Ultrasmall TiO2-Coated Reduced Graphene Oxide Composite as a High-Rate and Long-Cycle-Life Anode Material for Sodium-Ion Batteries.

Because of the low cost and abundant nature of the sodium element, sodium-ion batteries (SIBs) are attracting extensive attention, and a variety of SIB cathode materials have been discovered. However, the lack of high-performance anode materials is a major challenge of SIBs. Herein, we have synthesized ultrasmall TiO2-nanoparticle-coated reduced graphene oxide (TiO2@RGO) composites by using a one-pot hydrolysis method, which are then investigated as anode materials for SIBs. The morphology of TiO2@RGO has been characterized using transmission electron microscopy, indicating that the TiO2 nanospheres uniformly grow on the surface of the RGO nanosheet. As-prepared TiO2@RGO composites exhibited a promising electrochemical performance in terms of cycling stability and rate capability, especially the initial cycle Coulombic efficiency of 60.7%, which is higher than that in previous reports. The kinetics of the electrode reaction has been investigated by cyclic voltammetry. The results indicate that the sodium-ion intercalation/extraction behavior is not controlled by the semiinfinite diffusion process, which gives rise to an outstanding rate performance. In addition, the electrochemical performance of TiO2@RGO composites in full cells, coupled with carbon-coated Na3V2(PO4)3 as the positive material, has been investigated. The discharge specific capacity was up to 117.2 mAh g-1, and it remained at 84.6 mAh g-1 after 500 cycles under a current density of 2 A g-1, which shows excellent cycling stability.