Ultrafine Nb<sub>2</sub>O<sub>5</sub> Nanocrystal Coating on Reduced Graphene Oxide as Anode Material for High Performance Sodium Ion Battery

Ultrafine niobium oxide nanocrystals/reduced graphene oxide (Nb<sub>2</sub>O<sub>5</sub> NCs/rGO) was demonstrated as a promising anode material for sodium ion battery with high rate performance and high cycle durability. Nb<sub>2</sub>O<sub>5</sub> NCs/rGO was synthesized by controllable hydrolysis of niobium ethoxide and followed by heat treatment at 450 °C in flowing forming gas. Transmission electron microscopy images showed that Nb<sub>2</sub>O<sub>5</sub> NCs with average particle size of 3 nm were uniformly deposited on rGO sheets and voids among Nb<sub>2</sub>O<sub>5</sub> NCs existed. The architecture of ultrafine Nb<sub>2</sub>O<sub>5</sub> NCs anchored on a highly conductive rGO network can not only enhance charge transfer and buffer the volume change during sodiation/desodiation process but also provide more active surface area for sodium ion storage, resulting in superior rate and cycle performance. <i>Ex situ</i> XPS analysis revealed that the sodium ion storage mechanism in Nb<sub>2</sub>O<sub>5</sub> could be accompanied by Nb<sup>5+</sup>/Nb<sup>4+</sup> redox reaction and the ultrafine Nb<sub>2</sub>O<sub>5</sub> NCs provide more surface area to accomplish the redox reaction.