Activation of transport and local dynamics in polysiloxane-based salt-in-polymer electrolytes: a multinuclear NMR study

Abstract

We have investigated a new improved lithium ion conducting salt-in-polymer electrolyte system consisting of a polysiloxane backbone with oligoether side chains and added LiCF₃SO₃ (LiTf), which has a conductivity at 30 °C of up to 1.3 × 10⁻⁴ S cm⁻¹ and up to 6.9 × 10⁻⁵ S cm⁻¹ after cross-linking, which is employed to enhance mechanical stability. The mechanisms governing local dynamics and mass transport have been studied on the basis of temperature dependent spin–lattice relaxation time and pulsed field gradient diffusion measurements for ⁷Li, ¹⁹F and ¹H, respectively. The correlation times characterizing the local ion dynamics reflect the complexation of the cations by the polyether side chains of the polymer and show the anion as the more mobile species. In contrast, ⁷Li and ¹⁹F diffusion coefficients and their activation energies are rather similar, suggesting the formation of ion pairs with similar activation barriers for cation and/or anion long-range transport. In general, the activation energies describing local reorientation are significantly smaller than those characterizing long range diffusion, suggesting that the long-range transport of both cations and anions is a much more complex process than a simple succession of free ion jumps, and involves (1) the coupling of conformational side-chain reorientations to the cation movement, and (2) the correlated diffusion of cations and anions within dimers or clusters. An important practical conclusion from our results is that the relatively high ionic conductivity in polysiloxane-based polymer electrolytes could even be increased if salt dissociation could be enhanced further.