The properties of graphite/Li[(Ni0.5Mn0.5)(x)Co-y(Li1/3Mn2/3)(1/3)]O-2 (x + y = 2/3, y = 1/12 and 1/6) Li-ion cells are reported. There is an extended plateau near 4.5 V during the first charging of the cells that corresponds to the simultaneous removal of Li and oxygen from the Li[(Ni0.5Mn0.5)(x)Co-y(Li1/3Mn2/3)(1/3)]O-2 (x + y = 2/3, y = 1/12 and 1/6) electrodes. The release of this oxygen directly within a Li-ion cell has been a cause for concern. However, it was found that subsequent to 02 release, Li-ion cells delivered a high reversible positive electrode specific capacity near 250 mAh/g at C/30 between 2.5 and 4.8 V, the cells did not display increased irreversible capacity relative to counterparts having Li metal negative electrodes and the cells retained 85% of their initial capacity after 70 cycles at C/6 between 2.5 and 4.6 V. Therefore, the 02 released during the first charge does not significantly impact the electrochemical properties of graphite/Li[(Ni0.5Mn0.5)(x)Co-y(Li1/3Mn2/3)(1/3)]O-2 ( x + y = 2/3) lithium-ion cells. (c) 2005 Elsevier Ltd. All rights reserved.