First-principles study of structural stability and lithium storage property of Si_<i>n</i> clusters (<i>n</i> ≤ 6) adsorbed on graphene

Silicon/carbon composite is one of the most potential high-capacity anode materials for lithium-ion batteries. The interface state between silicon and carbon of silicon/carbon composite is an important factor affecting its electrochemical performance. In this paper, Si_<i>n</i> (<i>n</i> ≤ 6) clusters with different numbers of Si atoms are constructed on graphene as a structural unit of carbon material. The geometric configuration, structure stability and electronic property of Si_<i>n</i> clusters adsorbed on graphene (Si_<i>n</i>/Gr) are studied by the first-principles method based on density functional theory (DFT). The results show that when the number of Si atoms <i>n</i> ≤ 4, the Si_<i>n</i> clusters are preferentially adsorbed on graphene in a two-dimensional configuration parallel to graphene. When <i>n</i> ≥ 5, the Si_<i>n</i> clusters are preferentially adsorbed on graphene in a three-dimensional configuration. With the increase of the number of Si atoms <i>n</i>, the thermodynamic stability of Si_<i>n</i> clusters on graphene decreases significantly, the interface binding strength between Si_<i>n</i> clusters and graphene decreases, and the charge transfer between Si_<i>n</i> clusters and graphene becomes less. At the same time, the storage capacity of Li atoms in Si_<i>n</i>/Gr complex is also studied. Li atoms are mainly stored on the graphene surface near Si_<i>n</i> clusters and around Si_<i>n</i> clusters. The complex synergistic effect of Si_<i>n</i> clusters and graphene enhances the thermodynamic stability of Li adsorption. When <i>n</i> ≤ 4, storing two Li atoms is beneficial to improving the thermodynamic stability of <i>x</i>Li-Si_<i>n</i>/Gr system, and the thermodynamic stability decreases with the increase of Li atom number. When <i>n</i> ≥ 5, the thermodynamic stability of <i>x</i>Li-Si_<i>n</i>/Gr system decreases with the increase of Li atom number. In the <i>x</i>Li-Si₅/Gr system, the C-C bond and Si-Si bond are mainly covalent bonds, while the Li-C bond and Li-Si bond are mainly ionic bonds with certain covalent properties.