Transition Metal Doped MoS₂/Graphene Nanocomposites For Enhanced Lithium Storage Properties

In recent years,two-dimensional?2D?layered transition metal sulfides nanomaterials become hot topics due to its physical and chemical properties can be improved by adjusting their morphology,layers,and lattice structure.Among them,hexagonal MoS₂ nanosheets as one of important candidates of LIB anode materials,because its weak interlayer van der Waals forces and capable of storing Li ions by the four-electron lithization reaction.Graphene has unique two-dimensional layered structure,higher conductivity,larger specific surface area and better flexibility than other carbon materials.The combination with molybdenum disulfide can not only effectively limit its volume expansion effect in the process of charging and discharging,but also provide it with excellent conductive network.In this paper,L-cysteine,sodium molybdate and GO were used as precursors,MoS₂/rGO and X-MoS₂/rGO?X=Fe,Ni,Mn,Zn?composites were successfully synthesized by a simple one-step hydrothermal method,and their microstructure and electrochemical properties of lithium storage were characterized and analyzed.This paper comes to the following conclusions.Firstly,the MoS₂/rGO composites with different molybdenum-carbon ratios(m_Mo:m_C=1:3,1:2,2:3)were synthesized by controlling the dosing amount of sodium molybdate and GO.Compared with m_Mo:m_C is 1:3 and 2:3,the 1:2 composite material have better performance,the first discharge capacity can reach 1083 mAh/g at a current density of 100 mA/g,and a reversible capacity of 534.5 mAh/g after 50 cycles with coulombic efficiency is 98.6%.Secondly,according to the above best ratio m_Mo:m _C,X-MoS₂/rGO?X=Fe,Ni,Mn,Zn?composites were synthesis by a simple hydrothermal with introduction of ferrous sulfate,nickel acetate,manganese chloride and zinc acetate as dopant.The doping molybdenum disulfide which exhibit petal-like architecture sticks tightly to graphene,the Ni-MoS₂/rGO nanosheet is most small and uniform distribution,Fe-MoS₂/rGO times,compare with the former two composites,the sheet size of Mn-MoS₂/rGO and Zn-MoS₂/rGO are larger and more easer of stacking.The results which were proved by the analysis of XRD,Raman and XPS showed that Fe,Ni,Mn and Zn partially replaced Mo elements in the layered MoS₂ to form Fe-MoS₂,Ni-MoS₂,Mn-MoS₂ and Zn-MoS₂ composites.The results of SEM,TEM and HRTEM have displayed that the coiled MoS₂ nanosheets are well fixed on the surface of graphene sheets to construct a highly interconnected three-dimensional porous network;The basal surface of the MoS₂ nanosheet exhibits an exposed feature,which provides a larger surface area for electrolyte contact and lithium ion storage;The MoS₂ nanosheet is about 5-10 layers,and the interlayer spacing is 0.72 to 0.96 nm approximately,larger than 0.62 nm of?002?face of bulk MoS₂,which can be ascribed to the doping of Fe,Ni,Mn and Zn atoms or the insertion of other species.The adsorption/desorption isotherm test shows that the composite materials have a unique dual pore structure,the mesopores is about 33.4 nm,and the large pores can reach to 294.3 nm approximately,The type of two hole structure were derived from the composite material laminated composite material itself alternating between building,the dual pore structure can accelerate the diffusion rate of ions on the one hand,on the other hand can also be caused by the effective control of lithium volume expansion effect.The X-MoS₂/rGO exhibited excellent electrochemical performance.At a current density of100 mA/g,the reversible capacities of Ni-MoS₂/rGO,Fe-MoS₂/rGO,Mn-MoS₂/rGO and Zn-MoS₂/rGO after 100 cycles were 969.7,945.7,549.5 and 726.8 mAh/g,respectively.The specific capacity of 855.5,728.3,477.5 and 364.6 mAh/g are maintained even at a large current density of 1 A/g.As a result,compare with undoped materials,the electrochemical properties are improved to different degrees.