Surface Chemistry-Engineered Nb ₂ CT _x MXene Enables Reversible CO ₂ Redox in Li-CO ₂ Batteries

Li-CO 2 batteries hold potential due to the advantages in energy density and CO 2 utilization, with their application limited by the CO 2 reduction/evolution reaction kinetics (CRR/CER). Herein, Nb 2 CT x MXene is explored as an electrocatalyst for Li-CO 2 batteries, with its surface microenvironment engineered by functionalizing with -O, -F, -Cl, -Br, and -I terminations to optimize the catalytic activity. Theoretical and experimental analyses reveal that chlorine functionalization (Nb 2 CCl x ) enhances the adsorption of Li, CO 2, and Li 2 CO 3, promotes d-orbital electron delocalization in Nb atoms, and increases electron donation capability. Moreover, Li 2 CO 3 formed on Nb 2 CCl x displays an elongated Li–O bond and a reduced dissociation energy. Consequently, the Nb 2 CCl x -based Li-CO 2 battery delivers a discharge capacity of 15521.2 mAh g –1, an overpotential of 1.50 V, and an extended lifetime of over 157 cycles. These findings underscore the importance of surface chemistry in designing catalytically active MXene-based electrocatalysts for reversible Li-CO 2 batteries.