Stability of adsorption of Mg and Na on sulfur-functionalized MXenes

Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate the adsorption of Mg and Na adatoms on five M₂CS₂ monolayers (where M = Mo, Nb, Ti, V, and Zr) for battery applications. We assessed the stability of the adatom (i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo₂CS₂ cannot support a full layer of Na nor even a single Mg atom. Na and Mg exhibit the strongest binding on Zr₂CS₂, followed by Ti₂CS₂, Nb₂CS₂ and V₂CS₂. Using the nudged elastic band method (NEB), we computed promising diffusion barriers for both dilute and nearly full ion surface coverage cases. In the dilute ion adsorption case, a single Mg and Na atom on Ti₂CS₂ experience ∼0.47 eV and ∼0.10 eV diffusion barriers between the lowest energy sites, respectively. For a nearly full surface coverage, a Na ion moving on Ti₂CS₂ experiences a ∼0.33 eV energy barrier, implying a concentration-dependent diffusion barrier. Our molecular dynamics results indicate that the three (one) layers (layer) of the Mg (Na) ion on both surfaces of Ti₂CS₂ remain stable at T = 300 K. While, according to voltage calculations, Zr₂CS₂ can store Na up to three atomic layers, our MD simulations predict that the outermost layers detach from the Zr₂CS₂ monolayer due to the weak interaction between Na ions and the monolayer. This suggests that MD simulations are essential to confirm the stability of an ion-electrode system - an insight that is mostly absent in previous studies.