In this work, we use computer simulations to demonstrate that there may be limits to which polymer polarity alone can be used to influence the ionic conductivity of salt-doped polymer electrolytes. Specifically, we use coarse-grained molecular dynamics simulations to probe the effect of the polarity of the polymer electrolyte upon ion mobilities and conductivities of dissolved salts. At low polymer polarities, increasing the polymer dielectric constant reduces ionic aggregation and the resultant correlated ionic motion, and increases the ionic conductivity. At higher polymer polarities, polymer−polymer and polymer−ion interactions slows polymer segmental dynamics, leading to a reduction in the conductivity of the electrolyte. As a consequence, ionic conductivity achieves an optimum at an intermediate polymer polarity.

We use all-atom molecular dynamics simulations to study the effect of polymer polarity, as quantified by the dielectric constant, on the transport properties of lithium bis­(trifluoromethylsulfonyl)­imide (LiTFSI) doped polyethers. Our results indicate that increasing the host dielectric constant leads to a decrease in ionic cluster sizes and reduction in correlated motion of oppositely charged ions. This causes the ionic conductivity to more closely approach the Nernst-Einstein limit in which ionic conductivity is only limited by the diffusivities of Li+ and TFSI–. We compare our results to recent experimental observations which demonstrate similar qualitative trends in host polarity.

We report the results of atomistic molecular dynamics simulations on polymerized 1-butyl-3-vinylimidazolium-hexafluorophosphate ionic liquids, studying the influence of the polymer molecular weight on the ion mobilities and the mechanisms underlying ion transport, including ion-association dynamics, ion hopping, and ion-polymer coordinations. With an increase in polymer molecular weight, the diffusivity of the hexafluorophosphate (PF) counterion decreases and plateaus above seven repeat units. The diffusivity is seen to correlate well with the ion-association structural relaxation time for pure ionic liquids, but becomes more correlated with ion-association lifetimes for larger molecular weight polymers. By analyzing the diffusivity of ions based on coordination structure, we unearth a transport mechanism in which the PF moves by "climbing the ladder" while associated with four polymeric cations from two different polymers.

We probe the ion mobilities, transference numbers, and inverse Haven ratio of ionic liquids and polymerized ionic liquids as a function of their molecular weight using a combination of atomistic equilibrium and nonequilibrium molecular dynamics simulations. In contrast to expectations, we demonstrate that the inverse Haven ratio increases with increasing degree of polymerization (N) and then decreases at larger N. For a fixed center of mass reference frame, we demonstrate that such results arise as a consequence of the strong cation–cation correlated motions, which exceed (in magnitude) the self-diffusivity of cations. Together, our findings challenge the premise underlying the pursuit of pure polymeric ionic liquids as high transference number, single-ion conducting electrolytes.

In spite of significant interest toward solid-state electrolytes owing to their superior safety in comparison to liquid-based electrolytes, sluggish ion diffusion and high interfacial resistance

We study ternary polymer-polymer-salt blend electrolytes using coarsegrained molecular dynamics. We specifically examine the influence of the polymer hosts' incompatibility and polarity contrast on electrolyte ion transport characteristics. We find that, at moderate-to high-polarity contrasts, improving the miscibility of the polymer hosts by reducing their inherent incompatibility improves ionic transport, as measured by the ionic conductivity. However, contrary to expectations, ionic conduction slows with increased miscibility in low-polarity contrast electrolytes. Upon examining the underlying material properties, we find that ionic aggregation exhibits trends similar to ionic conductivity and is thus likely the controlling factor in these polymer-blend electrolytes. Our results suggest that ionic conduction can be improved in real polymer electrolytes by choosing chemistries that promote simultaneous miscibility and polarity contrast between the polymer hosts.

We used atomistic molecular dynamics simulations to study the properties of polymerized 1‐alkene‐3‐butylimidazolium‐hexafluorophosphate, a polymerized ionic liquid electrolyte, and characterized the influence of the linear alkene length on the mobility of the hexafluorophosphate ions. Consistent with experimental observations, our simulations indicate that as the alkene length increases, the diffusivity of hexafluorophosphate anion monotonically increases. We demonstrate that such a trend arises from the influence of linker segments on the intermolecular ion hopping rates, which is in turn modulated by intermolecular cationic separation distances. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1718–1723

We use coarse-grained molecular dynamics simulations to study the effect of salt concentration and host polymer molecular weight on ion transport in polymer electrolytes. We find that increasing salt concentration or molecular weight similarly slows polymer dynamics across a wide range of host polarities, and that the resulting relaxation times display a correlation to the product of the salt concentration and polymer molecular weight. However, we find that molar conductivity only decreases with polymer dynamics at high polarities but is uncorrelated with the latter at low polarities. We attribute such differences to the variation in ionic aggregation between high and low polarity electrolytes. At low polarity, ionic dissociation significantly increases with molecular weight and salt concentration, offsetting the slowdown in polymer dynamics and yielding the observed insensitivity of molar conductivity. However, at high polarity, ions are mostly dissociated, independent of either molecular weight or salt concentration, thereby strongly coupling molar conductivity to polymer dynamics.