Effect of Mg doping on the structural and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials

Highlights: • Improving the electrochemical performance by Mg doping. • The change of crystal structure by Mg doping are studied. • First-principles calculations indicate Mg doping increase the activation barriers. - Abstract: LiNi_0.6Co_0.2Mn_0.2O₂ and Mg-substituted LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials have been prepared by co-precipitation and high-temperature solid state method. The effects of partial substitution of Ni with Mg in LiNi_0.6Co_0.2Mn_0.2O₂ material on its chemical component, crystal structure, surface valence states, electrochemical properties, and Li⁺ diffusion barrier have been extensively studied. The Rietveld refinement results reveal that cation mixing has been suppressed effectively, and the lattice parameters a and c decrease while the c/a ratio increases induced by Mg doping. The X-ray photoelectron spectroscopy (XPS) indicates that Mg substitution reduces the relative ratio of Ni²⁺/Ni³⁺ ions on surface. Electrochemical studies indicate that the Mg-substituted material exhibits better cycling performance. First-principles calculations indicate that Mg doping increase the activation barriers of Li⁺ migration, thus excesses of Mg substitution lead to rate performance degradation.