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

摘要

The electronic structures of the nickel oxide electrode, NiO(x)(OH)(2-x) for x=0-1.6, and related nickel oxide materials, Li-substituted NiO, Li(y)Ni(2-y)O2 for y=0-1 and 'Ni(4+)' compounds, KNiO2(IO4) 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, Ni(2+)O(-)x(OH)(-)(2-x) X=1.0 AND 1.5, Li(+)Ni(2+)O(1.5-)2, K(+)Ni(2+)O(-)2(IO4)(-) and Ba(2+)Ni(2+)O(1.33-)3, over the conventional ionic electronic structures, Ni(2+x)(+)x(OH)(-)(2-x)X=1.0 AND 1.5, Li(+)Ni(3+)O(2-)2, K(+)Ni(4+)O(2-)2(IO4)(-) and Ba(2+)N(4+)O(2-)3. This finding suggests that the oxidation of the Ni(2+) ion is harder than oxidation of the O(2-) anion for these nickel oxide compounds studied here. p3.