Oxygen Redox in Alkali-Ion Battery Cathodes

Current high-energy-density Li-ion batteries use stoichiometric Li 3d transition metal oxides as positive electrodes, which are conventionally described purely by transition-metal redox during routine operating windows. Their practical specific capacities (mAh/g) may be increased by widening their operational voltage window, using Li-excess compositions, or a combination of the two, both of which have shown increasing evidence of O participation in the charge-compensation mechanism. Understanding how this influences the electrochemical performance of these cathodes has been of great interest. Therefore, this review summarizes the current understanding of O participation in alkali-ion battery cathode charge compensation. Particular scrutiny is applied to the experimental observations and theoretical models used to explain the consequences of O participation in charge compensation. The charge-compensation mechanism of LiNiO2 is revisited to highlight the role of O hole formation during delithiation and is discussed within the wider context of Li-excess cathodes.