‘Oxygen Bound to Magnesium’ as High Voltage Redox Center Causes Sloping of the Potential Profile in Mg-Doped Layered Oxides for Na-Ion Batteries

Abstract

The potential profile of layered oxides as cathodes for Na-ion batteries can be well tuned by cation doping. Doping typically leads to changes in the phase behavior and the redox chemistry, but the underlying mechanisms remain poorly understood, especially at potentials above ≈4 V vs. Na⁺/Na. Here, the influence of Mg-doping on the properties of layered O3-NaNi₀.₅Mn₀.₅O₂ is studied by means of synchrotron methods (XAS, RIXS, operando XRD) and DFT calculations. A strong oxygen redox activity is observed at high voltages, and Mg is found to induce an OP2 phase at low concentration (10%) and a solid solution type reaction at a high concentration (25%), thus greatly enhancing the cycle life. The change toward a more sloping charge profile with increasing Mg content is caused by a new redox center (arising from the ‘O bound to Mg') having a higher redox potential but being more irreversible compared to the redox activity of ‘O bound to Ni'. The appearance of an additional redox center explains findings obtained for other doped Ni- and Mn-based layered oxides, which also show a sloping potential in the high voltage region.