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

Layered LiNi_0.85Co_0.15O₂ cathodes have drawn much attention recently due to its higher capacity compared to the currently used layered LiCoO₂ cathode. However, the structural and chemical stabilities of the LiNi_0.85Co_0.15O₂ 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_1-xCoO₂ and Li_1-xNi_0.85Co_0.15O₂ 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⩽(1-x)⩽1. However, the nickel oxide system suffers from structural instability under mild heat (T>50°C) due to a migration of Ni³⁺ 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