Theoretical Study of Catalytic Performance of Pristine M 2 C and Oxygen-Functionalized M 2 CO 2 MXenes as Cathodes for Li-N 2 Batteries.

Li-N 2 batteries are a promising platform for electrochemical energy storage, but their performance is limited by the low activity of the cathode catalysts. In this work, density functional theory was used to study the catalytic activity of the pristine M 2 C and oxygen-functionalized M 2 CO 2 MXenes (M = Sc, Ti, and V) as cathodes for Li-N 2 batteries. The calculated results suggest that the pristine M 2 C MXenes (M = Sc, Ti, and V) show high electrical conductivity due to the Fermi level crossing the metal 3d states. The stable adsorption of N 2 occurs on M 2 C MXenes via a side-on model and strengthens gradually with decreasing metal atomic number. Furthermore, the kinetics of N 2 dissociation can be significantly accelerated by the coadsorption of Li on M 2 C MXenes. However, adsorption and dissociation of N 2 on the M 2 CO 2 surfaces are too difficult to occur due to strong electrostatic repulsion. The Li-mediated nitrogen reduction reaction during discharge proceeds favorably via (N + N)* → (LiN + N)* → (LiN + LiN)* → (Li 2 N + LiN)* → (Li 2 N + Li 2 N)* → (Li 3 N + Li 2 N)* → (Li 3 N + Li 3 N)* to form two isolated Li 3 N* on M 2 C MXenes. The calculated charge-discharge overpotentials decrease in the order of Sc 2 C < Ti 2 C < V 2 C. Notably, the Sc 2 C MXene has great potential as a cathode catalyst for Li-N 2 batteries because of its high electrical conductivity, strong N 2 adsorption, favorable Li-mediated N 2 dissociation, and ultralow discharging, charging, and total overpotentials (0.07, 0.06, and 0.13 V). This study offers a theoretical foundation for future research on Li-N 2 batteries.