Role of Long-Range Dispersion Forces in Modeling of MXenes as Battery Electrode Materials

We present a density functional theory study of stacked MXenes with the aim of assessing the accuracy of the most widely used exchange–correlation (xc) functionals for calculating properties relevant for battery electrode materials, namely, mechanical stability, open-circuit voltages, and lithium diffusion barriers. The open-circuit voltage results are nearly independent of the choice of xc-functional, while the lithium diffusion barriers are shown to be heavily dependent on the selected xc-functional, with diffusivities spanning 5 orders of magnitude. This is shown to be due to differences in the computed interlayer distances between the MXene monolayers, which are found to be linearly related to the diffusion barriers. This underlines the importance of starting from reliable crystal structures when modeling MXenes as battery electrode materials. Using an experimental reference materials database, we show a general trend in lattice parameter accuracy for 2D stacked materials with the chosen xc-functionals. Our results reveal that the optB88-vdW functional provides the best average accuracy for predicting the out-of-plane lattice parameter and hence also the best estimate for the transition-state barriers, while PBE with D3 damping performs only slightly worse on average.