Synthesis Of Selenium-hierarchically Porous Carbon Fiber Coiposites And Their Electrochemical Performance For Li-Se Batteries

Recently,Li-Se batteries have attracted a wide appeal to large energy storage system because of their high theoretical specific capacity.However,Li-Se batteries also have disadvantages,such as bad cycling stability and poor rate capability.In order to solve this problem,we started our research with the applicant for hierarchically porous carbon fibers(HPCF)in the lithium-selenium battery,a series of carbon and selenium-based composites were designed and synthesized,and the reversible capacity,cycling stability and rate performance were improved.We use sodium lignosulfonate(LN)as a green pore former,the HPCF with high specific surface area and large pore volume was synthesized by electrospinning.We fabricated the cathode electrode material of Se/HPCF-H2 by melt diffusion method of hydrogen reducing atmosphere.By solvothermal method,we made the Se/HPCF and the graphene self-assembled into Se/HPCF-RGO with a 3D structure.A good electrochemical performance of Se-HPCF(SeO2)was also synthesized by liquid-phase dissolution-diffusion method.The main contents of our research can be summarized as follows:(1)The Se/HPCF-H2 composites were obtained by melt diffusion method of hydrogen atmosphere with selenium powder and HPCF.As a comparison,the Se/MPCF-H2 composite was prepared by using microporous carbon fiber(MPCF)as the reactor,and the Se/HPCF-Ar composite was synthesized under argon inert atmosphere.When we use Se/HPCF-H2 composite as an cathode electrode for lithium-selenium battery,it exhibits a high reversible capacity of 587 mAh g-1 after 50 cycles at a current density of 0.2 C.And it also shows an outstanding rate performance,at a high current density of 5 C,the reversible capacity could maintain at 372 mAh g-1.The results show that the electrochemical performance of Se/HPCF-H2 is much better than Se/MPCF-H2 and Se/HPCF-Ar composites.In addition,when the preparation condition is the hydrogen reducing atmosphere,the composite has higher lithium storage capacity and cycle stability.(2)The Se-HPCF composite was compound with HPCF and selenium powder by melt diffusion method under the hydrogen reducing atmosphere.Finally,the Se/HPCF-RGO composite was synthesized with Se-HPCF composite and graphene dispersion by solvothermal method.The results show that the Se/HPCF-RGO composite with hierarchical porous structure and 3D structure has an excellent electrochemical property for lithium-ion.When used as an cathode material for lithium-selenium batteries,it demonstrates a high reversible capacity,impressive rate performance and excellent cycling stability.At the current density of 0.5 C,the Se/HPCF-RGO composite delivered a reversible capacity of 523 mAh g-1 after 200 cycles.At a high rate of 5 C,the specific capacity could maintain at 408 mAh g-1.When it comes to a super high rate of 10 C,the Se/HPCF-RGO composite still delivered a high capacity of 208 mAh g-1 even after 5000 cycles.This unique electrochemical property can be attributed to the synergistic effect of the microporous-mesoporous structure of the HPCF in composite and the 3D structure formed by the graphene coating.In addition,the doping of nitrogen can enhance the conductivity of the electrode,and nitrogen could generate N-Se bond with the selenide.This can effectively inhibit the dissolution of selenide during the cycle of lithium-selenium battery,which limits the occurrence of "shuttle effect" and improve the cycling stability of the material.(3)The Se-HPCF(SeO2)composite with good property was obtained by the combination of HPCF and SeO2 by a liquid-phase dissolution-difuusion method.We used Se-HPCF(SeO2)composite as the cathode material of lithium-selenium battery.The Se-HPCF(SeO2)composite shows a good cycling stability and rate performance,at a current density of 0.2 C,the capacity is maintained at 533 mAh g-1 after 50 cycles.Simutaneously,it also displays a large reversible capacity of 351 mAh g-1 at the high rate of 5 C.This excellent electrochemical performance can be attributed to the microporous-mesoporous structure of HPCF,which can effectively to accommodate large volume exchange,reduce the dissolution of the active substance selenium,maintaining the composite structural integrity and enhance the electronic/ionic transport effectively.