A new lithium-excess method is used for the synthesis of LiNi0.5Mn1.5O4 electrode materials at temperatures in the 600-800 degreesC range. Higher average manganese oxidation state and lower impurity contents are detected from X-ray diffraction in the spinet samples, as compared with stoichiometric synthesis. These properties cause the virtual elimination of the 4 V capacity, thus allowing a higher performance of these 5 V materials. A two-phase model of lithium extraction-insertion is detected by X-ray diffraction of electrodes prepared at different extensions of charge.