Studies on spinel cobaltites, FeCo₂O₄ and MgCo₂O₄ as anodes for Li-ion batteries

Abstract

The compounds, FeCo₂O₄ and MgCo₂O₄ are synthesized by the urea combustion and oxalate decomposition method, respectively. Powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS) have been used to characterize the phases. Scanning electron microscopy (SEM) showed the distinct features in the morphology, submicron-size of FeCo₂O₄ and needle-shaped particles of MgCo₂O₄. Galvanostatic cycling was carried out in the voltage range 0.005-3.0 V vs. Li at a current of 60 mA g⁻¹ (~ 0.08 C) up to 50 cycles. The observed first-cycle charge capacities are, FeCo₂O₄: 827 mA h g⁻¹ (corresponding to theoretical 7.3 mol of Li per mole of FeCo₂O₄) and MgCo₂O₄: 736 mA h g⁻¹ (5.7 Li). At the end of 50 cycles, FeCo₂O₄ retained ~ 90% of the first-charge capacity, and a coulombic efficiency of ~ 99% is noted between 20 and 50 cycles. MgCo₂O₄ showed extensive capacity-fading and retained only 0.9 mol of cyclable Li in the range 28-50 cycles. These results show that Fe is a better counter ion than Mg. The average charge and discharge voltages are ~ 2.0 V and ~ 1.2 V, respectively, for both the compounds. Cyclic voltammograms, up to 12 cycles on FeCo₂O₄ complement the galvanostatic cycling results. Impedance studies were carried out on FeCo₂O₄ at various voltages during the first cycle and after 10 cycles, and the data have been interpreted.