‘Oxygen bound to magnesium’ as high voltage redox center causes sloping of the potential profile in Mg-doped layered oxides for Na-ion batteries

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 processes and design rules are often unclear, especially at potentials above approximately 4 V vs. Na+/Na. Here, we study by means of synchrotron methods and DFT calculations how Mg- and Sc-doping influences the properties of layered O3-NaNi0.5Mn0.5O2. A strong oxygen redox activity is observed at high voltages and both dopants are found to be effective for decreasing the number of phase transitions and for improving cycle life. When comparing the effect of the dopants, notable differences in the first cycle are observed. While the Sc doping maintains the high voltage plateau, Mg doping causes a more sloping behavior. This difference can be understood from the appearance of 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.