Exploring the effect of layer spacing of graphene for lithium single-atom diffusion using first principles calculations

Abstract

Bilayer graphene is a representative two-dimensional carbon material and a promising anode material for lithium batteries.The layer spacing variation of bilayer graphene can affect the diffusion of lithium atoms, and thus affect the rate performance of lithium battery anode materials.To reveal the mechanism of the diffusion of the lithium atom in bilayer graphene at different layer spacing, the lowest energy configuration,formation energy, diffusion energy barrier, differential charge density and Mulliken population are calculated and investigated according to the first principles of density functional theory (DFT). The calculation results show that a single lithium atom is preferentially deposited at the hollow site of graphene, and the lithium atom embedded graphene gradually changes from A-B stacking mode (shifted graphene) to A-A stacking mode (aligned graphene) with the increase of layer spacing. For both A-A and A-B stacking mode bilayer graphene, the increase of layer spacing improves the formation energy of the system and reduces the stability of the structure. The increase of the layer spacing also improves the diffusion energy barrier of A-B stacking graphene and decreases that of A-A stacking. This means that the increase of layer spacing can make the diffusion of the lithium atom harder in A-B stacking while easier in A-A stacking. The results of differential charge density and Mulliken population verifiedthatwhen the layer spacing increases to 1.8 nm, the effect of bilayer graphene on the lithium atom is similar than that of monolayer graphene. The calculation results of OCV and the maximum capacity prove the potential of bilayer graphene as anode material. The results provide a new understanding of the lithium atom diffuse in bilayer graphene, and the diffusion properties of the lithium atom can be optimized by adjusting the layer spacing appropriately.