Oxygen Redox Intercalation Cathodes: The Fundamentals and Strategies to Resolve the Challenges

At its inception, it was realized that harnessing the redox of the “other” ion, namely, the anion, allows us to envisage cathodes with a significantly higher (if not double) capacity of currently available technologies. 1,2,3 In this perspective, we have decluttered the multitudes of hypotheses that have been proposed thus far to explain the origin of and processes involved in anion redox intercalation cathodes. By putting together different pieces of experimental evidence that have been gathered over the last two decades, we have shown that the most convincing mechanism observed in most Li-rich cathodes is that of oxidation of the lattice O 2– to molecular O 2 . 4 This is associated with irreversible in-plane TM migration, an order-to-disorder transition of the TM layer. The transition is aided by formation of vacancy clusters where the molecular O 2 gets trapped. By virtue of lying very close to a TM cation, the O 2 can be reduced back to lattice O 2– albeit at a lower voltage than the oxidation process. This leads to an irreversible voltage plateau during the first charge followed by an S-shaped discharge curve. The irreversible nature of O-redox had been the major obstacle until the discovery of compounds like Na 0.6 [Li 0.2 Mn 0.8 ]O 2, which show partially reversible high-voltage O-redox. More importantly, on studying this compound, it was found that it is possible to change the superstructure order of the TM layer such that the structural transitions and thereby the electrochemistry accompanying O-redox retain reversibility. 5 Taking clues from all of these studies, the perspective provides strategies that might in future enable us to harness O-redox for high-energy-density intercalation cathodes.