Electrochemical properties of hydrothermally obtained LiCo1-xFexO2 as a positive electrode material for rechargeable lithium batteries

Changes in the crystal structure and transition metal valency in LiFexCo1-xO2(0 less than or equal to x less than or equal to 0.25) positive electrodes during the charge-discharge of Li/LiFexCo1-xO2 cells were examined using ex situ X-ray diffraction measurements, Fe and Co K-X-ray absorption near edge specroscopy (XANES) and Fe-57 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 degrees C from a mixture of Co3+-Fe3+ 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 LiCoO2 prepared at low temperature in which Li ions are located at the interstitial 6c and octahedral 3a sites. Fe-57 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. (C) 2000 The Electrochemical Society. S0013-4651(99)08-040-4. All rights reserved.