Parasitic reactions taking place at the carbon anode are primarily responsible for the capacity loss that occurs during the "formation cycles" of a carbon/LiMn2O4 Li-ion battery. The additional amount of cathode material required to supplement this irreversible capacity leads to a reduction in the specific energy of the battery. This can be overcome with the use of the overlithiated cathode material, Li1+xMn2O4, in which the excess Li, x, is used to compensate the irreversible apacity at the anode. This investigation highlights the usefulness of n-BuLi reduction to synthesize Li1+xMn2O4 from LiMn2O4 and demonstrates the long term rechargeability of these materials in Li cells. Reaction of cubic spinel LiMn2O4 with BuLi to form overlithiated cathode materials of the general formula Li1-xMn2O4 (x = 0.1-1.0) was found to be quantitative under mild conditions at room temperature. The X-ray diffraction of each Li1-xMn2O4 appeared to represent a nominal composition of a two-phase material consisting of LiMn2O4 and Li2Mn2O4 at a-xx mole ratio, where x represents the number of moles of LiMn2O4 reacted with BuLi. Electrochemical characterization of Li1+xMn2O4 indicated that the chemically introduced Li(x in Li1+xMn2O4) could be extracted nearly 100% in a voltage plateau around 3.0 V vs. Li+/Li. Furthermore, the rate capability and cycle life of these materials when cycled between 4.25 and 3.0 V were identical to those of the baseline LiMn2O4. In balanced carbon/LiMn2O4 full cells, the chemically inserted Li could be fully utilized to compensate for the irreversible capacity loss occurring in their formation cycles.