Ion transport in silica nanocomposite electrolytes

The ion-transport properties of composite electrolytes composed of oligomers of poly(ethylene glycol) dimethyl ether, hydrophobic fumed silica, and Li(CF3SO2)(2)N (LiTFSI) are investigated using nuclear magnetic resonance (NMR), electrophoretic NMR (ENMR), ac impedance spectroscopy, and rheology. The effects of fumed silica and salt concentration on ionic conductivity, diffusivity of ions and oligomers, and lithium transference number (T-Li) are examined at 30degreesC. The fumed silica forms a self-supporting network with large pores such that the network, regardless of silica concentration, has little effect on ion-transport characteristics. Examination of the effect of salt on ion transport reveals a maximum ionic conductivity at around 1.06 M, which is attributed to a tradeoff of adding more charge carriers balanced against increased ion-ion interactions and reduced mobilities. T-Li with respect to salt concentration surprisingly passes through a minimum around 0.35 M. The increase in T-Li at higher concentrations is attributed to the mobilities of cations, anions, and solvating oligomer becoming constrained to the same value due to "loss of free volume." The values of T-Li at low salt concentrations (<0.35 M) are attributed to the ions existing in either a fully dissociated state or primarily as charged complexes. Results of T-Li from ENMR and from estimation via pulse field gradient NMR (pfg-NMR) are compared showing that pfg-NMR consistently overestimates T-Li. Finally, a comparison is presented of measured conductivity with that calculated from the Nernst-Einstein equation and diffusivities found from pfg-NMR measurement; we discuss possible reasons why it is inappropriate to estimate ion-pair formation by this comparison. (C) 2003 The Electrochemical Society.