Influence of the alkali metal salt on the properties of solid electrolytes derived from a Lewis acidic polyether

Abstract

A polyether containing Lewis acidic boron atoms in its branched chain architecture was synthesised by a condensation reaction of boron trioxide, triethylene glycol monomethyl ether and poly(ethylene glycol), the latter having a molecular weight of 300 g/mol. Electrolytes based on this polymer and several different alkali metal salts were prepared and investigated. The state of the ions in the electrolytes was studied by FTIR spectroscopy, which detected the presence of ion pairs in electrolytes containing lithium triflate and lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt. By thermogravimetry, all the electrolytes except those containing LiCl were found to be stable up to at least 150 °C. In general, an increased “hardness” of the basic anion gave rise to a decrease in the thermal stability. The reason behind this was thought to be an increasing destabilisation of the Lewis acidic boronate esters by interactions with the anions. The ionic conductivity of the electrolytes followed Vogel–Tammann–Fulcher (VTF) relationships, and was consistently found to increase with a decreased “hardness” of the basic anion. Consequently, the lowest conductivity at 30 °C for electrolytes with a salt concentration corresponding to [EO] : [Li] ∼46 : 1 was measured for LiCl, 4 × 10 − 6 S/cm, while the corresponding highest conductivity, 8 × 10 − 5 S/cm, was recorded for the LiTFSI electrolyte.