The cathode material Li1.5Ni0.25Mn0.75O2 5 was synthesized through a solid-state reaction. In-situ high energy X-ray diffraction, STEM and electrochemical characterizations confirmed the composite nature of the material. We mainly found that the layered components belonging to the R (3) over barm and C2/m like-phases formed stepwise following the thermal decomposition and reaction of Li2CO3. Solid diffusion at high-temperature with extended calcination times cannot change the composite nature. (C) 2013 The Electrochemical Society. All rights reserved.