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 Na x MnO 2 and Li x MnO 2 cathode materials are designed to investigate their ARR behaviors. It is found that the ARR can be activated in only Li-substituted Li x MnO 2 and not for Mg- and Zn-substituted ones, while all Mg/Li/Zn-substituted Na x MnO 2 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 Na x MnO 2 and Li x MnO 2 , 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.