MXene Electrode Materials for Electrochemical Energy Storage: First-Principles and Grand Canonical Monte Carlo Simulations

Abstract

MXenes are a novel class of two dimensional materials, discovered by Barsoum and Gogotsi [M. Naguib, J. Come, B. Dyatkin, V. Presser, P. Taberna, P. Simon, M. W. Barsoum, and Y. Gogotsi, Electrochemistry Communications 16, 61-64 (2012); B. Anasori, M. R. Lukatskaya, and Y. Gogotsi, Nature Reviews Materials vol. 2, 16098 (2017)]. Their large specific surface area and the tunability of their physicochemical properties as a function of the transition metal and surface terminal group make them a unique design platform for various applications, a primary example of which is pseudocapacitive energy storage. However, there is still incomplete understanding of how the transition metal chemistry and stoichiometry, and the surface termination affect charge storage mechanisms in MXene. In this study, we have performed systematic first-principles calculations for bulk MXene and found that the atomic charge of the metal cations, which is related to their valence, decreases across the d-electron metal series. Electronic-structure indicators of performance are examined to understand the energy storage behavior, whereby charges are stored between the terminal groups and adsorbing cations. Importantly, we found that the differential Bader charges show good agreement with theoretical capacitances and are useful in predicting charge storage trends in MXene-based pseudocapacitors. Furthermore, we have performed first-principles and grand canonical Monte Carlo calculations for the slab systems, finding that the solvent plays a critical role in enhancing the pseudocapacitive response.

Redox‐Active Metaphosphate‐Like Terminals Enable High‐Capacity MXene Anodes for Ultrafast Na‐Ion Storage

“…Nb 4 C 3 MXene, as a typical M 4 X 3 MXene, was selected as the research object in this study for its better chemical stability than M 3 X 2 and M 2 X MXenes according to the recent simulation effort. [ 13 ] Also, Nb 4 C 3 MXene has been proved with excellent conductivity [ 14 ] and large interlayer spacing, enabling it with considerably better cycling and rate performance than other MXene materials for lithium‐ion batteries. [ 15 ] The synthetic route of PO 2 ‐Nb 4 C 3 , relying on a targeted terminal conversion approach, is illustrated in Figure a.…”

Predicting the Pseudocapacitive Windows for MXene Electrodes with Voltage-Dependent Cluster Expansion Models

“…The predicted isotherms are consistent with other first-principles-based models of MXene electrode surfaces. 17,21 Relatively linear decreases in ion coverages as a function of voltage correspond to Faradaic charging and the characteristic rectangular cyclic voltammetry response for capacitors. The equilibrium charge/discharge curves can be contrasted with similar simulations of lithiumion battery materials, which are associated with a rapid decrease in Li content over a short voltage range, to show that many MXene compositions yield the pseudocapacitive charge/ discharge behavior.…”

“…Local averaging methods can also be used to predict equilibrium charge–discharge curves . These DFT models can provide electronic descriptors, such as shifts in the potential of zero charge or metal oxidation states, which reflect the ability to transfer electronic charge between the adsorbed species and the pseudocapacitive surface . Although the calculation of thermodynamic quantities over a range of adsorbate coverages can be generally accomplished with high accuracy in few-atom unit cells, the stability of larger surfaces with a high degree of configurational disorder at finite temperatures is rarely addressed.…”

“…20 These DFT models can provide electronic descriptors, such as shifts in the potential of zero charge or metal oxidation states, which reflect the ability to transfer electronic charge between the adsorbed species and the pseudocapacitive surface. 21 Although the calculation of thermodynamic quantities over a range of adsorbate coverages can be generally accomplished with high accuracy in few-atom unit cells, the stability of larger surfaces with a high degree of configurational disorder at finite temperatures is rarely addressed. Additionally, DFT-based models generally provide an incomplete representation of lateral adsorbate interactions due to the limited length scales accessible to electronicstructure calculations.…”

Applications of MXene-Containing Polypyrrole Nanocomposites in Electrochemical Energy Storage and Conversion

“…These include mono solid solution M-element type, M-element type, ordered double M-element type, and vacancy ordering type. They are particularly appealing for various applications, including electrochemical storage systems, due to their ease of property tuning. − ,− …”