Spherical (Ni0.5Mn0.5)(OH)2 with different secondary particle size (3μm, 10μm in diameter) was synthesized by co-precipitation method. Mixture of the prepared hydroxide and lithium hydroxide was calcined at 950°C for 20h in air. X-ray diffraction patterns revealed that the prepared material had a typical layered structure with R 3 ¯ m space group. Spherical morphologies with mono-dispersed powders were observed by scanning electron microscopy. It was found that the layered Li[Ni0.5Mn0.5]O2 delivered an initial discharge capacity of 148mAhg−1 (3.0–4.3V) though the particle sizes were different. Li[Ni0.5Mn0.5]O2 having smaller particle size (3μm) showed improved area specific impedance due to the reduced Li+ diffusion path, compared with that of Li[Ni0.5Mn0.5]O2 possessing larger particle size (10μm). Although the Li[Ni0.5Mn0.5]O2 (3μm) was electrochemically delithiated to Li0.39[Ni0.5Mn0.5]O2, the resulting exothermic onset temperature was around 295°C, of which the value is significantly higher than that of highly delithiated Li1−δ CoO2 (∼180°C).