A first-principles description of liquid BeF2 and its mixtures with LiF: 2. Network formation in LiF-BeF2

A polarizable ionic interaction potential, constructed from first-principles calculations, is used to examine the structure, vibrational spectra, and transport properties of molten mixtures of LiF and BeF2 across a range of compositions. The simulations reproduce the experimentally measured vibrational frequencies of the fluoroberyllate (BeF4(2-)) ions, which form in the melt, as well as conductivity and viscosity values across the composition range. Examination of the structures of the melts reveals the emergence of a slowly relaxing network of BeF4 units as the concentration of BeF2 is increased. The relationship between the appearance of the network and the composition dependence of the transport properties is explored.     

Internal mobilities and diffusion in an ionic liquid mixture

The internal mobility gives the rate at which one ionic species moves relative to the other species present in an ionic mixture, it mirrors the differential strength of the interactions between different ionic species. In this work we examine the dependence of the internal mobilities of the Li(+) and K(+) ions on the composition in molten mixtures of LiF and KF. We compare them to the behaviour of the individual diffusion coefficients and the self-exchange velocities, which measure the rate at which an ion separates from its nearest-neighbour coordination shell. The examination is made using molecular dynamics simulations with polarizable, first-principles parameterised interaction potentials which are shown to reproduce the limited available experimental data on the transport properties of these mixtures extremely well. The results confirm that the composition-dependence of the internal mobilities in LiF/KF follows the unusual type-II behaviour, which is not reflected in that of the diffusion coefficients or the self-exchange velocities.     

First-principles study of the effect of mechanical strength on ion transport in La-doped LiF-SEI on the Li (001) surface

The solid electrolyte interface (SEI) plays an important role in the lithium–sulfur battery system. It not only protects the stability of the lithium metal anode interface but also inhibits the growth of lithium dendrites during charge and discharge. The relationship between the shape of the SEI and the transport behavior of lithium ions affects the homogeneity of lithium dendrites. In this work, first-principles calculations are used to determine the stable structure and transport properties of the La-doped LiF solid electrolyte interface (La–LiF SEI) on the Li substrate. For the vertical transport of Li ions within the La–LiF SEI, the transport of Li ions in the grain boundary and that in the crystal grain was calculated separately. Regarding the plane diffusion behavior of Li ions between the La–LiF SEI and the lithium anode, the diffusion of Li ions on the surface and interface of the lithium anode were calculated. The effect of critical tensile strain on the diffusion of Li ions on the surface and interface was investigated. The results show that doping with La solves the problem of excessive periodic grain boundary gaps caused by the difference between LiF and Li lattices during the deposition process. The periodic gap is reduced from 0.478 nm to 0.306 nm after La doping. By comparing the migration energy barriers of each path, it is found that lithium ions are more likely to be inserted and extracted at the La–LiF SEI grain boundary. The reason is that the existence of the rare earth element La causes the grain boundary to have a more stable vacancy structure and a smaller transport energy barrier (0.789 eV). The critical tensile strain reduces the diffusion energy barrier (0.813 eV) of Li ions on the surface of the lithium metal anode, which promotes the fast diffusion and uniform deposition of Li ions between the interfaces. The establishment of SEI transport characteristics under the coupling conditions of mechanical stretching and ion transport is expected to improve the Li deposition behavior.     

Effect of rotational energy on the reaction Li + HF(upsilon = 0,j)-->LiF + H: an experimental and computational study

In a crossed molecular-beam study we have measured angular and time-of-flight distributions of the product LiF from the reaction Li + HF(upsilon = 0)-->LiF + H at various collision energies ranging from 97 to 363 meV for three markedly different rotational state distributions of HF obtained at nozzle temperatures close to 315, 510, and 850 K. Particularly, for the low and intermediate collision energies we observe significant effects of the varying j-state populations on the shape of the product angular distributions. At 315 K an additional feature appears in the angular distributions which is interpreted as being due to scattering from HF dimers. The experimental data are compared with simulations of the monomer reaction based on extensive quasiclassical trajectory calculations on a new state-of-the-art ab initio potential energy surface. We find an overall good agreement between the theoretical simulations and the experimental data for the title reaction, especially at the highest HF nozzle temperature.     

A first-principles description of liquid BeF2 and its mixtures with LiF: 1. Potential development and pure BeF2

The construction of an interaction potential for BeF2 and its mixtures with LiF on a purely first-principles basis is described. The quality of the representation of the forces on the ions obtained from ab initio electronic structure calculations by various potentials, which include many-body interaction effects to different extents, are considered. The predictions of the properties of pure BeF2 obtained in simulations with a polarizable potential are then compared with experimental values. In the subsequent paper, a more extensive comparison of the predicted properties of LiF-BeF2 mixtures with experiment is considered.     

有或没有Alq3参与情况下LiF和Al的化学反应

利用X光电子能谱（XPS）和高分辨电子能量损失谱（HREELS）研究了几种条件下LiF和Al的化学反应.在有Alq3参与的情况下,LiF和Al在室温下发生反应.在Al/LiF/Alq3系统中,Li 1s的峰发生了0.25 eV的位移,同时F 1s也发生了相应的位移.在没有Alq3参与的情况下,加热样品至350 K后,在Li 1s峰的低能端0.85 eV处出现了一个伴峰.XPS研究表明,这个伴峰对应的是金属态的Li 1s.HREELS的结果也验证了这一结论.     

Fragmentation of (LiF)(n)Li(+) clusters in the acceleration region of TOF spectrometers

The binary decay of ionized clusters in the extraction region of time-of-flight (TOF) spectrometers is analyzed. The dynamics of the fragments is studied and an analytical expression for the TOF peak shape is deduced; simulations are performed for linear spectrometers of different configurations. The questions addressed refer to the design of TOF spectrometers to improve their accuracy in the determination of metastable-state mean lives, the identification of precursor masses and the investigation of desorption mechanisms. As an illustration, the method is applied to the decay of positive ion clusters (LiF)(n)Li(+) for both spontaneous and collision-induced fragmentation processes. No clear evidence of delayed emission is found. The bumps observed in the TOF spectrum are due to tertiary ions emitted by the LiF target sputtered by secondary ions produced in the grid, a process that increases with higher target bias. The main cluster fragmentation observed is (LiF)(3)Li(+*) decaying preferentially into (LiF)Li(+); the data are compatible with a spontaneous decay of metastable clusters with mean lives of 20-30 ns.     

Ion mobilities and microscopic dynamics in liquid (Li,K)Cl

The dynamical properties of ionic melts formed from mixtures of LiCl and KCl have been studied across the full composition range in computer simulations of sufficient length to enable reliable values for such collective transport coefficients as the viscosity, conductivity, and internal mobilities to be determined reliably. Interest centers on the nontrivial concentration dependence exhibited by these transport coefficients, which agrees well with that observed experimentally, and in relating this to the strength of the association between an ion and its first coordination shell. The relationships between the various transport coefficients, such as those between the diffusion coefficient and the viscosity (Stokes-Einstein) and the conductivity (Nernst-Einstein) also exhibit composition dependences that reflect this association. The connection between the internal mobility and two measures of the coordination shell dynamics (the cage relaxation time and the self-exchange velocity) is explored; it is shown that the self-exchange velocity follows the composition and temperature dependence of the internal mobility very well. Finally, it is shown that allowing for anion polarization in the interaction model increases the mobility of all species without changing the structure of the melt discernibly, with the largest effect being found for the Li(+) ion.     

Quantum Dynamics of Li+HF/DF Reaction Investigated by a State-to-State Time-dependent Wave Packet Approach

Using the reactant coordinate based time-dependent wave packet method, on the APW potential energy surface, the differential and integral cross sections of the Li+DF/HF(v=0, j=0, 1) reactions were calculated over the collision energy range from the threshold to 0.25 eV. The initial state-specified reaction rate constants of the title reaction were also calculated. The results indicate that, compared with the Li+DF reaction, the product LiF of Li+HF reaction is a little more rotationally excited but essentially similar. The initial rotational excitation from j=0 to 1 has little effect on the Li+DF reaction. However, the rotational excitation of DF does result in a little more rotationally excited product LiF. The different cross section of both reactions is forward biased in the studied collision energy range, especially at relatively high collision energy. The resonances in the Li+HF reaction may be identifiable as the oscillations in the product ro-vibrational state-resolved integral cross sections and backward scattering as a function of collusion energy. For the Li+HF reaction, the rate constant is not sensitive to the temperature and almost has no change in the temperature range considered. For the Li+DF reaction, the rate constant increase by a factor of about 10 in the temperature range of 100?300 K. Brief comparison for the total reaction probabilities and integral cross section of the Li+HF reaction has been carried out between ours and the values reported previously. The agreement is good, and the difference should come from the better convergence of our present calculations.     

Transport coefficients, Raman spectroscopy, and computer simulation of lithium salt solutions in an ionic liquid

Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N(-), bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.     

An ab initio study of microsolvation of LiF in water: structures and properties of LiF-Wn,n = 1-9 complexes

Geometries of clusters of water molecules (W(n)) and those of the LiF-W(n) (n = 1-9) complexes were optimized using the B3LYP/6-31+G** method. Geometries of the complexes up to n = 7 were also optimized using the MP2/6-31+G** approach. Only one structure of each of W(n), n = 1-5 was considered to generate the complexes with LiF while two structures, one of a cage type and the other of a prism type, were considered for n = 6-9. The LiF-W(2) complex is found to be most stable among the various complexes. The LiF-W(6) complex, where W(6) is of a cage type, is predicted to be substantially less stable than that where W(6) is of a prism type. Certain existing ambiguities regarding the most stable structures of the LiF-W(n) (n = 1-3) complexes have been resolved. The LiF molecule seems to divide the W(n) clusters in the LiF-W(n) (n = 3-6) complexes into different fragments where at least one W(2)-like fragment is present. In LiF-W(6) (cage), there is one W(2)-like fragment while in LiF-W(6) (prism), there are three W(2)-like fragments. The LiF bond length is substantially increased in going from the gas phase to the different complexes, this increase being most prominent in LiF-W(6), where W(6) is of the cage or prism type. The LiF molecule, however, does not acquire the ionic structure Li(+)F(-) in any of the complexes studied here. An appreciable amount of electronic charge is transferred from LiF to the water molecules involved in the different complexes. In this process, the Li atom gains electronic charge in some cases, while the F atom considered separately, as well as the Li and F atoms taken together, lose the same in most cases.     

Atomistic insights into the conversion reaction in iron fluoride: a dynamically adaptive force field approach

Nanoscale metal fluorides are promising candidates for high capacity lithium ion batteries, in which a conversion reaction upon exposure to Li ions enables access to the multiple valence states of the metal cation. However, little is known about the molecular mechanisms and the reaction pathways in conversion that relate to the need for nanoscale starting materials. To address this reaction and the controversial role of intercalation in a promising conversion material, FeF(2), a dynamically adaptive force field that allows for a change in ion charge during reactions is applied in molecular dynamics simulations. Results provide the atomistic view of this conversion reaction that forms nanocrystals of LiF and Fe(0) and addresses the important controversy regarding intercalation. Simulations of Li(+) exposure on the low energy FeF(2) (001) and (110) surfaces show that the reaction initiates at the surface and iron clusters as well as crystalline LiF are formed, sometimes via an amorphous Li-F. Li intercalation is also observed as a function of surface orientation and rate of exposure to the Li, with different behavior on (001) and (110) surfaces. Intercalation along [001] rapid transport channels is accompanied by a slight reduction of charge density on multiple nearby Fe ions per Li ion until enough Li saturates a region and causes the nearby Fe to lose sufficient charge to become destabilized and form the nanocluster Fe(0). The resultant nanostructures are fully consistent with postconversion TEM observations, and the simulations provide the solution to the controversy regarding intercalation versus conversion and the atomistic rationale for the need for nanoscale metal fluoride starting particles in conversion cathodes.     

First-principles investigation on extraction of lithium ion from monoclinic LiMnO2

Changes with Li extraction in the crystal structure and electronic structure of monoclinic LiMnO₂ have been investigated by first-principle calculations using spin-polarized generalized gradient approximation method. It is found that the Li extraction has changed the oxidation state of Mn ion from 3+ to 4+ and thereby induced a phase transition from the monoclinic structure to rhombohedral symmetry, which opens channels for the migration of Mn ions from their own octahedral sites to the interlayer Li vacancies. It is the migration of Mn ions that responds for the phase transition to a spinel-like structure and the severe capacity loss during the electrochemical cycles of monoclinic LiMnO₂.     

Dynamics of harpooning studied by transition state spectroscopy. Part III. Li...FCH3

The Van der Waals complex Li...FCH3 has been formed in a crossed molecular beam apparatus. The transition state (TS) for the reaction Li*(2p 2P) + FCH3-->LiF + CH3 was accessed at various configurations by laser-excitation of the Li...FCH3 complex by tunable visible radiation, lambda 1. Photoinduced depletion of the complex by excitation to this TS was found to occur across a broad range of lambda 1 from 570 to 850 nm. This 'action spectrum' consisted of two broad regions located to either side of the atomic transition line of Li (2p 2P<--2s 2S). The first region, between 700 and 850 nm, was dominated by sharp maxima in the depletion intensity. A broad peak with weakly-resolved structure characterized the second region, between 570 and 680 nm. These findings were interpreted by means of high-level ab initio calculations of the potential-energy surfaces in the TS region. The peaks in the photodepletion spectrum were assigned to specific electronic transitions, their shapes and intensities being explained in terms of calculated transition-dipole moments and rovibrational wavefunctions.     

Differential reaction cross sections in the bending corrected rotating non-linear model: Li + HF → LiF + H

The 3D reaction Li + HF → LiF + H is studied using the bending corrected rotating non-linear model (BCRNM). Differential cross sections are computed and compared to experimental results at two energies, 0.38 and 0.63 eV. Results are found to be in good agreement with experiment at the lower energy, which is near the reaction threshold.     

A comparison of complexation of Li+ ion with macrocyclic ligands 15-crown-5 and 12-crown-4 in binary nitromethane-acetonitrile mixtures by using lithium-7 NMR technique and ab initio calculation

Lithium-7 NMR measurements were used to investigate the stoichiometry and stability of Li+ complexes with 15-crown-5 (15C5), benzo-15-crown-5 (B15C5), dibenzo-15-crown-5 (DB15C5) and 12-crown-4 (12C4) in a number of nitromethane (NM)-acetonitrile (AN) binary mixtures. In all cases, the exchange between the free and complexed lithium ion was fast on the NMR time scale and a single population average resonance was observed. While all crown ethers form 1:1 complexes with Li+ ion in the binary mixtures used, both 1:1 and 2:1 (sandwich) complexes were observed between lithium ion and 12C4 in pure nitromethane solution. Stepwise formation constants of the 1:1 and 2:1 (ligand/metal) complexes were evaluated from computer fitting of the NMR-mole ratio data to equations which relate the observed metal ion chemical shifts to formation constants. There is an inverse linear relationship between the logarithms of the stability constants and the mole fraction of acetonitrile in the solvent mixtures. The stability order of the 1:1 complexes was found to be 15C5·Li+>B15C5·Li+>DB15C5·Li+>12C4·Li+. The optimized structures of the free ligands and their 1:1 and 2:1 complexes with Li+ ion were predicted by ab initio theoretical calculations using the Gaussian 98 software, and the results are discussed.     

Determination of the distance-dependent viscosity of mixtures in parallel slabs using non-equilibrium molecular dynamics

We generalize a technique for determination of the shear viscosity of mixtures in planar slabs using non-equilibrium computer simulations by applying an external force parallel to the surface generating Poiseuille flow. The distance-dependent viscosity of the mixture, given as a function of the distance from the surface, is determined by analysis of the resulting velocity profiles of all species. We present results for a highly non-ideal water + methanol mixture in the whole concentration range between rutile (TiO(2)) walls. The bulk results are compared to the existing equilibrium molecular dynamics and experimental data while the inhomogeneous viscosity profiles at the interface are interpreted using the structural data and information on hydrogen bonding.     

First-principles calculations of migration energy of lithium ions in halides and chalcogenides

Migration of Li+ ions via the vacancy mechanism in LiX (X = F, Cl, Br, and I) with the rocksalt and hypothetical zinc blende structures and Li2X (X = O, S, Se, and Te) with the antifluorite structure has been investigated using first-principles projector augmented wave calculations with the generalized gradient approximation. The migration paths and energies, determined by the nudged-elastic-band method, are discussed on the basis of two idealized models: the rigid-sphere and charged-sphere models. The trajectories and energy profiles of the migration in these lithium compounds vary between these two models, depending on the anion species and crystal structure. The migration energies in LiX with both the rocksalt and hypothetical zinc blende structures show a tendency to decrease with increasing periodic number of the anion species in the periodic table. This is consistent with the widely accepted view that anion species with large ionic radii and high polarizabilities are favorable for good ionic conduction. In contrast, Li2O exhibits the lowest migration energy among Li2X compounds, although O is the smallest among the chalcogens, indicating that electrostatic attractive interactions play the dominant role in the inter-ion interactions in Li2O and, therefore, in the ion migration.     

Dissipative particle dynamics simulation study on the binary mixture phase separation coupled with polymerization

The influence of polymerization on the phase separation of binary immiscible mixtures has been investigated by the dissipative particle dynamics simulations in two dimensions. During polymerization, the bulk viscosity increases, which consequently slows down the spinodal decomposition process. The domain size growth is monitored in the simulations. The absence of 23 exponent for inertial hydrodynamic mechanism clearly reflects the suppressing effect of polymerization on the phase separation. Due to the increasing viscosity, the individual phase may be trapped in a metastable stage instead of the lamellar morphology identified for symmetric mixtures. Moreover, the polymerization induced phase separation in the binary miscible mixture has been studied. The domain growth is strongly dependent on the polymerization probability, which is naturally related to the activation energy for polymerization. The observed complex phase separation behavior is attributed to the interplay between the increasing thermodynamic driving force for phase separation and the increasing viscosity that suppresses phase separation as the polymerization proceeds.     

Viscosities for Binary Mixtures of 1-Decanol, Hexane, and Diethylamine at 10, 25, and 40°C

Experimental viscosities provide information on the structure of liquids and are required in the design of processes, which involve fluid flow, mass transfer, or heat transfer calculations. This work reports experimental viscosity data of the binary mixtures: 1-decanol + hexane, 1-decanol + diethylamine, and hexane + diethylamine at 10, 25, and 40°C and atmospheric pressure for the whole range of compositions. The viscosities of the pure liquids and their mixtures were determined using Cannon Fenske viscometers thermostated at ±0.01°C. The estimated error in the measured viscosities was less than ±0.005 mPa-s. The dynamic viscosity and the excess energy of activation for viscous flow were also calculated. The equation of Redlich–Kister was used for fitting the excess properties of the binary mixtures. The excess viscosity shows positive deviations from ideal behavior for the mixtures 1- decanol + hexane and 1-decanol + diethylamine and a small negative deviation for the binary system hexane + diethylamine. The experimental results have been also used to test some empirical and semiempirical equations adopted previously to correlate viscosity composition data.