A simple method for synthesizing polymeric lithium salts exhibiting relatively high cationic transference number in solid polymer electrolytes

Two polymeric lithium salts, lithium polyperfluorobutylene-1,4-bis-sulfonylimide (LiPBSI) and lithium polyperfluorohexylene-1,6-bis-sulfonylimide (LiPHSI), with different CF2 backbone lengths were synthesized by a new and easy method. These two polymeric lithium salts were successfully converted, via ion exchange and Li2CO3 treatment, from polymeric potassium salts which were prepared by condensation polymerizations of perfluoroalkylene-bis-sulfonyl fluorides (FSO2C4F8SO2F and FSO2C6F12SO2F) with 15% potassium hexamethyldisilizane in toluene. Both LiPBSI and LiPHSI were characterized by multi-nuclear (1H, 19F, or 13C) NMR, FT-IR, TGA, DSC, GPC, and so on. The ionic conductivity results of solid polymer electrolytes prepared by resulting lithium salts and poly(ethylene oxide) (PEO) showed that LiPBSI/PEO electrolyte had higher ionic conductivity. Furthermore, the lithium ion transference number of both LiPBSI/PEO and LiPHSI/PEO films were close to 0.4, which was twice that of film prepared from commercial lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).