The thermodynamic state functions and partition functions for adsorbates on solid surfaces are often treated with two-dimensional (2D) ideal gas and 2D ideal lattice gas models. These are idealized limits of the real situation for adsorbates on solid surfaces, which are more accurately described as hindered translators. We describe a simple extension of the ideal 2D gas model to a more realistic ideal hindered translator model based on our recent approximation for the partition function of ideal hindered translators [Sprowl et al., J. Phys. Chem. C., 2016 , DOI: 10.1021/acs.jpcc.5b11616]. Expressions for equilibrium constants and rate constants within transition-state theory (TST) are derived in a self-consistent formalism based on both partition functions and standard-state entropies. The mixing of these three adsorbate models (ideal 2D gas, ideal 2D lattice gas, and ideal hindered translator) within the same equilibrium or rate constant calculation is sometimes necessary but requires careful and consistent choices of standard-state concentrations. The formalism used here facilitates such mixing, using activities instead of concentrations to do so, and also to enable inclusion of nonidealities. We propose a standard state for 2D (and one-dimensional) ideal gases defined such that their translational entropy is 2/3 (or 1/3) that for the corresponding ideal three-dimensional (3D) gas, which offers intuitive advantages for estimating equilibrium and rate constants. This sets the standard-state concentration of the ideal 2D gas to be approximately the 2/3 power of the standard-state concentration of the corresponding 3D ideal gas (i.e., the concentration at 1 bar pressure). We show that in the derivation of the TST rate of elementary steps for ideal 2D lattice gases, the concentration of the transition state often increases as the adsorbate’s activity, θ/(1 – θ), rather than simply as θ, the fractional population of sites, and discuss the implications of this result.

With the recent explosion in computational catalysis and related microkinetic modeling, the need for a fast yet accurate way to predict equilibrium and rate constants for surface reactions has become more important. Here we present a fast and accurate new method to estimate the partition functions and entropies of adsorbates based on quantum mechanical estimates of the potential energy surface. As with previous approaches, it uses the harmonic oscillator (HO) approximation for most of the modes of motion of the adsorbate. However, it uses hindered translator and hindered rotor models for the three adsorbate modes associated with motions parallel to the surface, and evaluates these using an approach based on a method that has proven accurate in modeling the internal hindered rotations of gas molecules. The adsorbate entropies were calculated with this method for four adsorbates (methanol, propane, ethane, and methane) on Pt(111) using density functional theory (DFT) to evaluate the potential energy surface, and are shown to be in very good agreement with experiments, better than using only the HO approximation. The translational and rotational contributions to the entropy of a hindered translator / hindered rotor are very closely approximated by the corresponding harmonic oscillator entropy (within 0.46 R) when the barrier exceeds kT, and by the entropy of an ideal 2D monatomic gas of the same mass and a free 1D rotor with the same moment of inertia, respectively, (within 0.12 R) when the barrier is less than kT. However, the harmonic oscillator / lattice gas model severely overestimates the entropy when kT exceeds the barrier.

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Lithium-ion batteries are a leading energy storage technology. One challenge with lithium-ion batteries is the reductive decomposition of electrolyte on the surface, forming a passivating, solid−electrolyte interphase (SEI). The SEI prevents further electrolyte breakdown and consumption, but if not formed properly, may also consume lithium ions and prevent lithium-ion diffusion to the anode. Fluoroethylene carbonate (FEC) is currently one of the best electrolyte additives used to form a more robust SEI on silicon anodes. Herein, we use density functional theory (DFT) to investigate the spontaneous breakdown mechanisms, energies, and charge transfers of FEC on the surface of a silicon anode in a lesser lithiated LiSi and a more lithiated Li 15 Si 4 state. The reductive decomposition of FEC on LiSi and Li 15 Si 4 to F, CO 2 , and CH 2 CHO is energetically most favorable on both surfaces, but F, CO, and OCH 2 CHO can also be formed. The breakdown of FEC via either of the breakdown mechanisms is about 2 times more favorable on the Li 15 Si 4 surface than on the LiSi surface. The Bader charge transferred from the anode to the FEC breakdown products is larger when forming F, CO 2 , and CH 2 CHO than when forming F, CO, and OCH 2 CHO and is also larger on the Li 15 Si 4 surface than on the LiSi surface.

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Silicon anodes are a promising material for lithium ion batteries due to their large theoretical capacity for storing lithium. When the battery is charging, the anode works by alloying lithium ions with the silicon anode where the lithium ions are reduced. In addition to the lithium ions being reduced, unwanted side reduction reactions of the electrolyte also occur. When the electrolyte reductively decomposes at the anode surface, a solid-electrolyte interphase (SEI) is formed. Fluoroethylene carbonate (FEC) is added to the electrolyte to make a more robust SEI which prevents further electrolyte decomposition. From FEC reductive decomposition, the primary reduction products are CO2, CH2CHO, and F-. While the decomposition products have been discovered, the lowest energy surface facet which promotes the breakdown of FEC into those products is still unknown. Here FEC and its reduction products are being studied on different lithium silicide surfaces using density functional theory. The lithium silicide phases under consideration are the crystalline LiSi phase with a low lithium concentration and the crystalline Li15Si4 phase with a higher lithium concentration. All unique low index surface facets are considered for each lithium silicide phase. The interactions of FEC and its dissociation products are analyzed on the different anode surface facets to determine the surface sensitivity of FEC breakdown. From the binding energies of the FEC reactant and the CO2, CH2CHO, and F- products on the lithium silicide surface facets, FEC dissociation reaction energies are calculated. The facet on which SEI formation begins to form is determined from the reaction energies for both the low and high concentration lithium silicide anodes. This material is based upon work supported by the U.   S. Department of Energy,   Office of Science, Office of Workforce Development for Teachers and Scientists,   Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under contract number DE-SC0014664.   Use of the Center for Nanoscale Materials, an Office of Science user facility, is   supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under   contract number   DE-AC02-06CH11357.