Preparation And Electrochemical Performance Of ZIF-8 Derived Connected Ordered Macro-microporous Nitrogen-doped Carbon Based Lithium-sulfur Cathode

Lithium-sulfur batteries have attracted much attention due to their ultra-high specific capacity（1675 m Ah/g）and energy density（2600 Wh/kg）,but they still have problems to be resolved such as poor conductivity of the sulfur cathode and dissolution of polysulfide in organic electrolytes.The design of cathode structure,especially the application of novel matrix in encapsulating sulfur and immobilizing lithium polysulfide to solve the inherent problems of sulfur cathodes is a hot research topic.Aiming at the existing problem of sulfur cathode,this thesis has prepared connected ordered macroporous ZIF-8 nanoparticles and then carbonized them to fabricate nitrogen-doped connected ordered macro-microporous carbon（COM-MPC）which combines the advantages of both hierarchical porous carbon and heteroatom doping,the COM-MPC finally serve as sulfur host for cathodes to further improve the performance of lithium-sulfur batteries.The main research contents are as follows:（1）Firstly,this article proposes a double-solvent assisted in-situ crystallization preparation process in the face-centered cubic stacking template to obtain cuboctahedron shaped connected ordered macroporous ZIF-8 nanoparticle with high porosity.The regulation effects of reactant concentration,solvent type,face-centered cubic stacking structure on its nucleation rate and growth mode during the preparation process were analyzed,and the microscopic mechanism of the controllable growth of this material was further summarized.（2）By carbonizing the connected ordered macroporous ZIF-8,COM-MPC with hierarchical pore size was obtained,and the N atom in the precursor was uniformly doped into the carbon matrix during the carbonization process.As a comparison,a traditional ZIF-8derived carbon（C-ZIF-8）was prepared.Through a series of tests,properties of COM-MPC and C-ZIF-8 such as the microscopic morphology,surface element composition,pore structure were characterized and compared.indicating that COM-MPC is significantly better than traditional C-ZIF-8 in terms of capacity,pore richness,and specific surface area,showing good application potential.（3）The composite cathode is prepared by filling the sulphur element into the pores of the hierarchical porous carbon by the melting diffusion method and assembled into half-cell.The COM-MPC@S composite cathode exhibits a high initial discharge capacity of 1498.5m Ahg^-1 at 0.1C,and retains 82.3%of the reversible capacity within 10 to 50 cycles at 0.5C.Moreover,after 50 cycles at a high rate of 1C,it can still maintain a large capacity of 608.5m Ahg^-1.The electrochemical test demonstrate that compared with the traditional C-ZIF-8,the utilization rate of the active material in COM-MPC cathode is higher,and the cycle stability is significantly improved.Above all,benefits from the the introduction of macroporous structure,transportation of Li⁺become much easier and the internal resistance of the battery decrease a lot,significantly improving the battery’s high-rate performance.