First Principles Study of the Structure-Performance Relation of Pristine W n+1 C n and Oxygen-Functionalized W n+1 C n O 2 MXenes as Cathode Catalysts for Li-O 2 Batteries.

Li-O 2 batteries are considered a highly promising energy storage solution. However, their practical implementation is hindered by the sluggish kinetics of the oxygen reduction (ORR) and oxygen evolution (OER) reactions at cathodes during discharging and charging, respectively. In this work, we investigated the catalytic performance of W n+1 C n and W n+1 C n O 2 MXenes (n = 1, 2, and 3) as cathodes for Li-O 2 batteries using first principles calculations. Both W n+1 C n and W n+1 C n O 2 MXenes show high conductivity, and their conductivity is further enhanced with increasing atomic layers, as reflected by the elevated density of states at the Fermi level. The oxygen functionalization can change the electronic properties of WC MXenes from the electrophilic W surface of W n+1 C n to the nucleophilic O surface of W n+1 C n O 2 , which is beneficial for the activation of the Li-O bond, and thus promotes the Li + deintercalation during the charge-discharge process. On both W n+1 C n and W n+1 C n O 2 , the rate-determining step (RDS) of ORR is the formation of the (Li 2 O) 2 * product, while the RDS of OER is the LiO 2 * decomposition. The overpotentials of ORR and OER are positively linearly correlated with the adsorption energy of the RDS Li x O 2 * intermediates. By lowering the energy band center, the oxygen functionalization and increasing atomic layers can effectively reduce the adsorption strength of the Li x O 2 * intermediates, thereby reducing the ORR and OER overpotentials. The W 4 C 3 O 2 MXene shows immense potential as a cathode catalyst for Li-O 2 batteries due to its outstanding conductivity and super-low ORR, OER, and total overpotentials (0.25, 0.38, and 0.63 V).