The Oxidation State of Ni in the Nickel Oxide Electrode and Related Nickel Oxide Compounds: I. Spectroscopic Evidence.

The electronic structures of the nickel oxide electrode, NiOxOH2-x for x0-1.6, and related nickel oxide materials, Li-substituted NiO, LiyNi2-yO2 for y0-1 and Ni4 compounds, KNiO2IO4 and BaNiO3, have been investigated using Ni 2p XPS X-ray photoemission spectroscopy and Ni K-edge XAS X-ray absorption spectroscopy data. Three aspects of the Ni 2p XPS spectra the main peak energy, satellite energy separation, and main-line shape and three aspects of the Ni K-edge XAS spectra pre-edge peak energy, K-edge energy at the half-step height, and white-line peak energy have been analyzed by the aid of simplified models and theoretical calculations. The results of these analyses consistently favor the more covalent electronic structures, Ni2O-xOH-2-x X1.0 AND 1.5, LiNi2O1.5-2, KNi2O-2IO4- and Ba2Ni2O1.33-3, over the conventional ionic electronic structures, Ni2xxOH-2-xX1.0 AND 1.5, LiNi3O2-2, KNi4O2-2IO4- and Ba2N4O2-3. This finding suggests that the oxidation of the Ni2 ion is harder than oxidation of the O2- anion for these nickel oxide compounds studied here. p3