Understanding and optimizing temperature effects of Li-ion diffusion by analyzing crystal structures of layered Li(Ni_xMn_yCo_z)O₂ (NMC) (x + y + z = 1) materials are important to develop advanced rechargeable Li-ion batteries (LIBs) for multi-temperature applications with high power density. Combined with experiments and ab initio calculations, the layer distances and kinetics of Li-ion diffusion of NMC materials in different states of Li-ion de-intercalation and temperatures are investigated systematically. An improved model is also developed to reduce the system error of “Galvanostatic Intermittent Titration Technique” (GITT) with a correction of NMC particle size distribution. The Li-ion diffusion coefficients of all the NMC materials are measured from -25 to 50 °C. It is found that the Li-ion diffusion coefficient of LiNi_0.6Mn_0.2Co_0.2O₂ is the largest with the minimum temperature effect. Ab initio calculations and XRD measurements indicate that the larger Li slab space is benefited to Li-ion diffusion with minimum temperature effect in layered NMC materials.