A method for preparing a single-atom carbon material and its application in lithium-sulfur batteries

Abstract

The present invention discloses a method for preparing a carbon material containing a single atom and its application in a lithium-sulfur battery. A uniform carbon material containing a single atom with catalytic activity is prepared by directly carbonizing a zinc-based metal organic framework material with zinc oxide nanoparticles doped with metal elements adsorbed on the surface at high temperature. The method comprises the following steps: preparing a zinc-based metal organic framework material; preparing zinc oxide nanoparticles doped with metal elements (X‑ZnO); adsorbing the X‑ZnO nanoparticles on the surface of the zinc-based metal organic framework material by electrostatic action; and carbonizing at high temperature in an inert atmosphere to obtain a loaded carbon material with uniform dispersion of single atoms. The preparation method is simple, efficient, mild in conditions and easy to control. The present invention also discloses the application of a carbon material containing a single atom as a carrier material of the active substance sulfur in the positive electrode of a lithium-sulfur battery. The high catalytic activity of the single atom can improve the reaction kinetics, excellent capacity utilization, rate performance and cycle stability of the lithium-sulfur battery.

Description

Technical Field

[0001] The present invention relates to the technical field of energy materials, and in particular to a method for preparing a single-atom carbon material and application of the material in a lithium-sulfur battery. Background Art

[0002] With the progress and development of human society, we are facing problems such as the depletion of traditional petrochemical energy and environmental pollution. The development of clean and recyclable energy storage systems is the key to promoting the development of electric vehicles, smart grids and other fields. Currently, in electrochemical energy storage systems, lithium-ion secondary batteries have been widely studied and applied due to their high operating voltage, long service life and high power density. However, the maximum energy density of commercial lithium-ion battery systems can only reach 300Wh / kg, which is close to the energy density limit of existing lithium-ion battery systems and cannot meet people's requirements for performance improvement.

[0003] Lithium-sulfur batteries are composed of sulfur composite positive electrodes, metal lithium negative electrodes, separators and electrolytes. They are multi-electron reversible reactions based on the "solid → liquid → solid" reaction between sulfur and lithium. The overall reaction equation is: 2Li+S↔Li 2 S has a high theoretical energy density (2600Wh / kg), which is much higher than that of commercial lithium-ion batteries. At the same time, sulfur has the advantages of abundant resource reserves, low price, and environmental friendliness. It has good application prospects and is considered to be a new generation of high-energy-density secondary battery system with the most development potential. However, lithium-sulfur batteries still have many technical problems: on the one hand, sulfur, the active material of the positive electrode, is not conductive, and a large amount of carbon material with good conductivity needs to be added as a carrier. In addition, the volume change of the positive electrode sulfur during the charging and discharging process is large, which leads to the pulverization of the battery electrode, resulting in low utilization of active materials, poor cycle stability and poor rate performance. On the other hand, during the reaction of lithium-sulfur batteries, it is inevitable to generate discharge products polysulfides (Li 2 S n , 4 ≤ n ≤ 8), which diffuses and migrates between the positive and negative electrodes under the action of concentration gradient and electric field, reducing the active material of the positive electrode, resulting in rapid capacity decay and reduced coulombic efficiency, which is called the "shuttle effect".

[0004] In order to solve the problems of poor conductivity, volume expansion, and polysulfide shuttling in lithium-sulfur batteries, the main method at present is to load the positive electrode sulfur in a porous carbon matrix material matrix with good electronic conductivity and ionic conductivity while limiting the diffusion of polysulfides, such as porous carbon, carbon nanotubes and other materials. Or by introducing polar electrode materials that have mutual chemical forces with polysulfides, reducing the shuttling of polysulfides through surface adsorption and other forces, such as heteroatom-doped carbon, oxides and other materials. However, the strategy of using porous materials for "physical confinement" or polar groups for "chemical adsorption" can only improve the electrochemical performance of lithium-sulfur batteries to a certain extent, and cannot fundamentally improve the conversion efficiency of lithium-sulfur battery reactions. Recently, researchers have proposed the concept of "catalytic conversion" and synthesized a series of catalysts. Through catalysis, the reaction kinetics of lithium-sulfur batteries have been improved, which can accelerate the mutual conversion between sulfur-containing substances and reduce the concentration and conversion time of polysulfides in the electrolyte, thereby reducing the diffusion time and diffusion power of polysulfides, thereby achieving the purpose of inhibiting the "shuttle effect", such as precious metals, metal nitrides, metal sulfides, etc. However, these catalysts have problems such as complex synthesis, high cost, and poor conductivity. Summary of the invention

[0005] In order to solve the problems existing in the above-mentioned lithium-sulfur batteries, the present invention provides a method for preparing a single-atom carbon material with a simple synthesis process and low cost. The material is used in lithium-sulfur batteries as a positive electrode sulfur-carrying material and has a catalytic effect. The catalyst improves the conversion reaction kinetics of the lithium-sulfur battery and can significantly improve the specific capacity, cycle performance and rate performance of the lithium-sulfur battery.

[0006] The present invention solves the technical problem by adopting the following technical solutions:

[0007] A method for preparing a single-atom carbon material and its application in a lithium-sulfur battery, the method mainly comprising the following steps:

[0008] 1. Preparation of zinc-based metal organic framework materials (Zn-MOF);

[0009] 2. Preparation of metal-doped zinc oxide nanoparticles (X-ZnO, X refers to transition metal elements, such as Ni, Co, Fe, etc.);

[0010] 3. Adsorbing X-ZnO nanoparticles on the surface of Zn-MOF material through electrostatic action;

[0011] 4. Carbonizing the composite material at high temperature under an inert gas environment to obtain a single-atom-uniformly dispersed supported carbon material;

[0012] 5. Application of single-atom carbon materials in lithium-sulfur batteries.

[0013] The zinc-based metal organic framework material in step 1 is a zinc-based metal organic framework material composed of metal ions containing only zinc metal and organic ligands, such as ZIF-7, ZIF-8, ZIF-62, ZIF-90, MOF-5, MOF-74 (Zn), MOF-177, etc. The preparation method of the zinc-based metal organic framework material can be a conventional preparation method in the art, such as hydrothermal / solvothermal synthesis, ultrasonic method, microwave heating method, electrochemical method and mechanochemical synthesis method, etc.

[0014] Preferably, the prepared zinc-based metal organic framework material has a particle size of 50-500 nm, has a microporous and mesoporous structure, and a specific surface area of ​​more than 200 m 2 / g.

[0015] Preferably, the organic ligand of the prepared zinc-based metal organic framework material contains nitrogen.

[0016] In the step 2, metal element-doped zinc oxide nanoparticles are prepared. The metal element refers to one or more transition metal elements. Preferably, the transition metal element is nickel, cobalt, iron, manganese, chromium, etc. The specific preparation steps are as follows:

[0017] (a) dissolving a zinc salt and a doped metal salt in a dimethyl sulfoxide solvent in a certain proportion, wherein the zinc salt and the doped metal salt may be one or a mixed salt of two or more selected from acetate, nitrate, chlorate, chloride, etc., wherein the molar ratio of zinc in the zinc salt to the metal in the doped metal salt or the mixed salt is 1000:1 to 5:1.

[0018] Preferably, the molar ratio of zinc in the zinc salt to the metal in the doped metal salt or the mixed salt is 200:1 to 10:1.

[0019] (b) slowly adding an excess of 0.1 g / ml tetramethylammonium hydroxide ethanol solution under stirring, wherein the molar amount of the tetramethylammonium hydroxide used is 1 to 3 times the molar amount of all the salts used in step (a).

[0020] (c) adding an ethyl acetate solution having a volume twice that of the dimethyl sulfoxide solvent in step (a), collecting the precipitate by centrifugation or filtration, washing the precipitate, and drying the precipitate in a vacuum oven at 60° C. for 4 to 12 hours to obtain metal element-doped zinc oxide nanoparticles.

[0021] In the step 3, the X-ZnO nanoparticles are adsorbed on the surface of the Zn-MOF material by electrostatic action. Specifically, one or more X-ZnO nanoparticles and the Zn-MOF material are dispersed in a solution such as deionized water, ethanol or methanol in a certain proportion, and after magnetic stirring for 6-24 hours, the precipitate is collected by centrifugation or filtration, washed, and placed in a vacuum oven at 60° C. for drying for 4-12 hours.

[0022] The molar ratio of the zinc oxide nanoparticles X-ZnO to the zinc-based metal organic framework material Zn-MOF is 1:1000-1:5. Preferably, the molar ratio of the X-ZnO nanoparticles to the Zn-MOF material is 1:200-1:5.

[0023] In step 4, the inert gas is nitrogen or argon, the carbonization temperature is 800-1100° C., and the carbonization time is 1-4 hours. During the carbonization process, the zinc oxide nanoparticles are reduced to zinc by carbon in a high temperature environment, and the zinc in the Zn-MOF evaporates with the air flow at high temperature, leaving only the loaded single atomic metal and carbon.

[0024] Preferably, the carbonization temperature is 900-1000° C., and the carbonization time is 2 hours.

[0025] In the method for preparing the single-atom-containing carbon material, the prepared metal element is partially or completely dispersed on the carbon carrier in the form of single atoms, and the metal element can be one or more, and the mass percentage of the single atoms is 0.01% to 10%.

[0026] Preferably, the prepared carbon carrier material has a particle size of 50-200 nm, has a microporous and mesoporous structure, and a specific surface area of ​​more than 200 m 2 / g, pore volume greater than 0.5cm 3 / g, and the mass percentage of the loaded single atoms is 0.1%~5%.

[0027] An application of a single-atom carbon material in a lithium-sulfur battery, using the above-mentioned single-atom catalyst-containing carbon material as a positive electrode sulfur-carrying material of the lithium-sulfur battery, the specific steps are as follows:

[0028] (a) After the sublimated sulfur and the single-atom carbon material are uniformly mixed in a certain proportion, the sulfur powder and the single-atom carbon material are mixed by melt-filling sulfur under vacuum or inert gas protection atmosphere to obtain a carbon / sulfur composite positive electrode material, and the heating temperature is 155°C for 12 to 24 hours, wherein the mass proportion of the single-atom carbon material in the composite positive electrode material is 5% to 50%.

[0029] Preferably, vacuum sulfur loading is used, and the mass proportion of single-atom carbon materials in the obtained composite positive electrode material is 10% to 40%.

[0030] (b) The sulfur-carbon composite positive electrode material, conductive agent and binder obtained in step (a) are mixed in a certain proportion, using nitrogen methyl pyrrolidone (NMP) as a solvent, and the mixture is evenly mixed to form a slurry, which is then coated on a current collector and dried in a vacuum to prepare a positive electrode sheet for a lithium-sulfur battery.

[0031] Preferably, the certain proportion is that the sulfur-carbon composite positive electrode material accounts for 80-90%, the conductive agent accounts for 5-10% of the positive electrode active material, and the binder accounts for 5-10%.

[0032] The conductive agent is one or more of conductive carbon black, conductive graphite, carbon nanotubes, and graphene, and the binder is polyvinylidene fluoride.

[0033] Preferably, the homogenization mixing method adopts the ball milling method, the ball milling time is 2 to 3 hours, and the sulfur loading of the positive electrode active material is 0.8 to 4 mg / cm 2 .

[0034] (c) Assembling the positive electrode sheet obtained in step (b) and the lithium negative electrode, the separator, the electrolyte and the shell to obtain a lithium-sulfur battery.

[0035] The diaphragm is a microporous polyolefin diaphragm, a ceramic diaphragm or a non-woven fabric diaphragm, etc. Preferably, the polyolefin diaphragm is composed of one or more layers of polyethylene (PE) or polypropylene (PP).

[0036] The electrolyte is composed of a lithium electrolyte and a non-aqueous organic solvent. The electrolyte is lithium bistrifluoromethanesulfonyl imide (LiTFSI), and the non-aqueous organic solvent is one or two of dioxolane (DOL) and ethylene glycol dimethyl ether (DME).

[0037] The present invention also provides a lithium-sulfur battery, in which the carbon material containing single atoms is used as the sulfur-carrying material in the positive electrode.

[0038] Beneficial effects of the invention: The invention provides a method for preparing a single-atom carbon material containing one or more metal elements, by preparing transition metal element-doped zinc oxide nanoparticles (X-ZnO) and adsorbing them on the surface of a zinc-based organic framework material (Zn-MOF), and then carbonizing them in an inert atmosphere to finally prepare a carbon material with uniform metal single atom dispersion, high electrical conductivity and good catalytic activity. The method has low cost, simple process, and environmental friendliness, and can achieve large-scale production.

[0039] The single-atom-containing carbon material prepared by the present invention has a large pore volume and a large specific surface area. As a carrier of the positive electrode material of a lithium-sulfur battery, it plays a role in "physical confinement" and "chemical adsorption" of polysulfides, thereby reducing the "shuttle effect". In addition, the embedding of metal single-atom active sites increases the electron transmission capacity and the number of active sites of the carbon material, which not only improves the electrical conductivity of the carbon material, but also the catalytic properties of the single-atom metal can greatly improve the reaction kinetics of the lithium-sulfur battery, accelerate the electron transmission during the electrochemical reaction, reduce the shuttling of polysulfides, and reduce the polarization of the battery. The experimental results show that due to the introduction of single atoms, the lithium-sulfur battery can exhibit a higher gram capacity, good cycle performance, excellent rate performance, and faster reaction kinetics, which significantly improves the electrochemical performance of the lithium-sulfur battery, and is of great significance for the further commercialization of lithium-sulfur batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0041] Figure 1 The X-ray diffraction (XRD) diagram of the precursor ZIF-8 of the embodiments and comparative examples of the present invention;

[0042] Figure 2 The following are scanning electron microscope (SEM) images of the precursor ZIF-8 of the embodiments of the present invention and the comparative examples;

[0043] Figure 3 X-ray diffraction (XRD) patterns of the nickel and cobalt single-atom-loaded nitrogen-doped carbon material (Ni-Co-NC, curve 1) and the nitrogen-doped carbon material (NC, curve 2) prepared in the examples of the present invention and the comparative examples;

[0044] Figure 4 This is a scanning electron microscope (SEM) image of the nickel and cobalt single atom-loaded carbon material (Ni-Co-NC) prepared in an embodiment of the present invention;

[0045] Figure 5 This is a spherical aberration transmission electron microscope (STEM) image of the nickel and cobalt single atom-loaded carbon material (Ni-Co-NC) prepared in an embodiment of the present invention;

[0046] Figure 6 This is the full X-ray photoelectron spectrum (XPS) of the nickel and cobalt single atom supported carbon material (Ni-Co-NC) prepared in an embodiment of the present invention;

[0047] Figure 7 The graphs are the cycle performances of the carbon materials prepared in the examples of the present invention and the comparative examples after being loaded with sulfur in a lithium-sulfur battery at a current density of 0.5C;

[0048] Figure 8 The carbon materials prepared in the examples of the present invention and the comparative examples are shown in the performance diagrams of different rates in lithium-sulfur batteries after being loaded with sulfur;

[0049] Fig. 9 The carbon materials prepared in the embodiments of the present invention and the comparative examples are loaded with sulfur and correspond to the following in lithium-sulfur batteries: Figure 8 The first charge and discharge curve of the rate performance test;

[0050] Fig.10 The cyclic voltammetry curves of the carbon materials loaded with sulfur in the examples and comparative examples of the present invention in a lithium-sulfur battery;

[0051] Among them, the horizontal coordinate of the obtained XRD graph is the diffraction angle (2θ), and the vertical coordinate is the diffraction peak intensity (Intensity); the horizontal coordinate of the obtained XPS graph is the electron binding energy (Binding energy), and the vertical coordinate is the electron intensity (Intensity); the obtained cycle performance graph and rate performance graph, the horizontal coordinate is the cycle number (Cycle number), and the vertical coordinate is the gram capacity (Specific capacity); the obtained charge and discharge curve graph, the horizontal coordinate is the gram capacity (Specific capacity), and the vertical coordinate is the voltage (Voltage); the obtained cyclic voltammetry curve graph, the horizontal coordinate is the voltage (Pontential), and the vertical coordinate is the current (Current). DETAILED DESCRIPTION

[0052] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] Example 1

[0054] Preparation of double single-atom carbon materials containing nickel and cobalt and performance testing of their application in lithium-sulfur batteries.

[0055] The preparation method of the carbon material containing nickel and cobalt double single atoms in this embodiment is as follows:

[0056] 1. Preparation of zinc-based metal organic framework material ZIF-8

[0057] First, 0.28 mol (23 g) of 2-methylimidazole (C4 H 6 N 2 ) was added to 500 ml methanol (CH 4 O) solution, and then 0.035 mol (10.41 g) of zinc nitrate hexahydrate (H 12 N 2 O 12 Zn) was added to another 500 ml of methanol (CH 4 O) solution, and then add zinc nitrate solution to 2-methylimidazole solution. Stir magnetically at room temperature for 24 hours, collect the precipitate by centrifugation, the centrifugal speed is 9000r / min, the centrifugal time is 5min, and finally wash it with methanol solution for more than three times, place it in a vacuum oven at 60℃ and dry it for 6 hours to obtain ZIF-8 nanoparticles. Its XRD is as follows Figure 1 As shown, it can be determined that the material is ZIF-8; the scanning electron microscope image is as follows Figure 2 As shown, it can be seen that the approximate size of ZIF-8 nanoparticles is 50~60nm.

[0058] 2. Preparation of nickel and cobalt metal element doped zinc oxide nanoparticles (Ni-ZnO, Co-ZnO)

[0059] Preparation of Ni-ZnO: 0.5 mmol (124.4 mg) of nickel acetate tetrahydrate (C 4 H 14 NiO 8 ) and 10 mmol (2195.1 mg) of zinc acetate dihydrate (C 4 H 10 O 6 Zn) was added to 100 ml of dimethyl sulfoxide (C 2 H 6 OS) solution, and then stir it with a magnetic stirrer. Then add 30.8 ml of 0.1 g / ml tetramethylammonium hydroxide pentahydrate (C 4 H 13 NO 5 (H 2 The ethanol solution of 240 ml of ethyl acetate (C 4 H 8 O 2 ) is added to the above mixed solution, and finally the precipitate is collected by centrifugation at a speed of 6000r / min for 3min, and then washed with ethyl acetate solution for more than three times to obtain nickel-doped zinc oxide particles (Ni-ZnO).

[0060] Preparation of Co-ZnO: 0.5 mmol (124.5 mg) of cobalt acetate tetrahydrate (C4 H 14 CoO 8 ) and 10 mmol (2195.1 mg) of zinc acetate dihydrate (C 4 H 10 O 6 Zn) was added to 100 ml of dimethyl sulfoxide (C 2 H 6 OS) solution, stir it evenly with magnetic force. Then add 30.8 ml of 0.1 g / ml tetramethylammonium hydroxide pentahydrate (C 4 H 13 NO 5 (H 2 The ethanol solution of 240 ml of ethyl acetate (C 4 H 8 O 2 ) is added to the above mixed solution, and finally the precipitate is collected by centrifugation at a speed of 6000 r / min for 3 min, and then washed with ethyl acetate solution for more than three times to obtain cobalt-doped zinc oxide particles (Co-ZnO).

[0061] 3. Adsorbing Ni-ZnO and Co-ZnO nanoparticles on the surface of ZIF-8 material through electrostatic action

[0062] The Ni-ZnO and Co-ZnO nanoparticles obtained in step 2 were dispersed in 25 ml of ethanol solution respectively; the ZIF-8 material powder (15.68 mmol) obtained in step 1 was weighed and dispersed in 360 ml of ethanol solution, and magnetically stirred for 1 hour after ultrasonication for 10 minutes until the material was uniformly dispersed. Then, the ethanol solution of Ni-ZnO (containing 0.5 mmol Ni) and Co-ZnO (containing 0.5 mmol Co) nanoparticles was poured into the ZIF-8 ethanol solution, and magnetically stirred for 24 hours, and Ni-ZnO and Co-ZnO were adsorbed on the Zn-O surface by electrostatic action. Finally, the precipitate was collected by centrifugation, the centrifugal speed was 9000 r / min, the centrifugal time was 5 minutes, and then washed with ethanol for more than three times, placed in a vacuum oven at 60 ° C for 6 hours, and the ZIF-8 material adsorbed with Ni-ZnO and Co-ZnO nanoparticles was obtained.

[0063] 4. Carbonization of ZIF-8 composites to obtain single-atom uniformly dispersed loaded carbon materials

[0064] The ZIF-8 material adsorbed with Ni-ZnO and Co-ZnO nanoparticles was ground into powder and placed in a tubular furnace. The temperature was raised to 900°C at a rate of 5°C / min in a nitrogen atmosphere. After being kept at 900°C for 2 hours, it was cooled to room temperature with the furnace. During the high-temperature carbonization process, the zinc oxide nanoparticles were reduced to zinc by carbon in a high-temperature environment, and the zinc in the ZIF-8 evaporated with the air flow at high temperature, leaving only nickel, cobalt metal and nitrogen-doped carbon materials. The collected black powder material was the nickel / cobalt double single-atom uniformly dispersed loaded carbon material (Ni-Co-NC).

[0065] The XRD pattern of the nickel / cobalt double single atom supported carbon material (Ni-Co-NC) prepared in this example is as follows: Figure 3 As shown in the middle curve 1, the scanning electron microscope image is as follows Figure 4 As shown, the spherical aberration transmission electron microscope image is as follows Figure 5 The full XPS spectrum is shown in Figure 6 As shown. From the XRD graph, it can be seen that the diffraction peaks of carbon are around 23° and 43°, and no other characteristic diffraction peaks appear; from the scanning electron microscope image, it can be seen that the carbon material maintains the structure of ZIF-8 and does not collapse; the spherical aberration transmission electron microscope image shows that Ni and Co are dispersed on the carrier in the form of single atoms, proving the existence of single-atom materials; from the XPS full spectrum, it can be seen that nickel and cobalt are dispersed in the carbon material.

[0066] Application and testing of the carbon material containing nickel / cobalt double single atoms as a positive electrode sulfur-carrying material in this embodiment

[0067] The carbon material containing nickel / cobalt diatomic atoms and sublimated sulfur were weighed in a mass ratio of 3:7 and ground and mixed evenly. The mixed material was placed in a thin glass tube and evacuated until the vacuum degree in the glass tube reached below 10 mbar and maintained for 10 minutes. The glass tube was then melted and sealed with an acetylene spray gun. The glass tube was placed in an oven, heated to 155°C by molten sulfur filling, and then kept warm for 12 hours. After cooling, a sulfur-carbon composite material was obtained.

[0068] The obtained sulfur-carbon composite positive electrode material, conductive agent (conductive carbon black, Super P) and binder (polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 8:1:1, and methyl pyrrolidone (NMP) was used as a solvent. The mixture was evenly mixed by ball milling to form a slurry. The ball milling speed was 200r / min and the ball milling time was 2 hours. The slurry was then coated on the aluminum foil current collector with a scraper. The scraper blade height was 150um. The electrode was placed in a vacuum oven and baked at 60°C for 6 hours. After drying, the lithium-sulfur battery positive electrode was prepared. The coating surface density sulfur loading was about 1mg / cm 2 .

[0069] The positive electrode sheet of the lithium-sulfur battery was cut into a disc with a diameter of 14 mm as the working electrode (positive electrode), a metal lithium sheet as the counter electrode (negative electrode), a polyethylene / polypropylene composite separator (celgard 2400), and an electrolyte containing 1 mol / L lithium bistrifluoromethanesulfonyl imide (LTFSI) lithium salt and 1,3-dioxolane (DOL), ethylene glycol dimethyl ether (DME) (volume ratio of 1:1) solvent to assemble into a 2025 button lithium-sulfur battery. The battery was subjected to a cycle performance test. The charge and discharge current for the first three cycles was 0.1C, and then the test was performed at 0.5C, where 1C=1675mAh / g, and the test voltage range was 1.7~2.8V. The cycle performance of the carbon material containing nickel / cobalt double single atoms as the sulfur-carrying material for the positive electrode of the lithium-sulfur battery at a current of 0.5C is shown in the figure. Figure 7 As shown, the performance of different rates is as follows Figure 8 As shown in the figure, the first charge and discharge curve (0.1C) of the rate performance is as follows Fig. 9 The cyclic voltammetry curve is shown in Fig.10 shown.

[0070] Comparative Example

[0071] Preparation of carbon materials that do not contain any single atoms and performance testing of their application in lithium-sulfur batteries.

[0072] Different from the embodiment, the preparation process of the carbon material in the comparative example is to directly carbonize ZIF-8 at high temperature to obtain a nitrogen-doped carbon material (NC), and its XRD pattern is as follows: Figure 3 As shown in the middle curve 2, the other preparation steps and lithium sulfur test aspects are completely consistent with the embodiment. From the XRD diagram, it can be seen that the diffraction peaks of carbon are around 23° and 43°, and no other characteristic diffraction peaks appear. The cycling performance of the carbon material without any single atom as the positive electrode sulfur-carrying material of the lithium-sulfur battery at a current of 0.5C is shown in Figure 7 As shown, the performance of different rates is as follows Figure 8 As shown in the figure, the first charge and discharge curve (0.1C) of the rate performance is as follows Fig. 9 The cyclic voltammetry curve is shown in Fig.10 shown.

[0073] like Figure 7 As shown, the first discharge capacity of the embodiment is 1348.5 mAh / g, and that of the comparative example is 1172.2 mAh / g. After 250 cycles, the discharge capacity of the embodiment is 726.3 mAh / g, and that of the comparative example is only 469.6 mAh / g. The capacity and cycle stability of the embodiment are both higher than those of the comparative example. Figure 8As shown, the discharge gram capacities of the embodiment after 5 cycles at 0.1C, 0.5C, 1C and 2C rates are 1098.8 mAh / g, 833.6 mAh / g, 707.8 mAh / g and 573.6 mAh / g, respectively, and the corresponding discharge gram capacities of the comparative examples are 966 mAh / g, 701.4 mAh / g, 583.4 mAh / g and 464.2 mAh / g, respectively. The rate performance of the embodiment is significantly better than that of the comparative example. Fig. 9 for Figure 8 The first charge and discharge curve corresponding to the rate performance test shows that the material exhibits a typical charge and discharge platform curve unique to lithium-sulfur batteries. The discharge capacities are 1332.5 mAh / g and 1198.3 mAh / g, respectively. The overvoltage ΔE of the embodiment is much smaller than that of the comparative example, indicating that the conversion rate of polysulfide ions is improved and the embodiment has a certain catalytic effect. Fig.10 The cyclic voltammetry curves of the embodiment and the comparative example show that the reduction peak of the embodiment is at 2.288V and 2.07V, respectively, and the reduction peak of the comparative example is at 2.281V and 2.065V, respectively. The reduction peak of the embodiment is greater than that of the comparative example; while the oxidation peaks of the embodiment and the comparative example are at 2.327V and 2.42V, respectively, and the oxidation peak of the embodiment is less than that of the comparative example, indicating that the oxidation or reduction reaction of the embodiment occurs earlier than the oxidation-reduction reaction of the comparative example, further indicating that the single-atom material obtained in the embodiment improves the reaction kinetics and has a catalytic effect. Therefore, it can be seen from the experimental data that as a sulfur-carrying material in a lithium-sulfur battery, in terms of discharge capacity, cycle performance, rate performance, and reaction kinetics, the performance of the embodiment is significantly better than that of the comparative example, showing better electrochemical performance. Therefore, the application of the single-atom-containing carbon material proposed in the present invention in lithium-sulfur batteries is feasible and has a good performance improvement effect.

[0074] The present invention provides a method for preparing a carbon material containing single atoms of metal elements. The metal single atoms in the carbon material are evenly dispersed, have high electrical conductivity and good catalytic activity. The preparation method has low cost, simple process, and is environmentally friendly. It can achieve large-scale production and provides a new method and new idea for solving the problems of low specific capacity of lithium-sulfur batteries, poor cycle performance, and shuttle effect.

[0075] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a single-atom carbon material, It is characterized in that The following steps are involved: (1) Preparation of zinc-based metal organic framework material Zn-MOF; (2) Preparation of metal element doped zinc oxide nanoparticles X-ZnO; (3) The zinc oxide nanoparticles X-ZnO are adsorbed on the surface of the zinc-based metal organic framework material Zn-MOF by electrostatic action to obtain a composite material; (4) carbonizing the composite material obtained in step (3) at high temperature in an inert gas environment to obtain a single-atom-uniformly dispersed supported carbon material, i.e., a single-atom-containing carbon material; The zinc-based metal organic framework material in step (1) is a zinc-based metal organic framework material Zn-MOF composed of and only containing zinc metal ions and organic ligands; The metal element X in the metal element-doped zinc oxide nanoparticles X-ZnO in step (2) is one or more transition metal elements, and the transition metal elements are nickel, cobalt, iron, manganese, and chromium; In step (4), the high temperature carbonization temperature is 800-1100° C. and the time is 1-4 hours.

2. The method for preparing a single-atom-containing carbon material according to claim 1, It is characterized in that Step (2) The preparation steps of metal element doped zinc oxide nanoparticles X-ZnO are as follows: Dissolving a zinc salt and a doped metal salt in dimethyl sulfoxide, wherein the molar ratio of zinc in the zinc salt to the metal in the doped metal salt is 1000:1-5:1; the zinc salt is one or more of zinc acetate, zinc nitrate or zinc chloride; the doped metal salt is one or more of acetate, nitrate, chlorate and chloride containing a metal element X; Slowly add an excess of 0.1 g / ml tetramethylammonium hydroxide ethanol solution under stirring, wherein the molar amount of the tetramethylammonium hydroxide used is 1 to 3 times the molar amount of all salts used; Add ethyl acetate solution with 2 to 3 times the volume of dimethyl sulfoxide solvent, collect the precipitate by centrifugation or filtration, and dry it in a vacuum oven at 60° C. for 4 to 12 hours to obtain metal element-doped zinc oxide nanoparticles X-ZnO.

3. The method for preparing a single-atom-containing carbon material according to claim 1, It is characterized in that The preparation method of the composite material in step (3) is as follows: dispersing metal element-doped zinc oxide nanoparticles X-ZnO and zinc-based metal organic framework material Zn-MOF in deionized water, ethanol or methanol, stirring magnetically for 6-24 hours, collecting the precipitate by centrifugation or filtration, washing the precipitate, and drying the precipitate in a vacuum oven at 60° C. for 4-12 hours.

4. The method for preparing a single-atom-containing carbon material according to claim 3, It is characterized in that The molar ratio of the metal element doped zinc oxide nanoparticles X-ZnO to the zinc-based metal organic framework material Zn-MOF is 1:1000-1:

5.

5. The method for preparing a single-atom-containing carbon material according to claim 1, It is characterized in that In step (4), the inert gas is nitrogen or argon.

6. A single-atom carbon material obtained by the preparation method according to any one of claims 1 to 5, It is characterized in that The carbon material has a particle size of 50-500 nm, has microporous and mesoporous structures, and a specific surface area greater than 200 m 2 / g, pore volume greater than 0.5cm 3 / g, the metal elements are partially or completely dispersed on the carbon carrier in the form of single atoms, and the mass percentage of the single atomic metal elements on the carbon carrier is 0.01% to 10%.

7. Use of the single-atom-containing carbon material according to claim 6 in a lithium-sulfur battery, It is characterized in that The specific steps are as follows: 1) After uniformly mixing the sublimated sulfur and the single-atom carbon material, the sulfur powder and the single-atom carbon material are mixed by melting sulfur infusion under vacuum or inert gas protection atmosphere to obtain a carbon / sulfur composite cathode material, and the heating temperature is kept at 155°C for 12 to 24 hours, wherein the mass proportion of the single-atom carbon material in the carbon / sulfur composite cathode material is 5% to 50%; 2) The carbon / sulfur composite positive electrode material, the conductive agent and the binder obtained in step 1) are mixed, and nitrogen methyl pyrrolidone is used as a solvent. The slurry is evenly mixed and coated on the current collector, and the positive electrode sheet of the lithium-sulfur battery is prepared after vacuum drying; 3) Assemble the lithium-sulfur battery positive electrode sheet and lithium negative electrode, separator, electrolyte and shell obtained in step 2) to obtain a lithium-sulfur battery.

8. A lithium-sulfur battery, Features: The positive electrode of the lithium-sulfur battery contains the single-atom-containing carbon material according to claim 6 as a sulfur-carrying material.

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