Importance of metal−oxygen bond for stable oxygen-redox reaction in Li-excess layered oxides

Abstract Although nearly all oxygen redox reaction (ORR) research has been carried out based on Mn4+-containing layered oxides (Li[Li1−x−yMnxMy]O2) in Li-ion positive electrodes, the roles of Mn(t2g3)−O hybridized band in correlation with ORRs have yet to be understood. Considering the competition of covalent and ionic M O bond features, the two-type Li2MO3 cathode models (i.e., 3d3-Li2MnO3 and 3d0-Li2TiO3) operated by the pure ORRs were investigated using first-principles calculations to unlock veiled critical factors in activating ORRs, and enabling their reversibility upon charging. First, the thermodynamic formation energies showed that the oxygen instability induced by Li+-extraction is much larger for Li2TiO3 than that for Li2MnO3. Second, the [[EQUATION]]-type Mn(t2g3)−O hybridized band in the low energy state for the Mn oxide broadened the O 2p-bandwidth, resulting in weakening the chemical hardness of Mn O in comparison to that of Ti O. Third, the cooperative inter- and intra-layer O O dimerizations ( O dimers were present within the Mn intralayer for Li2MnO3. From this theoretical understanding, we have concluded that the presence of rigid Mn(t2g3)−O band upon charging plays a significant role in easily triggering the oxygen redox, and in suppressing the formation of very short O O dimers leading to O2 release, which are because of the non-overlap between the bonding states of O O and the Mn(t2g3)−O state. The revealed importance of Mn(t2g3)−O band provides an exciting direction for designing ORutilizing high energy density cathodes for lithium-ion batteries.