A novel method of preparing a highly homogenous spinel Li.sub.1+X Mn.sub.2-X O.sub.4+Y intercalation compound having a predetermined mean particle size and particle size distribution for 4 V secondary lithium and lithium ion cells is provided. The method comprises mixing at least one manganese compound having a predetermined particle size distribution with at least one lithium compound wherein the manganese compound has a mean particle size of between about 1 and 15 microns and the mean particle size of the lithium compound is less than that of the manganese compound. The mixture is then fired in one or more firing steps within specific temperature ranges to form the Li.sub.1+X Mn.sub.2-X O.sub.4+Y intercalation compound. Preferably, at least one firing step is at a temperature of between about 700.degree. C. and 900.degree. C. The Li.sub.1+X Mn.sub.2-X O.sub.4+Y intercalation compounds may be used in the positive electrodes of secondary lithium and lithium ion cells to provide cells having high specific capacity, cycleability, and charge-discharge rate capability.