Kristin Persson, LBNL
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. In this work, first-principles calculations are performed to investigate the electrochemical activity and structural stability of stoichiometric LixMnO3 (0 ≤ x ≤ 2) as a function of Li content. It is shown that the Li2MnO3 structure is electrochemically activated above 4.5 V on delithiation and that charge neutrality in the bulk of the material is mainly maintained by the oxidization of a portion of the oxygen ions: from O2− to O1−. While oxygen vacancy formation is found to be thermodynamically favorable for x < 1, the activation barriers for O2− and O1− migration remain high throughout the Li composition range, impeding oxygen release from the bulk of the compound. Furthermore, 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 the spinel-like structure transformation. Concurrently, the calculated energy barriers for Mn migration from the Mn-layer into the Li-layer suggests a Li2MnO3 structural instability for x < 0.5. Based on the observations, a critical phase transformation path is suggested for forming nuclei of spinel-like domains within the matrix of the original layered structure. Furthermore, the formation of defect layered structures during the first charge manifests in a significant depression of the voltage profile on the first discharge, providing one possible explanation for the observed ‘voltage fade’ of the Li-excess materials.
Two Mn migration paths – Edge path (continuous line) and Dumbbell path (dashed line), which both show feasible migration energies for Mn4+, in particular the Edge path for highly charged particles.