Impact of Potential\non the Energy Storage Mechanism\nin Chalcogen and Halogen Surface-Functionalized MXenes: Insights from\nthe Grand Canonical Fixed Potential Method

MXenes, as potential electrode materials for sodium-ion\ncapacitors,\nhave numerous chemically modifiable surface sites, which play a critical\nrole in determining MXenes’ electrochemical performance. However,\nconventional density functional theory calculations based on the fixed\ncharge method struggle to describe chemical processes involving electron\ntransfer. In this work, a grand canonical fixed potential method,\nwhich automatically adjusts the system’s total electron count\nto match a specified electrode potential, was performed to investigate\nthe potential effects on the Ti<sub>3</sub>C<sub>2</sub>T<sub>2</sub> (T = O, S, Se, F, Cl, Br) electrode for energy storage. Apart from\npreviously observed changes in the valence of Ti atoms, we discovered\nthat atoms of surface terminal functional groups also undergo charge\ntransfer when the electrode potential changes. Examinations of the\nelectronic structure with varying electrode potentials revealed that\nchanged potential not only shifts the Fermi level but also leads to\nthe renormalization of the density of states. This result differs\nsignificantly from the rigid band approximation. In addition, we found\nthat the origin of the strong energy storage capability of MXenes\ncould be due to the outermost p orbital of surface terminal functional\natoms with the help of the projected density of states analysis. By\nusing grand canonical fixed potential method, it is now feasible to\nestablish the relationship between electronic properties and the energy\nstorage performance in different electrode potentials for electrode\nmaterials.