Comparative study of density functionals for the description of lithium-graphite intercalation compounds.

The lack of description of van der Waals interactions in layered materials such as graphite and binary graphite intercalation compounds remains a main drawback of conventional density functional theory. Two fundamentally different approaches to overcome this issue are the employment of semiempirical dispersion correction scheme such as Grimme dispersion correction or nonlocal density functionals. We carefully compare these two approaches for the description of the geometric structure and the thermodynamic stability of pure graphite and Li-GICs at different lithium concentrations and stages. Based on the computed formation energies, we also evaluate the lithium-graphite intercalation potential. We find that PBE-D3(BJ) accurately reproduces the lattice parameters and the interlayer binding energy of graphite, although it underestimates the thermodynamic stability of stage-II Li-GICs mainly due to overbinding of carbon atoms in pure graphite. The nonlocal van der Waals functionals optB88-vdW, optB86b-vdW, and revPBE-vdW show a good agreement with experiments concerning stability of Li-GICs of different stages, although they overestimate the van der Waals interactions in graphite. The experimentally determined decreasing step-function behavior of Li-graphite intercalation potential can be qualitatively reproduced only by employing the revPBE van der Waals functional, whereas the other density functionals fail in the description. © 2019 Wiley Periodicals, Inc.

New insights into the origin of unstable sodium graphite intercalation compounds.

The thermodynamically unstable binary graphite intercalation compounds (GICs) with Na remain a main drawback preventing the implementation of Na-ion batteries in the market. In order to shed some light on the origin of Na-GICs instability, we investigate the structure and the energetics of different alkali metal (AM)-GICs by means of density functional theory (DFT) calculations with dispersion correction. We carefully consider different stages of AM-GICs for various AM concentrations and compare the results for Li, Na and K intercalation into graphite. In order to understand the compound stability, we investigated the interplay between the binding energy and the structural deformation due to the presence of AMs in graphite. Whereas the structural deformation energy linearly increases with the size of alkali metal ions, the binding energy passes through a maximum for Na-GIC. The analysis of different contributions to the binding energy allows to conclude that the alkali metal trend is broken for Li-GICs, not for Na-GICs. The high capacity for Li-GIC is a result of the small ion size of lithium. In addition to the mainly ionic binding nature, it allows to form a covalent contribution between lithium and graphite by orbital overlapping. In contrast, Na-GIC and K-GIC exhibit very small or hardly any covalent contribution. Furthermore, due to the small size of lithium the structural deformation energy cost also is small and allows van der Waals interactions between the graphite layers, which further enhance the stability of Li-GICs. For Na- and K-GICs, a higher energy amount for a structural deformation is needed and the stabilizing van der Waals interaction of graphite layers is weaker or hardly present.