We demonstrate fabrication of a two-dimensional Hf-containing MXene, HfCT, by selective etching of a layered parent Hf[Al(Si)]C compound. A substitutional solution of Si on Al sites effectively weakened the interfacial adhesion between Hf-C and Al(Si)-C sublayers within the unit cell of the parent compound, facilitating the subsequent selective etching. The underlying mechanism of the Si-alloying-facilitated etching process is thoroughly studied by first-principles density functional calculations. The result showed that more valence electrons of Si than Al weaken the adhesive energy of the etching interface. The MXenes were determined to be flexible and conductive. Moreover, this 2D Hf-containing MXene material showed reversible volumetric capacities of 1567 and 504 mAh cm for lithium and sodium ions batteries, respectively, at a current density of 200 mAg after 200 cycles. Thus, HfCT MXenes with a 2D structure are candidate anode materials for metal-ion intercalation, especially for applications where size matters.

Double-walled carbon nanotubes are coaxial nanostructures composed of exactly two single-walled carbon nanotubes, one nested in another. This unique structure offers advantages and opportunities for extending our knowledge and application of the carbon nanomaterials family. This review seeks to comprehensively discuss the synthesis, purification and characterization methods of this novel class of carbon nanomaterials. An emphasis is placed on the double wall physics that contributes to these structures' complex inter-wall coupling of electronic and optical properties. The debate over the inner-tube photoluminescence provides an interesting illustration of the rich photophysics and challenges associated with the myriad combinations of the inner and outerwall chiralities. Outerwall selective covalent chemistry will be discussed as a potential solution to the unattractive tradeoff between solubility and functionality that has limited some applications of single-walled carbon nanotubes. Finally, we will review the many different uses of double-walled carbon nanotubes and provide an overview of several promising research directions in this new and emerging field.

A novel high-purity V 2 C MXene two-dimensional carbide, was successfully synthesized by etching V 2 AlC with sodium fluoride and hydrochloric acid at 90 • C for 72 h. From the analysis of X-ray diffraction, energy dispersive spectra, and X-ray photoelectron spectroscopy, the purity of as-synthesized V 2 C MXene was >90 wt% with a few impurities of Na 5 Al 3 F 14 and V 2 AlC. The V 2 C MXene made by this method was much purer than those made by HF etching at room temperature. The as-prepared V 2 C MXene showed excellent electrochemical properties as anode of lithium-ion batteries. The capacity can be 260 mAh g −1 if discharged under 370 mA g −1 . The capacity was increased with charge cycles at high charge rate (500 mA g −1 ). It was suggested that V 2 C with high purity can be promising anode material with excellent performance.

The structure of the interface between a self-assembled monolayer (SAM) of alkanethiolates (AT) and the underlying Au(111) substrate is a longstanding puzzle in surface science. To cast further light on this problem, we took SAMs of hexanethiolate and dodecanethiolate on Au(111) as test systems and studied them by a combination of synchrotron-based high resolution X-ray photoelectron spectroscopy (HRXPS) and scanning tunneling microscopy (STM). The emphasis of the HRXPS characterization was put on the Au 4f emission of the substrate, which could be decomposed into the components related to the bulk and surface. The behavior of the surface component upon formation of hexanethiolate and dodecanethiolate SAMs was monitored in detail. We observed both a shift of this component and its branching into two independent emissions, which can be associated with two groups of Au atoms differently affected by the adsorption. This behavior, along with the relative intensity of both surface emissions, was correlated with most probable adsorption configurations of the thiolate headgroups. The analysis of the HRXPS data provides strong evidence for the involvement of the Au-adatoms, similar to the most recent models for short-chain AT monolayers on Au(111). The results indicate, however, that the structure of the long-chain systems is somewhat different and presumably more complex.

Improving the cyclic stability of lithium metal anodes is of particular importance for developing high-energydensity batteries. In this work, a remarkable finding shows that the control of lithium bis(fluorosulfonyl)imide (LiFSI) concentrations in electrolytes significantly alters the thickness and modulus of the related SEI layers, leading to varied cycling performances of Li metal anodes. In an electrolyte containing 2 M LiFSI, an SEI layer of ∼70 nm that is obviously thicker than those obtained in other concentrations is observed through in situ atomic force microscopy (AFM). In addition to the decomposition of FSI − anions that generates rigid lithium fluoride (LiF) as an SEI component, the modulus of this thick SEI layer with a high LiF content could be significantly strengthened to 10.7 GPa. Such a huge variation in SEI modulus, much higher than the threshold value of Li dendrite penetration, provides excellent performances of Li metal anodes with Coulombic efficiency higher than 99%. Our approach demonstrates that the FSI − anions with appropriate concentration can significantly alter the SEI quality, establishing a meaningful guideline for designing electrolyte formulation for stable lithium metal batteries.

Chemical and morphological structure of solid electrolyte interphase (SEI) plays a vital role in lithium-ion battery (LIB), especially for its cyclability and safety. To date, research on SEI is quite limited because of the complexity of SEI and lack of effective in situ characterization techniques. Here, we present real-time views of SEI morphological evolution using electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, fundamental differences of SEI formation from ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes during first lithiation/delithiation cycle on HOPG electrode surface were revealed.

Lithium dendrite growth is one of the most challenging problems affecting the safety performance of lithium-ion batteries (LIBs). It causes low Coulombic efficiency as well as safety hazards for LIBs. Understanding the evolution process of Li-dendrite growth at the nanoscale is critical for solving this problem. Herein, an in situ electrochemical atomic force microscopy (EC-AFM) investigation of the initial Li deposition in ethylene carbonate (EC)-based and fluoroethylene carbonate (FEC)-based electrolytes on graphite anodes is reported. These results show that the solid electrolyte interphase (SEI) formed from the FEC-based electrolyte can suppress Li-dendrite growth. The FEC-based electrolyte induces formation of LiF-rich SEI films, which are harder and denser than those formed in an EC-based electrolyte. Due to its better mechanical properties and larger resistance, the SEI layer formed from the FEC-based electrolyte is sufficient to prevent reduction of Li + ions and deposition of Li + ions on the anode surface. These results demonstrate that EC-AFM is a powerful in situ technique for the study of lithium-dendrite growth.

A layer of a metal-organic framework (SURMOF) was prepared on a thiol monolayer on Au. Charge transport across the insulating membrane could be established by using ferrocene as an immobilised redox mediator. Reversibility of the immobilisation and its role in the electrode kinetics are discussed.