Synthesis And Electrochemical Performance Of Graphene/C/SiO₂（Si） Porous Microspheres Load Sulfur/selenium Sulfide

Lithium-sulfur batteries are attractive in this regard due to their high energy density（2600 Wh/kg）and the abundance of sulfur.However,several hurdles such as poor cycle life,huge volume expansion and inferior sulfur utilization need to be overcome for them to be commercially viable.One of the most popular approaches to resolve these issues has been encapsulation of sulfur by carbon materials and polar material,which not only improve ion conductivity and relieve volume expansion of the sulfur particles,but also can mitigate the shuttle effect by physical and chemical adsorption.For selenium sulfide（Se_x S_y）,it combines the high stability of selenium and high capacity of sulfur,but there are still some problems,such as low conductivity and shuttle effect,and one of the effective solutions to solve this kind of problem is to load the Se_x S_y to conductive carbon/polar composite.In summary,based on graphene and carbon by spray-drying and melt impregnation method,S-G/C/SiO₂,G/C/SiO₂/Si/S and G/C/SiO₂/Se_x S_y composite electrode were prepared and studied on its lithium storage performance.The main results and new findings in this work are summarized as follows:（1）Preparation of G/C/SiO₂ porous microspheres loading sulfur and its electrochemical properties.The G/C and G/C/SiO₂ porous microspheres were successfully prepared by spray drying and carbonization process.Porous spherical morphology with a diameter ranging from 2μm to 5μm is found.Then it employed as a novel host for the sulfur cathode,which can improve sulfur utilization.Furthermore,the G/C/SiO ₂microspheres with different SiO₂ content were synthesized by controlling the ratio of graphene,C and SiO₂.In the unique structure,the microspheres with appropriate SiO₂content（20.1 wt%）facilitate to form optimized micro-meso-macro porous structure,which not only reinforce the structure stability,but also restrict the polysulfide via physical and chemical adsorption.As a result,the S-G/C/SiO₂ with 20.1 wt%SiO₂exhibits high discharge capacities of 1118.4 mAh g^-1 at 0.1 C,an excellent rate capability of 551.7 mAh g^-1 at 5 C,and an exciting cycle performance of 626.3 mAh g^-1 after 400 cycles at 0.5 C（ultralow capacity decay of only 0.062%per cycle）.（2）Preparation of G/C/SiO₂/Si porous microspheres loading sulfur and its electrochemical properties.The G/C/SiO₂/Si microspheres were successfully prepared by magnesiothermic reduction to house sulfur as a high performance cathode for lithium-Sulfur batteries.The conductivity of G/C/SiO₂/Si/S microspheres is improved by partial reduction,which can enhance rate performance of the composite.In this unique structure,the shuttle effect can be suppressed via the strong electrostatic interaction between negatively charged polysulfide anion and positively charged SiO₂/Si,improving the electrochemical performance.Moreover,the G/C/SiO₂/Si/S microspheres with different reduction time are compared in this paper.As a result,the G/C/SiO₂/Si/S-12electrode exhibits a high discharge capacity of 973.7 mAh g^-1 at 0.2 C,an excellent rate capacity of 670.8 mAh g^-11 at 2 C,and an outstanding cycle performance of 468mAh g^-11 after 400 cycles at 5 C.（3）Preparation of G/C/SiO₂ porous microspheres loading Se_x S_y and its electrochemical performance.3D hierarchical-porous G/C/SiO₂ hollow microspheres are used as a conducting matrix to encapsulate Se_x S_y as a cathode for high performance lithium-sulfur batteries.In this composite architecture,selenium doped sulfur can improve the conductivity and cycle stability of G/C/SiO₂/Se_x S_y composite electrode.Moreover,the G/C/SiO₂ acted as effective Se_x S_y carriers by confining the polysulfide/polyselenide and buffering volume changes during the charge-discharge processes.As results,the composite G/C/SiO₂/Se_x S_y with 25 wt%Se as cathode material for Li-S batteries exhibited high and stable discharge capacities of up to 724.3 mAh g^-1 over 100 cycles with initail discharge capacities of 860.6 mAh g^-1 at 0.2 C（high capacity retention of 84.2%）.In addition,the composite with different active material（S、Se_xS_y and Se）are compared.As results,the composite of G/C/SiO₂/Se_x S_y perform a better electric chemical property.The discharge capacity is up to 520.2 mAh g^-1 over 100 cycles with capacitiy of 988 mAh g^-1 for the first cycle at 2 C（capacity retention up to 90.5%）.