Polymer chain diffusion and Li⁺ hopping of poly(ethylene oxide)/LiAsF₆ crystalline polymer electrolytes as studied by solid state NMR and ac impedance

Highly crystalline polymer electrolytes exhibit remarkable ionic conductivity which is oppose to the view that crystalline phase are insulators. The crystal structures of the four PEO₆/LiAsF₆ (PEO refers to poly(ethylene oxide)) crystalline complexes are the same, but the ionic conductivity decreases almost 3 orders of magnitude when the molecular weight of PEO increases from 1000 Da to 6000 Da. The 2D exchange ¹³C NMR experiments demonstrate that the chain diffusion motion of the polymer PEO is too weak to efficiently trigger the Li⁺ hopping within the tunnels. Furthermore, the analysis of the conductivity spectra by the Almond-West (AW) model shows that the hopping process of Li⁺ ion rather than the concentration of the charge carriers controls the conductivity of the crystalline polymer electrolytes and the relaxation mechanism of Li⁺ ion is independent of temperature. By analyzing the peak widths of the powder X-Ray patterns, it is shown that the crystallite sizes decrease from 2800 Å to 900 Å with the increase of the molecular weight of PEO from 1000 Da to 6000 Da. On basis of the above results, we believe that the ionic conductive mechanism for highly crystalline polymer electrolytes with low molecular weight (ca. < 10 K) is different from polymer electrolytes with PEO of high molecular weight. We propose a hypothetical mechanism of the ionic conductivity in highly crystalline polymer electrolytes, that is, the polymer matrix is almost motionless while the Li⁺ ion hops between adjacent units. Furthermore, the bigger crystallite sizes will produce less inter-crystallite misalignments and boundaries between adjacent crystallites, which facilitate the efficient transport of Li⁺ ion and lead to higher conductivity.