Monatomic-loaded hollow porous carbon spheres and applications thereof

Abstract

The application belongs to the technical field of carbon material synthesis, and discloses single-atom supported hollow porous carbon spheres and application thereof. The preparation method provided by the application is as follows: sodium alginate, an organic ligand, a template and a solvent are mixed and uniformly heated and stirred, and then spray drying is performed to obtain spherical precursors; the spherical precursors are soaked in a transition metal salt solution for crosslinking, and then the crosslinked precursors are carbonized and acid washed to obtain single-atom supported hollow porous carbon spheres. When the single-atom supported hollow porous carbon spheres of the application are used as a carrier of a sulfur positive electrode material of a room-temperature sodium-sulfur battery, the interface impedance of the sulfur positive electrode can be effectively reduced through the synergistic effect of the porous structure of the carbon carrier and the single-atom catalytic sites, the capacity loss caused by the polysulfide shuttle effect can be significantly inhibited, the conversion process of sulfur and polysulfides can be accelerated, and the reversible capacity and cycle stability of the room-temperature sodium-sulfur battery can be greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon material synthesis, in particular to single-atom loaded hollow porous carbon spheres and application thereof. BACKGROUND

[0002] The continuous consumption of traditional non-renewable energy sources leads to a large amount of carbon emissions, resulting in global carbon cycle imbalance, which has become a global focus in recent years. Under this background, the development of green and clean energy storage system is of great significance for building a clean and efficient energy system. At present, although lithium ion batteries are the most widely used in the field of energy storage, their development is limited by the uneven distribution and relatively limited reserves of lithium resources, making it difficult to meet the needs of large-scale energy storage scenarios. Therefore, the development of sodium ion batteries based on abundant and widely distributed sodium resources as an important supplement to lithium ion batteries has become a research hotspot in the field of energy storage technology.

[0003] Among the many positive electrode materials of sodium ion batteries, sulfur positive electrode has an ultra-high theoretical specific capacity of 1675 mAh / g, and the sodium metal negative electrode (natural abundance of sodium ≈0.36%) has resource advantages, making sodium-sulfur batteries one of the most promising energy storage technologies. However, there are three major problems with sulfur positive electrodes, which restrict their application in sodium-sulfur batteries: first, the electronic and ionic conductivity of sulfur itself is extremely low, resulting in slow electrode reaction kinetics; second, the conversion efficiency of polysulfides is low during charging and discharging, affecting the sodium storage activity; third, the "shuttle effect" of polysulfides will cause rapid capacity decay. These problems make it necessary for sulfur positive electrodes to be combined with carbon materials to break through the performance bottleneck.

[0004] Under this background, the development of carbon carriers for sulfur positive electrodes has become a key link in improving the performance of sodium-sulfur batteries. Among them, biomass-derived hollow porous carbon spheres have unique application potential in the field of sulfur carriers due to their structural advantages of high specific surface area, which can effectively disperse active sulfur, avoid sulfur agglomeration, and accelerate the transmission rate of ions in the electrolyte, thereby improving the conversion efficiency of polysulfides.

[0005] At the same time, single-atom catalysts, as a promising type of catalytic material, not only have excellent catalytic activity and kinetic efficiency, but also have a negligible impact on the overall energy density of the battery due to their extremely low mass proportion. Based on this, by combining the dispersion advantages of biomass-derived hollow porous carbon spheres with the catalytic properties of single-atom catalysts, a "hollow porous carbon sphere-single atom catalysis" synergistic system can be constructed to address the core problems of low electronic conductivity, poor polysulfide conversion efficiency, and shuttle effect of sulfur positive electrodes, providing an ideal technical solution for improving the overall performance of room temperature sodium-sulfur batteries. SUMMARY

[0006] To solve the problems existing in the prior art, the present application provides single-atom loaded hollow porous carbon spheres and applications thereof, and the purposes include:

[0007] One of the purposes is to provide single-atom loaded hollow porous carbon spheres. The carbon spheres are derived from biomass as raw materials, and have super-high specific surface area and rich porosity due to the hollow porous structure, and a large number of efficient chemical catalytic sites are formed through the loading of single atoms, and the carbon spheres also have high electronic conductivity.

[0008] The second purpose is to provide a preparation method of the single-atom loaded hollow porous carbon spheres. The method uses biomass such as sodium alginate as a basic raw material, combines spray drying, crosslinking, carbonization and acid washing processes, and has a simple and efficient operation process, simple and easily obtained equipment, a green, non-toxic and safe preparation process, and can adapt to the loading requirements of different transition metal single atoms, and has strong universality.

[0009] The third purpose is to provide a room temperature sodium-sulfur battery based on the single-atom loaded hollow porous carbon spheres. The room temperature sodium-sulfur battery uses the single-atom loaded hollow porous carbon spheres as a sulfur positive electrode material carrier, realizes uniform dispersion of sulfur and rapid ion transmission through the hollow porous structure of the carbon spheres, accelerates the conversion of sulfur and polysulfides through the catalytic action of single atoms, can significantly reduce the impedance of the sulfur positive electrode, inhibit the capacity loss caused by the polysulfide shuttle effect, and ultimately greatly improve the reversible capacity and cycle life of the battery.

[0010] The preparation method of the single-atom loaded hollow porous carbon spheres provided by the present application comprises the following steps:

[0011] Step 1, mixing sodium alginate, an organic ligand, a template and a solvent and uniformly heating and stirring to obtain a mixed solution;

[0012] Step 2, spray drying the mixed solution obtained in step 1 to obtain a spherical precursor;

[0013] Step 3, immersing the spherical precursor obtained in step 2 in a transition metal salt solution, crosslinking, then centrifuging and washing and drying to obtain a sodium alginate crosslinked spherical precursor;

[0014] Step 4, carbonizing and acid washing the sodium alginate crosslinked spherical precursor to obtain single-atom loaded hollow porous carbon spheres.

[0015] As a preferred scheme, in step 1, the organic ligand is at least one of urea, thiourea, polypyrrole, dicyandiamide, polyaniline and tris(hydroxymethyl) phosphine, the template is at least one of sodium chloride, magnesium chloride, potassium chloride and lithium chloride, and the solvent is water or a mixture of water, ethanol and ethylene glycol.

[0016] As a preferred solution, in step 1, the mass ratio of sodium alginate, organic ligand, template and solvent is 1: (1-3): (5-20): (50-150).

[0017] As a preferred solution, in step 2, the parameters of the spray drying are as follows: the propelling speed of the peristaltic pump is 2-10 mL / min; the temperature of the air inlet is 150-200℃, and the temperature of the air outlet is 200-240℃.

[0018] As a preferred solution, in step 3, the solvent in the transition metal salt solution is a mixture of ethanol and water (the mass ratio of ethanol to water can be 1:9, 2:8 or 3:7), and the transition metal salt is at least one of ferric chloride, cobalt chloride, nickel chloride, copper chloride, zinc chloride, ferric nitrate, cobalt nitrate, nickel nitrate, copper nitrate and zinc nitrate; the concentration of the transition metal salt is 1wt%-10wt%.

[0019] As a preferred solution, in step 3, the crosslinking time is 5-30 minutes.

[0020] As a preferred solution, in step 4, the carbonization process is as follows: under the protection of an inert atmosphere (preferably nitrogen or argon), the temperature is raised to 450℃-500℃ at a temperature raising rate of 2-10℃ / min, and the temperature is kept for 1-3h, then the temperature is continuously raised to 800-900℃ at a temperature raising rate of 2-10℃ / min, and the temperature is kept for 1-3h, and then the temperature is naturally lowered.

[0021] As a preferred solution, in step 4, the pickling process is as follows: the carbonized material is soaked in an acid solution with a concentration of 1-3mol / L at 60-90℃ for 8-12h, and then taken out and dried. The acid solution is a sulfuric acid solution or a hydrochloric acid solution.

[0022] The single-atom loaded hollow porous carbon sphere prepared by the method can be used in a room-temperature sodium-sulfur battery, and based on this, the application further provides a room-temperature sodium-sulfur battery, which comprises a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, and the sulfur positive electrode material of the positive electrode sheet is prepared by using the single-atom loaded hollow porous carbon sphere as a sulfur carrier, mixing with elemental sulfur and then performing melt sulfur infiltration.

[0023] Compared with the prior art, the application has the following beneficial effects:

[0024] 1. The biomass-derived hollow porous carbon sphere provided by the application not only has an ultrahigh specific surface area and rich porosity, can efficiently disperse active substances and accelerate ion transmission, but also has high electronic conductivity, which can ensure rapid charge migration; at the same time, the carbon carrier forms a large number of efficient chemical catalytic sites through single-atom loading, realizes the synergistic effect of "structure support-electron transmission-catalytic reaction", and effectively breaks through the limitation that the existing carbon carrier is single in function and difficult to adapt to the demand of high-performance materials.

[0025] 2. The application provides a preparation method of single-atom loaded hollow porous carbon spheres, which is simple and efficient in operation process, simple and easy to configure in required equipment, safe and environmentally friendly in the preparation process, and suitable for the loading requirements of various transition metal single atoms, and has high universality.

[0026] 3. When the single-atom loaded hollow porous carbon spheres are used as a room-temperature sodium-sulfur battery sulfur positive electrode material carrier, the interface impedance of the sulfur positive electrode can be effectively reduced through the synergistic effect of the porous structure of the carbon carrier and the single-atom catalytic site, the capacity loss caused by the polysulfide shuttle effect can be significantly inhibited, the conversion process of sulfur and polysulfide can be accelerated, the reversible capacity and cycle stability of the room-temperature sodium-sulfur battery are greatly improved, and key technical support is provided for breaking through the performance bottleneck of the room-temperature sodium-sulfur battery and promoting practical application. BRIEF DESCRIPTION OF DRAWINGS

[0027] The application will be further described below in combination with the drawings and examples, in which:

[0028] Figure 1 A scanning electron microscope (SEM) image of a spray-formed spherical precursor provided for Example 1 of the application;

[0029] Figure 2 An SEM image of cobalt single-atom loaded hollow porous carbon spheres provided for Example 1 of the application;

[0030] Figure 3 A sorption-desorption curve of the cobalt single-atom loaded hollow porous carbon spheres provided for Example 1 of the application, wherein P is the gas pressure at the adsorption equilibrium, and P0 is the saturated vapor pressure of the adsorbate at the adsorption temperature;

[0031] Figure 4 An X-ray diffraction (XRD) pattern of the cobalt single-atom loaded hollow porous carbon spheres provided for Example 1 of the application;

[0032] Figure 5 A transmission electron microscope (TEM) image of the cobalt single-atom loaded hollow porous carbon spheres provided for Example 1 of the application;

[0033] Figure 6 A high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of the cobalt single-atom loaded hollow porous carbon spheres provided for Example 1 of the application;

[0034] Figure 7 A cycle performance diagram of the cobalt single-atom-carrier / sulfur composite electrode material provided for Example 1 of the application as a room-temperature sodium-sulfur secondary battery positive electrode material at a current density of 0.2 A / g;

[0035] Figure 8Rate performance chart of the cobalt monatomic-carbon support / sulfur composite electrode material provided for the embodiment 1 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery;

[0036] Figure 9 Electrochemical impedance spectroscopy (EIS) chart of the cobalt monatomic-carbon support / sulfur composite electrode material provided for the embodiment 1 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery, wherein Z' represents the real part of impedance and Z" represents the imaginary part of impedance;

[0037] Figure 10 XRD chart of the iron monatomic supported hollow porous carbon sphere provided for the embodiment 2 of the present application;

[0038] Figure 11 Cycle performance chart of the iron monatomic-carbon support / sulfur composite electrode material provided for the embodiment 2 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery under a current density of 0.2 A / g;

[0039] Figure 12 EIS chart of the iron monatomic-carbon support / sulfur composite electrode material provided for the embodiment 2 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery;

[0040] Figure 13 Cycle performance chart of the nickel monatomic-carbon support / sulfur composite electrode material provided for the embodiment 3 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery under a current density of 0.2 A / g;

[0041] Figure 14 EIS chart of the nickel monatomic-carbon support / sulfur composite electrode material provided for the embodiment 3 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery;

[0042] Figure 15 Rate performance chart of the carbon support / sulfur composite electrode material without monatomic support provided for the embodiment 4 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery;

[0043] Figure 16 EIS chart of the carbon support / sulfur composite electrode material without monatomic support provided for the embodiment 4 of the present application as a positive electrode material of a room temperature sodium-sulfur secondary battery. DETAILED DESCRIPTION

[0044] The concept and the technical effects of the present application will be described clearly and completely in combination with the drawings and the embodiments, so as to fully understand the purpose, features and effects of the present application. Obviously, the described embodiments are only some of the embodiments of the present application but not all the embodiments, and other embodiments obtained by those skilled in the art based on the embodiments of the present application without creative labor are within the protection scope of the present application.

[0045] The application provides a preparation method of single-atom loaded hollow porous carbon spheres, comprising the following steps:

[0046] Step 1, sodium alginate, organic ligand, template and solvent are mixed and heated and stirred uniformly to obtain a mixed solution;

[0047] Step 2, the mixed solution obtained in step 1 is spray dried to obtain a spherical precursor;

[0048] Step 3, the spherical precursor obtained in step 2 is soaked in a transition metal salt solution, crosslinked, then centrifuged, washed and dried to obtain a sodium alginate crosslinked spherical precursor;

[0049] Step 4, the sodium alginate crosslinked spherical precursor is carbonized and acid washed to obtain single-atom loaded hollow porous carbon spheres.

[0050] In some embodiments of the application, in step 1, the weight average molecular weight (Mw) of the sodium alginate is preferably 2000-20000, and specifically can be 2000, 5000, 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000 or 20000.

[0051] In some embodiments of the application, in step 1, the organic ligand is preferably at least one of urea, thiourea, polypyrrole, dicyandiamide, polyaniline and tris(hydroxymethyl) phosphine; and the mass ratio of the sodium alginate to the organic ligand is preferably 1:(1-3), and specifically can be 1:1, 1:1.5, 1:2, 1:2.5 or 1:3.

[0052] In some embodiments of the application, in step 1, the template is at least one of sodium chloride, magnesium chloride, potassium chloride and lithium chloride, and the mass ratio of the sodium alginate to the template is preferably 1:(5-20), and specifically can be 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19 or 1:20.

[0053] In some embodiments of the application, in step 1, the solvent is water, or a mixture of water and at least one of ethanol and ethylene glycol, and the mass ratio of the sodium alginate to the solvent is preferably 1:(50-150), and specifically can be 1:50, 1:60, 1:70, 1:80, 1:90, 1:100, 1:110, 1:120, 1:130, 1:140 or 1:150.

[0054] In some embodiments of the present application, in step 1, the rotation speed of the heating stirring is preferably 300-700 rpm, and can be specifically 300 rpm, 350 rpm, 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm, 650 rpm or 700 rpm; the temperature of the heating stirring is preferably 60-90℃, and can be specifically 60℃, 65℃, 70℃, 75℃, 80℃, 85℃ or 90℃; and the time of the heating stirring is preferably 6-24 h, and can be specifically 6 h, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, 22 h or 24 h.

[0055] In some embodiments of the present application, in step 2, the parameters of the spray drying are as follows: the propelling speed of the peristaltic pump is preferably 2-10 mL / min, and can be specifically 2 mL / min, 3 mL / min, 4 mL / min, 5 mL / min, 6 mL / min, 7 mL / min, 8 mL / min, 9 mL / min or 10 mL / min; the inlet temperature is preferably 150-200℃, and can be specifically 150℃, 160℃, 170℃, 180℃, 190℃ or 200℃; and the outlet temperature is preferably 200-240℃, and can be specifically 200℃, 205℃, 210℃, 215℃, 220℃, 225℃, 230℃, 235℃ or 240℃.

[0056] In some embodiments of the present application, in step 3, the solvent in the transition metal salt solution is a mixture of ethanol and water, and the mass ratio of ethanol to water is preferably 1:9, 2:8 or 3:7. The transition metal salt is at least one of ferric chloride, cobalt chloride, nickel chloride, copper chloride, zinc chloride, ferric nitrate, cobalt nitrate, nickel nitrate, copper nitrate and zinc nitrate, and the concentration of the transition metal salt is preferably 1wt%-10wt%, and can be specifically 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%.

[0057] In some embodiments of the present application, in step 3, the crosslinking time is preferably 5-30 min, and can be specifically 5 min, 7.5 min, 10 min, 12.5 min, 15 min, 17.5 min, 20 min, 22.5 min, 25 min, 27.5 min or 30 min.

[0058] In some embodiments of the present application, in step 3, the centrifugal rotation speed is preferably 1000-4000 rpm, and can be specifically 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm or 4000 rpm.

[0059] In some embodiments of the present application, in step 4, the carbonization comprises two stages.

[0060] The first stage temperature is preferably 450-500℃, and can be specifically 450℃, 460℃, 470℃, 480℃, 490℃ or 500℃; the heating rate is preferably 2-10℃ / min, and can be specifically 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min. The holding time is preferably 1-3h, and can be specifically 1h, 1.5h, 2h, 2.5h or 3h.

[0061] The second stage temperature is preferably 800-900℃, and can be specifically 800℃, 820℃, 840℃, 860℃, 880℃ or 900℃; the heating rate is preferably 2-10℃ / min, and can be specifically 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min or 10℃ / min. The holding time is preferably 1-3h, and can be specifically 1h, 1.5h, 2h, 2.5h or 3h.

[0062] In some embodiments of the present application, in step 4, the acid solution used in the pickling process is preferably a hydrochloric acid solution or a sulfuric acid solution; the concentration of the acid solution is preferably 1-3mol / L, and can be specifically 1mol / L, 2mol / L or 3mol / L; the temperature of the pickling process is preferably 60-90℃, and can be specifically 60℃, 70℃, 80℃ or 90℃; the time of the pickling process is preferably 8-12h, and can be specifically 8h, 9h, 10h, 11h or 12h.

[0063] The present application also provides a room-temperature sodium-sulfur battery, comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. The positive electrode material of the positive electrode sheet is prepared by mixing a sulfur carrier and elemental sulfur and then performing melt sulfur infiltration; the sulfur carrier is the single-atom supported hollow porous carbon sphere described in the above technical solution.

[0064] In the present application, the electrolyte in the room-temperature sodium-sulfur battery is preferably 1-2mol / L sodium bis(trifluoromethylsulfonyl)imide (NaTFSI) / propylene carbonate (PC)-fluoroethylene carbonate (FEC), and the volume ratio of PC to FEC is preferably 1:1.

[0065] For a clearer understanding of the present application, the following examples are provided for further illustration. The reagents and materials used in the following examples are commercially available.

[0066] Example 1

[0067] The cobalt single atom loaded hollow porous carbon spheres were prepared according to the following steps:

[0068] Step 1, 4 g of sodium alginate (SA, Mw=2000), 12 g of melamine and 40 g of sodium chloride were dissolved in 500 mL of water, and heated and stirred at 60°C at a speed of 600 rpm for 12 h to obtain a uniform transparent mixed solution.

[0069] Step 2, the mixed solution obtained in step 1 was introduced into the nozzle of a spray dryer under heating conditions with a rubber tube, the peristaltic pump was set to a pushing speed of 3 mL / min, the inlet temperature was set to 200°C, and the outlet temperature was set to 240°C, and the powder formed by spraying was collected in a sample collector; after running for 3 h, spherical precursors with a diameter of 2-5 microns were obtained, and the SEM image thereof is shown in Figure 1 .

[0070] Step 3, the spherical precursors obtained in step 2 were soaked in a 2wt% cobalt nitrate hexahydrate ethanol / water solution (the mass ratio of ethanol to water was 3:7) for 5 min. After taking out, it was washed by centrifugation with ethanol at a speed of 3000 rpm, and then the solid powder obtained by centrifugation was placed in a vacuum oven at 60°C for drying for 12 h to obtain sodium alginate cross-linked spherical precursors.

[0071] Step 4, the sodium alginate cross-linked spherical precursors obtained in step 3 were placed in a tube furnace and carbonized with argon as the protective gas: first, the temperature was raised to 450°C at a rate of 5°C / min and kept for 2 h, then the temperature was raised to 800°C at a rate of 5°C / min and kept for 2 h, and finally cooled to room temperature.

[0072] In order to remove the cobalt single atom clusters generated in the carbonization process and obtain a porous structure, the above carbonized precursors were soaked in a 2 mol / L hydrochloric acid aqueous solution at 60°C for 12 h, then washed and separated by centrifugation with ethanol and placed in a vacuum oven at 60°C for drying for 12 h to obtain cobalt single atom loaded hollow porous carbon spheres. The SEM of the cobalt single atom loaded hollow porous carbon spheres prepared in this example is shown in Figure 2 .

[0073] From the SEM characterization results of Figure 2 , it can be seen that the cobalt single atom loaded hollow porous carbon spheres prepared in Example 1 have a diameter of about 5 μm, and the formation of the hollow structure is due to the removal of the sodium chloride template in the preparation process. The removal of the template not only constructs the hollow morphology, but also significantly increases the specific surface area of the carbon carrier. At the same time, from the BET adsorption-desorption curve analysis results of Figure 3 , it can be seen that the hollow porous structure formed by the removal of the template in Example 1 has a large specific surface area of 584.0 square meters per gram. And according to Figure 4The XRD results showed that there was no Co phase in the metal single-atom catalyst in this embodiment.

[0074] Depend on Figure 5 TEM images revealed no obvious metal clusters in the material, preliminarily proving that cobalt exists in single-atom form within the carbon support, thus ruling out the possibility of metal agglomeration causing catalytic site failure. To further verify the dispersion state of cobalt, HAADF-STEM characterization was performed. Figure 6 As shown in the figure, numerous isolated and dispersed bright spots can be clearly observed. These bright spots correspond to individual cobalt atoms, directly confirming the uniform dispersion of cobalt single atoms in the carbon support. The uniformly dispersed single-atom cobalt can provide a large number of catalytic sites required for the conversion of sulfur and polysulfides. Combined with the physical confinement effect of the hollow porous structure of the carbon support on sulfur, a synergistic effect of physical confinement and chemical catalysis can be formed, effectively solving the problems of low conductivity and polysulfide shuttle in sulfur cathodes. Ultimately, this material becomes an excellent sulfur cathode support suitable for high-performance room-temperature sodium-sulfur batteries.

[0075] In this embodiment, a carbon support / sulfur composite cathode was prepared using cobalt single-atom-loaded hollow porous carbon spheres. The specific method was as follows: 40 mg of cobalt single-atom-loaded hollow porous carbon spheres were mixed with 40 mg of sulfur powder and placed in the inner liner of a reaction vessel. The reaction vessel was sealed under an inert atmosphere and heated to 155°C for 12 hours to melt, yielding carbon / sulfur composite cathode powder. Subsequently, this powder was mixed with CMC binder and water at a mass ratio of 7:2:1 to form a slurry, which was then coated onto aluminum foil. After drying in a vacuum oven at 60°C for 12 hours, the slurry was cut into small circular pieces with a diameter of 10 mm for later use.

[0076] Using a sodium metal sheet as the counter electrode, glass fiber as the separator, and 2 mol / L NaTFSI / PC-FEC (PC:FEC volume ratio of 1:1) as the electrolyte, a coin cell was assembled with the prepared positive electrode in an argon-filled glove box. Constant current charge-discharge tests were conducted using a Newway battery testing system at a current density of 0.2 A / g, with a charge-discharge voltage range of 0.8 V to 3.0 V. The test results are as follows: Figure 7 As shown.

[0077] Depend on Figure 7 As can be seen from the cycling performance curve, the composite electrode material exhibits excellent cycling stability: at a current density of 0.2 A / g, after 100 charge-discharge cycles, the electrode still maintains a high reversible capacity of 952.2 mAh / g.

[0078] Rate performance was tested at current densities of 0.2 A / g, 0.5 A / g, 1 A / g, 2 A / g, 5 A / g, and 10 A / g, respectively. The test results are as follows: Figure 8As shown in the figure, under the ultra-high current density of 10A / g, the high reversible capacity of 330.5mAh / g is still maintained.

[0079] The kinetics of the sulfur conversion process promoted by the cobalt monatomic catalyst was tested by EIS, and the test results are as shown in the figure Figure 9 The charge transfer resistance is 87Ω, indicating that the Co monatomic atom can effectively promote the conversion process of sulfur.

[0080] Example 2

[0081] This example is prepared according to the following steps:

[0082] Step 1: Dissolve 4g of sodium alginate (SA, Mw=2000), 12g of melamine and 40g of sodium chloride in 500mL of water, heat and stir at 600rpm for 12h at 60℃, to obtain a uniform transparent mixed solution.

[0083] Step 2: The mixed solution obtained in step 1 is introduced into the nozzle of the spray dryer under heating conditions with a rubber tube, the peristaltic pump is set to 3mL / min, the inlet temperature is set to 200℃, and the outlet temperature is set to 240℃. The powder formed by spraying is collected in a sample collector; after running for 3h, spherical precursors with a diameter of 2-5 microns are obtained.

[0084] Step 3: The spherical precursors obtained in step 2 are soaked in a 2wt% iron nitrate hexahydrate ethanol / water solution (ethanol to water mass ratio of 3:7) for 5min. After taking out, centrifugal cleaning is carried out with ethanol at a speed of 3000rpm, and then the solid powder obtained by centrifugation is placed in a vacuum oven at 60℃ for drying for 12h, to obtain sodium alginate crosslinked spherical precursors.

[0085] Step 4: The sodium alginate crosslinked spherical precursors obtained in step 3 are placed in a tube furnace for carbonization with argon as the protective gas: first, heat to 450℃ at a rate of 5℃ / min and keep for 2h, then heat to 800℃ at a rate of 5℃ / min and keep for 2h, and finally cool to room temperature.

[0086] In order to remove the iron single atom clusters generated in the carbonization process and obtain a porous structure, the above carbonized precursors are soaked in a 2mol / L hydrochloric acid aqueous solution at 60℃ for 12h, then washed, centrifuged and dried in a vacuum oven at 60℃ for 12h to obtain iron monatomic atom loaded hollow porous carbon spheres, and the XRD results are as shown in the figure Figure 10 As can be seen from the figure, there is no metallic phase iron in the material obtained in this example.

[0087] The hollow porous carbon spheres loaded with iron monatomic atoms were used to prepare carbon carrier / sulfur composite positive electrode sheets and assembled into button cells according to the method of Example 1. The constant current charge-discharge test was carried out at a current density of 0.2 A / g under the condition of the WEICell battery test system, and the charge-discharge voltage range was 0.8 V-3.0 V. The test results are shown in Figure 11 .

[0088] As can be seen from the cycle performance curve of Figure 11 , the composite electrode material can maintain a reversible capacity of 870.3 mAh / g after 100 charge-discharge cycles at a current density of 0.2 A / g.

[0089] The kinetics of the sulfur conversion process improved by the iron monatomic atom catalyst was tested by EIS, and the test results are shown in Figure 12 . The charge transfer resistance is 126 Ω, which is increased by 51 Ω compared with the impedance value of Example 1, indicating that the catalytic effect of iron monatomic atoms is reduced compared with cobalt monatomic atoms.

[0090] Example 3

[0091] In this example, the hollow porous carbon spheres loaded with nickel monatomic atoms were prepared according to the following steps:

[0092] Step 1: 4 g of sodium alginate (SA, Mw=2000), 12 g of melamine and 40 g of sodium chloride were dissolved in 500 mL of water, and heated and stirred at 600 rpm at 60°C for 12 h to obtain a uniform transparent mixed solution.

[0093] Step 2: The mixed solution obtained in Step 1 was introduced into the nozzle of a spray dryer under heating conditions using a rubber tube, and the peristaltic pump was set to a pushing speed of 3 mL / min, the inlet air temperature was set to 200°C, and the outlet air temperature was set to 240°C. The powder formed by spraying was collected in a sample collector; after running for 3 h, spherical precursors with a diameter of 2-5 microns were obtained.

[0094] Step 3: The spherical precursors obtained in Step 2 were soaked in a 2wt% nickel nitrate hexahydrate ethanol / water solution (the mass ratio of ethanol to water was 3:7) for 5 min. After taking out, the ethanol was centrifuged at a speed of 3000 rpm, and then the solid powder obtained by centrifugation was placed in a vacuum oven at 60°C for drying for 12 h to obtain sodium alginate cross-linked spherical precursors.

[0095] Step 4: The sodium alginate cross-linked spherical precursors obtained in Step 3 were placed in a tube furnace and carbonized with argon as the protective gas: first, the temperature was increased to 450°C at a rate of 5°C / min and held for 2 h, then the temperature was increased to 800°C at a rate of 5°C / min and held for 2 h, and finally cooled to room temperature.

[0096] In order to remove the elemental nickel clusters generated in the carbonization process and obtain a porous structure, the above-mentioned carbonized precursor is soaked in a 3 mol / L hydrochloric acid aqueous solution at 60°C for 12 h, then washed with ethanol, centrifuged and placed in a 60°C vacuum oven for drying for 12 h to obtain a hollow porous carbon sphere loaded with nickel monatomic atoms.

[0097] In this embodiment, a carbon carrier / sulfur composite positive electrode sheet is prepared using the hollow porous carbon sphere loaded with nickel monatomic atoms, and a button cell is assembled. The specific method is the same as that in Embodiment 1. The constant current charge-discharge test is carried out at a current density of 0.2 A / g using a new battery test system, and the charge-discharge voltage range is 0.8 V~3.0 V. The test results are shown in Figure 13 .

[0098] As can be seen from the cycle performance curve of Figure 13 , the composite electrode material can maintain a reversible capacity of 588.6 mAh / g after 100 charge-discharge cycles at a current density of 0.2 A / g.

[0099] The kinetics of the sulfur conversion process is tested by EIS for the nickel monatomic atom catalyst, and the test results are shown in Figure 14 . The charge transfer resistance is 175 Ω, which is increased by 100 Ω and 49 Ω compared with the impedance values of Embodiments 1 and 2, respectively, indicating that the catalytic effect of nickel monatomic atoms is lower than that of cobalt monatomic atoms and iron monatomic atoms.

[0100] Comparative Example 1

[0101] In this comparative example, a hollow porous carbon sphere without monatomic atom loading is prepared according to the following steps:

[0102] Step 1: 4 g of sodium alginate (SA, Mw=2000), 12 g of melamine and 40 g of sodium chloride are dissolved in 500 mL of water, heated and stirred at 600 rpm for 12 h at 60°C to obtain a uniform transparent mixed solution.

[0103] Step 2: The mixed solution obtained in Step 1 is introduced into the nozzle of a spray dryer under heating conditions using a rubber tube, the peristaltic pump is set to a pushing speed of 3 mL / min, the inlet air temperature is set to 200°C, and the outlet air temperature is set to 240°C. The powder formed by spraying is collected in a sample collector; after running for 3 h, spherical precursors with a diameter of 2-5 microns are obtained.

[0104] Step 3: The spherical precursors obtained in Step 2 are placed in a tube furnace and carbonized with argon as the protective gas: first, the temperature is raised to 450°C at a rate of 5°C / min and held for 2 h, then the temperature is raised to 800°C at a rate of 5°C / min and held for 2 h, and finally cooled to room temperature.

[0105] In order to remove the NaCl template in the obtained carbon spheres, the precursor after carbonization was soaked in water for 12 h, then washed with water, centrifuged and dried in a vacuum oven at 60°C for 12 h to obtain hollow porous carbon spheres without atom loading.

[0106] The comparative example 1 used the hollow porous carbon spheres without atom loading to prepare carbon carrier / sulfur composite positive electrode sheets and assemble button cells, and the specific method was the same as that of example 1. The rate performance was tested by using the new battery test system at a current density of 0.2 A / g, 0.5 A / g, 1 A / g, 2 A / g, 5 A / g and 10 A / g, respectively, and the charge-discharge voltage range was 0.8 V~3.0 V. The test results are shown in Figure 15 It can be seen from the figure that the rate performance is far worse than that of the carbon material with catalyst loading. At a current density of 10 A / g, it only shows a reversible capacity of 13.5 mAh / g.

[0107] The kinetics of the catalyst in improving the sulfur conversion process was tested by EIS, and the test results are shown in Figure 16 The charge transfer resistance is 600 Ω, which is increased by 525 Ω, 474 Ω and 425 Ω compared with the impedance values of example 1, example 2 and example 3, respectively, indicating that the conversion rate of the sulfur positive electrode is extremely low without single atom catalyst.

[0108] The above content is only an example and description of the concept of the present application. Those skilled in the art can make various modifications or supplements to the described specific embodiments or use similar ways instead, as long as the concept of the present application is not deviated, which shall belong to the protection scope of the present application.

Claims

1. A method for preparing single-atom-supported hollow porous carbon spheres, characterized in that, Includes the following steps: Step 1: Mix sodium alginate, organic ligand, template, and solvent in a mass ratio of 1:1-3:5-20:50-150 and heat and stir until homogeneous to obtain a mixed solution; the organic ligand is at least one of urea, thiourea, polypyrrole, dicyandiamide, polyaniline, and tris(hydroxymethyl)phosphine; the template is at least one of sodium chloride, magnesium chloride, potassium chloride, and lithium chloride; the solvent is water, or a mixture of water and at least one of ethanol and ethylene glycol; Step 2: Spray dry the mixed solution obtained in Step 1 to obtain spherical precursors; the parameters for spray drying are: peristaltic pump propulsion speed of 2-10 mL / min, inlet temperature of 150-200℃, and outlet temperature of 200-240℃. Step 3: Immerse the spherical precursor obtained in Step 2 in a transition metal salt solution for crosslinking, then centrifuge, wash, and dry to obtain a sodium alginate crosslinked spherical precursor; the solvent in the transition metal salt solution is a mixture of ethanol and water, and the transition metal salt is at least one selected from ferric chloride, cobalt chloride, nickel chloride, copper chloride, zinc chloride, ferric nitrate, cobalt nitrate, nickel nitrate, copper nitrate, and zinc nitrate, with a concentration of 1wt%-10wt%; the crosslinking time is 5-30 minutes; Step 4: Carbonize and acid-wash the spherical precursor cross-linked with sodium alginate to obtain hollow porous carbon spheres supported by single atoms; the carbonization process is as follows: under the protection of an inert atmosphere, heat to 450℃-500℃ at a heating rate of 2-10℃ / min, hold for 1-3h, then continue to heat to 800-900℃ at a heating rate of 2-10℃ / min, hold for 1-3h, and then cool naturally.

2. The preparation method according to claim 1, characterized in that, In step 4, the pickling process is as follows: the carbonized material is immersed in an acid solution with a concentration of 1-3 mol / L at 60-90℃ for 8-12 hours, and then removed and dried; the acid solution is a sulfuric acid solution or a hydrochloric acid solution.

3. A single-atom-loaded hollow porous carbon sphere prepared by the preparation method of claim 1 or 2.

4. The application of the single-atom-loaded hollow porous carbon spheres as described in claim 3 in a room-temperature sodium-sulfur battery.

5. A room-temperature sodium-sulfur battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, characterized in that, The positive electrode material of the positive electrode sheet is made by mixing the single-atom loaded hollow porous carbon spheres as described in claim 3 with elemental sulfur and then melting and infiltrating it.

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