Intercalation of Sodium Ions into Hollow Iron Oxide Nanoparticles

Cation vacancies in hollow γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles are utilized for efficient sodium ion transport. As a result, fast rechargeable cathodes can be assembled from Earth-abundant elements such as iron oxide and sodium. We monitored in situ structural and electronic transformations of hollow iron oxide nanoparticles by synchrotron X-ray adsorption and diffraction techniques. Our results revealed that the cation vacancies in hollow γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles can serve as hosts for sodium ions in high voltage range (4.0–1.1 V), allowing utilization of γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles as a cathode material with high capacity (up to 189 mAh/g), excellent Coulombic efficiency (99.0%), good capacity retention, and superior rate performance (up to 99 mAh/g at 3000 mA/g (50 C)). The appearance of the capacity at high voltage in iron oxide that is a typical anode and the fact that this capacity is comparable with the capacities observed in typical cathodes emphasize the importance of the proper understanding of the structure–properties correlation. In addition to that, encapsulation of hollow γ-Fe<sub>2</sub>O<sub>3</sub> nanoparticles between two layers of carbon nanotubes allows fabrication of lightweight, binder-free, flexible, and stable electrodes. We also discuss the effect of electrolyte salts such as NaClO<sub>4</sub> and NaPF<sub>6</sub> on the Coulombic efficiency at different cycling rates.