What Can We Learn from Ionic Conductivity Measurements in Polymer Electrolytes? A Case Study on Poly(ethylene oxide) (PEO)-NaI and PEO-LiTFSI

We explore in detail what information on ionic diffusivity and ion pairing can be exclusively gained from combining accurate direct-current conductivity data in polymer electrolytes with a novel evaluation model. The study was performed on two prototype systems based on poly(ethylene oxide) (PEO) with known disparate ion-association properties, which are due to the dissimilar salt components being either sodium iodide (NaI) or lithium bis(trifluoromethane-sulfonyl)-imide (LiN(CF3SO2)(2) or LiTFSI). The temperature dependence of the conductivity can be described by an extended Vogel-Tammann-Fulcher (VTF) equation, which involves a Boltzmann factor containing the pair-formation enthalpy Delta H-p. We find a distinct increase of the positive Delta H-p values with decreasing salt concentration and similarly clear trends for the pertinent VTF parameters. The analysis further reveals that PEO-NaI combines a high pair fraction with a high diffusivity of the I- ion. By contrast, PEO-LiTFSI appears to be characterized by a low ion-pairing tendency and a relatively low mobility of the bulky TFSI- ion. The observed marked differences between PEO-NaI and PEO-LiTFSI complexes of homologous composition are most pronounced at high temperatures and low salt concentrations.