Effect of oligo(ethyleneoxy)cyclotriphosphazenes, tetraglyme, and other small molecules on the ionic conductivity of the poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP)/lithium triflate system

Four small molecule cyclotriphosphazenes bearing linear and branched ethyleneoxy units have been synthesized as ion transport assistance (ITA) species to improve the ionic conductivity of poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP)-lithium triflate systems. The cyclotriphosphazenes were characterized by (31)p, H-1, and C-13 nuclear magnetic resonance spectroscopy and infrared spectroscopy. MEEP was complexed with lithium triflate and combined with different amounts of each of the ITA molecules. Room temperature conductivity measurements and differential scanning calorimetry experiments were carried out on each of these mixtures. The effect of concentration of the ITA molecules and salt on the ionic conductivity of MEEP is reported. The general trend observed is an increased conductivity with increasing ITA molecule concentration. The effect of branching in the ITA molecule on the ionic conductivity was studied by comparing influence of branched versus linear side groups; Differential scanning calorimetry indicate that these small molecules play a role in ionic cross-linking. The influence of high dielectric solvents on the ionic conductivity of MEEP/LiSO3CF3 systems was also studied. These solvents generated higher conductivities at the same concentration than did the cyclotriphosphazene ITA species.