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Structural correlations tailor conductive properties in polymerized ionic liquids

Publication Type
Journal Name
Physical Chemistry Chemical Physics
Publication Date
Page Numbers
14775 to 14785

Polymerized ionic liquids (PolyILs) are promising materials for applications in electrochemical devices
spanning from fuel cells to capacitors and batteries. In principle, PolyILs have a competitive advantage
over traditional electrolytes in being single ion conductors and thus enabling a transference number
close to unity. Despite this perceived advantage, surprisingly low room temperature ionic conductivities
measured in the lab raise an important fundamental question: how does the molecular structure
mediate conductivity? In this work, wide-angle X-ray scattering (WAXS), vibrational sum frequency
generation (vSFG), and density functional theory (DFT) calculations were used to study the bulk and
interfacial structure of PolyILs, while broad band dielectric spectroscopy (BDS) was used to probe
corresponding dynamics and conductive properties for a series of the PolyIL samples with tunable
chemistries and structures. Our results reveal that the size of the mobile anions has a tremendous
impact on chain packing in PolyILs that wasn’t addressed previously. Larger mobile ions tend to create a
well-packed structure, while smaller ions frustrate chain packing. The magnitude of these changes and
level of structural heterogeneity are shown to depend on the chemical functionality and flexibility of
studied PolyILs. Furthermore, these experimental and computational results provide new insight into the
correlation between conductivity and structure in PolyILs, suggesting that structural heterogeneity helps
to reduce the activation energy for ionic conductivity in the glassy state.