Li2Mn4Oy-delta spinal oxides with 0.36 < delta < 0.46 were synthesized by oxidation of aqueous Mn2+ with hydrogen peroxide or lithium peroxide in the presence of lithium carbonate or lithium hydroxide followed by firing the precursor in air at 300-400 degrees C. Li2Mn4O9-delta spinel is metastable and disproportionates at about 500 degrees C to give a lithium-rich spinel Li1+xMn2-xO4+delta and Mn2O3. The disproportionation temperature and the amount of Mn2O3 impurity formed depended on the synthesis procedure and raw materials used. Use of lithium peroxide and lithium hydroxide in the solution-based oxidation procedure produced the least amount of Mn2O3 impurity at intermediate temperatures. However, the value of x in the lithium-rich spinel Li1+xMn2-xO4+delta decreased with a further increase in filing temperature, and the stoichiometric spinel LiMn2O4 without Mn2O3 impurity was formed at 800 degrees C. Phase analysis as a function of firing temperature for a wider range of Li/Mn ratios in the system LiyMn3-yO4+delta (0.7 less than or equal to y less than or equal to 1.33) also revealed that single-phase spinel could be formed for the entire firing temperature range 300 less than or equal to T less than or equal to 800 degrees C only for a narrow value of 1.05 less than or equal to y less than or equal to 1.25. The lithium-intercalation properties of the metastable Li2M4O9-delta spinel were influenced by the firing temperature. A Li2Mn4O9-delta sample formed at an optimum temperature of 400 degrees C showed a capacity of 130 mAh/g in the range 3.8-2.0 V with excellent cyclability.