Well-crystallized and high-performance xLi2MnO3·(1-x)LiMn1/3Co1/3Ni1/3O2 cathode materials over a wide compositional range (0.1⿤ x ⿤0.5) were prepared by a facile co-precipitation strategy. All samples show small and uniform particle dimensions in the range of 200-500nm. 0.3Li2MnO3·0.7LiMn1/3Co1/3Ni1/3O2 (LMNC3) delivers the highest discharge capacities of 321.1mAhg⿿1 at 0.1C, 305.2mAhg⿿1 at 0.2 C, 243.2mAhg⿿1 at 0.5C, 205.9mAhg⿿1 at 1C, 182.1mAhg⿿1 at 2C, and 133.7mAhg⿿1 at 5C, respectively. LMNC3 also exhibits an excellent fast charge-discharge performance, and even at a charge-discharge rate of 2C in the 100th cycle its capacity still remains 128.2mAhg⿿1. The improved performance of LMNC3 can be ascribed to the improvement of electrochemical reversibility, lithium ion diffusion and ionic conductivity. First-principles calculation based on density function theory reveals that the strong covalent bonds formed between Mn-3d and O-2p orbits and stable Li2MnO3-enriched layer are helpful for stabilizing the LiMn1/3Co1/3Ni1/3O2 structure. The same strategy adopted in this work could be helpful to develop other Li-rich cathodes with long cycle life and high rate performance.