Synthesis and Ionic Conductivity of Polymer Electrolytes Based on a Polyphosphazene with Short Side Groups

Random copolymers of the polyphosphazene [NPR 2 ] n have been synthesized via living ionic polymerization with mixed substituents at the phosphorus atoms (i.e., R = bis(2-methoxy-ethyl)amino and n -propylamino). The polymers melt at 190 °C and start to decompose above 300 °C. Thin polymer electrolyte membranes were prepared by solution casting with dissolved lithium triflate (LiSO 3 CF 3 ) and with NaI. The transparent membranes showed favorable mechanical properties below 100 °C. T g values ranged between −50 and −36 °C. Membranes with 10 wt % LiSO 3 CF 3 (corresponding to the atomic ratio Li/(O + N) = 1/30) showed rather low conductivities between 3.2 × 10 - 7 S cm - 1 at 30 °C and 1.9 × 10 - 5 S cm - 1 at 100 °C as determined from impedance measurements. The dispersion of 4 wt % Al 2 O 3 nanoparticles in the polyphosphazene membranes with 10 wt % LiSO 3 CF 3, however, leads to an increase of the conductivities by 2 orders of magnitude, that is, 1.0 × 10 - 5 S cm - 1 at 30 °C and 1.5 × 10 - 3 S cm - 1 at 100 °C. The heterogeneously doped salt-in-polymer membranes thus combine good mechanical stability with a high ionic conductivity. The temperature dependence of the conductivity data was analyzed in terms of the MIGRATION model, which gives a consistent explanation of the non-Arrhenius dependence.