Solid polymer electrolytes are important materials in realizing safe and flexible energy storage devices. The present study looks at correlation between solvation structure and the ion-conductive behavior of poly­(ethylene carbonate) (PEC)/lithium bis­(fluorosulfonyl)­imide (LiFSI) electrolytes which have high Li transference number (t +) and show unusual salt-concentration dependence of conductivity. From FT-IR and Raman spectroscopy, we determined that Li ions interact with carbonyl (CO) groups and also with FSI ions, which can be referred to as contact ion pair or aggregate. 7Li magic-angle-spinning NMR spectroscopy and density functional theory calculations for model species suggest that a loose coordination structure, in which Li ions interact with CO groups and FSI ions with appropriate strength, allows the electrolytes to have both reasonable conductivity and high t + with a flexible and transparent character. A high salt dissociation rate is generally considered essential in SPEs, but the presence of aggregated ions having the loose coordination structure gives rise to favorable performance in highly concentrated PEC-based solid polymer electrolytes.

A new class of polymer electrolytes, consisting of poly(ethylene carbonate) (PEC) and metal salts, is expected to find application in all‐solid‐state batteries because of its excellent performance as an electrolyte. To study the ion‐conductive mechanism in PEC‐based electrolytes, broadband dielectric spectroscopy is used to analyze the correlation between dielectric relaxation and ionic conduction in PEC‐lithium bis‐(trifluoromethanesulfonyl) imide electrolytes over a broad range of salt concentration (0–150 mol%) at 40 °C. The PEC system has two relaxation modes, α and β, associated respectively with the segmental motion and the local motion of PEC chains. The conductivity increases exponentially with increasing salt concentration, while the α relaxation frequency (fα) decreases with increasing strength (Δεα) at low salt concentrations, whereas in contrast fα increases with Δεα being saturated at high salt concentrations above 10 mol%. It is believed that the mobility of PEC segment at high concentration is enhanced by two factors. The first is that intermolecular interactions decrease, given the existence of many ion pairs and aggregated ions around saturated PEC domains where the dissociated ions are highly concentrated. The second is that intramolecular interactions between CO and CH2 are lowered by the ion–dipole interaction.

Solid polymer electrolytes (SPEs) are compounds of great interest as safe and flexible alternative ionics materials, particularly suitable for energy storage devices. We study an unusual dependence on the salt concentration of the ionic conductivity in an SPE system based on poly(ethylene carbonate) (PEC). Dielectric relaxation spectroscopy reveals that the ionic conductivity of PEC/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte continues to increase with increasing salt concentration because the segmental motion of the polymer chains is enhanced by the plasticizing effect of the imide anion. Fourier transfer‐infrared (FTIR) spectroscopy suggests that this unusual phenomenon arises because of a relatively loose coordination structure having moderately aggregated ions, in contrast to polyether‐based systems. Comparative FTIR study against PEC/lithium perchlorate (LiClO4) electrolytes suggests that weak ionic interaction between Li and TFSI ions is also important. Highly concentrated electrolytes with both reasonable conductivity and high lithium transference number (t+) can be obtained in the PEC/LiTFSI system as a result of the unusual salt concentration dependence of the conductivity and the ionic solvation structure. The resulting concentrated PEC/LiTFSI electrolytes have extraordinary oxidation stability and prevent any Al corrosion reaction in a cyclic voltammetry. These are inherent effects of the highly concentrated salt. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2442–2447

To study the ion‐conductive and dielectric properties of polymer electrolytes based on poly(ethylene carbonate) (PEC) with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), the complex permittivity and conductivity were measured using broadband dielectric spectroscopy. The temperature dependence of the relaxation frequency and ionic conductivity for PEC‐LiTFSI electrolytes (1 – 200 mol%) indicates that the segmental motion of PEC chains decreases with the addition of just 1 mol% of Li salt and increases with increasing concentration above 10 mol%. According to the Walden rule for PEC‐based electrolytes, the value of deviation from the reference line increased, and the fragility and decoupling exponents decreased with increasing salt concentration. These results indicate that there are large numbers of ion pairs and aggregated ions, which imply low ionicity and reduced fragility in highly concentrated PEC‐based electrolytes. Copyright © 2016 John Wiley & Sons, Ltd.