Characterizations of a branched ester-type lithium imide in methoxy ether-substituted poly(organophosphazenes)-based solid polymer electrolytes

A branched ester-type lithium imide-lithium bis[(1,1,1,3,3,3-hexafluoro-2-propoxy) sulfonyl] imide (LiN[SO2OCH(CF3)2]2, LiHFPSI) was used as the salt for methoxy ether-substituted poly(organophosphazenes) {NP[(OCH2CH2)xOCH3]2}n (where x = 2-4, MExP) based polymer electrolytes. Their ionic conductivity, electrochemical stability and thermal stability have been characterized. The highest conductivity at 25 °C for MExP-LiHFPSI complexes (4.48 × 10−4 S/cm) was obtained when x = 4 and the molar ratio of Li+ to repeat unit was 3:8. The results of the FT-IR spectroscopy were examined to gain further insight into the structure of the electrolytes. The results of thermal measurements showed that these electrolytes were fully amorphous materials and were stable up to 200 °C. The electrochemical experiments showed the electrochemical stability window of these electrolytes were in excess of 6.0 V (versus Li+/Li).