| Rechargeable solid lithium batteries have been considered the ideal power source for electric vehicles, energy storage and supercapacitors. Especially, poly(ethylene oxide)(PEO) based polymer electrolytes gain more attention for the property of high-energy density, no leakage risk, and being chemically stable and easy to process. For over30years, it was widely believed that ionic conductivity occurred predominantly in the amorphous phase above the glass transition temperature Tg, driven by the local random Brownian motion of amorphous polymer chains, and the crystalline phase was considered to be insulators. However, Bruce et al. recently demonstrated that highly crystalline PEO/alkali polymer electrolyte exhibit remarkable ionic conductivity. However, the mechanism of ion transport in those highly crystalline polymer electrolytes is not closed.This thesis mainly focus on the conductive mechanism of highly crystalline PEO/alkali polymer electrolytes. Relatively high ionic conductivity PEO/lithium hexafluoroarsenate (LiAsF6) complexes were chosen to study with the molar ratio of ether oxygen atoms to lithium ions equaling to6:1, hereafter abbreviated to PEO6/LiAsF6. A series of different molecular weight PEO6/LiAsF6complexes were prepared.(1) It is reported that1000g/mol PEO6/LiAsF6complex existed two different crystal structure at different annealing temperature. The crystal structure formed at low annealing temperature called alpha phase which is composed of two PEO chains that fold to form double-chain helical structure. At high annealing temperature, the alpha phase will transform into a new phase. And the newly formed crystal structure only consists of one PEO chain, which is twisted as a single-helical structure, hereafter referred to beta phase. By studying the phase transition of different molecular weight PEO6/HAsF6complexes, we demonstrate for the first time that the alpha PEO6/LiAsF6complexes can transform into beta polymorph at high annealing temperature only with low molecular weight of PEO (Mn≤10000g/mol). On the contrary, when the molecular weight of PEO larger than100000g/mol, this kind of phase transition will not occur. By comparison of the crystallization behavior of the polymer chain, we found that the cause of the molecular weight dependence phase transition is the chain "entanglement" in the amorphous phase.(2) Both alpha and beta morphology exhibit high resolution13C NMR spectra, so we apply13C f p-RFDR DQ experiment and2D exchange13C experiments for the assignment of the13C NMR spectra. Peak assignments show that the10high resolution peaks in13C NMR spectrum of the alpha morphology correspond to12different carbon atoms in the crystal lattice. While there are only5high resolution peaks in13C NMR spectrum of the beta morphology which also correspond to12different carbon atoms in the crystal lattice.(3) By characterizing the polymer chain motion of different molecular weight PEO6/LiAsF6complexes, it is found that long-term chain diffusion (-Hz) and local segmental reorientation motion (-kHz) with different time-scale are co-existed in the crystalline region. And these two kind of motion are competing relationship. With increasing the molecular weight of PEO, the crystallinities decrease, local segmental reorientation motion decreases, but long-term chain diffusion motion is strengthened. On the contrary, with the decrease of the molecular weight of PEO, the crystallinities of complexes increase, long-term chain diffusion decreases, local segmental motion is enhanced.(4) The characterization for low molecular weight PEO6/LiAsF6complexes demonstrates that the ionic conductivity decreases almost3orders of magnitude with the molecular weight of PEO increasing from1000g/mol to6000g/mol, though the crystal structure are the same. The analysis of the conductivity spectra by the Almond-West (AW) model, shows that the hopping process of Li+ions rather than the concentration of the charge carriers controls the conductivity of the crystalline PEO/Li+polymer electrolytes and the relaxation mechanism of Li+ions is independent of temperature. By characterization for polymer chain motion, it is found that long-term chain diffusion motion is too weak to efficiently trigger the Li+ions motion within the complex crystals. As a result, we believe that the conductive mechanism of Li+ions in low molecular weight PEO/alkali complexes is different from high molecular weight PEO/alkali polymer electrolytes which studied by our group before. It could be imagined that local segmental reorientation motion will play more important role in triggering the Li+ions motion within the complex crystals in low molecular weight PEO/alkali polymer electrolytes. Meanwhile, it is showed that the crystallite sizes decrease from2800? to900? with the increase of the molecular weight of PEO from1000g/mol to6000g/mol. We believe that the bigger crystallite sizes will produce less inter-crystallite misalignments and boundaries between adjacent crystallites, which facilitate the efficient transport of Li+ions and lead to higher conductivity.In summary, we believe that the local segmental reorientation motion controls the ion transportation in low molecular weight PEO/Li+complexes. While both long-term chain diffusion motion and local segmental reorientation motion can trigger the Li+ions movement within the complex crystals in high molecular weight PEO/Li+complexes. In short, high-resolution solid-state NMR techniques can be a powerful and general tool for characterizing different time scale polymer chain motion and crystal structure, subsequently the conductive mechanism of ion transport in crystalline polymer electrolytes. |
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