57Fe Mossbauer Spectroscopy Study of the Electrochemical Reaction with Lithium of MFe2O4 (M = Co and Cu) Electrodes

Cycled electrodes of CoFe2O4 and CuFe2O4 were analyzed by Fe-57 Mossbauer spectroscopy. On decreasing the recording temperature, the spectra evidence the superparamagnetic-ferromagnetic transition of metallic iron formed during the reduction of iron oxides in a lithium cell. In addition, the full reoxidation of iron to the 3+ oxidation state is evidenced after cell recharge. As compared with CoFe2O4, the presence of copper led to three effects: a lower percentage of surface atoms, a higher average oxidation state of surface iron atoms, and smaller metallic particles in the reduced state. The relationships between the Mossbauer spectra and particle size and morphology of the raw spinel materials are studied. The charging process involves an important reordering of iron atoms between bulk and surface locations in CoFe2O4, which leads to minor differences in the spectra of the charged electrodes of both compounds.