Rethinking Oxygen Redox: Does Oxygen Dimerisation Occur without Oxidation in Li2NiO3?

In layered lithium transition metal oxide cathodes, high voltage operation is accompanied by the formation of oxygen dimers, which are widely used as an indicator of oxygen-redox activity. However, understanding the role that oxygen dimerisation plays in facilitating charge compensation is still needed. Here, Li2NiO3 (a 3d8L2-containing compound, where L is a ligand hole) is studied as a model system, where oxygen dimerisation is shown to occur without cathode oxidation. Electrochemical cycling results in a net reduction of the cathode structure, accompanied by structural transformations, despite spectroscopic features of oxygen dimers arising at the top of charge. Oxygen dimerisation is shown to coexist alongside a structurally transformed and electronically reduced cathode structure, thus raising questions about its origin with respect to delithiation.