Electrochemical properties of spinel Li{sub x}CoO{sub 2}: A first-principles investigation

We present the results of a first-principles study of the electrochemical and lithium ordering properties of spinel Li{sub x}CoO{sub 2} for {ital x} ranging from 0 to 1. The spinel form of Li{sub x}CoO{sub 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 by Li of both these sites. The calculated voltage intercalation curve of this phase exhibits a step of more than a volt at {ital x}=1/2. This step is a result of the large stability of ordered spinel Li{sub 1/2}CoO{sub 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 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 Li{sub x}CoO{sub 2}. A miscibility gapmore » 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 Li{sub x}MnO{sub 2} at room temperature for {ital x} in the vicinity of 1/4. {copyright} {ital 1999} {ital The American Physical Society}« less