The suppression effect of Mg doping on oxygen redox in Mn-based layered oxide cathodes for sodium-ion batteries

Layered transition metal oxides, owing to the presence of anion redox reactions (ARRs), are promising candidates for high-energy-density cathodes. However, ARR often leads to irreversible lattice degradation and a decline in electrochemical stability. Although numerous studies have demonstrated that doping with non-electrochemically active metals (such as Zn, Ti, Mg, etc.) can enable reversible ARR, the phenomenon of a sloping charge “plateau” in the modified materials when charged to high voltages has been overlooked. This work investigates the effect of Mg doping on the oxygen redox reaction in sodium-ion manganese-based layered oxide cathodes. By gradient doping Mg into Na0.67Ni0.33Mn0.67O2, we find that Mg doping suppresses ARR. Experimental and density functional theory analyses reveal that Mg forms strong covalent bonds with oxygen, localizing the electron distribution and reducing the participation of lattice oxygen in the charge compensation process. This work provides insights into the ARR mechanism in layered oxide cathodes.