Synthesis and electrochemical properties of LiCo2O4 spinel cathodes

The formation of LiCo2O4 spinel oxide has been investigated by chemically extracting lithium with the oxidizing agent Na2S2O8 from three samples of LiCoO2 that were synthesized at 400, 600, and 800degreesC to obtain Li0.5CoO2, followed by heat-treatment at 200degreesC. The samples have been characterized by X-ray diffraction, atomic absorption spectroscopy, and a redox titration to determine the oxygen content. While the Li0.5CoO2 sample obtained from the 400degreesC LiCoO2 crystallizes in the normal cubic spinel structure with a cation distribution of (Li)(8a)[Co-2](16d)O-4, that obtained from the 800degreesC LiCoO2 maintains the layer structure even after heating at moderate temperatures. The voltage profile of the LiCo2O4 spinel has been investigated experimentally. While the extraction of lithium from the 8a sites occurs at around 3.8-4 V, the insertion of lithium into the 16c sites occurs at around 3.5-3.6 V. The experimentally observed voltage difference of 0.3 V between the two processes is much smaller than that predicted from theoretical calculations (1.3 V) in the literature and observed for the LiMn2O4 system (1 V). (C) 2002 The Electrochemical Society.