The successful phase separation of P2- and O3-phase Na2/3Fe1/2Mn1/2O2 is demonstrated using co-precipitation method. Although P2-phase displays superior thermodynamic stability at 2/3Na and 1/2Fe:1/2Mn concentration, O3-phase can be stabilized by the means of kinetic control. The electrochemical performance evaluation with half- and full-cells prove that P2- and O3-phase exhibit comparable overall electrochemical performances when the concentrations of Na+ and transition metal constituents in the pristine state are the same. In fact, O3-phase exhibits a slightly larger reversible capacity while P2-phase exhibits slightly better average Coulombic efficiency and polarization under the same electrochemical testing conditions. Electrochemical impedance spectroscopy is used for an in-depth analysis of charge transfer rate and conductivity. The ex situ XRD experiments of both electrodes at 25th cycle show that the pristine crystal structure is maintained under the experimental conditions which indicates the structural stabilities of both electrodes although the gradual capacity degradation is observed due to the complex reactions during the cycling.