Peristaltic Ion-Conduction Mechanism in Sodium Polymer Electrolytes Based on Perfluoropolyethers, DMSO, and Sodium Bis(trifluoromethanesulfonyl)imide

The promise of sodium-ion conducting polymer electrolytes in rechargeable power sources is yet to be fulfilled. The possibility to combine the very attractive physical–chemical properties of perfluorinated polyethers (PFPEs) with sodium salts has not thus far been considered. Here we investigate a series of eight plasticized polymer electrolytes based on an α,ω-OH-terminated poly[PFPE-block-PEO] (hereafter called PEG-PFPE-PEG), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and dimethyl sulfoxide (DMSO) as plasticizer, having general formula PEG-PFPE-PEG/(NaTFSI)x(DMSO)y (0.004 ≤ x ≤ 0.610, 3.205 ≤ y ≤ 3.794). The thermal analysis results, carried out by means of DSC, indicate that the plasticizer remains intimately integrated in the polymer electrolytes up to the decomposition temperature of PEG-PFPE-PEG (180 °C ca.) and is not significantly released out of the polymer hosts in closed cells. The vibrational investigation, carried out by FT-IR spectroscopy in the medium IR region, shed light on NaTFSI–PEG-PFPE-PEG–DMSO interactions. In particular, the role of the OH functional groups in facilitating salt dissolution was elucidated, along with that of the CO and CF moieties. The ionic conductivity was investigated by impedance spectroscopy, in the frequency interval 100 mHz to 1 MHz. The conductivity at 25 °C is as high as 8.0 × 10–4 S·cm–1. Three of at least five polarization events (one electrode and four interdomain polarizations in total) were fully resolved and were associated with different interfacing domains in the polymer electrolytes with increasing DMSO content. The behavior of the corresponding conductivities and frequencies were fitted with Vogel–Tamman–Fulcher (VTF) and Arrhenius equations, respectively, highlighting the concomitant roles of segmental motion and ionic hopping in these hybrid systems. Furthermore, it was shown that the presence of DMSO, a well-known cryodepressant, significantly decreases the glass transition temperature of the electrolytes.