Preparation Of Molybdenum-based Composites And Its Applications In Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries have attracted considerable attention for large-scale electrochemical energy storage applications,which attribute to the high theoretical specific capacity(1675 mAh g^-1),high energy density(2600 Wh kg^-1),as well as the abundant and low cost of the sulfur element cathode.However,the practical applications of Li-S batteries suffered from several drawbacks:1.Highly soluble intermediates（Li₂S_n,4≤n≤8）result in irreversible capacity loss,shuttle effect and lithium plate corrosion;2.Inherent poor electrical conductivity of sulfur and its corresponding discharge products（Li₂S/Li₂S₂）lead to low sulfur utilization and sluggish reaction kinetics;3.Electrode structure was destoried due to volume change between sulfur and Li₂S.In this paper,the cathode and separator are modified to improve the performance of Li-S batteries by the strong adsorption of the cathode can inhibit the dissolution of polysulfide,and the separator as a barrier to block the diffusion of polysulfide to improve the utilization rate of active substances.The main research contents are as follows:（1）The Bi₂MoO₆/rGO composites were synthesized in situ by solventthermal method.The morphology and electrochemical performance of the Bi₂MoO₆/rGO composites with different the Bi₂MoO₆ loading were investigated.The 3D structure composites composed of hollow sphere-like Bi₂MoO₆ and rGO was prepared when the Bi₂MoO₆ loading was 25 wt%in the reactant.After the composite material was compounded with sulfur,which showed high specific capacity and superior cycling stability.The initial discharge capacity was 1032 mA h g^-1 at 0.2 C,and the capacity was 552 mA h g^-1 after 180 cycles.Because Bi₂MoO₆ hollow spheres assembled with nanosheets possess the strong adsorption for polysulfide,and the high conductivity of3D reticular structure carbon skeleton can facilitate the reaction kinetics of sulfur and its corresponding discharge products,which can improve the utilization ratio of sulfur,respectively.（2）The separator was modified by the the stratified structure Bi/MoO₂/rGO composites to improve the performance of Li-S batteries.The electrochemical performance of the Bi/MoO₂/rGO composites were investigated.When Bi/MoO₂/rGO composites were prepared at 450°C,Li-S batteries shows superior electrochemical performance.The initial discharge capacity was 985.7 mAh g^-1 at 0.5 C,and the discharge capacity was 629 mAh g^-1 after 100 cycles,which are attributed to the excellent electrical conductivity of composites.It can lower sulfur interface resistance between the cathode and the separator,so the deposition of"dead sulfur"was further activated to improve the utilization of active material.The strong chemical adsorption of MoO₂ for polysulfides can restrain the diffusion of polysulfides effectively.（3）The nitrogen-doped carbon coated Mo₂N nanomaterials were prepared by coordination compound decomposition method,which were used as coated separator material for Li-S batteries.The interaction between the composition of nitrogen-doped carbon coated Mo₂N nanomaterials and polysulfide were analyzed through XPS characterization in detail.The electrochemical performance of Li-S batteries assembled with the nitrogen-doped carbon coated Mo₂N modified separator were improved greatly.When the current density was 0.2 C,the initial discharge capacity was 1058.6 mAh g^-1,and discharge capacity was 597.9 mAh g^-1 after 400 cycles.The Li-S batteries showed excellent cycle stability,because the Mo₂N and Doped nitrogen atoms were benifit for chemically interacting with the polysulfid and effectively restrains polysulfides diffusion.