Julibeth M. Martinez de la Hoz

Reduction mechanisms of ethylene carbonate on si anodes of lithium-ion batteries: effects of degree of lithiation and nature of exposed surface.

Ab initio molecular dynamics simulations are used to identify mechanisms of reduction of ethylene carbonate on Si surfaces at various degrees of lithiation, where the low-coordinated surface Si atoms are saturated with O, OH, or H functional groups. The lowest Si content surfaces are represented by quasi-amorphous LiSi4 and LiSi2; intermediate lithiation is given by LiSi crystalline facets, and the highest Li content is studied through Li13Si4 surfaces. It is found that ethylene carbonate (EC) reduction mechanisms depend significantly on the degree of lithiation of the surface. On LiSi surfaces EC is reduced according to two different two-electron mechanisms (one simultaneous and one sequential), which are independent of specific surface functionalization or nature of exposed facets. On the less lithiated surfaces, the simultaneous two-electron reduction is found more frequently. In that mechanism, the EC reduction is initiated by the formation of a C-Si bond that allows adsorption of the intact molecule to the surface and is followed by electron transfer and ring-opening. Strongly lithiated Li13Si4 surfaces are found to be highly reactive. Reduction of adsorbed EC molecules occurs via a four-electron mechanism yielding as reduction products CO(2-) and O(C2H4)O(2-). Direct transfer of two electrons to EC molecules in liquid phase is also possible, resulting in the presence of O(C2H4)OCO(2-) anions in the liquid phase.

Size effect on the stability of Cu–Ag nanoalloys

Classical molecular dynamics (MD) simulations are used to study the phase stability of Cu–Ag nanoalloys based on the analysis of their thermodynamic mixing properties for both random and core-shell clusters as functions of nanoparticle size, temperature and composition. At 298 K, results for nanoalloys of increasing size at fixed composition suggest that alloying Cu and Ag is thermodynamically feasible only for a nanocluster size range, excluding very small ( < 1.8 nm) and large clusters (≳4 nm). In the size range of favourable alloy formation, Cu–Ag core-shell structures are more stable than random configurations, and the same conclusion holds for most of the composition range at fixed cluster size and 298 K. Varying temperature at fixed nanocluster size and fixed composition, core-shell structures are preferred up to the melting temperature of the nanoparticle. Also, we test an analytical model to predict the thermodynamic properties of mixing of nanoalloys using bulk enthalpies of mixing of the pure components and those of the corresponding bulk alloy. The enthalpies and Gibbs free energies of mixing obtained from the analytical model qualitatively agree with those obtained from MD simulations, especially when the nanoparticle size increases above 2.8 nm.

Structure and Reactivity of Alucone-Coated Films on Si and LixSiy Surfaces

Coating silicon particles with a suitable thin film has appeared as a possible solution to accommodate the swelling of silicon upon lithiation and its posterior cracking and pulverization during cycling of Li-ion batteries. In particular, aluminum alkoxide (alucone) films have been recently deposited over Si anodes, and the lithiation and electrochemical behavior of the system have been characterized. However, some questions remain regarding the lithium molecular migration mechanisms through the film and the electronic properties of the alucone film. Here we use density functional theory, ab initio molecular dynamics simulations, and Greens function theory to examine the film formation, lithiation, and reactivity in contact with an electrolyte solution. It is found that the film is composed of Al-O complexes with 3-O or 4-O coordination. During lithiation, Li atoms bind very strongly to the O atoms in the most energetically favorable sites. After the film is irreversibly saturated with Li atoms, it becomes electronically conductive. The ethylene carbonate molecules in liquid phase are found to be reduced at the surface of the Li-saturated alucone film following similar electron transfer mechanisms as found previously for lithiated silicon anodes. The theoretical results are in agreement with those from morphology and electrochemical analyses.

Theoretical infrared and terahertz spectra of an RDX/aluminum complex.

Density functional theory is employed to characterize the infrared and terahertz spectra of an explosive molecular species, RDX, deposited over an aluminum surface, modeled as a planar cluster of Al(16). Changes in the inter- and intramolecular vibrational modes are systematically analyzed starting from the isolated monomer, dimer, and tetramer and then considering the interactions of the monomer with an Al plate. The results are compared to available experimental information for RDX films on Al surfaces. It is found that the RDX molecule changes conformation because of the interaction with the model Al surface, becoming closer to an AAA conformation with the three NO(2) groups in nearly axial positions. The calculated spectra serve as an initial guideline to interpret the main peaks of previously reported RDX films on Al.

Modeling oxidation of Pt-based alloy surfaces for fuel cell cathode electrocatalysts

Nanoparticles (especially metals and metal-oxides) have been used as catalysts much earlier than the beginning of the era of nanotechnology. This is because small particles having a large surface/volume ratio and a large proportion of low-coordinated sites may be much more reactive than flat surface...