Transport mechanisms of ions in graft-copolymer based Salt-in-polymer electrolytes

Kunze M, Schulz A, Wiemhöfer H, Eckert H, Schönhoff M

Abstract

Salt-in-polymer electrolytes based on graft copolymers with oligoether side chains andadded LiCF3SO3 (LiTf) are investigated concerning the transport and dynamics of theionic species with respect to applications as Li ion conductors. Polymer architectures arebased on polysiloxane or polyphosphazene backbones with one or two side chains permonomer, respectively. NMR methods provide information about molecular dynamics ondifferent length scales: The mechanisms governing local dynamics and long range masstransport are studied on the basis of temperature dependent spin-lattice relaxation ratesand pulsed field gradient diffusion measurements for 7Li, 19F and 1H, respectively. Thecorrelation times characterizing local ion dynamics reflect the complexation of the cationsby the oligoether side chains of the polymer. 7Li and 19F diffusion coefficients and theiractivation energies are rather similar, suggesting the formation of ion pairs and clusterswith similar activation barriers for cation and/or anion long-range transport. Activationenergies of local reorientations are generally significantly smaller than activation energiesof long range diffusion. Long range transport is affected by (1) the coupling ofconformational side chain reorientations to the cation movement, and (2) the correlateddiffusion of cations and anions within ion pairs. Ion pairs and their dissociation playa major role in controlling the resulting conductivity of the material. Guidelines formaterial optimization in terms of a maximized conductivity can thus be derived byidentifying a compromise between high ionic mobility and good Li complexation by thecoordinating side chains. © by Oldenbourg Wissenschaftsverlag, München.