Ionic conductivity behavior of the liquid polymer electrolytes formed by dissolving the lithium salts in poly(ethylene oxide) and poly(propylene oxide) was investigated. A high resolution NMR(Nuclear Magnetic Resonance) relaxation technique was used to study the dynamics of the ions in polymer electrolytes. The $^7$Li NMR results revealed the coexistence of two separate lineshape components with substantially different spin-spin relaxation times($T_2$) in liquid polymer electrolytes. From the resolution of the two components, we could estimate the spin-spin relaxation time associated with the mobility of free mobile lithium ions and also the fraction of free lithium ions contributing to the ionic conductivity.
The temperature dependence of ionic conductivity in liquid polymer electrolyte followed Vogel-Tamman-Fulcher equation, which indicated the ionic mobility was dependent on the segmental motion of the polymer chain. From the $^7$Li relaxation data in PEO and PPO, it was found that there was an obvious difference between in their respective modes of the ion-polymer interaction and carrier migration. This difference probably arised from their structures and physical properties such as dipole moment or dielectric constant. The combination of the ionic conductivity and NMR data of the polymer electrolyte with the different molecular weights provided the evidence that the difference in conductivity was caused by the ionic migration process rather than the carrier generation process. And we could interpret the maximum conductivity phenomena observed in the plot of the ionic conductivity versus the salt concentration by using the $^7$Li relaxation parameters.
