We report on the synthesis of the first two-dimensional transition metal nitride, Ti4N3-based MXene. In contrast to the previously reported MXene synthesis methods - in which selective etching of a MAX phase precursor occurred in aqueous acidic solutions - here a molten fluoride salt is used to etch Al from a Ti4AlN3 powder precursor at 550 °C under an argon atmosphere. We further delaminated the resulting MXene to produce few-layered nanosheets and monolayers of Ti4N3Tx, where T is a surface termination (F, O, or OH). Density functional theory calculations of bare, non-terminated Ti4N3 and terminated Ti4N3Tx were performed to determine the most energetically stable form of this MXene. Bare and functionalized Ti4N3 are predicted to be metallic. Bare Ti4N3 is expected to show magnetism, which is significantly reduced in the presence of functional groups.

2D transition metal carbides and nitrides, named MXenes, are attracting increasing attentions and showing competitive performance in energy storage devices including electrochemical capacitors, lithium- and sodium-ion batteries, and lithium-sulfur batteries. However, similar to other 2D materials, MXene nanosheets are inclined to stack together, limiting the device performance. In order to fully utilize MXenes' electrochemical energy storage capability, here, processing of 2D MXene flakes into hollow spheres and 3D architectures via a template method is reported. The MXene hollow spheres are stable and can be easily dispersed in solvents such as water and ethanol, demonstrating their potential applications in environmental and biomedical fields as well. The 3D macroporous MXene films are free-standing, flexible, and highly conductive due to good contacts between spheres and metallic conductivity of MXenes. When used as anodes for sodium-ion storage, these 3D MXene films exhibit much improved performances compared to multilayer MXenes and MXene/carbon nanotube hybrid architectures in terms of capacity, rate capability, and cycling stability. This work demonstrates the importance of MXene electrode architecture on the electrochemical performance and can guide future work on designing high-performance MXene-based materials for energy storage, catalysis, environmental, and biomedical applications.

Two-dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition-metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS -on-MXene heterostructures through in situ sulfidation of Mo TiC T MXene. The computational results show that MoS -on-MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li S adsorption during the intercalation and conversion reactions. These characteristics render the as-prepared MoS -on-MXene heterostructures stable Li-ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications.

Herein we develop a chemical etching method to produce porous two‐dimensional (2D) Ti3C2Tx MXenes at room temperature in aqueous solutions. The as‐produced porous Ti3C2Tx (p‐Ti3C2Tx) have larger specific surface areas and more open structures than their pristine counterparts, and can be fabricated into flexible films with, or without, the addition of carbon nanotubes (CNTs). The as‐fabricated p‐Ti3C2Tx/CNT films showed significantly improved lithium ion storage capabilities compared to pristine Ti3C2Tx based films, with a very high capacity of ≈1250 mAh g−1 at 0.1 C, excellent cycling stability, and good rate performance (330 mAh g−1 at 10 C). Using the same chemical etching method, we also made porous Nb2CTx and V2CTx MXenes. Therefore, this study provides a simple, yet effective, procedure to introduce pores into MXenes and possibly other 2D sheets that in turn, can enhance their electrochemical properties.

materials to host sodium ions. Nanostructuring and hierarchical electrode architectures are required to facilitate transport of Na-ions within electrodes. [ 3 ] Recent studies have provided a leap forward in developing promising anode materials, such as carbonaceous materials, [ 4 ] phosphorus, [ 5 ] metallic alloys, [ 6 ] titanates, [ 7 ] and 2D metal carbides (MXenes). [ 8 ] MoS 2 , a layered material with S-Mo-S motifs stacked together by van der Waals forces, received extraordinary attention in the last few years. Many inroads have been made recently in developing MoS 2 based electrode materials for SIBs. For example, MoS 2 nanofl owers with expanded interlayers have been prepared as intercalationtype electrode materials in the voltage window of 0.4-3.0 V. [ 9 ] When the voltage window is expanded to 0.01-3.0 V, MoS 2 follows an intercalation-conversion mechanism for Na + storage. Due to the low conductivity and the huge volume variations of MoS 2 during charge/discharge processes, bare MoS 2 electrodes exhibited poor rate capability and fast capacity decay upon cycling. [ 10 ] To overcome this limitation, dispersing MoS 2 in carbon matrices with high electronic conductivity has been proved effective for improving the electrochemical properties in SIBs. Different MoS 2 -carbon hybrid materials have been tested as anodes for SIBs, such as MoS 2 nanodots embedded in carbon nanowires, [ 11 ] MoS 2 /graphene composites, [ 10,12 ] MoS 2 /CNT composites, [ 13 ] and MoS 2 /carbon nanospheres. [ 14 ] Usually, current collectors, conductive agents and binders are needed to fabricate fi lm electrodes, which inherently increases the total weight and cost of SIBs. Furthermore, they suffer from low initial Coulombic effi ciency (ICE <60%). The low ICE originates from: (i) the formation of solid electrolyte interfaces (SEI) caused by electrolyte decomposition; (ii) adverse side reactions between inactive components (conductive agent and binder) and sodium metal; (iii) electrical contact failure of electrode; and (iv) an excessive interface between carbon and electrolyte, which leads to considerable side reactions. [ 15 ] This low ICE requires a larger mass of the corresponding cathode material in full cells, thereby increasing the total weight and cost of SIBs. Rational geometrical design to give electrode materials a high ICE is a key research topic in SIBs. Moreover, anodeThe development of sodium-ion batteries for large-scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic effi ciency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium-ion batteries, consisting of molybdenum disulfi de (MoS 2 ) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables suffi cient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte,...