Electronic structure of CoO, Li-doped CoO, andLiCoO2

The electronic structure of ${\mathrm{Li}}_{\mathit{x}}$ ${\mathrm{Co}}_{1\mathrm{-}\mathit{x}}$O (0.01≤x≤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≤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), Δ (=5.5 eV), and (pdσ) (=1.3 eV).