Compositional dependence of free volume in PAN/LiCF3SO3 polymer-in-salt electrolytes and the effect on ionic conductivity

The free volume behaviour of the polyacrylonitrile/lithium triflate system is investigated over the composition range 0-75 wt % salt. The addition of salt, up to 45 wt %, to the PAN polymer substantially increases the free vo lume as measured by the orthopositronium pickoff lifetime, tau(3). Beyond t his salt concentration (i.e., 45-70 wt %) the free volume remains approxima tely constant. This constant free volume region corresponds to a region of high ionic conductivity in the glassy state, making these materials polymer -based fast ion conductors, that is, having a decoupling ratio R-tau much g reater than 1. The high salt content in these fast ion conductors results i n a high susceptibility to polar solvents such as water. For all compositio ns, water absorption results in a free volume increase attributed to plasti cization; however, in the fast ion conducting region, a significantly large r free volume response due to plasticization is measured and may be connect ed to a percolation morphology in these samples. Salt addition is shown to lower the T-g, as measured by positron annihilation lifetime spectroscopy ( PALS). T-g is 115 degrees C for PAN and 85 degrees C for 66 wt % lithium tr iflate. (C) 2000 John Wiley & Sons, Inc.