| As a new generation of energy storage devices,lithium-sulfur battery has high energy density and specific capacity,which is expected to solve the demand of high energy density energy storage system.However,the slow redox kinetics and shuttle effect lead to low cell capacity and coulomb efficiency as well as poor cycle stability,which hinder the practical application of Li-S batteries.Great progress has been made in the adsorption of lithium polysulfide（Li PSs）by polar materials,but the shuttle effect cannot be fundamentally inhibited.In recent years,it has been found that the introduction of a catalyst which can accelerate the conversion of Li PSs into lithium-sulfur battery system is a more effective strategy to suppress the shuttle effect.Transition metal compounds,including transition metal sulfides,nitrides and oxides,are commonly used as catalysts in the field of lithium sulfur batteries because of their catalytic activity for most reactions.Due to its strong interaction with Li PSs,the catalytic conversion of Li PSs can be accelerated.However,these catalysts have fewer active sites,and only a limited number of atoms on the surface and edges are reactive.By adjusting the surface electron and intrinsic structure of the material,it is hoped to improve the catalytic activity and accelerate the redox kinetics for improving the electrochemical performance of lithium sulfur batteries.In this paper,by constructing defect engineering and crystal structure to regulate the coordination structure of transition metal compounds,more active sites are exposed to promote the catalytic conversion of Li PSs,and obtain good lithium-sulfur electrochemistry.The specific research contents are as follows:（1）A flexible polypyrrole（PPy）coated rigid Mo S2 nanoflower structure（PPy@DR-Mo S2）with sulfur defects was synthesized by a simple method,which effectively improved the performance of the battery.The abundant sulfur defects in Mo S2 nanocrystals can not only effectively adsorb polysulfide,but also accelerate the reaction kinetics and effectively suppresse the shuttle effect.As a coating layer,PPy can physically adsorb the polysulfide,alleviate volume expansion,and carry out chemical adsorption on polysulfide through its functional groups.With PPy@DR-Mo S2/S as the positive electrode,the capacity decay rate of the lithium-sulfur battery is 0.17%after 300 cycles at 1 C.（2）Fe OOH nanosheets with Fe-rich vacancies were synthesized by wet chemistry and grown in situ on r GO/CNTs as sulfur positives（v-Fe OOH/r GO/CNTs）.A network of graphene and carbon nanotubes could provide pathways for electrons and lithium ions to conduct.Fe vacancies（Fe Vs）in Fe OOH nanosheets are negatively charged and can serve as Lewis base sites in the cathode.The Lewis base sites serve as lithium ion anchor and sulfur species accelerator through the strong SxLi...Fe Vs interactions,leading to good electrode kinetics.Adsorption experiments and DFT theoretical calculations further confirm that Fe vacancy can enhance the adsorption of polysulfide and promote the catalytic conversion of polysulfide.Thanks to these advantages,the v-Fe OOH shows superior electrochemical performance,with a high initial specific capacity of 1645.5 m Ah g-1 at 1 C and a discharge specific capacity of 320.8m Ah g-1after 800 cycles.Even at 5 C,after 300 cycles,the capacity decay rate is only 0.033%.（3）Compared with crystal nanomaterials,amorphous nanomaterials with long-range disordered atomic structures have a number of different properties,such as unsaturated electron configuration,atomic scale structural flexibility and unique chemical homogeneity,showing superior potential for catalyzing various electrochemical reactions.In view of this feature,amorphous Co Mo S4（Co Mo S4/r GO）composites grown on reduced GO were prepared by a simple hydrothermal method as catalysts for the modification of lithium-sulfur battery separator,coating on the positive side.The reduced GO provides sufficient reaction interface for the active sulfur to effectively mitigate the volume change during the reaction.The unique amorphous structure offers multiple advantages as follows:amorphous Co Mo S4 has a larger specific surface area and exposes more active sites,which is conducive to accelerate the transformation of polysulfide.In addition,the distortion of local structures and random electron configurations in amorphous Co Mo S4 can accelerate the charge transfer between the active site and the intermediate,which can speed up the reaction kinetics.Benefiting from the above advantages,Co Mo S4/r GO modified separator has an initial capacity of 1350 m Ah g-1at 1 C.And Co Mo S4/r GO battery discharge capacity still reached 390.2 m Ah g-1after 300 cycles,corresponding to the average capacity per cycle attenuation rate of 0.236%. |
