Nickel-based layered rock-salt cathode materials for lithium-ion secondary batteries have drawn considerable attention due to their high energy capacity, lower cost, and better environmental compatibility than other current potential materials. As the nickel content increases, however, these materials suffer from significant capacity fading during charge-discharge cycles at elevated temperatures. While the capacity deterioration and poor cycling stability are mainly because of the formation of NiO-like rock-salt structures at the surface of active materials, the degradation mechanisms have not been fully investigated. In this study, to clarify the process of formation of inactive NiO-like structures, the degradation mechanism of a cathode-active material, namely, LiNi0.80Co0.15Al0.05O2 (NCA) is studied after a single charge-discharge cycle, by using site-selective energy dispersive X-ray spectroscopy associated with transmission electron microscopy. It is confirmed that cation mixing occurs between the lithium- and transition metal-sites of the LiMO2 (M = transition metals) layered rock salt structures during the transition to partially ordered transition states and then to the final NiO-like disordered rock-salt structures. Further, the volume fractions of the partially/fully disordered phases in the NCA primary particles are quantitatively estimated.