Synthesis and ionic conductivity of polymer electrolytes based on a polyphosphazene with short side groups

Paulsdorf J, Kaskhedikar N, Burjanadze M, Obeidi S, Stolwijk NA, Wilmer D, Wiemhofer HD

Abstract

Random copolymers of the polyphosphazene [NPR2](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 degrees C and start to decompose above 300 degrees C. Thin polymer electrolyte membranes were prepared by solution casting with dissolved lithium triflate (LiSO3CF3) and with NaI. The transparent membranes showed favorable mechanical properties below 100 degrees C. T-g values ranged between -50 and -36 degrees C. Membranes with 10 wt % LiSO3CF3 (corresponding to the atomic ratio Li/(O+N) = 1/30) showed rather low conductivities between 3.2 x 10(-7) S cm(-1) at 30 degrees C and 1.9 x 10(-5) S cm(-1) at 100 degrees C as determined from impedance measurements. The dispersion of 4 wt % Al2O3 nanoparticles in the polyphosphazene membranes with 10 wt % LiSO3CF3, however, leads to an increase of the conductivities by 2 orders of magnitude, that is, 1.0 x 10(-5) S cm(-1) at 30 degrees C and 1.5 x 10(-3) S cm(-1) at 100 degrees 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.