Growth behavior and lithium storage performance of SnxCoy (x + y ≤ 7) clusters in double-layer graphene based on first-principles calculations

Abstract

The structural stability and lithium-storage properties of Sn_x and Sn_xCo_y (x + y ≤ 7) clusters intercalated between bilayer graphene were systematically investigated via first-principles calculations based on DFT. In the Sn_x/BLG systems, the Sn₄ cluster was found to exhibit superior structural stability through the combined analysis of formation energy and second-order difference energy. The Sn–C bond length in Sn_x clusters increased from 2.54 Å to 2.85 Å, accompanied by a marked intensification of carbon-layer distortion. Upon the introduction of Co atoms, among the Sn_xCo_y clusters, SnCo and Sn₄Co clusters were observed to exhibit the lowest formation energies and highest structural stability, thereby effectively mitigating atomic distortion of the bilayer graphene (BLG) carbon layers. The second-order difference energy (Δ²E) provides an effective criterion for evaluating the structural stability of intercalated clusters, enabling the screening of energetically stable Sn_x and Sn_xCo_y cluster models. Lithium-storage calculations for SnCo and Sn₄Co cluster intercalated BLG systems demonstrate that Li atoms are preferentially adsorbed around Sn_xCo_y clusters and transfer charge to graphene layers. With increasing Li content, per-Li charge transfer decreases and inter-Li repulsion strengthens, reducing stability at high Li concentrations. Furthermore, Co doping strengthens the electronic coupling between the clusters and the graphene layers. In the Sn₄Co/BLG system, Li insertion not only reduces the degree of localization of the density of states near the Fermi level but also enhances the conductivity of the system.