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 LiCF(3)SO(3) (LiTf), which has a conductivity at 30 degrees C of up to 1.3 x 10(-4) S cm(-1) and up to 6.9 x 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 (7)Li, (19)F and (1)H, 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, (7)Li and (19)F 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.

Transport Mechanisms of Ions in Graft-Copolymer Based Salt-in-Polymer Electrolytes

“…2b) is presented at two different salt concentrations, i.e. O : Li = 34 : 1 (filled symbols) and O : Li = 9 : 1 (open symbols), respectively [48]. The temperature dependence of diffusion coefficients is generally of Arrhenius type.…”

“…Viscosity experiments (not shown here) show that the overall fluidity of the polymer electrolyte monotonically decreases with salt content [58]. The salt may have a temporary crosslinking effect here, since the cations mediate an attractive coordination interaction between the chains [48] (see Sect. 4.3).…”

“…In the systems studied here, we showed that the BPP model can be applied to PSO and PSO-X for 19 F and 1 H, whereas for 7 Li it is not valid and data had to be evaluated with the Cole-Davidson model [48]. In the case of polyphospazenes, activation energies were extracted from the data using the model-free approach of Eq.…”

“…Pulsed-field-gradient NMR (PFG-NMR) was used to measure the selfdiffusion coefficients of 7 Li, 19 F, and 1 H as described previously [48]. The decay of the signal in dependence on the gradient strength g resulted in the diffusion coefficient by fitting the exponential decay function…”

“…Spin-lattice relaxation times T 1 of the cation ( 7 Li), the anion ( 19 F), and the polymer ( 1 H, in the oligo-ether side chain) were measured with the inversion recovery sequence as a function of temperature as described earlier [48].…”