Using a new electrolyte composition, which is stable. against oxidation up to 5 V, the full electrochemical deintercalation of lithium from the spinel LiMn2O4 is studied. The origin of two new reversible oxidation-reduction peaks near 4.5 and 4.9 V are examined. The capacity associated with these peaks depends on both the nominal composition x in LixMn2O4 and the synthesis conditions (annealing temperatures and cooling rates), and thereby can be used as an indicator for electrochemically optimized LiMn2O4 powders. We present evidence that these peaks are related to local structural defects. Thermogravimetric measurements (TGA) on LixMn2O4 Powders show a reversible loss of oxygen that can reach 5% at 1000-degrees-C. We find that some of this weight loss is associated with the conversion of cubic LiMn2O4 to a new tetragonal spinel phase and then to the decomposition of this phase into the orthorhombic LiMnO2 phase plus other products. This new tetragonal LiMn2O4 spinel is prepared as a single phase, and its electrochemical properties are reported.