Electrochemical Properties of Hydrothermally Obtained LiCo[sub 1−x]Fe[sub x]O[sub 2] as a Positive Electrode Material for Rechargeable Lithium Batteries

Changes in the crystal structure and transition metal valency in positive electrodes during the charge‐discharge of cells were examined using ex situ X‐ray diffraction measurements, Fe and Co K‐X‐ray absorption near edge spectroscopy (XANES) and Mössbauer spectroscopy. An iron‐doped host material with rhombohedral symmetry was obtained up to 25% Fe per formula unit in by hydrothermal reaction below 300°C from a mixture of coprecipitate (iron‐doped CoOOH) and LiOH. No symmetry changes were observed during electrochemical charge‐discharge tests up to the tenth cycle. The structures of Li‐extracted and reinserted samples were determined by a model similar to prepared at low temperature in which Li ions are located at the interstitial 6 c and octahedral 3 a sites. Mössbauer and Fe and Co K‐XANES spectra at 293 K indicated the presence of tetravalent Fe ions upon oxidation up to 4.3 V.