Preparation,Characterization And Electrochemical Performance Of Positive Electrode Materials For Lithium-sulfur Battery

With the rapid development of modern science and technology,energy and environmental issues have become increasingly prominent,and higher requirements have been put forward for the development and utilization of new energy.Lithium-sulfur batteries with a theoretical discharge specific capacity(1675 mAh g^-1)and theory energy density(2600 Wh kg^-1)has received wide attention from researchers across the world.Although,elemental sulfur as an active material has many advantages such as huge abundance,environmental friendliness and low cost.Lithium-sulfur batteries has become one of the most potential energy storage systems in the field of new energy research.However,the commercialization process of lithium-sulfur batteries has been greatly hindered,mainly due to the following problems:elemental sulfur is a poor conductor with the electronic conductivity,which tends to cause high rate performance of the battery;during the charging and discharging,the generated polysulfide is easily dissolved,resulting in a large loss of the positive active material and an increase in capacity attenuation;the inherent"shuttle effect"of the lithium-sulfur battery causes corrosion of the negative electrode material,the Coulombic efficiency and thus cycle performance is deteriorated.Therefore,the combination of a material having high conductivity and large specific surface area to improve the conductivity of sulfur with a reduction in the dissolution of polysulfide can accelerate the commercialization process of the lithium-sulfur battery.Polar metal sulfides?Co₉S₈,CoS?have a unique chemisorption effect on polysulfides and an electrocatalytic effect on redox reactions.When combined with sulfur,it can effectively reduce the diffusion of elemental sulfur and inhibit"shuttle effect".However,the metal sulfide has a low electrical conductivity at normal temperature,and it is necessary to add a material having excellent conductivity such as carbon nanotubes and to recombine it.Carbonization of hollow polyaniline nanofibers at high temperature can obtain hollow carbon nanofibers with larger specific surface area and stronger conductivity.It can effectively adsorb polysulfide produced during charge and discharge and inhibit its"shuttle effect",improving electrochemical performance to some extent.In this paper,three-dimensional interconnected Co₉S₈/MWCNTs composites,flower shape CoS/MWCNTs composites and hollow carbon nanofibers HCNFs were synthesized by chemical methods.Finally,the materials were respectively mixed with elemental sulfur were subjected to:X-ray diffraction,scanning electron microscopy,transmission electron microscopy,X-ray photoelectron spectroscopy,nitrogen adsorption and thermogravimetry analyses.Finally,the cells were galvanostatically charged and discharged at different C-rates.The cells were also analyzed by cyclic voltammetry,and AC impedance.Based on the experiments,the following conclusions are drawn:?1?The solvothermally synthesized Co₉S₈/MWCNTs composites exhibit a three-dimensional network structure.The carbon nanotubes and Co₉S₈ particles interlaced inside and outside the material are connected and tightly combined,further compounded with elemental sulfur,sulfur deposits inside the material.Polar Co₉S₈particles are effectively adsorbed to polysulfide,and carbon nanotubes increase the overall conductivity of the positive electrode material.And also reduced the loss of active material and the dissolution of polysulfides.Therefore,performance such as excellent discharge specific capacity,rate performance,and cycle life were synergistically enhanced.?2?The flower shape CoS/MWCNTs composite prepared by hydrothermal method has a large specific surface area,and the carbon nanotubes are connected between the flower shape CoS sheets.The three-dimensional conductive network structure not only shortens the path of lithium ion diffusion.It also improves the conductivity of the material.The carbon nanotubes also accommodate the volume expansion of the electrode reaction to a certain extent,improve the structural stability and safety of the material.The polar CoS can effectively adsorb the polysulfide,and accelerated the redox reaction,which subsequently improve the utilization of active substances and reduced the"shuttle effect"of polysulfides.The specific discharge capacities of CoS/MWCNTs/S at 0.1 C and 1 C were 1120 and 762 mAh g^-1,respectively,and the capacity retention was 78%at 0.5 C rate even after 200 cycles.?3?The hollow carbon nanofibers?HCNFs?synthesized by the method of oxidative polymerization and high-temperature carbonization have good electronic conductivity and large specific surface area.The unique structure can effectively adsorb elemental sulfur,reduce the loss of effective sulfur in the electrode reaction and inhibits the"shuttle effect"of polysulfides.Material has a higher discharge specific capacity,but still needs further optimization.