A lithium nickel manganese oxide, O3-Li2/3Ni1/3Mn2/3O2, is synthesized from the precursor, P3-Na2/3Ni1/3Mn2/3O2, by a Na+/Li+ ion exchange reaction using molten salt. Post-heating at 300, 400, 500, 600, and 700 degrees C is carried out for 5 h in air. The products are characterized by powder XRD, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), SEM, Li-6-magic-angle-spinning-NMR, and electrochemical measurements. The charge/discharge profiles of O3-Li2/3Ni1/3Mn2/3O2, thermally treated at 500 degrees C, show a high-potential plateau region at 4.8 V. Furthermore, sloping voltage profiles are observed at an average voltage of 3.21 V. An initial discharge capacity of 257 mA h g(-1) is obtained between 2.0 and 4.8 V with a current density of 15 mA g(-1) at 25 degrees C. This capacity corresponds to 0.90 electron transfers per formula unit. This study shows that Post heating of O3-Li2/3Ni1/3Mn2/3O2 is effective to improve its electrochemical properties. (C) 2015 Elsevier B.V. All rights reserved.