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Compositional dependence of free volume in PAN/LiCFɜSOɜ polymer-in-salt electrolytes and the effect on ionic conductivity
journal contributionposted on 2000-01-15, 00:00 authored by Maria ForsythMaria Forsyth, J Sun, D R Macfarlane, A J Hill
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 volume as measured by the orthopositronium pickoff lifetime, τ 3 . Beyond this salt concentration (i.e., 45-70 wt%) the free volume remains approximately 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 τ ≫1. The high salt content in these fast ion conductors results in a high susceptibility to polar solvents such as water. For all compositions, water absorption results in a free volume increase attributed to plasticization; however, in the fast ion conducting region, a significantly larger free volume response due to plasticization is measured and may be connected 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 °C for PAN and 85 °C for 66 wt% lithium triflate.