Study On Synthesis Of Nitrogen-doped MoS₂ Composites And Its Sodium Storage Performance

Two-dimensional transition metal dichalcogenides MX2(where M=Mo,Mn,Ni,Sn,Fe,Sb;and X=S)as analogues of graphene have recently attracted much interest of researchers and have been extensively explored in the field of sodium ion batteries(SIBs).Among different types of transitionmetal dichalcogenides,layered molybdenum disulfide(MoS2)has been proven to be a promising anode material because of its relatively high theoretical capacity(670 mAh g-1),low cost,and environmental benignity.Graphene-like MoS2 nanosheets appear as a S-Mo-S sandwich layered structure with strong in-plane covalent bonding and weak van der Waals force between layers.Specially,the large interlayered space of about 0.615 nm is beneficial to enhance the diffusion of active ions.The diversity of Mo oxidation states and large specific area from 2D structure promote the charge storage capabilities.2D nanomaterials experience severe restacking problem during electrode fabrication and galvanic charge/discharge process,which significantly reduce the actual surface area and accessible sites of electrode,resulting in inferior electrochemical performance.Besides,MoS2 also suffers from poor electrical conductivity and volume expansion during electrochemical cycles.It is therefore necessary to design and construct an efficient MoS2 electrode structure.Herein,a nitrogen doped MoS2 foam(NMF)is constructed by a novel templating-drying-calcination method using MoS2 nanosheets as building blocks.The as-obtained NMF contains large pore volume which is beneficial for electrolyte penetration,ion diffusion,and volume expansion buffering.More importantly,nitrogen containing polymeric template not only prevents the restacking of MoS2 nanosheets during drying process,but also acts as efficient nitrogen source to be doped in MoS2 and residual carbon.Because of the additional Mo-N bonds after high temperature treatment,the electrical conductivity of N doped MoS2 was enhanced by replacing S with N.Furthermore,owing to the large specific surface area of NMF achieved by the novel templating-drying-calcination procedure,pseudocapacitive storage of sodium ions on NMF surface,edges,and defective sites,was greatly enhanced,which is helpful in improving the stability and cyclability of NMF even at high current densities.Furthermore,nitrogen doped carbon from polymeric template was left on the surface of MoS2 in NMF,forming a protective layer which also effectively suppresses the aggregation of 2D MoS2 nanosheets,improves electrical conductivity,and enhances the overall structural integrity of the electrode structure.As a result,NMF exhibits excellent sodium storage performance,especially in term of rate capability.The NMF electrode delivered a high initial reversible capacity of 732 mAh·g-1 at a current density of 20 mA g-1 and showed 407 mAh·g-1 reversible capacity under 1 A g-1 after 100 cycles.Even at a high current density of 2A g-1,it can deliver an initial charge capacity of 401 mAh g-1 with a capacity retention of 80%after 100 cycles.Finally,in order to explore its practical application,NMF was assembled into a full sodium ion battery and successfully lit up a LED light.