Uncovering Local Correlations in Polymer Electrolytes by X-ray Scattering and Molecular Dynamics Simulations
Whitney S. Loo, Chao Fang, Nitash P. Balsara, Rui Wang
Abstract
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.