[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.
The Journal of Physical Chemistry B 04/2014; · 3.61 Impact Factor
[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.
The Journal of Chemical Physics 09/2013; 139(10):104503. · 3.12 Impact Factor
[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.
The Journal of Physical Chemistry B 09/2012; 116(42):12801-9. · 3.61 Impact Factor
[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.
The Journal of Chemical Physics 05/2012; 136(19):194506. · 3.12 Impact Factor
[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.
[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.
The Journal of Physical Chemistry C 03/2012; 116:7940–7951. · 4.84 Impact Factor
[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.
The Journal of Physical Chemistry A 02/2012; 116(11):2978-85. · 2.77 Impact Factor
[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: 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.
The Journal of Physical Chemistry C 01/2012; 116(1):1114. · 4.84 Impact Factor
[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.
The Journal of Physical Chemistry A 12/2011; 115(47):13896-905. · 2.77 Impact Factor
[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.
European Polymer Journal 10/2011; 47(10):2022-2027. · 2.56 Impact Factor
[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.
Journal of Chemical Theory and Computation 04/2011; 7(6):1902-1915. · 5.39 Impact Factor
[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.
The Journal of Physical Chemistry B 03/2011; 115(12):3073-84. · 3.61 Impact Factor
[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.
Journal of Non-Crystalline Solids 01/2011; 357(2):258-263. · 1.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations were conducted in order to improve our understanding of the forces that determine polyleucine chains conformations and govern polyleucine self-assembly in aqueous solutions. Simulations of 10 repeat unit oligoleucine in aqueous solution were performed using the optimized potential for liquid simulations (OPLS) – all atom force field using the canonical ensemble for a minimum of 1.3ns. These simulations provided information on conformations, chain collapse and intermolecular aggregation. Simulations indicate that single isotactic oligoleucine chains in dilute solution assume tightly packed, regular hairpin conformations while atactic oligoleucine assumes a much less regular and less compact structure. The regular, compact collapsed isotactic chain exhibited a greater degree of intramolecular hydrogen bonding and an increased level of hydrophobic t-butyl functional group aggregation compared to the atactic chain. This occurs at the expense of reduced leucine–water hydrogen bonding.
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulation studies of the structure and the differential capacitance (DC) for the ionic liquid (IL) N-methyl-N-propylpyrrolidinium bis(trifluoromethane)sulfonyl imide ([pyr(13)][TFSI]) near a graphite electrode have been performed as a function temperature and electrode potential. The IL exhibits a multilayer structure that extends 20-30 Å from the electrode surface. The composition and ion orientation in the innermost layer were found to be strongly dependent on the electrode potential. While at potentials near the potential of zero charge (PZC), both cations and anions adjacent to the surface are oriented primarily perpendicular to the surface, the counterions in first layer orient increasingly parallel to the surface with increasing electrode potential. A minimum in DC observed around -1 V(RPZC) (potential relative to the PZC) corresponds to the point of highest density of perpendicularly aligned TFSI near the electrode. Maxima in the DC observed around +1.5 and -2.5 V(RPZC) are associated with the onset of "saturation", or crowding, of the interfacial layer. The asymmetry of DC versus electrode polarity is the result of strong interactions between the fluorine of TFSI and the surface, the relatively large footprint of TFSI compared to pyr(13), and the tendency of the propyl tails of pyr(13) to remain adsorbed on the surface even at high positive potentials. Finally, an observed decreased DC and the disappearance of the minimum in DC near the PZC with increasing temperature are likely due to the increasing importance of entropic/excluded volume effects (interfacial crowding) with increasing temperature.
Journal of the American Chemical Society 10/2010; 132(42):14825-33. · 10.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulations of fluoroalkyl-derivatized imidazolium:bis(trifluoromethylsulfonyl)imide (TFSI) room temperature ionic liquids (FADI-RTILs) with cations of the structure 1-F(CF(2))(n)(CH(2))(2)-3-methyl imidazolium have been performed and compared with simulations of alkyl-derivatized 1-H(CH(2))(n+2)-3-methyl imidazolium analogs (ADI-RTILs). Simulations yield RTIL densities, viscosities and ionic conductivities for the FADI-RTILs and ADI-RTILs in reasonably good agreement with experimental data. Partial fluorination results in a larger increase in density than would be anticipated based upon the density difference between perfluoralkane and alkane melts. Similarly, the slowing down in dynamics upon partial fluorination is greater than would be expected based upon the increase in cation volume. Examination of cation-cation, anion-anion and cation-anion centers-of-mass radial distribution functions reveal remarkably little influence of partial fluorination on the spherically averaged intermolecular structure of the RTILs. Similarly, simulations reveal little change in tail conformations and the extent of tail-tail aggregation upon partial fluorination. The interaction of the TFSI anion with the positively charged imidazolium ring hydrogen and nitrogen atoms is also little influenced by partial fluorination. However, the partially fluorinated alkyl tail exhibits increased interaction with the TFSI anion due to the electron withdrawing character of the fluorinated groups. We believe this strong tail-anion electrostatic interaction largely accounts for the higher than expected density and slower than expected dynamics in the FADI-RTILs.
Physical Chemistry Chemical Physics 07/2010; 12(26):7064-76. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The pulsed-field-gradient spin-echo NMR measurements have been performed on 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([emim][FSI]) and 1-ethyl-3-methylimidazolium [bis[(trifluoromethyl)sulfonyl]imide] ([emim][TFSI]) over a wide temperature range from 233 to 400 K. Molecular dynamics (MD) simulations have been performed on [emim][FSI], [emim][TFSI], [N-methyl-N-propylpyrrolidinium][FSI] ([pyr(13)][FSI]), and [pyr(13)][TFSI] utilizing a many-body polarizable force field. An excellent agreement between the ion self-diffusion coefficients from MD simulations and pfg-NMR experiments has been observed for [emim][FSI] and [emim][TFSI] ILs. The structure factor of [pyr(13)][FSI], [pyr(14)][TFSI], and [emim][TFSI] agreed well with the previously reported X-ray diffraction data performed by Umebayashi group. Ion packing in the liquid state is compared with packing in the corresponding ionic crystal. Faster transport found in the FSI-based ILs compared to that in TFSI-based ILs is associated with the smaller size of FSI(-) anion and lower cation-anion binding energies. A significant artificial increase of the barriers (by 3 kcal/mol) for the FSI(-) anion conformational transitions did not result in slowing down of ion transport, indicating that the ion dynamics is insensitive to the FSI(-) anion torsional energetic, while the same increase of the TFSI(-) anion barriers in [emim][TFSI] and [pyr(13)][TFSI] ILs resulted in slowing down of the cation and anion transport by 40-50%. Details of ion rotational and translational motion, coupling of the rotational and translational relaxation are also discussed.
The Journal of Physical Chemistry B 05/2010; 114(20):6786-98. · 3.61 Impact Factor