Yu Xie

The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. Herein, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), M'2M″C2 and M'2M″2C3, where M' and M″ are two different early transition metals. In these solids, M' layers sandwich M″ carbide layers. By synthesizing Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo2TiC2Tx and Ti3C2Tx. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.

A combination of density functional theory (DFT) calculations and experiments is used to shed light on the relation between surface structure and Li-ion storage capacities of the following functionalized two-dimensional (2D) transition-metal carbides or MXenes: Sc2C, Ti2C, Ti3C2, V2C, Cr2C, and Nb2C. The Li-ion storage capacities are found to strongly depend on the nature of the surface functional groups, with O groups exhibiting the highest theoretical Li-ion storage capacities. MXene surfaces can be initially covered with OH groups, removable by high-temperature treatment or by reactions in the first lithiation cycle. This was verified by annealing f-Nb2C and f-Ti3C2 at 673 and 773 K in vacuum for 40 h and in situ X-ray adsorption spectroscopy (XAS) and Li capacity measurements for the first lithiation/delithiation cycle of f-Ti3C2. The high-temperature removal of water and OH was confirmed using X-ray diffraction and inelastic neutron scattering. The voltage profile and X-ray adsorption near edge structure of f-Ti3C2 revealed surface reactions in the first lithiation cycle. Moreover, lithiated oxygen terminated MXenes surfaces are able to adsorb additional Li beyond a monolayer, providing a mechanism to substantially increase capacity, as observed mainly in delaminated MXenes and confirmed by DFT calculations and XAS. The calculated Li diffusion barriers are low, indicative of the measured high-rate performance. We predict the not yet synthesized Cr2C to possess high Li capacity due to the low activation energy of water formation at high temperature, while the not yet synthesized Sc2C is predicted to potentially display low Li capacity due to higher reaction barriers for OH removal.

The 2D transition metal carbides or nitrides, or MXenes, are emerging as a group of materials showing great promise in lithium ion batteries and supercapacitors. Until now, characterization and properties of single-layer MXenes have been scarcely reported. Here, using scanning transmission electron microscopy, we determined the atomic structure of freestanding monolayer Ti3C2Tx flakes prepared via the minimally intensive layer delamination method and characterized different point defects that are prevalent in the monolayer flakes. We determine that the Ti vacancy concentration can be controlled by the etchant concentration during preparation. Density function theory-based calculations confirm the defect structures and predict that the defects can influence the surface morphology and termination groups, but do not strongly influence the metallic conductivity. Using devices fabricated from single- and few-layer Ti3C2Tx MXene flakes, the effect of the number of layers in the flake on conductivity has been demonstrated.

Rechargeable non-lithium-ion (Na(+), K(+), Mg(2+), Ca(2+), and Al(3+)) batteries have attracted great attention as emerging low-cost and high energy-density technologies for large-scale renewable energy storage applications. However, the development of these batteries is hindered by the limited choice of high-performance electrode materials. In this work, MXene nanosheets, a class of two-dimensional transition-metal carbides, are predicted to serve as high-performing anodes for non-lithium-ion batteries by combined first-principles simulations and experimental measurements. Both O-terminated and bare MXenes are shown to be promising anode materials with high capacities and good rate capabilities, while bare MXenes show better performance. Our experiments clearly demonstrate the feasibility of Na- and K-ion intercalation into terminated MXenes. Moreover, stable multilayer adsorption is predicted for Mg and Al, which significantly increases their theoretical capacities. We also show that O-terminated MXenes can decompose into bare MXenes and metal oxides when in contact with Mg, Ca, or Al. Our results provide insight into metal ion storage mechanisms on two-dimensional materials and suggest a route to preparing bare MXene nanosheets.