Li-rich nickel cobalt manganese oxide (NCM) materials with partial substitution of lithium by magnesium is synthesized by the Pechini process. Different synthesized Li100-2xMgx-NCM materials (x = 0, 1, 2.5, 5, and 10) are investigated by X-ray diffraction (XRD), neutron diffraction, and scanning electron microscopy to determine the role of Mg in the structure and its impact on the morphology and electrochemical properties. The chemical composition, crystal structure, and particle morphology are compared with those of the reference (x = 0) material. Mg substitution has a significant impact on the electrochemical properties. In comparison with the reference sample, the x = 1 sample exhibits a mitigation of the voltage drop owing to more stable structure during cycling, leading to a specific discharge of 210 mAh g−1 after 100 cycles at C/10 rate. However, compositions with x ≥ 2.5 exhibit larger voltage drop in discharge, due to the faster formation of a spinel-like structure during cycling.