Structure and Reactivity of Alucone-Coated Films on Si and Li(x)Si(y) 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 Green's 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.

Reduction mechanisms of additives on Si anodes of Li-ion batteries.

Solid-electrolyte interphase (SEI) layers are films deposited on the surface of Li-ion battery electrodes during battery charge and discharge processes. They are due to electrochemical instability of the electrolyte which causes electron transfer from (to) the anode (cathode) surfaces. The films could have a protective passivating role and therefore understanding the detailed reduction (oxidation) processes is essential. Here density functional theory and ab initio molecular dynamics simulations are used to investigate the reduction mechanisms of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on lithiated silicon surfaces. These species are frequently used as "additives" to improve the SEI properties. It is found that on lithiated Si anodes (with low to intermediate degrees of lithiation) VC may be reduced via a 2e(-) mechanism yielding an opened VC(2-) anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO(2-)(ads) anion and a radical anion ˙OC2H2O(2-). Additionally, in agreement with experimental observations, it is shown that CO2 can be generated from reaction of VC with the CO3(2-)anion, a product of the reduction of the main solvent, ethylene carbonate (EC). On the other hand, FEC reduction on LixSiy surfaces is found to be independent of the degree of lithiation, and occurs through three mechanisms. One of them leads to an adsorbed VC(2-) anion upon release from the FEC molecule and adsorption on the surface of F(-) and one H atom. Thus in some cases, the reduction of FEC may lead to the exact same reduction products as that of VC, which explains similarities in SEI layers formed in the presence of these additives. However, FEC may be reduced via two other multi-electron transfer mechanisms that result in formation of either CO2(2-), F(-), and ˙CH2CHO(-) or CO(2-), F(-), and ˙OCH2CHO(-). These alternative reduction products may oligomerize and form SEI layers with different components than those formed in the presence of VC. In all cases, FEC reduction also leads to formation of LiF moieties on the anode surface, in agreement with reported experimental data. The crucial role of the surface in each of these mechanisms is thoroughly explained.

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.

Local surface structure effect on reactivity of molecules confined between metallic surfaces.

Interactions between metallic surfaces separated by nanometer distances create an unusual reactivity environment. Here we evaluate the effect of the geometry given by differences in the structures of the interacting surfaces and by the presence of steps. Adsorption of an oxygen molecule and its dissociation is examined in gaps defined by interacting platinum surfaces that have separations between 5.36 and 4.70 Å, and by comparing the effect of the different gap geometries on the adsorption strength and barriers for dissociation. It is found that specific surface-surface configurations influence the electronic structure of the surface where the molecule is adsorbed, modifying the width of its d-orbital and therefore the adsorption strength due to changes in the overlap of the adsorbate molecular orbitals with the metal d-band. In addition, the degree of the molecule-metal interaction with the other surface may restrict the adsorbate mobility and its dissociation. The presence of defects may decrease the adsorbate-surface interaction strength, but the net result depends on the specific reaction and nature of the intermediates since in some cases weaker adsorptions may result in lower dissociation barriers.

Vibrational spectra of an RDX film over an aluminum substrate from molecular dynamics simulations and density functional theory.

We report calculated vibrational spectra in the range of 0-3,500 cm(-1) of RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) molecules adsorbed on a model aluminum surface. A molecular film was modeled using two approaches: (1) density functional theory (DFT) was used to optimize a single RDX molecule interacting with its periodic images, and (2) a group of nine molecules extracted from the crystal structure was deposited on the surface and interacted with its periodic images via molecular dynamics (MD) simulations. In both cases, the molecule was initialized in the AAA conformer geometry having the three nitro groups in axial positions, and kept that conformation in the DFT examination, but some molecules were found to change to the AAE conformer (two nitro groups in axial and one in equatorial position) in the MD analysis. The vibrational spectra obtained from both methods are similar to each other, except in the regions where collective RDX intermolecular interactions (captured by MD simulations) are important, and compare fairly well with experimental findings.

p-n Junction at the Interface between Metallic Systems.

Density functional theory is used to evaluate the electronic properties in a composite metallic material consisting of two subsystems made of interacting metallic thin films separated by a subnanometer gap. One of the subsystems, M/Pt-M/Pt, has a monolayer of metal M over a core of Pt atoms, and the other is Pt-Pt, where the interacting surfaces are made of pure Pt. At equilibrium, this composite material exhibits a potential barrier at the interface, resembling a semiconductor p-n junction. In the gap region of M/Pt-M/Pt, the amount of electrons correlates with the surface layer degree of polarization, which depends on electronegativity and number of unpaired electrons in the external shells. The electron density in the gap, the system work function, and the built-in potential at the interface of the composite system calculated for various metal skins correlate with the degree of reduction of the Pt atoms located at the junction area.

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.