(606d) Template-Free Synthesis of Fe3O4 Nanoparticles and Their Performance as Anode Materials in Lithium-Ion Batteries

The growing demand for small, light-weight, and inexpensive power sources for portable electronics and automotive uses is driving interest in lithium-ion battery (LIB) technologies that exceed performance of today's batteries. Metal oxides are attractive as anode materials for LIBs due to their high lithium intercalation capacity and generally high natural abundances. Magnetite, Fe3O4, is a particularly promising candidate because of its high theoretical capacity of 924 mAh/g (cf. 372 mAh/g for currently used graphite), low cost, and low environmental footprint. A major challenge that limits development of superior anode materials based on this material is the pulverization problem, namely, the breakdown of electrical contact between particles of the active material produced by the large, cyclic volume changes which accompany repeated lithium insertion/extraction. Herein, we report results from a systematic study of hollow Fe3O4 nanoparticles synthesized via a template-free solvothermal method. Temporal XRD and TEM characterization indicate that the growth follows an inside-out Ostwald ripening mechanism, that spontaneously produces hollow nanostructures irrespective of the particle chemistry. Electrochemical cyclic voltammetry studies show that both hollow and filled Fe3O4 nanostructures reversibly intercalate lithium at ~0.9 and 1.2 V vs. Li/Li+. When integrated in the LIB anode, we find a systematic improvement in performance as the internal void space is increased. We also study the effects of creating a carbon-Fe3O4 nanocomposite by compositing Fe3O4 with polyacrylonitrile (PAN) followed by calcination to convert PAN to carbon. This procedure was found to consistently improve the electrochemical performance of the particles. The origin of this improvement is investigated using post-mortem TEM and nanoindentation studies, as well as in-situ characterization of the lithium ion diffusion by impedance spectroscopy.