Understanding and further designing new LiNi_xMn_yCo_zO₂ (NMC) (x + y + z = 1) materials with optimized transition metals contents is an important way to meet high power density applications for rechargeable Li batteries (LIBs). Using ab initio calculations combined with experiments, we investigate the kinetics of Li-ion diffusion vs. selection of advantage paths and their effective diffusion barriers depended on activation energies and Li-slab space in NMC materials systematically. It is found that Li-ions tend to choose oxygen dumbbell hopping (ODH) at the early stage of charging (delithiation), and tetrahedral site hopping (TSH) begins to dominate when more than 1/3 Li-ions are extracted. Apart from this, in both ODH and TSH, the Li-ions surrounded by nickel (especially with low valence state) are more likely to diffuse with low activation energy and form an advantage path. The Li slab space, which also contributes to the effective diffusion barriers, is found to be closely associated with the delithiaton process (Ni oxidation) and the contents of Ni, Co, and Mn. Therefore, the kinetics of Li-ion diffusion in layered NMC materials can be tuned by the Li content (delithiation process) as well as the different contents of Ni, Co, and Mn in NMC materials. Based on these discovers, we finally propose that developing Li[Ni_0.5Mn_0.5]_1-xCo_xO₂ (0 < x ≤ 0.33) system is a most promising way to further optimize the performance of NMC materials.