Designing New Host Materials For Sulfur Cathode In Lithium-Sulfur Batteries

Sulfur is one of the most abundant elements in the earth and has a high theoretical energy density,which plays an important role in energy storage,especially in the research and application of lithium-sulfur batteries.However,due to the "shuttle effects" of polysulfide,the volume expansion in charge and discharge process and the poor conductivity of sulfur,the commercial application of lithium-sulfur batteries is still facing great challenges.Therefore,the development of new-type materials of lithium-sulfur batteries to improve the performances of lithium-sulfur batteries has become the focus of research.This thesis studied the composite of metal organic framework and reduced graphite oxide(MIL-100(V)/rGO)and uniformly dispersed metal fluorides(CaF2 and MgF2)derived from burdock root and their effects on lithium-sulfur batteries.The main research contents are listed as follows:(1)Mesoporous MIL-100(V)was prepared by a hydrothermal method,and MIL-100(V)/rGO complex was successfully synthesized by the introduction of graphene oxide during hydrothermal synthesis.In this thesis,these two materials were firstly used as sulfur host for lithium-sulfur batteries and their electrochemical properties were studied.MIL-100(V)with mesoporous structures and large surface area can provide the foundation for loading sulfur.Meanwhile,vanadium ions within MIL-100(V)not only exist in the form of V3+ but also V4+,which may form a stronger inhibitory effect on the dissolution of polysulfides.In addition,the introduction of reduced graphene oxide(rGO)can effectively accommodate the volume changes of sulfur during cycling process,and it can provide fast Li+ diffusion pathways and improve the electronic conductivities.Therefore,the addition of rGO is believed to be able to improve the electrochemical performance of MIL-100(V).(2)With burdock root as raw material,after calcinating in high temperature in air and immersing in hydrofluoric acid solution,a kind of uniformly dispersed and mesoporous mixture of CaF2 and MgF2(CMF)was obtained from burdock root which contains Ca and Mg elements.Metal fluorides derived from biomass were first prepared and applied as host for Li-S batteries.When the sulfur loading reached to 75 wt%,it delivers a reversible capacity of 650 mAh g-1 at 0.1 C(1C=1675 mA h g-1)after 100 cycles.It also exhibits an excellent cycling ability up to 530 mA h g-1 after 300 cycles at a current density of 0.5 C.The excellent performances of CMF reveal that metal fluorides are expected to become the new sulfur host for lithium-sulfur batteries.