Comparative Study of 2D Nb₂B-MBene and Nb₂C-MXene as Promising Anode Materials for Mg-Ion Batteries

Developing suitable two-dimensional (2D) anodes with both high capacity and fast kinetics represents an essential strategy for next-generation high-performance Mg-ion batteries (MIBs). This study employs density functional theory (DFT) calculations to systematically evaluate and compare the feasibility of Nb₂B-MBenes and Nb₂C-MXenes as promising candidate materials for MIBs, with particular emphasis on their theoretical capacity, ion diffusion kinetics, and structural stability during cycling. Electronic structure analysis confirms that both Nb₂B and Nb₂C monolayers retain metallic properties before and after Mg adsorption. Calculations reveal that along the optimal diffusion pathways, magnesium-ion migration barriers are exceptionally low at 0.06 eV for Nb₂B and 0.08 eV for Nb₂C, demonstrating outstanding ion transport dynamics. Furthermore, the theoretical specific capacities of Nb₂B and Nb₂C are 1636 and 1626 mAh g^-1, respectively, and both exhibit low open-circuit voltages. Ab initio molecular dynamics (AIMD) simulations reveal that fully magnesiated Nb₂B and Nb₂C maintain good thermal stability and structural integrity under full Mg loading, and that after Mg removal, both structures rapidly revert to configurations nearly identical to their initial optimized geometries within 2.5 ps, demonstrating robust structural recoverability. Overall, single-layer Nb₂B and Nb₂C exhibit significant advantages, including low diffusion energy barriers, high theoretical capacity, and intrinsic stability, which showcase their potential as high-performance anode materials for MIBs.