Preparation Of Metal Sulfide-Titanium Dioxide Heterostructures And Applications In Lithium Sulfur Batteries

Lithium sulfur battery（LSBs）is regarded as one of the most promising secondary energy storage systems,because of high theoretical specific capacity(1675mAh g^-1),much higher than the lithium ion battery.However,their commercial application is hindered by many problems,such as poor conductivity of sulfur（S）and its discharge product lithium sulfide（Li₂S）,large volume expansion（about 80%）during discharge and charge,"shuttle effect"of LiPSs and sluggish oxidation of discharge product Li₂S.The problems can be solved by selecting and desiging multifunctional sulfur host.In recent years,the application of heterostructural host have been widely applied.Heterostructures are comsist of different semiconductors with similar crystal structures,similar atomic spacings,and the coefficient of thermal expansions.Then the chemical composition and charge distribution in the heterointerface due to the difference band structure,carrier concentration and Fermi level.Heterostructures with synergistic effects,have the following characteristics:1)the performance of semiconductor heterostructure is much better than single semiconductor;2)The heterostructures have band gap,which improves the electrical conductivity;3)The internal electric field of the heterointerface can accelerate the diffusion kinetics of lithium ion（Li⁺）and reduce the diffusion energy barrier;4)The strong interaction of chemical force and van der Waals force,improves the stability of structure and enhances the cycle stability.5)The charge redistribution due to heterointerface will form more active sites.Titanium dioxide（TiO₂）is a low-cost polar oxide n-type semiconductor.By forming S-Ti-O bond,TiO₂ has strong adsorption to LiPSs due to the formation of S-Ti-O bonds,which adsorpting LiPSs efficiently.Moreover,TiO₂can also accelerate the oxidation of Li₂S.WS₂ and MoS₂ are two-dimensional metal sulfide semiconductor（n-type of WS₂,p-type of MoS₂）with stable catalytic interface,good LiPSs catalytic and electrical conductivity.The formation of metal sulfide-titanium dioxide heterostructure has a low band gap,and the internal electric field at the heterointerface accelerates the diffusion of Li⁺,and strong adsorption and catalytic ability of LiPSs,realizing the rapid"adsorption-diffuse-transformation",inhibiting the shuttle effect,and realizing the stable cycle of the battery.Details of the work are as follows:（1）Preparation and application of WS₂-TiO₂ hollow spheres（THSs）heterostructures and their application of LSBs.The THSs was prepared by hydrothermal method,and then the tungsten disulfide titanium dioxide hollow spheres（WS₂-THSs）were constructed by growing WS₂ nanosheets on the surface of THS.The results show that the heterostructure of WS₂-THSs have good conductivity,large specific surface area,good adsorption and catalytic ability of LiPSs and strong oxidation ability on Li₂S.Therefore,the initial capacity of WS₂-THSs heterostructure cathode provide initial specific capacity of 1091.16mAh g^-1at 0.3C(1C=1675mAh g^-1).Even at the high rate of 10C,there is still 378.71 mAh g^-1;After 800 cycles,the battery can still maintain the specific capacity of 321 mAh g^-1,with a low single-cycle decay rate of 0.075%,showing good rate performance and cycle stability.（2）Preparation of WS_2-TiO₂nanotubes（TNTs）heterostructures and their application of LSBs.TNTs were prepared by hydrothermal method.Then,WS₂nanosheets were grown on the surface to prepare tungsten disulfide-titanium dioxide nanotubes（WS₂-TNTs）as cathode.WS₂-TNTs heterostructues have good electrical conductivity,good adsorption and catalytic ability to LiPSs and strong oxidation ability to Li₂S.Compared with THSs,TNTs have larger specific surface,which provide more active sites.Therefore,WS₂-TNTs heterostructure electrode provide initial specific capacity of 1058.57 mAh g^-1at 0.3C,and 510 mAh g^-1at 8C even at a high rate of 8C.After 1100 cycles,the battery can still maintain 297 mAh g^-1capacity,with a low single-cycle decay rate of 0.041%,showing better cycle stability.（3）Preparation of MoS_2-TiO₂nanotubes（TNTs）heterostructures and their application of LSBs.TNTs were prepared by hydrothermal method.Then,MoS₂nanosheets were grown on the surface to prepare molybdenum sulfide-titanium dioxide nanotubes（MoS₂-TNTs）heterostructure.MoS₂-TNTs heterostructure have good electrical conductivity,and good adsorption and catalytic ability to LiPSs.WS₂-TNTs is an n-n type heterojunction with a lower internal electric field generated at its interface,while MoS₂-TNTs is a p-n type heterojunction with a stronger internal electric field.Therefore,the conversion ability of long-chain LiPSs to short-chain LiPSs（liquid-liquid conversion）of MoS₂-TNTs is stronger.MoS₂-TNTs heterostructure electrode provide initial specific capacity of 1086.2 mAh g^-1at 0.1C,and 589.68 mAh g^-1even at 4C.After 800 cycles,the battery can still maintain the specific capacity of 297.3mAh g^-1,with a low single-cycle decay rate of 0.065%,showing good rate performance and cycle stability.In conclusion,the metal sulfide-titanium dioxide heterogstructures were formed with different morphologies and configurations.All of them show high specific capacity and good cyclic stability.Therefore,this paper can provide an experimental idea for the commercialization of LSBs.