Polyethylene oxide with lithium trifluoromethane sulphonate; an NMR study

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.

A 7Li and 19F NMR relaxation study of LiCF3SO3 in plasticised solid polyether electrolytes

7Li and 19F NMR relaxation time (T1, T2, T1 rho) measurements have been used to probe the dynamics of LiCF3SO3 dissolved in an amorphous co-polymer poly(ethylene oxide-co-propylene oxide), and in particular the influence of the plasticising agents propylene carbonate and dimethyl formamide. The changes in relaxation behaviour of 19F and 7Li with increasing plasticiser concentration are very different, as is the effect of each plasticiser. These differences can be explained qualitatively in terms of the interaction between the plasticiser and the ions. Preliminary 7Li T1 rho measurements reveal two components at low temperatures.