A temperature-regulated synthesis of nitrogen-doped graphene loaded with different crystal phase cobalt telluride particle composite material, and a preparation method and application thereof

Abstract

The application provides a kind of based on temperature regulation synthesis nitrogen-doped graphene load different crystal phase cobalt telluride particle composite material and its preparation method and application. Its preparation method includes the following steps: adding cobalt salt into graphene oxide dispersion, stirring uniformly, to obtain dispersion A;Melamine is added to deionized water, heated to reaction temperature, add formaldehyde solution, after the system becomes transparent, add dispersion A, reaction;After cooling, filtering, washing, freeze-drying, calcination, to obtain precursor;The precursor and tellurium powder are placed in the downstream and upstream of the gas flow in the tube furnace respectively, and calcination is carried out to obtain the application. By temperature control, o-CoTe2 / NG and h-CoTe2 / NG composite materials can be obtained, which are applied in lithium-sulfur batteries and show excellent catalytic performance and cycle performance. The preparation process of the application is simple, the cost is low, the production efficiency is greatly improved, the needs of industrial production are met, and the application prospect is excellent.

Description

Technical Field

[0001] This invention relates to a temperature-controlled synthesis of nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases composite materials, its preparation method, and its application, belonging to the fields of new energy and nanomaterials technology. Background Technology

[0002] Lithium-sulfur batteries are recognized as one of the most promising next-generation energy storage technologies due to their high theoretical energy density (2600 Wh / kg). Furthermore, sulfur, the active material on the positive electrode side of lithium-sulfur batteries, has advantages such as abundant reserves, ease of mining, and environmental friendliness. However, researchers have discovered several problems that restrict the development of lithium-sulfur batteries during the research of positive electrodes: (1) The volume difference between active material sulfur and lithium sulfide, the final product of the discharge process, is large, which can easily lead to the collapse of the electrode structure during the charging and discharging process and affect the normal cycle of the battery; (2) Both active material sulfur and lithium sulfide belong to the insulating system, and their extremely low conductivity seriously affects the charge transfer efficiency, resulting in low utilization and poor rate performance; (3) During the operation of lithium-sulfur batteries, the discharge intermediate product lithium polysulfide is easily dissolved in the electrolyte. As the concentration difference diffuses to the negative electrode and reacts with lithium to generate short-chain lithium polysulfide, some of the lithium polysulfide shuttles to the positive electrode, and some generates insoluble lithium sulfide that covers the surface of the lithium negative electrode. This shuttle effect makes the battery self-discharge serious and leads to irreversible loss of active material.

[0003] The most common approach to solving these problems is to develop sulfur host materials with high specific surface area to provide sufficient space for the reaction; and to use highly conductive polar materials to accelerate charge transfer and catalyze the rapid conversion of lithium polysulfides, thereby limiting the shuttle effect. For example, Chinese patent document CN114751395A developed a method to regulate the coordination behavior of aqueous ZIF under low-temperature conditions, obtaining a nitrogen-doped carbon sphere with high porosity and high specific surface area. Its high specific surface area can meet the volume strain caused by charge and discharge products, its porosity ensures the transfer space between lithium polysulfides and lithium ions, and its high conductivity accelerates the charge transfer rate and suppresses the shuttle effect. When applied to lithium-sulfur batteries, it exhibits excellent electrochemical performance and stable cycle performance.

[0004] Transition metal tellurides have been widely reported as highly conductive catalytic materials in electrocatalysis. CoTe2, in particular, has shown excellent performance in the electrocatalytic splitting of water, but its application in lithium-sulfur batteries has been rarely studied. The metallic properties and moderate lithium polysulfide adsorption capacity of CoTe2 meet the requirements of lithium-sulfur battery catalysts, making it a promising cathode material for lithium-sulfur batteries. CoTe2 typically exists in orthorhombic (o-CoTe2) and hexagonal (h-CoTe2) phases. Due to their different structures, these phases undoubtedly exhibit different catalytic abilities for lithium polysulfides in lithium-sulfur batteries, a problem that urgently needs to be investigated. Synthetically, the preparation of both phases remains challenging, requiring continuous exploration of reaction temperature, time, and raw material selection. Loading different crystalline phases of CoTe2 onto high-specific-surface-area carbon materials presents an even greater challenge.

[0005] Therefore, developing a composite material based on nitrogen-doped graphene supporting cobalt telluride particles of different crystal phases for use as a cathode in lithium-sulfur batteries is of great significance. To this end, this invention is proposed. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a temperature-controlled synthesis method for nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases, along with their application. The invention first obtains a precursor through reflux freeze-drying and vacuum calcination. Then, by further controlling the tellurization temperature, nitrogen-doped graphene orthorhombic cobalt telluride composite materials (o-CoTe2 / NG) and nitrogen-doped graphene hexagonal cobalt telluride composite materials (h-CoTe2 / NG) are obtained. When used as the cathode material in lithium-sulfur batteries, these materials can effectively catalyze the conversion of lithium polysulfides, overcome the shuttle effect, and improve the cycle performance of lithium-sulfur batteries.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis includes the following steps:

[0009] (1) Cobalt salt was added to the graphene oxide dispersion and stirred evenly to obtain dispersion A; melamine was added to deionized water, heated to the reaction temperature, formaldehyde solution was added, and after the system became transparent, dispersion A was added to carry out the reaction; after the reaction was completed, the precursor was obtained by cooling, filtering, washing, freeze drying and calcining.

[0010] (2) The precursor and tellurium powder obtained in step (1) are placed in the downstream and upstream of the gas flow direction in a tube furnace, respectively, and calcined to obtain a composite material of nitrogen-doped graphene loaded with cobalt telluride particles of different crystal phases based on temperature control.

[0011] According to a preferred embodiment of the present invention, the cobalt salt in step (1) is one or a combination of two or more of cobalt nitrate hexahydrate, cobalt acetate, cobalt chloride, cobalt acetylacetonate, cobalt sulfate heptahydrate, and cobalt oxide; more preferably, it is cobalt nitrate hexahydrate, cobalt acetate, or cobalt sulfate heptahydrate.

[0012] According to a preferred embodiment of the present invention, the concentration of the graphene oxide dispersion in step (1) is 1-3 mg / mL; the graphene oxide is prepared by a modified Hummers method (reference: DC Marcano, DV Kosynkin, JMBerlin, A. Sinitskii, Z. Sun, A. Slesarev, LB Alemany, W. Lu, JMTour, ACS Nano 2010, 4, 4806.); the mass ratio of the graphene oxide to the cobalt salt is 0.2-0.6:1, more preferably 0.25-0.4:1.

[0013] According to a preferred embodiment of the present invention, the mass ratio of melamine to deionized water in step (1) is 0.02-0.04 g:1 mL; the mass ratio of melamine to cobalt salt is 2-20:1, and more preferably 3-10:1.

[0014] According to a preferred embodiment of the present invention, the concentration of the formaldehyde solution in step (1) is 40-50 wt%; the mass ratio of formaldehyde to melamine is 0.2-6:1, more preferably 0.4-1:1.

[0015] According to a preferred embodiment of the present invention, the reaction temperature in step (1) is 70-90°C; the reaction time is 2-10 hours, more preferably 5-8 hours.

[0016] According to a preferred embodiment of the present invention, the cooling in step (1) is natural cooling to room temperature; the washing is washing with deionized water 2-5 times; the freeze-drying temperature is -60 to -40°C, and the freeze-drying time is 5-30 hours.

[0017] According to the present invention, the calcination temperature in step (1) is preferably 450-950°C, more preferably 600-800°C; and the calcination time is 0.5-6 hours, more preferably 2-4 hours.

[0018] According to a preferred embodiment of the present invention, the calcination atmosphere in step (1) is a vacuum, Ar, N2, and an Ar / H2 mixture, wherein the volume percentage of H2 in the Ar / H2 mixture is 10%; more preferably, the calcination atmosphere is a vacuum with a vacuum degree of -0.1 MPa.

[0019] According to a preferred embodiment of the present invention, the mass ratio of the precursor to tellurium powder in step (2) is 1:0.5-10, and more preferably 1:1-5.

[0020] According to a preferred embodiment of the present invention, the calcination time in step (2) is 1-10 hours, more preferably 3-6 hours; the calcination is carried out in the presence of an Ar / H2 mixed gas, wherein the volume percentage of H2 in the mixed gas is 10%.

[0021] According to a preferred embodiment of the present invention, the calcination temperature in step (2) is 500-580℃ to obtain a nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material; the calcination temperature is 600-800℃ to obtain a nitrogen-doped graphene-supported hexagonal cobalt telluride composite material. This invention synthesizes nitrogen-doped graphene-supported CoTe2 particle composite materials with different crystalline phases by controlling the temperature, which are then applied to lithium-sulfur batteries. The rationally designed phase engineering provides valuable reference for the design of selective catalysts in other fields.

[0022] According to the present invention, the above-mentioned composite material of nitrogen-doped graphene loaded with cobalt telluride particles of different crystal phases based on temperature-controlled synthesis is applied in lithium-sulfur batteries.

[0023] The present invention also provides a lithium-sulfur battery cathode material, wherein the lithium-sulfur battery cathode material comprises a composite material based on temperature-controlled synthesis of nitrogen-doped graphene loaded with cobalt telluride particles of different crystal phases.

[0024] According to the present invention, the preparation method of the above-mentioned lithium-sulfur battery cathode material is prior art; more preferably, the lithium-sulfur battery cathode material is prepared according to the following method:

[0025] (i) The nitrogen-doped graphene-loaded cobalt telluride particles with different crystal phases synthesized based on temperature control were mixed with sulfur powder at a mass ratio of 3:7 and ground thoroughly. The resulting powder was transferred into an ampoule, sealed, and placed in a tube furnace. It was calcined at 155°C for 12 hours and then further calcined at 185°C for 2 hours. After the temperature dropped to room temperature, it was ground to obtain the sulfur-loaded composite material for later use.

[0026] (ii) The above sulfur-loaded composite material is mixed with acetylene black and polyvinylidene fluoride in a mass ratio of 7:2:1, and N-methylpyrrolidone is added and ball-milled; the total mass ratio of the sulfur-loaded composite material, acetylene black and polyvinylidene fluoride to the volume ratio of N-methylpyrrolidone is 1g:10mL.

[0027] (iii) The ball-milled slurry is coated onto carbon-coated aluminum foil and spread evenly with a scraper. It is then transferred to a vacuum oven at 60°C and dried for 12 hours. The dried aluminum foil is then formed into round sheets with a diameter of 12 mm as positive electrode material, wherein the sulfur loading in the positive electrode material is 1-1.5 mg.

[0028] Secondly, this invention discloses a method for preparing a composite material of nitrogen-doped graphene supported on CoTe2 particles of different crystal phases based on temperature-controlled synthesis for use as a cathode in lithium-sulfur batteries. This invention does not impose special requirements on the assembly method of the lithium-sulfur battery; assembly can be performed entirely by those skilled in the art familiar with the assembly process.

[0029] This invention provides the application of the above-mentioned lithium-sulfur battery cathode material in lithium-sulfur batteries. Preferably, the electrolyte of the lithium-sulfur battery is ethylene carbonate (EC), 1,3-dioxane (DOL), dimethyl carbonate (DMC), ethylene carbonate (DEC), biphenyl (BP), diethyl carbonate (EMC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene ethylene carbonate (VEC), 3-propanesulfonate lactone (PS), 1,4-butanesulfonate lactone (BS), 1,3-(1-propene)sulfonate lactone (PST), ethylene sulfate (ESA), ethylene sulfite (ESI), cyclohexylbenzene (CHB), or tert-amylbenzene (TP). B) A mixture of lithium salt and one or more of succinic anhydride (SN) and tert-butylbenzene (TBB); wherein the lithium salt is one or more of lithium hexafluorophosphate (LiPF6), lithium bis(SO2CF3)2, lithium tetrafluoroborate (LiBF4), lithium bis(fluorosulfonamide) (LiFSI), lithium trifluoromethanesulfonate (LiSO3CF3), and lithium dioxoborate (LiBOB); wherein the molar concentration of lithium salt in the electrolyte is 1-25 mol / L; wherein the negative electrode of the above lithium-sulfur battery is a lithium sheet, graphite, silicon carbide material or lithium titanate, and is further preferably a lithium sheet.

[0030] Compared with existing technologies, the present invention has the following outstanding advantages:

[0031] 1. This invention uses the condensation reaction of melamine and formaldehyde as a sacrificial template, which is also used as an ideal nitrogen source to dope into the graphene network to improve the material's polarity. The precursor obtained through calcination allows metal atoms to be uniformly dispersed on the carbon layer surface while maintaining the material's fluffy state, resulting in a composite material with a high specific surface area. It is crucial to control the temperature and time of the first calcination within the range specified in this invention; otherwise, the performance of the resulting material will be affected. The high specific surface area composite material not only alleviates the volume expansion problem during charge and discharge but also provides sufficient space for the conversion of lithium polysulfides. Orthorhombic and hexagonal CoTe2 phases are obtained through simple temperature control during tellurization. The orthorhombic cobalt telluride exhibits a more uniform distribution and smaller particle size, which is beneficial for the full exposure of catalyst active sites. Thanks to this superior structure, the orthorhombic cobalt telluride composite material is more effective than the hexagonal CoTe2 composite material in catalyzing the conversion of lithium polysulfides, overcoming the shuttle effect, and resulting in lithium-sulfur batteries with better cycle performance.

[0032] 2. The preparation process of this invention is simple and low-cost, which greatly improves production efficiency and can better meet the needs of industrial production, realize mass production, and has great application prospects.

[0033] 3. The catalyst prepared by this invention has high conductivity and strong practicality, and can be effectively applied to high-performance lithium-sulfur batteries. Attached Figure Description

[0034] Figure 1 The images show the XRD patterns of nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1. The left image shows the nitrogen-doped graphene-supported orthorhombic cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-supported hexagonal cobalt telluride particle composite material.

[0035] Figure 2 These are transmission electron microscope images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Example 1. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material.

[0036] Figure 3 These are scanning electron microscope (SEM) images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Example 1. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material.

[0037] Figure 4 The figures show the nitrogen adsorption-desorption curves and pore size distribution of the nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1. (a)-(b) are the pore size distribution and nitrogen adsorption-desorption curves of the nitrogen-doped graphene-supported hexagonal cobalt telluride particles composite material, respectively, and (c)-(d) are the pore size distribution and nitrogen adsorption-desorption curves of the nitrogen-doped graphene-supported orthorhombic cobalt telluride particles composite material, respectively.

[0038] Figure 5 The image shows the Raman spectra of the nitrogen-doped graphene-supported cobalt telluride particles with different crystal phases prepared in Example 1.

[0039] Figure 6 The cyclic voltammetry curves are those of the nitrogen-doped graphene-supported cobalt telluride particles with different crystal phases prepared in Example 1.

[0040] Figure 7 The impedance spectrum is shown for the lithium-sulfur battery assembled from nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1.

[0041] Figure 8The cycling performance of the lithium-sulfur battery assembled from nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1 at a current density of 0.5C is shown.

[0042] Figure 9 The image shows the charge-discharge curves of a lithium-sulfur battery assembled from nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1 at a current density of 0.5C.

[0043] Figure 10 The rate performance of the lithium-sulfur battery assembled from nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1 is shown.

[0044] Figure 11 These are transmission electron microscope (TEM) images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Example 2. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material.

[0045] Figure 12 The images are transmission electron microscope (TEM) images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Comparative Example 1. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material.

[0046] Figure 13 The images are transmission electron microscope (TEM) images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Comparative Example 2. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material.

[0047] Figure 14 The images are scanning electron microscope (SEM) images of nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases prepared in Comparative Example 3. The left image shows the nitrogen-doped graphene-loaded hexagonal cobalt telluride particle composite material, and the right image shows the nitrogen-doped graphene-loaded orthorhombic cobalt telluride particle composite material. Detailed Implementation

[0048] It should be noted that the following accompanying drawings and specific embodiments further illustrate the invention, but the detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0049] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0050] As introduced in the background section, the preparation of CoTe2 catalytic materials with different crystal phases in the prior art has problems such as complex processes and harsh conditions. In order to solve the above problems, the present invention provides a composite material for preparing nitrogen-doped graphene-supported CoTe2 particles with different crystal phases through a strategy of reflux and secondary calcination. The present invention will be further described below with reference to specific embodiments.

[0051] The graphene dispersions used in the examples were prepared by a modified Hummers method (see reference DC Marcano, DV Kosynkin, JMBerlin, A. Sinitskii, Z. Sun, A. Slesarev, LB Alemany, W. Lu, JMTour, ACS Nano 2010, 4, 4806.).

[0052] Example 1

[0053] A method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis includes the following steps:

[0054] (1) Measure 10 mL of graphene dispersion with a concentration of 8.4 mg / mL and disperse it in 40 mL of deionized water. Sonicate for 0.5 h to obtain graphene oxide dispersion. Add 0.3 g of cobalt nitrate hexahydrate to the obtained graphene oxide dispersion and stir for 1 h to obtain dispersion A. Weigh 1 g of melamine and place it in a 100 mL two-necked flask. Add 40 mL of deionized water and turn on the stirring and heating. After the temperature rises to 80 °C, add 1 g of formaldehyde solution (concentration of 50 wt%). After the mixture becomes transparent, add dispersion A to the two-necked flask and continue to react at 80 °C for 8 h. After the reaction is completed, cool the obtained reaction solution to room temperature naturally and filter it. Wash the obtained solid twice with deionized water and freeze-dry the obtained solid at -50 °C for 24 h to obtain a grayish-white product. Place the above grayish-white product in a tube furnace and calcine it at 700 °C for 2 h under vacuum conditions (vacuum degree of -0.1 MPa) to obtain the precursor.

[0055] (2) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:3. After calcination at 550℃ for 4 hours and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material, denoted as o-CoTe2 / NG composite material.

[0056] (3) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:3. After calcination at 650℃ for 4 hours and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported hexagonal cobalt telluride composite material, denoted as h-CoTe2 / NG composite material.

[0057] The resulting nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material and nitrogen-doped graphene-supported hexagonal cobalt telluride composite material are based on temperature-controlled synthesis of nitrogen-doped graphene-supported cobalt telluride particles with different crystal phases.

[0058] The XRD patterns of the nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases synthesized based on temperature control in Example 1 are shown below. Figure 1 As shown, from Figure 1 As can be seen, the XRD peaks of the sample correspond perfectly to the standard PDF cards for orthorhombic CoTe2 and hexagonal CoTe2, respectively; from Figure 2 Transmission electron microscopy images show that orthorhombic and hexagonal CoTe2 nanoparticles are uniformly dispersed on the nitrogen-doped graphene surface without agglomeration, and the o-CoTe2 nanoparticles are smaller and more uniform in size; from Figure 3 The scanning electron microscope (SEM) images show that the sample exhibits a highly curled and wrinkled morphology, which contributes to the increase in specific surface area. Figure 4 The nitrogen adsorption-desorption curves and pore size distribution curves show that the o-CoTe2 / NG composite material has a larger specific surface area than the h-CoTe2 / NG composite material. The larger specific surface area can alleviate the volume expansion problem during charging and discharging, and at the same time provide more space for the conversion of lithium polysulfides. The microporous structure helps the electrolyte to penetrate and bind the lithium polysulfides.

[0059] The Raman spectra of the nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases prepared in Example 1 are as follows: Figure 5 As shown, from Figure 5 As can be seen from this, the D peak (disordered carbon) and the G peak (sp) 2 The ratio of hybrid graphite carbon to nitrogen (NOC) is greater than 1, indicating that the incorporation of nitrogen introduces defects into the carbon matrix.

[0060] The nitrogen-doped graphene-supported cobalt telluride particle composite material with different crystal phases prepared in this embodiment was mixed with sulfur powder and used as the positive electrode material for lithium-sulfur batteries, and the batteries were assembled. The specific operation steps are as follows:

[0061] (1) Nitrogen-doped graphene-loaded cobalt telluride particles of different crystal phases were ground with sulfur powder in an agate mortar for 30 min at a mass ratio of 3:7. The mixed powder was transferred to an ampoule and placed in a tube furnace. After calcining at 155°C for 12 hours, it was calcined at 185°C for 2 hours. After cooling to room temperature, the sample was collected to obtain o-CoTe2 / NG / S and h-CoTe2 / NG / S composite materials.

[0062] (2) Mix 70 mg of o-CoTe2 / NG / S or h-CoTe2 / NG / S composite material with acetylene black and polyvinylidene fluoride at a mass ratio of 7:2:1 and grind them. Place the mixed material in an agate ball mill jar and add 1 mL of N-methylpyrrolidone. Then, ball mill the mixture on a planetary ball mill at a speed of 300 r / min for 4 h.

[0063] (3) The mixed slurry was coated onto carbon-coated aluminum foil with a scraper and scraped evenly with a 200μm scraper. Then it was transferred to a vacuum oven and dried overnight at 60℃. The dried aluminum foil was cut into discs with a diameter of 12mm as positive electrode plates with a sulfur loading of 1.5mg.

[0064] (4) Assembly of the lithium-sulfur battery must be carried out in an argon-filled glove box. The battery casing model is CR2016, with a 16mm diameter circular lithium sheet as the negative electrode and a Celgard 2400 separator. The electrolyte is a DOL / DME (volume ratio 1:1) mixed solvent, with LiTFSI as the lithium salt, and the concentration of lithium salt in the electrolyte is 1 mol / L. The assembled o-CoTe2 / NG / S composite electrode and h-CoTe2 / NG / S composite electrode need to stand for 12 hours to allow for full wetting by the electrolyte.

[0065] The o-CoTe2 / NG / S composite electrode and h-CoTe2 / NG / S composite electrode assembled in this embodiment were subjected to cyclic voltammetry tests on the Chenhua electrochemical workstation. The charge-discharge voltage range was 1.7-2.8V, and the results are as follows. Figure 6As shown, the o-CoTe2 / NG / S composite material exhibits a higher current response in both the oxidation and reduction peak regions compared to the h-CoTe2 / NG / S composite material, demonstrating that the o-CoTe2 / NG / S composite material is more effective in promoting the sulfur oxidation-reduction reaction. Electrochemical impedance spectroscopy was performed on the assembled composite electrode, and the results are as follows: Figure 7 As shown, the impedance spectrum consists of charge transfer impedance and diffusion impedance in the high-frequency region, and the smaller impedance value of the o-CoTe2 / NG / S composite material proves that the redox transformation of sulfur is easier on the o-CoTe2 / NG surface.

[0066] The assembled composite electrode was subjected to cycle performance testing. Figure 8 The cycling performance of the o-CoTe2 / NG / S composite electrode and h-CoTe2 / NG / S composite electrode prepared in this embodiment at a current density of 0.5C is given by... Figure 8 It can be seen that the capacity retention rate of the o-CoTe2 / NG / S composite electrode after 200 cycles at a current density of 0.5C is 86.6%, and that of the h-CoTe2 / NG / S composite electrode after 200 cycles at a current density of 0.5C is 72.2%.

[0067] Figure 9 These are the first charge-discharge curves of the o-CoTe2 / NG / S composite electrode and the h-CoTe2 / NG / S composite electrode prepared in this embodiment at a current density of 0.5C. Figure 9 It can be seen that the initial capacity of the o-CoTe2 / NG / S composite electrode at a current density of 0.5C is 1052.2 mAh / g, while that of the h-CoTe2 / NG / S composite electrode at a current density of 0.5C is 925.1 mAh / g. The voltage difference of the o-CoTe2 / NG / S composite electrode is smaller than that of the h-CoTe2 / NG / S composite electrode, proving that o-CoTe2 / NG is more effective in promoting the conversion of lithium polysulfides.

[0068] Figure 10 This section describes the performance tests of the o-CoTe2 / NG / S composite electrode and the h-CoTe2 / NG / S composite electrode prepared in this embodiment at different rate magnifications. Figure 10 The specific capacities of the o-CoTe2 / NG / S composite electrode at current densities of 0.2, 0.5, 1, 2, 3, 5, and 6 C are 1240.9, 1061.5, 971.3, 890.5, 833.9, 689.9, and 645.9 mAh / g, respectively. When the current density returns to 0.5 C, it still maintains a reversible capacity of 1009.8 mAh / g.

[0069] Example 2

[0070] A method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis includes the following steps:

[0071] (1) 8 mL of graphene dispersion with a concentration of 8.4 mg / mL was dispersed in 20 mL of deionized water and sonicated for 0.5 h to obtain graphene oxide dispersion. 0.2 g of cobalt nitrate hexahydrate was added to the obtained graphene oxide dispersion and stirred for 1 h to obtain dispersion A. 1.5 g of melamine was weighed and placed in a 100 mL two-necked flask. 40 mL of deionized water was added and the mixture was stirred and heated. After the temperature rose to 80 °C, 1.7 g of formaldehyde solution (concentration of 50 wt%) was added. After the mixture became transparent, dispersion A was added to the two-necked flask and the reaction was continued at 80 °C for 8 h. After the reaction was completed, the reaction solution was cooled to room temperature and filtered. The obtained solid was washed twice with deionized water and freeze-dried at -50 °C for 24 h to obtain a grayish-white product. The grayish-white product was placed in a tube furnace and calcined at 700 °C for 2 h under vacuum conditions (vacuum degree of -0.1 MPa) to obtain the precursor.

[0072] (2) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:2. After calcination at 550℃ for 4 hours and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material, denoted as o-CoTe2 / NG composite material.

[0073] (3) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:2. After calcination at 650℃ for 4h and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported hexagonal cobalt telluride composite material, denoted as h-CoTe2 / NG composite material.

[0074] The obtained nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material and nitrogen-doped graphene-supported hexagonal cobalt telluride composite material are synthesized based on temperature-controlled synthesis of nitrogen-doped graphene-supported cobalt telluride particles of different crystal phases. Transmission electron microscopy images of the o-CoTe2 / NG and h-CoTe2 / NG composite materials prepared in Example 2 are shown below. Figure 11 As shown, from Figure 11As can be seen, o-CoTe2 and h-CoTe2 nanoparticles are uniformly dispersed on the nitrogen-doped graphene surface without agglomeration.

[0075] Example 3

[0076] A method for preparing nitrogen-doped graphene-supported CoTe2 particle composites with different crystalline phases based on temperature-controlled synthesis includes the following steps:

[0077] (1) 8 mL of graphene dispersion with a concentration of 8.4 mg / mL was dispersed in 20 mL of deionized water and sonicated for 0.5 h to obtain graphene oxide dispersion. 0.25 g of cobalt nitrate hexahydrate was added to the obtained graphene oxide dispersion and stirred for 1 h to obtain dispersion A. 1.5 g of melamine was weighed and placed in a 100 mL two-necked flask. 40 mL of deionized water was added and the mixture was stirred and heated. After the temperature rose to 80 °C, 1.7 g of formaldehyde solution (concentration of 50 wt%) was added. After the mixture became transparent, dispersion A was added to the two-necked flask and the reaction was continued at 80 °C for 8 h. After the reaction was completed, the reaction solution was cooled to room temperature and filtered. The obtained solid was washed twice with deionized water and freeze-dried at -50 °C for 24 h to obtain a grayish-white product. The grayish-white product was placed in a tube furnace and calcined at 800 °C for 2 h under vacuum conditions (vacuum degree of -0.1 MPa) to obtain the precursor.

[0078] (2) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:2.5. After calcination at 550℃ for 4 hours and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material, denoted as o-CoTe2 / NG composite material.

[0079] (3) The precursor and tellurium powder were placed in two separate ceramic boats. The ceramic boats were placed in a tube furnace with Ar / H2 mixed gas (H2 volume percentage in the mixed gas was 10%). The ceramic boat containing tellurium powder was upstream of the gas flow direction, and the ceramic boat containing the precursor was downstream of the gas flow direction. The mass ratio of precursor to tellurium powder was 1:2.5. After calcination at 650℃ for 4h and natural cooling to room temperature, the sample was collected to obtain nitrogen-doped graphene-supported hexagonal cobalt telluride composite material, denoted as h-CoTe2 / NG composite material.

[0080] The resulting nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material and nitrogen-doped graphene-supported hexagonal cobalt telluride composite material are based on temperature-controlled synthesis of nitrogen-doped graphene-supported cobalt telluride particles with different crystal phases.

[0081] Comparative Example 1

[0082] A method for preparing nitrogen-doped graphene-supported CoTe2 particles with different crystal phases based on temperature control is described in Example 1, except that the amount of cobalt nitrate hexahydrate added in step (1) is 0.6g.

[0083] Transmission electron microscope (TEM) images of nitrogen-doped graphene-supported CoTe2 particle composites with different crystalline phases prepared in this comparative example are shown below. Figure 12 As shown, the results are from Figure 12 As can be seen, increasing the amount of cobalt source leads to significant aggregation of CoTe2 particles, and the size of CoTe2 particles increases.

[0084] Comparative Example 2

[0085] A method for preparing nitrogen-doped graphene-supported CoTe2 particle composites with different crystal phases based on temperature control is described in Example 1, except that the amount of cobalt nitrate hexahydrate added in step (1) is 0.08g.

[0086] Transmission electron microscope (TEM) images of nitrogen-doped graphene-supported CoTe2 particle composites with different crystalline phases prepared in this comparative example are shown below. Figure 13 As shown, the results are from Figure 13 As can be seen, reducing the amount of cobalt source only resulted in the observation of a small number of CoTe2 nanoparticles distributed on the surface of nitrogen-doped graphene, and their size also increased.

[0087] Comparative Example 3

[0088] A method for preparing nitrogen-doped graphene-supported CoTe2 particle composites with different crystal phases based on temperature control is described in Example 1, except that the product obtained by freeze-drying in step (1) of Example 1 does not need to be calcined under vacuum conditions, and steps (2) and (3) of Example 1 are carried out.

[0089] Scanning electron microscopy (SEM) images of nitrogen-doped graphene-supported CoTe2 particle composites with different crystalline phases prepared in this comparative example are shown below. Figure 14 As shown, the results are from Figure 14 As can be seen, the CoTe2 nanoparticles in the composite material obtained by reacting the product without further vacuum calcination with tellurium powder are unevenly distributed and larger in size.

[0090] Finally, it should be noted that this invention is not limited to the specific embodiments described above. This invention extends to any new features or combinations disclosed in this specification, as well as any new steps or combinations of any new methods or processes disclosed herein.

Claims

1. A method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis, comprising the following steps: (1) Cobalt salt was added to the graphene oxide dispersion and stirred evenly to obtain dispersion A; melamine was added to deionized water, heated to the reaction temperature, formaldehyde solution was added, and after the system became transparent, dispersion A was added to carry out the reaction; after the reaction was completed, the precursor was obtained by cooling, filtering, washing, freeze drying and calcining. (2) The precursor and tellurium powder obtained in step (1) are placed in the downstream and upstream of the gas flow direction in a tube furnace, respectively, and calcined to obtain a composite material of nitrogen-doped graphene loaded with cobalt telluride particles of different crystal phases based on temperature control.

2. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The cobalt salt mentioned in step (1) is one or a combination of two or more of cobalt nitrate hexahydrate, cobalt acetate, cobalt chloride, cobalt acetylacetonate, cobalt sulfate heptahydrate, and cobalt oxide; The concentration of the graphene oxide dispersion is 1-3 mg / mL; the mass ratio of graphene oxide to cobalt salt is 0.2-0.6:

1.

3. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The mass ratio of graphene oxide to cobalt salt in step (1) is 0.25-0.4:

1.

4. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, In step (1), the mass ratio of melamine to deionized water is 0.02-0.04 g: 1 mL; the mass ratio of melamine to cobalt salt is 2-20:

1. The concentration of the formaldehyde solution is 40-50 wt%; the mass ratio of formaldehyde to melamine is 0.2-6:

1.

5. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, In step (1), the mass ratio of melamine to cobalt salt is 3-10:1; the mass ratio of formaldehyde to melamine is 0.4-1:

1.

6. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The reaction temperature in step (1) is 70-90℃; the reaction time is 2-10 hours. The cooling process involves natural cooling to room temperature; the washing process involves washing with deionized water 2-5 times; the freeze-drying temperature is -60~-40℃, and the freeze-drying time is 5-30 hours.

7. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The calcination temperature in step (1) is 450-950℃; the calcination time is 0.5-6 hours; The calcination atmosphere is a vacuum, Ar, N2, and Ar / H2 mixture, wherein the volume percentage of H2 in the Ar / H2 mixture is 10%.

8. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The calcination temperature in step (1) is 600-800℃; the calcination time is 2-4 hours; the calcination atmosphere is vacuum, and the vacuum degree is -0.1MPa.

9. The method for preparing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis according to claim 1, characterized in that, The mass ratio of the precursor to tellurium powder in step (2) is 1:0.5-10.

10. The method for preparing a composite material of nitrogen-doped graphene supported on cobalt telluride particles of different crystal phases based on temperature-controlled synthesis according to claim 1, characterized in that, The mass ratio of the precursor and tellurium powder in step (2) is 1:1-5.

11. The method for preparing a composite material of nitrogen-doped graphene supported on cobalt telluride particles of different crystal phases based on temperature-controlled synthesis according to claim 1, characterized in that, The calcination time in step (2) is 1-10 hours; the calcination is carried out in the presence of an Ar / H2 mixed gas, wherein the volume percentage of H2 in the mixed gas is 10%; the calcination temperature is 500-580℃, to obtain a nitrogen-doped graphene-supported orthorhombic cobalt telluride composite material; the calcination temperature is 600-800℃, to obtain a nitrogen-doped graphene-supported hexagonal cobalt telluride composite material.

12. A composite material for synthesizing nitrogen-doped graphene-supported cobalt telluride particles of different crystalline phases based on temperature-controlled synthesis, characterized in that... It was prepared using the preparation method described in claim 1.

13. A lithium-sulfur battery cathode material, characterized in that, The lithium-sulfur battery cathode material includes the composite material of nitrogen-doped graphene loaded with cobalt telluride particles of different crystal phases based on temperature-controlled synthesis as described in claim 12.

14. The application of the lithium-sulfur battery cathode material according to claim 13 in lithium-sulfur batteries.

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