First-principles investigation of phase stability in the O2-LiCoO2 system

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. Only when both tetrahedral sites in T(#)2 are considered is the experimentally observed two-phase O2/T(#)2 region predicted. This indicates that this structural phase transformation is induced by enhanced configurational entropy in the T(#)2 phase and not by a metal-insulator transition as was previously proposed. We also predict that two ordered compounds are stable at room temperature: Li1/4CoO2 in the O2 structure and Li1/3CoO2 in the O6 structure. We show that the formation of the O6 phase is not related to Li staging.