Investigation Of Crystalline PEO/NaPF₆Solid Polymer Electrolytes Studied By Solid-state NMR Spectroscopy

Solid polymer electrolytes (SPEs) are the significant components in the solid batteries, which have been focused and studied extensively as recyclable energy sources with large energy density and long working life. SPEs based on poly (ethylene oxide)(PEO) complex with alkali metal ions, which play an important role in polymer batteries, are mostly studied owing to their high room temperature ionic conductivity and excellent mechanical properties. Other than the over-30-years-accepted concept that ionic conductivity is dominated by the ion transport occurring in amorphous regions, recently reported high-crystallinity PEO/Li+and PEO/Na+polymer electrolytes with high room temperature ionic conductivity open up new ways to explore new kinds of SPEs. In this dissertation, high-crystallinity PEO/NaPF6polymer electrolytes were prepared. The ionic conductivity, crystalline structure, phase transition, segmental motion and ionic dynamics have been studied by solid-state NMR, DSC and X-ray diffraction, in combination with quantum chemical calculations, and some interesting results were obtained.First, the segmental motion mode of pure PEO and PEO8:NaPF6with high Mw (6000g mol-1) and low Mw (1000g mol-1) was investigated by13C variable-temperature measurements of static powder spectra and static2D exchange spectra together with quantum chemical calculations. Considering the experimental and calculation results, we interpret that the segmental motion mode may be ascribed in terms of large angle reorientation similar to helical jumping. The large angle reorientation motion of polymer segments also onsets at low temperature (-243K) in PEOg:NaPF6.The segmental motion induce temperature-dependent13C chemical shift anisotropy powder pattern variations and brings out uniaxial chemical shift anisotropy δ33>δ22(δ11) at high temperature for high-Mw PEOg:NaPF6. In the low-Mw PEO8:NaPF6, however, no temperature-driving lineshape reversal occurs, and high-temperature13C powder pattern appears as a relatively symmetric and broadened peak. This can be attributed to the effects of short PEO chains, which result in heterogeneous coil structure and segmental motion. Because the motion of the end segments is less restricted, the diffusive rotation may become a significant motion mode in the short chain case which would broaden the diagonal signals and weaken the off-diagonal signals in2D exchange spectrum.Secondly, two types of high-crystallinity poly(ethylene oxide)/NaPF6electrolytes with ethylene oxide (EO)/Na molar ratios of8:1and6:1, termed as PEOg.NaPF6and PEO6:NaPF6with Mw=6000and1000g mol-1were prepared, and their ionic conductivity, structure and segmental motions were investigated and compared. For Mw=6000g mol-1samples, PEOg:NaPF6polymer electrolyte exhibits the room-temperature ionic conductivity7.7×10-7S cm-1which is about five times higher than the PEO6:NaPF6. By variable-temperature measurements of static powder spectra and1H spin-lattice relaxation time in rotation frame (1H T1p), we demonstrate that crystalline segments are more highly mobile in the crystalline PEOg:NaPF6with higher ionic conductivity than in the PEO6:NaPF6with lower ionic conductivity. The large angle reorientation motion of polymer segments in the PEO8:NaPF6onsets at lower temperature (～233K) with a low activation energy0.31eV that is comparable with that of the pure PEO crystal. Whereas, the large-angle reorientation motion of polymer segments in the PEO6:NaPF6starts around313K with a high activation energy of0.91eV. As a result of the temperature-enhanced large-angle reorientations, the13C static powder lineshape changes markedly from a low-temperature wide pattern with apparent principal values of chemical shift δ3<δ22<δ11to a high-temperature narrow pattern of uniaxial chemical shift anisotropy δ33>δ22(δ11). For Mw=6000g mol"1samples, however, no revearsal but a relatively symmetric and broadened peak appears as high-temperature13C powder pattern, due to the effect of short PEO chain. Besides, one sees remarkable increase of room temperature ionic conductivity (1.2×10-5S cm-1for PEO8:NaPF6and1.0×10-6S cm-1for PEO6:NaPF6respectively).It is suggested that the segmental motion in crystalline PEO-salt complex promotes ionic conductivity.Thirdly, PEO8:NaPF6and PEO6:NaPF6with Mw=6000and1000g mol-1were prepared, and ionic local environment and motions were investigated and compared. Different crystal structure brings out different23Na quadrupolar lineshape for PEO8:NaPF6and PEO6:NaPF6with Mw=6000g mol-1. From23Na quadrupolar lineshape at variable temperature, the temperature-dependent local environment variations of Na ions are closely correlated to the segmental motion. For PEO8:NaPF6and PEO6:NaPF6with Aw=1000g mol-1, no obvious23Na quadrupolar lineshape was observed as heterogeneous coil structure and segmental motion. Through23Na quadrupolar lineshape simulation combined with relaxation time study, it is suggested that the segmental motion in crystalline PEO-salt complex together with highly mobile Na ion relative motion to the PEO chain to enhance ion transportation in crystalline polyelectrolytes promotes ion conductivity.