β‑NMR Measurements of Lithium Ion Transport in Thin Films of Pure and Lithium-Salt-Doped Poly(ethylene oxide)

β-Detected nuclear spin relaxation of 8Li+ has been used to study the microscopic diffusion of lithium ions in thin films of poly­(ethylene oxide) (PEO), where the implanted lithium ions are present in extremely low concentration, and PEO with 30 wt % LiCF3SO3 over a wide range of temperatures both above and below the glass transition temperature. Recent measurements by Do et al. [Phys. Rev. Lett. 2013, 111, 018301] found that the temperature dependence of the Li+ conductivity was identical to that of the dielectric α relaxation and was well described by the Vogel–Fulcher–Tammann relation, implying the α relaxation dominates the Li+ transport process. In contrast, we find the hopping of Li+ in both samples in the high temperature viscoelastic phase follows an Arrhenius law and depends significantly on the salt content. We propose that the hopping of Li+ between cages involves motion of the polymer but that it is only for long-range diffusion where the α relaxation plays an important role.