Co, Mn and Mg co-doped lithium nickel oxides of the nominal composition LiNi0.9Co0.1−x [Mn1/2Mg1/2] x O2 (x =0.00, 0.02, 0.04, 0.06) are synthesized by a co-precipitation technique and solid state route. XPS results reveal that the oxidation states of Ni, Co, Mn and Mg are +3, +3, +4 and +2, respectively. An ordered α-NaFeO2 rock salt structure with R − 3 ¯ m symmetry has been confirmed by XRD. The cation mixing of Li/Ni and occupation of Mg ions have been analyzed by Rieveld refinement. Mg ions distribute between 3b and 3a sites in the crystal. Meanwhile a substantial reduction of cation mixing of Li–Ni has been confirmed by Mn–Mg substitution. Better cycling performance is observed in the compound with high Mn–Mg content compared with LiNi0.9Co0.1O2. Mn–Mg substitution for Co has been shown to suppress the increase of the cell impedance during cycles effectively.