Pristine, equivalently and non-equivalently Al substituted LiNi0.5Mn0.5O2 materials were prepared by a combination of co-precipitation and solid-state reaction. As shown by XRD and XPS, lattice volume shrinkage of LiNi0.5(Mn0.45Al0.05)O2 was attributed to the presence of Ni in both 2+ and 3+, while the lattice volume expansion of Li(Ni0.45Al0.05)Mn0.5O2 was caused by lowering the average oxidation state of Mn. Electrochemical performance of LiNi0.5Mn0.5O2 materials can be greatly affected by the change of oxidation states of the transition metals by Al substitution. Non-equivalent substitution of Al for Ni leads to deteriorated discharge performance and cyclic stability due to the reduction of the electrochemical active Ni2+ and structure supported Mn4+, while an increase in the amount of Ni2+ in LiNi0.5(Mn0.45Al0.05)O2 brings obvious improvement of the electrochemical properties. EIS analyses of the electrode materials at pristine and charged states indicate that the poor electrochemical performance of Li(Ni0.45Al0.05)Mn0.5O2 material can be ascribed to the higher charge transfer resistance and surface film resistance, and the observed higher current rate capability of LiNi0.5(Mn0.45Al0.05)O2 can be understood due to the better charge transfer kinetics.