Ab initiostudy ofLi+diffusion paths in the monoclinicLi0.5CoO2intercalate

Abstract

Band structure and ground-state total energy calculations were performed on layered ${\mathrm{LiCoO}}_{2}$ (rhombohedral $R3\ifmmode\bar\else\textasciimacron\fi{}m)$ and ${\mathrm{Li}}_{0.5}{\mathrm{CoO}}_{2}$ (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 ${\mathrm{Li}}_{0.5}{\mathrm{CoO}}_{2}$ along the [100] and [110] directions, by relaxing all atomic positions in Pm and $P1\ifmmode\bar\else\textasciimacron\fi{}$ space groups and obtaining the corresponding energy profiles as a function of the ${\mathrm{Li}}^{+}$ position. An a posteriori density-functional theory-based correction for the correlation energy was included. The two-step diffusion mechanism $(\mathrm{octahedra}\stackrel{\ensuremath{\rightarrow}}{l}\mathrm{tetrahedra}\stackrel{\ensuremath{\rightarrow}}{l}\mathrm{octahedral}\mathrm{ }\mathrm{site})$ 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.