Li[Co1−z Al z ]O2 (0≤ z ≤0.5) samples were prepared by co-precipitation and solid-state methods. The lattice constants varied smoothly with z for the co-precipitated samples but deviated for the solid-state samples above z =0.2. The solid-state method may not produce materials with a uniform cation distribution when the aluminum content is large or when the duration of heating is too brief. Non-stoichiometric Li x [Co0.9Al0.1]O2 samples were synthesized by the co-precipitation method at various nominal compositions x =Li/(Co+Al)=0.95, 1.0, 1.1, 1.2, 1.3. XRD patterns of the Li x [Co0.9Al0.1]O2 samples suggest the solid solution limit is between Li/(Co+Al)=1.1 and 1.2. Electrochemical studies of the Li[Co1−z Al z ]O2 samples were used to measure the rate of capacity reduction with Al content, found to be about −250±30 (mAh/g)/(z =1). Literature work on Li[Ni1/3Mn1/3Co1/3−z Al z ]O2, Li[Ni1−z Al z ]O2 and Li[Mn2−y Al y ]O4 demonstrates the same rate of capacity reduction with Al/(Al+M) ratio. These studies serve as baseline characterization of samples to be used to determine the impact of Al content on the thermal stability of delithiated Li[Co1−z Al z ]O2 in electrolyte.