Selenium,Layered Metal Sulfides/Functionalized Carbon Composite Materials:Preparation And Lithium/sodium Storage Properties

The lithium-ion battery?LIB?systems have been widely applied in the society owing to its high energy density,outstanding cycling performance and environmental friendliness.However,with the development of electric vehicle and large-scale energy storage equipment,the new demands for LIB systems are desired.The commercial lithium-ion batteries are restricted in the large scale energy storage application because of the shortage and local distribution of lithium resource.It is urgent to develop the new electrode materials and new battery systems.Among the cathode materials,selenium has obtained the increased attention owing to its high volume energy density and mass energy density.In recent years,sodium-ion batteries have become the research focus in view of the abundance and low-cost of sodium.The layered metal sulfides,such as SnS₂ and MoS₂,have been generally researched as the promising anode materials which have unique structure and high specific capacity for lithium-ion and sodium-ion storage.Although the layered metal sulfides and selenium have high theoretical specific capacity,the volume change,the low Coulombic efficiency of the first cycle and serious capacity decline make them disappoint to act as the electrode materials.The above problem can be effectively relieved when the layered metal sulfides and selenium are combined with carbon matrix.In this paper,different carbon materials including nitrogen doped porous carbon spheres and reduced graphene oxide were successfully synthesized to serve as the electric conductive matrix to support the active materials such as selenium and layered metal sulfides.Through the characterization analysis of the composites,the interaction between the structure and the electrochemical performance was clearly illuminated.The experimental contents were subdivided into the several sections as follows:?1?The prepared polypyrrole spheres were carbonized and chemically activated with KOH to prepare the nitrogen-doped porous carbon spheres.Then,the Se/N-MPCS composites were prepared by infusing the selenium into the micropores through melting method.The confine function of micropores and the existed Se-O bonding were beneficial for the transformation between Se and Li2Se.These features assured the outstanding electrochemical performance when Se/N-MPCS acted as the cathode material.The Se/N-MPCS can deliver 570 mAh g-1 at 0.5 C for 350 cycles.For Se/N-MPCS,the specific capacity as high as 460 mAh g-1 can be retained after 1600 cycles at 2 C.Even at 4.7 C,Se/N-MPCS can also deliver a reversible capacity of 440 mAh g-1.?2?The SnS₂ NC/EDA-RGO composites were synthesized through one-pot amine-thermal route at low temperature.When the reduced graphene oxide was functionalized with ethylenediamine?EDA?,the SnS₂ nanocrystals were grown on the EDA-functionalized graphene.The SnS₂ NC/EDA-RGO anode has some merits.First,the SnS₂ nanocrystals were ultrasmall with 2-4 nm in size.Second,the chemical bond existing between SnS₂ and ethylenediamine was demonstrated by the structure characterization and theoretical calculation.These merits were beneficial to deliver outstanding electrochemical properties.When acted as the anode materials for sodium-ion batteries,SnS₂ NC/EDA-RGO delivered 680 mAh g-1 at 200 mA g-1 after 100 cycles.The pure SnS₂ can only delivered 43 mAh g-1 at the same conditions.After being cycled at 1 A g-1 for 1000 times,the reversible capacity of SnS₂ NC/EDA-RGO was 480 mAh g-1 and the capacity retention was 85%.However,the reversible capacity of SnS₂/RGO was only 255.6 mAh g-1 after 1000 cycles,corresponding to the capacity retention of 43.8%.SnS₂ NC/EDA-RGO can still maintain a reversible capacity of 250 mAh g-1 at 11.2 A g-1.These results demonstrated that the ethylenediamine modified graphene was more competent than pristine graphene to improve the sodium storage properties.?3?The graphene oxide was thermally reduced in the argon atmosphere to prepare nitrogen-doped graphene sheets?NGSs?with the introduction of cyanamide.The SnS₂ nanocrystals were loaded on NGSs through the solvothermal method.In the contrast experiment,the graphene oxide was reduced with NaHSO₃ and then thermally treated to obtain the reduced graphene oxide.During this process,the accumulation was inevitable and the mass loading of SnS₂ was only 60 wt%.With the aid of cyanamide,the cyanamide can form the coating layer on the surface of graphene oxide to effectively prevent the restack of graphene.Benefiting from the good distribution,the SnS₂ load in SnS₂-NGS was 75 wt%.When serving as lithium-ion anode,SnS₂-NGS can sustain the capacity of 1407 mAh g-1 at 200 mA g-1 after 120 cycles.At the current density of 0.8 A g-1,SnS₂-NGS can deliver reversible capacity of 914 mAh g-1 after being cycled for 150 times.However,the reversible capacity of SnS₂-GN was 590 mAh g-1 under the similar conditions to that of SnS₂-NGS.This contrast showed that the nitrogen-doped graphene was significative in promoting the electrochemical performance.SnS₂-NGS can also act as the anode materials for sodium-ion batteries.SnS₂-NGS delivered 450 mAh g-1 at 200 mA g-1 after 100 cycles and 148 mAh g-1 at 10 A g-1.?4?SnS₂ and MoS_x nanoparticles were grown on reduced graphene oxide?MoS_x/SnS₂-GS?at the same time based on the principle of charge attraction.Both MoO42-and graphene oxide were negative.The MoS_x particles were difficult to attach to the surface of reduced graphene oxide because of the charge repulsion.This problem can be modified by the addition of Sn4+,which also acted as the raw material for SnS₂.The synergistic effect among MoS_x,SnS₂ and reduced graphene oxide and discrepant electrochemical reactions were beneficial to the excellent electrochemical performance.When acting as the anode material for lithium-ion batteries,MoS_x/SnS₂-GS can deliver better cycling stability than MOS₂-GS and SnS₂-GS.For sodium-ion battery?SIB?test,MoS_x/SnS₂-GS can deliver 655 mAh g-1,612 mAh g-1 and 546 mAh g-1 at 0.15 Ag-1,0.75 Ag-1 and 1.5 Ag-1,respectively.