Single-phase lithium nickel manganese oxide, LiNi0.5Mn0.5O2, was successfully synthesized from a solid solution of Ni1.5Mn1.5O4 that was prepared by means of the solid reaction between Mn(CH3COO)2·4H2O and Ni(CH3COO)2·4H2O. XRD pattern shows that the product is well crystallized with a high degree of Li–M (Ni, Mn) order in their respective layers, and no diffraction peak of Li2MnO3 can be detected. Electrochemical performance of as-prepared LiNi0.5Mn0.5O2 was examined in the test battery by charge–discharge cycling with different rate, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The cycling behavior between 2.5 and 4.4V at a current rate of 21.7mAg−1 shows a reversible capacity of about 190mAhg−1 with little capacity loss after 100 cycles. High-rate capability test shows that even at a rate of 6C, stable capacity about 120mAhg−1 is retained. Cyclic voltammetry (CV) profile shows that the cathode material has better electrochemical reversibility. EIS analysis indicates that the resistance of charge transfer (R ct) is small in fully charged state at 4.4V and fully discharged state at 2.5V versus Li+/Li. The favorable electrochemical performance was primarily attributed to regular and stable crystal structure with little intra-layer disordering.