Theoretical Evaluation of γ-MX (M = Ge, Sn; X = S, Se) Monolayers as Promising Anchoring Materials for Sodium–Sulfur Batteries

Sodium-sulfur (Na-S) batteries hold significant promise for next-generation energy storage devices due to their high theoretical energy density and environmental friendliness. However, the practical applications are impeded by the polysulfide shuttle effect and sluggish redox kinetics. In this work, we explore the two-dimensional (2D) γ-phase group IV monochalcogenides (γ-MX, M = Ge, Sn; X = S, Se) as potential anchoring materials in Na-S batteries to solve the above issues based on first-principles calculations. The results show that these γ-MX monolayers have moderate adsorption energies toward both S₈ and sodium polysulfides (NaPSs), which are conducive to mitigating the dissolution and shuttle effect of NaPSs. Notably, the γ-MX monolayers, particularly γ-GeS and γ-SnS, exhibit relatively low Gibbs free energy changes for the sulfur reduction reaction (SRR), as well as the energy barriers for Na₂S decomposition, ensuring a fast charge/discharge rate and high sulfur utilization in Na-S batteries. Furthermore, the performance of γ-GeS and γ-SnS monolayers as anchoring materials in Na-S batteries can be further enhanced by applying biaxial strain. Our findings indicate that the 2D γ-phase group IV monochalcogenides possess significant potential for applications in Na-S batteries.