CN111974430A - Preparation method of monoatomic copper catalyst and application of monoatomic copper catalyst in positive electrode of lithium-sulfur battery - Google Patents

Preparation method of monoatomic copper catalyst and application of monoatomic copper catalyst in positive electrode of lithium-sulfur battery Download PDF

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CN111974430A
CN111974430A CN202010629581.0A CN202010629581A CN111974430A CN 111974430 A CN111974430 A CN 111974430A CN 202010629581 A CN202010629581 A CN 202010629581A CN 111974430 A CN111974430 A CN 111974430A
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copper catalyst
nitrogen
sulfur
carbon fiber
copper
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孙振华
肖茹
陈克
杨珍珍
李峰
成会明
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Institute of Metal Research of CAS
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Abstract

The invention discloses a preparation method of a monoatomic copper catalyst and application of the monoatomic copper catalyst in a lithium-sulfur battery anode, and belongs to the technical field of battery materials. According to the invention, copper atoms on the foamy copper are captured and transferred to the carbon fiber substrate through high-temperature ammonia treatment, and the nitrogen-doped carbon fiber foam material loaded with monoatomic copper is prepared and used as a carrier material of anode sulfur in the lithium sulfur battery. The monoatomic copper catalyst prepared by the invention enables the lithium-sulfur battery to have faster reaction kinetics, excellent capacity exertion and cycling stability under high sulfur loading. The preparation process is simple, and provides a wide prospect for the application of the monatomic catalyst in the lithium-sulfur battery.

Description

Preparation method of monoatomic copper catalyst and application of monoatomic copper catalyst in positive electrode of lithium-sulfur battery
The technical field is as follows:
the invention relates to the technical field of battery materials, in particular to a preparation method of a monoatomic copper catalyst and application of the monoatomic copper catalyst in a lithium-sulfur battery anode.
Background art:
the sulfur has the characteristics of abundant natural reserves, environmental friendliness, high theoretical capacity (1675mAh/g) and the like, and is an ideal battery cathode material. The lithium-sulfur battery has high theoretical energy density (2600Wh/kg), and can meet the energy density requirement of future energy storage devices. However, sulfur still faces great challenges as a positive electrode material. Sulfur and discharge product Li2S/Li2S2Is an electronic insulator, thus leading to incomplete conversion of active substances in the electrochemical process and lower utilization rate. Furthermore, the intermediate product lithium polysulphide (Li)2SxAnd x is more than or equal to 3 and less than or equal to 8) can be dissolved in electrolyte and shuttles between the anode and the cathode under the action of electric field force and concentration gradient, so that the problems of rapid attenuation of battery capacity, low coulombic efficiency and the like are caused. The method widely used at present is to compound a carbon material with good conductivity with sulfur to increase the conductivity of the sulfur anode, and load a transition metal catalyst on a carbon material substrate. The catalyst can absorb soluble lithium polysulfide and accelerate the conversion kinetics of the lithium polysulfide, thereby inhibiting the shuttle effect. In addition, the catalyst can also accelerate Li in the charging process of the battery2S/Li2S2The oxidation kinetics of the catalyst, and the utilization rate of active substances is improved.
The size of the current supported transition metal catalysts is usually on the nanometer scale. The catalyst utilization is low because the adsorption and catalysis processes mainly take place on the surface of the catalyst particles, while the surface atoms of the nanoscale particles account for only a small fraction of the total number of atoms. The catalyst belongs to inactive components in the lithium-sulfur battery, and the increase of the atom utilization rate of the catalyst is beneficial to the increase of the overall energy density of the lithium-sulfur battery. The metal monatomic catalyst is dispersed in an atomic level and embedded on a substrate, and has a theoretical atom utilization rate of 100%, a large number of coordinatively unsaturated active sites, and maximized catalyst-substrate interaction. Metal monatomic catalysts offer great advantages over nanoscale catalysts, both in terms of catalytic efficiency and cell energy density. Therefore, the preparation method of the material is reasonably utilized, the metal monatomic catalyst is loaded on the carbon material substrate and is compounded with the sulfur, so that the conversion reaction kinetics of the sulfur anode can be greatly improved, and the electrochemical performance of the lithium-sulfur battery is improved.
The invention content is as follows:
the invention aims to provide a preparation method of a monatomic copper catalyst and application of the monatomic copper catalyst in a lithium-sulfur battery anode, wherein the monatomic copper catalyst uniformly loaded on a nitrogen-doped carbon fiber foam substrate is prepared, the catalyst accelerates the conversion reaction kinetics of a sulfur anode, reduces battery polarization, improves the utilization rate of active substances, and reduces inactive substance components. The prepared monoatomic copper/nitrogen-doped carbon fiber foam/sulfur composite positive electrode has excellent capacity exertion and cycling stability under high sulfur loading.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a preparation method of a monoatomic copper catalyst comprises the following steps:
(1) sequentially cleaning and drying natural cotton, putting the dried cotton and foamy copper into a tubular furnace, and sequentially performing high-temperature treatment in a protective atmosphere and an ammonia atmosphere to obtain nitrogen-doped carbon fiber foam loaded with a monatomic copper catalyst;
(2) uniformly mixing sulfur powder, multi-walled carbon nanotubes and carbon black in proportion, adding the mixture into a proper amount of solvent, and performing mixing grinding and ultrasonic dispersion to obtain an active substance suspension; and (3) immersing the nitrogen-doped carbon fiber foam loaded with the monatomic copper catalyst into the suspension for a period of time, then taking out the nitrogen-doped carbon fiber foam, and drying the nitrogen-doped carbon fiber foam to obtain the monatomic copper catalyst/nitrogen-doped carbon fiber/sulfur composite anode material.
In the step (1), the natural cotton is cleaned to remove impurities, and the cleaning process is as follows: soaking natural cotton in 0.5-5mol/L dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, magnetically stirring for 30-60 min, taking out, soaking in distilled water, magnetically stirring for 1-2 hr, and finally soaking in anhydrous ethanol, magnetically stirring for 1-2 hr, and taking out; and (3) drying the cleaned cotton in an oven at the temperature of 40-80 ℃ for 24-48 hours.
In the step (1), the cotton and the foamy copper which are put into the tube furnace are separately placed along the axial direction of a tube body of the tube furnace at a mass ratio of 1:5 to 1:20, wherein the foamy copper is close to the air inlet side of the tube furnace.
In the step (1), the high-temperature treatment process comprises the following steps: heating the tube furnace to 900-1100 ℃ in the protective atmosphere of argon or nitrogen, wherein the heating rate is 5-20 ℃/min, and keeping the temperature for 1-2 hours; then switching to ammonia atmosphere, preserving the heat for 30-90 minutes, and then cooling to room temperature along with the furnace.
In the step (1), copper atoms of the nitrogen-doped carbon fiber material loaded with the monatomic copper catalyst are uniformly dispersed on a nitrogen-doped carbon fiber substrate, and the copper content is 1-5 wt%.
In the step (2), the mass ratio of the sulfur powder, the multi-wall carbon nano tubes and the carbon black is (70-90): (5-15): (5-15); the solvent is absolute ethyl alcohol or N-methyl pyrrolidone.
In the step (2), the proportioned sulfur powder, the multi-wall carbon nano-tubes and the carbon black are mixed and ground for 30-120 minutes, then the mixture is added into a proper amount of solvent, and ultrasonic dispersion is carried out for 30-120 minutes; the ratio of the total amount of the sulfur powder, the multi-wall carbon nano-tubes and the carbon black added into the solvent to the solvent is 5-30 g: 1L of the compound.
In the step (2), the nitrogen-doped carbon fiber foam loaded with the monatomic copper catalyst is immersed in the suspension for 1 to 5 minutes.
And (2) drying the nitrogen-doped carbon fiber foam loaded with the monoatomic copper catalyst, which is taken out after being soaked in the suspension, in an oven at the temperature of 40-80 ℃ for 24-48 hours.
The monoatomic copper catalyst/nitrogen-doped carbon fiber/sulfur composite positive electrode material is used as a positive electrode material of a lithium-sulfur battery, and the sulfur capacity of the unit area of the positive electrode material of the lithium-sulfur battery is 3-15mg/cm2。
The design principle of the invention is as follows:
the method comprises the steps of firstly carrying out high-temperature carbonization treatment on natural cotton under the inert gas protective atmosphere to form carbon fiber foam. And then carrying out ammonia gas treatment at the same temperature, doping nitrogen atoms on the carbon fiber substrate, and simultaneously capturing and transferring copper atoms on the foamy copper to the nitrogen-doped carbon fiber substrate by ammonia molecules, thereby preparing the nitrogen-doped carbon fiber foam loaded with the monatomic copper catalyst. Mixing sulfur powder and a small amount of conductive additives (multi-walled carbon nanotubes and carbon black), adding the mixture into a proper amount of solvent to obtain an active substance suspension, and performing impregnation treatment to obtain the monatomic copper catalyst/nitrogen-doped carbon fiber/sulfur composite positive electrode material.
The self-supporting carbon fiber foam matrix helps to achieve high sulfur loading per unit area; the surface doped nitrogen atom can adsorb lithium polysulfide and inhibit shuttle effect; the uniformly dispersed monoatomic copper can catalyze the conversion reaction of the sulfur anode, reduce the polarization of the battery and improve the utilization rate of active substances. Compared with a nano copper particle catalyst, the monoatomic copper catalyst has more remarkable catalytic performance while reducing inactive substance components. By integrating the advantages of the components of the monatomic copper/nitrogen-doped carbon fiber foam/sulfur composite cathode material, the assembled lithium-sulfur battery realizes excellent electrochemical capacity exertion and cycle performance.
The invention has the following advantages and beneficial effects:
1. the monoatomic copper/nitrogen-doped carbon fiber foam/sulfur self-supporting composite anode prepared by the method can realize high sulfur loading.
2. The monoatomic copper catalyst prepared by the invention can accelerate the conversion process of the positive electrode sulfur, reduce the polarization of the battery and enable the lithium sulfur battery to have higher capacity and cycle performance.
3. The monatomic copper/nitrogen-doped carbon fiber foam/sulfur self-supporting composite anode prepared by the method has low catalyst content, and a binder and an aluminum foil current collector are not needed in the preparation of the electrode, so that the high energy density of the battery is realized.
4. The preparation process is simple, the raw materials required by the preparation are wide in source and low in cost, and the preparation method can be used for large-scale production.
Description of the drawings:
FIG. 1 is a graph of the electrochemical performance of a carbon fiber/sulfur (CNF/S) electrode; in the figure: (a) a first-turn charge-discharge curve at a current density of 0.1C; (b) specific capacity versus number of cycles and coulombic efficiency versus number of cycles at 0.1C current density.
FIG. 2 is a graph of the electrochemical performance of a nitrogen doped carbon fiber/sulfur (NCNF/S) electrode; in the figure: (a) a first-turn charge-discharge curve at a current density of 0.1C; (b) specific capacity versus number of cycles and coulombic efficiency versus number of cycles at 0.1C current density.
FIG. 3 is a schematic diagram of high temperature processing and temperature profiles for preparing nano copper particles/nitrogen-doped carbon fibers (NP-Cu/NCNF) and monoatomic copper/nitrogen-doped carbon fibers (SA-Cu/NCNF); wherein: (a) high temperature processing schematic; (b) preparing a temperature curve of NP-Cu/NCNF; (c) temperature profiles for SA-Cu/NCNF were prepared.
FIG. 4 is a transmission electron micrograph of NP-Cu/NCNF.
FIG. 5 is a graph of electrochemical performance of a nano-copper particle/nitrogen doped carbon fiber/sulfur (NP-Cu/NCNF/S) electrode; in the figure: (a) a first-turn charge-discharge curve at a current density of 0.1C; (b) specific capacity versus number of cycles and coulombic efficiency versus number of cycles at 0.1C current density.
FIG. 6 is a scanning transmission electron micrograph of SA-Cu/NCNF; wherein: (a) scanning a transmission electron microscope image; (b) and X-ray energy scattering spectra of corresponding carbon, oxygen, nitrogen and copper elements.
FIG. 7 is a graph of the electrochemical performance of a single atom copper/nitrogen doped carbon fiber/sulfur (SA-Cu/NCNF/S) electrode; in the figure: (a) a first-turn charge-discharge curve at a current density of 0.1C; (b) specific capacity versus number of cycles and coulombic efficiency versus number of cycles at 0.1C current density.
The specific implementation mode is as follows:
the invention is illustrated below with reference to comparative examples and examples, but the content of the patent protection is not limited to the following examples.
Comparative example 1
Comparative example 1 is the preparation of carbon fiber (CNF) and its use in a positive electrode of a lithium sulfur battery. Cleaning natural cotton with 10% dilute hydrochloric acid (about 3.1mol/L), distilled water and ethanol, and oven-drying the cleaned cotton in an oven at 60 deg.CDry for 24 hours. And (3) putting the dried cotton into a tube furnace, keeping the temperature at 25 ℃ for 30 minutes in an argon atmosphere, heating to 1000 ℃ at the speed of 5 ℃/minute, keeping the temperature for 2 hours, and cooling to room temperature along with the furnace to obtain the CNF foam. 450mg of sulfur powder, 25mg of multi-walled carbon nanotubes and 25mg of carbon black are mixed and ground in a mortar for 30 minutes, added into 35mL of absolute ethanol, and ultrasonically dispersed for 1 hour to obtain an active substance suspension. And (3) immersing the CNF foam into the suspension for 1 minute, taking out, and drying in an oven at 60 ℃ for 24 hours to obtain the CNF/S composite cathode material. The sulfur loading per unit area of CNF/S obtained by weighing was 6mg/cm2。
2032 button cells are used for the electrochemical performance test of the electrode material. The CNF/S foam was cut to a size of 5mm in length, 5mm in width, 2mm in height as a working electrode, a lithium plate (diameter 16mm, thickness 0.45mm) as a counter electrode, and a Celgard 2400 polypropylene film (diameter 19mm, thickness 25 μm) as a separator. The electrolyte is an electrolyte containing 1M lithium bistrifluoromethanesulfonylimide (LiTFSI) and 0.2M lithium nitrate (LiNO)3) The additive is a mixed solution of 1,3 Dioxolane (DOL) and ethylene glycol dimethyl ether (DME) (volume ratio is 1: 1). During discharge test, the potential interval is 1.8-2.8V (vs. Li/Li)+). As shown in fig. 1(a), the first-turn discharge capacity of the CNF/S electrode at a current density of 0.1C (1C 1675mA/g) is 950mAh/g, and the polarization voltage, i.e., the difference between the charging voltage plateau and the discharging voltage plateau, is large, and is about 0.3V. As shown in fig. 1(b), after 50 cycles, the CNF/S electrode maintains a specific capacity of 773mAh/g, the capacity retention rate is 75%, and the capacity decays rapidly.
Comparative example 2
Comparative example 2 is the preparation of nitrogen doped carbon fiber (NCNF) and its application in a positive electrode of a lithium sulfur battery. The natural cotton is washed by 10% dilute hydrochloric acid, distilled water and ethanol in sequence, and the washed cotton is put into an oven at 60 ℃ to be dried for 24 hours. And (3) putting the dried cotton into a tube furnace, keeping the temperature at 25 ℃ for 30 minutes in an argon atmosphere, heating to 1000 ℃ at the speed of 5 ℃/minute, keeping the temperature for 2 hours, switching to an ammonia atmosphere, keeping the temperature for 1 hour, and cooling to room temperature along with the furnace in the argon atmosphere to obtain the NCNF foam. 450mg of sulfur powder, 25mg of multi-walled carbon nanotubes and 25mg of carbon black are mixed and ground in a mortar for 30 minutes, and the mixture is added into 35mL of anhydrous ethyl acetateIn alcohol, ultrasonic dispersion for 1 hour gave a suspension of the active substance. And immersing the NCNF foam into the suspension for 1 minute, taking out, and drying in an oven at 60 ℃ for 24 hours to obtain the NCNF/S composite cathode material. The sulfur loading per unit area of NCNF/S was measured to be 4.3mg/cm2。
2032 button cells are used for the electrochemical performance test of the electrode material. NCNF/S foam was cut into a size of 5mm in length, 5mm in width, 2mm in height as a working electrode, a lithium plate (16 mm in diameter, 0.45mm in thickness) as a counter electrode, and a Celgard 2400 polypropylene membrane (19 mm in diameter, 25 μm in thickness) as a separator. The electrolyte is an electrolyte containing 1M LiTFSI and 0.2M LiNO3And DOL of additive and DME (volume ratio is 1: 1). During discharge test, the potential interval is 1.8-2.8V (vs. Li/Li)+). As shown in fig. 2(a), the first-turn discharge capacity of the NCNF/S electrode at 0.1C current density was 1073mAh/g, and the polarization voltage, i.e., the difference between the charging voltage plateau and the discharging voltage plateau, was about 0.32V. The adsorption effect of the doped nitrogen atoms on the lithium polysulfide on the carbon fibers inhibits the shuttle effect of the lithium polysulfide, so that the specific capacity of the electrode is improved. As shown in FIG. 2(b), the specific capacity of 920mAh/g was maintained after the NCNF/S electrode was cycled for 50 cycles, and the capacity retention rate was 81%.
Comparative example 3
Comparative example 3 is the preparation of NP-Cu/NCNF and its use in a positive electrode of a lithium sulfur battery. The natural cotton is washed by 10% dilute hydrochloric acid, distilled water and ethanol in sequence, and the washed cotton is put into an oven at 60 ℃ to be dried for 24 hours. As shown in fig. 3(a), the dried cotton was placed in a tube furnace together with copper foam. As shown in fig. 3(b), the temperature is maintained at 25 ℃ for 30 minutes under the argon atmosphere, the temperature is raised to 1000 ℃ at the rate of 5 ℃/minute, the temperature is maintained for 2 hours, the ammonia atmosphere is switched to, after the temperature is maintained for 3 hours, the NP-Cu/NCNF foam is obtained by furnace cooling to the room temperature under the argon atmosphere. As shown in fig. 4, the carbon fiber matrix is loaded with dispersed nano-copper particles. The copper content was 0.32 at% (atomic%) as measured by X-ray photoelectron spectroscopy.
Mixing 450mg of sulfur powder, 25mg of multi-walled carbon nano-tube and 25mg of carbon black in a mortar, grinding for 30 minutes, adding into 35mL of absolute ethyl alcohol, and performing ultrasonic dispersion for 1 hour to obtain an active substance suspension. And (3) immersing the NP-Cu/NCNF foam into the suspension for 1 minute, taking out, and drying in an oven at 60 ℃ for 24 hours to obtain the NP-Cu/NCNF/S composite positive electrode material. The sulfur-carrying amount per unit area of NP-Cu/NCNF/S obtained by weighing was 5mg/cm2。
2032 button cells are used for the electrochemical performance test of the electrode material. NP-Cu/NCNF/S foam was cut into a size of 5mm in length, 5mm in width, 2mm in height as a working electrode, a lithium plate (16 mm in diameter and 0.45mm in thickness) as a counter electrode, and a Celgard 2400 polypropylene membrane (19 mm in diameter and 25 μm in thickness) as a separator. The electrolyte contains 1mol/L LiTFSI electrolyte and 0.2mol/L LiNO3And DOL of additive and DME (volume ratio is 1: 1). During discharge test, the potential interval is 1.8-2.8V (vs. Li/Li)+). As shown in FIG. 5(a), the first discharge capacity of the NP-Cu/NCNF/S electrode at 0.1C current density is 1062mAh/g, and the polarization voltage, i.e., the difference between the charging voltage plateau and the discharging voltage plateau, is about 0.24V. The adsorption effect of the nano copper particles loaded on the carbon fibers on lithium polysulfide and the catalytic effect on the sulfur species conversion reaction improve the utilization rate and specific capacity of active substances of the electrode and reduce the polarization of the battery. As shown in fig. 5(b), after 50 cycles, the electrode maintains a specific capacity of 994mAh/g, the capacity retention rate is 87%, and the catalytic action of the nano-copper particles on the electrode reaction reduces the irreversible consumption of lithium polysulfide and improves the cycle performance.
Example 1
Example 1 is the preparation of SA-Cu/NCNF and its use in a positive electrode of a lithium sulfur battery. The natural cotton is washed by 10% dilute hydrochloric acid, distilled water and ethanol in sequence, and the washed cotton is put into an oven at 60 ℃ to be dried for 24 hours. As shown in fig. 3(a), the dried cotton was placed in a tube furnace together with copper foam. As shown in fig. 3(c), the temperature is maintained at 25 ℃ for 30 minutes under the argon atmosphere, the temperature is raised to 1000 ℃ at the rate of 5 ℃/minute, the temperature is maintained for 2 hours, the temperature is switched to the ammonia atmosphere, and after the temperature is maintained for 1 hour, the temperature is cooled to the room temperature along with the furnace under the argon atmosphere, so that the SA-Cu/NCNF foam is obtained. As shown in fig. 6, no distinct particles were observed locally in the carbon fiber, but the X-ray energy scattering spectrum showed a uniformly dispersed copper signal, with copper uniformly dispersed in a monoatomic form on the nitrogen-doped carbon fiber matrix. The copper content was 0.25 at% (atomic%) as measured by X-ray photoelectron spectroscopy.
450mg of sulfur powder, 25mg of multi-walled carbon nanotubes and 25mg of carbon black are mixed and ground in a mortar for 30 minutes, added into 35mL of absolute ethanol, and ultrasonically dispersed for 1 hour to obtain an active substance suspension. And (3) immersing the SA-Cu/NCNF foam into the suspension for 1 minute, taking out, and drying in an oven at 60 ℃ for 24 hours to obtain the SA-Cu/NCNF/S composite cathode material. The sulfur loading per unit area of SA-Cu/NCNF/S is 5mg/cm by weighing2。
2032 button cells are used for the electrochemical performance test of the electrode material. The SA-Cu/NCNF/S foam is cut into a size of 5mm in length, 5mm in width and 2mm in height to serve as a working electrode, a lithium sheet (16 mm in diameter and 0.45mm in thickness) serves as a counter electrode, and a Celgard 2400 polypropylene membrane (19 mm in diameter and 25 mu m in thickness) serves as a diaphragm. The electrolyte contains 1mol/L LiTFSI electrolyte and 0.2mol/L LiNO3And DOL of additive and DME (volume ratio is 1: 1). During discharge test, the potential interval is 1.8-2.8V (vs. Li/Li)+). As shown in FIG. 7(a), the first-turn discharge capacity of the SA-Cu/NCNF/S electrode at the current density of 0.1C is 1312mAh/g, and the polarization voltage, namely the difference between the charging voltage platform and the discharging voltage platform, is only 0.12V. Compared with CNF/S, NCNF/S and NP-Cu/NCNF/S electrodes, the SA-Cu/NCNF/S electrode has the fastest electrochemical dynamics, the lowest cell polarization and the highest capacity exertion, which is attributed to the remarkable catalytic action of the monoatomic copper catalyst on the sulfur anode conversion reaction. As shown in fig. 7(b), the electrode maintained a specific capacity of 1254mAh/g after 50 cycles, had a capacity retention of 88%, and had excellent cycling stability.
Therefore, based on the above description, the present invention provides a method for preparing a monatomic copper catalyst, which can effectively improve the electrochemical kinetics of a positive electrode of a lithium-sulfur battery. The prepared monoatomic copper/nitrogen-doped carbon fiber foam/sulfur composite cathode material has high capacity exertion and cycle performance under high sulfur-carrying capacity. The preparation method is simple, the raw materials are cheap, the expanded production is facilitated, and the method has a wide commercial prospect.
Furthermore, the above-described embodiments are merely illustrative descriptions of the present patent and are not to be construed as limitations of the present patent. Any improvements and modifications that may be made based on the principles and techniques of this patent are intended to be covered by this patent.

Claims (10)

1. A preparation method of a monoatomic copper catalyst is characterized by comprising the following steps: the method comprises the following steps:
(1) sequentially cleaning and drying natural cotton, putting the dried cotton and foamy copper into a tubular furnace, and sequentially performing high-temperature treatment in a protective atmosphere and an ammonia atmosphere to obtain nitrogen-doped carbon fiber foam loaded with a monatomic copper catalyst;
(2) uniformly mixing sulfur powder, multi-walled carbon nanotubes and carbon black in proportion, adding the mixture into a proper amount of solvent, and performing mixing grinding and ultrasonic dispersion to obtain an active substance suspension; and (3) immersing the nitrogen-doped carbon fiber foam loaded with the monatomic copper catalyst into the suspension for a period of time, then taking out the nitrogen-doped carbon fiber foam, and drying the nitrogen-doped carbon fiber foam to obtain the monatomic copper catalyst/nitrogen-doped carbon fiber/sulfur composite anode material.
2. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (1), the natural cotton is cleaned to remove impurities, and the cleaning process is as follows: soaking natural cotton in 0.5-5mol/L dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, magnetically stirring for 30-60 min, taking out, soaking in distilled water, magnetically stirring for 1-2 hr, and finally soaking in anhydrous ethanol, magnetically stirring for 1-2 hr, and taking out; and (3) drying the cleaned cotton in an oven at the temperature of 40-80 ℃ for 24-48 hours.
3. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (1), the cotton and the foamy copper which are put into the tube furnace are separately placed along the axial direction of a tube body of the tube furnace at a mass ratio of 1:5 to 1:20, wherein the foamy copper is close to the air inlet side of the tube furnace.
4. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (1), the high-temperature treatment process comprises the following steps: heating the tube furnace to 900-1100 ℃ in the protective atmosphere of argon or nitrogen, wherein the heating rate is 5-20 ℃/min, and keeping the temperature for 1-2 hours; then switching to ammonia atmosphere, preserving the heat for 30-90 minutes, and then cooling to room temperature along with the furnace.
5. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (1), copper atoms of the nitrogen-doped carbon fiber material loaded with the monatomic copper catalyst are uniformly dispersed on a nitrogen-doped carbon fiber substrate, and the copper content is 1-5 wt%.
6. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (2), the mass ratio of the sulfur powder, the multi-wall carbon nano tubes and the carbon black is (70-90): (5-15): (5-15); the solvent is absolute ethyl alcohol or N-methyl pyrrolidone.
7. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (2), the proportioned sulfur powder, the multi-wall carbon nano-tubes and the carbon black are mixed and ground for 30-120 minutes, then the mixture is added into a proper amount of solvent, and ultrasonic dispersion is carried out for 30-120 minutes; the ratio of the total amount of the sulfur powder, the multi-wall carbon nano-tubes and the carbon black added into the solvent to the solvent is (5-30) g: 1L of the compound.
8. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: in the step (2), the nitrogen-doped carbon fiber foam loaded with the monatomic copper catalyst is immersed in the suspension for 1 to 5 minutes.
9. The method for preparing a monatomic copper catalyst according to claim 1, characterized in that: and (2) drying the nitrogen-doped carbon fiber foam loaded with the monoatomic copper catalyst, which is taken out after being soaked in the suspension, in an oven at the temperature of 40-80 ℃ for 24-48 hours.
10. Monoatomic copper prepared by the method of claim 1The application of the catalyst in the positive electrode of the lithium-sulfur battery is characterized in that: the monoatomic copper catalyst/nitrogen-doped carbon fiber/sulfur composite positive electrode material is used as a positive electrode material of a lithium-sulfur battery, and the sulfur capacity of the unit area of the positive electrode material of the lithium-sulfur battery is 3-15mg/cm2。
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