Electrochemical Insertion of Li and Na Ions into Nanocrystalline Fe3O4 and α‐Fe2O3 for Rechargeable Batteries

Fe 3 O 4 powders with different particle sizes on average (400, 100, and 10 nm) were prepared and characterized by X-ray diffraction, transmission electron microscopy, Mossbauer spectroscopy, and electrochemical methods. To examine the electrochemical activity of Fe 3 O 4 in relation to the particle size effect, galvanostatic cycling tests in aprotic electrolytes containing lithium or sodium ions were conducted. The electrochemical activity was significantly enhanced as the mean particle size decreased. The nanocrystallized Fe 3 O 4 (10 nm) prepared by precipitation method delivered 190 mA h g ―1 of the rechargeable capacity in the voltage range of 2-3 V in a lithium-ion containing electrolyte, whereas the 400 and 100 nm Fe 3 O 4 powders showed 10 and 80 mA h g ―1 of the rechargeable capacity, respectively. An ex situ X-ray diffraction study for the electrochemically cycled samples suggested the partly reversible Fe ion migration from the tetrahedral sites to the octahedral sites with a retained spinel framework structure. The nanocrystallized Fe 3 O 4 as well as α-Fe 2 O 3 were highly electrochemically active in the sodium salt electrolyte. The rechargeable capacity of 160 or 170 mA h g ―1 with excellent capacity retention was obtained for nanocrystalline Fe 3 O 4 or α-Fe 2 O 3 , respectively.