New boron compounds as additives for lithium polymer electrolytes

A new class of difluoroalkoxyborane compounds ([RnOBF2]2) containing oligooxyethylene groups of various molecular weight in the form of a methyl monoether (Rn = CH3(OCH2CH2)n, n = 1, 2, 3 and 7) has been obtained in the reaction of BF3 etherate with appropriate glycols. 1H, 11B and 19F NMR spectral analysis of the derivatives obtained was carried out and the properties as Lewis acids of these derivatives have been compared with that of corresponding trialkoxyboranes and boron trifluoride in reaction with pyridine. The strength of the interaction of [R2OBF2]2 with the differing in “hardness” anions of various lithium salts has been analyzed on the basis of NMR spectra. The [RnOBF2]2 obtained were used as additives for polymer electrolytes containing PEO as polymer matrix and various lithium salts at an equimolar ratio of the boron compound to salt. The highest ionic conductivities, in the order 10−5 to 10−4 S cm−1 at 20–70 °C, were achieved for systems containing LiI and LiN(CF3SO2)2. The lithium transference number (t+) values, determined by the electrochemical method by steady-state technique for LiF and LiCF3SO3 are in the 0.6–0.8 range.