Layered Li(Ni0.5−xMn0.5−xM2x′)O2 (M′=Co, Al, Ti; x=0, 0.025) cathode materials for Li-ion rechargeable batteries

Layered Li(Ni0.5−xMn0.5−xM2x′)O2 materials (M′=Co, Al, Ti; x=0, 0.025) were synthesized using a manganese-nickel hydroxide precursor, and the effect of dopants on the electrochemical properties was investigated. Li(Ni0.5Mn0.5)O2 exhibited a discharge capacity of 120 mAh/g in the voltage range of 2.8–4.3 V with a slight capacity fade up to 40 cycles (0.09% per cycle); by doping of 5 mol% Co, Al, and Ti, the discharge capacities increased to 140, 142, and 132 mAh/g, respectively, and almost no capacity fading was observed. The cathode material containing 5 mol% Co had the lowest impedance, 47 Ω cm2, while undoped, Ti-doped, and Al-doped materials had impedance of 64, 62, and 99 Ω cm2, respectively. Unlike the other dopants, cobalt was found to improve the electronic conductivity of the material. Further improvement in the impedance of these materials is needed to meet the requirement for powering hybrid electric vehicle (HEV, <35 Ω cm2). In all materials, structural transformation from a layered to a spinel structure was not observed during electrochemical cycling. Cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) data suggested that Ni and Mn exist as Ni2+ and Mn4+ in the layered structure. Differential scanning calorimetry (DSC) data showed that exothermic peaks of fully charged Li1−y(Ni0.5−xMn0.5−xM2x′)O2 appeared at higher temperature (270–290 °C) than LiNiO2-based cathode materials, which indicates that the thermal stability of Li(Ni0.5−xMn0.5−xM2x′)O2 is better than those of LiNiO2-based cathode materials.