The Electrochemical Reduction of Cosub 3Osub 4 in a Lithium Cell

The first stage of the electrochemical reduction of crystallized spinel in lithium cells was investigated by means of in situ X-ray diffraction. Through the use of tailor-made materials prepared from Co-alkoxide precursors, we observed that the formation of the intermediate phase previously evidenced by several authors was highly dependent on the discharge rate, the texture of the active material (i.e., crystallite size, specific surface area), and the cycling temperature. When starting from a highly divided oxide and/or using a low current, we found that this plateau was actually associated with the formation of α-CoO, subsequently leading to metallic cobalt upon further reduction. Alternatively, was formed when using materials with a large crystallite size or/and applying a high discharge rate, later on similarly decomposing into divided metal. These findings and the related competition between different reaction paths represent an explanation for numerous electrochemical observations, and for the need of fast insertion in such host materials to stabilize intermediate lithiated compounds. This work illustrates the major influence of the initial texture as well as the temperature on the reactivity of the 3d-metal based oxides recently reinvestigated for their electrochemical performances as negative electrode materials. Also, it emphasizes the implications of the reactive grain size evolution upon cycling. © 2002 The Electrochemical Society. All rights reserved.