Theoretical Study of Catalytic Performance of WN MXenes As Cathodes for Li-O2 Batteries: Effects of Surface Functionalization and Atomic Layers

Promising Li-O-2 batteries suffer from high overpotentials of the oxygen evolution reaction (eta(OER))/oxygen reduction reaction (eta(ORR)) 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 - O-2 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 LixO(2) 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 eta(OER) on Wn+1NnO2. Moreover, the O-functionalized surfaces can also reduce the adsorption energies of LixO2, thereby lowering eta(ORR) and eta(OER). On the other hand, reducing atomic layers of Wn+1Nn and Wn+1NnO2 can further lower.ORR and.OER, because WN MXenes with fewer atomic layers have lower d-/p- band centers, which can weaken the adsorption of LixO(2). 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-O-2 batteries.