Prospective positive-electrode (cathode) materials for a lithium secondary battery, viz., LiNi0.8Co0.2−x Al x O2 (x = 0.00, 0.01, 0.03, and 0.05), were synthesized using a sol–gel method and the structural and electrochemical properties are examined by means of X-ray diffraction, cyclic voltammetry, and charge–discharge tests. The LiNi0.8Co0.2−x Al x O2 maintains the α-NaFeO2-type layered structure regardless of the aluminium content in the range x ≤ 0.05. On the other hand, as the aluminium content is increased, the capacity retention of LiNi0.8Co0.2−x Al x O2 is improved while initial discharge capacity is slightly decreased. Results also show that the current peaks on the cyclic voltammograms are diminished and merged on aluminium addition. This suggests that the improved cycle stability of LiNi0.8Co0.2−x Al x O2 is due to suppression of the phase transition.