Morphologies and dynamics in low-Tg single-ion conductors: Effects of comonomers, plasticizers, and functionalized nanoparticles

Single-ion conducting polymers, or ionomers, are under extensive investigation for applications as solid electrolytes in battery applications. Slow segmental dynamics of viscous ionomers make them inadequately poor conductors. Faster segmental dynamics are attained by decreasing the glass transition temperature (Tg) of the ionomer. Three compositional avenues are presented to reduce the Tg of a PEO-based lithium conducting ionomer (Tg ~ -12 °C): copolymers, blends, and nanocomposites. A fourth study employs weak-binding salts and flexible siloxanes to achieve a low T g ionomer.;Random multiblock copolymers with PEO and PTMO segments spaced by lithium sulfonate groups between each block are employed reveal that the enhanced segmental dynamics provided by PTMO (Tg ~ -70 °C) are insufficient to offset the poor ion solvation ability caused by low ether oxygen content. Segmental dynamics of the PEO-based lithium conductor (without PTMO) can be enhanced by polymeric and nanoparticle plasticizers. PEG oligomeric plasticizer and silica nanoparticles functionalized with PEO are both capable of depressing the glass transition temperature of the ionomer. Consequently, accelerated ion dynamics are observed for both systems without salt or solvent additives. With functionalized nanoparticles, these findings are of particular interest since the nanoparticles are solid fillers while the PEG oligomeric plasticizer is liquid-like. As an alternative to plasticizing an ionomer with additives, single-ion conductors based on highly flexible siloxane backbones and low binding energy salts can demonstrate very low Tgs (Tg ~ -80 °C). The charge-delocalized nature of tetrabutylphosphonium salt prevents ionic aggregation and ionic conductivity is independent of ion content. By establishing these correlations between accelerated segmental dynamics and ionic conductivity, it will be possible to explore new chemistries that decouple the two properties in single-ion conductors.