First-principles study of high performance lithium/sodium storage of Ti₃C₂T₂ nanosheets as electrode materials*

Abstract

Ti3C2 Tx nanosheet, the first synthesized MXene with high capacity performance and charge/discharge rate, has attracted increasingly attention in renewable energy storage applications. By performing systematic density functional theory calculations, the theoretical capacity of the intrinsic structure of single- and multi-layered Ti3C2 T 2 (T = F or O) corresponding to M (M = Li and Na) atoms are investigated. Theoretical volumetric capacity and gravimetric capacity are obtained, which are related to the stacking degree. The optimal ratios of capacity to structure are determined under different stacking degrees for understanding the influence of surface functional groups on energy storage performance. Its performance can be tuned by performing surface modification and increasing the interlayer distance. In addition, the reason for theoretical capacity differences of M atoms is analyzed, which is attributed to difference in interaction between the M-ions and substrate and the difference in electrostatic exclusion between adsorbed M-ions. These results provide an insight into the understanding of the method of efficiently increasing the energy storage performance, which will be useful for designing and using high performance electrode materials.

Pre-Sodiated Ti3C2Tx MXene Structure and Behavior as Electrode for Sodium-Ion Capacitors

“…The above discussion ignores the sodium coordination environments and its two-dimensional distributions, which are crucial to understand the ion intercalation and redox mechanism within the MXene layers. A survey of simulation studies − , indicates there are four possible surface sites for sodium ions between Ti 3 C 2 T x sheets, which are illustrated in Figure together with the distances and coordinates to the nearby atoms. Obviously, the hexagonal arrangement of terminal groups creates two different vacant sites (sites 1 and 2) that can accommodate sodium ions either right above the C atoms two layers below (site 1; Figure A) or right above the outer-Ti layer (site 2; Figure A).…”

“…MXenes, a family of two-dimensional (2D) transition-metal carbide and/or nitride materials, − are one of the promising pseudocapacitive materials for ion-intercalation-based energy storage. − They offer highly conductive layered structures that are capable of accommodating a wide variety of ions, and tunable surface chemistry with a potential to be adjusted for surface redox reactions . Since their introduction, there are numerous experimental and theoretical investigations on the structural responses and effects of intercalated ions to the electrochemical performances in Ti 3 C 2 T x (where T x is variable surface terminations of −OH, −O, and −F), the most studied MXene.…”

“…Recent density functional theory (DFT) simulations have indicated that the chemical reaction between the intercalated ions and the surface terminal groups is the primary mechanism determining capacitive behavior in Ti 3 C 2 T x MXenes ,,− …”

Design and Synthesis Strategy of MXenes-Based Anode Materials for Sodium-Ion Batteries and Progress of First-Principles Research

“…This provides a useful theoretical basis for understanding the MXene column effect and contributes to the development of anode materials for sodium-ion batteries. This was followed by a first-principles study of the Na storage capacity of Ti 3 C 2 T x by Bai et al [56], which, after comprehensive calculations for all configurations of Eads2, extended the calculations to their theoretical capacities and obtained the optimal capacityto-structure ratios in the stacked structure. The calculated CMNa value can be increased from 218.32 mAh g −1 to 413.13 mAh g −1 , which is in good agreement with the previously reported value of 413.0 mAh g −1 .…”

“…Bao et al [17] experimentally demonstrated that the S-modified Ti3C2 materi a strong adsorption of Na2Sn in the Na-S cell. Based on this, Lu et al [56] used a principles simulation to calculate the chemical properties and bond strengths betw modified Ti3C2 and Na2Sen, which showed that S-doped Ti3C2 has strong anchorin trapping effects on Na2Sen (Figure 8A). S-doped MXenes not only improve the che bonding energy and van der Waals force on Na2Sen, but also, more importantly, the action of S-doped MXene materials and the nano-scaled Na2Sen together can max the surface or near-surface charge storage of MXene materials, which is conducive proving the charging and discharging capacity of sodium-ion batteries.…”