Abstract
Composites of reduced graphene oxides (rGOs) with transition metal dichalcogenides have garnered considerable attention as promising anode materials for sodium-ion batteries (SIBs) because of their superior theoretical capacity and long-term stability compared with pure graphene. However, the underlying mechanism of how the oxygen functional groups improve the functionality of rGO remains unclear. In this study, we investigated the roles of functional groups in rGO-based heterogeneous bilayers using first-principles density functional theory calculations. The thermodynamic affinities to Na atoms, kinetic diffusion, and working potential behaviors of the Na atoms in various models, such as MoS(2)/graphene (Gr), MoS(2)/Gr-O, MoS(2)/Gr-OH, and MoS(2)/Gr-COOH, were accurately evaluated. It is clearly demonstrated that the noncovalent bonding nature is a predominant descriptor of Na affinity rather than the charge distribution around the intercalated Na atom. The activation barriers for Na atomic diffusion in the MoS(2)/Gr, MoS(2)/Gr-O, MoS(2)/Gr-OH, and MoS(2)/Gr-COOH models were estimated to be 0.24, 0.27, 0.35, and 0.31 eV, respectively. This indicated that the functional groups slightly delayed the Na motion. Notably, the obtained results demonstrated that the -COOH group not only enhanced the affinity towards Na intercalation but also induced a low working voltage at approximately 1 V. Therefore, the carboxyl functional group exhibits high material stability, making rGO a promising candidate for SIB anode materials.