Li[Ni1/2Mn3/2]O-4 having a spinel-framework structure was prepared by a two-step solid state reaction, i.e., crystallization at 1000 degreesC followed by an oxidation at 700 degreesC in air, and examined in nonaqueous lithium cells. Thus prepared Li[Ni1/2Mn3/2]O-4 showed low polarization and stability against electrolyte oxidation with rechargeable capacity of about 110-130 mAh/g at 4.7 V with 10-30 mAh/g at 4.0 V vs Li. Capacity in region of ca. 4.0 V vs Li, which was usually observed for the samples prepared by the conventional method, can be minimized by the oxidation process at 700 degreesC. A 3-volt lithium-ion battery with two insertion materials based on lithiated transition metal oxides having spinel-framework structures is demonstrated therefrom.