Phase Structures And Segmental Dynamics Of Poly（Ethylene Oxide）/LiCF₃SO₃Complexes As Studied By Solid-State High-Resolution¹³C Nuclear Magnetic Resonance

Solid polymer electrolytes (SPEs) have been a very active field of science since1970s due to their potential application in all-solid rechargeable batteries. Recently, a series of highly crystallized polymer electrolytes based on poly(ethylene oxide)(PEO) and alkaline metal salts have attracted great reseach interests for their high ionic conductivity at room temperature (1×10-6S·cm). In our work, we demonstrated that the solid-state13C nuclear magnetic resonance (NMR) spectra of PEO-alkaline metal salt complexes i.e. PEO/LiCF3SO3, PEO/LiClO4, PEO/LiAsF6, PEO/NaClO4all show very high resolution at ambient temperatures. Such a high-resolution nature of13C NMR spectra of these semi-crystalline complexes provides with not only the possibility of understanding the ionic conduction mechanism of crystalline polymer electrolytes, but also the opportunity of in-depth study over the helical jump motion, which is crucial for understanding many fundamental characteristics of semi-crystalline polymers.In this work, solid-state high-resolution13C NMR studies are carried out to deeply disscuss the phase structures and segmental dynamics of the PEO/LiCF3SO3system. A bief summary of this work is listed in the following.1. The phase structures of the PEOn/LiCF3SO3(n=3,6,12and30) complexes vary greatly with EO:Li molar ratios. Both the PEO3:LiCF3SO3crystalline complex and the PEO3:LiCF3SO3amorphous complex with EO:Li molar ratio of3:1exist in all of the samples. The amorphous PEO phase and the crystalline PEO phase start to appear when EO:Li molar ratio becomes larger than3and6, respectively. Therefore, there are four phase components coexisting in the complexes with EO:Li molar ratios larger than6. Such complicated phase structures of PEO based SPEs have not been disclosed before. Moreover, it is demonstrated that EO:Li molar ratio of the PEO3:LiCF3SO3amorphous complex remains to be3:1with the variation of EO:Li molar ratio of the sample.2. The solid-state high-resolution13C NMR spectrum of the PEO3:LiCF3SO3crystalline complex shows six peaks at room temperature. Peak assignment has been made successfully through the combination of the13C dipolar-INADEQUATE and the2D-exchange spectra. These six peaks are found to have simple correspondence with the six-CH2-groups in a repeat unit of the PEO3:LiCF3SO3crystalline complex. 3. The structure of the PEO3:LiF3SO3crystalline complex is independent of the molecular weight of PEO (Mn=1×103,2×103,4×103,6×103,1×104,1×105,5×106g·mol-1). However, phase structures as reflected in the crystallinities of the PEO3/LiF3SO3complexes change greatly with the molecular weight of PEO. The behaviours of these PEO3/LiF3SO3complexes with respect to different molecular weight of PEO are the same as those of neat PEO with different molecular weight. This seems to imply that the morphology of the PEO3/LiF3SO3complex is the same as that of neat PEO.4. The helical jump motion only exists for the complexes with the molecular weights of PEO larger than2×103g mol-1at293K, and more importantly the intensity of the helical jump motion dramatically increases with increasing molecular weight of PEO. It is demonstrated that the helical jump rate of the crystalline PEO segments depends on the relative content and the chain mobility of the amorphous structures for PEO-alkali metal salt complexes. The sufficient amount of amorphous structures is the necessary condition for the helical jump motion to happen. The amorphous structures not only play the role to receive the repeating units "stepping out" the crystallites, but also to provide repeating units that "step in" the crystallites in the process of the helical jump motion of the crystalline PEO segments. Based on the above recognition, we tend to believe that the helical jump motion is corresponding to the movement of an entire PEO chain embedded in the crystallites.5. The phase structures of the PEO-PPOn/LiCF3SO3(n=3,6,12and30) complexes vary greatly with EO-PO:Li molar ratios. The PEO3:LiCF3SO3crystalline complex exists in all of the samples. The poly(propylene oxide)(PPO) phase and the crystalline PEO phase start to appear when EO-PO:Li molar ratio becomes larger than3and6, respectively.Overall, The high-resolution nature of the13C NMR spectra of these polymer complexes provides with not only the possibility of understanding the ionic conduction mechanism of crystalline polymer electrolytes, but also the opportunity of in-depth study over the helical jump motion, which is crucial for understanding many characteristics of semi-crystalline polymers. More application of this technique on polymer complexes is expecting.