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The generalized gradient approximation (GGA) often fails to correctly describe the electronic structure and thermochemistry of transition metal oxides and is commonly improved using an inexpensive correction term with a scaling parameter U. The authors tune U to reproduce experimental vanadium oxide redox energetics with a localized basis and a GGA functional. The value for U is found to be significantly lower than what is generally reported with plane-wave bases, with the uncorrected GGA results being already in reasonable agreement with experiments. This computational set-up is used to calculate interstitial and substitutional insertion energies of main group metals in vanadium pentoxide and interstitial doping is found to be thermodynamically favored.
We investigate, from first principles, the electrochemical sodiation mechanism of rutile-type vanadium dioxide as a possible electrode material for sodium-ion batteries. The computed voltages versus sodium metal are low in comparison to current state-of-the-art sodium-ion battery cathodes, which we can relate to the large space demand of sodium ions in the compact rutile structure and the resulting severe lattice deformations compared to other working metals. Due to the same reason large anisotropic unit cell volume changes are predicted during cycling. We furthermore find a change of the preferred reaction mechanism during discharge, with a switching between insertion- and conversion-type reaction at higher degrees of sodiation. The predicted capacities on the other hand are appreciable, making a further consideration of this material as high-potential anode in combination with sodium working metal interesting.
Brookite titanium dioxide is investigated from first principles as possible insertion-type cathode material for Li, Na and Mg. Recently structural similarity of this phase and amorphous titanium dioxide was reported. Low-concentration insertion energies and the corresponding voltages, however, suggest poor electrochemical performance of brookite in comparison to e.g. layered titania phases such as B-TiO2. We argue that this behavior could be explained by local electronic structure leading to higher voltages in amorphous compounds, since the lattice strains induced by intercalation in brookite are not sufficient to explain the poor binding energies with the investigated metals.
We analyze charge density transfer from water to solvated transition metal (TM) ions in different formal oxidation states (FOSs) in aqueous solution by first principles and relate the degree of stabilization of the solvated cations to the charge donation from the water ligands. We find remarkable charge stability on the metal center regardless of FOSs. This effect is similar to what has previously been shown for charges on TM cations in inorganic crystals. This ligand-to-metal charge transfer results in softening of the ligand O–H bonds, which can be used to explain the formation of higher-FOS transition metalates and oxycations.
We present an ab initio study of dopant–dopant interactions in beryllium-doped InGaAs. We consider defect formation energies of various interstitial and substitutional defects and their combinations. We find that all substitutional–substitutional interactions could be neglected. On the other hand, interactions involving an interstitial defect are significant. Specially, interstitial Be is stabilized by about 0.9/1.0 eV in the presence of one/two BeGa substitutionals. Ga interstitial is also substantially stabilized by Be substitutionals. Two Be interstitials can form a metastable Be–Be–Ga complex with a dissociation energy of 0.26 eV/Be. Therefore, interstitial defects and defect–defect interactions should be considered in accurate models of Be-doped InGaAs. We suggest that In and Ga should be treated as separate atoms and not lumped into a single effective group III element, as has been done before. We identified dopant-centred states which indicate the presence of other charge states at finite temperatures, specifically, the presence of Beint+1 (as opposed to Beint+2 at 0 K).
We investigate the insertion energetics of Ca at low concentrations in four promising vanadium oxide phases (α and δ vanadium pentoxide (V2O5) polymorphs as well as rutile- (R) and bronze-type (B) vanadium dioxide (VO2)) using density functional theory (DFT). We find α-V2O5 to be the most suitable material for an application as cathode, driven by a stable coordinative environment, while VO2(R) does not exhibit a stable low-concentration CaxVO2 phase due to severe distortions of the host lattice due to the large calcium ion. The results provide insight into the possibility of employing these phases as active cathode materials of Ca-ion batteries.
Four different vanadium oxide phases [α-vanadium pentoxide (V2O5), β-V2O5, bronze-type vanadium dioxide [VO2(B)], and rutile-type VO2 [VO2(R)])] are investigated from first principles as potential electrode materials for potassium (K) ion batteries. Specifically, insertion energetics and diffusion barriers are computed. These phases are known as promising cathode materials for other types of metal ion batteries. Our results show that the metastable β-V2O5 provides the lowest (strongest) insertion energies for K and the lowest diffusion barriers compared with orthorhombic α-V2O5, VO2(B), and VO2(R). While three of these phases show energetically favorable potassiation and relatively small diffusion barriers, VO2(R) is predicted to be incapable of electrochemical K incorporation.
Doping is a potent and often used strategy to modify properties of active electrode materials in advanced electrochemical batteries. There are several factors by which doping changes properties critically affecting battery performance, most notably the voltage, capacity, rate capability, and stability. These factors have to do specifically with changes in structure, band gap and band structure, and structural instability induced by doping. We review our recent modeling works on the effects of doping of active electrode materials, notably for prospective materials for organic and post-lithium (Na ion, Mg ion) batteries, as well as present new results, to build a coherent view on the use of n- and p-doping to modulate Li, Na, and Mg storage properties, most notably voltage. Specifically, we clearly point out effects due to electronic structure and those due to strain (structural instability), which clears some confusion about the effects of n- versus p-doping and facilitates rational rather than ad hoc design of doped materials.
Diketopyrrolopyrrole (DPP) is a critically important building block that has gained importance in the organic electronics community because of its wide applicability in various devices. In this work, the thiophene flanked DPP moiety attached to alkyl chains of various lengths (this includes straight octyl and branched ethyl hexyl units) has been used as the monomer for electropolymerization. This paper focuses on the study of optical, thermal, solid state ordering and electrochemical properties of these electron deficient monomers using various characterization techniques such as UV–Vis spectrometry (UV), photo-luminescence spectroscopy (PL), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), X-ray diffraction (XRD), cyclic voltammetry (CV), as well as ab initio modeling. These monomers exhibit broad absorption spectra from the ultraviolet (280–400 nm) to visible (400–600 nm) regions and emission spectra between 560 and 610 nm. The band gaps of these monomers were calculated to be in the range of 2.00–2.20 eV. These monomers were electropolymerized by scanning the potential between −0.5 and 2.0 V versus ferrocene for up to 50 cycles on a glassy carbon electrode.
Electropolymerization is a promising approach to produce thin films of active organic conjugated materials on a desired conducting substrate. In this work, an electropolymerization study has been carried out on two diketopyrrolopyrrole (DPP)-based monomers 2,5-bis(2-butyloctyl)-3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BO-DPPF) and 2,5-bis(2-butyloctyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (BO-DPPT). These monomers consist of thiophene and furan heterocyclic moieties attached to a DPP core with a common solubilizing alkyl chain (butyl-octyl). The properties of these monomers were analyzed via differential scanning calorimetry, thermogravimetric analysis, UV–Vis spectrometry (UV) and photoluminescence. Cyclic voltammetry (CV) studies indicate the presence of irreversible oxidation and reduction reactions. The electropolymerization of BO-DPPF and BO-DPPT electron-deficient monomers to form polymer films on a glassy carbon electrode is achieved by applying a potential between −2 V and 2 V versus ferrocene for up to 50 cycles. The properties of the polymers were investigated using the cyclic voltammetry (CV) technique.
Using Density Functional Theory based modeling, we compare sodium attachment to disodium terephthalate (Na2Tph) and a related molecule disodium pyridine dicarboxylate (Na2PDC). We predict that substitution of the Na2Tph’s aromatic ring with pyridine will lead to an increased voltage by about 0.4 V vs Na2+xTph up to Na2+1PDC and a similar voltage to the terephthalate between Na2+1PDC and Na2+2PDC, i.e. a two-plateau behavior vs. a single plateau for Na2+xTph.
In ab initio modeling of doped semiconductors, estimation of defect formation energies involving substitutional sites of ternary compounds is ambiguous due to an approximate treatment of chemical potential of the substituted atoms. We propose a model of assigning fractions of the formation energy to individual atoms of a ternary semiconductor and test it on InGaAs. The accuracy of this approximation is on the order of 0.1 eV/atom and is expected to be sufficient for many practical purposes.
Disodium terephthalate (Na2TP), which is a disodium salt of terephthalic acid, is very promising organic electrode material for Na-ion batteries. We present an ab initio study of Na binding mechanism with Na2TP molecule. Specially, we provide the interaction energy of Na atom(s), effect of Na concentration on interaction energy, electronic properties of clean and Na attached Na2TP, and Na binding mechanism with Na2TP. We show that up to eight Na atoms can be attached to a single Na2TP molecule. The interaction energy of Na atoms varies from -0.79 to -0.66 eV with attachment of one to eight Na atoms. The adsorbed Na atom interacts with O atoms of carboxylate group and Na atoms of the salt molecule. The interaction between adsorbed Na and C atoms of the molecule is found to be not important for Na bindings. Attachment of a single Na atom generates a singly occupied orbital which becomes doubly occupied with attachment of second Na atoms. Attachment of more than two Na atoms leads to electron occupation of bonding orbitals formed between Na atoms and the carboxylate groups.
In search for effective negative electrodes for Mg- and K-ion batteries, we investigate the potential of glassy amorphous carbon by means of density functional theory calculations. Specifically, we provide the energetics for Mg and K insertion in two different structures of amorphous carbon. The insertion sites are found to be well distributed in energy, with insertion energies Ef vs. the cohesive energies of respectively Mg and K ranging from -1.1 to 2.8 eV for Mg and from -1.0 to 3.7 eV for K. To compare amorphous carbon to the most common structure of carbon (graphite), we study in addition the energetics associated with the insertion of Mg and K in graphite, for which two different stackings are considered for the two layers intercalating the Mg/K atom: the AB stacking which is most stable at the initial state of charge (in pure graphite) and the AA stacking which is most stable at the known final state of charge of K (KxC, x = 1/8). Already at the low concentration considered (x = 1/128), the insertion of Mg and K in graphite is found to favor the AA stacking, and to be thermodynamically unfavored (Ef positive with Ef = 1.9 eV for Mg and Ef = 0.7 eV for K). Amorphization appears therefore to provide insertion sites for Mg and K of lower (and negative) energies. The effect is of larger extent for Mg than for K, so much so that the insertion of Mg becomes more favored than that of K in amorphous carbon, although in graphite K is more easily inserted than Mg.
The adsorption 2-anthroic acid on titania has been shown to result in an interfacial charge transfer band, which makes this a promising interface for dye-sensitized solar cells with direct injection. Here, we model the adsorption of 2-anthroic acid on a TiO2 nanocluster exhibiting a (101)-like interface and compute light absorption properties of this system using for the first time a hybrid functional. The band alignment and the formation of interfacial charge transfer bands proposed in previous experimental and lower-level computational works are confirmed.
In this study, mode I debonding of the interface between silica and nylon-6 is examined using molecular dynamics, to predict the mechanical behavior of the interface between the polymer and silica. The effect of two types of surface treatment to the silica– Aminopropyltriethoxysilane and Hexamethyldisilazane (APTES and HMDZ) – on debonding is studied. Comparing the results for debonding between untreated, APTES and HMDZ modified surfaces suggests that the APTES treated surface provides a higher strength and toughness for surface debonding. The strength and toughness of the treated interfaces are higher than that of those of bare silica. The simulation results also show the formation of nano-sized voids in the polymer prior to separation with silica.
We present a computational density functional theory study of UF6 adsorption on ideal as well as hydrogenated and fluorinated graphene. We show that (i) the isotopic splitting in the vibrational spectrum of UF6 observed in vacuum is largely preserved in the adsorbed molecules. The existence of several adsorption configurations with competing Eads leads to overlaps in the vibrational spectra of isotopomers, but isotopomer-unique modes exist on all three surfaces. (ii) The adsorption energy of UF6 is of the order of 1.2 eV on ideal graphene, 1 eV on graphane, and 0.1 eV on fluorographene, i.e. the adsorption strength is moderate and can be controlled by surface modification. (i) and (ii) mean that it may be possible to cause desorption of a selected isotopomer by laser radiation, leading to isotopic separation between the surface and the gas.
Li attachment to free tetracyanoethylene (TCNE) molecules and TCNE adsorbed on doped graphene is studied using density functional theory. While TCNE is adsorbed only weakly on ideal graphene, we identified a configuration in which TCNE is chemisorbed on Al-doped graphene via its C atom and a surface oxygen atom. Up to four Li atoms can be stored on both free and adsorbed TCNE with binding energies stronger than cohesive energy of the Li metal. TCNE immobilized on the conducting graphene-based substrate could therefore become an efficient anode material for organic Li ion batteries.
We present rational computational design of phenothiazine dyes for dye-sensitized solar cells containing different five-membered rings (thiophene, furan, and selenophene) by a combined strategy of modified conjugation order and functionalization leading to the quinoidization of the ring. We predict that it is possible to lower the excitation energy by 20% vs. the parent dye by the combination of: change in the conjugation order of the methine unit, its functionalization by the CN group, and replacement of the thiophene ring by furan.
We present a comparative density functional theory study of Li, Na, and Mg storage energetics and diffusion in α-Sn, including the effects of temperature (vibrations). We study several concentrations corresponding to initial stages of insertion (number densities x= 1/64, 1/32, 1/16, and 1/8) as well as the final state of charge (Li17Sn4, Na15Sn4, and Mg2Sn). While final states of charge correspond to positive anode voltages for all three types of metal, insertion energetics is favorable for insertion for Li at all concentrations studied, for Na up to the concentration of x = 3/64, and Mg insertion is thermodynamically disfavored at all x. Diffusion barriers at dilute concentrations are computed to be 0.23, 0.51, and 0.44 eV for Li, Na, and Mg, respectively. Vibrations have a noticeable and temperature-, concentration-, and dopant-type dependent effect on voltages, of the order of 0.1 eV at room temperature.