Highly mobile segments in crystalline poly(ethylene oxide)8:NaPF_6 electrolytes studied by solid-state NMR spectroscopy

摘要

Two types of high-crystallinity poly(ethylene oxide)/NaPF_6 electrolytes with ethylene oxide (EO)/Na molar ratios of 8:1 and 6:1, termed as PEO_8:NaPF_6 and PEO_6:NaPF_6 with M_w = 6000 g mol~(-1) were prepared, and their ionic conductivity, structure, and segmental motions were investigated and compared. PEO_8:NaPF_6 polymer electrolyte exhibits the room-temperature ionic conductivity 7.7 × 10~(-7 S cm~(-1) which is about five times higher than the PEO_6:NaPF_6. By variable-temperature measurements of static powder spectra and 1H spin-lattice relaxation time in rotation frame (1H T_(1)ρ), we demonstrate that crystalline segments are more highly mobile in the crystalline PEO_8:NaPF_6 with higher ionic conductivity than in the PEO_6:NaPF_6 with lower ionic conductivity. The largeangle reorientation motion of polymer segments in the PEO_8:NaPF_6 onsets at lower temperature (~233 K) with a low activation energy 0.31 eV that is comparable with that of the pure PEO crystal. Whereas, the large-angle reorientation motion of polymer segments in the PEO_6:NaPF_6 starts around 313 K with a high activation energy of 0.91 eV. As a result of the temperature-enhanced large-angle reorientations, the ~(13)C static powder lineshape changes markedly from a low-temperature wide pattern with apparent principal values of chemical shift δ_(33) < δ_(22) < δ_(11) to a high-temperature narrow pattern of uniaxial chemical shift anisotropy δ_(33) > δ_(22) (δ_(11)). It is suggested that the segmental motion in crystalline PEO-salt complex promotes ionic conductivity.