Synthesis Of N,S Dual-Doped Carbon Materials And Co₃O₄ Nanotubes For The Applications In Lithium-Sulfur Batteries

The fast development of portable electronics has stimulated growing demand of advanced energy storage systems.Rechargeable lithium-sulfur battery has been regarded as one of the most promising rechargeable batteries because of its high theoretical capacity and ultra-high energy density.However,several obstacles still limit its large-scale applications:（1）the poor electrical conductivities of sulfur,Li₂S₂ and Li₂S,（2）the large volume changes of sulfur upon lithiation/delithiation of the cathode,（3）the shuttle effect of lithium polysulfides between the anode and the cathode.Thus,it is crucial to seek and design electrode materials with outstanding electrochemical performances for lithium-sulfur battery.In order to solve these issues,this thesis mainly focused on fabricating heteroatoms-doped（N and S）carbonaceous materials and metal oxide nanomaterials with fine structure,which used as cathode in lithium sulfur battery and exploring their electrochemical performance as the cathode in lithium sulfur battery.Details are as following:1.Synsthesis of heteroatoms-doped carbonaceous materials and the electrochemical performance as cathode in lithium sulfur battery.Urea and citric acid were firstly utilized to prepare a precursor by sol-gel method.Then,N,S-dual doped carbonaceous materials were prepared through calcining the as-prepared precursor under Ar and H₂S atmosphere.With N,S-dual doped carbonaceous materials as host materials and interlayers,a high sulfur loading content of 80 wt%was achieved,and the cell displayed a high initial discharge specific capacity of 1371.7 mAh g^-1 at 0.2 C.At the high current rate of 2 C and 5 C,the cell delivered high initial discharge specific capacities of 975.4 and 594 mAh g^-1,and remained at 647.1 and 393.6 mAh g^-1 after 500 cycles,respectively,indicating its excellent cycling stability.The N,S-dual doped carbonaceous materials exhibits superior electrochemical performance than those of N doped carbonaceous materials and bare carbon.The excellent electrochemical performances is attributed to the rich defect sites,high surface area and large pore volume of the N,S-dual doped carbonaceous materials with hierarchical pores,which provide sites for chemi-sorptions of lithium polysulfidesa and also mitigate the volume expansion.2.Synsthesis and electrochemical properties of Co₃O₄ nanotubes.In this thesis,a precursor of hydroxide was prepared via aging Co（OAc）₂·4H₂O for 72 h in a mixed solvent of water and ethanol（volume ratio of 1:2）.After calcined at 500℃,Co₃O₄ nanotubes were synthesized.When the Co₃O₄ nanotubes were used as the host material in the lithium-sulfur battery,an initial discharge specific capacity of 1049.3 mAh g^-1 and a low overpotential of 154 mV were obtained at the current density of 0.2 C.The hollow tube-like structure with high surface area and large pore volume can provide high space to encapsulate sulfur,and thus improve the utilization of sulfur.As the electrode materials in lithium-ion batteries,the Co₃O₄ nanotubes displayed a high specific capacity of 1081 mAh g^-1 at the 100 mA g^-1 after 50 cycles.The excellent electrochemical performance of the Co₃O₄ nanotube-based electrode is attributed to the unique hollow tube-like structure,which not only provides high space to remit huge volume changes arising from Li⁺insertion/extraction but also shortens the Li⁺diffusion distance,leading to fast reaction kinetics and high specific capacity.