Assessing (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CT₂(T = -O, -OH, and -F) i-MXenes as High-Performance Electrode Materials for Lithium and Non-Lithium Ion Batteries

Employing first-principles calculations, the energy storage properties and ion diffusion dynamics of Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺on bare (Mo_2/3Sc_1/3)₂C and surface-functionalized (Mo_2/3Sc_1/3)₂CT₂(T = -O, -OH, and -F) i-MXenes are predicted. The investigated i-MXenes show weak adsorption ability to the Zn²⁺ion regardless of the surface terminations, excluding their use as anodes for Zn ion batteries. The first-principles molecular dynamics simulations indicate that the adsorption of alkaline (earth) metal ions and Al³⁺on (Mo_2/3Sc_1/3)₂C(OH)₂and (Mo_2/3Sc_1/3)₂CF₂causes the surface reaction between metal ions and surface terminations, leading to the formation of metal hydride or fluorite overlayers covering the underneath remaining i-MXenes. We find that the bare (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CO₂are suitable candidates for use as anodes for alkaline (earth) metal and Al ion batteries. Specifically, both (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CO₂i-MXenes show good ion storage capacities, ideal open circuit voltages, and fast ion diffusion dynamics for alkaline (earth) metal ions. Notably, the predicted theoretical capacities of (Mo_2/3Sc_1/3)₂C ((Mo_2/3Sc_1/3)₂CO₂) for Li⁺, Mg²⁺, and Al³⁺are 291 mAh g^-1(254 mAh g^-1), 490 mAh g^-1(436 mAh g^-1), and 886 mAh g^-1(640 mAh g^-1), respectively. In addition, the calculated open circuit voltage profiles of Li⁺, Mg²⁺, and Al³⁺exhibit the small on-set voltage and the board plateau region. The climbing image-nudged elastic band predicts that the diffusion energies of Li⁺, Na⁺, K⁺, and Ca²⁺ions on bare (Mo_2/3Sc_1/3)₂C are extremely small (<0.05 eV), and the O-terminated surface show higher diffusion energies for various metal ions. Overall, (Mo_2/3Sc_1/3)₂C and (Mo_2/3Sc_1/3)₂CO₂are good electrode materials for fast charging alkaline (earth) metal ion batteries and supercapacitors.