Li-diffusion pathways in [formula omitted] and [formula omitted] MXenes using the Bond Valence Sum model

Abstract

Two-dimensional materials such as MXenes are being actively considered by the community for energy storage applications. Here, we employ Density Functional Theory (DFT) to model O and S terminated Zr2C MXenes. We find that the most energetically favourable positions for the termination atoms to sit are on top of the second-layer Zr atoms, in agreement with previous studies. Finally, arbitrarily placing a Li-ion on the surface of the MXenes, we apply the Bond Valence Sum (BVS) model to calculate Bond Valence Site Energies (BVSE). We show that BVS is a good substitute for DFT particularly for diffusion pathways, as it yields much faster results and with good accuracy, with the added advantage of not needing exact positions for the atoms. BVS can, therefore, be used as a quick filter when searching for low migration barriers in MXenes and two-dimensional materials.