A Two-Step Oxidation Mechanism Controlled by Mn Migration Explains the First-Cycle Activation Behavior of Li2MnO3-Based Li-Excess Materials
Julija Vinckevičiūtė, Daniil A. Kitchaev, Anton Van der Ven
Abstract
Li-excess materials created by incorporating Li2MnO3 into traditional layered transition-metal oxides promise to substantially increase the energy density of Li-ion batteries. The precise redox mechanisms that are responsible for the enhanced capacity of Li-excess materials, however, remain a mystery. In this work, we systematically enumerate a wide range of kinetically accessible oxidation mechanisms activated upon Li extraction from Li2MnO3 and calculate their reaction energies with first-principles electronic structure methods. The results of this study provide strong evidence for a two-step oxidation mechanism during the first charge of Li2MnO3. The first step involves partial Mn migration from octahedral to tetrahedral sites to enable an energetically favorable Mn redox process from Mn4+ to Mn6+/7+. This migration is followed by an electrochemically silent decomposition reaction whereby Mn reduces back to Mn4+, while oxygen oxidizes to form trapped O2 molecular species. This mechanism and its electrochemical consequences, as predicted from first principles, are consistent with available experimental observations, including the hysteresis and irreversibilities that continue to plague Li-excess materials. Crucially, the second decomposition step eliminates structural and electronic evidence of the original Mn redox process, providing an explanation for the absence of spectroscopic evidence for Mn6+/7+ species.