Lithium cobalt manganese oxide Li0.55Co0.5Mn0.5O2 with the O6-type layered structure was successfully prepared for the first time via Na+/Li+ ion-exchange from the parent Na0.58Co0.5Mn0.5O2 in molten LiNO3 salt at 280 degrees C. The precursor Na0.58Co0.5Mn0.5O2 sample with the P2-type layered structure was synthesized by solid-state reaction at 900 degrees C. Rietveld analysis using the powder X-ray diffraction data confirmed that the ion-exchanged Li0.55Co0.5Mn0.5O2 sample was the O6-type single phase with the rhombohedral crystal system, space group R-3m, and the lattice parameters of a = 2.8186(3) angstrom and c = 29.674(4) angstrom. The electrochemical measurements showed the reversible Li-ion insertion and extraction reactions. (C) 2014 Elsevier B.V. All rights reserved.