Layered LiNi0.6Co0.2Mn0.2-xMgxO2 (x = 0, 0.01, 0.03, 0.05) samples have been prepared by co-precipitation and high-temperature solid state methods. The influence of the Mg dopant on its chemical component, crystal structure, surface valence states, electrochemical properties, and Li+ diffusion barrier has been studied. Rietveld refinements confirm that when Mg was substituted for Mn in LiNi0.6Co0.2Mn0.2O2, the cation mixing decreased, the lattice parameters a, c and interslab space thickness decreased, while the c/a ratio increased. Electrochemical testing confirms that the initial coulombic efficiency and cycling stability increase by the substitution of Mg for Mn, and electrochemical polarization decreases. First-principles calculations show that the improvement of rate capability may be attributed to less cation mixing.