Heteroatom Driven Design Strategy of Zr3C2O2 MXenes for Alkali-Ion Batteries: Identifying Optimal Doping Regimes via Density Functional Theory Studies

Abstract

Zr3C2O2 MXene has garnered extensive interest as an anode material for alkali-ion batteries. Nevertheless, its inferior electrochemical performance compared to the well-established Ti- and V-based MXenes calls for targeted strategies to unlock its full potential. Heteroatom doping with nonmetals offers an effective way to overcome these limitations by tuning the electronic structure and surface chemistry while keeping the MXene framework intact. Herein, first-principles density functional theory calculations are used to systematically investigate nitrogen (N) and sulfur (S) substitution at the carbon site of Zr3C2O2 MXene. The effects of doping on its structural, electronic, and electrochemical properties as an anode for lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs) are examined. Three doping strategies were considered: N mono-doping, S mono-doping, and N–S co-doping. We evaluate each of these strategies for a range of dopant concentrations from single atom substitution to full site occupation within the supercell. Such a systematic study allows us to probe the isolated defect behavior at low concentrations and the dopant–dopant interaction and collective electronic modulation at higher doping levels. The calculations confirm that substitution at the carbon site is energetically favorable for both heteroatoms. N-doped systems consistently outperform S-doped and N–S co-doped counterparts in terms of structural stability, metallicity, and electrochemical response. N–S co-doping shows synergistic improvements than S doping. The 12N-substituted Zr3C2O2 exhibits the best performance with the highest density of states at the Fermi level, favorable Li/Na adsorption energetics, the lowest average open-circuit voltage, and reduced ion diffusion barriers among all the configurations. Theoretical specific capacities of 328 mA h g–1 for LIBs and 283 mA h g–1 for NIBs are achieved. These findings demonstrate that controlled heteroatom substitution is an effective design strategy for simultaneously optimizing structural integrity, electronic conductivity, energy density, and ion storage capacity in MXene-based anodes for next-generation alkali-ion batteries.