Quantifying the Capacity Contributions during Activation of Li₂MnO₃

Though Li₂MnO₃ was originally considered to be electrochemically inert, its observed activation has spawned a new class of Li-rich layered compounds that deliver capacities beyond the traditional transition-metal redox limit. Despite progress in our understanding of oxygen redox in Li-rich compounds, the underlying origin of the initial charge capacity of Li₂MnO₃ remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn⁴⁺, and surface degradation for Li₂MnO₃ cathodes displaying capacities exceeding 350 mAh g–¹. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn⁴⁺ and bulk oxygen redox during activation of Li₂MnO₃. Quantitative gas-evolution and titration studies reveal that O₂ and CO₂ release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. Lastly, these studies reveal that, although Li₂MnO₃ is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li₂MnO₃ by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn⁴⁺ valence.