Sulfur-Doped Graphene and Tin Sulfides as High Capacity Anode Materials for Sodium Ion Batteries

Abstract

Lithium ion batteries (LIBs) have been regarded as the most widely used power sources for many aspects such as portable electronics and electric vehicles, due to the demands for clean energy for the past few decades. However, the lithium resources are limited and non-uniformly distributed globally, so the cost of materials increased by the demands of LIBs. Thus the alternatives should be considered for substituting LIBs. <br/> Among many kinds of batteries, sodium ion batteries (NIBs) have been gaining great attention by the natural abundance and low cost of sodium resources. Na share similar chemical properties with Li, that the fundamental principle of the LIBs and NIBs are identical. The enormous previous researches of LIBs provide great guidance for NIB anode materials. However, it still needs to explore appropriate anode materials for NIBs. Therefore, a major focus has been to search for high capacity anode materials that will satisfy increasing demands.<br/> The overview of NIBs are introduced in chapter 1 including the benefits of NIBs. In addition, electrode materials currently being investigated as cathode and anode materials in NIBs are also introduced. The structure of the cathode can be divided into three categories while anode, although having a variety of different structures, all generally follow one of three reaction mechanisms. <br/> In chapter 2, the electrochemical properties of heteroatom-doped graphene is discussed. In this chapter the heteroatom-doped graphene with high doping levels and disordered structures were prepared through a simple and economical thermal process. These solvothermal-derived graphene showed excellent performance as an anode material for SIBs. It exhibited a high reversible capacity of 380 mAh g-1 after 300 cycles at 100 mA g-1, excellent rate performance 217 mAh g-1 at 3200 mA g-1 and superior cycling performance at 2.0 A g-1 during 1000 cycles with negligible capacity fade. The main reasons for the excellent cycle performance of SG are likely due to the large interlayer distances, highly disordered structures and greater number of active sites for Na ion storage than others. In addition, by doping with sulfur, as well as lowering the oxygen content, the capacity and cycle performance were significantly enhanced.<br/> In chapter 3, novel tin (II) sulfide/carbon (SnS/C) composites were prepared, through a ball-milling method, as anode materials for sodium ion batteries. Their electrochemical performance has been significantly improved when com-pared to bare SnS, especially in cycling stability and rate capabilities. SnS/C composites exhibit excellent capacity retention, at various current rates, and deliver capacities as high as 400 mAh g-1 even at the high current density of 800 mA g-1 (2C). Ex-situ transmission electron microscopy, X-ray diffraction and operando X-ray absorption near edge structure studies have been performed in order to better understand the reaction mechanism of SnS/C composites.<br/> <br/> <br/> Keywords: sodium ion batteries, anodes, sulfur doped graphene, tin sulfides, reaction mechanism <br/> Student number: 2014-30254<br/> <br/>