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

Abstract The ion mobility and ion coordination in PEO n LiX (X = N(SO 2 CF 3 ) 2 (NTf 2 ) or CF 3 SO 3 (Tf)) polymer electrolytes (2 ≤  n  ≤ 20) were investigated employing a range of advanced solid state NMR approaches. Temperature-dependent 7 Li and 19 F (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 Al 2 O 3 . Significant fractions of mobile ions only occurred in the salt-poor PEO n LiNTf 2 electrolytes, whereas virtually no ion mobility was observed in the PEO n LiTf electrolytes at ambient temperatures. The spatial relationship between cations and anions in PEO n LiNTf 2 structures was investigated employing 19 F–{ 7 Li} REDOR NMR spectroscopy. The results clearly allow for an unambiguous assignment of the signals observed in 19 F MAS spectra of PEO 3 LiNTf 2 to crystallographically non-equivalent trifluoromethyl residues, and to obtain clear information about the ion coordination motifs in PEO 2 - and PEO 6 LiNTf 2 , 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 Al 2 O 3 surface could be detected. Instead, the results indicate that the particles partially suppress the crystallization of the PEO 6 LiNTf 2 complex, thereby leading to increased ionic conductivities.