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.

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Rechargeable magnesium-ion batteries (MIBs) with Mg metal anodes have been attracting attention due to their potential safety, low cost, and high theoretical energy densities. Nevertheless, developing a high-energy-density MIB with long cycle life and reasonable rate capability is still a huge challenge due to the lack of high-performance cathodes beyond the Chevrel phases. Here, we investigate the mechanism of Mg-ion uptake and storage by MXenes, that have been theoretically predicted to be promising candidates for MIB cathodes. Flexible and conductive 3D macroporous Ti 3 C 2 T x MXene films were fabricated and tested as MIB cathodes after the incorporation of Mg ions from a Mg 2+containing electrolyte. The 3D MXene cathode exhibited promising cycling stability, accompanied by good rate performance. A 3D Mg 0.21 Ti 3 C 2 T x MXene electrode delivered, at 0.5, 1, and 5 C, capacities of ∼210, ∼140, and ∼55 mA h g −1 , respectively. A reversible intercalation charge-storage mechanism was demonstrated and a possible redox reaction mechanism proposed. Considering the large family of 2D transition metal carbides and nitrides, with over 30 different MXenes synthesized to date, this work suggests the availability of a variety of high-rate cathode candidates for MIBs.

Despite extensive research on the tribological properties of MoS2, the frictional characteristics of other members of the transition metal dichalcogenide (TMD) family have remained relatively unexplored. To understand the effect of the chalcogen on the tribological behavior of these materials and gain broader general insights into factors controlling friction at the nanoscale, we compared the friction force behavior for a nanoscale single asperity sliding on MoS2, MoSe2, and MoTe2 in both bulk and monolayer forms through a combination of atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations. Experiments and simulations showed that, under otherwise identical conditions, MoS2 has the highest friction among these materials and MoTe2 the lowest. Simulations complemented by theoretical analysis based on the Prandtl-Tomlinson model revealed that the observed friction contrast between theTMDs was attributable to their lattice constants, which differed depending on the chalcogen. While the corrugation amplitudes of the energy landscapes are similar for all three materials, larger lattice constants permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. These results emphasize the critical role of the lattice constant, which can be the determining factor for frictional behavior at the nanoscale.