Synthesis of Li1.20Mn0.54Co0.13Ni0.13O2 at different temperatures allows modifying particle sizes and specific surface areas while keeping very similar structures for the materials from the local scale to long distances. X-ray diffraction analysis reveals that irreversible structural reorganization of this lithium and manganese-rich layered oxide occurs during the 1st charge and continues during a few cycles. A mixture of two phases is formed on the high voltage plateau and is then maintained, showing that the material obtained upon cycling is metastable and out of equilibrium. Depending on the particles' size and on the cycling conditions (Cycling rate, temperature and number of cycles) the nature of the materials formed changes, with especially a difference in distribution between the two phases observed. We propose that these phenomena are intimately linked to oxygen participation to the redox processes, for one phase with (reversible) oxygen oxidation only and for the second phase up to (irreversible) oxygen loss.