Salt-Concentration Dependence of the Glass Transition Temperature in PEO–NaI and PEO–LiTFSI Polymer Electrolytes

We measured glass transition temperatures Tg as a function of the salt concentration in polymer electrolyte systems consisting of poly(ethylene oxide) (PEO) complexed either with sodium iodide (NaI) or lithium bis(trifluorosulfonylimide) (LiTFSI). At homologous compositions, Tg of PEO–NaI is found to be generally larger than that of LiTFSI. The present Tg values are markedly higher than previously reported reference data. Also the observed nonlinear concentration dependence differs from earlier studies. These findings are tentatively attributed to the more stringent preparation and measuring conditions maintained in the present work, thereby keeping organic solvent residues and water contamination at low levels. Also the high molecular weight of the polymer may have some influence. The measurements were performed by differential scanning calorimetry after quenching from the melt. We find that ex situ immersion quenching in liquid nitrogen leads to lower degrees of crystallinity than in situ quenching in the calorimeter environment. In addition, the strong decrease of the crystallinity with increasing salt content gives rise to pronounced steps in the heat capacity near Tg for the more concentrated electrolytes.