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(CF(3)SO(2))(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 ΔH(p). We find a distinct increase of the positive ΔH(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.

No abstract

We investigated mass and charge transport in amorphous salt-in-poly(ethylene oxide) (PEO) electrolytes with NaI and/or the ionic liquid (IL) EMImTFSI (1-ethyl-3-methylimizadolium bis(trifluoromethylsulfonyl)imide) as salt component. Combining the results of ion conductivity, pulsed field gradient nuclear magnetic resonance, and radiotracer diffusion measurements, it is found that over wide temperature ranges both the cation and anion diffusion coefficients and the charge diffusivity are distinctly larger in PEO(20)EMImTFSI than in PEO(20)NaI complexes, where the monomer-to-salt mole ratio equals 20. In the mixed-salt complexes PEO(20)NaI(1)EMImTFSI(1) and PEO(20)NaI(0.5)EMImTFSI(0.5), we observe a slowing down of the IL ions EMIm and TFSI along with a diffusivity enhancement of the I anion compared to the single-salt base complexes. For the cation Na, a diffusivity increase is only effected by IL substitution, but because of the concomitant decrease of the Na concentration, it does not predict more effective charge transfer in a battery cell configuration.

We find a strong impact of ion pairing on ionic transport in potential Grätzel-cell electrolytes based on poly(ethylene oxide) (PEO) and 1-propyl-3-methylimidazolium iodide (PMImI). Furthermore, the addition of free iodine enhances both mass and charge transport, which can be explained by the reduced pair-formation tendency of the bulky triiodide ion. These results arise from conductivity and diffusion measurements on amorphous complexes with EO/PMImI molar ratios of 20 and 30 and their evaluation in a comprehensive ion-transport model. In particular, the charge diffusivity D(σ) was compared with the PMIm diffusivity D(cat)* determined by pulsed-field-gradient nuclear magnetic resonance and the iodine diffusivity D(an)* obtained from radiotracer depth profiling. Simultaneous fitting of these diffusion coefficients in complexes with and without iodine additive yields best values for the model parameters. The results characterize not only the mobility of free ions and pairs as a function of temperature and composition but also the degree of ion pairing.

Conventional polymer electrolytes based on inorganic salts are commonly characterized and utilized over a small salt-poor composition range because of phase transitions accompanied by loss of ion conductivity at high salt concentrations. By contrast, well-chosen polymer-ionic-liquid (IL) systems offer the possibility to vary the IL content from the IL-in-polymer to the polymer-in-IL domain. We have investigated the temperature-dependent ionic conductivity in PEOyEMImI systems consisting of poly(ethylene oxide) complexed with 1-ethyl-3-methylimidazolium iodide for y = EO/IL ratios ranging from 0.6 to 60 and compared diffusivity data with that arising from (1)H pulsed-field-gradient nuclear magnetic resonance for EMIm and (125)I radiotracer diffusion for iodine. Surprisingly, the diffusivity of cations and anions vary at most by 50% at fixed temperatures over the entire composition range. The much larger changes in the charge diffusivity Dσ relate to ion pairing exhibiting a minimum near the intermediate composition y = 10. Altogether, the results are relevant to application in dye-sensitized solar cells and show that a high ion density is crucial to enhance the iodine transport capacity.