Slow recrystallization in the polymer electrolyte system poly(ethylene oxide)_n–LiN(CF₃SO₂)₂

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.