We prepare group VI transitional metal dichalcogenides (TMDs, or MX) from the 1T phase with quantum-sized and monolayer features via a quasi-full electrochemical process. The resulting two-dimensional (2D) MX (M = W, Mo; X = S, Se) quantum dots (QDs) are ca. 3.0-5.4 nm in size with a high 1T phase fraction of ca. 92%-97%. We attribute this to the high Li content intercalated in the 1T-MX lattice (mole ratio of Li:M is over 2:1), which is achieved by an increased lithiation driving force and a reduced electrochemical lithiation rate (0.001 A/g). The high Li content not only promotes the 2H → 1T phase transition but also generates significant inner stress that facilitates lattice breaking for MX crystals. Because of their high proportion of metallic 1T phase and sufficient active sites induced by the small lateral size, the 2D 1T-MoS QDs show excellent hydrogen evolution reactivity (with a typical η of 92 mV, Tafel slope of 44 mV/dec, and J of 4.16 × 10 A/cm). This electrochemical route toward 2D QDs might help boost the development of 2D materials in energy-related areas.

of the PO 2 -Nb 4 C 3 electrode was exemplified by assembling the PO 2 -Nb 4 C 3 //NHPC device with both high energy density (55 Wh L −1 ) and large power density (9765 W L −1 ). We hope that our results will encourage increasing efforts devoted to regulating the surface chemistry of MXenes and other 2D materials via terminal group engineering at the molecular level, which would contribute to the development of energy−power-balanced energy-storage devices.Research data are not shared.

Hard carbon attracts wide attentions as the anode for high-energy rechargeable batteries due to its low cost and high theoretical capacities. However, the intrinsically disordered microstructure gives it poor electrical conductivity and unsatisfactory rate performance. Here we report a facile synthesis of N-doped graphitized hard carbon via a simple carbonization and activation of a urea-soaked self-crosslinked Co-alginate for the high-performance anode of lithium/sodium-ion batteries. Owing to the catalytic graphitization of Co and the introduction of nitrogen-functional groups, the hard carbon shows structural merits of ordered expanded graphitic layers, hierarchical porous channels, and large surface area. Applying in the anode of lithium/sodium-ion batteries, the large surface area and the existence of nitrogen functional groups can improve the specific capacity by surface adsorption and faradic reaction, while the hierarchical porous channels and expanded graphitic layers can provide facilitate pathways for electrolyte and improve the rate performance. In this way, our hard carbon provides its feasibility to serve as an advanced anode material for high-energy rechargeable lithium/sodium-ion batteries.

MXenes are emerging layered materials that are promising for electrochemical energy storage and (opto-)electronic applications. A fundamental understanding of charge transport in MXenes is essential for such applications, but has remained under debate. While theoretical studies pointed to efficient band transport, device measurements have revealed thermally activated, hopping-type transport. Here we present a unifying picture of charge transport in two model MXenes by combining ultrafast terahertz and static electrical transport measurements to distinguish the short- and long-range transport characteristics. We find that band-like transport dominates short-range, intra-flake charge conduction in MXenes, whereas long-range, inter-flake transport occurs through thermally activated hopping, and limits charge percolation across the MXene flakes. Our analysis of the intra-flake charge carrier scattering rate shows that it is dominated by scattering from longitudinal optical phonons with a small coupling constant (α ≈ 1), for both semiconducting and metallic MXenes. This indicates the formation of large polarons in MXenes. Our work therefore provides insight into the polaronic nature of free charges in MXenes, and unveils intra- and inter-flake transport mechanisms in the MXene materials, which are relevant for both fundamental studies and applications.