Intercalation of Sodium\nIons into Hollow Iron Oxide\nNanoparticles

Abstract

Cation vacancies in hollow γ-Fe₂O₃ nanoparticles\nare utilized for efficient sodium ion transport. As a result, fast\nrechargeable cathodes can be assembled from Earth-abundant elements\nsuch as iron oxide and sodium. We monitored in situ structural and\nelectronic transformations of hollow iron oxide nanoparticles by synchrotron\nX-ray adsorption and diffraction techniques. Our results revealed\nthat the cation vacancies in hollow γ-Fe₂O₃ nanoparticles can serve as hosts for sodium ions in high voltage\nrange (4.0–1.1 V), allowing utilization of γ-Fe₂O₃ nanoparticles as a cathode material with high capacity\n(up to 189 mAh/g), excellent Coulombic efficiency (99.0%), good capacity\nretention, and superior rate performance (up to 99 mAh/g at 3000 mA/g\n(50 C)). The appearance of the capacity at high voltage in iron oxide\nthat is a typical anode and the fact that this capacity is comparable\nwith the capacities observed in typical cathodes emphasize the importance\nof the proper understanding of the structure–properties correlation.\nIn addition to that, encapsulation of hollow γ-Fe₂O₃ nanoparticles between two layers of carbon nanotubes\nallows fabrication of lightweight, binder-free, flexible, and stable\nelectrodes. We also discuss the effect of electrolyte salts such as\nNaClO₄ and NaPF₆ on the Coulombic efficiency\nat different cycling rates.