Preparation And Application Of Fe-N/CNT And V₂O₃@N,Ni-C Carbon Composite Nanostructures In Lithium-sulfur Batteries

With the development of society and the rise of 5G technology,3C products and new energy vehicles require more and more battery capacity.However,the energy density of commercial lithium-ion batteries is very limited,far from meeting the needs of life.Lithium sulfur battery is considered as one of the most promising candidates for future energy storage due to its high energy density(2600 Wh kg^-1)and high theoretical specific capacity(1675 m Ah g^-1).However,the shuttle effect of lithium polysulfide,the poor conductivity of sulfur,the slow redox kinetics of polysulfide,and the volume expansion in the process of sulfur reduction hinder the practical application of lithium sulfur battery.In this paper,Fe-N/CNT composite carbon nanotubes were synthesized by catalytic pyrolysis method,and V₂O₃@N,Ni-C composite hollow spheres were synthesized by solvothermal method and chemical vapor deposition method.The preparation of Fe-N/CNT composite carbon nanotubes and their application in cathode materials of lithium sulfur batteries were studied,and the preparation of V₂O₃@N,Ni-C composite hollow spheres and their application in diaphragm modification of lithium sulfur batteries were studied(1)The Fe-N/CNT composite carbon nanotubes synthesized by catalytic pyrolysis method have abundant active sites.The X-ray photoelectron spectroscopy(XPS)analysis and cyclic voltammetry(CV)test show that they can effectively adsorb polysulfides,promote the conversion of polysulfides,and improve the oxidation-reduction reaction kinetics of polysulfides.Further scanning electron microscopy(SEM)and electrochemical impedance spectroscopy(EIS)analysis also found that the pyrolysis temperature would affect the diameter,conductivity and impedance of the Fe-N/CNT composite carbon nanotubes.When the pyrolysis temperature is 800?,the cycling performance of lithium sulfur battery assembled by Fe-N/CNT composite carbon nanotubes as cathode sulfur host material is the best.When the current density is 0.5C,the capacity decreases by only 0.074%after 300 charge discharge cycles under75wt%sulfur load.At the current density of 1C(1C=1675 m Ah g^-1),the reversible capacity of 996.05 m Ah g^-1 was obtained in the first discharge,and the specific capacity remained 798.2 m Ah g^-1 after 150 cycles.(2)Carbon coated carbon films were synthesized by solvothermal and chemical vapor deposition methods V₂O₃@N,Ni-C composite hollow spheres.By means of XPS analysis,CV test and EIS test,it is proved that V₂O₃@N,Ni-C composite hollow spheres can effectively adsorb polysulfides,play a catalytic effect on the conversion of polysulfides,improve the oxidation-reduction reaction kinetics of polysulfides,and improve the conductivity of V₂O₃,providing an effective electron transport path for the conversion of polysulfides.Based on these advantages,the lithium sulfur battery can provide an initial discharge capacity of 1590.4 m Ah g^-1 at a current density of 0.1C.The initial discharge specific capacity of 1142.2 m Ah g^-1,corresponding to a lower capacity attenuation of 0.0617%per cycle,was demonstrated in 500 cycles at 1C current density.