The mechanisms of lithium cation (Li+) and bis(trifluoromethane)sulfonamide anion (TFSI-) transport in poly(ethylene oxide) (PEO, M w = 2380) melts were examined using molecular dynamics (MD) simulations over a wide range of salt concentrations and temperatures. MD simulations using a quantum-chemistry-based many-body polarizable force field yielded ion self-diffusion coefficients, electrolyte conductivity, ion aggregation, and the coordination environment of Li+ in good agreement with experiment. Lithium transport was found to arise from a combination of the subdiffusive Li+ motion along PEO chains, motion together with PEO segments and intersegmental Li+ hops from one PEO segment to another. The rate of intersegmental hops was found to correlate well with times at which Li+ motion crosses over from subdiffusive to diffusive behavior. The contribution of Li+ motion along PEO chains to the total Li+ transport was found to be approximately equal to the contribution from Li+ moving together with PEO segments. Diffusion of both Li+ and TFSI- was found to be strongly coupled to PEO ether oxygen atom displacements and PEO conformational dynamics.

Quantum chemistry studies of ethylene carbonate (EC) and dimethyl carbonate (DMC) complexes with Li+ and LiPF6 have been conducted. We found that Li+ complexation significantly stabilizes the highly polar cis-trans DMC conformation relative to the nearly nonpolar gas-phase low energy cis-cis conformer. As a consequence, the binding of Li+ to EC in the gas phase is not as favorable relative to binding to DMC as previously reported. Furthermore, quantum chemistry studies reveal that, when complexation of LiPF6 ion pairs is considered, the DMC/LiPF6 complex is about 1 kcal/mol more stable than the EC/LiPF6 complex. The EC3DMC(cis-cis)/Li+ complex was found to be the most energetically stable among ECnDMCm/Li+ (n+m=4) investigated complexes followed by EC4/Li+. Results of the quantum chemistry studies of these complexes were utilized in the development of a many-body polarizable force field for EC:DMC/LiPF6 electrolytes. Molecular dynamics (MD) simulations of EC/LiPF6, DMC/LiPF6, and mixed solvent EC:DMC/LiPF6 electrolytes utilizing this force field were performed at 1 M salt concentration for temperatures from 298 to 363 K. Good agreement was found between MD simulation predictions and experiments for thermodynamic and transport properties of both pure solvents and the electrolytes. We find increased ion pairing with increasing DMC content; however, both EC and DMC were found to participate in Li+ solvation in mixed EC:DMC electrolytes despite a huge difference in their dielectric constants. In contrast to previous NMR studies, where dominance of EC in cation solvation was reported, we find a slight preference for DMC in the cation solvation shell for EC:DMC (1 wt:1 wt) electrolytes and show that reanalyzed Raman spectroscopy experiments are in good agreement with results of MD simulations. Finally, analysis of solvent residence times reveals that cation transport is dominated by motion with solvating DMC and approximately equal contributions from vehicular motions with the first solvation shell and solvent exchange with respect to solvating EC.

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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.