Effect of anion size and salt concentration on ion transport in PEO-based solid polymer electrolytes at high temperatures

The ion transport mechanism in polymers plays a vital role in improving the ionic conductivity of solid polymer electrolytes. In this work, we adopt molecular dynamics simulation and density functional theory calculation to study the effect of anion size on the structural and dynamical properties of solid polymer electrolytes based on poly(ethylene oxide) (PEO) at high temperatures over a range of salt concentrations (x = [Li]/[EO]) from 0.02 to 0.2 and then analyzed the effect on ion transport properties. We selected three perfluorinated sulfonimide anions (FSI-, TFSI-, PFSI-) of different sizes to form three PEObased solid polymer electrolytes. Based on calculations of various salt concentrations, we revealed that x = 0.1 is a significant turning point in structural properties. Among the three solid polymer electrolytes we simulated, LiTFSI/PEO exhibits the best ion transport performance and the highest ionic conductivity, while LiFSI/PEO performs the worst. Through density functional theory calculations, we found that the dissociation of lithium salt and the interaction between anions and PEO chains are two key factors determining the ion transport performance at high temperatures. Our work indicates that larger anions are not always better at high temperatures.