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In this study, the amphiphilic salt lithium trifluoromethanesulfonylimide octadecane (C18LiTFSI) was used as a basis to investigate the effects of anion density and cation coordination sites within blended electrolytes with strong ionic aggregation. C18LiTFSI was previously reported as a single-component, ion-condensed electrolyte with a wide layered liquid crystalline phase regime. Three additive molecules with varyingly sized polar sulfonyl groups attached to an octodecane-tail were synthesized and mixed with C18LiTFSI. The thermal properties, morphology, and ionic conductivity of the blended electrolytes were characterized. It was found that the blended electrolytes exhibited layered liquid crystalline morphology over a narrower temperature range than the pure salt, and the ionic conductivity of the blended liquid crystalline electrolytes were generally lower than that of the pure salt. Surprising, the additives were found to have the greatest effect on the bulk ionic conductivity of the semicrystalline phase of the electrolytes. Addition of minor fractions of methylsulfonyloctadecane to C18LiTFSI resulted in increases in conductivity of over two orders of magnitude at room temperature, while addition of ethylsulfonyloctadecane or isopropylsulfonyloctadecane with the larger head group resulted in decreased ionic conductivity over the entire composition space and temperature range investigated.
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Ionic Liquid Solutions Show Anomalous Crowding Behavior at an Electrode Surface
X-ray reflectivity was used to study the several-nanometer-thick “crowded” layers that form at the interfaces between a planar electrode and concentrated solutions of ionic liquids. The ionic liquid [P14,6,6,6]+[NTf2]− was dissolved in either strongly polar propylene carbonate or weakly polar dimethyl carbonate. In the range of 19–100 vol % ionic liquid, between working electrode potentials +2 and +2.75 V, uniform 2–7 nm thick interfacial layers were observed. These layers are not pure anions but contain three to five times as many anions as cations and about the same percentage of solvent as the bulk solution. On the other side of the layer, the density is that of the bulk solution. These features are inconsistent with a picture of the crowded layer as a region of pure, close-packed counterions. Not only the layer thickness but also the charge density decrease with increasing dilution at any given applied voltage. This appears to indicate, counterintuitively, that a thinner layer with lower net charge density will screen an electric field as effectively as a thicker layer with higher charge density.
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- Award ID(s):
- 1665255
- PAR ID:
- 10325728
- Date Published:
- Journal Name:
- Langmuir
- ISSN:
- 0743-7463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.langmuir.2c00036
