| In this thesis, we studied the structures and dynamics of PEO/alkali metal salt complexes by multinuclear high-resolution solid-state NMR, X-ray diffraction, and Ac-impedance methods. The following conclusions are drawn from the experimental results:1) The 13C CP/MAS spectrum is very sensitive to the carbon's chemical environment. The number of peaks and their chemical shifts vary greatly with the species of alkali metal salt and its concentration. The obtained results turn out that 13C CP/MAS NMR spectroscopy can be a powerful tool for characterizing the structures, chain conformation as well as chain dynamics in the complex crystals. This potential of NMR has been underestimated in the past.2) The crystalline signal assignment of the 13C solid-state high-resolution NMR spectra of P(EO)6/LiCl04 and P(EO)3/LiClO4 are successfully achieved by combining the results of the solid-state C dipolar-INADEQUATE (e.g. fp-RFDR DQ) spectrum and the 2D exchange 13C spectrum under MAS. The methodology can also be applied to the spectral assignment of the other PEO/alkali metal salt complexes.3) At room temperature, marked conformation exchanges within the complex crystals are revealed by 2D exchange 13C spectra. The exchange rates increase with increasing temperature and the mixing time. Due to the strong coordination between the ether oxygens of PEO chain and Li+ ion, a novel conduction mechanism of PEO/LiClO4 complex crystals is proposed on molecular level: the conformational exchange of the PEO chains leads to Li+ ion long-range transportation in crystalline region. 4) From the high-resolution solid-state 13C,7Li and 6Li NMR spectra of P(EO)n/LiClO4 complexes with different LiClO4 concentration, we find that only the crystal structure of P(EO)6/LiC104 is formed within the complex samples, when EO/Li≥6; The 13C,7Li, and Li NMR spectra acquired by single pulse excitation with very short recycle delays show that the association status of Li+ ion in amorphous region is almost the same. When EO/Li<6, the P(EO)3/LiCl04 crystal starts to appear and becomes the dominant crystal when EO/Li= 3. With the decrease of the EO/Li ratio, the noncrystalline I3C signal becomes broader significantly, indicating that the chain mobility of the noncrystalline region decreases accordingly.5) The cross peak in 2D 7Li-7Li NOESY spectrum of P(EO)6:LiClO4 sample demonstrate that there exists exchange between Li+ ions engaged in different coordination status. In addition, through measuring the 6Li spin-lattic relaxation time, biexponential relaxation behavior of the upfield Li+ ion signal is observed. All those results indicate that there exist Li+ ions engaged in at least two different coordination structures and there exists motional inhomogeneity even in amorphous region.6) Comparative studies on the chain mobility of the complex crystals and the amorphous component indicate that for complex samples with marked amorphous content, the amorphous region plays a dominant role in ionic conduction.7) Based on the detailed understanding of the PEO/LiClO4 system, we development a new quantitative method for two-component complex.In summary, the structure and chain dynamics of PEO/LiC104 complexes were studied by employing the state of art solid-state 13C,7Li and 6Li NMR techniques. Remarkable helical jump motions of the PEO segments in the complex crystals at ambient temperatures have been clearly demonstrated. The jump motions are believed to induce the transportation of the coordinated Li+ ions along the crystallographic c axis, providing with a novel mechanism of ionic conductivity of the complex crystals. Meanwhile, the chain mobility and the Li+ ion coordination status in the amorphous region can also be clearly revealed, giving very useful information for understanding the conducting mechanism of the complexes. In addition, this work shows that solid-state high-resolution NMR technique can be a powerful and general tool for elucidating the phase structures, dynamics and subsequently the conduction mechanism of crystalline polymer electrolytes. |
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