First-principles calculations on the deposition behavior of LixNay (x + y ≤ 5) clusters during the hybrid storage of Li and Na atoms on graphene.

A new strategy of sodium ion battery with the hybrid storage of Li and Na ions has attracted much attention in the field of large-scale energy storage. For revealing the mechanism of hybrid storage of Li and Na atoms in carbon materials, the lowest energy configuration, adsorption energy, differential density charge and density of states of LixNay clusters on graphene, as a structural unit of carbon materials, were calculated and investigated based on the first principle of density functional theory. The calculation results show that the deposition behavior of single Li or Na atoms on graphene is similar, and both are preferentially deposited at the hollow of graphene (H-site). Li atom is deposited preferentially than Na atom, and the deposition height of Li atom is lower. When the total number of metal atoms x+y≥ 3, the LixNay clusters are deposited on graphene in the form of stereotyped atomic cluster, in which Li atom is usually at the bottom of LixNay cluster, while Na atom is usually at the top of cluster. The electronic structure analysis shows that the electrons of LixNay cluster are transferred to the anti-bonding π orbitals adjacent to graphene. The 2s orbitals of Li atoms, the 2s and 2p orbitals of Na atoms are hybridized with the 2p orbitals of C atoms. Therefore, The Li-C bond or Na-C bond formed between Li or Na atom and C atom of graphene is usually ionic bonds with partial covalent bond properties. Meanwhile, the Li-Li, Na-Na or Li-Na bond formed inside LixNay clusteris usually multiple metal-metal bond.