Snsb@CNT Nanostructures Rooted in Graphene and Its Application in Sodium Ion Batteries

The uniform SnSb alloy core/carbon-shell nanotubes growing on the graphene sheets were synthesized as the potential anode for sodium ion batteries by two steps. Typically, SnSb nanoparticles were firstly decorated on reduced graphene oxide by the hydrothermal method, and then the precursors were reduced by thermochemical vapour deposition with C₂H₂/Ar gas. In order to obtain uniform SnSb-core/carbon-shell nanotubes, the precursors were optimized with different C₂H₂/Ar gas flow rates (10:400, 20:400 and 40:400 sccm/sccm), and different annealing temperature varying from 550 to 700 ^oC. The morphologies and structures were characterized by SEM and TEM. Under the annealing temperature of 650 ^oC and C₂H₂/Ar gas flow rate of 20:400, the nanotubes distribute on graphene sheets more uniformly compared to that obtained from other temperatures and flow rates, and the average diameter of the nanotubes is around 100 nm. Moreover, the SnSb-core/carbon-shell nanotubes were used as anode materials for sodium ion batteries. The electrochemical performance for sodium ion batteries is significantly affected by the electrolyte. In this work, different electrolytes such as 1M NaClO₄ in PC, 1M NaPF₆ in PC with or without FEC and 1M NaClO₄ in PC/FEC have been applied in order to optimize the electrochemical performance of composite anode materials for sodium storage. It is found that cells using electrolyte without FEC exhibit a higher specific capacity compared to those using electrolyte with FEC, however, the coulombic efficiency of these cells is relatively lower. The cell delivers an initial reversible  capacity of 500 mAh/g with a coulombic efficiency of 66% in the electrolyte of 1M NaPF₆ in PC without FEC, but the capacity collapsed after about 15 cycles. When FEC is added in the electrolyte, however, the cycling performance is remarkably improved. Only slight decay can be observed after 80 charge/discharge cycles. The excellent electrochemical performances compared to other SnSb-based anodes can be attributed to the efficient prevention of graphene restacking by SnSb@CNT decoration, the protection role of CNT shell and the support of graphene sheets.