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 O2–/O2n–/O2 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 NaxNi0.25Mn0.68O2 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 O2 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 O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials

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 O2–/O2n–/O2 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 NaxNi0.25Mn0.68O2 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 O2 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 O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials

Investigating Light-Driven Hole Injection and Hydrogen EvolutionCatalysis at Dye-Sensitized NiO Photocathodes: A CombinedExperimental−Theoretical Study

International audienceDye-sensitized photoelectrochemical cells form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and easy processing of their constitutive photoelectrode materials. Such hybrid photoelectrodes consist of molecular dyes grafted onto transparent semiconducting metal oxides in combination with catalytic centers. The optimization of the performances of such hybrid photoelectrodes requires a detailed understanding of the light-driven electron transfer processes occurring first at the interface between the semiconducting material and the dye and then between the dye and the catalytic center. Here we address the first of these issues and use quantum chemistry to determine the structural and electronic features of the interfaces between a push-pull dye and the p-NiO (100) surface. We show that these calculations are in good agreement with transient absorption spectroscopic measurements on a prototypical dye-sensitized photocathode system able to evolve hydrogen in the presence of a cobaloxime catalyst in solution