Electronic Structure of Lithium Nickel Oxides by Electron Energy Loss Spectroscopy

Abstract

The electronic structures of NiO, LiNiO₂, and NiO₂ are studied by the electron energy loss spectroscopy at Ni L_2,3, Ni M_2,3, and O K edges. The Ni L_2,3 edge spectra suggest that the formal charge of nickel is +2 in NiO, +3 with a low-spin state in LiNiO₂, and +4 with a low-spin state in NiO₂. This is well confirmed by first-principles calculations. The Ni M_2,3 edge spectra show similar chemical shifts to those of the Ni L_2,3 edge. Superposition of the Li K edge spectrum, however, hinders further analysis. Although the formal charge of oxygen is −2 in all the three phases, the O K edge spectra indicate a more remarkable difference in the electronic structure of the oxygen in NiO₂ than that in either NiO or LiNiO₂. The spectra suggest that lithium extraction from LiNiO₂ reinforces the covalent bonding between the oxygen and nickel atoms and causes a notable reduction in electron density at the oxygen atoms.