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

摘要

beta-detected NMR (beta-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 beta-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 Li-8(+) was observed in all of the films above similar to 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. Published by AIP Publishing.