Intercalation of sodium ions into hollow iron oxide nanoparticles

Cation vacancies in hollow γ-Fe₂O₃ 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-Fe₂O₃ nanoparticles can serve as hosts for sodium ions in high voltage range (4.0-1.1 V), allowing utilization of γ-Fe₂O₃ 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-Fe₂O₃ 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 NaClO₄ and NaPF₆ on the Coulombic efficiency at different cycling rates.