Assessment of Sulfur-Functionalized MXenes for Li-Ion Battery Applications

Abstract

The surface termination of MXenes greatly determines the electrochemical properties and ion kinetics on their surfaces. So far, hydroxyl-, oxygen-, and fluorine-terminated MXenes have been widely studied for energy storage applications. Recently, sulfur-functionalized MXene structures, which possess low diffusion barriers, have been proposed as candidate materials to enhance battery performance. We performed first-principles calculations on the structural, stability, electrochemical, and ion dynamic properties of Li-adsorbed sulfur-functionalized groups 3B, 4B, 5B, and 6B transition-metal (M)-based MXenes (i.e., M2CS2 with M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W). We performed phonon calculations, which indicated that all of the above M2CS2 MXenes, except for Sc, are dynamically stable at T = 0 K. The ground-state structure of each M2CS2 monolayer depends on the type of M atom. For instance, while sulfur prefers to sit at the FCC site on Ti2CS2, it occupies the HCP site of Cr-based MXene. We determined the Li adsorption configurations at different concentrations using the cluster expansion method. The highest maximum open-circuit voltages were computed for the group 4B element (i.e., Ti, Zr, and Hf)-based M2CS2, which are larger than 2.1 V, while their average voltages are approximately 1 V. The maximum voltage for the group 6B element (i.e., Cr, Mo, W)-based M2CS2 is less than 1 V, and the average voltage is less than 0.71 V. We found that S functionalization is helpful for capacity improvements over the O-terminated MXenes. In this respect, the computed storage gravimetric capacity may reach up to 417.4 mAh/g for Ti2CS2 and 404.5 mAh/g for V2CS2. Ta-, Cr-, Mo-, and W-based M2CS2 MXenes show very low capacities, which are less than 100 mAh/g. The Li surface diffusion energy barriers for all of the considered MXenes are less than 0.22 eV, which is favorable for high charging and discharging rates. Finally, ab initio molecular dynamic simulations performed at 400 K and bond-length analysis with respect to Li concentration verify that selected promising systems are robust against thermally induced perturbations that may induce structural transformations or distortions and undesirable Li release.

Multiscale numerical simulation of in-plane mechanical properties of two-dimensional monolayers

“…Carrying out uniaxial tensile loading to investigate the mechanical properties of pristine and nanoporous graphene. An increase in system temperature results in a signicant reduction in the fracture stress and strain Siriwardane et al 140 Sulfur-functionalized MXenes Structural, stability, and ion dynamic properties…”

Strategies for Fabricating High‐Performance Electrochemical Energy‐Storage Devices by MXenes

“…[165] Furthermore, a recent theoretical study showed that sulphur-functionalised MXenes (Ti 2 CS 2 , V 2 CS 2 and Nb 2 CS 2 ) might provide higher gravimetric capacity than oxy-gen-terminated species (Ti 2 CO 2 , V 2 CO 2 and Nb 2 CO 2 ) for lithium storage (417.4, 404.5 and 284.4 mAh g À 1 versus 350, 335 and 250 mAh g À 1 ). [166] Specifically, the diffusion energy barrier for a single lithium ion on V 2 CS 2 (Zr 2 CS 2 ) was 0.199 eV (0.218 eV) (Figure 15 (d)), which was lower than that of V 2 CO 2 (0.24 eV) (Zr 2 CO 2 (0.5 eV)), indicating the improved ionic transport kinetics in M 2 CS 2 systems. [166][167][168] In another report, organic molecules (1-aminoanthraquinone (AQ)) were noncovalently and covalently integrated with Ti 3 C 2 T x MXenes to exploit the redox-active nature of the external species.…”

“…[166] Specifically, the diffusion energy barrier for a single lithium ion on V 2 CS 2 (Zr 2 CS 2 ) was 0.199 eV (0.218 eV) (Figure 15 (d)), which was lower than that of V 2 CO 2 (0.24 eV) (Zr 2 CO 2 (0.5 eV)), indicating the improved ionic transport kinetics in M 2 CS 2 systems. [166][167][168] In another report, organic molecules (1-aminoanthraquinone (AQ)) were noncovalently and covalently integrated with Ti 3 C 2 T x MXenes to exploit the redox-active nature of the external species. [169] Different from noncovalent integration by mixing AQ with Ti 3 C 2 T x , covalent functionalisation was realised by diazonium reaction of MXenes with tert-butyl nitrite (Fig- ure 15 (e)), which improved the capacitance by carbonyl moieties characteristic with the two-electron redox process.…”

“…(d) Energy barrier for a single lithium ion diffusing on various M 2 CS 2 species. [166] Reproduced with permission from Ref.. [166] Copyright 2020 American Chemical Society. (e) Schematic diagram illustrating the noncovalent/covalent functionalisation of AQ on Ti 3 C 2 T x MXenes.…”

“…Copyright 2019 American Chemical Society. (d) Energy barrier for a single lithium ion diffusing on various M 2 CS 2 species [166]. Reproduced with permission from Ref.…”