Investigating the Anionic Redox Mechanism of Lithium Rich Cathodes

Abstract Controlling anionic redox is the crucial factor for the commercialisation of Li‐Rich cathodes, being required to achieve high practical specific capacity of >250 mAh/g for long‐term cycling. However, the lack of generalizable understanding of the activation and anionic redox mechanisms complicates the rational design of robust Li‐rich cathodes towards practical applications. We find that the physical evolution during activation is only weakly correlated with performance, with structural change seemingly triggered by low‐voltage irreversible anionic redox. Structural evolution is undoubtedly important to the long‐term performance of the battery; however, we find that the electronic structure at the beginning of activation (~4.5 V) is the most important parameter for reversibility. Activation at low voltages triggers large scale structural change, which can in turn trigger more irreversible oxygen oxidation in a feedback loop. Our results suggest that three most cited activation mechanisms – the Reductive Coupling mechanism, the Reversible Transition Metal Migration mechanism, and the Transition Metal Layer Nanovoids theory – all play an important role in this feedback loop. Future optimisations of Li‐Rich cathodes must therefore consider the interactions between all mechanisms holistically, rather than designing around one activation mechanism exclusively.