| Due to the high theoretical specific capacity and energy density,large sulfur storage and low cost,lithium sulfur battery is regarded as the next generation energy storage technology.However,the poor conductivity of elemental sulfur and shuttle effect of the polysulfides which are originated from the electrochemical process as well as the volume expansion hinder the commercialization prospect of lithium sulfur battery.In recent years,researchers have designed sulfur host materials,modified membranes and optimized electrolyte additives to solve those above problems.The researchs show that the high performance sulfur host materials can effectively inhibit the shuttle effect and improve the conductivity of elemental sulfur.In this work,based on the electrode materials and the design of the micro-nano structure,we design two sulfur host materials that combine the high conductivity,strong adsorption capability and high catalytic activiy.Benefiting from multiple functions,the electrochemical performance of lithium sulfur battery is enhanced by carbon compound hollow cobalt oxide microspheres（C-Co O）and cobalt selenide microspheres（C-Co Se）.As the sulfur host materials enhanced electrochemical performance of lithium sulfur battery,the redox kinetics mechanism is systematically studied by in-situ Raman spectroscopy,density functional theory calculation and other experimental characterization methods.The specific research contents and results are presented as below:（1）The C-Co O microspheres were prepared by one-step hydrothermal and annealing method.The XRD,SEM and Raman characterization show that the material is hollow C-Co O microsphere.The polar Co O and hollow structure can effectively capture polysulfides and retain active materials by chemical adsorption and physical confinement,respectively.Moreover,the composite with conductive carbonaceous improves the conductivity and electrochemical performance of the sulfur cathode.The results of adsorption,symmetrical cell and impedance tests show that C-Co O can suppress the shuttle effect that orginates from the polysulfides and promote the conversion kinetics of polysulfides.As the sulfur host of the lithium sulfur battery,the prepared S@C-Co O cathode exhibits initial capacity of 1247.8m Ah g-1 at 0.2C;After cycling for 400 cycles at 1C,the capacity retention is 319 m Ah g-1.The research results indicate that C-Co O microspheres are potential sulfur host materials which can improve the electrochemical performance of lithium sulfur batteriesis.（2）Hierarchical carbon-encapsulated Co Se（designated as C-Co Se）hollow core-shell sphere is designed and constructed to relieve shuttle effect of the polysulfides,thereby leading to the performance enhancement of the lithium sulfur batteriesis.The prepared C-Co Se spheres comprise of spherical nano-aggregates as hollow core and N-doped carbon shell.Kinetics experiments and theoretical calculation as well as in situ Raman spectra reveal that the C-Co Se hollow core-shell spheres can effectively immobilize the polysulfides intermediates by chemical adsorption and physical confinement,and meanwhile accelerate conversion kinetics of the polysulfides owing to strong catalytic effect and superior electron conductivity during electrochemical reaction,thereby the shuttle effect being effectively inhibited.In addition,as a sulfur host,sufficient sulfur storage space related to hierarchical porous structure enable effective accommodation of volume change and full efficacy with sulfur loading of 73%.Benefiting from above advantages,the C-Co Se sulfur cathode exhibits impressive rate performance and excellent cycle stability.Even at a sulfur loading of 3 mg cm-2,a capacity of 584 m Ah g-1 at 0.5C is achieved and the capacity is maintained 450 m Ah g-1 after 200 cycles.Our results provide an effective strategy to design advanced sulfur hosts for the performance enhancement of the lithium sulfur batteries. |
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