Structural studies of lithium intercalation in a nanocrystalline α-Fe ₂O ₃ compound

A nanocrystalline Fe ₂O ₃, synthesized via a low-temperature aqueous synthesis route, is reported as a cathode material for rechargeable lithium batteries. This compound possesses a structure similar to that of hematite or α-Fe ₂O ₃ with crystallites ca. 5 nm in size. The electrochemical properties of this compound are seen to be dramatically superior to those of microcrystalline α-Fe ₂O ₃, with specific capacities of 200-250 mA·h/g and energy densities of 425-500 mW·h/g at different current rates. The compound also shows excellent reversibility upon discharge/charge cycling, much improved over that reported for most microcrystalline iron oxides. Detailed structural analysis of the compound and its lithium intercalation process has been conducted via X-ray diffraction and X-ray absorption fine structure spectroscopy (XAFS), and unique features associated with the nanocrystalline compound have been observed. First, owing to the very small crystallite size and the associated structural disorder, deviations in the local structure of the nanocrystalline compound in comparison to microcrystalline α-Fe ₂O ₃ are seen. Second, the nanocrystalline compound is observed to yield much higher single-phase capacity than that of the microcrystalline α-Fe ₂O ₃. The nanocrystalline compound allows accommodation of up to 0.47 Li per Fe ₂O ₃ whereas the microcrystalline compound is known to yield single-phase capacity up to only 0.03 Li per Fe ₂O ₃. Finally, upon further discharge the nanocrystalline compound shows transformation to a substantially disordered structure that can be indexed to a cubic lattice. Isosbestic points in the XAFS data clearly show the occurrence of this phase transformation and emerge as a strong tool for determining phase transformations incipient at the local scale. This study reveals the surprising electrochemical performance of the nanocrystalline iron oxide and the underlying novel structural and mechanistic characteristics and highlights the striking contrast between nanocrystalline intercalation compounds and their microcrystalline counterparts.