ELECTRONIC-STRUCTURE OF COO, LI-DOPED COO, AND LICOO2

The electronic structure of Li(x)Co(1-x)O (0.01 less-than-or-equal-to x less-than-or-equal-to 0.2), LiCoO2, and Co3O4(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 3T1g, similar to an intermediate-spin Co3+ state. For x less-than-or-equal-to 0.2 the local Co electronic structure is similar to that of CoO. However, LiCoO2 has a strongly reduced Co-O interatomic distance, resulting in a ligand field strong enough to stabilize a Co3+ low-spin ground state. LiCoO2 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), DELTA (= 5.5 eV), and (pd-sigma) (= 1.3 eV).