Layered cobalt and nickel oxide cathodes: a comparison of their structural and chemical stabilities

Abstract

Layered LiNi/sub 0.85/Co/sub 0.15/O/sub 2/ cathodes have drawn much attention recently due to its higher capacity compared to the currently used layered LiCoO/sub 2/ cathode. However, the structural and chemical stabilities of the LiNi/sub 0.85/Co/sub 0.15/O/sub 2/ cathodes during long-term cycling in lithium-ion cells have not been fully assessed. From a systematic investigation, the structural and chemical stabilities of the two systems are compared. Wet-chemical analysis of chemically delithiated Li/sub 1-x/CoO/sub 2/ and Li/sub 1-x/Ni/sub 0.85/Co/sub 0.15/O/sub 2/ establish that the former system tends to lose oxygen from the lattice for (1-x)<0.5, which limits its capacity to 140 mAh/g, while the latter system does not suffer from oxygen loss at least for 0.3/spl les/(1-x)/spl les/1. However, the nickel oxide system suffers from structural instability under mild heat (T>50/spl deg/C) due to a migration of Ni/sup 3+/ ions from the nickel layer to the lithium layer. The differences in the structural and chemical stabilities between the two systems are explained based on crystal field stabilization energies and the relative positions of metal:3d and O:2p bands.