Structural and electrochemical properties of LiNi_0.5Mn_{0.5 - x}Al_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_{0.5 - x}Al_xO₂ (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 LiNi_0.5Mn_{0.5 - x}Al_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_{0.5 - x}Al_xO₂ 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, LiNi_{0. 5}Mn_0.45Al_0.05O₂ 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 LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.