Design Of Micro-nanostructured Based On Metal Sulfides/selenides And Their Sodium Storage Properities

Lithium ion batteries（LIBs）have been widely applied in portable mobile electronic device and electric vehicles due to their high operating voltage,long cycle life and high energy density.However,the application of LIBs in large scale energy storage is restricted by the limitation and uneven distribution of lithium resources,resulting in high cost for LIBs.Sodium ion batteries（SIBs）,sharing the same working principles as LIBs,have attracted much attention for researchers because of the abundant nature of sodium resources and low cost,which is a choice as promising alternatives to the LIBs application in large scale energy storage due to the low cost.Nevertheless,the commercial LIBs graphite anode cannot directly used as anode material for SIBs owing to the low sodium storage capacity and poor cycle performance.Therefore,to find novel anode materials for SIBs with high capacity and excellent cycling performance is necessary.Metal sulfides/selenides have been considered as promising anode materials for SIBs because of their high theoretical capacity.However,their practical application is limited due to the unsatisfactory cycle and rate performance,which caused by serious volume change during the charge and discharge process.Therefore,in view of the above problems,the main aim of this paper is to improve the cycle and rate performance of metal sulfides/selenides,designing and preparing a series of metal sulfides/selenides with excellent electrochemical performance.The results are specifically illustrated as below:Fe₃O₄/Fe_1-xS@C@MoS₂ was prepared via combining with freeze drying and hydrothermal method using NaCl as hard template.In this designed architecture,carbon nanosheets as matrix can effectively facilitate ion/electron transport and avoid the volume change of Fe₃O₄/Fe_1-x-x S during the charge and discharge process;Fe₃O₄/Fe_1-x-x S nanoparticles and MoS₂ can offer more active sites for sodium storage and shorten the path for sodium ion diffusion.Benefiting from the synergistic effect of Fe₃O₄/Fe_1-xS nanoparticles,carbon nanosheets matrix and MoS₂ coating layers,Fe₃O₄/Fe_1-x-x S@C@MoS₂ shows high specific capacity,excellent cycling stability(403 mAh g^-11 after 1000 cycles at current density of 1.0 A g^-1)and rate performance(350 mAh g^-11 at current density of 2.0 A g^-1)as anode materials for SIBs.Therefor,the results indicate the synergy effect between MoS₂ and carbon matrix can improve the electrochemical performance of Fe₃O₄/Fe_1-xS@C@MoS₂ composite.Fe₂O₃ precursors with various morphology were synthesized via adding different additives,and Fe_1-x-x S@SC core-shell composites were prepared after subsequently the polymerization coating and one-step sulfurization process.When tested as anode materials for SIBs,the optimized S-Fe_1-xS@SC core-shell composites with core-shell microstructure exhibit high specific capacity,excellent rate performance and superior cycling stability.In-situ TEM analysis results shown that the synergistic effects of S-doped carbon shell and internal hollow structure can effectively buffer the serious volume change of Fe_1-xS during the sodiation/esodiation process,enhancing the structural stability of electrode.In addition,S-doped carbon shell can improve the conductivity and avoid the sulfur dissolution caused by the direct exposure of Fe_1-xS to the electrolyte during cycling process.Finally,a reversible capacity of 454.3 mAh g^-11 after 500 cycles at current density of 1.0 A g^-11 and high reversible capacity of 365 mAh g^-11 at current density of 10.0 A g^-11 can be obtained for S-Fe_1-x-x S@SC core-shell composites when tested as anode for SIBs.Finally,the results indicate that the rate performance and cycling stability can improve via design structural and morphology.Micro-nanostructure materials have many advantages as anode materials for SIBs.Hierarchical hollow tubular MoS₂/C microtubes（MoS₂/C MTs）composed of nanosheets were prepared using Sb₂S₃ as template.Formation of an intimate heterojunction between MoS₂ and C can effectively improve the conductivity of MoS₂,buffer the volume change of MoS₂during the charge and discharge process,stabilized on the discharge products as well as facilitate the reversible of MoS₂.In addition,the hierarchical hollow structure also can provide space to buffer the volume change of MoS₂ during the charge and discharge process.These analysis results can be confirmed and supported via in situ TEM and in situ XRD tests.As a result,MoS₂/C MTs exhibits excellent electrochemical performance when evaluated as anode material for SIBs.A reversible capacity of 563.5 mAh g^-11 was obtained at current density of 0.2 A g^-1.A specific capacity of 484.9 mAh g^-11 can be obtained after 1500 cycles at current density of 2.0 A g^-1,and a reversible capacity of 401.3 mAh g^-11 was obtained at 10.0 A g^-1.The results indicate micro-nanostructure design can achieve excellent electrochemical performance.In addition,this micro-nanostructure design idea can also be applied to other SIBs anode materials.Morphology design and carbon coating are two effective methods to improve the electrochemical performance of metal selenides,and combining the advantages of the two methods can greatly improve the cycle and rate performance of metal selenides.ZnSe/SnSe@NC core-shell composite with hollow structure have been prepared via in situ polymerization and selenization at high temperature using ZnSn（OH）₆ as precursor.Hollow structure and N-doped carbon can effectively buffer the volume change of ZnSe/SnSe during the sodiation/desodiation process.On the other hand,N-doped carbon with high conductivity can effectively improve the transport of sodium ions and electrons,and the ZnSe/SnSe nanoparticles can off more active sites for sodium storage.Thus,when tested as anode material for SIBs,ZnSe/SnSe@NC exhibits high initial coulombic efficiency（89.7%）,high sodium storage capacity(662.5 mAh g^-11 at 0.1 A g^-1),excellent rate performance(350 mAh g^-1at 10.0 A g^-1)and outstanding cycling stability(403.5 mAh g^-11 after 500 cycles at 1.0 A g^-1).the electrochemical tests indicate the hollow structure and N-doped carbon shell can improve the electrochemical performance.