Delocalized electron holes on oxygen in a battery cathode

Oxide ions in transition metal oxide cathodes can store charge at high voltage offering a route towards higher energy density batteries. However, upon charging these cathodes, the oxidized oxide ions condense to form molecular O-2 trapped in the material. Consequently, the discharge voltage is much lower than charge, leading to undesirable voltage hysteresis. Here we capture the nature of the electron holes on O2- before O-2 formation by exploiting the suppressed transition metal rearrangement in ribbon-ordered Na-0.6[Li0.2Mn0.8]O-2. We show that the electron holes formed are delocalized across the oxide ions coordinated to two Mn (O-Mn-2) arranged in ribbons in the transition metal layers. Furthermore, we track these delocalized hole states as they gradually localize in the structure in the form of trapped molecular O-2 over a period of days. Establishing the nature of hole states on oxide ions is important if truly reversible high-voltage O-redox cathodes are to be realized. The mechanism of oxygen redox in high-energy transition metal oxide cathodes is elusive. Here the authors illustrate the nature of the electron-hole states on oxide ions, offering insights for realizing reversible, high-voltage cathodes.

Automated summary

Oxide ions in transition metal oxide cathodes can store charge at high voltage. However, during charging, the oxidized oxide ions form trapped O-2, resulting in undesirable voltage hysteresis. By studying ribbon-ordered Na-0.6[Li0.2Mn0.8]O-2, the authors discovered the delocalized electron holes on oxide ions before O-2 formation. The understanding of these hole states is crucial for realizing reversible high-voltage O-redox cathodes.