Fast ion transport phenomena in oriented semicrystalline LiI-P(EO)_n-based polymer electrolytes

We have employed a variety of experimental methods, including DC and AC conductivity, scanning electron microscopy (SEM), atomic force microscopy (AFM), differential scanning calorimetry (DSC), Fourier Transform Infrared (FTIR) spectroscopy, and pulsed field gradient nuclear magnetic resonance (NMR), to investigate the poly(ethylene oxide):LiI system. The effect of stretching the polymer electrolyte on its DC conductivity is dramatic, resulting in up to a 40-fold increase in the LiI P(EO)₇ composition. Structural ordering imposed by the stretching is observed in SEM and AFM images, and the cation solvation sheath (i.e., the helical PEO structure) is also affected by stretching in a manner believed to favor enhanced transport, according to the FTIR results. The NMR results demonstrate unambiguously that Li⁺ diffusivity is anisotropic and enhanced along the stretch direction. Although the cation transport mechanism in polyether-salt polymer electrolytes is believed to rely heavily on polymer segmental mobility, this investigation suggests that other factors also contribute significantly. Such factors which can be augmented by stretching are modest changes in the cation solvation sheath and alignment of the helical structural units characteristic of PEO and its salt complexes.