Synthesis and characterization of lithium-salt complexes with difluoroalkoxyborates for application as lithium electrolytes

We present a modified method for the synthesis of a Lewis acid of acidity lower than that of BF3: ROBF2, (where R is an oligooxyethylene substituent containing 1 to 7 ethylene oxide (EO) monomeric units). The synthesis consisted of thermal decomposition of trifluoroalkoxyborate salts under reduced pressure. The derivatives synthesized were applied in complexation reactions with low-molecular-weight lithium salts, such as LiF, LiI, LiPF6 CH3COOLi, CF3COOLi and (COOLi)2. The salts obtained show properties of ionic liquids at ambient temperature. On the basis of NMR, FTIR and EIS spectroscopy, the course of the complexation reaction has been proposed and the properties of the salts obtained have been determined. The system with CF3COOLi has the highest ambient-temperature ionic conductivity of the carboxylic complex salts studied (on the order of 10−4 S cm−1). The product of the reaction of CH3(OCH2CH2)2OBF2 and LiI, applied in a solid polymer electrolyte (10 mol% in PEO), was characterized by ionic conductivity of the order of 10−6 − 10−3 S cm−1 over a temperature range between 20 − 90 °C, and high lithium transference number (0.80 − 0.87).