Theoretical study of catalytic performance of WN MXenes as cathodes for Li-O2 batteries: Effects of surface functionalization and atomic layers

Abstract

Promising Li–O2 batteries suffer from high overpotentials of the oxygen evolution reaction (ηOER)/oxygen reduction reaction (ηORR) at cathodes during charging/discharging. Herein, the first-principles calculations were performed to investigate WN MXenes, including the pristine Wn+1Nn and oxygen − functionalized Wn+1NnO2 (n = 1, 2, and 3), as the potential cathode catalysts for Li − O2 batteries. Both Wn+1Nn and Wn+1NnO2 exhibit high electrical conductivity. Compared to the electrophilic W surfaces of Wn+1Nn, the nucleophilic O surfaces of Wn+1NnO2 can not only bind to Li of LixO2 to activate the Li − O bond, but also promote electron transfer to form Li+, which facilitates to the delithiation of LixO2 in the OER, thus lowering ηOER on Wn+1NnO2. Moreover, the O-functionalized surfaces can also reduce the adsorption energies of LixO2, thereby lowering ηORR and ηOER. On the other hand, reducing atomic layers of Wn+1Nn and Wn+1NnO2 can further lower ηORR and ηOER, because WN MXenes with fewer atomic layers have lower d-/p- band centers, which can weaken the adsorption of LixO2. The W2NO2 MXene not only has a high electrical conductivity but also exhibits ultra-low ORR, OER, and total overpotentials (0.12, 0.17, and 0.29 V), indicating it possesses an ultra-high catalytic performance as cathodes for Li-O2 batteries.