Staging phase transitions in LixCoO2

Recently, there has been more and more interest in charging LiCoO2 to potentials above 4.2 V to increase its specific capacity for commercial use. Unlike the situation for LixCoO2 below 4.2 V, the electrochemistry and structure of LixCoO(2) above 4.2 V is not completely clear. In this study, two reversible plateaus at 4.55 and 4.63 V were observed in the voltage profile of Li/L(i)xCoO(2) cells, which were then studied by in situ X-ray diffraction (XRD). The intermediate phase, formed at 4.6 V with x = 0.12 in LixCoO2, was determined to be a stage two phase, that is with lithium occupying only every second space between CoO2 "sandwiches.'' The structure of this phase was determined using Rietveld refinement of an ex situ XRD pattern of Li0.12CoO2. We find that adjacent CoO2 sandwiches are stacked with aa transition metal alignments if there are no Li atoms between them (as in O1-CoO2) and with ab alignments if there are Li atoms between them (as in O3-LixCoO2). This leads to a six-layer unit cell. The structure we find agrees well with that predicted by Ceder's group for the stage two phase that they call H1-3. (C) 2002 The Electrochemical Society.