Unveiling Oxygen Redox Activity in P2-Type NaxNi0.25Mn0.68O2 High-Energy Cathode for Na-Ion Batteries

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications.Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides.High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O 2-/O 2 n-/O 2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered.Here, we dissect this intriguing oxygen redox activity in Mn-deficient Na x Ni 0.25 Mn 0.68 O 2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation.We identify low-energy intermediates that release molecular O 2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe.These new atomistic insights on O 2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials.