Design And Synthesis Of Transition Metal/Nitrogen-doped Porous Carbon Materials For The Application In Lithium-Sulfur Batteries

Lithium-sulfur batteries（LSBs）are expected to replace traditional lithium-ion batteries as the next generation of efficient energy storage device due to their high theoretical energy density(2600 Wh kg^-1),low cost of sulfur cathode and environmental friendliness.However,the practical application of LSBs is still hindered by a series of issues:（1）the large volume change of the sulfur cathode during the lithiation/delithiation process（the volume difference between S₈ and Li₂S is 80%）will cause cracking and pulverization of the electrodes;（2）the insulating properties of sulfur and its discharge products（Li₂S₂/Li₂S）will result in high overpotentials and slow reaction kinetics;（3）the soluble intermediates（Li₂S_x,2<x≤6）generated during the charge and discharge processes will dissolve,diffuse and decompose in electrolytes and/or at interfaces,causing loss of active materials and interface instability.In this thesis,transition metal and nitrogen-doped porous carbon materials are designed and synthesized for breaking the above limitations in LSBs.The capacity and cycle stability of LSBs can be improved through the effect of porous carbon framework on mitigating volume expansion and increasing conductivity,and the function of transition metal/nitrogen doping sites on the catalytic conversion/adsorption of polysulfides（Li PSs）.Details are as following:1.Study on nickel/nitrogen-doped graphene nanosheets-carbon nanotubes（Ni@NG-CNTs）modified separator as a high-efficiency Li PSs barrier layer for LSBs.Using urea,citric acid and nickel acetate as feedstocks,the precursor prepared by a sol-gel method was treated through pyrolysis and then washed with acid to obtain the Ni@NG-CNTs.The cell with Ni@NG-CNTs modified separator showed high discharge specific capacity and good cycle stability in the electrochemical performance test（S loading:77 wt%）.At a current density of 1.0 C,the initial discharge specific capacity of the cell was as high as 1144 m Ah g^-1,and after 800cycles,it obtained a high discharge specific capacity of 655 m Ah g^-1.At high current densities of 5.0 and 10.0 C,the cell still maintained high discharge specific capacities of 309 and 265 m Ah g^-1 after 500 cycles.These excellent electrochemical properties are attributed to the strong adsorption of Li PSs by N-doped graphene nanosheets and CNTs with rich carbon nanocages,and the synergistic effect of Ni nanoparticles catalyzing the redox reaction of Li PSs.These results show that Ni@NG-CNTs modified separators can be used as high-efficiency Li PSs barrier layers for improving the capacity and cycling performance of LSBs.2.Study on two-dimensional cobalt/nitrogen-doped hollow carbon nanosheets（Co/N-HCNSs）modified separator as a high-efficiency Li PSs barrier layer for LSBs.First,Co-ZIF-8@Glu-CN-180@Co-ZIF precursor was obtained using g-C₃N₄ as the template and nitrogen source,glucose（Glu）as the carbon source,and ZIF-8molecular cage used to confine metallic Co.Subsequently,the precursor was calcined to form nitrogen/doped hollow carbon nanosheets with uniform cobalt distribution（Co/N-HCNSs）.The Co/N-HCNSs-modified Celgard PP separator was used as the interlayer material in LSBs.The assembled LSBs showed high discharge specific capacity and good cycle stability in electrochemical performance tests.The cell achieved an initial discharge specific capacity of 1537 m Ah g^-1 at a current density of0.2 C,and maintained a low cycle attenuation rate of 0.047%/cycle after 1000 cycles at a current density of 1.0 C.At high current densities of 5.0 and 10.0 C,the battery still maintained high discharge specific capacities of 484 and 327 m Ah g^-1 after 500cycles.The excellent electrochemical performance is attributed to the unique sandwich structure of Co/N-HCNSs,which not only can alleviate the volume expansion during charging and discharging,but also acts as a physical barrier for impeding the shuttle of Li PSs and improving the cycle stability of the electrode material.In addition,the synergistic effect of nitrogen-doped sites and metallic cobalt on chemical adsorption and catalytic convertions of Li PSs makes the battery system possess fast reaction kinetics and high charge-discharge specific capacity.