Design And Synthesis Of Hollow Sulfur/Oxide Cathode Materials And Their Application For Lithium-sulfur Batteries

With the rapid growth of electric vehicles and the gradual depletion of non-renewable fossil fuels such as coal,oil and natural gas,there is an urgent need to find alternative energy sources that are compatible with sustainable development.However,the energy density and cycling performance of lithium-ion batteries currently under researches have not been able to meet the demand for large-scale energy storage devices in actual production.Therefore,the development of energy storage materials with higher energy density,power density and long cycle life has become a hot spot in the world.In the new energy storage system,lithium-sulfur batteries have a high theoretical specific capacity of 1675 mAh g^-1 and a high theoretical specific energy of up to 2500 Wh kg^-1,which is five times that of today's commercial lithium-ion batteries.Meanwhile sulfur is low cost,in natural abundance and environmentally friendly.Therefore,lithium-sulfur batteries have become one of the most promising candidates for the next generation of energy storage devices and have received extensive attention.Carbon/sulfur composite materials are commonly used as cathode materials for lithium-sulfur batteries.However the effect of various carbon materials to fix polysulfides through physical confinement is limited.Meanwhile the polysulfides can only be temporarily confined near the carbon material and will diffuse into the electrolyte for a long time,which seriously affects the cycle life.Nanostructured oxides and sulfides have proven to be effective polar host materials,since these inorganic materials containing such as O and S atoms have a stronger interaction with lithium polysulfide and have proven to be effective polar host materials.The hollow materials can accommodate more active sulfur through the macroporous structure while providing more adsorption sites.Therefore,hollow oxides and sulfides are expected to become the supporting skeleton of sulfur,suppressing the shuttle effect and improving the performance of Li-S batteries.The copper sulfide,which is catalytically active for polysulfide conversion,was firstly selected as a candidate material.Cu₂S hollow cubic crystals and hollow CuS microspheres were designed and synthesized.The types and amounts of vulcanizing agents were explored separately,and suitable vulcanizing agents were found.The quantity of vulcanizing agents not only has a crucial impact on the maintenance of the shape,but also maintains a high specific capacity.When 10%TAA is added to prepare Cu₂S cubes,the initial specific capacity can reach 701 mAh g^-1 and after50 cycles,the specific capacity can still maintain 412 mAh g^-1.Vanadium sulfide has stronger chemical affinity and adsorption for lithium polysulfide than other metal sulfides,and can improve the utilization rate of active sulfur.Herein,a variety of vanadium sulfide structures and their graphene composites were synthesized then their electrochemical properties were measured after sulfur loading.It was found that the vanadium sulfide materials get involved with lithium intercalation and extraction in the voltage window of the lithium-sulfur battery,which increase the capacity of the lithium-sulfur battery.However,the open structure is not beneficial to the adsorption of polysulfide,which limits the improvement of its cycling stability.Combined with the above work,the hollow microsphere VO₂@S composite cathode material was designed and synthesized based on the VO₂ intercalation and S conversion reactions.The microsphere VO₂hollow sphere provides enough space for sulfur loading and increases the tap density of the VO₂@S(1.64 g cm^-3).The in-situ generated VS_x can not only promote the conversion of polysulfide and improve the utilization and,but also enhance the interaction between the shell and polysulfide and inhibit the shuttle effect of polysulfide.Finally,the initial discharge specific capacity of VO₂@S composite cathode material can exhibits a high initial discharge capacity of 930 mAh g^-1,corresponding to the initial volume discharge capacity of 1084 mAh cm^-3.