Li‐rich cathodes are extensively investigated as their energy density is superior to Li stoichiometric cathode materials. In addition to the transition metal redox, this intriguing electrochemical performance originates from the redox reaction of the anionic sublattice. This new redox process, the so‐called anionic redox or, more directly, oxygen redox in the case of oxides, almost doubles the energy density of Li‐rich cathodes compared to conventional cathodes. Numerous theoretical and experimental investigations have thoroughly established the current understanding of the oxygen redox of Li‐rich cathodes. However, different reports are occasionally contradictory, indicating that current knowledge remains incomplete. Moreover, several practical issues still hinder the real‐world application of Li‐rich cathodes. As these issues are related to phenomena resulting from the electronic to atomic evolution induced by unstable oxygen redox, a fundamental multiscale understanding is essential for solving the problem. In this review, the current mechanistic understanding of oxygen redox, the origin of the practical problems, and how current studies tackle the issues are summarized.

Activation of oxygen redox during the first cycle has been reported as the main trigger of voltage hysteresis during further cycles in high‐energy‐density Li‐rich 3d‐transition‐metal layered oxides. However, it remains unclear whether hysteresis only occurs due to oxygen redox. Here, it is identified that the voltage hysteresis can highly correlate to cationic reduction during discharge in the Li‐rich layered oxide, Li 1.2 Ni 0.4 Mn 0.4 O 2 . In this material, the potential region of discharge accompanied by hysteresis is apparently separated from that of discharge unrelated to hysteresis. The quantitative analysis of soft/hard X‐ray absorption spectroscopies discloses that hysteresis is associated with an incomplete cationic reduction of Ni during discharge. The galvanostatic intermittent titration technique shows that the inevitable energy consumption caused by hysteresis corresponds to an overpotential of 0.3 V. The results unveil that hysteresis can also be affected by cationic redox in Li‐rich layered cathodes, implying that oxygen redox cannot be the only reason for the evolution of voltage hysteresis. Therefore, appropriate control of both cationic and anionic redox of Li‐rich layered oxides will allow them to reach their maximum energy density and efficiency.

Li-rich layered oxides are promising high-energy-density cathodes for lithium-ion batteries. However, their ultimate energy density remains obscure due to an incomprehensive understanding of the first-cycle irreversible capacity. Here, we report...

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