Lithium manganese nickel oxides Li{sub x}(Mn{sub y}Ni{sub 1{minus}y}){sub 2{minus}x}O{sub 2}. 2: Electrochemical studies on thin-film batteries
- Oak Ridge National Lab., TN (United States). Solid State Div.
- Univ. of Kentucky, Lexington, KY (United States). Dept. of Chemistry
In a lithium thin-film battery, the reversible discharge capacity of a Li{sub 1.12}Mn{sub 0.44}Ni{sub 0.44}O{sub 2} cathode deposited by rf magnetron sputtering and postannealed at 750 C under O{sub 2} could be increased by 80% to 136 {micro}Ah/mg when cycled between 4.8--2.5 V instead of 4.2--2.5 V. An 82 atom % Li{sub 1.03}Mn{sub 0.42}Ni{sub 0.55}O{sub 2}/18 atom % Li{sub 2}O (14 vol %) cathode prepared from a rf magnetron sputter-deposited film that was annealed at 700 C under N{sub 2} supplied a reversible discharge capacity of 146 {micro}Ah/mg between 5.3--1.5 V. 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 Li{sub x}Mn{sub 0.44}Ni{sub 0.44}O{sub 2} at 4.6 V where about 0.4 Li{sup +} per formula units remained on the lithium layers. Such a high lithium concentration between the MO{sub 2} 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 NiO{sub 2} and CoO{sub 2} and is believed to be an important prerequisite for the good cycle stability of Li{sub x}Mn{sub 0.44}Ni{sub 0.44}O{sub 2} between 4.8--2.5 V.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC05-96OR22464
- OSTI ID:
- 316275
- Journal Information:
- Journal of the Electrochemical Society, Vol. 145, Issue 12; Other Information: PBD: Dec 1998
- Country of Publication:
- United States
- Language:
- English
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