Prediction of Li intercalation and battery voltages in layered vs. cubic Li_xCoO₂

It is now possible to use a quantum-mechanical electronic structure theory of solids and derive, completely from "first-principles," the voltage of a battery based on intercalation reaction energetics. Using such techniques, we investigate the structural stability, intercalation energies, and battery voltages of the two observed ordered phases ("layered" and cubic) of LiCoO₂. We perform calculations for not only fully lithiated LiCoO₂, but also fully delithiated □CoO₂ and partially delithiated Li_0.5CoO₂. Our calculations demonstrate that removal of Li from the cubic phase results in movement of the Li atoms from their original octahedral sites to tetrahedral sites, forming a low-energy LiCo₂O₄ spinel structure. The energetics of the spinel phase are shown to account for the observed marked differences in battery voltages between the cubic and layered phases of LiCoO₂. A small energy barrier exists for Li motion between octahedral and tetrahedral sites, thus indicating the metastability of the high-energy octahedral sites. Finally, we point out a possible pressureinduced layered → cubic transition in LiCoO₂.