FIRST-PRINCIPLES EVIDENCE FOR STAGE ORDERING IN LIXCOO2

We have investigated phase stability in the layered LixCoO2 intercalat ion compound for x < 0.4 from first principles. By combining a lattice model description of the Li-vacancy configurational degrees of freedo m with first-principles pseudopotential calculations, we have calculat ed the free energy of the material as a function of Li concentration i n three different host structures: (i) the rhombohedral form of LixCoO 2, (ii) the hexagonal form of CoO2, and (iii) a stage II compound of L ixCoO2 in which the host structure can be considered as a hybrid of th e rhombohedral and hexagonal host structures. The first-principles fre e energies indicate that the stage II compound is the most stable of t he three phases for Li concentrations ranging between 0.12 and 0.19. T his result is consistent with the experimental observation by Ohzuku a nd Ueda(1) and Amatucci et al.(2) that the rhombohedral form of LixCoO 2 transforms to a new phase at Li concentrations around x = 0.15. We f ind that the calculated X-ray powder diffraction patterns of the stage II structure agree qualitatively with those observed experimentally a t low Li concentration.(1,2)