Preparation And Electrochemical Study Of Transition Metal Oxide Cathode Materials For Lithium-Sulfur Batteries

As one of the new-generation electrochemical energy storage systems,lithium-sulfur batteries（LSBs）have outstanding advantages such as environmental friendliness,low material cost,high theoretical specific capacity(1675 m Ah g^-1)and high energy density(2600 Wh kg^-1),which can be widely applied in aerospace,unmanned aerial vehicles,electric vehicles and various portable electronic devices.However,practical applications of lithium-sulfur batteries face a series of challenges due to series of issues within sulfur cathode,such as the low conductivity of sulfur and Li₂S,serious volume expansion of sulfur during charge and discharge processes,and the shuttle effect of polysulfides.These problems lead to the decrease of coulombic efficiency,sulfur utilization and cycling stability of lithium-sulfur batteries,hindering their further development.As one kind of sulfur cathode modifiers,transition metal oxides（TMO）exhibit strong polarity,low cost,and simple preparation processes.They can effectively adsorb polysulfides to suppress the shuttle effect,thereby improving sulfur utilization efficiency and enhancing electrochemical performance of Li-S batteries.Therefore,this paper focuses on the electrochemical properties of diverse TMO materials applied as additives,hosts,and separator decorators in lithium-sulfur batteries.The influences of TMO material types,crystal structures,and microstructural morphologies on electrochemical performance of Li-S batteries are also analyzed.Furthermore,by integrating TMO with the high-conductivity and mesoporous carbon materials（such as carbon nanotubes（CNTs）,porous carbon spheres（CS）,etc.）,the conductivity,porosity,distribution homogeneity of pores,and specific surface area of the S cathode are improved,effectively suppressing the shuttle effect of polysulfides and enhancing sulfur utilization.Additionally,by utilizing the tunability of pore sizes of transition metal oxide and the spontaneous polarization property of ferroelectric materials,heterogeneous structures of TMOs are constructed to increase active sites,accelerate the adsorption and conversion of polysulfides,and further suppress the shuttle effect of polysulfides,thereby enhancing the cycling stability of lithium-sulfur batteries.The specific researches are as follows:1.In order to reduce the shuttle effect of polysulfides and improve the electrical conductivity of sulfur cathode,a composite material（Ni O@CNTs）via the combination of nickel oxide（Ni O）and carbon nanotubes（CNTs）with a high aspect ratio was designed.The effects of electrical conductivity and microstructure of Ni O@CNTs on the adsorption performance and electrochemical properties of polysulfides were investigated.The Ni O@CNTs/S composite maintained a specific capacity of 642 m Ah g^-1 after 300 cycles at 1.0 C,with a coulombic efficiency over 99.3%.Ni O@CNTs improved the conductivity of the sulfur cathode,suppressed the dissolution of polysulfides,and enhanced the electrochemical performance of lithium-sulfur batteries.2.By leveraging the advantages of high specific surface area and high porosity of porous carbon spheres（CS）and the high adsorption capacity and chemical stability of transition metal oxide titanium dioxide（TiO₂）,a core-shell structured porous TiO₂@CS/S was synthesized to significantly reduce the shuttle effect of polysulfides.Electrochemical performance of Li-S batteries and characterizations of material revealed that TiO₂@CS/S exhibited high conductivity,stability and excellent adsorption properties for polysulfides.After 400 cycles at 1.0 C,TiO₂@CS/S still retained 81%of its initial capacity(726 m Ah g^-1).Additionally,TiO₂@CS/S demonstrated excellent rate performance,providing a specific capacity of 366 m Ah g^-1 at a high current density of 4.0 C.This effectively suppressed the shuttle effect of polysulfides and improved the utilization of the sulfur.3.By utilizing the advantages of heterogeneous structures that combine different TMO materials,high mesoporous TiO₂ hollow spheres were chosen as templates for the in-situ growth of ferroelectric material BiFeO₃ with spontaneous polarization.This resulted in the formation of a BiFeO₃@TiO₂ heterogeneous structure.The BiFeO₃material exhibited excellent adsorption properties for polysulfides,while the TiO₂template provided a unique pore size distribution.This combination significantly suppressed the shuttle effect of polysulfides.Based on the BiFeO₃@TiO₂ heterogeneous structure,the lithium-sulfur battery demonstrated an extremely high initial specific capacity of 1303 m Ah g^-1 at a current density of 0.1 C.The residual capacity still retained85%of the initial capacity after 800 cycles at 1.0 C,reaching 754 m Ah g^-1 with good cycling stability.