A first-principles investigation of the phase stability in the O2-LiCoO2 system is performed to better understand the unusual layered phases obtained upon Li deintercalation (i.e., T#2 and O6). First-principles pseudopotential calculations within the local density approximation and thermodynamic models extracted from these calculations both show that two tetrahedral sites for the Li ions need to be considered in the T#2 structure for qualitative agreement with experiment to be obtained...