Ab initio study of Li+ diffusion paths in the monoclinic Li0.5CoO2 intercalate

Band structure and ground-state total energy calculations were performed on layered LiCoO2 (rhombohedral R $(3) over bar $ m) and Li0.5CoO2 (monoclinic P2/m) by periodic Hartree-Fock methods, using effective core pseudopotential basis sets. Insulating and conducting behaviors are correctly predicted for the former and latter compound, respectively, on the basis of nonmagnetic closed-shell wave functions. Lithium diffusion was simulated in Li0.5CoO2 along the [100] and [110] directions, by relaxing all atomic positions in Pm and PT space groups and obtaining the corresponding energy profiles as a function of the Li+ position. An a posteriori density-functional theory-based correction for the correlation energy was included. The two-step diffusion mechanism (octahedral-->tetrahedral-->octahedral site) is predicted to be favoured with respect to the direct jump between octahedral sites, with an activation energy of 0.13 against 0.27 eV. A significant cobalt-oxygen charge transfer is observed during the diffusion process, and its meaning is analyzed.