[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations of N-methyl-N-propylpyrrolidinium (pyr13) bis(trifluoromethanesulfonyl)imide (Ntf2) ionic liquid [pyr13][Ntf2] doped with [Li][Ntf2] 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 mol% 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 Li-Ntf2 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+- NNtf2 residence times were dramatically reduced with addition of solvents, indicating an increasing contribution from structural diffusion of the Li+ cations.
Full-text · Article · Jan 2015 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: The influence of low-molecular-weight polyethylene 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.
Full-text · Article · Apr 2014 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Sep 2013 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The double layer composition and structure of the mixed-solvent electrolyte tetramethylene sulfone/dimethyl carbonate (TMS/DMC) doped with LiPF6 near the graphite surface have been investigated using molecular dynamics simulations as a function of applied potential between the electrodes ranging from 0 to 6 V. Three solvent compositions, with TMS/DMC ratios of 1:2, 1:1, and 2:1 doped with LiPF6 salt, were investigated. At uncharged electrodes, electrolyte composition at the interfaces was found to be similar to that of bulk electrolyte for TMS/DMC ratios of 1:1 and 1:2 systems but deviated from the bulk for a TMS/DMC ratio of 2:1. At negative electrodes the polar solvent TMS preferentially adsorbs at the electrode surface displacing the almost nonpolar DMC solvent. The preferential partitioning of TMS relative to DMC to the negative electrode surface is consistent with the stronger binding of the former with Li+ that partitions to the anode surface as potential becomes more negative as well as with the ability of relatively polar TMS to better respond to the electrostatic potential near a charged surface. At the positive electrode, TMS/DMC ratios were found to be similar to bulk compositions that is different to the behavior observed in ethylene carbonate (EC)/DMC/LiPF6 electrolyte where preferential partitioning of a more polar EC molecule was observed on both electrodes. Our results also show that, in TMS/DMC/LiPF6 electrolyte, DMC is located approximately 0.8 Å further way from the positive electrode than in EC/DMC/LiPF6 indicating that it might be more difficult to oxidize DMC in the TMS-based electrolytes that is consistent with experimentally reported increased oxidative stability of the latter. Finally, changes of the Li+ solvation shell and double layer capacitance were analyzed as a function of electrode potential.
Full-text · Article · Oct 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Sep 2012 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · May 2012 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Mar 2012 · Journal of Physical Chemistry Letters
[Show abstract][Hide abstract] ABSTRACT: The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1-alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][TFSI], n = 2, 4, 6, and 8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within ±2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.
Full-text · Article · Mar 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: 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.
No preview · Article · Feb 2012 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: Using a soft, coarse-grained model and a Lennard-Jones bead-spring
model, we study the morphology of random block copolymers in the bulk
and in contact with a hard wall that preferentially attracts one
component. We show that both coarse-grained models yield similar
equilibrium morphologies at intermediate and long length scales, and
identify a mapping between the parameters of the two models. For most
parameters we observe a disordered, microemulsion-like morphology. We
study the single-chain dynamics in the bulk and in contact with a
preferential surface. The relaxation times of the soft, coarse-grained
model is about two orders of magnitude faster than the Lennard-Jones
bead-spring model. In both models the relaxation time increases with
segregation but the Lennard-Jones bead-spring model is additionally
slowed down by the densification of the local packing at low
temperatures. We employ the soft, coarse-grained model to generate
starting configurations for the bead-spring model. Then, the bead-spring
model is quenched below its glass transition temperature, and we
investigate the local mechanical properties of the disordered, yet
[Show abstract][Hide abstract] ABSTRACT: The equilibrium structure and ordering kinetics of random AB block copolymers is investigated using a Lennard-Jones bead–spring model and a soft, coarse-grained model. Upon increasing the incompatibility a disordered microemulsion-like structure is formed, whose length scale slightly increases with segregation. The structure factor of composition fluctuations, molecular conformations, single-chain dynamics and collective ordering kinetics are investigated as a function of the segregation between A and B blocks. The harsh repulsion of the Lennard-Jones potential gives rise to pronounced fluid-like packing effects that affect the liquid structure on the length scale of the bead size and, upon cooling and increase of the local density, result in an additional slowing down of the dynamics. The soft, coarse-grained model does not exhibit pronounced packing effects and the softness of the potential allows for a faster equilibration in computer simulation. The structure and dynamics of the two different models are quantitatively compared. The parameters of the soft, coarse-grained model are adjusted as to match the long-range structure of the bead–spring model, and it is demonstrated that the soft, coarse grained model can be utilized to generate starting configurations for the Lennard-Jones bead–spring model.
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics (MD) simulations of an electrolyte comprised of ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF6 salt near the basal face of graphite electrodes have been performed as a function of electrode potential. Upon charging of the electrodes, the less polar DMC molecule is partially replaced in the interfacial electrolyte layer by the more polar EC. At negative potentials, the carbonyl groups from the carbonate molecules are repelled from the surface, while at positive potentials, we find a substantial enrichment of the surface with carbonyl groups. PF6– rapidly accumulates at the positive electrode with increasing potential and vacates the negative electrode with increasing negative potential. In contrast, Li+ concentration in the interfacial layer is found to be only weakly dependent on potential except at very large negative potentials. Hence, both composition of the electrolyte at the electrode surface and solvent environment around Li+ are observed to vary dramatically with the applied potential with important implications for oxidation/reduction of the electrolyte and the process of Li+ intercalation/deintercation.
Full-text · Article · Jan 2012 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF(6)(-) and ClO(4)(-) anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant ε = 1) and employing the polarized continuum model with a dielectric constant ε = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF(6)(-) and ClO(4)(-) anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC-PF(6)(-) and PC-ClO(4)(-) were CO(2) and acetone radical. Formation of HF and PF(5) was observed upon the initial step of PC-PF(6)(-) oxidation while HClO(4) formed during initial oxidation of PC-ClO(4)(-). The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC-PF(6)(-) decomposition, while acetic acid, carboxylic acid anhydrides, and Cl(-) were found among the decomposition products of PC-ClO(4)(-). The decomposition pathways with the lowest barrier for the oxidized PC-PF(6)(-) and PC-ClO(4)(-) complexes did not result in the incorporation of the fluorine from PF(6)(-) or ClO(4)(-) into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC-PF(6)(-) oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions.
Full-text · Article · Dec 2011 · The Journal of Physical Chemistry A
[Show abstract][Hide abstract] ABSTRACT: Rheological evidence is provided demonstrating that covalent grafting of monodisperse isotactic poly(L-leucine) branches onto linear hyaluronan (HA) polysaccharide chains yields comb-branched HA chains that self-assemble into long-lived physical networks in aqueous solutions driven by hydrophobic interactions between poly(L-leucine) chains. This is in stark contrast to native (unmodified) HA solutions which exhibit no tendency to form long-lived physical networks.
Full-text · Article · Oct 2011 · European Polymer Journal
[Show abstract][Hide abstract] ABSTRACT: Molecular simulations reveal that the shape of differential capacitance (DC) versus the electrode potential can change qualitatively with the structure of the electrode surface. Whereas the atomically flat basal plane of graphite in contact with a room-temperature ionic liquid generates camel-shaped DC, the atomically corrugated prismatic face of graphite with the same electrolyte exhibits bell-shaped behavior and much larger DCs at low double-layer potentials. The observed bell-shaped and camel-shaped DC behavior was correlated with the structural changes occurring in the double layer as a function of applied potential. Therefore, the surface topography clearly influences DC behavior, suggesting that attention should be paid to the electrode surface topography characterization in the studies of DC to ensure reproducibility and unambiguous interpretation of experimental results. Furthermore, our results suggest that controlling the electrode roughness/structure could be a route to improving the energy densities in electric double-layer capacitors.
[Show abstract][Hide abstract] ABSTRACT: We have developed a quantum chemistry-based polarizable potential for poly(ethylene oxide) (PEO) in aqueous solution based on the APPLE&P polarizable ether and the SWM4-DP polarizable water models. Ether–water interactions were parametrized to reproduce the binding energy of water with 1,2-dimethoxyethane (DME) determined from high-level quantum chemistry calculations. Simulations of DME–water and PEO–water solutions at room temperature using the new polarizable potentials yielded thermodynamic properties in good agreement with experimental results. The predicted miscibility of PEO and water as a function of the temperature was found to be strongly correlated with the predicted free energy of solvation of DME. The developed nonbonded force field parameters were found to be transferrable to poly(propylene oxide) (PPO), as confirmed by capturing, at least qualitatively, the miscibility of PPO in water as a function of the molecular weight.
No preview · Article · Apr 2011 · Journal of Chemical Theory and Computation
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations were performed on N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (pyr(13)FSI) room temperature ionic liquid (RTIL) confined between graphite electrodes as a function of applied potential at 393 and 453 K using an accurate force field developed in this work. The electric double layer (EDL) structure and differential capacitance (DC) of pyr(13)FSI was compared with the results of the previous study of a similar RTIL pyr(13)bis(trifluoromethanesulfonyl)imide (pyr(13)TFSI) with a significantly larger anion [ Vatamanu, J.; Borodin, O.; Smith, G. D. J. Am. Chem. Soc. 2010, 132, 14825]. Intriguingly, the smaller size of the FSI anion compared to TFSI did not result in a significant increase of the DC on the positive electrode. Instead, a 30% higher DC was observed on the negative electrode for pyr(13)FSI compared to pyr(13)TFSI. The larger DC observed on the negative electrode for pyr(13)FSI compared to pyr(13)TFSI was associated with two structural features of the EDL: (a) a closer approach of FSI compared to TFSI to the electrode surface and (b) a faster rate (vs potential decrease) of anion desorption from the electrode surface for FSI compared to TFSI. Additionally, the limiting behavior of DC at large applied potentials was investigated. Finally, we show that constant potential simulations indicate time scales of hundreds of picoseconds required for electrode charge/discharge and EDL formation.
No preview · Article · Mar 2011 · The Journal of Physical Chemistry B
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations of linear polymer melts represented using simple bead-necklace models showed for the first time a distinct separation between primary α- and secondary Johari–Goldstein β-processes. The split is observed only for models where the bead diameter is much larger than the bond length connecting the beads. The overlap of neighboring (along the chain) beads results in a mismatch between local intramolecular correlations and intermolecular packing (cage size), which leads to two processes in segmental relaxation characterized by torsional autocorrelation function. The observed β-process shows all characteristics and correlations expected for the true Johari–Goldstein process.
No preview · Article · Jan 2011 · Journal of Non-Crystalline Solids