Structural and Chemical Transformations in Li2MnO3

Li₂MnO₃ is a critical component in the family of the so-called `Li-excess' materials, which are attracting attention as advanced cathode materials for Li-ion batteries. We present first-principle calculations to investigate the electrochemical activity and structural stability of stoichiometric Li<sub>x</sub>MnO<sub>3</sub> (0 < x < 2) as a function of Li content. We find that the Li<sub>2</sub>MnO<sub>3</sub> structure is electrochemically activated above 4.5 V on delithiation and that charge neutrality in the bulk of the material is mainly maintained by anion oxidization. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O migration remain high throughout the Li composition range, impeding any significant oxygen release from the bulk of the compound. Furthermore, we show that, defect layered structures, where some Mn resides in the Li layer, become thermodynamically favorable at lower Li content (x < 1), indicating a strong tendency towards local spinel-like domain transformation. Concurrently, the calculated energy barriers for Mn migration from the Mn-layer into the Li-layer suggests a Li<sub>2</sub>MnO<sub>3</sub> structural instability for x < 0:5. Based on our observations, we suggest a critical phase transformation path for forming nuclei of spinel-like domains within the matrix of the original layered structure. We also show that formation of defect layered structures during the first charge manifests in a significant depression of the voltage profile on the first discharge, providing a possible background for the observed `voltage fade' of the Li-excess materials.