Li+ dynamics in a polymer nanocomposite:: An analysis of dynamic line shapes in nuclear magnetic resonance

Characterization of dynamics of the charge-carrying species in polymer electrolytes has proven difficult. In this work we focus on a nanocomposite polymer electrolyte created when poly(ethylene oxide) (PEO) is intercalated into a layered silicate, Li-montmorillonite. We characterize both the Li+-silicate distance and the cation dynamics by analysis of the changes in Li-7 nuclear magnetic resonance (NMR) line shape observed as the temperature is changed and cation diffusion is enabled. The observed spectra are compared to spectral simulations which emphasize the role of dipolar fields, associated with the static paramagnetic Fe3+ ions randomly distributed at the Al3+ lattice sites, interacting with the mobile cations. Low temperature line shapes are asymmetric, and not simply related to line shapes of more typical NMR interactions. Simulation of Li-7 NMR spectra and comparison to experimental spectra shows that the Li+ interacts most strongly with the silicate surface layer, and all our evidence indicates that the cation diffusion is restricted to the surface. Line shape narrowing is observed over the temperature range 270 less than or equal to T less than or equal to 420 K reflecting diffusion along the silicate surface. At higher temperatures motional narrowing leads to a limiting linewidth which depends on the spacing between silicate planes and not on the spacing between Li+ and those planes. The high temperature line shape has the same orientation dependence as chemical shift anisotropies. Li+ diffusion rates appear consistent with values reported previously for this system and with a simplified line shape analysis. (C) 1999 American Institute of Physics. [S0021-9606(99)70111-X].