Nanocrystalline lithium manganese oxide spinel (LMOS) has been prepared by ballmilling LMOS powders prepared by solid-state reaction. X-ray diffraction and transmission electron microscopy showed that significant strain was introduced and nanometer-scale grains were generated in the spinel particles. The ballmilled spinel cathodes exhibited excellent cyclability in the 3 V range at room temperature and at elevated temperatures in spite of an apparent Jahn-Teller distortion; stable discharge capacities in the 3 V range of 2.4 to 3.4 V at a current density of 0.5 mA/cm(2) were ca. 125 and 150 mAh/g at room temperature and 60 degreesC, respectively.