Theoretical Study of Catalytic Performance of Pristine M2C and Oxygen-Functionalized M2CO2 MXenes as Cathodes for Li-N2 Batteries.

ABSTRACT

Li-N2 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 M2C and oxygen-functionalized M2CO2 MXenes (M = Sc, Ti, and V) as cathodes for Li-N2 batteries. The calculated results suggest that the pristine M2C MXenes (M = Sc, Ti, and V) show high electrical conductivity due to the Fermi level crossing the metal 3d states. The stable adsorption of N2 occurs on M2C MXenes via a side-on model and strengthens gradually with decreasing metal atomic number. Furthermore, the kinetics of N2 dissociation can be significantly accelerated by the coadsorption of Li on M2C MXenes. However, adsorption and dissociation of N2 on the M2CO2 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)* → (Li2N + LiN)* → (Li2N + Li2N)* → (Li3N + Li2N)* → (Li3N + Li3N)* to form two isolated Li3N* on M2C MXenes. The calculated charge-discharge overpotentials decrease in the order of Sc2C < Ti2C < V2C. Notably, the Sc2C MXene has great potential as a cathode catalyst for Li-N2 batteries because of its high electrical conductivity, strong N2 adsorption, favorable Li-mediated N2 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-N2 batteries.