Ru-Doping of P2-NaxMn0.75Ni0.25O2-Layered Oxides for High-Energy Na-Ion Battery Cathodes: First-Principles Insights on Activation and Control of Reversible Oxide Redox Chemistry

Design and development of high-energy, efficient, and structurally stable positive electrodes for Na-ion batteries (NIBs) have recently been focusing on layered transition metal oxides (NaxTMO2, 0 < x < 1). When doped with late transition metals, the redox reactions ensuring the charge compensation upon Na+ removal can rely on the direct participation of anionic chemistry, with encouraging outcomes in terms of both energy density and power. However, the control of reversible O2–/O– reactions is still a major issue, as the undesired release of O2 can lead to detrimental effects on cathode capacity and overall cell stability. The fine-tuning of metal–oxygen bond covalency has recently emerged as a promising strategy toward the reversible access of oxygen redox. Following the route paved by Ru-based Li-rich cathode materials, we hereby present a first-principles investigation of a Ru-doped NaxTMO2 (TM = Ru, Ni, and Mn) system and the related structural and electronic properties of interest for NIB applications. We aim to dissect the specific role of each element sublattice in compensating the electronic charge along desodiation, with a major focus on the anionic contribution. The oxygen activity is addressed in the high-voltage range (i.e.,xNa = 0.25), and the underlying mechanism is derived from PBE + U(-D3BJ) calculations. We also discuss the effects of Mn deficiency as a suitable site for the formation of low-energy superoxide species via preferential breaking of the Ni–O bond. Conversely, breaking a Ru–O bond is unlikely to occur, which assesses the key role of the Ru dopant in stabilizing the oxide lattice and enabling the desired reversible conditions. Our results also highlight the oxygen vacancy formation energy as an effective descriptor for different activities toward the O2–/O–/O2 evolution. All these theoretical insights can be useful to drive further experimental efforts toward the optimal design of efficient and high-energy NIB cathodes with enhanced practical reversible capacity.