A N / O co-doped carbon nanotube and its preparation method and application

Abstract

The present invention discloses a N / O co-doped carbon nanotube and a preparation method and application thereof, wherein the preparation method comprises the following steps: S1. placing MOF powder in a container, covering a liquid gallium layer on the MOF powder, and evacuating; S2. covering the container with a liquid gallium layer again, evacuating the container, performing a pyrolysis reaction, and taking out the lower layer product Co@NCNTFs; S3. acid-washing the lower layer product Co@NCNTFs obtained in S2, and obtaining the N / O co-doped carbon nanotube after drying. The present invention discloses a novel, simple and high-yield pyrolysis strategy, which is based on liquid gallium-assisted pyrolysis of ZIF‑67 to obtain CNTs, and prepares a synthesis of CNTs rich in N (7.7at%) with a high yield (77.8wt%). After appropriate oxidation treatment, a large amount of O element (34.31at%) is introduced into the carbon nanotube, and the obtained N / O-rich NOCNTF‑15 shows excellent sodium storage performance.

Description

Technical Field

[0001] The present invention relates to the technical field of nanomaterials, and in particular to an N / O co-doped carbon nanotube and a preparation method and application thereof. Background Art

[0002] In order to achieve the goal of sustainable development, various renewable energy and clean energy are constantly emerging, leading to the rapid development of electrochemical energy storage technology. As a one-dimensional nanomaterial, carbon nanotubes (CNTs) have the characteristics of large specific surface area, excellent conductivity, good chemical stability and excellent flexibility, and are a promising energy storage material. At present, there are many methods for preparing CNTs, mainly arc discharge method, chemical vapor deposition method, laser ablation method, etc., but these methods have high requirements for temperature and may produce impurities (Science, 2009, 323, 760-764. Chem. Rev., 1999, 99, 1787-1800.). Due to economic and safety reasons, people are increasingly interested in sodium ion batteries and use CNTs in sodium ion batteries, but due to the small interlayer spacing of CNTs (d = 0.34nm), its sodium storage performance is severely limited. Many studies have shown that the properties of sodium storage active sites can be effectively improved by expanding the interlayer spacing and introducing heteroatoms, and the electron distribution of the carbon matrix can be adjusted, thereby alleviating the drastic volume expansion phenomenon and slow kinetics during the charge and discharge process of sodium-ion batteries (SIBs).

[0003] Metal organic framework (MOF) is a porous material formed by self-assembly of organic ligands and metal ions or clusters through coordination bonds. It contains a large number of C and N atoms, as well as a high content of metal ions, and is a good precursor for preparing CNTs. CNTs can be prepared by pyrolysis of MOF, but its pyrolysis method also has a crucial influence on the preparation of CNTs. Lou et al. obtained a nitrogen-doped carbon nanotube framework by pyrolysis of ZIF-67. Co nanoparticles were rapidly formed in an Ar / H2 pyrolysis atmosphere, and a hollow structured carbon nanotube framework was obtained (Nat. Energy, 2016, 1, 15006.). Li et al. obtained a hollow polyhedron of Co nanoparticles embedded in N-doped carbon nanotubes by pyrolysis of ZIF-8@ZIF-67 (J. Am. Chem. Soc., 2018, 140, 2610-2618). Although CNTs can be obtained by pyrolyzing ZIF-67 with existing technology, the small molecules formed by the decomposition of ligands diffuse outward during the pyrolysis process, resulting in CNTs growing only on the surface of the framework with low yield. Currently, MOFs-derived carbon-based materials are attracting people's interest in energy applications, however, the limitations of the pyrolysis method restrict further development. Therefore, it is still a daunting challenge to develop a simple and efficient method for preparing CNTs based on the pyrolysis method and successfully apply it to sodium storage. Summary of the invention

[0004] In order to solve the above technical problems, the primary purpose of the present invention is to provide a method for preparing N / O co-doped carbon nanotubes, which is simple to operate and easy to implement by pyrolyzing MOF assisted by liquid gallium and subsequently treating with a strong acid to obtain N / O co-doped carbon nanotubes.

[0005] A further object of the present invention is to provide a N / O co-doped carbon nanotube prepared by the above method.

[0006] The third object of the present invention is to provide an application of the above-mentioned N / O co-doped carbon nanotubes in the preparation of sodium ion batteries.

[0007] A fourth object of the present invention is to provide a sodium ion battery.

[0008] The above object of the present invention is achieved through the following technical solutions:

[0009] A first aspect of the present invention provides a method for preparing N / O co-doped carbon nanotubes, comprising the following steps:

[0010] S1. placing MOF powder in a container, covering the MOF powder with a liquid gallium layer, and evacuating the container;

[0011] S2. Cover the container with a liquid gallium layer again, evacuate the container, perform a pyrolysis reaction, and take out the lower layer product Co@NCNTFs;

[0012] S3. The lower layer product Co@NCNTFs obtained in S2 is acid-washed and dried to obtain the N / O co-doped carbon nanotubes (NOCNTF).

[0013] The present invention adopts liquid gallium to assist the pyrolysis of MOF, which can effectively limit the carbon source and nitrogen source decomposed by the ligand in the pyrolysis process in the reaction framework, and the obtained product has high yield, high nitrogen content and a non-hollow structure formed by the interweaving of carbon nanotubes. The carbon nanotube framework is treated with acid washing, and the Co nanoparticles at the ends of the carbon nanotubes are etched to form an open carbon nanotube. At the same time, a large number of C=O groups are introduced on the surface of the carbon nanotube, and finally a non-hollow three-dimensional framework material (NOCNTF) composed of open high-content N / O co-doped carbon nanotubes is obtained.

[0014] Specifically, x can be used to represent the total thickness of the two liquid gallium layers, in mm, and the lower layer product prepared in step S2 can be expressed as Co@NCNTFs-x, and the N / O co-doped carbon nanotubes prepared in S3 can be expressed as NOCNTF-x.

[0015] In a specific implementation, in step S1, MOF powder is placed at the bottom of a container and compacted, a liquid gallium layer is covered on the MOF powder, and a vacuum drying oven is used for evacuation.

[0016] Furthermore, in step S1, the thickness of the liquid gallium layer is 2 to 8 mm.

[0017] The role of the liquid gallium layer is to prevent the carbon source from escaping and to confine the carbon source and nitrogen source decomposed by the ligand during the pyrolysis process within the reaction framework, thereby increasing the yield and nitrogen content. However, when the first liquid gallium layer is too thick, the oxygen in the contact interface between the MOF powder and the liquid gallium layer cannot be extracted, which will affect the formation of carbon nanotubes.

[0018] Preferably, in step S1, the thickness of the liquid gallium layer is 5-6 mm.

[0019] Furthermore, in step S1, the MOF is ZIF-67, ZTF-8@ZIF-67 or ZIF-67@ZIF-8.

[0020] Furthermore, in step S1, the vacuuming time is 1 to 4 hours.

[0021] Furthermore, in step S1, the temperature of the vacuum drying oven is 40-80°C.

[0022] In a specific implementation, in step S2, the container is again covered with a liquid gallium layer, vacuumed in a vacuum drying oven, and pyrolysis reaction is carried out in a muffle furnace. After cooling, the upper layer of liquid gallium is recovered, and the lower layer product Co@NCNTFs-x is taken out.

[0023] Furthermore, in step S2, the vacuuming time is 1 to 4 hours.

[0024] Furthermore, in step S2, the temperature of the vacuum drying oven is 40-80°C.

[0025] Furthermore, in step S2, the thickness of the liquid gallium layer is 0 to 30 mm.

[0026] Furthermore, the pressure difference between the contact surface of the liquid gallium layer and the MOF powder is 115-2200Pa.

[0027] The present invention can change the diameter and crystallinity of CNTs by changing the thickness of liquid gallium during the pyrolysis process.

[0028] Furthermore, in step S2, the temperature of the pyrolysis reaction is 400-1000°C.

[0029] Furthermore, in step S2, before the pyrolysis reaction, the heating rate from room temperature to the pyrolysis reaction temperature is 2-5°C / min.

[0030] Furthermore, in step S2, the pyrolysis reaction time is 2 to 10 hours.

[0031] In a specific embodiment, in step S3, Co@NCNTFs-x is ultrasonically dispersed in a strong acid solution, heated and stirred under condensation reflux, the product is collected by centrifugation, washed with deionized water and ethanol for 3 to 4 times, and dried to obtain the N / O co-doped carbon nanotubes NOCNTF-x.

[0032] Furthermore, in step S3, pickling is performed using a strong acid, and the strong acid is preferably nitric acid.

[0033] Furthermore, in step S3, the concentration of the nitric acid is 2-10M.

[0034] Furthermore, in step S3, the heating and stirring is performed at a temperature of 40 to 90° C. for a time of 1 to 4 days.

[0035] Furthermore, in step S3, the ultrasonic dispersion time is 10 to 30 minutes.

[0036] Furthermore, in step S3, drying is performed in a vacuum drying oven at 40 to 80°C.

[0037] The second aspect of the present invention provides N / O co-doped carbon nanotubes prepared by the method described in the first aspect.

[0038] The third aspect of the present invention provides the use of the N / O co-doped carbon nanotubes described in the second aspect in the preparation of sodium ion batteries.

[0039] A fourth aspect of the present invention provides a sodium ion battery comprising the N / O co-doped carbon nanotubes described in the second aspect as a negative electrode material.

[0040] The present invention uses N / O co-doped carbon nanotubes NOCNTF-x to prepare negative electrode materials for sodium ion batteries. NOCNTF-x exhibits excellent rate performance, outstanding long cycle stability and high discharge specific capacity.

[0041] The present invention uses the pre-sodiumized NOCNTF-x electrode as the negative electrode and pairs it with the Na3V2(PO4)2O2F (NVPOF) positive electrode to assemble a sodium ion full battery, which is denoted as NOCNTF-x||NVPOF. The NOCNTF-x||NVPOF full battery exhibits excellent rate performance and long cycle stability in the voltage window of 1.25 to 4.3V.

[0042] Beneficial effects of the present invention:

[0043] 1. The present invention uses liquid gallium to assist the pyrolysis of ZIF-67, which can effectively confine the carbon source and nitrogen source decomposed by the ligand during the pyrolysis process in the reaction framework. The obtained product has a high yield (77.8wt%), a high nitrogen content (7.7at%) and a non-hollow structure formed by interweaving carbon nanotubes. By changing the thickness of the liquid gallium during the pyrolysis process, the diameter of CNTs and the crystallinity of CNTs can also be changed.

[0044] 2. The present invention uses acid washing to treat the carbon nanotube framework, and the Co nanoparticles at the ends of the carbon nanotubes are etched to form an open carbon nanotube. At the same time, a large number of C=O groups are introduced on the surface of the carbon nanotubes, and finally a non-hollow three-dimensional framework material NOCNTF-x composed of open high-content N / O co-doped carbon nanotubes is obtained.

[0045] 3. The N / O co-doped carbon nanotubes NOCNTF-15 prepared by the present invention exhibit excellent rate performance as anode materials for sodium ion batteries at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A·g -1 The discharge specific capacities under these conditions are 371.4, 348.9, 337.2, 322.4, 304, 278.5 and 252.7 mAh g -1 , it showed excellent rate performance even at high current density. When the current density was restored to 0.1 A g -1When the capacity of NOCNTF-15 electrode is rapidly restored to 338.9 mAh g -1 , and remained stable in subsequent cycles. In addition, NOCNTF-15 also exhibited excellent long-cycle stability and high discharge specific capacity. -1 After 400 cycles, the reversible capacity still remained at 286.8 mAh g -1 , with a high capacity retention rate of 86.5%, and an average capacity decay of only 0.11 mAh g per cycle. -1 In addition, the NOCNTF-15 electrode has a -1 After 20,000 cycles at a current density of 1.5 mAh g -1 The high reversible specific capacity and coulombic efficiency are close to 100%.

[0046] 4. The present invention uses the pre-sodiumized NOCNTF-15 electrode as the negative electrode and pairs it with the Na3V2(PO4)2O2F (NVPOF) positive electrode to assemble a sodium ion full battery, which is denoted as NOCNTF-15||NVPOF. The NOCNTF-15||NVPOF full battery exhibits excellent rate performance and long cycle stability in the voltage window of 1.25 to 4.3 V. At 0.1, 0.2, 0.5, 1, 2, 5 and 10 A·g -1 The discharge specific capacities at the current densities are 547, 439, 393, 372, 308, 202 and 124 mAh·g, respectively. -1 When the current density returns to 0.1A·g -1 When the capacity of NOCNTF-15||NVPOF is rapidly restored to 381 mAh g -1 , and remain stable in subsequent cycles. NOCNTF-15||NVPOF full cell at 10A·g -1 After 15400 cycles, the capacity is still 38 mAh g -1 In addition, the maximum energy density of NOCNTF-15||NVPOF can reach 233Wh·kg -1 (Based on the total active mass of the two electrodes). BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 These are the scanning electron microscope (SEM) and transmission electron microscope (TEM) images of Co@NCNTF-15 obtained in the preparation process of Example 2; among them, (a~c) are SEM images, and (d~f) are TEM images.

[0048] Figure 2These are the SEM and TEM images of Co@NCNTF-5 obtained in the preparation process of Example 1; among them, (a~c) are SEM images, and (d~f) are TEM images.

[0049] Figure 3 These are the SEM and TEM images of Co@NCNTF-25 obtained in the preparation process of Example 3; among them, (a~c) are SEM images, and (d~f) are TEM images.

[0050] Figure 4 These are the SEM and TEM images of NOCNTF-5 prepared in Example 1; (a-b) are SEM images, and (c-d) are TEM images.

[0051] Figure 5 These are the SEM and TEM images of NOCNTF-15 prepared in Example 2; wherein (a to c) are SEM images, and (d to f) are TEM images.

[0052] Figure 6 These are the SEM and TEM images of NOCNTF-25 prepared in Example 3; wherein (a-b) are SEM images, and (c-d) are TEM images.

[0053] Figure 7 The X-ray powder diffraction (XRD) patterns and Raman spectra of NOCNTF-5, NOCNTF-15 and NOCNTF-25 prepared in Examples 1 to 3; wherein (a) is the XRD pattern and (b) is the Raman spectrum.

[0054] Figure 8 This is the X-ray photoelectron spectrum (XPS) of NOCNTF-15 prepared in Example 2.

[0055] Fig. 9 These are the N 1s and O 1s XPS spectra of NOCNTF-15 prepared in Example 2; wherein (a) is the N 1s XPS spectrum, and (b) is the O 1s XPS spectrum.

[0056] Fig.10 These are the SEM and TEM images of Co@NPCF obtained during the preparation process of Comparative Example 1; wherein (a-b) are SEM images, and (c-d) are TEM images.

[0057] Fig.11 This is the SEM image of NOPCF prepared in Comparative Example 1.

[0058] Fig.12 This is the XRD spectrum of NOPCF prepared in Comparative Example 1.

[0059] Fig.13 These are the SEM and TEM images of the ETCNTs prepared in Comparative Example 2; wherein (a-b) are SEM images, and (c-d) are TEM images.

[0060] Fig.14 Figure 2 shows the rate performance of NOCNTF-5, NOCNTF-15 and NOCNTF-25 at different current densities. -1 and 10A·g -1 Long cycle performance diagram at current density of 1A·g -1 Long cycle performance diagram at current density of (c) 10A·g -1 Long cycle performance diagram at current density of .

[0061] Fig.15 This is the rate performance diagram of NOPCF at different current densities.

[0062] Fig.16 The rate performance diagram of NOCNTF-15||NVPOF at different current densities is shown in Figure 2. -1 The charge and discharge curves at current density and at 10A -1 Long cycle performance diagram at current density of ; (a) is the rate performance diagram; (b) is the long cycle performance diagram at 0.1Ag -1 The charge and discharge curves under current density, (c) is at 10Ag -1 Long cycle performance diagram at current density of . DETAILED DESCRIPTION

[0063] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0064] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0065] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods, and the materials, reagents, etc. used are all available from commercial sources unless otherwise specified.

[0066] Example 1

[0067] A method for preparing N / O co-doped carbon nanotubes (NOCNTF-5) comprises the following steps:

[0068] S1. Place 42 mg of ZIF-67 at the bottom of a quartz bottle with a radius of 5 mm, cover the ZIF-67 with 5 mm thick liquid gallium, and then place the quartz bottle in a vacuum drying oven at 60°C and evacuate for 2 h.

[0069] S2. Place the quartz bottle in a muffle furnace and heat it to 600°C at a heating rate of 2°C / min and pyrolyze it for 4 hours. After cooling to room temperature, recover the liquid gallium on the upper layer to obtain the black powder at the bottom of the quartz bottle, which is denoted as Co@NCNTF-5, and record the yield; the pressure difference ΔP at the contact surface between the liquid gallium layer and the ZIF-67 powder is 289.296 Pa (approximately 0.0029 standard atmospheric pressure).

[0070] S3. Place Co@NCNTs-5 in a 9M HNO3 solution, ultrasonicate for 30 min, then condense and reflux it at 70°C with stirring for 4 days, collect the product by centrifugation, and wash the product by centrifugation 4 times with deionized water and anhydrous ethanol, and finally dry it in a vacuum drying oven at 80°C to obtain NOCNTF-5.

[0071] Example 2

[0072] A method for preparing N / O co-doped carbon nanotubes (NOCNTF-15) comprises the following steps:

[0073] S1. Place 42 mg of ZIF-67 at the bottom of a quartz bottle with a radius of 5 mm, cover the ZIF-67 with 5 mm thick liquid gallium, and then place the quartz bottle in a vacuum drying oven at 60°C and evacuate for 2 h.

[0074] S2. Add 10 mm of liquid gallium to the quartz bottle, and place the quartz bottle in a vacuum drying oven for 2 h. Place the quartz bottle in a muffle furnace and heat it to 600 °C at a heating rate of 2 °C / min and pyrolyze it for 4 h. After cooling to room temperature, recover the upper layer of liquid gallium to obtain a black powder at the bottom of the quartz bottle, which is denoted as Co@NCNTF-15, and record the yield; the pressure difference ΔP between the liquid gallium layer and the ZIF-67 powder contact surface is 867.888 Pa (ΔP = 5.904 × 103 kg / m 3 ×9.8N / kg×15×10 -3 m=867.888N / m 2 , that is 867.888 Pa, about 0.0086 standard atmospheric pressure).

[0075] S3. Place Co@NCNTs-15 in a 9M HNO3 solution, ultrasonicate for 30 minutes, then condense and reflux it at 70°C with stirring for 4 days, collect the product by centrifugation, and wash the product by centrifugation 4 times with deionized water and anhydrous ethanol, and finally dry it in a vacuum drying oven at 80°C to obtain NOCNTF-15.

[0076] Example 3

[0077] A method for preparing N / O co-doped carbon nanotubes comprises the following steps:

[0078] S1. Place 42 mg of ZIF-67 at the bottom of a quartz bottle with a radius of 5 mm, cover the ZIF-67 with 5 mm thick liquid gallium, and then place the quartz bottle in a vacuum drying oven at 60°C and evacuate for 2 h.

[0079] S2. Add 20 mm of liquid gallium to the quartz bottle, and place the quartz bottle in a vacuum drying oven for 2 hours. Place the quartz bottle in a muffle furnace and heat it to 600°C at a heating rate of 2°C / min and pyrolyze it for 4 hours. After cooling to room temperature, recover the upper layer of liquid gallium to obtain a black powder at the bottom of the quartz bottle, which is recorded as Co@NCNTF-25, and record the yield; the pressure difference ΔP between the liquid gallium layer and the ZIF-67 powder contact surface is 1446.78 Pa (about 0.0143 standard atmospheric pressure).

[0080] S3. Co@NCNTs-25 was placed in a 9M HNO3 solution and ultrasonicated for 30 min. It was then condensed and refluxed at 70°C with stirring for 4 days. The product was collected by centrifugation and washed 4 times with deionized water and anhydrous ethanol. Finally, it was dried in a vacuum drying oven at 80°C to obtain NOCNTF-25.

[0081] Figure 1 The scanning electron microscope (SEM) and transmission electron microscope (TEM) images of Co@NCNTF-15 obtained in the preparation process of Example 2 are shown in FIG. Figure 1 As can be seen in the figure, after pyrolysis, Co@NCNTF-15 retains the skeleton morphology of the original ZIF-67, and a large number of CNTs are distributed on its surface. Unlike the previously reported hollow structure, it can be seen from the TEM image that Co@NCNTF-15 has a non-hollow structure. In addition, the ends of the CNTs are filled with Co nanoparticles, which is consistent with the description of the tip growth mechanism.

[0082] Figure 2 The scanning electron microscope image and transmission electron microscope image of Co@NCNTF-5 obtained in the preparation process of Example 1 are shown in FIG. Figure 3The scanning electron microscope and transmission electron microscope images of Co@NCNTF-25 obtained in the preparation process of Example 3 show that changing the thickness of the liquid gallium layer significantly affects the outer diameter of the produced CNTs. The increase in the thickness of the liquid gallium layer causes the outer diameter of the CNTs to increase from about 8 nm for Co@NCNTF-5 to about 27 nm for Co@NCNTF-25.

[0083] Figures 4 to 6 The scanning electron microscope images and transmission electron microscope images of NOCNTF-5, NOCNTF-15, and NOCNTF-25 prepared in Examples 1 to 3 are shown respectively. After Co@NCNTF-5, Co@NCNTs-15, and Co@NCNTs-25 were treated with concentrated HNO3 (9M) at 70°C, the morphology of the products NOCNTF-5, NOCNTF-15, and NOCNTF-25 did not change much. From the framework destroyed during the acid treatment, it can be seen that CNTs are densely grown inside the framework. In addition, TEM results show that the Co nanoparticles wrapped at the ends of CNTs in Co@NCNTF-5, Co@NCNTs-15, and Co@NCNTs-25 are etched to form an open CNTs.

[0084] Figure 7 The X-ray powder diffraction patterns and Raman spectra of NOCNTF-5, NOCNTF-15 and NOCNTF-25 prepared in Examples 1 to 3 are shown. The crystallinity of the carbon materials is confirmed by X-ray powder diffraction (XRD). Figure 7 a), there is a sharp peak at 26.1°, corresponding to the (002) crystal plane of graphite, which further confirms that NOCNTF-5, NOCNTF-15 and NOCNTF-25 are composed of CNTs with high crystallinity. No characteristic peak of metal Co was found in the XRD spectrum, indicating that metal Co was etched by HNO3, which is consistent with the TEM results. Raman spectrum ( Figure 7 b) shows that when the thickness of the liquid gallium layer increases from 5 mm to 25 mm, the I D / I G The value decreases from 0.96 to 0.73, indicating that the increase in the thickness of the liquid gallium layer leads to an increase in the graphitization degree of CNTs.

[0085] Figure 8 The X-ray photoelectron spectrum (XPS) of NOCNTF-15 prepared in Example 2, the full spectrum of XPS ( Figure 8 ) showed that after high concentration HNO3 treatment, a large amount of O (34.31at%) and N (7.7at%) elements were introduced into NOCNTF-15.

[0086] Fig. 9The N 1s and O 1s XPS spectra of NOCNTF-15 prepared in Example 2. The high-resolution N1s XPS spectrum reveals the presence of two types of nitrogen: pyridinic N at ∼397.88 eV and pyrrolic N at ∼399.43 eV ( Fig. 9 a). The O1s spectrum shows two peaks at ~531.47eV and ~532.96eV, which are attributed to C=O and C-OH, respectively. Fig. 9 b).

[0087] Comparative Example 1

[0088] A method for preparing porous carbon comprises the following steps:

[0089] 42 mg of ZIF-67 was placed in an Ar atmosphere, heated to 600 ° C at a heating rate of 2 ° C / min and pyrolyzed for 4 hours, and a black powder (Co@NPCF) was obtained after cooling, and the yield of the product was recorded. Co@NPCF was placed in a 2M dilute HNO3 solution, ultrasonicated for 30 minutes, and then condensed and refluxed at 45 ° C for 4 days. The product was collected by centrifugation, and the product was centrifuged and washed 4 times with deionized water and anhydrous ethanol, and finally placed in a vacuum drying oven at 80 ° C to obtain NOPCF.

[0090] The yields of pyrolysis of ZIF-67 in Examples 1 to 3 and Comparative Example 1 are shown in Table 1:

[0091] Table 1

[0092]

[0093] Fig.10 The SEM and TEM images of Co@NPCF obtained in the preparation process of Comparative Example 1 are shown in Figure 1. Fig.10 It can be seen that without the liquid gallium layer, the Co@NPCF obtained by pyrolysis in an Ar atmosphere roughly retains the original framework of ZIF-67. Due to the outward diffusion of the gas produced by the rapid decomposition of 2-methylimidazole, the ZIF-67 particles shrink sharply inward to form a hollow structure. In this case, no CNTs were observed in the framework, which may be due to the rapid escape of the carbon source produced by the decomposition of the ligand during the pyrolysis process, which cannot provide sufficient material for the growth of CNTs. Compared with the experiments carried out under liquid gallium, the yield of Co@NPCF in Table 1 (43.2wt%) is much lower than the yield of Co@NCNTF-x (x = 5, 15 and 25) (77.8wt%), which further confirms this.

[0094] Fig.11 This is the SEM image of NOPCF prepared in Comparative Example 1. Fig.11It can be seen that after Co@NPCF was treated with dilute HNO3 (2M) at 45°C, the product NOPCF can maintain its original morphology.

[0095] Fig.12 is the XRD spectrum of NOPCF prepared in Comparative Example 1, Fig.12 The peak at ∼26° corresponds to the (002) crystal plane of graphitic carbon. In addition, no characteristic peak of metallic Co was found in the XRD spectrum, indicating that metallic Co was etched by HNO3.

[0096] Comparative Example 2

[0097] A method for preparing carbon nanotubes comprises the following steps:

[0098] 42 mg of ZIF-67 powder was evenly mixed with 10 mL of anhydrous ethanol and placed in an Ar atmosphere. The mixture was heated to 600 °C at a heating rate of 2 °C / min and then pyrolyzed for 4 h. After cooling, black powder ETCNTs were obtained.

[0099] Fig.13 (a-b) SEM images and (c-d) TEM images of ETCNTs prepared in Comparative Example 2. Fig.13 It can be seen that in the same pyrolysis environment, the introduction of an additional carbon source (anhydrous ethanol) into the reaction system can grow CNTs, thus confirming the role of the liquid gallium layer in preventing the escape of the carbon source.

[0100] Test Example 1

[0101] The sodium storage performance of NOCNTF-5, NOCNTF-15, NOCNTF-25 prepared in Examples 1 to 3 and NOPCF prepared in Comparative Example 1 was tested. The test method was as follows: the active material (NOCNTF-5, NOCNTF-15, NOCNTF-25 or NOPCF), conductive carbon black and sodium carboxymethyl cellulose (CMC) were mixed in a mass ratio of 7:2:1 and deionized water was used as a solvent to prepare a working electrode. The mixed slurry was coated on a Cu foil and dried under vacuum at 60°C for 12 hours. The mass loading of the active material was about 0.5 to 1.0 mg. 1M sodium hexafluorophosphate (NaPF6) was dissolved in ethylene glycol dimethyl ether as an electrolyte, glass fiber purchased from Whatman was used as a diaphragm, and a sodium sheet with a diameter of 12 mm was used as a counter electrode to assemble a CR 2023 button cell in a glove box (O2<0.01ppm, H2O<0.01ppm). The Land CT 2001 A instrument was used to measure the + / Na) voltage window for charge and discharge tests.

[0102] Fig.14 a shows the rate performance of NOCNTF-5, NOCNTF-15, and NOCNTF-25 electrodes at different current densities, among which NOCNTF-15 exhibits the best performance at 0.1, 0.2, 0.5, 1, 2, 5, and 10 A·g -1 The discharge specific capacities under these conditions are 371.4, 348.9, 337.2, 322.4, 304, 278.5 and 252.7 mAh g -1 , showing excellent rate performance even at high current density. In addition, when the current density is restored to 0.1A·g -1 When the capacity of NOCNTF-15 electrode is rapidly restored to 338.9 mAh g -1 , and remain stable in subsequent cycles. Fig.14 As shown in b, NOCNTF-15 electrode at 1A·g -1 After 400 cycles, the reversible capacity remained at about 286.8 mAh g -1 , with a high capacity retention rate of 86.5%, and an average capacity decay of only 0.11 mAh g per cycle. -1 .like Fig.14 c) In order to evaluate the long-term cycling stability of NOCNTF-15 electrode at high current density, the NOCNTF-15 electrode was subjected to a 10 A·g -1 The NOCNTF-15 electrode can still provide 182.4 mAh g -1 The high reversible specific capacity and the coulombic efficiency remain close to 100% during cycling. In summary, the excellent sodium storage performance of NOCNTF-15 electrode may be attributed to the special structure of NOCNTF-15. The three-dimensional framework assembled by cross-linking of open CNTs from the inside to the outside provides a channel for rapid transmission of electrons and avoids electrical contact losses, resulting in excellent rate performance, especially at high current density. In addition, the high content of N / O co-doping, especially C=O located on the surface of carbon nanotubes, can not only promote rapid charge transfer at the electrode / electrolyte interface, but also help enhance the Na + Adsorption / desorption and surface redox reactions on the CNTs surface, thus exhibiting a high sodium storage capacity.

[0103] like Fig.15 As shown, the NOPCF electrode has a high conductivity at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A·g -1 The discharge specific capacities under these conditions are 264.1, 235.1, 212.2, 191.3, 169.6, 141.7 and 116.1 mAh·g, respectively. -1In addition, when the current density returned to 0.1A·g -1 When the capacity of NOCNTF-15 electrode is restored to 248.5 mAh g -1 , and remain stable in subsequent cycles.

[0104] Test Example 2

[0105] The NOCNTF-15 prepared in Example 2 was subjected to a NOCNTF-15||NVPOF sodium ion full battery performance test. The test method was as follows: Na3V2(PO4)2O2F, conductive carbon black and polyvinylidene fluoride (PVDF) were stirred and mixed in a mass ratio of 7:2:1 with N-methyl-2-pyrrolidone as solvent to prepare a positive electrode slurry. It was then coated on Al foil and vacuum dried at 60°C for 12 hours. The mass loading ratio of active materials of the positive electrode to the negative electrode was (4.5-5):1 to ensure that the prepared full battery had optimal performance. Before assembling the full battery, the positive electrode was charged at 0.1A·g in a sodium ion half-cell. -1 The NOCNTF-15 electrode was pre-sodiumized by three cycles of charge and discharge at a current density of . Subsequently, the half-cell was disassembled in a glove box to obtain the pre-sodiumized negative electrode. Na3V2(PO4)2O2F was used as the positive electrode, the pre-sodiumized NOCNTF-15 electrode was used as the negative electrode, and the full cell was assembled using the same electrolyte and separator. The constant current charge / discharge test of the full cell was carried out between the voltage window of 1.25 and 4.3 V (based on the mass of the active material of the negative electrode).

[0106] like Fig.16 a, at 0.1, 0.2, 0.5, 1, 2, 5, 10 A·g -1 The discharge specific capacities of NOCNTF-15||NVPOF at different current densities are 547, 439, 393, 372, 308, 202, and 124 mAh·g, respectively. -1 When the current density returns to 0.1A·g -1 When the capacity of NOCNTF-15||NVPOF is rapidly restored to 381 mAh g -1 , and remain stable in subsequent cycles. Fig.16 b shows that NOCNTF-15||NVPOF has a high operating voltage of 4.0V. More importantly, Fig.16 c shows the NOCNTF-15||NVPOF full battery at 10A·g -1 After 15400 cycles, the capacity is still 38 mAh g -1 In addition, the maximum energy density of NOCNTF-15||NVPOF can reach 233Wh·kg -1 (Based on the total active mass of the two electrodes).

[0107] The present invention discloses a novel, simple and high-yield pyrolysis strategy, which is based on liquid gallium-assisted pyrolysis of ZIF-67 to obtain CNTs. In this strategy, liquid gallium plays a role in isolating air, promoting heat transfer, and promoting the reduction of cobalt ions to form catalytically active Co nanoparticles by limiting the escape of H2 produced by the thermal decomposition of 2-imidazole to catalyze the formation of CNTs. In addition, liquid gallium is conducive to retaining carbon atoms and nitrogen atoms in ZIF-67, resulting in the synthesis of CNTs rich in N (7.7at%) with a high yield (77.8wt%). After appropriate oxidation treatment, a large amount of O element (34.31at%) is introduced into the carbon nanotubes, and the resulting N / O-rich NOCNTF-15 shows excellent sodium storage performance. The pyrolysis strategy of the present invention provides a new way to prepare low-cost CNTs and MOFs-derived carbon materials by pyrolysis using MOFs as precursors and apply them to negative electrode materials for sodium ion batteries.

[0108] Obviously, the above embodiments of the present invention are only examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art should understand that other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preparing N / O co-doped carbon nanotubes, characterized in that: The following steps are involved: S1. The MOF powder is placed in a container, a liquid gallium layer is covered on top of the MOF powder, and the container is evacuated; the thickness of the liquid gallium layer is 2 to 8 mm; S2. Covering the container with a liquid gallium layer again, evacuating the container and performing a pyrolysis reaction, and taking out the lower layer product Co@NCNTFs; the thickness of the liquid gallium layer is 0~30 mm; S3. The lower layer product Co@NCNTFs obtained in S2 is acid-washed and dried to obtain the N / O co-doped carbon nanotubes.

2. The preparation method according to claim 1, characterized in that: In step S1, the MOF is ZIF-67, ZTF-8@ZIF-67 or ZIF-67@ZIF-8.

3. The preparation method according to claim 1, characterized in that The pressure difference between the liquid gallium layer and the MOF powder contact surface is 115~2200 Pa.

4. The preparation method according to claim 1, characterized in that In step S2, the temperature of the pyrolysis reaction is 400-1000°C.

5. The preparation method according to claim 1, characterized in that: In step S3, pickling is performed using a strong acid.

6. N / O co-doped carbon nanotubes prepared by the method according to any one of claims 1 to 5.

7. Use of the N / O co-doped carbon nanotubes according to claim 6 in preparing sodium ion batteries.

8. A sodium ion battery comprising the N / O co-doped carbon nanotubes according to claim 7 as a negative electrode material.

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