In a lithium thin-film battery, the reversible discharge capacity of a Li1.12Mn0.44Ni0.44O2 cathode deposited by rf magnetron sputtering and postannealed at 750 degrees C under O-2 could be increased by 80% to 136 mu Ah/mg when cycled between 4.8-2.5 V instead of 4.2-2.5 V. An 82 atom % Li1.03Mn0.42Ni0.55O2/18 atom % Li2O (14 vol %) cathode prepared from a rf magnetron sputter.-deposited film that was annealed at 700 degrees C under N-2 supplied a reversible discharge capacity of 146 mu Ah/mg between 5.3-1.5V. For a given lithium concentration in the cathode during cycling, the magnitude of the chemical potential of sites on the lithium layers (3a sites) in both rhombohedral cathode phases decreased whenever the charge cutoff voltage was raised. This thermodynamic change is attributed to the migration of transition metal ions from the 3b layer sites to vacancies on the lithium layers at high potentials. These transition metal ions also explain the kinetic limitations the cathodes exhibited at higher current densities. Only one rhombohedral phase could be detected by ex situ X-ray diffraction (XRD) measurements over the voltage range 4.6-1.5 V. At 1.5 V, however, possible additional phases might have been present but not detectable due to their low concentration and/or their X-ray amorphousness. The maximum valence state of the transition metal ions of +4 was reached in rhombohedral LixMn0.44Ni0.44O2 at 4.6 V where about 0.4 Li＇ per formula units remained on the Lithium layers. Such a high lithium concentration between the MO, slabs (M = metal ion on 3b layer sites) prevented the phase from developing a unit cell. with the extremely small c axis parameter found for "NiO2" and CoO2 and is believed to be an important prerequisite for the good cycle stability of LixMn0.44Ni0.44O2 between 4.8-2.5 V.