Uncovering Local Correlations in Polymer Electrolytes by X-ray Scattering and Molecular Dynamics Simulations

Understanding the distribution of ionic species in electrolytes is important for predicting the ion-transport properties. Here, a quantitative analysis of wide-angle X-ray scattering (WAXS) profiles was conducted for the first time on a series of mixtures of poly(ethylene oxide) (PEO) and lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI), PEO/LiTFSI, as a function of salt concentration in the melt state. Abnormal scattering signatures were observed: while WAXS data showed a single peak (Peak 1) in the absence of salt, a second peak (Peak 0) appeared at lower scattering angles with added salt. Molecular dynamics simulations with the standard TraPPE-UA force field were used to uncover the molecular origins of the WAXS peaks. Qualitative agreement was found between the experimental and simulated scattering profiles. Simulations indicated that Peak 1 arises from correlations between EO segments as well as correlations between TFSI- ions and EO segments, while Peak 0 arises from correlations between neighboring TFSI- ions. There were, however, quantitative disagreements between experiment and simulations, which were resolved by the introduction of a charge rescaling factor, Rf, to account for the polarization of ions and polymers. Simulations with charge rescaling predicted that the formation of anion-rich clusters occurs at a higher salt concentration than the emergence of Peak 0. While the WAXS data did not directly reflect the presence of anion-rich clusters, they provided a basis for more refined calculation of short-range correlations between ions, correlations that directly affect clustering and ion-transport properties such as ionic conductivities and transference numbers.