Stoichiometric Li4Mn5O12 spinel oxide has been synthesized by an oxidation of Mn2+ by lithium peroxide in the presence of excess lithium hydroxide followed by firing at T less than or equal to 500 degrees C. The sample has been characterized by X-ray diffraction, elemental and oxidation state analyses, and thermogravimetric analysis. X-ray results indicate that Li4Mn5O12 begins to disproportionate at T> 500 degrees C to give Li1+xMn2-xO4 (x < 0.33) and Li2MnO3. The samples fired at 400 and 500 degrees C show in the voltage range 3.3-2.3 V, an initial capacity of 160 and 153 mAh/g, respectively, which is close to the theoretical value. The sample fired at 500 degrees C shows excellent cyclability with <2% capacity decline over 40 cycles.