Lithium storage mechanisms in oxygen-functionalized Nb-Ta double transition metal MXenes controlled by metal ordering

The effects of surface functionalization and metal ordering on the structural, mechanical, and electrochemical properties of two-dimensional transition metal carbide MXenes remain insufficiently understood, particularly for double-metal systems. Nb₂TaC₂ and Ta₂NbC₂ monolayers, in bare and oxygen-functionalized forms, are studied using density functional theory to determine how these factors affect lithium adsorption, diffusion, and storage performance. Oxygen functionalization increases in-plane stiffness (Young's modulus up to 384.7 N/m) and stabilizes Poisson's ratios, while lithium preferentially adsorbs at metallic sites on bare MXenes and at carbon sites on O-functionalized surfaces, with adsorption energies reaching -3.70 eV. Nudged elastic band calculations show that oxygen functionalization slightly increases diffusion barriers, from 0.05 to 0.21 eV for Nb₂TaC₂ and from 0.06 to 0.24 eV for Ta₂NbC₂, while fast Li-ion mobility is maintained. Open-circuit voltage analysis indicates average voltages of 0.74 to 0.95 V with capacities up to 412 mAh/g. These results provide design guidelines for tuning surface chemistry and metal ordering in double-metal MXene anodes for Li-ion batteries.