Two-dimensional NbMCO2 (M = Ge, Sn, Sb and Bi) monolayers as a novel anode material for sodium-ion batteries by simulation insight

The development of novel anode materials is of paramount importance for next-generation rechargeable metal-ion batteries. In this contribution, the potential of Nb₂CO₂ and NbMCO₂ (M = Ge, Sn, Sb and Bi) monolayers as electrode materials for sodium-ion batteries is investigated upon the basis of first-principles within the framework of density functional theory. The five monolayers exhibit cohesive energies of 1.922, 0.935, 0.983, 0.910, and 1.051 eV/atom, suggesting that they are all energetically stable. Simultaneously, ELF analysis combined with AIMD simulations further confirmed that the five monolayers are structurally stable. It is found that except for the Sb-C terminal, the adsorption energy of other structures at each terminal T_C site is more stable than that at other sites. Upon adsorption of Na atoms, the characteristics are modified from semiconductors to metallic, denoting a significant enhancement of the electrical conductivity. Notably, the most favorable diffusion barriers for NbGeCO₂, NbSnCO₂, NbSbCO₂ and NbBiCO₂ monolayers at the M–C (Nb–C) termination are 0.376 eV (2.210 eV), 0.269 eV (0.443 eV), 0.518 eV (0.463 eV) and 0.246 eV (0.483 eV), respectively. Interestingly, the M–C terminals in the NbSnCO₂ and NbBiCO₂ monolayers have relatively lower migration barriers than the Nb₂CO₂ surfaces, implying that the incorporation of Sn and Bi makes the migration more favorable and helps to promote fast charge/discharge capability. These results demonstrate that the NbMCO₂ monolayers can provide potential applications as an anode material for sodium-ion batteries.