Layered LiNi0.5Mn0.5-xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge-discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi0.5Mn0.5-xAlxO2 structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi0.5Mn0.5-xAlxO2 materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219,169,155, and 129 mAh g(-1) at 10, 100, 200, and 400 MA g(-1) current density respectively. Cycled under 40 mA g(-1) in 2.8-4.6 V, LiNi0.5Mn0.45Al0.05O2 shows the highest discharge capacity of about 199 mAh g(-1) for the first cycle, and 179 mAh g(-1) after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi0.5Mn0.45Al0.05O2 can be understood due to the better charge transfer kinetics. (C) 2009 Elsevier B.V. All rights reserved.