Electrochemical properties of Li-excess electrode materials, Li1.2Co0.13Ni0.13Mn0.54O2, with different primary particle sizes are studied in Li cells, and phase transition behavior on continuous electrochemical cycles is systematically examined. Although the nanosize (<100 nm) sample delivers a large reversible capacity of 300 mAh g–1 at the initial cycle, capacity retention is not sufficient as a positive electrode material. Moreover, unfavorable phase transition, gradual enrichment of trivalent manganese ions, and lowering structural symmetry is not avoidable on electrochemical cycles for a nanosize sample, which is confirmed by combined techniques of synchrotron X-ray diffraction, X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. A submicrosize sample also delivers a large reversible capacity of 250 mAh g–1 even though a slow activation process is observed accompanied with partial oxygen loss and migration oxide ions in the crystal lattice coupled with transition metal migration on the initial charge process. Such an unfavorable phase transition at room temperature is effectively suppressed by the use of a submicrosize sample with low surface area. However, suppression of the phase transition is found to be a kinetically controlled phenomena and is, therefore, unavoidable at elevated temperatures.