Role of Oxygen and Halogen Functionalization in Tuning the Surface Properties of Zr3C2T2 MXene for Lithium Storage: A Density Functional Theory Study

We constructed computational models of bare Zr₃C₂ and surface-functionalized Zr₃C₂T₂ (T = O, S, F, Cl), and utilized first-principles calculations to systematically explore the effects of these surface-functionalized groups on the structural stability, electronic properties, and lithium storage performance of Zr₃C₂T₂. Compared to halogen functional groups (e.g., F, Cl), the structure and electronic properties of Zr₃C₂ are more profoundly influenced by oxygen group functional elements (O, S). The formation energy of Zr₃C₂T₂ (T = O, S) functionalized by the same periodic oxygen group elements is lower than that of Zr₃C₂T₂ (T = F, Cl) functionalized by the same periodic halogens. Regarding electronic properties, the oxygen and sulfur functional groups have strong hybridization with Zr₃C₂ in the valence band and generate a new band structure, which makes the DOS move toward the conduction band. The adsorption energy calculations reveal that lithium ions exhibit stable adsorption on bare Zr₃C₂ and O/S-functionalized Zr₃C₂T₂ surfaces, whereas no stable adsorption occurs on Zr₃C₂F₂ or Zr₃C₂Cl₂. In terms of adsorbing lithium atoms, bare Zr₃C₂ tends to adsorb at the HCP position, while Zr₃C₂O₂ and Zr₃C₂S₂ tend to adsorb at the CCP position. First-principles calculations indicate distinct theoretical lithium storage capacities for Zr₃C₂-based materials: monolayer adsorption yields capacities of 180.13 mAh/g (bare Zr₃C₂), 162.64 mAh/g (Zr₃C₂O₂), and 148.20 mAh/g (Zr₃C₂S₂); bilayer adsorption significantly increases these values to 360.25, 325.29, and 296.41 mAh/g, respectively.