LITHIUM MANGANESE NICKEL OXIDES LI-X(MNYNI1-Y)(2-X)O-2 - II - ELECTROCHEMICAL STUDIES ON THIN-FILM BATTERIES

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 prepare d from a rf magnetron sputter.-deposited film that was annealed at 700 degrees C under N-2 supplied a reversible discharge capacity of 146 m u Ah/mg between 5.3-1.5V. For a given lithium concentration in the cat hode during cycling, the magnitude of the chemical potential of sites on the lithium layers (3a sites) in both rhombohedral cathode phases d ecreased whenever the charge cutoff voltage was raised. This thermodyn amic change is attributed to the migration of transition metal ions fr om the 3b layer sites to vacancies on the lithium layers at high poten tials. These transition metal ions also explain the kinetic limitation s the cathodes exhibited at higher current densities. Only one rhomboh edral phase could be detected by ex situ X-ray diffraction (XRD) measu rements 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 th eir low concentration and/or their X-ray amorphousness. The maximum va lence state of the transition metal ions of +4 was reached in rhombohe dral 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 betw een the MO, slabs (M = metal ion on 3b layer sites) prevented the phas e from developing a unit cell. with the extremely small c axis paramet er found for ''NiO2'' and CoO2 and is believed to be an important prer equisite for the good cycle stability of LixMn0.44Ni0.44O2 between 4.8 -2.5 V.