Li 2 MnO 3 is a critical component in the family of “Li-excess” materials, which are attracting attention as advanced cathode materials for Li-ion batteries. Here, fi rst-principle calculations are presented to investigate the electrochemical activity and structural stability of stoichiometric Li x MnO 3 (0 ≤ x ≤ 2) as a function of Li content. The Li 2 MnO 3 structure is electrochemically activated above 4.5 V on delithiation and charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions from O 2− to O 1− . While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O 2− and O 1− migration remain high throughout the Li com position range, impeding oxygen release from the bulk of the compound. Defect layered structures become thermodynamically favorable at lower Li content ( x < 1), indicating a tendency towards the spinel-like structure transformation. A critical phase transformation path for forming nuclei of spinel-like domains within the matrix of the original layered structure is proposed. Formation of defect layered structures during the fi rst charge is shown to manifest in a depression of the voltage profi le on the fi rst discharge, providing one possible explanation for the observed voltage fade of the Li-excess materials.