Theoretical Insights into the Favorable Functionalized Ti₂C-Based MXenes for Lithium–Sulfur Batteries

Abstract

Because of the high specific surface area, excellent electronic conductivity, facile Li diffusion, and rich functional groups, Ti 2 C-based MXenes have been widely used to improve the electrochemical property of lithium−sulfur batteries. The complex surface functionalization (such as −OH, −S, −F, and −O) of MXenes boosts the performance but also causes controversies about the favorable functionalized surface in the electrochemical reaction during the charge and discharge process. In the present work, a theoretical study based on density functional theory has been carried out to clarify the favorable functionalized surface by comparing pristine Ti 2 C and −OH-, −S-, −F-, and −Ofunctionalized Ti 2 C surfaces from the aspects of adsorption ability, electronic conductivity, and kinetic conversion ability. It is found that compared with severe polysulfide deformation on pristine Ti 2 C and Ti 2 C(OH) 2 surfaces, Ti 2 CO 2 , Ti 2 CS 2 , and Ti 2 CF 2 have effective polysulfide adsorption. Ti 2 CO 2 has the largest surface adsorption energy, followed by Ti 2 CS 2 , and Ti 2 CF 2 is the weakest. Meanwhile, the narrow-band gap semiconductor property of Ti 2 CO 2 during adsorption indicates worse electronic conductivity than metallic Ti 2 CS 2 and Ti 2 CF 2 . In addition, for the kinetic conversion ability, the Ti 2 CS 2 surface has the fastest polysulfide conversion and Li diffusion, followed by Ti 2 CF 2 , and Ti 2 CO 2 represents the slowest conversion and diffusion. Accordingly, because of the medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion, Ti 2 CS 2 is revealed to be the favorable functionalized surface. More importantly, the origin for the Ti 2 CS 2 surface with medium adsorption ability represents the fastest polysulfide conversion, and Li diffusion is further clarified. The great affinity of the Ti 2 CS 2 surface to the product Li 2 S leads to facile polysulfide conversion. The uniform charge distribution on the Ti 2 CS 2 surface contributes to the fast Li diffusion.

High-Performance Flexible Supercapacitor Device Composed of a Hierarchical 2-D MXene-Ni(OH)2 Nanocomposite and Biomass-Derived Porous Carbon Electrodes

“…MXene has attracted the researchers’ attention because it is a novel material in the 2-D material family due to its exclusive properties. MXenes are an important family of 2-D materials having more than 20 kinds, including Ti 3 C 2 , Ti 2 C, Nb 2 C, Nb 4 C 3 , Mo 2 CT, V 2 CT x , − etc., and basically it can be represented as M n +1 X n T x (M denotes early transition metal, n = 1, 2, 3, etc., X denotes carbon (C) and/or nitrogen (N), and T indicates -OH, -O, and -F functional groups, such as Ti 3 C 2 T x ). , Owing to its unique 2-D structure with abundant functional groups, MXene has been selected as a potential component to design a nanocomposite for supercapacitor application. MXene is mostly synthesized by chemical etching of the A-group element layers from the M n +1 AX n called as the MAX phase (in this, A is usually a group 13 or 14 element and Ti 3 AlC 2 is a typical example of the MAX phase).…”

Properties of S-Functionalized Nitrogen-Based MXene (Ti2NS2) as a Hosting Material for Lithium-Sulfur Batteries

“…The exchange-correlation functional is described by the Perdew-Burke-Ernzerhof (PBE) functional within generalized gradient approximation (GGA) [38]. The Grimme of DFT-D2 is used to describe the van der Waals (vdW) interaction between the substrate and LiPSs [39,40]. The models of MXenes are constructed using 3 × 3 super cells.…”

Computational screening of functionalized MXenes to catalyze the solid and non-solid conversion reactions in cathodes of lithium–sulfur batteries

“…46 The effects of electrocatalysts have also been demonstrated by introducing electrocatalytic cobalt atoms into nitrogen-doped graphene, which can obviously reduce the reaction barriers from Li 2 S 2 to Li 2 S as confirmed by density functional theory (DFT) calculations and experimental results. 48 Among them, MXenes with two-dimensional (2D) structures have been regarded as promising candidates to host polysulfides, since many experimental and DFT calculated results have confirmed their excellent anchoring abilities for the reduction products followed by enhanced cycling stabilities, [49][50][51][52][53][54][55] such as the carbon nanotubes between the Ti 3 C 2 /Ti 3 CN layers combined with sulfur species that can achieve a long-term cycling performance, with a fading rate as low as 0.043% per cycle in 1200 cycles. 56 Nevertheless, MXenes have a large number of family members with similar structures, as usually expressed as M n+1 X n T x (n = 1, 2, or 3), where M represents early transition metals (Ti, Nb, V, etc.)…”