Investigating the Anionic Redox Mechanism of Lithium Rich Cathodes

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.