Preparation And Lithium Storage Performance Of SnS₂@MWCNT Nanostructured Anode For Lithium Ion Batteries

With growing advance in science and technology,portable electronic devices and electric vehicles currently develops rapidly.Since there is increasing requirements for energy storage,Li-ion batteries draw wide attention for their high energy density,long circling life,low cost and high security.The capacity of widely commercial Graphite negative electrode only reaches 372 mAh·g-1.The low capacity and energy density cannot keep pace with fast developing electronic devices,fail to meet requirements of energy storage and get limited improvement in the future.Consequently,the key to research of Li-ion anode electrode is to find materials with higher specific capacity and longer cycle life.SnS₂ gains more and more attention among varieties of Li-ion anode materials for its special structure and high specific capacity.Compared with commercialized graphite electrode,the theoretical capacity of SnS₂ is 645 mAh·g-1.SnS₂ exhibits CdI2 hexagonal layered structure.In each crystal cell,there is a Sn layer boned to two Sulphur layers by covalent bond and Sulphur layers bonded to each other by van der Waals-type forces.The sandwich-like structure contains large space for Li-ion insertion and extraction,which makes SnS₂ as a research highlight in Li-ion anode materials.Though SnS₂ owns specific structure,there are many drawbacks including poor conductivity and high electron transfer impedance existed in SnS₂ anode materials.Large volume effect easily appears during lithium insertion and extraction of SnS₂ anode,destroying electrode structure and finally leading to degradation of batteries property.Among different problems of SnS₂ anode,our research focuses on the improvement of conductivity and buffer of volume expansion.Therefore,we create multi-walled carbon nanotube constraint SnS₂?SnS₂@MWCNT?nanostructure through a 2-step process.Firstly,we fabricate Sn@MWCNT nanoparticles as the precursor by the means of DC arc-discharge method.In this method,the bulk Sn is evaporated in CH4 reactant gas and Ar protectant gas.Secondly we get Sn@MWCNT nanoparticles and sulfur together under high temperature and hermetic environment to go through vulcanization reaction.Sublimed Sulfur gets access to internal multi-walled carbon nanotube through carbon defects and thus we get SnS₂@MWCNT structure.We characterize SnS₂@MWCNT by TEM and the result shows the SnS₂@MWCNT nanoparticles exhibit a structure with the average size of 10 nm in thickness and about 400 nm in length and the lattice fringe is visible.The subsequent powder X-ray diffraction?XRD?and Raman spectroscopy experiment results reveal good crystallization degree.Through Fourier transform infrared spectroscopy?FTIR?and x-ray photo electron spectrum?XPS?,we analyses the existence of C-C bond and SnS₂ functional group in the surface.Finally,the electrochemical characterization demonstrates the excellent electrochemical properties of SnS₂@MWCNT for Li-ion anode.During the first discharge,the primary discharge capacity and the reversible capacity are much higher than pure SnS₂.After 50 cycles,the capacity maintains 703 mAh·g-1.In cycling process,MWCNT significantly improves the conductivity of the anode due to its good electron transfer property.Meanwhile,as constraint frame,MWCNT buffers the severe volume expansion.Various active components participate in the reaction and contribute to the capacity,accounting for good capacity properties.Different lithiation/delithiation reaction platforms of various active components lead to the step-shaped reaction platforms of the anode,avoiding electrode damage from volume expansion and ensuing the long cycle life of the battery.