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 (T g ) increase. Ethylene oxide spacers (B2) lower T 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 T g can be lowered.

Cluster-continuum quantum mechanical models to guide the choice of anions for Li+-conducting ionomers

“…These values are much larger than those for the other 10 Li-anion systems considered because they have a very diffuse negative charge on the anion. 53 From the dielectric relaxation spectroscopy measurement, the conducting ion fraction for Li + ionomers at room temperature is: perfluorinated tetraphenylborate (15%) 53 > tetraphenylborate (5%) 53 > TFSI (0.1%) 65 > benzene sulfonate (0.009%), 18,20 in quantitative agreement with our CCM values in Table IV. Figure 9(a) suggests that trends in locally charged species fractions do not differ between the gas phase and CCM.…”

“…The CCM energies of the four ion states can be used to guide the choice of Li single-ion conductors, as has been done previously in choosing tetraphenylborate and perfluorotetraphenylborate anions attached to siloxane backbones. 53 The gas-phase ordering of anion-Li pair interaction energies is preserved in both the PCM (Table I) and CCM (Table II) calculations, suggesting gas phase calculations suffice for screening anions of interest.…”

“…Of the anions considered, the perfluorotetraphenylborate anion was found to have the weakest binding energy with Li + , owing to delocalizing the anionic charge. 48,53 Johansson et al employed ab initio methods to do extensive gas-phase calculations for the dissociation reaction LiA → Li + + A − . 37 The difference between their ion-pair binding energies and our counterparts in Table I is negligible (±2%) despite their use of a slightly larger basis set (6-311+G*).…”

“…The Li-perfluorotetraphenylborate system has an insignificant CCM pairing energy that should be expected to facilitate ion dissociation. 48,53 The ratio of the CCM quadrupole energy and twice the CCM pair energy is calculated to evaluate the propensity of ions to cluster. The large value of E CCM quad /(2 E CCM pair ) for Li-F is consistent with the fact that this salt does not dissolve in PEO.…”

Ion Transport in Solvent-Free, Crosslinked, Single-Ion Conducting Polymer Electrolytes for Post-Lithium Ion Batteries

“…Anions with delocalized electron densities have been shown to enable high ionic conductivities, because the delocalized charge promotes dissociation of the ion pair and increases free cation concentration in the solvating polymer (such as the PEO chain) [13,18,[23][24][25][26]. One such highly delocalized tethered anion is (4-styrenesulfonyl)(trifluoromethanesulfonyl)imide (STFSI).…”