John W. Lawson

Computational and experimental investigation of Li-doped ionic liquid electrolytes: [pyr14][TFSI], [pyr13][FSI], and [EMIM][BF4].

We employ molecular dynamics (MD) simulation and experiment to investigate the structure, thermodynamics, and transport of N-methyl-N-butylpyrrolidinium bis(trifluoromethylsufonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosufonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]), as a function of Li-salt mole fraction (0.05 ≤ xLi(+) ≤ 0.33) and temperature (298 K ≤ T ≤ 393 K). Structurally, Li(+) is shown to be solvated by three anion neighbors in [pyr14][TFSI] and four anion neighbors in both [pyr13][FSI] and [EMIM][BF4], and at all levels of xLi(+) we find the presence of lithium aggregates. Pulsed field gradient spin-echo NMR measurements of diffusion and electrochemical impedance spectroscopy measurements of ionic conductivity are made for the neat ionic liquids as well as 0.5 molal solutions of Li-salt in the ionic liquids. Bulk ionic liquid properties (density, diffusion, viscosity, and ionic conductivity) are obtained with MD simulations and show excellent agreement with experiment. While the diffusion exhibits a systematic decrease with increasing xLi(+), the contribution of Li(+) to ionic conductivity increases until reaching a saturation doping level of xLi(+) = 0.10. Comparatively, the Li(+) conductivity of [pyr14][TFSI] is an order of magnitude lower than that of the other liquids, which range between 0.1 and 0.3 mS/cm. Our transport results also demonstrate the necessity of long MD simulation runs (∼200 ns) to converge transport properties at room temperature. The differences in Li(+) transport are reflected in the residence times of Li(+) with the anions (τ(Li/-)), which are revealed to be much larger for [pyr14][TFSI] (up to 100 ns at the highest doping levels) than in either [EMIM][BF4] or [pyr13][FSI]. Finally, to comment on the relative kinetics of Li(+) transport in each liquid, we find that while the net motion of Li(+) with its solvation shell (vehicular) significantly contributes to net diffusion in all liquids, the importance of transport through anion exchange increases at high xLi(+) and in liquids with large anions.

Comparison of ReaxFF, DFTB, and DFT for phenolic pyrolysis. 1. Molecular dynamics simulations.

A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (~30%) and small C(m)H(n)O molecules (~70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (~60%) and fused carbon rings (~70%) relative to DFT. Heterocyclic oxygen-containing five- and six-membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.

Transport in Molecular Junctions with Different Metallic Contacts

Ab initio calculations of phenyl dithiol connected to Au, Ag, Pd, and Pt electrodes are performed using non-equilibrium Greens functions and density functional theory. For each metal, the properties of the molecular junction are considered both in equilibrium and under bias. In particular, we consider in detail charge transfer, changes in the electrostatic potential, and their subsequent effects on the IV curves through the junctions. Gold is typically used in molecular junctions because it forms strong chemical bonds with sulfur. We find however that Pt and Pd make better electrical contacts than Au. The zero-bias conductance is found to be greatest for Pt, followed by Pd, Au, and then Ag.

Comparison of ReaxFF, DFTB, and DFT for phenolic pyrolysis. 2. Elementary reaction paths.

Reaction paths for the loss of CO, H2, and H2O from atomistic models of phenolic resin are determined using the hybrid B3LYP approach. B3LYP energetics are confirmed using CCSD(T). The energetics along the B3LYP paths are also evaluated using the PW91 generalized gradient approximation (GGA), the more approximate self-consistent charge density functional tight binding (SCC-DFTB), and the reactive force field (ReaxFF). Compared with the CCSD(T)/cc-pVTZ level for bond and reaction energies and barrier heights, the B3LYP, PW91, DFTB(mio), DFTB(pbc), and ReaxFF have average absolute errors of 3.8, 5.1, 17.4, 13.2, and 19.6 kcal/mol, respectively. The PW91 is only slightly less accurate than the B3LYP approach, while the more approximate approaches yield somewhat larger errors. The SCC-DFTB paths are in better agreement with B3LYP than are those obtained with ReaxFF.

Ab Initio Simulations and Electronic Structure of Lithium-Doped Ionic Liquids: Structure, Transport, and Electrochemical Stability

Density functional theory (DFT), density functional theory molecular dynamics (DFT-MD), and classical molecular dynamics using polarizable force fields (PFF-MD) are employed to evaluate the influence of Li(+) on the structure, transport, and electrochemical stability of three potential ionic liquid electrolytes: N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([pyr14][TFSI]), N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide ([pyr13][FSI]), and 1-ethyl-3-methylimidazolium boron tetrafluoride ([EMIM][BF4]). We characterize the Li(+) solvation shell through DFT computations of [Li(Anion)n]((n-1)-) clusters, DFT-MD simulations of isolated Li(+) in small ionic liquid systems, and PFF-MD simulations with high Li-doping levels in large ionic liquid systems. At low levels of Li-salt doping, highly stable solvation shells having two to three anions are seen in both [pyr14][TFSI] and [pyr13][FSI], whereas solvation shells with four anions dominate in [EMIM][BF4]. At higher levels of doping, we find the formation of complex Li-network structures that increase the frequency of four anion-coordinated solvation shells. A comparison of computational and experimental Raman spectra for a wide range of [Li(Anion)n]((n-1)-) clusters shows that our proposed structures are consistent with experiment. We then compute the ion diffusion coefficients and find measures from small-cell DFT-MD simulations to be the correct order of magnitude, but influenced by small system size and short simulation length. Correcting for these errors with complementary PFF-MD simulations, we find DFT-MD measures to be in close agreement with experiment. Finally, we compute electrochemical windows from DFT computations on isolated ions, interacting cation/anion pairs, and liquid-phase systems with Li-doping. For the molecular-level computations, we generally find the difference between ionization energy and electron affinity from isolated ions and interacting cation/anion pairs to provide upper and lower bounds, respectively, to experiment. In the liquid phase, we find the difference between the lowest unoccupied and highest occupied electronic levels in pure and hybrid functionals to provide lower and upper bounds, respectively, to experiment. Li-doping in the liquid-phase systems results in electrochemical windows little changed from the neat systems.

Structure and energetics of Li(+)-(BF4(-))n, Li(+)-(FSI(-))n, and Li(+)-(TFSI(-))n: ab initio and polarizable force field approaches.

The Li(+)-BF4(-) and BF4(-)-BF4(-) interactions are studied using second order perturbation theory (MP2) and coupled cluster singles and doubles approach, including the effect of connected triples, CCSD(T). The MP2 and CCSD(T) results are in excellent agreement. Using only the MP2 approach, the interactions of Li(+) with bis(trifluoromethane)sulfonimide anion (TFSI) and Li(+) with bis(fluorosulfonyl)imide anion (FSI) are studied. The results of these high level calculations are compared with density functional theory (DFT) calculations for a variety of functionals and with the APPLE&P force field. The B3LYP approach well reproduces the accurate calculations using both a small and large basis set. The M06 and M06L functionals in the larger basis set are in good agreement with the high level calculations. While the APPLE&P force field does not outperform the best functionals, the APPLE&P results agree better with the accurate results than do some of the functionals tested.

Lattice Thermal Conductivity of Ultra High Temperature Ceramics ZrB2 and HfB2 from Atomistic Simulations

Atomistic Green-Kubo simulations are performed to evaluate the lattice thermal conductivity for single crystals of the ultra high temperature ceramics ZrB2 and HfB2. Recently developed interatomic potentials are used for these simulations. Heat current correlation functions show rapid oscillations, which can be identified with mixed metal-Boron optical phonon modes. Results for temperatures from 300K to 1000K are presented.

On the current flow for benzene-1,4-dithiol between two Au contacts

The current flow for a benzene-1,4-dithiol molecule between two Au contacts is computed. For most configurations considered, the computed current is an order of magnitude larger than experiment, as found by other groups. When the benzene-1,4-dithiol is bonded to the top of an fcc cluster of Au atoms, the current flow is significantly reduced, bringing it into better agreement with experiment. This represents an alternative to the previously proposed two molecule mechanism, as explanation of the experimental results.

Evaluation of molecular dynamics simulation methods for ionic liquid electric double layers

We investigate how systematically increasing the accuracy of various molecular dynamics modeling techniques influences the structure and capacitance of ionic liquid electric double layers (EDLs). The techniques probed concern long-range electrostatic interactions, electrode charging (constant charge versus constant potential conditions), and electrolyte polarizability. Our simulations are performed on a quasi-two-dimensional, or slab-like, model capacitor, which is composed of a polarizable ionic liquid electrolyte, [EMIM][BF4], interfaced between two graphite electrodes. To ensure an accurate representation of EDL differential capacitance, we derive new fluctuation formulas that resolve the differential capacitance as a function of electrode charge or electrode potential. The magnitude of differential capacitance shows sensitivity to different long-range electrostatic summation techniques, while the shape of differential capacitance is affected by charging technique and the polarizability of the electrolyte. For long-range summation techniques, errors in magnitude can be mitigated by employing two-dimensional or corrected three dimensional electrostatic summations, which led to electric fields that conform to those of a classical electrostatic parallel plate capacitor. With respect to charging, the changes in shape are a result of ions in the Stern layer (i.e., ions at the electrode surface) having a higher electrostatic affinity to constant potential electrodes than to constant charge electrodes. For electrolyte polarizability, shape changes originate from induced dipoles that soften the interaction of Stern layer ions with the electrode. The softening is traced to ion correlations vertical to the electrode surface that induce dipoles that oppose double layer formation. In general, our analysis indicates an accuracy dependent differential capacitance profile that transitions from the characteristic camel shape with coarser representations to a more diffuse profile with finer representations.

Recent Developments in Ultra High Temperature Ceramics at NASA Ames

NASA Ames is pursuing a variety of approaches to modify and control the microstructure of UHTCs with the goal of improving fracture toughness, oxidation resistance and controlling thermal conductivity. The overall goal is to produce materials that can perform reliably as sharp leading edges or nose tips in hypersonic reentry vehicles. Processing approaches include the use of preceramic polymers as the SiC source (as opposed to powder techniques), the addition of third phases to control grain growth and oxidation, and the use of processing techniques to produce high purity materials. Both hot pressing and field assisted sintering have been used to make UHTCs. Characterization of the mechanical and thermal properties of these materials is ongoing, as is arcjet testing to evaluate performance under simulated reentry conditions. The preceramic polymer approach has generated a microstructure in which elongated SiC grains grow in the form of an in-situ composite. This microstructure has the advantage of improving fracture toughness while potentially improving oxidation resistance by reducing the amount and interconnectivity of SiC in the material. Addition of third phases, such as Ir, results in a very fine-grained microstructure, even in hot-pressed samples. The results of processing and compositional changes on microstructure and properties are reported, along with selected arcjet results.