Effect of equivalent and non-equivalent Al substitutions on the structure and electrochemical properties of LiNi0.5Mn0.5O2

Pristine, equivalently and non-equivalently Al substituted LiNi_0.5Mn_0.5O₂ materials were prepared by a combination of co-precipitation and solid-state reaction. As shown by XRD and XPS, lattice volume shrinkage of LiNi_0.5(Mn_0.45Al_0.05)O₂ was attributed to the presence of Ni in both 2+ and 3+, while the lattice volume expansion of Li(Ni_0.45Al_0.05)Mn_0.5O₂ was caused by lowering the average oxidation state of Mn. Electrochemical performance of LiNi_0.5Mn_0.5O₂ 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 LiNi_0.5(Mn_0.45Al_0.05)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(Ni_0.45Al_0.05)Mn_0.5O₂ material can be ascribed to the higher charge transfer resistance and surface film resistance, and the observed higher current rate capability of LiNi_0.5(Mn_0.45Al_0.05)O₂ can be understood due to the better charge transfer kinetics.