Fluorine-free lithium-based flexible gel electrolytes: stretchability versus diffusivity

We report on the translational dynamics of ions in fluorine-free gels prepared using a flexible lithium salt comprising a (2-methoxyethoxy)acetate (MEA) anion, ethylene glycol (EG) and polyvinyl alcohol (PVA). ¹H and ⁷Li Pulsed-Field-Gradient (PFG) NMR were performed on thin (0.2 mm) and thick (0.7 mm) films of the gels at different orientations with respect to the external magnetic field and magnetic gradients. Two diffusional decay components are observed: a slow mode linked to PVA network oscillations and a fast mode from mobile ions with diffusivities up to three orders of magnitude higher. It is found that Li⁺ cations are not directly bound to the network, but they have a distribution of diffusion coefficients. A similar trend is observed for diffusivities of the organic anion, (MEA), revealed from the fast component of diffusional decays in ¹H PFG NMR. The diffusivities of ions have an orientational dependence on the films and are higher in thick films in the normal orientation with respect to the external magnetic field. The temperature dependence of Li⁺ diffusivities does follow the Arrhenius behavior. An interesting observation is that an elongation of the gel films by stretching reduces Li⁺ cation diffusivities and leads to broadening of the ⁷Li NMR resonance lines, suggesting that the transport properties of the ions are strongly governed by the structural constraints and internal stresses in the gel network.