Predicting the pseudocapacitive windows for MXene electrodes with voltage-dependent cluster expansion models

TL;DR

This paper uses voltage-dependent cluster expansion models to predict the charge storage performance of MXene pseudocapacitors, revealing that group-VI MXenes have significantly larger energy densities and voltage windows than titanium-based MXenes.

Contribution

The study introduces a novel predictive modeling approach for MXene pseudocapacitors, highlighting the superior energy density and voltage windows of group-VI MXenes compared to titanium-based ones.

Findings

Group-VI MXenes have up to 80% higher areal energy densities.

Predicted pseudocapacitive voltage windows are 1.2 to 1.8 times larger for group-VI MXenes.

Multiple ions in solvent can significantly increase charge capacity.

Abstract

MXene transition-metal carbides and nitrides are of growing interest for energy storage applications. These compounds are especially promising for use as pseudocapacitive electrodes due to their ability to convert energy electrochemically at fast rates. Using voltage-dependent cluster expansion models, we predict the charge storage performance of MXene pseudocapacitors for a range of electrode compositions. $M_3C_2O_2$ electrodes based on group-VI transition metals have up to 80% larger areal energy densities than prototypical titanium-based ( e.g. $Ti_3C_2O_2$) MXene electrodes. We attribute this high pseudocapacitance to the Faradaic voltage windows of group-VI MXene electrodes, which are predicted to be 1.2 to 1.8 times larger than those of titanium-based MXenes. The size of the pseudocapacitive voltage window increases with the range of oxidation states that is accessible to the MXene transition metals. By similar mechanisms, the presence of multiple ions in the solvent (Li$^+$ and H$^+$) leads to sharp changes in the transition-metal oxidation states and can significantly increase the charge capacity of MXene pseudocapacitors.