The electronic structures of NiO, LiNiO2, and NiO2 are studied by the electron energy loss spectroscopy at Ni L-2,L-3, Ni M-2,M-3, and O K edges. The Ni L-2,L-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,M-3 edge spectra show similar chemical shifts to those of the Ni L-2,L-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.