In-Depth First-Principles Study of High-Performance M2XO2 MXene Cathode Catalysts for Sodium-Oxygen Batteries

Na−O2 batteries are plagued by high cathodic oxygen reduction (ORR)/oxygen evolution (OER) overpotentials during discharging/charging. Herein, we constructed six carbide/nitride MXenes (M2XO2, M = Ti, Zr, and Hf, X = C, and N) and investigated their performance as cathodes for Na−O2 batteries by first-principles calculations. M2CO2 MXenes have a pseudogap, showing semiconducting properties, while M2NO2 MXenes are conductive. The nucleophilic O on the M2XO2 surfaces prefers to bind with the Na atoms of NaxO2 intermediates to activate the Na−O bonds, improving the sodium deintercalation. For all M2XO2 MXenes, the OER overpotential is higher than the ORR overpotential, forming a performance bottleneck of Na−O2 batteries. The overpotentials originate from the too-strong adsorption of NaxO2 on M2XO2 MXenes. Lowering the O p-band center of the M2XO2 MXenes can weaken the NaxO2 adsorption, thereby reducing the overpotential. Consequently, the overpotentials of the M2CO2 carbides are lower than those of the M2NO2 nitrides and further decrease with a decreasing M atomic number. The Ti2CO2 MXene shows extremely low ORR, OER, and total overpotentials (0.23, 0.32, and 0.55 V), suggesting a huge potential as cathodes in Na−O2 batteries.