A five-volt lithium insertion material of Li[Ni_1/2Mn_3/2]O₄, having a spinel-framework structure, was prepared from nickel manganese double hydroxide with LiOH and characterized by XRD, FT-IR, TG, SEM and electrochemical methods. Heating a reaction mixture below 600°C did not give good results in terms of polarization and cycleability during charge and discharge in lithium nonaqueous cells. Higher heating temperatures gave lower polarization and better cycleability, while rechargeable capacity at 4.75V vs. Li reduces as heating temperature rises. After several trials, an optimum condition to prepare a five-volt lithium insertion material of Li[Ni_1/2Mn_3/2]O₄ is found to be a two-step solid state reaction, i.e., crystallization at 1000°C followed by oxidation at 700°C. By this process Li[Ni_1/2Mn_3/2]O₄ showed an excellent performance in a five-volt nonaqueous lithium cell and also in a three-volt lithium-ion cell with a Li[Li_1/3Ti_5/3]O₄-negative electrode.