Functional Modification Of Nano-Sulfur And Its Performance In Lithium-Sulfur Batteries

In the era of new energy,it is imminent to develop rechargeable batteries with high energy density and long life.Because of its excellent theoretical specific capacity(1675m Ah g^-1)and extremely high energy density(2600Wh Kg^-1),lithium-sulfur batteries are considered to be commercialized high-energy power sources.However,the inherent insulating properties of sulfur,large volume expansion（～80%）,and"shuttle effect"hinder the commercial development of lithium-sulfur batteries.To this end,this paper is based on the research on the functionalization of nano-sulfur,and the modification of nano-sulfur anchored on carbon-based materials to construct a composite cathode material with high performance.Various characterization methods were used to explore its electrochemical performance.The specific research content and analysis results are as follows:（1）Phosphorized nano-sulfur（P-NS）particles were dispersed and anchored on monolayer graphene oxide（MGO）by a hydrothermal method.Among them,phosphating is beneficial to improve the conductivity of highly electrochemically active nano-sulfur.Taking advantage of the good dispersion of graphene quantum dots（GQDs）and good biocompatibility with MGO,the P-NS is firmly anchored on MGO through chemical bonding with GQDs as the intermediary.Meanwhile,combined with density functional theory（DFT）calculations,the phosphating and robustly anchored P-NS effectively inhibit the dissolution of intermediates in ether-based electrolytes.Thus,the synergistic effect of phosphating and interfacial interactions leads to high energy density and ultralong cycle life,with specific discharge capacities of 1117 and 838 m Ah g^-1 at 0.1 and 1.68 A g^-1,respectively.After charging and discharging 1000 times at 1.68A g^-l,the capacity fading rate per cycle is 0.041%.（2）By a solution self-assembly method,polypyrrole（PPy）-coated monodisperse nanosulfur spheres（NS@PPy）were uniformly bonded to the surface of acidified carbon nanotubes（CNTs）,and a novel coaxial cable structure was designed（CNT/NS@PPy）.Polymer conductive coating polypyrrole can not only provide an effective electron conduction path,but also have strong physical and chemical constraints on polysulfides.NS@PPy is firmly anchored on the CNT surface through interfacial chemical bonds,and the two synergistically positive electrode has good reversible performance,fast reaction kinetics and excellent mechanical flexibility.The results show that the CNT/NS@PPy cathode has a specific discharge capacity of 1065m Ah g^-1 in the first cycle at a rate of 0.1C,and a discharge capacity of 675 m Ah g^-1after 1000 cycles at a constant current density of 1C.The capacity decay rate per turn is 0.016%.（3）Manganese dioxide（MnO₂）nanosheets were grown on the surface of polyethylenedioxythiophene（PEDOT）coated nanosulfur spheres（NS）by in-situ growth method,and a core-shell cathode material（CNT/NS/PEDOT@Mn O₂）,PEDOT conductive coating is used to solve the problem of low conductivity of sulfur and acts as a protective layer to prevent polysulfide dissolution.The functionalized Mn O₂nanosheets on PEDOT further provide a high active contact area,enhance the wettability of the electrode material with the electrolyte,and further interconnect the polymer chains to improve the conductivity and stability of the composite.The results show that the CNT/NS/PEDOT@Mn O₂ cathode has an initial specific capacity of1082m Ah g^-1 at a rate of 0.1C.At a high current density of 1C,the discharge capacity after 1000 cycles is 408 mAh g^-1,and the capacity retention rate is 50.3%.