Computational design of GeC4 and SnC4 monolayers as high-capacity fast-kinetics anodes for sodium-ion batteries

Abstract

The development of high-capacity anode materials with rapid ion transport kinetics remains a critical challenge for sodium-ion batteries (SIBs). Herein, density functional theory calculations are employed to design and evaluate two-dimensional (2D) GeC₄ and SnC₄ monolayers as potential SIB anodes. Phonon dispersion and ab initio molecular dynamics simulations confirm dynamic and thermal stability at 300 K. These heteroatom-integrated carbon monolayers exhibit theoretical specific capacities of 944 mAh g⁻¹ (GeC₄) and 683 mAh g⁻¹ (SnC₄), substantially surpassing those of experimental Ge/C (160 mAh g⁻¹ after 700 cycles) and Sn/C (∼300 mAh g⁻¹) composites. Na⁺ binding energies of −1.28 eV (GeC₄) and −1.21 eV (SnC₄) at pentagonal ring sites, combined with diffusion barriers as low as 0.18 eV (GeC₄) and 0.20 eV (SnC₄), indicate favorable thermodynamics and kinetics for reversible sodiation. Electronic structure analysis reveals preserved metallic character across all Na⁺ loadings, ensuring electronic conductivity. Average open-circuit voltages of 0.10 V (GeC₄) and 0.04 V (SnC₄) versus Na/Na⁺ position these materials as low-voltage, high-power anode candidates. Notably, minimal in-plane lattice expansion (2.32% for GeC₄ and 2.38% for SnC₄) at full Na loading further underscores the exceptional structural stability of these 2D frameworks during electrochemical cycling.