Synthesis and Lithium Ion Conduction of Polysiloxane Single-Ion Conductors Containing Novel Weak-Binding Borates

Abstract

Three borate monomers: lithium triphenylstyryl borate (B1), a variant with three ethylene oxides between the vinyl and the borate (B2) and a third with perfluorinated phenyl rings (B3) were synthesized and used to prepare polysiloxane ionomers based on cyclic carbonates via hydrosilylation. B1 ion content variations show maximum 25 °C conductivity at 8 mol %, reflecting a trade-off between carrier density and glass transition temperature (<i>T</i>_g) increase. Ethylene oxide spacers (B2) lower <i>T</i>_g, and increase the dielectric constant, both raising conductivity. Perfluorinating the four phenyl rings (B3) lowers the ion association energy, as anticipated by ab initio estimations. This increases conductivity, a direct result of 3 times higher measured carrier density. The ∼9 kJ/mol activation energy of simultaneously conducting ions is less than half that of ionomers with either sulfonate or bis­(trifluoromethanesulfonyl) imide anions, suggesting that ionomers with weak-binding borate anions may provide a pathway to useful single-ion Li⁺ conductors, if their <i>T</i>_g can be lowered