Construction Of Single-Atom Catalyst Based On Three-Dimensional Carbon Material And Application Of Lithium-Sulfur Battery

With the rapid development of new energy technology,lithium-sulfur（Li-S）batteries have attracted much attention in recent years.Because of its high theoretical energy density and low cost,Li-S batteries are expected to become a new type of secondary battery.However,there are some problems in the development of Li-S batteries,that hinder their commercialization process:（1）The lithium polysulfides produced during the cycle of lithium-sulfur batteries is dissolved in the electrolyte and shuttled to the lithium anode under the action of concentration gradient and electric field.This can cause irreversible loss of the active substance;（2）The volume expansion of the cathode material and the poor conductivity of S and Li₂S cause the reduction of battery capacity;（3）The uneven growth of lithium dendrites in the negative electrode may pierce the separator and affect the safety of the battery.In view of the above problems,this paper is divided into two parts to study the application of metal single-atom catalysts in lithium-sulfur batteries.Single atom catalysts（SACs）have high catalytic activity and close to 100%atomic utilization,which can effectively inhibit the shuttle effect of polysulfides and improve the cycle stability of batteries.Moreover,the synergistic effect of bimetallic single atoms is expected to further improve the catalytic performance of single-atom catalysts.The specific research is as follows:（1）The zeolite imidazolate skeleton structure（ZIF-8）was used as the precursor coated with phenolic resin（RF）,V metal atoms were introduced by impregnation method,and V metal single atoms and hollow nitrogen doped porous carbon nanostructure composite materials（V-N/HNPC）were obtained after high-temperature carbonization and acid treatment,which was used as a sulfur carrier for lithium-sulfur batteries.Near-edge X-ray Absorption Spectroscopy（XANES）and Extended X-ray Absorption Fine Structure Spectroscopy（EXAFS）show the coordination form of V-N₄.It was found that the highly active V-N₄ site had strong adsorption of polysulfides and could efficiently catalyze the conversion of polysulfides.In addition,the three-dimensional porous structure of V-N/HNPC can alleviate the volume expansion problem of cathode materials;high specific surface area and rich mesoporous,which can improve sulfur utilization and provide more defect sites to anchor polysulfides.Therefore,the discharge specific capacity of 605 m Ah g^-1 is still present after 200 cycles at S@V-N/HNPC electrode 1 C.Compared with the control group S@HNPC and S@Ni-N/HNPC electrodes have lower capacity attenuation.After 100 cycles at 0.2 C at sulfur loading of 3.7 mg cm^-2,the discharge specific capacity of the S@V-N/HNPC electrode was 685 m Ah g^-1 and had good rate performance.（2）Considering the uneven growth of lithium dendrites,hollow nitrogen-doped porous carbon nanospheres（Ni/Fe-HNCS）modified by Ni and Fe metal monatoms were prepared by the traditional hard template method and impregnation method for the positive and negative electrodes of lithium-sulfur batteries.The aberration corrected high-angle annular darkfield scanning transmission electron microscopy test（AC-HAADF-STEM）image can observe that both Ni atoms and Fe atoms are dispersed in the form of single atoms.XANES and EXAFS prove the coordination forms of Ni-N₄ and Fe-N₄.It was found that Ni-N₄ and Fe-N₄ sites had strong adsorption of polysulfides and could efficiently catalyze the conversion of polysulfides.The tests of Li-Cu half-cells and Li-Li symmetric cells showed that Ni/Fe-HNCS could effectively induce uniform deposition of Li and inhibit the growth of lithium dendrites.Therefore,the lithium-sulfur battery using Ni/Fe-HNCS as the positive and negative electrode carrier shows the best battery performance.It can be stable for 500 cycles at 2 C,even at a high current density of 5 C,the specific capacity of the first turn discharge can reach 741.2 m Ah g^-1,and there is still a reversible capacity of 615.6 m Ah g^-1 after 500 cycles.In this paper,the catalytic effect of single-atom catalysts in lithium-sulfur batteries was studied by constructing single-atom catalyst materials with different three-dimensional carbon substrates,demonstrating the excellent catalytic performance of single-atom catalysts.Moreover,the preparation and application of bifunctional catalytic materials provide ideas for the design of cathode and anode materials for high- performance lithium-sulfur batteries.