Assessing carbon-based anodes for lithium-ion batteries: A universal description of charge-transfer binding

Many key performance characteristics of carbon-based lithium-ion battery anodes are largely determined by the strength of binding between lithium (Li) and sp2 carbon (C), which can vary significantly with subtle changes in substrate structure, chemistry, and morphology. Here, we use density functional theory calculations to investigate the interactions of Li with a wide variety of sp2 C substrates, including pristine, defective, and strained graphene; planar C clusters; nanotubes; C edges; and multilayer stacks. In almost all cases, we find a universal linear relation between the Li-C binding energy and the work required to fill previously unoccupied electronic states within the substrate. This suggests that Li capacity is predominantly determined by two key factors -- namely, intrinsic quantum capacitance limitations and the absolute placement of the Fermi level. This simple descriptor allows for straightforward prediction of the Li-C binding energy and related battery characteristics in candidate C materials based solely on the substrate electronic structure. It further suggests specific guidelines for designing more effective C-based anodes. The method should be broadly applicable to charge-transfer adsorption on planar substrates, and provides a phenomenological connection to established principles in supercapacitor and catalyst design.Comment: accepted by Physical Review Letter

Equilibrium at the edge and atomistic mechanisms of graphene growth

The morphology of graphene is crucial for its applications, yet an adequate theory of its growth is lacking: It is either simplified to a phenomenological-continuum level or is overly detailed in atomistic simulations, which are often intractable. Here we put forward a comprehensive picture dubbed nanoreactor, which draws from ideas of step-flow crystal growth augmented by detailed first-principles calculations. As the carbon atoms migrate fromthe feedstock to catalyst to final graphene lattice, they go through a sequence of states whose energy levels can be computed and arranged into a step-by-step map. Analysis begins with the structure and energies of arbitrary edges to yield equilibrium island shapes. Then, it elucidates how the atoms dock at the edges and how they avoid forming defects. The sequence of atomic row assembly determines the kinetic anisotropy of growth, and consequently, graphene island morphology, explaining a number of experimental facts and suggesting how the growth product can further be improved. Finally, this analysis adds a useful perspective on the synthesis of carbon nanotubes and its essential distinction from graphene

Tellurium: Fast Electrical and Atomic Transport along Weak Interaction Direction

In anisotropic materials, the electrical and atomic transport along the weak interaction direction is usually much slower than that along the chemical bond direction. However, Te, an important semiconductor comprised of helical atomic chains, exhibits nearly isotropic electrical transport between intra-chain and inter-chain directions. Using first-principles calculations to study the bulk and few-layer Te, we show that this isotropy is related with similar effective mass and potential for charge carriers along different transport directions, benefiting from the delocalization of the lone-pair electrons. This delocalization also enhances the inter-chain binding, although it is still significantly weaker than the covalent intra-chain bonding. Moreover, we find a fast diffusion of vacancies and interstitial atoms along and across the chains, enabling rapid self-healing of these defects at room temperature. Interestingly, the interstitial atoms diffuse along the chain via a concerted-rotation mechanism. Our work reveals the unconventional properties underlying the superior performance of Te, while providing insight into the transport in anisotropic materials

Understanding the Intention to Use Commercial Bike-sharing Systems: An Integration of TAM and TPB

Commercial bike-sharing system is growing rapidly as a critical form of the sharing economy. Although past research has discussed the design and operation of commercial bike-sharing systems, there have been few studies examining the factors motivating the use of such systems. This study integrates the technology acceptance model (TAM) and the theory of planned behavior (TPB) to develop a holistic model to explain the intention to use commercial bike-sharing systems. The PLS-SEM results from a survey with 286 users reveal that the intention to use commercial bike-sharing systems is positively affected by perceived usefulness of the system, attitude toward bike-sharing and perceived behavioral control. Further, we find that attitude toward the bike-sharing is positively affected by perceived usefulness and perceived ease of use of the system. Beyond our expectation, subjective norm has no significant effect on the intention to use. Implications and directions for future research are also discussed

Schottky-barrier-free contacts with two-dimensional semiconductors by surface-engineered MXenes

Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. Here we demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. Based on first principles calculations, we find that the surface chemistry strongly affects the Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, while F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weak Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control the surface terminations based on the calculated formation energies. This study enhances the understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives practical predictions for improving 2D electronics