Study On Performance Of Lithium-Sulfur Battery Based On Co₃O₄ Composite Cathode Material

Lithium-sulfur?Li-S?batteries have received increased attention in recent years based on the high theoretical specific capacity?1675 m Ah g-1?of sulfur cathodes and the high energy density?2600 Wh kg-1?achievable with lithium-metal anodes.Additionally,sulfur occupies abundant natural reserves and has low environmental impact that enable it to be applied to energy sustainable development.However,the commercialization of Li-S batteries is hindered by several significant challenges,including the shuttle effects of intermediate polysulfide,large volumetric expansion upon lithiation,and poor electronic conductivity.In the thesis,the S@Co₃O₄,S@CNT/Co₃O₄,and S@C/Co₃O₄ composite were desigined to study the effects of different components and structures.The physical and electrochemical properties of the matrials were systematicly examined using a variety of characterization methods.The major research contents and result of this thesis are as follows:?1?Co₃O₄ hollow microspheres with a diameter of approximately 1.95 ?m were synthesized through a template-free solvothermal method.The as-abtained Co₃O₄ hollow microspheres were characterized by scanning electron microscope?SEM?,X-ray diffraction?XRD?,and Thermogravimetric Analysis?TG?.Additionally,we studied the effects of different experimental factors on the morphology of Co₃O₄ and determined the optimal experimental conditions for the preparation of Co₃O₄ precursor: 375 mg of cobalt acetate?Co?CH3COO?2.4H2O?,22.5 mg of polyvinyl pyrrolidone?PVP,Mw = 40000 g mol-1?,30 m L of ethylene glycol,reaction time of 12 h,The heating mode is to start heating at 180 oC.S@C/Co₃O₄ composite cathode material was prepared by using Co₃O₄ hollow spheres as carrier and sublimation sulfur by melt-irrigation method.The electrochemical performance test results show that the S@C/Co₃O₄ hollow spheres with a sulfur content of 70 wt.% have an initial specific capacity of 1074.8 m Ah g-1 at a current density of 0.2 C.After 200 cycles,the capacity was 483.7 m Ah g-1.?3?Carbon nanotubes-warpped Co₃O₄ spheres composite?CNT/Co₃O₄?was synthesized by CTAB modified conducting Carbon nanotubes and Co₃O₄ hollow spheres.When the mass ratio of CNT to Co₃O₄ is 4:10,the interweaving degree of carbon nanotubes is moderate.The network structure of carbon nanotubes is well coated on the surface of the Co₃O₄ hollow spheres.By measuring the electrochemical performance of the S@CNT/Co₃O₄ electrode,the S@CNT/Co₃O₄ composite electrode has the inital discharge specific capacity of 929.7 m Ah g-1 at 0.5 C current density,and still retains 48.3% capacity after 200 cycles,which showing better cycle performance and rate performance.?3?a novel S@C/Co₃O₄ composite with sulfur encapsulated by sericin-derived carbon/Co₃O₄ hollow spheres was developed as a cathode for Li-S batteries in this study.Co₃O₄ hollow microspheres were synthesized utilizing a template-free solvothermal method.A C/Co₃O₄ composite was formed by immersing the Co₃O₄ spheres into a sericin solution.With a sulfur content of 67.3 wt%,the S@C/Co₃O₄ cathode exhibited excellent electrochemical performance with a high initial discharge specific capacity of 1171.6 m Ah g-1 at a rate of 0.2 C.High reversible capacity was maintained at 495.4 m Ah g-1 over 700 cycles at 0.2 C with a low decay rate of 0.049%.Under high constant currents of 1 C and 2 C,S@C/Co₃O₄ also provides excellent long-term cyclability over roughly 1000 cycles.The excellent electrochemical properties of the S@C/Co₃O₄ composite are mainly attributed to strong polar chemical bonding with polysulfides from Co₃O₄,the encapsulation of sulfur by hollow spheres,and the conductive characteristics of sericin-derived carbon materials.Throught the research in this thesis,sericin is used as a conductive material to modify Co₃O₄ hollow microspheres which are composed of flakes and then coated with sulfur.As the cathode of lithium sulfur battery,it has excellent cycle stability,which provides a better experimental idea for solving the problems faced by lithium sulfur battery.