High-performance transition metal mixed oxides in conversion electrodes: a combined spectroscopic and electrochemical study

Cobalt-spinel ferrites with a CO3-xFexO4 (X = 0, 1, 2) stoichiometry have been prepared by a sol-gel process involving solvent sublimation by a freeze-drying procedure. SEM micrographs showed the formation of flake-like aggregates yielding a macroporous system that ensures good electrolyte-particle contact in iron-containing samples. The analysis of the electrochemical curves evidenced changes in the first discharge profiles ascribed to sequential reduction of trivalent cobalt ions prior to the main reduction to the metallic state in Co2FeO4. The changes in the bulk and surface of the electrodes during electrochemical reaction were monitored by ex situ EPR and IR spectroscopy. An enhanced electrochemical performance was achieved for the Co2FeO4 sample, yielding capacity values above 630 mA h/g after 76 cycles and excellent rate performance.