Structural and Chemical Evolution of the Layered Li‐Excess LixMnO3 as a Function of Li Content from First‐Principles Calculations

Li₂MnO₃ is a critical component in the family of “Li‐excess” materials, which are attracting attention as advanced cathode materials for Li‐ion batteries. Here, first‐principle calculations are presented to investigate the electrochemical activity and structural stability of stoichiometric Li_xMnO₃ (0 ≤ x ≤ 2) as a function of Li content. The Li₂MnO₃ 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 com­position 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 first charge is shown to manifest in a depression of the voltage profile on the first discharge, providing one possible explanation for the observed voltage fade of the Li‐excess materials.