Comparison of the Chemical Stability of Li1−xCoO2 and Li1−xNi0.85Co0.15O2 Cathodes

The chemical stability of the layered Li_1−xCoO₂ and Li_1−xNi_0.85Co_O.15O₂ cathodes is compared by monitoring the oxygen content with lithium content (1−x) in chemically delithiated samples. The Li_1−xCoO₂ system tends to lose oxygen from the lattice at deep lithium extraction while the Li_1−xNi_0.85Co_0.15O₂ system does not lose oxygen at least for (1−x)>0.3. This difference seems to result in a lower reversible (practical) capacity (140 mA h/g) for LiCoO₂ compared to that for LiNi_0.85Co_0.15O₂ (180 Ma h/g). The loss of significant amount of oxygen leads to a sliding of oxide layers and the formation of a major P3 and a minor O1 phase for the end member CoO_2−δ with δ=0.33. In contrast, Ni_0.85Co_0.15O_2−δ with a small amount of δ=0.1 maintains the initial O3 layer structure.