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

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.