Ion‐Migration Mechanism: An Overall Understanding of Anionic Redox Activity in Metal Oxide Cathodes of Li/Na‐Ion Batteries

The anionic redox reaction (ARR) has attracted extensive attention due to its potential to enhance the reversible capacity of cathode materials in Li/Na-ion batteries (LIBs/SIBs). However, the understanding of its activation mechanism is still limited by the insufficient mastering of the underlying thermodynamics and kinetics. Herein, a series of Mg/Li/Zn-substituted NaxMnO2 and LixMnO2 cathode materials are designed to investigate their ARR behaviors. It is found that the ARR can be activated in only Li-substituted LixMnO2 and not for Mg- and Zn-substituted ones, while all Mg/Li/Zn-substituted NaxMnO2 cathode materials exhibit ARR activities. Combining theoretical calculations with experimental results, such a huge difference between Li and Na cathodes is closely related to the migration of substitution ions from the transition metal layer to the alkali metal layer in a kinetic aspect, which generates unique Li(Na)–O–□TM and/or □Li/Na–O–□TM configurations and reducing reaction activation energy to trigger the ARR. Based on these findings, an ion-migration mechanism is proposed to explain the different ARR behaviors between the NaxMnO2 and LixMnO2, which can not only reveal the origin of ARR in the kinetic aspect, but also provide a new insight for the development of high-capacity metal oxide cathode materials for LIBs/SIBs.