In situ X-ray absorption spectroscopy study of Li(0.98-x)Ni1.02O2 phases shows that the Ni K edge continuously shifts to higher energies with a decrease in Li content in a manner consistent with oxidation of Ni(III) to Ni(IV). The Ni K-edge energy for Ni1.02O2 is consistent with that observed for chemically prepared quadrivalent Ni in KNiIO6. Variations in the coordination numbers, bond lengths, and disorders as a function of state of charge (i.e., the value of x) are consistent with the following facts: (i) a Jahn-Teller distortion for Ni(III), (ii) an undistorted environment for Ni(IV), and (iii) an electrochemical oxidation of Ni(III) to Ni(IV).