Values for the lithium ion transference number (t + 0 ) are reported for the solid polymer electrolyte system poly(ethylene oxide) (PEO) complexed with Li(CF 3 SO 2 ) 2 N (LiTFSI). t + 0 ranges from 0.17 ( 0.17 to 0.60 ( 0.03 in the salt concentration (c) region of 742 to 2982 mol/m 3 at 85 °C. The concentration dependence of t + 0 and the molar ionic conductivity (Λ) are shown to be in good agreement with a free volume approach over the salt-rich composition range investigated. The present t + 0 results were obtained using an electrochemical technique based on concentrated solution theory. This experimentally straightforward method is herein demonstrated to give accurate results for a highly concentrated SPE system, without relying on any dubious simplifications regarding the state of the electrolyte.

Thermal and ion-transport properties of the salt-in-polymer system poly(ethylene oxide)n–LiN(CF3SO2)2 [P(EO)nLiTFSI] were investigated for compositions ranging from n = 5 to n = 50. Particular attention was paid to the region n = 8 to 10 where a crystallinity gap previously had been reported. We concluded that the absence of distinct melting transitions for salt-rich compositions (n = 5 to 12) was attributable to the extremely slow kinetics of recrystallization of this system following a heat treatment. The results further indicated that it was primarily the nucleation process that was inhibited by the [(bis)trifluoromethanesulfonate imide] (TFSI) anion. As a corollary, the ionic conductivity was strongly dependent on the thermal history of samples, and an enhancement of up to 300% was observed in the ambient temperature ionic conductivity for pre-heated salt-rich samples.

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