The lithium-rich transition metal oxides show a larger first charge capacity and larger cycling capacities than the non-lithium-rich transition metal oxides. The disadvantages of the lithium-rich transition metal oxides include relatively poor rate capabilities and relatively large irreversible capacities. In this report, the irreversible capacity loss of the lithium rich oxide Li[Li1/5Ni1/5Mn3/5]O-2 was investigated. Stepwise traverse of the oxygen-release plateau increased the cycling capacity of Li/Li[Li1/5Ni1/5Mn3/5]O-2 cells and gave evidence that lithium was removed from the transition metal layer at the start of the oxygen release plateau. The irreversible capacity loss was attributed to the diffusion of transition metals into the lithium vacancies in the transition metal layer and the subsequent inability for lithium reinsertion into the transition metal layer. Isothermal calorimetry of Li/Li[Li1/5Ni1/5Mn3/5]O-2 cells cycled from 2.5 to 4.4 V (no oxygen loss) supported the view that lithium is not deintercalated from the transition metal layer at the start of charge. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3579418]