(140h) Enhancing the Performance of Mxene Supercapacitor in Ionic Liquid By Expanding Interlayer Spacing

In the last decade, MXenes have become a fast-growing family of electrically conductive two-dimensional transition metal carbides and nitrides. Benefitting from their unique layered structures, MXenes are highly conductive and exhibit the capability to host various organic and inorganic molecules. Supercapacitors using MXenes as electrode materials have achieved high specific capacitances at high scan rates in aqueous solutions.^[1] Nonetheless, the limited electrochemical window of aqueous electrolytes remains an obstacle for maximizing the energy density of MXene supercapacitors. In order to produce supercapacitors that have comparable energy densities with conventional batteries, widening the potential window is an effective approach considering the specific energy equation for capacitive systems. Ionic liquids (ILs) have been widely used as electrolytes in supercapacitors due to wide electrochemical windows and excellent chemical stabilities However, ILs are typically comprised of large, bulky ions which limit the accessibility and transportation in porous materials.

Experiments have shown that pre-intercalation of organic ions (specifically, tetraalkyl ammonium cations) with varying chain lengths into MXenes can widen the interlayer spacing (d-spacing) up to ~1.5 nm, which results in significantly increased capacitances.^[2] In this work, classical molecular dynamics (CMD) simulations were used to analyze the capacitive performance of these pre-intercalated MXenes at a molecular-level and try to find the underlying charge storage mechanism. In these CMD simulations, the constant potential method (CPM) was used to apply partial charges on MXene atoms and maintain the potential at the MXene surfaces. In comparison with the constant charge method, the CPM is able to more accurately describe the charge distribution at solid surfaces. CPM is particularly useful for IL-MXene systems which consist of solid surfaces of varied functional groups and ions with complicated structures.

Through the use of CMD, systems of varying d-spacings and surface groups are considered. Structural properties of ILs within the MXene layers are analyzed to compute capacitive properties, whereas the dynamic properties are described through self-diffusion coefficients and the time evolution of the total charge on the MXene surfaces. By analyzing both capacitive and dynamic performances, we hope to find a guideline for simultaneously improving the energy density and power density of supercapacitors.

Reference

1. Lukatskaya, M. R. et al. Ultra-High-Rate Pseudocapacitive Energy Storage in Two-Dimensional Transition Metal Carbides. Nat. Energy 2, 17105 (2017).
2. Liang, K. et al. Engineering the Interlayer Spacing by Pre-Intercalation for High Performance Supercapacitor MXene Electrodes in Room Temperature Ionic Liquid. Submitted (2021).