Computational Investigation of Li-Ion Adsorption on Charged Graphene

Abstract

Two-dimensional (2D) materials with very high specific surface areas are considered as promising electrode materials for lithium-ion batteries (LIBs). Graphene, in particular, is a good candidate for anode material for LIBs due primarily to its tunable electronic properties, and high mobility. However, this allotrope of carbon has low storage capacity due to its inert characteristic that leads to a poor adsorption of lithium-ion. Recently, there has been several suggestions made to enhance the capacity of graphene by doping the structure with the same group elements that result in improvement of the lithium-ion adsorption.1 In this study, electronic structure investigations of the lithium-ion adsorption on charged graphene single layers doped with germanium have been made by employing a modified first principles technique in the density functional theory (DFT) formalism. The effective screening medium (ESM) extension of the DFT method involves building a non-repeated slab model placed in between semi-infinite domain of media as macroscopic description of surfaces in contact with vacuum/conductor/solution with a prescribed permittivity.2 With this approach, it becomes possible to flexibly model the adsorption characteristics of charged graphene surfaces under potential bias. As a test case, the electrical charge (+0.026e) is applied to the graphene slab to simulate the negative electrode of LIBs, where Li-ion is placed on the hallow site above the carbon ring. In Figure 1a, the local accumulation and depletion of charges are shown in yellow and blue isosurfaces respectively. Charge density difference plot, shown in Figure 1b, depicts the variation of planar averages of electron density along the z-coordinate of the surface slab. Additionally, in Figure 1c, electrostatic potential energy difference between charged and neutral slabs is shown with the Li atom existence on the hollow site. The result confirms the accumulation of induced charge on the adsorbed Li-ion on the hallow site of the graphene layer. Furthermore, the results of electronic structure calculations for charged graphene single layers doped with germanium will be presented and the results will be compared with the neutral slab models.