First-Principles Study of Sodium Intercalation in Crystalline Na x Si24 (0 ≤ x ≤ 4) as Anode Material for Na-ion Batteries.

The search for Si-based anodes capable of undergoing low volume changes during electrochemical operation in rechargeable batteries is ample and active. Here we focus on crystalline Si24, a recently discovered open-cage allotrope of silicon, to thoroughly investigate its electrochemical performance using density functional theory calculations. In particular, we examine the phase stability of Na x Si24 along the whole composition range (0 ≤ x ≤ 4), volume and voltage changes during the (de)sodiation process, and sodium ion mobility. We show that Na x Si24 forms a solid solution with minimal volume changes. Yet sodium diffusion is predicted to be insufficiently fast for facile kinetics of Na-ion intake. Considering these advantages and limitations, we discuss the potential usefulness of Si24 as anode material for Na-ion batteries.

Unveiling the electrochemical mechanisms of Li2Fe(SO4)2 polymorphs by neutron diffraction and density functional theory calculations.

The quest for new sustainable iron-based positive electrode materials for lithium-ion batteries recently led to the discovery of a new family of compounds with the general formula Li2M(SO4)2 with M = transition metal, which presents monoclinic and orthorhombic polymorphs. In terms of electrochemical performances, although both Li2Fe(SO4)2 polymorphs present a similar potential of ∼3.8 V vs. Li(+)/Li(0), the associated electrochemical processes drastically differ in terms of polarization and reaction redox mechanisms. We herein provide an explanation to account for such a behavior. While monoclinic Li2Fe(SO4)2 directly transforms into Li1.0Fe(SO4)2 upon oxidation, the orthorhombic counterpart forms a distinct intermediate Li1.5Fe(SO4)2 phase leading to a two-step delithiation process involving an unequal depopulation of the two Li sites pertaining to the structure as deduced by neutron powder diffraction experiments and confirmed by both density functional theory and Bond Valence Energy Landscape calculations. Moreover, to access band gap information, both polymorphs are studied by UV/Vis spectroscopy. Lastly, the possibility of transforming the monoclinic phase to the orthorhombic phase under pressure is explored.

Disjoining pressure and the film-height-dependent surface tension of thin liquid films: new insight from capillary wave fluctuations.

In this paper we review simulation and experimental studies of thermal capillary wave fluctuations as an ideal means for probing the underlying disjoining pressure and surface tensions, and more generally, fine details of the Interfacial Hamiltonian Model. We discuss recent simulation results that reveal a film-height-dependent surface tension not accounted for in the classical Interfacial Hamiltonian Model. We show how this observation may be explained bottom-up from sound principles of statistical thermodynamics and discuss some of its implications.

Capillary fluctuations and film-height-dependent surface tension of an adsorbed liquid film.

Our understanding of both structure and dynamics of adsorbed liquids heavily relies on the capillary wave Hamiltonian, but a thorough test of this model is still lacking. Here we study the capillary wave fluctuations of a liquid film with short-range forces adsorbed on a solid exhibiting van der Waals interactions. We show for the first time that the measured capillary wave spectrum right above the first order wetting transition provides an interface potential consistent with independent calculations from thermodynamic integration. However, the surface tension exhibits an oscillatory film thick dependence which reveals a hitherto unnoticed capillary wave broadening mechanism beyond mere interfacial displacements.

Semi-infinite boundary conditions for the simulation of interfaces: the Ar/CO2(s) model revisited.

We propose a method to account for the long tail corrections of dispersive forces in inhomogeneous systems. This method deals separately with the two interfaces that are usually present in a simulation setup, effectively establishing semi-infinite boundary conditions that are appropriate for the study of the interface between two infinite bulk phases. Using the wandering interface method, we calculate surface free energies of vapor-liquid, wall-liquid, and wall-vapor interfaces for a model of Lennard-Jones argon adsorbed on solid carbon dioxide. The results are employed as input to Young's equation, and the wetting temperature located. This estimate is compared with predictions from the method of effective interface potentials and good agreement is found. Our results show that truncating Ar-Ar interactions at two and a half molecular diameters results in a dramatic decrease of the wetting temperature of about 40%.

Electron microscopy characterization of nanostructured carbon obtained from chlorination of metallocenes and metal carbides.

In this work we report some new well-defined carbon nanostructures produced by direct chlorination of metallocenes (ferrocene and cobaltocene) and NbC, at temperatures from 100 to 900 degrees C. Thus, amorphous carbon nanotubes with variable dimensions depending on reaction temperature were produced from ferrocene. When cobaltocene is the carbon precursor the main product are solid amorphous nanospheres. The high refractory metal carbide NbC as carbon source favours the growth of nanospherical cabbage-like particles with a higher degree of graphene sheets order. Besides, NbC crystallites encapsulated in an amorphous carbon shell were also found at lower temperatures (T< or =700 degrees C).