, and add 3 g of polyvinylpyrrolidone under stirring to obtain the first solution; dissolve 1 g of 1,3,5-benzenetricarboxylic acid in a mixture of 20 mL of deionized water and 20 mL of ethanol to obtain the second solution, add the second solution dropwise to the first solution, stir for 3 h, age at 30 °C for 24 h, centrifuge, wash and dry to obtain Co-BTC.

[0043] (2) Dissolve 500 mg of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 500 mg of NiCl2•6H2O, 500 mg of ammonium fluoride and 1 g of urea while stirring. Stir for 40 min, transfer to a reaction vessel, and hydrothermally react at 200 °C for 12 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, dry, and obtain the composite precursor. (3) Weigh the composite precursor and dicyandiamine at a mass ratio of 1:28 and place them in two quartz boats respectively. Then place the dicyandiamine on the gas inlet side of the tube furnace and the composite precursor on the gas outlet side of the tube furnace. Calcinate in a nitrogen atmosphere. First, raise the temperature to 430°C at 10°C / min and hold for 2 hours. Then raise the temperature to 850°C at 10°C / min and hold for 4 hours. Cool to room temperature with the furnace to obtain the Ni / Co2N@NCNTs composite material.

[0044] Example 4; (1) Dissolve 100 mg of cobalt salt in a mixture of 20 mL of deionized water and 20 mL of ethanol, and add 3 g of polyvinylpyrrolidone while stirring to obtain the first solution; dissolve 1 g of 1,3,5-benzenetricarboxylic acid in a mixture of 20 mL of deionized water and 20 mL of ethanol to obtain the second solution, add the second solution dropwise to the first solution, stir for 2 h, age at 30 °C for 21 h, centrifuge, wash and dry to obtain Co-BTC.

[0045] (2) Dissolve 100 mg of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 500 mg of NiCl2•6H2O, 500 mg of ammonium fluoride and 1 g of urea while stirring. Stir for 30 min, transfer to a reaction vessel, and hydrothermally react at 170 °C for 11 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, dry, and obtain the composite precursor. (3) Weigh the composite precursor and dicyandiamine at a mass ratio of 1:25 and place them in two quartz boats respectively. Then place the dicyandiamine on the gas inlet side of the tube furnace and the composite precursor on the gas outlet side of the tube furnace. Calcinate under a nitrogen atmosphere. First, raise the temperature to 410℃ at 10℃ / min and hold for 2h. Then raise the temperature to 830℃ at 10℃ / min and hold for 3.5h. Cool to room temperature with the furnace to obtain the Ni / Co2N@NCNTs composite material.

[0046] Example 5; (1) Dissolve 200 mg of cobalt nitrate in a mixture of 20 mL of deionized water and 20 mL of ethanol, and add 2 g of polyvinylpyrrolidone while stirring to obtain the first solution; dissolve 200 mg of 1,3,5-benzenetricarboxylic acid in a mixture of 20 mL of deionized water and 20 mL of ethanol to obtain the second solution, add the second solution dropwise to the first solution, stir for 3 h, age at 30 °C for 24 h, centrifuge, wash and dry to obtain Co-BTC; (2) Dissolve 200 mg of Co-BTC in 40 mL of deionized water and 40 mL of ethanol. Add 400 mg of NiCl2•6H2O, 250 mg of ammonium fluoride and 200 mg of urea while stirring. Stir for 40 min, transfer to a reaction vessel, and hydrothermally react at 200 °C for 12 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, dry, and obtain the composite precursor. (3) Weigh the composite precursor and dicyandiamine at a mass ratio of 1:15 and place them in two quartz boats respectively. Place the dicyandiamine boat on the gas inlet side of the tube furnace and the composite precursor boat on the gas outlet side. Calcinate in a nitrogen atmosphere. First, heat to 400℃ at 10℃ / min and hold for 2h. Then heat to 800℃ at 10℃ / min and hold for 4h. Cool to room temperature with the furnace to obtain Ni / Co2N@NCNTs composite material.

[0047] Comparative Example 1; This comparative example aims to verify the preparation of composite materials without using Co-BTC as a template. (1) Dissolve 485 mg of NiCl2•6H2O in 40 mL of deionized water and 40 mL of ethanol. Add 232 mg of ammonium fluoride and 546 mg of urea while stirring. Stir for 30 min, transfer to a reaction vessel, and hydrothermally react at 180 °C for 11 h. After cooling to room temperature, centrifuge, wash, collect the precipitate, and dry to obtain the composite precursor. (2) Weigh the composite precursor and place it in the first quartz boat, and weigh dicyandiamine and place it in the second quartz boat. The mass ratio of the composite precursor to dicyandiamine is 1:20. Place the second quartz boat on the gas inlet side of the tube furnace and the first quartz boat on the gas outlet side of the tube furnace. Calcinate under a nitrogen atmosphere. First, raise the temperature to 400℃ at 10℃ / min and hold for 2h. Then raise the temperature to 800℃ at 10℃ / min and hold for 3.5h. Cool to room temperature with the furnace to obtain the Ni@NCNTs composite material.

[0048] Comparative Example 2; The difference between Comparative Example 2 and Example 2 is that step (2) is omitted, and step (3) is changed to: weighing Co-BTC and placing it into the first quartz boat, weighing dicyandiamine and placing it into the second quartz boat, wherein the mass ratio of Co-BTC and dicyandiamine is 1:20, placing the second quartz boat on the gas inlet side of the tube furnace and the first quartz boat on the gas outlet side of the tube furnace, calcining under a nitrogen atmosphere, first heating to 400°C at 10°C / min and holding for 2 hours, then heating to 800°C at 10°C / min and holding for 3.5 hours, and cooling to room temperature with the furnace to obtain the Co2N@NCNTs composite material; the remaining steps are the same as in Example 2.

[0049] Test and Results Analysis The composite materials prepared in each embodiment and comparative example were used as positive electrode catalyst materials for lithium-oxygen batteries and assembled with lithium sheets to form coin-type lithium-air batteries. The assembly method is as follows: A slurry containing 60% Ketjen Black (KB), 30% catalyst material and 10% PVDF was coated onto a carbon paper current collector and then dried in a vacuum drying oven at 60°C for 12 hours. The net weight of the dried catalyst on the carbon paper was about 0.3 to 0.5 mg. Using lithium foil as the anode, a glass fiber separator was laid flat, 80 μL of electrolyte was added, and then the carbon paper with the catalyst was added. Finally, the cathode instrument was covered with nickel foam as filler, and the battery assembly was completed in an argon-filled glove box.

[0050] 1. Material morphology characterization The Ni / Co2N@NCNTs composite material prepared in Example 2 was characterized by scanning electron microscopy (SEM), as follows: Figure 1 As shown, the material exhibits a typical one-dimensional carbon nanotube structure, with interwoven carbon nanotubes and a uniform diameter distribution. This unique metal alloy encapsulating the carbon nanotube structure serves two purposes: firstly, it utilizes the high conductivity of the carbon nanotubes to construct a three-dimensional conductive network, forming an efficient electron transport channel; secondly, the outer Ni / Co2N metal alloy acts as a catalytic active layer, providing abundant catalytic active sites for the oxygen reduction and oxygen evolution reactions in lithium-oxygen batteries; and thirdly, the tight bond between the carbon nanotubes and the metal alloy contributes to improved structural stability and electron transfer efficiency.

[0051] 2. Overpotential Comparison Analysis At a current density of 100 mAg - ¹Under the condition of limiting specific capacity to 1000mAh / g: like Figure 2 As shown, the overpotential of the Ni / Co2N@NCNTs composite material in Example 2 of this application is only 0.76V; Figure 4 As shown, the overpotential of the Ni@NCNTs composite material in Comparative Example 1 is as high as 1.32V; Figure 6 As shown, the overpotential of the Co2N@NCNTs composite material in Comparative Example 2 is 1.18V.

[0052] As can be seen from the comparison, the overpotential of the composite material in this application is reduced by 42.4% compared with the single metallic Ni material and by 35.6% compared with the single cobalt-based nitride material. This indicates that the synergistic catalytic effect of Ni / Co2N is an important reason for the significant reduction in overpotential. The bimetallic component optimizes the adsorption energy for LiO2 and Li2O2 by regulating the electronic structure; at the same time, the outer metal alloy is directly exposed to the reaction interface, providing abundant active sites. The combined effect of these two factors synergistically reduces the reaction energy barriers of ORR and OER, thereby significantly reducing the charge and discharge overpotential.

[0053] 3. Discharge specific capacity test analysis Test method: Constant current discharge tests were performed on the coin cells assembled in each embodiment and comparative example. The test temperature was 25±1℃. Before the test, the assembled cells were placed in a high-purity oxygen atmosphere and left to stand for 6 hours to ensure that the electrolyte fully wetted the electrodes and stabilized the open circuit potential. During the test, oxygen was continuously introduced to maintain a pressure of 1 atm, and the current density was 100 mAg. - ¹ Under constant current conditions, discharge was carried out until the cutoff voltage was 2.0V; the discharge specific capacity was calculated according to the formula (discharge current × discharge time / catalyst mass), and the results are shown in Table 1.

[0054] Table 1

[0055] As shown in Table 1, the discharge specific capacity of Example 2 is as high as 23568.2 mAh / g. This is because the outer Ni / Co2N provides a high density of catalytic active sites through a synergistic effect, optimizes the adsorption energy of oxygen intermediates, and effectively promotes the oxygen reduction reaction. The three-dimensional conductive network constructed by the inner carbon nanotubes ensures rapid electron transport, allowing the deep active sites to fully participate in the reaction. At the same time, the tight combination of the metal alloy and the carbon nanotubes improves the utilization rate of the active sites, jointly promoting the efficient generation and uniform deposition of the discharge product Li2O2, thereby maximizing the release of the discharge specific capacity.

[0056] Although the discharge specific capacity of Examples 1 and 3-5 is slightly lower than that of Example 2, it still remains at a high level, indicating that the technical solution of the present invention has good universality and adjustability, and can achieve excellent catalytic performance under different process parameters.

[0057] Comparative Example 1 shows that the single metal Ni has limited catalytic active sites and lacks the synergistic effect of the cobalt component, making it difficult to efficiently catalyze the oxygen reduction reaction to generate the discharge product Li2O2, resulting in insufficient capacity release.

[0058] Comparative Example 2 shows that the introduction of the Co2N active component improves the catalytic efficiency, but since no nickel source is introduced, the Ni / Co2N bimetallic synergistic effect cannot be formed, and the active site density and catalytic activity are still limited. Therefore, the increase in discharge specific capacity is limited, and the maximum capacity release cannot be achieved.

[0059] 4. Electrochemical impedance spectroscopy analysis To further investigate the electrode reaction kinetics and electron transport capabilities of each material, electrochemical impedance spectroscopy (EIS) tests were performed on the lithium-oxygen batteries assembled in Example 2, Comparative Examples 1 and 2. The tests were conducted at open-circuit potential with an AC amplitude of 5 mV and a frequency range of 100 kHz to 0.01 Hz. Before the tests, the batteries were allowed to stand until the open-circuit potential stabilized (potential fluctuation less than ±1 mV / min). The specific results are shown in Table 2.

[0060] Table 2

[0061] As shown in Table 2, the charge transfer resistances of each component in Example 2 of the present invention are reduced compared to Comparative Examples 1 and 2. The impedance of the charge transfer resistance increases from 70Ω to 383Ω after the discharge process and recovers to 123Ω after the charging process, which shows that the present invention has efficient charge transfer and discharge product decomposition capabilities, which together improve the reversibility of the lithium-oxygen battery.

[0062] For Comparative Example 1, the charge transfer resistance increased much more than in Example 2. After the charging process was completed, the resistance was still 204Ω, indicating that most of the Li2O2 on the material surface was not decomposed after the charging process. The residual insulating layer caused the charge transfer resistance to remain high and the reversibility to be poor.

[0063] For Comparative Example 2, the resistance increased to 624Ω after discharge and only recovered to 233Ω after charging, which was significantly higher than the initial value. This indicates that the absence of a nickel source led to severe accumulation and uneven distribution of the discharge product Li2O2 on the electrode surface. The incomplete decomposition during charging resulted in a large amount of residual insulating products covering the active sites, causing the charge transfer resistance to increase significantly after cycling and the reversibility to deteriorate.

[0064] 5. Cyclic Stability Comparison Analysis like Figure 3 As shown, the composite material of Example 2 can stably cycle 220 times under the conditions of a current density of 500 mA / g and a limited specific capacity of 1000 mAh / g.

[0065] like Figure 5 As shown, the Ni@NCNTs composite material of Comparative Example 1 can cycle 70 times under the conditions of a current density of 500 mA / g and a limited specific capacity of 1000 mAh / g.

[0066] like Figure 7 As shown, the Co2N@NCNTs composite material of Comparative Example 2 can cycle 110 times under the conditions of a current density of 500 mA / g and a limited specific capacity of 1000 mAh / g.

[0067] The Ni@NCNTs composite material in Comparative Example 1 did not introduce a cobalt source, so it could not generate a Co2N component with a synergistic catalytic effect. It relied solely on the single metallic Ni to provide active sites, resulting in insufficient catalytic activity. At the same time, the metal particles were directly loaded on the carbon nanotube surface without the encapsulation and anchoring of an outer metal alloy, which made them prone to agglomeration and detachment during long-term cycling, leading to a significant deterioration in cycling stability.

[0068] Although the Co2N@NCNTs composite material of Comparative Example 2 generated the Co2N active component, it did not introduce a nickel source and could not form a unique structure in which the outer Ni / Co2N metal alloy tightly wrapped the carbon nanotubes. The binding force between the active component and the carbon nanotubes was weak, and it was easy to detach from the conductive framework during repeated charge and discharge. As a result, although the cycle life was better than that of Comparative Example 1, it was still not ideal.

[0069] In contrast, this invention successfully constructs a unique structure in which a Ni / Co2N metal alloy tightly encapsulates carbon nanotubes by introducing a nickel-cobalt bimetallic compound. This achieves a balance between bimetallic synergistic catalysis, active site anchoring, and conductive network stabilization, thus maintaining excellent performance stability during 220 long cycles.

[0070] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No markings in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for preparing a self-grown nitrogen-doped carbon nanotube composite material based on a MOF template, characterized in that, Includes the following steps: (1) Using cobalt salt as the metal center, a coordination self-assembly reaction was carried out in the presence of organic ligands and polyvinylpyrrolidone to synthesize MOF templates; (2) The above MOF template is mixed with nickel salt, ammonium fluoride and urea and subjected to hydrothermal reaction to form Co-MOF / Ni(OH)2 composite precursor; (3) Place the composite precursor obtained in step (2) into the first quartz boat and place it on the gas outlet side of the tube furnace. Weigh dicyandiamine and place it into the second quartz boat and place it on the gas inlet side of the tube furnace. Calcinate in stages under an inert atmosphere to obtain Ni / Co2N@NCNTs composite material.

2. The method according to claim 1, characterized in that, The cobalt salt mentioned in step (1) is at least one of cobalt acetate, cobalt nitrate and cobalt sulfate.

3. The method according to claim 1, characterized in that, The organic ligand mentioned in step (1) is 1,3,5-benzenetricarboxylic acid.

4. The method according to claim 1, characterized in that, The mass ratio of cobalt salt to organic ligand in step (1) is 1:(1~10).

5. The method according to claim 1, characterized in that, The nickel salt mentioned in step (2) is one of NiCl2•6H2O, Ni(NO3)2·6H2O and NiSO4·6H2O.

6. The method according to claim 1, characterized in that, The mass ratio of MOF template, nickel salt, ammonium fluoride and urea in step (2) is 1:(1~5):(0.5~5):(1~10).

7. The method according to claim 1, characterized in that, The hydrothermal reaction temperature in step (2) is 160-200℃, and the hydrothermal reaction time is 10-12h.

8. The method according to claim 1, characterized in that, The segmented heating and calcination in step (3) involves first heating to 380~430℃ and holding for 1.5~2.5h, then heating to 760~850℃ and holding for 3~4h.

9. The application of a composite material prepared by the method according to any one of claims 1-8 in a cathode catalyst of a lithium-oxygen battery.

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