Nanoparticulated spinel-type iron oxides obtained in supercritical water and their electrochemical performance as anodes for Li ion batteries

Nanoparticulated iron oxide-hydroxide samples are synthetized in supercritical water in continuous regime. Freshly prepared iron(II) acetate solutions are used as precursors and experiments at different flow rates and reactor temperatures are carried out. Samples composition is determined by thermogravimetric and energy loss spectroscopy. The obtained powdered samples are structurally characterized through Rietveld refinements of X-ray diffraction data. They mainly consist of FeO1.5-z spinel-type phases, though minority oxyhydroxide and hematite phases are obtained at low temperatures and total flow rates. Analysis by electron microscopy reveals mean particles sizes around 30 nm and roughly spherical morphology in most cases. The electrochemical response, cycling performance and Coulombic efficiency are evaluated from discharge-charge and cycling experiments at different current densities. The obtained response seems to depend on the presence or absence of oxyhydroxide phases and on both microstructural homogeneity at nanoscale and relative concentration of supercritical water at the mixing point. The 7-3-200 electrode sample shows superior rate capability, with capacity values higher than graphite theoretical one, even at high rates (500 Ah kg(-1) at 2C after 100 cycles). Besides, the capacity values are restored when returning to low rates, reaching 1200 Ah kg(-1) at C/5 and 800 Ah kg(-1) at C/2. (C) 2016 Elsevier B.V. All rights reserved.