Ball-like LiMn0.4Ni0.4Co0.2O2 particles composed of flakes were synthesized by a simplified co-precipitation method followed by a solid-state reaction at temperatures of 600–900°C. The relationship between the flake thickness and the electrochemical performance of the layered oxides was investigated in this work. The layered oxide with a flake thickness of 80–100nm synthesized at 800°C has the best electrochemical performance among these cathode materials, especially the rate capability. An initial discharge capacity of 160mAhg−1 was obtained at 5C (1400mAg−1) in the voltage range of 2.5–4.5V, and the capacity retention was 80% after 50 cycles. The excellent rate capability is attributed to the well formed structure, short diffusion distance and good crystallinity. In addition, a detailed study of diffusion coefficient of Li+ ( D Li + ) was carried out to further understand this material. The value of D Li + calculated is in the range of 10−11 to 10−12 cm2 s−1.