Influence of Host Polarity on Correlating Salt Concentration, Molecular Weight, and Molar Conductivity in Polymer Electrolytes

Abstract

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.

Structures of cationic and anionic polyelectrolytes in aqueous solutions: the sign effect

“…For example, based on Stockmayer-type coarse-grained models, 45 Ganesan et al have studied ion transportation and conductivity in polymer electrolytes. 46,47 We have developed a molecular dynamics simulation algorithm based on the Stockmayer-fluid model to investigate the multi-body correlations between ion–ion, ion–dipole, and dipole–dipole interactions in ion-containing liquids with and without the application of the electric field. 24 Using this simulation algorithm, we have also illustrated the microscopic mechanisms for the diminishing of ionic clusters caused by higher dielectric solvent addition in solution, which clarifies the experimentally observed noise reduction of ionic current in nanopore detection for ionic polymers.…”

Ion Conductivity and Correlations in Model Salt-Doped Polymers: Effects of Interaction Strength and Concentration

“…Importantly, recent computational studies show that the fractional deviation of conductivity from the Nernst-Einstein equation (denoted as Λ/Λ N E in the present work and often called the degree of uncorrelated ion motion in simulation work) can be enhanced by increasing polymer dielectric constant (polarity), which can improve the overall conductivity. [27][28][29] Some work also investigated the separate contributions from the various types of cross correlation terms to the total ionic conductivity (overall correlations can be divided into cation-cation, anion-anion, and cation-anion correlations). 18,30,31 Despite the abundance of computational work on conductivity, accurate and efficient measurement of conductivity from molecular dynamics simulations remains challenging.…”

“…Unphysical overall Λ/Λ N E values larger than 1.0 were sometimes encountered, apparently due to the poor statistics. 28,29,32,33 Several methods have been used to reduce statistical noise in the conductivity calculation such as 1) using only the short-time scale data to allow for significant averaging and because the collective ion migration terms tend to fluctuate wildly at long time scales, 7,[27][28][29]32,33 2) considering ion clusters as noninteracting charge carriers and applying the Nernst-Einstein equation with the diffusion constants and net charges of the different types of ion clusters, 34 and 3) measuring ion mobility from nonequilibrium simulations with an external electric field. 31,[35][36][37][38] Prior studies have compared conductivities estimated by different methods for several specific systems.…”

Transport Mechanisms Underlying Ionic Conductivity in Nanoparticle-Based Single-Ion Electrolytes

“…h(t 0 )h(t 0 + t) h (19) where h(t) takes a value of one if a cation-solvent association is present at time t and zero otherwise. 17 The autocorrelation function C(t) shows on average how long a cation-solvent pair remains associated.…”