Electrochemical properties of spinel LixCoO2:: A first-principles investigation

We present the results of a first-principles study of the electrochemical and lithium ordering properties of spinel LixCoO2 for x ranging from 0 to 1. The spinel form of LixCoO2 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 by Li of both these sites. The calculated voltage intercalation curve of this phase exhibits a step of more than a volt at x=1/2. This step is a result of the large stability of ordered spinel Li1/2CoO2 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 x=1/2 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 LixCoO2. A miscibility gap is predicted between x = 0.5 and x = 1.0. At x = 1/4, 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 LixMnO2 at room temperature for x in the vicinity of 1/4. [S0163-1829(99)06201-3].