First Principles Study of the Structure-Performance Relation of Pristine W_n+1C_n and Oxygen-Functionalized W_n+1C_nO₂...

Li-O₂ 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+1C_n and W_n+1C_nO₂ MXenes (n = 1, 2, and 3) as cathodes for Li-O₂ batteries using first principles calculations. Both W_n+1C_n and W_n+1C_nO₂ 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+1C_n to the nucleophilic O surface of W_n+1C_nO₂, 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+1C_n and W_n+1C_nO₂, the rate-determining step (RDS) of ORR is the formation of the (Li₂O)₂* product, while the RDS of OER is the LiO₂* decomposition. The overpotentials of ORR and OER are positively linearly correlated with the adsorption energy of the RDS Li_xO₂* intermediates. By lowering the energy band center, the oxygen functionalization and increasing atomic layers can effectively reduce the adsorption strength of the Li_xO₂* intermediates, thereby reducing the ORR and OER overpotentials. The W₄C₃O₂ MXene shows immense potential as a cathode catalyst for Li-O₂ batteries due to its outstanding conductivity and super-low ORR, OER, and total overpotentials (0.25, 0.38, and 0.63 V).