Electronic structure of CoO, Li-doped CoO, and LiCoO2.

Abstract

The electronic structure of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{Co}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$O (0.01\ensuremath{\le}x\ensuremath{\le}0.2), ${\mathrm{LiCoO}}_{2}$, and ${\mathrm{Co}}_{3}$${\mathrm{O}}_{4}$(1% Li) has been investigated using x-ray photoemission spectroscopy (XPS), bremsstrahlung isochromat spectroscopy (BIS), and x-ray-absorption spectroscopy. The experimental results are compared with model cluster calculations. We find that the CoO band gap is of an intermediate character, between Mott-Hubbard-like and charge-transfer-like. The first ionization state of CoO is therefore of strongly mixed Co 3d and O 2p character. Its local symmetry corresponds to $^{3}$${\mathit{T}}_{1\mathit{g}}$, similar to an intermediate-spin ${\mathrm{Co}}^{3+}$ state. For x\ensuremath{\le}0.2 the local Co electronic structure is similar to that of CoO. However, ${\mathrm{LiCoO}}_{2}$ has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a ${\mathrm{Co}}^{3+}$ low-spin ground state. ${\mathrm{LiCoO}}_{2}$ is an insulator with a gap of 2.7 eV. From a comparison of the XPS and BIS CoO spectra to the cluster calculations, we find values for U (=5.3 eV), \ensuremath{\Delta} (=5.5 eV), and (pd\ensuremath{\sigma}) (=1.3 eV).