Controllable Synthesis Of Co₃O₄-based Composite Cathodes And Performances Of Lithium-sulfur Batteries

The limited energy density of lithium-ion batteries is increasingly difficult to meet the requirements of portable electronic devices,electric vehicles and new energy power stations for the endurance of energy storage power sources.Lithium sulfur battery is one of the ideal choices for developing high energy secondary battery because of its high theoretical specific capacity(1675m Ah g^-1),abundant positive sulfur reserves and environmental friendliness.However,problems such as poor conductivity,serious volume expansion and shuttle effect caused by the dissolution of lithium polysulfide on the positive electrode side restrict the development of large-scale commercialization of lithium polysulfide.In this paper,sulfur/Cobalt tetroxide（Co₃O₄）hollow microspheres,sericin-derived Carbon（SC）coated sulfur/Co₃O₄and reduced graphene oxide（r GO）coated sulfur/Co₃O₄composites were synthesized by solvothermal,solution soaking and high-temperature calcination techniques.The effects of different morphology and composition of positive electrode composites on electrochemical performance of lithium sulfur batteries were investigated.The main research contents and conclusions are as follows:（1）We optimized the synthesis process of Co₃O₄hollow microspheres to improve the shuttle effect caused by the dissolution of polysulfide in the reaction process,namely,3 mmol cobalt acetate tetrahydrate and 30 mmol anhydrous sodium acetate as reactants,adding 3 mmol cetyltrimethyl ammonium bromide（CTAB）surface activator,Co3O4 hollow microspheres with a diameter of about 2μm were prepared by the reaction of 30 m L glycol solvent at 180℃for 12 h.The S/Co₃O₄composite anode material with a sulfur ratio of 60 wt%was prepared by the combination of the sublimated sulfur and the lithium-sulfur battery was assembled together with the lithium sheet as a negative electrode,and the electrochemical performance was good.The initial discharge capacity is 953.4 m Ah g^-1at 0.2 C,and 330.7 m Ah g^-1remains after 200 cycles.The average capacity decay rate per cycle is about 0.22%.However,due to the poor conductivity of elemental sulfur and the final product Li₂S,its rate performance is still not ideal,and it is necessary to add other conductive materials on the basis of S/Co₃O₄microsphere composite to improve it.（2）Sericin was selected as biomass carbon source,S/Co₃O₄microspheres were coated by solution immersion method,and then the S/Co₃O₄@SC composite was formed by high temperature calcination.The surface of sericin carbon was modified to form a conductive network,and the electrochemical performance was improved.The initial discharge capacity of the lithium-sulfur battery with S/Co₃O₄@SC composite as positive electrode is 911.8 m Ah g^-1,and it can retain 435.4m Ah g^-1after 500 cycles.Compared with S/Co₃O₄,its specific capacity and cyclic stability are greatly improved.At the same time,its rate performance has been significantly improved,and it can withstand the maximum charge and discharge current density of 4 C,among which,301 m Ah g^-1capacity output can be guaranteed at 2 C,showing the overall improvement of electrochemical performance.（3）In order to continue to verify the role of conductivity additives,this paper also used reduced graphene oxide（r GO）to cover S/Co₃O₄microspheres,and fabricated S/Co₃O₄@r GO composite as a positive lithium sulfur battery.The initial discharge capacity of S/Co₃O₄@r GO composite is 1131.6 m Ah g^-1at 0.2 C,and 545.6 m Ah g^-1still remains after 500 cycles.Moreover,it can charge and discharge at a maximum current density of 5 C,which proves its excellent cycle performance and rate performance.In conclusion,in this paper to Co₃O₄based composite cathode material,according to a study using Co₃O₄hollow structure contribute to solving the positive more volume expansion and the dissolution of sulfide sulfur,supplemented by conductive additives,which can effectively overcome the existing lithium sulfur batteries battery anode materials technical barriers,so that its comprehensive performance is the overall ascension,It provides necessary theoretical and practical basis for the market application of lithium sulfur battery.