Ab initio investigations of orthogonal ScC2 and ScN2 monolayers as promising anode materials for sodium-ion batteries

Abstract

Sodium-ion batteries (SIBs) have attracted widespread intensive attention all over the world owing to their low cost and relatively high safety. The present work reports the first-principles calculations on orthogonal ScC₂ and ScN₂ monolayers to investigate their feasibility as SIB anode materials. The results show that both o-ScC₂ and o-ScN₂ monolayers are thermodynamically stable and have metallic features even during the sodiation process. The adsorption energy of one Na ion on 2 × 2 × 1 cells of the monolayer is −0.28 eV for o-ScC₂ and −0.75 eV for o-ScN₂. Each of o-ScC₂ and o-ScN₂ has the capacity to accommodate two sodium atoms to form Na₂ScC₂ and Na₂ScN₂, which gives low open circuit voltages of 0.08 and 0.10 V, and high Na storage capacities of 777 and 735 mA h g⁻¹, respectively. The ab initio molecular dynamics simulations exhibit the anisotropic diffusion behaviors of Na ions on both o-ScC₂ and o-ScN₂ monolayers with low energy barriers of 0.050 and 0.269 eV, respectively, which are comparable with those of other outstanding SIB anode materials such as Sc₂C (0.012 eV) and MoS₂ (0.28 eV). Given the thermodynamic stability, metallic feature, low Na ion diffusion energy barrier, and high specific capacity, both o-ScC₂ and o-ScN₂ monolayers have great potential to be excellent SIB anode materials.