Some new aspects of low-temperature lithium cobalt oxides prepared through citric acid precursor route

Low-temperature (LT) lithium cobalt oxides were prepared at 300 degrees C by the solid-state reaction between Li2CO3 and Co3O4, having various starting compositions Li1+xCoO2 with x = -0.2, 0.0, 0.2, and 0.4. A finely mixed precursor of the reacting compounds was obtained in molten citric acid. The morphology observed by scanning electron microscopy (SEM) showed the homogeneous and fluffy nature of the specimens, with a BET specific surface as high as 19 m(2)g(-1). The cubic crystal lattice was found to decrease from a 7.990 to 7.984 Angstrom against Li/Co = 0.62 to 0.90 in molar ratio of the water-leached products. The resistivity and open circuit voltage (OCV) against lithium metal were found to be sensitive to the initial lithium content of the samples. Chemical titration showed that all of the samples contained an appreciable amount of Co2+ in addition to trivalent cobalt. XANES spectra supported the presence of tetrahedrally coordinated divalent cobalt. A model is proposed in which oxidation of divalent cobalt explains the electrochemical charge-discharge irreversibility in the initial cycle. (C) 1999 Elsevier Science Ltd.