Impact of Potential on the Energy Storage Mechanism in Chalcogen and Halogen Surface-Functionalized MXenes: Insights from the Grand Canonical Fixed Potential Method

MXenes, as potential electrode materials for sodium-ion capacitors, have numerous chemically modifiable surface sites, which play a critical role in determining MXenes' electrochemical performance. However, conventional density functional theory calculations based on the fixed charge method struggle to describe chemical processes involving electron transfer. In this work, a grand canonical fixed potential method, which automatically adjusts the system's total electron count to match a specified electrode potential, was performed to investigate the potential effects on the Ti3C2T2 (T = O, S, Se, F, Cl, Br) electrode for energy storage. Apart from previously observed changes in the valence of Ti atoms, we discovered that atoms of surface terminal functional groups also undergo charge transfer when the electrode potential changes. Examinations of the electronic structure with varying electrode potentials revealed that changed potential not only shifts the Fermi level but also leads to the renormalization of the density of states. This result differs significantly from the rigid band approximation. In addition, we found that the origin of the strong energy storage capability of MXenes could be due to the outermost p orbital of surface terminal functional atoms with the help of the projected density of states analysis. By using grand canonical fixed potential method, it is now feasible to establish the relationship between electronic properties and the energy storage performance in different electrode potentials for electrode materials.