Quantifying the Capacity Contributions during Activation of Li₂MnO₃

Abstract

Though Li<sub>2</sub>MnO<sub>3</sub> 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 Li<sub>2</sub>MnO<sub>3</sub> remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn<sup>4+</sup>, and surface degradation for Li<sub>2</sub>MnO<sub>3</sub> cathodes displaying capacities exceeding 350 mAh g<sup>–1</sup>. Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn<sup>4+</sup> and bulk oxygen redox during activation of Li<sub>2</sub>MnO<sub>3</sub>. Quantitative gas-evolution and titration studies reveal that O<sub>2</sub> and CO<sub>2</sub> 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 Li<sub>2</sub>MnO<sub>3</sub> is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li<sub>2</sub>MnO<sub>3</sub> by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn<sup>4+</sup> valence.