Synthesis and characterization of new trifluoroalkoxyborates lithium salts of ionic liquid properties

Abstract

A new class of lithium trifluoroalkoxyborate salts Li[ROBF 3] containing as the R substituent oligooxyethylene groups of various molecular weights in the form of a methyl monoether (R CH 3(OCH 2CH 2) n ) ( n = 1, 2, 3 and 7) or propionitrile, has been obtained in the reaction of BF 3 etherate with appropriate alcohol and butyllithium. The obtained salts are characterized by properties of room temperature ionic liquids (RTIL) of viscosity dependent on the type of substituent in the alkoxy group. The room temperature ionic conductivity of neat salts is of the 10 −4 S cm −1 order, and the best properties are characterized by the salt containing n = 3 oxyethylene monomeric units. Optimal concentrations in aliphatic carbonates of the obtained salts were determined achieving ionic conductivities of the order of 10 −3 S cm −1, slightly higher than that of analogous LiBF 4 solutions. Solid polymer electrolytes containing 10 and 50 mol% of borate salts and PEO in the form of flexible films have been determined. On the basis of DSC analysis it was found that borate salts at low concentration do not limit the tendency of PEO to crystallization, the share of the crystalline phase was over 60% which resulted in low ionic conductivity. The best properties were characterized by the system with the salt of n = 7 oxyethylene monomeric units ( T g = −53 °C) and the system of high salt concentration ( σ RT 10 −5 S cm −1).