β-NMR measurements of molecular-scale lithium-ion dynamics in poly(ethylene oxide)-lithium-salt thin films

β-detected NMR (β-NMR) has been used to study the molecular-scale dynamics of lithium ions in thin films of poly(ethylene oxide) (PEO) containing either lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) or lithium trifluoroacetate (LiTFA) salts at monomer-to-salt ratios (EO/Li) of 8.3. The results are compared with previous β-NMR measurements on pure PEO and PEO with lithium triflate (LiOTf) at the same loading [McKenzie et al., J. Am. Chem. Soc. 136, 7833 (2014)]. Activated hopping of ⁸Li⁺ was observed in all of the films above ∼250 K, with the hopping parameters strongly correlated with the ionicity of the lithium salt rather than the polymer glass transition temperature. The pre-exponential factor increases exponentially with ionicity, while the activation energy for hopping increases approximately linearly, going from 6.3±0.2 kJ mol^-1 in PEO:LiTFA to 17.8±0.2 kJ mol^-1 in PEO:LiTFSI. The more rapid increase in the pre-exponential factor outweighs the effect of the larger activation energy and results in ⁸Li⁺ hopping being fastest in PEO followed by PEO:LiTFSI, PEO:LiOTf, and PEO:LiTFA.