The Synthesis Of Porous And Hollow Carbon Nanomaterials And Their Applications In Lithium-Sulfur Batteries

With the increasing energy demands from electric vehicles and portable electronic devices,the development of next-generation secondary batteries with high energy density,low cost,and good safety performance becomes urgently.Owing to its high theoretical specific capacity(?1672 mAh g-1),resource abundant and environment friendly,lithium-sulfur battery is deemed to be one of the most promising next-generation secondary batteries.However,three major issues still prevent the practical application of Li-S batteries,including:(1)the huge volumetric change during cycling,(2)the insulating property of sulfur and lithium sulfide(Li2S),and(3)the side shuttle effect of the high-order lithium polysulfides Li2Sx(4<x?8)dissolved in the organic electrolyte.These issues may lead lithium-sulfur batteries to be poor cycle life,low active material utilization and poor rate performance.Over the last few years,many works on design of sulfur cathode has been intensively investigated by the research teams world-wide.An effective strategy is to design smart carbon nanomaterials as the host for sulfur.In order to improve the electrochemical performance of lithium-sulfur batteries,we carried out a series of research work.Some carbon nanomaterials with unique porous and hollow structure have been synthesized and used as the host for sulfur.The main content of this thesis includes:In chapter 1,it summarizes the main characteristics of lithium-sulfur batteries,as well as recent developments.In chapter 2,it states the main reagents,experimental apparatus and testing measures in this thesis.In chapter 3,a carbon material with special structure of 3D porous and interconnected hollow nanospheres array has been successfully designed and synthesized.The 3D-HCNA/S composite with high sulfur content of 76 wt%was prepared by a facile melt-diffusion approach.Due to the special porous and hollow structure of 3D-HCNA,The 3D-HCNA/S composite delivered a high initial capacity of 1318 mAh g-1 at 0.05 C and retained a reversible capacity of 757 mAh g-1 at 0.5 C after 200 cycles.Moreover,excellent cycling performance was also observed with a capacity of 708 mAh g-1 maintained after 150 cycles at 1 C,which suggests that the 3D-HCNA/S composite could be a promising cathode material for lithium-sulfur batteries.In chapter 4,a tube-in-tube MWCNTs@SnO2 nanomaterial has been successfully designed and used as a host for sulfur.Owing to the unique hollow structure,the MWCNTs@SnO2@S composite delivers high sulfur content of 73 wt%.Nanostructure metal oxides have much stronger adsorption ability to polysulfides and render the lithium-sulfur batteries with long life span and high sulfur utilization.After 300 cycles,the as-prepared MWCNTs@SnO2@S cathode still remained a high capacity of 514 mAh g-1 at 0.5 C with a low decay rate of 0.122%per cycle.Although the cycle performance of MWCNTs@SnO2@S cathode is great,the low initial capacity seems to be a fly in the ointment.In chapter 5,we designed MOFs-derived Co-N dual-doped porous carbon material/carbon nanotubes(Co-NPC/CNTs)as the host of sulfur.The dual-doped material has stronger adsorption ability to polysulfides than the nonpolar carbon materials.Owing to its porous architecture,as-prepared Co-NPC/CNTs@S cathode with a high sulfur content of 72 wt%exhibited excellent electrochemical performance.It showed high initial capacity of 1403 mAh g-1 at 0.05 C.Moreover,excellent cycling performance was also delivered with a capacity of 612 mAh g-1 maintained after 300 cycles at 0.5 C.