Preparation And Electrochemical Performance Of Carbon Materials For Cathode In Lithium-Sulfur Batteries

Large-scale energy storage and electric vehicles have increasingly high requirements for the energy density of battery.Lithium-Sulfur batteries with specific capacity of 1675 mAh g-1 and energy density of 2600 Wh kg-1 which are 8-10 times of the actual energy density of the current commercial lithium-ion batteries,are seen as the main technical route for high-energy density storage system.Unlike the intercalation reaction of lithium-ion batteries,the following problems remain to be solved due to the conversion reaction in the charge-discharge process of lithium-sulfur batteries.First,the shuttle effect of polysulfides will cause the loss of active substances,resulting in irreversible capacity loss,reduced cycle stability and rapid capacity attenuation of batteries.Second,one of the key problems to be overcome in realizing the practical application of lithium-sulfur batteries is the cycle stability of the batteries under high areal loading.Designing and constructing practical sulfur host materials is an effective strategy to solve the problems of lithium-sulfur batteries.In view of the above problems,this paper constructed two kinds of carbon materials as sulfur hosts to improve the electrochemical performance of lithium-sulfur batteries.The main research details are as follows:(1)There is a strong adsorption and catalytic effect between metal phosphide and polysulfides.Molybdenum phosphide nanoparticles which are embedded in N-doped carbon hollow nanostructure(MoP@NC)was designed.The electrochemical performance of this material as host for preparing sulfur electrode was studied,and the mechanism of inhibiting polysulfides was revealed from the aspects of adsorption,catalysis and theoretical calculation.The results show that MoP@NC nanoparticles have good electrical conductivity and abundant pore structure,which can effectively capture and catalyze the conversion of polysulfides and accelerate the kinetics of liquid-liquid and liquid-solid reactions.The hollow structure provides more diffusion paths for ions,which is conducive to high rate charge and discharge.Theoretical calculation shows that the formation of Mo-S bond increases the binding energy between polysulfides and MoP@NC,which is beneficial to accelerate the REDOX reaction kinetics.In a word,the prepared MoP@NC/S electrode shows great specific capacity and cycle stability.(2)Biomass carbon nanofibers were prepared as sulfur host materials by high temperature carbonization using the three-dimensional interconnecting structure of bacterial cellulose itself,and the electrochemical performance of sulfur electrode was studied.The electrochemical performance of high areal loading electrode prepared by using nickel foam as collector was studied,and the application of this material in the preparation of high areal loading electrode was explored.The results show that the carbon nanofibers obtained by high temperature carbonization have good electrical conductivity,maintain the original interconnected three-dimensional network structure of bacterial cellulose,rich pore structure,and contain nitrogen,oxygen and other hybrid elements on the surface,which can prevent the diffusion of polysulfide due to firmly integrate with polysulfides.Carbon nanofibers are considered as desired sulfur host materials.The prepared CNFS electrodes showed good cycle performance,and the high areal loading electrode CNFSP/S-Ni showed excellent discharge areal capacity(13.9 mAh cm-2)and stable cycle performance.