Regulating Anionic Redox via Mg Substitution in Mn‐Rich Layered Oxide Cathodes Enabling High Electrochemical Stability for Sodium‐Ion Batteries

Abstract With the limited resources and high cost of lithium‐ion batteries (LIBs) and the ever‐increasing market demands, sodium‐ion batteries (SIBs) gain much interest due to their economical sustainability, and similar chemistry and manufacturing processes to LIBs. As cathodes play a vital role in determining the energy density of SIBs, Mn‐based layered oxides are promising cathodes due to their low cost, environmental friendliness, and high theoretical capacity. However, the main challenge is structural instability upon cycling at high voltage. Herein, Mg is introduced into the P2‐type Na 0.62 Ni 0.25 Mn 0.75 O 2 cathode to enhance electrochemical stability. By combining electrochemical testing and material characterizations, it is found that substituting 10 mol% Mg can effectively alleviate the P2–O2 phase transition, Jahn‐Teller distortion, and irreversible oxygen redox. Moreover, structural integrity is greatly improved. These lead to enhanced electrochemical performances. With the optimized sample, a remarkable capacity retention of 92% in the half cell after 100 cycles and 95% in the full cell after 170 cycles can be achieved. Altogether, this work provides an alternative way to stabilize P2‐type Mn‐based layer oxide cathodes, which in turn, put forward the development of this material for the next‐generation SIBs.