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Organic ionic plastic crystals (OIPCs) appear as promising materials to replace traditional liquid electrolytes, especially for use in solid state batteries. However, OIPCs show low conductive properties relative to liquid electrolytes, which presents an obstacle for their widespread applications. Recent studies revealed very high ion mobility in solid phases of OIPCs, yet the ionic conductivity is significantly (~100 times) suppressed because of strong ion-ion correlations. To understand the origin of the ion-ion correlations in OIPCs, we employed broadband dielectric spectroscopy, light scattering and NMR diffusion measurements in liquid and solid phases of Hexafluorophosphate - Diethyl(methyl)(isobutyl)phosphonium [PF6][P1,2,2,4]. The results confirmed significant decrease in conductivity of solid phases of this OIPC through ion-ion correlations. Surprisingly, these ionic correlations suppress charge displacement on rather long time scales comparable to the time of ion diffusion on the ~1.5 nm length scale. We ascribe the observed phenomena to momentum conservation in motion of mobile anions and emphasize that microscopic understanding of these correlations might enable design of OIPCs with strongly enhanced ionic conductivity.
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This content will become publicly available on March 28, 2026
The ion-ion correlations in organic ionic plastic crystal
Organic ionic plastic crystals (OIPCs) are emerging as promising electrolyte materials for solid-state batteries. However, despite the fast ionic diffusion, OIPCs exhibit relatively low DC conductivity in solid phases caused by strong ion-ion correlations that suppress charge transport. To understand the origin of this suppression, we performed a study of ion dynamics in the OIPC 1-Ethyl-1-methylpyrrolidinium bis (trifluoromethyl sulfonyl) imide [P12][TFSI] utilizing dielectric spectroscopy, light scattering, and Nuclear Magnetic Resonance diffusometry. Comparison of the results obtained in this study with the published earlier results on an OIPC with a completely different structure (Diethyl(methyl)(isobutyl)phosphonium Hexafluorophosphate [P1,2,2,4][PF6]) revealed strong similarities in ion dynamics in both systems. Unlike DC conductivity, which may drop more than ten times between melted and solid phases, diffusion of anions and cations remains high and does not show strong changes at phase transition. The conductivity spectra in the broad frequency range demonstrate unusual shapes in solid phases with an additional step separating fast local ion motions from suppressed long-range charge diffusion controlling DC conductivity. We suggested that in solid phases, anions and cations can jump only between the specific ion sites defined by the crystalline structure. These constraints lead to strong cation-cation and anion-anion correlations strongly suppressing long-range charge transport.
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- Award ID(s):
- 2417963
- PAR ID:
- 10600058
- Publisher / Repository:
- OAE publisher; open access journal
- Date Published:
- Journal Name:
- Energy Materials
- Volume:
- 5
- Issue:
- 7
- ISSN:
- 2770-5900
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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