To develop a novel energy storage device with high power density, the Li-rich layered oxide (LLOs) is a promising cathode material. A previous work reported, a unique charge-discharge performance for this material, i.e., first run exhibits slope-change and a plateau but not observed in the following runs. However in literature, interpretations of the mechanism based on experiments are controversial. First-principles calculation based on DFT were employed to simulate the lattice stability of xLi2MnO3•(1-x)Li(Ni1/3Mn1/3Co1/3)O2 composite-layered cathode materials on the first charging. The atomistic models based on both monoclinic (C2/m) and rhombohedral (R3m) structures for the pristine Li2MnO3 and Li(Ni1/3Mn1/3Co1/3)O2 phases were constructed, respectively. The calculated XRD patterns based on the optimized structures xLi2MnO3•(1-x)Li(Ni1/3Mn1/3Co1/3)O2 with x = 0.0, 0.3, 0.5, and 0.7 agree closely with experimental data. Based on the most energetically favorable atomistic models, convex hull analyses were performed to investigate the mechanism of Li-ion intercalation during charging/discharging.