Characterizations of a branched ester-type lithium imide in poly(ethylene oxide)-based 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 salt for poly(ethylene oxide) (PEO)-based polymer electrolytes. Its conductivity, electrochemical stability, cationic transference number and thermal stability were characterized. The conductivities of these electrolytes correlated with the concentration of the lithium salt and the most conductive electrolyte (3.97×10–5 S/cm, 30 °C) was obtained when the molar ratio of EO unit to lithium salt was 8. The results from thermal analysis also exhibited that the lithium salt decreased the crystallinity of PEO efficiently. The electrochemical experiments showed that the electrochemical window of the polymer electrolyte was in excess of 5.8 V vs. Li+/Li and the transference number of cation could reach up to 0.47. Compared with the traditional lithium imide-lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), LiHFPSI exhibited the better comprehensive properties as the lithium salt in polymer electrolytes.