Dielectric Relaxation Behavior of a Poly(ethylene carbonate)-Lithium Bis-(trifluoromethanesulfonyl) Imide Electrolyte

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 degrees C. The PEC system has two relaxation modes, and , associated respectively with the segmental motion and the local motion of PEC chains. The conductivity increases exponentially with increasing salt concentration, while the relaxation frequency (f) decreases with increasing strength (epsilon) at low salt concentrations, whereas in contrast f increases with epsilon 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 CO and CH2 are lowered by the ion-dipole interaction.