Li- and Mn-rich layered oxide (LMRO) cathode materials deliver a reversible capacity over 280 mAh g−1, which is almost two times higher than that of traditional cathode materials. However, the improved mechanism of high capacity for the LMRO cathode material has not been elaborated clearly so far. In this work, the relationship between the structures and the electrochemical performance in the Li(2x+2)/(2+x)Ni(2-2x)/(6+3x)Co(2-2x)/(6+3x)Mn(2+4x)/(6+3x)O2 (XLNCMO) (0 ≤ X ≤ 1) cathode materials is systematically studied. Electrochemical performance is evaluated by a galvanostatic charge and discharge test and electrochemical impedance spectroscopy (EIS). Structure and morphology are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) technologies. Results demonstrate that there is an obvious synergistic effect between the Li2MO3(C2/m) phase and the LiMO2(R-3m) phase in the XLNCMO cathode with a mutual doped composite structure. In this composite structure, the LiMO2(R-3m) phase plays a role of an “activator” to activate the electrochemical activity of the Li2MO3(C2/m) phase and also provides electrochemical capacity, and the Li2MO3(C2/m) phase can be seen as an “stabilizer” to enhance the cycling stability of the LiMO2(R-3m) phase. Moreover, the Li2MO3(C2/m) phase can also be seen as a major contributor for enhancing the electrochemical capacity of the XLNCMO cathodes.