Cation–anion coordination, ion mobility and the effect of Al2O3 addition in PEO based polymer electrolytes

The ion mobility and ion coordination in PEOnLiX (X = N(SO2CF3)2 (NTf2) or CF3SO3 (Tf)) polymer electrolytes (2 ≤ n ≤ 20) were investigated employing a range of advanced solid state NMR approaches. Temperature-dependent 7Li and 19F (MAS) NMR experiments were used to study the mobility of both anions and cations in the polymer electrolyte complexes as a function of the nature of the anion and the presence of nanoscaled Al2O3.Significant fractions of mobile ions only occurred in the salt-poor PEOnLiNTf2 electrolytes, whereas virtually no ion mobility was observed in the PEOnLiTf electrolytes at ambient temperatures. The spatial relationship between cations and anions in PEOnLiNTf2 structures was investigated employing 19F–{7Li} REDOR NMR spectroscopy. The results clearly allow for an unambiguous assignment of the signals observed in 19F MAS spectra of PEO3LiNTf2 to crystallographically non-equivalent trifluoromethyl residues, and to obtain clear information about the ion coordination motifs in PEO2- and PEO6LiNTf2, whose structures have not been solved so far. Furthermore, the influence of alumina particles on the local Li coordination and dynamics within the nano-composites was studied. In contrast to e.g. polyphosphazene based nano-composites, no interaction between Li cations and the Al2O3 surface could be detected. Instead, the results indicate that the particles partially suppress the crystallization of the PEO6LiNTf2 complex, thereby leading to increased ionic conductivities.