Electrochemical properties of hydrothermally obtained LiCo_1-xFe_xO₂ as a positive electrode material for rechargeable lithium batteries

Changes in the crystal structure and transition metal valency in LiFe_xCo_1-xO₂ (0 ≤ x ≤ 0.25) positive electrodes during the charge-discharge of Li/LiFe_xCo_1-xO₂ cells were examined using ex situ X-ray diffraction measurements, Fe and Co K-X-ray absorption near edge spectroscopy (XANES) and ⁵⁷Fe Mossbauer spectroscopy. An iron-doped host material with rhombohedral symmetry was obtained up to 25% Fe per formula unit in LiCoO₂ by hydrothermal reaction below 300°C from a mixture of Co³⁺-Fe³⁺ co-precipitate (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 LiCoO₂ prepared at low temperature in which Li ions are located at the interstitial 6c and octahedral 3a sites. ⁵⁷Fe Mossbauer and Fe and Co K-XANES spectra at 293 K indicated the presence of tetravalent Fe ions upon oxidation up to 4.3 V.