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Tungsten-doped vanadia-based catalysts supported on anatase TiO2 are used to reduce hazardous NO emissions through the selective catalytic reduction of ammonia, but their exact atomistic structure is still largely unknown. In this computational study, the atomistic structure of mixed tungsta-vanadia monolayers on TiO2 support under typical operating conditions has been addressed by periodic density functional theory calculations. The chemical environment has been taken into account in a grand-canonical approach. We evaluate the stable catalyst structures as a function of the oxygen chemical potential and vanadium and tungsten concentrations. Thus we determine structural motifs of tungsta-vanadia/TiO2 catalysts that are stable under operating conditions. Furthermore, we identify active sites that promise high catalytic activity for the selective catalytic reduction by ammonia. Our calculations reveal the critical role of the stoichiometry of the tungsta-vanadia layers with respect to their catalytic activity in the selective catalytic reduction.

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li2O2 (lithium peroxide) and to a lesser extent, Li2O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO2 (lithium superoxide) considered to be an intermediate product. Bulk Li2O2 is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li2O2 compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li2O and Li2O2 particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li2O and Li2O2 surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li2O2, and discuss the consequences for the observed morphology of the reaction products.

We have performed density functional theory calculations to explore the possibility to overcome the linear scaling relations in the oxygen reduction reaction (ORR) using local inhomogeneities on Pt...

Recently, a surprisingly fast diffusion of O atoms on a fully CO-covered Ru(0001) surface has been observed using video-rate scanning tunneling microscopy. This finding has been explained by a so-c...

To strain or not to strain: We calculate the strain dependence of metal anode surface properties from first principles to explore whether surface stress can be related to dendrite growth in metal-ion batteries on the atomistic level. We only find a weak dependence of these properties on the applied strain, which suggests that strain does not influence dendrite growth on the atomistic scale, but rather dendrite-related morphologies on longer-length scales. Dendrite growth poses a significant problem in the design of modern batteries as it can lead to capacity loss and short-circuiting. Recently, it has been proposed that self-diffusion barriers might be used as a descriptor for the occurrence of dendrite growth in batteries. As surface strain effects can modify dendritic growth, we present first-principles DFT calculations of the dependence of metal self-diffusion barriers on applied surface strain for a number of metals that are used as charge carriers in batteries. Overall, we find a rather small strain dependence of the barriers. We mainly attribute this to cancellation effects in the strain dependence of the initial and the transition states in diffusion., publishedVersion

Ion mobility is a critical performance parameter in electrochemical energy storage and conversion, but also in other electrochemical devices. Based on first-principles electronic structure calculations, we have derived a descriptor for the ion mobility in battery electrodes and solid electrolytes. This descriptor is entirely composed of observables that are easily accessible: ionic radii, oxidation states and the Pauling electronegativities of the involved species. Within a particular class of materials, the migration barriers are connected to this descriptor through linear scaling relations upon the variation of either the cation chemistry of the charge carriers or the anion chemistry of the host lattice. The validity of these scaling relations indicates that a purely ionic view falls short of capturing all factors influencing ion mobility in solids.The identification of these scaling relations has the potential to significantly accelerate the discovery of materials with desired mobility properties.

CO adlayers on Pt(111) electrode surfaces are an important electrochemical system and of great relevance to electrocatalysis. The potential‐dependent structure and dynamics of these adlayers are complex and still controversial, especially in the CO pre‐oxidation regime. We here employ in situ high‐speed scanning tunneling microscopy for studying the surface phase behavior in CO‐saturated 0.1 m H2SO4 on the millisecond time scale. At potentials near the onset of CO pre‐oxidation local fluctuations in the (2×2)‐CO adlayer are observed, which increase towards more positive potentials. Above 0.20 V (vs. Ag/AgCl), this leads to an adlayer where COad apparently reside on every top site, but still exhibit a (2×2) superstructure modulation. We interpret this observation as a dynamic effect, caused by a small number of highly mobile point defects in the (2×2)‐CO adlayer. As shown by density functional theory calculations, the CO lattice near such defects relaxes into a local (1×1) arrangement, which can rapidly propagate across the surface. This scenario, where a static (2×2) COad sublattice coexists with a highly dynamic sublattice of partially occupied top sites, explains the pronounced COad surface mobility during electrooxidation., Adsorbed CO on Pt(111) electrodes can become highly mobile at the onset of pre‐oxidation, despite a high adsorbate surface density. Origin of this behavior is the formation of defects in the adlayer, which enable density fluctuations and by this rapid quasi‐collective CO transport.

A structural analysis of solvating water layers on a Pt(111) electrode has been performed based on extensive ab initio molecular dynamics simulations. We have emulated different electrochemical conditions by varying the concentration of hydrogen ions in the water layers, which effectively corresponds to a variation in the electrode potential. We present a detailed analysis of the arrangement and orientation of the water molecules and also address their mobility in the solvation layer.

Apart from high capacity and high energy density, fast kinetics and thus low barriers for lithium diffusion are essential for the functioning of Li-ion batteries. Recent studies have shown that disordered rock salt (DRS) compounds can meet these requirements, which is surprising as disordered materials typically are assumed to exhibit low ion mobility. In our computational study, we investigate the structural stability as well as the diffusion kinetics of Li-rich transition metal (TM) oxides and oxyfluorides with a special focus on vanadium- and titanium-based compounds. By investigating different model systems, we move from α-layered LiTMO2 type compounds to the disordered rock salt structures of Li-rich Li2TMO3 and Li2TMO2F, proving that the DRS oxides can only be stabilized by Li-excess. High fluorine contents, on the other hand, are found to call for a certain degree of ordering to avoid unfavorable TM–F bonds. In Li-rich DRS phases, we find that diffusion barriers may, in spite of having the same local environment, differ strongly, consequently pointing towards important factors beyond the so-called gate site configuration. Moreover, the substitution of oxygen by fluorine in DRS-type structures is observed to have a rather negative impact on the kinetics, in contrast to expectations from oxidation state arguments. The diffusion barriers in DRS-type materials turn out to be non-trivial quantities that are determined by the exact configuration, extending beyond the gate site occupancy.

Aiming at a molecular-level understanding of the processes at the electrode|electrolyte interface (EEI), we investigated the interaction between the battery-relevant ionic liquid (IL) 1-butyl-1-met...

Summary The description of electrode–electrolyte interfaces is based on the notion of the formation of an electric double layer (EDL). Most of the concepts underlying its structure and properties have been developed more than one hundred years ago based on continuum approaches. Still, a complete atomistic theoretical description is missing. Here, first the traditional models of the EDL will be briefly reviewed before recent atomistic first-principles approaches using either explicit aqueous electrolytes or a grand-canonical formalism will be presented. Finally, the importance of the formation of an EDL at electrode/electrolyte interfaces in batteries will be discussed.

Due to their thermoelectric properties, intermetallic clathrates are technologically highly interesting materials. Especially their strongly reduced lattice thermal conductivity has been largely investigated and strongly debated. Although density-functional-theory-based lattice dynamics calculations have helped clarify the thermal transport in clathrates, significant discrepancies remain between experiment and theory, especially in Ge-based clathrates. In this work, we show that the recently released meta-generalized-gradient approximation functional SCAN (strongly constrained and appropriately normed) is able to overcome these issues and provide a much improved agreement between the experimental and the theoretical phonon spectra, thus enabling quantitative predictions of the clathrate phases with a high potential for thermoelectric applications.

In the area of sustainable energy storage, batteries based on multivalent ions such as magnesium have been attracting considerable attention due to their potential for high energy densities. Furthermore, they are typically also more abundant than, e.g., lithium. However, as a challenge their low ion mobility in electrode materials remains. This study addresses the ionic conductivity in spinel host materials which represent a promising class of cathode and solid-electrolyte materials in Mg-ion batteries. Based on periodic density functional theory calculations, we identify the critical parameters which determine the mobility and insertion of ions. We will in particular highlight the critical role that trigonal distortions of the spinel structure play for the ion mobility. In detail, we will show that it is the competition between coordination and bond length that governs the Mg site preference in ternary spinel compounds upon trigonal distortions. This can only be understood by also taking covalent interactions into account. Furthermore, our calculations suggest that anionic redox plays a much more important role in sulfide and selenide spinels than in oxide spinels. Based on our theoretical study, we rationalize the impact of the metal distribution in the host material and the ion concentration on the diffusion process. Furthermore, cathode-related challenges for practical devices will be addressed. Our findings shed light on the fundamentional mechanisms underlying ionic conductivity in solid hosts and thus may contribute to improve ion transport in battery electrodes.

Based on first-principles electronic structure calculations, we have derived an efficient physical descriptor for the ion mobility in battery electrodes and solid electrolytes which is a critical performance parameter in electrochemical energy storage and conversion. This descriptor is entirely composed of observables that are easily accessible: ionic radii, oxidation states and the difference in the Pauling electronegativities of the involved species. Within a particular class of materials, the activation barriers for migration are connected to this migration parameter through linear scaling relations both as far as the variation of the cation chemistry of the charge carriers and the anion chemistry of the host lattice are concerned. The validity of these scaling relations indicates that a purely ionic view of ion mobility in solids falls short of capturing all factors influencing this mobility. The identification of these scaling relations has the potential to significantly accelerate the discovery of materials with desired mobility properties.

Conductive polymers represent a promising alternative to semiconducting oxide electrodes typically used in dye-sensitized cathodes as they more easily allow a tuning of the physicochemical properties. This can then also be very beneficial for using them in light-driven catalysis. In this computational study, we address the coupling of Ru-based photosensitizers to a polymer matrix by combining two different first-principles electronic structure approaches. We use a periodic density functional theory code to properly account for the delocalized nature of the electronic states in the polymer. These ground state investigations are complemented by time-dependent density functional theory simulations to assess the Franck-Condon photophysics of the present photoactive hybrid material based on a molecular model system. Our results are consistent with recent experimental observations and allow to elucidate the light-driven redox chemical processes -- eventually leading to charge separation -- in the present functional hybrid systems with potential application as photocathode materials.

Vacancies matter: metal atoms can typically only change places in metals through vacancy-assisted diffusion. Here we address the diffusion in single-atom surface alloys by performing kinetic Monte Carlo simulations based on density functional theory calculations. We illustrate how the separation and recombination with a surface vacancy determine the trajectory of the foreign atom in such a surface alloy. Bimetallic surfaces can exhibit an improved catalytic activity through tailoring the concentration and/or the arrangement of the two metallic components. However, in order to be catalytically active, the active bimetallic surface structure has to be stable under operating conditions. Typically, structural changes in metals occur via vacancy diffusion. Based on the first-principles determination of formation energies and diffusion barriers we have performed kinetic Monte-Carlo (kMC) simulations to analyse the (meta-)stability of PtRu/Ru(0001), AgPd/Pd(111), PtAu/Au(111) and InCu/Cu(100) surface alloys. In a first step, here we consider single-atom alloys together with one vacancy per simulation cell. We will present results of the time evolution of these structures and analyse them in terms of the interaction between the constituents of the bimetallic surface., publishedVersion

The development of suitable electrode materials remains a great challenge for alternative battery technologies such as sodium-ion and potassium-ion batteries; especially with regard to the negative...

We report results of a combined experimental and computational model study on the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]−) with Li on pristine highly oriented pyrolytic graphite (HOPG), which aims at a molecular-/atomic-level understanding of the processes at the electrode|electrolyte interface of Li-ion batteries. Employing mainly X-ray and ultraviolet photoelectron spectroscopy as well as dispersion-corrected density functional calculations (DFT-D), we find intact anion–cation pairs for adsorbed [BMP]+[TFSI]− (sub)monolayers on HOPG at 300 K and also on lithiated HOPG at 80 K, that is, under conditions where the mobility of Li+ in the bulk is low. Vapor deposition of [BMP]+[TFSI]− on lithiated HOPG at 300 K results in rapid accumulation of Liδ+ at the surface or in the surface region, indicating that deintercalation is activated under these conditions. This is explained by a dynamic equilibrium between bulk Li+ ...

Dendrite formation is one of the most pressing issues in current battery research. Lithium based batteries are prone to forming short-circuit causing dendrites, while magnesium based batteries are not. Recently it was proposed that the tendency towards dendrite growth is related to the height of the self-diffusion barrier with high barriers leading to rough surface growth which that subsequently cause dendrite formation, which was supported by density functional theory calculations for Li, Na and Mg [J. Chem. Phys., 2014, 141, 174710]. We now extend this computational study to zinc and aluminum which are also used as battery anode materials, and we additionally consider diffusion barriers that are relevant for three-dimensional growth such as barriers for diffusion across steps. Our results indicate, in agreement with experimental observations, that Li dendrite growth is an inherent property of the metal, whereas Zn dendrite growth results from the loss of metallic properties in conventional Zn powder electrodes.

V2O5 with a layered van der Waals (vdW) structure has been widely studied because of the material's potential in applications such as battery electrodes. In this work, microelectronic devices were fabricated to study the electrical and optical properties of mechanically exfoliated multilayered V2O5 flakes. Raman spectroscopy was used to determine the crystal structure axes of the nanoflakes and revealed that the intensities of the Raman modes depend strongly on the relative orientation between the crystal axes and the polarization directions of incident/scattered light. Angular dependence of four-probe resistance measured in the van der Pauw (vdP) configuration revealed an in-plane anisotropic resistance ratio of ∼100 between the a and b crystal axes, the largest in-plane transport anisotropy effect experimentally reported for two-dimensional (2D) materials to date. This very large resistance anisotropic ratio is explained by the nonuniform current flow in the vdP measurement and an intrinsic mobility anisotropy ratio of 10 between the a and b crystal axes. Room-temperature electron Hall mobility up to 7 cm2/(V s) along the high-mobility direction was obtained. This work demonstrates V2O5 as a layered 2D vdW oxide material with strongly anisotropic optical and electronic properties for novel applications.

Advanced energy materials 10(25), 2000783 (2020). doi:10.1002/aenm.202000783, The development of alternative anode materials with higher volumetric and gravimetric capacity allowing for fast delithiation and, even more important, lithiation is crucial for next-generation lithium-ion batteries. Herein, the development of a completely new active material is reported, which follows an insertion-type lithiation mechanism, metal-doped CeO$_2$. Remarkably, the introduction of carefully selected dopants, herein exemplified for iron, results in an increase of the achievable capacity by more than 200%, originating from the reduction of the dopant to the metallic state and additional space for the lithium ion insertion due to a significant off-centering of the dopant atoms in the crystal structure, away from the original Ce site. In addition to the outstanding performance of such materials in high-power lithium-ion full-cells, the selective reduction of the iron dopant under preservation of the crystal structure of the host material is expected to open up a new field of research., Published by Wiley-VCH, Weinheim

Based on the results of periodic density functional theory calculations, we have recently proposed that the height of self-diffusion barriers can serve as a descriptor for dendrite growth in batteries [M. Jackle et al., Energy Environ. Sci. 11, 3400 (2018)]. However, in the determination of the self-diffusion barriers, the electrochemical environment has not been taken into account. Still, due to the presence of electrical double layers at electrode/electrolyte interfaces, strong electric fields can be present close to the interfacial region. In a first step toward including the electrochemical environment, we have calculated barriers for terrace-diffusion on lithium, magnesium, and silver surfaces and across-step self-diffusion on lithium in the presence of electric fields. Whereas the electric field effect is more pronounced on a stepped surface than on flat terraces, overall we find a negligible influence of electric fields on self-diffusion barriers which we explain by the good screening properties of metals.

In this work, we present and apply schemes to determine from first-principles calculations the relevant effective parameters used in phase field theory simulations of battery compounds. In particular, we derive that a consistent free energy density can be obtained by mean-field sampling, which is especially suited for materials with different configurations on a lattice, such as alloys or Li intercalation batteries. In addition, it is demonstrated that mean-field sampling can be performed very effectively with the use of special quasi random structures and that experimentally determined free energy density parameters for ${\mathrm{LiFePO}}_{4}$ are reproduced by density functional theory calculations. The additional computation of interfacial and strain energy parameters allows us to present a consistent phase field parametrization of ${\mathrm{Li}}_{2}{\mathrm{FeSiO}}_{4}$ without relying on experimental data.

Over the past years, density functional theory (DFT) became a widely approved and successful method for calculating properties of various materials and molecules. Especially suited for systems with delocalized electrons like metals, the efficient treatment of the van der Waals interaction remained a problem for DFT functionals within the generalized gradient approximation (GGA). Combining Grimme’s D3 correction with the RPBE functional and using a previously published data set, we show that this yields a functional that is well-suited for an accurate and balanced description of adsorption energies. The RPBE-D3 approach performs comparably to higher-level methods such as the BEEF-vdW and the SW-R88 method. Even for oxide systems, which traditionally are not well-described by GGA functionals, RPBE-D3 leads to satisfactorily results when combined with the +U approach, as demonstrated with respect to the energetic ordering of the three TiO2 polymorphs rutile, anatase, and brookite.

Electrochemical interfaces between an electrode and an electrolyte are often covered by ions from the solution. These adsorbed ions can strongly modify the properties of the interfaces. Furthermore, in electrocatalysis, the reacting species typically have to get into contact with the surface of the electrocatalyst, where the reaction then proceeds. Hence the understanding of the interaction of solvated species with electrode surfaces and the determination of the resulting adsorbate structures as a function of electrochemical control parameters such as electrode potential and electrolyte concentrations are crucial in electrochemistry. Here, grand-canonical theoretical approaches to derive adsorbate structures at electrochemical interfaces from an atomistic perspective will be reviewed. Special emphasis will be put on the distinction between the validity of the approach and the approximations that are typically made when using this approach.

One of the difficulties limiting the development of high capacity Li-O2 batteries is the positive electrode passivation by the discharge product Li2O2 which is deposited mostly due to the second electron transfer of oxygen reductionwhich requires the presence of Li+ in the Stern layer. To suppress the passivation and shift the reaction zone of Li2O2 formation towards the electrolyte bulk, we propose to use additional cations in the electrolyte. Using molecular dynamics simulations, we investigate the ability of various cations to replace Li+ ions in the first cation layers near the electrode, with EMI+ (1-ethyl-3-methylimidazolium) and PP13+ (N-methyl-N-propylpiperidinium) showing pronounced effects. However, our experimental studies including cycling voltammetry and discharge capacity measurements in high and low donor number solvents reveal practically no effect of such addition. Therefore, Li+ should be fully eliminated from electron transfer zone, and this is possible by anchoring of additional cations according to the simulations. We optimized the surface density for these cations, although the experimental support of this approach looks challenging.

The deposition of zinc from aqueous solutions is of great practical importance, and it also serves as a prototype for the deposition of divalent ions. Both experiment and theory agree, that it takes place in two steps. Previous theoretical work [1] had suggested that the step, Zn + + + e - → Zn + takes place in the outer sphere, but gave a prohibitively high energy of activation of the order of 1.4 eV, in accord with the enigma of metal deposition postulated by Gileadi [2] . In this work the treatment of the reactant – solvent interaction is substantially improved by introducing nonlinear terms based on molecular dynamics. Our calculations suggest that the first steps follows an inner sphere path with a much lower energy of activation, which results in a physically adsorbed Zn + ion. The second step then occurs on the electrode surface. These findings are in line with experimental data.

Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM2+0.5V0.54+O2 stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM2+0.5V4+0.5O2 (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g-1, respectively. In comparison, for the disordered rock-salt Li2VO3, an average discharge voltage of ∼2.2 V with V5+/4+ redox couple has been reported. However, detrimental electrode-electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode-electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

The present study shows the electrochemical performance of a room-temperature magnesium/sulfur (Mg/S) battery with a newly designed sulfur (3–0.5 mgsulfur cm−2) composite cathode. Operando Raman spectroscopy is employed to investigate the formation of polysulfide species at the cathode of Mg/S cells during the charge/discharge process, while density functional theory (DFT) calculations are used to correlate the Raman modes with a series of polysulfide species (Sxn−, x = 1–8). Operando Raman spectroscopy proves the chemical transformation from elemental sulfur (S8) via long and short chain polysulfides to magnesium sulfide upon discharge and conversion back to elemental sulfur during charging. Furthermore, the spectral measurements indicate the formation of a nanocrystalline magnesium sulfide with a cubic zinc blende phase at the end of discharge. The changes in the open circuit potential of the Mg/S cell during the resting period are investigated with the help of various spectroscopic techniques. These studies conclude that cell impedance is dominated by the anode side impedance due to the formation of a passivation layer under static conditions. Finally, the impedance studies under dynamic conditions verify that the applied electrode potential plays a significant role on the evolution of the anode interfacial impedance.

The description of electrode-electrolyte interfaces is based on the concept of the formation of an electric double layer. This concept was derived from continuum theories extended by introducing point charge distributions. Based on ab initio molecular dynamics simulations, we analyze the electric double layer in an approach beyond the point charge scheme by instead assessing charge polarizations at electrochemical metal-water interfaces from first principles. We show that the atomic structure of water layers at room temperature leads to an oscillatory behavior of the averaged electrostatic potential. We address the relation between the polarization distribution at the interface and the extent of the electric double layer and subsequently derive the electrode potential from the charge polarization.

We report on a many-electron wavefunction theory study for the reaction energetics of hydrogen dissociation on the Si(100) surface. We demonstrate that quantum chemical wavefunction based methods using periodic boundary conditions can predict chemically accurate results for the activation barrier and the chemisorption energy in agreement with experimental findings. These highly accurate results for the reaction energetics enable a deeper understanding of the underlying physical mechanism and make it possible to benchmark widely used density functional theory methods.

Aiming at a better understanding of the solid–electrolyte interphase formation in Li-ion batteries, we have investigated the interaction of ultrathin films of ethylene carbonate (EC), which is a key solvent of battery electrolytes, with pristine and lithiated highly oriented pyrolytic graphite (HOPG) and with postdeposited Li. Employing X-ray and ultraviolet photoelectron spectroscopy as well as Fourier transform infrared spectroscopy under ultrahigh-vacuum conditions, in combination with density functional theory (DFT)-based calculations, we find that EC adsorbs molecularly intact on pristine HOPG in the entire temperature range between 80 K and desorption at 200 K. Features in the ultraviolet photoelectron spectra could be related to the molecular orbitals of EC obtained from DFT calculations, and a similar adsorption/desorption behavior is obtained also on lithiated HOPG. In contrast, stepwise postdeposition of ∼0.5 and one monolayer of Li0 on a preadsorbed EC adlayer leads not only to stabilization of...

Cation-disorderedoxides have been ignored as positive electrode material for a long time, due tostructurally limited lithium insertion/extraction capabilities. In this work, acase study is carried outon nickel based cation-disordered Fm-3m LiNi0.5M0.5O2 positive electrode materials.The present investigation targets tailoring the electrochemical properties fornickel basedcation-disordered rock-salt by electronic considerations. The compositionalspace for binary LiM+3O2 with metals active for +3/+4 redoxcouples are extended to ternary oxides with LiA0.5B0.5O2 with A=Ni+2 and B=Ti+4 , Zr+4 and V+4 in order to assess the impact of the differenttransition metal in the isostructural oxides. The direct synthesis of variousnew unknownternary nickel based Fm-3m cation-disordered rock-salt positiveelectrode materials ispresented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode materialfor Li ion batteries displays an average voltage of ~ 2.55 V and a highdischarge capacity of264 mAhg-1 corresponding to 0.94 Li. For appropriate cut-off voltages, a long cyclelife is achieved. The charge compensation mechanism is probed by XANES,confirming thereversible oxidation and reduction of V4+/V5+. The enhancement in theelectrochemical performanceswithin the presented compounds stresses the importance of mixedcationdisordered transitionmetal oxides with different electronic configuration.

While the technological importance of carbon-based anodes for sodium-ion batteries is undebated, the underlying mechanism for sodium insertion and storage is still strongly disputed. Here, we present a joint experimental and theoretical study that allows us to provide detailed insights into the process of Na insertion in nongraphitizable (hard) carbon. For this purpose, we combine data from in situ Raman scattering of Na insertion in hard carbon with density functional theory-based lattice dynamics and band structure calculations for Na insertion in graphitic model structures used for a local description of graphitic domains in hard carbon. The agreement of experimental results and computational findings yields a clear picture of the Na insertion mechanism, which can be described by four different stages that are dominated by surface morphology, defect concentration, bulk structure, and nanoporosity. On the basis of the resulting model for sodium insertion, we suggest design strategies to maximize the cap...

We study the transport properties of single-atom-thick Cu wires submerged in an electrochemical solvent containing HCl. As a first step, we investigate the stability of hydrogen coadsorbing with chlorine on the Cu(111) surface in an implicit electrochemical environment. We find that adding hydrogen to a Cl-covered Cu surface is energetically unfavorable. The result serves as an estimate for the number of Cl atoms that adsorb near the single-atom wire. We use it to construct model junctions (Cu wire plus adsorbates), the electron transport properties of which we investigate with density functional theory. We find that the Cl and H adsorbates tend to deplete the density of states of the Cu wire near the Fermi energy. As a consequence, the transmission is reduced. Interestingly, we observe that in the case of H-adsorption, the amount of depletion is quite sensitive to the wire geometry (relaxed vs unrelaxed), but this is not the case with Cl.

The structure of water on metal electrodes is addressed based on first-principles calculations. Special emphasis is placed on the competition between water-metal and water-water interaction as the structure determining factors. Thus the question will be discussed whether water at metal surfaces is ice- or rather liquid-like. The proper description of liquid phases requires to perform thermal averages. This has been done by combining first-principles electronic structure calculations with molecular dynamics simulations. After reviewing recent studies about water on flat, stepped and pre-covered metal electrodes, some new results will be presented.

We studied the adsorption of 4,4′-dithiodipyridine (PySSPy) on Au(111) under ultrahigh vacuum conditions and at low-temperature both experimentally by means of scanning tunneling microscopy and spectroscopy (STM and STS) and theoretically by density functional theory (DFT). We find PySSPy molecules, characterized by their elongated appearance in STM, assembled in islands that reside exclusively in fcc regions of the herringbone reconstructed Au (HB) terraces. A triangular structure motif dominates the local arrangement of the PySSPy. DFT calculations reveal a virtually planar adsorption geometry of the PySSPy with the S–S bond of the molecule almost parallel to the Au–Au bonds of the substrate underneath. Though van der Waals forces dominate the stability of the adsorbed PySSPy, there is also a covalent contribution to the PySSPy/Au interaction. As a consequence, the PySSPy structures are relatively unstable. As found by STS, the highest occupied molecular orbital (HOMO) is located at around −0.7 eV below...

The effect of solvation on the adsorption of organic molecules on graphite at room temperature has been addressed with force-field molecular dynamics simulations. As a model system, the solvation of a bis(terpyridine) isomer in water and 1,2,4-trichlorobenzene was studied with an explicit solvation model. The inclusion of solvation has a noticeable effect on adsorption energies. Although the results of the various considered force fields differ quite significantly, they all agree that the adsorption of BTP from the TCB solvent is almost thermoneutral. The substrate simply acts as a template to allow a planar arrangement of the network, which is stabilized by the intermolecular interaction. Using an atomic thermodynamics approach, the order of the stability of various network structures as a function of the chemical potential is derived yielding a sequence in agreement with the experiment.

The adsorption of water on bimetallic PtRu/Pt(111) surface alloys has been studied based on periodic density functional theory calculations including dispersion corrections. The Ru atoms of the PtRu surface alloy interact more strongly with water than Pt atoms, as far as both single water molecules and ice-like hexagonal structures are concerned. Within the surface alloy layer, the lateral ligand effect reducing the local reactivity of the surface atoms with increasing Ru content is more dominant than the opposing geometric effect due to the tensile strain. The structural preference for the Ru atoms also prevails at room temperature, as ab initio molecular dynamics simulations show.

The electronic and structural properties of oligo- and polythiophenes that can be used as building blocks for molecular electronic devices have been studied by using periodic density functional theory calculations. We have in particular focused on the effect of substituents on the electronic structure of thiophenes. Whereas singly bonded substituents, such as methyl, amino or nitro groups, change the electronic properties of thiophene monomers and dimers, they hardly influence the band gap of polythiophene. In contrast, phenyl-substituted polythiophenes as well as vinyl-bridged polythiophene derivatives exhibit drastically modified band gaps. These effects cannot be explained by simple electron removal or addition, as calculations for charged polythiophenes demonstrate.