Though Li 2 MnO 3 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 2 MnO 3 remains hotly contested. To resolve this issue, we review all possible charge compensation mechanisms including bulk oxygen redox, oxidation of Mn 4+ , and surface degradation for Li 2 MnO 3 cathodes displaying capacities exceeding 350 mAh g −1 . Using elemental and orbital selective X-ray spectroscopy techniques, we rule out oxidation of Mn 4+ and bulk oxygen redox during activation of Li 2 MnO 3 . Quantitative gas-evolution and titration studies reveal that O 2 and CO 2 release accounted for a large fraction of the observed capacity during activation with minor contributions from reduced Mn species on the surface. These studies reveal that, although Li 2 MnO 3 is considered critical for promoting bulk anionic redox in Li-rich layered oxides, Li 2 MnO 3 by itself does not exhibit bulk oxygen redox or manganese oxidation beyond its initial Mn 4+ valence.

Lithium-excess layered oxide cathode materials (Li(1+x)TM(1–x)O2) for lithium-ion batteries achieve high specific capacities (≥250 mA h/g) via redox participation of both transition metals and oxygen anions. While oxygen is initially present as O2– in the cathode, oxidized oxygen species such as peroxo-like oxygen (O2 2–) and oxygen gas (O2) are known to form on charge. In this work, differential electrochemical mass spectrometry (DEMS) is used to study the mechanisms by which lithium carbonate, a common impurity, influences how these oxygen species form and react within the battery. We first show, in agreement with prior studies, that Li2CO3 oxidizes electrochemically on charge to evolve CO2, but not O2, implying that reactive oxygen species form instead that then react with cell components to form nonvolatile products. To study the effect of Li2CO3 on degradation processes at the cathode surface, a Li-excess cathode material Li1.17(Ni0.2Mn0.6Co0.2)0.83O2 (NMC) is synthesized using a method that prevents formation of carbonate impurities in the synthesized material. Isotopically tagged lithium carbonate is then deposited on the NMC surface through controlled exposure to 13CO2 or C18O2 gas. DEMS results show that when lithium carbonate is present on the cathode surface, organic fragments containing diatomic oxygen are formed on the cathode surface during charge above 4.2 V versus Li/Li+. Isotopic analysis indicates that the diatomic oxygen within these fragments primarily originates from the NMC lattice, with only a minor fraction originating from the Li2CO3 itself. Our results therefore suggest that reactive oxygen released from the NMC lattice is triggered by the oxidation of surface lithium carbonate.

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By altering the surface of LiNi0.6Mn0.2Co0.2O2 (NMC622) we show that surface defects and contaminants dominate the outgassing and irreversible surface transformations during the first electrochemical cycle. To alter the surface defects and contaminants without changing the bulk structure of the NMC622, we perform mild methanol and water rinses, a water soak, a water rinse and subsequent heat treatment, as well as purposeful increase of the surface Li2CO3. By combining isotopic labeling; gas analysis; and peroxide, hydroxide, and carbonate titrations we observe that these alterations change the surface Li2CO3, surface hydroxides, and the local defects, which in turn alter the nature and extent of the outgassing to O2 and CO2. Our results highlight that outgassing of Li-ion cathode materials is highly dependent on the synthesis and storage routes and comparison of varying compositions must take into account these differences to make any meaningful conclusions. We also show that simple rinsing procedures may be an effective route to controlling interfacial reactivity of Li-ion active materials.