Deciphering the Oxygen Absorption Pre-Edge: Universal Map of Transition Metal Redox Potentials in Batteries

The well-defined low-energy features, so-called pre-edges, of oxygen K-edge X-ray absorption spectra in transition metal (TM) oxides correspond to the hybridized states of TM 3d and O 2p orbitals. The evolution of these features has been broadly used for studying the oxygen oxidation states in battery electrodes. However, critical questions remain on the validity of such an application as well as on the origin of the pre-edge evolution upon cycling. Here, combined with theoretical calculations, we investigated the O-K pre-edge collected from 14 groups of electrodes and a total of 55 oxides that cover all 3d TMs with different elements, structures and electrochemical states. Our detailed analysis shows that the O-K pre-edge variation is dominated by the change of TM states. More importantly, the O-K pre-edge enables a unique opportunity to project the lowest unoccupied states of all TMs onto one common energy window, which corresponds to the relative potentials of TM reduction reactions (electron filling). The summary delivers a universal map of the relative TM redox potentials, which reveals an unusual Cr3+/6+ redox that is further confirmed by both experiments and theory. This work provides a critical clarification on interpreting the O-K pre-edge of TM oxides and a benchmark for oxygen state studies. Our finding demonstrates a simple but effective method to determine the relative TM redox potentials of known or unknown, usual or
novel electrode materials.