We present the results of a first-principles study of the electrochemical and lithium ordering properties of spinel Li x CoO 2 for x ranging from 0 to 1. The spinel form of Li x CoO 2 belongs to the Fd3m space group and can accommodate Li ions in two crystallographically distinct sites: the tetrahedrally coordinated 8a sites and the octahedrally coordinated 16c sites. We have investigated a lattice model for this system derived from first principles that allows for the simultaneous occupation

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... Li of both these sites. The calculated voltage intercalation curve of this phase exhibits a step of more than a volt at xϭ1/2. This step is a result of the large stability of ordered spinel Li 1/2 CoO 2 in which the Li ions fill all the tetrahedral 8a sites. The remarkable stability of this ordered phase produces a sharp curvature in the free energy at xϭ1/2 that translates into an abrupt change in the voltage curve. As the temperature is raised, the voltage step gradually disappears as a result of the increased excitations of Li ions into energetically less favorable octahedral sites. We have also calculated the phase diagram for spinel Li x CoO 2 . A miscibility gap is predicted between xϭ0.5 and xϭ1.0. At xϭ1/4, Li is predicted to order below approximately 150 K. This ordering causes a small step of about 0.2 V similar to that observed in spinel Li x MnO 2 at room temperature for x in the vicinity of 1/4.