Influence of Solvating Plasticizer on Ion Conduction of Polysiloxane Single-Ion Conductors

Lithium ion conduction is investigated for a polysiloxane-based single-ion conductor containing weak-binding borates and cyclic carbonate side chains, plasticized with poly(ethylene glycol) (PEG). The addition of PEG increases the conductivity by up to 3 orders of magnitude compared to the host ionomer. A physical model of electrode polarization is used to separate ionic conductivity of the ionomers into number density of simultaneously conducting ions and their mobility. A reduction in T-g with increasing PEG content boosts ion mobility owing to an increase in polymer chain flexibility. Further, the PEG ether oxygens lower the activation energy of simultaneously conducting ions (from 14 to 8 kJ/mol, significantly increasing conducting ion content by 100X, suggesting that ion aggregates observed in the host ionomer are solvated by PEG. This directly reflects the disappearance of an ion aggregation peak observed in X-ray scattering, and an initial large increase in static dielectric constant (epsilon(s)), upon addition of PEG, suggesting that ionic aggregation is significantly reduced by a small amount of PEG. Further dilution with lower dielectric constant PEG gradually reduces epsilon(s).