A methodology for performing phase-field simulations of plating and stripping in the presence of a solid electrolyte interphase (SEI) is presented and applied to a lithium metal electrode. Material point method (MPM) simulations are performed assuming a homogeneous SEI layer and are compared with experiment. Results are consistent with experiment for two electrolytes and confirm the dominance of the SEI layer in determining cell impedance. Notably, in some instances, the SEI potential drop greatly affected activation overpotential, differing from the applied potential. To accurately depict stripping current vs potential for SEI-free systems, the model considered non-ideal electrolyte effects: concentration-dependent salt activity coefficient, salt diffusion coefficient, and electrolyte conductivity. Conversely, systems with SEI layers displayed minimal non-ideal concentration-related electrolyte effects due to impedance originating primarily from the SEI. In plating scenarios, low SEI salt concentration negated the need for non-ideal SEI effects. However, for stripping, non-ideal salt-concentration dependent SEI effects were crucial in reproducing experimental behavior, owing to high salt concentration at the electrode/SEI interface.

Quasi-static uniaxial tension and compression, three-point bending, and Hopkinson bar measurements have been carried out on low-porosity Al/Ni compacts (pressed powders) as well as compacts of the constituent Al and Ni. Concurrently, mesoscale material point method (MPM) simulations of fully resolved Al, Ni, and Al/Ni compact microstructures and engineering scale MPM simulations of homogenized Al/Ni compacts have been conducted. These simulations, in conjunction with the experimental measurements, have allowed us to parameterize mesoscale particle/particle contact models and engineering scale anisotropic damage mechanics models for the materials. The goal of this coordinated experimental and simulation study of Al/Ni and its constituent materials was to better understand their mechanical properties and gain insight into the nature of Al–Ni and Al–Al interfaces in Al/Ni compacts and their impact on mechanical properties of the compacts. We found that brittle tensile failure of Al/Ni compacts was dominated by the strength of the Al–Al interface while ductile compressive behavior of Al/Ni compacts was largely determined by the behavior of the Al phase. The properties of both Al–Al interface and Al phase appear to be altered by the presence of Ni. Finally, it was found that softening resulting from shear damage offset underlying strain hardening of the Al and Ni phases during compressive loading of the Al/Ni compacts.

The material point method (MPM) is increasingly being used in multimaterial contact problems. Despite MPM’s advantages for such problems, some aspects of current contact methods are prone to instabilities. The main causes of instabilities are errors in the methods used to calculate normal vector and material separation. This paper presents new methods for these tasks by applying logistic regression to material point clouds contributing to each contact node. The new logistic regression process determines the most probable plane between two materials and finds their separation. The separation calculation explicitly accounts for particle deformation for increased accuracy in large deformation problems. Another improvement to multimaterial MPM is to do contact mechanics corrections twice each time step — first after initial velocity extrapolation to the grid and second after updating grid momenta. Two corrections are needed to find the correct acceleration when updating particle velocities and to extend methods that filter null-space noise to account for contact mechanics. Several examples validate the new methods and demonstrate their advantages over prior methods.

Molecular dynamics simulations of reactions in Al/Ni layered systems have been carried out under isothermal conditions for a wide range of temperatures and several system sizes. An embedded atom method potential, known to reasonably reproduce the phase behavior of Al/Ni, was employed. Simulations revealed reaction mechanisms involving an initial fast process and much slower more complex longer-time reactions. The initial reaction process consists of diffusion of Ni from the pure solid Ni phase into the molten Al phase, resulting in the formation of an Al-rich Al/Ni liquid. The initial reaction ends when the Al/Ni liquid becomes saturated in Ni and solid Al/Ni phases begin to form at the interfaces between the pure solid Ni phase and the Al/Ni liquid. The growth of these solid phases is intrinsically slow compared to the formation of the liquid and is further slowed by the need for Ni to diffuse through the growing interfacial Al/Ni solid phases. Analysis of the initial Al/Ni liquid forming process indicates Fickian behavior with the Ni diffusion coefficient exhibiting Arrhenius temperature dependence. The longer-time slow reaction process(es) resulting in the growth of Al/Ni solid phases do not lend themselves to detailed numerical analysis because of the complex dependence of the Ni transport on the detailed nature of the interfacial layers.

Material point method (MPM) simulations of a model reactive structure material (RSM) have been performed at the mesoscale. The MPM methodology employed allows us to generate representative microstructures of the RSM comprised of highly compacted powders of aluminum and nickel. MPM simulations of the RSM microstructures under shock and compressive loading utilizing EOS and constitutive models for the component materials as well as advanced contact models for particle-particle interactions were performed, allowing us to predict the mechanical and thermal behavior of the RSMs for various loading scenarios. The role of voids and particle-particle friction on the mechanical and thermal response of Al/Ni and the potential for mechanical loading (e.g., shock and impact) to initiate intermetallic reactions between aluminum and nickel are discussed.

Continuum level Material Point Method (MPM) simulations have been carried out on low porosity (around 90% or greater of theoretical maximum density) microstructures of Al/Ni granular composites (Al/Ni). The Al/Ni microstructures were subjected to uniaxial (strain) compressive loading and shock compression up to 25 GPa. The MPM model accounted for frictional heating between grains in addition to plastic work and compressional heating effects. The distributions of stresses and temperature in the composite materials were found to be highly heterogeneous due to the heterogeneous nature of the composite microstructure. The manner in which interfaces between grains were treated (sliding vs. non-sliding) was found to influence both mechanical and thermal responses to loading. Plastic deformation, mechanical work, and grain/grain frictional effects led to modest increases of mean temperature and local hotspots with maximum temperature not higher than 800–850 K for loadings investigated. For all scenarios investigated, heat generation was insufficient to cause local Al melting believed to be a precursor for shock initiation.

Morphometric assessments, such as muscle density and body fat distribution, have emerged as strong predictors of cardiovascular risk and postoperative morbidity and mortality. To date, no study has examined morphometric mortality risk prediction among kidney transplant ( KT ) candidates. KT candidates, waitlisted 2008‐2009, were identified (n=96) and followed to the earliest of transplant, death, or administrative end of study. Morphometric measures, including abdominal adipose tissue, paraspinous and psoas muscle composition, and aortic calcification, were measured from CT s. Risk of waitlist mortality was examined using Cox proportional hazard regression. On adjusted analyses, radiologic measures remained independently and significantly associated with lower waitlist mortality; the addition of radiologic measures significantly improved model predictive ability over models containing traditional risk factors alone (net reclassification index: 0.56, 95% CI : 0.31‐0.75). Higher psoas muscle attenuation (indicative of leaner muscle) was associated with decreased risk of death ( aHR : 0.93, 95% CI : 0.91‐0.96, P <.001), and for each unit increase in lean paraspinous volume, there was an associated 2% decreased risk for death ( aHR : 0.98, 95% CI : 0.96‐0.99, P =.03). Radiologic measures of lean muscle mass, such as psoas muscle attenuation and paraspinous lean volume, may improve waitlist mortality risk prediction and candidate selection.

Submitted for the MAR17 Meeting of The American Physical Society Chemical substrate transfer of topological insulator thin films for novel characterization and interfaces1 GRANT SMITH, ANTHONY RICHARDELLA, NITIN SAMARTH, Pennsylvania State University — Molecular beam epitaxy (MBE) is a widely used technique for synthesizing wafer scale samples of bismuth chalcogenide topological insulator (TI) thin films. Importantly, the deposition technique allows the fabrication of heterostructures wherein a TI is interfaced with symmetry breaking phases of matter such as ferromagnets, antiferromagnets and superconductors. However, the MBE of TI films on such substrates yields films of widely varying structural quality.1 An alternative approach is to grow a TI film with optimized structural quality on an appropriate substrate and then lift it off for transfer onto an arbitrary substrate of interest [Bansal et al., Nano Lett. 14, 1343 (2014)]. This method vastly expands the range of possible TI heterostructures. We describe experiments wherein large area MBE-grown TI thin films are lifted off from sapphire substrates and then transferred onto other materials such as ferromagnetic insulators. These samples are characterized using electrical transport measurements, atomic force microscopy, and x-ray diffraction. We also describe plan view transmission electron microscopy (TEM) of the TI films by transfer to TEM grids, as well as attempts to fabricated TI thin films suspended over deep valleys in a substrate. 1This work is funded by the The Pennsylvania State University Two-Dimensional Crystal Consortium Materials Innovation Platform (2DCC-MIP) which is supported by NSF cooperative agreement DMR-1539916. Grant Smith Pennsylvania State University Date submitted: 11 Nov 2016 Electronic form version 1.4

Molecular dynamics simulations of N-methyl-N-propylpyrrolidinium (pyr(13)) bis(trifluoromethanesulfonyl)imide (Ntf(2)) ionic liquid [pyr(13)][Ntf(2)] doped with [Li][Ntf(2)] salt and mixed with acetonitrile (AN) and ethylene carbonate (EC) organic solvents were conducted using polarizable force field. Structural and transport properties of ionic liquid electrolytes (ILEs) with 20 and 40 mol % of organic solvents have been investigated and compared to properties of neat ILEs. Addition of AN and EC solvents to ILEs resulted in the partial displacement of the Ntf2 anions from the Li+ first coordination shell by EC and AN and shifting the LiNtf(2) coordination from bidentate to monodentate. The presence of organic solvents in ILE has increased the ion mobility, with the largest effect observed for the Li+ cation. The Li+ conductivity has doubled with addition of 40 mol % of AN. The Li(+)NNtf(2) residence times were dramatically reduced with addition of solvents, indicating an increasing contribution from structural diffusion of the Li+ cations.

The influence of low-molecular-weight poly(ethylene glycol) (PEG, Mw ≈ 550 Da) plasticizers on the rheology and ion-transport properties of fluorosulfonimide-based polyether ionic melt (IM) electrolytes has been investigated experimentally and via molecular dynamics (MD) simulations. Addition of PEG plasticizer to samples of IM electrolytes caused a decrease in electrolyte viscosity coupled to an increase in ionic conductivity. MD simulations revealed that addition of plasticizer increased self-diffusion coefficients for both cations and anions with the plasticizer being the fastest diffusing species. Application of a VTF model to fit variable-temperature conductivity and fluidity data shows that plasticization decreases the apparent activation energy (Ea) and pre-exponential factor A for ion transport and also for viscous flow. Increased ionic conductivity with plasticization is thought to reflect a combination of factors including lower viscosity and faster polyether chain segmental dynamics in the electrolyte, coupled with a change in the ion transport mechanism to favor ion solvation and transport by polyethers derived from the plasticizer. Current interrupt experiments with Li/electrolyte/Li cells revealed evidence for salt concentration polarization in electrolytes containing large amounts of plasticizer but not in electrolytes without added plasticizer.

Molecular dynamics (MD) simulations of mixtures of the room temperature ionic liquids (ILs) 1-butyl-4-methyl imidazolium [BMIM]/dicyanoamide [DCA] and [BMIM][NO3(-)] with HNO3 have been performed utilizing the polarizable, quantum chemistry based APPLE&P(®) potential. Experimentally it has been observed that [BMIM][DCA] exhibits hypergolic behavior when mixed with HNO3 while [BMIM][NO3(-)] does not. The structural, thermodynamic, and transport properties of the IL/HNO3 mixtures have been determined from equilibrium MD simulations over the entire composition range (pure IL to pure HNO3) based on bulk simulations. Additional (non-equilibrium) simulations of the composition profile for IL/HNO3 interfaces as a function of time have been utilized to estimate the composition dependent mutual diffusion coefficients for the mixtures. The latter have been employed in continuum-level simulations in order to examine the nature (composition and width) of the IL/HNO3 interfaces on the millisecond time scale.

We have conducted quantum chemistry calculations and gas- and solution-phase reactive molecular dynamics simulation studies of reactions involving the ethylene carbonate (EC) radical anion EC(-) using the reactive force field ReaxFF. Our studies reveal that the substantial barrier for transition from the closed (cyclic) form, denoted c-EC(-), of the radical anion to the linear (open) form, denoted o-EC(-), results in a relatively long lifetime of the c-EC(-) allowing this compound to react with other singly reduced alkyl carbonates. Using ReaxFF, we systematically investigate the fate of both c-EC(-) and o-EC(-) in the gas phase and EC solution. In the gas phase and EC solutions with a relatively low concentration of Li(+)/x-EC(-) (where x = o or c), radical termination reactions between radical pairs to form either dilithium butylene dicarbonate (CH(2)CH(2)OCO(2)Li)(2) (by reacting two Li(+)/o-EC(-)) or ester-carbonate compound (by reacting Li(+)/o-EC(-) with Li(+)/c-EC(-)) are observed. At higher concentrations of Li(+)/x-EC(-) in solution, we observe the formation of diradicals which subsequently lead to formation of longer alkyl carbonates oligomers through reaction with other radicals or, in some cases, formation of (CH(2)OCO(2)Li)(2) through elimination of C(2)H(4). We conclude that the local ionic concentration is important in determining the fate of x-EC(-) and that the reaction of c-EC(-) with o-EC(-) may compete with the formation of various alkyl carbonates from o-EC(-)/o-EC(-) reactions.

Electrostatic double-layer capacitors (EDLCs) with room-temperature ionic liquids (RTILs) as electrolytes are among the most promising energy storage technologies. Utilizing atomistic molecular dynamics simulations, we demonstrate that the capacitance and energy density stored within the electric double layers (EDLs) formed at the electrode-RTIL electrolyte interface can be significantly improved by tuning the nanopatterning of the electrode surface. Significantly increased values and complex dependence of differential capacitance on applied potential were observed for surface patterns having dimensions similar to the ions' dimensions. Electrode surfaces patterned with rough edges promote ion separation in the EDL at lower potentials and therefore result in increased capacitance. The observed trends, which are not accounted for by the current basic EDL theories, provide a potentially new route for optimizing electrode structure for specific electrolytes.

Molecular dynamics simulations of N-methyl-N-propylpyrrolidinium (pyr(13)) bis(trifluoromethanesulfonyl)imide (Ntf(2)) ionic liquid [pyr(13)][Ntf(2)] mixed with [Li][Ntf(2)] salt have been conducted using a polarizable force field. Mixture simulations with lithium salt mole fractions between 0% and 33% at 363 and 423 K yield densities, ion self-diffusion coefficients, and ionic conductivities in very good agreement with available experimental data. In all investigated electrolytes, each Li(+) cation was found to be coordinated, on average, by 4.1 oxygen atoms from surrounding anions. At lower concentrations (x ≤ 0.20), the Li(+) cation was found to be, on average, coordinated by slightly more than three Ntf(2) anions with two anions contributing a single oxygen atom and one anion contributing two oxygen atoms to Li(+) coordination. At the highest [Li][Ntf(2)] concentration, however, there were, on average, 3.5 anions coordinating each Li(+) cation, corresponding to fewer bidendate and more monodentate anions in the Li(+) coordination sphere. This trend is due to increased sharing of anions by Li(+) at higher salt concentrations. In the [pyr(13)][Ntf(2)]/[Li][Ntf(2)] electrolytes, the ion diffusivity is significantly smaller than that in organic liquid electrolytes due to not only the greater viscosity of the solvent but also the formation of clusters resulting from sharing of anions by Li(+) cations. The ionic conductivity of the electrolytes was found to decrease with increasing salt concentration, with the effect being greater at the higher temperature. Finally, we found that the contribution of Li(+) to ionic conductivity does not increase proportionally to Li(+) concentration but saturates at higher doping levels.

Atomistic molecular dynamics simulations were performed on 1-butyl-3-methyl-imidazolium azide [bmim][N(3)], 1-butyl-2,3-dimethylimidazolium azide [bmmim][N(3)], and 1-butynyl-3-methyl-imidazolium azide [bumim][N(3)] ionic liquids. The many-body polarizable APPLE&P force field was augmented with parameters for the azide anion and the bumim cation. Good agreement between the experimentally determined and simulated crystal structure of [bumim][N(3)] as well as the liquid-state density and ionic conductivity of [bmmim][N(3)] were found. Methylation of bmim (yielding bmmim) resulted in dramatic changes in ion structuring in the liquid and slowing of ion motion. Conversely, replacing the butyl group of bmim with the smaller 2-butynyl group resulted in an increase of ion dynamics.