Unraveling Oxygen Evolution in Li-Rich Oxides: A Unified Modeling of the Intermediate Peroxo/Superoxo-like Dimers.

Abstract

Peroxo/superoxo is a key intermediate in oxygen evolution/reduction reactions in (electro)catalysis. However, peroxo/superoxo analogues have aroused controversies relevant to the origin of oxygen-anion redox. Specifically, some characteristics such as the magnitude of the O-O bond length in bulk materials have been puzzling during oxygen oxidation, as has the relationship between the peroxo/superoxo intermediate and the release of oxygen. The latter is a major safety concern to the application of oxygen-anion redox in lithium ion batteries. Herein, we present a unified modeling of the full delithiation process for model system Li2MnO3 by using first-principles calculations. We find that the cationic antisite defects and the electron deficiency are two major limiting factors in the anionic oxidation whose state can evolve, as the degree of delithiation increases, from the electron/hole, through peroxo-like O2δ- dimer formation, to the eventual release of gas-phase oxygen molecule. During the delithiation process, the dangling oxygen (i.e., singly coordinated with Mn) pairs play a critical role in intermediate dimer formation. Meanwhile, we identify five generic binding patterns of O2δ- dimers with Mn ions for which the O-O bond length varies from 1.45 Å in the peroxo state to 1.22 Å in the gas-phase oxygen molecule. Moreover, the dominant features of the three molecular orbitals, σc, πa, and πb, are distinguished, with the corresponding energy levels being highly delocalized and mixed as a result of the interplay with the host lattice. This work provides a deep understanding of the intermediate states of the anionic redox and suggests new strategies that mitigate oxygen release for the design of highly efficient and safe Li-rich cathode materials.