Iron-containing lithium-rich layered manganese oxides are promising as cathode active materials for fabricating high energy density and low-cost lithium-ion batteries. We synthesized Li1.26Mn0.52Fe0.22O2 (or 0.7Li(2)MnO(3)center dot 0.3LiFeO(2)) by using the co-precipitate-hydrothermal-calcination method. We then tested a graphite/Li1.26Mn0.52Fe0.22O2 cell after a stepwise pre-cycling treatment. The pre-cycling treatment drastically increased the specific capacity of the Li1.26Mn0.52Fe0.22O2 from 167 to 236 mAh g(-1) at a current rate of 5 mA g(-1). Other cell performances, such as operation voltage, discharge rate capability, and cyclability, also improved with this treatment. An analytical study using gas chromatography revealed that these improvements were derived from the suppression of oxygen release from the oxide lattice. X-ray absorption spectroscopy and X-ray photoelectron spectroscopy analyzes indicated that the main charge compensation mechanism of pre-cycling treatment was not the redox reaction of transition metals, such as Mn3+/Mn4+ and Fe3+/Fe4+, but the redox reaction of O2- accompanying O-2(2-) formation in the oxide lattice. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.014209jes] All rights reserved.