Oxygen redox activity in battery cathodes and the role of underbonded oxygen

It is widely recognised that oxygen redox makes a significant contribution to the high capacity of some cathode materials in addition to transition metal redox. We attribute this to the presence of underbonded oxide ions, either in bulk structures or at surfaces, which are the initial source of oxygen redox during charging. Underbonded oxide ions experience departures from local electroneutrality; examples include Li4Mn5O12 spinel and Li‐rich layered rock salt structures but not stoichiometric LiCoO2 or Li2MnO3. Underbonded oxide ions ionise more readily than fully‐stabilised lattice oxide ions since, using an ionic bonding description, they are associated with a deficiency of positive charge in the coordination sphere of surrounding cations. Pauling’s electrostatic bond strength criterion gives a simple guide to structures or compositions that show departures from local electroneutrality and provides the starting point for a new way to consider oxygen redox. Li­­4Mn5O12 behaves as an anode due to Mn redox below 3V and as a cathode due to reversible oxygen redox above ~3.8 V. Underbonded oxygens are also identified in other structures, including high capacity Li‐rich layered rock salt structures.