An Na/TM-site Mg substituted P2-Na2/3[Fe1/3Mg1/12Mn7/12]O2 cathode with extremely high capacity for sodium-ion batteries

The anionic redox reaction (ARR) has become a hot topic in battery research due to its ability to provide high energy density. Nevertheless, there are still many issues in Na-based layered oxides with the ARR, such as large voltage hysteresis, lattice oxygen loss, irreversible structural changes, and cation migration in the TM layer, resulting in structural collapse and poor electrochemical performance. Herein, a series of Na2/3[Fe1/3MgxMn2/3−x]O2 cathodes are synthesized using a traditional solid-state reaction method. The effectiveness of Mg substitution amounts and site occupancy in regulating the reversibility of the ARR has been explored using various experimental techniques. Surprisingly, the well-designed Na/TM-site Mg substituted P2-Na2/3[Fe1/3Mg1/12Mn7/12]O2 exhibits an extremely high initial reversible capacity of ∼253.21 mA h g−1, equivalent to ∼0.94 e− transfer, which is contributed by both cationic and anionic redox reactions as confirmed by hard X-ray absorption spectroscopy (hXAS) and soft X-ray absorption spectroscopy (sXAS) analyses. In addition, the improved cycling and high-rate performance of the P2-Na2/3[Fe1/3Mg1/12Mn7/12]O2 are achieved by a well-maintained crystal structure and the highly reversible anionic redox reaction of O2−/On−. These in-depth studies provide crucial knowledge for the development and understanding of cathode materials with a highly reversible ARR for low-cost and high-energy sodium-ion batteries.