On the foundation of Li���Ni���Co���Mn quaternary phase diagram, a series of manganese-based cathode materials Li �� Ni 0.25-z Mn 0.75-z Co 2z O y (0 ��� �� ��� 1.75, z = 0, 0.05, 0.15 and 0.25) have been designed and systematically studied. These materials are synthesized via a carbonate co-precipitation route and followed by a solid-state reaction. The effects of cobalt and lithium contents on phase transformations morphologies, and electrochemical behaviors of the cathode materials are overall compared and evaluated. The results reveal that, with �� increases, structures of the materials present phase evolution from spinel phase (�� ��� 0.5) to integrated composite of spinel and layered phase (Fd3��m and R3��m, 0.5 < �� < 1.5) and then to a pure layered phase (R3��m and C2/m, �� ��� 1.5). In addition, size of primary particle and roughness of the secondary particles are also affected by �� value. Moreover, it is found that the initial discharge capacity and cycle stability of the cathode materials can be improved by a proper amount of cobalt substitution for both spinel and layered structure. These studies on the Li���Ni���Mn���Co quaternary phase diagram provide a new insight into the research and development of the cathode materials for advanced lithium-ion batteries.