Conduction Mechanism in 70Li2S-30P2S5 Glass by Ab Initio Molecular Dynamics Simulations: Comparison with Li7P3S11 Crystal.

The 70Li2S-30P2S5 glass is a promising solid-state electrolyte for all solid-state lithium-ion batteries. Nevertheless, understanding the Li+ conduction mechanism is limited because of the complex amorphous nature of the glass. Herein, we present an ab initio molecular dynamics study of the 70Li2S-30P2S5 glass using a long simulation run (800 ps), improving the general understanding of its structure, dynamics, and electronic polarizability by comparing the results to those of the Li7P3S11 crystals. The shape difference of the P2S74- between the Li7P3S11 crystal and 70Li2S-30P2S5 glass is clearly observed in P-S-P bond angle, indicating that the P2S74- units in the 70Li2S-30P2S5 glass are relatively free from stress for crystallographic ordering. From the Li trajectories for 800 ps, the diffusion within the limited space in the unit cell is derived as the effective porosity. The lower effective porosity in the 70Li2S-30P2S5 glass compared to the Li7P3S11 crystal implies that a part of volume in the 70Li2S-30P2S5 glass cannot contribute to Li+ conduction. This reduction is attributed to the rotational motion of PS43-, which is observed only in the 70Li2S-30P2S5 glass. The sulfur polarizability is thoroughly analyzed for isotropic and anisotropic span through the Born effective charge tensor. The uniformly anisotropic polarizability of sulfur in the 70Li2S-30P2S5 glass is a characteristic property, which cannot create fast Li+ conduction paths as that in Li7P3S11 crystal.

Effect of Elevated Temperatures on the States of Water and Their Correlation with the Proton Conductivity of Nafion.

For the first time, we report the effects of elevated temperatures, from 80 to 100 °C, on the changes in the states of water and ion-water channels and their correlation with the proton conductivity of Nafion NR212, which was investigated using a Fourier transform infrared spectroscopy study. Experimentally, three types of water aggregates, protonated water (H+(H2O) n ), nonprotonated hydrogen (H)-bonded water (H2O···H2O), and non-H-bonded water, were found in Nafion, and the existence of those three types of water was confirmed through ab initio molecular dynamics simulation. We found that the proton conductivity of Nafion increased for up to 80 °C, but from 80 to 100 °C, the conductivity did not increase; rather, all of those elevated temperatures showed identical conductivity values. The proton conductivities at lower relative humidities (RHs) (up to 50%) remained nearly identical for all elevated temperatures (80, 90, and 100 °C); however, from 60% RH (over λ = 4), the conductivity remarkably jumped for all elevated temperatures. The results indicated that the amount of randomly arranged water gradually increased and created more H-bonded water networks in Nafion at above 60% RH. From the deconvolution of the O-H bending band, it was found that the volume fraction f i (i=each deconvoluted band) of H-bonded water for elevated temperatures (>80-100 °C) increased remarkably higher than for 60 °C.

Ab Initio Molecular Dynamics Simulations and GIPAW NMR Calculations of a Lithium Borate Glass Melt.

The atomic structure of a molten 0.3Li2O-0.7B2O3 glass at 1250 K was investigated using ab initio molecular dynamics (AIMD) simulations. The gauge including projector augmented wave (GIPAW) method was then employed for computing the chemical shift and quadrupolar coupling constant of (11)B, (17)O, and (7)Li from 764 AIMD derived structures. The chemical shift and quadrupolar coupling constant distributions were directly estimated from the dynamical structure of the molten glass. (11)B NMR parameters of well-known structural units such as the three-coordinated ring, nonring, and four-coordinated tetrahedron were found to be in good agreement with the experimental results. In this study, more detailed classification of B units was presented based on the number of O species bonded to the B atoms. This highlights the limitations of (11)B NMR sensitivity for resolving (11)B local environment using the experimentally obtained spectra only. The (17)O NMR parameter distributions can theoretically resolve the bridging and nonbridging O atoms with different structural units such as nonring, single boroxol ring, and double boroxol ring. Slight but clear differences in the number of bridging O atoms surrounding Li that have not been reported experimentally were observed in the theoretically obtained (7)Li NMR parameters.

An adaptive finite-element method for large-scale ab initio molecular dynamics simulations.

We present the current status of the finite-element method for large-scale atomistic simulations based on the density-functional theory. After a brief overview of our formulation, we describe recent developments, including the optimal choice of adaptive coordinates, an efficient implementation of the ground-state calculations, and a remedy for the eggbox effect. As a new application of our formulation, we present ab initio molecular dynamics simulations on sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (SPPBP), which is a typical example of polymer electrolyte membranes for fuel cells.

Ab initio studies on the proton dissociation and infrared spectra of sulfonated poly(ether ether ketone) (SPEEK) membranes.

SPEEK is known to possess high proton conductivity at high water content, being comparable with other popular membranes used in fuel cells, such as Nafion and sulfonated polyethersulfone (SPES). However, much less is known about its fundamental properties, including the status of proton dissociation and spectral features. In this work, the properties of two model molecules of SPEEK, M1 (20 atoms), M2 (50 atoms) and their hydrated systems, M1 + nH2O and M2 + nH2O (n = 1-9), have been investigated using static electronic structure calculations and the ab initio molecular dynamics (MD) method. Optimized structures for all of the systems and the trajectories of M1 + nH2O (n = 3-6) at finite temperatures have been computed using density functional theory at the B3LYP level of theory. Proton dissociation has been discussed in detail, especially for n = 3 and n = 4. In addition, the infrared spectra of SPEEK and its hydrated systems have been studied using a combination of theory and experiment. The characteristic bands of SPEEK and the surrounding water clusters have been assigned with emphasis on their relationship with the degree of proton dissociation. We have found that the hydronium ion stretching modes, which appear in the 2000-3000 cm(-1) region in static electronic structure calculations, are not observed experimentally. This discrepancy is explained by the stationary structure and the temperature effect.

First-principles molecular dynamics simulation and conductivity measurements of a molten xLi2O-(1- x)B2O3 system.

The electronic properties and atomic structure of a molten xLi2O-(1 - x)B2O3 system were investigated by measuring conductivity and using first-principles molecular dynamics (MD) simulations. The conductivities obtained were converted to a Li self-diffusion coefficient Dσ, using the Nernst-Einstein equation to assess charge transfer mechanisms. Dσ was compared with a Li self-diffusion coefficient, DNMR, which we measured in a previous study using high-temperature pulsed field gradient NMR. The DNMR/Dσ of xLi2O-(1 - x)B2O3 (0.2 ≤ x ≤ 0.5) at 1250 K ranged from 2.5 to 3.2, following the same trend as room temperature ionic liquids. First-principles MD simulations were performed using our own finite element density functional theory code, FEMTECK (finite element method-based total energy calculation) for molten xLi2O-(1 - x)B2O3 systems at 1250 K. We found that the O-B-O angle distribution functions were characterized by a peak at approximately 120°. Although the electron number from the electronic radial distribution function was arbitrary with regard to the cutoff distance, the net Li charge calculated from the integrated electron number surrounding Li was approximately 0.9 at 0.085 nm. The mean square displacement (MSD) of Li as a function of time was evaluated from the atomic configuration. Li self-diffusion coefficients calculated from the MSD were in better agreement with experimental results than they were using classical MD.

Ab initio mass tensor molecular dynamics.

Mass tensor molecular dynamics method was first introduced by Bennett [J. Comput. Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of generalized atomic masses. Here, we show how to apply this method to ab initio molecular dynamics simulations with minimal computational overhead. Test calculations on liquid water show a threefold reduction in computational effort without making the fixed geometry approximation. We also present a simple recipe for estimating the optimal atomic masses using only the first derivatives of the potential energy.

An ab initio modeling study on a modeled hydrated polymer electrolyte membrane, sulfonated polyethersulfone (SPES).

The nature of proton dynamics as well as a pendant side chain's ability for proton dissociation and capture in low-hydration sulfonated polyethersulfone (SPES) (lambda = 2, 4) have been studied theoretically by means of quantum chemical calculations and first-principles molecular dynamics simulations. A detailed comparison of results on SPES with those on Nafion has been made. It is found that the sulfonic groups of Nafion tend to dissociate protons more easily than do those of SPES. Hydration by four water molecules allows the dissociation of a proton from the sulfonic groups in both SPES and Nafion. The results of the first-principles MD simulations on SPES show that the nature of proton transfer kinetics for both hydration levels is very similar. Compared with low-hydration Nafion, however, hydration around the sulfonic groups in SPES is not sufficient to fully dissociate protons from the sulfonic groups, which results from the fact that some of the water molecules participate in hydrating SO(2) groups in SPES rather than SO(3)(-). Such a feature affects the performance of SPES under low-hydration conditions.

Nature of proton dynamics in a polymer electrolyte membrane, nafion: a first-principles molecular dynamics study.

First-principles molecular dynamics simulations have been carried out to investigate the nature of proton dynamics in Nafion, a representative polymer electrolyte membrane (PEM) widely used in PEM fuel cells. From the trajectories of the simulations, diffusion coefficients for the protonic defects were calculated to be 0.3 x 10(-5) cm(2) s(-1) and 7.1 x 10(-5) cm(2) s(-1) for lambda = 4.25 and 12.75, respectively, where lambda denotes hydration levels inside Nafion defined as a number of water molecules per sulfonic group. Our simulations show that proton hopping probability does not depend much on the water content inside Nafion. This finding indicates that the classical vehicular (or en masse) diffusion model, which has been employed to account for the slow diffusion process of protons in low water-content Nafion, is an oversimplification and does not correctly describe proton dynamics. Furthermore, it is found that difference in the value of the proton diffusion coefficient with respect to water content inside Nafion is related to the different character of proton hopping occurring in the water hydrogen bond network. When the water content is low, the proton hopping occurs in a manner that does not contribute constructively to proton mobility, while when the water content is high, it occurs in a manner which is beneficial to overall proton mobility. Such a different nature of proton hoppings arises mainly from the difference in the connectivity of water hydrogen bond network. Our results broadly support earlier simulation studies and provide the molecular level origin of properties arising from the proton dynamics in Nafion.

Nature of water transport and electro-osmosis in nafion: insights from first-principles molecular dynamics simulations under an electric field.

The effects of water content on water transport and electro-osmosis in a representative polymer electrolyte membrane, Nafion, are investigated in detail by means of first-principles molecular dynamics (MD) simulations in the presence of a homogeneous electric field. We have directly evaluated electro-osmotic drag coefficients (the number of water molecules cotransported with proton conduction) from the trajectories of the first-principles MD simulations and also explicitly evaluated factors that contribute to the electro-osmotic drag coefficients. In agreement with previously reported experiments, our calculations show virtually constant values ( approximately 1) of the electro-osmotic drag coefficients for both low and high water content states. Detailed comparisons of each factor contributing to the drag coefficient reveal that an increase in water content increases the occurrence of the Grotthuss-like effective proton transport process, whose contribution results in a decrease in the electro-osmotic drag coefficient. At the same time, an environment that is favorable for the Grotthuss-like effective proton transport process is also favorable for the transport of water arising from water transport occurring beyond the hydration shell around the protons, whose contribution results in an increase in the electro-osmotic drag coefficient. Conversely, an environment that is not favorable for proton conduction is also not favorable for water transport. As a result, the electro-osmotic drag coefficient shows virtually identical values with respect to change in the water content.

First-principles molecular dynamics study on aqueous sulfuric acid solutions.

The properties of aqueous sulfuric acid have been studied employing density functional theory-based molecular dynamics simulations in conjunction with norm-conserving pseudopotentials. The simulations were carried out for two different concentrations whose molar concentrations were fixed at 0.84 and 10.2 mol/l. The structural features of aqueous sulfuric acid solutions show a strong dependency on the concentration. The Grötthuss-type proton transfer mechanism is not effectively operative at the higher concentration because of the broken hydrogen bond network of water induced by ions generated by the dissociation of sulfuric acid. In addition, to evaluate electrical properties, we carried out a simulation that takes an electric field into account. Results are compared with those of the simulation undertaken with no external electric field.

Synonymy of Calyptogena solidissima with Calyptogena kawamurai (Bivalvia: Vesicomyidae) and its population structure revealed by mitochondrial DNA sequences.

Nucleotide sequences of part (1,101 bp) of the mitochondrial cytochrome oxidase c subunit I (COI) gene were determined for two specimens of Calyptogena kawamurai collected in Kashima Nada and Suruga Bay, respectively. These sequences were identical to each other and to those from many individuals of Calyptogena solidissima, i.e., 11 of 12 specimens from a seep area in Nankai Trough, two of 20 from hydrothermal-vent fields in Okinawa Trough, and one of 14 from a seep area on Kuroshima Knoll. The nucleotide sequences of the 5' part (about 700 bp) of the first internal transcribed spacer (ITS-1) also showed a close relationship between C. kawamurai and C. solidissima. The radiating threads on the shell surface that were emphasized in describing C. solidissima are not consistent throughout these local populations. Variation in cardinal dentition was confirmed to be intraspecific by observations of a series of specimens. The shell length-height and shell length-width relationships of both species all fit a single regression line. These results suggest that C. solidissima is a junior synonym of C. kawamurai. The populations of Nankai Trough, Okinawa Trough, and Kuroshima Knoll were shown to be diverging genetically from each other. Populations of Okinawa Trough and Kuroshima Knoll are suggested to have derived independently from the most common haplotype of Nankai Trough.

Ab initio molecular-dynamics study of liquid formamide.

Properties of neat liquid formamide (HCONH2) have been studied by the combination of gradient-corrected density-functional theory, norm-conserving pseudopotentials, and the adaptive finite-element method. The structural and dynamical quantities have been calculated through molecular dynamics simulations under the Born-Oppenheimer approximation. Satisfactory agreement with experimental data was obtained for both intramolecular and intermolecular properties. Our results are also compared with those of the empirical potential functions to clarify their accuracies.