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 O 2− before O 2 formation by exploiting the suppressed transition metal rearrangement in ribbon-ordered Na 0.6 [Li 0.2 Mn 0.8 ]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.